The influence of different twisting degrees on yarn
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The impact of different twisting degrees on yarn
The twisting degree of yarn not only affects the appearance quality such as diameter and gloss of the yarn, but also affects the intrinsic quality and feel of the yarn such as strength, elasticity, elongation and so on. Therefore, the determination of yarn twist degree is also particularly important in textile technology.
(1) Effect on yarn diameter and density
Twisting makes the fibers in the single yarn denser, reducing the gaps between fibers, increasing the density of the single yarn, and reducing the diameter. When the twist coefficient increases to a certain value, the compressibility between fibers in a single yarn becomes very small, and the density does not change much as the twist coefficient increases. On the contrary, the diameter of the yarn may increase slightly because the fibers are too tilted.
The diameter and density of the strands are also related to the twist direction of the strands and single yarns. When the twist direction of the strands is the same as that of the single yarn, the relationship between the twist coefficient, density and diameter is similar to that of the single yarn. When the twist direction of the strands is opposite to that of the single yarn, and the twist coefficient of the strands is small, the density of the strands will decrease and the diameter will increase due to the untwisting effect of the single yarn; when the twist coefficient reaches a certain value, the density of the strands will decrease and the diameter will increase. The density of the strands increases with the increase of the twist coefficient, while the diameter decreases with the increase of the twist coefficient. As the twisting continues, the density does not change much, while the diameter gradually increases.
(2) Effect on yarn strength
For short fiber yarns, the direct function of twisting is to obtain strength, but it does not mean that the greater the degree of twisting, the greater the strength of the yarn. The reason is that twisting has advantages that are beneficial to improving the strength of the yarn. factors, and there are factors that are not conducive to the strength of the yarn.
1. Favorable factors
(1) As the twist coefficient increases, the centripetal pressure of the fiber on the yarn uranium increases, the frictional resistance between fibers increases, and the possibility of the yarn breaking due to slippage between fibers decreases.
(2) Twisting reduces the uneven strength of the yarn in the length direction.
Under the action of tensile external force, the yarn will always break at the weak point of the yarn. The strong point of the yarn is the external force that the weak link can withstand. As the twist coefficient increases, more twists are allocated to the weak loop, which increases the strength at the weak loop more than other places, thereby increasing the yarn strength.
2. Disadvantages
(1) Twisting makes the fibers in the yarn tilt, which reduces the axial component of the fiber, thereby reducing the strength of the yarn.
(2) During the twisting process of the yarn, the fibers are prestressed. When the yarn is stressed, the fiber’s ability to withstand external forces is reduced.
The effect of twisting on yarn strength is the unity of the above favorable and unfavorable factors. When the twist coefficient is small, favorable factors play a dominant role, which is manifested in that the yarn strength increases with the increase of the twist coefficient. When the twist coefficient reaches a certain value, unfavorable factors play a dominant role, and the strength of the yarn decreases as the twist coefficient increases, as shown in Figure 2-9. The twist coefficient when the yarn strength reaches a maximum value is called the critical twist coefficient (ak in the figure), and the corresponding twist is called the critical twist. In process design, a twist less than the critical twist coefficient is generally used to improve the production efficiency of the spinning machine while ensuring the strength of the spinning yarn.
(3) Effect on yarn elongation at break
For single yarns, twisting reduces the possibility of fiber slippage in the yarn and increases fiber elongation deformation, which is manifested as a decrease in the elongation at break of the yarn. However, as the twist coefficient increases, the degree of inclination of the fiber in the yarn increases. When stretched, the degree of inclination of the fiber decreases and the yarn becomes thinner, thereby increasing the elongation at break of the yarn. Generally speaking, within the generally adopted range of twist coefficient, the favorable factors outweigh the disadvantageous factors, so as the twist coefficient increases, the elongation at break of the single yarn increases.
For strands twisted in the same direction, the effect of twist coefficient on yarn elongation at break is the same as that of single yarn. For strands twisted in different directions, when the twist coefficient is small, the twisting of the strand means the untwisting of the single yarn. The average twist amplitude of the strand decreases with the increase of the twist coefficient, so the breaking elongation of the strand The rate decreases slightly. When the twist coefficient reaches a certain value, the average twist amplitude increases as the twist coefficient increases, and the breaking elongation of the strands also increases.
(4) Effect on yarn elasticity
The elasticity of yarn depends on both the elasticity of the fiber and the yarn structure. The yarn structure is mainly formed by the twisting of the yarn. For single yarns and co-twisted strands, twisting makes the yarn The thread structure is tight, the fiber slippage is reduced, and the stretchability of the fiber is increased. Within the general twist coefficient range, as the twist coefficient increases, the elasticity of the yarn increases.
(5) Effect on yarn gloss and feel
For single yarns and strands twisted in the same direction, the twisting causes the fibers on the surface of the yarn to tilt and makes the surface of the yarn rough and uneven. The luster of the yarn becomes worse and the feel becomes harder. For twisted strands in opposite directions, when the ratio of the strand twist coefficient to the single yarn twist coefficient is equal to 0.707, the outer layer twist width is zero and the surface fibers are parallel to the yarn axis. At this time, the strands have good luster and soft feel.
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