THE TWIST-RELATED STRUCTURE OF YARNS WITH FEW FILAMENTS

False-twist-textured yarns of 22 dtex and below with few filaments are commonly used in stretch stockings and tights. Because of the relativ ely high ratio of filament diameter to yarn diameter in such yarns, a concentric-helix model cannot be assumed, and the geometrical relation ships describing their structure have to take filament diameter into a ccount. For example, the helix angle of the yarn surface in false-twis ting may differ significantly from that of the axes of the surface fil aments. These twist angles may be calculated from a knowledge of the t wist level of the twisted yarn, its diameter, and the diameters of the filaments. Measurement of the twist by untwisting or by the untwist-t wist method is often difficult, especially for intermingled and therma lly set twisted yarns. Surface-filament-centre and yam-surface twist a ngles are defined for yarns with few filaments, and a satisfactory pra ctical method of determining these factors and the twist levels of fin e hosiery yarns, as used in the false-twist-texturing process, is deve loped through observation of the packing of the filaments within the y arn structure and the filament crimp. Based on these observations and for idealised yarn structures, expressions relating these parameters a nd the twisted-length ratio are developed.