Patent Publication Number: US-8532546-B2

Title: Developer supply device and image forming apparatus having the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority under 35 U.S.C. §119 from Japanese Patent Application No. 2010-171718 filed on Jul. 30, 2010. The entire subject matter of the application is incorporated herein by reference. 
     BACKGROUND 
     1. Technical Field 
     The following description relates to one or more developer supply devices configured to supply charged powdered development agent to an intended device. 
     2. Related Art 
     A developer supply device has been known, which includes a developer carrying member and a transfer board. 
     The developer carrying member is configured as a roller-shaped member that has a cylindrical circumferential surface facing the intended device. The transfer board includes a plurality of transfer electrodes arranged along a developer transfer path. The transfer board is configured to transfer development agent in a developer transfer direction along the developer transfer path by the action of an electric field generated when a predetermined voltage is applied to the transfer electrodes. 
     The transfer board includes a vertical transfer board and a bottom transfer board. The vertical transfer board extends vertically so as to transfer the development agent vertically upward in the developer transfer direction. The developer carrying member is disposed to face an upper end of the vertical transfer board. The bottom transfer board forms a bottom surface of a developer storage section. The bottom transfer board is connected with a lower end of the vertical transfer board. Thus, the bottom transfer board is configured to charge the development agent by friction with the development agent and transfer the charged development agent to the lower end of the vertical transfer board. 
     In order to generate an electric field to transfer the charged development agent from the upper end of the vertical transfer board to the development carrying member, a predetermined voltage is applied between the vertical transfer board and the developer carrying member. 
     In the developer supply device configured as above, the development agent is transferred by the vertical transfer board, vertically upward in the developer transfer direction along the developer transfer path. Then, in a position where the upper end of the vertical transfer board faces the developer carrying member, the charged development agent is transferred onto the developer carrying member by the action of the electric field generated in response to the predetermined voltage being applied between the vertical transfer board and the developer carrying member. Thus, the development agent is held and carried on the circumferential surface of the developer carrying member. 
     SUMMARY 
     In the developer supply device of this kind, when transferring of the development agent is once stopped, the development agent being transferred may be left behind on the transfer board (the vertical transfer board). In the case where the development agent left behind on the transfer board is transferred as is toward the developer carrying member when the transferring of the development agent is restarted, inadequately-charged development agent is carried on the developer carrying member. It might have a negative influence on quality of a formed image. 
     Aspects of the present invention are advantageous to provide one or more improved techniques for a developer supply device, which techniques make it possible to supply adequately-charged development agent to an intended device. 
     According to aspects of the present invention, a developer supply device is provided, which is configured to supply charged development agent to an intended device. The developer supply device includes a developer holding member including a developer holding surface formed as a cylindrical circumferential surface parallel to a first direction, the developer holding surface being disposed in closest proximity to and opposite the intended device in a first position, the developer holding member being configured to rotate around an axis parallel to the first direction such that the developer holding surface moves in a second direction perpendicular to the first direction, and to supply the development agent carried on the developer holding surface to the first position, a casing formed as a box-shaped member, the casing including a developer storage section configured to accommodate the development agent, and an opening provided in a position facing the intended device, the casing rotatably supporting the developer holding member at the opening such that the developer holding surface faces the intended device through the opening, and a transfer board disposed inside the casing, the transfer board including a plurality of transfer electrodes arranged along a substantially vertically-extending developer transfer path perpendicular to the first direction, the transfer board being configured to convey the development agent up toward a second position where the transfer board is in closest proximity to and opposite the developer holding surface, from the developer storage section along the developer transfer path and transfer the development agent onto the developer holding surface around the second position, under a traveling-wave transfer electric field generated when a transfer bias voltage containing a multi-phase alternating-current voltage component is applied to the transfer electrodes, and the transfer board further including an activating section disposed in a position close to the developer holding member and substantially lower than the second position, the activating section being supplied with an activating voltage for making the development agent leave the developer transfer path and drop into the developer storage section during an activating process performed at an initial stage of a developer transferring operation of transferring the development agent, the activating section being supplied with the transfer bias voltage after the activating process, and an always-transferring section disposed across at least a range of a position facing the developer storage section to a position lower than and adjacent to the activating section, the always-transferring section being always supplied with the transfer bias voltage during the developer transferring operation. 
