The design of pressure diecastings is limited by the requirement for it to be removable from the die and for any undercuts to be formed by separate die pieces that retract (usually but not always in a straight line). Despite this fact it is possible to make surprisingly complex geometries without resorting to post casting machining operations. The designer must weigh the functional, manufacturing or aesthetic advantages that can be realised with a design that demands extra moving die members against the extra costs that will be incurred. The most obvious extra cost is for die manufacture. The effect on piece costs may or may not be significant, depending on whether the die utilisation is reduced (possible extra maintenance requirements) or extra casting trimming costs are incurred. It is by no means certain that significant extra costs will result, but it is very unlikely that specifying a design requiring extra sliding cores will reduce these costs.
The often-stated rule that dies with sliding cores run more slowly than “open and shut” dies is not generally true. Sliding cores retract quickly, often simultaneously with the die opening and hence their effect on cycle time is negligible. In fact moving cores can often be utilised to avoid heavy casting sections, which will result in faster cycle times. Occasionally extra delay may be necessary before die opening to enable the casting to cool sufficiently to prevent it distorting when the sliding die member is retracted. These situations can often be avoided by providing casting or die features that prevent such distortion.
Complex die parting lines add significantly to tooling costs but can provide advantages that are realised in secondary operations or in function or aesthetics. Again the designer must weigh the benefits against the costs. In general a well-designed complex parting line should not incur much extra casting and trimming cost.
The necessity to remove the casting from the die reliably and without damaging or distorting it must be remembered. Firstly it must be arranged that when the die opens the casting will move back with the die half on the moving platen. With simple open and shut dies this is usually achieved by orienting the casting such that it tends to shrink on to the features of the moving die half more tightly than on the fixed half. Casting geometries that cannot be arranged to achieve this should either be modified accordingly or other measures will need to be taken (such as Z pins or side cores) to ensure the required retention on the moving half. A situation to guard against is where the opposite ends of the casting tend to grip on separate die halves. Even if the grip on the moving half is much tighter than that on the fixed half the casting will almost certainly be grossly distorted during die opening.
An effort should be made to produce a casting design which will not require a die geometry which is vulnerable to damage or which has stress raising features which are likely to crack under the influence of thermal fatigue. Zinc alloys are much easier on the die than most other metals but these points should still be considered.