DE20202780U1 - Device for removing foreign substances - Google Patents

Description

MFR-1221-02 21.02.2002

Device for the removal of foreign substances

The invention relates to a device for detecting and separating foreign substances from a fiber web formed on a carding machine, wherein the fiber material removed from a drum via a doffer is transferred to a subsequent pair of squeezing rollers.

Several such devices are known from the patent literature which show the detection and separation of foreign substances on a card. For example, DE-A1-196 27 194 describes an embodiment in which the fiber material removed from a card drum via a doffer (here: doffer roller) is transferred to the subsequent nip roller. The fiber material deposited on the doffer is released from the doffer roller set via a nozzle comb and transferred into a channel shown schematically, via which the fiber material is transported to the clamping line of the subsequent nip rollers. A sensor is assigned to this conveyor channel following the nozzle comb, which examines the fiber material, or the fiber fleece, for foreign fibers or impurities. The sensor reacts to a color that differs from cotton. Compressed air nozzles are arranged following this sensor, which work together with opposite channels that are kept under pressure. As soon as foreign fibers are detected by the sensor, the following compressed air nozzles are controlled via corresponding valves, whereby the foreign substances are fed to a disposal facility via the opposite suction channel. The fibers removed by the customer

The material is transferred to the subsequent squeezing rollers in the form of a fiber fleece. This fiber fleece is very thin and has a low adhesive force. As described in the state of the art shown, the exposure of the fiber fleece to compressed air to remove the foreign fibers creates holes which can affect the band number of a fiber band to be formed subsequently and reduce the adhesive force within the fiber fleece. This can lead to subsequent problems in the further transport of the fiber fleece until the band is formed if the fleece is not guided optimally.

In addition, the fiber fleece does not have a compact and uniform structure due to the removal from the doffer via a nozzle comb. The same publication also shows a design in which the fiber material is removed from the doffer via a scraper roller that works together with a guide element shown schematically and is transferred to a subsequent pair of squeegee rollers. In this design, the sensors for detecting foreign fibers and a subsequent separation device (suction nozzles) are assigned to the outer circumference of the doffer.

Furthermore, an embodiment is known from EP-A1-606 626, whereby the fiber material taken over by a pickup is delivered to a subsequent pair of rollers. How the fiber material is taken from the pickup and transferred to the subsequent pair of rollers is not described here. The fiber fleece emerging from the pair of rollers is guided over a free space in which a monitoring device for detecting undesirable particles in the fiber fleece is provided. The fiber fleece then passes into a channel-shaped guide element, which is assigned a correspondingly designed separating device in order to remove the detected foreign parts. As already described, this embodiment does not show a solution for how the fiber material can be taken from the pickup and delivered in a controlled manner to a subsequent conveyor.

sensor for monitoring. The fiber fleece is not guided, particularly in the area of the monitoring device. This can cause problems in accurately separating the detected foreign parts with a time delay by the subsequent separating device. This design also shows another possibility, whereby the detection and separation can be carried out on the receiver's clothing.

DE-A1-196 04 499 shows a design in which the fiber material on a pickup is removed via a stripping roller and transferred via a guide element (fleece bridge) to a subsequent pair of squeezing rollers. With this design it is possible to present a compact and defined fleece to a monitoring device, which in this case is provided within the guide element designed as a profile in the form of a camera. However, a device for separating detected foreign substances is not provided in this design.

Furthermore, a design is known from DE-A1-39 28 279, whereby the fiber material removed from a pickup via a stripping roller is transferred to a subsequent pair of nip rollers. From this pair of nip rollers, the fiber material in the form of a fiber fleece passes via corresponding guide elements to a subsequent pair of transport rollers, via which the fiber material is delivered for subsequent band formation. In the area between the pair of nip rollers and the pair of transport rollers, a detection device for detecting disruptive particles in the fiber fleece is arranged. These disruptive particles are detected with this detection device and then fed to an evaluation device for evaluation. A separation device for separating the detected, disruptive particles is not provided here.

The invention is based on the object of eliminating the known disadvantages of the prior art and of providing a device for detecting

and separation of foreign substances, which on the one hand can be carried out in a compact design and on the other hand ensures gentle treatment of the fibre fleece on which the separation of foreign substances takes place.

