Abstract:
In order to prolong life of a charger, there is provided an image forming device including a photosensitive member, a charger, an exposing unit, a developing unit and a control unit. The charger charges the photosensitive member at a charging position, and the exposing unit exposes the photosensitive member to light and forms an electrostatic latent image on the photosensitive member. The developing unit develops the electrostatic latent image formed on the photosensitive member at a developing position, the developing unit having a developing member that carries thereon developer charged to a polarity, the developing member having a surface potential. The control unit controls the charger so that a first voltage having a first absolute value and a second voltage having a second absolute value are selectively applied to the charger, the first absolute value being greater than the second absolute value, the surface potential of the developing member having a third absolute value smaller than the second absolute value, the control unit applying the first voltage to the charger at least during development while applying the second voltage to the charger during operations other than development, the first voltage, the second voltage and the surface potential having a polarity same as the polarity of the developer.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority from Japanese Patent Application No. 2008-302343 filed Nov. 27, 2008. The entire content of the priority application is incorporated herein by reference. 
       TECHNICAL FIELD 
       [0002]    The present invention relates to an electrophotographic image forming device and also to a method of controlling such an image forming device. 
       BACKGROUND 
       [0003]    In a conventional electrophotographic image forming device, an electrostatic latent image is formed on a surface of a photosensitive drum after the surface is charged and then exposed to light. Toner is subsequently supplied to the electrostatic latent image, thereby forming a visible toner image to be transferred onto sheets. For charging the surface of the photosensitive drum, there has been used a charger, such as a charging wire for generating a corona discharge. 
         [0004]    When rotating the photosensitive drum for the purpose of warming up or cleaning, a voltage is applied to the charger (charging member) for making the surface of the photosensitive drum charged with a potential higher than that of a developing roller in order to prevent toner adhesion to the photosensitive drum. The voltage applied to the photosensitive drum for a purpose other than development is conventionally the same as a voltage applied for development, which has a relatively high absolute value. As a result, given a fact that the life of the charging member is dependent on the level of voltage applied thereto, continuous applications of such a high voltage to the photosensitive drum may result in encouraging adherence of foreign materials to the charging member, thereby leading to a malfunction of the charging member and a shorter life of the charging member. 
       SUMMARY 
       [0005]    In view of the foregoing, it is an object of the present invention to provide an image forming device capable of prolonging life of a charging member, and a method of controlling the image forming device. 
         [0006]    In order to attain the above and other objects, there is provided an image forming device including a photosensitive member, a charger, an exposing unit, a developing unit and a control unit. The charger charges the photosensitive member at a charging position. The exposing unit exposes the photosensitive member to light and forms an electrostatic latent image on the photosensitive member. The developing unit develops the electrostatic latent image formed on the photosensitive member at a developing position, the developing unit having a developing member that carries thereon developer charged to a polarity, the developing member having a surface potential. The control unit controls the charger so that a first voltage having a first absolute value and a second voltage having a second absolute value are selectively applied to the charger, the first absolute value being greater than the second absolute value, the surface potential of the developing member having a third absolute value smaller than the second absolute value, the control unit applying the first voltage to the charger at least during development while applying the second voltage to the charger during operations other than development, the first voltage, the second voltage and the surface potential having a polarity same as the polarity of the developer. 
