Powder feeding apparatus, pressing apparatus using the same, powder feeding method and sintered magnet manufacturing method

Abstract
A powder pressing apparatus comprises a powder feeding apparatus. The powder feeding apparatus includes a container having a bottom portion provided with a powder holding portion formed with openings, and an impactor. The impactor is hit against the container to give an impulsive force, thereby feeding the powder contained in the container into the cavity formed in a die via the openings. The powder fed in the cavity is pressed, and the obtained compact is sintered into a sintered magnet. The powder feeding apparatus may include a feeder box containing the powder, and the feeder box may be provided therein with a rod member, and an opening of the feeder box may be provided with a linear member. In this case, the powder is fed into the cavity while moving the rod member in the horizontal direction in the feeder box, when the feeder box is above the cavity.
Description




BACKGROUND OF THE INVENTION




1. Field of the Invention




The present invention relates to a powder feeding apparatus, a pressing apparatus using the same, a powder feeding method and a sintered magnet manufacturing method. More specifically, the present invention relates to a powder feeding apparatus for feeding a powder into a cavity formed in a die, a pressing apparatus using the same, a powder feeding method and a sintered magnet manufacturing method.




2. Description of the Related Art




Currently, as sintered rear-earth alloy magnets, two kinds, i.e. a samarium-cobalt magnet and a rare-earth-iron-boron magnet, are used extensively in many fields. Of the two, the rare-earth-iron-boron magnet is appreciated in application to variety of electronic devices and apparatuses. (Hereinafter, the rare-earth-iron-boron magnet will be called “R-T-(M)-B magnet”, where R represents a rare-earth element including yttrium, T represents iron or iron partially substituted by a transition metal element, M represents a doped element, and B represents boron.) A reason for this is that the R-T-(M)-B magnet is the most superior of many kinds of magnets in terms of magnetic energy product and relatively inexpensive in terms of price. The transition metal included as T may be cobalt for example. Boron can be partially substituted by carbon.




In manufacture of such a rare-earth magnet, first, a magnetic alloy powder made by milling a rare-earth alloy is pressed into a compact (green compact) by a pressing apparatus. When making the compact, the magnetic alloy powder is fed into a cavity formed by a die hole (through hole) provided in a die and a lower punch inserted into the die. The magnetic alloy powder fed in the cavity is pressed by an upper punch. The compact thus obtained is then sintered at a temperature of 1000° C. -1100° C. approx., and then finished as the sintered rare-earth magnet.




Conventionally, a variety of methods are proposed for feeding the magnetic alloy powder into the cavity in the pressing apparatus.




For example, Japanese Utility Model Publication (of examined Application for opposition) No. 59-32568 and Japanese Patent Laid-Open No. 61-147802 each discloses a technique of vibrating a container which holds the powder and thereby supplying the power into the cavity in sieving action through a metal net.




According to Japanese Patent Laid-Open No. 61-147802, there is described an apparatus comprising a feeder cup (the powder container) having a bottom portion provided with a metal net. The feeder cup is vibrated relatively rigorously by using a solenoid coil, thereby feeding the granular magnetic powder through the metal net into the cavity in a short time.




However, according to the conventional apparatus disclosed in Japanese Patent Laid-Open No. 61-147802, the vibration is generated by means of attracting force between the solenoid coil and an iron core, and of restoring force provided by a spring, and the vibration is given to the feeder cup itself which holds the powder. The iron core (moving part) is fastened to the feeder cup by a connecting hardware. With this arrangement, the vibrating force transmitted to the powder in the feeder cup is only a reciprocating force, and the transmitted force is still not sufficient to break down a lump of powder. In such an apparatus, in order to supply the granular powder into the cavity while preventing bridge formation, one possibility is to use the metal net having a fine grid (mesh). However, use of such a fine-mesh metal net poses another problem that the powder is not quickly sieved and there is a significant increase in the time for feeding the powder.




Another problem with the above conventional apparatus is that it is difficult to increase the stroke (amplitude) of vibration given to the feeder cup. If the feeder cup is moved only in a short stroke, it is difficult to feed the powder uniformly in the cavity.




There is still another problem. Specifically, corner and/or edge regions of the cavity is more difficult to feed with the powder than a center region of the cavity. According to the conventional apparatus therefore, when the rare-earth alloy powder is supplied through the metal net which is provided at a position relatively high above the die surface, the powder tends to form a high portion in the center region. If the powder is fed in such a non-uniform density in the cavity, the compact formed by the pressing operation has an unacceptably large difference in its pressing density, between the corner and/or edge regions and the center region. This density difference can cause a crack in the compact.




This problem is presumable also in an apparatus disclosed in Japanese Utility Model Publication (of examined Application for opposition) No. 59-32568.




Other techniques for feeding the powder into the cavity are proposed in Japanese Patent Laid-Open No. 11-49101 and Japanese Patent Laid-Open No. 2000-248301.




According to the technique disclosed in Japanese Patent Laid-Open No. 11-49101, a feed is fed into a container by means of pneumatic tapping and via a supplying hopper. An arrangement is made so that the feed is present in both of the supplying hopper and the container after the pneumatic tapping. Then, of this mass of the feed present in both of the supplying hopper and the container, a portion of uniform density formed in the container is separated from the feed remaining in the supplying hopper.




Japanese Patent Laid-Open No. 2000-248301 discloses a supplying apparatus, in which a feeder box having an opening in a bottom is moved to above a cavity formed in a die tooling, allowing a rare-earth alloy powder to be supplied into the cavity from the opening. The supplying apparatus comprises rod members which are moved at the bottom portion horizontally within the feeder box. The rod members are reciprocated when the rare-earth alloy powder in the feeder box is supplied to the cavity.




However, according to the technique disclosed in Japanese Patent Laid-Open No. 11-49101, since the feeding into the container is performed by the pneumatic tapping, the feeding density of the feed in the container becomes higher than by means of natural gravitational fall. For example, a rare-earth alloy powder fed by means of natural gravitational fall has the feeding density of 1.8 g/cm


3


approx., versus the feeding density of 3.4 g/cm


3


approx. by means of pneumatic tapping. The feed packed to such a high density does not allow particles of the powder to move easily, requiring a stronger magnetic field in order to orient the powder, leading to increase in manufacturing cost.




According to the technique disclosed in Japanese Patent Laid-Open No. 2000-248301 on the other hand, as shown in

FIG. 21A

, a feeder box


2


is moved toward a cavity


1


. Then, as shown in

FIG. 21B

, when the feeder box


2


is positioned above the cavity


1


, a powder


3


is supplied into the cavity


1


by the weight of the powder


3


itself. The feeding thus performed is not even, and therefore the powder


3


is not distributed uniformly. Thereafter, as shown in FIG.


21


C and

FIG. 21D

, a shaker


4


is activated to fill the cavity


1


with the powder


3


. The shaker


4


forces the powder


3


in, to the density of 2.3 g/cm


3


approx., thereby uniformalizing the feeding density. As a result, a stronger magnetic field is necessary in order to obtain a desired level of orientation.

FIG. 22

shows state changes in the feeding operation performed by this conventional apparatus.




Further, if the cavity is shallow in a direction of the pressing operation provided by the punches, the feeding density inconsistency in the cavity is not easily corrected by the pressing operation, leading to occasional crack development in the compact.




SUMMARY OF THE INVENTION




It is therefore a primary object of the present invention to provide a powder feeding apparatus, a pressing apparatus using the same and a sintered magnet manufacturing method, capable of feeding the powder uniformly and in a short time into the cavity of the pressing apparatus.




Another object of the present invention is to provide a powder feeding apparatus, a pressing apparatus using the same, a powder feeding method and a sintered magnet manufacturing method, capable of providing a desired orientation and a high magnetic characteristic at a low cost.




According to an aspect of the present invention, there is provided a powder feeding apparatus for feeding a powder into a cavity formed in a die, comprising: a container including a bottom portion provided with a powder holding portion formed with a plurality of openings capable of allowing the powder to pass through; and an impactor capable of hitting against the container; wherein the impactor is hit against the container to give an impulsive force to the container, thereby feeding the powder contained in the container into the cavity via the openings.




According to this invention, by having the impactor hit against the container, a lump of the powder contained in the container can be broken down and the powder in the broken state can be supplied into the cavity.




According to another aspect of the present invention, there is provided a pressing apparatus comprising: the above described powder feeding apparatus; and pressing means which presses the powder fed in the cavity by the powder feeding apparatus.




According to still another aspect of the present invention, there is provided a sintered magnet manufacturing method comprising: a first step of applying an impulsive force to a container which includes a bottom portion provided with a powder holding portion formed with a plurality of openings capable of allowing the powder to pass through, thereby feeding the powder contained in the container via the openings into a cavity formed in a die; a second step of forming a compact by pressing the powder fed in the cavity; and a third step of manufacturing a sintered magnet by sintering the compact.




By pressing the powder which is fed uniformly in the cavity, a compact which has a uniform density, and a small inconsistency in size and weight can be manufactured.