This object is achieved on the one hand by the fiber material located on the doffer being stripped off via a stripping roller and being delivered to a guide element, via which the fiber material is transferred in the form of a fiber fleece to the subsequent pair of nip rollers and at least the separation device is arranged between the stripping roller and the clamping line of the pair of nip rollers.

With this design, it is possible to transfer the fiber fleece from the stripping roller via the guide element in a compact form to the pair of squeeze rollers, whereby a targeted and safe separation of detected foreign substances can take place in this area.

It is further proposed that the detection device for detecting the foreign substances is located opposite the surface of the doffer in the area of the fiber material removal. Since the fiber material on the doffer is completely stripped off by the subsequent stripping roller and transferred to the guide element, the time that the foreign fibers travel between detection and the separation device can be precisely defined. This makes it possible to carry out precise separation of the foreign substances.

It is also proposed that the detection device be arranged in the area of the guide element to monitor the fiber fleece guided over the guide element. This device enables the foreign matter to be detected and separated precisely, whereby the fiber fleece is also compact for the detection device and disruptive influences from clothing tips - for example when detecting on the doffer - do not have to be taken into account.

It is proposed to provide the detection device, which is connected upstream of the separating device, between the stripping roller and the clamping line of the squeeze roller pair.

This also provides good accessibility for maintenance purposes for the detection and rejection devices, for example.

If, as further proposed, the guide element is transparent in the area of the detection device, transmitted light can be used, with a light source being arranged on the opposite side of the detection device.

In order to ensure that the fiber fleece is gently transferred above the guide element between the stripping roller and the pair of squeezing rollers, it is proposed to install an additional guide roller in this area. The speed of this guide roller is coordinated with the speed of the doffer and the subsequent squeezing roller, so that the fiber fleece transported between the guide roller and the guide element is transported in a controlled and gentle manner. With this arrangement, it is possible to carry out a controlled detection and separation of foreign substances, especially if the detection device is arranged between the stripping roller and the guide roller and the separation device is arranged between the guide roller and the clamping line of the pair of squeezing rollers. The fleece can be held under a slight pre-tension, which supports precise separation.
The guide element is preferably provided with a guide surface for guiding the fiber fleece.

With regard to the separating device, it is proposed that the guide surface in the area of the separating device be provided with at least one opening.

via which the detected foreign substances can be fed to a collection device.

The collecting device can be provided in the form of a closed cavity which is connected to a negative pressure source, wherein the opening of the guide element opens into this cavity.

Another possibility for the separation of foreign matter can be provided in the area between a suggested distance of the front end of the guide surface and the clamping line of the squeeze roller pair.

In order to remove the foreign particles removed from the nonwoven fabric using compressed air or other media, it is proposed that a suction channel be provided underneath the guide surface and the separating device, which is connected to a vacuum source. This suction channel preferably has an opening that points in the direction of the separating device.

Advantageously, this suction channel, or its opening, can be arranged or designed in such a way that it can also be used to receive components which are stripped by a stripper from the lower roller of the squeeze roller pair.

Furthermore, the invention can also be achieved in that the fiber material is removed from the collector by a stripping roller and transferred to a subsequent pair of squeezing rollers, which is followed at a distance by a pair of transport rollers and a guide element is provided between the pair of squeezing rollers and the pair of transport rollers, to which a separation device for foreign substances that have been detected by an upstream detection device is assigned.

• *

The detection device is advantageously installed upstream of the separating device in the area of the guide element. This makes it possible to carry out a targeted and precise separation of the foreign parts based on the detected position. This is ensured in particular by the fact that the fiber fleece is guided precisely over the guide element.

In the area of the detection device, transmitted light can be used if the guide element is designed as a light table. This enables optimal detection of these foreign substances, as they can be detected very well in the transmitted light process by appropriate light-sensitive sensors, for example by a camera.

In order to eliminate the area of the guide element, it is proposed to provide it with at least one opening.

Another possibility for separation can be provided in the free space created by the formation of a gap between the front end of the guide element and the clamping line of the transport rollers.

In order to detect foreign parts in the nonwoven fabric which do not differ in colour from the good fibres, it is suggested that the fibre material be subjected to a chemical treatment before reaching the detection device. This causes the colour of these components to change, allowing them to be detected. Such components or foreign parts can be, for example, shreds or fibre parts made of polypropylene which still come from the packaging of the delivered cotton bales. Since these polypropylene parts behave completely differently in terms of colour absorption during a subsequent dyeing process than, for example, the cotton fibre, this would be immediately visible in

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a fabric made with it. In order to avoid this loss of quality, care must be taken to ensure that these foreign substances are removed.

