Machinery's Handbook, 31st Edition
586 MANUFACTURING WITH PLASTICS Processing of Thermosets.— While some thermosetting materials have been adapted to injection molding processes, most are still processed by compression molding, transfer molding, prepreg molding, sprayup, and hand layup. Commonly used compression mold ing is a process in which precompounded and catalyzed resins, still thermoplastically formable, are preformed into buns or bricks of known weight. One or more bricks are preheated in convection or electronic ovens and quickly positioned in the female half of a horizontal mold. It and the hovering male half are at the molding temperature. The mold is closed and brought to high pressure, squeezing the softened bricks, which, ideally, flow into all parts of the mold cavity. Pressure is maintained for the time required to complete the chemical cross-linking reaction and to solidify the molding, after which it is ejected from the mold, any slight leakage at the parting line ( flash ) is trimmed, and it is set aside to cool. With the correct choice and dose of catalyst, time in the mold need not be long. As with injection molding, the high pressures dictate massive molds and presses. Compression molding is also an efficient way to process unsaturated polyesters, per haps the most versatile and user-friendly resins available for reinforced plastics process ing. When reinforced with chopped glass and mixed to a dough-like consistency, these compounds, known as premix or “gunk,” have been widely used in the automotive and boat industries. Compression molding is also used with sheet-molding compounds. Glass-fiber cloths are preimpregnated with resin and need only heat and pressure to cure them into finished products. If a mold is provided with both heating and cooling, thermoplastics, too, may be com pression molded, an obviously slow process, but sometimes convenient for making proto types and test pieces. The use of heated and cooled molds with compression is also used in thermal imprint lithography to form micro- and nano-sized features on the cavity surface. In transfer molding , also called plunger molding , the thermosetting compound is pre heated in a separate cylinder (the pot) until it is fully fluid, then forced by a plunger through a runner into the final mold. This process is capable of molding complex parts that cannot be made by compression molding, and also offers closer dimensional tolerances and glossier surfaces than the latter. Multi-cavity molds and short cycle times are feasible with transfer molding. Transfer molding has been used widely in the electronics industry and in molding and overmolding of silicone products, including medical devices. Another popular method of processing thermosets into unidirectionally reinforced prod ucts is pultrusion , in which continuous glass roving or mat is pulled through a resin bath or impregnator and then into a preforming fixture where the cross section is partly shaped. The preliminary profile continues through dies where curing mechanisms are applied, and finally to a cut-off saw to produce shapes such as channels, rods, and tubes. Ladder rails, fishing rods, tool handles, tent poles, and numerous other parts are made by this process. Polyurethanes.— Polyurethanes (PUR) are among the most versatile of polymers, both as thermoplastic and thermosetting, as well as the most ubiquitous in everyday products. For example, the flexible thermoplastic members of the family are mainly elastomers that are favorites for athletic shoes, while the rigid, transparent thermoplastics are vying with polycarbonate for the high-value market in eyeglass lenses. Cast rigid PUR is available in rod, tube, and bar stock form. The thermosets are formed by linking two reactive components, an isocyanate and a polyol. Toluene diisocyanate and diphenylmethane diisocyanate are the most used isocyanates, while a variety of polyols are commonly used. Short-chain, low-molecular- weight types result in rigid plastics, and longer-chain types form flexible materials. About 70 percent of all PURs worldwide go into automotive, building and construction, mat tresses, and furniture including frame members produced by reaction injection molding. Densities of rigid PUR foams range from 1 to 8 lb/ft 3 (16–128 kg/m 3 ) and much higher in furniture members. Flexible PUR foams range from near that of the parent plastic down to 1 lb/ft 3 (16 kg/m 3 ). Since the reactive components must be mixed instantly and thoroughly at the moment of application, a special industry of PUR processing machinery has devel oped to deal with that requirement.
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