Patent Publication Number: US-8526852-B2

Title: Electrostatic charging apparatus, image forming apparatus, electrostatic charging method and image forming method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2010-209268 filed on Sep. 17, 2010. 
     BACKGROUND 
     Technical Field 
     The present invention relates to an electrostatic charging apparatus, an image forming apparatus, an electrostatic charging method and an image forming method. 
     SUMMARY 
     According to an aspect of the invention, there is provided an electrostatic charging apparatus including: an electrostatic charging member that has a discharge face causing discharge between the electrostatic charging member and a to-be-charged member having a charge retention part to hold a charge, and comes in contact with the to-be-charged member so as to charge upon the application of voltage; and an adjusting unit that adjusts an angle between the to-be-charged member and the discharge face. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiment(s) of the present invention will be described in detail based on the following drawings, wherein: 
         FIG. 1  is a view illustrating an image forming apparatus according to a first exemplary embodiment of the present invention; 
         FIG. 2  is a view illustrating an electrostatic charging apparatus provided in the image forming apparatus of  FIG. 1 ; 
         FIG. 3  is a view explaining about an angle θ between an outer periphery of the electrostatic charging member in the electrostatic charging apparatus and a photoconductor; 
         FIGS. 4A ,  4 B and  4 C are views illustrating the electrostatic charging apparatuses shown in  FIG. 2 :  FIG. 4A  is a view illustrating the electrostatic charging member being positioned in a first position;  FIG. 4B  is a view illustrating the electrostatic charging member being positioned in a second position; and  FIG. 4C  is a view illustrating the electrostatic charging member being positioned in a third position; 
         FIG. 5  is a block diagram of a control in the image forming apparatus shown in  FIG. 1 ; 
         FIG. 6  is a flow-chart explaining about the operations of the image forming apparatus shown in  FIG. 1 ; 
         FIGS. 7A and 7B  are views explaining about discharge occurred between the electrostatic charging member provided in the electrostatic charging apparatus and the photoconductor in  FIG. 2 :  FIG. 7A  is a view in the state that discharge is stable, and  FIG. 7B  is a view in the state that discharge is unstable; 
         FIGS. 8A and 8B  are views explaining about processes for electrostatic charging to the photoconductor with the electrostatic charging apparatus shown in  FIG. 2 :  FIG. 8A  is a view explaining about a process that the photoconductor is electrostatically charged in a case of a thick charge retention part in the photoconductor; and  FIG. 8B  is a view explaining about a process that the photoconductor is electrostatically charged in a case of a thin charge retention part in the photoconductor; 
         FIG. 9  is a diagram illustrating the occurrence of a fine black line and an uneven potential on the images formed corresponding to the number of image formations and the number of rotations of the photoconductor when the electrostatic charging apparatus shown in  FIG. 2  charges the photoconductor; 
         FIG. 10  is a diagram illustrating the occurrence of a fine black line and an uneven potential on the images formed corresponding to the number of image formations and the number of rotations of the photoconductor when the electrostatic charging apparatus according to a first comparative example charges the photoconductor; 
         FIG. 11  is a diagram illustrating the occurrence of a fine black line and an uneven potential on the images formed corresponding to the number of image formations and the number of rotations of the photoconductor when the electrostatic charging apparatus according to a second comparative example charges the photoconductor; 
         FIG. 12  is a diagram illustrating the occurrence of a fine black line and an uneven potential on the images formed corresponding to the number of image formations and the number of rotations of the photoconductor when the electrostatic charging apparatus according to a third comparative example charges the photoconductor; 
         FIG. 13  is a view illustrating an electrostatic charging apparatus provided in an image forming apparatus according to a second exemplary embodiment of the present invention; 
         FIGS. 14A ,  14 B and  14 C are views of the electrostatic charging apparatus shown in  FIG. 13 :  FIG. 14A  is a view of the electrostatic charging member being in a first position;  FIG. 14B  is a view of the electrostatic charging member being in a second position; and  FIG. 14C  is a view of the electrostatic charging member being in a third position; 
         FIG. 15  is a view illustrating an electrostatic charging apparatus provided in an image forming apparatus according to a third exemplary embodiment of the present invention; and 
         FIGS. 16A ,  16 B and  16 C are views the illustrating electrostatic charging apparatuses shown in  FIG. 15 :  FIG. 16A  is a view of the electrostatic charging member being in a first position;  FIG. 16B  is a view of the electrostatic charging member being in a second position; and  FIG. 16C  is a view of the electrostatic charging member being in a third position. 
     
    
    
     DETAILED DESCRIPTION 
     Next, an exemplary embodiment of the present invention will be described based on the drawings. 
