Abstract:
A developing unit in an electro-photographic apparatus such as a printer, a facsimile and a copier for actualizing an image employing colored particles such as toner, and more particularly to a developing unit for forming a toner image on the surface of a photosensitive body. The developing unit includes a plurality of developing rollers each having a core and a sleeve. The sleeve accommodates the core. The plurality of developing rollers include a first developing roller and a second developing roller. The sleeve for the first developing roller is rotated to move in the same direction as a moving direction of the surface of the photosensitive body. The sleeve for the second developing roller is rotated to move in an opposite direction to the moving direction of the surface of the photosensitive body. The core for at least one of the plurality of developing rollers is driven in rotation.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a developing unit and an electro-photographic apparatus such as a printer, a facsimile and a copier for actualizing an image employing colored particles such as toner, and more particularly to a developing unit and an electro-photographic apparatus having a feature in a developing process of forming a toner image on the surface of a photosensitive body.  
           [0003]    2. Background Art  
           [0004]    Conventionally, the printers of electro-photographic method have a developing process for actualizing an image on the surface of photosensitive body using colored particles, and a transfer process for transferring the actualized image of colored particles onto a recording medium. The colored particles may be a powder called a toner exclusively used for electrophotography.  
           [0005]    The photosensitive body is evenly charged over its entire surface, and then partially discharged by radiating light. At this time, a potential contrast with a charged region and a discharged region is formed on the surface of photosensitive body to form an electrostatic latent image.  
           [0006]    In the developing process, first of all, the toner particles are charged using a developer. The developer is classified into a two-component type consisting of a mixture powder of the toner and carrier beads as magnetic particles and a one-component type of the toner alone. The developer is received in the developing unit, and agitated.  
           [0007]    Specifically, a two-component developer is composed of a mixture of the toner as colored particles of resin powder having a particle diameter of about 10 micrometers, and the carrier of magnetic particles having an average particle diameter of 30 to 100 micrometers such as ferrite, magnetite, or iron powder. A one-component developer contains a magnetic powder in the toner and not using the carrier.  
           [0008]    In the two-component type developer, the toner is charged due to the friction with carrier beads. Also, in the developer of one-component type, the toner is charged due to the friction with members.  
           [0009]    The developer is conveyed to a developing position opposed to an electrostatic latent image on the surface of photosensitive body by a magnet roller called a developing roller. At this time, a “magnetic brush” in which the developer is arranged like a brush along the magnetic force lines is formed in a region opposed to the photosensitive body.  
           [0010]    In the manner, a developing method in which the developer is conveyed to the electrostatic latent image on the photosensitive body by the magnetic roller is called a magnetic brush development.  
           [0011]    On one hand, a method for actualizing the electrostatic latent image, called a bias development, is often employed. In the bias development, a bias voltage is applied to the developing roller, and the charged toner particles are separated from the developer on the surface of the developing roller, owing to an electric field generated between a latent image potential on the surface of photosensitive body and the developing roller, and moved to the surface of photosensitive body, thereby performing the image formation.  
           [0012]    The charging potential or discharging potential may be employed as the latent image potential, or a potential of an image forming portion of the photosensitive body. Generally, the method of using the charging potential as the latent image potential is called a normal development, or the method of using the discharging potential is called a reversing development.  
           [0013]    One of the charging potential and the discharging potential that is not used as the latent image potential is called a background potential. The bias voltage of the developing roller is set at a certain value between the charging potential and the discharging potential, and a difference from the latent image potential is called a developing potential difference.  
           [0014]    Similarly, a difference from the background potential is called a background potential difference. Usually, the developing potential difference for governing a development performance itself is set to be greater than the background potential difference. With greater developing potential difference, the formed electric field (developing electric field) is increased, so that the development performance (toner developing amount) is enhanced.  
           [0015]    Also, increasing the rotating speeds of the developing roller and the photosensitive body, reducing the distance, and decreasing the electrical resistance of the developer have effect on strengthening the developing electric field, making it possible to increase the toner developing amount.  
           [0016]    A developing method of using the magnetic brush development to convey the developer, and the bias development to actualize the electrostatic latent image (hereinafter referred to as a “magnetic brush bias development”) is widely employed.  
           [0017]    A relative moving direction of the developing roller and the photosensitive body may be the same or opposite. Also, one developing unit may employ a plurality of developing rollers. In some developing unites, the plurality of developing rollers are rotated in the same direction, but in other developing unites, they are rotated in different directions.  
           [0018]    In this case, a developing unit is well-known in which the adjacent developing rollers are rotated in different rotations from the opposite position of developing rollers toward the photosensitive body, and the developer is branched and conveyed like a fountain from the opposite position of developing rollers to the photosensitive body (hereinafter referred to as a “fountain-type developing unit”).  
           [0019]    An example of this developing unit was disclosed in JP-A-54-10869.  
           [0020]    The magnetic brush bias development has a problem that the image edge is difficult to develop in the rotation direction of the developing roller. This problem occurs due to a mechanical factor that the magnetic brush rubs against the surface of photosensitive body, and because the potential of the photosensitive body contacted by the magnetic brush is drastically changed from the background potential of non-image part to the developing potential of image part to change the electrical characteristics of the developer.  
           [0021]    Thus, using the fountain-type developing unit, two developing rollers rotated in different directions compensate each other, thereby solving this problem.  
           [0022]    The developing roller includes a core having the magnetic poles and a metallic sleeve containing this core, in which the developer is conveyed by rotating the sleeve with the core fixed (hereinafter referred to as a “sleeve rotating method”) or rotating both the core and the sleeve (hereinafter referred to as a “core rotating method”).  
