underspun

Underspun yarn has received fewer twists per inch (TPI) than the specification calls for, leaving the fibers insufficiently bonded along the length of the strand. This creates a yarn that is mechanically weak, prone to breaking during weaving or knitting, and prone to shedding loose fibers during fabric formation. The problem shows up immediately on the loom or knitting frame as frequent thread breaks and higher waste rates.

Twist is the foundational property that converts loose fibers into usable yarn. During spinning, fibers are drafted to the required linear density while simultaneously rotated around their own axis; this rotation is what creates twist. A yarn is underspun when the number of rotations falls below target, typically defined in turns per inch or, in metric systems, turns per meter. The severity ranges from marginally low (which may only cause problems under tension) to extremely low (where the yarn barely holds together at rest).

Underspun yarn results from spinning frame malfunction or operator error: a worn or slipping spindle bearing, incorrect frame speed, or a twist lever set too low. The most reliable detection happens with a twist tester, which untwists a yarn sample and measures the actual TPI; visual inspection can catch severe cases, where the yarn appears fuzzy or shows visible fiber separation. Microscopic examination reveals fibers that lie at a shallow angle rather than tightly helixed around the yarn core.

The name reflects the simple mechanical reality: the yarn has received less spinning than required. In the broader production chain, underspun yarn is a quality defect that compounds downstream; a weaver cannot correct inadequate twist, only accept slower production or higher breakage. The relationship between twist and yarn properties is not linear: modest underspinning may reduce breaking strength by 10 to 20 percent, while severe underspinning can halve it.