by Ahmed Raza, MSc. Textiles C Text ATI BSc.

With the reduction of weaving activity in developed countries for various reasons, including their high labour costs and stringent environmental pollution standards, it is essential for our weaving sector not only to continue to meet the domestic demand but to also gear to itself up to meet the demand from quality conscious export markets.

However, to be competitive our quality and pricing has to be acceptable to the export buyer. For this purpose it will be essential to remember the following main points, which contribute to profitability of any set up:

1. Optimum utilization of installed looms.

2. Maximum operating efficiency.

3. Minimum fabric defects.

4. Convenience for post-weaving operations like wet processing etc.,

In order to achieve the optimum in the objectives while 1 will depend mainly on equipment and their condition, 2 and 3 will depend to a large extent on the raw material supplied to the looms both for warp and weft. Every loom stoppage apart from reducing loom efficiency is a potential damage in the fabric and should be avoided. Loom stoppages can be caused mainly by warp breakages, weft breakages, weft running out or malfunctioning of some loom mechanism and some other causes.

Prerequisites for Yarn Selection

The choice of the yarn for weaving will necessitate purchase of yarn with CSP in the range of 2000 - 2500 depending on count even though yarn with a lesser CSP will also run. It is better to invest a little more in better raw material and benefit by lesser loom stoppages, better loom efficiency and reduced fabric damages.

Tests should be taken on the yarn for CV% of lea count and CV% of lea strength and yarns not exceeding 2.5 % for the former and 6% for the latter can be expected to perform satisfactorily. Similarly single yarn strength variability should not exceed 8 % and variability of single yarn twist should not exceed 6% if optimum performance is required. Whilst these factors affect loom performance, yarn unevenness affects fabric appearance and should preferably be around 12% - 15 %, U% depending on whether we are using combed yarn or carded yarn. Another quality affecting fabric appearance is yarn imperfections - particularly “neps”. These should not exceed 1000 - 1200 per km for carded yarns and 300 per km for combed yarns. Doubled yarns should have significantly lesser U% and lesser number of yarn defects. Doubled yarn CSP in the range 2500 -3300 will provide proper weaving performance.

Requirements for quality yarn and existing spinning process

In the last two decades, spinning mills have begun to experience a quite distinct intensification of business pressures.The international yarn trade tends to give preference to the producers of better and cheaper products, and there are many producers who can offer yarn of a satisfactory quality. The reaction time (quick response) is being given higher priority. The yarn quality has to be accurately adapted to the conditions of further processing and the end product. In order to be able to compete, one must continuously invest. Although there is a certain amount of unemployment,there is an absence of competent technologists with good knowledge of the complete textile process.In order to satisfy the expectations of the customer, the quality of the raw material and of the yarn, in many cases, is considerably over-dimensioned, for safety reasons, and also on the basis of the motto “Quality is when the customer returns and not the goods”. Quality, however, can be achieved with an efficient quality management system.

Within the spinning process, the limiting conditions have changed during the last few years:

• Personnel have less knowledge about the raw material than was earlier the case. Furthermore,particularly with cotton, the spinning value is often much worse than previously; alternatively, many important factors necessary for yarn manufacture are not taken into consideration during classification.
• New machines in the spinning mill run quicker, produce more, and stress the fibre much more than was previously the case.
• The spinning process is shorter, and the number of fault-reducing doublings is much less.
• The yarn processor requires a much longer fault-free length of yarn or a better quality of yarn.
• Competition has a negative effect on profit margins. As raw material prices reach approximately up to 72% of the overall yarn production costs, the spinning value of a raw material and variations in the raw material have to be regarded as much more important.
• The yarn buyer not only demands a yarn quality adapted to the requirements of further processing and the appearance of the end product, but he is also interested in close tolerances and a reduction in the number of so-called ‘rogues’. In other words, his aim is zero defects according to specifications.

Warp yarn properties

Warp stoppages are attributable mainly to raw material quality and/or its preparation for weaving. It is therefore essential the right quality of yarn is purchased for the preparation of the warp beam. If weaving of single yarn is resorted to, the factors affecting performance are yarn strength and its variability and the quality of warp sizing. While lea strength is another important factor for the purchase of yarn for weaving variability of the single yarn strength i.e. CV% of single yarn strength influences warp stoppages more than any other factor.

(a) “Effect of Sizing”

Any yarn subject to the sizing operation where it is dipped in a size solution and dried results in an improvement in strength and a loss in elongation and this improvement in single yarn strength should be a minimum of 20 % - 25 % and the loss in elongation of the single yarn should not exceed 20%.

