Artilces By Dr. N.Balasubramanian
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Spinning
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Intimacy of Mixing and Blend Variation
N.Balasubramanian
Retd. Jt. Director, BTRA and Consultant *
Intimacy of mixing and Blend variation are important parameters to be controlled in blends in view of their critical influence on the appearance, fault incidence, grade and sale value of fabric. Not only the variability in strength, feel and abrasion resistance of fabric is affected but also streakiness, shade variations, weft bars and weft way defects are critically influenced by them. The problem becomes more critical when the fibres blended together are of different colours or the fabric is cross-dyed to different shades for the different components. Considerable value loss and fabric rejections are encountered in the mills because of defects associated with improper blending. Blend irregularity is broadly of two types: 1. Variations in the blend proportion of the component fibres in each cross-section along the length of yarn. The variation can be short term or long term type. 2. Inadequate intermixing of the components within a cross-section. This is termed as degree of lateral mixing. This arises because of grouping of fibres of one type. The best possible mixing that can be obtained with present system of spinning is that obtained by random dispersion h such a mixing a certain minimum variability is to be expected both in respect of longitudinal blend variation and intimacy of lateral mixing. The scope for improvement in mixing in a given situation is therefore ascertained by comparing the actual level of mixing with that due to random arrangement.- Longitudinal Blend Variation
- Short Term Blend Variability
Short term Blend Variability
Short term blend variability results in straky or marl appearance of fabric leading to seconds. Undyed and differently dyed specks, thick places and slubs also come from short term blend variation and lead to rejections. For simplicity, we will we will consider a blend made of polyester and viscose fibres. Let b = number of polyester fibres in yarn cross-section w = number of viscose fibres in yarn cross-section and n = total number of fibres in a yarn cross-section Then p = b/n and q = (1-p) = w/n If polyester and viscose fibres are randomly mixed, number of polyester fibres in a cross-section will vary as per Binomial distribution1. Out of n fibres, the probability that b will be polyester will be given by (n! / (b!* (n-b)!) ´ pb ´ qn-b p has a mean equal to np and variance equal to npq. Let N cross-sections of yarns be examined for number of polyester and viscose fibres contained in each of them. Let bi, wi, and ni denote the number of polyester, viscose and all fibres in the ith section then as proposed by Coplan and Klein2.
Index of Blend Irregularity (IBI) = √(1 / N)∑(bi - pni)2/pqni
Thus, IBI compares the observed deviation in blend proportion in each section with the theoretically estimated deviation of blend proportion of that section and determines the average of this over a number of sections. This is nothing but the chi-square test used in statistics for determining the significance of the observed deviation from a known theoretical distribution3. IBI can therefore used to find out if the mixing is random and if there is scope for bringing about further improvement in blend regularity. Thus from the above it is clear that a certain minimum variability in blend proportion is inevitable and this depends among other things on number of fibres in yarn cross-section. This will be clear by considering a 40s p/v blend yarn made out of 50% polyester and 50% viscose. If 1.5 denier of polyester and viscose are used, then the average number of fibres per cross-section will be 88. The blend proportion of polyester fibres will have a frequency distribution shown in Fig. 1( by ) when mixing is random. The blend proportion will vary with a standard deviation of 0 .0532. 2.5% of yarn cross-sections will have a blend composition lower than 0.394 and 2.5% will have a blend composition higher than 0.606. So even with ideal mixing, 5% of cross-sections will have a blend composition beyond +/- 0.106 of the average.Increase in the number of fibres per cross-section by using finer denier fibre is obviously one of the ways of reducing blend variability. This will be clear from frequency distribution of blend proportion of polyester fibre, shown in Fig. 1 ( by ) when 1 denier fibre is used. Fig 1 shows a significantly lower variability in blend proportion with 1 denier fibre. The standard deviation is reduced from 0.0532 to 0.0425 as fibre denier is reduced from 1.5 to 1. 5% of cross-sections will have a blend proportion beyond +/- 0.085 of average with 1 denier fibre. This is significantly lower than +/- 0.106 obtained with 1.5 denier fibre. The actual blend variability is usually higher than that due to random mixing. The extra component of variability comes from 1. uncontrolled movement of short fibres in one of the components during drafting. In polyester/cotton blend, cotton has a significant short fibre content. The uncontrolled movement of short fibres results in a higher cotton content in thick places and a lower cotton content in thin regions in the yarn. The same argument holds good in polyester/viscose blend though to a lesser extent.2. movement of component fibres in groups instead of as single fibres. Seed coat beards in cotton will move as a group. Seed portion gets dissolved in bleaching and the tuft of cotton beard dye to a different shade and show as specks. Fused synthetic fibres are caused by melting during cutting due to defects in cutting blade. Such fibres move as group. Highly entangled fibres, both in cotton and polyester move together and thorough individualisation of fibres during carding is therefore essential to minimise blend variation 3. Very short wavelength irregularity present in roving due to torsional vibration of back or middle rollers can also cause blend variation in yarn4. Polyester being of constant staple length, rear ends are also grouped and so drafting at ring frame ( which entails a reversal) results in a high irregularity of polyester component. But with cotton there is no such grouping of rear ends because of variability in fibre length and so the irregularity (due to short wavelength periodicity in roving) is much reduced or almost absent in cotton component in yarn. As a result, thick places will have more polyester content and thin places lower polyester content in yarn.
Long term Blend Variability
Long term blend variability leads to weft bands and undyed or differently dyed warp threads which show as lines. The contribution to long term blend variability from random mixing is small and can be neglected. Major contributions to long term blend variations come from 1 day to day or shift to shift variations in hank of pre blend drawing sliver hank in draw frame blend. This can be minimised by having1. autoleveller and count alarm systems in draw frame. 2.by minimising variation in moisture content in cotton or viscose component in blow room blending. This can be overcome by checking moisture content in the component before blending. 3.by minimising weighing errors in blow room blending. This can be minimised by regular calibration checks of balance( once a week). 4. Flat strips removed in carding contain disproportionately more cotton in p/c blend and more viscose in p/v blend. So flat strip variations between cards should be kept to a minimum. Flat wires used should be of the same type in all cards. 5. fibre droppings under the doffer at the web doffing junction consist predominantly of cotton or viscose. So such droppings should be minimised by accurate settings, by prompt replacement of scraper blades and effective control of relative humidity and temperature in card room.Intimacy of lateral mixing
Extent of intermingling of components within a cross-section is another equally important parameter of degree of mixing. Grouping of fibres of one type could also lead to streaks and uneven appearance in dyed fabric. In the extreme case, the two different coloured components remain completely separate which would occur if the rovings made out of these fibres are blended together at ring frame. Less extreme cases will be found if doublings after blend drawing are not adequate in draw frame blending or if opening of fibres is not adequate in blow room blending. Let us consider a polyester/cotton blend as an example. To assess intimacy of mixing it is convenient to examine the ribbon of fibres as they emerge from front roller nip of ring frame. Two measures of intimacy of mixing have been proposed. 1 Index of mixing Π = proportion of polyester fibres having a polyester right hand neighbour. 2. Measure of mixing g = Number of groups of polyester fibres in the strand
It is easy to see that Π = (b-g)/g
where b = number of polyester fibres in a section.
Cox5 was the first to theoretically examine the dependence of intimacy of lateral mixing on doublings. If d denotes total doublings after blending, then the ribbon of fibres emerging from the front roller of ring frame may be divided into d groups, some possibly empty corresponding to d original slivers. Let the average number of fibres in yarn cross-section be n and the proportion of polyester fibres be p. The number of fibres in a group will vary as per Poisson distribution with a mean n/d. the probability that a group contains r fibres is
qr = (e-n/d × (n/d)r) /r!
Π = 1 - ((1-p) × (1- e-n/d ))/(n/d)
For completely random arrangement d → ∞ , Π = p. Fig 2 shows how P varies with number of doublings for 50/50 and 67/33 blends with 100 and 200 fibres per cross-section in yarn. With increase in doublings, P decreases first rapidly and thereafter at progressively slower rate and attempts to reach asymptotically the value given by perfectly random mixing. Thus Π decreases from .9 to .547 as doublings increase from 20 to 500 and from .547 to .524 with a further increase in doublings to 1000( in 50/50 blend with 100 fibres in yarn). For a 50/50 blend if P has to be within .55, number of doublings has to be above 480 for 100 fibres in yarn and above 940 for 200 fibres in yarn. Thus, broadly doublings should be greater than 5 times the number of fibres in yarn cross-section if intimacy of mixing has to close to random. De Barr and Walker6 have shown that
g = d × p × (1-p) × (1 - e-n/d )
For random mixing , d → ∞
gr = np(1-p)
Degree of mixing = gactual/gr
Fig 3 shows how g varies with number of doublings for 50/50 and 67/33 blend with 100 and 200 fibres in yarn cross-section. As with Π, g increases rapidly with doublings initially, but later at progressively slower rate attempting to reach the value for random mixing at very high doubling.Studies were made by De Barr and Walker6 to find how degree of mixing varies with doublings. This was done by collecting the untwisted strand of fibres emerging from front rollers of ring frame on a velvet pad and counting the number of group of black fibres across the width in a blend of black/white fibres. A series of yarns were made by varying the number of doublings. This showed that 1 Degree of mixing approaches unity with increase in doublings but at a decreasing rate with increase in doublings 2. More doublings are required in a coarser yarn than a finer yarn to obtain a given degree of mixing 3. Number of doublings required to achieve a given degree of mixing is much less than that as per the theory. 4. The last finding indicates that a certain amount of intermingling of components takes place at each process. This arises from condensation of web into sliver at trumpet in drawing and conversion of strand into roving at speed frame. A good amount intermixing of fibres also takes place if blending is done prior to carding. De Barr and Walker showed that blending prior to carding has an equivalent number of doubling of about 1500. So, for achieving thorough intermingling of components doubling should be done prior to carding. This means Blow room blending has an edge over Draw frame blending particularly in sensitive sorts where small differences in colour and shade are perceivable.
