Patent Publication Number: US-2016228932-A1

Title: Extrusion die for forming hollow material

Description:
TECHNICAL FIELD 
     The present invention relates to an extrusion die for forming a hollow material and, more specifically, to an extrusion die for forming a hollow material having a partition wall provided in the interior thereof by extruding a high-strength alloy, particularly a high-strength aluminum alloy such as the so-called 7000 series. 
     BACKGROUND ART 
     In general, extrusion processing of an aluminum alloy or the like is broadly employed nowadays since it can provide highly versatile sectional views and is excellent in acquiring hollow materials formed by extrusion. 
     That is, products formed by extrusion processing have come to be used broadly as strength members of structural materials, mechanical components, and the like, and demands for extrusion members formed with a high-strength alloy, particularly high-strength aluminum alloys of the so-called 7000 series such as 7075, 7N01, 7003, and the like have been increasing. Further, as hollow materials formed by extrusion, not only the angular columnar shapes but also those with complicated sectional views such as a type in a sectional view with a single lateral partition wall, a type in a sectional view with two lateral partition walls, and the like have recently been produced. 
     As an example of conventional extrusion die for molding hollow materials of complicated sectional views, known are a metal-made three-dimensional extrusion material manufacturing method and a manufacturing device thereof (e.g., see Patent Document 1). 
     The metal-made three dimensional extrusion material manufacturing method and the manufacturing device thereof are structured to be able to form a three-dimensional extrusion member in which a hollow part and a solid part exist in a mixed manner in the lengthwise direction. 
     Further, also known is a hollow material extrusion die for forming a hollow material having a partition wall (e.g., see Patent Document 2). 
     This extrusion die is structured to be able to form a hollow material having a laterally-long sectional view with a single partition wall and a hollow material having a sectional view with two partition walls. 
     Further, also known are an extrusion processing method and a device for a metal-made extrusion material with different lateral sectional views in the lengthwise direction (e.g., see Patent Document 3). With the extrusion processing method and the device, it is possible to extrusion-mold aluminum-made extrusion materials having different lateral sectional views in the lengthwise direction. 
     Patent Document 1: Japanese Unexamined Patent Publication Hei 4-305312 
     Patent Document 2: Japanese Examined Patent Publication Hei 5-9169 
     Patent Document 3: JP No. 3095916 
     Incidentally, in a case of forming a hollow material of a complicated sectional view such as a sectional view with two partition walls by using a high-strength alloy, particularly 7000 series high-strength aluminum as a hollow member molding material, a pair of opposing outer circumferential walls and two parallel partition walls are formed. Those partition walls are in a straight-line form, so that it is considered that billets can flow relatively easily. 
     However, recently, not only the hollow materials having the sectional view with two partition walls and the like but also hollow materials having still more complicated sectional views such as a hollow material having a sectional view with a cross-like partition walls, a hollow material having a sectional view with a curve-shaped partition wall, and the like are desired due to the reason for improving the strength of the hollow materials, for example. 
     In a case of the hollow material having a sectional view with a cross-like partition wall, the cross-like partition walls forming the sectional view with a cross-like shape intersect with each other at the centers thereof. Thus, the billet formed with an aluminum alloy that is fed from the upstream side and extruded is not easily flown to the direction orthogonal to each other from the intersection. This causes such an issue that the cross-like partition walls having the intersection cannot be formed sufficiently. 
     Further, even in a case where there is no intersection and in a case of a hollow material having a complicated curve-shaped partition wall, flow of the billet tends to slow down at the curved part. This causes such an issue that the curve-shaped partition wall cannot be formed sufficiently. 
     Further, while the manufacturing method and the manufacturing device of the metallic three-dimensional extrusion material disclosed in Patent Document 1 described above are structured to be able to mold the three-dimensional extrusion material in which the hollow part and the solid part exist in a mixed manner in the lengthwise direction, it is not possible with the device disclosed in Patent Document 1 to mold the hollow material having an intersection constituted with partition walls. 
     Further, while the hollow-type material extrusion die having the partition walls disclosed in Patent Document 2 can mold the hollow material having a laterally long sectional view with a partition wall and the hollow material having a sectional view with two partition walls, the intersection is formed with the partition walls as described above. Thus, the extruded billet can flow in one direction but cannot flow easily from the intersection in the direction orthogonal to that direction. Therefore, it is difficult to mold the hollow material having the sectional view with a cross-like form inside therein. 
     Further, while the extrusion processing method and the device for the metallic extrusion materials having different lateral sectional views in the lengthwise direction disclosed in Patent Document 3 described above can extrusion-mold aluminum-made extrusion materials having different lateral sectional views in the lengthwise direction, it is not possible with the device disclosed in Patent Document 3 to mold a hollow material having an intersection constituted with partition walls. 
     In order to overcome the above-described issues, it is an object of the present invention to provide an extrusion die for forming a hollow material capable of easily forming a hollow material having a partition wall provided inside thereof through extruding a billet formed with a high-strength alloy with a large extrusion processing force, particularly a high-strength aluminum alloy such as the so-called 7000 series. 
     DISCLOSURE OF THE INVENTION 
     In order to achieve the foregoing object, the extrusion die for forming a hollow material according to the present invention is an extrusion die for forming a hollow material, which includes: a male-type member which forms an inner shape of the hollow material while guiding a billet constituted with an aluminum alloy fed from an upstream side toward a downstream side; and a female-type member which holds the male-type member with an outer circumferential part and forms an outer shape of the hollow material, wherein: the male-type member includes a mandrel section for forming the inner shape, and a holder section connected integrally to an outer circumferential part of the mandrel section via a plurality of bridge sections; a billet guide hole for guiding a part of the billet toward the downstream side is provided in a center region of the mandrel section; an upstream-side opening area of the billet guide hole is formed larger than a downstream-side opening area; and a plurality of inner formation pieces are fixedly mounted on a downstream side of the billet guide hole and at positions for forming continuous partition walls inside the hollow material while keeping a billet flow-in gap space forming a merging space of the billet flowing in from each of the bridge sections toward the downstream side. 
     The extrusion die for forming a hollow material according to the present invention is structured in the manner described above, so that a part of the billet from the billet guide hole provided in the center region of the mandrel section is mixed with the billet flown-in from the bridge section side and extruded out from the billet flow-in space maintained toward the downstream side. The billet extruded out from the billet guide hole is extruded out toward the position of the partition wall part of the hollow material formed by a plurality of inner formation pieces, so that it can sufficiently reach even into complicated sectional views such as the intersection of the partition walls, the curved part of the curve-shaped partition walls, etc. As a result, it becomes possible to easily form the hollow material having partition walls provided therein through extruding out the billet formed with a high-strength alloy with a large extrusion processing force, particularly a high-strength aluminum alloy such as the so-called 7000 series. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross sectional perspective view showing a main part of a first embodiment of an extrusion die for forming a hollow material according to the present invention; 
         FIG. 2  is a plan view showing the entire extrusion die for forming the hollow material according to the first embodiment; 
         FIG. 3  is a cross sectional view taken along a line III-III of  FIG. 2 , which is a cross sectional view showing a state where a holder and a bridge outer circumferential face are formed integrally by a bridge section press-fit structure and where a billet guide hole is in a two-stage structure; 
         FIG. 4  is a fragmentary sectional arrow view taken along a line IV-IV of  FIG. 3 ; 
         FIG. 5  is a sectional perspective view of a male-type member and a female-type member of the first embodiment, which is the entire view of  FIG. 1 ; 
         FIG. 6  is an overall plan view showing the surface of the female-type member of the first embodiment; 
         FIG. 7  is a perspective view showing a hollow material having a sectional view with a cross-like shape formed by the extrusion die for forming the hollow material according to the first embodiment; 
         FIG. 8  is a second embodiment of the extrusion die for forming a hollow material according to the present invention, which is a cross sectional view corresponding to  FIG. 3  showing a state where a holder and a bridge outer circumferential face are formed integrally by a bridge section press-fit structure and where a billet guide hole is in a tapered shape; 
         FIG. 9  is a sectional perspective view of a male-type member and a female-type member shown in  FIG. 8 ; 
         FIG. 10  is an overall plan view showing a third embodiment of the extrusion die for forming a hollow material according to the present invention; 
