JP2005203555A - Manufacturing method of sintered magnet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a thin sintered magnet of high orientation by raising the fluidity of a compound and reducing the disturbance of surface orientation caused by friction, while restraining oxidation due to the rise of a formation temperature and an increase in remaining carbon amount due to an increase in resin amount. <P>SOLUTION: In the manufacturing method of the sintered magnet, the compound is formed by mixing a thermoplastic resin and magnetic material powder, the compound is formed to a molded item by an extrusion or injection molding machine, and the thermoplastic resin is degreased from the molded item and sintered. A supercritical fluid is dissolved inside the extrusion or injection molding machine and mixed with the compound, and the molded item is formed by extrusion or injection molding in a magnetic field. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method for producing a sintered magnet in which a compound in which a thermoplastic resin and a magnetic material powder are mixed is extruded or injection-molded and the thermoplastic resin is degreased and then sintered. The present invention relates to a method for producing a thin-walled sintered magnet having a high orientation and excellent magnetic properties, for producing a highly oriented magnetic material powder-containing thermoplastic resin molding.

  In recent years, R-Fe-B permanent magnets have been used in a wide range of industrial fields as materials for magnetic circuits such as various motors, actuators, and sensors. In particular, focusing on the high energy product of this magnet, it has played an important role in reducing the size and weight in various fields from the viewpoint of environmental conservation and energy saving. Furthermore, in the information and communication field, the size and weight have been reduced, and there has been an increasing demand for magnets to be small and thin.

Unlike bonded magnets, Nd-Fe-B sintered magnets that are press-molded and manufactured by sintering do not contain a binder and can obtain high magnetic properties by densification by sintering. However, in press molding, if a certain amount of magnetic powder is not always supplied into the mold cavity, the thickness will vary, and if processing is not performed after sintering, it is not possible to supply magnets within the required dimensional tolerance in mass production. Impossible. The processing cost is extremely high in the entire manufacturing process, and it is difficult to obtain a priced product that satisfies customer requirements unless it is a small, thin product that is not processed.
In addition to press molding, as described in Patent Document 1, etc., a manufacturing method in which magnet powder and a thermoplastic resin are mixed and injection molded, or as described in Patent Document 2, magnet powder, water-soluble polymer and There has been proposed a production method in which water is mixed, injection molded, and sintered after dehydration and binder removal treatment. However, mass production of thin magnets having a uniform wall thickness of 1 mm or less without processing has not yet been studied and, of course, has not been implemented.
JP-A-9-283358 ((0005), FIG. 1) JP-A-7-130516 ((0042))

  As a result of the challenge of the present inventors to this compact, thin-walled, unprocessed sintered magnet, the use of a water-soluble binder has a problem with the compact strength and shape retention, so the dimensional accuracy of the sintered compact is low. It turned out that it was hard to put out. In compression molding, there is a limit to reducing the shape and thickness. On the other hand, when a thermoplastic resin is used, it is possible to produce a small or thin sintered magnet with high orientation and high dimensional accuracy by using thermoplastic resin, degreasing in hydrogen, setter, getter, etc. I found sex. However, as the wall thickness is reduced, the influence of the orientation disorder on the surface of the molded body due to the friction between the surface of the molded body and the mold becomes more prominent and the characteristics deteriorate. Trying to lower the friction by increasing the molding temperature or increasing the amount of resin and increasing the fluidity of the compound, but increasing the molding temperature promotes oxidation of the compound, and increasing the amount of resin causes an increase in the amount of residual carbon. In order to produce a thin magnet, it was necessary to increase the fluidity of the compound by other methods.

  Therefore, the present invention suppresses oxidation due to an increase in molding temperature and increases the residual carbon amount due to an increase in the amount of resin, while increasing the fluidity of the compound and reducing the disturbance of the surface orientation due to friction with the mold. It is an object of the present invention to provide a method for producing a sintered magnet.

