WEAVABILITY MEASUREMENT SYSTEM

Transcription

1 USTER TENSOJET 4 APPLICATION REPORT Successful yarn buying in the weaving mill by using modern high-performance tensile testing THE WEAVABILITY MEASUREMENT SYSTEM Ch. Färber, W. Söll January 1997 SE 550

2 Copyright 2005 by Uster Technologies AG All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, translated or transmitted in any form or by any means, electronically, mechanically, photocopying, recording or otherwise, without the prior permission in writing of the copyright owner. veronesi\tt\schulung_dokumente\off-line\zugprüfungutr4_utj4\ SE_550_Successful yarn buyingin the weaving.doc 2 (20) USTER TENSOJET 4

3 Contents 1 Questions regarding the economical and technological importance of yarn buying in the weaving mill Traditional yarn buying Solution with modern test methods the high-performance tensile test Basics of high-performance testing Practical examples Warp yarn (50/50 PES/CO, Ne 35/1) Warp yarn (100% CV, Nm 50/2) Summary Litarature USTER TENSOJET 4 3 (20)

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5 1 Questions regarding the economical and technological importance of yarn buying in the weaving mill What is the importance of the yarn selection in the weaving mill or of the decision in favour of a yarn supplier according to strictly objective criteria which are well-founded, both technologically and economically? Is it true that yarns which are offered on the market at low prices appear to be particularly attractive at first? Which is the finally decisive argument for the purchase, the concrete requirements of the article to be produced or the price of the yarn? Are the quality parameters of the yarn which are necessary for the production of an article of market-oriented quality and price range part of the purchase order or are they provided in the form of test certificates by the supplier at the delivery of the first lot? Are perhaps such quality parameters regarding a requirement profile not even known, or are there only inadequate descriptions based on practical experience or certain data from the past? 2 Traditional yarn buying A tried and tested support for yarn purchasing are certainly the USTER STATISTICS, which allow the user to objectively realize the procurement of specific qualities. The user thereby takes the 25%-line, for example, as guidance and then defines the test parameters which appear to be relevant for the intended purpose. Concerning the classical yarn quality parameters such as evenness, imperfections (thick places, thin places and neps), hairiness and Classimat faults, extensive experience has been gained over the years. The final quality of the fabric to be expected, the characteristics disturbing the subsequent processing and yarn faults resulting in visible deficiencies in the grey cloth or the finished fabric are usually known (Fig. 1 and Fig. 2) Fig. 1 Effect of differences in yarn hairiness. On the visual fabric appearance USTER TENSOJET 4 5 (20)

6 Fig. 2 Disturbing yarn fault (thick place) in a fabric However, the determination of the required dynamometric properties of a yarn is still a problem. The strain on the yarn in warp or weft direction depends on numerous conditions which, on the one side, are process-specific (air-jet, gripper or projectile technology) and, on the other side, are subject to far stronger influences through machine settings or design characteristics of the fabric than, for example, the appearance of the fabric surface. Especially in the case of the weft, the strain on the yarn is directly dependent on the machine performance. On top of this it is a fact that a calculation or a verification by measurement of the actual loads is not possible without a considerable effort. In some cases, for example, it is required that the breaking force and the elongation at break of a yarn should be at the highest level (e.g. 5%-line of the USTER STATISTICS). This involves a certain risk, in so far as that the decisive factors for the behaviour in subsequent processing are not only the average tensile strength or elongation of a yarn but mainly the frequency of seldom-occurring extreme weak places. These are, as it is well known today, independent from raw material, spinning method, yarn construction and yarn price. There is certainly a great disadvantage in today's tensile tests according to the existing DIN standard with a testing speed of 250 mm/min. With a reasonable time effort, only a few tests on the sample material are possible under such measuring conditions. The resulting mean values and coefficients of variation of the test sample provide general information on the average characteristics of the yarn and on the expected tensile strength of the grey cloth. With this type of tensile testing, however, it is not possible to determine the yarn strength over a greater yarn length, for example, over entire bobbins or cones, let alone to identify the seldom-occurring weak places, which cause avoidable production stoppages and therefore affect the weaving efficiency. Even an apparently high yarn quality can occasionally fall short of the process requirements as a result of weak places [1] and thereby cause production stoppages. 6 (20) USTER TENSOJET 4

