Manufacturing Flexible Packaging
Manufacturing Flexible Packaging
Materials, Machinery, and Techniques
Thomas Dunn
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Introduction
Efficient and profitable delivery of quality flexible packaging
to the marketplace requires the design and manufacture of products that are both “fit-for-use” and “fit-to-make.” The engineering function for a flexible packaging operation must design
products and processes that deal with both challenges. The
alternative finds the company facing a stalemate in which
“Manufacturing can’t make what Sales can sell” and “Sales
can’t sell what Manufacturing can make.”
The scope of engineering knowledge inherent in designing
“fit-for-use” and “fit-to-make” products is broad:
• Chemical engineering and chemistry: underlying
principles of the resins, adhesives, inks, etc., that
serve as the raw materials for the industry’s products and of the foods, pharmaceuticals, and other
products packaged in them;
• Mechanical engineering: basics of both the equipment used to manufacture flexible packaging materials and the machines used by its customers to
form and/or fill and/or seal that material;
• Materials science: principles explaining how flexible packaging materials respond to environmental
forces during their manufacture and use.
Ideally, the engineering function company of a company
takes on the characteristics of a “learning organization”. . .
. . .organizations where people continually expand their
capacity to create the results they truly desire, where new
and expansive patterns of thinking are nurtured, where
collective aspiration is set free, and where people are continually learning to see the whole together [1].
xiii
xiv
INTRODUCTION
This book attempts to both introduce the engineering student
or novice to basic manufacturing steps used to manufacture
flexible packaging and suggest how to find and use additional
learning resources. Effective participation in the industry
requires continual learning. While the pace of change in the
industry does not match those based in newer technologies,
such as telecommunications and electronics, competitive
advantage lies in early recognition and application of material
and equipment innovations. Failure to do so leads to professional and organizational stagnation.
The book focuses on “fit-to-make” design challenges of flexible packaging manufacturing. It addresses various manufacturing processes individually, but the reader should appreciate that
a great many flexible packages require multiple processes, carried out in sequence, to manufacture a fully functional fit-foruse product. Each process has an input material(s) to which it
adds some value. That value-added material may well serve as
the input to subsequent processes, and so on until a fit-for-use
product is ready for market. The industry uses various “secondary quality characteristics” (Chapter 32) to measure interim
and final products. Such test methods attempt to predict the
ability of the final product to function successfully for its customer. The methods often provide the basis for contractual
“specifications.”
Background
Flexible packaging manufacturing is part of the “converting”
industry. The expression comes from the paper industry’s use
of the term to describe secondary processes associated with a
paper mill that “converted” the mill’s large rolls of paper into
smaller products such as envelops, bags, and sheets for writing
or printing. This book approaches the industry as essentially a
manufacturing process. Its physical inputs are rolls of paper,
plastic film, and aluminum foil, various fluids such as inks and
adhesives, and some plastic resins. Its outputs are functional
packaging materials ready to contain various products. The
INTRODUCTION
xv
simple input/output model of a process is expanded by the
“5Ms” model of manufacturing. Manufacturing requires materials, manpower, machines, methods, and measures.
Value-added
benefits
CPG
Co.
Appearance
barrier
containment
The book is organized to present “methods” first (Chapters
1 9), then “machines” (Chapters 10 18), “materials”
(Chapters 19 29), and finally measures (Chapters 30 32). The
subject matter of each chapter could take up an entire academic
term of study, but the objective here is to provide enough
familiarity with the subject that its relationship to the others is
appreciated and a deeper study of its details is possible.
The larger converting industry includes manufacturing processes to produce products other than flexible packaging such
as paper and plastic bags, pressure-sensitive tapes and labels,
articles fabricated from nonwoven fabrics, and decorative laminates. These all involve “web handling processes,” some basics
of which Chapter 1 addresses. The book uses the terms “web,”
“substrate,” and “roll stock” interchangeably. The below
figure provides a very simplified summary of manufacturing
processes for raw material roll stock used in flexible
manufacturing. In depth, teaching resources exist for these raw
materials processes. This book only considers raw material
manufacturing in the context of how the converting industry
uses them.
xvi
INTRODUCTION
Web
••Wood pulp
••Ore
••Plastic resin
Raw material
••Col fabrication
molding
••Rolling
••Extruding
Roll stock
••Clay coat
••Print prime coat
••Orient/metallize
••Foil
••Film
Enhancement
(examples)
Much of the appeal of flexible packaging comes from the
versatility of the industry’s products, from simple plastic bags
that serve as little more than a container to keep out dust, to
complex “retort pouches” in which microbe-susceptible food is
sterilized under intense heat and pressure and then preserved
for years in a safe and suitable state. The application range of
the industry’s products results in large part from the ability to
combine multiple materials in laminating processes (Chapters 4
and 5).1 The term “structure” is used to describe the resultant
material.
Reference
[1] P. Senge, The Fifth Discipline: The Art and Practice of the
Learning Organization, second ed., Doubleday, New York, NY,
1990, 413pp.
1
Although such composite materials can provide a great deal of package functionality, their
nature precludes simple recycling of their components into subsequent use in identical form.
1 Basics of Web Processes
Chapter Outline
Web Tension
Web Winding
Cross-Web Variation
Web Dimensional Analysis
Industry Units of Measure
Web Length Estimation
Roll Rewind Designation
2
3
5
9
9
11
12
Essentially all flexible packaging converting processes involve
rolls of web materials (thin materials, manufactured and
processed in the form of a continuous, flexible strip). The full
length of the strip represents the “machine direction” and its
width, the “cross direction”. Equipment pulls material from the
roll and then modifies it in some way that increases its suitability
for use as a package. If the eventual fit-for-use packaging material
requires several converting processes, the equipment will rewind
the modified material into roll form again. The basic flexible
packaging converting processes are printing, laminating, and slitting. The modifications at each stage are generically called
“value-adding” processes and they form the basis for converters’
selling margins over their costs of purchased raw material.
Web handling in general reflects a dynamic, but otherwise
simple, model of Newton’s laws of motion:
1. Any object in a state of uniform motion tends to
remain in that state of motion unless an external
force is applied to it.
2. Force applied to a material accelerates it in direct
proportion to its mass; the direction of acceleration is the same as that of the applied force.
Manufacturing Flexible Packaging. DOI: http://dx.doi.org/10.1016/B978-0-323-26436-5.00001-1
© 2015 Elsevier Inc. All rights reserved.
1
2
MANUFACTURING FLEXIBLE PACKAGING
3. For every action there is an equal and opposite
reaction.
These and related “laws” of classical mechanics make web
handling a model system for mechanical engineering science to
understand and control web processes. The physical and mathematical relationships developed by this science to describe and
design web processes involve several sophisticated concepts.
This chapter can only highlight some of the powerful insights
of the science when designing and operating web handling
equipment, but the interested reader can find more detailed
sources in the further reading chapter.1
Web Tension
“Pulling” a web off an unwinding roll and rolling it onto a
rewinding one presents major mechanical issues. A force must
be applied to the unwinding web. The general term for this force
is called “tension.” Tension in web converting is often expressed
in terms of “pounds per linear inch (PLI).” The units reflect the
actual force pulling the web divided by its width (without regard
to the thickness of the web material). Web process conditions
typically report only this value. To better understand the physical effects of tension on a material, its “tensile stress” must be
recognized. Instead of force per unit width, this measure
addresses force per unit area, “pounds per inch2.” This value
relates directly to laboratory measures of tensile properties, an
“Intrinsic Property” of the web material (Chapter 31).2
Tension applied to a web may not only pull it off the unwind
roll, but also stretch it, or even break it (depending on the
web’s tensile properties). Flexible plastic films in particular
have tensile and elongation properties that can result in
diversion of some of the applied unwind force to stretching the
film (Figure 1.1). When cross-web length variation (called
1
The teaching and research facilities of the Oklahoma State University Web Handling
Research Center provide best-in-class access to the theory and practice of web handling;
http://webhandling.okstate.edu/.
2
http://www.webhandling.com/WHMain.
1: BASICS
OF
WEB PROCESSES
3
Figure 1.1 Distortion of plastic film in response to applied force.
Force is applied to web at an unwind, the web resists, the roll turns,
releasing web, at the same time the web itself deforms and reshapes.
“bagginess”) is present, the stretching force can sometimes
“pull out” the bagginess, so that the web appears to lie in a flat
plane to observers as well as to the mechanics of the valueadding processes.
In addition to moving the web through the equipment to the
unwind, tension on the web helps to resist side to side movement, to reduce drooping (“catenary” effect) in horizontal
spans between supports, and to establish friction against rollers
along the web path and in the rewinding roll itself.
Web Winding
The rewind roll of a web process represents a protective
means of storing the web for subsequent use in converting or at
an end user. Consideration of the winding step itself reveals
many of the additional mechanical considerations critical to
successful web processes. Consistent winding of an excellent
roll involves three critical factors at the rewind: Tension of the
web as it wraps onto the roll; Nip pressure of drum or roller
that presses down on the winding web; Torque of the rotating
roll as it winds more web material onto itself. Controlling various combinations of “T N T ” factors at different points along
the whole web process provides the essence of its design and
operation. Tension was described above. Torque is simply a
4
MANUFACTURING FLEXIBLE PACKAGING
“turning” force, which is the one acting in a clockwise or
counterclockwise, rather than a linear direction. Nip represents
a point along the process at which two rollers contact the web at
the same time. One or both of these rollers are “driven,” that is,
having torque applied to them (using a DC motor, a fluid motor,
or a slip clutch). Surface friction between the roll(s) and web’s
surface controls the web’s speed, lateral position, tension, etc.
Excellence for web winding (called “good roll formation”)
implies an overall cylindrical shape (i.e., circular crosssection), lateral alignment of web edges on both sides (i.e.,
even from core to top of roll), and centered placement on the
core. Because the roll itself represents a convenient interim
storage state for the web, it must of course be “unwindable.”
“Blocked” is the term used to describe the condition in
which one wrap of the web on a roll adheres to an adjacent
one. Blocking prevents unwinding and often tears the web.
Block-prone webs require a thin layer of air between wraps of
a roll, generally referred to as winding a “soft” roll. Tensile
and surface properties of the web mean different TNT combinations provide optimum roll formation for a particular web.
Three types of winding processes can adapt to the range of
properties anticipated in a particular industry:
1. Center winding: a rotating rewind shaft turns the
core that holds the winding roll in order to apply a
tension to the web.
2. Surface winding: rotating drum(s) on the surface
of the winding roll apply tension to the web.
3. Centersurface winding: both rotating rewind
shaft and rotating drum apply a tension to the web.
Table 1.1 summarizes how these winding processes apply
the TNT factors to webs.
All along web processes, TNT factors control movement of
the web through the overall process. Many value-adding steps
themselves represent nip points (e.g., applying inks, adhesives;
laminating webs to one another). The combination of friction,
adhesion, and lubrication between a web and any roller
1: BASICS
OF
WEB PROCESSES
5
Table 1.1 Comparison of Winding Processes
Winding
Process
Center
Winding
Surface
Winding
CenterSurface
Winding
Generic
winding
type
Nip present
Torque
Gap
Torquegap
Lay on roller 1/2 driven drum
rollers
Spindle shaft Drum roller(s)
Driven lay on
roller
Torque
Spindle/drum
applied to
roller
Web tension Torque from Nip with drum(s) Nip lay on roller
source
spindle
Roll hardness Tension
Nip at drum
Torque from
control
and nip
spindle
Roll hardness Softer
Harder
Softer
range
Typical web Plastic films Inelastic
mslip and
materials
mdiameter
surfaces is called “traction.” Traction between a web and a
roller along the process can transfer some of the web’s energy
to the roller and cause it to rotate if the roller is properly lubricated. Traction at a nip changes the tension of the web relative
to the force between nip rollers, the torque on driven rollers
and friction between web and rollers.
Cross-Web Variation
Rollers and webs with exactly uniformly flat surfaces across
their widths do not exist in industry. Plastic films in particular
are prone to “gauge bands,” that is, a machine direction strip
with thickness consistently higher than to either side of it.
When wound in a roll, this thicker strip will wind on itself,
wrap after wrap, until a noticeable “hard” band appears across
the width of the roll. As Figure 1.2 suggests, if forces acting on
6
MANUFACTURING FLEXIBLE PACKAGING
Figure 1.2 Thick band in a web can stretch adjacent areas into
“Pucker lanes” if would tightly in roll form.
Figure 1.3 Image of parallel pucker lanes in a plastic web. (thick band
between these is not visible.)
the film near this strip become great enough, the elastic limits
if the film (see Chapter 31) of the material can be exceeded
causing permanent deformation of the material on either side
of the strip. Figure 1.3 provides an image of this defect. In
effect, the greater roll circumference at the hard band forces
adjacent material to stretch so much that it becomes permanently deformed (i.e., effectively wider than the nominal width
and thinner than the nominal thickness).
Such deformed areas result in “baggy” film. Uniform tension
across the width of a web can lead to folds in these baggy
areas. In the case of elastic films, increasing machine direction
tension forces can “pull out” the bagginess within limits. In this
case the material with nominal dimensions is elastically
stretched enough match the dimensions of the baggy regions.
1: BASICS
OF
WEB PROCESSES
7
υ1
υ1
rpm
rpm
υ2
(A)
d c
(B)
d′
(D)
(C)
C = πd
υ1 = RC per min.
c′
C′ = πd′
υ2 = RC′ per min.
d′>d, so C′>C, and υ2>υ1
Speed of film on taped side is greater
Figure 1.4 Band of tape around a roller causes edge of web at that
location to travel faster than in un-taped areas (used if web has a
“Baggy” edge).
This provides consistent tension at critical value-adding locations. Down-machine of such locations, tension can be lowered.
Several techniques are available to vary cross-direction tension.
These utilize transfer of energy from rollers to webs through traction forces. Figure 1.4 presents the principle in a simple model. In
“part A,” a web travels past a roll at velocity ν 1 while the roll
rotates at a certain frequency, “R.” Part B indicates that ν 1 equals
the circumference of the roll (C): π times its diameter (d) times R
(revolutions—i.e., circumferences—per minute). A strip of tape
placed on the roll (part C) increases the diameter of the roller for
the width of the tape to d0 , its circumference to C0 (part D) and
because the rotational frequency of the roll with tape on it does not
change, the velocity of the web over the tape increases to ν 2. If the
edge of the film with velocity ν 2 is baggy, this additional speed
exerts additional force and pulls out the baggy edge. This analysis
assumes that web to roller traction matches that of web to tape.
The lack of cross-direction uniformity causes traction forces
that move the web laterally. Cross-web variations and roller
shafts not parallel to the equipment’s centerline result in traction forces that cause moving webs to move to one side or the
other. In effect, a web’s entry angle relative to a roller will be
parallel to direction of the roller’s surface motion. This angle
8
MANUFACTURING FLEXIBLE PACKAGING
Flat expander roller
Flex expander roller
Bowed roller
Figure 1.5 Shaped rollers to “pull out” wrinkles or baggy lanes in webs
may not be perpendicular to equipment centerline for
several reasons: roller misalignment, roller diameter variation,
web bagginess, nip pressure variations, and forces external to
the equipment (e.g., airflow, coatings). Tension resulting traction causes such lateral shifts to occur as the web enters the
(down-line) roll rather than exiting the (up-line) roll.
Passive techniques deliberately intended to guide a web use
roller surfaces not parallel to the equipment’s centerline both to
spread webs so to eliminate wrinkles or baggy lanes as a web
enter value-adding locations (Figure 1.5) and to direct webs
into different directions (e.g., otherwise parallel lanes of web
slit from as master web; see Chapter 6 and Figure 1.6).
Active monitoring and correction of the web’s lateral position
is critical. “Web guides” (1) detect the edge of the material with
web guide sensors, (2) calculate the deviation from the actual
edge position to a reference position, and (3) move the material
to the required position with a “steering frame actuator” (i.e., a
movable frame able to vary the axis of a steering roller relative
to the centerline of the equipment). In practice additional web
guiding occurs by moving a roll’s unwind stand to keep the web
close to a reference position when entering the equipment and
by moving the rewind to “chase” actual position of the web as it
exits the line. While all such movements could of course be
adjusted manually, integrated system using hydraulic positioners
or electric drives here provide appropriate adjustments.
1: BASICS
OF
WEB PROCESSES
9
Figure 1.6 Passive direction control with shaped rollers.
Web Dimensional Analysis
The convenience and utility of web storage in roll form has
given rise to a variety of metrics used in web converting industries. The weight, outside diameter, and width of a well-formed
roll are readily measured.3 From these three roll measurements,
two unique web metrics are derived: (1) yield (area of web per
unit weight) and (2) basis weight (weight of web per standard
area). These two are the arithmetic inverse of each other, but
industry usage conventions require close attention to dimensional values (Table 1.2 reflects terms used in the flexible
packaging industry).
Industry Units of Measure
The importance of “yield” and “basis weight” measures in
the industry result from manufacturing and commercial considerations. Historically, most raw materials, including rolls of
webs, for the industry are bought and sold by weight (lb or kg)
while linear (feet or meters) measures quantify manufacturing
production. When value-added products are sold by area
3
“Good roll formation” allows ready measurement of a web’s width on a roll (lateral alignment of web edges on both sides), and a circular cross-section permits accurate estimation of
the volume of material on the roll.
Table 1.2 “Yield” and “Basis Weight” Units of Measure for Flexible Packaging
System
US Units (Abbreviation)
US to Metric
Conversion
Metric Units
(Abbreviation)
Metric to US
Conversion
Thickness
1023 inch (mil) and
1025 inch (gauge)
103 square inches
per pound (msi/lb)
500 sheets each 24 by
36 inches (.432,000
square inches 5 3000
square feet)
pounds/ream (lb/rm)
mil 3 25.4 5 micron
1026 meter (micron)
micron 3 0.394 5 mil
msi/lb 3 1.422 5
m2/kg
N/A
square meter/kilogram
(m2/kg)
N/A
m2/kg 3 0.703 5 msi/lb
lb/rm 3 1.627 5 gsm
grams/square meter
(gsm)
gsm 3 0.6145 5 lb/rm
Yield
Ream
Basis
weight
N/A
1: BASICS
OF
WEB PROCESSES
11
(square feet or meters), planning raw material purchases and
necessary manufacturing times requires reliable basis weight to
yield conversions. For example:
1. 14.4 pounds of low-density polyethylene “yields”
about 3000 square feet of 1 mil thick film (0.2 kg of
low-density polyethylene “yields” about 1 square
meter of 25.4 µ thick film).
2. The web width (in feet or meters) times the production rate (in feet or meters per minute) indicates the
area (square feet or square meters) of product produced per minute. The total area of product ordered
divided by this production rate estimates the length
of production time needed for the order.4
In Step 1 here, the area to weight conversion assumes that
the actual thickness of the web matches its nominal thickness.
If it is thicker than nominal, a given roll weight will yield less
area than expected, that is, material purchased by weight will
produce less product to sell by area.
Web Length Estimation
Figure 1.7 reviews how web length on a roll can be estimated
from the outside radius of the roll, the outside radius of its core,
and the web thickness. The area of the torus (ring-shape) represented by a roll’s cross-section (i.e., the difference in area of the
circle defined by the roll’s outer surface minus the outer circumference of the roll’s core) divided by the web’s thickness estimates the length of web wound on the roll. The calculation does
not take into account air entrapped between layers or compressible webs such as paper or nonwoven fabrics. The relationship is
also useful in estimating the average thickness of a web by measuring the width (using a simple rule or tape), weight (using a
scale of sufficient capacity), and length (usually recorded by the
web processing equipment) of a roll.
4
In practice, such estimates must include considerations of waste and nonproductive time
(see Chapter 8).
12
MANUFACTURING FLEXIBLE PACKAGING
Length of web = (area of roll minus area of outside of core) divided
by thickness
=π* ([rR2]-[rC2])/t
rR = outside radius of the roll
rC = outside radius of the core
t = thickness of web
rR
rC
Figure 1.7 Web length calculation from roll values.
3
Y
6
COPY
7
C
O
PY
8
PY
COPY
5
CO
COP
COPY
4
PY
2
CO
1
COPY
Figure 1.8 Numerical designations for various rewind configurations.
Roll Rewind Designation
Subsequent use of material in a roll requires recognizing the
spatial relationship of the web (its beginning, end, and any differences of one surface to the other) to the roll. Standard terminology across all web processes describes this relationship.
Figure 1.8 indicates that when a web surface with a special
condition (e.g., coated, electrostatically treated, or printed) is
wound with that surface out, it has a designation 1 through 4,
or simply “coated’ (etc.) side-out). That surface wound toward
the inside of a roll has a designation 5 through 8, or simply
“coated’ (etc.) side-in). Numerical designations are particularly
needed for printed webs so that the intended image is properly
oriented when the material is fabricated into a bag or package.
2
Rotogravure Printing
Chapter Outline
Gravure Process
Gravure Cylinders
Halftone Image Reproduction
Ink Metering
Gravure Process Innovation
Cylinder Cost and Cycle Time
Work Practices
Reference
14
14
16
20
22
23
24
25
At present, most flexible packaging is printed using either the
rotogravure or the flexographic (Chapter 3) printing method.
Rotogravure (or simply, gravure) is the more mature package
printing process. It enjoys widespread use around the world.
Basic gravure process technology is also used for magazines
and other publications, catalogs, newspaper supplements,
labels, cartons, gift wrap, wall/floor coverings, and a variety of
precision coating applications (see Chapter 4).
Gravure printing for packaging provided the ability to reproduce detailed text information in small font sizes as well as
excellent photographic-like reproduction of products, serving
suggestions. The process gained widespread appeal for opaque
flexible packaging (especially materials incorporating paper
and aluminum foil). Its durable metal printing media were well
suited for large production runs (500,000 impressions and
more). The time and expense to produce or modify these media
also favored stable package graphics with infrequent changes.
Greater market segmentation (i.e., smaller target market sizes)
and changeable product messaging for global consumer product
goods have stimulated significant recent innovation in gravure
Manufacturing Flexible Packaging. DOI: http://dx.doi.org/10.1016/B978-0-323-26436-5.00002-3
© 2015 Elsevier Inc. All rights reserved.
13
14
MANUFACTURING FLEXIBLE PACKAGING
Figure 2.1 Gravure printing basics.
press equipment. The innovation has done much to lower the economic order size for gravure-printed packages.
Gravure Process
Gravure printing requires a cylinder engraved with hollow
“cells” (precisely shaped depressions in the cylinder’s surface)
on the order of 0.001 0.002 inch (0.025 0.05 mm) in diameter. Cells’ distribution and volume determines the lightness/
darkness of particular image area. The printing equipment fills
the cylinder’s cells with ink. Ink transfers to the substrate in a
pattern matching the cell pattern as the cylinder presses against
(i.e., “nips”) the substrate (Figure 2.1).
Gravure Cylinders
Because the cylinder directly contacts both ink supply and
printed substrate, storing, handling, and using them require
extreme care. Traditional cylinders have a steel core with a
layer of copper electroplated on it. The cells for carrying ink
are engraved into this relatively soft copper layer and a layer
of protective chrome electroplated over the entire surface.
The printed image consists of hexagonal-shaped (or similar)
cells that are engraved into the copper cylinder using a computerdriven engraving head with diamond tip (electromechanical
2: ROTOGRAVURE PRINTING
15
Figure 2.2 Magnified view of gravure-printed lower case “h”. Note
scalloped edges reflecting the engraved cells of the printing cylinder.
method). The former “chemical etching” process is rarely used
now. In it, the cylinder is covered with a gelatin photo-resist (a
water-sensitive, fibrous paper, coated with a smooth gelatin layer,
which has been sensitized to light by submerging it in a bath of
potassium bichromate and water). The cylinder is then exposed to
UV light to harden the gelatin resist. Finally corrosive ferric chloride solutions of varying strengths etch printing cells into the copper layer of the cylinder where it is not protected by the hardened
gelatin. These two traditional engraving methods leave distinct
semicircular (scalloped) edges on lines (Figure 2.2). Recently,
direct laser engraving into metallic cylinders with high reproducibility and pulse energy stability is possible with high power laser
systems. The method can vary the diameter, depth, aspect ratio,
and shape of each cell independently using digital image data [1].
Engraving thin-walled metallic sleeves that slide over reusable
press cylinders represents the latest cylinder preparation
technology.
16
MANUFACTURING FLEXIBLE PACKAGING
Halftone Image Reproduction
This discussion on halftone image reproduction applies to all
printing processes. Its introduction in this rotogravure chapter
reflects the history that the gravure process itself utilized the concept
for mass production of “photographic quality” images for packaging
and many other products well before other roll-to-roll techniques.
The requirements for halftone printing presented here provided an
ideal match to gravure’s cell-by-cell application of ink to substrates.
Reproducing photographic-like images presents all printing
processes with a crucial test: producing a continuous range of
grey tones or color values from a discrete number of printed colors. Printing methods meet the challenge by tricking the eye into
perceiving a pattern of discrete dots as a blended range of
smooth shade transitions (Figure 2.3). At any given distance, an
Figure 2.3 Right: Gray-tone variation accomplished with black dots of
varying size printed on white background. Left: Magnified view of
varying black dot sizes that cause the effect at right.
2: ROTOGRAVURE PRINTING
17
observer loses the ability to distinguish individual dots from one
another as the size of those dots decreases. If arranged properly,
the dots appear in the aggravate as a shaded continuum.
Black dots on white backgrounds appear as shades of grey
(Figure 2.4). Colored dots on white backgrounds appear as lighter or darker shades of that color. Printing different colored dots
in close proximity appears to the eye as a wholly differently
color depending on the effects of light filtering by the ink.
“Properly arranging” in this sense implies dependably printing
the planar location and thickness of those dots. A rigorous prepress analysis of an image by a “screening” process determines
this arrangement, given the colors of ink available and the
Figure 2.4 Top: Photograph-like image of mango accomplished by
Halftone gravure printing. Bottom: Magnified view of dot-pattern at
border of leaf and mango fruit in top image.
18
MANUFACTURING FLEXIBLE PACKAGING
Table 2.1 Secondary Additive Colors (5Primary Subtractive
Colors)
Additive Color
Plus Additive Color Appears
Red
Green
Blue
Yellow
Magenta
Yellow
Magenta
Cyan
Cyan
Table 2.2 Primary Additive Colors from Subtractive Colors
Subtractive Color
Minus Subtractive Color Appears
Cyan
Magenta
Yellow
Red
Green
Blue
required range of tonal variation. The term reflects its historical
development involving literal use of cross-line screens to subdivide the image into a regular grid of dots varying in size. In this
sense, the dots are analogous to the concept of the film grain
resulting from the small particles of metallic silver, or colored
dye clouds, in emulsion-coated photographic film. The printed
images are generically called “halftone images.”
The additive system of color involves mixing light of the
three primary colors (red, green, and blue—“RGB”) in various
proportions to obtain specific color values. The combination of
two of the standard three additive primary colors in equal proportions produces an additive secondary color—cyan (green
1 blue), magenta (blue 1 red), or yellow (red 1 green)
(Table 2.1). This second group of three colors (abbreviated
“CMY”) comprises the standard primary colors of the subtractive color system. Only one of the primary colors (red, green,
or blue) remains when one of the subtractive primary colors
(cyan, magenta, or yellow) has been removed from an image
(Table 2.2). Simply inserting a filter of the subtractive color to
be removed between the source of light and its receptor
2: ROTOGRAVURE PRINTING
19
Figure 2.5 White light passing though colored layers of transparent
ink reflect off a white background before seen by a viewer's eyes.
The two transits through CMYK (subtractive) ink colors (first incident,
then reflected) absorb wavelengths of light except for the
corresponding (primary) colors, RGBK.
accomplishes this. A printed layer of transparent ink serves as
such a filter for halftone images.
Combining halftone printing with subtractive color theory
provides the ability to print “continuous-tone” images with
almost photographic quality. Ambient (white) light from the
environment passes through the layers of C, M, and Y colored
inks, reflects from a white (e.g., paper) surface though the ink
layers again giving the intended effect of the additive RGB colors and tones in between. In practice, a black (K) ink provides
the image’s zero-light reflecting regions because the subtractive
ability of the other three is less than complete (Figure 2.5).
The optics of Figure 2.3 demand careful control over three
dimensions of the printed ink: (1) machine direction and
(2) cross-direction of ink on the substrate, which define its
contribution to the halftone effect, while the (3) depth of the ink
layer itself (along with the ink’s “color value”—discussed in
Chapter 3) determines the quality and quantity of white light filtering occurring while light travels into and back out of the ink
20
MANUFACTURING FLEXIBLE PACKAGING
layer. Reducing variability of these three dimensions serves as a
major objective for increasing the productivity, quality, and output of rotogravure presses. Ink layer depth of course depends on
the volume of liquid ink deposited from cells onto the substrate.
Depending on ink and substrate, “electrostatic” pinning may be
employed to create an electric potential between engraved cylinder and substrate to assure complete release of the cell’s ink volume onto the substrate. While quality of both ink and substrate
affects print results, a gravure press itself has only minimal ability to adjust for variability in these raw materials.
Ink Metering
The term “meter” in printing industries generically refers to the
controlled application of fluid ink to the print media that is to eventually transfers the ink to a substrate. In this sense, a fluid is a substance that continually flows under an applied “shear” stress (i.e.,
at least partially lateral, as distinct from compressive or tensile
stresses, which act perpendicularly to a surface). An ink’s tendency
to flow (from high, e.g., a “liquid” ink, to low, e.g., a “paste” ink)
depends on its composition. Components include three categories:
1. Pigment1: the color itself.
2. Binder: a chemical matrix capable of adhering the
ink to a substrate and holding the pigment in a
three-dimensional matrix.
3. Diluent: a chemical capable of (i) causing the
complete ink mixture to be sufficiently fluid during the printing process and then (ii) changing
physically (e.g., evaporating) or chemically (e.g.,
cross-linking) so as to render the printed image a
solid layer (“ink film”) on the substrate.
1
Printing inks utilize either pigments or dyes for providing color. Dyes are soluble in a solvent (typically water). When printing a porous substrate (e.g., paper) with a dye-based ink, the
colored solution can penetrate into the substrate to some depth. In contrast, pigments are dry,
solid colorants, usually ground into a fine powder. This powder is added to a binder that suspends the pigment in an ink’s fluid form. Variations of both exist, but generally, pigments provide more color stability.
2: ROTOGRAVURE PRINTING
21
Figure 2.6 Cross-section of doctor blade metering ink on gravure
cylinder.
Additional modifiers may be added to the ink formulation to
give desired functionality to either the fluid or the dried ink.
Each combination of printing process, substrate, and print durability presents its own challenge in formulating an ink from
components optimal for that use. The industry uses a trial and
error process, guided by chemical and physical theory as well
as experience to match an ink formulation to any given application. The industry has advanced to a point allowing quantitation of critical fluid ink characteristics in order to predict
consistent print quality results from the printed ink.
Metering ink into the cells of a gravure cylinder requires that
it flows into the cells, filling them completely. Gravure inks
are liquids that will flow readily in response to gravity, electrostatic forces, or capillarity. Figure 2.6 indicates how a gravure
press controls the amount of ink in each engraved cell:
• The engraved roller rotates though an “ink pan”
filled with one ink color.
22
MANUFACTURING FLEXIBLE PACKAGING
• Liquid ink flows into each cell. A convex meniscus
results over the top edges of cells because the particles in the ink have a stronger attraction to each
other (cohesion) than to the material of the cell
(adhesion). Ink may also adhere to un-engraved
portions of the cylinder.
• Excess ink over the top edges of cells and on the
un-engraved cylinder surfaces is “wiped” from the
cylinder by a “doctor blade” (“doctor” here is a
corruption of the German “ductor”).
• Ink wiped by the doctor blade falls back into the
ink pan.
A “chambered doctor blade” assembly provides a more
refined and controllable configuration of the basic ink metering
as shown in Figure 2.6. The assembly isolates the ink and doctor blade from the environment, reducing the amount of ink
solvent that can evaporate into the atmosphere. In a chambered
doctor blade system, coating is pumped into the chamber and
disbursed to the engraved roll with the help of a pair of doctor
blades: the metering blade and the containment blade. The
metering blade wipes the excess coating from the roll while the
containment blade keeps the coating from leaking out of the
chamber (Figure 2.7). End seals keep the coating from leaking
out the ends of the chamber.
Gravure Process Innovation
Markets for packaged consumer goods and improved competing printing technology have taken market share from gravure in recent years. The industry has responded with major
redesigns of equipment and work practices to increase affordability of the process. Gravure’s inefficiencies principally result
from cylinder costs and job changeover times (see Chapters 8
and 9). Both add to the fixed costs of any given job. By
decreasing either or both, the productivity and yield of gravure’s variable costs can keep the process competitive.
2: ROTOGRAVURE PRINTING
23
Figure 2.7 Chambered doctor blade assembly.
Cylinder Cost and Cycle Time
The cross-section of a gravure cylinder. In Figure 2.8 provides a reference for innovation efforts to lower gravure costs:
•
•
•
•
Steel base
Copper layer
Engraved cells
Chrome layer
The reusable steel base has remained relatively constant in the
industry.2 Traditionally, copper and chrome layers from a used
set of cylinders must be removed before engraving a new set of
images. However, the new direct laser engraving process is
designed to image a zinc layer on the copper plating. This process
2
Note that the “repeat length” (also known as the “cutoff”) of one package impression (usually the top-to-bottom length of a package) must correspond to the circumference of a finished,
engraved cylinder (Note: Integral multiples of the repeat length may also be used) so that each
rotation of a cylinder corresponds to printing that color for one package cutoff (although multiple images are often printed across the web with each rotation). As a consequence sets of steel
cylinder bases, of various circumferences, must be maintained by the printer or his gravure cylinder supplier.
24
MANUFACTURING FLEXIBLE PACKAGING
Figure 2.8 Cross-section of gravure cylinder.
allows reuse of the expensive copper plating with only zinc and
chrome removed for each new job. The metal plating processes
have been automated with robots and electronic controls, allowing essentially 24/7 production of cylinders with no labor costs.
Electromechanical engraving, itself faster than the previous
photo-resist/chemical etching sequence, is now subject to challenge to direct laser etching because of engraving speed and control of cell geometry and size. The laser technology is analogous
to laser imaging technology used for photocopiers and digital
“laser-jet” printing. The compatibility aids in reducing the time
needed for “prepress” design, review, revision, and approval.
New thin-walled metallic sleeve engraving processes offer
both cost and time advantages. Sets of base cylinders of given
diameter can be in use in press while engraving sleeves for a
new job requiring that diameter. This lessens the inventory cost
of cylinder bases. The ability to proceed to engrave sleeves
while base cylinders are in use shortens turnaround time
between artwork approval and actual production.
Work Practices
The flexible packaging converter’s “uptime” for any given
value-adding line is a critical measure of productivity (Chapter 9).
Historical equipment design required an essentially sequential
approach to the job of changing from one print job to the next:
•
•
•
•
Finish current production
Clean and remove print cylinders
Clean press parts of previous job’s ink remnants
Remove and/or reposition reservoirs of various ink
colors.
2: ROTOGRAVURE PRINTING
(A)
(B)
1-Finish
former
job
i-Clean/
remove
cylinders
2-Clean/
remove
cylinders
3-Clean
ink
remnants
ii-Clean
ink
remnants
iii-Remove
ink
reservoirs
25
4-Remove
ink
reservoirs
iv-Install
new ink
reservoirs
Trolley preparation
5-Install
new ink
reservoirs
6-Install
engraved
cylinders
v-Install
engraved
cylinders
7-Thread
web in
press
8-Register
each color
to others
-Produce!
2-Thread
web in
press
3-Register
each color
to others
-Produce!
1-Install
prepared
trolleys
1-Finish
former
job
Figure 2.9 Removable trolley system to reduce changeover times.
• Install ink reservoirs and related hardware for new inks
• Install new cylinders
• “Thread” new job’s substrate though press (each
print station)
• “Register” each color to each of the others
• Begin new production
Figure 2.9 suggests how modern gravure press design allows
converters to perform the demanding cleanup and setup of each
ink station for a press. In parallel while the current job is still in
press. Specialized “setup crews” work on interchangeable trolley assemblies while the press itself continues to print. A given
trolley holds a print cylinder and its corresponding ink reservoir,
ready to be rolled into place, as soon as the current job finishes.
With trolleys in place only threading the web through the press
and registering colors to one another remain before new production begins. Such arrangements reduce down-time between jobs
from as much as 90 min per color station (12 h for an 8-color
press) to 90 min between final good print from the previous job
and initial good print on the next job.
Reference
[1] G. Hennig, K. Selbmann, S. Mattheus, R. Kecke, S Bruning,
Laser precision micro fabrication in the printing industry,
J. Laser Micro/Nanoeng. 2 (1) (2006) 89 98.
3
Flexographic Printing
Chapter Outline
The Flexo Process
Numerical Color Space
Flexo Ink Metering
Flexo Halftone Printing (Process Printing)
Flexo Process Innovation
Reference
28
28
32
33
35
37
At present, most flexible packaging in the USA is printed using
the flexographic printing method. Flexography (or simply,
flexo) is a rapidly growing package printing process. It currently is taking market share from gravure. Basic gravure process technology is also used for corrugated boxes, flexible
retail and shopping bags, food service bags and sacks, milk and
beverage cartons, pressure-sensitive labels, disposable cups and
containers, and envelopes and wallpaper.1
Flexo printing for packaging provided the ability to reproduce basic text information and large, single-color areas. The
process gained widespread application for transparent flexible
packaging (initially cellophane and eventually plastic films). Its
inexpensive rubber or polymeric printing plates were well
suited for smaller production runs (100,000 impressions and
less). The minimal time and expense to produce or modify
plates also favored regional product brands and promotional
graphics for short-term use with larger brands.
Forces that led to innovation in gravure processes and equipment also stimulated flexo changes that dramatically increased
quality and reproducibility. The changes increased the cost of
flexo printing plates but still maintained an economic advantage over gravure for all but very large press runs.
1
Western Michigan University provides an overview of flexo printing at its Overview of
Printing Processes site: http://www.wmich.edu/pci/flexo/pp1.htm.
Manufacturing Flexible Packaging. DOI: http://dx.doi.org/10.1016/B978-0-323-26436-5.00003-5
© 2015 Elsevier Inc. All rights reserved.
27
28
MANUFACTURING FLEXIBLE PACKAGING
The Flexo Process
Flexo printing involves a “plate cylinder” covered in whole or
in part with raised flexible print media that carry ink for an
image. The media then deposit that ink onto the substrate.
Originally, the media was made of rubber sheets pressed into
rigid molds and vulcanized. A metal (e.g., magnesium) plate
exposed with the negative of the image to be printed and
developed in an acid bath creates the initial mold. This metal
relief plate makes the mold for commercial use (usually in
Bakelite plastic). A rubber or plastic sheet material is vulcanized
(by pressing it into the metal mold under controlled temperature
and pressure) to create the printing plate. Alternatively, a
“photopolymer” material exposed with the negative of the image
to be printed creates the print media directly. Light not excluded
by the negative image cross-links the polymer. In the following
step, a liquid bath removes the polymer not cross-linked during
the exposure process. Raised surfaces of (crossed-linked) photopolymer material provide the raised surfaces to carry ink.
Recently, direct laser light exposure of photopolymer material
has been able to eliminate the need for the intermediate negative
providing high reproducibility and quality.
As in gravure printing, the diameter at the surface of the print
media must match (i.e., equal or represent an integral multiple of)
the package’s cutoff length. As indicated in Figure 3.1, this diameter is affected by the core “plate cylinder,” the thickness of the
flexo plate material (0.030 0.112 inch (0.76 2.84 mm)), and the
“sticky back” material (0.015 0.062 inch (0.38 1.57 mm)) used
to adhere the latter to the former. The circumference of the printing surface would be more than 0.25 1 inch (7 25 mm) greater
than the plate cylinder circumference.
Numerical Color Space
Understanding and application of quantitative color theory has
greatly aided the practice of flexographic printing. Primary colors
(red, green, blue; see Chapter 2) provide the basis of this theory by
which three values define any color perceived by the human eye.
3: FLEXOGRAPHIC PRINTING
29
Plate cylinder
Sticky back
Flexo plate
Backup roller
Figure 3.1 Flexo printing basics.
L = 100
White
+b
Yellow
–a
Green
+a
Red
–b
Blue
L=0
Black
Figure 3.2 Geometric model of three dimensional color space:
“L”, white to black; “a”, green to red; “b” blue to yellow.
Figure 3.2 suggests the three values, generically called
“L , a , b ” modeled in three-dimensional space, such that:
1. An “L ” dimension (from 0 to 100) measures a
white to black continuum (“relative luminosity”).
2. An “a ” dimension measures a red (positive numbers) to green (negative numbers) continuum.
3. A “b ” dimension measures a blue (positive numbers) to yellow (negative numbers) continuum.
While not the only system for color quantification, L a b
color space is the one widely used to communicate color values
when managing prepress dialogue for commercial printing
files, and for controlling color on-press. Automated instruments
use carefully controlled light sources and receptors to measure
the three values using complex mathematical algorithms.2
2
Formally called “CIE 1976 color space,” or CIELAB (CIE for Commission internationale
de l’éclairage or International Commission on Illumination), the system ultimately originates in
the sensitivity of the three types of color-perceiving cells of the human eye.
30
MANUFACTURING FLEXIBLE PACKAGING
When properly calibrated and applied the instruments provide
precise and reliable “color values.” If a color difference is
measured, the difference is quantified as the Euclidian distance between the two values (i.e., the square root of the sum
of the square of the difference between each set of the three
values). This distance is referred to as ΔE (pronounced “delta
E”). The ΔE value of about 2.3 is considered a “just noticeable difference.”
Control of print quality requires that the L a b values of
each of the printed ink colors correspond to specified standards.
Isolated blocks of each color can be printed on any out of the
way part of the web and monitored during the press run for
compliance to standard. If ΔE values for a given color exceed,
for example, 3, the ink can be “adjusted.” The trend in flexography is to use single pigment ink colors so that such adjustments deal primarily with L (e.g., changing the ratio of
transparent binder to color pigment to make the color lighter or
darker).
The other dimensions of print quality control (i.e., the plane
defined by down-web and cross-web directions) require that
the converter predicts and controls the area of pigmented ink
applied by the flexo plate. The plate material itself will compress as it nips the web against the backup roller. Rotary
motion of the cylinders causes one edge of the raised printing
surface (the leading edge) to compress before the rest of
that surface. Similarly the trailing edge is the last part of the
raised surface to decompress as the cylinder rotates. Figure 3.3
shows the effect of this pattern of compression and decompression. Ink is “squeezed out” to the edges of the raised surface
leaving thicker ink deposits at the edges and thinner ones in
the center. Halftone flexo printing requires a “screening” process analogous to that used for halftone gravure printing. This
results in many (85 133 per inch (3 5 per mm)) small printing surfaces, called “dots.” Each dot presents a leading and a
trailing edge to the web during the printing process. As shown
in Figure 3.4, this can result in a “doughnut effect” in which
the raised dot leaves little or no ink pigment in the center of
the dot.
3: FLEXOGRAPHIC PRINTING
31
Figure 3.3 Magnified view of flexo-printed “oe.” Note compressible
flexo plate leaves heavy ink deposits on edges.
Figure 3.4 Magnified view of flexo-printed process area.
Cyan, magenta, and yellow ink dots on a white background
appear to an observer as an additional color.
Current flexo plate material development focuses on optimizing the balance between ink release and plate compressibility.
Available photopolymer materials offer minimal squeeze out
with consistent, predictable compression performance. The latter
is generically referred to as “dot gain.” In order to control the
area of pigmented ink applied by the flexo plate, an empirical
process called “press fingerprinting” (described below) allows
the converter to systematically control liquid ink volumes and
color values, plate material, and press conditions in order to
select dot sizes for use when printing future halftone images.
32
MANUFACTURING FLEXIBLE PACKAGING
Flexo Ink Metering
Metering for current flexo printing processes utilizes chambered doctor blade assemblies very similar to those for the gravure process. Instead of an engraved image roll, the assembly
deposits ink into the cells of an “anilox roll.” The surface of an
anilox roll is covered with a uniform pattern of cells. The
metering doctor blade wipes excess ink from the anilox much
as it does for the engraved image pattern of a gravure image
cylinder. The anilox, with its cells filled uniformly with ink,
then transfers that ink to raised surfaces on the plate cylinder
where the two comes into contact as suggested in Figure 3.5.
The pattern of cells on the anilox roll represents a critical
element of color control. Cell volume is the initial consideration, but the shape and location of cells relative to one another
also affect how much ink transfers to plate cylinders. Anilox
cell volumes are typically reported as BCM (Billion Cubic
Microns per square inch). One BCM equals to 0.645 cubic centimeters per square meter. Cell volumes vary from about 0.5 to
over 10 BCM (1 5 15 cc/m2) and the number of cells per inch
ranges from about 100 to 2000 (50 800 cells per cm). The cylinder itself may be the typical gravure type, steel base with
copper plating and chrome plating on that, or one made of an
industrial ceramic. Both types can be engraved mechanically
with a diamond stylus or optically with lasers.
Press fingerprinting determines how a controlled set of ink
colors, anilox rolls, and plate materials on a given flexo press
Ink out
Anilox cylinder
Plate cylinder
Ink in
Figure 3.5 Flexo ink metering: chambered doctor blade/anilox/plate
cylinder.
3: FLEXOGRAPHIC PRINTING
33
will reproduce a standard arrangement print images. The purpose of the analysis is to determine an optimum combination
of flexo printing materials in that press for future press runs.
The printing typically combines photographic-type images and
color blocks with ink coverage ranging from 5% to 100% (one
set for each ink color). Additionally, the anilox roll can be
engraved with bands of different cell volumes and patterns in
order to provide an additional variable for the analysis.
Without fingerprinting data, each new press run requires timeconsuming on-press adjustments until satisfactory print results
are obtained.
Flexo Halftone Printing (Process Printing)
Precision needed for accurate L a b measurement is too
great for direct real-time measurement of halftone color regions
of a printed image (Chapter 2). However, the fingerprinting
process allows off-line measurements of the color blocks with
varying ink coverage. Comparison of the theoretical ink coverage percentages in the series of blocks with the measured coverages indicates how efficient the ink transfer process is for
that combination on that press. The measured values of ink
coverage are used to determine dot size for halftone areas of
images on subsequent runs for that press (while exactly matching the L a b ink color values, and using the flexo plate material, and anilox roll pattern used while fingerprinting the press).
Flexo printers refer to halftone printed images as “process”
areas, and to uniform colored areas as “Line work” (the latter
whether the area is a line or a relatively large area). “Process”
reflects the considerable effort (generically called “prepress”)
required to develop appropriate printing media before any
printing can even be started. The CMYK color reproduction
strategy falls short of completely matching the range of human
color perception. Figure 3.6 presents the range of colors in a
two-dimensional “gamut.” The large circle represents the range
of human color perception and the smaller areas outlined
within the circle represent the range of colors in color systems.
“Pantones” is a proprietary color matching system comprised
34
MANUFACTURING FLEXIBLE PACKAGING
Visible
Color gamut
RGB
Color gamut
Green
Pantone
Color gamut
Red
CMYK
Color gamut
Blue
Figure 3.6 Two-dimensional, gray-scale representation of color space
showing normal visible human range and ranges achievable by
printing techniques.
of a set of specific colors and corresponding numbers. It is
widely used by graphic designers in the consumer goods industry to specify colors to printers of advertising, packaging, etc.
A standard computer or television display reproduces the
“RGB” gamut. The “CMYK” gamut shown represents the optimum color range the four colors can produce if each color has
maximum color intensity. Note that the three primary additive
colors almost reach the limits of human perception of red,
green, and blue. The maximum distance (from the white center
of the figure) reached by the CMYK gamut corresponds to the
mid points of the RGB space. Specific brand colors often lay
outside the CMYK gamut. Similarly, this gamut frequently
cannot portray the full tonal range of halftone printed areas. As
suggested by Figure 3.7, expanding the CMYK gamut with the
addition of red, green, and blue inks greatly increases the color
values that flexography can print. Such “expanded gamut” sets
of ink color may deviate from standard CMY and RGB colors
in order to print a particular brand color.
