Yarn Requirements for Denim
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.
Ring Spinning
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
COTTONS USED
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.
Open End
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]
Ring Spun
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
Notes:
Yarn Production
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%.
Managing Waste
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
Blow Room
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.
Cards
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.
SFC(w) SFC(n)
Significant Increase >3% >6%
Increase in short fibre content less than these values are not significant and should not be considered a processing problem.
Draw frames
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.
SPINNING PROCESS
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 Selection:
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.
Opening Zone
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
.
ON SHUTTLE LESS LOOMS
Traditional denim weaving had been done with the rapier looms and projectile looms for a long time. But with the development of the air jet weaving technology, the main flow of the denim weaving has been changed to Air Jet Loom due to its suitability to mass production.
The general guidelines in selection of various types of shuttle less weaving machines are as follows:
Rapier is known for its versatility in weaving fancy fashion materials of different constructions like suiting, upholstery, furnishing etc.
Air Jet weaving machine is most suitable for mass production.
Projectile is suited for dress material, industrial fabrics, heavy denim and geotextiles in single or multiple widths.
An evaluation on the application of various methods of weft insertion carried out by the Textile Machinery Society of Japan is given in Table 5.1
As can be seen from the Table that Air Jet Weaving scores in high productivity and labour savings however suffers in high energy costs and yarn wastage. Projectile scores well on both parameters, that is energy saving and fabric quality. More over, running cost will be the lowest for projectile during extended machine’s economic life beyond capital life. However, these ratings may be different in Indian context and for specific denim applications.
ON AIR JET LOOM
The important features of Denim weaving on Air Jet Loom are as follows:
a. High Productivity: As an actual production in the mill, 900 rpm is achieved already with the latest models of different makes.
b. Start Marks: The rush motor emits 1200% torque at the time of start and avoids start marks. Electronic let off, ensures the even warp tension from the full beam to the empty beam. Electronic let off is also equipped with the programmable kick back function which controls the cloth fell position at the start according to the loom stop duration. The second feeler is equipped to detect blow off of weft yarn.
c. Special arrangements for the Denim Weaving of Coarse Yarns:
I. Reinforced loom structure for heavy duty weaving.
II. Reinforced let-off motion for heavy warp.
III. Intermediate rocking supporter to make a strong beat without bending for heavy fabric.
IV. Positive cam motion for high speed operation for the heavy load of warp.
d. Stop Market Prevention:
i) Automatic Leveling Device: It automatically closes the shed after a loom stop by leveling all the shafts and prevents warp strain which may create corrugate marks.
ii)One Pick Insertion System: To minimize stop marks, a special weft insertion system functioning at the time of restart can be equipped with. When the loom is started first pick is automatically inserted just before the first beat. By avoiding the beat- up without yarn at reverse rotation, this system prevents the corruption of the fabric construction and prevents corrugate marks.
e. Devices for a coarse count weft:
The following are some of the arrangements equipped to use a coarse count yarn for weft.
i) Balloon Breaker reduces the weft tension due to ballooning.
ii) Auxiliary main nozzle is installed before the main nozzle to insert a coarse weft with less air pressure.
iii)Stretch nozzle is furnished to give adequate stretch to the weft for a perfect insertion of a coarse weft.
f. Labour Saving:
This is realized with a large size packages and loom automation. Automation includes automatic pick finding for easier weft repair and automatic defective pick remover.
REQUIREMENTS FOR HIGH INSERTION RATES IN AIR JET WEAVING
weaving profitability is the result of weaving productivity. The higher the weaving machine speed and efficiency the higher the productivity. As soon as speed is increased however, weaving machine efficiency, which is affected by loom down time on warp and product changing on repairs and yarn break repair times and also quit markedly by the number of warp and weft stoppages falls a rule.
The standards for loom stops per hour is given in Table-5.2
In air jet weaving, the proportion of weft stoppages to total is high, and their reduction is therefore of great importance for increased efficiency in air jet weaving processes. In addition, fewer warp yarn breaks, their faster repair and short setting times are the key for high production. It is not the speed of the machine, but the number of picks actually produced that is the deciding factor in modern weaving. The important points are i)the factors which guarantee high insertion rates ii) the humidity levels iii) the factors which lead to a high fabric quality.
Influence of Yarn:
The yarn breaks in spinning are correlated to the yarn breaks on the weaving machine. It is important to maintain count and strength variability to the lowest. The standards for Count CV% and Strength CV% is given in Table- 5.3.
The count CV% will be higher by +0.5 where auto levelers are not existing on finisher draw frame. Therefore it is necessary to use two passage draw frames where finisher draw frame is equipped with auto levelers particularly fine, ring and characteristic yarns.
In case of OE spinning there is an interesting correlation between high residual trash- content and increased yarn breaks in weaving. The norms for residual trash content at feed sliver are 0.5%.
The weft insertion performance depends to a large extent on the level of weak spots. The standards for Uster Tensojet Tensile properties are given in Table-5.4. Percentile 0.1% values given in Col 5 and 6 for Elongation% and Force in cN is a tool for identifying the weak spots in the yarn lot and estimated performance on loom.
Influence of Weaving Preparatory on the Cloth Production System:
Tight ends, lost ends, pieces of yarn or lint dragged onto the warper beam are performance killers in air jet weaving. Therefore the accurate functioning of the stop motion on the creel plays a key role.
Warper beams should not contain any grooves, high edges or crossed ends. They should be made with bobbins of adjusted length and run off the creel without crossing ends and at uniform tension. In this manner yarn breaks on the sizing machine can be almost eliminated.
During sizing, the lengths run at creep speed should present less than 4% and a moderate size application control system should be used. For air jet weaving, two size boxes and real wet splitting should be used in case of warps with more than 70% cover density, in order to reduce the hairiness of the yarn.
The use of size add on control seems to be essential when operating with two size boxes, because there are no two size boxes, because there are no two size boxes which, in spite of exactly identical setting, produce exactly the same size add on. The differences measured may reach as much as 4% in absolute figures.
Over drying, particularly when sizing cotton yarns should absolutely be avoided.
After waxing, (0.2 to 0.5%) of the warp ends generally improves its performance.
Finally the warp ends should be fixed by tapes in such a way that they can be entered into the harness without crossing.
Factors influencing the performance of the warp in the weaving machine:
The shed geometry in front of the reed is to a large extent designed by the builder of the machine. When setting the back shed and harness stroke the weaver is required to choose the proper setting, also when determining the warp tension. These settings have an important impact on the performance of the machine. It is therefore essential that these settings are optimized with utmost care and by using modern measuring instruments.
Following two factors have a particular influence on the behaviour of the warp:
i) yarn traction force
ii) yarn hairiness
A too high warp yarn traction force is leading to overloading the warp ends. As the yarn traction force is not constant during weaving, the peak tensions which generally appear in the bottom shed, particularly in the middle of the warp should be taken into consideration. These peak values should not exceed 5 to 6 cN/tex, depending on the quality of the yarn.
The importance of the setting of the warp stop motion is often under estimated. Its position has a direct influence on the back shed and thus on the yarn traction forces in the bottom and top sheds. The vertical position of the warp stop motion should therefore be set very precisely. It changes the asymmetry of the shed.
