Causes and ways to reduce neps
Causes and ways to reduce neps
Neps are an important indicator for yarn quality assessment. The technical requirements for cotton yarn grades not only stipulate the total number of neps and impurities in 1g of yarn, but also specify the number of neps. The nep content not only affects the appearance quality of yarn and fabric, but also affects the yarn structure and evenness as well as the yarn breakage rate. High neps content is currently a problem faced by many manufacturers in improving yarn quality. The author conducted a preliminary study and discussion on the causes of neps formation, changing patterns, detection methods and technical measures to reduce neps.
1Causes 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 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 harvesting process. . 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, and some fibers experience fatigue, resulting in a decrease in their own strength and bending stiffness, 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, the neps can increase by 10%; after a holding blow, the neps can increase by 20%; therefore, violent holding blows to the fiber 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 and 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 and 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 is the main site where neps are produced, and most of these neps are large, loose 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 waste.
In the free carding function, 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 of the fibers on the surface of the cylinder card clothing, the fibers are easy to separate from the needles when the relative needle/tooth surface distance is large. / Teeth, located in the gap between cylinder card clothing and 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 on 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, causing fibers to twist and rub 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. 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. Likou.
During drafting, fibers with poor separation and parallel straightness in the slivers 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 hook part of the fiber is held by the doffer, while the straight part is exposed outside the doffer clothing. The straight part will be formed by the air formed by the rotation of the doffer.Flowering and combing waste removes more neps, which can reduce neps in the yarn.
Reversing the cover or increasing the running speed of the cover can effectively reduce neps.
Select process parameters according to 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 should not be used, but three-wing comb needle beaters with low rotation speed should be used; licker-ins and doffers should be opened at low speed, Control the speed ratio between the licker-in roller and the cylinder; a small speed ratio means more neps; a large speed ratio means less neps; pure cotton should be controlled at about 1:2.
4 Detection methods and changes of neps
Among my country’s cotton textile enterprises, most manufacturers currently use the traditional hand-visual method to detect the nep impurity content of semi-finished products 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//m, and the diameter of the small neps formed by a single fiber is nearly 60/Jm. Due to the limitation of human eye resolution, many objectively existing small neps cannot be visually detected. The weight of individual neps varies greatly. Test reports indicate that small sliver neps are only 1.4% of the weight of large neps in cotton laps. 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). Small neps formed by two fibers that are not easily recognized by the naked eye can be detected. It is measured that such neps are much smaller than those detected by visual inspection, so that both large and small neps can be measured and 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, while the number of neps measured by AFlS is large; making the test The number of neps varies 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 neps in the card sliver. It has decreased (the combed products have increased in the preparation process, and the combed sliver has decreased because the production is less than the elimination), and has increased in both combing and coarseness. The spun yarn has more neps than the ones produced due to the wrapping. Use visual inspection. The surface neps of the measured yarn are reduced.
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 1 () times more than cotton lap neps; some believe that carding is the main process that produces neps. Cotton is the main process for reducing neps. It can reduce nearly 80% of the neps in the cotton roll. We believe that the key to the increase or decrease of neps in the carding process lies in the carding quality. Normal carding produces less and more neps. If the carding is eliminated, the sliver neps will be reduced; if carding is abnormal, with more production and less elimination, the carding sliver neps will increase.
5Conclusion
Neps are mainly produced by cotton fibers during the ginning and carding processes. Holding and loosening are the main parts of producing neps. The key to the increase or decrease of neps lies in the carding quality of the card, and the core lies in the card clothing. The changes in neps have only ideal trends but no fixed rules. In actual production, they vary with the performance of processed raw materials and spinning machines. kRriSwQuOs
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