Abstract:
The invention relates to a device which is used to continuously filter material mixtures, in particular for separating impurities from plastic melts. The device includes a hollow cylindrically-shaped filter element ( 2 ) which is arranged inside a housing ( 1 ), an annular chamber ( 22 ) which is defined from the outside of the filter element ( 2 ) and an inner wall of the housing, and at least one stripper ( 23 ) which can be pressed onto the filter body by an adjusting device. The stripper is used to remove the impurities detained on the filter element ( 2 ) due to a relative movement of the filter element ( 2 ) and the stripper ( 23 ). The adjusting device contains a pressure sensor ( 42, 53 ) which is used to detect the pressure of the material mixture upstream from the filter body and an actuator ( 43 ) which is connected to the pressure sensor, the actuator being used to adjust the pressure of the stripper ( 23 ) according to the pressure detected by the pressure sensor.

Description:
FIELD OF THE INVENTION 
     The invention relates to a device for continuously filtering material mixtures, particularly for separating impurities from plastic melts. 
     BACKGROUND OF THE INVENTION 
     Used plastics or plastic waste typically have high percentages of foreign materials, e.g., metal parts, paper residue, glass, secondary plastics, and the like. Usually, these foreign materials or impurities must be removed before the plastics are reused. This is realized in several ways such that the used plastics are first plasticized by heating and the plastic melts are then filtered. For this purpose, so-called melt filters are used, through which the metallic or non-metallic foreign materials or higher melting point plastics are separated. However, to enable continuous and uninterrupted filtering, the melt filter must be cleaned continuously. 
     From U.S. Pat. No. 4,470,904, a separating device according to the class is known, in which the contaminated plastic melts are pressed into the interior of a hollow, cylindrically-shaped filter body arranged in a housing. In the interior of the filter body, there is a rotationally driven stripper shaft, which is arranged coaxial to this filter body and which defines an inner annular space with the inner wall of the filter body and carries on its outer side several strippers at an angle to the axial direction and expanding into a spiral. The residue detained on the filter body on its inner side is transported to a material outlet opposite the inlet end of the inner annular space in the axial direction along the filter body by the strippers through the rotation of the stripper shaft. The strippers are elastically pressed from their inner side onto the inner surface of the filter body. However, in such elastic contact of the strippers, there is the problem that the strippers can be lifted from the surface of the filter body due to the pressure of the plastic melts and thereby lose their effectiveness. On the other hand, too high a contact pressure leads to increased friction between the filter body and the strippers, which is associated with accelerated wear. 
     SUMMARY OF THE INVENTION 
     The problem of the invention is to create a device of the type named above, which enables improved removal of the residue detained on the filter element. 
     This problem is solved by a device with the features of the invention. Various advantageous embodiments of the invention are also provided herein. 
     An essential advantage of the device according to the invention is that the contact pressure of the stripper can be automatically modified to the actual conditions without intervention from the outside. For example, if the pressure of the fed material increases, thereby increasing the risk of the stripper being lifted, according to the invention, the contact pressure of the stripper is also automatically increased without intervention from the outside. In contrast, if the pressure of the fed material drops, then the contact pressure of the stripper is also reduced accordingly, thereby decreasing the friction between the stripper and the filter element. 
     In a preferred embodiment of the invention, the pressure sensor is a hydraulic pressure transducer, which detects the pressure of the fed material upstream from the filter element and converts it into a hydraulic control signal. The actuator consists of an adjusting cylinder connected to the hydraulic pressure transducer via a hydraulic line, through which the control pressure is converted into a contact pressure for the stripper. 
     However, the pressure sensor can also be an electronic pressure transducer, which delivers corresponding control signals for a pressure control valve or an electronic actuator. 
     In the device according to the invention, the filter residue is lifted from the filter surface in the radial direction and thus discharged on the quickest path from the filter surface. The residue is not pushed axially to the filter surface, so that the wear decreases and the stability of the device can be improved. Through the lower abrasive loads of the filter, simpler and more economical filters can also be used. 
