PROJECT: Prusa i3 MK3S + MMU2S

So... a couple of months ago we got the MMU2S (multi-material) upgrade for our Prusa i3 MK3S, and boy oh boy has it been a wild ride trying to get the thing calibrated... 
The process reminded me of our old Prusa i3v, as it too required a bit of work to get a print up and running, which in the end you still had to keep an eye on to make sure everything was ok

Calibration Troubles

As you might have guessed, the MMU2S did not work out of the box for us. This was mostly due to stringy tips during unloading, which eventually lead to load blockages
I tried numerous things to improve the tip shape, from adjusting the number cooling moves/unload speed/printing temperature, to varying the unload temperature using a Python script. Sadly, none of this helped as much as changing to PrusaSlicer 2.2.0 DRIBBLING by antimix. Which is a custom slicer that adds a "dribbling" motion (think basketball) during the unload

Here is a snapshot of the 30 tests I did trying to improve the tip shape:
Default PrusaSlicer Settings (with & with out the Python script)

HCD profile (with & with out the Python script)

Reliability Mods

I soon found that using a custom slicer was not enough, as I constantly ran into unload issues even when the tips were nice and pointy. The culprit turned out to be the PTFE tube running from the MMU2S to the extruder, which had too small an inner diameter (2mm ID) for the tips to travel freely
Why? Well it turns out that the PTFE tube inside the hotend (different to the one above) expands over time, and this tube also controls the tip diameter (think molten tips being squished into a cylinder). This results in the unloaded tips being 2-2.5mm in diameter which believe it or not have trouble fitting through a 2mm hole ;^)

With that said, here is a list of mods I installed to improve reliably:
  1. Stock (2mm ID) PTFE tubes replaced with 3mm ID PTFE tubes. Left the 5 coming out of MMU2S as stock 2mm ID (to make sure filament does not "fall out" during unload), but did cut them down so that ~5mm sticks out (see below)
  2. Stock MMU2S selector replaced with magnet variant
  3. Chamfered inner edge of Festo fitting (PTFE holder) at extruder to help with flat tips
  4. Replaced stock filament buffer with "gravity assisted buffer" (using M16 nuts as weights). I found that the stock buffer & long PTFE tubes added too much resistance to filament movement, which made printing finer details that much harder (as in areas where small amounts of plastic was meant to be deposited were nearly cavities)

Printing Profile

After heaps (o_o) of tests here is what I settled on:
NOTE: This is for single-material (PLA) printing
  1. PrusaSlicer 2.2.0 DRIBBLING kby antimix
  2. Dribbling temperature -15°C of normal print temperature. For example, if your printing temperature is 200°C then I would suggest setting the dribbling temperature to 185°C
  3. Minimum temperature same as dribbling temperature
  4. Maximum temperature same as printing temperature
  5. Number of dribbling moves to 3
  6. Purge volume at 250mm³

The Outcome

Our first successful print!!!
  • Smiling owl by cipis
  • 5 colours
  • 448 tool changes with only 2 issues requiring intervention:
    • Once due to filament debris from previous print
    • Other due to bad tip with glow in the dark filament (load error)

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