Process for spinning naphthalate polyester fibers

A process for producing a naphthalate polyester filament, fiber or yarn having a diffraction intensity ratio (R) between a Bragg reflection angle 2.theta. = 18.7.degree. and 2.theta. = 15.6.degree. , as determined by the X-ray diffraction method, being in the range of more than 1.73 and up to 5.00, which comprises melt-spinning a naphthalate polyester containing at least 85 mol % of ethylene-2,6-naphthalate units and having an intrinsic viscosity of 0.45 to 1.0, using a spinning nozzle having a cross sectional area of 0.049 to 3.14 mm.sup.2 per hole at a spinning temperature expressed by the following equation:

• 28.6 [.eta.] + 301.4 .gtoreq. T .gtoreq. 35.7 [.eta.] + 279.3
• wherein T is the spinning temperature in .degree. C. and [.eta.] is the intrinsic viscosity of the polyester,
• said spinning being at a draft ratio of 50-20,000 and the draft ratio satisfying the following equation:
• -7.43 .times. 10.sup.-.sup.5 W + 2.37 .ltoreq.log D .gtoreq.2.27 .sqroot.A + 1.98
• wherein W is the take-up speed in meters per minute, D is the draft ratio, and A is the cross-sectional area in square millimeter per hole of the spinning nozzle, and then
• cooling the extruded filaments, and taking up the extruded filaments by Godet rollers at a speed of 3,000 to 8,000 meters per minute.