EBB 220/3
POLYMER PROCESSING
METHODS
Introduction

The beauty of polymeric compounds  it can be
shape to the desirable shape with the desirable
characteristics.

There are three things that involved in all basic
processing methods:
1.
Mixing of the compound ingredient to get the
physical forms that can be shape.
2.
Changes the compound shape to the desirable
shape and size.
1.
To ensure the products retained their shape and
dimension after shaping process.
Types of Mixing Process
Based on 2 basic mixing functions;
 Blending
 Compounding

•
•
Blending mixing is used when the fabrication process will
be followed by compounding process (pigments must be
mix into granules/powder followed by injection molding
process), thermosetting powders and fillers are often
blends which disperse upon fusion of the resin during
molding
Compounding mixing is used when accurate distribution
& dispersion of ingredients is required (e.g. in rubber
compounding, 4-5 additives have to act together for
efficient cross-linking of the rubber)
Blending
Stirring together/blending of a number of
solids, e.g.polypropylene powder, pigment,
antioxidant, etc.
 The results is a mixture of powders; the
individual powder remain and can be
separated (in principle)

Compounding
Involves more intimate dispersion of the
additives into the polymeric matrix
 It requires;

 A physical
change in the component
 High shear force to bring about the change
 The polymer to be in the molted or rubbery
state during mixing
Some Processes and Machine
(Blending)
Vary from the simplest to sophisticated
high speed machine
 The simplest- is to tumble together dry
ingredients, e.g. using a twin-drum tumbler

Twin-drum tumbler
Some Processes and Machine
(Blending)

High speed mixer




More sophisticated & rapid machine for blending
Widely used for PVC dry blends, drying, incorporated pigments,
antioxidant, etc.
Run at several thousand rpm, and form a circulating powders which
becomes heated by friction (150-200ºC)
Mixing tank can be single wall or jacketted for temp. control
Mixing tank
High Speed Mixer
Some Processes and Machine
(Blending)

Ball Mill
 Comprises
of cylindrical vessel containing large
number of steel or ceramic balls
 It rotates, the balls tumble inside together with the
powder
 Agglomerates of powder are broken down by the
grinding action of the tumbling balls
Ball Mill
Some Processes and Machine
(Compounding)
Involve high shear process & much more
powerful machinery
 The simplest technique is two-roll mill

Two roll mill

Two roll mill consists of two adjacent,
smooth,
hardened-steel
rolls
set
horizontally.

They rotate in opposite directions (i.e
towards each other)  produce friction or
grinding action between them.

The ratio between the operating speed of
the front and back roll is referred to as
friction ratio.

Mixing was achieved by the shearing
action induced in the space between the
roll which is adjustable.

During the mixing operation, cutting and
blending is carried out in order to obtain a
thorough and uniform dispersion of the
ingredients in the polymer mix
Internal Mixer

Consists of an enclosed chamber and
two rotors with small clearance
between them and the enclosing wall.

The rotors rotate in opposite directions.

The mixing chambers can be cooled by
circulating water or heated by steam.

Widely used for masticating rubbers
and incorporating other ingredients.

The mixed batch is dropped from
bottom of internal mixer onto a
sheeting mill and taken off for storage
until further processing.

Internal mixer  fast, clean and works
to mixed the large quantity of
compounds.
Forming techniques

There are various methods to form the polymeric
materials.

Methods that can be used to polymeric materials
depends on various factors :

1.
Whether the materials is thermoset, themoplastic or
elastomer
2.
If themoplastics  the temperature at which it softens
3.
The atmospheric stability of the materials being formed
4.
Geometry and size of the finished products.
There are numerous similarity between some of the
techniques and those utilized for fabricating metals
and ceramic

The production of polymeric materials normally
done at elevated temperature and pressure.

Thermoplastic :

Normally been form at temperature above the
Tg  to form polymeric products

As maintained during cooling process to
maintained the shape.

The advantages  it can recyclable.

