Analysis of Permanent Mold Distortion in Aluminum Casting

Mold distortion in permanent mold casting campaigns results in loss of mold productivity and shorter mold life, and can result in out-of-dimensional tolerance castings. Remedies to avoiding distortion during a casting campaign include back face mold machining, bolting, and clamping, but permanent deformation occurs when these restraints are removed. An AFS research program was undertaken using finite element thermal and stress analysis to better understand the causes of permanent mold distortion. Phase 1 of this research, using a simple mold geometry, indicated mold–metal interface thermal shock, and continued mold cycling resulted in cumulative permanent plastic strain. Phase 2 work using a commercial mold insert resulted in good agreement reached between the model plastic strain and the actual measured mold insert distortion. The study showed that mold material properties are significant, with higher mold thermal conductivity (better heat diffusivity beyond the mold–metal interface); and higher creep strength results in less plastic strain per casting cycle and less distortion of the permanent mold.