Originally Posted by
JonB_in_Glencoe
Ditto for me (highlighted in red)
I call it shrinkage. (I haven't seen many others call it that, so if I am misusing a term, forgive me.)
What is happening is, as the boolit freezes, the alloy shrinks. As the alloy is shrinking, it needs to pull more alloy into the cavity from the molten sprue puddle. If the sprue puddle is small, it will freeze before the boolit inside the mold freezes and shrinks. If there is no molten alloy to pull into the mold, the boolit will just stay shrunken...typically in the center, just like the boolits in the photo shown.