Stress whitening appears as pale, opaque, or silver-white zones where the polymer has been strained beyond a stable condition. It is common in impact-modified and semicrystalline materials, although many polymers can whiten when temperature, strain rate, and geometry combine unfavorably.
The process stage at which the mark first appears is the most useful clue. Whitening visible immediately after forming usually indicates an underheated region, sharp geometry, a cold plug, excessive plug speed, or poor material distribution. Marks created during release suggest insufficient cooling, undercuts, high friction, or excessive eject force. Whitening exposed during trimming often reveals molded-in stress.
Corrective action
Sheet temperature uniformity should be confirmed before the overall heat setting is raised. Plug temperature, finish, alignment, and motion should be checked. Sharp transitions should be given suitable radii, and deep features may require better pre-stretch or plug-assisted distribution.
The part must remain supported until it has enough stiffness for release. More cooling will not correct whitening that was already created by cold forming or excessive local strain.