Scrap from thermoplastics can be melt and
reshape to new shape

Thermoset :

Involved two stages of production
1.
2.
Preparation of linear polymer (prepolymer) 
liquid form with low molecular weight
The liquid form then will change to the hard and
rigid products with desirable shape  Curing
process will start during heating or with addition
of catalyst with pressure.

During curing  chemical and structural changes
occur  crosslinking or structural network were
formed.

It can not be recycle, not melting and can be used
at high temperature compared with thermoplastics.
Plastic Molding Methods
Forming techniques: Calendering

The calender is a machine equipped with two,
three or more heavy internally heated or cooled
rolls  usually placed above each other

The rolls rotate in opposite directions and
operate at even or uneven speeds as the
applications requires

The calender is used essentially for producing
rubber sheets of various lengths and thickness.

It is also widely used for frictioning or skin
coating of fabrics including shower curtains,
tablecloths, laminating film
Calendering process
Common roll configurations for
four roll calenders
Forming techniques: Extrusion &
Calendering



Alternate method for making
sheets or flat film
In this process, the extrudate
is extruded directly into the nip
area between two rolls.
The roles have small gap
between them, and plastics is
forced through this gaps by
the counterrotating of the rolls.
Forming techniques:Extrusion

Extrusion is the continuous semi-finished
product with uniform cross section.

The range of products extends from simple
semi-finished products  tubes, sheets and film
to complicated profiles.

In principle  the extrusion process comprises
the forcing of molten material through a shaped
die by means of pressure.
Forming techniques:Extrusion

What is extrusion?
 The
word extrusion comes from Greek roots-means
‘push out’
 Continuous process
 Process which forcing a molten materials (plastic)
through a shaped die by means of pressure- e.g.
melting of plastic resin + adding mixing fillers
 In this process, screws are used to progress the
polymer in the molten or rubbery state along the
barrel of the machine
 Single screw extruder is widely used, however twin
screw extruder are also used where superior mixing is
needed
Typical extrusion line showing
major equipment
Materials fed into hopper, falls through a hole in the top the extruder (feed throat)
Onto the screw. The screw moves the molten plastic forward until the end of the
Extruder barrel to which die has been attached. Die gives shape to molten plastics,
Cooled in water tank.
There are 3 zones; feed zone, compression zone and metering zone
The channel depth decreases from
feed end to die end
Solid polymer is fed in at one end, inside the polymer melts and
Homogenizes and molten extrudate emerges from the other
Main features of a single Screw extruder
Screw Extruder

The screw extruder has one or two flights spiraling
along its length.

The diameter of the flight is constant along the length
channel varies in depth to allow close fit in the barrel

The root or core  is varying diameter and so the
spiraling channel varies in depth ( the channel depth
decreases from feed end to the die end)

A consequences of the decreasing channel depth 
increasing pressure along the extruder and drives the
melt through the die.

There are three zones whose functions as follows:
1. Feed zone

Function to preheat the polymer and convey it to subsequent
zones
2. Compression zone

Function  to expels air trapped between the original granules

Heat transfer from the heated barrel walls is improved as the
materials thickness decreases

The density change during melting is accommodated
3. Metering zone

The functions is to homogenize the melt and hence to supply to
the die region materials which is of homogeneous quality at
constant temperature and pressure
Forming techniques: Compression
Molding
The principle of the compression molding can
be outlined as follows:


The mould is held between the heated platens of
the hydraulic press

A prepared quantity of molding compound is
placed in the mold and the mould placed in the
press

The press closes with sufficient pressure to
minimize flash at mould parts line

The compounds softens and flows to shape,
chemical cure then occurs as the internal mould
temperature becomes high enough

The press is opened and the molding removed.
Molding press
Before
After

The advantages of compression molding:

The low scrap arising (2-5 %)

The low orientation in the molding  the product
advantages:

Fibrous fillers are well distribute are not disturbed or oriented
during processing.