         [0007]    According to another aspect of the present invention, there is provided a method for controlling operations of an image forming device. The image forming device includes a photosensitive member; a charger that charges the photosensitive member at a charging position; an exposing unit that exposes the photosensitive member to light and forms an electrostatic latent image on the photosensitive member; and a developing unit that develops the electrostatic latent image formed on the photosensitive member at a developing position, the developing unit having a developing member that carries developer thereon, the developing member having a surface potential, the developer being charged to a polarity. The method for controlling the image forming device includes: applying a first voltage to the charger at least during development, the first voltage having a first absolute value; and applying a second voltage to the charger during operations other than development, the second voltage having a second absolute value smaller than the first absolute value, the surface potential of the developing member having a third absolute value smaller than the second absolute value, the first voltage, the second voltage and the surface potential having a polarity same as the polarity of the developer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    In the drawings: 
           [0009]      FIG. 1  is a cross-sectional view illustrating an entire configuration of a laser printer according to an embodiment of the present invention; 
           [0010]      FIG. 2  is a flowchart showing steps of an image forming operation controlled by a CPU of the laser printer; 
           [0011]      FIG. 3  is a time-chart showing how voltages are applied to a developing roller, a charging wire and a transfer roller, along with output states of a motor; 
           [0012]      FIG. 4  is a schematic diagram of an essential portion of the laser printer; and 
           [0013]      FIG. 5  is a flowchart showing steps controlled by the CPU of the laser printer for warm-up and cleaning operations. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    First, a general configuration of a laser printer  1  according to an embodiment of the present invention will be described with reference to  FIG. 1 . Note that, in the following description, orientations are referred to assuming that the laser printer  1  is disposed in an orientation in which it is intended to be used. In other words, the right side of the laser printer  1  in  FIG. 1  will be referred to as a “front side”, while the left side of the printer  1  in  FIG. 1  will be referred to as a “rear side.” Also, the near side in  FIG. 1  with respect to the paper width direction will be referred to as a “left side”, while the far side in  FIG. 1  will be referred to as a “right side.” 
         [0015]    As shown in  FIG. 1 , the laser printer  1  is a monochrome printer that includes a main casing  2 . Within the main casing  2  disposed are a sheet accommodation section  3 , an exposure device  4 , a process cartridge  5 , a fixing section  8 , a main motor  9  and a control device  10 . The laser printer  1  is formed with an opening at the front side thereof, and a front cover  21  is pivotably movably provided on the main casing  2  for covering the opening. A sheet discharge tray  23  is formed on an upper surface of the laser printer  1 . 
         [0016]    The sheet accommodation section  3  is disposed in a lower portion of the main casing  2 . The sheet accommodation section  3  includes a sheet tray  31  that accommodates sheets P in a stacked state, a sheet-pressing plate  32 , a sheet-feeding roller  33 , a sheet-feeding pad  34 , paper dust rollers  35  and  36 , and a pair of registration rollers  37 . The sheet tray  31  is detachably mounted in the main casing  2 , and the sheet-pressing plate  32  is pivotably movably provided within the sheet tray  31 . The sheet-feeding roller  33  is disposed above and forward of the sheet tray  31 . 
         [0017]    In the sheet accommodation section  3 , each sheet P is urged upward to reach the sheet-feeding roller  33  by the sheet-pressing plate  32 , conveyed by the sheet-feeding roller  33  and the sheet-feeding pad  34  while being pinched therebetween, transmitted to the registration rollers  37  via the paper dust rollers  35  and  36 , and then conveyed to the process cartridge  5 . 
         [0018]    The exposure device  4  is disposed in an upper portion of the main casing  2 . The exposure device  4  includes a laser emission section (not shown), a rotatable polygon mirror  41 , lenses  42  and  43 , and reflection mirrors  44 ,  45  and  46 . The laser emission section emits a laser beam based on image data. After being reflected off or passing through the polygon mirror  41 , the lens  42 , the reflection mirrors  44  and  45 , the lens  43  and the reflection mirror  46  in this order, the laser beam scans a surface of a photosensitive drum  61  (described later) at a high speed, as shown by a dotted chain line in  FIG. 1 . 
         [0019]    The process cartridge  5  is disposed below the exposure device  4 . The process cartridge  5  is detachably mounted in the main casing  2  through the opening when the front cover  21  is opened. The process cartridge  5  includes a drum cartridge  6  and a developing cartridge  7 . 