7 3 Solution with modern test methods the highperformance tensile test With the USTER TESTER for yarn evenness testing, a testing unit has been available for many years, which provides reliable and convincing results, especially as a consequence of its high testing capacity. With the USTER TENSOJET, the new high-performance tensile testing unit, it is now also possible to obtain results from a representative sample within a very short time, and this with the same testing capacity. At the maximum testing speed of 400 m/min, the USTER TENSOJET (UTJ) offers the possibility of performing 30,000 tensile tests in one hour. In comparison with the standard testing speed, this results in a much higher testing capacity (factor 238); even in comparison to the quick test at 5 m/min, it still leads to an increase by a factor of 42 (Fig. 3). TENSO JET tensile tests/ h at 400 m min -1 TENSORAPID tensile tests/ h a t 5 m min -1 TENSORAPID tensile tests/ h a t 5 m min -1 DIN tensile tests/ h a t 0,25 m min Fig. 3 Testing capacities of different tensile testing methods In the area of yarn testing, therefore, it has now become possible to offer the weaving mill a high-performance tensile testing unit which allows the user to perfectly organize the yarn buying process and to adapt it to the sometimes extremely high requirements of the industrial production of woven fabrics. Prior to placing the purchase order, the most suitable and, at the same time, most economical yarn for the respective application can be determined from a selection of yarns from different manufacturers. USTER TENSOJET 4 7 (20)

8 4 Basics of high-performance testing Fig. 4 illustrates that not the mean breaking force of a yarn is the decisive criterion, but rather the seldom-occurring events or weak places, which in the most unfavourable case coincide with the highest loads on the weaving machine and in that way lead to a warp or weft end break [2]. Only a sufficiently large sample size will ensure that enough measurement data are produced to be able to clearly evaluate these conditions. Load generated by the weaving process Frequency Tensile strength of the yarn Critical overlap area Tensile force (cn) Fig. 4 Relationship between tensile load and tensile strength during the weaving process With its evaluation possibilities, USTER TENSOJET takes a step towards the future by summarizing all the information which is generated from this sizeable amount of test data on one single page. Fig. 5 shows such a test protocol (USTER Quality Report). A colored force/elongation scatter plot with color code provides information about the testing sequence. The window marked with ' + ' in the center of the point cloud indicates the mean value of the measurement. In addition, the scatter plot shows the frequency distribution of the breaking force and the elongation at break. The number of samples (e.g. bobbins) tested and the total number of breaks are indicated on the right above the force/elongation diagram. The stroke diagram provides a time history of the measurement. It also supplies information on trends and possible periodic faults. The statistics block contains the mean values of the breaking force, of the elongation at break, of the tenacity and of the breaking work as well as the corresponding standard deviations, coefficients of variation and confidence ranges. In addition to the maximum and minimum values, the statistics block also includes the so-called percentiles, which are divided into five classes. Percentile P 0.01 signifies that 0.01% of all measured values are smaller than or equal to the indicated force, elongation or work. The figure in parentheses indicates the exact number of measured values in that class. In our example, there are 10 measured values, which is 0.01% of the test volume with a total of 50,000 measured values. 8 (20) USTER TENSOJET 4

9 Each part of the test protocol can be printed out separately. It is therefore possible, for example, to examine the scatter plot and the stroke diagram, the frequency distribution and the statistics block individually. Individual printouts of each tested yarn sample (bobbin, etc.) are also possible. Fig. 5 USTER Quality Report Research has shown that the correlation between the P 0.01 percentile values and the weft-specific stoppages in the weaving mill is quite good (Fig. 6 and Fig. 7). The analysis is based on extensive records of stoppages [3] in processing of different ring-spun cotton yarns 13.4 tex to a plain weave fabric (L 1/1) on a single-width air-jet weaving machine at 755 rpm (weft material PES dtex 76f22). USTER TENSOJET 4 9 (20)

10 8,00 6,00 r = ,00 2,00 0, Tensile strength percentile F P= 0,01 [cn] Fig. 6 Relationship between the percentile values of peak force and the weft breakage frequency 8,00 6,00 r = ,00 2,00 0,00 2,00 2,50 3,00 3,50 4,00 4,50 5,00 Elongation percentile εp= 0,01 [%] Fig. 7 Relationship between the percentile values of the elongation at peak force and the weft breakage frequency The software of the TENSOJET allows the definition of lower limits for the breaking force and the elongation at break so that weak places can be clearly highlighted by colored lines in the scatter plot (see Fig. 5). These limits represent the minimum processing requirements which result from the loads exerted on the yarn during the shed formation or the weft insertion. This allows the user to adapt the weak place analysis exactly to the specific loads of his weaving process. Extensive research for cotton yarns, which has been carried out in close cooperation with weaving machine manufacturers, shows that weak places below 4.0 cn/tex almost inevitably cause a weft end break and elongations less than 2.0% a warp end break. In many cases, however, even less distinct weak places will result in an end break. For specific conditions in the subsequent processing, the limits have been placed at 10.0 cn/tex and 2.5% (ring-spun yarn 5.5 tex, 100% cotton) as indicated in Fig. 5. In the specified critical range, there are individual weak places which will lead to stoppages in subsequent processing. The graphic representation allows a quick and easy interpretation of the test protocol even by an inexperienced user. The high testing capacity of the USTER TENSOJET alone will guarantee a sufficiently high statistical reliability in yarn testing. Only with an adequate sampling size is it possible to detect these seldom-occurring but very dangerous weak places in the yarn (Fig. 8) [4]. An additional advantage of using the TENSOJET in a weaving mill lays in the high testing speed. Particular research has shown that a cotton yarn is loaded in approx. 3 ms until it breaks. 10 (20) USTER TENSOJET 4

