In the simplest case a die consists of two halves into which the impression of the part to be moulded is cut. The mating surfaces of the die halves are accurately machined so that no leakage can occur at the split line. It may by seen that in order to facilitate mounting the mould in the machine and cooling and ejection of the moulding, several additions are made to the basic mould halves. Firstly, backing plates permit the mould to be bolted on to the machine platens. Secondly, channels are machined into the mould to allow the mould temperature to be controlled. Thirdly, ejector pins are included to that the moulded part can be freed from the mould. In most cases the ejector pins are operated by the shoulder screw hitting a stop when the mould opens. Two side cores are incorporated in fixed half as each component requires it. Two finger cams are provided in fixed half for the actuation of these side cores. The mould cavity is joined to the machine nozzle by means of the sprue. The sprue anchor pin then has the function of pulling the sprue away from the nozzle and ensuring that the moulded part remains on the moving half of the mould, when the mould opens. The impressions are joined to the sprue by runners - channels cut in core insert of the mould through which the plastic will flow without restriction. A narrow constriction between the runner and the impressions allows the moulding to be easily separated from the runner and sprue. This constriction is called the gate.
Fixed half and moving half
The various mould parts fall naturally into two section or halves. Hence that half attached to the stationary platen of the machine is termed the fixed half. The other half of the mould attached to the moving platen of the machine is known simply as the moving half.
Generally the core is situated in moving half and the overriding reason is, the molding, as it cools, will shrink on to the core and remain with it as the mould opens. This will occur irrespective of whether the core is in the fixed half or the moving half. However, this shrinkage on to the core means that some form of ejector system is almost certainly necessary. Motivation for this ejector system is provided if the core is in moving half. Moreover, in the case of single-impression basic mould, where a direct sprue feed to the underside of the molding is desired the cavity must be in the fixed half and core in the moving half.
