Crucible Molds

[Jammer]

I've made 2 crucible molds but I've only tried out the small one. I trying to use a castable refractory rated for over 3000F. The first one cracked at the top. Apparently I have a slight undercut in the mold. I'm going to try to fill it in with some hotglue or a little plaster. I'm considering casting it in Aluminum so it doesn't get damaged from demolding. Also, had a lot of bubbles on the outside, I'll try it again with a thinner mix. The larger one is about an A-10, well it's molded from a #10, 10 kilos of copper, crucible from Legend Mining Supply.






The inside came out pretty rough, I coated it with plaster and sanded it down some. It's a lot better now. I'll seal it with some paint or varnish and try it out.

[mite5255]

When you de-mold Jammer make sure you remove the inner piece first, if you de-mold the outside first the inner mold can put pressure on the casting and crack it. For the air bubbles get a piece of rod that will fit in the mold cavity, round off the end so its bull nosed and use that as a tamper , I would fill the mold about a 1/4 and them tamp it with the bull nosed rod repeat till full, that should get rid of a lot of your air bubbles


Mike

[Rasper]

Be sure to put something (graphite?) in your castable mix that will conduct heat. I made one of pure Mizzou that would not melt bronze. It would never get hot enough inside the crucible. Dense castable is something of an insulator.

Richard

[Jammer]

That's a good point Mike, I just popped out the easiest first. Should have known that wasn't right. I tamped it down with a wooden stick. The inside was real smooth, most of the problem was it was just too thick. I know the more water, the weaker it will be. I wonder if I could add a small amount of Sodium Silicate to act as a defloculent (sp?). Also, there are some chunks of ganister in it that are a little big. I want to sift the larger chunks out and break them up or replace it with Graphite and/or Silica Carbide. Probably the latter to answer Richard's suggestion. I planned on adding something, I don't know why I just used the castable, I guess I just wanted to see if it would come out. I didn't want any other variables.

[mite5255]

That's a good point Mike, I just popped out the easiest first. Should have known that wasn't right. I tamped it down with a wooden stick. The inside was real smooth, most of the problem was it was just too thick. I know the more water, the weaker it will be. I wonder if I could add a small amount of Sodium Silicate to act as a defloculent (sp?). Also, there are some chunks of ganister in it that are a little big. I want to sift the larger chunks out and break them up or replace it with Graphite and/or Silica Carbide. Probably the latter to answer Richard's suggestion. I planned on adding something, I don't know why I just used the castable, I guess I just wanted to see if it would come out. I didn't want any other variables.

I'm using reference to my years in the concrete industry and I'd say the refractory looks wet enough just needs more or maybe better tamping, I would try maybe filling less tamp add more tamp and so on, maybe fill a 1/4 of the hight at a time, oh and if you can tamp at an angle not straight up and down. Over to you harry and other concreting specialist

Mike

[Jammer]

I might get out there and try another one. I still need to smooth out the lip area and maybe find something better than vegetable shortening for the release, I know in concrete they use some kind of thick grease. It's been hot here lately, I had to cool the mold down so the grease didn't run off.