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How I (Almost) 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 15c before it would unload the filament. After a week of use I can successfully report that this fixed the jamming issue!

How To Drop Temp Before Unloading

  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] - 15}
  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] - 15} is a PrusaSlicer macro that’ll be replaced with the nozzle temperature of the final filament being used minus 15c. So for a print where the last filament had a profile with a nozzle temp of 245c the instruction would be M109 R235.

Update: 26-03-17 - Still Jamming With Other Filaments

I’m somewhat bummed to report that this did not fully address my jamming issue and I’m still getting jams with some filaments. I suspect each filament needs to be fine tuned with how much to drop the temperature by, and also explore adjusting the ramming settings within PrusaSlicer to really dial everything in.

Since I was primarily using the MMU3 to let me print my 5 most common colors without manually loading / unloading spools and don’t print multi color prints I’ve decided to remove the CoreBoxx from my printer and go back to the normal single spool setup.

I had high hopes for the MMU3 and really thought it was the future but maybe I’ll try again when INDX comes out.

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.

  • If you still experience jams after adding a temperature drop before unload but they happen less frequently try increasing the drop in temperature. For example I started off with a 10c drop but found I was still getting a jam once in awhile so I stepped it up to 15c.