Processing of Plastic Materials

Processing of Thermoplastic materials Injection Molding: A Molding process where a heat-softened plastic material is forced by a screw-drive cylinder into a relatively cool mold cavity that gives the plastic the desired shape. More uniform delivery of melt for injection. High quality, low labor cost, but high initial cost.

Extrusion: The forcing of softened plastic material through an orifice, producing a continuous product ( plastic pipe and sheet )

Blow molding - Compressed air is blown into heated cylinder or pipe of plastic to press it against the wall of mold.

Processing of Thermosetting materials Compression molding: Pressure is applied on heated plastic by upper mold and the molten plastic fills the cavities. Low initial cost, simple. Less wear and abrasion of molds. Difficult to mold complex parts Creates flash (spills)

Transfer Molding A plunger forces plastic resin, placed outside mold, into mold cavities through runners and gate. No flash formed. Multiple parts at a time. Can be used for small and intricate parts. Injection molding is also used to process thermosetting plastics. Special heating-cooling jackets are added to standard injection molding machine. 7

Elastomers (Rubbers) H CH3 H H C C C C H H n Natural rubber: Produced from latex of Havea Brasiliensis tree. Vulcanization: Heating rubber with sulfur and lead carbonate. Increases tensile strength. Restricts molecular movement by crosslinking of molecules High modulus Can be elastically deformed when a force is applied to them and can return to their original shapes (or almost) when the force is released. H CH3 H H C C C C H H n 8

Natural rubber

Natural Rubber - Properties 10

Samples and Application of Rubber Elastomeric materials include natural rubber, synthetic polyisoprene, styrene-butadiene rubber, nitrile rubbers, polychloroprene, and the silicones.

synthetic polyisoprene nitrile rubbers polychloroprene styrene-butadiene rubber Silicon