IE 337: Materials & Manufacturing Processes IE 337 Lecture 7: Polymer Processing IE 337: Materials & Manufacturing Processes Lecture 10: Polymer Processing Sections 3.4-3.5 and Chapters 8, 13 S.V. Atre

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

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

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

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

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

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

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

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

Classification: Structure Linear thermoplastic Branched thermoplastic Crosslinked thermosetting Network thermosetting

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

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

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

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

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

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)

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

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 “Non-Newtonian fluid” “Shear thinning”

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Compression Molding Thermosets with axisymmetric shapes

Blow Molding Hollow shapes

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

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

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