C. Twist. If the yarn section is regarded as a circle, the angle between the fiber and the axial direction of the yarn at different radii is different. In order to show this situation, the twist index is introduced.
D. Twist factor. Twist can not be used to compare the degree of twist of different yarns, because the inclination of coarse yarn is greater than that of fine yarn with the same twist. In practical production, twist factor is often used to express the degree of twist of yarn. Twist factor is the relative value of yarn twist degree combined with linear density, which can be used to compare the twist degree of different thickness yarns. Twist factor can be calculated according to yarn twist and yarn linear density.
E. Twist direction. Twist direction refers to the inclined direction of single yarn in single yarn or single yarn in strand after twisting. It can be divided into Z-twist and S-twist. After twisting, the twisting direction of the yarn from the lower right corner to the upper left corner and the inclination direction consistent with the middle of "s" is called S-twist or free hand twist; the twisting direction of the yarn from the lower left corner to the upper right corner and the inclination direction consistent with the middle of "Z" is called Z-twist or backhand twist. In general, single yarn usually adopts Z-twist and strand yarn adopts S-twist. The twist direction of strands is indicated by the twist direction of successive twists. For example, if the single yarn is Z-twist, the first twist is S-twist and the second twist is Z-twist, the twist direction is represented by zsz. The twist direction of yarn has a great influence on the appearance and handle of fabric. By combining the twist direction of warp and weft yarn with fabric structure, fabrics with different appearance and handle can be woven. In plain weave fabrics, if the warp and weft yarns of the same twist direction are used, the fabric will have greater strength, but poor luster and hard handle. If the direction of yarn twist is opposite to that of twill, the twill is clear and full. When Z-twist yarn and S-twist yarn are arranged at intervals in the fabric, the effects of hidden lattice, hidden stripe and hidden twill can be obtained. The combination of Z-twist yarn and S-twist yarn can form wrinkle effect. How to judge the twist direction of a yarn? It's very simple. When the left hand is fixed, the direction of the right hand is S-twist; when the left hand is fixed, the direction of the right hand is Z-twist.
F. Twist and shrink. After twisting, the length of yarn is shortened due to the inclination of fiber, resulting in twist shrinkage. The twist shrinkage is usually expressed by the twist shrinkage ratio, that is, the percentage of the difference between the length before and after twisting in the length before twisting. The twist shrinkage directly affects the yarn density and twist, which must be considered in the process design of spinning and twisting. The twist shrinkage of cotton yarn is generally 2-3%. The twist shrinkage is not only related to twist factor, but also related to spinning tension, workshop temperature and humidity, yarn thickness and other factors. Twist shrinkage can make the fabric produce different texture and style, and it is also a common means of textile fabric innovation.
2、 True twist and true twist.
1. True twist. After the true twist of the fiber bundle is obtained, the outer layer of the fiber will twist back, the fiber will twist and deform, and the yarn will close together, which will change the structure and mechanical and physical properties of the fiber collective.
When there is an encircling angle, the fiber has centripetal pressure on the bundle. The larger the encircling angle is, the greater the centripetal pressure is. Due to the existence of centripetal pressure, the outer fiber is squeezed to the inner layer, which increases the yarn tightness and the friction between the fibers, thus changing the yarn structure and its physical and mechanical properties. This is the essence of true twist yarn.
2. False twist. False twist yarn is often used in textile fabric design. Hold the two ends of the multifilament and twist them in the middle of the two ends of the multifilament through the twisting device. The multifilament gets the same number of twists in opposite directions at both ends of the twisting device: one end is S-twist, the other end is Z-twist. The total twist of the whole yarn is 0. Generally speaking, the two ends of a rope are fixed and twisted in the middle. One side is more and more tightly twisted, while the other side is also more and more tightly twisted, but the direction of twisting is opposite. Therefore, the total number of twists on the rope does not change, it is still 0, so the added twists are called false twists. (as shown in the figure below)