           [0023]    In addition, it is called a sleeve rotation developing roll in which the magnetic roll inside the developing roll is fixed. It is called a double rotation developing roll in which the internal magnet roll is also rotated.  
           [0024]    The core rotation method is more effective in agitating the developer in a developing area opposed to the photosensitive body than the sleeve rotation method, and has a greater toner developing amount. Therefore, even if the rotating speed of the sleeve is decreased, a sufficient toner developing amount can be assured to reduce the scraped toner image on the photosensitive body caused by mechanical sliding of the developing roller itself, thereby improving the image quality.  
           [0025]    The core rotating developing unit was described in JP-A-58-142358.  
           [0026]    Further, a developing method using two double rotation developing rolls was described in JP-A-58-142358. As its contents, a first double rotation developing roll rotates the sleeve and the magnet roll in opposite directions, or rotates the sleeve alone, while a second double rotation developing roll rotates the sleeve and the magnet roll in the same direction.  
         SUMMARY OF THE INVENTION  
         [0027]    When the fountain-type developing unit having the feature that no defective image edge is caused due to the electrical factor employs at least one of the developing rollers based on the core rotating method (hereinafter referred to as a “core rotating fountain-type developing unit”), the scraped toner image due to the mechanical factor can be reduced, whereby the very high quality image with less disorder can be produced.  
           [0028]    However, the core rotating fountain-type developing unit has a problem that the image has jitter because two magnet rollers are rotated to cause vibration due to a magnetic field interference between the magnetic poles.  
           [0029]    Also, in the two-component development, there is a problem that the magnetic force is temporally varied in strength in the developing area, and the carrier is likely to be attached onto the photosensitive body when the magnetic force is weak.  
           [0030]    The temporal magnetic force changes in the developing area appear as “fogging” in the one-component development.  
           [0031]    The present invention has been achieved in the light of the above-mentioned problems. It is a first object of the invention to provide a developing unit and an electro-photographic apparatus that can print the high quality image by preventing jitter, splashing of carrier, and fogging from occurring due to vibration.  
           [0032]    Further, in the previous method using two double rotation developing rolls, since a rotating magnetic field or a fixed magnetic field generated by the magnet roll of the first roll and a rotating magnetic field generated by the second roll pass each other in opposite directions at the most proximate point of two developing rolls, a magnetic interference between the first roll and the second roll occurs, or the magnetic poles attract or repel each other, so that the developing unit itself is vibrated around the rotation axis of the double rotation developing roll. Vibration of the developing unit causes the jitter, resulting in nonconformity that the striped unevenness appears on the image. With this constitution, two developing rollers must be fully separated from each other to avoid vibration due to interference of magnetic poles, resulting in a problem that the developing machine is larger in size.  
           [0033]    In order to solve this problem, it is a second object of the present invention to provide an electro-photographic printer that can print a high definition image in a developing unit having a developing roll composed of a sleeve and a magnet roll that are both rotated by reducing vibration due to an interference between one magnetic pole of a rotating magnetic roll and the other magnetic pole.  
           [0034]    In order to attain the first object, the invention provides a developing unit for developing an electrostatic latent image formed on a photosensitive body by supplying colored particles to the surface of the photosensitive body, which includes a plurality of developing rollers each having a core and a sleeve. The sleeve accommodates the core. The plurality of developing rollers includes a first developing roller and a second developing roller. The sleeve for the first developing roller is rotated to move in the same direction as a moving direction of the surface of the photosensitive body. The sleeve for the second developing roller is rotated to move in an opposite direction to the moving direction of the surface of the photosensitive body. The core for at least one of the plurality of developing rollers is driven in rotation.  
           [0035]    In the developing unit as above constituted, one core is driven in rotation, thereby solving the problem that two cores are separately driven in rotation to give rise to vibration due to magnetic field interference between the magnetic poles.  
           [0036]    Preferably, rotation of the core for one of the plurality of developing rollers follows rotation of the core for another one of the plurality of developing rollers.  
           [0037]    In the developing unit as above constituted, one core is driven in rotation, and another core is freely rotated by following the rotation of the one core, whereby a balance point with the least force applied due to magnetic field interference is automatically generated to prevent the periodical vibration from occurring.  
           [0038]    Preferably, the core for one of the plurality of developing rollers is fixed.  
           [0039]    In the developing unit as above constituted, the sleeve is rotated with the core for one developing unit fixed to make the sleeve rotating method, whereby the fixed magnetic poles exist in the developing area opposed to the photosensitive body to exert a strong magnetic force at any time. Therefore, the carrier is attracted and not splashed, and the toner is not splashed to cause fogging on the background portion.  
           [0040]    Also, the rotating sleeve cleans the carrier attached onto the photosensitive body or the toner of fogging on the background portion.  
           [0041]    Preferably, the core is fixed via a damper.  
           [0042]    In the developing unit as above constituted, since the core for the developing roller is fixed, there is the fear that the vibration occurs due to magnetic field interference of the core for the opposed developing roller, but since the core is fixed via the damper, an abrupt change in the torque is prevented by absorbing the rotation of the fixed core along with the rotation of opposed core.  
           [0043]    The invention provides a developing unit for developing an electrostatic latent image formed on a photosensitive body by supplying colored particles to a surface of the photosensitive body, which includes first and second developing rollers each having a core and a sleeve. The sleeve accommodates the core. The core has magnetic poles. The cores of the first and second developing rollers have the same arrangement of the magnetic poles. The magnetic pole of the core of one of the first and second developing rollers is placed at an opposing position where first and second developing rollers mutually oppose, while the core of the other one of the first and second developing rollers is disposed such that a intermediate portion between the magnetic poles thereof is placed at the opposing position. The sleeve of the first developing roller is driven to rotate in the same direction as a moving direction of the surface of the photosensitive body. The sleeve of the second developing roller is driven to rotate in an opposite direction to the moving direction of the surface of the photosensitive body.  
