The denim units have a high production output. According to international standards a unit of 50 million metres per annum capacity is considered to be of an economic size. In India however units of 10 million metres per annum capacity have been operating.
The spinning capacity is usually determined by the performance in the dyeing range. For a 10 million metre capacity unit producing 12 ozs average fabric weight with a standard width of 150cms require denim yarn of 16 to 17 tonnes per day. Extending it further, the raw material requirement will be about 19 to 20 tonnes of cotton per day. Thus, spinning of denim yarn is really a large scale business because of heavy yarns and fabrics.
PLANT & MACHINERY
The following features in plant and machinery have become prerequisites for producing top quality denims:
A. Modern Blow room line with 4 to 5 beating points with micro dust extractor and very efficient automatic waste evacuation system.
B. Modern High Production Chute feed cards with autolevellers and efficient automatic waste evacuation system.
C. High Speed autoleveller Draw frame at finisher passage.
Open End Spinning
D. Modern Open End Spinning machine with automatic piecing and yarn monitoring devices like Corolab or Uster Polyguards.
E. High speed frames with bigger doff packages.
F. Ring Spinning machines with Longer Lift and Larger Ring dia.
G. Cone winding machines with efficient yarn clearers
Let us first start with the raw material. Cotton fibre specifications of important Indian varieties used in denim yarns are given in Table-3.1.Various cottons are used, ranging from irrigated J-34 from North, hybrid Shankar-6 from Gujarat, windy V-797 of Saurashtra, lean season rescuer NHH-44 from federation, quilty Bengal desi to cosmic bunny to name a few from the Table [Table 3.1].
Often recycled waste, purchased comber noil of other mixings is also added.
Cotton fibre specifications of some imported cottons used in denim yarns are given in Table-3.2. Here again cotton used, ranging from 32s to 36s of US upland cottons, trashy CIS cottons, to sticky Sudanese cotton.
This is because, denim fabrics that, whether we like it or not, are somehow getting close to commodity status, cannot ignore the very high raw material cost. In addition, over the years, process and technology improvements allowed the operational flexibility to use various cottons, leading to cost savings still meeting the quality standards required by customers. In addition, India has the sole distinction of producing many varieties covering all four species and their hybrids, each with limited availability. For example, V-797, [which is considered to be the seismic test for installing a wind mill, in the lighter sense] with higher Uniformity Ratio translating to better yarn, has limited availability geographically. With the increased demand from large scale denim businesses, the relative price cannot go beyond the better value cotton, say J-34. Cost, proximity and availability over-ride the variety preference.
Cotton Mixing Requirements For Denim Yarns
Cotton mixing specifications of important fibre characteristics for denim yarns engineered to meet seven major end-uses are given in Table- 3.3.
Upper Half Mean Length, Uniformity Index, Strength at 1/8” and Micronaire values as tested in HVI mode are to be maintained in a lay down. [Row 1 in Table 3.3]
Uniformity of average fibre properties in successive lay downs is far more important and desirable than a level of certain properties, that being difficult to achieve, is not easily maintained day in and day out.
Coarse count yarn in denim refers to less than 8s Ne. and fine count is 9s to 16s. Mixing requirements for rope dyed warp open end yarns are higher and normally made from “U” rotors. [Row 2 in Table 3.3]
Mock-ring yarns are made in open end by the various devices available (amsler, caipo etc). Strength variability due to character (slubs) demands slightly better mixing. [Row 3 in Table 3.3]. Strength uniformity is far more important for Weft yarns used in very high speed air jets.
For fine counts and ring spinning, no doubt a good length and appropriate micronaire are a must. [Row 4, 6 and 7 in Table 3.3]
Quality fabrics can also be produced from Value mixings. This is because of the realization that heavy denim has strength far above realistic requirements. However the denim fabrics produced out of such yarns should not be meant for elaborate destructive garment washes. [Row 5 in Table 3.3]
In spite of open end yarns have lower strength than ring spun yarns it has been enjoying mass market denim products due to cost angle. The raw material costs and the cost differential between ring and open end is discussed in “Engineering of Denim Products”
For aesthetic reasons, at least in the warp, several denim products are actually ring spun. Also core-spun elastic yarns for weft will always be made in rings.
