Yarn Evenness / Unevenness / Irregularity | Imperfections of Yarn

Yarn Evenness
Non-uniformity in variety of properties exists in yarns. There can be variation twist, bulk, strength, elongation, fineness etc. Yarn evenness deals with the variation in yarn fineness. This is the property, commonly measured, as the variation in mass per unit length along the yarn, is a basic and important one, since it can influence so many other properties of the yarn and of fabric made from it. Such variations are inevitable, because they arise from the fundamental nature of textile fibres and from their resulting arrangement.

Accordingly, there are limits to the achievable yarn evenness.

Unevenness / Irregularity
The mass per unit length variation due to variation in fiber assembly is generally known as "IRREGULARITY" or "UNEVENNESS". It is true that the diagram can represent a true reflection of the mass or weight per unit length variation in a fiber assembly. For a complete analysis of the quality, however, the diagram alone is not enough. It is also necessary to have a numerical value that represents the mass variation. The mathematical statistics offer 2 methods 
 
Irregularity of yarn
1. The irregularity U%: It is the percentage mass deviation of unit length of material and is caused by uneven fiber distribution along the length of the strand.

2. The coefficient of variation C.V.%

Imperfections
Yarns spun from staple fibers contain imperfections. They are also referred to as frequently occurring yarn faults. They can be subdivided into three groups

1. Think places
2. Thick places
3. Neps

The reasons for these different types of faults are due to raw material or improper preparation process. A reliable analysis of these imperfections will provide some reference to the quality of the raw material used.

The standard sensitive levels are as follows :
  • Thin place: -50%
  • Thick place: +50%
  • Neps: +200%
Thin places and thick places in a yarn can, on the one hand, quite considerably affect the appearance of a woven or knitted fabric. Furthermore, an increase in the number of thin places and thick places refer to a particularly valuable indication that the raw material or the method of processing has become worse.

On the other hand, it cannot be concluded from the increased number of thin place faults that this yarn, the downtime of weaving or knitting machines will be increased to a similar degree. Thin places usually exhibit a higher yarn twist, because of fewer fibres in the cross-section resulting in less resistance to torsion. The yarn tension does not become smaller proportionally with reduced number of fibres. With thick place faults the contrary is the case. More fibres in the cross-section result in a higher resistance to torsion. Thick places have therefore, in many cases, a yarn twist which is lower than the average. The yarn tension in the yarn at the position of the thick place is only in very few cases proportional to the number of fibres. These considerations are valid primarily for ring-spun yarns.

Neps can influence the appearance of woven or knitted fabrics quite considerably. Furthermore neps of a certain size can lead to processing difficulties, particularly in the knitting machines. Therefore the avoidance of neps in the production of spun yarns is a fundamental textile technological problem.

Neps can be divided, fundamentally, into two categories:
  1. Raw material neps  
  2. Processing neps
The raw material neps in cotton yarn are primarily the result of vegetable matter and immature fibers in the raw material. The influence of the raw material with wool and synthetic fibers in terms of neps production is negligible. Processing neps are produced at ginning and also in cotton, woolen and worsted carding. Their fabrication is influenced by the type of card clothing, the setting of the card flats, workers and strippers, and by the production speeds used. 
 
 

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