PROCESS FOR PRODUCING RARE EARTH BOND MAGNET

Abstract: A process for producing a rare earth bond magnet comprising the steps of first mixing rare earth magnetic powder, a binding resin and an anti-oxidant at a predetermined ratio, kneading the mixture at a temperature higher than a thermal deformation temperature of the binding resin, granulating or classifying the kneadate to a mean particle diameter of, e.g., about 0.01 to 2 mm, press-molding the resulting granular material at a first temperature at which the binding resin is softened or molten, and cooling the resulting molded article under a pressure to a second temperature lower than at least the first temperature. Consequently, a rare earth bond magnet having a low porosity, high dimensional accuracy and excellent magnetic characteristics can be obtained.

Description: The present invention relates to a method of manufacturing a rare earth bonded magnet by binding magnet powder containing rare earth elements by a binder. Bonded magnets are manufactured from mixtures (compounds) of magnet powders and binding resins (organic binders), by pressure-molding the mixtures into desired magnet shapes. Among such bonded magnets, a special type of bonded magnet referred to as a rare earth bonded magnet is made up from a magnet powder which is composed of a magnetic material containing a rare earth element or elements. Methods of manufacturing rare earth bonded magnets are disclosed, for example, in JP-B-53-34640, JP-B-46-31841, JP-B-04-74421, JP-A-59-136907, JP-A-59-213104, JP-A-02-153509, JP-A-60-211908, JP-A-60-216523, JP-A-61-164215, JP-A-59-103309 and JP-A-03-108301. The methods of manufacturing rare earth bonded magnets are broadly sorted into compaction molding, injection molding and extrusion molding. In compaction molding, the aforesaid compound is packed in a press mold and compacted at a room temperature so as to form a green body. Subsequently, when the binding resin is a thermosetting resin, the resin is hardened, whereby a magnet is obtained. This method enables the molding to be carried out with a smaller amount of binding resin than other methods, resulting in a smaller resin content in the product magnet, thus advantageously contributing to improvement in the magnetic characteristics of the magnet. Extrusion molding is a method in which a heated molten compound extruded from an extruder die is solidified by cooling and then cut at a desired length, whereby a magnet is obtained. This method on the one hand offers an advantage in that it permits easy production of thin-walled or elongated magnets by virtue of a comparatively large molding versatility on the shape of the product magnet, but on the other hand suffers from a problem in that it requires, in order to ensure a sufficiently high fluidity of the molten compound during the molding, a greater amount of binding resin to be used as compared with the compaction molding method, with the result that the magnetic characteristics are impaired due to the increased resin content in the product magnet. In injection molding, the aforesaid compound, which has been heated and molten to exhibit sufficiently high fluidity, is poured into a mold so as to form a magnet of a desired shape. This method offers a molding versatility on the magnet shape even greater than that offered by the extrusion molding method, enabling easy fabrication of magnets having irregular configurations. However, this method requires a higher level of fluidity of the molten compound and, hence, a greater content of the binder resin than required in the extrusion molding method, resulting in poor magnetic characteristics of the product magnet due to the increased content of the binder resin in the product magnet. Among these methods, the compaction molding method enables production of magnets having superior magnetic performance as compared with other methods. Manufacture of bonded magnets by the known compaction molding method, however, suffers from the following disadvantages. Firstly, it is to be pointed out that rare earth bonded magnets manufactured by this method tend to exhibit high porosity, which reduces mechanical strength and corrosion resistance of the product magnet. Hitherto, therefore, countermeasures have been taken in the compaction molding, such as use of high-pressure molding technique which employs a compaction pressure as high as 70 kgf/mm 2 and anti-corrosion coating on the molded product. Elevated compaction pressure, however, heavily burdens the mold and molding machine, which in turn requires greater dimensions of the mold and molding machine, incurring a rise of the production costs. In addition, the anti-corrosion coating does not achieve sufficient improvement in the resistance to corrosion. A second problem is as follows. The compound is pelletized before subjected to the molding. It is often difficult, however, to smoothly charge the pellets of the compound into the mold and to completely fill up the mold cavity. In addition, the pelletized compound does not permit delicate control of the rate of supply of the compound into the mold. A third problem is as follows. When compaction molding is conducted on a compound containing a thermosetting resin, the compaction is effected at room temperature, regardless of whether the thermosetting resin is of the type which is in solid phase at the room temperature or of the type which is in liquid phase at the room temperature. Therefore, when the former type of thermosetting resin, i.e., the solid-phase resin, is used, moldability of the material is impaired, tending to exhibit greater porosity than that obtained when a thermoplastic resin is used. In addition, mechanical strength also tends to be reduced due to inferior dispersibility of the resin and the magnet powder. When the later-mentioned resin, i.e., the liquid-phase resin, is used, physical properties of the resin tend to be sensitively varied in accordance with the molding environment, e.g., temperature and humidity, often resulting in inferior charging of the mold, although a green body of high density is obtainable. The second and third problems mentioned above cause the dimensions of the product magnets to substantially fluctuate from the target dimensions. Namely, the dimensional precision is impaired and the molding cannot be conducted at high degree of stability. These deficiencies are serious particularly when small-sized magnets are to be manufactured. In order to obtain a product magnet in conformity with the target dimensions despite the substantial fluctuation in the dimensions, it is necessary that the molded article has dimensions greater than the target dimensions and that such molded article is subjected to a secondary processing such as milling or grinding into final shape and dimensions. Such a secondary work increases the number of steps of the manufacturing process, and increases the risk of production of unacceptable products, with the results that the production efficiency is lowered and the cost of production is raised. The present inventors have discovered that one of the causes of the first to third problems described above is impropriety of factors such as method and conditions of preparation of the compound, molding conditions such as temperature, and post-molding conditions such as cooling condition. JP-A 6314605 discloses a method according to the pre-characterizing portion of claim 1, in which the pressure molding step is followed by a heating step to cure the resin. EP-A-0 561 445 discloses a compound for manufacturing a rare-earth bonded magnet. The compound comprises a rare-earth magnet powder of a particle size of 0.1 to 0.3 mm. To prevent the particles from oxidizing during subsequent processing each particle is separately coated with at least two layers. The first layer includes antioxidant constituents and the second, organic layer provides the adhesive binder for a subsequent compaction molding. It is an object of the present Invention is to provide a method of manufacturing a rare earth bonded magnet which has a low porosity and which excels in moldability, mechanical and magnetic characteristics and dimensional stability. This object is achieved with a method as claimed in claim 1. Preferred embodiments are subject-matter of the dependent claims. This method enables production of a rare earth bonded magnet which has a low porosity and which excels in moldability, mechanical characteristics, magnetic characteristics, and dimensional stability. In particular, it is to be noted that these advantageous features are obtainable with a reduced amount of the binder resin. Preferably, the binder resin is a thermoplastic resin. The use of a thermoplastic resin further offers further improvement in the moldability and further reduction of the porosity. It is also preferred that the kneading is conducted at a temperature not lower than the thermal deformation temperature of the binder resin, such that the surfaces of the rare earth magnet powder particles are coated with molten or softened binder resin component. This offers an improvement in the kneading efficiency so as to ensure a more uniform kneading, contributing to the reduction in the porosity. Preferably, the content of the rare earth magnet powder in the kneaded material ranges from 90 wt% to 99 wt%. This contributes to an improvement in the magnetic characteristics, because the binder resin content in the magnet is reduced correspondingly. It is also preferred that the kneaded material contains an antioxidant. The use of an antioxidant suppresses oxidation, degradation and denaturation of the rare earth magnet powder and of the binder resin in the course of the manufacture, thus contributing to the improvement in the magnetic characteristics. Preferably, the average granule size of the granular material ranges from 0.01 to 2 mm. This ensures quantitative feed of the material while maintaining low porosity, offering a high degree of dimensional precision of the rare earth bonded magnet. Preferably, the second temperature is the melting temperature or the thermal deformation temperature of the binder resin. It is also preferred that the pressure applied to the molded body during cooling under pressure is maintained constant at least in the period until the temperature falls to a temperature between the first and second temperatures. These features render more remarkable the effects of reduction in the porosity and improvement in the dimensional precision. A detailed description will now be given of the method of manufacturing a rare earth bonded magnet and the rare earth bonded magnet resulting from the method. The method of the present invention for manufacturing a rare earth bonded magnet has the following major steps. <1> Preparation of Composition as Material of Rare Earth Bonded Magnet The first step of the process is to prepare a composition which is to be used as the material of a rare earth bonded magnet (this composition will be referred to simply as "composition", hereinafter). The composition is mainly composed of a rare earth magnet powder and a binder resin, and preferably contains an antioxidant. Other additives are also contained as necessitated. These components are mixed together by means of a mixer such as a Henschel mixer or a blender, followed by kneading so as to form a kneaded compound. A description will be given of each of these components. 