Yarn Strength from Fibre Selection
Since the yarn strength is of special significance when processing denim yarns, especially open end yarns, it is important to know the effects fibre strength, fibre length and fibre fineness have on the yarn strength.
The relationship between fibre properties and yarn strength of open end yarns for various counts are expressed in the form of equation.
The equation for calculating
CSP =735 x [UHML x Str - 3 x Str -255 -6.6 x UHML]^0.22– [72.5 x Mc/UHML + 16] x C
Where, UHML is Upper Half Mean Length in mm and
Str is Strength in grams per tex as tested using HVI Mode
Mc is Micronaire and
C is Count in Ne
The above equation which uses Fibre characteristics as tested using HVI Mode is deduced from earlier ATIRA equation for predicting CSP of Open-end yarns using Fibre characteristics using ICC Mode.
CSP =720 x [2.5% SL x S - 300]^0.22 – [72.5 x Mc/2.5% SL + 16] x C
Where, 2.5% SL is 2.5% Span Length in mm
S is Strength in grams per tex as tested using ICC mode
Mc is Micronaire and
C is Count in Ne
Cotton From Dyer’s Perspective
The colour of cotton fibre is important in gradation of cotton fibres. The calorimeter of HVI measures two colour components of cotton – lightness and yellowness. Lightness is expressed as a percent reflectance (%Rd), and yellowness is expressed in Hunter’s scale (+b) values.
These values affect dye ability of denim warp yarn. With cost savings, proximity and availability taking more focus on selection of cotton, the dyer has to leverage operational flexibility to use various cottons and still hits the target shade.
The % Rd, +b values of typical Indian and imported cottons are given in Table-3.4.
TABLE -3.4 : COLOUR COMPONENTS OF TYPICAL INDIAN AND IMPORTED COTTONS
As can be seen from the Table, that V-797 is yellowier than J-34 and necessary corrections need to be done when switching over from yarn set made from J-34 to V-797.
Norms For Yarn Realization
The percentage yarn realization depends primarily on the process waste taken out at the blow room and cards. Of these the waste taken out in the blow room depends on the trash content of the mixing, the waste in cards and on the type of cards. It is therefore, necessary to use a separate norm for yarn realization for each mixing. Over and above the type of cotton used in denim mixings range from J-34 to trashy V-797. The norms for yarn realization which depends on the trash content in the mixing is given in the Table as a guideline.
A one percent reduction in yarn realization has almost the same economic impact on the mill’s profit as an increase of one percent in the mixing cost. The control of yarn realization is thus as important to a mill as the control of cotton and mixing costs. One may find the detailed pcedures for the control of yarn realization in Chapter 3, ATIRA Silver Jubilee Monograph “Process Control in Spinning”.
TABLE – 3.5 : NORMS FOR YARN REALIZATION
1.Yarn realization (%)= ——————————————————————– x 100
Bale Cotton +Waste of other mixings +Purchased Wastes
2. Usable wastes of the same mixing, added back are not considered while determining the yarn realization.
3. Yarn realization can also be obtained from the following formulae. These can be used for quick estimates.
Yarn Realization (%) = 93.44 – 1.30 x Trash in mixing (%)
4. The values given in the table are arrived at by considering the waste levels as follows:
Invisible loss: 1.5% on mixing fed
Sweepings : 1.5% on mixing fed
Usable Waste: (% is based on material fed at each process)
Card Sliver 0.5%,
Draw frame Sliver: 0.6%,
Speed frame: 0.5%.
Ring frame Pnemafil 1.0%,
Roving ends: 0.4%
Open End Sliver: 0.3%
5. The percentage trash in the mixing is for the mixing inclusive of usable waste and waste of other mixings added.
6. When comber noil is added to the mixing, the yarn realization will be lower than that given in the Table. For every 10% comber noil addition in the mixing, the yarn realization will come down by 1%
7. The yarn realization values given the Table are for ring doffs weighed without conditioning, wherein the time lapse between doffing and weighing is very short. If doffs are conditioned and weighed, the realization will be higher by about 1.0 to 1.5%.
There was a time when blow room and carding non-re workable wastes used to be just that: non-re workable, except perhaps for surgical cotton. Attempts have always been made to re-use flat strips.