Atmospheric Conditions
Relative humidity have a great impart on the performance of the weaving machines. The optimal dissipation of temperature and humidity over the machines, that is the warp, is generally not reached, because the sources of heat within the weaving machine disturb the climatic conditions. Numerous yarn breaks are caused by dust, lint and fibre accumulations. The best experiences have been made with air conditioning systems whereby humidified air is conducted over the machine, whilst the dust – loaded exhaust air is evacuated through floor opening under the back shed, and because the descending flow of the conditioned air is assisting the sedimentation of the lint. At the same working conditions for the personnel are improved. It is surprising how much cleaner the machines and the whole weave room are, compared with systems whereby the exhaust air is evacuated in the alleys, or worse, through the outside walls.
Table -7.6: Weaving - Typical Denim Constructions
TABLE-7.7 : PRODUCTION CALCULATION
The following information is an estimated performance for weaving 14.5 ozs indigo denim.
a)Fabric : Indigo Denim 14.5 ozs/square yard
Warp Yarn : 100% Cotton Ne 7s Indigo Dyed
Weft Yarn : 100% Cotton Ne 6s Grey
Warp Density: 60 ends/inch
Weft Density : 40 picks/inch
Width : 66.5 inches
Weave : 3/1
b)Reed Space : 68.5 inches
2 Colour Weft Mixing
Positive cam shedding Motion, 4 shafts
-Denim Weaving Arrangement
c)Estimated Loom Speed : 900 rpm
d)Estimated Production Per Loom:
i)Daily Production (linear length)
= 900 rpm x 60min x 24h x 0.92 eff x 0.0254 / 40 ppi
= 757 metres/day/loom
ii)Monthly Production (linear length)
= 757 m x 30 days
=22713 metres/month/loom
The following standards for denim fabric quality in terms of inspection, packing, shade and physical requirements are of general nature considering varied requirements of end-uses / customers, however holds good for major universe.
This chapter details classifying fabrics according to color and defects so that the fabrics are made to garments at ease with defects eliminated, grouped according to shade and perform satisfactorily.
INSPECTION REQUIREMENTS
Fabric Group Classification According to Yarn Type
Group One
All Basic Denims (OE/OE)
Group Two
All Denims (RING/OE) & (RING/RING)
Group Three
Specialty Denims: Linen Denims, Vintage
Acceptable Defect Point Levels According to Yarn Type
TABLE-11.1:DEFECT POINTS PER 100 SQUARE YARDS
TABLE-11.2:DEFECT POINTS PER 100 SQUARE METRES
TABLE 11.3: QUALITY CATEGORISATION OF DENIM FABRICS
Defect Grading:
A. Point Grading System: Denim fabrics are normally graded using the “4-Point” system. This numeric grading system is endorsed by ASTM, AAMA (American Apparel Manufacturer’s Association) and ECMA (European Cloth Manufacturers Association). All defects which are clearly visible from one meter (three feet) are scored as defects and demerit points assigned according to severity.
B. Length of Defect Points: Demerit points are assigned to warp and filling defects as follows (Defects in any direction)
v 1 point - Defects 7.5 cm(3”) or less
v 2 points - Defects exceeding 7.5 cm(3”) up to 15 cm(6”)
v 3 points - Defects exceeding 15cm (6”) up to 22.5cm(9”)
v 4 points - Defects exceeding 22.5 cm(9”)
C.Counting of Lengthwise 4 Point Defects: No linear meter is penalized more than 4 points.
v Defect length 22.6cm (9”) to 100cm (40”) – 1 no of 4 point defect
v Defect length 100.1cm (40.1”) to 200cm (80”)–2 no of 4 point defects
v Defect length 200.1cm (80.1”) to 300cm (120”)–3 no of 4 point defects
v If length of defect is more than 3 meters (120”) the length containing the defect is removed.
D. Full Width Defects:
v A full width defect running over 6” in length shall be removed.
v More than four full width defects per one hundred linear meters shall not be accepted as first quality.
v A full width defect in first or last three meters of roll shall not be allowed.
E. Flagging of Defects:
v Only 4 point defects are flagged with a metallic sticker. One may find the metallic sticker at the start of the defect. Metallic flags should be a minimum of 2 cm wide and 7.5 cm length.
v Defect points 1, 2, 3 shall be counted but not flagged.
v All splices shall be flagged.
v All holes shall be removed. There must be two or more yarns broken at the same place to consider a defect as a hole.
F. Roll Length and Splicing:
v Roll length tolerance should be agreed upon the first delivery. Say maximum 135 meters and minimum 85 meters.
v No pieces shall be accepted as first quality with length less than 30 meters.
v No roll shall be accepted as first quality containing more than one splice.
v The shade continuity between parts must allow for the mixing of garment components within a garment.
v Two part rolls should be identified as such, No more than 25% two part roll should be allowed in any shipment.
G.Shading:
v No pieces shall be accepted as first quality that exhibits a noticeable degree of shading from side- to -side or side- to –centre.
v No pieces shall be accepted as first quality that exhibits a noticeable degree of shading from end– to –end.
Skew
A. All 3/1 and some 2/1 twill fabrics for bottoms require skewing. Since the optimum skew varies depending on the fabric (anywhere from 5% to 9%), this must first be determined by the supplier and agreed upon before bulk fabric orders are placed. Target for individual rolls = +/- 3%
B. No fabric shall be accepted as first quality exhibiting more than 3% skew movement.
Waviness
No roll shall be accepted as first quality exhibiting a noticeable degree of looseness (waviness) or tightness along either or both selvages.
Ripples or puckers in the body of the fabric, which prevents the fabric from lying flat when spread in a conventional manner, is unacceptable.
The woven selvage or fringe should be the same on both sides of the fabric.
Tolerance wavy selvage: 2% in length
Width
Cuttable width variations must meet the minimum fabric width specification. In addition width variation within a roll should not exceed 2 cm.
PACKING REQUIREMENTS:
Put- up- Specifications
A. Fabric shall be rolled onto a spiral-wound tube with necessary wall thickness [OD: 60 mm; ID: 45 mm] and strength [Radial Crushing Strength > 500 Kgf for 8” length] to insure it reaching to customers in good condition, provided it is handled by reasonable and acceptable methods.
B. Rolls are wrapped in such a manner that will protect the fabric from all types of damage during transportation and storage.
C. The outer edge of each roll shall be taped down to prevent unrolling during shipment and storage.
D. Roll Identification
Two stickers must be attached one to the end of the roll and another on side of the roll. The stickers shall contain the following information.
1. Roll /Bale Number.
2. Fabric Style number
3. Shade Group
4. Roll length in meters
5. Roll length in yards
6. Fabric width in inches
7. Total square meters
8. Demerit points per 100 square Yards
9. No of fabric pieces
10. Gross weight in Kg
11. Net weight in Kg
12. Quality Category
13. Total number of 4 point defects
14. Total number of Demerit points
15. Security code
COLOUR EVALUATION / SHADE GROUPING
Option 1: Instrumental Shade Evaluation Procedure of Wash Swatches
Instrumental Color Measurement: The color measurement shall be carried out using Spectrophotometer.
