Print TPU with MMU  

RE:

Unless you are a masochist or aim for extreme challenges, just don't. Don't print TPU with MMU. 
If you insist, at least have an MMU which is flawlessly dialed in with regular materials and has a very reliable track record. 

TPU may require a different extruder gear tension than your other filaments. Have a look if the filament sensor is giving the right signals also with TPU, not just with PLA, PETG etc. There could be a problem there. Generally the MMU absolutely hates flexibles, I would recommend turning off or sidenlining the MMU when printing with flexibles. There are mods for on/off switches for the MMU on printables if I am not mistaken. 

RE: Print TPU with MMU

Thank you for your reply.  The MMU works just fine for PLA and PETG.  I don't need nulticolor TPU so I would happily bypass it if I knew how.  The TPU pushes through all the tubing very easily.

When you talk of bypassing the MMU do you mean inserting the TPU directly into the top of the extruder?  I tried searching for some ideas, but wasn't getting helpful information.  Any links greatly appreciated.

RE:

The makeshift method to bypass the MMU without modifying anything is to start a print (but with no filament loaded into the channel) and trigger the two sensors manually during load as needed so that the MMU and MK3s believe the filament is being loaded. (unscrew the tube between MMU and extruder and use a piece of filament to trigger the upper sensor when needed, the lower sensor can be triggered by softly pressing/bending the lever inwards until it triggers.

Then when the printer starts the priming line, pause the print. manually lead the TPU to the extruder, manually set the extruder to move forward and flush the TPU sufficiently through the nozzle that way. Movement is very slow manually but that also helps a lot with TPU as it prevents a tangling up. It really helps with flexibles also if you have removed all previous filament via cold pull before.

When you loaded the TPU manually and flushed, just hit "continue print".

By "bypassing" I did not mean physically bypassing it, merely trick the loading mechanism. If problems persist one can also delete the parts of the initiating g-code related to loading and priming (don't remove the temperature setting parts though). Then the printer doesn't do anything of that loading stuff and starts printing without loading anything and without prime line. The printer might complain about the lack of that g-code or not but that's how I print with TPE.

The more elegant option is to build a kill switch for the MMU but for that you have to modify your printer.

RE: Print TPU with MMU

I spent about an hour moving my filament sensor tower on top of the extruder by fractions of a mm (or at least I think I was moving it) and adjusting the tensioning screw until I can load TPU fairly reliably with the MMU. I expect that were I to sneeze too loudly, it might stop working so I don't really recommend this.

@thejiral gave a solution for getting the printer going with TPU. While I have no doubt that it works, it looks overly complex to me. Before I had the adjustments made, I simply waited by the printer until the TPU was loaded and about to start printing. I'd simply push on the idler door at the top (the long skinny part) so that when the TPU was in the extruder, it'd trip the filament sensor. When printing started, I'd simply let go. Once printing actually starts, the filament sensor in the extruder is ignored, so it is OK if the extruder's filament sensor reports that filament isn't present. The printer only looks at the FINDA sensor in the MMU to detect a runout.

RE: Print TPU with MMU

The problem I had with my 90A TPE was that the loading even by menu regularily failed because the printer simply loads it too fast and also the flushing at the very start lead to clogging or messing up in the extruder. If one loads by manual extruder movement however, I had no problems so far.

RE: Print TPU with MMU

I don't really understand the instructions.  Is "priming line" the line at the front of the bed where it sweeps left to right and back while extruding?  I never get that far.

I separated the MMU tube from the extruder and put a piece of PLA into the tube from the rear so to look like regular filament position.  When it starts to load that filament, I put the TPU into the extruder.  The MMU stops feeding the "fake" filament.  The lower feed grabs the TPU and starts extruding a ~300 mm string and then says MMU needs attention.  The filament in the MMU advances 10mm and the cycle repeats itself.

I don't know what it is looking for to know that the filament is loaded and ready to go.

RE:

I am not sure we talk about the same thing, the way I do it is not to load that other filament in place of the TPU from the rear, I load nothing, but in order to let the MMU believe that it loads something I insert a short piece of filament from the side right next to the MMU filament sensor. Just far enough to trigger the ball of the filament sensor, which is a maybe 1 cm in or so. Once you feel that you triggered the ball you can remove the filament again. It is enough for the MMU to blieve that it is loading something. Then it will wait until the extruder is at target temp and then one has to trick the filament sensor on the extruder as well. You can either do that by pressing the lever just slightly inwards enough to trigger the sensor or if you don't want to bend anything, one can also unscrew the tensioner and trigger the light sensor with the unmounted lever. 

Then the MMU also believes that the filament (which there is none at that point) has also loaded into the extruder and will continue with flushing at the starting point and then the priming line. At the priming line would be the point to pause the print, manually load the TPU into the arranged channel and then load it into the extruder by manual extruder move command. Let some more TPU through the extruder to make up for the missed flushing/priming. Keep in mind that when you pause a print, before doing anything you have to manually set the hot temp to the right value again, otherwise it will cool down which you don't want. Once you are done, simply continue. 

In case you have troubles with that flushing and priming, one can delete the g-code for that and then one would need some brim or something so that one can do that pausing and manual loading. 

PS: I know, this is a bit labourious, other solutions are more straight forward, but this works and doesn't need printer mods, nor removing the MMU or interrupting its cables. 

RE:

So with my method there is no point at which the MMU grabs anything and there is no point at which the Extruder loads anything (for real). It is you who loads it manually, by pausing the print, manually setting hotend temperature, manually setting extruder movement and manually moving the filament to the extruder gears and wait until some filament has been extruded that way (manual movement of the extruder gears is much slower than anything the extruder does normally during loading procedure, which means it might take a minute or two, but that is a good thing, it means much lower risk of the TPU getting all messed up at the gears and).

It is a while that I did it myself. I have to look it up again if one can even save oneself the troubles of faking the MMU loading process if one simply deletes the Starting g-code, except for the heating up commands of course. 

RE: Print TPU with MMU == Solved

IT'S PRINTING!  Thank you very, very much.

RE: Print TPU with MMU

My steps to bypass MMU for TPU printing:

1. preheat nozzle to the temperature of the previous material (if previous material print temp >  TPU print temp) or to TPU temp (if TPU print temp > previous material print temp)
2. unscrew bowden tube that connects MMUunit and the extruder,  from both ends 
3. prepare a 10 cm long PLA or PETG filament string
4. from the menu, load to nozzle and after 2-3 secs after the MMU tries to load the filament, insert the prepared filament string until it engages the ball of the PINDA probe
5. then, after a few seconds, insert the tpu into extruder, and purge multiple times