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Staying pure at the cotton mill

Compressed air keeps cotton free of foreign fibers.

Edited by Mary C. Gannon,
senior associate editor

Staying pure
Select figure to enlarge.

Raw cotton often contains foreign objects, such as polypropylene scrap, or dirt. Plants that spin, weave, knit, and assemble garments spend enormous amounts of money each year to rectify damage caused by such objects, so these contaminants must be removed early in cotton processing.

Trtzschler GmbH & Co. KG, which was founded in Mnchengladbach, Germany in 1888, specializes in the equipment used by spinning mills and the non-woven industry — and it has developed a proprietary solution for immaculately clean cotton. Engineer Robert Tbben, in charge of machine development and engineering, explains how it 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
The cotton and foreign fibers it contains pass over a drum rotating at speeds up to 16 m/sec. Trtzschler has developed technology for this high-speed precision work: compressed air is used to shoot the foreign fibers away from the cotton. A nozzle beam ejects what high-speed cameras have detected, and what image processing software has identified as an imperfection. If the camera and software recognize a foreign fiber in the path of pneumatic valve number six, for instance, the software calculates when this foreign object will pass over valve number six — which opens to blast away the foreign object. “We developed this nozzle beam in close cooperation with Bosch Rexroth,” explains Konrad Temburg, Group Manager at Trtzschler.

Sorting out the chaff
To help minimize the amount of good cotton fiber that is blown out with the contaminant, the software processing the images must determine the exact moment and position needed to blow the foreign object away. “Drum speed, rotation angle, the valves’ response, and opening times all 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 are used, depending on the application and machine model. “Rexroth technicians have managed to install two versions in the same basic aluminum body so the only external differences are the orifice diameters and the space between the nozzles,” says Temburg. In the version with 64 valves, this spacing is 1 mm; where 32 valves are used, the spacing is 11 mm.

To keep machine design uniform, width is always 1.6 m. Temburg sees the major advantage, however, in the valves’ quick opening and closing actions, at just about 10 msec. “We use quick-acting diaphragm valves here,” he says. “They are twice as fast as gate valves.”

Tougher than tough
The system must not only be 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°C — is not simple. “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.

Tbben concludes, “Our users 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 downtime. Once all that’s been clarified, and everything is up and running, our users 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.”

For more information, contact Heinz Gottstein, Bosch Rexroth, Germany, at [email protected] or visit