When considering yarn production from the economical point, it is advisable to use medium volume instruments (MVI) and low volume instrument (LVI) to achieve the same quality with medium cost, says V. Parthasarathi.
The global spinning industry is currently undergoing big changes, not least to wards the use of more modem and faster spinning machinery and computer system. These high speed machines require much cleaner raw material, and therefore they constantly challenge the testing machinery developers to provide faster and more accurate testers and quality controllers. The techniques de cribed in this article are mainly related with colton fibre testing.
In this article, the author has dealt with the following:
- Concepts in bale management
- Latest fibre testing instrument
- Principle of fibre testing using HVI
- Advanced fibres information system (AFIS)
- Fibre contamination system (FCS)
- Integrated indices for fibre quality
- Different methods of maturity measurement
- Image analysis
- Different methods of fineness measurement
- Advantageous of latest fibre testing
In this, HVI, AFIS & FCT play an important role, so their principle working method, working time and sample requirements are given. It gives various new technologies employed in printing, which will give an idea about the new testing methods and procedures.
Machine speeds and settings need to be matched to the properties of the fibre in process. With the annual production of cotton lint approaching 100 million bales, with spinner's demands & for increasingly high quality, and with the testing of every bale becoming almost universal in export mills, cotton testing has never been more important.
The following observations have made the cotton testing an important one.
The following observations have made the cotton testing an important one:
- Raw material typically accounts for half of the cost of making cotton yarn.
- Competitive man-made fibres can be made to more precise specifications.
- Scientific blending, based on knowledge of each bale, leads to cost & quality effective processing.
- Machine speeds and settings need to be matched to the properties of the fibre in process.
- Demands on the yarn quality are increasing.
Concepts in bale management
This is based on the categorising of cotton bales according to their fibre quality characteristics. It includes the measurement of the fibre characteristics with reference to each individual bale, separation of bales into classes and lying down of balanced bale mixes based on these classes. The reason for undertaking this work lies in the fact that there is sometimes a considerable variation in the fibre characteristics from one bale to another, even within the same delivery. This variation will result in the yarn quality variation if the bales are mixed in an uncontrolled manner.
Latest fibre testing instruments
Latest trend in the fibre testing was the developments of a single instrument in which all the major testing parameters can be tested. The instruments we discuss in this category are:
- High volume instrument (HVI)
- Advanced fibre information system (AFIS)
- Fibre contamination tester (FCT)
High volume instrument
High volume instrument systems are based on the fibre bundle strength testing, ie, many fibres are checked at the same time and their average values determined. Traditional testing using micronaire, pressley, stelometre, and fibro graph are designed to determine average value for a large number of fibres, the so called fibre bundle tests. In HVI, the bundle testing method is automated. Here, the time for testing is less and so the number of samples that could be processed is increased, quite considerably. The influence of operator is reduced.
The HVI testing is attractive due to the classing of cotton and the laying down of a mix in the spinning mill. This HVI testing is suitable for the extensive quality control of all the bales processed in a spinning mill. The mill is in a position to determine its own quality level within a certain operating range. The time for testing per sample is 0.3 minutes. It is best applied to instituting optimum condition for raw material. About 180 samples per hour can be tested and that too with only 2 operators.
Principles of fibre testing using HVI
The fibro gram method is preferred while preparing the sample for fibre length estimation. The sample has to be presented to the measuring zone by clamping the fibres at a random catch point. Here the fibro sampler is used.
The test specimen obtained using the fibro sampler/comb combination is a beard of fibres with individual fibres projecting to different length from the clamping point. In HVI, the strength testing is also done on the same beard of fibres with individual fibres projecting to different lengths from the clamping point.
In HVI, strength testing is also done on the same beard of fibres prepared for length measurement. While using the low volume instrument -fibro stelo for strength measurement, the sampling is done on the separate fibre bundles, of which 15 mm long is prepared after remounting the short fibres by combing. For micronaire testing, a sample of cotton weighing approximately 10 grams is used. For colour testing, random mass of fibres sufficient to cover the test window is used for measurement.
