A resin bound magnet and its production process

Abstract: This invention relates to a resin bound type magnet and its production process, especially a cylindrical or a thin plate state magnet and its production process. A resin bound type magnet comprising a magnetic powder and an organic resin or further with an additive and its production process wherein it is moulded as a single body in a cylindrical form satisfying a relation of 2DL / d² ≧ 1 among the outer diameter (D), the inner diameter (d) and the length (L) of the said magnet, and it also has a radiated anisotropy in the diameter direction. Further this invention is a resin bound type magnet and a production process of a resin bound type magnet by an extrusion moulding of the said magnet with a die in a magnetic field wherein an average particle size r of the magnetic powder satisfies r ≦ 0.1 t (t ≦ 1mm) with the thickness t of the moulded article of an anisotropic resin bound type magnet comprising the said magnetic powder and the resin.

Description: This invention relates to a resin bound type magnet which is used for a miniature motor, an encoder, a linear actuator, etc. which are applied for electronics instruments, etc. and its production process, especially for a resin bound type magnet, and its production process by using an extrusion moulding method. The resin bound type magnet is generally produced by (1) an injection moulding method, (2) a press moulding method and (3) an extrusion moulding process. Among these moulding methods, the injection moulding method is to mould a predetermined shape by packing a magnet composition comprising a magnetic powder and a thermoplastic resin into a die by heating it at a temperature at which a sufficient fluidity is attained. The press moulding method is a moulding method by pressing after packing a magnet composition comprising a magnetic powder and a thermosetting resin into a die of a press machine. The extrusion moulding method is a moulding method by charging a magnetic composition in a fluidized state by heating a mixture of a magnetic powder and a resin to make it in a molten state by a screw, a ram or a plunger into a die, and by converging it there. Among these moulding methods, the injection moulding and the press moulding can mould a magnet having an anisotropy by applying a magnetic field in the die at the moulding step. However as for the injection moulding method and a press moulding method for a moulding of a long sized magnet of which demand is recently being increased, in case of the injection moulding method because of an impossibility of packing of a magnet composition in a cavity and of taking out a moulded article, etc. and in case of the press moulding method since a length of a moulded article is determined by a stroke of a moulding punch, they have a defect that the length of the moulded article is limited. Especially in case of a moulding of a cylindrical magnet having a radiated anisotropy, there is a limitation on the length of the moulded article, and a magnet, of which an outer diameter (hereinafter called as D), an inner diameter (hereinafter called as d) and a length (hereinafter called as L) satisfy a below-listed equation, can only be moulded. 2DL/d 2 < 1 (Reference literature: Masaaki Hamano, an abstract book on the 9th meeting of Injection Moulding Technology of High Performance Plastic Magnet and Its Application Development, Plastic Industry Technical Committee 1986) Accordingly as to a cylindrical magnet which satisfies 2DL/d 2 > 1, only 2 kinds were obtainable i.e. one was a single moulded magnet with an isotropy or with a poor magnet performance and the other was a one which was prepared by sticking plural number of the magnets having a radiated anisotropy. However the aforementioned cylindrical resin bound type magnet and its production process have problems listed below. (1) In order to get a high performance motor or an actuator by using an isotropic magnet or a magnet having a performance close to the isotropic magnet, the volume of the magnet to be used has to be large, and a miniaturisation and a light weighing of these instruments can not be satisfied. (2) In case that plural number of the magnets having a radicated anisotropy is stuck, there are following problems. [1] The sticking is weak against delamination and there is a danger of destroying the bond by a repeated hot and cold cycle. It has an inferior reliability such as a variation of the sticking strength, etc. [2] A production cost is raised because of a step of the sticking in a production process. Furthermore the stuck magnet has to be finished by a cutting fabrication, etc. to ensure a dimensional accuracy of the moulded article resulting a further increase of the production cost. [3] In case of the finish by the cutting fabrication of the moulded article, there is a possibility of a deterioration of the magnet performance by the finish. (3) As for the injection moulding method and the press moulding method, it requires a fixed cycle of a packing, a moulding and a taking out of a magnet composition to/from a die in the moulding process, and the productivity is limited because it is basically a batch type production system. It is, therefore, difficult to achieve a reduction of the production cost. (4) As for the conventional extrusion moulding method, it is a major method not to apply a magnetic field at the moulding stage. However an isotropic magnet can only be attained by this moulding method. Accordingly the extrusion moulding method, which has a very high productivity due to an ability of a continuous operation from a supply of a raw material to a receipt of a moulded article, and is able to easily mould a long sized magnet, becomes popular. Particularly there are many researches to improve the magnet performance, especially researches on an extrusion moulding method in a magnetic field in order to reform the magnetic property which has been considered to be poor. Concerning a method to charge a magnetic field at the moulding step. as for a column type magnet, there is a report by R.E. Johnson ("Development in The Production of Bonded Rare Earth-Cobalt Magnets." 5th International Workshop on Rare Earth-Cobalt Magnets and Their Applications. 1981), and as for a cylindrical magnet, there are methods shown in Japan Patent Laid-Open Sho 58-219705 and Japan Patent Laid-Open Sho 61-121307. Both of these methods were to form a magnet by orientating an axis of easy magnetisation of a magnetic powder to a direction of a magnetic field by charging a magnetic field in a die of an extrusion machine while a magnet composition was being passed through the die. However for example in a method described in Japan Patent Laid-Open Sho 61-121307, a cylindrical magnet magnetised and orientated in a die was cooled by a cooling unit outside the die, a direction of the anisotropy was only formed in one direction and a moulded article having a radiated anisotropy in a diameter direction could not be obtained. Moreover, when the moulded article was extruded from the die, its temperature was still high resulting a deterioration of a magnet performance by a disorder of an orientation of a magnetic powder. As a results, it is not attainable to get a cylindrical magnet having a high magnet performance radiated anisotropy even if this method is applied. Moreover there are below-listed problems in the aforementioned production method. (1) In the conventional extrusion moulding method in a magnetic field, a electromagnetic coil is merely installed on the die and no consideration is paid for a demagnetisation of a moulded article produced. If there is a residual magnetism in a moulded article, it is very difficult to handle in the post process such as an adhesion at a cut stage to a cutter or other magnetic materials. Further when a predetermined magnetisation is carried out for the magnet, the residual magnetism give an unfavourable influence to a balance of The magnetisation. (2) As an example of a moulding method in which the demagnetisation of the extrusion moulded article is considered is disclosed in Japan Patent Laid-Open Sho 60-217617. However since the demagnetisation coil is equipped at a front end of a die in this case, it results an extremely large die and a mouldability is poor. Especially since a passage of a raw material compound in the die is long, the moulding speed is slow and the moulding itself is also difficult. (3) When a cylindrical magnet having a radiated anisotropy is moulded, a length of an orientation section on which a magnetic field is charged due to a constitution of a magnetic circuit is determined by an inner diameter of the moulded article. However since a considerably high magnetic field is generally required to orientate a rare