模具设计外文翻译塑料成型过程.docx

1、模具设计外文翻译塑料成型过程外文翻译原文 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 pro

2、cesses are thermoplastics 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 ExtrusionE

3、xtrusion is 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 ba

4、sically a process 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 commo

5、nly used machines. 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 possib

6、ilities in terms 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.

7、 Almost all 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 mouldingThis process begins wi

8、th the preparation 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 in

9、jection. Blow 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

10、other fluid 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

11、 in size, 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 moulding3 Rotational mouldingLike blow moulding, r

12、otational 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 heate

13、d, the 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 s

14、imple and 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 inje

15、ction moulding. 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,

16、 thick-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 Zealand4 Compression mouldingC

17、ompression 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 app

18、lications. 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 i

19、t 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 common products made by compression moulding.FIGURE 4.Compression moulding.5 Transfer mouldingTransfer moulding is similar to compression moulding

20、 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 impro

21、ves the temperature distribution in the material and hence accelerates the cross-linking 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

22、inserts like electric appliance parts, electronic components and connectors that may enclose coils, integrated circuits, and plugs can also be easily made.6 ThermoformingSheet thermoforming was developed in the 1950s. The limitations such as poor wall thickness distribution and large peripheral wast

23、e 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,

24、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 mater

25、ial.Products made by thermoforming can be small as well as large. Smaller products are made in high output machines, using multi-cavity 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

26、 can have relatively complex shapes with reasonably even thickness. Large products are generally made from cut sheets at a lower though-put rate, and they are usually of simple shapes. Fisher & Paykels vacuum form moulding machines produce the majority of pre-forms for refrigerators and freezers. Ma

27、ny other interior parts are also made by the same or similar processes. FIGURE 5.Three basic methods of thermoforming.7 Injection mouldingInjection moulding has always been one of the most common processing methods for plastics. Nowadays countless parts in many electrical appliances, automobiles and

28、 office equipment are injection moulded. The most common 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 t

29、he 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

30、, 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 ra

31、pidly enough to ensure complete filling of the cavity. A high holding or packing pressure is normally exerted, to partially compensate 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 conduc

32、tivity 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 productivity and allows the products to have many fine details such as bosses, loca