1. Rare Earth Magnet Powder It is preferred that the rare earth magnet powder comprises an alloy of a rare earth element and a transition metal. More specifically, the following Samples [1] to [5] are preferred. [1] A magnetic alloy powder containing, as basic elements, a rare earth element R which is mainly constituted by Sm and a transition metal which is mainly constituted by Co (referred to as "R-Co type alloy", hereinafter). [2] A magnetic alloy powder containing, as basic elements, R (R is at least one element which is selected from rare earth elements and which includes Y), a transition metal constituted mainly by Fe, and B (referred to as "R-Fe-B type alloy", hereinafter). [3] A magnetic alloy powder containing, as basic elements, a rare earth element R which is mainly constituted by Sm, a transition metal which is mainly constituted by Fe and an interstitial element which is mainly N (referred to as "R-Fe-N type alloy", hereinafter). [4] A magnetic alloy powder containing, as basic elements, R (R is at least one element which is selected from rare earth elements and which includes Y), and a transition metal constituted mainly by Fe, and having magnetic phases on the size of nanometers (referred to as "nano-crystalline magnet", hereinafter). [5] A mixture of at least two of the compositions [1] to [4] set forth above. Use of such a mixture permits easy achievement of excellent magnetic characteristics by virtue of the combination of advantages offered by different types of magnet powder contained in the mixture. Typical examples of the R-Co type alloy are SmCo 5 and Sm 2 TM 17 , wherein TM represents a transition metal constituted mainly by Co. Typical examples of the R-Fe-B type alloy are Nd-Fe-B alloys, Pr-Fe-B alloys, Nd-Pr-Fe-B alloys, Ce-Nd-Fe-B alloys, Ce-Pr-Nd-Fe-B alloys and other alloys which are obtained by substituting part of Fe of the foregoing alloys with another transition metal such as Co, Ni or the like. A typical example of the R-Fe-N type alloy is Sm 2 Fe 17 N 3 which is prepared by nitriding Sm 2 Fe 17 alloy. Examples of the rare earth elements in the magnet powder are Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and misch metals. One, two or more of the elements listed above may be contained. Examples of the transition metal are Fe, Co, Ni and the like. One, two or more of these metals may be contained. In order to improve the magnetic characteristic, the magnet powder may contain, as required, an element or elements such as B, Al, Mo, Cu, Ga, Si, Ti, Ta, Zr, Hf, Ag, Zn or the like. Although not exclusive, the average particle size of the magnet powder preferably ranges from 0.5 to 100 µm and more preferably from 1 to 50 µm. The average particle size of the magnet powder can be measured by, for example, the F.S.S.S.(Fischer Sub-Sieve Sizer) method. The particle sizes may be uniform around the average size or may be distributed widely to some extent. Wide particle size distribution is preferred when it is desired to obtain excellent moldability during the molding with the use of a reduced amount of binder resin, as will be described later. Such a wide distribution of the particle size provides a reduced porosity of a bonded magnet as the product. In case of the mixture material [5] mentioned before, different kinds of magnet powder as the mixture components may have different average particle sizes. No restriction is posed on the method of preparing the magnet powder. For instance, magnet powder may be prepared by melt casting an alloy ingot, and pulverizing the alloy ingot into an appropriate size, followed by sieving. In an alternative method, a quenched ribbon (micro-fine polycrystalline structure) is formed by means of a quenching ribbon production apparatus of the type which is normally used for production of amorphous alloys, and pulverizing the ribbon into particles of suitable sizes, followed by sieving. The content of the magnet powder in the composition to be obtained preferably ranges from approximately 90 to 99 wt%, more preferably from approximately 92 to 99 wt% and most preferably from approximately 95 to 99 wt%. A too small magnet powder content hampers improvement in the magnetic characteristics, in particular the maximum magnetic energy product, and tends to deteriorate the dimensional precision. Conversely, a too large magnet powder content causes a reduction in the moldability, due to a consequent decrease in the binding resin content. 2. Binder resin The binder resin may be either a thermoplastic resin or a thermosetting resin. The use of a thermoplastic resin as the binder resin provides an advantage over the use of a thermosetting resin in that the magnet can have a smaller porosity. According to the present invention, a further reduction in the porosity can be achieved, by virtue of the molding temperature condition and cooling condition which will be described later. Examples of usable thermoplastic resins include polyamide resins (e.g., nylon 6, nylon 46, nylon 66, nylon 610, nylon 612, nylon 11, nylon 12, nylon 6-12 and nylon 6-66); liquid crystal polymers such as thermoplastic polyimide resins and aromatic polyester resins; polyphenylene oxide resins; polyphenylene sulfide resins; polyolefin resins such as polyethylene resins, polypropylene resins and ethylene-vinyl acetate copolymers; modified polyolefin resins; polyether resins; polyacetal resins; and copolymers, mixtures, and polymer alloys of the foregoing resins. One of these resins is used alone or two or more of these resins are used in the form of a mixture. Among these resins, polyamide resins or their copolymers are preferably used from the view points of moldability and mechanical strength, materials mainly constituted by liquid crystal polymers and polyphenylene sulfides are used from the view point of heat resistance, and materials mainly constituted by polyolefins are preferably used from the view points of ease of molding and economy. These resins also exhibit superior kneadability with the magnet powder. Preferably, the thermoplastic resin has a melting point of 120°C or higher, more preferably from 122°C to 400°C and most preferably from 125°C to 350°C. A melting point below the lower limit of the range specified above degrades the heat resistance of the molded magnet, making it difficult to obtain required levels of temperature characteristics (magnetic or mechanical). Conversely, a melting point exceeding the upper limit of the range specified above requires elevated molding temperature, tending to allow oxidation of the magnet powder. In order to further improve the moldability, the thermoplastic used in the invention preferably has an average molecular weight (polymerization degree) of from approximately 1000 to approximately 60000, more preferably from approximately 12000 to 35000. Examples of thermosetting resins suitably used are epoxy resins; phenol resins; urea resins; melamine resins; polyester resins (unsaturated polyester resins); polyether resins (e.g., polyether nitrile); polyurethane resins; and so forth. One of these resins may be used alone or two or more of them may be used in the form of a mixture. Among these resins, epoxy resins and phenol resins, in particular epoxy resins, are preferably used because they provide remarkable improvement in moldability, as well as superior mechanical strength. These thermoplastic resins excel also in the kneadability with magnet powder. The thermosetting resins, when used, are added in a state in which they have not yet been hardened, and may be in solid (powdered), liquid or semi-liquid phase at room temperature. The content of the binding resin in the composition preferably ranges from approximately 1 wt% to approximately 10 wt%, more preferably from approximately 1 wt% to approximately 8 wt%, and most preferably from approximately 1 wt% to approximately 5 wt%. A too large content of the binder resin does not contribute to improvement in the magnetic characteristics (in particular, maximum magnetic energy product) but, rather, tends to deteriorate the dimensional precision. Conversely, a too small binder resin content impairs moldability. 3. Antioxidant Antioxidant is an additive which is employed in the preparation of the composition of the present invention for the purpose of preventing oxidation deterioration of the rare earth magnet powder and denaturation of the same due to oxidation of the binder resin (this denaturation occurs as a result of catalytic action of the metallic components of the rare earth magnet powder). The addition of the antioxidant is effective in preventing oxidation of the rare earth magnet powder and contributes to improvement in the magnetic characteristic of the magnet, as well as to improvement in the thermal stability of the composition under kneading and molding. Thus, the antioxidant plays an important role of ensuring good moldability with a reduced amount of the binder resin. Since the antioxidant tends to be lost by evaporation or denaturated during kneading and molding into a magnet, only part of the antioxidant added remains and exists in the rare earth bonded magnet as the product. Any type of anti-oxidant which can prevent or suppress oxidation of the rare earth magnet powder may be used. For instance, suitably used are chelating agents which form chelate compounds with the metallic ions, in particular Fe ions, such as amine type compounds, aminoacid type compounds, nitrocarboxylic acid type compounds, hydrazine type compounds, cyanide type compounds, sulfide type compounds and so forth. Obviously, antioxidants mentioned above are only illustrative and antioxidants of other types and compositions than those listed above may be used. The content of the antioxidant when it is used in the composition preferably ranges from approximately 0.1 wt% to approximately 2 wt%, more preferably from approximately 0.5 wt% to 1.5 wt%. At the same time, the ratio of the antioxidant content to the binder resin content preferably ranges from approximately 2 percent to approximately 150 percent, more preferably from approximately 30 percent to approximately 100 %. In the present invention, the degree of oxidation of the magnet powder, as well as the degree of denaturation of the same due to oxidation of the binder resin, is ruled by the ratio of the content between the magnet powder and the binder resin. Needless to say, therefore, the