MMU2S PETG Success!
I have had a MK3S with MMU2S for well over a year and have finally got it working reliably printing 2 colour PETG at 260 degrees Celsius with no print fan. The key to success with the MMU2S is understanding the Prusa Slicer setttings and how they relate to the hardware.
If you have a stock MK3S you might need to adapt these settings, I have upgraded mine with a Mosquito v2 hot end from Slice Engineering. The heat break in the Mosquito is a little narrower than the PTFE tube between it and the extruder cog. This means when retracted for a filament change the hot tip must stay inside the narrower metal heat break so that it stayed squished to fit back inside later. It must also not be pulled up through the extruder cog before it has cooled enough not to deform. Finally, the speed at which is it pulled out of the melt zone and the distance, number and time for the cooling moves are what stops long stringy tips or blobs.
All default for Prusament PETG except for:
1. Nozzle: First layer 250, Other layers 260 (this is to produce really good interlayer adhesion, at this temp your linear advance needs to be K=0.07)
2. Cooling: All cooling disabled (if your print object is going to be prone to curling though then default cooling settings are just fine, the thing I print to sell is shaped in a way where it will never curl)
3. Advanced: Max volumetric speed: 12mm3/s, Minimal purge on wipe tower: 0mm3, Unloading speed at the start: 200mm/s, Delay after unloading: 8s, Number of cooling moves: 3, Speed of first cooling move: 1mm/s, Speed of the last cooling move: 20mm/s, Ramming settings: Total ramming time (s): 0.50, Ramming line width (%): 120, Ramming line spacing (%): 140, Volumetric speed (mm3/s): 15mm/s
1. Single extruder MM setup: Cooling tube position: 15mm, Cooling tube length: 11mm, Extra loading distance: -18 (everything else default)
Explanation: This is what the above settings do...
1.First a 0.5s ram is done at 15mm3/s (or at least 12mm3/s if the cap applies). This helps the filament break cleanly from the melt zone. Just a very quick, hard ram is all you need at 260 degrees Celsius.
2. The filament is retracted 15mm at 200mm/s, or maybe the printer can't actually move the filament that fast so it will just be retracting at maximum speed. You want this so that the filament necks and makes a very thin string only which is easy to get rid of with cooling moves later...
3. The high speed retraction stops after moving 15mm, and this means the hot tip will be right in the middle of the Mosquito heat break heatsink fins where maximum cooling will happen.
4. The extruder cog then pauses for 8 seconds so the heat break can absorb the heat from the hot filament tip to cool it solid while also ensuring the tip is no fatter than the tube it is inside (which is the narrowest spot the filament has to pass through on it's way to the melt zone.
5. The cooled tip then moves up and down 3 time gradually increasing speed from 1mm/s to 20mm/s. The filament will move upward 5.5mm and downward 5.5mm, this means on the downward dip the tip comes to 1.5mm of the melt zone and this melts off any stringy tip.
6. Finally now that the tip is cool, thin enough to slide in the narrowest spot it will encounter and has no more than a very short stringy tip, it retracts at 20mm/s until it exits above the extruder cog and the MMU2S drags it back up to wait in the MMU2S.
I should say I have the latest upgraded MMU2S razor blade holder and it is activated. With these latest settings I've done thousands of filament changes and not had a single jam where the razor has had to cut a fat tip. I have seen the razor in action while developing these settings though and it cuts pretty good.
So what I have learned, is that the MMU2S can work very well, but you really have to engineer your own settings for the filament type and temperature you are printing. An advantage of printing at higher temperatures is that the colour change purge needs less wasted filament to completely change colour.
As we all know the internet if full of MMU2s tears! So I hope this post helps some of you. If you have a stock e3d hotend then you will need to work out if the heat break or the PTFE tube is the narrower part and do the cooling moves in whatever is narrower. You will have to adapt the cooling tube length and cooling tube position if you have different hardware to me.
My MMU2S is used to print a commercial PETG product and I am now so happy with it that when my one MMU2S unit is at capacity I am going to buy another one for my second MK2S.
I also cannot speed highly enough of the Mosquito v2 hot end, it is so easy to change nozzles. The e3d hotend always gets damages after a while from nozzle changes and then leaks as well. A tip for the Mosquito, if the heat break ever gets clogged, use the extra long hex driver Slice Engineering recommend to remove it and then torch it with a little cheg's propane gas torch. It gets it red hot within seconds and totally burns out any clogged plastic. The Mosquito heat break cools so efficiently the usual unclogging methods such as disabling the heat break fan will not work, a Mosquito can actually work with no head break fan it cools so well (but I would not actually recommend printing like that). I wish Prusa offered a Mk3S without a hot end so you can just buy the Mosquito from the start.