Prepared By Amirul Eahsan
Prepared By Amirul Eahsan
WHAT IS IRREGULARITY OR UNEVENNESS OF FIBER?
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
TYPES OF METHODS
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 place
2. Thick place
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 thin places and thick places refer to a particularly valuable indication that the raw, on the one hand, quite considerably affect the appearance of a woven or knitted fabric. Furthermore, an increase in the number of material or the method of processing has become worse
IRREGULARITY OF YARN IS RECOGNISED AS:
1. Variation of liner density 2. Variation of linear thickness 3. Variation of linear twist 4. Variation of linear strength 5. Variation of linear color
Periodic variation:
Inspection of some traces express definite sequences of thick
place in the strand of material. These forms of irregularity refer to
as periodic variation.
Classification of periodic variation:
Classified according to their wavelength, using the fiber length as
a length unit,
i. Short terms variation:1 to 10 times the fiber length.
ii. Medium term variation: 10 to 100 times the fiber
length.
iii. Long term variation: 100 to 1000 times the fiber
length.
Classification of the causes of yarn
irregularity given by Martindle:
Properties of raw materials.
Inherent shortcomings in yarn making & Preparatory
machinery.
Mechanically detective machinery.
External causes due to working conditions & inefficient
operation.
Properties of raw materials:
There are some variables in the properties of natural fibre which
causes variety of problems for the spinners. Fibre length is the
major variable of them. There are others variables which have
some effect on the yarn properties such as; surface character,
fineness, shape of cross-section, maturity, crimp etc.
Inherent shortcoming in yarn making & preparatory
machinery:
Variation in fibres hamper the fibre manipulation by rollers,
aprons, gills & other machine parts. To give the best result the
machines is set within the limitations imposed by the materials.
The ‘Drafting Wave’ is one example of irregularity due to the
inability system to control each fibre. Short fibres are formed due
to this kind of irregularity which float in the drafting zone & move
forward in an irregular but cyclical manner which results in the
drafted strand having thick & thin places.
Mechanically detective machinery:
An efficient maintenance system in machinery is essential to
keep the level of irregularity within bounds/minimum. Many
defects hampers the mechanism of machinery such as; machine
draft out of adjustment, bearings become worn, components get
damaged, lubrication systems clog/stops, and dirt etc.
External causes due to working condition & insufficient
operation:
Men & women working in a mill must be aware of their duties,
their lack of responsibilities may decrease the quality control.
Operatives can spoil yarn too, by poor piecing, careless oiling &
clearing, & general slack work.
Effect of irregularity in weight per unit
length:
1. Strength
2. Fabric appearance
3. Stripy Knitted goods
4. Dyeing & finishing faults
1. Strength:
Thin places in slivers, roving or yarns are the weakest places.
During the process when the strand has to support itself during its
passage from one point to another, more irregular strand will have
the greater chance to breaks usually means the introduction of
other faults e.g. thicker sliver or roving of the piecing or knots in
yarn.
2. Fabric appearance:
A certain amount of irregularity is unavoidable & these causes
irregularity in appearance of fabric. Excess variation in irregularity
will produce an objectional appearance on the fabric surface. This
effect of irregularity depends on some factors like, the types of
variations(periodic or non-periodic), fabric structure, fabric
dimensions, use of yarn(as warp or weft),& fabric finish. Fabric
appearance can be classified as below-
i. Diamond bars &
ii. Weft bars or block bars
3. Stripy Knitted goods:
Weaver fares are better than the knitter the irregularity in one
set of threads may concealed/hid by other side. Knitting m/c are
set to produce a given quality of fabric from a known count to
cone or where one cone contains yarn of different count, the
result is stripy knitted fabric.
4. Dyeing & finishing faults:
The thicker & soft parts of the yarn take up more size than the
thinner & harder regions. As a result during the dyeing process
the distribution of the residual size may be uneven & cause
difficultly in achieving a level dyeing.
Methods of measuring irregularity:
Visual method
Cutting & weighing method
Variation in thickness under compression
-W.I.R.A roving evenness tester
-LINRA roller yarn diameter tester
Electric capacitance tester
-Fielden-walker yarn evenness
recover
-USTER evenness tester
Photo electric tester
-W.I.R.A photo electric tester
-LINRA tester
Miscellaneous method:
-Airflow
-Mercury displacement.
Premier evenness Tester:
Imperfections/km- No.of mass increases (Thick place).Mass
reduction (Thin place) & short mass increase (Neps) measured
simultaneously at the following values-
Thin places: -30%,-40%,-50%,-60% etc.
Thick places:+30%,+50%,+70%,+100%etc.
Neps places:+140%,+200%,+280%,+400%etc.
Recommended Values:
For ring spun yarn:-50%,+50%,+200%
For open end:-50%,+50%,+280%
Fig. Premier Evenness Tester
The ‘USTER’ evenness tester
Definition : The instrument or M/C by which unevenness (U%) ,
co-efficient of variation of mass (CV%), yarn hairiness,
imperfection index (IPI) and thick, thin place, neps etc of yarn,
roving sliver can be measured or calculated is called Uster
Evenness Tester or Uster Tester .
