When the uniformity of the semi-finished product is reduced, the uniformity of the spun yarn is also reduced accordingly; if the spun yarn is not even well, the strength of the yarn will decrease and affect the strength of the fabric. When weaving with uneven spun yarn, various defects and streaks will appear on the fabric, which will affect the appearance quality. The requirements for the uniformity of the spun yarn in knitting production are generally stricter than that of weaving. In the knitting process, the unevenness of the spun yarn or the existence of yarn defects will damage the normal loop-forming process and sometimes cause needle breakage. In the manufacture of tire cord, excessive unevenness of the spun yarn will cause spiral defects in the production process, that is, adjacent yarns are entangled with each other, which affects the processing process and product quality. In addition, uneven yarn evenness will increase the rate of yarn breakage in spinning and weaving, and even reduce labor productivity.
1.Differences in raw material properties
The yarn is uneven due to the difference in the properties of fiber raw materials. All kinds of natural fibers have unevenness in length, fineness or other properties, and the same mechanical and technological processing will cause differences and form unevenness.
2.Random arrangement of fibers
The yarn is uneven due to the random arrangement of fibers. According to the principle of short fiber spinning, the ideal yarn can be assumed to be composed of fibers randomly arranged. This randomly arranged yarn has a certain unevenness, which is called random unevenness. The value is equal to the average number of fibers in the cross section of the yarn. The square root of is inversely proportional, and random unevenness will increase when the number of fibers is small.
3.Poor selection of parameters
The yarn is uneven due to poor selection of spinning process parameters. For example, improper selection of process parameters such as the gauge and pressure of the drafting mechanism will result in poor fiber movement control, which will result in the phenomenon of thick and thin knots, resulting in uneven thickness.
4.Spinning machinery defects
The yarn is uneven due to the defects of spinning machinery. For example, eccentric rollers or top rollers, gear defects, etc., will cause obvious periodic unevenness of the yarn, often showing undulating wave changes. Shorter wavelengths are called uneven short segments, and long ones are uneven. Generally, the short-segment unevenness produced by the front spinning machine, due to the longer draft, shows the long-segment unevenness in the yarn; the unevenness produced on the spinning frame is the short-segment unevenness.
Uneven yarn composition
Due to the limitation of the measurement method, the length of the test fragment is always limited, set to L. The total yarn unevenness is composed of CV (L) and CB (L). CV(L) is called the internal unevenness, which is the value of the unevenness in the L-length yarn, expressed by the coefficient of mean square error. The larger the L value of the sampled sliver length, the greater the probability of unevenness, that is, the CV(L) value gradually increases with the increase of L, and approaches the total unevenness CV(∞) value. The CV(L) curve passes through the origin, and the curve has an almost linear relationship to the value of L at the initial section, and the length of the sample L increases further. 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 total unevenness. CB(L) is called the external unevenness, which is the average weight mean square deviation unevenness among multiple L-long yarns. The shape of the curve is the opposite of the CV(L) curve. When L approaches 0, CB(0) is equal to the total unevenness. As L increases, CB(L) gradually tends to 0, that is, the unevenness between long segments decreases as the length of yarn segments increases. When the length of the test segment is L, there is the following relationship among the inner unevenness CV(L), outer unevenness CB(L) and total unevenness of the same yarn: [CV(L)]2+ [CB(L)]2=[CV(∞)]2=[CB(0)]2. Various yarns, including spun yarn and semi-finished products of pre-spinning processes, have different forms of CV) and CB(L) curves. The changes in the process parameters during the spinning process, especially the changes in the combination number and draft ratio, all lead to corresponding changes in the shape of the curve.
Spectrum Analysis
The yarn unevenness curve can be assumed to be composed of many periodic waves, and each harmonic has its wavelength and spectrum. By plotting wavelength versus amplitude, a spectrum can be drawn. The abscissa is expressed as the logarithmic value of the wavelength, and the ordinate is the average amplitude value of the periodic unevenness. In the spectrogram of an ideal yarn, the corresponding wavelength of the highest peak amplitude value is located at 2.5 to 3 times the average fiber length. The wavelength values of the two adjacent channels are arranged in a proportional series, and the proportionality constant is 1.15, that is, the spectrogram For the two adjacent values of the abscissa, the wavelength on the right is 15% more than the wavelength on the left. Since the abscissa of the spectrogram uses logarithmic values, the spectrum curve has the characteristic of lateral displacement. After the yarn is drafted, if it is assumed that no additional unevenness is introduced, the spectrogram of the drafted yarn can be obtained by shifting the spectrogram of the original yarn to the right by a distance corresponding to the draft multiple. In the actual yarn sliver, if there is a draft wave generated due to process factors, the spectrogram will show peak-shaped protrusions; if there is a periodic unevenness caused by mechanical defects in the yarn, the corresponding wavelength will be changed on the spectrogram. The amplitude is 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 as to improve or eliminate it in actual production.
