Process and apparatus for molding article

Abstract

 A process for molding an article made of an inductive material is provided, wherein the inductive material is molded in a mold by means of high frequency induction heating caused by a high frequency magnetic field which is established by a high frequency induction coil surrounding the outer periphery of the mold. The process comprises the steps of: (a) supplying the inductive material continuously or intermittently from the top of the mold; (b) subjecting the inductive material in the mold to the high frequency induction heating to melt the same thereby to form a molten zone; and (c) moving the mold downwards relative to the high frequency magnetic field; the steps (a), (b) and (c) being done simultaneously to move the molten zone upwards from the bottom to the top of the mold, whereby the inductive material is once melted and then solidified to form a molded article. The inductive material may be an inductive metal or a composite matrial composed of an inductive metal and a ceramic material. The condition of the material in the mold may be monitored by a sensor and the lowering speed of the mold or the electric power for energizing the high frequency induction coil is controlled in response to the signal from the sensor. The mold may be vibrated to improve the production efficiency and the property of the molded article. Also provided is an apparatus for accomplishing the process described above.

Description

BACKGROUND OF THE INVENTION:

Field of the Invention;

[0001] The present invention relates to a directional solidification process and an apparatus used therefor. In detail, it relates to a process and apparatus for molding an article made of an inductive material, in which a high frequency induction heating is utilized for zone-melting and subsequent solidification of the used inductive material.

Prior Art;

[0002] Known processes for molding metal articles include casting, forging, machining and powder metallurgical processes. The casting processes have been used most commonly in view of the cost performance. However, conventional casting processes have disadvantages in that gases contained in the cast material or emitting during the processing steps are left in the cast product to form blow holes or pin holes resulting in internal defects in the cast product or to deteriorate the smoothness of the surface of the cast product. As a result, the yield rate of the process is reduced. In addition, due to the change in cooling condition after the molten mass of the cast material is filled in the mold, the structure in the solidified cast product becomes uneven. In the conventional casting process, it is difficult to control the crystalline structure of the cast product and thus it is almost impossible to form an oriented structure of solidified cast metal, a single crystal structure or a microcrystalline structure.

[0003] The cost for preparing a molded article by the forging or machining process is relatively high since an article of complicated shape requires multiple skillful operations. The powder metallugical process has a disadvantage in that an article prepared therethrough apts to become porous to have a lower density.

[0004] On the other hand, in order to improve a certain property of the cast product, it has been proposed to use a composite cast material which is composed of a metal and a ceramic material. It has been known in the art to sinter such a composite material to form a sintered article. In the known sintering process, a metal powder and a ceramic material powder are mixed to form a mixture which is consolidated under pressure and then sintered. However, this known process has disadvantages in that the finished artilce becomes porous to have a lower density and that a large size sintered article or a sintered article having a complicated shape cannot be prepared by this known process, in addition to the lower production efficiency.

OBJECTS AND SUMMARY OF THE INVENTION:

[0005] Accordingly, an object of this invention is to provide a molding process in which degassing is effected to a satisfactory extent to uniformalize the structure of the finished product.

[0006] Another object of this invention is to provide a molding process by which the crystalline structure of the cast article can be controlled as desired to prepare, for example, a cast article having a unidirectional or oriented crystalline structure.

[0007] A further object of this invention is to provide a molding process for preparing a large size article having a presize dimensions at low cost.

[0008] A further object of this invention is to provide a molding process for preparing a large size article or an article having a complicated shape while using a composite material composed of a metal and a ceramic material at a high production efficiency.

[0009] A still furthr object of this invention is to provide a molding process in which contamination with impurities from the surface of the mold is prevented to improve the purity of the molded article.

[0010] Yet a further object of this invention is to provide an apparatus used for accomplishing such a molding process.

[0011] The present invention provides a process for molding an article made of an inductive metal material, or a composite material composed of an inductive metal material and a ceramic material, in a mold by means of high frequency induction heating caused by a high frequency magnetic field which is established by a high frequency induction coil surrounding the outer periphery of said mold, comprising the steps of:

(a) supplying said metal or composite material continuously or intermittently from the top of said mold;

(b) subjecting said metal or composite material in said mold to said high frequency induction heating to melt the same thereby to form a molten zone; and

(c) moving said mold downwards relative to said high frequency magnetic field;
   said steps (a), (b) and (c) being done simultaneously to move said molten zone upwards from the bottom to the top of said mold, whereby said metal or composite material is once melted and then solidified to form a molded article.

