Neps are a big problem in spinning. How do they occur? How to deal with it?

Neps in cotton are fiber knots made up of cotton fibers that are tightly intertwined. The big one is called “silk ball” and the small one is called “white star”.

1. Reasons for the formation of neps

Neps are small round or granular fiber knots formed by entangled single fibers or multiple fibers. The cause is that cotton fibers, immature cotton or stiff cotton are aggregated due to improper handling during ginning or spinning. The fundamental reason for the formation of neps is the rubbing and friction between fibers. Neps can be divided into two major categories based on their formation causes.

The first category is caused by raw materials. The neps formed by raw materials include neps caused by impurities and defects, such as neps formed by fibers attached to the cotton seed skin, neps formed by the adhesion of cotton wax, and neps formed during the cotton collecting process. Neps formed, etc. The number of neps is an indicator for evaluating the grade of raw cotton. Low-grade raw cotton has high impurity and defect content, fine fibers, poor maturity, and more neps formed during the production process. Therefore, the number of neps and impurities in yarn spun from different raw materials is incomparable and cannot be used as an indicator to measure the performance of spinning machines.

The second category is caused during processing, including rough processing, ginning and spinning of raw cotton. The neps formed by ginning are mainly the neps produced by the sawtooth gin; the neps caused by the spinning process include the neps formed when the fiber is loosened, the friction resistance of the fiber channel and the sticking, entanglement, blocking and hanging. neps, as well as neps produced by hook fibers during the drafting process.

Currently there are two ways to loosen fibers (opening and carding), namely free loosening and holding loosening. During the process of loosening, the fibers are constantly subjected to axial and radial stress to produce strains. Some fibers suffer from fatigue, which causes their own strength and bending stiffness to decrease, resulting in bending deformation and kinking with each other to form neps. The free loosening effect is gentle, causing small deformation and few neps; the holding loosening effect is violent, causing large deformation and many neps. Tests have shown that after a free blow, neps can increase by 10%; after a holding blow, neps can increase by 20%; therefore, violent holding blows to the fibers should be minimized during the process.

The beaters of the cleaning machine and the lick-in rollers of the carding machine have a strong loosening effect on the held fibers, which are the main parts that produce fiber deformation and form neps. For example, when the licker-in roller combs the cotton roll held by the cotton feed plate ~ the cotton feed roller, it leaves behind the held long fibers in the cotton roll that are aligned with the direction of the cotton flow; and the disordered, inconsistently arranged, and The unheld fibers are taken away by the teeth of the licker-in, which is equivalent to extracting the ordered fibers from the disordered cotton bundles. During the extraction process, the messy fibers are easily pumped, pulled, rubbed and turned to form a large number of neps. This produces nepsThe main part of the cotton neps are mostly loose large neps.

The combing machine alternately holds and combs both ends of the cotton bundle, which not only produces a small number of neps, but also eliminates many neps through noil.

Free carding action, such as when the cylinder is carded between the movable flat plate, the fixed flat plate, the doffer, or the fibers are condensed. Due to the large centrifugal force on the surface of the cylinder card clothing, the relative needle/tooth surface spacing is large when the fibers are It is easy to break away from the needles/teeth, stay in the gap between the cylinder card clothing and the adjacent card clothing, lose control, and become floating fibers. Because there is a large speed difference between the opposite card clothings, the floating fibers are easily rubbed and formed into knots; the number of floating fibers affects the number of neps formed by rubbing and rubbing.

The friction of fiber channels can easily cause hairiness at the edges of the whiskers to form neps. Since the tumbling and friction of fibers in the cleaning and cotton conveying channels will cause the fibers to twist and form neps, the fiber channels should be made smooth and clean, and the negative pressure in the cotton conveying pipelines should be increased to ensure smooth cotton flow and no blockage.

Abnormal phenomena such as sticking, entangling, blocking, and hanging can often cause severe friction, which can lead to fiber twisting and rubbing to form neps. For example, when the cylinder, cover plate and doffer needle/teeth are blunt or counter-thorned, the fibers cannot be transferred smoothly. Some fibers float between the needle/tooth surfaces and are rubbed by other fibers on the two needle/tooth surfaces. , more neps will be formed. When the distance between the lick-in roller and the cylinder is too large and the teeth are not smooth, it will cause poor peeling between the lick-in rollers, and the lick-in roller will bring the fibers back to the cotton feeding board, and rub them with the cotton whiskers. The neps increase significantly; the teeth of the cylinder metal card clothing are crushed and rough, the needles/teeth have oil stains and rust spots, and the distance between the cylinder and the doffer is too large, and the transfer rate is low, which will cause the cylinder to wrap around, resulting in an increase in neps. .

During drafting, fibers with poor separation and parallel straightness are easily entangled and pulled into neps. Hook fibers are the main reason for the increase in neps during drafting. The exposed ends of the fibers condensed on the surface of the doffer card clothing and grasped are combed by the cylinder. When they follow the doffer out of the cohesion area and are stripped by the stripping roller to form a cotton net, the combed end is in front and is Hold one end at the back, and the fibers on the card clothing will have more hooks. The hooked part of the fiber is held by the doffer, while the straightened part is exposed outside the doffer card clothing. The straightened part will be bent by the airflow formed by the rotation of the doffer, causing some of the rear hooked fibers to become hooked fibers. The more hooked fibers there are, the more hooked fibers there are. The more neps are formed during the drafting process.

