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.