The full recycling of all opening and carding wastes, using a new line of machinery from Trutzschler and others, is attempted by few with a success . Its obvious importance in Denim manufacture lies in the overall weight on the final cost represented by the cost of cotton .Because of heavy yarns and fabrics, if one can save 3 or 4% on cotton costs, the impact on the bottom line can be remarkable.
This clean material has some residual trash in it not too different from the cotton used. Naturally there are more short fibres. The yield will be approximately 50%, in other words from each 2 kgs of raw waste we get 1 kg of clean recycled cotton. This material is baled again and fed to the mix at the lay down. Normally 10% is used. A loss of some 0.5 to 1.0 cN/tex is then unavoidable, but with 10% it will be manageable.
SPINNING PREPARATORY PROCESSES
The raw material for Denim yarn is cotton, the type and quality of which varies tremendously, not only from country to country but within India and even within regions. Because of this variation it is difficult to envisage a standard line to meet all cases and manufacturers of opening and cleaning machinery have to be able to configure a line to meet the requirements of each individual customer. In general, Modern Blow room line with 4 to 5 beating points with micro dust extractor and very efficient automatic waste evacuation system will be adequate to ensure satisfactory opening and cleaning.
In the case of variations within regions it is not uncommon to have fluctuation of colour from bale and therefore blending becomes very critical if shade variations from lot to lot in the finished cloth are to be kept to a minimum.
It is already mentioned in the beginning that denim units have a high production output This necessitates, the modern method of opening bales. That is to use an automatic bale plucking machine which removes small size tufts of raw material from a line of bales and feeds these pneumatically to a blending machine. This ensures a uniform tuft size being fed to the blending and cleaning machines. However, the disadvantage is that removal of foreign material. In traditional blow rooms it is the practice to pre-open the bales and create a stack mixing. This ensures easy removal of foreign matters. It is important to have both options available to maintain the ability to satisfy all the variables.
For feeding recoverable waste and small blends of fibre material it is recommendable to integrate a pre-mixer.
V-797 and CIS cottons have considerable amount of trash, as high as 15%, as a result of geographic conditions and bad picking practices. In addition to trash, the fine clay of the soil, on which the cotton grows, permeates everything. It is also important to use pre cleaners to handle trashy cottons. Tandem cards used in the past, was successful in yarn quality considerations, but cost considerations turned it into an impossible proposition. The yarn realization as well as the clean cotton cost will be impacted by the use of high trashy cottons. [See 3.2.3 Norms for Yarn Realization]
It is also possible to integrate a waste recycling line to recycle non re workable wastes from blow room line and cards to bring down cotton cost [See 3.2.4 Managing Waste]
For denim yarns produced from open end spinning systems, probably the most important characteristic of the sliver is its cleanliness with particular care to be given to dust removal.
Particularly in ring spun Denim yarn, a high incidence of nep in the yarn will cause uneven dye uptake during the warp yarn preparation. It is observed that modern blow room lines create neps, upto an increase of 100% over neps in raw cotton that will still allow the carding machine to be able to remove most of these objectionable faults.
The card is absolutely decisive for the quality of the denim yarns and for the efficiency of the production. It was possible to double the card production during the past 25 years.
The carding quality is decisively influenced in the areas of cylinder, flat. Cylinder speed, clothing fineness and distance between cylinder and flat are deciding factors.
Monitoring and controlling the carding process is critical in ensuring minimal fibre damage. Instruments such as AFIS [Advanced Fibre Information System] can be used to generate information of processes that precede yarn manufacturing. The carding process is very aggressive and, if not adjusted properly, can reduce fibre length causing short fibre. This phenomenon is especially true in the new high speed carding machines. If card flats are adjusted too close to the main cylinder, the nep and trash removal is improved, however the result can also be a reduction in fibre length and an increase in short fibre content. Replacing new cylinder wire or regrinding the wire will cause a temporary increase in short fibre content but this should return to normal in a relatively short time.
The measurement of short fibre content will have some variability due to the natural variation in raw cotton. The following guideline will give an idea on how much increase is significant.
Significant Increase >3% >6%
Increase in short fibre content less than these values are not significant and should not be considered a processing problem.
