Core One MMU3 – Filament not retracting fully (possible buffer push-back)
New MMU3 on Core One with buffer mounted to the printer. During prints, filament doesn’t retract deep enough into the MMU3, leaving it sticking out and causing jams. Not sure if this is a retraction issue or push-back from the buffer.
Steps I took so far:
- Checked FINDA sensor alignment (moved closer, now detects correctly).
- Verified filament unload length in settings.
- Inspected buffer mounting and PTFE path for friction.
- Tested with multiple filaments/spools → same issue.
Added pictures for clarity:
RE: Core One MMU3 – Filament not retracting fully (possible buffer push-back)
I just had the exact same issue my my MMU3 on my CoreONE.
MMU3 was just assembled as per the assembly instructions. I was having constant filament jams within the MMU. My filaments weren't being retracted fully, and would get caught/jammed/bent in the MMU. This would cause the MMU to jam, stop working, and caused numerous "homing errors". I was getting jams multiple times per layer.
It turns out, I had the buffer mounted on the side of the CoreONE (with the magnetic brackets) as per the instruction manual, with the the bottom of the buffer sitting flat on the desk/table. This decreased the amount of room for the unloaded to filament, and was causing the buffer to be "full" and would push the filament back towards the MMU.
I moved the buffer higher on the side of the printer (the standard magnetic mounts are barely strong enough to hold it up), which allows the filament in the buffer to use all of the available space.
Buffer mounted higher up the side of the printer (bottom of buffer above surface of the desk):
Buffered filament can now use the space below the buffer, preventing the buffer from being "full" and pushing the filament back towards the MMU:
Since I moved the buffer up, I have had ZERO issues with the MMU. I am now over a 100 filament changes in with zero issues.
The base of the buffer really should have legs/stands/supports so that the buffer can't be set directly on the surface of a desk/table. I'm going to design a replacement part that clips on and provides a base or legs to hold the buffer up off of the desk.
Hope this helps.
RE: Core One MMU3 – Filament not retracting fully (possible buffer push-back)
Wow @dumalsco! Thanks for the tip, I didn't notice that the buffer shouldn't be on the floor. I now moved it and it works better, but still not perfect.
I see you're using PolyTerra PLA, are your tips "pointy" as suggested in the instructions? Did you change the temperature or the ramming?
Thanks!
RE: Core One MMU3 – Filament not retracting fully (possible buffer push-back)
You're welcome @alonmln!
To be fair, the instructions show it sitting on a table. Maybe that works with the enclosed MMU version (I think it has a shorter PTFE tube, so there would be less filament being unloaded). A note in the instruction manuals would be handy.
Right now, most of my PLA is PolyTerra. Most of my PETG is Prusament.
I usually cut a sharp tip on the filament before loading it into the buffer and mmu. I used the default "Generic PLA" profile in the PrusaSlicer with all of my PolyTerra PLA, seems to work well. I haven't tweaked the the temperature or ramming settings at all with this PLA. My last print had around 50 filament changes.......the MMU had zero problems and completed the print just fine.
After getting your buffer up off the floor with adequate space below it, do you still have filament sticking out of the MMU after it's unloaded from the nozzle? How long is your PTFE tube between the MMU and extruder? The PTFE tube length is set correctly in the printer? Your filament sensors are working correctly (both of them)?
Hoping you get things working smoothly soon!
RE: Core One MMU3 – Filament not retracting fully (possible buffer push-back)
FYI, I've created a quick modification to the bottom plate holders for the buffer. I've added a 90 degree support to allow the buffer to sit securely on the frame of the CoreONE. The buffer is much better supported this way, and maintains adequate space below the buffer to prevent filament pushback towards the MMU (causing jams).
MMU3 Buffer Plate Holder - Bottom Support Bracket Upgrade
RE: Core One MMU3 – Filament not retracting fully (possible buffer push-back)
@dumalsco It’s working perfectly now!
I printed your MMU3 Buffer Plate Holder – Bottom Support Bracket Upgrade, and it works like a charm. I’m surprised this isn’t a standard part of the kit.
Yesterday, I printed a part with 170 color changes—not a single one had an issue. Admittedly, the tip still isn’t shaped perfectly, but it doesn’t seem to affect the process.
Thank you again - you’re a lifesaver!
RE:
Awesome, that's great to hear! If the tip isn't perfect and it doesn't load correctly into the extruder, after three failed loading attempts (assuming you have the "cutter" setting enabled), the MMU will slide the carriage all the way to the right, push out a small amount of filament, and slide the carriage across cutting it with the blades. I've seen this happen once. After the fresh cut, the filament loaded just fine, and the print continued.
Glad I could help! Happy printing.