[Solved] MMU2S jamming on unload for certain filaments  

RE: MMU2S jamming on unload for certain filaments

I have the same problem since months. I observed several times, that the extruder is running in "load" direction while the MMU2S tries to unload the filament. Thus, the filament is drawn into the extruder instead of unloading.

RE: MMU2S jamming on unload for certain filaments

Try to look at the screws tensioning the MMU assembly ( https://help.prusa3d.com/en/guide/5-pulley-body-assembly_43152, step 32). If they are loose, the MMU does not pull the filament enough and the extruder pulley could do this glitch. 

RE: MMU2S jamming on unload for certain filaments

As someone who used to have a lot of issues with my MMU2S not unloading filament. Not sure if this is was just a coincidence but I read somewhere about adding a heatsink (had some explanation about causing it to bulge unevenly) to the extruder motor when I was trying to figure out how to fix issues with the unloading. So I added two heat sinks and this was like a year ago, and since then I have not had any unloading issues. I use the stock profiles on PrusaSlicer. 

The only changes I have done to the MMU2S, other than adding the heatsinks are using the Auto Rewinder for the filaments. The only change I've made to the printer is using BuildTak sheet to the plate. 

I just set up another printer at work with an MMU2S, and after the first print, it had issues unloading the filament too (single colour). So going to try the same thing by adding heatsinks to the extruder on it and report back in case it helps.

RE: MMU2S jamming on unload for certain filaments

Hey. I have the Same Problems with my MMU2S. Every unload fails due to a little hook that gets stuck above the extruder gears. Have you been successful with these heatsinks on your second printer?

RE:

Still, no one has offered a solution for this, I am also facing the same problem although I have tried to fix it but nothing works.

RE:

Yes, both printers have had way better results with the heatsinks. I've noticed some filaments need their number of cooling moves to be changed to 2 or 3, especially after sitting out for a while, typically don't have any issues when they are new. Overall, the heatsinks reduce the flat heads from forming every time at least. So that's a win. 

If you pull out the filament and it looks like a bulge on either side caused by the extruder gear or the filament typically is jamming at the extruder, the heatsinks will definitely help. If it's some other issue, like the PTFE tube gap, it may not help as much.

Check out this video too:

I haven't tried everything here myself, but adjusting the cooling moves did help with the older filaments. I've been too lazy to bake the filaments or keep them sealed, really need to work on that.

RE:

I have this issue using the single profile in PrusaSlicer and when adjusting Z.  Both end up with the hook pancake that jams.  I have heat sinks, but even with short prints it jams with the same filament oddity.  I think the ramming gcode may be causing the issue.

Looking at the gcode in PrusaSlicer 2.4.2 it does load once more prior to the unload.  I may delete it in the End Code under Machine settings in PrusaSlicer and see what it does with the filament that had the issue.  When using the Mulimaterial profile is unloads 15 mm of filament prior to the unload, there is not a load before it using that profile (except the wipe tower).  I'm not sure if this will help anyone, but it was fun digging into Gcode for me 🙂

;Single Filament using MMU2s Unload
; Filament-specific end gcode
G1 Z3 F720 ; Move print head up
G1 X0 Y210 F7200 ; park
G1 Z51 F720 ; Move print head further up
G1 E2 F5000 ; Feed rate 5000
G1 E2 F5500 ; Feed rate 5500
G1 E2 F6000 ; Feed rate 6000
G1 E-15 F5800 ; Unload 15mm @ Feed Rate of 5800
G1 E-20 F5500 ; Unload 20mm @ Feed Rate of 5500
G1 E10 F3000 ; Load 10mm of filament @ Feed Rate of 3000
G1 E-10 F3100 ; Unload 10mm of filament @ Feed Rate of 3100
G1 E10 F3150 ; Load 10mm of filament @ Feed Rate of 3150
G1 E-10 F3250 ; Unload 10mm of filament @ Feed Rate of 3250
*Load before unload*
G1 E10 F3300 ; Load 10mm of filament @ Feed Rate of 3300


M140 S0 ; turn off heatbed
M107 ; turn off fan
M702 C ; Unload Filament


;Multimaterial Profile using the MMU2s Unload (there is more prior to this due to the wipe tower, etc - end gcode is diferent if not using a wipe tower and has the same load 10mm filament as the single profile).

G1 E-15 F3000 ; Unload 15mm @ Feed Rate of 3000

M140 S0 ; turn off heatbed
M107 ; turn off fan

; Unload filament
M702 C ; Unload filament

RE: MMU2S jamming on unload for certain filaments

Thanks for the update. What is the type/model of heatsink you used?

RE:

Same issue. PETG was printing without issue. I swapped to PLA and it clogs every time. I'm also having the gears eat through the PLA filament. Originally I was printing with a steel nozzle, and I swapped to bronze because I thought that may be part of the problem. The clogging got worse with the bronze nozzle. After changing to a bronze nozzle, I lowered the print temp and that did not help. I raised the temps back to the steel nozzle temp and it still clogged.

To me, it looks like the PLA is solidifying and breaking off in the extruder gear. If I manually pull it out the filament forms a good tip and doesn't break/clog. Allowing the mmu2s and extruder to remove filament is breaking it off nearly every time.

Is there a way to increase the nozzle temp prior to removing filament and also slowing down the removal speed?

P.S. I think I have answered my own question about the heatsink.

RE: MMU2S jamming on unload for certain filaments

You can change the unload speeds in PrusaSlicer per filament. It's in the Filament Settings Tab -> Advanced. You need to have expert mode enabled to see the settings. This is also where you change the number of cooling moves. Try increasing this also to 2.

As for the heatsinks, I got these https://www.amazon.com/dp/B07D4FVC61/ not sure if they are the best, but they work.

RE: MMU2S jamming on unload for certain filaments

Thanks! The number of default moves was 4. I've decreased it to 2. I've also slowed down the unload speed as I'm having the bondtech gears chew into PLA.

RE: MMU2S jamming on unload for certain filaments

Hi.

I had same problem. Maybe this will help you:  https://forum.prusa3d.com/forum/original-prusa-i3-mmu2s-mmu2-assembly-and-first-prints-troubleshooting/the-hook-of-death/#post-617034

RE: MMU2S jamming on unload for certain filaments

I don't know if this will help anyone.  I re-printed the extruder parts for the MK3s+/MMU2s in ABS.  I'm not sure what was happening with the PETG parts (something may have worn out over time or an assembly issue from the last bad jam), but after changing parts and re-assembling it is no longer getting stuck.  The extruder is also quieter.  Something must have been out of alignment.  I'm not sure why that would affect the single profile more than the multiple profile.  I've been running it all day with no issues and the tip on the filament even looks better.  The PTFE tube looked fine and undamaged, so I did not replace it. (I'm not saying reprint in ABS - that was just me dialing in ABS with nothing better to do.  Check assembly and see if anything has shifted over time). 🙂

RE: MMU2S jamming on unload for certain filaments

Two hours into a print and I suddenly started getting unload/load issues.  I noticed my extruder motor was extremely hot to the touch.  I am going to try some heatsinks as suggested by George P and see if anything changes.  I think it's possible the motor starts out cool and that's why I don't have the issue at the start of prints.

RE: MMU2S jamming on unload for certain filaments

I found the problem causing the filament unloading jams to be due to a worn extruder PTFE tube. There are pictures and a detailed description from @xuis here:

https://forum.prusa3d.com/forum/original-prusa-i3-mmu2s-mmu2-assembly-and-first-prints-troubleshooting/the-hook-of-death/#post-621493