Abstract: Disclosed is a polyethylene terephthalate untwisted multifilament which satisfies the following requirements (A), (B), (C) and (D);(A) the intrinsic viscosity [IV] is 0.97 to 1.15;(B) the amorphous orientation function [fa] is not larger than 0.55;(C) the tenacity [T] (g/d), the shrinkage [.DELTA.S] (%) as measured after standing in dry air at 150.degree. C. for 30 minutes, the medium elongation [ME] (%) under a load of 4.5 g/d, and the dimensional stability index [Y] expressed by the formula: Y=ME.sup.0.81 +.DELTA.S+1.32 are within ranges defined by the following formulae (a), (b), (c), (d) and (e):0.33Y+5.55.ltoreq.T.ltoreq.0.33Y+6.50 (a),8.0.ltoreq.T.ltoreq.9.5 (b),8.5.ltoreq.Y.ltoreq.10.5 (c),5.ltoreq.ME.ltoreq.10 (d),and2.ltoreq..DELTA.S.ltoreq.g (e);and (d) the elongation at break is at least 11% and the product of the tenacity and elongation, which is defined by: ##EQU1## is 30 to 36.

Abstract: A process for producing polyester fiber highly suitable for a raw yarn for a woven or knitted fabric, having a residual elongation of not higher than 60%, a Young's modulus of 60 to 100 g/d, a boiling water shrinkage of 4 to 10% and a dry heat shrinkage of 5 to 12%, the peak stress temperature in a dry heat shrinkage stress curve of said fiber being lower than 100.degree. C. The process comprises melt spinning a thermoplastic polyester through a spinneret into a filament and taking up the filament after solidifying the spun filamentary polymer, characterized in that the whole process from spinning through take-up is carried out without heating the filament, the filament is subjected to stretch treatment at a stretch ratio of not more than 20% after the solidification of the filament but before take-up, and take-up is carried out at a take-up speed of not less than 5,000 m/min.

Abstract: A polyester fiber highly suitable for a raw yarn for a woven or knitted fabric, having a residual elongation of not higher than 60%, a Young's modulus of 60 to 100 g/d, a boiling water shrinkage of 4 to 10% and a dry heat shrinkage of 5 to 12%, the peak stress temperature in a dry heat shrinkage stress curve of said fiber being lower than 100.degree. C. The fiber can be produced by a process comprising melt spinning a thermoplastic polyester through a spinneret into a filament and taking up the filament after solidifying the spun filamentary polymer, characterized in that the whole process from spinning through take-up is carried out without heating the filament, the filament is subjected to stretch treatment at a stretch ratio of not more than 20% after the solidification of the filament but before take-up, and take-up is carried out at a take-up speed of not less than 5,000 m/min.

Abstract: This invention relates to a method and apparatus for imparting false twists to two yarns simultaneously. The significance of the invention resides in that an industrially utilizable method for false twisting yarns by frictional contact of two yarns with 4 rotating frictional discs has been provided. The apparatus of the present invention for practising the false twisting method is characterized in that the angle of contact of yarns with the frictional discs is maintained within a specific range and the frictional discs are attached to four shafts in specific order at specific heights. Various processes for preparing a variety of false twist yarns by utilizing the false twisting method and apparatus of the present invention have been established.

Abstract: A process for producing a highly oriented polyester undrawn yarn comprising melt-spinning a polyester from a spinneret, solidifying the melt-spun yarn and taking up a solidified yarn at a take-up velocity from 2,500 to 4,500 m/min, wherein the spinning length is set within a specific range in accordance with the take-up velocity, yarn denier and maximum allowable tension of the yarn.