Moulding processes thermoset

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.

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

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. 

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 ... 

Open-mould processes are thermoset moulding processes with open moulds in the male or female form. In this process the mould is made from a pattern and the reinforced thermoset is applied to the mould to reproduce the shape and finish of the mould. Typical examples would be boat hulls made from fibreglass-reinforced gel coats. Figure 6.10 shows the basic contact-moulding process. 

This process is frequently used for more intricate mouldings. The thermoset powder is placed in a heated chamber, where it is liquefied and then transferred by a piston through a channel into a second mould, where the material is kept heated under pressure while the curing cycle is completed. 

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. 

This technique is a more recent process, designed for thermosetting resins. The method takes the principle of the ordinary injection moulding process, together with features of the traditional polyester laminating procedures mentioned later in this chapter. It makes large mouldings, for example, vehicle body panels. 

The shell-moulding process uses a thin expendable mould or shell made from a fine silica or zircon sand bonded with a thermosetting resin. The fineness of the sand influences the surface of the finished casting. 

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. 

Molding compound is transferred in the cold state by pressure from the material hopper into the main injection chamber. Here it can be preheated before injection into the heated mould tool. Injection, through a special nozzle, can be either by ram or screw pressure. If screw feed is used, the screw must be of the type designed to process thermosets as opposed to thermoplastics. 

Sheet Moulding Compound (SMC)—A mixture of fibres, fillers and thermosetting resin in sheet form similar to DMC, but having longer fibres and higher fibre content. It is used in hot press moulding processes. 

These thermosetting resins contain a urethane prepolymer in methyl methacrylate monomer. Although they are cured in a similar fashion to polyesters, their initial viscosity is much lower and the cure reaction is more rapid. The speed of cure along with the odour of the monomer preclude the use of open mould processes, but these resins are especially suitable for fast processing by methods including pultrusion and resin... 

A less-important process is compression moulding of thermosetting resin compounds. These are often phenol-formaldehyde resins reinforced with fibrous fillers. 

In the case of moulding thermosets, the uncured (initial) compound is usually placed first in the cavity of the mould, or the transfer pot of a transfer moulding process, and heated (to about 150 °C) to provide sufficient flow for mould filling. Pressure (approximately 13 MPa) is then applied for sufficient time to allow the resin to cure. 

New glass fibre-reinforcement products for u.se in fibre-directed preform processes are another focus of development, designed specifically for liquid composite moulding applications such as resin transfer moulding with thermoset polyesters. 

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