By Aj.Ball , Siam University
083 910 4584, anan.kem@siam.edu
The use of flexography is growing
in popularity in today’s printing
markets, where short run, low-cost,
and high-quality are crucial to success.
A look at the development of
flexography reveals a printing
technology and culture that has been
willing and capable of change in
response to the needs of the
packaging industry. An evolutionary
process of improvements in materials
and equipment, rather than a single
discovery, has led to flexography as it is
known today.
The first use of soft compressible plates can
be traced back to the late 1880s, when
letterpress printers needed to find a way to
print Kraft paper grocery bags and
corrugated boxes. The materials were rough
in texture, and did not respond well to the
ink transfer pressure of hard letterpress
plates. To solve the problem, printers began
creating plates from rubber, rather than
from wood or lead. This quickly became the
dominant method for printing corrugated,
and also grew steadily in the bag printing
industry. In 1914, the Interstate Commerce
Commission approved the corrugated box
as a shipping container, a decision that
began a new growth period for flexography.
Clear Packaging
In the early-to-mid 1920’s, flexographic
printers responded quickly to the
introduction of cellophane as a clear
packaging material. White inks were
developed, drying systems were improved
for cellophane’s non-absorbent surface,
and press tension systems were refined to
handle its caliper and weight.
Approximately ten years later, polyethylene
was introduced, and went on to become
the most commonly used material for clear,
flexible packaging.
Origins of the Name
Originally, flexography was known as aniline printing, a name taken from the aniline dyes used as colorants for the ink. Aniline dyes, however,
were considered toxic and eventually banned for use on food packaging. New inks were developed as replacements, but the name remained
until 1951, when Franklin Moss, a leader in package printing, started a campaign to change it. He asked aniline printers and suppliers for
suggestions, receiving over two hundred. Of those, three finalists were selected: permatone process, rotopake process, and flexography.
After a vote, the process came to be known as flexography in 1952.
anan.kem@siam.edu, 083 910 4584
No.01
The use of flexography is growing in popularity in today’s printing markets, where short run, low-cost, and high-quality are crucial to success.
A look at the development of flexography reveals a printing technology and culture that has been willing and capable of change in response to
the needs of the packaging industry. An evolutionary process of improvements in materials and equipment, rather than a single discovery, has
led to flexography as it is known today.
Corrugated
Folding Cartons
Plastic Carrier Bags
By far the largest market for flexography,
corrugated is printed on sheet-fed presses.
Fast drying water-based inks, the soft,
conforming plate, and light impression
pressure make flexography well suited for
printing corrugated boxes in large quantities.
Although folding cartons, including cereal,
detergent, and cosmetic boxes, are printed
by both the lithographic and gravure processes,
flexography has recently increased
its market share due to improved quality.
Flexography can add advertising and
graphics to plastic bags that are carried
by customers in stores.
Envelopes
Most flexible packaging uses non-absorbent
polymer film, including bread bags, snack
food bags, candy wrappers, pouches, and
textile wrap.
Flexography is used to print many kinds
of envelopes, including those for direct
mail, sweepstakes, general mailing, and
overnight delivery. The security printing
on the inside of many envelopes is often
applied by flexography.
Gift Wrap and Wallpaper
Paper Grocery Bags
Flexible Packaging
A continuous repeat allows the printing of
products such as wallpaper and gift wrap.
Design rolls, which do not have a plate
seam, are used to print a continuous
background color.
The paper bag is the original flexographic
product. In the late 1800’s flexography
evolved from the need to apply graphics
to plain brown grocery sacks. The evolution
continues today.
Newspapers
Rigid Paper Boxes
Rigid paper boxes, or pre-formed boxes
are used for bakery products, shoes, and
neckties.
Milk Cartons
Approximately 90% of all milk cartons
are printed using flexography.
Tags and Labels
The fast drying fluid inks used by flexographers
allow inline die cutting immediately
after printing. The quality of many flexographic
labels is equal to or better than
that offered by lithography or gravure.
Pre-printed Linerboard
Pre-printing linerboard allows high-quality
graphics to be placed on corrugated containers.
In the United States alone there are
between 35 and 40 newspapers using
the flexographic printing process for the
entire paper. Many newspapers use flexography
to print the Sunday comics.
anan.kem@siam.edu, 083 910 4584
No.02
The three most widely used printing processes in use today are offset lithography, gravure, and flexography. Printing presses for each method differ
primarily in design of the image carrier or printing plate, the ink, and the ink delivery system to the printing plate.
Offset Lithography
Gravure
Flexography
Widely used in the publication industry, offset lithography presses
print magazines, catalogs, and daily newspapers, as well as annual
reports, advertising, and art reproduction. Offset lithography can
also print paper-based packaging, such as cartons, labels, and bags.
Offset lithography is a planographic process, meaning that the
printing plate holds both the image and non-image areas on one
flat surface or plane. On most offset presses, image areas on the
plate are chemically treated to attract the lithographic paste ink,
while a fountain solution or ink repellent chemical treatment
protects non-image areas from inking. From the plate, the image is
first transferred to a blanket (hence the term offset), and then to
the paper or other material, known as the substrate. To dry, most
lithographic inks require a certain period of time or an application
of heat.