Stretch during the sizing operation should not exceed 2 % if the sized warp beams should behave properly. This sized warp should absorb the required quantity of moisture before it is gaited on the loom so that better weaving results as beams just off the sizing machine do not have the required moisture content and hence the yarn can be brittle.

The warp threads running from the warp beam all the way to interlacing in a loom come into contact with the back rest roller,the drop wires,the healds and the reed,and also sometimes with the picking block and weft yarn,where yarn forces or stresses occur in the positions where the warp yarn passes from the right side to the left side and vice versa.These forces are effective in the warp and weft until counter balanced.In addition to the above-mentioned stresses,the warp yarns are subjected to variable forces arising at shedding,then to the forces of weft yarn beat-up to the fabric fell;fabric let-off is actuated by the cloth regulator. In order to protect weak places in the warp yarns from destruction,and not to classify them into the group of weak places in the yarn,the fibers within the thread should be bonded together; in this way their further destruction will be prevented.A thinner place in the yarn absorbs less size,and if it is a weak place,it requires more intensive sizing,demanding sizing agents of higher quality and requiring continuous control.If the frequency of weak places is higher and the yarn more uneven with a higher coefficient of variation,it is necessary to size it with a size coat of higher quality to meet the requirements of weaving.By sizing,warp yarns are provided with necessary strength,elasticity,smoothness,and acquire resistance to abrasion and static charge.Quality sizing is deep sizing,where fibers are fixed in the position in which they were before sizing.Besides deep sizing,it is also important to apply size on the surface of the thread in the form of a film providing outer protection of the threads. Modern sizing technology increases the efficiency of looms,brings savings of sizing agents and energy, and improves the quality of the sized warp.In spite of huge progress in sizing technology, there is a series of theoretical and practical problems concerning the optimization of size.Since no standardized test methods yet exist, which could show to what extent end breakage rate would decrease on a loom after sizing,difficulties exist in planning the efficiency of looms and the quality of the end product.It is believed that size  will enable a minimum end breakage rate on a loom. According to Table-1,the optimum size is slightly higher than minimum size , because it is very difficult to attain its precision and uniformity through out the yarn.Due to an insufficient penetration of size into fibre inter-spaces, the central portion of the cross section remains unsized.The partly sized thread will not obtain the necessary strength,and by destruction of the inside fibre portion the fibers will be stretched, the protective size layer will break and be removed from the thread surface,an increasing friction will occur.The consequence of this occurrence is a decrease in productivity,the efficiency of the loom and the quality of the end product.One of the essential pre-requisites for good and efficient sizing is that the size should be uniform from the beginning to the end of sizing of one lot.It is necessary to keep some of the influential parameters constant or within the function of the size ,such as concentration,viscosity,the temperature of the size in the size box,the through-put speed of the warp through the box,the inlet moisture of the warp before sizing,the outlet moisture of the warp after sizing,and the squeezing force and yarn tension in the size box.To optimize the size quantitatively and qualitatively, it is necessary to apply the following parameters:the raw material composition of yarn and its properties such as yarn count,yarn twist,fibre length,spinning method,the type of weaving machine,the article,the seasonal and climactic conditions of processing natural fibers,and so on .The yarn to be used as warp comes from a spinning mill with properties which,in most cases without sizing, do not guarantee satisfactory quality and productivity during weaving .Due to its insufficient strength and surface hairiness,frequent end breakage rates and great yarn deformations occur.

Where doubled yarn warps are involved the effect of sizing is marginal but even though weaving is possible without sizing it is recommended we subject it to a minimum size application so that weaving and any problems connected with snarling and hairiness are neutralized as the protruding fibers get laid down to blend with the parent yarn in this process. Further this helps in the reduction of free floating fibers in the weave room that might be caused by abrasion during the weaving process. However, when dealing with double yarns this sizing process cannot be expected to improve the yarn strength to any great extent.

Where colour warp is used it would be advisable to opt for the use of beam dyed warp for sizing rather than dyed cones or cheeses as variation in dye uptake causing streaky warp can be avoided. This, however, reduces warping and sizing efficiency as the set length is only about 2500 metros against much longer lengths possible using cones or cheeses. This expense will, however, be justified by better warp quality and fabric quality.

