Main Causes of Bubble Defects
Moisture in raw materials PP pellets absorb tiny moisture from the environment. Wet raw materials vaporize under high extrusion temperature, forming air bubbles trapped inside the sheet.
Improper extrusion temperature setting Too‑high barrel temperature will over‑melt PP material and generate volatile gas. Too‑low temperature causes incomplete plasticization, bringing air into the melt.
Screw & feeding system issues Unstable feeding speed, screw wear or improper back‑pressure cannot fully exhaust gas inside molten PP, and air will be wrapped into the hollow sheet.
Mold and calibration problems Blocked mold vents, dirty die surface or incorrect calibration vacuum will introduce air during sheet forming.
Practical Solutions to Remove Bubbles
✅ Dry raw materials before production Dry PP granules properly to remove moisture. Keep drying temperature and time according to material grade. Do not use damp recycled materials directly.
✅ Optimize extrusion temperature profile Adjust barrel and die temperature. Avoid excessive heating. Ensure full but not over plasticization for PP melt.
✅ Adjust screw back pressure and feeding rate Set suitable back pressure to improve melt homogeneity. Keep consistent feeding speed to prevent air suction. Inspect screw wear regularly.
✅ Check die vents and vacuum calibration Clean extrusion die frequently, keep vent channels unobstructed. Stabilize vacuum level for sheet calibration to avoid air infiltration.
✅ Control recycled material proportion High‑content regrind material may carry gas and impurities. Mix recycled pellets with virgin PP within a reasonable ratio.
Quick On‑site Check List
- Are raw materials damp?
- Is barrel temperature too high?
- Is screw back‑pressure sufficient?
- Are die vents clean and open?
- Does vacuum calibration work normally?
Summary Bubble faults on PP hollow sheets mostly come from moisture, volatile gas and entrapped air. Start checking from raw material drying, temperature parameters, screw condition and die vacuum. Step‑by‑step troubleshooting can effectively eliminate bubble defects and improve finished‑product yield.
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