Air Streaks
Matte, silvery or white marks left on the surface by air that cannot escape during filling. Solved with decompression and venting settings.
Air streaks form when air drawn in between the granules during plasticizing mixes into the melt and comes out on the part surface. They usually appear as lines following a particular path from the gate. They resemble moisture streaks, but their source is air, not water; that is why drying does not fix them.
How to Recognize It
- Matte, silvery or white marks; often AROUND domes, ribs and sudden changes in wall thickness.
- Air hooks are seen near lettering/embossing and recesses.
- If reducing decompression or lowering the injection speed reduces the defect, the diagnosis is correct.
- Marks near the gate usually come from air drawn in by decompression.
Physical Causes
- Air that cannot escape in time during filling is drawn to the surface and stretched in the flow direction.
- Excessive or too-fast decompression sucks air in front of the screw; this air is carried into the mold with the injection.
- Insufficient back pressure: air drawn in during plasticizing cannot be expelled back toward the hopper.
- Unvented areas in the mold (bases of ribs, blind holes).
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.
- Reduce the decompression amount and the screw retraction speed.
- Increase the back pressure and adapt the screw speed.
- Adapt the injection speed according to the defect (usually reduce it).
- If bubbles are visible in the injected melt, check the granulate feed and plasticizing capacity (the metering stroke should be 1-3 times the screw diameter).
Mold, Design and Material Solutions
- Add venting to the problem areas; round off the sharp edges of transitions.
- Reduce the depth of engraving/embossing or move the gate.
- Using a shut-off nozzle reduces the need for decompression.
- Check the sealing of the nozzle.
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 |
|---|
| Moisture Streaks | A moisture streak disappears completely when the raw material is dried correctly. An air streak is not affected by drying; it changes with the back pressure and feed zone settings. |
| Air Bubbles | A bubble leaves a void inside the part. An air streak remains as a line on the surface; both can arise from the same source. |
Frequently asked questions
Why does back pressure work?
Back pressure compresses the melt as the screw comes back and lets the air inside it escape back to the hopper. Increase it gradually; too much raises the shear heat and produces burn marks.
Does the feed zone temperature have an effect?
Yes. If the feed zone is too hot, the granulate softens early, bridges and traps the air between the granules. Lowering this zone's temperature a little is an often-overlooked but effective correction.
Prepared by:Uğur Çamlıca· Teknik Part · Source: manufacturer documentation and field practice