       FIG. 1  illustrates an image forming apparatus  10  according to a first exemplary embodiment of the present invention. As shown in  FIG. 1 , the image forming apparatus  10  has an image forming apparatus main body  12 , and the image forming apparatus main body  12  has an image forming unit  100 , a paper feeder  300 , and a controller  400  serving as a control unit therein. The image forming apparatus main body  12  further has a transport path  350  through which a paper as a recording medium is delivered. Additionally, the image forming apparatus  12  has an exit  14  for ejecting a paper. 
     The image forming unit  100  is used to form an image having e.g. a single color and employs electro-photography. The image forming unit  100  has a photoconductor  102 , an electrostatic charging apparatus  200  which charges the photoconductor  102 , a latent image forming device  110  which emits light on the surface of the photoconductor  102  charged by the electrostatic charging apparatus  200  so as to form an electrostatic latent image on the surface of the photoconductor  102 , a developing device  114  which develops the latent image formed on the photoconductor  102  with a developer so as to form a developer image on the surface of the photoconductor  102 , a transfer device  116  which transfers the developer image formed on the surface of the photoconductor  102  by the developing device  114  onto a paper, a cleaning device  120  which cleans up the photoconductor  102  after the transferring of the developer image onto the paper by the transfer device  116 , and a fixing device  126  which allows the developer image transformed onto the paper by the transfer device  116  to be fixed on the paper. 
     The photoconductor  102  is e.g. cylindrical, and is employed as a to-be-charged member as well as an image carrier. Further the photoconductor  102  has a base  104  which is made of e.g. aluminum and is e.g. cylindrical. The base  104  is covered with a film  106  on the surface thereof. The film  106  is employed as a charge retention part to hold a charge, consists of e.g. an organic photosensitive layer, and for example, includes a charge generation layer having a charge generation material used for generating a charge and a resin as a binder and a charge transport layer having a charge transport material used for transporting a charge and a resin as a binder. Additionally a under coat layer, a protective layer and the like may be provided. Further, a rotation counter  410  to count the number of rotations of the photoconductor  102  is arranged in the photoconductor  102 . 
     The developing device  114  has a developing device main body  136 , and the developing device main body  136  has a developer carrying member  138  being e.g. cylindrical. The developing device main body  136  further accommodates developer consisting of toner and carrier therein. The toner of the developer is carried toward the photoconductor  102  by the developer carrying member  138 . 
     The cleaning device  120  has a cleaning member  122 , whose the one end presses the photoconductor  102  to remove e.g. a toner from the surface of the photoconductor  102 , for cleaning up the photoconductor  102 . Due to the pressing force by the cleaning member  122 , the film  106  of the photoconductor  102  is subjected to abrasion, thus becoming thinner, as the number of rotations of the photoconductor  102  or the number of formed images by the photoconductor  102  increases. In addition, the film  106  is also subjected to abrasion by contact with the developer carrying member  138 , the transfer device  116  and the electrostatic charging apparatus  200 . 
     The fixing device  126  has a heating roller  128  having internally a heat source and a pressure roller  130  being in connect with the heating roller  128 . The transferred toner image on a paper is fixed by heated and pressurized at the contact region between the heating roller  128  and the pressure roller  130 . 
     The paper feeder  300  feeds the image forming unit  100  with a paper. The paper feeder  300  has a paper container  302  in which papers are stacked and a feed roller  304  to forward papers from the paper container  302 . 
     The transport path  350  is defined as a transport path through which a paper is fed to the paper feeder  300 , the transfer device  116 , the fixing device  126 , and subsequently the exit  14  for ejecting from the image forming apparatus main body  12 . Along the transport path  350 , the above feed roller  304 , a conveyance roller  354 , a registration roller  356 , the above transfer device  116 , and the above fixing device  126  are arranged sequentially from the upstream in the paper transporting direction. 
     The registration roller  356  temporarily stops the movement of a paper toward the transfer device  116  at the top of a paper, and subsequently releases the movement of the paper toward the transfer device  116  in the stopping state at the top of a paper so as to adapt to the timing where a developer image is formed onto the photoconductor  102 . 
       FIG. 2  illustrates the electrostatic charging apparatus  200 . As shown in  FIG. 2 , the electrostatic charging apparatus  200  has an electrostatic charging member  204  and a support  210  supporting the electrostatic charging member  204 . The electrostatic charging member  204  is employed as a charging electrode to charge the photoconductor  102 . Additionally, the electrostatic charging member  204  has an outer periphery  206  employed as a discharge face causing discharge around the photoconductor  102 , and is employed as an electrostatic charging member which comes in contact with the photoconductor  102  to be charged after the application of voltage. The electrostatic charging member  204  is flexible, endless-shaped, cylindrical, and semi-conductive film-shaped or rubber-like. 
     The support  210  being conductive, and e.g. cylindrical, has a smaller outer diameter than the inner diameter of the electrostatic charging member  204 , and thus is arranged inside the electrostatic charging member  204 . An adjusting mechanism  280  served as an adjusting unit which adjusts an angle θ between the photoconductor  102  and the outer periphery  206  of the electrostatic charging member  204  is attached to the support  210 . Additionally, a voltage application device  270  is attached to the support  210 . The angle θ will be described later. 