earth magnet powder, the length of the orientation has to be relatively short in order to charge a sufficient magnetic field at the orientation section. Consequently it is essentially not able to mould a cylindrical magnet with a high magnet performance having a small inner diameter. (4) When a column type or a sheet type magnet is moulded, the length of the orientation section can be made long in a certain degree. However a gap between pole pieces can not be made too short due to a mechanical strength of the die, and thus it is not able to increase a magnetic field to be charged at the orientation too high. The magnet performance of the moulded article therefore drops. In case that a thermoplastic resin is used as a resin in the aforementioned extrusion moulding method, the moulding is carried out by a solidification with cooling of a molten mixture after orientating at the front end of the die. When a thermosetting resin is used as a resin, there are methods to mould by a solidification with cooling after orientating at the front end of the die as same to the thermoplastic resin, and to mould by a curing with heating after the orientating. In case of a method with the solidification with cooling by using the thermosetting resin, it is necessary to heat it to cure the resin after the moulding. Anyhow even if the moulding is done by either methods, a magnet moulded is extruded continuously, and it is necessary to cut the magnet moulded in a predetermined length. For the cut, mechanical cutting methods namely a guillotine cutter system or a rotary saw-tooth system were utilised in the conventional method. However the conventional cutting methods had below-listed problems. In case of a mechanical cutting method such as the guillotine cutter system or the rotary saw-tooth system, a force and a vibration are charged to a magnet to be cut. When an uncured resin bound type magnet moulded by a solidification with cooling by using a thermosetting resin is cut and a thin thickness magnet characterised by an extrusion moulding is cut, a crack, a breakage and/or a deformation of the magnet are taken place during the cutting because of a brittleness and a weakness of the magnet to be cut. Particularly when a volume ratio of the magnetic powder in a resin bound type magnet is increased in order to improve a performance of the magnet, the volume ratio of the resin decreases and the aforementioned problems tends to happen further easily because of a reduction of bonding force of the resin with the magnetic powder. Moreover in case of the mechanical cutting method, a cut dust is unavoidably produced. In case of a rare earth magnet, especially a R-Co type magnet, a treatment of the cut dust is extremely important because cobalt has a bad effect to a human body, and it requires a recovery unit of the cut dust. Further among the conventional moulding methods by using a thermosetting resin, a press moulding was a general, but an injection moulding and an extrusion moulding were not widely used and a thermoplastic resin was commonly used. Accordingly only a few methods were available to cure an uncured magnet by heating moulded by the injection moulding and by the extrusion moulding, and the method was to secure a cylindrical shape of the magnet with a centrifugal force by rotating a jig after fixing its outer diameter to the jig. The problems of the aforedescribed conventional technology are summarised as below. Firstly it is a use of a thermoplastic resin for moulding a magnet by an injection moulding and the extrusion moulding. In order to make a magnet moulded by a thermoplastic resin to be usable even at a temperature at around 150°C, its moulding temperature has to be at 200°C or more. Therefore a magnetic powder blended with the resin is exposed at such a temperature. When a rare earth magnet, especially a R-Fe-B type magnet is used as a magnetic powder, a deterioration of a magnet performance of the magnet happens due to an oxidation of the magnetic powder at a high temperature above 200°C since the magnetic powder is easily to be oxidised. Moreover, the thermoplastic resin has problems on heat resistance and solvent resistance when compared with a thermosetting resin. Next, methods of the curing by heating after moulding by using the thermosetting resin are pointed out. For a moulding by using the thermosetting resin, it requires that the resin possesses a thermoplastic property in a certain temperature region. Nevertheless this temperature region is lower or higher than the thermosetting temperature, it is necessary to secure the shape once moulded in order to cure it. In order to achieve it, methods in the conventional technology are available. However in a conventional production process of a cylindrical resin bound type magnet, since the inner diameter is not fixed, a fixing jig of the outer diameter is rotated with a sample to secure the shape during the curing, and the jig for this purpose is thus required. Furthermore it is difficult to operate the curing treatment for lots of magnets by this method, and it has problems such as a curing treatment for a long sized magnet moulded by an extrusion moulding method is also difficult, etc. Further in the past, as to a particle size of a magnetic powder in a resin bound type magnet, a concern of an oxidation when the magnetic powder was fining was considered, but no consideration of the particle size of the magnetic powder from a viewpoint of a thickness of a magnet moulded article was considered. Further in order to mould a magnet with a thickness of 1mm, it was necessary to make the desired thickness by a cutting fabrication after moulding one with the thickness more than 1mm by a press moulding or an injection moulding in advance. However the particle size of a magnetic powder gives a large influence to the thickness of a moulded article of an anisotropic resin bound type magnet. Namely if the average particle size of a magnetic powder does not change, an orientation of one magnetic particle affects more to a degree of the orientation of the magnet by thinning the thickness of the moulded article. For example, when an anisotropic magnet with a thickness of 0.5mm is moulded, if the average particle size of the magnetic powder is 50µm, the influence that one magnetic particle gives to the orientation is around 10%. Although the influence is reduced if the thickness of the moulded article becomes 0.5mm or more, the influence is enlarged if the thickness becomes thinner. Accordingly as to the average particle size of the magnet, a problem is generated that it should relate to the thickness of the magnet moulded article. Further by a conventional method to mould a magnet with the thickness 1 mm or less, it resulted a high production cost, etc. because of the fabrication process required. Further, a rare earth magnet, especially a rare earth-iron-boron type magnet was easily oxidised, and there was a problem of a formation of a rust during its service. In order to solve the problem, a coating method of a resin on the magnet moulded, a metal plating and a coating of a ceramic or a resin on the magnetic powder were investigated in the past. However following problems are enumerated in the aforementioned conventional rust resistance technology of a magnet. The former method in which a resin is coated on a magnet moulded does not have an effect to an oxidisation of the magnetic powder during the moulding. In other words, when an injection moulding or an extrusion moulding is performed, the magnetic powder is exposed under a high temperature during kneading of the magnetic powder and the resin or moulding and the magnetic powder can be oxidised at this stage resulting an impossibility of the moulding and a deterioration of a magnet performance. Moreover if a slight pin hole is present in a coating film after the moulding, there is a problem that the magnet inside it is oxidised from it. From considering those problems, the latter method in which a metal plating or a coating with a ceramic, a resin, etc. on the magnetic powder may be a method to solve the aforementioned problems. However the latter method still has a problem. The average particle size of a magnetic powder is several ten microns, if a film is coated on it, its thickness has to be 1 micron or less, and therefore there is a problem that the film coated has to be extremely tough and strongly adhesive or it has to establish a production process not to remove the film coated. Further in the past, a thin plate state resin bound type magnet was mainly produced by a calendar moulding method, an extrusion moulding method and an injection moulding method. For each moulding method, a mixture kneaded of a magnetic powder and a thermoplastic resin is used and in