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Discussion Starter #1
Hey all,

Having a little trouble with molding and wondering if I could get some feedback.

Here is what I did.

I made a shrunken head out of air dry clay. I then let it dry for about 5 days. To the touch it seemed very hard at this point. I then covered it in vaseline, then tooks some plaster of paris from Michaels (havnt found that 30 cal stuff locally). Gave it a good coat on both sides, using a clay wall as barrier. Let it sit for about an hour and tried to pull it off.

And this is what happened. :confused:

When I pulled it apart, first the back of the shrunken head came off in parts. Asuming I may need to do a couple coats here? But when I pulled off the front, it came in one piece, but it also took the nose, part of an ear, mouth, and an eye with it!

So I am kind of at a loss to what I should do here. It seemed that the plaster of paris, either the fact that it was wet or the heat made the clay loose again.

Is it as simple as needing to let it dry alot longer, or maybe I need to find some sort of sealer to apply to the clay sculpture? Im sure the oil based stuff is better for things like this, but really trying to stick to a budget.

Any help would be appreciated. Dont want to keep losing my clay creations!
 

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Grave Rigger
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982 Posts
I think your main issue is that it sounds like you only used one coat of PP. You really need to layer it up thick, as PP is not that durable as a mold. you might even want to add some fiberglass gauze between layers. Ultra Cal 30 is definately the way to go, though. You can find it cheap online, and if you buy smaller quantities the shipping isn't as bad.
As for the clay seperating, yes a better release agent might help, but you have to realize that your original may be distroyed in the molding process. Unfortunately it happens. Just carefully scrape the clay out of your new mold and make as many as many replacements as you want!:D
 

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I'm not sure what kind of airdry clay you used, but it's possible the vaseline may have softened the clay up a bit.

As for plaster of paris, coats of it REALLY are not going to work for a mold without some reinforcement. The individual layers don't cross-bond well, and bandages won't hold the detail you need.

You should try a method more similar to this:

http://www.opusframing.com/library/pdf/2part_mold.pdf

Then you get two solid blocks of plaster. I'ld say you want at least 1/2" thick on all sides.

good luck, and have fun.
 

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Grave Rigger
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Ohhh...expandable foam will most likely blow out a PP mold. Definately agree with Scourge. Mother mold with UltraCal.
 

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Discussion Starter #11
Actually the end result hasnt been that bad. I did a PP cast of a plastic skull I had. So far I have created 7 skulls with expanding foam. It seems that the middle of the line stuff works best, the cheapest on the shelf leaves some odd formations, but so far thats been a great addition as it really adds to the uniqueness of each skull.
 

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Reaper
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No idea about the vaseline method :( I've only made molds with clay then coated them with latex paint and then filled that with plaster.
 

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Discussion Starter #13
Sealer worked the best, Modge Podge to be exact. Two coats. Made a plaster cast of my alien head sculpture the other night. Did the first layer with a thin fabric, then a second layer of burlap. (Both soaked in PP). Pealed that off, then added a final layer of straight PP to give it strength. Finally I sealed the inside.

About 2 hours out from pulling my first alien foam head, hoping it looks good!
 

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Join my Doomsday cult!
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Sorry to respond so late, but here's my input anyway.

Your biggest mistake was in trying to make a rigid mold from a rigid original. You got lucky when the clay softened up and fell apart, otherwise you would not have been able to remove the sculpt from the mold.

If you had molded the original when it was only "surface dry", then you would not need any release. The sculpt is destroyed in the removal process, but any undercuts in a rigid/rigid molding will result in damage to either the sculpt or the mold or both.

The final casting material is really what determines the mold material and process. If you're using one-part poly foam a strong, rigid, well sealed and coated with release, mold is needed. Since the foam you are using expands and is not self-skinning, provide it an escape route, like the neck hole, to relieve stress on the mold. Self-skinning two-part poly foams need to build pressure to create a good thick skin. (Better if under pressure, at least)

As for releases, I would apply a good coating of car wax, something with carnuba, to the mold. Buff it into the mold really well. This is to seal the pores and provide a smooth surface. Then I would apply a very thin layer of Vaseline before each casting.

Another thing you may want to keep in mind is that one-part polyurethane foam is a water cure foam. It uses the moisture in the air to cure. You may find that after a few weeks your castings will deform. Shrivel, shrink, and collapse. This is because the foam in the center is finally curing. It takes so long because it wasn't getting any moisture. One way to avoid this, is to apply some foam, then very lightly mist water into the mold, before adding the rest of the foam.

As was mentioned before, PoP just doesn't layer well. Your mold won't last long. If you do though, don't use separate batches for the same mold, and try to build the layers quickly. Not so much layering, as simply reinforcing with strips of PoP soaked cloth.

Good luck!
 
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