Abstract:
A center heated die plate for an underwater pelletizer radiates heat outwardly to the extrusion orifices and die faces of the die plate thereby maintaining the die plate and extrusion orifices at an elevated temperature to obtain optimum flow of molten polymer through the extrusion orifices. In one embodiment, a cylindrical heating coil is placed in a hollow central core of the die plate and a plurality of peripheral heating elements are inserted in radial recesses distributed around an outer perimeter of the die plate to create an inner heat zone and an outer heat zone which are separately controllable.

Description:
[0001]     This is a continuation-in-part application of co-pending U.S. application, Ser. No. 11/041,865 filed Jan. 25, 2005. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention generally relates to heated die plates for underwater pelletizers and, more specifically, to center heated die plates for underwater pelletizers.  
         [0004]     2. Description of the Related Art  
         [0005]     Underwater pelletizers are well known and include a die plate having a plurality of extrusion orifices extending from an upstream side of the die plate to a downstream side for extruding strands of molten polymer or other extrudable materials. The extrusion orifices are arranged in one or more circular patterns concentrically of and in radially spaced relation to the center of the die plate and in spaced relation to each other. The extruded strands of polymer or other material are cut into pellets by rotating cutter blades in a water box. The blades cut the strands into pellets which have been cooled and solidified by the water in the water box. A slurry of pellets and water is discharged from the water box for subsequent removal of moisture from the exterior surface of the pellets.  
         [0006]     It is desirable to maintain the die plate and orifices at an elevated temperature for optimum extrusion of the molten polymer through the extrusion orifices. Various efforts have been made to heat the die plate including the use of passageways in the die plate through which a heating fluid is circulated. Also, radial cavities have been formed in the periphery of die plates into which electric cartridge heaters are inserted from the external periphery of the die plate. Such known structures are typically illustrated in the following U.S. patents owned by the assignee of this application and include disclosures of heated die plates: Pat. Nos. 4,123,207; 4,621,996; 4,728,276; 5,403,176; and 5,624,688.  
         [0007]     As illustrated in the above patents, heated die plates with concentric circular patterns of extrusion orifices are generally well known as well as passageways for heated fluid and cavities or pockets for receiving cartridge heaters. Such die plates require that portions of the die plates through which extrusion orifices would normally extend be occupied by the passageways for heating fluid or cavities for cartridge heaters. The passageways or cavities thus reduce the number of extrusion orifices that can be incorporated into the die plate since the passageways carrying the heating fluid or the cavities or pockets receiving the cartridge heaters require that the extrusion orifices be omitted from a portion of the area of the die plate. Also, the heated fluid passageways and heat cartridge cavities tend to concentrate heat radiation in areas closely adjacent to the passageways or cavities.  
       SUMMARY OF THE INVENTION  
       [0008]     The center heated die plates of the present invention are installed in a conventional manner between the extruder and the water box of an underwater pelletizer. The upstream side of the die plate receives molten polymer from the extruder with the molten polymer being extruded through a plurality of orifices extending from the upstream side to the downstream side of the die plate to form extruded strands of polymer. The extruded polymer strands are cooled by water passing through the water box and are cut into pellets by rotating cutter blades associated with the downstream side of the die plate in a manner well known in the art of underwater pelletizing.  
         [0009]     In the present invention, the circular orifice pattern of the die plate is maintained, and the central portion of the die plate is provided with a electrical resistance heating element. In one embodiment of the present invention, the center of the upstream face of the die plate, adjacent the nose cone, is removed or cut-out to provide a hollow or recessed circular central area inwardly of the nose cone. A circular heating plate is positioned in the recessed central area inwardly of the nose cone and between the nose cone and the die plate for heating the center of the die plate. The flat bottom surface of the circular heating plate is positioned flush with the bottom of the recessed central area to provide full surface contact so that heat from the heating plate flows directly to the die plate.  
         [0010]     The die plate is retained in position in a conventional manner by fasteners that connect the extruder, die plate and water box. The nose cone is retained in position by the normally provided nose cone anchor bolt which extends through a circular opening in the center of the circular heating plate. Heater leads or heater wiring extend through a single radial hole in the die plate for attaching to a heater lead connector associated with the circular heating plate for providing power to the plate. The heater wiring can be affixed to the heater lead connector or detachable therefrom. This embodiment of the present invention can be used on one piece die plates, as well as removable center die plates.  
         [0011]     In a second embodiment of the present invention, the entire inner diameter of the die plate is cut out to form a centrally located, cylindrical hollow area in the die plate. A cylindrically coiled electric resistance heating element is placed in the hollow inner area. The cylindrically coiled electric resistance heating element is flexible and is designed so that its outer surface is in contact with the inner surface of the die plate hollow inner area so that heat from the heating element flows directly into the die plate. The nose cone is secured by a conventional anchor bolt to close off the upstream side of the hollow inner area; and the other end is closed off by a circular insulation plug of the type disclosed in U.S. Pat. No. 6,824,371, the disclosure of which is expressly incorporated by reference as if fully set forth herein (hereinafter the “&#39;371 patent”), or similar insulation plate, which forms a closure for the hollow downstream end of the die plate.  
