BALE MANAGEMENT SYSTEMS & MIXING STRATEGY

  Cotton yarn: Quality depends on mixing strategy

BALE MANAGEMENT    The function of Bale Management is very much unique to the Spinning Industry. The module should have a function in such a way that the system should automatically generate the issues for Mix or a Count, where all the Lay down or Issues should have a consistent quality parameter both in terms of average and SD.The system should be capable of generating the Issues or Lay down as per the User requirements like: Standard Distribution – The average Quality of all Parameters are same from the first issue to the final issue. Normal Distribution – The Issue should contain the required average for the selected Quality parameters and it should follow the theory of normal distribution. Moving point distribution – If the user prefers to have a very narrow working range for the selected Quality parameter, the system should generate the issue and according to the availability the shift in the parameter should happen gradually till all the bales are exhausted in the Lot. For example if the User would like to have the Micronaire working range of 4.2 to 4.6. After exhausting the bales in 4.6 Micronaire, the system should automatically shift to 4.1 to 4.5. After exhausting 4.5, the system should go for 4.0 to 4.4.  Factors considered for efficient Bale management system: Origin (Country of origin) Length & Length uniformity Fibre fineness (MIC value, ugm/inch) Strength & Elongation SIC (Spinning Consistency Index) Maturity Color Trash% etc. Which should be Consider in a particular lot? Micronaire range of the cotton bales used should be same for all the mixings of a lot. Micronaire average of the cotton bales used should be same for all the mixings of a lot. Range of color of cotton bales used should be same for all the mixings of a lot. Average of color of cotton bales used should be same for all the mixings of a lot. Range of maturity coefficient of cotton bales used should be same for all mixings of a lot. Average of maturity coefficient of cotton bales used should be same for all mixings of a lot. It is important to have an effective bale management system to ensure correct fiber combinations. In cotton lines we employ automatic bale management systems to certify correct mixing percentages for consistent fibre properties to avoid shade variation during fabric dyeing. Cotton bales are stocked up to 6 months in order to provide our repeat customers with consistent quality from the same batch of fibers. In synthetic lines, automatic weighing blenders are used to measure exact blending percentages. Blended fibers are maintained up to 20 days to ensure that every customer receives a homogeneous blend of yarns with low barrenness. Mixing Strategy  A systematic mixing plan by maintaining the lowest possible variation in cotton quality parameters ensures trouble-free yarn production with consistent yarn quality. In a spinning mill control over cotton quality is involved in the three steps stated below: 1. Approval of cotton bales: It is very important to purchase the right quality cotton bales according to defined quality standards. As cotton is a natural fibre and also a commercial commodity, its availability, prices and quality vary time to time based on supply and market demand, and therefore it is very important to select the required quality cotton bales at reasonable prices which should be viable for spinning industries. Approval of cotton bales should be very strict against its standards, otherwise this will result in stock of huge quantity of off-standard bales. 2. Grading of cotton bales: After reaching cotton bales in mill, these should be tested and properly graded according to mill standards. If possible it should be stacked in warehouse grade-wise. 3. Mixing planning: According to grade-wise, cotton bales stock and spinning count pattern, cotton bales must be selected for mixing plan with minimum possible variation. Critical cotton properties The following are the most common cotton properties, which are measured at the time of selection of cotton bales: 1. Length: Measured either in inches or in mm. 2. Fineness: Cotton fibre fineness usually measured in micronaire, ie, weight of fibres in microgram per inch. 3. Strength: Measured as fibre bundle breaking force in gms per tex. 4. Maturity: Measured either in term of maturity coefficient or in maturity ratio. 5. Uniformity: Measured either in % or as an index. 6. Short fibre content: Measured as %. 7. Trash content: Measured as %. 8. Moisture content: Measured as %. 9. Brightness: Represents as Rd value against degree of reflection. 10. Yellowness: Represents as +b value. 