         FIG. 11  is a cross sectional perspective view taken along a line X 1 -X 1  of  FIG. 10 , which is a cross sectional view showing a state where a holder and a bridge outer circumferential face are formed integrally by a bridge section shrink-fit structure and where a billet guide hole is in a two-stage structure; 
         FIG. 12  shows a fourth embodiment of the extrusion die for forming a hollow material according to the present invention, which is a cross sectional perspective view showing a state where a holder and a bridge outer circumferential face are formed integrally by a bridge section shrink-fit structure and where a billet guide hole is in a tapered shape; 
         FIG. 13  shows a fifth embodiment of the extrusion die for forming a hollow material according to the present invention, which is an overall plan view showing a state where a holder and a bridge outer circumferential face are formed integrally by a bridge section shrink-fit structure; 
         FIG. 14  is a cross sectional perspective view taken along a line XIV-XIV of  FIG. 13 ; 
         FIG. 15  is a fragmentary sectional arrow view taken along a line XV-XV of  FIG. 14 ; 
         FIG. 16  is a perspective view showing a hollow material having a sectional view with a lattice form which is formed by the extrusion die for forming a hollow material according to the fifth embodiment; 
         FIG. 17  is an overall plan view showing a sixth embodiment of the extrusion die for forming a hollow material according to the present invention; 
         FIG. 18  is a cross sectional view taken along a line XVIII-XVIII of  FIG. 17 ; 
         FIG. 19  is a fragmentary sectional arrow view taken along a line XIX-XIX of  FIG. 18 ; 
         FIG. 20  is a perspective view showing a hollow material having a sectional view with a lattice form which is formed by the extrusion die for forming a hollow material according to the sixth embodiment; 
         FIG. 21  shows a seventh embodiment of the extrusion die for forming a hollow material according to the present invention, which is an overall plan view showing a state where a holder and a bridge outer circumferential face are formed integrally by a bridge section shrink-fit structure; 
         FIG. 22  is a cross sectional perspective view taken along a line XXII-XXII of  FIG. 21 ; 
         FIG. 23  is a fragmentary sectional arrow view taken along a line XXIII-XXIII of  FIG. 22 ; 
         FIG. 24  is a perspective view showing a hollow material having a sectional view with a lattice form constituted with partition walls of different thicknesses, which is formed by the extrusion die for forming a hollow material according to the seventh embodiment; 
         FIGS. 25A and 25B  show schematic views of hollow materials having curve-shaped partition walls formed by utilizing eighth and ninth embodiments of the extrusion die for forming a hollow material according to the present invention, in which  FIG. 25A  is a view showing a hollow material having curve-shaped partition walls formed by the extrusion die of the eighth embodiment and  FIG. 25B  is a view showing a hollow material having curve-shaped partition walls formed by the extrusion die of the eighth embodiment; 
         FIG. 26  is a schematic view of a hollow material having curve-shaped partition walls formed by utilizing a tenth embodiment of the extrusion die for forming a hollow material according to the present invention, in which two wave-like curve-shaped partition walls are provided; 
         FIG. 27  is a cross sectional perspective view showing a modification example of the fifth embodiment ( FIG. 14 ) according to the present invention; and 
         FIGS. 28A and 28B  show perspective views, in which both  FIG. 28A  and  FIG. 28B  show modification examples of a hollow material having partition walls with intersections formed by utilizing the extrusion die for forming a hollow material according to the present invention. 
     
    
    
     BEST MODES FOR CARRYING OUT THE INVENTION 
     Hereinafter, a first embodiment of an extrusion die for forming a hollow material (simply referred to as an extrusion die hereinafter) of the present invention will be described by referring to  FIG. 1  to  FIG. 6 . 
     An extrusion die  10  of the first embodiment is for forming a hollow material formed with a high-strength alloy, particularly a high-strength aluminum alloy that is the so-called 7000 series. Further, among hollow materials having complicated sectional views such as curve-shaped partition walls, partition walls forming an intersection, and the like, the extrusion die  10  forms a hollow material  1  having a sectional view with a cross formed with partition walls  1   b  and  1   b  provided in a cross-like form having an intersection X inside thereof as shown in  FIG. 7 . 
     As shown in  FIG. 1  and  FIG. 3 , the extrusion die  10  is constituted by including: a male-type member  20  having a mandrel section  23  which forms the inner shape of the hollow material  1  while guiding a billet B formed with an aluminum alloy extruded out from the upstream side toward the downstream side; and a female-type member  30  which holds the male-type member  20  with the outer circumferential part and forms the outer shape of the hollow material  1 . 
     The male-type member  20  is constituted by including: the mandrel section  23 ; and a holder section  25  which is integrally connected to the outer circumferential part of the mandrel section  23  via a plurality of bridge sections  24 . 
     The holder section  25  is formed in a disc shape as a whole with a prescribed thickness. On the end face of the upstream side of the extrusion direction thereof, formed is a billet introduction opening  25 B in a disc shape as a whole while being sectioned by each of the bridge sections  24 . 
     As will be described later in details, a billet guide hole  28  for guiding a part of the billet B toward the downstream side is provided in the center region of the mandrel section  23 . 
     Further, an upstream-side opening area  28 A of the billet guide hole  28  is formed to be larger than the opening area of a downstream-side opening  28 B. 
     Further, a plurality of (four in this embodiment) inner formation pieces  23 B are fixed on the side opposing to the downstream-side opening of the billet guide hole  28  and at positions for forming partition walls having an intersection X inside the hollow material  1  while keeping a billet insertion hole BH 1  (a billet flow-in gap space) forming a merging space with the flow-in billet B from each of the bridge sections  24  on the downstream side, i.e., towards the female-type member  30  side. The inner formation pieces  23 B are provided to the mandrel section  23  via a connecting section  23 M. 
     Further, in the female-type member  30 , an outer formation die hole  30 B for forming outer shapes of a plurality of the hollow materials  1  is provided by opposing to the entire outer circumferential face of the four inner formation pieces  23 . 
     Hereinafter, each structure will be described in more details. 
     First, the entire extrusion die  10  will be described by referring to  FIGS. 2 and 3 . 
     As shown in  FIG. 2 , the extrusion die  10  is formed in a disc shape as a whole. Further, as shown in  FIG. 3 , the extrusion die  10  is constituted by including the male-type member  20 , the female-type member  30 , and a back die  70  for holding the female-type member  30 . 
     Further, the billet B is housed within a billet extrusion device  80  constituted with a chamber and the like disposed in the upstream side of the male-type member  20 , and extruded out by the billet extrusion device  80 . 
     The male-type member  20 , the female-type member  30 , and the back die  70  are connected integrally. 
     That is, as shown in  FIG. 2  and  FIG. 3 , after the male-type member  20  and the female-type member  30  are placed at the positions by two positioning pins  71 , for example, the male-type member  20 , the female-type member  30 , and the back die  70  are connected and fixed by two connection bolts  72 , for example. 
     As shown in details in  FIGS. 3 and 5 , the male-type member  20  is constituted by including a spider  22 . The spider  22  is constituted by including: the mandrel section  23  which forms the inner shape of the hollow material  1 ; the bridge section  24  which supports the mandrel section  23  and is projected substantially in an X-letter shape toward outer side from the periphery of the mandrel section  23 ; and the holder section  25  which is connected integrally via the bridge section  24 . 
     As shown in  FIG. 2 , the bridge section  24  is constituted with four pieces of a first bridge  24   a , a second bridge  24   b , a third bridge  24   c , and a fourth bridge  24   d  disposed clockwise. Further, the spaces between each of the bridges  24   a  to  24   d  form the billet introducing spaces S. 
     A top face  23 A of the mandrel section  23  is formed in a disc-like flat surface, and the top faces of the bridges  24   a  to  24   d  are connected to the top face  23 A. The top faces of the bridges  24   a  to  24   d  are formed in a downward sloping shape from the top face  23 A of the mandrel section  23  toward the internal circumferential face of the holder section  25 . 
     Further, the top face  23 A of the mandrel section  23  is formed as the same height with a top end face  25 A of the holder section  25  (see  FIGS. 3 and 5 ) when the spider  22  and the holder section  25  are assembled integrally. 
     As shown in  FIGS. 1, 3, and 5 , tip external circumferential faces  24 A of the bridges  24   a  to  24   d  are formed to be engaged with bridge receiving faces  26 B of a bridge holding section  26  of the holder section  25 . 
     That is, in the holder section  25 , provided by corresponding to each of the bridges  24   a  to  24   d  are: bridge presser sections  26 A which are formed on the top end of the holder section  25  for pressing the tip upper faces  24 B (see  FIG. 3 ) of the bridges  24   a  to  24   d ; and the bridge receiving faces  26 B which are formed continuously to the bridge presser sections  26 A and formed to be substantially equal (preferably a little wider) with respect to the width of the bridge  24   a  and the like. 
     Further, fixing members  27  in a flat square columnar shape, for example, are knocked into the bridge presser sections  26 A so that each of the bridges  24   a  to  24   d  does not rotate. 
     As shown in  FIG. 5 , the fixing members  27  are knocked from the above into knock-in holes for the fixing members  27  opened over the upper-side parts of the bridge presser sections  26 A and the bridges  24   a  to  24   d  after precisely positioning each of the bridges  24   a  to  24   d  and the bridge presser sections  26 A. 