Master Roll: Master roll shall be established during initial stages and will remain during the entire life cycle of the product. The procedure for selecting the master roll is given under the head “Procedure for Selecting the Master Roll – Instrumental Evaluation”
Blanket Preparation: The blanket preparation procedure is given under the head “Blanket Preparation”
Shade Evaluation: The color measurement shall be carried out using Spectrophotometer and shade grouping shall be carried out according to wash color reference of master roll. Before assessment, all fabric swatches must be conditioned for at lease 1 hour in a controlled atmosphere. Samples must be conditioned from a dry state. All measurements must be done as soon as possible after conditioning.
Classifying Rolls by Color Groups: LMD grouping shall be followed. (L= Light, M = Medium, Dark = D), the taper standards are defined as color difference (dEcmc) by Standard, Average, Range and Roll to Roll. L: C [Lightness: Chroma]
The tolerance to be followed is
Average : 1.2
Standard : 2.0
Range : 1.5
Roll : 0.5
L: C Ratio : 2.0
Visual Evaluation: All roll sequences shall be evaluated visually by laying the swatches in standard lighting conditions and remove abnormal rolls.
Light Conditions: Only D65 lamps are to be used to assess color. Make sure appropriate Number of lamps is used to obtain luminosity of minimum 1500 lux.
Identifying Color Groups: To facilitate classification of rolls, the rolls shall be identified with appropriate color stickers of L, M and D and the same shade group shall be mentioned in the packing list by tracking via roll number.
Swatch Service: The system shall be to send two set of unwashed and washed swatches to customers. The washed swatches are being sent so that customer can review the shade at their end and the unwashed swatches will be used for additional use if any.
Option 2: Visual Shade Evaluation Procedure of Wash Swatches
Visual Color Assessment: Each wash swatch representing one roll in the shipment needs to be compared to the master swatch under standard light conditions.
Master Roll: Master roll shall be established during initial stages and will remain during the entire life cycle of the product. The procedure for selecting the master roll is given under the head “Procedure for Selecting the Master Roll – Visual Evaluation”
Blanket Preparation: The blanket preparation procedure is given under the head “Blanket Preparation”
Shade Evaluation: Each wash swatch shall be evaluated versus reference shade swatch on shade, color, wash-down, abrasion and overall appearance. Based on this comparison swatches representing rolls shall be accepted/ rejected.
Light Conditions: Only D65 lamps are to be used to assess color. Make sure appropriate Number of lamps is used to obtain luminosity of minimum 1500 lux. Make sure a matt grey background is used with reference of Munshell grey N5 or N7 to assess colors.
Identifying Color Groups: To facilitate classification of rolls, the rolls shall be identified with appropriate color stickers of L, M and D and the same shade group shall be mentioned in the packing list by tracking via roll number.
Swatch Service: The system shall be to send two set of unwashed and washed swatches to customers. The washed swatches are being sent so that customer can review the shade at their end and the unwashed swatches will be used for additional use if any.
Option 3: Visual Shade Evaluation Procedure of Unwashed Swatches
Visual Color Assessment: Each wash swatch representing one roll in the shipment needs to be compared to the master swatch under standard light conditions.
Master Roll: Master roll shall be established during initial stages and will remain during the entire life cycle of the product. The procedure for selecting the master roll is given under the head “Procedure for Selecting the Master Roll”
Shade Evaluation: Each unwashed swatch shall be evaluated versus reference shade swatch on shade, color and overall appearance. Based on this comparison swatches representing rolls shall be accepted/ rejected.
Light Conditions: Only D65 lamps are to be used to assess color. Make sure appropriate Number of lamps is used to obtain luminosity of minimum 1500 lux. Make sure a matt grey background is used with reference of Munshell grey N5 or N7 to assess colors.
Identifying Color Groups: To facilitate classification of rolls, the rolls shall be identified with appropriate color stickers of L, M and D or D+, D, D- and the same shade group shall be mentioned in the packing list by tracking via roll number..
Swatch Service: The system shall be to send three set of unwashed swatches to customers.
Blanket Preparation
Swatch Size: Use uniform swatch size of 8” x 6”
Preparation of Blankets: All blankets for washing shall have standard shade swatch from Master Roll. Swatch shall be cut from every roll of a shipment each 8” x 6”. Identify each roll no at the back of the swatch. In case of light weights identify by serial nos at the backside corner with indelible ink.
Waste material of same style
Wherever less number of swatches is available use dummy swatches of same style nos.
Do not use washed swatches for dummies.
v Dummy swatch size shall be of same size as that of blanket swatches.
v To make wash load if required, use dummy swatches of same style only.
v Sew the two panels together to avoid entangled panels in washing.
v All panels from one shipment to be washed in same wash load.
Washing of the Blankets: Wash blankets according to the recipe for various classes of products.
Procedure for Selecting the Master Roll
Instrumental Evaluation
v From the average of the washed lot select about five roll which are near to the average values.
v Check their UNWASHEDED L*a*b* values, these must belong to 555 block. If not then select 5 more rolls near to average values from the washed lot.
v The roll which belongs to 555 shade block, check the preceding and succeeding rolls L*a*b* values, these must also belong to 555 shade block. This will ensure that there is no shade variation within the roll.
v Now check the consistency of the complete beam. There should not be much variation within the beam.
v The dyer will use the yarn latti of this particular beam as shade reference for dyeing.
Visual Evaluation
v Select about five rolls which represent middle of the washed lot.
v Check unwashed swatches as well, for their true representation of the middle of the lot.
v Check the preceding and succeeding swatches. These must also belong to the same shade. This will ensure that there is no shade variation within the roll.
v Now check the consistency of the complete beam. There should not be much variation within the beam.
v The dyer will use the yarn latti of this particular beam as shade reference for dyeing.
TESTING REQUIREMENTS - PERFORMANCE STANDARDS
Product development concerns itself with modifications or extensions provided to ideas so as to improve the functioning, the cost, and the value-for-money of the product. Development efforts improve the performance of the product, add options and additional features and even add variants of the basic product. On the whole, development effort is innovative vis-à-vis research which is more inventive – the thrust being on developing new product ideas, technologies and processes.
The tradeoff between research and development is an important strategic decision for most organizations.
Variety or Standardization
There are two distinctly different priorities that can affect the product development.
By adding variety, an organization attempts to satisfy the varied needs and tastes of customers and competes on non-price considerations as well.
On the other hand standardization / simplification offer:
-reduction in total inventory of yarn
-reduction in extra or unutilized ends
-reduction in accessories/ reeds
-reduction in change over times
-simplifies many operational procedures
-reduces need for many controls
-lower unit costs
Now the two caveats to trade off are
# a family of similar products is much simpler to produce than a family of dissimilar products.
# it is not enough to produce a product but it has to be produced so that there is an added value as perceived by the market
Modularization: Middle Path – Buddha’s Way
One method used to obtain variety or perceived variety and yet hold down cost is through modularization. A product is deigned using modules or sub assemblies that are interchangeable and each different combination of modules gives a new variety of the product.
1 Warp sheet
X 3 Colors
X 3 Weaves
X 3 Wefts
--------------------------
1x3x3x3= 27 Products
-----------------------------
X Tints
X Garment Washes
X Blasting/Brushing
X ….
