Manufacturing Processes Chapter 20 Forming and Shaping of

Manufacturing Processes Chapter 20 Forming and Shaping of Plastics

Plastics Review • Thermoplastics (TPs) – Polymers in which, after heating and cooling, properties return to original values. – Behavior depends on: structure, composition, temperature, rate of deformation. – Glass transition temperature (Tg): the temperature at which the transition in behavior (solid to liquid) takes place. – Below Tg, behavior is like an elastic solid.

Plastics Review • Thermoplastics (TPs) – Plastics have a high strain-rate sensitivity exponent (m). – This enables them to undergo high deformation before fracture. – When a plastic undergoes tensile stresses, the plastic changes in color due to the formation of microvoids. – The material becomes translucent. – This phenomena is referred to as Stress Whitening.

Commonly Known Thermoplastics – Acrylics – Polystyrene – Polycarbonates – Polyesters – Polytetrafluoroethylene (Teflon) – Polyethylene – Polyurethane – Polypropylenes – Polyvinyl Chloride (PVC) Vinyl See last slide for commercial names.

Plastics Review • Thermosets (TSs) – Long molecules are cross-linked three-dimensionally during polymerization – This creates one big molecule and a complex 3 D structure. – The name thermoset is due to the fact that the polymerization is complete and the final shape is set. – The reaction, in this case is IRREVERSIBLE. – Thermosets are rarely used in injection molding.

Plastics Review • Thermosets (TSs) – Thermosets have no set glass Transition temperature (Tg). – Temperature and Rate of Deformation do not affect the strength and hardness of thermosets. – Sample products: knobs, handles, switches.

Commonly Known Thermosets • Epoxy • Melamine • Natural Rubber • Polyester • Silicone. See last slide for more info.

Plastics Review • Biodegradable Plastics – The disposal of plastic products is a major concern, given the limited landfill space available. – Degrade over time with exposure to microorganisms in soil and water. • Some bioplastics available: – Starch-based: a pelletized form of starch is added to the plastic during processing. – Lactic-based: Lactic acid is polymerized to form a polyester resin. – Sugar-based.

Plastics Review • Elastomers (Es) & Rubbers – Amorphous polymers having a very low Tg. – Can undergo large elastic deformation without rupture. – Soft. – Very low Elastic Modulus – Return to original shape after an applied load is removed. – Rubber is a good example of cross-linked polymer. Its hardness increases with increased cross-linking of molecular chains.

Forming and Shaping of Plastics • Plastics can be shaped easily into many shapes with few operations. • Plastics melt at relatively low temperatures. • Properties of resulting parts depend on manufacturing method, process parameters.

Most common methods for Plastic Forming 1. 2. 2 a. 3. 4. 5. Extrusion Injection Molding Reaction Injection Molding Structural Foam Molding Blow Molding Rotational Molding 6. Thermoforming 7. Compression Molding 8. Transfer Molding 8 a. Foam Molding 9. Casting 10. Cold Forming

1. Extrusion • Raw Material: thermoplastic pellets, granules or powder. • Material is placed in hopper and fed into extrusion barrel. • Screw inside barrel blends pellets and conveys them down barrel. • Heaters and friction heat pellets and liquify them.

1. Extrusion (cont. ) • Screw – Feed section: conveys material from hopper to center of barrel. – Transition region: where melting begins. – Pumping section: where shearing, melting and pressure buildup begins.

1. Extrusion (cont. ) • You can adjust the length of these sections depending on the melting characteristics of different plastics. • Once molten plastic is fed through die, it is air-cooled or via cooling tubes. • Cooling rate is critical to avoid shrinkage, distortion. • Can make long sections with no problem due to continuous feed from hopper.

1. Extrusion (cont. ) • Sheet film extrusion – Performed with a flat extrusion die. (See figs. 18. 3, 4) – Plastic bags are made with an extruder (blown film process). – Thin walled tube is extruded vertically upward. – Air is blown through center and expand into a balloon shape. – Balloon is air cooled, blown film is wound up.

2. Injection Molding • A barrel is heated to promote melting. • Friction promotes heating dramatically. • Pellets are fed into heated cylinder. • Melt is forced into split die chamber by hydraulic plunger or rotating screw. • Thermosets require that mold be heated as well to promote polymerization & cross-linking. • Part cools (if thermoplastic) or cures (if thermoset). • Mold is opened, part is ejected.

2. Injection Molding (cont. ) • Mold components: – Runners – Cores – Cavities – Cooling Channels – Inserts – Knockout pins – Ejectors

2. Injection Molding (cont. ) • Other metal components can be placed inside the mold. • They become integral part of the product. – Example: electrical connectors/components. • Multi-component injection molding allows forming of parts having different shapes/colors. – Example: tail light lenses for automobiles.

2. Injection Molding (cont. ) • Process capabilities – Can achieve good dimensional accuracy, complex shapes. – Cycle time ranges 5 -60 seconds. – Molds made of tool steels -cost: hundreds of thousands. – Must justify expenditure with high volume production. – Cycles: • Steel molds: 2 million cycles - Aluminum 10, 000 cycles.

