Quote:
Originally posted by nicozeg
it don't need to handle temperatures. 
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I don't quite understand that. It will have to be able NOT to melt when the molding material is applied. The ony way to make a mold is to have something in liquid form that will cool off and harden. This will also not be able to melt when the silver is applied. So in other words it seems to me this plastic prototype needs to have a higher melting temp than silver, because the stuff that the mold will be made of needs to have a higher melting point than silver (obviously). And in order to make the mold the stuff that the mold will be made of will have to be molten, which will have to be at higher temp then what silver will turn to liquid from, or the mold will melt when the silver is cast into it. So in other words this protoype needs to have a very high melting point.
Finally found it. Melting point of pure silver, 1763.2 °F. This means the mold needs to be able to withstand this and a few hundered degrees more as getting silver into a fluid state has to be hotter than melting point. So the mold needs to be able to handle that temp, which means this plastic prototype needs to be able to handle around 2000F.
Of course this is from my very limited knowlegde of casting, the above process could be totally wrong, but I would look into it before you try it.