WO 00/44542 PCT/KROO/00066 -1 APPARATUS FOR MANUFACTURING THERMOPLASTIC RESIN FOAM PELLETS Technical Field The present invention relates, in general, to an 5 apparatus for manufacturing thermoplastic resin foam pellets and, more particularly, to an apparatus having both a cylindrical die designed to continuously and radially extrude thermoplastic resin foam lines through a plurality of radial resin extrusion ports and means for 10 continuously cutting the extruded foam from the die into pellets having a predetermined size and shape, thus continuously producing desired thermoplastic resin foam pellets from a thermoplastic resin material having a high degree of viscosity in commercial quantity at a low 15 production cost. Background Art As well known to those skilled in the art, thermoplastic resin foam pellets are easily and effectively molded into the form of complicated products. 20 Such thermoplastic resin foam pellets also have a tough and smooth surface, a high degree of cushioning effect, a high degree of durability against repeated impact, a high degree of resistance to chemicals, and a high degree of moisture resistance. Therefore, the thermoplastic 25 resin foam pellets are preferably used as materials of molded shock absorbing containers, impact-resistant packaging materials, cushion materials for automobiles, buoys, etc. In order to produce such thermoplastic resin foam 30 pellets in the prior art, linear resin, produced by melting and extruding polystyrene or a copolymer prepared by a polymerization of polystyrene and another resin, is primarily cooled with water and is secondarily cut into pellets, thus producing intermediate products or foamable 35 resin pellets. The foamable resin pellets are, thereafter, subjected to a foaming and molding process WO 00/44542 PCT/KR00/00066 -2 including a heating and pressurizing step, thus manufacturing desired thermoplastic resin foam pellets. The above method has been generally used in the prior art since it effectively and easily produces the desired 5 thermoplastic resin foam pellets. However, this method is problematic in that it necessarily performs a foaming and molding process after the production of foamable resin pellets, thus being undesirably complicated. In addition, waste products, made of the thermoplastic resin foam 10 pellets produced through the above method, undesirably generate poisonous gases during an incineration of the waste products, and so many nations restrict the use of products made of such resin foam pellets in recent years. Therefore, some thermoplastic resins free from 15 emitting poisonous gases during an incineration, for example, low-density polyethylene (LDPE), linear low density polyethylene (L-LDPE), polypropylene (PP) and etc., are recently used as the materials of thermoplastic resin foam pellets. 20 However, since the thermoplastic resins, such as LDPE and L-LDPE, have a high degree of viscosity, it is very difficult to produce desired resin foam pellets through a conventional cutting technique. Therefore, the desired thermoplastic resin foam pellets are produced through a 25 small-quantity foaming process wherein a foaming agent is mixed with plastic particles of the resins prior to a heating and pressurizing process and a foaming process. In a detailed description, foamable resin pellets, made in accordance with both a selected foaming ratio and 30 the size of desired foam pellets, are poured into a drum and are mixed with a foaming agent prior to being highly pressurized and heated at a temperature of not higher than a melting point of the resin pellets, and are sufficiently mixed together in the drum for 5 to 30 35 minutes. When the foaming agent completely permeates the resin pellets, the pellets are foamed to intermittently produce desired thermoplastic resin foam pellets in small quantity. However, such a small-quantity foaming process fails to continuously produce desired resin foam pellets 40 in commercial quantity, thus increasing the production WO 00/44542 PCT/KROO/00066 -3 cost while manufacturing the resin foam pellets and failing to produce resin foam pellets having a uniform size. Generally, it is easy to produce resin foam pellets 5 from polystyrene. However, it is very difficult to cut the thermoplastic resins, such as LDPE, L-LDPE and PP, into pellets since the thermoplastic resins have a high degree of viscosity. In addition, the thermoplastic resin foam pellets have burrs at the cut-sections, thus reducing 10 the quality and uniformity of the resin foam pellets. Therefore, it is necessary to provide a means for effectively and uniformly producing desired resin foam pellets from such thermoplastic resins having a high degree of viscosity through a continuous foaming process 15 at a low production cost. In an effort