The rate at which a casting cools affects its microstructure, quality
, and properties is termed as cooling rate. It is largely controlled by the molding media used for making the mold. When the molten metal
is poured into the mold, the cooling down begins. This happens because the heat within the molten metal flows into the relatively cooler parts of the mold. Molding materials transfer heat from the casting into the mold at different rates. For example, some molds made of plaster may transfer heat very slowly, while a mold made entirely of aluminium would transfer the heat very fast. This cooling down ends with solidification
when the liquid metal turns to solid metal. Intermediate cooling rates from melt result in a dendritic microstructure.
At its basic level a foundry may pour a casting
without regard to controlling how the casting cools down and the metal freezes within the mold. However, if proper planning is not done the result can be gas porosities and shrink
porosities within the casting. To improve the quality
of a casting and engineer how it is made, the foundry engineer studies the geometry of the part and plans how the heat removal should be controlled. Where heat should be removed quickly, the engineer will plan the mold to include special heat sinks to the mold, called chills. Fins may also be designed on a casting to extract heat, which are later removed in the cleaning (also called fettling) process. Both methods may be used at local spots in a mold where the heat will be extracted quickly. Where heat should be removed slowly, a riser or some padding may be added to a casting
. A riser is an additional larger cast piece which will cool more slowly than the place where is it attached to the casting.