the slow cold rollers 41 and the fast cold rollers 42 viz the ratio between the peripheral speed of rollers 42 and that of rollers 41 is comprised between 2 5 and 3 6 preferably between 2 8 and 3 3 while the difference of the peripheral speeds between the fast cold rollers and the take up rollers 43 is only that required to permit the contraction of the yarn due to the texturization and therefore said take up rollers have a peripheral speed that is less than that of the fast cold rollers by an amount in the order of 7 12 when a second setting operation is carried out as in fig5 the peripheral speed of the take up rollers 52 is 14 18 lower than that of the rollers 14 and the air temperature in the oven 51 or the temperature of the hot plate which may be used in place thereof which will called 34 setting 34 temperature without further precision thus to distinguish it from the aforesaid 34 texturization 34 temperature is 150 u00b0 200 u00b0 c for the yarn linear speeds set forth with reference to fig1 all the numerical data set forth hereinbefore are preferred but not binding for a further illustration of the invention the numerical values of the process variables in some concrete embodiments of the invention.