Magnet roll and method for manufacturing the same
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a magnet roll and method for manufacturing the same, and particularly to a magnet roll suitable for use in a developing device of electrophotographic copier as disposed inside of a developing sleeve along the circumference of which is transported toner. 
     2. Description of the Prior Art 
     In a developing device for developing an electrostatic latent image formed on an imaging surface, such as the surface of an electrophotographic member, by application of toner to the latent image, a developing sleeve is rotatably provided as a toner carrier and a magnet roll is disposed inside of the sleeve thereby producing a magnetic field at the surface of the sleeve to have the toner magnetically attracted to the peripheral surface of the sleeve. Such a magnet roll typically includes a magnet formed by sintering a magnetic material such as ferrite. In this case, however, since ferrite is brittle, there is difficulty in forming various shapes using ferrite and ferrite must be provided at those locations where unnecessary so as to maintain its integrity when manufactured. For this reason, the prior art magnet roll tended to be heavier in weight and higher in manufacturing cost thereby requiring the provision of costly associated parts for supporting and driving to rotate the magnet roll. Moreover, since the prior art magnet roll was manufactured by sintering, there was a difficulty in dimensional control due to distortions caused by sintering, which could also hinder to attain an intended performance. It is true that a secondary processing may be carried out to the magnet roll after sintering to attain a desired dimensional accuracy, but such a secondary processing will push up the manufacturing cost. 
     FIG. 1 shows a prior art doughnut-shaped magnet roll 1. In this case, the doughnut-shaped magnet 1a was first formed by sintering and then fitted onto and fixed to a shaft 1b, for example, by an appropriate adhesive. In this example, the outer peripheral surface of the magnet 1a required to be abraded as a secondary step so that there was a difficulty in manufacture. Besides, the overall structure was quite heavy and thus it required a large amount of driving power for rotation. 
     FIG. 2 shows another prior art magnet roll 2 which is suited for providing a plurality of magnetic poles at unequal spacing as different from the structure of FIG. 1. In this case also, the overall structure is doughnut-shaped with intermediate portions A interposed between adjacent magnetic portions 2a in order to increase integrity of the structure. This magnet roll is also disadvantageous because the portions A must be provided to compensate the physical weakness of ferrite magnets 2a thereby increasing not only weight but also cost. 
     FIG. 3 shows a further prior art magnet roll 3 which was proposed to overcome the shortcomings of the previous two prior art magnet rolls shown in FIGS. 1 and 2. That is, in this case, in order to make the overall structure light in weight by removing unnecessary portions, a plurality of elongated, rectangularly shaped magnets 3a were first manufactured and fixedly attached to a cylindrical support 4 at those locations where required. In this case, it is true that the overall weight may be minimized, but the manufacturing steps are increased because a plurality of magnets 3a must be fixedly attached to the support 4 one after another. Moreover, the shape of support 4 is rather complicated because of required positioning and secure holding of the individual magnets 3a, so that there is produced another difficulty in forming the support 4. On the other hand, since the magnet 3a is rectangular in cross section, the largest gap G is formed between the magnet 3a and the inner peripheral surface of a developing sleeve 5 at the midpoint G1 of the magnet 3a where the largest magnetic force is normally required when assembled, as shown in FIG. 4. Thus, the structure of FIG. 3 is disadvantageous also from the viewpoint of performance. It is true that one side of the magnet 3a may be formed into a shape to be in compliance with the inner peripheral surface of the sleeve 5; however, such a secondary processing can be a cause of pushing up the manufacturing cost. 
     SUMMARY OF THE INVENTION 
     It is therefore a primary object of the present invention to provide an improved magnet roll and method for manufacturing the same. 
     Another object of the present invention is to provide a magnet roll which is light in weight and sturdy in structure. 
     A further object of the present invention is to provide a magnet roll which is suitable for use in a developing device for developing an electrostatic latent image with the application of toner to the latent image. 
     A still further object of the present invention is to provide a method for manufacturing a magnet roll with ease and at low cost. 
