Abstract:
A solid face die plate for an underwater pelletizer includes a carrier or holding plate having a circular slot for holding a hard anti-wear element of highly wear-resistant material through which the extrusion orifices open for extruding polymer. The solid face die plate eliminates the need for insulation or plugging material in the center of the die plate and, by embedding the hard anti-wear element within the carrier, protects the edges of the hard anti-wear element for longer wear life.

Description:
This application is entitled to and hereby claims the priority of co-pending U.S. Provisional application, Ser. No. 60/800,030 filed May 15, 2006. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is related to the field of underwater pelletizers and, more particularly, to a solid face extrusion die plate for an underwater pelletizer. 
     2. Description of the Related Art 
     Known underwater pelletizers include an extrusion die or die plate having extrusion orifices extending therethrough and through which molten polymer is extruded. A cutter hub with cutter knives is oriented in rotatable facing relationship to the face of the extrusion die plate to cut the strands of polymer into pellets. A water box with a water inlet and outlet provides circulation of water to cool and solidify the strands of extruded polymer to enable the knives on the cutter hub to cut the strands into pellets. The water circulating through the water box also entrains the pellets into a slurry of water and pellets which are discharged through the outlet of the water box. 
     A known practice is to provide a recess in the die face of the extrusion die plate and place insulation material in the recess to reduce heat transfer from the extrusion die plate and molten polymer being extruded through the extrusion orifices to the water circulating through the water box. The insulation material inserted into the recess is typically a gasket material mounted in place by a suitable mastic, glue or the like and covered with a thin metal plate in the recess. The thin plate is secured to the die plate by suitable fasteners to maintain the insulation material in place in the recess. The insulation material is subjected to substantial degradation due to the temperatures encountered in the recess and, in some instances, does not effectively insulate the central recess area of the die face. Ineffective insulation can result in excessive cooling of the molten polymer as it is being extruded through the extrusion orifices causing freeze off of the molten polymer at the die face. 
     One solution to these problems is to provide an insulation plug in the form of a circular plate that substantially fills the recess in the die face, as set forth in U.S. Pat. No. 6,824,371, the entire disclosure of which is hereby expressly incorporated by reference in its entirety as though fully set forth herein. The circular plate of the &#39;371 patent, referred to herein as a gasket eliminator plug (GEP), reduces the transfer of heat from the molten polymer being extruded through the orifices in the extrusion die plate to the water circulating through the water box of the underwater pelletizer, and further is not subject to deterioration and degradation as encountered with conventional insulation techniques. 
     The tolerance between the GEP of the &#39;371 patent, or conventional insulation materials, and the die face is very small. Hence, it is easy to damage the die face when installing or removing the GEP during routine maintenance. This is a problem in that the GEP is relatively expensive to manufacture and yet, to work correctly, must have a tight fit within the die plate. 
     SUMMARY OF THE INVENTION 
     In order to overcome the known problems associated with the insulation techniques of the prior art, the present invention provides a solid face die plate for an underwater pelletizer. The solid face die plate includes a die plate base member and a removable die insert body. The die insert body has a solid face plate on its cutting face side which is made up of a carrier or holding plate with a hard anti-wear element (or elements) surrounding the extrusion die holes. The carrier is made of one piece of material and has a generally circular slot for holding the hard anti-wear element. Once fitted within the slot, the hard anti-wear element is held, supported and protected by the carrier. 
     The solid face die plate is affixed to the die insert body so as to be in contact with cutter knives and the water flowing through the cutting chamber. Because it is solid, the die plate prevents water flowing through the cutting chamber from leaking behind the die plate and also ensures that molten polymer behind the die plate cannot leak into the cutting chamber. 
     It is, therefore, an object of the present invention to provide a solid face die plate for an underwater pelletizer in which the hard anti-wear element of the die plate is held, supported and protected by a carrier having a unitary construction. 
     Another object of the present invention is to provide a solid face die plate that eliminates the need for a gasket eliminator plug or insulating materials that are otherwise necessary with conventional die plate designs having an open center. This solid face design also avoids the need for insulation holes and nose cone bolt holes. 
     A further object of the present invention is to provide a solid face die plate with no insulation changing requirements or other maintenance needs associated with the center of the die. 
     Still another object of the present invention is to provide a unitary carrier that protects the hard anti-wear element of the die plate, especially the edges, with the hard anti-wear element fitting flush within the carrier. 
     A still further object of the present invention is to provide a solid face die plate that enables quicker die changes to be made between products being pelletized. 
     Yet another object of the present invention is to provide a solid face plate that can be permanently or removably attached to the die insert body, with removable attachment allowing the same die insert body to be assembled with multiple solid face die plates. Removable attachment also results in cost and time savings as only the solid face die plate needs to be replaced instead of the entire die insert body. 
     A still further object of the present invention is to provide a solid face die plate which reduces heat transfer from the molten polymer and the housing of an underwater pelletizer to water circulating through the water box associated with the cutter hub, cutter knives and die face. 
     Yet another object of the present invention is to provide a solid face die plate for an underwater pelletizer which will conform to conventional forms of manufacture, be of simple construction and easy to use so as to provide a device that will be economically feasible, long lasting and relatively trouble free in operation. 
     These together with other objects and advantages which will become subsequently apparent reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial sectional view of an underwater pelletizer illustrating a solid face plate mounted on a die insert body in accordance with the present invention. 
         FIG. 2  is a perspective view of the die insert body and solid face plate of  FIG. 1 . 
         FIG. 3  is a rear view of the die insert body and solid face plate of  FIG. 2 . 
         FIG. 4  is a front view of the die insert body and solid face plate of  FIG. 2 . 
