1、外文翻译原文 Plastics forming processes There is a wide range of processing methods that may be used for plastics. Nevertheless, they all involve three or four basic stages: softening, shaping, solidifying and cooling of the moulds (for thermoplastics only). Common materials for moulding processes are the
2、rmoplastics and thermoset polymers. Principal methods of processing thermoplastics include extols ion, blow moulding, rotational moulding, thermoforming and injection moulding； but as for thermosets, compression, transfer and reaction injection moulding are frequently used. 1 Extrusion Extrusion is
3、one of the most important forming processes for the reason that pellets, which are used for many other moulding processes, are normally produced by this process. In fact, some moulding processes are post-extrusion operations, such as blow moulding and thermoform moulding. Extnlsion is basically a pr
4、ocess of continuously shaping a fluid polymer through the orifice of a die, and subsequently solidifying it into a product of a uniform cross-section. An extruding machine may have one or two screws, or no screw (screwless). Single-screw extruders, as seen in Figure 1, are the most commonly used mac
5、hines. Screwless (ram) extruders allow a precise control of the melt flow rate and are gaining popularity. They are particularly suited for high viscosity. In recent years, there has been a steady increase in the use of twin screw extruders. These machines permit a wider ranger of possibilities in t
6、erms of output rates, mixing efficiency and heat generation. They are, however, considerably more expensive.Common extrusion products include filaments of circular cross-section, profiles of irregular cross-section, axisymmetric tubes and pipes, and flat products such as films or sheets. Almost all
7、types of intricate cross-sectional shapes with large lengths are made by extrusion moulding, which many other discrete forming processes, such as compression, transfer and injection moulding, are incapable of producing. FIGURE 1.Single-screw extruder. 2 Blow moulding This process begins with the pre
8、paration of a soft, extruded and preformed thermoplastic tube over a core pin.As the mould halves close, air pressure inflates the thinwalled preform and forces it outwards against the mould sides. Figure 2 shows the process at two stages. The preform can be made by either extras ion or injection. B
9、low moulds are subjected to moderate pressures and clamping forces, compared to injection moulds. Thus, they can be made of a light material such as aluminmm, which has advantages of light weight and high heat conductivity. Blown-ware containers are commonly used for packaging beverage and other flu
10、id food, e.g. narrow neck plastic bottles for mineral water, milk, alcoholic beverage and carbonated beverages. Other non-food products packed in the blown-ware containers include cosmetics, pharmaceuticals, paint and powder products. Blow moulding is also used to produce some huge products in size,
11、 such as shipping drums and stationary storage tanks whose volumes may reach as high as 10 000 litres 5. These tanks are used for underground fuel storage and septic tanks. Stage 1: Preform extrusion Stage 2: Blowing FIGURE 2.Extrusion blow moulding 3 Rotational moulding Like blow moulding, rotation
12、al moulding is also used to produce hollow plastic articles, though the principles in each method differ a lot. During the process, a carefully weighed charge of plastic powder is placed in one half of a metal mould. The mould halves are then clamped together and heated on an oven. When heated, the
13、mould rotates about two axes at right angles to each other. After a time the plastics will be sufficiently softened 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 low-pressure process, the moulds are relatively simple a
14、nd inexpensive. Secondly, the product is virtually strata-free. 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 compared to blow or injection m
15、oulding. The mould-handling 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, thick-
16、walled articles which could not be produced economically by any other processes. The largest capacity of a rotational-moulding made tank is recorded at about 75 000 litres 4. FIGURE 3.Shuttle cart rotational moulding. The Institution of Professional Engineers New Zealand 4 Compression moulding Compr
17、ession 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 compression moulded thermoset products can be found in electrical and electronic applica
18、tions. Glass-fibre 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 compression moulded. A useful feature of it is
19、 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 common products made by compression moulding. FIGURE 4.Compression moulding. 5 Transfer moulding Transfer moulding is similar to compression 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