2. Injection Molding (cont. ) • Machines – Injection molding machines are typically horizontal (fig. 18. 7) – Typical pressures in injection molding: 10, 000 30, 000 psi. – Forces range from 100 to 250 tons. – Largest machine: 5000 tons - makes parts up to 55 lbs.

3. Injection Molding (cont. )

2 a. Reaction Injection Molding • Resin and catalysts are forced into mold cavity at high speed. • Reaction is fast, part solidifies into a thermoset. • Can add reinforcing fibers to increase product strength & stiffness. – Typical parts: bumpers, fenders, water skis.

3. Structural Foam Molding • Used to make plastic products with a solid outer skin and a cellular inner structure. • Injection Foam Molding uses a blowing agent (Nitrogen gas) to expand the material. • Can create skin thicknesses as much as 2 mm. • Good stiffness to weight ratio. – Typical parts: furniture parts, TV shells, computer printer housings.

3. Structural Foam Molding

4. Blow Molding • Extrusion Blow Molding – A tube is extruded & then clamped into a mold with larger cavity cross section. . – Tube is blown outward to fill cavity (50 - 100 psi). – All ends of mold close in as air is injected into tubular piece. – Part is cooled and ejected.

4. Blow Molding • Injection Blow Molding – A short piece tubular piece is mold-injected (called the parison). – Dies open, parison is transferred to blowmolding die. – Hot air is injected, expands to walls of cavity. • Typical products: plastic bottles, hollow containers, shampoos.

4. Blow Molding

5. Rotational Molding • Can form large hollow parts by this method. – Two piece mold is designed such that it can be rotated about 2 perpendicular axes. – Pre-measured amount of powder plastic is placed inside warm mold. – Mold is heated further and rotated about the two axes. – Powder tumbles against mold walls, causing it to fuse without melting. – Additional cross-linking occurs via a chemical agent. • Typical products: trash cans, buckets, toys, boat hulls.

6. Thermoforming • Used forming TP sheet or film over a mold. • Done by applying heat and pressure. – Sheet is heated to the sag point (soft but not to he point of melting) – Sheet is taken form oven, placed over mold, forced against it via a vacuum. – Mold is at room temp. , and plastic takes shape of mold immediately.

6. Thermoforming (cont. ) – Material undergoes drawing (vertically) and stretching (horizontally). – It must exhibit high uniform elongation (high m). – Parts with holes cannot be made by this method. – Typical products: refrigerator liners, packaging, appliance housings.

7. Compression Molding • Very similar to forging. • A pre-measured amount of material is placed into heated mold cavity. • Dies are moved/pressed together, forming the part. • An amount of flash results, which is trimmed off.

7. Compression Molding (cont. ) • Primarily for TSs: material is partially polymerized prior to molding. • Cross linking is finished in heated die: times range from ½ to 5 minutes. • Typical parts: dishes, container caps, handles, washing machine agitators.

8. Transfer Molding • Similar to Compression Molding, but dies are not preheated. • Friction caused by ram forcing material into mold generates frictional heat. • Heat makes material homogenous. • Curing takes place by cross linking – Typical parts: electrical connectors, rubber and silicone parts.

8 a. Foam Molding • Polystyrene beads are placed in mold cavity. • Heat is added via steam and air. • Beads expand as much as 50 times to take shape of mold. • Expansion is controlled by varying time and temperature. – Typical products: styrofoam cups, food containers, packaging materials for appliances.

8 a. Foam Molding (cont. ) • Can also use polyurethane foam. • Mix two or more chemicals to form a cellular structure. • This fills and solidifies in mold. – Typical uses: auto seat cushions, insulation, packaging.

9. Casting • Some TPs (nylons, acrylics) and TSs (epoxies, polyester) can be cast in rigid or flexible molds. • Conventional Casting: – TPs: create mixture of monomer, catalyst, heat and pour into mold – Polymerization occurs at ambient pressure.

9. Casting (cont. ) • Potting & Encapsulation: – Involves casting the plastic around another component to embed it in the plastic. – Typical applications: electrical plugs, coils, solenoids, electronic ignitions.

10. Cold Forming Processes • Processes used to form many TPs at room temperature (cold forming) (10 - 20 deg C) • Processes include rolling, extrusion, deep drawing, others. • Materials include ABS, PVC, polypropylene, polycarbonate.

10. Cold Forming Processes • Materials for cold working must: – be ductile at room temp. (This eliminates polystyrene, acrylics, thermosets. ) – not recover from deformation. • Cold forming of plastics creates products with higher strength, toughness.

10. Cold Forming Processes • Elastomers – Usually by extrusion or injection molding, but also blow molding and thermoforming – Typical products: tubing, hoses, molding, inner tubes. • Rubber gloves are made by dipping a metal form (a hand) many times in a liquid compound that adheres to form, which is later vulcanized and stripped.

Summary

Some Common Thermoplastics

Some Common Thermosets