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YARN-ENGINEERING BASED ON THE SIMULATION OF KNITTED FABRICS -

INNOVATIVE CHANCES FOR THEENGINEERING OF BLENDED YARNS -

M. Gerig and B. WulfhorstInstitut für Textiltechnik (ITA)

Rheinisch Westfälische Technische Hochschule(RWTH)

Aachen, Germany

Abstract

Differences in yarn structure influence the properties andthe characteristics of fabrics produced from these yarns.Therefore the securing and controlling of quality in spinningmills aims at providing the basis for a yarn productionprocess that takes the final article into consideration. Afundamental support for the engineering of blended cottonyarns is the possibility to simulate knitted and wovenfabrics, taking the yarn characteristics as a starting point.

The yarn evenness is the dimension with the most influenceon the appearance of textiles, especially on that of knittedfabrics. It can be measured capacitively as well as optically.Both these industrially used measuring methods can beapplied online and offline. They were the basis for thedevelopment of different simulation systems haveconsequently been put into industrial practice.

All simulation systems do not yet provide a quantitative orqualitative evaluation of the optics of simulated knittedfabrics. The new ITAOPT System which has been evolvedat the Institut für Textiltechnik der RWTH Aachen (ITA)allows, for the first time, an objective final evaluationdeciding on quality. Thus it provides experts with a tool tomake comparing judgements more objective andindependent of external influences.

This substitution of subjective, visual judgment of the opticsof simulated knitted fabrics with the newly developedobjective, qualitative and quantitative analysis realized bythe ITAOPT System opens up new economic perspectivesfor the yarn-engineering, especially of blended cotton-yarnsthat focus on the final article.

Introduction

Especially for spun yarns that consist of fiber-blends suchas for example cotton and man-made fibers, it is desirableto achieve an even distribution of the components across theyarn cross section. A homogeneous distribution of thesingle fiber components across the cross-section of the yarnhas a positive influence on the structural parameters as wellas on the dyeing and wearing properties.

For the OE-rotor-spinning, particularly varying fibercharacteristics such as the rigidity of the fibers have aninfluence on the tying in behavior of the fibers into the yarn.Therefore they can lead to a more uneven fiber distributionalong the cross-section of the yarn which has a negativeeffect on the appearance of knitted fabrics. Fiber-blends thatare supposed for manufacturing should therefore becoordinated with respect to their characteristics so that thefinal article can be specifically designed, starting from thechoice of suitable fibers.

Simulation of Knitted Fabrics

The yarn feature that has the most influence on theappearance of especially knitted fabrics is the yarnevenness. It is decisively affected by the fiber characteristicsof the single blend components. The yarn evenness can bemeasured capacitively and optically. Proceeding from thesetwo measuring methods that are both used industrially -either on-line or off-line - the following three independentsimulation programs have been developed and madeaccessible for industrial use (Fig. 1).

When simulating knitted fabrics with the USTER®EXPERTSystem it is the evenness of mass that is evaluated, while thesimulation with the CYROS®System, respectively theOASYS®System, evaluates the evenness of the opticallyefficient yarn diameter. With all three systems it is possibleto simulate various textile surfaces such as knitted andwoven fabrics with varying constructions as well as yarnboards.

USTER®EXPERT System The USTER®EXPERT System provides the means for theelectronic generation of yarn boards and knitted and wovenfabrics with given machine parameters, using the measuringvalues delivered by the USTER-Tester 3. This system aimsat the visualizing of yarn defects in knitted and wovenfabrics.

The system supplies a monitor display of the simulated yarnboards and fabrics which can be printed out. The evaluationof the appearance of fabrics is not yet possible. Now asbefore the subsequent judging of the simulated yarnboardsand the knitted and woven fabrics is performed in the sameway as the judging of the veritable surfaces: it is performedvisually and therefore subjectively by experts.

CYROS®SystemThe simulation with the CYROS®System (Cotton YarnRating through Online Simulation) that is offered by CiS isbased on the optically efficient yarn evenness that ismeasured with the yarn structure tester G580 from Zweigle.

