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):
- IMG_2290.jpg (418.73 KiB) Viewed 508 times
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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