     A still further object of the present invention is to provide a method for manufacturing a magnet roll with high dimensional accuracy without increasing cost. 
     Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 through 3 are schematic illustrations showing several typical prior art magnet rolls using ferrite as a magnetic material; 
     FIG. 4 is a fragmentary, enlarged view showing in detail part of the structure of FIG. 3; 
     FIGS. 5 and 6 are schematic illustrations showing two magnet rolls embodying the present invention; and 
     FIG. 7 is a perspective, schematic view showing a further embodiment of the present invention in which magnets 8 are first manufactured separately and then fixedly attached to its support. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIG. 5, there is shown in cross section a magnet roll constructed in accordance with one embodiment of the present invention when applied to a developing device for developing an electrostatic latent image in an electrophotographic copier. As shown, the structure of FIG. 5 includes a rotary shaft 6 onto which is tightly fitted a support 7. A driving shaft 6a of a motor (not shown ) is integrally fitted into the hollow portion of support 7 so that the support 7 may be set in rotation around the central axis of the rotary shaft 6 as driven by the motor. As shown, the support 7 for securely supporting magnets 8 thereon includes a cylindrical portion 7a and a plurality of fins 7b extending generally radially from the outer peripheral surface of the cylindrical portion 7a at predetermined positions. In the embodiment illustrated in FIG. 5, six fins 7b are provided as extending radially outwardly from the outer peripheral surface of the cylindrical portion 7a and three magnet members 8a, 8b and 8c are provided in spaces S1, S3 and S5, respectively, with five magnetic poles arranged at the outer periphery of magnet roll at unequal spacing. 
     The support 7 is thus generally in the shape of an impeller, and, thus, it may be manufactured easily by extrusion. Accordingly, the impeller-shaped support 7 may preferably be manufactured by extrusion from a light weight material such as aluminum. In this manner, in the present invention, the support 7 may be fabricated quite easily from an inexpensive and light weight material, thereby contributing to keep the manufacturing cost low and the overall structure light in weight. The support 7 thus fabricated is then tightly fitted onto the rotary shaft 6 thereby forming an integrated structure. 
     As described before, the impeller-shaped support 7 includes six fins 7b extending radially outwardly from the cylindrical portion 7a so that there are defined six regions S1 through S6 between any two adjacent fins. In the present embodiment, magnet members 8a, 8b and 8c are only provided in the regions S1, S3 and S5, respectively, with the other regions S2, S4 and S6 left unused. These magnet members 8a, 8b and 8c are comprised of a composite material containing a matrix component, such as nitrile-butadiene rubber, and a magnetic component, such as ferrite. The magnet member 8 thus formed has a specific weight of approximately 3.5 which is significantly smaller as compared with the specific weight of 6 or more possessed by the prior art sintered-ferrite magnet; moreover, the present magnet member 8 is far less brittle. In addition, the present magnet member 8 is easily formable to any desired shape and it deforms very little once formed into a desired shape, thereby allowing to secure accurate dimensional control and performance. As the matrix component, instead of a nitrile-butadiene rubber family material, use may also be made of various resin materials, such as chlorinated polyethylene, which may be uniformly mixed with ferrite particles. 
     As described above, in accordance with the present invention, the impeller-shaped support 7 is comprised of aluminum, which is a light weight material, and the magnet 8 is comprised of a composite material including an appropriate matrix component and a magnetic component and provided only where required, and, thus, the magnet roll of the present invention may be made far lighter in weight than the prior art magnet rolls. For example, the overall weight of the present magnet roll may be reduced to half of that of the magnet roll shown in FIG. 2. When compared with the magnet roll shown in FIG. 3, the overall weight may be reduced approximately by 30%. These comparisons were made with the magnet rolls of 40 mm in outer diameter; however, the larger the diameter, the more the differences in weight. 
     As shown, in the embodiment of FIG. 5, there is also provided a heat-shrinkable tubing 9 which encloses the magnet roll structure including the impeller-shaped support 7 and the magnet members 8a through 8c as a unit thereby allowing to increase the integrity between the support 7 and the magnets 8. 
