The cooling phenomenon of a Poly (Ethylene Terephthalate) (PET) yarn under the false-twist texturing treatment has been compared with that of a yarn running without rotation around its own axis. The effect has been investigated of the yarn temperature during the yarn untwisting on yarn properties.
The temperature of a running yarn in a cooling stage has been determined by deducting a raised temperature from a measured one by using a thermocouple. The raised temperature caused by the yarn friction has been estimated from a deviation of a measured value from a theoretical one.
If a yarn passes without rotation, a twisted yarn tends to be more difficult to be cooled down than a non-twisted yarn. By contrast, in the false-twist texturing process, the cooling rate of a twisted yarn is considerably higher than that of a non-twisted yarn running without rotation. This would presumably be caused by a yarn rotation even if the yarn is twisted.
The coefficient of heat transfer in the cooling stage can be calculated from the formula about a fine filament yarn proposed by Kase and Matsuo. Some modifications are needed; a yarn rotating surface velocity should be used in stead of a cross air flow velocity, and a coefficient of velocity should be corrected to a little smaller value.
The properties of the textured yarn untwisted with an incomplete cooling effect have the same tendency as those of the yarn treated in a low false-twisting number. They are low in crimp contraction and number of crimps, and high in residual shrinkage and yarn torque.
From the view point of the yarn properties, it is necessary to the yarn temperature to the glass transition temperature of a crystalline region of a PET yarn. If possible, it is preferable to decrease it below the glass transition temperature of amorphous region.
Ballooning phenomenon of the yarn in false-twist texturing processing gives an effect on the yarn cooling. Therefore it might be better to allow the ballooning of a running yarn if it has no obvious troubles in the manufacturing process.

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