X …
X ...
For example one warp sheet, three colors, three weaves and three wefts will give
1 x 3 x 3 x 3 =27 varieties, yet making large quantities of standard modules.
This is illustrated with two typical basic denim constructions. The grey construction details are given in Table-13.1.
Volatility in Cotton prices
Denim mills are searching for ways to produce denim fabrics of the highest quality at competitive prices. The engineering from raw fiber to finished fabric results in superior denims when modern fiber and manufacturing technologies are consistently applied throughout the processing chain. However, the cost of these engineered products delivered to the market place demands, in addition the prudence in tackling the volatility associated with the raw material, mainly cotton.
The Raw Material Cost [RMC] per metre of 150 cm width basic OE denim, for various denim weights over a decade is depicted below in equation, for the sake of simplicity:
RMC/metre = 2.3 x ozs per sq.yd. + 8.4
+/-0.4
That is cotton cost per metre for a 10ozs OE denim is Rs.31.4 [Ranging 27.4 to 35.4]
The engineering of denim fabrics utilizes, ring spinning or open end rotor spinning or their differentiated innumerable characteristic yarns + modern fabric forming and finishing techniques and commonly available but selected cotton fibers. The mixing specifications for different yarns are given in Table-13.3. Cotton Mixing Specifications of Important Fibre Characteristics for Denim Yarns (HVI Mode).
The following equation depicts the cost differentials between Ring denim and Rotor denim in a simplified way:
RMC Ring per metre = RMC OE per metre x [1+ozs per sq.yd/100]
That is cotton cost for a 10 ozs ring denim is Rs.31.4 + [1+10/100] = Rs.31.4 x 1.1 = Rs34.54.
Engineering
Hence the engineering of denim fabrics begins with the selection of appropriate combination of cotton fibre characteristics for different yarn types with the aid of modern HVI instruments. The mixing specifications for different yarns are given in Table-13.3. Cotton Mixing Specifications of Important Fibre Characteristics for Denim Yarns (HVI Mode).
Once fibre characteristics are known, yarn quality and processing performance can be predicted. Last column in the Table-13.3 provides an iNDEX for various yarn types which can be used as a composite measure to arrive at a variance with respect to actual yarn quality of various yarn types including individual characteristic yarns.
The equation for calculating
iNDEX=735 x [UHML x Str - 3 x Str -255 -6.6 x UHML]^0.22
Where UHML is Upper Half Mean Length in mm and
Str is Strength in grams per tex as tested using HVI Mode
The above iNDEX 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
The spinning parameter Twist Multiplier and important yarn parameters, such as strength, count and their variability’s of various yarn types are provided in Table-13.4. Spinning Parameters and Yarn Quality.
Rope dyeing technology demands less torque yarns in warp which are made from “U”rotors. The H values of Yarns made from “U” rotors as tested in Uster evenness testers are higher by 2.0 to 2.5 numbers in comparison with the yarns produced from “T” rotors
Cover Factor
To obtain better performance on the loom and fabric yield, a guideline for cover factor in developing new fabric constructions is given in Table-13.5 and classified under 3 categories for
i) Rings
ii) Bulky Open-ends and
iii) Shrinking Stretches.
B.Grey construction is used for calculations.
C.The formula used = Threads per inch
--------------------------- -------- X 100
[28 X Square root of Count]
Changing Trend in Yarn Preferences
Beginning of twenty first century saw the explorations in different yarn options. The below list is arranged by their usage in volume, starting with warp followed by their use in weft.
Crosshatch / Streak / Rain Denims
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. Aggressive slub parameters tend to lower Yarn Strength and poor performance at subsequent processes. The minimum tenacity values as tested using Uster Tensorapid for satisfactory performance level is given in Table-10.4 in Row 3 and 7. Beyond which compromise needs to be made among Slub parameters or performance levels or cotton fibre characteristics.
Chinos
Two ply chino denims in indigo dyed shade have a unique soft hand feel, Fabric cover and a luxurious appeal.
Tencel Denim
Woven century luxury cellulosic fibre made from specially grown woods and transformed in non-chemical process which give feel of silk and comfort of cotton.
Stretch Denims
For stretch denims, Core-Spun Cotton Spandex, Poly Spandex and Type -400 yarns are generally employed. Core-Spun Cotton Spandex yarns in denims range 10s to 16s Ne and uses generally 70 denier filament; though 40, 55 and 120 denier filaments are also available. The Spandex % of Core-spun cotton spandex yarn ranges from 3.5 to 5.5 %. The twist multiplier is 4.4.
Union Denims
Denims are differentiated with weft yarns to create Union Fabrics. The union denims produced in large volumes uses the following weft yarns in the same order – Polyester Texturised Filament, Stretch yarns of Poly Spandex and Type -400 yarns, and Pre-bleached Linen Yarns.
Poly Denim
Polyester Texturised Yarns in denim applications range 300 to 600 denier, with a tenacity of 4.0 g/tex and 20 to 27% extension. Bulk which is expressed in %HCC (Hot Crimp Contraction) is about 40 and nips per metre ranges from 70 to 90 for better performance.
Grey as well as dyed yarns are being used.
Poly Stretch Denim
Poly spandex yarns used in denims range 150 to 300 denier with 6 to 12 % spandex. Nips per metre for better weaving performance is 100 to 130. Type -400 yarns from Invista used in denim applications range 150 to 600 denier with a bulk of 50% HCC and nips per metre of 35 to 50.
Linen Vintages
Pre-bleached Linen yarns range 9s to 16s Ne [or 25 to 44 Lea]. Though these yarns have very high strength of over 3000 csp and 20 cN/tex, due to low elongation of 2% and the natural variation in yarn, the loom performance as well as full width defects is poor in comparison with normal denim.
Value Engineering
In accordance with the value engineering, quality fabrics can also be produced from Value mixings given in Table -11.3 Row 5. However the denim fabrics produced out of such yarns should not be meant for elaborate destructive garment washes.
Addition of 10% Recycled Waste
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.
Control of Yarn Realization
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 procedures for the control of yarn realization in Chapter 3, ATIRA Silver Jubilee Monograph “Process Control in Spinning”.
Mock Rings
Various attempts to duplicate superior denims made from ring yarns with rotor yarns of mock or slubby have always failed in fabric strength properties, fabric hand and appearance due to differences in yarn structure and yarn properties. Still one wonders how much of the present so-called ring spun Denims are such and how many are mock ring made in open end.
Other Value Offerings
Cotton rich polyester denims are with superior hand feel, luster and colour contrast for fashion market. One may find a deep value in using dyed polyester texturised filament yarn in place of costly yarn dyed cotton weft for high fashion denim. Poly Spandex yarns are replacing core spun lycra yarns in the value universe. Within Poly Denims, the denier is getting coarser day by day from 300 to 450 to 600. Though, Linen Vintage denims are not in high volume, there are efforts to replace it with Jute (Indian Linen) in the value proposition.
Successful strategies in denim mean profit, often (now) even survival. Engineering the fabrics on continual basis provides a way to achieve both quality and cost benefits of substantial proportions. At the same time full manufacturing flexibility through modularization is being maintained, enabling the denim mill to meet new and changing trends in raw material and fashion.
