During the entire casting and solidification process, it comes to mold wall movement within the sand molds, which may cause changes in the mold cavity dimensions.

Causes for this process may be expansion or change in modification of the basic mold material, the metallostatic pressure in the liquid casting material, gas diffusion and graphite expansion in cast iron.

Counteracting factors are mold resistance and atmospheric pressure. Mold material additives that increase compressive stresses in the outer mold wall, as well as excessive compaction, excessive quantities of binding agents, and exceeded casting temperatures are aspects that promote mold wall movement. As soon as solidification is completed and the material performs spontaneous movement due to contraction or expansion, any mold wall movements do not have an influence on dimensional accuracy any more. 

Additional references:
Mold wall