I use solid wood rather than plywood. The end-grain is much better for surface quality. This is a trade-off; solid wood forms also will tend to warp over time. But I rarely reuse my forms; I cast once and if I need another copy the next time around I use the casting rather than the wooden form.
What works for me is to dip all the forms in hot wax, to reduce surface imperfections. This has the added effect of dealing with the casting equivalent of Xerox burn. When you make a mold you add excess material for machining tolerance, and excess material for shrinkage. Once you've cast something and cut the sprue off, you can use it as a form. It has shrunk, already, so if you want to make the next one larger, to prepare for its shrinkage allowance, you can dip it in wax and fill in any pits.
Drip a very small amount of water around the edges of the wooden form before removing it from the sand -- this strengthens the sand. You can also blow out bits of sand that have fallen loose into the form void.
It will also help to provide a fairly thick sprue that connects to the side of the form, which somewhat prevents the aluminum messing up the void during pouring, by lifting/eroding sand from the face of the image itself. To get the air to smoothly leave the void when the aluminum is flowing in, it can help to use a long thin straight wire to poke holes into the sand, from the form out through the top. A good cast will have aluminum run out these little air holes.
Sprues and risers: Make them huge. I use 2"x2" sections that I've put a slight taper on with a radial arm saw. (This also works really well as a ramming-up tool.) In the sprueing, the height of the sprue top, compared to the top of the form, determines to a large extent the solidity of the casting. The more hydrostatic pressure you can generate in the sprue, the better the detail and solidity of the final casting. So one thing you can do is to use a sprue extender. I use a 6-oz steel tomato can with both ends cut out, that I place over/around the sprue hole just before I pour. It's nice to have tall enough risers, such that when you pour, the aluminum can come out of the risers. This floats some of the slag and dross out the top. The risers also vastly increase the quality of the casting; when the casting is solidifying it contracts at the top, and if you have a big riser off the top of your form, the contraction will all happen in the riser, which you then cut off, rather than in the top part of your form.
I'll mention this over on the casting forms page as well, but it is very important that you make casting boxes which positively attach to one another -- ie they can be pinned or hooked together. Aluminum is very heavy stuff. Just because the mold frame weighs a hunnert zillion pounds, means nothing when you pour the hot metal in; it *will* float the top frame full of sand right off the lower one and spill metal ALL OVER the floor and your shoes and everything else in sight, and you will not get a good casting. One of my recent castings, I was casting shot for a coilgun. The pieces were 3" outer diameter, 2.5" inner diameter, maybe 3/4" high, weighed a couple ounces each. I thought, "Hey, no problem, I don't have to worry about doing a full mold" and did only the upper part of the mold, which I set on a 2x2" frame that I'd packed full of sand for a nice flat surface. I poured, and even this little bit of aluminum floated a half a cubic foot of water-filled sand right on up and let aluminum come pouring out the sides of the frame. The casting turned out good, as it happens, but that's amazing -- and I had a sheet of 1/2" thick aluminum that connected all the individual rings (I cast six at a time) to each other like a huge web, and it was very difficult to break them out of this web. Don't cast without pinning your frames together. It won't work. Trust me on this one. Or try it out yourself and see. Just wear heavy leather boots.
This page written on 12/15/00, last modified 23 May 2023.