The gravure method, sometimes known as roto-gravure, is used on a
wide variety of substrates, including vinyl flooring, woodgrain
desktops, and paneling. It is the second most often used process in
Europe and the Far East, and the third in the United States. Gravure is
used to print high-volume products such as packaging, magazines and
the advertising inserts found in newspapers. In addition, offset
versions of gravure presses are used to print labels or logos on
medicine capsules and the “M” on M&M® candy. Gravure is an intaglio
process, in which the image area is recessed below the level of the
non-image areas. The image is etched or engraved in a cell format onto
a copper plate or copper-plated cylinder. By varying the size and depth
of each cell, a printer using a gravure press can vary tones. Often, after
the copper is etched or engraved, the plate or cylinder is plated with
chrome to add durability and increase its run-length. A fast drying ink
fills the recessed cells, a thin metal strip called a doctor blade clears
the non-image area of ink, and the image is transferred directly to the
substrate under heavy pressure from a rubber covered impression
cylinder.
Because its soft compressible plate conforms to uneven surfaces,
flexography is often used for printing on packaging materials,
such as corrugated and paperboard. The fast-drying fluid inks
used in flexography are ideal for such materials as polyethylene
films, used for plastic grocery bags. Flexographic presses are
often part of a manufacturing process, in which packaging is
printed, folded, shaped, and die-cut. Flexography is characterized
as a relief process. The image areas on the compressible plate are
raised above the surrounding non-image areas. An ink metering
cylinder called an anilox roll applies ink to the raised areas. The
plate is then moved into light contact with the substrate to
transfer the image. The minimal pressure during image transfer
allows printing on material, such as corrugated board, that may
be adversely affected by impression pressure..
anan.kem@siam.edu, 083 910 4584
No.03
Typical
Uses
Substrates
Impression
Pressure
Plate
Run-Length
Press
Width
Repeat
Length
Press Speed
(feet per
minute)
Ink
Screen
Ruling
Minimum
Printed
Highlight Dot
Dot Gain
Offset
Lithography
Magazines,
newspapers,
advertising
pieces,
annual
reports,
cereal boxes,
bags, tags &
labels
Coated & uncoated
papers, newsprint,
some polymer
packaging films
Moderate
to heavy
300,000 to
400,000
maximum
impressions
Sheet-fed :
to 60“
Web-fed :
11" to 60"
Standard
format
with
limited
repeat
length
Magazines
: 2,500 fpm
Sheet fed
: 12,000
impressions
per hour
Paste ink
Oil & soy
based
Heat set &
non-heat set
Wet
trapping
65-300 lpi
Most
common:
133-150 lpi
3-5%
Midtones
: 20%
Gravure
Magazines &
catalogs,
Sunday
supplements
, candy
wrappers,
cereal boxes,
snack food
bags, vinyl
flooring
Coated & uncoated
papers, newsprint,
paperboard, foil,
metallized paper,
polyethylene vinyl,
polypropylene,
cellophane,
polystyrene
Heavy
6 to 7
million
impressions
average;
longer with
rechromed
cylinder
2" to 144"
Infinitely
variable
repeat
length
Magazines
: 3,000 fpm
Bread bags
: 500-900 fpm
Vinyl flooring
: 50 fpm
Fast drying
fluid ink
Solvent &
water-based
Dry trapping
120-300 lpi
Most
common:
150 lpi
3%
Midtones
: 23-26%
Flexography
Newspapers,
phone
directories,
corrugated
containers,
bread bags,
cereal boxes,
milk cartons,
gift wrap
Coated & uncoated
papers, newsprint,
paperboard,
corrugated board,
foil, metallized paper,
polyethylene vinyl,
polypropylene,
cellophane,
polystyrene
Light
1 to 2
million
impressions
average
Narrow
web
: 6" to 24“
Wide web
: 24" to 90“
corrugated
Presses
: 120"
Variable
repeat
length
Toilet tissue
: 3,000 fpm
Bread bags
: 500-900 fpm
Pressure
sensitive
Labels
: 100-300 fpm
Fast drying
fluid ink
Solvent &
water-based
UV curable
Dry trapping
45-150 lpi
Most
common:
100-133 lpi
8-12%
Midtones
: 20-25%
anan.kem@siam.edu, 083 910 4584
No.04
Flexographic printing units in use today consist of three basic types: the
two roll unit, the two roll unit with a doctor blade, and the dual doctor
ink chamber system.
The two roll unit with a doctor blade
The dual doctor ink chamber system
Two roll units are usually found on older flexographic presses, and on
narrow web presses. Narrow web presses equipped for process colors
often use the two roll unit with a doctor blade, and more modern wide
web presses use the dual doctor ink chamber system.
Each type of flexographic press uses an anilox roll. The surface of every
anilox roll is engraved with a pattern of tiny cells, so small they can only
be seen under magnification. The size and number of these cells
determine how much ink will be delivered to the image areas of the
plate, and ultimately to the substrate. An anilox roll is either copper
engraved and then chrome-plated, or ceramic coated steel with a laser
engraved cell surface.
