shrinkage cavity[′shriŋ·kij ‚kav·əd·ē]
a void in an ingot or shaped metal casting, formed during the solidification of the molten metal as a result of shrinkage. Shrinkage cavities are usually located in the upper portion of an ingot or in those spaces within a casting where the molten metal is last to enter during pouring. During solidification, a shrinkage cavity serves to supply molten metal to all the voids formed underneath the cavity; therefore, steps are taken to maintain a temperature above the point of solidification for as long as possible in the area where the shrinkage cavity is located.
Before ingots are rolled or forged, the upper portion with the shrinkage cavity is generally cut off and discarded. In order to minimize scrap losses, a shrinkage cavity must be as wide and short as possible and extend only a little into the ingot. This is achieved by using an ingot mold that widens toward the top, which gives the shrinkage cavity the shape of a short cone with a peak facing the bottom of the ingot. Steps are also taken to heat the feeder part of the ingot. If solidification of the last portions of the molten metal is not retarded and the shrinkage cavity is not localized, the cavity will spread down into the ingot and its lower, narrower end will branch out; as a result, additional large cavities or small holes (porosity) may form near the shrinkage cavity, usually underneath.