Polymer fundamental

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“Polymer Engineering Fundamentals”
Course Code: PE-2102
Credit Hours: Theory (3)
Course Title:
Lecture: Polymer Processing
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Course Outline
• Techniques of polymerization
• Solid state of Polymers
• Polymer Additives and Blends
• Polymer Rheology
• Polymer Processing
• Polymer Composites
• Testing and characterization of polymeric products
• Recycling of Polymers
• Applications of polymers
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Polymer Processing
The conversion of polymers into useful polymeric products is generally termed as Polymer
Processing and methods used to convert polymers into products is called Polymer Processing
Techniques. The main goal of polymer processing is to produce a usable object.
In general, Polymer processing methods have three phases:
a) Heating to soften or melt
b) Shaping/forming under constraint
c) Cooling to retain shape
The most common processing techniques are:
1. Extrusion (mostly useful for thermoplastics)
2. Injection molding (mostly useful for thermoplastics)
3. Blow molding (mostly useful for thermoplastics)
4. Compression molding (more useful for thermosets)
Extrusion Process
Principle:
It comprises of forcing of a plastic material through an orifice(die) by means of
pressure.
• Extrusion can be defined as the act of shaping a material by forcing it through a die.
• Plastics extrusion is a normally high-volume manufacturing process where a polymer
material, enriched with the desired additives, is melted and formed in a continuous process.
• The raw material (polymer) in the form of granulates, is gravity fed into the hopper and
through the feed throat, drops on a rotating screw.
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Extrusion Process
• Polymer material in form of pellets is fed into an
extruder through a hopper.
• The material is then conveyed forward by a
feeding screw and forced through a die,
converting to continuous polymer product.
• Heating elements, placed over the barrel, soften
and melt the polymer.
• The temperature of the material is
monitored/controlled by thermocouples.
• The product going out of the die is cooled by
blown air or in water bath.
• Extrusion of polymers is continuous process
lasting as long as raw pellets are supplied.
Extrusion Process
EXTRUDER
Extruders are basically helical screw pumps that convert solid polymer material into melt,
which is then delivered to the die.
TYPES:
✓ Single screw extruder
✓ Twin screw extruder
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Extrusion Process
✓ Single screw extruder ✓ Twin screw extruder
Extrusion Process
ZONES/SECTIONS IN Single Screw Extruder (S.S.E)
❖ Feed zone
❖ Compression/ plasticizing zone
❖ Melt/Metering zone
❖ Die zone
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Extrusion Process
1. FEED ZONE
 In this 1st zone, the polymer pellets are conveyed
to the subsequent zone.
 Main function is to preheat the polymer.
 Here the screw depth is constant.
2. COMPRESSION ZONE
 Here the screw Tapers.
 It repels the air gap between the granules.
 Heat transfer from the barrel wall is improved.
 Density change is accommodated.
3. METERING ZONE
 Here screw depth is constant.
 It homogenizes the melt and supply it
to the die.
Extrusion Process
4. DIE ZONE
 The final zone where the screen pack is located.
 It usually comprises of the perforated steel
plate called breaker plate and a sieve pack.
 It sieves out the extraneous materials(foreign
particles).
 It removes the turning memory from the
polymer melt.
 It allows head pressure to develop by
providing a resistance for pumping
action of metering zone.
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Extrusion Process
Extrusion Process
VENTED EXTRUDER
 Vented or two-stage screws are used when volatiles gases are produced by polymer which are
supposed to be removed in order to make a satisfactory product.
 As a result, the air bubbles are removed.
 It is nothing but an addition of a decompression zone to the already existing screw.
• It reduces the melt pressure to
atmospheric, so there is no tendency
for the polymer to be pumped/leaked
out the vent opening.
• That is accomplished by increasing the
channel volume so that the output of
the extruder only partially fills the
flights under the vent.
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Extrusion Process
ADV
ANTAGES
 Low-cost tooling and short lead time.
 Low-cost products.
 Reinforcement can be done easily.
 Use of multiple parts in varieties.
 Slow maintaining.
 Also used in co-extrusion , tubular flows,
crossway extruder in similar process.
• Applications
• Drainage and irrigation pipe,
• Medical fluid and IV tubing,
• weather-stripping, fencing, deck railings, vinyl
siding, window frames,
• Home décor and furniture trim,
• Automotive components, appliance trims and
seals,
• Filtration parts, drinking straws,
• The “zip” strip on re-sealable bags,
• Food processing tubing and rails,
• Plastic films and sheeting,
• Thermoplastic coatings, electrical conduit and
cable protectors and wire insulation.
