between the plug outlet 24 and the cooling groove 26 a free spacing a extends to permit an unobstructed deposit of the yarn plug 13 in the cooling groove 26 during the crimping process the spacing a remains unchanged in the crimping device 7 a heated conveying fluid enters the yarn feed channel 20 via the injector 19 this causes a suction effect to develop at the upper end of the yarn feed channel 20 which sucks the filament bundle 10 into the crimping device 7 the conveying fluid advances the filament bundle 10 through the yarn feed channel 20 into the stuffer box chamber 22 in the stuffer box chamber 22 the filament bundle 10 compacts to a yarn plug 13 in so doing the filament bundle 10 opens up and the individual filaments come to lie on top of one another in loops and coils in this process the formation of the yarn plug 13 is largely defined by the quality of the conveying fluid and by the pressure of the conveying fluid as conveying fluid it is preferred to use hot air to decrease the pressure of the conveying fluid the upper region of the stuffer box chamber 22 is made air permeable in the form of air slots or lamellas so that the conveying fluid is able to escape into a pressure relief chamber 21 and from there to the outside the yarn plug 13 advances at a defined adjusted speed v f through the stuffer box chamber 22 to the plug outlet 24 from there the yarn plug 13 enters the cooling groove 26 at the yarn advancing speed v f the cooling groove 26 moves at a cooling speed v k which is defined by.