Page 34 - Basic Principles of Textile Coloration

PRODUCTION AND PROPERTIES OF YARNS 23

preliminary, mechanical operations open up the compressed fibrous mass,
eliminate non-fibrous debris, and blend the fibres in preparation for carding. All
natural fibres have inherent variations in their properties because of growth
differences, and blending of the fibres is vital to ensure constant quality of the
yarns produced. Good opening and separation of clumps of fibres is essential for
level dyeing of loose fibre in a dyeing machine with circulating liquor.

The objective of carding is to make a continuous band of parallel fibres called
card sliver. This process also removes any residual debris and those fibres that are
too short for spinning. During carding, the wide band of fibres passes around a
large, rotating roller, with metal pins projecting from its surface. Other small
rotating rollers on its periphery have similar pins, and comb and align the fibres
held on the pins of the larger roller. The natural wax in raw cotton provides
sufficient lubrication for carding. In the case of scoured or degreased raw or
recycled wool, additional lubricating oil is necessary to avoid excessive fibre
breakage and to control the development of static electricity during carding.
Scouring removes this oil before dyeing.

Combing is a process similar to carding. It removes more short fibres from card
sliver, leaving the longer staple fibres even more parallel to each other. Longer
staple length allows greater drawing out of the combed fibres and thus the
production of finer yarn. Spinning of carded wool gives the coarse, low-twist yarns
for woollen articles, whereas drawing and spinning of combed wool produces the
much finer and stronger high-twist yarns for worsted materials.

After carding, the sliver passes to the drawing or drafting process. Several bands
of sliver are combined and gradually drawn out by passing them between pairs of
friction rollers of increasing speed. The fibres slide over each other increasing their
alignment. This produces a finer band of fibres. It is quite weak and a slight twist
helps to hold it together. Further drawing and twisting produce a coarse yarn
called roving.

Spinning involves drawing out the band of fibres even more, gradually reducing
its thickness, but simultaneously twisting the fibres around each other. Twisting
increases the number of contact points between fibres so that their natural
adhesion provides sufficient strength to avoid breaking the yarn. The yarn will be
stronger the longer the staple length of the fibres, the greater the degree of twist
inserted, and the higher the fibre adhesion. The latter is greater if the fibres have a
natural or artificially-made crimp.

The various spinning technologies give yarns with quite different
characteristics. Classical ring spinning requires a pre-formed roving, which is