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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