     The electrostatic charging member  204  is in contact with the photoconductor  102  in a contact position N in the electrostatic charging apparatus  200 . The electrostatic charging member  204  is subjected to a voltage by the voltage application device  270  via the support  210 . During the application of voltage, the electrostatic charging member  204  sticks fast to the photoconductor  204  due to static electricity. The electrostatic charging member  204  thus rotates in the direction of the arrow b in conjunction with the rotation of the photoconductor  102  in the direction of the arrow a. 
     Further, in the electrostatic charging apparatus  200 , there is a wedge-shaped region S which is formed in a position of the upstream side of the contact position N in the direction of rotation of the photoconductor  102  and between the outer periphery  206  of the electrostatic charging member  204  and the photoconductor  102 . Upon the application of voltage to the electrostatic charging member  204  by the voltage application device  270 , an electrical discharge occurs in the region S and thereby the film  106  of the photoconductor  102  is charged. 
       FIG. 3  is a cross-sectional view illustrating enlarged the neighborhood of the contact position N and explaining the angle θ between the outer periphery  206  of the electrostatic charging member  204  and the photoconductor  102 . The angle θ indicates an angle between the photoconductor  102  and the outer periphery  206  of the electrostatic charging member  204  in the neighborhood of the contact position N of the region S, specifically the angle being the upstream side of the contact position N in the direction of the movement of the photoconductor  102 . More specifically, the angle θ indicates an average angle between individual tangent line L 2  of the outer periphery  206  to a tangent point between the point P 10  and the point P 12  and a tangent line L 1 . Note that, the point P 10  is the central point in the width of the contact position N in the direction of the movement of the photoconductor  102 ; the tangent line L 1  is a tangent line of the photoconductor  102  to the point P 10 ; and the point P 12  is another end opposite to the point P 10  in the region S where discharge occurs around the outer periphery  206  of the electrostatic charging member  204 . The region. S includes a unstable discharge region as well as a stable discharge region. 
     The size of the angle θ may be determined by a distance d between the photoconductor  102  and the outer periphery  206  of the electrostatic charging member  204  at a point P 14  located at a given distance from the point P 10  on the tangent line L 1  in the direction of the movement of the photoconductor  102  is positioned within the region S. For example, the size of the angle θ can determined by the distance d between the photoconductor  102  and the outer periphery  206  of the electrostatic charging member  204  at a point located 800 μm away from the point P 10  on the tangent line L 1  upstream in the direction of the movement of the photoconductor  102 . 
       FIG. 4  illustrates the operation of the electrostatic charging apparatus  200 . As shown in  FIG. 4 , the support  210  of the electrostatic charging apparatus  200  is arranged so as to move between positions shown in  FIGS. 4A ,  FIG. 4B , and  FIG. 4C  using the adjusting mechanism  280  (Refer to  FIG. 2 ). The electrostatic charging member  204  moves between positions shown in  FIGS. 4A ,  FIG. 4B , and  FIG. 4C  corresponding to the movement of the support  210 . The positions of the support  210  and the electrostatic charging member  204  shown in  FIG. 4A ,  FIGS. 4B , and  FIG. 4C  will be referred to as a first position, a second position, a third position, respectively below. 
     As the support  210  and the electrostatic charging member  204  move, the angle θ varies. When the support  210  and the electrostatic charging member  204  are in the first position, the angle θ is θ 1 ; when the support  210  and the electrostatic charging member  204  are in the second position, the angle θ is θ 2 ; and when the support  210  and the electrostatic charging member  204  are in the third position, the angle θ is θ 3 . In the case, θ 1  is greater than θ 2 , and θ 2  is greater than θ 3 . 
     In the point located upstream 800 μm away from the contact position N in the direction of the movement of the photoconductor  102 , when the angle θ is θ 1 , the distance between the photoconductor  102  and the outer periphery  206  is 120 μm; when the angle θ is θ 2 , the distance between the photoconductor  102  and the outer periphery  206  is 80 μm; and when the angle θ is θ 3 , the distance between the photoconductor  102  and the outer periphery  206  is 40 μm. 
     The adjusting mechanism  280  (Refer to  FIG. 2 ) is controlled by the controller  400  (Refer to  FIG. 1 ) in the image forming apparatus  10 . After the completion of a series of operations for forming an image, the support  210  and the electrostatic charging member  204  move so as to be in the states of the first position. Therefore, when an operation for forming an image starts, the support  210  and the electrostatic charging member  204  are located in the first position and the angle θ is θ 1 . 
