Shrinkage in Casting: Causes and Solutions
Metal shrinks as it transforms from liquid to solid (known as solidification shrinkage) and undergoes additional thermal contraction as it cools to room temperature. Cast parts are therefore designed with shrinkage allowances to result in parts of the desired dimensions. Cast steel, for example, will shrink about ¼ in. per foot and produce rough-looking castings. Shrinkage allowances are known for various metals and an experienced mold designer will take the shrinkage into account when designing a mold. For more information on casting in general please refer to our article on Types of Casting Processes.
Shrinkage can also cause defects in cast products which can lead to failure, leakage, and so on. Sometimes these defects appear on the surface of the casting and can be detected visually or through dye penetrant or similar non-destructive procedures. Sometimes the defects are internal to the casting and require X-ray inspection or destructive tests to uncover. These defects are collectively known as open- and closed-shrinkage defects.
Open Shrinkage Defects
If the metal cools and shrinks with insufficient liquid available to fill any voids, pipes may form in the surface and extend into the body of the casting. Similarly, defects that form on the surface and spread across the face are sometimes called cave defects, or sinks. In both instances, the defects are open to atmosphere and air takes the place of molten metal.
Cracks and hot tears usually form in the final stages of solidification and can be localized around abrupt changes where stress concentration exists such as a thin web connecting two heavy sections. They can also occur where insufficient draft in the part exists and in heavy sections where heat pools.
Closed Shrinkage Defects
Porosity is one of the main defects in castings and is caused by both trapped gases in the molten metal and as the casting shrinks during cooling. Shrinkage porosity is by far the most common type and can usually be detected on the surface of a cast part by what appear to be small holes or cracks. These holes may seem round but are actually angular in shape and tend to form branching internal fractures. Thick multi-angled parts are most susceptible to such shrinkage, which occurs as the metal cools and solidifies in a non-uniform pattern. Porosity can exist in the interior of a casting as well, without necessarily showing on the surface of a part. This occurs when liquid metal is surrounded by solidified metal and molten metal is unable to fill in behind the liquid as it cools and shrinks.
Read more: Shrinkage in Casting: Causes and Solutions