Fig. USTER Evenness Tester
Two oscillators A and B have equal frequencies when there is no
material in the measuring capacitor C. When the two frequencies
are superimposed the difference in frequency is zero.
The presence of material in the capacitor causes its capacity to
change and so alter the frequency of the oscillator. A there will be
a difference between the two frequencies which changes
according the to the amount of material between the capacitors
plates. The suitable circuits D translate these frequencies
differences into signals which are indicated on the meter M , drive
the pen of recorder , and are fed into the integrator which
indicates the average irregularity either as percentage mean
deviation or coefficient of variation according to the model used.
The actual tester is illustrated following external features:
The ‘comb` of eight measuring capacitors of different sizes.
The creel and guides to control the material.
The traverse roller which can control the material speed over
a range from 2-100 yd per minute.
The control switches.
The mater on the main unit which indicate the momentary
variations in the material.
The indicator which indicates P.M.D or C.V.
The high speed pen recorder whose chart speed can be
varied between 1 and 40 in/min.
Advantage of Uster Evenness Tester:
The CV% measured by USTER give a measure of variation of weight per unit length.
This instrument measure the irregularity of material at high speed (2-100ft/min)
It can show both % of M.D & C.V. of material. The recorder of pen can work at a high speed of 100yds/min
Disadvantages of Uster Evenness Tester:
The result obtained by this M/C is affected by moisture content of material.
This M/C can't detect the thick and thin place. In this instrument material up to a certain thickness can be
tested.
Uses of Uster Evenness Tester:
1. Evenness measurement of yarn, roving and sliver. 2. Measurement of imperfection ( thick, thin place) 3. Mass analysis. 4. Spectrogram analysis/frequency analysis. 5. Yarn hairiness measurement 6. Fabric simulation i,e before making fabric.The yarn gathering
knowledge about the quality of the fabric. 7. Variation of trend analysis.
PHOTOELECTRIC EVENNESS TESTER:
When a beam of light is directed on to a photoelectric cell an
electric current is produced. The magnitude of the current is
directly proportional to the intensity of light falling on the light-
sensitive part of cell. If part of the light is cut off by a yarn passing
between its source and the photoelectric cell, the current following
will vary as thickness of yarn varies.
Irregularity testers based on this principle usually consist of the
following parts:
Fig: schematic diagram of a photoelectric evenness tester
1. A stabilized source of light (A).
2. An optical system to control the light (B).
3. A slit of variable width through which the yarn passes (C).
4. A photoelectric cell (D).
5. Suitable meters and recording instruments to measure the
current variations and produce an irregularity trace (E).
6. A yarn-control device.
The coefficient of variation measured by photoelectric methods
does not give a measure of the variation of weight per unit length.
In the papers referred to, the coefficients of variation obtained by
photoelectric testers and the cutting weighing of 1 in. lengths have
been compared for a number of yarns. The conclusions reached
that yarns of comparable cont and construction are graded by a
photoelectric tester in the same order as by cutting and weighing
1 in. lengths, and that the numerical value of the coefficient of
variation measured optically is greater than that of short term
weight variation.
Difference between the Electronic Capacitance
Tester and Photo Electric Tester:
Electronic Capacitance Tester Photo Electric Tester
1) Main part of this tester is a
capacitor.
1) Main part of this tester is photo
electric cell.
2) In this tester the evenness is
measured by the variation in
frequencies between the two
oscillators.
2) Evenness is measured by the
variation in the intensity of light of
different yarns.
3) This tester is based on the
mechanism of the capacitor.
3) Based on the optical system.
4) Consists of a yarn controlling
device, light source etc.
4) Consists of yarn controlling
device, light source and suitable
meter.
5) Comparatively slow in giving the
value of irregularity in PMD or CV.
5) Gives quick and accurate value
of irregularity in terms of CV.
6) The results are affected by the
m/c of material.
6) The results are not affected by
the m/c of material.
7) Figure
7)Figure
The LINRA roller:
An instrument developed by the Linen Industry Research
Association.
The yarn passes round a series of rollers so that it lies between
four nips.
The pressure of each roller being 5g.
The four thickness so measured are equally spaced within 1
inch.
The top roller supports a small anvil on which the plunger of
micrometer rests.
A series of measurements may be taken along the yarn.
Modifications to such an apparatus can convert from a hand
operated instrument with intermittent measurement to a
continuous tester with the movement of the top roller being
used to actuate mechanical or electrical pen recorders.
This instrument works satisfactorily with coarse and medium
flax yarns up to about 60s lea (27.5tex) and on rayon staple
yarns with twist factor of 1.5 or more.
Conclusion
The evenness of yarn is an important index of quality control of
textiles, so the researches about the yarn evenness test method
have been the hotspot in the textile measurement for recent
years. From this assignment we got an opportunity to learn more
about the importance of evenness. We have discussed about the
factors which affects the evenness of yarn. Most important thing is
that, we have gathered a clear concept about some of the major
methods and machines or tester to determine the evenness of
yarn. All the knowledge we have acquired from our analysis which
we made to perform this assignment successfully will help us to
understand more about the necessity of various textile testing
especially the evenness testing.