monomer gases plates 16 a 16 b and 18 a 18 b respectively abut each other and contain air passages 27 29 therebetween air passages 27 29 are respectively formed by pairs of recesses 30 32 and 34 36 that align with each other in abutting faces of the plates 16 a 16 b and 18 a 18 b as shown best in fig1 these recesses 30 32 and 34 36 take the form of so called coat hangar recesses which become wider in dimension from the inlet portion 40 located proximate input ports 26 28 to an outlet portion 42 located proximate respective distribution passages 44 distribution passages 44 extend respectively through plates 16 b and 18 b and lead to extrusion die 14 plates 16 c and 18 c respectively abut central plate 20 as shown respective liquid passages 54 56 are formed between plates 16 c 20 and 18 c 20 and again are formed by respective pairs of coat hangar recesses 58 60 and 62 64 that align with each other in abutting surfaces of these plates 16 c 20 and 18 c 20 as shown in fig1 a these recesses 58 60 and 62 64 are also formed with a coat hangar configuration between inlet portions adjacent respective liquid input ports 66 68 and outlet portions which form elongate liquid outlet slots 70 72 for abutting the top surface of the extrusion die 14 and aligning with coextensive liquid inlet slots 73 75 in this embodiment the two liquid input ports 66 68 and coat.