Measurement of different parameters using HVI & LVI
Both the high volume instrument and the low volume instrument use an optical principle of determination of fibre length. A narrow rectangular beam of light is allowed to fall on the specimen beard. The attenuation of Iight through the specimen at different areas of the beard is measured and used to obtain the different span length values. In the HVI, the tip of the beard is scanned first and scanning gradually proceeds towards the clamp while in the LVI, the beard is scanned in the opposite direction. In both the instruments, the span length values are obtained by actual measurement.
In the LVI, the fibrostelo is used. This module uses the pendulum lever principle of loading the specimen to estimate the fibre strength characteristics. A random sample of cotton fibres is prepared, short fibres being removed by combing so that all the fibres in the test specimen extend all the way through the jaws.
Breaking tenacity in g/tex = (breaking load in kg/ sample weight in mg) x 15 mm.
HVI 9000 Strength Measurement
HVI uses the "Constant rate of elongation" principle while testing the fibre sample. The available conventional methods of strength measurement are slow and are not compatible to be used with the HVI. The main hindering factor is the measurement of weight of the test specimen, which is necessary to estimate the tenacity of the sample. Expression of the breaking strength in terms of tenacity is important to make easy comparison between specimens of varying fineness.
The problem is overcome in the HVI 9000 by positioning the jaws and breaking the fibres at a constant "Amount" location across the beard. By breaking the fibres at a constant amount location, it is made sure that the samples are broken with a constant number of fibres between the jaws.
Therefore, raw data strength is directly proportional to the force to break the fibres. The raw data so obtained are then adjusted to desired Ievels by testing samples of designated values. In order to make the estimation of the specimen linear density accurate enough, a micronaire correction factor is normally introduced so that the strength values are not affected by variations in micronaire.
The micronaire module of HVI 9000 and the low volume fineness tester use the airflow method to estimate the fineness value of cotton. A sample known weight is compressed in a cylinder to known volume and subjected to an air current at a known pressure. The rate of airflow through this porous plug of fibre is taken to be a measure of the fineness of cotton.
The number of fibres in a given weight of cotton will be more in the case of finer fibres than in the case of finer fibres than in the case of coarser fibres. If air is blown through these samples, the plug containing finer fibres will be found to offer a greater resistance than the plug with coarser fibres. This is due to the fact that the totaI surface area in the case of the former will be greater than the latter and hence the drag on the air flowing past will be more. This differentiating factor is made use of to indirectly measure the fineness of cotton.
The instrument operates as follows. The chamber lid is closed; a piston at the chamber bottom compresses the fibre to a fixed and known volume. A regulated stream of air is then forced through the sample and the pressure drop across the sample is applied to a differential pressure transducer. The transducer outputs an analog signal voltage proportional to the pressure drop.
This analogue voltage is applied to an analogue to digital converter, which outputs a digital signal representing the voltage. Cotton with known fineness values is tested and the voltages obtained are used to obtain the calibration curve, which is used for all subsequent testing to display the cotton fineness.
The fineness is expressed in the form of a parameter called the micronaire value, which is defined as the weight of one inch of the fibre in micrograms. Maturity of cotton also influences the micronaire value.
The HVI colour module utilises optical measuring principles to define colour. The colour module has a photodiode, which collects the reflected light from the sample. The photodiode output is converted into meaningful signals using signal conditioners. The illumination of the sample is done with the help of two lamps connected in parallel.
Light from the lamps is reflected from the surface of a cotton sample on the test window. The reflected light is diffused and transmitted to the Rd and +b photodiode. These two signals are conditioned to provide two output voltages, which are proportional to the intensity of light falling on the respective photodiodes. These voltages are converted to digital signals from which the computer derives Rd and +b readings to be displayed on the screen.
HVI calibration principles
HVI uses a unique calibration principle by which routine calibration is performed by presenting samples bf known value to the instrument, which then makes adjustment to the raw instrument values, obtained so that measurements will agree with the calibration cotton designated values. The general principle by which this calibration is done involves a simple, two-point regression analysis.