         [0012]     This second embodiment also utilizes heater leads or heater wiring which extend through a single radial hole in the die plate powering the coiled electric resistance heating element. The heater wiring is preferably formed integrally with the heating element. This embodiment of the present invention can also be used on one piece die plates, as well as removable center die plates.  
         [0013]     Positioning the heating elements in direct contact at the center of the die plate provides more effective and more uniform radiation of heat outwardly from the center of the die plate to more effectively elevate the temperature of the die plate and extrusion orifices and more evenly heat all of the extrusion orifices which are arranged in concentric circular patterns about the center of the die plate. The present arrangement which positions the circular or cylindrical heating element in the center of the die plate also avoids elimination of extrusion orifices in certain areas of the die plate which is otherwise required when heating fluid passageways are utilized or when cavities are provided for radially extending cartridge heaters. The center heated die plate is especially beneficial when pelletizing micro pellets which utilize extrusion orifices of very small diameters in which the extruded strands and micro pellets typically have a maximum outside dimension of about 50 microns or less.  
         [0014]     In a third embodiment of the present invention, an inner heat zone is created by a cylindrically coiled electric resistance element as in the second embodiment. However, the inner heat zone is augmented by an outer heat zone created by a plurality of rod-type heaters inserted within radial recesses formed adjacent the outer perimeter of the die plate. Each of the inner and outer heat zones is preferably controlled separately by its own thermocouple or temperature reading device, allowing the inner heating element to be run at a different temperature than the outer heaters. This embodiment of the present invention can also be used on one piece die plates, as well as removable center die plates. As illustrated herein, removable center die plates are depicted that include a die plate body and a separate tapered die plate insert of the type disclosed in U.S. Pat. No. 5,403,176, the disclosure of which is expressly incorporated herein by reference as if fully set forth (hereinafter the “&#39;176 patent”).  
         [0015]     It is therefore an object of the present invention to provide a die plate for an underwater pelletizer having an electrical resistance heating element associated in direct heat exchange contact with a central portion of the die plate between the upstream side and the downstream side to provide heat that radiates outwardly from the center of the die plate and evenly to the extrusion orifices which are oriented in concentric circular patterns about the center of the die plate in order for the entire extrusion orifice pattern to be maintained close to an optimum temperature for extrusion of the molten polymer or other extrudate.  
         [0016]     Another object of the present invention is to provide a center heated die plate in which the electric resistance heating element is in the form of a cylindrical coil positioned internally in a hollow die plate between the upstream and downstream sides with the hollow interior of the die plate being closed by a nose cone at the upstream side and closed by an insulation plug or plate at the downstream side with both the insulation plug and nose cone being held in position by an anchor bolt.  
         [0017]     A further object of the present invention is to provide a center heated die plate for underwater pelletizers in which the central location of the heating element provides effective and even heating of the extrusion orifices and associated areas of the die plate to maintain all of the extrusion orifices and the molten polymer passing therethrough at a constant and even temperature to produce higher quality pellets of a more uniform size.  
         [0018]     Still another object of the present invention is to provide a center heated die plate in which heater leads or heating wiring connected to the central heating element extend through a single radial hole in the die plate for connection to a power source outside of the die plate.  
         [0019]     Yet a further object of the present invention is to provide a center heated die plate in accordance with the foregoing objects in which the cylindrical coil heating element forms an inner heat zone and a plurality of radially-extending, rod-type heaters are provided in the outer perimeter of the die plate to form an outer heat zone, the inner and outer heat zones being separately controllable.  
         [0020]     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  
       [0021]      FIG. 1  is a schematic vertical sectional view of one embodiment of the center heated die plate of the present invention in which a circular heating plate is disposed in a recess in the upstream side of the die plate.  
         [0022]      FIG. 2  is an exploded group perspective view of the structure of  FIG. 1  illustrating the relationship of the circular heating plate to the die plate and pelletizer components.  
         [0023]      FIG. 3  is an exploded group perspective view of a second embodiment of the present invention in the form of a coil type center heated die plate and illustrating the association of the components prior to assembly.  
         [0024]      FIG. 4  is a perspective view, partially in section, illustrating the association of the components of the second embodiment of the center heated die plate of the present invention.  
         [0025]      FIG. 5  is a partial exploded group perspective view of the second embodiment of the present invention, similar to  FIG. 3 , but showing the heater wiring formed integrally with the cylindrical coil electric resistance heating element.  
