a process is disclosed for producing composite sheeting of polystyrene having a thin protective layer of vinylidene fluoride polymer . the process comprises joining a molten stream of each polymeric material in a single extruder discharge conduit such that there is produced a single molten stream having a reasonably well defined interface between the two types of polymeric material , and thereafter , passing the resulting single stream of material through a sheet - form extrusion die having its die lips generally aligned with the foregoing interface .