Thermal degradation begins when a polymer is exposed to more heat than its chemistry and grade can tolerate. It may show as fine bubbles, yellowing, dark spots, loss of gloss, a sharp odor, surface pitting, or visible charring. Mechanical properties can decline before severe cosmetic damage appears.
What causes it
Excessive sheet temperature is only one route. A long dwell at a lower radiant setting can also create damaging thermal exposure. Failed heater controls, hot spots, incorrect material identification, contaminated regrind, and repeated processing history reduce the available margin. PVC, styrenics, and some transparent materials may signal degradation early through odor or color shift.
Diagnosis and correction
Actual sheet temperature should be mapped across the forming area. The heater output, exposure time, element-to-sheet distance, and material thickness must be considered together. A defect fixed by a shorter cycle at the same final forming quality is strong evidence that heat exposure was excessive.
Material batch and regrind percentage should be checked when the process is within its validated window. Degraded scrap should not be returned to appearance-critical or structural sheet. Ventilation must control fumes, but ventilation is not a substitute for correcting the thermal cause.