Patent Application
20060219323
The present invention relates to a magnetic field molding device, method for producing a ferrite magnet, and die which can be used for them.
Ferrite (sintered) magnets as prevailing magnets are produced by a series of steps of calcining a raw material mixture with a given composition into a ferrite state, milling the resulting calcined body into a fine ferrite powder of submicron size, compression-molding the powder using a die in a magnetic field (hereinafter referred to as magnetic field molding), and sintering the molded body into a ferrite magnet.
The processes for magnetic field molding fall into two general categories; dry process wherein the powder is molded as a dried material and wet process wherein the powder is molded as a slurry.
The wet magnetic field molding involves a problem of decreased production yield resulting from cracking or the like of the molded body, unless the slurry is dehydrated enough to remove its water content.
Therefore, there has been proposed a technique for improving the dehydration properties of the slurry in which the slurry be heated before it is injected into a die to reduce its viscosity and thereby to improve its dehydration properties, as disclosed in, e.g., Patent Documents 1, 2 and 3.
Patent Document 1 proposes a technique in which a heating device for heating a slurry is provided between a die assembly and a pressure pump for pumping the slurry to the die assembly.
This technique, however, which uses an electric heater tube or water bath as the heating device, involves a problem of needing a long heating time. Patent Document 2, in an attempt to solve the above problem, proposes a technique in which microwaves are used to uniformly heat the slurry in a shorter time.
Patent Document 3 proposes the following techniques. That is, the slurry in a tank is directly heated by a pipe heater or the like before being injected into the die; or indirectly heated by hot water or the like circulating over the tank; or the slurry is flowing in a pipe connecting the tank to the die, into which it is to be automatically injected, and the slurry is heated by heating the pipe periphery. Thereby, the slurry is kept from 40 through 90° C.
[Patent Document 1] Japanese Patent Publication No. 1-54167 (Claims)
[Patent Document 2] Japanese Patent Laid-Open No. 6-182728 (Claim 1)
[Patent Document 3] Japanese Patent Publication No. 2-13924 (Claims and page 3)
Problems to be Solved by the Invention
However, the inventors of the present invention have found that injection of the heated slurry into a die causes problems resulting from decreased temperature of the slurry and consequently increased viscosity of its dispersion medium, because it is quenched by the die or the like.
The technique disclosed by Patent Document 3 has the essence of keeping the slurry from 40 through 90° C. in a die, for which it is heated while it is held in a tank before being sent to the die directly by a pipe heater of the like or indirectly by hot water or the like circulating over the tank, or while it is flowing in a pipe to the die, into which it is to be automatically injected, by heating the pipe periphery, as described above. It is however practically difficult to keep the slurry from 40 through 90° C. in a die by the above-described heating procedure, because it is quenched when injected into the die, as described above. This has been experimentally confirmed.
These problems are particularly noted in such a case where a large-size die is used for providing a plurality of cavities therein, etc, in order to produce a plurality of molded bodies by one die, because the heat capacity of such a die is very large. In these cases, the conventional techniques are difficult to effectively solve the problem of cracking of molded bodies. Moreover, in a die provided with a plurality of cavities, slurry temperature may fluctuate cavity by cavity, depending on their positions in the die. This may cause fluctuation of dehydration properties of the slurry, cavity by cavity, and eventually density itself of the finally obtained molded bodies.
In addition, die temperature may change with ambient temperature and consequently viscosity of the dispersion medium in the slurry may change in a die, season by season, not to stabilize product quality.
The present invention has been developed to solve these technical problems. The objects of the present invention are to provide a magnetic field molding device, capable of improving yield in a production line and stabilizing product quality, method for producing a ferrite magnet and the like.
Means for Solving the Problems
The present invention provides a magnetic field molding device used in producing a ferrite magnet to solve the above problems, comprising a die into which a molding slurry, produced by dispersing a powder mainly composed of ferrite in a dispersion medium, is injected to be compression-molded; a magnetic field generating source which applies a magnetic field in a given direction to the slurry in the die; and a temperature control unit for controlling die temperature.
The temperature control unit can comprise a heater provided in the die for heating the die and a controller for controlling the heater. Alternately, the temperature control unit can also be constructed so as to comprise a flow path provided in the die, a pump for sending the liquid medium into the flow path and a heat source for heating the liquid medium.
The molding device can control slurry temperature by heating the die by the temperature control unit or the like to reduce viscosity of its dispersion medium and thereby to keep dehydration properties of the slurry at a high level during the magnetic field molding process.
The die is preferably kept from 40 through 120° C. by the temperature control unit, more preferably from 40 through 100° C., still more preferably from 40 through 80° C.
The above configuration is particularly effective for a large-size die or a die provided with a plurality of cavities for producing a plurality of ferrite magnets by one die.
