The Cotton Textile Industry is essentially a conversion industry i.e. unlike in other industries, raw material does not undergo fundamental change in basic manner but here the raw material exists in the same form but undergoes a structural change only. Thus cotton or synthetic staple fibre is stretched into yarn and then woven into cloth. The former process is called ‘Spinning’ and the latter is called ‘Weaving’. Then to follow is the ‘Chemical Processing in deference to modern fashion which transforms the ordinary piece of cloth into an acceptable and marketable product by various talented chemical operations. The earlier process ‘Spinning’ and ‘Weaving’ as stated above can be named as Mechanical Processing.
The entire sequence of operations that is exercised as above can be summarised under three distinct headings namely
(1) Spinning Preparatory & Spinning Process.
(2) Weaving Preparatory & Weaving Process.
(3) Chemical Processing of Cloth.
The brief details of various processes are as under :-
(1) Spinning Preparatory & Spinning Process.
The above process which converts cotton into yarn necessitates the following process –
(a) Blowing & Mixing.
(b) Carding & Combing.
(c) Drawing Slubbing & Interframes, i.e. Preparatory for Spinning.
(d) Ring Spinning.
1.1 Blowing & Mixing Process.
It may be seen from the earlier chapter that loose cotton is packed in fully compressed bales for easy transport. These bales have to be broken, cleaned and mixing of cotton has to be done. The object of Mixing Cotton is to secure uniformity of quality in the yarn product. This is of great important from the commercial point of view as successful spinning depends to a very large extent upon suitable blends of various qualities of cotton.
By means of spiked lattices, cylinders Rollers and Beaters, the cotton is loosened and dashed against grids in machines through which sand and other impurities are expelled. The modern machine doing the above job is known as “Hopper Bale Breaker”. However additional machines are interposed such as “Hopper Openers” and “Hopper Feeders” for gradual opening of the lumpy cotton.
In the standard Bin System of Mixing, the cotton is conveyed on the Bins by lattices of the Hopper Bale Breaker or by air current in a metal duct. Cotton expands in its loose form and becomes conditioned. The further process is called “Blowing” where matted fibres are separated to a greater degree. The Blowing Process itself consist of (1) Opening (2) Scutching operations. In Opening Process the cotton is opened to a fleecy condition and impurities are removed further. In Scutching Operation, apart from removing the impurities further, a uniform sheet of cotton is made and the same is formed into a compact roll which is technically called a “Lap”. The machinery are called Buckley openers and porcupine openers which have cylindrical beaters in a horizontal position, creighton openers with conical beaters, scutcher which consists of lap rollers.
The dust and fluff formed during the entire process are exhausted to the underground cellar beneath the Blow Room through filter grids which is cleaned at regular intervals.
1.2 Carding Process.
The purpose of this process is to further loosen and lay the fibres into a straight position. Despite the previous cleaning, some impurities remain because of entanglement of the fibres. This process eliminates such imperfections and arranges the fibres to the nearest parallel order. This is the first step in the construction of yarn, the fibres composing the lap being in a confused mass.
The Revolving Flat Carding engine is the most popular machine for the above process which consists of a large main cylinder covered with card clothing (thickly wire set comb) which revolves fairly rapidly. The cylinder is surrounded by ‘card flats’ wire fillets (all connected) travelling at a slow speed but in the same direction. Between the flats and the cylinder a continuous stream of fibres will be combed. A ‘Doffer’ strips the resulting filmy sheet of fibres and drops it into a ‘sliver’, a filmsy rope of cotton, composed of more or less parallel fibres. The same is deposited in a slowly revolving tall can (drum). Because of difference of speed, short or broken fibres in the cotton will be removed. The other type of machine, called “Roller & Cleaner Card” is used for spinning of waste and low counts where quality is scarcely necessary. The “sliver”, a result out of above process is in the form of rope-like untwisted strand.
