Blobbing on weep tower after filament run out detected  

Eminent Member

Hi there, 

I will update this topic as soon as I know what's needed. Anyway, the PRUSA with MMU2s is running great, but occasionally has a filament run out detected. Im running the latest firmware here. 

It asks for the unload and reload procedure, squeezes a bit of filament out (parking position front right), then moves back to the weep tower. While moving I can already see some additional filament being squeezed out (if you are quick it can be removed on the fly). It is clearly no oozing. On the weep tower, while the head doesn't move but is already lowered to its surface, another big material blob is generated. The weeping starts and usually ends up at best in a recoverable crash detection. Quite often the weep tower becomes loose.

My first theory was: The fake filament run out was detected by a piece of broken off filament so that the retracted length to the FINDA did not match, hence run out detected (seems to happen during load/unload). The tip would then be in the extruder being pushed by the new filament. BUT, this cannot be true, not if you do another run aka filament color not correct. The additional volume of filament should at some point squished out and the (hopefully) retracted filament length match the normal length. 

There simply seems to be no way to actually ensure no blob being generated on the weep tower. 

So the question is: Is that a common problem?

And the second question: How do I suggest to make a change in the firmware?

Posted : 07/11/2019 9:52 am
Trusted Member

There is a reported issue on github that looks like it's what you are encountering. Bugs can (and should) be reported there. Devs rarely read in the forums.

Posted : 07/11/2019 6:21 pm
Honorable Member

your problem is almost certainly a slightly out of calibration IR sensor or finda that is momentarily indicating no filament.

once it triggers a manual filament change the trick is to not remove the first bit as it changes filaments but to hold on to it and let the 2nd bit extrude as it moves to the purge tower and pull it away before it gets to the tower

Posted : 08/11/2019 3:43 pm
Eminent Member


Ok, for the second tip: Right, this is the way to go and works well. 

The first one is the common cause, but with my setup I wouldn't suspect it. I control the IR and FINDA via separate LEDs and my MMU runs 2000 filament changes without intervention easily. So, I would say it detects what it is supposed to detect. Anyway, I will check the LED more closely next time. It could trigger late, but that would rather result in 0.05 mm of additional filament being extruded. But what it actually does, the extruder sits idle for 3-4 seconds and happily extrudes, but not wiping around. I could understand if it tries to position the filament within the extruder, but it could do that while moving around. And, this is the routine it already does while changing the filament not being on the wipe tower.  So, I still think it is a programming issue. GitHub is informed 🙂 Let's see... 

Posted : 09/11/2019 1:38 pm
Eminent Member

Ok, little update: I still don't believe it is the usual non calibrated IR sensor, BUT I gave it a bit of priority and found lots of debris in the extruder chamber, that could potentially lead to a falls "filament detected" sensor reading. Now, after another 2 day operational period I cannot report anything useful back in terms of the blobbing as I hasn't happened yet again as no false filament runout was detected. The former is unfortunate, the latter might be an effect of a clean chamber (I have no idea about the programmed logic in the background).

Posted : 12/11/2019 12:18 pm
Reputable Member

I have found the IR flag/sensor to not always be "digital". It is possible to have the sensor so that a little filament makes it read 1, the putting it in further makes it read 0.


Similarly, the gears can be out of round, and they can move further (if out of phase) than loaded filament causes, creating a high noise-to-signal ratio.

I maintain an informal list of San Diego, CA 3D printing enthusiasts. PM me for details. If you include a contact email and I can add you to the informal mailing list....
Posted : 12/11/2019 10:28 pm
Honorable Member

garbage in the bondtech area of the extruder can get caught in the bondtech teeth or gears and bounce the door

Posted : 12/11/2019 11:32 pm

Please Login or Register