Patent Publication Number: US-2018029111-A1

Title: Metal molded body manufacturing apparatus by electromagnetic stirring

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an apparatus for manufacturing a metal molded body by stirring molten metal in a die. 
     Description of Related Art 
     Conventionally, as one of technologies for molding aluminum alloy or the like, a die casting method for pressurizing and injecting molten metal into a die to obtain a molded article having a predetermined shape is used, and in a case where molten metal is used, problems, such as a short life of the die, insufficient quality of a product due to generation of shrinkage, mold cavity, or the like, are pointed out. 
     Additionally, conventionally, as a method for molding a metal molded body while stirring molten metal in a die, and a method for further machining an obtained metal molded body by a press machine, a rheocasting process is known. In this case, the die is generally provided with a draft angle, and therefore work for extracting a manufactured metal molded body to transpose the metal molded to a molding die such as a press machine in a next process is easy. Of course, the metal molded body is sometimes used as a product without change. 
     Furthermore, an aluminum structure used in a vehicle body or the like is often thin and long, and is bent. In a case where such a thin and long structure is molded with aluminum, the shape of a stirring die is made to be thin and long in order to approximate the shape of a metal molded body before press molding into the structure. 
     JP 5352236 B1, JP 2009-74103 A, and JP 2007-144501 A each describe an apparatus for performing electromagnetic stirring for molten metal. An inner peripheral surface of a melting furnace (pot) of the apparatus is formed in a cylindrical shape, and molten metal is stirred along the circular inner peripheral surface. 
     In addition, an apparatus described in JP 2006-289448 A is used for a continuous casting facility, and linear type electromagnetic stirring apparatuses are provided on an outside so as to face both linear side walls. 
     Additionally, JP 2006-289448 A and JP 2006-289476 A each disclose a structure of a linear type electromagnetic stirring apparatus. 
     Furthermore, for example, relation between a gap and electromagnetic force is considered in “Electromagnetic Coil Designed by Magneto-Hydro-Dynamic Simulation, shinnittetsu giho vol. 379”, and “Development of a Simulation Model for Electromagnetic Stirring in Melting Furnace, Furukawa-sky Review No. 3, 2007”. 
     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     Molten metal which is put into a die generally starts cooling from a moment of this putting. Therefore, time for stirring until the molten metal is solidified is about 10 seconds at most. That is, it is important to how efficient the molten metal is stirred for a short time until the molten metal is solidified. 
     Herein, the above die is provided with the draft angle for extracting the manufactured metal molded body. 
     However, thrust of electromagnetic stirring which acts on the molten metal is significantly changed at a position in the depth direction in the die, depending on this draft angle. For example, as illustrated in  FIG. 1B , when an effective gap G 2  at a lower end of the die is twice an effective gap G 1  at an upper end of the die, magnetic flux density is about 30%, there is a possibility that the thrust falls to ten percent (square of 0.3) (refer to “Electromagnetic Coil Designed by Magneto-Hydro-Dynamic Simulation, shinnittetsu giho vol. 379”, and “Development of a Simulation Model for Electromagnetic Stirring in Melting Furnace, Furukawa-sky Review No. 3, 2007”). Therefore, a large difference in thrust is generated between the upper end and the lower end of the die, and efficiency of stirring becomes low in the vicinity of the lower end. 
     The difference in thrust cannot be ignored, when the stirring time of about 10 seconds 10 is considered. In addition to the above, particularly, in a case of the metal molded body of such a thin and long, and bent structure as to be used for a vehicle body or the like, the shape of the die is shapeless or is not a usual shape, and has a branch section or the like in the middle. The shape of such a die is one of causes of hindering a flow of stirring, and cannot be ignored in view of efficiency of stirring, along with the gap of the draft angle as described above, when all the stirring processes for about 10 seconds are considered, 
     Therefore, an object of the present invention is to provide a metal molded body manufacturing apparatus capable of efficiently stirring molten metal. 
     Means to Solve the Problem 
     (1) A metal molded body manufacturing apparatus of the present invention is a metal molded body manufacturing apparatus for electromagnetically stirring molten metal, and molding a metal molded body, the metal molded body manufacturing apparatus including: a die including a side wall having an inclined inner surface; and a moving magnetic field generation section that is disposed along an outer periphery of the die, and stirs the molten metal in the die, wherein the moving magnetic field generation section includes a magnetic body, and a coil wound around the magnetic body as a center, and an end surface of the magnetic body is disposed such that a gap between the end surface of the magnetic body and the inner surface of the side wall becomes uniform. 