In addition, the cotton clumps, cords, fiber impurities, short fibers and harmful defects formed during the blowroom process can also be easily converted into neps during the carding process. Neps formed by flying flowers falling on the whiskers, mechanical defects such as worn rubber rings or poor assembly, spindle eccentricity, worn wire rings, etc. may also cause neps.

2. Methods to eliminate neps

Through noil removal and carding, neps are loosened. Opening noil, carding noil and cover board waste, and combing noil are the main areas for removing neps; carding can effectively eliminate fiber entanglement. , Reduce neps; through careful combing of the cylinder and cover, some loose neps are decomposed into single fibers, and large neps are decomposed into single fibers or small neps; at the same time, short lint also increases.

3. Measures to reduce neps

Turn “messy” fibers into orderly arranged single fiber strips through opening, carding, and pulling. Neps will gradually appear after continuous removal. The generation of neps is inevitable; but neps should be minimized as much as possible. The generation of knots and the removal of multiple neps. There are many reasons for the formation of neps, which should be considered systematically, so that the good condition of each spinning component and equipment is the basis, and the reasonable configuration of process parameters is the guarantee. Good mechanical condition and reasonable process design can significantly reduce the neps in the yarn. .

The carding process separates the fiber bundles into single fibers, which will inevitably add many new neps; and the careful carding of the cylinder-cover plate can comb the loose neps into small neps or single fibers; therefore, , Carding is a key process for generating, removing, and carding neps. Because the neps and impurities in the card sliver directly affect the yarn knots and cloth surface defects, the movement of the fibers during drafting in the doubling, thickening, and thinning processes, and the normal movement of the traveler when the spun yarn is twisted and wound. Impact, resulting in uneven sliver and yarn defects; therefore, it is necessary to control and reduce the number of knots and impurities in the card sliver. Improving carding quality is an effective way to reduce neps.

The key to reducing neps in the card sliver is to achieve “precise flat lathe, tight gauge, sharp needle/teeth; achieve strong carding, control floatation, reduce rubbing and friction, produce less and eliminate more”. That is, the precision flat-packing machine performs special parts repairs to ensure the loading process, improve the static and dynamic balance, roundness and flatness of the cylinder, licker-in, and doffer and the flatness of the cover plate needle surface, and keep the carding distance tight and accurate. This is the basis. The key is to select high-quality card clothing and grind the needles in a timely manner to make the needles/teeth of the card clothing sharp, smooth, and intact, and improve the ability of the tooth tips to grab, hold, comb, and transfer fibers. By achieving tight spacing and sharp needles/teeth, the quality of carding can be improved, fiber floating can be controlled, and knotting caused by rubbing can be reduced; the straightening and parallelism of fibers can be improved, and neps can be reduced during the drafting process; in cylinder The tight distance between the licker-in roller and the licker-in roller can increase the transfer of fiber bundles on the licker-in roller to the cylinder, and reduce the lick-in roller’s backsplash from rubbing with the feed cotton layer at the cotton feeding plate to form knots. The tight spacing between the cylinder and the doffer can improve the doffer transfer rate and reduce the return load on the cylinder, thus reducing the load between the cylinder and the flat plate, improving the one-time carding capacity between the cylinder and the flat plate, and reducing the number of neps. . At the same time, through the cylinder and cover roomDetailed combing can detangle and eliminate loose neps in the cotton flow and reduce neps.

The key to improving carding quality lies in card clothing. Card clothing is the core component in the carding process to improve output and quality and reduce neps. Its tooth shape specifications, manufacturing quality, specification matching, sharpness and wear resistance directly affect the results. Yarn quality; using high-quality sharp card clothing can significantly reduce neps. The new card clothing rack has the characteristics of short, shallow, sharp, thin, dense, and small. It has strong carding ability and fine carding, which improves the ability to card fibers and comb out neps. It has good fiber transfer and reduces the number of floating neps. Neps formed by fiber rubbing. Tests have shown that after applying the new card clothing, the neps content of the card sliver is reduced by 2% to 60%, the straightening, parallelism and separation of the fibers in the card sliver are improved, and the neps generated during the drafting process in the post-process are reduced, making the finished product Yarn neps are reduced.

As the service life of card clothing prolongs, its wear and tear increases day by day, and the tooth tips become blunt and not sharp; the needles/teeth are not smooth due to grinding; resulting in an increase in neps. Regular gun tests should be conducted to analyze the performance of the card clothing, and the needle grinding and replacement cycles should be reasonably determined to keep the needles/teeth sharp. Timely needle grinding or replacement of the card clothing is one of the important measures to control neps.

