Friday, July 29, 2005
I thought I'd try a little experiment to see how well PLA will take on small detailed shapes.
This evening I cast a gear into a plaster of paris mold of a spur gear. I put PLA granules into the mold. I heated everything to a little over 200°C and waited. The PLA melted and formed an amorphous blob but it wouldn't reach into the teeth of the gear. A little disappointing, so I removed it from the heat and left it to cool. Interestingly in the process of cooling, the plastic was drawn into the finest details of the gear. Something about the cooling process really helped. I don't know what this means for extrusion or if it helps at all.
The result is extremely accurate and detailed and you can see very fine details of the involute tooth shape (not very apparent from the photo). There were a couple of little bubbles but they were well away from the centre and the teeth and it seems very hard and strong. It meshes very nicely with matching gears with no slop.
I coated the plaster of paris with a cooking oil spray to act as a release agent for the mold, but it failed pretty abismally. It was too hot for the oil I guess, and it led to the brown discolouration as the oil denatured (along with an unpleasant smell that filled the house). The plaster didn't release as it should and I had to break it off and scrape and brush the remaining plaster off.
The mold making was probably fairly critical to the detailed result, but we're not really interested in molding parts right now, so I won't go into detail.