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Define Silk Yarns | Process Flow Chart of the Production of Silk Fibre | Manufacturing Process for Silk Yarns

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Silk:
Silk is a natural protein fiber which is obtained from the cocoons of the silkworm. It is a natural filament produced by the salivary glands of silkworms, which are a type of moth that feeds on the mulberry bush. They spin their cocoons using a complex set of mechanisms within their bodies when they change from larva into pupa. The origin of the silk dates back to 2600 B.C and the country producing the best quality of silk is China which is also known to be the birth place for the silk fibre. It is protein in nature and so exhibits the natural characteristics which resemble the wool fiber. The cultivation and manufacturing process of silk fiber is a costly affair, making silk a costly fiber. The different varieties of silk produced in India from different species of silkworm are mulberry silk, muga silk, eri silk and tussar silk. Let us discuss the manufacturing and evaluation of silk.
Silk
Manufacturing Process for Silk Yarns:
Silk is a fine translucent fibre produced from the silkworm. The manufacturing process of silk starts with the rearing of cocoons. There are many varieties of the silkworm from which the silk fibre can be obtained. However, it is found that the fibre obtained from the larva of Bombyx mori is of commercial value. The process of obtaining silk fibre from Bombyx mori requires careful nourishment of the cocoons which is put through the spinning process. The flow chart of the production of silk fibre is as follows:

Cultivation of Cocoons: 
The process of cultivating the silkworm for the production of raw silk is called as sericulture. Silk fibre is a continuous filament fibre consisting of the fibroin, which is connected together with the silk gum, sericin. This natural protein or fibroin is secreted from two salivary glands. There are four stages in the life cycle of the moth which are as follows:

1. The egg, which develops into a larva, or caterpillar – the silkworm
2. The silkworm, which spins its cocoon for protection, to permit development into the pupa, or chrysalis
3. The chrysalis, emerges from the cocoon as the moth.
4. Female moth lays eggs, so continuing the life cycle.

The silk moth lays eggs, which hatch into an ant called as larva about 1/8 inches (3mm) in length. The larva at this stage has voracious appetite and requires careful nourishment. They are fed 5 times a day on chopped mulberry leaves. After four changes of skin, or moltings, the worm reaches full growth of about 3 ½ inches (9cm) long. At this stage the interest in the food ceases and is ready to spin its cocoon. The silkworm begins to secrete a protein like substance through a small opening under the caterpillar’s jaws which is called as the spinneret. The silk solidifies when it comes in contact with the air.

The silk worm is hidden from view within twenty – four hours and in three days the cocoon is complete to a size and shape of a peanut shell. The filament is in the form of a double strand of fibroin, which is held together by a gummy substance called sericin, or silk gum. As this cutting through damages the cocoon, the filament cannot be unwound in one long thread. The life cycle is terminated at this point by a process known as stoving, or stifling. The cocoons are heated to suffocate the chrysalis, but the delicate silk filament is not harmed.
Boiling of cocoons
If left undisturbed, the chrysalis inside the cocoon develops into a moth within two weeks. To emerge, the moth breaks open the cocoon by secreting an alkaline liquid that dissolves the filament.

Filature Operations: 
The cocoons are delivered to a factory called filature, are subjected to different process to unwind the silk. The strands are collected into skeins. The different stages of operation are as follows:
  1. Sorting cocoons
  2. Softening the sericin
  3. Reeling the filament
1. Sorting Cocoons: 
The cocoons which are obtained are first sorted according to the colour, size shape, and texture. Cocoons from China are white; Japanese cocoons are creamy white and yellow in colour whereas Italian cocoons are yellow.

2. Softening the Sericin: 
The sorted cocoons are heated in boiling water to soften the gummy substance that holds the cocoon filament in place. This permits the unwinding of the filaments as one continuous thread. Raw silk consists of about 80 percent fibroin and 20 percent sericin. At this stage, only about 1 percent of the sericin is removed because this silk gum is a needed protection during the further handling of the delicate filament.

3. Reeling the Filament: 
Reeling is the process of unwinding the filament from the cocoon. Silk filaments are unwound in the reeling process and combined together to make a thread of raw silk. As filament of a single cocoon is too fine, the filaments from four to eight cocoons are joined and twisted and are then combined with a number of other similarly twisted filaments to make a thread that is wound on a reel. As the reeling of the filament from each cocoon nears completion, it is replaced with another cocoon. The resulting is called as raw silk, which consists usually of 48 individual silk fibres. The sericin acts as an adhesive in holding the several filaments together while they are combined to form the single thread. The length of the reeled filament is approximately a quarter of a mile long. The term reeled silk is used to the raw silk strand obtained by combining several filaments from separate cocoons. The silk filaments are reeled into skeins which are packed in small bundles, weighing about 5 to 10 pounds.

Throwing: 
Throwing is a process of twisting of one or more threads of the raw silk into a strand sufficiently strong for weaving or knitting. It is a process of preparing the raw silk for loom by twisting and doubling it to the required strength and thickness. Throwing is derived form the Anglo – Saxon word “thrown”, meaning “to twist”.

The raw silk skeins are sorted according to size, colour, and length or quantity. These are then soaked in warm water with soap or oil. This softening of the sericin aids in handling the thread. After mechanical drying, the skeins are placed on light reels from which the silk is wound on bobbins. During this winding operation, single strands may be given any desired amount of twist. If two or more yarns are to be doubled, they are twisted again in the same direction or in reverse direction, depending on the kind of thread to be made.

Four different types of silk thread may be produced by the throwing process which includes thrown singles, tram, crepe, and organzine.

Thrown singles are individual raw-silk threads that are twisted in only one direction, the number of turns depends on the quality of thread desired. Usually, three to eight strands of silk filaments are twisted together in one direction to form a yarn called a single. The loose twist single consists of two or three twist per inch and are used for weft yarns. The hard twist singles have a greater number of twists per inch and are used in the sheer fabrics.

Tram is made of two or more singles, twisted and doubled and is used for the weft of various fabrics.

Organzine is made of singles twisted one way, then doubled and twisted in the opposite direction. This is used for the warp of heavy fabrics.

Crepe is similar to organzine but is twisted to a much greater extent. Crepe thread is employed in the weaving of characteristic crinkly fabrics, and single thread is used for sheer fabrics.

Degumming of Thrown Silk: 
The process of degumming involves putting thrown silk yarns through final soap bath to remove the sericin. This process brings out the natural lustre and the soft feel of the silk. As much as 25 percent of the weight is lost by the degumming process. When the gum has been removed, the silk fibre or fabric is a creamy white colour, beautifully lustrous, and luxuriantly soft.

Let us discuss the production of spun silk – shorter lengths of inferior silk which also have value in the preparation of apparel.

Spun Silk: 
Short lengths of inferior silk are not used in producing the reeled silk. These are carded and combed and are spun as cotton, linen or wool yarns are spun. Spun silk is used for shantung and pile fabrics; for dress trimmings, linings, elastic webbing, and sewing silk, for wash silks, velvets, umbrella fabrics, and insulative materials. Spun silk yarns are soft, but they are less lustrous than reeled silk and are not strong or elastic. Spun silk fabric tends to become fuzzy after wearing because the yarn is made of short staple.

There are several sources of staple silk,

1. Pierced cocoons, the result of breeding moths that have emerged from their cocoons;
2. Double cocoons, the result of two cocoons having been spun by two silkworms too close together (some times called douppioni silk);
3. Floss, brushed from cocoons before reeling;
4. Frison, the coarse and uneven silk fibre at the beginning and end of each cocoon;
5. Scrap, the machine waste left from reeling, throwing.

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