The requirements of Draw Frames are as follows:
i) Drawing mixes stock coming from different cards, producing a more even sliver of a more uniform quality.
ii) The high friction between fibres, especially in modern high speed frames, tends to eliminate remaining dust adhering to fibre surfaces.
iii) The fibre parallelization induced by drawing (remember a card web has fibres randomly oriented) is essential for ring roving and spinning and is helpful in open end to achieve a combination of high draft at the opening roll and good yarn uniformity.
One or Two Process of Drawing:
No doubt, the making of ring yarn with good staple cotton needs the parallelization at two drawing passages.
Several studies have been conducted to see the effect of the number of draw frame passages on the yarn properties of rotor yarn. The original general rule suggested is
i) If you make fine counts, maintain the two processes.
ii) If you go to coarse counts, one process all you required.
iii)If you are processing very short fibre material you are better off straight from the card.
Because most cotton drawing system need a fairly uniform fibre length diagram to produce slivers of good short term evenness, it is obvious that materials with short fibre contents have little to gain from even a single passage.
On the other hand, use of draw frames is also advantageous from the point of view of blending and cleaning.
It has already been several years since cards have been offered with auto-levellers. Most of these can be classified as:
-truly short term auto-levellers
-long and short term auto-levellers
The truly short term auto-regulation, now virtually abandoned, measures the sliver thickness coming out of the card by means of short variable drafting zone which attempts to adjust sliver weight to the nominal value. But as card speeds increased, and also can sizes and weights, the increased inertia of the system made accurate corrections very difficult. Even the speed of the coiler has to follow what the correction dictates. Also even if the average sliver weight could be maintained, the short term sliver evenness does deteriorate from the lack of fibre control at the small drafting zone.
Nowadays, all long and medium term auto levelers operate by adjusting the rate of feed to the card, in fact adjusting the total draft of the machine. It is sound in principle, but does not guarantee short term uniformity and is not so easy in many cases to have every card adjusted to exactly the same average value.
This is one reason why the averaging effect of at least one passage of drawing is still necessary. The new high response short term auto-levellers provide an ideal complement and very even slivers are now possible, totally free of count drift and with very acceptable short term irregularity.
It has also been observed that the direction of majority hooks has little influence whereas the number of draw frame passage has a marked effect on yarn strength. It is also important to maintain count and strength variability to the lowest as the yarn breaks in spinning are correlated to the yarn breaks on the weaving machine.
Thus, we can modify the original rule as
i) Two passages for ring spinning and
ii)one passage for coarse count open end spinning provided, the Count CV% of yarns produced is about 1.0% [or not exceeding 1.2%] and warping breaks per million metres of warp is about 0.3 [or not exceeding 0.5]
Quality of Feed Sliver
Standards for Trash and Nep Levels
In ring spinning, trash particles are thrown harmlessly off the yarn as it balloons whereas in rotor spinning, they tend to become embedded along with dust particles in the rotor groove. The impurities and dust which accumulate in the rotor groove not only impair the operating conditions of the machine and cause rapid wear of the rotor surface but are also responsible for lower yarn quality and increased yarn breakage rate. In order to minimize end breaks, the total trash percentage of sliver fed to the rotor machine should not exceed 0.5% as tested using MDTA.
Particularly in ring spun Denim yarn, a high incidence of nep in the yarn will cause uneven dye uptake during the warp yarn preparation. It is observed that modern blow room lines create neps, upto an increase of 100% over neps in raw cotton, that will still allow the carding machine to be able to remove most of these objectionable faults. The nep content expressed in neps/gram as tested using AFIS should not exceed 150.
Mill Studies: Health Checks at Spinning Preparation Processes
In addition to level of neps at feed sliver, AFIS [Advanced Fibre Information System] can also be used as a powerful process control tool for scanning the processes that precede yarn manufacturing periodically.-
One such analysis, done Product wise and Process wise in seven different mills, four open end spinning and three ring spinning, is summarized in Table – 3.6.
TABLE – 3.6 : NEP LEVELS AT SPINNING PREPARATION AND PROCESS ANALYSIS
The product wise analysis refers to Level of neps at mixing, Blow room and Card sliver. The process wise analysis refers to nep increase at Blow room and nep reduction at card.
It is important to arrive the product standards, considering the end use requirement for card sliver. Once the level of card sliver neps required are decided, the level of neps required at blow room material and target level at mixing can be arrived. This is by working backwards with the use of process standards for nep increase in blow room and nep reduction at cards.
