Continuous filament of yarn structure | Yarn structure

Continuous filament of yarn structure
In a steel or in a rope structure, one strand or component remains in the centre, and all other components have a specific and constant radius of spirality (Helix), remaining in an annular envelope about the center or cable axis. In examining a segment of the cable, one would find the longest components at the surface and the shortest component in the centre. This is possible because each component is individually controlled by tension or rate of feeding during formation of the cable or rope.
During the twisting of multi continuous filaments, however, there exists no control of the tension or rate of feeding of the individual filaments resulting a self-equalization tension phenomenon wherein the individual filaments exchange positions radially among the various annular zones in the assemblage. Those filaments going to the outer layers of the yarn develop more tension and, being tight, force themselves to the center of the yarn to relieve the tension. The filaments in the core of the yarn is under less tension are pushed to the yarn surface. This means that every filament is entangled periodically at both surface and core of the yarn.

Yarns with minimal twist or entanglement will have a very long fiber segment length between points of entanglement, the filaments are free to spread, and there is no definite or stable yarn cross-sectional shape or diameter.

Yarns with more twist, the degree of linearity of filament  segment lengths in the yarn decreases slightly leading to reduced strength. This reduction in yarn strength occurs because the more linear filaments are strained immediately on loading, whereas the less linear filaments tend to straightened and then strained. However, greater twist also reduces the average length of the filament segments between points of entanglement and the tendency for filaments to spread out and snag .With sufficient more twist, the cross section of a filament yarn can be given more support so that, under a compressive or bending deformation, an elliptical shape occurs rather than a total collapse or flattening out of the filament assemblage. Increased twist in filament yarn also leads to greater bending stiffness in the yarn structure primarily because of greater entanglement and friction among the filaments.

STAPLE YARN STRUCTURE

The structure of staple (Spun) yarns is an order of magnitude more complex than the structure of continuous filaments. Although a high-quality staple yarn may appear to be as uniform as a filament yarn, closer examination would indicate a random grouping of the fiber and a variation in twist and in linear density, resulting in thick and thin spots along the length of yarn. Also noticeable would be a concentration of fiber forming a core

in the centre of the yarn and an outer fringe of fiber known as fuzz zone caused by protruding fiber ends. An untwisting of the spun yarn structure would indicate a more thorough and frequent tying-in of the surface and core layers than found in filament yarns. Examination of low-quality staple yarn would show many localized fiber entanglements or hard spots known as neps that are contained in the structure.

A tremendous variation in the quality of staple yarns and uniformity of yarn structure is possible because of the manner in which the fibers are processed previously. Staple fibers are handled as a mass rather than as individuals and therefore tend to behave and to process in groups and sub-groups. The fibers tend to participate to a much greater extent in their own processing than in the case of filament processing. Each staple has two free ends that may protrude, contributing to fuzz or may bend, hook, buckle, or roll on themselves or in conjunction with other fibers. Intermediate staple fiber assemblages (sliver and roving) contain a slight internal entanglement caused mainly by the condensation of fiber web into linear assemblages. Also some twist is normally incorporated into the roving structures. In the final stage of staple yarn formation, a rather greater amount of twist is superimposed onto the existing looser entanglement.

In blends of fibers with substantially different processing characteristics, an abnormal fiber migration may occur that is known as preferential radial migration or coring; one component of the blend is found primarily in the core and the other component mostly near the surface.

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