Quality Assessment of Man made fibre Filament and Yarn
Length Staple length determines the strength of yarn and
optimum twist factor. Extra long staple fibres require a lower twist factor and
enable higher spindle speeds. At the same time, they are more prone to nep
generation and result in inferior yarn appearance. In the case of cut staple
fibres, fibre length is measured by laying the fibre on a glass plate smeared
with oil to enable straightening of fibre. Length of fibre is measured on a
scale. Roughly about 10-15 fibres are thus tested and average determined. In
the case of variable staple fibre and synthetic tops used in worsted spinning
system, automatic fibre diagram machine by SDL or WIRA is used. Cut square
method is used to draw a tuft of fibres from the sliver and tuft is passed
between the plates of capacitor. A measure is thus obtained of number of fibres
in the tuft from the base to tip, from which cumulative fibre distribution is
obtained. The measurement is fully automated and the fibre diagram together with
results of mean length, upper half mean and CV are automatically displayed.
Fibre Fineness Fineness determines the count to which the fibre can be
spun. It also determines the strength, evenness and imperfections of yarn and
also the end breaks in spinning. Fibre fineness is determined gravimetrically or
by vibroscope.
Gravimetric method About 25 fibres are cut to a specified length
under tension by means of a template and weighed in a sensitive balance from
which weight per unit length is determined. The method is time consuming.
Vibroscope The fibre, held between two clamps, is tensioned by a
pre determined weight and subjected to transverse vibrations at variable
frequency. The frequency at which maximum amplitude is obtained, which is
resonance frequency, is determined, from which fineness is determined. Vibromat
by Textechno Herbertstein and Vibroscop by Lenzing are based on this principle.
The equipment is fully automatic and fibre fineness and CV are
displayed.
Tensile properties Fibre strength determines the strength of yarn
and fibre elongation determines yarn elongation. Single fibre testing is
normally done. Fibre is clamped preferably pneumatically between two jaws at a
predetermined tension. Lower jaw is traversed at a constant rate while upper jaw
is attached to a sensitive load cell of 50-100cN capacity. The load extension
curve, breaking load, elongation, work of rupture and modulus are determined and
displayed. Facility is also offered in some instruments to carry out tests under
liquid. Fafegraph and Favimat by Textechno Herbert stein and Lenzing are some
commonly used instruments. Bundle strength in stelometer is not recommended
because of fibre slippage under the jaws.
Crimp Crimp is an imprtant property that determines processing
behaviour in carding, drafting and fault incidences in yarn. Crimp frequency,
amplitude, crimp stability,crimp elongation, decrimping point are some of the
important properties that determine crimp. Crimp frequency and amplitude may be
determined by projecting a magnified image of fibre on screen. Opto electronic
sensor is used in some equipments to provide digital representation of fibre
held between two clamps at very low tension. Tensile tests using an extremely
sensitive force measuring system enables the measurement of the curve of crimp
force vs elongation, crimp extension, decrimping point and crimp stability.
Spin Finish Nature and quantum of spinfinish has critical
influence on performance of fibre. Lap licking, cylinder loading and roller
lapping are often traced to improper spinfinish.Spinfinish is traditionally
estimated by extraction with a solvent like carbon tetrachloride or benzene on
Soxhlet's apparatus. This is time consuming though accurate. ALFA 200 by Lenzing
and Rapid extraction apparatus by SDL are rapid methods for determining
spinfinish
Static Generation One of the problems encountered with synthetic
fibres is generation of static electricity. static generation is a cause for lap
licking, cylinder loading, web falling incidices and coiler chokeup in carding
and roller lapping in drawframe to ringframe. High hairiness in yarn, fabric
defects like stitches and floats are also attributed static electricity.
Sparking may take place due to static in synthetic carpets. Accurate instruments
are available for estimating the amount static charge and half decay time. Half
decay time denotes the time taken for static charge to come down to half its
level.
Abnormalities in fibre Presence of extraneous agglomerations on
the surface of fibre, fused and undrawn fibres, overlength fibres are some of
the abnormalities in fibre. Projectina is useful in to detect such abnormalities
in fibre. Baer sorter can be used for estimating over length fibres.
Filaments and Yarns Draw force is an important property in POY
yarns. It determines the performance of yarns and fabric defects like weft bars.