The standards used in the analysis are as follows:
Nep/gram at Card Sliver : 120
Card Feed : 400
Mixing : 200
Nep increase at Blowroom: 200%
Nep reduction at Card : 70%
As can be seen from the Table, the blow room nep generation was high at OE Mill1, OE Mill2 and OE Mill 4 Feeder 2. It is observed that Long and bent material transport duct lines were the reasons for nep generation in OE Mill 4 Feeder 2.
Slivers made from OE Mill1, OE Mill2, OE Mill 4 Feeders 1, 2 and , RS Mill 5 feeder1 had excessive neps. It is also observed, that the overall rep removal efficiency of cards in OE Mill 1 and 4 were very poor due to poor condition of wire points.
In addition, the cards who’s performance was too poor than the rest were identified and given in remark – though individual values could not find place in this summarized report.
Denim plants have both open-end and ring denim capabilities, although a majority of the capacities are open-end, which historically cater to mass-market denim products. However, open-end plants are also able to manufacture specialty denim fabric with changes to their processes. Ring spun denim is of a higher quality and commands higher prices in the denim fabric market.
The following pages describe the merits and limitations of the systems in general, inclusive of non denim yarns, so as to understand and derive maximum benefit from both systems.
Ring Spinning: Merits and Limitations:
Ring spinning had remained unchallenged for almost 150 years, since its inception. However its limitation in regard to production speeds was well realized which made its position quite vulnerable to new spinning technologies like rotor spinning. Subsequent to this realization, renewed attempts made the maximum production speed has increased to 25000 rpm by i)by extending the maximum traveller speed to 45m/sec and using ii)smaller ring diameter and bobbin lift. However, this has not prevented rotor spinning applications in coarse denim yarns as these yarns demand higher ring diameter and bobbin lift. Today the problem of excessive knots due to smaller ring package is of little consequence due to efficient splicing systems available at winding. Another serious problem of excessive initial end breaks due to greater number of doffs has been solved by the employment of efficient automatic piecing devices. Additionally there are support systems such as automatic roving transport to the ring frame, automatic roving rupture if the yarn is not pieced in three successive attempts in order to reduce incidence of roller lapping.
The merits which makes it unique ever are i)It produces the strongest yarn, it is the bench mark among all 100% staple fibre spinning systems for various types of fibres and their blends. ii)It can produce yarns with a large range of twist, density from very low to very high. No other spinning system can match this unique capability of catering from knit to voile yarns. iii) It can be used for all types of fibres and can spin from very coarse to extremely fine yarns. iv) The desired hand, crisp or soft as per requirements can be imparted using the ring yarns.
Open End Rotor Spinning: Merits and Limitations:
The rotor spinning system has the following merits and possibilities:
As can be seen from the process sequence, i)the speed frames and cone winding machines can be dispensed with. ii)the productivity per position is 6 to 8 times that of ring spinning. iii) It is extremely amenable to automation – viz auto piecing, auto cleaning and auto doffing. Features like Online Quality Monitoring can also be opted for. iv) The rotor yarns are extremely regular and have much lower levels of imperfections and faults as compared to ring yarns.
The inherent drawbacks / limitations of the rotor spinning system are:
i)The rotor yarn strength is lower than ring yarns.
ii)Longer fibres (>32mm) offer no advantage in regards to yarn quality and /or productivity.
iii) the yarn twist required for optimum strength is higher than ring yarns.
iv)the biggest drawback of rotor yarns is the harsh feel of the fabrics made out of them.
v) The minimum number of fibres required in the cross section of rotor yarn is around 100 to 110 compared to 50 required for ring yarns. Therefore the quality of rotor yarn deteriorates when finer yarns are spun on this system.
Rotor size, rotor groove configuration, rotor speed and rotor surface treatment all have a decisive influence on the structure and properties of a rotor yarn.
30, 33 and 36mm rotor diameters are used for finer yarns and 40 and 46mm rotors are useful for coarser yarns. A small rotor cannot accommodate the fibre mass needed for a coarse count in its narrow groove, and a possible overfeeds in case of yarn rupture would quickly choke the rotor cup.
It is a common misconception that yarn quality deteriorates with small smaller rotors and higher rotor speeds.