As a result of the study by the present inventors, it has been recognized that it is most important to reduce the friction between the surface of the molded body and the mold in order to obtain a highly oriented thin magnet by extrusion or injection molding in a magnetic field. It is. As the wall thickness decreases, the influence of the friction of the mold increases, and the orientation of the surface of the extruded / injected molded body is disturbed, resulting in a decrease in magnetic properties.
That is, in order to solve the above-mentioned problem in the present invention, it is a compound in which a thermoplastic resin and a magnetic material powder are mixed, the compound is formed into a molded body by an extrusion or injection molding machine, and the thermoplastic resin is degreased from the molded body, A method for producing a sintered magnet to be sintered, which is a technical means in which a supercritical fluid is dissolved in the extrusion or injection molding machine and mixed with a compound to form a molded body by extrusion or injection molding in a magnetic field. It was adopted. Dissolving the supercritical fluid improves the fluidity of the compound, reduces the friction between the molded product and the mold when molding a thin molded product in a magnetic field, and forms a molded product with less surface disorder. It is possible. Further, the density of the supercritical fluid is lower than that of the thermoplastic resin. For this reason, when a supercritical fluid is mixed with a compound, the orientation of the magnetic material powder can be improved by the magnetic material powder being easily rotated in the magnetic field direction in a magnetic field. Furthermore, it is possible to lower the molding temperature and the amount of resin by improving the fluidity, and there are effects of reducing the production cost and the molding cycle. Supercritical fluid is a substance at pressure and temperature above the critical point, and has properties different from gas and liquid, such as diffusibility close to gas and solubility like liquid, and properties at temperature and pressure. It has features such as being able to control.

  When a fluid in which nitrogen or carbon dioxide is brought into a supercritical state together with a compound is injected into the molding machine and dissolved, the fluidity of the compound is drastically improved depending on the injection amount. Accordingly, the extrusion and injection pressures can be reduced, and the friction between the molded body and the mold surface can be reduced, so that the disturbance of the surface orientation that has greatly affected the thin-walled product can be greatly reduced. Further, a pressure drop occurs during extrusion and injection molding, and the fluid begins to foam. If the average diameter of the foamed cells exceeds 5 μm, it remains as a crack in the sintered body after degreasing and sintering, resulting in a decrease in mechanical strength and a decrease in magnetic properties. These effects can be improved by reducing the cell diameter to 5 μm or less (not including 0 μm). In particular, it is preferable to suppress to 3 μm or less. The cell diameter tends to decrease as the thickness of the compact is reduced, and the supercritical fluid behaves more favorably as the thinner magnet is made. The cell disappears due to shrinkage during sintering, and there is almost no influence on mechanical strength and properties, and only the advantage of improved fluidity can be used. Regarding the relationship between pressure, thickness and cell diameter, the greater the pressure drop during extrusion, the higher the cell density and the finer the cell. However, the thinner the sample, the less the cell growth and Become a cell.

  As the magnetic material powder, R-TM system (R is one or more rare earths including Y, T is Fe, or Fe and Co, M is Cu, S, Ni, Ti, Si, V, Nb, Ta, Cr if necessary. , Mo, W, Mn, Al, Sb, Ge, Sn, Zr, Hf, Ca, Mg, Sr, Ba, Be)), R-TM-B system, R-TM-N system rare earth magnet, Sr ferrite magnets, Ba ferrite magnets, LaCo-added ferrite magnets, and the like can also be used. Considering productivity and cost, it is most effective to use R-TM-B magnet powder. It is currently used as a magnet having the largest energy product, and there is a strong demand for downsizing and thinning, and there is a demand for a manufacturing method with high characteristics and low cost.

  The R content is preferably 32.5 wt% or more and 37.5 wt% or less as the composition of the R-TM-B rare earth magnet powder. If it is 32.5 wt% or less, R reacts with carbon and oxygen in the thermoplastic resin, the effective amount of R is depleted, and sufficient coercive force cannot be obtained. If it exceeds 37.5 wt%, the residual magnetic flux density decreases and effective magnetic properties cannot be obtained.

  Degreasing is preferably performed in a hydrogen atmosphere. Carbon is reduced by heating in a hydrogen atmosphere during degreasing. A resin mainly composed of carbon and hydrogen is heated in hydrogen to reduce the molecular weight due to cracking, so that degreasing can be performed more completely. Cracking means that the resin molecular weight is lowered by heating in the presence of hydrogen. Sintering can be performed in Ar or vacuum to suppress oxygen contamination in the compact.