[0012] The present invention also provides an apparatus for molding an article in a mold by means of high frequency induction heating caused by high frequency magnetic field, comprising:

(a) a support for holding said mold in said high frequency magnetic field;

(b) a high frequency induction heating coil surrounding the outer periphery of said mold, said coil establishing said high frequency magnetic field;

(c) a feeder for continuously or intermittently supplying an inductive material into said mold; and

(d) a driver for moving said support together with said mold downwards reletive to said high frequency induction coil along the vertical plane;
   whereby said inductive material is once melted to form a molten zone and then solidified from the bottom to the top of said mold to form said molded article.

[0013] The mold may be made of a non-inductive material, such as a ceramic material. Alternatively, the mold may be made of a metallic material, wherein the metallic material is partitioned into plural segments extending along the magnetic field and these plural segments are joined together while respective segments being separated through insulating layers. Although it is desirous that these segments are electrically connected through a wire, these segments may be electrically connected with each other through the inductive material supplied in the mold.

[0014] In a preferred embodiment, the state of the supplied material subjected to high frequency heating is sensed by a sensor, for example, for sensing the temperature of the material, and the moving speed of the mold or the current supplied to energize the high frequency induction coil is controlled. In a further preferred embodiment, the mold may be made of a transparent quartz glass so that the state of the supplied material can be visually monitored to analyze melting or solidification state of the material, or the temperature of the material in the mold is detected by an external infrared thermometer. Cooling means may be provided below the high frequency induction coil so that the molten mass of the supplied material is cooled rapidly to be solidified.

[0015] It is desirable, depending on the properties of the used cast material, that the process is performed in an oxidating, inert or reducing atmosphere or in vacuum or under a reduced pressure. It is also possible to subject the cast material to hot isostatic pressing during it is processed through the process of the invention. The mold may be prepared through the powder molding process. A mold prepared through the powder molding process is particularly preferred when the mold is collapsed after the completion of molding or the mold is used as a core mold.

[0016] The mold may be vibrated by a vibrator during the steps of melting and solidifying the material. The vibrator may be directly connected to the mold, or a holder for holding the mold is provided and connected to the vibrator through a connecting rod or other proper means. The frequency of vibration for vibrating the mold ranges from several tens to several hundreds hertzes. Ultra-sonic wave of 20 KHz or more may be utilized for this purpose.

[0017] According to the invention, the material supplied into the mold is melted at the lower portion in the mold by high frequency indcution heating to form a molten zone (molten pool) from which gases escape readily to upper void cavity in the mold to effect satisfactory degassing. Additional material is supplied onto the molten pool while the mold is moving downwards relative to the high induction magnetic field. Thus, the material supplemented through the material supplying port is melted by high induction heating to be intimately dispersed in the existing molten pool without forming pore or like void portions. This leads to an advantageous effect that pores, chinks or other void portions are not left in the solidified product, although the material is supplied in the form of powders or granules. The portion moving below the high induction magnetic field is solidified to form the portion of a molded article having a structure oriented unidirectionally. The material is continuously or intermittently supplemented from the port of the material feeder and then melted by high induction heating to form the molten pool above the solidified portion of the molded article. The crystalline structure of the molded article can be controlled by controlling the relative moving speed of the high frequency magnetic field or by controlling the cooling rate.

[0018] Eddy currents are induced in the molten portion (molten bath or molten pool of melted material) by the AD magnetic field, so that electromagnetic force towards the center is caused to spontaneously agitate the molten mass. Concurrently, the molten mass is subjected to floating force and the force for separating the molten mass from the inner periphery of the mold is applied to the portion of the molten mass vicinal to the inner periphery of the mold. As a result, the contacting area between the molten mass and the inner periphery of the mold is decreased to prevent contamination of impurities from the mold. Particularly when the mold is forcibly cooled by means of a cooling medium, the portion of the molten mass contacting with or vicinal to the inner periphery of the mold is rapidly cooled to prevent contamination with impurities to a further reduced extent.

[0019] When a metal mold is used, it it preferred that the mold comprises plural metal segments extending along the magentic field and these segments are joined together while respective segments being separated by interposed insulating layers. With such construction, eddy currents induced in the mold wall are reduced to improve the efficiency for heating the material in the mold. These segments are electrically connected to maintain the electric potentials of these segment at a constant level to eliminate the occurence of arc disharge which might cause breakdown of the insulating layers to lead contamination of impurities generated from the breakdown of the insulating material.