Relation between lateral mixing and longitudinal blend variation
Balasubramanian7 showed that longitudinal variability ( caused by non random variations) is critically influenced by lateral mixing of components. With intimate lateral mixing the blend drafts more as a single species and irregularities due to drafting wave, mechanical faults and other causes have an equal influence on both components. The relationship between index of blend irregularity and index of irregularity was theoretically analysed7 in a blend yarn of 50% black and 50% white fibres and the following relationship was arrived at
r≈ (k12 - k22)/(k12 + k22)
Where K1 = Index of irregularity ( Actual irregularity / Irregularity due to random fibre arrangement) K2 = Index of blend irregularity r = Correlation coefficient between number of white and black fibres
If K1 is equal to K2 then correlation between number of white and black fibres will be zero. The black and white fibres draft independent of each other. When K1 is > K2 then there will be a positive correlation between number of white and black fibres. The irregularity added in drafting affects irregularities added in both fibres in the same way under such conditions.
When blending is random, K2 = 1 and
r ≈ (k12 - 1)/(k12 + 1)
To confirm the influence of lateral mixing on longitudinal blend variation, Balasubramanian7 prepared 2 blends of 50% black and 50% white viscose fibres. In the first blend, 100 doublings were given to drawing sliver after blend drawing. In the second blend, black and white fibres were blended at the final drawing keeping white slivers together on one side and black fibres on the other side. The number of doublings were kept the same as before. A more intimate mixing is obviously obtained by the first method than in the second method. The two blends were spun into yarn and index of irregularity, index of blend irregularity and correlation between number of white and black fibres were estimated. The results showed a significant correlation between number of white and black fibres only in the first blend where there is intimate lateral mixing. The index of blend irregularity is much lower than index of irregularity in this case and is close to unity. In the second blend where lateral mixing is poor, there is no correlation between number of white and black fibres and index of blend irregularity was close to index of irregularity. This is in conformity with the theoretically derived equations given above. These results clearly show that with increase in intimacy of lateral mixing, blend variability comes down and approaches that due to random mixing even though yarn irregularity is higher than that due random fibre arrangement.Blend Proportion in Yarn Faults
Thick places and slubs are major defects that degrade the fabric even in 100% cotton goods. With blends they have even more detrimental effect on yarn appearance mainly because the blend proportion at the defective portion is different from that at normal portion, as a result of which they dye to different shade. Blend proportion in Classimat A type of faults was checked by Balasubramanian8 et al and compared with normal portion in 30s p/c in 3 mills. For this purpose, A type of faults was removed by running the yarn on Classimat in the cut mode. Blend proportion in the fault portion and normal region was estimated by chemical methods. Cotton content in fault region was found to be higher by 3-6%. Similar results were also obtained in the case of C and D faults. Gupte9 too found the cotton content to be higher in p/c yarns in faults cleared by electronic clearer. Cotton content increased from 35 to 43% in fault region. Kumaraswamy and Sheriff10, on the other hand, found that majority of objectionable faults contain either a higher amount of cotton or polyester. Townend11 et al examined the blend composition in streaky portion compared to normal portion in polyester/wool blends. Blend proportion of wool was either too low or too high in streaky portion compared to normal portion. These findings can be explained from the way objectionable thick faults, slubs and streaks are formed. Thick faults and slubs are partly due to uncontrolled movement of very short fibres in drafting and partly from incorporation of overhanging fluff from clearer rollers and from fluff liberated at the time of roof or machine cleaning into yarn. Cotton and to a lesser extent viscose contains very short fibres and slubs caused by uncontrolled movement of short fibres in drafting are likely to have more cotton (or viscose) content. Clearer roller accumulations will also be preponderant in short fibres and so will have a high cotton content. Moreover seed coat beards consist of bunches of fibres, which move together as group in drafting and so make a significant contribution to short thick faults in yarn. This is another reason why cotton content is higher in thick faults. The reason why some slubs and streaks consist predominantly of polyester fibres is because of fused and over length fibres in polyester due to cutter defects. Fused and over length fibres result in drafting faults like slubs, due to high drafting force and fibre movement in groups. Presence of very short wavelength irregularity in fibre ends in roving due to torsional vibration of back rollers could also contribute to thick faults and slubs with higher polyester content. A high amplitude periodicity develops in polyester portion of the blend after drafting under such conditions due to reasons discussed earlier. Townend12 et al found that streakiness in polyester/wool blends is influenced to great extent by the contrast in lightness between the dyed component fibres and to a lesser extent by the contrast in hue. Streakiness increases with fibre diameter and with reduction in number of fibres in cross-section. For minimum streakiness, blending should be done at the earliest possible stage.
References
1. A.G. Hampson and W.J.Onions, J. Textile Institute, 1956, 47, T234 2. M.J.Coplan and W. J. Klein, Textile Research J., 1955, 25, 743 3. P.G. Walker, J.Textile Institute, 1957, 48, T133 4. S.K.Sett and N. Balasubramanian, J. Textile Association, 1985 Nov, 183 5. D.R. Cox, J. Textile Institute, 1954, 45, T113 6 A.R. De Barr and P.G. Walker, J. Textile Institute, 1957, 48, T 405 7. N. Balasubramanian, Textile Research J., 1970, 40, 129 8. N. Balasubramanian, R. Krishnawamy and T.L. Paradkar, Proceedings of 38th Joint Tech conference, SITRA, 1995, 18. BTRA Survey Report No 31, 1994 Sept. 9. A.A. Gupte, Ph. D. Thesis, University of Bombay, 1991 10. K.K. Kumaraswamy and I. Sharieff, Proceedings of 20th Joint Tech Conference, SITRA, 1979, 7.1 11. P.P. Townend, R. Harper and J.D. Watt., J.Textile Institute, 1964, 55, T352 12. P.P. Townend, R. Harper and J.D. Watt., J.Textile Institute, 1964, 55, T365 Controlling Count Variation in Yarn
N.Balasubrmanian
Retired Joint Director BTRA and Consultant
Importance of count variability in yarn hardly needs any emphasis. Higher count variability invariably leads to higher strength variability. The weak patches in the yarn lead to frequent end break in further processing, which often reaches annoying levels leading to rejection of bobbins and cones. In latest autoconers, which have settings for rejecting bobbins with count of yarn exceeding beyond certain limits of the nominal, processing of yarns with even slightly high count variations becomes extremely difficult. Winding efficiency reaches unacceptably low levels with such yarns. Higher count variability especially of medium to long length range results in moire like appearance in fabric and increases warp way streaks and weft bars. Ring cuts and soiled ring packages is another problem with higher count CV. To overcome this, wider clearance is kept between ring diameter and full package leading to lower doff weights. With higher count variability, percentage of bobbins exceeding tolerance limits of nominal count increases, leading to sales rejections and market complaints. In shuttleless looms, problem of weft tear is encountered during weaving when count of weft changes abruptly beyond certain limits at the time of pirn change. High count variations in weft are also a cause of warp way fabric creases in processed fabrics like dyed poplins1. Dependence on Length Variability of count depends upon the length of the yarn used for estimating count. Though 120 yd. or lea is normally used for estimating yarn count, sometimes half leas are used especially in coarse polyester blend counts to keep strength measurement within the capacity of strength tester. In very fine counts like 120s, two leas are weighed together to estimate count to achieve better accuracy in weighment. It is well known that CV of count decreases with increase in length but the rate of reduction decreases with increase in length. CV of half lea will be1.414 (i.e.; √ 2) of full lea, if there is no serial correlation between adjoining leas. But there is usually a positive serial correlation because of long term variations. So CV of half lea will be 1.2 to 1.3 times the CV of full lea. The same rule holds good in the case of slivers and rovings. CV of 5yd wrapping of roving is much lower than 1.732 (i.e.; √3) times CV of √15 yd. wrapping in inter. The former is usually around 1.4 to 1.5 times that of latter. Tracing the Source of Count Variation Location of source of count variability will be greatly facilitated if wrappings and estimate of CV from the same are based on corresponding wrapping lengths of material at different stages. Thus wrappings and CV of wrappings may be based on 5yd instead of the traditional 15 yd length at Inter. 5 yd length at Inter after drafting will be closer to 120 yd length in yarn than 15 yd length and CV estimates based on the former will be more helpful to show if ringframe is contributing to additional variation. At drawframes, CV of wrappings based on 0.5 yd length will be more useful from the same consideration. Estimation of CV of such lengths can be obtained from modern Evenness testers like Uster tester 3. Norms for CV It is beneficial to set norms for CV not only at Ringframes but also at different stages of processing. Two sets of Norms, one with and another without autoleveller at Drawframe are given in Table 1