composition of the invention may have an antioxidant content which falls below the range set forth above or may even be completely devoid of such antioxidant. The contents of the binder resin and the antioxidant are determined taking into account, for example, the following factors. A comparatively small binder resin content causes the magnet powder content to increase correspondingly, so that the viscosity of the mixture under kneading is enhanced to require a greater kneading torque, resulting in enhanced generation of heat and resultant promotion of oxidation. If the content of the antioxidant is small, oxidation of the resin cannot be sufficiently suppressed, which serves to increase the viscosity of the material under kneading (molten resin), with the results that the kneadability and moldability are impaired to hamper production of magnets which have low porosity and which excel in mechanical strength and dimensional stability. In contrast, when the content of the antioxidant is comparatively large, the binder resin content decreases correspondingly, tending to reduce the mechanical strength of the molded green body. Conversely, a greater binder resin content correspondingly decreases the content of the magnet powder, which in turn reduces the influence of the magnet powder on the resin, thus suppressing oxidation tendency of the resin. It is therefore possible to suppress oxidation of the resin even with a reduced amount of the antioxidant. Thus, a comparatively large content of the binder resin permits a reduction in the content of the antioxidant, whereas a smaller binder resin content requires the use of a greater amount of antioxidant. Thus, the total content of the binder resin and the antioxidant in the composition preferably ranges from 1.0 wt% to 8.0 wt%, more preferably from 2.0 wt% to 6.0 wt%. A total content falling within the range specified above contributes to improvement in the moldability and to enhancement of the anti-oxidation effect, thus offering advantages in the production of magnets having low porosity and being excellent both in mechanical strength and magnetic characteristics. 4. Other additives The composition may contain, as necessitated, other type or types of additives such as a plasticizer (e.g., aliphatic acid salt such as zinc stearate, and aliphatic acid such as oleic acid), a lubricant (e.g., silicone oil, wax, aliphatic acid, and inorganic lubricant such as alumina, silica and titania), a hardening agent, a hardening assistant, and other molding assistants. Addition of the plasticizer improves fluidity during the molding, so that desired characteristics can be achieved with the use of a reduced amount of binder resin. In addition, pressure molding can be carried out with a lower compacting pressure. These advantages are also obtainable through the use of a lubricant. Preferably, the content of the plasticizer is from approximately 0.01 wt% to approximately 0.2 wt%, while the content of lubricant is preferably from approximately 0.05 wt% to approximately 0.5 wt%. The rare earth magnet powder and the binder resin, preferably together with an antioxidant and together with other additives as required, are mixed to form a mixture which is used as the magnet molding composition. The mixing is conducted by using, for example, a mixer such as a Henschel mixer, or a blender. Preferably, the mixture is further kneaded to form a kneaded material which is used as the magnet molding composition. Such kneading may be performed by, for example, a twin-screw extrusion kneader, a roll-type kneader, or an ordinary kneader. The kneading may be conducted at a room temperature but is preferably conducted at a temperature not lower than the thermal deformation temperature of the binder resin (measured by a method specified by ASTM D648) or not lower than the softening point of the same, more preferably not lower than the melting point of the binder resin. For instance, when a polyamide resin (thermal deformation temperature 145°C, melting point 178°C) is used as the binder resin, the kneading temperature specifically preferred is from approximately 150°C to approximately 280°C, whereas, when a phenol novolak resin (softening point 80°C) is used, the specifically preferred kneading temperature is from approximately 80°C to approximately 150°C. The kneading time varies according to conditions such as the kind of the binder resin, type of the kneader used and the kneading temperature. Usually, however, the kneading time preferably is from about 3 minutes to about 120 minutes, more preferably from about 5 minutes to about 40 minutes. The kneading should be conducted sufficiently to obtain such a state that the surface of the powder particles of the rare earth magnet powder is coated with the molten or softened binder resin component. In order to obtain the above-mentioned state, when kneading is conducted at a temperature falling within the range specified above, the kneading time preferably ranges from approximately 5 minutes to 90 minutes, more preferably from approximately 5 minutes to 60 minutes, although the kneading time may vary according to factors such as the kind of the binder resin, type of the kneading machine used and the kneading temperature. The described kneading conditions serve to improve kneading efficiency and permit uniform kneading in a shorter time than that required when the kneading is conducted at room temperature. At the same time, since a comparatively low viscosity of the binder resin is maintained during the kneading, the particles of the rare earth magnet powder are uniformly coated by the binder resin. This contributes to a reduction in the porosity of the composition and, hence, the porosity in the magnet as the product. When n types of thermoplastic resins (n being an integer) are used in the form of a mixture, the aforesaid "thermal deformation temperature (or melting point) of the binder resin used" can be determined through the following calculation. It is assumed that the total amount of the thermoplastic resins is one weight part. Amounts of the respective thermoplastic resins are expressed by A 1 , A 2 , ...., A n , respectively, while the thermal deformation temperatures (or melting points) of the respective thermoplastic resins are expressed by T 1 , T 2 , .... T n . Then the thermal deformation temperature (or melting point) of the thermoplastic resin mixture is given by A 1 T 1 + A 2 T 2 + ... A n T n . This calculation method applies also to the cases where n types of thermoplastic resins are used in the form of a mixture. <2> Preparation of granules The mixture or kneaded composition prepared through composition preparation step <1> described above is then granulated or pelletized so as to become granules of a predetermined grain size. Although no restriction is posed on the method of granulation or pelletizing, the granulation or pelletizing is preferably conducted by crushing the kneaded composition. The crushing may be effected by, for example, a ball mill, a vibration mill, a breaker, a jet mill or a pin mill. The granules also may be prepared by using an extrusion pelletizer or even by a combination of the pelletizing and crushing. The grain size of the granules may be regulated through classification by means of, for example, a sieve. A description will now be given of the suitable size of the granules. Preferably, the maximum size of the granules is not greater than the minimum size of the mold gap (space to be filled with the granules). Specifically, the maximum size of the granules is not smaller than 0.2 mm, preferably not smaller than 0.5 mm. A maximum granule size exceeding the mold gap size hinders the filling of the mold with the granules or makes it difficult to control the amount of the granules to be charged, thus hampering improvement in the dimensional precision of the bonded magnet. Conversely, a too small maximum size of the granules tends to increase the porosity of the bonded magnet as the product. Preferably, the granules have an average size of from approximately 0.01 mm to approximately 2 mm, more preferably from approximately 0.02 mm to approximately 2 mm, and most preferably from approximately 0.05 mm to approximately 2 mm. An average granule size exceeding 2 mm makes it difficult to delicately control the amount of the granules charged in the mold, thus impairing quantitative control, failing to achieve the desired improvement in the dimensional precision, particularly when the dimensions of the magnet to be manufactured are small, i.e., when the size of the mold gap is small. On the other hand, granules of an average size less than 0.01 mm is not easy to prepare or requires a troublesome work in the preparation. In addition, a too small average granule size tends to increase the porosity of the bonded magnet as the product. The granule size is preferably uniform, although it may have a distribution to some extent. Such a uniform granule size enhances the packing density in the mold, leading to low porosity and high dimensional precision of the product bonded magnet. The term "granules" is used here to mean granular matter with a distinction from "pellets" which generally have greater sizes than granules. <3> Pressure Molding Pressure molding is performed on the rare earth bonded magnet composition, in particular the granules prepared in the preceding step <2> of preparing granules. A description will now be given of a pressure molding which is a representative example of the pressure molding. A predetermined amount of granules of the rare earth bonded magnet composition, obtained through a volumetric measurement such as, for example, grading method (a method in which a powder-like material is charged in a vessel by gravity and any surplus material protruding above the top of the vessel is scraped off) or through weighing, is charged in a mold gap of a pressure molding machine. The granular material thus charged in the mold is then molded by pressure with or without application of an aligning magnetic field. (The aligning magnetic field intensity may be, for example, from 5 to 20 KOe, and the aligning direction may be longitudinal, transverse or radial.) The pressure molding ,here, is warm molding. Namely, suitable treatment such as heating of the mold is conducted so as to elevate the temperature of the material under molding to a predetermined temperature (referred to as "first temperature", hereinafter) at which the binder resin, specifically a thermoplastic resin, is softened or molten. Thus, the first temperature is a temperature which is not lower than the thermal deformation temperature or not lower than the softening temperature of the binder resin used. More specifically, when the binder resin used is a thermoplastic resin, the first temperature is preferably not lower than the melting temperature of the resin, more preferably it ranges from