Re: Hydroperm Plaster

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Posted by Jesse Brennan on January 12, 19100 at 13:45:39:

In Reply to: Re: Hydroperm Plaster posted by joe wizard on December 29, 1999 at 17:17:36:

Hydroperm is not intended for lost wax casting. It is meant to be used with either a permanent rubber or metal master mold to make production or limited production metal casting molds.

The mixing ratio specified is 1 pound plaster to 1 pound water.

The low density is produced by frothing with air during mixing. Idealy this will produce a dry density of 54.5 pounds per cu ft. 62.4% void volume and a dry compressive strength of 650 psi.

The master mold needs to be coated with a mold release and the mold should be vibrated to remove bubbles from the surface.

For aluminum the mold needs to be baked to 500 degrees F. The interior of the mold must be above 220 degrees F. For copper based alloys the mold must be baked to 1200 degrees F .

Molds must not be allowed to readsorb water from the atmosphere after baking. Baking above 1200 F will damage the mold.

There are other metal casting plasters with a higher adeded water ammount. These can produce molds with a void volume to 74.3% at 37.3 pounds per cubic ft. The compressive strength for such a mix is only 50 psi. The USG rep cpuldn't tell me more about this higher void stuff.

I was(am?) intending to use this material as a better venting core in sand casting, or as a lower density core inside a ceramic shell lost wax mold. The cores will get addequate support with pins during handling and casting.

Jesse


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