In actual practice, raw values from the instrument will not match exactly with the designated values due to the variety of reasons including component ageing, atmospheric conditions and other sources of calibration drift. In such cases, the computer software makes adjustments to the raw data by using regression analysis. For the calibration of length, strength and micronaire, ICC cotton can be used while the calibration of colour has to be performed using the standard colour tiles supplied along with the instrument.
Advanced Fibre Information System (AFIS)
Advanced Fibre Information System is based on the single fibre testing. There are two modules here, one for testing the number of neps and the size of neps, while the other one is used for testing the length and the diameter. Both modules can be applied separately or together.
With the introduction of AFIS, it is possible to determine the average properties for a sample, and also the variation from the fibre to fibre. The information content in the AFIS is more. The spinning mill is dependant on the AFIS testing method, to achieve the optimum conditions with the available raw material and processing machinery. The AFIS-N module is dealt here and it is basically used for counting the number of neps and the size of neps. The testing time per sample is 3 minutes in AFIS-.N module.
This system is quick, purpose oriented and reproducible counting of neps in raw material and at all process stages of short staple spinning mill. It is thus possible, based on forecasts supervisory measures and early warning information to practically eliminate subsequent complaints with respect to finished product. The lab personnel is freed from the time consuming, delicate and unpopular, proceeding of nep counting. Personnel turnover and job rotation no more affects the results of the nep counting. The personnel responsible for quality can now at least deal with the unpopular neps in a more purpose-oriented manner than ever before.
AFIS -Working principle
Advanced Fibre Information System (AFIS)
Source: International Textile Centre
A fibre sample of approximately 500 mg is inserted between the feed roller and the feed plate of the AFIS-N instrument Opening rollers open the fibre assembly and separate off the fibres, neps, trash and dust. The trash particles and dust are suctioned off to extraction. On their way through the transportation and acceleration channels, the fibres and neps pass through the optical sensor, which determines the number and size of the neps.
The corresponding impulses are converted into electrical signals, which are then transmitted to a microcomputer for evaluation purposes. According to these analyses, a distinction is made between the single fibres and the neps. The statistical data are calculated and printed out through a printer. The measuring process can be controlled through a PC-keyboard and a screen.
It can be evidenced that the resu Its are provided very quickly using the AFIS measurement method and this with reference to approximately the same weight of sample material. This to be particularly the case when one compares the AFIS-N method with the frequently- used manual/ visual ASTM method. The savings are 150 with raw cotton and 1:25 with draw frame sliver.
Fibre Contamination System (FCT)
The FCT system is used for testing stickiness, neps, trash and seed coat fragments. The sample in the form of bundle is fed into a self-cleaning, micro carding device integrated in the FCT, to produce about 10m of transparent web in order to expose the impurities and contaminants in the best way possible. An area of 1 sq m per sample is tested.
Firstly, the web is analysed by a machine vision system for the presence of trash, neps and seed coat fragments and then they are pressed between the 2 stickiness crush rollers in the same manner as with the crush rolls of the commercial cards. The cotton web removed by the vacuum is then deposited on the stickiness crush rollers. They are examined by a laser signal analysis system for determining the stickiness content. Time taken for one sample is 40 seconds.
The advantages are:
1) Major contaminants like stickiness, seed coat neps and fibre neps are also considered as parameters for trading cotton.
2) Evaluation of performance of the precleaning systems such as cards, comber etc, for determination of their optimum operational setting and efficiency are done.
Integrated indices of fibre quality
If a versatile measure of cotton quality can be quantified and universally accepted, it would be enormously valuable in both technical and commercial applications. Numbers of different cottons, such as bales in a warehouse, are much easier to compare if each has one descriptive number rather than several. A single-figure index should meet the following criteria:
- It is based on common HVI results, and should not require purchase of additional instruments.
- It reflects a balance between market forces and technical considerations.
- Is, as often happens, more or fewer properties than usual are tested, the estimate is unbiased, ie for average cottons the premium or discount (p/d) is unchanged.