Parallel technology advances in plate materials, anilox cylinder
engraving, ink formulation, and printing press equipment itself
have combined to make flexography as much a science as a craft.
3: FLEXOGRAPHIC PRINTING
35
Yellow
Red
Green
Magenta
Cyan
Blue
Figure 3.7 Expanded gamut: “CMY” 1 “RGB.”
Members of the volunteer industry group, The Flexographic
Technical Association, have cooperated in writing a comprehensive guide to these principles entitled “Flexographic Image
Reproduction Specifications & Tolerances” (abbreviated to
“FIRST”). The publication is now in its fourth edition, reflecting
the steady record of industry insights and improvements [1].
These advances lead to the loss of flexible packaging printing
market share referenced in Chapter 2, as well as quality and productivity improvements for flexo printers.
Flexo Process Innovation
Flexo’s inefficiencies principally result from job changeover
times, including plate cylinders, anilox rolls, and ink colors.
All add to the fixed costs of any given job. By simplifying
requirements for cleanup and setup from one job to the next,
the productivity and yield of flexo increase.
The modern flexo press described in Chapter 12 requires that
the doctor blade assemblies, aniloxes, and plate cylinders may
36
MANUFACTURING FLEXIBLE PACKAGING
be more than 8 ft (2.5 m) above the factory floor. Access to
these heavy and delicate components often requires mechanical
hoists with a crew of 2 3 replacing one. As an example of why
a replacement might be required, consider changing from surface printing an opaque (e.g., metallized) film to reverse printing
a clear one. In the former case, a heavy layer of white ink must
be printed before any other colors in order to reflect light
through the colored images subsequently printed. For reverse
printed films, all of the colors will be printed before a white
layer is applied under them. As viewed through the film, the colors will be visible as a result of the last-down white reflective
layer. To achieve that heavy layer of white ink, whether printed
first or last, a high-volume anilox is needed to deliver the large
amount of ink to the plate cylinder. Switching from surface to
reverse printing in such cases involves removing a high-volume
anilox from the first station, and installing one in the last station
for the next run. The basic hoist-assisted manual effort to move
these heavy aniloxes in and out can be replaced by special
robotic units (which like gravure trolleys can be prepared while
one job is still running). Flexo plates can be premounted on variable thickness sleeves that slide on and off between jobs.
Automatic ink cleaning systems are available to flush one color
of ink from a doctor blade assembly and its associated circulation hoses before another one is introduced.
The ideal job cleanup/setup sequence for flexo involves using
the newfound color management tools to eliminate as much
change from one job to the next for all color stations. The ideal
involves using a set of extended color gamut inks in a given color
station on a press for any and all jobs. Each ink is matched to a
specific anilox in the fingerprinting process. The empirically
derived color values from that combination are reflected in all of
the prepress effort to transform the designer’s original artwork
into press ready color-separated printing plates. Computerized
prepress systems assist with this combination of creative artistic
design and predictable technical execution.
The operational advantages of printing sequential jobs on a
press by changing only the printing plates are obvious. The
computerized prepress systems also improve basic business
3: FLEXOGRAPHIC PRINTING
37
systems by facilitating real-time review and revision of artwork
using images on computer screens and digital proofs. Physical
exchange of image proofs (e.g., using overnight air delivery
services) becomes unnecessary, with time and money savings.
The industry practice of “press approvals,” wherein the brand
owner’s representative physically traveled to the printing press
location and waited until the printed output matched his/her
approval, also becomes unnecessary.
Reference
[1] Foundation of Flexographic Technical Association, FIRST 4.1,
Bohemia, NY, 2013, pp. 867.
4
Adhesive Lamination
Chapter Outline
Adhesive Laminating Process
Adhesive Lamination Strength
Other Coating Processes
Adhesive Laminating Innovation
Reference
41
43
44
45
47
The basic functionality of all packages includes:
1.
2.
3.
4.
Contain product(s)
Protect and preserve product(s)
Convey product(s)
Inform about/sell product(s)
Providing all of these functions presents challenges to thin
materials used for flexible packaging. A flexible package may
use more than one flexible material depending on the product,
its storage and distribution experience, and its customer interaction requirements. Laminating materials together allows
them to function as a complete packaging material. Generally
flexible packaging can be visualized as having four operational
layers: surface, bulk, barrier, and sealant (Table 4.1). The surface carries printing and interacts with consumers and packaging machinery. Bulk layers add stiffness for shelf appeal and
machining. Barrier layers prevent desirable product elements
(e.g., flavor and aroma compounds) from escaping the package
and prevent undesirable environmental factors (e.g., oxygen
and moisture) from entering the package and harming the product. The sealant layer serves to close up the package and make
it a container rather than a simple wrapper.
Manufacturing Flexible Packaging. DOI: http://dx.doi.org/10.1016/B978-0-323-26436-5.00004-7
© 2015 Elsevier Inc. All rights reserved.
39
MANUFACTURING FLEXIBLE PACKAGING
40
Table 4.1 Package Functions Distributed into Flexible
Packaging Layers
Basic
Package
Functions
Flexible Package Layer
Surface
Contain
Keep
product(s)
product
in
Protect and Keep bad
preserve
out and
product(s)
good in
Convey
product(s)
Inform
Printed
about/sell
Info
product(s)
Bulk
Barrier
Sealant
Resist
puncture
Keep
Keep
product
product
in
in
Resist
Keep bad Keep bad
puncture
out and
out and
good in
good in
Resist puncture
Close
package
General
impression
Depending on the packaged product, the four operational
layers may consist of one to more than four actual layers of
materials. A bread bag made of a single layer of film (usually
polyethylene) protects sliced bread intended for either food service or retail use. Spices and dried soup are often packaged in
a four layer pouch material comprised of paper (surface layer),
polyethylene (bulk layer), aluminum foil (barrier layer), and a
second layer of polyethylene (sealant layer).
One of the most effective approaches to reducing the cost of
flexible packages consists of incorporating multiple operations
into a single physical web. Examples include webs coated with
barrier materials, and films simultaneous extruded with multiple materials each bringing its specific value to the composite
web. (Figure 4.1 provides illustrations of each.)
Laminating multiple flexible webs together provides the
means of building a packaging material able to provide all of
the functions required of it by its product. This process requires
a laminating adhesive to adhere the two layers to another. If
one layer is porous (typically paper), a “wet bond laminating”
4: ADHESIVE LAMINATION
41
1 µ polyvinylidene
chloride coating
12 µ oriented polyester
2 µ polypropylene
copolymer coex
15 µ oriented polypropylene
(A)
Material
Uncoated
Coated
Relative barrier
Oxygen
Water vapor
1.0
1.0
0.04
0.4
(B)
Material
Not coexed
Coexed
Heat real range
°F
°C
Shrinks at 300°F (149°C)
250–300
121–149
Figure 4.1 Enhanced value of single webs with multiple operational
abilities.
process can be used. In this case, the liquid adhesive is applied
to the nonporous layer; the porous one nipped against it; and
the adhesive’s solvent (typically water) dried through the
porous material. Alternatively, a “dry bond laminating process”
involves applying an adhesive to the either layer (usually the
more dimensionally stable one); drying the adhesive’s solvent
from the coated material; and then nipping, with heat and pressure nipping the other layer to the dried, but tacky adhesive
surface. Dry bond adhesives (Chapter 28) usually undergo
hardening (also known as curing or cross-linking) after the two
materials are laminated so that the initial tackiness disappears
and adhesion between the two materials increases. “100%
solids laminating” is similar to the wet bond process except
that the adhesive has no solvent to dry. It simply hardens from
a fluid as applied to a solid when aged between the two materials. The dry bond process is much more common in flexible
packaging because plastic films and foil are nonporous.
Extrusion lamination (Chapter 5) is a kind of 100% solids process in which the adhesive itself is a molten fluid because it is
heated and then it solidifies as it cools.
Adhesive Laminating Process
The familiar gravure cylinder usually applies adhesives (wetor dry bond) to a web. In the process, the cylinder resembles
42
MANUFACTURING FLEXIBLE PACKAGING
an anilox roll used in flexo printing. The cell volume and pattern determine the volume of fluid adhesive applied to the substrate. Standard industry practice for determining the amount
of adhesive applied per unit area, called “coating weight,”
involves:
1. Subtracting the volume percent of volatiles from
the volume applied to determine volume of adhesive solids applied.
2. Converting the volume of adhesive solids to
weight of adhesive solids by multiplying by a density factor.
3. Reporting the solid weight of adhesive per unit
area of web coated as “pounds per ream”1 or
“grams per square meter”.2 The two are abbreviated “ppr” and “gsm.”
Coating weight can be calculated either by measuring the
volume consumption of adhesives during a run and the total
area of material in the run (roll length times roll width) or by
weighing a piece of known area cut from the web before and
after the adhesive is removed by rubbing a solvent.
100% solids adhesives require an alternate metering process.
Although these are fluid as applied, they are relatively viscous
or pasty. Their application usually involves a “stack” of four
counter-rotating rubber rollers. The pressure between adjacent
rollers is controlled to leave some of the pasty adhesive on the
initial roller while a smaller amount strips away onto the next
roller. (This is the basic ink metering concept used for the offset printing process.)
1
In the paper industry, any “ream” represents 500 sheets of some standard length and
width. The North American flexible packaging industry references a “tag stock” sheet, 24
inches by 36 inches (610 mm 3 914 mm). In effect then, for North America flexible packaging,
a ream equals 432,000 square inches or 0.56 square meters (i.e., 24 inchesx36 inchesx500).
2
One pound per ream 5 814.5 gram/square meter (1 gram/square meter 5 0.0012 pound per
ream).
4: ADHESIVE LAMINATION
43
Adhesive Lamination Strength
Sufficient interlaminar adhesion determines how well the
layers of a lamination contribute to the overall properties of the
flexible packaging material. This strength is a common performance requirement for multilayer materials (Chapter 32).
While this quantitative measure serves to qualify the material for use, qualitative identification of the lamination’s failure
mode importantly adds to understanding how to solve
lamination problems and design functional laminations. As
Figure 4.2A suggests a lamination has at least three layers: the
primary substrate (the one to which the liquid coating was
applied), the secondary substrate (the one nipped to the dry
adhesive surface on the primary substrate), and the layer of
adhesive itself. The three layers result in two interfaces: adhesive to primary substrate and adhesive to secondary substrate.
Recognizing and describing a failure mode involve reference to
these three layers and two interfaces. For “primary adhesive
failure” (Figure 4.2B), the adhesive leaves the primary substrate and adheres to the secondary. For “secondary adhesive
failure” (Figure 4.2C), the opposite is the case. When the adhesive layer itself fails (Figure 4.2D), the failure mode is
Secondary substrate
A: Original lamination
Adhesive
Primary substrate
Secondary substrate
B: Primary adhesive failure
Adhesive
Primary substrate
Secondary substrate
C:Secondary adhesive failure
Adhesive
Primary substrate
Secondary substrate
D:Cohesive failure
Adhesive
Adhesive
Primary substrate
substrate
Secondary
E: Material failure
Adhesive
Primary substrate
Figure 4.2 Lamination failure mode designations.
44
MANUFACTURING FLEXIBLE PACKAGING
“cohesive failure.” This is the case when the adhesive solids
themselves fail to properly cure (Chapter 28). With very strong
adhesion, failure can result because one of the substrates
breaks. The break can be either the primary or secondary substrate. If following the usual practice of using the more dimensionally stable substrate as the primary one, the secondary
substrate will break (Figure 4.2E).
Other Coating Processes
Adhesive laminating is a subset of the broader converting
process called “coating.” Coating refers to any process in which
a fluid material of some sort is applied over the width of a
web. The coating material functions in many ways including:
barrier improvement, heat sealability, adhesion when pressed to
another surface deliberate release from such “pressure-sensitive”
surfaces. The fluid dynamics of the coating material dictate one
of the many metering methods available to web converters for
coating processes.3 Fluid-coated substrates lend themselves to
dedicated high-volume manufacturing processes, unlike the jobshop traditions of flexible packaging converting. With few
exceptions, such products are marketed by rollstock suppliers to
converters as raw materials.
The “pattern coating” process is better suited to the
manufacturing environment of the flexible packaging converter.
The process imparts the coating’s functionality to only a part
of the substrate, usually registered to a printed pattern. For
example, a frame of pressure-sensitive adhesive coating applied
to the inside of chocolate bar wrappers allows packaging of the
product with pressure only. This avoids heat sealing that could
melt the product. As digital electronic control of web processes
allows easier, more reliable registration of a coating to a
printed image, additional opportunities for converters to add
value to flexible packaging emerge.
3
For a more complete review of metering methods for various coating methods, see Ref. [1].
4: ADHESIVE LAMINATION
45
Adhesive Laminating Innovation
In general, the many industrial processes involving web coating were developed for coatings formulated with organic solvents. These all involved applying a fluid material to a
substrate, drying the volatile solvent of the coating, and using
the properties of the solid material left on the substrate. The
evaporated solvents, formerly simply released into the atmosphere, react in the air to form the respiratory irritant, ozone (a
major component of “smog”). Air quality improvement laws
(e.g., the US Clean Air Act of 1970) mandated the control of
solvents (called “Volatile Organic Compounds,” VOCs) from
dried coatings. That change initiated four decades of innovation
to change flexible packaging adhesive chemistry, laminating
equipment, and the basics of the process as well.
VOC control options included reformulating coatings to use
no or less solvent and “end-of-pipe” approaches in which evaporated solvent is incinerated to carbon dioxide and water vapor.
Reformulating options included increasing the volume percent
of solids in the adhesive as applied on a laminator (“high solids
adhesives”) and using a non-VOC solvent (e.g., water).
Incinerating evaporated VOC’s added capital investment and
operating costs to a laminating process.
Chemical approaches to increasing the ratio of solids to
VOCs in an adhesive provided interim VOC emissions reductions, but were ultimately unable to satisfy allowable emissions
rates (2.9 pounds of VOC per gallon of solids) without radical
changes to the adhesive metering devices on laminators. The
traditional chemistry of solvent-borne flexible packaging adhesives reacted with water so that no cure would occur. In
response, water-based adhesive chemistry was evaluated.
Water-based adhesives presented two operational challenges
in converting equipment. First, the heat of vaporization of
water exceeds that of most solvents traditionally used. As a
result, compared to the same volume of VOC, water requires
more energy input (from higher temperatures and/or longer
exposure) before evaporating. Second, the surface energy of
water is higher than VOCs. Because of this, water-based
46
MANUFACTURING FLEXIBLE PACKAGING
adhesives would not “wet out” the many low-surface energy
plastic films used in the industry. Instead, the adhesives would
bead-up leaving uneven layers of solids when finally dried.
These challenges in turn stimulated other innovations. To the
options of higher temperatures and longer exposure to heat
were added infrared preheaters able to raise the temperature of
the still liquid coating before entering hot air ovens, and ovens
with high efficiency air-flows to add more heat to the coatings
by circulating more air at higher speeds. Converters retrofitted
in-line “corona treaters”4 to existing laminators that increased
the surface energy of plastic films to match the water-based
adhesives.
The operational challenges were actually exceeded by functional challenges that continue to limit the scope of application
of water-based adhesives. Adhesive chemistry compatible with
water does not provide the range of temperature, chemical, and
moisture resistance achievable with solvent-borne adhesives.
While water-based adhesives found and continue to find use in
flexible packaging applications for which such resistance is not
required, traditional adhesives with VOC content are still used.
Reformulating for higher solids adhesives did lead to 100%
solids adhesives, but only after the metering process described
above for them was available. Because the process compares
more closely to the wet bond process, retrofitting existing dry
bond equipment is not feasible:
• Dry bond equipment is not configured to nip the
secondary substrate to the just coated substrate, but
literally at the other end of the machine.
• The primary and secondary substrates should have
minimum tension on them when nipped. Adhesive
strength of 100% solids adhesives is initially very
low with the result that any high levels of tension
in a substrate when nipped will cause delamination
forces when the tension is reduced.
4
Corona treaters create an ionizing atmosphere around high-voltage electrodes which can
both burn-away small surface contaminants on a substrate surface and cause chemical reactions between the substrate surface and other molecules in the atmosphere.
4: ADHESIVE LAMINATION
47
• 100% solids adhesives require two components: adhesive solids and cross-linking (or catalyst) solids.
Ultimate adhesive strength depends on providing the
correct ratio of the two components, and, once combined, the components react together rapidly. To manage the chemistry, mixing is done in real time by an
automated “meter mixer” that dispenses both components in the correct ratio directly onto the stack of
metering rollers.
While different than dry bond laminators, 100% solids (or
“solvent-free”) laminators represent a simpler machine design
and can cost accordingly less. Forty years after beginning the
effort to reduce VOC emissions from the industry, most new
adhesive laminating equipment uses the nonpolluting formulations and lowers product costs.
Reference
[1] E. Cohen, E. Gutoff (Eds.), Modern Coating and Drying
Technology (Advances in Interfacial Engineering Series), Wiley
& Sons, Hoboken, NJ, 1992.
5 Extrusion Lamination
and Coating
Chapter Outline
Extrusion Laminating Process
Promoting Adhesion: Melt Curtain
Promoting Adhesion: Substrate
Extrusion Coating Process
Extrusion Laminating Innovation
References
49
52
54
55
57
59
The power of the extrusion laminating and coating process lies
in its incredible flexibility in adding layers to flexible packaging structures. Although by definition, the process involves one
or more additional substrates, as Table 5.1 suggests, it can add
any one of flexible packaging’s operational layers to substrates
to complete a fully functional flexible packaging material. The
flexibility is all the more powerful because the process directly
incorporates undifferentiated polymer into specific functional
layers without the need (and cost) of separate processes to fabricate a roll of value-added rollstock.
With such flexibility, comes an opportunity to develop and
market proprietary products. Unique extrusion-coated/laminated
products engage options presented by both the machinery and the
material choices available to the process. Harnessing this power
does require broad understanding of basic process principles and
creative responses to challenges. The essential principle to understand involves how extruded layers come to adhere to rollstock.
Extrusion Laminating Process
Extrusion laminating is analogous to the wet bond and 100%
solids laminating processes described in Chapter 4. The laminating
Manufacturing Flexible Packaging. DOI: http://dx.doi.org/10.1016/B978-0-323-26436-5.00005-9
© 2015 Elsevier Inc. All rights reserved.
49
50
MANUFACTURING FLEXIBLE PACKAGING
Table 5.1 Flexible Packaging Layers (See Chapters 23–25 for
descriptions of the listed polymers)
Polymer
Added Using
Surface
Extrusion
Laminate/
Coat
Slip-modified
LDPE
LDPE
EVOH
LDPE
copolymer
Flexible Package Layer
Bulk
Barrier
Sealant
Protect
print
Add
thickness
Add oxygen
barrier
Add
sealant
layer in this case is a form of 100% solid adhesive. Specifically,
a molten curtain of polymer drops from a slot in the bottom of a
die. The width of the slot and die is greater than or equal to that
of the web. Molten polymer enters the die from a plastic
extruder,1 and flows through the die and out through the slot
(primarily in response to gravity). The polymer entering the die
may be a single polymer, a blend of polymers, or a layered set
of polymers able to maintain its layered configuration (called a
“coextrusion”) as it exits the slot. As Figure 5.1 shows, the
primary and secondary webs are introduced on either side of the
curtain and all three layers brought together at a nip where a
large chill roll presses against a backup roll. The chill roll lowers
the temperature of the hot polymer so that a solid, three-layer
lamination quickly develops.
As with adhesives for the various adhesive laminating processes, extrusion laminating and coating layers are measured in
terms of “coating weight.” Adjusting coating weight on an
extrusion laminating line requires careful coordination of
1
For a more in-depth understanding of the extrusion process upstream of a die, see Refs.
[13].
AND
COATING
51
DIE
5: EXTRUSION LAMINATION
Molten
polymer
Primary
web
Secondary
web
Backup
roll
Chill roll
Laminated
web
Figure 5.1 Cross-section view of extrusion lamination.
process conditions. Constant coating weight on the substrate
implies that the entire mass of polymer delivered through the
die to the slot will coat the substrate2:
MassðlbÞ5coating widthðftÞ3coating weightðlb=3000 sq: ft reamÞ2
3line speedðfpmÞ3run timeðmÞ
Obviously, if line speed increases the rate of delivery of polymer to the slot must increase if coating weight is to remain constant. The equipment’s control system typically controls the
polymer delivery rate by varying the speed at which the extruder’s
screw rotates in the extruder barrel. Alternatively, if line speed
increases with no increase in the rate of delivery of polymer to the
slot, coating weight must decrease. This increases the “draw down
ratio” (DDR), defined as machine-direction length of the die’s
slot divided by coating thickness. The molecular size and geometry of a given polymer (its “morphology”) limits its range of functional DDRs and influences other product characteristics.
2
Actual coating weight distribution across the web is not uniform. At a minimum, cohesive
forces on the edges of the falling melt result in a thicker strip (called an edge bead) on each
edge. See Ref. [4].
52
MANUFACTURING FLEXIBLE PACKAGING
Insuring adhesion between the molten polymer and the two
substrates involves choosing appropriate processing conditions
and/or materials. Adhesion requires energy that can come from
chemical and/or physical linkages, the latter being reversible
when enough energy is applied. To adhere a molten polymer
curtain with physical linkages requires that the substrate surface
has (1) surface energy sufficient to attract the melt to that surface area while still fluid and (2) surface area sufficient to entangle the melt when it solidifies. Fibrous substrates (paper and
nonwoven fabrics) rely on such linkages, but often require pretreatment (e.g., primers, corona or flame3 treatment) to provide
the surface energy that allows the melt to wet-out the fibers.
Promoting adhesion with chemical linkages provides wideranging options, some of which are generic for the process while
others are specifically designed for a specific polymersubstrate
combination. The approach can address either or both the melt
and the substrate. Chemical adhesion involves atomic interaction at the interface of the two. A hydrogen atom from one molecule attracted to an atom of nitrogen, oxygen, or fluorine in
another molecule forms relatively weak bonds (called hydrogen
bonds). Stronger adhesion results when atoms of two materials
swap electrons (ionic bonding) or share them (covalent bonding). If surface molecules can bond, then the surfaces will be
bonded together by a network of such interatomic forces. None
of these forces act over more than a nanometer, so surfaces must
have intimate contact (e.g., combined with the pressure of a nip
point) and isolated from subsequent forces that might disrupt
them. For example, high DDR can impart strain to the solidifying melt that can exceed some adhesive forces.
Promoting Adhesion: Melt Curtain
The typical extrusion laminating process uses polyethylene
and to a lesser extent polypropylene. Both are polyolefins
3
Flame treaters rely on the ionizing plasma created by fuel burning in oxygen to increase
the electrostatic attraction of areas on a substrate’s surface. Flame treating on web processes
requires that a heat sink (i.e., cooling roll) prevent the flame from burning or melting the
substrate.
5: EXTRUSION LAMINATION
AND
COATING
53
(Chapter 23) which implies that in their natural state, those
molecules have little or no electro-polar charge. Adhesion promotion involves exposing the melt to conditions that introduce
charges by causing chemical changes to the melt surface. The
combination of melt temperature (600 F or 315 C) and ambient
atmosphere (B19% oxygen) favors oxidation of the melt curtain. The distance between the slot at the bottom of the die and
the point at which the melt curtain touches a substrate (or roll
in the case of extrusion coating) is called the “air gap.” Most
modern extrusion laminating and coating lines have adjustments to raise or lower the bottom of the die relative to the
substrate location. This adjustment to the air gap also changes
the time the melt curtain spends in the air gap and can undergo
oxidation.
This “time in air gap (TIAG)” measure provides a good indication of relative adhesion promotion potential of a process
given its running speed and air gap4:
TIAG 5 L=vf
for “L” 5 air gap and “vf ” 5 line speed
For example, a 12 inch (1 foot) air gap on a line running at
600 feet per minute (fpm), provides a TIAG of 1.67 milliseconds (ms), raising the air gap to 15 inches increases TIAG to
2.1 ms and reducing the speed to 300 fpm increases it to
3.3 ms. The effect of TIAG on adhesion is not simple. The
basic expectation that a longer TIAG provides more time to
oxidize the melt surface takes with it the assumption that melt
temperature is constant. In reality, ambient conditions begin to
cool the curtain as soon as it exits the die. All else being equal,
a cooler melts impair adhesion for two reasons: (1) less energy
is available for surface oxidation and (2) the thicker, more viscous fluid cannot wet-out and flow onto the substrate surface
as completely.
Direct exposure of the melt curtain to ozone (O3) provides
an alternative way to oxidize the melt curtain surface. Ozone is
a strong oxidizing form of oxygen that severely weakens some
4
(Note: if the units of air gap and distance of substrate per unit time are not identical, a
conversion factor is required.)
54
MANUFACTURING FLEXIBLE PACKAGING
polymers with prolonged exposure. Controlled exposure to an
ozone source in the air gap provides the melt curtain with a
greater degree of oxidation than possible in the ambient atmosphere. In addition to better adhesion, ozone treatment can
allow lower melt temperatures5 and higher line speeds.
Instead of adding chemical activity to polyolefins in order to
promote adhesion, ethylene copolymers bring with them specific chemical activity to promote melt curtain to substrate
adhesion (see Chapter 24). Such copolymers result by adding
“comonomers6” to ethylene monomer polymerization reactions.
Such copolymers may have functional value for flexible packaging structures in their own right, but their role as adhesion
promoters in extrusion laminating and coating represents significant commercial importance. They can be used as an entire
layer, blended with a compatible polyolefin, or coextruded as a
thin adhesive layer with a thicker bulk layer of polyolefin.
Promoting Adhesion: Substrate
Corona and flame surface treatment methods can impart
surface chemical activity to the surface of substrates. Both of
these involve oxidation reactions with the substrate material.
Alternative reactions causing chemical activity other than oxygen
compounds are possible using “atmospheric plasma” treatment
[5]. Nitrogen and ethylene gases have successfully promoted
adhesion to low-density polyethylene melt curtains.
A special coating technique termed “priming” provides a
means of covering a substrate’s surface with chemical activity
that will adhere to a melt curtain (see Chapter 29). In some
case, the chemistry of the primer matches that of functional
laminating adhesives. Others are simple bifunctional, one end
5
For a given resin, lower melt temperature may result in thinner edge bead areas and wider
melt curtains. Lower temperatures may also favor better melt curtain integrity and uniformity,
lower odor, and better heat sealing.
6
Common comonomers in the flexible packaging industry include vinyl acetate; methyl,
ethyl, and butyl acrylates; and acrylic and methacrylic acid. Comonomers may be present from
a few weight percent (wt%) to more than 20 wt%. Acid copolymers (negative polarity) partially
neutralized with inorganic cations (positive polarity) are called “ionomers.”
5: EXTRUSION LAMINATION
AND
COATING
55
of which has an affinity for the substrate, the other for the melt
curtain molecules (i.e., nonpolar on one end and polar on the
other). In this type, the molecules serve as a bridge to link the
two materials to each other.
Adhesion promotion often requires incorporating surface
chemical activity for both the substrate and melt curtain.
Adhesion involves two surfaces intended to form one interface,
so both must have mutual affinity for the other.
Extrusion Coating Process
Extrusion laminating evolved from the simpler extrusion coating process. Development work in the mid-1940s resulted in
commercial applications involving extrusion coating paper and
paperboard substrates with low-density polyethylene. Improved
wet strength and barrier to insects for multiwall bags provided
the initial motivation. By 1951, a major supplier of gabletop cartons for milk and other beverages began substituting
polyethylene-coated paperboard for the wax-dipped paperboard
previously used. The coating served not only to waterproof the
board, but also to seal and to hold the carton’s shape [6].
Extrusion coating a substrate may serve to protect its surface,
but more frequently, the extruded layer provides the heat seal
layer of a packaging material. In this role, the strength and
integrity of heat seals made with extrusion-coated layers have a
complex relationship to the manufacturing process:
1. Substrate adhesion is critical to seal strength. Low
adhesion leads to adhesive failure analogous to the
mechanism shown in Figure 4.2B and C.
2. Melt curtain oxidation can weaken the degree of
molecular intermingling needed for strong heat
seals. The result can be cohesive failure (analogous to Figure 4.2D).
3. Many of the characteristics of the polyethylene
grades making them desirable for extrusion coating (see Chapter 25) translate into low seal
56
MANUFACTURING FLEXIBLE PACKAGING
strength when even adhesion is high and oxidation
is low. The measured strength of such seals can be
low even if material failure occurs (Figure 4.2E).
A heat seal layer of necessity is an exterior layer of the
packaging structure. In this position, the surface must slide relatively easily over itself and other surfaces on machines that
fabricate bags and filled packages. The “slip” (see Chapter 32)
of this surface must be sufficiently high to permit this. Organic
“slip additives” as used in blown and cast films7 may be
blended into the molten polymer. Additionally, the chill roll
used for an extrusion coating operation will have significant
impact on the layer’s slip. The extrusion coating line’s nip
presses the melt curtain onto the chill roll as it solidifies. Any
pattern or other surface irregularity on the roll’s surface will
mold its mirror image into the polymer’s surface. Patterns chosen to reduce the surface area contact percentage on the film
increase the slip of the surface. Shallow cavities on the chill
roll also hold a layer of air under the polymer surface that
helps to lift the polymer off the roll before the substrate wraps
completely around it. Table 5.2 lists the names given to the
patterns of cavities (surface “finishes”) used on chill rolls. All
except the “mirror pocket” finish have a regular, repeating pattern of cavities. Manufacture of these surfaces is similar to gravure roll, with engravings in a soft metal layer (e.g., copper)
and chrome plating over it. A “mirror” surface has few such
cavities (if any) and the extruded layer will tend to adhere to it.
A “matte” finish has so many cavities that the extruded polymer’s surface will appear hazy in an otherwise transparent
structure. Mirror pocket finish was developed to provide the
low-haze clarity of a mirror finish roll while delivering enough
randomly shaped and spaced cavities on the roll to provide the
lifting air for release and reduced surface area on the coated
substrate for high slip.
7
These additives are generically “fatty acid amides” of animal, vegetable, or synthetic origin.
Chemically, they are long chain (1220 carbons long) carboxylic acids reacted with ammonia.
These small modules tend to migrate to a plastic’s surface and there provide lubrication that lowers the coefficient of friction of the surface. They may also be vaporized during extrusion if temperatures are high enough.
5: EXTRUSION LAMINATION
AND
COATING
57
Table 5.2 Chill Roll Finishes
Finish
Average Roughness (Ra)a
Matte
Gloss
Mirror
Mirror pocket
Optical mirror
30125
410
24
B1 (see text)
#1
a
Ra measures roughness as the average of vertical deviations from the
average vertical height as measured along a roughness “profile line.”
Extrusion Laminating Innovation
The range and complexity of packaging structures manufactured using the extrusion coating and laminating process continue to grow as a result of both equipment and material
innovation. Increased value adding by the converter motivates
much of this growth. Cost reduction imperatives justify some
of this effort, but adding package functionality with resin rather
than with purchased rollstock creates true added value for the
market. Although multifunctional resins are the exception, multiple layers of resin allow multifunctional additions to structures at a single extrusion die. Chapter 14 covers these
coextrusion technology innovations in more detail.
Coextruded layers from extrusion coating or laminating provide
many creative and innovative structures. This enables one converter to distinguish a product line from his competitors. The following are some examples of design possibilities (see Chapters 24
and 25 for more detail on the resins mentioned here):
1. As described in Chapter 3, a white reflective layer
in a structure makes halftone color printing possible. Using a white pigment in an extrusion layer
under printed inks provides such a reflective layer
that can provide better color fidelity than a layer
of white ink (Table 5.3). “Skin” layers of clear
resin may be needed on either side of the
58
MANUFACTURING FLEXIBLE PACKAGING
Table 5.3 Comparison of White Opacifying Ability of Ink and
Extrusion Layers
Source of Reflective
Surface
White Ink Layer
White Extruded
Layer
Whitener material
Titanium dioxide
(TiO2)
2
Titanium dioxide
(TiO2)
10
B50
1
B25
2.5
Thickness of
white layer (µ)
Weight % of TiO2
Effective thickness
of TiO2 (µ)
Traditional structure
Barrier extrusion lamination
12 µ barrier-coated OPET
12 µ plain OPET
12 µ barrier coextrusion
36 µ Ionomer
2 µ adhesive resin
8 µ barrier resin
2 µ adhesive resin
25 µ Ionomer
Barrier coextrusion detail
Figure 5.2 Barrier resin replaces barrier-coated film.
pigmented core to prevent abrasion of the dies surfaces from the pigment particles.
2. Figure 5.2 suggests how a layer of barrier resin
(ethylene vinyl alcohol) in the extruded layer can
replace the barrier coating of a purchased film.
Here, skin layers of adhesive resins tie the barrier
resin to films in the lamination.
3. Actually changing a structure from an extrusion
lamination to an extrusion coated one represents significant savings opportunities (Figure 5.3). A new
generation of copolymer resins provides extrusion
coating lines with options previously available only
to producers of coextruded blown film.
All of these options require a combination of machines having suitable features and grades of material compatible with
those features. A machine purchase is an infrequent occurrence,
so understanding what products fit the converter’s competitive
5: EXTRUSION LAMINATION
AND
COATING
Traditional structure
Barrier extrusion lamination
16 µ OPP
12 µ extruded LDPE
16 µ OPP
25 µ copolymer film
36 µ sealant coextrusion
59
26 µ extruded LDPE
10 µ copolymer resin
Sealant coextrusion detail
Figure 5.3 Specialty sealant resin replaces specialty film.
strategy is critical. Material innovation happens frequently,
allowing continual experimentation and product and process
improvement. A working knowledge of extrusion coating and
laminating principles beyond this brief introduction (e.g., from
the resources cited here) makes both activities more efficient
and effective.
References
[1] T. Bezigian (Ed.), Extrusion Coating Manual, fourth ed., TAPPI
Press, Atlanta, GA, 1999.
[2] B.H. Gregory, Polyethylene Extrusion Coating and Film
Lamination: The Complete Process Manual, Trafford Publishing,
Bloomington, IN, 2012, 380 pp.
[3] B.H. Gregory, Extrusion Coating: A Process Manual, Trafford
Publishing, Bloomington, IN, 2009, 216 pp.
[4] T. Bezigian, Extrusion Coating Manual, fourth ed., TAPPI Press,
Atlanta, GA, 1999 (Chapter 8).
[5] R. Wolfe, Promoting Adhesion—Corona, Flame, Ozone, and
Plasma Surface Treatment, TAPPI 2010 PLACE Conference,
TAPPI Press, Atlanta, GA, pp. 44.
[6] History in Bezigian, Extrusion Coating Manual, fourth ed.,
TAPPI Press, Atlanta, GA, 1999 (Chapter 1).
6
Finishing and Slitting
Chapter Outline
Communicating Slit Roll Requirements
Slitting Options
Rewind Options
References
61
65
65
69
Fast line speeds and wide widths provide productivity and efficiency for converting processes that minimize the cost of flexible packaging. Packaging equipment uses web widths narrower
than those produced on converting equipment (called a “master
web”). The process that separates one packaging line-width of
material from the rest of the master web is slitting. While the
concept may seem minimal and straightforward, slitting brings
to light any irregularities in the master web as produced and
reveals the physical forces involved in basic web handling.
These “slit rolls” become the actual product shipped to a customer, so their quality and conformance to specifications serve
as critical sources of a first impression.
Communicating Slit Roll Requirements
Packaging equipment unwinds materials from its roll, forms
it into the shape of a container, fills it with product, and seals
the container. Printed material should, of course, reflect the
container’s length (the “cutoff ”) and its width (the “web”).
Web and cutoff can be no more precise than as initially printed,
but any subsequent web handling can introduce additional
variability to both. Slitting defines a roll’s edge that must parallel to the machine direction of the printing process. Figure 6.1
suggests the challenge. Three impressions on the right have a
Manufacturing Flexible Packaging. DOI: http://dx.doi.org/10.1016/B978-0-323-26436-5.00006-0
© 2015 Elsevier Inc. All rights reserved.
61
62
MANUFACTURING FLEXIBLE PACKAGING
SPF 25
SPF 25
SPF 25
SPF 50
SPF 50
SPF 50
Sun
screen
Sun
screen
Sun
screen
Sun
screen
Sun
screen
Sun
screen
SPF 25
SPF 25
SPF 25
SPF 50
SPF 50
SPF 50
Sun
screen
Sun
screen
Sun
screen
Sun
screen
Sun
screen
Sun
screen
SPF 25
SPF 25
SPF 25
SPF 50
SPF 50
SPF 50
Sun
screen
Sun
screen
Sun
screen
Sun
screen
Sun
screen
Sun
screen
1
2
3
4
5
Figure 6.1 Slitting precision for multiple lanes, multiple colors.
light-colored border and three on the left have a darker border.
Slitting between the third and fourth impressions (cut No. 3) is
likely to produce a wavy line with alternating strips of either
color. They would appear on the right edge of the third roll and
left edge of the fourth roll. When print extends to (and through)
a slit edge as in Figure 6.1 (called a “bleeding edge”) its color
must be uniform for a width at least as great as the side-to-side
tolerance of the slitting operation.
The forming process on a packaging machine requires conformance of printing to the expected machine sequence. The issue
is a recurring one for printed rolls of any material for any handling process. To communicate expectations, the printing industry
(not only flexible packaging) uses standard designations for
6: FINISHING
AND
SLITTING
63
Figure 6.2 Standard industry rewind chart.
print orientation on a roll (Figure 6.2). Eight orientations are
described. Numbers one through four refer to printing wound to
read from the outside of the roll; five through eight refer to
printing wound inside of the roll. The four numbers of each set
refer to the orientation of the printing relative to the leading
edge of the unwinding roll, top, bottom, right, or left.
The inside diameter (“ID”) of the core on which the slit web
is wound must match the packaging machine equipment. These
are usually 3 or 6 inches (75 or 150 mm). The outside diameter
(“OD”) of the entire slit roll depends on many relatively arbitrary factors. The thickness of the material determines the length
of web wound to a given OD.1 Its basis weight determines the
weight of the entire roll at an OD.2 Figure 6.3 provides reference
for predicting the length and the weight of a roll given its OD.
In Figure 6.3, the area of the cross-section of a roll as shown
is:
Aroll 5 π ð02cÞ2
(6.1)
All of the cross-section outside of the OD of the core represents the web material.3 The OD of the core is the core ID,
plus twice the thickness of the core’s wall (often 1/2 inch12 µ).
1
Packaging machinery productivity can depend on roll length, as replacing an empty roll
requires stopping the equipment and perhaps rethreading the web path.
2
Roll weight limitations may result from the lifting capacity of packaging room equipment
or safety limits on how much a worker can physically lift in the work place.
3
This statement ignores the very thin layer of entrapped air wound with each layer of material on the roll.
64
MANUFACTURING FLEXIBLE PACKAGING
0
ab
c
Figure 6.3 Roll length calculation.
In Figure 6.3, the area of the cross-section inside the OD of the
core is
Acore 5 π ð02bÞ2
(6.2)
The area of web material on the core is
Aroll 2 Aroll 5 Amaterial
(6.3)
The area of web material is equal to its length times its
thickness, so the length of material is estimated as:
Material length 5 Amaterial =material thickness
(6.4)
The material length times web width represents the area of
material in the roll. That area times its basis weight is the
weight of material on the roll.
Customer requirements for slit rolls may also address the
allowable number of splices per roll, the length, if any, of core
extending beyond the stacked edge of web material, and general roll appearance. The last factor results from air entrapped
on the web surface and wound into the roll.4 This layer of air
can escape at low speeds from the ends of the slit rolls. At high
speeds, it is wound as a layer between wraps of web into the
roll where it can act as a lubricant between the layers of
4
Web materials trap layer of air on heir surface as they move through a machine. When
films are slit, the layer of air is carried into the rewinding roll.
6: FINISHING
AND
SLITTING
65
material. Material layers may shift to positions in which the
edge of the web cannot align with the edge of the roll defined
by previously wound material. In extreme cases, the condition
is called “roll telescoping” and can cause damage to significant
lengths of material.
Slitting Options
Basic slitting techniques include:
• razor blade (or burst) slitting: one knife (rotary or
linear) extends through the product to slit it,
• shear slitting: two sharpened knives work together
in a scissor action to slit the web,
• score (or crush) cut slitting: one knife (rotary or
linear) pinches the product against a platen or roller
to slit it.
Minimal value is added to flexible packaging material in the
slitting process. Its input consists of material with all printing,
laminating, coating, labor, and raw material value. The cost of
waste at this stage of converting is substantial, so careful selection of a slitting option must consider how to accomplish the
greatest number of customer-ready rolls in the shortest time
with the least waste. Table 6.1 provides generic comments on
the relative advantages of the options.
Rewind Options [1]
Chapter 1 presents the basics of winding for all web
processes (Tension, Nip, Torque) and describes three winding
processes [2]:
1. Center winding: a rotating rewind shaft turns the
core that holds the winding roll in order to apply a
tension to the web.
MANUFACTURING FLEXIBLE PACKAGING
66
Table 6.1 Considerations for Slitting Types
Method
Typical Application
Razor
Cast films; oriented films Low cost for equipment
under 2 mils (50 µ)
and consumables; quick
set up; slit width
variability/waving
Oriented films 2 mils
Shear force can bend
(50 µ) thick and over,
shaft; match optimum
paper, foil, laminates
blade rotation speed to
web speed; higher
speeds improve edge
quality for some webs
Set up speed critical;
Causes paper dust; plastic
webs too abrasive for
film perpendicular
other methods; edge
cracking; preferred for
quality secondary
pressure sensitive
adhesive-coated paper
Shear
Score
Considerations
2. Surface winding: rotating drum(s) on the surface
of the winding roll apply tension to the web.
3. Centersurface winding: both rotating rewind
shaft and rotating drum apply a tension to the web.
The primary process for flexible packaging, including slitting,
is center winding, sometimes with surface winding assistance.
This technique provides optimal control of roll “hardness,” also
described as “roll density” or “in-wound tension.” This reflects the
amount of air wound into the roll between layers of web. If too
soft, air can escape allowing rolls to lose their roundness, causing
difficulty for smoothly unwinding on packaging machinery. If too
tight, wound-in tension can cause wraps to adhere to adjacent
ones in the roll. Extensible films will stretch in response to the
high tension and then try to return to their original length in roll
form. This can actually crush cores and exaggerate existing web
gauge variations by stretching and deforming film around them.
Tension on the winding web largely determines final roll hardness. Much empirical assessment with web handling has
6: FINISHING
AND
SLITTING
67
T
T =
R
TQ
R↑
TQ
Regulator
Figure 6.4 Center winder—Tension/Torque/Radius.
determined that the material’s modulus of elasticity (see
Chapter 31) has a linear relationship to toll hardness. The basic
center wind process uses the rotating rewind spool to pull the web
from the final nip of slitter onto the rewinding roll. That “pull”
represents a stress on the film. The film’s reaction to the stress is
to elongate (stretch). Figure 6.4 depicts a winding roll with
increasing radius, “R.” As R increases, tension “T ” decreases if
the torque “TQ” remains constant. The center wind “torque” is
controlled by various mechanical and/or electronic mechanisms.
The simple relationship “T 5 TQ/R” programs the regulator
that in turn changes the center winding torque to provide a
desired web tension. This strategy to control tension in order to
obtained desired roll hardness has demonstrated that tapering
tension from a high level at the core of a roll to a lower value
at its top produces appropriate roll hardness. A top of roll value
of 25% of the core setting is typical. The general limit is to
keep tension to less than 1.5% of the material’s elastic modulus. That value reflects intrinsic material properties, the method
used to fabricate the material and material thickness. Table 6.2
summarizes the maximum tension settings for webs of simple
materials that are 12 inches wide.
The relationship for these calculations is as follows5:
Web tension # 0:015 film cross-section area secant modulus
(6.5)
5
For plastic films, the 1% or 2% secant modulus is extrapolated linearly to 100% elongation to estimate elastic modulus.
Table 6.2 Maximum Tension Calculations for Certain Films (See Chapters 2122 for description of
Material names)
Material
LDPE
LLDPE
HDPE
BOPP
OPET
Average Secant
Modulus
Gauge
Web Width
kg/cm2
k lb/in2
micron
inch
cm
inch
1400
1750
11,200
15,850
38,500
20
25
160
230
550
25
25
25
18
12
0.0010
0.0010
0.0010
0.0007
0.0005
30.5
30.5
30.5
30.5
30.5
12
12
12
12
12
Cross-Section
Area
Max.
Tension
cm2
in2
kg
lb
0.0774
0.0774
0.0774
0.0542
0.0372
0.0120
0.0120
0.0120
0.0084
0.0058
1.6
2.0
13.0
12.9
21.5
4
5
29
29
48
6: FINISHING
AND
SLITTING
69
Table 6.2 entries indicate the need to give critical attention
to units of measurement. For example, many machine tension
readings in the USA are denominated in “pounds per linear
inch (PLI).” In such a case, equipment settings for “tension”
would be 1/12 the right column of Table 6.2.
References
[1] D. Whiteside, Basics of web tension control, PLACE Division
Conference, TAPPI, Norcross, GA, 2007, pp. 23.
[2] R.D. Smith, Challenges in winding flexible packaging films,
PLACE Division Conference, TAPPI, Norcross, GA, 2007,
pp. 78.
7
In-Line Processes
Chapter Outline
Equipment Requirements
Operational Considerations
Availability
Performance
Quality
Success Criteria
71
73
73
74
74
74
The essential business model for the converting industry
involves (1) unwinding a web, (2) adding value to it, and
(3) rewinding it. The interval between Steps 1 and 3 costs the
converter money: labor, equipment depreciation, and allocated
plant costs. In general, converter profitability benefits by
increasing the value added during such intervals. More specifically, profitability increases when linking converting processes
in series (“in-line processes”) only if the incremental value
added during the interval exceeds any additional costs (added
waste, extra time, etc.) experienced.
Equipment Requirements
Gravure and flexographic printing are essentially in-line processes. Gravure presses have individual print stations arrayed
linearly. Central impression (CI) flexo presses organize their
stations in a circular array. In both cases, the rotation of print
media from one station to the next must be synchronized with
the speed of the web through the equipment in order to register
colors to each others’ positions. With mechanical systems, a
common drive system with geared linkages to each print station
Manufacturing Flexible Packaging. DOI: http://dx.doi.org/10.1016/B978-0-323-26436-5.00007-2
© 2015 Elsevier Inc. All rights reserved.
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MANUFACTURING FLEXIBLE PACKAGING
accomplishes this. Electronic systems link otherwise independent drives at each station with signals to control rotation.1
One or more gravure “print” stations can be linked to the
drive system of a CI flexo press in order to include printing or
coating functions not otherwise available on the primary flexo
drum. So-called “pattern cohesive coatings” are applied his
way. Such coatings are applied to the inside of printed packaging material intended to wrap heat-sensitive products (e.g.,
chocolate bars). The pattern of the coating consists of a border
printed inside the web and registered to the perimeter of the
printed image on its other side. The coating in this border acts
not only to make pressure-sensitive seals (protecting the sensitive product from heat) but also releasing from the outside of
the material as it unwinds from its roll.
Alternatively, the in-line gravure “print” station can be configured to apply and dry an adhesive on one side of a film that
was just printed on the CI press. Then with an unwind station
for a second web and a nip to join the two, a two-layer printed
laminate is produced with the in-line process.2
Theoretically, two concepts could be combined using two inline gravure stations geared to the flexo press. More commonly, the CI process itself is modified to use the final station
on the CI drum to apply an adhesive, dry it in the overhead
dryer, and laminate it to a second web before a single in-line
gravure station applies the cohesive border.
Extrusion laminating and coating lines are often configured
with two dies and at least two unwinds (called a “tandem”).
This in-line sequence provides an efficient means of laminating
a printed web to a barrier one and then extrusion coating the
sealant layer at the second die. A third unwind provides the
ability to laminate a third web as a specialty sealant or other
high-performance layer.
1
Mechanical systems address the need to make fine machine-direction adjustments for color
register, by providing a braking mechanism to slightly retard rotation at subsequent stations.
