MK3(S) & MMU print errors - filament is getting clogged and extruder motor is hot
Hi all,
First thank you for excellent printers - I have hundreds of hours over many years of printing on my (old) MK2, MK3(S) and Mini!
I upgraded my MK3 with the MK3S+MMU2S kit several years ago, it's run fine since. However just recently I've had some issues with the filament jamming. These started after I upgraded to the latest firmware a couple of weeks ago however that may be a coincidence. (I used the ORIGINAL PRUSA I3 MK3S firmware update). All issues were using a single filament with the MMU.
The first two times the print finished and I came back to "MMU Error", and the filament had not retracted from the nozzle. I detached the tubing from the print head, heated the nozzle and manually pulled the filament up and out. I printed several times after that just fine. Then it happened again and I fixed it the same way.
This last time in the middle of the print I noticed the filament was not extruding and I heard a clicking sound coming from the extruder. I stopped the print but could not pull the filament out this time as I did the last two times. The machine cooled down and I left the power on and accidentally noticed that the extruder seemed very hot. I then powered it down. Laster I partially took apart the extruder to get to the bondtech gears and after heating up the nozzle was able to use needle-nose plyers to pull the filament up and out, although it was very difficult. With the nozzle still heated I hand-fed some filament down into the nozzle and filament did extrude, although it seemed much harder to force it through than previous times. Then I cooled the nozzle down and noticed that, with the printer doing nothing the extruder motor seemed very hot again.
So I'm not sure if the issue may be:
- The nozzle is clogged/jammed causing the extruder motor to not be able to feed filament, which would cause the gears to "notch" out the filament so it can not be fed forward or retracted back.
- There is some issue with the extruder motor, as it seems to get hot doing nothing.
- There is some issue with the MMU2S, perhaps mistakenly preventing the filament from feeding at some point during the print, causing the extruder gears to notch out the filament (same as above).
- It is related to the firmware update somehow?
My next step was to reassemble the print head and manually extrude filament to see how that works, and at the same time see how the extruder motor gets so hot doing nothing. I was also thinking if the nozzle is clogged I could change it.
Any thoughts welcome!
If you can touch the motor for over a second it is not going outside design limits...
Try a cold pull: https://help.prusa3d.com/article/cold-pull-mk3s-mk2-5s_2075
What is the state of the bondtech gear bearings?
Are the gears themselves badly worn - unlikely unless you have been printing abrasive filaments.
Note that stepper motors can get hot 'doing nothing' if they have been stopped and are braking in a part step (microstep) position where they are energising coils to hold in place. Turn the printer off, allow everything to cool, turn on again but do nothing else - does the motor get hot now?
The most likely damage to a stepper motor is the failure of one of the internal bearings. With the printer off and the Bondtech gears removed attempt to turn the stepper by hand. You should feel the regular bump of the steps but otherwise expect it to be smooth. If you feel grating or if you cannot turn it then the motor is failing.
It won't hurt to change the nozzle, they are cheap and it would remove one variable from the problem.
Cheerio,
RE: MK3(S) & MMU print errors - filament is getting clogged and extruder motor is hot
So I thought I would update this post in case anyone is having similar issues since I have discovered what the problem was.
The issue above all stemmed from the filament sensor in the MK3S not working reliably. In the MK3S extruder the filament physically pushes a lever that drives a sensor, and the tolerance involved is VERY tight. There is some "adjustment" but it is very tiny, and over time with any tiny movement due to vibration, etc. the filament sensor can become unreliable (which is worse than it not detecting the filament at all!).
Often the print would start fine, meaning the sensor was working, but at some time during the print the filament sensor stopped working. Of course the print finished fine however now the coordination between the MMU and the printer is broken when it's time to extract the filament. Normally the printer would extract the filament until it's sensor indicated it was out of the extruder, then the MMU would take over and extract it the rest of the way. However with the sensor not working and indicating no filament the first step is skipped and the MMU tries to pull the filament out, and of course it can't because its locked into the extruder's Bondtech gears. This explains the gnawing of the filament about a foot from the end as the MMU grinds away trying to remove the filament.
Another symptom was at the very beginning of a print when the filament is loaded for the first time. The MMU would send the filament down, wait, and then retract it and try again. I didn't realize what was happening at the time: after the MMU sent the filament down the extruder never detected it, it timed out and tried again. Occasionally the filament would come down two or even three times before the print started.
I should note that the sensor didn't stop working entirely, it stopped only occasionally making this issue very intermittent and hard to debug. There is also no debug information from the printer that helps identify errors like this.
I resolved the issue by calibrating the extruder's filament sensor, following the PRUSA instructions. Once it read the filament status reliably everything was back to normal.
Overall the MMU is a neat idea but the integration with the printer is very poor. It feels like the MMU has it's firmware, the printer has it's own, and the two only work together when everything goes perfectly right. When it fails it's never clear why, I've never been able to recover during a print, it's not clear what your options are when it stops, etc.