If you are using a stone mold that has been thoroughly dried, it must be sealed to make it as nonporous as possible before applying the release agent. New molds or molds that have not been used in a while should receive a second coating of release. All coats of release should be thin and dry before proceeding. Care must be taken in demolding the foam to prevent tearing the skin. Stone molds can make removal of urethane foam difficult, so beware. Every surface that
the foam touches must be released because of the foam’s tendency to stick to anything it comes in contact with when it sets up.
Like foam latex, finding the right amount of material to pour into the mold will take some experimentation before you become adept at just "knowing." It will partly be a factor of calculating the volume of the appliance and partly knowing how much the foam will expand as the mixed liquid components turn to foam. When you mix foam latex, the volume is dependent on whipping speed and time. With urethane foam, it rises of its own accord; under the best of circumstances, the cold foam will expand seven times its original liquid volume. The molds used for urethane foam must have escape holes drilled for excess foam to get out. If too much foam is put into the mold and the mold is then clamped tightly shut, the resulting foam will be dense and less flexible than if just enough is added and expands into all the cavities of the mold.
Though cold foam can be a pain in the neck because of its high sensitivity to any moisture, what I like about it is how tough it is compared to foam latex. It isn’t as soft as foam latex, though a softer foam can be achieved by increasing the ratio of part A (I’m talking mere drops for a small mold; too much A could prevent the foam from skinning at all); an appliance made of cold foam can be used repeatedly because it is less susceptible to tearing during removal. I have had pieces made that have lasted for as many as 40 stage performances, without wear noticeable from the audience. In fairness, I must attribute at least some of that success to conscientious actors.
According to Kryolan, the optimum working temperature for the foam is 113°F (45° C). You can achieve this by placing the mold halves, the positive and the negative, in an oven set on warm—about 120°F (about 49° C) in most ovens. Heat the molds until they’re warm to the touch. If that’s not possible, try using a blow drier on high for a few minutes.