Abstract:
The invention relates to a rotary cutter for the pelletization of plastic extrudates, said rotary cutter being provided with knives projecting axially away from its end surface, said knives being individually affixed to supporting surfaces of the rotary cutter by means of fastening elements. The supporting surfaces are formed by the side walls of radial grooves in the peripheral surface of the rotary cutter and by radial transverse walls in the radial grooves, said radial grooves being limited by the radial transverse walls, the knives each being inserted in hook-like manner with a projecting portion into said radial transverse walls, wherein one side of the projecting portion forms an abutment for the fastening element, said fastening element penetrating a radial transverse wall and pressing on the abutment.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority under 35 U.S.C. §119 to German Patent Application No. DE 10 2004 049 862.8 (filed Oct. 13, 2004), and International Patent Application No. PCT EP 2005/010023 (filed Sep. 16, 2005), the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a rotary cutter for the pelletization of plastic extrudates, said rotary cutter being provided with knives projecting axially away from its end surface, said knives being individually affixed to supporting surfaces of the rotary cutter by means of fastening elements. 
     2. Description of Prior Art 
     Such a rotary cutter is described and presented in U.S. Pat. No. 3,196,487. Said rotary cutter is used in an underwater pelletizer having a die plate with circularly disposed extrusion orifices, said extrusion orifices being skimmed over in circular motion by the knives of the rotary cutter, said knives widely projecting in the manner of blades. The rotary cutter rotates about a shaft extending through the centre of the circle along which are disposed the extrusion orifices in the die plate. As they move over the extrusion orifices, the knives cut the extruded plastic strands issuing in molten form from the extrusion orifices into individual pieces which form the desired pellets or granules. This cutting operation takes place under water, said water flowing over the die plate during rotation of the rotary cutter. Owing to their considerable length, the knives, which project in the manner of blades, tend to flutter somewhat, this having an adverse effect on the close contact between knives and die plate. However, constant close contact between the knives and the die plate is a prerequisite for the uniform cutting of the plastic strands. 
     In another known underwater pelletizer according to DE-PS 10062113, the knives supported by the rotary cutter are individually screwed onto supporting surfaces affixed to the rotary cutter and provide a more compact, flutter-free design. The supporting surfaces extend towards the die plate in the rotary cutter, for which purpose the rotary cutter is provided with supporting surfaces extending in identical direction. The individual knives are pressed onto said supporting surfaces by means of screws which serve as fastening elements, said screws penetrating the knives, with the heads of the screws protruding from the knives such as to be easily accessible for a tool. Provided between the individual knives and the individual supports, forming the supporting surfaces, on the rotary cutter are intermediate spaces through which the water and pellets are able to flow away in the axial direction. It has emerged that the screw heads, which protrude from the individual knives, are, firstly, not easily accessible on account of the oppositely positioned supports and, secondly, obstruct the passage of water and pellets as the latter flow past them. 
     SUMMARY AND OBJECTS OF THE INVENTION 
     The object of the invention is to improve the fastening of the knives to such a rotary cutter such that, firstly, there is good accessibility of the fastening elements which support the knives and that, secondly, said fastening elements do not obstruct the flow in the aforementioned intermediate spaces. 
     The object of the invention is achieved in that the supporting surfaces are formed by the side walls of radial grooves in the peripheral surface of the rotary cutter and by radial transverse walls in the radial grooves, said radial grooves being limited by the radial transverse walls, the knives each being inserted in hook-like manner with a projecting portion into said radial transverse walls, wherein one side of the projecting portion forms an abutment for the fastening element, said fastening element penetrating a radial transverse wall and pressing on the abutment. 
     On account of this hook-like manner of fastening of the individual knives and the insertion of the fastening elements through a radial transverse wall, said fastening elements, as seen from one end surface of the rotary cutter, are directly and easily accessible. They are disposed at a distance from the intermediate spaces between knives and supports and are therefore unable to obstruct the flow in said intermediate spaces. Furthermore, the hook-like design of the knives provides a high degree of security with regard to their position in relation to the rotary cutter, with the overall consequence that the rotary cutter according to the invention is of a flutter-free design which is particularly stable for operation. 
