Final answer:
A chill in sand moulding and casting serves to increase the rate of heat extraction from the casting, reducing solidification time and improving mechanical properties.
Step-by-step explanation:
In the context of sand moulding and casting processes, a chill is introduced into the mold to modify the cooling characteristics of the metal as it solidifies. The purpose of a chill is to increase the rate of heat extraction from the casting. This is done with the aim of achieving a finer grain structure, reducing the solidification time, and to enhance certain mechanical properties of the casted part such as hardness and strength. A chill can be made from various materials including iron, copper, or graphite depending on the desired effect and the type of metal being cast. Its function is critically important in areas of the casting that are thick or where rapid cooling is required to refine the microstructure or to accommodate for thermal contractions.