       FIG. 5  illustrates the controller  400 . The controller  400  is employed as a control unit which controls the adjusting mechanism  280  to allow the angle θ to be small in response to increasing the number of charging of the electrostatic charging apparatus  200  and increasing the number of image formations by the image forming unit  100 . As shown in  FIG. 5 , the controller  400  also has a control circuit  402  to which image signal is input via a communication interface  404  and an output signal is input from a rotation counter  410 . The control circuit  402  is connected to a rotation number storage  406  storing data for the number of rotations of the photoconductor  102 . The image forming unit  100  and the adjusting mechanism  280  are controlled by output of signal from the control circuit  402 . 
       FIG. 6  illustrates the operation of the image forming apparatus  10 , i.e. the control of the adjusting mechanism  280  using the controller  400 . As shown in  FIG. 6 , on starting a series of controls by inputting image signal by the communication interface  404 , the controller  400  discriminates whether or not the number of rotations of the photoconductor  102  since the attachment of the photoconductor  102  onto the image forming apparatus main body  12  is equal to or more than predetermined times i.e. a first number of rotations N 1  in the step S 10 . If the number of rotations of the photoconductor  102  is less than the first number of rotations N 1 , the controller  400  terminates a series of controls. If the number of rotations of the photoconductor  102  is equal to or more than the first number of rotations N 1 , the operation proceeds to the next step S 12 . 
     The predetermined first number of rotations N 1  is e.g. 333K rotations, and the term K is X1000. The first number of rotations N 1  is determined in consideration of influence on abrasion of the film  106  caused by the rotation of the photoconductor  102 , and the abrasion of the film  106  is dependent on factors such as hardness of the film  106 , hardness of the cleaning member  122  in the cleaning device  120 , and a pressing force of the cleaning member  122  against the photoconductor  102 . For example, the number of rotations N 1  of the photoconductor  102  is determined so as to be the number of rotations that the thickness of the film  106  reduces from 27 μm i.e. starting thickness to 22 μm. 
     In the step S 12 , the controller  400  controls the adjusting mechanism  280  so as to move the electrostatic charging member  204  and the support  210  to the second position (Refer to  FIG. 4B ). Therefore, the angle θ between the photoconductor  102  and the outer periphery  206  of the electrostatic charging member  204  reduces from θ 1  to θ 2 . 
     In the next step S 14 , the controller  400  discriminates whether or not the number of rotations of the photoconductor  102  since the attachment of the photoconductor  102  onto the image forming apparatus main body  12  is equal to or more than predetermined times i.e. a second number of rotations N 2 . If the number of rotations of the photoconductor  102  is less than the second number of rotations N 2 , the controller  400  terminates a series of controls. If the number of rotations of the photoconductor  102  is equal to or more than the second number of rotations N 2 , the operation proceeds to the next step S 16 . 
     The predetermined second number of rotations N 2  is e.g. 666K rotations, and the term K is X1000. The second number of rotations N 2  is determined in consideration of influence on abrasion of the film  106  caused by the rotation of the photoconductor  102  as well as the first number of rotations of N 1 , and the abrasion of the film  106  is dependent on a factor such as hardness of the film  106 , hardness of the cleaning member  122  in the cleaning device  120 , and a pressing force of the cleaning member  122  against the photoconductor  102 . For example, the second number of rotations N 2  of the photoconductor  102  is determined so as to be the number of rotations that the thickness of the film  106  reduces to 17 μm. 
     In the step S 16 , the controller  400  controls the adjusting mechanism  280  so as to move the electrostatic charging member  204  and the support  210  to the third position (Refer to  FIG. 4C ) Therefore, the angle θ between the photoconductor  102  and the outer periphery  206  of the electrostatic charging member  204  reduces from θ 2  to θ 3 . 
     As described above, the image forming apparatus  10  is configured so that the angle between the photoconductor  102  and the outer periphery  206  of the electrostatic charging member  204  varies in response to increasing the number of charging and increasing the number of image formations, and the angle θ between the photoconductor  102  and the outer periphery  206  of the electrostatic charging member  204  becomes smaller in response to increasing the number of charging and increasing the number of image formations. 
     Additionally, the image forming apparatus  10  is configured so that the angle between the photoconductor  102  and the outer periphery  206  of the electrostatic charging member  204  varies in response to variation of the thickness of the film  106  in the photoconductor  102 . Further, in the image forming apparatus  10 , the angle θ between the photoconductor  102  and the outer periphery  206  of the electrostatic charging member  204  becomes smaller in response to reducing the thickness of the film  106  in the photoconductor  102 . 
       FIGS. 7A and 7B  explain discharge occurred between the electrostatic charging member  204  and the photoconductor  102 .  FIG. 7A  cross-sectionally illustrates discharge in the case that the distance d between the photoconductor  102  and the outer periphery  206  of the electrostatic charging member  204  is d 1  in the region S, and  FIG. 7B  cross-sectionally illustrates the same in the case that the distance d is d 2  in the region S. As shown in  FIGS. 7A and 7B , d 2  is longer than d 1 . 
