About cotton yarn drying

Yarnlevelness (English: yarnlevelness) 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. There are inhomogeneities in the length, fineness or other properties of various natural fibers. Processing by 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 are caused by 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 the short segment unevenness.

3. Methods for measuring evenness

Commonly used methods for measuring evenness unevenness in spinning production include the section weighing method, the blackboard evenness visual method and There are three types of instrumental detection methods. 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. capacitanceThe mechanical detection is suitable for slivers, rovings and spun yarns, while the 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 perform spectral analysis on the composition of yarn unevenness and draw a spectrum chart to display Significant periodic unevenness in the yarn. 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 composition of uneven yarn strips

Due to the limitations of the measurement method, the length of the test segment is always limited and is set to L. The total yarn unevenness consists of two parts, CV(L) and CB(L). CV(L) is called internal unevenness, which is the unevenness value within the length of yarn L, expressed by the mean square error coefficient. The larger the length L of the yarn sampling segment, the more likely it is that unevenness will occur, that is, the CV (L) value gradually increases as L increases, and approaches the total unevenness CV (∞) value. In Figure 1, the CV (L) curve passes through the origin, and the curve has an almost straight-line relationship with the L value in the initial section.system, as the specimen length L further increases. The growth rate of CV value gradually decreases. For general yarn, when the length of the sample taken is more than 10 meters, the CV value is close to the fixed value of the total unevenness. CB(L) is called external unevenness, which is the average weight mean square error unevenness among multiple L-long yarn strips. The curve shape is exactly opposite to the CV(L) curve (Figure 1). When L approaches 0, CB(0) is equal to the total unevenness. As L increases, CB(L) gradually approaches 0, that is, the unevenness between long segments decreases as the length of the yarn segments increases. When the length of a given test segment is L, there is the following relationship between the internal unevenness CV(L), external unevenness CB(L) and total unevenness of the same yarn strip: [CV(L)]2+ [CB(L)]2=[CV(∞)]2=[CB(0)]2. Various yarn slivers, including spun yarn and semi-finished products from various pre-spinning processes, have different shapes of CV) and CB (L) curves. Changes in process parameters during the spinning process, especially changes in the number of combinations and draft multiples, will lead to corresponding changes in the curve shape. 5. Spectral analysis of yarn unevenness The yarn unevenness curve can be assumed to be composed of many periodic waves. Each harmonic has its own wavelength and spectrum. By plotting the amplitude against the wavelength, a spectrum diagram can be drawn (Fig. 2). The abscissa in the figure represents the logarithm of the wavelength, and the ordinate represents the average amplitude of the periodic unevenness. In the spectrogram of an ideal yarn strip, the corresponding wavelength of the peak amplitude value is located at a position 2.7 to 3 times the average length of the fiber. The wavelength values ​​of the two adjacent channels are arranged in a geometric series, and the proportionality constant is 1.15, that is, the horizontal axis in the spectrogram is For two adjacent values ​​of coordinates, the wavelength on the right is 15% greater than that on the left. Since the abscissa of the spectrum chart adopts logarithmic values, the spectrum curve has the characteristics of lateral displacement. After the yarn is drafted, assuming no additional unevenness is introduced, the spectrum of the yarn after drafting can be obtained by shifting the spectrum of the original yarn to the right by a distance corresponding to the drafting multiple. If there are draft waves caused by process factors in the actual yarn, the spectrum will show peak-shaped protrusions; if there are periodic unevenness caused by mechanical defects in the yarn, the corresponding wavelength will appear on the spectrum. The amplitudes are superimposed to form a chimney-shaped protrusion. Therefore, according to the shape of the yarn spectrum, the cause of the defect can be determined, so that it can be improved or eliminated in actual production. 1. Improvement of evenness of spun yarn. In recent years, with the continuous progress and development of textile technology, the quality level of cotton yarn has been continuously improved. High-end textiles have continuously improved requirements for cotton yarn quality. Compared with the Usler statistics in 2001 and 1997, all quality requirements for cotton yarn have improved to a certain extent. In particular, the evenness of spun yarn is an important indicator of yarn quality. It not only affects the single yarn strength and strength variation coefficient, but also affects preparation, weaving breakage and cloth surface appearance quality. The spinning process is a key process that affects the evenness, so optimizing the spinning process and reducing the CV value of the finished yarn is an urgent quality issue that enterprises need to solve. 1 The influence of spinning drafting form on yarn evenness. Most of the new spinning machines use pneumatic pressurization and V-shaped drafting. The front area process implements the “three small” process, that is, small floating area,With small jaw spacing and small roller center distance, its rear area adopts curved drafting, with a longer jaw holding distance and a shorter non-control area length, which not only increases the friction boundary strength of the rear area, but also strengthens the The control of whiskers and good adaptability to uneven fiber lengths reduce the drafting waves generated by displacement drafting. 2. The influence of roving ration on yarn evenness is known from the drafting theory. The greater the drafting multiple, the greater the additional unevenness. If the number of spun yarns is the same, the greater the specific number of rovings fed, the required drafting multiple The larger the draft, the greater the additional unevenness. For the new V-type draft, after testing, the roving basis weight can be larger than that of the traditional draft.

For the traditional drafting type, as the basis weight of roving increases, the CV of spun yarn increases, and the thick places and neps also increase. For pneumatic pressurized V-type drafting, due to the unique additional friction boundary setting of V-type drafting, the ability to control fibers is greatly enhanced, and the structural uniformity and fiber straightness of the yarn in the front drafting area are improved. With good density and tightness, the roving basis weight is increased and the evenness of the spinning yarn does not change significantly, so the roving basis weight of the FA507 spinning frame can be appropriately increased. 3. The influence of drafting process on yarn evenness. The configuration of drafting process should consider the adaptability of drafting process and drafting mechanism as well as the mutual configuration of various parameters of drafting process. Long-term production practice and process testing are better. Process optimization method.

　3.1 Front area process

The front area process mainly includes front area roller spacing, jaw spacing, front rubber roller pressure, etc. 3.1.1 The holding distance of the front roller is determined based on the principle that the holding distance of the roller should be as small as possible without damaging the fiber and maintaining a balance between the drafting force and the holding force. Because the holding distance of the front area is related to the length of the drafting floating area in the front area, the configuration of the holding force and the drafting force, and is closely related to the Cy value of the yarn evenness. In traditional technology, the center distance between the front and center rollers is mostly 43mm for pure cotton varieties. In recent years, new domestic spinning machines have made improvements in the pin support form under the roller seat. The surface distance between the front and middle rollers can be as small as 16.5mm. We have improved the roller seat of the FA507 spinning machine and tried spinning. The small surface The distance is changed to 17.0mm, which greatly shortens the length of the floating area in the front area, which is beneficial to improving the evenness level and the difference rate of the evenness CV value. 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.