Over in the Primo Thread folks were discussing the filament extrusion, filament recycling and whether someone could do it at home. I fell down that particular rabbit hole a few weeks ago and after some gentle prodding am writing up a brief treatment of the subject.
tl;dr: Unless you are made of money or have the patience of a saint and the precision of swiss watch you can’t expect to extrude filament at home that will in any way compete with what is available commercially.
Desktop extruders, whether scratch, kit, or commercially built come in 2 variants which I will call (for simplicity) the Filastruder and the Filabot.
These all have a length of (generally black iron) pipe with a window cut in it to feed plastic and a closely fitted wooden auger bit inside to convey the plastic towards the meltzone. Material is melted at/past the end of the screw by heating elements surrounding the pipe and then forced out through a nozzle at the far end. Filament is either pulled down by gravity in vertical configurations or drawn by a set of traction rollers in a horizontal configuration.
CNC Kitchen’s build/review of Filastruder+Filawinder
Lyman’s extruder won a $40k prize in 2013
YouTube playlist of two attempts at a DIY build
The Filabot uses a proper extrusion screw rather than a auger bit. The auger bit of the Filastruder and friends just moves material along much like the gears in a 3d printer’s extruder just push filament into the melt zone. The extrusion screwhowever _moves material, compresses, mixes, and meters out material in a much more controlled fashion (more info). These machines often will have one or more heaters along the barrel to better manage the melting process. Like the machines above they will have some means to control the filament drawn out of the machine (either gravity or traction) and will often (always?) have an air or water cooling stage immediately after the nozzle to make sure the filament cools quickly and evenly which helps retain proper shape and diameter.
Filabot EX2 Extruder line, $4,500
Filaboe EX6 ($9,000) line running
Precious Plastic Extruder (plans)
AdaFruit tries the Filabot extruder by itself. This is part 1, there is no part 2
Recent example of a DIY extrusion line
German language video (captioned) of a 5kg/hr extrusion line
You will notice something about all of the links for Filastruder-like machines: they are all >6years old. Also notice how nobody has posted any follow up videos saying how great their extruder is working and how much money they are saving recycling their prints.
The Filabot-type machines appear to have better success actually producing a product, but are not what anyone would call affordable, it seems.