When provided with a path for injecting the molding slurry into the individual cavities, the die can provides heat beforehand to the molding slurry while it is flowing in the delivery path towards the cavities.
The present invention can also be considered to be a method for producing a ferrite magnet. This method can comprise a molding step in which a slurry, e.g., that produced by dispersing a powder mainly composed of ferrite in a dispersion medium, is injected into a die kept from 40 through 120° C., to be compression-molded in a magnetic field of a given direction to obtain a molded body; and a sintering step in which the molded body is sintered into a ferrite magnet. It is preferable, also in this case, to keep the die from 40 through 100° C., more preferably from 40 through 80° C.
The present invention can also be considered to be a method for producing a ferrite magnet, comprising a slurry producing step in which a powder mainly composed of ferrite are dispersed in a dispersion medium to produce a molding slurry; a molding step in which the molding slurry with the dispersion medium controlled to have a viscosity of 0.70 [mPa·s] or less is compression-molded in a magnetic field of a given direction using a die; and a sintering step in which the molded body is sintered into a ferrite magnet. The dispersion medium is more preferably at a viscosity of 0.65 [mPa·s] or less.
It is preferable to keep the dispersion medium at a viscosity of 0.70 [mPa·s] or less by heating the die in the molding step thereby heating the molding slurry injected into the die.
The die of the present invention is used to compression-mold a molding slurry in which a powder mainly composed of ferrite is dispersed in a dispersion medium to produce a molded body of a given shape in the ferrite magnet production process. It is characterized by being provided with one or more cavities for obtaining a molded body, delivery path for injecting the molding slurry, supplied from the outside of the die, into the individual cavity (cavities) and a heater-holding mechanism provided to hold a heater for heating the die. The heater-holding mechanism is not structurally limited, but is preferably in the form of concavity, e.g., groove or hole, through which the heater is inserted into the die.
The die can have a heater assembled in the heater-holding mechanism, i.e., built-in type heater.
The die of the present invention can also be characterized by being provided with a liquid medium flow path in which the liquid medium can be heated by an external heat source to give a heat to the die when the liquid medium flows through the flow path.
In the die structure provided with a plurality of cavities, the delivery path preferably has a volume at least the same as the volume of molding slurry to be injected into a plurality of the cavities for one molding cycle, wherein the molding slurry volume for one molding cycle is a volume of a molding slurry including the materials corresponding to a total weight (dry basis) of the molded bodies produced by one molding cycle. This allows the slurry to be totally heated before it is injected into the cavities while the slurry previously charged in the cavities is compression-molded.
Moreover, the heater-holding mechanism is preferably provided along the delivery path. This allows the slurry flowing in the delivery path to be efficiently heated, when the heater is set in the heater-holding mechanism.
Still more, when a plurality of cavities are provided in the die, the individual delivery paths are preferably designed in such a way to have an almost equal length towards the individual cavities, thereby the slurry to be injected into the individual cavities can be uniformly heated.
The present invention heats a slurry to be injected into a die by controlling temperature of the die to reduce viscosity of the dispersion medium in the slurry. This allows the slurry to keep its dehydration properties at a high level during the molding process in a magnetic field, and to be efficiently dehydrated even in a large-size die or die provided with a plurality of cavities for producing many ferrite magnets by one die, to bring favorable effects, e.g., improved and stabilized product quality resulting from equalized density of the finally obtained molded body, reduced defective products, and improved yield in the production process.
The present invention is described in detail based on the embodiments by referring to the attached drawings.
As shown in
Next, the calcined body is milled by a preliminary milling step (Step S103) to produce a calcined powder composed of ferrite particles. It is then milled to a submicron size by a fine milling step (Step S104), after being added additives, as required, to produce a fine powder mainly composed of magnetoplumbite type ferrite. The preliminary and fine milling steps may be carried out by a wet or dry process. It is however preferable that the preliminary milling step is carried out by a dry process and fine milling step is carried out by a wet process, because the calcined body is generally composed of granules. In the above case, the calcined body is preliminarily milled to a given size or less in the preliminary milling step, and then made up into slurry with water and finely milled to a given size or less in the fine milling step.
Then, the finely milled powder is dispersed in a dispersion medium to produce the slurry (molding slurry) of given concentration, and the slurry is molded in a magnetic field. When the fine milling step is carried out by a wet process, the slurry may be concentrated in a dehydrating step (Step S105) to a given concentration.
The suitable dispersion media include water and liquids having a viscosity of 0.70 [mPa·s] or less at normal temperature (20° C.). These liquids include hexane, toluene, p-xylene and methanol or the like. Other dispersion media may also be used, so long as they have a viscosity of 0.70 [mPa·s] or less when injected into a heated die mentioned below.
