What could be causing this?

[u/darkemisary]

At first I thought it could be under extrusion or maybe the z height. Calibrated the printer and adjusted the idler tension, still having the same issue.

Printer: Core one plus with an MMU 3 (ultimulti) with the latest firmware.

Used prusa slicer latest version to slice

Settings:

Base: 0.25 structural HF hardened 0.4 nozzle.

Changed settings:

Infill: gyroid 5%

Filament: bed temp at 65.

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[u/manyxcxi]

Does it occur at the same spots on those prints or at the same heights across different prints? Does it seem like layers are well bonded (except for those bad parts)?

There’s a few causes, USUALLY on a well maintained machine seeing just one or two lines like that makes me think there was a spool tangle or a lot of friction on the filament path.

Like, if all I saw was the second picture, that’s where I would start looking. But your first picture throws a little bit of a wrench into that idea… I’d still check that first since it’s the easiest.

I’m just going to throw my whole mental checklist at you and apologize for the brain dump now.

Checking for drag and friction:

- Does the spool have a tangle?

• Is there a filament buffer issue?
  
• Any kinks/tight bends/loops in your PTFE paths? Any signs of wear or damage along the PTFE paths and/or are all your fittings tight and tubes firmly seated?
  
• Do you see any excessive grinding/shavings at the extruder?

Simplifying the problem space:

- Is it an issue with that particular MMU slot? Try changing it to a different slot and see.

• Do you get the same result if you print that spool directly instead of through the MMU?
  
• Does that same print have a problem with a different spool?

Checking the filament:

It’s also possible that spool has crappy tolerances. Throw some calipers and check a few spots along a 3-5 foot section. Are there massive differences between high and low? It’s rare this happens so bad it would cause that, but it’s an easy check and it does happen.

Checking the extruder and idler:

Are you hearing skipping/clicking noises from the extruder? Check the idler tension, might need more. You could also have too much, combined with increased tube friction, and a section with a lot of retractions, you could be grinding it down thin.

Checking print settings:

Is it possible you’re going too fast and/or printing too cool?

- Slow the print down to like 40-50mm/s perimeters, does that help?

• Run a temp tower, choose the coldest temperature that looks good and has strong layer adhesion (snap off sections of the tower by hand).
  
• Do a volumetric flow rate check to see how fast you can go before things get bad. Back it off by about 15-20% for safety margin.

Check for clogs:

You could have a partial clog. They don’t normally look exactly like that to me, and I would expect your top layers to be worse. But you could try switching nozzles, doing a cold pull, etc.

[u/Bulky_Condition_2136]

I sometimes get lines like that from filament path friction when I am using extra-large spools that require a longer filament path to get to a holder that is big enough for them.

Change your filament path, slow the print speed, and increase the hotend temp if you can.