What Does Shrinkage Mean?
Shrinkage is an internal or external change in volume that occurs during a phase change in a metal's transition from a liquid state to a solid state at the exposed surface. This phenomenon occurs in processes like casting and concrete solidification. It is visible during a nondestructive test, and can be used to analyze plastic shrinkage cracks and void formation in castings and concrete.
Corrosionpedia Explains Shrinkage
Metals undergo contraction when solidifying and cooling, because the specific volume is larger in the liquid state than in the solid state for standard casting metals. Therefore, it is a function of the casting material.
In metals, shrinkage occurs in three phases:
This phenomenon leads to voids and cavities when there is not enough molten metal to feed the shrinkage when casting. Areas with higher modulus (the volume of the section divided by the surface area) are the last to solidify, so the probability of shrinkage is increased as the modulus of the section increases. Areas that are hotter for a long time (e.g., a thick section or tight corner) solidify later than the surrounding sections.
Shrinkage results in several casting defects such as porosity and stains in the product. This can be avoided by altering the casting geometry and using risers.