TECHNICAL FIELD 
     The present invention relates to underwater pelletizers, and more particularly to an improved extrusion die and die assembly for use in an underwater pelletizer. 
     BACKGROUND 
     Pelletizers have been known and used for a number of years. They are used to process rubber compounds, and molten thermoplastics and other polymers into pellets, which may in turn be used in other processes to manufacture various plastic materials. 
     In an underwater pelletizer, molten plastic is typically extruded through orifices in a die, forming continuous strands which are cut by knives at the die&#39;s cutting face. The cutting takes place underwater in a water chamber or housing. Plastic pellets are formed which cool and harden in the water contained in the water chamber. Typically, the water in the water chamber is much cooler than the molten polymer, allowing very quick cooling of the polymer and quick solidification of the pellets. 
     Typically, an underwater pelletizer is constructed so that a constant stream of water passes over the die&#39;s cutting face, conveying the hardened pellets along to equipment which separates out the pellets from the water and dries them. 
     It is very important that the pellets formed by such a pelletizing process be uniform in size and shape, and that they be made to a specific geometry. There are, however, a number of difficulties which may be encountered in trying to form uniform pellets. 
     To form uniform pellets of a specific geometry, the orifices in the extrusion die must remain free and clear of solidified polymer material. If partially plugged, an orifice will allow only a small or irregular strand of polymer through it; this leads to small, irregularly-shaped pellets. If an orifice is completely clogged, of course, no polymer may pass through it, and no pellets are formed, decreasing production of the pelletizer. 
     The rate of solidification of a polymer exiting an extrusion die in an underwater pelletizer depends upon the nature of the polymer and also on the temperature of the water in the water chamber and the speed of its flow. For any particular polymer, of course, the rate of solidification will depend mostly upon the water temperature. For some polymers, the water temperature must remain fairly cool to cause the polymer to solidify quickly. However, the die plate must not be allowed to become so cooled by the passing water that the polymer solidifies before it has exited the extrusion orifices. 
     In respect of other polymers, a very hot water temperature is required to keep the polymer from solidifying before exiting the extrusion orifices. Of course, high water temperatures can be hazardous if the water chamber bursts or otherwise leaks. 
     Rather than attempting to controlling solidification of polymer material merely by adjusting water temperatures, the die plates in underwater pelletizers are typically heated. Past efforts at heating pelletizer dies have focussed upon heating of the die plate by steam or hot oil, as shown in U.S. Pat. No. 4,123,207, which issued in 1978. Such steam and oil-heated die plates are costly, complicated, and require the addition to the pelletizer of a separate complicated system for heating the steam or oil. They are also hazardous. 
     Other die plates aimed at solving the problem of orifice clogging have employed complicated “sealed heat transfer tubes”, as shown in U.S. Pat. No. 5,629,028. These tubes allegedly transfer heat from the molten polymer to the die. They are problematic since they are difficult to adjust for different polymers, which, as noted above, have different properties. 
     Still other prior art die plates are heated by inserting electric heating rods radially into chambers formed around their circumference, as shown in U.S. Pat. Nos. 4,621,996 and 5,403,176. At least one of the drawbacks of these systems is that the heat is concentrated at a limited number of points on one side of the die hole pattern, and not around the entirety of each of the orifices. To heat the portion of the orifice furthest away from the heating rod enough to prevent clogging, one might increase the heat in the heating rods, but this can cause polymer degradation nearest the heating rod. 
     There remains, accordingly, a need for an improved extrusion die having a heating system which provides better heating to the extrusion orifices, and to the entirety of each of them, without being of such complicated construction nor as dangerous as currently-employed systems. 
     SUMMARY OF THE INVENTION 
     The present invention is an extrusion die, and die assembly, for use in an underwater pelletizer. More particularly, the die preferably comprises a die plate having an upstream extruder face and a downstream cutting face, an inner annular groove formed in one face of the die plate, an outer annular groove formed in one face of the die plate, a plurality of spaced-apart extrusion orifices extending through the die plate from the extruder face to the cutting face, the orifices arranged in a pattern occupying a space between the inner and outer annular grooves and a coiled heating element contained within each of the inner and outer annular grooves, the coiled heating elements having leads extending outwardly from the die plate for connection to an electrical source. 
     In one embodiment of the invention, the pattern of extrusion orifices is a circular pattern, and the inner and outer annular grooves and the circular pattern of extrusion orifices are all concentric. In another embodiment, all are coaxial with the central axis of the die plate. 
     A thermally-conductive paste may be contained within each of the inner and outer annular grooves, occupying the volume not occupied by the heating coils, to assist heat transfer. Also assisting heating of the die are a plurality of electrical heating rods occupying chambers arranged radially in the die plate, the chambers extending inwardly from the circumference of the die plate. 
