模具設(shè)計外文翻譯---塑料成型過程-模具設(shè)計(編輯修改稿)
2025-02-24 05:42 本頁面
【文章內(nèi)容簡介】 as poor wall thickness distribution and large peripheral waste restricted its use to simple packaging applications. In recent years, however, there have been major advances in machine design and materials, which have resulted in a wide range of products being made by thermoforming. There are three types of thermoforming processes (Figure 5): vacuum moulding, air pressure moulding, and mechanical moulding. The moulds, which are not subjected to high pressure, are often made from cast or machined alumininm for small and medium sizes, and they do not require a good surface finish. The product surface quality is largely dependent upon that of the sheet material. Products made by thermoforming can be small as well as large. Smaller products are made in high output machines, using multicavity moulds. Such products are often found in the food industry and medical applications, for example, jelly or cream containers, cups, robs and trays. These small items can have relatively plex shapes with reasonably even thickness. Large products are generally made from cut sheets at a lower thoughput rate, and they are usually of simple shapes. Fisher amp。 Paykel39。s vacuum form moulding machines produce the majority of preforms for refrigerators and freezers. Many other interior parts are also made by the same or similar processes. FIGURE basic methods of thermoforming. 7 Injection moulding Injection moulding has always been one of the most mon processing methods for plastics. Nowadays countless parts in many electrical appliances, automobiles and office equipment are injection moulded. The most mon injection moulding machinery is the reciprocating screw machine, whose process can be divided into several stages as seen in Figure 6. At the plastication stage, the feed unit operates as an extntder, melting and homogenising the material in the screw/barrel system. The screw, however, is allowed to retract in order to make room for the molten material in a space at the cylinder head, called material reservoir, between the screw tip and a closed valve or an obstntction of solidified material from the previous shot. At the injection stage, the screw is used as a ram (piston) for rapid transfer of the molten material from the reservoir to the cavity between the two halves of the closed mould. Since the mould is kept at a temperature below the solidification temperature of the material, it is essential to inject the molten material rapidly enough to ensure plete filling of the cavity. A high holding or packing pressure is normally exerted, to partially pensate for the thermal contraction of the material upon cooling. The cooling of the material in the mould often limits the production time because of the low thermal conductivity of polymers. The mould, after being cooled, can be opened and the solid product ejected. Although the screw machine is by far the most popular, plunger injection machines are also used to give products some unique features. There is no shearing or mixing action, as a plunger does not rotate. The resulting moulded part can take on a marbled appearance with swirls of two or more colours. This may be the desired finish for certain products. Regardless of different machines, injection moulding yields a high p
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