Twist Testing Method for Short-Fiber Single Yarns

The number of turns within a unit length of yarn is called twist. Twist is expressed as twists per meter (t/m) or twists per 10 cm (t/10 cm). Twist is an important physical property of yarn. The physical and mechanical properties of yarn are determined by the properties of the constituent fibers and the yarn structure, among which the twisting action is the most important factor affecting yarn structure. Short fibers can form yarns with certain physical and mechanical properties precisely because of twisting. Twist is not only the fundamental condition for yarn formation, winding, and strength development, but is also closely related to yarn strength and elongation at break. Therefore, twist testing must accurately reflect the true inherent twist of the yarn while being simple, fast, and accurate to operate.

Different twist testing methods apply to different yarns. Some yarns can be tested by several methods, but differences among methods may lead to varying results. At present, the main twist testing methods for short-fiber single yarns are the direct untwist method, the single untwist–retwist method, and the double untwist–retwist method. By using these three methods to test the twist of short-fiber single yarns, experiments can be conducted to compare differences in testing procedures and results.

Experimental Section

The twist measurement methods for short-fiber single yarns include the direct untwist method, the single untwist–retwist method, and the double untwist–retwist method. The applicable standards are GB/T 2543.1-2015 “Textiles—Yarns—Determination of Twist—Part 1: Direct Counting Method” and GB/T 2543.2-2001 “Textiles—Yarns—Determination of Twist—Part 2: Untwist–Retwist Method.

Principles of the Methods

The direct untwist method consists of holding a specimen of a specified length under a prescribed tension and rotating one end to remove the twist until the constituent elements of the yarn become parallel. The number of turns required to remove the twist is used to calculate the yarn’s twist.
The untwist–retwist method is an indirect method. In the single untwist–retwist method, the specimen is untwisted and then twisted in the opposite direction until it returns to its original length. It is assumed that the number of reverse twists equals the original twist of the specimen, so half of the number recorded by the counter represents the twist of the yarn. In the double untwist–retwist method, the first specimen is tested according to the single untwist–retwist method. For the second specimen, one-quarter of the twist count obtained from the first specimen is removed, after which the yarn is retwisted to its initial length to correct the error caused by the pre-tension.

Instruments and Testing Environment

Instrument: Y331 yarn twist tester. Conditioning and testing atmospheres follow GB/T 6529-2008 “Standard Atmospheres for Conditioning and Testing of Textiles.”

Testing Plan

Because short-fiber yarns exhibit uneven fineness and uneven twist, in order to avoid differences among samples, two bobbins of yarn are randomly selected from a package of 18.0 tex cotton yarn with a designed twist of 96.5 t/10 cm. These are designated as Sample 1 and Sample 2, tested separately using the direct untwist method, the single untwist–retwist method, and the double untwist–retwist method. The pre-tension is 9.0 cN.
Specimen lengths: 25 mm for the direct untwist method; 500 mm for both the single and double untwist–retwist methods.

Results and analysis cna be found in ​https://fyitester.com/twist-testing-method/.