Yarn Evenness Testing | USTER Evenness Tester

Evaluation of Evenness of Yarn: 
This could be defined as the variation in a yarn’s weight or thickness per unit length. Evenness of yarn is measured by the following methods. 

Visual Examination: 
In this method, yarn evenness is checked by wrapping it onto a black board in uniformly spaced turns to reduce the effect of optical illusions caused by irregularity. These boards are then checked under proper lighting using a uniform and unidirectional light. Normally visual examination is done without any comparison with a standard, though comparison could also be made with the ASTM standard if it is available. Nowadays more uniformly spaced yarn boards are prepared with the help of motorized wrapping machines. By way of these wrapping reels the yarn moves gradually along the tapered black board as it is revolved. Tapered black boards are preferred for evaluating or determining periodic faults. If there are periodic faults in the yarn they produce a woody pattern, which is clearly visible. This visibility of the yarn faults on the tapered boards is due to the equal spacing of the yarns on the board. 

For determining the mass variation of yarn, the cut and weigh method is considered to be the simplest. In this method consecutive lengths of yarn are cut and weighed. For this testing, we need a precise way of cutting the yarn as all the lengths should be same. The small error in cutting the lengths of yarn results in wrong and inaccurate measurements. To avoid this problem, the yarn is wrapped around a grooved rod with a circumference of exactly 2.5 cm. Then yarn with equal lengths of 2.5 cm is cut by running a razor blade along the groove. These lengths of yarn are then weighed on a sensitive weighing balance. By plotting the mass of each length, we produce the graph shown in Figure-1. 

Figure-1: Variation in mass per unit length of a yarn.

The visual indication of unevenness of yarn can be found by plotting the line showing mean value and thus comparing the scatter of individual readings. A mathematical measurement is also necessary for this unevenness of yarn, which can be expressed in the following two ways. 

The first term is used by USTER Technologies to designate U%. The average value for all the deviations from the mean is calculated and then expressed as a percentage of the overall mean, which is known as the percentage mean deviation (PMD). The value of the standard deviation is calculated by squaring the deviations from the mean, which is then expressed as a percentage of the overall mean. The deviations having a normal distribution about the mean are correlated as: 

CV = 1.25PMD

USTER Evenness Tester: 
The evenness tester of USTER Technologies finds the variations in thickness of a yarn by using capacitive techniques. The yarn to be examined is passed through a pair of parallel plates of a capacitor whose capacitance is continuously measured electronically. The presence of yarn between the capacitor’s plates continuously changes the capacitance of the system. The capacitance depends on the mass of yarn between the plates and the type of raw material used. For the same dielectric constant, the signals are directly related to the mass of yarn present between the capacitor’s plates. To get the same relative permittivity for yarn, it should be made up of the same type of fiber and it must have uniform moisture content throughout its length. The varying moisture content, or an uneven blend of two or more fibers, will vary the dielectric constant in different parts of the yarn and this variation will be signaled as unevenness. The readings made by the USTER tester are equivalent to weighing successive 1 cm lengths of the yarn. 

Figure-2: USTER evenness tester

The following can be possible reasons for yarn unevenness: 

• The number of fibers in the yarn’s cross-section is not constant but varies widely depending upon the fiber parameters. This is the most significant reason of yarn unevenness.
• The staple spun yarn is made up of natural fibers having a variable fineness. This variation leads to a difference in yarn thickness even when the number of fibers in the cross-section remains the same.

For more info.

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