The yarn diameter values are transformed into a three-dimensional presentation of the yarn surface. Like theEXPERT System, the CYROS System offers a monitordisplay of the simulations, which can be printed out. The

Reprinted from the Proceedings of the Beltwide Cotton ConferenceVolume 1:805-809 (1998)

National Cotton Council, Memphis TN

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judging of the simulated surfaces is again carried outvisually by a selected circle of experts.

OASYS®SystemThe simulation with the OASYS®System that is offered byZweigle, is based, similar to that of the CYROS System, onthe optically efficient yarn evenness, which is measuredwith the yarn structure tester G580 respectively G585 fromZweigle.

The OASYS System delivers, just as the EXPERT Systemand the CYROS System, monitor displays of thesimulations which can subsequently be printed out. Thecomparing judgement of the simulated displays is alsoperformed visually and therefore subjectively by a selectedcircle of experts.

Comparison of Simulated and Real Knitted Fabrics

The following example will enlighten the possibilities thatare given with the simulation of the appearance of knittedfabrics from various rotor-spun blended yarns which consistof PES and Cotton (50/50 %). The objective judging ofsuch simulations with the ITAOPT System will be shownexemplarily by means of two different yarns. The two yarnsdiffer through the choice of the fineness of the fibercomponents. Therefore the blend ratio related to the numberof fibers is different. The yarn F/G(63/37) was producedfrom a fine PES (number related: 63 %) and a coarsercotton (number-related: 37 %). The yarn G/F(50/50) wasproduced from a coarse PES (number-related: 50 %) and afine cotton (number-related: 50 %) (Fig. 2).

Both batches were spun into knitting yarns under the sameconditions and then processed into single-jersey knittedfabrics. The gray knitted fabrics were then dyed andfinished. The blue dye that was used has only affinity tocotton and does not dye the PES-fibers (differential dyeing).

The evenness of the two yarns F/G(63/37) and G/F(50/50)deviates especially as far as the yarn mass is concerned. Theunevenness of the yarn diameters, however, is not asobvious. It is therefore to be expected that thedifferentiation of the two yarns, with regard to theappearance of the fabrics made from them, can be realizedwith the EXPERT System because this system is based onthe evenness of the yarn mass. Nevertheless, the specificstructure of OE-rotor-yarns suggests that the distinctappearance of the two fabrics can also be simulated bymeans of the optically efficient yarn diameter, that is to sayby means of the CYROS and the OASYS Systems.

When regarding the appearance of the simulated knittedfabrics it is essential to notice that the representationcorresponds to the undyed, raw-white fabric knitted fromthese two yarns. In contrast to this, the real fabric is coloredwith differential dyeing. Therefore the appearance of the

real and the simulated fabric is only conditionallycomparable.

It is striking that all the simulated fabrics do differ fromeach other, even those that are based on the evaluation ofthe optically efficient yarn diameter. The simulated fabricsshow the same features as the genuine, dyed fabrics. Thesimulated knitted fabric made from F/G(63/37) is altogethermore homogeneous than that made from G/F(50/50). It isalso brighter and even the subjective judging reveals that thebright areas are more wale-oriented and smaller than thoseof the simulated fabric made from G/F(50/50). The fabricknitted from F/G(63/37) exposes a constant unevennesswhich affects the appearance less than the more course-oriented distribution of the very contrastive bright and darkareas of the simulated fabric knitted from G/F(50/50).

Objective Evaluation of the Appearance

So far, all three systems do not deliver the possibility tojudge the appearance of the simulated knitted fabricsquantitatively or qualitatively. The judgement of theappearance of simulated fabrics which has still has to becarried out subjectively entails the same difficulties as thesubjective, visual judgement of the appearance of realfabrics. The latter one, however, can already be replaced bythe optical analysis of the appearance with the aid of theITAOPT System, which has been developed at the ITA andwhich has been represented at the Beltwide CottonConference in 1996. Only since recently the ITAOPTSystem can also be used for an objective final evaluationthat decides on the quality of simulated knitted fabrics. Bythis means the experts can be equipped with a new toolwhich will help to turn the judgement more objective andindependent from external influences.

