If one end of the spinning end is held and the other end is turned, yarn can be formed. This process is called twisting.
Twisting design affects the physical and mechanical properties and appearance of tows, yarns and threads, which in turn has a great impact on the properties, appearance and use value of fabrics.
For short fiber yarns, twisting is a necessary means for the yarn to obtain strength and other properties; for filament yarns and strands, twisting can form a layer that is not easily Tight structures destroyed 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 intrinsic quality of the fabric.
The purpose and requirements of twisting
The purpose of twisting is to turn fiber strips into yarns The necessary means is that before twisting, the loose fibers generally need to be condensed into fiber strips. After twisting, the outer fibers of the fiber are squeezed into the inner layer to generate centripetal pressure, so that the strips obtain friction along the length of the fiber.
Requirements: The yarn should obtain the best strength, elongation, elasticity, flexibility, gloss and feel; the structure of the yarn should be diversified; and the twisting efficiency of the yarn should be improved.
Indicators indicating the degree of twisting of the yarn
Indicators indicating the degree of twisting of the yarn are Twist, twist angle, twist width and twist coefficient. The index indicating the direction of twist is called twist direction.
The two cross sections of the yarn produce an angular displacement of 360°, which becomes a twist, which is commonly known as a turn.
The number of twists per unit length of yarn is called twist. my country’s cotton yarn uses a special number system for twist, which is expressed by the number of twists within 10 cm of yarn length; combed wool yarn and chemical fiber filament use a metric system of twist, that is, expressed by the number of twists per meter; in addition, There is also an imperial count system of twist expressed as turns per inch.
Before twisting, the fibers in the yarn are parallel to each other. After twisting, the fibers become tilted. The greater the degree of twisting of the yarn, the greater the tilt of the fiber. The degree of twist can be expressed by the tilt angle of the fiber in the yarn – the twist angle β.
Two yarns with the same twist have different twisting degrees due to different thicknesses. The thicker yarn has a greater degree of twisting and a larger twist angle β.
If the yarn cross-section is regarded as a circle, then the fibers at different radii are in the axial direction of the yarn The included angles are different. In order to express this situation, the twist width index is introduced.
Twist cannot be used to compare the degree of twist of yarns of different thicknesses, because with the same twist, the fibers of thicker yarns have a greater degree of inclination than Thin yarn strips. In actual production, the twist coefficient is often used to express the degree of twist of the yarn. The twist coefficient is a relative value that combines linear density to express the twist degree of yarn, and can be used to compare the twist degree of yarns of different thicknesses. The twist coefficient can be calculated based on the twist of the yarn and the linear density of the yarn.
The twist direction refers to the inclination of the fibers in the single yarn or the single yarn in the strand after the yarn is twisted. direction. It is divided into two types: Z twist and S twist. After twisting, the twisting direction of the yarn is from the lower right corner to the upper left corner, and the tilting direction is consistent with the middle of the “S”, which is called S twisting or smooth twisting; the twisting direction of the yarn is from the lower left corner to the upper right corner, and the tilting direction is consistent with the “S” If the middle part of “Z” is consistent, it is called Z twist or backhand twist. Generally, single yarns often use Z twist, and strands use S twist.
The twist direction of the strands is represented by the twist directions of successive twists. For example, if the single yarn is Z-twisted, the first twist is S-twist, and the second-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. By matching the twist direction of the warp and weft yarns with the fabric structure, we can weave fabrics with different styles such as appearance and feel. Fabric.
In plain weave fabrics, if the warp and weft yarns use yarns with the same twist direction, the resulting fabric will have greater strength, but poor gloss and a harder feel. For twill weave fabrics, if the yarn twist direction is opposite to that of the twill line, the twill line will be clear and full.
Z-twisted yarns and S-twisted yarns are arranged at intervals in the fabric to obtain hidden grid and hidden stripe effects. Z-twisted yarn and S-twisted yarn are combined and twisted to create wrinkle effects, etc.
How to judge the twist direction of a yarn
The left hand is fixed, and the direction of tightening the screw with the right hand is S twist
Fixed with the left hand, the direction of counter-tightening the screw with the right hand is Z twist
After twisting, due to the tilt of the fiber, the yarn The length is shortened, causing twisting. The size of twist shrinkage is usually expressed by the twist shrinkage ratio, that is, the difference in yarn length before and after twisting accounts for the percentage of the length before twisting.
The twist rate directly affects the linear density and twist of the spun yarn, and must be considered in the design of spinning and twisting processes. 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 factors such as spinning tension, workshop temperature and humidity, and yarn thickness.
True twist plus twist and false twist plus twist
After the true twist is obtained on the slivers, the outer fibers will produce inclined spiral twists, the fibers will twist and deform, and the yarn slivers will tightly embrace, changing the overall structural form and mechanical physics of the fibers.Properties are shown in Figure 9-3 (A).
When there is an enveloping angle for the yarn sliver, the fibers will have centripetal pressure on the yarn sliver. The larger the enveloping angle, the greater the centripetal pressure. Due to the existence of centripetal pressure, the outer layer of fibers is squeezed into the inner layer, which increases the tightness of the yarn and the friction between the fibers, thereby changing the structural form of the yarn and its physical and mechanical properties. This is true twist. The essence of yarn.
The above-mentioned twist degree, twist angle, twist width, twist coefficient, twist direction and twist shrinkage are all indicators of the degree of true twist and twist.
Hold both ends of the multifilament and twist it through the twisting device in the middle of the two ends of the multifilament. The two ends of the twisting device get twist diagrams with opposite twist directions and the same number: one end is S twist and the other end is Z twist. The sum of the twists of the entire yarn is 0.
In layman’s terms, the two ends of a rope are fixed and twisted in the middle. One side is twisted tighter and tighter, while the other side is also twisted tighter and tighter, but in the opposite direction. Therefore, the total number of twists on the rope has not changed and is still 0, so the added twist is called false twist.