Injection Molds 模料模具毕业设计外文翻译.docVIP

2.3 Injection Molds 2.3.1 Injection Molding Injection molding is principally used for the production of thermoplastic parts, and it is also one of the oldest. Currently injection-molding accounts for 30% of all plastics resin consumption.Typical injection-molded products are cups, containers, housings, tool handles, knobs, electrical and communication components (such as telephone receivers), toys, and plumbing fittings. Polymer melts have very high viscosities due to their high molecular weights; they cannot be poured directly into a mold under gravity flow as metals can, but must be forced into the mold under high pressure. Therefore while the mechanical properties of a metal casting are predominantly determined by the rate of heat transfer from the mold walls, which determines the grain size and grain orientation in the final casting, in injection molding the high pressure during the injection of the melt produces shear forces that are the primary cause of the final mole cular orientation in the material. The mechanical properties of the finished product are therefore affected by both the injection conditions and the cooling conditions within the mold. Injection molding has been applied to thermoplastics and thermosets, foamed parts, and has been modified to yield the reaction injection molding (RIM) process, in which the two components of a thermosetting resin system are simultaneously injected and polymerize rapidly within the mold. Most injection molding is however performed on thermoplastics, and the discussion that follows concentrates on such moldings. Chapter 2 Plastics Molds A typical injection molding cycle or sequence consists of five phases (see Fig. 2-1): (1) Injection or mold filling; (2) Packing or compression; (3) Holding; (4) Cooling; (5)Part ejection. Plastic pellets (or powder) are loaded into the feed hopper and through an opening in the injection cylinder where they are carried forward by the