     Further, the engaging faces between the tip outer circumferential faces  24 A of the bridges  24   a  to  24   d  and the bridge receiving faces  26 B of the holder section  25  are formed as sloping faces approaching toward the center of the die from the upstream side of the extrusion direction toward the downstream direction. Thus, moment generated at the application point of extrusion by the inner formation pieces  23 B to be described later can be decreased, thereby making it possible to increase the strength of each of the bridges  24   a  to  24   d . As a result, breakage of each of the bridges  24   a  to  24   d  can be prevented. 
     Note that the tip outer circumferential faces  24 A of the bridges  24   a  to  24   d  and the bridge receiving faces  26 B of the holder section  25  are integrated by press fitting by a bridge section press-fit structure M. 
     As shown in  FIG. 1  in details, the bottom ends of the bridge sections  24   a  to  24   d  are located at positions distant from the holder receiving face  30 A of the female-type member  30  toward the upper side by a prescribed distance, and formed in a shape connected therefrom to a plurality (four in this embodiment) of the inner formation pieces  23 B for forming the inner shape of the hollow materials  1  (see  FIG. 7 ) via the connecting section  23 M of the mandrel section  23 . 
     Further, at each bottom end of each of the bridges  24   a  to  24   d , a tunnel-like billet insertion hole BH is formed with the bottom end of each of the bridges  24   a  to  24   d  at the bottom end part of the bridge receiving face  26 B of the holder section  25  and the holder receiving face  30 A of the female-type member  30 . As shown with an arrow, the billet insertion hole BH forms a billet merging space where the billets B introduced from the billet introduction spaces S for introducing each billet B are merged. 
     Thereby, the billets B are introduced from the billet introduction space S for introducing each billet B, merged in the billet insertion hole BH, and extruded out to the downstream side. 
     As shown in  FIGS. 1, 3, and 5 , the four inner formation pieces  23 B are provided at the downstream-side end of the flow of the billet B of the mandrel section  23 . 
     Each of those inner formation pieces  23 B is formed substantially in a square columnar shape and provided at the end of the mandrel section  23  via the connecting section  23 M as described above (also see  FIG. 4 ). Further, the four inner formation pieces  23 B are projected out toward the female-type member  30  side and, as shown in  FIGS. 4 and 6  in details, formed to be inserted into the outer formation die hole  30 B formed in the female-type member  30 . 
     The female-type member  30  is formed to oppose to the outer circumferential face of the entire four inner formation pieces  23 B and in a size securing a gap L 1  of a prescribed size. 
     Further, each of the inner formation pieces  23 B is designed to be inserted into the outer formation die hole  30 B of the female-type member  30 , and the gap L 1  in the prescribed size set between the outer circumference of each of the inner formation pieces  23 B and the outer formation die hole  30 B forms a material outer formation hole  50  (see  FIGS. 1, 3, and 5 ). Furthermore, as shown in  FIG. 3 , the outer formation die hole  30 B is formed with a straight-line part in a small size and a clearance hole  30 C expanded from the straight-line part toward the outer circumference direction of the female-type member  30 . 
     Each of such inner formation pieces  23 B is designed to form each of the four inside spaces  1 S of the hollow material  1  in a sectional view with a cross inside thereof as shown in  FIG. 7 , and the four inner formation pieces  23 B are disposed so that the entire shape thereof forms substantially a square shape as shown in  FIG. 4 . 
     As described above, each of the inner formation pieces  23 B is provided at the end on the downstream side of the extrusion direction of the material inner forming section  23  via the connecting section  23 M. 
     As shown in  FIG. 1 , on the upstream side of the extrusion direction of each of the inner formation pieces  23 B, a band-like flange section  23 F projected toward the outer side from the respective outer circumferences is provided by being wrapped around the outer circumference of each of the inner formation pieces  23 B. 
     As shown in  FIGS. 1 and 4 , a gap L 2  in a prescribed size is formed between the opposing flange sections  23 F of the inner formation pieces  23 B that are neighboring to each other. Further, with those gaps L 2 , the material formation inner hole  51  for forming the cross-like partition walls  1   b  and  1   b  of the hollow material  1  is constituted. 
     Further, the outer circumference of each of the flange sections  23 F of the inner formation pieces  23 B is disposed to oppose to the outer formation die hole  30 B formed in the female-type member  30 . Further, the gap L 1  in the prescribed size is formed between the both, and the material formation outer hole  50  for forming outer circumferential walls  2   a  and  2   a  of the hollow material  1  is formed with those gaps L 1 . 
     As shown in  FIGS. 1, 3 , and the like, the top faces of the flange sections  23 F of the respective inner formation pieces  23 B are on the same flat surface as that of the holder receiving face  30 A of the female-type member  30 . Thus, the billet B is also extruded out along the top end faces of the flange sections  23 F via the side face of the connecting section  23 M of the mandrel section  23  from the billet insertion hole BH. 
     As described above, in the hollow material  1 , the single intersection X is formed with the two partition walls  1   b  and  1   b . Thus, the billets B extruded out only from the billet introduction space S for introducing the billet B, the billet insertion hole BH, and the billet insertion hole BH 1  may not be sufficient to fill up to the intersection X. 
     Thus, as shown in  FIGS. 1 to 4 , the die  10  of the first embodiment is structured to include the billet guide hole  28  which guides a part of the billet B toward the downstream side provided in the center region of the mandrel section  23 . 
     The billet guide hole  28  is provided by corresponding to the intersection X of the partition walls  1   b  and  1   b . Further, the upstream side opening area located on the billet B flow-in side of the mandrel section  23  is formed larger than the opening area of the downstream side opening located on the billet B flow-out side. 
     Further, on the side opposing to the downstream-side opening of the billet guide hole  28  and also between the opposing surfaces of the connecting section  23 M, the billet insertion hole BH 1  constituting the billet introduction gap space is provided. The billet insertion hole BH 1  is for constituting the billet merging space where the billets B introduced into the billet introduction spaces S for introducing the billets B are merged with each other, and the billets B introduced from the billet guide hole  28  are extruded out via the billet insertion hole BH 1 . 
     That is, as shown in details in  FIGS. 1, 3, and 5 , the billet guide hole  28  of the first embodiment is constituted with a large opening hole  28 A formed with a diameter φ 1  on the upstream side including a step part in the midway of the mandrel section  23  and a small opening hole  28 B formed with a diameter φ 2  which introduces a part of the billet B on the lower side of the mandrel section  23 , i.e., on the side of the intersection X of the partition walls  1   b  and  1   b.    
     Thus, a part of the billet B fed from the upstream side and extruded out is securely introduced into the small opening hole  28 B by being guided by the large opening hole  28 A. 
     Further, since the small opening hole  28 B is provided on the lower side of the mandrel section  23 , it is possible to secure thickness of the small opening hole  28 B to be large in the mandrel section  23 . Therefore, the strength of the die for the stress at the time of extrusion can be increased. As a result, cracking of the die can be prevented. 
     The region where one corner each of flange sections  23 F of the four inner formation pieces  23 B gathers, the position of the intersection point P corresponds to the intersection X formed with the partition walls. Further, the position of the small opening hole  28 B is set so that the position of the intersection point P and the center of the small opening hole  28 B of the billet guide hole  28  to be described later in details become consistent. 
     Next, a forming method of the hollow material  1  by the extrusion die  10  in the above-described structure will be described. 
     When the billet B is fed and extruded out by the billet extrusion device  80  provided on the upstream side of the extrusion direction of the billet B for the male-type member  20 , the billet B is first introduced into the billet introduction space S for introducing the billet B constituted with the gap formed in the mandrel section  23  of the male-type member  20 , the bridge section  24 , and the holder section  25 , and a part thereof is introduced into the larger opening hole  28 A of the billet guide hole  28 . 
     The billet B introduced into the billet introduction space S is introduced into the material formation outer hole  50  from the side faces of the first to fourth bridges  24   a  to  24   d , the side face of the material inner formation section  23 , the billet insertion hole BH, the billet insertion hole BH 1 , and the top face of the flange section  23 F of each of the inner formation pieces  23 B, and extruded out from the material formation outer hole  50 . 
     Meanwhile, a part of the billet B introduced into the large opening hole  28 A of the billet guide hole  28  is securely introduced into the small opening hole  28 B by being guided by the large opening hole  28 A. At that time, the billet B from the billet insertion hole BH 1  is also merged and extruded out. 
     Then, the extruded and molded hollow material  1  is fed out from a material feed-out hole  70 A formed in the back die  70 . Thereafter, it is held by a holding mechanism, not shown, and conveyed into a prescribed stockyard or the like. 
     Next, the hollow material  1  molded by the above-described extrusion die  10  according to the above-described first embodiment will be described by referring to  FIG. 7 . 