     When the distance d is less than d 1 , discharge occurs stably between the photoconductor  102  and the electrostatic charging member  204  in the image forming apparatus  10 . When the distance d is equal to or more than d 1  and is less than d 2 , discharge occurs unstably between the photoconductor  102  and the electrostatic charging member  204 . Further, when the distance d is equal to or more than d 2 , discharge does not occur between the photoconductor  102  and the electrostatic charging member  204 . 
     A region that the distance d is less than d 1  in the wedge-shaped region S (Refer to  FIG. 2  also) and discharge occurs stably between the photoconductor  102  and the electrostatic charging member  204  will be referred to as a stable discharge region below. Further, a region that the distance d is equal to or more than d 1  and less than d 2  in the wedge-shaped region S and discharge occurs unstably between the photoconductor  102  and the electrostatic charging member  204  will be referred to as an unstable discharge region below. 
       FIGS. 8A and 8B  explain processes that the electrostatic charging apparatus  200  charges the photoconductor  102 .  FIG. 8A  illustrates the step of the electrostatic charging on the photoconductor  102  in the case that the photoconductor  102  has the thickness of the film  106  being equal to or more than 22 μm and the electrostatic charging member  204  and the support  210  are located in the first position (Refer to  FIG. 4A ).  FIG. 8B  illustrates the step of the electrostatic charging on the photoconductor  102  in the case that the photoconductor  102  has the thickness of the film  106  being equal to or less than 17 μm and thereby the electrostatic charging member  204  and the support  210  move to the third position (Refer to  FIG. 4C ). In  FIGS. 8A and 8B , for convenience of illustration, the curved surfaces of the photoconductor  102  and the outer periphery  206  of the electrostatic charging member  204  are illustrated as a plane respectively. The arrow a in  FIGS. 8A and 8B  illustrates the direction of the movement of the photoconductor  102 . 
     In  FIGS. 8A and 8B , a region S 1  that the distance d between the photoconductor  102  and the outer periphery  206  of the electrostatic charging member  204  is less than d 1  is a stable discharge region, and a region S 2  that the distance d is equal to or more than d 1  and is less than d 2  is an unstable discharge region. A region that the distance d is equal to or more than d 2  is no discharge region. The region S 1  will be referred to as a stable discharge region S 1 , and the region S 2  will be referred to as an unstable discharge region S 2  below. 
     As shown in  FIG. 8A , when the electrostatic charging member  204  is located in the first position, a certain portion of the photoconductor  102  passes through points P 1  and P 2  and subsequently comes at a point P 3  during the movement of the photoconductor  102  along the direction of the arrow a. Therefore the certain portion of the photoconductor  102  passes through the unstable discharge region S 2  during passing from the point P 1  to the point P 2 , and passes through the stable discharge region S 1  during passing from the point P 2  to the point P 3 . Consequently, the certain portion of the photoconductor  102  is electrostatically charged by passing through the unstable discharge region S 2  and the stable discharge region S 1  and thus being subjected to discharge occurred between the photoconductor  102  and the electrostatic charging member  204 . 
     Further as shown in  FIG. 8B , when the electrostatic charging member  204  is located in the second position, a certain portion of the photoconductor  102  passes through points P 4  and P 5  and subsequently comes at the point P 3  during the movement of the photoconductor  102  along the direction of the arrow a. Therefore the certain portion of the photoconductor  102  passes through the unstable discharge region S 2  during passing from the point P 4  to the point P 5 , and passes through the stable discharge region S 1  during passing from the point P 5  to the point P 3 . Consequently, the certain portion of the photoconductor  102  is electrostatically charged by passing through the unstable discharge region S 2  and the stable discharge region S 1  and thus being subjected to discharge occurred between the photoconductor  102  and the electrostatic charging member  204 . 
       FIG. 9  is a diagram illustrating the observations of images formed in the image forming apparatus  10  for occurrence of a fine black line and occurrence of an uneven potential when the electrostatic charging apparatus  200  of the image forming apparatus  10  according to the first exemplary embodiment of the present invention charges the photoconductor  102 . The observations describes the formed image in each the number of rotations since starting use of the photoconductor  102  attached to the image forming apparatus main body  12 . The first column of the diagram in  FIG. 9  indicates the number of rotations of the photoconductor  102  since starting use of the photoconductor  102 . The second column of the diagram in  FIG. 9  indicates the thickness of the film  106 . The third column of the diagram in  FIG. 9  indicates the angle between the photoconductor  102  and the electrostatic charging member  204  in the wedge-shaped region S (Refer to  FIG. 4 ). The fourth column of the diagram in  FIG. 9  indicates whether or not a fine black line occurs on the formed image, and a circle in the fourth column means that a fine black line does not occur, that is, there is no problem on image quality. The fifth column of the diagram in  FIG. 9  indicates the degree of the uneven potential occurring on the photoconductor  102  by electrostatic charging the photoconductor  102 , and a circle in the fifth column means that the degree of the uneven potential on the photoconductor  102  is less than 10 V, that is, there is no problem on image quality in the formed image. An uneven potential causes uneven density in the formed image on the photoconductor  102 . 