They have no provision to speed up rotation if needed. Electronic drives in contrast can
respond either way to input signals. As a result, electronic drives on presses provide quicker
(and better) setup times than mechanical ones.
2
The adhesive coating on top of the color image printed on the press, and its application
method, must be chosen so as not to “rewet” the inks and blur the image.
7: IN-LINE PROCESSES
73
At some point, mechanical variability (e.g., as a result of
vibration, roll alignment) inherent with large equipment limits
the number of in-line processes than can be linked, but material
and operational factors usually prevent expansions before those
are reached. For example, lateral forces from cross-direction
web adjustments can move two laminated webs out of parallel
alignment with solventless adhesives that generate low initial
bond strength. As a result the long web path necessary for two
in-line solventless laminators could cause the two initial webs
to separate before laminating them to a third.3
Given the many possible combinations, equipment design
represents only part of the challenge for implementing in-line
processes. Material limitations play an important role. Often
the converter lacks the ability to confirm materials’ fitness for
use in an in-line process until after the equipment is built and
installed, adding significant risk to the effort to improve
profitability.
Operational Considerations
Even with machines that are physically, mechanically, and
electronically feasible and with materials that are fit-for-use, an
in-line process does not guarantee increased profitability. The
availability, performance, and quality considerations of the OEE
model (Chapter 8) suggest other factors that must be managed.
Availability
Approaching job setup and cleanup requirements for in-line
processes in sequence rather than in parallel manner eliminates
much of the profit enhancing potential of a combined process.
Neither process step can start until the other is ready to begin.
Maximum availability requires that cleanup of the previous job
3
Such tandem adhesive laminators are available for two solvent-adhesive processes and one
solvent-adhesive process followed by a solventless one.
74
MANUFACTURING FLEXIBLE PACKAGING
and setup for the next take place concurrently at each step.
Unplanned equipment outages at one step also reduce the availability of equipment for the other step.4
Performance
Electronic drives and control systems eliminate some, but
not all of the natural mechanical variability in complex in-line
systems. Sheer size of these lines (e.g., 100 m long and
1.5 2 m wide) produces multiple opportunities for mismatch
of speeds, over adjustment, misalignment of rollers, etc. Such
effects are less pronounced and more easily monitored and controlled at low speeds, with the obvious impact on performance.
Stoppages or slowdowns for one step upset the performance of
both steps. If one process is not readily stopped, its output
becomes scrap (including wasted raw materials) until the other
is placed back on-line.
Quality
Natural mechanical variability of complex in-line systems
can lead to quality losses, particularly if lower speeds are not
used to recognize and correct them. The option to remove off
quality material (e.g., bad print) before adding first-quality raw
material to it (e.g., laminating a sealant film to it) does not
apply when the processes run in-line. In-line system complexity
brings with it additional secondary quality characteristics and
process variables that must be verified, understood, and controlled in a quality management program.
Success Criteria
The challenge of envisioning, equipping, mangling, and
controlling in-line processes is difficult but not impossible.
4
The potential to bypass one step of an in-line process increases availability, if alternate
equipment is available to duplicate the process of the malfunctioning equipment.
7: IN-LINE PROCESSES
75
Table 7.1 Success Criteria for In-Line Processes
System
Element
Success Criteria
Machines
Designed, installed, and maintained to minimize
sources of variability
Supplied with features and accessibility to permit
quick cleanup and setup
Properties consistent with minimizing variability
Compatibility with each other in context of all inline processes
Processes and materials compatible with others at
anticipated speeds
Access for adjusting one process without disturbing
other(s)
Training in quick changeover techniques for these
processes
Cross-training for all in-line processes
Maximized line-side verification for rapid decision
making
Optimized communication linkages between
processes
Materials
Methods
Manpower
Measures
A converter who intends to use this approach for improving
profitability must recognize the difficulties and plan to deal
with them. Table 7.1 lists some factors important for making
this planning successful.
8
OEE Effectiveness
Chapter Outline
Overall Equipment Effectiveness
Availability
Performance
Quality
OEE Calculation
References
79
81
81
83
83
85
Overall Equipment Efficiency (OEE) is a hierarchy of metrics
to measure how effectively a manufacturing operation is utilized. OEE provides a particularly useful framework for the
converting industry because of its dependence on capital
equipment and high cost of raw materials (relative to sales
revenue).
The system is based on the pioneering manufacturing
efficiency theories of the self-taught business management
authority, Harrington Emerson (1858 1931). According to a
contemporary report Emerson, who early in his career was a
Professor of Modern Languages, credited French character, and
German military efficiency (witnessed as a young man during
the Franco-Prussian War), for producing his strongest insight—
the need for standards. The discipline evidenced in producing
orchestral music, breeding horses and surveying railroad routes
also persuaded him to seek similar planning and control for
manufacturing processes [1].
When Emerson focused his eclectic interests on manufacturing, he sought to determine product characteristics and costs
(compared to planned outcomes), and losses occurring in the
use of raw materials, while planning, scheduling, and dispatching work through a large factory. The result of his theoretical
Manufacturing Flexible Packaging. DOI: http://dx.doi.org/10.1016/B978-0-323-26436-5.00008-4
© 2015 Elsevier Inc. All rights reserved.
77
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MANUFACTURING FLEXIBLE PACKAGING
Table 8.1 Origins of OEE Methods
Harrington Emerson, 1913 The Twelve Principles of Efficiency
1 Clearly Defined Ideals
2 Common Sense
3 Competent Counsel
4 Discipline
5 The Fair Deal
6 Reliable, Immediate, and Accurate records
7 Despatching (sic)
8 Standards and Schedules
9 Standardized Conditions
10 Standardized Operations
11 Written Standard Practice Instructions
12 Efficiency Reward
The Engineering Magazine; New York, 456 pp.
and hands-on manufacturing efforts became his “Twelve
Principles of Efficiency” (Table 8.1).1 Present-day systems for
manufacturing operations called “Enterprise Resource Planning
(ERP)” and “Quality Management Systems (ISO 9000)” firmly
reflect these principles.
OEE was first described—as a central component of the
“Total Productive Maintenance” methodology—in Seiichi
Nakajima’s book [1a].2 The system has become so effective
around the world that scores of books, consultants, even smart
phone apps are available to help the practitioner implement it.
This description will only highlight its main principles in order
to motivate in-depth understanding.
1
Emerson and Frederick W. Taylor were contemporaries and both are regarded as significant contributors to the origins of scientific management for manufacturing. While Taylor
focused on management’s training and enforcement of standard methods because front-line
workers were not to be trusted, Emerson’s approach included more subject matter expert control along with financial incentives.
2
http://www.oeefoundation.org/origin-of-oee/ retrieved January 15, 2014.
8: OEE EFFECTIVENESS
79
Total time (365 days x 24 h)
Not
scheduled
Total operations time
Loading time
Unscheduled
Failures
idling
Running time (production time)
Line
restraint
Theoretical output
Reduced
speed
Actual output
Good!
Scrap
Minor
stoppages
Rework
OEE solitaire
OEE (top)
Operations effectiveness
Asset utilization
Net utilization (=TEEP)
Capacity utilization
Figure 8.1 Time components of Overfall Equipment Effectiveness.
Overall Equipment Effectiveness
The concept is best understood by imagining equipment
operating continuously at its maximum output rate (i.e.,
product per hour) for 365 days per year and 24 h per day
(8760 h per year). The equipment will essentially never
achieve this theoretical annual output, because of two types of
issues: (1) time during which the equipment does not operate
and (2) product failing to meet its requirements for commercial acceptability. As Figure 8.1 suggests, unscheduled time
decreases the maximum available 8760 h per year (e.g.,
3.8 hour shifts per day for 5 days each week provides only
6257 h per year). Figure 8.1 shows that other nested deductions reduce “Total Operations time” until reaching a theoretical output (equal to production during the actual running
80
MANUFACTURING FLEXIBLE PACKAGING
time). Quality issues (scrap and reworking off specification
product) reduce that theoretical amount to the “Good” (saleable) output.
The transition in Figure 8.1 from units of time to units of
production produces a critical system measure. Executing the
process to produce product requires consuming resources (time,
labor, raw material, utilities, etc.). “Efficiency” is defined as
the amount of resources needed to manufacture a given product
output. “Effectiveness” is defined as the ratio between the
Good output and the theoretical output, and “Productivity” as
the ratio between efficiency and effectiveness. Alternatively,
“Productivity” is the ratio between the amount of resources
needed to manufacture the realized good output and the amount
of resources needed to manufacture the theoretical output.
The quantity (1 2 Productivity) represents waste (time, labor,
material, etc.) for which the operation receives no income.
Efforts to measure, understand, and reduce some or all of the
causes that reduce total time of equipment operation form the
basis of improving manufacturing margins, lowering waste,
improving quality, and increasing value addition. In keeping
with the standardization of manufacturing management efforts,
all of the terms in Figure 8.1 and elsewhere in this discussion
have strict definitions.3 OEE breaks the performance of a
manufacturing unit into three separate but measurable components: availability, performance, and quality. The tool also
allows for specific analysis at the level of product, raw material, shift, etc. It is unlikely that any manufacturing process can
run with all three components at maximum (100% OEE).
Many manufacturers target world class performance, considered to be 85% [2]. The following descriptions are derived
from the OEE foundation, a volunteer industry organization of
OEE users that maintains OEE standards, exchanges experiences in using the method, and improves the overall practice
and understanding of OEE metrics.4 At their core, the OEE calculations pose three questions.
3
4
http://oeeindustrystandard.oeefoundation.org/ retrieved January 15, 2014.
http://oeeindustrystandard.oeefoundation.org/oee-calculation/ retrieved January 15, 2014.
8: OEE EFFECTIVENESS
81
Availability
“Is the machine running or not?” The availability rate
(0 100%) indicates the relationship between the time that the
machine could theoretically have been in operation and the
time that there was actual output. Common reductions to availability include mechanical breakdowns, waiting to define job
requirements, set up and clean up between jobs and waiting for
raw materials.
For example, if a machine produces 420 min (regardless of
speed and quality) during an 8 h (480 min) shift, its availability
is (420/480) or 87.5%. The two other OEE factors in fact
account for the speeds attained and product quality experienced
during these 420 min.
In this metric the origin of OEE in Seiichi Nakajima’s
“Total Productive Maintenance” investigations becomes evident. Nakajima’s insight replaces the short-sighted alternative:
“If it’s not broken, don’t fix it!” with a more sustainable
attitude “Fix the equipment before it breaks your entire
process!” Equipment maintenance, if considered in isolation,
generally represents nonproductive time, producing no saleable
product. Such isolated consideration does not do justice to the
intricacies of any type of equipment. Mechanical and electronic
systems (as well as their physical components) experience
failure rates at statistically expected intervals. “Preventative
maintenance” methods attempt to estimate intervals between
failures and replace susceptible elements before the next failure
during planned maintenance sessions rather than after the failure has taken place. “Predictive maintenance” substitutes active
dynamic monitoring of elements in place of statistical intervalbased maintenance. Either maintenance approach represents an
improvement in future reliability and productive time compared
to the frenzied response of an emergency maintenance outage.
Performance
“How fast is the machine running?” In the performance rate,
“theoretical output” is the output that the machine could have
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MANUFACTURING FLEXIBLE PACKAGING
made in theory if the machine produced at maximum speed
during the time that it actually operated. Minor stoppages and
reduced speed reduce theoretical to “actual” output. For example, an operator might stop a printing process to remove a contaminant from the surface of printing media or that operator
might sense that the equipment experiences excessive vibration
when operating above a particular speed and choose to run
slower than the standard. (In fact vibration represents a common dynamic factor monitored by a predictive maintenance
system.)
In the example above, a machine produces 420 min at the
rate of 8 pieces per minute (regardless of quality) during the
shift (3360 pieces) while the expected output is 4200 pieces
(10 pieces per minute). Its performance is (3360/4200) or 80%.
It should be obvious that the rate of 8 pieces per minute in
the example corresponds to an average production rate for the
420 min of available time. Running at the reduced rate of 80%
of the standard speed would generate the 3360 pieces, as
would running 84 min during the shift with zero output and
336 min at the standard rate of 10 pieces per minute. Knowing
which actually happened during the shift prompts preventive
maintenance attention in one case and attention to the condition of ink formulations and/or environmental conditions in
the other.
Business management expert, Peter Drucker is credited with
saying “What gets measured gets managed.”[3] Monitoring
the performance of a manufacturing operation provides a valuable perspective on how additional production might be
realized without significant additional capital investment.
A simple operator’s log that notes the cause and beginning and
ending times of a minor stoppage can provide important insight
into circumstances that managers and supervisors might
otherwise overlook. Control systems typical for any modern
electronic-driven equipment provide automated Supervisory
Control and Data Acquisition records of a process’ operation
(Chapter 10). Used together, both the manual and the automated records provide the basis for troubleshooting and process
improvement.
8: OEE EFFECTIVENESS
83
Quality
“How many products met specifications?” The relationship
between the number of units produced and the number of the
units produced that meet specifications is the “quality rate.”
The latter number includes deductions for products that will
never be sold (scrap) and those for which additional resources
(time at a minimum) must be invested to bring into specification compliance.
To the example, of the 3360 pieces produced during the
shift, 168 are rejected (3192 meet specifications). The quality
rate is (3192/3360) or 95%.
The financial and competitive advantages to manufacturing
in transitioning from quality control systems, to statistical process control, to total quality management to audited quality
management systems are well documented.5 This sequence
reflects increased understanding of the importance (and cost) of
variability (Table 8.2). An original focus on specifications
described in terms of the secondary quality characteristics of
products (Chapter 32) has grown to include whole business systems much as Emerson’s early principles suggested.
OEE Calculation
The single measure of overall equipment efficiency is the
product of these three measures: Availability 3 Performance 3
Quality. In the example, 87.5 3 80 3 95% 5 66.5%. Figure 8.2
provides a graphic summary of the cumulative impact of loss
of available time, performing below standard rates, and failure
to meet quality requirements.
Because computing OEE is the simple product of the three
component values, increasing any one value provides the same
proportionate increase in OEE itself. In the example used
here, Performance has the lowest value, suggesting if the
5
ASQ Global, http://asq.org/learn-about-quality/history-of-quality/overview/total-quality.
html retrieved January 16, 2014.
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MANUFACTURING FLEXIBLE PACKAGING
Table 8.2 Evolution of Variability Concerns in Quality
Management
Quality Approach
Variation Focus
Primary Quality
Activity
Quality control
Variation beyond
allowable
specification
tolerances
Variability of
secondary quality
characteristics over
time
Variability of critical
process conditions
over time
Variation of
organizational
behavior from
optimum norms
Measure secondary
quality
characteristics
Statistical product
control
Statistical process
control
Total quality
management
(Quality is
everyone’s job)
Monitor secondary
quality
characteristics
Monitor critical
process conditions
Integrate efforts of
organizational
groups into
optimum
economic solutions
for customers
Quality
Deviation from
Documentation of
management
formal work
business and
system (audited
practices and
manufacturing
conformance to a
expected outcomes
processes
global standard)
cause of reduced performance can be determined (in the
example, dirty print media or equipment vibration) and
remedied, OEE can be increased accordingly. Each combination of equipment and organization generates a different set of
priorities for process improvement (i.e., increasing OEE).
Systematically addressing each priority to increase the associated value affected by it provides the basis for increased
quality, productivity, and profit.
8: OEE EFFECTIVENESS
Availabilty
Performance
Quality
A
Potential production time
(480 min)
B
Actual production time
(420 min)
C
85
Availability losses
• Breakdowns
• Wait/changeover
• Line restraint
Theoretical output
(420 min x 10 part/min = 4200 parts)
D
Actual output (3360) parts
E
Actual output (3360) parts
F
Good product (3192) parts
Quality losses
• Minor stoppage
• Reduced speed
Quality losses
• Scrap
• Rework
Effectiveness loss
Figure 8.2 Components of OEE losses matched to potential output
(not to scale).
References
[1] H.B. Drury, Economy Scientific Management, second ed., rev.,
Columbia University Press, New York, NY, 1918, p. 125 126.
[1a] TPM tenkai. Japan Institute of Plant Maintenance, Tokyo, 1982.
[2] F. Wauters, J. Mathot, Overall Equipment Effectiveness, ABB,
Inc. Wickliffe, Ohio, 2002, pp. 27.
[3] L. Prusak, Harvard Business Review Blog. http://blogs.hbr.org/
2010/10/what-cant-be-measured/, 2010 (accessed 15.01.14).
9
Efficiency and Cost Accounting
Chapter Outline
Efficiency
Material Waste
Time Waste
Cost Accounting
Minimum Order Size
References
88
90
92
93
98
101
The business of manufacturing flexible packaging has impressive benefits compared to many other industries:
1. The product is typically consumed steadily and soon
after its manufacture, providing good cash flow and
manageable finished goods inventory costs.
2. As a business-to-business industry, the customer
buys the packaging products in order to move its
product to market. Assurance of ongoing delivery of
the packaged product to its market involves switching costs that deter impulsive supplier changes.
3. Access to equipment and materials used in the
industry generally remains open, allowing one converter to invest financial and intellectual assets to
match its competitors’ innovations with reasonably
quick alternative of its own.
Any converter in the industry can develop business plans and
strategies to exploit—or ignore—one or more of these benefits.
• Reducing the “economic order quantity” or job size
is an effective means of both adding productive
capacity and reducing inventory costs. Supply chain
Manufacturing Flexible Packaging. DOI: http://dx.doi.org/10.1016/B978-0-323-26436-5.00009-6
© 2015 Elsevier Inc. All rights reserved.
87
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MANUFACTURING FLEXIBLE PACKAGING
integration, both backward (e.g., extruding films for
converting operations) and forward (e.g., fabricating bags or pouches for form/seal packages), captures that much more value-adding opportunity. It
creates opportunities for subtle product differentiation
that makes competitive products less interchangeable.
Patented products can also create barriers to competition, but require constant market surveillance for
infringement and the willingness to pursue legal
redress whenever it’s found.
• Inventory costs and write-offs of finished goods
can expand dramatically if the converter tries to
minimize setup and cleanup time by running quantities of product too large for the customer’s order
pattern. Some flexible packaging product segments
have become so generic that the cost of switching
suppliers is minimal. Imitating a competitor’s new
product takes time, money, knowledge, and the
will to learn something new.
Efficiency
Annual surveys by the Flexible Packaging Association in the
USA indicate that on average about 60% of the selling price of
flexible packaging represents expenditures for its raw materials
(Figure 9.1) [1]. “Other manufacturing costs” and direct
“labor” add another 25% of the selling price of the industry’s
product to its cost. Obviously, increasing the efficiencies of
manufacturing processes represents the major opportunity of
profit management for the converter.
The elemental industry formula for business success is 1.
Buy raw materials; 2. Add value to them; 3. Sell the resulting
product for greater than the costs of No. 11 No. 2.1 The
1
“Contract converting” is an alternative method. In this case the customer buys the raw
material and then buys back the converted value-added product at an agreed upon premium.
Usually some provision is made for an amount of “wasted” scrapped during converting and
the converter takes the risk that he can deliver the negotiated finished quantity without excessive waste.
9: EFFICIENCY
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89
Material
Other mfg costs
Direct labor
Sales/Adm/R&D
Profit before tax
Figure 9.1 Cost components US flexible packaging industry 2012
(FPA).
industry gives much emphasis to buying raw material at prices
lower than competitors can (e.g., volume discounts or dedicated
freight arrangements). Raw material suppliers themselves have
both raw material costs and competitors, so price concessions
are limited. The raw materials more closely reflect the swings
in market prices of commodities (e.g., the prices of resins used
to make an extruded plastic film closely follow market prices
of oil and gas), limiting the ability of suppliers to offer price
concessions. Those suppliers also have the need to sell as
much of their manufacturing capacity as possible, so the risk of
losing a customer who learns that another one enjoys special
pricing considerations tends to keep prices level across the
industry.
Those limitations on managing the “delivered” price of raw
materials make focus on the efficiency of manufacturing processes critical. That focus in practice includes two critical
objectives:
1. Minimize the amount of delivered raw material
that is wasted and as a result never sold to a customer as a value-added product.
2. Minimize the amount of time that the converter’s
machines and manpower are no actively adding
value to those raw materials.
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MANUFACTURING FLEXIBLE PACKAGING
Material Waste
The 60% of selling price represented by raw materials in
Figure 9.1 does not represent the delivered price of raw materials as a percentage of selling price. Rather, Figure 9.1 is an
aggregate representing the total raw material expense divided
by the total sales revenue. The difference is the material wasted
to which value is never added. A loss of 10% by weight of the
material purchased by a converter compared to the weight of
raw materials purchased is not uncommon. This waste experience implies that if $0.60 of a dollar of sales revenue reflects
raw material costs, the supplier sold the material in the finished
product for $0.54 (i.e., $0.60 (10.1)) and $0.06 worth of raw
material was wasted.
Waste in converting has two primary sources: programmed
waste and operations waste. The need to use wider input material in the printing, laminating, and slitting processes than the
sum of the widths of the output webs represents “programmed”
waste. Variation in the lateral position of a web as it moves
from unwind to rewind (Chapter 1) in a process requires the
use of wider material so that the applied inks, adhesives, etc.
remain within the width of the web at all times. The web
guides that actively track and control a web’s lateral position
can use not only the physical edge of a web, but alternatively a
printed line parallel to the centreline of graphics. Subsequent
operations may trim and discard the narrow strip of material
between such “guidelines” and physical edges of webs. That
strip, perhaps 0.125 inch (3 mm) wide, called a “trim allowance” represents programmed waste. Multiple converting
processes may each involve its own trim waste. The result
makes the trim waste required by each process additive when
determining starting widths for web materials. As the width of
material being converted increases, the weight percent of this
linear trim material decreases in comparison to the total finished weight of web needed. Electronic drives (in place of
mechanical gearing) as well as other equipment improvements
have reduced the total lateral position variability and the width
of trim allowance required.
9: EFFICIENCY
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91
Operations waste reflects a very different picture. It certainly includes the quality deduction discussed as part of OEE
measurement (Chapter 8), but usually more results from setup
and cleanup of jobs (see minimum order quantity below).
Statistical process control assumes a steady-state process.
When equipment operation requires frequent start-ups and
shut-downs, a steady-state condition is the exception. In the
cleanup process, changing the materials from the previous job
to those for the next one usually entails waste because used
material from the previous job must be discarded or is too
little to reinventory. Simplifying a product line (e.g., using
the same adhesive, extrusion laminating resin, or ink types
and colors on a piece of equipment) can minimize waste of
these materials. Sequencing jobs with the same web materials
(even similar widths) reduces waste. Many of the innovations
highlighted in Chapters 27 address machine design modifications intended to reduce waste of both materials and time
between jobs.
Job setup, if not well managed, can generate waste more
costly than the value added by the converter. This occurs as the
process is set up, run for 50 min, stopped, and the last material produced checked by quality control inspectors (often
called “QCed”) for conformity to specifications. When specifications are not met, all of the material produced in that cycle is
scrap, and the sequence restarted until satisfactory results are
attained. The printing process often generates the most scrap
with the number of colors that must be checked individually,
as well as between-color registration.
The cost of wasted material can be minimized by using
“make-ready” stock. This is web material that in some way is
not suitable for customer product (e.g., excessive film haze or
gauge variation). Suppliers sell such “off-spec” material at
reduced prices to salvage some value from quality losses.
The most effective means of controlling process waste
during job setup involves “standard operating procedures
(SOPs)” developed by operators and reviewed and accepted by
other operator teams. All operators receive SOP training and
document their use of them during each setup. In effect, the
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MANUFACTURING FLEXIBLE PACKAGING
nonsteady-state activities during job setup are systematized into
repeatable behaviors able to produce predictable results.
Time Waste
Figure 9.1 includes labor costs as well as the opportunity
cost of owning and running converting machines (as well as
inventory—raw, in-process, and finished materials—“carrying”
costs). The converter must cover all of these costs whether or
not producing saleable material. If equipment is not running,
no value is added to raw materials but he still incurs these
costs. A converter adds his time and institutional knowledge to
raw material to accomplish the addition of value. As one point
of reference, the US EPA in consultation with flexible packaging industry representatives considered 2000 h per year as the
normal operating time for printing presses in the industry while
developing its “reasonably available (emissions) control technology” guidelines in 2006 [2]. That is about one-half, onethird, or one-fourth of staffed production hours for 2, 3, or 4
shift operations, respectively. In 1977, a Flexible Packaging
Association poll of its members for an earlier US EPA emissions control recommendation found that this equipment actually operated one-third of staffed production time [3].
Much of the activity that wastes material also wastes time,
so attention to one provides improvement for both. Some
wasted time can be minimized by giving the operators the tools
and authority to confirm specification conformance machineside instead of relying on quality department inspectors in a
centrally located lab to measure product characteristics.
The most effective means of minimizing time waste during
job setup involves “setup reduction/quick changeover technique.” Manufacturing processes in general face the challenge
of minimizing wasted equipment time (the more customized
and shorter the jobs, the more critical the challenge). Quick
changeover is a tool of lean manufacturing, which focuses on
eliminating wasted time. Benefits of the technique require
changing the behavior of operators during changeover, so its
9: EFFICIENCY
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93
implementation usually requires in-depth participation by
operators in understanding the need, recognizing the opportunities, and helping design the new behaviors. Designing the
new SOPs starts by analyzing setup activities and separating
them into “external” and “internal” categories. External tasks
can be performed when the machine is running while internal
refers to tasks that must be performed when the machine is
shut down. (Examples of time wasted for internal tasks include
searches for equipment, tools, and cleaners with the machine
down.) The goal is to minimize internal elements as much as
possible, reducing time the machine is shut down and not producing. SOPs identify the necessary external elements and
direct they are performed before the current job ends.
Setup reduction also delivers other benefits such as shorter
lead times, higher productivity, increased capacity, more flexibility, fewer defects, and postponing capital investment for
new capacity.
Cost Accounting
The flexible packaging industry considers itself a “job
shop,” make-to-order business, distinct from the “process
industry,” make-to-stock nature of many of its suppliers and
customers. The distinction places emphasis on a “job”: defining
it, preparing for it, executing it, storing it, shipping it, and
being paid for it. A “job” for cost-accounting purposes represents unique material manufactured for a specific customer at a
particular time. Managing and accounting for “time” in a converting operation becomes the major focus of maintaining and
enhancing profitability.
Scheduled production time in a converting operation comprises:
1. Setup time
2. Running time
3. Cleanup time
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MANUFACTURING FLEXIBLE PACKAGING
Availability
Availability
A
Potential
production time
(4160 h)
B
Running time
(2080 h)
C
Production
planned
(2080 h x 600
fpm)
= 75 millon feet
D
Setup and
cleanup
time (2080 h)
Setup and
cleanup
time (0 fph )
Effective feet per
minute: 300 fpm Setup and cleanup
time (50% of
= (0.5 at 600
potential)
fpm + 0.5 at 0 fpm)
Production
ft/min
ft/h
h/day
day/week
h/week
week/year
h/year
ft/year
Annual potential
600
36,000
16
5
80
52
4160
149,760,000
8
5
80
52
2080
74,880,000
Annual planned
Figure 9.2 Effective feet per minute calculation.
As discussed above, only running time adds value to raw
materials, but all three types have a common opportunity cost
structure. This cost includes workers staffing a piece of equipment; baseline consumption of utilities (particularly electricity
and natural gas); allocated costs of plant management and staff;
general facility costs (e.g., taxes, insurance, heating/airconditioning); and depreciation. The size of the US retail food
market2 historically supported long-running jobs (e.g., 23
days running on one machine). The frequency of such jobs
decreased as retailers expanded regional market segmentation,
with segment-specific packaging needed to support their
efforts. At the same time, faster and wider converting equipment produces more product volume in a unit of “running
time.” The proportion of “unproductive” cleanup and setup
time increases as these factors lower running time.
Figure 9.2 introduces the powerful notion of “effective feet
per minute” (efm) for the converting industry. The value is
simply the weighted average of setup and cleanup time (no production or 0 feet per minute) and the average run speed during
production (600 fpm in the example). The calculation recognizes that during some portion of the time that equipment is
staffed and scheduled to run, it remains unproductive. The
2
The FPA State of the industry Report for 2013 indicates that just over 50% of the $27.7 billion of flexible packaging sold in the US packaged retail food products.
9: EFFICIENCY
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95
relationships in Figure 9.2 demonstrate the power of eliminating time waste in raising output and effective capacity. In the
example, reducing setup and cleanup time for two lines (e.g.,
printing presses) effectively adds the equivalent output of
another line with no capital investment.
Cost accounting must deal with many types of cost
elements:
• Direct and indirect costs
• Current and capital expenses
• Local facility and general company costs
Financial accounting for all such costs follows “Generally
Accepted Accounting Principles” (GAAP). These combine
authoritative standards (set by policy boards, often national)
and simply the commonly accepted ways of recording and
reporting accounting information (e.g., textbook booking). The
system provides the means of calculating taxes, reporting to
stock-holders and regulatory agencies, and negotiating loans
and financial instruments. While financial accounting is certainly not entirely rigid in its practice, cost accounting in general presents a much flexibility and managerial discretion. It
serves to capture a company’s costs of production by assessing
the input costs of each production step as well as fixed costs.
Cost accounting allows management to both plan (budget) for
the future and review past performance for its conformance to
expectations and opportunities for improvement.
One model for using cost accounting principles to predict
expected and measure actual costs of jobs is shown in
Figure 9.3. Although cost accounting has many commonly
accepted ways of recording and reporting information, no
“authoritative standards” control its practice. The primary purpose of the information is to help manage the operation in
order to improve profitability. Costs are divided into two major
kinds: direct and indirect. Indirect costs reflect the “cost of
doing business” at a particular location of a specific company.
Figure 9.3 considers “plant” and “company” overhead factors.
This distinction depends very much on how a company is
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MANUFACTURING FLEXIBLE PACKAGING
Materials
Machine hour rate ($/h) =
labor + capital + indirect
1-Labor
Direct
Freight
Packaging
2-Capital*
Job cost
Utilities
Taxes
* “Allocated
costs"
Plant
overhead
Insurance
3-Indirect*
Company
overhead
Plant staff
Figure 9.3 Buildup of job cost estimate/calculation.
organized and managed, but it should serve to emphasize where
and by whom various costs are authorized and controlled. In
Figure 9.3, this includes costs for utilities (real estate), taxes,
insurance, and employees not assigned to operate a specific
machine (e.g., managers and supervisors, and works in maintenance, quality, and process engineering departments). In practice, estimated budgets (usually annual) for these plant and
company factors are produced and approved. This total expense
is then “allocated” to machines on some basis, often the total
potential production time (Figure 9.2).
The direct costs types listed in Figure 9.3 include one for
which a similar allocated cost factor is needed. Financial
accounting has various techniques (called depreciation schedules) for assigning the current cost of using a long-term asset
that was purchased in another period. The simplest is to divide
its actual cost by its expected useful service life years and
9: EFFICIENCY
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97
assign this value to the asset in the current year. This cost unitized to an hourly figure by the total potential production time
may be the allocated figure. Alternatively, the calculation can
use the replacement cost of the asset for computing allocations.
In the former case, the allocated cost of the asset drops to zero
(if its actual service life exceeds its expected life). In the latter
case, the allocated cost increases over time if replacement costs
inflate. Management expectations and plans for the business’s
future affect which allocation is used. Added together, all of
the allocated costs plus the costs of labor to operate the equipment represents the cost of a “machine hour.” In this way, the
cost-accounting effort establishes a “machine hour rate” ($/h):
effectively the minimum “rent” that a job must pay the manufacturer for the machine resources used for its production.3
The cost accounting model provides the basis for estimating
the cost to produce a job. With the number of impressions printed
across a web established, the length of material represents:
Total number of impressions 3 impression lengthðftÞ
Number of impression across web
This nominal “run length” (in feet) of the job divided by
the efm rating for the equipment that will run it estimates the
machine hours needed to run the job, and, with the machine
hour rate, the “cost” to use that equipment.4 If the job is not run
(i.e., no variable costs for material, packaging, or freight incurred)
the company nevertheless will experience these costs. If the job is
run, the time it requires is not available to run another job.
Figure 9.3 provides a basis not only for estimating the cost
to run a job, but also for “taking out costs.” In addition to the
material and time waste factors discussed above, overhead factors may provide an opportunity to reduce costs. Using operators to perform conformance testing for specifications could
reduce the cost of quality control inspectors. Using process
3
Multiple production lines and machines complicate actual cost accounting and budgeting
effort. Managerial preferences for budgeting and controlling costs also influence costaccounting practices.
4
Adjustment for process waste allows procurement of sufficient material for the job.
Program waste must be added to the nominal web width (impression width 3 number of impressions across).
98
MANUFACTURING FLEXIBLE PACKAGING
engineers to establish Standard Operations Conditions could
replace conformance testing entirely with process monitoring.
Minimum Order Size
Figure 9.2 demonstrates the significant impact that job setup
and cleanup time has on the converter’s revenue potential.
Requiring long-length jobs reduces the number of nonproductive setup/cleanup cycles, but it also restricts access to the
many shorter length jobs in the marketplace. Chapter 8 (OEE)
and this chapter emphasize time as the critical success metric
for the converter:
• Equipment and inventory use capital (for which
interest per unit of time—i.e., investors’/owners’
opportunity cost—is charged),
• Labor receives wages for time on the job,
• Raw materials are converted into product and/or
waste during the time a job is running,
• Other costs (sales, administration, utilities, supplies, etc.) accumulate over time in keeping the
business going.
The basics of cost-accounting presented here help in making
real-time decisions about what jobs to undertake under what
circumstances (e.g., accept or reject a small-size job; raise or
lower a selling price; break a large job into smaller ones corresponding to a customer’s delivery patterns). This involves a
high-level cost-accounting computation, the “value-add rate”
(VAR). At the simplest level, VAR equals “budgeted expenses”
for an entire operation divided by the “scheduled production
hours” for the production equipment.
For a simple example—a one-press operation is scheduled to
operate two 8 h shifts per day, 5 days a week all year:
ð5 days=week 3 52 weeks=yearÞ 5 260 days
At 16 h per day this is 4400 production hours per year. If the
operation is budgeted to spend $1.6 million on all costs, direct
9: EFFICIENCY
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99
and indirect, the press must produce on average $363 in
sales per hour in order to generate enough sales income to
cover the expenses of 1.6 million. $363 in sales per hour
becomes the break-even VAR for the simple operation.
A profitable operation must average more than this minimum
VAR. For example, 10% return on sales requires $400 per
hour.5 The latter figure is the “Target” VAR (TVA). Decisions
described above depend on the value of this inequality:
ðSales valueÞ 2 ðJob costÞ
$ Target value 2 Add rate
Timeconsumed
Cost accounting provides values for all the variables used in
the calculation:
• Sales value: the product of the unit price (e.g.,
$ per impression) times the number of units sold
(impressions).
• Job cost: the sum of fixed and variable costs
assigned to producing the job (as discussed above).
• Time consumed: the elapsed scheduled production
time between starting this job and starting the next
scheduled one.
• Target VAR: a company-specific value determined
on a periodic basis (usually annually), based on
“budgeted expenses” for the entire operation divided
by the “scheduled production hours” for the production equipment adjusted for an expected return on
sales value.
Table 9.1 summarizes an example job to demonstrate the use
of VAR.
• Sales value: Sell 25,000 impressions (8.67 in web
width with a 11.75 in cutoff (102.3 si/impression))
for $0.15/impression. The sales value is $3750.
5
The 2013 FPA State of the Industry Report indicates that industry average ROS ranges
from 4% to 6%.
MANUFACTURING FLEXIBLE PACKAGING
100
Table 9.1 Example Job Details for VAR Calculation
Printing Details
Width
(in.)
Cutoff (in.)
Setup (h)
Cleanup
(h)
Across
(number)
8.66
11.82
6
4
5
Order Size Sell Price
(imp)
($/imp)
Sales
Value
($)
Run
Run Speed
Length (ft) (fpm)
25,000
3750
4925
Job Details
0.15
350
Time Calculations
Run Length Run
(min)
Length (h)
14
0.25
• Job cost: Raw material costs (the major portion of
variable costs) are included as part of the annual
budget. Estimate job cost relative to the TVA as
6 h setup time, 4 h cleanup time, and run time
equal to ((run length in feet)/(run speed in fpm))/
60 min/h.
• Time consumed: Assume the press operates at
350 fpm and the job is run at 5 across. Then the
press produces the 25,000 impressions in 14 min
(about 0.25 h). The time consumed to estimate for
the job is 4 1 6 1 0.25 h or a total of 10.25 h.
• Covers VAR: The estimated job VAR is ($3750/
10.25) 5 $366. This production costs ($363/h) but
falls short of the $400/h target VAR. If all other
assumptions are held constant, the order quantity
necessary to match or exceeds the target VAR is
demonstrated graphically in Figure 9.4 suggesting
9: EFFICIENCY
AND
COST ACCOUNTING
101
Figure 9.4 Graphic solution for Minimum Order Quantity in VAR
Example.
increasing the order by about 10% to 27,400
impressions would meet the target. Increasing selling price by 10% ($0.165 per impression) also
increases the VAR above target. The latter illustrates the rationale of the industry’s practice of
quoting prices in “quantity brackets,” by which
unit costs decrease as job sizes increase.
References
[1] Flexible Packaging Association, State of the US Flexible
Packaging Industry Report, Linthicum, MD, 2013, 122.
[2] US Environmental Protection Agency, Control Techniques
Guidelines for Flexible Package Printing, Office of Air Quality
Planning and Standards, Research Triangle Park, NC, 2006, 33 pp.
[3] US Environmental Protection Agency, Control Techniques
Guidelines for Graphic Arts (rotogravure printing and flexographic printing), Office of Air Quality Planning and Standards,
Research Triangle Park, NC, 1978, 63 pp.
10 Basics of Control Systems
Chapter Outline
Distributed Control Systems
Data Inputs
Process Feedback
Open-Loop Control System
Closed-Loop Control System
PID Controls
References
103
105
106
106
107
108
110
Web-converting processes represent relatively continuous
manufacturing systems with many subsystems that must remain
within expected limits for consistent product output. The state of
those subsystems (e.g., the outside diameter of unwinding and
rewinding rolls) constantly changes requiring mechanical
changes to continue consistently. The purposes of a control system include monitoring such states, storing the data in a timeseries database, automatically adjusting conditions according to
preprogrammed decision logic, and responding to operator input.
Distributed Control Systems [1]
Distributed control systems (DCSs) are computer-software
packages communicating with control hardware and providing
a centralized human machine interface (HMI) for controlled
equipment.1 Programmable logic controllers (PLCs) form the
core of DCSs and other computer control systems. These
replace hard-wired relay circuits and allow easy programming
and reprogramming; easy diagnostics and repair; and communicating with central data collection systems feeding a DCS. The
device included a power supply, processor, communication
1
SCADA systems have traditionally described control systems for multiple independent systems, distributed over large geographic areas while DCS is the term more properly describing
systems on converting equipment.
Manufacturing Flexible Packaging. DOI: http://dx.doi.org/10.1016/B978-0-323-26436-5.00010-2
© 2015 Elsevier Inc. All rights reserved.
103
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MANUFACTURING FLEXIBLE PACKAGING
module, and input/output module. They are physically placed
close to equipment sensors sending data and devices receiving
commands from them. Different functionality can be combined
according to the requirements of the system to be controlled
(e.g., temperature, tension). The development and implementation of PLCs was the first step toward the highly interconnected DCSs in use today. General information technology
systems issues (e.g., security, communication protocols, programming languages, operating systems) make such software a
critical part of a company’s overall IT policy and a key building block for high-level resource planning systems.2
A DCS provides operators and others with a centralized
overview of conditions on a piece of equipment. Depending on
the process, dozens, even hundreds of machine set points and
process variables may be observed through the interface.
Observations may lead to a decision that set point changes are
necessary or to simple confirmation that the process in fact is
operating in control. When the latter is the case, those set
points may be recorded and stored as a “recipe” for the next
time the product is run. Figure 10.1 presents a DCS screen
A
2nd COATING
1st COATING
2nd LAMI
1st LAMI
90Ø
1st UW & INFEED
A
65Ø
B
TENSION
1st UNWINDER
INFEED
1st COATING
90Ø
2nd UNWINDER
A
Total
3rd UNWINDER
B
1st
LAMI
3,690
0
B
A
1st
SANDWICH U/W
2nd
COATING
2nd
SANDWICH U/W
32,767
REWINDER
B
A
B
REWINDER
TAPER TENSION
SET (kg)
8.0
8.0
9.0
12.0
8.0
11.0
9.0
14.0
8.0
8.1
9.0
11.8
7.8
11.0
9.0
12.3
35
%
Figure 10.1 DSC screen of tension settings and readings on tandem
extrusion laminator/coater.
2
Enterprise resource planning (ERP) is a software-supported business management process
intended to provide visibility of business conditions, including manufacturing conditions, to
various levels of oversight, including, procurement, scheduling, payables/receivables, inventory,
and performance to plan. For example, while oven temperature and tension data from a DSC
may have little use beyond the immediate equipment’s operation, data on its speed and raw
material consumption roll up into higher level metrics for managing the operation.
10: BASICS
OF
CONTROL SYSTEMS
105
image from a tandem extrusion line. This indicates tension settings for the three unwinds and one rewind as well as four
intermediate nip points. The physical distance from first
unwind to rewind can be as much as 100 ft (30 m). The challenge of monitoring numerous process conditions over such
large distances underscore the utility of a DCS.
Data Inputs
Process conditions, whether observed at a sensor, a PLC, or a
DSC screen, represent physical devices interacting with environmental states as a function of some measurement principle and
communicating that degree of interaction through some information channel to a human observer. The validity of data that
appear reliable and precise on screens such as Figure 10.1 is in
fact contingent on multiple factors in the design, installation, and
maintenance of the equipment. Operators and others attempting
to control—or understand—a process must understand that data
provided by control systems is only as good as the entire network
from sensor to display. Calibration, with validation and verification, of sensors must be part of a periodic maintenance. The supply of electrical power must have reliability and repeatability
suitable for the sophisticated signal processing involved in communicating a sensor’s state to a centralized system.
For example, each tension reading from Figure 10.1 represents a physical set of strain gauges. Figure 10.2 depicts the
general arrangement with two dual cantilever beams, one at
Figure 10.2 Physical configuration of tension transducer.
106
MANUFACTURING FLEXIBLE PACKAGING
each end of the roll, and strain gauges mounted on the top and
bottom surfaces of each. The bearings are attached to the free
end of each beam. When web tension is applied the beam
deflects a small amount, causing an electrical output from the
strain gauges. The electrical output is an analogue signal that a
PLC processor translates into a digital “read-out” for the DCS
display.
DCS designs on the whole are robust and provide reliable
data on which to base process decisions. Installation and maintenance of equipment, including system subcomponents and
new sensors, represents occasions for which regular calibration
and verification procedures must be observed.
Process Feedback
The DCS provides an overview of process conditions at a
given time. A process that is in-control will not require frequent modifications, but if a process variable shifts to an out of
control value, the DSC facilitates operator intervention (based
on predetermined decision rules) to return to an in-control
state. When developing and verifying a process, a DCS provides the ability to vary process variables for conducting
designed experiments and optimizing product performance.
Both types of change require control system access to the
equipment and the means to bring about the desired change.
For example, Figure 10.1 indicates that the rewind taper tension (Chapter 1) be set to 35%. The screen presents only this
set point for taper tension while providing both set point and
measured values for other tensions. The former is an example
of an “open-loop” control system while the latter is a “closedloop” control system.
Open-Loop Control System
An open-loop control system provides initial instructions to
equipment but provides no means of gathering data and comparing assumptions underlying those instructions to the reality
10: BASICS
OF
CONTROL SYSTEMS
107
of what the equipment actually produces. Such control techniques are acceptable (and lower cost) if equipment operations
are reasonably predictable, and the consequences of deviation
from assumed output conditions are minimal and acceptable.
For the example of rewind taper tension in Figure 10.1,
Chapter 1 discussed the dependence of rewind tension on the
(constantly increasing) outside diameter of a rewinding roll.
With operator input of starting and taper functions of winding
tension, the equipment’s electronic rewind drive can be programmed to calculate the complex mathematical dependency
of desired rewind tension on roll diameter or winding speed.
The basic relationship is complicated by inevitable nonsteadystate conditions (e.g., torque losses from rolling resistance or
inertial losses during acceleration and deceleration).
Closed-Loop Control System
In contrast, a closed-loop control system provides initial
instructions to equipment and includes means of (1) gathering
data and (2) comparing assumptions underlying those instructions to the reality of what the equipment actually produces,
and (3) adjusting the equipment to bring production in line
with expectations. These three steps are repeated until a satisfactory steady state for the equipment output is achieved.
Figure 10.3 provides a schematic of a closed-loop control
system. Its lower half portrays the critical system function of
comparing a measured value to the desired one (the reference
value). The difference prompts the controller (DCS) to calculate whatever physical adjustment is appropriate to modify
S
i
g
n
a
l
2.
Processor
Re
fe
re
nc
e
va
lv
e
Measured
deviation
3.
Regulator
Regulate
equipment
Equipment
1.
Sensor
Figure 10.3 Functional Schematic of closed loop control logic.
S
T
s
t
e
m
O
u
t
p
u
t
C
l
o
s
e
d
l
o
o
p
108
MANUFACTURING FLEXIBLE PACKAGING
system output and signal the appropriate regulator to cause the
change. For the rewind taper tension, this would require a tension measuring roller between the second laminator nip and the
rewind roll. Data from this measurement could be used to verify the conformance of process output to theoretical values. If
measured tension is consistent with expected values for a specific product, the open-loop strategy would be consistent. The
data set can be used to adjust the initial instructions to provide
an optimum solution for the product using an open-loop
system.
PID Controls [2]
In the closed-loop system description here, the DCS must
“calculate whatever physical adjustment is appropriate to modify system output.” That calculation must take into account the
physical limitations of the equipment.3 A thermostat (e.g., in a
drying oven or extruder barrel) provides a simple illustration.
A set point temperature would appear as a precise, unique
number on system display screens. The logic is to increase heat
input (e.g., more gas flow to burners or electrical current to
band heaters) if the measured temperature is too low or
decrease it if measured values are too high.
Difficulty arises when changing units of heat input does not
result in a direct, discrete temperature change. The minimum
increase in heat may cause temperature to overshoot its set
point. Temperatures in equipment may be less responsive to a
signal to decrease because of the efficiency of cooling systems or heat capacity of equipment components. With enough
knowledge of physical constraints, an appropriate range about
a set point may represent a “neutral zone” in which the control system does not respond to a deviation from the discrete
set point.
3
Analog variables (e.g., temperature and airflow) are particularly subject to such limitations. Electronic drives have much more precise controls compared to the clutch and brake
controls provided on mechanical drives. The consequences of such precision include higher
product quality (less output variation) and faster process setup (exact settings for system
variables).
10: BASICS
OF
CONTROL SYSTEMS
109
Without such knowledge, control system logic called a “proportional-integral-derivative” controller (PID controller) is effective.
This is a generic control-loop feedback mechanism. The essential
advantage of a PID controller is to provide incremental changes
to system inputs without prior knowledge of how much or how
rapidly such change will affect process conditions.
A controller first calculates an “error” value as the difference
between a measured process variable and a desired set point. A
small error value should result in a response leading to a small
change. The process variable will overshoot the set point if that
change is too large. “Small” and “large” are of course relative
values. For a system in which physical constraint is not well
known, a PID controller uses recent history to provide context
for choosing the size of desired change. The controller determines process inputs using three separate parameters: the proportional, the integral, and derivative values, denoted P, I, and
D. In terms of time, these values suggest these interpretations:
• “P” depends on the present error,
• “I” on the accumulation (i.e., integration) of past
errors,
• “D” is a prediction of future errors, based on
current rate of change (i.e., the difference between
the most recent measured value and the value
measured immediately before that one).
