Iron-based particles have long been used as a base material in the manufacture of structural components by powder metallurgical methods. The iron-based particles are first molded in a die under high pressures in order to produce the desired shape. After the molding step, the structural component usually undergoes a sintering step to impart the necessary strength to the component.
Magnetic core components have also been manufactured by such power metallurgical methods, but the iron-based particles used in these methods are generally coated with a circumferential layer of insulating material.
Two important characteristics of an iron core component are its magnetic permeability and core loss characteristics. The magnetic permeability of a material is an indication of its ability to become magnetized, or its ability to carry a magnetic flux. Permeability is defined as the ratio of the induced magnetic flux to the magnetizing force or field intensity. When a magnetic material is exposed to a rapidly varying field, the total energy of the core is reduced by the occurrence of hysteresis losses and/or eddy current losses. The hysteresis loss is brought about by the necessary expenditure of energy to overcome the retained magnetic forces within the iron core component. The eddy current loss is brought about by the production of electric currents in the iron core component due to the changing flux caused by alternating current conditions.
Early magnetic core components were made from laminated sheet steel, but these components were difficult to manufacture and experienced large core losses at higher frequencies. Application of these lamination-based cores is also limited by the necessity to carry magnetic flux only in the plane of the sheet in order to avoid excessive eddy current losses. Sintered metal powders have been used to replace the laminated steel as the material for the magnetic core component, but these sintered parts also have high core losses and are restricted primarily to direct current operations.
Research in the powder metallurgical manufacture of magnetic core components using coated iron-based powders has been directed to the development of iron powder compositions that enhance certain physical and magnetic properties without detrimentally affecting other properties. Desired properties include a high permeability through an extended frequency range, high pressed strength, low core losses, and suitability for compression molding techniques.
When molding a core component for AC power applications, it is generally required that the iron particles have an electrically insulating coating to decrease core losses. The use of a plastic coating over the iron particles (see U.S. Pat. No. 3,935,340 to Yamaguchi) and the use of doubly-coated iron particles (see U.S. Pat. No. 4,601,765 to Soileau et al.) have been employed to insulate the iron particles and therefore reduce eddy current losses.
Recently, it has been found that the insulating polymeric material need not be present in the powder composition as a full coating of the individual iron particles, but rather can be present in the form of discrete particles that are integrally admixed with the iron particles. The present invention is directed to a method of forming this admixture in a manner that ensures homogeneity and thereby leads to improved magnetic properties of pressed parts made with the powder composition. The invention eliminates the need to provide the iron-based particles with a circumferential coating of the polymeric material, which coating generally required the use of more expensive fluidized bed processes.