         [0020]    The drum cartridge  6  includes a drum casing  60  constituting an outer frame of the drum cartridge  6 . Within the drum casing  60 , a photosensitive drum  61 , a charger  62 , a transfer roller  63  and a cleaning brush  64  are provided. 
         [0021]    The photosensitive drum  61  is rotatably supported to the drum casing  60 . The photosensitive drum  61  includes a cylindrical drum main body having conducive characteristics whose circumferential surface is covered with a photosensitive layer with charging properties. 
         [0022]    The charger  62  is disposed above and in opposition to the photosensitive drum  61  with a space kept therebetween so that the charger  62  and the photosensitive drum  61  can be spatially separated from each other. The charger  62  spans along an axial direction of the photosensitive drum  61 , and has a charging wire  62 A that applies stepwise voltages (charging bias) to generate a corona discharge for uniformly charging the surface of the photosensitive drum  61 . A grid electrode may or may not be provided in the charger  62 . 
         [0023]    The transfer roller  63  is also rotatably supported to the drum casing  60 . The transfer roller  63  is disposed below and in contact with the photosensitive drum  61 . A transfer bias is applied to the transfer roller  63  for transferring a toner image formed on the photosensitive drum  61  onto the sheets P. 
         [0024]    The cleaning brush  64  is disposed in opposition to and in contact with the photosensitive drum  61  at a position rearward of the photosensitive drum  61 . A cleaning bias is applied to the cleaning brush  64  during cleaning operations or development operations, thereby removing toner remaining on the surface of the photosensitive drum  61  and paper dust of the sheets P deposited thereon. 
         [0025]    The developing cartridge  7  is detachably mounted in the drum cartridge  6  (the drum casing  60 ). The developing cartridge  7  has a developing casing  70  within which a developing roller  71 , a supply roller  72 , a thickness-regulating blade  73  and a toner accommodation section  74  are provided. 
         [0026]    The developing roller  71  is disposed in opposition to and in contact with the photosensitive drum  61  at a position forward of the photosensitive drum  61 . The developing roller  71  supplies toner to an electrostatic latent image formed on the surface of the photosensitive drum  61 . A developing bias VD is applied to the developing roller  71  during development operations. 
         [0027]    The supply roller  72  is disposed so as to be in contact with the developing roller  71  for supplying toner thereto at a position forward of the developing roller  71 . The thickness-regulating blade  73  slidingly contacts the developing roller  71  and controls thickness of toner carried on the surface of the developing roller  71 . The toner accommodation section  74  is arranged at a position forward of the supply roller  72  for accommodating toner. An agitator  74 A is provided within the toner accommodation section  74  for agitating the toner accommodated therein as well as for supplying the toner to the supply roller  72 . 
         [0028]    In the process cartridge  5  having the above-described configuration, the charger  62  (the charging wire  62 A) applies a charging bias V 1  to the surface of the photosensitive drum  61  uniformly. Subsequently, the high-speed scanning of the laser beam emitted from the exposure device  4  exposes the surface of the photosensitive drum  61 , lowering a potential of the scanned area. As a result, an electrostatic latent image is formed on the surface of the photosensitive drum  61  based on image data. 
         [0029]    In the meantime, the toner within the toner accommodation section  74  is supplied to the supply roller  72 , and subsequently to the developing roller  71  when the developing roller  71  and the supply roller  72  are in contact with each other. In accordance with rotation of the developing roller  71 , the toner is then carried on the surface of the developing roller  71  as a thin layer of uniform thickness because of the thickness-regulating blade  73 . While the developing roller  71  rotates, the toner carried on the developing roller  71  is then supplied to the electrostatic latent image formed on the surface of the photosensitive drum  61 . In this way, a visible toner image is formed on the surface of the photosensitive drum  61 . When the sheet P is conveyed between the photosensitive drum  61  and the transfer roller  63 , the toner image is transferred onto the sheet P. 
