METHOD OF PRODUCING PERMANENT MAGNETS BY POLAR ANISOTROPIC ORIENTATION MOLDING

Abstract: A method of producing permanent magnets by polar anisotropic orientation molding in which a material containing a ferromagnetic powder in a plastic binder is molded directly against the plurality of long thin magnetic pole pieces which make up a portion of the mold cavity.

Description: METHOD OF PRODUCING PERMANENT MAGNETS BY POLAR ANISOTROPIC ORIENTATION MOLDING Technical Field of the Invention The present invention relates to methods for producing permanent magnets by molding a plastic material containing a ferromagnetic powder in the presence of a magnetic field, and more particularly to improved methods of performing polar anisotropic orientation molding in orientation magnetic field equipment. Background of the Invention Bonded cylindrical magnets having a plurality of N and S poles arranged alternately and extending parallel to the cylinder axis are employed in such diverse applications as rotors for electric motors and magnetic brushes for handling magnetic toner carrier particles in electrographic copiers. Figure 2 shows such a cylindrical magnet 10 having alternating N, S pole faces 11 extending parallel to the cylindrical axis of the magnet. The magnetic flux lines 12 internal to the magnet enter the body of the magnet at the S poles, extend into the body, and emerge at the N poles. U.S. Patent No. 4,678,616 issued July 7, 1978 to Kawashima notes that it is known to produce such cylindrical plastic magnets by polar anisotropic orientation molding in orientation magnetic field equipment. However, Kawashima points out that in performing polar anisotropic orientation molding, in orientation magnetic field equipment, it is disadvantageous in that the mold is complicated in structure, and due to the high temperature of the molding process, the electromagnetic coils employed in generating the magnetic field are not stable. Summary of the Invention It is the object of the present invention to provide improved methods of producing long cylindrical magnets by polar anisotropic orientation molding. According to one aspect of the present invention, the cylindrical magnets are produced in a mold having electromagnets comprising long thin bars of high permeability magnetic material, wrapped with current conducting coils. One edge of the bars defines a magnetizing pole tip that is aligned parallel with the long axis of a cylindrical molding cavity, and forms a portion of the wall of the cavity, whereby the magnets are molded in contact with the magnetizing pole tips. According to a further aspect of the invention, the present inventors have found that if high temperature insulated wire is employed in the mold, and the mold is cooled with chilled water, excessive temperature within the coils is avoided. According to another aspect of the invention, the pole tips are narrowed at the surface of the cavity to concentrate the magnetic flux in a narrow band, thereby causing the magnetic field lines to deeply penetrate into the mold cavity. The space between the pole tips is either filled with a low permeability material, or is of the same material as the electromagnetic pole but is sufficiently thin such that the material is readily saturated, thereby causing the magnetic flux to penetrate the mold cavity. According to a still further aspect of the present invention, it has been discovered by the inventors that the alignment of magnetic particles in the mold takes place within a few seconds and therefore very large currents can be applied to the coils for these few seconds without destroying the coils in the mold. According to a further feature of the present invention, long cylindrical magnets are molded inside a tube with the pole tips of the orientation magnets in contact with the outside of the tube. Brief Description of the Drawings FIG. 1 is a cross section of a mold for performing polar anisotropic orientation molding according to the present invention; FIG. 2 is a schematic diagram of a cylindrical magnet having polar anisotropic orientation; FIG. 3 is a partial cross section showing an area in the region of a pole tip of the mold of FIG. 1; FIG. 4 is a cross section of a mold having an alternative configuration; FIG. 5 is a cross sectional schematic of the mold cavity of FIG. 1 illustrating an alternative method of molding in a sleeve; and FIGS. 6 and 7 are plots showing the strength of the magnetic field measured at the surface of magnets produced according to the present invention. Modes of Carrying Out the Invention FIG. 1 is a cross sectional view of a mold employed to carry out the polar anisotropic orientation molding method of the present invention. The mold, generally designated 14, comprises a frame 16 having a top half 16' and a bottom half 16". The mold parts along line 17 in FIG. 1. The frame 16 is made of high magnetic permeability material such as low carbon steel, and holds 6 radially arranged long thin bars 18. The bars 18 form the pole pieces of an electromagnet, and are wound with conductor coils 20, such as 10 to 30 turns of 8 to 12 guage copper wire with high temperature insulation (e.g. 220 C) . One edge of each bar 18 and 24 defines a portion of a cylindrical mold cavity 22. Alternating with the bars 18 of high permeability magnetic material, are bars 24 of low permeability material such as non-magnetic stainless steel or brass. FIG. 3 shows a partial section of one of the bars 18 at the pole tip. The pole tip 26 is narrowed to concentrate the magnetic field at the tip. The spaces between the pole tips 26 at the surface of the mold cavity are filled with the low magnetic permeability material 24, thereby preventing a magnetic short circuit between the pole tips, and causing the magnetic field to penetrate deeply into the mold cavity 22. The coils 20 around the pole pieces 18 are connected so that the pole tips 26 alternate in polarity between N and S, and generating magnetic field lines as shown by lines 28 in FIG. 1. For most applications, it is required that the poles be evenly spaced around the periphery of the mold cavity. It can be appreciated however that the spacing of the poles may easily be arranged to provide an asymmetrical pattern if desired. As can be seen from FIG. 1, the construction of the mold is relatively simple, employing a number of identical long thin bars that are inserted into slots in the frame 16. The mold can be cooled with chilled water through pipes 29 flowing through channels (not shown) inside the mold. It has been discovered by the present inventors that the magnetic orientation of the particles in the molten binder in the mold takes place in a relatively short time with respect to the mold cycle time. It has been found that with the water cooling and relatively short magnetization cycle time, the coils 20 can be driven with very high currents without encountering any deterioration of the coils due to overheating. A large (3-6 K watt) direct current power source (not shown) supplies current to the coils 20 in the mold. Preferably the coils in the mold are impedance matched to the power source. A current of between 100 to 600 amps is typically applied for several seconds to form a cylindrical magnet approximately 30 cm long by 3 cm in diameter. For smaller magnets, such as those employed in miniature motors, the time may be reduced to a fraction of a second. One or more ejector pins 30 may be provided in the center of the pole pieces, as shown in FIG. 1, or alternatively between the coils through the low permeability material 24. In operation, power is applied to the coils 20 slightly before injecting the molten material into the mold cavity through one or more gates (not shown) at one end of the mold. This allows the full magnetic field strength to be established prior to introducing the molten material into the mold. Typical nozzle temperatures for injecting nylon or similar polymer binder heavily loaded with anisotropic ferromagnetic powder such as barium ferrite into the mold are 250° to 290°C. After several seconds, current to the coils 20 is terminated. After a sufficient cooling period (15 seconds to 2 minutes depending on the size of the part), the mold is opened and the magnet is removed. Although the process has been found to produce magnets having sufficient strength that no further magnetization is required, post molding magnetization employing a fixture similar to the mold itself can be performed, and will provide a marginal increase in the external magnetic field of the magnet. Such a fixture may be connected to a capacitor discharge magnetizer which provides several thousands of amps for a small fraction of a second. FIG. 4 shows an alternative way of constructing the mold cavity according to the present invention. In this embodiment, spaces between the tips 24 of the magnetic pole pieces 18 around the mold cavity 22 are occupied by thin walled sections 32 of the material comprising the pole piece. When the current in the coils 20 is sufficiently high, the thin walled sections 32 of the mold cavity will magnetically saturate, thereby causing the magnetic field to extend into the mold cavity as shown by flux lines 34 in FIG. 4. The low carbon steel that is employed in the pole pieces for example saturates at 19,000 Gauss, which is readily achieved with the arrangement described. The pole pieces and mold cavity shown in FIG. 4 can be readily produced using the technique of wire electro-discharge machining (EDM) . According to another embodiment of the present invention, as illustrated in FIG. 5, the cylindrical magnets may be formed by molding inside a thin walled tube 36, such as non-magnetic stainless steel. Although non-magnetic stainless steel is presently preferred for the tube, other materials may be employed, such as low carbon steel or brass. In this embodiment, it is not necessary to provide spacers between the pole tips 26. The tube 36 is inserted into the mold prior to injecting the magnetic material into the mold. It has been found that the molded magnetic roller is easily removed from the tube 36 after cooling. Different size magnets over a limited range, can be produced in the same mold by varying the thickness of the tube. The relative width of the pole tips 26 determines the strength of the magnetic field produced at the surface of the magnets. The narrower the pole tips, the more concentrated the magnetic field, and hence the higher the magnetization produced. FIG. 6 is a plot showing the magnetic field measured around the circumference of a magnetic roller produced in a mold having wide pole tips (e.g. slightly less than 60°). FIG. 7 is a similar plot showing the increased magnetic field intensity resulting from a mold having narrowed pole tips as shown in FIG. 1. As can be seen by comparing FIGS. 6 and 7, an increase in field strength of between 20 to 30% was achieved by narrowing the pole tips. Both the broad pie and the narrow pole shape are useful depending on the application. Industrial Applicability and Advantages The magnet molding methods according to the present invention are useful for making rotors for electrical motors and magnetic brushes for electrographic printers. The methods are advantageous in that magnets exhibiting very strong uniform fields are produced.

