RepRap: Blog

OK, so as always I cheat a bit. I used the ancient art of lost-wax casting, substituting PLA for wax. It burns/vapourises away cleanly at low temperature and is relatively benign stuff. So, how do you go about turning your PLA RepRap output into metal?

I took one standard-issue Mighty Reprap Power Ring in PLA as a test subject, and stuck on a few bits of 3mm PLA where I thought it looked useful. At the highest point, I stuck a simple printed cone of PLA to act as a filling hole. Then I painted it with thinnish plaster of Paris mix. While that dried off a bit, I mixed up more plaster and snipped into it about 30cc of uncompressed household fibreglass insulation. They only need to be about 5mm long, and are mixed in to disperse as well as you can. This will help stop the mould cracking (or, more to the point, hold it together if it does). This stiff mix is packed smoothly around the object, taking great care to not trap air bubbles like I did.

I wrapped masking tape around the bottom of a jar (initially sticky-side out) to make a disposable paper casing for our test subject. It looks black in the photo 'cos it's well-baked. Then, a couple more spoonfuls of plaster bedded said plastered bundle into place in this little paper pot. The pot was dried overnight in a desiccator, then baked at 150C for 15mins to drive off moisture. Next, the pot was inverted over a wire stand and foil drip-tray - out of deference for me not being in trouble for mucking up the oven. The temperature was raised to a scientific "11", and stayed that way until the conical filler hole looked absolutely clean, though slightly browned.

During that time, I fired up the ol' furnace (renewable fuels, etc.) and melted a load of pewter. With the mould still really hot (and thus safely dry) I poured in the pewter and left it for a 1/2 hr or so to cool off gently. I definitely did not impatiently have-to with a can of "Freeze Spray."

With the aid of tongs, the flat side of a 160g pein hammer was strategically deployed multiple times (Adrian will remember my skill with this tool from an earlier occasion) to reveal an amazingly faithful reproduction of the ring. The pegs sticking up are where pewter got into the vent holes. You want this to happen - just trim the excess off and file down. Same goes for the filler hole, just pop the scraps back into the clearly-labelled "Scrap Pewter" box.

After a stiff brushing under hot water, I was allowed in the house - no, I mean the ring was cleaned up and presentable. An air bubble was trapped inside the ring, resulting in a blister of pewter. But pewter is soft stuff, and you can easily correct problems like that with woodworking tools and a file.

If you do all this, do take basic safety precautions. Also be wary of using metals that melt at a higher temperature like aluminium. They cause plaster of Paris to decompose in undesirable ways and you need to research it properly.

Vik :v)

Labels: casting, lost, pewter, plaster, reprap, ring, wax

The valve mechanism in the experimental paste extruder was a bit difficult to make as it needed some machining. I've now done an alternative that uses rapid prototyping to make a blank and then soft casting in a silicone mould to produce the final item. You can't quite use rapid prototyping for the whole thing because RP is not good at threads, and this has an M14 one.

The advantage of this approach is that once you have set it up you can make a lot of these devices quickly and easily by repeated casting.

It uses the trick of trapping nuts in the resin to give good retention of screwed-in parts - see the soft casting page again for details.

Labels: casting, resin, soft tooling

The last gear problem for Darwin, casting a 12 mm gear to fit on a 6.35 mm motor shaft. A flat was needed to ensure no slippage on such a thin shaft. Final solution was to use the same kit used to make the Y gears (see previous post). The collar was prepacked again, but this time, becasue there was a bit more wall thickness, the hole was simply started by working a screwdriver into the capa. The collar was then put into the pot with a new blade part which fitted the motor shaft profile. The motor shaft was then poked in from the back. Worked first time.

This was then complemented with Vik's superb idea of a washer + bit of capa for the second lip.



Next: a repeatabiltiy test for the X. Though I need to solder up a universal PCB first...

Labels: casting, darwin, gear, toothed pulley, x-axis