11. Colour grade: Colour grade is determined with conjunction of Rd & +b value. All in one single value - SCI Spinning Consistency Index (SCI) is a calculated value based on a regression equation. This equation takes into account all HVI properties and calculates one value to be used on each sample tested. The SCI is an index derived with data from a large number of cotton samples having a wide range in properties that is related to test data from yarn spun from each sample. By multiple regression analysis the contribution of each fibre property to yarn properties is revealed. Consequently, a single SCI value is influenced by inherent relationships of cotton micronaire, length, uniformity, strength, Rd and +b. The following are SCI equation for the most important HVI measurements including colour: For HVI Calibration Mode: SCI = - 414.67 + (2.9 x Strength) - (9.32 x Mic) + (49.17 x Length in inch) + (4.74 x Uniformity Index) + (0.65 x Rd) + (0.36 x +b) If no colour module is installed then SCI equation is as follows: SCI = - 322.98 + (2.89 x Strength) - (9.02 x Mic) + (43.53 x Length) + (4.29 x Uniformity Index) For ICC Calibration Mode: SCI = - 414.67 + (2.9 x Strength) - (9.32 x Mic) + (49.17 x Length) + (8.61 x Uniformity Ratio) + (0.65 x Rd) + (0.36 x +b) If no colour module is installed then SCI equation is as follows: SCI = - 322.98 + (2.89 x Strength) - (9.02 x Mic) + (43.53 x Length) + (7.79 x Uniformity Index) Effect of fibre properties Each cotton characteristic imposes its own influence on spinning process and ultimately on the yarn quality explained as below: Fibre length: Fibre length is the most important factor for spinning. Selection of cotton fibre is usually based on fibre length only and it is also the main deciding factor for count range to be spun. Hence this impacts each and every process of spinning operation. The following are the main influencing areas where fibre length impacts on spinning performance and yarn quality. Count pattern and spinning production rate: Fibre length is the main deciding factor for count pattern in a spinning mill. Production speeds of spinning machinery also very much depend on fibre length of cotton. In modern high production spinning set-up cotton fibre length cannot be selected as per old traditional systems. For modern high production spinning set-up cotton fibre properties required are as below: Definition of modern high production spinning set-up: Spinning set-up running with more than 50 kg/hr carding production rate, getting 40s converted GPSS above 110 with a HOK level of below 12 in spinning, equipped with modern Autoconer. Working performance: Higher length variation causes poor working performance of spinning processes, which lead to lower production and huge disturbance in process. Only achieving average fibre properties as per standard is not guarantee for better spinning performance; hence it should be ensured that each cotton lot should have minimum length variation. Cotton lots having higher variation should run in controlled quantity, which is given in next chapter, ie, Mixing plan. Higher irregularity or unevenness: If cotton lengths are not suitable to the spin plan or have higher length variation, eg, to produce the 30s NE count if cotton selected below 28 mm length, it will lead to high irregularity or unevenness in yarn and will also cause higher autoconer cuts and fabric defects. High hairiness: Cotton selected of lower fibre length will cause high yarn hairiness, leading to high fluff generation in department, more tendency of traveller loading, excessive EYC cuts and cops rejection at autoconer and pilling or barre problem in fabric. Fineness/Micronaire of fibre: The second most important factor for spinning mills to decide the count pattern is fineness of cotton fibre that is commonly known as micronaire. Same as fibre length, it is also the deciding factor for cotton selection and spin plan of a spinning mill. Number of fibres in yarn cross-section: Spinning of a particular fineness of yarn is based on the fibre length of cotton and its fineness which decide how many fibres will be in the cross-section of yarn. It seems that below 90 numbers of fibres in cross-section of ring spun yarn (for coarse & medium count range) can lead to major working problem at spinning with lower yarn strength while more than 300 numbers of fibres in cross-section of yarn lead to higher unevenness in ring spun yarn. Hence it is recommended to use right micronaire value for a particular spin plan. Table 1 recommends guideline for micronaire value for different count range. Higher neps generation: Low micronaire value causes high neps generation in blow room and carding, which lead to higher neps and imperfection in final yarn and white spots or dots in dyed fabric. Low fineness cotton fibres always have tendency to form fibre entanglement or neps, hence it is recommended to reduce blow room beaters speed and carding production rate whenever low micronaire value cotton is being processed. Barre or shade variation: Too much variation in micronaire values can lead to barre problem or shade variation in fabric. Genetic character: Micronaire value of any cotton fibre strongly exhibits its origin variety genetic character. If a cotton fibre fineness value does not belong to normal distribution