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.
Ring Spinning
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
COTTONS USED
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.
Open End
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]
Ring Spun
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
Notes:
Yarn Production
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%.
Managing Waste
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
Blow Room
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.
Cards
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.
SFC(w) SFC(n)
Significant Increase >3% >6%
Increase in short fibre content less than these values are not significant and should not be considered a processing problem.
Draw frames
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.
SPINNING PROCESS
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 Selection:
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.
Opening Zone
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
.
ON SHUTTLE LESS LOOMS
Traditional denim weaving had been done with the rapier looms and projectile looms for a long time. But with the development of the air jet weaving technology, the main flow of the denim weaving has been changed to Air Jet Loom due to its suitability to mass production.
The general guidelines in selection of various types of shuttle less weaving machines are as follows:
Rapier is known for its versatility in weaving fancy fashion materials of different constructions like suiting, upholstery, furnishing etc.
Air Jet weaving machine is most suitable for mass production.
Projectile is suited for dress material, industrial fabrics, heavy denim and geotextiles in single or multiple widths.
An evaluation on the application of various methods of weft insertion carried out by the Textile Machinery Society of Japan is given in Table 5.1
As can be seen from the Table that Air Jet Weaving scores in high productivity and labour savings however suffers in high energy costs and yarn wastage. Projectile scores well on both parameters, that is energy saving and fabric quality. More over, running cost will be the lowest for projectile during extended machine’s economic life beyond capital life. However, these ratings may be different in Indian context and for specific denim applications.
ON AIR JET LOOM
The important features of Denim weaving on Air Jet Loom are as follows:
a. High Productivity: As an actual production in the mill, 900 rpm is achieved already with the latest models of different makes.
b. Start Marks: The rush motor emits 1200% torque at the time of start and avoids start marks. Electronic let off, ensures the even warp tension from the full beam to the empty beam. Electronic let off is also equipped with the programmable kick back function which controls the cloth fell position at the start according to the loom stop duration. The second feeler is equipped to detect blow off of weft yarn.
c. Special arrangements for the Denim Weaving of Coarse Yarns:
I. Reinforced loom structure for heavy duty weaving.
II. Reinforced let-off motion for heavy warp.
III. Intermediate rocking supporter to make a strong beat without bending for heavy fabric.
IV. Positive cam motion for high speed operation for the heavy load of warp.
d. Stop Market Prevention:
i) Automatic Leveling Device: It automatically closes the shed after a loom stop by leveling all the shafts and prevents warp strain which may create corrugate marks.
ii)One Pick Insertion System: To minimize stop marks, a special weft insertion system functioning at the time of restart can be equipped with. When the loom is started first pick is automatically inserted just before the first beat. By avoiding the beat- up without yarn at reverse rotation, this system prevents the corruption of the fabric construction and prevents corrugate marks.
e. Devices for a coarse count weft:
The following are some of the arrangements equipped to use a coarse count yarn for weft.
i) Balloon Breaker reduces the weft tension due to ballooning.
ii) Auxiliary main nozzle is installed before the main nozzle to insert a coarse weft with less air pressure.
iii)Stretch nozzle is furnished to give adequate stretch to the weft for a perfect insertion of a coarse weft.
f. Labour Saving:
This is realized with a large size packages and loom automation. Automation includes automatic pick finding for easier weft repair and automatic defective pick remover.
REQUIREMENTS FOR HIGH INSERTION RATES IN AIR JET WEAVING
weaving profitability is the result of weaving productivity. The higher the weaving machine speed and efficiency the higher the productivity. As soon as speed is increased however, weaving machine efficiency, which is affected by loom down time on warp and product changing on repairs and yarn break repair times and also quit markedly by the number of warp and weft stoppages falls a rule.
The standards for loom stops per hour is given in Table-5.2
In air jet weaving, the proportion of weft stoppages to total is high, and their reduction is therefore of great importance for increased efficiency in air jet weaving processes. In addition, fewer warp yarn breaks, their faster repair and short setting times are the key for high production. It is not the speed of the machine, but the number of picks actually produced that is the deciding factor in modern weaving. The important points are i)the factors which guarantee high insertion rates ii) the humidity levels iii) the factors which lead to a high fabric quality.
Influence of Yarn:
The yarn breaks in spinning are correlated to the yarn breaks on the weaving machine. It is important to maintain count and strength variability to the lowest. The standards for Count CV% and Strength CV% is given in Table- 5.3.
The count CV% will be higher by +0.5 where auto levelers are not existing on finisher draw frame. Therefore it is necessary to use two passage draw frames where finisher draw frame is equipped with auto levelers particularly fine, ring and characteristic yarns.
In case of OE spinning there is an interesting correlation between high residual trash- content and increased yarn breaks in weaving. The norms for residual trash content at feed sliver are 0.5%.
The weft insertion performance depends to a large extent on the level of weak spots. The standards for Uster Tensojet Tensile properties are given in Table-5.4. Percentile 0.1% values given in Col 5 and 6 for Elongation% and Force in cN is a tool for identifying the weak spots in the yarn lot and estimated performance on loom.
Influence of Weaving Preparatory on the Cloth Production System:
Tight ends, lost ends, pieces of yarn or lint dragged onto the warper beam are performance killers in air jet weaving. Therefore the accurate functioning of the stop motion on the creel plays a key role.
Warper beams should not contain any grooves, high edges or crossed ends. They should be made with bobbins of adjusted length and run off the creel without crossing ends and at uniform tension. In this manner yarn breaks on the sizing machine can be almost eliminated.
During sizing, the lengths run at creep speed should present less than 4% and a moderate size application control system should be used. For air jet weaving, two size boxes and real wet splitting should be used in case of warps with more than 70% cover density, in order to reduce the hairiness of the yarn.
The use of size add on control seems to be essential when operating with two size boxes, because there are no two size boxes, because there are no two size boxes which, in spite of exactly identical setting, produce exactly the same size add on. The differences measured may reach as much as 4% in absolute figures.
Over drying, particularly when sizing cotton yarns should absolutely be avoided.
After waxing, (0.2 to 0.5%) of the warp ends generally improves its performance.
Finally the warp ends should be fixed by tapes in such a way that they can be entered into the harness without crossing.
Factors influencing the performance of the warp in the weaving machine:
The shed geometry in front of the reed is to a large extent designed by the builder of the machine. When setting the back shed and harness stroke the weaver is required to choose the proper setting, also when determining the warp tension. These settings have an important impact on the performance of the machine. It is therefore essential that these settings are optimized with utmost care and by using modern measuring instruments.
Following two factors have a particular influence on the behaviour of the warp:
i) yarn traction force
ii) yarn hairiness
A too high warp yarn traction force is leading to overloading the warp ends. As the yarn traction force is not constant during weaving, the peak tensions which generally appear in the bottom shed, particularly in the middle of the warp should be taken into consideration. These peak values should not exceed 5 to 6 cN/tex, depending on the quality of the yarn.
The importance of the setting of the warp stop motion is often under estimated. Its position has a direct influence on the back shed and thus on the yarn traction forces in the bottom and top sheds. The vertical position of the warp stop motion should therefore be set very precisely. It changes the asymmetry of the shed.