         [0030]    The fixing section  8  is disposed at a position rearward of the process cartridge  5  (downstream in a sheet conveying direction). The fixing section  8  includes a heat roller  81 , a pressure roller  82  disposed in opposition to the heat roller  81  and a pair of conveyor rollers  83 . While the sheet P passes between the heat roller  81  and the pressure roller  82 , the toner transferred onto the sheet P is thermally fixed on the sheet P. The sheet P is then conveyed by the conveyor rollers  83 , and finally discharged onto the sheet discharge tray  23  by a pair of discharge rollers  22  provided on the main casing  2 . 
         [0031]    The main motor  9  is a well-known motor that transmits driving force, via a transmission mechanism (not shown), to each section of the laser printer  1 , such as the sheet-feeding roller  33 , the photosensitive drum  61 , the transfer roller  63 , the developing roller  71 , the agitator  74 A and the heat roller  81 . The main motor  9  is disposed appropriately within the main casing  2 . 
         [0032]    The control device  10  is suitably disposed within the main casing  2  and includes components not shown in  FIG. 1 , such as a CPU, a RAM, a ROM and an input-output circuit. The control device  10  controls operations of the laser printer  1  in accordance with programs and data stored in the ROM and outputs from a sensor (not shown). 
         [0033]    Next, how the control device  10  controls image forming operations in the laser printer  1  will be described with reference to  FIGS. 2 to 4 . Note that, a time-chart of  FIG. 3  illustrates timings, not exact time, at which each voltage is applied and the main motor is driven. Also note that, toner used in the present embodiment is assumed to be positively charged and therefore tends to move toward a side whose potential is lower. 
         [0034]    As shown in  FIGS. 2 and 3 , when a personal computer (not shown) connected to the laser printer  1  transmits a command requesting to print together with image data, the control device  10  receives the image data (S 101 ). Upon receipt of the image data, the control device  10  starts to apply a charging bias V 2  to the charging wire  62 A (S 102 ). The charging bias V 2  is set to be lower than the charging bias V 1 , which is applied to the charging wire  62 A during development, as well as to be higher than a surface potential of the developing roller  71  carrying positively charged toner thereon. 
         [0035]    The control device  10  then starts driving the main motor  9  (S 103 ), resulting in rotation of the photosensitive drum  61 . At this time, the surface of the photosensitive drum  61  is charged by the charging wire  62 A to which the charging bias V 2  is applied. The charging bias V 2  is higher than the surface potential of the developing roller  71 . As a result, electrical attraction of toner to the photosensitive drum  61  can be suppressed, thereby preventing the surface of the photosensitive drum  61  from being contaminated by the toner before development. 
         [0036]    The control device  10  then executes warm-up operations before development for a predetermined period of time (S 104 ). Warm-up operations may include controlling the agitator  74 A to agitate toner within the toner accommodation section  74  and to supply the toner to the supply roller  72 , and supplying power to a heating source of the heat roller  81 . The control device  10  then electrically connects the main motor  9  and the sheet-feeding roller  33  via a transmission mechanism (not shown). As the main motor  9  powers up, the sheet-feeding roller  33  are made to rotate. In this way, the sheets P accommodated in the sheet tray  31  start to be fed (S 105 ). 
         [0037]    Subsequently, the control device  10  changes the charging bias applied to the charging wire  62 A from the charging bias V 2  to the charging bias V 1  (S 106 ). More specifically, as shown in  FIG. 3 , the control device  10  steps up the charging bias V 2  to the charging bias V 1  at a timing T 1  in the present embodiment. The period of time from the timing T 1  to a timing T 4  is a duration for the photosensitive drum  61  to make one turn. Upon uniformly charging the entire surface of the photosensitive drum  61  by the charging wire  62 A to which the charging bias V 1  is applied, the developing bias VD is applied to the developing roller  71  at the timing T 4 . 
