APPAREL-INDUSTRY

(COTTON FABRIC-2)

                 

Hello Everyone!!! Till now we have seen about fundamental of cotton crop its basics .Now we will see complete fibre formation process and then its conversion into yarns. So lets continue…..

COTTON FABRIC PROCESING

There are six stages:

·         Cultivating and Harvesting

·         Preparatory Processes

·         Spinning

·         Weaving or Knitting

·         Finishing

·         Marketing

Cultivating and harvesting

Cotton is grown anywhere with long, hot dry summers with plenty of sunshine and low humidity. Indian cotton, Gossypium arboreum, is finer but the staple is only suitable for hand processing. American cotton, Gossypium hirsutum, produces the longer staple needed for machine production.^[3] Planting is from September to mid-November and the crop is harvested between March and June. The cotton bolls are harvested by stripper harvesters and spindle pickers that remove the entire boll from the plant. The cotton boll is the seed pod of the cotton plant; attached to each of the thousands of seeds are fibres about 2.5 cm long.

·         Ginning

The seed cotton goes into a cotton gin. The cotton gin separates seeds and removes the "trash" (dirt, stems and leaves) from the fibre. In a saw gin, circular saws grab the fibre and pull it through a grating that is too narrow for the seeds to pass. A roller gin is used with longer staple cotton. Here, a leather roller captures the cotton. A knife blade, set close to the roller, detaches the seeds by drawing them through teeth in circular saws and revolving brushes which clean them away. The ginned cotton fibre, known as lint, is then compressed into bales which are about 1.5 m tall and weigh almost 220 kg. Only 33% of the crop is usable lint. Commercial cotton is priced by quality, and that broadly relates to the average length of the staple and the variety of the plant. Longer staple cotton (2½ in to 1¼ in) is called Egyptian, medium staple (1¼ in to ¾ in) is called American upland, and short staple (less than ¾ in) is called Indian. The cotton seed is pressed into a cooking oil. The husks and meal are processed into animal feed, and the stems into paper.

Preparatory processes - preparation of yarn

·        Ginning, bale-making and transportation is done in the country of origin.

·        Opening and cleaning

Platt Bros. Picker

Cotton is shipped to mills in large 500 pound bales. When the cotton comes out of a bale, it is all packed together and still contains vegetable matter. The bale is broken open using a machine with large spikes, called an opener. In order to fluff up the cotton and remove the vegetable matter, the cotton is sent through a picker or a similar machine. In a picker, the cotton beaten with a beater bar in order to loosen it up. It is then fed through various rollers, which serve to remove the vegetable matter. The cotton, aided by fans, then collects on a screen and gets fed through more rollers till it emerges as a continuous soft fleecy sheet, known as a lap.

·         Blending, Mixing and Scutching

Scutching refers to the process of cleaning cotton of its seeds and other impurities. The first scutching machine was invented in 1797, but did not come into further mainstream use until after 1808 or 1809, when it was introduced and used in Manchester, England. By 1816, it had become generally adopted. The scutching machine worked by passing the cotton through a pair of rollers, and then striking it with iron or steel bars called beater bars or beaters. The beaters, which turn very quickly, strike the cotton hard and knock the seeds out. This process is done over a series of parallel bars so as to allow the seeds to fall through. At the same time, air is blown across the bars, which carries the cotton into a cotton chamber.

·         Carding

Carding machine

A Combing machine

In the carding process, the fibres are separated and then assembled into a loose strand (sliver or tow). The cotton comes off of the picking machine in laps, and is then taken to carding machines. The carders line up the fibres nicely to make them easier to spin. The carding machine consists mainly of one big roller with smaller ones surrounding it. All of the rollers are covered in small teeth, and as the cotton progresses further on the teeth get finer (i.e. closer together). The cotton leaves the carding machine in the form of a sliver: a large rope of fibres.

Note: In a wider sense carding can refer to these four processes: Willowing- loosening the fibres; Lapping- removing the dust to create a flat sheet or lap of cotton; Carding- combing the tangled lap into a thick rope of 1/2 inch in diameter, a sliver; and Drawing- where a drawing frame combines 4 slivers into one, repeated for increased quality.

·         Combing is optional, but is used to remove the shorter fibres, creating a stronger yarn.

·         Drawing the fibres are straightened

Several slivers are combined. Each sliver will have thin and thick spots, and by combining several slivers together a more consistent size can be reached. Since combining several slivers produces a very thick rope of cotton fibres, directly after being combined the slivers are separated into rovings. These rovings (or slubbings) are then what are used in the spinning process.

Generally speaking, for machine processing, a roving is about the width of a pencil.

·         Drawing frame: Draws the strand out

·         Slubbing Frame: adds twist, and winds onto bobbins

·         Intermediate Frames: are used to repeat the slubbing process to produce a finer yarn.

·         Roving frames: reduces to a finer thread, gives more twist, makes more regular and even in thickness, and winds onto a smaller tube.

Spinning - yarn manufacture

• Spinning

Most spinning today is done using Break or Open-end spinning, this is a technique where the staples are blown by air into a rotating drum, where they attach themselves to the tail of formed yarn that is continually being drawn out of the chamber. Other methods of break spinning use needles and electrostatic forces. This method has replaced the older methods of ring and mule spinning. It is also easily adapted for artificial fibres.

The spinning machines takes the roving, thins it and twists it, creating yarn which it winds onto a bobbin.

In mule spinning the roving is pulled off a bobbin and fed through some rollers, which are feeding at several different speeds. This thins the roving at a consistent rate. If the roving was not a consistent size, then this step could cause a break in the yarn, or could jam the machine. The yarn is twisted through the spinning of the bobbin as the carriage moves out, and is rolled onto a cylinder called a spindle, which then produces a cone-shaped bundle of fibres known as a "cop", as the carriage returns. Mule spinning produces a finer thread than the less skilled ring spinning.

·         The mule was an intermittent process, as the frame advanced and returned a distance of 5ft.It was the descendant of 1779 Crompton device. It produces a softer less twisted thread that was favoured for fines and for weft.

·         The ring was a descendant of the Arkwright Water frame 1769. It was a continuous process, the yarn was coarser, had a greater twist and was stronger so was suited to be warp. Ring spinning is slow due to the distance the thread must pass around the ring, other methods have been introduced.

Sewing thread, was made of several threads twisted together, or doubled.

• Checking

This is the process where each of the bobbins is rewound to give a tighter bobbin.

• Folding and twisting

Plying is done by pulling yarn from two or more bobbins and twisting it together, in the opposite direction that in which it was spun. Depending on the weight desired, the cotton may or may not be plied, and the number of strands twisted together varies.

• Gassing

Gassing is the process of passing yarn, as distinct from fabric very rapidly through a series of Bunsen gas flames in a gassing frame, in order to burn off the projecting fibres and make the thread round and smooth and also brighter. Only the better qualities of yarn are gassed, such as that used for voiles, poplins, venetians, gabardines, many Egyptian qualities, etc. There is a loss of weight in gassing, which varies' about 5 to 8 per cent., so that if a 2/60's yarn is required 2/56's would be used. The gassed yarn is darker in shade afterwards, but should not be scorched.

                                     THE END