case of the calendar moulding method, the aforedescribed magnet raw material is made in a thin plate state by rolling with hot rollers. However the production process of the aforementioned thin plate state resin bound type magnet has below-listed problems. (1) In case of the calendar moulding method, an isotropic moulded article can only be obtained when a rare earth magnetic powder is used since it is not possible to charge a magnetic field during the moulding, and thus a magnet performance of the magnet is low. Further because of a heating capacility of the rollers, it is not possible to use a resin having a very high melting point as a binder, and the heat resistance of the moulded article is inferior. Furthermore it requires a certain flexibility in the moulded article for the moulding, and therefore an amount of a magnetic powder in a raw material can not be considerably large resulting a low magnet performance of the moulded article. (2) In case of a moulding of a thin plate state magnet by an extrusion moulding method, an unevenness of the moulded article is likely to happen due to a difference of the extrusion rate between a central point and an outer point of the outlet of the die. Accordingly in order to produce an even moulded article, a complicated design of the passage in the die is required and the die becomes extremely expansive. Further when the moulding is carried out in a magnetic field, the gap between pole pieces can not make very small because of a concern of mechanical strength of the die, and thus the magnetic field to be charged at the moulding can not be very high. Consequently a magnet performance of the moulded article is deteriorated. (3) In case of a moulding of a thin plate state magnet by an injection moulded, it is difficult to mould one having a thickness of 1 mm or less. This is due to a generation of a poor moulding by an insufficient packing of the moulding raw material into the cavity of the die if the thickness is thin, since the moulding raw material has a poor fluidity containing a large amount of a magnetic powder. Further when an anisotropic magnet is moulded and if the thickness is thin, a moulding with a substantially isotropy can only be obtained because of an effect of a skin layer (a section where the ratio of the resin is high) of the magnet surface.Further a big moulding machine is required to mould a magnet having a large area, and it is disadvantageous from a moulding cost. It is an object of the present invention to mitigate at least some of the above mentioned problems. Document EP-A2-0318251 describing a resin bound type cylindrical magnet according to the preamble of claim 1, and a production process for such a magnet according to the preamble of claim 9, discloses various compositions for the production of magnets, including rare earth magnetic powders mixed with a cross-linkable organic material. According to a first aspect of the present invention, there is provided a resin bound type cylindrical magnet formed of magnetic powder and an organic resin, wherein the magnetic powder makes up at least 90% by weight of the magnet the magnetic powder being a rare earth magnetic powder having a coercive force of 0.56 MA/m (7kOe) or greater and having an average particle size (r) and the magnet having a thickness (t), characterised in that; the magnet has radiated anisotropy, fulfils the requirements of: r ≤ 0.1t, with t ≤ 1mm the resin is a thermoplastic. According to a second aspect of the present invention, there is provided a production process of a resin bound type cylindrical magnet wherein a raw material comprising a magnetic powder and an organic resin with the magnetic powder making up at least 90% by weight of the magnet, is moulded by extrusion moulding by passing the material through a die in a magnetic field, the magnetic powder being a rare earth magnetic powder having a coercive force of 0.56 MA/m (7kOe) or greater and having an average particle size (r) and the moulded material forming a body having a thickness (t); characterised in that; the body has radiated anisotropy, satisfies the requirement of r ≤ 0.1t, with t ≤ 1 mm the organic resin is a thermoplastic. Embodiments of the present invention will now be described, by way of example only and with reference to the accompanying drawings, in which:- Figure 1 is a sketch of an extrusion moulding machine used in Example of this invention; Figure 2 is a sketch of a die structure for an extrusion moulding in a magnetic field of a cylindrical resin bound type magnet used in Example of this invention; Figure 3 is a sketch of an extrusion moulding machine used in Example of this invention; Figure 4 is a sketch of a pulse magnetisation unit used in Example of this invention; Figure 5 is a graph showing a relation between a magnetic field for the moulding and a residual magnetic flux density of the moulded article for cases with and without a magnetisation of a magnetic powder prior to the moulding; Figure 6 is a sketch of a die structure for an extrusion moulding in a magnetic field of a thin plate state resin bound type magnet; Figure 7 is a graph showing a relation between a magnetic field for magnetisation before the moulding and a residual magnetic flux density of the moulded article; Figure 8 is a sketch of an extrusion moulding machine; Figure 9 is a sketch of a die structure for an extrusion moulding in a magnetic field of a cylindrical resin bound type magnet; Figure 10 is a sketch of an extrusion moulding machine used in Examples of this invention; Figure 11 is a sketch of Example to cut and to make a thin plate state of a cylindrical resin bound type magnet; and Figure 12 is a sketch of a press unit for moulding of a thin plate state magnet. As for a magnetic powder which can be applied in this invention, a so-called rare earth magnetic powder such as a magnetic powder composing of a rare earth metal and transition metals mainly constituting cobalt and iron as a basic composition, or a magnetic powder composing of a rare earth metal, boron or transition metals mainly constituting iron as a basic composition, etc. are enumerated. As for an organic resin which can be applied in this invention, the thermoplastic resin can be a plastic such as polyamide, polypropylene, polycarbonate, polyphenylenesulphide (PPS), etc., an elastomer such as chlorinated polyethlene, ethylene vinylacetate copolymer (EVA), etc. and synthetic rubber are enumerated. Further as for the additive, a lubricant to reduce an extrusion resistance at the moulding such as a metal soap (zinc stearate, calcium stearate), wax, etc. can be used. The magnetic powder is sufficiently mixed with the organic resin and the additive if it is necessary. Then the mixture is sufficiently kneaded in a kneading machine with heating above a temperature at which the organic resin is molten, and it is granulated. The magnetic composition granulated is charged in an extrusion machine, it is heated in a cylinder to make it in a fluidized state and is sent into a die by a screw or a plunger. The magnetic composition injected in the die is moulded by uniforming (orientating) an axis of easy magnetisation of the magnetic powder in the raw material to a direction of a magnetic field by passing through a die in which a magnetic field is charged. The magnetic composition is solidified with cooling while it is in the magnetic field formed in the die, and it is extruded. The moulded article is then cut into a suitable length. Further the moulded article extruded from the die is demagnetised by charging a magnetic field of a reverse direction to the magnetic field charged in the die at the moulding at a front end of the mandrel. The strength of the magnetic field is adjusted by a distance between the mandrel and a yoke of the electromagnetic coil. The moulded article extruded from the die is also demagnetised by charging a magnetic field for an attenuation by passing it through an electromagnetic coil for the demagnetisation. A cylindrical resin bound magnet is thus produced. Further this invention is useful to facilitate an orientation of a magnetic powder, to improve the magnetic property and to reduce an extrusion resistance at the moulding. According to this invention, by defining an average particle size of a magnetic powder with a thickness of an anisotropic resin bound type magnet moulded article, and the magnet is moulded by an extrusion, it is possible to mould a thin thickness anisotropic magnet without a postfabrication, and further a high performance magnet is able to mould. Next, in order to make a thin plate state magnet, one or plural points on the circumference of the moulded article in a cylindrical form is cut in parallel with the central axis of the moulded article. Then the