     Herein, “the gap between the end surface of the magnetic body and the side wall inner surface is uniform” means that the end surface of the magnetic body of the moving magnetic field generation section is parallel to the inner surface of the side wall, and is a concept including a case where the end surface of the magnetic body is not parallel to an outer face. 
     Furthermore, the metal molded body is a concept including metal that is obtained by cooling molten metal to be brought into a solid-liquid coexistent state, includes metal that is further cooled to be solidified, including metal that is obtained by further heating solid metal to be brought into a solid-liquid coexistent state, and including a primary molded body that is machined in a next process to become a product, and a final molded body that becomes a product without change. 
     (2) In such a metal molded body manufacturing apparatus, an inclination of the side wall inner surface preferably serves as a draft angle for extracting the metal molded body. 
     (3) The draft angle is preferably 1° to 9°. 
     (4) The side wall preferably has a pair of facing walls that face each other, the moving magnetic field generation section is preferably provided along each of the facing walls, and the moving magnetic field generation sections preferably generate respective moving magnetic fields in reverse directions from each other so as to generate a vortex in the molten metal. 
     (5) The metal molded body manufacturing apparatus preferably further includes: a base; an inclined plate to which the moving magnetic field generation section is fixed; and an inclination adjusting mechanism that adjusts an inclination of the inclined plate to the base so as to uniform the gap between the end surface of the magnetic body and the side wall inner surface, the inclination adjusting mechanism being disposed between the inclined plate and the base. 
     (6) The end surface of the magnetic body of the moving magnetic field generation section preferably extends beyond an upper surface of the molten metal put into the die from below a bottom of the die. 
     (7) The die preferably has a curved shape in plan view, an additional magnetic body member is preferably provided in the end surface of the magnetic body of the moving magnetic field generation section, and an end surface of the additional magnetic body member is preferably a curved surface along the curved inner surface of the die. 
     Effect of the Invention 
     (1) In the metal molded body manufacturing apparatus of the present invention, the end surface of the moving magnetic field generation section is disposed in parallel to the inclined inner wall surface of the die, and the gap (distance) between this end surface and the inner surface of the die is made the same, and therefore a difference of thrust in the vertical direction of the wall surface is unlikely to be generated. 
     Therefore, while the molten metal is stirred by a die having a shape close to a final product, residual gas is discharged, and growth of a crystal is prevented, the molten metal can be isothermally cooled as a whole to be solidified. 
     (2) In such a manufacturing apparatus for the metal molded body, in a case where the inclination of the side wall serves as the draft angle for extracting the metal molded body, the metal molded body can be easily extracted, which is effective. 
     Additionally, the draft angle is provided, and therefore the penetration depth of the moving magnetic field approaches the center of the die toward a lower surface of the die. Therefore, a strong vortex generates closer to the center toward the lower surface of the die, and a downward flow which draws the molten metal from the upper surface generates. Dislocation (convection) of an upper layer and a lower layer of the molten metal proceeds by this vortex to facilitate discharge of residual gas. 
     (3) In a case where the draft angle is 1° to 9°, such an angle is easy to use as the draft angle. In addition, the convection can be generated. 
     (4) In a case where the side wall has a pair of the facing walls that face each other, the moving magnetic field generation section is provided along each of the facing walls, and these moving magnetic field generation sections generate respective moving magnetic fields in reverse directions from each other so as to generate a vortex in the molten metal, it is possible to enhance efficiency of stirring even in an oblong die. 
     Additionally, growth of a metal crystal is hindered by the vortex, resulting in a dense structure. 
     (5) In a case where the metal molded body manufacturing apparatus further includes a base, an inclined plate to which the base and the moving magnetic field generation section are fixed and the inclination adjusting mechanism that adjusts inclination of the inclined plate to the base such that the end surface of the magnetic body are parallel to the side wall, the inclination adjusting mechanism being disposed between the inclined plate and the base, the inclination of the moving magnetic field generation section can be adjusted to the draft angle when the draft angle of the die is changed. 
     (6) In a case where the end surface of the magnetic body of the moving magnetic field generation section extends beyond the upper surface of the molten metal put into the die from below the bottom of the die, the whole of the molten metal can be reliably moved, and therefore stirring efficiency is high. 
