Natural fibres have been used since thousands of years but there are not enough of them to meet the demands of today’s world population [1]. Therefore the invention of synthetic fibres was undoubtedly one of the most important discoveries of the twentieth century.

Another point is the competition in the textile sector. In order to satisfy new demands of the customer, various improvements in the production technology of filament yarns and their characteristics have been made and the filament yarn industry has become more and more important.

This article deals with the measurement of mass variations of filament yarns and with the interpretation of test results. In spun yarns, yarn evenness is inevitable because it arises from the fundamental nature of the fibres, their manufacturing methods and the arrangement of fibres in the yarn. In case of filament yarns, granulate heterogeneity, spinning process irregularity, faults in subsequent yarn cooling and winding operations, together with machine defects and drafting faults quickly lead to mass variations which can affect the mass of the yarn over thousands of meters of material, because the production speed is very high. Such variations often cause difficulties in the subsequent processes like draw-twisting, texturing or dyeing. These mass variations reduce the quality of the yarn, and a comprehensive evenness testing is a must for a quality control system [2].

Uster Technologies has been manufacturing textile testing systems for measuring filament yarns since 1955. The USTER® TESTER 5-C800 for filament yarns was introduced in the market as the 5th generation in 2005. The quality characteristics of filament yarns can be quickly assessed by means of this testing system. The test results can also be used to judge the consequences on subsequent processes.

Frequent sources of faults in the melt spinning process

Filament yarns can be manufactured according to different spinning methods. In Table 1 these methods are mentioned below.

Table 1. Different spinning methods of filament yarns [2]

In this article we will concentrate on the melt spinning process. Melt-spinning is described as the simplest method of filament yarn manufacturing because it does not involve problems associated with the use of solvents [2]. 

Many scientists have worked on the analyses of melt spinning, both theoretically and experimentally[2,4,5,6,7] .The spinning of synthetic fibres is sensitive to any variation of process parameters, as for example, the temperature of the polymer melt or the temperature of the quench air of the spinning unit[6]. Particularly, the conditions in the quenching zone influence the formation of a suitable filament yarn[5].

As we can see in Figure 1, in melt spinning there are mainly three stages; hopper, spinning and wind-up[3]. 

In the hopper stage, the raw material is stored, melted and then processed. The starting material for melt-spinning is polymer granules or chips and they are first dried and then melted in the extruder. Today in modern plants, polyester and polyamide are produced in continuous polymerisation units in which the melt is directly transported from the final polymeriser to the melt-spinning unit. Polypropylene is different because polymerisation leads to a solid product. For this reason it is separated from the spinning process[2]. 

In the spinning stage, every time the same amount of homogenised and filtered melt, which is transferred from extruder to the spinning pumps, is pressed through the orifices in the same amount of time. After the spinning heads, at a distance of 5 to 20 cm below the orifices, in the quench air duct, the filaments spun from the melt are cooled by a jet of air and freeze. When using a multiple of orifices in the form of a spinneret, the bundle of filaments can be drawn off as an undrawn or partially drawn filament yarn[3]. 

In the wind-up stage, after leaving the quench air duct, the filament material is drawn over preparation rollers through an oil-fat-emulsion (spin finish). The filament bundle, which has passed the spin finish application, is wound on spinning packages and can be transported in this state to other processing machines[3]. 

The localisation of faults during this spinning process will be illustrated by taking the example of polyamide yarn manufacturing. 

In Figure 1, the faults resulting from certain machine groups as indicated with a “Circle” and a “Triangle” can be determined with the help of the diagram, spectrogram, evenness value, variance-length curve and relative count. The faults at machine groups which are indicated with a “triangle” can also be determined with measurements undertaken throughout one full package with the help of the diagram, spectrogram, evenness value, variance-length curve and relative count[3]. 

The maximum mass deviations from the nominal value within the test length can also be measured[3]. 

Evenness testing of filament yarns and USTER® STATISTICS for filament yarns 

As we have mentioned before, the quality characteristics of filament yarns can be quickly determined by using USTER® TESTER 5-C800 for filament yarns. The test results which are obtained from this measuring system, can be evaluated both in graphical and numerical form. 

Diagram

The diagram is an important part of evenness test and provides an enormous amount of details on the spinning process for a filament yarn specialist. Figure 2 shows diagrams of two different filament yarns. The lower filament yarn has a very high mass variation compared to the upper yarn. 

The evenness of the yarn of the upper diagram in Figure 2 was CVm = 1,15%, the evenness of the lower diagram was CVm = 2,60%. The source of the high mass variation of the lower diagram was a significant problem with the air intensity and air guiding in the quench air duct which causes intensive vibration during the solidification of the filament yarn. 

Numerical results of filament yarns 

Table 2 shows a selection of the result columns of a filament yarn test. Yarn: Polyester, dtex 76f100. The test was carried out at 10 POY packages; test length was 1000 m per package. The value U% is the evenness; the value CVm is the coefficient of variation of the yarn mass while the measuring system was set to "Normal test". The values CVm 1 m, 3 m, 10 m and 50 m represent the coefficient of variation of the yarn mass of various “cut lengths”. 