     The hollow material  1  is constituted with the outer circumferential walls  1   a ,  1   a  having a square sectional view and the partition walls  1   b ,  1   b  in a cross-like form provided inside the outer circumferential walls  1   a ,  1   a . The center part where the partition walls  1   b  and  1   b  intersect with each other is the intersection X. Thus, the hollow material  1  is formed in a sectional view with a cross having four spaces  1 S inside thereof. 
     The hollow material  1  having such sectional view with a cross is molded by continuously extruding out the billet B from the material formation outer hole  50  and the material formation inner hole  51  of the extrusion die  10 . 
     The extrusion die  10  of the first embodiment is structured in the manner described above, so that following effects can be acquired: 
     (1) A part of the billet B fed from the upstream side is extruded out from the billet guide hole  28  provided in the center region of the mandrel section  23  toward the intersection point P where the one corner each of the flange sections  23 F of the four inner formation pieces  23 B gathers. The position of the intersection point P corresponds to the intersection X formed by the partition walls and, further, the intersection point P and the center of the small opening hole  28 B of the billet guide hole  28  are consistent with each other on the same line, so that the billet B fed through the small opening hole  28 B is extruded out via the intersection point P. Thus, the intersection X can be molded easily. As a result, it becomes possible to easily mold the hollow material in which complicated-shape partition walls are provided through extruding out a billet formed with a high-strength alloy of a high extrusion processing force, particularly a high-strength aluminum alloy such as the so-called 7000 series. 
     (2) The Billet guide hole  28  is in a two-stage structure constituted with the large opening hole  28 A formed on the upper side of the mandrel section  23  and the small opening hole  28 B formed in the lower side of the mandrel section  23 . A part of the billet B extruded out from the upstream side is securely introduced into the small opening hole  28 B by being guided by the large opening hole  28 A. This makes it possible to secure the sufficient billet for forming the partition walls. 
     (3) The engaging face between the tip outer circumferential face  24 A of the first to fourth bridges  24   a  to  24   d  of the bridge section  24  and the bridge receiving face  26 B of the bridge holding section  26  is formed as a sloping surface approaching the center of the die toward the downstream side of the extrusion direction. Therefore, the distance between the base end of the bridge receiving face  26 B of the holder section  25  to the application point in the direction orthogonal to the extrusion direction at the inner formation piece  23  from the base end can be made shorter. Thus, the moment generated at the application point of the inner formation piece  23  can be decreased, so that the strength of the first to fourth bridges  24   a  to  24   d  can be increased. This makes it possible to prevent breakage of the first to fourth bridges  24   a  to  24   d . As a result, it becomes possible to perform high-speed extrusion and to extend the life of the die even when extrusion-molding the billet B formed with a high-strength alloy of a high extrusion processing force, particularly a high-strength aluminum alloy such as the so-called 7000 series. 
     (4) Each of the bridges  24   a  to  24   d  and the bridge receiving face  26 B are formed integrally by the bridge section press-fit structure M, so that the strength of each of the bridges  24   a  to  24   d  and in its turn the strength of the mandrel section  23  can be secured. Thus, the pressure at the time of extrusion of the billet B can be received by the entire male-type member  20 . As a result, it becomes possible to perform high-speed extrusion and to extend the life of the die even when extrusion-molding the billet B formed with a high-strength alloy of a high extrusion processing force, particularly a high-strength aluminum alloy such as the so-called 7000 series. 
     (5) Each of the bridges  24   a  to  24   d  and the bridge presser section  26 A is fixed by the whirl-stop fixing members  27  knocked into the knock-in holes opened over the spaces therebetween, so that rotation of each of the bridges  24   a  to  24   d  can be prevented. Thereby, the hollow materials  1  of high precision can be molded. 
     Next, a second embodiment of the extrusion die of the present invention will be described by referring to  FIG. 8  and  FIG. 9 . 
     The shape of the billet guide hole  28  of the extrusion die  10  according to the first embodiment is in a two-stage structure of the large opening hole  28 A and the small opening hole  28 B, whereas it is formed as a billet guide hole  38  in a tapered shape in an extrusion die  11  of the second embodiment. However, other members, structures, and the like are completely the same as those of the extrusion die  10  of the first embodiment. 
     Therefore, the same reference numerals are applied to the same structures and the same members as those of the first embodiment, and only the different points will be described. Note here that the mandrel section  23  is different only in terms of the shapes of the billet guide holes  28  and  38 , so that the reference numeral  23  that is the same as the case of the first embodiment is also applied in the second embodiment for explanation. 
     The shape of the billet guide hole  38  of the extrusion die  11  according to the second embodiment is formed in a tapered shape which becomes smaller toward the downstream-side opening side from the upstream-side opening of the mandrel section  23 . 
     Note here that the diameter φ 1  of the upstream-side opening of the mandrel section  38  is substantially equivalent to the diameter φ 1  of the large opening hole  28 A of the first embodiment, and the diameter φ 2  of the downstream-side opening of the tapered-shape hole is substantially equivalent to the diameter φ 2  of the small opening hole  28 B of the first embodiment. 
     With the billet guide hole  38  of the extrusion die  11  according to the second embodiment, as in the case of the first embodiment, in the region where one corner each of flange sections  23 F of the four inner formation pieces  23 B gathers, the position of the intersection point corresponds to the intersection X formed with the partition walls. Further, the position of the billet guide hole  38  is set so that the position of the intersection point and the center of the of the billet guide hole  38  become consistent with each other. 
     As in the case of the extrusion die  10  of the first embodiment, it is possible with the extrusion die  11  of the second embodiment described above to mold the hollow material  1  having a sectional view with a cross inside thereof as shown in  FIG. 7 . 
     With the extrusion die  11  of the second embodiment described above, it is possible to acquire substantially the same actions as those of the extrusion die  10  of the first embodiment and substantially the same effects as those described in (1) to (5). In addition, it is possible to achieve the following effect: 
     (6) The flow of the billet B can become smooth since the billet guide hole  38  is formed in a tapered shape which becomes smaller from the upstream-side opening of the mandrel section  23  toward the downstream-side opening side. 
     Next, a third embodiment of the extrusion die according to the present invention will be described by referring to  FIG. 10  and  FIG. 11 . 
     In the extrusion dice  10  and  11  of the first and second embodiments, the lower part of the first to fourth bridges  24   a  to  24   d  of the bridge section  24  and the lower part of the bridge receiving face  26 B are sloping in the direction approaching the dice center side as going toward the female-type member  30 , and those are engaged by the bridge section press-fit structure M. 
     In the meantime, in an extrusion die  12  of the third embodiment, a tip outer circumferential face  34 A of first to fourth bridges  34   a  to  34   d  of a bridge section  34  for supporting a mandrel section  33  and a part of a holder section  125  for holding each of the bridges  34   a  to  34   d  are formed as an integrated structure by the bridge section shrink-fit structure N. 
     Note here that shrink fitting is a method for acquiring strong bonding by utilizing heat, which is a method with which a member such as a disc with a hole is thermally expanded, a shaft formed slightly larger than the diameter of the hole is fitted therein, and it is cooled thereafter to be fixed. The method is used as bonding of fastening type. Further, the both (the disc and the shaft in the above case) are tightly fixed by shrink fitting. 
     A male-type member  120  of the extrusion die  12  according to the third embodiment is substantially the same in terms of the entire shape as those of the extrusion dice  10  and  11  of the first and second embodiments, and the only difference is that there is no bridge presser section  26 A that is formed in the extrusion die  10  and the like. 
     A spider  32  of the third embodiment is constituted with: the mandrel section  33  corresponding to the inner shape of the hollow material  1 ; and the bridge section  34  which supports the mandrel section  33  and supports the mandrel section  33 . 
     The bridge section  34  is constituted with a plurality of, i.e., four bridges of the first bridge  34   a , the second bridge  34   b , the third bridge  34   c , and the fourth bridge  34   d  projected substantially in an X-letter shape from the periphery of the mandrel section  33  to the outer side, and the space between each of the bridges  34   a  to  34   d  forms the billet introduction space S. 
     In the center region of the mandrel section  33 , the billet guide hole  28  for guiding a part of the billet B toward the downstream side is provided. The billet guide hole  28  is formed by corresponding to the intersection X of the partition walls  1   b  and  1   b . Further, the billet guide hole  28  is constituted with the large opening hole  28 A and the small opening hole  28 B. 
     The billet guide hole  28  is in a similar structure as that of the billet guide hole  28  of the extrusion die  10  according to the first embodiment. 
     A supporting member  36  that is a bridge section supporting mechanism for supporting each of the bridges  34   a  to  34   d  is interposed between the lower end of the outer circumference of each of the bridges  34   a  to  34   d  and the holder receiving face  30 A of the female-type member  30 . The both ends of the supporting member  36  are fixed over the lower end of each of the bridges  34   a  to  34   d  and the holder receiving face  30 A of the female-type member  30 . 
