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Author: Subject: A mould material for tin casting - brainstorming!
Biochemicus Energeticus

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[*] posted on 3-6-2007 at 16:57
A mould material for tin casting - brainstorming!

I'd like to cast tin (95%) into a mould that has the following properties:
- it can resist temperatures up to 400 deg C
- it does *not* release steam when heated up to that temp
- it is porose and brittle enough to chisel/dissolve/hammer/break it off after the tin cast has cooled

Now, you go, simple, why do you ask?
Yes. Tell me. What is it?
I tried a variety of plasters of paris (CaSO4 with additives), and even after heating the mould (and melting out the wax positive which is where the tin goes in) to 250 deg C for many many hours (10+), making sure it is dehydrated completely (the weight remained constant for several hours), filling the plaster mold with 330 deg C tin still produced steam - so vociferous that the actual tin cast looks horrible.
Concrete you'll say. Well I can't knock it off easily, can I?
What other materials can you think of? It will have to be some inorganic compound, because they are usually cheap, and they can resist high temp. Temp-resistent silicone is another option, but a) HUGELY expensive (40$/kg), but also, the casts are not good - tried and tested. Gypsum/PoP works very nicely for small casts (cork sized), but the steam is the issue for anything bigger.
Can you think of a material that nicely surrounds a positive (i.e. a wax imitate of a chestnut including the shell& thorns), which I can melt out at higher temp, and where the mold is filled after with tin? I can get the approximate mold with almost any material, but the steam, at least with gypsum, screwed up the detail, which is what I am after.

Any thoughts?

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Post Harlot

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[*] posted on 3-6-2007 at 17:09

Did you cut the PoP with sand?

I never had a problem with the stuff after cooking it in the oven at 450°F for a few hours.


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[*] posted on 3-6-2007 at 17:19

Originally posted by chemoleo Concrete you'll say. Well I can't knock it off easily, can I?

Perhaps you can just make a weaker concrete by adding some sand into the mix?
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[*] posted on 3-6-2007 at 18:23

When we do lost-wax investment casting of silver or gold, we use a plaster that contains silica. The mold is heated up to about 800°C in a 10hr cycle. You can easily cast 100mls of metal into one mold, with details as fine as 0.25 mm reproduced. (All the stamps 9k 14k 18k etc are reproduced on the articles inn this casting process)

It's reasonably expensive thought [EDIT: though].

Depending on the piece, you can use two pieces of cuttle fish bone, though this wont allow particularly fine detail, and will allow for practically no undercutting.

Industrially, some places use molds made from silicone rubber. IIRC, you can get 50+ pieces from one mold. I'll check through my books and report back anything else that might help.

Just what kind of a shape are you trying to cast? I just ask cos it'll make a difference to the best way of going about this.


[Edited on 4-6-2007 by enhzflep]
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[*] posted on 4-6-2007 at 10:02

Why do you want it as hot as 400C?
I have used wooden molds for tin; they smoke a bit, but they work. The stuff I was doing was shallow so the fumes and steam didn't matter too much.
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[*] posted on 4-6-2007 at 11:01

Lost Wax Casting

Comercial Investments

Economy Investment Formula

You could try 1:1:1 (by volume) fine sand, celite (diatomaceous earth, filter aid), plaster.

Try baking out with oven cleaning cycle in self cleaning oven. It should get hotter than your pouring temperature.
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[*] posted on 4-6-2007 at 13:33

how about using uncented cheap cat has bentonite for making your molds.....along with some sand.......solo

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[*] posted on 4-6-2007 at 15:56

Actually solo, very fine ground bentonite (accomplish this with a blender or ball mill) and motor oil make for a very good casting material for flat items with fine detail. Almost zero green strength (resistance to deformation after the mold is made) but if offers amazing detail rendering, just as good as investment. Bentonite is used all the time in making greensand, used for metal casting. The objective of it being in there is to serve as a binder by encapsulating the sand grains and to keep moisture in so the mold is easily rammed up (flowability). It does dry out though.

I think lost wax casting is your best option unless you want to try with a resin (furan) + acid catalyzed "no bake" foundry sand. That cures at room temperature for a hard mold with good shakeout properties. Not ideal for tin, mainly designed for higher temperature metals.

If you do lost wax, clean your model thoroughly, paint on your investment (essentially cristobalite, a form of quartz, and Plaster of Paris, hydrated gypsum) with a small paint brush. Just make a few milliliters of investment in a disposable and flexible cup as it sets up in 7-12 minutes depending on moisture and water temperature. Using a brush gets it into all of the crevices. Then dust if over with dry investment powder, let it dry, then repeat the cycle til it is 1/3 of a centimeter thick. Then I dewax it with steam or a hot air gun and wrap it up with wire, suspend it in a clay triangle like you use for heating a crucible, and get the whole thing red hot with a Bunsen. Now take your ''shell'' and put it in a clean, dry can and pour in hot sand around it. This method gives very good surface detail and uses less investment, so there's less water to steam up and the sand offers better porosity. From personal experience, it works very well, better than filling a whole flask with investment.

You can find investments from Kerr Inc. (I use KerrCast) or Shor International. If you want to pay shipping, I can send you a few pounds of it, I have plenty, U2U.

[Edited on 4-6-2007 by Fleaker]

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[*] posted on 4-6-2007 at 16:03

Bentonite tends to form water-tight muds, even with coarse sand would likely be hard to dry out.

Plaster of Paris, with or without additives, works pretty well. Drying is not real fast, a slow ramp-up of temperature over many hours is best, large molds can take a day or more to bake dry. You'll need to finish the baking at 300 C or higher. Don't let the bake mold sit after cooling, it will pick up moisture again; if you can try casting into the warm mold.

For fairly simple shapes people have used soapstone, carving the mold cavity into 1 or 2 pieces, then strapping the pieces or single mold piece and a flat piece together for casting. When it's cooled down, just pry the two pieces apart.
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[*] posted on 4-6-2007 at 16:39

I'll echo what not_important says: Baking the mold with a temp program that holds for 300 C for several hours then pouring immediately into the hot mold should ensure decent castings.

PoP is ideal: you can add some cryolite and silica flour to the mix, although it isn't necessary for the low temp Sn is poured at. Adding courser silica can improve the porosity, although typically porosity is not the objective for an investment-type mold.

Soapstones and steatites can also be used, as mentioned. Another option, for small work, is cuttlefish bone.

Finally, another option is to make a so-called baked mold, much like that used for cores. Use molasses or linseed oil as a binder for plain dry sand somewhere around 6-10% by wt. You can add a little wood flour or sawdust too. Bake it at ~200 C to cure and you will have a hard mold that should be weak enough to break up once cast. Another core recipe uses water glass that is fixed with CO2... Depending on your application, a baked mold might work.


[Edited on 4-6-2007 by jove]
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