Contribution from various processes Ring frame Contribution to count variation from ringframe comes from Variation in mechanical draft between frames Slippage of top roller Stretch of material in creel Variation in Mechanical Draft Variations in mechanical draft come from the use of different change pinions on frames of the same make and drafting system. Some common causes for this defective practice are Lack of sufficient stock of change pinions Using change pinions differing by one tooth on the two sides of a frame, ostensibly to achieve an average count close to nominal. This should be discouraged as it increases variability in count between frame sides. Frames of different makes and drafting systems are used for spinning the same count but the same mechanical draft is not kept on them, Slippage of strand under top rollers arises because of inadequate weighting or improper grip. Count variation is therefore reduced upon conversion of older version of top arms to later versions with higher pressures2, 3. Higher frequency for cot buffing, higher starting diameter for cot up to 30 cm are therefore helpful to reduce count variation4, 5. Disturbances to weighting also comes from worn out springs, leakage of air and improper seating of plunger on rib in pneumatic drafting, leading to count variation. One of the reasons for stretch of strand in the creel is low roving twist. The level of creel breaks can assess this. Misalignment of creel roving bobbin in relation to the creel roving guide is another contributory factor to stretch. Improper location of creel guide rod in relation to the bobbin can also cause stretch. If located too high or too low, stretch takes place when roving unwinds from top most or bottom most portion of roving bobbin. Group Control of Count Group control of count should invariably be preferred to individual frame control as a basis for changing the change pinion except under special circumstances. For this purpose, average count from wrappings taken from all ringframes working on a count should be determined and if the count exceeds the preset limits even after a repeat wrapping, pinion change should be done on all frames. The only exception for this is when frames or drafting systems differ widely in age. Under such conditions, slippage of strand occurs under drafting rollers with older frames and drafting systems leading to a lower actual draft at the same mechanical draft5. In one study2 the mechanical draft had to be reduced from 36.5 to 32.6 to get the same yarn count with the same back material when drafting system was changed from SKF PK211E to SKF PK225. Speedframe Most of the problems enumerated above like variations in mechanical draft between frames and slippage of strand under top roller are sources of count variation at speedframe. In addition, the variation in stretch from start to full position is an added source of variability. Stretch variation during build can be estimated accurately, only if a correct method of estimating wrapping is followed. Traditionally roving bobbin is placed on the top of self-weighted top roller driven by wrapping block, with a spindle inside while preparing wrapping. This method has the drawback of slippage of roving between the self- weighted top roller and wrapping drum. The amount of slippage increases from full bobbin to empty bobbin, often to the extent of 5 - 6 %, leading to errors in estimation of wrapping variation from start to full. A better and more accurate method, free from such error, is to suspend the bobbin from a bobbin holder located at the back of wrap block. The roving drawn from the bobbin is passed through the nip of self-weighted top roller and wrapping drum. Actual studies in the mill showed that wrapping estimated by this method is 5 - 10 % coarser than the one obtained by the traditional method. Draw Frame Major contribution to yarn count variation comes from drawframe. Primarily there are two sources of variability 1. Medium term variations in the sliver and 2. Long term variations in the average hank between frames and between shifts. Medium Term Variations Variations in the length region between 0.2 to 0.7 m, depending on the yarn count and hank organisation influence directly count CV as it corresponds to length of lea divided by draft between drawframe and ring frame. Contribution to this variability comes from irregularities in breaker sliver, which do not get reduced by doubling in finisher. Long Term Variations Long term variability in the hank of drawing sliver arises from variation in average weight of lap weight in lap feed systems and weight of feed sheet to card in chute feed systems, that occur from shift to shift. The piano feed regulating motion in scutcher and pressure switch control in chute feed to card operate on the basis of volumetric control and the degree of openness of the material therefore affects the extent of control. In piano feed belt shift would occur when the sheet becomes bulkier (because of better openness) even though weight per unit length remains the same. Likewise, the pressure developed in the chute would be higher for the same weight with well-opened material. Thus it is necessary to standardise the evenness roller to inclined lattice setting in scutcher and speed of beaters and pressure switch setting in the chute, based on factors like type of cotton, whether the feed is directly from bales or from a sandwich mixing from pre-opened material. This will minimise variations caused by openness of material. The variation in average hank of card sliver from shift to shift have less chance of getting reduced from doubling at draw frame, as slivers made in one shift do not get doubled with those in the next shift. But the variability due to this can be kept down by increasing storage capacity of laps and card sliver cans so that material made at longer intervals of time get doubled. But most of the mills do not have such storage space. Moreover material stored for long lengths run the risk of contamination with dust and fly. Autoleveller Modern draw frames are equipped with autoleveller and sliver monitor to reduce both type of variability mentioned above. The autoleveller brings down the variability in sliver lengths beyond 0.3 - 0.5 m. Further, the rate of reduction in CV of wrapping with increase in wrapping length is steeper with autolevelled product. This will be clear from Fig.1, where the variance length curves of Finisher drawing slivers made on the same drawframe with and without autoleveller are compared. The amount reduction in CV with autoleveller increases as wrapping length increases from 0.5 to 3 m. Variance length curve of Breaker drawing sliver (without autoleveller) fed to this draw frame is also shown plotted in the same Fig. The figure shows that doubling in the draw frame brings down the CV with length 0.5 m and about and the order of reduction increases with length. So the variance length curves become steeper from Breaker to Finisher. The short term irregularity in sliver is however not reduced by doubling or autolevelling. Norms for CV of wrapping of drawing sliver at various lengths with and without Autoleveller are given in Table 1. Functioning of autoleveller gets affected by zero setting, obstruction to free movement of scanning roller, improper functioning of servo motors, drives, solenoids and related parts. Leveling action of autoleveller has therefore to be checked at regular intervals. A method usually followed in the mills is to feed 9 ends and 7 ends of sliver and check the wrapping of outgoing sliver. The extent of deviation of hank from nominal under such circumstances is taken as leveling sensitivity of autoleveller. In normal practice, however, such wide variations in input hank seldom occur and the effectiveness of autoleveller in leveling lower order of variations would be of greater interest. The best method of judging this is by comparing CV of 0.5, 1 and 3 m lengths of material with and without autoleveller. Sliver Monitor Sliver monitor consists of a sensor fitted to the calendar rollers of draw frame to check the hank of the outgoing sliver If the actual hank deviates from the nominal by more than the preset limits the drawframe will be stopped. The limits of deviations from nominal hank at which drawframe will be stopped can be varied. It is normally advisable to set it at +/- 2 %. In some makes, sliver monitor can also be set to stop drawframe if CV of wrapping goes beyond certain limits. If the drawframe stoppage due count exceeding preset limits is high then the hank of breaker sliver should be checked to find if it is deviates considerably from normal. If this is not the case, then the functioning of autoleveller has to be checked. Thus a step by step approach should be followed. Mechanical Condition of Draw Frame Poor mechanical condition of draw frame, coupled with inadequate weighting for top rollers have sometimes been traced as a source of count variation. When older versions of Whitin drawframes were replaced by later versions of Laxmi Rieter drawframes, among other noticeable benefits found by the mills, was a significant reduction in count CV of yarn6 as shown in Table 2.Table 1: Norms for CV at different stages Material Wrapping Length Yd Without Autoleveller With Autoleveller Yarn 120 2.5 - 3 1.5 - 2 Roving 15 1.5 - 1.8 0.9 - 1.2 Roving 5 2.2 - 2.7 1.3 - 1.8 Drawframe 5 0.7 - 0.8 0.4 - 0.5 Drawframe 1.0 - 1.2 0.5 - 0.7 Drawframe 1 1.5 - 1.6 0.9 - 1.1 Drawframe 0.5 1.9 - 2.1 1.3 - 1.5
Card Sliver Though card sliver is characterised by high variability of wrapping, its contribution to yarn count CV is not that significant because doublings in drawframe reduce the variations substantially. Autolevelling at card is therefore not found very beneficial in bringing down count CV of yarn. This will be clear from Table 3 where card slivers made with and without autoleveller were processed up to ringframe and variability at different stages assessed.Table 2: Effect of mechanical condition of Drawframe on Count and Strength CV of yarn (30s) Drawframe Type Age Mechanical Condition CV of
Count%CV of
Strength%Mixing1 Mixing2 MIxing1 Mixing2 Whitin J5 Old Unsatisfactory 4.9 3.8 11.4 8.2 Laxmi Rieter DO2S New Good 4.4 3.0 8.1 4.7
Though autolevelling at card brings down CV of 1 yd wrapping of card sliver from 7 to 1.2, CV of 1 yd of finisher drawing sliver comes down only marginally from 1.6 to 1.3 and count CV of yarn drops marginally from 3.2 to 2.6. U % of card sliver also does not have much effect on count CV of yarn. This will be clear from a study where cards giving low U % and wrapping CV % and those giving high U % and wrapping CV % were separately channelised up to yarn. The results (Table 4) show that even with large differences in U % and wrapping CV % of card sliver, there is no significant difference in yarn count CV %.Table 3: Effect of Autolevelling at Card on Yarn Count CV % (20s) U % of Card sliver CV % of 1 yd of Card sliver CV % of 1yd of Finisher Drawing sliver Count CV % of Yarn With Autolevelling 3.8 1.2 1.3 2.6 Without Autolevelling 4.1 7.0 1.6 3.2
Comber Sliver High comber sliver U % arising from piecing wave can affect yarn count CV, particularly if single post comber drawframe passage is used. This will be clear from the following study where combers giving low and high U % were channelised up to ring frame separately and checked for yarn count CV. The results given in Table 5 show that yarn count and strength variations are higher with yarns made from combers with higher variability.Table 4: Effect of card sliver irregularity on Yarn count CV % (20s) Property Cards with low sliver variability Cards with high sliver variability Card sliver U % 4.3 7.2 CV of 6 yd wrapping of card sliver 6.1 7.3 CV of Yarn count % 2.8 2.9