     Therefore, a gap of the height of the supporting member  36  is to be formed between the lower end of each of the bridges  34   a  to  34   d  and the holder section receiving face  30 A of the female-type member  30 . This gap forms the tunnel-like billet insertion hole BH where the billets B introduced within the neighboring bridge insertion holes BH are merged with each other. The billet insertion hole BH has the same function as that of the billet insertion hole BH of the first embodiment, which constitutes the billet merging space and the billet flow-in gap space. 
     Further, four inner formation pieces  33 B substantially in the same shape as that of the inner formation piece  23 B are provided in the downstream-side end of the flow of the billet B of the mandrel section  33 , and respective flange sections  33 F are provided to those inner formation pieces  33 B. 
     Each of the inner formation pieces  33 B is projected toward the female-type member  30  side and inserted into the outer formation die hole  30 B formed in the female-type member  30 . 
     Such inner formation pieces  33 B form each of the four inside spaces  1 S of the hollow material  1  having a sectional view with a cross inside thereof as shown in  FIG. 7 . Further, those inner formation pieces  33 B are formed in a square shape substantially in the same shape as that of the inner formation pieces  23 B of the first and second embodiments and also disposed in a square form. 
     With the extrusion die  12  of the third embodiment described above, it is possible to mold the hollow material  1  having the sectional view with a cross inside thereof as shown in  FIG. 7 . 
     With the extrusion die  12  of the third embodiment described above, it is possible to acquire substantially the same actions as those of the extrusion die  10  of the first embodiment and substantially the same effects as those described in (1), (2), and (6). In addition, it is possible to achieve the following effect: 
     (7) The tip outer circumferential face  34 A of each of the bridges  34   a  to  34   d  and a part of the inner circumferential face of the holder section  125  are formed integrally by the bridge section shrink-fit structure N, so that the strength of each of the bridges  34   a  to  34   d  and in its turn the strength of the mandrel section  33  can be secured. Thus, the pressure at the time of extrusion of the billet B can be received by the entire male-type member  20 . As a result, it becomes possible to perform high-speed extrusion and to extend the life of the die even when extrusion-molding the billet B formed with a high-strength alloy of a high extrusion processing force, particularly a high-strength aluminum alloy such as the so-called 7000 series. 
     (8) At the lower end of each of the bridges  34   a  to  34   d , the supporting member  36  is fixed over the lower end of each of those and the holder receiving face  30 A of the female-type member  30 , so that a gap of the height of the supporting member  36  is to be formed between the lower end of each of the bridges  34   a  to  34   d  and the holder section receiving face  30 A of the female-type member  30 . The supporting member  36  can form the tunnel-like billet insertion hole BH where the billets B introduced into the neighboring bridge insertion spaces S merge with each other and also can support each of the bridges  34   a  to  34   d . Therefore, the supporting member  36  can serve two roles, so that it is possible to effectively utilize the member. 
     Next, a fourth embodiment of the extrusion die according to the present invention will be described by referring to  FIG. 12 . 
     In an extrusion die  13  of the fourth embodiment, the shape of the billet guide hole  38  is formed different from that of the billet guide hole  28  of the extrusion die  12  of the third embodiment. Further, the shape of the billet guide hole  38  is the same as that of the billet guide hole  38  of the extrusion die  11  of the second embodiment. 
     Other members, structures, and the like are completely the same as those of the extrusion die  12  of the third embodiment. Therefore, the same reference numerals are applied to the same structures and the same members as those of the third embodiment, and only the different points will be described. 
     The shape of the billet guide hole  38  of the extrusion die  13  according to the fourth embodiment is formed in a tapered shape which becomes smaller toward the downstream-side opening from the upstream-side opening of the mandrel section  33 . 
     In the extrusion die  13  of the fourth embodiment, the four inner formation pieces  33 B are disposed in a square form, so that it is possible to mold the hollow material  1  having the sectional view with a cross inside thereof as shown in  FIG. 7 . 
     With the extrusion die  13  of the fourth embodiment described above, it is possible to acquire substantially the same actions as those of the extrusion die  12  of the third embodiment and substantially the same effects as those described in (1), (2), (7), and (8). 
     Next, a fifth embodiment of the extrusion die according to the present invention will be described by referring to  FIG. 13  to  FIG. 16 . 
     As in the cases of the third and fourth embodiments, in an extrusion die  14  of the fifth embodiment, a tip outer circumferential face  44 A of each of first to fourth bridges  44   a  to  44   d  and a part of the inner circumferential face of the holder section  125  are fixed integrally by the bridge section shrink-fit structure N. Thus, the strength of each of the bridges  44   a  to  44   d  and a mandrel section  43  is secured. 
     In the extrusion die  14 , the structure of a billet guide hole  48  is designed to be different from those of the billet guide holes  28 ,  38  of the extrusion dice  12 ,  13  to the third and fourth embodiments. However, other members, structures, and the like are completely the same as those of the extrusion dice  13  and  14  of the third and fourth embodiments. 
     Therefore, the same reference numerals are applied to the same structures and the same members as those of the third embodiment, and only the different points will be described. 
     The extrusion die  14  of the fifth embodiment is structured to be able to mold a hollow material  2  having a sectional view with a lattice form having four intersections X as shown in  FIG. 16 . 
     The male-type member  120  includes a spider  42  which is constituted with: the mandrel section  43  for molding the inner shape of the hollow material  2 ; and a bridge section  44  which supports the mandrel section  43  and is projected substantially in an X-letter form toward the outer side from the periphery of the mandrel section  43 . The spider  42  is integrally connected with the holder section  125  via the bridge section  44 . 
     In  FIG. 13 , the bridge section  44  is constituted with four bridges disposed clockwise, which are a first bridge  44   a , a second bridge  44   b , a third bridge  44   c , and a fourth bridge  44   d . Further, spaces between each of the bridges  44   a  to  44   d  form the billet introduction spaces S for introducing the billet B. 
     As shown in  FIGS. 13 and 14 , the billet guide hole  48  is in a two-stage structure constituted with a large opening hole  48 A formed on the upstream side of the mandrel section  43  and a small opening hole  48 B formed in the downstream side of the mandrel section  43 , i.e., formed to correspond to the position where the intersection X of the partition walls  1   b  and  1   b  of the hollow material  2  can be formed. 
     The large opening hole  48 A is substantially in a square shape on a plan view and is formed in a recessed shape that is recessed by a prescribed size into the lower-part side of the mandrel section  43 . A plurality (four in this embodiment) of the small opening holes  48 B are formed at the large opening hole  48 A. The large opening hole  48 A is provided by opening the holes toward the downstream side of the mandrel section  43  from the bottom face of the large opening hole  48 A. 
     In the extrusion die  14  of the fifth embodiment, nine inner formation pieces  43 B are provided to be able to correspond to four intersections X. Those inner formation pieces  43 B are formed substantially in the same square shape as that of the inner formation pieces  23 B of the extrusion die  10  of the first embodiment, and are provided in the lower part of the mandrel  43  via a connecting section that is in the same structure as that of the connecting section  23 . 
     Further, those inner formation pieces  43 B are disposed to form a square shape as a whole as shown in  FIG. 15 . Furthermore, the material formation inner hole  51  is formed by the gaps L 2  between each of the inner formation pieces  43 B. Further, the nine inner formation pieces  43 B are inserted into an outer formation die hole  130 B of the female-type member  130 . 
     In the extrusion die  14  of the fifth embodiment, the mandrel  43  is substantially in the same size as that of the mandrel sections  33  and  33  of the extrusion die  13  of the fourth embodiment, and the nine inner formation pieces  43 B are provided in the mandrel section  43 . Thus, the size of each of the inner formation pieces  43 B is formed to be smaller than the size of each of the four inner formation pieces  33 B of the extrusion die  13  of the fourth embodiment. The mandrel  43  may be formed larger in a case where each of the inner formation pieces  43 B is to be formed larger. 
     Further, the four regions where one corner each of flange sections  43 F of the nine inner formation pieces  43 B gathers, the positions of each of the intersection points P correspond to the intersections X formed with the partition walls. Further, the positions of the small opening holes  48 B are set so that the positions of the four intersection points P and the centers of each of the small opening holes  48 B of the four billet guide holes  48  become consistent with each other. 
     Next, the hollow material  2  molded by the extrusion die  14  of the above-described fifth embodiment will be described by referring to  FIG. 16 . 
     The hollow material  2  is formed substantially in a square sectional shape and constituted with two pairs of outer circumferential walls  2   a ,  2   a  disposed to oppose to each other and two each of partition walls  2   b ,  2   b  provided laterally and vertically inside thereof. The sectional view thereof is a lattice form having nine spaces  2 S inside thereof. Further, there are four intersections X where the partition walls  2   b  and  2   b  intersect. 
