Hello Tom & Doris:
Just a quick note to add to Tom's comments:
1) Plaster Molding or rubber-plaster molding is different from the solid flask
investment casting that Tom described.
Rubber-Plaster is used to prototype diecasting (among other methods) and is based
upon rubber take-offs of a master pattern. It does not use a wax or a
"lost-pattern" and also requires a permeability to be structured in the plaster
(more so than solid flask).
2) It is my understanding of solid flask that it is used for non-ferrous
applications for two reasons: a) Sulfer in the gypsum reacts with ferrous materials
and b) if you preheat the plaster much above the 1400f that tom mentioned you will
destroy it and thus you can't heat it to the typical 1800f that is used in ceramic
shell.
have fun
martin
Tom Richards wrote:
> Mine preceeded by %%%%%%%%%%%%%%
>
> At 11:48 AM 12/3/99 -0500, you wrote:
> >Hello,
> >My name is Doris Walters. I am a technical writer & researcher at Perry
> >Johnson Registrars in Southfield, Michigan. I am currently doing a project
> >on SIC Code 3321: Gray & Ductile Iron Foundries. I wonder if you could
> >answer a nagging question for me regarding investment casting.
>
> %%%%%%%%%%%%%%%% Gray and ductile irons are seldom investment cast for many
> reasons, but are usually sand cast.
> >
> >What is the "refractory slurry" that is poured around the wax pattern? That
> >is, what material is it made of?
>
> %%%%%%%%%%%%%%%%% In shell molding for investment casting, the wax patterns
> are dipped into a slurry of colloidal silica binder with suspended
> refractory grains typically of chrystallobolite (silica) or zirconia.
> Successive layers are built up followed by drying. The mold is dewaxed and
> fired at 1800F and cooled. The molds are reheated to the proper tenperature
> for the pouring in of the molten nonferrous or ferous alloy (not irons)
> which is allowed to solidify.
>
> I take it that the material is not
> >plaster, because that would be plaster molding.
>
> %%%%%%%%%%%%%%%%%% In solid molding for investment casting, a slip of gypsum
> (CaSO4.xH2O) binder with suspended refractory grains of typically
> chrystobalite (silica) is poured into a flask containing the wax patterns,
> and allowed to solidify. The mold is dewaxed and fired at 1400F, cooled to
> the proper temperature at which molten nonferrous alloy is poured into the
> mold and allowed to solidify.
>
> Is the mold generally made
> >in two halves, or all in one piece?
>
> %%%%%%%%%%%%%%%% Solid molds are in one piece, plaster molds are in two halves.
> >
> >One other question that has been bothering me: regarding surface finish,
> >what does "RMS" stand for?
>
> %%%%%%%%%%%%%%% root/mean/square
>
> Which would be the smoothest finish, a high RMS
> >number or a low RMS number?
>
> %%%%%%%%%%%%%% Low numbers are smoother. Investment castings are generally
> 80 to 100 RMS(sand blasted).
> >
> >I hope you or someone you work with can take a couple of minutes to answer
> >these questions. I would really appreciate it. My e-mail is:
> >dorisw@pjr.com. My phone number here is 1-800-800-7910, extension 34.
> >
> >Sincerely,
> >
> >Doris Walters
> >
> >
>
> For more information about the rp-ml, see http://ltk.hut.fi/rp-ml/
For more information about the rp-ml, see http://ltk.hut.fi/rp-ml/
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