Test Methods and Precautions

Cotton Fibre

• Length
  
  • Balls Sorter-Determines frequency length distribution of fibres from which mean,CV are etimated. This is a classical method, time consuming and is not commonly used
  • Baer Sorter Determines cumulative fibre length distribution. Effective length, Mean length and % short fibres are important parameters determined. Merits-1.Effective length is close to Grader's staple length 2.Provides accurate estimate of short fibre content Limitations. 1. Time consuming(2hrs per sample 2. Calls for considerable operator skill in sampling and preparing the diagram
  • Fibrograph - Fibres are randomly clamped at any point along their length on a comb, the beard thus obtained optically scanned from which 2.5%, 50% span length, uniformity ratio and short fibre% are determined. Merits 1. Simulates beard formed by fibres held by back or front roller nip of a drafting system 2. Very rapid(abou 15 min per sample 3. Does not depend much upon operator skill
  • Effective Length by Baer Sorter = 4 + 2.5% Span length in mm
• Fibre fineness
  • Gravimetric method - Fibres are counted either as whole fibres or after being cut to a finite length and weighed in a sensitive balance. Time consuming
  • Air Flow Methods
  • Micronaire Determines rate of flow of air through a known mass of fibres packed in a cylinder of known dimensions under constant pressure. This is expressed in mirograms/inch. Micronaire value is dependent upon not only liner density but also on maturity of fibres. Micronaire Mc = K/ MH where M = Maturity Ratio H = Fibre weight/unit length and K=constant
  • Micromat Fineness and Maturity Tester by SDL- Provides independent estimates of fineness and maturity of cotton
  • Arealometer-Determines specific surface area, fineness and maturity of fibres

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 inf;uences 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 variance 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
    This 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.