The controller computes D and I values simply using the
subtracted difference(s) between successive error calculations
of the recent past.4
The power and value of current control systems provides the
basis of much of the quality and productivity of modern converting equipment. Operators and process engineers rely on control
systems for making critical production decisions. To avoid costly
4
The adjustment involves first-order difference calculations. In simple terms: (the current
calculated error factor) 1 (accumulated error measurements over some period of time) 1 (difference of the two most recent two error measurements). In use, linear “gain constants” specific for the system modify the PID response in order to dampen ossifications of the error
above and below the set point.
110
MANUFACTURING FLEXIBLE PACKAGING
mistakes, including potential equipment damage, the people who
use the systems must appreciate their physical realities5:
1. Where is the actual location of a sensor for which
a measured value is output on a screen?
2. What is the physical principle used by the sensor
to make its measurement?
3. Does the time-series data set produced by the sensor represent reasonable process variation?
4. Do values reported by the system respond with
expected speed and direction when set points are
changed?
When control system users recognize inconsistent output
from a control system, immediate response is essential in order
to protect time, materials, and machinery. Standard operating
procedures, appropriately documented and trained, should
guide such responses. As with any complex information technology, control system output is only as good as its input.
(Figure 10.3, “1. Sensor”); its output represents an electronic
signal to a device (Figure 10.3, “3. Regulator”) that actually
causes a physical change, and its control logic (Figure 10.3, “2.
Processor”) is a complex coded program with many interconnected parts. Diagnosing the root cause of an inconsistency
may require more information (i.e., answers to more questions),
but proper diagnosis is essential to correcting the issue.
References
[1] B. Galloway, G.P. Hancke, Introduction to industrial control
networks, Commun. Surv. Tutorials, IEEE 15 (2) (2013)
p860 880.
[2] M. Araki, PID control; control systems, robotics and
automation-V2, in: H. Unbehauen (Ed.), Encyclopedia of Life
Support Systems, Developed under the Auspices of the
UNESCO, Eolss Publishers, Oxford, UK, 2011, 22pp.
5
These questions assess the “general health” of the system.
11 Rotogravure Presses
Chapter Outline
Press Components
Ink Viscosity
Electrostatic Assist
Image Monitoring
111
113
114
114
The traditional packaging gravure press involves an unwind,
usually eight printing stations (each comprised of engraved
print cylinder, ink chamber with circulating ink, doctor blade
assembly, a dryer unit, and idler rollers as required), and a
rewind. A single rotating solid line shaft delivers power to each
station through appropriate gearing. The dryers involve an
angled path over rollers (first upward then downward) in a triangular compartment 2 3 m above the print station. The triangular sheet metal shell that encloses the rollers is hinged at the
top apex allowing the shell to swing open from its bottom to
permit access to the rollers. The unit is accordingly called a
“clamshell” dryer. Some presses are designed with a longer
path dryer (i.e., a horizontal length between the upward and
downward legs) at the last station. This assures complete drying of the entire depth of ink film and enhances the drying of
100% coverage overprint coatings on the web (Figure 11.1).
More recent press designs use an “electron line shaft,” efficient
compact dryers, and quick changeover trolleys described in
Chapter 2. The improvements support running speeds of
1300 ft/min (400 m/min) and total changeover times of less than
30 min.
Press Components
Table 11.1 summarizes the primary elements of a gravure
press. Engraved gravure cylinders still represent the limiting
Manufacturing Flexible Packaging. DOI: http://dx.doi.org/10.1016/B978-0-323-26436-5.00011-4
© 2015 Elsevier Inc. All rights reserved.
111
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MANUFACTURING FLEXIBLE PACKAGING
Figure 11.1 Flexible packaging rotogravure press (8 colors). (Courtesy
Sung An Machinery Co., Ltd. (www.sungan.net))
Table 11.1 Rotogravure Press Components
Consumable materials
Print web
Inks
Replaceable materials
Printing cylinders
Hardware subsystems
Unwind
Printing stations (up to 12)
Rewind
Electrostatic assist
Ink reservoirs
Ink pumps
Monitoring/regulation subsystems
Web guide
Web length
Web tension(s)
Color register
Repeat length
Ink viscosity
Nip pressure at cylinders
11: ROTOGRAVURE PRESSES
113
time and cost factors for gravure printed flexible packaging.
Equipment for the process is expensive, its footprint is large,
and staffing requirements, both in numbers and skills, relatively
high. Some of these limitations are decreased with new
basic design themes (interchangeable print stations “trolleys”),
advanced digital monitoring and control (see image viewing
discussion below), but such features tend to increase the purchase price of new equipment. Even so, productivity and efficiency gains from new equipment often make replacing old
presses with new ones a good investment. These gains can be
so considerable that one new press can replace two or three
older ones. The lower operating costs combined with higher
productivity provide the positive return on investment.
Ink Viscosity
In addition to basic web control components, an ink circulation system is critical for maintaining print consistency. This
system includes subsystems to monitor and control ink viscosity. Viscosity control for solvent inks involves replacing the
organic solvent that has evaporated from the ink circulating
system with “makeup” solvent.
Manual techniques for measuring ink viscosity involve an
indirect assessment of viscosity using a small (1.5
ounce 44 ml) stainless steel “Zahn” cup with hole in its center
and a long handle attached to the sides. The cup is dipped in
the ink and the time required for it to empty (“efflux” time)
noted. Zahn cups are designated “#1” through “#5” based on
the diameter of the bottom hole (larger holes for more viscous
liquids). The size of the cup is chosen to ensure sufficient
efflux time for a precise measurement (usually at least
10 15 s). Packaging gravure inks are considered press-ready at
about “20 30 s #2 Zahn cup” (21 56 centistokes 5 mm2/sec).
Automated ink viscosity measurement typically uses the falling body technique. This consists of a tube containing the ink
and a smooth ball. The ball is placed in the fluid and the time
that it takes to fall the length of the tube noted.
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MANUFACTURING FLEXIBLE PACKAGING
In both manual and automated cases, the assumption
involved is that viscosity is too great (as a result of evaporation
of volatiles). The response to high viscosity is to dilute the ink.
If viscosity is in fact low, solids (pigment and/or vehicle) must
be increased.
If the original ink solvent is a blend (e.g., ethyl alcohol and
ethyl acetate) replacement or “makeup” solvent will contain
more of the solvent with the fastest evaporation rate in order to
keep the composite ink formulation as close to the original
blend as possible. Water-based inks can evaporate ammonia
and experience viscosity changes. Monitoring the pH of such
inks and adjusting it with concentrated ammonia solution control its viscosity. Energy-cured inks have essentially no volatile
components, so they have stable viscosity on-press.
Electrostatic Assist
Transfer of ink from engraved cells to web depends on the
substrate’s ability to attract the ink and “pull” it from the cylinder. With the uneven surface of paper, intimate contact with
the ink in a cell by the substrate may not occur. As a result the
“dot” from that cell’s ink will not appear on the web.
“Electrostatic assist” (ESA) generates an electric field of
about 300 1000 V on the backup roller at the printing nip.
This field “pulls” inks out of the cells toward the backup roller
(and the web in the nip) whether or not the substrate makes
intimate contact with the ink. Solvents now considered “hazardous air pollutants” would “lubricate” inks to assist in their
release from engraved cells. Their use presents such operational
compliance challenges for printers that they have largely been
formulated out of packaging gravure inks. Without this lubricating help for releasing ink, ESA improves print consistency
even when printing on relatively smooth-surfaced plastic films.
Image Monitoring
A gravure press must provide real-time viewing of image
quality (color value, color to color registration, print quality,
11: ROTOGRAVURE PRESSES
115
etc.) by incorporating either manual or digital print web viewers. These allow the operator to perform detailed, in-process
inspections of the web surface while running at full production
speed.
Manual devices (called “scan-a-webs”) are typically assemblies that are as wide as the press’ maximum web width. They
are installed either after each print cylinder or after all colors
have been printed. Their position allows illumination of the
web at the point of viewing. A set of horizontal mirrors rotate
in synchrony with web speed such that an observer sees a
“frozen” image of the moving web. If each print station has
an individual scan-a-web, nearby operator controls allow
adjustment of print register at the station to previously printed
colors.
Digital devices involve one or more stations at which a digital video camera can transverse the web’s width (with various
programmed paths), capture an image of the printing as a
strobe light illuminates it, and transmit this to an operator (for
manual control) or an automated controller. Integrating digital
video data with electronic press controls provides powerful
productivity and quality advantages to state-of-the-art presses.
With electronic drives to control color register adjustments, the
press operator can bring all stations into register from a central
video console. With standardized registration marks along the
edge of a web, programmed register control systems can fully
automate the entire registration process.
After setup and while the job runs, digital vision systems
provide print defect detection by finding, notifying, and recording various faults (e.g., color variations, doctor blade streaks,
hazing, mis-registration, spots, splashes). By “mapping” (i.e.,
recording down-web and cross-web position) such systems do
the “flagging” job previously done by the operator. Digital
record of the defects can “communicate” with controls on subsequent processes (e.g., slitting) for removal. More productively, defect recognition can alert operators to the condition
and prompt then to correct the situation in order to minimize
the waste. With proper programming, the advanced digital systems are able to monitor print quality and perform tasks that no
116
MANUFACTURING FLEXIBLE PACKAGING
manual system operator interface can reasonably perform. For
example:
• Automatic bar code verification1
• Quantitative color specification
compliance
(“delta E” measurement)
• Impression cutoff length monitoring
• Impression counting
• Integration with manufacturing control systems
(e.g., production planning, inventory).
Vision systems provide powerful means of documenting the
production and further converting of printed packaging materials in both flexible packaging manufacturing and customer
packaging operations. This data, and related digital information
captured during manufacturing, provide effective (and almost
effortless) records for “tracing” materials as required by law
(e.g., the US Food Safety Modernization Act of 2011) and voluntary third-party certification schemes (e.g., ISO 9001 Quality
Management Systems and the Global Food Safety Initiative).
1
Machine readable bar codes link package graphics to human readable warnings such as
allergens and nutritional information (typically to a producer specific “stock keeping unit”—
SKU number). Such information is mandatory for some consumer products and can result in
marketplace recalls if a product is merchandized with improper package graphics.
12
Flexographic Presses
Chapter Outline
Press Components
Plate Cylinder Pressure
Plates, Mounting Tape, and Plate Sleeves
Drying Technology
Reference
117
118
120
121
122
The modern packaging central impression (“CI”) flexo press is
configured very much like a gravure press: an unwind stand,
usually eight printing stations (each comprised of engraved
plate cylinder, ink chamber with circulating ink, doctor blade
assembly, a between-color (“BC”) dryer unit), and a rewind.
The obvious differences are the arrangement of printing stations around a large (8 10 ft to 21/2 3 m) diameter drum and
an extended overhead dryer. The footprint of a CI flexo press
is relatively compact with the drum surrounded by print stations under one end of the dryer and the unwind/rewind stands
under the other (Figure 12.1). More recent press designs tend
to enclose the entire drum and print stations area in a sheet
metal cabinet that can collect any fugitive solvent emissions
and exhaust them to a pollution control device.
Press Components
Table 12.1 summarizes the primary elements of a flexo
press. The principles of image quality and ink viscosity monitoring and control discussed for gravure presses (Chapter 11)
are essentially identical to flexo press subsystems. It should be
obvious that no space is available for manual observation of
print quality between the flexo print stations. Controls for
adjusting print register for each print station must be provided
Manufacturing Flexible Packaging. DOI: http://dx.doi.org/10.1016/B978-0-323-26436-5.00012-6
© 2015 Elsevier Inc. All rights reserved.
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MANUFACTURING FLEXIBLE PACKAGING
29
20
31
22
24
25
25
32
28
23
28
W
32
21
27
23
24
30
Key to numbers in figure
“W” web
20 flexographic press
27 unwind
21 central impression drum
28 rollers
22 side frames
29 overhead tunnel dryer
23 print stations “color decks” (8)
30 rewind
24 anilox roll
31 rollers
25 plate cylinder
32 between-color dryer
Figure 12.1 8 color CI flexo press, US patent No. 6,176,184.
after the web has exited the overhead dryer (i.e., at the opposite
end of the machine). Electronic access provides critical quality
and productivity advantages here.
Plate Cylinder Pressure
The CI drum serves as the impression (backup) roller for
each print station. Ideally web tension is uniform from the start
of its travel on the CI drum until it leaves to enter the overhead
dryer.1 Working against this ideal are the 6 10 nip points
(i.e., one for each print station) as images are printed around
the drum. This balance must be addressed by controlling “plate
impression” at each print station. The constrained geometry
1
This tension control favors uniform color to color register, and less overall probability of
stretching an elastic web causing curl and/or improper package cutoff length.
12: FLEXOGRAPHIC PRESSES
119
Table 12.1 Flexographic Press Components
Consumable materials
Print web
Inks
Replaceable materials
Printing plates
Mounting tape
Anilox rolls
Plate sleeves
Hardware subsystems
Unwind
Printing stations (up to 10)
Rewind
Overhead dryer
Ink reservoirs
Ink pumps
Monitoring/regulation subsystems
Web guide
Web length
Web tension(s)
Color register
Repeat length
Ink viscosity
Pressure of plates on drum
around the drum prevents measuring web tension with standard
techniques (see Chapter 10, Figure 10.2). Rather, the pressure
holding the plate cylinder against the drum is set and monitored
(i.e., the pressure from hydraulic cylinders pushing one roll
against the other represents the only control variable). Actual
“nip pressure” is lower than the web tension, and the fluid ink
on flexo plates allows the web to slip through the nip without
affecting its tension. Although print quality can suffer, effects
on web handling are negligible. If the plate cylinder pressure is
too high, the compressible plate material will “mash” its image
onto the web. When pressure is too low, the web can wipe ink
from the plates causing steaks of color instead of carefully controlled images.
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MANUFACTURING FLEXIBLE PACKAGING
Plates, Mounting Tape, and Plate Sleeves
Developments in this area represent the current priority for
converter-friendly press design improvements. The objectives
are to reduce the prepress effort and costs to prepare and mount
flexo plate cylinders; to minimize the time required on press
for plate cylinder cleanup and setup; and to minimize job to
variability in the dynamics of plate cylinder pressure (called
“bounce” in the industry).
Traditionally, flexo plate material is mounted directly to steel
cylinders with diameter chosen to match the package impression’s cutoff. The process developed with elastomeric rubber
plates vulcanized in molds on a job-by-job basis. A given setting of plate cylinder pressure results in variable pressure at the
interface of plates with web depending on the surface area of
the image and the compressibility of that batch of vulcanized
rubber. Although nothing can be done about differences in the
surface area of the images, photopolymer plate material provides much more consistent and predictable compressibility.
This consistency reduces the need for the structural reliability
of steel cylinders and supports use of sleeve systems manufactured from lightweight composite materials.
Advantages of a sleeve system for flexo plate cylinders
depend in large part on product mix and job sizes. The potential for improved profitability can be found in these factors [1]:
•
•
•
•
•
Decreases setup times from hours to minutes.
Price competitive when compared to steel.
Lightweight (can be handled by one person).
Easy to clean and maintain.
Requires less storage space, allowing for a wider
range of sizes in a smaller space.
• Plate sleeves allow repeated jobs to be mounted,
stored, and ready to use on a moment’s notice.
• Reduces plate damage by keeping plates mounted
between jobs.
• Scribe lines on the sleeve’s face improve plate
mounting speed and precision.
12: FLEXOGRAPHIC PRESSES
121
The flexo finger printing process (Chapter 3) also supports
empirical determination of the optimum plate, mounting tape
(“sticky back”), and sleeve material combinations. With this
knowledge, more of the unique print quality requirements for
any given can be managed offline without delaying press
production.
Drying Technology
The CI flexo press design has traditionally used forced air
dryers (gas or electric) with centralized air heating and ductwork as required to convey air to the web at desired locations,
that is, the BC dryers and the overhead dryer. This convective
drying process uses a moving volume of air at a set temperature to volatize solvents from printed inks.2 The rate of drying
depends on the air’s temperature and the amount of unsaturated
air available to hold the evaporated vapors. The temperature
sensitivity of plastic films places an upper limit on the dryer’s
air temperature (dependent on the thermal properties of the specific plastic).
Several approaches have been applied to the constrained BC
space of the flexo press with good results. Infrared heaters
before the dryers can raise the temperature of solvents before
moving air completely evaporates them and carries them off.
High-velocity air increases the exposure of solvent to a volume
of air with available vapor carrying capacity during the short
BC dryer residence time.
The overhead dryer must thoroughly cure all of the printed
ink from the first color printed the last. In a normal convection
dryer, air flow is aimed at the top of the web. The first color
may well be covered by several layers of subsequent colors,
but only the top layer directly experiences the evaporative force
of the air flows. As Figure 12.1 suggests, the web is supported
2
Nominally, the BC dryer cures the ink just printed quickly enough so that the next applied
ink will not blot that printed image. The overhead dryer then removes remaining solvent
printed at all print stations. If an overhead dryer is too hot, it can cure a thin layer of ink on
top of a web and leave considerable solvent dissolved in the partially cure ink below. That
“retained solvent” can eventually volatize, causing unacceptable odors and flavors in a packaged product.
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MANUFACTURING FLEXIBLE PACKAGING
through the dryer from its unprinted side on a series of rollers.
As an alternative, “flotation dryers” support the web with specially designed nozzles called air bars. These provide flotation
for web conveyance as well as a high heat transfer coefficient.
When the system is correctly designed, the converter realizes
several key advantages including higher drying speeds, web
stability, proper exhaust removal, and efficient energy use
without the potential for leaving marks or scratches on the
unprinted side of the web.
Reference
[1] S. Garduno, The Benefits of Printing Sleeves, FLEXO
Magazine, 2013, pp. 100 104.
13 Adhesive Laminators
Chapter Outline
Dry Bond Laminators
Solventless Laminators
Online Coating Measurement
123
125
125
Machinery for adhesive lamination is relatively straight forward. Unwinds for at least two webs are required. An application station for the adhesive, a means of curing the adhesive,
and a rewind for the lamination complete the system. The type
of adhesive, liquid (solvent or water-based) or solventless,
determines the configuration of equipment (Table 13.1).
Dry Bond Laminators
Solvent or water-based adhesives are both applied using a
“doctored” method. This is usually a chamber doctor blade and
sometimes a wire-wound (Mayer) rod. Both techniques involve
applying an excess of coating solution to the applicator
(engraved cylinder or rod) and then removing (doctoring) the
excess solution with another device before the web encounters
the applicator. This two-stage process delivers a set volume of
liquid coating to the solid web surface. The liquid is recirculated from a large reservoir (about 40 gallons; 150 l) into and
out of the application device as its viscosity is monitored and
controlled. Some reactive two-part adhesive chemistry will
begin to cure even in this liquid form. This limits “pot life”
and may necessitate delivery of smaller and more frequent
batches of the mixed adhesive to the laminator.
The volatile solvent or water must be dried so that the
adhesive solids will cure into a film strongly adhered to the
substrate. Ovens with circulating hot air provide energy for
Manufacturing Flexible Packaging. DOI: http://dx.doi.org/10.1016/B978-0-323-26436-5.00013-8
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MANUFACTURING FLEXIBLE PACKAGING
Table 13.1 Adhesive Laminator Components
Consumable materials
Printed web
Adhesive
Laminate web
Replaceable materials
Engraved roll
Hardware subsystems
Unwinds (21)
Coating station
Rewind
Overhead dryer
Adhesive reservoir
Adhesive pump
Monitoring/regulation subsystems
Web guide
Web length
Web tensions
Coating weight (infrequent)
Nip pressure at applicator
evaporation. As with inks, the drying must not “skin over”
the surface of the liquid coating and trap liquid in the
layers beneath. The infrared preheating and flotation oven techniques discussed in Chapter 12 also apply to making this
curing process efficient. Traditional industry practice has been
to stop running during a job setup and then test the web (after
solvent drying) for coating weight determination and retained
solvents.
After exiting the drying oven, the coated surface is pressed
against the second web in a heated nip. Here, the recently dried
adhesive coating acts with a pressure sensitive-like mechanism
to wet out this second surface and adhere to it. Typical flexible
packaging webs and adhesives provide enough adhesive
strength at this point for rewinding and then slitting shortly
afterward.
13: ADHESIVE LAMINATORS
125
Solventless Laminators
Because their curing mechanisms are different, solventless
adhesives do not require laminators with drying ovens. The
equipment can be simpler, less expensive, and smaller than solvent or water-based adhesive laminators. In exchange for this
curing ease, solventless laminators require a much more complicated and sensitive application system. When the adhesive
finally coats the web, it is releasing from the last of a series of
transfer rollers that successively reduce the amount of fluid
adhesive transferred at each roller. The two (or more) parts of
the adhesive must be precisely measured and mixed immediately before these rollers start to control the adhesive layer.
The device is called a “meter mixer.” It must be calibrated to
the temperature and viscosity of each adhesive part so that an
exact ratio of one part to the other(s) is maintained. The pot
life of these adhesives is minutes in the mixed state, so no
batch formulation is feasible. The mixer’s outlet travels back
and forth across the stack of rollers pumping the mixture
directly onto it in a continuous flow.1
As soon as the adhesive is applied to the first web, the second web is nipped to it so that adhesion to both webs and
development of internal, cohesive strength occurs simultaneously, but not quickly. Several days may be required before
bond strength is high enough to permit even slitting. The curing process can be accelerated by storing laminated rolls in a
hot room (about 100 F; 37 C).
Online Coating Measurement
Recent online coating weight monitoring technology (similar
to the systems used for extrusion coating and laminating;
Chapter 14) eliminates process interruptions for coating weight
determination and provides a more efficient process with less
waste.
1
Any excess adhesive not soon transferred to the web cures in place as a mostly insoluble
coating on the machine parts where it has cured.
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MANUFACTURING FLEXIBLE PACKAGING
With a relatively uniform web, the applied coating weight
can be measured using a total weight method, beta transmission, gamma backscatter, or X-ray transmission. Direct weight
techniques can measure a unique chemical characteristic of the
coating, which is then converted to coating weight. These
include infrared absorption, beta backscatter, and X-ray
fluorescence.
Because the function of the adhesive coating process is to
adhere webs to one another, precise coating weight control is
not as quality critical as it is for coated products (e.g., barrier
films, photograph papers, heat seal coated foil), but the advantages of reduced loss in small volume coating runs, rapid correction of out of limits material, and improved uniformity
provide opportunities for productivity and efficiency advantages that may make online systems good investments.
14
Flexible Packaging Extrusion
Coating/Laminating Line
Chapter Outline
Line Configuration
Gauge Measurement and Control
129
129
The purpose of this process is to join together (“laminate”) two
substrates and/or cover (“coat”) one substrate or with a molten
film of thermoplastic resin. In both instances adhesion of the
resin, once it solidifies, to one or both substrates is the critical
success factor.
Table 14.1 lists the basic systems of an extrusion coating
and laminating line. This equipment typically represents
the most complex machinery in a converting application.
“Tandem” lines, with two extrusion dies, are typical. Three
(in rare cases more) flexible webs may be combined into a
multifunctional packaging material that rivals rigid plastic containers. The two dies are each supplied with streams of molten
plastic resins from one to three extruders. Distributed Control
Systems (Chapter 10) are critical for monitoring and controlling the scores of process variables involved. This discussion
will focus on the machine-direction flow of materials from
unwind to rewind. Extruding resins for laminating and coating
layers is covered extensively in other resources.1
The flexibility of the industry’s extrusion coating and
laminating lines matches their complexity. Complex equipment
designed to prime two webs and laminate them to a third with
resin from as many as six extruders will also produce a simple
material representing one web coated with one resin. The
value of the process derives from a converter’s knowledge of
1
See the references by Bezigian and Gregory listed in Chapter 5.
Manufacturing Flexible Packaging. DOI: http://dx.doi.org/10.1016/B978-0-323-26436-5.00014-X
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MANUFACTURING FLEXIBLE PACKAGING
Table 14.1 Extrusion Laminating/Coating Equipment
Components
Consumable materials
Printed web
Rein (adhesive or coating)
Laminate web
Replaceable materials
None
Hardware subsystems
Unwinds (21)
Priming station
Die
Rewind
Resin hopper
Extruder
Monitoring/regulation subsystems
Web guide
Web length
Gauge monitor/die lip control
Back pressure
Melt temperature
Nip pressure at chill drum
Web tension
Extrusion profile(s)
Screw amperage
Screw speed (rpm)
Chill roll temperature (in/out)
Nip pressure at primer applicator
numerous materials that are able to come together in fit-to-make
combinations for various fit-for-use products. Process leverage
lies in the ability to incorporate layers of resin (usually polyolefins, but necessarily) directly from their bulk commodity form
without the cost of a separate plastic film fabrication process.
Product features, such as stiffness, toughness, barrier, and seal
integrity, are often improved (relative to adhesive laminations)
by relatively thin (5 14 ppr; 8 23 gsm) layers of resin. Such
thin polyolefin layers (35 gauge; 9 µ) may be prohibitively
14: FLEXIBLE PACKAGING EXTRUSION COATING/LAMINATING LINE
129
Figure 14.1 Single extrusion laminator.
expensive or practically unachievable as a separate web, offering
additional leverage to the process.2
Line Configuration
Figure 14.1 presents the cross-section of a simple single die
extrusion laminating line (the equipment can also be configured
to coat a web unwound from the primary unwind unit). The
extruder and associated resin handling hardware are in profile
behind the laminating unit. The chill drum and backup roller
under the die require extensive and sizeable hydraulic apparatus so that they can be moved into and out of a nip position
depending on the stage of job setup. These assemblies can also
move forward and backward relative to the die in order to vary
the time the melt curtain resides on one of the webs before the
chill drum cools it. The die itself can be adjusted up or down
to adjust the amount of oxidation of the melt in the air gap.
The priming unit in Figure 14.1 is essentially a coating station with clamshell drying unit. The primer coating method
itself may be direct gravure, reverse gravure, smooth roll, etc.
Gauge Measurement and Control
Layers of resin that laminate or coat webs on these lines will
reflect variation in the gap between die lips across the die.
2
At the other extreme, thick polyolefin layers (e.g., more than 2.5 mil; 65 µ) often approach
the maximum output of an extruder on an extrusion coating and laminating line. This requires
low run speed unless a larger extruder is provided, or the output of the existing one is augmented with a coextruder feeding the same die.
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MANUFACTURING FLEXIBLE PACKAGING
While these can be adjusted when the line is not running using
“shims” of known thickness and adjusting bolts that press more
or less against the lips, the dynamic pressure of the exiting
resin will change the gap. Such pressure itself is not uniform
across a die because of the combined effects of internal die
geometry and variation of the viscosity of molten resin. The
effect of this variability can cause machine-direction “gauge
bands” in the finished lamination or coated web. The bands are
strips parallel to the web’s edges that are thicker or thinner
than the desired thickness.3
Controlling product quality requires controlling this crossweb variation during the run itself. Traditionally, the industry
uses a “beta gauge” to measure real-time thickness variation.
This consists of a beta-ray4 emitting source and a detecting
device. Beta particles do not travel far, and they are easily
stopped by several feet of air or thin plastic. This makes it quite
easy to shield the gauge for the safety of operators or others
working nearby. It also provides the basic measurement technique. When the particles strike any material, some of them pass
through, while others will be absorbed. The thicker (or denser)
the material, the greater the chance a particle will be stopped.
With the assumption of uniform density, the ratio of the number
of particles passing through the material to the number without
any material is proportional to the thickness of the material. The
detector for these gauges consists of a chamber filled with an
inert gas such as argon and given an electric potential of about
500 V. Beta particles entering the chamber release electrons
from the atoms of gas. These are attracted by the electric potential of the chamber to a counter. The loosed electrons are proportional to the number of beta particles entering the detector and
in turn to the thickness of the material previously encountered
by the beta particles.5
3
Ideally all points on a line across a web will have a thickness within 6 3 to 5% of the
average thickness of the web along that line.
4
Beta rays are high-speed electrons or positrons emitted by certain radioactive nuclei such
as potassium-40.
5
Beta gauges are used in many continuous industrial processes for controlling not only
thickness, but also other material variables such as moisture content (e.g., of paper on a paper
machine or processed food on a conveyor belt).
14: FLEXIBLE PACKAGING EXTRUSION COATING/LAMINATING LINE
131
The dynamic nature of the web process is monitored by a
mechanical assembly that moves the beta source (above the
web) and the detector (below it) simultaneously across the web
and back again. (The web is passed through a structural frame
that spans the width. Source and sensor are supported on the
cross-machine legs of the frame and able to move back and
forth.) While the elements do not trace a true orthogonal track
across the web, signal processing translates the angled sample
path into thickness estimates for 1 2 inches (21/2 5 cm) segments across the web. (That is cross-web variability greatly
exceeds down-web variability during the duration of sampling.)
Calibrated to the material and machinery, the segments correspond to specific adjustment bolts at the die lips.
Recent die design allows actual closed-loop control systems
to adjust die gap at such adjustment points by changing the
thermal expansion/contraction of an “automatic die bolt” in
response to the thickness measurement. The algorithm used
requires even more sophistication that the PID system
described in Chapter 10 because of the nonlinear response of
pressure exerted by resin in adjacent areas when a spot change
is made.
Signal process and precision of these measurements allow
differential and subtractive determination of base web variability, and determination of a second coating’s thickness. Other
methods have been adapted to monitoring and controlling
extrusion coating and laminating lines and other processes
(Chapter 13). These include X-ray transmission and backscatter, gamma ray backscatter and infrared (IR) reflectance and
transmission techniques. Each has different strengths and weaknesses. The radioactive materials used in beta systems, for
example, may require permits and licensing fees that the others
(based on various forms of electromagnetic energy) do not. IR
systems can be calibrated to measure the thickness of multiple
layers of individual coextruded resins when derived from a
single die.
15
Slitters
Slitting equipment is as varied as the flexible packaging materials that a converter may have to be slit. From a machine’s
unwind stand to its rewinds, the right machine depends on the
requirements of the jobs in the operation. In the converting
industry, web widths have increased to almost 80 inches (2 m).
A 40 inches OD laminated roll can weigh over 3200 pounds
(1500 kg). Obviously, the materials handling capabilities in a
converting operation must be designed to handle such loads,
and the slitter at the end of the processes must be built to deal
with the weight, width, and diameter. Although these components may seem simple (Table 15.1), the need to minimize
time and waste costs in this process places a premium on automation and precision for the equipment. Basic industrial engineering work practices yield major rewards at this seemingly
simple, inconspicuous point in the whole manufacturing
process. This, the last work done for the product by the
manufacturing supplier, is responsible for making the first
impression when the packaging user receives his shipment.1
The unchanging principle in the industry is that while slitting
cannot make bad material good, it will make good material bad
if not properly done.
Consider the 80 inches wide, 40 inches OD roll above. At 5
mills thick (125 µ), the lamination is about 20,000 ft (6000 m)
long and will have a basis weight of about 70 ppr (114 gsm). If
the material is printed with eight 10 inch wide impressions,
industry terminology describes this cross-directional figure as
1
Current food safety law makes the slitting operation critical in providing the product labeling and documentation necessary to trace a retail food package from the market place back to
the raw material bought to manufacture it.
Manufacturing Flexible Packaging. DOI: http://dx.doi.org/10.1016/B978-0-323-26436-5.00015-1
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MANUFACTURING FLEXIBLE PACKAGING
Table 15.1 Finishing and Slitting Equipment Components
Consumable materials
Complete material
Customer cores
Replaceable materials
Knives
Hardware subsystems
Unwind
Knife holders
Rewinds
Monitoring/regulation subsystems
Web guide
Web length
Web tension(s)
eight “lanes.” The customer may have a 40 pound (18 kg)
per slit roll weight limit.2 This is about 610 ft (185 m) per slit
roll. If the converter wants to run his slitter at 1000 ft/min,
he needs to change slit rolls every 36 s. This involves nine
cycles of stopping and setting up another up another eight
new cores for every roll from a laminator. The machine directional figure represents 10 “doffs.” The familiar desire for faster run speeds in converting operations has diminishing returns
for overall productivity. As Table 15.2 suggests, increasing the
slitting running speed by 700% (from 250 to 2000 fpm) reduces
the elapsed time between rolls by only 5%. Decreasing the
setup time between one doff and the next by 50% from 10 to
5 min, reduces the elapsed time between rolls by 47%. While
the figures are arbitrary assumptions, the relationships hold
true: When slitting relatively small customer rolls from large
converting equipment rolls, the advantage lies more in improving productivity between doffs than by increasing run speeds.
Such productivity gains may be realized by standard operating procedures for operators and/or introducing automation into
the slitting operation. To the extent possible, queuing cores on
2
Other slit roll specification may be stated in terms of roll length or number of impressions
per roll, depending on the material tracking systems they use.
15: SLITTERS
135
Table 15.2 Efficiency Effects for Slitting Operating
Quadruple slitting speed
Roll Length Slitting Speed
(ft)
(fpm)
Setup Time
(minutes)
160
160
160
160
10
10
10
10
250
500
1000
2000
min for One
Doff
10.64
10.32
10.16
10.08
Halve setup time
Roll length Slitting Speed
(ft)
(fpm)
Setup Time
(minutes)
160
160
160
160
10
8
6
5
250
250
250
250
min for One
Doff
10.64
8.64
6.64
5.64
shafts for the next doff set and transferring full customer rolls
from the rewinds to pallets, boxes, or other appropriate customer delivery formats greatly increases output from slitters.
When considered in the context of OEE (Chapter 8), quality
at the slitting operation represents the number of rolls ready for
customer shipment as a percent of total slit roll setups. All of
the handling constraints that apply to wide web converting also
affect slit roll quality. Equipment configuration, including programmable process controls (e.g., taper tension, acceleration
and deceleration rates), should be flexible enough for the full
range of product length, widths, and thickness the operation’s
other equipment can produce. The converse is also true: for a
specialized product line, dedicated slitter capability optimized
for the material is critical for profitable, quality operation.
16 Preventative Maintenance
versus Available Production Time
Chapter Outline
Availability
Preventative Maintenance
Calibration
Actual Operating Time
137
138
139
140
Chapters 16 18 revisit the components of Overall Equipment
Efficiency (OEE) (Chapter 7) in the context of converting
equipment and flexible packaging products. Table 16.1 summarizes the components and their meanings.
Availability
“Is the machine running or not?” The availability rate
(0 100%) indicates the relationship between the time that the
machine should theoretically have been in operation and the
time that there was actual output. Mechanical breakdowns,
waiting to define job requirements, and waiting for raw material detract from 100% performance.
Converting equipment for flexible packaging is relatively
expensive, usually quite large and an intricate combination of
mechanical, electrical/electronic, and control systems. Flexible
packaging itself is a “make-to-order” business, requiring ongoing flexibility in the setup and operation of that equipment. All
of the job start and stop creates wear on the machinery and leads
to predictable breakdowns if not anticipated and addressed
before problem occurs when production is scheduled. Perhaps
worse, sensors and gauges can drift and lose calibration. They
Manufacturing Flexible Packaging. DOI: http://dx.doi.org/10.1016/B978-0-323-26436-5.00016-3
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MANUFACTURING FLEXIBLE PACKAGING
Table 16.1 Overall Equipment Effectiveness
Component
Concern
Metric
Availability
(Chapter 16)
Performance
(Chapter 17)
Quality
(Chapter 18)
“Is the machine running Time: running versus
or not?”
scheduled
“How fast is the machine Product: produced
running?”
versus planned
“How many products met Product: accepted
specifications?”
versus produced
appear to operate properly, but in fact provide inaccurate information about the process and the product it produces.
Nothing in this description of machinery and its components
should cause controversy or debate, but an organization’s
response to these facts is not so obvious. The unconsidered
response might involve ignoring the obvious, running the
equipment until it stops working, and hoping the work at hand
will be finished before it does. Best practice involves regular,
planned professional maintenance that is documented1 and
used to schedule subsequent maintenance routines.
Preventative Maintenance
No viable manufacturing operation can stay in business long
unless it tends to its maintenance responsibilities. In flexible
packaging operations, this usually involves duties distributed
from line operators on a daily basis to specialized professionals
on an annual rotation. Maintenance requires planning and
attention to detail:
• Matching training of the staff members to their
assigned maintenance responsibilities is of course
necessary.
1
Maintenance and servicing activities, especially those performed infrequently, can present
hazardous energy danger (electrical, mechanical, hydraulic, pneumatic, chemical, thermal, or
other sources) in areas not unsafe to workers at other times. The US Occupational Health and
Safety Administration has issued specific regulatory practices and procedures (“lockout/tagout”) for disabling machinery or equipment to prevent the release of hazardous energy.
16: PREVENTATIVE MAINTENANCE
139
• All activities are scheduled, and their completion
recorded.
• Maintenance managers oversee the schedule and
insure that training and necessary supplies and
materials are available when and where needed.
• Special requirements of equipment manufacturers
and customers must be recognized (e.g., cleaning
metal surfaces that contact raw material or product
may require tooling made of brass or plastics to
avoid scratching; equipment making flexible packaging for food must use “food grade” lubricants to
avoid potential adulteration of food after packaging).
The time and other resources dedicated to maintenance are
necessary investments in the ongoing productivity and reliability of the operation. Activities can be coordinated with parallel
work areas to minimize total time away from production. Often
representatives of the equipment manufacturers assist in
conducting major overhauls. Equipment operators can work
under the direction of maintenance specialists. Most converting
equipment has subsystems requiring periodic cleaning and
maintenance as well as the primary operating unit. While an
extrusion coating and laminating line is down for annual maintenance, cleaning of the air ducts feeding resin pellets from
delivery cars and intermissive storage areas can be done.
Calibration
Sensors and gauges on equipment, in addition to operational
maintenance, require periodic calibration.2 Sophisticated control systems (Chapter 10) can critically damage operating
equipment if a “bad” sensor sends faulty data to it (e.g., controls may interpret a broken temperature gauge as a location
requiring process heat). The result: a resistance heater activated
until temperatures melt components or initiate a fire.
2
Standard Quality Management Systems (ISO 9001) require routine calibration plans and
documentation for any equipment that measures product attributes and variable that a customer
considers critical.
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MANUFACTURING FLEXIBLE PACKAGING
Calibration can require certified technicians with the training
and equipment to assure staff that data used to control their
process are accurate and reliable. Annual calibration is the
usual minimum for equipment required for critical quality
measurements.
Actual Operating Time
The time remaining after scheduled maintenance time is
deducted from all available production time represents “available time”. Ongoing production logs, maintained by operators
on site and in real time, should record and identify any time
not available for production because of unplanned maintenance
or interruptions to the delivery of material and/or instructions.3
Such logs provide important feedback to maintenance planning
and shop-floor control systems. For example, unplanned maintenance may reflect equipment components with a “mean time
to failure” less than the interval between major line overhauls.
In such cases, replacement can be scheduled for minor maintenance periods (i.e., before failure is likely to occur again).
Spare parts inventories for each major production line represent some of the best investments a converter can make. In
what has become a global market for equipment, replacement
parts, even if available from the equipment manufacturer, may
have transit times of several days. Expedited shipments must
still await customs clearance. The converter’s sense of urgency
to resume production may not be matched by others with their
own priorities.
Shop-floor control systems are currently evolving into integrated management systems that extend from initial order taking through product delivery and invoicing. The cost of
inventory and inventory storage may require “just in time”
3
This analysis includes job setup and cleanup time in “Performance” (Chapter 17) rather
than here in “Availability” because the job-shop nature of the industry makes the efficiency of
job changeover a management factor for the ongoing management of operations rather than an
inclusive scheduling issue for equipment usage.
16: PREVENTATIVE MAINTENANCE
141
delivery of raw materials.4 More typically, the industry uses
forecasts to predict the demand for raw materials over time.
For example, a rough 12-month forecast may be maintained. It
has less precision for periods farther into the future, but more
for those within one or two delivery lead time intervals.
The premium on time in flexible packaging manufacturing
operations normally works against assuming high risk of nondelivery of raw materials, but systems that can minimize both
raw material inventories and raw material lead times have competitive advantage.
4
On the sales side of the industry, “Vendor managed inventory” (VMI) represents a growing
trend to expect packaging materials suppliers to inventory their finished product near a product’s packaging operation and deliver packaging on a daily, even shift, basis.
17 Setup/Cleanup versus
Scheduled Production Time
Chapter Outline
Performance
Setup and Cleanup
Decreased Speeds and Minor Stoppages
Increased Speeds
143
143
145
146
Chapters 16 18 revisit the components of Overall Equipment
Efficiency (OEE) (Chapter 7) in the context of converting
equipment and flexible packaging products. Table 17.1 summarizes the components and their meanings.
Performance
“How fast is the machine running?” In the performance rate
metric, “theoretical output” is the output that the machine
would have made if the machine produced at maximum speed
during the entire time that it actually operated. Minor stoppages
and reduced speed reduce theoretical to “actual” output. Best
practices in the industry prescribe standard operating procedures (SOPs), including speeds, for a given product for a specific machine (Chapter 18).
Setup and Cleanup
The boundaries of manufacturing performance are simple:
1. Converting equipment does not add value if it
does not run.
Manufacturing Flexible Packaging. DOI: http://dx.doi.org/10.1016/B978-0-323-26436-5.00017-5
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MANUFACTURING FLEXIBLE PACKAGING
Table 17.1 Overall Equipment Effectiveness
Component
Concern
Metric
Availability
(Chapter 16)
Performance
(Chapter 17)
Quality
(Chapter 18)
“Is the machine
Time: running versus
running or not?”
scheduled
“How fast is the
Product: produced
machine running?”
versus planned
“How many products Product: accepted
met specifications?”
versus produced
2. If equipment runs the wrong product (or the right
product poorly) it actually costs money (time lost
and the materials wasted).
This inescapable reality of the flexible packaging industry
imposes on the converter the opportunity and responsibility to
make setup for the next job and cleanup of the last one a competitive advantage.1 “Time to first quality product” is perhaps
the single most important success metric for flexible packaging
manufacturing. Much of the current innovation discussions in
Chapters 1 9 deals with this critical aspect of the industry. No
doubt, equipment automation helps minimize time invested in
(without return from) setups and cleanups, but work practices
provide as much if not more savings opportunities as available
from capital equipment.
The challenge is an intrinsic one for any make-to-order manufacturer. Chapter 9 reviews the “setup reduction/quick changeover technique” as developed in other industries. It also
addresses “minimum order size” considerations for the industry. Chapter 9 included this example:
Assume the press operates at 350 fpm and the job is run at
5 across. Then the press produces the 25,000 impressions
1
Other web converting product lines, for example, film coating, tend to minimize product
grade changes in order to minimize waste and maximize productivity. Those businesses in the
long run compete as commodity markets where competitors’ access to equivalent equipment
and raw materials blur the lines distinguishing vendors and their products.
17: SETUP/CLEANUP VERSUS SCHEDULED PRODUCTION TIME
145
Table 17.2 Effective Feet per Minute
Activity
Duration
(minutes)
Speed
(fpm)
Material
(feet)
Effective
Speed (efm)
Setup
Cleanup
Run
Job total
360
240
14
614
0
0
350
0
0
4900
4900
0
0
8
in 14 min (about 0.25 h). The time consumed estimate for
the job is 4 1 6 1 0.25 h or a total of 10.25 h.
Table 17.2 summarizes the situation and introduces the metric “effective feet per minute (efm).” In the example, the efm
is 8, when the actual operating speed was 350 ft/min!2 This
reflects the short job length that does not economically underwrite the 10 h dedicated to setup and cleanup. Cutting setup/
cleanup in half doubles the efm while doubling the run speed
has essentially no effect on the efm.
Reducing setup and cleanup times for jobs of any size
improves the performance dimension of the OEE of a flexible
packaging operation, adds revenue to the bottom line, and creates opportunities to serve new customers with smaller order
quantities.
Decreased Speeds and Minor Stoppages
Automatic roll indexing mechanisms on web converting
equipment have done much to eliminate the inefficiencies of
stopping the process to replace emptied rolls of raw material
and remove full rolls of finished product.3 Although indexing
2
The efm of a job is the length of product run (feet) divided by the time consumed by the
job. The measurement is useful for planning production, reporting it, and recognizing underutilized capital in the enterprise.
3
Care is necessary to mark the location of splices representing new rolls of raw material
added to the process.
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MANUFACTURING FLEXIBLE PACKAGING
may require slowing the process to as much as 50% of the
nominal operating speed, its duration is so short that the overall
effect on average speed is usually minimal. Thicker materials,
having shorter lengths on rolls of the same weight and
diameter, will compound the effect of indexing on average
speed. Proper staging of materials and staffing of the preparation efforts help to insure continuous operation through roll
changes.
During process stops, uncontrolled effects (e.g., solvents drying in cylinders, films overheating in dryers, resins decompose
in extruders) can cause significant downtime for additional
cleanup or replacement of components. Web converting works
most reliably and efficiently at steady state.
Current flexible packaging converting speeds can exceed
1000 ft/min (305 m/min). Manual process adjustments at those
rates are inherently unsafe. The potential for injury is lessened
(though certainly not eliminated) at slower speeds. As a result
any necessary adjustments (e.g., dirty printing media, clogged
coating cylinders) will occur at reduced speeds if not with the
entire process stopped. The flagging process (described in
Chapter 18) first marks the start of the problem in the roll and
then its end.
Increased Speeds
Attempts to increase speeds should require additional process
control studies. Studies must consider seasonal, even diurnal,
variations in ambient temperature and humidity conditions.
Such environmental factors play a large role in determining
capable conditions for processes involving drying the solvents
from inks coatings and adhesives. When conditions favor rapid
evaporation of solvents, so-called “slow solvents” operate in
small amounts to keep inks sufficiently fluid for metering and
transfer to substrates. As suggested in Table 17.3, staying fluid
in ambient conditions translates into “drying slowing” in
dryers. If not evaporated in drying ovens, the remaining solids
of inks coatings and adhesives can retain these solvents and
17: SETUP/CLEANUP VERSUS SCHEDULED PRODUCTION TIME
147
Table 17.3 Relative Evaporation Rates of Converting Solvents
Family
Name
Rate Ether 5 1
KETONE
GLYCOL ETHER
ESTERS
ESTERS
ALCOHOL
ALCOHOL
ALCOHOL
ALCOHOL
GLYCOL ETHER
ESTERS
KETONE
GLYCOL ETHER
GLYCOL ETHER
Methyl ethyl ketone
Ethyl acetate
Isopropyl acetate
n-propyl acetate
Ethyl alcohol
Isopropyl alcohol
n-propyl alcohol
Methoxy propanol
Ethoxy propanol
Methoxy propyl acetate
Cyclohexanone
Diacetone alcohol
Methoxy butanol
2.7
2.9
4.2
6.1
8.3
11
16
25
33
34
40
147
160
cause problems (e.g., odors, poor adhesion) in subsequent
operations and uses.4
In lieu of verified process control studies, ad hoc efforts to
increase run speeds create the potential for adding production
issues and quality problems at other manufacturing steps. As
described above the pay back for reducing nonproductive time
is greater and as Chapter 18 discusses, production that is out of
specification causes its own reduction in OEE.
4
Although the industry regularly uses a gas chromatographic method (ASTM F1884—
Standard Test Method for Determining Residual Solvents in Packaging Materials), reasonable
temperature conditions (about 190 F; 88 C) for the laboratory assessment do not match heat
exposure during extrusion laminating/coating, heat sealing, etc. when the problems are
experienced.
18
Saleable Product versus
Product Produced
Chapter Outline
Quality
Reference
149
154
Chapters 16 18 revisit the components of Overall Equipment
Efficiency (OEE) (Chapter 7) in the context of converting
equipment and flexible packaging products. Table 18.1 summarizes the components and their meanings.
Quality
Industry practice involves testing material from the end of a
roll just removed from a machine’s rewind. The tests measure
secondary quality properties (Chapter 32) that are used to predict that product will perform for its customer.1 If all tests are
satisfactory, the product is released to the next operation. If
not, the roll is “on quality hold” pending further testing (and
perhaps processing). Properties such as coefficient of friction
or bonds that are expected to “age up” will be retested after an
appropriate period (usually at least 24 h) based on a sample
again taken from the end of a roll. If the test results are still
unsatisfactory, the roll will be sampled at additional locations
along its length. If the samples suggest that some good material
is in the roll, results from confirming samples are required. In
this process, a portion of that roll will be scrapped as “offspec” or otherwise unusable.
