Plastics are usually made by a process called polymerization, in which many molecules of the same kind are combined to make networks of giant particles. All plastics can be formed or shaped; some become pliable under heat, some at elevated room temperatures. When treated, some plastics become hard, some incredibly strong, some soft like putty.
Plastic objects are formed using several different methods. Each method produces a different type of plastic. In compression molding, plastics compounds are compressed and treated inside a mold to form them. In injection molding, liquid plastic is injected into a mold and hardened. Blow molding is like glass blowing—air is forced into plastic to make it expand to the inner surface of a mold. In extrusion, hot plastic is continuously forced through a die to make products like tubing. Laminating involves fusing together resin-soaked sheets, while the calender process, or calendering, forms sheets by forcing hot plastic between rollers. Finally, in fabrication, workers make items out of solid plastic pieces by heating, sawing, and drilling.
While plastics compounds may be mixed in plastics-materials plants, plastics manufacturers sometimes employ blenders, or color mixers, and their helpers to measure, heat, and mix materials to produce or color plastic materials. Grinding-machine operators run machines that grind particles of plastics into smaller pieces for processing. Pilling-machine operators take plastics powder and compress it into pellets or biscuits for further processing. Other workers are responsible for making the molds (plastic form makers) and patterns (plastics patternmakers) that are used to determine the shape of the finished plastics items. Foam-machine operators spray thermoplastic resins into conveyor belts to form plastic foam.
Many plastics products plants make goods according to clients' specifications. When this is the case, job setters, using their knowledge of plastics and their properties, adjust molding machines to clients' instructions. They make such adjustments as changing the die through which the plastic flows, adjusting the speed of the flow, and replacing worn cutting tools when necessary. Then the machine is ready to accept the plastic and produce the object.
Injection molders operate machines that liquefy plastic powders or pellets, inject liquid plastic into a mold, and eject a molded product. Compact discs, toys, typewriter keys, and many other common products are made by injection molding. Injection workers set and observe gauges to determine the temperature of the plastic and examine ejected objects for defects.
One common plastic is polystyrene, which when molded using heat and pressure makes cast foam products such as balls, coolers, and packing nests. Polystyrene-bead molders operate machines that expand these beads and mold them into sheets of bead board. Polystyrene-molding-machine tenders run machines that mold pre-expanded beads into objects. At the end of the molding cycle, they lift the cast objects from the molds and press a button to start the machine again.
Extruder operators and their helpers set up and run machines that extrude thermoplastics to form tubes, rods, and film. They adjust the dies and machine screws through which the hot plastic is drawn, adjust the machine's cooling system, weigh and mix plastics materials, empty them into the machine, set the temperature and speed of the machine, and start it.
Blow-molding-machine operators run machines that mold objects such as bleach bottles and milk bottles by puffing air into plastic to expand it. Compression-molding-machine operators run machines that mold thermosetting plastics into hard plastic objects. Thermosetting plastics are those that harden because of a chemical reaction rather than by heating and cooling.
Casters make similar molded products by hand. Strippers remove molded items from molds and clean the molds. Some molded products must be vacuum cured and baggers run machines that perform this task.
Plastic sheeting is formed by calender operators, who adjust the temperature, speed, and roller position of machines that draw plastic between rollers to produce sheets of specified thickness. Stretch-machine operators stretch plastic sheets to specified dimensions. Preform laminators press fiberglass and resin-coated fabrics over plaster, steel, or wooden forms to make plastic parts for boats, cars, and airplanes.
Other common plastics products are fiberglass poles and dowels. Fiberglass-dowel-drawing-machine operators mount dies on machines, mix and pour plastics compounds, draw fiberglass through the die, and soak, cool, cure, and cut dowels. Fiberglass tube molders make tubing used in fishing rods and golf club shafts.
Plastics that are not molded may be cut into shapes. Shaping-machine operators cut spheres, cones, blocks, and other shapes from plastic foam blocks. Pad cutters slice foam rubber blocks to specified thicknesses for such objects as seat cushions and ironing board pads.
Many products undergo further processing to finish them. Foam-gun operators reinforce and insulate plastic products such as bathtubs and auto body parts by spraying them with plastic foam. Plastic-sheet cutters use power shears to cut sheets, following patterns glued to the sheets by pattern hands. Sawyers cut rods, tubes, and sheets to specified dimensions. Trimmers trim plastic parts to size using a template and power saw. Machine finishers smooth and polish the surface of plastic sheets. And plastics heat welders use hot-air guns to fuse together plastic sheets.
Hand finishers trim and smooth products using hand tools and sandpaper. Buffers remove ridges and rough edges from fiberglass or plastic castings. Sponge buffers machine-buff the edges of plastic sponges to round them, and pointing-machine operators round the points on the teeth of plastic combs. Edge grinders tend machines that square and smooth edges of plastic floor tile.
Assemblers and laminated plastics assemblers and gluers assemble pieces to form certain products. These may include skylights and wet suits. Plastics inspectors inspect and test finished products for strength, size, uniformity, and conformity to specifications.
Experienced workers supervise plastics-making departments, and the industry also employs unskilled workers such as laborers to help haul, clean, and assemble plastics materials, equipment, and products.
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