Abstract: An apparatus used to form a three-dimensional composite structure is disclosed having an initially rigid heat-softenable layer of ABS plastic wherein uncured portions of the composite structure are first assembled between the layer of ABS plastic and a hard mold surface. Prior to curing of the composite structure, pressure and heat are applied to the ABS layer to soften the layer and thereafter compact the uncured portions of the structure against the hard mold surface. The ability to pre-shape the ABS layer allows stiffening beams to be incorporated into the structure so as to yield a lightweight, strong composite structure that can be used as a pressure vessel in an aerospace vehicle.