1. Field of the Invention
This invention relates to a ferrite magnet suitable for use in electronic copying apparatus, audio equipment, rotary machines, etc., and a method of producing such ferrite magnet by means of extrusion compacting.
2. Description of the Prior Art
Heretofore, a ferrite magnet used in speakers, small DC motors, magnet type rotary machines, etc., has usually been produced by compacting, and when an anisotropic ferrite magnet of high magnetic properties is produced, magnetic field compacting in which compacting is carried out in the magnetic field has been in use. For compacting a cylindrical or columnar ferrite magnet of elongated type (having a length at least twice as large as the diameter) suitable for use as a magnet roll used in a developing device of an electronic copying apparatus, for example, what is generally referred to as a rubber press method is used which consists in compacting a ferrite magnet by applying pressure thereto at a right angle to the axis. Such magnet roll is described in specifications of U.S. Pat. Nos. 4,166,263, 4,168,481, 4,266,328, 4,167,718 and 4,169,998 (granted to Yamashita et al.). However, it is only an isotropic ferrite magnet that is produced by the rubber roll method.
In many applications, an ordinary magnet roll is required to have the magnetic properties of Br.gtoreq.3300 G and of the magnetic flux density Bo.gtoreq.700 G on the sleeve. In addition, it is also required that a compact size be obtained (having an outer diameter of below 24 mm) in a magnet roll. In the aforesaid rubber press method, however, difficulties would be experienced in obtaining improved uniformity of the magnet in a major axis direction because of the fact that compacting is carried out through a rubber mold. Another disadvantage in the rubber press method is that when sintered bodies of about 25 mm in diameter are ground to a size of about 24 mm in outer diameter, the Br is in the range 2000 and 2100 G at most because the ferrite magnet obtained in isotropic.
In one method known in the art for producing an anisotropic ferrite magnet, anisotropy is imparted beforehand prior to compacting, as described in Japanese Patent Publication No. 47043/78, for example. Another known method is an extrusion compacting which is used for producing a cylindrical rubber magnet or a plastic magnet used in a small motor, for example. It is disclosed in Japanese Patent Application Laid-Open No. 75536/74 that an elongated cylindrical ferrite magnet for use as a magnet roll is produced by extrusion compacting.
The extrusion compacting methods referred to hereinabove have high forming efficiency, so that productivity can be greatly increased. Besides they are capable of imparting anisotropy to a certain degree to the magnet when it is produced, thereby enabling an improvement in magnetic properties to be expected. However, the ferrite magnet produced by the extrusion compacting methods of the prior art is unable to meet the properties as an anisotropic ferrite magnet. To obviate this disadvantage of the extrusion compacting methods of the prior art, it has been usual practice to carry out extrusion compacting in a magnetic field and it is proposed, as shown in Japanese Patent Publication No. 34192/72, to use a special nozzle in addition to the application of a magnetic field. Thus, compacting of a ferrite magnet in a magnetic field is generally practised to obtain an anisotropic ferrite magnet by extrusion compacting. One encounters the problems, when this method is used, that a static magnetic field of a current of a high value is required and hence the apparatus becomes large in size, and in addition the magnetic field must have a considerably high value of about 5000 G.