Plaster Mold

From molding systems to gating, what goes on at the molding bench will make or break a casting.
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Jeff Clark
Posts: 85
Joined: Mon Dec 05, 2011 3:40 pm
Location: Woods Cross UT

Plaster Mold

Post by Jeff Clark »

I want to try and make some plaster molds, Im curious who has done this and what is the best plaster combination plaster and talk mix? is any brand plaster better?
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Rocco
Posts: 98
Joined: Thu Jun 02, 2011 8:35 am

Re: Plaster Mold

Post by Rocco »

Both Georgia-Pacific and U.S. Gypsum make plasters they market specifically as metal casting plasters. The USG product for example, is called Hydroperm, it's more gas permeable than standard plaster of Paris, you might find it useful to check out the related product literature on their websites.

Personally, I have no real experience with plaster molds, I've only done one lost wax casting in plaster, I dipped my pattern in ordinary POP to create a skin coat, then embedded it in a 50/50 POP and sand mixture to create the mold, baked off the mold for a couple of hours and poured into it while still hot from the oven, it worked very well for me but of course with only the one casting under my belt, I can't really say my results are typical of what one could expect from my method.
Rasper
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Re: Plaster Mold

Post by Rasper »

It depends on what you are going to do with the molds. For a mold to cast wax for a lost wax pour, use 100% plaster of paris. For an investment of the wax into which to pour molten metal, I use one part plaster and two parts brick dust (silica sand is fine too).

Richard
Jeff Clark
Posts: 85
Joined: Mon Dec 05, 2011 3:40 pm
Location: Woods Cross UT

Re: Plaster Mold

Post by Jeff Clark »

I tried to get some hydroperm, but the foundry supply does not supply it here they use to but it was not a big seller.. I looked all over locally with no luck, Ill try in Los Angeles area we do alot of business there and easy for me to get things picked up.
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F.C.
Posts: 560
Joined: Sat Jun 04, 2011 10:28 am

Re: Plaster Mold

Post by F.C. »

If you're going to be reproducing a lot of wax blanks from an original I use HydraCal Plaster, it's used in patching swimming pools. This is a very durable, carveable, plaster that will hold up to some abuse. For investment castings (lost wax) I use straight regular plaster, with 80 grit sand mix of about 60/40 solution. Mix only to a consistancy equal to a pourable milk shake... 60 being the plaster. Pour your investment mold on a surface you can vibrate all the air bubbles out of after you top off the mold. If it's a big mold include a mesh of chicken wire around the perimeter of the mold embedded within the first inch of the exterior of the mold. Typically, when you burn out a regular plaster (only) mold there will be fractures throughout its body. Go to lift it and remove it from the kiln and it'll fall apart. Depending on your burn out skills you may still create fractures in the mold during burnout regardless of the precautions (such as chicken wire) used. Still, those precautions will enable the cast to happen without loss to the mold and metal. Afterward, you can recover the burned out investment and use it in a different mix solution to create other investment molds. When you get to that point let me know and i"ll clue you in on the altered mix.

Depending on the metal you intend to cast, the temperature of melt, etc., you may be able to do a piece mold out of hydracal plaster you assemble, surround in a bucket backfilled with dry sand. As long as the mold is chemically dry..( this has nothing to do with moisture content, btw)... you may be able to get a few casts from the original mold before it's surface begins to degrade. Zinc is a good example here... low melting point, durable cast, shines to a chrome finish and does not tarnish. I have created piece molds and duplicated cast morgan silver dollars that look as new and perfect as any mint made dollar made by the US. The only difference is in the "ring" when you flip it in the air. Anyone who knows silver will understand that "ring". I made these replicas solely for presentation sake. I keep my originals in a safe deposit. I shit you not, you can reproduce a frog hair detail on the surface of your mold impression with this material... including a finger print (which I found out by accident). The coins come out as pretty and mint fresh as any brand new coin you've ever seen. You can only get about three, maybe four per mold if you're lucky.
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