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Enabling Net Shape Casting

Zinc alloy diecasting is capable of being amongst the most consistent of three dimensional forming processes, frequently eliminating the need for secondary operations, even when used to manufacture precision components. However achieving this desirable and highly cost effective goal does require careful planning on behalf of the product designer, the mould designer, the toolmaker and the diecaster. If the product design is unsuitable no amount of effort by the others will be able to compensate. The parties need to jointly consider how any proposed component geometry will change due to thermal contraction both in the die and after ejection, taking into account the physical constraints and the effects of the differences in temperature that will occur between one part of the casting and another. They also need to be aware of the potential effects of ejection forces resulting from any proposed ejector layout and any other forces likely to be experienced during processing, such as slight impacts it might experience when falling from the die after automatic casting. The design can then be modified to avoid forms that lead to unpredictable dimensional changes between the die cavity and the resulting casting for the critical dimensions.

A common example of a design that is difficult to produce to close tolerances is a long hole with varying sections and constraints around it. The diecaster may well be able to produce a consistent part but a round straight core will probably produce a slightly out of round slightly out of straight hole. The out of roundness may be compensated for in some cases by producing an out of round core (at significant cost), but in most such cases the out of straightness will prove insurmountable. In contrast a hole surrounded by an even wall section without external constraints, leads to a round straight hole with very consistent dimensions.

It is emphasised that considerable time and effort should be spent on this aspect of the design and will usually be recouped many times over during subsequent manufacture.

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