Twist per inch

TPI (twists per inch or turns per inch) is a term used in the textile industry. It measures how much twist a yarn has, and can be calculated by counting the number of twists in an inch of yarn.

Twist is needed in yarn to hold the fibres together, and is added in both the spinning and plying processes. The amount of twist varies on the fibre, thickness of yarn, preparation of fibre, manner of spinning, and the desired result. Fine wool and silk generally use more twist than coarse wool, short staples more than long, thin more than thick, and short drawn more than long drawn.

The amount of twist in a yarn helps to define the style of yarn- a yarn with a lot of air such as a woollen yarn will have much less twist than a yarn with little air, like a worsted yarn. It also affects the stretchiness of the yarn, strength, the halo of the yarn, and many other attributes. Filling or weft yarns usually have fewer twists per inch because strength is not as important as with warp yarns, and highly twisted yarns are, in general, stronger. Warp yarns have to be stronger so that they can withstand the tension of the loom. Filament fibers, such as silk, or many synthetics, need only to be twisted slightly to create a yarn.

Handspinners use the number of twists per inch often. Because the amount of twist defines a lot about a yarn, the number of twists per inch is an important measure to recreate a yarn. As a spinner spins, they will often stop every few minutes to check to see that the number of twists per inch is the same throughout the yarn, as well as that the number of wraps per inch (the thickness of the yarn) is the same. Measuring the number of twists per inch while spinning singles also helps the spinner create a balanced yarn when plying. Yarns that have relatively few twists per inch tend to have a softer hand but are not as strong as yarns with more twists per inch, such as medium twist or hard twist yarns. Yarns that have a very hard twist, enough so that the yarn will double back onto itself when released from tension, are called crepe-twist yarns.

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