The injection moulding process is suitable for all materials, only the tool design having to be changed depending on the shrink characteristics of the polymer/polymer combinations. Injection moulding is a process which converts polymer granules into one of the most dimensionally accurate moulded thermoplastic parts possible. It does so using a reciprocating or ram, plasticating extruder. Dry plastic granules are added to the plasticating extruder in the same way as described earlier in the chapter for film and sheet extrusion. Most importantly, the drying time of the polymer and its masterbatch significantly affect the process time, with PET, PA and PLA needing between 6 and 12 hours for complete drying. To overcome this problem, pre-hoppers are used to pre-dry the polymer and liquid colorant is used to negate the need to dry the masterbatch.
Once heated in the extruder, the homogeneous, molten mass is then injected into a mould through a gate, known as the injection point. A predetermined mass of polymer, designed to completely fill the mould, is metered out within the injection moulding machine, by controlling the stroke of the reciprocating screw or the ram piston. The polymer (PP has a much higher shrinkage than HDPE, for example) and the colour used significantly affect the shrinkage of the moulding. This can result in a separate die being required or changes made to the cycle time and cooling conditions used, to ensure the moulding once cooled meets its dimensional specification.
The mould consists of two or more steel parts, one with a cavityaccurately cut away to form the female section of the moulding, the other with a corresponding profiled section. When the two halves are clamped together, the gap between the male and female sections of the mould corresponds to the shape, finish and thickness of the moulding required after taking shrinkage into consideration. The parts are locked together with a clamping forcesufficient to ensure that both singleand multi-cavity moulds stay closed, until the moulding is cool enough to be ejected. Multi-cavity moulds are used to increase the number of units produced over a given time and therefore reduce the unit cost. The number of cavities possible is dependent on the surface area of the cavity and the locking force of the injection moulding machine, the larger the locking force the greater the number of cavities possible.
The mould is cooled with temperature controlled liquid to ensure the mouldings cool as evenly as possible. Injection points are positioned so that the flow of material into the mould is as even and thus stress free as possible, and the whole moulding is free of weak areas where the polymer has flowed together in the mould. Injection points can be visible on the mouldings, therefore consideration must be given to this at the mould design stage. With care, they can be hidden from view.