Extrusion Process
MAJOR PROBLEMS
 Die swelling
 Melt Fracture
➢ Spiraling
➢ Bam booing
➢ Regular ripple
➢ Random fracture
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Melt fracture
• The appearance of rough surface of extrudate with short cracks in
machine line direction or helically around the extrudate.
• Often signaled by surface roughness and/or surface irregularities in
the extrudate
• Reasons
• Tensile forces overcome shear stress
• Improper streamlining of die
• Barrel temperature fluctuations
• Low temperature of a material of high molecular weight
Die
Machine Direction
Cross Direction
Melt fracture
• Troubleshooting
• Increase the melt temperature
• A small temperature increase may sufficiently reduce the polymer’s
viscosity, lowering the shear stress imparted
• Streamline the die
• Resin should be of low molecular weight
• Reduce the Land length
• This will decrease the shear stress in the extrudate but will increase the
swelling and decrease the molecular alignment along the direction of
material flow
• Lower the extruder speed
Slip-stick Palm tree Spiral Random
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Sharkskin or Alligator Hide
• The roughness of extrudate due to lines appearing perpendicular
to the flow direction
• Less severe form of shark skin is the occurrence of matted nonglossy
surface
• Bambooing is a severe form of sharkskin
• Formed in the die land or at the exit
• Reasons of sharkskin
• Stresses due to laminar flow
• High Extrusion speed (high extrusion pressure)
• Rapid acceleration of the surface layers of the extrudate when the polymer
leaves the die
• Presence of high modulus resin
• Low temperature in the die
Sharkskin or Alligator Hide
• Troubleshooting:
• Reduce the speed of extruder
• In continuous extrusion sharkskin results from extruding the resin too fast
• Increase the temperature in die section
• Increasing the temperature at the land section
• Use material of broad molecular weight resin
• High viscosity polymers with narrow MWD are more susceptible to sharkskin
• Use processing aids to reduce the viscosity (internal slip)
• External lubricants
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Surging
• A cyclic variation in the extrudate thickness with a cycle time
between 30 sec. to 3 min.
• This is a condition of an unsteady-state extrusion operation
• The screw is not delivering a constant melt flow
• Reasons and solutions for surging are
• Resin bridging in hopper
• Eliminate bridging by using proper hopper
• Incorrect melt temperature
• Inadequate control of screw speed or screw design
• The screw designed for denser materials will face problem when will deal with
less dense materials
Surging
• Undersize design of drive motor
• Raise the temperature to reduce the load on screw
• Presence of metal contaminants
• Pullout the screw and clean the barrel and screw
• Improper metering section length
• Use proper screw design
• Plugging of screw channel with solids
• Screw design – evaluate
• Raise temperatures
• Raise polymer temperature (Preheat)
• Screw/barrel wear – repairing required
• Excessive fillers, i.e. inadequate % of polymer to allow for fluxing
Fluxing is the continued flow of material or fluid
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Extrusion Process
WAYSTO REDUCE DIE SWELLING
 Decrease the extrusion rate
 Increase the length of the die end
 Increase the draw down ratio without affecting the output
 By improving design factors
Modified Extrusion Based Processing Techniques:
1. Co-extrusion
2. Tubular Blown Films
3. Cross Head
Extrusion Process
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Extrusion Process
CO-EXTRUSION
 Simultaneous extrusion of two or more polymers
through a single die.
 Allows multiple layers extrusion of
films,sheets,pipes and mostly in packing industry.
ADVANTAGES:
• Reduced material
• Processing cost improved
• More uniform distribution temperature.
Extrusion Process
Blown Film Extrusion
➢ Used for specialized plastic films for packing
industry.
➢ It consists of extrusion tube with thermoplastic
material and is inflated to form thin tubular
product.
➢ There is fixed orientation due to inflation and
pulling.
• Advantage is that film is very strong in
machine direction but trade off is that it weak
in transverse direction.
➢ Commonly used HDPE,LLDPE.,
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Extrusion Process
Crosshead Extrusion
• Crosshead extrusion process is widely
used to coat wires and cables with a
polymer.
• The basic procedure includes pulling of
the wire / cable to be coated at a
uniform rate via a crosshead die, where
it is covered with the molten plastic.
Injection Molding
Basic Principles:
To inject the molten polymer into a closed cooled mould where it solidifies to give the product. The
moulding by opening the mold to release it.
