Gabe,
I'm not sure that I can give a definitive answer to the question of how many shots can be had from a mold used in resin casting but will attempt something with the following. While no scientific method was used to come up with these opinions here are some of the situations I've observed and think might have an effect on mold life. First, I think it very important to have a slick smooth pattern for making molds from, the theory is that this makes a mold that is less pores than one made from a pattern that is itself pores. This is one of the reasons why I gloss coat most of my patterns before mold making. It is especially critical if you are using brass patterns as the reaction between the metal and the rubber will surly damage the rubber during the curing process and limit the number of castings you can get from the mold. Next on the list of things you may or may not be able to control yourself is that, a larger heavier casting will generate much more heat than a smaller casting and I believe that this heat also contributes to mold deterioration. As most of the casting I've done over the past decade has consisted of small parts, or small scale figures I think that this may be the most influential of the factors determining mold life. Right now I am using a pair of molds for making 1/48 scale armature parts that have produced well over one hundred castings each. I don't know the exact number of castings gotten from each mold and know that one may have been used more than the other, but do know it is a pretty high number because I have had these molds in action for nearly four years (I made the patterns and molds in May of 2006), and last year alone I sculpted 47 figures using these armature bits, while at the same time sending two or three dozen out to people on PF, also last year. Were I to guess right now I'd say that each mold has seen an average of 100-110 castings, and I expect to get an additional 20-30 from each as no sign of resin sticking has been noted yet.
Two or three final possibilities remain (possibilities that I can think of, though more may exist), first is I think it important that a mold be de-gassed very thoroughly, my own vac-pump will pull 24.25 inches at about a mile above sea level, more at lower elevations. Finally, I'm certain that the choice of rubber will have a direct effect on its usable life once made into a mold. In my experience the harder rubbers seem to have a shorter working life than softer ones becoming impregnated with residual resin more quickly than the softer types. Concerning rubbers I think it also important that the rubber to hardener ratio be as exact as possible, a mold that cures to rapidly I think has a shorter working life than one that cures more slowly. Finally, choice of resin is sure to have some effect on mold life. Some types cure fast and hot, while others are a bit slower and generate much less heat in the process. My personal choice of Rubber is Dow Corning’s HS 2, and resin is Alumilite. The rubber I choose because it is soft enough to be forgiving of deep undercuts, de-gasses quickly without trouble and lasts well as a mold. The choice of resin is part, because I'm familiar with it (used to get it at the hobby shop for simple drop and open face casting) and it works well enough and reliably enough for my purposes, which are not to make production castings.
As previously stated no scientific method was used to come up with the above so feel free, anyone, to find fault or disagree with it. These answers are all the result of haphazard observation, even the numbers of castings per mold are not 'actual', but rather the product of trying to recall how many of a certain casting I've used over the years and when the mold(s) was made. The nice thing about the example used is that I've only ever made the two original molds made from one set of dated patterns-I always date my patterns but never the molds.
Ray
