Metal Casting Zone

In years past, I have cast some silver and gold jewelry successfully, techniques and equipment that are FAR from "by-the-book". Until this week, I had never used a vacuum de-bubbler. My burnout oven was a metal coffee can on top of a hotplate that surely never reached so much as 700 deg F (flask never glowed orange). And my centrifugal caster appears to be home-made (I bought it used), and only spins for about 15 seconds.... slowly. With all this... I have cast some pretty nice stuff. But I'm trying to do better. So...


I made a vacuum de-bubbler and it really sucks the bubbles out of my investment. I bought a small burnout oven that hits 1200 deg F in 45 minutes and really makes that flask glow orange hot! I bought a nice, clean new crucible, and glazed it with Borax. All should be in order.... but my castings are having huge flaws for the first time ever. It seems to me that the metal might be flowing back out of the mold/sprue because the centrifuge doesn't spin long enough. Does that seem possible to anyone?

I melt the silver, release the centrifuge and it spins.... everything looks perfect, the metal was measured perfectly... no slop. Then, as the centrifuge starts to slow - PLOP - a glowing, red, blob of molten silver drops to the ground, and my casting looks like the one in the picture below. Also see the drip mark on the mold.

For the first time, I'm setting that flask/mold on the centrifuge glowing red hot, right out of the kiln... is that TOO hot? The metal isn't freezing fast enough? Or that is a good thing, but it needs to spin longer? At this point, it seems like just pouring it in there would do the trick. I can't afford an $800 vacuum casting machine, but given the ease with which I made a vacuum de-bubbler, that seems like something I could do as well. Thanks for any advice!


Huge flaw right where the sprue attached:




More of my pics: www.flickr.com/photos/11168745@N06/

You would know more about this than I, but I think your mold was too hot. The silver didn't have a chance to cool and set up. I don't know what the optimal temp would be for the mold, I would guess about half the melting point of whatever metal you are casting. Just a guess.

There is some of information missing.
Many things could have gone wrong, starting with how your sprue was designed?
What percent of metal did the button use?
It almost looks like an incomplete casting!
Are you saying that you compete your burnouts in 45 minutes?

Hi Daverham
Welcome to the world of lost wax casting. That casting arm has to spin and spin , when you wind the arm up to put the tention into the spring and let the arm go you should be up around the 400 RPM and spin very easliy for a few minutes. I can remember the casting days of my apprenticeship (GREAT MEMORIES) my boss would set the last flask of the mornings casting up and set it spinning. I would be cleaning the previous flask which took several miniutes ,it was my job to remove the last flask out of the arm , quench in the bucket and clean it. The speed the casting arm was still at when I went to remove the flask was amazing. I always had to wait little while untill i could safely bump the counter weight end with my hand to slow it down a bit to get on with the cleaning. The gold or silver had well solidified in the flask but shows how well these arm spin.
It certainly sounds to me Daverham you need some work on the casting arm there, the arm needs to spin rapidly for a good minuite or so, at least long enough to allow your alloy your casting with to solidify.
If you want to improve the casting results you will need a good burnout furnace,a proper burnout is very important in lost wax casting. Casting flask temp for silver , depends on the design of the articles as well, around 620oC , 500oC. Jamma I think hit the nail on the head, about half the melt temp of the metal is a good enough place to start.
Best of result with your casting give it another go once you adjust the arm.
Johncast

mold's too hot. cast at 850-900 f

it looks like you're not using enough metal, but that's because the sprue is falling off with, I think.

Very useful information especially to people who are new in the precious metals industry. Two thumbs up!!