I need to make a bunch of molds in sandcast bronze and would like to know the kind of sand to get for finest detail/smoothest finish . They won't be complex , or even 2 part sand molds, probably just bronze poured onto a flat cope that has an depression in it and a dam built around the depression .
It's good to make a point of the fact that you have to know what you're doing to use plaster molds for bronze temperatures, or you're in for an explosion.
It will need a high-temperature mixture, probably containing vermiculite or some modern ceramic replacement, and it will have to be calcined. "Bone dry" isn't dry enough. You have to drive off some of the chemically retained water.
Another option is green sand dusted with plumbago (graphite powder). That is said to be capable of reproducing a person's fingerprint. I've seen some aluminum castings done that way and they look almost like investment castings.
I'm thinking plaster is like gypsum and under high heat will give up water vapor. So the mould would have to be baked to drive it off.
My experience with bullet casting is any water combined with hot metal is catastrophic. I was using Marvelux for flux, it has an affinity for water and I didn't preheat the spoon I used to flux with before putting it in the pot. BANG! OUCH!
Right. And when you get up to bronze-casting temperatures, the problem is
*much* more dangerous.
I have a big collection of literature from US Gypsum on casting plasters, which I haven't consulted for, oh, maybe 20 years , and it's buried deep in my research material somewhere, but I think there's enough information around the Web to put the story together. Using plaster (which is gypsum) to cast aluminum is one thing -- although you still have to calcine it for that. For bronze, you have to cook it a lot more, at a higher temperature, and you need to have heat-resistant additives in it. Vermiculite (expanded mica) is a traditional one. I don't recall how much you're supposed to use.
Since the OP is talking about an open mold, the one thing you don't have to worry about here is venting. That's another big issue when you use plaster for casting molds.
Personally, I'd first try sand with a dusting of graphite powder. If you dust it just right, it fills in the texture of the sand and gives you a really smooth finish. You can sprinkle it in from an old stocking or a piece of pantyhose. Don't let your wife see it...
On Fri, 31 Oct 2008 22:59:53 -0600, the infamous F. George McDuffee scrawled the following:
Good discourse.
Some of the sculpture there is really interesting, Unk. But what's with those painter "artists"? My gawd, man. Does that shit really SELL? I guess they produce the equivalent (or worse) of Rap in painting. This reminds me why I don't support the National Endowment for the Arts.
Oliveira paintings, Staprans, Moses, Frohsin, any paint/poetry by freakin' Rodefer: Ptui! I feel like washing my eyes out.
-- Everything I did in my life that was worthwhile I caught hell for. -- Earl Warren
It's not just "seasoning." First you have to dry out the free water. Then you have to drive off a percentage of the chemically retained water. That's called "calcining." For casting zinc or aluminum, it doesn't take much. For casting brass or bronze, it's critical, and requires more heat.
It's not something to do by guess and by gosh. Those things can explode in a serious way. Furthermore, ordinary Plaster of Paris will not do for casting brass or bronze. You need special high-temperature casting plaster.
Do you know of any references on how to do it right? I've got a project in mind (that probably needs to wait 10 years) that could be done quite well with plaster cast aluminum, I think.
I had collected some info years ago, mostly from US Gypsum, that I think is no longer available at all. In fact, around eight years ago I decided to update what I have by calling them to see if there was anything lurking in their archives, and one of their people sent me some more photocopies.
There also is information about it in old British MAP books, which I have. You may want to check the listings for those Brit model books and see what's in print. And plaster is being used today in some high-tech casting operations. The materials must be available commercially.
I saved all of this stuff because my intention is to write a pamphlet or short book about it. Gathering it and summarizing it will be a very big job, or I'd volunteer to send copies of what I have. Before I do so I will contact current suppliers to see what updates are available.
I should mention, though, that I have several such projects hanging around for a rainy day, and they may never see the light, because the market for this kind of info is so small that it may not be worth it to do it right. I always have three times as many projects in the hopper than I can possibly complete. It's just a characteristic of my type of work.
US Gypsum (now USG) is, I think, out of that business. But you may find they would dig into their archives for you, if you luck upon the right person. It's usually somewhat frustrating work. That's how I make a living. d8-)
Google on investment casting. Jewellers do this routinely with high melting point metals like gold, silver and even platinum.
The stuff to use is investment casting plaster.
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It can be found at jewellers' supply places. Once the mold is made, it needs to be "burned out" or calcined at about 1200F long enough to "burn out" the wax and also to drive off excess moisture.
With ally you'll probably need quite a tall sprue to get enough pressure to get molten metal into the smaller internal features -- or, use a centrifugal approach.
Just be aware that investment molds typically have much less of a venting problem than cast plaster molds do, because they're a lot thinner. I should say "gas porosity" rather than venting, actually.
So, back to sand , green sand... I've heard that a lot but never seriously looked into it. Is it mixed w/oil , but with extra fine grain, or what?.
I eschewed lost wax investment casting because I don't have the gear anymore and don't want to pay someone else for these molds, and plaster sounds like just an extra bit too much trouble . Oil sand I can do down and dirty .
First, green sand is not mixed with oil. It's mixed with clay and water. It is based on sharp, fine-grained sand, but your Googling should give you a better idea than we could in a short message.
Regarding the oil, there are two types of molding sand that use oil. One is the very popular Petrobond, which most users say is easier to use than green sand. It's a combination of sand, motor oil and a processed clay that is "organophilic," which means it absorbs oil rather than water.
Just to avoid confusion, this is NOT the same thing as the old-style core-molding mixes that used linseed oil as a binder. Those materials had to be baked before use, and the point of their formulation was to stand up better to crushing forces that occur when a casting cools around a core. There also are baked-sand molds as well as cores, but, today, they don't use linseed. I'm not sure if they ever did.
It's supposed to be really nice for hobby-scale operations.
Interestingly, I found a green sand , Diamond Green Sand , and the guy there said I could mix a particular one, formulated for detail, with oil or water . Now, to find a place that will sell less than a truckload, lol !
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