ArtMetal

I've done no metal casting yet myself.

Aluminum will probably cause a steam explosion or at least mold failure with plaster. When you heat plaster, the water goes out and the material turns back into gypsum.
http://en.wikipedia.org/wiki/Plaster

You may mean investment. Investment is refractory (heat resistant). It's like comparing house cats and tigers.

I don't know if a two-part investment mold will work for aluminum or if you need something watertight. If it does work, you could make multiples using casting from a plaster master. You'd probably have to dry them in a furnace out to drive out the excess water.

If two-part molds don't work, you could do lost wax casting.

I'd probably try do do it via sand casting myself, perhaps using plaster masters.

You may want to find someone local to show you the tricks of the craft. You may also want to use pewter or plaster as a test material before switching over to pouring aluminum.

If you better mechanical strengh than pewter, have you looked at Zamak, or other zinc-aluminum alloys, aka pot metal?
http://en.wikipedia.org/wiki/Zamak
You can melt it on a burner (outside), as with pewter.

You can pour zamak (or pewter) into certain forumulations of silicone molds. I know you can pour pewter into plaster, dunno about zamak. Make sure to wear protection, the zinc fumes are quite fucking* toxic.
http://en.wikipedia.org/wiki/Zamak
http://www.instructables.com/id/E3M8YB8RIAEP2865W9/

*profanity used for emphasis, so that people who are careless about health and safety don't blow this part off.