The best type of rubber for making concrete molds is polyurethane rubber. Other rubbers, such as polysulfide and silicone, are too fragile to handle the stress of large, heavy concrete molds, although they can be used for smaller, fragile concrete molds.
Polyurethane rubber is generally sold as two components, a resin and a hardener. The resin is the base of the rubber, while the hardener determines the flexibility and durability of the rubber once it has cured. The hardener liquid is measured on a scale from 30 to 90 where 30 is the softest and 90 is the hardest. A polyurethane rubber that has a hardener of 30 has a consistency that is comparable to a rubber band. A polyurethane rubber with a hardener of 60 to 90 is as dense as a car tire or a rubber stamp. The adaptability of polyurethane rubber means that it can be used for intricate and detailed concrete molds as well as heavier ones.
Once the resin and the hardener have been mixed, the polyurethane rubber generally stays workable between 15 and 60 minutes. Once it has been applied to the concrete mold, it takes about 16 hours to cure and harden. The rubber can either be poured into a mold or brushed on, depending on the viscosity of the resin.