[GreenKeys] [External] M33 Type drums

[Jones, Douglas W]

From: hwhall at aol.com [hwhall at compuserve.com] -- Saturday, August 17, 2024 10:03 PM

> To sidestep the printer resolution, maybe do a silicon mold of the drum, cast a wax (etc.)

When doing precision work with lost wax, you need to take into account the thermal expansion and contraction, and adjust the scale of you wax original accordingly.

a) When you invest the wax original, you're working at room temperature, but then, you heat the mold to casting temperature to burn out the wax and make ready for casting.  That means the mold is thermally expanded when you pour the metal.

b) The metal shrinks as it cools from casting temperature back to room temperature.

Depending on the thermal coefficients of the investing material and the metal, the was original will have to be either larger or smaller than the target dimensions.  Only if the investing material (usually a paintable ceramic) has exactly the same thermal coefficient as the metal you're using for casting will the dimensions of the wax original be the same as the target dimensions.

Note that low-temperature alloys can be cast in plaster of paris, but it doesn't work for high temperature alloys -- heat it much above 100C, and the plaster converts to bassanite.  Above 200C, it converts to anhydrite.  Both transitions release water vapor.  The strengh of anhydrite falls by a factor of 10 as you heat it from 200C to 450C, and by an additional factor of 10 as you heat it from 450C to 650C.  That's about the melting point of aluminum.

Remember that at room temperature, anhydrite is soft enough to crumble under fingernail pressure.  Hence the use of ceramics for lost-wax casting, even casting aluminum.  Gee, the things you learn in high-school art class -- where we did lost-wax casting with zinc.

          Doug Jones