The degree of twisting of the yarn not only affects the appearance quality such as the diameter and gloss of the yarn, but also affects the inherent quality and feel of the yarn such as strength, elasticity and elongation. Therefore, the determination of the degree of twisting of the yarn in the textile process is also of particular importance.
(1) Effect on yarn diameter and density
The twisting causes the fibers in the single yarn to be dense, the voids between the fibers are reduced, the density of the single yarn is increased, and the diameter is reduced. When the enthalpy coefficient is increased to a certain value, the compressibility between the fibers in the single yarn becomes small, and the density does not change much as the enthalpy coefficient increases. On the contrary, the diameter of the yarn may be slightly increased due to the excessive inclination of the fibers.
The diameter and density of the strands are also related to the twist of the strands and single yarns. When the strand twist direction is the same as the single yarn twist direction, the relationship between the twist coefficient and the density and diameter is similar to that of the single yarn. When the strands are opposite to the single yarn twist, when the twist coefficient of the strand is small, the density of the strands is reduced and the diameter is increased due to the untwisting effect of the single yarn; when the twist coefficient reaches a certain value, The density of the strand increases with the increase of the enthalpy coefficient, and the diameter decreases with the increase of the enthalpy coefficient. As the density continues to increase, the diameter gradually increases.
(2) Effect on yarn strength
For short-staple yarns, the most direct effect of twisting is to obtain strength, but the greater the degree of twisting, the greater the strength of the yarn, because the twisting has both factors that contribute to the strength of the yarn. There are also factors that are not conducive to yarn strength.
1. Favorable factor
(1) The enthalpy coefficient is increased, the centripetal pressure of the fiber on the uranium is increased, the frictional resistance between the fibers is increased, and the possibility that the yarn is broken due to slippage between the fibers is reduced.
(2) Twisting reduces the strength unevenness of the yarn in the longitudinal direction.
Under the external force of the yarn, the break always occurs at the minimum strength of the yarn, and the strength of the yarn is the external force that the weak ring can withstand. As the enthalpy coefficient increases, more rounds are distributed at the weak ring, which makes the weak ring stronger than other places, thus making the yarn stronger.
2. Unfavorable factors
(1) Twisting causes the fibers in the yarn to be inclined, so that the axial component of the fiber is reduced, thereby reducing the strength of the yarn.
(2) The fiber is prestressed during the twisting process of the yarn, and when the yarn is stressed, the ability of the fiber to bear an external force is lowered.
The effect of twisting on the strength of the yarn is the unity of the above favorable factors and unfavorable factors. When the enthalpy coefficient is small, the favorable factors play a leading role, as the yarn strength increases with the increase of the enthalpy coefficient. When the enthalpy coefficient reaches a certain value, the unfavorable factor plays a leading role, and the strength of the yarn decreases with the increase of the æ» coefficient, as shown in Figure 2-9. The enthalpy coefficient at which the strength of the yarn reaches a maximum is called the critical enthalpy coefficient (ak in the figure), and the corresponding enthalpy is called the critical enthalpy. In the process design, the twist is less than the critical enthalpy coefficient, so as to improve the production efficiency of the spinning machine under the premise of ensuring the strength of the spun yarn.
(3) Effect on elongation at break of yarn
For a single yarn, twisting reduces the likelihood of fiber slippage in the yarn and increases the elongation of the fiber, manifesting as a decrease in the elongation at break of the yarn. However, as the enthalpy coefficient increases, the degree of inclination of the fiber in the yarn increases, and when stretched, there is a tendency to reduce the inclination of the fiber and the yarn becomes thinner, thereby increasing the elongation at break of the yarn. In general, the favorable factor is greater than the unfavorable factor in the range of the commonly used enthalpy coefficient, so as the enthalpy 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 elongation at break of the yarn is the same as that of the single yarn. For the strands twisted in the opposite direction, when the twist coefficient is small, the twisting of the strand means the untwisting of the single yarn, and the average width of the strand decreases with the increase of the twist coefficient, so the elongation at break of the strand The rate decreases slightly. When the enthalpy coefficient reaches a certain value, the average æ» amplitude increases with the increase of the æ» coefficient, and the elongation at break of the strand increases.
(4) Influence on yarn elasticity
The elasticity of the yarn depends on both the elasticity of the fiber and the structure of the yarn. The structure of the yarn is mainly formed by the twisting of the yarn. For single yarns and strands twisted in the same direction, twisting makes the yarn structure tight. The fiber slip is reduced and the fiber's stretchability is increased. Within the range of the general twist coefficient, the elasticity of the yarn increases as the twist factor increases.
(5) Influence on yarn gloss and hand feeling
The single yarn and the twisted strands in the same direction are twisted by the twisting of the yarn surface, and the surface of the yarn is roughened, the gloss of the yarn is deteriorated, and the hand feels hard. When the ratio of the strand twist coefficient to the single yarn twist coefficient is equal to 0.707, the outer twist is zero, and the surface fiber is parallel to the yarn axial direction. At this time, the strand has the best luster and soft handfeel. .
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