Abstract: An improved mold for molding composite structures inside the closed cavity of the mold that uses a projecting tool member, a compression plate and at least two side plates located inside an expansion cavity formed in the mold. The mold includes a base made up of two mold sections that when jointed form a partially enclosed mold cavity. Located over the mold cavity are two side plates each with an inside beveled edge. The side parts are aligned inside the expansion cavity so that the two beveled edges are adjacent a form a large V-shape void. The compression plate includes a V-shaped projection that fits into the V-shaped void. The improvement includes a U-shaped, heat expandable boot located between the inside surfaces of the projecting tool member, the inside surfaces of the two side plates, and the inside surfaces of the mold that evenly distributes the external forces and evenly distributes the forces created by expansion during the curing stage to product structures with uniform side walls.