Yarn Unevenness

1. YARN UNEVENNESS &
ITS IMPACT ON QUALITY
By Engr. Shan Imtiaz
+92 3006600484
Lecturer
The University of Faisalabad

2. Yarn Unevenness
• Yarn unevenness deals with the variation in yarn
fineness. This is the property, commonly measured as
the variation in mass per unit length along the yarn.
• In other words, it refers to the variation in yarn count
along its length.
• It is the evenness of staple spun yarn that is of concern
here.
• It is a basic and important one, since it can influence so
many other properties of the yarn and of fabric made
from it.

3. Perfectly Even Yarn
• For a spun yarn to be perfect even, we need two
conditions:
• The constituent fibres are uniform in thickness
• The yarn has the same number of fibres in all cross
sections along its length

4. Methods to express Unevenness
• For a complete analysis of quality we use two methods
to express irregularity or unevenness.
1. The Irregularity U%
2. The Coefficient of Variation CV%

5. The Irregularity U%
• It is the % mass deviations of unit length of material and
caused by uneven fibre distribution along length of
strand.
• The irregularity U% is proportional to the intensity of the
mass variations around the mean value.
• The U% is independent of the evaluating time or tested
material length with homogeneously distributed mass
variation.

6. Coefficient of Variation CV%
• The coefficient of variation CV is a well known value for
the determination of the evenness of slivers, rovings and
yarns.
• The Coefficient of Variation (CV) is the standard
Deviation (SD) expressed as a percentage of the mean
CV % = (SD ÷ mean) x 100
• Each process in a spinning mill contributes a part to the
evenness.

7. Comparison of two Fabrics with different CV%
The lower the CV-value, the more even is the material and the more even it will
look in the end-product. Fig shows a comparison between two fabrics with a
low (CV = 11.48%) and a high coefficient of variation (CV = 17.76%).

8. Classification of Variation
• Random Variation
Variation occurring without definite pattern of repetition
or definite sequence of Thick and Thin Places.
• Periodic Variation
Variation show definite sequence of thick and thin
places in strand of material.
1. Short Term Variations
Wave length 1-10 times of fibre length.
1. Medium Term Variations
Wave length 10-100 times the fibre length
1. Long Term Variations
Wave length 100-1000 times the fibre length

9. Imperfections
• Imperfections referred to as frequently occurring yarn
faults.
• They can be subdivided into three groups

10. Causes of Unevenness
• The reason for yarn irregularity is based on the fact that
it is not possible for staple fiber yarns to keep a
constant number of fibers in the cross-section.
• Unevenness is unavoidable but we can minimize
unevenness.
1. Variations in Raw material
2. Variation in Process

11. Reason (Spinning Process)
• In order to achieve an even yarn quality, the spinning
process must be controlled at every stage. This
underlines the fact that an even yarn is the result of a
controlled preparation process.
Up to that point, the origin of faults can often be
found in:
– Incorrect draft distributions in drawing, roving and
spinning
– Incorrect settings of the auto-leveling system at the
finisher drawframe
– Worn-out machine parts at any production stage

12. Unevenness Testing
• The evenness of each preparatory product before
spinning must be monitored and machine settings
adjusted according to the raw material parameters, the
machinery used and the required yarn quality level, as
defined in the yarn quality profile agreed between the
spinner and the retailer.
1. Visual Examination
2. Gravimetric Method (Cut and weigh method)
3. Capacitive Method
4) Mechanical method of sliver/roving evenness measurement
5) Optical method
6) Pneumatic Method
7) Acoustic Method

13. Visual Examination
• Yarn to be examined is wrapped onto a matt black
surface in equally spaced turns. The black boards are
then examined under good lightening conditions using
uniform non-directional light.

14. Uster Tester
• Measuring device (Uster Tester) is a parallel plate air
capacitor.
• The effect of introducing a nonconducting material such
as sliver, roving or yarn into the space between the
plates is to change the capacity of the capacitor.

15. • The change in mass per unit length is proportional to the
change in capacitance.
• These changes in capacitance are detected, amplified
and processed to obtain results in different forms.

16. Impact of Unevenness on Quality
• Irregularity is the most important quality characteristic of
yarn.
• In quality monitoring it plays a crucial role nowadays.
Importance of irregularity arises from the following
factors
• Strength of Yarn
• Better strength realisation from fibre can be achieved if
regularity of yarn is improved.
• The uneven one should have more thin regions than the
even one as the result of irregularity, since the average
linear density is the same. Thus an irregular yarn will
tend to break more easily during spinning, winding,
weaving, knitting or any other process.

17. Impact of Unevenness on Quality
• Fabric Appearance
• Irregularity has a profound influence on appearance of
yarn and fabric.
• More regular the yarn, better will be the appearance and
aesthetic value of the product. As a result, better sale
value can be achieved.
• Neps can influence the appearance of woven or knitted
fabrics quite considerably.

19. When the yarn evenness exceeds certain limits, it will result in a
cloudy fabric appearance.

20. Impact of Unevenness on Quality
• Fabric Defects and Rejections
• Fabric defects and rejections are critically influenced by
irregularity of yarns.
• Defects such as streaks, stripes, barre, or other visual
groupings develop in the cloth. Such defects clearly
apparent to eye and are usually compounded when the
fabric is dyed or finished, as a result of the twist
variation accompanying them.
• Periodic irregularities in yarn result in warp way streaks
and weft bars in woven fabrics leading to fabric
rejections.

21. • Image of Fabric with Strippes

22. Impact of Unevenness on Quality
• Fabric Feel
• Regularity contributes to a smoother feel. In apparel and
most of other textiles, smoothness is most desired
characteristic. Sale value of fabric is dependent, among
other things, on smoothness.
• Luster Effect
• Other fabric properties such as abrasion or pill-
resistance, soil retention, drape, absorbency,
reflectance, or luster may also be directly influenced by
yarn evenness.

23. Impact of Unevenness on Quality
• Productivity
• During winding even yarns give more production
efficiency. Uster cuttings less.
• Because of the lower incidence of weak places, fewer
end breaks are encountered with regular yarns in
weaving preparatory, weaving and knitting.
• Efficiency in these processes is improved leading to
higher productivity.

24. References
• Technical Consultancy Spinning
www.balu1.exactpages.com/unevenness.html
www.bala.bravehost.com/yarnirregularity.html
• Other Websites
www.scribd.com/yarn_unevenness
www.textile-technology.blogspot.com
www.yarnmanufacturing.blogspot.com