Abstract:
An insulated die plate assembly for use in underwater pelletizing and other granulation processes includes a thin, continuous air chamber formed across the plate assembly generally parallel to the die face such that the heated upstream portion of the die plate assembly is thermally insulated from the downstream portion. The air chamber is atmospherically equilibrated by venting the air chamber to the atmosphere. The plurality of extrusion orifices, either individually or in groups, are formed in extrusion orifice extensions that extend through the insulation chamber so that the process melt to be granulated can pass therethrough. The orifice extensions and the components forming the air chamber around the orifice extensions channel heat along said extensions to maintain the process melt therein at a desired temperature, to help rigidify the die plate assembly and to better seal the air chamber.

Description:
[0001]    This application is a continuation application of Ser. No. 12/222,669, filed Aug. 13, 2008, and hereby claims the priority thereof to which it is entitled. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention generally relates to an insulated die plate assembly for use in underwater pelletizers and other granulation processes that include hot-face or non-fluidic pelletization. More specifically, the present invention relates to an insulated die plate assembly that includes a thin continuous air pocket or chamber formed across the plate assembly such that the upstream portion of the die plate assembly is thermally insulated from the downstream portion of the same assembly, thus allowing the respective portions to co-exist at different temperatures. The plurality of extrusion orifices, individually or in groups, extend through extrusion orifice extensions that project through the insulation air pocket or chamber so that the material to be pelletized or granulated can pass therethrough. 
       BACKGROUND OF THE INVENTION AND PRIOR ART 
       [0003]    Underwater pelletization equipment and its use following extrusion processing have been implemented for many years by Gala Industries, Inc. (“Gala”), the assignee of the present invention. Pelletization dies and die plates, in particular, are demonstrated in prior art disclosures including, for example, U.S. Pat. Nos. 4,123,207, 4,500,271, 4,621,996, 4,728,276, 5,059,103, 5,403,176, 6,824,371, 7,033,152, U.S. Patent Application Publication Nos. 20060165834 and 20070254059, German Patents and Applications including DE 32 43 332, DE 37 02 841, DE 87 01 490, DE 196 51 354, and World Patent Application Publications WO2006/081140 and WO2006/087179. These patents and applications are all owned by Gala and are expressly incorporated herein by reference as if set forth in their entirety. 
         [0004]    As well understood by those skilled in the art, die plates used with rotating cutter hubs and blades, such as in underwater pelletizing, have the extrusion orifices or through die holes arranged in a generally circular pattern, or groups of multiple die holes arranged (as in pods or clusters) in a generally circular array. As so arranged, the rotating blades can cut the extrudate as it exits the die holes along a circular cutting face. 
         [0005]    U.S. Pat. No. 4,378,964 and World Patent Application Publication No. WO1981/001980 disclose a multi-layer die plate assembly for underwater pelletization of polymeric materials in which an insulation layer, preferably zirconium oxide, is fixedly positioned between the body of the die plate and the layers comprising the cutting face of the die. Adjacent or proximal to the insulation layer is a heating chamber through which is circulated a heating fluid for maintenance of the temperature of the die. 
         [0006]    U.S. Pat. No. 4,764,100 discloses a die plate construction specifically described for underwater pelletization of plastic extrudate including a closed insulating space formed between the baseplate and the cutting plate through which penetrates the extrusion nozzles, and optional inserts serve to further strengthen and support the structure. 
         [0007]    Vacuum heat insulating cavities between extrusion nozzles are disclosed in U.S. Pat. No. 5,714,713 in a multi-step process that includes electron beam welding while the die components are maintained under high vacuum. This disclosure is extended to vacuum heat insulation portions in areas peripherally external to the extrusion nozzles for enhanced insulation performance in U.S. Pat. No. 5,989,009. 
         [0008]    Similarly, closed continuous thermal stabilization cavities filled with air or gas are disclosed in U.S. Pat. No. 6,976,834. Additionally, brazing in a furnace at high temperature, 900° C. to 1200° C., under vacuum is disclosed as a manufacturing process with controlled cooling under argon to prevent oxidation thusly presenting an opportunity to introduce vacuum into the thermal stabilization cavities. 
         [0009]    German Patent Application No. DE 100 02 408 and German Patent Utility Model No. DE 200 05 026 disclose a hollow space or a multiplicity thereof in the inner region of the nozzle plate and the nosecone extension to enhance temperature control by virtue of the reduction of mass necessitating temperature maintenance and thusly introducing thermal insulation. Use of solid, liquid, or gas as insulating materials is disclosed therein. 
         [0010]    World Patent Application Publication No. WO2003/031132 discloses the use of ceramic plates for insulation of the die face from the heated portion of the die body. 
         [0011]    Finally, Austrian patent application AT 503 368 A1 discloses a thermally insulated die plate assembly with a detachable face plate that is sealed to the discharge end of the extrusion orifice nozzles by an O-ring or metal seal. This die plate assembly is very fragile and highly susceptible to process melt leakage, thus requiring considerable maintenance. 
         [0012]    There is, therefore, a need for a thermally insulated die plate assembly which is robust in construction, retains the air pocket in a sealed condition, requires low maintenance and provides high performance. 