     In order to provide secure support for the knife, the side of the projecting portion facing the knife edge may extend at such an angle to the radial direction that the fastening element, pressing on the projecting portion and being in the form of a screw, exerts a force component such as to pull the projecting portion into the rotary cutter. However, it is also possible for the projecting portion to be provided with a notch, said notch being engaged by a correspondingly shaped end surface of the fastening element, said fastening element being in the form of a screw. Screws are preferably used as the fastening elements. In this case, it is advantageous for the screw head to be provided with a recess in the rotary cutter, the screw head being countersunk in the rotary cutter. The screw head is provided in conventional manner with a recess for receiving a turning tool. The fact that the screw head is accommodated in a recess in the rotary cutter has, above all, the advantage that the screw heads are unable to obstruct any cleaning operations. Preferably, the screws are disposed such that they penetrate into the rotary cutter from the side facing the knife edge. Said side is particularly well accessible for repair work. However, it is also possible for the screws to be provided in the opposite direction, i.e. such that they penetrate into the rotary cutter from the side facing away from the knife edge. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changed and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows the rotary cutter in a view onto its end surface with screws as fastening elements; 
         FIG. 2  shows the same rotary cutter in a view onto the rear side of the knives; 
         FIG. 3  shows a portion of the rotary cutter with some of the radial grooves unoccupied; 
         FIG. 4  shows a section through the rotary cutter in the region of an inserted knife; 
         FIG. 5  shows a variation on the design of the rotary cutter from  FIG. 4 , with a notched projecting portion; 
         FIG. 6  shows a further variation with double knife; 
         FIG. 7  shows a further variation with a screw from the rear side; 
         FIGS. 8 and 9  show a further variation with a wedge as fastening element. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The rotary cutter  1  presented in  FIG. 1  is provided on its peripheral surface  2  with the individual knives  4 , the cutting edges  5  of which extend radially with respect to the central axis C of the rotary cutter  1  and thus contact a die plate (not shown), as presented, for example, in U.S. Pat. No. 3,196,487, wherein, as the rotary cutter  1  rotates with the knives  4 , the extruded plastic strands are then pelletized in the above-described manner. The knives  4  are exchangeably fastened to the rotary cutter  1  by means of the fastening elements in the form of screws  12  (see, for example,  FIG. 4 ). As is apparent from  FIG. 1 , the screws  12  are easily accessible from the end surface  3  of the rotary cutter  1 , where they are unable to obstruct a flow in the intermediate spaces between the individual knives  4 . 
       FIG. 2  shows the same rotary cutter  1  with the knives  4  which project from the peripheral surface  2  of the rotary cutter  1 . The fastening of the knives  4  to the rotary cutter  1  is presented in  FIGS. 4 to 7 . 
     The rear side of the rotary cutter  1  presented in  FIG. 2  has three receiving holes  17  which serve to fasten the rotary cutter  1  to a rotary shaft (not shown). 
       FIG. 3  presents a view of the rotary cutter looking perpendicularly onto the central axis C of the shaft thereof, said view showing the design of the radial grooves  7 , some of which are without knives. As can be seen in FIGS.  1  and  3 - 6 , the radial grooves  7  has an opening  7   o  which is substantially rectangular in shape and which opens in a radial direction R of the rotary cutter  1 . As can be seen in  FIG. 3 , the rotary cutter  1  is provided in the region of its peripheral surface  2  with a plurality of radial grooves  7 , of which the radial grooves  7  have been left unoccupied for the purpose of better representation of the design of the rotary cutter  1 . Only one radial groove (not visible) is occupied by the knife  4 . The radial grooves  7  are limited at the sides by the side walls  8  and  9  and also by the radial transverse walls  10  and  11 . As can be seen in  FIGS. 1-3 , the side walls  8  and  9  and the transverse walls  10  and  11  of each of the radial grooves  7  formed on the peripheral surface  2  of the rotary cutter  1  have surfaces arranged at an angle that is neither parallel nor orthogonal to the end surface  3  of the rotary cutter. Thus, the radial groove  7  is substantially rectangular in shape. Consequently, the radial grooves  7  are of a length (i.e., have a depth  7   d ) which, except for a remaining gap, corresponds to the length of the projecting portions  6  (see  FIGS. 4-6 ). 
