PVC has a limited thermal-processing margin compared with many common thermoforming sheets. Excessive temperature or dwell can produce discoloration, odor, surface damage, loss of properties, and corrosive decomposition products.
Stabilizer package, color, recycled content, sheet construction, and grade determine the usable window. A local control failure can degrade one zone while the rest of the sheet appears normal.
Correction and safety
Sheet temperature and residence time should remain within the supplier’s limits. Heater controls, reflectors, switching devices, and hot spots must be checked when discoloration repeats by machine position. Ventilation and corrosion-resistant maintenance practices are required where PVC is processed.
A degraded sheet should be removed from the process. Lowering the next cycle without identifying a stuck heater or incorrect recipe leaves the equipment and product at risk. Fumes must never be treated as a normal indicator that PVC is ready to form.