Further, by sintering the compact, a magnet which has a small inconsistency in size and weight can be obtained.




Preferably, the apparatus further comprises a vibrating mechanism connected to an upper portion of the container. The impactor is provided so as to hit against a lower portion of the container, and the vibrating mechanism vibrates an upper portion of the container, thereby allowing the impactor to hit against the lower portion of the container. In this way, by connecting the vibration mechanism with the container and by separating the impactor from the vibration mechanism, it becomes possible to reduce whirling up of the powder, thereby reducing binding of the powder in the vibrating mechanism. Further, by hitting the impactor on the lower portion of the container, the impact can be transmitted more directly to the opening of the container, making possible to transmit the impact to the entire mass of the powder present at the opening, thereby feeding the cavity with the powder uniformly.




Further, preferably, the powder holding portion is formed of a net having a mesh size of 2-14. More preferably, the powder holding portion is formed of a net having a mesh size of 2-8. By using a relatively coarse net as the above, the powder can be fed uniformly into the cavity while remarkably reducing the time necessary for the powder feeding.




Preferably, the powder holding portion is provided at a height smaller than 2.0 mm from a surface of the die. More preferably, the powder holding portion is provided at a height smaller than 1.0 mm from the surface of the die. This arrangement makes possible to allow only a small amount of the powder to project from within the cavity above the surface of the die. Therefore, an amount of the extra powder to be wiped is small, and a lump produced in the wiping operation by the container is not unwontedly fed into the cavity at the next cycle of powder feeding.




Further, preferably, the container can move when the impulsive force is given to the container by the hitting of the impactor against the container. With this arrangement, it becomes possible to have the moving container be hit by the impactor, and to give a reverse impact to the container, and therefore to feed the cavity with the powder more uniformly.




Preferably, the apparatus comprises a plurality of the impactors disposed outside of the container in an opposing relationship, with the container in between. With this arrangement, the impulsive force can be given continuously to the container.




Further, preferably, the apparatus further comprises a partition plate provided inside the container. With this arrangement, when the impactor hits a side wall of the container, the impulsive force can be transmitted dispersively to the powder inside the partitioned container, making possible to feed the powder more efficiently. This arrangement can remarkably reduce feeding time of the powder into the cavity.




Further, preferably, a size of the openings provided in the powder holding portion is in accordance with a location of the opening. By changing the coarseness according to the location of the opening in this way, the amount of powder to be fed into the cavities can be controlled according to region.




If the powder is a rare-earth alloy powder, the powder particles are angular, and with addition of a lubricant, the powder decreases its flowability and forms a lump, into a state not to easily drop from the opening of the powder holding portion. However, according to the present invention, even if the powder is a rare-earth alloy powder mixed with a lubricant and poor in flowability, the powder can be fed in the cavity uniformly and efficiently in a short time.




According to another aspect of the present invention, there is provided a powder feeding apparatus for feeding a powder into a cavity formed in a die, comprising: a feeder box movable to above the cavity, including a bottom portion formed with an opening, and containing the powder; a rod member provided inside the feeder box and pushing the powder downwardly; a linear member provided at the opening of the feeder box; and orienting means which aligns the powder fed from the feeder box in the cavity.




According to still another aspect of the present invention, there is provided a powder feeding method for feeding a powder into a cavity formed in a die, the method comprising: a step of moving a feeder box to above the cavity of the die, with the feeder box containing the powder, being provided inside thereof with a rod member movable in a horizontal direction, and having an opening provided with a linear member; a step of feeding the powder into the cavity while moving the rod member in the horizontal direction within the feeder box, when the feeder box is above the cavity; and a step of orienting the powder by applying a magnetic field to the powder in the cavity.




According to this invention, by providing the linear member at the opening of the feeder box, the powder does not fall into the cavity even when the feeder box has moved to above the cavity. The powder can be fed into the cavity thereafter, by activating the rod member in the feeder box. In this feeding, the powder can be fed into the cavity uniformly at a natural feeding density (1.7 g/cm


3


-2.0 g/cm


3


for example). Since the powder is not fed at a high density, the powder particles can move easily, and a desired orientation can be achieved by an orienting magnetic field of a relatively low strength. This makes possible to prevent manufacturing cost from increasing. Further, since the density distribution in the feeding can be made uniformly, a product having a superb magnetic characteristic can be obtained by orienting the powder in the cavity.




Preferably, the rod member is spaced from the linear member by a distance not smaller than 0.5 mm and not greater than 10 mm. With this arrangement, flow of the powder near the linear member is assisted, making possible to smoothly feed the powder into the cavity at a density suitable for the orientation.




According to still anther aspect of the present invention, there is provided a pressing apparatus comprising: the powder feeding apparatus described above; and pressing means which presses the powder fed in the cavity by the powder feeding apparatus.




According to this invention, by pressing the powder which is fed in the cavity by the above powder feeding apparatus, a compact high in density uniformity can be obtained, and thus crack and fracture development due to inconsistent density can be prevented.




If the powder is produced by using a rapid quenching process, and a particle distribution pattern of the powder is made narrow, the powder has an extremely poor flow ability. However, according to the present invention, since the powder flowablity can be improved by the natural gravitational feeding, density consistency of the powder in the cavity can be improved even if the powder is produced by using the rapid quenching process and the particle distribution pattern of the powder is made sharp. Further, each powder particle can be easily moved, and therefore it becomes possible to form a magnet having a high magnetic anisotropy for example.




Preferably, the interval between the linear members is not smaller than 2 mm and not greater than 12 mm.




According to still anther aspect of the present invention, there is provided a sintered magnet manufacturing method comprising: a step of obtaining a compact by pressing a powder in a cavity, the powder being fed by the above described powder feeding method; and a step of manufacturing a sintered magnet by sintering the compact.




According to this invention, by pressing the powder fed into the cavity by means of the above described method, a compact high in density uniformity can be obtained, and thus crack and fracture development in the compact can be reduced. As a result, sintered magnet obtained by sintering the compact has a decreased rate of defects due to cracking and/or fracturing, and a decreased rate of deformation. Therefore, it becomes possible to improve yield in manufacturing process, to improve productivity of the sintered magnet, and to manufacture a sintered magnet having a favorable magnetic characteristic.




The above objects, other objects, characteristics, aspects and advantages of the present invention will become clearer from the following description of embodiments to be presented with reference to the accompanying drawings.











BRIEF DESCRIPTION OF THE DRAWINGS





FIG. 1

is a perspective view showing a principal portion of a pressing apparatus as an embodiment of the present invention;




FIG.


2


A and

FIG. 2B

are views showing a principal portion of a powder feeding apparatus used in the embodiment in FIG.


1


;

FIG. 2A

is a plan view with a lid removed, whereas

FIG. 2B

is a sectional view with a powder present;




FIG.


3


A and

FIG. 3B

are sectional views showing a fall of the powder from a net member caused by an impact force;

FIG. 3A

illustrates a state before applying the impact force, whereas

FIG. 3B

illustrates a state right after the application of the impact force;





FIG. 4

is an enlarged sectional view of a part of a powder container for illustrating a gap between a die surface and the net member;





FIG. 5

is a graph showing a relationship of the gap between the die surface and the net member with a thickness inconsistency;





FIG. 6

is a schematic diagram showing the pressing apparatus in

FIG. 1 and a

surrounding setting;





FIG. 7

is a sectional view of a powder container in a powder feeding apparatus according to another embodiment;




FIG.


8


A and

FIG. 8B

are plan views each showing a variation of the net member;




FIG.


9


A and

FIG. 9B

are views each showing a principal portion of a powder feeding apparatus used in still another embodiment;

FIG. 9A

is a plan view with a lid removed, whereas

FIG. 9B

is a sectional view with a powder present;





FIG. 10

is a perspective view showing a principal portion of the pressing apparatus according to another embodiment of the present invention;





FIG. 11

is a side view showing a section of a principal portion of the embodiment in

FIG. 10

;





FIG. 12

is an end view taken in line C—C (shown in FIG.


11


), showing a principal portion of the embodiment in

FIG. 10

;





FIG. 13

is a side view showing a principal portion of a powder feeding apparatus used in the embodiment in

FIG. 10

;





FIG. 14

is a perspective view showing a feeder box provided with a shaker and linear members;




FIG.


15


A through

FIG. 15D

are views illustrating a powder feeding operation according to the embodiment in

FIG. 10

;





FIG. 16

is a diagram illustrating state changes in the powder feeding according to the embodiment in

FIG. 10

;





FIG. 17A

is a view showing a compact formed in an experiment, whereas

FIG. 17B

is a table showing a result of the experiment;





FIG. 18

is a schematic diagram showing another embodiment of the present invention;





FIG. 19

is a schematic diagram showing still another embodiment of the present invention;




FIG.


20


A and

FIG. 20B

are graphs showing a result of another experiment;




FIG.


21


A through

FIG. 21D

are diagrams illustrating a powder feeding operation performed by a conventional apparatus; and





FIG. 22

is a diagram illustrating state changes in the powder feeding according to the conventional apparatus.











DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS




Hereinafter, embodiments of the present invention will be described with reference to the drawings.




Referring to FIG.


1


and

FIG. 2

, a powder pressing apparatus


10


as an embodiment of the present invention comprises a pressing portion


12


and a powder feeding apparatus


14


.




The pressing portion


12


includes a die set


16


and a die tooling


18


. The die tooling


18


includes a die


20


, lower punches


22


and upper punches


24


(See FIG.


6


). The die


20


has a saturated magnetism not smaller than 0.05 T and not greater than 1.2 T for example. The die


20


is fitted into the die set


16


. Each of the lower punches


22


is disposed so as to be inserted into a die hole


26


from below. The die hole


26


is a through hole running vertically through the die


20


. An upper end surface of the lower punch


22


and an inner circumferential surface of the die hole


26


provide a cavity


28


(See

FIG. 2B

) of a variable volume. With this arrangement, the upper punch


24


is inserted into the cavity


28


to press a powder m (to be described later) fed in the cavity


28


into a compact. Further,a magnetic field generating coil


29


is provided near the die


20


. By using the coil


29


for generation of magnetic field, an orienting magnetic field, having a strength of 1.2 T for example, is applied to the powder m in parallel with the pressing direction.




The powder feeding apparatus


14


includes a base plate


30


disposed in abutment on the die set


16


. On the base plate


30


, a feeder box


32


is disposed. The feeder box


32


is moved by a cylinder rod


36


of a cylinder


34


which is driven hydraulically or pneumatically for example (or by an electric servo motor), in a reciprocating pattern between a predetermined position on the die


20


and a stand-by position. Near the stand-by position of the feeder box


32


, there is provided a replenishing apparatus


38


for replenishing the feeder box


32


with the powder m.




The replenishing apparatus


38


includes a weighing scale


40


, a feeder cup


42


disposed thereon, and a vibrating trough


44


which drops the powder m by a small amount into the feeder cup


42


. The weighing operation is performed while the feeder box


32


is moved onto the die


20


. When the weight of the powder m in the feeder cup


42


reaches a predetermined level, a robot


46


grasps the feeder cup


42


, and when the feeder box


32


returns the stand-by position, the robot


46


replenishes the feeder box


32


with the powder m in the feeder cup


42


. The amount of the powder m in the feeder cup


42


replenishes an amount of the powder in the feeder box


32


used in a cycle of pressing operation. Therefore, the feeder box


32


holds a constant amount of the powder m. Because of the constancy in the amount of the powder m held in the feeder box


32


, pressure in gravitational fall of the powder m into the cavity


28


is constant, and an amount of the powder m fed into the cavity


28


is constant. The powder m may be a rare-earth alloy powder for example.




Reference is now made to FIG.


2


A and

FIG. 2B

, and description will be made for a principal portion of the powder feeding apparatus


14


.




The feeder box


32


of the powder feeding apparatus


14


includes an enclosing member


48


and a lid


50


which is disposed on an upper surface of the enclosing member


48


and can be opened and closed. Inside the enclosing member


48


, a powder container


52


is disposed. The powder container


52


is disposed between a pair of opposed impactors


54


. The feeder box


32


, with the powder container


52


containing the powder m, is moved to above the cavity


28


formed in the die


20


of the pressing apparatus


10


, allowing the powder m to be supplied into the cavity


28


.




The lid


50


provided on the upper surface of the enclosing member


48


can seal the inside of the enclosing member


48


. Preferably, inside the enclosing member


48


an inert gas such as nitrogen gas is supplied, preventing the powder m contained in the powder container


52


from oxidization by the atmosphere. The lid


50


can be opened and closed automatically by an air cylinder for example.




The powder container


52


has a bottom portion provided with a net member


56


which is capable of holding the powder m and of allowing the powder m to pass through upon impact from the impactor


54


. Preferably, the net member


56


is made of a stainless steel such as SUS


304


, and has a mesh size of 2-14 (sieve aperture not smaller than 1.8 mm and not greater than 12.7 mm). More preferably, the mesh size is 2-8 (sieve aperture not smaller than 3.2 mm and not greater than 12.7 mm). For example, the net member of a mesh size of


8


can be made of a metal wire having 0.6 mm diameter weaved into a net having 3.0 mm grids. The net member


56


preferably is plated with nickel for example. This decreases surface coarseness of the net member


56


, making possible to improve flowability of the rare-earth alloy powder at the time of feeding.




Each of the impactors


54


is provided with and driven by an air cylinder


58


, independently of the other. The impactor


54


can be moved quickly by the air cylinder


58


toward the powder container


52


, to hit on a side wall of the powder container


52


thereby applying an impulsive force (an impacting force). By this impact, the powder m contained in the powder container


52


is supplied into the cavity


28


through the net member


56


. Preferably, the impactors


54


are driven by the air cylinders


58


to hit the powder container


52


at a rate of 50-120 times per minute. Each of the impactors


54


has a reciprocating stroke of 10 mm-20 mm for example.




Preferably, upon impact from one of the impactors


54


, the powder container


52


can move toward the other impactor


54


. In order to allow this, the enclosing member


48


is provided with a pair of guide members


60


extending in parallel with each other in the direction in which the impactors


54


are moved. The powder container


52


can move linearly in the enclosing member


48


along the guide members


60


. With this arrangement, the other impactor


54


can be hit against the approaching powder container


52


, and it becomes possible to give the powder container


52


an impact in the reverse direction of the direction of the container movement. This makes possible to feed the powder m in the cavity


28


uniformly.




The powder container


52


has a bottom edge provided with a sliding member


62


(thickness: 5 mm approx. for example) made of such material as a thin plate of fluororesin or felt. The sliding member


62


reduces chance for the powder m to be caught between the powder container


52


and the die


20


, making possible for the powder container


52


to slide smoothly on the die


20


. A similar sliding member


64


is provided at a bottom edge of the enclosing member


48


. The sliding member


64


reduces chance for the powder m to be caught between the enclosing member


48


and the die


20


, making possible for the enclosing member


48


to slide smoothly on the die


20


. With these arrangements, the feeder box


32


can slide smoothly on the die


20


of the pressing apparatus


10


.




Next, reference is made to FIG.


3


A and FIG.


3


B.

FIG. 3A

shows a state before the impactor


54


gives an impact. If the powder m is a rare-earth alloy powder produced by using a strip cast process, each powder particle is angular. Further, if a lubricant is added to the powder m, the powder m decreases in its flowability and forms a lump. In this case, the powder m, i.e. the rare-earth alloy powder, is in a state not to easily drop from the opening


56




a


(grid) of the net member


56


. For this reason, the net member has a relatively coarse grid of 2-14 mesh approx., with the opening


56




a


having a relatively large width (gap) d


1


, which is a few millimeters through ten plus a few millimeters.




Thereafter, as shown in

FIG. 3B

, the impact is given by the impactor


54


, to break up the lump, allowing the powder m or particles smaller than the mesh to fall through the opening


56




a


of the net member


56


. A note should be made here that in FIG.


3


A and

FIG. 3B

, the illustrated particles of the powder m are relatively oversized. In reality however, the particle of the powder m provided by a rare-earth alloy powder typically has a diameter not greater than 10 μm, which is by far smaller than the width d


1


(a few millimeter through a ten plus a few millimeter) of the opening


56




a.






As has been described, according to the present embodiment, unlike the prior art in which the container itself is vibrated, the impactors


54


are hit against the powder container


52


as shown in FIG.


2


A and FIG.


2


B. This makes possible to break down the powder m, which is poor in flowablity and subject to lump formation in the powder container


52


, and to supply the cavity


28


with the powder m under a broken state. Use of the impactors


54


makes possible to apply the powder container


52


with a very large force which acts in a significantly short period of time (instantaneous force), which transmits to the powder m and effectively breaks the lump of powder m into finer state. According to the present embodiment, by using a relatively coarse net of 2-14 mesh size approx., it becomes possible to uniformly feed the powder m in the cavity


28


in a remarkably reduced time.




Next, reference is made to FIG.


4


. According to the powder feeding apparatus


14


, after supplying the cavity


28


with the powder m, and when the feeder box


32


is moving away from above the cavity


28


, a bottom edge of the powder container


52


wipes a top portion of the fed powder. This makes possible to accurately feed a predetermined amount of powder m which is to be pressed into compact, into the cavity


28


. In order to properly adjust the amount of the powder by the wiping operation, the net member


56


is attached closely to the surface of the die


20


, at the bottom portion of the powder container


52


. The net member


56


is spaced from the surface of the die


20


by a distance d


2


, which is preferably smaller than 2 mm, and more preferably smaller than 1 mm.




