Method for producing a magnet roll

Abstract: This invention relates to a magnet roll and a producing method therefor in which a magnetic property comparable to that obtained by an injection molding can be obtained in spite of an extrusion molding. According to a first method, the yoke width of the magnetic-field extrusion die is varied along an extrusion direction. According to a second method, a pipe filled with resin bonded magnet material is used as a shaft. According to a third method, a shaft having a groove which is filled with magnet is used. According to a fourth method, a shaft provided with surface magnetic charge is used. According to a fifth method, a magnetized region of a shaft is set to be shorter than the length of a main magnet. According to a sixth method, an extruded matter is moved while stored in a sizing metal mold having a magnetic circuit structure. According to a seventh method, a sizing metal mold for storing an extruded matter therein is installed into a heating device and a cooling device equipped with a magnetic circuit structure while storing an extruded matter therein. According to an eighth method, a preliminary magnetization is carried out with magnetic poles whose number is smaller than a finally-required number of magnetic poles, and then a re-magnetization is carried out with the finally-required number of magnetic poles.

Description: This invention relates to a magnet roll for use in a developing machine using an electrophotographic technique such as a copying machine, a facsimile, a laser beam printer (LBP) or the like and a cleaning machine, and a method for producing the same. A magnet roll has been broadly utilized in a developing machine using an electrophotographic system such as a copying machine. a facsimile or the like. The magnet roll has a function of picking up developing agent from a developing agent box and delivering the pick-up developing agent to a photoreceptor. Figs. 1 and 2 schematically show the construction of an embodiment of the magnet roll as described above. As shown in Figs. 1 and 2, a metal sleeve 2 is rotatably provided around a magnet roll 1 to form a developing cylinder 3, and the developing cylinder 3 is disposed so as to confront a photosensitive drum 4. The delivering operation of the developing agent by the magnet roll 1 is performed with the assist of plural magnetic poles which are magnetized on the surface of the magnet roll, and the intensity of magnetic force (surface magnetic flux density) and magnetic pattern of each magnetic pole is beforehand set in accordance with the role to be performed by the magnetic pole. The following magnet rolls have been known as a conventional magnet roll 1. One is a magnet roll comprising a cylindrical magnet body which is formed by conducting an extrusion molding on resin bonded magnet material, and a metal shaft inserted into a hollow of the cylindrical magnet body over its whole longitudinal body, another is a magnet roll comprising a single body having both end shaft portions which is integrally formed using no metal shaft by conducting an injection molding on resin bonded magnet material, and the other is a magnet roll comprising a solid cylindrical magnet body and short shaft rods inserted into both end portions of the magnet body. The magnet rolls as described above have been generally produced through the following processes, that is, I) a molding process, II) a shaft-inserting process, III) a sizing process, IV) a process for demagnetizing magnets (hereinafter referred to as "demagnetizing process"), and V) a process for re-magnetizing magnets (hereinafter referred to as "re-magnetizing process"). The details of works of the respective processes are as follows. I) Molding Process This process is a process for molding resin bonded magnet material containing magnetic material powder dispersed therein in correspondence to the shape of a magnet roll, and both of the extrusion molding and the injection molding have been mainly used. The extrusion molding is mainly used to form a magnet roll having an uniform sectional shape over its whole longitudinal length such as a cylindrical body, a columnar body, etc., and the injection molding is mainly used to form a magnet roll having its whole body which is integrally formed of resin bonded magnet material so as to contain a shaft portion. The extrusion molding has more excellent producibility than the injection molding. The resin bonded magnet material has a lower magnetic flux density near the surface of a magnet product (hereinafter referred to as "surface magnetic flux density") than a sintered magnet, and thus various devices are made to improve the surface magnetic flux density. Such devices are made in the molding