Post on 24-Dec-2015
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
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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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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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
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