     According to aspects of the present invention, further provided is an image forming apparatus including a photoconductive body configured such that a development agent image is formed thereon, and a developer supply device configured to supply charged development agent to the photoconductive body. The developer supply device includes a developer holding member including a developer holding surface formed as a cylindrical circumferential surface parallel to a first direction, the developer holding surface being disposed in closest proximity to and opposite the photoconductive body in a first position, the developer holding member being configured to rotate around an axis parallel to the first direction such that the developer holding surface moves in a second direction perpendicular to the first direction, and to supply the development agent carried on the developer holding surface to the first position, a casing formed as a box-shaped member, the casing including a developer storage section configured to accommodate the development agent, and an opening provided in a position facing the photoconductive body, the casing rotatably supporting the developer holding member at the opening such that the developer holding surface faces the photoconductive body through the opening, and a transfer board disposed inside the casing, the transfer board including a plurality of transfer electrodes arranged along a substantially vertically-extending developer transfer path perpendicular to the first direction, the transfer board being configured to convey the development agent up toward a second position where the transfer board is in closest proximity to and opposite the developer holding surface, from the developer storage section along the developer transfer path and transfer the development agent onto the developer holding surface around the second position, under a traveling-wave transfer electric field generated when a transfer bias voltage containing a multi-phase alternating-current voltage component is applied to the transfer electrodes, and the transfer board further including an activating section disposed in a position close to the developer holding member and substantially lower than the second position, the activating section being supplied with an activating voltage for making the development agent leave the developer transfer path and drop into the developer storage section during an activating process performed at an initial stage of a developer transferring operation of transferring the development agent, the activating section being supplied with the transfer bias voltage after the activating process, and an always-transferring section disposed across at least a range of a position facing the developer storage section to a position lower than and adjacent to the activating section, the always-transferring section being always supplied with the transfer bias voltage during the developer transferring operation. 
    
    
     
       BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS 
         FIG. 1  is a side view schematically showing a configuration of a laser printer in an embodiment according to one or more aspects of the present invention. 
         FIG. 2  is an enlarged cross-sectional side view of a toner supply device for the laser printer in the embodiment according to one or more aspects of the present invention. 
         FIG. 3  is an enlarged cross-sectional side view of each transfer board for the toner supply device in the embodiment according to one or more aspects of the present invention. 
         FIGS. 4A and 4B  exemplify respective waveforms of voltages output from four power supply circuits for each transfer board in the embodiment according to one or more aspects of the present invention. 
         FIG. 5  shows a simulation result for verifying an operation and an effect in a modification employing an activating voltage as illustrated in  FIG. 4B  in accordance with one or more aspects of the present invention. 
         FIG. 6  shows a simulation result for verifying an operation and an effect in a modification employing a different configuration from that shown in  FIG. 5  in accordance with one or more aspects of the present invention. 
         FIG. 7  shows a simulation result for verifying an operation and an effect in a comparative example employing a different configuration from those shown in  FIGS. 5 and 6 . 
         FIG. 8  is an enlarged cross-sectional side view of a toner supply device for the laser printer in a modification according to one or more aspects of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     It is noted that various connections are set forth between elements in the following description. It is noted that these connections in general and, unless specified otherwise, may be direct or indirect and that this specification is not intended to be limiting in this respect. 
     Hereinafter, an embodiment according to aspects of the present invention will be described with reference to the accompanying drawings. 
     &lt;Configuration of Laser Printer&gt; 
     As illustrated in  FIG. 1 , a laser printer  1  includes a sheet feeding mechanism  2 , a photoconductive drum  3 , an electrification device  4 , a scanning unit  5 , and a toner supply device  6 . 
     A feed tray (not shown), provided in the laser printer  1 , is configured such that a stack of sheets P is placed thereon. The sheet feeding mechanism  2  is configured to feed the sheets P placed on the feed tray, on a sheet-by-sheet basis along a predetermined sheet feeding path PP. 
     On a circumferential surface of the photoconductive drum  3 , an electrostatic latent image holding surface LS is formed as a cylindrical surface parallel to a main scanning direction (i.e., a z-axis direction in  FIG. 1 , hereinafter which may be referred to as a width direction). The electrostatic latent image holding surface LS is configured such that an electrostatic latent image is formed thereon in accordance with an electric potential distribution. Further, the electrostatic latent image holding surface LS is configured to hold and carry toner T (see  FIG. 2 ) in positions corresponding to the electrostatic latent image. The photoconductive drum  3  is driven to rotate in a predetermined direction (counterclockwise in  FIG. 1 ) around a central axis parallel to the main scanning direction. Thereby, the photoconductive drum  3  is configured to move the electrostatic latent image holding surface LS along an auxiliary scanning direction perpendicular to the main scanning direction. 
     The electrification device  4  is disposed to face the electrostatic latent image holding surface LS, so as to evenly and positively charge the electrostatic latent image holding surface LS. The scanning unit  5  is configured to generate a laser beam LB modulated based on image data and to scan the laser beam LB, which is converged in a scanned position SP on the electrostatic latent image holding surface LS, along the main scanning direction. Namely, the scanning unit  5  is configured such that an electrostatic latent image is formed on the electrostatic latent image holding surface LS. 
