Can i use cement as a mold for molten aluminum? urethane?

Question

can i use cement as a mold for molten aluminum? or if not, can i use poly-urethane? i think urethane works cause we used it in my class but i dont know where to buy poly-urethane

Answer 1

no to either.

Molten aluminum will melt your polyurethane and while cement probably will hold up it will give you a bad surface and it can be even dangerous unless you are sure it contains absolutely no moisture. Moisture in the cement will cause gas/vapour to form at the surface when the hot metal hits it and you cast won't look good at all. The water turning into steam within the cement can even cause the cement to explode (depends a bit on the volume of your metal, but the more metal you have, the more likely this is going to happen).

You can make a sand/plaster mold, but you would also have to dry this e.g. in a kiln to make sure all water is gone.

Answer 2

the cement/concrete may explode when exposed to the intense heat of the aluminum and the urethane will more than likely melt.

Answer 3

I would recommend urethane because of the porous nature of cement and the way that the aluminum would fill in the pores.

Answer 4

No, it should be plaster.  Never did it with poly.  plaster is cheap and I like the art of pouring and creating the plaster as part of the process.

Answer 5

You can make molds of a product called investment, which requires specific baking steps (up to 1350F) to make it work the way it is supposed to (wash off) for aluminum, brass, bronze, gold, and silver.  Within limits a 50-50 mix of plaster of paris and silica flour from pottery supply places will also work, but it has to be dried at a lower temp to drive off water and then at a higher temp (900F) on the way to casting to deal with chemically held water after which it is pretty weak.  Aluminum also does well with sand casting.

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