Filament gets caught in the extruder during unload?  

Re: Filament gets caught in the extruder during unload?

You could try reducing ramming.

But you must be aware that the issue you describe has knock-on consequences. The filament in the MMU will be ground down and you may then experience a load fail later in the print.

When this happens, you should pull out the filament and cut off any ground down parts.

Peter

Re: Filament gets caught in the extruder during unload?

You could try reducing ramming.

Okay, what difference would that make? (Ramming is a bit like voodoo to me....)

Re: Filament gets caught in the extruder during unload?

When I have seen the behavior you describe, it is generally because the tip of the filament is too wide and too hard. It's had time to cool a bit, and is too wide to fit through some part of the extruder (generally above the bondtech, as the bondtech gears are pretty good at yanking the filament out).

As to how reducing ramming helps, I need to walk through the unload sequence:
- shift extruder over to the purge block
- rapidly extrude some filament (ramming)
- move the filament up/down a few times (cooling cycles)
- withdraw the filament using the bondtech
- unload the filament using the MMU2 gears

The ram step introduces a bunch of filament (rapidly) to the melt area, where it melts. That phase change requires energy, which is obtained by pulling heat from the already melted filament and the nozzle/heat-block. The effect is to cool the tip of the filament in the nozzle a bit. That melt zone can be wider than your PTFE tubing, so if you go too far it'll solidify and you won't be able to pull it out (which is why the mk3 won't let you unload filament with a cold nozzle). However, if you lower the temp a bit, you'll get a melted tip, but slightly cooler. A cooler tip is less likely to leave a long string when you withdraw it.

So: if you are ramming too much, the temp may drop too far, and the tip may be too fat/too hard. It'll then get stuck. Ramming for a shorter period of time (and implicitly less filament shoved into the melt zone) is a potential solution.

Re: Filament gets caught in the extruder during unload?

Paul, that is amazingly interesting! 😀 Thank you! I'll probably give it a go, but it's hard to know where to start.
You are right in that the tip gets insanely hard; I could not break it off with my fingers...

Re: Filament gets caught in the extruder during unload?

The longer tip plug comes from the melt zone. The more you ram, the further you push into the melt zone and cool the melted filament around the new filament and therefore the longer the tip plug.

When retracted, the filament starts to cool; as it cools it shrinks lengthwise, but expands in diameter. It's at this point where you need the filament to be slowly passing through the smallest diameter of the PTFE tubing - and that happens to be near the top of the tube, where the locking collet is. So too much cooling in the lower part of the PTFE is also bad 🙁

Peter

Re: Filament gets caught in the extruder during unload?

So what setting do you use to get cooling to happen at the top of the ptfe tube?

Re: Filament gets caught in the extruder during unload?

Is the solution here to lower the hotend temp? This is happening to me with ninjaflex. Currently set to 230

Re: Filament gets caught in the extruder during unload?

Hi,

in my case the filament stuck in ptfe between mmu2 and extruder in loading phase. And i think the problem is a too wide tip.
How i can reduce ramming ? Filament Settings -> Advanced -> Ramming Settings -> : i should reduce 'Total raming time' ? Now is 2.50 sec.

Thanks

Re: Filament gets caught in the extruder during unload?

... and why firmware dont use filament sensor on extruder to check the filament correct loading ? It's a BUG ?

Re: Filament gets caught in the extruder during unload?

... and why firmware dont use filament sensor on extruder to check the filament correct loading ? It's a BUG ?

Indeed. It's there, so use it!

Re: Filament gets caught in the extruder during unload?

... and why firmware dont use filament sensor on extruder to check the filament correct loading ? It's a BUG ?

I think I'd call this a "buglike feature."

Re: Filament gets caught in the extruder during unload?

... and why firmware dont use filament sensor on extruder to check the filament correct loading ? It's a BUG ?

OK, so it's not just me. I've been watching my MMU2 fail over and over again and was wondering why the printer didn't seem to detect the lack of filament in the extruder....

Re: Filament gets caught in the extruder during unload?

Hi!

