Bronze Casting. Can You Bake the Investment Twice??

I found out the answer to this question today, unfortunately.

This is the PLA + wax tree which I wanted to cast. There are 3 bevel gear cases, for the Armstrong model cannon.

There are internal cavities for the bevel gears, holes for the shafts, and external/internal surfaces, quite a complex shape.

Yesterday I mixed the investment, and poured into the flask, around the tree. It was a tight fit in the flask. I had miscalculated the flask diameter, and some parts were probably touching the metal cylinder. So I was not too confident about a good outcome. But I went ahead and commenced the drying out, burning out, and baking cycles. About 7-8 hours altogether.

Then started the bronze melt in the electric melting furnace.


The temperature was not rising.

Usually, 1100ºC was reached in 30 minutes or so.

But it remained at 19-20ºc. Ambient temperature.

Penny dropped. Bummer. The Melting furnace was RS.

6 months old, still under warranty. But how long would it take to get parts or replacement from China? Email sent.

And what to do about the already prepared investment flask, happily baking away? So I turned off the investment oven, and let it slowly cool down.

Discussed the situation with Stuart. Probably burnt out the heating coil, but could be the relay or the control unit. Whatever, it will take days/weeks/months to sort out. Bummer.

Stuart: “You can borrow my spare unit”.

Me: “you have a spare melting furnace?”

Stuart: “sure”. “you can borrow it”.

Me: “wonderful, thank you”

So today I wondered if the investment flask could be reheated to 710ºc, and accept the molten bronze. Stuart: “Don’t know. Never done it”.

So today, I reheated the investment flask to 710ºc, over 3-4 hours, held there for an hour, melted some bronze in Stuart’s loan furnace, and poured.

Here is the result.

3 almost perfect gear cases. I have removed the sprues and funnel and most of the investment. Not showing up well in the photo, but I have demonstrated that the investment mold CAN be reheated. I will machine off the unwanted bits and surfaces tomorrow.

Casting with bronze seems a bit more forgiving than with aluminium. And Investment mold, at least with Goldstar Omega Plus, CAN be cooled and reheated without cracking up.

And a replacement heating coil will be sent from China. Maybe I should buy a spare melting furnace.