the melting temperature to the temperature which is 200°C above the melting temperature, and most preferably from the melting temperature to the temperature which is 130°C above the melting temperature. For instance, when the thermoplastic resin used is a polyamide resin (melting temperature 178°C), the preferred material temperature at molding, i.e., the first temperature, should range from about 180°C to about 300°C. In contrast, when the binder resin used is phenol novolak resin (softening temperature 80°C) which is a thermosetting resin, the first temperature preferably falls within the range of from 80°C to 280°C. Pressure molding, when conducted at such a temperature, ensured high fluidity of the material under molding, thus enabling mass-production of magnets having low porosity, high mechanical strength and high degrees of shape and dimensional stability, even when the magnet is of a shape having a thin-walled portion, e.g., ring-shape, tabular shape or curved tabular shape, and even when the magnet is small in size or elongated, not to mention ordinary cylindrical or block shape. The compacting pressure employed in the pressure molding is preferably 60 kgf/mm 2 or less, more preferably from approximately 2 kgf/mm 2 to approximately 50 kgf/mm 2 , and most preferably from approximately 5 kgf/mm 2 to approximately 40 kgf/mm 2 . According to the present invention, it is possible to shape and mold bonded magnets having the foregoing advantageous features, even at such comparatively low levels of molding pressure, by virtue of the fact that the molding is conducted at the first temperature set forth above. Such low levels of molding pressure correspondingly reduces loads on the mold and the molding machine, eliminating necessity of use of mold and molding machines having greater size and power, while extending the lives of the same, thus offering an advantage in the manufacturing process. <4> Cooling After pressure molding, the molded body should be cooled. Preferably, the cooling is conducted down to a predetermined temperature (referred to as "second temperature") which is below the first temperature, while keeping the molded body under pressure. This cooling step, therefore, will be referred to as "cooling under pressure", hereinafter, Such cooling under pressure maintains the low porosity established in the course of the molding without change, thus ensuring low porosity, high dimensional stability and superior magnetic characteristics of the rare earth bonded magnet as the product. In order to achieve low porosity and high dimensional stability of the product bonded magnet, it is preferred that the second temperature (pressure releasing temperature) is set to be as low as possible. According to the present invention, the second temperature is preferably not higher than the melting temperature of the binder resin (specifically thermoplastic resin), and more preferably not higher than the thermal deformation temperature (softening temperature) of the resin used as the binder resin. Preferably, the difference between the first temperature and the second temperature is 20°C or greater, more preferably 50°C or greater. The greater the difference, the greater the effects in reducing porosity and enhancing dimensional precision. A comparatively large magnet powder content in the composition permits easier production of low-porosity bonded magnets, even if the second temperature is set to a comparatively high level. For instance, when the magnet powder content in the kneaded material is 94 % or greater, it is possible to obtain a low porosity (not greater than 4.5 % or not greater than 4.0 %), even when the second temperature is set to a temperature around the melting point or a temperature which is about 10°C higher than the melting temperature of the binder resin. The cooling under pressure may be executed after a temporal dismissal or reduction of the pressure from the molding pressure. For the purpose of simplification of the process and improvement in the dimensional precision, however, it is preferred that the cooling is conducted without dismissing the molding pressure. The pressure may be changed during the cooling. It is, however, preferred that the pressure is maintained constant at least in the period in which the temperature falls from the first temperature to the second temperature. More preferably, the pressure is maintained constant at least in the period in which the material temperature falls down to the melting temperature, still more particularly down to the thermal deformation temperature, of the resin used as the binder resin. Such control of the pressure offers a remarkable effect in the reduction of the porosity and enhancement of the dimensional precision. The change of the pressure during the cooling may be controlled in accordance with such a pattern that the pressure increases or decreases linearly or stepwise. The pressure (average pressure when the pressure changes by time) applied during the cooling under pressure should be equal to or below the molding pressure applied during the pressure molding. More preferably, the same level of pressure as the molding pressure is maintained, at the shortest, until the temperature comes down to the melting temperature of the binder resin. When any pressure is applied in the period in which the material cools from the melting temperature to the thermal deformation temperature of the binder resin, the pressure in such a period is preferably set to range from approximately 40 % to approximately 100 %, more preferably from approximately 50 % to 80 % of the molding pressure. The effects of reducing porosity and enhancing dimensional precision are rendered more remarkable by the described control of pressure. Needless to say, the present invention does not exclude continuation of the cooling without application of pressure, i.e., under normal atmospheric pressure, subsequent to the cooling under pressure, i.e., after the release of the pressure. It is also possible to conduct, subsequent to the cooling without application of pressure, a later-mentioned heat treatment, followed by another cooling under pressure. There is no restriction in the cooling rate (average cooling rate when the rate changes by time) during the cooling under pressure. The cooling rate, however, ranges preferably from 0.5°C/sec to 100°C/sec, more preferably from 1°C/sec to 80°C/sec. A too high cooling rate involves a risk of fine cracking in the molded body due to rapid contraction caused by the quick cooling, resulting in a reduction of the mechanical strength. Too quick cooling also enhances the internal stress which, when the molded body is removed from the mold, causes distortion or deformation of the body due to relaxation of the stress, leading to an inferior dimensional precision. Conversely, a too low cooling rate prolongs the cycle time, resulting in a reduction of the productivity. Cooling after release of the pressure, if such cooling is conducted, may be done at any desired rate. Thus, the foregoing ranges of the cooling rate may apply to such cooling conducted after release of the pressure. The rate of cooling under pressure, as well as the rate of cooling executed after release of pressure, may be constant or varied in relation to time. The cooling may be effected by any suitable method such as natural cooling, forced air cooling, water cooling, oil cooling, or a combination of water cooling and air cooling. <Heat Treatment> A heat treatment (baking) may be conducted after warm molding or after the cooling subsequent to such molding. The primary purpose of the heat treatment is to thermally cure the binder resin when the latter is a thermosetting resin, while the secondary purpose is to soften or melt the binder resin so as to enhance the bonding force thereby increasing the mechanical strength. The heat treatment intended for the primary purpose is to heat the thermosetting binder resin to a temperature not lower than the hardening temperature, and requires a comparatively long heating time of, for example, from 30 minutes to 4 hours. The heat treatment intended for the secondary purpose may be executed to produce appreciable effect. This heat treatment is conducted by heating the binder resin to a temperature not lower than the softening temperature, preferably not lower than the melting temperature, and the heating time may be as short as from, for example, 1 to 30 minutes or so. This heat-treating step may be executed in an overlap with the foregoing step <3> or subsequent to the step <3>, or even after the cooling step <4>. Obviously, the invention does not exclude any heat treatment which is intended for purposes other than the foregoing primary and secondary purposes. The rare earth bonded magnets of the present invention, manufactured by the method as described, exhibit the following excellent characteristics. More specifically, according to the Invention, the rare earth bonded magnet of the present Invention can have a low porosity, preferably 4.5 vol% or less, more preferably 4.0 vol% or less and most preferably 3.0 vol% or less. Such low porosity, i.e., high density, offers greater mechanical strength, as well as improved corrosion resistance, and provides a high dimensional precision which ensured reduced fluctuation of dimensions, thus realizing superior dimensional stability. The rare earth bonded magnet in accordance with the present invention also excels in the magnetic characteristics. Even isotropic magnets exhibit superior magnetic characteristics by virtue of the composition of the magnet powder and the large content of the magnet powder. The content of the rare earth magnet powder in the rare earth bonded magnet of the present invention ranges preferably from approximately 92 wt% to approximately 99 wt%, more preferably from approximately 94 wt% to approximately 99 wt%, and most preferably from approximately 96 wt% to approximately 99 wt%. A too small content of the magnet powder does not contribute to an improvement in the magnetic characteristic (in particular maximum magnetic energy product) and tends to impair the dimensional precision. Conversely, a too large content of the magnet powder causes the binder resin content to decrease correspondingly, resulting in an inferior moldability. The rare earth bonded magnet of the invention, when manufactured without magnetic field, preferably possesses a maximum magnetic energy product (BH)max of 6 MGOe or greater, more preferably 8 MGOe or greater. When manufactured under the influence of a magnetic field, the magnet possesses a maximum magnetic energy product (BH)max of 12 MGOe or greater, more preferably 13 MGOe or greater. The rare earth bonded magnet of the present invention, which has the superior magnetic characteristics and high dimensional precision as described, offers excellent performance of a device such as a motor