These foreign particles (e.g. polypropylene) can be removed from the fiber material, for example, by equipping a roller involved in the fiber fleece transport with a heating device to burn off these undesirable components. By heating the roller to a certain temperature, these particles stick to the roller surface and are stripped off and disposed of in a lower area of the roller using appropriate stripping devices.

The treatment of the fibre material with chemical agents to make certain undesirable components visible for a detection device is not only limited to the claimed embodiment on the carding machine, but can also be used at other points in the processing of the fibre material.

The use of a roller with a heating device for burning off or separating out undesirable components, such as polypropylene, can also be used at other points in the fiber processing process and can also be claimed in general. Further advantages of the invention are shown and described in more detail using the following embodiments.

Show it:

Fig. 1 is a schematic side view of a carding machine with a foreign

substance detection and separation

Fig. 2 is an enlarged partial view according to Fig. 1 in the area of the

taker

Fig. 3 shows another embodiment according to Fig. 1

Fig. 4 shows another embodiment according to Fig. 1

Fig. 5 another embodiment of a foreign substance detection

and separation in the area of the card collector
Fig. 6 is a partial view of Fig. 5 in modified version

Fig. 1 shows a carding machine 1 with a drum 2 (tambour) to which a circumferential cover 3 is assigned. The fiber material is fed in flake form to a shaft 5 via transport lines (not shown) and finally reaches an infeed table 6, on which the fiber material in the form of a batting W is fed to a feed roller 8 with a feed trough 9. Fibers are torn out of the end of the batting W via a licker-in (licker-in) 11 and delivered to the clothing (not shown) of the following drum 2.

As indicated schematically, carding segments are provided distributed around the circumference of the drum 2, which perform the carding work in conjunction with the cover 3. The fiber material, which has now been broken down into individual fibers, is removed from the clothing of the drum 2 (not shown) via a subsequent doffer 13. The doffer 13 is also provided with a clothing 14, which is shown schematically in Fig. 2.

The fiber material taken over by the doffer 13 is transferred anti-clockwise to a stripping roller 16, which completely strips off the fiber material on the doffer 13 and delivers it as a fleece onto a guide element 18. The guide element 18 runs in its rear area approximately tangentially to the circumference of the doffer 13 and is also generally referred to as a fleece bridge. The fiber fleece V is transferred to a subsequent pair of squeeze rollers 22 via a guide surface 20 of the guide element 18, which runs approximately horizontally. A detection device 24 is mounted below the guide surface 20 in the area of the stripping roller 16, which serves to detect the foreign substances F carried in the fleece V. Foreign substances are particularly understood to mean

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This includes fabrics, cords, films and fibres that do not belong to the fibre material specified in the yarn. However, it also includes substances that, in connection with plant fibres, belong to the plant but are undesirable in the subsequent processing of the fibres, for example trash pieces, nits, seed capsules, etc.

The detection device 24 can, for example, consist of one or more cameras, to which a lighting device is assigned and which are arranged so as to be displaceable over the width of the guide element 18 via corresponding transport devices. Such an arrangement is known, for example, from

DE-A 1-196 04 499. The guide element 18 is equipped with a transparent glass plate 52 in the area of the detection device 24, for example. Instead of moving monitoring devices, fixed optical sensors can also be used, which are distributed over the width of the guide element 18. The use of one or more line cameras (CCD cameras) or inductive sensors, ultrasonic sensors and the like is also conceivable.

The data determined by the detection device 24, or data on the precise positional determination of existing foreign substances, are transmitted via the connecting line 25 to a subsequent control unit S. Based on these data, a valve 28 of a specific nozzle 30 is activated via the line 26, whereby the corresponding nozzle 30 is supplied with compressed air from a compressed air source 32 via the line 31, whereby the foreign substance or substances F are blown downwards out of the fiber fleece V through an opening 34 in the guide element 18 via the pressure pulse generated.

••••TI ··· i ·

This device can be seen in particular from the enlarged illustration in Fig. 2, where it can be seen that the separated foreign particles F pass through a channel 36 into a suction pipe 38.

