Abstract: Composite structures are fabricated and cured without applying external heat from an autoclave or oven. This technique uses composite preforms to form high strength, three-dimensional co-bonded joints, and thermoplastic conformal mandrels such as rota-molded vacuum bags. The bags form internal tooling within the structure and provide molding with integral fittings that circulate heated air within the bags. The exteriors of the bags are simultaneously under vacuum and exert pressure on the composite elements of the structure that are being cured. This internally-applied heat initially causes to the bags to soften and fully conform to the internal shapes of the structure that are being co-bonded. The heat then transfers into the uncured materials, causing them to cure without unnecessarily heating the entire structure or any required tooling.