       SUMMARY OF THE INVENTION 
       [0013]    The thermally insulated die plate assembly of the present invention is installed in a conventional manner between the melting and/or mixing devices and the pellet transport components including mechanical, pneumatic, and/or fluid conveyance. The upstream side of the insulated die plate assembly receives molten polymer or other fluidized material from the melting/mixing devices that is subsequently extruded through the multiplicity of orifices extending from the upstream side to the downstream side of the die plate assembly to form extruded strands of material. The extruded strands, with at least marginal cooling, are cut into pellets by rotating cutter blades engaging a cutting surface or cutting die face associated with the downstream side of the die plate in a manner well known in the art of pelletizing. 
         [0014]    The thermally insulated die plate assembly of the present invention is retained in position in a conventional manner by fasteners that connect the melting and mixing components, the die plate, and the pellet transport components. The nose cone, optionally a separate component, is retained in position as required by the normally provided nose cone anchor bolt as is understood by those skilled in the art. Similarly, thermal regulation fluid as required enters and exits chambers in the die plate through conventional inlet and outlet orifices, respectively. 
         [0015]    The thermally insulated die plate assembly in accordance with the present invention is essentially formed by machining a cutout in the downstream side or die face side of a die plate body, preferably forming a generally circular cavity. The periphery of the cutout cavity should extend beyond the circular pattern or array of extrusion orifices or die holes with a raised circular ridge which matches and encompasses the circular pattern or array of extrusion orifices or die holes. The raised circular ridge thus divides the cutout cavity into, preferably, an annular outer section and a circular inner section. The raised circular ridge is preferably trapezoidal in vertical cross-section with the extrusion orifices extending centrally therethrough. Orifice protrusions project from the upper surface of the raised ridge at the extrusion orifice locations so that the extrusion orifices extend through the orifice protrusions. 
         [0016]    Finally, a cover plate with holes matching the orifice protrusions is sized to fit over and into the cutout cavity in the die plate body to complete the downstream side of the die plate assembly and form a generally planar die face. In addition, the upstream side of the cover plate is machined with a counterbore which conforms to the configuration of the orifice protrusions and defines the outside wall of the air cavity around the orifice protrusions and the raised circular ridge. The cover plate is attached around its periphery to the die plate body and attached around its matching holes to the distal end of the orifice protrusions adjacent the die face. 
         [0017]    The thickness of the cover plate is less than the depth of the cutout cavity so that when the cover plate is in place a thin, generally flat, continuous air pocket or air chamber is formed around the raised circular ridge and associated orifice protrusions, which air chamber is generally parallel to the die face. The thickness of the air chamber is on the order of about 0.05 millimeters (mm) to about 6.0 mm, and preferably about 0.5 mm to about 1.0 mm. Stated another way, the thickness of the air chamber is preferably about 5% to about 10% of the thickness of the die plate assembly. 
         [0018]    The raised circular ridge and associated orifice protrusions which encompass and extend the extrusion orifices from the base of the cutout cavity to the matching holes of the cover plate are together referred to herein as the “extrusion orifice extensions”. The extrusion orifice extensions for each of the extrusion orifices or die holes extend fully through the air chamber so that the orifice extensions are surrounded by the thermally insulating air. 
         [0019]    The air chamber is preferably vented to the atmosphere outside the die plate assembly, such as through one or more channels in the die plate body to provide for atmospheric equilibrium of the air chamber. The air chamber thus forms a thermally insulating air pocket or chamber between the typically heated upstream side of the die plate assembly and the downstream side forming the die face, which contacts the cooling water of the waterbox in an underwater pelletizer, or other cooling medium associated with a rotating cutter hub and blade assembly. 
         [0020]    The cover plate should be made of a chemical, corrosion, abrasion, and wear-resistant metal. The cover plate can contain at least one circumferential expansion groove on at least one face and preferably contains a multiplicity of circumferential expansion grooves on at least one face. When expansion grooves are formed on both faces, they are preferably arranged in a staggeringly alternating configuration. Preferably, the cover plate is welded in position with nickel steel. More preferably, the cover plate is attached by welding with nickel steel at peripheral grooves circumferentially surrounding the cover plate and at weld locations between the distal end of the orifice protrusions and the inside of the cover plate holes. 
         [0021]    The die plate body of the thermally insulated die plate assembly according to the present invention can be thermally regulated by any suitable heating system known to those skilled in the art, such as thermal regulation fluid as required to enter and exit heating chambers in the die plate body to conventional inlet and outlet orifices, respectively. Alternatively, the die plate body can be thermally regulated by at least one of electrical resistance, induction, steam, and thermal transfer fluid. Preferably, the die plate body is heated by electric heaters in techniques known to those skilled in the art. 
         [0022]    In a first embodiment of the present invention, the thermally insulated die plate assembly is configured with a one-piece die plate body. In a second embodiment of the present invention, the thermally insulated die plate assembly is configured with a two-piece die plate body having a removable center die insert thermally insulated in accordance with the present invention which is peripherally surrounded by a die plate outer ring thermally regulated by at least one of electrical resistance, induction, steam, and thermal transfer fluid. 