           [0044]    In the developing unit as above constituted, one sleeve is rotated to move in the same direction as the moving direction of the surface of photosensitive body, and the other sleeve is rotated to move in the opposite direction to the moving direction of the surface of photosensitive body, whereby the colored particles can be supplied onto the photosensitive body from two directions, and surely attached to the edge portion of the latent image formed on the surface of photosensitive body.  
           [0045]    In the rotation of two cores having the same arrangement of magnetic poles, the magnetic pole of one core is placed between the magnetic poles of the other core, so that a constant magnetic force is applied on both the developing rollers to prevent vibration from occurring.  
           [0046]    The invention provides an electro-photographic apparatus, which includes a photosensitive body, a charger for charging a surface of the photosensitive body, an exposing unit for exposing the surface of the photosensitive body charged by the charger to form an electrostatic latent image having different potential levels of at least two values on the surface of the photosensitive body, a developing unit for forming a visible image on the photosensitive body by supplying colored particles to the surface of the photosensitive body; and a transfer unit for transferring the visible image onto a recording medium. The developing unit includes a plurality of developing rollers each having a core and a sleeve. The sleeve accommodates the core. The plurality of developing rollers includes a first developing roller and a second developing roller. The sleeve for the first developing roller is rotated to move in the same direction as a moving direction of the surface of the photosensitive body. The sleeve for the second developing roller is rotated to move in an opposite direction to the moving direction of the surface of the photosensitive body. The core for at least one of the plurality of developing rollers is driven in rotation.  
           [0047]    In the electro-photographic apparatus as above constituted, the sleeve for at least one developing roller is rotated to move in the same direction as the moving direction of the surface of photosensitive body, and the sleeve for at least one of the other developing rollers is rotated to move in the opposite direction to the moving direction of the surface of the photosensitive body, whereby colored particles are supplied onto the photosensitive body from two directions.  
           [0048]    At this time, since only one core is driven in rotation, it is possible to solve the problem that two cores are separately driven in rotation as conventionally to give rise to vibration due to magnetic field interference between the magnetic poles.  
           [0049]    Preferably, rotation of the core for one of the plurality of developing rollers follows rotation of the core for another one of the plurality of developing rollers.  
           [0050]    In the electro-photographic apparatus as above constituted, one core is driven in rotation, and the other core is freely rotated by following the rotation of the one core, whereby a balance point with the least force applied due to magnetic field interference is automatically generated to prevent the periodical vibration from occurring. Preferably, the core for one of the plurality of developing rollers is fixed.  
           [0051]    In the electro-photographic apparatus as above constituted, the sleeve is rotated with the core for one developing unit fixed to make the sleeve rotating method, whereby the fixed magnetic poles exist in the developing area opposed to the photosensitive body to exert a strong magnetic force at anytime. Therefore, the carrier is attracted and not splashed, and the toner is not splashed to cause fogging on the background portion.  
           [0052]    Also, the rotating sleeve cleans the carrier attached onto the photosensitive body or the toner of fogging on the background portion.  
           [0053]    Preferably, the fixed core is fixed via a damper.  
           [0054]    In the electro-photographic apparatus as above constituted, since the core for the developing roller is fixed, there is the fear that the vibration occurs due to magnetic field interference of the core for the opposed developing roller, but since the core is fixed via the damper, an abrupt change in the torque is prevented by absorbing the rotation of the fixed core along with the rotation of opposed core.  
           [0055]    The invention also provides an electro-photographic apparatus, which includes a photosensitive body, a charger for charging a surface of the photosensitive body, an exposing unit for exposing the surface of the photosensitive body charged by the charger to form an electrostatic latent image having different potential levels of at least two values on the surface of the photosensitive body, a developing unit for forming a visible image on the photosensitive body by supplying colored particles to the surface of the photosensitive body, and a transfer unit for transferring the visible image onto a recording medium. The developing unit includes first and second developing rollers each having a core and a sleeve. The sleeve accommodates the core. The core has magnetic poles. The cores of the first and second developing rollers have the same arrangement of the magnetic poles. The magnetic pole of the core of one of the first and second developing rollers is placed at an opposing position where first and second developing rollers mutually oppose, while the core of the other one of the first and second developing rollers is disposed such that an intermediate portion between the magnetic poles thereof is placed at the opposing position. The sleeve of the first developing roller is driven to rotate in the same direction as a moving direction of the surface of the photosensitive body. The sleeve of the second developing roller is driven to rotate in an opposite direction to the moving direction of the surface of the photosensitive body.  
           [0056]    In the electro-photographic apparatus as above constituted, one sleeve is rotated to move in the same direction as the moving direction of the surface of photosensitive body, and the other sleeve is rotated to move in the opposite direction to the moving direction of the surface of photosensitive body, whereby the colored particles can be supplied onto the photosensitive body from two directions, and surely attached to the edge portion of the latent image formed on the surface of photosensitive body.  
           [0057]    In the rotation of two cores having the same arrangement of magnetic poles, the magnetic pole of one core is placed between the magnetic poles of the other core, so that a constant magnetic force is applied on both the developing rollers to prevent vibration from occurring.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0058]    [0058]FIG. 1 is across sectional view showing a first embodiment of an electro-photographic apparatus according to the present invention.  
         [0059]    [0059]FIG. 2 is a cross sectional view showing a developing unit for the electro-photographic apparatus according to the first embodiment of the invention.  