Timely grind the heel and toe surface of the cover plate so that the needle surface of the cover plate maintains an appropriate heel-toe difference (0.56 mm) to reduce the overall distance between the cylinder and the needle surface of the cover plate, strengthen carding, and reduce the number of needles. The floating fibers between the surfaces are twisted to form neps, which can achieve good results. At present, the heel-toe surface of the carding machine cover in many cotton spinning mills has been seriously worn, and the heel-toe difference is large, which makes the distance between the cylinder and the cover plate small in the process. In fact, the gap between the cylinder and the cover plate needle surface is However, the gauge is large, and tight gauge and strong carding cannot be achieved, so the neps of the card sliver increase. In some factories, the flat plate chain has been worn and lengthened, and the distance between the cylinder and the flat plate is easily changed and inaccurate, which affects the carding quality and increases neps. For these factories, timely grinding of the heel and toe surfaces of cover plates and replacement of chains are effective measures to reduce neps.

For carding machines without additional carding parts, the front and rear fixed covers with air suction devices can be appropriately added to improve carding quality, eliminate fine dust and reduce the generation of neps. Reasonable control of car belly noil, flat waste and combing noil will eliminate more neps and reduce neps in the yarn. Reversing the cover or increasing the running speed of the cover can effectively reduce neps.

Choose process parameters based on the characteristics of raw cotton. For example, when the fiber is thin and the maturity is poor, low speed and soft opening should be used. Three-wing blades and porcupine beaters are not used, and three-wing comb needle beaters with low rotation speed are used; licker-ins and doffers Low speed, control the speed ratio of licker-in roller and cylinder; small speed ratio, more neps; large speed ratio, less neps; pure cotton should be controlled at about 1:2.

IV. Neps detection methods and changes

Among my country’s cotton textile enterprises, most manufacturers currently use the traditional hand-feel visual inspection method to detect the nep impurity content of semi-finished and finished products in each textile process; a few manufacturers use the AFlS single fiber testing system for detection. Neps by visual inspection are defined as one or more fibers that are tightly combined to form a small round or granular fiber knot, also known as a white star. The sample to be tested is manually loosened in the fiber inspection room and placed on a black glass plate. On the black glass plate, using the light projected from above, white spots that cannot be loosened are considered neps. This kind of neps is a fiber mass that is made up of many fibers twisted together and can only be identified visually. One large nep may also be broken down into multiple small neps. The detection result depends on the eyesight of the tester. The diameter of a single fiber is nearly 20/μm, and the diameter of the small neps formed by a single fiber is nearly 60/μm. Due to the limitation of the resolution of the human eye, many objectively existing small neps cannot be visually detected. The weight of individual neps varies greatly. Test reports indicate that small neps in the card sliver are only 1.4% of the weight of large neps in the cotton roll. Only large neps are measured, not small neps, which cannot reflect the true situation of neps. .

The AFIS single fiber testing system defines neps as non-single fibers whose core projection diameter exceeds the specified value (100 μm). Neps are small neps formed by two fibers that are not easily recognized by the naked eye. Neps can be detected, and such neps are much smaller than those detected by visual inspection, so that both large and small neps can be measured, which can reflect the true situation of neps.

Due to the different definitions and testing methods of neps, one nep visually detected can be measured as many neps by AFlS; therefore, the number of neps visually detected is small, but the number of neps measured by AFlS is large. ; makes the number of neps tested vary greatly. Since the nep data measured by different methods are incomparable, the nep data must indicate the test method.

Since the raw materials, equipment, processes, etc. of each factory vary greatly, the occurrence of neps also varies. Current literature reports believe that the changes in neps detected by the two detection methods are different; the number of neps detected by the eye detection method shows an increasing trend with the increase of processing procedures; and the AFIS single fiber testing system detects The neps tend to decrease with the increase of processing steps; the results of the two trends are opposite.

We believe that from the perspective of the generation of neps, regardless of exclusion, wrapping or covering, regardless of the size of the neps and whether they can be detected, the number of neps in the fiber increases with the increase in processing steps. . The ideal result is to reduce the generation of neps, and the removal of neps should be strengthened, so that the neps will gradually increase during the cleaning process. A large number of neps will be produced during the carding process, but more than will be removed, resulting in sliver neps. Reduced (combed products increased in preparation process, combed stripsThe neps on the surface of the yarn measured by visual inspection have decreased because the neps covered by the spun yarn are more than the neps produced.

Recently, literature reports have been very controversial about the changes in neps in the carding process. Some scholars believe that carding is the main process that produces neps. Card sliver neps are more than 10 times more numerous than cotton lap neps; some believe that carding is the main process that produces neps. Carding is the main process for reducing neps. It can Reduce nearly 80% of the neps in the feeding cotton roll; we believe that the key to the increase or decrease of neps in the carding process lies in the carding quality; if carding is normal, less production and more removal will result in fewer sliver neps; carding If it is abnormal, if there is more production and less elimination, the neps of the sliver will increase.

5. Conclusion

Neps are mainly produced by cotton fibers during the ginning and carding processes. Holding and loosening are the main parts that produce neps. The key to the increase or decrease of neps lies in the carding quality of the card, and the core is the card clothing. The changes in neps have only ideal trends but no fixed rules. In actual production, they change with the performance of processed raw materials and spinning machines.