you mention that some metal can be casted in silicone. Is that possible with aluminum?
I'm afraid aluminum has too high of a melting point for silicone molds to withstand.
There are several types of RTV silicone rubber that you can use for casting low-temperature metal/alloys like tin and pewter (SmoothOn's Mold Max 60 has the highest heat resistance that I know of at 560° F / 294° C), but RTV silicone is NOT recommended for higher melting temperature metals.
To cast aluminum, you'll need to do either sand casting or lost wax casting in a plaster mold. Here are two instructables and an online example that go over the lost wax casting process:
Hope this was helpful!
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There have been a variety of aluminum casting done with PLA material used on 3D printers. Results seemingly have been phenomenal. Bismuth is a very popular low melt point metal that many uses. Best of Luck to you.