Anilox rolls are carefully selected for specific types of printing, substrates,
and customer requirements. Often the flexographic printer will perform
test runs to determine the ideal anilox for producing the desired ink
distribution for halftones, spot color, and solids. The design of the
flexographic printing unit enables press manufacturers to build presses in
any one of three configurations: the stack press, the inline press
(including corrugated presses), and the common impression cylinder
press. Each configuration can be equipped with any of the basic printing
units, depending upon the needs of the flexographic printer.
On a two-roll flexographic printing unit, the rubber covered fountain roll rotates in a fluid ink
bath, dragging ink from the pan to cells of the anilox roll. The soft rubber fountain roll is held in
tight contact with the anilox roll. As the anilox rotates past the nip point, the fountain roll wipes
excess ink from non-cell areas. Once past the nip point, each cell is filled with ink, and a
measured, repeatable amount of ink is available to the printing plate. The metered anilox roll is
moved into light kiss contact with the image areas of the plate, and the plate cylinder is moved
into kiss contact with the substrate to transfer the image. The steel impression cylinder
supports the substrate. When a thin metal or polyethylene doctor blade is used with a two-roll
unit, the nip point between the fountain and the anilox roll is opened to allow ink to flood the
anilox and fill the cells. The doctor blade comes into contact with the anilox to clear excess ink
from non-cell areas. With a dual doctor ink chamber, the fountain roll and inking pan can be
eliminated; ink is delivered directly to the anilox through an enclosed chamber.
anan.kem@siam.edu, 083 910 4584
No.05
Characteristics of Anilox Rolls
Cell Per Inch
Range: 140 to 1200 CPI. As cell count increases, ink
delivered to plate decreases. As line screen resolution
increases, CPI should also increase.
(CPI)
Range: 1.8 to 17 BCM (Billion Cubic Microns per
square inch of cells). As CPI increases, cell volume
decreases.
Cell Angle
Typical anilox cell angles are 30°, 45°, and 60°. A 60°
angle allows for more complete ink transfer, and is the
preferred cell angle. The screen angle of the printing
plate and the cell angle can combine to cause a moiré
pattern, even with one color halftones. Moiré is
avoided by angling separation screens.
Application
line art
halftones at 65 lpi
4/c halftones at 133 lpi
Substrate
Cells Per Inch
corrugated board
200 - 280
corrugated board
360 - 400
polyethylene bags
600 - 900
Cell Volume
7 - 8.5 BCM
4.0 - 5.5 BCM
1.8 - 2.0 BCM
Common Impression Cylinder (CIC) Press
• 4-8 color units
• Limited to one-sided printing
• Ideal press for hairline register at high speeds on
stretchable films
• Longer make-ready times required because printing
units are more difficult to access
Stack Press
• 1-8 color units
• Some presses can print on both sides
• Traps should be no less than 1/64" for thin films
• Often used inline with other converting operations
such as lamination, rotary and flatbed die cutting,
and sideseal bag converting.
Inline Press
• Up to 12 color units
• Can print two sides with the aid of a turn-bar
• Used for printing thick substrates such as paperboard
• Not recommended for printing thin packaging film
• Often used inline with other converting operations such as
lamination, rotary and flatbed die cutting, and sideseal bag
converting.
Corrugated Press
Plate cylinders with different diameters can be mounted on many flexographic presses, allowing for
variable repeat lengths. Printing a roll of packaging, such as gift wrap, uses a continuous repeat, where
the same set of images is repeated many times on a continuous stream of substrate. To avoid the plate
seam, images may require nesting, an arrangement that creates a staggered effect. Staggering images
gives the appearance that the design is continuous, no matter where the substrate is cut.
• Same configuration as the inline press
• Sheet-fed; widths up to 120"
• Usually no more than 4 colors
• Limited to one-sided printing
• Less accurate registration capabilities
anan.kem@siam.edu, 083 910 4584
No.06
Printing plates used on modern flexographic presses are
produced in three different ways: molding rubber, exposing
and processing photopolymer, and imaging with lasers.
While molded rubber plates have been used since the
1930’s, photopolymer plates, introduced in the 1970’s,
generally provide higher resolution and more accurate color
registration. Direct-to-plate laser imaging, called ablating, is
available for both materials. Selection of a particular type of
plate depends on the press, the plate cylinder inventory, and
the customer’s requirements, such as resolution,
registration, and cost.
Molded Rubber
Molded rubber plates are created in a multistep process that
involves exposing and etching a magnesium plate, making a
mold, and then placing the image on the rubber plate using
a molding press.
Photopolymer
Light-sensitive photopolymer is supplied in either solid
sheets or in a thick liquid state. The image area of the plate
is exposed through a film negative. Liquid photopolymer,
about the consistency of honey, solidifies when exposed to
ultraviolet light. After exposure, the nonimage area is
removed by processing.
Laser Ablated Plates
Some platemaking machines can transfer images directly
from the computer to the plate, a process known as
directto-plate that avoids the production of film. Some
photopolymer plates can be directly imaged by lasers and
then conventionally processed. A design roll is a cylinder
covered with rubber or photopolymer and molded or
imaged by a laser. Design rolls can provide a true continuous
repeat with a continuous background color. Laser ablated
plates must be nested to hide the plate seam for the
appearance of continuous repeat
Photopolymer
• Line screening is at least 150 lpi and can
be as high as 200 lpi.