of its origin variety, it means either this fibre is immature or it does not belong to the said variety. This type of cotton lot should be avoided. Table 2 shows a general trend for critical cotton characteristics for most common Indian cotton varieties. Fibre strength: Fibre strength is an important factor for working of spinning process and yarn strength. Fibre strength is directly proportionate to yarn strength and working performance of spinning machines. However it seems that strength of fibre is also related with others cotton properties like fibre length, micronaire and maturity. Fibre bundle strength is a deciding factor for spinning machine speeds. If fibre is not adequately stronger, then there has to be a compromise with production rate and force to reduce speed of machines. Similarly ultimate yarn strength will be less as required. Maturity: Maturity of cotton fibre is related with its growing process. A fully mature fibre means the fibre has achieved its complete growth process and has developed in all respects. Mature fibres achieve and exhibit better fibre properties in all respect as per their origin of variety characteristics, as described in Table 2. Fibres which are not grown in a normal condition or picked up before their complete growth will fail to have their specific characteristics and will be shorter, weak & fine as compared to mature fibres. In HVI testing, the maturity index is a relative value that is calculated by using a sophisticated algorithm including other HVI measurements, such as micronaire, strength and elongation. It indicates the degree of cell wall thickness within a cotton sample. Table 3 shows the average test results of total 98241 number of bales wrt degree of maturity index. Hence maturity is a key parameter for cotton selection and spinning process. In various testing methods it is represented as maturity coefficient, maturity index or as maturity ratio. Higher the numerical value means higher will be maturity of fibres. Low maturity value impact at spinning process or yarn quality as below: Fibre rupture: Immature fibres breaks into multiple pieces in blow room and carding section during metallic action of beaters and carding wires cause increase in short fibres & micro-dust which further reduce the effective length of fibre and strength. This fibre rupture increases yarn breaks, yarn defects, spinning waste and yarn imperfection level, subsequently reducing the working efficiency of spinning machinery causing lower production, yarn recovery with lower yarn strength. Dead cotton neps: Immature fibres create heavy neps during spinning process, which ultimately reflect as white dots in dyed fabric, causing fabric rejection. Table 3 and Figure 1 showing that fibre maturity is directly proportionate to fibre length & bundle length of fibres. Uniformity: Settings of spinning drafting rollers depends on the length of fibres. To obtain optimised and correct spinning draft roller settings fibre uniformity is very important factor. Cotton fibre with low uniformity causes very high variation in process and creates difficulties for spinners to set the right setting for particular cotton mixing. Short fibre content: Fibre lengths below half inch, ie, below 12.5 mm are considered as short fibres. High short fibre % in cotton leads to many problems in spinning process such as high end breaks, excessive yarn defects, very high fluff generation, lower machine efficiency, lower yarn realisation and higher worker turnover. Trash content: High trash content in mixing causes higher waste extraction, high end breaks rate at ring frame, higher yarn imperfection and Classimat faults. It also appears as black dots in greige fabric, usually known as kitti particles. Moisture content: Cotton is a hygroscopic fibre, it absorbs moisture from a high humid atmosphere and evaporates it when stored in dry atmosphere. For smooth spinning operation, the cotton fibre should have moisture in the range of 6 to 8 per cent. Moisture content below this limit causes high fluff generation and higher end breaks, while cotton having high moisture content is difficult to open and clean in blowroom-carding, hence creating very high imperfection and yarn slubs which again cause end-downs at ring frame stage, lowering the production efficiency with increase in yarn imperfection and defects. Colour Grade: Variation in colour grade is a major responsible factor for shade variation in cotton yarn and fabric. Grading of cotton bales It is very difficult to grade a cotton lot based on all 11 parameters mentioned above, and hence we can decide four major critical parameters for grading of cotton bales, which represent the overall grading of cotton. These parameters may be utilised for cotton bales procurements, stacking and finally for selecting the cotton bales for Mixing Plan. If a mill's cotton purchasing is strictly from a particular region then there will be less chances of too much variation in colour grade, and so critical parameters for them might be as below: 1. Fibre length: A must requirement to decide the count pattern of spin plan. 2. Micronaire: A must requirement to decide the count pattern of spin plan. 3. Maturity: Required to control the quality of cotton, it also represents to fibre strength. 