Air Bubbles
Bubbles formed inside or on the surface of the part by air trapped during injection. Solved within the triangle of decompression, back pressure and venting.
Air bubbles are air trapped in the melt leaving voids inside the part. The difference from a vacuum void is that the void is formed not by cooling shrinkage but by air coming in from outside. The source is usually air drawn from the hopper during plasticizing, excessive suck-back, or air that cannot escape inside the mold.
How to Recognize It
- If the bubbles are also visible in the injected melt, they are caused by air.
- If they decrease when decompression is switched off, the diagnosis is confirmed.
- If changes to holding DO NOT AFFECT the bubble size it is air; if they do, it is a vacuum void (the critical distinction).
- The defect is already visible in partial fills.
Physical Causes
- Excessively high/fast decompression sucks air in front of the screw.
- Insufficient back pressure: the pressure gradient inside the screw cannot push the air back toward the feed.
- An excessive metering stroke (it should not exceed 3 screw diameters) or feed problems.
- The melt traps air in unvented pockets in the mold.
Solution Steps at the Machine
Apply the steps in order and one at a time: after each change, wait a few cycles and assess the result. If there is no improvement, undo the change and move on to the next step.
- Slow down and/or reduce the decompression.
- Increase the back pressure and adapt the screw speed.
- If the defect started suddenly, check the granulate charge, the feed and the throat temperature.
- Check the barrel temperature profile against the manufacturer's instructions.
Mold, Design and Material Solutions
- Check and improve the mold venting.
- If necessary, a larger plasticizing unit (so that the metering stroke comes down into the 1-3D range).
- Check the unit for function, cleanliness and wear.
Important Notes
- ⚠️ Parts with voids are weaker than parts without; carry out internal checks by cutting non-transparent parts at random.
Which defect is it confused with?
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 |
|---|
| Vacuum Voids | A vacuum void forms in a thick section through cooling shrinkage and gets smaller when holding is increased. An air bubble is air coming in from outside and does not get smaller with holding. |
| Moisture Streaks | Moisture vapor makes silvery lines on the surface. An air bubble sits inside the part as a round void. |
Frequently asked questions
Does increasing the back pressure reduce bubbles?
Usually yes, because back pressure compresses the air in the melt and pushes it back to the hopper. But too much raises the shear heat and leads to burn marks; increase it gradually and assess the result at each step.
What does it mean if the bubble always appears in the same area?
A bubble in a fixed position shows air trapped inside the mold; the venting at that point is insufficient. Scattered bubbles in varying positions, on the other hand, come from plasticizing.
Prepared by:Uğur Çamlıca· Teknik Part · Source: manufacturer documentation and field practice