The product has low residue stresses

Mechanical and electrical properties are retained because there
is little shearing flow

Mould maintenance is low  little erosion from low shear forces

Capital and tooling costs are relatively low, plant and tooling are
comparatively simple
Forming techniques:
Compression Molding

The process employs thermosetting resins
in a partially cured stage, either in the form
of granules, putty-like masses, or preforms
Transfer moulding

Transfer molding  is development of compression
molding in which reservoir of the molding compound is
located in the mold and upon closure is transferred via
runner to the cavities.

Transfer molding is used :



To give many small parts easily
To reduce the risk of damage or movement of thin or delicate
mold parts
It is claimed to be faster due to better heat transfer trough
runners
The disadvantages of transfer molding:





The flow usually gives unwelcome orientation on the products
It also increases wear and maintenance cost
Tooling is rather complex and more expensive
The runner are scrap  not economical
Before
After
Injection Molding

The basic principle of injection molding is to  inject
molten polymer into a closed mold where it is
solidifies to give the product.

The molding is recovered by opening the mold to
release it

An injection molding machine has two main section
to it

The injection unit  plasticizing part of the process

The clamping unit or press which houses the mold 
essentially press closed by a hydraulic or mechanical
systems
Injection Molding

The molding cycle:

The mould is closed  a short of melt is ready in the injection
unit

Injection occurs  the valve opens and the screw acting as a
plunger forces the melt through the nozzle into the mold

The hold on stage when pressure is maintained during the early
stages of cooling to counteract contraction

The valves closes and screw rotation starts  pressure
develops against the closed off nozzle and the screw moves
backwards to accumulate a fresh shot of melt in front

Meanwhile the molding in the mould has continue to cool 
when ready the press and the mould open and the molding is
removed.

The mould closes again and the cycle repeats
Principles of injection molding
Blow molding and blown film extrusion
Blow molding and blown film extrusion  similar processes that utilize
air to shape softened polymer tubes.

1.
2.
Blow molding  to produce hollow items such as milk and soda
bottles.
Blown film  to produce relatively thin film and bag

In both cases tube is formed in a shape that approximates the article to
be made

In the case of bottle like objects

the tube called parison is either extruded or injection molded and is then
clamped in a split mold with a blow pin inside.

Air inflates the polymers until it conforms to the shape of the mold

The polymer cools on contact with cold mold walls  the mold is then open
and the part is ejected
In blown film extrusion  no mold is required


A continuous tube is extruded clamped between rollers at one end and
blown with compressed air to form large hollow tube having a very thin wall.

The tube continuously flattened and placed on a roll for bag production or it
can split opened and used for film.
Blow molding
Stages in blowing a bottle
Forming techniques: Blown Film
extrusion




The extruded materials
flow through a tubular die
The melt flow around a
mandrel and exits the die
as a tube
A cooling ring is placed at
the exit of the die to give
the tube some
dimensional stability
Air is introduced through
the back of the die and
flow upwards inside the
middle of the tube of
materials
Forming techniques: Blown Film
extrusion
The tube or bubble, continues to expand,
cool and crystallize until the radial (tensile)
strength of the plastics equals the
pressure of the air inside.
 The bubble is then forced into a flat sheet
by the collapsing guides, and moved into
the nips rolls
 Then it travels down over some rollers and
it was sealed

Blown film extrusion
Forming techniques: Blow Molding
Plastic formation process-manufacture of
bottles and hollow-shaped parts
 Competitive with other processes, injection
molding
 Advantages; cycle is very short (low cost),
lower mold cost than injection molding,
high production rates in producing hollow
parts with small or large opening

Blow molding process; general
steps
Melting the resin- done in extruder
 Form the molten resin into a cylinder or
tube (this tube is called parison)
 The parison is placed inside a mold, and
inflated so that the plastic is pushed
outward against the cavity wall
 The part is allowed to cool in the mold and
is then ejected
 The part is trimmed