Medium Term Variations in Yarn Count variation is traditionally used to indicate variability in 120 yd lengths. Variability in shorter lengths are also equally important though they do not form part of quality monitoring activity in most of the mills. Variability of weights in lengths in the region of 0.5 to 10 m affect the appearance of fabric, lead to moire like defects and contribute to weft bars and warp way streaks. U % of drawing sliver has a direct influence on the variations in these lengths. This will be clear from the results of a study in a mill given in Table 6.Table 5: Effect of comber sliver U % on variability of count and strength of Yarn (40s) U % of comber sliver CV of yarn count % CV of yarn lea strength % 3.7 1.75 4.12 6.9 2.78 7.96
Drawing sliver U % was brought down substantially in the mills by optimising roller settings. When this material was channelised up to ring frame, medium term variations in the yarn as indicated by CV of 1 to 10 m lengths came down markedly though CV of 120 yd length showed little improvement. The fabric appearance also improved markedly. This shows the importance of monitoring CV of yarn count in length regions 1 to 10 m. This is conveniently obtained in later model Uster Evenness testers like UT3. References: 1. Warpway Creases in fabrics N.Balasubramanian and C.Chatterjee, Indian Textile J., 1996 Oct, p66 2. Benefits from modernisation of ring frames from second generation Top arms M.Balakrishnan and N.Balasubramanian, BTRA Scan 1980 Sept., XI, p4 3 Yarn quality improvements from second generation Top arms at Ring Frame G.Janakiraman and N.Balasubramanian, BTRA Scan, 1987 Dec, XVIII, p 9 4.Cot buffing at Speed Frame - A useful measure to reduce count variation S.K.Sett, G.V.Aras and N.Balasubramanian, BTRA Scan, 1984 March, XV, p 4 5.Contribution of Ring Frame drafting condition to yarn count variability in fine counts N.Balasubramanian and G.Janakiraman, Indian J. of Fibre & Textile Research, 1990, 15, p 198 6. Influence of mixing characteristics and Draw Frame condition on yarn quality G.V. Aras and N.Balasubramanian, BTRA Scan, 1982 June, XIII, p 10Table 6. Contribution of drawing sliver U % on medium term variations in yarn (20s) Draw Frame
settings,
mm Fr/BackU % of Drawing sliver CSP U % CV of 1 m % CV of 3 m % CV of 10 m % CV of 120 yd % 49 - 56 6 2044 15.6 8.0 6.5 4.6 2.5 44 - 53 4.6 2116 13.8 6.2 5.2 4.0 2.6 Relative merits of Polyester and Polypropylene Polypropylene (PP) and Polyester(PES) are the two major fibres mainly used in Nonwovens, Industrial yarns and fabrics. It is therefore useful to have an understanding of their relative merits and limitations. Polyester is made from Dimethyl terepthalate (DMT) and Mono Ethylene Glycol. Modern processes use pure Terepthalic acid (PTA) in place of DMT. Polypropylene is a polyolefin made from a polypropylene monomer obtained from naphtha. Both fibres are available as virgin and bottle grade (from regenerated material). Virgin fibre is used for apparel purposes and regenerated fibre is used in nonwovens for making carpets, floor coverings, blankets and filters. Regenerated bottle grade fibre is cheaper, has lower strength and higher variability in fibre characteristics. Further it contains more fused fibres and molten chips and requires accurate control over humidity and temperature in carding room. o The two fibres are nearly comparable in tenacity though PES is available in higher tenacity grades. For industrial fabrics with higher stipulated strength, PES will be able to meet the requirements. o Elongation is higher in PP. This gives better elasticity for material and improved moulding in moulded automobile carpets. o Density of polypropylene (0.91g/cc) is much lower than that of polyester (1.38 g/cc). As a result thicker, bulkier yarns and loftier fabrics and more comfortable carpets are made with the former for a given count of yarn and area density of fabric. o Polypropylene is dope dyed and is available in an extensive range of colours and shades. It is therefore much easier to achieve colour and shade matching by mixing a minimum number of shades of fibres. Dope dyed polyester, on the other hand, is available only in a limited number of colours and shades. The required shade has often to be developed through R&D work or fibre has to be often dyed. With dyed fibre fastness to rubbing and washing will not be as good as dope dyed fibre. o Melting point of polypropylene (165oC) is much lower than that of polyester (260oC). Material made from this fibre is therefore not suitable in fire fighting and similar clothing where temperatures are high. Heating time, temperature and pressing time are therefore more critical in moulding with polypropylene. o Flame retardancy by burning rate is inferior with polypropylene than with polyester. A flame retardant compound has to be added to the binder to meet the flammability requirements in Exports with polypropylene. This adds to the costs. o Resistance to UV light is inferior with PP compared to PES. Photo degradation takes place upon exposure to sunlight with polypropylene. UV stabiliser has to be added during manufacture of polypropylene to improve its resistance to UV light. Carbon black is the usually added UV stabiliser. With geotextiles and upholstery material continuously exposed to sunlight, PES is more suitable than PP. o Polypropylene tends to form beads during carding. The beads get deeply loaded on the cylinder wire and also on the needles. There has to be a regular programme for removing the beads from the wire and needle. Achievable production rates are therefore lower with polypropylene than polyester because of card loading. Card processing problems are more acute with lower deniers and recycled fibres with polypropylene. o PP is highly inert to chemicals and is suitable as fishing nets and geotextiles in alkaline and acidic soils. PES on the other hand loses its strength in alkaline soils and should not be used. Leaching also takes place with polyester in aqueous solution. Strength retention of PES after immersion in aqueous solution at 95oC is given below.
N.Balasubramanian
Retired Jt. Director (BTRA) and Consultant
o PES has higher creep resistance (retention of tensile properties over long duration of time) than PP and this is a distinct advantage in geotextiles used for anchoring of soils and similar applications. Because of its low glass trangition value (Tg) PP has less creep resistance. Creep elongation increases from 10 - 20% within one week with PP. o Polyester/ cotton and Polyester/viscose blends are very popular as apparel material. Polypropylene is on the other hand not usually blended with cotton or viscose.Strength Retention of PES straps after immersion in aqueous solution at 950c Number of days Alkaline solution Neutral solution 30 76% 90% 300 38% 45% Merits of Lea CSP and Lea Ratio Lea and single thread strength are the usual measures used to estimate strength of a yarn. Many authors have discussed their relative merits and limitations. Lea CSP is based on a larger sample size and so has less sampling variation. It is also a much faster test, involving about 30 min. Further the count of the yarn is estimated from the same specimen used for testing strength and so strength can be corrected for any variation of actual count from nominal. Traditional lea test has been considerably improved by using a load cell to determine the strength in place of the pendulum. The instrument is also pc operated. Further, soft wares have been developed to determine CV, maximum and minimum values of Lea Strength, count and CSP. Single thread is a more scientific measure of strength and is amenable to mathematical treatment and interpretation. Another advantage of single thread test is that it provides estimates of breaking elongation and load elongation curve. However, count is not estimated from the same specimen used for testing strength and so accurate corrections for deviation of actual count from nominal is not possible. The test is moreover time consuming and is based on a much smaller sample size. Operative involvement has been considerably reduced in modern single yarn strength testers by automating it and by operating the instrument through a pc. Single thread strength is the currently recognised method of testing strength of yarns in international market. Standards for single thread strength or tenacity (RKM) are prescribed in Export trade in developed countries. Lea test is more similar to a multiple strand test, though slippage of thread is allowed at the clamp. When the weakest thread in the strand breaks, the break spreads to the whole lea in a catastrophic manner leading to breakage of lea. So the strength of lea is determined by the strength of the weakest strand and lea strength is therefore much lower than the combined strength of threads forming the lea. Lea strength is therefore affected by variability in strength and elongation. Yarns, which have more variability, will have lower lea strength in spite of having comparable single thread strength. Lea CSP therefore will provide a better indication of how the yarn will perform in subsequent processes where weak places account for breakages. Further, lea ratio, which is a ratio of predicted CSP (from single thread strength) and actual lea CSP, is a useful indicator of the variability in yarn. The present article attempts to show through various examples the merits of lea CSP and lea ratio in determining strength characteristics of yarn. Lea Ratio Lea Ratio denotes the ratio of Lea count strength product (CSP) to predicted lea CSP from single thread tenacity (RKM). Lea is composed of 160 strands and if we omit the slippage around the clamps, maximum lea strength will be 160 × single thread strength. So Predicted Lea CSP =(S ×160×590.6) /(453.6×T)
N.Balasubramanian,
Retd. Jt. Director and Consultant*
= Single Thread Tenacity ��.3 where, S = Single thread strength in gms T = Count of yarn in Tex units Actual CSP Lea Ratio = ———————---------------- Predicted CSP = (CSP/Single thread tenacity) × 0.0048 Optimum Twist Factor Optimum twist factor is higher by 0.2 to 0.5 units for single thread than for lea strength. This is because twist flows to thin place in a yarn and so twist at the thinnest place in a lea will be higher than that in a single thread. Since yarn usually breaks at the thinnest place, optimum twist factor is lower in lea strength than in single thread strength. In processes like warping or sizing or weaving or knitting, long length of yarn is subjected to tension and strength of thinnest place determines the breaking point. Use of optimum twist factor based on lea test (which is generally lower than that from single thread test) will therefore ensure better performance of yarn in these machines. Moreover, use of lower twist factor will improve cover factor, feel, dye uptake and absorbency of fabric. Single and Double Feed It is well known that double feed at ring frame improves yarn strength and regularity, particularly in medium and coarse counts1. Lea CSP, single thread tenacity and lea ratio of yarns from single and double feed are compared in 2 counts in Table 1.
