Easiest/Cheapest/Reusable Metal Casting Method?

I'm starting to get into metal casting, mostly aluminum for now. I've tried a couple casting methods, but I'm still inconclusive as to which
I should use.
I've tried the lost foam method, It's rather annoying. It works well, but I lose my form in the process. :( If the cast fails, I start over at square 1. It's also no good for making duplicates of things.
I also today just tried plaster of paris. I formed a mold in a can I lined with vegetable oil(to help the mold slide out when it's ready), 2 halves with pins in the top half to align the bottom half. I let it dry for an hour or so, then tried to get it out. It slid out easily. Too easily. I cracked the bottom 1/2 of the mold in two. I took my form out of the mold, and tried to fix the bottom part of the mold by pouring more plaster around the form filling the damaged part. I also added pins into the undamaged part to hold the newly formed hack-fix. I let it dry, it seemed to hold. I threw the mold in the oven at 500F for a couple hours, I had made the plaster for the fix a little thin so it could fill cracks, and that part was still rather damp. Oh well. I threw the mold into my forge and let it cook at ~1800F for 10 minutes or so. The entire mold was glowing red. Oops. Some fissures had formed on both halves. I guess it wasn't a good idea to fire it. The mold was very very dry, upon removing the bottom 1/2 the seam where I had fixed it broke and it fell apart. Doh. I only bought a little plaster, since I wanted to see how well it would work with casting aluminum before I tried anything. I fired up my foundry and melted some aluminum, and placed the top 1/2 of the mold(still fairly intact, but fissured) onto a flat plane of sand. I put on some gloves and goggles, and poured the aluminum into the sprue. I was expecting the mold to shatter, hiss, or something. I've heard bad things about using plaster of paris as a mold, since it retains water. Nothing happened, the pour went perfectly. Unfortunatly, the mold wasnt divided at the sprue, so I had to ruin the rest of the mold to get the cast out. It came out very nice, smooth edges and no bubbles. I learned a few things today, what to do and what not to do, but I'm still undecided as to weather its a good idea to use plaster of paris as a mold. Does anyone have any idea about the following questions?
1) If I divide the mold at the sprue, will the aluminum come out cleanly so the mold can be reused? I'm leaning towards yes. The form I used came out cleanly when making the original mold. Aluminum shrinks when it cools, so if anything I'd imagine it coming out easier.
2) How dry does the plaster have to be? This cast was fairly small, maybe 3x4 inches. The pour did nothing to furthur damage the mold I had made, despite it was already cracked from drying it in the forge. I''m really hoping I can make a reusable mold.
I'm still wary of the cope and drag method. I don't really have access to greensand readily, I could use used motor oil as a bonding agent for the sand but I'd prefer not to breathe that in, plus, the method seems a bit complicated, having to pack and smooth out the sand, as well as pinning the cope and drag together. Also, when casts get too big, you need additional supports in the cope.
It would be NICE if I could make plaster work right, it's very cheap and hopefully reusable.
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d13f00l wrote:

I saw a very cool episode of This Old House where they visited a doorknob factory. They used lost wax and plaster to cast parts in brass. They cast the wax patterns in latex molds, trimmed them a bit, then made a plaster investment (after degassing the plaster in a vacuum chamber). The investment went into a furnace for preheating (_not_ to red hot!) where the wax was melted out and the plaster dried. Then they poured brass.
They just broke off the plaster and threw it out -- if you were really thrifty you could heat it up and get it back to plaster dust I suppose.
My understanding of the lost wax or foam processes is that you use cheap tooling to generate the pattern, then depending on your volume you may do some cleanup on the wax or foam, then you cast. You _don't_ spend forever on the pattern unless you're doing a 1-off.
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Tim Wescott
Wescott Design Services
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Tim Wescott wrote:

THe correct term is investment - as said. It looks like plaster, but is high temp material that when heated, the water drives out. Plaster keeps water and thereby explodes under high heat.
All pottery, furnace, firebrick type places sell investment materials.
Martin
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Martin Eastburn
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d13f00l wrote:

Generally speaking, shrinkage of the cast will make removal easier, unless restricted by the mold. Plaster based molds are weaked by the heat and cast removal, so are seldom reused.

What kind of plaster? Plaster without a refractory begins to weaken and change when heated beyond 135 F. Adding a refractory such as silica, will extend the range. There are commercially available plaster compounds designed for this purpose, but are still considered usable for only one cast. Compounds for aluminum are heated to 500 F, and for bronze about 1100 F, as I recall.

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Billy Hiebert
HIEBERT SCULPTURE WORKS
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