[0020] When a composite material composed of a mixture of a metal and a ceramic material is used as the material to be molded, a portion or zone of a mixture is heated by high frequency induction heating so that metal particles contained in this zone are melted and ceramic material particles are dispersed in the molten mass of the thus melted metal. By preparing a uniform mixture of metal particles with ceramic material particles and supplying the uniform mixture into the mold cavity continuously or intermittently, the ceramic material particles can be evenly dispersed in the molten zone which is sufficiently thin or narrow. The ceramic material particles can be homogenously dispersed in the cast article.

[0021] When the mold is vibrated, the agitating action by the electromagnetic force is further promoted so that th molten mass is flown towards the direction for moving remoter from the inner periphery of the mold to reduce the contact area between the molten mass and the inner periphery of the mold to suppress the reaction (mold reaction) between the molten mass and the material forming the mold. Thus, contamination of impurities from the inner periphery of the mold is suppressed to improve the quality of the molded article and to improve the casting skin or surface condition of the molded article.

DESCRIPTION OF THE DRAWINGS:

[0022] The above and other objects and advantages of the invention will become apparent from the following detailed description of presently preferred embodiments of the invention with reference to the appended drawings in which:

Fig. 1 is a sectional view showing an embodiment of the invention;

Fig. 2 is a cross sectional view showing the embodiment of Fig. 1 while the mold being moved downwards relative to the high frequency induction coil;

Fig. 3 is a sectional view showing another embodiment of the invention;

Fig. 4 is a perspective view showing a further embodiment of the mold;

Fig. 5 is a sectional view showing the molfd of Fig. 4 when it is in use;

Figs. 6A and 6B are perspective views showing further embodiments of the mold; and

Fig. 7 is a sectiona view showing a still further embodiment of the mold.

DESCRIPTION OF PREFERRED EMBODIMENTS:

[0023] A first embodiment of the invention is shown in Figs. 1 and 2, in which a mold is denoted by 10 and made of a non-inductive material, for example, a ceramic material. Examples of the non-inductive ceramic material, which may be used to form the mold 10 in the present invention, include zirconia, fused silica, fused alumina, calcined clay and mullite. Stucco particles of any of these materials are mixed with a binder, such as colloidal silica or ethyl silicate, to form a slurry which is applied on a lost model. After the lost model is lost or dewaxed, the stucco particles are sintered to form the mold 10.

[0024] The mold 10 is placed on a holder or mold holding frame 12 serving as a support. The mold holding frame 12 is guided by not-shown guide rails to be movable in the upward or downward direction. The movement of the mold holding frame 12 is controlled by driving means 14 which comprises a rack 16 secured to the outer periphery of the frame 12, a pinion 18 meshing with the rack 16 and an electric motor 20. The pinion 18 is rotated by the motor 20 so that the mold holding frame 12 is moved vertically by means of the rack 16.

[0025] The mold is cylindrical and a material supplying pipe 22 having a feed port is inserted in the cylindrical mold 10. The pipe 22 serves as a feeder for feeding therethrough the cast or moled material, and may be substituted by chute, or other convenient conveyning means. The material supplying pipe 22 is fixed to a base (not shown) and has a bottom end forming a material supplying outlet or port. A sensor 24, for example an infrared temperature sensor, is attached on the outer periphery of the lower end of the pipe 22. The sensor 24 is used to detect the temperature of the molten pool or zone A, as will be described hereinafter.

[0026] Reference numeral 26 designates a high frequency induction heating coil, reference numeral 28 designates a heat insulating plate disposed beneath the coil 26, and reference numeral 30 designates cooling means or block disposed below the heat insulating plate 28. Cooling water is recirculated through the cooling means or block 30 so that the molten mass formed by heating the cast material by the induction heating coil 26 is cooled to be solidified. The coil 26, the heat insulating plate 28 and the cooling block 30 are fixed to the not-shown base, similar to the material supplying pipe 22.

[0027] Reference numeral 32 designates a controller for controlling the lowering speed of the mold holding frame 12 or the current supplied to energize the coil 26 in response to the output signal from the sensor 24. Reference numeral 36 designates a vibrator which is secured to the upper portion of the inner wall of the mold holding frame 12. The mold 10 is vibrated by the vibrator 36 through a connecting rod 38. The frequency of vibration by the vibrator 36 may be the same as of the commercial frequency or may be the frequency (ranging from several tens to several hundreds Hz) obtained by multiplying the commercial frequency by an integer.

[0028] The vibrator may generate a vibration having the frequency out of the range as described above, and may be a ultra-sonic vibrator for generating ultra-sonic wave having a frequency of about 20 KHz or more. The vibrating frequency may be determined depending on the sort and volume of the mold 10.

