The process by which molten metal freezes in a shaped casting is a complex process.  The first metal to freeze at the mold’s surface is typically fine grained and dense due to chilling from the mold surface.  As molten metal solidifies, it shrinks.  To If sufficient liquid metal is not delivered to the solidification front, pores will form.  This dentritic solidification resembles a forest of branched trees; which is the most common mode of solidification in commercial castings.  During cooling, these dentrites grow together making it difficult for molten metal to flow into the remaining unfilled interdentritic regions.  Once the liquid trapped in these isolated areas freezes, porosity is formed.  This formation of ‘shrinkage’ porosity is a common occurrence in alloy castings.

Solidification shrinkage becomes more pronounced as shape complexity increases.  The casting engineer can design gates and risers that will maximize the amount of liquid metal fed to the internal regions of the casting and minimize shrinkage porosity.  However, nature and the shape of real parts make the complete elimination of shrinkage porosity an impossible task.