a process for the manufacture of composite sheeting having a layer of polystyrene and at least one layer of polycarbonate polymer firmly united thereto , comprising joining a molten stream of each of said polymeric materials in a conduit to form a single stratified stream of molten material conforming to the cross - section of said conduit and having a relatively sharply defined interface between said polymeric materials , and passing said stratified stream of molten material through a sheet - form extrusion die having its die lips generally aligned with said interface between the two polymeric materials .