During the solidification process, the sizes of steel castings are generally reduced, and the reduction rate is known as casting line shrinkage rate or casting shrinkage rate. So, the mold (including the core box) will be enlarged according to casting shrinkage rate, to make sure that sizes of steel castings meet the requirements after cooling. The shrinkage can be calculated by the following formula:

 

Shrinkage value = dimensions of casting part× shrinkage rate

 

Main factors affacting shrinkage rate are as follows:

 

1. Raw material

Different materials result in different shrinkage rates. For example, the shrinkage rate of steel casting is larger than that of gray iron castings. For gray iron castings, the more sulfur contents, the larger shrinkage rate; while the more silicon, the smaller shrinkage rate.

 

2. Structure of casting

Complex structure makes shrinkage difficult. 

 

3. Deformability of mould

The better deformability if mould, the larger shrinkage rate of castings. For example, the shrinkage rate of astings using wet and water glass sand mould is larger than that of castings using dry type mould. In the same way, with the increase of the size of castings, the deformability of the mold becomes worse, and the shrinkage of the casting reduces.

 

So the casting shrinkage rate should be choosen based on the actual situation. For high precision castings, shrinkage rate should be revised according to the dimension of the casting.

 

For the same casting part, its shrinkage rates from different directions may be inconsistent due to structural reasons. For castings with higher precise requirements, different shrinkage should be given in all directions. In the actual production, general casting, especially small castings, all directions have the same rate of shrinkage rate, although this will cause some errors, but the errors in general is not large. That will be good for the mold manufacturing process.