From measurement of draw force variations in molecular orientation, shrinkage
and dyeing chareteristics of the material can be assessed. Testing of draw force
under dynamic conditions has the merit of high speed testing and continuous
recording of variations in draw force. crimp force or crimp rigidity of textured
yarns and shrinkage force of flat and textured yarns are also important quality
characterics that determine in fault incidence in fabric. For measuring draw
force, the yarn is passed between two godets at high speed. The measuring roll
of force measuring system senses the yarn in between the two godets. Below the
measuring system, the yarn is heated by a heating system to high temperature.
There is usually facility to run the tester either with constant extension or
contraction for continuous measurement of yarn tension, or with constant tension
for continuous measurement of extension or contraction.
POY yarns The yarn is run at constant extension and draw force is
continuously measured and recorded.
Textured yarns Overfeed is kept between two godets to determine
shrinkage.The test is carried out at low speed and high temperature to determine
shrinkage. Alternatively the yarn is tested at high speed and low yarn
temperature to determine crimp contraction or crimp rigidity.
Flat yarns Shrinkage is tested by running yarn at high yarn
temperature
Broken Filaments Broken filaments arise from disturbances in
process, unsatisfactory manufacturing conditions and ineffective process
control. Broken filaments on running threads are determined by sensor consisting
of optical and infra red emitter and transmitter.
Tensile properties Breaking load, elongation and work of rupture
are some of the important characterics of filaments and staple fibre yarns. Lea
and single thread strength are the commonly used measures of yarn strength.
Single thread strength Tensile testers may be classified as
Constant rate of loading type
Constant rate of elongation
Constant rate of traverse
Constant rate of loading Rate of loading of yarn is constant in
these instruments throughout the test period. Inclined plane testers belong to
this category. Though this system of loading has many merits, this type of
tester is not used now a days
Constant rate of Elongation This is most popular system these
days. Tensile testers using a range of sensitive load cells are used for
determining breaking load and load elongation curve of yarn. The yarn is held
between two jaws with upper jaw connected to load cell and lower jaw traversed
downwards at a constant rate of traverse. Insertion of a new specimen into the
clamps and clamping of the specimen at a pre determined tension are done
automatically. Automatic package changers( with upto 20 packages) are also
provided with the tester so that after a prescribed number of tests are caried
from a package, the package is automatically changed to a new package and
insertion of new yarn to the clamps is automatically done. A series of high
resolution load cells enable testing of yarn with strength between 100cN to
1000cN. Software is provided for determining mean, maximum, minimum, S.D., CV,
Confidence limits and a host of other useful information. A high resolution opto
electronic sensor measures the elongation of yarn.
Constant rate of traverse Pendulam type testers belong to this
category. These have become obsolete because of long operting time and higher
errors.
Factors affecting strength
Gauge Length Gauge length has considerable influence on strength.
With increase in gauge length, strength will decrease because of increased
chances of occurrence of more weak places and the weak place being weaker. So
gauge length has to be standardised. Normally 50 cm gauge length is used except
in POY yarns. In the case POY yarns, 20cm gauge length is used because of the
high elongation of these yarns.
Rate of loading or elongation Rate of loading or elongation
infuences test results. With increase in rate of loading or elongation, time for
break decreases and a higher strength will be obtained. Rate of loading or
elongation is usually adjusted so that time for break is around 20sec.
Regular calibration of equpiment is essential to avoid erroneous
results. Calibration is done by hanging standard weights from upper
clamp.
Lea strength Lea strength test is common in staple fibre and
cotton yarns. The yarn is wound on a wrap reel of 54inch circumferance for 80
wraps to prepare a lea of 120 yards length. The lea is tested for strength in a
pendulam type of tester in olden days. Now a days a load cell is used in place
of pendulam to measure strength. Load cell is attached to the top jaw. Lea
strength has the merit of larger sampling length. It also takes into account
variability in the yarn and so can give a better indication of the performance
of yarn at later stages
Cyclic Loading The yarn is seldom extended upto breaking point. It
is more often subjected to repeated loads of small value. So performance of
material under cyclic loading may give better information about durability. The
specimen is loaded upto a certain load or elongation and brought back to its
original level. This action is repeated cyclically a number of times till the
specimen breaks. Number of cycles of loading withstood by specimen, is taken as
a measure of its strength. Alternately, the specimen is tested for strength
after a known number of cycles of loading.
