What is the effect of different twist levels on yarn quality?
The degree of twisting of a yarn not only affects the appearance quality such as the diameter and luster of the yarn, but also affects the internal quality and feel such as the strength, elasticity, and elongation of the yarn. Therefore, it is particularly important to determine the degree of yarn twist in textile processes.
（1） Effect on yarn diameter and density
Twisting causes the fibers in a single yarn to become dense, resulting in a decrease in the gap between the fibers, an increase in the density of the single yarn, and a decrease in 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 changes little with the increase of the twist coefficient. On the contrary, due to the excessive inclination of the fibers, it is possible to slightly increase the diameter of the yarn. The diameter and density of the ply yarn are also related to the twist direction of the ply yarn and single yarn. When the twist direction of the ply yarn 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 ply yarn is opposite to that of the single yarn, when the twist coefficient of the ply yarn is small, due to the untwisting effect of the single yarn, the density of the ply yarn decreases and the diameter increases; When the twist coefficient reaches a certain value, the density of the strand increases with the increase of the twist coefficient, while the diameter decreases with the increase of the twist coefficient. As the twist continues, the density does not change much, but the diameter gradually increases.
（2） Effect on yarn strength
For staple fiber yarns, the most direct role of twisting is to obtain strength, but it is not that the greater the degree of twisting, the greater the strength of the yarn. The reason is that there are both factors conducive to improving the yarn strength and factors that are not conducive to the yarn strength. 1. The advantageous factor (1) is an increase in twist coefficient, which increases the centripetal pressure of the fibers on the yarn, increases the frictional resistance between the fibers, and reduces the possibility of yarn breakage due to fiber slippage. (2) Twisting reduces the strength unevenness of the yarn in the length direction. Under the action of tensile external forces, yarn breakage always occurs at the place where the yarn strength is the smallest, and the strength of the yarn is the external force that the weak ring can withstand. As the twist coefficient increases, more twist is distributed at the weak loop, which increases the strength at the weak loop compared to other areas, thereby increasing the yarn strength. 2. Unfavorable factor (1) Twisting causes the fibers in the yarn to tilt, reducing the axial force borne by the fibers, thereby reducing the strength of the yarn. (2) During yarn twisting, fibers are prestressed, and when the yarn is stressed, the ability of the fibers to bear external forces decreases. The influence of twisting on yarn strength is a unity of opposites between the above favorable and unfavorable factors. When the twist coefficient is small, favorable factors play a leading role, as the yarn strength increases with the increase of the twist coefficient. When the twist coefficient reaches a certain value, unfavorable factors play a leading role, and the strength of the yarn decreases with the increase of the twist coefficient. The twist coefficient when the yarn strength reaches the 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 factor is generally used to improve the production efficiency of the spinning frame while ensuring the yarn strength.
（3） Effect on yarn breaking elongation
For single yarns, twisting reduces the possibility of fiber slippage in the yarn and increases fiber elongation and deformation, manifested by a decrease in yarn elongation at break. However, with the increase of twist coefficient, the inclination degree of fibers in the yarn increases, and there is a tendency to reduce the inclination degree of fibers and make the yarn thinner when stretched, thereby increasing the elongation at break of the yarn. In general, within the range of commonly used twist coefficients, favorable factors outweigh unfavorable factors, so as the twist coefficient increases, the elongation at break of a single yarn increases. For a ply yarn twisted in the same direction, the effect of twist coefficient on yarn breaking elongation is the same as for a single yarn. When the twist coefficient is small for a ply yarn with opposite twist, twisting the ply yarn means untwisting the single yarn. The average twist amplitude of the ply yarn decreases with the increase of the twist coefficient, so the elongation at break of the ply yarn decreases slightly. When the twist coefficient reaches a certain value, the average twist amplitude increases with the increase of the number of twists, and the elongation at break of the ply yarn also increases.
（4） Effect on yarn elasticity
The elasticity of a yarn depends on both the elasticity of the fiber and the structure of the yarn, which is mainly formed by twisting the yarn. For single and co directional strands, twisting tightens the yarn structure, reduces fiber slippage, and increases the extensibility of the fiber. Within the general twist coefficient range, as the twist coefficient increases, the elasticity of the yarn increases.
（5） Effect on yarn gloss and feel
Single yarn and co directional twisted plied yarn, as a result of twisting, inclines the fibers on the surface of the yarn, making the surface of the yarn rough and uneven, resulting in poor luster and a hard feel. When the ratio of the twist coefficient of the plied yarn to the twist coefficient of the single yarn is equal to 0.707, the outer twist amplitude is zero, and the surface fibers are parallel to the yarn axis. At this time, the plied yarn has the best luster and a soft feel.
