Moulding compression

Compression moulding is one of the most common methods used to produce articles from thermosetting plastics. The process can also be used for thermoplastics but this is less conunon - the most familiar example is the production of LP records. The moulding operation as used for thermosets is illustrated in Fig. 4.62. A pre-weighed charge of partially polymerised thermoset is placed in the lower half of a heated mould and the upper half is then forced down. This causes the material to be squeezed out to take the shape of the mould. The application of the heat and pressure accelerates the polymerisation of the 

During compression moulding, the charge of material may be put into the mould either as a powder or a preformed cake . In both cases the material is preheated to reduce the temperature difference between it and the mould. If the material is at a uniform temperature in the mould then the process may be analysed as follows. 

Consider a cake of moulding resin between the compression platens as shown in Fig. 4.63. When a constant force, F, is applied to the upper platen the resin flows as a result of a pressure gradient. If the flow is assumed Newtonian then the pressure flow equation derived in Section 4.2.3 may be used 

For the annular element of radius, r, in Fig. 4.63 it is more convenient to use cylindrical co-ordinates so this equation may be rewritten as 

Now if the top platen moves down by a distance, dH, the volume displaced is (nr dH) and the volume flow rate is nr dHjdt). 

The compression moulding process exerts a much lower level of shear on the material than injection moulding, which is a high shear method. 

Large UHMWPE sheets are commercially produced using compression moulding. Once the central temperature has reached 180 °C, the mould is slowly cooled to below the polymer crystalline melting point before the mould is opened and the part removed. 

PE grade Melting (°C) Preliminary compression Final compression Cooling  

Fibre reinforcement and resin are premixed, and other additives are added prior to compression moulding in a heated mould which triggers the hardening of the resin. Many forms of premix are available, making a variety of fibre arrangements possible  

This process has been described in a previous chapter as a means to process unreinforced resins. Fibre-reinforced composites can be fabricated in a similar way. The resin and fibre can be combined by usual hand lay-up or spray-up techniques. Glass fibre content up to 70 wt% or 50 vol% can be achieved for fibre composites processed by this technique. Press moulding offers obvious advantages like fast cycle time, good finish, structural integrity and high reinforcement content. 

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Both thermoplastics and thermosets can be formed by compression moulding (Fig. 24.5). The polymer, or mixture of resin and hardener, is heated and compressed between dies. The method is well suited to the forming of thermosets (casings for appliances, for instance) and of composites with a thermosetting matrix (car bumpers, for example). Since a thermoset can be removed while it is still hot, the cycle time is as short as 10 seconds for small components, 10 minutes for large tliick-walled mouldings. Pressures are lower than for injection mouldings, so the capital cost of the equipment is much less. 

Fig. 24.5. Compression moulding the thermoset is squeezed to shape and cured (by heating) at the some time. Once it has cured it con be token from the mould while still hot - so the process is faster than the moulding of thermoplastics.

Injection/compression moulding - Thermoplastics and thermosets Blow moulding - Thermoplastics Rotational moulding - Thermoplastics... 

Injection moulding - Elastomers Compression moulding - Composites Hand/spray lay-up - Composites... 

Note The injection/compression moulding process capability maps 1, 2 and 3 are used for large parts with a major dimension greater than 50 mm typically and/or for large production volumes. Map 4 is for injection moulded parts that have a major dimension less than 150 mm and which are produced in small volumes. 

The setting of these materials after shaping occurs via a chemical process, that of cross-linking. The most common process is moulding but some extrusion, sintering and other miscellaneous processes are also used. A typical compression moulding process is illustrated in Figure 8.12. 

Figure 8.12. Typical process for the compression moulding of thermosetting plastics, (a) Load, (b) Mould, (c) Eject, stripper in. (d) (After Groves )...

Compression moulding is used only occasionally with polyethylene. In this process the polymer is heated in a mould at about 150°C, compressed to shape and cooled. The process is slow since heating and cooling of the mould must be carried out in each cycle and it is employed only for the manufacture of large blocks and sheets, for relatively strain-free objects such as test-pieces and where alternative processes cannot be used because of lack of equipment. 

Table 16.5 Typical mechanical properties of compression-moulded polystyrene plastics as measured by appropriate ASTM tests...

Internal stresses occur because when the melt is sheared as it enters the mould cavity the molecules tend to be distorted from the favoured coiled state. If such molecules are allowed to freeze before they can re-coil ( relax ) then they will set up a stress in the mass of the polymer as they attempt to regain the coiled form. Stressed mouldings will be more brittle than unstressed mouldings and are liable to crack and craze, particularly in media such as white spirit. They also show a characteristic pattern when viewed through crossed Polaroids. It is because compression mouldings exhibit less frozen-in stresses that they are preferred for comparative testing. 

The polymer is not easy to process and in injection moulding melt temperatures of 300°C are employed. In order to prevent excess embrittlement by shock cooling of the melt, mould temperatures as high as 150°C may be used. The polymer may also be compression moulded at temperatures of 250-260°C. 

The first commercial grades were introduced by Phillips Petroleum in 1968 under the trade name Ryton. These were of two types, a thermoplastic branched polymer of very high viscosity which was processed by PTFE-type processes and an initially linear polymer which could be processed by compression moulding, including laminating with glass fibre, and which was subsequently oxidatively cross-linked. 

The thermosetting materials are said to be initially linear but are cross-linked by heating in air to a temperature of at least 345°C. It is claimed that they have a useful working range up to 315°C. The materials may be used in compression mouldings powders, as the binder resin in glass cloth laminates and as the polymer base in heat-resistant metal coatings. 

Fabrication is most conveniently carried from solution to produce insulating film, wire lacquers and even fibres. The polymers have also been compression moulded. Some properties of two experimental Exxon grades are given in Table 21.8. 

Table 21.8 Some properties of compression moulded polyfparabanic acid) materials at 23°C22...

It is likely that the quinone methide and related structures formed at these temperatures account for the dark colour of phenolic compression mouldings. It is to be noted that cast phenol-formaldehyde resins, which are hardened at much... 

Conventional U-F moulding powders originally formulated for compression moulding are often satisfactory but special injection grades have been developed. Ideally the moulding material should have good flow characteristics between 70... 

Whilst the injection moulding process has now been widely accepted for phenolics the transition from compression moulding has been less extensive with U-F materials. The basic reason for this is that the U-F materials are more difficult to mould. This has been associated with filler orientation during moulding, which can lead to stress peaks in the finished product which the somewhat brittle resin in less able to withstand than can a phenolic resin. 

In order to overcome such disadvantages the injection-compression process has been developed. A conventional compression press is coupled to a screw preplasticising unit which can deliver preheated and softened material direct to a compression mould cavity. 

One limitation of epoxide moulding compositions is their short shelf life (typically 1-3 months), which necessitates strict stock control. The compounds may be compression, transfer or injection moulded, although compression moulding is preferred for long-fibre grades. 

One resin, however, can be considered as an exception to this. Although rarely recognised as a plastics material it can be fabricated into pipe mouthpieces, cigarette holders and various forms of jewellery. It may also be compression moulded and extruded. It is the fossil resin amber. 

In this section a selection procedure will be developed for injection moulding, since this process is used for the widest range of materials. The choice available for other processes such as, for example, compression moulding, filament winding and vacuum forming, is much more restricted. The approach described will be less mechanistic than the systems described in the two previous sections, requiring the prospective user to be aware of the properties of the various materials available. Because the approach is somewhat different, it would be instructive to run it parallel to the above processes and compare the results. 

Plastics processing sheet No. 9 safety at compression moulding machines Safety at granulators... 

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