     The die assembly of the present invention comprises the die described above and also a die cone attached to the central portion of the extruder face of the die plate, the die cone covering and enclosing the inner annular groove but not the extrusion orifices. Also, an adaptor for attaching the die plate to the extruder may be attached to the die plate, the adaptor when bolted to the die plate covering the outer annular groove but not the extrusion orifices. In another embodiment, the die assembly can further comprise a circular flange attached to the cutting face of the die plate, the flange allowing said die plate to be attached to said water chamber, a circular cap attached to the cutting face of said die plate, and thermally-insulative gaskets fitted between the die plate and both the flange and cap. 
     In yet another embodiment of the invention, a die plate is provided having a plurality of coiled heaters. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     In the accompanying drawings which illustrate specific embodiments of the invention, but which should not be construed as restricting the spirit or scope of the invention in any way: 
     FIG. 1 is an exploded perspective view of an extrusion die assembly made in accordance with one embodiment of the invention; 
     FIG. 2 is a front view of the extrusion die of the die assembly illustrated in FIG. 1; 
     FIG. 3 is a cross-sectional view of the extrusion die illustrated in FIG. 2, taken along line A—A; 
     FIG. 4 is a cross-sectional view of the die assembly shown in FIG. 1; and 
     FIG. 5 is a front view of another extrusion die made in accordance with the invention. 
    
    
     DESCRIPTION 
     Referring first to FIG. 2, an extrusion die made in accordance with one embodiment of the invention, denoted generally by the numeral  10 , comprises a disc-shaped die plate  12  having an extruder face  18  (FIG. 3) which faces an extruder when the die is in place in an underwater pelletizer, and a cutting face  20  on the opposite side of die plate  12 . Die plate  12  may be made of any suitable material, but is typically made of steel. 
     Die plate  12  may have bolt holes  14  for mounting plate  12  to an extruder (not shown), or an adapter (as discussed below), and may also have bolt holes  16  for mounting plate  12  onto the water chamber or a suitable adaptor (also discussed below) of an underwater pelletizer. 
     A plurality of extrusion orifices  22  extend through die plate  12  from extruder face  18  to cutting face  20 . One such extrusion orifice  22  is shown in cross-sectional detail in FIG.  3 . As is known in the art, orifice  22  may be tapered, as shown, to assist molten polymer material in passing therethrough. 
     Orifices  22  may be arranged in any suitable pattern, but as shown in FIG. 2, the plurality of orifices  22  may be arranged in a circular pattern or “ring” about the central axis of die plate  12 , in a manner well known in the art. In one embodiment of the invention, however, the ring of orifices  22  is slightly offset from the central axis of die plate  12 , allowing the cutting knives (not shown) of a pelletizer, which are typically arranged coaxially with the die  10 , to cut each strand of material which passes through orifices  22  at a slightly different point along their respective blade lengths, thereby reducing knife wear. 
     In the preferred embodiment of the invention, the ring of orifices  22  is surrounded by an inwardly adjacent annular groove  24  formed in one face of the die plate, preferably the extruder face  18 , and an outwardly adjacent annular groove  26  formed in a face of the die plate, also preferably extruder face  18 . Annular grooves  24 ,  26  preferably extend nearly the entire width of die plate  12 , as shown in FIG.  3 . Grooves  24 ,  26  are preferably coaxial with the ring of orifices  22 , but need not be. 
     Contained within each of the grooves  24 ,  26  is a heater. In a preferred embodiment of the invention, coiled heating elements  28   a ,  28   b  (FIG. 3) are employed. Heating elements  28   a ,  28   b  are conveniently made of a coil of an electrically resistive metallic material such that they become hot when electric current is passed through them. 
     Electrical leads  30  are attached to heating elements  28   a ,  28   b , and extend outwardly from grooves  24 ,  26  to the exterior of die plate  12  through chambers  32  formed therein. Leads  30  are attached to an electrical source which provides enough current to suitably heat heating elements  28   a ,  28   b . It will be appreciated, given this description, that when heated, heating elements  28   a ,  28   b  provide heat to the ring of orifices  22 , both on the inside of the ring, and also on the outside of the ring, thereby maintaining an almost-uniform amount of heat around the entirety of each of orifices  22 . This allows polymer material passing through each of orifices  22  to remain molten until it has exited the orifice. 
     Heating elements  28   a ,  28   b  may be made of a wire material which expands when heated, and if so, it is desirable to fill grooves  24 ,  26  with a thermally-conductive paste so that heat from heating elements  28   a ,  28   b  may be properly conducted to orifices  22  no matter how much the wire of elements  28   a ,  28   b  has expanded. The paste may be conveniently introduced into grooves  24 ,  26  under pressure through grease nipples communicating with the grooves, or through other chambers (not shown) specifically fashioned for this purpose. 