Claims: Cla ims 1. A method of producing a long cylindrical magnet by polar anisotropic orientation molding, characterized by: the mold having electromagnets comprising long thin bars of high permeability magnetic material, wrapped with current conducting coils, one edge of each bar defining a magnetizing pole tip aligned parallel with the cylindrical axis of the mold, and defining portions of the wall of the mold cavity, whereby the magnet is molded in contact with the pole tips of the orienting magnets, thereby increasing the magnetic field strength of the molded magnet. 2. A method of producing a long cylindrical magnet by polar anisotropic orientation molding, characterized by: portions of the walls of the mold cavity being defined by a plurality of alternating N and S pole tips of the electromagnets employed to orient the magnetic material, and the pole tips being spaced apart by low permeability material, whereby the magnetic flux from the pole tips is directed into the cavity of the mold, thereby increasing the magnetic field strength of the molded magnets. 3. A method of producing a long cylindrical magnet by polar anisotropic orientation molding, characterized by: portions of the walls of the mold cavity being defined by the pole tips of the electromagnets employed in the orientation, and the electromagnets being driven with a large current for a relatively short time during the mold cycle. 4. A method of producing a long cylindrical magnet by polar anisotropic orientation molding, characterized by: molding the magnet in a tube with the pole tips of the orientation magnets in contact with the outside of the tube. 5. A method of producing a long cylindrical magnet by polar anisotropic orientation molding, characterized by: molding the magnet in a tube with the pole tips of the orientation magnets in contact with the outside of the tube, wherein the tube is non-magnetic material. 6. A method of producing a long cylindrical magnet by polar anisotropic orientation molding, characterized by: cooling the mold with chilled water during the molding cycle, and driving the electromagnets in the mold for a small fraction of the mold cycle.

Inventors: FURLANI EDWARD P, LEE J KELLY, VIGODA ABRAHAM M, FURLANI, EDWARD, P., LEE, J., KELLY, VIGODA, ABRAHAM, M.
Applicants: EASTMAN KODAK CO, EASTMAN KODAK COMPANY
Application year: 1990
Main classification: H01F41
IPC codes: H01F41