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IE 337: Materials & Manufacturing Processes

IE 337: Materials & Manufacturing Processes

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IE 337: Materials & Manufacturing Processes

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

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

  3. smaller M larger M w w Engineering Plastics • Chain of organic molecules • Properties: • Lightweight • Corrosion-resistant • Low strength • Low stiffness • Relatively inexpensive • Very formable • Temperature concerns Giant molecules with repeating units (monomer)

  4. What are polymers? polypropylene (PP) polyethylene (PE) polyvinyl chloride (PVC) polytetrafluoroethylene (PTFE)

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

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

  7. Two Types of Plastics • Thermoplastics • 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 • 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

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

  14. 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

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

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

  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. Mold Cavity 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

  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: PlasticShrinkage, mm/mm (in/in) Nylon‑6,6 0.020 Polyethylene 0.025 Polystyrene 0.004 PVC 0.005

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

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

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

  42. Compression Molding Thermosets with axisymmetric shapes

  43. Blow Molding Hollow shapes

  44. Stereolithography Additive Manufacturing “Rapid prototyping”

  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)