Atmospheric Conditions
Relative humidity have a great impart on the performance of the weaving machines. The optimal dissipation of temperature and humidity over the machines, that is the warp, is generally not reached, because the sources of heat within the weaving machine disturb the climatic conditions. Numerous yarn breaks are caused by dust, lint and fibre accumulations. The best experiences have been made with air conditioning systems whereby humidified air is conducted over the machine, whilst the dust – loaded exhaust air is evacuated through floor opening under the back shed, and because the descending flow of the conditioned air is assisting the sedimentation of the lint. At the same working conditions for the personnel are improved. It is surprising how much cleaner the machines and the whole weave room are, compared with systems whereby the exhaust air is evacuated in the alleys, or worse, through the outside walls.
Table -7.6: Weaving - Typical Denim Constructions
TABLE-7.7 : PRODUCTION CALCULATION
The following information is an estimated performance for weaving 14.5 ozs indigo denim.
a)Fabric : Indigo Denim 14.5 ozs/square yard
Warp Yarn : 100% Cotton Ne 7s Indigo Dyed
Weft Yarn : 100% Cotton Ne 6s Grey
Warp Density: 60 ends/inch
Weft Density : 40 picks/inch
Width : 66.5 inches
Weave : 3/1
b)Reed Space : 68.5 inches
2 Colour Weft Mixing
Positive cam shedding Motion, 4 shafts
-Denim Weaving Arrangement
c)Estimated Loom Speed : 900 rpm
d)Estimated Production Per Loom:
i)Daily Production (linear length)
= 900 rpm x 60min x 24h x 0.92 eff x 0.0254 / 40 ppi
= 757 metres/day/loom
ii)Monthly Production (linear length)
= 757 m x 30 days
=22713 metres/month/loom
The following standards for denim fabric quality in terms of inspection, packing, shade and physical requirements are of general nature considering varied requirements of end-uses / customers, however holds good for major universe.
This chapter details classifying fabrics according to color and defects so that the fabrics are made to garments at ease with defects eliminated, grouped according to shade and perform satisfactorily.
INSPECTION REQUIREMENTS
Fabric Group Classification According to Yarn Type
Group One
All Basic Denims (OE/OE)
Group Two
All Denims (RING/OE) & (RING/RING)
Group Three
Specialty Denims: Linen Denims, Vintage
Acceptable Defect Point Levels According to Yarn Type
TABLE-11.1:DEFECT POINTS PER 100 SQUARE YARDS
TABLE-11.2:DEFECT POINTS PER 100 SQUARE METRES
TABLE 11.3: QUALITY CATEGORISATION OF DENIM FABRICS
Defect Grading:
A. Point Grading System: Denim fabrics are normally graded using the “4-Point” system. This numeric grading system is endorsed by ASTM, AAMA (American Apparel Manufacturer’s Association) and ECMA (European Cloth Manufacturers Association). All defects which are clearly visible from one meter (three feet) are scored as defects and demerit points assigned according to severity.
B. Length of Defect Points: Demerit points are assigned to warp and filling defects as follows (Defects in any direction)
v 1 point - Defects 7.5 cm(3”) or less
v 2 points - Defects exceeding 7.5 cm(3”) up to 15 cm(6”)
v 3 points - Defects exceeding 15cm (6”) up to 22.5cm(9”)
v 4 points - Defects exceeding 22.5 cm(9”)
C.Counting of Lengthwise 4 Point Defects: No linear meter is penalized more than 4 points.
v Defect length 22.6cm (9”) to 100cm (40”) – 1 no of 4 point defect
v Defect length 100.1cm (40.1”) to 200cm (80”)–2 no of 4 point defects
v Defect length 200.1cm (80.1”) to 300cm (120”)–3 no of 4 point defects
v If length of defect is more than 3 meters (120”) the length containing the defect is removed.
D. Full Width Defects:
v A full width defect running over 6” in length shall be removed.
v More than four full width defects per one hundred linear meters shall not be accepted as first quality.
v A full width defect in first or last three meters of roll shall not be allowed.
E. Flagging of Defects:
v Only 4 point defects are flagged with a metallic sticker. One may find the metallic sticker at the start of the defect. Metallic flags should be a minimum of 2 cm wide and 7.5 cm length.
v Defect points 1, 2, 3 shall be counted but not flagged.
v All splices shall be flagged.
v All holes shall be removed. There must be two or more yarns broken at the same place to consider a defect as a hole.
F. Roll Length and Splicing:
v Roll length tolerance should be agreed upon the first delivery. Say maximum 135 meters and minimum 85 meters.
v No pieces shall be accepted as first quality with length less than 30 meters.
v No roll shall be accepted as first quality containing more than one splice.
v The shade continuity between parts must allow for the mixing of garment components within a garment.
v Two part rolls should be identified as such, No more than 25% two part roll should be allowed in any shipment.
G.Shading:
v No pieces shall be accepted as first quality that exhibits a noticeable degree of shading from side- to -side or side- to –centre.
v No pieces shall be accepted as first quality that exhibits a noticeable degree of shading from end– to –end.
Skew
A. All 3/1 and some 2/1 twill fabrics for bottoms require skewing. Since the optimum skew varies depending on the fabric (anywhere from 5% to 9%), this must first be determined by the supplier and agreed upon before bulk fabric orders are placed. Target for individual rolls = +/- 3%
B. No fabric shall be accepted as first quality exhibiting more than 3% skew movement.
Waviness
No roll shall be accepted as first quality exhibiting a noticeable degree of looseness (waviness) or tightness along either or both selvages.
Ripples or puckers in the body of the fabric, which prevents the fabric from lying flat when spread in a conventional manner, is unacceptable.
The woven selvage or fringe should be the same on both sides of the fabric.
Tolerance wavy selvage: 2% in length
Width
Cuttable width variations must meet the minimum fabric width specification. In addition width variation within a roll should not exceed 2 cm.
PACKING REQUIREMENTS:
Put- up- Specifications
A. Fabric shall be rolled onto a spiral-wound tube with necessary wall thickness [OD: 60 mm; ID: 45 mm] and strength [Radial Crushing Strength > 500 Kgf for 8” length] to insure it reaching to customers in good condition, provided it is handled by reasonable and acceptable methods.
B. Rolls are wrapped in such a manner that will protect the fabric from all types of damage during transportation and storage.
C. The outer edge of each roll shall be taped down to prevent unrolling during shipment and storage.
D. Roll Identification
Two stickers must be attached one to the end of the roll and another on side of the roll. The stickers shall contain the following information.
1. Roll /Bale Number.
2. Fabric Style number
3. Shade Group
4. Roll length in meters
5. Roll length in yards
6. Fabric width in inches
7. Total square meters
8. Demerit points per 100 square Yards
9. No of fabric pieces
10. Gross weight in Kg
11. Net weight in Kg
12. Quality Category
13. Total number of 4 point defects
14. Total number of Demerit points
15. Security code
COLOUR EVALUATION / SHADE GROUPING
Option 1: Instrumental Shade Evaluation Procedure of Wash Swatches
Instrumental Color Measurement: The color measurement shall be carried out using Spectrophotometer.
