Rubber Mold technique

[Mack]

I'm molding a large 5' clay figure in sections in order to cast it in plastic so I can ship it to my foundry across the country and not worry that it will be damaged in the process. There it will be molded again for a production mold. For this "waste" mold, I'm using Polytek's polyurethane rubber for the mold and backing it with a mother mold of liquid plastic and the casting is Polytek's EasyFlo-60. Some of the areas of the cast are very thin, how do you keep the rubber from collapsing away from the mother mold? How do you keep the rubber in the mother mold? One thing that I've tried is to pour a couple of small rubber cylinders 3/4" wide and i/2" high and attach them to the final coat of the rubber. When I add the mother mold over the rubber mold, I let the top surface of these cylinders be visible (flush) at the finished surface of the plastic mother mold so that when I reassemble the mold after removing the clay section, I can see that the mold rubber is fully seated in it's corresponding mother mold section. But because of it's weight it can slide away from the mother mold and collapse onto the the opposite side making it impossible to pour a cast. Any suggestions? I suppose I could glue the rubber into the mother mold at points since this is a one-off proposition...but I wonder how other's have handled this.
Thanks

[raspero]

I find that thick polyurethane molds don't collapse nearly as badly as thin ones. Also, after the first two thin coats of #30, I would use #50 or probably even harder as a stiffener.

I generally just add Cab O Sil to the #30 for the build up coats, but I think this just makes the mix thicker so I can trowel it on; I don't think it makes it much harder or stiffer after it cures, which is what you want.

Richard

[evaldart]

Believe it or not I have a project right now that will be having me make a rubber mold with a plaster mother mold. IT is big (a 36 inch duck sculpture...dont ask) and I'll be facing some very similar problems as the mold (not the sculpture...which has been modeled in water based clay) will have to travel across country to the foundry.
I am planning to support the initial "skins" of brushed on latex rubber with some thickened rubber (I'll use fumesilica, unless someone has a better product) and then support that with the plaster. A two piece mold that will hopefully survive the shipping.

[KatyL]

Mack, if the rubber mold is so thin that it collapses into the other side when you turn it over it is too thin.

I also wonder how this will be poured. Do you intend to rotate it as you pour the skin in? It will be 5 foot tall, A(huge). It sounds to me that you will need to cast it in pieces then patch it together--

Muek, it seems, has his rubber molds for his large sculptures actually laminated to the mother mold -- which he uses a fiberglas mother mold with a wood frame support. The wood frame is "registered" so that the mold sides go together tightly. The fiberglass mother mold and rubber is bolted to metal brackets and then to the wood. These are taken off/put on using screws. The mold is further separated, but all the parts fit together on the wood frame.

[Mack]

Thanks very much for the replies...I think that I have been spreading the rubber a bit thin to save on cost...Katy I've cut up the piece into 2' sections and pouring them solid with Easy-flo 60 liquid plastic...E. i would reccomend for your project: Polytek's poly fiber 11; it's not as poisonous as the stuff you mentioned as a thickener for the rubber (it comes in a 5 gallon bucket) and for the mother mold ,instead of plaster, their poly 15-6 liquid plastic. It' a one to one ratio mix and has several minutes of working time and is stronger and lighter than plaster. You can mix the poly fiber 11 with it to thicken it as much as you need as well as with the rubber. And you can cut some strips of fiberglass fabric and lay it on a coat of the wet plastic and strengthen the mother mold that way.