Improved uniformity
Evenness
In recent years, with the continuous progress and development of textile technology, the quality of cotton yarn has been continuously improved. High-end textiles have continuously improved the quality of cotton yarn. Compared with the statistics of Usler in 2001 and 1997, the quality requirements of cotton yarn have been improved to a certain extent. In particular, the evenness of spun yarn is an important indicator of yarn quality. It not only affects the strength of single yarn and the coefficient of variation of strength, but also affects the quality of preparation, weaving breakage and appearance of the cloth surface. Spinning process is a key process that affects the evenness of evenness, so optimizing the spinning process and reducing the CV value of yarn evenness is a quality problem that enterprises urgently need to solve.
The effect of spinning draft on yarn evenness:
Most of the new spinning frames are pneumatically pressurized and V-shaped drafting. The front area process implements the “three small” processes, namely, small floating area, small jaw gap,
The center distance of the small roller, its rear zone adopts curved drafting, has a longer gripping distance between the jaws and a shorter non-control zone length, which not only increases the strength of the frictional force in the rear zone, strengthens the control of the beard, It also has the characteristics of good adaptability to fiber length unevenness, which reduces the draft wave generated by the displacement draft.
Roving quantitative influence
According to the drafting theory, the greater the drafting ratio, the greater the additional unevenness. If the same number of spun yarns, the greater the number of rovings fed, the greater the required drafting ratio and the greater the additional unevenness. For the new type V-draft, after testing, the roving weight can be larger than the traditional draft.
For the traditional drafting type, as the roving weight increases, the spun yarn evenness CV increases, and the thick knots and neps also increase. For the pneumatically pressurized V-draft, the unique additional friction setting of the V-draft greatly enhances the ability to control the fiber, and the structure uniformity and fiber straightness of the yarn in the front drafting zone The tightness is better, the roving weight is increased, and the change of the spun yarn evenness is not obvious, so the roving weight of the FA507 spinning frame can be appropriately increased.
Drafting process influence
The configuration of the drafting process should consider the adaptability of the drafting process and the drafting mechanism, as well as the mutual configuration of the various parameters of the drafting process. Long-term production practices and process tests are a better process optimization method.
1. Front zone process
The front zone process mainly includes the front roller gauge, the jaw gauge, and the front rubber roller pressurization.
(1) The front roller gripping distance
The principle of determining the gripping distance of rollers is that under the condition of not damaging the fibers and maintaining a balance between the drafting force and the gripping force, it is advisable to hold it small. Because the front zone gripping distance is related to the length of the drafting and floating zone of the front zone, the configuration of the gripping force and the drafting force, and the yarn evenness Cy value. In traditional craftsmanship, the center-to-center distance of the front and middle rollers is mostly 43mm cotton. The domestically-made new spinning frame has improved the pin support form under the roller seat. The minimum surface spacing of the front middle roller can reach 16.5mm. We have improved the roller seat of the FA507 spinning frame for trial spinning, and the minimum surface distance has been changed to 17.0 mm, which greatly shortens the length of the floating zone in the front zone, which is beneficial to improve the level of the sliver and the level of the difference rate of the sliver CV value. Theoretically, when the raw material and the drafting mechanism are constant, there is an approximately linear relationship between the roller gripping distance and the additional draft unevenness. Therefore, reducing the roller gripping distance and shortening the floating zone can make the shifting point of the drafting zone move forward. The jaws are close together to improve the evenness of yarn evenness.
In production, it is difficult to measure the gripping distance of rollers, so the surface distance of the rollers is changed to adjust the gripping distance of rollers, and the process optimization test is carried out. The smaller front roller gauge enables the front drafting area to take on a larger draft ratio, reducing the floating area and the movement of the floating fiber, thereby reducing the deviation of the fiber travel distance and improving the evenness of the yarn This is very obvious in spinning pure cotton varieties, especially pure cotton carded varieties. It can be seen from Table 3 that the gripping distance of the front rollers is reduced, and the yarn evenness is obviously improved.
