COMBING PROCESS

COMBING PROCESS

The combing process is normally used to produce smoother, finer, stronger and more uniform yarns. Therefore, combing is commonly confined to high grade, long staple natural fibers. In recent years, combing has been utilized for upgrading the quality of medium staple fibers. In addition, a yarn made of combed cotton needs less twist than a carded yarn. However, these quality improvements are obtained at the cost of additional expenditure on machines, floor-space and personnel, together with a loss of raw material. Yarn production coast is increased by something under 1 US$/Kg of yarn (depending on the intensity of combing). To improve the yarn quality, the comber must perform the following operations:

➢ Elimination of precisely pre-determined quantity of short fibers;

➢ Elimination of the remaining impurities;

➢ Elimination of a large proportion (not all) of the neps in the fiber material;

 ➢ Formation of a sliver

Having maximum possible evenness;

➢ .Producing of more straight and parallel fibers.

Elimination of short fibers produces an improvement mainly in staple length, but also affects the fineness of the raw material. The micronaire value of combed sliver is slightly higher than that of feedstock (elimination of dead fibers). Also the degree of parallelization might reduce the inter-fiber adhesion in the sliver to such an extent that fibers slide apart while being pulled out of the can – i.e. sliver breaks or false drafts might be caused.

Types of comber:

The major types of combers include:

➢ Rectilinear comber (with stationary or oscillating nippers),

➢ Circular combers (English worsted process),

➢ Rotary comber (production of Schappe spun yarns) and

➢ Hackling machines (bast fibers). The short staple spinning mill uses only the rectilinear comber with swinging nippers and stationary detaching rollers, as originally developed in 1902 by the Englishman Nasmith and in 1948 by whitin company. Machine layouts used in practice comprise single sided machines with eight heads.

Combing machine

The basic elements of the combing machine are shown in figure below. These are the feeding element, the nipper plate, the combing system and the detaching rollers. The feeding element consists of a feed plate and feed roll. The main function of the feeding element is to feed the comber lap in a series of short lengths. The nipper plate grips the fibers as a means of holding long fibers while the short fibers, neps, and trash are being removed. The combing system consists of two combs. The first one is a rotating bottom circular comb that performs the main combing action. The second one is a linear top comb that completes the function of the bottom comb through vertical combing movement. The detaching rolls are two pairs of gripping rolls that rotate forward and backward in intermittent fashion to hold and move the combed web for a net forward travel.

The objectives of combing mentioned earlier are accomplished by a precise sequence and synchronized series of actions performed by the combing elements. The following text will review this sequence of actions, or the combing cycle, in a very simplified manner to demonstrate the function of each comber component.

The Combing Cycle:

The following figures illustrate the different actions involved in the combing cycle. The principle of combing is to advance a pre-determined portion of the fiber lap to the combing station. This portion is then gripped by a pair of nipper plate while a toothed half (bottom comb) is combing the fiber fringe and removing the short fibers, neps and trashes. This waste (noil) is later removed from the needles of the bottom comb using a revolving brush. The detailed actions are illustrated in the figures:

Effect of combing machine on the yarn quality

Parallelization of fibers in the sheet:

➢ Lake of longitudinal orientation, i.e. noticeable fiber disorder, leads to elimination of longer fibers, and hence overloading the cylindrical comb (Thick sheet).

➢ At same machine settings, noil quantity decreases linearly with increasing parallelization of the fibers without any reduction in yarn quality (see figure 17.)

➢ It is not always follow that more noil is automatically associated with better yarn quality. The correct goal is always a predetermined waste elimination level.

➢ The self cleaning effect of the sheet, will be greater the more random is the disposition of the fibers making up the sheet. If the fibers have a very high degree of parallelization, the retaining power of the sheet can be so strongly reduced that it is no longer also able to hold back the neps as it usually does. Some of the sheet neps also pass through the top comb. Neppiness of the web is increased.

➢ If the degree of order of fibers is too high, the sheet does not hold together well.

➢ High degree of parallelization always leads to marked hairiness of the lap.

➢ The degree of parallelization depends on the total draft between the card and the comber.

Sheet thickness:

➢ A thick sheet always exerts a greater retaining power than a thin one.

➢ Also, a thick sheet always applies a strong load on the comb and this can lead to uncontrolled combing.

➢ In case of very thick sheet, the fibers farthest from the cylinder comb may escape the combing operation, because the combs are no longer able to pass through the whole layer.

➢ Optimal sheet fineness now normally lies between 55 and 75 ktex. Typical values can be derived.

Evenness of the lap sheet:

➢ Evening of the lap is of considerable significance “better clamping”.

➢ High degree of evenness is due to higher doubling.

➢ This explains the effect of doubling on the ribbon lab machine.

The disposition of the hooks:

➢ Fibers should be presented to the comber so that leading hooks predominate in the feedstock.

➢ If the sheet is fed in the wrong direction, the number of neps rises markedly.

➢ Quantity and form of fiber hooks depend mainly upon the stiffness of the fibers; this rises to the second or third power with increasing the coarseness of the fibers.

➢ Fine and long fibers, will always exhibit more and longer hooks (horseshoe shape) than short fibers, coarse fibers (hokey stick form).

➢ Accordingly the role of fiber hooks in spinning process becomes more significant as fibers become finer.

Influence of combing operation on quality:

Combing can be applied to a wide range of spinning processes. Following is the classification of quality of combed yarns:

➢ Semi-combed (upgrading to higher grade) with noil percentage of 5 -10% (-12%)

➢ Normally combed, with a noil percentage between 10 and 20 %.

➢ Super combed, with noil percentage over 20%.