Detection and Removal of Contaminations

The Degree of Contamination is an important Quality Parameter

The success of Asian spinning mills depends more and more on the quality of their product. Only if they reach the world class quality level, they will be able to defend or even expand their exports. In these circumstances it cannot surprise that substantial investments have been made in order to improve the quality of the yarn produced. One of the most important measures taken was the purchase of systems to reduce foreign contamination to the absolute minimum. This was particularly necessary since the contamination present in the local raw material unfortunately is quite high.

Sorting Machines and Yarn Clearers

Two solutions are proposed to fight contamination in the raw cotton once it has arrived in the spinning mills and has to be processed: Removal systems (sorting machines) in the blow room and electronic yarn clearers equipped with the appropriate FF (Foreign Fiber) channel.

Removal systems in the blow room line focus on the bulk of the contamination. Such systems have not been designed to detect and remove small particles (in the range of several millimeters such as individual fibers). The removal of such parts is the task of yarn clearers.

The use of only yarn clearers can only be considered in the event of very clean cotton. Normal cotton (such as Asian and African cotton) will contain so much contamination that it cannot be removed without an excessive number of clearer cuts ie. without a loss of machine efficiency which cannot be accepted.

Depending on the quality requirements one system in the blow room line may, however, be sufficient particularly where manual picking is still used during the opening process.

In case of stringent requirements and of heavily contaminated cotton both measures are recommended: The sorting machine in the blow room will act as a coarse filter removing the bulk of the contamination, the yarn clearer focusing on the remaining particles which can be eliminated with an acceptable number of cuts.

Loptex Sorter

One of the leading manufacturers of sorting machines is the company Loptex srl, located in Italy with design and manufacturing facilities in Switzerland. The LOPTEX SORTER has been introduced to Asia starting in 1999. Since then about 200 Sorters have been sold throughout Asia.

The success of the Loptex Sorter is due to a combination of several key features:

• The most effective position within the blow room line

• No interruption of the fiber flow

• Detection of coloured contamination by optical means

• Detection of colourless contamination by ultrasonic means

• Ejection of the contamination by individually controlled pneumatic valves.

The effect of this combination has been so successful that most suppliers of competitive systems have adapted similar solutions in the meantime.

The Most Effective Position in the Blow Room Line

The degree of opening of the cotton tufts is the controlling parameter for selecting the optimal position in the bow room line. The better the degree of opening, the smaller the risk that contamination is hidden within the tufts. Experience shows that a system at the beginning of the blow room line will detect only about 20% of the particles which can be detected at the end of the line.

The degree of opening is important not only in respect to the detection performance but also in respect to the loss of good fibers. If the cotton is not well opened, larger cotton tufts will have to be removed. The loss of good fibres at the beginning of the line is about 5 times higher than the loss at the end of the blow room line (about 3 to 8 gr per removal cycle as compared to 0.5 to 1 gr). In case of a system at the beginning of the line, it will be necessary to reduce the loss of good fibres by subsequently manually sorting the removed material.

Blow room lines frequently start with one bale plucker and are afterwards divided in two lines. In such cases two removal systems would be necessary at the end of the line whereas only one system would be needed at the beginning.

To minimize the financial consequences Loptex suggests its "Tandem" version. This has been designed using the standard version, but dividing it in two autonomous halves. Each half can be set at different sensitivities which permits the processing of two different qualities, each with a production of 400 kg/h.

Detection by Optical Means

All systems detect contamination by optical means. Yarn clearers and the Sorter of Loptex use photosensors and detect the contamination as being darker than the cotton. Other sorting machines use colourcameras and detect contamination as being different in colour. The difference in practical performance is insignificant. Cameras, however, are more delicate and costly (for repairs and replacements) and, if they fail, will cause a total break down of the system. In the opposite case an array of photosensors is used, in case of the Loptex Sorter 256 units are arranged in two arrays. The default of a photosensor will not lead to a total break down, but only slightly reduce its performance. The defective photosensor will be signalled by the Sorter. Since the photosensors are arranged on separate modules, the replacement will be easy and cost effective. In some cases cameras will not monitor the fiber flow directly but only indirectly through mirrors. Mirrors attract, however, dust and need to be cleaned.

The critical point, however, is that an optical sensor can only see what is visible, meaning that it cannot detect contamination which is hidden within the cotton tufts. To compensate this handicap most systems,like the Loptex Sorter, use two optical sensors each positioned at the opposite side of the pipe. This permits to detect contamination which is located on the back of a cotton tuft. In most cases this will be sufficient. However, if the opening of the raw material is poor, it can happen that the contamination is hidden inside of the tuft.contamination which offers no contrast to the cotton ie. colourless contamination or contamination of the same shade. Unfortunately one of the most harmful contamination, namely packing material in polypropylen, comes in whitish shades which does not offer a sufficient contrast to the raw material.

Detection by Ultrasonic Means

Everyone knows the phenomena of echo. Louds will be reflected by a rock wall. The degree of reflectance of accoustic waves depends on the surface structure of the object in their path. The Loptex Sorter makes use of this principle. It detects contamination independent of it colour on the basis of its surface structure. Most contamination has a denser surface structure than loose cotton tufts, in particular plastics. The sensor consists of a number of emitters of ultrasonic therefore not hearable waves. The receiver will receive waves which are reflected by the contamination contained in loose cotton. If no contamination is present, the ultrasonic waves will be absorbed in the absorber box located on the other side of the pipe.

Ejection by Pneumatic Valves

In case of the detection of a contamination being by the optical or the accoustical system the electronic control will activate pneumatic valves. It will take into account the transportation speed of the raw material and release the air blow after the necessary delay. The number of valves which will be activated is variabel. It depends on the size of the detected contamination. The air blow will be targeted since only the valves are activated which are located in front of the passing contamination. The contamination will be deviated through an opening in the pipes into the waste container of the machine.

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