     FIG. 6 shows another embodiment of the present invention, which is directed to provide a magnet roll which is, in principle, structurally the same as the previous embodiment shown in FIG. 5 but larger in diameter. As shown, in this embodiment, the magnet members 8 are made as small as practicably possible and the impeller-shaped support 7&#39; includes a pair of cylindrical portions 7&#39;a and 7&#34;a which are different in diameter and arranged concentrically and are connected by a plurality of ribs 7c extending therebetween and a plurality of fins 7&#39;b, six in the illustrated example, which extend radially outwardly from the outer periphery of the outer cylindrical portion 7&#34;a. Thus, there are defined a plurality, six in the illustrated example, of regions for mounting the magnet member 8. In the present embodiment, the impeller-shaped support 7&#39; may also be formed from aluminum. It will be appreciated that the magnet roll of FIG. 6 may be made extremely light in weight from the viewpoint of a material to be used and from the viewpoint of physical structure it provides. 
     Now, a method for manufacturing the present magnet roll having the above-described structure will be described below with particular reference to FIG. 5. In the first place, the impeller-shaped support 7 is formed from aluminum by extrusion. Then, the thus formed support 7 is tightly fitted onto the rotary shaft 6 which has been prepared previously. In this instance, if the resulting magnet roll is to be used under relatively light load condition, then the rotary shaft 6 and the support 7 may be formed integrally from the same material at the same time by extrusion. After formation of the support 7, the magnet members 8 are provided only in the regions S defined between any two adjacent fins 7b and 7b as fixedly attached to the support 7. In the present embodiment, a flowable composite material which is prepared by uniformly mixing a molten matrix component of nitrile-butadiene rubber with a magnetic component of ferrite particles is directely poured into the selected regions S1, S3 and S5 with the application of a magnetic field, and, then, the thus supplied composite material is solidified, thereby providing the magnet members 8a, 8b and 8c as fixedly mounted in the selected regions S1, S3 and S5, respectively. In this instance, it is preferable to use an appropriate shape-forming member, such as a cylindrical cover which may be fitted onto the support 7, thereby allowing to form the magnet members 8a, 8b and 8c as shown in FIG. 5 when hardened. In this case, the present magnet roll may be manufactured with a minimum number of processing steps and a high dimensional accuracy. The above-described method may be equally applied to the case in which use is made of a resin as the matrix component. 
     An alternative method for providing the magnet members 8 to the impeller-shaped support 7 will now be described with particular reference to FIG. 7. In this case, the composite material is not directly flowed into the selected regions; instead, the individual magnet members 8a, 8b and 8c are separately formed by using respective molds. Then, the separately formed magnet members 8a, 8b and 8c are fixedly attached to the support 7 at their respectively selected regions using an adhesive or the like. In this case, since the support 7 is impeller-shaped, the positioning of each of the magnet members 8a, 8b and 8c is extremely easy and accurate. 
     In the illustrated example, after provision of the magnet members 8a, 8b and 8c in the respectively selected regions S1, S3 and S5, the heat-shrinkable tubing 9 is fitted onto the entire structure and heat is applied to the tubing 9 so that the tubing 9 may be brought into contact with the entire outer surface of the magnet roll. If the heat-shrinkable tubing 9 can keep the magnet members 8a, 8b and 8c in position during the operation of the magnet roll, then the use of adhesive may be omitted. 
     As described in detail above, in accordance with the present invention, a magnet roll comprises an impeller-shaped support and an appropriate number of magnet members formed from a composite material which is a mixture including a matrix component, such as rubber and resin, and a magnetic component, such as ferrite particles, and provided as mounted on the support only where necessary. Accordingly, the present invention may provide a magnet roll which is extremely light in weight and yet sturdy structurally. 
     While the above provides a full and complete disclosure of the preferred embodiments of the present invention, various modifications, alternate constructions and equivalents may be employed without departing from the true spirit and scope of the invention. Therefore, the above description and illustration should not be construed as limiting the scope of the invention, which is defined by the appended claims.