1.3 Preparatory (Spinning)
The various processes under this heading are in short, as under. Preparing embraces all the remaining processes upto spinning. These are :
(a) Drawing – This is a process in which several card slivers are combined and attenuated into one, more perfecting the parallel nature of fibres and giving a more uniform sliver.
(b) Combing – This is an optional process when fine and super fine varieties of cloth are achieved. This is done by a complex machine, combing out any blemishes which still remain and giving a finished sliver coiled in a can.
(c) Slubbing & Intermediate Slubbing – This process is closely on the lines of Drawing, producing a slubbing or thread several times finer than the sliver. In intermediate slubbing, two threads are combined into one drawing out each to twice its original length.
(d) Roving – This process is similar to that of intermediate slubbing, two strands being doubled into one, strong enough to undergo treatment in the next process i.e. spinning.
Items ‘c’ and ‘d’ above are done on machines known as speed or fly frames on which the material is transferred on the bobbins for the first time. Combination of all processes above varies according to counts being spun, while all combinations are required for finest counts. The machines basically have pairs of rollers, through which, yarn passes each successive pairs revolving faster than the previous ones. By doing this, the resulting yarn not only gets attenuated but also imparted with a little twist.
Practically, the only difference between the machines which perform various operations consists in their relative strength and in the size of Bobbins and spindles – as he twisting and blending operations are carried a stage further by the respective machine machines, the diameter of sliver becomes less and the spindles and bobbins are correspondingly smaller.
Two methods of spinning known respectively as Mule Spinning and Ring Spinning are employed in Modern Mills. The object of each of this machine is
(1) To reduce the roving to a final yarn count.
(2) To impart required twist to give strength and eslasticity besides other qualities such as exten – sibility, lustre and smoothness.
The mule spinning is becoming obsolete in cotton spinning and is of much more complicated nature. The essential difference between Mule Spinning and Ring Spinning is that operation of twisting in Ring Spinning is continuous while that of twisting in Mule Spinning is intermittent. In this way, Mule Spinning produces a more softer and more elastic type of yarn.
Ring frame, which is an all metal machine resembling the Fly-frame with the usual three pairs of draft-rolls, the spun yarn being wound on to a pirn, tube or bobbin. Spinning is done intermittently on Mule and continuously on the Ring. The Mule is the larger machine containing about 1200 spindles on which yarn is wound by means of a traversing machinism. The Ring has between 300 to 500 spindles.
Mule frame, worked in pairs, each consisting of long frame. The peculiarity is that the three pairs of draft rollers that attenuate the yarn and the creel containing the bobbins are in fixed position while the carriage supporting the spindles has a slow to and fro motion from the rollers. The yarn is would either on the bare spindle and small paper tube slipped over it. The finished spool is called a “cop”.
Mule works on the principle of ‘charka’. The rove is drawn out, the necessary twist is imparted and yarn is wound on the spindle. On Ring, the drafted material is imparted with the required twist as it emerges from “Drafting Rollers” and is simultaneously wound on to the Bobbins. All the above are achieved by means of the revolving spindle, the ring and its traveller.
1.5 It become necessary to define the meaning of “Count”.
The quality of yarn is usually measured in counts, the index number being determined by the number of hanks which make up a pound in weight. A bank is a loosely coiled bundle of yarn of thread measuring 840 yards in length. Thus if ten hanks of uniform quality weigh exactly 1 1b., the yarn of which the hank consists will be classified as ten counts.
After Spinning, Weaving is the next step in Mechanical Processing and consists of following sections :-
(a) Winding & Doubling.
(b) Warping or Beaming.
(d) Drawers & Reachers.
(e) Weaving and Loom Shed.
The brief details above processes are as under :-
Winding & Doubling.
Winding is a process to have maximum yarn length on one single package so as to economise handling and also to clean yarn of some of its defects.
The bobbins from the Ring Spinning are wound on the Winding Machine and the Process is a simple operation of guiding the yarn (thread) by a suitable traverse mechanism to build up bobbins or cones or cheeses. The also enables to clear the yarn, to build of any spacks, as well as to detect sections of uneven thickness and piecing up unbroken threads. ‘Pirn Winding’ is the method by which yarn is prepared, ready for use, in the shuttle of the loom (weft).
