From project forum post:
Low cost filament extruder made from 10 mm ID stainless steel tube, silver soldered inside an ordinary plumber 15 mm copper tube (melt zone) Auger is a 10 mm wood drill bit with the end cut off 5-10 mm in the melt zone. 1 mm hole in a brass endstop at the end of the copper pipe. Aluminium block with a 15 mm hole to fit around the copper pipe. Fixed to the outside of the aluminium two 220 volt 200 degree PTC heaters( I tried 12 volt 230 degrees but the temperature was to high) Insulated with glass fibre cloth. The ptc heaters are available on aliexpress for $1 each)
System driven with a car windscreen motor at the low speed (I have a 2 speed motor - salvaged from scrap yard)
I use the white plastic that is in the inside of the doors of scrap refrigerators. Cut it up with a circular saw and extrude that to a filament of 1.75 mm
I use this filament in my Delta printer on a daily basis.
The trick to have a nice filament is to let the filament run over a copper pipe that is suspended in front of the extruder and let natures G force take over. I fix a weight(clothes-peg) at the end of the filament as it comes out of the hole untill it reaches the floor and then move the end of the filament to the outside of a circle approximately 1 meter away.From then on the G force will take over and produce a nice circular movement of the filament to roll it on the floor .
The slightest air movement or other external force that you apply to the extruded filament will result in a kink in the filament - no fans required.
No fancy electronics involved.
Material cost to should be less than $100
This extruder was only tested with the white refrigerator plastic ( I think it is ABS because it will desolve in acetone ) - no ABS pellets or any other plastic tested.