Shrink Porosity Vs. Gas Porosity

There are basically two types of porosity classifications in die castings. Shrink porosity and gas porosity are the two types of air inclusions in die casting.

Die castings are subject to shrink holes that are primarily caused by insuffiecient feeding of the die casting. For instance when the die casting freezes 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 metal in an intermediate section between the gate and the section of distress.

Gas holes are another type of defect in die castings.Thet are distinguishable from shrink holes in that they are smooth and round, wheras shrink holes are ragged and irregular shaped. Gas holes are caused by the release of gas or vapors from the metal as it is cast. These die cast defects occur when the metal is not scavenged after it has been exposed to humid atmosphere and fuel gases in the melting or holding die cast furnace.

Doehler, H.H. Die Casting. New York, New YorkMcGraw-Hill Book Company Inc., 1951. Print.