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Thread: Drywall Mud Shell Casting

  1. #1
    Senior Member Fredo's Avatar
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    Drywall Mud Shell Casting

    I'm so glad this isn't a crackpot free zone, because I've got another off the wall idea that sounds really good in my head... I'm gonna call this one Drywall Mud Shell Casting... Same skull, this time it's only the top half.
    First a coat of mud w/o sand.. Next three light coats of sanded mud. About 75% mud, 25% window screen sifted play sand, dried between coats... Next came a thin covering of fiber blanket... The dried mold was first painted with sodium silicate, next the fiber blanket applied, treat it just like it was fiberglass, using a brush to remove all bubbles... The outside of the mold dried rock hard over night... It'll go in the oven tonight without melting out the wax beforehand, and cast early in the morning... Hoping the FB will prevent any cracking from wax expansion. Success or failure, I'll post after casting.
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    Moderator DavidF's Avatar
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    If it works out I think I will have to do the same thing with a large printed part so I dont loose my "crack pot" status...
    A calm sea does not make a skilled sailor...
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  3. #3
    Senior Member Fredo's Avatar
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    Just started the melt. The burn out went rather fast, the wax was out of the mold in about 20 min in at 500*f oven temp.. I think it only got to about 1000*f Was a bit concerned with a couple spots where the wax was weeping through to the out side.... It was very little and didn't amount to much, just a little staining at the spots.. Since wax was melting on surface on the inside and expanding, I think the pressure forced the wax into the permeable mud.. It must have been at a few places that wasn't sealed by the SS and blanket..
    The melt was fast and the casting went smooth... Time seems to stretch when you're waiting for a casting to cool Time to make a cup of coffee... It came out better than expected... After the finish on the full skull, I was prepared for anything.... The last pic is for comparison of the wax, to the finished build up of the mold.. I guess it isn't crackpot if it works...
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    Senior Member Jammer's Avatar
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    When you pour your wax, you may try to insert a tapered dowel or something to make a hollow core to reduce shrink damage. Looks like it came out very good.

  5. #5
    Senior Member Fredo's Avatar
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    I'm not sure a hole in the center would help control the expansion of the wax since it's being heated very fast from the outside..
    I was thinking to contain the expansion with the reinforcement of a hard ceramic shell of blanket on the out side.... It's a supposition on my part as to how the mud may be helping control the expansion by absorbing some of the wax into the mud.
    These pics of the mud w/o sand show a clean burnout... I'm only guessing at the burnout temp reached.. I think the time in the oven for this piece could be as little as 3 hours or less.. The pics show the mud w/o sand... It was the key to good detail. This was a shot in the dark for me. I'm sure better technique will show itself with more experimenting.. maybe one more thin layer of blanket... I also want to see how fast of a mold burnout I can get.....
    The discoloration on the outside of the mold is caused by the binder in the blanket not getting hot enough to burn out....
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    Senior Member Fredo's Avatar
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    Here's the skull after 5 minutes clean up at the bench... The pouring cup sprue is the just right size to hold on to when cleaning.... Weight with sprue is 254gm...
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    Senior Member Fredo's Avatar
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    Here's a couple small pieces.... calling one BFF, and the other, Monday morning.... Wax Doodles... Used straight joint compound, slightly thinned, for the first coat.. Fast dried in front of a fan, then another coat applied for even coverage... perlite sifted through window screen was substituted for sifted sand in the last 3 layers of mud this time... It's important that the mud be completely dry before applying FB and SS, two thin layers..... This time I allowed the first layer of FB/SS to harden before the second layer was applied... Best to hit the FB with some sand paper to knock down any silica needles.. not as bad as fiberglass, but there will be some..

    Reexamining pieces of the first mold, I found places where the thickness of the blanket was uneven, there, unsaturated FB was next to the mud w/o sand... Those were the places where there was a weeping out of wax in the first mold...
    I'm replacing the sand with perlite this time because it's an insulating refractory and even when reduced in size it retains some porosity.. Seems to me the mud should key into the perlite better than sand.... I'm just making this up and tweaking it as I go..: Finished with the mud part, next time, if the perlite works, I'll pulse it in the blender for a finer grit... It was a little to lumpy.. Last coat this time was plain mud to smooth it out some....

    Needed some more sprue cups, but my balloons are old and might break... Filled a plastic Easter egg with clay, put a long screw in the top for a handle.... Put it in the freezer for half an hour, sprayed with mold release and dipped twice in the hot wax, not over half way.. After the wax sets you'll need to drill a little hole in the wax on top to prevent a vacuum when pulling the wax off the egg...

    I'm satisfied with the results I've gotten as far as casting quality goes... Although the lay up of the FB/SS can be a pia.. Once you get ss on your gloves it gets a little like working with honey and feathers..... I'm moving on to another brain fart I've had... I'm less optimistic about this one...
    Decided to do a 2 hour burnout total time. Put the molds in 5-600*f..... Wax was out in 20 min then molds placed on their sides and heated until red heat.... Wanted to see if wax expansion would break the molds. It didn't..
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    Senior Member Jammer's Avatar
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    Easter eggs are great, I used one for the interior mold for a small crucible. Seems to be just the right contour. A yogurt cup was the outside.

    Castings are coming out very well. Are you going to try any Bronze?

  9. #9
    Senior Member Fredo's Avatar
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    The use of sodium silicate as a binder really makes this type mold only suitable for aluminum or other low melt temp metals... The ss melts at about 1600*f so I'm thinking the higher melt temp of brass and bronze would cause problems....
    When I get a crucible to use for bronze I'll make a mold and use Silicon Dioxide, SiO2, in place of the ss.. It vitrifies around 1600*f and melts around 2900*f, which means I'll need to fire the mold to at least 1600*f to vitrify the SiO2.... Don't know if the drywall mud will take those higher temps and stay intact... Although, vitrifying the SiO2 would make it a true ceramic shell mold, of sorts...
    Why do I feel like I should duck after making a statement like that????

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    Senior Member TRYPHON974's Avatar
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    I'm impressed by the results and I like the way you thought out of the box.
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