process. For example, in order to allow the resin bonded magnet material to have the maximum surface magnetic flux density when it is magnetized after molding, magnetic particles are beforehand oriented in a predetermined direction in the molding process. As a method of orienting the magnetic particles in a predetermined direction is adopted a "molding in the presence of magnetic field (hereinafter referred to as "magnetic-field molding")" that a magnetic field generator for magnetizing magnet products (hereinafter referred as "magnetizer") is embedded into a portion of an extrusion molding die or a metal mold for injection molding which is to be magnetized, and a magnetic field is applied to melted resin bonded magnet material stored in the die or mold by the magnetizer to thereby conduct the focus orientation on the magnetic particles in a predetermined direction. II) Shaft-Inserting Process This process is a process for inserting a metal shaft into a cylindrical plastic bonded magnet immediately after molded, and a round bar of iron is used as the shaft. III) Sizing Process This process is a process for adjusting the external shape of the molded resin bonded magnet in which the semi-hardened magnet product is accommodated in the die serving as a magnet roll accommodating space to be subjected to a clamping, so that the external shape of the magnet product is adjusted while it is cooled. For a shaft-inserting type of magnet roll, the sizing process is performed immediately after the shaft-inserting process is carried out. On the other hand, for a shaft-integral type injection molding magnet roll, the sizing process is performed immediately after the molding process is carried out. The conventional extrusion molding in the presence of magnetic field has a problem that the degree of magnetic particle orientation in resin bonded magnet material are disturbed or lowered due to difference in magnetic field distribution between the inside of the die and the outlet thereof and existence of magnetic Maxwell stress and self-demagnetizing field until the resin bonded magnet material is cooled and solidified. The extrusion molding in the presence of magnetic field has been known as having high producibility and being suitable for producing a longitudinal resin bonded magnet (JP-A-62-025 410). However, this molding has an inherent disadvantage that instantaneous cooling of an extruded matter is impossible, and thus has the following problems (1), (2) and (3). (1) The extruded matter is deformed by magnetic Maxwell stress from the time when it is discharged from the die till the time when it is cooled and solidified, and thus it is difficult to obtain a magnet product having a desired shape. (2) Since the magnetic field distribution in the neighborhood of the outlet of the die is different from that inside of the die, the orientation state of the magnetic particles which are oriented in a desired direction by the magnetic field inside of the die is disturbed at the outlet portion of the die, and thus a resin bonded magnet having unfavorable magnetic pattern is unintentionally liable to be obtained. (3) The degree of magnetic particle orientation is lowered by the demagnetizing field (self-demagnetizing field), so that the magnetic property is degraded. These phenomena are remarkable particularly in the process of producing a longitudinal magnet formed with a number of magnetic poles. Therefore, a trial-and-error must be carried out many times to develop such products, and it is difficult to produce a magnet roll which has high magnetic property to such an extent as to be matched with a high-quality requirement of a electronic copying machine, a facsimile and an LBP, and is produced in low cost. As a basic method of solving the above problem, the extruded matter which is extruded from the extrusion die for the magnetic field orientation is cooled and solidified under magnetic field which does not disturb the orientation (including a process of cooling and solidifying the extruded matter while it is disposed adjacent to or in contact with magnetically soft material such as iron to erase the demagnetizing field), and this method itself has been well known. The inventors of this application have proposed a method that an endless-shaped magnetically soft material block and/or another permanent magnet block is moved in synchronism with an extruded matter while it is in contact with the extruded matter from the time when the extruded matter is discharged from the extrusion die for magnetic field orientation till the time when it is cooled and solidified, as disclosed in Japanese Laid-open Patent Application No. 62-273707. However, this method is unsuitable for the production of a cylindrical