incorporating this magnet. No restriction is posed in regard to the shape and dimensions of the rare earth bonded magnet of the present invention. Thus, the magnet can have any desired shape or configuration such as cylindrical shape, polygonal columnar shape, tubular shape, arcuate shape, flat tabular shape or curved tabular shape. The size may vary from large to small. Furthermore, there is no limitation in the purpose of use of the rare earth bonded magnet of the present invention. Examples of the present invention will now be described. (Example 1) The following magnet powder, binder resin (thermoplastic resin) and additive were mixed together to form a mixture which was subjected to kneading, and the kneaded matter was granulated (grained) into granules. The granular material was charged in a mold of a molding machine and was pressure-molded (warm molding) without application of a magnetic field. After the molding, cooling was executed while maintaining the same level of pressure as the molding pressure, whereby Sample Nos. 1a to 9a of rare earth bonded magnets, having magnet powder particles bonded together by the solidified binder resin, were obtained. The amounts of the matters shown below are expressed in terms of the contents in the kneaded composition. Composition: Nd-Fe-B type magnet powder: Nd 12.0 Fe 77.8 Co 4.3 B 5.9 96.0 wt% (content in product magnet is almost the same as this value) Thermoplastic resin: Each of resins A to G shown in Table 1, 2.8 wt% Antioxidant: hydrazine-type antioxidant, 1.2 wt% Mixing: Mixed by means of Henschel mixer Kneading: Kneaded by a twin-screw kneader at temperatures shown in Table 2. Screw speed 100 to 300 r.p.m., kneading time (time of stay in the kneader) 5 to 15 minutes Granulation (Graining): Kneaded composition was granulated into granules of average size of 0.8 mm, though grinding and classification. Molding: Granulated material was charged into a mold at room temperature, and pressure-molded when temperature had been raised to a predetermined molding temperature (first temperature). As to the molding temperature and the molding pressure, reference is made to Table 2. Cooling: The molded body was cooled down to a predetermined pressure releasing temperature (second temperature) while the pressure was maintained. After the release of the pressure, cooling was continued down to the room temperature. Samples were thus obtained and removed from the molds. As to the pressure releasing temperature, reference is made to Table 2. The cooling rate in the cooling under pressure was 1°C/sec. Product shapes: Cylindrical shape (outside diameter 30 mm x inside diameter 28 mm x height 7 mm); Flat tabular shape (20 mm wide x 20 mm long x 3 mm thick) (For use as a mechanical strength test piece) The values of the thermal deformation temperature appearing in Table 1 were those obtained through measurement conducted in accordance with ASTM D 648 set forth below. ASTM D648: A test piece in an oil bath is supported at its both ends and loaded at its center by a loading bar so as to have a bending stress of 4.6 kgf/cm 2 . Oil temperature is raised at a rate of 2°C/minute, and the temperature at which the deflection reaches 0.254 mm is measured. Magnetic performance (magnetic flux density Br, coercive force iHc, maximum magnetic energy product (BH)max), as well as density, porosity, mechanical strength and corrosion resistance were examined with the samples of the rare earth bonded magnet thus obtained. The results are shown in Table 3. The items of evaluation appearing in Table 3 were measured in accordance with the following methods. Magnetic performance: Each sample was magnetized by a 40 KOe pulse and subjected to measurement by a D.C. magnetometer under application of a maximum magnetic field of 25 KOe. Alternatively, a magnet piece of 5 mm square and 1 mm thick was cut out of each sample and was subjected to measurement by a vibration specimen magnetometer (VSM). Density: Density was measured in accordance with the Archimedean method (submerging method). Porosity: Porosity was calculated from the weighed composition and the measured density of the molded body. Mechanical strength: Mechanical strength was measured through a punching shearing test. A machine named autograph, manufactured by Shimadzu Corporation, was used as the testing machine. The test was conducted at a shearing rate of 1.0 mm/min, by using a circular punch of 3 mm diameter. A flat tabular magnet was used as the test piece. Corrosion resistance: Molded magnets was placed in a thermo-humidistat oven maintaining an atmosphere of 80°C and 90 % humidity, and the time until rust was generated was measured. The sample magnets were taken out of the oven at every 50 hours for observation of their surfaces with an optical microscope (magnification 10). After elapse of 500 hours, the observation interval was changed to 500 hours. As will be clear from Table 3, Sample Nos. 1a to 9a of the rare earth bonded magnet of the present invention, manufactured by using a thermoplastic resin as the binder resin, have porosity values as small as 1 % or less, despite the low molding pressure employed in the molding. Thus, the bonded magnet in accordance with the invention has a high density which approximates the stoichiometric density and, accordingly, an extremely high mechanical strength. Furthermore, these samples of the bonded magnet showed sufficiently high corrosion resistance, even with no coating thereon. This is considered to be attributed to the small porosity which ensures that the binder resin uniformly covers the magnet powder particles. Sample Nos. 1a to 9a of the bonded magnet were cut and the cut surfaces were observed through electron-microscopic photography (SEM). The cut surface of each sample showed almost no pore, and it was confirmed also that the binder resin was uniformly distributed around the particles of the magnet powder particles. It will also be seen that the samples of the bonded magnet of the invention exhibit high levels of magnetic flux density Br, coercive force iHc and maximum magnetic energy product (BH)max, thus demonstrating excellent magnetic characteristics. (Example 2) The following magnet powder and binder resin (thermosetting resin) were mixed together to form a mixture which was subjected to kneading, and the kneaded matter was granulated (grained) into granules. The granular material was charged in a mold of a molding machine at the room temperature and was pressure-molded (warm molding) without application of a magnetic field. After the molding, the binding resin was hardened, whereby Sample Nos. 12a to 15a of rare earth bonded magnets were obtained. The amounts of the matters shown below are expressed in terms of the contents in the kneaded composition. Composition: Nd-Fe-B type magnet powder: Nd 12.0 Fe 77.8 Co 4.3 B 5.8 96.0 wt% (content in product magnet is almost the same as this value) Thermosetting resin: Each of resins shown in Table 4, 4.0 wt% (inclusive of hardening agent) Mixing: Mixed by means of a twin-cylinder mixer when the binder resin used was solid at room temperature. A blender was used when the binder resin was in liquid phase at the room temperature. Kneading: Kneaded by a kneader at temperatures shown in Table 5. Screw speed 50 -to 250 r.p.m., kneading time 30 minutes Granulation (Graining): Kneaded composition was granulated into granules of average size of 0.8 mm or smaller, through grinding and classification. Molding: Granulated material was charged into a mold at room temperature, and pressure-molded at a predetermined molding temperature. As to the molding temperature and the molding pressure, reference is made to Table 5. Cooling: The molded body was cooled down to a predetermined pressure releasing temperature and, after release of the pressure further cooled down to the room temperature. Samples were thus obtained and removed from the molds. The cooling was conducted by air cooling. As to the pressure releasing temperature, reference is made to Table 5. The cooling rate was 2°C/sec. Heat treatment: The tentative molded samples were placed in a thermostat oven, for the purpose of hardening the thermosetting resin. As to the hardening conditions, refer to Table 4. Product shapes: Cylindrical shape (outside diameter 30 mm x inside diameter 28 mm x height 7 mm); flat tabular shape (20 mm wide x 20 mm long x 3 mm thick) (For use as a mechanical strength test piece) Surface treatment: A 10 µm thick coating of epoxy resin was formed by spraying on each sample of the molded bodies to be subjected to a corrosion resistance test. Magnetic performance (maximum magnetic energy product (BH)max), as well as density, porosity, mechanical strength and corrosion resistance were examined with the samples of the rare earth bonded magnet thus obtained. The results are shown in Table 6. The method of evaluation of each item is the same as that in Example 1. Sample Nos. 12a to 15a of the bonded magnet were cut and the cut surfaces were observed through electron-microscopic photography. It was confirmed also that the binder resin was uniformly distributed around the magnet powder particles, although not few pores were observed in the core of the magnet. It will be seen from Tables 5 and 6 that the rare earth bonded magnet (in particular Sample Nos. 12a to 15a), manufactured by using thermosetting binder resins, requires higher levels of compacting pressure than those employed in the manufacture of the magnet using thermoplastic binder resins and exhibits correspondingly increased porosity. The increased porosity, however, is still as small as from 5 to 6 % or so. As to the corrosion resistance test, although the samples required a surface treatment, such a surface treatment may be light one, i.e., only a thin coating film had to be formed, and the samples showed extremely excellent corrosion resistance, even with such light surface treatment. This is attributable to the still relatively small porosity, as well as to the uniform distribution of the binder resin around the particles of the magnet powder as confirmed through the SEM observation. Namely, the magnet powder particles are uniformly coated with the binder resin which also serves to increase adhesion of the magnet powder particles. Since the degree of the required surface treatment is not substantial, high dimensional precision is maintained on the whole magnet as the product. (Example 3) The following magnet powder, binder resin (thermoplastic resin) and additive were mixed together to form a mixture which was subjected to kneading, and the kneaded matter was granulated (grained) into granules. The granular material was charged in a mold of a molding machine and was pressure-molded (warm molding) under the influence of a magnetic field. After the molding, cooling was executed while maintaining the same level of pressure as the molding pressure, whereby