Seams under-extruded on overhang on a round item using TPU for AMS, any ideas?

Bambu Lab X Series Bambu Lab X1C
1

Does anyone have any ideas on how to tune this in Bambu Studio to solve this problem?

I’m using TPU for AMS. I’m very happy with it, except for this issue with seams on an overhang!

It’s a round part and as you can see from my photo, the seams all look under-extruded.

NOTE: The photo shows the parts upside-down as printed. I have them bottom up so you can see the issue.

I’ve tried adjusting many things in Bambu Studio, but I can’t seem to affect how this seam prints.

If I print this in PLA, it looks perfect!

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To me, it looks like nozzle pressure is dropping at the seam and can’t catch up fast enough. It’s less pronounced at the shorter end of the part next to the bed.

The part is round, but has a taper, so it takes longer around the top, than at the bottom, next to the bed.

I’d appreciate ideas anyone has to solve this! I’ve spent a good part of the day trying various adjustments with not much change to fix this.

Printer is an X1C and using Bambu Studio, both using up-to-date firmware.
TPU for AMS, dried 8 hours.

2

I had to calibrate my spool of TPU for AMS. The pressure advance changed significantly. It wouldn’t hurt to run flow ratio as well.

3

To reduce rapid flow changes, I would vastly reduce acceleration values, at least for the walls. I haven’t used TPU for AMS yet, but for softer TPU, pressure advance wasn’t able to fully compensate fast accelerations for me satisfyingly.

With reduced acceleration, pressure advance / flow dynamics calibration was able to make the results perfect.

4

I found part of the problem.

I was using scarf seams. When i turn scarf seams off, then the problem goes away.

But I thought scarf seams were not supposed to apply to overhangs?

Perhaps this is a bug in Studio? Or I need to tune the scarf settings, which otherwise look good everywhere else on the print.

5

That is the definition of incorrect pressure advance. Flow is not changing in sync with velocity (speed and direction).

6

Does anyone know which settings can tune scarf overhang settings?

PLA may have looked perfect if I didn’t have scarf seams turned on in the filament settings.

Scarf seams look great, except on this circular overhang area.