If the gap d


2


between the net member


56


and the surface of the die


20


is small as described, only a small amount of the powder m is allowed to project from within the cavity


28


above the upper surface of the die


20


. Therefore, an amount of the extra powder m to be wiped is small, and a lump of the powder resulting from the wiping operation by the powder container


52


is not fed into the cavity


28


in the next cycle of powder feeding. Further, it becomes possible to reduce an amount of powder m dropped between the surface of the die


20


and the net member


56


in a region other than the cavity


28


, making possible to prevent this extra amount of powder m from being fed (pushed) into the cavity


28


at the time of wiping. Further, even if the cavity


28


has corner and/or edge regions which are difficult to supply with the powder m as compared with a cavity center region, it is possible to prevent the powder m from projecting in the center region (i.e. to prevent extra amount of powder from being fed), and to uniformly feed the powder m in the corner and/or edge regions of the cavity


28


up to the surface of the die


20


.




As has been described, by attaching the net member


56


closely to the surface of the die


20


, it becomes possible to feed the powder m uniformly in the cavity


28


. It should be noted here that if the net member


56


is provided closely to the surface of the die


20


as described above, in order to prevent the net member


56


from contacting the surface of the die


20


, it is preferable that the net member


56


does not easily sag down. For this reason, the net member


56


is preferably made of a rolled mesh which is not distorted easily.





FIG. 5

is a graph showing a relationship of the distance (gap) d


2


between the net member


56


and the surface of die


20


with thickness inconsistency of the sintered compact (sintered body). The thickness inconsistency was measured as follows: First, block-like compacts each having a size of 55 mm width, 45 mm length and 16 mm height were manufactured by the pressing apparatus


10


. The compacts were then sintered, and then thickness measurements were made at a total of five locations, i.e. four locations near respective corners as well as one center location, on an upper surface of the sintered body. The thickness inconsistency (percent) was calculated by dividing a difference between a maximum measurement and a minimum measurement of the five measurements, by an average of the five measurements. For each setting of the gap d


2


, the thickness inconsistency was obtained for thirty sintered bodies, an average of which is then plotted on the graph as the thickness inconsistency (percent) at each particular gap d


2


.




As understood from the graph, the thickness inconsistency could be reduced to not greater than 4% when the gap d


2


is smaller than 2 mm, and compacts of a desired shape having a relatively uniform thickness could be manufactured. Also, it was learned from the graph that in order to reliably manufacture a compact having a small thickness inconsistency, the gap d


2


should preferably be smaller than 1 mm, and further, if the gap d


2


is set to not greater than 0.5 mm, it becomes possible to manufacture a highly accurate sintered body having a remarkably reduced thickness inconsistency.




As has been described, in the powder feeding apparatus


14


according to the present embodiment, the impactors


54


provide impulsive force to break down the lump of powder m in the powder container


52


, and to allow the powder m to be supplied into the cavity


28


through the relatively coarse net member


56


provided closely to the surface of the die


20


, whereby it became possible to feed the powder m uniformly regardless of the depth or region in the cavity


28


. Further, it became possible to remarkably reduce the time necessary for the powder supply. The powder feeding apparatus


14


according to the present embodiment was applied to the feeding operation of a rare-earth alloy powder which had poor flowability due to addition of a lubricant made of raw material to be described later, and was found to have a significant effect. Further, the effect was particularly remarkable when the depth of the cavity


28


to which the powder m was fed was not greater than 30 mm.




Now, description will cover an operation of the pressing apparatus


10


.




An inert gas such as nitrogen gas is supplied to the powder container


52


in the feeder box


32


. Under this state, the lid


50


of the feeder box


32


is opened, and the robot


46


supplies the powder container


52


with a predetermined amount of powder m measured in the feeder cup


42


. After supplying the powder m, the lid


50


is closed so as to maintain the inside of the powder container


52


filled with the inert gas. The supply of the inert gas into the powder container


52


is continuous, not only when the feeder box


32


is moving above the cavity


28


, in order to prevent the powder from catching fire. The inert gas may alternatively be argon or helium gas.




Under the above condition, the feeder box


32


containing the powder m is moved to above the cavity


28


, and then the powder supply is performed. As shown in FIG.


2


A and

FIG. 2B

, the powder supply is performed by driving the air cylinders


58


connected with the impactors


54


thereby applying impulsive force to the powder container


52


. By using the impactors


54


and thereby applying the impact multiple times continually, the powder m contained in the powder container


52


is supplied into the cavity


28


through the net member


56


.




A hitting pattern of the impactors


54


can be varied in many ways. For example, the pattern may be that the left impactor


54


hits the powder container


52


whereupon the right impactor


54


leaves the powder container


52


, and then the right impactor


54


hits the powder container


52


whereupon the left impactor


54


leaves the powder container


52


. Along with the hitting action, it is preferable that the powder container


52


is allowed to reciprocate on the die


20


, so that the powder container


52


itself is finely vibrated. By providing the impactors


54


to oppose each other, on the left and right sides, it becomes possible to supply the powder m into the cavity


28


in an appropriate hitting pattern that allows the powder m to easily enter the cavity


28


uniformly.




Reference is made to FIG.


6


. Now that the powder m is fed, the upper punches


24


begin to lower, and the coil


29


generates a magnetic field for orientation, which is applied to the powder m in the cavities


28


. The upper punches


24


and the lower punches


22


press the powder m in the cavities


28


, thereby forming compacts


66


in the cavities


28


. Thereafter, the upper punches


24


are raised, and the lower punches


22


are raised to push (to take) the compacts


66


out of the die


20


.

FIG. 6

shows a state in which the lower punches


22


have held up the compacts


66


entirely above the die


20


.




After the pressing operation is complete, the compacts


66


which are elevated by the lower punches


22


are placed onto a sintering plate


68


(thickness: 0.5 mm-3 mm) by an unillustrated transporting robot. The plate


68


is made of a molybdenum material for example. The compacts


66


are transported on the conveyer


70


, together with the plate


68


, into a sintering case


72


which is placed in a space filled with inert gas atmosphere such as nitrogen atmosphere. The sintering case


72


is preferably made of a thin molybdenum plate (thickness: 1 mm-3 mm approx.).




The sintering case


72


is provided with a plurality of molybdenum rods (supporting rods)


74


extending horizontally. The rods


74


support the plate


68


, on which the compacts


66


are placed, generally horizontally in the sintering case


72


.




Use of the sintering case


72


as described above allows a plurality of compacts


66


to be sintered efficiently in the sintering furnace while preventing the compacts


66


from being exposed within the furnace during the sintering, making possible to prevent such problems as oxidization of the compacts


66


.




Hereinafter, description will cover a method of manufacturing an R-T-(M)-B rare-earth magnet by using the powder feeding apparatus


14


.




In order to manufacture an R-T-(M)-B magnet, first, an R—Fe—B alloy is made by using a strip cast process, which is a known method of making an alloy by means of rapid quenching process (quenching speed: not slower than 10


2


° C./s and not faster than 10


4


° C./s). The strip cast process is disclosed in the U.S. Pat. No. 5,383,978 for example. Specifically, an alloy having a composition comprising


26


weight percent Nd, 5.0 weight percent Dy, 1.0 weight percent B, 0.2 weight percent Al, 0.9 weight percent Co, 0.2 weight percent Cu, with the rest of ingredient being Fe and unavoidable impurities is melted by a high-frequency melting process into a molten. The molten is maintained at 1,350° C., and then quenched on a single roll, yielding a flaky alloy having a thickness of 0.3 mm. Cooling conditions at this time include a roll peripheral speed of about 1 m/s, a cooling rate of 500° C./s, and a sub-cooling of 200° C. for example.




The obtained alloy flake is coarsely pulverized by means of a hydrogen occlusion milling, and then further milled in an nitrogen atmosphere by a jet mill, into a fine alloy powder having an average particle diameter of 3.5 μm approx. It is preferable that the amount of oxygen in the nitrogen atmosphere should be maintained at a low level, at around 10000 ppm for example. Such a jet mill as the above is disclosed in Japanese Patent Publication (of examined Application for opposition) No. 6-6728. Preferably, concentration of oxidizing gas (such as oxygen and moisture) contained in the atmosphere during the fine milling should be controlled, whereby oxygen content (weight) in the finely milled alloy powder is controlled not greater than 6000 ppm. If the oxygen content in the rare-earth alloy powder is excessive, beyond 6000 ppm, then the magnet contains non-magnetic oxide at a high rate, which deteriorates magnetic characteristic of the resulting sintered magnet.




Next, a lubricant is added to and mixed with the rare-earth alloy powder at a rate of 0.3 weight percent, for example, in a rocking mixer, so that particle surfaces of the alloy powder are coated with the lubricant. Preferably, the lubricant is a fatty acid ester diluted with a petrol solvent. According to the present embodiment, capronic acid methyl can be used as the fatty acid ester, and isoparaffin can be used as the petrol solvent, suitably. Weight ratio of the capronic acidmethyl to isoparaffin is 1:9 for example.




The kind of the lubricant is not limited to the above-mentioned. For example, besides capronic acid methyl, usable fatty ester includes capric acid methyl, lauryl acid methyl, and lauric acid methyl. As for the solvent, isoparaffin is representative but many others can be selected from petrol solvents, as well as naphthene and other solvents. The solvent may be added at a discretionary timing, i.e. before, during or after the fine milling. Further, a solid (dry) lubricant such as zinc stearate can be used together with the liquid lubricant.




