Abstract:
To provide a way to use a ceramic alloy as a material for producing a cutter knife to be mounted on a thermoplastic resin pelletizer easily, with reduced costs, and also eliminating the possibility of breakage.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 08/866,734 filed May 30, 1997 now anandoned. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a cutter knife to be mounted on a thermoplastic resin pelletizer for pelletizing thermoplastic resin. The invention also relates to a production method of said cutter knife. 
     BACKGROUND OF THE INVENTION 
     For the sake of convenience of transportation, reducing costs of later processing and other reasons, thermoplastic resin is normally produced and distributed in the form of pellets, which are small, approximately rice-grain sized particles. These pellets are produced industrially by means of a kneading pelletizing (extrusion) machine incorporated in a chemical apparatus. Examples of types of a pelletizing device used in such a pelletizer include a hot cut type and a underwater cut type. There are also various blades used for this purpose; a typical example of fixed blades is a die plate, while there is a cutter knife  1  shown in FIG. 8 as an example of movable blades. Nowadays, because of their superior abrasion resistance and resin cutting capability, ceramic alloys are most widely used as a material of cutter knife  1 . 
     However, ceramic alloys used as a material of such a cutter knife present various problems; not only are they expensive and hard to acquire but also prone to being chipped or broken because they have lower resistance to a transverse load, in other words are more fragile, compared with steel which is the most typical blade material. Although ceramic alloys require a thorough countermeasure to solve the above problems, it cannot be said that a sufficiently effective countermeasure exists at present. Furthermore, a ceramic alloy is difficult to process by using machine, which means that processing a ceramic alloy is costly. Therefore, using a ceramic alloy, which is itself an expensive material, to produce cutter knives substantially increases their costs. 
     A prior art method of producing a ceramic alloy cutter knife uses a process of sintering powdered ceramic alloy directly to two large base metal bodies by using a special container and then subjecting the contents to a hot isostatic pressing (HIP) process. The bodies are removed from the container and separated from each other. Each body is then cut along its longitudinal axis which is directly in the middle of the ceramic alloy material. Each half is then cut again into individual pieces or blanks, which are in turn cut again and formed into individual cutter knives. In addition, the process requires that the ceramic alloy be ground into a knife edge. The powdered ceramic alloy HIP process is complicated and presents various technological difficulties, for example, the air must be removed from the container prior to sintering which naturally results in the discharge of some of the powdered material. The entire production method is expensive and does not alleviate any problems associated with fragility of the ceramic alloy blade or that of avoiding unnecessary processing. In addition, powdered ceramic alloy material is less available and must be specially procured from a powder manufacturer. In short, the prior art method does not address the problems associated with producing a ceramic alloy cutter knife in terms of expense, the delicate nature of the material, availability of ceramic alloy material and the reduction of processing of the blade. 
     SUMMARY OF INVENTION 
     In order to solve the above problems presented by conventional cutter knives, an object of the invention is to provide a cutter knife for a thermoplastic resin pelletizer and its production method by using a ceramic alloy as a material for said cutter knife, wherein said cutter knife can easily be produced with reduced costs and without danger of breakage in spite of using a ceramic alloy as a material. A feature of the invention is a cutter knife to be mounted on a thermoplastic resin pelletizer for pelletizing thermoplastic resin, wherein the main body and the knife edge of the cutter knife are integrally formed, the main body formed out of steel and the knife edge formed out of ceramic alloy. According to the second feature of the invention, a groove is formed in a corner of a rectangular steel body in such a manner as to extend in the longitudinal direction of the steel body which has an appropriate thickness, a ceramic alloy plate is inserted into the groove and bonded thereto by means of diffusion bonding, and the material is then cut into a specified shape. According to the third feature of the invention, a steel block is shaped as to have a step when viewed from the side and, after having a ceramic alloy plate bonded to the upper surface of the lower step thereof by means of diffusion bonding, is diagonally cut at specified intervals, and then, each cut piece is cut into a specified shape. A fourth feature of the invention is the use of a functionally gradient material between the bonding surfaces of the ceramic alloy plate and the steel body prior to the diffusion bonding which acts as a binder during the diffusion bonding process to improve the strength, defacement resistance and chipping resistance of the cutter knife and in particular, the ceramic alloy blade. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a cutter knife according to the invention; 
     FIG. 2 is a perspective view of cutter knife of FIG. 1 before the cutting processing; 
     FIG. 3 is a perspective view to explain another method of processing a cutter knife according to the invention, illustrating the state where ceramic alloy plate is bonded by means of diffusion bonding; 
     FIG. 4 is a perspective view of the embodiment of FIG. 3 illustrating a cut piece; 
     FIG. 5 is a perspective view of the embodiment of FIG. 3 after the cutting processing; 
     FIG. 6 is a schematic partial cross sectional view illustrating the relationship between a thermoplastic resin pelletizer and its cutter knives; 
     FIG. 7 is a front view of the arrangement shown in FIG. 6; 
     FIG. 8 is a perspective view of a conventional cutter knife; 
     FIG. 9 is a perspective view of an alternate embodiment illustrating the state where ceramic alloy plate is bonded by means of diffusion bonding using a thin plate of functionally gradient material; 
     FIG. 10 is a perspective view of the embodiment of FIG. 9 illustrating a cut piece; 
     FIG. 11 is a graph of test data to show the strength characteristics of a ceramic alloy bar; and 
     FIG. 12 is a graph of test data to show the strength characteristics of a diffusion bonded ceramic alloy/steel body bar. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Embodiments of the present invention are explained in detail hereunder referring to the drawings. 
