Classification and Causes of Neps in Yarn

Neps are a crucial indicator of yarn quality. Technical requirements for cotton yarn grades specify not only the total number of neps and impurities per gram of yarn, but also the specific number of neps. The amount of neps affects not only the appearance quality of the yarn and fabric, but also the yarn structure, evenness, and yarn breakage rate.

Causes of Neps

Neps are small, round knots or granular fiber knots formed by the disordered arrangement and entanglement of single or multiple fibers. They are formed when cotton fibers, immature cotton, or stale cotton aggregate due to improper handling during ginning or spinning. The fundamental cause of neps is the rubbing and twisting between fibers. From the perspective of their formation, neps can be divided into two main categories: neps caused by raw materials and neps formed during processing.

Neps Caused by Raw Materials

Neps formed by raw materials include those caused by impurities and defects, such as neps formed by fibers attached to cottonseed husks, neps formed by cotton wax adhesion, and neps formed during cotton harvesting. The number of neps is an indicator for evaluating the grade of raw cotton. Lower-grade raw cotton has a higher content of impurities and defects, finer fibers, and poorer maturity, resulting in more neps formed during production. Therefore, the number of neps and impurities in yarns spun from different raw materials is not comparable and cannot be used as an indicator of spinning machine performance.

Neps Caused During Processing

Neps caused during processing include those generated during the rough processing of raw cotton (ginning) and during spinning production. Neps formed by ginning are mainly those produced by saw gins; neps generated during spinning include those formed during fiber opening and carding, neps formed by frictional resistance in fiber channels and adhesion, entanglement, blockage, and snagging, and neps generated by hooked fibers during drafting.

Neps Formed During Fiber Opening and Carding

Currently, there are two methods for opening and carding fibers: free unwinding and grip unwinding. During the opening and carding process, fibers are constantly subjected to axial and radial stresses, resulting in strain. Some fibers experience fatigue, leading to a decrease in their strength and bending stiffness, causing bending deformation and knot formation. Gentle free-release action results in minimal deformation and fewer knots; vigorous gripping and release action results in significant deformation and more knots.

The beaters of the cotton cleaner and the licker-in rollers of the carding machine are the main sites of fiber deformation and knot formation due to their gripping and carding action. These knots are mostly loose, large knots. The combing machine alternately grips and combs the ends of the cotton bundle, producing a small number of knots while removing many through waste cotton. When the cylinder separates or aggregates fibers between the moving and fixed top plates and the doffer, the high centrifugal force on the surface of the cylinder's card clothing makes fibers prone to detachment when the spacing is large. These fibers become uncontrolled and float in the gaps between the cylinder's card clothing and adjacent card clothing. Because of the significant speed difference between the relative needle cloths, floating fibers are easily rubbed and knotted; the number of floating fibers affects the amount of knots formed by rubbing and friction.

Knots formed by adhesion, entanglement, blockage, and hanging

Abnormal phenomena such as adhesion, entanglement, blockage, and hanging easily cause intense friction, leading to fiber rubbing and knot formation. This is mainly manifested in the following aspects:

(1) When the cylinder, top plate, and doffer needle cloth teeth are blunt or have reverse barbs, fibers cannot transfer smoothly. Some fibers float between the teeth and are rubbed and twisted by other fibers on the two tooth surfaces, forming knots.

(2) When the gap between the licker-in roller and the cylinder is too large, and the tooth surface is rough, poor stripping between the cylinder and the licker-in roller causes the licker-in roller to return the fibers to the feed plate, where they rub and twist with the cotton fibers, significantly increasing the number of knots.

(3) Damaged or rough teeth on the cylinder wire mesh, oil stains and rust spots on the teeth, and an excessively large cylinder-doffer gap resulting in low transfer rate will cause tangling in the cylinder, leading to an increase in neps.

Neps formed by frictional resistance in the fiber channel

Because the fibers tumble and rub against each other in the cleaning and conveying channels, they will twist and form neps. Therefore, the fiber channel should be kept smooth, and the negative pressure in the conveying pipe should be increased to ensure smooth and unobstructed cotton flow.

Neps formed by hooked fibers during drafting

During drafting, fibers with poor separation and parallel straightness in the sliver are prone to entanglement and pulling into neps. Among them, hooked fibers are the main cause of increased neps during drafting. During drafting, the hooked part of the fiber is held by the doffer wire mesh teeth, while the straight part is exposed outside the doffer wire mesh teeth. The straight part is bent by the airflow generated by the doffer's rotation, resulting in more hooked fibers and thus more neps formed in the sliver during drafting.

Other causes of neps:

Cotton clumps, fibrous strands, and unremoved fibrous impurities, short fibers, and harmful defects formed during the opening and cleaning process can easily transform into neps during the carding process. Neps can also form from fly ash landing on the sliver. Mechanical defects in equipment, such as worn or poorly assembled rubber travelers, spindle eccentricity, and worn traveler wires, can also contribute to neps formation.

The number of neps is one of the most important indicators determining yarn quality. To reduce neps, it's crucial to understand neps. Blindly talking about reducing neps and haphazardly implementing technical measures is ineffective. The most effective, direct, and cost-efficient methods should be chosen based on the specific circumstances of the textile enterprise.