Processing, thermoplastics barrel materials

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). 

Injection molding is a cyclic discontinuous process used to form three-dimensional plastic parts (25. 26). Both thermoplastics and thermosets are injection molded. The process is shown schematically in Figure 11 for a reciprocating screw injection molder. This type of a machine has a screw rotating in a barrel similar to an extruder. However, here the melt flow is discontinuous and controlled via a check valve at the tip of the screw. Material is melted in the extruder, and the melt is accumulated in front of the screw tip until sufficient melt is at hand to fill the mold cavity (cavities). 

While injection molding machines designed specifically for processing thermoset materials are available, a number of manufacturers offer replacement screw and barrel assemblies and stuffer hoppers to convert conventional thermoplastic injection moulding machines to process thermosets. 

Injection moulding has been briefly described in Chapter 8. The use of filled materials does not significantly effect the moulding process although higher barrel temperatures and greater injection pressures may be required. Table 7 compares the properties of some reinforced and unreinforced thermoplastics and illustrates how the presence of reinforcing fibre improves most of the mechanical and physical characteristics of polymers. 

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