This material allows parts with deep undercuts to be produced from a very simple mold. The part pictured below is a fuel tank bulkhead that fits into a molded part to isolate a fuel tank compartment. The pattern has been fabricated from Rohacell foam then painted.
Wood also works well to build patterns.
On this part only the front and sides of the part needed to be duplicated so a single part mold was used. First we must build a simple box to contain the rubber molding compound while it is curing around the pattern. This box was constructed quickly by using balsa sheets and cyanoacrylate glue. Make sure that all the seams in the box are sealed with glue. The bottom of the box must follow the back surface of the pattern.