magnet which is magnetized so as to have multi-poles. As another method as disclosed in Japanese Laid-open Patent Application No. 60-182710, the melted resin bonded magnet material is not subjected to the magnetic field orientation in the die, and it is subjected to the magnetic field orientation when it is passed through a sizing mold for both of a cooling and magnetic-field orientation which is disposed at the rear stage of the die. However, in this method. the melted resin bonded magnet material is rapidly cooled and thus its viscosity is heightened. Therefore, there occurs a problem that the degree of magnetic particle orientation is not heightened irrespective of the application of the magnetic field. In addition, since the sizing mold is left static, the orientation of the magnetic particles is obstructed by friction between the inner wall of the sizing mold and the extrusion. As a result, there occurs another problem that no resin bonded magnet product having high magnetic property can be obtained. Accordingly, in many cases, a magnetizing process must be added in a fabricating process after the molding process, and sufficient magnetic field can not be necessarily obtained by the magnetizing process. In view of the foregoing, there have been required a method for producing a resin bonded magnet and a producing apparatus suitable for the method in which the degree of magnetic particle orientation of the resin bonded magnet material is prevented from being disturbed or reduced, and a magnetic property comparable with that obtained through the injection molding can be obtained with keeping high producibility obtained through the extrusion molding. This invention has an object of solving the problem that the magnetic property of the magnet roll is reduced due to the phenomenon that the degree of megnetic particle orientation are disturbed or reduced due to the difference in magnetic field distribution between the inside of the die and the outlet of the die or due to existence of magnetic Maxwell stress and self-demagnetizing field until the resin bonded magnet material is cooled and solidified. According to the invention, the above problem is solved with the features of the claims. In order to attain the above object, in a process of producing a resin bonded magnet, magnet raw material obtained by dispersing and mixing ferromagnetic powder having magnetic anisotropy into thermoplastic resin is subjected to the extrusion molding in the presence of magnetic field, heated under magnetic field in a sizing metal mold having yoke structure which has substantially the same pole arrangement as the extrusion die, and then is cooled together with the metal mold in the presence of magnetic field. According to this construction, the magnetic particles whose orientation has been fixed with being disturbed in the extrusion molding in the presence of magnetic field are re-oriented in the heating process under magnetic field, and then cooled and solidified with keeping the re-oriented state. Consequently, a resin bonded magnet product which has high degree of orientaion at the surface portion thereof, where a function inherent to the manget is substantially developed, and be oriented. That is, the resin bonded magnet which has been once oriented in the extrusion molding in the presence of magnetic field is installed into the heating apparatus while sandwiched by the sizing metal mold having the yoke structure, and the resin at the surface portion is melted by heat of the heating apparatus. The magnetic particles at the surface portion of the melted resin bonded magnet material are in a movable state, and thus re-oriented by applied magnetic field. Thereafter, the resin bonded magnet material is installed together with the sizing metal mold into the cooling apparatus having a built-in magnetic circuit, so that the re-oriented magnetic particles in the resin bonded magnet material are fixed without being disturbed. According to this process, the orientation state of the magnetic particles at the surface portion which are disturbed in the extrusion molding is improved, and a magnet roll having high surface magnetic flux density can be obtained. The product obtained through the extrusion molding according to the invention is demagnetized and then re-magnetized, or re-magnetized without being demagnetized. The invention will be described now with reference to the accompanying drawings, in which : Fig. 1 is a cross-sectional view of a developing apparatus in which a magnet roll is installed; Fig. 2 is a cross-sectional view of the construction of a developing cylinder in which a magnet roll is installed; Fig. 3 is a schematic diagram showing the idea of a heating process under magnetic field and a