Hello again.
Here comes a mixed bag update. I did some calculation and measurements on my molds and knowing that 1 cubic centimeter = 1 ml of volume, I estimated my RTV requirements and came up with a volume greater than what I had in stock. Past experience has taught me the hard way that new RTV will bond to old perfectly, so I decided to chop up some old molds from old projects and mix them into my supply to "boost" it. This reminded me of movies when I saw coke dealers cutting their supply with baby formula, but I forged onward. It was tedious, cutting up the old molds, until I realized I could raid the kitchen for a spice cutting tool of my wife's. Even though she will ultimately benefit from this, she does not need to know.
Then disaster struck. My RTV, which has been in the basement for perhaps a year has gone bad. No molding today. No need for boosting a supply either - I'll just buy more and in enough quantity. The bad part is that the store I get it at is a 9 to 5 M-F affair, and it's hard to get there from work. This could be a multi-week setback. I'm a little gutted.
Anyway, that was my Sunday morning. Time for a run.
To Ged's questions:
Lego is cheap, quick and easy for building a mold box, but a properly constructed plastic or wooden one is every bit as good, probably better.
Clay: for 2 part molds, one half of the part being molded must be buried in the clay during the pouring of the first half of the rubber. Most people make a clay bed and push the parts halfway into that and then build a mold box around the whole thing. I do that, but then trim away most of the clay so that the part is on a raised "island" of clay. This way, the parting line of the mold itself is not in line with the parting line on the part. This allows for a large depression to exist in the final mold, into which I pour my resin for the part before putting the "lid" on. It makes the mold less leaky and easier to use. This is probably confusing without photos - I'll have some when I get there though.
The clay is just children's modeling clay. The particular stuff I'm using is Crayola brand (same as the crayons) from the crafts section of Walmart. I've heard some clays are not compatible with RTV, but I don't know the reason and this stuff is fine. Unlike the other products you mention, this stuff does not ever cure and stays flexible for years. It's cheap, too.
RTV: Room Temperature Vulcanizing (Rubber). It's the mold product itself. The stuff I'm using, which I like a lot (spoilage notwithstanding) is an A + B system by Smooth-On. It is mixed from 2 equal amounts of 2 components. Other systems are catalyzed via a the addition of a small amount of catalyst to a larger volume of rubber, say 10:1.
Anyway, here are some pictures of what turned out to be a disappointing day.
Cheers,
K.