aforementioned moulded article cut is made in a thin plate state by using, for example, 2 rollers, etc. The moulded article is then solidified with cooling, and is cut into a suitable length. Further as an alternative method, after cutting a cylindrical moulded article extruded into a suitable length, one or plural points on the circumference of the moulded article is cut in parallel with the central axis of the moulded article. The moulded article cut is heated and it is spread when a viscosity of the moulding article drops to make a thin plate state. As described above, a magnet of this invention is superior to a conventional magnet, in which plural magnets are sticked, from a viewpoint of a reliability. Furthermore by an application of a production process of this invention:- (1) A cylindrical resin bound type magnet with a high magnet performance can be produced with a high productivity and an economic cost. (2) Since no excess force or vibration is charged in the magnet at a cutting step by cutting an extruded resin bound type magnet with a wire heated by an electric resistance, it is possible to cut on uncured magnet which is very brittle and a thin thickness magnet without a crack and a breakage. (3) It has a significant effect for moulding a cylindrical magnet having a radiated anisotropy in a diameter direction though it is difficult to get a strong magnetic field at the moulding. (4) It can be widely used for a magnetic sensor, an encoder, an actuator, a linear actuator which requires a miniaturisation, a precision and a high performance. Further according to this invention by defining the average particle size of the magnetic powder by the thickness of the anisotropic resin bound type magnet moulded article and by moulding the magnet by an extrusion, it is possible to mould a thin thickness anisotropic magnet without a postfabrication, and it is also possible to mould a magnet with a high performance. Furthermore according to this invention by kneading a mixture of a magnetic powder and a resin after heating it to make its viscosity at 10 kPa s (100 kpoise) prior to the kneading, the resin in a molten state is absorbed on the surface of the magnetic powder, and thus it has effects to prevent the coated film on the surface of the magnetic powder by relieving a mechanical stress and to improve an oxidation resistance of the magnet moulded. EXAMPLE This invention is explained in details in accordance with examples shown below. Example 1 Raw materials were molten to make a composition as Sm (Co 0.672 Cu 0.08 Fe 0.22 Zr 0.028) 8.35, after casting, an ingot produced was magnetically cured by a heat treatment, and then a magnetic powder of its average particle size 10µm was prepared by crushing the said ingot. The magnetic powder, nylon 12 powder and zinc stearate powder were mixed to make a ratio of 92 wt%, 7.9 wt% and 0.1 wt% respectively. The mixture was kneaded by an 2 axes extrusion kneading machine at 260°C. The kneaded material was granulated to make granules of an outer diameter 1 - 10mm, they were used as a raw material compound 111, and a cylindrical magnet was produced by an extrusion machine. The moulding method is explained in accordance with Figure 1. The moulding machine consists of a hopper i.e. a raw material charging section 101, a cylinder 102, a screw 103, an adapter plate 104 to equip a die at the cylinder 102, a die 105 and a driving motor of the screw (which is not shown in Figure), and further an electromagnetic coil 109 to charge a magnetic field in the die is positioned at the outside of the die 105. A yoke 110 comprising a magnetic material is installed around the electromagnetic coil 109. The aforementioned granulated raw material compound was charged in the extrusion machine. The raw material compound 111 was heated at 260°C in the cylinder 102 to make it in a fluilized state, and it was passed through the die 105. The die structure is shown in Figure 2. The die is constituted by an outer die 201 and a mandrel 202. Although the outer die is made of a non-magnetic material, a ring shaped outer die section magnetic material 201a to induce a magnetic flux is installed at the front end. The mandrel 201 is also made of a non-magnetic material, and further a mandrel section magnetic material 202a is installed at its front end. When a current is passed through the electromagnetic coil 109 installed outside of the die, the magnetic flux generated flows alongside of a magnetic flux flow H shown in Figure by an arrow since it tends to pass through a magnetic material with a high magnetic permeability. Accordingly a radiated shape magnetic field is generated in a space (hereinafter called as an orientation section) between the front end 202a of the mandrel 202 and the ring shaped outer die 201a of a magnetic material installed in the outer die 201. Therefore when a magnetic composition passes through the orientation section, it is moulded with a progress of the orientation of the magnetic powder. In this Example, the magnetic field for moulding was 1.2 MA/m (15 kOe), the temperature of the die at the moulding was 250°C and the cooling was performed by a forced air cooling at the outlet section of the die. Accordingly, an orientated raw material compound was moulded by an extrusion with a solidification with cooling at the outlet of the die. The size of the moulded article was the outer diameter 32mm, the inner diameter 30mm, and the length was cut into 22mm. The magnetic property of the moulded article produced was Br = 0.58T (5.8 kG) and (BH) max = 58 kJ/m 3 (7.3 MGOe). The magnets thus produced were assembled in 25 units of DC motor, and a continuous operation test for 500 hours Test 1 was carried out. As for a comparative example for Test 2, the raw material having the same composition was moulded by an injection in a magnetic field and a cylindrical magnet of the outer diameter 32.5mm, the inner diameter 30mm and the length 6mm was moulded. The magnetic property of the moulded article produced was Br = 0.57 T (5.7 kG) and (BH) max = 56 kJ/m 3 (7.0 MGOe). 4 pieces of the magnet were sticked by an epoxy type adhesive, and then the outer diameter and the length were made at 32mm and 22mm by a cutting fabrication. The magnets were assembled in 25 units of DC motor and the 500 hour continuous operation was carried out as same as Test 1. The test results were shown in Table 1. DC Motor Tested (units) DC Motor Exhibited a Deteriorated Property (units) Test 1 (Example) 25 0 Test 2 (Comparative example) 25 2 In the Table 1, those of which the motor showed a deteriorated property were cases that the rotation of the motor stopped or the torque was dropped due to an insufficient magnetic flux available by a delamination at the adhesive area of the magnets sticked. Consequently by an application of the magnet of this invention, a reliability of the motor can be improved. Example 2 A magnetic powder of an average particle size of 10µm was prepared by the same composition and procedure as Example 1. A coercive force iHc of this powder was 0.64 MA/m (8 kOe). This is called as Powder A. Further as for the other kind of powder, a magnetic powder of an average particle size of 20µm was prepared with the following composition and procedure. Raw materials to make a composition Nd13 Fe82.7 B4.3 were molten, were cast and a quenched ribbon was prepared in an argon atmosphere by using a quenching and a ribbon rolling machine from the ingot obtained. The quenched ribbon was coarsely crushed, was charged in a mould and a high temperature press moulding was performed by applying a pressure of 20 kg/mm 2 for a short time at 700 - 800°C in an argon atmosphere. A density of the consolidated article was almost 100%. The consolidated article was again processed with a high temperature press moulding in a vertical direction to the first pressing direction with a pressure of 10 kg/mm 2 at 700 - 800°C in an argon atmosphere (namely it was treated with a die upset). A bulk magnet obtained was crushed to make a magnetic powder of an average particle size of 20µm. A coercive force iHc of this powder was 0.95 MA/m (12 kOe). This is called as Powder B. Powder A, nylon 12 powder and zinc stearate powder were mixed to make a ratio of 92 wt%, 7.9 wt% and 0.1 wt% respectively. Further Powder B was also mixed with the aforementioned resin powder and the additive to make a ratio of 91 wt%, 8.8 wt% and 0.2 wt%. These mixtures were kneaded by a 2 axes extrusion kneading machine at 260°C. The kneaded mixture was granulated to granules of an outer diameter of 1 - 10mm to make a raw material compound 111, and a cylindrical magnet was moulded by a procedure explained in Example 1 by an extrusion moulding machine as shown in Figure 1. The moulding method was the same method