Influence of Different Twisting Degrees on Yarn

The degree of twisting of the yarn not only affects the appearance quality of the yarn such as diameter and luster but also affects the inner quality and hand feel of the yarn such as strength, elasticity, and elongation. Therefore, the determination of the degree of yarn twisting in the textile process is also very important.

Influence on Yarn Diameter and Density

The twisting makes the fibers in the single yarn 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 the fibers in the single yarn becomes very small, and the density does not change much with the increase of the twist coefficient. On the contrary, the diameter of the yarn may increase slightly due to the excessive inclination of the fibers.

The diameter and density of strands are also related to the twist direction of 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 strand is opposite to that of the single yarn when the twist coefficient of the strand is small, the density of the strand will decrease and the diameter will increase due to the untwisting effect of the single yarn; when the twist coefficient reaches a certain value, it will make 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 twist continues, the density does not change much, but the diameter gradually increases.

Influence on Yarn Tenacity

For staple fiber yarn, the most direct effect 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 the improvement of yarn strength. Some factors are unfavorable for yarn strength.

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Favorable factors

• As the twist coefficient increases, the centripetal pressure of the fiber on the yarn increases, the frictional resistance between the fibers increases, and the possibility of the yarn breaking due to slipping between the fibers decreases.
• Twisting reduces the uneven strength of the yarn in the length direction.
  When the yarn is stretched by an external force, the fracture always occurs at the point where the yarn strength is the smallest, and the strength of the yarn is the external force that the weak link can bear. With the increase of the twist coefficient, more twists are assigned to the weak ring, so that the strength of the weak ring is higher than that of other places so that the yarn strength is improved.

Unfavorable factors

• The twisting makes the fibers in the yarn tilt, which reduces the axial component force on the fibers, thereby reducing the strength of the yarn.
• During the twisting process of the yarn, the fiber is prestressed. When the yarn is stressed, the ability of the fiber to bear the external force is reduced.

The influence of twisting on yarn strength is the unity of opposites between the above favorable factors and unfavorable factors. When the twist coefficient is small, favorable factors play a dominant role, showing that the yarn strength increases with the increase of the twist coefficient. When the twist coefficient reaches a certain value, it shows that 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 strength of the yarn reaches the maximum is called the critical twist coefficient (ak in the figure), and the corresponding twist is called the critical twist. In the process design, the twist less than the critical twist coefficient is generally used to improve the production efficiency of the spinning frame under the premise of ensuring the strength of the spun yarn.

Influence on yarn elongation at break

For single yarn, twisting reduces the possibility of fiber slippage in the yarn and increases the elongation and deformation of the fiber, which is manifested as a decrease in the elongation at the break of the yarn. However, with the increase of the twist coefficient, the degree of inclination of the fiber in the yarn increases, and when stretched, the degree of inclination of the fiber decreases and the yarn becomes thinner, thereby increasing the elongation at the break of the yarn. In general, within the generally used twist coefficient range, the favorable factors outweigh the unfavorable factors, so as the twist coefficient increases, the elongation at the break of the single yarn increases.

For strands twisted in the same direction, the effect of the twist coefficient on the elongation at the break of the yarn is the same as that of the single yarn. For strands twisted in opposite directions, when the twist coefficient is small, the twisting of the strands means the untwisting of the single yarn, and the average twist amplitude of the strands decreases with the increase of the twist coefficient, so the elongation at break of the strands The twist rate decreases slightly. When the twist coefficient reaches a certain value, the average twist amplitude increases with the increase of the twist coefficient, and the elongation at the break of the strands also increases.

Effect on Yarn Elasticity

The elasticity of the yarn depends on the elasticity of the fiber and the yarn structure, and the yarn structure is mainly formed by twisting the yarn. For single yarn and twisted strands in the same direction, twisting makes the yarn structure tight, the fiber slippage decreases, the stretchability of the fiber increases, and within the general twist coefficient range, the elasticity of the yarn increases with the increase of the twist coefficient.

Influence on yarn luster and hand

For single yarn and co-twisted strands, due to twisting, the fibers on the surface of the yarn are inclined, and the surface of the yarn becomes rough, the luster of the yarn becomes poor, and the handle becomes hard. For counter-twisted strands, when the ratio of the twist coefficient of the strands to the twist coefficient of the single yarn is equal to 0.707, the outer twist width is zero, and the surface fibers are parallel to the axial direction of the yarn. At this time, the luster of the strands is the best and the hand feels soft, the polyester sewing thread produced is also the best. Read More: Yarn Wikipedia