Compression moulding thermosets

Figure 14.20 These objects are made from compression-moulded thermosetting plastics.

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... 

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

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. 

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... 

Fairbanks [100] has also used ultrasound (75 W) to compression mould both thermoplastic powders (acrylics and vinyls) and thermoset powders (phenolics and allylics). The optimum conditions to produce maximum fusion are given it Tab. 5.26. 

Tab. 5.26 Optimum conditions for ultrasonic compression moulding of thermosets and thermoplastics.

Thermoplastics are easily moulded or formed into useful articles. Once they are molten they can be injected or blown into moulds, and a variety of different-shaped items can be produced (Figure 14.19). Thermosetting plastics can be heated and moulded only once, usually by compression moulding (Figure 14.20). 

Thermoplasts, thermosets and rubbers can be processed by compression moulding. For thermoplasts this technique is an exception the best known example is the manufacture of gramophone records from a copolymer of vinyl chloride and vinyl acetate (chosen to enable a very good flow into all details of the mould). 

Compression moulding of rubbers is not essentially different from that of thermosets. The starting material is a blend of a rubber, vulcanisation ingredients and... 

Thermosetting caps are compression moulded on either platen or rotary-type equipment. Pressure, dwell time and temperature are critical to the curing operation, otherwise brittle or soft caps may be produced. 

ISO 295. Plastics—Compression moulding of test specimens for thermosetting materials. International Organization for Standardization, 1991. 

BS2782. Part 9, Method 902A, Methods of testing plastics Compression moulding test specimens of thermosetting materials. British Standards Institute Publication. 1992. 

ISO 295. Compression moulding test specimens of thermoset materials. 

In compression moulding, both thermoplasts as well as thermosets can be processed. Polymer granules or resin materials are placed between two complementary moulds as indicated in Fig. 21.16a. 

Compression moulding The disk is moulded directly onto the machined recess on the cathode sheet using a thermosetting plastic with silica fiber fillers. 

Compression moulding is used chiefly with thermosetting plastics such as urea and melamine formaldehyde, epoxies and phenolics. 

The main limitation of the process is the loss of material in the sprue, runners and gates - as thermosetting materials cure during moulding, this cannot be reused. Moulding tools are usually more complex and therefore more costly than compression-moulding tools. 

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