MMU2S Print Help  

RE: MMU2S Print Help

Hi Alex,

First of all please describe what happens if you do a small MMU test print. More detailed error description might help to provide better hints.

Post some images of your filament tips and measure the diameter of them. Slightly stringy tips are not a big issue for MMU2s. Once you start battle that you might run into other issues. That's why I'm asking for more details first. 

RE: MMU2S Print Help

I've also been having some trouble with the tips. I seem to have tails, bulging and it even seems a bit burnt. I've been playing with temp, unloading speeds, cooling moves and even ramming but can't seem to get it right. 

RE: MMU2S Print Help

we are going to need more details what exactly is not working for you.

the vast majority of people that have problems do so because the filament sensor is not positioned correctly 

RE: MMU2S Print Help

Thanks for the reply and the tip. What I'm seeing that the first few Filament changes happen ok but eventually the filament is unable to load into the bontec gears at the extruded. My guess is that the bulging you see in the picture is causing the filament to get stuck and not able to load? 

Ill check the sensor tonight. Also I'm on the latest firmware.

Thanks! 

RE: MMU2S Print Help

I adjusted the sensor so it is properly registering 0 and 1 now. https://help.prusa3d.com/article/wprwemqsmj-ir-filament-sensor-calibration

That did seem to help but I do still have some long strings that will start to clogg up after a few changes. Going to experiment with lowering the temp and cooling moves etc. 

RE: MMU2S Print Help

Internal stringing (what we see here vs on the printed part) is because the tip is not properly cooled when it is unloaded. There can be two reasons for this. The first being temp and the second being an issue with friction that does not allow the filament to move as expected.

If you have not messed with any settings in PS and have a factory configuration for your hot end then either you are using a filament that has a lower than average melting point or the issue is friction. Trying a different manufacturer's filament is a good test for temp/filament issues. If you are using an average filament then I would start looking at your filament path first.

How the filament moves from the spool all the way to the extruder can have a significant (and frustrating) impact on MMU performance. I started off with all kinds of pain and frustration too, but once I got my filament path under control strings of significance disappeared and my failure rate when to <1% of tool changes. You can see more info/suggestions in the link in my signature, but the top points are:

• The Prusa supplied tubes are stiff and prone to causing drag if not laid out just right.
  • Some of us have switched to Capricon, but it is pricey.
  • Others have converted to using tubes with a larger interior diameter.
• Too much curvature in the tubes will cause binding. Ideally it would all be straight, but focus on making any curves that must be there are as gentle as possible.
  • Sorting this out may mean rethinking how you have your spools positioned in relation to the printer.
• The entries and exits of the buffer and entry to the MMU use a compression fit which if not adjusted just right can cause binding.
  • There are mods for both to use pass through PTFE festos. These are very worthwhile mods.
• It could be your filament.
  • I have never had issues with it, but I've seen a number of complaints about the rough texture of Amazon/Overture PLA being a problem for people. They switch to a different brand and their problems go away.

Basically my rule of thumb is that if it takes any kind of force or intervention (e.g. holding/moving the tube) to move the filament from one point to the next, then you have friction that needs to be addressed.

RE: MMU2S Print Help

Thanks very much for this info. I did totally re-position the spool holder and buffer and put them both into a large plastic container to keep them dry. I'm sure I have some friction along the lines somewhere. I'm going to re think this whole setup now.  

RE: MMU2S Print Help

So I've looked for any friction points and added compression fittings as you mentioned. I still have the issue unfortunately. The issue does seem to be very predicable. After 3 or 4 tool changes one filament can't seem to load. It will make three short grinding noises then back the filament out. The tips often look pretty good too, not perfect but not bad. The printer will display MMU failure and If I cut the tip off the filament then the printer is able to continue. If I look in the "window" of the extruder I can see the Bontec Gears load the filament but then back the filament right back out. I've removed the hotend to look for any friction. I put in a new PFTE tube, I noticed the ID was kind of small. I had an extra tube so I tried drilling out to make the ID bigger. Unfortunately that also made no difference.  

RE: MMU2S Print Help

@michael-w19

I've been going through similar trials getting a new MMU2S setup working satisfactorily. 

> It will make three short grinding noises then back the filament out. The tips often look pretty good too, not perfect but not bad.

I experienced the same or something similar.  For me, it turned out to be the IR sensor was just occasionally sensitive to motions of the extruder during a print, and that caused unnecessary retractions because the MMU thought the filament wasn't reaching the extruder.

• Doing the IR sensor test in the troubleshooting manual, everything looked fine. 
  • FESTO removed from the extruder, and no filament in place, the IR sensor read 0.
  • Pushing in filament or an Allen key into the extruder Bondtech gears, the IR sensor would change to a 1.
  • Both of those were the target behaviour so I thought everything was okay with the sensor.
• However, at the start of prints, I would consistently get that attempt-to-load-then-retraction and MMU failure to load error.  (There were some bad words said...)
• What helped to diagnose this was starting a print, then moving the MK3S display immediately to the Support -> Sensor Info screen. I kept a piece of filament in the extruder, with the sensor showing IR = 1 as desired.  But then as the extruder moved around during the Z calibration tests I saw that sometimes that IR would flick to a 0.  That flick-to-0 triggered a reset of the filament loading process, and eventually the MMU fail to load error. 

I think I have successfully fixed that issue now by moving the 'chimney'/IR sensor around until I got more reliable IR sensor readings , subjecting the filament to different tugs and pulls to simulate the extruder moving.  Once I had stable 0 or 1 readings, then tightening the IR/chimney bolts down hard locked that in place.  Since then, at least all my single-colour prints have been starting successfully.  (Multi-colour prints are still a mess because I'm suffering from the same bulbous filament tips on retraction that bind in the PTFE tubes that others are struggling with.)