     Although heating elements  28   a ,  28   b  may be suitable in and of themselves to properly heat orifices  22 , another embodiment of the invention additionally employs a plurality of electrical heating rods  40  placed in chambers  42  (one is shown in dotted outline in FIG. 3) arranged radially in die plate  12 , the chambers  42  extending inwardly from the circumference of die plate  12 , as known in the art. However, unlike heating rods used in prior art dies, the rods of the present invention are preferably formed such that the heating element of each rod is concentrated near the bottom of the rod and the normally concave bottom is filled with copper to direct most of the rod&#39;s heat through its end and towards the die hole pattern rather than to the rest of the die plate  12 . 
     To further improve the die&#39;s heat retention capabilities for the purpose of improved polymer flow, the die  10  of the present invention may be combined with other elements which essentially serve as thermal barriers to form a die assembly as shown in FIG. 1, which is denoted generally herein  100 . 
     In typical underwater pelletizing systems, there are a number of different arrangements of extruders, and adaptors are commonly bolted between an extruder and the extrusion die of choice. On the “upstream” side of die  10 , die assembly  100  of the present invention has such an adaptor, labelled  54  in FIG. 1, which may be bolted between the die  10  described above and an extruder. 
     Die assembly  100  of the present invention also has a die cone  50  attached to the extruder face  18  of die plate  12  to better direct molten thermoplastic material from the extruder directly to orifices  22 . Die cone  50  may be attached with bolts extending through cone bolt holes  52  (FIG. 2) in die plate  12 . A bolt hole  52  is shown in dotted outline in FIG.  3 . 
     What is unique about this aspect of the present invention is that the base of die cone  50  of die assembly  100  is just large enough that it covers almost the entire surface area of the extruder face  18  inwards of the inside edge of the ring of orifices  22 , as shown in FIG.  4 . Die cone  50 , accordingly, covers and encloses inner groove  24 , thereby keeping polymer material out of groove  24 , and tending also to moderate temperature fluctuations in the immediate area adjacent inner groove  24 . 
     Further, as shown in FIG. 4, adaptor  54  may be fashioned to have tapered inner surfaces  55  which also serve to direct molten material directly to orifices  22 . Adaptor  54  thereby covers the surface area of die plate  12  outward of the ring of orifices  22 , and in particular, encloses outer groove  26 , similarly moderating temperature in this area of die plate  12 . 
     This combination of die plate  12 , cone  50  and adaptor  54  serves to present only a small surface area of extruder face  18  to molten material extruded by the extruder to which adaptor  54  is bolted; specifically, only the area presenting the upstream ends of orifices  22 . This tends to moderate the temperature in the die  10 . 
     On the “downstream” side of die  10 , a circular flange  56  may be bolted to die plate  12  and may serve to allow die plate  12  to be attached to a water chamber in an underwater pelletizer in a quick-disconnect system. A thermally non-conductive gasket  58  may be inserted between die plate  12  and flange  56 , thereby thermally shielding the outermost portion of die plate  12  from the cold water passing by die plate  12  when it is in use. 
     Further, a thermally-non-conductive gasket  60  and cap  62  may be attached to the inner portion of die plate  12 , thermally shielding the central portion of die plate  12  from the cold water. 
     Similar to the arrangement of die cone  50  and adaptor  54  on the upstream side of die plate  12 , cap  62  and flange  56  may be of such diameters as to allow only a small surface area forming a raised cutting surface  21 of the cutting face  20  of die plate  12  to be presented to the cold water of the water chamber of the pelletizer. Specifically, only the portion of the cutting face presenting the downstream end of orifices  22  may be exposed directly to the water in the water chamber. 
     In a preferred embodiment, flange  56  and cap  62  are made from a material of lower thermal conductivity than tool or mild steel, such as stainless steel. 
     As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. For example, as shown in FIG. 5, this invention is not limited to dies having only one ring of extrusion orifices, and 2 heater elements. Rather, this invention also encompasses dies and die assemblies having a plurality of orifice rings, each having an inner and outer heating element, as shown in FIG.  5 . Accordingly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.
A die and a die assembly are disclosed for use in association with an underwater pelletizer. The die has unique coiled heating elements which allow for uniform heating of the extrusion orifices which pass through the die, reducing or eliminating polymer solidification within the die, thereby allowing the manufacture of uniform pellets of a specific geometry. The heating elements are contained within the die plate of the die, and in a preferred embodiment occupy annular grooves formed therein which are concentric with the extrusion orifice pattern. The die assembly includes thermally-insulating adaptors and gaskets which cover the annular grooves containing the heating elements.