         [0023]    As used herein the term “die plate body” is intended to include the body of the die plate when the assembly of the present invention is configured as a one-piece construction and the removable center die insert in combination with the die plate outer ring when the assembly is configured in a two-piece construction. 
         [0024]    In addition to having the die face of uniform planarity, the annular cutting face containing the distal ends of the orifice protrusions, and through which penetrate the multiplicity of extrusion orifices, can be raised a certain distance above the remaining portion of the die face, as known to those skilled in the art. The rotating cutting blades thus engage the raised annular cutting face. The raised annular cutting face should be at least 0.025 millimeters higher than the surrounding die face and preferably is at least 0.50 millimeters above the surrounding die face. 
         [0025]    Preferably, at least the surface of the annular cutting face engaged by the cutting blades is subjected to an enhancing surface treatment. The enhancing surface treatment includes at least one of nitriding, carbonitriding, electroplating, electroless plating, electroless nickel dispersion treatments, flame spraying including high velocity applications, thermal spraying, plasma treatment, electrolytic plasma treatments, sintering, powder coating, vacuum deposition, chemical vapor deposition, physical vapor deposition, sputtering techniques and spray coating. These surface treatments result in metallizing, attachment of metal nitride, metal carbides, metal carbonitrides, and diamond-like carbon and can be used singly and in any combination. Different surface treatments can be applied to different circumferential planes on the cutting face and should be at least approximately 0.025 millimeters in thickness. Preferably, the treatments are at least approximately 0.50 millimeters in thickness. 
         [0026]    The raised circular ridge and associated orifice protrusions are formed in at least one annular ring, and each orifice protrusion can contain at least one to a multiplicity of extrusion orifices arranged in groups, pods, and clusters. The orifice protrusions can be of any geometry including at least one of oval, round, square, triangular, rectangular, polygonal, and in many combinations. Similarly, the orifice protrusions can be arranged concentrically, alternating, in a staggering configuration, and linearly, and can be positioned parallel to the arc of rotation of the cutting blades, perpendicular to the arc and including kidney to comma-like configurations. 
         [0027]    In addition, the extrusion orifices can be of any geometry including but not limited to round, oval, square, rectangular, triangular, pentagonal, hexagonal, polygonal, slotted, radially slotted and any combination thereof. A multiplicity of extrusion orifices can be of different geometry in a particular orifice protrusion or die face. 
         [0028]    In view of the foregoing, it is an object of the present invention to provide a die plate assembly in which the typically heated upstream portion of the assembly is thermally insulated from the typically cooled downstream portion adjacent the die face by an internal insulation air pocket or air chamber extending substantially parallel to the die face. 
         [0029]    A further object of the present invention is to provide a thermally insulated die plate assembly in accordance with the preceding object in which the insulation air pocket or air chamber surrounds extrusion orifice extensions configured as a raised circular ridge and associated orifice protrusions, through which the extrusion orifices extend to the die face. 
         [0030]    Another object of the present invention is to provide a thermally insulated die plate assembly in accordance with the preceding object in which the insulation air pocket or air chamber is formed by machining or cutting out a cavity in the downstream side of a die plate body leaving in place the raised circular ridge. The cavity is closed by a cover plate having a counterbore sized to match the extrusion orifice extensions and with holes to match the distal ends of the orifice protrusions. 
         [0031]    Still another object of the present invention is to provide a thermally insulated die plate assembly in accordance with the two preceding objects in which the raised ridge has a trapezoidal shape in vertical cross-section to aid in channeling heat to the orifice protrusions and thus maintain the process melt at a desired temperature in the extrusion orifice at the die face. 
         [0032]    A still further object of the present invention is to provide a thermally insulated die plate assembly in accordance with the preceding three objects in which the insulation air pocket or air chamber is configured to follow and surround the raised circular ridge and associated orifice protrusions so as to retain the heat in the raised ridge and orifice protrusions in order to maintain the process melt at a desired temperature in the extrusion orifices at the die face. 
         [0033]    It is another object of the present invention to provide a thermally insulated die plate assembly in accordance with the preceding objects in which the insulation air pocket or air chamber is vented to the atmosphere outside of the die plate assembly to maintain the temperature and pressure conditions inside the cavity or chamber equilibrated to the atmosphere. 
         [0034]    It is a further object of the present invention to provide a thermally insulated die plate assembly in accordance with the preceding objects in which the die plate body is configured in a single-body construction. 
         [0035]    Yet another object of the present invention is to provide a thermally insulated die plate assembly in accordance with the preceding objects in which the die plate body is configured in a two-piece construction including a removable center die insert surrounded by a die plate outer ring. 
         [0036]    Still yet a further object of the present invention is to provide a thermally insulated die plate assembly in accordance with the preceding object in which the removable insert and the die plate outer ring can be individually and/or separately heated or thermally regulated. 
         [0037]    A final object to be set forth herein is to provide a thermally insulated die plate assembly which will conform to conventional forms of manufacture, will have improved strength and robustness, will maintain the insulating air pocket tightly sealed to provide improved thermal insulation in operation, and will be economically feasible, long-lasting and relatively trouble-free in use. 
