Abstract:
A device for continuously filtering material mixtures, particularly for separating contaminants out of plastic melts, includes a filter ( 3 ), which is rotatably arranged in a filter chamber ( 2 ) of a housing ( 1 ), at least one wiper ( 17 ) for lifting off contaminants retained by the filter ( 3 ), and a discharge device ( 18 ) for removing the contaminants lifted off the filter ( 2 ) by the wiper ( 17 ) from the housing ( 1 ). The discharge device ( 18 ) includes a discharge shaft ( 19 ), which is rotatably arranged at the housing ( 1 ) and which has at least one continuous opening ( 20 ) having at least one piston ( 22 ) which is movably guided therein.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a United States National Phase Application of International Application PCT/EP2011/071464 filed Dec. 1, 2011 and claims the benefit of priority under 35 U.S.C. §119 of German Patent Application DE 10 2010 055 167.8 filed Dec. 18, 2010, the entire contents of which are incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention pertains to a device for continuously filtering mixtures of materials, especially for separating impurities from plastic melts. 
     BACKGROUND OF THE INVENTION 
     Used plastics or plastic wastes usually contain high percentages of foreign substances, e.g., metal parts, paper remnants, glass, secondary plastics and the like. These foreign substances or impurities must, as a rule, be removed before the recycling of the plastics. This is frequently performed by the used plastics being at first plasticized by heating and by the plastic melt being subsequently filtered. So-called melt filters, by which the metallic or nonmetallic foreign substances or higher-melting plastics are separated, are used for this. However, the melt filters must be constantly cleaned to make continuous and trouble-free filtration possible. 
     A device of this type with a filter arranged in a filter space of a housing with a stripper for lifting off impurities retained by the filter and with a discharge means for removing the impurities lifted off from the filter by the stripper from the housing is known from DE 202 10 115 U1. The discharge means comprises in this prior-art device a feed screw, which is arranged within the housing in the immediate vicinity of the stripper and extends in parallel to the axis of rotation of the tubular filter. The feed screw is arranged such that the residues or impurities lifted off by the stripper from the outside of the filter are transferred directly to the feed screw and are removed by this from the housing. One drawback of this conveying means is that the impurities must be transported over the entire length of the feed screw and hence over a relatively long conveying path. In addition, there is a direct connection from the filter space in the housing via the feed screw in such a conveying means, so that, on the one hand, overflow of the material being conveyed into the feed screw intended for removing the impurities may occur. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a device of the above-mentioned type with a simple design, which makes it possible to remove the impurities or residues retained by the filter rapidly and reliably. 
     According to the invention, a device is provided for the continuous filtering of material mixtures, especially for separating impurities from plastic melts. The device comprises a filter arranged rotatably in a filter space of a housing. At least one stripper is provided for lifting off the impurities retained by the filter. A discharge means is provided for removing the impurities lifted off by the stripper from the filter from the housing. The discharge means comprises a discharge shaft, which is arranged rotatably relative to the housing and has at least one continuous opening with at least one piston guided displaceably in the opening. 
     In the device according to the present invention, the discharge means contains a discharge shaft, which is arranged rotatably at the housing and has at least one continuous opening with at least one piston guided displaceably in same. The material lifted off from the filter by the stripper can be removed to the outside relatively rapidly without long conveying paths by the piston, which is displaceable at right angles to the axis of rotation or even in the direction of the axis of rotation of the discharge shaft, and the discharge shaft with the piston arranged therein seals off the pressurized filter space to the outside. Since there is no continuously open connection between the filter space and a discharge opening for the discharge of the impurities retained by the filter, overflow of the material mixture to be filtered into the area intended for the removal of the filter residues can be avoided. The device according to the present invention has a simple and cost-effective design and is designed especially for filtering mixtures of materials with a lower degree of contamination. 
     The opening may be designed, e.g., as a continuous slot, in which an individual, pusher-like piston is arranged. However, a plurality of pusher-like pistons may also be arranged in the slot. Tilting about the longitudinal axis occurring because of an unfavorable width-to-length ratio can be prevented hereby in case of individual pushers. A plurality of slot-like or round openings, which are separated from each other and are located at spaced locations from one another in the axial direction of the discharge shaft, may also be arranged in the discharge shaft with a pusher-like piston or round piston each arranged therein, and the openings may have the same orientation or may be angularly offset in the circumferential direction. 
     The discharge shaft is advantageously arranged in a longitudinal hole of the housing, which said hole is open towards the filter space, such that a part of the discharge shaft extends through a recess of the housing that is open towards the filter space. A discharge opening is provided for the discharge of the impurities being removed by the discharge means on the side of the housing located opposite the recess. 
