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
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.
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
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
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.
Two ply chino denims in indigo dyed shade have a unique soft hand feel, Fabric cover and a luxurious appeal.
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.
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.
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.
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.
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.
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”.
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.