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How I Fixed My MMU3 Jamming When Finishing Prints

Shop and Tools 3D Printing

I run a CoreBoxx on one of my Prusa Core Ones and overall I’ve been happy with it but for awhile I kept running into issues with the MMU3 jamming when attempting to unload the filament at the end of a print.

I primarily print PETG and found through lots of testing that the likelihood of my MMU3 jamming on final unload would increase the hotter I was printing at. The jam would always occur down in the Nextruder due a to a blob or curve on the filament tip so to try and address it I fiddled with the ramming settings in PrusaSlicer but wasn’t able to make any major improvements.

Another option I explored was lowering my nozzle temperature and by dropping the nozzle about 10c - 15c the jams went away, however the layer adhesion of my prints suffered greatly.

Since I knew that dropping the nozzle temp down would fix the jams but I couldn’t lower the nozzle temp until the print had finished I decided to tweak my printer’s end print gcode to drop the nozzle temp by 10c before it would unload the filament. After a week of use I can successfully report that this fixed the jamming issue!

How To Fix

  1. Within PrusaSlicer, navigate to your Printer’s profile and click on Custom G-code.
  2. Scroll down to the End G-code section.
  3. Look for the M702 ; unload the current filament line and just above it add: M109 R{temperature[current_extruder] - 10}
  4. Save your changes and reslice any files as needed.

How it works

M109 is the wait for hotend temperature instruction and R<temp> tells it to update the target extruder temp of the printer and to wait for the new temperature to be reached (via heating up or cooling down) before proceeding.

{temperature[current_extruder] - 10} is a PrusaSlicer macro that’ll be replaced with the nozzle temperature of the final filament being used minus 10c. So for a print where the last filament had a profile with a nozzle temp of 245c the instruction would be M109 R235.

Notes

It’s possible this fix may not address your jamming issues. Some jams are due to too much friction from excessive bends on the PTFE tube or from improper tension on the idler in the MMU3. I would only suggest trying this fix if you are experiencing bad filament tips getting stuck in the nextruder.