Gloss variation appears as bright and dull regions on a part that should have a uniform finish. The sheet may be copying different tool conditions, reaching different temperatures, or making incomplete contact.
Hotter regions can reproduce fine tool texture more strongly. Trapped air can leave a dull area, while a polished contact zone may become glossier. Sheet extrusion direction, masking, contamination, and local stretch can also change how light reflects from the surface.
Diagnose the pattern
The defect should be compared with the thermal map, tool finish, and vent locations. A patch that repeats in one tool position suggests surface or air evacuation. A band that follows the sheet direction suggests incoming stock or heating response.
Correction may require heater balance, tool refinishing, vent improvement, or sheet-quality control. Adding texture to the tool can improve uniformity, but it will intentionally change the final surface and should be approved against the product requirement.