Now that I have a small supply of food grade HDPE granules, my next task is to find a way of extruding it. I should point out that I’m not concerned with building an extruder that’s compact enough to go directly on a Mendel. At least not at this stage. Instead, I am looking to build something that is reliable enough that it can produce a filament of plastic which can then be spooled and later used for a traditional pinch feed extruder. This gives me the freedom to make it any size or shape that works best.
Not wanting to reinvent the wheel, I did some quick research on the topic of granular extrusion to see what stage the rest of the reprap community is up to.
The most useful links that I found were;
- Adrian’s discussion on the topic
- Adrian’s prototype
- Forrest’s experimental work
- Some students work from Delft University of Technology
The general concept is to use some mechanical means to compress the plastic granules into a heated chamber and forcing them through a nozzle. Two different methods of compressing the granules was discussed; compression by a piston and by a auger.
Some good points were made about the piston method in the comments from Adrian’s discussion. Most importantly, with a single piston it would be difficult to produce a filament of even thickness and it would be unable to let the air escape from around the granules as they were compressed. So I decided to pursue the idea of the auger, which is used extensively in industry for plastic extrusion.
Neither method seemed very backyard shed friendly so I decided to do my own experiments using a drill bit as an auger. In my shed I found a large drill bit and a section of pipe that fit it snugly. I then curt a half section out the pipe to allow granules to enter.
Using an empty plastic cordial bottle I made an impromptu granule hopper and by turning the drill by hand tried to get the granules to pass through. Despite the granules of HDPE having an extremely low coefficient of friction the drill bit was very hard to turn by hand. This seemed to be due to the granules entering the pipe getting compressed against the opening.
This set-up obviously wasn’t working so I instead used an empty soda bottle bottle top as a hopper to try and copy the method of feeding the granules used by the Delft students.
This worked very well, with near no resistance and a constant flow of granules. By placing the opening of the pipe flush against the bottom of a bucket I was able to feel the upwards pressure created as the granules collected at the base.
From playing around with this drill bit and reading other peoples work I think my own design will have the following:
- Use a longer, shallower threaded auger bit such as this one.
- Use a stainless steel pipe for the auger housing for its low thermal conductivity and ware resistance.
- Use a copper or brass hot end for its high thermal conductivity and high temperature corrosion resistance.
- Use an all metal feeder such as the Delft students one.
- Have the drill bit only extend as far as the stainless steel pipe so that it stays well below the HDPE’s Tg.
- Use some form of heat sinking on the stainless steel pipe to help keep the drill bit cool and provide a sharp thermal gradient at the copper/stainless steel pipe interface.
- Use an old cheap drill press that I have to old the drill bit and turn it by hand during testing.
So once my exams are over next week I will be off to the local tip shop to try and rustle up these parts.
As a final note, I have also started to order parts relating to my Mendel build which I have listed on a new page here.