• Nesting is required for the appearance
of continuous repeat.
• Positioning and register devices on most
modern flexographic presses are designed
for one-piece photopolymer plates.
• Direct-to-plate laser imaging is available.
Molded Rubber
• Molded rubber plates shrink when they are
removed from the molding press. Plate films should
be adjusted to compensate for shrinkage, which is
typically 1.5%-2.0% in the direction of the rubber
grain, and .5%-1% across it. Exact shrinkage amounts
should be communicated between production artists
and platemakers.
• Line screening is limited to 120 lpi.
• Registration can be more difficult than with
photopolymer plates.
• Nesting is required for the appearance of
continuous repeat.
• It is difficult to mold accurate rubber plates larger
than 24" X 36". Larger designs must be placed on
multiple plates for each color.
Laser Ablated Plates
• Line screening is limited to
100 lpi for tone reproduction,
but can be 200 -300 lpi for tints.
• Film is not required.
• Design rolls can provide a true
continuous repeat with a
continuous background color.
• Plates or design rolls imaged
directly on the cylinder do not
require compensation for plate
elongation.
• Laser ablating is available for
both rubber and photopolymer.
anan.kem@siam.edu, 083 910 4584
No.07
Plate Elongation
As the soft plate wraps around the cylinder, it can
elongate, stretching images, halftones, and text across
the curve dimension. Without compensation for plate
elongation, images will not print as designed. In the
example, if no compensation is applied, the sun image
is printed as an oval and the vertical lines on each side
are lengthened.
Distortion
Special flexographic software can compensate for plate elongation by adding distortion,
using a basic formula, as shown in the following example.
When designing images for flexographic
printing, it is important to understand the
effects of plate elongation. Because
flexographic plates are made with soft
material, they tend to stretch when
mounted on the plate cylinder, sometimes
distorting images and text. A circle, for
example, may be stretched to look more
like an oval. Fortunately, special
flexographic software can compensate for
plate elongation by slightly distorting
images.
Distortion is usually performed within the
flexographic software application or at the
RIP stage before the film is imaged. The
amount of distortion depends on the
thickness of the plate and the mounting
tape used to fasten it, and on the
circumference of the cylinder (the repeat
length). In general, thicker plates and
shorter repeat lengths increase the
elongation. To be sure that images will be
printed with the correct size and shape,
the design should be output to film after
plate thickness has been determined and
the proper distortion factor has been
applied. Improperly calculated distortion
may also cause misregistration. Direct-toplate imaging, which is becoming more
widely available, avoids the need for
distortion if the imaging is applied directly
on a design roll, or on a plate already
mounted on the cylinder. Because the
image is applied to a curved surface, no
stretching occurs.
anan.kem@siam.edu, 083 910 4584
No.08
Paper/Paperboard
Polymer Films
Multilayered/ Laminations
kraft linerboard: corrugated, for boxes
coated kraft: corrugated, for boxes
solid bleached sulfate (SBS): folding cartons
recycled paperboard: folding cartons
coated paper: labels, gift wrap
uncoated freesheet paper: paperback books
polyethylene (PE): dry cleaner bags, bakery,
textile bags
polypropylene (PP): snack packages, candy
wrappers, cookie packaging
labels
polyvinyl chloride: vinyl films, labels, wall Coverings
metallized papers: gift wraps
metallized film: snack food bags
polyethylene coated solid bleached sulfate: milk
cartons
White, brown kraft, a variety of colored papers.
Clear, white or colored.
Determined by the top-most layer.
Increases with bleached & coated papers.
Decreases with greater amounts of recycled
fiber. Optical brighteners can be added.
Determined by the opacity of white film. Clear films
require the use of a white plate.
Determined by the top most layer. Foil &
metallized surfaces require the use of a white
plate.
Amount of light transmitted through the
substrate. A lower opacity allows more light
to pass through.
Thin, lightweight papers have lower opacity &
are more likely to have ink show through.
Low for thin, lightweight papers, which are more
likely to let ink show through on reverse side.
Higher with multiple layers of material.
Smoothness
Newsprint, corrugated linerboard &
paperboard are relatively rough; calendered &
coated papers smoothest.
Smooth printing surfaces; ink adhesion is sometimes a
problem.
Determined by the top-most layer.
Newsprint, corrugated linerboard &
paperboard are very absorbent, calendered;
coated papers are less absorbent & exhibit high
ink hold-out.
Non-absorbent, with no dot gain.
Usually low, but determined by the substrate used
as a printing surface.
Calendered & coated papers are high gloss;
corrugated linerboard, newsprint & paperboard
are low gloss.
High for most films, but films can be produced with a
matte finish.
Determined by the top-most layer.
Range: .002" to .010"; paperboard > .010". Thin
papers more consistent in caliper; paperboard
more inconsistent.
Ranges from .00065" to .006". Thin films may stretch;
inconsistency in caliper can cause misregistration and
wrinkling.
Increases as layers are added. Thin layers may be
laminated together to obtain the required
thickness.
