With the continuous development of modern textile industry, the increase of textile international trade volume, the diversification of textile products and the continuous improvement of quality level, the international market demands more and more high-quality cotton yarn, which puts forward higher requirements for the quality of domestic cotton yarn. More neps not only affect the quality and appearance of yarn and fabric surface, but also affect the continuity of dyeing and finishing, spinning and weaving process. Therefore, the "93 standard" increases the examination of NEP number compared with "78 standard", and the neps become one of the main indexes to measure the yarn quality. How to strictly control the neps in the production process to ensure the quality of the final products has become one of the problems to be solved urgently.
1. Definition and test method of Neps
A large number of experiments show that 60-70% of neps are caused by poor properties of raw cotton, but also by poor spinning process. According to the GB / T 398-93 standard, neps are made of fibers, immature cotton or stiff cotton due to poor rolling quality or poor treatment during spinning. The inspection method is as follows: under the illumination of no less than 400lx, the light is injected from the left rear side, the installation angle of the inspection surface is 45 °± 5 ° with the horizontal plane, the inspector's line of sight is in a vertical line with the yarn sliver, and the inspection distance is based on the principle that the inspector's eyesight is easy to identify defects.
2. Relationship between properties of raw cotton and neps of yarn
As for the relationship between physical properties of raw cotton and neps, in recent years, a large number of studies at home and abroad have shown that the mature cotton fiber has small moisture absorption capacity, large single fiber strength, more natural crimp, good elasticity and rigidity. Therefore, it has strong tensile, compressive and bending resistance, and is not easy to be damaged in the process of processing, and the fiber is rubbed by surface friction At the same time, the damage and fracture of the fiber under mechanical force in the process of opening, striking, carding and drafting are less, and the increase of short fiber rate is less. Therefore, the chance of NEP formation is less. The relationship between maturity coefficient and NEP number of yarn is shown in Figure 1.
3. Relationship between carding process and neps
A large number of studies show that the Neps on the cotton net basically determine the neps of the yarn. Therefore, how to reduce the number of neps and improve the carding effect of carding machine is very important to control neps. Through the practice of factory production in our school, we have done the following work.
3.1 reasonable distribution of blowing and carding noil to improve the structure and quality of cotton lap
The large impurities which are easy to be separated and eliminated are removed by the opening and cleaning process, and the small impurities with fibers with larger adhesion force are removed by carding process. For the carding process itself, the sterile seeds, stiff cotton and dead fibers in the lap are excluded from the taker in part, while those with fiber seed chips, fine fiber impurities, neps and short fibers are excluded from the cylinder cover plate. At the same time, attention should be paid to improving the structure of the cotton coil and reducing the number of hook shaped and loose cotton bundles which are easy to form cotton web neps in the cotton roll. In general, the impurity removal efficiency of taker in roller is controlled between 50-60%, while that of cylinder cover is controlled at 3-10%, and the impurity content of green strip is controlled below 0.15%. Therefore, the licker in part is the key point of impurity removal in carding process. We can adjust the technology of licker drop zone, change the carding and impurity removal elements, length and main spacing, and strengthen the elimination of neps and impurities.
3.2 improve carding efficiency of carding machine
The strong carding of carding machine can improve the degree of fiber single fiber, which is not only conducive to the separation of fibers and impurities, reduce the entanglement of fibers, but also loosen some neps in the cotton roll. Therefore, we choose new clothing with good carding effect for cylinder, cover plate, doffer and taker in roller clothing, and adopt smaller spacing for cylinder cover plate, cylinder doffer, cylinder taker in roll and taker in roller feeding board. It not only enhances the holding carding ability of taker in roller, but also improves the free carding and repeated carding efficiency of cylinder and cover plate.
Through the test, we can see that the number of Neps in low-speed tight gauge sliver is more than that in high-speed tight gauge sliver; the number of Neps in sliver increases with the increase of cover gauge; the carding degree increases with the increase of cylinder speed, which makes the fiber fully carding and easy to eliminate neps. Meanwhile, the average carding force on the unit needle surface and fiber decreases, and the fiber is not easy to be stressed and the elongation is too large Reduce neps due to fiber rebound and fiber breakage. The tight gauge makes the needle teeth contact with the fiber more, the fiber is easy to be held by the needle tooth, the needle tooth penetrates into the fiber deeply, the carding effect is enhanced, and the neps are easy to be eliminated.
3.3 improve fiber transfer and reduce the formation of new neps
The basic reason for the formation of new Neps on the carding machine is the rubbing and rubbing between fibers, and the abnormal phenomena such as returning flower, winding flower and hanging flower are easy to cause violent friction, which leads to the formation of neps by rubbing and rubbing of fibers. The main reasons for returning, winding and hanging flowers are improper speed ratio or spacing, or insufficient sharpness and smoothness of opening carding elements. Therefore, in view of the above reasons, we mainly take the following corresponding measures to eliminate the phenomenon of fiber rubbing and severe friction. Firstly, the speed ratio of the cylinder licker in roller should be correctly configured to ensure the normal transfer of fibers and reduce the return of flowers; secondly, it is necessary to pay attention to the status of carding elements, and only by "four points and one accuracy" can the taker in roller, cylinder, cover plate and doffer needle teeth be kept sharp with good puncture performance to ensure the accurate spacing of each part; thirdly, ensure the smooth and smooth passage of fibers, so as to reduce the hanging flowers of fibers in the process of motion Phenomenon.
3.4 strengthen temperature and humidity management to control moisture regain of fiber
Under high temperature and high humidity, cotton fiber has high plasticity, poor bending resistance, easy adhesion between fibers, and easy to form neps. Especially for raw cotton with poor maturity, it is easier to absorb moisture and form neps under high temperature and high humidity. Due to the poor elasticity of fiber under high temperature and high humidity, cotton fiber in the working area of cover plate often turns into neps because it is not combed. Therefore, the relative humidity of carding process is kept at a low level, generally 55-60%, so that the fibers are in continuous dehumidification state, so as to control the moisture regain of semi-finished products, reduce the cohesion between fibers, maintain good elasticity and rigidity, which is conducive to opening, impurity removal, carding and transfer, and reduce the friction between fibers and needle teeth and filling the needle gap.
4. Conclusion
Through the above analysis, we can draw the following conclusions:
(1) The probability of forming neps in the process of processing for the fiber with poor maturity is higher than that of normal cotton fiber.
(2) The process configuration of carding process is the key to affect the number of neps. To strengthen the effect of removing neps and impurities in the taker roller, we should implement the process principle of "tight spacing and strong carding", appropriately increase the cylinder speed, and pay attention to "four front and one accuracy", improve the fiber transfer, and reduce the formation of new neps.
(3) Temperature and humidity control should be strengthened in carding process to reduce neps.
