All the matels shrink when cooling except perhaps bismuth. This is, because of the inter-atomic vibrations which are amplified by an increase in temperature. However, there is a distinction to be made between liquid shrinkage and solid shrinkage.
Liquid Shrinkage refers to the reduction in volume when the matel changes from liquid to solid state at the solidus temperature.
Solid shrinkage is the reduction in volume caused, when metal loses temperature in solid state. The shrinkage allowance is provided to take care of this reduction.
The Shrinkage allowance of casting metals are as follows:
Grey cast iron: 0.7-1.05%, Mallable Iron: 1.5%, Steel: 2.0%, Aluminum alloys: 1.3-1.6%, Brass: 1.4%, Bronze 1.05-2.1%
- When cooling solidification, the casting with perfect packaged will be smaller shrinkage allowance.
- The thin-walled casting’s shrinkage allowance is smaller than the thick-walled casting’s.
- The percentage of large casting’s shrinkage is smaller than the small castings.
- The shrinkage allowance of complex-shape casting is smaller than the simple one.
- Even on the same casting parts, the shrinkage allowance is different on different sizes.
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