         FIG. 5  is a sectional view taken along line A-A of  FIG. 4 . 
         FIG. 6  is an enlarged view of detail B of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In describing a preferred embodiment of the invention illustrated in the drawings, specific terminology will be resorted to for the sake of clarity. However, the invention is not intended to be limited to the specific terms so selected, and it is to be understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar purpose. 
       FIG. 1  of the drawings illustrates an underwater pelletizer including a solid face die plate in accordance with the present invention, generally designated by reference numeral  10 , mounted on a housing, generally designated by reference numeral  12 , of an extruder. The housing  12  includes an inlet passageway  14  receiving molten polymer from upstream equipment. The molten polymer is diverted outwardly by a nose cone  16  and passes through a plurality of extrusion orifices  18  in the die plate  10 . The die plate  10  preferably has radial heating elements  22  extending inwardly from the periphery thereof and a plurality of apertures  24  receiving bolts  26  which are threaded into threaded apertures  28  in housing  12 . 
     The solid face die plate  10  may be unitary in construction or, as shown in  FIG. 1 , may preferably include a die plate base member  21  and a removable die insert body  30  fitted therein. In either case, the solid face die plate  10  has an attaching surface  32  on its downstream side. Mounted on the attaching surface  32  is a solid face plate, generally designated by reference numeral  34 , that includes a carrier  36  and a hard anti-wear element  38 . The solid face die plate  10  also includes a bore  40  to accommodate a fastening element  42  used to hold the nose cone  16  in place on the die plate  10 . In the embodiment shown in  FIG. 1 , the bore  40  is in the die insert body  30  portion of the solid face die plate  10 . 
     A water box, generally designated by reference numeral  50 , in the form of a hollow housing includes a flange  52  secured to the die plate  10  and housing  12  by the bolts  26 . A seal gasket  27  ensures a watertight seal between the die plate  10  and carrier  36  and the water box  50 . 
     The water box  50  includes a water inlet  54 , a water and pellet slurry outlet  56  and a flange  58  associated with a similar flange on a drive unit (not shown). Water circulating through the water box cools and solidifies the polymer and entrains the pellets into a water and pellet slurry for discharge through outlet  56 . Other standard components associated with the water box and pelletizer such as the drive shaft, cutter hub and cutter knives for cutting polymer strands extruded through the orifices  18  into pellets are not shown but are of conventional design and function in a well known manner to extrude molten polymer through extrusion holes or orifices  18  in the solid face die plate  10  as would be known by persons of ordinary skill in the art. 
     As shown in  FIG. 2 , the solid face plate  34 , including the hard anti-wear element  38  fitted within the carrier  36 , sits flush with the die insert body  30  and may be attached thereto by welding, brazing or other comparable technique. The embedded position of the hard anti-wear element  38  within the carrier  36  protects the edges  60  (see  FIGS. 5 and 6 ) of the hard anti-wear element  38 , preventing them from being damaged. The one-piece construction of the carrier  36  also eliminates the need for an insulation plate, or GEP such as that described in the &#39;371 patent. Given the cost of manufacturing a GEP, this results in cost savings to the user. 
     The die insert body  30  is typically made of carbon steel while the hard anti-wear element  38  is made of a high-wear-resistant material such as tungsten carbide. In a preferred construction, the carrier  36  is made of a nickel-iron alloy such as HASTELLOY C-276 alloy or INVAR 36 alloy. As shown in  FIG. 2 , the hard anti-wear element  38  may be formed as a solid tungsten carbide ring or, alternatively, may be composed of a plurality of tungsten carbide tiles as is known in the art. Generally, smaller dies are made with a solid face ring while tiles are used in the construction of larger die plates. 
     As shown in the rear view of  FIG. 3 , machined ramps  70  formed in the die insert body  30  funnel the extrusion material to the attaching surface  32  and into the upstream side of the solid face plate  34 . The bore  40  is drilled and tapped in the solid center plate  72  for connection of the nose cone  16  to the die plate  10 . The die insert body  30  has a flange  73  provided with holes  74  for fastening of the die insert body  30  to the die plate base member  21  with fastening elements  75 . 
     A front view of the solid face plate  34  as coupled to the die insert body  30 , with the exit openings  19  of the plurality of extrusion orifices  18 , is shown in  FIG. 4 . Sectional views of the die insert body  30  with solid face plate  34  are provided in  FIGS. 5 and 6 . 
     In addition to being brazed onto the die insert body  30 , the solid face plate  34  according to the present invention may also be bolted in position. In this embodiment, fastening elements such as socket head cap screws are fitted into counter-bored holes provided in the inner surface of the hard anti-wear element (not shown). With this construction, the solid face plate  34  is interchangeable and can be replaced separately from the die insert body  30 , resulting in cost savings to the user. The user can also use the same die insert body with multiple carriers which provides a benefit for maintenance purposes between product changes. 
     With the solid face plate  34  according to the present invention, the need for an insulation plate or GEP is eliminated. This results in the elimination of three drill and tap holes, as well as the associated bolts which can become eroded and broken. There is also no need for RTV insulation sealer. Hence, maintenance time is reduced and safety of the operator is improved. 
     The solid face plate  34  may further be provided with a wear limit indicator hole that indicates wear life remaining on the hard anti-wear element. This hole can be machined into the carrier so as to be exposed when grinding the die plate to its maximum. 
     The foregoing descriptions and drawings should be considered as illustrative only of the principles of the invention. The invention may be configured in a variety of shapes and sizes and is not limited by the dimensions of the preferred embodiment. Numerous applications of the present invention will readily occur to those skilled in the art. Therefore, it is not desired to limit the invention to the specific examples disclosed or the exact construction and operation shown and described. Rather, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.