Next, the pressing apparatus


10


is used to form compacts from the alloy powder described above.




First, the rare-earth alloy powder is fed in the feeder box


32


of the powder feeding apparatus


14


, and then the alloy powder is supplied from the feeder box


32


into the cavities


28


formed in the die


20


of the pressing apparatus


10


. By using the powder feeding apparatus


14


, the powder can be fed uniformly without forming a bridge for example, in the cavities


28


. Next, the rare-earth alloy powder in the cavities


28


is pressed (press formation) within a magnetic field, into compacts of a predetermined shape. The compacts are made to have a density of 4.3 g/cm


3


for example. According to the present embodiment, the powder feeding apparatus


14


feeds a predetermined amount of the rare-earth alloy powder uniformly in each of the cavities


28


. Therefore, by pressing the rare-earth alloy powder thus fed, compacts having a uniform density can be formed. Further, since the powder feeding apparatus


14


can uniformly feed a plurality of cavities at one time, crack development in the compact during the pressing operation can be prevented and therefore yields can be improved.




Particularly, if the depth of the cavity is not greater than 30 mm, inconsistent feeding of the rare-earth alloy powder into the cavity allows bridge formation by the rare-earth alloy powder, and can increase density inconsistency in the resulting compact. The powder teeing apparatus


14


can feed the powder uniformly even if the cavities are of such a shallow depth.




Thereafter, as shown in

FIG. 6

, the compacts placed on the sintering plate


68


are encased in a sintering case


72


, transported to a sintering apparatus, and then placed in a preparation chamber at an entrance of the sintering apparatus. The preparation chamber is then sealed, and atmosphere inside the preparation chamber is partially vacuumed to 2 Pa approx., in order to prevent oxidization. Next, the sintering case


72


is transported into a de-wax chamber, where a de-wax process (Temperature: 250° C.-600° C., Atmospheric pressure: 2 Pa, Time: 3 hours-6 hours) is performed. The de-wax process allows the lubricant (wax) that coats the particle surfaces of the magnetic powder to evaporate before the sintering process. In order to improve orientation of the magnetic powder at the time of pressing operation, the lubricant is mixed with the magnetic powder before the pressing operation, and is present between the particles of the magnetic powder. During the de-wax process, different gases such as organic gases, vapor and so on are released from the compacts. Therefore, it is preferable that a getter which can absorb these gases should be placed in advance in the sintering case


72


.




After completion of the de-wax process, the sintering case


72


is transported into a sintering chamber, where the compacts undergo a sintering process in an argon atmosphere at a temperature of 1000° C. -1100° C. for 2 hours-5 hours approx. During the process, the compacts are sintered while shrinking, into sintered bodies.




During the above process, since the compacts have a uniform density according to the present embodiment, the shrinkage inconsistency of the compacts in magnetically anisotropic directions is favorably small. Therefore, the sintered bodies can be finished into a predetermined size in a reduced working time, making possible to improve productivity.




Thereafter, the sintering case


72


is transported into a cooling chamber, and cooled to a room temperature. The sintered bodies thus cooled are then placed in an aging furnace to undergo a known aging process. The aging process is preformed under such conditions as within an argon atmosphere of 2 Pa approx., at a temperature of 400° C. -600° C. for 3 hours-7 hours. The sintered bodies may be taken out of the sintering case


72


onto a stainless steel mesh container before the aging process.




The sintered bodies of the rare-earth magnet thus manufactured to have a desired magnetic characteristic are then cut and polished into a desired shape. Since the sintered bodies have a favorably small size-inconsistency, working time for shaping operation can be reduced. Thereafter,the shaped magnets undergo surface treatment in order to improve weather resistance as necessary, including formation of a protective coating with such material as Ni and Sn, to be rare-earth magnets as a final product.




It should be noted that the rare-earth magnet manufactured by the method according to the present invention is not limited to the magnet of the composition described above. For example, the rare-earth element R can be provided by a raw material that includes at least one of the following elements: Y, La, Ca, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm and Lu. In order to attain a satisfactory level of magnetization however, it is preferable that at least 50 atomic percent of the rare-earth element R is provided by Pr or Nd, or combination of both.




The transition metal element T that can include Fe and Co may only include Fe. However, addition of Co raises Curie temperature and improves heat resistance. Preferably, at least 50 atomic percent of the transition metal element T should be provided by Fe, since the rate of Fe lower than 50 atomic percent decreases saturation magnetism of Nd


2


Fe


14


B type composites.




Addition of B is indispensable in order to allow stable crystallization of the tetragonal Nd


2


Fe


14


B type composites. The amount of B smaller than 4 atomic percent allows crystallization of R


2


T


17


phase, which reduces coercive force, resulting in excessive deformation of a desirable square pattern in demagnetizing curve. For this reason, it is preferable that B should be added at a rate not smaller than 4 atomic percent.




Other elements may be doped in order to further increase magnetic anisotropy of the powder. At least one selected from the following group of elements, Al, Ti, Cu, V, Cr, Ni, Ga, Zr, Nb, Mo, In, Sn, Hf, Ta, W can be preferably used as the doping element. The doping element M is not necessary for obtaining magnetically isotropic powder, but addition of Al, Cu, Ga and so on can increase intrinsic coercive force.




Next, reference is made to

FIG. 7

, and description will cover a powder container


76


used in a powder feeding apparatus


14




a


according to another embodiment. A plurality of partition plates


78


are provided inside the powder container


76


. With such a provision as the partition plates


78


, when the impactor


54


hits a side wall of the powder container


76


, the impulsive force can be transmitted dispersively to the powder m that is partitioned by the partition plates


78


in the powder container


76


, making possible to feed the powder m more efficiently. With such an arrangement, the time necessary for the powder feeding into the cavity


28


can be remarkably shortened. Vertical positions (along the height of powder container


76


) of the partition plates


78


are adjustable. By adjusting the position of partition plates


78


in accordance with the volume of the powder m held in the powder container


76


, the force can be distributed appropriately to the entire mass of the powder.




The net member provided at the bottom portion of the powder container may be varied. FIG.


8


A and

FIG. 8B

show such variation as a net member


80


and a net member


82


. As shown in

FIG. 8A

, the net member


80


includes two kinds of net assemblies


80




a


and


80




b


each having a different grid coarseness from each other. Likewise, as shown in

FIG. 8B

, the net member


82


includes two kinds of net assemblies


82




a


and


82




b


each having a different grid coarseness from each other. By changing the grid coarseness as the above, in accordance with locations in the net member, it becomes possible to control the amount of powder m to be fed into the cavities


28


according to region.




As has been described earlier, sometimes, corner and/or edge regions of the cavity


28


can receive a smaller amount of powder supply than a center region of the cavity


28


. In such a case, in order to supply the entire cavity


28


with the powder uniformly, it is preferable to make an arrangement to supply a greater amount of the powder m in the corner and/or edge regions of the cavity


28


.




For this reason, according to the net members


80


and


82


in FIG.


8


A and

FIG. 8B

, portions corresponding to the edge regions of the cavity


28


are respectively provided with coarser net assemblies


80




b


and


82




b


, whereas the portions corresponding to the center region are respectively provided with finer net assemblies


80




a


and


82




a


. With such an arrangement, it becomes possible to feed the edge regions of the cavity


28


with a greater amount of powder m than the center region.




Further, according to the net member


82


shown in

FIG. 8B

, the finer net assembly


82




a


is provided at a rear portion with respect to the moving direction (indicated by an arrow A in the figure) of the net member


82


during the wiping operation which is performed after the powder feeding. The region beneath the finer net assembly


82




a


gets less supply of the powder m. This is because the powder m scattered on the die


20


may be wiped into the edge region of the cavity


28


(the region corresponding to the finer net assembly) during the wiping operation, so the amount of the supply to the region is reduced in advance. Such an arrangement allows the entire cavity


28


to have uniformly fed with an appropriate amount of the powder m upon completion of the wiping.




Table 1 shows a result of experiment conducted to the embodiments of the present invention and a comparative example.




In Embodiment 1, the powder feeding apparatus


14


shown in

FIG. 2

was used to feed the cavities


28


with a rare-earth alloy powder, and then a pressing operation was performed to form compacts. In Embodiment 2, the powder feeding apparatus


14




a


shown in

FIG. 7

was used to form compacts. In Comparison 1, compacts were formed by using a shaker type powder feeding apparatus disclosed in Japanese Patent Laid-Open No. 2000-248301.




Each of the compacts formed as the above was sintered, and measurements were made to see thickness inconsistency and weight inconsistency of the sintered body. The thickness inconsistency was calculated as follows: First, for each of the sintered bodies, the thickness was measured at nine locations. Then, a difference between a maximum measurement and a minimum measurement of nine measurements was obtained, and the difference was divided by an average of the nine thickness measurements to obtain the thickness inconsistency. Note that the thickness inconsistency value given in Table 1 is an average of the thickness inconsistency values (percent) obtained for 200 sintered bodies. The weight inconsistency was calculated by first obtaining a difference between a maximum weight and a minimum weight of the 200 sintered bodies, and then dividing the difference by an average weight of the 200 sintered bodies. The feed time is a length of time needed for feeding the cavities with a certain amount of the powder.

















