Molding and PLA extrusion
I tried vaseline as an alternative release agent and the whole molding/casting process worked a lot better. The plastic removes cleanly and there is no discolouration. The method involved heating the vaseline to make it a thin liquid. It was then painted onto the surface. Heating the plaster of paris absorbs some of the vaseline, so another application was added to ensure some at the surface.

The plastic is very viscous when melted and extremely hard when set. This leads to two concerns I currently have. One is that it may not flow through a narrow diameter extrusion nozzle. The other is that it is so hard that the screw-based feed mechanism may not be able to reliably grip onto rods of PLA (unlike EVA, which is quite soft so easy to feed).

I've had a slightly different idea for extruding PLA. It probably won't be useful in a reprap situation but it may be useful as an interim method in the repstrap. The idea is to have a narrow tube (15mm copper tubing) that opens in a larger diameter tube above it, which serves as a hopper. A nozzle would be attached to the 15mm copper tubing. A 12mm auger bit running in reverse goes through the hopper and down into the tube. The whole thing is heated via nichrome wire to melt PLA. Because the PLA is quite viscous, rotating the auger downwards should push the melted plastic downwards, forcing it out of the nozzle. The advantage of this approach is that you can feed almost any form of PLA into the hopper and it will be extruded. This means you don't need to prepare rods and you can use granules, chips or powder. It also makes recycling easy because you just break up the original plastic part and drop it in the hopper.
- posted by Simon McAuliffe @ 10:22