Shrinkage (metal casting)

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Caliper and depth gauge with shrinkage
Shrinkage standards

In foundry technology , shrinkage is understood as the ratio between the volume of a casting at the solidification temperature or melting temperature (i.e. the volume of the mold ) and the volume at room temperature . Since it is not the volume but the constructively given length that is used in production ( model making ), one speaks of the shrinkage. It is given in relation to the substance by the coefficient of linear expansion [1 / K ].

Under shrinkage thus refers to the extent to which shortens a measure of a casting part with respect to the extent of the mold.

The reason for this is the thermal expansion of the solid metal, as well as possible changes in the crystal structure during cooling . The original dimensions change as the metal contracts as the solidified casting cools. This is called "fixed shrinkage". It is not to be confused with the "liquid shrinkage" or the "solidification shrinkage", which by suitable feeder technology is taken into account: from the feeder passes liquid metal according to this shrinkage balance.

Shrinkage dimensions must be taken into account in the construction dimensions in model and mold making by means of oversizes, i.e. H. the mold is made larger so that the solidified casting gets the right size. In order to reduce the effort in production, special rulers and other measuring tools are used that take shrinkage into account. A length of 100mm in a model for gray cast iron is also manufactured as 100mm as indicated by the measuring tool. The “real length”, however, is 101 mm.

However, the alloy-specific guide values ​​for the degree of shrinkage often deviate due to shrinkage hindrances.

Typical alloy-specific values ​​for the shrinkage are: