Twisting: If one end of the spinning yarn is held and the other end is turned, a yarn is formed, a process called twisting.
Twisting design affects the physical and mechanical properties and appearance of tow, yarn, and thread, and then has a great impact on the properties, appearance, and use value of fabrics.
For staple yarns, twisting is a necessary means for yarns to obtain strength and other characteristics; for filament yarns and strands, twisting can form a compact structure that is not easily damaged by lateral external forces. Twisting can also form textured yarn and fancy yarn. The amount of twisting and the direction of twisting not only affect the feel and appearance of the fabric, but also affect the inner quality of the fabric.
1. The purpose and requirements of twisting
Twisting is a necessary means to make fiber strips into yarns. Before twisting, it is generally necessary to condense loose fibers into fiber strips. After twisting, the outer layer fibers of the fibers are extruded to the inner layer to generate centripetal pressure, so that the strands along the The length direction of the fiber acquires friction.
Requirements: obtain the best strength, elongation, elasticity, flexibility, gloss and hand feeling of the yarn; diversify the structure of the yarn; improve the twisting efficiency of the yarn.
2. The indicators of yarn twisting degree include twist degree, twist angle, twist amplitude and twist coefficient. The index indicating the direction of twist is called twist direction.
Twist: The two sections of the yarn produce a 360° angular displacement, which becomes a twist, which is commonly referred to as a turn.
The number of twists per unit length of yarn is called twist. my country's cotton-type yarn adopts the twist of the special number system, which is expressed by the number of twists within a yarn length of 10 cm; the twist of the combed wool yarn and chemical fiber filament adopts the metric count system, which is expressed by the number of twists per meter; in addition, There is also an imperial count system twist expressed in the number of twists per inch.
Twisting angle: Before twisting, the fibers in the yarn are parallel to each other, and after twisting, the fibers are inclined. The greater the degree of twisting of the yarn, the greater the inclination of the fiber, and the degree of twisting can be expressed by the inclination angle of the fiber in the yarn—the twist angle β.
Two yarns with the same twist have different degrees of twist due to their different thicknesses. The thicker yarn has a greater degree of twist and a larger twist angle β.
Twist amplitude: If the cross section of the yarn is regarded as a circle, the angles between the fibers at different radii and the axial direction of the yarn are different.
Twist coefficient: Twist cannot be used to compare the degree of twisting of yarns of different thicknesses, because with the same twist, the degree of inclination of the fibers of thick yarns is greater than that of thin yarns. In actual production, the twist coefficient is often used to indicate the degree of twisting of the yarn. Twist coefficient is the relative value of yarn twisting degree combined with linear density, which can be used to compare the twisting degree of different thickness yarns. The twist coefficient can be calculated according to the twist of the yarn and the linear density of the yarn.
Twist direction: The twist direction refers to the inclined direction of the fibers in the single yarn or the single yarn in the strand after the yarn is twisted. It is divided into Z twist and S twist. After twisting, the twist direction of the yarn is from the lower right corner to the upper left corner, and the inclination direction is consistent with the middle of "S", which is called S twist or hand twist; the twist direction of the yarn is from the lower left corner to the upper right corner, and the inclination direction is the same as "S". The middle part of Z" is called Z twist or backhand twist. Generally, single yarn often adopts Z twist, and ply yarn adopts S twist.
The twist direction of the strands is represented by the twist direction of successive twists. For example, if the single yarn is Z twist, the primary twist is S twist, and the double twist is Z twist, the twist direction is represented by ZSZ.
The twist direction of the yarn has a great influence on the appearance and feel of the fabric. Using the twist direction of the warp and weft yarns to cooperate with the fabric structure, fabrics with different styles such as appearance and feel can be woven.
In the plain weave fabric, if the warp and weft yarns are twisted in the same direction, the fabric formed will be stronger, but the gloss will be poor and the handle will be harder. For twill weave fabrics, if the twist direction of the yarn is opposite to that of the twill lines, the twill lines will be clear and full.
Z-twisted yarns and S-twisted yarns are arranged at intervals in the fabric, which can obtain hidden grid and hidden stripe effects. Z-twisted yarn and S-twisted yarn are combined and twisted to form wrinkle effect and so on.
How to judge the twist direction of a yarn:
The left hand is fixed, and the direction of screwing the screw with the right hand is S twist.
The left hand is fixed, and the direction of turning the screw with the right hand is Z twist;
Twist shrinkage: After twisting, the length of the yarn is shortened due to the inclination of the fibers, resulting in twist shrinkage. The size of the twist shrinkage is usually expressed by the twist shrinkage rate, that is, the percentage of the difference in the length of the sliver before and after twisting to the length before twisting.
The size of the twist shrinkage directly affects the linear density and twist of the spun yarn, which must be considered in the design of the spinning and twisting process. The twist shrinkage of cotton yarn is generally 2%~3%. The size of the twist shrinkage is not only related to the twist coefficient, but also related to spinning tension, workshop temperature and humidity, yarn thickness and other factors.
3. True twist twisting and false twist twisting
(1) True twist
After the true twist is obtained on the sliver, the outer fiber will produce an inclined helical twist, the fiber will be twisted and deformed, and the sliver will be tightly entangled, changing the structural shape and mechanical physical properties of the fiber collective.
When there is an enveloping angle to the sliver, the fiber has a centripetal pressure on the sliver, and the larger the encircling angle, the greater the centripetal pressure. Due to the existence of centripetal pressure, the outer layer of fibers is squeezed towards the inner layer, increasing the tightness of the sliver and the friction between the fibers, thus changing the structural shape and physical and mechanical properties of the sliver, which is true twist The essence of yarn.
The above-mentioned twist degree, twist angle, twist width, twist coefficient, twist direction and twist shrinkage are indicators of true twist twist degree.
(2) False twist
Hold the two ends of the multifilament, and twist through the twisting device in the middle of the two ends of the multifilament, and the multifilament will get the twist diagram with the opposite twist direction and the same number at the two ends of the twisting device: one end is S twist, and the other is twisted. One end is Z twist. And the twist sum of the whole yarn is 0.
In layman's terms, it is to fix the two ends of a rope and twist it in the middle. One side is twisted tighter and tighter, and although the other side is twisted tighter and tighter, the twist direction is opposite. Therefore, the total number of twists on the rope has not changed, it is still 0, so the added is called false twist.