IE 337: Materials & Manufacturing
Processes
Lecture 10: Polymer
Processing
Sections 3.4-3.5 and
Chapters 8, 13
This Time
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What are plastics and polymers?
Polymer Rheology
Major Plastics Molding Processes
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Extrusion
Injection Molding
Thermoforming
Compression Molding
Molding Machine
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Engineering Plastics
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Chain of organic molecules
smaller M w
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Properties:
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larger M w
Lightweight
Corrosion-resistant
Low strength
Low stiffness
Relatively inexpensive
Very formable
Temperature concerns
Giant molecules
with repeating units
(monomer)
What are polymers?
polyethylene
(PE)
polytetrafluoroethylene
4
(PTFE)
polypropylene
(PP)
polyvinyl chloride
(PVC)
Classification: Chemistry
polyethylene
(PE)
polyvinyl chloride
(PVC)
polytetrafluoroethylene
(PTFE)
polypropylene
(PP)
polymethyl methacrylate
(PMMA)
polystyrene
(PS)
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Classification: Chemistry
polyhexamethylene adipamide
(Nylon)
polyethylene terephthalate
(Polyester, PET)
polycarbonate
(PC)
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Two Types of Plastics
1.
Thermoplastics
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2.
Thermosets
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Chemical structure remains unchanged during heating and
shaping
More important commercially, comprising more than 70% of
total plastics tonnage
Undergo a curing process during heating and shaping,
causing a permanent change (called cross-linking) in
molecular structure
Once cured, they cannot be remelted
Families of Plastics
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Thermoplastics
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Acetals
Acrylic
Cellulose (Acetates)
Fluorocarbons
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Teflon
 Nylon
 Polycarbonate
 Polyethelene
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Density
 Polystyrene
 Vinyl
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Thermosets
 Epoxies
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Bonding
 Melamines
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Resistant
 Phenolics
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Bakelite
 Polyesters
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Resistant
 Silicones
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Sealant
 Urea-formaldehyde
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Environmental concerns
Plastic Family Properties
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Thermoplastics
 Reversible softening &
hardening
 Softening range (not
melting point)
 Weak bonds between
molecules
 Properties inverse with
temperature:
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Stiffness
Hardness
Ductility
Solvent resistance
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Thermosets
 Irreversible hardening
reaction
 Strong bonds between
molecules (cross-linking)
 Compared with
Thermoplastics:
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Stronger
Rigid
Heat resistant
Brittle
Low impact toughness
Lower ductility
Classification: Structure
Linear
thermoplastic
Crosslinked
thermosetting
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Branched
thermoplastic
Network
thermosetting
Classification: Structure
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random coil
partially extended
(amorphous)
(semi-crystalline)
Elastomers
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Exceptional elastic deformation
 Near-complete* recovery
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Viscous deformation is permanent
 Twisted/coiled molecular chains
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Can be cross-linked (vulcanization)
Degradable
Insulative
Chemical forms
 Natural
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Rubber
 Synthetic
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Polyisoprene (Santoprene)
Silicone rubber
Urethanes
Elastomers
polyisoprene
(natural rubber)
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polychloroprene
(Neoprene rubber)
polydimethylsiloxane
polyisobutylene
(silicone rubber)
(butyl rubber)
Plastic Utility
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Degradable
 UV Light
 Flammable, Oxidation
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Modifiable Properties
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Additives Make Polymers into Plastics
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Color
Conductivity
Adhesiveness
Mechanical
Stabilizers, Flame retardants
Dyes (translucent), Coloring Agents (opaque)
Anti-statics, Anti-microbials
Plasticizers (improve flow), Lubricants (improve moldability)
Reinforcements, Fillers
Classification: Structure
a) random
b) alternating
COPOLYMERS
more than one
“mer”
c) block
d) graft
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Economics of Plastics
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Compared with Metals (+):
 Lower fabrication tooling
costs
 Higher production rate
 Greater DFA (Design For
Assembly) potential
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Snap fits/fastener-less
assembly
Friction/ultrasonic/solvent
welding
Self-tapping fasteners
 Lower reuse cost (scrap)*
 Lower finishing costs
 Lower density
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Compared with Metals (-):
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Higher cost / weight
Lower impact resistance
Lower strength
Lower stiffness
Smaller operational
temperature range
 Lower resistance to:
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Flame
Solvents
Light (UV)
Plastic Shaping Processes
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Almost unlimited variety of part geometries
Plastic molding is a net shape process; further
shaping is not needed
Less energy is required than for metals
because processing temperatures are much
lower
 Handling of product is simplified during production
because of lower temperatures
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Painting or plating is usually not required
Viscosity of Polymer Melts
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A fluid property that measures the resistance to flow –
quotient of shear stress to shear rate within a fluid
Due to its high molecular weight, a polymer melt is a
thick fluid with high viscosity
Important because most polymer shaping processes
involve flow through small channels or die openings
High flow rates lead to high shear rates and shear
stresses, so significant pressures are required to
process polymers
Viscosity
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Like liquid metals, polymer viscosity is
