From reading other peoples blog’s it appears the best extruder is one that has a sharp transition zone from hot to cold. On the Mendel this is achieved most often with a hollow cylinder of PTFE to act as a thermal barrier.
Unfortunately this isnt a practical option for a auger based granular extruder. As such my first attempted at a granular extruder just involved a tapered pipe with nickel chromium wire rapped around the last few cm. Although this did extrude a little by manually compressing the graduals with the handle end of a screwdriver, it proved to much for the auger.
It took most of my body weight to extrude just this small amount. As the exposed end of the pipe was removing a lot of the heat I suspect it was not getting hot enough and/or jamming at the point where the granules first melt. I order to try and overcome this problem I decided to remake the barrel in two parts, one insulated from the other and joined by a bell shaped insert.
I took some photos along the way showing the process. I first polished and cut a pipe to length.
I then cut this to length and using the rounded end of a hammer made a bell shape in one end. A photo showing how here.
I then took this same pipe, cut the other end length ways in 8 sections to a depth of about 1.5cm. I then folded these cut sections in on them selves to create a taper. This was ground down to make it smooth before being soldered with a heat shrink gun. A photo showing this process here. This filled all the gaps making it air tight. Lastly this soldered end was cleaned up with more grinding and the end drilled out again with a 2.5mm drill bit.
For the other end of the pipe in which the auger sits in, I used a file to taper the pipe so that it would mate smoothly with the bell shaped end of the other pipe. Both ends were then rapped in Teflon tape. A photo showing both ends here. After some testing with the heat shrink gun I was satisfied that the thin layer of teflon (PTFE) tape now separating the two pipes would be enough for my purposes to insulate them from each other.
The hot end was then electrically insulated with polyimide tape, wrapped with nichrome wire, fitted with a thermistor and heat insulated with roofing insulation.
Lastly, I made up a bracket out of aluminium to hold the lot together on the drill press.
This mounted to the drill press to hold the extruder all together.
The next task is to work on gearing down the small electric motor so the auger will have a fighting chance of producing enough pressure to extrude the HDPE.