Was browsing about ye ole internet and came across a site that might be the answer to various questions posted around the forums regarding Metal Casting using Shapeways model. Just for signing up they are offering two free guides to metal casting techniques. They cover far more than just metal casting.
Thought a few folks about here would like to take a look at them.
Let me give you a bit of advice, though. I've taken classes and cast aluminum, bronze and even iron. Those processes are inherently dangerous and really should be done under supervision the first few times.
Pewter, however, is relatively safe if you use the lead-free variants. For that matter, there are numerous resins that present no danger of death-by-fire.
What I'm saying is that you should consider starting out on low temperature metals or resins, refine your process, then proceed to aluminum. After that you can do the higher temperature alloys.
Safety first, and believe me... casting can go bad quickly. I have a shirt that is covered in pewter because of my lack of care one afternoon. I keep that shirt as a reminder that if I hadn't been wearing glasses that day I would have probably lost an eye.
"Just this once" is the most dangerous phrase on the planet.