Two Sections:
I. Injection unit
II. Clamp /Press unit
TWO PHASES:
I. Plasticizing phase
II. Injection phase
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Injection Molding
Injection Molding-Mechanism
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Melting
Injection of
Polymer/Resin
Screw retraction
& part cooling
Part Removal
Injection Molding
Step wise Working
Injection Molding
 The difference here is that the screw does not rotate
but it reciprocates to and fro thereby injecting the
melt into the mold.
 In addition, here there is no breaking plate or sieve
plate instead there is check valve.
 The check or non-return valve (NRV) on the screw
head of injection machine plays the role of
preventing the melt back discharge during injection
process.
 The sensitivity of non-return valves opening and
closing affects the quality of the product.
 Gating is important in flow systems as they resist
the flow.(Edge,disk,ring,submarine gates)
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Injection Molding
Parts:
Extruder +
i. Cavity or impression
ii. Channels
iii. Cooling channels
iv. Ejector pins
Injection Molding
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Injection Molding
MOULDING CYCLE
1. Mold is closed and a shot of melt is
ready in injection unit.
2. Valve opens and screw forces the
melt through nozzle into mold.
3. Hold on stage,where pressure is
maintained and released when freezing
commences.
4. Valve closes and reciprocation starts.
5. Pressure develops against nozzle and
screw accumulates new slot of melt.
6. When mold is cooled, press is opened,
and moulding removed.
Injection Molding
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Injection
Molding
CONTROL PARAMETERS IN Injection Molding
• Temperature of melt
• Temperature of mold
• Pressure and speed of injection
• Hold on pressure
• Timings of various parts of process cycle
Defects In Injection Molding:
• Short shots
• Voids and sinks
• Weld lines
• Orientation
• Shrinkage
Injection Molding
Modified Injection Molding Processes
1. Spruless moulding:
• Inorder to reduce scrap:
• The nozzle locates directly to mold cavity.
• For complex single impression molding.
2. Sandwich moulding:
• The technique injects skin and core melt seperately.
• Skin polymer is generally normal one,
• Core polymer is onewhich contains lubricating
agents.
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Injection Molding
2. Sandwich Injection Molding Steps:
Injection Molding
Modified Injection Molding Processes
3. Structural Foam Molding
 SFM is development is used where stiffness
becomes prerequisite. Stiffness ,S = E(t)3
 Suitable for full product.
 Here the melt is expandable, contains dissolved gas
which decomposes at melt temperature to give foam.
 A short shot leaves space into which foam
expands.The pressure becomes driving force to fill
mold, which is less than 3Mpa.
Advantages:
 Producing large and flexible products;
 low process energy:
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Injection Molding
4. Reaction Injection Moulding [RIM]
 The technique combines two metered ,well mixed
reactive sreams.(urethane)
 One stream contains polyether backbone,a catalyst
and a cross linking agent.
 Another stream contains isocynate, blowing agent
ADVANTAGES:
▪ Conducted at low temperature(60-90C)
for polyvinyl.
▪ Processing and preparing takes
place simultaneously.
Injection Molding
5. Reinforced Reaction Injection Moulding[RRIM]
 It is an extesion of RIM where there is inclusion of a
reinforced glass fibre.
 Here, two fast reacting liquid streams are precisely
metered and mixed using high pressure.
impingement mixing with polymer blowing agents.
Advantages:
❑ Molding ranges from rubbery flexible to semi
flexible materials to rigid unyielding products by
suitable deformation of reactants.
❑ Internal mould pressures are low.
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Applications of Injection Molding
Extrusion Process
Definition:
It represent the processing operation in which fluid/melt is pumped
through a die orifice to produce a shape of constant cross-section.
➢ The term has a Greek root which means
‘Push out’.
➢ Squeezing a tube of toothpaste is an extrusion process.
➢ Extrusion is the most efficient process
for melting plastics as well as
mixing fillers, colorants and other
additives into the molten plastics.
➢ When the extruder is used to shape the parts
directly then the process is called extrusion.
e.g. manufacturing of pipes
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Extrusion Process
The heart of extruder is ‘The Screw’
➢ The polymer granules are fed from hopper onto the screw, conveyed along the barrel and heated by conduction
from barrel heaters & shear due to the movement of screw.
➢ The depth of the screw channel is reduced along the length of the screw for compaction.
➢ At the end of the extruder, the melt is passed through the die to produce an extrudate of the desired shape.
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Extrusion Process
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Extrusion Process
Extruder with vented zone
Extrusion Process-Screw geometry
It consists of helical screw rotating inside the cylindrical barrel.
The screw geometry is as follows;
Shank: Fitted into the quill of screw drive
Flight: Each turn of the helix
is called a Flight.
Root dia: It is the measure of the
dia of the shaft of the screw.
