1. IE 337: Materials & Manufacturing Processes
IE 337 Lecture 7: Polymer Processing
IE 337: Materials & Manufacturing Processes
Lecture 10: Polymer Processing
Sections and
Chapters 8, 13
S.V. Atre

2. This Time What are plastics and polymers? Polymer Rheology
Major Plastics Molding Processes
Extrusion
Injection Molding
Thermoforming
Compression Molding
Molding Machine

3. IE 337 Lecture 7: Polymer Processing
Engineering Plastics
Chain of organic molecules
Properties:
Lightweight
Corrosion-resistant
Low strength
Low stiffness
Relatively inexpensive
Very formable
Temperature concerns
smaller M w
larger M
Giant molecules with repeating units (monomer)
Mw is molecular weight
S.V. Atre

4. IE 337 Lecture 7: Polymer Processing
What are polymers?
polypropylene
(PP)
polyethylene
(PE)
Repeats of “mer” units
polyvinyl chloride
(PVC)
polytetrafluoroethylene
(PTFE)
S.V. Atre

5. Classification: Chemistry
polyethylene
(PE)
polyvinyl chloride
(PVC)
polytetrafluoroethylene
(PTFE)
polypropylene
(PP)
polymethyl methacrylate
(PMMA)
polystyrene
(PS)

6. Classification: Chemistry
polyhexamethylene adipamide
(Nylon)
polyethylene terephthalate
(Polyester, PET)
polycarbonate
(PC)

7. Two Types of Plastics Thermoplastics Thermosets
Chemical structure remains unchanged during heating and shaping
More important commercially, comprising more than 70% of total plastics tonnage
Thermosets
Undergo a curing process during heating and shaping, causing a permanent change (called cross‑linking) in molecular structure
Once cured, they cannot be remelted

8. Families of Plastics Thermoplastics Thermosets Acetals Acrylic
Cellulose (Acetates)
Fluorocarbons
Teflon
Nylon
Polycarbonate
Polyethelene
Density
Polystyrene
Vinyl
Thermosets
Epoxies
Bonding
Melamines
Resistant
Phenolics
Bakelite
Polyesters
Silicones
Sealant
Urea-formaldehyde
Environmental concerns

9. Plastic Family Properties
Thermoplastics
Reversible softening & hardening
Softening range (not melting point)
Weak bonds between molecules
Properties inverse with temperature:
Stiffness
Hardness
Ductility
Solvent resistance
Thermosets
Irreversible hardening reaction
Strong bonds between molecules (cross-linking)
Compared with Thermoplastics:
Stronger
Rigid
Heat resistant
Brittle
Low impact toughness
Lower ductility

10. Classification: Structure
Linear
thermoplastic
Branched
thermoplastic
Crosslinked
thermosetting
Network
thermosetting

11. Classification: Structure
random coil
(amorphous)
partially extended
(semi-crystalline)

12. Elastomers Exceptional elastic deformation Chemical forms
Near-complete* recovery
Viscous deformation is permanent
Twisted/coiled molecular chains
Can be cross-linked (vulcanization)
Degradable
Insulative
Chemical forms
Natural
Rubber
Synthetic
Polyisoprene (Santoprene)
Silicone rubber
Urethanes

13. polydimethylsiloxane
Elastomers
polychloroprene
(Neoprene rubber)
polyisoprene
(natural rubber)
polydimethylsiloxane
(silicone rubber)
polyisobutylene
(butyl rubber)

14. Plastic Utility Degradable Modifiable Properties
UV Light
Flammable, Oxidation
Modifiable Properties
Color
Conductivity
Adhesiveness
Mechanical
Additives Make Polymers into Plastics
Stabilizers, Flame retardants
Dyes (translucent), Coloring Agents (opaque)
Anti-statics, Anti-microbials
Plasticizers (improve flow), Lubricants (improve moldability)
Reinforcements, Fillers

15. Classification: Structure
a) random
b) alternating
COPOLYMERS
more than one “mer”
c) block
d) graft

16. Economics of Plastics Compared with Metals (+):
Lower fabrication tooling costs
Higher production rate
Greater DFA (Design For Assembly) potential
Snap fits/fastener-less assembly
Friction/ultrasonic/solvent welding
Self-tapping fasteners
Lower reuse cost (scrap)*
Lower finishing costs
Lower density
Compared with Metals (-):
Higher cost / weight
Lower impact resistance
Lower strength
Lower stiffness
Smaller operational temperature range
Lower resistance to:
Flame
Solvents
Light (UV)

17. Plastic Shaping Processes
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
Painting or plating is usually not required

18. Viscosity of Polymer Melts
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

19. Viscosity
Like liquid metals, polymer viscosity is dependent on temperature
Unlike liquid metals, polymer viscosity depends on shear rate
“Non-Newtonian fluid”
“Shear thinning”

20. Viscoelasticity 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

21. Extruder Sectional View
Components and features of a (single‑screw) extruder for plastics and elastomers

22. Extruder Screw Divided into sections to serve several functions:
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

23. Dies and Extruded Products
The shape of the die orifice determines the cross‑sectional shape of the extrudate
Common die profiles and corresponding extruded shapes:
Solid profiles
Hollow profiles, such as tubes
Wire and cable coating
Sheet and film
Filaments

24. Extruding a Coated Wire
Side view cross‑section of die for coating of wire by extrusion

25. Injection Molding
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

26. Injection Molded Parts (Moldings)
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
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

27. Polymers for Injection Molding
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

28. Injection Molding Machine
Two principal components:
Injection unit – melts and delivers polymer melt, operates much like an extruder
Clamping unit – opens and closes mold each injection cycle

29. Injection Molding Machine
A large (3000 ton capacity) injection molding machine
(courtesy Cincinnati Milacron)

30. Injection Molding Cycle: Stage 1
Typical molding cycle:
(1) mold is closed

31. Injection Molding Cycle: Stage 2
Typical molding cycle:
(2) melt is injected into cavity

32. Injection Molding Cycle: Stage 3
Typical molding cycle:
(3) screw is retracted

33. Injection Molding Cycle: Stage 4
Typical molding cycle:
(4) mold opens and part is ejected

34. The Mold 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
Cavity
Mold

35. 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 Shrinkage, mm/mm (in/in)
Nylon‑6,
Polyethylene
Polystyrene
PVC

36. Compensation for Shrinkage
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

37. Shrinkage Compensation Factors
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

38. 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

39. Thermoforming Process - Step 1
Vacuum thermoforming: (1) a flat plastic sheet is softened

40. Thermoforming Process - Step 2
Vacuum thermoforming: (2) sheet is placed over mold cavity

41. Thermoforming Process - Step 3
Vacuum thermoforming: (3) vacuum draws sheet into the cavity

42. Compression Molding
Thermosets with axisymmetric shapes

43. Blow Molding
Hollow shapes

44. IE 337 Lecture 7: Polymer Processing
Stereolithography
Additive Manufacturing
“Rapid prototyping”
S.V. Atre

45. You should have learned
The difference between plastics and polymers
Viscoelastic properties of polymers
Key plastics molding processes
Extrusion
Injection Molding
Thermoforming
Compression Molding

46. Next Week
Mid-Term Exam (Tuesday)
Forming (Thursday)