For the best fabrics, we have to choose the best yarns. Then how to know yarn quality? You must understand quality parameters. In this article, we’ll introduce one of the most important yarn quality parameters – yarn evenness.
1. Yarn evenness:
Yarn evenness is a measure of the level of variation in yarn linear density or mass per unit length of yarn. In other words, it refers to the variation in yarn count along its length.
A yarn with poor evenness will have thick and thin places along yarn length, while an even yarn will have little variation in mass or thickness along length.
2. Importance of Yarn evenness
While a yarn may vary in many properties, evenness is the most important quality aspect of a yarn, because variations in other yarn properties are often a direct result of yarn count irregularity.
For example, as we all know, twist tends to accumulate in the thin places in yarn, so irregularity in yarn linear density will cause variations in twist along yarn length. This preferential concentration of twist in thin places along a yarn also exacerbates the variations in yarn diameter or thickness, which often adversely affects the appearance of the resultant fabrics.
An irregular yarn will also vary in strength along the yarn.
The following yam properties are usually subject to yarn evenness:
→ Weight per unit length
→ Twists per inch
3. Yarn Faults’ Classification
Evenness, Unevenness, regularity, and irregularity are common terms used to describe the degree of uniformity of a textile product. In the textile ﬁeld, the uniformity of products like the lap, sliver, roving or yam is expressed in terms of evenness or regularity or in terms of unevenness or irregularity.
A yarn which is not uniform is said to be irregular or to contain yarn defects or faults. These faults vary in their cross-sectional size and length. Following figure shows a plot of fault cross-sectional size against fault length.
In the above plot, three distinct categories of yarn faults are represented based on their size, length and their frequency of occurrence:
a. Unevenness or irregularity: This is yarn evenness we usually talk about.
• Thick places: It is the region of yarn where the local linear yarn mass is well above the average value normally by at least 35%.
• Thin places: It is the region of yarn where the local linear yarn mass is well below the average value normally by at least – 30%.
• Neps: There are regions of yarn where local linear mass increases at least 100% of the normal region. These regions are called as neps.
c . Objectionable yarn faults: This is not acceptable and it will bring problems on fabrics.
4. Express of yarn evenness: U% & CV%:
The irregularity or unevenness of a yarn is commonly deﬁned as the variation in ﬁneness along its length and more appropriately as the variation in mass per unit length along the yam’. It is expressed as U% or CV%.
Generally two parameters are used to express the irregularity of yarns:
1). Percentage of mean deviation (PMD) or Unevenness (U%)
U%: The average value of all the deviations from the mean which is expressed as a percentage of the overall mean is called percentage of mean deviation (PMD). This is termed U% by the uster.
2). Coefficient of variation CV%
Coefficient of mass Variation (CV%):
The coefficient of mass variation CV % is the ratio of standard deviation of mass variation divided by average mass variation.The higher the CV value is the more irregular the yarn.
5. Relationship between CV% AND U%:
A modern instrument, such as the Uster Evenness Tester, can measure the U and CV values of a fiber assembly at a high speed. The larger deviations from the mean value are much more intensively taken into consideration in the calculation of CV% rather than in U% (due to the squaring of the term). For this reason, the Coefficient of Variation CV% has received more recognition in modern statistics than the irregularity value U%.
6. Classification of Yarn Unevenness (Irregularity)
Generally two types of variation are shown in yarns (especially spun yarns).
1). Random Variation
Random variation is a variation that occurs randomly in a textile material without any deﬁnite pattern or order is called random variation. This is caused mainly due to the natural variations in the ﬁber properties.
2). Periodic Variation
Periodic variation is a variation that occurs at deﬁnite length sequences in the textile material.
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