Hard Rubber Mold? Any Ideas?

I have a friend that is making pouring lead peices and has a need to build his own custom molds to do this with. He has had one made for him that is made out of hard rubber (like a tire) its 2 peices. He was told that its a special kind of rubber and u take the part you want to make and force the two peices together for a set amount of time , then you take apart and cut pouring holes to pour the lead through. Any ideas of the materials to do this with and instructions? Or any other ideas to do this with. Making fishing lures.