     As can be seen in  FIGS. 1-9 , knives  4  have flat front and back side surfaces  4   s   1  and  4   s   2  which are parallel to each other.  FIG. 4  presents a sectional view along the centre of the knives  4  from  FIGS. 1 and 2  and shows how the individual knives  4  are fastened. The knives  4  are each provided with a projecting portion  6  which projects away in hook-like manner with respect to the portion  4   k  with a knife edge  5 . As such, the projecting portion  6  and the portion  4   k  with the knife edge  5  form an L-shaped portion of the knife  4 . The projecting portion  6  projects into the radial groove  7  in the rotary cutter  1  (see  FIG. 3 ). The radial groove  7  is of a width between side walls  8  and  9  which corresponds, with a small tolerance, to the thickness T between front and back side surfaces  4   s   1 ,  4   s   2  of the knives  4 , with the result that an inserted knife  4  is securely supported laterally on the side walls  8  and  9  of the radial groove  7 . The knife  4  is secured by the screw  12  which is screwed into a corresponding penetration in the rotary cutter  1  and presses onto the abutment  18  on the projecting portion  6 , thereby securely fastening the knife  4  to the rotary cutter  1 . The side of the projecting portion  6  facing the knife edge  5  extends, as is clearly shown in  FIG. 4 , at such an angle to the radial direction that the screw, pressing on the projecting portion  6 , subjects the projecting portion  6  to a force which pulls the projecting portion  6  into the rotary cutter  1 , 
     thereby guaranteeing the secure fastening of the projecting portion  6 . Further, as can be seen in  FIGS. 3-6 , the bottom surface  4   b  of the projection portion  6  of the knife  4  rests on the bottom  7   b  of the groove and extends in a direction that is substantially orthogonal to the knife edge  5 . 
       FIG. 5  presents a variation on the design shown in  FIG. 4  in which the knife  4  is secured to the rotary cutter  1  by means of a notch  14  in the projecting portion  6 , said notch  14  serving as an abutment  18 . For this purpose, the screw  15  is provided on its end surface with a corresponding conical portion which fits into the notch  14  and, with the screw  15  tightened, secures the knife  4  to the rotary cutter  1 . 
     As in the design shown in  FIG. 4 , also in the design shown in  FIG. 5  the screw  15  is provided with a hexagonal recess into which it is possible to insert an appropriate key or wrench with which to tighten the screw  15 . 
       FIG. 6  presents a variation on the design shown in  FIG. 5  in which the knife  4  is double-edged. In other words, the knife  4  of  FIG. 6  has two L-shaped portions  4 L. This design has the advantage that, when one of its cutting edges  5  is worn, the knife  4  can simply be turned round by 180°, as a result of which a new cutting edge  5  comes into the region of the rotary cutter  1  which then performs the pelletizing operation. 
       FIG. 7  shows a design which is essentially the same as that shown in  FIG. 4 , it merely being the case that the screw  16  presses against the projecting portion  6  from the side facing away from the knife edge  5 , thereby securing the knife  4  to the rotary cutter  1 . 
     It should also be pointed out that, in the methods of fastening presented in  FIG. 4  to  FIG. 7 , the head of the screw in each case disappears into a countersink, said head being adapted to receive a turning tool. This has the advantage that, when it occasionally becomes necessary to clean off firmly adhering pelletized material from the rotary cutter  1 , the scraping off of said material does not result in damage to and therefore breaking off of the screw head, which would then subsequently make it impossible to unscrew the screw. 
     In the illustrative embodiments shown in  FIGS. 1-7 , screws are used as the fastening elements for the knives. As initially indicated, however, it is also possible to employ alternative fastening elements.  FIGS. 7 and 8  present an illustrative embodiment in which a wedge piece is used as the fastening element,  FIG. 8  showing a radial section through the rotary cutter  1 , wherein, in order to simplify the representation, only one knife has been inserted into the rotary cutter  1 , while  FIG. 9  shows a section along line IX-IX from  FIG. 8 . 
     As shown in  FIGS. 8 and 9 , the wedge piece  19  penetrates into the rotary cutter  1 , for which purpose the rotary cutter  1  is provided with a corresponding penetration  20  into which the wedge piece snugly fits with its rear cylindrical neck. With its front wedge-shaped portion, the wedge piece  19  presses laterally against the projecting portion  21 , which projecting portion  21  is in this case provided on its side facing the wedge piece  19  with a corresponding beveled portion  22 . On account of the friction between it and the beveled portion  22 , the wedge piece  22  is securely held in its position as presented in  FIGS. 8 and 9 . It should also be pointed out that the use of a wedge piece may be provided from both sides of the rotary cutter  1 , as was already presented in the above-described drawings in connection with the use of screws as fastening elements. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.