TABLE 1













Weight




Thickness








Feed




Inconsistency




Inconsistency







Method




Time




(R/Av)




(R/Av)




























Embodiment




Hitting-type




12 s




2.67%




1.54%






1




Feeding







Apparatus






Embodiment




Hitting-type




10 s




2.35%




1.12%






2




Feeding







Apparatus







plus







Partition







Plates






Comparative




Shaker-type




15 s




5.40%




2.74%






Example 1




feeding







Apparatus














From Table 1 given above, it is clear that as compared with the shaker-type powder feeding apparatus (Comparative Example 1) disclosed in Japanese Patent Laid-Open No. 2000-248301, the powder feeding apparatuses


14


and


14




a


(Embodiments 1 and 2) shown in FIG.


2


and

FIG. 7

respectively can feed more quickly and can decrease dimensional and weight inconsistency of the sintered body.




Next, reference is made to FIG.


9


A and

FIG. 9B

, which show a principal portion of a powder feeding apparatus


14




b


according to another embodiment. The powder feeding apparatus


14




b


comprises a vibration mechanism


84


connected to an upper portion of a powder container


52


. The vibration mechanism


84


is connected to a cylinder


86


such as an air cylinder. Further a pair of impactors


88


is attached to the enclosing member


48


so as to hit a lower portion of the powder container


52


. Each of the impactors


88


has a tip


90


made, for example, of a hard resin so that the hitting with the powder container


52


does not produce a spark. Other arrangements including mesh size of the net member


56


, the distance from the surface of the die


20


to the net member


56


are the same as in the powder feeding apparatus


14


shown in FIG.


2


A and FIG.


2


B.




According to the powder feeding apparatus


14




b


, the cylinder


86


drives the vibration mechanism


84


, and the vibration mechanism


84


vibrates the upper portion of the powder container


52


, whereby the impactors


88


are hit against the lower portion of the powder container


52


. The powder container


52


is moved in a stroke of 1 mm-15 mm for example.




According to the powder feeding apparatus


14




b


, the vibration mechanism


84


is disposed at an upper portion whereas the impactors


88


are disposed at a lower portion. By such separation, the impactors


88


can be disposed closer to the surface of the die


20


, making possible to apply the impact force more uniformly to the opening


56




a


of the powder container


52


which contains the powder m. Therefore, the powder m can be fed more uniformly and stably into the cavity


28


.




Further, if the powder m is provided by a very fine powder having, for example, an average particle diameter not greater than 10 μm, it becomes possible to reduce whirling of the powder m in a feeder box


32




b


out of the powder container


52


, making possible to prevent the powder m from being caught by sliding part between the enclosing member


48


and the air cylinder


86


for example.




Further, the powder m fed in the cavity


28


by using the powder feeding apparatus


14




b


can be pressed in the same way as in the embodiment shown in

FIG. 1

, and then sintered into a sintered magnet. In this way, a sintered magnet having a small inconsistency in size and weight can be obtained.




The powder feeding apparatus


14




b


offers generally the same effects as offered by the Embodiment 2 shown in the above Table 1.




Next, reference is made to FIG.


10


through

FIG. 14

, and description will cover a pressing apparatus


100


according to another embodiment of the present invention.




The powder pressing apparatus


100


comprises a pressing portion


112


and a powder feeding apparatus


114


.




The pressing portion


112


includes a die set


116


and a die tooling


118


. The die tooling


118


includes a die


120


, a lower punch


122


and an upper punch


124


. The die


120


has a saturated magnetism not smaller than 0.05 T and not greater than 1.2 T for example. The die


120


is fitted into the die set


116


. The lower punch


122


is disposed so as to be inserted into a die hole


126


from below. The die hole


126


is a through hole running vertically through the die


120


. An upper end surface of the lower punch


122


and an inner circumferential surface of the die hole


126


provide a cavity


128


of a variable volume. With this arrangement, the upper punch


124


is inserted into the cavity


128


, to press a powder m fed in the cavity


128


into a compact.




The powder feeding apparatus


114


includes a base plate


130


disposed in abutment on the die set


116


. On the base plate


130


, a feeder box


132


is disposed. The feeder box


132


is moved by a cylinder rod


136


of a cylinder


134


which is driven e.g. hydraulically or pneumatically (or by an electric servo motor), in a reciprocating pattern between a predetermined position on the die


120


and a stand-by position. Near the stand-by position of the feeder box


132


, there is provided a replenishing apparatus


138


for replenishing the feeder box


132


with the powder m. The replenishing apparatus


138


includes a weighing scale


140


, a feeder cup


142


, a vibrating trough


144


and a robot


146


. The operation of the replenishing apparatus


138


is the same as of the replenishing apparatus


38


described earlier, and therefore repetitive description will not be made.




As shown in FIG.


11


and

FIG. 12

, a shaker (may also be called agitator)


148


is provided inside the feeder box


132


. The shaker


148


includes a plurality of rod members


150


disposed in parallel with an upper surface of the die


120


and with an upper surface of the base plate


130


, and a plurality of generally U-shaped supporting members


152


. Each of the rod members


150


is made for example of a bar material having a circular section of a diameter not smaller than 3 mm and not greater than 10 mm. The bar material may be a square bar. The rod members


150


and the supporting members


152


are each made of a stainless steel (SUS


304


) for example. According to the present embodiment, three rod members


150


and three supporting members


152


are used. Each of the rod members


150


has its two end portions connected with one of the supporting members


152


, so that three sets of generally rectangular frame-like structure are provided. Two supporting rods


158


extend in parallel with each other, penetrating two side walls


154


,


156


, which are the walls across moving directions of the feeder box


132


. Each of the supporting members


152


has its upper portion connected to the two supporting rods


158


, whereby the supporting members


152


and the rod members


150


are fastened. Each supporting rod


158


has two ends respectively fastened by e.g. screws to connecting members


160


,


162


provided by e.g. strip-like pieces, and is connected with each other. The side wall


156


has an outer surface provided with a fixing hardware


164


, to which an air cylinder


166


is fixed. The air cylinder


166


has a cylinder shaft


168


fastened to the connecting member


162


. With this structure, the air cylinder


166


has two ends each supplied with air through an air supply tube


170


. This causes the cylinder shaft


168


to reciprocate, thereby reciprocating the shaker


148


. The rate of reciprocation is determined in accordance with the volume of powder to be fed.




Further, a gas supply pipe


172


is provided at a center upper portion of the side wall


156


of the feeder box


132


, for supplying an inert gas such as nitrogen gas into the feeder box


132


. The inert gas such as nitrogen gas is supplied into the feeder box


132


at a higher pressure than the normal atmospheric pressure in order to maintain the inside of the feeder box


132


filled with the inert gas. Because of this arrangement,even if friction is generated between the feeder box


132


and the powder by the reciprocating movement of the shaker


148


, this does not cause catching fire. Likewise, the feeder box


132


is moved, with the powder caught between a bottom surface of the feeder box


132


and the base plate


130


, but friction in this movement does not cause ignition either. Further, movement of the feeder box


132


generates friction among powder particles in the feeder box


132


, but this does not lead to ignition of the powder, either.




Further, a powder containing portion


174


of the feeder box


132


is maintained air tight by a lid


176


. When replenishing the powder m, in order to open an upper surface of the powder containing portion


174


, the lid


176


must be moved toward the cylinder


166


(in a rightward direction as in FIG.


13


). For this purpose, an air cylinder


178


which opens the lid


176


is provided on a side wall


180


. The lid


176


and the air cylinder


178


are connected with each other by a hardware


182


and fastened together by screws. In order to maintain the inside of the feeder box


132


filled with the inert gas, the lid


176


is disposed to cover the powder containing portion


174


of the feeder box


132


at normal times, and is moved toward the cylinder


166


only when the powder is replenished. The side wall


180


of the feeder box


132


is opposed by a side wall


184


, which is provided with guide means (not illustrated) so that the lid


176


can move smoothly during the open/close operation by the air cylinder


178


. With this arrangement, the air cylinder


178


has two ends each supplied with air through an air supply tube


186


. The air drives the cylinder shaft (not illustrated), thereby opening and closing the lid


176


.




The feeder box


132


has a bottom surface provided with a plate member


188


. The plate member


188


, made for example of a fluororesin, has a thickness of 5 mm and is fastened by screws. The feeder box


132


slides on the base plate


130


via the plate member


188


, which prevents the powder m from being caught between the feeder box


132


and the base plate


130


.




