Twisting is a key step in the spinning process, and the degree of twist-quantified as "twist level"-is a crucial indicator of yarn performance. Twist level is defined as the number of twists (or helical turns) per unit length of yarn. The specific unit length used for measurement varies by yarn type: "twists per 10 cm" is typically used for cotton and cotton-type chemical fiber yarns, while "twists per meter" is used for worsted wool yarns and chemical filament yarns. Twist level influences properties such as yarn strength, stiffness/flexibility, elasticity, and shrinkage.
While yarn strength generally increases with twist level, there is a limit beyond which strength actually declines; this threshold is known as the "critical twist." Critical twist levels vary depending on the raw material. Twist level also closely affects yarn softness, elasticity, and shrinkage. Generally, provided strength requirements are met, a lower twist level is preferable; higher twist levels tend to make the yarn feel stiffer, reduce elasticity, and increase shrinkage-which is why filament yarns are typically produced with little to no twist.
Additionally, twist level affects yarn bulk density and diameter, as twisting increases yarn compactness. Within a certain range, increasing the twist level raises the yarn's bulk density and reduces its diameter, thereby altering fabric characteristics such as coverage and comfort.
In summary, the appropriate twist level should be selected based on the intended use of the fabric. For instance, warp yarns require high strength and thus benefit from a higher twist level, whereas weft yarns and knitting yarns need to be soft and should have a lower twist level. Yarns for woven or knitted pile fabrics require lower twist levels to facilitate the raising of the pile. Conversely, lightweight, crisp crepe fabrics-which require a smooth, stiff, and cool-to-the-touch feel-call for yarns with a higher twist level.