There are two methods of Winding, namely precision winding and Open Winding. The yarn from a single ring bobbin or multiple bobbins is threaded through a slub-catcher and a tension device. Then it passes through stop-motions device before it is guided on to a tube. On modern high speed winders, the cones/cheeses contract a drum which has continuous spiral groove which helps both winding and guiding the yarn.
Doubling is an optional operation (like that of combing) applied to quality yarns to produce stronger and more regular thread. This is a comparatively new process and takes place after spinning but before weaving. It consists drawing one or more threads from the bobbins, twisting them round each other in the opposite direction to the original twist, and rewinding them on to a bobbin. The above process can be done both wet and dry.
Warping or Beaming.
This process becomes necessary to bring together a number of ends of yarn to form a beam of sufficient width. In this process a number of Bobbins obtained after winding and/or Doubling are placed in a frame called “Creels”., the number of them varying from 400 to 600. The threads from the Bobbins are taken separately through a tensioning device, a detector wire to stop the machine when any yarn breaks’ and is finally wound on a beam at constant tension.
This process is necessary to protect the warp from stresses during weaving. The warped yarn is passed through a chemical process where it is impregnated in a paste containing starches, gums and softners at about 100*C temperature’ and then dried by passing over a number of heated cylinders. This process is to round up the yarn and give it an additional strength to enable it withstand tension during the subsequent process.
The size, thus applied, also binds the protuding fibres and prevents them from catching on the adjacent yarns. Also it protects the yarn from scrapping on the loom.
Drawers and Reachers.
In this process, sized ends are drawn out according to the pattern of cloth to be woven. The drawing-in frame used for this process is a simple one, consisting appliances for holding the yarn i.e. the set of ‘healds’ and ‘reeds’.
The ‘heald’ controls and guides the wrap threads in the loom according to the pattern. It is responsible for lifting or separating the warp threads to allow to an fro motion of the shuttle as the weft threads are built up. The ‘read’ is a steel, comb-like frame through which the yarns pass in parallel. At the same time, it forces the weft yarn upto the woven cloth.
In the above process, the workman called ‘drawer’ sits in front of the reed and selects threads from the Beam, drawing them through both ‘heald and reed’. A hook implement is then inserted through a hole in the reed and then on through the correct eye of the heald. The assistant called ‘reacher’ who will be behind, places a thread on the hook which will be drawn to the front again.
Warp tying is a process of joining new warp threads to the old ones of a running-out beam. By means of selectors on the machine, one thread is taken from the warp beam and another thread from the heald. Both are tied together by a knotting device, either manual or automatic.
Having prepared warp beams, it becomes necessary to prepare the weft. This is comparatively simple operation to build a pirn (or a tube) to suit the shuttle of the loom. This has been discussed earlier under ‘winding’ – (Please see Pirn Winding).
Weaving Shed or Loom Shed
It is in the weaving shed or loom shed where cotton is finally converted into a woven fabric. It is a process of interlacement of warp and weft threads to form a united surface. The process is effected by machines called looms.
In weaving the loom alternately raises and lowers the sections of the warp threads and passes the weft thread through the space (known as shed) between the raised and lowered warp. This is achieved by throwing the shuttle (which contains the weft) through the shed across the loom.
The main function of weaving done on a loom are as under :
(a) To release warped yarn from the beam – known as “letting-off”. Letting-off is simply a tension motion, putting strain upon the yarn during whole weaving.
(b) To open the shed by lifting up certain warp threads and leaving others down to allow cross-word movement of shuttle – known as “shedding”.
(c) To throw the shuttle between the two sets of opened threads leaving a line of weft on its track – known as “picking”.
(d) To best in the weft up to the fell of the cloth – known as “beating up”.