It will be seen that both CSP and single thread tenacity are improved by double feed but the improvements are more prominent in lea CSP. This is clearly brought out by lea ratio results. Lea ratio is found to be significantly higher with double feed, which is because of better regularity of yarns. Lea CSP and lea ratio therefore give a more complete picture of strength characteristics of yarn. Ring and Rotor Yarns Rotor yarn is known to be weaker but at the same time more uniform, in terms of short and medium term variations, than ring spun yarns. This is reflected in lea ratio results as shown in Table 2. In this Table lea CSP, single thread tenacity and lea ratio of ring and rotor yarns spun from the same mixing are given. Results are given for a normal virgin mixing and a waste mixing.Table 1: Influence of type of feed on Lea Ratio Cotton Count(Tex) Mode of Feed CSP Single thread tenacity Lea Ratio Kalyan 24s 1Ne Single 1632 10.41 0.753 2Ne Double 1930 11.69 0.792 Digvijay 30s 1.2Ne Single 1708 10.10 0.811 2.4Ne Double 1911 10.63 0.863
As is well known, strength of rotor yarn is lower than that of ring yarn. But what is interesting is that the difference in strength is more in single thread tenacity than in lea CSP. This is borne out by the higher lea ratio found in rotor yarns compared to ring yarns. Variability in strength is lower in rotor yarns than ring yarns2 and as a result higher lea ratio is found in rotor yarns. This again shows that lea CSP and lea ratio provide more comprehensive information about strength characteristics and processing behaviour of yarn at subsequent stages. Polyester/Cotton Blends Blending of cotton with polyester usually results in lower strength (than the weighted average). This is because of elongation difference between the two fibres. As a result, polyester fibre is far removed from breaking point at the time rupture of cotton fibre and does not contribute fully to strength. Lea CSP, single thread tenacity and Lea Ratio of 100% cotton and 67/33 polyester cotton blended yarns spun from the same mixing and processes are compared for 60s single and 2/60s double yarn in Table 3.Table 2: Comparison of Lea Ratio of Ring and Rotor Yarns Mixing Count(Tex) Spinning system CSP Single thread Tenacity Lea Ratio Normal 12s (49.2) Ring spun 1819 11.50 0.759 Rotor spun 1580 9.63 0.788 Waste 11s(53.7) Ring Spun 1656 11.19 0.710 Rotor spun 1524 9.20 0.795
Reduction in both lea CSP and single thread tenacity is seen upon blending polyester with cotton in both single and double yarns. But the reduction is more prominent in lea CSP. As a result, lea ratio is found to be lower in blend than in 100% cotton yarn. The CV of strength and elongation are higher in blend yarn and this is the reason for the lower Lea Ratio. Lea ratio is therefore a useful indicator of variability in strength and elongation of yarn. The merits of lea ratio are easy to appreciate as variability has considerable influence on performance of yarn. Conclusion Lea Ratio and Lea CSP give more comprehensive information on strength characteristics of yarn as they take into account variability in strength and elongation of yarn. Lea ratio and Lea CSP therefore provide a better indication of the performance of yarns in winding, warping and loom shed. Therefore, testing yarns for lea strength and lea ratio should form an essential part of quality monitoring program of quality conscious mills. References 1. “Effect of feeding conditions at Ring frame upon Yarn quality and Spinning performance”, N.Balasubramanian and G.K. Trivedi, Textile Research Journal, 1973, 43, p 1 2. “Influence of Rotor Speed, Rotor Diameter, and Carding conditions on Yarn quality in Open End Spinning”, J.S. Manohar, A.K. Rakshit and N. Balasubramanian, Textile Research Journal, 1983, 53, p 497Table 3:Comparison of Lea CSP, single thread tenacity and Lea Ratio of Cotton and P/C Blend yarns Mixing and Count CSP Single thread Tenacity Lea Ratio CV of strength% CV of Elongation% 60s Cotton 3814 22.7 0.806 11.4 6.5 60s P/C 2389 15.39 0.745 13.8 29.5 2/60s Cotton 4230 23.96 0.847 8.8 10.6 2/60s P/C 3093 19.50 0.761 13.8 16.1 Tips to mills for getting full benefits of Modernisation
N.Balasubramanian
Retired Joint Director BTRA and Consultant
Raw material and technology up gradation are often cited as the main factors determining the quality of product, performance and profitability of a textile mill. There is however a limited scope available in regard to raw material selection as it is dependent crop availability and price levels in the country, crop cultivation conditions, monsoon, lot to lot and station to variations in quality, ginning conditions and government policies. Further relationship between measurable characteristics of cotton and yarn quality is not precise, with only 70-80% of yarn quality being determined by measurable fibre properties. Technology up gradation and modernisation on the other hand are more rewarding in the long run for improving the mills’ product quality and performance. However in actual practice, the gains of modernisation are not fully and in some cases not even partially realised by mills. There is also lot of conflicting views in regard to priority areas of modernisation and technology choice among the mill executives and technicians. Returns from modernisation also vary markedly from mill to mill. In this article, the important aspects that should be addressed by mills planning for modernisation to get full benefits from it are discussed.Lack of understanding of Machine
Shop floor technicians and even quality control personnel do not have full understanding of the features and facilities available from the newly installed modern machine. In some cases, they do not even know how to achieve the various settings and what to do when quality or production claims are not achieved. This arises primarily because the technicians are not given proper hands on training on the new machine. They are not encouraged from making changes in the machine parameters, as it is felt that this may damage the machine. These problems can be overcome, if shop floor technician is intimately associated with the machine right from the time of erection to trial runs made on the machine after erection. It is not adequate if the mills technician is sent to manufacturer’s end for training. Mills should insist that the erector or technical representative of machine manufacturer stay in the mills at least for a week after erection and train the mill personnel with an open mind and transparency on the various functions, features and facilities available on the machine. They should also guide them on how to tackle a problem when it arises. This will give them hands down training and confidence in the new machine.Non-Optimisation of parameters
This again arises from inadequate knowledge of machine and lack of experience. There is a tendency in mills to rely wholly on manufacturer’s representative for deciding the parameters and settings. This is not advisable in the long run as mill staff do not get the required confidence in optimisation and getting the best results. Moreover, process optimisation is not a one time affair and requires to be reviewed and modified depending upon the changes in mixing sorts, type of sorts, customer requirements etc. Manuals of the machine should be readily available to technician. For fear of loss of the manual, it is often kept under the custody of head of department. A good practice will be to make Xerox copies of the same and give them to the supervisors handling the machine.Choice of Technology
Choice of technology has to be based on the location of mill, type of raw material and infra structure. Mills going for second and third generation machinery should be preferably located at a place where spare parts and modern workshop facilities are readily available. Otherwise, production losses and disturbance to processes reaches alarming levels. Many times it is found mills run blow room without continuous feed in operation and bale plucker kept idle, cards without automatic waste evacuation system, draw frames without auto leveller or sliver monitor, ring frames without auto doffer and overhead blower, because of lack of spare parts.
Indian cottons are characterised by high level of contamination, trash and full seeds and broken seed fragments and contaminant is one of the major sources of rejection. Contaminants consist of coloured chindies (cloth pieces)Hessian, coir and polypropylene threads, sand, tar and metallic particles. Extent and type of contamination varies from station to station. If the mills procure cottons with high contamination, installation of bale plucker in blow room should be given a second thought. Though instrumental sorting equipments like Uster Optiscan and Truetshler Securomat claim to remove contaminants, their level of accuracy needs to be still established. Often many export oriented mills, though equipped with bale pluckers, open the cotton first through a bale opener, manually remove the contaminants and repack the material into a heap for action by bale plucker. Alternately, opened material from bale plucker is made to fall from a condenser on to a slow moving transport lattice of about 8-12 feet long. Operatives sitting on either side of lattice pick up the contaminants. This increases labour employment. Further, lot sizes should be large while operating with bale plucker. With larger lot sizes, long term stability in the quality and colour of the mixing is ensured. Chute feed to card has come to be accepted as a standard feature of modern technology. Major advantage of chute feed is labour saving in respect of scutcher tenters and lap carriers. Chute feed should be adopted only with large lot sizes and with single mixing running on the line. Chute feed creates problems with shorter lots and when more than one mixing is used on the same line. Important speeds and settings except for doffer speed cannot be optimised to suit the characteristics of two mixings. There is also risk of material from one mixing contaminating other and this becomes serious when coloured material is run. Modern cards operate at very high speed. The metallic wire on cylinder, doffer and licker in are therefore liable to be damaged even with slight amount of contaminant and foreign matter in cotton. Moreover, the wires are likely to wear out fast and mills have to provide for a more frequent wire replacement program. The same holds for cots in draw frame, speed frame and ring frame. Replacement schedules for critical parts are discussed later. Because modern cards and draw frame operate at high speed, breakdown should be kept at very low level as otherwise production at subsequent stages suffer from lack of back stuff. Hank variation in sliver should be kept at very low levels if mills draw frame is equipped with sliver monitor. Sliver monitor can be set to stop the draw frame when count/hank exceeds certain preset limits and draw frame stoppages will rise to unmanageable levels if hank variation is high. Modern winding machines are equipped with clearers, which reject the bobbin if repeated breaks occur due to count exceeding limits. If count variation, is high bobbin rejections will increase leading to low winding efficiency. Roller lapping incidences should be kept at low levels · By avoiding low Micronaire, sticky and honey dew infested cottons · Through use of right amount and type of antistatic spray on polyester fibre · By acid treatment and Berkolising treatment to cots · By accurate control of RH and temperature in department · By restricting the addition of soft waste to 2-3% Roller Lapping will cause extensive damage to cots in high production machine and contribute to defects in yarn and to low productivity.Utilisation
Seven days twenty-four hour per day working is the norm for mills with latest technology. This is required not only to ensure rapid payback of investment but also to minimise starting day problems. With 6 day working, working problems like web sagging at card, roller lapping at ring frame is encountered on the starting day with high production machines. Further, faults like slubs and crackers are found on the starting day. Bottom apron breaks and spindle tape breaks are also encountered on this day. These problems are also encountered to a lesser extent after recess if machines are stopped for recess every shift. Ring frame productivity is found to be 8-10% lower on staring day, while fault level in yarn is 12-15% higher. Machine parts like card wire, ring frame cots get damaged and wear out fast because of these problems in 6 day working.Marketing
Marketing has to be strong in mills with latest technology to minimise stoppages due to excess stock accumulation. Intensive survey of local and export market has to be done for different types of counts and sorts and profitable areas with high demand should be continuously assessed. Specific quality requirements and standards set by customer should be procured and forwarded to technical staff. There has to be a close interaction between sales and technical personnel so that demands of the customer in terms of quality can be fully met. Samples of competitor mills yarn should be obtained and compared with the mills yarn. New sorts and products should also be developed in collaboration with quality control/ product development personnel.Power Consumption
