Injection moulding machine section

PP-structural foam mouldings were produced on an injection moulding machine in a pre-pressurised mould cavity by the classical low-pressure process and an alternative low-pressure process. Melt temperature, injection direction and sprae diameter were varied. Cross-sections cut from the middle of the small cylinder in longitudenal orientation were investigated by site-resolved X-ray scattering. Morphological properties were investigated. 4 refs. 

Preparation of the compound for the moulding process, as for all injection processes, needs to be carefully controlled. The stock needs to be presented to the mould in a clean, fresh state. The strip must be free from excessive amounts of anti-tack agents and should be of a continuous and uniform section that will feed without hindrance into the injection moulding machine screw. If the compound is based on butadiene-acrylonitrile copolymer, or similar polar materials, then it is necessary to ensure that the anti-tack agent used is based on zinc stearate. 

Section of injection moulding machine with mould at right. 

The injection moulding machine is an 80-ton clamp Negri-Bossi. Temperatures in plasticizing and injection sections are 185 C. The mould cooling water is at 70°C. 

Multi-shot processes, as the name implies, require multiple shots of material to make a single component. For each one of these materials an injection unit is required. To mould these multiple shots also requires special tooling and equipment. Multi-shot capability can be built either into the injection tool or controlled by the injection moulding machine. To enable multi-shot, multiple injection units can be arranged to feed machines in a number of ways as the next section will explain. 

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). 

The pressure exerted on the rubber in the cavity is related to the available pressure from the moulding machine, the speed of injection and the size of the runner through which it must flow to reach the mould cavity. The smaller the cross-sectional area of the runner, the higher is the pressure gradient between the point of injection and the cavity. If the length of the runners is the same to each cavity, there will be an equal rate of fill to all the cavities. If the path lengths are unequal, some cavities will fill before the others. Once a cavity has filled, it, and the related runner system, will rapidly achieve the full available... 

If we imagine a tub-shaped moulding fed directly from the sprue (Fig. 2.3) as the example of the spool end was, the injection speed becomes controlled by the thinnest section, i.e. the gate, when a simple machine is used. This means that the sprue is filled relatively slowly, as is the main part of the moulding beyond the gate. The position would be even worse in a two-impression mould with runners. Fig. 2.4, because the sprue and runner system is quite extensive, and ideally should be filled quickly. 

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