plenum chamber 156 in this manner axial momentum of the fluid subportions entering the plenum chamber 156 is dissipated and therefore is not free to accentuate a circumferential maldistribution moreover by providing the circumferential plenum chamber 156 with a volume substantially greater than the volumetric flow rate of the second portion for a unit of time material entering the plenum chamber 156 preferentially moves circumferentially with a comparatively slow velocity in this fashion circumferential uniformity of flow properties is obtained within the plenum chamber 156 from the radially outermost portion of the plenum chamber 156 the cross section decreases in the radially inward direction see fig3 causing an acceleration of the second synthetic resinous material toward the radially inwardly convergent channel 158 the acceleration which occurs in the radially inner portion of the plenum chamber 156 and in the channel 158 is effective to further erase any circumferential maldistribution which might otherwise be present when four equiangularly spaced channels are used to apply a flowing stratum the flow from the channel 158 is applied to the outside of the first stratum or core layer at the downstream end of the discharge zone 50 thus at the downstream end of the discharge zone 50 a radially stratified annular flow results in which a core layer of a first material has inner and outer skins of a second material it will be noted that the plenum chamber 156 and the acceleration channel 158 are each defined in part by the forming die 26 and in part by the second distribution plate 38 in this manner the axial length of the distribution plates can remain small and fabrication is facilitated by eliminating complex internal cavities as.