With high-speed machines, power is major cost of yarn manufacture. So careful attention should be paid to horse power and type of motor for ring frames. High performance motors, which have a higher efficiency, should be invariably used. More number of bearing supports for tin roller pulley shaft will help minimise power. Instead of 4 spindle tape drive, 8 or 16 spindle tape drive will reduce power consumption. Manufacturer should be asked if he can provide this. Spindle wharve diameter should be kept low up to 18mm. Energy efficient spindle oils which contain dispersion should be preferred. Fluid coupling or soft start motion should be used in speed frames to minimise starting torque. Electronic ballast should be fitted on tube lights. It is prudent to equip the mill with generator or any other source of power generation as power failure and shutdowns cause marked loss. Additives are available for diesel used in generator for reducing the cost of power generation. Power factor should be kept high up to .97 - .98. Walls should be painted with white paint. Suitable sensors should be fitted to automatically switch of light in sections like wash room and toilet after use. Pnuemafil motor should be stopped when ring frame is stopped for long duration.On Line quality and process monitoring
Modern machines have on line quality and process monitoring systems. These should be effectively used to get full benefits. Cards and Draw frames have monitoring systems for continuous assessment of hank, evenness, CV of hank, frequency of thick places in sliver. Later models also display spectrogram and variance length curve of sliver and nep, trash and seed coat level in web in card. Draw frames are fitted with sliver monitor, which measures sliver hank and evenness, thick places and spectrogram. This can be set to stop the draw frame if the hank exceeds nominal by more than a preset %age or irregularity exceeds beyond a limit. To get full benefits from such systems, technicians should review sliver quality values from on line monitoring system at frequent intervals and take prompt corrective measures. Attempt should also be made to improve hank variations in card and breaker draw frame, so that setting limits on sliver monitor in draw frame for hank variation can be brought down, which will eventually lead to lower yarn count CV. Modern ring frames are invariably equipped with ring date systems which give among other things spindle wise speeds, end breakage rates. This should be effectively put to use to detect · Spindles running at slower speed · Spindles that give repeated occurrence of end breaks because of defective items or defective back material · Train the tenter to attend promptly to sides with higher end breaks · Reduce ends down losses by more effective patrolling making use of this system Modern winding machines have on line classimat, which gives continuous estimates various short and long length faults in yarn. This data should be related to the ring frame and production line from which doffs come, so that corrective actions are possible if some doffs give higher faults. Winding machine is also equipped to detect repeated breaks due to count variation and to reject the ring cops which give breaks exceeding a preset limit. This system should be put to use to find out ring frames or process sequences that give high count variation so that corrective actions can be taken.Maintenance
Maintenance infrastructure in mills should be geared up to the needs of modern machinery. As production rates are high, repairs and breakdowns have to be kept at very low levels by organising and rigidly implementing a preventive maintenance program. Otherwise, not only considerable production losses are encountered but also imbalances in production between different machines will be created. Considerable waste and defective material will be produced at the time of breakdowns. Since modern machines have many electronic items, mills should equip themselves with competent and experienced electronic engineers to service them and maintain them in good working condition. Overall knowledge of spinning technology will greatly help electronic engineer to understand the significance of various electronic controls and enable him to service them more effectively. Electronic engineer should therefore be given some orientation in spinning technology. Lack of competent and trained electronic engineers is a major factor hampering the utilisation of on line quality and process monitoring systems in the mills with latest technology.Replacement Schedules
Modern machines run at very high speed and as a result parts, which work on the material, get worn out fast and need to be replaced at more rapid intervals. Guidance in this regard is given in Table below.Table I Machine Part Frequency of Replacement/Maintenance Blow Room Saw tooth, pin or lag covering of beater 4 years Blades and pegs of disc type beater 1.Polishing every 2years
2.Replacement every 8 yearsCard Licker-in wire 9 months Cylinder, Doffer, Flat wire, transfer and redirecting roll wire, Stationary Flats, Combing segments 1.Grinding – 6 months
2.Replacement – 3yearsStripping Brush, Cleaning brush, Scavenger rod clothing 12 months Wire covering of Feed Roller 3 years Feed Plate 5 years Comber Half Lap 3 years Top Comb 9 months Stripping Brush 12 months Cot on detaching roller 1.Buffing – 6 months
2.Replacement – 12 monthsCot on Draw Box 1.Buffing – 3 months
2.Replacement – 12 monthsDraw Frame Cots 1.Light Buffing – 15 days
2.Replacement – 12 monthsSprings, plungers and other elements used in weighting 5 years Pneumatic hose pipe for weighting 1.Reversal – 6 months
2.Replacement – 4 yearsSpeed Frame Cots 1.Buffing – 3months
2.Replacement – 15 monthsAprons 1.Top – 18 months
2.Bottom – 9 monthsClearer Cloth 12 months Positively driven clearer cloth 18 months Top arm 1.Reconditioning – 5 years
2.Replacement – 10 yearsFlyer 10 years Ring Frame Cot 1.Buffing – 3 months
2.Replacement – 15 monthsApron 1.Top – 18 months
2.Bottom – 9 monthsClearer cloth 12 months Top Arm (Spring Weighting) 1.Reconditioning – 5 years
2.Replacement – 10 yearsTop Arm (Pneumatic Weighting) 1.Hose pipe reversing – 6 months
2.Hose pipe replacement – 4 years3.Weighting plunger and ribs – 4 yearsPneumafil pipe 1. Reconditioning – 5 years
2. Replacement – 10 yearsRing 3 years Traveller 1.Coated – 20 days
2.Normal – 5 daysSpindle and Bolster 10 years Quality Control
Quality control plays an even more crucial role in getting the best of modern machines. There will be difference in the estimates of quality between on line quality measuring systems and off line systems. This is because of difference in speed and mode of measurement. So studies should be made to work out the correlation between the two. This will help to make better utilisation of on line measurements. Apart from routine quality measurements and reports based on them, quality control personnel should undertake take studies for optimisation of process parameters like beater speeds, settings, card settings, break draft, roller settings, noil level in combing, trumpet size, spacer, top roller pressure etc. They should also take rounds of the department and carry out special investigation for controlling · Defective items – Spectrogram of material will help to bring out drafting defects and their place of origin · Disturbances in processes and analyse the reasons for them · Cleaning and work practices of operatives · Cause wise analysis of detention of machine · Check the functioning of on line quality measuring systemsControl of Defective items
With high production machinery, control of defective items plays a crucial role in producing defect free goods and in maintaining required productivity. Critical observations of processes and machinery at regular intervals should be instituted for this purpose and prompt corrective action should be taken. Supervisor who is entrusted with such work should be adequately trained in spotting the defect. Some critical defects, which should be looked for and attended, are discussed below
Blow Room
· Effective sorting and removal of contaminants from material – Effectiveness can be cross checked by examining the feed sheet to card for presence of hessian, coir, dirty and oily material. Dirty oily material often comes from soft waste and can be minimised by training the operatives in handling and sorting of soft waste. · Choke up of interspaces of grid bars with seed coats and seeds · Damaged spikes, saw tooth and pins in beaters – damages arise from ingress of wood or hard material into the line. · Sharpness of striking edges of beater · Good lint droppings under any beater – Can be minimised by optimising speeds of beater and condenser and settings
Carding
· Damaged strips in cylinder, doffer, licker-in and Flats – Damages arise from passage of wood, broom stick and hard substances · Strip loading of cylinder, doffer and flats – Strip loading can be because of damage to wire or due to deposition of oily and sticky material on wire. The latter can be removed by petrol washing · Defective seating of flats on flexible bends – Defective seating is mostly because of rigidity in links and can be set right by proper lubrication of flat chain · Excessive build up of waste on scavenger rod and uneven build across the width – arises from defective setting of licker- in bonnet in relation to feed plate · Improper functioning of flat end cleaning and Philipson brush · Examination of wire points of cylinder and doffer for striations, sharpness and freedom from burrs every 3 months – Use of a low power illuminated microscope will be useful for this purpose · Excessive flat strip waste with long fibres – disturbed front plate settings cause long bridging fibres · Worn out or enlarged trumpets – Trumpet gauge will help to estimate enlargement · Leakages in chute feed – Arises from worn out liners. · Unsatisfactory working of automatic waste extraction system – Arises from faulty sensors and plungers which operate the doors in ducting.
Combing
· Damaged and loaded half laps and top combs – Arises from double laps, lap licking and splitting, plucking under nipper and disturbed settings · Worn out and enlarged trumpets · Lap licking and splitting – Arises from improper setting of anti lap licking device, incorrect spacing of slivers in finger guides in Super lap, and high or low humidity in combing room · Web defects like curled fibres, holes, prominent piecing wave – Defects in nipper grip, feed roller grip, inadequate weighting of detaching roller and incorrect setting of detaching roller starting time should be looked into. · Noil variation between heads and long fibre loss in noil - Defective settings, inadequate nipper grip, damaged half lap and insufficient fibre parallelisation in input lap should be looked into.
Draw Frame
· Effective functioning of stop motion · Unaccounted stoppages – Stoppages can be because hank or evenness or CV is beyond preset limits · Robbing of fibres into suction system – Can be due to excessive suction or improper setting of web condensing/supporting guides · Eccentricity in drafting rollers and gears · Tension draft between front roller and calendar roller – should kept minimum without causing crumpling of material.
Speed Frame
· Torsional vibration of back bottom roller – This manifests itself in the form of intermittent rotation of back bottom roller and results in a very short length wave. The wave is masked in roving but shows up prominently in yarn particularly with man made staple fibre and blends. Defect arises because static friction in bearing is higher than dynamic friction. Defective movement can be minimised by perfect alignment of bottom roller and frequent lubrication of bottom roller bearings. Long length speed frames are more prone to this defect. · Eccentricity in top and bottom roller – Arises from misalignment of roller stands, worn out bearings, defects in joints, inferior quality of steel etc. · Alignment of middle and back sliver guide – Can be minimised by use of a proper checking device while fixing the guides · Disturbed top roller setting – Frequent roller lapping can lead to disturbances · Top roller defects like flute marks, flattened portions, oval shape and taper – Flute marks arise when frame is kept stopped for a long period with top arm weighted. Flattening arises from melting of cots particularly when top roller movement is jammed by heavy roller lapping · Defective loading of cradle – Front edge of cradle stays in a lifted condition and arises from defective or broken cradle retention spring · Choke up in presser eye · Tension in the roving in front – Excessive tension can lead to stretch and hank variation · Free flow of twist to front roller – With blends untwisted material issues out from some spindles because of worn out cots, absence of front condenser etc. · Sliver splitting in creel – Common causes are excessive fibre parallelisation, improper trumpet size at draw frame.
