Blisters are enclosed bubbles that appear inside a sheet while it is being heated or immediately after forming. The same visual defect can come from absorbed moisture, gas generated by thermal breakdown, or voids already present in the extruded sheet. Those causes require different corrective actions.
Identify the source
Moisture-related blisters usually multiply as the sheet approaches forming temperature. They are often widespread and may become cloudy or rupture at the surface. A few bubbles that return in the same positions on every sheet are more likely to be extrusion voids, gels, or contamination. Fine bubbles concentrated near one heater zone point toward local overheating or degradation.
Corrective action
The material grade, storage history, and supplier drying instructions should be checked before heater settings are changed. Hygroscopic sheet must be dried with controlled temperature, airflow, and time, then kept protected from ambient moisture until forming. Sheet temperature should be measured directly; oven percentages do not show whether the polymer has exceeded its stable processing range.
If blistering remains after verified drying, the incoming sheet, regrind level, heater map, and residence time should be investigated. Do not keep increasing drying temperature in an attempt to remove bubbles caused by extrusion defects or degraded polymer.