  Examples of the thermoplastic resin include polyethylene (PE), active polypropylene (APP), polystyrene (PS), polypropylene (PP), amorphous polyolefin (APO), naphthalene, porestyrol (GPPS, HIPS), polycarbonate (PC), Acrylic (PMMA), polyacetyl (PC), acrylic (POM), polyacetyl (PC), vinyl chloride (PVC), ethylene-vinyl acetate copolymer (EVA) and the like can be used as appropriate.

  The present invention is effective when applied to the production of thin magnets having a molded body thickness of 1 mm or less. If the thickness is 1 mm or more, the influence of the surface orientation disturbance is reduced, and the effect of using the supercritical fluid is not sufficiently exhibited. In addition, as the wall thickness increases, the cell diameter of the supercritical fluid tends to increase, resulting in a decrease in characteristics. It is particularly preferable to use the film with a thickness of 0.6 mm or less where the influence of the surface orientation disturbance is large.

  By dissolving the supercritical fluid in the compound, the fluidity of the compound is improved, and even if a thin molded product is extruded / injected in a magnetic field, the friction force between the molded product and the mold is reduced. It is possible to mold a molded body with less surface disorder. In addition, the magnetic material powder is more easily rotated in the magnetic field direction than the thermoplastic resin, and the degree of orientation is improved. Further, by improving the fluidity, the amount of resin can be reduced even if the molding temperature is lowered, and the effects of reducing production costs and molding cycles can be obtained.

  EXAMPLES Next, although an Example demonstrates this invention concretely, this invention is not limited by these Examples.

(Example 1)
The details of the present invention will be described with reference to the drawings. FIG. 1 shows an example of an extrusion molding apparatus applied to the present invention. The injection molding apparatus has a hopper 1 for containing the compound 5, a valve 2 for controlling the flow rate of the supercritical fluid, a screw 3 for extruding the compound 5, and a compound 5 for molding the compound 5 into a predetermined shape. Mainly from the molding die 4 and the magnetic field generator 6 for orienting the magnetic powder in the compound in a certain direction.
First, an alloy consisting of Nd 32.5 wt%, Dy 3.0 wt%, B 1.05 wt% and the balance substantially Fe in weight percentage was dissolved by high frequency melting, and coarsely pulverized powder was prepared by strip casting. The coarsely pulverized powder was subjected to hydrogen storage and dehydrogenation treatment, and then made into a coarse powder of 500 μm or less by a randem mill. Paraffin wax was added to and mixed with 0.05 to 0.10 wt% of the coarse powder, and then finely pulverized by a jet mill. The grinding media for the jet mill was nitrogen gas, and the gas pressure was 0.69 MPa (7 kgf / cm ² ). The average particle diameter of the obtained alloy powder raw material is 4.2 μm. Compound 5 was prepared by kneading 93 wt% of alloy powder raw material and ethylene-vinyl acetate copolymer (EVA), polyethylene (PE), and paraffin wax (PW) as a thermoplastic resin balance at a resin weight ratio of 28:20:52. The supercritical fluid of nitrogen injected from the compound 5 and the valve 2 introduced into the barrel through the hopper 1 is mixed by the rotation of the pair of screws 3 and a shearing force is applied. The compound 5 was conveyed to the molding die 4 while being heated and melted at a temperature of 110 to 130 ° C., where it was narrowed down to a predetermined cross-sectional area in a magnetic field and passed through the molding space. The molded body is made anisotropic by the magnetic field generator 6, passes through a cooling mold, is extruded, and is discharged outside. The molded body was molded with a thickness of 0.6 mm and an extrusion speed of 1 to 2 cm / s and a cooling mold temperature of 70, 80 and 90 ° C.
This molded body was punched into a size of 7 mm square to produce a thin-walled molded body. Thereafter, the molded body was degreased in hydrogen at a heating rate of 20 ° C./h and a degreasing temperature of 615 ° C. Sintering was performed in Ar at a heating rate of 200 ° C./h and a sintering temperature of 1100 ° C.
The residual magnetic flux density, coercivity, and maximum energy product of the sintered thin-walled R-Fe-B sintered magnet were measured by VSM. Table 1 shows the results.