Table 2. Numeric results of a filament yarn test 

The column "Rel. Count" describes the relative fineness of the yarn. The testing system calculates the mean of the yarn fineness for the entire measuring series and always prints out zero as a mean value. Afterwards, the system calculates the deviation of each individual package relative to the mean. The basis for this calculation is the capacitive measurement of the mass over the entire test length. The columns mMin and mMax describe the maximum deviation from the mean value during the tests. 

Spectrogram 

The spectrogram as shown in Figure 3 is a representation of mass variation in the frequency domain, ie, the measuring system detects periodic mass variations. Figure 3 shows the spectrogram of the polyester filament yarn, dtex 76f100, described in Table 2.

The spectrogram taken from the USTER® TESTER 5-C800 shows a significant periodic fault with a wavelength of 1.2 m, which was caused by an eccentric spinning package during wind-up. The second severe quality problem is shown in Figure 3 as an increase of the spectrogram between 10 and 80 m. Such mass variations sometimes lead to misinterpretations if one only checks the diagram because the variations look like strictly periodic faults in the diagram. Only the spectrogram shows precisely what is happening. The origin is a non-optimised air stream in the quench air duct (Figure 4) as already mentioned above. 

The quenching zone of the spinning machine is very important. A non-optimised air system in the quench air duct is one of the most frequent sources of considerable mass variations of filament yarns. Since the take-off of filament yarns takes place at very high speed, the cooling process in the quench air duct has to be efficient. If the air stream is not conducted properly the individual filaments start to vibrate. Because the filaments are not solidified at this point of the manufacturing process, the vibrations cause mass variations[3]. 

Figure 5 is the recording of 10 spectrograms of the described filament yarn of 10 packages from the same spinning machine. All the spectrograms show that the faults are common to all packages. 

Spectrograms of filament yarns frequently have many peaks, which have to be interpreted correctly. Several peaks in the spectrogram do not necessarily mean that there are several manufacturing problems. The correct interpretation of the peaks, however, can provide detailed information where manufacturing problems exist. In order to find the correct origin of the manufacturing problem, the USTER® TESTER 5-C800 also has a Knowledge Based System which simplifies the interpretation of the spectrograms. 

Benchmarking for polyester and polyamide filament yarns 

The USTER® TESTER 5-C800 also supports the user with experience values which can be used for benchmarking and evaluation. Figure 6 shows the USTER® STATISTICS of polyester and polyamide filament yarn tests. It represents the evaluation of mass variations of various packages. The coefficient of variation depends on the fineness (dtex) of the individual filaments in the filament bundle and on the amount of periodic and non-periodic mass variations. Figure 6 can be used for partially oriented yarn as well as for fully oriented yarn. 

Conclusion 

As a result of continuous improvements in the filament yarn industry, the demand of reliable and reproducible test methods for the filament yarn industry has also increased. Especially the yarn evenness is still a very important quality parameter in the area of filament yarns since small mass variations can already have a considerable effect on the appearance of fabrics, particularly after dyeing. As it is well-known, the evenness values of filament yarns can drop below CVm = 1%. This means that even the smallest deviations can have an adverse effect on the product quality in the subsequent processing. Uster Technologies began to test the evenness of filament yarns at a very early stage. With the USTER® TESTER 5-C800 for filament yarns, the quality characteristics of yarns can be assessed quickly and worldwide compatibility of the same types of yarn can be guaranteed. This is especially important because, in the high-performance production of filament yarn spinning mills even a small reduction in quality can result in disastrous financial losses. 

Literature 

1. Schenek A: Chemical Fibres – The Replacement of Cotton? – Facts, Limits and Trends”, STF Anniversary Colloquium ‘99, Swiss Textile, Clothing and Fashion College, Switzerland,1999. 
2. Gupta V B, Kothari V K: Manufactured Fibre Technology, Chapman & Hall, 2-6 Boundary Row, London, First Edition, 1997. 
3. Uster Technologies AG, Application Manual, Testing of Filament Yarns,V1.0, No.410 107-04020, August 2006. 
4. Fourné F: Synthetische Fasern: Herstellung, Maschinen und Apparate, Eigenschaften; Handbuch für Anlagenplanung, Maschinenkonstruktion und Betrieb, Carl Hanser Verlag, München, Wien, 1995. 
5. Harder C: Finite Element Analysis of Melt Spun Yarn, Journal of Materials Processing Technology, 2001,118, 454-459. 
6. IDESAKI A, et al: Fine SiC Fibre Synthesised From Organosilicon Polymers: Relationship Between Spinning Temperature and Melt Viscosity of Precursor Polymers, Journal of Materials Science, 36, 5565 – 5569, 2001. 
7. TAE HWAN OH, et al: Numerical Simulation of the Melt Spinning of Hollow Fibres, Textile Research Journal, 68, 6, pp 449-56, 1998. 

Dr Serap Dönmez Kretzschmar 
Uster Technologies AG, 
Uster/Switzerland. 

Richard Furter 
Uster Technologies AG,
Uster/Switzerland.