Sample Nos. 16a to 19a of rare earth bonded magnets were obtained. The amounts of the matters shown below are expressed in terms of the contents in the kneaded composition. Composition: Sm-Co type magnet powder: Sm(Co bal. Pe 0.32 Cu 0.06 Zr 0.016 ) 7.8 95.0 wt% (content in product magnet is almost the same as this value) Thermoplastic resin: PPS resin, 4.2 wt% Antioxidant: hydrazine-type antioxidant, 0.8 wt% Mixing: Mixed by means of twin-cylinder mixer Kneading: Kneaded by using various types of kneading machines. As to kneading conditions, refer to Table 7. Granulation (Graining): Kneaded composition was granulated into granules of average size of 0.8 mm, through grinding and classification. Molding: Granulated material was charged into a mold at room temperature, and pressure-molded under the influence of a transverse magnetic field (15 KOe) when temperature had been raised to a predetermined molding temperature (first temperature). The molding temperature was 320°C, while the molding pressure was 20 kgf/mm 2 . Cooling: The molded body was cooled down to a predetermined pressure releasing temperature (second temperature) which was 150°C while the pressure was maintained. After the release of the pressure, cooling was continued down to the room temperature. Samples were removed from the molds after demagnetization. The cooling was conducted by air cooling. The cooling rate in the cooling under pressure was 5°C/sec. Product shapes: Rectangular parallelopiped (11 mm long x 8 mm wide x 7 mm high, aligned in heightwise directionl; flat tabular shape (20 mm wide x 20 mm long x 3 mm thick) (For use as a mechanical strength test piece) Magnetic performance (maximum magnetic energy product (BH)max), as well as density, porosity, mechanical strength and corrosion resistance were examined with the samples of the rare earth magnet powder thus obtained. The results are shown in Table 8. Methods of evaluations of the items are the same as those in Example 1. As will be clear from Table 8, Sample Nos. 16a to 19a of the rare earth bonded magnet of the present invention have porosity values as small as 1 % or less. Thus, the bonded magnet in accordance with the invention has a high density and, accordingly, high levels of mechanical strength and corrosion resistance. Furthermore, Sample Nos. 16a to 19a were subjected to SEM observation similar to those of the preceding Examples. Almost no pore was observed, and it was confirmed also that the binder resin was uniformly distributed around the particles of the magnet powder particles. It is also understood that the maximum magnetic energy product (BH)max is large, thus demonstrating superior magnetic characteristics. (Comparative Example 1) The following magnet powder, binder resin (thermoplastic resin) and additive were mixed together to form a mixture which was then charged in a mold of a molding machine and was pressure-molded (warm molding) under the influence of a magnetic field. After the molding, cooling was executed without applying pressure, whereby Sample Nos. 20a and 21a of rare earth bonded magnets were obtained. The amounts of the matters shown below are expressed in terms of the contents in the mixture. Composition: Sm-Co type magnet powder: Sm(Co bal. Fe 0.32 Cu 0.06 Zr 0.016 ) 7.8 95.0 wt% (Sample No. 20a) 96.0 wt% (Sample No. 21a) Thermoplastic resin: PPS resin, 4.2 wt% (Sample No. 20a), 3.2 wt% (Sample No. 21a) Antioxidant: hydrazine-type antioxidant 0.8 wt% Mixing: Mixed by means of twin-cylinder mixer Molding: The mixture was charged into a mold at room temperature, and pressure-molded under the influence of a transverse magnetic field (15 KOe) when the temperature had been raised to a predetermined molding temperature. The molding temperature was 320°C, while the molding pressure was 20 kgf/mm 2 . Cooling: The molded body was demagnetized in the mold and was taken out of the mold, followed by cooling down to room temperature under atmospheric pressure. The cooling was conducted by air cooling. The cooling rate in the cooling was 5°C/sec. Product shapes: Rectangular parallelopiped (11 mm long x 8 mm wide x 7 mm high, aligned in heightwise direction); flat tabular shape (20 mm wide x 20 mm long x 3 mm thick) (For use as a mechanical strength test piece) Sample Nos. 20a and 21a of the magnet could not be formed into the desired shapes due to recession of the molded body and breakage of edges of the same, caused by deposition of the edge and end surfaces of the molded body to the punch of the molding machine, as a result of leakage of the resin during the molding. SEM observation of the molded bodies showed non-uniform distribution of the binder resin, allowing mixing of discrete magnet powder and binding resin. Many pores were also observed. Both Sample Nos. 20a and 21a were defective as stated above, and could not effectively be subjected to measurement of characteristics such as mechanical strength. (Comparative Example 2) The following magnet powder, binder resin (thermosetting resin) and additive were mixed together to form a mixture which was then charged in a mold of a molding machine and was pressure-molded (cold molding) under the influence of a magnetic field. After the molding, the resin was hardened, whereby Sample No. 22a of rare earth bonded magnets was obtained. The amounts of the matters shown below are expressed in terms of the contents in the mixture. Composition: Sm-Co type magnet powder: Sm(Co bal Fe 0.32 Cu 0.06 Zr 0.016 ) 7.8 96.0 wt% Thermosetting resin: Bisphenol A novolak resin (melting temperature 60°C) 3.6 wt% (inclusive of hardening agent) Antioxidant: Hydrazine-type antioxidant 0.4 wt% Mixing: Mixed by means of twin-cylinder mixer Molding: The mixture was charged into a mold at room temperature, and pressure-molded under the influence of a transverse magnetic field (15 KOe). The molding was conducted at the room temperature, while the molding pressure was 20 kgf/mm 2 . Heat treatment: The molded body was demagnetized in the mold and then taken out of the mold. The molded body was then subjected to a heat treatment conducted at 170°C for 4 hours, so as to harden the thermosetting resin. Product shapes: Rectangular parallelopiped (11 mm long x 8 mm wide x 7 mm high, aligned in heightwise direction); flat tabular shape (20 mm wide x 20 mm long x 3 mm thick) (For use as a mechanical strength test piece) Sample No. 22a of the magnet could not be formed into the desired shapes due to dropping of the magnet powder, recession of the molded body and breakage of edges of the same. This is attributed to a small bonding force between the magnet powder particles, insufficient distribution of the binder resin and presence of a residual magnetic flux in the powder even after demagnetization although the intensity is low, as a result of the molding conducted on the mixture at the room temperature by using a slid epoxy resin. SEM observation of the molded bodies showed non-uniform distribution of the binder resin, allowing mixing of discrete magnet powder and binding resin. Many pores were also observed. Sample No. 3a were defective as stated above, and could not effectively be subjected to measurement of characteristics such as mechanical strength. (Example 4) The following two types of magnet powder, binder resin (thermoplastic resin) and additive were mixed together to form a mixture which was subjected to kneading, and the kneaded matter was granulated (grained) into granules. The granular material was charged in a mold of a molding machine and was pressure-molded (warm molding) under the influence of a magnetic field. After the molding, cooling was executed while maintaining the same level of pressure as the molding pressure, whereby Sample Nos. 23a to 31a of rare earth bonded magnets were obtained. The amounts of the matters shown below are expressed in terms of the contents in the kneaded composition. Composition: Sm-Co type magnet powder: Sm(Co 0.672 Fe 0.22 Cu 0.08 Zr 0.028 ) 8.35 70.5 wt% (content in product magnet is almost the same as this value) Sm-Fe-N type magnet powder: Nd 12.0 Fe 77.8 Co 4.3 B 5.9 23.5 wt% Thermoplastic resin: Polyamide resin (nylon 12) 5.0 wt% Antioxidant: Phenol-type antioxidant 1.0 wt% Mixing: Mixed by means of Henschel mixer Kneading: Kneaded by a twin-screw kneader at 150°C to 300°C. Screw speed 100 to 300 r.p.m., kneading time (time of stay in the kneader) 5 to 15 minutes Granulation (Graining): Kneaded composition was granulated into granules of average sizes shown in Table 9, through grinding and classification. Molding: Granulated material was charged into a mold at room temperature by a grading method, and pressure-molded under the influence of a transverse magnetic field (15 KOe) when temperature had been raised to 220°C (first temperature). The molding pressure was 10 kgf/mm 2 . Cooling: The molded body was cooled down to a predetermined pressure releasing temperature of 100°C (second temperature) while the pressure was maintained. Samples were then taken out from the molds. The cooling was conducted by water cooling. The cooling rate in the cooling under pressure was 20°C/sec. Product shapes: Flat tabular shape (30 mm wide x 2.5 mm thick and x mm high, aligned in the heightwise direction) The weight, density, porosity, and height were examined with the samples of the rare earth bonded magnet thus obtained. The results are shown in Table 9. From Table 9, it will be seen that superior quantitative control can be achieved by virtue of the suitable selection of the granule size, thus enabling production of bonded magnets having low porosity and high dimensional precision. In particular, when the granule size falls within the range of from 0.01 mm to 2 mm as is the case with Sample Nos. 23a to 30a, both low porosity (1.5 wt% or less, in particular 1 % or less) and high dimensional precision (dimensional error being within ±5/100 mm) could be simultaneously obtained. (Example 5) The following type of magnet powder, binder resin (thermoplastic resin) and additive were mixed together to form a mixture which was subjected to kneading, and the kneaded matter was granulated (grained) into granules. The granular material was charged in a mold of a molding machine and was pressure-molded (warm molding) under the influence of a magnetic field. After the molding, cooling was executed while maintaining the same level of pressure as the molding pressure, whereby Sample Nos. 32a to 42a of rare earth bonded magnets were obtained. The amounts of the matters shown below are expressed in terms of the contents in the kneaded composition. Composition: Nd-Fe-B type magnet powder: Nd 12.6 Fe 69.3 Co 12.0 B 6.0 Zr 0.1 97.0 wt% (content in product magnet is almost the same as this value) Thermoplastic resin: Resin A or F shown in Table 1, 1.5 wt% each Antioxidant: Hydrazine-type antioxidant 1.4 wt% Lubricant: Zinc stearate 0.1 wt% Mixing: Mixed by means of