模具設(shè)計(jì)外文翻譯---塑料成型過程-模具設(shè)計(jì)-展示頁
2025-01-31 05:42本頁面
【正文】 d to form a homogeneous layer on the surface of the mould. The process is attractive for a number of reasons. Firstly, as it is a lowpressure process, the moulds are relatively simple and inexpensive. Secondly, the product is virtually stratafree. Thirdly, a uniform thickness can be easily achieved. Finally, it is possible to introduce reinforcement into the products, and their surface can be textured as desired. However, the cycle times are longer pared to blow or injection moulding. The mouldhandling device, capable of imparting double rotations, is the central element of rotational moulding equipment. There are two major types of equipment: shuttle cart system, as shown Figure 3, and swing/rotary arm system. Rotational moulding is good at producing very large, thickwalled articles which could not be produced economically by any other processes. The largest capacity of a rotationalmoulding made tank is recorded at about 75 000 litres [4]. FIGURE cart rotational moulding. The Institution of Professional Engineers New Zealand 4 Compression moulding Compression moulding is often used to produce articles from thermoset materials, though it can also be used for thermoplastics. The moulding operation used for thermosets is illustrated in Figure 4. A large number of pression moulded thermoset products can be found in electrical and electronic applications. Glassfibre reinforcement can be easily added to meet the heat resistance requirement. However, the limitation with this process is that the product has to be simple in shape and without thin walls or fragile inserts. Numerous rubber products are pression moulded. A useful feature of it is its ability to have metal inserts that form strong bonds with the product and are often used to attach the product to structures. Tyres are the most mon products made by pression moulding. FIGURE moulding. 5 Transfer moulding Transfer moulding is similar to pression moulding except that, instead of the moulding material being pressurised in the cavity, it is pressurised in a separate chamber and then forced through an opening and into a closed mould cavity. The advantage of transfer moulding is that the preheating of the material injected through a narrow orifice improves the temperature distribution in the material and hence accelerates the crosslinking reaction in thermosets. As a result the cycle time is reduced and there is less distortion in the product. The improved flow of material also means that more intricate shapes can be produced. Parts with fragile inserts like electric appliance parts, electronic ponents and connectors that may enclose coils, integrated circuits, and plugs can also be easily made. 6 Thermoforming Sheet thermoforming was developed in the 1950s. The limitations such 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 ofte
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