cooling process under magnetic field according to the invention; Fig. 4 is a perspective view of a sizing metal mold used in an embodiment of the invention; Fig. 5 is a diagram showing a heating apparatus used in the embodiment of the invention; Fig. 6 is a diagram showing a cooling apparatus having no source of magnetic field, which is used in the embodiment of the invention; Fig. 7 is a diagram showing a cooling apparatus having a source of magnetic field, which is used in the embodiment of the invention; The invention has an object of solving the problem that the magnetic property (surface magnetic flux density) of a magnet roll is deteriorated due to disturbance or deterioration in degree of magnetic particle orientation until a molded material is cooled and solidified because of the difference in magnetic field distribution between the inside of the die and the outlet of the die or existence of magnetic Maxwell stress and self-demagnetizing field, and relates to the improvement of a sizing metal mold to attain the above object. Fig. 3 is an explanatory diagram of the producing method of this invention. In Fig. 3, a reference numeral 4 represents a sizing metal mold, a reference numeral 5 represents a resin bonded magnet, a reference numeral 6 represents a heating device, and a reference numeral 7 represents a cooling device. A reference numeral 12 represents a heater serving as a heating source, a reference numeral 10 represents an exciting magnet, and a reference numeral 11 represents a cooling pipe into which cooling water is circulated. Fig. 4 is a perspective view of the sizing metal mold, Fig. 5 is an explanatory diagram of a magnetic circuit of the heating device, and Figs. 6 and 7 are explanatory diagrams of a magnetic circuit of the cooling device. As shown in Fig. 4, the sizing metal mold comprises two or more separatable sub-units, and has a yoke structure having the substantially same magnetic-pole arrangement as the molding die in the presence of magnetic field. The heating device 6 has a sizing metal mold accommodating space 13 at the center thereof, and has a structure that a heater 12, a yoke 8 and an exciting electromagnet 10 are disposed around sizing metal mold accommodating space. As the cooling device 7 may be used a device in which a yoke structsure having the substantially same magnetic pole arrangement as the molding die in the presence of magnetic field is eqipped with a cooling pipe 11 and thus which has no source of magnetic field (Fig. 6), a device in which a yoke structure having an exciting permanent magnet 9 is equipped with a cooling pipe 11 as shown in Fig. 7, or the like. All cooling devices have at the center thereof a sizing metal mold accommodating space 14 capable of accommodating the sizing metal mold in which magnet material is supported. According to this invention, the resin bonded magnet 5 which is molded by the magnetic-field orientation extruding machine is sandwiched by the sizing metal mold 4 having the resin bonded magnet 5 therein is installed into the heating device 6 where the resin bonded magnet 5 is supplied with magnetic field while heated. The resin bonded magnet 5 is softened with heat at the surface portion thereof, and magnetic particles which are allowed to move are re-oriented by the applied magnetic field. After a predetermined time elapses, the resin bonded magnet is moved together with the sizing metal mold to the cooling device. The magnetic particles which have been re-arranged by the heating device are fixed by the magnetic circuit of the cooling device with no great disturbance and no deformation. In the heating device, a magnetic field which is equal or approximate to the magnetic field for orientation in the extrusion molding is generated and applied to the resin bonded magnet, so that the re-orientation of the magnetic particles is promoted by the magnetic field thus generated On the other hand, a source of magnetic field is not necessarily required in the cooling device, and by providing a yoke structure formed of soft magentic material to the magnetic pole portion of the molded product, the self-demagnetizing field can be offset by mirror image effect of the yoke structure, so that the orientation disturbance can be prohibited. Of course, by providing a source of magnetic field, the magnetic particles can be fixed with further strongly keeping the orientation state. This invention will be described in more detail on the basis of the embodiments thereof, however, no limitation is imposed on this invention. SS41, S25C, SUS410, permendur (FeCo alloy) or the like may be used as metal mold yoke in this invention. The embodiments of this invention will be next described. Embodiment -(1) The pellet was formed