Color
Dependent upon substrate material; substrate
color will significantly influence ink.
Whiteness/Brightness
The strength of white or color of a substrate.
Opacity
Smoother substrates allow higher lpi; rough,
irregular surfaces require much lower lpi.
Absorption
Determines how ink dries and spreads. Low
absorption produces drying at the surface,
increasing color saturation and decreasing
dot gain. Higher absorption increases dot gain.
Gloss
Reflective quality of the substrate. Gloss can be
increased with varnish or lamination and can be
decreased with matte or low-gloss finishes.
Caliper
Thickness of a substrate, as measured by
a micrometer.
anan.kem@siam.edu, 083 910 4584
No.09
Multicolor Options
In flexography, opaque spot colors are printed in the
order of lightest to darkest. Process color inks are made
from transparent pigments and can be applied in any
sequence. For transparent substrates, white ink is printed
first to provide a background for colors.
Reverse-Side Printing
An exception to the rule of lightest to
darkest printing occurs when a spot
color or line art job calls for reverseside printing, sometimes called “back
printing.” Styrofoam products are often
decorated by laminating a reverse-side
printed film. Some snack food
packaging is also done this way.
White Plate
In order to place colors on a transparent
substrate, a solid white ink is printed first
to create a reflective background that
improves the color intensity. The colored
inks are then printed on top of the white
background.
Because the appeal of packaging is significantly enhanced by
color, flexographic presses commonly offer six and eight colors,
and even as many as twelve for limited applications.
Designers can choose from a number of different
combinations, including multiple spot colors and HiFi printing,
which is a method of increasing the color gamut by printing six
or seven process colors. Most flexographic inks consist of
opaque or semi-opaque pigments. To ensure proper ink
coverage, the spot colors are usually printed from lightest to
darkest. Process color inks are made from transparent
pigments and can be applied in any sequence. Transparent
substrates, such as polypropylene, require a white backup
plate provided by a “choke” plate, as a background for colors;
otherwise, colors would appear flat and translucent. Another
technique used for applying ink to transparent substrates is
reverse-side printing. The image is laterally reversed, and
colors are printed instead from darkest to lightest; the
packaging is then displayed from the non-ink side of the
substrate. This provides a scratch proof surface to the ink layer,
and a glossy finish. In some cases, a water-based ink is used
for reverse-side printing lamination. Paper or styrofoam plates
are often decorated by laminating a reverse-side printed film
layer to the plate. Corrugated containers have historically been
only one color on brown kraft paper linerboard, but more
designers are taking advantage of multiple spot and four-color
process capabilities. In some cases, printing is done directly on
a white or clay coated corrugated linerboard, avoiding the
costly practice of applying labels to achieve high quality
graphics.
anan.kem@siam.edu, 083 910 4584
No.10
Choking and Spreading
Overprints and Traps
Traps are created by spreading or choking graphic elements,
depending on adjacent colors. When a lighter element appears against
a darker background, the lighter color is spread into the darker color,
slightly increasing the size of the graphic element. When a darker
element appears against a lighter background, the knockout is choked
by slightly filling it with the lighter color. Both methods create a small
overlap of ink colors, called the trap width. This width depends on the
thickness and size of type, the adjacent colors, and the registration
capabilities of the press. Script, serif fonts, or small type can be spread
or choked only small amounts before the shapes are compromised. In
general, light and dark colors allow for greater trap widths than colors
that are similar.
Printing inks on top of each other is called overprinting. In
flexography, one ink can be printed on another only after the first
has been completely dried or cured. For this reason, flexographic
presses use dry trapping, where the printer must allow for drying.
For substrates like clay-coated liner, or non-absorbent packaging
such as polymer film, drying time can be a problem. Overprinting a
second color when the first is not dry creates an unattractive
blotchy effect (often called nailheads), especially on large solids.
Excessive trapping can cause such problems and can slow down
the printing process.
Trapping is a technique of overlapping colors
to avoid unsightly gaps created by
misregistration. Small variations in the
placement of color, called misregistration,
can be caused by substrate handling and
tension problems on the press, irregular
plate elongation from one color to the next,
inaccuracies in plate mounting, plate bounce,
and limited register capabilities, especially
with molded rubber plates. A test run, called
a fingerprinting analysis, can determine the
registration tolerances. When designing
packaging for flexographic printing, it is best
to avoid the requirement for tight
registration, to design images with dominant
colors printed on top of lighter ones, and to
avoid trapping on gradations. Typically, a
designer will build traps into the file if the
design is simple, using options in publication
or illustration software; more complicated
designs require the help of service bureaus
and special trapping software programs,
such as TrapwiseTM from Luminous
Corporation or DK&A Island TrapperTM. Trap
widths on narrow web presses should be set
at a minimum of 0.005"; some presses
require as much as 1/32“ (0.031"), which is
large compared to average traps of 0.002" 0.005" for offset lithography. A typical trap
width for polyethylene printed on a wide
web press is 1/72" (0.014"), though if an
objectionable dark trap line is created, the
width may need to be cut in half. Trapping
for linerboard or corrugated cardboard may
require a trap width of 1/64" to 1/8".
anan.kem@siam.edu, 083 910 4584
No.11
Typographic Guidelines
Type Weight
Letterspacing and/or linespacing may
increase slightly from plate elongation.