     Note that the thickness of the partition walls  2   b  and  2   b  is the same. 
     With the extrusion die  14  of the fifth embodiment described above, it is possible to acquire substantially the same actions as those of the extrusion dice of the third and fourth embodiments and substantially the same effects as those described in (1), (2), (7), and (8). In addition, it is possible to achieve the following effect: 
     (9) A part of the billet B fed from the upstream side is extruded out from the large opening hole  48 A of the billet guide hole  48  provided in the center region of the mandrel section  43  via the small opening hole  48 B toward the four intersection points P where the one corner each of the flange sections  43 F of the nine inner formation pieces  43 B gathers. The positions of each of the intersection points P correspond to the four intersections X formed by the partition walls and, further, each of the intersection points P and the centers of the small opening holes  48 B of each of the billet guide holes  48  are consistent with each other on the same line, so that the billet B fed through each of the small opening holes  48 B is extruded out via each of the intersection points P. Thus, the four intersections X can be molded easily. 
     Next, a sixth embodiment of the extrusion die according to the present invention will be described by referring to  FIGS. 17 to 20 . 
     In the extrusion die  14  of the fifth embodiment, there are the four small opening holes  48 B provided in the bottom face of the large opening hole  48 A. However, in an extrusion die  15  of the sixth embodiment, nine small opening holes  58 B are provided in the bottom face of a large opening hole  58 A. 
     As described above, the extrusion die  15  of the sixth embodiment is different from the extrusion die  14  of the fifth embodiment only in terms of the shapes of the billet guide holes  48 B and  58 B. Other members, structures, and the like are completely the same as those of the extrusion die  14  of the fifth embodiment. Therefore, the same reference numerals are applied to the same structures and the same members as those of the fifth embodiment, and only the different points will be described. 
     The extrusion die  15  of the sixth embodiment is structured to be able to form a hollow material  3  having a sectional view with a lattice form as shown in  FIG. 20 . Further, in the hollow material  3 , nine intersections X are provided. 
     The male-type member  120  of the extrusion die  15  includes a spider  52  which is constituted with: a mandrel section  53  for molding the inner shape of the hollow material  2 ; and a bridge section  54  which supports the mandrel section  53  and is projected substantially in an X-letter form toward the outer side from the periphery of the mandrel section  53 . The spider  52  is integrally connected with the holder section  125  via the bridge section  54 . 
     Further, a tip outer circumferential face  54 A of each of first to fourth bridges  54   a  to  54   d  and a part of the inner circumferential face of the holder section  125  are fixed integrally by the bridge section shrink-fit structure N. Thus, the strength of each of the bridges  54   a  to  54   d  and the mandrel section  53  is secured. 
     In  FIG. 17 , the bridge section  54  is constituted with four bridges disposed clockwise, which are the first bridge  54   a , the second bridge  54   b , the third bridge  54   c , and the fourth bridge  54   d . Further, spaces between each of the bridges  54   a  to  54   d  form the billet introduction spaces S for introducing the billet B. 
     As shown in  FIGS. 18 and 19 , the billet guide hole  58  is constituted with a large opening hole  58 A formed on the upstream side of the mandrel section  53  and a small opening hole  58 B formed in the downstream side of the mandrel section  53 , i.e., formed to correspond to the positions where the intersections X of the partition walls  1   b  and  1   b.    
     The large opening hole  58 A is substantially in the same shape as that of the large opening hole  48 A of the extrusion die  14  of the fifth embodiment. That is, the large opening hole  58 A is formed substantially in a square shape on a plan view and is formed in a recessed shape that is recessed by a prescribed size into the lower-part side of the mandrel section  53 . The small opening holes  58 B are formed in the bottom face of the large opening hole  58   a . Nine small opening holes  58 B are provided by opening the holes toward the downstream side of the mandrel section  53  from the bottom face of the large opening hole  58 A. 
     Further, those small opening holes  58 B are designed to correspond to the nine intersections X for forming the hollow material  3  having the sectional view with a lattice, and sixteen inner formation pieces  53 B are provided in the lower part of the mandrel  53  to be able to form the intersections X. In the nine regions where one corner each of flange sections  53 F of the sixteen inner formation pieces  53 B gathers, the positions of each of the intersection points P correspond to the intersections X. Further, the positions of the small opening holes  58 B are set so that the positions of the nine intersection points P and the centers of each of the small opening holes  58 B of the nine billet guide holes  58  become consistent with each other. 
     There are the nine small opening holes  58 B provided in the bottom face of the large opening hole  58 A, so that the plan shape of the large opening hole  58 A is formed larger than that of the large opening hole  48 A of the fifth embodiment. Further, the size of each of the inner formation pieces  53 B is substantially the same as the size of each of the inner formation pieces  43 B of the extrusion die  14  of the fifth embodiment, so that the size of the mandrel  53  of the extrusion die  15  is formed larger than the size of the mandrel  43  of the extrusion die  14  of the fifth embodiment. 
     Therefore, the size of the outer formation die hole  130 B of the female type 130 for housing the sixteen inner formation pieces  53 B is formed larger than the size of the outer formation die hole  30 B of the extrusion die  14  of the fifth embodiment. 
     Further, each of the sixteen inner formation pieces  53 B is in the same square shape and disposed to form a square shape as a whole as shown in  FIG. 19 . Furthermore, the material formation inner hole  51  is formed by the gaps L 2  between each of the inner formation pieces  53 B. Further, the sixteen inner formation pieces  53 B disposed in a square shape are inserted into the outer formation die hole  130 B of the female-type member  130 . 
     Next, the hollow material  3  molded by the extrusion die  16  of the above-described sixth embodiment will be described by referring to  FIG. 20 . 
     The hollow material  3  is formed substantially in a square sectional shape and constituted with two pairs of outer circumferential walls  3   a ,  3   a  disposed to oppose to each other and three each of partition walls  3   b ,  3   b  provided laterally and vertically inside thereof. The sectional view thereof is a lattice form having sixteen spaces  3 S inside thereof. Further, there are nine intersections X where the partition walls  3   b  and  3   b  intersect. 
     The three each of the partition walls  3   b  and  3   b  provided laterally and vertically are formed in the same thickness. 
     With the extrusion die  15  of the sixth embodiment described above, it is also possible to acquire substantially the same actions as those of the extrusion die  14  of the fifth embodiment and substantially the same effects as those described in (1), (2), (7), and (8). In addition, it is possible to achieve the following effect: 
     (10) A part of the billet B fed from the upstream side is extruded out from the large opening hole  58 A of the billet guide hole  58  provided in the center region of the mandrel section  53  via the small opening holes  58 B toward the nine intersection points P where the one corner each of the flange sections  53 F of the sixteen inner formation pieces  53 B gathers. The positions of each of the intersection points P correspond to the nine intersections X formed by the partition walls and, further, each of the intersection points P and the centers of the small opening holes  58 B of each of the billet guide holes  58  are consistent with each other on the same line, so that the billet B fed through each of the small opening holes  58 B is extruded out via each of the intersection points R Thus, the hollow material  3  with the lattice-form sectional view having the nine intersections X can be molded easily. 
     Next, a seventh embodiment of the extrusion die according to the present invention will be described by referring to  FIGS. 21 to 24 . 
     In an extrusion die  16  of the seventh embodiment, the structure of a billet guide hole  68  is formed different from that of the billet guide hole  58  of the extrusion die  15  of the sixth embodiment. However, other members, structures, and the like are completely the same as those of the extrusion die  15  of the sixth embodiment. Therefore, the same reference numerals are applied to the same structures and the same members as those of the sixth embodiment, and only the different points will be described. 
     The male-type member  120  of the extrusion die  16  includes a spider  62  which is constituted with: a mandrel section  63  for molding the inner shape of the hollow material  3 ; and a bridge section  64  which supports the mandrel section  63  and is projected substantially in an X-letter form toward the outer side from the periphery of the mandrel section  63 . The spider  62  is integrally connected with the holder section  125  via the bridge section  64 . 
     Further, a tip outer circumferential face  64 A of each of bridges  64   a  to  64   d  and a part of the inner circumferential face of the holder section  125  are fixed integrally by the bridge section shrink-fit structure N. Thus, the strength of each of the bridges  64   a  to  64   d  and the mandrel section  63  is secured. 
     In  FIG. 21 , the bridge section  64  is constituted with four bridges disposed clockwise, which are the first bridge  64   a , the second bridge  64   b , the third bridge  64   c , and the fourth bridge  64   d . Further, spaces between each of the bridges  64   a  to  64   d  form the billet introduction spaces S. 
