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Dave Parvin
Joined:
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16-Mar-2005
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Started On : 23-Aug-2009 at 07:43:43 PM, #Views : 6157
Topic Subject : More on rubber molds |
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There has been a lot of good information on mold (or mould) making on this site in the last few days. I am always impressed with the fact that there are many different ways to accomplish the same thing. I agree with all of you and respect your experience but want to add my 2 cents' worth as well.
In most cases, my life casts are made with alginate from which a plaster positive is cast. But the plaster is almost never the end casting. I make a silicone rubber mold of the plaster heads, torsos, and hands and feet into which I will cast Forton MG, resin, wax for lost wax casting, or water based clay for raku firing. I refer to the silicone mold as a secondary mold which I make for three reasons. The first is so that I can make additional copies either for my own editions or for other family members increasing my potential income. Secondly, a silicone rubber mold opens up the possibility of using materials which would be difficult or impossible to cast in an alginate such as resin or water based clay. (I realize that one can add "Water Trap" to resin in order to cast in alginate, but the results are not as good as if cast in a silicone mold.) The third is that any repairs or additions made to the plaster will be by far more invisible if a secondary mold is constructed after the repairs have been made and then the finished piece is cast. Opening eyes is a perfect example. If all one had to do to realistically open eyes were to peel off part of the closed eyelids, it wouldn't matter. But for the eyes to look right , some reconstruction of the eye structure is essential for the eyes to look right.
I have rarely used fast setting skin safe thixotropic platinum cured silicone in place of alginate. It has had two disadvantages. The first is cost. The second is that it would stick to hair and removing the rubber from the model had been almost like a waxing. If you put enough release on the the hair and skin, I felt that you lost so much detail that an an alginate mold was actually superior. However, there are some skin safe alginates now available which do not stick to hair at all and I suspect that I will find them more useful in the future.
A thorough discussion on silicone rubber is for too complicated for this site. I would refer anyone to the article in the Library by Ed MCCormick. Also, my friend Michael Sisbarro who is a Silicone, Inc. rep. wrote a very informative article on silicones in the May 2005 issue of "Sculpture Journal." I will try to get it posted in the ALI Library. In the meantime, I would be glad to email it to anyone interested.
The two silicones that I most often use are made by Silicone Inc. and are GI 1110 and 184B. The 1110 is similar to GI 1000 except that its Shore A is a very soft 10 while 1000's is a 30. I use the green 179 activator which according to SI give the same library life as 2020 but also improves the elongation and the tear and tensile strengths. The only disadvantage to the 179 is that it is very reactive to humidity and can set off surprisingly fast on a high humidity day. I use the 1110 for pour or block molds. For skin molds, I use 184B with its thixotropic activator. However, for the first layer, I often use 184B with the 179 activator which is not thixotropic. I keep handy a thixotropic additive, silicone oil for thinning, and Ultra Fast Catalyst, which is very effective in accelerating the cure, to use as necessary. Michael Sisbarro is getting me a sample of GI384 which he claims has a better resistance to urethane resin. In addition, it is has a low viscosity (very runny) but can be effectively thickened with SI 661 thickener. He thinks that I will like it so much that I will use it in place of both the 1110 and 184.
As I said, a complete discussion on selecting and using silicone rubber is beyond the space allowed here. There are excellent rubbers made by other companies. The trick is to know the characteristics that you need and order accordingly.
One more thing, mother molds. I make my mother molds of Forton MG and cotton batting with scrim binder. My mother molds are mostly about 1/8 inch thick and are very light yet very strong. I describe how I make them in three articles on making secondary molds in the Library. I have also come up with a very slick way of attaching the rubber to the mother mold so securely that the rubber only needs to be about 1/8 inch as well. I have also explained this in an article titled "Another Little Trick For Some Molds."
Thanks for sharing your ideas and your considerable experience.
Dave
Ed, if I went over my one MB, sorry.
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