         [0038]    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 
         [0039]      FIG. 1  is a schematic vertical sectional view of a first embodiment of a thermally insulated die plate assembly in accordance with the present invention in which the assembly is of a single body construction. 
           [0040]      FIG. 2  is an enlarged schematic vertical sectional view illustrating further details of the components around an upper extrusion orifice for the embodiment shown in  FIG. 1 . 
           [0041]      FIG. 3  is a partial cut-away perspective view of the die plate assembly shown in  FIG. 1 , illustrating the association of the various components. 
           [0042]      FIG. 4  is a schematic vertical sectional view of a second embodiment of a thermally insulated die plate assembly in accordance with the present invention in which the assembly is of a two-piece construction, including a removable center die insert and die plate outer ring. 
           [0043]      FIG. 5  is a schematic vertical cut-away side perspective view of one-half of the removable center insert of the die plate assembly shown in  FIG. 4 . 
           [0044]      FIG. 6  is an enlarged view of the components shown in  FIG. 5 , illustrating the detail of the air chamber around the raised circular ridge and the orifice protrusion. 
           [0045]      FIG. 7  is a schematic top perspective view of one-half of the removable center insert of the die assembly shown in  FIG. 4 , showing the design of the raised circular ridge and the orifice protrusions associated therewith. 
           [0046]      FIG. 8  is a schematic bottom perspective view of a cover plate which, when turned over, is assembled onto the top of the removable center insert shown in  FIG. 7  to form the air pocket or air chamber of the die plate assembly shown in  FIG. 4 . 
           [0047]      FIG. 9  is an enlarged schematic vertical section view showing the cover plate of  FIG. 8  assembled onto the removable insert shown in  FIG. 7  with the welds in place around the periphery of the cover plate and around the extrusion orifices, together with a hard face on the downstream surface of the cover plate. 
           [0048]      FIG. 10  is an exploded schematic vertical section view of a thermally insulated die plate assembly similar to  FIG. 4  in which the removable center insert includes a separate center heating coil. 
           [0049]      FIGS. 11   a - g  are a composite perspective view illustrating various configurations for the heat conducting protrusions in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0050]    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. 
         [0051]    Referring to the drawings,  FIGS. 1 ,  2  and  3  illustrate one embodiment of the present invention associated with components of a pelletizer, such as an underwater pelletizer. The pelletizer includes an inlet housing  12  from a melting and/or mixing apparatus (not shown). The inlet housing includes a passageway  14  for molten material or other extrudate (hereinafter collectively referred to as “process melt”) that can include organic materials, oligomers, polymers, waxes, and combinations thereof without intending to be limited. Nose cone  16  directs the process melt to the upstream side of the single-body or one-piece die plate assembly constructed in accordance with the present invention and generally designated by reference numeral  10 . The nose cone  16  can be attachedly connected to the die plate assembly by a threaded rod (not shown). The threaded rod is screw threaded at one end into threaded bore  18  of nose cone  16  and at its distal end into threaded bore  20  of die plate  10 . Alternately, the nose cone  16  can be rigidly affixed to or unitary with the die plate  10  and need not be attachedly connected as herein described. 
         [0052]    The single-body die plate assembly  10  contains a multiplicity of extrusion orifices  22  concentrically arranged singly or in multiples thereof in at least one annular ring that extends from the upstream face  24  to the downstream face or die face  26  of the die plate assembly  10 . A plurality of cutter blades  28  mounted on a rotatably driven cutter hub  30  in a cutting chamber (not shown) cut the extruded and at least partially solidified process melt extruded through orifices  22  into pellets at the cutting surface of the die face  26 . The pellets thusly formed are transported mechanically, pneumatically, hydraulically, or in combinations thereof to downstream processing, such as a dewatering system, drying equipment and the like. 
         [0053]    The die plate assembly  10  is made up with two major components, die plate body  36  and cover plate  38 . A thin, continuous air pocket or air chamber  32 , parallel to die face  26 , is formed between the downstream side of the die plate body  36  and the upstream side of the cover plate  38 . In order for the extrusion orifices  22  to pass through the air chamber  32 , the extrusion orifices  22  extend through a raised circular ridge  34  formed in the downstream face of the die plate body and orifice protrusions  35  positioned on top of the ridge  34  (see  FIG. 2 ), which together define the extrusion orifice extensions, generally designated by reference numeral  31 . 
         [0054]    The upstream side of the cover plate  38  is provided with a generally circular counterbore  76  which conforms to and receives the circular array of orifice protrusions  35 . The counterbore  76  has outlet holes  39  which match the orifice protrusions  35  and form the distal ends  68  of the extrusion orifices  22 . The distal ends  70  of protrusions  35  then fit into the matching holes  39  in the cover plate  38 . The raised circular ridge  34  and associated heat conducting protrusions  35 , which encompass and provide heat to the distal end  68  of the extrusion orifices  22 , thus extend through and are surrounded by the air chamber  32 . 
