Stress relief in extruded sheet: why your part moved after machining

Nearly every complaint about a plastic plate that 'warped' after machining is not a warping problem. It is a stress relief problem, and the material was already out of balance before the first cut.

Extruded sheet is cooled under tension. That leaves a stress gradient through the thickness, with the outer surfaces in a different state from the core. Machining removes one surface, and the remaining material is now unbalanced — so it moves to find a new equilibrium. Thinner parts move more because there is less material to resist the imbalance.

The practical controls are three. Buy thicker stock and machine down to size, because the outer skin is where the gradient is steepest. Rough machine, stress relieve with a controlled heat cycle, then finish. And for parts where flatness is critical, use cast or press-quenched sheet rather than extruded. If you are seeing movement and cannot explain it, send us the drawing and the process sequence and we can usually identify which step is releasing the stress.