     The material lifted by the stripper is preferably transported away by a spiral conveyor or the like. The Filter element and the spiral conveyor can be driven separately, so that a separate control of cleaning and foreign material discharge speed is enabled. Through such control, a very high foreign material concentration can be effected and thus also a high yield of the primary material. In a preferred configuration, the conveying device comprises a rotationally motor-driven spiral conveyor. The rpm values of the filter and the spiral conveyor can be controlled separately, whereby a very high impurity concentration for an optimally active filter surface can be achieved. According to the type of plastic, the filter and the spiral conveyor can have the same or opposite direction of rotation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further special features and advantages of the invention emerge from the following description of a preferred embodiment with reference to the drawing. Shown are: 
         FIG. 1 , a first embodiment of a separating device in a longitudinal section; 
         FIG. 2 , a cross section of the separating device from  FIG. 1 ; 
         FIG. 3 , a cross section of a second embodiment of a separating device; 
         FIG. 4 , a first embodiment of a contact device for pressing a stripper onto the filter tube, and 
         FIG. 5 , a second embodiment of a contact device. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The filter device shown schematically in  FIG. 1  for filtering contaminated plastic melts includes a housing  1 , in which a hollow, cylindrically-shaped filter element  2  is rotatably arranged about a center axis  3 . The filter element  2  is mounted on a motor-driven carrier shaft  4 . This includes a narrow driving part  5  mounted in the housing  1 , a wider holding part  6  for the melt filter  2 , and a narrow bearing journal  7 , which is rotatably mounted in a corresponding bore  8  of a bearing cover  9  fixed on the housing  1 . 
     The filter element  2  comprises a filter tube  11  provided with a plurality of radial through-holes  10  and a hollow, cylindrically-shaped support body  12 , which is connected to the carrier shaft  4  with a positive fit and onto which the filter tube  11  is shrunken. The sieve-like filter tube  11  can be produced, e.g., from a steel sheet, which has through-holes  10 , which is bent and then welded into a tube. Preferably, it is made from a wear-resistant and corrosion-resistant steel and hardened. The filter tube  11  can also be provided with surface coatings, through which the wear resistance and other properties can be improved. The through-holes  10  are configured as bores with a cross section expanding in the flow direction. The through-holes  10  can taper conically, e.g., outwardly. On its outer side, the hollow, cylindrically-shaped support body  12  has several collection channels  13  configured as circular grooves or flat threads. Several radial outflow bores  14  spaced apart at equal angular intervals in the peripheral direction lead inwards from these channels. 
     As is clear from  FIGS. 1 and 2 , the radial outflow bores  14  open into axial collection slots  15 , which are distributed within the carrier shaft  4  at the same angular intervals as the outflow bores across the periphery of the expanded holding part  6  and which form an inner space for collecting the filtered material. The collection slots  15  expanding in the flow direction lead to a central collection channel  16 , which opens via an inclined section into a first annular channel  17  within the housing  1 . From the first annular channel  17 , a first side bore within the housing  1  leads to an outlet channel  18  of a connection piece  19 . In the connection piece  19  there is also an inlet channel  20 , which leads via a second side bore within the housing  1  to a second annular channel  21  in the housing  1 . This annular channel  21  connects to an annular space  22 , which is limited between the inner wall of the housing  1  and the outer wall of the filter tube  11 . 
     As is clear from  FIG. 2 , a stripper  23 , in the form of a blade or a doctor, running in the axial direction over the entire length of the filter tube  11  and contacting the outer side of the filter tube is arranged in the lower part of the housing  1 , such that the residue or impurities detained on the filter element  2  are discharged in the radial direction on the shortest path without being entrained over the filter element. The stripper  23  is arranged at an angle to the outer surface of the filter element  2  and towards the direction of rotation of the filter element. In the shown configuration, the stripper  23  is arranged, e.g., at a contact angle at in the area of 45° to a center plane  40  of the filter element  2  and is pressed against the outer wall of the filter tube  11  by a contact device, described in more detail below and shown schematically in  FIG. 4 . In the direct vicinity of the stripper  23 , within the housing  1  there is a spiral conveyor  24 , which is parallel to the center axis  3  of the filter element  2  and which is led along the outer side of the filter element  2  to an outlet opening. The spiral conveyor  24  is arranged such that the residue stripped radially by the stripper  23  is transferred directly to the spiral conveyor  24  and transported away to the outside from this conveyor in the direction of the arrow  25  of  FIG. 1 . In the configuration shown in  FIG. 2 , the stripper  23  is mounted on a hollow shaft  26 , which surrounds the spiral conveyor  24  and which can rotate within the housing  1  and by means of an adjustment lever  27 . Therefore, the contact angle α and the pressure force of the stripper  23  can be changed. In the hollow shaft  26 , there are cooling channels  29  in the area of the material outlet of the spiral conveyor  24 . Via these channels, the material transported through the spiral conveyor  24  can be cooled in order to form a thermal barrier. 
     The stripper  23  can also be mounted at a given angular position in the housing  1 , as shown in  FIG. 3 . The stripper  23  is guided displaceably there in a diagonal slot  31  in the housing  1  and is pressed against the outer side of the filter tube  11  by an actuator of a contact device, described in more detail below. 