1
Quality management systems procedures (e.g., certified ISI 9001) will specify appropriate
sampling methods and decision requirements. The sequence described here is for illustrative
purposes.
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MANUFACTURING FLEXIBLE PACKAGING
Table 18.1 Overall Equipment Effectiveness
Component
Concern
Metric
Availability
“Is the machine running Time: running versus
(Chapter 16)
or not?”
scheduled
Performance
“How fast is the machine Product: produced
(Chapter 17)
running?”
versus planned
Quality
“How many products met Product: accepted
(Chapter 18)
specifications?”
versus produced
Line operators observe the web as it runs through a machine
to watch for transient defects (e.g., foreign matter on printing
plates or cylinders or a pattern in a lamination caused by cells
on application rollers clogged with dried adhesive). The practice is to inset a “flag” (a small brightly colored tag) into the
roll as soon as the defect is recognized, remedy the cause of
the problem, and then insert a second flag as soon as product
quality has been restored. Some organizations may rewind rolls
with such quality issues, removing defective material between
flags, and splicing good material back to good material. Others
will simply pass the roll onto the next process and accumulate
additional flags as necessary throughout all processes.2 At the
last process (e.g., slitting or bag/pouch making) remaining
defective material is removed and first-quality product prepared
for the customer.
Both the roll-by-roll and in-line quality assessment issues
generate waste that reduces the amount of product produced to
product actually accepted and fit for sale to the customer.
Various “rework” methods may salvage some material, but
unless the value of labor and raw materials involved is substantial, the effort and incremental material waste may have diminishing returns.
Of course, the best protection against poor effectiveness
from this factor is a well-designed product produced with incontrol processes having a high degree of reliability. Statistical
2
The first case takes “extra” time” and the second requires “extra” material. The choice
depends on the cost structure of the product and the process manufacturing it.
18: SALEABLE PRODUCT
VERSUS
PRODUCT PRODUCED
151
process control (SPC) provides the approach for managing
such a manufacturing system. It requires:
1.
2.
3.
4.
5.
Realistic specifications
Understanding of process relationships
Verified measurement and gauging methods
Characterizations of the process
Indicated responses
Figure 18.1 illustrates the relationship of these components
in an SPC system. The critical challenge is establishing the
relationships between process conditions and the product characteristics required by the specification. Designed experiments
(“DOEs”) serve as the standard tool for this effort. Basic process and raw material property understanding helps optimize
the duration and cost of such “DOEs.”
The core of using SPC to manufacture product is the knowledge that keeping relevant process conditions at their optimum
state produces product meeting specification. Those process
conditions include raw materials that satisfy their required
input specifications. Those specifications should reference a
“target value” and “allowable variation” rather than a mean
value with a “plus or minus tolerance.” Figure 18.2 illustrates
the difference. Both lots of raw material, “Normal” and
“Extreme,” have 10 elements (“n 5 10”). The specification
2. Process-to-product relationships
3. SPC feedback
5
Input
• Materials
• Set points
4
Process
• Conditions
• Environment
Product feedback
Figure 18.1 SPC feedback system.
Output
• Value
• Presence
3
Specifications
• Variable
• Attribute
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MANUFACTURING FLEXIBLE PACKAGING
Raw material variable
5
4
3
2
1
0
1
2
3
4
5
Normal-n = 10
6
7
8
9
Extreme-n = 10
Figure 18.2 Influence of raw material variation.
variable for both lots has an average value of 5. The values for
the elements in the Normal lot vary closely around that average. However, elements in the extreme lot have values that represent the tolerance limits (i.e., 5 6 4 5 1 and 9). By way of
example:
• if the raw material specification requires an average heat seal value of 5 pounds per inch width,
• the lamination process adds an average 2 pounds
per inch to the heat seal strength of a lamination,
• on average, the normal lot of raw material will produce product with heat seals ranging from 6 to 8
pounds per inch,
• on average, the extreme lot of raw material will
produce product with heat seals ranging of 3 11
pounds per inch.
The normal raw material lot can produce product with a
specified heat seal range (pounds per inch) of 5 minimum to
10 maximum (production protection on one end and easy open
seals at the other). The extreme lot produces product that fails
to meet the specified range.
Reliable and precise measurement and gauging of the process assume regular maintenance and calibration. When the
18: SALEABLE PRODUCT
VERSUS
PRODUCT PRODUCED
153
process is quantitatively characterized, standard operating procedures for out-of-control process conditions can be
established.3
The application of SPC to web processes is well studied, but
it may require statistical analyses different than the usual methods used for discrete object manufacturing [1]. Table 18.2
summarizes basic considerations for applying SPC to web
processes. It notes that if a quality requirement involves the
presence or absence of an “attribute” rather than a “variable”
(i.e., an “either/or” condition rather than a measured value), the
Poisson distribution rather than the Normal distribution is
appropriate.
The value of SPC to the flexible packaging industry should
be obvious even if only considering material lost from further
testing (and perhaps reprocessing) rolls that are placed “on
quality hold.” The “cost of quality” in a manufacturing operation includes more elements than just this reduction of total
production by out-of-specification products. Other factors
include customer allowances, expedited freight charges, loss of
good will, excess inventory, billing disputes, and eventually
Table 18.2 SPC Considerations for Web processes
SPC Assumptions
Web Process Considerations
Samples: from the same
Cross-web variation constant in
population
that position?
Samples: from normal
Defects with binomial (Poisson)
distribution
distribution?
Process: in statistical control Assignable cause or inherent
process variation?
Process: has constant mean Nonrandom trends, cycles, etc.
over time
over time?
3
Best practices (as required for ISO 9001 Quality Management Systems) require that SOPs
be documented in written form and trained to operators who have responsibility for them.
Records of that training should be kept so that future staffing changes at the process result in
operators having the appropriate training.
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MANUFACTURING FLEXIBLE PACKAGING
lost sales. Actual quality costs can well reach into double digits
for an industry with return on sales performance measured in
single digits.
Reference
[1] P.J. Frost, E.B. Gutoff, The Application of Statistical Process
Control to Roll Products, second ed., PJ Associates, Quincy,
MA, 1991, 201pp.
19 Paper
Chapter Outline
Paper Dimensioning
Paper Grades
Paper Coatings
Paper for Flexible Packaging
References
155
156
158
159
160
Paper is easily considered the oldest flexible packaging material. The material itself does not provide barrier to gases or
heat sealability (but it serves as a rudimentary barrier to dirt
and bacteria), but enhancements to composition and surfaces
greatly extend its functionality.
Paper Dimensioning
Various fibers (usually from wood pulp) comprise a web of
paper. The pulp is cast onto a drying belt or drum that extracts
liquid water leaving a nonwoven mat of fibers to define the
paper’s intrinsic structure. The fibers can be more or less
densely compressed in the mat as a result of chemical and
mechanical treatments before and during the actual paper laying process. The result is a web with an average thickness, usually compressible by weight or roll forces.
A particular paper making processes with its chemical and
mechanical specifics will produce a web with an average “caliper.” Measuring caliper using traditional methods generates
high variability. Given the elusiveness of this determination,
weight per unit area (“basis weight”) defines paper bulk. In the
USA, units of measurement are “pounds per ream (ppr).”
Metric units are “grams per square meter (gsm).” The US measure simply expresses the weight of 500 sheets (a “ream”) of a
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MANUFACTURING FLEXIBLE PACKAGING
certain length and width. Not so simply, confusion results
from differences in the specified sheet dimensions in various
paper markets. The flexible packaging converting industry
uses a 24 inches 3 36 inches (i.e., 864 square inches or 6
square feet) sheet as its sheet size.1 In the industry, a ream
represents 3000 (3 3 2 3 500) square feet (432,000 square
inches). Flexible packaging converting uses paper in the range
of 15 50 ppr (25 80 gsm).
Paper Grades
Most of the paper used by flexible packaging is produced
from the “kraft process.” With many improvements over time
in pulping chemistry and process controls, the kraft process has
evolved to provide strong paper with less cellulosic degradation
(stronger individual fibers), and lower lignin content (greater
between fiber cohesion) compared to other processes. The
pulp can be bleached to provide white paper for good print color
reliability. On the predominant Fourdrinier-type paper machine,
the pulp is cast onto a moving loop of wire or plastic mesh belt.
The mesh allows excess water to drain off. Additional drying by
suction, pressure, and heat forms a continuous sheet. Calenders
(basically large diameter nip rollers in series) can smooth the
paper and impart gloss or other desired finish to the surface.
The work done to the paper while forming and drying it tends
to orient the pulp fibers in the machine direction. At a minimum,
differences between machine and transverse physical properties
result. At the extreme, the paper can have very weak crossdirection strength. Chemical and mechanical design options help
to avoid such problems. The choice of materials and process conditions for the suction, pressure, and heat drying methods greatly
influences the smoothness and surface finish of the paper.
Table 19.1 lists some designations used to characterize grades of
paper produced with various methods. The nature of paper
1
The size is called a “Tag” or “Bag” sheet. In contrast, typical “20 pound” office stationary uses a 17 inches 3 22 inches (“bond”) sheet as its basis sheet size (note: this is exactly
four 8 1/ 2x11 “letter-size” sheets).
19: PAPER
157
Table 19.1 Grade Designations for Paper
Designationa
Meaning
Use
BL
MF
MG
C1s
C2s
Bleached
Machine-finished
Machine-glazed
(“clay”)-coated one side
(“clay”)-coated two sides
“White” paper surface
Fair printing surface
Good printing surface
Best printing surface
(Magazines)
a
May be used in combination, for example, “MG Bl”: “machine-glazed
bleached.”
making brings about high-volume specialization in a few grades
of paper from any given paper machine. Any change, even in
basis weight, causes line waste and productivity losses.
Glassine represents the extreme in mechanical handling of
pulp on a paper machine. It is a very thin (e.g., 25 ppr) and
smooth paper resistant to water and air flow. Translucent unless
dyed for color or opacity, it is manufactured by “supercalendering.” After pressing and drying, the paper web is passed
through a stack of alternating heated steel and fiber-covered
rolls called a supercalender at the end of the paper machine.
The forces flatten paper fibers and force them into the same
plane, eliminating the nonwoven fibrous structure that causes
the paper to refract incident light.
Designations for the surface finish on various grades of paper
are listed in Table 19.1. Physical properties for paper grades
other than surface finish must be specified to insure ongoing fitness for use of composite materials with paper in them.
Adding chemicals to the pulp before forming the paper sheet
(i.e., the “wet end”) provides some flexibility to a paper
machines’ product line. “Sizing” is the broad industry term for
imparting various degrees of water resistance to a finished
paper. The range extends from virtually none (e.g., blotter
paper) to substantial (e.g., butcher wrap), called “waterleaf” to
hard sized, in the industry. Sizing agents, including natural and
synthetic resins (low molecular weight), starches, and waxes,
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MANUFACTURING FLEXIBLE PACKAGING
coat paper fibers to give them the water resistance desired. Such
resistance may involve absorbing water to slow its penetration
through the paper and to the opposite side (e.g., printing inks
on one surface) or repelling the water at the point of entry (e.g.,
dry wall liner). Whatever the primary function, subsequent
functioning of the paper in composite form (i.e., adhesion) must
not suffer. This intent determines if the sizing chemical is uniformly mixed into the pulp itself, or added to one surface at a
“size press” station near the rewind of a paper machine. Other
“wet-end” chemicals, such as flame retardants or mold inhibitors, may be added for specific paper applications.
Paper Coatings
Coated paper typically refers to an optical coating applied
to one or more surfaces [1]. Inorganic materials such as clay,
bentonite, talc formulated with various binders are applied as
liquids using common metering methods and dried. The variations on “clay” coatings are many, from one-side-coated papers
with single clay layers to double-sided clay-coated liners with
multiple top coatings and high internal cohesion. Available
surfaces range from high gloss to matte finish. Figure 19.1
suggests the magnitude of the difference in reflective properties
between uncoated and high gloss-coated papers. Magazines
and other periodicals with frequent printed halftone images
use coated papers for their reliable high-quality printing
reproductions.
Paper may receive other functional coatings as part of paper
making operations, but most frequently this value addition
takes place in converting operations. Heat seal coatings for
paper proved especially challenging before polymer extrusion
coating became available, because coatings would block to the
Coating
Paper
Figure 19.1 Reflective surface of coated paper.
19: PAPER
159
uncoated sides of the paper sheet. Coating son paper’s surface
would also stick on package and bag making equipment [2].
A solution of sorts was reached by using two sheet materials
laminated together with a thermoplastic adhesive (e.g., waxes)
that would only permeate one sheet, forming heat seals while
the other (on the outside of the package) would not seal. The
combination of tissue paper and aluminum foil (Trademarked
Rey-seals by the Reynolds Metal Company) proved quite effective for tobacco and some food packaging for many years [3].
Paper for Flexible Packaging
As the flexible packaging material of longest standing, paper
applications in the industry have long experienced efforts at
substitution. Cellophane, called “transparent paper” in its early
days, provided product visibility not available with paper
packages. Voided plastic films (Chapter 22), particularly OPP,
displaced glassine paper used for oily savory and sweet snacks
on the basis of cost, both by area and by weight.
Paper combined with aluminum foil by in-line extrusion laminating and coating lines produced dependable “paper/poly/foil/
poly” packaging for spices, dry-mixes, and moisture sensitive
bar wraps. If products require the barrier of foil, paper opacity
provides no problem. Coated paper proves an excellent print surface for the “use suggestion” images that accompany pouches of
nondescript spices and mixes. Original high-speed form-fill seal
pouch equipment utilized the combined stiffness of paper/foil
laminations to provide clean, reliable packaging operations.
Two unique properties of paper preserve its use in the packaging applications that still use it, (1) its tear properties and (2)
its dead fold. Tear propensity allows small packets (e.g., sugar)
of granular products to open easily while the thin plastic sealant layer provides sufficing burst strength and barrier properties. Dead fold allows twist wraps around single pieces of hard
candy, etc. to keep the product safely inside its wrapper.
In recent years, consumer goods companies may prefer a “less
plastic” look for some food products. While paper laminations
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MANUFACTURING FLEXIBLE PACKAGING
Figure 19.2 Paper/plastic lamination (left); all plastic lamination
(right).
have been used to provide these, various plastic film technologies have emerged to make plastic packages look like paper. The
matte finish OPP finds application here (Chapter 22). The casual
consumer is hard pressed to tell the difference between the two
approaches on sight alone (Figure 19.2).
Paper maintains its distinction, along with cellophane, as flexible packaging material produced using renewable resources.
The fibers in composite paper/plastic laminations, while theoretically recyclable, the commercial ability for doing so does not
presently exist. Sustainable packaging initiatives may favor one
or both of these features, label claims about them in retail displays require careful documentation [4].
References
[1] I. Endres, M. Tietz, Blade, film and curtain coating techniques
and their influence on paper surface characteristics, TAPPI J. 6
(11) (2007) 24 32.
[2] R.A. Farrell and C.L. Wagner, Heat-sealable sheet material; US
patent No. 2474619 A. (1949) 7pp.
[3] F.A. Grant, Method of laminating, US patent No. 2726979 A.
(1955) 3pp.
[4] US Federal Trade Commission, Guides for the use of environmental marketing claims, Federal Register Vol. 77 (2012)
62121 62132.
20
Foil
Chapter Outline
Production
Converting
Commercial Trends
References
161
163
164
165
Aluminum foil (very thin sheeting of thickness from 0.00017
inch 4 µ to 0.0059 inch 150 µ) became a commercial reality in
the early twentieth century as it replaced the stiffer tin foil used
previously as a barrier wrapper for cigarettes. Use as a wrapper
for food (chocolate and tea) followed shortly after that [1]. The
material is unique in providing essentially complete barrier to
light, oxygen, or water vapor transmission with a single web.
Foil has low tear resistance, no heat sealability, and for thicknesses less than 0.001 inch 25 µ, a tendency to form “pinholes”
when twisted or folded. Coating or laminating foil with other
materials overcomes many of these limitations, and those forms
have allowed foil to enjoy widespread use as a barrier-flexible
packaging material. Usual flexible packaging thicknesses range
from 0.00028 inch 7 µ to 0.00035 inch 9 µ. For applications in
which high abuse forces are likely and barrier loss is unacceptable, 0.0005 inch 12 µ to 0.001 inch 25 µ is used.
Production
Figure 20.1 provides a schematic summary of the “foil rolling” process [2]. The rolled strip material serving as the input
is 0.08 0.16 inch (2 4 mm) thick. The chemical composition
of the typical “1100” alloy is over 99% aluminum with about
0.12% copper. Different alloys are needed for specialty packaging formats, especially “cold-formed” foil materials.
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MANUFACTURING FLEXIBLE PACKAGING
Cold rolling Doubling
Cold rolling Separating and cutting Annealing
Foil
Figure 20.1 Foil cold-rolling process from thicker “strip.” From Gunter
Schubert, 2002, Adhesion of Coatings to Aluminum foil.
The “doubling” and “annealing” process steps for foil present challenges for subsequent converting operations. As thicker
layers are rolled together, rolling oil is sprayed between the
two layers as a release agent and the “twin foil” is rolled down
to the desired final thickness. By itself, the oil on the foil’s surface impairs adhesion in subsequent coating and laminating
steps. Nominally, the oil evaporates from the surface during
the heated annealing process. The foil is in roll form during
annealing, so evaporation takes place at roll edges. Residual oil
may remain on the surface of foil at the center of the web.1
Table 20.1 indicates test solutions of water and ethyl alcohol
used to test the surface energy of foil. More residual oil results in
lower surface energy and lower grade designation. “A-wettable”
foil should be specified for converting uses.
The annealing process (at 300 C) also provides energy to the
foil in ways that cause chemical and physical changes at the
surface where atmospheric oxygen is present (Figure 20.2).
The result is a chemically diverse foil surface with distinct
cross-web variability.
Table 20.1 Foil Wettability Designations and Test Solutions
Class
Water (%)
Ethyl Alcohol (%)
mN/m (5dyne/cm)
A
B
C
D
100
90
80
70
0
10
20
30
72.0
47.0
38.5
33.0
1
The twin-foil rolling process results in different surface appearances on either side. The
sides touching the rolls of the equipment are polished to a smooth “bright” or “glossy” sheen.
Sides folded together have a duller, “matte” finish. Additional machining of the web can polish
the matte side.
20: FOIL
163
Before annealing
T
B
After annealing
T
2–10 nm
2–6 nm
1–2 nm
2–8 nm
T
B
H
H
Al2O3 growth at
interface
Al
H
Al
H
H = Heterogeneous region
T = Top layer
B = Barrier layer
Figure 20.2 Annealing effects on foil microstructure.
Converting
With foil’s fragility and limited functionality, its role as a
packaging material requires laminating and/or coating with
other materials. These same physical properties of foil demand
that converting equipment adding value to foil be maintained to
handle it properly. Foil’s tendency to wrinkle makes it intolerant
of rollers on equipment that are not aligned exactly perpendicular to the web direction. The wrinkles become creases pressed
and laminated into place at downstream nips. Nicks on the edge
of a roll of foil often causes tearing across the whole web if web
tensions are not even across the web. Foil converting is manageable with experience and attention to detail.
The chemical activity on foil’s surface favors good adhesion
to coatings and resins with their own electrochemical activity
(e.g., ethylene copolymers and oxidized homopolymers). With
effective wet out (intimate surface to surface contact), the
adhesion to aluminum will increase over time at room temperature. High heat and humidities (e.g., those required for inpackage thermal processing) can cause product compounds
such as moisture, acids, or fats to migrate through a sealant
layer to its adhesive interface with foil. There, these migrants
can react at the interface and replace adhesive forces needed
for package integrity. A wide range of special foil adhesives
and primers are available for the demands of thermal and
chemical resistance needed for specific applications. Careful
consultation with both a packaged product’s developers and
suppliers of adhesives and primers is indicted.
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MANUFACTURING FLEXIBLE PACKAGING
Commercial Trends
When laminated to bulky plastic layers, foil gives stiffness
to a filled package.2 Its metallic gleam has connoted quality
packaging for quality products and allows interesting printed
effects with transparent inks. Its barrier properties are unique.
In spite of these advantages, its disadvantages for flexible packaging applications have allowed alternate materials to replace
much of volume in the industry over the past 30 years [3]:
1. The price of foil is closely tied to aluminum ingot
commodity prices, which vary with many global
factors (e.g., energy prices, geopolitical unrest,
and export/import regulations). Demand for aluminum in durable goods also causes swings in its
price for packaged consumables.
2. The converting challenges with foil limited supply
to a relatively few manufacturers who specialized
in converting it.
3. Foil’s relative fragility requires that heavy gauges
or considerable amounts of other materials be
combined with it in order to function as a durable
packaging material.
4. The converting time needed to laminate additional
layers when making foil packaging materials adds
considerable expense even when foil’s price itself
is low.
As with most material replacements, foil substitution began
with functionally similar materials. A process to deposit aluminum vapor onto plastic films (“metallized films”) for decorative purposes had been developed in the mid-1950s. The
technique was then refined beginning about 1970 so that the
deposited vapor provided a consistent and repeatable barrier
coating on the plastic film. Metallized oriented polyester
2
Thinner laminations may appear wrinkled and “shop worn” after distribution and handling
at retail.
20: FOIL
165
(48 ga 12 µ) and oriented nylon (60 ga 15 µ) films were initially used for a one-for-one replacement for foil. Metallized
nylon adhesive laminated to (or extrusion coated with) LDPE
became standard material for decorative helium-filled balloons,
and a heat-sealable coextruded (50 ga 12 µ) oriented polyester
structure laminated to a reverse printed film provided a truly
economical single web replacement for a two layer foil barrierplastic sealant component in a packaging material.
Certain oriented polypropylene films, coextruded with heat
seal layers, have evolved into significant moisture and oxygen
barrier films when metallized. These packaging materials have
replaced foil in many savory and sweet snack packaging applications. In doing so, significant cost savings were realized and
additional converting options took form.
Metallized films have continued to replace foil in many flexible packaging applications in North America and Europe. In
the USA, the Flexible Packaging Association reports for 2012
that the industry used about one million tons of foil [4]. These
uses were mainly for pouches (dry mixes, ground coffee a bag,
stand-up beverage pouches, and retort pouches). Globally,
especially where distribution systems require extended shelf
lives and humid climates require high moisture barrier, foil
maintains its uses for many dry products.
References
[1] The Aluminum Association, Aluminum Foil, second ed.,
Washington, DC, 1981, 78pp.
[2] G. Schubert, Adhesion of Coatings to Aluminum foil, PLACE
Conference Proceedings, 2002, 11pp.
[3] J.R. Newton, Metallized Polyester Challenges Aluminum Foil,
ICI Americas, Wilmington, DE, 1985, 16 pp.
[4] Flexible Packaging Association, US Flexible Packaging Industry
Report, Linthicum, MD, 2013, 122 pp.
21
Unoriented Plastic Films
Chapter Outline
Flexible Films
Cast
Tubular
General Film Property Effects
References
167
169
170
172
175
A process called extrusion manufactures plastic films. It
involves melting a thermoplastic resin with a combination of
heat and friction generated by a spiral screw turning in a long
barrel filled with resin. The turning screw pushes molten resin
out of the far end of the barrel and through a die with a long,
narrow gap. A linear die drops (“casts”) the molten resin downward onto a rotating water-chilled drum. It solidifies there and
is then wound as a film into roll form. Alternately, an annual
die pushes out a ring of molten resin that over time becomes a
large plastic tube. If the tube ascends as it leaves the die and
air pressure inflates it as it cools in air and a nip pulls it
upward, the process is called “blown film.” A tube exiting an
annual die downward where a water bath cools it is called
“water-quenched film.” Other resources present details of these
processes (see “Resources”). This discussion will deal with the
influences of the extrusion processes on the films themselves.
Flexible Films
Flexible packaging films are considered by standard industry
usage to have thicknesses less than 10 mils (0.01 inch or
250 µ). A material’s rigidity varies with the cube (third power)
of its thickness and with the first power of its modulus.
Increasing thickness quickly overcomes the inherent flexibility
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MANUFACTURING FLEXIBLE PACKAGING
of “soft” materials. Thick materials are considered “sheeting.”
With sheet thicknesses, cooling a tube of plastic from an annual
die severely limits process outputs, so sheeting is typically manufactured with the cast process. The next chapter, on “oriented
plastic films,” addresses films usually extruded in sheet thicknesses, but thinned by stretching to less than 10 mils thick.
Many flexible packaging polymers, with the notable exception of the polyethylenes and its copolymers, are used in both
oriented and unoriented film forms. Although equipment and
process condition vary greatly, the same basic processes manufacture films with widely different characteristics and uses. The
stretching and annealing steps in the orientation process result
in films that are stiffer (higher modulus), less elastic (lower
elongation), and ready to shrink (back to their original length,
width, and thickness) when heated. Figure 21.1 illustrates the
Machine direction
Load, pounds
25
125 gauge cast PP
20
125 gauge OPP
15
10
5
0
0
2
4
6
8
10
12
14
16
Displacement, inches
Cross direction
125 gauge cast PP
60
Load, pounds
50
125 gauge OPP
40
30
20
10
0
0
2
4
6
8
10
12
14
16
Displacement, inches
Figure 21.1 Modulus and elongation of cast and oriented PP compared
(See chapters 22 23 for descriptions of PP and OPP).
21: UNORIENTED PLASTIC FILMS
169
dramatic elongation and modulus differences that result from
orientation of a 125 gauge polypropylene film. Such property
differences are inherently neither good nor bad. Value in use
provides the context for choosing one film form or the other.
Cast
The cast film manufacturing process is very similar to extrusion coating and lamination (Chapter 5). Both utilize a slot die
in which molten resin enters in the middle of the top, flows out
to the full width of the die, and fills up the center cavity of the
die before exiting a narrow die-wide slot at the bottom. To
manufacture a cast film, no web-based substrate serves to carry
extruded resin away from the die. Rather, the molten resin
must solidify quickly on a chill roll (Figure 21.2). Then, in a
fraction of a second, the newly formed film releases from the
chill roll as nip rollers pull it toward a rewind at the end of the
line. “Chill-roll-release” additives are used for copolymers with
adhesive affinity for metal. Between chill roll and rewind,
corona or flame treatment of film surfaces (particularly polyolefins) may be necessary. Such treatment not only enhances
adhesion of the surface in subsequent converting processes, but
it also provides a relatively long-lived increase in surface
energy so that (for example) aqueous inks and adhesives will
flow out evenly on the surface [1].
The cast film process adapts itself to a significant variation
(“coextrusion”) that produces a multilayer film. Different extruders can output their respective resins to a “combining adaptor”
before they enter the die. The adapter can stack the outputs as
delivered, or split the some of the input streams into two or
more internal ones and place these on either side of other
layers. For example, an adaptor might (1) receive resin
from three extruders, “A” with low-density polyethylene, “B”
with adhesive tie resin, and “C” with ethylene vinyl alcohol;
(2) split the “A” and “B” flows each into two separate streams;
and (3) rearrange the flows into a five-layer barrier film of
low-density polyethylene/adhesive tie resin/ethylene vinyl
alcohol/adhesive tie resin/low-density polyethylene. Using the
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MANUFACTURING FLEXIBLE PACKAGING
Feed
Nip rolls
Die
ldler roll
Extruder
Chill roll
Rewind
Figure 21.2 Schematic of cast film line.
extruders’ designations to identify layers, the film structure is
described as “A/B/C/B/A.” Adaptor technology has developed
equipment configurations able to rearrange flows from the various extruders with the result any given line can produce many
different coextruded products.
Tubular
Tubular film manufacturing reflects a distinctly different
process (Figure 21.3). In the “blown film” variation, molten
resin output from an extruder enters the bottom of an annularshaped die. The die receives the resin and delivers it to the circular opening at its top using an internal channel that spirals
upward. At this point, an operator must collect the extruded
resin from around the ring and lift it to elevated nip rolls scores
of meters above. As the nip rolls begin to carry away the plastic, internal air pressure inflates the tube into a “bubble.” The
diameter of the bubble is typically several multiples (up to 8)
of the die’s diameter. The ratio of bubble diameter to die diameter is called a “blow up ratio” or BUR. It represents a critical
determinant of film properties.
As the molten resin forms a tube, the surrounding air cools it
to a solid. In some equipment the air used to create the positive
internal air pressure is itself cooled to provide “internal bubble
cooling.” At some point above the die, called the “frost line,”
21: UNORIENTED PLASTIC FILMS
171
Figure 21.3 Schematic of blown film line.
the transition from transparent molten resin to a hazier plastic
film is usually visible. This hazy plastic appearance does not
indicate that the film has completely cooled. In fact, along its
entire height, polymer crystallization and other morphological
features take place. When sufficiently cooled, a “collapsing
frame” compresses opposite side of the bubble together to form
a double-layer web. The web can be rewound in this form, or
its folded edges slit to produce two single-layer webs each
rewound at its own rewinder. Obviously, if the film remains
warm and tacky going though the nip at the top of the collapsing
frame, the two layers will effectively seal together and cannot be
separated into individual rolls. This effect often represents the
rate-limiting step in producing blown film (particularly with
high ambient temperatures). It can also deliberately produce a
double-layer film.
Producing multilayer coextruded films with the blown process is possible, but the technique is not as adaptable as the
cast extrusion one. The different extruders must output their
respective resins directly to dedicated spiral channels within
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MANUFACTURING FLEXIBLE PACKAGING
the die. Resin flows in these channels directly to the circular
opening at its top. Only then do they join in the order prescribed by the channels. Changing the order of layers, or their
thicknesses relative to each other, involves using different die
equipment. Much of the blown film used for flexible packaging
is in fact coextruded, but the structure of the films made on
any particular line remains essentially constant.
The tubular process for producing “water-quenched film”
closely resembles the one for blown film except that a water
bath (not air flows) cools the newly extruded film. The ratelimiting influence of ambient air temperature on film cooling
does not apply. This often allows higher process throughput.
General Film Property Effects
Cast, blown, and water-quenched film manufacturing processes place very different demands on the molten resins at the
molecular level. The ability to maintain a bubble form during
blown extrusion demands that the polymer molecules hold
tightly to one another and resist the downward force of gravity.
A low MFI (i.e., high viscosity) resin provides such behavior.
The same kind of gravitational forces tend to pull resin straight
down through the die from its entrance point. Again, intramolecular forces and adhesive forces between resin and the die metal
must resist gravity. This geometry favors moderate MFI resins
with a high degree of side-chain branching.1
The difference in film properties resulting from these different processes exceeds even the scope of differences in the choice
of resins. Much of the effects develop as a result of the
molecular-level crystallization processes that take place. Crystal
formation depends on the external factors of temperature and
time. Temperature reflects the energy available to molecules for
moving around within amorphous parts of the polymer. Longer
times in molten form allows moving molecules to align with
1
In actuality, each of these extrusion processes exposes resins to many dynamic forces
beyond the obvious ones addressed here. Optimum resin selection often requires trial and error
testing to determine which resin grade works best.
21: UNORIENTED PLASTIC FILMS
173
“matching” molecular structures, then stop moving when constrained by the intermolecular forces of the crystal structures
themselves. Thermal analysis of films can quantify the energy
stored in a film’s crystalline structure [2]. The balance of crystalline and amorphous regions in films greatly affects film properties. The relatively slow cooling experienced during blown
film formation favors crystal formation. The polymers commercially available as oriented films (Table 21.1) continue to form
crystalline structures even at room temperature. Crystallization
results in greater stiffness (higher modulus) and more haziness
(interference of light by crystals in the film), and less moisture
vapor transmission (amorphous areas have higher inherent permeability to moisture). Blown high-density polyethylene film
liners (usually coextruded with an EVA (See Chapter 24) sealant
layer) provide very economical moisture protection for boxed
cookies and crackers. Cast high-density polyethylene film liners
twice as thick would not provide as much moisture protection.
Cast films, their molecules quickly quenched by the cold chill
roll, demonstrate the opposite trends: soft and pliable to the
touch with very low haze.2 The cooling of water-quenched films
makes these products similar to cast films in these areas.
In addition to the influences of cooling rate and crystallization, these different extrusion processes impose various directional forces during film formation. While these are present to
nowhere as great a degree as found in oriented plastic films
(Chapter 22), they do affect properties of unoriented films.
While crystallization behavior reflects gravitational forces during extrusion, these directional forces lead to variation between
machine and cross-direction film properties.
All of the extrusion dies expel molten resin in a form thicker
than the intended final film gauge. The ratio of the thicknesses
at the die (called “die gap”) to film thickness is termed the
“draw down ratio” (DDR). This thinning effect results from
force of winding up film at a faster linear rate (feet per minute)
2
Extrusion process selection is not the only variable that influences the film properties discussed here. For example, blending 5 10% of a 3 5% EVA copolymer with a homopolymer
polyethylene reduces crystallization, modulus, and have of blown films enough to match cast
film levels.
Table 21.1 Examples of Different Applications of Various Polymers in Oriented and Cast Film Formats
Polymer
Polypropylene
Polyester
Polyamide
Example
Cast Film Benefit
Application
Oriented Film Benefit
Application
High-temperature
sealant
Oil/grease-resist
structural film
Thermoformable
structural film
Retort pouch
High moisture-barrier
print film
High heat-resist
print film
High puncture-resist
layer
Salted snacks
Microwavable
food
Processed meat
Beverage stand-up
pouch
Military rations
21: UNORIENTED PLASTIC FILMS
175
than material leaving the die gap. Surface friction on various
rolls in the web path (starting at the chill drum) restricts the
ability of the warm cast film to become narrower, and the overpressure inflating the tube in that process has the same effect.
On the macro level, film thickness trades off for film length,
while at the molecular level, the force extends polymer chains
and untangles side chains. Linear polymers actually array
themselves in the (machine) direction (MD) of the force.
Blown films also experience MD forces proportional to DDR,
but, unlike cast, they experience cross-machine forces represented by the BUR. These effects result in unbalanced tensile
properties in cast films. In Figure 21.1, the MD modulus (i.e.,
slope of the stress strain curve) of the cast polypropylene is
much higher than its CD value. The molecules align parallel to
the machine direction and resist bending against that alignment.
In the cross-direction, the lack of molecular alignment makes
bending very easy. These same molecular arrangements allow
the film to tear more easily in the MD, but resist tearing in the
cross-direction.3 The difference can be exploited in stand-up
pouches that are fabricated with the sealant film MD across top
of the pouch. Once started, a tear across the top should propagate relatively easily and reliably.
References
[1] W. Eckert, Corona- and Flame Treatment of Polymer Film, Foil
and Paperboard, PLACE Division Conference Proceedings;
TAPPI, Norcross, GA, 2004, 6 pp.
[2] T. Dunn, Use of Differential Scanning Calorimetry in Developing
and Applying Films for Flexible Packaging, American Society for
Testing and Materials, Special Technical Publication 912, 1986,
75 87.
3
In both MD and TD, the slopes of the cast curves are substantially less than the corresponding oriented film values.
22
Oriented Plastic Films
Chapter Outline
Film Orientation
Oriented Film Applications
Cast (Tenter)
Tubular (Bubble)
Special Oriented Film Effects
References
178
180
180
182
183
185
In the first half of the Twentieth century, cellophane was the only
available transparent material for flexible packaging materials.
This is a cellulosic (e.g., wood pulp) based material involving
chemically refined fibers dissolved in alkali and carbon disulfide.
The solution is called viscose.1 Cellophane is formed by casting
thick viscose solution though a slot die and carrying the sheet
though chemical baths to convert the viscose to cellulose, remove
sulfur, bleach the film, and add glycerin to plasticize it.
Along with opaque aluminum foil and paper transparent cellophane supplied the early flexible packaging market. All of
the substrates required coatings of various sorts to add functionality, in particular, heat sealability. Both paper and cellophane absorb and desorb water in response to environmental
changes of relative humidity. Research to develop coatings to
“moisture-proof” the materials devised wax coatings for paper
and eventually nitrocellulose (Chapter 26) and polyvinylidene
chloride (Chapter 25) ones for cellophane.
These coatings are also heat sealable. This encouraged the
development of equipment to make bags using heat seals rather
than glues. For there, the concept of integrated form-fill-seal
packaging processes would emerge. Early bag making and package filling machinery design relied on the stiffness of the flexible
materials to “push” them through the equipment. Plastic films,
1
Viscose is also the raw material for “rayon” textiles.
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MANUFACTURING FLEXIBLE PACKAGING
primarily polyethylene, introduced in the mid-twentieth century
lacked the stiffness of cellophane, and even though they were
transparent and heat sealable, they were unable to replace much
of cellophane’s packaging volumes. That challenge could not
develop until oriented plastic films became a reality.
Film Orientation [1]
Orientation involves the precise use of mechanical and thermal energy at a macro level to literally arrange the polymer
structure of films at a molecular level. Not all polymers lend
themselves to an orientation process. Polyethylenes in general
do not, although recent machine-direction oriented (“MDO”)
high-density polyethylene has received attention [2]. The primary commercial oriented films used in flexible packaging are
oriented in both machine and cross-machine (or “transverse”)
direction (biaxially oriented). The primary polymers involved
are polypropylenes, polyesters, and polyamides (nylon). Other
products have specialty application, but do not enjoy large volume use (e.g., polyvinyl alcohol and poly lactic acid). Shrinks
films, especially those made of polyvinyl chloride and polystyrene, use a machine-direction orientation (“TDP”) process.2
All orientation processes involve initially heating a relatively
thick layer of film above the temperature at which crystals in
the polymer maintain their well-ordered assemblies (called the
“glass transition temperature, TG”). By definition, at this temperature, the polymer is pliable and malleable. In this state, the
film is stretched (either simultaneously or sequentially) in the
machine and/or cross-direction so that it becomes thinner, but
wider and longer. While the film remains under tension in the
stretched direction, it is “heat set” or crystallized at an elevated
temperature (below the TG). This step locks in the orientation
of the molecules arranged in the form of crystals (resulting
from the tension forces) in the plane of the film.
2
Such shrink films take on the shape of a tube that fits around the perimeter of an object
(e.g., bottle). When heated, the diameter of the tube shrinks, usually until constrained by the
object itself, but the length is largely unaffected.
22: ORIENTED PLASTIC FILMS
179
Table 22.1 Physical Property Differences: 1 mil CPP vs 1 mil OPP
Property
Machine direction tensile
Cross direction tensile
Machine direction elongation
Cross direction elongation
Haze
Gloss
Water Vapor barrier
Oxygen barrier
Yield
OPP vs CPP: % Increase
208
482
(78)
(91)
67
6
167
64
0
Orientation of the polymer chains changes essentially all of
the physical properties of oriented films compared to unoriented
ones. Table 22.1 lists the percentage difference of specific properties for 1 mil OPP films compared to 1 mil CPP (CPP and
OPP described in Chapters 21 and 22 respectively.). Elongation
of the OPP is less because the molecules can stretch less before
they break (and break at much higher stress levels). Water vapor
barrier is good for CPP, but much better for the OPP because of
the dense crystalline structure. Oxygen barrier is not great for
either form, but the effect of crystallinity is evident here as well.
If the polymers in an oriented film rise to a temperature above
TG, they have enough energy to break out of their crystal arrangements and oriented positions. In doing so, the heated area of the
film shrinks to its previous (untensioned) length, width, and thickness. At the TG the polymer is not heated enough to make heat
seals. The result is that oriented films are not heat sealable. Much
as cellophane’s coatings added to its sealability and other functionalities, early commercial oriented plastic films included many
coated grades using the familiar cellophane coatings.
Oriented films revealed the versatility and scope of coextruding film technology. Given the dilemma of oriented films that
cannot heat seal and unoriented films that lack stiffness, coextrusion provides the means of achieving both. An amorphous
polymer coextruded as the sealant layer of oriented film that is
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MANUFACTURING FLEXIBLE PACKAGING
primarily compromised of a polymer that will crystallize makes
a stiff oriented film that heat seals. Such amorphous layers are
usually restricted to about 10% of the total film thickness.
They must have strong adhesion to the primary polymer in
order to provide strong heat seals and keep the layer in place
against the primary layer.
Oriented Film Applications
The strength and dimensional stability of oriented films
makes them ideal substrates for printing and coating. They are
typically used as the reverse-printed outer web of laminations.
As such, they experience the high temperature of heat seal jaws
directly. If high heat seal temperatures are necessary (e.g., in
order to rapidly seal relatively thick laminations), oriented polyester’s higher TG makes them the print film of choice.3
Oriented nylon is used less frequently as a print film for
laminations, but it has puncture and burst strength properties
that find use in critical applications in which even minute holes
are unacceptable.
Cast (Tenter) [3]
Biaxial orientation of films casts from a slot die involves a tenter (sometimes called stenter) frame. The process (Figure 22.1)
involves:
1. Casting a relatively thick sheet of plastic from a
slot die and rapidly cooling it on a chill and roll
and water bath.
2. Drawing (stretching) the sheet in the machine
direction using a stack of heated rollers (to increase
the temperature of the plastic above its TG). The
stack consists of a series of nips that rotate at
speeds progressively faster than each previous one.
3
Oriented polypropylene will shrink at temperatures around 300 F (150 C) while oriented
polyester can withstand temperatures of 390F (200 C).
22: ORIENTED PLASTIC FILMS
181
Figure 22.1 Generic tenter frame (Ref: US Patent Appn. 20030151162).
3. Drawing the sheet in the cross (transverse) direction, by grasping each edge of the film with clips
rotating on a continuous chain. As the clips pull
the sheet forward, the track carrying them diverges
to pull the plastic in the cross-direction.
4. Clips continue to carry the now relatively thin
film (under uniform MD and TD tension) through
a warm oven to anneal the plastic.
5. After annealing, any required surface treatment is
applied to the film. The still thick edges held by
the clips gripping the sides of the film are trimmed
off, and the film rewound.
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MANUFACTURING FLEXIBLE PACKAGING
Newer digitally controlled drives have been developed that
control the speed of the clips and simultaneously tension the
sheet in MD while the physical divergence of the track tensions
the TD direction. The ratio of MT to TD draw depends on the
use of the film, but is limited by molecular and morphological
configurations of the polymer [4].
The same geometric advantages that slot die extrusion provides for coextruding unoriented films allow the tenter process
to produce sophisticated (three- to seven-layer) biaxially oriented coextrusions. The ability to regrind, reprocess, and coextrude the thick edge trim into subsequent film production as a
center core layer provides more uniform surface properties
when the “skin” layers are comprised of virgin resin.4
Tubular (Bubble)
Biaxially orienting film using the tubular process involves
essentially identical thermal and tension effects for a thick film,
but the mechanical means of accomplishing these is very different. Figure 22.2 depicts one configuration of a “double bubble”
process.5 In the tubular process, orientation is done simultaneously. The exit roller (pulling faster than the inlet rollers feed
film) draws the film in the machine direction while the radial
expansion of the tube imparts to transverse direction tensioning.
Tubular biaxially oriented films have often provided “balanced” properties. This refers to the physical properties in
machine and transverse directions (MD and TD respectively)
having essentially the same values, as a result of similar MD
and TD orientation ratios. Tubular lines generally have lower
outputs than tenter film lines because ambient air temperature
determines its cooling rate.
4
Such edge “reclaim” must provide a homogenous melt in even the core layer to avoid visible gels and surface eruptions in these thin films.
5
Other, somewhat simpler process mechanics are possible (including extruding a tube downwards or horizontally). The example provided here provides reasonable presentation of the
heat and tension effects on the film.
22: ORIENTED PLASTIC FILMS
183
Figure 22.2 Schematic of double bubble process for biaxially oriented
film.
Special Oriented Film Effects
The significant and uniform physical forces imposed on
films during orientation provide the basis for surprising visible
effects. At the molecular level, these forces literally elongate
polymer chains and move them to new positions. Additive particles (Chapter 24) intimately incorporated the polymer matrix
will not respond to orienting force with the same stress/strain
responses. The particles (with diameter ranging from about 0.1
to about 10 µ) attempt to adhere to surrounding polymer until
the force of elongating polymer exceeds these adhesive forces.
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MANUFACTURING FLEXIBLE PACKAGING
Ref: US PAtent no. 4377616 A
lustrous satin appearing, opaque film compositions and method of preparing same
Void
Immiscible particle
Polymer
3000x
5µ
Figure 22.3 Cross-section view of micro-structure of voided film.
As that happens, the particles form a sort of nucleus of a void
within the polymer (Figure 22.3). Multiplied many times over
within any unit volume of film, the effects are dramatic:
• The film, literally “full of holes,” covers much
more area per unit weight (the film’s “yield”) than
a uniform plastic cross section.
• Light incident on the film is refracted at each polymer/
air interface, resulting in a film that appears white
opaque with the luster of pearls’ surfaces. (Similar
light refraction effects at interlayer interfaces cause
the pearlescent sheen.)
Coextruded layering allows use of uniform clear-or whitepigmented skins of void layers that provide smoother surfaces
for printing and coating layers. Similar in some ways to the
microstructure of paper, these white-voided films function in
some ways like that material and have in fact replaced paper
packaging material for confectionery bar wraps, snack food
packaging, and some bottle labels.
The same effect if restricted to a thin surface layer of an oriented film produces a “matte finish” film. Such surfaces contain microscopic raised areas in the form of fibers (e.g.,
22: ORIENTED PLASTIC FILMS
185
elongated ridges) and/or in the form of nodules (e.g., rounded
mounds). The surface irregularities reflect incident light in various directions, but are sufficiently shallow to allow a high
degree of light transmission. A reverse-printed image on such a
film will appear distinctly thorough the film, while the film’s
surface will not appear “shiny.”
References
[1] M.T. DeMeuse (Ed.), Biaxial Stretching of Film, Elsevier,
Cambridge, 2011, 288pp.
[2] D. Ryan Breese, Economic Benefits of Utilizing MDO Films in
Flexible Packaging, PLACE Division Conference, TAPPI,
Norcross, GA, 2007, 23pp.
[3] J. Breil, Oriented film technology, in: J. Wagner (Ed.),
Multilayer Flexible Packaging, Elsevier, Cambridge, 2009,
p. 119 237.
[4] Y.J. Lin, et al., Relationship between biaxial orientation and
oxygen permeability of polypropylene film, Polymer 49 (2008)
2578 2586.
23
Bulk Polyolefin Resins
Chapter Outline
Polymer Structure
Functional Description
Intrinsic Material Characteristics
Value Provided
Forms Used
Reference
187
189
190
194
195
196
The polyolefin resins, polyethylene, and polypropylene are the
major plastic family used in flexible packaging. Annual polyethylene usage for consumer flexible packaging is estimated at
over 10 million metric tonnes and polypropylene usage at about
6.5 million metric tonnes. Together they comprise about twothirds of annual flexible packaging material production [1].
These plastics are polymers (poly-) comprised of olefins (Latin
“oleum” for oil). The preferred chemical name for “olefin” is
“alkene,” relatively simple compounds of carbon and hydrogen.
The simplest carbon and hydrogen compounds are termed
“alkanes.” In alkanes, each carbon atom bonds to one other carbon
atom and two or three hydrogen atoms so that each carbon has
four bonds to other atoms. The alkenes are similar except for the
presence of one pair of carbon atoms joined by a “double bond.”
These carbon atoms have bonds to two other atoms, so that, again,
each carbon has four bonds to other atoms (i.e., one “double
bond” plus two “single bonds”). Figure 23.1 displays standard
chemical formulas for two and three carbon alkanes and alkenes.
Polymer Structure
The process of polymerizing alkenes into polyolefins involves
replacing the carbon carbon double bonds of alkenes with
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187
188
MANUFACTURING FLEXIBLE PACKAGING
Figure 23.1 Chemical structure of olefins and polyolefins.
carbon carbon single bonds (and the addition of hydrogen atoms
so that every carbon has a total of four bonds). On an industrial
scale, the process requires high temperatures and pressures and the
use of special catalysts to energize the double-bonded carbon
atoms and replacing them with single bonds to hydrogen and other
carbons. Industry uses several variations of the basic process with
different combinations of pressure, temperature, catalysts, and
hardware in order to produce polymers with comparable chemical
composition but very different physical properties. Figure 23.1 also
depicts simple polyethylene and polypropylene molecules made
from ethylene and propylene “monomers” (industrially called
“feedstocks”).