Henschel mixer Kneading: Kneaded by a twin-screw kneader at 150°C to 350°C. Screw speed 100 to 300 r.p.m., kneading time (time of stay in the kneader) 5 to 10 minutes Granulation (Graining): Kneaded composition was granulated into granules of average size of 0.3 mm through grinding and classification. Molding: Granulated material was charged into a mold at room temperature, and pressure-molded under the influence of a radial magnetic field (15 KOe) when the temperature had been raised to molding temperature shown in Table 10 (first temperature). The molding pressure was 15 kgf/mm 2 . Cooling: The molded body was cooled down to a predetermined pressure releasing temperature of 100°C (second temperature) while the pressure was maintained. The pressure was released after demagnetization, followed by cooling down to room temperature. Samples were then taken out from the molds. The cooling was conducted by water cooling. The cooling rate in the cooling under pressure was 30°C/sec. Product shapes: Cylindrical shape (20 mm outside diameter x 18 mm inside diameter x 5 mm height, compacted in the heightwise direction); Flat tabular shape (20 mm square x 3 mm thick (for use as mechanical strength test piece) Magnetic performance (maximum magnetic energy product (BH)max), as well as density, porosity and mechanical strength were examined with the samples of the rare earth bonded magnets thus obtained. The results are shown in Table 10. Methods of evaluations of the items are the same as those in Example 1. As shown in Table 10, the binder resin was softened or molten when the molding temperature was set to be not lower than the thermal deformation temperature of the binder resin as is the case with Sample Nos. 32a to 42a, so that the molding could be successfully carried out. In particular, further reduction in porosity and, hence, further improvement in the magnetic performance cold be achieved when the molding was conducted at a temperature not lower than the melting temperature of the binder resin, as is the case with Sample Nos. 33a to 36a and 40a to 42a. (Example 6) The following type of magnet powder, binder resin (thermoplastic resin) and additive were mixed together to form a mixture which was subjected to kneading, and the kneaded matter was granulated (grained) into granules. The granular material was charged in a mold of a molding machine and was pressure-molded (warm molding) without application of a magnetic field. After the molding, cooling was executed while maintaining the same level of pressure as the molding pressure, whereby Sample Nos. 43a to 52a of rare earth bonded magnets were obtained. The amounts of the matters shown below are expressed in terms of the contents in the kneaded composition. Composition: Nano-crystalline Nd-Fe-B type magnet powder: Nd 5.5 Fe 6.6 B 18.5 Co 5 Cr 5 98.0 wt% (content in product magnet is almost the same as this value) Thermoplastic resin: Resin A or G shown in Table 1, 1.0 wt% each Antioxidant: Hydrazine-type antioxidant 1.0 wt% Lubricant: Zinc stearate 0.1 wt% Mixing: Mixed by means of Henschel mixer Kneading: Kneaded by a twin-screw kneader at 150°C to 350°C. Screw speed 100 to 300 r.p.m., kneading time (time of stay in the kneader) 10 to 15 minutes Granulation (Graining): Kneaded composition was granulated into granules of average size of 0.1 mm through grinding and classification. Molding: Granulated material was charged into a mold and pressure-molded when the temperature had been raised to a predetermined molding temperature. More specifically, the molding temperature was 200°C for the resin A and 300°C for the resin G. The molding pressure was 25 kgf/mm 2 . Cooling: The molded body was cooled down to a predetermined pressure releasing temperature shown in Table 11 (second temperature) while the pressure was maintained. Samples were then taken out from the molds. The cooling was conducted by water cooling. The cooling rate in the cooling under pressure was 50°C/sec. Product shapes: Cylindrical shape (10 mm outside diameter x 7 mm inside diameter x 7 mm height, compacted in the heightwise direction) Magnetic performance (maximum magnetic energy product (BH)max), as well as density, porosity and the outside diameter were examined with the samples of the rare earth bonded magnet thus obtained. The results are shown in Table 11. Methods of evaluations of the items are the same as those in Example 1. As demonstrated by Sample Nos. 43a to 52a shown in Table 11, the product magnets exhibit low porosity, high density and high dimensional precision (dimensional error within ± 5/100 mm), as well as superior magnetic performance, when the pressure releasing temperature is not higher than the melting temperature of the binder resin or when the difference between the pressure releasing temperature and the molding temperature is not smaller than 20°C. These advantages are more remarkable when a lower pressure releasing temperature is employed. In particular, when the pressure releasing temperature is not higher than the thermal deformation temperature of the binder resin as is the case with Sample Nos. 46a, 47a, 50a, 51a and 52a, it is possible to obtain a high density which approximates the stoichiometric density, so that the product magnet exhibits an extremely superior performance by making full use of the characteristics of the magnet powder. (Example 7) The following types of magnet powder, binder resin (thermoplastic resin) and additive were mixed together to form a mixture which was subjected to kneading, and the kneaded matter was granulated (grained) into granules. The granular material was charged in a mold of a molding machine at the room temperature and was pressure-molded (warm molding) without application of a magnetic field. After the molding, cooling was executed while maintaining the same level of pressure as the molding pressure, whereby Sample Nos.1b to 6b of rare earth bonded magnets were obtained in which the magnet powder particles were bonded together by the solidified binder resin. Nd-Fe-B type magnet powder: Nd 12.0 Fe 77.8 Co 4.3 B 5.9 97 wt% (content in product magnet is almost the same as this value) Polyamide resin (PA12): Melting temperature 178°C, thermal deformation temperature 145°C 1.6 wt% Antioxidant: Hydrazine-type antioxidant 1.0 wt% Mixing: Mixed by means of Henschel mixer Kneading: Kneaded by a twin-screw kneader at 150°C to 250°C. Screw speed 100 to 250 r.p.m., Granulation (Graining): Kneaded composition was granulated into granules of average size of 1 mm through grinding. Molding: The composition (granulated material) was charged into a mold and pressure-molded by means of a punch when the temperature had been raised to a temperature shown in Table 12 (first temperature). The molding pressure was 10 kgf/mm 2 . Cooling: The molded body was cooled while the pressure was maintained. The pressure was released when the temperature had come down to 100°C (second temperature). Samples were then taken out from the molds. The cooling was conducted by air cooling. The cooling rate in the cooling under pressure was 0.5°C/sec. Product shapes: Solid cylindrical shape (10 mm diameter x 7 mm height); flat tabular shape (20 mm square x 3.0 mm thick) Magnetic performance (magnetic flux density Br, coercive force iHc, maximum magnetic energy product (BH)max), as well as density, porosity and mechanical strength were examined with the samples of the rare earth bonded magnet thus obtained. The results are shown in Table 12. Methods of evaluations of the items appearing in Table 12 are the same as those in Example 1. As will be seen from Table 12, the density and porosity of the product bonded magnet tend to increase and decrease, respectively, when the molding temperature is elevated. Specifically a low porosity is obtained so as to provide high levels of mechanical strength and magnetic performance, when the molding temperature is not lower than the melting temperature of the thermoplastic resin, as in the case of Sample Nos. 3b to 6b. (Example 8) Each of the magnet powders shown in Table 13, a binder resin (thermoplastic resin) and additives were mixed together to form a mixture which was subjected to kneading, and the kneaded matter was granulated (grained) into granules. The granular material was charged in a mold of a molding machine and was pressure-molded (warm molding) without application of a magnetic field. After the molding, cooling was executed while maintaining the same level of pressure as the molding pressure, whereby Sample Nos.7b to 31b of rare earth bonded magnets were obtained in which the magnet powder particles were bonded together by the solidified binder resin. Composition: Magnet powder: Each of the powders 1 to 5 shown in Table 13 96.0 wt% (content in product magnet Is almost the same as this value) Polyamide resin (PA 12): Melting temperature 178°C, thermal deformation temperature 145°C 2.55 wt% Antioxidant: Hydrazine-type antioxidant, 1.4 wt% Other additive: Higher fatty acid (Stearic acid), 0.05 wt% Mixing: Mixed by means of Henschel mixer Kneading: Kneaded by a twin-screw kneader at 150°C to 250°C. Screw speed 100 to 250 r.p.m., Granulation (Graining): Kneaded composition was granulated into granules of average size of 0.5 mm through grinding and classification. Molding: The composition (granulated material) was charged into a mold and pressure-molded by means of a punch when the temperature had been raised to 230°C (first temperature). The molding pressure was 15 kgf/mm 2 . Cooling: The molded body was cooled while the pressure was maintained. The pressure was released when the molded body had been cooled to the temperatures shown in Table 14 (second temperature). Samples were then taken out from the molds. The cooling was conducted by air cooling. The cooling rate in the cooling under pressure was 2°C/sec. Product shapes: Cylindrical shape (20 mm outside diameter x 18 mm inside diameter x 10 mm height) The values of the average granule sizes appearing in Table 13 were obtained through measurement in accordance with F.S.S.S. (Fisher Sub-Sieve Sizer). The density, porosity and circularity (dimensional precision) were examined with the samples of the rare earth bonded magnet thus obtained. The results are shown in Tables 14 and 15. Methods of evaluation of the items appearing in Tables 14 and 15 were as follows. Density: The same as that in Example 1 Porosity: The same as that in Example 1 Circularity: Outside diameter of the molded body was measured at 10 points and the circularity was determined through the following calculation based on the maximum and minimum measured outside diameters. Circularity = (maximum value - minimum value)/2 As will be seen Tables 14 and 15, such a tendency was observed with each of the magnet powder compositions that a lower pressure releasing temperature in the course of cooling provides a greater density and smaller porosity of the