         [0060]    [0060]FIG. 3 is a cross sectional view showing an example of a rotating mechanism for a core, taken along the line III-III in FIG. 2.  
         [0061]    [0061]FIG. 4 is a cross sectional view showing a magnetic pole arrangement for the core.  
         [0062]    [0062]FIG. 5 is a cross sectional view showing another example of the rotating mechanism for the core.  
         [0063]    [0063]FIG. 6 is a cross sectional view showing another embodiment of the electro-photographic apparatus according to the first embodiment of the invention.  
         [0064]    [0064]FIG. 7 is a cross sectional view showing a further example of the rotating mechanism for the core.  
         [0065]    [0065]FIG. 8 is a schematic view of a developing unit according to a second embodiment of the present invention.  
         [0066]    [0066]FIG. 9 is a schematic view showing the constitution of an electro-photographic printer.  
         [0067]    [0067]FIG. 10 is a schematic view of two developing rolls in the electro-photographic printer according to the second embodiment of the invention.  
         [0068]    [0068]FIG. 11 is a schematic view of two developing rolls in the electro-photographic printer according to another embodiment of the second embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     The First Preferred Embodiment  
       [0069]    First preferred embodiments of the present invention will be described below in detail with reference to the accompanying drawings. In the embodiments as described below, the same or like parts are designated by the common numerals throughout the drawings, and the description of those parts is simplified or omitted.  
         [0070]    Referring to FIGS.  1  to  4 , a first embodiment will be described below.  
         [0071]    As shown in FIG. 1, an electro-photographic apparatus  10  of the first embodiment comprises a drum-like photosensitive body  11  for forming a print image on the surface, a charger  12  for charging the surface of the photosensitive body  11 , an exposing unit for exposing the surface of the photosensitive body  11  charged by the charger  12  to form an electrostatic latent image having different potential levels of at least two values on the surface of the photosensitive body  11 , a developing unit  15 A for developing the electrostatic latent image by attaching a toner  14  as colored particles onto the surface of the photosensitive body  11  with the electrostatic latent image carried thereon, a transfer unit  17  for transferring the toner  14  attached on the surface of the photosensitive body  11  onto the sheet  16  as a recording medium, a fixing unit  18  for fixing the toner  14  transferred onto the sheet  16 , and a cleaner  19  for cleaning the surface of the photosensitive body  1  after printing.  
         [0072]    Accordingly, the exposing unit  13  consisting of a semiconductor laser and an optical system for emitting light under the control of exposure control means such as a laser driver forms an electrostatic latent image on the surface of the photosensitive body  11  charged uniformly by the charger  12 , and the developing unit  15 A develops the electrostatic latent image by attaching the toner  14 . The toner  14  attached on the surface of the photosensitive body  11  is transferred onto the sheet  16  by the transfer unit  17 , and then heated, melted and fixed on the sheet  16  by the fixing unit  18 . The toner  14  not transferred onto the sheet  16  but remaining on the surface of the photosensitive body  11  is withdrawn by the cleaner  19 , whereby a series of processes are ended.  
         [0073]    In this first embodiment, the magnetic brush bias method with two-component developer is employed as an example of the developing method, and a discharging potential is used as a latent image potential for forming the latent image on the surface of the photosensitive body  11 , in which the reversing development is made.  
         [0074]    [0074]FIG. 2 shows the constitution of the developing unit  15 A for use with the electro-photographic apparatus  10  according to the invention.  
         [0075]    In this developing unit  15 A, a developing unit case  22 , which is shaped like a box and has a rectangular cross section, having an opening  21  on one face, is provided to cover the opening  21  with a part of the photosensitive body  11 , with a slight gap.  
         [0076]    The developing unit case  22  is provided over the entire width (width as seen in the orthogonal direction to the view of FIG. 2) of the photosensitive body  11 .  
         [0077]    As shown in FIG. 2, a plurality of (two in this case) developing rollers  25 U,  25 L having the cores  23 U,  23 L and the sleeves  24 U,  24 L are provided inside the developing unit case  22 .  
         [0078]    In these developing rollers  25 U and  25 L, at least one core  23 U ( 23 L) is rotated in counterclockwise direction in FIG. 2 so that the surface of one developing roller  25 L is moved in the same direction (up to down in FIG. 2) as the moving direction of the surface of the photosensitive body  11 , or is rotated in clockwise direction in FIG. 2 so that the surface of the other developing roller  25 U is moved in the opposite direction (down to up in FIG. 2) as the moving direction of the surface of the photosensitive body  11 .  
         [0079]    That is, the sleeve  24 U of the upper developing roller  25 U is rotated in clockwise direction, and the core  23 U is rotated in counterclockwise direction or stopped, while the sleeve  24 L of a lower developing roller  25 L is rotated in counterclockwise direction, and the core  23 L is rotated in clockwise direction or stopped.  
         [0080]    Thereby, the toner  14  is moved on the surfaces of the sleeves  24 U and  24 L, and supplied through an interstice between the developing rollers  25 U and  25 L onto the surface of the photosensitive body  11 .  
         [0081]    A doctor blade  26  for regulating the flow of the toner  14  is provided between the developing rollers  25 U and  25 L. A scraper  27  for scraping the toner  14  and carrier attached on the surface of the sleeve  24  for the developing roller  25 U,  25 L is provided behind the developing roller  25 U,  25 L.  
         [0082]    A toner hopper  28  for supplying the toner  14  is provided in the rear upward inside the developing unit case  22 . Beneath this toner hopper  28 , a pair of auger screws  29  for agitating and equalizing the toner  14  in the width direction (orthogonal direction to the view of FIG. 2) are provided rotatably. A toner conveying roller  30  for conveying the toner  14  to the developing rollers  25 U and  25 L is provided between the auger screw  29  and the lower developing roller  25 L.  
