"Cold Shuts"

As liquid metal fills the die, it flows in complicated patterns from multiple directions until the die is filled. So there will always be meeting places where metal flowed from multiple directions and came together. As long as the metal is fully liquid it simply joins together.

However, sometimes the leading edges or fronts of metal coming together have become slightly cooler than the rest of the metal and have hardened. Thus, when those two fronts are pushed together, hardened metal is pushed against hardened metal, and there is no true joining, but a tiny hairline crack between them which is called a 'cold shut' because the gap was shut after the metal was cold.

A cold shut allows porosity on the surface, and most visible cold shuts will not coat properly because they may outgas at some future time, or absorb pretreatment chemicals which later leach out.