     As shown in  FIG. 9 , in the image forming apparatus  10  according to the first exemplary embodiment, when the number of rotations of the photoconductor  102  is up to 333K rotations, a fine black line does not occur. The angle θ is θ 1  during electrostatic charging the photoconductor  102  (Refer to  FIG. 8A ), and thus a certain portion of the photoconductor  102  has shorter time to pass through the unstable discharge region S 2  as compared with the case with the angle θ 3  (Refer to  FIG. 8B ). 
     Further in the image forming apparatus  10  according to the first exemplary embodiment, when the number of rotations of the photoconductor  102  is up to 333K rotations, an uneven potential does not significantly affect image quality. The angle θ is θ 1  during electrostatic charging the photoconductor  102 , and thus though a certain portion of the photoconductor  102  has shorter time to pass through the stable discharge region S 1  as compared with the case with the angle θ 3 , the film  106  is subjected to the electrostatic charging by the application of relatively-low charge because of a relatively-thick film. 
     Further in the image forming apparatus  10  according to the first exemplary embodiment, when the number of rotations of the photoconductor  102  is between 333K to 666K rotations, a fine black line does not occur and an uneven potential does not significantly affect image quality. 
     Further in the image forming apparatus  10  according to the first exemplary embodiment, when the number of rotations of the photoconductor  102  is between 666K and 1000K rotations, a fine black line does not occur. The angle θ is θ 3  during electrostatic charging onto the photoconductor  102  (Refer to  FIG. 8B ), and thus a certain portion of the photoconductor  102  has longer time to pass through the unstable discharge region S 2  as compared with the case with the angle θ 1 . However, since having a relatively-thin film, the film  106  needs a large amount of electric charge in order to be subjected to electrostatic charging as well as the case with a thick film, and consequently a fine black line little occurs. 
       FIG. 10  is a diagram illustrating the observations of images formed in an image forming apparatus  10  according to a first comparative example for occurrence of a fine black line and occurrence of an uneven potential as the electrostatic charging apparatus  200  of the image forming apparatus  10  charges to the photoconductor  102 . The observations describe the formed image in each the number of rotations since starting use of the photoconductor  102 . In the contrast to the change of the angle θ in response to the number of rotations of the photoconductor  102  in the above image forming apparatus  10  according to the first exemplary embodiment, the image forming apparatus  10  according to the first comparative example keeps the angle θ at θ 1  (Refer to  FIG. 8A ) regardless of the number of rotations of the photoconductor  102 . The image forming apparatus  10  according to the first comparative example has same configuration as the image forming apparatus  10  according to the first exemplary embodiment except that the angle remains at θ 1 . 
     Items in the first to fifth columns of  FIG. 10  are the same as  FIG. 9 , respectively. Further, an open triangle in the fifth column means that the uneven potential on the photoconductor  102  ranges from 10 V to 20 V, and the formed images has acceptable uneven density for image quality. Furthermore, a cross in the fifth column means that the uneven potential on the photoconductor  102  ranges from 20 V to 30 V, and the range is problematic with uneven density of the formed images for image quality. 
     In the image forming apparatus  10  according to the first comparative example, when the number of rotations of the photoconductor  102  is between 333K and 666K rotations, an uneven potential occurs on the photoconductor  102  and thus the formed image has uneven density with acceptable image quality. Since the angle θ remains at θ 1  (Refer to  FIGS. 8A and 4A ) during electrostatic charging the photoconductor  102 , a certain portion of the photoconductor  102  has shorter time to pass through the stable discharge region S 1  as compared with the case of θ 2 . Further, the film  106  has a thickness between 17 μm and 22 μm i.e. a medium thickness, and thus is not subjected to enough electric charge to be subjected stable electrostatic charging. 
     Further in the image forming apparatus  10  according to the first comparative example, when the number of rotations of the photoconductor  102  is between 666K and 1000K rotations, an uneven potential occurs on the photoconductor  102  and the formed image has a problematic uneven density with image quality. Since the angle θ remains at θ 1  (Refer to  FIG. 8A ) during electrostatic charging on the photoconductor  102 , a certain portion of the photoconductor  102  has shorter time to pass through the stable discharge region S 1  as compared with the case of θ 3  (Refer to  FIG. 8B ). Further, the film  106  has a thickness between 12 μm and 17 μm i.e. relatively-thin, and thus is not subjected to enough electric charge to be subjected stable electrostatic charging. 
