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| Issue date:01/12/2008 |
| ATA Journal for Asia on Textile & Apparel - Dec 2008 Issue |
| Source:Journal for Asia on Textile & Apparel |
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by Dr Serap Dönmez Kretzschmar and Richard Furter
To make a thorough assessment of yarn’s quality level, various physical yarn characteristics should be measured in the laboratory. The results based on laboratory tests can be used to predict the performance of a yarn in subsequent processes. Two of the most important yarn characteristics are the tensile strength and the elongation.
Tensile force and elongation values can be used for prediction of the suitability of a yarn for a designated textile process. For example, yarns used on a weaving machine require a considerable strength, whereas the yarns processed on modern knitting machines demand better elongation properties.
Ring yarn, carded, Ne 24, cotton 100%, 10,000 tests, with a high amount of weak places |
A high-speed strength testing system is thus used to give a detailed analysis of yarns quickly. It can be used for both knitting and weaving yarns for detection of weak places in the yarn which cannot be detected with evenness testing but later can cause problems during the weaving and knitting production.
Such a tester, e.g. Uster Tensojet, brought a new understanding to the textile community, particularly in the area of isolated weak places. It was demonstrated that these weak places correlate with the end breaks of yarns on weaving machines. In order to find these weak places it was necessary to increase the testing speed considerably. Therefore, it is not only required to know the mean strength and mean elongation of yarns, but also the weak places.
Ring yarn, carded, Ne 24, cotton 100%, 10,000 tests, with hardly any weak places |
With two examples of yarns shown in the following two tables, a scatter plot of strength and elongation with many weak places is demonstrated, while the second table shows the opposite.
Each dot represents a strength test with a defined force and elongation value. The high number of single events on the left hand side of the scatter plot in the first figure indicates that the spinning process is not under control. There is a high risk that these weak places will lower the efficiency of the weaving machines considerably. There is a high probability that the weaving process will be time-consuming because a high number of end breaks have to be repaired.
Knittability measured through strengthtesting
Spinning mills might think that strength testing is not required for knitting yarns, because the force applied on yarns during the knitting process is considerably lower than for weaving processes. Contrary to this perception, researchers studied the physical characteristics of yarns (including strength and elongation) and found that these characteristics play an important role in the knitting performance of a yarn.
Physical properties of 100 % cotton ring-spun yarns |
According to researchers, the strength of the yarn must be sufficient to resist tensile strain occurring during the knitting process, especially during withdrawal from the cone and in the region of needles, sinkers and cams during stitch formation. They also stated that yarn elongation is necessary, so that it can resist the bending strains or neutralize them by getting extended in such a way that the yarn does not break. The high speed of needles of modern knitting machines increases the tenacity requirements of yarn. In staple fiber yarns the strength is to a very large extent directly proportional to the level of twist inserted during spinning to a twist multiplierαe of about 4,5.
When we consider knitting performance of a yarn, we have to explain also the term “knittability”. The knittability of a yarn expresses if the knitting process with a yarn is easy or not.
Knittability is a factor that affects directly stop-motions of a knitting machine, productivity of knitting process, quality of knitted fabric, and production costs. This property does not only depend on the characteristics of fiber from which yarns produced but also on its own physical and structural properties. It is very important to determine yarns having lower degree of knittability before knitting.
Evaluating the relation between yarn properties, friction values and fabric faults |
Other researchers have also investigated the knittability of a yarn before knitting.
About 30 different cotton yarns, many of them of commercial value, were used in the experiments. 12 of the yarn samples (100% cotton, ring-spun yarns) were selected and given (see tables on top right).
All the yarns were knitted into 1x1 rib structures and fabric take-down and machine speed values were kept constant. Samples were knitted at three different loop length values, representing a range of tight, medium, and loose fabrics. The knitting process was observed during 2000 revolutions, and the number of machine stops, yarn breaks, and holes were recorded. By using multiple regression analysis, yarn-yarn and yarn-needle friction equations that depend on yarns characteristics were found. In the last stage, they used these equations, machine tightness factor, and machine speed values in order to determine knittability.
According to statistical analyses, researchers have found that yarn characteristics have a significant effect not only on the fabric quality, but also on the friction values. During the knitting process, from bobbin to knitted fabric, yarn-yarn and yarn-needle friction is occurring between yarn and parts of the knitting machine. This causes an increase in yarn tension. The increase in yarn tension causes a higher number of yarn breakages and a decrease in productivity of the knitting process. For this reason, it is expected to retain yarn-yarn and yarn-needle friction at the lowest possible level.
Moreover, some spinning deficiencies cannot be found by measuring the evenness alone, but can be detected with a high-speed strength testing system, e.g. Uster Tensojet. Defects like a reduction of strength and elongation cannot be detected by evenness testing, as the defect does not lead to a mass variation.
Results of a Uster Tensojet, some bobbins of slow spindles |
The graphics on the right demonstrates an example of cones with bobbin from the same spinning machine with a poor preventive maintenance. The ring-spinning machine had various contaminated spindles with the result that these spindles did not produce yarns with nominal twist (slow spindles). (Yarn: Cotton 100%, carded, Ne 16, ring-spun yarn.)
The two stroke diagrams show the variation of the strength and the elongation. The manufacturing problems are mostly more pronounced in the stroke diagram of elongation. A stroke diagram represents each single test as a vertical line. The length of the line is equivalent to the force and elongation. The diagrams also indicate that nine bobbins were tested, and 1000 tests were carried out at each bobbin. Due to the manufacturing problems, the scatter plot in the bottom graph is very long.
With a high-speed strength testing system, strength tests can be synchronized with evenness tests so as to give a comprehensive picture of the production, by recognizing weak places in the knitting and weaving yarns, as well as their physical yarn characteristics.
Source: Uster Technologies AG
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| Copyright © Adsale Publishing Limited. Any party needs to reprint any part of the content should get the written approval from Adsale Publishing Ltd and quote the source "ATA Journal for Asia on Textile & Apparel", Adsale Textile English Website - www.AdsaleATA.com. We reserve the right to take legal action against any party who reprints any part of this article without acknowledgement. For enquiry, please contact Editorial Department. |
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| Copyright © Adsale Publishing Limited. Any party needs to reprint any part of the content should get the written approval from Adsale Publishing Ltd and quote the source "ATA Journal for Asia on Textile & Apparel", Adsale Textile English Website - www.AdsaleATA.com. We reserve the right to take legal action against any party who reprints any part of this article without acknowledgement. For enquiry, please contact Editorial Department. |
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