Twist is a rotational distortion in which opposite corners move in different directions. It reflects an asymmetric stress or shrinkage field across the part.
Uneven wall thickness, mold-temperature gradients, directional sheet orientation, off-center loading, unequal clamp tension, and asymmetric part geometry are typical causes. A part may remain flat in the surrounding web and twist as soon as it is trimmed free.
Diagnosis
Corner heights should be measured at several stages: on the tool, after demolding, after full cooling, and after trim. The sheet can be rotated to determine whether the bias follows extrusion direction. A thermal map and thickness map often reveal the same diagonal pattern as the twist.
Correction may require heater zoning, plug alignment, cooling balance, or geometric stiffening. Forcing the part flat in a fixture can be useful during cooling, but repeated springback after fixture removal indicates unresolved internal stress.