Physical property differences result from two major factors:
(1) different molecular weights (i.e., the length of the polymer
chains) and (2) patterns of “side-chain branching” (i.e., carbon
atoms within one chain that bond to one hydrogen and three other
carbon atoms rather than two hydrogen atoms and two carbons,
here the additional carbon bond begins another “side” chain).
Additional physical differences result from the use of chemicals
(called “comonomers”) closely related in size and carbon bonding
patterns to the primary alkene monomers (Chapter 24).
Ethylene and propylene are themselves flammable gases derived
from fossil fuels, both oil and natural gas. The oil-derived gases
represent individual “fractions” produced during the petroleum
“cracking” process. As by-products of the refining process, olefins
contribute to the overall profitability of the oil industry, but must
reflect at the same time market fluctuations in commodity oil
prices.
23: BULK POLYOLEFIN RESINS
189
Ethylene production from natural gas involves “dehydrogenation” of ethane (i.e., removal of two hydrogen atoms leaving
a carbon carbon double bond). Dehydrogenating propane
(H3C-CH2-CH3) to produce propylene (H3C-CHQCH2) is also
possible. With abundant low-cost propane in North America
from shale gas it has become a much more common propylene
source. Both alkenes are commodity chemicals with industrial
uses other than plastics production. As a result, feedstock
prices strongly influence the market prices of polyolefin resins.
Polyolefins are thermoplastics having a solid form at room
temperatures, but fluid at some higher temperature. In solid
form, the polymers have a “semicrystalline” nature. Some parts
of the large molecule align in an orderly pattern with other parts,
while some parts of the solid represent a random, unstructured
molecular maze, called its “amorphous” region. The balance
between crystalline and amorphous regions in a solid polyolefin
depends on both the physical properties of the polymer and
the method used to fabricate the solid (more discussion in
Chapters 22 and 23).
Functional Description
Thermoplastic properties of polyolefin resins provide the
basic heat sealing properties of flexible packaging. The sealing process uses resin properties in an integrated, multistep
process to weld together two previously separate plastic
surfaces:
1. The surfaces are heated to temperatures that soften
the plastic to an amorphous, high energy state.
2. At the same time, the surfaces are pressed into
intimate contact with the softened surfaces conforming closely to each others’ microsurfaces.
3. At the molecular level, the ends of heated, mobile
polymer chains can cross over the very small distance now separating surface where they become
entangled with chains from the other surface.
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MANUFACTURING FLEXIBLE PACKAGING
4. With sufficient time, pressure, and temperature,
the amount of entanglement increases until the
original interface between the two surfaces disappears and they become welded to each other.
As the two most common materials used in flexible packaging, polyethylene and polypropylene function in many other
ways to assist a flexible package in protecting, preserving, containing, transporting, and describing its product (Chapters 21
and 22).
Intrinsic Material Characteristics
The fitness for use (value) of a flexible package relative to
the product packaged in it ultimately depends on the intrinsic
characteristics of the materials in it (Chapter 31). For polyolefin resins, its thermal and related physical properties are critical. Its melt point temperature is the most obvious one, but this
in turn depends on molecular descriptions (e.g., density, short
chain branching, molecular weight).
As a molten fluid, thermoplastics do not respond to forces in
the same manner as familiar room-temperature liquids (especially water). The tendency to flow in response to a force (such
as gravity) is generally described as a fluid’s “viscosity.”
Contrasting how water and molasses behave when unrestrained
on a flat surface illustrates a range of high viscosity (very fluid)
to low viscosity (much lower tendency to flow). In general, the
viscosity of molten thermoplastics depends on their temperature and the amount of shear force they experience. In this
way, the low viscosity of a softened plastic surface allows
molecules to cross the distance to the other sealing surface
without entirely losing its form.
This temperature and shear force dependency of thermoplastics’ viscosity complicates both measuring and reporting this
characteristic of the material. Measuring requires quantifying
the resistance to flow (centipoise) at a set of controlled
temperatures. Rotating metal surfaces typically impart the
shear force in well-defined test geometries. The result is data
23: BULK POLYOLEFIN RESINS
191
reported as centipoise at a given temperature per second (or
simply “reciprocal seconds” at the temperature). Data for a
resin is reported in graphical form with isothermal lines showing viscosity as a function of shear rate.
Because of the complications of measuring and reporting viscosity, the polyolefin industry has developed a secondary quality
measure. The test involves testing a resin in a vertical cylinder
of specific geometry at a specified temperature with a given
load (weight) pushing it down and out of the cylinder. The more
resin that exits the cylinder in a specific time, the lower is its
viscosity (given the test conditions). The measured property is
called “melt flow index” (MFI), “melt flow rate” (MFR), or simply “melt index” (MI). The test is precise and repeatable, but
attention must be given to the temperature and load used in any
measurement. With their usually higher melt temperatures, polypropylene resins typically use a 230 C temperature, while polyethylene resin more often uses 230 C. As Table 23.1 suggests,
the choice of load varies to provide a reasonable time required
for ejecting a statistically sufficient amount of resin. The geometry of test apparatus may also cause confusion between measurements. With all of this variability in measuring and reporting
MFR, MFI, or MI, direct numerical comparisons are not valid
unless temperature, load, and geometry are the same. With
matching test conditions, higher MFR (MFI or MI) values signify lower viscosity polymers, but short of having complete sets
of data specified temperatures showing viscosity as a function of
shear rate, values representing different test conditions are not
directly comparable. Table 23.2 lists the standard test protocols
for thermal properties of polymer materials.
Density is the mass of a material per unit volume (expressed
as “gm/cm3 or gm/cc”). The density of water at one atmosphere and 4 C is considered 1.000. Polymer density has critical financial importance for products manufactured with
plastic. Undifferentiated polymers are sold by weight, while
their value in use results in large part from the volume used for
a particular product.1 Less dense materials provide greater
1
In the case of flexible packaging, area (cm2) of a specific thickness (cm1).
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MANUFACTURING FLEXIBLE PACKAGING
Table 23.1 Melt Flow Rate Condition
Designationsa
Designation Temperature ( C) Load (kg)
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
a
125
125
150
190
190
190
200
230
230
265
275
230
190
190
300
190
235
235
235
250
310
210
285
315
0.325
2.16
2.16
0.325
2.16
21.6
5
1.2
3.8
12.5
0.325
2.16
1.05
10
1.2
5
1
2.16
5
2.16
12.5
2.16
2.16
5
Identified in past revisions of ASTM D1238.
product volume per unit of weight than higher density products.
The polyolefins provide good economic value in use with densities all less than 1.0 (Figure 23.2).
Figure 23.2 makes no attempt to differentiate among various
kinds of polypropylene with molecular differences that result in
the wide density range indicated. The indicated density classes
(high, medium, and low) of polyethylene represent distinctions
23: BULK POLYOLEFIN RESINS
193
Table 23.2 Major Intrinsic Thermal Characteristics of Resins
Thermal
ASTM
ISO
Brittleness temperature
ASTM D746
Coefficient: linear thermal
expansion
Deflection temperature
under load (DTUL)
1160 psi (8.0 MPa)
264 psi (1.80 MPa)
66 psi (0.45 MPa)
Ductile/brittle transition
temperature
Glass transition temperature
Melt flow index (MFI, MI)
Melting temperature
Peak crystallization
temperature
Specific heat
Thermal conductivity
Vicat softening temperature
ASTM D696;
E831 and E228
ISO 812, ISO
974
ISO 11359-1, -2
ASTM D648
ASTM D648
ASTM D648
ASTM C351
ISO
ISO
ISO
ISO
ASTM E1356
ASTM D1238
ASTM E794
ASTM D3418
ISO 11357-2
ISO 1138
ISO 3146
ISO 3146 &
11357-1, -3
ASTM C351
ASTM C177
ASTM D1525
75
75
75
6603-2
ISO 8302
ISO 306
1.00– – Water
0.99–
0.98–
0.97–
0.96–
0.95–
High-density polyethylene
0.94–
Medium-density polyethylene
0.93–
0.92–
Low-density polyethylene
0.91–
0.90–
Polypropylene
0.89–
0.88–
0.87–
0.86–
0.855–
Figure 23.2 Density ranges (gm/cc) of bulk polyolefins.
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MANUFACTURING FLEXIBLE PACKAGING
Key molecular trend
Higher polyethylene
crystallinity
Intrinsic characteristic
Secondary property
Higher density
More moisture barrier
Unchanged oxygen
Higher melt point
Lower seal initiation temperature
More temperature resistance
Higher modulus
More stiffness
Figure 23.3 Density effects on intrinsic and secondary properties of
polyethylene.
with important differences for flexible packaging. Molecularlevel differences in polymers (in particular, the molecular
weight and side-chain branching mentioned above) result in
different balances of crystalline and amorphous areas in solid
plastic formed using comparable processes. With less sidechain branching, high-density grades2 have greater crystallinity.
Crystallinity indicates that individual molecules have aligned
themselves closely together in response to atomic-level forces
so that at the macro level higher density results. Figure 23.33
summarizes how crystallinity differences result in changes in
intrinsic characteristics and secondary (use-related) properties.
Polypropylene responds similarly, but molecular differences
tend to complicate describing cause and effect (in particular,
polypropylene’s “CH3” groups, Figure 23.1, take on different
forms and positions in the polymer molecule).
Value Provided
The significant portion of flexible packaging comprised of
polyolefins suggests that they provide significant value in use.
In addition to the many benefits provided, the relatively low
cost of polyolefins combined with their low density make them
the first choice when designing flexible packaging materials.
2
Density is measured using ASTM Plastic Test Standard 1505 “Density” (ISO Test Standard
1183 “Density”).
3
The format of Figure 23.3 is used in other chapters to review molecular-level effects on
macro-level measurements.
23: BULK POLYOLEFIN RESINS
195
The benefits result from these functions of the polymers’
features:
1. Melt points around 180 F (82 C) serve well for
many packaging applications. If more heat resistance is needed (e.g., boil-in-bags or hot fill
pouches), a higher density resin may work.
2. The relative hardness of polyolefins allows them
to move smoothly over metal surfaces that form
flat films into three-dimensional packages. If less
friction is necessary, the amorphous portions of
polymers allow small “slip additive” molecules to
migrate to the surface where they lubricate moving interfaces.
3. Polyolefin films are waterproof and largely unaffected by water. This feature and their low in-use
cost makes them the common choice for packaging
material for keeping moist products moist and dry
products dry. Moisture transmission through the
films varies with crystallinity and thickness, providing design latitude when specifying specific grades.
4. Tensile properties of polyolefins provide generally
robust packages reasonably able to withstand
puncture, tear, impact, and compressive forces
encountered by packages during distribution and
merchandising.
Forms Used
Both blown and cast film processes are used to manufacture
polyolefin films (Chapters 22 and 23). Thicknesses range from
0.0005 inch (12 µ) to 0.01 inch (250 µ). About two-thirds of
the polypropylene used is biaxially oriented, while machine
direction-oriented polyethylene film remains a specialty
product.
Polyethylene’s economy and versatility are evident in its use
as an adhesive and coating in the extrusion laminating/coating
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MANUFACTURING FLEXIBLE PACKAGING
process. This process delivers package benefits described above
directly from resin form without the need of separate
processes.
Reference
[1] D. Beard, The Future of Global Flexible Packaging to 2018,
Smithers Pira, Surrey, UK, 2013, 186pp.
24 Specialty Sealant and Adhesive
Resins and Additives
Chapter Outline
Polymer Structure
Alpha-Olefin Comonomers
Additives
Functional Advantages
Ethylene Vinyl Acetate
Ethylene Methyl Acrylate
Ethylene Acrylic Acid
Ionomer
Alpha-Olefin Copolymers (LLDPE and mLLDPE)
198
199
200
201
202
202
203
203
204
Chapter 21 described how the range of polyethylene densities
provided options for designing performance profiles of flexible
packages. Secondary properties of polymers critical to various
uses determine the degree of heat resistance, stiffness, and
toughness that packages need. The available ranges of such
properties are expanded by specialty polymers and polymer
additives. Such specialty resins often represent polymers in
which ethylene is polymerized win the presence of chemicals
with similar chain-building chemical bonds but different overall chemical identity. Polymers comprised of only one chemical
are called “homopolymers” and those with two chemical components, “copolymers.”1
1
Ethylene propylene copolymers (and ethylene propylene butylene “terpolymers”) are
often used in heat-sealable biaxially oriented polypropylene films. Just a few weight percent of
ethylene significantly impairs the ability of the orientation process (Chapter 22) to tesilize
(i.e. to leave residual strain after annealing) film layers made from those copolymers allowing
them to heat seal without shriveling the other layers. The amorphous terpolymer compositions
lower seal initiation temperatures and provide significant functionality to such products.
Manufacturing Flexible Packaging. DOI: http://dx.doi.org/10.1016/B978-0-323-26436-5.00024-2
© 2015 Elsevier Inc. All rights reserved.
197
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MANUFACTURING FLEXIBLE PACKAGING
Polymer Structure
Figure 24.1 presents several comonomers used with ethylene
for enhanced functionality. All have a carbon carbon double
bond at the end of the molecule (called the “alpha” position).
These double bonds incorporate the comonomers into the polymer chain along with ethylene. The relative amounts vary from
a few percent comonomer to about half of the polymer composition. Uses of the various comonomers are described in the
functional section below. Figure 24.2 indicates how general
characteristics and properties follow comonomer content
regardless of comonomer chemistry. Comonomer chemistry
does influence the choice of copolymers and grades for specific
adhesion and sealant purposes.
Physical blending of homopolymers and high-percent copolymers may provide benefits similar to single materials of a
lower percent copolymer. For example, a blend of two parts
polyethylene to one part of 10% ethylene acrylic acid (EAA)
H H
| |
H—C=C—H
ethylene
H H O
| | ||
H—C=C—C—O—H
acrylic acid
H H H
| | |
C=C—C—H
| | |
H H H
propylene
H H
| |
H—C=C—
generic “vinyl” group
O
H CH3
| |
||
H—C=C—C—O—H
methacrylic acid
H H
O
| |
||
H—C=C—O—C—CH3
vinyl acetate
H H O
| | ||
H—C=C—C—O+Na–
ionomer
H H O
| | ||
H—C=C—C—O—CH3
ethylene methyl acrylate
Figure 24.1 Chemistry of comonomers.
Key molecular trend
Higher comonomer
content
Intrinsic characteristic
Secondary property
Lower crystallinity
(and density)
Less moisture barrier
Unchanged oxygen*
Lower melt point
Lower seal initiation temperature
less temperature resistance
Lower modulus
Less stiffness
Figure 24.2 Comonomer effects on intrinsic and secondary properties
of copolymers. See Chapter 25 “Barrier Resin” discussion of ethylene
vinyl alcohol copolymer.
24: SPECIALTY SEALANT
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199
copolymer may adhere as well as a uniform layer of 3% EAA
(e.g., a lower weighted price of the blend than the lower of
copolymer itself). Care must be taken in matching the thermal
behavior (particularly viscosity characteristics) of blended
resins. While chemical methods are hard pressed to distinguish
between the two, melt point determination (ASTM E794) will
indicate a bimodal melt temperature for the blend and a simple
melt point for the unmixed resin.
Alpha-Olefin Comonomers
A series of alkenes (Chapter 21) with increasing number of
carbons with a carbon carbon double bond at the end of the
molecule (called the “alpha olefins”) form a singular group of
copolymers. In the presence of special catalysts, these
comonomers will react with ethylene to produce low-density
polyethylenes at conditions similar to those used to polymerize
high-density polyethylene.
Figure 24.3 shows “n-butene” and “n-hexene,” where butene
has four carbons and hexene has six.2 In effect, the singlebonded carbons beyond the carbon carbon double bonds
represent short side chains on what is otherwise a linear
polyethylene. The resins are called “linear low-density polyethylenes” for this reason. These regular side chains have a
secondary structure at the molecular level as they become
entangled and hold adjacent molecules together. The lower
temperature and pressure conditions needed to polymerize linear low-density resins can result in lower production costs. The
savings may be overcome by extra costs for the necessary polymerization catalysts and comonomers themselves.
H H
| |
—
C—C
| |
H H
ethylene
H H H
| |
|
C—
—C—C—H
| |
|
H H H
propylene
H H H H
| | |
|
C—
—C—C—C—H
| | |
|
H H H H
n-butene
H H H H H
| | |
| |
C—
—C—C—C—C—H
| | |
| |
H H H H H
n-hexene
Figure 24.3 Chemistry of alpha olefins (alkenes).
2
The “n-” (for normal) in the designation with the alkene identifier indicates an alpha position of the carbon carbon double bond.
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MANUFACTURING FLEXIBLE PACKAGING
New polymerization catalysts, generically called metallocene
catalysts, make it possible for polymer processes to assemble
alpha-olefin copolymers and ethylene monomers into polymers
with densities as low as 0.870. In some uses, such products
offer performance enhancements over both traditional homopolymer grades and specialty copolymers.
Additives
Plastic additives refer to a broad class of special chemicals added to plastic films in order to provide additional
functionality to the film. In extrusion processes (Chapters 21
and 22), film additives may be included in the grade of
resin as purchased from the supplier or from concentrated
“masterbatch” concentrates dry blended into the primary
resin prior to extruding. A masterbatch consists of a “carrier” resin with a high concentration (10 60%) of the additive mixed into it. Special compounding extruders mix the
chemicals uniformly into the resin, sometimes after the
chemicals have received pretreatments that enhance the uniformity of their distribution within the polymer matrix. Dry
blending a masterbatch resin with the primary resin requires
attention to the thermal properties of each. This assures that
the final distribution of the additive in the film will be uniform as initially extruded.
Common additives and purposes for their use are listed in
Table 24.1. The table makes a distinction between “migratory”
and “nonmigratory” types. For nonmigratory types, uniform
distribution within the overall matrix of the film is expected.
Migratory types may start out uniformly distributed, but their
function anticipates their movement through the polymer to a
film’s surfaces. The general descriptions of “Additive
Chemistry” in the table in fact refer to compounds with similar
chemical structure.3 The “amphiphile” term implies a molecule
3
Table 24.1 emphasizes additives for polyolefin homopolymer and copolymer films. Other
films (e.g., polyester and polyamides) often require different additive chemistries to accomplish
intended functionality.
24: SPECIALTY SEALANT
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201
Table 24.1 Additives for Flexible Packaging Films
a
Additive Chemistry
Function
Fatty acid amidesa
Esters/amines/amidesa
Amphiphilesa
Inorganic particlesb
Titanium dioxideb
Pigmentb
Slip modifiers
Anti-static
Anti-fog
Anti-block
White opacity
Color
Migratory additives.
Nonmigratory additives.
b
with two different chemical sections, one drawn to water
(e.g., the “acid” end of a “fatty acid”) and the other (e.g., the
“fatty” end of a “fatty acid”) to olefin chemistry. The latter
allows the additive to disperse uniformly within the polymer.
Chemical functionality on the film’s surface attracts the former,
resulting in the migration tendency. The water-attracted part of
the molecule can represent chemical activity able to react with
other chemistry in the packaging material causing unintended
consequences.
Functional Advantages
This chapter title suggests that the copolymers presented
here function to stick resin layers to other materials within
composite flexible packaging and to make strong heat seals
quickly. Some copolymers also produce plastic films with
added strength or flexibility. Such enhancements originate at
the molecular level where electrochemical, physical, and thermal factors act together to provide desirable results.
The discussion here addresses the various copolymers separately in order to explain how such factors determine where
and when use of that material is appropriate. The discussion
includes frequent reference to various secondary quality
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MANUFACTURING FLEXIBLE PACKAGING
factors. Chapter 32 provides descriptions of these. This interaction of electrochemical, physical, thermal, and other factors
demonstrates how a material’s molecular level features function as benefits in a product (adding to its value) when the
product is used in a particular context. This concept also
explains how one material can replace a different one if its features provide the benefits at lower cost to the user.
Ethylene Vinyl Acetate
The primary uses of ethylene vinyl acetate (EVA) in flexible
packaging involve improved heat seal performance, by lowering seal initiation temperatures and increasing hot tack. Small
percentages (2 5%) of vinyl acetate (VA) comonomer can
provide significant advantages over homopolymers. Moderate
ones (12 24%) provide the ability to seal through fine particulates that contaminate seal areas. High VA content (50%) polymer acts as a pressure-sensitive sealing surface to itself and to
surfaces such as glass and coated metal.
EVA can serve as an economical adhesive resin, but other
copolymers usually outperform comparable priced grades.
EVA serves as an initial component in producing the most
common grades of adhesive “tie” resins for coextruded films
(Chapter 21). Grafting a maleic anhydride molecule onto an
EVA polymer (in a “reactive extrusion” process similar to
compounding additives) produces a hybrid amphiphile molecule able to adhere to polyolefins and barrier resins with
oxygen chemistry (Chapter 25). Polymers other than EVA
serve as initial components for anhydride-grafted tie resins, but
EVA grades are the most versatile and economic.
Ethylene Methyl Acrylate
Ethylene methyl acrylate (EMA) and related acrylate copolymers based on ethyl- and butyl-acrylate comonomers (EEA and
EBA) find use to enhance adhesion in coextrusions and extrusion coatings to resins and films with oxygen chemistry.
24: SPECIALTY SEALANT
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AND
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203
Films made with moderate MA content (B20%) are soft,
pliable, and tough over a wide temperature range. They find
use where film stiffness (and sounds made when handling it)
must be avoided.
Ethylene Acrylic Acid
EAA and related acid copolymers based on methacrylic acid
provide excellent adhesion to metals and paper. They also have
low sealing temperatures, good hot tack, and toughness. The
comonomers’ carboxylic acid functionality actively contributes
to these advantages, but also causes acid-corrosive related challenges when processing such resins. Many extrusion laminations containing foil use the metal adhesion advantages of acid
copolymers having 3 12% comonomer content.
Ionomer
Acid copolymers, much as any carboxylic acid, can
exchange electro-positive hydrogen ions for similarly charged
inorganic ions (e.g., sodium or zinc). The exchange is called
“neutralization.” The term “Ionomer” refers to acid copolymers
that are at least partially neutralized. Ionomers have many of
the same properties as acid copolymers, as well as additional
ones. The differences result from amplified electrochemical
forces within ionomer resins. The tendency of the resin to
absorb atmospheric humidity is one immediate result, requiring
distribution of ionomers in foil-lined packaging (bags or box
liners) to separate the resin from ambient conditions. The
forces impede migration of some of the common amphiphile
additives that function in homopolymers and other copolymers.
Additives with a different balance of water-attracted and
olefin-attracted functionality are required.
Those same forces provide enough increased value in use to
motivate packaging manufactures to deal with them. Ionomers
have outstanding melt strength, toughness, stiffness, adhesion
properties, broad seal range, high not tack, and the ability to seal
through contaminants (e.g., dry particulates and oily residues).
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MANUFACTURING FLEXIBLE PACKAGING
The advantages result from the ions that bind adjacent molecules
together with a force augments crystalline attractions and physical entanglement experience by other resins.4
Alpha-Olefin Copolymers (LLDPE and mLLDPE)
Explanation of alpha-olefin copolymers requires classifying
them into two categories, the older ones (late-1970s) are made
with traditional coordination catalysts (“Ziegler-Natta” and
“Phillips”) and the more recent ones (mid-1990s) are made
with metallocene catalysts. The former are generally called
LLDPE and the latter mLLDPE.
The greater strength and toughness of LLDPE resins made
them attractive means for down-gauging the thickness of polyethylene used in existing applications. Initially subtle processing differences favored blending newer resins with the
traditional ones, but as new equipment capable of processing
unmixed linear resins became available, LLDPE grades
completely displaced the older technology in some markets.
Linear geometry with reliable side-chain branching made
LLDPE grades preferred for some applications. In particular,
n-octene (i.e., eight carbon atoms long) copolymers provide
linear molecules with elevated melting points and six-carbon
long side chains that entangle with one another resulting in
very strong films at a given thickness. These provide an optimal combination for packaging liquids in flexible pouches.
Such liquids often must be filled hot in order to inactivate
pathogens and they generate significant hydraulic forces
against pouch sides and seals if the pouches are dropped.
Commercial uses of mLLDPE resin grades have taken a very
different direction. The metallocene catalyst costs are several
multiples of the traditional ones, and so the initial applications
of mLLDPE resins were in specialty niches. As production
costs declined (from additional sources and improved production efficiencies), this specialty focus continued with the result
4
Water absorbed by ionomers becomes incorporated into these ionic bonding areas where it
is held strongly until extrusion temperatures release it as steam as the molten resin expands as
it encounters atmospheric pressure.
24: SPECIALTY SEALANT
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205
that mLLDPE resins have taken significant market share from
copolymers. This share comes primarily from applications for
which low seal initiation temperature favored copolymer (and
ionomer) use over homopolymer use. Enhancing adhesion,
especially if the result of chemical interaction, offers only marginal opportunity for mLLDPE polymers to compete against
copolymers on the basis of that functionality.5 Many former
applications that were dependent on ionomer resins now use
mLLDPE grades for the simple reason that the processing ease
and low seal initiation properties of the two match up well, but
mLLDPE has a cost advantage. If the additional functionality
of ionomers’ ability to seal through oil-contaminated seals adds
no benefit (while adding cost) its continued use is unlikely in a
given application.
With 20% comonomer content or more, some mLLDPE
grades exhibit properties similar to elastomers (rubber-like
polymers). When blended into other polyolefins, they impart
significant toughness to films.
5
Although mLLDPE copolymers are olefins with no inherent chemical attraction to other
materials, their softness (low modulus) results in intimate surface contact of molten mLLDPE
with other materials. This favors opportunistic bonding to active sites resulting from local oxidation or other treatment methods.
25
Barrier Resins
Chapter Outline
Barrier Kinetics
Polyvinylidene Chloride
Ethylene Vinyl Alcohol
Nylon
Coextrusion
References
207
214
215
216
216
217
“Barrier resin” is a relative term in the flexible packaging industry,
but it typically includes polyvinylidene chloride (PVDC), ethylene
vinyl alcohol (EVOH), and nylon (PA) resins.1 In this regard, oxygen transmission rate (OTR) is the permeability of concern.
Polyolefins (particularly biaxially oriented polypropylene) provide
low water vapor transmission rates (WVTRs). Figure 25.1 summarizes general barrier properties for these barrier resins.
Figure 25.1 uses a logarithmic scale on the vertical axis for
the respective OTR and WVTR values. It indicates that the OTR
performance of EVOH varies with relative humidity (RH). The
OTR of both PA and EVOH depend on temperature and RH.
The effect in use is minimal under most ambient conditions, but
becomes limiting when in-package processing conditions stress
the polymers (Figure 25.2).
Barrier Kinetics
Understanding polymer barrier properties requires considering
polymer permeability. Permeability is itself a well-understood
Another polymer, poly-chloro-tri-fluoro-ethylene (PCTFE) film (trade named Aclars) provides very good barrier performance. While used for various pharmaceutical products, its cost
generally excludes its application in food packaging. Impact-modified acrylonitrile-methyl
acrylate copolymers (trade named Barexs) provide very high oxygen barrier and chemical
resistance. They are used to package a variety of personal care, cosmetic, and industrial chemicals, usually as rigid containers.
1
Manufacturing Flexible Packaging. DOI: http://dx.doi.org/10.1016/B978-0-323-26436-5.00025-4
© 2015 Elsevier Inc. All rights reserved.
207
208
MANUFACTURING FLEXIBLE PACKAGING
(Units-see below)
100
10
1
OTR -80%RH
0.1
0.01
PVDC
EVOH-32
WVTR
N6
OTR -70%RH
WVTR: (g-mil) / (100in2-day) at 100°F and 90%RH
OTR: (cc-mil) / (100 in2 -day -atm) at 20°C (RH as indicated)
Figure 25.1 OTR and WVTR performance for “barrier” resins.
(From Down Chemical.)
Oxygen transmission rate of various polymers versus
relative humidity at 20°C
02GTR, cc20 µ m/m2 day atm
10,000
LDPE
BOPP
1000
UPVC
100
OPET
ON
10
BAREX210
SARAN EX
1
EVAL-E
EVAL-F
EVAL EF-XL
0.1
0
20
40
60
80
100
Relative humidity, %
Figure 25.2 Oxygen barrier of various polymers and films. (From
Kuraray.)
25: BARRIER RESINS
209
property of small molecules as they move around in matter with
free spaces at least as large as those molecules. The gasses,
vapors, and volatiles (taste and aroma chemicals) of concern in
packaging applications are the small molecules (see Table 25.1).
Although polymers are themselves typically large molecules (having molecular mass of tens of thousands), they are not typically
very dense (see Table 25.2). Aluminum and glass are two to three
times denser than polymers. All molecules move because of their
kinetic energy (at temperatures above absolute zero). The higher a
molecule’s temperature, the higher is its speed. Importantly, that
movement is random unless the matter is subject to a particular
attractive or repulsive force, such as an electric field.
On average, that random motion can appear to be directional—
not random—if areas with different concentrations of mobile
molecules are connected. By definition in the high-concentration
area, more molecules per unit volume are present and moving. If
molecules from both high- and low-concentration areas move
with the same random paths, a greater number of molecules from
Table 25 1 Permeating Molecules Relative Size
Molecule
Chemical Formula
Molecular Mass
Gas & vapor permeants
oxygen
Water vapor
carbon dioxide
nitrogen
O2
H2O
CO2
N2
32
18
44
28
Flavor/aroma chemistry
limonene
octane
capsaicin
cinnamaldehyde
C10H16
C8H18
C18H27NO3
C9H8O
Mass used here as a substitute for volume
136
114
305
132
210
MANUFACTURING FLEXIBLE PACKAGING
Table 25.2 Packaging Material Densities
Packaging Material
Density (gm/cm3)
aluminum
glass
Water
polyethylene terephthalate (crystalline)
polyethylene terephthalate (amorphous)
nylon-6 (crystalline)
nylon-6 (amorphous)
High density polyethylene
polypropylene (crystalline)
medium density polyethylene
low density polyethylene
polypropylene (amorphous)
2.7
2.4-2.8
1.0
1.5
1.3
1.2
1.1
0.95
0.95
0.93
0.92
0.85
the high-concentration area will move themselves to the lowconcentration area than vice versa. Given enough time, the concentrations of the two areas will be the same. The result is a mass
transfer of molecules from one area to the other, and an equalization of molecules per unit area in the two regions.
These very general observations about motion at the molecular
level serve as the basis for a more quantitative discussion of the
permeability of gasses, vapors, and volatiles through polymers.
Hansen (1998) provides the basis for the following explanation [1].
For a generic kind of molecule (“solute”) in a polymer, its
net movement to one side of the polymer or the other (the solute as a “permeant”) depends on (1) an intrinsic polymer property, D (its “Diffusion Coefficient”), (2) L the distance from
one side to the other, and (3) the difference in concentration of
the molecule from one side to the other (C1 and C2, respectively). This mathematical relationship is called “Fick’s first
law.” If time enough is allowed to reach the point at which the
net transfer, Q, is constant, Fick’s first law is expressed as:
Q 5 2DðC2 2 C1 Þ=L
(25.1)
25: BARRIER RESINS
211
In the more general case, for any time before the net transfer
is constant and for any position “x” (from point 0 to point L),
Fick’s law expresses transfer, q, as2:
q 5 2Dðδc=δxÞ 5 2Dðc2 2 c1 =x2 2 x1 Þ
(25.2)
Together with mass movement (assuming δc/δx ¼
6 0), molecu3
lar motion results in pressure, p. With the introduction of another
constant, S (“solubility coefficient”), Henry’s law holds that:
c 5 Sp
(25.3)
Substituting for c in Eq. 25.2 gives4:
q 5 2DSðδp=δxÞ 5 DSðp1 2 p2 =x2 2 x1 Þ
(25.4)
Rearranging terms suggests a new term, P5
qðx1 2 x2 =p2 2 p1 Þ 5 DS 5 P
(25.5)
The relationship P 5 DS (Eq. 25.5) describes the movement
of gas and vapor molecules through polymers with three fundamental factors6:
1. P: “Permeation coefficient” (mass transfer, q, over
distance (x1 2 x2) with pressure (p2 2 p1));
2. D: “Diffusion coefficient” (the speed at which permeant passes through the polymer);
3. S: “Solubility coefficient” (the amount of permeant that can be dissolved in the polymer).
The three factors in Eq. 25.5 have the benefit of being reasonably tangible notions which describe interactions among
2
For the equations, capital letters designate physical constants or the value of variables at
steady state conditions; lower case letters, variables’ values at any other conditions.
3
For gaseous and volatile molecules, pressure is more easily quantified than mass. The
introduction of Henry’s law relationships into Fick’s first law equations allows experimental
measurement and prediction of mass movement.
4
The reversal of the pressure difference relative to the location difference allows elimination
of the “2” sign in the equation.
5
Note: In this case “P” is not the steady state value of pressure “p”.
6
Reference to the units of measurement of factors and other variables in Table 25.3 may
provide a more tangible meaning.
MANUFACTURING FLEXIBLE PACKAGING
212
Table 25.3 Units of Measure for Gas Transport in Polymers
Symbol
Factor
P
R
D
S
q
Q
c
x
l
c1
c2
p1
p2
R1
R2
Permeation Coefficient
Resistance
Diffusion Coefficient
Solubility Coefficient
rate of mass transfer
rate of mass transfer (Steady state)
concentration
length
Overall layer thickness
concentration Side 1
concentration Side 2
vapor partial pressure Side 1
vapor partial pressure Side 2
surface resistance Side 1
surface resistance Side 2
å resistances
RTOT
Units (SI)
gm/cm s
cm s/gm
cm2/s
gm/cm3
gm/cm2s
gm/cm2s
gm/cm3
cm
cm
gm/cm3
gm/cm3
(dimensionless)
(dimensionless)
cm s/gm
cm s/gm
cm s/gm
solids, liquids, and gases in other contexts (e.g., carbon dioxide
gas dissolved in carbonated beverages, small solid particulates
in smoke diffusing through the air; moisture permeating
through textiles). Those can be discussed as theoretical and
mathematical relationships beyond the scope of this book, but a
few comments here help to anticipate barrier behaviors in plastic
packaging applications. The concepts add to an understanding
of other observations in plastic packaging applications (e.g.,
“blooming” of slip additives, flavor scalping, extractables in
packaged food).
The diffusion of a solute in a polymer depends on characteristics of both. The size, shape, and polarity of the solute are
critical, as are the structure and mobility of polymer chains.
Polymer chains arranged in crystalline form provide less free
volume for solute diffusion. Some solutes in a polymer can
embed themselves between the chains of polymers, pushing
them apart (increasing the “free volume”), and significantly
25: BARRIER RESINS
213
lower the glass transition temperature (TG—see Chapter 22) of
the plastic and make it softer.7
Similarly, the solubility of a solute in a polymer depends on the
size, shape, and polarity characteristics of solute. Additionally,
hydrogen bonding and related van der Waals forces in the polymer can attract-hold or repel solutes. Solubility has surface dependencies as well as volume considerations. Localized surface
effects (e.g., stagnant air, a layer of condensed moisture or other
coating, convection) may decrease the probability that a given solute molecule will enter the polymer and begin permeating at a
polymer specific rate of P 5 DS.
The inverse of a permeation coefficient, represented as R
“Resistance,” begins to agree with the familiar notion of barrier. R, by inverting Eq. 25.5, equals:
R 5 ðp2 2 p1 =x1 2 x2 Þ=q 5 1=P
(25.6)
Solving for q with a unit distance (i.e., x1 2 x2 5 1), R (just
as the usual notion of barrier) is seen to be inversely related to
mass transfer, so that q equals “driving force” divided by
resistance:
q 5 ðp2 2 p1 Þ=R
(25.7)
Equation 25.7 suggests that mass flow through a multilayer
film may behave in a manner analogous to the flow of an electric current through a circuit of resistors connected in parallel
(cf. Ohm’s law). Two implied assumptions must be addressed:
(1) normalizing Eq. 25.6 to a unit distance must be readjusted
for actual thicknesses of various film layers and (2) the diving
force through each layer i (pi2 2 pi1) must be known at best a
mathematical challenge and at worst an experimental
impossibility.
Neither adjustment is needed if (see Eq. 25.6) 1/P is substituted for R values and the thickness, li, for each layer i is
substituted for (xi1 2 xi2). Additionally each surface of the composite film may present “surface resistance” (R1 and R2) equal
to or greater than 0 for any permeating solute. The total
7
Such an effect of water in EVOH leads to the dependence of EVOH barrier on RH.
214
MANUFACTURING FLEXIBLE PACKAGING
resistance, RTOT, equals two surface resistance values as “R,”
plus the thickness of each of “n” layers in an n-layered material, divided by its layer permeability:
RTOT 5 R1 1 l1=P1 1 l2=P2 1 ? 1 ln =Pn 1 R2
(25.8)
With these adjustments for layer thicknesses and the internal
permeant vapor pressure at each interface, total mass transport
for a multilayered film (see Eq. 25.7) can be expressed as:
qTOT 5 ðp2 2 p1 Þ=RTOT
(25.9)
Equation 25.9 stresses that mass transport of a gas or vapor
through a multilayered film depends on both the total resistance
of the film to molecular movement and the existing pressure
differential of the permeant.
Polyvinylidene Chloride
PVDC (also known by its trade name Sarans) has long
served as an important role as a flexible packaging material.
The material used typically represents a copolymer consisting
of percent vinylidene chloride and vinyl chloride, or methyl
methacrylate. Its hydrophilic nature allows its formulation into
water- and solvent-based coatings. Its polarity favors high crystallinity, providing a dense moisture barrier microstructure. The
high chlorine content of the polymer attracts (and slows) any
oxygen molecules attempting to diffuse through it.
The material itself is stable, safe to handle, and not environmentally harmful under normal circumstances. PVDV resin
presents significant handling challenges for extruding. If the
resin reaches high temperatures 392 F 2 200 C, it will degrade,
evolving hydrogen chloride gas (hydrochloric acid) at concentrations that may cause eye, skin, and respiratory irritation and/
or injury. Such degradation limits its use in extrusion equipment that is not especially designed and fabricated for PVDV
use. Hydrogen chloride gas evolving from PVDC-containing
packaging incinerated in energy-from-waste facilities has also
discouraged its use in the industry.
25: BARRIER RESINS
215
Ethylene Vinyl Alcohol
Polyvinyl alcohol (PVA) is not prepared by polymerization
of the corresponding monomer, vinyl alcohol. Such monomers
are unstable in the presence of acetaldehyde. PVA is prepared
instead by first polymerizing vinyl acetate, and the resulting
polyvinylacetate is converted to the PVA.8 Typically supplied
as beads or aqueous solutions, PVA itself enjoys great demand
in papermaking, textiles, and a variety of coatings. Ethylene
vinyl acetate (EVA), having greater than 50% vinyl acetate
content, can similarly be converted to EVOH. EVOH copolymer is defined by its mole % ethylene content: lower ethylene
content grades have higher barrier properties; higher ethylene
content grades have lower extrusion temperatures.
EVOH resins used as a barrier layer within rigid and flexible
plastic packaging have become a global standard for polymeric
oxygen barrier.9 The sensitivity of its barrier performance to
RH (Figure 25.2) often requires unique design approaches, but
helps illustrate some of the nuance of barrier kinetics. Moisture
readily dissolves in EVOH resin (just as liquid water and
alcohols form intimate mixtures).10 The dissolved moisture
becomes intimately bound within EVOH crystals, the result of
“hydrogen bonds” as the hydrogen atom of the hydroxyl group
attracts the oxygen molecule. This causes general disruption of
the crystalline structure and more free space within the polymer
for oxygen to permeate. The effect is termed plasticizing (i.e.,
increasing the fluidity of a polymer).
When packaging aqueous liquids (e.g., condiments, juices) a
desiccant can be compounded in a polyolefin layer between the
product and EVOH layer to absorb water vapor migrating
8
The reaction is usually conducted by base-catalyzed trans-esterification using ethanol.
Other synthesis routes are also possible.
9
The resin’s excellent barrier to taste and odor compounds has found use in liquid packaging designed to minimize the loss of such volatiles from juices. It has found industrial application in plastic fuel tanks of vehicles by preventing leakage of harmful hydrocarbons given off
by gasoline.
10
Chlorine atoms occupy the analogous space in PVDC that hydroxyl groups (oxygen bound
to hydrogen) take up in EVOH. Chlorine in the PVDC polymer persistently repels oxygen while
hydroxyl chemistry of EVOH attracts and holds it.
216
MANUFACTURING FLEXIBLE PACKAGING
through the polyolefin [2]. High temperature accelerates and
increases the amount of water vapor dissolving in the EVOH.
The effect on polymer structure and barrier performance is
reversible to some degree as ambient temperature and RH
decrease. Foods in EVOH barrier packaging can be heat sterilized (called “retort” processed) if sufficient design considerations and postprocess controls are available.
Nylon
Nylon actually represents a large family of resins comprised
of two different monomers, a diacid and a diamine, generically
called “polyamides.” Typically, the number of carbon atoms in
the two monomers defines a type of nylon. Nylon 6,6 (the most
common polyamide used in flexible packaging) consists of the
polymer made from adipic acid [HOOC(CH2)4COOH] and hexamethylene diamine (H2N(CH2)6NH2). The carboxyl groups
(COOH) and the amine groups (NH2) of adjacent polymer
chains in solid nylon attract one another to form crystals. Much
as in EVOH crystals, these areas of molecular interaction will
attract and hold water vapor. Nylon can absorb so much water
vapor that drying the resin with hot air before extrusion is common. The plasticizing effect of moisture impairs oxygen barrier
of nylon in flexible packaging uses.
Coextrusion
Coextruded flexible films with cores of barrier resins have
addressed some of the limitations posed by handling difficulties
of PVDC and moisture sensitivity of EVOH and nylon. In the
former case, “encapsulation” of PVDC by polyolefins separates
corrosion-sensitive parts of extrusion equipment from hot
hydrochloric acid. The acrylate copolymers provide adequate
adhesion to both core and skin layers to deliver a film with
structural integrity. In the latter case, the good water vapor barrier of polyethylene and polypropylene can sufficiently isolate
25: BARRIER RESINS
217
EVOH and/or nylon layers from moisture to maintain effective
oxygen barrier.11 To this end, a diverse array of specialty tie
resins has been developed to provide interlayer adhesion
(Chapter 24).
Barrier resins can reinforce frail oxygen barrier of other
films. Metallized oriented polypropylene films provide good
oxygen barrier if the vapor-deposited coating is not cracked or
crazed (the plain film, with no coating, has poor oxygen barrier). Coextruding the polypropylene with an adhesive layer to
an EVOH layer allows the oriented film to provide robust and
high oxygen barrier performance in spite of handling during
filling and distribution [3].
References
[1] C. Hansen, Permeability of polymers, Pharmaceutical and
Medical Packaging 8 (1998) 17.
[2] C. Farrell, B. Tsai, J. Wachtel, Drying agent in multi-layer polymeric structure; US Patent No. 4,407,897 A, 1983, 9pp.
[3] J.R. Wagner, Jr., Barrier films having vapor coated EVOH surfaces; US Patent No. 5,688,556 A, 1997, 6pp.
11
Good design suggests that if the inside of a multilayer plastic film does eventually allow
moisture to plasticize EVOH or nylon, the outside should be chosen to allow rapid moisture
transmission out of the system.
26 Inks
Chapter Outline
Ink Vehicles
Ink Pigments
Ink Curing
Ink Selection
References
219
221
222
223
225
Printed images represent a critical part of packages that both
market the packaged product and convey important information
about it. The physical demands placed on the ink used to print
the images and texts are as strenuous as those expected of the
rest of the package material. In addition, the message must
remain clear and legible throughout a package’s useful life. Ink
pigments, the colorants, and inks’ “vehicle,” the matrix holding
pigment in the packaging comprise its two major components.
Special additives may be included for special performance
features, such as coefficient of friction. Some means must be
provided to cure the ink, change it from a fluid to a solid. And
once in place, the ink must adhere to one or more substrates
and resist environmental and product challenges to its printed
form and layer adhesion [1]. The ink chemistry basics presented here provide only a cursory overview of ink technology
for flexible packaging. Small amounts of additives (e.g.,
1 3%) in an ink can change the fit-for-use performance of a
packaging material.
Ink Vehicles
An ink vehicle must cure on a substrate to form a waterclear film layer with good cohesive strength and the ability to
Manufacturing Flexible Packaging. DOI: http://dx.doi.org/10.1016/B978-0-323-26436-5.00026-6
© 2015 Elsevier Inc. All rights reserved.
219
220
MANUFACTURING FLEXIBLE PACKAGING
“wet out” pigments suspended in them. Liquid inks required
for flexographic and gravure ink vehicles frequently use “nitrocellulose” (“NC”). This is formed by “nitrating” cellulose by
exposing it to nitric acid or another powerful nitrating agent.
Fully nitrated, it serves as a propellant or low-grade explosive.
If the cellulose is nitrated to only about 10 12% and dissolved
in alcohol, it will dry to form a film layer. Nitrocellulose-based
inks find extensive application in inks for films and foils.
Nitrocellulose is also a very good pigment-dispersing resin. NC
“bases” represent high concentrations of pigment intimately
blended in the resin and minimal solvent. These are later
diluted to press-ready concentrations using un-pigmented nitrocellulose “extender.”
Other vehicles for liquid inks are thermoplastic, lowmolecular-weight polymers, produced from a wide range of
feedstocks. The major source of feedstocks is various petrochemicals with average molecular weight of resins below 2000.
Polyamide vehicle resins soluble in ethanol, n-propanol,
isopropanol are used for surface-print polyolefin film inks.
These resins are compatible with alcohol-soluble nitrocellulose.
This provides faster drying and the ability to use economical
NC bases.
Recent solvent-based ink developments include polyurethanebased, flexographic and rotogravure printing ink system designed
for use on multiple flexible packaging lamination structures and
substrates. These provide high bond strengths (B500 gm/inch
width) for extrusion and adhesive laminations on multiple substrates, but may not be compatible with NC bases.
Styrene acrylic aqueous emulsions serve paper printing
needs and some film printing markets for which solventbased printing inks are not an option. Their ability to wet
out and adhere to plastic film surfaces limits plastic printing
applications elsewhere.
Seldom are ink systems for flexible packaging applications
formulated with a single vehicle resin. Vehicle design provides
significant competitive advantages for ink formulators, and
much ink development work progresses on a pragmatic empirical basis.
26: INKS
221
Ink Pigments
The colorants in flexible packaging inks are much more standardized than vehicles. Although common names may vary with
location, a reasonable uniform global pigments identification
system identifies pigments by “Generic Name” and “Constitution
Number.” Such pigments are well-defined (although not always
totally disclosed) organic chemicals and they also have a unique
Chemical Abstracts System (CAS) Registry Number. Table 26.1
summarizes these designations for the four process colors.
Pigment chemistry is very complex. The absorption of color
provided by a given pigment results from the pigment’s pattern
of chemical bonds.1 Different chemical bonds arranged in a
chemical molecule absorb the energy of specific waves lengths
of light (i.e., “subtract” color). Some 13,000 Generic Names
are maintained in the global index [2]. Each of these represents
a distinct chemical formula, a distinct color value, and its own
production process. The quest for plentiful and affordable
Table 26.1 Pigments for Four-Color CMYK Inks
Process Pigment Name Generic
Color
Name
Constitution CAS No.
No.
Yellow
C.I. 21090
86349-57-7
C.l. 15850
5281-04-9
C.l. 74160
147-14-8
C.I. 77266/
42765:1
98615-67-9/
1324-76-1
Diarylide
C.l. Pigment
Yellow AAA
Yellow
12:1
Magenta Lithol Rubine C.l. Pigment
Red 57:1
Cyan
Phthalocyanine C.l. Pigment
Blue GS
Blue 15:3
Black
Furnace Carbon C.I. Pigment
Black toned
PBK-7/
with Alkali
Alkali
Blue
Blue 61
1
The subtractive color system is discussed in Chapter 2.