     In a preferred embodiment, the pusher-like piston has, when viewed in the direction of displacement, a smaller width than the diameter of the discharge shaft. As a result, the pusher-like piston can be displaced radially within the slot during a rotation of the discharge shaft and form receiving chambers for the impurities thereby. 
     The speed of the carrier shaft with the filter arranged thereon and the speed of the discharge shaft can be advantageously controlled separately, as a result of which separate regulation of the speed of cleaning and of the speed of discharge of foreign substances is made possible. A very high concentration of foreign substances and hence high yield of the primary material are achieved by such a regulation. 
     Further peculiarities and advantages of the present invention appear from the following description of a preferred exemplary embodiment on the basis of the drawings. The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIG. 1  is a cross-sectional view showing a filter device according to the present invention with a cleaning means; 
         FIG. 2  is a partial enlarged view of the cleaning means from  FIG. 1 ; 
         FIG. 3  is a sectional view of the filter device along line A-A in  FIG. 1 ; 
         FIG. 4  is a view showing different exemplary embodiments of a discharge shaft; 
         FIG. 5  is an enlarged view from  FIG. 2 ; and 
         FIG. 6  is a sectional view showing a discharge shaft with further exemplary embodiments of a piston. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings in particular, the device for filtering contaminated plastic melts shown in various views in  FIGS. 1 through 3  contains a housing  1  with a filter space  2 , in which a hollow cylindrical filter  3  is arranged rotatably about an axis of rotation  4 . Housing  1  contains a material inlet  5  with a feed opening  6 , via which the material to be filtered is introduced into the filter space  2 . Filter  3 , which consists, e.g., of a tube with a plurality of radial holes, is seated on a carrier shaft  7 , which is rotationally driven by means of a motor and contains, according to  FIG. 3 , a more slender drive pin  9  mounted rotatably in a hole (bore)  8  of housing  1 , a broadened mounting part  10  for filter  3 , which said mounting part is arranged in filter space  2  of housing  1 , and a bearing journal  13  mounted rotatably in a corresponding hole (bore)  11  of a bearing cap  12  fastened to the housing  1 . The filter  3  seated on the mounting part  10  is rotatable within the filter space  2  of housing  1  by means of the drive pin  9  coupled with a rotating drive, not shown. 
     The tubular filter  3  may be manufactured, e.g., from a steel plate provided with passage openings, which is bent into a tube and then welded. It is advantageously manufactured from a wear-resistant and corrosion-resistant steel and hardened. The passage openings advantageously have a cross section expanding in the direction of flow and may be, e.g., conical holes prepared by laser treatment, Filter  3  may also be provided with surface coatings, by which the wear resistance and other properties are improved. 
     As is apparent from  FIG. 3 , the carrier shaft  6  contains a plurality of collection slots  14 , which are arranged distributed over the circumference of the widened mounting part  10  and form an interior space for collecting the filtered material. The collection slots  14  lead to a central collection channel  15 , via which the filtered material can be removed to the outside through a radial opening  16  and a ring channel, not shown, in housing  1 . 
     A stripper extending over the entire length of the tubular filter  2  in the axial direction and being in contact with the outside thereof is arranged in the form of a blade, a scraping knife or the like, is arranged in housing  1  such that the residues or contaminants retained on filter  2  can be lifted off from filter  2  and removed by a discharge means  18  to be explained in more detail later in the radial direction. Stripper  17  is arranged obliquely to the outer surface of filter  2  and sloped towards the direction of rotation thereof. Stripper  17  may be mounted stationarily in a preset angular position in housing  1 , as this is shown in  FIGS. 1 and 2 . The stripper  17 , designed as a scraping knife or blade, may, however, also be guided displaceably in an oblique recess in housing  1  and pressed onto the outside of filter  2  by means of a spring or another pressing element. 
     Discharge means  18  comprises a discharge shaft  19 , which is arranged rotatably at housing  1  in the area of stripper  17 , is driven by a motor, extends in parallel to the carrier shaft  7  and contains in the area of filter  2  a continuous opening  20  with a piston  22  guided displaceably in same at right angles to the axis of rotation  21  of discharge shaft  19 . Opening  20  is designed as a continuous slot with an individual pusher as a piston  22  in the embodiment being shown in  FIGS. 1 through 3 . 
       FIG. 4  shows various exemplary embodiments for embodying the discharge shaft  19 . In the view on the left, the discharge shaft  19  is shown with a continuous, slot-like opening  20  and with an individual pusher as a piston  22  according to  FIGS. 1 through 3 . A discharge shaft  19  with a continuous, slot-like opening  20  and with a plurality of separate, pusher-like pistons  22  is shown next to it. A discharge shaft  19  with a plurality of separate rectangular openings  20  and with a plurality of pusher-like pistons  22  arranged displaceably in the respective openings  20  is shown in the view in the center. Next to it is shown another exemplary embodiment of the discharge shaft  19  with a plurality of slot-like openings  20  offset angularly in the axial direction and in the circumferential direction. Four openings  20 , which are offset in the axial direction and are offset by 45° each in relation to one another, are shown in the exemplary embodiment being shown. The right-hand part of  FIG. 4  shows an embodiment with a plurality of round openings  20  following each other in the axial direction and corresponding round pistons  22 . 
