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