Steam Casting Method by Alan Shinn
11/14/2000 copyright Alan Shinn
I mentioned steam casting earlier and thought that I would do a little write-up of the method.
This is a first draft, any and all comments; clarification or general wordsmithing is welcome. I have some sketches (as crude as this write-up) but will not post them here. (Where??) perhaps I'll stick them on my website somewhere.
I have read that the ancient Egyptians practiced steam casting. I shall describe how I have done steam casting in the much more recent past. I shall assume that you know how to do centrifugal or vacuum investment casting. Steam casting is easy to understand once you realize that (most?) molten metals have high surface tension. This surface tension both necessitates some sort of force to push the metal into fine details and facilitates the practice of steam casting. Imagine melting a couple of tablespoons of lead in the bottom of a 1" radius cup made of some high temp screen. The puddle would be maybe 1/4" deep?? and would not flow through the screen, it would just sit there. If you make the puddle deeper, at some point the pressure would be great enough to force the metal through the screen. The smaller the holes the greater the pressure needed. Sooo, how does this apply to casting? Well, here's the idea, If you incorporate a screen sort of system above the sprue system of a regular investment mold, you can melt the metal in an integral cup/crucible above the sprue system and it will sit there nice and molten till you apply a pressure (steam for instance) to it, at which point it will squirt into the mold and be forced to fill all the fine details. Here is a step by step using the minimum equipment, but be warned that I did use a burnout kiln. I shall not try to discuss alternatives to that now. I used about a 2" waxed hemisphere as a mold base/ crucible former (like a sprue former in other casting methods) ( I have used a well waxed light bulb but they can break, so I made a plaster hemisphere) At the top of the hemisphere wax down one end of 6 or 9 or 12 or so 1/32" dia X 1/2" long wax wires in a 3/8" pattern. join these together as close as you can to the hemisphere. Trim the resulting stalk (It now looks like a weird chopped off plant with roots growing into a hemisphere.) and if you like to use a reservoir ball, wax one on to the top of stalk. To this, attach a regular fat sprue and to that, the wax model. Place a ~1 1/2" dia X 2" long steel pipe nipple over your wax, onto the hemisphere and adjust the height of the model. The total length of the model/sprue/crucible system should place the top of the model ~ 1/4" from the top of the pipe. (which will be the bottom when you cast) now seal all around where the pipe meets the hemisphere with wax or such. At this point you fill the pipe with investment. I am going to leave out the many details and methods of getting a good investment, You should adjust the above order of things to suit your favorite way of investing. At this point, after the investment plaster has set, you hold your tongue just right and with a good twist, separate the hemisphere from the flask (the steel pipe). If all worked well, you will see a semispherical depression about 3/4"(??) deep in the end view of the investment with a bunch of little wax "dots" (the ends of the wax wires) at the bottom. When the wax is gone, these "dots" will form the above mentioned screen. Burn out your investment till just red hot if you look down into the holes where the wax wires were (details left out here) and place it crucible side up on some sort of high temperature insulated surface like a light weight fire brick. Place your pre measured metal into the crucible and melt it. Now, the idea is to rapidly cap this glowing crucible with a board covered in a layer of wet newspaper, which flashes into steam, creating a pressure on top of the metal. If you coordinate this well, and achieve a good seal with your full body weight in a rapid fashion, you will have the metal being forced in with like 100psi. Hold that pose for about a minute and then toss the flask into a bucket of cold water. Swish around a bit and then behold your masterpiece. I found that too often, I would slam the "lid" down off center or something like that and thus get poor pressure and poor castings so I made a little lever gizmo to help my aim and coordination. I must admit though, that when a centrifugal caster was available I used it instead of my steam caster.. Whew, what a long writing, I hope it conveys the idea.
Here is a drawing for Steam Casting
Here is a sketch of my casting gizmo
Here is a sketch of a water powered vacuum aspirator pump made with hardware store stuff