as Example 1 and the die structure was also the same to Figure 2 as explained in Example 1. If a current flows in the electromagnetic coil equipped outside of the die, the magnetic flux generated flows in the arrow H in Figure since it tends to pass through a magnetic material with a high magnetic permeability. Consequently a radiated magnetic field is generated in the orientation section which is a space between a magnetic material 202a of the mandrel and a magnetic material ring 20/a installed on the outer die. Therefore when a magnetic composition is passing through the orientation section, it is being moulded with a progress of an orientation of the magnetic powder. Furthermore a magnetic field with a reverse direction to the magnetic field in the orientation section is generated in a space between the front end of the mandrel and the yoke 110 of the coil. Therefore a demagnetisation of the moulded article can be achieved by making the magnetic field of this space at a suitable strength with an adjustment of a distance between the mandrel and the yoke 110. In this Example, the magnetic field for moulding was 1.1 MA/m (14 kOe), the temperature of the die at the moulding was 250°C and the cooling was applied by a forced air cooling to the outlet section of the die. By the above, the orientated raw material compound 111 was moulded by an extrusion with a solidification with cooling at the outlet of the die. A strength of the demagnetisation magnetic field was adjusted to almost the same to the coercive force iHc of the magnetic powder in the moulded article. The size of the moulded article was the outer diameter 30mm and the inner diameter 29mm. In Table 2, surface magnetic flux densities for cases Test 3 and 4 in which the demagnetisation was performed, and for cases Test 5 and 6 without the demagnetisation and shown. Powder Surface Magnetic Flux Density (mT) (Flux Density (G)) Test 3 Example A 2.0 (20) Test 4 Example B 3.5 (35) Test 5 Comparative example A 15 (150) Test 6 Comparative example B 22 (220) Example 3 A magnetic powder of an average particle size 10µm and iHc 0.64 MA/m (8 kOe) was prepared by the same composition and procedure as Example 1. This powder is called as Powder A. As for the other kind of powder, a magnetic powder of an average particle size 20µ m and a coercive force iHc 0.95 MA/m (12 kOe) was prepared by the same composition and procedure as Powder B of Example 2. This powder is called as Powder B. As same to Example 2, the Powder A was mixed with nylon 12 powder and zinc stearate powder to make a ratio of 92 wt%, 7.9 wt% and 0.1 wt% respectively. Powder B was also mixed with the aforementioned resin powder and additive to make a ratio of 91 wt%, 8.8 wt% and 0.2 wt% respectively. These mixture were kneaded by a 2 axes extrusion kneading machine at 260°C. The kneaded mixture was granulated to granules of the outer diameter 1 - 10mm to make a raw material compound and a cylindrical magnet was moulded by an extrusion moulding machine. The moulding method is explained in accordance with Figure 3. An extrusion moulding machine of Figure 3 is composed of a similar constitution to an extrusion moulding machine of Figure 1, and an electromagnetic coil 109 is positioned outside a die to charge a magnetic field in the die, but there is a difference i.e. an electromagnetic coil for demagnetisation is installed in front of it. The aforementioned granulated raw material compound 111 was charged in the extrusion moulding machine. The raw material compound 111 was heated at 260°C in the cylinder 102 to make it in a fluidized state, and it was passed through the die 105. The die structure was the same as explained in Example 1. According to this invention, when the magnetic composition is passed through the orientation section, it is moulded with a progress of an orientation of the magnetic powder as same to Example 2. In this Example, the magnetic field for moulding was also 1.1 MA/m (14 kOe), the temperature of the die at the moulding was 250°C, and the cooling was given by a forced air cooling to the outlet section of the die. The orientated raw material compound 111 was moulded by an extrusion by a solidification with cooling at the outlet of the die. The demagnetisation was carried out by generating a magnetic field for the demagnetisation by turning on an attenuated pulse current in the electromagnetic coil 113. The strength of the magnetic field for the demagnetisation was 2.4 MA/m (30 kOe), and it was attenuated with 800m sec. The magnetic field was generated in the electromagnetic coil 113 in every 15 sec, and the demagnetisation was carried out continuously. The size of the moulded article was the outer diameter 30mm and the inner diameter 29mm. Surface magnetic flux densities of the moulded articles for cases in which the demagnetisation was performed (Test 7 and 8) and for cases without the demagnetisation (Test 9 and 10) are shown in Table 3. Powder Surface Magnetic Flux Density (mT) (Flux Density (G) ) Test 7 Example A 1.0 (10) Test 8 Example B 1.5 (15) Test 9 Comparative example A 15 (150) Test 10 Comparative example B 22 (220) As it is clear from Table, the surface magnetic flux density which remains in the moulded article could be dropped at 6 - 7% by an application of the process of this invention. Consequently the workability at the cutting step, etc. was greatly improved. Example 4 A magnetic Powder A or Powder B as the same compositions to Example 1 and 2, and a thermoplastic resin Nylon 12 were weighed to make a desired volume ratio, were mixed and a sheet state compound was prepared by kneading the mixture by passing it through a gap of a twin roller mill repeatedly after charging it in the mill. The kneading temperature of the mixture was at 250°C. Then the compound was crushed into particles and was moulded by an extrusion by passing through a cylindrical die by charging it in a screw type extrusion moulding machine. A barrel temperature of the extrusion moulding machine was at 250°C and the die temperature was the moulding temperature. The extrusion rate was 1 mm/sec. The outlet temperature of the die was set at a solidification temperature of the composition moulded. The process used was a process to solidify with cooling at the outlet of the die. The magnets thus produced were cut by a rotation saw tooth system. In this test, the magnet cut was a cylindrical magnet with the outer diameter 30mm and the inner diameter 29mm, and the volume ratio of the magnetic powder was 60 vol. %. Although the magnetic Powder A and B were used, both showed the same results. Example 5 Raw materials to make the same composition of Powder A in Example 2, Sm (CO 0.672 Cu 0.08 Fe 0.22 Zr 0.028) 8.35 were molten, an ingot produced was magnetically cured by a heat treatment after casting, and a magnetic powder having the average particle size of 10µm and the coercive force iHc of 0.80 MA/m (10 kOe) was obtained by crushing the said ingot. The powder was mixed with nylon 12 powder and zinc stearate powder to make a ratio of 92 wt%, 7.9 wt% and 0.1 wt% as same to Example 2. The mixture was magnetised by a pulse magnetisation unit shown in Figure 4 with using a magnetic field of 2.0 MA/m (25 kOe), and it was then kneaded by a two axes extrusion kneading machine at 260°C. In Figure 4, 301 is an electromagnetic coil, 322 is a pulse current generation power source, 303 is a table to adjust a height of a sample and 305 is a raw material magnetic powder. The kneaded mixture was granulated to particles of the outer diameter of 1 - 10mm to make a raw material compound, and a cylindrical magnet was moulded by using the extrusion moulding machine shown in Figure 1 and the die shown in Figure 2 with a same procedure described above. In this Example, the die temperature at the moulding was 250°C, and a cooling was carried out by a forced air cooling at the outlet of the die. Accordingly, an orientated raw material compound was moulded by an extrusion by a solidification with cooling at the outlet of the die. The size of the moulded article was the outer diameter 33mm and the inner diameter 32mm. In Figure 5, a comparison of changes of residual magnetic flux density (Br) with magnetic fields for moulding for cases with and without magnetisation of the raw material powder before the moulding is shown. From Figure 5, it is understood that a magnet with a high Br ie with a high degree of the orientation can be prepared by an adoption of the magnetisation for the powder prior to the moulding even if the magnetic field for the moulding is low. Furthermore as for the magnet performance at the high magnetic field (around 1.2 MA/m (15 kOe) for the moulding, the cases with the magnetisation showed higher values, and it was understand that the magnetisation