Plaster and Polyester Resin

Xylophone

New Member
I have a plaster mold that I brushed polyurethane wood varnish onto in order to reinforce/seal it. I'm not sure if it worked, but I went ahead and poured some polyester resin inside anyway (Castin' Craft). Now that the resin is almost done curing, I'm wondering how I can get it out of the mold. I was thinking that I could thicken the resin a little more and then try to pry or break the plaster mold to retrieve the resin. Any thoughts?
 
Well... You might hope that the polyester resin shrinks away from the mold walls. This type of resin does shrink considerably, so maybe that will help. What kind of plaster is your mold? some plasters will soften when wet (and many won't!). How thick is the mold? Maybe a picture or two of what you are molding will help. Your idea of breaking the plaster off of the casting might work out, under the right circumstances. And then again, maybe not. Tell us more, and we'll see what we can do to help!
 
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I don't think the brand of plaster of paris really matters, but I sprayed some water on a piece of plaster and it softened just enough for me to rub some powder off. The mold is about half an inch thick on average, and as you can probably tell, I was trying to cast a mask. The little grey bits you might notice are small pieces of oil-based clay that was left over from the original sculpt.
 
Okay, the mold is plaster of paris. You can soften, maybe even dissolve it in a mixture of warm water and lots of baking soda. Some net research might even yield specific instructions for this. Now I'd imagine that you will want that casting to be as strong as possible. I don't believe that the Castin Craft resin is the best material for your purpose, but it could work. You might want to consider laying in a few layers of fiberglass cloth to strengthen the mask. A few layers of resin/cloth will be much stronger than just the resin itself. When ready, soften or dissolve the plaster. You will likely need to spend some time cleaning softened plaster out of the little detail areas of the mask.
 
Where would I look for the fiberglass cloth?
You should be able to find small quantities of fiberglass cloth at most hardware stores, or better yet, at any automotive parts/supply stores. Used for small repairs to cars, boats, etc. Should be easy enough to find. Something like this: Bondo 499 Fiberglass Cloth : Amazon.com : Automotive

If you search here on the rpf you will find many builds that use these materials, probably will give you some idea of how to use the stuff. It's not hard, but there are a few "tricks" to getting it right. Check out the pep builds. Those guys use it to reinforce their paper pep constructions. This seems like a good project to dabble with and learn some of these materials and techniques. I only hope that after all your effort you do get that out of the mold! (I think that you will...)
 
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