to accomplish the above object, the inventor of this invention proposed, in Korean Patent No. 135,899, an apparatus for continuously and directly producing desired thermoplastic resin foam pellets without 20 performing an intermediate process of forming foamable intermediate products. In the above Korean apparatus, the desired resin foam pellets are produced by continuously foaming a selected thermoplastic resin in the form of pellets while extruding the resin mixed with a foaming 25 agent. The object of the present invention is to further improve the above Korean apparatus and to further improve productivity while producing desired thermoplastic resin foam pellets. The above Korean apparatus comprises a cylindrical 30 die designed to receive pressurized and molten thermoplastic resin mixed with a foaming agent in a direction from a resin inlet of the die. A plurality of radial resin extrusion ports are formed at the outlet portion of the die. A spindle-shaped torpedo is fixed to 35 the outlet end of the die at one end thereof, with the other end of the torpedo extending into the middle portion of the interior of the die. A rotary cutting means is rotatably locked to the outlet portion of the die and is used for cutting the thermoplastic resin foam 40 lines continuously coming out of the radial resin WO 00/44542 PCT/KROO/00066 -4 extrusion ports of the die into pellets, thus producing desired thermoplastic resin foam pellets. The rotary cutting means comprises a plurality of panel-shaped blades, which are brought into tangential contact with the 5 external surface of the die at positions around the radial resin extrusion ports. However, the above Korean apparatus has the following problems since it uses the rotary cutting means, with the panel-shaped blades designed to come into axial and 10 tangential contact with the external surface of the cylindrical die at positions around the radial resin extrusion ports. First, the panel-shaped blades are installed on the cylindrical die, with the sharpened edges of the blades 15 coming into axial and tangential contact with the external surface of the die at positions around the radial resin extrusion ports. It is thus very difficult to precisely install the blades on the die so as to allow the blades to perform a smooth and effective cutting operation. In 20 addition, the blades waste excessive time while being changed with new blades when the existing blades are broken. Second, the sharpened blade edges are rotated on the die while being brought into close contact with the 25 external surface of the die and are repeatedly impacted by the extruded foam coming out of the die during a cutting operation. The blade edges are thus easily abraded and are easily broken while undesirably generating operational noise and vibration. 30 Third, such panel-shaped blades undesirably generate frictional heat between the die and the blades while forming an air current undesirably acting on the die and the extruded resin foam lines, thus unstably changing the resin foam extruding conditions and failing to accomplish 35 desired foaming conditions. The above air current also makes a nonuniform shape of resulting resin foam pellets. Disclosure of the Invention Accordingly, the present invention has been made WO 00/44542 PCT/KROO/00066 -5 keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide an apparatus for manufacturing thermoplastic resin foam pellets, which is designed to more effectively and 5 continuously produce desired thermoplastic resin foam pellets from a thermoplastic resin material having a high degree of viscosity in commercial quantity at a low production cost. In order to accomplish the above object, the primary 10 embodiment of the present invention provides an apparatus for manufacturing thermoplastic resin foam pellets, comprising: a hollow cylindrical die receiving pressurized and molten thermoplastic resin mixed with a foaming agent in a direction from a resin inlet of the die, the die 15 having a plurality of radial resin extrusion ports at its outlet end portion; a torpedo locked to the outlet end of the die at a first end thereof, with a second end of the torpedo extending into the middle portion of the interior of the die; and a means for continuously cutting 20 thermoplastic resin foam lines coming out of the radial resin extrusion ports of the die into pellets having a predetermined size, thus forming desired thermoplastic resin foam pellets, the cutting means being installed outside the die at a position around the radial resin 25 extrusion ports, the cutting means comprising a plurality of steel wires spirally extending along a rounded external surface of the cylindrical die at a position around the radial resin extrusion ports while coming into close contact with