         [0055]    In order to form the air pocket or air chamber  32 , the central area of the downstream face  26  of die plate body  36  is machined or cut out to provide a circular recess or cavity  33 . The cavity  33  extends beyond the extrusion orifices  22  and is preferably formed with the raised circular ridge  34  in place, although the ridge could be formed as a separate piece and welded or otherwise attached to the bottom of the cavity  33 . The raised ridge thus divides the cavity  33  into an annular outer section  72  and an inner circular section  74 . The orifice protrusions  35  can also be formed during the machining process and thus be integral with the raised ridge  34 . However, preferably, the protrusions  35  are configured as separate collars of the same material as the die plate body  36  (and ridge  34 ) and are adhered to the top of ridge  34  as by welding or the like. 
         [0056]    Circular cover plate  38  with holes  39  matching the distal ends  70  of the orifice protrusions  35  overlays the recess cavity  33  and is attachedly connected to die plate body  36  and to orifice protrusions  34  by brazing, welding, or similar technique known to those skilled in the art. Preferably, the cover plate  38  is constructed of an abrasion and corrosion resistant metal and, more preferably, is constructed of nickel steel. Similarly, attachment of the cover plate  38  to the die plate body  36  and to the distal ends  70  of orifice protrusions  35  is preferably achieved by welding and, more preferably, is achieved by nickel steel welding. Weldments  40  and  42  are preferentially made at circumferential grooves  77  peripherally about the cover plate  38  and into the cover plate holes  39  which are sized to expose a portion of the distal end  70  of protrusions  35  for welding or the like. To assist in rigidifying the cover plate  38  to the die plate body  36 , the peripheral edge  80  is designed to rest on ledge  82  cut into the downstream face of the die plate body. The peripheral edge  80  and the die plate body  36  have opposing chamfers which form groove  77  for receiving the peripheral weld  40  and maintain the peripheral edge  80  solidly against the ledge  82 . 
         [0057]    The surface of the cover plate  38  and thus the downstream face  26  is preferably coated with a chemical, abrasion, corrosion, and wear resistant coating  60  as described hereinbelow. Once weldment  42  is in place, along with wear resistant coating  60 , if included, the distal end  68  of the extrusion orifices  22  can be completed by machining from the downstream side of the die plate assembly, such as with an EDM machine or otherwise as known by those skilled in the art, thus clearing any weld  42  and coating  60  from the extrusion orifice distal end  68 . 
         [0058]    The raised circular ridge  34  is preferably trapezoidal in vertical cross-section to aid in channeling heat to the orifice protrusions  35 , which transfer the heat from the raised ridge to the die face  26 , thus maintaining the process melt at a desired temperature in the extrusion orifice distal end  68 , and to assist in creating a robust thermally insulated die plate assembly. While a trapezoidal cross-section for the raised circular ridge is preferred, other shapes for the ridge cross-section could be designed by those skilled in the art in order to achieve the foregoing goals, as contemplated by the present invention. 
         [0059]    The assemblage as heretofore described encloses the circular recess  33  to form the thin, continuous thermal air pocket or air chamber  32  which is preferably connected to the surrounding atmosphere by at least one vent tube  44 . Variation in temperature and/or pressure within the die plate body  10  equilibrates by expansion or contraction of air into and through vent tube  44  thus avoiding vacuum formation and/or pressure build-up which could potentially lead to undesirable deformation of the downstream face  26 . Raised ridge  34  and orifice protrusions  35  through-penetrate the atmospheric air pocket  32  to provide continuous and more uniform heating along the length of the through-penetrating extrusion orifices  22 , and the weldment of their distal ends  70  to the openings  39  in the cover plate  38  serve to strengthen and maintain the planar shape of the cover plate. 
         [0060]    As best seen in  FIG. 2  the air pocket or chamber  32  is generally parallel to the die face  26 , but extends into the counterbore  76 , as at  78 , in order to surround the outer periphery of each orifice protrusion  35 . The thickness of the air chamber  32  can vary at different locations but should be at least about 0.05 mm to no more than about 6.0 mm deep, and preferably is about 0.5 mm to about 1.0 mm deep. Stated another way, the thickness of the air chamber  32  in the dimension parallel to the die face is preferably about 5% to about 10% of the thickness of the die plate assembly  10 . 
         [0061]    Cover plate  38  preferably includes at least one circumferential expansion groove  62  on the portion of the cover plate  38  that extends beyond the circular array of extrusion orifices  22 . More preferably, at least one circumferential expansion groove  62  is on each side of cover plate  38  outside the array of extrusion orifices. Still more preferably, a multiplicity of circumferential expansion grooves  62  are positioned staggeringly on opposite sides of the cover plate  38 . The circumferential expansion grooves  62  can be of any geometry in profile including but not limited to square, angular, rounded, and hemispherical and the multiplicity of grooves on cover plate  38  can be of similar or differing geometries. Preferably, the circumferential grooves are rounded in profile as shown in  FIG. 2 . 