In addition, as shown in

FIG. 14

, a plurality of linear members


192


are disposed at an opening


190


of the feeder box


132


, in parallel with a direction of movement of the feeder box


132


. The opening


190


is larger than an upper opening of the cavity


128


. The linear members


192


are made of a nonmagnetic metal wire having a diameter of 0.15 mm approx. The linear members


192


are spaced at an interval not smaller than 2 mm and not greater than 4 mm. The rod members


150


are spaced from the linear members


192


by a distance not smaller than 0.5 mm and not greater than 10 mm. The diameter of the linear members


192


and the distance between the rod members


150


and the linear members


192


are adjusted in accordance with the size of the cavity


128


.




Further, a pair of magnetic field generating coils


194


is provided to sandwich the die set


116


, as orienting means. At a center of each coil


194


, a core


195


made of permendur for example is provided. By energizing the magnetic field generating coils


194


, an orienting magnetic field having a strength for example of 1.2 T is applied to the powder m in the cavity


128


, in a direction indicated by Arrow B, and the powder m is oriented.




Description will now cover an operation of the pressing apparatus


100


.




An inert gas such as nitrogen gas is supplied through the gas supply pipe


172


to the inside of the powder containing portion


174


of the feeder box


132


. Under this state, the lid


176


of the feeder box


132


is opened, and the robot


146


supplies the powder containing portion


174


with a predetermined amount of powder m from the feeder cup


142


. After supplying the powder m, the lid


176


is closed so as to maintain the inside atmosphere of the powder containing portion


174


filled with the inert gas. The supply of the inert gas into the powder containing portion


174


is continuous, not only when the feeder box


132


is moving above the cavity


128


, in order to prevent the powder from spontaneous ignition. The inert gas may alternatively be argon or helium gas.




Under the above condition, the air cylinder


134


is activated to move the feeder box


132


to above the cavity


128


of the die


120


. Then, the rod members


150


in the feeder box


132


are reciprocated 5 times-15 times for example in horizontal directions to allow the powder in the feeder box


132


to drop through a screen of linear members


192


into the cavity


128


, in the inert gas atmosphere. The above process allows supplying of the powder into the cavity


128


at a remarkably uniform feeding density, without any risk of ignition. During the process, the powder in the feeder box


132


does not drop naturally when the feeder box


132


comes above the cavity


128


. When the shaker


148


begins its pushing action, the powder begins to pass through the screen of the linear members


192


, being placed in the cavity


128


at a density suitable for the orientation.




After the powder m is fed in the cavity


128


, the feeder box


132


is receded, and then the upper punch


124


is lowered. Under this state, while the magnetic field generating coils


194


generate the orienting magnetic field, the powder m in the cavity


128


is pressed. During this process, the feeder box


132


which has been receded is replenished with the powder m. By repeating the above described cycle, the pressing operation of the powder m is performed continually.




According to the pressing apparatus


100


, even when the feeder box


132


is moved toward the cavity


128


as shown in

FIG. 15A

, and even after the feeder box


132


has moved above the cavity


128


as shown in

FIG. 15B

, the powder m does not fall into the cavity


128


since the powder m is in the state of bridging due to the linear members


192


provided at the opening


190


of the feeder box


132


. Thereafter, as shown in FIG.


15


C and

FIG. 15D

, each reciprocating stroke of the shaker


148


in the feeder box


132


allows a constant amount of the powder m to be placed in the cavity


128


generally uniformly. Specifically, the powder m is fed in the cavity


128


as illustrated in

FIG. 16

, and the powder m can be fed uniformly in the cavity


128


at a natural feeding density (1.7 g/cm


3


-2.0 g/cm


3


for example). As described, since the powder m is not fed at a high density, the powder particles can easily move, allowing a desired orientation by an orienting magnetic field of a relatively low strength. This makes possible to prevent manufacturing cost from increasing. Further, since the feeding can be made generally uniformly, a product having a superb magnetic characteristic can be obtained by orienting the powder m in the cavity


128


.




It should be noted that preferably the reciprocating operation of the shaker


148


should allow at least one of the rod members


150


to move from one side above the cavity


128


to the other side thereof. This setting allows more uniform feeding of the powder m into the cavity


128


.




By setting the distance between the rod members


150


and the linear members


192


to be not smaller than 0.5 mm and not greater than 10 mm, flow of the powder m near the linear members


192


is assisted, making possible to smoothly feed the powder m into the cavity


128


at a density suitable for the orientation. If the distance between the rod members


150


and the linear members


192


is smaller than 0.5 mm, the powder m between the rod members


150


and the linear members


192


develops intense friction with the rod members


150


and the linear members


192


, and the friction can cut the fine liner members


192


. On the other hand, if the distance between the two members exceeds 10 mm, it becomes impossible to let the powder pass through the screen of linear members


192


by the pushing action of the rod members


150


, and therefore a feeding suitable for orientation cannot be achieved.




Further, feeding by means of natural gravitational fall performed by the pressing apparatus


100


can improve flowability of the powder m at the time of magnetic orientation. Therefore, even if the powder m is made by a rapid quenching process, particles of the powder m can move easily in the cavity


128


. This makes possible to easily orient the powder m in a given magnetic direction, and to form a magnet having a high magnetic anisotropy for example.




Further, the interval between the linear members


192


should preferably be 2 mm-12 mm. If the interval is smaller than 2 mm, it becomes impossible to push the powder m in by the moving action of the rod members


150


. If the interval is grater than 12 mm, the feeding density becomes higher than the natural feeding density, since the bridging force above the cavity


128


is weak.




Further, by pressing the powder m which is uniformly fed in the cavity


128


, a compact of a highly uniform density can be obtained. Also, crack and fracture development and deformation due to inconsistent density can be prevented.




The compact is then transported to a sintering furnace and sintered in an argon atmosphere at a temperature of 1050° C. for two hours, and then aged in an argon atmosphere at a temperature of 600° C. for an hour, to be the sintered magnet. In this stage of sintered magnet, again, rate of defective products due to cracking and/or fracturing is decreased, and rate of after-sintering deformation is also decreased. Therefore, machining margin reserved for dimension correction can be decreased, which makes possible to improve yield in manufacturing process, to improve productivity of the sintered magnet, and to manufacture a sintered magnet having a favorable magnetic characteristic.




Further, by performing the pressing operation using the die


120


which has the saturated magnetism of not smaller than 0.05 T and not greater than 1.2 T, a uniform distribution of magnetic flux density is provided in the cavity


128


, and, it becomes possible to manufacture a sintered magnet without deformation.




Next, description will cover an experiment. The experiment was conducted to the pressing apparatus


100


and the pressing apparatus disclosed in Japanese Patent Laid-Open No.2000-248301 (conventional apparatus), and results were compared.




The experiment was conducted under the following conditions:












TABLE 2









Experiment conditions
























Compacts




Size: 80 mm × 52.2 mm × 20 mm







Number of compacts made per press: one







Raw material: Nd-Fe-B alloy powder







Produced by a strip cast process







(Average particle diameter: 2 μm-5 μm)







Capronic acid methyl was added as a lubricant.







Pressed density: 4.1 g/cm


3









Feeding density:







Pressing apparatus 10; 1.8 g/cm


3









Conventional Apparatus; 2.3 g/cm


3








Feeder Box




Shaking: 10 reciprocations in parallel with the







die surface (in both apparatuses)







Size of rod members: 3 mm diameter







Material of rod members: stainless steel







Size of linear members: 0.15 mm diameter







Material of linear members: copper







Spacing between liner members: 2 mm







Spacing between rod and linear members: 2 mm-4 mm






Pressing




Pressing method: Pressing in a magnetic field







Pressing was made while applying a magnetic field







perpendicularly to the pressing direction.







Die hole size: 80 mm × 52.2 mm







Depth of powder feeding: 50 mm






Measurement




Formed compacts were sintered, aged, cut and then







measured. Measurement was made for only one







sintered magnet which was sliced out of the center







portion. Measurement was made on a main surface







of the sintered magnet.














In this experiment, a compact as shown in

FIG. 17A

, which can be used in manufacturing a voice coil motor for example, was manufactured. The size of the compact was 80 mm×52.2 mm×20 mm. One compact was made per cycle of the pressing operation. Pressing was performed in a magnetic field, and the pressing was made while applying the magnetic field perpendicularly to the pressing direction (indicated by Arrow S in FIG.


17


A). The feeder box was a single-cavity feeding type. The shaker was reciprocated ten times in horizontal directions. The powder was a rare-earth alloy powder (Nd—Fe—B alloy powder). A strip cast process was used to produce the alloy powder having an average particle diameter of not smaller than 2 μm and not greater than 5 μm. A lubricant (capronic acid methyl) was added to the alloy powder. The compact shown in

FIG. 17A

was then sintered, aged, and then cut into sintered magnets. Of these sintered magnets, magnetic characteristic was measured for only one sintered magnet obtained from the center portion (corresponding to the shaded piece P in FIG.


17


A). The measurement was made on a main surface of the sintered magnet.




