We've had our MMU2S for a while now and have used it a few times to make multicolored print but mostly we've mostly just used one color. But lately I seem to be running into a problem with a ball forming at the end of the filament when it is retracted from the extruder. The filament I'm using is the silver Prusament PLA that came with the printer. This is causing the filament to be very hard or jam in the tubes when changing to a different filament. It is also causing the occasional failure to load to the extruder. I've done some reading about adjusting some settings like temperature but no one really specifies up or down. It seems to me that hotter would lead to stringing at the tip and colder would lead to more of a ball shape forming at the tip. Does anyone know if this is correct? Also are there any other settings I can adjust that could help remedy this problem?
Here is a pic of tip shapes I'm getting along with the size of the ends I measured
Sorry for the upside-down Pic couldn't figure out how to rotate it.
The Prusa heat break is a modified version of the E3D-V6 heat break. Prusa has the bore drilled to 2.2 mm down to the neck of the heat break. Prusa says this is to help the MMU load filament. Folsk without the MMU find the heat break causes more problems than is solves because on high activity prints retractions cause the filament to pump melt up into this 2.2 mm region where it cools and then jams.
Some MMU users have replaced the heat break and not had loading problems.
And the stock E3D-V6-175:
I can see why they did that. You can imagine that trying to feed filament with a 2mm diameter tip into a 2mm hole could be tough, you'll have a line on line situation. But I think that they probably shouldn't have bored it that deep. Maybe only half the distance.
Now the strange thing is it seems to be getting worse as time goes by. It used to be that the filament was a little difficult to rewind through the tubes when swapping out a spool but now I almost always have to cut the tip off while its still in the MMU before I rewind it. I don't know if its due to wear on the machine (haven't had it that long so I doubt it) or if it has something to do with firmware or slicer updates. But there's got to be a way to make this thing function reliably.