     (7) In a case where the die has the curved shape in plan view, the additional magnetic body member is provided in the end surface of the magnetic body of the moving magnetic field generation section, and the end surface of the additional magnetic body member is the curved surface along the curved inner surface of the die, it is possible to form a smooth flow along the curved surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a sectional view illustrating a metal molded body manufacturing apparatus according to an embodiment of the present invention, and  FIG. 1B  is a sectional view illustrating an example for comparison with the apparatus of  FIG. 1A . 
         FIG. 2A  is a plan view illustrating of a die according to the embodiment, and  FIG. 2B  is a sectional view taken along a A-A of the die of  FIG. 2A . 
         FIG. 3  is a schematic plan view illustrating moving magnetic field generation sections and the die. 
         FIG. 4A  and  FIG. 4B  each are a schematic view illustrating a state where a coil is wound around a core back. 
         FIG. 5A  and  FIG. 5B  each are a schematic view illustrating a state where the coil is wound between slots. 
         FIG. 6  is a side view illustrating a metal molded body manufacturing apparatus according to another embodiment. 
         FIG. 7A  is a front view illustrating a moving magnetic field generation section according the other embodiment and  FIG. 7B  is a bottom view of the moving magnetic field generation section of  FIG. 7A . 
         FIG. 8  is a plan view illustrating a manufacturing apparatus according to yet another embodiment. 
         FIG. 9A  is a plan view illustrating a state where an additional magnetic body is detached from the moving magnetic field generation section of  FIG. 8 , and  FIG. 9B  is a plan view of an additional magnetic body member of  FIG. 8 , and  FIG. 9C  is a plan view illustrating an additional magnetic body member according to another embodiment. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     First Embodiment 
     1. Summary 
     A manufacturing apparatus for a metal molded body of the present invention (hereinafter simply referred to as a manufacturing apparatus) will be described with reference to  FIG. 1 . A manufacturing apparatus  1  illustrated in  FIG. 1  is an apparatus that stirs molten metal, which is put into a die  2 , by moving magnetic field generation sections  3 , and molds a metal molded body in the die. 
     The metal molded body manufactured by this manufacturing apparatus  1  is subjected to machining for making a press-formed or press-molded article by, for example, a press apparatus (not illustrated) in a next process, or is used as a final molded body without change. 
     2. Manufacturing Apparatus  1   
     More specifically, the manufacturing apparatus  1  includes the die  2  having a side wall  5  having an inclined inner surface, and a moving magnetic field generation section  3  that stirs the molten metal in the die  2 . An end surface  3   a  of a magnetic body of the moving magnetic field generation section  3  is disposed such that a gap G (refer to  FIG. 1A ) with an inner surface  5   c  of the side wall (hereinafter referred to as an inner wall surface) becomes uniform. The end surface  3   a  and the inner wall surface  5   c  are parallel or substantially parallel to each other. 
     (As to Die  2 ) 
     The die  2  is preferably formed of, for example, a material having low magnetic permeability and a higher melting point than that of the molten metal which is metal put into the die. In this embodiment, stainless steel is used. 
     The die  2  (refer to  FIG. 2B ) has a bottom  4 , and a side wall  5  extending upward from a peripheral edge of the bottom  4 . The side wall  5  inclines outward toward an opening side so as to extract the metal molded body. The thickness of the side wall  5  is generally uniform, but may not be uniform. In a case where the thickness is not uniform, at least the inner surface only needs to incline outward toward the opening side. The angle of the inclination only needs to be a draft angle for extracting the metal molded body to an axis of the die  2  (an axis perpendicular to a surface of the bottom, or a vertical axis). For example, the draft angle is 1° to 9°, and preferably 5° to 7°. 
     (Shape of Die) 
     As illustrated in  FIG. 2A , in this embodiment, the die  2  has, for example, a long oval shape or an athletics track shape in plan view. That is, the side wall  5  of the die  2  includes a pair of facing walls  5   a ,  5   a  facing each other. Respective both ends of the straight facing walls  5   a ,  5   a  are connected to the semicircular sections  5   b ,  5   b , so that the above long oval shape or athletics track shape is formed. 
       FIG. 2B  illustrates a sectional view of  FIG. 2A . The side wall  5  inclines so as to expand toward the opening side. In this embodiment, the bottom  4  is flattened. 
