Internal voids and gels originate during sheet extrusion, casting, compounding, or contamination control. Forming stretches and magnifies them, making a small defect in flat stock appear as a bubble, streak, hard inclusion, or optical distortion.
These defects often recur in the same location relative to the sheet roll or extrusion direction rather than the mold. They may not respond to drying or normal heater changes.
Confirm the incoming defect
Flat sheet from the affected batch should be inspected with transmitted and reflected light. Samples should be marked before forming so the same defect can be tracked into the part. Rotating or shifting the sheet relative to the tool helps separate stock from machine position.
Affected material should be quarantined and documented with batch, roll position, and photographs. Process settings should not be driven outside the validated window in an attempt to erase an inclusion that is physically present in the sheet.