for the powder prior to the moulding had a bigger effect. Example 6 Table 4 shows how much degree of the thickness is mouldable without a post fabrication in each moulding method of an extrusion moulding method, a press moulding method and an injection moulding method. Thickness [mm] Moulding Method 1.0 0.9 0.7 0.1 0.01 Extrusion Moulding P P P P P Press Moulding P P I I I Injection Moulding P P I I I P = possible I= impossible Magnets prepared here was ring shaped with the outer diameter of 30mm, and the mouldings were performed with the thickness shown in Table 4. The magnetic powder used was Sm - Co family rare earth magnetic powder and as for the resin, nylon 12 was used for the extrusion moulding method and the injection moulding method, and an epoxy resin was used for the press moulding method. The mixing ratio of the magnetic powder and the resin was 90 wt% :10 wt% for the extrusion moulding method and the injection moulding method, and 98 wt% : 2 wt% for the press moulding method. As it is understood from Table 4, the moulding was not be able to perform if the thickness of the magnetic moulded article became thin for the press moulding method and the injection moulding method. This was due to a difficulty to fill the magnetic powder in a cavity if the thickness became thin for a case of the press moulding method, and is case of the injection moulding method, it could also not be moulded because of a difficulty to inject a molten mixture of the magnetic powder and the resin in a cavity. On the other hand, in case of the extrusion moulding method, a thin thickness magnet can be moulded because it is moulded by continuously flowing a molten mixture of the magnetic powder and the resin and by gradually converging the molten mixture. According it is clear that the extrusion moulding method is an effective method to mould a thin thickness magnet having the thickness of 1mm or less. Example 7 Next effects on moulding radial magnet with a thin thickness by changing the magnetic powders are shown in Table 5. The magnet moulded was a ring shaped magnet of the outer diameter 32.8mm, the inner diameter 31.8mm and the thickness 0.5mm, and it was moulded by an extrusion. The compound used comprised 60 vol. % of a magnetic powder and 40 vol. % of a resin and nylon 12 was used for the resin. Further as for the magnetic powder, a rare earth magnet having a composition of Sm (Co 0.672 Cu 0.08 Fe 0.22 Zr 0.028) 8.35 was used, it was adjusted to make the average particle size r for each test of Test 11 - 15 and for comparative examples Test 16 - 18, and the results are shown in Table 5. As shown in Table 5, there was a change of the magnet performance of the magnet moulded by changing the average particle size r of the magnetic powder. By making the average particle size r of the magnetic powder small, the magnet performance of the magnet is improved. And a sufficient magnet performance was obtained when the average particle size r was not more than 1/10 of the thickness of the magnet moulded article. However in case when the average particle size is not less than 1/10 of the thickness of the magnet, the magnet performance is low. Test No r [µm] Br [T] [kG] (BH) max [kJ/m 3 ] [MGOe] 11 50 0.588 (5.88) 60 (7.5) 12 20 0.595 (5.95) 62 (7.8) 13 10 0.618 (6.18) 64 (8.1) 14 1 0.633 (6.33) 68 (8.5) 15 0.1 0.616 (6.16) 64 (8.0) 16 70 0.560 (5.60) 52 (6.5) 17 100 0.501 (5.01) 41 (5.2) 18 150 0.470 (4.70) 36 (4.5) The reason why the above results were obtained was, in case of the extrusion moulding, the magnetic powder in the molten mixture of the magnetic powder and the resin was orientated at a section in which a magnetic field was charged by passing through the passage of a die, and the moulded article was extruded outside the die after solidifying it with cooling in the die by keeping the state as it is. Accordingly the molten mixture received a friction force at the contact section with the die. Thus the orientation of the magnetic powder at the contact surface of the molten mixture with the die might be disordered by the friction force while it was being solidified with cooling even though the magnetic powder was once orientated in the die. As it is clear from Table 5, in order to reduce the influence, it is appropriate that the average particle size r of the magnetic powder is 1/10 or less of the thickness of the magnetic moulded article. Example 8 An alloy having a composition of Nd 14 Fe 81 B 5 was molten in a crucible, it was cooled rapidly by a melt-span method and a thin piece was prepared. The thin piece was crushed till it had an average particle size of 35µm, and treatments shown in Table 6 were processed thereafter. Treatment Treatment 1 Cobalt-phosphorus•chromium plating Treatment 2 SiO 2 coating Treatment 1 was that after crushing the magnetic powder, a cobalt-phosphorus plating was carried out in a sodium hypophosphite reduced ammonia, alkaline cobalt plating bath, then a chromate treatment was performed by putting the magnetic powder in a potassium dichromate solution and a cobalt plating layer was formed on the magnetic powder. Treatment 2 was that pure water adjusted its pH with hydrochloric acid was mixed with tetramethoxysilane to make an approximate molar ratio of 4:1, and a hydrolysis was carried out by adding ethanol to it. After the decomposition and an addition of a surfactant, the magnetic powder was added and was stirred for a predetermined time. Then the magnetic powder was separated from the solution, was dried and a heat treatment was performed to form SiO 2 film on the magnetic powder. After the surface treatment, the magnetic powder and the resin to make a ratio of 60 vol. % and 40 vol. % were weighed and were mixed, and after the mixing, it was charged in a kneading machine to knead it and a compound was prepared. The kneading machine herewith used was a roller mill. Further as for the resin a thermoplastic polyamide resin (nylon 12) was used. After crushing the compound produced, it was mould by an extrusion moulding machine. Firstly in order to check in which step the film tended to be removed, a coverage rate was determined by taking a sample before and after each step in the total process. The magnetic powder used here was a plating-treated one with the plating thickness of 1 µm, and the moulding was carried out by an extrusion moulding machine. The results are shown in Table 7. From Table 7, it is clear that the film coverage rate suddenly dropped between before and after the kneading. In other steps, the drop of the coverage ratio was small, and it was considered that it was due to a protection of the coated film by the mixed resin especially after the kneading. Step Coverage Rate (%) Before mixing 100 After mixing (Before Kneading) 95 After kneading (Before crushing) 50 After crushing (Before moulding) 48 After moulding 46 On the other hand, before kneading, the solid resin and magnetic powder are merely mixed, the resin does not give the protection for the coated film, and therefore the coated film is removed by a strong stress to the magnetic powder applied during the kneading. Next an oxidation resistance of a magnet produced, by the production process of this invention was investigated. The results of Test 19 and 20 as this Examples and Test 21 and 22 as comparative examples are shown in Table 8. Surface Treatment Oxidation Resistance Test 19 Treatment 1 A 20 " 2 A 21 " 1 C 22 " 2 B A = Excellent B = Good C = Bad Test 21 and 22 of the comparative examples were the cases in which the kneading was carried out without heating before the kneading, and Test 19 and 20 of Examples were carried out by heating at 10 kPa s (100 kpoise). The oxidation resistance was a result after storing the sample in a constant temperature and constant humidity oven at 80°C x 95 % for 100 hours. It is clear that the oxidation resistance is improved by the heat treatment before the kneading. Example 9 Raw materials to make a composition of Sm (Co 0.672 Cu 0.08 Fe 0.22 Zr 0.028) 8.35 were molten, were casted, an ingot produced was magnetically cured by a heat treatment, and then a magnetic powder having an average particle size of 10µm was obtained by crushing it. The powder was mixed with nylon 12 powder and zinc stearate powder to make a ratio of 92 wt%, 7.8 wt% and 0.1 wt%, respectively. The mixture was then kneaded by a two axes extrusion kneading machine at 260°C. The kneaded mixture was granulated to particles of the outer diameter of 1 - 10mm to make a raw material compound, and a cylindrical magnet was moulded by an extrusion moulding machine. The moulding method is explained in accordance with Figure 10. As shown in Figure 10, the extrusion moulding machine is constituted by a hopper 101 which is a section of charging material, a cylinder 