Ring Frame
· Disturbed top roller setting – Arises from heavy roller lapping · Defective loading of cradle with cradle front edge in lifted position – Is caused by broken or worn out cradle retention spring. · Top roller defects like flute marks, flattened portions, oval shape and taper – discussed earlier · Laterally shifted aprons – Common causes are slack aprons, misalignment of apron tensioning roller with knurled middle bottom roller, wrong ID of apron in relation to cradle. · Defective bottom apron tensioning arrangement – Defective or jammed springs, worn out cradles which hold the apron tensioning roller · Grooved Cots – Improper roving traverse movement · Bursted cots – Inner diameter of cot being too low in relation bare top roller diameter. · Slipped cots – Improper mounting of cots, unsatisfactory glue used for mounting. · Choke up of roving guide with slubs or thick material. · Defective movement of back bottom and top clearers – Improper or disturbed holding pins or clamps, lap up of roving on back bottom clearer roller. · Disturbed spindle and lappet centring · Worn out or damaged rings – Frequent ring cut bobbins is one cause for premature wear. · Vibrating spindles and bobbins · Over filled and under filled bobbins – Arises from improper fit of bobbin, vibrating bobbin, defective doffing practices · Riding of tape over wharve rim and twisted tapes – alignment of jockey pulley, missing spindle latches should be checked. · Oozing of grease from bottom roller bearing into bottom roller · Unaccounted idle spindles – Caused by repeated occurrence of end breaks or back material shortage or worker negligence.
![[http://www.geocities.com/ba1ja2/energysaving.jpg]](article_files/energysaving.jpg)
N.Balasubramanian
Retd. Jt. Director (BTRA) & Consultant
I, Rajeswari, 36, 17th Road, Chembur, Mumbai 400071 (Tel- 25280767
Specifications and quality requirements vary from one automobile manufacturer to another. The specifications for moulded carpets for some of the major manufacturers are given in Table 1.
| Sr. No. | Physical Property | Maruti Zen | Maruti Esteem | GMI Opel Astra | Tata Indica | Hyundai Santro | Fiat India Uno | Toyota Plain | Toyota Velour | |
|---|---|---|---|---|---|---|---|---|---|---|
| 1 | Material Nonwoven felt | Blend of 30%PES &70%PP | Blend of PES & PP of different shades | Blend of 2 different shades of PP& 2 different shades of PES | PES of single shade | PES of single shade | Blend of PES of 2 different shades | Blend of 3 different shades of PP | Blend of PP of 3 different shades | |
| 2 | Binder back coating | Acrylic | Acrylic | Thermo bondable | Acrylic | Acrylic | Acrylic | Acrylic | Acrylic | |
| 3 | Lamination | LDPE Powder & Film | LDPE Powder & Film | Nil | LDPE Powder | LDPE Powder | LDPE Powder | LDPE Powder | LDPE Powder | |
| 4 | Namda | Mixture of waste fibres | Mixture of waste fibres | |||||||
| 5 | GSM (gms/sq.m) NonWoven Binder LDPE Total | 330 50 260 640 Namda – 600 | 650BR>100 225 975 Namda – 1200 | 650 150 - 800 | 500 100 150 750 | 400 100 150 650 | 500 100 250 850 | 380 40 310 730 | 450 50 400 900 | |
| 6 | Tolerance GSM | +/_ 10% | +/_ 10% | Min – 800 | +/_ 10% | +/_ 10% | +/_ 10% | +/_ 10% | +/_ 10% | |
| 7 | Surface of Non Woven | Plain | Rib | Velour | Velour | Plain | Plain | Plain | Velour | |
| 8 | Thickness, mm | 4 max | 5 | 5-7 | 3 min | 3.25 +/- .75 | 3.2 | 3.2 | 3.5 | |
| 9 | Tensile Strength(20 * 5 cm) kgf M/C Dir Cross Dir | >35Ø >45 | Min 25 Min 25 | 40 60 | >18 >35 | >31 >41 | >40 40 | >40 >40 | ||
| 10 | Abrasion Resistance | 10000Cycles | 3000 cycles | Weight Loss after 1000 cycles – 20 g/sq m | Weight Loss after 100 cycles - < 0.5% | Weight Loss after 100 cycles - < 0.5% | Weight Loss after 100 cycles – < 0.5% | Weight Loss after 100 cycles - < 0.5% | Weight Loss after 100 cycles – < 0.5% | |
| 11 | Burning Rate ISO 3795 | Conform to FMVSS 302 | Conform to FMVSS 302 | |||||||
| 12 | Colour Fastness - To Light - To Washing - To Rubbing | >6Ø >4 –5 >3 – 4 | >5 >4 >4 | 6 | ||||||
| 13 | Temperature stability Dimensional stage M/c direction Cross direction | +/- 1.5% +/- 1.5% |
The following important features emerge from Table I
- PP and PES are the fibres mainly used in making needle punched fleece which forms the top surface of the carpet. In Maruti Zen, Esteem and GMI Opel Astra, blend of PP and PES is used.
- While single shade of fibre is used in manufacturing non woven in Tata Indica and Hyundai Santro, a blend of 2 to 3 shades of fibres is used to get the shade requirements in Zen, Esteem, Opel Astra, Fiat Uno and Toyota. When a blend of fibres is used, more precautions are needed to ensure uniformity in blend proportion and colour of carpet.
- GSM of the non woven fleece as well as final moulded carpet vary widely from car to car. GSM is lowest in Zen at 640 and highest in Esteem at 975.
- Plain, Rib and Velour surfaces are found. For velour type, Dilour machine of Dilo would give the best appearance because of random and uniform lay of the piles. As most of the nonwoven manufacturers in India are not equipped with this machine, Diloop type machine is used with a suitable arrangement of fork needles in the board to get velour surface.
- Though abrasion resistance is an important specification, different manufacturers prescribe different methods of estimating the same. For Zen and Esteem, the number of cycles to rupture or cause a hole in top surface is prescribed. For other manufacturers weight loss after 100 or 1000 cycles of abrasion are prescribed.
- For assessing flammability, burning rate on a horizontal flammability tester as per ISO 3795 is prescribed by all. All manufacturers require conformance to FMVSS 302 specifications, which is 4 in/min.
Apart from those specified in Table I, specifications for temperature stability, static compressibility, peeling force, fogging resistance, dielectric weldability, and freedom from odour are prescribed by some manufacturers like GMI. Maruti prescribes specifications for static electricity generation and half decay time. Since nonwoven manufacturers do not have facilities for these tests, they are required to take the assistance of Research institutions like BTRA. Some of the test methods are peculiar to automobile industry and have to be developed and standardized even in Research institutes. Raw material for needle punched top surface
Polyester and polypropylene are the commonly used fibres for manufacture of needle-punched fleece, which forms the top surface of carpet. Their relative merits are discussed below
1. The two fibres are nearly comparable in tensile strength though polyester fibre is available in higher tenacity grades.
2. Elongation is higher in polypropylene. This gives some advantages in terms of reduced tear during moulding. 3. Density of polypropylene (0.91g/cc) is much lower than that of polyester (1.38 g/cc). As a result thicker, loftier and more comfortable carpets are made with the former for a given area density.
4. Polypropylene is dope dyed and is available in an extensive range of colours and shades. It is therefore much easier to achieve colour and shade matching by mixing a minimum number of shades of fibres. Dope dyed polyester, on the other hand, is available only in a limited number of colours and shades. The required shade has often to be developed through R&D work or fibre has to be dyed.
5. Melting point of polypropylene (165oC) is much lower than that of polyester (260oC). Heating time, temperature and pressing time are therefore more critical in moulding with polypropylene.
6. Flame retardancy by burning rate is inferior with polypropylene than with polyester. A flame retardant compound has to be added to the binder to meet the flammability requirements in Exports with polypropylene. This adds to the costs.
7. Resistance to UV light is inferior with polypropylene compared to polyester. UV stabiliser has to be added during manufacture of polypropylene to improve its resistance to UV light.
