O-ring to combat extruder backflow/jamming

Hi there!

I had a quick thought and wanted to bounce it off the community.

It sounds like one of the major issues with the extruders is molten plastic back flowing into the lower temperature zone. Once there, it solidifies and causes jamming issues.

Has anyone experimented with using a high temperature o-ring, captured in the heated barrel and sealing against the filament. It may require some tricky machining but it seems like it could be worthwhile- I don't know though yet, as my build isn't complete.

Thanks

its a idea worth trying i think, my only worry is it getting jammed up in there some how.

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[mike-mack.blogspot.com]

Yes an interesting idea. You would have to get it at just the right point to ensure all the plastic above was below its Tg and the ring itself would have to be cooler than Tg. The problem then is that backflow doesn't tend to go that high anyway.

Backflow is not an issue if you have a slippery seamless filament path, e.g. PTFE. You have to make sure the PTFE does not bulge though and there must not be a gap at the join to the barrel big enough or plastic to flow into.

With less slippery materials like stainless steel, or PEEK, a taper at the transition point solves the problem.

I have two extruders one using each of these techniques and they never jam. My mendel has clocked up over 1000 hours of almost continuous operation using a PTFE insulator constrained in a copper tube.

They only time I get backflow problems is with undersized filament in my oversized extruder on HydraRaptor. I.e. it has a 3.6mm bore and I have trouble with US filament that is only about 2.8mm.

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[www.hydraraptor.blogspot.com]

Nophead, I have looked at your extruder designs but fail to understand how the taper helps in your SS barrel. I believe you have created the thin section to reduce the heat transfer. Is the idea behind the taper that the diameter increases so filament is not in contact and thus no friction? Is the friction really that bad? I think I need to get my hands wet to fully understand all that is going on here.

Yes the friction is bad because the forward pressure on the plastic past its Tg causes it to swell outwards (as if it were rubber) and grab the sides of the tube. If the coefficient of friction is above a certain threshold it will jam solid because the more you push, the more outwards force you get and hence more friction to counter the push.

The taper prevents this happening.

[hydraraptor.blogspot.com]

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[www.hydraraptor.blogspot.com]