the rounded external surface of the die. 30 Another embodiment of this invention provides an apparatus for manufacturing thermoplastic resin foam pellets, comprising: a hollow cylindrical die; a torpedo concentrically set within the die, with a plurality of axial resin passages being formed between the external 35 surface of the torpedo and the cylindrical internal surface of the die while being regularly spaced apart from each other at an angular interval; a plurality of radial resin extrusion ports regularly formed on the sidewall of the cylindrical die at positions corresponding 40 to the axial resin passages; and a means for continuously WO 00/44542 PCT/KROO/00066 -6 cutting thermoplastic resin foam lines coming out of the radial resin extrusion ports of the die into pellets having a predetermined size, thus forming desired thermoplastic resin foam pellets, the cutting means being 5 installed inside the die at a position around the radial resin extrusion ports, the cutting means comprising a plurality of movable rods and a drive unit for actuating the movable rods, the movable rods being axially arranged inside the die to be axially reciprocable along the die, 10 with a plurality of pairs of holes being formed on each of the movable rods so that they each alternately communicate with one of two neighboring radial resin extrusion ports in accordance with an axial reciprocating action of each movable rod. 15 Brief Description of the Drawings The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 20 Fig. 1 is a sectional view of an apparatus for manufacturing thermoplastic resin foam pellets in accordance with the primary embodiment of the present invention; Fig. 2 is a sectional view of a resin foam pellet 25 cutting means included in the apparatus of Fig. 1; Fig. 3 is a view of an apparatus for manufacturing thermoplastic resin foam pellets in accordance with the second embodiment of the present invention; Fig. 4 is a sectional view of a die included in the 30 apparatus of Fig. 3; Fig. 5 is a sectional view, showing the radial resin extrusion ports formed in the die of Fig. 4 in more detail; and Fig. 6 is a sectional view of the radial resin 35 extrusion ports taken along the line A-A of Fig. 4. Best Mode for Carrying Out the Invention WO 00/44542 PCT/KROO/00066 -7 In the present invention, thermoplastic resins are preferably used as the material of desired resin foam pellets. Examples of the thermoplastic resins include ordinary polystyrene, styrene-acrylonitrile copolymer, 5 styrene-ethylene copolymer, low-density polyethylene, linear low-density polyethylene, polypropylene, ethylene propylene copolymer, ethylene-vinyl acetate copolymer, polyvinyl chloride and mixtures thereof. On the other hand, the foaming agent used in the 10 present invention may be selected from the group consisting of volatile foaming agents and decomposable foaming agents. Examples of volatile foaming agents include aliphatic hydrocarbons, such as propane, butane, isobutane and pentane, cyclic hydrocarbons, such as 15 cyclobutane, cyclophentane and cyclohexane, methyl chloride, methylene chloride, dichloro fluoromethane, chloro trifluoromethane and dichloro difluoromethane. As for the decomposable foaming agents, they are exemplified b y dinitrosopentamethylene tetramine, 20 trinitrosotrimethylene triamine, benzene sulfonylhydrazide, azodicarbonamide, etc. The present invention preferably uses any one of the above-mentioned foaming agents or mixtures thereof. The above-mentioned thermoplastic resin, foaming 25 agent and the other additives form a desired foamable thermoplastic resin mixture when they are processed by an extruding machine. That is, in an extruding machine with a kneader installed between a main extruding part and a cold 30 extruding part, thermoplastic resin is supplied into the main extruding part and is molten therein while being fed to the kneader by a screw feeder. A foaming agent is added to the molten and pressurized thermoplastic resin while being kneaded, and so the foaming agent is 35 uniformly dispersed in the molten thermoplastic resin. In the above extruding machine, the main extruding part is heated to a desired temperature in accordance with a selected thermoplastic resin, for example, 100 2000C in the case of low-density polyethylene. On the 40 other hand, the kneader is heated to a maximum WO 00/44542 PCT/KROO/00066 -8 temperature and is pressurized at a maximum pressure. The above thermoplastic resin mixture from the extruding machine is, thereafter, continuously foamed in the form of pellets by an apparatus of this invention as 5 shown in Figs. 1 and 2 or Figs. 3 to 6. Figs. 1 and 2 show an apparatus for manufacturing thermoplastic resin foam pellets in accordance with the primary embodiment of this invention. As