Monday, January 29, 2007


Mk 2.1: Defining the envelope

Okay. I got it working. Now that hard part, defining its operating parameters.

I first established that it would work at all.

It did with HDPE. The extrusion rate wasn't nice, but these are early days.

First I did a series of tests with HDPE. It's a fairly tough, but soft surfaced engineering plastic. With hand feed experiments that I did a while back it was slippery to a point where I had to grip it with pliers to feed it into the extruder barrel.

My first instinct was to really tighten the springs that press the filament into the threaded polymer pump. You can do that, but if you try to make the pump run too fast, that is, develop some pressure in the extruder barrel, it just excavates a groove in the side of the filament and you get nowhere. That was discouraging.

I then shifted over to polypropylene (HPP). It is a harder surfaced plastic and the threaded pump drive engaged it much better than it did the HDPE. After faffing around with HPP for a while I began to plan an experimentation campaign.

Right now I'm running the polymer pump with HDPE at 2 rpm and 2 amps power. I'm going to keep power constant and run the rpm up step by step and see what happens.

Very nice. Did you use filament from a vendor or make some yourself?

I wonder if HDPE would require more contact area with the drive. What would happen if you had an extruder with, say, four threaded rods in counter-rotating pairs? Probably a lot harder to build than the Mk 2 and the maximum diameter of the rods would be, what, twice that of the filament. Something like that.
Got mine from a plastic welding rod company.

I've thought about the counter-rotating thing. It's a rather difficult design problem. I keep running into a wall figuring out how to get rotational energy to the threaded drive rods.
One other thought for making it easier to grip is to figure out how to add radial grooves to the feed stock to give the thread something to hold onto. Make the feedstock look like a big plastic threaded rod, for example.
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