Flight depth/Channel depth:
the difference between the top of
flight and the root dia.
The flight depth varies with the
variation of root dia.
Helix angle: angle of the flight with the shaft
(most common pitch angle = 17.5o)
Pitch: The distance between the crests of two flights next to each other.
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Extrusion Process-Screw geometry
It consists of helical screw rotating inside the cylindrical barrel.
The screw geometry is as follows;
Clearance:
The distance between
the tip of flight and the barrel wall
It is constant over the length of the screw.
Flighted Length: It consists of
80% of the length of the screw.
(This is the portion of the screw
which receives the pellets that are
conveyed to the metering zone by
the rotation of the screw)
Extrusion Process
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Extrusion Process
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Extrusion Process
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Injection Molding
Injection Molding machine (IMM) is a simple process in which,
➢ A Polymer in the form of granules/powder passes from feed hopper into the barrel
➢ It is heated in the screw/barrel assembly to melt the granules
➢ It is then forced through nozzle into the mold which is tightly closed
➢ When polymer becomes solid in the mold, it is opened and the article is ejected from the mold
Major Advantages:
➢ Versatility in molding wide range of products
➢ Manufacturing of products with close tolerances
➢ High production rate
Injection Molding-Equipment
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Injection Molding
Other IMM making companies are,
Arburg, Boy, Kraus Maffai etc
Injection Molding
Now a days in Injection molding,
➢ The ‘Reciprocating Screws’ are used, performing dual role;
i) On one hand, it rotates in the normal way like the extruder screw to transport, pressurize
and melt the polymer material in the barrel
ii) On the other hand, it is also capable of moving like a plunger to inject the melt into the mold
Due to the availability of ‘Reciprocating Screws’, the IMM with plunger types are seldom used.
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Melting
Injection of
Polymer/Resin
Screw retraction
& part cooling
Part Removal
Injection Molding-Terminology
Shot Size:
It is the quantity of the melt injected into the mold. (measured in g or ounces)
Cycle Time:
The time required to complete one cycle is called cycle time. (It can vary from few
Seconds to several minutes)
Stroke:
The specified distance which is allowed for the screw to move back & forward to
act as a plunger/ram
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Injection Molding-Clamping Unit
The Clamping unit performs the following functions;
i) Holds the mold
ii) Closes the mold
iii) Keeps the mold closed under injection pressure
iv) Opens the mold to allow the ejection of the part
v) Accommodate the ejector system which ejects the part
The Clamping mechanism provides the force to keep the mold closed during the injection &
holding pressure stages of the machine cycle.
Types of Clamping Mechanism:
i) Hydraulic clamping
ii) Toggle clamping
iii) Combination of both hydraulic and toggle
Injection Molding-Clamping Force
The amount of clamping force (F) is calculated using the expression;
Clamping Force = F = P * A
Where,
P = Injection Pressure
A = surface area of the cavity (widest area of the cavity or part)
The size of the injection molding machine is rated on the basis of clamping force.
In metric units, the force is given in Newton (N)
In English units, the force is given in Tons.
e.g. 200 tons machine means the clamping force which can be applied.
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Injection Molding-Platen
➢ Designing molds for injection molding is far more complicated than for extrusion dies.
➢ The mold is between the stationary platen and the moveable platen of the IMM.
➢ The connection from the injection unit to the mold is through nozzle.
The injection Mold is the element that,
1) Receives the molten polymer from injection unit
2) Forms the shape of the desired polymer part
3) Provides the necessary cooling to solidify the part
Injection Molding-Ejector System
➢ To facilitate part removal, molds have mechanism to knock out the parts.
➢ Knock-out pins are mounted on an Ejector plate attached to a piston called the Ejector rod.
➢ When the mold is open and part is to be removed, the ejector rod advances, moving the
ejector plate and ejector pins.
➢ The pins move through holes and enter the back of the part to push the parts out of the mold.
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Injection Molding-Ejector System
Injection Molding
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Injection Molding-Gate System
Polymer melt flows into the individual mold cavities through one or more restricted gates.
Gate: It is the point where melt enters into the cavity of the mold.
Functions of Gate:
1) To allow polymer melt to pass into the mold at relatively high flow rates.
2) To provide a site to separate the molding from the material
3) To promote rapid freezing once filling and packing phases are completed.
Rheological Aspects of Gate:
As the gate dimensions are very small in comparison to both the runners & mold cavities.
A combination of rapid injection and restricted gate leads to extremely high shear rates,
Typically in the range of 104 to 106 sec-1.
Gate Freezing:
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