These air pockets damage transparency, weaken sheet strength, and cause failures in downstream thermoforming, printing and lamination. For sheet producers and processors, identifying root causes fast helps cut waste and improve final product quality.
What bubble defects look like
- Tiny scattered micro‑bubbles inside the sheet
- Large visible voids or surface blisters
- Hazy, cloudy appearance instead of clear transparency
- Cracks or broken spots during thermoforming processing
Main Causes & Practical Solutions
| Root Cause | Detailed Explanation | Practical Solution |
|---|---|---|
| Excessive moisture in PET pellets | PET material easily absorbs humidity from air. Undried moisture turns into steam under high extrusion temperature and forms bubbles inside molten plastic. | Control raw material moisture below 50‑100 ppm. Optimize drying temperature & time; seal raw material storage bags against damp air. |
| Poor vacuum venting of extruder | Volatiles and trapped air cannot be fully drawn out from the barrel. Gas stays mixed with PET melt before sheet forming. | Inspect vacuum system, keep vent ports clean and unblocked. Check for vacuum leakage. |
| Improper extrusion temperature setting | Over‑heating leads to PET degradation, releasing small gas molecules. Too‑low temperature causes incomplete melting and air entrapment. | Adjust barrel & die temperature profile according to material grade; avoid local overheating. |
| Air entrapped during feeding | Too‑fast feeding speed brings extra air into screw plasticizing zone. | Lower feeding rate properly, adjust screw back‑pressure for better melt compaction. |
| Impurity & contaminated raw material | Dust, dirty recycled flakes bring foreign gas and volatile substances. | Filter raw material strictly; use qualified recycled PET flakes; keep hopper clean. |
Quick Troubleshooting Checklist (On‑site production)
✅ Confirm pellet moisture test result first (No.1 killer of bubble‑free PET sheets)
✅ Check vacuum pump working status and vent blockage
✅ Verify temperature curve of extruder barrel and T‑die
✅ Check hopper sealing against moist workshop air
✅ Replace or clean melt filter screen if contaminants are suspected
Final Note
Most PET sheet bubble issues come from moisture and poor degassing. Follow standard drying and extrusion parameters, perform regular equipment inspection, and you can largely avoid this annoying defect.
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FAQ
Q: My bubbles only appear in thermoforming step, not on original sheet rolls. Why?
A: It usually means the sheet has absorbed moisture during storage. Keep PET sheets well‑sealed; pre‑dry sheets before thermoforming processing.
Q: Can high‑quality extrusion equipment reduce bubble risk?
A: Yes. Well‑designed vented extruder systems, stable vacuum units and precise temperature control greatly lower bubble rate for both virgin and recycled PET raw materials.