         [0038]    Referring to  FIG. 4 , it is assumed that a position C on the photosensitive drum  61  is immediately below the charging wire  62 A at a timing T 2  and is moved to a developing position D after expiration of a period of time T from a timing T 2  to the timing T 4 . At the developing position D, the developing roller  71  supplies toner to the surface of the photosensitive drum  61 . At some time during the period of time T, the photosensitive drum  61  is exposed to the laser beam emitted from the exposure device  4  to form an electrostatic latent image. When the position C on the photosensitive drum  61  has reached to the developing position D at the timing T 4 , the developing bias VD is applied to the developing roller  71  to thereby enable development of the latent image. 
         [0039]    In this way, the charging bias V 2  is stepped up to the charging bias V 1  at a timing some time before the timing T 2 . Therefore, after the developing bias VD has been applied to the developing roller  71  at the timing T 4 , the developing roller  71  applied with the developing bias VD and the photosensitive drum  61  charged with the charging bias V 1  come into contact with each other. In other words, the electrostatic latent image is to be formed on the circumferential surface of the photosensitive drum  61  which has been uniformly charged with the charging bias V 1 . Hence, compared to a case where the surface of the photosensitive drum  61  has been charged partially with the charging bias V 1  and partially with the charging bias V 2  when the electrostatic latent image is to be formed thereon, this configuration enables quality of the image to be developed on the sheet P to be high and stable. 
         [0040]    Although the charging bias V 2  is set to be lower than the developing bias VD (V 2 &lt;VD) in the present embodiment, the surface of the photosensitive drum  61  has been in a state charged only with the charging bias V 1  when brought to development. This configuration can prevent electrical attraction of the toner to non-exposed portions on the surface of the photosensitive drum  61  at the time of development, thereby suppressing degradation in quality of the image transferred on the sheet P. 
         [0041]    Preferably, the charging bias V 2  should be set higher than the developing bias VD. Under this configuration, toner attraction to the surface (especially to the non-exposed portions on the surface) of the photosensitive drum  61  can be reliably suppressed regardless of the timing at which the charging bias V 2  is switched to the charging bias V 1 . As a consequence, the surface of the photosensitive drum  61  will not be stained with the toner, thereby preventing the image from getting deteriorated. 
         [0042]    After the charging bias V 2  has been changed to the charging bias V 1 , the control device  10  applies a transfer bias VT to the transfer roller  63  at a timing T 3  (S 107 ). After a predetermined period of time has passed from the timing T 3 , the control device  10  applies the developing bias VD to the developing roller  71  at the timing T 4  (S 108 ). With this application of the developing bias VD, a toner image is formed on the photosensitive drum  61  (development is done) and then transferred onto the sheet P. 
         [0043]    The control device  10  then stops applying the developing bias VD at a timing T 6  after development has been completed (S 109 ). When the toner image has been transferred onto the sheet P, the control device  10  stops applying the transfer bias VT at a timing T 7  (S 110 ). Subsequently, after cleaning the photosensitive drum  61 , the control device  10  finally terminates driving the main motor  9  and application of the charging bias V 1  at a timing T 8  (S 111 ). 
         [0044]    Next, how the control device  10  controls operations of the laser printer  1  during warm-up or cleaning operations will be described with reference to  FIG. 5 . 
         [0045]    Note that the warm-up operations are executed when the laser printer  1  is powered, or when the process cartridge  5  is replaced with new one and the front cover  21  is closed thereafter. The cleaning operations are executed when so instructed by a user. As shown in  FIG. 5 , when any of the above operations is initiated, the control device  10  receives such information on starting the operation (S 201 ). In response, the control device  10  applies the charging bias V 2  to the charging wire  62 A (S 202 ). 
         [0046]    The control device  10  subsequently starts driving the main motor  9  (S 203 ), thereby rotating the photosensitive drum  61 . In this way, the surface of the photosensitive drum  61  is charged with the charging bias V 2  which has a potential higher than the surface potential of the developing roller  71 . Since the toner is positively charged in the present embodiment, the surface of the photosensitive drum  61  can be kept away from the toner. 
