Metal Casting Zone

Hi. I'm a dental student, so I have some experience with centrifugal lost wax casting. We cast crowns from gold alloy (around 18K) using a centrifugal casting machine with a gas/oxygen torch for melting the gold. I have had good success with my dental castings, but I am about to cast an exgagement ring, and I have a few questions. I have made the wax pattern out of purple Matt wax with Renfert classic sculpturing wax overlaid on the outside of the band for detail. I am casting with 18K xtra white casting grain from G&S metals(www.gsgold.com/castinggrain/whitecastinggrain.htm). When we cast crowns, we pour the investment (whipmix fastfire 15, stable to 2200 F(1200 C)), allow it to set for 30 minutes, trim the glazed end to allow air to escape, place the casting ring/investment upside down in the oven at casting temp (about 1550 F) for 30 minutes, melt the gold, remove the ring/investment from the oven, place it in the cradle and let the casting arm fly. The Matt wax says that I should place the investment in a burnout oven at 300 F for 1 hour, then 450 F for 1 hour. Do you think that this is necessary? I don't have a seperate oven for wax burnout at a lower temperature. If I put the flask in the oven at casting temp (1850 F) for 30 min - 1 h will the
Matt wax burn out completely like the Renfert wax does? I would expect that at that temperature, whatever doesn't drain out by gravity would be forced out by gasses or be burned to gas, but I've never worked with this Matt wax before. Any suggestions or opinions?

Thanks,
Alec

I usually just put my flasks into a cold oven and then turn it on and let it come to temp, my oven is a fairly large kiln so it takes a bit for it to get up to tempature.
you could all ways let the oven to get to a temp and turn it off for a bit then turn it back on and let it heat up more then turn it off and so on until you get it up to the temp the investment mfg recommends. Your ring is very nice and it looks like you have invested alot of time into it, so you might want to make a dummy ring and try the burn out and pour on it first to make sure all goes well.

Thanks for the response.

I did a bit more research, and found some useful info. I guess a standard way to do a burnout is at lower temperatures for a longer time. I guess maybe this has to do with investment material, because I read that if you heat the flask too quickly, the interior of the investment will break. I guess this isn't an issue with the investment I was using, because we've been taught to do a quick burnout at flask temp (30 min at about 1350F - 1500F). I guess we use a higher flask temp because we're most concerned about accurately casting the thin edges where a crown meets the tooth (the margin). I guess higher temp increases chances of porosity, so jewelers cast at a lower flask temp. Anyway, I put the flask in one oven at 1350F for 30 min. When I checked, the sprue entrances were glowing red with no sign of black or grey residues. I then moved the flask to an oven at 1050F (recommended flask temp for the alloy was 1000F - 1100F) for 45 min, and then cast it, melting the gold with a gas/oxygen torch. Per alloy instructions, I quenched as soon as the red had left the button. It came out nearly perfectly, and after finishing and setting the stones, I'm very satisfied with the result.

Alec

Hi falec-illmore
Very interested in the casting you did for the ring pictured,I must say looks very nice indeed.you certainly didn't leave much to chance with those big sprues to the sides of the ring. The burn out is so so fast. Its amazing the flask didn't explode and crack. The moister in the investment mix needs to be evaporated slowly to my knowledge and I do this to my flask for my brass casting over 12 to 14 hours, my furnace clicks on at 3am in the morning and I am ready to cast 4pm in the afternoon. Im am casting 3 to 4 flacks, 6 inch by 3 inch. My boss also burned out over 12 hours to do his 9ct and silver rings.
However,I have read in one of my books that flasks can be placed into a furnace on high temp to burn out quickly.

JOHNCAST www.orncastings.com

Yeah, the rapid burn-out is I guess pretty standard in dental casting. I don't know if its success has more to do with the size of the casting and flask ( single unit - I don't think we do multiple units on trees, at least I haven't been taught to do that; some lab techs may) or if it has more to do with the specific investment. What you said about the moisture needing to get out slowly makes sense; I don't know why, but with the materials I used, we've actually been told not to let the investment dry out; if you're not casting immediately after casting, keep it moist until you do.

Hi, have you given any thought to using a standard kitchen micro-wave oven to dewax .

The refractory gets warm, but the wax is invisible to micro-waves.

Use low temps and short times until the wax is out.
Then pop it in a high temp oven.