ABS has a useful thermoforming range, but prolonged or excessive heat can change both appearance and performance. The sheet may show gloss variation, yellowing, odor, unstable sag, surface waviness, or reduced impact strength.
Pigment, cap layer, recycled content, sheet thickness, and grade formulation affect the visible response. A textured ABS sheet may become locally glossier where it is hottest because the softened surface reproduces the tool more closely.
Correction
Actual sheet temperature and exposure time should be compared with the supplier’s data for the grade. Heater balance is often more important than reducing every zone. Material storage and any required drying should be controlled, especially after humid exposure.
Repeatedly reheated regrind can narrow the process window. If acceptable appearance requires unusually low temperature or short cycles, incoming sheet condition and formulation should be reviewed rather than treated as an oven-only issue.