This substitution of the subjective judgement of the yarnappearance of the simulated knitted fabrics with anobjective, qualitative and quantitative analysis of theappearance of the simulated knitted fabrics can be realizedirrespective of the physical principle, on which the structureanalysis of the yarn is based.

The simulated knitted fabrics and the digitized images of thereal fabrics (REAL) are represented for all three systems asgray value images, consisting of values between 0 for blackand 255 for white.

The differences in brightness that are visible both in thesimulated as well as in the real knitted fabric are allocatedto various gray value levels by means of the digitizedoriginal image. These gray values are allocated to one ofthree relative classes. The three gray value classes contain,according to their brightness the gray values of the bright,respectively dark „clouds“ and of the areas that are inbetween, that is to say the „standard areas“.

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The allocation of every single measuring point to one of thethree gray value classes, leads to a representation of theknitted fabric which is composed of only three gray values,that is white, gray and black. Bright clouds are representedas white areas, dark clouds are represented as black areasand the remaining part of the knitted surface is representedas gray area. The bright and dark areas which have thusbeen isolated, are subsequently evaluated with regard totheir characteristics. In Fig. 3 the transformation of theappearance of the surface into a digitized image and thesubsequent isolation of the „clouds“ is illustrated. For thisexemplified representation the genuine knitted fabric hasbeen used.

Evaluation of the Standardized Gray-ValuesThe mean graded gray values characterize the overallbrightness and the standardized gray values characterize theresulting contrast between the the graded values of theknitted fabric examined. The higher contrast between thebright respectively dark areas and the standard areas in thereal and the simulated fabric knitted from G/F(50/50)reveals that areas within the fabric, which are significantlydarker or brighter than the mean gray value, are rare, but allthe more striking. The more contrastive bright and darkareas become optically more prominent (Fig. 4).

The fact, that the difference between the bright areas andthe standard areas is the smallest in both the real and thesimulated fabrics knitted from F/G(63/37), proves that thesefabrics are not only altogether brighter, but also morehomogeneous than the fabrics knitted or simulated fromG/F(50/50).

For both knitted fabrics it is obvious that the standardizedgray-values of the fabrics simulated with the EXPERTSystem are quantitatively comparable to the genuine fabrics.In opposite to this, the contrast is more salient for thefabrics simulated with the CYROS System and the OASYSSystem. This higher contrast is due to the divergentrepresentation of the simulated knitted fabrics. Through thechoice of an optimized background design, this contrast canbe reduced, so that it is no longer limited to qualitativecomparability, but can also be consulted for quantitativelystatements. Thus the propositional power of the overallrepresentation of the simulations can be increased.

Evaluation of the Variation of the Standardized Gray-ValuesThe visual evenness of the appearance of various knittedfabrics is depicted by the distribution of the graded gray-values. Similarly it is in particular the standardizedcoefficient of variation of the gray values that is suited fora qualitative statement. The higher evenness of thedistribution of gray-values of the fabric knitted fromF/G(63/37) appears especially when comparing thecoefficient of variation of the distribution of gray-values ofthe bright respectively dark and the standard areas.

For Fig. 5 a standardized representation of the coefficient ofvariation of the graded gray-values has been chosen. Apartfrom the fact that both the real and the simulated fabricknitted from G/F(50/50) are altogether darker, they are alsomore contrastive. By choosing a standardizedrepresentation, it becomes obvious that the higher contrastis distributed similar to the more homogeneous and brighterfabrics knitted and simulated from F/G(63/37).

The comparison of the standardized coefficient of variationconfirms the suitability of the CYROS System and theOASYS System for the simulation of knitted fabrics madefrom OE-rotor-yarns, although they differ only minimally intheir structural parameters. For the special application of thesimulation of knitted fabrics from OE-rotor-spun blendedyarns the EXPERT System is also suitable. Differences canbe simulated realistically, that is to say the appearance ofknitted fabrics can be predicted in a realistic way.

Summary

The different yarn structure of the two yarns F/G(63/37) andG/F(50/50), which is caused by the composition of thefiber-blends, is mirrored much more obviously in theevenness of the yarn mass than in the evenness of thevisible yarn diameter. Nevertheless it is possible to simulaterealistic knitted fabrics from OE-rotor-yarns in using thesimulation based on the optically efficient yarn diameter.The reason therefore is the specific structure of OE-rotor-yarns.

