john.s32
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Filament gets caught in the extruder during unload?

Oct 24, 2018 2:20 pm

I’ve had a problem for a while now: the MMU2 stops, I read (for instance) ”1>3” on the display, the selector is in position #1, and the red LED above #3 blinks rapidly.
I discovered that this happens when the filament gets caught in the extruder during unload of filament. If I open the lower FESTO coupling, and pull hard on the filament, it comes loose and it’s easy to reset the printer so that the print can continue.

— But how do I avoid the problem??
There is no stringing, and my tips look like this (PLA):
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Best Answer by paul.m27 » Oct 24, 2018 2:41 pm
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.

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PJR
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Re: Filament gets caught in the extruder during unload?

Oct 24, 2018 2:22 pm

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
Please note: I do not have any affiliation with Prusa Research. Any advices given are offered in good faith. It is your responsibility to ensure that by following my advice you do not suffer or cause injury, damage or loss.

john.s32
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Re: Filament gets caught in the extruder during unload?

Oct 24, 2018 2:32 pm

PJR wrote:
Oct 24, 2018 2:22 pm
You could try reducing ramming.
Okay, what difference would that make? (Ramming is a bit like voodoo to me....)

paul.m27
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Re: Filament gets caught in the extruder during unload?

Oct 24, 2018 2:41 pm

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.

john.s32
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Re: Filament gets caught in the extruder during unload?

Oct 24, 2018 2:49 pm

Paul, that is amazingly interesting! :D 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...

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PJR
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Re: Filament gets caught in the extruder during unload?

Oct 24, 2018 4:25 pm

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
Please note: I do not have any affiliation with Prusa Research. Any advices given are offered in good faith. It is your responsibility to ensure that by following my advice you do not suffer or cause injury, damage or loss.

rod.l2
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Re: Filament gets caught in the extruder during unload?

Nov 07, 2018 12:26 am

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

mike.k12
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Re: Filament gets caught in the extruder during unload?

Feb 01, 2019 7:09 am

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

vincenzo.d
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Re: Filament gets caught in the extruder during unload?

Feb 01, 2019 10:46 pm

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

vincenzo.d
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Re: Filament gets caught in the extruder during unload?

Feb 01, 2019 10:47 pm

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

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