I have cast a lot of aluminum, both industrially and in smaller home-based shops. Here are some things to consider:
Is it possible?- Yes, we see cast-aluminum products all around us.
Is it practical for the casual DIY to accomplish?,- in nearly all cases, no, but look at some of the things that we do here, it is possible, but not practical.
The Molds:
The molds are usually made from "packed-sand", you pack the sand into the proper-shape using a pattern, usually made of wood to produce a void in the sand, into which the molten aluminum is poured and allowed to cool. A steel-mold may also be used, but you will need to preheat the mold and likely use more than gravity to push the molten-metal into the steel-mold, (injection-molding). Even industry uses "sand-casting" for nearly all of the aluminum-casting that is done.
The Temperature:
Aluminum melts at over twice the heat required for lead, or gold, jeweler's equipment will likely not be rated for high enough temperature, but I can ask my jeweler-friend to see what he thinks. The temperature will also require a fairly serious kiln, or oven, while in contrast lead may be melted over a hot-plate, the tempertaure-difference between the two are worlds apart.
The Results:
1).Density-The cast aluminum will be less-dense than "milled" aluminum, which I would think would be a disadvantage for use as a heat-sink, but I would leave this to the physics-experts here to determine.
2) Accuracy/finish-Your part, out of the mold, will have a fairly rough-surface, check out an engine-block, or cast-aluminum-wheel surface to get an idea of what it will look like. Cast-parts are almost always finish-machined to improve finish, at least on any of the critical-dimensions/finish surfaces. Even when machined, cast-aluminum will not be as nice as milled-aluminum. Another reason cast-parts are finish-machined is that because of shrinkage of the cast-material, (alum. is one of the worst for this), you would have to finish-machine just to get decent accuracy of your dimensions. For a heatsink in a pointer, all dimensions are fairly critical, as is the surface-finish, so you will most likely be machining the whole-thing when you are done anyway.
3) Your shape- The desired-shape for most host's would be a cylinder with a hole through it, You will not likely be able to "cast" the hole into the part, due to limitations of the molding-process (draft-angle is required on all surfaces to get you pattern out of the sand), so you will be machining the ID-features, casting will not help you here, unless you are injection-molding, so really you would be casting a solid-cylider, and then machining everything, including the OD of the cast-part. You would be better-served just starting with an aircraft-grade milled-aluminum bar-stock at this point.
Summary:
It would seam to me, that this would be an impracticle-idea from start to finish, however that being said, anyone one who tries something new, will learn a great-deal regardless of the success in the end. You may not end up with a heastink-casting process, but you will learn a hell of a lot, which is really the point, right? You may even somehow be able to overcome all of the above obstacles and come up with something new, [highlight]let's remember, it would seam impractical for most people to buy a DVD-burner and make a high-powered laser from it's harvested-diode, but we all share a passion for doing just this.[/highlight] The above information is not meant to discourage, rather it is meant to prepare anyone wishing to pursue aluminum-casting, with some starting information of what you will be up against.
Good luck to all. If there is anything I can do to help, just let me know
DH