Different methods of maturity measurement
- The notion that micronaire is a measure of fineness still appears frequently. Double compression airflow measurement.
- Polarised light analysis
- Causticaire, ie, micronaire of conditioned specimens before and after soaking in concentrated caustic soda solution
- Centrifugal methods
- Near infrared spectrometry
- Image analysis
Image analysis of both longitudinal images and sections was applied successfully to the measurement of maturity; Less exactly when the minimum projected width was used. In general image analysis can measure the mean and distribution of several fibre transverse dimensions:
- The apparent fibre diameter, a nebulous concept in a fibre having such a non circular shape, but a useful measure of micronaire, ribbon width is a better term in most image analysers.
The other properties are more apparent when cross sections are made:
- Martin radius, the mean of 8 equidistant radii from the centre of gravity
- Total cross sectional area, including both fibre wall and the hollow lumen
- Cross sectional area of fibre wall an exact measure of linear density and considered along with the above area, a measure of maturity
- Perimetre, nearly a constant for a given variety regardless of the degree of thickening and perhaps the best available measure of standard linear density
Degree of thickening, usually defined as the cross sectional area divided by the area of a circle having the same perimetre as the fibre. A double compression instrument, where estimates of linear density and maturity depend on small differences between airflow properties measured at two level of compression. It showed the effect of varying sample mass necessary for automated HVI lines and found drift in the micromat rather than the step changes of earlier models.
Different methods of fineness measurement
Direct measurements of mass divided by length, the later is the limiting factor in accuracy; this has been developed by image analysis, where it is commonly expressed as cross-sectional wall area. Measurement of resistance to airflow, by single or double compression
Advantages of latest fibre testing techniques:
- The results are practically independently of the operator.
- The tests are based on the large volume of samples and so they are more significant.
- The results are summarised and are immediately available.
- The raw material data is utilised in the best manner.
- As a result of the fibre material, the problems can be predicted and corrective measures taken before such problems can occur.
Attempts are being made to express fibre quality by a single index. As the number of properties tested in HV lines continues to increase, so too, does the average person's confusion about the meaning of results.
- Cannot tell readers the relative importance of properties for their own circumstances.
- Do not distinguish fully between the different measures of the same property, such as uniformity index and uniformity ratio.
- Do not address the diversity of ways of measuring some properties, such as maturity or measuring some properties, such as maturity or stickiness, and the different results obtained from different instruments or techniques.
- FCT is used at the gin as a quality controller and immediate cotton classing tool.
The traditional test of counting neps is so labor intensive and subjective that it invites innovation; the first step alone, is to prepare a card web, for which few laboratories have the machinery. A major response has been the development of the NEP module, AFIS-N, of the Advanced Fibre Information System.
As the fibres are separated aerodynamically then passed across a photocell, the electro-optical sensor distinguishes the wave forms of single fibres, trash and neps. Whether the sugar is metabolic or entomological and whether the secretion is from white fly or aphids matters little to the person trying to process sticky cotton.
Model liquid chromatography makes it possible to separate characterise and quantify the sugars. The sensitivity of rotor spinning to accumulations of trash and dust are led to the development of more sensitive instruments for testing small impurities in sliver. The AFIS apparatus lends itself to measurement of trash, dust and neps from lint through the roving stages and provides the entire size distribution.
The above discussion gives an idea about main latest fibre testing techniques using HVI, AFIS, FCT and this concludes that one can achieve higher accuracy with least time in this system. Fibre testing is an important part in the final product, so it is clear one can achieve great quality with accurate testing techniques, which were discussed in this paper. Apart from this, various methods are also included which will give proper results in fibre testing.
When considering from economical point, it advisable to u medium volume instruments (M\ & low volume instrument (LVI) to achieve the same quality with medium cost. With present depression in textile field, it is essential to achieve good quality raw material by good testing techniques and achieve good growth in textile field. It is essential in this competitive global market survival with this latest fibre testing techniques.
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Originally Published in: The Indian Textile Journal April 2008
About the Author:
The author is with the Kumaraguru College of Technology, Coimbatore.
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