the invention relates to a process for manufacturing a composite molded structure including the steps of : depositing first and second members in a mold ; inserting at least one flexible plastic membrane between the first and second members so as to form a sheath when the mold is closed ; closing the mold ; injecting into the sheath reactants intended to react to form a foam which expands in situ and stretches the sheath until it forces the members flat against the walls of the mold ; cooling the contents of the mold ; and opening the mold to withdraw the composite molded structure .