Extruded sheet retains molecular and mechanical history. During heating, residual orientation can relax and cause directional shrinkage, curl, twist, or different elongation along and across the extrusion direction. The effect is often strongest near edges or in highly oriented thin sheet.
A controlled heat-shrink test and marked sheet orientation can separate material memory from uneven oven temperature. If the defect rotates with the sheet, the sheet is the stronger suspect. If it remains fixed in the machine, heating, tooling, or clamping should be investigated.
Production loading direction should be standardized when orientation affects the part. Supplier profile, extrusion direction, storage, and annealing history may need tighter control. Heater compensation should not be the first response to a material property that changes between batches.