IE 337: Materials & Manufacturing Processes

Lecture 10: Polymer Processing

Sections 3.4-3.5 andChapters 8, 13

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This Time

What are plastics and polymers? Polymer Rheology Major Plastics Molding Processes

Extrusion Injection Molding Thermoforming Compression Molding

Molding Machine

Engineering Plastics

Chain of organic molecules

Properties: Lightweight Corrosion-resistant Low strength Low stiffness Relatively inexpensive Very formable Temperature concerns

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smaller Mw larger Mw

Giant molecules with repeating units

(monomer)

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What are polymers?

polyethylene(PE)

polyvinyl chloride(PVC)

polypropylene(PP)

polytetrafluoroethylene(PTFE)

Classification: Chemistry

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polyethylene(PE)

polyvinyl chloride(PVC)

polytetrafluoroethylene(PTFE)

polypropylene(PP)

polystyrene(PS)

polymethyl methacrylate(PMMA)

Classification: Chemistry

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polycarbonate(PC)

polyethylene terephthalate(Polyester, PET)

polyhexamethylene adipamide(Nylon)

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Two Types of Plastics

1. Thermoplastics Chemical structure remains unchanged during heating and

shaping More important commercially, comprising more than 70% of

total plastics tonnage

2. 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

Families of Plastics

Thermoplastics Acetals Acrylic Cellulose (Acetates) Fluorocarbons

Teflon Nylon Polycarbonate Polyethelene

Density Polystyrene Vinyl

Thermosets Epoxies

Bonding Melamines

Resistant Phenolics

Bakelite Polyesters

Resistant Silicones

Sealant Urea-formaldehyde

Environmental concerns

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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

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Classification: Structure

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Linearthermoplastic

Branchedthermoplastic

Crosslinkedthermosetting

Networkthermosetting

Classification: Structure

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random coil(amorphous)

partially extended(semi-crystalline)

Elastomers

Exceptional elastic deformation 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

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Elastomers

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polyisoprene(natural rubber)

polychloroprene(Neoprene rubber)

polydimethylsiloxane(silicone rubber)

polyisobutylene(butyl rubber)

Plastic Utility

Degradable 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

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Classification: Structure

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c) block

a) random

b) alternating

d) graft

COPOLYMERSmore than one

“mer”

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)

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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

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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

Viscosity

Like liquid metals, polymer viscosity is dependent on temperature

Unlike liquid metals, polymer viscosity depends on shear rate

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“Shear thinning”

“Non-Newtonian fluid”

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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

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Extruder Sectional View

Components and features of a (single‑screw) extruder for plastics and elastomers

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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

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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

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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

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

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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

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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

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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

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A large (3000 ton capacity) injection molding machine (courtesy Cincinnati Milacron)

Injection Molding Machine

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Typical molding cycle: (1) mold is closed

Injection Molding Cycle: Stage 1

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Typical molding cycle: (2) melt is injected into cavity

Injection Molding Cycle: Stage 2

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Typical molding cycle:

(3) screw is retracted

Injection Molding Cycle: Stage 3

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Typical molding cycle:

(4) mold opens and part is ejected

Injection Molding Cycle: Stage 4

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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

A side, micro arrays cavity

Mold

Cavity

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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‑6,6 0.020

Polyethylene 0.025

Polystyrene 0.004

PVC 0.005

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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

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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

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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

Vacuum thermoforming: (1) a flat plastic sheet is softened

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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

Compression Molding

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Thermosets with axisymmetric shapes

Blow Molding

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Hollow shapes

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

Extrusion Injection Molding Thermoforming Compression Molding

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Next Week

Mid-Term Exam (Tuesday) Forming (Thursday)