     Numeral  11  denotes a cutter knife used as a movable blade of a thermoplastic resin pelletizer  20 . Cutter knife  11  comprises a main body  12  made of steel and a knife edge  13  which is formed out of a ceramic alloy as an integral body with main body  12 . 
     Although the ceramic alloy used for the present embodiment is a titanium carbide (TiC)—type alloy, it is to be understood that ceramic alloys applicable for the present invention are not limited to a TiC alloy. 
     A groove  14  is formed in a corner of main body  12  in such a manner as to extend in the longitudinal direction of main body  12  and intruding or extending towards the middle of main body  12 , which is formed out of steel into a rectangular shape having an appropriate thickness, and a ceramic alloy plate that constitutes knife edge  13  is inserted into groove  14  and bonded thereto by means of diffusion bonding (see FIG.  2 ). Then, by cutting the material into a specified shape by a method known to those skilled in the art, cutter knife  11  is formed (see FIG.  1 ). The ceramic alloy plates  13  are easily produced from larger blocks of sintered ceramic alloy material. This allows the plates to be easily and relatively inexpensively produced into thin sheet plates  13 . The ceramic alloy plates  13  produced in this manner are preferably, but not limited to, about 2 mm thick. The ceramic alloy plates  13  produced in this manner have a considerably high yield which significantly reduces material and production costs. An advantage of this embodiment of the invention is that the groove  14  provides increased support for the ceramic alloy plate and knife edge  13 . The groove  14  also provides protection by surrounding a majority of the fragile ceramic alloy plate  13  during handling, processing as well as use of the cutter knife  11 . 
     In another embodiment of the invention, a steel block serving as the material of main body  12  is so shaped as to have a step when viewed from the side, and then, after bonding a ceramic alloy plate that constitutes knife edge  13  to the upper surface of the lower step by means on diffusion bonding, is diagonally cut at specified intervals (see FIGS.  3  and  4 ). Then, by cutting the material into a specified shape by a method known to those skilled in the art, cutter knife  11  is formed, in the same manner as the previous embodiment (see FIG.  5 ). The diagonal cut results in a sharpened knife edge  13  that requires little or no post processing. This is important feature due to the problems (fragility, expensive, machining difficulty) associated with the ceramic alloy as described above. 
     In an alternate embodiment of the invention, the bonding of the ceramic alloy plate  13  to the main body  12  can be strengthened and improved by using a thin plate  20  of a functionally gradient material placed between the ceramic alloy plate  13  and the main body  12 . Typical functionally gradient materials include, but are not limited to, nickel, silver and copper which all have relatively high melting temperatures. The diffusion bonding of the ceramic alloy plate  13  to the main body  12  is accomplished by hot isostatic pressing (HIP). This process uses high temperature and high pressure to create the diffusion bond. When the thin plate  20  of nickel, for example, is placed between the ceramic alloy plate  13  and the main body  12 , the temperature in the HIP furnace is increased to just below the melting temperature of the nickel while at the same time subjecting the parts to high pressure. At this point, the nickel diffuses into the ceramic alloy plate  13  and the main body  12 . The thin plate  20  acts as a binder to improve the uniform consistency and strength of the bond by promoting the diffusion of the ceramic alloy plate  13  into the main body  12  and the diffusion of the main body  12  into the ceramic alloy plate  13 . The method of producing the cutter knife is altered to take advantage of the properties of the functionally gradient material and thereby improving the strength, defacement resistance and chipping resistance of the cutter knife  11  and in particular, the ceramic alloy blade  13 . The thickness of the thin plate  20  of the functionally gradient material used in this process is typically, but not limited to, the range of about 0.1-0.3 mm and preferably 0.1 mm. 
     Referring now to FIGS. 11 and 12, the advantage of using a thin ceramic alloy plate  11  in producing a cutter knife  11  is illustrated. In FIG. 11, a bar of ceramic alloy having a 12 mm thickness was subjected to a compression and bending test. Due to the brittleness of the ceramic alloy material, the bar failed (broke) at 5.01 kN after a transformation/displacement of less than 2.6 mm. In FIG. 12, a bar of ceramic alloy plate of 2 mm thickness diffusion bonded to a steel body 10 mm thick was subjected to the same compression and bending test. Due to improved strength and ductility of the diffusion bonded materials, the bar attained a maximum load of 9.01 kN after a transformation/displacement of about 9.0 mm and did not break after achieving transformation/displacement of over 22.0 mm. 
     Effect of Invention 
     As described above, the present invention substantially improves breakage resistance of a cutter knife to be mounted on a thermoplastic resin pelletizer for pelletizing thermoplastic resin by integrally forming the main body and the knife edge of the cutter knife out of steel and ceramic alloy respectively. Further, the invention is capable of reducing costs of material by forming the main body out of steel, as well as considerably reducing production costs of cutter knives by simplifying their production method.