Bad layers at certain heights, SOMETIMES consistent

Bambu Lab P1 Series Troubleshooting
1

For a while, this was a rare error. But recently It’s been happening almost every print. The weird thing is that, once the bad layers happened exactly the same way with the same gcode file (I have the two prints next to each other in the photo…from two different print sessions, but the same gcode file). For a while, I thought it was something going wrong with the slicer. However, I looked at two different failed prints, from two different STLs, and noticed that both had the error at the same specific height.

pauldrons
pauldrons1920×1083 198 KB

axe failure
axe failure1920×1440 249 KB

I’ve attached a 3MF file that had the error happen (weirdly not at the typical heights, but the same error):

Hawks goggle buck ERROR.3mf (857.5 KB)

I cleaned the z-axis screws, to see if something was jamming there. But the print I did right after that still had the failure. Everyone on Reddit says that i need to clean all the belts, and I will, but for the life of me I cannot understand why on earth having the XY belts dirty would cause failures at specific heights, at a semi-consistent rate (I have a feeling most of those people just looked at the photos and didn’t bother reading anything I wrote).

Also, I’ve ruled the slicer out because a gcode file that previously printed perfectly just failed (at one of the heights that the failures usually occur).

Assorted details:

  • I exported the prints as gcode to the SD card that came with the printer. My printer is too far away from my wifi signal to connect to the app, so getting diagnostic data is tricky.
  • I’m printing in PETG with a 6mm nozzle.
  • Bambu Studio is up to date
  • I’m feeding my PETG from a 3rd party heated drying box (I originally thought the filament might be jamming somewhere between this and the hot end).
2

This issue is well known and it generally occurs when there is a change in speed of the nozzle. One way to test it is to slow down the print using quiet mode and see if the printer behaves differently. Note that the printer speed could be at the edge of tolerance and just a slide change in model density, temperature or humidity can contribute.

This YouTube video while not directly applicable to the example you provided, does show this principle in effect. Look at his remedy at time index 2:20.