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Plaster Mold From Eps Foam


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I am having mold negatives machined out of 2 lb. EPS foam which I will pour plaster into to make my mold. Have any of you done this? If I have to break the foam away it not a big deal, but it would be nice if I didn't have to. Should I seal the foam with anything? I was thinking about just using house paint to try to seal the foam a little then using foam soap over that. I don't expect the finish of the plaster to come out perfect, but this is kind of a one shot deal and I would rather not screw it up. Thanks!

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I agree thgat a smooth surface then mold release is the best way to go-house paint is not as smooth or hard as sheliac so I would avoid it its to soft as well.

Use the mold release as well.

Mark

I never have poured plaster onto foam myself.

most master molds are cast around a rubber- compound. I assume you are making either a master or one working mold?

Mark

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Thanks for the advice, I will give the shellac a try. I am planning on making a working/master mold. I want to test it out one or two times and if everything works as expected I will make a rubber mother mold of it. Is this okay or is there really a difference between a master mold and a working mold? I have a few bags of #1 pottery plaster and a few bags of Hydrocal 11-a and am not sure which I am going to use

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The master is as Becky said above.

The master is a positive of a form-you pour your working molds from that to make a negative (hollow) mold

I think you need to read up on this 1st. Find a mold making book and it will become clear.As a master is not made from a used working mold you tried a few times.

Mark

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Since the shellac is a solvent based material, I would definitely test it on another piece of  the foam before using on the machined mold. The shellac may melt the foam. I would try a water-based material like Diamond Varathane. It goes on milky white and dries to a hard, clear finish, sealing the pores in the foam.

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Something else to consider when you make the mold is the finished size of your final product. You must take into account clay shrinkage, which means your mold will have to be larger than the final product you plan to produce.

There is an excellent article in the current issue of Pottery Making Illustrated​, "Reverse Engineering for the Potter" ​by Roger & Pauline Graham in which they discuss how to determine the starting size of your piece from measurements made on your finished piece.  Check it out.

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