8. Polypropylene tends to form beads during carding. The beads get deeply loaded on the cylinder wire and also on the needles. There has to be a regular programme for removing the beads from the wire and needle. Achievable production rates are therefore lower with polypropylene than polyester because of card loading. Card processing problems are more acute with lower deniers and recycled fibres with polypropylene. Manufacturing Process
Moulded carpet manufacturing can be broadly divided into 4 sections 1. Needle punching 2. Back coating of needle-punched fleece with binder 3. Lamination, Blank cutting and Moulding 4. Trimming, checking and packing. Needle punching
Needle punching line is similar to that in a normal nonwoven plant. The following steps may be beneficial in getting good quality at lower manufacturing cost. The opening line consists of a Bale opener followed by 2 beating points. Use of high capacity mixing bin with automatic laying of material and simultaneous spraying of antistat such as found in Empto mix of Hergeth is helpful in ensuring intimacy of mixing and in minimising shade variations from place to place. Further performance in carding is improved because of uniform application of antistat. Feeding chute to card should have controls like photocells and vibrator or pressure switch to ensure uniform density of feed sheet. Incorporation of micro weighing system at the top of chute whereby small pre weighed tufts are dropped into the chute will improve batt weight uniformity further. Card – Cross lap sequence is preferred to air laying because of long length of fibres used for making the felt. Since the fibre denier ranges from 6 to 15 d, card should be mounted with appropriate wire of coarser gauge. The card room should have a good humidification system with some control over temperature. This will minimise loading of cylinder, doffer and stripping rollers especially with finer denier. Loading of rollers not only causes production loss but also is a major source of variations in lenier density (gsm). Needle punching machine of 2m width would be adequate, as the length and width of the carpets of automobiles is below 2m.The needle punching may be done in 2 stages i.e., Pre needling line and a Finish needling line with a break in between. This has the advantage that if the gsm of the material is lower than the requirements, correction is possible in finish needling by the addition of a thin felt of required gsm. A continuous line with preneedling followed by finish needling does not provide such correction facility. Pre needling
With Die pressed needles with regular barb spacing should be preferred as it minimises fibre damage and achieves uniform fibre interlocking. Further 3.5 inch needle should be used, as it allows wider gauge between stripper and stitching plate which in turn minimises distortion of the voluminous batt as it enters the needle loom. 32-34 gauge with a punch density of 90 – 100 punches /sq cm and depth of penetration of 11 – 13 mm depending on thickness of felt is normally used. Finish needling
3 inch needle with close barb spacing has the merit that it allows all the barbs to enter the fleece with a relatively low depth of penetration. This improves consolidation and minimises fuzzy surface at the bottom. The lower needle length is also less susceptible to breakage. RB needle however gives more smooth surface. The penetration depth is between 10 – 12 mm with punch density of 150 – 250 punches/sq cm in case of plain carpets. In the case of ribs and velours, a lower punch density of 90 – 100 punches/ sq cm is used to enable easy penetration of fork needles at structure needle machine Structure Needling
In case carpets with rib or velour surface, a structure needle loom is used after finish needling. Diloop loom or its equivalent is used by majority of manufacturers though as pointed out earlier, Dilour machine by Dilo gives a more random and uniform pile surface. According to the position of fork in the needle board, rib or velour surface is obtained. For products with rib surface, penetration side of the fork needle will be reverse side of the finish-needled material, as it will give compact ribs. On the other hand, penetration side of fork needle will be the same side of finish needling in case of velour surface as this improves velour effect. Penetration depth is 7 – 8 mm in ribs and 8 – 10 mm in velours. The normally used fork needle is of 25 gauge. Back Coating
Back coating with a binder in the form of foam, followed by curing in a stenter gives dimensional stability and strength to the carpet. An auto foaming unit will be helpful as it permits automatic adjustment of foam ratio so as to get uniform pick-up. Acrylic binder is normally used as it gives the required strength. The binder pick-up is kept low as the material is going to be subsequently laminated with LDPE. As seen from Table 1, binder pick-up varies from 10 – 20 % for carpet where subsequent lamination is done. Foam ratio is one of the important characteristics to be monitored and kept under control during the process to minimise variations in binder pick- up which ultimately leads to variations in linier density of carpet. Curing of binder is carried out in a stenter through heating by electrical heaters kept on both sides of the material. Temperature on the binder face is kept at 140oC and on the fibre side at 110oC. Material speed is adjusted as per the material and binder pick up and is usually between 2 – 3 m/min. To cut down processing time and costs, thermo bondable binder is sometimes used. With such binder there is no need for lamination with LDPE powder/film. Thermo bondable binder is special styrene/ butadiene latex. The latex not only binds the fibres but also provides mouldability to the material. The binder film is thermoplastic with little softening in the region of 35 – 90oC. On further heating the softening increases very rapidly in the region of 120oC enabling moulding. The ratio of styrene to butadiene in the binder determines the stiffness and thermo plasticity of the binder. Lamination
Carpets back coated with normal binder (acrylic) require lamination with LDPE powder or film or with both to enable moulding. The properties of LDPE powder have considerable influence on moulding. The average micron size of powder is 70 microns. For satisfactory moulding, the size of the granules should not be uniform throughout but should be variable from 30 to 100 with a peak value of 70. A distribution consisting of coarse and finer particles on either side of the average improves mouldability. The powder is dusted on the moving carpet on the back-coated side by an applicator, which is automatically controlled to achieve the required powder application. The material is then preheated in steps by electrical heaters to softening temperature and calendared by a pair of high-pressure calendars. Lamination of the powder on the carpet takes place as a result of softening and pressure application. With some products a pre-heated film is also introduced into the nip of calendar rollers along with the carpet, which causes lamination of film on the carpet. Blank Cutting Laminated carpets are cut to required length on a blank cutting machine. The length requirements vary with the car and is standardised after initial trials. Lengths are marked on the Table of bank cutting machine with different colours ( for identification) for the different cars. Moulding The laminated and blank cut carpets are gripped on either side by grippers and moved to a pre-heating zone. The LDPE lamination is softened during pre heating. After preheating for a specified time the material is taken forward to a moulding press consisting of a male and female mould. The moulds are fabricated accurately as per the design of car body. Since car body is a patent of the car manufacturer, the moulds have to be prepared after a formal agreement with them to protect secrecy. Moulds are made either of aluminium or resin bonded fibreglass. The former though more expensive has a longer life and has a cooling arrangement, which reduces moulding time and increases productivity. Time for pre heating and pressing in moulding are critical factors affecting quality of moulding. They are determined by type of fibre, gsm of carpet, type of binder and lamination. Carpets made out of high proportion of polyester require in general more preheating than those with high proportion of polypropylene. Heavier carpets require more pressing time. Time for pre heating and pressing are standardised for each type of car after initial trials. With some cars like Maruti, Namda, which is a bulky material made out of waste fibres, is attached to the underside of the carpet either during moulding itself or subsequently by an adhesive. Namda improves insulation and helps to absorb vibrations. Trimming
After moulding the carpets are trimmed to shape on a trimming unit. The extra material at the sides used for gripping the carpet during moulding is cut out and holes are punched at different place to accommodate car parts like seat legs, break, clutch and accelerator pedals. Trimming unit is made to match car body perfectly in minutes detail so that trimmed moulded carpets fit perfectly in the car. Inspection
The moulded and trimmed carpets are checked for fitment on car body or a template provided by car manufacturer. Minor defects in the carpets are mended. Carpets with major defects and improper fitment are rejected. As majority of problems come from improper fitment, a great care has to be exercised in trimming and inspection. Too reduce labour costs, some manufacturers get trimming and inspection done on contractual basis by an outside party. Heel Pad Fixing
In some cases, heel pad is welded to carpet on a high frequency plastic welder. The object of heel pad is to prevent wear of carpet due to foot pressure from driver of car. Heel pad is usually of PVC material and car manufacturer gives specification. Final Checking
Final checking for fitment is done on car body. As this is time consuming, it will not be possible to have 100% checking. So random checking of 15 – 25% of production is usually done. It would be advantageous to carry out checking some carpets on car body with seats in position. Major Quality Problems
Some major quality problems in moulded carpets which lead to rejection of the material and precautionary measures to overcome them are discussed below. Colour Mismatch
Non matching of colour of the carpet with the master approved sample and variations in colour and shade from place to place and lot to lot form a major source of rejection. The following precautionary measures will help to overcome this problem.
1. Raw material (fibre) should be matched in colour against master sample in a colour cabinet. It is advisable to have two master samples representing the extremes in tolerable colour variation.
2. Intimacy of mixing of components can be improved by increasing the number of layers in the sandwich and by having one more toppling before processing.
3. Dope dyed fibre should be preferred to dyed fibre as better consistency in colour and higher fastness to rubbing and light is obtained with the former.
4. Scope for improvement in fibre opening in the line should be explored by addition of Kirschner beater. Contamination
Contamination with foreign material and fibres from previous lot arises because of inadequate cleaning of card wires and needle board and stripper and stitching plate. Molten plastic beads and nep like clusters in needle board also result in contamination. Card wires should be washed with petrol once a shift and during lot change. Needle board should be removed and cleaned with compressed air. Fibre clusters firmly adhered to the barbs should be removed with a wire hook. Needle board cleaning should be done once every 4 hours and at the time of lot change. Weight variations.
High variations in weight (gsm) and thickness from place to place cause rejections. The variations in weight can be at needling stage or during back coating. Stoppages of card due to loading of material on cylinder, workers, strippers and transfer rollers is one of the causes of variations in gsm. This problem is more often encountered with fine deniers and with polypropylene. Optimisation of antistatic spray applied on the fibre and maintenance of proper humidity and temperature in card room will help minimise loading. The card wire should be washed with petrol and damages in wire rectified. Lower doffer speed with higher web density will also be helpful. Control of foam ratio in the binder during back coating will also minimise variations in binder pick-up. Lines in the Fabric
Two types of defects are common.
1. Longitudinal lines running along machine direction of needle punching line, arise from broken or worn out needles. A proper schedule for replacement of needle should be drawn up and adhered to. Life of needle depends on type of needles, barb size, type of product, punch density, gsm etc. and it would be difficult to give a schedule for needle replacement that will hold good at all times. Phased replacement of needles in the needle board as suggested by Foster Needle Co., will help minimise lines caused by needle wear and breakage. Needle board should be divided into 3 parts(A, B, C). All needles occupying one third of the width of the board(A) is replaced at a time after about 10-15 million strokes. Broken needles in other 2 halves of board (B & C) are replaced by good needles removed from the board (A) at this time. After another 10-15 million strokes, all needles in section B are replaced and broken needles in section A & C replaced by good needles retrieved from B The same process is continued after another 10-15 million strokes in Section C. 2.Horizontal lines in cross direction of fabric at regular intervals arise because needles are penetrating in the same hole as fabric advances. This arises because net advance of fabric during a stroke matches with the distance between two adjacent rows of needles. The defect can be overcome by altering the slowly the stroke frequency while keeping fabric delivery speed constant. Shape Retention
Rejection due to deformation or non retention of the shape of moulded carpet is another source of rejection. Moulded carpets laminated with LDPE film in general retain he shape much better than those laminated with LDPE powder. But lamination with film alone is more difficult and wrinkles are encountered if the speed of film is not properly controlled or if the film is not properly rolled. So a good compromise is to laminate with both powder and film. Improper packing and transportation of carpets also leads to loss of shape. If a large number of carpets are stacked one over the other and transported, those at the bottom get deformed. Carpets should be laid on specially prepared rack with number of trays conforming to the shape of carpet. The carpets in different layers should be separated from another. Improper Fitment
Improper fitment is one of the major complaints. This is mostly due to improper trimming arising from wrong methods or improper tools. Use of pre heated knife helps in getting sharp cuts. The tools used for cutting holes should be checked for wear and tear and sharpness. The car body used for checking carpets in moulding plant must match accurately in shape with the cars in automobile manufacturing unit. Holes in Carpet
Excessive pre heating is one of the causes of holes particularly at corners. Excessive tensioning of material during moulding may also contribute to holes. Mould should be free from burrs or scratches or extraneous matter. Wrinkles in Carpet
Wrinkles are caused because of inadequate or excessive pre heating or inadequate tensioning of material during moulding. Pre heating time has to optimised for different types of fibres and area density of carpet.