Bubbles formed by the thermal degradation products of the material. Prevented by managing temperature and residence time.
Gas bubbles are gas released through thermal degradation of the material leaving voids inside the part. The difference from moisture-related bubbles is that the source is not water coming in from outside but the breakdown of the material itself. That is why drying the raw material does not solve the problem; you need to look at the temperature and residence time side.
A wrong diagnosis leads to turning the right parameter in the wrong direction. This defect can be confused with the ones below; the right-hand column gives the practical way to tell them apart.
| Confused with | How to tell them apart |
|---|---|
| Moisture Streaks | A moisture streak leaves a silvery line on the surface and disappears with correct drying. A gas bubble is inside the part and does not change with drying. |
| Air Bubbles | An air bubble is air coming in from outside; it can be reduced by increasing the back pressure. A gas bubble arises from degradation of the material. |
In materials with low thermal stability: POM, PVC and flame-retardant compounds come first. With these materials the processing temperature range is narrow, and approaching the upper limit quickly starts degradation.
Reduce cycle pauses and check your shot volume against the machine capacity. If you are working below 20% of the screw capacity, the material waits in the barrel longer than necessary; a smaller injection unit is the right solution.