222
MANUFACTURING FLEXIBLE PACKAGING
synthetic dyes and pigments provided much of the motivation
for chemical discovery in the nineteenth century.2
Because of the sensitive dependence of color value on the
molecular structure of pigments, any change to the molecule
changes the color perceived when viewing it. Chemical stressors, such as ultraviolet light and environmental pH and temperature, can change color values, sometimes irreversibly.
Ink Curing
Ink printed on an impermeable substrate takes the form of a
very thin (0.04 0.8 milil 1 20 µ) film of cured vehicle with its
embedded pigment.3 This “film-forming” ability represents a
major criterion for ink vehicles. Such a film, formed from a fluid
with solids dissolved or suspended in it, has physical properties
just as any extruded thermoplastic film. The properties depend on
the effects of the liquid solvent that is evaporated from the ink
and on the presence of various other additives in the fluid.
Film forming may result from molecules of the vehicle simply
arranging themselves into relatively cohesive patterns as a result
of the chemical forces (hydrogen bonds and van der Waals
forces) involved in interlayer adhesion (Chapters 28 and 29).
Such films redissolve in solvents and lack resistance to similar
chemicals that may challenge them while a package is in use.
Heat resistance may also be minimal, both with respect to adhesive and cohesive strength and color stability.
Other vehicles (e.g., polyurethanes) form films in which crosslinked chemical bonds bind molecules together. Applications
requiring heat and chemical resistance require these more durable
inks.
2
Some early pigment chemistry relied on heavy metal atoms (i.e., lead, mercury, cadmium,
hexavalent chromium) interacting with organic molecules. About half of the states in the USA
prohibit the deliberate use of such pigments in packaging. The European Union has similar
restrictions.
3
In printing porous webs (e.g., paper), their fibers can wick the fluid vehicle away from the
surface, leaving pigment disseminated over a relatively large area of the uneven surface. The
light reflecting the web has not encountered concentrated areas of pigment and color will not
appear as “sharp.” Coating on paper serves to isolate printing inks from the porous fibers’
effects and provides better graphic reproduction.
26: INKS
223
Traditionally fluid inks rely on evaporating the solvent
(organic or water) that holds the vehicle in solution or suspension and allowing the remaining solid materials to form films as
their chemistry dictates.4 “Energy-cured” inks involve 100%
solids (vehicle plus pigment) formulation in which external
energy (UV light or electron beam energy) causes chemical
bonds between vehicle molecules to form the ink film. These
cure using the mechanisms described for energy-cured adhesives
(Chapter 28). Formulations include a mix of small reactive molecules (making them sufficiently low viscosity for ink transfer)
and larger molecules (reducing the amount of energy needed to
cure the ink). “Dark” pigments (in the sense of absorbing UV
energy) can impair UV curing by absorbing curing energy before
it reaches the lowest layers of the ink coating.
Ink Selection
Choosing the set of inks that matches a converter’s equipment, processes, and product mix may well be the most critical
purchase decision faced by the organization. Often only experience serves to anticipate interactions between an ink and the
surrounding chemistry of the packaging structure (e.g., chemical forces changing color values, ink additives migrating
through adjacent layers or offsetting to unprinted surfaces in
roll form). Figure 26.1 suggests a few initial considerations for
choosing an ink with its associated additives and functionality
for a particular process and product. When the need is recognized, a small amount of the proper additive (e.g., 1 3%) may
satisfy the need, but often more than one technique is available
to satisfy the need. In the figure “jaw release” for surfaceprinted webs refers to the need to have a printed area contacted
4
Printing with solvent inks relies on a hierarchy of “slower” and “faster” solvents used to
optimize the transfer of inks from their fluid reservoir to their desired position on printed
material. The transfer is often dependent on ambient temperature and humidity resulting in
press-side adjustments of an ink’s solvent blend. The slow solvents may not completely evaporate
in the press dryer and impair the cohesive or adhesive functionality of the ink.
224
MANUFACTURING FLEXIBLE PACKAGING
Press, Process, Product
what ink to use?
COF
requirement
Slip
additive
Cross-linking
Heat
resistance
Release
additive
Jaw
release
Surface
print
Slip
Hard
Overprint
varnish
Cross-linking
Chemical
campatibility
Buried
print
Adhesive
compatibility
Adhesive
lamination
Extrusion
lamination
Extrudate
compatibility
Figure 26.1 Ink choice is determined by Press, Process, and Product.
by hot sealing jaws on packaging machinery release from the
jaws after a heat sealing cycle after its heat, pressure and length
of time tends to soften and attach to the ink film. The solution
may be to add a release additive that will free the ink from the
26: INKS
225
jaw after pressure is relieved, or an ink film hardener that will
reduce the ink’s tendency to attach to the jaw. Scuff resistance
of a surface-printed ink can be addressed with similar tactics:
additives to make the ink slide without disruption or to harden
the ink to withstand greater forces. The choice often depends
on other concurrent requirements (e.g., coefficient of friction or
gloss).
References
[1] The Kerley News. What’s in an ink? http://www.kerleyink.com/
tech_corner/whats_in_an_ink.htm/, 2014 (accessed 26.03.14).
[2] Colour Index International. About Colour Index Fourth Edition
2014
Online. http://www.colour-index.com/about/about/,
(accessed 26.02.14).
27
Overprint Varnishes
and Coatings
Chapter Outline
Overprint Varnish
Coating Integrity
Vacuum Deposition
Reference
227
228
229
232
Coating, much as laminating, adds functionality to single material
webs. The exact functionality required dictates the chemistry of
the coating material (e.g., barrier, heat sealability, gloss, and scuff
resistance). Figure 27.1 summarizes some basic techniques for
roll coating.1 Functional coatings for paper, foil, and cellophane
did much to advance the range of flexible packaging applications
before plastic films (and extrusion coatings) were available and
continue to provide economical utility to various substrates,
including plastic films.
Overprint Varnish
An overprint “varnish” represents a coating applied over a
surface-printed image.2 One obvious function of such coatings
is to protect the printed image, but the opportunity for a converter to add value while a web runs through a printing press
can reduce his raw material costs and increase the versatility of
his product. For example, a heat-sealable overprint varnish
(with appropriate heat-seal jaw release properties) provides the
1
Many other coating methods exist. The choice depends on equipment configuration, viscosity of the coating, and desired coating weight.
2
“Varnish” is a generic term for a transparent, hard, protective finish often, but not always
referring to a wood finish.
Manufacturing Flexible Packaging. DOI: http://dx.doi.org/10.1016/B978-0-323-26436-5.00027-8
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227
228
MANUFACTURING FLEXIBLE PACKAGING
Coating methods
C
B
B
B
A
A
A
Direct Gravure-fountain
A
Meyer Rod
gravure roll
Reverse Gravure-fountain
A
Offset Gravure-Dr. Blade
C
D
C
D
B
C
D
C
D
A
Direct Gravure-Dr. Blade
B
A
e
A
Reverse angle-Dr. Blade
B
D
Knife over roll
backup roll
C docking roll
e= Meyer (wire-wound rod)
Reverse Gravure-kiss
D
hard roll
Figure 27.1 Coating methods.
opportunity for heat seals made between the inside film surface
and this varnish on the outside. Such “in-to-out” seals are often
used to make machined direction seals on flexible packages
while the end seals represent sealing “in-to-in” surfaces. If the
heat-sealable overprint varnish can provide added barrier to the
web (i.e., some modified polyvinylidene chloride resins), even
more value may be added.
The overprints can be formulated to provide different
degrees of gloss, varying from matte finish to high gloss.
A glossy surface requires both a uniform coating and a
very smooth dried film surface on the printed surface. Much as
oriented film can be produced with a paper-texture surface
(Chapter 22), overprint varnishes can give paper-based packaging a plastic-like appearance.
Coating Integrity
Achieving a desired level of functionality often requires
careful process design. Voids in a coating’s surface will impair
27: OVERPRINT VARNISHES
AND
COATINGS
229
visual and barrier performance. The voids may result from
physical abuse to the web after coating (e.g., creasing, package
forming forces) or from irregularities in the film formed as the
coating cures. Foaming (small air bubbles trapped the liquid) in
the coating is the typical cause of these. Low-shear pumps help
minimize the effect and chemical defoaming agents can
decrease the liquid’s surface tension. When voids are possible,
but their consequences unacceptable, multiple coating layers
provide the best process response. As the initial layer forms a
film, a subsequent layer applied to its surface adheres to it with
good cohesive strength.
Heat seal coatings have long expanded the functionality of
foil and paper webs (Chapters 19 and 20). Such coated webs
(printed or not) often serve as heat-sealed “lidding” for openmouth containers. The coatings are by nature thermoplastic
materials. They soften on the coated substrate when heated.
They must then “wet out” and adhere to the opposing heat seal
surface (e.g., the flange or metal rim of a container) as they
cool. Unlike polymer-to-polymer seals, such seals may involve
dissimilar surfaces, including ones that do not seal themselves.
The heat sealing process then resembles the dry bond adhesive
lamination process (Chapter 4). The inherent tensile strength of
the coated material (directly related to its thickness) and adhesion to the opposing sealing surfaces limit such seal strengths.
In liquid form, heat seal coatings can be formulated within limits using a variety of adhesion promoters, plasticizers, cross linkers, etc., to increase seal strength.
Vacuum Deposition [1]
Vacuum deposition of a thin (,1 µm) layer of aluminum
onto plastic films transformed the concept of “barrier plastic
films” more than any other technology since 1980. The
process (Figure 27.2) requires a deep vacuum (about
one-millionth of an atmosphere), provided by a series of
230
MANUFACTURING FLEXIBLE PACKAGING
Metalizing
chamber
Rewind
Unwind
Pumping/
cooling
system
Aluminum
wire spool
Shields
Boat
Figure 27.2 Vacuum deposition chamber.
strong vacuum pumps. At these very low pressures, the
vaporizing temperature (i.e. “boiling point”) of materials is
lowered. Aluminum in the form of wire is melted in a heated
crucible or “boat,” and aluminum vapor rises from the pool
of molten metal. The vapor condenses on the cool surface of
a passing plastic film and forms a uniform metallic coating
on the film.3 A “dual chamber metallizer” maintains a deeper
vacuum in the chamber volume that immediately surrounds
the molten aluminum. The outer chamber removes most of
any gases and vapors adsorbed on the film surface before it
enters the deep vacuum region. The process is essentially a
batch one. It requires that the pumps draw down pressure in
the chamber(s) before metalizing can begin. The areas holding the uncoated and coated webs must return to atmospheric
pressures to remove product and load a new roll.
Productivity of the process depends heavily on the efficiency
3
As it condenses, the vapor “gives up” the heat of condensation (resulting from it vapor-tosolid phase change). This heat must be absorbed by the “system.” In some metallizing equipment, the web rests on a chill drum as it passes through the aluminum vapor. The drum serves
to cool the film and presents melting or shrinking.
27: OVERPRINT VARNISHES
AND
COATINGS
231
of vacuum creation, so equipment design to minimize
changeover time is critical (Chapter 17).
The thin layer of aluminum on a plastic film behaves much
the same as does the surface of aluminum foil (Chapter 20).
The actual surface is a layer of transparent aluminum oxide.
Process controls to deliver optimum formation of the microstructure of this thin layer can have dramatic effects on the ultimate barrier performance of metallized film. In the USA,
oriented polyester and oriented polypropylene are the primary
metallized packaging films. In other parts of the world,
metallized cast polypropylene is popular.
The vacuum-deposited layer of aluminum on metallized
films provides barrier properties equal to aluminum foil only
when the integrity of the coating in length, width, and depth
is absolute. . . an occurrence not yet experienced in the
packaging industry. Just as thin gauge aluminum foil has
statistically predictable incidence of pinholes, metallized
film coatings will not be perfect. Web handling of metallized films is critical. Soft, cork-covered rollers are recommended to avoid scratching. Higher tensions and extrusion
melt temperatures, suitable for oriented films, can stress the
metal layer causing cracks. Experience has demonstrated
that enhancing the adhesion of the aluminum to the film
(e.g., using specialized surface treatments) allows the plastic
to reinforce the metal layer, allowing greater elongation
before break.
Vacuum deposition technology includes variations beyond
“simply” melting metals and depositing their vapor (called
“physical vapor deposition—PVD”). These techniques now
provide innovative composite materials to replace other
material technologies, e.g., solar panels, displays on electronic
devices, and energy-efficient building windows. Different
chemicals introduced into the vacuum can react with each other
to form a third chemical that then deposits on a film (so-called
“chemical vapor deposition—CVD”). This process augmented
with plasma energy to drive the chemical reaction is called
“plasma-enhanced chemical vapor deposition—PECVD.” This
process development, often justified for manufacturing durable
232
MANUFACTURING FLEXIBLE PACKAGING
products with greater value than disposable packaging, in turn
has provided “transparent barrier-coated films.” At present use
of these is limited by their cost, but process improvements are
underway to lower production costs.4
Reference
[1] V. Ataya, Inline Coating and Metallizing as a Way to Improve
Barrier and Reduce Carbon Footprint, TAPPI PLACE
Conference, Norcross, GA, 2012, 27 pp.
4
The transparent coatings are layers of ceramic materials such as aluminum oxide and silicon oxides. The weakness of these compared to even vacuum-deposited aluminum requires an
adhesion promoting coating to avoid cracking during converting and use.
28 Adhesives
Chapter Outline
Polyurethane Adhesives
Acrylic-Based Adhesives
Energy-Cured Adhesives
References
234
236
237
238
The adhesive laminating process (Chapter 4) involves adhering
two surfaces to one another with a layer of a third substance
that (1) provides high adhesion to both surfaces and (2) has
internal strength (cohesiveness) greater than either interface.
Both the adhesive and cohesive strength of a suitable flexible
packaging material must resist any expected thermal, chemical,
or physical exposure that would negate those strengths over the
life of the packaged product.
The adhesion to other surfaces requires that the adhesive “wet
out” that surface. If complete wetting does not occur, then liquid
of will form a bead, having a contact angle relative to the surface
(See Figure 28.1. The value of the angle results from the surface
energies of the solidliquidair system). The surface is solid and
the adhesive must intimately cover the plane of its surface with no
air (or other substances) trapped between the solid and the fluid.
Cohesion of the adhesive layer itself results from “curing” the
adhesive. The curing mechanism differs for different adhesive
types but often involves a chemical reaction [1].1 The reaction
may require multiple chemical “parts” (details follow) and may
originate a gaseous by-product that must escape the structure.
Adhesives, coatings, and inks often involve chemical reactions.
As a result, regulatory control over flexible packaging
adhesives, particularly for food packaging, is considerable. The
1
The chemistry of these adhesives is complex, requiring basic organic chemistry knowledge.
The discussion here provides general reference to chemical names. More detail is available in
Ref [1].
Manufacturing Flexible Packaging. DOI: http://dx.doi.org/10.1016/B978-0-323-26436-5.00028-X
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233
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MANUFACTURING FLEXIBLE PACKAGING
Contact
angle
Solid–liquid–
air point
Figure 28.1 Contact angle: drop of liquid on solid surface.
converter as well as the supplier of these materials must comply
with these both in formulating the materials and using them.
Adhesion by a coated adhesive to the second substrate
requires that the coating still has a fluid nature, either because
it has not yet begun to cure, or the partial curing that has
occurred still allows the adhesive surface to soften when heated
and conform to the other surface under pressure.
Polyurethane Adhesives
A urethane is formed by the reaction of the hydroxyl (i.e.,
“-OH” or alcohol functionality) of a “polyol” (two or more
hydroxyl functions) with a suitable isocyanate chemical. If that
chemical is a “diisocyanate” it reacts with another hydroxyl
group on another polyol, lengthening a polymer chain and
increasing molecular weight of the “polyurethane.” Specific
polyol and diisocyanate chemistry result in different adhesive
tendencies, environmental resistance, and health implications
(Tables 28.1 and 28.2) [2].
In any specific adhesive formulation, the polyol can be lowto high-molecular weight of singular or mixed materials, polyurethane “prepolymer,” hydroxyl terminated, or mixtures of the
two. These are reacted with low- to medium-molecular weight
diisocyanate prepolymer. They cross-link to form the urethane
link and the final cured adhesive. This would be called a twopart adhesive system.
There is always the potential that water will react with the
isocyanate as a result of humidity in the air and water in
28: ADHESIVES
235
Table 28.1 Influence of Polyol Choice on Polyurethane Adhesive
Performance
Polyol Used
Advantages
Toluene diisocyanate
(TDI)
Moisture resistance
Flexible at low
temperatures
Microbial resistance Soft/rubbery feel
Chemical resistance
Brittle at low
Diphenylmethane 4,40
diisocyanate (MDI)
temperatures
Better metal adhesion Design options:
crystallinitybased
Isophorone diisocyanate Balance hardness and
(IPDI)
low temperature
flexibility
Greater tensile
Hexamethylene
properties
diisocyanate (HDI)a
a
Used as modifier with polyether/polyester polyols.
Table 28.2 Influence of Diisocyanate on Polyurethane Adhesive
Performance
Diisocyanate
Type
Species
Advantages
Aromatic
TDI
MDI
IPDI
HDI
Fast reaction
Low cost/health concerns
UV light resistant; lowest
food packaging concerns
Aliphatic
solvents.2 Though in this case the goal is to create polyurethane
adhesive, A reaction can occur between the diisocyanate and
water to form an amine, considered carcinogenic. When the
adhesive is fully cured, the amine is completely reacted and the
laminate is safe to use. (If the laminate is brought into contact
2
Industrial solvents can be specified “-UG” for specific low moisture content of water miscible solvents (e.g., acetates).
236
MANUFACTURING FLEXIBLE PACKAGING
Table 28.3 Polyurethane Adhesives: Solvent and Solvent-Free
Issue
Solvent-Based
Urethanes
Solvent-Free
Urethanes
Industry
experience
Solvent
Well-known
Relatively new
Dedicated equipment
Solvent-free
Versatility
Financial
Operations
Air pollution source
Fire hazard
Widest (food types,
resistance,
substrates, etc.)
Higher solids content
Cure time required
Once mixed, limited
pot life
Solvent drying may
limit output
Wide (highest
performance not
demonstrated)
Low-coating weights
Bond strength about
zero until curing well
underway
Mixed on demand
(Adapted from Jopko, 2004)
with food before curing is complete, the amine formed may
migrate into the packaged product) [3]. The actual chemistry of
adhesive reactions is complex and a converter must wait sufficient time to assure the reaction of the amine into the cured
adhesive before the food is packed in a laminated material.
The breadth of urethane chemistry provides the flexible
packaging industry with a wide range of adhesive choices. The
historical solvent-based grades are now giving way to solventfree (100% solids) versions because of their financial and pollution control advantages (Table 28.3).
Acrylic-Based Adhesives
Acrylic is a general term for polymers made from acrylic
acid. Acrylic esters (“acrylates”) such as methyl, ethyl, butyl
and 2-ethylhexyl acrylates, copolymers, and various blends of
are used to formulate acrylic adhesives.
28: ADHESIVES
237
The acrylic-based polymers usually have mid- to highmolecular weights and are emulsified in water. This produces
very small particles dispersed in water and allows high percent
solids at very low viscosity. Many of these adhesives are used
as one-part with either no cross-linking or some self-crosslinking. In essence, they remain pressure sensitive and rely on
their inherent molecular weight for the properties of adhesion,
tack, and intrinsic bond strength and shear resistance.
For higher performance, a suitable cross-linker can be used
to create a two-part system. There are water-dispersible isocyanates that can form acrylic urethane polymers when cured.
This allows the performance and benefits of both the acrylic
and the urethane chemistry. Another approach is to use
amineepoxy cross-linking for the cure. The acrylic polymer
is terminated either in amine or epoxy, and the opposite prepolymer is used for the cure. Many other cross-linking materials are available for acrylic-based adhesives, but few have
status in the food laws.
Acrylic chemistry is naturally crystal clear and relatively
low cost. One-part acrylic adhesives provide limited bond
strength, and chemical and heat resistance. Two-part systems
are somewhat better but are not able to perform in demanding
chemical and thermal environments.
Energy-Cured Adhesives
This type of adhesive cross-links acrylics and polyurethanes
with ultraviolet light or electron beam energy (streams of electrons). UV cross-linking relies on “photoinitiator” chemicals
that release “free radical” species (an atom, molecule, or ion
that has unpaired electrons). Free radicals provide energy to
cross-link adhesive prepolymers and to in-turn release more
free radicals to continue the process. Electron beam energy
cross-links the chemicals directly.
Photoinitiators are small molecules and if not completely
reacted in the curing process remain mobile in the adhesive
matrix from which they can migrate into food. Electron beam
238
MANUFACTURING FLEXIBLE PACKAGING
Table 28.4 Summary of Available Adhesive Chemistry
Adhesive
Type
Snack General Chemical Hot Boil-CookLow
Purpose Resistance Fill in Retort
Demand
Medium High
Solvent PU
OK
100% solid PU OK
Acrylic
OK
Energy cure
(OK)
OK
OK
OK
(OK)
OK
OK
(OK)
(OK)
OK
(OK)
NO
NO
OK
OK
OK
(OK)
OK
(OK)
NO
NO
(OK), careful testing/experience required.
(Adapted from Jopko, 2004)
energy averts the photoinitiator requirement, leaving less concern about chemical contamination.
Energy-cured adhesives systems are not in widespread use in
the industry, even though the process eliminates the need to
wait for adhesives to cure.
Proper selection of flexible packaging adhesives challenges
the industry regularly. New chemical options, new regulatory
limitations, new product/package interactions, long product
shelf life requirements all make finding the optimum adhesive
solution difficult. Table 28.4 provides directional guidance, but
any choice must be carefully designed to meet all fit-for-use
requirements.
References
[1] K. Svensson, Adhesives in Food Contact Materials and Articles:
Proceedings from a Nordic Seminar, Nordic Council of
Ministers, Copenhagen, Denmark, 2001, 120pp.
[2] L. Jopko, Flexible Packaging Adhesives: The Basics, PLACE
Division Conference, TAPPI, Norcross, GA, 2004, 7 pp.
[3] Flexible Packaging Europe, FPE Guideline on Use of
Isocyanate-Based Adhesives in Packaging Laminates;
Düsseldorf, DE, 2009, 3pp.
29
Primers
Chapter Outline
Polyethylene Imide Primers
Ethylene Acrylic Acid Copolymer Primers
Other Primers
Primer Selection
Reference
239
241
242
242
243
A primer is a liquid coating applied to a web surface to promote
the adhesion of another liquid (or molten resin) that is to be
applied onto the primed surface. The general notion of “adhesion
primer” or “anchor coat” refers to coatings applied to a variety of
substrates for which subsequent converting operations (e.g., printing, adhesion lamination) may lack equipment or materials
necessary to achieve satisfactory bonding. “Primed foil” is an
example in which a foil rolling operation will apply a thin shellac
or vinyl coating to the foil to provide anchorage for inks. Many
coated films employ a prime coat to enhance the functionality of
the primary functional coating (e.g., to provide higher adhesion—
and in turn higher heat seals strength—of a heat seal coating to a
base film than otherwise possible). Converters may use primers
for such purposes with appropriate equipment, but the major use
of the term for converting refers to adhesion promoting material
for an extrusion coating. In this sense, polyurethane primers
(essentially adhesives) are used as extrusion primers, but simpler
systems are available.
Polyethylene Imide Primers [1]
A primer acts as a surface modifier to:
1. increase the surface energy of the substrate;
2. facilitate “wet out” of a subsequent coating on the
substrate;
Manufacturing Flexible Packaging. DOI: http://dx.doi.org/10.1016/B978-0-323-26436-5.00029-1
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MANUFACTURING FLEXIBLE PACKAGING
3. add chemically reactive sites to the surface (polarity);
4. clean and remove contaminants.
In providing these functions, the primer must differ from both
substrate and subsequent coating but at the same time share some
characteristics in common with them. Adhesion promotion from
primers depends on chemical attractions. From strongest to weakest, these are ionic bonds, hydrogen bonds, and Van der Waal
(see Chapter 25).
Figure 29.1 suggests a generic structure of the polyethylene
imide (PEI) molecule.1 The nitrogen hydrogen bonds represent
the source of chemical activity for hydrogen bonds. The hydrogen
atoms on “primary nitrogen” locations in the molecule are most
available, those on “secondary nitrogen” sites less so. While
“tertiary nitrogen” atoms have no bonds to hydrogen, they do
possess some polarity as a result of differential electron attraction
in carbon bonds. The tertiary sites are physically inaccessible for
bonding with other surfaces.
The PEI molecule readily dissolves in water (as a consequence of hydrogen bonding). If the solution is acidic (low pH),
the surplus of hydrogen ions tends to turn the PEI molecules
inwards where they can find their lowest energy state. High pH
solutions cause the molecules to spread out in response to the
NH2
H
N
N
H
N
NH2
H
N
NH2
N
H
N
H
N
N
NH
NH
N
N
H
N
N
H
N
NH2
Figure 29.1 Example PEI molecule; primary amine R1NH2;
secondary amine R1R2NH; tertiary amine R1R2R3N.
1
For those not familiar with organic chemistry, in Figure 29.1, the intersections of short
lines indicate the presence of a carbon atom. Such carbon carbon bonds are strong and do
not tend to generate adhesive forces with adjacent surfaces.
29: PRIMERS
241
forces attracting its hydrogen atoms. The same molecular forces
attract the PEI to oxidized surfaces (e.g., those treated with
ozone or corona discharge).
Applying a PEI primer to a substrate during extrusion laminating or coating represents applying a layer of molecules to a
surface to which it adheres and then applying the molten polymer material on those molecules.2 This accomplishes all four
of the surface modification effects listed above. The internal
cohesion of PEI itself is minimal, so a very dilute layer of the
primer is best.
Ethylene Acrylic Acid Copolymer Primers
Figure 29.2 suggests a generic structure of the ethylene
acrylic acid (EAA) molecule. The hydrogen atom from the acid
(“-COOH”) functionality represents the source of chemical activity for what in this case are ionic bonds.
The EAA molecule readily dissolves in water (also as a consequence of hydrogen bonding). With a high pH (basic) solution, acids’ hydrogen ions tend to leave the copolymer, but
with drying, water and the basic chemistry (e.g., ammonia)
restore the hydrogen to the acid part of the polymer. This ionic
acid functionality can adhere to polar portions of other
EAA polymer in high pH solution
R1–CH2–CH2–CH2–CH–CH2–CH2–CH2–R2
||
COO–H+
EAA polymer after drying
R1–CH2–CH2–CH2–CH–CH2–CH2–CH2–R2
||
COOH
Figure 29.2 Example EAA primer molecule.
2
Chapter 5 highlighted the importance of oxidizing both substrate surfaces (e.g., with
corona treatment) and extrudate curtains (e.g., with time in air gap). This chemical activity
interacts with hydrogen atoms in the PEI primer to increase adhesive strength.
MANUFACTURING FLEXIBLE PACKAGING
242
materials (e.g., paper, nylons, and metals) and to acid groups
from elsewhere in the EAA. Unlike the PEI, EAA has good
cohesive strength.
Other Primers
Urethane formulations, similar to those used for adhesive
laminating may also be used as extrusion primers. They require
higher coating weights than PEI and EAA grades, though less
than when adhesive laminating. At these low coat weights, loss
of cross-linking ability (i.e., capped by environmental moisture)
presents particular concerns.
Primer Selection
Table 29.1 summarizes implications for using these extrusion
primers. In all cases, an oxidized melt curtain (e.g., time-in-air-gap,
melt temperature, air gap, and ozone treatment) promotes adhesion at the melt-primer interface. The chemistry of the substrate
drives the choice of primer.
Table 29.1 Selection of Extrusion Primers
Consideration
EAA
PEI
Good adhesion
• Metals with high
•
•
•
•
Target coat weight
(dry ppr)
Temperature of dry
web: F ( C)
Handling
bonds to oxide layer
(Al foil)
• Paper
• Cellophane
• Nylon
0.10 0.20
140 180 (60 80)
• Avoid reactive metals
• Minimize agitation
Polyethylene
Polypropylene
Polyester
Paper
0.02 0.03
140 180
(60 80)
Avoid reactive
metals
29: PRIMERS
243
Table 29.2 Resistance to Aggressive Products (e.g., Hair
Shampoo, Hair Conditioner, Dishwashing Liquid, Spicy/Sweet
Sauces, Dry Salted/Fried Snacks, Acidic Fruit Juices, Liquid
Fabric Conditioner, Vinegar, Soy Sauce, Cooking Oil, Salt,
Sugar, Oily Gummy Snacks, Oily Meat Snacks, Cookies and
Biscuits, Coffee W/Creamer)
Primer Adhered to
PEI Primer
EAA Primer
Foil
OPET film-direct
contact with
substance
OPET film-separated
by sealant
OPP film
Not recommended
Low
High
Moderate
Moderate
Moderate
Low
Moderate
Fit-for-use product design must consider not only initial bond
strength but also potential chemical interference at the substrate
primer interface as a result of migration through polymer layers.
Some products listed in Table 29.2 may contain ingredients that
can interfere with the primer-to-substrate bonds. The ionic bonding of the EAA primer in general has more resistance than that
afforded by hydrogen bonding with the PEI primer, but in all
cases, product resistance testing is indicated. Accelerated aging at
elevated temperatures aids the assessment.
Primer application and drying uses general coating application
methods (Chapter 27). Primers are dilute and required coating
weights low. This favors simple smooth roll application methods,
but direct and indirect gravure application with an engraved
cylinder may also be used.
Reference
[1] Robert Hammond, Chemical Primers, PLACE Conference,
TAPPI, Norcross, GA, 2010, 41 pp.
30 Conditioning
Chapter Outline
Standard Conditioning
Special Conditioning
References
246
247
248
The basic purpose of all measures is to obtain information
about a “sample” and to use that information for deciding to do
(or not do) something. One measurement (a “datum” typically
presents very little information, but considered in the context
of other similar measurements (“data”), the information supports a decision. “Similar” here includes many important relationships including:
1. Is a sample representative of what the decision
will affect?
2. Does the measure use techniques and devices
common to other data?
3. Have environmental factors influenced the value
of this measure different than value of other data?
Conditioning consists of efforts to control environmental factors
that can influence the value of a measure. Temperature and humidity represent the usual factors requiring control, but they interact
with others, such as time and pressure, that must be standardized
for some measures as well. Measuring physical and chemical
changes over time (e.g., migration through films and cross-linking
reactions) in particular demand precise control of environmental
factors. Standard conditioning environments are used if representative measures of materials are sought. Standard conditioning
environments provide information about how materials—and their
measures—can change if their use environments change.
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MANUFACTURING FLEXIBLE PACKAGING
Standard Conditioning
“Standard conditioning and testing atmospheres for paper,
board, pulp handsheets, and related products” (Test Method
TAPPI/ANSI T 402 sp-13) details the standard paper industry
conditioning environment for temperature and humidity.1 It
also specifies procedures for handling these materials so that
they may reach equilibrium with the respective atmosphere.
This standard practice does not include special conditioning
and testing atmospheres, such as those that attempt to simulate
tropical or arctic environments.
The “conditioning” environment of the TAPPI Standard
requires 50% 6 2% RH and 23.0 6 1.0 C (73.4 6 1.8 F). Webs
of and containing paper at 50% RH can have different physical
properties depending on whether the sample was stabilized at
50% from higher or lower relative humidities, For this reason,
a standard “preconditioning” environment of 10 35% RH and
22 40 C (72 104 F).
ISO 291: 2008 provides conditioning environments for plastics [1]. . . the conditions specified here are same as the TAPPI
standard. Special atmospheres applicable to a particular test or
material or simulating particular climatic environments are not
presently included in this International Standard, but the 1997
version suggested 65% RH and 27 C (80.6 F) for tropical
country use. Both temperature and humidity can influence plastics’ properties. Properties of polyolefins themselves do not
vary greatly with humidity, although copolymer polyolefins,
and condensation polymers may (Chapters 23 25). As thermoplastics, ambient temperature variation can significantly
influence polymer properties.
Table 30.1 shows the dependence on relative humidity of the
equilibrium water content of Nylon 6 [2]. Conditioning of
nylon-containing samples with standard relative humidity
1
Standard conditioning and testing atmospheres for paper, board, pulp handsheets, and
related products, Test Method TAPPI/ANSI T 402 sp-13; (also ISO 187:1990-paper, board and
pulps—standard atmosphere for conditioning and testing and procedure for monitoring the
atmosphere and conditioning of samples).
30: CONDITIONING
247
Table 30.1 Influence of Relative Humidity (RH%) on
Equilibrium Water Absorption (in %, at 23 C-Air) of Nylon 6
Type
30% RH
50% RH
62% RH
100% RH
Nylon 6
1.1
2.75
3.85
9.5
conditions is critical for producing comparable physical testing
results.2
The industry around the world has standardized and 50% RH
and 23 C for its testing environments and simply refers to these
as “TAPPI Conditions.”
Special Conditioning
In particular contexts, the effect of temperature and humidity
on flexible packaging materials may be of interest themselves.
For example, the surface of films made from “soft” resins (e.g.,
high copolymer content EVA) will deform and conform to
adjacent surfaces under pressure. This can lead to interlayer
adhesion between layers of a roll of material (called “blocking”),
particularly if high temperatures soften the plastics even further.
Rather than test whole rolls of film wound with different
tensions, individual sheets of film can be stacked on one another
and compressed with known weights.3 Conditioning of the test
stack in elevated temperatures and humidities (e.g., 37 C 100 F
and 90% RH) provides insight into the effects of storage conditions on film blocking.
At the other extreme, cold temperature can embrittle plastic
films making them much less durable than at 73 F.
2
High levels of moisture in the nylon surface layer of thin flexible packaging films can
migrate quickly to the surface when ambient temperature and/or humidity change and cause
reduced slip tendencies.
3
The applied weight divided by the surface area of its “footprint” on the stack of film represents a pressure value (pounds per square inch or kilogram per square meter). This can be
related to the tension with which a roll is wound.
MANUFACTURING FLEXIBLE PACKAGING
248
Conditioning films at temperatures they can expect in their use
situations allow quantification of the effect.
Additional special conditioning environments have been
adapted to assist in measuring the performance of flexible
packaging.
• Packaging films that discharge accumulated electrostatic charge before this force can become high
enough to damage electronic products within usually required adsorption of moisture onto their
surface to be effective. To measure surface resistivity of these products, the US military requires
conditioning samples at 12 6 3% RH and
73 6 5 F for at least 48 h prior to testing in the
same environment [3].
• Films packaging moist products stored in refrigerated conditions will appear hazy in use because of
moisture from the products condenses in small droplets on the inside of the film, unless a special
“antifog” additive lowers the surface energy of the
film so much that the droplets coalesce into a uniform thin layer. To assess this effect, film is conditioned at 4 C (39 F) in low humidity before it is
exposed to cold water at this temperature.
Special conditioning environments are appropriate when a
measure attempts to model the behavior of flexible packaging
materials when used in such conditions. Standard conditions
are appropriate for general measurement of material behavior
under average conditions.
References
[1] N. Jia, V. Kagan, Tensile Properties of Semi-Crystalline
Thermoplastics: Performance Comparison under Alternative
Testing Standard, Society of Plastics Engineers, ANTEC 1998
Proceedings, 1998, 1706 1713.
30: CONDITIONING
249
[2] ISO 291:2008-Plastics: Standard Atmospheres for Conditioning
and Testing, European Standard EN ISO 291, DIN (Deutsches
Institut für Normung e.V.), Berlin, 11 pp.
[3] MIL-PRF-81705E w/AMENDMENT 1 (February 8, 2010),
Performance Specification: Barrier Materials, Flexible,
Electrostatic Discharge Protective, Heat-sealable; Naval Air
Warfare Center Aircraft Division, Lakehurst, NJ; 25pp.
31 Intrinsic Material Properties
Chapter Outline
Standards
Intrinsic Property Influences
Case Study: Intrinsic Property Influences
253
253
255
Materials used for flexible packaging frequently may find application for other purposes. For example, low density polyethylene (LDPE) in a plastic bag may also serve as the plastic
sheathing on conductive metal wires, the resin in a plastic bowl,
or the waterproof material for a protective tarp. Structures of
materials at atomic or molecular scales dictate much of their fitness for use at the macroscale of common place articles and
operations. “Materials engineering” is the study of the relationship of the microscale chemical and physical nature of materials
to these macroscale uses. Table 31.1 suggests the diversity of
this interdisciplinary engineering science.
This list of property categories applies across the full range
of the atomic table, organic and inorganic chemicals as well as
the abundance of devices in the cultural environment. A particular use of a material exploits some, but not all of its intrinsic
properties. Consider, in the LDPE applications above:
• The plastic’s electrical insulating property is critical for sheathing wires but irrelevant in other uses.
• Its water impermeability is adequate for the tarp,
but may be too permeable as a package to keep dry
products crisp.
Its stiffness at the thickness used in the bowl gives a rigid
three-dimensional container, but at the thickness of a film provides for a flexible bag.
Manufacturing Flexible Packaging. DOI: http://dx.doi.org/10.1016/B978-0-323-26436-5.00031-X
© 2015 Elsevier Inc. All rights reserved.
251
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MANUFACTURING FLEXIBLE PACKAGING
Table 31.1 Material Properties (Example)
Acoustical
Atomic
Chemical
Electrical
Environmental
Magnetic
Mechanical
Optical
Radiological
Thermal
These intrinsic properties contrast with the “Secondary
Quality Properties” addressed in Chapter 32. The latter are
derived measures typically of a fabricated article, intended to
predict the article’s fitness for use in a specific application.
Although intrinsic properties contribute to the values of the
derived measures, test protocols, for the specific application,
dictate actual numerical values. For this reason, values for
“Intrinsic Material Properties” are constant across applications,1 but those for “Secondary Quality Properties” are often
quantitatively different in different applications.2 Online databases (e.g., http://www.ides.com/) provide information about
the intrinsic properties of specific grades of material.
Understanding exactly how a change in extrinsic properties
will influence secondary quality properties requires in-depth
knowledge of and experience with both. Professional societies
1
If grades of a material used differ from one application to the next, intrinsic material properties may differ as well (e.g., in the LDPE example used, the demands of different fabrication
techniques may require different LDPE grades). This reflects the need for microscale material
differences (i.e., molecular) in order to provide macroscale compatibility for a specific
application.
2
For example, the paper industry measures “coefficient of friction (COF)” as “slide angle,”
reflecting the degrees of inclination from the horizontal required to slide a specified mass on
the paper down a plane surface. This suggests how stable a stack of bags or boxes made of the
paper will be when vibrated (e.g., during shipment). The plastic film industry measures COF as
the dimensionless ratio of a downward force (a given mass accelerated by gravity) to the perpendicular force required to start it moving (“static” COF) and keep it moving at constant
speed (“kinetic” COF) along a horizontal plane. This reflects movement on various metal surfaces of packaging materials through the intermittent motion of packaging machinery.
31: INTRINSIC MATERIAL PROPERTIES
253
in the industry regularly present conferences and symposia
with experimental studies of these influences.3
Standards
Measuring either intrinsic or secondary properties must follow
and document established test methods. Various national
(ASTM, ANSI, DIN, BSI)4 and International (ISO) organizations publish relevant standards of both types. The development
process for the standards involves “voluntary consensus”
decision-making. In this process, interested parties (e.g., from
academia, supplier, user, and public interest organizations—or
individuals) volunteer to serve on specific committees. Each
committee focuses on either specific materials (intrinsic) or commercial uses (secondary). The committee and its subcommittees
propose draft standards using established formats. Following
within-committee negotiations to reach consensus, the proposal
undergoes a prescribed round of balloting to interested parties
who do not participate in committee work. Only after resolution
of any revisions or rejections suggested during balloting does the
standard become official. Standards receive periodic (5 years or
more) review with essentially the same process originally used.
In some industries, certification of products to relevant
national or international standards (i.e., for secondary properties) becomes legally required. Only a few flexible packaging
applications (i.e., some medical and pharmaceutical ones) have
properties measured by such standards incorporated into law,
but contractual specifications frequently incorporate them.
Intrinsic Property Influences
Understanding how intrinsic properties influence the manufacture and use of flexible packaging is important, but often
not necessary. Initial familiarity with secondary properties
3
TAPPI (www.TAPPI.org) and SPE, the Society of Plastic Engineers (www.4SPE.org), provide
such valuable studies. Reports from past sessions held around the world are available online.
4
ASTM (American Society for Testing and Standards), ANSI (American National Standards
Institute), DIN (Deutsches Institut für Normung [German Institute for Standardization], BSI
(British Standards Institute), ISO (International Standards Organization).
254
MANUFACTURING FLEXIBLE PACKAGING
serves to explain performance differences among products and
how to troubleshoot, reduce variability, and substitute one
material for another. Addressing advanced challenges in such
areas benefits from applying knowledge of intrinsic material
properties. This discussion cannot speak to all of these influences, but rather provide examples. In practice, raw material
suppliers often offer technical service support to converter customers dealing with advanced challenges. Their supplier-level
perspective deals more closely with intrinsic properties of
materials because of the less differentiated nature of the product they sell before the converter adds value. The converter
organization able to internalize knowledge of intrinsic material
properties enjoys the advantages of:
• avoiding problems, variability, and excessive costs
in the initial design and production of products;
• addressing problems, variability, and excessive
costs quickly without involving others;
• communicating with supplier technical service support more quickly and effectively.
The intrinsic mechanical properties of materials used for flexible packaging greatly affect both their fitness for manufacturing
and their fitness for use. For example, those reflecting the material’s reaction to stretching or pulling are measured by a tensile
test. In this, a force is applied to the material (stress) while the
deformation (strain) experienced. Figure 31.1 depicts a simple
Stress
X
Fracture
Plastic region
Elastic region
Strain
Figure 31.1 Simple stress strain curve.
31: INTRINSIC MATERIAL PROPERTIES
255
stress strain curve with three strain regions important to plastic
materials: “elastic”: reversible elongation; “plastic”: irreversible
elongation (i.e., permanent deformation); and, “fracture”: complete material failure (Table 31.2).
Intrinsic thermal properties of plastics are also critically
important in both make and use contexts:
• Flexible plastic films are made by melting polymer
in solid form and extruding them through narrow
dies into films of desired width and thickness.
• Hermetic seals of flexible packages require heating
the surface of at least one thermoplastic material,
pressing it into intimate contact with the surface of
a compatible material, and allowing it to cool so
that the two surfaces adhere.
Atomic, chemical, and electrical properties of liquids (i.e.,
inks, adhesives, primers, and coatings) have great influence on
their behavior during converting process because the forces
that hold them together are much weaker than those experienced by solid materials. Gravity of course acts to level a liquid unconstrained by a container, but on any given surface
electrical and chemical forces can resist the leveling tendency
leaving three-dimensional drops.
Case Study: Intrinsic Property Influences
As discussed in Chapter 4, air quality programs beginning in
the late 1970s required the industry to reduce solvent emissions
from adhesive drying on laminators. One compliance option
involved 100% solids adhesives with all of the equipment
changes they involve. At the time, neither the adhesive supplier
nor converter industries appreciated all of the fit-for-use limitations that the technology presented. In particular, these
materials based on urethane chemistry are “reactive” systems.
As such, they require a “catalyst”, something to begin the
chemical reaction of the adhesive which causes them to cure.
MANUFACTURING FLEXIBLE PACKAGING
256
Table 31.2 Mechanical Properties of Materials (measurement
requires standard sample specimens and test methods)
Mechanical Properties of Materials
Property
Compressive
strength
Ductility
Description
Maximum stress before
compressive failure
Ability of a material to
deform under tensile
load (elongation)
Flexural modulus The tendency for a beam
of material to bend
Flexural strength
Stress experienced by
beam of material at its
moment of rupture
while bending
Fracture toughness Energy absorbed by unit
area before the fracture
of material
Hardness
Ability to withstand
surface indentation
(relative number)
Plasticity
Ability of a material to
undergo irreversible
deformations
Poisson’s ratio
Ratio of lateral strain to
axial strain
Shear modulus
Ratio of shear stress to
shear strain
Shear strain
Change in the angle
between two
perpendicular lines in
a plane
Shear strength
Maximum shear stress a
material can withstand
Specific modulus
Modulus per unit volume
Specific strength
Strength per unit density
Units
SI
Ft-lb
Pa
psi
%
%
Pa
psi
Pa
psi
J/m2
lbf in/in2
No
units
No
units
No
units
MPa
psi
Radian Degree
MPa
psi
m2/s2
M
ft2/s2
ft
31: INTRINSIC MATERIAL PROPERTIES
257
Table 31.2 Mechanical Properties of Materials (measurement
requires standard sample specimens and test methods)—cont’d
Mechanical Properties of Materials
Units
Property
Description
SI
Ft-lb
Specific weight
Tensile strength
Weight per unit volume
Maximum tensile stress a
material can withstand
before failure
The stress at which a
material starts to yield
Ratio of linear stress to
linear strain
Ratio of vertical force
(weight) to the
horizontal force
required to pull a
material surface against
another
Variability of sampled
surface heights from
average height
N/m3
MPa
lb/in3
psi
MPa
psi
MPa
psi
Yield strength
Young’s modulus
Coefficient of
restitution
Roughness
No
units
No
units
A “moisture cure urethane” uses moisture (liquid water) or
humidity (water vapor) to cure (Figure 31.2):
In adhesives, the isocyanate compound used is typically a diisocyanate (i.e., a generic compound “R” with two “NQCQO”
(“cyanate”) groups bonded to it).5 In this way, the “polyurethane”
suggested in the figure becomes a macromolecule with potentially
thousands of the R N C(QO) N R sequence shown repeated
(i.e., this repeating group replaces the hydrogen atom—“H”—
bonded to either side of the “R” compound).
The challenge encountered when water-cured urethanes were
first introduced to flexible packaging converting, involved
complete lack of cure at Stage II (i.e., the reaction marked “a”
5
In the interest of readers who are inexpert in the conventions of chemical notation, such
specialized representation is minimized here in favor of simpler graphic and alphabetic entries.
258
MANUFACTURING FLEXIBLE PACKAGING
Stage I
b
C O + H2O → R–NH2 + CO2 ↑ (gas)
||
||
R–N
Isocyanate
Water
Amide Carbon dioxide
Stage II
a
||
C O + R–NH2 →
||
R–N
Isocyanate
Amide
C
||
R–N–C–N–R
|
|
H
H
Polyurethane
Figure 31.2 Moisture-cured urethane reaction.
in the figure seemed to not occur). With this limited amount of
knowledge, the usual troubleshooting response involved making water more available, so that Stage I would produce sufficient RNH2.
The intrinsic chemical property of this isocyanate reaction
allows the Stage II-type reactions to occur with any chemical
with ( NH2), “amide” functionality. This is exactly the chemical nature of the fatty-acid-amides used as slip additives to
lower the surface friction for slip-modified plastic films
(Chapter 24). This intrinsic isocyanate chemical property rendered this adhesive system essentially unfit-for-use with the
commercial slip-modified films of that time. Similarly, more
advanced 100% solids adhesives will undergo the Stage I reaction with water in high humidity environments, releasing the
gaseous carbon dioxide (i.e., the reaction marked “b”). If the
layers being laminated do not readily allow this gas to permeate through one of them, it remains forming visible “bubbles”
between the layers.
32 Secondary Quality
Characteristics
Chapter Outline
Containment Integrity Characteristics
Protection/Preservation Characteristics
Transportation Integrity Characteristics
Communication Integrity Characteristics
Flexible Packaging Material Specifications
References
264
266
272
274
274
276
The development and ongoing manufacture of flexible packaging require tools to conclude that the flexible packaging material as designed and made will function as fit-for-use product
in the context of the package to be packaged. Any packaging,
flexible or rigid, functions to benefit the product packaged in it
and the product’s ultimate consumer in a number of ways. The
official textbook for the Institute of Packaging Professionals’
certification examination [1], lists these as:
1. “Contain”: Define a relevant unit of product:
Packaging defines a relevant unit of product; it
delimits a unit of sale or use or separates one component from others; it maintains the integrity of
the unit from the point of product manufacture to
its intended use consumed; it can serve as an
integral part in the product’s manufacture, for
example, the can or special pouch in which food
is heat sterilized, and then stored as a “shelfstable product” for years. Other packaging functions must be considered in the context of the
packaging’s essential role as a container: “[This]
is the most complex and perhaps the most critical.