The investigations, that have been represented here byexamples, show that a realistic simulation of the appearanceof knitted fabrics is possible on the base of the evenness ofthe yarn diameter as well as on the base of the evenness ofthe yarn mass. All three simulations can profitably beapplied as prediction devices for the industrial use and forresearch or developmental purposes. The USTER EXPERTSystem, which is used for the simulation of the appearanceof knitwear, differs from the OASYS System and theCYROS System with regard to the structural attributes thatare measured.

For the EXPERT System it can be Stated:The simulation of the appearance of fabrics with theEXPERT System, which is based on the evenness of theyarn mass is suited for those OE-rotor-yarns that differ intheir composition with regard to the fiber fineness and thenumber of fibers across the cross-section. With the aid ofthe simulation it is possible to emphasize the morehomogeneous and the brighter appearance of the fabricknitted from F/G(63/37) as opposed to the fabric knittedfrom G/F(50 /50).

For the CYROS System and the OASYS System it canbe stated:The simulation of the appearance of various OE-rotor-yarnsfrom varying fiber-blends is possible on the base of the

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optically efficient yarn diameter. Both Systems takes eventhose differences into consideration, that have only a smallinfluence on the values of the yarn structure, but aconsiderable influence on the appearance of the fabric.

Final Conclusions

Hitherto the simulation of the appearance of knitted fabricswas used exclusively for the off-line measurement of theevenness of either yarn mass or visible yarn diameter. Thismeasuring device was only used for yarns that were actuallymanufactured on production facilities. It is, however, alsopossible to apply it for an on-line measurement of the yarnevenness, which enables the simulation of the knitted fabricand can thus be used for quality control. Anotherapplication could be the prediction of the quality which is tobe expected under given spinning conditions. Optical andcapacitive sensors for the on-line control of the yarn qualityare offered by various companies.

Very interesting for this use is the optical sensor Corolab®,offered by Barco. This sensor is on one hand alreadyapproved industrially as a yarn clearer at OE-rotor-spinningmachines, on the other hand it is applied for the yarnstructure tester G580 and G585 from Zweigle, that is to say:it serves as a sensor for the simulations with the OASYSSystem and the CYROS System. The integration of thesimulation systems into the production sector thereforeseems realistic. Furthermore it is imaginable to connect thesensor with the spinning unit of the QUICKSPINN-Systemthat is offered by Zellweger Uster. Thus it would bepossible to predict - on the base of the optically efficientdiameter - the probable appearance of knitted fabrics at astage where the fiber components of the yarns are not evenyet determined.

The values that are provided for example by Corolab, can beused just as before as basic values for the simulationsCYROS and OASYS and the subsequent objective analysisbased on the selection algorithms of the ITAOPT System.In this way decisions on optimum fiber-blends can be madequicker and more economically. This expanded range ofuses of the simulation systems in combination with theobjective and reproducible evaluation of the simulatedknitted fabrics with the ITAOPT System opens up newperspectives for the fiber to yarn to fabric engineering.

Acknowledgements

We give special thanks to Gaby Peters and Chris Färber atZellweger Uster (CH) as well as to Claus Müller at CiSGraphik & Bildverarbeitung (D) just as to Dieter Zweigleand Herbert Krupp at Zweigle Textilprüfmaschinen (D) fordata processing necessary to development of thismanuscript.

Figure 1. Engineering of knitted fabrics

fineness offibers [dtex]

length of fibers [mm]50 % Sp.l. [mm]

number offibers

PES F/G(63/37) 1.11 25.4 91 G/F(50/50) 1.56 25.4 64

CO F/G(63/37) 1.92 13.3 52 G/F(50/50) 1.57 13.2 64Figure 2. Properties of fibers

F/G(63/37)

G/F(50/50)

appearance ofknitted fabrics

digitized knittedfabrics

„cloud-isolation“

Figure 3. Evaluation of the appearance of knitted fabrics

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Figure 4. Standardized gray-valueFigure 5. Coefficient of variation of standardized gray-value