What is shrinkage in metal casting?

Most molten metals will shrink as they solidify. For stainless steels and nickel alloys this might typically be 2-3% by volume, whereas for copper alloys it can be 5-6%. Shrinkage during casting is an issue that must be carefully managed.

The art of the foundry is to therefore compensate for this anticipated shrinkage when designing the mould. For simple shapes, the dimensions of the mould can be increased over and above the final parts size to accommodate some of this shrinkage. However, if molten metal were simply poured directly in to a mould of the same dimension as the required component some voids may be expected. Therefore, the careful design of the gates, runners and feeders that manage the flow of the molten metal in to the mould can also restrict the effect of shrinkage. But, as these items are not part of the finished component, and will be removed and recycled as scrap after casting, then over-compensating will introduce unnecessary costs.

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