     The extrusion die  16  of the seventh embodiment is structured to be able to form a hollow material  4  having a sectional view with a lattice form as shown in  FIG. 24 . Further, in the hollow material  4 , there are nine intersections X that are formed by partition walls of different thicknesses. The billet guide hole  68  is structured to be able to correspond to those intersections X. 
     That is, the billet guide hole  68  is constituted with a large opening hole  68 A provided on the upper side of the mandrel section  63  and nine small opening holes  68 B formed in the lower side of the mandrel section  63  by corresponding to each of the nine intersections X. The large opening hole  68 A is substantially in the same shape as that of the large opening hole  58 A of the billet guide hole  58  of the extrusion die  15  of the sixth embodiment. 
     Further, the small opening holes  68 B are formed by opening the holes from the bottom face of the large opening hole  68 A toward the intersection X side, i.e., toward the female-type member  130  side. 
     The small opening holes  68 B are constituted with three kinds having different opening areas as shown in  FIGS. 21 and 23 . 
     That is, among three each of the small opening holes  68 B disposed laterally and vertically in an equivalent manner, a single first small opening hole  68 B 1  of a largest opening area is disposed in the center, and second small opening holes  68 B2 of a second largest opening area are provided on cross-like lines with respect to the first small opening hole  68 B 1  on both sides thereof. 
     Further, on lines in parallel to the cross-like lines of the second small opening holes  68 B 2 , one each of third small opening holes  68 B 3  of a smaller opening area than that of the second small opening hole  68 B 2  is provided on the outer side of the second small opening holes  68 B 2 . That is, the third small opening holes  68 B 3  are disposed in the four corners of the bottom face of the large opening hole  68 A. 
     Further, those small opening holes  68 B are designed to be able to correspond to the nine intersections X for forming the hollow material  4  having the sectional view with a lattice form, and sixteen inner formation pieces  63 B are provided in the lower part of the mandrel  63  to be able to form the intersections X. 
     In the nine regions where one corner each of flange sections  63 F of the sixteen inner formation pieces  63 B gathers, the positions of each of the intersection points P correspond to the intersections X 1 , X 2 , and X 3 , respectively. Further, the positions of each of the small opening holes  68 B 1 ,  68 B 2 , and  68 B 3  are set so that the positions of the nine intersection points P and the centers of each of the small opening holes  68 B 1 ,  68 B 2 , and  68 B 3  of the nine billet guide holes  68  become consistent with each other. 
     Further, the size of each of the inner formation pieces  63 B is substantially the same as the size of each of the inner formation pieces  53 B of the extrusion die  15  of the sixth embodiment. 
     Each of the sixteen inner formation pieces  63 B is in the same square shape, and disposed in an equivalent manner to form a square shape as a whole as shown in  FIG. 23 . 
     Note here that each of the inner formation pieces  63 B is disposed with different spaces provided with each other. That is, four each of the sixteen inner formation pieces  63 B are disposed by sandwiching a cross-like gap L 3 , and the four each of the inner formation pieces  63 B are disposed by sandwiching a cross-like gap L 4 . 
     Among the sixteen inner formation pieces  63 B, the side faces of the outermost twelve inner formation pieces  63 B oppose to the outer formation die  130 B formed in the female-type member  130  with the gap L 1  provided therebetween. 
     Note here that the gap space of the gap L 4  is designed to be a larger width gap space than the gap space of the gap L 3 , while the gap L 1  is set to be the gap space that is between the gap L 4  and the gap L 3 . Further, the material formation inner hole  52  is formed by the gap L 3 , the material formation inner hole  53  is formed by the gap L 4 , and the material formation outer hole  50  is formed by the gap L 1 . 
     Furthermore, those sixteen inner formation pieces  63 B are to be inserted into the outer formation die hole  130 B of the female-type member  130 . 
     Next, the hollow material  4  molded by the extrusion die  16  of the above-described seventh embodiment will be described by referring to  FIG. 24 . 
     The hollow material  4  is formed in a lattice-form sectional shape, and constituted with outer circumferential wall  4   a ,  4   a  in a square columnar sectional shape, cross-like first partition walls  4   b   1 ,  4   b   1  continuing from the outer circumferential walls  4   a ,  4   a , and cross-like second partition walls  4   b   2 ,  4   b   2  provided in the center part in the length direction of the first partition walls  4   b   1 ,  4   b   1 . 
     Further, the first partition walls  4   b   1 ,  4   b   1  are formed to be thicker compared to the second partition walls  4   b   2 ,  4   b   2 . 
     Furthermore, the part where the first partition walls  4   b   1  and  4   b   1  intersect with each other is the intersection X in the greatest thickness, and the part where the first partition wall  4   b   1  and the second partition wall  4   b   2  intersect with each other is the intersection X 2  that is in the second greatest thickness. Further, the part where the second partition walls  4   b   2  and  4   b   2  intersect with each other is the intersection X 3  that is in the smallest thickness. 
     As shown in  FIGS. 21 and 23 , the first small opening hole  68 B 1  corresponds to the thickest intersection X 1 , the second small opening hole  68 B 2  corresponds to the second thickest intersection X 2 , and the third small opening hole  68 B 3  corresponds to the thinnest intersection X 3 . 
     As a result, it becomes possible to form intersections of arbitrary thicknesses through changing the diameters of each of the small opening holes  68 B 1 ,  68 B 2 , and  68 B 3 . 
     With the extrusion die  16  of the seventh embodiment described above, it is also possible to acquire substantially the same actions as those of the extrusion die  15  of the sixth embodiment and substantially the same effects as those described in (1), (2), (6), (7), and (9). In addition, it is possible to achieve the following effect: 
     (10) The large opening hole  68 A to the small opening holes  68 B of the billet guide hole  68  are constituted with three kinds of the holes with different diameters, i.e., the first small opening hole  68 B 1 , the second small opening hole  68 B 2 , and the third small opening hole  68 B 3 . Each of those corresponds to the intersections of different thicknesses, i.e., the intersection X 1  where the first partition walls  4   b   1  and  4   b   1  intersect, the intersection X 2  where the first partition wall  4   b   1  and the second partition wall  4   b   2  intersect, and the third intersection X 3  where the second partition walls  4   b   2  and  4   b   2  intersect. Therefore, the hollow material  4  in a lattice-form sectional shape having the partition walls of different thicknesses can be molded easily. 
     Next, eighth to tenth embodiments of the extrusion die of the present invention will be described by referring to  FIGS. 25 and 26 . 
     In the first to seventh embodiments, the hollow materials  1  to  6  molded by each of the extrusion dice  10  to  16  have complicated sectional shapes with the intersections X formed by the partition walls  1   b  and the like. However, the hollow materials are not limited to such cases. With extrusion dice  17  to  19  of the eighth to tenth embodiments of the present invention, it is possible to mold hollow materials  7 ,  8 , and  9  having curve-shaped partition walls  7   b ,  8   b , and  9   b  inside thereof as shown in  FIGS. 25A, 25B  and  FIG. 26 , through changing the shapes of the inner formation pieces, respectively. 
     That is, as shown in  FIG. 25A , the hollow material  7  molded by the extrusion die  17  of the eighth embodiment is constituted with: outer circumferential walls  7   a ,  7   a  in a square sectional shape; and curve-shaped partition walls  7   b ,  7   b  provided inside those outer circumferential walls  7   a ,  7   a . Those partition walls  7   b ,  7   b  are formed to connect the center parts in the length direction of the orthogonal outer circumferential walls  7   a ,  7   a  with curve-shaped lines. 
     The partition walls  7   b  and  7   b  are constituted with a single inner formation piece  73 B 1  and two inner formation pieces  73 B 2  disposed by sandwiching the inner formation piece  73 B 1 . Those inner formation piece  73 B 1  and the inner formation pieces  73 B 2  are designed to be inserted into the outer formation die hole  30 B formed in the female-type member  30 . 
     Note here that the material formation outer hole  51  of the gap L 1  is formed between each of the inner formation piece  7381 , the inner formation piece  73 B 2 , and the outer formation die hole  30 B. Further, the thickness of the partition walls  7   b  and  7   b  is set to be the gap L 2 , and the material formation inner hole  51  is formed by the gap L 2 . 
     At the part where the curved sections of the partition walls  7   b  and  7   b  come closest to each other, the small openings  78 B and  78 B of the billet guide hole  78  are disposed opposing to each other. The large opening hole  28 A of the billet guide hole  78  is connected to those small openings  78 B and  78 B. 
     Note that the billet guide hole  78  is provided in the center part of the mandrel, not shown. Further, the entire structure of the extrusion die  17  is substantially the same as the entire structure of the extrusion die  10  and the like of the first embodiment. 
