Control features in injection moulding

We have now observed the importance of control of moulding conditions in the manufacture of two injection-moulded products. Next let us examine these process variables, and the way in which a modern machine gives control over them. In the account of the moulding of the support bracket and the film-spool end it became clear that control of injections, speed and pressure, were vital to the production of satisfactory mouldings. One other feature, not specifically discussed previously, but clearly also of vital importance, is the supply of consistent polymer melt delivered by the plasticizing unit at the specified rate. 

The control parameters for the injection are also those of speed and pressure. Now, however, the speed is the linear speed of the screw (not turning), acting as a plunger, and the pressure is applied, via the hydraulic system, to the screw, and hence the melt in the mould. 

The overall injection profile divides into two distinct sections the injection and the packing (holding pressure) sections. The injection section is primarily speed-dependent, and packing is pressure-dependent. The point in the overall cycle at which the change is made from the one to the other can often be critical, as we saw in the case of the spool-end moulding in forming satisfactory welds at the outer rim of the product. 

The conventional machine is basically run at a single speed throughout the injection stroke, with the option of a boost in the first section, supplied by a second hydraulic pump or a pressure accumulator. The basic speed is adjustable, but is then fixed during operation. 

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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