[0029] The vibrator 36 may be secured to any desired position as far as the mold is vibrated thereby. The position at which the vibrator 36 is secured is not limited to the position shown in Figs. 1 and 2. For example, the vibrator 36 may be disposed vicinal to the center of the mold 10 or below the coil 26 to vibrate the center or bottom of the mold 10 or to vibrate the mold 10 at the position vicinal to the coil 26.

[0030] At the initial stage of using the illustrated apparatus, the mold holding frame 12 is raised to the uppermost position, as shown in Fig. 1, so that the bottom of the mold 10 is positioned vicinal to the coil 26. Then, a material 34 is supplied continuously or intermittently through the material supplying pipe 22.

[0031] Preferable examples of the material 34 include fluidizable metal powders, particularly sintering metal powders, specific examples being powders of iron, ferroalloys such as ferronickel and ferrocromiuin alloys, copper, copper alloys, cromium alloys and stainless steel such as SUS 304 and SUS 316. Particularly preferred metal powders are sintering powders each having a particle size of from 0.7 to 1.0 mm. Powders of composite alloys may also be used as the metal powder in the present invention, a specific example being a mixure of SUS 316 powder containing 33% of TiC powder.

[0032] Composite materials which may be used in this invention include mixtures each composed of a ferroalloy powder mixed with 10 to 50 wt% of fine particles of a ceramic material such as silicon carbide, silicon boride, silicon oxide or silicon nitride. It is desirous that such a cast material is processed through the process of this invention in vacuum or under a reduced pressure in an oxidating, inert or reducing atmosphere, depending on the specific properties of the used cast material 34. Particularly, processing in vacuum or under a reduced pressure is recommendable since degassing effect is promoted.

[0033] Referring back to Figs. 1 and 2, a high frequency induction coil 26 surrounds the mold 10 to be moved along the longitudinal direction of the mold 10, and the coil 26 is connected to a high frequency current source which is contained in the controller 32. For example, when a stainless steel is used as the cast material, a current of 450 KHz, 40 KW is supplied to the coil 26. As a current is flown through the coil 26, a high frequency magnetic field is established across the section surrounded by the coil 26 so that the portion of the inductive material 34 contained in this section is heated by currents induced by high frequency magnetic flux and is melted to form a molten zone A. The molten zone A is heated, for example, to 1700°C.

[0034] As the material 34 is supplemented through the material supplying pipe 22 while moving the mold 10 downwards, the thus supplemented material 34 is piled over the molten zone A to form a zone B which is not yet melted. Then the zone B is heated by the high frequency induction coil 26 to be melted and admixed to the molten zone A, or the material 34 in the zone B is dispersed into the molten zone A to be melted into the molten zone A to form a uniform molten pool. A small metal plate or block may be placed on the bottom of the mold 10 before the initiation of molding processs to form the initial molten zone A, whereby the subsequent processing may be carried out more smoothly.

[0035] As the mold 10 moves downwards, the lower portion of the molten zone A is moved into the space surrounded by the cooling block 30 (serving as cooling means) to be solidified. The solidified portion or zone is denoted by C in Fig. 2. By supplying the material 34 from the top while moving the mold 10 downwards, as described above, to proceed casting or molding while solidifying the lower portion of the molten zone A, smooth supply of the material 34 to the molten zone A is realized to prepare a high quality cast article with no defect at high yield.

[0036] The molten mass in the molten zone A is smoothly flown under the vibrating action by the vibrator 36. Since the vibrated molten mass becomes readily flown, it is smoothly flown under the electromagnetic force caused by the coil 26 to be agitated more intimately.

[0037] The molten mass is thus smoothly flown along the direction directing from the inner periphery of the mold towards the center of the mold, whereby the molten mass contacts with the inner periphery of the mold for a shortened time to suppress the reaction between the molten mass with the material of the mold. Accordingly, contamination with impurities from the mold is eliminated to improve the purity of the finished product and to improve the condition of the skin or surface of the molded article.

[0038] Another embodiment of the invention is shown in Fig. 3, wherein hot isostatic pressing is effected simultaneously with high frequency induction heating and subsequent solidification.

[0039] Referring to Fig. 3, a pressure container 50 is made of a steel and closed with a lower steel lid 52 and an upper steel lid 54. A support 56 is contained in the container 50 and supports a mold 58. The outer periphery of the mold 58 is surrounded by a high frequency induction coil 60. The induction coil 60 is splitted into plural segments along the longitudinal direction of the mold 58, these segments being energized independently by flowing a current through each of them.