(Comparative Example 1)
The experiment was performed under the same conditions as in Example 1 except that no nitrogen supercritical fluid was used. Molded bodies were prepared at cooling mold temperatures of 70, 80, 90, 100, 110, and 120 ° C. Thereafter, a sample was prepared in the same manner as in Example 1, and the residual magnetic flux density, coercive force, and maximum energy product were measured by VSM. Table 1 shows the results. It was found that the mold temperature needs to be raised by about 30 ° C for molding because the fluidity is reduced by not using a supercritical fluid. Further, the characteristics tend to be improved by raising the mold temperature, but it has been found that even if the mold temperature is set to 120 ° C., characteristics equivalent to those of the thin magnet of the present invention cannot be obtained.

(Reference example)
A cooling part temperature was set to 100 ° C. in the same manner as in Example 1 to prepare a sample, and the residual magnetic flux density, coercive force, and maximum energy product were measured by VSM. Table 1 shows the results. If the temperature is raised and the fluidity is raised, the properties will drop. This is because the magnet powder oriented in the magnetic field is disturbed by the extrusion pressure if the fluidity is too high in the non-magnetic field immediately before being extruded to the outside.

(Example 2)
In the same manner as in Example 1, the mold cooling part temperature was fixed at 80 ° C., and the molded body thickness was set to 0.4, 0.8, 1.0, 1.2, and 2.0 mm. Residual magnetic flux density, coercive force, and maximum energy product were measured by VSM. The cell diameter was measured by SEM. The results are shown in Table 2 and FIG. As the thickness increases, the cell diameter also increases, cracks remain during sintering, the coercive force decreases rapidly, and the residual magnetic flux density also decreases.

(Comparative Example 2)
A thin-walled sintered magnet was produced in the same manner as in Example 1 without using a nitrogen supercritical fluid. Samples were prepared with the mold cooling part temperature fixed at 110 ° C. and the molded body thicknesses of 0.4, 0.8, 1.0, and 1.2 mm, and the residual magnetic flux density, coercive force, and maximum energy product were measured by VSM. The results are shown in Table 1 and FIG. The thinner the film becomes, the lower the characteristics are. However, when the supercritical fluid is not used, the decrease is remarkable.

(Reference Example 2)
Change the in-machine pressure near the die from 20MPa to 10MPa and 7MPa by changing the supercritical fluid injection pressure and molding pressure at the cooling part temperature of 80 ℃ and the molded body thickness of 0.6mm in the same way as in Example 1. Different samples were prepared, and the residual magnetic flux density, coercive force, and maximum energy product were measured by VSM, and the cell diameter was measured by SEM. The results are shown in Table 2. The data of Example 1-2 is also shown.

(Example 3)
The sample of Example 1-2 and the sample of Comparative Example 1-5 were observed by X-ray while polishing the surface, and the degree of orientation was compared at the peak intensity of (006). As a result, it was found that the sample of Example 1-2 had a higher degree of orientation near the surface. The use of supercritical fluid seems to have reduced the friction between the surface of the molded body and the mold and suppressed the disorder of orientation.

It is an example of the extrusion molding apparatus used for this invention. It is a figure which shows the relationship between the magnet thickness and magnetic characteristic in a sample for this invention and a comparison.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Hopper 2 Valve 3 Screw 4 Molding die 5 Compound 6 Magnetic field generator

Claims (6)

A method for producing a sintered magnet comprising a compound in which a thermoplastic resin and a magnetic material powder are mixed, the compound is formed into a molded body by an extrusion or injection molding machine, the thermoplastic resin is degreased from the molded body, and sintered. ,
A method for producing a sintered magnet, wherein a supercritical fluid is dissolved in an extrusion or injection molding machine and mixed with a compound to form a molded body by extrusion or injection molding in a magnetic field. The method for producing a sintered magnet according to claim 1, wherein the supercritical fluid is nitrogen or carbon dioxide. The magnetic material powder is an RTB alloy (R is one or more rare earth elements including Y, T is Fe and / or Co), and the R amount is 32.5 wt% or more and 37.5 wt% or less. 3. The method for producing a sintered magnet according to claim 1, wherein the sintered magnet is produced. The method for producing a sintered magnet according to any one of claims 1 to 3, wherein a foamed average cell diameter of the molded body is 5 µm or less. The method for producing a sintered magnet, wherein the molded body is a thin plate having a thickness of 1.0 mm or less. A method for producing a sintered magnet, wherein the compact is degreased in hydrogen gas and sintered in a vacuum or Ar gas atmosphere.

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