11 This value compares very well to the dynamics of a weaving machine where the peak loads during weft insertion extend over a period of 3 6 ms depending on the type of weft insertion system [5]. These findings clearly show that the development of the USTER TENSOJET high-performance tensile testing unit has made it possible to realistically simulate the dynamic loads which affect the yarn during the weft insertion. Relative frequency (%) 46'000 Tensile tests 1'300 Tensile tests Tensile strength (cn) Fig. 8 Determination of the minimum values of strength by increasing the sample size 5 Practical examples The following examples are intended to show the benefit of the highperformance tensile testing with respect to yarn buying in a weaving mill and with regard to the behaviour in subsequent processing. 5.1 Warp yarn (50/50 PES/CO, Ne 35/1) Table 1 shows the mean values of the tensile strength and the evenness properties as well as the USTER STATISTICS percentile values for four different yarns which are procurable as warp material for the intended production. From these values, we will try to establish a ranking: Yarn 1 will not be considered because it has the worst value in each category. The best yarn of this assortment appears to be yarn 3, whereby the differences between the yarns 2, 3 and 4 are not obvious. Each of these three yarns has strong and weak points. In this case, one might be inclined to make a decision based on the price. USTER TENSOJET 4 11 (20)

12 Yarn 1 Yarn 2 Yarn 3 Yarn 4 Tenacity R H cn/tex 20,5 20,5 22,1 21,7 USTER TENSOJET 4 CV(R H ) % 14,2 10,4 9,6 10,8 Elongation ε H % 8,7 8,7 8,4 8,8 CV(ε H ) % 13,5 10,4 10,2 11,6 Tenacity R H cn/tex 18,2 18,4 19,5 19,5 USTER STATISTICS % USTER TENSORAPID 3 CV(R H ) % 12, ,5 10,8 USTER STATISTICS % Elongation ε H % 8,8 8,9 8,7 9,3 USTER STATISTICS % CV(ε H ) % 14,6 10,2 8,7 8,9 Evenness CV m % 22,0 19,2 18,8 19,7 USTER TESTER 3 USTER STATISTICS % > Thin places 1) 1/km Thick places 2) 1/km Neps 3) 1/km ) (-35%, -40%, -50%) 2) (+35%, +50%) 3) (+140, +200%) Table 1 Quality parameters of the yarns to be selected for purchase Fig. 9 shows a comparison of the warp end break frequencies related to 10 5 picks, and the force/elongation scatter plots of the USTER TENSOJET [6], which are based on 50,000 measurements per yarn lot. The limits of the critical range indicated in the scatter plot are set at 12.0 cn/tex and 5% elongation. It can be seen quite clearly that yarn 1 has far more weak places within the critical range than the other yarns, whereas yarn 3 shows the most compact scatter plot. With the aid of the mean values in Table 1, it is possible to make a preselection, but only the weak place analysis will provide reliable information about the expected behaviour in subsequent processing. 12 (20) USTER TENSOJET 4

