Glossary
Hot tearing — definition
A crack that forms while the part is still solidifying, because contraction is restrained by the tool.
In the transition between liquid and solid the metal is at its weakest. If the part contracts while a core or a die wall holds it back, the section can tear before it has gained strength.
Hot tears almost always start in sharp internal corners and where a thin section meets a thick one.
The remedies are the same as for shrinkage porosity: radii, even wall thickness, and a geometry that lets the part contract freely where it can.