Gas bubble on worm thread surface

[CP Yu]

Hi,
I have a molding issue is bubble on worm thread surface.
It is not on same position & cavity, and the defect rate is less than 0.5%.
We have do many testing on molding parameter (Reduce injection speed, increase back pressure), material drying and mold venting.
Have improvement but still cannot eliminate.
Do you have any suggestion? Thanks.

342

[Pilot]

please check the sprue and nozzle conditions. wearing, sharp edges.. etc. Do you have hot tips?

[chrisprocess]

What material?

[CP Yu]

No hot tips.
Why spure and nozzle will cause bubble?

[CP Yu]

PA46, Stanly TW-341.

[chrisprocess]

Strange it looks like blistering of some sort. Usually bubbles I've seen don't blister out like that.
Material says it uses a release agent, heat stabilizers, and is lubricated.
Maybe delamination? Only thought would be to lower melt temps and shear rates (lower screw recovery, lower back pressure).
Also seen improvement in delamination with various fill times - slow or fast.
Would be interesting to see if the blister(?) is in the same spot or if it moves with various fill speeds.

Or perhaps lack of cooling on core/cavity. This material runs rather hot, would need to check thermal stability, and add mold closing delay (a few seconds?) to cool down before injecting plastic to see if this helps. The steel will cool faster without the part on it.

Also couldn't hurt to reach out to the material manufacturer!

[rickbatey]

Where is defect relative to gate? How is the resin gated into the office part? I believe it is a product of the melt front flow in the cavity. I think your melt is jetting into the cavity and results in gas trap in the thread feature. But localized hot spots on the core could cause this too. Is the defect seen on the same parts all the time? Could be a tool issue like venting or melt front being sheared in the runner.
Rick.

[Joel JS]

When you get something that has such a small amount of defects, the best way to figure it out is to try to make it worse so you can actually see what is causing it...... so, try no back pressure for example, it it gets worse quickly,....you'll know that back pressure needs to be moderate to high. Try some poorly dried material....etc...

Joel

[CP Yu]

If by jetting, it may have high defect rate and bubble will be on same position, but seems not.
Have considering on the material decomposited then caused bubble, this may related to high shear rate & heat generated by small gate size.