Thermoset injection moulding

The BMC materials are fed from a hopper into the plasticization section of a relatively low-temperature (to minimize cure) single-screw extruder, where the material is mixed, metered and sheared into a homogenous mixture and collected in an injection cavity is the end of the screw. Once this cavity has been filled the entire screw is hydraulically plunged forwards and the material in the extruder cavity is injected into the mould under high pressures (typically 15 000-30 000 psi). The mould is at high temperature to facilitate curing of the part in the mould. 

Quality-control tests and important process variables 

Typical systems include BMC and DMC polyester systems, which are usually filled with long ( lmm) glass fibres. 

Gibson and Williamson (1985b, 1985a) examined the shear and extensional flow of BMC materials in injection moulding. Extensional and shear viscosities are found to be important in injection moulding, and here they use the following power-law relationships  

Kamal and Ryan (1980) presented a review of early kinetic and chemorheological models of thermoset materials for injection moulding. They highlight the following model  

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Parker, F.J, The Status of Thermoset Injection Moulding Today, Progress in Rubber and Plastic Technology, October 1985, vol 1, No 4, pp 22-59. 

Thermoset injection moulding is a high-pressure process typically used for DMC or BMC polyester materials, and is shown in Figure 6.18. 

Although the above are the major moulding processes for thermosets, injection moulding can now be used for the selected thermoset moulding operations. The major use of thermosets still remains with screw closures. These are made on either a multicavity platen tool (up to 96 cavities) or a rotary type press which resembles a rotary tableting machine. 

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. 

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

Deformation of a polymer melt—either thermoplastic or thermosetting. Processes operating in this way include extrusion, injection moulding and calendering, and form, in tonnage terms, the most important processing class. 

In ntosl respects the process is similar to the injection moulding of thermoplastics and the sequence of operations in a single cycle is as described earlier. For thermosets a special barrel and screw are used. The screw is of approximately constant depth over its whole length and there is no check value which might cause material blockages (see Fig. 4.50). The barrel is only kept warm (80-110°C) rather than very hot as with thermoplastics because the material must not cure in this section of the machine. Also, the increased viscosity of the thermosetting materials means that higher screw torques and injection pressures (up to 200 MN/m are needed). 

For the same part, injection moulding of thermosets can offer up to 23% production increase and lower part-costs than compression. 

The success of transfer moulding prompted further developments in this area and clearly it was only a relatively small step to an injection moulding process for thermosets as described in Section 4.3.10. 

Reaction injection moulding (RIM), commonly used with thermosets, is sometimes used for specific thermoplastics such as Nyrim. Nyrim is supplied in two parts ... 

Injection moulding is by far the most frequently applied technique for manufacturing end-products directly from a thermoplastic raw material. In this process articles are formed by injecting the molten polymer into a cooled mould. Also on rubbers and thermosets this method is applied, though at a smaller scale in these cases the mould is heated rather than cooled in order to promote the vulcanisation or curing reaction. 

The metathesis polymerisation of dicyclopentadiene, an inexpensive monomer (commercially available cyclopentadiene dimer produced by a Diels-Alder addition reaction containing ca 95 % endo and ca 5 % exo form), leads to a polymer that may be transformed into a technically useful elastomer [144-146, 179] and thermosetting resin [180,181]. The polymerisation has characteristics that make it readily adaptable to the reaction injection moulding ( rim ) process [182], The main feature of this process comes from the fact that the polymerisation is carried out directly in the mould of the desired final product. The active metathesis catalyst is formed when two separate reactants, a precatalyst (tungsten-based) component and an activator (aluminium-based) component, are combined. Monomer streams containing one respective component are mixed directly just before entering the mould, and the polymerisation into a partly crosslinked material takes place directly in this mould (Figure 6.5) [147,168,183-186],... 

In recent years a new type of thermosetting polymer based on polydicyclopen-tadiene has been developed and marketed. Usually this is amber in colour and it is made by a reaction injection moulding process employing various catalysts ... 

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