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The process of developing short fibres into long continuous yarns involves several steps and fibres assume forms of lap, sliver, roving and finally yarn.
- Lap formation through blending and opening
- Lap to card sliver by carding process
- Card sliver to comb sliver by combing process
- Sliver to roving by drawing out/ drafting process
- Roving to a yarn by twisting process
- Reeling of yarn on bobbins, spools or cones
Cotton fibre lends itself to all the above processes, hence taken as example for the description of mechanical spinning process.
Opening, cleaning and blending: Different varieties of cotton after ginning arrive at the manufacturing unit in the form of compressed bales. These bales are opened and blended together to accomplish uniform quality in the final yarn. The fibre is fed into the opener and blender. These machines separate or open the hard lumps into staple fibres and blend the fibres from different bales. Cleaning of the fibres also takes place during this process as trash separates from the fibre.
Several opening and blending machines with different technologies are prevalent in the industry. A typical machine consists of mechanical bale pickers that pick up layers of cotton from matted bales. The fibre is mixed and passed to an opener. Cylinders with protruding fingers are capable of opening up the lumps and partially separate the remaining seeds, burs and trash from the fibre. The type and number of cylinders depend on the type and fineness of the fibre used. The commonly employed porcupine beater revolves at a speed of 1000 revolutions per minute. Cotton that emerges out of this process is slightly uniform and free from 1/3rd of the trash content. The sheet of cotton called ‘lap’ is conveyed over the belt ready for carding. Each lap is a loosely tangled mass of fibres of one inch thick and 40” wide.
Carding The purpose of carding is two fold
- Removal of further impurities/ trash in fibre mass.
- Arranging the tangled mass to assume parallel alignment, so that the longitudinal axes are somewhat parallel.
Carding machine consists of cylinders anchored with very fine hooks or wire brushes. A belt with anchored wire brushes moves slowly over the moving cylinder in concentric magnitude. The lap is passed through a beater section and drawn over the fast revolving cylinder. The two sets of pins/ brushes on cylinder and belt move in the same direction, but at different speeds to tease fibres into a filmy layer, so that a thin web of fibres is formed on the cylinder. The teasing action also removes remaining trash, disentangles the fibres and arranges them in a relatively parallel manner. The web thus formed is passed through a funnel shaped device that molds it in the form of soft round rope like mass called ‘card sliver’, about the diameter of ½ " to ¾ ". Inexpensive carded yarns are produced from these slivers directly. For more serviceable yarns, further processing is done.
Doubling The card sliver is not completely uniform in diameter and the fibres are not totally parallel. This necessitates doubling i.e., combining several card slivers together to make compact slivers.
Combing When high quality cotton yarns with superior evenness, smoothness, fineness and strength are required, the fibres are combed. Combing process is an additional process that straightens the fibres, makes them parallel to each other and removes short fibres called noils.
Combing machine consists of several fine toothed combs with increasing speed. These combs continue straightening the fibres and arrange them with high degree of parallelism and separate the noils below ½ " length. This process finally produces combed slivers made of long fibres, which, in turn produces a smoother and more even yarn. Above 25% of cotton is eliminated due to this process and this becomes a valuable byproduct used in non-woven industry. As combing influences the evenness, smoothness and strength of yarn, it is often indicated on fabric labels as ‘combed cotton’.
Drawing/ Drafting The slivers from different combing units or carding units are processed through the drafting frame. Eight slivers are drawn together to produce an attenuated ‘drawn sliver’ having the diameter of one card (comb sliver). In this process, fibres of different types can be blended together to form blended yarns. For example, to produce cotton polyester blend of 50:50, 4 cotton slivers and 4 polyester slivers are combined to form a single blended sliver.
The draw frame has several pairs of rollers; each advanced set revolves at a progressively higher speed. By pulling action of these cylinders, the slivers are attenuated into thinner and longer slivers, where in fibres assume parallel alignment. These drafted slivers are taken to a slubber where similar rollers attenuate the sliver further. The slub thus formed is passed to the spindles where first twist is imparted to the sliver to withstand the strain on roving frame and wound on bobbins.
Roving Roving frame facilitates further drawing and twisting of the slivers from bobbins till the sliver attenuates to the size of a pencil lead. Two stages of roving are apparent: intermediate and fine. Even though operations are same, each operation produces a finer product than the received stock. Roving is the final product of the several drawing out operations and preparatory stage for final insertion of twist. Roving has no strength, as it is given only enough twist to keep the fibres together. It will break easily if pulled apart.
Spinning This is the final operation for formation of yarn of desired size. The spinning frames complete the manufacture of yarns by performing operations such as drawing the roving, inserting twist and winding yarn on bobbins, all in one operation.
The roving on bobbins is fed through sets of rollers on spinning frame. Just as in roving, the front set of rollers rotates faster than the back set. This difference attenuates the yarn and makes it even, smooth and uniform.
Two types of spinning frames are commercially employed. For relatively high speed production and making coarser yarns, ring frame is used. For finer yarns,mule frame is used, which operates at low speed. On ring frame, the attenuated yarn is fed down, guided through a ‘U’ shaped guide called a ‘traveller’ on to a take up package or bobbin. The traveller moves around the take up package at a slower speed than the take up package. Hence the name ‘ring spinning’. The movement of the traveller and the turning of the spindle on which the bobbin is held combine to introduce twist into yarns. The spindle revolves about 13,000 revolutions per minute. The size of the yarn and the amount of twist (turns per inch) can be controlled. The yarn on bobbins may be reeled into skeins or wound on spools for weaving and further processing.
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