Cutter knife for thermoplastic resin pelletizer and production method of said cutter knife

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.

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.

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.