how to make a dimensionally correct casting ?

Can plaster be made to expand in a controlled manner? I need to reproduce an aluminum casting (in aluminum) that is as dimensionally close to the original as possible. Since Aluminum shrinks in the mold as it cools the pattern used must be a little larger than the original. My idea was to use silicone rubber to pull a mold off the original then use plaster or some other mix that will expand in the original mold to make a pattern that is a little larger. I was told that by varying the temperature of the plaster mixing water you can change the amount it expands when cureing. The aluminum will shrink approximately 5/32 per foot so I need the plaster to expand about the same. Is it possible to achieve these results, what type of plaster should be used? Any other methods to do this?

HillbillyCaster, welcome to the forum

At the following tutorial different values of plaster are being presented in a numeric fashion. Perhaps this can be of help. I also would like to mention that in this tutorial it says that plaster always expands... and, it is advisable that you -if at all possible- experiment a bit as well.

Please click here.