“Beating up” is the process of carrying the reed forward with the line of weft in front of it to the edge of cloth that is being woven. The reed then retraces for another pick of weft which in turn is beaten up and so the weaving proceeds.
(e) To make up the cloth on to a roller in front – known as “Take up”.
If during weaving, a warp thread breaks up, the weaver joins the broken ends by a thread from a tuff of short threads called “thrums”. Any breakage of weft thread, stops the loom automatically.
There are two types of looms namely ordinary and automatic looms. The essential and automatic between them is that in the automatic loom when the weft breaks or is exhausted, the shuttle is automatically re-charged with the weft and threaded without having to be removed from the reed.
There are different types of looms available for weaving. Tappet looms suffice for weaving simple designs while Dobby Looms are employed for alightly more complicated patterns. Jacquard type of looms are required for any fabric where the pattern is developed by inter-weaving the weft threads with the warp in a number of different ways and different colours.
Processing of cotton and staple fibres and maintenance of comfortable working conditions require control of temperature and relative humidity in textile mills. These plants are needed to give artificial humidity which is very necessary for Man-made and Cotton fibres. There are three different types of providing humidity which are mostly followed by textile mills, namely (a) Central Station Plant or Carrier Plant; (b) Unit Type and (c) Plants with all air system.
In central station plant, air handling equipment is located at a central point and departments are served through ducting system in conjunction with air washer units. The air supplied can be sucked through return air ductings and exhaust. The same air can be recirculated depending on outside temperature. In Unit type system, the air circulating fan, humidifiers and ducts etc. are all assembled in one unit. In all air system, relative humidity upto 85% is achieved without supplementing. Also the advantage is, irrespective of outside weather, the humidity is controlled at uniform level.
CHEMICAL PROCESSING OF COTTON CLOTH
The chemical processing of woven cloth covers the gradual transition of fabric delivered by weaving section into a final shape of finished fabric. This is achieved by chemically treating the cloth on different machines to produce an acceptable and marketable product as seen in the end. The chemical processes can be described in brief under following headings:
(1) Bleaching Process
(2) Finishing Process
(3) Dyeing Process
(4) Printing Process
(1) BLEACHING PROCESS
Purpose for the above process is two-fold. The first one is to produce an absolute white cloth which is absolutely necessary to get light and bright shades in dyeing or pure whites in Printing. The second one is to obtain a pure cotton fabric by removing the impurities which have not been cleared in the mechanical process – such as foreign matters, starch fillings or sizing and incidental traces of dirt and oil etc. the process involves boiling the Grey Cloth using chemicals which may ‘Bleach’ liquor or hydrogen peroxide or others selected as per the composition of Grey Cloth to avoid damage to the fibre. In the process, the cloth is washed vigerously to give white cloth. Bleaching and Washing is done on the latest type of machines called ‘J’ box or continuous open width machines which have replaced the convention 1 ‘Kier’ boiling.
There are two processes called “shearing and cropping” and “singeing” which are carried out before Bleaching.
‘Shearing Cropping’ is done to clean off from the cloth a certain amount of dust, shall or scale. For this purpose, the cloth is passed through a machine which consists of revolving knives with spiral blades which do the necessary smothening without weakening the cloth. The cloth is brushed by a spiral revolving brush before leaving the machine.
‘Singeing’ is an additional process employed for obtaining a clean surface on the finished cloth. The most popular method is by “Gas Singeing Machines”. The other methods are by “Plate Type” and “Rotary Type Machines”. The three advantages of the former method are (a) Less space is required (b) More economic fuel consumption and (c) Suitability for singeing Cloth of uneven surface.
In the method the cloth is drawn rapidly through LPG gas kerosene/petrol flames issueing from narrow slits spread over whole width of cloth. The machine can be so set that during ore passage of fabric, both sides of same can be singed once or one side can be singed twice. The cloth will be passed through water after Singeing. Suitable exhaust devices are provided to exhaust singed fibres and other dirt.