     With the structure described above, the billet is fed from the upstream side, a part thereof is introduced into the large opening hole  78 A of the billet guide  78 , and extruded out from the gap between the inner formation piece  73 B 1  and the inner formation pieces  73 B 2  via the small openings  78 B,  78 B. At that time, the billet introduced into the small openings  78 B,  78 B is extruded out from the material formation inner hole  51 . Therefore, the curved-shape partition walls  7   b  and  7   b  can be formed easily. 
     Next, the extrusion die  18  of the ninth embodiment will be described. 
     As shown in  FIG. 25B , the hollow material  8  molded by the extrusion die  18  is constituted with: outer circumferential walls  8   a ,  8   a  in a square sectional shape; and curve-shaped partition walls  8   b ,  8   b  provided inside those outer circumferential walls  8   a ,  8   a . Each of those partition walls  8   b  and  8   b  is formed in a curved shape projected toward the center part of the sectional shape of the hollow material  8  from the opposing outer circumferential walls  8   a  and  8   a.    
     The partition walls  8   b  and  8   b  are constituted with a single inner formation piece  83 B 1  and two inner formation pieces  83 B 2  disposed by sandwiching the inner formation piece  83 B 1 . Those inner formation piece  83 B 1  and the inner formation pieces  83 B 2  are designed to be inserted into the outer formation die hole  30 B formed in the female-type member  30 . 
     Note here that the material formation outer hole  50  of the gap L 1  is formed between each of the inner formation piece  83 B 1 , the inner formation piece  83 B 2 , and the outer formation die hole  30 B. Further, the thickness of the partition walls  8   b  and  8   b  is set to be the gap L 2 , and the material formation inner hole  51  is formed by the gap L 2 . 
     Further, at the part where the curved sections of the partition walls  8   b  and  8   b  come closest to each other, the small openings  88 B and  88 B of the billet guide hole  88  are disposed opposing to each other. The large opening hole  88 A of the billet guide hole  88  is connected to those small openings  88 B and  88 B. 
     Note that the billet guide hole  88  is provided in the center part of the mandrel, not shown. Further, the entire structure of the extrusion die  18  is substantially the same as the entire structure of the extrusion die  10  and the like of the first embodiment. 
     With the structure described above, the billet is fed from the upstream side, a part thereof is introduced into the large opening hole  88 A of the billet guide  88 , and extruded out from the gaps between the inner formation piece  83 B  1  and the inner formation pieces  83 B 2  via the small openings  88 B,  88 B. 
     At that time, the billet introduced into the small openings  88 B,  88 B is extruded out from the material formation inner hole  51 . Therefore, the curve-shaped partition walls  8   b  and  8   b  can be formed easily. 
     Next, the extrusion die  19  of the tenth embodiment will be described. 
     As shown in  FIG. 26 , the hollow material  9  molded by the extrusion die  19  is constituted with: outer circumferential walls  9   a ,  9   a  in a square sectional shape; and wave-shaped partition walls  9   b ,  9   b  provided inside those outer circumferential walls  9   a ,  9   a . Each of those partition walls  9   b  and  9   b  is formed in a wave-like shape connecting between the opposing outer circumferential walls  9   a  and  9   a.    
     The partition walls  9   b  and  9   b  are constituted with a single inner formation piece  93 B 1  and two inner formation pieces  93 B 2  disposed by sandwiching the inner formation piece  93 B 1 . Those inner formation piece  93 B 1  and the inner formation pieces  93 B 2  are designed to be inserted into the outer formation die hole  30 B formed in the female-type member  30 . 
     Note here that the material formation outer hole  50  of the gap L 1  is formed between each of the inner formation piece  93 B 1 , the inner formation piece  93 B 2 , and the outer formation die hole  30 B. Further, the thickness of the partition walls  9   b  and  9   b  is set to be the gap L 2 , and the material formation inner hole  51  is formed by the gap L 2 . 
     Further, at substantially the center parts in the respective length directions of the partition walls  9   b  and  9   b , the small openings  98 B and  98 B of the billet guide hole  98  are disposed opposing to each other. The large opening hole  98 A of the billet guide hole  98  is connected to those small openings  98 B and  98 B. 
     Note that the billet guide hole  98  is provided in the center part of the mandrel, not shown. Further, the entire structure of the extrusion die  19  is substantially the same as the entire structure of the extrusion die  10  and the like of the first embodiment. 
     With the structure described above, the billet is fed from the upstream side, a part thereof is introduced into the large opening hole  98 A of the billet guide  98 , and extruded out from the gaps between the inner formation piece  93 B  1  and the inner formation pieces  93 B 2  via the small openings  98 B,  98 B. 
     At that time, the billet introduced into the small openings  98 B,  98 B is extruded out from the material formation inner hole  51 . Therefore, the curve-shaped partition walls  9   b  and  9   b  can be formed easily. 
     While the present invention has been described above by referring to the embodiments, the present invention is not limited only to the embodiments. Various changes and modifications occurred to those skilled in the art can be applied to the structures and details of the present invention. Further, the present invention also includes mutual and proper combinations of a part of or a whole of each of the embodiments. 
     For example, in the fifth embodiment, the large opening hole  48 A of the billet guide hole  48  of the extrusion die  14  is formed substantially in a square shape on a plan shape and in a dented recessed shape toward the downstream side. However, as shown in  FIG. 27 , the large opening hole  78 A in the extrusion die  14 A of a modification example is formed as a tapered-shape hole that becomes narrower from the top face of the mandrel section  43  toward the bottom face of the large opening hole  78 A. 
     Further, the large opening holes  58 A and  68 A of the billet guide holes  58  and  68  of the sixth and seventh embodiments may also be formed as a tapered-shape hole that becomes narrower from the top face of the mandrel sections  53  and  63  toward the bottom face of the large opening holes  58 A and  68 A as in the above case. 
     Further, while the billet guide hole  28  is formed as a two-stage structure of the large opening hole  28 A and the small opening hole  28 B and the bottom face of the large opening hole  28 A is formed as a flat face in the first and third embodiments, the structure thereof is not limited to such case. The bottom face of the large opening hole  28 A may be formed as a bottom face constituted with an angular part with a sloping face of 45°, for example. With this, flow of the billet B can become still smoother. 
     Further, while each of the extrusion dice  10  to  13  of the first to fourth embodiments can mold the hollow material  1  having a sectional shape with a cross inside thereof and each of the extrusion dice  14  to  16  of the fifth to seventh embodiments can mold the hollow materials  2  to  4  having a sectional shape with a lattice form inside thereof, the structures are not limited only to such cases. For example, as shown in  FIG. 28A , it is possible to employ a structure capable of molding the hollow material  5  in which two intersections X are formed laterally with two vertically disposed outer circumferential walls  5   a ,  5   a  and a single laterally disposed partition walls  5   b ,  5   b  through changing the shapes of a plurality of inner formation pieces. 
     Further, as shown in  FIG. 28B , it is also possible to employ a structure capable of forming the hollow material  6  having an external shape in which partition walls  6   c ,  6   c  are provided in an X-letter form at the four corners of the external circumferential walls  6   a ,  6   a  formed in a square columnar shape through changing the shapes of a plurality of inner formation pieces. 
     INDUSTRIAL APPLICABILITY 
     The extrusion die of the present invention is utilized when molding hollow materials having partition walls inside thereof by using a high-strength alloy, particularly a high-strength aluminum alloy such as the so-called 7000 series. 
     REFERENCE NUMERALS 
     
         
         
           
               1  Hollow material having sectional shape with cross inside (hollow material formed by first to fourth embodiment) 
               2  Hollow material having sectional shape with lattice inside (hollow material formed by fifth embodiment) 
               3  Hollow material having sectional shape with lattice inside (hollow material formed by sixth embodiment) 
               4  Hollow material having sectional shape with lattice inside (hollow material formed by seventh embodiment) 
               10  Extrusion die for forming hollow material (first embodiment) 
               11  Extrusion die for forming hollow material (second embodiment) 
               12  Extrusion die for forming hollow material (third embodiment) 
               13  Extrusion die for forming hollow material (fourth embodiment) 
               14  Extrusion die for forming hollow material (fifth embodiment) 
               15  Extrusion die for forming hollow material (sixth embodiment) 
               16  Extrusion die for forming hollow material (seventh embodiment) 
               20  Male-type member 
               22  Spider 
               23  Mandrel section 
               23 B Inner formation piece 
               24  Bridge section 
               24   a  to  24   d  First to fourth bridges 
               24 A Bridge tip outer circumferential face 
               25  Holder section 
               26  Bridge holding section 
               26 B Bridge receiving face as bridge abutting/engaging face 
               30  Female-type member 
               30 B Outer formation die hole 
               50  Material formation outer hole 
               51  Material formation inner hole 
             BH Billet insertion hole (billet merging space) 
             BH 1  Billet insertion hole (billet merging space) 
             S Billet introducing part 
             M Bridge section press-fit structure 
             N Bridge section shrink-fit structure