The slurry is kneaded in Step S106, and injected into a die, where it is compression-molded in a magnetic field of a given direction in Step S107.
The molded body is sintered into the ferrite magnet in Step S108. It is then processed into a given shape to produce the ferrite magnet as the final product in Steps S109 to S110.
The magnetic field molding device 10 compression-molds a slurry of given concentration in a magnetic field to orient the ferrite particles to produce the molded body of given shape. As shown in
The mortar-shaped die 19 may also be stationary or vertically movable.
As illustrated in
In addition, as shown in
As shown in
The filter cloth 18 is provided over the mating surfaces between the upper die 11 and mortar-shaped die 19, to discharge moisture in the slurry from the cavity 13. It allows moisture in the slurry to trickle from the mating surfaces between the upper die 11 and mortar-shaped die 19 to the outside.
A magnetic field generating coil (not shown) or the like is provided in the vicinity of the upper die 11, to apply the magnetic field to the slurry in a given direction.
As shown in
Moreover, the concavities 19a are preferably arranged along the delivery path 14. This allows the heater members 20, set in the concavities 19a, to efficiently heat the slurry flowing in the delivery path 14.
The heater member 20 is connected to the heater power source 21. The heater member 20 generates heat, when a voltage is applied thereto from the heater power source 21, to heat the mortar-shaped die 19. The heater member 20 and heater power source 21 constitute the heater.
Still more, the sensor 22 of thermocouples or the like is provided to sense the temperature of the mortar-shaped die 19, and the controller 23 is also provided to control the heater power source 21 based on temperature sensed by the sensor 22.
The example of heating the mortar-shaped die 19 is described above. However, the die can be structured to heat the upper die 11 or the lower die 12 in a similar manner.
As the heater, it may employ a construction in which a liquid medium is heated. In this case, the mortar-shaped die 19 is provided with the flow path 30 for supplying the liquid medium, in place of the heater member 20, as shown in
In the magnetic field molding device 10 of the above structure, the slurry kneaded in the above-mentioned Step S106 is distributed/supplied by the pump 16 from the material container 15 to each of the cavities 13 between the upper die 11 and the lower die 12 via the delivery path 14. When the cavities 13 are filled with a given quantity of the slurry, the lower die 12 is driven to press the slurry at a given pressure between the upper die 11 and the lower die 12, while a magnetic field generated by the magnetic field generating coil (not shown) or the like is applied to the slurry. This molds the slurry into a given shape while it is dehydrated, with moisture in the slurry trickling to the outside via the filter cloth 18.
On completion of the molding, the upper die 11 is opened to release the molded body formed into a given shape from the lower die 12.
In the molding in a magnetic field, described above, the mortar-shaped die 19 is heated (regulated) by the heater member 20 to a given temperature level under the control by controller 23. It is preferable to keep the mortar-shaped die 19 at 40° C. or higher as temperature T1 sensed by the sensor 22. If the temperature T1 of the mortar-shaped die 19 is lower than 40° C., it is difficult to assuredly realize the slurry heating effect. If the temperature T1 of the mortar-shaped die 19 is more higher than 120° C., on the other hand, moisture in the slurry may boil, although depending on the cavity 13 internal pressure, i.e., slurry pressure. Therefore, the upper limit of temperature T1 of the mortar-shaped die 19 is preferably at 120° C. or lower, more preferably at 100° C. or lower, still more preferably at 80° C. or lower. It is therefore preferable to control heater power source 21 by the controller 23, based on the temperature level sensed by the sensor 22.
When, for example, the mortar-shaped die 19 is heated to T1 of 50° C., slurry temperature T2 will be at 43° C. in the cavity 13. T2 will be at 49° C. at T1 of 60° C.
Heating the mortar-shaped die 19 can increase slurry temperature in the cavity 13 more assuredly than heating the slurry before it is injected into the die, and consequently more efficiently reduces viscosity of the dispersion medium in the slurry and improves the dehydration properties of the slurry, thereby improving product yield. As discussed above, the cavities 13 can be uniformly heated even in a die provided with a plurality of cavities 13 or large-size die, to equalize density itself of the molded body as a result. Moreover, heating the mortar-shaped die 19 makes the magnetic field molding device 10 less sensitive to seasonally fluctuating ambient temperature, allowing it to produce a ferrite magnet of stable quality.