Count Count of the yarn is determined along with the lea test by
weighing the broken leas. Auto sorter is used to determine mean, minimum,
maximum, S.D., CV, Confidence limits and other statistics from count tests of
leas from within and from different packages. The instrument consists of an
electronic balance to measure the weight of leas. This is equipped with a
software for determining the various statistics. Some manufacturers like
Textechno Herberstein have developed an instrument 'Autocount' for automatically
determining count. This consists of drive rollers which withdraw a known length
of yarn from a package and deposit it on an electronic balance for weighing and
estimating count and related statistics.
Irregularity Irregularity is an imprtant quality characteristic of
staple fibre and cotton yarns. It determines the appearance of yarn and fabric,
feel of fabric, performance of yarn in further processes and the strength
realisation of fibre in yarn. Irregularity is commonly measured by capacitance
type irregulaity tester. Uster Eveness tester is the widely used instrument for
measuring irregulariy. The yarn is passed between two capcitance plates at a
constant speed. The capacitance of the condenser varies according to weight per
unit length of yarn. The variations in capacitance are converted into voltage
and amplified. A continuous record of variations is obtained. Instantaneous
values of Mean Deviation%(U%) or Coefficient of variation(CV%) of the variations
is computed by an integrator and displayed. A number Condenser slots of
different sizes are used for tesing slivers, rovings and yarns as per their
count. Apart from short term variation, the instrument has also facility for
determining medium term term variations and variaance length curve of the yarn.
Imperfection Tester This is an attachment for determining the
extreme places. Thick places, Thin places, Neps, each of 4 categories based on
their size, are measured by the
instrument. Spectrograph Spectrogrpah carries out a fourier
analysis of mass variations in the material and displays a curve showing the
amplitude of different wavelengths present in the material. This is useful to
detect the presence of periodic irregularities and their wavelength in the
material. Periodic irregulaities are a source of weft bars and warp way dects in
woven and knitted fabrics. Spectrograph is a useful aid in minimising
periodicities caused by mechanical faults in machinery.
Precautions in testing
Conditioning Conditioning of material to test room is important,
when test room humidity varies considerably from manufacturing room. One hour in
the case of yarns and 2-3 hours in the case of rovings of conditioning would be
adequate. The bobbins should be laid in such a way that moisture enters from all
sides to ensure uniform conditioning. In the case of sliver, uniform
conditioning may take several days. So it is recommended that testing be carried
out immediately after the material is brought to testing room, after removing a
few top layers of sliver
Material Speed Material speed has to be standardised. With
increase in material speed, irregulaity will increase.Condenser slot
size Condenser slot size should be properly chosen. It is advisable to
use a slot that gives a lower material to air space.
Calibration Regular calibration has to be carried out to ensure
accuracy
Entanglement Tester Presence of entanglements affects the
performance of textured yarns. Traditionally, this done by piercing the yarn
with a needle and pulling it through the yarn till it encounters a compaction.
The distance covered by needle is then measured. This is however a time
consuming test and has been rplaced by automatic testers. Here the piercing unit
is automatically inserted into the yarn and the yarn advances at a speed
adjustable as required with its tension being measured. As soon the piercing
unit encounters a compaction, the tension in yarn increases and at preselected
tension levels, the length between two compction points is determined and
displayed. In addition, soft entanglement points are determined.
Friction Friction is an important property particularly in
filaments and sewing threads. The yarn is taken around a guide at constant speed
and the tension in the yarn before and after passing over the guide is measured
by a sensitive electronic tension meter. Coeffcient of friction is estimated
from the two tension values
Moisture Content Maintenance of moisture to the standard level is
important in yarn packages before they are packed, weighed and sold. If moisture
level is lower than standard by even 0.5%, considerable losses will be incurred
by mills beacause of wrong invoicing. Moisture is measured in the whole package
by a non contacting electrode senser which passes a harmless electromagnetic
field through the package. The high dielectric constant of water enables the
estimation of moisture content.
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information presented herein intended to substitute for the advice provided to you by any health care or other professional
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