         [0062]    As described previously, the raised circular ridge  34  of the extrusion orifice extensions  31  is preferably unitary with die plate body  36  and therefore of the same chemical composition. The orifice protrusions  35 , on the other hand, are formed as separate collars and attachedly connected to the top of the raised ridge as by brazing, welding, and any similar mechanism known to those skilled in the art. The protrusions  35  can be of similar or differing composition to the ridge  34  and die plate body  36  of which the composition can include but is not limited to tool steel, hardened tool steel, stainless steel, nickel steel, and the like. 
         [0063]    Turning to  FIGS. 4 through 9  there is shown a two-piece die plate assembly, generally designated by reference numeral  100 , in accordance with a second embodiment of the present invention. The die plate assembly  100  includes a die plate outer ring  105  and removable center die insert  106 . Since many of the components of the die plate assembly  100  are the same as or very similar to the components of the die plate assembly  10 , the same reference numerals are carried forward from the latter for corresponding components in the former, but preceded by the “1” digit. 
         [0064]    Similarly to the  FIG. 1  embodiment, the die plate assembly  100  is attachedly connected to an inlet housing  112  from a melting and/or mixing apparatus (not shown). The inlet housing  112  includes a passageway  114  for process melt as heretofore described. Nose cone  116  directs the process melt to the upstream side  124  of the removable insert  106  to which it is attachedly connected by threaded rod (not shown). The threaded rod is screw threaded at one end into threaded bore  118  of nose cone  116  and at its distal end into threaded bore  120  of removable insert  106 . 
         [0065]    The removable center die insert  106  includes a multiplicity of extrusion orifices  122  concentrically arranged singly or in multiples thereof in at least one annular ring that extends from the upstream face  124  to the downstream face  126  of removable insert  106 . A plurality of knife blade assemblies  128  mounted on a rotatably driven cutter hub  130  in a cutting chamber (not shown) cut the extruded and at least partially solidified process melt into pellets. The pellets thusly formed are transported mechanically, pneumatically, hydraulically, or in combinations thereof to downstream processing as before. 
         [0066]    The central areas of the downstream face  126  of insert  106  are machined or cut out to provide a central circular recess or cavity  133  in the same manner as described above for the first embodiment, including raised circular ridge  134  and orifice protrusions  135 , which together define the extrusion orifice extensions  131  and encase the extrusion orifices  122  through the cavity  133 . A circular cover plate  138  with holes  139  matching the distal ends  170  of the orifice protrusions  135  overlays the recess cavity  133  to form a thin, continuous thermal air pocket or air chamber  132  across the insert  106  generally parallel to the die face  126 . The upstream side of cover plate  138  is also provided with a generally circular counterbore  176  which includes the outlet holes  139  and conforms to and receives the circular array of orifice protrusions  135 . The extrusion orifice extensions  131  made up of the raised circular ridge  134  and orifice protrusions  135  serve to channel and provide heat from the insert body  136  to the distal end  168  of the extrusion orifices  122 , while at the same time the extensions  131  are thermally insulated from cover plate  138  by the air chamber  132  which surrounds the orifice extensions  131 . 
         [0067]    The cover plate  138  is attachedly connected to the periphery of the insert body  136  and to orifice protrusion distal ends  170  by brazing, welding, or similar technique known to those skilled in the art. Preferably, the cover plate  138  is constructed of an abrasion and corrosion resistant metal and more preferably is constructed of nickel steel. Similarly, attachment of the cover plate  138  to the insert body  136  and orifice protrusion distal ends  170  is preferably achieved by welding and, more preferably, is achieved by nickel steel welding. Weldments  140  and  142  are preferentially made at circumferential grooves  176  peripherally about the cover plate  138  and onto protrusion distal ends  170  at weldment locus  142  (see  FIG. 9 ). The surface of the cover plate  138  and thus the downstream face  126  of die insert  106  is preferably coated with a chemical, abrasion, corrosion, and wear resistant coating as described hereinbelow. 
         [0068]    The circular cavity  133  is preferably connected to the surrounding atmosphere by at least one vent tube  144  which passes through both the removable die insert  106  and the die plate outer ring  105 . Variation in temperature and/or pressure within the air chamber  132  equilibrates by expansion or contraction of air into and through vent tube  144 , thus avoiding vacuum formation and/or pressure build-up which could potentially lead to undesirable deformation of the downstream face  126 . Raised ridge  134  and orifice protrusions  135  through-penetrate the atmospheric air pocket  132  to provide continuous and more uniform heating along the length of the extrusion orifices encompassed therewithin. The configuration of the raised circular ridge  134 , preferably trapezoidal in vertical cross-section, serves to channel heat to the orifice protrusions  135  in order to assist in maintaining the process melt in protrusions  135  at the desired temperature prior to exit from the distal end  168  of extrusion orifices  122 . Weldment of the periphery of the cover plate  138  to the insert  106  and of the distal ends  170  of the orifice protrusions  135  in the holes  139  of the cover plate  138  serve to strengthen and rigidify the cover plate in its planar shape, as further described in the next paragraph. 