         [0026]      FIG. 6  is an exploded group perspective view, partially in section, of a third embodiment of the present invention having a coil type center heated die plate and a plurality of radially-extending rod-type heater elements positioned adjacent the outer periphery of the die plate.  
         [0027]      FIG. 7  is a perspective view, partially in section, illustrating the association of the components of the third embodiment of  FIG. 6 , as assembled.  
         [0028]      FIG. 8  is an end view of the assembled components of  FIG. 7 .  
         [0029]      FIG. 9  is a side perspective view of the assembled components of  FIG. 8 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]     Although only preferred embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its scope to the details of construction and arrangement of components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or carried out in various ways. Also, in describing the preferred embodiments, specific terminology will be resorted to for the sake of clarity. It is to be understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar purpose.  
         [0031]     Referring to the drawings,  FIGS. 1 and 2  illustrate one embodiment of the center heated die plate of the present invention designated generally by reference numeral  10  associated with components of an underwater pelletizer. The underwater pelletizer includes an inlet housing  12  from a molten polymer extruder (not shown). The inlet housing  12  includes a passageway  14  for molten polymer or other extrudate (hereinafter collectively referred to as “polymer”) and a nose cone  16  which directs the polymer to the upstream side  18  of the die plate  10 . The die plate  10  includes one or more concentric rings of extrusion orifices  20  which extend from the upstream face  18  of the die plate  10  to the downstream face  21  of the die plate  10 . A plurality of knife blade assemblies  22  mounted on a rotatably driven cutter hub  23  in a water box (not shown) of the underwater pelletizer cut the extruded cooled and solidified strands of the extruded polymer into pellets which are entrained with the cooling water passing through the water box and discharged as a water and pellet slurry with the pellets subsequently being dried by removing moisture in a dryer.  
         [0032]     As illustrated in  FIGS. 1 and 2 , the upstream face or side  18  of the die plate  10  is cut out to provide an annular recess or cavity  24  inwardly of the extrusion orifices  20 . The recess  24  is of circular configuration and receives a circular heating plate  26  which is secured in the bottom of the recess  24  by fasteners  28  threaded into internally threaded bores  30  to anchor the heating plate  26  in the interior of recess or cavity  24 . The heating plate  26  includes a lateral connecting member  32  that connects to electrical leads  34  which extend outwardly through a radial hole or passageway  36  for connection to a source of electrical energy. The circular heating plate  26  is designed to fit neatly within recess  24  with the bottom  27  of plate  26  fitting flush against the bottom  25  of the recess  24  to provide full surface contact between surfaces  27  and  25 . This full surface contact insures that heat from the circular heating plate  26  flows directly to the die plate surface  25  and thus into the die plate  10 .  
         [0033]     Once the heating plate  26  has been anchored in place and connected to the electrical leads  34 , the nose cone  16  is positioned over the heating plate  26 . The electrical leads  34  can be firmly attached to connector  32  or detachably connected thereto. A shallow recess  38  in the base of the nose cone  16  fits within opening  24  of the die face  18  to seal the nose cone  16  against the die face  18 .  
         [0034]     A nose cone anchor bolt  40  extends through a circular insulation plug  42 , such as that disclosed in the &#39;371 patent, that is positioned in a recess  44  on the downstream side of the die plate  10 . The anchor bolt  40  extends through a central hole  46  in the die plate and a central hole  39  in the heating plate  26  and is screw threaded into a threaded bore  48  in the base of the nose cone  16 .  FIG. 1  illustrates the assembled condition of the components of the center heated die plate  10 . The circular insulation plug  42  and anchor bolt  40  form a closure for the central bore  46 . Molten polymer can then be extruded through the extrusion orifices  20  with the heat plate  26  maintaining an elevated temperature of the die plate by radiating heat outwardly from the recess or cavity  24  at points intermediate the upstream die face  18  and the downstream die face  21 . The insulation plate  42  forms a closure for the downstream side of the die plate and the nose cone  16  forms a closure for the upstream side of the die plate so that the heat produced by the heat plate  26  is conducted to the die plate between the upstream and downstream faces or sides of the die plate.  
         [0035]     The heating plate  26  of the present invention is preferably a conventional plate heater, such as commercially available from Watlow Electric Manufacturing Company, St. Louis, Mo., Model HT Foil Heater.  
         [0036]      FIGS. 3, 4  and  5  illustrate a second embodiment of the present invention in which the die plate, generally designated by reference numeral  60 , includes a cylindrical hollow interior  62  extending from an upstream side or face  64  to a downstream side or face  66 . The die plate  60  includes extrusion orifices  68  extending from the upstream side  64  to the downstream side  66  and arranged in a circular pattern of one or more circles. A heating device  70  in the form of a cylindrical coil is positioned in the cylindrical interior  62  of the die plate  60  and includes a connector  72  adjacent the nose cone  74 . The nose cone  74  is secured in place with its base inwardly of the circular pattern of the extrusion orifices  68 . The connector  72  is connected to electrical leads  76  which extend out through a radial hole or passageway  78  for connection to a source of electrical energy.  