I have had an MMU for a few months and it's... mostly been a paperweight because of this issue. The blob on the end of the filament is noticeably larger than the original filament. I'd say it jams... one in every 4 attempts? It feels like every time we want to use the machine we have to open the filament changer and the extruder, clear the line, reset everything, and then we can attempt another print. To date we have not succeeded at printing a single complete multicolor print, tho a few single nozzle prints have been OK. FWIW this unit is my fifth official Prusa machine.

I reread the documentation on the MK3 upgrade (because that's what we did) and currently feeling paranoid about the PTFE tube in the nozzle - surely I replaced it wrong and that's where it's blobbing?

If you have some idea or smart question to get this thing going, please share.

Re: Filament gets caught in the extruder during unload?

Hey there,

I've heard that if the PTFE tube is not installed correctly into the hot-end you can get blobbing like this. I was having an issue with my hot-end misbehaving and jamming on my MK2/S, and then I had read that when inserting the PTFE tube into the hot-end, to push it all the way, then while holding the PTFE tube into the hot-end you also pull back the black plastic locking ring so that it continues to "force" the PTFE tube to have constant tension inside the hot-end. That solved some of my issues. If that doesn't make sense, I'll take some pictures of my spare hot-end and visually show you what I mean... sometimes words fail me. 😆

Was your printer an MK2/S originally? I've also read that the internal geometry of the heat break is different on the MK2/S hot-end vs an MK2.5/MK3 hot-end. From what I understand the MMU1 kit for MK2/S came with the "new" hot end that's currently used in MK3 printers, which has the different internal geometry that's supposed to make the tips smaller to prevent the blobbing and jamming inside of the PTFE tubes.

I made the mistake of putting the old style hot-end into my printer when going from MK2/S w/MMU1 to MK2.5 w/no MMU. When I eventually installed the MMU2 upgrade on my MK2.5, I was having huge issues with PTFE tube jams and blobbing. The other day I installed the hot-end that came with the original MMU1 kit (which is the same hot-end used in MK3 printers). I've not completed the testing yet, but this seems to have alleviated many of the issues I was having so far. I've only done a few single-color prints but I was having issues with that too. I'll be doing a multi-color benchy again to see if it's gotten better.

so... Might it be possible that you have an old non MK3 hot-end installed in your printer?

Re: Filament gets caught in the extruder during unload?

Agree with what previous poster said.

Also, I, and several others, have found that the PTFE tube can be installed wrong, bent, etc. Several people saw improvements just swapping it out. I used Capricorn, with a smaller ID and higher temperature resistance. I'd like to think it helps (making the blob smaller), but regardless of mechanism, making sure everything is put together just so is a good idea!

Re: Filament gets caught in the extruder during unload?

I have been having the same problems with my MMU2. Have stopped using it until I can figure out what the problem is. I will have to try swapping out the PTFE tube with some Capricorn

Re: Filament gets caught in the extruder during unload?

I used Capricorn, with a smaller ID and higher temperature resistance. I'd like to think it helps (making the blob smaller), but regardless of mechanism, making sure everything is put together just so is a good idea!

I'm still having issues with the tip looking right - although I do admit that I haven't done anything in the terms of tuning yet. I'm interested in the capricorn idea - in fact haven't even heard of it until you mentioned that so thanks!

I have been having the same problems with my MMU2. Have stopped using it until I can figure out what the problem is. I will have to try swapping out the PTFE tube with some Capricorn

Please let us know how it goes.

Re: Filament gets caught in the extruder during unload?

I have been having the same problems with my MMU2. Have stopped using it until I can figure out what the problem is. I will have to try swapping out the PTFE tube with some Capricorn

Have you tried this yet? I'm curious as to your results.

Re: Filament gets caught in the extruder during unload?

Been having similar issues, not tried checking/replacing the PTFE into the extruder yet.

I have had some success by removing the PTFE just before the gears. That section of PTFE gets squished and so has a narrower internal diameter. Seems to be where it was always getting stuck.

I also swiched to a passthrough connector (off thingyverse) but found the PTFE would gradually get pulled in and cause the same problem. So, I reduced the diameter of the printed part at the end just enough to prevent that.