         [0069]    The insert body  136  and cover plate  138  are designed with a multitude of complementary abutting surfaces to improve the effectiveness of the weldments  140  and  142 . This in turn increases the rigidity of the assembled cover plate  138  onto the insert body  136 , improves the sealing of the air chamber  132  and provides an overall robust die plate assembly  110 . First, the machined cutout  133  includes peripheral ledge  182  (see  FIGS. 6 and 7 ) which receives a peripheral edge  184  of the cover plate  138  to define the periphery of the air chamber  132 . The complementary abutting surfaces of the insert body peripheral ledge  182  and cover plate peripheral edge  184  are then held together by weldment  140 . Second, holes  139  of cover plate  138  include a countersunk section  186  on their upstream side (see  FIG. 8 ) which forms a ledge  188  that engages the outer periphery of the distal ends  170  of the orifice protrusions  135  (see  FIG. 9 ). These complementary abutting surfaces  170  and  188  are adhered together by weldments  142  at each extrusion orifice  168 . 
         [0070]    The circular counterbore  176  in cover plate  138  differs from the circular counterbore  76  in cover plate  38  in that the former is contoured with tapered side walls  190  to more closely follow the contour of the tapered sides  192  of the raised ridge  134 . By more closely following the contour of raised ridge  134 , the counterbore  176  and resultant air chamber  132  provide additional insulation about the ridge  134  and the associated orifice protrusions  135 . In contrast, the circular counterbore is more rectangular in cross-section and is positioned adjacent the raised ridge  34  without contouring dimensionally with its tapered sides  92 . It is understood that the contours of the circular counterbore  176  adjacent raised circular ridge  134  and of the counterbore  76  adjacent raised ridge  34  are only two non-limiting examples and other designs comparable to and intermediate between these two configurations are encompassed by the present invention. Use of the rectangular counterbore  76  and tapered counterbore  176  can be applied to the solid-body die plate assembly  10  as well as to the two-piece die plate assembly  100 . 
         [0071]    If desired, cover plate  138  can be provided with circumferential grooves, such as grooves  62  illustrated and described above for cover plate  38 . 
         [0072]    Heating and/or cooling processes can be provided by electrical resistance, induction, steam or heat transfer fluid as has been conventionally disclosed for the single-body die plate  10  as well as the two-piece die plate assembly  100 . As shown in  FIGS. 1 and 4 , the die plate body  36  and insert body  136  are each respectively heated by radial electric heaters  46  and  146  positioned in radial slots  47  such as shown in  FIG. 3 , as well known in the art. In the two-piece die plate assembly  100  shown in  FIG. 4 , the removable insert  106  and the die plate outer ring  105  can each be separately heated by similar or differing mechanisms. 
         [0073]    For example,  FIG. 10  illustrates a partially exploded view of a die plate assembly, generally designated by reference numeral  200 , which includes a center-heated removable insert  208 . Since many of the components of the die plate assembly  200  are the same as or very similar to the components of the die plate assembly  100 , the same reference numerals are carried forward from the latter for corresponding components in the former, but preceded by the “2” digit instead of the “1” digit. 
         [0074]    The die plate assembly  200  thus includes a die plate body, generally designated by reference numeral  212 , comprised of die plate outer ring  205  surrounding center-heated removable insert  208 . The electrical resistance coil  250  is contained in an annular recess or cavity  252  centrally located within the insert  208  adjacent to the upstream face  224 . Nose cone  216  is attachedly connected to removable insert  208  through use of a threaded rod (not shown) that is screw threaded at one end into threaded bore  218  of nose cone  116  and at its distal end into threaded bore  220  of removable insert  208  in a manner similar to that shown in  FIGS. 1 and 4 . When attached, nose cone  116  closes off cavity  252  with coil  250  positioned therein. Other methods of fastening are well-known to those skilled in the art. The removable insert  208  can thus be heated separately as by electric radial heaters  146  hereinbefore described in connection with the die plate assembly  100  shown in  FIG. 4 . 
         [0075]    The downstream face  26 ,  126  of die plate assembly  10 ,  100 ,  200  can be in one plane as shown in  FIG. 1  but preferably is of two parallel planes as indicated by the encircled area  66 ,  166  in  FIGS. 2 and 9 , wherein the area adjacent to the outlets  68 ,  168  of extrusion orifices  22 ,  122  is raised in a plane parallel to that of the downstream face  26 ,  126 . The elevation of the plane above that of the downstream face  26  should be at least approximately 0.025 mm, and preferably is at least approximately 0.50 mm. 
         [0076]    Similarly, the recess cavity  33 ,  133  is at least approximately 1.05 millimeters in depth, preferably on the order of 5.0 mm to 7.0 mm. The thickness of the cover plate  38 ,  138  should be on the order of 1.0 mm to 8.0 mm, preferably about 6.0 mm in order to provide a thickness of the air chamber  32 ,  132  on the order of about 0.05 mm to about 6.0 mm, and preferably about 0.5 mm to about 1.0 mm. 
