Abstract: An apparatus for molding hollow composite articles having a plurality of internal webs. A mandrel assembly apparatus includes slidable sidewall members, an inflatable pressure membrane surrounding the length of the slidable sidewall members and actuator means mounted on the interior of the sidewall members for lateral displacement thereof together with the inflatable membrane. Adjacent mandrel assemblies accurately position the internal webs of the composite article mechanically, while the inflatable membranes serve to compact the web laminates. The expandable sidewall members also prevent bridging or tearing of the pressure membrane associated with the formation of inner corner radii.