         [0037]     The heating coil  70  is disposed interiorly of the cylindrical interior  62  in direct heat exchange contact with the surface  63  of the cylindrical interior  62 . The heating coil  70  is flexible so that it can be twisted with its outer diameter reduced so as to fit within the cylindrical interior  62 . When released, the diameter expands so that the outer surface  71  of the coil  70  seats in direct contact with the surface  63  of the cylindrical interior  62 . Thus, with the spring tension fit of the coil to the inside diameter of the interior  62 , heat from the heating coil  70  passes directly into the die plate  10  through the surface  63 . The downstream end  75  of the coil  70  rests against a shoulder  80  in the inner end of the cylindrical interior  62 . An insulation plug or closure plate  82  is inserted into the downstream side  66  of the die plate and includes a sealing edge  84  which engages surface  86  of shoulder  80  on the die plate. A nose cone anchor bolt  88  extends through the plate  82 , heating coil  70  and threads into the nose cone  74  to maintain the structure in assembled relation as illustrated in  FIG. 4 .  
         [0038]     In the form of the second embodiment shown in  FIG. 5 , the electrical leads or heater wiring  76 ′ which extend out through the radial hole or passageway  78 ′ are formed integrally with the upstream end of the cylindrical coil heating device  70 ′. This integral structure reduces the cost and improves the quality of the cylindrical coil heater assembly.  
         [0039]      FIGS. 6, 7 ,  8  and  9  illustrate a third embodiment of the present invention having multiple heating zones. Many of the components of the third embodiment are the same as or comparable to those already discussed in connection with the second embodiment. Therefore discussion of these components, which are identified by common reference numerals, will not be repeated here.  
         [0040]     Also, while the previous embodiments of  FIGS. 1-5  depicted only the die plate insert, the omission of the die plate body was only for purposes of ease in illustration. Similarly, general reference to the die plate insert as the “die plate  60 ” was also for ease of discussion. However, in each of the embodiments disclosed herein, the “die plate” more precisely includes a die plate insert  90  and a die plate body  92  into which the insert  90  is fitted in use.  
         [0041]     Hence, as shown in  FIGS. 6 and 7 , the die plate of the third embodiment includes a die plate body  92  and a die plate insert  90  positioned in a tapered opening  94  in the die plate body  92 . The insert  90  includes an outer surface  96  which has a taper corresponding to the taper of the die plate body opening  94 . The enhanced heat transfer capacity obtained with these correspondingly tapered surfaces is discussed in the &#39;176 patent previously incorporated by reference herein.  
         [0042]     According to the third embodiment, the outer ring  98  of the die plate body  92  includes a plurality of radial recesses  100  into which are respectively fitted a plurality of peripheral heaters  102  in a manner as is known in the art. These peripheral heaters  102  create an outer heat zone and are controlled by a thermocouple  104  or other type temperature reading device positioned near the outer perimeter of the extrusion orifices  20 . Heat generated by the peripheral heaters  102  in the outer heat zone is transferred inwardly to the die plate insert  90  and provides additional heating to prevent polymer freeze off in the extrusion holes  20  of the die plate insert  90 . The peripheral heaters  102  may be embodied as rod or cartridge heaters as is known by persons of ordinary skill in the art.  
         [0043]     The inner coil heater  70  according to the present invention creates an inner heat zone that is separately controllable from the outer heat zone. This separate control is provided by a second thermocouple  106  which, in the embodiment shown, is located in a recess  108  on the tapered outer surface  96  of the die plate insert  90 . Having two separate thermocouples  104 ,  106  and two resulting heat zones is advantageous as it allows the inner heat zone to be run at a temperature higher than that of the outer heat zone to improve the running conditions of the die plate insert. However, the independent control of the two heat zones can be used to effect any relationship between the temperatures of the two zones as may be desired.  
         [0044]      FIGS. 8 and 9  show the assembled die plate of the third embodiment with the die plate insert  90  inserted within the die plate body  92 . The electrical lead  76  from the cylindrical coil  70  passes outwardly through a radial hole or passageway  36 ′ in the die plate body, while respective electrical leads  110  for each of the rod heaters  102  also extend outwardly from the periphery of the die plate body  92  for connection to a source of electrical energy.  
         [0045]     The heating coil  70  and  70 ′ of the present invention is preferably a conventional coil heater, such as that commercially available from Gebhard &amp; Castiglia GmbH &amp; Co., Waldbroel, Germany, Model GC Coil, which includes integral electrical leads  76 ′.  
         [0046]     The foregoing should be considered as illustrative only of the principles of the invention. Since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described. Accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.