         [0077]    The surface of the downstream face  26 ,  126  is preferably subjected to a chemical, abrasion, corrosion, and/or wear resistant treatment, i.e., “surface treatment,” in the annular area generally defined by the array of extrusion orifice outlets  68 ,  168  and identified by the numeral  60 ,  160  in  FIGS. 2 and 9 . This annular area includes the cutting face  63 ,  163  where the cutting blades engage the die face. The surface treatment should be at least approximately 0.025 mm, and preferably is at least approximately 0.50 mm. The composition of the surface treatment  60 ,  160  can be different in the planar area surrounding the extrusion orifice outlets  68 ,  168  than that on other parts of the downstream face  26 . Preferably, the surface treatment  60 ,  160  is the same on all faces and can involve one, two, or a multiplicity of processes inclusive and exemplary of which are cleaning, degreasing, etching, primer coating, roughening, grit-blasting, sand-blasting, peening, pickling, acid-wash, base-wash, nitriding, carbonitriding, electroplating, electroless plating, electroless nickel dispersion treatments, flame spraying including high velocity applications, thermal spraying, plasma treatment, electrolytic plasma treatments, sintering, powder coating, vacuum deposition, chemical vapor deposition, physical vapor deposition, sputtering techniques, spray coating, and vacuum brazing of carbides. 
         [0078]    Surface treatment for all surfaces, other than the cutting face, includes flame spray, thermal spray, plasma treatment, electroless nickel dispersion treatments, high velocity air and fuel modified thermal treatments, and electrolytic plasma treatments, singly and in combinations thereof. These surface treatments metallize the surface, preferably fixedly attach metal nitrides to the surface, more preferably fixedly attach metal carbides and metal carbonitrides to the surface, and even more preferably fixedly attach diamond-like carbon to the surface, still more preferably attach diamond-like carbon in an abrasion-resistant metal matrix to the surface, and most preferably attach diamond-like carbon in a metal carbide matrix to the surface. Other ceramic materials can be used and are included herein by way of reference without intending to be limiting. These preferred surface treatments can be further modified optionally by application of conventional polymeric coating on the downstream face  26 ,  126  distal from the extrusion orifice outlet  68 ,  168 . The polymeric coatings are themselves non-adhesive and of low coefficient of friction. Preferably the polymeric coatings are silicones, fluoropolymers, and combinations thereof. More preferably the application of the polymeric coatings requires minimal to no heating to effect drying and/or cure. 
         [0079]      FIG. 11  illustrates additional configurations of extrusion orifices and orifice protrusions projecting from the raised circular ridge.  FIG. 11   a  illustrates concentric rings of orifice protrusions  302  projecting from ridge  303  in staggered formation, each protrusion having a single extrusion orifice  304 . The extrusion orifices can be oriented in a multiplicity of groups or pods  306  as illustrated in  FIG. 11   b  for a grouping of two extrusion orifices  308 ,  FIG. 11   c  for a grouping of three extrusion orifices  310 ,  FIG. 11   d  for a cluster of four extrusion orifices  312 ,  FIG. 11   e  for a pod of sixteen extrusion orifices  314 ,  FIG. 11   f  for a multiplicity of thirty-seven extrusion orifices  316 , and  FIG. 11   g  for a multiplicity of sixteen extrusion orifices  318 . 
         [0080]    Groups, clusters, pods, and a multiplicity thereof can be arranged in any geometric configuration including but not limited to oval, round, square, triangular, rectangular, polygonal, and combinations thereof. The geometries of the orifice protrusions can be further rounded, angled, and chamfered and can contain any number of a multiplicity of orifices. Orientation of the geometries containing the multiplicity of orifices can be circumferentially and parallel to the arc, circumferentially and perpendicular to the arc, staggered and alternatingly circumscribing the arc and any combination thereof. Furthermore, the geometric orientation may conform to the arc as in a kidney shape or comma-shape. A multiplicity of concentric rings, at least one or more, of extrusion orifices can include extrusion orifices, singly or a multiplicity thereof, that can be arranged in a linear array, alternatingly, staggeredly, and any combination thereof relative to the other concentric rings in accordance with the instant invention. 
         [0081]    Further, while the outlet of the extrusion orifices  22 ,  122 , such as outlet  68  in  FIG. 2  and outlet  168  in  FIG. 9 , is preferably round, the outlets can be of any geometry including but not limited to round, oval, square, rectangular, triangular, pentagonal, hexagonal, polygonal, slotted, radially slotted and any combination thereof. A multiplicity of extrusion orifice outlets  68  can be of different geometry in a particular protrusion  35 . 
         [0082]    Further, the extrusion orifice extensions may include more than one raised circular ridge  34 ,  134 , depending upon the arrangement of the extrusion orifices and the width of the cutting blade. In addition, although at least one raised circular ridge  34 ,  134  is preferred to form the base of the extrusion orifice extensions  31 ,  131 , it may be possible to design the extensions  31 ,  131  without any raised ridge. In such circumstances, the orifice protrusions  35 ,  135  would extend from the base of cutout  33 ,  133  to the respective opening  68 ,  168  of the cover plate  38 ,  138 . 
         [0083]    The foregoing is considered as illustrative only of the principles of the invention. Numerous modifications and changes will readily occur to those skilled in the art. Therefore, it is not desired to limit the invention to the exact construction and operation shown and described, and, accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.