Forming techniques: Blow Molding
Image of Parison
The parison can be formed by
Extrusion process
Injection molding process
Blowing step in the blow molding
process
Stretch blow molding




the plastic is first molded into a "preform" using
the Injection Molded Process. These preforms
are produced with the necks of the bottles,
including threads (the "finish") on one end.
These preforms are packaged, and fed later
(after cooling) into an EBM blow molding
machine.
In the SBM process, the preforms are heated
(typically using infrared heaters) above their
glass transition temperature, then blown using
high pressure air into bottles using metal blow
molds.
Usually the preform is stretched with a core rod
as part of the process. The stretching of some
polymers, such as PET (PolyEthylene
Terepthalate) results in strain hardening of the
resin, allowing the bottles to resist deforming
under the pressures formed by carbonated
beverages, which typically approach 60 psi.
Thermoforming

Term thermoforming  reshaping of plastics under
the influence of heat and pressure or vacuum.

There are many techniques for performing this
process  the use of compressed air and/or
vacuum has become preferred methods for
reshaping thermoplastics.

Thermoforming is a manufacturing process for

Thermoplastic sheet or film

Used on large scale for the manufacture of packaging

For large parts such as swimming pool or motor
vehicle parts.
Thermoforming Products
Thermoforming
Process used to shape thermoplastic
sheet into discrete parts
 Basic principles

 Heat
a thermoplastic sheet until it softens
 Force the hot and pliable material against the
contours of a mold by using either
mechanical, air or vacuum pressure
 Held against mold and allow to cool, and
plastics retains its shape


Thermoforming process is differed from other
plastic processing because the material is not
melted, lower pressures are required to
thermoform (the mold materials is less sturdy
materials compared to other processings)
Disadvantage; generates greater amount of
scrap, cost of the sheet materials is raised
(because of separate sheet-forming step),
limited design parts (parts with sharp bends and
corners are difficult to be produced), process
results in internal stresses
Summary of advantages &
disadvantages of thermoforming process
Forming Process: Fundamental
Vacuum forming



The material is
clamped into a frame
Material is heated,
begin to sag, the
center of materials
moves downward and
seal against the mold
Vacuum is applied to
the back of the mold
(outside pressure of
the air pushes the
plastic against the
mold)
Hand methods
a)
-
-
Hand lay-up
Mould is treated with a release agent-to
prevent sticking
Gel coat layers are placed on the mold- to give
decorative and protective surface
Put the reinforcement (woven rovings or
chopped strand mat)
The thermosetting resin is mixed with a curing
agent, and applied with brush or roller on the
reinforcement
Curing at room temperature
Concept of hand lay-up process
Filament Winding/Pelilitan Filamen
Process to produce a hollow (usually
cylindrical) shape
 End products; pressure vessel, storage
tanks, and aerospace parts such as
helicopter blades.
 Wet winding (fiber + resin) & dry winding
(prepreg)

Schematic diagram of filament
winding process
Helical
Circumferential
Polar
Methods;
 The fibers are passed through the resin
bath
 Then continuously wound onto the
mandrel
 After number of layers, curing is carried
out in an oven or room temperature.
 Mandrel is removed

Pultrusion
Used to manufacture components with
continuous lengths and constants crosssectional shape (rod, tubes, beam, etc)
 Continuous process, easily automated
(production rates are high, hence it is vey
cost effective), variety of shapes are
possible
 Starting materials can be in the forms of
Prepreg or fiber + resin

Schematic Diagram of pultrusion
process
Methods;
 Continuous rovings of the reinforcement
are impregnated with the resins (passing
through a bath of resin)
 Then pulled through the heated die
(compact & give shape to the composites)
 Curing takes place in the heated die &
Oven
 Puller used to pull the whole system with
certain speed (Pultrusion- pulling action)

Example of the exams question

Discuss
the
difference
between
compression molding and extrusion blow
molding.

What is the most suitable processing
method to produce a film

Discuss
two
polymeric
methods that you know.
processing