MMU2S - mixed results and some oberservations
I had a well running Mk3 and it was doing its job. After reading many posts in the forum I was happy to give it a shot. Upgraded Mk3 to Mk3S and MMU2S in one go. A very first two-color print (sheep) seemed to work with one two load failures but things were downhill from there.
I followed the valuable advice in this forum. First, get it to print in single mode. I tried and it would stop in the middle of a print. After countless hours of reading and checking, using different filaments I found out that the drive gear on the extruder motor had become loose.
Then came many hours of loading failures to the extruder. Once again I followed a lead from the forum and printed a new extrusion idler door (PLA, 0.2mm). That made the IR sensor stable and improved the situation on the single color prints but would still fail every now and then.
You can read about friction anywhere. Some recommend tubing (from MMU to extruder) with larger inner diameter (e.g. 3mm) , others recommends smaller diameters (1.9mm) or high end PTFE tubing (e.g. Capricorn XS).
Many hours of research later and the MMU2S happily grinding the filament I printed a new MMU2S selector from https://www.thingiverse.com/thing:3337120
That immediately made things better and reduced the MMU loading failures.
I never had issues with the filament tips but still adjusted the ramming setting.
The biggest issue seems to be the tightness of the spring of the extrusion idler. Some people have it very loose, others (including recommendation of Prusa's manual) have the screw quite tight. I have mine quite tight (in combination with the new door).
I fired up another print of the two color sheep and in the middle of the print the selector on the MMU would go to a wrong position (between two filament paths). I did not find a way to recover (how can one home that thing ?) and had to cancel. Started the print again and it worked.
While looking harder into the "grinding filament issue" I noticed a gear on the MMU idler had become loose 🙁
I am now running on two filaments but had trouble loading a third and fourth. Third (Prusa Silver PLA) and fourth work in single color mode but do not get loaded to the extruder properly in multi-color mode. Here I experience the issue where the filament gets load but it is then retracted and sent back to the MMU.
I will likely try two more mods
1) https://www.prusaprinters.org/prints/3676 - this one could be the most important. It uses a pass-through coupling to feed filament through the PTFE tube right up to the Bondtech gears)
2) https://www.thingiverse.com/thing:3233579 - replaces the PTFE holder (screw on compression thingy) with M10 couplers.
Summary:
1) Make sure the printer works fine in single color mode, test, test, test, extruder gears, IR sensor (new door), tension of spring
2) always start with fresh tip, remove sections which were grinded, doing this will save you from a lot of pain
3) start with the updated selector on MMU2. It is easy to print and exchange. https://www.thingiverse.com/thing:3337120
4) Don't waste time by using the "load to nozzle function". Try the combined tip/load/unload test from https://www.thingiverse.com/thing:3217915
Hopefully that will give you nice start into multi filament printing.
Check out this thread for more information.