13 Weaving machine TENSOJET Stops/100K picks Y1 Y2 Y3 Y4 3 End break rate for yarns Y1 to Y4. = Forbidden zone / Range of peak load on weaving machine Source: R. Hines, ITT Fig. 9 Weaving machine stops and stress/strain scatter plot The fabric manufactured in this example is a plain weave article with the mentioned ring-spun yarn 50/50 PES/CO, Ne 35/1 in the warp (4800 threads, 30 threads/cm) and a ring-spun yarn 100% CO, Ne 35/1 in the weft (26 threads/cm, 160 cm weaving width). The speed of the weaving machine is 700 rpm. The warp stoppage costs related to 10 5 picks amount to USD 0,66 per stop. With a production time of 6'000 hours per year, this results in yearly costs of USD 1'666.- per weaving machine. It is really necessary to be aware of the fact that especially a warp end break will always be a visible fault in the finished textile fabric, which will have to be tolerated or repaired. This will either result in a loss of quality or an increase in costs, which we will not discuss further at this point. Table 2 shows a comparison of the stoppage frequencies and the corresponding yearly stoppage costs of the four yarns under evaluation. The line below indicates by how much yarns 1, 2 and 4 would have to be cheaper in order to fully compensate the lower stoppage costs of yarn 3 via the yarn price. In this calculation, the article is running on 30 weaving machines, which results in a warp yarn requirement of tons per year. Based on the TENSOJET weak point analysis, the decision would be made in favour of yarn 3 for technological reasons and as a result of the lower stoppage costs. However, should the yarn 2 actually be offered with a price reduction of 0,23 USD/kg, then it would be a cost-neutral alternative. Under the assumption that the price for yarns 2 and 3 is the same and that the decision, based on the present facts, has actually been made in favour of the yarn 3, we can compare the savings of 0,23 USD per kg of yarn with the investment cost of a TENSO- JET. USTER TENSOJET 4 13 (20)

14 Yarn 1 Yarn 2 Yarn 3 Yarn 4 Warp stops per ,5 2,2 1,1 3,0 Warp stop costs per year and machine Price differential per kg yarn required for identical production costs USD 20' ' ' '998.- USD 2,42 0,23 0,00 0,40 Table 2 Warp stoppages, stoppage costs and price differentials The yearly savings, after deducting the operating cost of USD 5'000.- and the labor cost for a part-time employee of USD 8'000.- and assuming a capital interest rate of 10%, result in a pay-back time of only 2.8 years. This practical example, based on only one single article, proves quite clearly that the USTER TENSOJET allows the user to ideally organize the yarn buying in the weaving mill, and this from both a technological and an economical point of view. Concerning the applied warp material, it should be noted that the tests were carried out with untreated yarn. It has been observed that, as a rule, the size application will increase the breaking force and decrease the elongation at break. Weak places, however, will neither be caused nor eliminated by sizing. 5.2 Warp yarn (100% CV, Nm 50/2) Fig. 10 and Fig. 11 show the scatter plots of two ply yarns of viscose Nm 50/2, which are offered by different suppliers. The testing included 22 packages of each yarn, with 10'000 tensile tests per package. From the force/elongation scatter plots, it can be seen quite clearly that the yarn of supplier B has far more weak places than the yarn of supplier A. Fig. 10 Scatter plot of the yarn manufacturer A 14 (20) USTER TENSOJET 4

15 Fig. 11 Scatter plot of the yarn manufacturer B Table 3 shows a comparison of the TENSOJET measurements and the warp end break frequencies related to 10 5 picks for the yarns of suppliers A and B. The material was woven into a jacquard fabric with a width of 224 cm, machine speed 340 rpm, weft material PP. The evaluation showed obvious differences with regard to both, yarn characteristic and stoppage frequency. Manufacturer A Manufacturer B Tenacity R H cn/tex 24,1 22,0 CV (R H ) % 6,1 7,7 R P=0.01 (n=22) cn/tex 17,8 12,9 R P=0.05 (n=110) cn/tex 20,5 15,0 Elongation ε H % 15,7 14,4 CV(ε H ) % 6,0 9,4 ε P=0.01 (n=22) % 10,5 4,8 ε P=0.05 (n=110) % 11,5 7,3 Warp stops per 10 5 picks 4,9 8,2 Table 3 Quality parameters and warp stoppage frequencies of the yarn to be selected for purchase With the following calculation, we would like to prove that it is not possible to determine the really dangerous out-layers, that is to say weak places, with statistical methods. For that purpose, we look at the mean values of the tenacity and the corresponding coefficients of variation which are used in this example (Table 3). These are the common parameters which are used for describing the tensile properties of a yarn and which can be determined with every tensile testing unit. In addition, the table contains the percentile values of the USTER TENSOJET and the determined stoppage figures of the weaving machine. USTER TENSOJET 4 15 (20)

