5. Shrinkage. These are cavities ca

5. Shrinkage. These are cavities caused by contraction of metal and show on radiographs as piping, filamentary, or a fine branching network (Fig. 9.8). They are usually extensive cavities and their form depends on the metal thickness and the rate of cooling. They are most typical in medium and large castings. In filamentary shrinkage there is usually a tree-like structure with trunk and major and minor branches, and the finer arms may degenerate into sponginess. A special form of this defect which occurs in large areas of uniform thickness such as cast plates, often occurs on a plane near the centreline of the wall thickness and is called 'centreline shrinkage' (Fig. 9.9).