has little affect if any on the properties of fiber made from the resulting 6ia 6ia copolymer since most of the plasticizer is removed by vaporization from the copolymer during polymerization and processing thereof into fiber thus if desired a plasticizer may be used in preparing any of the copolymers of the invention even though it presence is not required to prevent phase out during polymerization for example in preparing 6ta 6ia copolymers wherein the mole ration of 6ta to 6ia is between 60 40 and 70 30 preferably the 6ta and 6ia salts used in conducting the polymerization should be of the highest possible purity the presences of even trace amounts of impurities in the salts may be detrimental to the quality of the resulting copolymer and fiber made therefrom for example the presence of 10 ppm of carboxybenzaldehyde has been found to apparently catalyzes formation of bishexamethylenetriamine and other materials such as ammonia and hexamethylenimine bishexamethylenetriamine can cause branching and eventual cross linking of the resulting copolymer under these circumstances the copolymer may not process easily if at all into useful fiber or fiber processed therefrom may be discolored and or of non uniform denier and low quality following the melt polymerization cycles the resulting molten mass may be directly melt spun into fiber or it may be extruded and made into flake that may subsequently be melted and extruded into fiber preferably when melt spinning is conducted using flake the flake is first dried for example in a vacuum oven for 12 hours at 70 u00b0 to.