Distortion refers to profile deviations by forming exceeding constructive tolerance limits. As distortion is not always visible with the naked aye, a dimensional inspection is required to provide absolute certainty. This dimensional inspection is to be carried out after the last production step. For example, after blasting following heat treatment.
In castings, distortion is usually caused by stress, e.g. due to shrinkage constraints in the mold. If these stresses are released when emptying the mold, they are sufficient to deform the components. If the casting is removed at increased temperatures, these stresses may lead to cracks. This also explains the fact that distortion may occur independently of the molding process.
Design defects, resulting in irregular and constraint shrinkage of the casting during solidification and cooling may lead to distortion and respectively to cracks (see Cold crack). However, distortion may also be “inevitable” due to the provision of the pattern, die or mold and respectively at the gate system and the feed system.
At die casting, it is distinguished between offset due to ejection forces due to insufficient separating agents or inappropriate separating agents, damaged molds which leads to back drafts or insufficient demolding inclinations and thermal offset after ejection. For example, this includes high demolding temperatures, high melting temperatures, high mold temperatures and short cycle times.
Respectively, large die castings often show distortion as as the casting system, particularly the casting residue in the shot sleeve, gets stuck and the component is distorted when the mold is opened. Incorrect positioning of the ejectors may also be the case. In this case, the ejector forces are not applied where the component is shrunk on, during removal of the component from the contour-defining surface of the moveable half of the mold. Excessive cooling time (solidification time) in the die casting mold and irregular cycles may lead to distortion.
Distortion often occurs during welding and heat treatment, the two most important casting post-treatment processes. Stresses and resulting distortion or cracks always occur if too much heat is applied too quickly or if larger castings with critical stress properties are not pre-heated.
Distortion may occur during heat treatment if the castings are too quickly cooled down after annealing or in a higher temperature range.