Glossary
Shrinkage porosity — definition
Voids that form where the section is thickest, because the metal contracts as it solidifies and receives no feed.
Shrinkage porosity is not gas bubbles. It is empty space created because liquid metal occupies more volume than solid metal, and because the last melt in a heavy section has nowhere to draw material from.
It always lands in the same place: where the section is thickest and solidifies last. Often in a boss, in a corner without a radius, or where a rib meets a wall at full thickness.
The remedies are all design-side: even wall thickness, cored bosses, ribs instead of solid metal. In gravity casting, feeders that stay liquid longer than the part are used as well.
This is therefore one of the clearest examples of casting quality being decided at the drawing board rather than at the machine.