Trying to balance friction for extruder vs rewinder  

Trusted Member

I just finished building my MMU2S for my MK3S. I haven't got it running yet. I'm at the point where I'm getting my rewinder dryboxes connected to the MMU2S. I didn't assemble the filament buffer or the spool holders because I don't have room for them on the desk where my printer sits. So I have an arrangement of dryboxes with spools and rewinders inside.

After reading about people having trouble with friction putting a lot of load on the extruder, I bought some PTFE tubes with 3mm inner diameter to connect my rewinding spools to the MMU2S. These have much lower friction than the PTFE tubes that came with the MMU2S (which I never even used). I also don't need long tubes; about 40cm is sufficient to span the distance between my rewinder spools and the MMU2S. So the friction in those tubes is really small.

The problem is, now that friction is reduced, it's seems easy to pull the filament out of the MMU2S, if enough backward torque builds up on the rewinder. There is nothing holding the filament in the MMU2S except friction... but I eliminated that friction.

So I am wondering if it would be best to use my 3mm inner-diameter PTFE tube as that single bowden tube going to the extruder to reduce friction in that path, and use the tighter-fit PTFE tubes that came with the MMU2S.

Has anyone tried replacing the bowden tube with a larger inner diameter? I have plenty 4mm OD / 3mm ID tube left over. If I do this, will I run into any problems? Or will doing this help prevent jams?

This topic was modified 11 months ago by Anachronist
Posted : 10/04/2020 7:44 am
Trusted Member

Following up to my own post... Now ALL my tubes are PTFE tubing with 3mm inner diameter. Including the the extruder tube. I re-printed my rewinder springs wind up less and lose more energy during the first couple of turns.

This fixed my problems. The rewinders no longer pull filament out of the MMU2S, and filament no longer gets stuck in the tubes.

While trying my first multi-color print, the filament kept getting stuck in the stock extruder feed tube. It wasn't the pointy end of the filament getting stuck, it was just overall friction. The MMU2S would strain to push and pull the filament through the tube, and end up stripping it. It is possible that my filament was not perfectly round, or had some diameter variability (it was advertised as 0.05 tolerance), or maybe swollen from moisture (unlikely, I keep it in a sealed bag with fresh dessicant).

Replacing the feed tube cured that problem. I am concerned that the looseness of the filament in the tube might cause a misfeed, but that hasn't happened yet.

I have 3 out of 3 successful color prints so far.

This post was modified 10 months ago 2 times by Anachronist
Posted : 05/05/2020 4:22 pm

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