As schematically indicated in Fig. 2, the suction pipe 38 is connected to a vacuum source 40 in order to feed the separated foreign particles to a disposal device (not shown). In the area of the suction pipe 38, a scraper 42 is also mounted, the scraper edge of which is on the circumference of the lower roller

22 b of the pair of squeeze rollers 22 and serves to keep this surface clean. The components stripped off by the scraper 42 also reach the suction pipe 38 and are disposed of with the foreign matter F.

A plurality of nozzles 30 are arranged next to one another across the width of the guide element 18, whereby each nozzle can be controlled separately via corresponding valves 28. Such an arrangement of nozzles is known, for example, from
EP-B1-606 626.

As can also be seen from Fig. 1, the fiber fleece freed of foreign matter reaches the clamping line KL of the pair of squeeze rollers 22 and is then discharged onto a cross conveyor belt QF. The fiber fleece V is pulled off laterally via the cross conveyor belt QF, whereby a fiber strip FB is formed at the end of the cross conveyor belt, which is transferred to a strip deposit 46 via a subsequent pair of stepped rollers 44. At the strip deposit 46, the fiber strip FB is transferred via calender rollers 48 to a rotatably mounted funnel wheel 49, by means of which the fiber strip is deposited in a loop-like manner in a can K. Solutions are also known, whereby the fiber fleece discharged from the clamping line KL is discharged onto two cross conveyor belts conveying to the center of the card, which bring the fiber fleece together in the middle, which

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is then formed into a fiber tape via a funnel with subsequent take-off rollers. Furthermore, a design is also known in which the fiber fleece is brought together via a fleece bowl and fed to a funnel with subsequent take-off rollers.

As indicated by dash-dotted lines in Fig. 2, instead of the detection device 24, a detection device 24' can be provided below the stripping roller 16, via which the scanning for foreign matter on the circumference of the pickup 13 takes place. The detection device 24' is mounted in the area shortly before the pickup point 15. This detection device 24' is also connected to the control unit S via the line 25'.

As indicated by the arrow Z at the control unit S, it is also possible to transmit the data on the presence of foreign substances determined by the detection device 24, 24' to an evaluation unit (not shown in detail) in order to monitor, record and display the quality of the fiber material.

However, it is also conceivable to provide a further monitoring device directly after the separation device in order to carry out subsequent monitoring of the quality of the fibre material, which has essentially been freed of foreign substances, while it is still in the fibre fleece.

In Fig. 3, a further embodiment is shown, wherein the detection device 24 is arranged in the area between the stripping roller 16 and the top roller 22a of the pair of squeeze rollers 22. With this form of arrangement, it is also possible to work with transmitted light, wherein in the present case a light source 51 is mounted below the guide surface 20 of the guide element 18/which is opposite the detection device 24. In this area, the guide element 18 is provided, for example, with a transparent glass plate 52 to ensure the passage of the light from the light source 51. The guide element 18

ends at a distance A from the clamping line KL of the pair of squeeze rollers 22 and has a suction pipe 38 a below this distance A, which is provided with a vacuum source 40 for removing the separated foreign substances. The nozzles 30, which have already been described above, are also located within the distance A and above the suction pipe 38 a.

This arrangement of the detection device 24 also enables the use of other detection devices which, for example, cannot be used with the installation shown in Fig. 2 below the stripping roller. In particular, line cameras which work with backlight can be used here. Such designs can be found, for example, in EP-B 1-824 607.

The design shown in Fig. 4 corresponds essentially to the design according to

Fig. 3, wherein in addition a guide roller 55 is rotatably mounted in the area between the stripping roller 16 and the upper roller 22a of the pair of squeeze rollers 22. The detection device 24 is mounted between the stripping roller 16 and the guide roller 55 and the nozzles 30 of the separation device are mounted between the guide roller 55 and the upper squeeze roller 22a. The conveyor roller 55 interacts with the surface 20 of the guide element 18 and supports the precise and gentle guidance of the fiber fleece V on the guide surface 20. The guide roller 55 can, for example, be provided with an appropriate coating, for example rubber. The disposal of the detected foreign substances takes place in accordance with the embodiment of Fig. 2, wherein the foreign substances are discharged by a compressed air jet from the corresponding nozzle 30 via a channel 36 to a suction pipe 38 which is connected to a vacuum source 40.