     (Molten Metal) 
     The molten metal which is put into the die  2  is metal such as aluminum alloy that is melted. 
     (Moving Magnetic Field Generation Section  3 ) 
     Returning to  FIG. 1A , the moving magnetic field generation sections  3  each include, for example, a magnetic body  6  (hereinafter referred to as a core), and coils  7  wound around the core  6  as a center. As illustrated in  FIG. 1A , the moving magnetic field generation sections  3 ,  3  (refer to two-dot chain lines of  FIG. 2A ) are disposed such that the end surfaces of the cores  6  are parallel to the inner surfaces of the facing walls  5   a ,  5   a  of the side wall and there is clearances between the end surfaces of the core  6  and the outer surfaces of the side wall. 
     The end surfaces of the cores  6  of the moving magnetic field generation sections  3  each extend from a position below the bottom  4  of the die  2  to a position above an upper surface of the molten metal put into the die. Therefore, it is possible to reliably move the whole of the molten metal. 
     (Magnetic Bodies  6  and Coils  7 ) 
     Returning to  FIG. 1A , the magnetic bodies  6  and the coils  7  are conventionally known ones. For example, each magnetic body  6  employs, for example, a silicon steel sheet, and is formed by stacking thin sheets because of reduction in influence of an eddy current or the like. 
     In each magnetic body  6 , a plurality of slots  8  are formed on the side wall side of the die  2  at equal intervals (refer to  FIG. 4A ). Each slot  8  has a coil  7  wound therearound. 
     (Principle and the Like) 
       FIG. 3  schematically illustrates the manufacturing apparatus  1 . In the figure, the three moving magnetic field generation sections  3  are lined to be connected on one side of the die  2 . A power source is a three-phase alternating current (AC). Three-phase ACs U, W, V are sequentially applied to coils of terminal symbols R, S, T. Then, a magnetic field (direction of an arrow H) that moves in parallel to the facing wall  5   a  of the side wall  5  is generated in the die  2 . Specifically, a +U-phase current, a −W-phase current, a +V-phase current, a −U-phase current, a +W-phase current, and a −V-phase current are applied to the respective coils  7  illustrated in  FIG. 3  in this order from the left of the figure. In these three-phase ACs, phases of the +U-phase, the +V-phase, and the +W-phase are AC currents shifted by 120° in order, and currents of the −V-phase, the −W-phase, and the −U-phase are opposite directions. As to the current flow direction, it is defined that the flow direction from the front to the back of the paper sheet is positive. When a current flows in the positive direction, a clockwise magnetic flux with the coil as the center generates, and when a current flows in the reverse direction, a counterclockwise magnetic flux generates. The magnitude of the magnetic flux density increases as a current value of the coil increases. 
     Accordingly, when a current is applied to the coil, a combined magnetic flux density distribution moves from the left to the right (see an arrow φ) with lapse of time. That is, a moving magnetic field that moves from the left to the right along the longitudinal direction of the core  6  is formed. Consequently, an induced current is generated in the molten metal, and an electromagnetic force (Lorentz force) is generated in the molten metal. Then, driving force (refer to arrows H 1 , H 2 , H 3 ) that provides flows to follow the motion of the moving magnetic field is applied in the molten metal. 
     Symbol O denotes a neutral point. 
       FIG. 4A  is a plan view of the moving magnetic field generation section  3  of  FIG. 1A . A winding method illustrated in the figure is a core back winding method. The cores  6  of the moving magnetic field generation sections  3  each are formed in a comb-tooth shape, and the coils  7  are wound between the teeth of each core (the above slots  8 ). 
     The slots are filled with the coils (coil conductors)  7 . Each core  6  is composed of a rectangular column shaped yoke (core back)  6   a , and teeth  9  each having an end fixed to dovetail-shaped grooves  6   b  formed in the yoke. The number of the teeth  9  is seven, the number of the coils  7  to be wound around the yoke between the teeth is six, and all the coils are wound in the same direction. Therefore, in a case where a current enters from an initially wound coil, and a case where a current enters from a finally wound coil, the polarities are reverse. Each slit  8   a  that is opened toward the die  2  is formed between the adjacent teeth  9 ,  9 . The yoke  6   a  and the teeth  9  are each formed of a core formed by superimposing a large number of thin sheets of silicon steel sheets, and these may be integrated. 
