Plastic Resin Casting and Molding advice


I'm making my first plastic resin cast and the mold for it using the tutorial here:

One difference to that tutorial however is that the object I am making a mold of is something I've make in air-dry modelling clay. I've spent quite a bit of time making the original so bit worried about pouring the silicone for the mold onto the clay object - will it break the clay down or is there some way to prepare the clay (varnish or something?). I've put a test of dried clay in water and it has broken down in five minutes. Can anyone who has done this sort of thing before advise please? Thanks in advance.


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89esprit6 years ago
Silicone will pour just fine, sulfer free clay is preferred. in case you are worried, use a mold release spray. Spray Pam works too. Just make sure it's not a silicone spray or it will become a bonding agent for the silicone you are about to pur.
garrettlynch (author)  89esprit6 years ago
So the silicone won't dissolve the clay? Not sure if the clay is sulfer free or not it's just labelled air-dry moulding clay. I'm going to get some petroleum jelly to use as mold release, I'm told that works. Thanks for suggestions.
clay is often used to seal a mold box so silicone, etc wont leak. Petroleum jelly tip i picked up on that was to cut it with some mineral spirits. has some good tips too. Good luck, have fun!
garrettlynch (author)  89esprit6 years ago
Thanks for your help, think I've sorted it now and will make the mold today. The test I did worked, the clay came out fine but your right petroleum jelly would have helped as a release agent. To make doubly sure the clay stays intact someone suggested I coat the clay in pva which I've done.
Never tried this so take it as free advice, you get what you pay for. If your clay brakes down that fast in water it makes sence that it is too pourous and is absorbing the water. Anything you use a sealant should slowdown if not stop the process if done right. Varnish may work, may not. I would look more into something that is used specifily for clay hardening like a glaze or something. ModPodge or wax might even work but experiment first with some clay you don't mind if it breaks up.