Re: CASTING Plaster Investments vs. Ceramic Shell?????


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Posted by bpfink on February 10, 1999 at 00:28:36:

In Reply to: Re: CASTING Plaster Investments vs. Ceramic Shell????? posted by John Stricklin on February 09, 1999 at 22:15:51:

There may be a misunderstanding with the temperature John.

Sure that was'nt C instead of F?

Years ago we used to burn out all the plaster based investment molds at near 1500 F for a 2 to 3 day period and it worked fine. It also exhibited more slight cracking though which may have been due to a number of things such as the heat, extended time, wax presures when remelting prior to being able to flow out,the sand types or proportions to the mixes, etc. etc..

These minor cracks caused flashing to be part of the metal casting. I don't like any extra work especially when it is flashing.

Thus I slowly reduced the burnout temperatures until getting to my present system of using fully water WET molds

(water in the plaster is a better heat conductor and normalizer than dry plaster)

and at a temperature of 600 to 700 F right off and then up to near 900 F for the last 2 days. This also means it is from a 5 to 7 day burnout but I have many playtoys to kjeep me amused while that happens (like brooms, shovels and sand buckets or just listening to music and dancing or writing on this... everyone has their alternatives.).

So I am not sure where they got that figure unless it was maybe Centigrade and not F???

Plain plaster has no ability to withstand the shock though and if you put a small block of it in the kiln you will find it becomes a maze of small cracked up pieces. The sand and fillers are the strengths that are like the Grog to hold it together and distribute the shearing lines... like stone and sand is to concrete whern used with cement.

And yes it will become a powder again if kept at a high enough temperature long enough as both the mechanical and chemical water is driven off and it then is back to it's normal cycle.

It is also the key to the way it is made in the first place.

Fire away. bpfink

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