     In the moving magnetic field generation section  3 , as a minimum unit for smoothly generating a moving magnetic field by a three-phase power supply, the six slots  8  are used as the minimum unit. In a case where the further reduced number of the slots is used as the minimum unit, balance is kept by mutual magnetic coupling. 
       FIG. 5A  illustrates a moving magnetic field generation section  3  according to another embodiment. A winding method illustrated in the figure is an inter-slot winding method. A U-phase coil passes through a leftmost slot and a fourth slot so that the coil is wound around three teeth between the leftmost slot and the fourth slot. Similarly, a V-phase coil passes through a second slot and a fifth slot so that the coil is wound around three teeth between the second slot and the fifth slot. A W-phase coil passes through a third slot and a sixth slot so that the coil is wound around three teeth between the third slot and the sixth slot. These three kinds of coils are overlapped to intersect with each other on the lateral side of the core  6 , as illustrated in  FIG. 5B . 
     In this method, the coils  7  do not wound around a core back  6   a , and therefore the core back  6   a  can be used to fix the moving magnetic field generation section  3 . 
     Conclusion 
     Returning to  FIG. 1A , the moving magnetic field generation sections  3  are disposed such that the end surfaces of the cores  6  are parallel to the inner surface of the side walls inclining at draft angle. Therefore, a difference in thrust in the vertical direction of the side walls is unlikely to be generated. That is, the whole of a material along the wall surfaces flows, and therefore stirring efficiency is high, growth of crystals is prevented. As illustrated in  FIG. 2A , a pair of the moving magnetic field generation sections  3 ,  3  generate moving magnetic fields in the reverse directions from each other so as to generate a vortex in the molten metal, and therefore stirring efficiency is high. 
     Second Embodiment 
     3. Another Embodiment 
     Now,  FIG. 6  illustrates a manufacturing apparatus according to another embodiment. The manufacturing apparatus  10  includes a base  11  installed on a floor surface of a building such as a factory, and inclined plates  12 , which are provided in the base  11 , and to which the moving magnetic field generation sections  3  are fixed. The two inclined plates are used as one set, and are provided with the respective moving magnetic field generation sections  3 . These inclined plates  12 ,  12  are disposed so as to face each other. In the base  11 , inclination adjusting mechanisms  13  that adjust the inclination of the inclined plates  12  are provided. 
     (Base  11 , Inclined Plates  12 , Inclination Adjusting Mechanisms  13 ) 
     The base  11  includes a base plate  11   a  installed on the floor surface, a placing stand  11   b  that is erected from the vicinity of a center of the base plate  11   a , and allows a die  2  to be placed on an upper surface thereof, and a pair of supporting sections  11   c ,  11   c  erected from the base with the placing stand  11   b  therebetween. 
     The inclined plates  12  each are a flat plate shape, and formed of metal such as stainless steel. Each inclined plate is rockingly pivoted to the vicinity of an upper end of the supporting section  11   c . A shaft of the rocking is a shaft  12   a  parallel to a longitudinal axis of the die  2 . 
     The inclination adjusting mechanisms  13  each include a screw mechanism  13   a . The screw mechanism  13   a  is disposed below the shaft  12   a . In the screw mechanism  13   a , a male screw member is screwed in the female screw member provided in the supporting section  11   c  toward the inclined plate  12 , so that a lower part of the inclined plate  12  is pressed, and the inclined plate  12  is rotated about the shaft  12   a  as a rotation center. 
     In each inclination adjusting mechanism  13 , the moving magnetic field generation section  3  can be disposed such that an end surface of the core  6  is parallel to an inner surface  5   c  of the side wall by changing the inclination of the inclined plate  12 . Therefore, a difference in thrust is unlikely to be generated in molten metal in the vertical direction of the side wall. The whole of the material along the wall surfaces flows, and therefore growth of metal crystals is prevented, and minute metal molded body is formed. 
     In the manufacturing apparatus  10 , moving mechanisms  14  that allow the supporting sections  11   c  to be freely movable to the bases plate  11   a  may be provided. As the moving mechanism  14  (two-dot chain lines), conventionally known mechanisms such as slide mechanisms can be used. By use of the moving mechanisms  14 , the inclined plates  12  are brought closer to or away from the side wall  5 , that is, are moved with parallel translation to adjust the gaps. 
