TPU with mm2s in single mode
Tried printing with TPU for the first time (sainsmart tpu). Printer is set in single mode, so there is no filament swapping, it only need to load once.
Sliced file, loaded filament (to mmu), cleaned bed, hit print. mmu advences filament to extruder. Extruder detects. Printer starts warming bed. When bed warms, printer goes thru the bed leveling routine, then gets to the point where it should do a purge line. mmu removes filament from extruder, then feeds it back in. For a while nothing happens. Looking at the gears, extruder is trying to push filament in, while simultaneously mmu is trying to pull filament out. After a while extruder gives up, mmu retracts filament, feeds it back into the extruder, and the tug of war with mmu and extruder pulling in opposite directions begins again.
Reset things multiple times. Took things apart, opened covers, cleaned gears. Check gear alignment. All is well. Sometimes one of the gears ends up chewing the filament and can't pull anymore, but usually not. Tried adjusting extruder tension. If tension is increased a lot (far past reasonable) got a gigantic blob of plastic out of the extruder while they were doing their tug of war. Finally got filament wrapped all around the extruder gears and called it a night.
I think there is some kind of handoff going on, where the mmu tries to make sure that the extruder has control of the filament. I think that's the point of the tug of war (Are they really checking to see if one of the motors starts skipping? surely not. Maybe what looks like a smooth tug of war is actually a series of starts and stops). Normally, this works fine, but with a flexible filament, it stretches and so confuses the handoff routine.
How can I get around it? Does anyone understand how the hand off work?
Thanks in advance
RE: TPU with mm2s in single mode
Well, I figured it out. Sort of.
The solution is always RTFM, right? On TPU's prusa suggests loosening the extruder tension screw. I played around with extruder tension, but I figured, if they said loosen, I would set it as loose as I can. So I set the screw half a thread into the nut. And everything worked great.
Well, it printed anyway. It's very, very stringy, but aside from that, it's ok. I'll need to play around with temperatures, and it seems that my filament is a tad wet (there are little light spots in the print, resembling splay in injection molding. I think it's a sign of moisture). Hopefully this will help someone with the same problem.