RepRap: Builders

Like some others, I'd thought about a feed mechanism similar to Vik's but never pursued it. In search for a mechanism that would also allow backing out the filament (as a ratcheting mechanism like Vik's does not), I thought about how other cylindrical things move... namely snakes. Here's a hastily drawn diagram of a snake-inspired friction-based feed mechanism.

It seems pretty obvious that the closer you could come to bending the filament through full 180 degree arcs, the more force you could apply. There's almost certainly some great math out there for averaging force vectors and friction coefficients... Anyone?

Labels: FDM extruder

When I first tested my extruder I found that the filament diameter varied with extrusion speed. The filament expands after it passes through the nozzle aperture and this is known as die swell. I seem to have variable die swell whereas others have reported constant die swell. I have done some fairly detailed experimental work to get to the bottom of it and I think I have a simple explanation and a mathematical model to exploit it.

Further details here: http://hydraraptor.blogspot.com/2007/09/equations-of-extrusion.html

Incidentally, I read that die swell in polypropylene can be eliminated by the addition of carbon nanotubes: http://www.azom.com/details.asp?ArticleID=2601

Labels: Die swell, FDM extruder, HDPE