Surface Defects caused by Speed Irregularities (Especially Overhangs)

Bambu Lab Software Bambu Studio
1

Machine: P1S
Hotend: Bambu Lab 0.4mm Hardened Steel
Gears: Bambu Lab Hardened Steel
Outer Wall Speed: 100, 150, 175, 200, 225, 250mm/sec (happens on all)
Wall type: Classic
Wall Order: Inner/Outer (as there are overhangs)
Detect overhangs: Enabled
Calibrations completed: Temp Tower, Flow Dynamics, Flow Rate (Bambu Studio) plus Adaptive Pressure advance (30 values) in Orca/Anycubic Next. Tolerance Test (within 0.01mm).

Filament: Elegoo PETG PRO, RAPID, Flash Forge PETG CF, Tinmorry PETG CF, etc… (all filaments I run show the same issues, just to a different extent, including active-drying filaments in my ACE PRO).

Humidity in my AMS 2 PRO: 18-22% (not a wet filament issue or the defects would appear on the entire model, not repeatedly on the irregular speed side).

I’ve been running countless calibration tests to fine-tune my profiles for my P1S on a specific part I will begin producing en-masse, once I finally get the issues resolved with the print. For whatever reason, defects only occur on one side of the print, and only on exterior walls (I printer inner/outer using classic wall generator as I often have overhangs). On all other sides, the surface quality is perfect. Here is an example of what I’m talking about where I printed two copies of a section of the part (which has the most defects) on the same plate and am showing the two different sides of said part:

IMG_8293

This surface defect happens no matter what settings I activate/deactivate, calibrate or tune, or speed settings I’ve applied (even slowing it down substantially). The defect does not happen on the interior wall layers (I’ve watched countless times as it prints). Although I’ve been able to reduce the defects significantly with various settings in the software on various test prints, I have not been able to eliminate them entirely, and they always occur on the same area, no matter how the part is oriented on the print plate.

Today I was watching another comparison round of 4 of these parts while printing and noticed “the side that has the defects is printing dramatically faster than the other side of the part”. On the left side it prints nice and slow, then as it rounds the corner to do the opposite side of the part, it speeds up like crazy and boom, defects occur. Here is an example from the slicer showing this speed irregularity (blue is slow, green is faster, yellow is even faster, orange is fastest):

Defects caused by speed irregularities

I have Slow Down for Overhangs enabled as well as slow down for curved perimeters. I have my overhang speed set to 5mm/sec for 50%, 75% and 100%. Despite both sides having the same overhang, the software is only applying the proper speeds to once side. I also tried the same part in Orca and the same irregularity exists there as well using the same exact settings imported from Bambu Studio.

I would appreciate any advice on how to modify my print settings (and which ones specifically to modify) in order to have both sides of the part printing at the same speed, without slowing down the entire model. If I can achieve this, I am almost 100% positive that all remaining defects will be eliminated from the print and I can move into production.

Thank you in advance for any guidance you can offer.

2

Do you have an Aux fan and is it blowing on the side with the defects?

This looks like a temperature issue.

I don’t see any mention of extrusion temperature. If you don’t have an Aux fan to turn off, try bumping extrusion temp up by 10º. Or if you’re really patient, run a temp tower cal.

No mention of the plastic you’re using, either. Whatever it is, is it dried out? Especially if you’re in the northern hemisphere, ambient humidity levels are generally quite a bit higher now that it’s springtime. Plastics like ABS, ASA, PA and PETG rapidly absorb moisture from the air, and that will effect flow. Doesn’t look like this is the issue from the pictures you posted, but it’s worth eliminating as a contributing factor if you can.

3

As I mentioned, it doesn’t matter what way I orient the part, the defects always occur on the same side, no matter if it is facing the auxiliary fan or away from it. Typically speaking the side the has no defects is facing the auxiliary fan, which is turned on.

A temp tower is the first calibration test I run. That comes before the flow rate and pressure advance testing, always. I also have this on Orca with adaptive pressure advance enabled with 30 readings (5 acceleration values @ 6 speeds per value). I am not some amateur here - my profiles are more tuned than 99% of users.

If you’re asking about dried or non-dried filament, you’re ignoring the fact that this defect only happens on one-side and is speed-related. Wet filament would not cause defects on only one side of a model - this is a very amateur comment on your part, ignoring what is already stated/displayed. This happens on all filaments, including ones I have actively drying during the print on my other machines, as all machines run a very similar slicer (Bambu Studio, Orca, Anycubic Next).

4

For what it’s worth, I’ve found a way to balance out the speed discrepancy created by the slicer. It can only be controlled via the “scarf joint seam (beta)” function, by reducing the scarf joint speed which effectively balances out (not perfectly, but a lot better than the software on its own) the speed on both sides of the part, which have identical overhangs. No more defects/artifacts.

Take a look:
Scarf joint seam beta work-around