Shrinkage Allowance for Metal Casting

A shrinkage allowance for metal casting is something that must be figured into a design from the very beginning. As the molten metal cools and solidifies it will begin to contract. This means that although the molten metal completely filled up a mold, by the time the casting was cold, the casting is smaller than the mold.

What this mean is that a pattern must be made larger than the design drawing. The difference between the size or dimensions of the desired casting and the size of the pattern used to create the mold is called a shrinkage allowance.

The shrinkage allowance for metal casting varies by the type of metal. It takes experience in metal casting to be able to accurately judge the proper shrinkage allowance to be built into a pattern. Below is a basic shrinkage allowance and pattern oversize factor chart based on metal type.

Metal	Pattern Oversize Factor*	Finish Allowance**	Win Wall mm/(in)
Aluminum	1.08 – 1.12	0.5 to 1.0%	4.75 (0.187)
Copper Alloys	1.05 - 1.06	0.5 to 1.0%	2.3 (0.094)
Gray Cast Iron	1.10	0.4 to 1.6%	3.0 (0.125)
Nickel Alloys	1.05	0.5 to 1.0%	N/A
Steel	1.05 – 1.10	0.5 to 2%	5 (0.20)
Magnesium Alloys	1.07 – 1.10	0.5 to 1.0%	4.0 (0.157)
Malleable Irons	1.06 – 1.19	0.6 to 1.6%	3.0 (0.125)

*(each direction)      **(lower number for larger sizes)

The shrinkage allowance for metal casting is linear meaning that these allowances apply in every direction. In addition, the shrinkage allowances in the chart above are approximations. The actual size and shape of the casting actually determines the actual shrinkage allowance.

To complicate things further, different parts of a casting may require a totally different allowance. This is often found in the portion of the casting that is at the very top of the mold where impurities and air bubbles end up when the molten metal is poured.

If a casting is to be sand cast, then often the rough surface of the final casting will require surface finishing. Thus a machining allowance or “finish allowance” must also be configured into the casting pattern.

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