shown in the drawings, the apparatus of the primary embodiment of this 10 invention comprises a hollow cylindrical die 11, which is locked to the outlet end of the extruding machine 10 at its inlet end 13 and has an open interior 14. A torpedo 17 is set within the die 11 while being locked to the outlet end of the die 11 at the first end thereof, with 15 the second end of the torpedo 17 extending into the middle portion of the interior 14 of the die 11. The apparatus also has a means 25 for continuously cutting thermoplastic resin foam lines continuously coming out of a plurality of radial resin extrusion ports 23 of the die 20 11 into pellets having a predetermined size, thus forming desired thermoplastic resin foam pellets. The above cutting means 25 is installed outside the die 11 at a position around the outlet end of the die 11. In the primary embodiment, the die 11 comprises a 25 first body 11A, a second body 11C, a third body 11D and a locking ring 11E. In the above die 11, the first body 11A is locked to the outlet end of the extruding machine 10 at its inlet end 13 and has an annular interior groove 11B seating a centering support part 27 of the torpedo 17 30 therein. The second body 11C axially receives the torpedo 17 and has the radial resin extrusion ports 23 at an appropriate portion. The object of the third body 11D is to lock the first and second bodies 11A and 11C to each other and to hold the centering support part 27 of 35 the torpedo 17. The third body 11D is thus inserted into the outlet end of the first body 11A at its inlet end and is bolted to the outlet end surface of the first body 11A at its radial flange, with the front end of the second body 11C being locked to the interior of the inlet end of 40 the third body through a screw-type engagement. The WO 00/44542 PCT/KROO/00066 -9 locking ring 11E engages with the exterior surface of the inlet end of the first body 11A through a screw-type engagement and is bolted to the outlet end surface of the extruding machine 10. 5 As shown in Fig. 2, the cutting means 25 comprises two rings 26 and 27, which are fitted over the outlet end portion of the die 11 and are rotatably supported by bearings 28. The two rings 26 and 27 are connected to each other through a plurality of connectors 29. A 10 plurality of steel wires 30 spirally extend along the rounded external surface of the cylindrical die 11 at a position around the radial resin extrusion ports 23 of the die 11 between the two rings 26 and 27 while coming into close contact with the rounded external surface of 15 the die 11. In the primary embodiment of Figs. 1 and 2, each of the steel wires 30 is preferably locked to an appropriate position of the first ring 26 at one end thereof, with the other end of each steel wire 30 being locked to the second ring 27 at a position deviated from 20 the steel wire locked position of the first ring 26 at a deviation angle of about 1800. In the preferred embodiment, the cutting means 25 preferably comprises four steel wires 30, each locked to the two rings 26 and 27 at both ends thereof. 25 In order to rotate the cutting means 25 around the second body 11C of the die 11, a drive means (not shown), for example, a drive motor, is installed in back of the cutting means 25 on the apparatus. The rotating force of the drive means is transmitted to the cutting means 25 30 through a power transmission unit 31. The above apparatus is operated as follows. That is, when a foamable thermoplastic resin mixture is supplied from the extruding machine 10 into the inlet end 13 of the die 11, the mixture is fed to the outlet end of 35 the die 11 through the passage between the die 11 and the torpedo 17 while being compressed, expanded and recompressed. At the outlet end portion of the die 11, the mixture is continuously, radially and linearly extruded to the 40 outside of the die 11 through the radial resin extrusion WO 00/44542 PCT/KROO/00066 -10 ports 23 while being foamed, thus forming thermoplastic resin foam lines. The thermoplastic resin foam lines are continuously cut into pellets having a predetermined length by the steel wires 30 of the cutting means 25 5 operated by the drive means (not shown), thus continuously forming desired thermoplastic resin foam pellets. When the cutting means 25 is rotated by the rotating force of the drive means, the steel wires 30, which are individually locked to the two rings 26 and 27 at both 10 ends and spirally extend along the rounded external surface of the cylindrical die 11 at the position around the radial resin extrusion ports 23 of the die 11, spirally move on the rounded external surface of the die 11 while coming into close contact with the rounded 15 external surface of the die 11. The steel wires 30 thus continuously cut the