         [0083]    A toner concentration sensor  31  for sensing the amount of toner  14  is mounted on a bottom face  22   a  of the developing unit toner  22 , whereby the toner  14  is supplied from the toner hopper  28  in accordance with a sensing value of this toner concentration sensor  31 . At the top end (left end in FIG. 2) of the bottom face  22   a  of the developing unit case  22 , a carrier catch roller  32  for capturing the carrier transferred and attached from the developing unit onto the surface of the photosensitive body  11  is provided.  
         [0084]    [0084]FIG. 3 is a cross sectional view of the developing unit, taken along the line III-III in FIG. 2. The upper developing roller  25 U is rotatably provided with the core  23 U inside the sleeve  24 U, in which the sleeve  24 U can be driven in rotation by a motor  33  and the core  23 U can be driven in rotation by a motor  34 . The lower developing roller  25 L is also rotatably provided with the core  23 L inside the sleeve  24 L, in which the sleeve  24 L can be driven in rotation by a motor  35  and the core  23 L can be driven in rotation by a motor  36 .  
         [0085]    In this manner, the toner  14  supplied from the toner hopper  28  is agitated with the carrier by one pair of auger screws  29 ,  29 , and charged at an appropriate level. The charged toner  14  is conveyed to a back portion of the blade  26  by the toner conveying roller  30 .  
         [0086]    Thereafter, the toner  14  passes through an interstice (called a doctor gap) between the doctor blade  26  and the upper and lower developing rollers  25 U,  25 L, and is branched into a toner flow conveyed upward on the surface of the photosensitive body  11  by the upper developing roller  25 U and a toner flow conveyed downward on the surface of the photosensitive body  11  by the lower developing roller  25 L, so that the toner  14  is supplied onto the surface of the photosensitive body  11 .  
         [0087]    Therefore, the surface of the sleeve  24 L for one developing roller  25 L is rotated to move in the same direction as a moving direction of the surface of the photosensitive body  11 , and the surface of the sleeve  24 U for the other developing roller  25 U is rotated to move in the opposite direction to the moving direction of the surface of the photosensitive body  11 , so that the toner  14  is supplied to the photosensitive body  11  from two directions, whereby the toner  14  can be surely attached to the edge of electrostatic latent image formed on the surface of the photosensitive body  11 . Thereby, the image quality can be improved by preventing the defective image edge that conventionally occurred.  
         [0088]    [0088]FIG. 4 shows the arrangement of magnetic poles for the cores  23 U,  23 L in the upper and lower developing rollers  25 U,  25 L. Both the cores  23 U,  23 L have the same arrangement of magnetic poles, in which both the cores  23 U and  23 L are rotated so that the magnetic pole (N-pole here) of one core  23 U is located at a position P where two developing rollers  25 U,  25 L are opposed, and the other core  23 L is located at the position P between magnetic poles.  
         [0089]    The cores  23 U and  23 L can be rotated in various ways, but may be rotated at the same rotating speed in a predetermined direction by the motors  34  and  36 , as shown in FIG. 3.  
         [0090]    Thereby, the cores  23 U and  23 L are rotated so that the magnetic pole of one core  23 U of the cores  23 U,  23 L with the same arrangement of magnetic poles may be located between magnetic poles of the other core  23 L, where by a constant magnetic force is applied to the developing rollers  25 U and  25 L to prevent the vibration from occurring.  
         [0091]    Along with this, the jitter is prevented, whereby the high quality image can be obtained. Also, vibration due to magnetic field interference between the cores  23 U and  23 L can be prevented, whereby the very high quality image can be produced without disorder, jitter, the defective image edge, and the scraping of toner  14 .  
         [0092]    [0092]FIG. 5 shows a second embodiment for rotating the cores  23 U and  23 L. In this embodiment, one core  23 U is driven in rotation by the motor  34 , and the other core  23 L is supported to be freely rotatable, in which the arrangement of magnetic poles for the cores  23 U and  23 L is the same as shown in FIG. 4. In this state, if one core  23 U is driven in rotation by the motor  34 , one magnetic pole of one core  23 U is located between the magnetic poles of the other core  23 L, so that the other core  23 L is dragged due to interference between the magnetic fields, and rotated as a follower at the same rotating speed in the opposite direction, as described previously and shown in FIG. 4.  
         [0093]    Thereby, the cores  23 U and  23 L can take the same effect as in the first embodiment as previously described (as shown in FIG. 3) to produce the high quality image. In addition, a balance point at which the least force is applied due to a magnetic field interference is automatically generated, making it possible to avoid a manufacturing error in the arrangement of magnetic poles for the cores  23 U and  23 L, an adjusting error in the phase of magnetic poles, and a slight vibration caused by the rotational vibration of the cores  23 U and  23 L, whereby the very high quality image can be produced with less periodical vibration such as jitter.  
         [0094]    If one core  23 U is followed by the other core  23 L, as above described, the balance point at which the least force is applied due to magnetic field interference is automatically generated, the arrangement of magnetic poles is not limited to that as shown in FIG. 4, but the arrangement or strength of magnetic poles for the upper and lower cores  23 U and  23 L maybe varied (e.g., see FIG. 6) to obtain a sufficient vibration preventing effect.  
         [0095]    Referring to FIGS. 6 and 7, a third embodiment of a developing machine will be described below. FIG. 6 shows the constitution of the developing unit  15 B. This developing unit  15 B is the same as the developing unit  15 A of FIG. 2 as previously described, except for the arrangement of magnetic poles for the lower core  23 L. The same or like parts are designated by the common numerals, and the duplicate description is omitted.  