[0040] A material 64 to be molded is contained in a feeder 62, and the supply rate of the material 64 from the outside of the pressure container 50 into the mold 58 is controlled by means of a magnetic valve 66.

[0041] A gas, such as argon, is charged in the interior of the container 50 to serve as a pressing medium. The walls of the container 50 are thermally insulated by an insulator 68 interposed between the walls of the container 50 and the coil 60.

[0042] The interior space within the container 50 is pressurized to have a pressure of more than 100 MPa (about 1000 kgf/cm²), and respective segments of the coil 60 are energized serially, for example, from the lowermost segment to the uppermost segment; and simultaneously the magnetic valve 66 is opened for a predetermined time period to supply the cast material 64 from the hopper 62 into the mold 58. As a result, the material 64 is melted to form a molten zone which traverses from the bottom portion to the top portion of the mold 58, whereby the same effect as attainable by moving the mold in the embodiment shown in Figs. 1 and 2 is attained by this embodiment.

[0043] Since the material 64 is subjected to hot isostatic pressing at high pressure, the finished cast product can be densified to have a denser structure so that the properties of the finished cast product can be improved. It is preferable that the material 64 is subjected to such hot isostatic pressing simultaneously with the high frequency induction heating and subsequent solidification. However, hot isostatic pressing may be effected after the processing by high frequency heating has been completed.

[0044] When an elongated product is prepared, the mold 58 may be placed in the inclined posture and the material is supplied from the bottom towards the top so that the molten zone is moved continuously from the bottom to the top of the mold 58 while permitting to solidify the lower portion of the molten pool. However, it is preferred that the mold 58 is placed in the upstanding posture so that gases or other impurities migrate more easily into the molten zone which traverses from the bottom to the top of the mold 58 to leave a solidified cast product having an improved purity.

[0045] The material 64 supplied to be molded may be changed during the molding step to change the material forming a part of the molded article. Otherwise, the material forming the molded article may be varied continuously. For example, by supplying different materials from plural feeders to form the surface and the inner portion of the molded article from different materials, or to mold a molded article in which the composition of the material is continuously varied to be used as a product exhibiting a gradually varying performance characteristics.

[0046] Within the scope of the present invention, the high frequency induction heating and subsequent solidification steps may be repeated for plural times, by moving the coil 60 relative to the mold 58, to ensure further improvement in purity of the finished product having more improved unidirectional structure. The mold 10 or 58 is not limited to those having straight cylindrical shape as shown in Figs. 1 and 3, but may be arcuated or otherwise defomed to have a more complicated shape.

[0047] Fig. 4 shows a further embodiment of the mold, and Fig. 5 shows the thus modified mold in use. The modified mold 70 is made of a metal, such as copper. The mold 70 is a cylinder having a closed bottom 72 and an opened top, and has a cylindrical wall 74 composed of plural segments 76 extending in the longitudinal direction and separated from each other by slits which are filled with insulating layers 78. These segments 76 are electrically connected with each other by the bottom 72.

[0048] A passage 80 for flowing a cooling medium is provided through each segment 76, as will be seen from Fig. 5 in which two segments are cut and the structures thereof are shown, and a pipe 82 is contained in each passage 80. These pipes 82 are connected within the bottom 72 so that the cooling medium is fed into respective pipes 82 and floods over the top ends of respective pipes 82 to flow down along the inner peripheries of the passages 80 and then discharged from a discharge port provided on the bottom 72.

[0049] As shown in Fig. 5, the mold 70 is lowered relative to a high frequency induction coil 88, and a material 94 to be molded is continuously or intermittently supplied from a feeder 90 through an electromagnetic valve 92. The material 94 is melted by high frequency heating and then solidified to form a solidified zone C which contacts with the mold 70. Electromagnetic force caused by the coil 88 is applied to the molten zone A and the unmelted zone B so that the material within the zones A and B is agitated as shown by the arrows in Fig. 5. The unmelted zone B is in the condition wherein unmelted material 94 is intermingled with molten material.

[0050] In the portion of the molten zone A vicinal to the inner periphery of the mold 70, a force for moving the molten mass remoter from the inner periphery of the mold 70 is generated to move the molten mass upwards. Accordingly, the outer periphery of the molten zone A scarcely contacts with the inner periphery of the mold 70, and the portion thereof contacting with the periphery of the mold 70 is solidified rapidly. Therefore, impurities contained in the mold 70 does not contaminate the molten mass.