Changes in the weaving process

In the last few  years, there has been an increase in production of 2 to 3 times the area of woven cloth per weaving machine as a result of increases in weaving machine speed and fabric width. The weaving speeds that were previously considered impractical have become normal production conditions within only a few years. However, high production alone is no guarantee of economic conditions and product quality. Unproductive downtimes, such as stops due to end breaks, are of fundamental importance in terms of manufacturing costs, as “material structure and short stops at weaving determine the productivity at a weaving machine and not the maximum and theoretically possible machine speed”.. Here, more than ever, the law of any business undertaking is applicable:
 “Not taking a step forward is in fact the same as taking a step backward”. If the rate of weft insertion of a weaving machine is doubled, then the yarn will have to contain half the number of weak places than was previously the case, in order to achieve the same weaving efficiency. Those weak places that will later produce stops at weaving and knitting machines. However, in many cases, weak places only exhibit a certain percentage of the mean breaking force of a yarn. Considered statistically, weak places in yarns are rare events. Nevertheless, they are gaining more and more importance in high-production spinning and weaving mills, for the following reasons:

• It is particularly the weak places that produce end breaks, which can negatively influence the running efficiency. When making a forecast of machine stops as a result of weak places, the following must be taken into consideration:
• Tensile tests in the laboratory are carried out under different conditions than those which are encountered during weaving and knitting.
• Yarns can break during subsequent processing as a result of package defects, poor winding conditions, clinging of two threads, poor warping and beaming conditions, unsuitable sizing, too much yarn hairiness, too many thick and thin places and neps, poor splices or knots, etc.

Analysis of the requirements of the weaving mill show that no more than one stop per 100,000 m of weft yarn should occur. As soon as two or more stops per 100,000 m of yarn are available in the high production weaving mill, costs will increase as a result of a reduction in efficiency. If one takes as an example a ring-spun yarn with a yarn count of 20 tex (Ne 30) and a cop weight of 50 g, then at the most one weak place every 41 cops (which could result in a stopping of the machine) would be acceptable.

Reasoning  of  warp stops

Today, less than 0.4 stops per 1,000 warp threads and 100,000 picks are considered acceptable.Only when one considers that (in each case according to the style) 20,000 m of yarn can be available in the weaving zone for 5 to 10 minutes, does the severity of this requirement really become comprehensible.Yarn hairiness, an important warp characteristic, has always been a factor which has influenced the appearance of the cloth, which was true even long before it could be measured. Varying yarn hairiness, e.g., from package to package, results in weft stripes in a woven fabric, or a cloudy appearance in a knitted fabric. Hairiness is also increasing in importance with respect to the running conditions at all processing stages subsequent to spinning. A high amount of hairiness of the warp yarns can negatively influence the movement of the weft yarn through the shed with air-jet weaving machines, and the weft transfer with rapier weaving machines. The result is usually a stop as a result of a warp. Hairy yarns and structure faults in the yarn, such as neps, often produce threads which cling to each other, particularly with the much smaller shed openings in modern weaving machines. If one increases the warp tension in order to avoid these clinging fibers, more end breaks can result due to weak places. The result of this is that hairiness, as well as hairiness variation and particularly periodic hairiness variation, is of increasing importance with warp yarns, especially with respect to their application on high-production weaving machines.

Requirement of warp yarn for weaving

In contrast to a weft yarn,  weak places in the warp yarn which slip apart due to too little twist, as for instance with a slub-type yarn fault. Furthermore, with warp yarn, hairiness is becoming increasingly important, i.e., there should be little hairiness and it should be evenly distributed. The requirement profile of a cotton warp yarn is indicated in Table 3.

Reasoning of stops during beaming

The yarn is placed under stress not only during weaving but also during weaving preparation such as warping, beaming and sizing, which is particularly true in terms of yarn elongation.Beaming machines operate today with drawing-off speeds of up to 1,200 m per minute.Only with an end break frequency of less than 0.4 end breaks per 1,000,000 m are conditions considered as very good when beaming a combed ring-spun yarn, and conditions of less than 0.8 end breaks per 1,000,000 m are considered as very good when beaming a carded ring-spun yarn.

Weft yarn properties

(a) Importance of Automatic Pirn Winding

In shuttle weaving the size of the shuttle will determine the maximum size of the pirn that can be used and we should use the pirn that has the longest length so that replenishment, which can cause damages, are less frequent. If the yarn breaks and is not sensed in time or if the pirn runs out and the feeler does not actuate stoppage or transfer in time a potential fault occurs and hence pirns must be wound to maximum density and in a shape that will permit tension free and or uniform tension withdrawal. It will, therefore, be necessary to use a suitable automatic pirn winding equipment if we are not using shuttleless looms. This problem is compounded further if multi colour weft is used. In addition to the production of proper pirns it will be necessary to have them colour graded properly. This is essential as it is usual to have some shade variation between the end and beginning of the dyed cone or cheese when package dyeing is used for the dyeing operation. Hank dyed yarn can show still greater variation between lots and has to be guarded against.