         [0096]    [0096]FIG. 7 is a cross sectional view of the developing unit  15 B, taken along the line VII-VII in FIG. 6. As shown in FIG. 7, one core (here, lower core  23 L) is fixed to permit no rotation. Namely, the upper developing roller  25 U relies on a core rotating method, and the lower developing roller  25 L relies on a sleeve rotating method.  
         [0097]    In the sleeve rotating method in which the core  23 L is fixed, the fixed magnetic poles exist in the developing area opposed to the photosensitive body  11 , so that a strong magnetic force is always applied, making it possible to suppress the carrier from splashing over the photosensitive body  11 , and fogging on the background portion. Also, there is the effect of cleaning the carrier attached on the photosensitive body  11  and the toner  14  of fogging on the background.  
         [0098]    Therefore, the upper developing roller  25 U employs the core rotating method to enhance the image quality, and the lower developing roller  25 L employs the sleeve rotating method, whereby the upper developing roller  25 U withdraws the splashed carrier attached to the photosensitive body  11  and the fogging on the background.  
         [0099]    Because the core  23 L of the lower developing roller  25 L is fixed, there is the fear that vibration occurs due to magnetic field interference of the core  23 U in the upper developing roller  25 U. The core  23 L is fixed via the damper  37  to absorb the rotation of the core  23 L caused by rotation of the core  23 U to prevent an abrupt change in the torque, whereby the image quality can be improved by preventing jitter from arising on the image.  
         [0100]    The damper  37  may be an elastic absorber such as a rubber damper to achieve the sufficient effect, but preferably employs a viscous absorber capable of absorbing vibration energy in addition to the elastic absorber to further achieve the vibration proof effect.  
         [0101]    The developing unit and the electro-photographic apparatus of the invention are not limited to the above embodiments, but appropriate variations or improvements may be made within the scope or spirit of the present invention, regarding the core, sleeve, developing roller, damper, photosensitive body, charger, exposing unit, developing unit, and transfer unit in terms of the material, shape, size, form, number, and arrangement.  
         [0102]    As above described, with the present invention as defined in claim 1, the sleeve for at least one developing roller is rotated to move in the same direction as the moving direction of the surface of photosensitive body, the sleeve for at least one of other developing rollers is rotated to move in the opposite direction to the moving direction of the surface of photosensitive body, whereby colored particles can be supplied onto the photosensitive body from two directions, and surely attached to the edge portion of the latent image formed on the surface of photosensitive body. Accordingly, the image quality can be improved by preventing the defective image edge that conventionally occurred. Also, since only one core is driven in rotation, the high quality image can be produced by avoiding the vibration due to magnetic field interference between the magnetic poles which conventionally occurred when two cores were separately driven in rotation.  
         [0103]    Also, according to the invention, the rotation of one core is followed by the rotation of another core, whereby a balance point with the least force applied due to magnetic field interference is automatically generated. Therefore, it is possible to avoid a manufacturing error in the arrangement of magnetic poles for the cores, an adjusting error in the phase of magnetic poles, and a slight vibration caused by the rotational vibration of the cores, whereby the very high quality image can be produced with less periodical vibration such as jitter.  
         [0104]    According to the invention, the sleeve rotating method is employed with the core for one developing roller fixed, whereby the fixed magnetic poles exist in the developing area opposed to the photosensitive body to exert a strong magnetic force at any time. Therefore, it is possible to suppress the splashing of carrier to be attached on the photosensitive body, and the fogging on the background portion. Also, there is the effect of cleaning the carrier attached onto the photosensitive body or the toner of fogging on the background portion.  
         [0105]    According to the invention, the core for the developing roller is fixed, whereby there is the fear that the vibration occurs due to magnetic field interference of the core for the opposed developing roller, but since the core is fixed via the damper, an abrupt change in the torque is prevented by absorbing the rotation of the fixed core along with the rotation of opposed core. Therefore, the image quality can be improved by preventing the jitter from occurring on the image.  
         [0106]    According to the invention, one sleeve is rotated to move in the same direction as the moving direction of the surface of photosensitive body, and the other sleeve is rotated to move in the opposite direction to the moving direction of the surface of photosensitive body, whereby the colored articles can be supplied onto the photosensitive body from two directions, and surely attached to the edge portion of the latent image formed on the surface of photosensitive body. Also, in the rotation of two cores having the same arrangement of magnetic poles, the magnetic pole of one core is placed between the magnetic poles of the other core, so that a constant magnetic force is applied on both the developing rollers to prevent vibration from occurring. Thereby, the high quality image can be produced by preventing jitter from occurring.  
         [0107]    Also, according to the invention, the sleeve for at least one developing roller is rotated to move in the same direction as the moving direction of the surface of photosensitive body, and the sleeve for at least one of the other developing rollers is rotated to move in the opposite direction to the moving direction of the surface of the photosensitive body, whereby colored particles can be supplied onto the photosensitive body from two directions, and surely attached to the edge portion of the latent image formed on the surface of photosensitive body. Accordingly, the image quality can be improved by preventing the defective image edge that conventionally occurred. Also, since only one core is driven in rotation, the high quality image without jitter can be produced by preventing the vibration due to magnetic field interference between the magnetic poles which conventionally occurred when two cores were separately driven in rotation.  
         [0108]    Also, according to the invention, the rotation of one core is followed by the rotation of the other core, whereby a balance point with the least force applied due to magnetic field interference is automatically generated. Therefore, it is possible to avoid a manufacturing error in the arrangement of magnetic poles for the cores, an adjusting error in the phase of magnetic poles, and a slight vibration caused by the rotational vibration of the cores, whereby the very high quality image can be produced with less periodical vibration such as jitter.  
