/diy/ - Do It Yourself

I'm not sure if this is even the right board to ask but I recently wanted to make some molds and casts but the way sites lay their products out makes this some of the most confusing shit that I've ever encountered. What exactly is the difference between a rubber used for molding and one used for casting? For example Smooth-On sells tin cure rubber molds that are opaque and transparent. Lets say that I want to make something transparent. Why could I not use one of their opaque molds to make a mold of the object and then use one of their transparent molds to cast the object? Wouldn't you just have to apply a release agent to make sure they don't stick? Like why would I have to buy one of their clear urethane casting rubbers?