prattcm Posted November 24, 2015 Report Share Posted November 24, 2015 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! Link to comment Share on other sites More sharing options...
Mark C. Posted November 24, 2015 Report Share Posted November 24, 2015 Need more info on the whole deal-not famliar with esp foam-how about a photo? Mark Link to comment Share on other sites More sharing options...
prattcm Posted November 24, 2015 Author Report Share Posted November 24, 2015 I don't have the molds yet, but this gives you a basic idea of what the surface will be. It's styrofoam, liek the stuff often used in packaging, but a little more dense. Link to comment Share on other sites More sharing options...
Colby Charpentier Posted November 24, 2015 Report Share Posted November 24, 2015 2-3 coats of shellac and a light application of universal mold release should serve you well. Remember to brush the UMR into the detail after spraying for proper application. Link to comment Share on other sites More sharing options...
Mark C. Posted November 25, 2015 Report Share Posted November 25, 2015 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 Link to comment Share on other sites More sharing options...
prattcm Posted November 25, 2015 Author Report Share Posted November 25, 2015 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 Link to comment Share on other sites More sharing options...
BeckyH Posted November 25, 2015 Report Share Posted November 25, 2015 The master is what you make your mold from. The working mold is what you make your cast items with. Link to comment Share on other sites More sharing options...
Mark C. Posted November 25, 2015 Report Share Posted November 25, 2015 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 Link to comment Share on other sites More sharing options...
JohnnyK Posted November 26, 2015 Report Share Posted November 26, 2015 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. Link to comment Share on other sites More sharing options...
JohnnyK Posted November 26, 2015 Report Share Posted November 26, 2015 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. Link to comment Share on other sites More sharing options...
Mark C. Posted November 26, 2015 Report Share Posted November 26, 2015 JonnyK brings up a good point on eatint the foam. mark Link to comment Share on other sites More sharing options...
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