Yarn levelness (English: yarn levelness) is a textile term, that is, the main thread of yarn and sliver. The uniformity of thickness or weight within a short segment of yarn, sliver or roving along the axial direction is called evenness. The quality of textiles is closely related to yarn evenness. If the spun yarn is not evenly dry, the strength of the yarn will be reduced and affect the strength of the fabric. When weaving with uneven fine yarns, various defects and bars will appear on the fabric, affecting the appearance quality. 1. Impact on textiles When the uniformity of semi-finished products decreases, the uniformity of spun yarns also decreases accordingly; if the spun yarns are not evenly dry, the strength of the yarns will be reduced and affect the strength of the fabric. When weaving with uneven fine yarns, various defects and bars will appear on the fabric, affecting the appearance quality. The requirements for the uniformity of fine yarns in knitting production are generally more stringent than those in weaving. In knitting processing, uneven spun yarn or yarn defects will damage the normal knitting process and sometimes cause needle breakage. In the manufacture of tire cord, excessive unevenness of the spun yarns will cause spiral defects during the production process, that is, adjacent yarns are entangled with each other, thus affecting both the processing process and product quality. In addition, uneven spun yarn will increase the breakage rate in spinning and weaving, and even reduce labor productivity. 2. Reasons for unevenness ① Uneven yarns are caused by differences in the properties of fiber raw materials. Various natural fibers are uneven in length, fineness or other properties. Processing with the same machinery and technology will cause differences and cause unevenness. ② Uneven yarns occur due to random arrangement of fibers. According to the principle of short fiber spinning, the ideal yarn can be assumed to be composed of randomly arranged fibers. This randomly arranged yarn has a certain unevenness, which is called random unevenness. The value is the same as the average number of fibers in the cross section of the yarn. is inversely proportional to the square root of , and random unevenness will increase when the number of fiber roots is small. ③Uneven yarn slivers occur due to poor selection of spinning process parameters. For example, improper selection of process parameters such as drafting mechanism distance and pressure will lead to poor control of fiber movement, resulting in thick and thin joints, resulting in uneven thickness. ④ Uneven yarn sliver caused by defects in spinning machinery. For example, roller or top roller eccentricity, gear defects, etc. will cause obvious periodic unevenness in the yarn, often showing undulating wavy changes in thickness. Shorter wavelengths are called short segment unevenness, and long lengths are called long segment unevenness. Generally, the short segment unevenness produced by the pre-spinning machine is due to the longer draft, and long segment unevenness appears in the yarn; the unevenness produced by the spinning frame is short segment unevenness. 3. Evenness determination method There are three main methods for measuring evenness unevenness in spinning production, including the section weighing method, the blackboard evenness visual inspection method and the instrumental detection method. The cutting and weighing method can be used for each semi-finished product and spun yarn; the blackboard dry visual inspection method is mainly used for spun yarn; the instrumental detection method can be divided into capacitive detection and mechanical detection. Capacitive detection is suitable for slivers, rovings and spun yarns, while mechanical detection is only suitable for slivers and rovings. ① Cutting and weighing method: Cut the yarn into sections according to the specified length, weigh them separately, and then calculate the unevenness. The length and number of fragments taken depend on the actual production situation and test accuracy requirements. The disadvantage of this method is that it is time-consuming and is only suitable for measuring the weight unevenness of longer segments of pre-spun semi-finished products with lower count. ② Blackboard evenness visual inspection method: It is a method commonly used in production to check and evaluate the evenness level of spun yarn. By winding the spun yarn evenly around a rectangular (or trapezoidal) blackboard at equal intervals, you can visually analyze the uneven composition of the spun yarn and rate the evenness of the spun yarn against the standard sample. Grading standards and sampling assessment methods vary from country to country. The blackboard visual inspection method has certain requirements for blackboard specifications, lighting during inspection, and observation distance. ③Instrumental detection method: The capacitive evenness tester is suitable for testing the evenness unevenness of slivers, rovings and spun yarns spun from various short fibers. For filament, a false twisting device must be installed to eliminate the “cross-section effect” of the yarn (that is, the detection error caused by the variation of the cross-sectional shape of the yarn between the detection capacitor slots). The test method is to pass the yarn through the middle of the capacitor plate. The thickness of the yarn segments causes capacitance changes, and the unevenness of the yarn is calculated through the electronic circuit. When applying a capacitive uniformity tester, samples with high humidity or uneven humidity should be avoided to avoid excessive test errors. Generally, the sample is first subjected to humidity balance under standard temperature and humidity conditions (temperature 20±3°C; relative humidity 65±3%). When applying the capacitive evenness meter to test yarn unevenness, it can simultaneously measure the details, thick spots and number of knots in the spun yarn. In addition, it can also conduct spectral analysis on the composition of yarn unevenness and draw a spectrum chart to display Significant periodic unevenness in the yarn sliver. According to the spectrum, the causes of defects in each process can be found and improved or eliminated. The unevenness of the yarn measured by the capacitive uniformity meter can be expressed by the average difference coefficient unevenness U% value or the mean square error coefficient unevenness CV% value. The mechanical evenness tester is suitable for testing the evenness of slivers and rovings. Feed the sliver or roving into a groove of a certain specification, apply a certain pressure on it, and measure the thickness change of the sliver. Unevenness is often expressed as the average range coefficient per meter of yarn. Because the range coefficient cannot represent the composition of the uneven structure of the yarn, it is rarely used. 4. The yarn is not�Improvement, the minimum surface distance of the front middle roller can reach 16.5mm. We have improved the roller seat of the FA507 spinning machine. The minimum surface distance is changed to 17.0mm, which greatly shortens the length of the floating area in the front area, which is beneficial to Improve the level of evenness level and evenness CV value difference rate. Theoretically, when the raw material and drafting mechanism are constant, there is an approximately linear relationship between the roller holding distance and the additional drafting unevenness. Therefore, reducing the roller holding distance and shortening the floating zone can move the speed change point of the drafting zone forward. The closer jaws will help improve the evenness of the yarn. It is difficult to measure the roller holding distance during production, so the roller surface distance is changed to adjust the roller holding distance and conduct process optimization tests.
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