of the composition of the table 1, and was subjected to the extrusion molding in the presence of magnetic field to obtain a cylindrical resin bonded magnet having an average outer diameter of 16 mm⊘. A shaft was inserted into the cylindrical resin bonded magnet thus obtained, and the cylindrical resin bonded magnet having the shaft therein was heated and cooled in the presence of magnetic field in the processes as shown in Fig. 3. The inner diameter of the sizing metal mold was set to 16 mm  and SS41 and SUS 316 were used as magnetic material of a yoke portion and non-magnetic material of portions between the magnetic poles, respectively. The heating process was carried out at 160 °C for three minutes, and the same magnetic field as the molding magnetic field was applied during the heating process. The device having no source of magnetic field as shown in Fig. 6 was used as the cooling device, and the cooling process was carried out for two minutes. The peak value of the surface magnetic flux density of the magnet roll was measured at the position 19 mm ⊘ away from the center of the magnet roll thus obtained, and the measurement result is shown in a Table 2. Strontium ferrite (OP71, produced by Nihon Bengara Co., Ltd 90.0 weight% Polyvinyl chloride (s1001, produced by Kaneka Corporation) 7.0 weight% Di-2-ethylhexyl phthalate (DOP) 2.8 weight% Tri-basic lead sulphate (Tribase) 0.1 weight% Lead stearate 0.1 weight% Embodiment -(2) A magnet roll was formed by carrying out the same processes as "Embodiment -(1)" except that a device having a source of magnetic field as shown in Fig. 7 was used as the cooling device, and the peak value of surface magnetic flux density of each magnetic pole of the magnet roll is shown in the Table 2. Samarium-cobalt based sintered magnet was used as the source of magnetic field. Comparative Example -(1) A shaft was inserted into a molded sample obtained by the conventional extrusion molding in the presence of magnetic field to obtain a magnet, and the peak value of surface magnetic flux density of each magnetic pole of the magnet roll is shown in the Table 2. Comparative Example -(2) A shaft was inserted into a molded sample obtained by the conventional extrusion molding in the presence of magnetic field, and the result was subjected to pulse magnetization to obtain a magnet roll. The peak value of surface magnetic flux density of each magnetic pole of the magnet roll is shown in the Table 2. As is apparent from the table 2, the producing method of the embodiments of this invention can provide a resin bonded magnet having more improved surface magnetic flux density in comparison with the conventional comparative examples. As described above, according to this invention, in the process of producing a magnet roll of resin bonded magnet, the magnet roll is heated and cooled in the presence of magnetic field while accommodated in the sizing metal mold after the molding process in the presence of magnetic field. Through this process, the magnetic particles on the surface portion of the magnet roll are allowed to be in a high orientation state, and the magnet roll having remarkably improved surface magnetic flux density can be obtained. Therefore, the yield of products is improved, and pattern control of surface magnetic flux density distribution can be facilitated, so that the degree of freedom for development of products is improved.

Claims: A magnet roll producing method characterized by the steps of: a) subjecting a resin magnet material obtained by dispersing and mixing a ferromagnetic material powder having magnetic anisotropy in a thermoplastic resin to extrusion molding in the presence of a magnetic field; b) heating the extruded matter in the presence of a magnetic field in a sizing metal mold; and then c) cooling the extruded matter in the presence of a magnetic field in the sizing metal mold. The magnet roll producing method as claimed in claim 1, wherein in step a) and b) the sizing metal mold storing the extruded matter therein is accommodated in a heating device or a cooling device, respectively. The method as claimed in claim 1 or 2, wherein the magnetic field in step a) is produced by means of a magnetic-field orienting die. The method as claimed in claim 3, wherein said sizing metal mold has the substantially same magnetic-pole structure as the magnetic-field orienting die.

Inventors: ARAI HIDEKO, MOHRI FUMIHITO, SAKATA YOSHIO, ARAI, HIDEKO, MOHRI, FUMIHITO, SAKATA, YOSHIO, Arai, Hideko, c/o Kanegafuchi kagaku Kogyo, Mohri, Fumihito, c/o Kanegafuchi kagaku Kogyo
Applicants: KANEGAFUCHI CHEMICAL IND, KANEGAFUCHI KAGAKU KOGYO KABUSHIKI KAISHA
Application year: 1992
Main classification: H01F7
IPC codes: H01F7, B29C47, G03G15, B29C70, H01F41