Abstract: A tubular, laminar, plastic sheet material having a core layer of oriented plastic and two outer layers of approximately isotropic plastic is prepared in extrusion apparatus whose die block has three channels wider than they are thick, and merging at acute angles to form an extrusion nozzle. The channels are continuously supplied with molten resin compositions webs of which are superimposed on each other in area contact at the junction of the channels. They move as an initially fluid, but coherent, layered structure from the nozzle into an immediately adjacent duct of a cooling head in which the structure is cooled from a first temperature above the melting temperature of the highest-melting resin composition of the central layer to a temperature intermediate that melting temperature and the temperature at which the outer, lower melting layers become solid.