Master Roll: Master roll shall be established during initial stages and will remain during the entire life cycle of the product. The procedure for selecting the master roll is given under the head “Procedure for Selecting the Master Roll – Instrumental Evaluation”
Blanket Preparation: The blanket preparation procedure is given under the head “Blanket Preparation”
Shade Evaluation: The color measurement shall be carried out using Spectrophotometer and shade grouping shall be carried out according to wash color reference of master roll. Before assessment, all fabric swatches must be conditioned for at lease 1 hour in a controlled atmosphere. Samples must be conditioned from a dry state. All measurements must be done as soon as possible after conditioning.
Classifying Rolls by Color Groups: LMD grouping shall be followed. (L= Light, M = Medium, Dark = D), the taper standards are defined as color difference (dEcmc) by Standard, Average, Range and Roll to Roll. L: C [Lightness: Chroma]
The tolerance to be followed is
Average : 1.2
Standard : 2.0
Range : 1.5
Roll : 0.5
L: C Ratio : 2.0
Visual Evaluation: All roll sequences shall be evaluated visually by laying the swatches in standard lighting conditions and remove abnormal rolls.
Light Conditions: Only D65 lamps are to be used to assess color. Make sure appropriate Number of lamps is used to obtain luminosity of minimum 1500 lux.
Identifying Color Groups: To facilitate classification of rolls, the rolls shall be identified with appropriate color stickers of L, M and D and the same shade group shall be mentioned in the packing list by tracking via roll number.
Swatch Service: The system shall be to send two set of unwashed and washed swatches to customers. The washed swatches are being sent so that customer can review the shade at their end and the unwashed swatches will be used for additional use if any.
Option 2: Visual Shade Evaluation Procedure of Wash Swatches
Visual Color Assessment: Each wash swatch representing one roll in the shipment needs to be compared to the master swatch under standard light conditions.
Master Roll: Master roll shall be established during initial stages and will remain during the entire life cycle of the product. The procedure for selecting the master roll is given under the head “Procedure for Selecting the Master Roll – Visual Evaluation”
Blanket Preparation: The blanket preparation procedure is given under the head “Blanket Preparation”
Shade Evaluation: Each wash swatch shall be evaluated versus reference shade swatch on shade, color, wash-down, abrasion and overall appearance. Based on this comparison swatches representing rolls shall be accepted/ rejected.
Light Conditions: Only D65 lamps are to be used to assess color. Make sure appropriate Number of lamps is used to obtain luminosity of minimum 1500 lux. Make sure a matt grey background is used with reference of Munshell grey N5 or N7 to assess colors.
Identifying Color Groups: To facilitate classification of rolls, the rolls shall be identified with appropriate color stickers of L, M and D and the same shade group shall be mentioned in the packing list by tracking via roll number.
Swatch Service: The system shall be to send two set of unwashed and washed swatches to customers. The washed swatches are being sent so that customer can review the shade at their end and the unwashed swatches will be used for additional use if any.
Option 3: Visual Shade Evaluation Procedure of Unwashed Swatches
Visual Color Assessment: Each wash swatch representing one roll in the shipment needs to be compared to the master swatch under standard light conditions.
Master Roll: Master roll shall be established during initial stages and will remain during the entire life cycle of the product. The procedure for selecting the master roll is given under the head “Procedure for Selecting the Master Roll”
Shade Evaluation: Each unwashed swatch shall be evaluated versus reference shade swatch on shade, color and overall appearance. Based on this comparison swatches representing rolls shall be accepted/ rejected.
Light Conditions: Only D65 lamps are to be used to assess color. Make sure appropriate Number of lamps is used to obtain luminosity of minimum 1500 lux. Make sure a matt grey background is used with reference of Munshell grey N5 or N7 to assess colors.
Identifying Color Groups: To facilitate classification of rolls, the rolls shall be identified with appropriate color stickers of L, M and D or D+, D, D- and the same shade group shall be mentioned in the packing list by tracking via roll number..
Swatch Service: The system shall be to send three set of unwashed swatches to customers.
Blanket Preparation
Swatch Size: Use uniform swatch size of 8” x 6”
Preparation of Blankets: All blankets for washing shall have standard shade swatch from Master Roll. Swatch shall be cut from every roll of a shipment each 8” x 6”. Identify each roll no at the back of the swatch. In case of light weights identify by serial nos at the backside corner with indelible ink.
Waste material of same style
Wherever less number of swatches is available use dummy swatches of same style nos.
Do not use washed swatches for dummies.
v Dummy swatch size shall be of same size as that of blanket swatches.
v To make wash load if required, use dummy swatches of same style only.
v Sew the two panels together to avoid entangled panels in washing.
v All panels from one shipment to be washed in same wash load.
Washing of the Blankets: Wash blankets according to the recipe for various classes of products.
Procedure for Selecting the Master Roll
Instrumental Evaluation
v From the average of the washed lot select about five roll which are near to the average values.
v Check their UNWASHEDED L*a*b* values, these must belong to 555 block. If not then select 5 more rolls near to average values from the washed lot.
v The roll which belongs to 555 shade block, check the preceding and succeeding rolls L*a*b* values, these must also belong to 555 shade block. This will ensure that there is no shade variation within the roll.
v Now check the consistency of the complete beam. There should not be much variation within the beam.
v The dyer will use the yarn latti of this particular beam as shade reference for dyeing.
Visual Evaluation
v Select about five rolls which represent middle of the washed lot.
v Check unwashed swatches as well, for their true representation of the middle of the lot.
v Check the preceding and succeeding swatches. These must also belong to the same shade. This will ensure that there is no shade variation within the roll.
v Now check the consistency of the complete beam. There should not be much variation within the beam.
v The dyer will use the yarn latti of this particular beam as shade reference for dyeing.
TESTING REQUIREMENTS - PERFORMANCE STANDARDS
Product development concerns itself with modifications or extensions provided to ideas so as to improve the functioning, the cost, and the value-for-money of the product. Development efforts improve the performance of the product, add options and additional features and even add variants of the basic product. On the whole, development effort is innovative vis-à-vis research which is more inventive – the thrust being on developing new product ideas, technologies and processes.
The tradeoff between research and development is an important strategic decision for most organizations.
Variety or Standardization
There are two distinctly different priorities that can affect the product development.
By adding variety, an organization attempts to satisfy the varied needs and tastes of customers and competes on non-price considerations as well.
On the other hand standardization / simplification offer:
-reduction in total inventory of yarn
-reduction in extra or unutilized ends
-reduction in accessories/ reeds
-reduction in change over times
-simplifies many operational procedures
-reduces need for many controls
-lower unit costs
Now the two caveats to trade off are
# a family of similar products is much simpler to produce than a family of dissimilar products.
# it is not enough to produce a product but it has to be produced so that there is an added value as perceived by the market
Modularization: Middle Path – Buddha’s Way
One method used to obtain variety or perceived variety and yet hold down cost is through modularization. A product is deigned using modules or sub assemblies that are interchangeable and each different combination of modules gives a new variety of the product.
1 Warp sheet
X 3 Colors
X 3 Weaves
X 3 Wefts
--------------------------
1x3x3x3= 27 Products
-----------------------------
X Tints
X Garment Washes
X Blasting/Brushing
X ….
