Spinning short fibers into continuous yarn requires twisting. The synthetic fibers aggregate together to maintain a certain cohesion, thereby giving the yarn a certain strength. The filaments in composite yarn also need to be lightly twisted. If there is no cohesive force to connect them together, they will fall apart or be scratched during use. In addition, according to the needs of different uses, several single yarns can be combined and twisted to form strands and cables, making them more uniform and structurally stable to withstand higher loads. If two or more yarns are fed at different speeds and tensions during twisting, or yarns of different colors or fancy patterns are combined and twisted, a fancy thread can be formed. Twisting is a method of bringing fiber bundles, filaments or single yarns together so that the textile can withstand stress, strain and friction during manufacturing and use, and gives the textile a unique appearance.
01 The concept of twist
Twist not only affects the tension of the yarn In addition to the elongation properties, the twist size also affects the diameter, specific volume, softness and stiffness of the yarn, and affects the covering performance, warmth retention, crease recovery ability, dye absorption rate and permeability of the fabric. Twisting also affects the hairiness of the yarn surface. Therefore, twist is an important indicator of the structural characteristics of the yarn.
In addition to using twist and twist coefficient to express the twisting characteristics of the yarn, the twist direction in the yarn is also very important. The twist direction refers to the fibers or fibers in the single yarn after twisting. The slanted direction exhibited by individual yarns in a strand. There are two types of twist directions in yarn: S twist or clockwise twist; Z twist or counterclockwise twist. Single yarns generally use Z twist, while S twist is often used for plied yarns.
02 Twist measurement method
The commonly used twist test methods in my country are: There are two types: direct counting method and untwisting and twisting method. Short fiber and strand tests generally use the direct counting method, while spun yarns use the untwisting and twisting method. In addition, there are secondary and triple untwisting and twisting methods, slipping methods, etc.
2.1 Direct counting method
One end of the sample is fixed, and the other end Rotate in the untwisting direction until the fibers in the yarn are completely straightened and parallel. The untwisted twist is the number of twists in the length of the sample. The direct counting method is the most basic method for measuring yarn twist. The measurement results are relatively accurate and are often used as a standard to evaluate the accuracy of other methods.
However, this method has low efficiency. If the fibers in the yarn are kinked, the fibers will not be easily decomposed into parallel lines, and the decomposed fibers will make the yarn easy to break. The direct counting method is generally used to count the number of twists of rovings or strands, while the black and white yarn point method can be used to test spun yarns, that is, a black roving and a white roving are fed into the same spindle to spin black and white spun yarns, and the sample is clamped. Hold it on a twist meter and perform untwisting in the same way as the direct counting method. As the untwisting increases, the distance between black and white on the yarn becomes larger, making it easier to count manually. Generally, after untwisting to about half of the total twist number, the remaining twist number of manual points can be entered. The total twist number on the yarn is the sum of the untwist number of the twist meter and the remaining twist number of manual points.
2.2 Untwisting and twisting method
Untwisting and twisting method It is tested under the assumption that under a certain tension, the yarn elongation after untwisting is the same as the yarn shortening when reverse twisting. A typical test setup is shown in the figure.
03 Factors affecting twist test
3.1 Pre-tensioning
There are different regulations at home and abroad for the pre-tensioning of the untwisting and twisting method. Excessive pre-tension will cause the pointer to slowly return to zero during reverse twisting, thereby increasing the twist in reverse twisting and causing fiber slippage in advance in the slip method; conversely, if the pre-tension is too small, reverse twisting will occur. The degree of twist will be relatively reduced during twisting, resulting in lower values during the test using the untwisting and twisting method. In the slip method and the secondary untwisting and twisting method, since the error caused by this tension change is taken into account, the method of subtracting the errors of the two samples is adopted, so the influence of tension is small.
3.2 Allowable elongation value
Test after untwisting the yarn The allowable elongation value set by the elongation limit prevents the slippage of yarn fibers during the untwisting process. Experiments show that in the untwisting and twisting method test, the larger the allowable elongation value, the greater the measured twist value. At the same time, if the allowable elongation value is too large, the fiber slippage at the weak ring of the yarn will significantly affect the accuracy of the experiment. sex. The allowable elongation value is related to the twist coefficient. The larger the twist coefficient of the yarn, the larger the allowable elongation value can be; conversely, it should be smaller. Usually the twist coefficient of the yarn changes within a small range, so the difference in allowable elongation values will not be too large.
3.3 Uneven yarn dryness
Yarn dryness Unevenness will have an impact on twist testing. If there are defects such as neps and slubs in the yarn, reverse twisting will begin before the original twist on the yarn is fully detwisted, which will increase the test error. If there are long uneven segments on the yarn, it will affect the accuracy of the applied tension and eventually cause additional testing errors.
Conclusion
In the yarn twist test, if the data deviates The above three factors that affect the test can be used as a reference to find out the reasons. It is not ruled out that there are human factors that cause the data to be inaccurate. No matter what kind of test, the inspector must…�Really careful to ensure the scientific accuracy of the data. </p