102, a screw 103, an adaptor plate 104 to install a die to the cylinder section 102, a die 105 and a driving motor for the screw (which is not shown in Figure). The aforementioned granulated raw material compound 111 was charged in the extrusion moulding machine. The raw material compound 111 was heated in the cylinder 102 at 260°C to make it in a fluidized state, and it was passed through the die 105. In this Example, the die temperature at the moulding was 250°C, and the cooling was carried out by a forced air cooling at the outlet section of the die. The size of the moulded article produced was the outer diameter of 33mm and the inner diameter of 32mm. The moulded article was made in a thin plate state by a unit shown in Figure 11. Figure 11 was a drawing viewed from upside, the cylindrical moulded article 112 extruded from the die 105 was split into two equal sections of the upside and the downside by the cutter 501 installed in front of the die 105, and the bisected moulded articles were moulded into thin plate state magnet by passing between 2 sets of 2 rollers 502 positioned in the rearward of the cutter 501. Then the end surfaces of the thin plate state magnet 503 was cut to make it in a desired size. The size of the moulded article was the width of 50mm and the thickness of 1 mm. The magnetic property of the moulded article obtained is shown in Table 9. As a comparative example, a thin plate state moulded article was moulded by an extrusion by using a die which was generally used for an extrusion moulding of a thin plate state plastic, and its magnetic property is shown as Test 24. As for the raw material compound, the same magnetic powder, nylon 12 powder and zinc stearate powder as Test 23 of Example were mixed to make a ratio of 91.5 wt%, 8.3 wt% and 0.2 wt% respectively, they were kneaded, were granulated, and were moulded for the determination of the magnetic property. The size of the moulded article was the same as Test 23. In Table 9, a mouldability of this Example (Test 23) and the comparative example (Test 24) is also shown with the magnetic property. (BH) max [kJ/m 3 ] (MGOe) Mouldability Test 23 (Example) 20 (2.5) G Test 24 (Comparative example) 19 (2.4) O G = Good O = Ordinary As it is clear from Table 9, there is no significant difference on the magnetic property between Test 23 of Example and Test 24 of the comparative example, and a magnet, having its magnet performance equal to or above those obtained by the conventional moulding method, can be moulded. On the other hand, from a viewpoint of the mouldability, it was difficult to achieve a stable moulding for Test 24 of the comparative example, and a rate of an rejected article was high. Furthermore the fabrication cost of the die used in Test 24 of the comparative example was approximately 3 times higher than the die of Test 23 of Example, and it was more expensive than a total cost of the die and the press unit of Example. Accordingly, a high performance thin plate state resin bound type rare earth magnet can be produced with a high productivity by using the moulding method of this invention. Example 10 Raw materials to make a composition of Sm (Co 0.672 Cu 0.08 Fe 0.22 Zr 0.028) 8.35 were molten, were casted, an ingot produced was magnetically cured by a heat treatment, and then a magnetic powder having the average particle size of 10µm was prepared by crushing the ingot. This powder was mixed with nylon 12 powder and zinc stearate powder to make a ratio of 92 wt%, 7.8 wt% and 0.2 wt% respectively. Then the mixture was kneaded by a two axes extrusion kneading machine at 260°C. The kneaded mixture was granulated to particles having the outer diameter of 1 - 10mm to make a raw material compound, and a cylindrical magnet was moulded by an extrusion moulding machine shown in Figure 1. The moulding method was the same method to Example 1. As same to Figure 1, the extrusion moulding machine consists of a hopper 101 i.e. a raw material charging section, a cylinder 102, a screw 103. an adaptor plate 104 to equip a die at the cylinder, the die 105 and a driving motor for the screw (which is not shown in Figure), and further an electromagnetic coil 109 to charge a magnetic field in the die 105 is positioned at the outside of the die 105, 106, 107 and 108 are heaters. The aforementioned granulated raw material compound 111 was charged in the extrusion moulding machine. The raw material compound was heated in the cylinder 102 at 260°C to make it in a fluidized state, and was passed through the die 105 of which structure was shown in Figure 2. The die is constituted by an outer die 201 and a mandrel 202. The outer die is made of a non-magnetic material, but a ring shaped magnetic material 201a is installed at the front end to induce a magnetic flux. The mandrel 202 is also made of a non-magnetic material, and at its front end a magnetic material 202a is installed as well. When a current is turned on in the electromagnetic coil 109 installed outside the die, the magnetic flux generated flows in a direction of the arrow H in Figure since it tends to pass in a magnetic material with a high magnetic permeability. Therefore a radiated magnetic field is generated in a space (hereinafter called as an orientation section) between the front end 202a of the mandrel and magnetic material ring 201a installed in the outer die. Thus while a magnetic composition passes through the orientation section, it is moulded with a progress of an orientation of the magnetic powder. In this Example, the magnetic field for the moulding was 1.2 MA/m (15 kOe), the die temperature at the moulding was 250°C, and the cooling was carried out by a forced air cooling at the outlet section of the die. Thus the orientated raw material compound 111 was moulded by an extrusion by a solidification with cooling at the outlet of the die. The size of the cylindrical moulded article was the outer diameter of 33mm and the inner diameter of 32mm. The moulded article was cut into a suitable length, was demagnetised, and further it was divided into two equal sections in parallel with the central axis of the moulded article. The moulded article was then made in a thin plate state by heating at 180°C with a press unit shown in Figure 12. The press unit is to press a moulded article 603 by moving the press plate 601 located in upper position downward as shown by an arrow. The thickness of the moulded article is adjusted by a spacer 602. The press unit was placed in a firing furnace, was heated and the moulded article was set on the press plate. A press was carried out to make a thin plate state when the viscosity of the moulded article dropped, the edges were cut, and finally a thin plate state magnet with the desired size was obtained. The size of the moulded article was the width of 50mm and the thickness of 1 mm. The magnetic property of the moulded article obtained in Test 25 is shown in Table 10. As a comparative example, Test 26 was carried out by an extrusion moulding by using a die shown in Figure 6, and the magnetic property of the moulded article in a thin plate state is also shown. As for the raw material compound, the same one to Test 25 was used. In the die, when a current is turned on in the electromagnetic coil 403, a magnetic field vertical to the passage of the compound in the die is formed between upper and lower pole pieces 404. Therefore a thin plate state magnet of which the magnetic powder is orientated in the thickness direction can be moulded. The magnetic field at the moulding was 0.88 MA/m (11 kOe), and the size of the moulded article was the same to Example of Test 26. A mouldability is also shown in Table 10 with the magnetic property. (BH) max [kJ/m 3 ] (MGOe) Mouldability Test 25 (Example) 60 (7.5) Good Test 26 (Comparative example 20 (2.4) Ordinary As it is clear from Table 10, though the same moulding raw material was used, the comparative example Test 26 showed lower magnetic property. It was considered that it was due to an impossibility to enlarge the magnetic field for the moulding because of the structural problem of the die in case of the comparative example Test 26 resulting an insufficient orientation of the magnetic powder. Furthermore it was very difficult to achieve a stable moulding in case of Test 26 of the comparative example, and it had a high rejection rate. Moreover the fabrication cost of the die used in the comparative example Test 26 was approximately 3 times more expensive than the die used in Test 25 of Example, and it was more costly than the total cost of the die and the press unit in Example of Test 25. Consequently, by using the moulding method of this invention, a high performance thin plate state resin bound type rare earth magnet can be produced with a good productivity.