Singeing is done only for unbleached cloth. However, when a final cropping is required on a dyed cloth, shearing is preferred as Singeing imparts a brown colour to the cloth and can be used only before the cloth is dyed.
There are other processes associated with Bleaching such as Grey soaking, De-Sizing, Lime Boiling, Piling, Souring and Washing which are either optional or outdated by the modern Bleaching Machinery.
(2) FINISHING PROCESS
All the cloth which is bleached requires to be finished and dried. This is done in sophisticated machinery such as stenters mercirisers, drying ranges, sanforizers, calenders etc.
The cloth which comes out of bleaching in a rope form is opened out by means of a scutcher and dried by passing it over “Drying ranges” either horizontally or vertically. The cloth is passed through heated cylinders for drying purpose. After drying, the cloth is stretched in order to ensure an even width and tension. This is achieved by passing the cloth through ‘stretching stenters’ machines. By ‘Mercerising’ process, softness and lustre is added to the cloth. The qualities attained will be permanent and cloth will stand up to any amount of washing thereafter. The process involves passing of cloth through strong, cold caustic soda and impregnating the same with the solution. Not only does the cloth take on a silky lustre but its colour absorbing qualities are increased. To prevent shrinkage of cloth, the fabric is ‘sanforized’. In this way, the fabric becomes denser and softer. The process called ‘calendering’ is similar to ironing of the fabric and imparts to the cloth the feel or touch required. The cloth rolls between different heated bowls and gets compressed very hard. The cloth after calendering gets automatically folded as it leaves the machine.
‘Raising’ is a process to impart fluffy, surface on the face of the cloth eg. Blankets, turkish towels etc. The cloth is passed through a revolving cylinder made of wire teeth which teases the surface of the cloth when it passes over.
Again, there are many incidental processes which are either optional or one similar to the other depending upon the quality of marketable fabric required.
(3) DYEING PROCESS
In this process the cloth is Dyed in different attractive shades in machines such as Jiggers, Continuous Dyeing Machines, High Temperature Pressure Beam Dyeing, Yarn Dyeing, Cheese Dyeing etc. Synthetic Dyes are available in powder form many of which are coal-tar derivatives.
Cotton can be dyed in yarn or fabric form.
In the process of Dyeing Cotton in the finished cloth form, ropes or sheets of cloth are passed through Dye-vats. Surplus water from the cotton cloth will be removed by ‘Hydro Extractor’ after Dyeing is complete. The yarn Dyeing is done mainly for producing different colours for use in weft for design of cloth. It is done by either continuous dyeing or Pressure Dyeing or Hank Dyeing or Cheese Dyeing Machines. Beam Dyeing (warp) can also be done in the same way. The Dyeing is done in a pressure vessel for penetration of colour through the yarn.
The Dyeing Machine is known as “Jigger”.
(4) PRINTING PROCESS
Printing is done on Bleached or Dyed cloth to provide the cloth with catchy prints of varying designs to produce a competitive product. The process is similar to printing on paper. Any desirable, single or multi coloured pattern can be printed on the fabrics. The various machinery available are Roller Printing, Flat bed and Rotary type Screen Printing Machines.
The cloth is passed through engraved copper rollers for reproduction of designs in case of Roller Printing and in case of Screen Printing, fabric is required to be spread and prints are applied by means of stencils. The stencils are prepared by a sealing compound in such a way that the pattern area only remains to be porous for penetration of Printing Colour. The colour is applied by means of a small board and distributed on the stencil and pressed on the cloth. This will be done automatically on Rotary and Flat bed Screen Printing Machines while Hand Screen Printing is also being done in the same method.
After printing, the fabric is dried, heat cured to polymerise the resins (print) in order to fix the colour. The volatile material must be driven off the cloth before passing it through a curing machine. As the fabric is to be dried by passing it through heated cylinders, the voltalite ingredients will be removed. Exhaust ingredients will be available to remove solvent fumes and other by products.
After the curing process, the finished fabric is folded, bundled and baled for marketing.