[0051] The molten mass is flown more smoothly particularly when the mold 70 is vibrated by a vibrator (not shown). As a result, contact with the outer periphery of the molten zone A with the inner periphery of the mold 70 is diminished to prevent contamination by the intermingling impurities into the molded product.

[0052] Since contaimination of impurities from the mold 70 is prevented more effectively by this embodiment, purity of the product is ensured more reliably. A metal article made of, for example, titanium, zirconium, hafnium, molybdenium chromium, niobium or alloys of these metals can be molded or casted by this embodiment. In order to ensure smooth removal of the solidified product from the mold 70 and to ensure prevention of contamination with impurites, the inner wall of the mold 70 may be coated with boron nitride or another releasing agent.

[0053] A further modified mold is shown in Figs. 6A and 6B. The modified mold 100 is prepared from a block 106 (Fig. 6A) which is prepared by laminating plural copper plates 102 with alternately laminated plural insulating layers 104. The block 106 is machined to form a cavity 108. The material for the insulating layer 104 may be a glass fiber layer impregnated with a resin which is sandwitched between the copper plates 102 and hardened. Although it is desirable that the mold 100 is provided with passages for flowing a cooling medium therethrough, but the mold 100 may be cooled from the outer periphery thereof by a cooling medium.

[0054] A mold 100 having variable shape can be produced from the block 106 at a low production cost with a short time for production. The copper plates 102 of the mold 100 are not electrically connected. However, when the conductive material to be molded is supplied in the mold cavity 108 to be melted and solidified therein, the copper plates 102 are electrically connected with each other by the thus supplied material.

[0055] Although the materials 34, 64 and 94 used in the preceding embodiments are in the form of powder or granule, the present invention is not limited to the use of the material in the powder or granule form. For example, material to be molded may be used in the form of a wire, rod, plate or block, as far as it is supplied continuously or intermittently into the mold. The material to be molded may be supplied in the form of long continuous wire or thin plate.

[0056] The mold used in the invention may be formed through the powder molding process. Since the graviational force and heat shock generated in the mold are mainly applied to the molten zone so that the force and shock applied to the residual portion of the mold are extremely small, it becomes possible to use a relatively week mold prepared through the powder molding process. It is preferred that the powder used for the preparation of the mold is a non-inductive material, such as a powder of ceramics material.

[0057] Considerable reduction in production cost can be realized by using a mold prepared through the powder molding process as a mold, such as a consumable mold or core mold, which is broken for every operation cycle. Such a consumable mold or core mold may be prepared by solidifying a material of powder form by heating under pressure in a mold. Sintering is not essential, and powders of the material are bound together by the intermolecular force (van der Waals' force), capillary force or electrostatic force.

[0058] In order to increase the strength of the mold, it may be subjected to hardening by using a variety of gasses. For example, when foundry sands are used together with sodium silicate acting as a binder, hardening of the mold can be accerelated by blowing carbon dioxide gas (the CO₂ process). When a ethyl silicate binder is used, hardening of the mold can be accerelated by using gaseous ammona.

[0059] Although a mold having no hollow portion can be, of course, prepared through the powder molding process, a mold having an inner hollow portion or a mold having a core therein can also be produced through the powder molding process. A process for preparing a core mold having an inner hollow portion may comprise the steps of holding a resilient or elastic bag (i.e. balloon) in a splittable outside mold, supplying a powder in-between the bag and the outer mold, and blowing a high pressure gas within the balloon-bag to inflate the balloon-bag thereby to press the powder by the balloon-bag and the outer mold.

[0060] Although more detailed description of the process described in the preceding paragraph will not be given herein, description in my pending Japanese Patent Application No. 144381/1990 (Unexamined Japanese Patent Publication No. 37437/1992) will be incorporated herein as a reference. It is desirable that the thickness of the core mold is as thin as possible in order to easy collapse of the core mold.

[0061] When the mold having core portion is prepared through the powder molding process, the mold thus prepared through the powder molding process may be combined with a metal mold. Fig. 7 is a sectional view showing an example of such mold. Referring to Fig. 7, a mold is generally denoted by 120 and comprises a main mold 122 and an extending core portion 124 integral with the main mold 122.

[0062] In order to improve the efficiency of high frequency induction heating, the main mold 122 preferably comprises interposed insulating layers filled in the slits or a laminated structure having a alternately laminated metal plates and insulating layers. Although it is not essential that the core portion 124 has a construction same as that of the main mold 122, it is preferred that the core portion 124 has the same construction as that of the main mold 122. Branching passages 126, 128 for flowing a cooling medium or water are provided in the wall of the mold 120.