4. Short fibre %: Required to control the working performance of spinning mill and yarn realisation. It also represents to uniformity of cotton. Less the SFC more the uniformity of fibre. If cotton procurement of mill is from different regions, then there might be a possibility of variation in cotton colour grade. Hence in that case, colour grade also is to be taken into consideration. Mixing planning Object: To mix cotton fibres of different bales in a homogeneous form to overcome the variations of cotton properties and maintain the uniformity and consistency throughout the spinning process and in yarn quality. Making a mixing plan may be a tedious job, but spinning performance totally depends on it. Hence investing time and skill on mixing plan will pay consistency in spinning wrt working and quality as a result. A practical mixing plan may be prepared as below: 1. First check the availability of grade-wise cotton bales and spin plan. 2. Then select grade-wise number of cotton bales in ratio of present cotton bales stock and as required for spin plan. Stock of cotton bales to be utilised in such a manner that long staple cotton (ie, A ++ in Table 4) to be used for fine count pattern. 3. If present spin plan is not supporting fine count range then it should be preserved for future, if there is a possibility in future. 4. To control the length variation in a selected mixing plan either we have to consume "A++" grade cotton first or we have to preserve it till all the bales of "B" grade cotton are exhausted. 5. Refer to cotton grading system as per Table 4, in a mixing plan either there should be top three grade bales (ie, A++, A+ & A Grade) to be used or lower three grade bales (ie, A+, A & B Grade) to be used. 6. In a mixing plan fibre length range should not exceed more than 2.5 mm and micronaire range to be maintained below 0.60. (Refer Table 6.) 7. Low variation in fibre length is required to set right the correct gauges of spinning drafting rollers and to avoid the fibre rupture and drafting waves. Variation in fibre length causes poor quality and working of spinning along with high fly generation. 8. Low micronaire range must require maintaining the uniform number of fibres in cross-section of yarn, which ensures better yarn evenness and single yarn strength. 9. Number of cotton lots in a mixing plan to be selected in such a manner that participation of each cotton lot bales should not be more than 5 per cent. For example, if we are making a mixing plan for 100 number of bales than there should be at least 20 number of cotton lots contributing 5 bales each for each batch. This is required to avoid wide changes in the mixing plan, even if there is change of one single cotton lot then there will be only 5 per cent change in particular mixing plan. 10. Replacement of cotton lot should be in decreasing and increasing order. If one cotton lot is going to be exhausted in mixing that should be run out slowly and replacement lot to be inserted in same way, which ensures minimum variation in process during replacement of cotton lots. Table 5 shows how one cotton lot should run out and another should replace it. 11. This system ensures very little change in mixing on day-to-day basis. Only one bale replacement out of 100 bales mixing will be only 1 per cent change in mixing plan, which is almost negligible. 12. To further minimise the variation during lot change care should also be taken so that same grade cotton or cotton lot with same characteristics are replaced. 13. If cotton is procured from different station, then try to replace the cotton lot with same origin or station. Monitoring of colour Grade: Monitoring of colour grade is also a very important job to control the shade variation or barre problem in resultant yarn and fabric. The following guidelines might be useful for better control on colour grade of cotton. Rd value: Usually in Indian cotton Rd values lie between 72 and 82. For better control it is advisable to maintain the Rd value range below 5 in daily mixing plan. (Refer Table 6.) +b: +b value significantly differs region-wise and variety-wise. In Indian cotton +b lies in range of 6.0 and 11.0. It is better to maintain the +b range below 2.5 in one variety cotton mixing. (Refer Table 6.) Colour grade: The colour grade is determined by locating the quadrant of the colour chart in which the Rd and +b values intersect. For example, a sample with Rd value of 72 and +b value of 9.0 would have a colour code of 41-3. Colour grade is represented in three digits as xx-y. First digit represents brightness of cotton. Lower the number higher will be the brightness of cotton. Second digit represents yellowness of cotton; yellowness increases with increase