METHOD OF MANUFACTURING HIGH ENERGY RARE EARTH ALLOY MAGNETS

Abstract: High energy rare earth magnets are formed by injection molding and magnetizing (14, 20) in the mold (12) at a temperature sufficient to substantially reduce the coercivity of the magnetic material, whereby high energy fully magnetized rare earth magnets are formed in a single molding step without the need for post magnetization.

Description: METHOD OF MANUFACTURING HIGH ENERGY RARE EARTH ALLOY MAGNETS Field of the Invention This invention relates to methods of making plastic bonded permanent magnets and more particularly to methods of injection molding such plastic bonded magnets employing isotrophic high energy rare earth alloy magnetic materials. Background of the Invention High energy rare earth alloy magnetic materials require high magnetizing field strength to magnetize the materials to saturation. For example, rapid solidified NdFeB magnetic material typically requires fields on the order of 3183 KA/m or more to substantially fully saturate the magnetic materials. See published European patent application 0 125 752 A2 published 21.11.84. It is known to form permanent magnets by injection molding a mixture of ferromagnetic powder and polymer binder to form a body of magnetic material, and to magnetize the magnetic material in the mold by employing electromagnets within the mold. See for example U.S. patent no. 4,678,616 issued July 7, 1987 to Kawashima. The high magnetic fields required to fully saturate the high energy rare earth magnetic alloy materials are difficult to achieve in an injection mold, particularly for small parts, where the pole pieces are relatively small, and there is little room for large magnetizing coils. Summary of the Invention It is the object of the present invention to provide a method of making permanent magnets by injection molding while magnetizing in the mold, that overcomes the difficulty noted above. The present inventors have discovered that under the elevated temperatures and pressures experienced in the injection molding environment, a surprising reduction in the coercivity of high energy rare earth magnetic materials occurs, and it is therefore possible to fully saturate a permanent magnetic material that ordinarily requires fields in acess of 3183 KA/m at room temperature and pressure with fields of only 7.9 KA/m or less. Thus, according to the present invention, a plastic bonded permanent magnet is formed by injection molding a mixture of high energy rare earth alloy magnetic material (preferably rapid solidified NdFeB) requiring magnetization fields on the order of 3183 KA/m or greater at room temperature and pressure, and thermoplastic powder in a mold having means for providing magnetizing fields of 7.9 KA/m or less, and at a sufficiently elevated temperature and pressure to allow the field to fully magnetize the injection molded magnet, whereby high energy bonded magnets are provided without the need for further magnetization of the finished product. Brief Descri ion of the Drawings Figure 1 is a schematic perspective view of a permanent magnetic roller made according to the method of the present invention; Figure 2 is a cross sectional view of an injection mold useful in practicing the method of the present invention; and Figure 3 is a partial cross sectional view of the injection port of the mold shown in Figure 2. Modes of Carrying Out the Invention Referring to Figure 1, a bonded plastic cylindrical magnet 10 produced according to the present invention is shown. The magnet has longitudinal alternating N and S poles, and is of the type found in rotating motors and employed as toner transport rollers in electrographic apparatus. According to the present invention, the roller 10 comprises a mixture of rare earth alloy permanent magnetic powder, preferably rapid solidified NdFeB, in a plastic binder such as nylon. The magnetic alloy material has a coercivity at room temperature and pressure of 3183 KA/m or greater. According to the present invention, the magnet is formed in injection molding apparatus having means for molding the material in the presence of a magnetic field of 7.9 KA/m or less, preferably about 477 KA/m, and at an elevated temperature and pressure to fully magnetize the magnetic alloy material. Figure 2 is a cross section of a mold useful in practicing the present invention. The mold comprises a mold body 12 supporting a plurality of soft iron pole pieces 14. The pole pieces 14 are in the shape of rectangular blades that fit into slots in the mold body 12. The mold parts along the line 16. At the center of the mold body 12 is a tube 18 that forms a cylindrical mold cavity. The tube may be nonmagnetic material such as stainless steel or aluminum. The pole pieces 14 are in contact with the outside of the tube 18. The pole pieces 14 are wrapped with high temperature insulated wire coils 20, and sufficient current is supplied to the coils during the molding operation to generate a magnetic field at the pole tips of less than 7.9 KA/m, preferably about 477 KA/m. The mixture of magnetic powder and plastic is heated in the injection molding machine and injected under pressure into the mold cavity through a heated barrel 22 shown in Figure 3. Figure 3 is a vertical cross section of the molding apparatus shown in Figure 2 at one end thereof. Current is maintained in the coils 20 while the mold fills, and then is terminated. After a short cooling time, the tube 18 is removed from the mold and the magnet 10 is forced from the tube 18. It has been found that the finished magnets can be produced in this manner with no need for further processing or post magnetization. Working Example A mixture of rapid solidified NdFeB magnetic powder (obtained from General Motors Corporation) 80-90% by weight and nylon 12, polyamid thermoplastic resin purchased from LNP Engineering Plastics Inc., Malvern, PA. was injection molded in apparatus as shown in Figures 2 and 3 at a barrel temperature of 280-290°C and a mold temperature of 90°C to produce a magnetic roller shown in Figure 1. A field of 477 KA/m was applied for 2 seconds while the mold was filled. The resulting magnetic roller exhibited a magnetic field at the surface of 5000 gauss indicating substantially complete magnetization, and a coercivity of 1114 KA/m. Industrial Applicability and Advantages The magnet forming method of the present invention is useful in forming high energy injection molded magnets, and is advantageous in that fully magnetized rare earth magnets are formed in a single injection step with low magnetizing field, without the need for subsequent magnetization steps.

Claims: We claim: 1. A method of forming a bonded plastic magnet comprising the steps of: a. providing a mixture of rare earth alloy magnetic powder requiring a magnetic field of 7.9 KA/m at room temperature and pressure of 3183 KA/m greater to substantially fully magnetize the magnetic powder, and a thermoplastic binder; and b. injection molding the mixture in an injection molding apparatus having means for providing a magnetizing field of less than 7.9 KA/m at a sufficiently elevated temperature and pressure effective to fully magnetize the rare earth alloy magnetic material. 2. The method claimed in claim 1, wherein the rare earth alloy magnetic material is rapid solidified NdFeB. 3. The method claimed in claim-2, wherein the mixture is 80-90% by weight NdFeB powder and polyamid thermoplastic. 4. The method claimed in claim 3, wherein the injection molding is performed at a barrel temperature of 280-290 C and a mold temperature of 90°C. 5. The method according to any of claims 1-4 characterized in that a magneticaing field of about 477 KA/m is applied to said mixture.

Inventors: LEE J KELLY, REZNIK SVETLANA, LEE, J., KELLY, REZNIK, SVETLANA
Applicants: EASTMAN KODAK CO, EASTMAN KODAK COMPANY
Application year: 1991
Main classification: H01F1
IPC codes: H01F1, H01F13, H01F41