dependent on temperature
Unlike liquid metals, polymer viscosity depends
on shear rate
“Non-Newtonian fluid”
“Shear thinning”
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Viscoelasticity
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Viscous and elastic (pseudoplastic) properties
Possessed by both polymer solids and polymer
melts
Example: die swell in extrusion, in which the
hot plastic expands when exiting the die
opening
Swell ratio, rs = Dx/Dd
Extruder Sectional View
Components and features of a (single-screw) extruder
for plastics and elastomers
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Extruder Screw
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Divided into sections to serve several
functions:
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Feed section - feedstock is moved from hopper
and preheated
Compression section - polymer is transformed into
fluid, air mixed with pellets is extracted from melt,
and material is compressed
Metering section - melt is homogenized and
sufficient pressure developed to pump it through
die opening
Dies and Extruded Products
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The shape of the die orifice determines the
cross-sectional shape of the extrudate
Common die profiles and corresponding
extruded shapes:
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Solid profiles
Hollow profiles, such as tubes
Wire and cable coating
Sheet and film
 Filaments
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Extruding a Coated Wire
Side view cross-section of die for coating of wire by extrusion
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Injection Molding
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Polymer is heated to a highly plastic state and
forced to flow under high pressure into a mold
cavity where it solidifies; molded part is then
removed from cavity
Produces discrete components almost always
to net shape
Typical cycle time 10 to 30 sec, but cycles of
one minute or more are not uncommon
Mold may contain multiple cavities, so
multiple moldings are produced each cycle
Injection Molded Parts (Moldings)
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Complex and intricate shapes are possible
Shape limitations:
 Capability to fabricate a mold whose cavity is the
same geometry as part
 Shape must allow for part removal from mold
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Part size from  50 g (2 oz) up to  25 kg
(more than 50 lb), e.g., automobile bumpers
Injection molding is economical only for large
production quantities due to high cost of mold
Polymers for Injection Molding
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Injection molding is the most widely used
molding process for thermoplastics
Some thermosets, elastomers, metals and
ceramics are also injection molded
 Modifications in equipment and operating
parameters must be made
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Injection Molding Machine
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Two principal components:
 Injection unit – melts and delivers polymer melt, operates
much like an extruder
 Clamping unit – opens and closes mold each injection cycle
Injection Molding Machine
A large (3000 ton capacity) injection molding machine
(courtesy Cincinnati Milacron)
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Injection Molding Cycle: Stage 1
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Typical molding cycle:
(1) mold is closed
Injection Molding Cycle: Stage 2
Typical molding cycle:
(2) melt is injected into cavity
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Injection Molding Cycle: Stage 3
Typical molding cycle:
(3) screw is retracted
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Injection Molding Cycle: Stage 4
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Typical molding cycle:
(4) mold opens and part is ejected
The Mold
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Custom-designed and fabricated for the part to
be produced
Various types of mold for injection molding:
 Two-plate mold
 Three-plate mold
 Hot-runner mold
A side, micro
arrays cavity
Cavity
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Mold
Shrinkage
Reduction in linear size during cooling from molding to
room temperature
 Polymers have high thermal expansion coefficients,
so significant shrinkage occurs during cooling in mold
 Typical shrinkage values for selected polymers:
Plastic
Nylon-6,6
Polyethylene
Polystyrene
PVC
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Shrinkage, mm/mm (in/in)
0.020
0.025
0.004
0.005
Compensation for Shrinkage
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Dimensions of mold cavity must be larger than
specified part dimensions:
Dc = Dp + DpS + DpS2
where Dc = dimension of cavity;
Dp = molded part dimension, and
S = shrinkage value
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Shrinkage Compensation Factors
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Fillers in the plastic tend to reduce shrinkage
Injection pressure – as pressure is increased, it
forces more material into the mold cavity, and
shrinkage is reduced
Compaction time - similar effect - forces more
material into cavity during shrinkage
Molding temperature - higher temperature
lowers the polymer melt viscosity, allowing
more material to be packed into mold and
reducing shrinkage
Thermoforming
Flat thermoplastic sheet or film is heated and
deformed into desired shape using a mold
 Heating usually accomplished by radiant
electric heaters located on one or both sides of
starting plastic sheet or film
 Widely used in packaging of products and to
fabricate large items such as bathtubs,
contoured skylights, and internal door liners for
refrigerators
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Thermoforming Process - Step 1
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Vacuum thermoforming: (1) a flat plastic sheet is softened
Thermoforming Process - Step 2
Vacuum thermoforming: (2) sheet is placed over mold cavity
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Thermoforming Process - Step 3
Vacuum thermoforming: (3) vacuum draws sheet into the cavity
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Compression Molding
Thermosets with axisymmetric shapes
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Blow Molding
Hollow shapes
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Stereolithography
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Additive Manufacturing
“Rapid prototyping”
You should have learned
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The difference between plastics and polymers
Viscoelastic properties of polymers
Key plastics molding processes
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Extrusion
Injection Molding
Thermoforming
Compression Molding
Next Week
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46
Mid-Term Exam (Tuesday)
Forming (Thursday)