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Yarn twist and fiber packing process | Textile yarn | Textile Fiber


TWIST AND FIBER PACKING IN YARNS:
Yarn structure can be described in a simple idealized form is the packing of fibers in yarns. Schwarz has theorized that the packing of circular fibers can be described by two basic forms; I) Open packing and  2) Hexagonal close packing.
OPEN PACKING:

In this form the fibers lie in layers between successive concentric circles, as shown in figure below. In this assembly, the first layer is a single core fiber around which six fibers.
arranged so that the fibers first touch the circle that circumscribes the second layer. Additional layers are added between the successive circumscribing circles. The number of fibers in each layer and the total number of fibers in an ideally packed open structure are given in table below:

CONCENTRATING FACTORS:

1)      One of the concentrating features is the general tendency of the fibers to follow the path of minimum energy. In addition, they tend to settle down and cohere like any other materials.
2)      The other factor is twist, which has stronger effect. Twist causes the development of tangential and radial forces in fibers that tend to bind the fibers together. Any imposed tension in a yarn will generate inward pressure from the outer layers on those inside, leading to a close packing of filaments.
3)      High tension and twist can cause deformation in the fiber shape, especially in
      thermoplastic yarns.
DISTURBING FACTORS:

1)      Ideal packing is not achieved if the total number of fibers in  the yarn cross-section differ from the open-packed and hexagonal-packed yarns. Any deviation from the appropriate number would make the fibers pack unevenly, resulting in an irregularly shaped cross-section.
2)      The simple ideal forms are true for a circular fiber shape. Other fiber shapes will modify the ideal packing arrangement.
3)      Twist has a profound effect on the arrangement of fibers in the yarn cross-section. Because of twist, the fibers in the outer layers of the yarn follow a helical path; this introduces ellipticity in the cross-section of the fibers. The elliptical cross-section further affects the packing, thus modifying the ideal arrangement of fibers in the yarn cross-section.
4)      The other disturbing effect of twist is the variation in the path length between fibers at different radial positions in the yarn cross-section. These differences in the path length must be nullified either by buckling of fibers in the centre or by an interchange of radial position of fibers (migration), which results in the effective averaging of path lengths.
5)      Sometimes irregularities in the arrangement of fibers prior to yarn formation (such as in roving) may persist in the final yarn structure. Irregularities in the arrangement of fibers can also be result from the variations in fiber length supplied to the twisting zone in staple yarn spinning.
Other processing factors, such as the passing the yarns over guides or between feed rolls during winding, may cause changes in the yarn shape. In addition, the form of twisting, that is, whether the fibers are presented in a ribbon form or in cylindrical shape, may introduce asymmetry and irregularity in the shape of the yarn.

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