As schematically indicated, the lower roller 22 b of the pair of squeeze rollers 22 is connected to a heating coil 60, via which the outer circumference of the roller 22 b can be heated to a certain temperature. Due to the outer casing of the roller 22 b being heated to a certain temperature, the films, for example made of polypropylene, which are carried along in the fiber material and have not yet been separated, change their state and remain adhered to the surface of the roller 22 b and are thus removed from the fiber fleece V. These parts can be stripped off the surface 22 b again via a stripper 62 and delivered to a disposal channel 64.

It would also be possible to burn these parts in the nonwoven fabric V directly at a sufficiently high temperature, although of course no damage should be caused to the good fibres in the nonwoven fabric V. In addition, appropriate safety precautions must be taken during this process.

Such separation mechanisms for such described foreign substances (e.g. polypropylene) are appropriate if they cannot be detected by the detection system used because they are the same colour as the good fibres.

A further embodiment of a detection and separation device on a card is shown in Fig. 5, wherein the separation and detection takes place after the squeezing rollers. The fiber fleece V delivered by the squeezing rollers 22 reaches a guide surface 66 of a guide element 65 and is transferred to a subsequent transport roller pair 70. From this transport roller pair, the fiber fleece V reaches a cross conveyor belt QF, via which, as already described, a fiber band FB is formed, which is fed via the step rollers 44

a subsequent belt deposit 46. _L

A detection device 24 and a separation device with nozzles 30 are arranged above the guide surface 66 and between the roller pairs 22 and 70. In the example shown, the detection device 24 (for example a line camera) works together with light sources 58 that operate using the incident light method.

If a glass plate is provided in the guide element 65 in the area of the detection device 24, it is also possible to work with transmitted light - as shown schematically with the light source 51. The detection and separation device essentially works according to the same principle as was already explained in more detail in the previous examples in Fig. 3 and Fig. 4.

In order to make certain foreign substances, such as polypropylene, visible to the detection device, it is proposed that a device 75 be provided between the stripping roller 16 and the squeezing roller 22a in order to chemically treat the nonwoven fabric V. The device 75 essentially consists of several nozzles 76 arranged next to one another, which draw the chemical agent from a tank 77. The device 75 is essentially controlled via the path 72 from the control unit S. Of course, the device 75 is also provided with a pressure source and with corresponding valves, which are not shown in more detail here.

By spraying certain chemical agents, the foreign substances that would otherwise not be detectable by the detection system change their color or shape and can thus be detected by the subsequent detection device.

For example, it is possible to spray certain chemical substances onto the fibre material using this device, whereby these substances, due to their properties, only come into contact with the foreign substances to be eliminated, e.g. from polypropylene.

form a bond, while no bond is formed with the cotton fibers. This means that these foreign substances wetted with the chemical substance can be detected using a special device, for example, if the fiber material is irradiated with UV light in this area, so that only the wetted foreign fibers are visible for monitoring.
Such a device for chemically treating fiber material in order to make certain foreign substances visible is of course possible with all the other designs shown and described above. In principle, such a device for treating fiber material can generally be used anywhere where it is necessary to make certain substances visible in a fiber mixture.

The use of previously described devices for heating rollers in order to filter out certain substances from a fiber mixture can also be applied generally and is not limited to the embodiments shown above.

The partial view in Fig. 6 shows a further embodiment in the area of the transport rollers 70, wherein (as shown in Fig. 3) the guide element 65 ends at a distance B from the clamping line KT of the transport rollers 70. The nozzles 30 are mounted in this free space and separate the captured foreign matter downwards into the upper opening of a suction pipe 38, via which they are fed to a disposal facility (not shown) by means of the connected vacuum source 40.

Claims (20)

