Is the MMU -> printer PTFE still length specific?  

RE: Is the MMU -> printer PTFE still length specific?

Yes and no.

It is true that the MMU2S uses the IR sensor to detect when the filament reaches the hot end. So it doesn't need to know precisely how long the feed tube is any more.

But it still needs to know approximately how long the tube is, because it errors out if it loads or unloads for too much distance without getting the expected signal that the filament has reached the end of the tube. This lets the operator fix the jam and resume the print if the load or unload fails.

RE: Is the MMU -> printer PTFE still length specific?

I suppose that makes sense. Thanks

RE: Is the MMU -> printer PTFE still length specific?

It really doesn't seem to care *too much*.  I have been single filament printing disconnecting the bowden from the MMU end leading to the extruder and manually feeding into the extruder when the MMU starts spitting filament into the air.  I doubt I'm within any sensible tolerance here, so I'd say it's likely to work unless you've gone silly 🙂

RE: Is the MMU -> printer PTFE still length specific?

Hmmm.. Come to think of it, I did have one load failure where it got it part way into the PTFE tube and then dug a groove into the and it just kept driving until I got the idler cover open so I could pull the filament out. It was in the high speed mode so it would have spit out a good 5' of filament or so in the time it took me to get the cover off and filament back out past the FINDA (where it then stopped trying to feed and reported a load error).

Now that I think about it, I think I have 1m of PTFE tube running around here somewhere. I guess I'll dig it out and give it a shot to see what happens. I can try one of the buffer -> MMU tubes too as I believe those are longer and I'm not using them right now.

RE: Is the MMU -> printer PTFE still length specific?

Well...

I put the 1m tube on and told it to load to nozzle. It worked in so far as it got the filament to the nozzle without complaint. Unloading, however, is not going well and I think I may have just made my perfectly find MMU not so fine any more 😥 

During the load it went through the process as normal, but after running the fast load for what should have gotten it to the IR sensor it then stopped and started moving it ahead incrementally until it finally did trip the IR sensor.

I had last been printing with PETG, but did this test with PLA so on the first eject it did come back with a black tail (PETG) on my yellow filament. It only pulled the filament back past the extruder gears, but even when I manually pulled it back out past the FINDA it still didn't register as fully unloaded and ultimately gave me a fast blink alternating between green and red. I had to reset the printer itself to get it out of the "Unloading filament" screen.

Thinking some PETG remnants got pulled up and stuck in the extruder gears I disconnected the PTFE tube and used the Move Axis functions to feed a bit of the PLA through at PETG temps to clean things out. I did have to apply some force to get it moving after it had the gears which I take to mean that there was a bit of a clog. Once it got going, however, everything was fine and I purged it out until it was clear yellow.

I put the original PTFE back on and again it loaded fine, but again even though it drew the filament back farther (about 3" before it got to the selector) it did the same thing.

Apparently with the MMU attached it does not like my Live Z test that I sliced for the plain MK3 test so I'll have to slice a quick test later and see if this is just an issue with manual unloading or if I screwed something up.

RE: Is the MMU -> printer PTFE still length specific?

Everything was fine when I gave it a single layer "life Z" type print. I also made up a second similar print, but it instead does 5 20x20x0.2mm prints each set to an extruder. That also ran well.

Feeling lucky I gave the 1m tube a try again with the 5 color test. The first color change worked well enough, but the second had issues though I *think* there is a tangle in the spool. We'll see how changes 3 (had a bit of trouble with the normal tube) and 4 go before I claim success.

Two things that are blatantly apparent:

1. It adds considerable time to each tool change as it stops speed loading part way through and then "feels" it's way to the end. This happens for both loading and unloading. Based on the comment in PrusaSlicer for the "Filament parking position" about needing to match what is in the firmware I suspect that updating the firmware with a better number would improve the speed of a tool change. Give that the distance from the FINDA to the "park" position is a known constant I'd like to see the MMU firmware be smart enough to calibrate itself on the first tool load and then just remember it for the entire print.
2. The auto-rewind spools are useless with such a long tube. Basically they can't rewind enough so you get a lot of extra filament flailing around after the unload. Either a reworked rewind spool or a properly sized buffer are critical for such a long PTFE tube.

RE: Is the MMU -> printer PTFE still length specific?

So while finishing my previous post, it finished the print with no further issues. So I'll call this successful if not ideal. 

Note that the ugly spot in the wipe tower was created during the uncooperative 2nd tool change. It did actually cause a crashing while wiping on the 4th change, but the printer recovered with no help from me. 

Stupid forum rotating images...

RE: Is the MMU -> printer PTFE still length specific?

You can definitly calibrate the tube length (mmu <-> printer) for each filament.

And it also has a maximum adjustable length. If you reach that limit, you cannot move the filament towards the extruder anymore during calibration.  (I've had to cut about 20cm of my tube in order to stay below the limit)

The filament moves super fast after calibration and slows down right above the extruder. Also the mmu got super reliable after I did that.     

(Check the help websites from prusa, somwhere is a guide for this calibration)

RE: Is the MMU -> printer PTFE still length specific?

I know that is how it was done before the S, but it doesn't appear to work that way any more. Or at least not entirely. 

I'm hoping to get back to it this weekend to understand (and change) the code better, but while the limts are still in there they are not being used for more than hints. 

My stock tube length, for example, is closer to the "min" length than it is the default length. I think I read somewhere that the tube was shortened for the S, but I'm not positive. 

1m is also WAY over the "max" length set in the firmware, yet other than slowing things way down it does work just fine. 

I'm not confident that I fully understand the firmware code yet, but hopefully I'll be able to spend time with it this weekend and get a better understanding so I can figure out how to improve the process. 

RE: Is the MMU -> printer PTFE still length specific?

@gnat

Were you able to change the tube length and get it to work reliably? I have my printer in an enclosure and would like to remote mount the selector to the top of the enclosure for better access.