       FIG. 11  is a diagram illustrating the observations of images formed in an image forming apparatus  10  according to a second comparative example for occurrence of a fine black line and occurrence of an uneven potential as the electrostatic charging apparatus  200  of the image forming apparatus  10  charges the photoconductor  102 . The observations describe the formed image in each the number of rotations since starting use of the photoconductor  102 . In the contrast to the change of the angle θ in response to the number of rotations of the photoconductor  102  in the above image forming apparatus  10  according to the first exemplary embodiment, the image forming apparatus  10  according to the second comparative example keeps the angle θ at θ 2  (Refer to  FIG. 43 ) regardless of the number of rotations of the photoconductor  102 . The image forming apparatus  10  according to the second comparative example has same configuration as the image forming apparatus  10  according to the first exemplary embodiment except that the angle remains at θ 2 . 
     Items in the first to fifth columns of  FIG. 11  are the same as  FIG. 9 , respectively. Further, a cross in the fourth column illustrating whether or not a fine black line occurs on a formed image means that a fine black line occurs on the image and the degree of a fine black line is problematic for image quality. Furthermore, an open triangle in the fifth column means that the uneven potential on the photoconductor  102  ranges from 10 V to 20 V, and the formed image has acceptable uneven density for image quality, as well as  FIG. 10 . 
     In the image forming apparatus  10  according to the second comparative example, when the number of rotations of the photoconductor  102  is up to 333K rotations, a fine black line occurs on an image and the degree of a fine black line is within a problem of an image quality. Since the angle θ remains at (Refer to  FIG. 4B ) during electrostatic charging the photoconductor  102 , a certain portion of the photoconductor  102  has longer time to pass through the unstable discharge region S 2  as compared with the case of θ 1  (Refer to  FIGS. 4A and 8A ). 
     Further in the image forming apparatus  10  according to the second comparative example, when the number of rotations of the photoconductor  102  is between 666K and 1000K rotations, an uneven potential occurs on the photoconductor  102  and thus the formed image has uneven density with acceptable image quality. Since the angle θ remains at  92  (Refer to  FIG. 4B ) during electrostatic charging on the photoconductor  102 , a certain portion of the photoconductor  102  has shorter time to pass through the stable discharge region S 1  as compared with the case of θ 3  (Refer to  FIGS. 4C and 8B ). Further, the film  106  has a thickness between 12 μm and 17 μm i.e. relatively-thin and thus is not subjected to enough electric charge to be subjected stable electrostatic charging. 
       FIG. 12  is a diagram illustrating the observations of images formed in an image forming apparatus  10  according to a third comparative example for occurrence of a fine black line and occurrence of an uneven potential as the electrostatic charging apparatus  200  of the image forming apparatus  10  charges the photoconductor  102 . The observations describe the formed image in each the number of rotations since starting use of the photoconductor  102 . In the contrast to the change of the angle θ in response to the number of rotations of the photoconductor  102  in the above image forming apparatus  10  according to the first exemplary embodiment, the image forming apparatus  10  according to the third comparative example keeps the angle θ at θ 3  (Refer to  FIG. 8B ) regardless of the number of rotations of the photoconductor  102 . The image forming apparatus  10  according to the third comparative example has the same configuration as the image forming apparatus  10  according to the first exemplary embodiment except that the angle θ remains at θ 3 . 
     Items in the first to fifth columns of  FIG. 12  are the same as  FIG. 9 , respectively. Further, a cross in the fourth column illustrating whether or not a fine black line occurs on a formed image means that a fine black line occurs on the image and the degree of the fine black line is a problematic image quality. Furthermore, double-crosses in the fourth column means that a fine black line occurs on an image and the degree of the fine black line is a serious problematic image quality. 
     In the image forming apparatus  10  according to the third comparative example, when the number of rotations of the photoconductor  102  is up to 333K rotations, a fine black line occurs on an image and the degree of the fine black line is within a serious problem of an image quality. Since the angle θ remains at  83  (Refer to  FIG. 8B ) during electrostatic charging on the photoconductor  102 , a certain portion of the photoconductor  102  has longer time to pass through the unstable discharge region S 2  as compared with the case of θ 1  (Refer to  FIG. 8A ). 
     In the image forming apparatus  10  according to the third comparative example, when the number of rotations of the photoconductor  102  is between 333K and 666K rotations, a fine black line occurs on an image and the degree of the fine black line is problematic for image quality. Since the angle θ remains at  83  (Refer to  FIGS. 8B and 4C ) during electrostatic charging the photoconductor  102 , a certain portion of the photoconductor  102  has longer time to pass through the unstable discharge region S 2  as compared with the case of θ 2  (Refer to  FIGS. 4B ). 
