Why do yarns need to be twisted? What are twisting and false twisting?

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Twisting: If one end of the sliver 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 the tow, yarn, and line, and thus has a great impact on the properties, appearance, and use value of the fabric.

For short-staple yarns, twisting is a necessary means for the yarn to obtain strength and other characteristics; for filament yarns and ply yarns, twisting can form a compact structure that is not easily destroyed by lateral external forces. Twisting can also form deformed yarns and fancy yarns. The amount of twisting and the direction of twisting not only affect the feel and appearance of the fabric, but also the intrinsic quality of the fabric.

Purpose and requirements of twisting

Purpose:

Twisting is a necessary means to make fiber strips into yarns. Before twisting, the loose fibers generally need to be condensed into fiber strips. After twisting, the outer layer of the fiber is squeezed toward the inner layer to generate centripetal pressure, so that the slivers obtain friction along the length of the fiber.

Requirements:

The yarn has the best properties such as strength, elongation, elasticity, flexibility, gloss and feel; the structure and morphology of the yarn are diversified; the twisting efficiency of the yarn is improved.

Indicators indicating the degree of yarn twist

Indicators indicating the degree of yarn twist include twist, twist angle, twist width and twist coefficient. The indicator indicating the twisting direction is called twist direction.

Two cross sections of the yarn produce an angular displacement of 360°, which becomes a twist, which is usually called a turn.

The number of twists per unit length of the yarn is called twist. Chinese cotton yarn uses special number system twist, that is, it is expressed by the number of twists within 10 cm yarn length; combed wool yarn and chemical fiber filament use metric count system twist, that is, it is expressed by the number of twists per meter; in addition, there is also British count system twist expressed by the number of twists per inch.

Two yarns with the same twist have different twisting degrees due to different thicknesses. The thicker yarn has a larger twisting degree and a larger twist angle β.

If the cross section of the yarn is regarded as a circle, the angle between the fiber at different radii and the yarn axis is different. In order to express this situation, the twist width is introduced.

Twist cannot be used to compare the degree of twisting of yarns of different thicknesses, because the fiber inclination of thick yarns is greater than that of thin yarns with the same twist. In actual production, the twist coefficient is often used to indicate the degree of twisting of the yarn. The twist coefficient is a relative value that combines the linear density to indicate the degree of twisting of the yarn, which can be used to compare the degree of twisting 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.

Twist direction refers to the tilt direction of the fiber in a single yarn or a single yarn in a ply yarn after the yarn is twisted. It is divided into Z twist and S twist. After twisting, the twist direction of the yarn tends from the lower right corner to the upper left corner, and the tilt direction is consistent with the middle of the "S" is called S twist or hand twist; the twist direction of the yarn tends from the lower left corner to the upper right corner, and the tilt direction is consistent with the middle of the "Z" is called Z twist or backhand twist. Generally, single yarns are often twisted in Z twist, and ply yarns are twisted in S twist.

The twist direction of ply yarns is expressed by the twist direction of twisting in sequence. For example, a ply yarn with Z twist, S twist in primary twist, and Z twist in secondary twist is expressed as ZSZ.

The twist direction of yarn has a great influence on the appearance and feel of the fabric. By combining the twist direction of warp and weft yarns with the fabric structure, fabrics with different styles such as appearance and feel can be woven.

In plain weave fabrics, if the warp and weft yarns use yarns with the same twist direction, the resulting fabric will be stronger, but the gloss will be poor and the feel will be harder.

For twill weave fabrics, if the yarn twist direction is opposite to the twill line direction, the twill line will be clear and full. Z-twisted yarns and S-twisted yarns are arranged alternately in the fabric to obtain hidden grid and hidden stripe effects. Z-twisted yarns and S-twisted yarns are twisted together to form a wrinkling effect.

After twisting, the length of the yarn is shortened due to the inclination of the fiber, resulting in twist shrinkage. The size of the twist shrinkage is usually expressed by the twist shrinkage rate, that is, the difference in the length of the yarn before and after twisting accounts for the percentage of the length before twisting.

The size of the twist shrinkage 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 rate of cotton yarn is generally 2%~3%. The size of the twist shrinkage rate is related to the twist coefficient, as well as factors such as spinning tension, workshop temperature and humidity, and yarn thickness.

True twist twisting and false twist twisting

After the true twist is obtained on the sliver, its outer fiber will produce an inclined spiral twist, the fiber will twist and deform, and the yarn will be tightly held, changing the collective structural morphology and mechanical and physical properties of the fiber.

When there is an enveloping angle on the yarn, the fiber will have a centripetal pressure on the yarn. The larger the enveloping angle, the greater the centripetal pressure. Due to the existence of the centripetal pressure, the outer fiber is squeezed toward the inner layer, increasing the compactness of the yarn and the friction between the fibers, thereby changing the structural morphology and physical and mechanical properties of the yarn. This is the essence of true twist yarn.

Hold the two ends of the multifilament and twist it through the twisting device in the middle of the two ends of the multifilament. The multifilament will get the same twist diagram with opposite twist directions at both ends of the twisting device: S twist at one end and Z twist at the other end. The sum of the twists 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, while the other side is also twisted tighter and tighter, but the twist direction is opposite. Therefore, the total twist number on the rope does not change and remains 0, so the twist is called false twist.