Main processes of spandex covered yarn

　　In the spinning process, the pre-draft ratio of spandex (Spandex Stretch) is an important process parameter, affecting the elasticity of covered yarn (Covered Yarn) and fabrics, as well as the strength, elongation, evenness, and creep performance of covered yarn. If the pre-draft ratio of spandex is too small, the advantage of good elasticity in elastic fabrics cannot be fully utilized. When the draft is too high, spinning becomes difficult, and it is prone to thread breakage, leading to a decline in product quality. Additionally, the percentage content of spandex yarn also affects the elasticity of the covered yarn; a higher content leads to better elasticity. Under certain specifications of spandex yarn, increasing the draft ratio reduces the percentage content of spandex yarn, meaning the pre-draft ratio of spandex is inversely proportional to the percentage content of spandex in the finished yarn.

　　The strength and elongation of air-covered yarn increase with the increase of the spandex pre-draft ratio. After drafting to a certain value, the retraction of the spandex filament causes the outer-wrapped fibers to become crimped. Appropriately increasing the pre-draft ratio reduces the spandex content, leading to a greater degree of crimping of the outer-wrapped fibers. However, when the draft is too large, and the deformation amplitude of the spandex filament approaches the critical value of deformation, the strength and elongation of the covered yarn will decrease, but it is beneficial for the evenness of the covered yarn.

　　The phenomenon where deformation changes over time under a constant tensile force is called creep. Conventional covered yarns show good creep resistance when the spandex filament is pre-drafted 3.5 times. Specific adjustments should be made according to the application and the style of the downstream fabric.

　　For mechanical covered yarn, in addition to the spandex draft ratio requirements for air-covered yarn, another important process parameter is twist. Twist affects yarn quality, strength and elongation, and evenness. Increasing the twist can enhance the cohesion between the outer-wrapped fibers and the spandex filament, thereby improving the strength of the covered yarn. If the twist is too low, the outer-wrapped fibers become loose, affecting the covering effect and leading to core exposure. Therefore, the twist should be relatively high during mechanical covering. However, if the twist is too high, the fabric will feel stiff and have poor drape. Generally, twist needs to be adjusted accordingly with changes in yarn denier and different fabric style requirements. For example, for N40D+20D mechanical covered yarn, a twist of around 600 is preferable, while for N70D+40D or T150D+40D mechanical covered yarn, controlling the twist around 480 is sufficient. The elongation of mechanically covered yarn is also primarily determined by the performance of the spandex core filament. If the twist is too high, and the outer layer fibers are wrapped too tightly, the elastic effect of the spandex filament cannot be fully utilized, leading to a decrease in elongation. As twist increases, the evenness improves. If the twist is too small, the outer-wrapped fibers become loose, and during the spinning process, due to friction and other reasons, the fibers may slightly move along the yarn axis, thus deteriorating the evenness.