Holes / waves after overhang with PETG-HF

[GM_Frank]

I’m experiencing a weird issue with PETG-HF printing. I’ve narrowed it down to a super simple model: a cube with an overhang wall:

And this is how it prints with PETH-HF vs PLA:

Machine is almost brand new and calibrated multiple times. Bambu studio on latest version. I’ve tried many different settings, but none of them solved the issue. Tried drying filament, using different spool, using different toolhead (stainless vs hardened steel), using different layer height (0.12, 0.16, 0.20), different infill percentage and pattern. I even tried tweaking filament settings, changing temperature (+5 degrees), changing flow ratio (+0.01), which is not suggested for official filament.

The only setting that seems to help a little bit is slowing down print speed dramatically (change outer wall & inner wall to 50 mm/s). This is how it compares:

Any idea why this is happening? Is it a machine or software issue?
I see similar discussions on PETG-HF, so I tend to believe it’s because the new filament profile is not optimized.

[GM_Frank]

Change the smooth coefficient to lowest setting (1). This also seems to print better, as it smooths out the speed change. And it does not slow down the print time very much (less than 1% slow down).

I think the reason might be due to the PETG-HF filament not actually being “high flow”, and sudden speed changes will cause extrusion problem?

[GM_Frank]

A final update here, submitted support ticket and they can also reproduce this issue so it’s definitely not a machine failure.

For now the solution is to use smooth coefficient around 0.1, or slow down acceleration to 200 mm/s2 as I shared in this post

And they promised to look into this and improve in future software. But no guarantee on the timeline