A Word on Dimensional Stability of a Die Casting


“The dimensional stability of die castings depends on several factors; the degree to which aging effects the volume of the material that is used; the number of locked-up stresses that are present in the die cast part; the temperature to which the parts are subjected during finishing and service; the severity of the subsequent machining operations; and the tendency of the metal to creep under load.”

“Dimensional changes resulting from locked-up stresses appear generally as distortion, warpage and misalignment. They are, as a rule, caused by failure to freeze the die casting in a progressive manner. As a result, internal stresses are set up between sections of unequal thickness because of unbalanced contraction during solidification, or because the natural path of solid contraction is forcibly restricted by sudden direction changes.

When the parts are cast from aluminum many times the die cast skin or outer surface is sufficiantly strong enough to restrain the internal stresses; if the parts are to be machine and the skin broken, distortion can be minimized by low temperature stress relief before putting the die casting into service.

Warpage and distortion resulting from the application of baked finishes are a straight temperature effect. If the baking cycle is too long, the restraining force of the skin and the inherent strength of the core are decreased to a point where internal stresses cause dimensional changes; therefore, finishing cycles involving baking of aluminum die castings must be carefully adjusted in order to prevent such troubles.

Creep under load is rarely encountered in aluminum die castings unless the castings are overstressed or operated at excessive temperatures.”

 

 

Doehler, H.H. (1951) Die Casting. New York, New York. McGraw-Hill Book Company, Inc.