(2) Use small jaws
In order to prevent the fiber from changing speed early in the drafting process, it is advantageous to appropriately reduce the pin nip, which can improve the straightness of the fiber in the front zone. In the five gears of 2.5~3.5mm used in conventional varieties, each gear is increased by 0.25mm. The number of gears is adjusted slightly, which is convenient for fine adjustment of different varieties and different drafting processes to receive good process results.
(3) Pressure on the cradle
The rubber roller pressurization is the condition to produce enough holding force for the beard, so that the drafting can be carried out normally. The FA507 spinning frame is equipped with pneumatic pressurization, the static pressure is stable and reliable, and the pressurization is fully adjusted and convenient. Through our experiments, the pressure of the spinning cradle, especially the pressure of the front rubber roller, has a significant effect on the yarn evenness CV value. Different rubber roller pressures spin CJ18.2rex yarn evenness CV.
2. Back zone process
The rear zone process mainly includes the rear zone draft ratio, the rear zone center distance and the roving twist coefficient. These important drafting process parameters need to be closely matched and reasonably configured to stabilize and improve the yarn quality.
(1) Roving twist
The drafting of the back zone of the spun yarn is simple roller drafting, and it is effective to use the additional friction generated by the twisting of the roving to control the fiber movement. When the draft ratio is small and the draft force is large, the control force of the rear fiber on the floating fiber is greater than the guiding force of the front fiber to the floating fiber, and the fiber shift point moves forward and tends to be stable, when the rear draft ratio increases At the same time, the twist coefficient must be appropriately increased to make the yarn tight, the friction strength is enhanced, and the fiber shift point is moved forward and stabilized. However, when the draft ratio of the rear zone is larger, the guiding force is increased, the control force is weakened, and the floating fiber is shifted earlier, which deteriorates the yarn evenness. In the knitting process, the elastic drafting effect is strong, which can not only prevent The difference in yarn tightness in the drafting zone before feeding is increased, and it is beneficial to improve the unevenness of twist distribution, which is very beneficial to drafting in the front zone. Therefore, the rear zone of the knitting yarn process adopts small draft and high twist of roving. After drawing, the drafting sliver enters the apron drafting zone with a certain amount of twists. The twists on the drafting sliver in the front zone can also be used as a supplement to the frictional force in the middle of the apron drafting zone. The sliver width has a certain control effect, which is beneficial to the control of fiber movement and is beneficial to improving the yarn evenness.
In actual production, we combine theory with reality, and the determination of roving twist coefficient combines roving quantitative, post draft multiple, middle and rear roller gauge, pressure, temperature and humidity, etc. Table 6 shows the CJ14.5rex spun yarn evenness CV under different roving twist coefficients when other factors are basically unchanged.
(2) The holding distance of the rear roller and the draft multiple of the rear roller
In spun yarn drafting, the front zone has a strong ability to control fiber movement, and the number of fibers in the controlled section is small. The change in the draft ratio of the front zone has less influence on yarn unevenness. Therefore, the spun yarn The draft distribution focuses on the influence of the draft in the back zone on the unevenness of the yarn. In production, a small draft ratio of the rear zone is generally used, and the rear zone gauge is appropriately enlarged. Generally, the yarn for weaving is between 1.25 and 1.40 times, and the yarn for knitting is generally between 1.07 and 1.20 times. This process is not only The evenness is good, and if the rear zone gauge is appropriately increased, the rear zone draft ratio changes within a small range or the fiber length does not change much, the gauge can be left unadjusted, which is convenient for management, reduces the time for changing vehicles, and reduces the labor intensity of workers .
With the increase of the post-draft ratio, the evenness of the spun yarn is obviously deteriorated.
In the case of normal drafting, the CV value of the spun yarn evenness of the roller holding distance after changing the zone does not change much.
glue roll
The rubber roller is an important drafting component in the spinning process and has a significant impact on the quality of the yarn. The general quality requirements for rubber rollers are: uniform hardness, smooth surface, consistent color, small diameter difference and outer circle eccentricity, antistatic, and certain adaptability to temperature and humidity.