(b) Effect of Winding Tension

Tightly wound pirns with material content of about 25 gms in the case of spun yarns and 15 gms in the case of filament yarns will have to be used in the interest of reduced transfers. In the case of filaments, excess winding tension can caused elongation and result in bright picks which are fabric defects.

In earlier years it was usual to be satisfied with yarn of poorer quality characteristics for weft in view of the lesser tensions involved during the process of weaving. Modern high speed weaving machines and methods require higher tensions on both warp and weft, hence they do not permit the use of low quality of yarn. These modern looms also require that yarn quality characteristics for warp and weft must be similar.

(c) Yarn Quality and Type of Loom

As the weaving industry starts modernizing and installing more modern automatic looms in place of the old non-automatic looms the choice of raw material becomes more critical as the higher investment necessitates keeping the looms running round the clock at efficiencies greater than 90% and this will be possible only with proper raw material quality selection. We cannot afford to take the risks that might have been taken so far as these more expensive looms with their greater production potential can cause greater losses also if they are kept idle or allowed to produce faulty fabric. Modern looms have suitable safeguards to prevent warp breakages and weft breakages from causing defects in the fabric but if the raw material is not selected properly there is a loss in efficiency, which can be costly and has to be avoided.

Requirements of  weft yarn for weaving

In contrast to earlier considerations, a weft yarn today must have a requirement profile as high as that of a warp yarn in order to satisfy the requirements of high-production weaving machines.According to the scientifically-based investigations, a weft yarn must exhibit at least the following quality characteristics as indicated in Table 2. What has become particularly important, for instance, is the yarn elongation as well as the variations in breaking force and elongation. The Classimat faults can also be considered as weak places, because a thick place fault usually contains less twist than the rest of the yarn, and can easily break when a tensile force is applied. It should be mentioned here that, with a higher variation (i.e., a higher coefficient of variation value of elongation), this can only be compensated by a elongation at break value in order to achieve equivalent running conditions. In terms of the spinning process, this means “Better raw materials, higher yarn twist, etc., certainly result in increased yarn manufacturing costs”.

Measures to produce quality yarns

Better yarn properties do not necessarily result in increasing costs as normally spinners in Pakistan think, but in the long term they usually provide a reduction in costs.

The following possibilities:

• Detection of raw material variation and its reduction (a basic and fundamental requirement).
• Reduction in the coefficient of variation values of the yarn quality characteristics.
• Elimination of rare yarn faults (weak places), which is becoming more and more important.
• Increase of the yarn breaking force and elongation at break.
• Application of high-quality preparation machines and spinning units.
• Supervision and control at all the spinning processes (also a basic requirement).
• Obtaining optimum conditions with all machines.
• Use of automation (according to existing mill conditions).
• Elimination of any negative ambient influences.

In the spinning mill, it is never advisable to obtain maximum conditions based on one single quality characteristic alone. As is normally the case with the spinning mills of Pakistan,it should be noted that success always lies in a balanced compromise. What is the use, for instance, of a nep-free yarn whose fibers during carding are so stressed that yarn tensile force and elongation are reduced as a result of the increase in short fibre content? For this reason one should always assess the complete process, and not try to obtain maximum conditions on a single machine.

Blend Variations

Major portions of the looms are suitable for all cotton fabrics.
There is also a significant segment using blended yarn. Popular blends in Pakistan are with polyester - 65/35/ P/C, 52/48 PC, 80/20 PV, etc. With the cost of cotton progressively increasing, manufacturers have a tendency to substitute this with polyester. However, it will be necessary to make sure that the yarn is made from not too fine polyester fibre as these yarns have a tendency to pill after use and this affects fabric appearance. Pilling is more pronounced with high tenacity fine denier fibre. Hence yarn quality assessment has to be made for pilling resistance also.Yarn CSP values are higher and in the range of 3000 - 3500 depending on counts, fibre fineness and blend proportion. Unevenness levels are also about 2% lower than all cotton yarns due to the proportion of uniform fibers in the blend.

Two fold yarns made with blends weave better than all cotton warps and can provide very satisfactory levels of weaving efficiencies with lesser fabric damages.
 