Manufacturing Flexible Packaging. DOI: http://dx.doi.org/10.1016/B978-0-323-26436-5.00032-1
© 2015 Elsevier Inc. All rights reserved.
259
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MANUFACTURING FLEXIBLE PACKAGING
If the material fails to contain its product properly,
its intended appearance and barrier performance are
quickly compromised. Containment relies on the
physical strength of the composite material to
deliver a product securely to its intended market. It
also depends on the ability to seal flexible packages
by using heat and or pressure to weld opposing surfaces of a two-dimensional leaf of material into the
three-dimensional container (e.g., pouch, bag)” [2].
2. “Protect/Preserve”: Maintain acceptable quality of
a product until used: Packaging keeps in “good”
elements of a product and keeps out harmful ones
from the environment. The package protects and
preserves, in the case of food, essentially maintaining acceptable taste, texture, aroma, and safety
until consumed. Light can degrade foods vitamins
and flavors and fade colored fabric. Oxygen can
rust metal products and chemically change the
taste/aroma components of food. Moisture can
leave a package allowing a product to dry out or it
can enter a package making its contents soggy and
susceptible to mold growth. Contamination by
pathogens, filth, and pests through the body of a
pouch or its seals must be prevented.
3. “Transport”: Withstand abuses encountered during
storage and distribution: Packaging must protect
its products from abuses encountered in warehouses and on its way to the place of use. Sudden
acceleration/deceleration can destroy products.
Vibration causes abrasion or piercing holes in the
packaging material, which invalidates the protective properties of the packaging.
4. “Inform”: Explain details about the product it contains to a potential user or consumer: Packaging
informs a would-be user/consumer about the product. A whole range of information is needed, for
example, contents, usage instructions, ingredients,
nutritional information, and the manufacturer.
32: SECONDARY QUALITY CHARACTERISTICS
261
Converting flexible packaging material usually occurs significantly removed in distance and time from the product it will
contain, and that packaged product similarly distant from its
ultimate purpose and consumption. Secondary quality characteristics represent measures of the material during or shortly
after its manufacture intended to predict its future ability to
function successfully. They also provide the objective means of
communicating expectations (e.g., as specifications) about the
material’s suitability for its intended use, from converter to
product manufacturer to product distributer to product user.
Figure 32.1 summarizes the continuum of flexible packaging
material deploying from rollstock to its packaged product’s consumer. The range of experiences encountered by the material has
been divided into arbitrary steps to highlight how they correspond
to the four packaging functions. The duration of the steps and their
proximity in time to one another vary greatly according to specifics of the packaging’s use. For example, an integrated formfill-seal packaging machine may perform steps 1 3 in less than
1 s. Alternatively, pouches (or bags) may be premade (Step 1) following slitting, and remained packaged and stored for months at
some distance from a fill and seal operation (Steps 2 and 3).
Table 32.1 arrays the steps on its vertical axis and the functions
horizontally. These axes define cells that list secondary quality
characteristics related to the material’s ability to provide that function at that step of its commercial existence. In the column for any
one function, characteristics often appear multiple times, indicating
their continued role in providing that function over multiple steps.
In a row (one particular step), characteristics do not repeat
indicating the cumulative characteristics required at the step.
Table 32.2 lists the characteristics, the context of their importance
and the standard ASTM test method used to quantify a value for it.
1. Form 3D
pouch
2. Fill
pouch
Cases
Product
Roll stock
3. Seal
pouch
4. Pack
pouches
5. Store
cases
6. Ship
cases
7. Distribute
product
8. Consume
product
Figure 32.1 Deploying flexible package material from rollstock to
product consumer.
Table 32.1 Secondary Quality Characteristics Critical to Package Functioning Over the Steps That Take
Material from Roll Form to Product Consumption (see Table 32.2 for Context and Test Methods for
Characteristics)
Package Function
Step
Containment
Protect/preserve
1. Form 3D pouch/bag
CoF
Seal strength
Seal strength
Hot tack strength
Seal strength
SIT
CoF
Seal strength
Thickness
Seal strength
Bond strength
Thickness
Seal strength
Bond strength
Thickness
Seal strength
Thickness
Flex resistance
2. Fill pouch/bag
3. Seal pouch/bag
4. Pack pouches/bags
5. Store in cases
6. Ship cases
7. Distribute product
8. Purchase/consume product
Seal strength
Tear strength
Transportation
Communication
Scuff resistance
Scuff resistance
Moisture barrier
Oxygen barrier
Light barrier
Moisture barrier
Oxygen barrier
Light barrier
Moisture barrier
Oxygen barrier
Light barrier
Puncture resistance
Burst strength
Scuff resistance
Puncture resistance
Burst strength
Scuff resistance
Puncture resistance
Burst strength
Scuff resistance
Scuff resistance
Whiteness
32: SECONDARY QUALITY CHARACTERISTICS
263
Table 32.2 Performance Considerations and Test Methods for
Secondary Quality Characteristics
Characteristic
Bond strength
Consideration
How much force holds layers
together before separating?
Burst strength
How much compressive force
can a filled package
withstand?
CoF (coefficient How easily does the material
of friction)
slide against another surface?
Flex crack
How robust is the structure to
resistance
bending and twisting?
Hot tack
How quickly do seals cool to
strength
some minimum strength?
Light barrier
How much light energy does
the material absorb and
reflect?
Moisture barrier How much water vapor
permeates through the
material?
Oxygen barrier How much oxygen permeates
through the material?
Puncture
How much force can material
resistance
withstand before perforating?
Scuff resistance How much force can material
withstand before abrading?
Seal strength
How much force can heat seals
withstand before separating?
SIT (seal
Lowest temperature that seals
initiation
reach minimum strength?
temperature)
Tear strength
How much force can material
withstand before ripping?
Thickness
How thick is the material from
outside to inside?
Whiteness
How much visible light does the
material reflect?
ASTM Test
Method
F904-98
F2054
ASTM D1894
F392/F392M
F1921
D1746
F1249
F1927
D7192
D5264
F88/F88M
F2029
D1938
F2251
E1347
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MANUFACTURING FLEXIBLE PACKAGING
Containment Integrity Characteristics
A package must hold its product. As simple and natural as
this statement seems, its importance consumes much of the
effort dedicated to testing or verifying materials fitness for use.
Acceptable performance each of the other functions requires
preserving the integrity of the package as a container.
Coefficient of friction describes how easily the material
slides against another surface. Many surfaces are important, for
example:
• How well the material slides against itself: outside
to outside as filled packages slide into corrugated
cases and as they vibrate during transportation; and
inside to inside as edges are aligned to make seals;
• How well the material (usually the outside) slides
against metal surfaces on machines that transform
webs into three-dimensional shapes;
• How well the outside of the material grips machine
mechanisms intended to push it in a necessary
direction;
• How easily filled packages move along conveyor
surfaces.
CoF is measured as the ratio of a vertical force (weight) to
the horizontal force required to move one surface against
another. Values below 0.25 indicate that one surface slides easily against another. Test results must report the mass of the
applied vertical force, because subtle physical surface changes
(e.g., “creep”) increase with increasing weight and affect resistance to horizontal movement.
Seal strength quantifies the force required to breach a heat
seal. That amount reflects many factors inherent in the flexible
packaging material. The simplest seal, a uniform layer of thermoplastic material sealed to itself, has a strength that approximates the tensile strength of one of the layers (“Material
failure” mode Figure 32.2). A seal that separates at the original
sealing surfaces is called “Cohesive failure.” This may reflect
32: SECONDARY QUALITY CHARACTERISTICS
265
Adhesive failure
Cohesive failure
Heat sealed
Material failure
Figure 32.2 Heat seal failure mode descriptors.
inadequate temperature or pressure when the seal was made.1
Heat seals made with multilayer materials may fail by forcing
the actual sealed area to break the sealant layer adjacent to it
and delaminate the sealant from the rest of the structure, called
“adhesive failure.” These modes of failure generate diverse
stress curves as the heat seal strength is measured. Seal strength
may be reported as an initial peak, maximum, or average value
depending on the material and its seal failure mode design.
Reporting the mode of failure as suggested in Figure 32.2 provides understanding of seal integrity in additional to any single
quantitative figure.
Seal strength remains critical to the container integrity of a
package throughout its life. Seals must withstand transient
increases of internal pressure as a package is compressed during transport and storage or as gas-filled packages transitions
from low to high altitudes. Vacuum-packed products must
resist allowing air at ambient pressure moving through seals to
increase pressure inside the package to surrounding levels.
Hot tack strength measures the strength that newly made
heat seals will achieve a short period (e.g., 0.5 s) after removing a heat source. The value is especially important for integrated form-fill-seal packaging equipment in which heat seals
are stressed with forces shortly after they have been made (e.g.,
product dropping vertically onto the bottom seals of bags, or
positive air pressure inflating three-side seal pouches).
Specially instrumented test equipment is needed to model these
1
Sealant layer resin blends may be used to provide this effect at a particular seal strength,
providing a package user with an “easy open” package.
266
MANUFACTURING FLEXIBLE PACKAGING
dynamic forces that cycle rapidly on high-speed packaging
equipment.
Seal initiation temperature assesses the temperature needed
to make heat seals of a minimum strength for given seal pressures and time durations. The value provides an effective measure of comparative heat seal performance among different
materials. It fails to reflect the dynamic relationship of material
and machine that hot tack strength provides, but if the assumption of “all else being equal” applies, a lower SIT value indicates higher throughput for a particular packaging machine.
Thickness provides a general measure of a material’s
strength (burst strength, tear strength, and puncture resistance
reflect forces encountered as packages function at particular
steps on their way to eventual use.)
Bond strength reflects the packaging material’s ability to
integrate the functional contributions of its individual layers
into the fully functional package itself. Bonds affect seal
strength. The interface of sealant with its adjacent layer contributes directly to measured seal strength, but bond strengths
between other layers influence the material’s ability to absorb
whole-package impact, as modeled by burst strength tests.
Tear strength predicts how well the material with withstand
cutting and ripping forces imposed on the material from objects
inside or outside of a package. Different values (characterizing
different use conditions) result from tears for which an initial
notch is provided compared to those initiated from a clean
edge of a package. Quantifying this strength usually requires
specialized test equipment and sample sizes designed to position material in a manner that imitates a package’s use.
Measured differences in tear force between machine and cross
directions can provide package users with an method of opening a package without breaching its seals.
Protection/Preservation Characteristics
Once in its package, the quality and safety of a product
depends on the protective and keeping abilities of the
32: SECONDARY QUALITY CHARACTERISTICS
267
packaging material. Product quality involves preserving its own
fitness for use from the time of packaging to the time of consumption. Food products have obvious sensory characteristics
(e.g., smell, taste, appearance, and texture) what define their
acceptability to consumers. Nonfood products may also be susceptible to environmental factors such as oxygen and moisture
that can corrode metal parts and static electricity that can
destroy delicate electronic devices. Safety concerns for food,
pharmaceutical, and medical device products basically involve
the integrity of a physical barrier (i.e., the containment functions) separating a packaged product from environmental
pathogens.
Flex crack resistance addresses a material’s ability to maintain its unstressed barrier qualities following twisting and folding forces. These forces are greatest as the material is formed
into its three-dimensional container shape but may also occur
because of handling though transportation and storage.
A relatively arbitrary, but repeatable set of twisting and compressing forces stress material and the barrier performance (see
below) of this conditioned sample is compared to the material’s
unstressed performance. “Robust” materials experience little or
no loss of barrier functionality while more “brittle” materials
may lose most of theirs. The measure is particularly useful for
thin vacuum-deposited barrier layers on plastic films. Such
layers (e.g., metal, inorganic oxides) have tensile properties
significantly different (i.e., more brittle) from the plastic films
they coat. Those differences result in coating cracks and voids
when stressed, particularly if the adhesion of the coating to the
plastic is not sufficient.
Oxygen barrier protects chemicals in food from reactions
that generate off-odors and tastes (rancidity). The gaseous
“headspace” of packages for such products is often filled with
inert nitrogen gas that does not cause rancid reactions. The
oxygen barrier of a packaging material functions to keep out
external oxygen (21% of “air”) and maintain this 100% internal
nitrogen atmosphere.
Aluminum foil in flexible packaging material provides
essentially complete barrier to oxygen (and moisture). Virtually
N2
O2
N2
Stage II
N2
Plastic film
Stage I
N2
Plastic film
Plastic film
O2
Migrates through film body
O2
Evaporates from film surfaces
MANUFACTURING FLEXIBLE PACKAGING
Dissolves into film surfaces
268
O2
Stage III
Figure 32.3 Dynamics of oxygen permeation through a plastic film.
all plastics used for flexible packaging materials do allow passage of these gases in response to partial pressure differences
between the package’s interior (e.g., 0% oxygen) and the surrounding environment (21% oxygen). Material density, approximately 1 g/cm3 for plastics and 2.7 g/cm3 for foil explains the
difference.2 The low density of plastics reflects space available
within their matrix of polymer molecules which can hold gases.
Various chemical and electrostatic forces from the polymers
themselves determine which gases and how much of them will
take up residence (i.e., dissolve) in the polymer. Partial pressures of gases on either side of a plastic film determine the net
direction and speed of movement of the gases from a film.
Figure 32.3 suggests how plastic barrier materials might
function. In Stage I, gas molecules dissolve into the plastic.
Once inside the plastic matrix, the small gaseous molecules
vibrate and move in all random directions (i.e., “Brownian
motion”). In this “second stage,” Stage I (“solution”) and Stage III
(“evaporation”) continue. On average, more gas in high concentration on a given side of the film dissolves into the plastic
then evaporates out, and vice versa. At equilibrium, gas concentrations on either side of the film are equal and these random
2
Only foil’s pinholes allow transmission of these gases (Chapter 20). Paper, with its network
of fibers, is “porous” to gases, presenting essentially no barrier to gas transmission at all.
32: SECONDARY QUALITY CHARACTERISTICS
269
Test film
100%
O2 Out
O2
N2 and O2
Out
Meter
100%
O2 In
O2
100%
N2 In
Figure 32.4 Permeation cell for oxygen permeation.
movements result in no net change. Enhancing the barrier properties of a film usually involves introducing a coating (often one
with higher density or chemical resistance to a gas) on one side of
the film that prevents the solution or evaporation of the gases on
that side, in this way preventing equilibrium concentrations from
random motions.
Figure 32.4 presents the basic concept for measuring the barrier of a film to any gas.3 The quantification involves maintaining essentially zero partial pressure of the test gas on the side
of the film from which it evaporates by using a constant excess
flow of an inert carrier gas. In effect, the test gas evaporating
at Stage III of Figure 32.3 can never match the high pressure
on the opposite side. As the processes of Stages I and II reach
equilibrium, that rate at which the test gas evaporates into the
inert gas reaches a maximum. A sensor appropriate to the gas
(an electrode-based one for oxygen) quantifies this rate.
Ambient temperature and the partial pressure of the gas on
its high pressure side determine this value for any specific plastic. This barrier value, also called the film’s “permeability” is
reported as mass (or volume) per unit area of film per day per
3
The figure shows an oxygen barrier arrangement, but the configuration remains the same
for other gases. The gas of interest challenges the film in high concentration on the left while
an inert carrier gas sweeps any of it that evaporates from the other side. An appropriate sensor
measures the amount of test gas in the carrier gas.
270
MANUFACTURING FLEXIBLE PACKAGING
partial pressure (in atmospheres) at the test temperature.4 The
value is normalized to a standard film thickness (e.g., per mil
or 25 µm) if the film is a uniform polymer, but reported
directly with reference to the grade of film for coated and multilayer webs. US units for oxygen barrier are cubic centimeters
of oxygen (at standard temperature and pressure)/100 square
inches/day (cc/100 in2/day). Metric units are cubic centimeters
of oxygen (at standard temperature and pressure)/square meter/
day (cc/m2/day). Comparing values usually requires common
test method temperatures and partial pressures (and relative
humidity for the challenge gas if relevant). Unless a packaged
product is stored at these conditions, the reported barrier value
provided only a relative—not quantitative—indication of gas permeating from a high external partial pressure into the package.
Moisture barrier maintains the texture of food within a package: crisp foods kept crisp and moist food kept moist. Nonfood
items may also require a dry package environment to prevent
corrosion of critical components. Complex food chemistry
greatly influences the rate at which a food picks up or loses
moisture. Even with this influence, the partial pressure of water
vapor in a gas-filled space (i.e., the relative humidity) provides
the driving force for water vapor movement in one direction or
another through a film.
A test cell similar to Figure 32.4 also measures water vapor
(moisture) barrier. In this case, care is taken to insure that the
carrier gas is dry (by passing it through a desiccant) and the
absorption of infrared light serves as the basis of the quantitative sensor. US units for moisture barrier are grams of water
vapor/100 square inches/day (cc/100 in2/day). Metric units are
grams of water vapor/square meter/day (cc/m2/day). The same
cautions about test conditions apply to comparing moisture barrier values as were mentioned for oxygen barrier.
Light barrier for a product may reflect its inherent sensitivity
to light energy (e.g., packaging for photographic film or pigmented products) or the role played by light in chemical degradation reactions. In all cases, the flexible packaging material
4
“Permeability” properly refers only to films. “Solubility” and “diffusivity” of gasses in polymers represent the underlying “primary material features” that are independent of film formats.
32: SECONDARY QUALITY CHARACTERISTICS
271
will (1) reflect, (2) absorb, or (3) transmit light energy. The relative amounts of incoming light energy that experience some
or all of these outcomes depends in large degree on the wavelengths of light energy. Understanding what effects from light
energy the package must control determines design of the
material and the tests appropriate to measure its effectiveness.
In the example, photographic film is sensitive to any visible
light, so precluding transmission of light of any sort is critical.
Only ultraviolet light energy may fade pigments in textiles and
other colored products, so less energetic, visible light, transmission may be appropriate. Food systems are often more complex. Light can act as a catalyst or initiator for some pathways
of food chemistry degradation if other chemicals (e.g., oxygen)
needed for the degradation reactions are present. In the absence
of the necessary chemicals, light energy may present no threat.
Measuring light barrier involves comparison of the amount
of transmitted light energy to the original (“incident”) amount.
The comparison is often expressed as the log10 of the ratio,
called “optical density” (“OD”):
LT
OD 5 2 log VV
L0
Here LT and L0 represent transmitted light and incident light,
respectively. For example, film with a vacuum-deposited aluminum layer that allows 1% (0.01 or 1022) of visible light to
pass though it has an OD of 2.
Sensitive instrumentation and controlled test conditions are
necessary to obtain precise measurement of “light” barrier. The
light source and detectors must of course represent the light
energy relevant to the product’s protection.
Product shelf life is influenced by these barrier functions,
but other factors, initial product conditions, and storage/transportation conditions are often of equal or greater importance.
The three combines to determine how long a product remains
acceptable after its initial processing and packaging, i.e., its
“shelf life.” Much can be done to manage the three in order to
influence shelf life, but their interdependence is inevitable.
272
MANUFACTURING FLEXIBLE PACKAGING
Frozen storage and transportation slows chemical reactions
that oxygen and light might cause, but the low relative humidity
of these conditions requires that packaging prevents product
moisture from evaporating into the cold air resulting in “freezer
burn.” Moist air packed in the headspace of a delicate electronic
device may corrode its circuitry before any permeation of environmental water vapor. In such cases, “modifying” a package’s
internal atmosphere can standardize the product’s initial conditions and allow it to take full advantage of the packaging’s barrier properties. Placing a package inside a secondary barrier
“package” of dry inert gas reduces the partial pressure deriving
force for permeability through the primary package.
A food’s shelf life often depends on many factors. For example, consumers will reject crisp fried snacks (e.g., potato chips)
as “soggy” if their moisture content exceeds 3 4%. If off flavors and smells from oxidized (rancid) frying oils reach certain
levels, consumers reject the product even if it is still crispy. As
technology advanced to provide high moisture barrier packaging materials for such products, packaging practice for them
required replacing air (21% oxygen) in the packages’ headspace atmospheres with 100% nitrogen and improving the
oxygen barrier functionality of the packaging material.
Transportation Integrity Characteristics
The functions discussed previously address containing a
product and protecting and preserving it until its consumption.
Between product packaging and product consumption, the
filled packages often must be stored over considerable times
and transported over considerable distances. During these intervals, the package itself must withstand both random and
predictable abuses encountered in a distribution system.5 The
cases in which filled flexible packages are themselves packed
5
Standard “shock and vibration” testing for such abuses has been developed in the packaging industry. Such tests typically address the integrity of transport packaging (filled corrugated
cases as well as stacks of such cases secured on shipping pallets) when subjected to forces
when dropped or vibrated. Damage to the primary packages contained in these aggregates
often serves to rank the acceptability of the transport packages.
32: SECONDARY QUALITY CHARACTERISTICS
273
provide some protection to the package material, but shipping
and handling forces can test the integrity of the primary package, and render functionality discussed above ineffective.
Puncture resistance protects packaging material from sharp
objects inside and outside of a package. Sudden deceleration,
as when a dropped package hits a floor, can press a pointed
object into the wall of a flexible package. Rhythmic vibration
during ground transportation (rail of highway) represents
another potential source of puncture abuse. Test methods for
both high- and slow-rate puncture resistance are available. The
shape of the puncture force can be selected to match objects
encountered in distribution.
Puncture itself results from a combination of many primary
material features including, tensile strength, elongation, and
modulus. Strong interlaminar bonds integrate various strengths
of individual component layers in ways that can overcome limitations of any one layer. Foil, for example, has very low puncture resistance but lamination with high elongation sealants and
high tensile strength-oriented films protects foil from otherwise
ruinous forces.
Burst strength is also tested by sudden deceleration of a
dropped flexible package. The shock of impact compresses the
volume of the package and increases internal pressure at all
points in the package.6 Steady downward pressure, as from
weight stacked on packages, may also increase internal pressure. Compressive strength of corrugated cases should be specified to avoid such an effect. Internal package pressure
increases if external atmospheric pressure decreases, as is the
case when packages packed at low altitudes are moved to or
shipped at high altitudes.
Quantifying burst strength involves compressing the package
between horizontal plates (1) with increasing force until complete package failure results or (2) at constant pressure with no
change in vertical displacement observed for a minimal time
period. Alternatively, a package can be submersed in a tank of
6
Vacuum packaging eliminates any package headspace subject to these volume changes.
This method is preferred if rough handling is likely (e.g., for military rations), but is not always
possible if a product cannot withstand the compressive forces of the vacuum itself.
274
MANUFACTURING FLEXIBLE PACKAGING
water and a vacuum pulled in the tank. This increases internal
pressure in the package and allows observations similar to
compression forces reactions.
Communication Integrity Characteristics
A package must often communicate to a potential or actual
consumer many details about the product. Printed messages
communicate most of this information, but some details may
be inherent in a package’s color or shape.
Scuff resistance is particularly important for surface-printed
flexible packages, but scuffing the surface of packaging in
which printing is buried under a transparent surface film may
also hamper an individual’s ability to see what the printing
intends to communicate. Both surface friction and the relative
hardness of a material influence its resistance to scuffing.
Measuring scuff resistance involves abusing the surface of a
sample with a particular substance under indicated pressure for
an indicated number of cycles. A special test device allows
repeatable abuse exposure with a simple pass/fail criterion for
message legibility.
Whiteness of a surface behind the transparent pigments used
for package printing inks determines the color value of the
printed image perceived by an observer. The whiteness may
result from bleached paper, a printed layer of white ink or a
white pigmented layer of plastic. All will have different influence on the perception of colors they reflect. Yellowing effects
from aging or environmental exposure by some plastics and
coatings (for paper or films) will also influence color perception. Standard values for the white layer under printing provide
the foundation for color matching.
Flexible Packaging Material Specifications
Secondary quality characteristics provide a common language for flexible packaging material suppliers and users to
32: SECONDARY QUALITY CHARACTERISTICS
275
Table 32.3 ASTM F99 Specification Guide for Flexible Barrier
Material
Physical Properties
Thickness
Yield
Seal strength
Barrier
Application Requirements
Sterilization compatibility
Toxicity
Application specific requirements
Appearance
Cleanliness and particulates
Aesthetics
Functional interference
Rollstock dimensions
Width
Core inside diameter
Roll outside diameter
Splices: maximum number
Splices: color and type
Telescoping
Roll edge profile
Printing requirements
Surface/reverse
Repeat specification
Chemical resistance
Color variance
Legibility
Placement
Packaging/marking
Identification
Shipping protection/compatibility
Other
Quality system
Traceability
Change management
276
MANUFACTURING FLEXIBLE PACKAGING
manage expectations about the materials. They provide the
basis for “specifications” to ensure the purchaser receives
exactly what is ordered based on what has been found to
function properly for his product. The requirements document
both initial material qualification descriptors, and routine production and receipt requirements. The statements may take the
form of minimum and maximum ranges or targets with
acceptable variations. Verification can include any combination
of in-process testing or certificates of analysis by the producer
and statistical sampling of incoming shipments or spot testing
by the user. The objective is to minimize variation and accompanying waste while packaging product.
Even with the diversity of materials, there are still basic
requirements that all flexible barrier materials should exhibit.
ASTM F99 Standard Guide for Writing a Specification for
Flexible Barrier Rollstock Materials defines requirements and
considerations for flexible barrier materials. It addresses some
critical printing requirements for flexible barrier materials and
provides guidance on specification requirements and considerations for flexible barrier materials intended to be purchased as
rollstock. Table 32.3 lists the specification components suggested by the ASTM Guide. The guide cautions that values
required by a specification have relevant test methods associated with them. In particular, “Application specific requirements” may reference an additional body of methods and
specifications associated with the materials’ uses. Military and
food contact packaging requirements are very common for
flexible packaging.
References
[1] W. Soroka, Fundamentals of Packaging Technology, fourth ed.,
Institute of Packaging Professionals, Naperville, IL, 2009, 623pp.
[2] T.J. Dunn, Multilayer Flexible Packaging, third ed., The Wiley
Encyclopedia of Packaging Technology, Hoboken, NJ, 2009,
pp. 799 806.
Index
Note: Page numbers followed by “f” and “t” refer to figures and
tables, respectively.
A
Acid copolymers, 203
Acrylates. See Acrylic esters
Acrylic chemistry, 237
Acrylic esters, 236
Acrylic polymer, 237
Acrylic-based adhesives,
236 237
Actual operating time, 140 141
Adaptor technology, 169 170
Additives, 200 201, 219
chill-roll-release, 169
Adhesion primer, 239
Adhesive(s), 40 41
acrylic-based, 236 237
energy-cured, 237 238
failure, 43 44, 233, 264 265
lamination, 39
coating processes, 44
innovation, 45 47
process, 41 42
strength, 43 44
laminators, 123
dry bond laminators,
123 124
online coating
measurement, 125 126
solventless laminators, 125
polyurethane, 234 236
Air gap, 52 53
Alkanes, 187
Alkene, 187 188, 199f
Alpha olefins, 199, 199f
Alpha-olefin comonomers,
199 200
Alpha-olefin copolymers,
204 205
Aluminum alloys, 161
Aluminum foil, 161, 267 268
Amphiphiles, 200 201
Anchor coat, 239
Anilox cylinder, 34 35
Annealing, 162, 163f
ASTM F99 Specification Guide
for Flexible Barrier
Material, 263t, 276
Attributes, 153
Automatic die bolt adjustment,
131
Automatic ink cleaning systems,
35 36
Automatic roll indexing,
145 146
Availability, 73 74, 81, 137 138
B
Barrier kinetics, 207 214
Barrier layer, 39
Barrier resins, 207
barrier kinetics, 207 214
coextruded flexible films,
216 217
277
278
Barrier resins (Continued)
ethylene vinyl alcohol,
215 216
nylon, 216
OTR performance for, 208f
polyvinylidene chloride
(PVDC), 214
WVTR performance for, 208f
Basis weight, 9 11, 10t,
155 156
Benefits of manufacturing
flexible packaging, 87
Beta gauge, 130
Beta systems, 131
Beta transmission, 126
Beta-ray, 130
Between-color (BC) dryers,
117, 121
Bleeding edge, 61 62
Blow up ratio (BUR), 170
Blown film, 167, 171f, 173 175
Bond strength, 266
Bulk polyolefin resins
density ranges of, 193f
forms used, 195 196
functional description,
189 194
intrinsic material
characteristics, 190 194
polymer structure, 187 189
value provided, 194 195
Burst strength, 273
Business plans, 87 88
n-butene, 199
C
Calibration, 139 140
Cast (tenter), 180 182
Cast films, 169 170, 172 173,
174t
INDEX
Catalysts, 187 188
Cellophane, 159, 177
Center surface winding, 4, 66
Center winding, 4, 65
Central impression (CI) flexo
press, 71, 117
Chemical Abstracts System
(CAS), 221
“Chemical etching” process,
14 15
Chemical vapor deposition
(CVD), 231 232
Chill drum, 129
“CIE 1976 color space”, 29 30
Clamshell dryer, 111, 129
Clay-coated paper, 158
Cleanup time, 94 95, 98
“Closed-loop” control system,
106 108
Coated paper, 158 159
Coating integrity, 228 229
Coating processes, 44
Coating weight, 42, 50 51
Coefficient of friction, 149, 264
Coextruded film, 171 172,
179 180
Coextruded flexible films,
216 217
Coextrusion, 49 50, 216 217
Cohesive failure, 43 44,
264 265
Collapsing frame, 170 171
Color register, 25
Communication integrity
characteristics, 274
scuff resistance, 274
whiteness, 274
Comonomers, 188, 198, 198f
Conditioning, 245
antifog, 248
INDEX
electrostatic charge, 248
humidity sensitivity, 246 247
measurement, 245, 248
sample, 245
special conditioning, 247 248
standard conditioning,
246 247
temperature sensitivity,
246 247
Contact angle, 234f
Containment integrity
characteristics, 264 266
bond strength, 266
coefficient of friction, 264
hot tack strength, 265 266
seal initiation temperature, 266
seal strength, 264 265
tear strength, 266
thickness, 266
Continuous tone images, 19
Contract converting, 88
Control systems, 82, 103
data inputs, 105 106
distributed control systems
(DCSs), 103 105
process feedback, 106 110
closed-loop control system,
107 108
open-loop control system,
106 107
PID controls, 108 110
shop-floor, 140 141
Converting operation, scheduled
production time in, 93 94
running time, 94
setup and cleanup time,
94 95
Copolymers, 54, 197, 201 202
acid, 203
α-olefin, 204 205
279
Cost accounting, 93 101
direct and indirect costs, 95 96
local facility and general
company costs, 95
minimum order size, 98 101
Covalent bonding, 52
Covalent bonds, 52
Cross-direction, 1, 156 157
Cross-web variation, 5 8
Crystalline structures, 172 173
Curing, 40 41, 233
ink, 222 223
D
Dedicated slitter capability, 135
Depreciation schedules, 96 97
Designed experiment, 151
Die gap adjustment, 131
Diffusion coefficient, 210 211
Digital devices, 115
Diisocyanate, 234, 235t
Direct costs, 96 97
Distributed control systems
(DCSs), 103 106
Distributed Control Systems, 127
Doctor blade, 21f, 22
Double bonds, 187 188
Down web variation, 7 8
Draw down ratio (DDR), 51,
173 175
Dry bond laminating, 40 41
Dry bond laminators, 123 124
Drying technology, 121 122
E
Effective feet per minute (efm),
94 95, 94f, 145
Effectiveness, defined, 80
Efficiency, 88 93
defined, 80
280
Efficiency (Continued)
material waste, 90 92
time waste, 92 93
Elastic plastic materials,
254 255
Electron beam, 237
Electron beam energy, 223
Electronic drives, 74, 115
Electrostatic assist (ESA), 114
Emerson, Harrington, 77 78, 78t
Energy-cured adhesives, 237 238
Energy-cured inks, 223
Enterprise resource planning
(ERP), 77 78, 103 104
Equipment automation, 144
Ethylene acrylic acid (EAA),
198 199, 203, 241 242
copolymer primers, 241 242
Ethylene methyl acrylate,
202 203
Ethylene propylene copolymers,
197
Ethylene vinyl acetate (EVA),
202, 215
Ethylene vinyl alcohol (EVOH),
200 201, 207
Extended gamut, 36
Extrusion
coating, 55 56
laminating process, 49 55
innovation, 57 59
promoting adhesion
melt curtain, 52 54
substrate, 54 55
Extrusion coating/laminating
line, 127
equipment components, 128t
gauge measurement and
control, 129 131
line configuration, 129
INDEX
Extrusion laminating, 49 55
Extrusion primers, 242t
F
Falling body technique, 113
Feedstocks, 187 188, 220
Fibers, 155 156, 184 185
Fick’s first law, 210 211
Film orientation, 178 180
Financial accounting, 95 97
Finishing and slitting equipment
components, 135t
Fit-for-use product design, 243
Fit-for-use products, 127 129
Flagging rolls, 115 116
Flex crack resistance, 267
Flexible films, 167 172
Flexible packaging layers, 50t
package functions distributed
into, 40t
Flexible packaging material
specifications, 274 276
Flexo ink metering, 32 33
Flexo process, 28 37
innovation, 35 37
Flexographic presses, 117
finger printing process, 121
press components, 117 122
drying technology,
121 122
plate cylinder pressure,
118 119
plates, mounting tape, and
plate sleeves, 120 121
Flexographic printing, 27
halftone printing, 33 35
ink metering, 32 33
numerical color space, 28 31
process, 28 37
process innovation, 35 37
INDEX
Flotation dryers, 121 122
Foil, 161, 273
annealing process, 162
“A-wettable” foil, 162
commercial trends, 164 165
converting, 163
production, 161 162
rolling process, 161
Form-fill-seal packaging,
177 178, 182
Fourdrinier-paper machine, 156
Fracture plastic materials,
254 255
Free radicals, 237
Frozen storage, 272
Functional coating, 158 159, 227
Functions of packaging, 40t
G
Gamma backscatter, 126
Gauge bands, 129 130
Gauge measurement and control,
129 131
Generally Accepted Accounting
Principles (GAAP), 95
Glass transition temperature, 178
Glassine, 157
Gloss coating, 228
“Good roll formation”, 4
Gravure press. See Rotogravure,
presses
Gravure. See Rotogravure,
printing
H
Halftone image reproduction,
16 20
Hazardous air pollutants, 114
Heat seal coating, 158 159,
177 178, 227 229
281
n-hexene, 199
High-density polyethylene, 199
Homopolymers, 197 199
Hot tack strength, 265 266
Human machine interface
(HMI), 103 104
Hydrogen bonds, 52, 215, 240
Hydrogen chloride gas, 214
I
Image monitoring, 114 116
Image screening, 16 18
Impression cutoff, 61 62
Indirect costs, 95 96
Infrared preheating, 121
Inks, 219
curing, 222 223
metering, 20 22, 32 33
pigments, 221 222
selection, 223 225
vehicles, 219 220
viscosity, 113 114
In-line processes
equipment requirements,
71 73
operational considerations,
73 74
availability, 73 74
performance, 74
quality, 74
success criteria, 74 75
In-line quality assessment, 150
Inside diameter (ID), 63
Integrated form-fill-seal
packaging, 177 178
Intrinsic material properties,
190 194, 251
intrinsic property influences,
253 255
case study, 255 258
282
Intrinsic material properties
(Continued)
materials, mechanical
properties of, 256t
standards, 253
Ionic bonding, 52
Ionomer, 203 204
ISO 291: 2008, 246
J
Job cost estimate/calculation,
buildup of, 96f
K
Kraft process, 156
L
Lamination strength, 43 44
Light barrier, 270 271
Linear low-density
polyethylenes, 199
Linear polymers, 173 175
Linear/slot extrusion die, 167
LLDPE, 204 205
Low-density polyethylene
(LDPE), 199, 251
M
“Machine direction”, 1
Machine hour rate, 96 97
“Make-ready” stock, 91
“Make-to-order” business, 137
“Makeup” solvent, 113 114
Manual devices, 115
Master web, 61
Material traceability, 116
Material waste, 90 92
Materials engineering, 251
Matte finish, 56, 184 185
Mean time to failure, 140
Measuring/controlling gauge, 130
INDEX
Mechanical linkages, 71
Melt flow index (MFI), 191
Melt flow rate (MFR), 191, 192t
Melt index (MI), 191
Melt point, 198 199
Metallized films, 164 165, 231
Metallocene catalyst, 200,
204 205
Meter mixer, 125
MFI resins, 172
Migratory additives, 200 201,
201t
Minimum order size, 98 101
Minor stoppages, decreased
speeds and, 145 146
MLLDPE, 204 205
Moisture barrier, 270
Moisture-cured urethane
reaction, 258f
“Moisture-proof”, 177
Monomers, 187 188
N
Nakajima, Seiichi, 78, 81
Newton’s laws of motion, 1 2
Nip pressure, 3 4, 7 8, 118 119
Nitrocellulose (NC) bases,
219 220
Nonmigratory additives,
200 201, 201t
Numerical color space, of
flexographic printing,
28 31
Nylon, 178, 207, 216, 246 247
Nylon 6, 246 247, 247t
O
Olefins, 187 188
chemical structure of, 188f
Online coating measurement,
125 126
INDEX
Online databases, 252
“Open-loop” control system,
106 107
Operations waste, 90 91
OPP films, 179
Oriented films, 172 173,
178 180
Oriented nylon, 164 165, 180
Oriented plastic films, 177
applications, 180 182
cast (tenter), 180 182
crystalline, 179
double bubble process, 183f
film orientation, 178 180
special oriented film effects,
183 185
tubular (bubble), 182
Outside diameter (OD), 9, 63,
103, 107
Overall Equipment Efficiency
(OEE), 77, 79 80, 135
availability, 81
calculation, 83 84
effectiveness, 138t, 144t, 150t
origins of OEE methods, 78t
performance, 81 82
quality, 83
Overhead dryer, 72, 121 122
Overprint varnish, 227 228
Oxygen barrier, 216 217, 267
of polymers and films, 208f
Oxygen permeation
dynamics of, 268f
permeation cell for, 269f
Oxygen transmission rate
(OTR), 207
P
Paper, 177, 246
coatings, 158 159
283
dimensioning, 155 156
for flexible packaging,
159 160
grades, 156 158
paper/plastic lamination, 160f
“Pattern coating” process, 44
Pattern cohesive coatings, 72
Performance rate, 81 82
Performance rate metric, 143
Permeability, 207 209,
213 214, 269 270
Permeant, 210 211
Permeation coefficient, 211,
213
Photoinitiator, 237
Photopolymer plates, 120
Physical vapor deposition
(PVD), 231 232
Pigment, 20, 219, 221, 221t
chemistry, 221 222
constitution number, 221
generic name, 221
Pinholes, 161, 231
Plasma-enhanced chemical vapor
deposition (PECVD),
231 232
Plastic, 254 255
additives, 200
extruder, 49 50
films, 200, 227, 258, 267
lamination, 160f
“Plate cylinder”, 28, 32, 117
pressure, 118 120
Plate mounting tape, 121
Plate sleeves, 120 121
Polyamides, 178, 216
Poly-chloro-tri-fluoro-ethylene
(PCTFE) film, 207
Polyethylene imide (PEI)
primers, 239 241
284
Polyethylenes, 11, 39 40, 55,
168 169, 178, 187,
198 199
Polymer chains, 173 175,
183 184, 198, 212 213,
234
Polyolefin resins, 187
chemical structure of, 188f
Polyolefins, 207, 246
Polypropylene, 52 53, 174t,
187, 195, 216 217
Polyurethane adhesives,
234 236
diisocyanate, influence of,
235t
polyol, influence of, 235t
solvent-based urethanes, 236t
solvent-free urethanes, 236t
Polyvinyl alcohol (PVA), 215
Polyvinylidene chloride (PVDC),
207, 214
Pounds per linear inch (PLI),
2, 69
Pounds per ream (ppr), 42,
155 156
Predictive maintenance, 81 82
Prepolymer, 234, 237
Prepress, 23 24, 33 34, 36, 120
Press fingerprinting, 31 33
Preventative maintenance, 81,
138 139
versus available production
time, 137
actual operating time,
140 141
availability, 137 138
calibration, 139 140
Primary additive colors, 18 19,
18t
Primary adhesive failure, 43 44
INDEX
Primed foil, 239
Primers, 239, 242
ethylene acrylic acid
copolymer primers,
241 242
hydrogen bonding, 240 241,
243
ionic bonding, 241, 243
polyethylene imide primers,
239 241
primary nitrogen sites, 240
selection, 242 243
Priming, 54 55
unit, 129
Print trolley, 111 113
Process, 44
of adhesive lamination, 41 42
feedback, 106 110
of extrusion coating, 55 56
of extrusion lamination, 49 55
of flexographic printing,
28 37
innovation
of flexographic printing,
35 37
of rotogravure printing, 22
printing, 33 35
of rotogravure printing, 14
Process printing, 33 35
Product shelf life, 238, 271
Production logs, 140
Productivity, defined, 80
Programmable logic controllers
(PLCs), 103 104
Programmed waste, 90
Proportional-integral-derivative
(PID) controls, 108 110
Protection/preservation
characteristics, 266 272
flex crack resistance, 267
INDEX
light barrier, 270 271
moisture barrier, 270
oxygen barrier, 267
product shelf life, 271
Pulp, 156
Puncture resistance, 273
Q
Quality, 27, 83, 115, 149 154
secondary characteristics, 259
Quality control, 30, 84t
Quality management, 74
system (ISO 9000), 77 78,
84t
variability concerns in, 84t
Quality rate, 83
Quick changeover, 92 93, 111
technique, 144 145
R
Raw materials, 9 11, 44,
77 78, 89 90, 98,
150 152
and market prices, 88 89
Razor blade (or burst) slitting, 65
Razor slitting, 65, 66t
Ream, 10t, 42, 155 156
Relative humidity (RH), 177,
207, 246 247, 270, 272
Renewable resources, 160
Resin blends, 198 199,
264 265
Reverse printing, 35 36
Rewind, 1, 65 69, 111,
134 135
chart, 63f
designation, 12
options, 65 69
Rework methods, 150
Roll doffs, 133 134
285
Roll length estimation, 11
Roll quality, 135
Roll rewind designation, 12
Roll telescoping, 64 65
Rolling oil, 162
Roll-to-roll quality assessment,
16, 150
Rotogravure
presses, 111
electrostatic assist, 114
image monitoring, 114 116
ink viscosity, 113 114
press components, 111 116
printing, 13
cylinder cost and cycle
time, 23 25
cylinders, 14 15
halftone image
reproduction, 16 20
ink metering, 20 22
process, 14
process innovation, 22
work practices, 24 25
Running time, 94
S
Saleable product versus product
produced, 149
overall equipment
effectiveness, 150t
quality, 149 154
Scan-a-webs, 115
Score (or crush) cut slitting, 65
Scuff resistance, 227, 274
Seal initiation temperature,
204 205, 266
Seal strength, 55, 264 266
Sealant layer, 39, 72
Secondary adhesive failure,
43 44
286
Secondary colors, 18 19, 18t
Secondary quality
characteristics, 259
communication integrity
characteristics, 274
containment integrity
characteristics, 264 266
flexible packaging material
specifications, 274 276
performance considerations
of, 263t
protection/preservation
characteristics, 266 272
to product consumption,
262t
test methods for, 263t
transportation integrity
characteristics, 272 274
properties, 149, 252
Sensor calibration, 139
Setup reduction, 92 93
Setup/cleanup, 143 145
overall equipment
effectiveness, 144t
versus scheduled production
time, 143
decreased speeds and minor
stoppages, 145 146
increased speeds, 146 147
performance, 143
Shear slitting, 65
Shop-floor control systems,
140 141
Shrinks films, 178
Side-chain branching, 172, 188,
192 194
Single bonds, 187
Single extrusion laminator, 129f
Slit roll
inside diameter (ID), 63
INDEX
outside diameter (OD), 63
requirements, 61 65
Slitters, 133
Slot die extrusion, 182
Slow solvents, 146 147
100% solids laminating, 40 41,
49 50
Solubility coefficient, 211
Solute, 210 211
Solvent-based adhesives, 123
Solventless laminators, 125
Spare parts inventory, 140
Special conditioning, 247 248
Special oriented film effects,
183 185
Specialty sealant and adhesive
resins, 197
and additives, 200 201
functional advantages, 201 205
alpha-olefin copolymers
(LLDPE and mLLDPE),
204 205
ethylene acrylic acid, 203
ethylene methyl acrylate,
202 203
ethylene vinyl acetate, 202
ionomer, 203 204
neutralization, 203
polymer structure, 197 200
alpha-olefin comonomers,
199 200
Standard conditioning, 246 247
Standard industry rewind
chart, 63f
Standard operating procedures,
91 93, 110, 143,
152 153
Statistical process control (SPC),
84t, 150 151, 153 154
feedback system, 151f
INDEX
to manufacture product,
151 152
value of, 153 154
to web processes, 153, 153t
Statistical product control, 84t
Sticky back, 28, 121
Strength, of adhesive lamination,
43 44
Subtractive colors, 18 19, 18t
Supercalendering, 157
Supply chain integration, 87 88
Surface printing, 35 36
Surface treatment, 54, 181
Surface wet-out, 53, 233
Surface winding, 4, 66
T
“Tag” or “Bag” sheet, 156
Tandem laminators, 72
Taper tension, 67, 106
TAPPI conditions, 247
TAPPI standard, 246
Taylor, Frederick W., 77 78
Tear strength, 266
Tensile properties, 195, 254 255
Tension transducer, 105f
Tenter frame, 180 182
Test methods, 253, 273
Thermoplastic properties,
189 190
Thickness, 266
Time in air gap (TIAG), 53
“Time to first quality product”,
144
Time waste, 92 93
Torque units of measure, 3 4
“Total Productive Maintenance”
methodology, 78, 81
Total quality management, 83, 84t
TPM tenkai, 78
287
Traction, 4 5
Transfer rollers, 125
Transient defects, 149 150
Transportation integrity
characteristics, 272 274
burst strength, 273
puncture resistance, 273
Tubular biaxially oriented
films, 182
Tubular film manufacturing, 170
Twin-foil rolling process, 162
U
Ultraviolet light, 222, 237
Unoriented plastic films, 167
cast, 169 170
chill-roll-release, 169
extrusion, 167
flexible films, 167 172
frost line, 170 171
general film property effects,
172 175
internal bubble cooling, 171f
modulus, 168 169
sheeting, 167 168
surface treatment, 169
tubular, 170 172
Unplanned maintenance, 140
Unwind stand, 117, 133
Urethane formulation, 242
V
Vacuum deposition, 229 232
Vacuum-deposited coatings, 231
Value-add rate (VAR), 98 99
calculation, 99 101, 100t
Van der Waals force, 213, 222
Variables, 127, 153
Vendor managed inventory
(VMI), 140 141
288
Vinyl acetate (VA), 202, 215
Viscose, 177
Viscosity, 190
Vision systems, 115 116
Voided films, 184
Volatile Organic Compounds
(VOCs), 45
Voluntary consensus standards,
253
Vulcanized rubber plates, 120
W
Waste in converting, 90
Water vapor transmission rates
(WVTRs), 207
Water-based adhesives, 45 46,
123
“Waterleaf”, 157 158
Water-quenched film, 167, 172
Web dimensional analysis, 9 12
industry units of measure, 9 11
Web length estimation, 11
Web materials, 1, 64 65
Web processes
cross-web variation, 5 8
INDEX
roll rewind designation, 12
web dimensional analysis,
9 12
industry units of measure,
9 11
web length estimation, 11
web tension, 2 3
web winding, 3 5
Web width, 64
Web winding, 3 5
Web-converting processes, 103
Wet bond laminating, 40 41
Whiteness, 274
Winding processes, comparison
of, 5t
Wire-wound (Mayer) rod, 123
X
X-ray transmission, 126
Y
“Yield” measure, 9 11, 10t
Z
Zahn cups, 113