Claims: A resin bound type cylindrical magnet formed of magnetic powder and an organic resin, wherein the magnetic powder makes up at least 90% by weight of the magnet, the magnetic powder being a rare earth magnetic powder having a coercive force of 0.56 MA/m (7kOe) or greater and having an average particle size (r) and the magnet having a thickness (t), characterised in that; the magnet has radiated anisotropy, fulfils the requirements of: r ≤ 0.1t, with t ≤ 1mm and the resin is a thermoplastic. A resin bound type magnet as claimed in claim 1 wherein the magnet is an extrusion moulded magnet. A resin bound type magnet as claimed in claim 1 or claim 2 in the form of a hollow cylinder, the magnet having an outer diameter (D), an inner diameter (d), and a length (L), the magnet fulfilling the requirements of: 2DL/d 2 ≥ 1. A resin bound type magnet as claimed in claim 3 wherein t = (D-d)/2≤ 1mm. A resin bound type magnet as claimed in any preceding claim wherein the magnet comprises an additive in addition to the magnetic powder and the organic resin. A resin bound type magnet as claimed in any preceding claim wherein the rare earth magnetic powder is selected from magnetic powders of a magnet having a composition comprising a rare earth metal and transition metals mainly constituting cobalt and iron or a magnet having a composition comprising a rare earth metal, boron, and a transition metal mainly constituting iron. A resin bound type magnet as claimed in any preceding claim wherein the thermoplastic resin is selected from one or more of polyamide, polypropylene, polycarbonate, polyphenylenesulphide and chlorinated polyethylene. A resin bound type magnet as claimed in claim 5 wherein the additive is selected from one or more of zinc stearate, calcium stearate, wax and peroxides. A production process of a resin bound type cylindrical magnet wherein a raw material comprising a magnetic powder and an organic resin, with the magnetic powder making up at least 90% by weight of the magnet, is moulded by extrusion moulding by passing the material through a die in a magnetic field, the magnetic powder being a rare earth magnetic powder having a coercive force of 0.56 MA/m (7kOe) or greater and having an average particle size (r) and the moulded material forming a body having a thickness (t); characterised in that; the body has radiated anisotropy, satisfies the requirement of r ≤ 0.1t, with t ≤ 1 mm and the organic resin is a thermoplastic. A production process of a resin bound type magnet as claimed in claim 9 wherein the moulded material forms a hollow cylinder having an outer diameter (D), an inner diameter (d), a length (L), wherein the material is formed to a body satisfying the requirements of 2DL/d 2 ≥ 1. A process as claimed in claim 10 wherein t = (D-d)/2 ≤ 1mm. A production process of a resin bound type magnet as claimed in any one of claims 9 to 11 wherein the raw material comprises an additive in addition to the magnetic powder and the organic resin. A production process of a resin bound type magnet as claimed in any of claims 9 to 12 wherein a die structure (105) for moulding having one end of a mandrel section (202a) which projects longitudinally beyond an edge of an outer die (201) is constituted, a magnetic circuit is formed among the said mandrel section (202a), the outer die (201a) and an electromagnetic coil (109) installed at an outer circumference of the said die structure (105); the magnet is moulded in a cylindrical form by charging a magnetic field in the magnetic circuit by the electromagnetic coil (109), and the moulded article extruded is demagnetised at the said one end of the mandrel (202a). A production process of a resin bound type magnet as claimed in claim 13 wherein an electromagnetic coil (113) with an air-core is also installed in front of the aforementioned electromagnetic coil (109) to generate a magnetic field for demagnetisation in the said electromagnetic coil (113), and the moulded article extruded is demagnetised. A production process of a resin bound type magnet as claimed in any one of claims 9 to 14, wherein the surface of the magnetic powder is coated with a metal plating or ceramics. A production process of a resin bound type magnet as claimed in any of claims 9 to 15, wherein the magnetic powder is selected from magnetic powders of a magnet having a composition comprising rare earth metal, transition metals mainly constituting cobalt and iron or a magnet having a composition comprising a rare earth metal, boron, and a transition metal mainly constituting iron. A production process of a resin bound type magnet as claimed in any of claims 9 to 16, wherein the thermoplastic resin is selected from one or more of polyamide, polypropylene, polyphenylenasulpyhide and chlorinated polyethylene. A production process of a resin bound type magnet as claimed in any of claims 9 to 17 wherein the said additive is selected from one or more of zinc stearate, calcium stearate, wax and peroxides.

Inventors: AKIOKA KOJI, IKUMA KEN, SAKATA MASAAKI, SHIMODA TATSUYA, AKIOKA, KOJI, IKUMA, KEN, SAKATA, MASAAKI, SHIMODA, TATSUYA, AKIOKA, KOJI, ROOM 336 KANAZAWASEIWA-SO, IKUMA, KEN, ROOM 101 MEZON SHANDORO, SAKATA, MASAAKI, ROOM 401 KANAZAWASEIWA-SO, SHIMODA, TATSUYA, NO 10017-16 OCHIAI
Applicants: SEIKO EPSON CORP, SEIKO EPSON CORPORATION
Application year: 1990
Main classification: H01F41
IPC codes: H01F41