         [0109]    Also, according to the invention, the sleeve rotating method is employed with the core for one developing roller fixed, whereby the fixed magnetic poles exist in the developing area opposed to the photosensitive body to exert a strong magnetic force at any time. Therefore, it is possible to suppress the splashing of carrier to be attached on the photosensitive body, and the fogging on the background portion. Also, there is the effect of cleaning the carrier attached onto the photosensitive body or the toner of fogging on the background portion.  
         [0110]    Also, according to the invention, the core for the developing roller is fixed, whereby there is the fear that the vibration occurs due to magnetic field interference of the core for the opposed developing roller, but since the core is fixed via the damper, an abrupt change in the torque is prevented by absorbing the rotation of the fixed core along with the rotation of opposed core. Therefore, the image quality can be improved by preventing the jitter from occurring on the image.  
         [0111]    Also, according to the invention, one sleeve is rotated to move in the same direction as the moving direction of the surface of photosensitive body, and the other sleeve is rotated to move in the opposite direction to the moving direction of the surface of photosensitive body, whereby the colored particles can be supplied onto the photosensitive body from two directions, and surely attached to the edge portion of the latent image formed on the surface of photosensitive body. Also, in the rotation of two cores having the same arrangement of magnetic poles, the magnetic pole of one core is placed between the magnetic poles of the other core, so that a constant magnetic force is applied on both the developing rollers to prevent vibration from occurring. Thereby, the high quality image can be produced by preventing jitter from occurring.  
       The Second Preferred Embodiment  
       [0112]    A second preferred embodiments of the present invention will be described below with reference to the accompanying drawings.  
         [0113]    Referring to FIG. 9, first of all, an image forming process of the electro-photographic printer will be described below. FIG. 9 is a typical view showing the electro-photographic printer.  
         [0114]    A photosensitive body  101  rotated in clockwise direction has its surface uniformly charged by a charger  102 , an exposing unit  103  blinks light in accordance with the image data, and an illuminated portion on the photosensitive body  101  is conductive so that a charge on the surface disappears.  
         [0115]    A toner image is formed on the photosensitive body  101  by a developing machine  104 . The toner on the photosensitive body  101  is transferred onto the sheet  107  by a transfer unit  105 . The toner image transferred onto the sheet  107  is melted by heating by a fixing unit, though not shown, and fixed on the sheet  107 . Thereafter, the toner remaining on the photosensitive body  101  is removed by a cleaner  106 . The image formation is continued in the same way after this. The consumed toner is refilled from a toner hopper  108  by the rotation of a toner refill roll  181 .  
         [0116]    In an area where the toner is not developed on the photosensitive body  101 , a slight amount of carrier may be developed. Therefore, a developing roll  143  for withdrawing the carrier is provided within the developing machine  104  or in its neighborhood.  
         [0117]    [0117]FIG. 8 shows the details of the constitution of the developing machine  104 . The developing machine  104  has the internal constitution as shown in FIG. 8, containing a two-component developer that is a mixture of the toner and the carrier. The toner and the carrier are mixed by the rotation of the agitators  146  and  147 , and charged by contact charge. Each of the developing rolls  141  and  142  has a magnetic roll internally, the developer conveyed by a paddle  148  is adsorbed onto the surface of sleeve by a magnetic force, and conveyed along with the rotation of the sleeve. The conveyed developer is regulated in amount through a gap between a doctor blade  145  and the developing roll  141 ,  142  in passing by the doctor blade  145 . The developer having passed by the doctor blade  145  is conveyed to a developing area that is a gap portion between the photosensitive body  101  and the developing roll  141 ,  142 , and the toner is developed on the surface of the photosensitive body  101  by the amount in accordance with a bias voltage applied to the developing roll  141 ,  142 , a photosensitive surface potential, and a strength of electric field determined by a development gap that is a gap between the photosensitive body  101  and the developing roll  141 ,  142 .  
         [0118]    A developing roll  141  is a double rotation developing roll in which an internal magnet roll  412  is also rotated. The rotation direction is indicated by the arrow. Furthermore, a developing roll  142  is a sleeve rotation developing roll in which a magnet roll  422  is fixed. The magnet roll  412  of the developing roll  141  magnetically interferes with the magnet roll  422  of the developing roll  142 . Specifically, they attract or repel magnetically. For example, a magnetic pole N 1  of the developing roll  142  repels the N pole of the developing roll  141 , and attracts the S pole. Because the magnet roll  412  of the developing roll  141  is being rotated, repulsion and attraction alternately occur, resulting in variations in the rotational torque of the magnet roll  412 . The variations in the rotation torque are those in the kinetic moment around the central axis, generating vibration in a rotational direction around the rotation axis. In this case, the developing unit is vibrated. Also, since a motive power for the rotation is generated by a motor, variations in the rotation torque have effect on the motor to cause vibrations of the motor in the rotation direction. Those variations have effect on the travel speed of the sheet  107  and the exposing unit  103 , giving rise to striped unevenness in a direction perpendicular to a feed direction of the sheet  107  on the print image.  
         [0119]    Specifically, the circumferential speed of the photosensitive body  101  is 200 mm/s, and the magnet roll  412  of the developing roll  141  has eight poles and a rotating speed of 20 rps, the frequency of magnetic attraction and repulsion is equal to 8 poles÷2×20=80 Hz, so that striped unevenness appeared on the print image at a spatial period of 200÷80=2.5 mm, where “÷” denotes a division sign and “×” denotes a multiple sign. (These signs hereinafter denote the same meanings.)  