In flexography, the soft plate compresses in the printing nip,
causing the fluid ink to flow slightly outward from the image
area. Thus the weight of type may appear to increase, and
reverse type may fill-in.
Avoid placing fine type on the same color plate
with line work and solid printing areas.
Avoid reversing type out of two or more colors
unless a dominant color outline is used.
In flexography, printing consistently well-defined
type is complicated by the soft plate, irregular
substrate surface, and the fluid ink. Ink tends to
spread outward, sometimes obscuring the
definition of small point sizes or the fine detail of
certain letter shapes. Reverse type, which uses the
substrate or a background color to define the
letter, tends to get filled in.
To help compensate for the typographic weight
gain, it is possible to use the trapping techniques of
spreading on positive type and choking on reverse
type. Software programs, such as MacroMedia
FreeHandTM and Adobe IllustratorTM, let the
designer adjust the thickness of type. Some
compensation can be done by choosing either a
lighter or bolder face. For example, if medium
positive type is desired, use a lighter weight face; if
a medium reverse type is desired, specify a bold
face. When possible, sans-serif fonts should be
used. In general, larger point sizes produce more
consistent type.
Letterspacing must also be considered. Letters
squeezed together for a denser appearance with
offset lithography may merge together
unacceptably in flexography. Ideal letterspacing
keeps letters close enough together so that they
lend support to each other while under the
pressure of the printing nip, but separated enough
to avoid merging.
Specify type accurately to the service
bureau or prepress department.
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No.12
Impression Pressure
Even though ink is
transferred under
relatively light pressure
in the printing nip, the
soft flexographic plate
deforms slightly and
compresses during
image transfer. This
causes ink to spread,
increasing dot gain.
Plate Durometer
An instrument called a Shore A scale measures
plate hardness, which is called durometer. The
image on the left shows the scale measuring a
soft plate; on the right it measures a harder plate.
Because harder plates do not
compress as much as softer plates,
they produce less dot gain. Softer
plates, however, transfer solid images
more completely. Dot gain can be
minimized by using a thin (0.002"0.005") capping layer surface with a
higher durometer than the supporting
plate material. Dot gain can also be
reduced by mounting the plate with
compressible tape or a blanket that
absorbs pressure.
A higher viscosity ink will not spread as quickly as
one with lower viscosity. The spreading, or flow-out,
of a low viscosity flexographic ink occurs as it is
transferred to the substrate and before it dries,
contributing to dot gain. By comparison, lithographic
ink is a thicker, paste consistency, and is not prone to
excessive flow-out.
All printing processes are subject to the
unavoidable occurrence known as dot gain.
As dots are transferred from film to plate,
they tend to grow in size during light
exposure. When an ink dot is transferred
from the plate to the substrate, it can
increase in size once again as the ink spreads
during absorption. A dot that began as 50%
on film can grow to 51% on the plate, and
eventually print on a flexographic press as a
65% or greater dot. The fluid ink and
compressible plates used in flexography
tend to increase dot gain, but it varies
according to the type of press and the
substrate. Smooth non-absorbent films and
coated papers will have less dot gain than
absorbent and irregular surfaces, such as
uncoated papers, newsprint and corrugated
liner board. Dot gain, however, is often
consistent and predictable. Image or color
separation software can adjust dots based
on measurements supplied by the printer.
Typically, the printer performs a
fingerprinting analysis, which provides dot
gain information to the color separator or
desktop designer.
The printing surface or finish of a substrate also influences dot gain. When ink is applied to smooth
non-absorbent films and coated papers it tends to spread very little, preserving the dot shape. With
more absorbent and irregular printing surfaces, such as uncoated paper, newsprint, and corrugated
liner board, the paper fibers act as a wick, absorbing the fluid ink and causing it to spread beyond
the dot shape and pattern.
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No.13
Screen Tints and Gradations
Halftone Dot Shape
Offset Lithography
To minimize density jumps in halftones,
round dots are preferred. Round dots do
not touch until coverage is nearly 65%
for the symmetrical round dots and 75%
for asymmetrical dots. Dot gain is less
at these higher coverages, and is more
easily controlled or compensated for.
Flexographic
Highlights
Most photopolymer plates are
capable of holding a 2% highlight
dot. If plates aren’t properly
exposed, however, screen tints
and highlight dots less than 3%
tend to drop out. Because the
small dots of highlight areas are
subject to relatively large dot gain,
it is important that any highlight
limitations are discussed with the
printer before separations are
made. Although printers in offset
lithography can print acceptable
highlights using 5% film dots,
flexographic printers may have to
reduce a similarly bright highlight
to 2%, taking into account the
additions of dot gain.
Gradations and Vignettes
Flexographic dot gain on highlights
makes it difficult to print a fadeto-white gradation without a
harsh break at the highlight edge.
When designing images for
flexography, it is best to fade off
the end of the design (rather than
to white), or place a border at the
highlight end of a vignette.