thermoplastic resin foam lines coming out of the radial resin extrusion ports 23 of the die 11 into pellets having a predetermined length, thus continuously forming desired thermoplastic resin foam 20 pellets. In such a case, the thermoplastic resin foam lines coming out of the ports 23 are free from a vertical shearing force of the cutting means, but a diagonally acting shearing force is applied from the steel wires 30 to the thermoplastic resin foam lines different from a 25 conventional apparatus. The thermoplastic resin foam lines coming out of the ports 23 are thus smoothly cut into desired pellets by the steel wires 30. Therefore, the thermoplastic resin foam line cutting operation of this apparatus is almost free from operational noise or 30 operational vibration different from a conventional apparatus having panel-shaped cutting blades. The apparatus of this invention also effectively produces uniform-sized and uniform-shaped thermoplastic resin foam pellets. In addition, since the steel wires 30 of this 35 invention have a diameter of 0.3 mm - 2.0 mm and spirally move on the rounded external surface of the die 11 while coming into close contact with the rounded external surface of the die 11 during a thermoplastic resin foam line cutting operation, the apparatus of this invention 40 is almost free from a generation of frictional heat or WO 00/44542 PCT/KROO/00066 -11 air current different from the conventional apparatus having the panel-shaped cutting blades. Since the apparatus is free from frictional heat, desired resin foam extruding conditions in addition to desired foaming 5 conditions are accomplished. This apparatus free from the air current effectively makes a uniform shape of resulting resin foam pellets. Figs. 3 to 6 show an apparatus for manufacturing thermoplastic resin foam pellets in accordance with the 10 second embodiment of this invention. As shown in the drawings, the apparatus of the second embodiment comprises a hollow cylindrical die 1. A torpedo 2 is concentrically set within the die 1, with a plurality of axial resin passages 3 being formed between the external 15 surface of the torpedo 2 and the cylindrical internal surface of the die 1 while being regularly spaced apart from each other at an angular interval. A plurality of radial resin extrusion ports 4 are regularly formed on the sidewall of the cylindrical die 1 at positions 20 corresponding to the axial resin passages 3. The apparatus also has a cutting means for continuously cutting the thermoplastic resin foam lines coming out of the radial resin extrusion ports 4 of the die 1 into pellets having a predetermined size, thus forming desired 25 thermoplastic resin foam pellets. In the second embodiment, the cutting means is installed inside the die 1 at a position around the radial resin extrusion ports 4, and comprises a plurality of movable rods 5 and a drive unit for actuating the movable rods 5. The above 30 movable rods 5 are axially arranged inside the die 1 to be axially reciprocable along the die 1. A plurality of pairs of holes 6a and 6b are formed on each of the movable rods 5 so that they each alternately communicates with one of two neighboring radial resin extrusion ports 35 4 in accordance with an axial reciprocating action of each movable rod 5. The drive unit actuates the movable rods 5 so as to allow the rods 5 to reciprocate on the external surface of the die 1 in an axial direction. The external surface of the die 1 has a plurality of 40 cutting surfaces 7, which cooperate with the outside ends WO 00/44542 PCT/KROO/00066 -12 of the holes 6a and 6b of the movable rods 5 so as to continuously cut the thermoplastic resin foam lines coming out of the holes 6a and 6b. In order to allow a smooth cutting action and a smooth pellet distribution, a slope 5 surface 8 extends from the outside edge of each of the cutting surfaces 7. In the present invention, it is possible to use a variety of mechanisms as the drive unit for the movable rods 5. In the embodiment of Fig. 3, the drive unit 10 comprises a circular eccentric cam 32 that is installed on a rotating shaft 31 rotated by a drive motor 9. A crank 33 is coupled to the eccentric cam 32 at one end thereof, with a bearing rotatably supporting the crank 33 on the eccentric cam 32. The outside ends of the movable 15 rods 5, close to the crank 33, are commonly held by a support panel 34, with the center of the support panel 34 being connected to the end of the crank 33 through a connecting rod 35. In Fig. 3, the joint between the connecting rod 35 and the support panel 34 is 20 schematically shown. However, it should be understood that the joint between the connecting rod 35 and the support panel 34 is preferably formed by an appropriate joint, for example, a universal joint capable