What is the running out of filament experience like?
I tested running out of filament on my bare MK4 and saw it was smart enough to push the last bit of filament up out of the extruder. What is the experience when you run out of filament with the MMU3 in use? Does it notice up at the FINDA when you run out? What is the procedure for replacing the filament with a new spool to resume printing? (If I'm stuck on an error screen, how do I get to the load filament screen?) I seem to remember noticing that the end of the filament on the spool is often bent or grabbed solidly on the spool itself? Will it get a filament stuck error before getting an out of filament error? Just trying to plan ahead to have some idea what to do when it inevitably happens :-).
RE: What is the running out of filament experience like?
From my (admitily poor) memory it works quite well.
Yes the logic for filament run-out seenms to be: if the FINDA changes to no-filament state whilst printing (whilst Fsensor is still sensing filament)
The filament then retacts out of the extruder and if all works correctly a stub of filament is left poking out of the mmu3 for you to pull out by hand. It then alerts you to feed new filament, and then when this is pulled through the mmu into the extruder it peforms a purge asking you the usual "purged enough?" question.
More intervention is required if the end of the filament is tightly curved, hence it doesn't seem to work perfectly every time (as you aluded to)
RE:
I've not had great experiences all the time. With the MK4/MMU3 I would expect it to unload to MMU and then move the selector aside and purge frontward from the MMU. My experience has been that I end up with an error and the remnant is not sticking out anywhere, I pull the buffer cartridge, but the filament is in the PTFE and unreachable (between the MMU and the cartridge). Or the filament is still in the hotend and I have to take off the MMU-to-hotend PTFE and pull the remnant that way.
I've done things like load new filament through the buffer cartridge and just push the remnant through.
I've spent a lot of time removing the MMU-to-hotend PTFE tube for various issues.
The MMU3 is great, but it is very picky about perfect and straight filament.
One of the things I tend to forget is that the MMU3 has manual Move and Load buttons, those are nice, but it's not always what is needed,
X marks the spot.
RE: What is the running out of filament experience like?
The main problem (no matter whether MMU or not) is if the manufacturer has created a big kink at the end of the filament, where it kind of hooks into a hole in the spool. Without the MMU (on a MK3S+), I once had to disassemble the extruder because that end got sucked in in such a bad way that it got stuck while pushing out.
And in any of the PTFE tubes of the MMU setup, a badly mangled end will get stuck for sure.
What helps extremely for these kinds of issues is to add Festo fittings to the back of the MMU, for example with this part https://www.printables.com/model/669311-mmu3-ptfe-holder-for-pc4-m10-fitting . Then you can easily disconnect one of the long PTFE tubes from the back, and can freely pull the filament out of the back if something goes wrong like @ig mentioned. Highly recommended low-effort mod.