Most of the spandex core-spun yarns are made by adding spandex yarn feeding and drafting mechanism on the traditional ring spinning frame, which is made by ring spinning and twisting. Usually, the spandex thread density and draft ratio are selected according to the yarn linear density. The linear density of yarn is 7.3tex~83.3tex, the linear density of spandex yarn is 22 dtex~78 dtex, and the draft ratio is 2.5 times to 4.5 times.
Due to the pre-draft of the spandex yarn, the corresponding internal stress (elastic potential energy) is stored in the fiber, so that the fiber is at a higher energy level. According to the "least energy principle", the state of spandex filaments in the yarns or fabrics (especially the fabrics before being shaped) is relatively unstable. They are always under their own tension and friction with the outer fiber (in the fabric structure Inner) is a kind of dynamic balance, so the spandex yarn always has a tendency to shrink.