bonded magnet and reduces the risk of deformation of the sample when the same is separated from the mold, thus offering improved circularity (dimensional precision). An extremely high degree of circularity (dimensional precision) is obtained when the pressure releasing temperature is not higher than the melting temperature (178°C), in particular not higher than the thermal deformation temperature (145°C) of the thermoplastic resin used as the binder resin. (Example 9) The following type of magnet powder, each of the binder resins (thermoplastic resins) shown in Table 16 and additive were mixed together to form a mixture which was subjected to kneading, and the kneaded matter was granulated (grained) into granules. The granular material was charged in a mold of a molding machine and was pressure-molded (warm molding) under the influence of a magnetic field. After the molding, cooling was executed while maintaining the same level of pressure as the molding pressure, whereby Sample Nos.7 to 31b of rare earth bonded magnets were obtained in which the magnet powder particles were bonded together by the solidified binder resin. Magnet powder: Sm(Co 0.672 Fe 0.22 Cu 0.08 Zr 0.028 ) 8.35 96.5 wt% (content in product magnet is almost the same as this value) Thermoplastic resin: One of resins A to G shown in Table 16, A + B, each 2.3 wt% Antioxidant: Phenol-type antioxidant 1.2 wt% Mixing: Mixed by means of Henschel mixer Kneading: Kneaded by a twin-screw kneader. As to the kneading temperature, reference is made to Table 5. Screw speed 100 to 250 r.p.m., Granulation (Graining): Kneaded composition was granulated into granules of average size of 0.5 mm through grinding and classification. Molding: The composition (granulated material) was charged into a mold at the room temperature and pressure-molded by means of a punch when the mold temperature had been raised to a temperature shown in Table 16 (first temperature). The molding pressure was 10 kgf/mm 2 . A radial magnetic field (aligning magnetic field of 15 KOe) was applied immediately before the pressurizing. Cooling: The molded body was cooled while the pressure was maintained. Demagnetization was conducted at a temperature shown in Tables 17 to 19 (second temperature). Samples were then taken out from the molds. The cooling was conducted by water cooling. The cooling rate in the cooling under pressure was 10°C/sec. Product shapes: Cylindrical shape (30 mm outside diameter x 27 mm inside diameter x 5 mm height) The values of thermal deformation temperature appearing in Table 16 were obtained through measurement in accordance with the aforementioned ASTM D648. The density, porosity and the circularity (dimensional precision) of the samples of rare earth bonded magnet thus obtained were measured to obtain the results as shown in Tables 17, 18 and 19. Evaluations of the measurement items appearing in Tables 17, 18 and 19 were the same as those in Examples 1 and 8. As will be seen Tables 17 to 19, such a tendency was observed with each of the binder resin compositions that a lower pressure releasing temperature in the course of cooling provides a greater density and smaller porosity of the bonded magnet, thus offering improved circularity (dimensional precision). An extremely high degree of circularity (dimensional precision) is obtained when the pressure releasing temperature is not higher than the melting temperature, in particular not higher than the thermal deformation temperature of the thermoplastic resin used as the binder resin. Magnetic performance was examined with each of the samples shown in Tables 17 to 19. All these samples showed superior magnetic characteristics: namely, magnetic flux densities Br not lower than 7.0 KG, coercive force iHc not smaller than 7 KOe and maximum magnetic energy product (BH)max not smaller than 13 MGOe. (Example 10) Magnet powders, binder resins (thermoplastic resins) and an additive were mixed to form compositions as shown in Table 20. The mixture was kneaded and the kneaded composition was granulated (grained) to form granules which were then charged in a mold of a molding machine and pressure-molded (warm molding) without application of a magnetic field. Then, cooling was conducted in which the pressure of the same level as the molding pressure was maintained until the temperature had come down to the melting point and thereafter the cooling was continued with progressive reduction of the pressure to about 50 % the compacting pressure, whereby Sample Nos. 63b to 80b of the rare earth bonded magnet were obtained in which magnet powder particles were bonded together by the solidified binder resin. The contents of the magnet powder in the sample magnets thus obtained were almost the same as those In Table 20. Magnet powder: Nd 12.0 Fe 77.8 Co 4.3 B 5.9 Polyamide resin (PA12): Melting temperature 178°C Thermal deformation temperature 145°C Antioxidant: Hydrazine-type antioxidant Mixing: Mixed by means of a Henschel mixer Kneading: Kneaded by a twin-screw kneader, Kneading temperature 100 to 250°C, Screw speed 100 to 250 r.p.m. Granulation (Graining): Kneaded composition was granulated into granules of an average size of 0.5 mm through grinding and kneading. Molding: The composition (granules) was charged into a mold at room temperature and pressure-molded by means of a punch when the mold had been heated to 220°C (first temperature). The molding pressure was 20 kgf/mm 2 . Cooling: Cooling was conducted while keeping the molded body under pressure (pressure progressively decreased in relation to time), and the pressure was completely removed when the temperature had come down to the temperatures shown in Tables 21 and 22 (second temperature). The samples were then taken out of the mold. The cooling was conducted by water cooling. The rate of cooling under pressure was 50°C/sec. Product shape: Roof-tile shape (8 mm outside curvature radius (R) x 7 mm inside curvature radius (r) x 120° sector angle x 8 mm height) The density, porosity, circularity (dimensional precision) and the magnetic characteristic (maximum magnetic energy product (BH)max) were measured on each of these samples to obtain the results as shown in Tables 21 and 22. Methods of evaluation of the items appearing in tables 21 and 22 were the same as those in Examples 1 and 8. As will be seen in Tables 21 and 22, such a tendency was observed with each of the magnet powder contents that a lower pressure releasing temperature in the course of cooling provides a greater density and smaller porosity of the bonded magnet, thus offering improved circularity (dimensional precision). An extremely high degree of circularity (dimensional precision) is obtained when the pressure releasing temperature is not higher than the melting temperature, in particular not higher than the thermal deformation temperature of the thermoplastic resin used as the binder resin. As will be understood from the foregoing description, according to the present invention, it is possible to obtain, even with a reduced amount of binder resin, a rare earth bonded magnet which excels in moldability, porosity, mechanical strength, corrosion resistance (durability), dimensional stability (dimensional precision) and magnetic performance. An extremely low porosity and further improved dimensional stability are obtainable when the size of the granular material falls within a desired range. The molding is be carried out by warm molding, so that rare earth bonded magnets having advantageous features set forth above can be manufactured with comparatively low molding pressure, thus offering ease of manufacture. This offers a reduction in the production costs and enhances adaptation to mass-production. Specifically low porosity and extremely high dimensional stability of the rare earth bonded magnet are achieved when the second temperature (pressure releasing temperature) in the cooling step is not higher than the melting temperature, in particular not higher than the thermal deformation temperature, of the thermoplastic resin used, or when the difference between the first and second temperatures is greater than a predetermined value. The improvement in the corrosion resistance and dimensional stability, not to mention the magnetic characteristics, greatly contributes to improvement in the performance of a device such as a micro-motor Incorporating the magnet of the invention. Industrial Applicability By virtue of the advantages described hereinbefore, the present invention finds extensive use such as, for example, use as permanent magnets of various motors, e.g., stepping motors, brushless motors and so forth, permanent magnets of solenoids and actuators, permanent magnets of automotive sensors, permanent magnets of finders of VTRs or the like, and permanent magnets incorporated in various types of meters.

Claims: A method of manufacturing a rare earth bonded magnet, comprising the steps of: (a) mixing a rare earth magnet powder and a binder resin and kneading the mixture so as to prepare a kneaded material; (b) granulating or graining the kneaded material to form the kneaded material into a granular material; and (c) conducting a pressure molding on said granulated material at a first temperature at which said binder resin is softened or molten; characterized by the step of (d) cooling the molded body resulting from step (c) while keeping said molded body under pressure at least in the period in which said molded body is cooled down to a second temperature which is below said first temperature. The method according to Claim 1, wherein said binder resin is a thermoplastic resin. The method according to Claim 1 or 2, wherein the kneading is conducted at a temperature not lower than the thermal deformation temperature of said binder resin, such that the surfaces of the rare earth magnet powder particles are coated with the molten or softened binder resin component. The method according to Claim 1 or 2, wherein the content of said rare earth magnet powder in said kneaded material ranges from 90 wt% to 99 wt%. The method according to Claim 1 or 2, wherein said kneaded material contains an antioxidant. The method according to Claim 1 or 2, wherein the average granule size of said granular material ranges from 0.01 to 2 mm. The method according to Claim 1 or 2, wherein said second temperature is the melting temperature or the thermal deformation temperature of said binder resin. The method according to Claim 1 or 2, wherein the pressure applied to said molded body during cooling under pressure is maintained constant at least in the period until the temperature falls to a temperature between said first and second temperatures.

Inventors: IKUMA KEN, SHINOHARA TAKATOMO, SHIOHARA KIYOSHI, IKUMA, KEN, SHINOHARA, TAKATOMO, SHIOHARA, KIYOSHI
Applicants: SEIKO EPSON CORP, SEIKO EPSON CORPORATION
Application year: 1997
Main classification: H01F41
IPC codes: H01F1