Shrink holes are caused primarily by insufficient feeding of metal to the casting. For instance, when the metal in the gate freezes before the metal in the die casting has had a chance to solidify, volume changes attending the liquid shrinkage cannot be made up by supplying reserve liquid metal through the gate, and shrink holes are apt to occur. Another condition that contributes to the formation of shrink holes is premature freezing of the metal in an intermediate section between the gate and the section in distress.
Gas holes are another type of defect in die casting. They are distinguishable from shrink holes in that they are smooth and round, whereas shrink holes are ragged and irregular in shape. Gas holes are caused by the release of gasses or vapors from the metal as it is cast.These defects occur when the metal is not scavenged after it has been exposed to humid atmosphere and fuel gases in the melting or holding furnace.
Doehler, H.H. Die Casting. New York,New York: McGraw-Hill Book Company Inc., 1951. Print.