When the two parts are mixed and cast in a silicone rubber mold, they solidify. In silicone rubber mold casting, the PU takes from one to two hours to cure. The cost of a silicone mold is usually several times higher in unit price compared to the cast, but this is normally factored into the unit price of the cast itself. The PU casting method is preferred over the machining method in following scenarios:

• Lower the unit part cost when the volume goes higher;
• Make complex shapes that are not feasible by machining;
• Have special material requirements, such as transparency.

The PU casting process is ideal for low volume plastic parts. It saves clients from expensive and long lead-time production tooling. Also, PU casting allows certain complex shapes that are not attainable by injection molding because the silicone rubber mold is soft to allow large deformation to release the cast from the mold itself.

The above picture shows the mold and the cast. The part has a large special undercut that cannot be done by injection molding.