Yesterday when we conducted part 2 of the casting class/lab the blue casting machine failed, despite the best efforts of @nausser915 and @darrent to troubleshoot. (It is now working again and we’re ordering some repair parts for the future since it’s a 22 year old machine. Thank you @kobin !)
Instead we cast using the small furnace and the vacuum casting table. It was a learning experience, to say the least. Joseph did a great job of getting some castings through the process and the rest of us learned a lot from it. I wanted to share what we learned so we can fine tune the process going forward.
We melted the bronze in a large crucible in the “small furnace”. When it “looked ready” (obviously this is subjective), we transferred the investment flask from the hot kiln to the vacuum table, engaged the vacuum and then Joseph poured from the crucible into the investment flask. This is more challenging than it sounds because there was a tiny little slug of metal in the very large crucible and he was pouring blind.
In the photo you can see the vacuum table in the foreground and the small furnace on the RHS in the background.
Results:
Pours 1 & 2: Successful pour. Heavy surface scale but no pitting.
Pour 3: Failed. Not enough metal into investment mold.
Pour 3: Failed. We lowered the temp and it was too low.
Pour 5: Success. Surface quality sub-optimal due to investment failure around 3D-printed wax. Not an artifact of the casting process.
I’ve cleaned the scale off of pours 1 & 2 just so we could evaluate the results but the class participants still need to polish.
In the future, Joseph says he has two perforated flasks we could use on the vacuum caster that should enhance the vacuum draw. They are a little larger than the small flasks we use in Blue.
After Action Review:
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Temperature. Five pours. The first three were too hot and produced much fire scale. We slightly reduced the temp between pours 2 and 3 but it wasn’t enough. The fourth pour was too cold and didn’t melt the bronze enough to get a good pour. The fifth pour was set to 1900oF and seemed about right for the standard “pink bronze” from Rio.
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Metal quantity. Better to pour with a little extra metal (pour 5). The vacuum doesn’t seem to pull the metal into the flask quite as vigorously as the centrifuge does, and since you’re pouring blind you might miss the pouring hole a little.
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Surface quality. Seems fine with the wax masters (DMS logos). We are still trying to figure out how to get good surface quality from 3D-printed wax resin (pour 5). This is a result of the investment process, not the pouring process.