In the spinning process, the spandex stretch is an important process parameter, which affects the elasticity of the covered Yarn and fabric, as well as the strength, elongation, evenness, and creep properties of the covered yarn. The pre draft ratio of spandex is too small to fully exploit the advantages of good elasticity of elastic fabrics. When the draft is too high, it is difficult to spin, and it is easy to cause yarn breakage, resulting in a decline in product quality; In addition, the percentage content of spandex fiber also affects the elasticity of the covered yarn, with high content and good elasticity. Under certain conditions of spandex fiber specifications, increasing the drafting ratio decreases the percentage content of the spandex fiber, which means that the pre drafting ratio of the spandex fiber is inversely proportional to the percentage content of the spandex fiber in the yarn.

The strength and elongation of air wrapped yarn increase with the increase of pre draft ratio of spandex. After the draft reaches a certain value, the retraction property of spandex yarn causes the wrapped fiber to be in a curly state; Increasing the pre draft ratio appropriately will reduce the percentage content of spandex and increase the degree of crimping of the wrapped fiber. However, when the draft is too large, the deformation amplitude of the spandex fiber is close to the critical value of deformation, which will reduce the strength and elongation of the covered yarn, but is beneficial to the evenness of the covered yarn.

The phenomenon of deformation changing with time under constant tensile force is called creep. Conventional coated yarns exhibit good creep resistance when pre drawn by 3.5 times the spandex filament. Specific adjustments should be made according to the usage and the style of the downstream fabric.

In addition to the requirements for spandex drafting ratio in air covered yarns, there is also an important process parameter for mechanical yarn covering, which affects yarn quality, strength, elongation, and evenness. Increasing the twist can increase the cohesion between the wrapped fiber and the spandex yarn, making the wrapped yarn stronger. The twist is too low and the outer fiber is loose, which affects the coating effect and causes core exposure. Therefore, the twist should be high during mechanical coating. However, the twist is too high, the fabric feels stiff and has poor drapability. Generally, the twist should be adjusted accordingly depending on the yarn denier number and the style requirements of the fabric. For example, for N40D+20D machine bags, the twist should be around 600 twists, while for N70D+40D or T150D+40D machine bags, the twist should be controlled around 480. The elongation of the machine bag is also mainly determined by the performance of the polyurethane core yarn. If the twist is too large and the outer fiber is wrapped too tightly, the elastic effect of the polyurethane yarn cannot be fully exerted, resulting in a decrease in elongation. As the twist increases, the evenness of the yarn is improved. The twist is too small and the outer fiber is loose. During the spinning process, due to friction and other reasons, the fiber will slightly move along the axial direction of the yarn, resulting in deterioration of the evenness.