       FIG. 13  illustrates an electrostatic charging apparatus  200  in an image forming apparatus  10  according to a second exemplary embodiment of the present invention. The above electrostatic charging apparatus  200  according to the first exemplary embodiment of the present invention has the endless-shaped electrostatic charging member  204  and the support  210  to support the electrostatic charging member  204  with being arranged inside the electrostatic charging member. The adjusting mechanism  280  attaches to the support  210  and allows the support  210  to move, and thereby the angle θ between the photoconductor  102  and the outer periphery  206  of the electrostatic charging member  204  in the wedge-shaped region changes. However, the electrostatic charging apparatus  200  of the image forming apparatus  10  according to the first modified example has the electrostatic charging member  204 , the support  210 , and further a contact member  220 . 
     The contact member  220  is in contact with the outer periphery  206  of the electrostatic charging member  204  from the photoconductor  102  side. The contact member  220  has the adjusting mechanism  280  to adjust the angle θ by moving the contact member  220 . 
     In the second exemplary embodiment of the present invention, the electrostatic charging member  204  is employed in belt-like form, that is, the electrostatic charging member  204  is not necessarily endless-shaped, and thus may be belt-like. 
     Except the above description, the image forming apparatus  10  according to the second exemplary embodiment has the same configuration as the image forming apparatus  10  according to the first exemplary embodiment. 
       FIG. 14  illustrates the operation of the electrostatic charging apparatus  200  in the image forming apparatus  10  according to the second exemplary embodiment. In the above electrostatic charging apparatus  200  according to the first exemplary embodiment, the adjusting mechanism  280  adjusts the angle θ between the photoconductor  280  and the outer periphery  206  of the electrostatic charging member  204  near the contact position N in the wedge-shaped region S by moving the support  210 . However, in the electrostatic charging apparatus  200  according to the second exemplary embodiment, the adjusting mechanism  280  adjusts the angle θ by moving the contact member  220 . 
       FIG. 14A  illustrates the electrostatic charging apparatus  200  in which the contact member  220  and the electrostatic charging member  204  is in the first position and the angle θ is θ 1 .  FIG. 14B  illustrates the electrostatic charging apparatus  200  in which the contact member  220  and the electrostatic charging member  204  is in the second position and the angle θ is θ 2 .  FIG. 14C  illustrates the electrostatic charging apparatus  200  in which the contact member  220  and the electrostatic charging member  204  is in the third position and the angle θ is θ 3 . 
       FIG. 15  illustrates an electrostatic charging apparatus  200  provided in the image forming apparatus  10  according to the third exemplary embodiment of the present invention. The above electrostatic charging apparatus  200  in the first exemplary embodiment of the present invention has the endless-shaped electrostatic charging member  204 . However, in the third exemplary embodiment, the electrostatic charging apparatus  200  has a plate-like electrostatic charging member  230  attached with the adjusting mechanism  280 . Except the above description, the image forming apparatus  10  according to the third exemplary embodiment has the same configuration as the image forming apparatus  10  according to the above first exemplary embodiment. 
       FIG. 16  illustrates the operation of the electrostatic charging apparatus  200  in the image forming apparatus  10  according to the third exemplary embodiment. In the above electrostatic charging apparatus  200  according to the first exemplary embodiment, the adjusting mechanism  280  adjusts the angle θ between the photoconductor  280  and the outer periphery  206  of the electrostatic charging member  204  near the contact position N in the wedge-shaped region S by moving the support  210 . However, in the electrostatic charging apparatus  200  according to the third exemplary embodiment, the adjusting mechanism  280  adjusts the angle θ by moving the electrostatic charging member  230 . 
       FIG. 16A  illustrates the electrostatic charging apparatus  200  in which the electrostatic charging member  230  is in the first position and the angle θ is θ 1 .  FIG. 16B  illustrates the electrostatic charging apparatus  200  in which the electrostatic charging member  230  is in the second position and the angle θ is θ 2 .  FIG. 16C  illustrates the electrostatic charging apparatus  200  in which the electrostatic charging member  230  is in the third position and the angle θ is θ 3 . 
     In each of the exemplary embodiments described above, although described the angle θ regarding the three types, the angle θ may be changed to e.g. two types or more than four types in response to the number of rotations of electrostatic charging apparatus  200  or in response to the number of image formations of the image forming unit  100 . 
     In each of the exemplary embodiments described above, the image forming unit  100  used for forming a single color image is described as one example. However each of the exemplary embodiments of the present invention may be applied to a multiple colors image forming apparatus as the image forming unit  100 . For example, the present invention may be applied to an image forming apparatus to form an image using developers with colors of yellow, magenta, cyan and black and rotating a photoconductor four times. In this case, approximate quarter of the number of rotations of the photoconductor represents the number of image formations in the image forming apparatus. 
     As described above, the present invention is available for an image forming apparatus such as a printer and a fax machine, an image forming method using the image forming apparatus, an electrostatic charging apparatus applicable to e.g. the above image forming apparatus, and an electrostatic charging method using the electrostatic charging apparatus. 
     The foregoing description of the exemplary embodiment of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The exemplary embodiment was chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.