When the short fibers are twisted together, they take the form of yarn. Thus, yarn is a continuous filament of interconnected fibers used for making textile. Yarn can be made by one of the several techniques of spinning. Ring spinning is the oldest and the most preferred method of manufacturing yarn. Open-end spinning is another major technique of spinning fiber into yarn.
The process of twisting together of fibers to form yarn is called spinning. In the ancient time, fiber was spun by hand using such simple tools like spindle, distaff and spinning wheel. It was only during the Industrial Revolution that industrial spinning through various machines gained importance. Although newer methods are continuously being invented for spinning, certain methods adopted earlier are popular till today. Ring spinning and Open-end spinning are such two methods.
The basic processes included in spinning can be listed as carding, combing, drafting, twisting and winding. When the fibers pass through these processes, they are subsequently formed into lap, sliver, roving and finally yarn. These stages can be explained according to their occurrence.
- The carding process converts lap into card sliver.
- The combing process converts card sliver to comb sliver.
- The drafting or drawing out process converts sliver into roving.
- Further drafting and twisting process converts roving into yarn.
- The winding process reels the yarn on bobbins, spools or cones.
It should also be understood here that although all the fibers pass through spinning process, there are certain differences when long strands or filaments , such as silk are used rather than staple or short fibers , such as cotton. In case of long strands, the required number of filaments is simply twisted together in a ropelike fashion.
The spinning processes explained below are generally adopted for short fibers.
The compressed mass of raw fiber at a spinning mill goes through the processes of blending, opening and cleaning. Blending is essential for obtaining uniformity of fiber quality. Opening loosens the hard lumps of fiber and disentangles them. Cleaning removes the trash such as dirt, leaves, burrs and any remaining seeds. Carding, the initial straightening process, puts the fiber into a parallel lengthwise alignment. Now the fiber is called 'Lap' . The lap is treated for removing the remaining trash, disentangling and molding it into a rope like mass called 'Sliver'. The sliver is then straighten again which is called Combing. In this process, the fine-toothed combs continue straightening the fibers until they are arranged in such a parallel manner that the short fibers are completely separated from the longer fibers. This procedure is not required for man made staple fiber because they are cut into predetermined equal lengths. Thus, it produces 'comb sliver' made of the longest fibers. This process of combing is required for better quality as long staple yarn produces stronger, smoother and more serviceable fabrics.
After combing, drawing is done which pulls the staple lengthwise over each other. After several stages of drawing out, the sliver is passed to the spindles where it is given its first twist and is then wound on bobbins. These bobbins are placed on the roving frame where further drawing out and twisting is done until the fiber is about the diameter of a pencil lead. 'Roving' is the final product of the several drawing-out operations. It is the preparatory stage for the final insertion of twist. Till now, enough twist is given for holding the fibers together but it has no tensile strength. It can break apart easily with a slight pull. The roving, on bobbins, is placed in the spinning frame, where it passes through several sets of rollers running at high speed and finally the 'Yarn' is produced of the sizes desired.
For Ring spinning, two kinds of spinning machines are used: ring frame and mule frame. The ring frame is a faster process but produces a relatively coarse yarn. For very fine yarns, such as worsted, the mule frame is needed because of its slow, intermittent operation. Fine yarns require more twists than coarse yarns. The amount of twists is an important factor for determining the appearance and longevity of a fabric. To retain the twist in the yarns and prevent any tendency to untwist, the yarns are given a twist setting finish with heat or moisture, depending upon the kind of fiber used.
When the yarn is hold in a vertical position, the direction of its twist may be observed. If the direction of twist is identical to the direction of the slope of the central part of the letter S, the yarn has an S twist and if they are identical to the slope of the letter Z, the yarn has a Z twist. Tightness of twist is measured in TPI (twists per inch or turns per inch).
The staple of two or more different kinds of fibers may be combined or blended at any one of the several stages viz. opening, carding, drawing out or roving
Open- End Spinning
Open- End Spinning is a relatively recent development. It is generally used for spinning coarser yarns. It begins with the carded sliver which is put into rollers. These rollers revolve at a faster speed. As a result, the sliver is completely opened up. The separated fibers are moved by an air stream and are collected as a thin layer in a groove on the inner surface of a funnel- shaped rotor, which rotates at a very high speed. The centrifugal force of the rotor builds up a multilayer of fibers which is peeled away from the collecting groove as it is simultaneously twisted by the rotation of the rotor and withdrawn continuously, thus being formed into a yarn.
Open- End Spinning can produce yarn at a higher rate than that of the conventional ring spinning. Although it provides better fiber elongation, the produced yarn is only as even as a good ringspun yarn. Blending of fibers can also be done even better than ring spinning but it has certain problems too. When spinning 100% manmade staple ( except rayon staple), the fibers get deposited in the rotor and causes logging. It is also not possible to spin combed yarn.
