100 % cotton
High-quality cotton has to be entirely free of foreign fibers. That’s why Trützschler, a German company specializing in textiles processing machinery, aims compressed air at these unwanted contaminants.
Raw cotton often contains foreign objects such as polypropylene scrap or simple dirt. To achieve a quality product, these contaminants have to be removed in a basic cleaning process. Firms involved in spinning, weaving and knitting and in assembling garments spend enormous sums each year to rectify damage caused by such objects – and thus to avoid customer complaints.
The Trützschler GmbH & Co. KG was founded in Mönchengladbach, Germany, in the year 1888. It is the market and technology leader in machinery used to prepare fiber for spinning. The company specializes in the equipment used by spinning mills and the non-wovens industry – and it has developed a proprietary solution for immaculately cleaned cotton. Engineer Robert Többen, in charge of machine development and engineering at Trützschler, explains how that works. “We use two cleaning steps. In the first pass the colored and high-contrast foreign objects – bits of fabric, twine, film, wood, paper and the like – are ejected. In the second step the white, colorless and transparent polypropylene will also be detected and removed.”
Nozzles shoot “live ammunition”
What sounds fairly simple is in fact a real challenge since the cotton and the foreign fibers it contains pass over a drum rotating at speeds of up to 16 meters per second. Trützschler has developed special technology for this high-speed precision work: compressed air is used to “shoot” the foreign fibers away from the cotton. Here the so-called “nozzle beam” ejects what high-tech cameras have detected and the image processing software has identified as an imperfection. If the camera and software recognize a foreign fiber in the path of pneumatic valve number 6, for instance, the software will calculate when this foreign object will pass over valve number 6, which is then opened to blast away the foreign object. “We developed this nozzle beam in close cooperation with Rexroth,” explains Konrad Temburg, Group Manager at Trützschler.
Sorting out the chaff
To keep down the amount of good cotton fiber that is blown out at the same time, the software processing the images will have to determine the exact moment and position needed to blow the foreign object away.
“Drum speed, rotation angle, the valves’ response and opening times – all these have to be calculated in a split second and then coordinated. If the valve does not close quickly enough after the blast, then the air stream will also remove good fibers that ought to remain in the process. And that”, explains Temburg, “is exactly the effect that customers complained about again and again in the past.
Two in one
Nozzle beams with 32 or 64 valves will be used, depending on the application and the machine model. “Rexroth technicians have managed to install two versions in the same basic aluminum body so that the only external differences are the orifice diameters and the space between the nozzles.” In the version with 64 valves this spacing is one millimeter; where 32 valves are used, the spacing is eleven millimeters. The installed, 1.6 meters in width, is always the same. This is in the interest of uniform machine design. Temburg sees the major advantage, however, in the valves’ quick opening and closing actions, at just about ten milliseconds. “We use quick-acting diaphragm valves here. They are twice as fast as gate valves.”
Tougher than tough
The system has to be not only fast and precise, but rugged, as well. Extended service life – given the adverse environment found in spinning mills with fiber-laden air and temperatures of up to 50 degree Celsius – is hardly a simple matter of fact. “In such settings we have to depend on the components working all the time, without fail, and that is true for Rexroth Pneumatics,” Temburg emphasizes.
Robert Többen summarizes: “Our customers are interested in how effectively foreign objects can be recognized in the process and eliminated without removing desirable fibers. They also want to know how reproducible the sorting process is and whether the machine will run without downtimes. Once all that’s been clarified and everything is up and running, our customers boast this fact in their advertising, stressing that this detection technique lets them offer yarns and woven and knit materials free of any foreign objects.”
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