Thanks Matt.
I use aluminum only because good molds must be large to get rid of the heat, and maintain precision. That means heavy with brass iron or steel. The latter two have severe rusting problems, and none of these three will outlive my properly made aluminum, which are nice and light to handle for long casting sessions, even in 4 cavity, and produce bullets faster than any other molds I know of. It would require several paragraphs to give all the other advantages of aluminum, which add up to great benifits overall.
Please don't try a Lee mold to see how an 'aluminim' mold works.