Blend yarn, if single, has to be sized with proper ingredients and while starch sizes with 18/20 % add-on may be adequate, it will be better to opt for beams produced with water soluble size ingredients like poly vinyl alcohol (PVA) as these have better adherence at lesser concentrations and are easy to make up, apply and remove with minimum environment pollution problems. Further as many of the weaving units may not be able to afford efficient humidification systems and have to operate under varying conditions of temperature and humidity these PVA sized beams will operate efficiently at 70/75 % relative humidity levels instead of the 80% and greater humidity levels required for starch sizes. Lesser shed humidification costs will also help in reducing the cost of production.

Inventory Control

Sized beams have a length of around 1000/1200 metros depending on count and flange diameter. Since looms can be expected to produced around 100 metros per day the loom beam will last for 10 days. Depending on looms installed the beam fall can be worked out and only the beams required for 2 to 3 days need be maintained in the shed. Since sizing is done in the interest of economy it will be better to size a set of about 8000 metros which will be a sizing machine’s production in a shift.

As the sorts woven will have a bearing on the beam run out pattern this will also have to be taken into consideration. The actual number of beams that can be gaited in a shift - whether knotted or fresh gaiting with healds and read will also influence the inventory.
While no loom should stop waiting for a warp beam we should also not carry too many sized beams in stock as prolonged storage can result in soft size affecting loom performance when this beam is used in addition to blockage of funds.

In respect of weft the inventory will depend on whether the weft is grey or coloured and the number of colours required for the assortment. However each loom should have a full battery - 25 pirns - plus a container full of pirns - say 80 numbers. If colour weft is used all colours required or generally asked by the market must be carried in stock to ensure assortments are ready for despatch without any delay. Fabric doffed from the looms as per lengths stipulated by the customer should be inspected and got ready for despatch and can be expected to be around 2 days production. Inventory of warp and weft must be stored safely under room temperature and humidity so that wastage is avoided. Fashion changes are frequent and markets require smaller lengths in different colour/weave combinations and to cater to this requirement a larger inventory than that for weaving a standard sort becomes necessary. The yarn supplier has to be identified based on the yarn characteristics specified and 3 days stock will be adequate. Long term arrangements could also be made with the yarn supplier to deliver the yarn when required and save on inventory carrying costs. Inventory carrying will depend on ability to service the outlay but excess safety inventory is wasteful and has to be avoided.

Conclusion

The choice of the yarn for efficient single yarn weaving will therefore necessitate purchase of yarn with CSP in the range of 2000 - 2500 depending on count even though yarn with a lesser CSP will also run. It is better to invest a little more in better raw material and benefit by lesser loom stoppages, better loom efficiency and reduced fabric damages.

Tests should be taken on the yarn for CV% of lea count and CV% of lea strength and yarns, not exceeding 2.5 % for the former and 6% for the latter, can be expected to perform satisfactorily. Similarly single yarn strength variability should not exceed 8 % and variability of single yarn twist should not exceed 6% if optimum performance is required. Whilst these factors affect loom performance, yarn unevenness affects fabric appearance and should preferably be around 12% - 15 % U% depending on whether we are using combed yarn or carded yarn. Another quality affecting fabric appearance is yarn imperfections - particularly “neps”. These should not exceed 1000 - 1200 per km for carded yarns and 300 per km for combed yarns. Doubled yarns should have significantly lesser U% and lesser number of yarn defects. Doubled yarn CSP in the range 2500 -3300 will provide proper weaving performance.

 The future of the weaving sector of Pakistan will depend on its ability to meet the quality demands of the domestic and export markets. In both markets by reason of fashion changes smaller runs and larger assortments are becoming the rule. For this purpose better looms have to be installed - Shuttle, Automatic and Shuttleless Looms - and these have to be operated round the clock at over 90% efficiencies and utilization. This will be feasible only with the use of good quality yarn which conforms to specifications for efficient weaving on the basis of low variability in count, strength, low unevenness and with minimum defects which affect performance and fabric visual appearance.

Supervision of the sizing operation is vital and must be performed in a manner that will facilitate efficient weaving with minimum crossed and missing ends. The weaving operation itself must be aided by the creation of proper weaving conditions - particularly humidity - and proper training of personnel in all weaving operations and strict maintenance practices, which is badly lacking in weaving sector of Pakistan. Proper material handling has to be insisted upon, so that the material moves from spinning mill to warping / sizing and, thereafter to looms without damage to material, if one has to meet the stringent quality demands. As blend yarns acquire oil stains easily and fabric can be damaged it is essential that proper weft and warp handling procedures are followed at all stages.