注塑指导机械工程英语unit06ME

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'注塑指导机械工程英语unit06ME'
Unit 6 Injection Molding Injection molding 喷射造型法(Fig.6.1) is the predominant process for fabrication of thermoplastics热塑性塑料 into finished forms, and is increasingly being used for thermosetting plastics热固性塑料, fiber-filled composites, and elastomers人造橡胶.It is the process of choice精选的 for tremendous variety of parts ranging in weight from 5g to 85kg. It is estimated that 25% of all thermoplastics are injection molded. If newer modifications, such as reaction injection molding真空成型, and the greatly increased rate of adoption of plastics as substitutes for metals are considered, it is likely that the worldwide industrial importance of injection molding will continue to increase. Currently, probably close to half of all major processing units is injection molding machines. In 1988, a dollar sale of new injection molding machinery in the U. S. was approximately 65% of total major polymer machinery sales volume; this included 4,600 injection-molding units. The machines and their products are ubiquitous到处存在的 and are synonymous with plastics for many people. A reciprocating screw injection-molding machine往复螺杆式注塑机 combines the functions of an extruder and a compressive molding press. It takes solid granules of thermoplastic resin热塑树脂, melts and pressurizes them in the extruder section, forces the melt at high velocity and pressure through carefully designed flow channels into a cooled mold, then ejects the finished part(s), and automatically recycles. This machine is a descendant of the plunger type "stuffing machine" patented by the Hyatt brothers in 1872 to mold celluloid赛璐珞. In 1575, the Hyatts developed the first multicavity mold, but it was not until 1938 that Quillery (France) patented a machine incorporating a screw to plasticize the elastomer being molded. ----In 1956, Ankerwerk Nuremberg commercialized the modem (modern?) reciprocating screw injection molding machine for thermoplastics. Today, over 50 machine manufacturers are listed in Modern Plastics Encyclopedia, offering machines to the U.S. market ranging from 2 to 6,000 tons clamping capacity. (A machine with a 10,000-ton capacity has been built to mold 264-gallon HDPE (high-density polyethylene 高密度聚乙烯) trash containers.) A host of suppliers of auxiliary equipment, molds, instruments, and controls service this major segment of the polymer industry.Injection molding is particularly worthy of intensive study because it combines many areas of interest extrusion, mold design, rheology 流变学, sophisticated hydraulic and electronic controls, robotic accessories自动机械, design of complex products, and, of course, the integration of materials science and process engineering. The objectives of injection molding engineers are simple enough: to obtain minimum cycle time with minimum scrap, to attain specified product performance with assurance, to minimize production costs due to downtime or any other reasons, and to steadily increase in expertise专门技术 and competitiveness. Profit margins利润率 for custom injection molders are said to be generally skimpy不足的, 吝啬的; an established way to improve profits is to be selected for more demanding, higher margin利润 jobs that demand the highest level of efficiency and competence能力.This text will concentrate on the reciprocating screw machine for thermoplastics, which has largely replaced the older reciprocating plunger types except for very small-capacity machines.Injection Molding Materials It is not possible to injection-mold all polymers. Some polymers like PTFE 聚四氟乙烯(Poly 聚乙烯-tetra]脂鲤-fluoro氟石 ethylene乙烯) cannot be made to flow freely enough to make them suitable for injection molding. Other polymers, such as a mixture of resin and glass fiber in woven or mat席子form, are unsuitable by their physical nature for use in the process. In general, polymers which are capable of being brought to a state of fluidity can be injection-molded. The vast majority of injection molding is applied to thermoplastic polymers. This class of materials consists of polymers which always remain capable of being softened by heat and of hardening on cooling, even after repeated cycling. This is because the long-chain molecules of the material always remain as separate entities实体and do not form chemical bonds to one another. An analogy car, be made to a block of ice that can be softened (i. e. turned back to liquid), poured into any shape cavity, and then cooled to become a solid again. This property differentiates thermoplastic materials from thermosetting ones. In the latter type of polymer, chemical bonds are formed between the separate molecule chains during processing. In this case the chemical bonding referred to as cross-linking is the hardening mechanism. In general, mo
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