     As the manufacturing apparatus  10 , both the moving mechanisms  14  and the inclination adjusting mechanisms  13  are preferably provided. 
     (Transposing Mechanism of Laminar Flow) 
     For example, as illustrated in  FIG. 7A  and  FIG. 7B , a moving magnetic field generation section  15  that generates a inclined vertical flow P can be used. In this case, the inclined vertical flow is generated in molten metal, and molten metal in upper and lower layers in a die is transposed. Consequently, residual gas venting is facilitated, more efficient stirring is attained, and therefore it is possible to make crystal grains fine. 
     More specifically, in the moving magnetic field generation section  15 , slits  8   b  obtained by inclining (skewing) two central slits in the advancing direction from a bottom to a top are formed. Consequently, coil end portions are the same appearance as a general coil end, and therefore there is no waste in assembling appearance. 
     As the shape of the bottom of the die  2 , a hemispherical bottom for smoothly reversing a downward flow of axially moving molten metal  1  to an upward flow is preferably provided (not illustrated). Reference numeral  8   c  in  FIG. 7B  denotes a wedge or plate for holding the coil  7  in the slot  8 . 
     Third Embodiment 
       FIG. 8  illustrates a manufacturing apparatus according to another embodiment. This embodiment includes a large number of parts which are the same as the parts described in the above embodiments, and therefore the same parts are denoted by the same reference numerals, and description thereof will be omitted. 
     A manufacturing apparatus  16  illustrated in  FIG. 8  includes a die  17  having an inverted T-shape in plan view, and a plurality of pairs of the moving magnetic field generation sections  3  disposed along a wall surface of the die so as to sandwich the die  17 . 
     Near a root of the T shape of the die  17 , curved side walls  17   a  having curved inner surfaces are provided. In this embodiment, moving magnetic field generation sections  18  having curved surfaces parallel to the curved inner surfaces are provided so as to face the curved inner surfaces of the curved side walls  17   a  in order to maintain uniform gaps. Additional magnetic body members  19  are provided on end surfaces of cores  7  of the moving magnetic field generation sections  18 . End surfaces  19   a  of the additional magnetic body members are curved surfaces along the curved side walls  17   a  of the die  17 . The additional magnetic body members  19  are mounted on teeth  9  of cores  6  that generate magnetic fields, are designed so as to be parallel to the inner surfaces of the curved side walls  17   a  by magnetically extending the teeth  9 . 
     A moving magnetic generation section of this embodiment includes twelve moving magnetic field generation sections  3  provided on liner parts, and the two moving magnetic field generation sections  18  provided at corners. Thus, the moving magnetic field generation section is divided, and two kinds of the moving magnetic field generation sections are further combined and disposed, so that it is possible to correspond to various shaped of dies. Consequently, it is possible to uniformly stir molten metal in various shaped dies (refer to arrows in the figure). 
     Now, details of the curved moving magnetic field generation sections  18  will be described with reference to  FIG. 9A  and  FIG. 9B .  FIG. 9A  illustrates a state where an additional magnetic body member  19  is detached from the moving magnetic field generation section  18  of  FIG. 8 . Insertion ports  18   a  are formed in distal end surfaces (surfaces facing the side wall of the die) of the teeth  9  of the moving magnetic field generation section in  FIG. 9A . On the other hand, as illustrated in  FIG. 9B , insertion sections  19   b  that slide to be inserted into the insertion ports  18   a  are formed in proximal end surfaces of the additional magnetic body member  19 . The additional magnetic body member  19  is freely engaged and disengaged by sliding. Therefore, an additional magnetic body member in accordance with a curved shape of a die only needs to be prepared with respect to the single moving magnetic field generation sections  18  as a base, and therefore this configuration is economical. 
       FIG. 9C  illustrates an additional magnetic body member  20  having a recessed curved surface. 
     The additional magnetic body member may be curved in the height direction (not illustrated). 
     4. Modification 
     As a material of the die  2 , metal having heat resistance withstanding of the high temperature of molten metal, and having magnetic permeability (for example, stainless steel), ceramics, and the like can be used. 
     The die may have a shape such as an arc shape or a forked shape in plan view. 
     The upper part of the die  2  is opened in this embodiment, but may be closed by an openable lid. In this case, the lid is opened, and molten metal is put into the die or extracted a metal molded body.