1. Device for detecting ( 24 ) and separating ( 30 ) foreign substances (F) on a fiber fleece (V) formed on a card ( 1 ), wherein the fiber material removed from a drum ( 2 ) via a doffer ( 13 ) is transferred to a subsequent pair of squeezing rollers ( 22 ), characterized in that the fiber material located on the doffer ( 13 ) is stripped off via a stripping roller ( 16 ) and delivered to a guide element ( 18 ), via which the fiber material in the form of a fiber fleece (V) is transferred to the subsequent pair of squeezing rollers ( 22 ), and at least the separating device ( 30 ) is arranged between the stripping roller ( 16 ) and the clamping line (KL) of the pair of squeezing rollers ( 22 ). 2. Device according to claim 1, characterized in that the detection device ( 24 ') is opposite the surface of the pickup ( 13 ) in the region of the fiber material pickup ( 15 ). 3. Device according to claim 1, characterized in that the detection device ( 24 ) is arranged in the region of the guide element ( 18 ) for monitoring the fiber fleece (V) guided over the guide element. 4. Device according to claim 3, characterized in that the detection device ( 24 ) is arranged between the stripping roller ( 16 ) and the clamping line (KL) of the squeezing roller pair ( 22 ) and is connected upstream of the separating device ( 30 ). 5. Device according to claim 4, characterized in that the guide element ( 18 ) is transparent ( 52 ) in the region of the sensors ( 24 ) of the detection device and the guide element ( 18 ) is arranged between the sensors ( 24 ) and a light source ( 51 ). 6. Device according to claim 4 or 5, characterized in that a guide roller ( 55 ) is arranged in the region between the stripping roller ( 16 ) and the pair of squeezing rollers ( 22 ) above the guide element ( 18 ). 7. Device according to claim 6, characterized in that the sensors ( 24 ) of the detection device are arranged between the stripping roller ( 16 ) and the guide roller ( 55 ) and the separating device ( 30 ) is arranged between the guide roller ( 55 ) and the clamping line (KL) of the squeezing roller pair ( 22 ). 8. Device according to one of claims 1 to 7, characterized in that the guide element ( 18 ) is provided with a guide surface ( 20 ) for guiding the fiber fleece (V). 9. Device according to claim 8, characterized in that the guide surface ( 20 ) is provided with at least one opening ( 34 ) in the region of the separating device ( 30 ). 10. Device according to claim 9, characterized in that the guide element ( 18 ) has a closed cavity ( 36 , 38 ) below the guide surface ( 20 ), into which the opening ( 34 ) opens and which is connected to a negative pressure source ( 40 ). 11. Device according to claim 8, characterized in that the front end of the guide surface ( 20 ) ends at a distance (A) from the clamping line (KL) of the squeeze roller pair ( 22 ) and the separating device ( 30 ) is arranged within this distance (A) above the guide surface ( 20 ). 12. Device according to claim 11, characterized in that below the guide surface ( 20 ) and the separating device ( 30 ) a suction channel ( 38 , 38a ) is provided, which is connected to a negative pressure source ( 40 ) and has at least one opening ( 36 ) pointing in the direction of the separating device. 13. Device according to claim 12, characterized in that the opening of the suction channel ( 38 ) is also provided for receiving components which are stripped by a stripper ( 42 ) from the lower roller (22b) of the pair of squeeze rollers ( 22 ). 14. Device for detecting ( 24 ) and separating ( 30 ) foreign substances (F) on a fiber fleece (V) formed on a card ( 1 ), wherein the fiber material removed from a drum ( 2 ) via a doffer ( 13 ) is transferred to a subsequent pair of nip rollers ( 22 ), characterized in that the fiber material is removed from the doffer ( 13 ) by a stripping roller ( 16 ) and transferred to a subsequent pair of nip rollers ( 22 ), which is followed at a distance by a pair of transport rollers ( 70 ), and between the pair of nip rollers ( 22 ) and the pair of transport rollers ( 70 ) a guide element ( 65 ) is provided, to which a separation device ( 30 ) for foreign substances (F) is assigned, which have been detected by an upstream detection device ( 24 ). 15. Device according to claim 14, characterized in that a detection device ( 24 ) is arranged upstream of the separating device ( 30 ) in the region of the guide element ( 65 ). 16. Device according to claim 14 or 15, characterized in that the guide element ( 65 ) is designed as a light table. 17. Device according to claim 14 or 15, characterized in that the guide element ( 65 ) is provided with at least one opening ( 34 ) in the region of the separating device ( 30 ). 18. Device according to claim 14 or 15, characterized in that the guide element ( 65 ) ends at a distance (A) from the clamping line (KL) of the transport rollers and the separating device ( 30 ) is arranged within this free distance (B) above the guide surface ( 66 ) of the guide element ( 65 ). 19. Device according to one of the preceding claims, characterized in that the detection device ( 24 ) is preceded by a device ( 76 ) for chemically treating the fiber material in order to make foreign substances (F) (e.g. polypropylene) visible to the detection device ( 24 ). 20. Device according to one of claims 1 or 14, characterized in that at least one roller ( 22b ) involved in the fiber fleece transport, which follows the doffer ( 16 ), is provided with a heating device ( 60 ) in order to burn off undesirable components (e.g. polypropylene) in the fiber material.