The grove configuration determines whether a yarn is bulky or compact, weak or strong, more or less ring-yarn-like etc. Most manufacturers of rotor spinning machines offer an array of different rotors. The grooves normally used in Denim applications are as follows:
“S” Groove Rotors produces a bulky yarn which is weaker than yarn spin in any other rotor. It yields an excellent uniformity and is suitable for cotton with above-average trash content and for all synthetic fibres.
“U” Groove Rotors possesses good self-cleaning properties as far as dust is concerned, but trash particles can still jam the groove and cause moiré. The yarn strength is higher than that from an S-rotor. For these reasons U-rotor is preferred for denim yarns.
“T” Groove furnishes the strongest yarn due to its narrow, recessed groove, especially in fine counts. It is also the leanest, most compact yarn, having a low number of hairs per yarn cross section. Yarn torques is also higher, indicating a more ring-yarn-like structure.
It is susceptible to initial deposits, but then the self cleaning effect sets in, maintaining uniform yarn properties. It is unsuitable for trashy cottons. It may also not be useful for denim warp yarns for rope dyeing where the higher yarn torque may cause problems in rebeaming.
The opening roller wire specifications should be chosen as per fibre specifications, The general principle to be followed for deciding opening roller speed is that the higher the speed, the lower the yarn unevenness, the faults, the yarn strength and the breaking extension, Lower opening roller speeds should be used normally be used for the following:
i) longer, finer and crimped fibres
ii) coarser and cleaner sliver
iii) lower rotor speed
The range of speeds normally ranges from 5000 to 8000 rpm.
The influence of other machine and process parameters on rotor spinning yarn properties in general are well documented in two references given at the end which we suggest for further reading.
YARN QUALITY PROFILES
Yarn quality profiles can be presented in many different ways. E.g. in table or graphic form. One example, shown here in Tables 3.7, 3.8 and 3.9 details the properties of yarn spun on both the rotor spinning and ring spinning systems and tabulate the yarn quality levels achieved with different yarn counts. The specific information on rotor groove and end use, whether it is meant for warp/weft, rope/ slasher are provided for interesting comparisons. Each of these Tables corresponds to the Fibre characteristics of different mixings, coarse count open end, fine count open end and ring spinning mixings , as provided earlier in Table -3.3 in this treatise.
If one studies critically, the data shown in these Tables confirm many of the statements made throughout this book.
COTTON, PROCESS AND YARN TESTING
In order to monitor yarn quality consistently and efficiently, a testing laboratory equipped with modern instruments is an indispensable tool for the spinner. A good sampling plan (size and frequency of test samples) keeps track of sliver quality and yarn quality in such a way that in case of deviation from set quality standards, corrective action can be taken immediately. The sampling plan usually varies from plant to plant and should represent a careful balance between the prevailing quality requirements and the cost of testing. The Table – 3.10 lists the most commonly used testing instruments to determine specific cotton, process tests and yarn properties. These instruments are fairly well standardized throughout the world; they are not operated not only in yarn spinning plants, but also in research laboratories, textile institutes, universities and at machinery manufacturers. The following brief describes the uncommon tests – yarn torque and fancy yarn testing.
Determination of Yarn Torque
A simple and fast method to evaluate yarn kinkiness, bulk, and hand is the so-called Torque Test.
Two leas (skeins) of the same yarn are connected by slipping an O-ring in each end as the skeins are pulled off the reel. This double skein, suspended in a vertical position with the lower ring released turns due to its own torque. The number of turns reveals very useful information about the yarn structure, internal dynamics and its behavior in subsequent processes, such as kinking and snarling, as well as fabric properties such as skew, bulk and hand.
Torque measurements range from near 0 (no torque) to 6 (very high torque). Subjectively comparing the soft or harsh feel of yarn skeins provides good clues to the hand of the fabric.
Determination of Fancy Yarn Parameters:
Recently, the usage of characteristic yarns, such as slubby and multi count yarns, both in ring spinning and open end rotor spinning system in denims are on the rise. These yarns need additional monitoring of Slub parameters. Fancy yarn module of UT-5 serves this purpose with testing of Slub parameters -Slub frequency, Slub Length and Mass increase in addition to yarn diameter, yarn density and shape. The fancy yarn properties of typical characteristic yarns are given in Table-3.11