Problem overview
Bubbles, voids and blisters are caused by gas or vapour trapped in the melt. The gas usually comes from moisture or volatiles in the compound, air that was not removed by venting, or degradation of the PVC itself.
The location and shape of the bubbles is a useful clue: fine bubbles throughout the wall usually point to moisture or venting, while bubbles with discolouration usually point to overheating.
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Use PVC Problem SolverCommon symptoms
- Small bubbles on the outer or inner surface
- Voids visible in the cut wall section
- Blisters with yellow / brown discolouration
- Bubbles that appear mostly in humid weather
- Material coming out of the vent port
Possible causes
| What you see | Possible causes |
|---|---|
| Fine bubbles in humid season | Moisture in compound, filler or resin; insufficient hot-mix time; poor storage |
| Voids across the wall | Vacuum vent not working properly; blocked vent; low vacuum |
| Bubbles with discolouration | Degradation — barrel / die temperature too high, insufficient stabilizer |
| Melt or powder in vent | Incorrect fusion before vent zone; temperature profile or lubrication imbalance |
What to check
- Moisture in calcium carbonate and the mixed compound
- Hot-mixer discharge temperature and mixing time
- Vacuum level and cleanliness of the vent port
- Barrel and die temperature profile
- Whether the compound is stored covered and in a dry area
Processing and formulation factors
Moisture
Fillers absorb moisture. If the hot mix does not drive it off, it turns to steam in the extruder.
Venting
The vent must remove air and volatiles. A blocked or low-vacuum vent leaves gas in the melt.
Degradation
Overheated PVC releases gas. Bubbles with colour change usually mean temperature or stabilization should be reviewed.
Practical troubleshooting checklist
- Check vacuum gauge reading and clean the vent
- Check hot mix temperature and time
- Test filler and compound moisture
- Verify actual barrel and die temperatures
- Store compound covered and use it within your normal time
When to contact a technical team
- Bubbles continue after vent and moisture are corrected
- Bubbles appear together with yellowing
- Problem started after a raw-material change
This guide gives general starting points. Results depend on your formulation, raw materials and machine; changes should be confirmed with controlled trials.
Frequently asked questions
What causes bubbles on PVC pipe surfaces?
Usually moisture, poor vacuum venting, or degradation of the compound from excess temperature.
Why do bubbles increase during the monsoon?
Fillers and compound pick up more moisture in humid weather. Mixing and storage conditions become more important.
Can high temperature cause bubbles in PVC pipe?
Yes. Overheating degrades PVC, which releases gas and often shows discolouration with the bubbles.
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