in second digit number. There are 25 colour grades and five categories of colour grades. Indian cotton usually lies in white and light spotted categories with middling to good middling colour grade. To avoid the shade variation or barre problem it is advisable that only four adjacent cotton grades be selected for mixing plan, eg, 11, 12, 21, 22 may run together but running 11 with 31 or 11 with 13 should be avoided. Colour grade categories of spotted, tinged and yellow stained, ie, represented by 3, 4 & 5 numbers in second digit of colour grade should be strictly avoided. Use of off-standards bales: In spinning mills there should be strict control on purchasing of cotton bales or passing for quality specification, otherwise a lot of off-standard cotton bales accumulate in mill godown, which will never allow spinners to prepare a controlled mixing plan. However even after better control there might be chances of some off-standard cotton bales lying, which do not belong to normal standard either for any single parameter or due to failure of multiple parameters. These bales are to be utilised in very controlled manner so that their bad quality should not affect spinning process and yarn produced. Based on quality parameters of off-standard bales these may be consumed in controlled manner (ie, in a range of 1 to 5 per cent according to quality parameter) as below: 1. If there is only a slight variation in any one or two parameters from standard, then such cotton bales may be considered in mixing with 3 per cent contribution. For example, if standard for fibre length is above 29 mm and the micronaire is above 3.6 and actual values of a cotton lot found are 28.5 mm and 3.5 mic value then such cotton bales may be considered with 3 per cent ratio in mixing. 2. If there is a significant deviation in a single value, such as instead of 29 mm fibre length it is 27 mm while rest other parameters observed are within range, in that case such cotton bales may be consumed with below 2 per cent ratio. 3. If 2 - 3 parameters fail to meet the standard specs, then such cotton bales should be strictly consumed with below 1 per cent ratio. Mixing Plan monitored through Spinning Consistency Index: Instead of monitoring of several parameters for a mixing plan, it might be more easy to monitor the "SCI" value of cotton properties obtained from HVI test report. The following guidelines may be adapted for using "SCI" value in cotton grading and mixing planning: 1. Cotton may be graded in five grades and stored in warehouse according to their SCI value, ie, as below: 2. According to stock position a mixing plan must be prepared and average, minimum, maximum and range should be drawn for SCI value. 3. Average SCI value of daily mixing should be maintained constant and there should not be more than +/-2 deviations on daily basis. 4. It should be tried that maximum and minimum range of SCI value should not be more than 30, for an ideal mixing plan it should be maintained below 20. 5. The total range of maximum and minimum SCI value should also be monitored on a daily basis and there should not be too much variation on daily basis. Conclusion Cotton is the most favourable fibre for spinning industries. As cotton is a natural fibre, hence variation in its properties is also an inherent characteristic. To overcome these variations, scientific and systematic control is required on cotton quality. Control over cotton quality in a spinning mill is recommended in three stages, ie, approval of cotton bales, grading of cotton bales and finally selection of cotton bales for mixing plan. Major cotton fibre properties and their impacts on spinning processes and yarn quality have been explained in details with examples, data and graphical representations. More emphasis has been given on SCI and maturity of fibres. Maturity of cotton fibre is an important parameter and also impacts on others parameters such as length, strength, elongation, etc. All in one single value derived from regression equation Spinning Consistency Index (SCI) is a value influenced by inherent relationships of cotton micronaire, length, uniformity, strength, Rd and +b, which may help spinners to monitor only one parameter for decision making. Mixing planning is a very important function for spinning operation and investing the time and skill for systematic mixing plan pays good returns such as consistency in productivity and quality. A systematic mixing plan by maintaining the lowest possible variation in cotton quality parameters ensures trouble-free yarn production with consistent yarn quality. Variation in cotton colour grade is a major responsible factor for shade variation and barre problem in fabric, and hence control over cotton colour grade is one of the most important functions of mixing plan, which cannot be ignored. Consuming the off-standard quality bales is a tricky job and it should be used in a very controlled manner so that performances of spinning process and yarn quality are not affected.