Compositions – Magnetic – With wax – bitumen – resin – or gum
Reexamination Certificate
2000-12-28
2002-09-17
Koslow, C. Melissa (Department: 1755)
Compositions
Magnetic
With wax, bitumen, resin, or gum
C252S062550, C428S404000, C428S403000
Reexamination Certificate
active
06451221
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to extrudable magnets. More particularly the present invention relates to an extrudable magnet compound with improved flow properties in the extrusion processing equipment. Improved flow properties are provided by adding an external surface additive to the extrudate compound. The present invention also related to methods and materials for producing permanent extruded magnets.
2. Description of Related Art
Permanent magnets of a wide variety of design and made by a wide variety of methods are known in the art, and have been employed in a wide range of applications. For example, permanent magnets are known for use with various types of electric motors, holding devices and transducers, including loudspeakers and microphones. For many of these applications, the permanent magnets have a circular cross section constituting a plurality of arc segments comprising a circular permanent magnet assembly. Other cross-sectional shapes, including square, pentagonal and the like may also be used. With magnet assemblies of this type, and particularly those having a circular cross section, the magnet is typically characterized by anisotropic crystal alignment.
Various designs of permanent magnets have been produced by a number of different methods, including compression molding, injection molding and extrusion molding.
Compression molding is generally a method wherein a magnet composition comprising a magnetic powder and a thermosetting resin is filled into a mold in a press at room temperature. The composition is then compressed and heated to cure the resin, thereby molding a magnet. In the case of the compression molding method, since the resin content of the magnet composition is lower than that for the other molding methods, the freedom of shape in molding a magnet is smaller although the magnetic properties of the resultant magnet are often better.
Injection molding is a method wherein a magnet composition comprising a magnet power and a resin component is heat-melted to prepare a melt having sufficient fluidity. The melt is then injected into a mold where the melt is molded into a desired shape. In the case of the injection molding, in order to impart sufficient fluidity to the magnet composition, the resin content of the magnet composition is higher than that for the compression molding method, which often results in lowered magnetic properties. The freedom in molding, however, is generally higher than that for the compression molding method.
Extrusion molding is a method wherein a magnet composition comprising a magnet powder and a resin component is heat-melted to prepare a melt having sufficient fluidity. The melt is then formed into a desired shape by extruding the melt through a die. The melt is then set by cooling, thereby providing a product having a desired shape. In the extrusion method, like the injection molding method, the resin content needs to be high enough to impart the magnet composition the desired and necessary fluidity. An advantage of the extrusion method is that a thin-walled and long magnet can be easily produced.
Among the above methods, injection molding and extrusion generally use a thermoplastic resin as the resin. These are disclosed in Japanese Patent Laid-Open Nos. 123702/1987, 152107/1987, 194503/1985 and 211908/1985. These methods, and others, are also disclosed in U.S. Pat. Nos. 5,888,416, 5,047,205 and 4,881,984, the entire disclosures of which are incorporated herein by reference.
Accordingly, for various permanent magnet applications, it is known to produce a fully dense rod or bar of a permanent magnet alloy, which is then divided and otherwise fabricated into the desired magnet configuration. Such a bar can be formed by any of the above-described processes, although the extrusion method is particularly well suited for forming such a bar, as the process can be operated on a continuous basis.
A problem generally encountered in using the extrusion method, however, is that the material being extruded must exhibit good flow properties both in the granular material state and during processing, i.e., in the dynamic state wherein mechanical and thermal forces are applied to melt, mix, and move the “binder resin” and other additives through the processing equipment. For example, in the granular state, the material must be able to flow from the feed hopper into the extruder in a stable or uniform manner over time. If the flow is not stable and uniform, a problem can arise that material can cake on or adhere to the walls of the hopper. If the hopper is heated, as it often is, the caked material can solidify into solid chunks. These chunks can in turn further deteriorate or completely impede the flow of material into the extruder, and can result in unuseable extruded products. Similarly, and equally important, in the melted state the compound must flow properly in order to extrude into the proper shape.
To address the problem of flow properties, it is generally known to add various flow additives to the extrusion material. For example, it is known to add various calcium and/or zinc stearates to improve the flow properties of the material, in both the granular and the melted state. However, this approach is not always suitable in all applications due to potential unwanted side effects. For example, in the case of calcium stearates, a high temperature is required subsequent to extrusion to burn off any residual calcium from the product. However, these high burn-off temperatures can exceed the processing temperatures of the material, and thus either excess calcium must be tolerated, or the extruded material is degraded by the high temperatures. Similarly, in the case of zinc stearates, the zinc can be detrimental to the mechanical and magnetic properties of the material. Furthermore, zinc can be incompatible with some product materials, and can result in out gassing of chlorine gas. Additionally, incorporating the aforementioned flow additives into the bulk of the compound prior to granulation has little or no impact on the granular flow properties.
SUMMARY OF THE INVENTION
There is thus a need in the art for improved manufacturing methods and materials for producing high quality permanent magnets in a reliable process. In meeting this need, it is required that the process and materials be compatible with the processing equipment, without degrading the magnetic and other properties of the magnets being produced.
These and other objects are satisfied by the present invention, by adding an external additive to the extrudate material, to improve the flow properties of the material in the processing equipment, while at the same time not degrading performance characteristics. The additive thus improves the flow properties of the material, both in the granular and in the melted state. This is achieved in the present invention by using, as an external flow additive, materials such as, for example, extremely small particle size fumed silicon dioxide, commonly sold under the trade name AEROSIL, or a fine particle size ferrite.
More particularly, in embodiments, the present invention provides an extrudable magnet composition, comprising:
composite particles comprising thermoplastic resin and magnetic material, and
a surface additive selected from the group consisting of fumed silicon dioxide (or AEROSIL) particles and ferrite particles,
wherein said surface additive is applied to a surface of said composite particles.
The present invention also provides methods of making extrudable magnets using such a composition, and magnets made thereby and therefrom.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Broadly, the method of the present invention provides for the production of a fully dense, or substantially so, permanent magnet alloy article by producing a particle charge of a permanent magnet alloy composition from which the article is to be made. The charge is placed in a container and the container is evacuated, sealed and heated to elevated temperature. It is then extruded to compact the charg
Eckstrom Lois A.
Hart Steven C.
Koslow C. Melissa
Oliff & Berridg,e PLC
Xerox Corporation
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