of smoothly transmitting the rotating force of the shaft 31 to the 25 movable rods 5 while converting the fluctuating action of the crank 33 caused by the eccentric rotating action of the cam 32 into a desired reciprocating action of the movable rods 5. The apparatus of Figs. 3 to 6 is operated as 30 follows. That is, when a foamable thermoplastic resin mixture is supplied into the interior of the die 11 through the axial resin passages 3 formed between the external surface of the torpedo 2 and the cylindrical internal surface of the die 1. Thereafter, the mixture 35 is extruded to the outside of the die 1 through both the radial resin extrusion ports 4 of the die 1 and the holes 6a and 6b of the movable rods 5. In such a case, the holes 6a and 6b of the rods 5 are designed to each alternately communicate with one of two neighboring radial 40 resin extrusion ports 4 of the die 1 in accordance with WO 00/44542 PCT/KROO/00066 -13 an axial reciprocating action of each movable rod 5. When the first hole 6a of each rod 5 communicate with one port 4 of the die 1 as shown in Fig. 3, the second hole 6b of each rod 5 is closed by the sidewall of the die 1 5 between the two neighboring radial resin extrusion ports 4. Therefore, the extrusion of resin foam lines through the second holes 6b of the rods 5 is temporarily stopped. When the drive motor 9 is started in the above position, the rotating shaft 31 is rotated by the 10 rotating force of the motor 9, thus allowing the cam 32 to be eccentrically rotated around the central axis of the shaft 31. Such an eccentric rotating action of the cam 32 causes a fluctuating action of the crank 33. The fluctuating action of the crank 33 is, thereafter, 15 transmitted to the support panel 34 through the connecting rod 35 and makes the movable rods 5 simultaneously reciprocate inside the die 1 in an axial direction of the die 1. Due to such a reciprocating action of the movable rods 5, the holes 6a and 6b of each movable rod 5 each 20 alternately communicate with one of two neighboring radial resin extrusion ports 4 of the die 1. Therefore, the foamable resin mixture within the die 1 is extruded to the outside of the die 1 through the holes 6a or 6b of the rods 5 communicating with the ports 4 of the die 1 25 while being foamed, thus forming thermoplastic resin foam lines. The thermoplastic resin foam lines coming out of the holes of the rods 5 are continuously cut by the cutting surfaces 7 of the die 1 into pellets having a size determined by the reciprocating velocity of the rods 30 5, thus producing desired thermoplastic resin foam pellets. Since a plurality of radial resin extrusion ports 4 are regularly formed on the sidewall of the cylindrical die 1 at positions corresponding to the axial resin passages 3, the apparatus of the second embodiment 35 effectively produces a desired number of thermoplastic resin foam pellets at each reciprocating action of the movable rods 5. During the thermoplastic resin foam line cutting operation of the above apparatus, the cutting surfaces 7 40 of the die 1 cut the thermoplastic resin foam lines WO 00/44542 PCT/KROO/00066 -14 coming out of the holes 6a and 6b of the movable rods 5 while forcing the cut sections of the resin foam lines inwardly. Therefore, the resulting thermoplastic resin foam pellets, produced by the apparatus of the second 5 embodiment, are desirably uniformized at both cut sections thereof while being almost completely free from a slit or a crack on the cut sections. The apparatus of the second embodiment thus effectively produces thermoplastic resin foam pellets having a desired surface. 10 Industrial Applicability As described above, the present invention provides an apparatus for manufacturing thermoplastic resin foam pellets. The apparatus of this invention is designed to continuously and directly produce desired thermoplastic 15 resin foam pellets in commercial quantity without performing an intermediate process of forming foamable intermediate products. The apparatus of this invention easily, effectively and continuously produces desired thermoplastic resin foam pellets from high viscosity 20 thermoplastic resin, which is difficult to be produced into foam pellets due to the high degree of viscosity in the prior art. In the apparatus of this invention, the cutting means for continuously cutting the thermoplastic resin foam lines into desired pellets is structurally 25 improved to be almost completely free from operational noise, operational vibration, frictional heat or undesirable air current during a cutting operation, and so the apparatus produces uniform thermoplastic resin foam pellets having improved quality. 30 Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the 35 invention as disclosed in the accompanying claims.