     On the connection port  19 , in the area of the inlet channel  20  there is an input-side mass pressure sensor  35 , and in the area of the outlet channel  18  there is an output-side mass pressure sensor  34 . These are connected to control electronics  36  for controlling the filter device. Thus, e.g., the rotational movement of the filter body  2  and the spiral conveyor  24  can be controlled as a function of a detected differential pressure by means of the control electronics  36 . Therefore, it is possible to allow the filter element  2  and the spiral conveyor  24  to turn intermittently according to two given pressure values (max-min) and thus to reduce wear. Between the inlet channel  20  and the outlet channel  18  there is a drainage channel  41  through the connection port  19  and the housing  1 . In this way, portions of foreign material can be prevented from reaching the side of the goods via the bearing position. 
     In  FIG. 4 , a first embodiment of a contact device for pressing the stripper  23  configured in the form of a blade or an edge against the outer side of the filter tube  11  as a function of the pressure of the fed material mixture is shown. The contact device comprises a pressure sensor  42  for detecting the pressure of the material mixture upstream from the filter element  2  and an actuator  43  connected to the pressure sensor  42  for setting the contact pressure of the stripper  23  as a function of the pressure detected by the pressure sensor  42 . For the hydraulic contact device shown in  FIG. 4 , the pressure sensor  42  is a hydraulic pressure transducer with a pressure piston  45 , which can move within a piston housing  44  and which is connected on one of its ends to a pressure bolt  46  extending opposite the piston housing  44 , A pressure chamber  47  filled with hydraulic fluid is defined by the other end of the pressure piston  45  and the piston housing  44 . The pressure sensor  42  is attached to the connection port  19 , such that the pressure bolt  46  projects into the inlet channel  20 . 
     The actuator  43  consists of an adjusting cylinder, which contains a pressure piston  33  displaceable within a cylinder housing  32  with an outwardly projecting piston rod  48 . The front end of the piston rod  48  is connected to the stripper  23 . With the cylinder housing  32 , the rear end surface of the pressure piston  33  borders a pressure chamber  49 , which communicates with the pressure chamber  47  of the pressure sensor  42  via a hydraulic line  50 . Within the cylinder housing  32 , there is a compression spring  51  for generating a restoring force acting on the pressure piston  33 . The adjusting cylinder can also be configured as a double-acting differential cylinder with an additional pressure connection  52  for the return movement. 
     Via the pressure bolt  46 , the pressure of the material fed through the inlet channel  20  is transferred to the pressure piston  45 , which generates a corresponding control pressure in the pressure chamber  46 . This control pressure is also in the pressure chamber  49  of the actuator  43  via the hydraulic line  50  and ensures that the stripper  23  is pressed against the filter tube  11  via the pressure piston  33  and the pressure rod  48 . If the pressure in the inlet channel  20  rises, the stripper  23  is also pressed more strongly against the filter tube  11 . 
     In  FIG. 5 , another possibility for a contact device is shown. There, an electric pressure transducer  53 , which detects the pressure of the plastic melts within the inlet channel  20  and converts them into proportional electrical signals, is provided on the connection port  19 . The signals delivered by the pressure transducer  53  are converted in control electronics  54  into corresponding control signals for a pressure control valve  55 . The pressure control valve  55  is connected to the actuator  43  via a pressure line  56 . Then, through the pressure control valve  55 , the control pressure of the hydraulic actuator  43 , and thus the contact pressure of the stripper  23 , can be set as a function of the pressure detected by the pressure transducer  53 . 
     In another embodiment, the actuator can also be configured as an electrical actuating drive, through which the contact pressure of the stripper  23  is set automatically as a function of the pressure detected by the electric pressure transducer  53 . 
     In the previously described device, the non-treated material mixture (predominantly plastic mass) according to  FIG. 1  is pressed at the inlet opening  20  in the direction of arrow  37  under pressure into the annular space  22  and through the fine through-holes  10  in the filter tube  11  of the rotating filter body  2 . The filtered material is led via the filter tube  11  and the support body  12  with the collection grooves  13  and the discharge bores  14  via the carrier shaft  6  to the outlet opening  18  and there can be removed in the arrow direction  38 . The residue retained at the filter tube  11  is lifted away by the stripper  23  when the filter tube  11  rotates and transferred directly to the rotating spiral conveyor  24  without further contacting the filter. The residue can then be transported from the spiral conveyor  24  to an output and can there be discharged in the arrow direction  25 . 
     The invention is not restricted to the previously described embodiment. Thus, filtering can also be performed, e.g., with a flow direction directed from the inside outwards, wherein the stripper is then attached to the inner side of the hollow, cylindrically-shaped filter body. The filter element can also be stationary and the stripper can rotate.