Throughout its history, flexography has been printing
quality line art and spot colors on a wide variety of
substrates. However, it is the recently improved
capability of high-quality, economical four-color
process printing that has given flexography an edge
over other processes for packaging applications.
The Halftone Dot
In flexography, the shape of the halftone dot used to
reproduce a continuous tone image can significantly
affect the density of the image. Halftone dots can be
generated in a number of shapes, including square,
elliptical, octagonal, and both symmetrical and
asymmetrical dots. At 50% coverage, for example,
square dots produce a pattern resembling a
checkerboard, with individual dots just beginning to
join at their corners. When plates are created from
film, dot gain increases the joining of the dots, which
causes sudden jumps in density in the printed image,
rather than a smooth, continuous transition. To
minimize the density jumps, printers can use other
kinds of dots that remain discreet and retain their
shape, even at coverages of sixty and seventy percent.
A round dot, or octagonal dot are often used. Though
most design software can specify round dots, selection
of dot shape should occur early in the process to avoid
choosing a shape not available in the RIP, imagesetter,
or platesetter.
Conventional Screen Ruling
Selection of proper screen ruling, which is critical to
four-color process flexography, is often dictated by the
type of substrate. Anilox cell count and screen ruling
for separations should be correlated for best results.
The cells of the anilox, which ink a halftone plate,
should be large enough to produce appropriate
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No.14
Line Screens and Cells Per Inch
Anilox Cell Angles and Halftone Screen Angles
color strength, but not so large that the
halftone dots will dip into them like an
inkwell. For conventional halftones,
resolution is expressed as the number
of lines of halftone dots per inch, also
called screen ruling or lpi. Higher
screen rulings produce higher
resolution images because there are
more dots per square inch used to
reproduce detail. Compare the images
in a newspaper (low screen ruling) to a
picture in a monthly magazine (high
screen ruling).
Stochastic Screening
Stochastic, or frequency modulated
(FM) screening, can offer advantages
over conventional halftone screening.
Stochastic screening eliminates the
possibility of moiré, and also allows the
flexographic printer to use HiFi color,
which involves the application of six or
seven process colors.
Dot size used for stochastic screening,
however, is extremely small,
comparable in size to the highlight dot
of conventional screening. Since
flexography is subject to significant dot
gain, stochastic screening should only
be used after the printer and color
separator have performed press
fingerprints to determine the ideal dot
size and accurate compensation for dot
gain.
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No.15
Step-and-Repeat
To save material costs and maximize productivity,
the packaging industry uses a printing technique
called step-and-repeat. Different images, such as
labels, are arranged on the plate to fill its repeat
length and use the entire width of the substrate
when printing. Often, a technique called nesting
will be required. Images are strategically
staggered in an arrangement that maximizes
the repeat length and avoids the plate seam. The
plate ends are cut to accommodate the
arrangement. Nesting can give the appearance of
a continuous repeat, and is used for jobs where all
the images are part of a single roll, such as gift
wrap.
Plate Bounce
In addition to maximizing substrate area,
staggering images can also avoid a problem called
plate bounce. In flexography, images are on the
raised areas of the plate, and can produce a
bounce when coming into jarring contact with an
anilox roll or impression cylinder. Bouncing can
cause print areas to skip or misprint if the bounce
is away from the impression, or areas of excess
inking if the movement is toward the anilox roll.
Nesting images can maximize the plate area and avoid the plate seam
for step-and-repeat printing. Layout and imposition software offer
step-andrepeat options for specific repeat lengths and web widths.
Die-Cutting
Staggering images on the plate can help keep continuous contact
between cylinders, minimizing plate bounce. Sometimes it is
necessary to place nonprinting bearer bars on non-image areas to
maintain contact.
Bleeds To specify bleeds, the designer
must know where the packaging will be
cut, folded, and joined. In general, bleeds
extend beyond fold and cut lines, but the
precise amount of bleed depends on the
press.
Cut Areas When flexographic printers are
connected to inline flatbed or rotary
diecutting, the die must be held in register
with the printed colors. Graphic elements
should not be placed too close to cut areas.
Glue & Seal Areas To ensure sealing, glue
areas should be free of ink and varnish,
especially those sealed by heat.
Score Lines Die-cut folding cartons usually
fold at score lines, where the designer
should make sure that registration is precise.
Varnish-Free Areas Areas that contain
variable information, such as freshness
dates and product coding, must be free of
varnish.
Windows Die-cut windows for folding cartons
or labels should be clearly indicated,
but may not be available on all die-cutting
machines. Always check with the printer
before including them in the design.
Bar Coding To help keep bar codes precise
for lasers, they should be printed
parallel to the direction of the web, and
must allow for dot gain.
Die-Cut Templates Templates can be
exported from CAD systems to illustration
programs, providing the designer with a
two dimensional layout of the job.
After printing, most substrates are cut, folded,
scored, sealed, or glued, often inline. In order for
packaging to be properly constructed, die-cutting
requirements must be exactly specified for both
the designer and the printer. A die-cut label or
folding carton, for example, must have all
graphical elements in the correct positions.
Packaging engineers often use Computer Aided
Design (CAD) systems to design folding cartons,
corrugated containers, or rigid paper boxes;
designers may also import the CAD layout to use
as template for design.