1. The effect of rubber roller hardness on evenness
In long-term production practice, we know that soft rubber rollers can improve spun yarn evenness. One of the soft rubber rollers, under the action of pressure, widens the jaw line formed by the roller grip correspondingly, which can significantly enhance the gripping performance of the jaw to the beard. Moreover, the jaw line extends to both ends, causing both forward and backward movement. The jaw line moves back, which relatively reduces the gripping distance in the front zone, which is beneficial to control the movement of floating fibers and improve the evenness of the evenness. Second, the nip line composed of soft rubber roller and roller is relatively stable. The soft rubber roller has the characteristics of good elasticity, large surface deformation and strong vibration absorption ability, which keeps the dynamic holding force of the jaw relatively stable. The three soft rubber rollers hold evenly in the transverse direction, which has a strong ability to control the edge fibers of the whiskers and is beneficial to improve the spun yarn evenness.
In the production, we use WRC-965 untreated rubber rollers. The even CV value is good, but because its winding resistance is not ideal, when spinning polyester cotton varieties, the high-speed rotating rubber rollers are caused by static electricity generated by friction with the fibers. Winding the rubber roller phenomenon. In order to be able to effectively solve this problem, we reduce the coating ratio and then apply coating treatment to the surface of the rubber roller, and carry out a vehicle tracking test.
The test results show that due to the low concentration of the paint in the desalination process, the rubber roller does not lose its original low-hardness and high-elasticity characteristics. After pressure, the characteristics of good resilience are basically unchanged, so the rubber roller can still Effectively control fiber and improve quality. After the WRC-965 untreated rubber roller is treated with paint desalination, the conductivity is enhanced and static electricity is eliminated, thus making the rubber roller resistant to winding. The third kind of paint is slightly uneven but even better. In comprehensive consideration, we chose the third method of coating desalination treatment for WRC-965 untreated rubber rollers, coupled with strict rubber roller replacement cycle management, popularized use, and the evenness level is maintained at 25%. After long-term use and the test of high temperature and humidity, it is verified that after the third coating desalination treatment of WRC-965 type untreated rubber roller, the phenomenon of rubber roller winding has basically disappeared. It is worthwhile for the car worker to make a good life and the product quality is stable.
2. Rubber roller diameter and yarn quality
The larger diameter rubber roller has good elasticity under the same pressure, the arc contact surface with the roller is large, the frictional force boundary is enlarged, the floating area is reduced, the control of the floating fiber is strengthened, and the shift point is conducive to the forward jaw concentration. At the same time, the large-diameter rubber roller has good vibration absorption, which can reduce the fluctuation caused by the roller groove or dust, so that the uniformity of the lateral holding force of the rubber roller is improved, and it is beneficial to control the movement of the edge fibers of the strand. The large-diameter rubber rollers fully, evenly and stably hold the drafting strands, so that the drafting state is improved, which is beneficial to improve the quality of the yarn.
It is theoretically analyzed
of large diameter rubber rollers is better than that of small diameter rubber rollers, but the diameter of the middle and rear rubber rollers must be matched during spinning. The diameter of the front rubber roller increases the height of the rear cradle, so the diameter of the middle iron roller and the rear rubber roller must be increased accordingly. In the production, we use the eliminated front rubber roller to process the rear rubber roller, the diameter of which is between 27mm and 28.5mm. Cots with different diameters are tested on FA507 spinning frame (pneumatic pressure, V-shaped drafting) with the same spindle and the same roving,
Based on the above tests, we have uniformly regulated the rubber rollers on the spinning frame. The diameter of the rubber rollers is required to be between 29.5 and 30.5mm. The rubber rollers smaller than this range are used as the rear rubber rollers, and the rear rubber roller must be smaller than the front rubber roller. The rolls are cut and the rubber rolls have the same diameter.
Conclusion
(1) The new V-shaped drafting forms a unique additional frictional force boundary by raising the rear roller, which improves the structural uniformity, compactness and fiber straightness of the yarn entering the front zone, which is higher than that of ordinary drafting. The drafting ability, spun yarn evenness is better than ordinary drafting.
(2) The use of smaller front roller surface distance, smaller roller nip and larger front rubber roller for pressure can make the fiber shift point close to the front nip, thereby reducing the deviation of the shift distance and improving the yarn quality Evenness of evenness.
(3) A small draft ratio in the back zone is beneficial to the spun yarn evenness. Under the condition of normal drafting, an appropriate increase in the roving twist coefficient can improve the spun yarn evenness.
(4) Cots are the key equipment for spinning yarn drafting. The use of untreated soft rubber rolls increases the diameter of the rubber rolls, which can effectively control the floating fibers, make the shifting points stable and concentrated, which is beneficial to improve the evenness of the yarn.