         [0047]    The control device then executes warm-up or cleaning operations for a prescribed period of time in accordance with the received information (S 204 ). Here, the warm-up operations may include instructing the agitator  74 A to agitate the toner accommodated in the toner accommodation section  74  and to supply the toner to the developing roller  71 . The cleaning operations may include instructing the cleaning brush  64  to clean the surface of the photosensitive drum  61 . Once such an operation ends, the control device  10  stops driving the main motor  9  and applying the charging bias V 2  (S 211 ). 
         [0048]    As described above, according to the laser printer  1  of the present embodiment, the charging bias V 1  is applied to the charging wire  62 A during an image formation from the timing T 1  to the timing T 8 , while the charging bias V 2 , which has the potential lower than the charging bias V 2 , is applied to the charging wire  62 A for a period of time other than the image forming period. With this configuration, the current flowing into the charging wire  62 A can be decreased. As a result, such decreased current contributes to prevention of foreign matters from being attached to the charging wire  62 A, leading to prolonging life of the charging wire  62 A. 
         [0049]    Further, since the charging bias V 2  is set to be lower than the developing bias VD in the present embodiment, the current flowing into the charging wire  62 A can be made even smaller, thereby preventing attraction of foreign matters to the charging wire  62 A. As a consequence, the life of the charging wire  62 A can be made even longer. 
         [0050]    While the present invention has been described in detail with reference to the first embodiment thereof, it would be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention. 
         [0051]    In the present embodiment, the charging bias V 2  is switched to the charging bias V 1  at the timing T 1 . As a variation, this change in the charging bias may take place at the timing T 2 . With this configuration, the period of time during which the charging bias V 1  (a voltage having an absolute value larger than that of the charging bias V 2 ) is applied to the charging wire  62 A can be made even shorter than the present embodiment. As a result, the current flowing into the charging wire  62 A can be reduced even more, resulting in further prolongation of life of the charging wire  62 A. 
         [0052]    Further, as shown in  FIG. 3 , the charging bias V 1  is applied to the charging wire  62 A during a period of time from the timing T 1  to the timing T 8  (including the development period) in the present embodiment. Note that, in the present invention, the development period is specifically meant to span from the timing T 2  to the timing T 5 , i.e. from the timing T 4  (when the developing bias VD is applied) minus the period of time T until the timing T 6  (when application of the developing bias VD is ended) minus the period of time T. However, as a variation, the charging bias V 1  may be applied to the charging wire  62 A only during the development period (i.e., from the timing T 2  to the timing T 5 ). With this configuration, the current flowing into the charging wire  62 A becomes further smaller, leading to much longer life of the charging wire  62 A. 
         [0053]    Further, instead of the charger  62  provided with the charging wire  62 A, there may be employed a so-called sawtooth AC corona charger having needle electrodes arranged in line. 
         [0054]    Further, the photosensitive drum  61  is used as an example of photosensitive members in the present embodiment. However, a photosensitive belt may also be employed instead. 
         [0055]    Further, a laser is used for exposing the photosensitive drum  61  in the exposure device  4 . Alternatively, light emitted from LEDs, EL elements or a fluorescent material may be used for exposing the photosensitive drum  61 . 
         [0056]    An image forming device according to the present invention encompasses not only a laser printer but also a copier, a multifunctional device or the like. Moreover, an image forming device according to the present embodiment (the laser printer  1 ) is configured to print in monochrome, but an image forming device that prints colored images may well be applicable to the present invention. 
         [0057]    Further, in the present invention, since the positively charged toner is employed, the charging biases V 1  and V 2 , the developing bias VD, and the surface potential of the developing roller  71  are all assumed to have a positive polarity, as shown in  FIG. 3 . However, as a variation, negatively charged toner may also be used. In this case, the charging biases V 1  and V 2 , the developing bias VD and the surface potential of the developing roller  71  should be of a negative polarity, yet maintaining the absolute value relation described above.