16 For the further calculation of this example, we use the minimal requirements for the breaking force of 500 cn (12.5 cn/tex) and 5.0% for the elongation at break. For the standard normal distribution with µ = 0 and σ = 1, the standard normal variable is z = (x - μ) / σ In diese Gleichung setzt man für μ den Mittelwert der Garnfestigkeit oder - In this equation, µ is to be replaced by the mean value of the yarn strength or elongation, x represents the minimal requirement and σ can be calculated from the coefficient of variation. With one-sided z-tests and with the aid of the respective statistical tables [7], one determines the probability that the standard normal variable will assume a value which is smaller than or equal to z. This value describes the probability of the occurrence of a weak place with a value below the minimal requirement x. In Table IV, the calculated z-values and probabilities are listed and compared to the weak point analysis of the TENSOJET. These figures clearly show how important the new form of representation of the force/elongation scatter plot and the calculation of the percentile values are. It is the only way to carry out a really reliable weak point analysis since the statistics and the theory of probabilities have their obvious limitations. With respect to our example, this means that the use of conventional statistical methods for decisionmaking would have made it quite acceptable to buy the yarn B because the tests show practically no weak places. With USTER TENSOJET, however, it is possible to reliably detect those seldom-occurring events which do not have a normal distribution. z P(z) one-sided Weak places according to probability calcuation USTER TENSOJET weak places Minimum requirement tensile strength: 500 cn (12,5 cn/tex -1 ) Manufacturer A -7,9 1, practically no weak places No weak places in the scatter plot Manufacturer B -5,6 1, practically no weak places 15 weak places per 220'000 tensile tests Minimum requirement elongation: 5% Manufacturer A -11,3 6, practically no weak places No weak places in the scatter plot Manufacturer B -6,9 2, practically no weak places 35 weak places per 220'000 tensile tests Table 4 Weak place analysis by probability calculus and by USTER TENSOJET 16 (20) USTER TENSOJET 4

17 6 Summary The tensile testing of staple fiber yarns with USTER TENSOJET is specifically designed to meet the requirements of the subsequent processing on modern high-performance machines. This is reason enough from a technological and an economical point of view to examine the use of a highperformance tensile testing installation also in the modern weaving mill. With the preceding practical examples, it has been shown that the use of the high-performance tensile testing allows the user to reliably identify the yarn which is perfectly suitable for the intended application and, at the same time, has a reasonable price. The example of a pay-back analysis shows an amortization time which is remarkably short for testing equipment. Considering that modern weaving mills are usually equipped with at least 50 to 250 weaving machines, which quite often produce an immense variety of articles, the actual amortization time will probably be much shorter. Any attempts to obtain information on the behaviour in subsequent processing and on weak places or machine stoppages with the aid of traditional sampling and test procedures will fail to provide the desired results, even though it is still considered state of the art. The generally known and applied theory of probabilities and mathematical statistics offer no reliable data for the evaluation of seldom-occurring events which do not have a normal distribution. The traditional tensile test will still play an important role in the foreseeable future. This applies in particular to the testing of filament yarns, highstrength technical yarns, fancy yarns, etc. Furthermore, special trials such as piecing or splicing strength and tensile elasticity are clearly reserved to the far more flexible, traditional tensile testing. Besides, if someone insists on a test according to DIN , then he will of course not be able to benefit from the enormous evaluation capacity of the modern highperformance tensile testing. In the weaving sector, however, it is only a question of time until the high-performance tensile testing will be firmly established and eventually replace traditional systems. USTER TENSOJET 4 17 (20)

18 7 Litarature [1] Frey, M.; Furter, R.: Qualitätsmanagement in der Spinnerei USTER News Bulletin Nr. 39, Uster Technologies, August 1993 [2] Krause, H. W.: Werden als Folge der höheren Tourenzahlen bei Webmaschinen bessere Garne benötigt? [ETH Zürich] Textil Praxis International März 1977 [3] Bergold, K.: Einfluss von on-line-geprüften Qualitätsmerkmalen in der Spulerei auf die Garn- Weiterverarbeitung Abschlussarbeit (1994), Fachhochschule Reutlingen, Deutschland [4] Krause, H. W.; Erfassung und Prognose seltener Nöbauer, H.; Shaheen, A.: Störereignisse an Webmaschinen [ETH Zürich] Melliand Textilberichte, August 1990 [5] Weissenberger, W.: Prozessübergreifende Qualitätssicherung aus der Sicht vom Gewebe zum Garn Melliand Textilberichte 4/93 [6] Hines, R.: The Effect of Tensile Variation on Warp Yarn Performance [ITT] Graduate Thesis (1994), University of North Carolina at Chapel Hill, USA [7] Abramowitz, M.: Handbook of Mathematical Functions with Stegun, I. A.: Formulas, Graphs and Mathematical Tables Dover Publications Inc., New York USA 18 (20) USTER TENSOJET 4

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