Moreover, the mortar-shaped die 19 is provided with the delivery path 14, by which the slurry is supplied to fill the cavities 13. The mortar-shaped die 19 is heated by the heater member 20, and the slurry is also heated while it is flowing in the delivery path 14. In other words, the slurry can be heated before being injected into the cavities 13. As a result, slurry temperature T2 in the cavities 13 can be increased. The heater member 20 for heating the mortar-shaped die 19 also works as the heat source for the slurry flowing in the delivery path 14, dispensing with any additional heat source to obtain the effects with simplifying the structure. In particular, the total volume of the delivery path 14 is set to be at least the same as the slurry volume to be injected for one molding cycle, and the slurry can be assuredly and efficiently heated in the delivery path 14 before being injected into the cavities 13 while the previous charge is molded in the cavities 13 and the above-mentioned effects are assuredly obtained. When, for example, 16 molded bodies each having a weight of 40 g (on a dry basis) are to be produced in one cycle, i.e., by the die provided with 16 cavities, the delivery path 14 preferably has a volume of 325 cm3 or more when the slurry has a concentration of 76% and density of 2.59 g/cm3.
When the total volume of the delivery path 14 is smaller than the slurry volume for one molding cycle, it is preferable to pre-heat the slurry by a heater or the like before it is sent into the delivery path 14 by the pump 16 from the material container 15.
The relationship between slurry temperature and cavity internal pressure was investigated. The results are described below.
First, the molding slurry was prepared by the process flow illustrated in
The slurry kept at a varying temperature level was injected into a disk-shape cavity (diameter: 30 mm) under constant conditions. Then, it was molded in a magnetic field under constant molding conditions, where the magnetic field molding device used was the same as the above-described magnetic field molding device 10, except that it was provided with one cavity (cavity 13), and provided with none of the heater member 20, heater power source 21, sensor 22 and controller 23. The highest pressure determined by a pressure sensor, provided in the close vicinity of the delivery path 14 and on the slurry injection route outside of the mortar-shaped die 19 was recorded as cavity internal pressure. The slurry was measured for its temperature 20 minutes after it was injected into the cavity, and was recorded as slurry temperature. Cavity internal pressure can be used as a measure of slurry dehydration properties; lower pressure being considered to indicate higher dehydration properties. The results are given in
As illustrated in
Next, the relationship between die temperature and cavity internal pressure was investigated. The results are described below.
First, the molding slurry (solid content in the slurry is 76%) was prepared by the process flow illustrated in
Then, the slurry was molded in a magnetic field using the magnetic field molding device 10, illustrated in
As shown in
Slurry temperature was at 36° C. when die temperature was set at 40° C. The dispersion medium (water) had a viscosity of 0.70 [mPa·s] at the above temperature level.
The present invention was compared with the conventional technique which heats the slurry beforehand. The results are described below.
First, a molding slurry (solid content in the slurry was 76%) was prepared by the process flow illustrated in
Then, the slurry was molded into a ferrite magnet of given shape (having an essentially arc-shape cross-section) and size under the following conditions.
The slurry was molded in a magnetic field using the magnetic field molding device 10, illustrated in
The slurry was molded in a magnetic field using the magnetic field molding device 10 into a molded body and the obtained molded body was sintered into a ferrite magnet, where temperature T1 of the mortar-shaped die 19 was kept at 60° C. by the heater member 20.
The slurry was molded in a magnetic field using the magnetic field molding device 10 into a molded body and the obtained molded body was sintered into a ferrite magnet, where temperature T1 of the mortar-shaped die 19 was kept at 100° C. by the heater member 20.
The slurry was molded in a magnetic field using the magnetic field molding device 10 into a molded body and the obtained molded body was sintered into a ferrite magnet, where the mortar-shaped die 19 was not heated by the heater member 20 and hence left at normal temperature.
The slurry was molded in a magnetic field using the magnetic field molding device 10 into a molded body and the obtained molded body was sintered into a ferrite magnet, where the mortar-shaped die 19 was not heated by the heater member 20 but the slurry was heated to 50° C. by a heater provided over the hose by which the slurry was supplied to the die from the material container 15 (this corresponds to a conventional technique).
The slurry was molded in a magnetic field using the magnetic field molding device 10 into a molded body and the obtained molded body was sintered into a ferrite magnet, where the mortar-shaped die 19 was not heated by the heater member 20 but the slurry was heated to 70° C. by a heater provided over the hose by which the slurry was supplied to the die from the material container 15 (this also corresponds to a conventional technique).
Slurry temperature T2 in the cavity 13 and cavity internal pressure were measured in each of EXAMPLES 1 to 3 and COMPARATIVE EXAMPLES 1 to 3. The results are given in
As shown in
It was also confirmed that cavity internal pressure was apparently lower in EXAMPLES 1 to 3 than in COMPARATIVE EXAMPLES 1 to 3, which coincides with the above results. Decreased pressure in the cavity indicates improved water releasing rate (i.e., dehydration properties), and allows the slurry to be molded in a shorter time.
The ferrite magnets prepared were tested. The results are given in
As shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| 2004-103415 | Mar 2004 | JP | national |
| 2004-375507 | Dec 2004 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP05/06026 | 3/30/2005 | WO | 2/7/2006 |