         [0120]    An example of means for reducing magnetic interference will be described below.  
         [0121]    If the magnet roll  412  of the developing roll  141  and the magnet roll  422  of the developing roll  142  have the almost same diameter, and the magnetic pole angle between N 1  pole and S 1  pole for the magnet roll  422  and the magnetic pole angle between N pole and S pole for the magnet roll  412  are roughly equal, repulsion and attraction are emphasized depending on the rotational position of the magnet roll  412  of the developing roll  141 .  
         [0122]    For example, when the N pole and S pole of the rotatable magnetic roll  412  comes closer to the magnetic poles N 1  and S 1  of the non-rotatable magnet roll  422 , the N 1  pole and the N pole are repelled, and the S 1  pole and the S pole are repelled, resulting in a greater effect in a direction to suppress the rotation of the magnet roll  412 . On the other hand, when the magnet roll  412  is rotated, and the S pole and the N pole of the rotatable magnet roll  412  comes closer to the N 1  pole and the S 1  pole of the magnet roll  422 , the N 1  pole and the S pole are attracted, and the S 1  pole and the N pole are attracted, resulting in a greater effect in a direction to promote rotation of the magnet roll  412 . In this manner, when two magnetic poles of N pole and S pole of the rotatable magnet roll  412  and two magnetic poles of N pole and S pole of the non-rotatable magnet roll  422  come closer to each other, repulsion or attraction occurs at the same time, resulting in greater variations in the rotation torque.  
         [0123]    Thus, in this invention, the angle or distance between magnetic poles of the rotatable magnet roll was made smaller than the angle or distance between magnetic poles of the non-rotatable magnet roll so that two magnetic poles of N pole and S pole of the rotatable magnet roll and two magnetic poles of N pole and S pole of the non-rotatable magnet roll  422  might not come closer to each other. Thereby, two magnetic poles of each magnet roll did not interfere at the same time, so that vibration was reduced.  
         [0124]    The angle between magnetic poles for the rotatable magnet roll is made smaller than the angle between magnetic poles for the non-rotatable magnet roll, but from the experimental results, it has been found that it is preferably three-fourth or less the angle between magnetic poles for the non-rotatable magnet roll.  
         [0125]    [0125]FIGS. 10 and 11 are typical views of the developing rolls  141  and  142  extracted to explain this invention in detail. In FIGS. 10 and 11, the developing roll  141  has the sleeve  411  and the magnet roll  412  that are rotatable, while the developing roll  141  has the rotatable sleeve  421  and the non-rotatable magnet roll  422 . Reference numerals  413  and  423  denote the center of the magnet rolls  412  and  42102 , respectively. The rotation direction is indicated by the arrow in the figure.  
         [0126]    In FIG. 11, the rotatable magnet roll  412  has eight poles, so that the magnetic pole angle is 45 degrees. If the angle between pole N 1  and pole S 1  of the magnet roll  422  around the center of the magnet roll  422  is changed, the magnitude of vibration is changed. If the angle was increased from 45 degrees, 56 degrees to 60 degrees, the N pole and S pole of the magnet roll  412  and the N 1  pole and S 1  pole of the magnet roll  422  were less likely to interfere at the same time, so that vibration was reduced. Observing the influence on the print image, striped unevenness due to vibration was not seen in a case of 60 degrees. In a case of 56 degrees, appreciable striped unevenness appeared. In a case of 45 degrees, striped unevenness was easily appreciated.  
         [0127]    In FIG. 11, the rotatable magnet roll  412  has twelve poles, so that the magnetic pole angle is 36 degrees. In the case where the angle between pole N 1  and pole S 1  of the magnet roll  422  is 56 degrees, striped unevenness was not seen. In a case of 45 degrees, appreciable striped unevenness appeared.  
         [0128]    From the above results, it has been found that if the angle of magnetic poles of the rotatable magnet roll is smaller than, or preferably three-fourth or less the magnetic pole angle of the magnetic pole for the non-rotatable magnet roll closer to the rotatable magnet roll, it is possible to prevent striped unevenness from occurring.  
         [0129]    In the example of FIG. 8, the magnetic pole angle between N pole and S pole with respect to the center of the magnet roll of the developing roll  141  is set to be three-fourth or less the magnetic pole angle between N 1  pole and S 1  pole with respect to the center of the magnet roll of the developing roll  142  so that interference between the magnet rolls of the developing roll  141  and the developing roll  142  may not be emphasized. In this manner, the developing machine is less subjected to vibration, so that the high definition print image can be printed without striped unevenness.  
         [0130]    Since the positional relation between magnetic poles determines the magnetic interference, if the distance between magnetic poles of the rotatable magnet roll is smaller, or preferably three-fourth or less the distance between magnetic poles of the non-rotatable magnet roll for the magnetic pole, closer to the rotatable magnet roll, it is possible to prevent the striped unevenness from occurring. When the developing roll diameters are different, the magnetic poles may be opposed at the same location to interfere with each other even if the angles between magnetic poles are different, whereby the distance between magnetic poles is set in the condition of avoiding interference and the vibration due to interference, thereby preventing the striped unevenness from occurring.  
         [0131]    As described above, since this invention allows the magnetic poles of the N-pole and S-pole of the rotatable magnet roll not to interfere with the magnetic poles of N-pole and S-pole of the non-rotatable magnet roll at the same time, the high definition image can be printed with the developing unit having the developing rolls composed of the sleeve and the magnet roll that are both rotated by reducing vibration due to interference between one magnetic pole of the rotatable magnet roll and the other magnetic pole.