The degree of twisting of the yarn not only affects the appearance quality such as diameter and gloss of the yarn, but also affects the strength, elasticity, Intrinsic qualities such as stretch and feel. Therefore, the determination of yarn twist degree is also particularly important in textile technology.
Effects on yarn diameter and density
Twisting makes The fibers in the single yarn are dense, the gaps between the fibers are reduced, the density of the single yarn is increased, and the diameter is reduced. 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.
Effects on yarn strength
For short fiber yarns In other words, the most 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 factors that are conducive to improving the strength of the yarn, and there are also factors that are not conducive to improving the strength of the yarn. Line strength factors.
Favorable factors
a) The twist coefficient increases, and the fiber twists the yarn The centripetal pressure of the shaft increases, the friction resistance between fibers increases, and the possibility of yarn breakage due to slippage between fibers decreases.
b) 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 point where the yarn has the smallest strength. The strength 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.
Disadvantageous factors
a) Twisting causes the fibers in the yarn to tilt , which reduces the axial component of the fiber, thereby reducing the strength of the yarn.
b) 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 1.
The twist coefficient when the strength of the yarn reaches the maximum is called the critical twist coefficient, 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.
Effects on yarn elongation at break
On single yarn Generally speaking, 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 the twist coefficient on the 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.
Influence on yarn elasticity
The elasticity of yarn depends on It depends on both the elasticity of the fiber and the yarn structure, and the yarn structure is mainly formed by twisting the yarn. For single yarns and strands twisted in the same direction, twisting makes the yarn structure tight and fiber slippage is reduced. Small, the stretchability of the fiber increases. Within the general twist coefficient range, as the twist coefficient increases, the elasticity of the yarn increases.
Effects on yarn gloss and feel
Single yarn and For strands twisted in the same direction, the fibers on the surface of the yarn are tilted due to twisting, and the surface of the yarn becomes 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 twist width is zero and the surface fibers are parallel to the yarn axis. At this time, the strands have the best luster and soft feel. .
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