     According to  FIG. 2 , discharge shaft  19  is arranged in a longitudinal hole  23  of housing  1 , which said hole is open towards the filter space  2 , such that a part of the discharge shaft  19  extends through a recess  24  that is open towards the filter space  2 . A slot-like discharge opening  25  for discharging the impurities retained by filter  2  and removed by and through the discharge means  18  is provided on the side of housing  1  located opposite the recess  24 . As is apparent especially from  FIG. 5 , piston  22  has, when viewed in the direction of displacement, a smaller width than the diameter of discharge shaft  19 . Piston  22  can thus be displaced radially within the continuous opening  20 . The discharge opening  25  in housing  1  is arranged at an angle offset in relation to the central axis  26  of the recess  24  open towards the filter space  2  such that the continuous opening  20  opens during rotation of the discharge shaft  19  towards the discharge opening  25  only when opening  20  is already fully open at the recess  24 , which is open towards the filter space  2 . 
     Carrier shaft  7  and discharge shaft  19  can be driven such that they are regulated by different drives. For example, the speeds of the carrier shaft  7  and discharge shaft can be regulated as a function of the intended use or the prevailing pressures by a corresponding electronic unit. 
     In the above-described device, the contaminated material mixture (mainly plastic mass) is pressed via the feed opening  6  of housing  1  into the filter space  2  under pressure and enters the annular space formed between the outside of filter  3  and housing  1 . The impurities present in the material mixture are retained by filter  3 , while the filtered material reaches the collecting channel  15  through the corresponding passage openings in filter  3  via the collection slots  14  and can be removed to the outside via radial opening  16 . The residues retained in filter  3  are lifted off by stripper  23  during the rotation of filter  3  and collected between stripper  17 , housing  1  and discharge shaft  19 . Since piston  22  has a length that is shorter than the diameter of discharge shaft  19  and is displaceable within the opening  20  of discharge shaft  19 , it can form a receiving chamber for the impurities. When opening  20  reaches the recess  24  in housing  1  that is open towards the ring channel  2  during rotation of the discharge shaft  19  on one side according to  FIG. 2 , piston  22  is pressed radially outwardly by the internal pressure in filter space  2  and the impurities located in front of stripper  17  are pressed into the receiving chamber formed thereby. At the same time, the impurities collected on the opposite side of opening  20  are transported to the surrounding space by the motion of piston  22  via the slot-like discharge opening  25  of housing  1 . Simple and nevertheless effective removal of the impurities retained by filter  2  can be guaranteed hereby. 
       FIG. 6  shows further possibilities for arranging piston  22  within the discharge shaft  19  rotatable about an axis of rotation  21 . In the embodiment shown in the top part of  FIG. 6 , opening  20 , which extends at right angles through the discharge shaft  19 , is arranged obliquely in relation to the axis of rotation  21  of the discharge shaft rather than at a right angle. Piston  22  is therefore also displaceable obliquely in relation to the axis of rotation  21  of discharge shaft  19  within opening  20 . In the embodiment shown in the bottom part of  FIG. 6 , the continuous opening  20  extends at an angle with an inner area  27  extending in the longitudinal direction and two outer areas  28  extending radially outwardly at right angles to the axis of rotation  21 . The two radially outwardly extending areas  28  are offset in relation to one another in the longitudinal direction of the axis of rotation and piston  22  is arranged in the inner area  27  displaceably in the longitudinal direction of discharge shaft  19 . 
     Pressure is alternatingly admitted to piston  22 , which is displaceable within the discharge shaft  19  and whose stroke is limited and said piston is connected over the entire piston area or via flow admission channels to the pressurized space of the filter and to a discharge opening via a geometrically defined control edge such that piston  22  performs a cleaning stroke by dirt located under the stripper being pressed at the same time by overpressure in the filter space into the piston space formed and by the dirt located on the discharge side being pressed into the discharge opening without a direct connection being established between the interior space of the filter and the outer space of the filter. Piston  22  can perform its emptying stroke when the pressure-side inlet opening reaches the optimal cleaning point. 
     The present invention is not limited to the exemplary embodiments described above. Filtering may also be carried out, e.g., with a direction of flow directed from the inside to the outside. In addition, the filter may also have a disk-shaped design or may have another, suitable shape. 
     While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.