Why Pattern Making is Complex
The pattern needs to incorporate suitable allowances for shrinkage; these are called contraction allowances, and their exact values depend on the alloy being cast and the exact sand casting method being used. Some alloys will have overall linear shrinkage of up to 2.5%, whereas other alloys may actually experience no shrinkage or a slight “positive” shrinkage or increase in size in the casting process. The shrinkage amount is also dependent on the sand casting process employed, for example clay-bonded sand, chemical bonded sands or other bonding materials used within the sand.
The pattern needs to incorporate suitable allowances for draft, which means that its sides are tapered so that when it is pulled from the sand, it will not drag sand out of place with it. This is also known as taper which is normally between 1 and 3 degrees.
Sprues, gates, risers, cores and chills
The patternmaker or foundry engineer decides where the sprues, gating systems and risers are placed with respect to the pattern. Where a hole is desired in a casting, a core may be used which defines a volume or location in a casting where metal will not flow into. Sometimes chills may be located on a pattern surface, which are then formed into the sand mold. Chills are heat sinks which enable localized rapid cooling. The rapid cooling may be desired to refine the grain structure or determine the freezing sequence of the molten metal which is poured into the mold.