Geometrical shapes of cross-setions of nylon-6 or polyester false twist textured yarns are classified as those of polysides plus original arcs, those of polysides without arcs, and polygons. Observing the shapes of the vertical cross-sections cut at equal intervals along a single nylon false twist textured filament, it was found that the filament migrates from outside to inside and vice versa during heat setting. Nylon and polyester false twisted yarns are relaxed at 120 and 140°C in water or

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... pressure water containing carrier at 100°C. The number of sides around the cross-section seems to decrease by relaxation, except nylon of feed ratio of -1.2 % and relaxed at 120°C. Single false twisted filaments before and after relaxation were observed with a scanning electron microscope to find the followings : The filament has polysides or polygonal shape at the convex part of the bending, and the original round shape at the concave part. Filaments which have polysides or polygonal cross-sections have been twisted in straight form, and bent rather abruptly. Smaller dye penetration length from a round side than from a straight side is observed for a partialy dyed single filament. Easiness for dye penetration into nylon filament can be ranked as follows: 2.0% feed > ratio > -1.2 % feed ratio > yarn before false twisting. Amount of moisture regain for the corresponding yarns has the same order as above mentioned.