X …
X ...
For example one warp sheet, three colors, three weaves and three wefts will give
1 x 3 x 3 x 3 =27 varieties, yet making large quantities of standard modules.
This is illustrated with two typical basic denim constructions. The grey construction details are given in Table-13.1.
Volatility in Cotton prices
Denim mills are searching for ways to produce denim fabrics of the highest quality at competitive prices. The engineering from raw fiber to finished fabric results in superior denims when modern fiber and manufacturing technologies are consistently applied throughout the processing chain. However, the cost of these engineered products delivered to the market place demands, in addition the prudence in tackling the volatility associated with the raw material, mainly cotton.
The Raw Material Cost [RMC] per metre of 150 cm width basic OE denim, for various denim weights over a decade is depicted below in equation, for the sake of simplicity:
RMC/metre = 2.3 x ozs per sq.yd. + 8.4
+/-0.4
That is cotton cost per metre for a 10ozs OE denim is Rs.31.4 [Ranging 27.4 to 35.4]
The engineering of denim fabrics utilizes, ring spinning or open end rotor spinning or their differentiated innumerable characteristic yarns + modern fabric forming and finishing techniques and commonly available but selected cotton fibers. The mixing specifications for different yarns are given in Table-13.3. Cotton Mixing Specifications of Important Fibre Characteristics for Denim Yarns (HVI Mode).
The following equation depicts the cost differentials between Ring denim and Rotor denim in a simplified way:
RMC Ring per metre = RMC OE per metre x [1+ozs per sq.yd/100]
That is cotton cost for a 10 ozs ring denim is Rs.31.4 + [1+10/100] = Rs.31.4 x 1.1 = Rs34.54.
Engineering
Hence the engineering of denim fabrics begins with the selection of appropriate combination of cotton fibre characteristics for different yarn types with the aid of modern HVI instruments. The mixing specifications for different yarns are given in Table-13.3. Cotton Mixing Specifications of Important Fibre Characteristics for Denim Yarns (HVI Mode).
Once fibre characteristics are known, yarn quality and processing performance can be predicted. Last column in the Table-13.3 provides an iNDEX for various yarn types which can be used as a composite measure to arrive at a variance with respect to actual yarn quality of various yarn types including individual characteristic yarns.
The equation for calculating
iNDEX=735 x [UHML x Str - 3 x Str -255 -6.6 x UHML]^0.22
Where UHML is Upper Half Mean Length in mm and
Str is Strength in grams per tex as tested using HVI Mode
The above iNDEX 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
The spinning parameter Twist Multiplier and important yarn parameters, such as strength, count and their variability’s of various yarn types are provided in Table-13.4. Spinning Parameters and Yarn Quality.
Rope dyeing technology demands less torque yarns in warp which are made from “U”rotors. The H values of Yarns made from “U” rotors as tested in Uster evenness testers are higher by 2.0 to 2.5 numbers in comparison with the yarns produced from “T” rotors
Cover Factor
To obtain better performance on the loom and fabric yield, a guideline for cover factor in developing new fabric constructions is given in Table-13.5 and classified under 3 categories for
i) Rings
ii) Bulky Open-ends and
iii) Shrinking Stretches.
B.Grey construction is used for calculations.
C.The formula used = Threads per inch
--------------------------- -------- X 100
[28 X Square root of Count]
Changing Trend in Yarn Preferences
Beginning of twenty first century saw the explorations in different yarn options. The below list is arranged by their usage in volume, starting with warp followed by their use in weft.
Crosshatch / Streak / Rain Denims
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. Aggressive slub parameters tend to lower Yarn Strength and poor performance at subsequent processes. The minimum tenacity values as tested using Uster Tensorapid for satisfactory performance level is given in Table-10.4 in Row 3 and 7. Beyond which compromise needs to be made among Slub parameters or performance levels or cotton fibre characteristics.
Chinos
Two ply chino denims in indigo dyed shade have a unique soft hand feel, Fabric cover and a luxurious appeal.
Tencel Denim
Woven century luxury cellulosic fibre made from specially grown woods and transformed in non-chemical process which give feel of silk and comfort of cotton.
Stretch Denims
For stretch denims, Core-Spun Cotton Spandex, Poly Spandex and Type -400 yarns are generally employed. Core-Spun Cotton Spandex yarns in denims range 10s to 16s Ne and uses generally 70 denier filament; though 40, 55 and 120 denier filaments are also available. The Spandex % of Core-spun cotton spandex yarn ranges from 3.5 to 5.5 %. The twist multiplier is 4.4.
Union Denims
Denims are differentiated with weft yarns to create Union Fabrics. The union denims produced in large volumes uses the following weft yarns in the same order – Polyester Texturised Filament, Stretch yarns of Poly Spandex and Type -400 yarns, and Pre-bleached Linen Yarns.
Poly Denim
Polyester Texturised Yarns in denim applications range 300 to 600 denier, with a tenacity of 4.0 g/tex and 20 to 27% extension. Bulk which is expressed in %HCC (Hot Crimp Contraction) is about 40 and nips per metre ranges from 70 to 90 for better performance.
Grey as well as dyed yarns are being used.
Poly Stretch Denim
Poly spandex yarns used in denims range 150 to 300 denier with 6 to 12 % spandex. Nips per metre for better weaving performance is 100 to 130. Type -400 yarns from Invista used in denim applications range 150 to 600 denier with a bulk of 50% HCC and nips per metre of 35 to 50.
Linen Vintages
Pre-bleached Linen yarns range 9s to 16s Ne [or 25 to 44 Lea]. Though these yarns have very high strength of over 3000 csp and 20 cN/tex, due to low elongation of 2% and the natural variation in yarn, the loom performance as well as full width defects is poor in comparison with normal denim.
Value Engineering
In accordance with the value engineering, quality fabrics can also be produced from Value mixings given in Table -11.3 Row 5. However the denim fabrics produced out of such yarns should not be meant for elaborate destructive garment washes.
Addition of 10% Recycled Waste
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.
Control of Yarn Realization
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 procedures for the control of yarn realization in Chapter 3, ATIRA Silver Jubilee Monograph “Process Control in Spinning”.
Mock Rings
Various attempts to duplicate superior denims made from ring yarns with rotor yarns of mock or slubby have always failed in fabric strength properties, fabric hand and appearance due to differences in yarn structure and yarn properties. Still one wonders how much of the present so-called ring spun Denims are such and how many are mock ring made in open end.
Other Value Offerings
Cotton rich polyester denims are with superior hand feel, luster and colour contrast for fashion market. One may find a deep value in using dyed polyester texturised filament yarn in place of costly yarn dyed cotton weft for high fashion denim. Poly Spandex yarns are replacing core spun lycra yarns in the value universe. Within Poly Denims, the denier is getting coarser day by day from 300 to 450 to 600. Though, Linen Vintage denims are not in high volume, there are efforts to replace it with Jute (Indian Linen) in the value proposition.
Successful strategies in denim mean profit, often (now) even survival. Engineering the fabrics on continual basis provides a way to achieve both quality and cost benefits of substantial proportions. At the same time full manufacturing flexibility through modularization is being maintained, enabling the denim mill to meet new and changing trends in raw material and fashion.
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