It was found that the conventional apparatus fed the cavity at a feeding density of 2.3 g/cm


3


approx. On the other hand, the pressing apparatus


100


according to the present invention fed at a desired feeding density of 1.8 g/cm


3


approx. Therefore, as understood from

FIG. 17B

, the sintered magnet obtained from the compact manufactured by the pressing apparatus


100


has an improved residual magnetic flux density Br and a maximum energy product (BH)max than the sintered magnet obtained from the compact manufactured by the conventional apparatus.




It should be noted that the pressing apparatus


100


may use the die


20


shown in

FIG. 1

, which is formed with a plurality of cavities


28


.




In this case, as shown in

FIG. 18

, an arrangement may be made so that each of the cavities


28


is fed with the powder m by one of the rod members


150




a.


In such an arrangement, preferably, a mutually adjacent pair of the rod members


150




a


should be spaced from each other by a distance generally equal to a center-to-center distance between the corresponding rows of the cavities


28


. In the above arrangement, in order for each rod member


150




a


to move from one side to the other side above the corresponding row of cavities


28


, the rod member


150




a


should only move in a stroke L


1


which is generally as wide as the cavity. Further, in the shaking action of the rod members


150




a


, none of the rod members


150




a


stops above an unrelated row of the cavities


28


, making possible to prevent non-uniform powder feeding. Further, weight inconsistency in the powder feeding can be decreased if a distance between each rod member


150




a


and the die


20


is set equally.




Further, as shown in

FIG. 19

, each of the cavities


28


may be fed with the powder m by all of the rod members


150




b


(three rod members according to this embodiment: The number of the rod members can be one or more). In this case, a stroke L


2


of the rod members


150




b


is set to allow all of the rod members


150




b


to move from one side to the other side above all the rows of cavities. In this case again, weight inconsistency in the powder feeding can be decreased if a distance between each rod member


150




b


and the die


20


is set equally.




Next, another experiment will be described.




In this experiment, a die formed with two cavities in a row in a direction of the feeder box movement was used to form two compacts (s intered bodies) per pressing cycle. The sintered body was for manufacture of a VCM (voice coil motor). During the pressing operation, a pressing direction of the powder was perpendicular to an orienting direction of the powder. The sintered bodies were manufactured respectively by using the powder feeding apparatus


114


shown in FIG.


10


and the conventional powder feeding apparatus disclosed in Japanese Patent Laid-Open No. 2000-248301, and comparison was made in terms of the weight distribution. Experiment conditions were as follows: The size and weight of the sintered body to be manufactured were set as 58.63 mm×36.9 mm×18.13 mm, and 217.7 g. The linear members used were provided by a wire of a 0.6 mm diameter made into a metal net having a sieve aperture of 6 mesh. A total of 300 blocks of compacts (sintered bodies) were manufactured in 150 continual stroke cycles of feeding and pressing.




A result of the experiment is shown in FIG.


20


A and FIG.


20


B. The weight inconsistency was improved by about 30%, from 9.22 gas achieved by the conventional apparatus to 6.04 g, proving improvement in feeding accuracy. As exemplified, use of the shaker


148


and the linear members


192


in the pressing apparatus formed with a plurality of cavities can also improve the weight inconsistency in feeding to the cavities, as compared with the conventional apparatus.




It should be noted that the die


120


should preferably be a low-magnetic metal die disclosed in Japanese Patent Laid-Open No. 2000-248301, or a metal die including a nonmagnetic die and highly magnetic yokes disposed on die hole side surfaces which are perpendicular to a direction of magnetic field application. By using such a metal die, it becomes possible to uniformalize magnetic flux density in the cavity


128


, and therefore to prevent the obtained compact from deforming when sintered.




The linear members


192


may be provided perpendicularly to the direction of movement of the feeder box


132


or may be made like a net, at the opening


190


of the feeder box


132


.




The present invention being thus far described and illustrated in detail, it is obvious that these description and drawings only represent an example of the present invention, and should not be interpreted as limiting the invention. The spirit and scope of the present invention is only limited by words used in the accompanied claims.



Claims
  • 1. A powder feeding apparatus for feeding a powder into a cavity formed in a die, comprising:a container including a bottom portion provided with a powder holding portion formed with a plurality of openings capable of allowing the powder to pass through; and an impactor capable of hitting against the container; wherein the impactor is hit against the container to give an impulsive force to the container, thereby feeding the powder contained in the container into the cavity via the openings.
  • 2. The apparatus according to claim 1, further comprising a vibrating mechanism connected to an upper portion of the container,wherein the impactor is provided so as to hit against a lower portion of the container, the vibrating mechanism vibrating an upper portion of the container, thereby allowing the impactor to hit against the lower portion of the container.
  • 3. The apparatus according to claim 1, wherein the powder holding portion is formed of a net having a mesh size of 2-14.
  • 4. The apparatus according to claim 1, wherein the powder holding portion is formed of a net having a mesh size of 2-8.
  • 5. The apparatus according to claim 1, wherein the powder holding portion is provided at a height smaller than 2.0 mm from a surface of the die.
  • 6. The apparatus according to claim 1, wherein the powder holding portion is provided at a height smaller than 1.0 mm from the surface of the die.
  • 7. The apparatus according to claim 1, wherein the container can move when the impulsive force is given to the container by the hitting of the impactor against the container.
  • 8. The apparatus according to claim 1, comprising a plurality of the impactors disposed outside of the container in an opposing relationship, with the container in between.
  • 9. The apparatus according to claim 1, further comprising a partition plate provided inside the container.
  • 10. The apparatus according to claim 1, wherein a size of the openings provided in the powder holding portion is in accordance with a location of the opening.
  • 11. The apparatus according to claim 1, wherein the powder is provided by a rare-earth alloy powder.
  • 12. The apparatus according to claim 11, wherein a lubricant is added to the powder.
  • 13. A sintered magnet manufacturing method comprising:a first step of applying an impulsive force by an impactor to a container which includes a bottom portion provided with a powder holding portion formed with a plurality of openings capable of allowing a powder to pass through, thereby feeding the powder contained in the container via the openings into a cavity formed in a die; a second step of forming a compact by pressing the powder fed in the cavity; and a third step of manufacturing a sintered magnet by sintering the compact.
  • 14. The method according to claim 13, wherein an upper portion of the container is vibrated, thereby applying the impulsive force to a lower portion of the container, in the first step.
  • 15. The method according to claim 13, wherein the powder is a rare-earth alloy powder,the method further comprising a step of adding a lubricant to the rare-earth alloy powder before the first step.
  • 16. A powder feeding apparatus for feeding a powder into a cavity formed in a die, comprisinga container for containing the powder, including a bottom portion provided with a net, wherein the net is provided at a height smaller than 2.0 mm from a surface of the die.
  • 17. A powder feeding apparatus for feeding a powder into a cavity formed in a die, comprisinga container including a bottom portion provided with a net, wherein a size of an opening of the net varies in accordance with a location of the opening.
  • 18. A pressing apparatus comprising:the powder feeding apparatus according to any one of claims 1 through 12, 16 or 17; and pressing means which presses the powder fed in the cavity by the powder feeding apparatus.
  • 19. A powder feeding apparatus for feeding a powder into a cavityformed in a die comprising: a feeder box movable to above the cavity, including a bottom portion formed with an opening, and for containing the powder; a rod member provided inside the feeder box for positioning the powder for the downward movement; a linear member provided at the opening of the feeder box; and orienting means which orients the powder fed from the feeder box in the cavity.
  • 20. The apparatus according to claim 19, wherein the rod member is spaced from the linear member by a distance not smaller than 0.5 mm and not greater than 10 mm.
  • 21. A pressing apparatus comprising:the powder feeding apparatus according to claim 19; and pressing means which presses the powder fed in the cavity by the powder feeding apparatus.
  • 22. A powder feeding method for feeding a powder into a cavity formed in a die, the method comprising:a step of moving a feeder box to above the cavity of the die, with the feeder box containing the powder, being provided inside thereof with a rod member movable in a horizontal direction, and having an opening provided with a linear member; a step of feeding the powder into the cavity while moving the rod member in the horizontal direction within the feeder box, when the feeder box is above the cavity; and a step of orienting the powder by applying an orienting magnetic field to the powder in the cavity.
  • 23. The method according to claim 22, wherein the powder is made by a rapid quenching process.
  • 24. The method according to claim 22, wherein the interval between the linear members is not smaller than 2 mm and not greater than 12 mm.
  • 25. A sintered magnet manufacturing method comprising:a step of obtaining a compact by pressing a powder in a cavity, the powder being fed by the method according to claim 22 or 23; and a step of manufacturing a sintered magnet by sintering the compact.
Priority Claims (2)
Number Date Country Kind
2000-276508 Sep 2000 JP
2001-020785 Jan 2001 JP
US Referenced Citations (2)
Number Name Date Kind
6299832 Kohara et al. Oct 2001 B1
6481993 Kohara et al. Nov 2002 B1
Foreign Referenced Citations (6)
Number Date Country
59-32568 Sep 1984 JP
59-40560 Oct 1984 JP
61-147802 Jul 1986 JP
6-136403 May 1994 JP
11-49101 Feb 1999 JP
2000-248301 Sep 2000 JP