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Importance of twist structure | Yarn twist Structure


Twist plays an important role in affecting the arrangement of fibers or filaments in the cross-section. In staple yarns, twist is essential to hold the fibers together and to impart some degree of cohesiveness to the structure. On the other hand, the filaments in a multifilament yarn would fray away if there were no cohesive forces holding them together. Furthermore, the formation of plied yarn or cabled yarns is also achieved by twisting single or plied yarns together to produce a coherent linear structure. In other words, twist is a means by which a bundle of fibers, filaments, or yarns in a plied yarn is held together so that the ultimate structure is made capable of withstanding the stresses and strains
and the chafing action of many processes involved in the manufacture and use of textile fabrics. The role of twist is very profound in determining the properties of finished yarns, vis-à-vis fabrics.
When a number of components (fibers or filaments) in a continuous strand are twisted, radial forces develop which in turn affect the relative position of the components in the yarn structure, leading to close packing of all the components in a given cross-section.
Thus the insertion of twist in fiber assemblies (yarns) affects, in addition to the tensile properties (Strength-Extension), the diameter and the specific volume (in other words, softness or hardness) of yarns. The change in the fiber packing in turn determines the cover of a fabric and such other properties as warmth, crease recovery, permeability, and various other related property characteristics. Twist also affects the hairiness of yarn, which is very important property in determining the pilling behavior and the economics of cropping/shearing and singeing process.
  
YARN TWIST:
Twist is the measure of spiral turns given to a yarn in order to hold the constituent fibers or threads together. Twist is also the spiral disposition of the components of a thread which is usually the result of the relative rotation of the two ends of fibers.
When two ends of s straight strand (yarn) are rotated relative to one another, he fiber on the surface of the yarn lie in helices about the yarn axis. In other words, a yarn is twisted when fibers on the surface, which were originally parallel to the axis of the yarn, are now deformed (rotated) so that they make an angle O with the axis, and the amount of twist is a function of this angle. This definition of twist is applicable for the ideal case of an originally straight and parallel fiber assembly. 

However, in actual yarns, the variability of yarn diameter, contraction because of twist, migration of  fibers from one zone to the other, radial compression of the yarn, and fiber slippage are some of the factors that tend to make the yarn geometry depart from the ideal geometry.
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