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PLA Foot Jamming nozzle on unload  

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PLA Foot Jamming nozzle on unload

I have rebuilt the hot end (again), with a brand new prusa-bought heatbreak. I added a new PTFE tube while I was at it (Prusa bought for the i3mk3s+ hotend) I was able to print multiple (single color) PETG objects without incident.

PLA, however, is another story.  PLA on the i3mk3s(+) has always been an issue for me, and I was really hoping this would fix the problem (extruder clicking issues).

I was able to successfully print a small cube (10mm x 10mm x 10mm) in pla, and it came out well.  The problem is when I attempt to unload the pla.  It always starts to unload for about 15-20mm, then everything seizes up.

I pulled the door off the extruder and took a look inside... what I found was surprising.  The filament developed a plastic "foot", it went off on about a 90 degree angle from the unused filament.   This foot hits the adapter on the unload (this part:, and keeps it from unloading from the hot end.  The bondtech gears on the MMU then proceed to shred the filament back at the MMU.  I have to pull apart the MMU and the hotend to clear everything.


I'm not sure what to do here, everything looks good to me, and I took my time in re-assembly of the hot end.  I was methodical, read every syllable of the directions, and followed them. I'm sure its right...

It still does this, even if I just use "load to nozzle" and "unload" from the controller menu.  Happens every time with PLA (PETG is fine).

Obviously, there can be no hope for a multi-color print if it cannot unload.




This topic was modified 12 months ago 5 times by moonglum
Posted : 17/03/2023 7:36 pm
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Topic starter answered:
RE: PLA Foot Jamming nozzle on unload

Wow.  Nobody has a clue, huh?

I spent a few hours online with support today -- and the technician didn't either.  I couldn't see how this was happening... but now I think I do...

I took a video of the filament unloading during this support session.  I did a first layer calibration.  As usual, I got the "foot" blocking the unload.  After the first layer cal, the i3mk3s+ just automatically unloads the filament.  You get no choice.  Upon reviewing the video I noticed something.  The filament gets pulled out of the hot end... but then the filament direction reverses and it is pushed back in again, after which it goes for the true unload.  I don't know if this is a "cooling move" or what (I cannot find a definition of what a cooling move is -- I know what it achieves, but not sure how it does it, I'm guessing this is it).. .but I'm confident the unload would work fine if it didn't do this.

I think what is happening is that it is pulling the filament out of the hot end, and it is coming out very stringy, then it jams the filament back down into the extruder.  It hits the motionless bondtech gears, and smashes the soft, gooey strings into that "foot" shape, which cools and hardens prior to the real unload.  It cannot get through the hole with that foot, so the MMU tries to unload it, but it can't, so it just grinds out a huge notch in the filament, and I get jams at both ends.

There is nothing I can do... I cannot stop the machine from unloading in this fashion.

Again, I'm using prusament PLA.  Everything I have and use on this is Prusa brand (in an attempt to eliminate confounding factors).  It does not do this with PETG.  ONLY with PLA.

This whole foot thing started after reinstalling a prusa brand, i3mk3s heat break.  I also added the firmware update that came out early in March.... could one of these things be the culprit?  I had a nice looking cold pull, I see nothing to suggest there is a problem with the heatbreak, and I know it is installed properly and with an ample amount of thermal grease... so was it the firmware?  I notice they just released another the other day... I'll try it, but I won't hold my breath...

Posted : 01/04/2023 10:10 pm
Reputable Member
RE: PLA Foot Jamming nozzle on unload

I have seen that on my Printer several times.  I am thinking the PLA is still melted when it comes out of the hotend and gets stuck to the gears?  Just guessing.  I was able to recently print the MMU torture test which has 1ooo filament changes.  I upgraded the extruder plastic parts, When from 2mm id to 3mm id on the extruder to MMU tubing. Did some upgrades on the MMU.  Here are my upgades.

Changed all the bowden tubes going into the MMU and from the MMU to Extruder to 4mm od x 3mm id

Since I did all these updates about a week ago I have much less issues so I am only about 1 see in but I printed this torture test for the first time successfully





Posted : 03/04/2023 5:32 am