Newbie Questions on MMU3  

RE: Newbie Questions on MMU3

With this filament removal algorithm, never rely on it throwing the rest of the filament up when it's done. Even in this case, the extruder performs the downward extrusion as if it were shaping the tip when removing the filament. This will get the rest of the filament out of the idlers and you have to remove it manually after opening the idlers. I don't know why the developers left the movement there, but they probably forgot to put the condition that it is an emergency ejection at the end of the filament and there is no need to shape anything and flush the nozzle on the rest. To prevent these situations, keep a sufficient supply on the spool at all times. This will save you a lot of work. And @lukeeeskywalker: I also have my first MMU, but I've already figured out these nuances.

RE: Newbie Questions on MMU3

When filament is running via MMU3 and gets empty this is working rather well, also the SpoolJoin works great
But I understand why it is causing trouble when it runs out in the extruder since the modified filament sensor doesn't actually detect the filament anymore (like it would do before the MMU modification), it rather detects the filament's presence at the gear wheel and upper idler in the extruder which is rather close to the nozzle already which reduces the chance to successfully eject it.
I totally agree with @miroslav-h4 any ramming or further extrusion of whatever kind shouldn't take place if the filament sensor is set to MMU.

RE: Newbie Questions on MMU3

Ok, I was curious today and tried. Turned off the MMU3 on the MK4S screen, confirmed the filemant sensor on the display is still set to MMU, printed some dummy model and I cut the PLA filament during print.
I did not notice any ramming after empty filament was detected so if there was some extrusion it must have been very short.

The MK4S tried to eject the remaining filament but this was unsuccessful which is no surprise considering it can be only detected when the filament already is past the upper idler so really late.
I had zero chance to grab the remining filament piece after opening the idler, less than a millimeter left, and I really have a lot of tools at home to try (by far too tools many if you would ask my wife 😉) so only option to remove the remaining filament would have been to remove the nozzle which I didn't want of course and it would have ended the print because the nozzle was too far down to be able to pull it out.
So I did something different: I simply cut a 20 cm piece of the "new" filament and pushed half of it through the nozzle hole (which can be easily reached from the top/side when the idler is open) and then i pulled it back rather fast. After that I closed the idler, confirmed the removal of the filament and I was successfully able to load the new filament.
So overall not that difficult. But it doesn't make it any better that the whole thing should work better.

Anyway... I still think the way the MMU3 modification of the Nextruder works is not very well engineered. It's really difficult for me to understand why Prusa didn't come up with a better solution for this considering their experience in 3D printing and considering that they use their own printers, too.

RE: Newbie Questions on MMU3

Ok maybe I'm being really thick here, but what would stop you from manually feeding in the new filament above the old?

So one filament spool ends - the printer stops with it's warning. 

You then push in the start of a new filament spool (with a cut flat end, not cut at an angle) and manually keep pushing downwards as you resume the print. 

The lever style filament sensor should detect the filament, and once its gone far enough to be grabbed the extruder that will do the rest and keep pushing down, using up all the old filament and then moving on to the new......

As I said, I'm sure there is some reason why this wouldn't work, as otherwise it would be the easiest thing to do?? Or is teh old filament too far down so feeding in new filament from the top wont 'meet' with it in the first instance?