eat227-lecture 8- shaping process
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Lecture 8
SHAPING PROCESSES FOR POLYMERS
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PolymersA polymer is a compound consisting of long-chain molecules, each molecule made up of repeating units connected together.Polymers can be separated into three major groups: Thermoplastic polymers (thermoplastics) Thermosetting polymers (thermosets) Elastomers (rubbers)
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Thermoplastic polymers (thermoplastics)Solid materials at room temperature, become viscous liquids when heated.The most important thermoplastics are:
Acrylics (Plexiglas): lenses, window glazingFluorocarbons (Teflon): nonstick coatings, bearings, sealsPolyamides (Nylons, Kevlar): fibersPolycarbonates (Lexan): helmets, bullet-resistance windows, wind shieldsPolyesters (Dacron, Mylar, Kodel): gears, cams, rollersPolyvinyl chloride (PVC): pipes, cable insulation, packaging, flooring, toysPolyethylene: bottles, cans, packaging materials
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Thermosetting polymers (thermosets) When initially heated, soften and flow for molding. After cooling, harden into an infusible solid. No repeated heating cycle is possible. The most important thermosets are: Epoxies: fiber-reinforced materials Phenolics (Bakelite): knobs, handles, cases Polyesters: fiber-reinforced materials Silicones: waterproof and heat resistance materials
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Elastomers (rubbers) Exhibit extreme elastic extensibility under low mechanical stresses.The most important rubbers are: Natural rubber (Latex): tires, shoes, seals Silicones: seals, thermal insulation, electronics Polyurethane: seals, gaskets
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The difference in properties of the polymers are attributable to so-called cross-linking, which occurs in thermosets and partially in elastomers.Cross linking
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Manufacturing processes for polymersExtrusionIn twin-screw extruders both screws are parallel and side-by-side inside the barrel. These extruders aresuitable for extrusion of difficult-to-extrude polymers, and for materials that require greater mixing.
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Extrusion
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Dies for extrusion
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Injection MoldingInjection Molding is a process, in which a polymer is heated to a highly plastic state and forced to flow under pressure into a mold cavity, where it solidifies. The part, called a molding, is then removed from the cavityThe production molding cycle time is in the range 10 to 30 sec
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Typical molding cycle
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Mold Features for Injection Molding
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Blow MoldingBlow molding is a modified extrusion and injection molding process, wherein a tube is extruded, clamped into a mold with a cavity much larger than the tube diameter, and then blown outward to fill the moldBlowing is done with a hot-air blast at a pressure of 350~700 kPa.
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Compression MoldingIn compression molding, the workpiece (pre-shaped part, volume of powder, mixture of liquid resin and fillers) is placed in the heated mold and is formed under pressure.Compression molding of thermosets: (1) charge is loaded, (2) charge is compressed and cured, and (3) part is ejected and removed.
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Transfer MoldingTransfer molding is a similar to compression molding process, but the charge is placed not in the die cavity but into a chamber next to the die cavity. Pressure is then applied to force the material to flow into the heated mold where curing occurs.
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ThermoformingThermoforming is a process in which a flat thermoplastic sheet is heated and deformed into desired shape. The classical process involves the use of vacuum and is called vacuum forming.
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Calendering- used for shaping high melt viscosity thermoplastic sheet 1-feed pass2-metering pass3-sheet formation-gauging-finishing
Crown-crossing-bending