The Nurdle Extruder is still slowly progressing. Just to reiterate, the purpose of this extruder is to take food grade HDPE nurdles and produce a 3mm filament which can be spooled for later use on a geared reprap extruder.
As previously mentioned, the feed barrel was getting too and so the HDPE nurdles were passing above their Tg before entering the melt chamber. This was solved by cutting down a CPU heatsink and drilling it out to fit the feed barrel. I also added a fan.
After this addition all the nurdles will still fall freely out of the barrel when turned upside down after use, indicating that they did not get too hot.
The next problem I had is that the auger was turning two fast. This led to the nurdles being fed into the barrel faster than they could be extruded and so the whole thing would jam up.
The obvious solution was to further reduce the gearing of the drill presses AC motor. I moved the output belt off its small wheel and directly onto the middle shaft. I also added a bigger pulley wheel onto the output.
This has taken the ratio from 8.5:1 down to around 20:1. So the auger now turns at about 70RPM.
Unfortunately this turned out to still be too fast. Not being able to decrease the RPM any further with out starting from scratch I looked for other solutions. The most obvious of which would be to increase the size of the hole in the die, currently 2.5mm, but this would also increase the size of the filament. So instead I added a second 2.5mm hole in the die so that two 3mm filaments (the HDPE swells to 3mm after exiting the 2.5mm hole) could be extruded at once. This ad-hock fix will have to do for the time being.
The next problem was that after a few trial runs I was unable to get back up to the temperature required for extruding the HDPE. Removing the roofing insulation showed that the nichrome wire, although rapped tightly around the barrel, was expanding as it heated. This expansion lifted the wire just off the surface of the barrel and so with out the heat sinking the barrel provided, began to glow red hot. This had become hot enough to destroy the polyimide tape that was holding it down.
You can see the wire expanding and contracting in the video below.
I could possibly get by this problem by encasing it it in fire cement, adding groves for the wire to sit in to increase contact area, or possibly adding some sort of tensioner. However I also have a nichrome ribbon heating element on hand so I gave that a try instead.
The ribbon was given to me by the same friend who gave me the HDPE nurdles. He said its used on the industrial feed barrels that leads into an extruder to make milk bottles. It has a resistance of 3Ω per meter. I gave it a very quick google and you may be able to buy it by the foot if your interested.
Due to its very low resistance I couldnt use short lengths as the current draw would be too high for my power supply. So instead I placed a layer of polyimide on just one side of the ribbon and then rapped all 1.1m of it onto the barrel to make a single band.
Its discoloured in the photo because I increased the power a little too high when testing it out, but it still works. Being rapped around the barrel seems to help resist it from expanding. Also the increased surface area in contact my also stop it from getting too hot, also reducing expansion.
Overall the progress is slow, but I’m nearly at the point where I can have a constant feed in of nurdles and a constant feed out of filament. Then at last I can start work on a automated collection spindle.