[0063] Pipes 130, 132 extend within the passages 126, 128. These pipes 130 and 132 are gathered at the converged portion 134 within the bottom of the mold wall so that the cooling medium is fed through the converged portion 134 to the pipes 130, 132. Cooling medium flooding over the top open ends of the pipes 130, 132 cools the inner periphery of the passages 126, 128 to be discharged through a discharge port 136 provided at the bottom of the mold wall.

[0064] The center core portion 124 is covered with a core mold 138 which is formed through the powder molding process. The core mold 138 is prepared by placing the core 124 of the mold 122 in an outer mold for molding the core mold 138 to mold the core mold 138 through the powder molding process. By the use of the mold 120, the core mold 138 is also cooled to control solidification under well-controlled condition. By the provision of a vibrator which vibrates each of the molds shown in Figs. 3 to 7, a similar effect as obtained in the embodiments of Figs. 1 and 2 can be attained.

[0065] As will be understood from the foregoing, according to the present invention, a high frequency inductive metal or similar composite material is supplied continuously or intermittently into a mold while heating the thus supplied material from the outside of the mold by high frequency induction heating in a manner such that the molten zone is moved upwards from the bottom, followed by subsequent directional solidification. By controlling the moving or travelling speed of the mold or induction heating coil or by controlling the supply rate of the cast material, smooth casting can be effected. The material can be smoothly supplied without the fear that the material is coagulated in the portion above the molten zone.

[0066] The melting and solidification steps can be controlled properly throughout the process by controlling the current supply to the induction coil or the speed of relative movement between the mold and the coil to promote degassing and to control crystalline structure of the resulted product. Accordingly, it becomes possible to prepare a molded product having an oriented crystalline structure, single crystal structure or controlled microcrystalline structure. As a result, a molded product having a satisfactory strength equivalent to that of the product produced through machining and complicated shape can be produced at low cost.

[0067] The mold may be made of a non-inductive material such as a ceramic material, or may be made of a metal, such as pure copper. It is desirable that the mold is splitted into plural segment extending along the magnetic field, when the mold is made of a metal. By constructing the mold from plural splitted metal segments, eddy currents generated within the mold are diminished to heat the material supplied in the mold more efficiently.

[0068] By electrically connecting the metal segments, arc discharge between the splitted segments is prevented to exclude breakdown of insulation between the segments due to arc discharge, to prevent contamination of impurities caused by intermingling impurities onto the surface of the molded article, and to prevent breakdown of the mold. The segments may be electrically connected by the material supplied in the mold.

[0069] Particularly when a material composed of a mixture of a metal powder and a ceramic material powder is supplied continuously or intermittently as the material to be molded, the ceramic material particles are distributed evenly throughout the finished solidified mass since the cast material is melted little by little. Accordingly, it becomes possible to prepare a molded product having desired properties at high efficiency. Consolidation of powdered material by press molding prior to the molding step can be eliminated to make it easier to prepare a large size cast article.

[0070] The material may be selected from various inductive sinterable metal powders and sinterable composite materials composed of inductive metal powders and ceramic materials. The material may be supplied in the form of granule, powder, wire, rod, plate or block. It is desirable that the material is molded in an oxidating, inert or reducing atmosphere or in vacuum or under reduced pressure depending on the material used.

[0071] By subjecting the material to repeated zone-melting and subsequennt solidification steps, the properties of the molded product can be further improved. The properties of the molded product may be further improved by subjecting the molded material to hot isostatic pressing during it is melted by high frequency induction heating and subsequently solidified.

[0072] By subjecting the mold to vibration during the molding process, the molten mass is flown more smoothly within the molten zone. The molten mass in the molten zone is allowed to flow by the action of the electromagnetic force induced by the high frequency induction coil, and the flowing and agitation of the molten mass is further promoted since the molten mass is in the state ready for flowing by the thus applied vibration.

[0073] The material is thus flown from the vicinity of the inner periphery of the mold towards the center as soon as it is melted. Even when it contacts with the inner periphery of the mold, the time for the material to contact with the inner periphery of the mold is shortened to suppress the reaction between the molten mass and the inner periperal wall of the mold to diminish migration of impurities into the molded product. Accordingly, the purity of the molded product is improved and the skin condition of the molded product is improved.

[0074] An apparatus adapted for use to practice the process of the invention is also provided. When the mold is made of a transparent quartz glass, the condition of the material in the mold can be visually inspected The steps of melting and solidification can be monitored, for example, by the provision of an infrared thermometer to obtain data for analyzing the solidification step. The mold may be prepared through the powder molding process, by which a consumable mold or a core mold can be prepared efficiently at low cost.