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No.16
Flexographic Imagesetting Requirements
Following design, production, and ripping, a job may take different paths through the prepress output and platemaking
stages. For conventional photopolymer or rubber plates, jobs are first processed through an imagesetter, creating film
output, which is used for plate exposure and processing. Plates are then placed on a mounting machine where, in some
cases, a plate proof is made. A platesetter device essentially follows the same flow with the exception that the film
processing step is eliminated. With direct-to-plate (or cylinder), devices, shown below as laser ablation, both platemaking
and processing steps are eliminated.
Design/Layout
Station
Production/RIP
Station
Laser Ablation
Imagesetter
Film
Processor
Platesetter
Proofer
Platemaker
Platemounter
After a design has been trapped,
distorted to compensate for plate
elongation, and electronically
imposed (considering step-andrepeat requirements), the file is
processed by the raster image
processor (RIP) and output to film
or plate. The RIP converts
PostScriptTM data into a series of
bitmapped images. The laser output
device records this visual
information received from the RIP
onto film or plate material. Larger,
more complex files will take longer
to process through the RIP than
simple text files.
Accuracy To optimize registration, output devices
should meet a minimum standard of 1 mil over
multiple separations. Geometric and absolute
accuracy capabilities are also important
imagesetters considerations.
Size The imagesetter format size should be large
enough to make the most effective and economic
use of film, given the particular application.
Film All film for soft photopolymer plates should
be output to matte emulsion film (minimum
thickness: 0.004"; 0.007" is preferred). This helps
avoid trapping air between the film and plate
during exposure.
Calibration Film dot percentages below 10%
should not vary by more than 1%; areas over 10%
should not vary by more than 3%.
Uniformity Screen tints should be a uniform dot
percentage, with no variation in size between
individual dots.
Dot Shape The imagesetter should be capable of
outputting a “hard” round dot.
Resolution Resolution should be between 1200
and 3600 dpi. For line art, solids and type, 1200
dpi is adequate; halftones require a minimum of
2400 dpi.
Density Film density is an important factor;
imagesetter exposure levels and film processing
chemistry should provide DMax areas of 3.5-4.0.
Plate Processor
Platemounter
Flexographic Press
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No.17
Before a flexographic job is printed, a number of factors need to be checked to ensure accuracy. Makeready on a flexographic press includes installation of
the required plate cylinders and inks, setting levels for impression and ink pressure, register adjustment, and any setup for inline finishing, such as cutting,
folding, or gluing. Pressure levels are particularly critical for accurate printing. Finally, after press adjustments are complete, the press operator checks
colors for the job, running a sample at (or near) production speeds. Designers should also make sure to perform a press check for the first actual print run.
Type
Is the type sharp? Has the weight of the type changed?
With a loupe, look for outline halos.
Substrate
Is the width, caliper, and type of substrate as
specified? If the substrate is an opaque film, is the
opacity appropriate?
Registration
Are the colors in register? Make sure colors line up and
check areas where inconsistent elongation may have
caused misregistration. For a four-color process with
traditional screening, color-to-color register should not
vary by more than a single row of halftone dots.
External Register
Does the print line-up with the specified finishing
operation, such as diecuts, sideseals, slots, scores, and
glue areas? Request a mock-up container to check
accuracy of external register.
Density/Color Intensity
Is the density of the color appropriate, especially in
situations where screens and solids are printing from
the same cylinder? Is the color strength consistent
from side to side?
Color Match
Do spot colors match? Each color should be verified
under 5000° Kelvin lighting conditions. When
appropriate, use color measurement instruments to
verify acceptable match.
Ink Laydown
Is the ink laydown consistent, without mottle? Are
there pinholes or voids? Under a loupe, halftone dots
should appear sharp, not slurred.
Ink and Impression Levels
Look at the edges of solids for an outline halo, which
is the result of excess ink or impression pressure. The
press operator may be able to relieve some pressure;
some plates may have spot color inaccuracies that
require a plate remake or remount.
Dot Gain
Dot gain levels should be comparable to those
achieved during fingerprint trial.
UPC and Bar Coding
Over-impression can change the width of bars and
spaces, potentially making the bar code unreadable.
Wind Direction
Has the job been installed in the right direction on
press? If the press is running roll-to-roll, is the print
rewind in the proper direction?
These are some of the elements that
should be verified prior to printing.
Type
Is the point size and font correct? Is the
typography what was expected? Has
plate elongation affected leading,
letterspacing, or word spacing?
Line Art
Have all of the images elongated and
reproduced accurately?
Register
Internal (color to color) and external
(images to die-cut, sealing areas,
perforations, etc.) If all colors have
been accurately distorted, and the
plates accurately mounted, the job
should be in register.
Traps
Have plate mounting and plate
elongation maintained accurate trap
areas?
Bar Coding
Have the bar codes been positioned to
allow proper open area surrounding the
code for scanning? Has the bar code
been positioned with the bars parallel
or perpendicular to the web direction?
When mounted perpendicular to the
web direction, bar codes will be
affected by plate elongation.
Wind Direction
Has the job been set-up to print in the
right direction on press? Will the print
direction match the packaging or
labeling operation?
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No.18