Friction Spinning is a variation of Open- End Spinning. In this process, a stream of carded fibers in the roll nip of the perforated drums is passed along the longitudinal drum axis. Both of them move in the same direction. This action forces the fibers to be wedged along the nip, thereby becoming compressed. The friction of the rollers cause the fibers to twist around each other.
In this technique, the fiber preparation costs are lower as direct feed of card slivers for spinning is possible. Yarn breakage during spinning is also avoided because their is no tension in the spinning area. However, the yarns produced may lack uniform distribution of twist through the cross section. They also have a low tensile strength.
Vortex Spinning is also a variation of Open- End Spinning. In this process, instead of rotor, a tangential air inlet is used that draws fibers down a tube. As a seed yarn is rotated in the air stream, a twist is inserted causing the fibers to grasp onto each other forming a continuous yarn. Although the procedure is cost saving as the moving parts in the machine are eliminated, the yarns produced are weak and irregular.
Air- Jet Spinning
The Air- Jet Spinning technique drafts the sliver to a predetermined size and passes it through rollers over a friction plate (to prevent backtwist) into a cylindrical pneumatic twisting chamber. Jets are set in the walls of the chamber which release compressed air to the central axis of the tube and the fibers are whirled around each other giving them either an S or Z twist. With the help of air suction, the strand is passed to a second chamber where it is first stabilized and then given an equal amount of twist in the opposite direction. This technique produces a yarn of uniform diameter without thick or thin areas. Although the tensile strength of air- jet spun yarn is said to be less than that of ring spun yarn, it may be greater than that of open- end yarn.
Formation of Filament Yarns
The manmade filaments are produced by different methods and they can be formed directly into yarn without the use of techniques to connect fibers together to form the required lengths.
In this process of wet spinning, an appropriate liquid solution is pumped through small nozzle, called spinneret, into a chemical bath that coagulates the extruded solution of endless strands of filaments. These coagulated continuous fibers are drawn out of the bath, purified by washing, dried and then wound onto spools. Spinnerets are also of two types. One type of it has one extrusion hole and it produces a monofilament yarn. The other type has many holes which produces a multifilament yarn.
Dry spinning involves the use of an appropriate liquid solution which is pumped through a spinneret into an air chamber. The air reacts with the extruded streams causing them to solidify. These coagulated continuous fibers are then drawn out of the chamber, twisted or processed further and then wound onto spools.
Melt Spinning is another method of manufacturing manmade fibers into yarn. An example of this is the process of making polyester. Polymer chips obtained from previously reacted chemical combinations are melted and then pumped through a spinneret into an air chamber. The extruded streams cool and solidify into continuous filaments and are then drawn out of the chamber, twisted or processed further and then wound onto spools.
In this bicomponent spinning technique, two polymers of different chemical and/or physical structures which are connected with each other are extruded through a spinneret. There are three different methods of bicomponent fiber production- side- by-side, core cover and matrix.
In the first method, two varieties of the same polymer are extruded side by side through one spinneret hole. As the two variants cool and solidify, they adhere together as one filament, one side of which is composed of one type of the polymer and the other side of which is composed of another type of the same polymer.
In the second method, one spinneret is used inside another. One variant of the polymer is pumped through the center, or core, spinneret while another is pumped through the outer or surrounding spinneret. Both varieties of polymers emerge as one through the common hole of the surrounding spinneret so that the fiber is formed of a core of one polymer variant surrounded by a ring of the other variant.
In the third method, drops of one molten polymer variant are distributed into another molten form of the same polymer. This mixture is then extruded through the single spinneret hole. It results in a matrix, or conglomerate, of minute, short strands of one fiber suspended inside another variety of the same fiber.
Formation of Integrated Multicomponent Yarns
Yarns are also produced by combining staple and filament fibers in order to obtain certain of the characteristics of each. These combinations are known as Integrated Multicomponent Yarns.
Integrated Composite Spinning
In the Integrated Composite Spinning, monofilament or multifilament strand is passed through a molten proprietary polypropylene alloy. A part of the length of the staple fiber is immediately embedded into the polymeric resin. Then a predetermined variety and length of staple fiber is superimposed and press bonded by compression to the polymeric coated continuous filament core. The ICS yarn is then given a twist at high speed, cooled and wound on spools.
Another type of integrated multicomponent yarn spinning is coverspun. This technique wraps fine continuous filament manmade fiber around a core of untwisted natural or manmade staple fiber to produce yarns in a wide range of counts. Coverspun yarn is made by drafting conventional roving to the desired fineness and passing it directly into the top of a hollow vertical spindle that has a spool of filament mounted on it. The filament is fed into the top of the hollow spindle and as the spindle turns, the filament wraps around the untwisted core.