Claims

1. A process for molding a metal article made of an inductive metal material in a mold by means of high frequency induction heating caused by a high frequency magnetic field which is established by a high frequency induction coil surrounding the outer periphery of said mold, comprising the steps of:

(a) supplying said metal material continuously or intermittently from the top of said mold:

(b) subjecting said metal material in said mold to said high frequency induction heating to melt the same thereby to form a molten zone: and

(c) moving said mold downwards relative to said high frequency magnetic field;
   said steps (a), (b) and (c) being done simultaneously to move said molten zone upwards from the bottom to the top of said mold, whereby said metal material is once melted and then solidified to form a molded article.

2. The process according to claim 1, wherein said mold is made of a non-inductive material.

3. The process according to claim 1, wherein said mold comprises plural metal segments which are joined together through an insulating layer.

4. The process according to claim 1, wherein said metal material is an iron or stainless steel powder which is used to prepare a sintered article.

5. The process according to claim 1, wherein said metal material is supplied in the form of granule, powder, wire, rod, plate or block.

6. The process according to claim 1, wherein said metal material is supplied in an oxidating, inert or reducing atmosphere or in vacuum or under reduced pressure.

7. The process according to claim 1, wherein said metal material is subjected to induction heating while said mold is vibrated.

8. A process for molding an article of a composite material composed of an inductive metal and a ceramics material in a mold by means of high frequency induction heating caused by a high frequency magnetic field which is established by a high frequency induction coil surrounding the outer periphery of said mold, comprising the steps of:

(a) supplying said composite material continuously or intermittently from the top of said mold;

(b) subjecting said composite material in said mold to said high frequency induction heating to melt the same thereby to form a molten zone; and

(c) moving said mold downwards relative to said high frequency magnetic field;
   said steps (a), (b) and (c) being done simultaneously to move said molten zone upwards from the bottom to the top of said mold, whereby said composite material is once melted and then solidified to form a molded article.

9. The process according to claim 8, wherein said mold is made of a non-inductive material.

10. The process according to claim 8, wherein said mold comprises plural metal segments which are joined together through an insulating layer.

11. The process according to claim 8, wherein said composite material comprises a metal powder and a ceramics powder, the composite material being used to prepare a sintered article.

12. The process according to claim 8, wherein said composite material is supplied in the form of granule, powder, wire, rod, plate or block.

13. The process according to claim 8, wherein said composite material is supplied in an oxidating, inert or reducing atmosphere or in vacuum or under reduced pressure.

14. The process according to claim 8, wherein said composite material is subjected to induction heating while said mold is vibrated.

15. An apparatus for molding a molded article by means of high frequency induction heating caused by high frequency magnetic field, comprising:

(a) a support for holding a mold in said high frequency magnetic field;

(b) a high frequency induction heating coil surrounding the outer periphery of said mold, said coil establishing said high frequency magnetic field;

(c) a feeder for continuously or intermittently supplying an inductive material into said mold; and

(d) a driver for moving said support together with said mold downwards reletive to said high frequency induction coil along the vertical plane;
   whereby said inductive material is once melted to form a molten zone and then solidified from the bottom to the top of said mold to form said molded article.

16. The apparatus according to claim 15, further comprising a sensor for sensing the molten state of said inductive material to generate an output signal, and a controller for receiving said output signal from said sensor to control at least one of the lowering rate of said support and the energizing electric power of said induction heating coil.

17. The apparatus according to claim 15, further comprising a cooler for cooling said mold and disposed below said induction heating coil.

18. The apparatus according to claim 15, wherein said mold is made of a non-inductive material.

19. The apparatus according to claim 18, wherein said mold is made of a transparent quartz glass so that supply, melting and solidification of said inductive material is visually inspected from the outside of said mold.

20. The apparatus according to claim 15, wherein said mold is prepared through the powder molding process.

21. The apparatus according to claim 15, wherein said mold comprises plural metal segments extending along the magnetic field established by said induction heating coil and joined together through an insulating layer.

22. The apparatus according to claim 21, wherein a passage for flowing a cooling medium is formed through each of said plural metal segments so that said cooling medium is recirculated through said passage.

23. The apparatus according to claim 15, wherein said mold is prepared by machining a block which is formed by laminating plural metal plates joined together through an insulating layer.

24. The apparatus according to claim 15, further comprising a vibrator for vibrating said mold.

Drawing