     The toner supply device  6  is disposed under the photoconductive body  3 , so as to face the photoconductive body  3  in a development position DP which is downstream relative to the scanned position SP in a moving direction of the electrostatic latent image holding surface LS moving in response to rotation of the photoconductive drum  3 . The toner supply device  6  is configured to supply the positively charged toner T (see  FIG. 2 ) from underneath to the electrostatic latent image holding surface LS in the development position DP. Subsequently, a detailed explanation will be provided about a specific configuration of each element included in the laser printer  1 . 
     The sheet feeding mechanism  2  includes a pair of registration rollers  21  and a transfer roller  22 . The registration rollers  21  are configured to feed a sheet P toward a transfer position TP (which is downstream relative to the development position DP in the moving direction of the electrostatic latent image holding surface LS moving in response to rotation of the photoconductive drum  3 ) between the photoconductive drum  3  and the transfer roller  22  at a predetermined moment. 
     The transfer roller  22  is disposed to face the electrostatic latent image holding surface LS across the sheet feeding path PP (the sheet P) in the transfer position TP. Additionally, the transfer roller  22  is driven to rotate in a direction (clockwise in  FIG. 1 ) opposite to the rotational direction of the photoconductive drum  3 . Further, the transfer roller  22  is connected with a transfer bias power supply circuit (not shown), such that a predetermined transfer bias voltage is applied for transferring, onto the sheet P, the toner T (see  FIG. 2 ) adhering onto the electrostatic latent image holding surface LS. 
     &lt;&lt;Toner Supply Device&gt;&gt; 
     As depicted in  FIG. 2  that is a cross-sectional side view showing the toner supply device  6  in an enlarged manner, the toner supply device  6  is configured to transfer the positively charged toner T along a toner transfer path TTP under a traveling-wave transfer electric field and supply the toner T to the photoconductive drum  3 . 
     A toner box  61 , which forms a casing of the toner supply device  6 , is a box-shaped member that is formed in an oval shape when viewed in the z-axis direction. Further, the toner box  61  is disposed to have a longitudinal direction parallel to an up-to-down direction (i.e., the y-axis direction in  FIG. 2 , which may be referred to as a vertical direction). The toner box  61  is configured to accommodate the toner T that is powdered dry development agent. Namely, the toner box  61  includes a toner storage section  61   a , which is an internal space formed in a half-cylinder shape at the bottom of the toner box  61 . In the embodiment, it is noted that the toner T is positively-chargeable nonmagnetic-one-component black toner. Further, the toner box  61  includes an opening  61   b  provided at the top thereof, i.e., in the position to face the photoconductive drum  3 . 
     There is a development roller  62  housed in the toner box  61 . The development roller  62  is rotatably supported by the toner box  61 . The development roller  62  is a roller-shaped member having a toner holding surface  62   a  that is a cylindrical circumferential surface. The development roller  62  is disposed at the opening  61   b  so as to face the photoconductive drum  3 . Namely, the development roller  62  is supported by the toner box  61  such that in the development position DP, the toner holding surface  62   a  of the development roller  62  is in closest proximity to and opposite the electrostatic latent image holding surface LS of the photoconductive drum  3  across a predetermined distance. 
     &lt;&lt;&lt;Transfer Board&gt;&gt;&gt; 
     There is a transfer board  63  provided along the toner transfer path TTP inside the toner box  61 . The transfer board  63  is fixed on an inner wall surface of the toner box  61 . In the embodiment, the transfer board  63  includes a bottom transfer board  63   a , a vertical transfer board  63   b  (containing an always-transferring section  63   b   2  and an activating section  63   b   1 ), and a retrieving transfer board  63   c . It is noted that a detailed explanation will be provided below about an internal configuration of the transfer board  63  (the bottom transfer board  63   a , the vertical transfer board  63   b , and the retrieving transfer board  63   c ). 
     The bottom transfer board  63   a  is disposed at a bottom portion of the internal space of the toner box  61 , so as to form a bottom surface of the toner storage section  61   a . The bottom transfer board  63   a , which is a concave curved surface formed in a half-cylinder shape when viewed in the z-axis direction, is smoothly connected with a lower end of the vertical transfer board  63   b . Specifically, the bottom transfer board  63   a  is connected with the lower end of the vertical transfer board  63   b  so as to transfer the toner T stored in the toner storage section  61   a  toward the lower end of the vertical transfer board  63   b.    
     The vertical transfer board  63   b  is formed in a flat plate shape and provided to extend vertically so as to transfer the toner T, received from the bottom transfer board  63   a , vertically upward. In a toner carrying position TCP where the upper end of the vertical transfer board  63   b  is in closest proximity to and opposite the development roller  62  (the toner holding surface  62   a ), there is a predetermined distance of gap provided between the vertical transfer board  63   b  and the toner holding surface  62   a . In the embodiment, as described above, the vertical transfer board  63   b  includes the activating section  63   b   1  and the always-transferring section  63   b   2 . 
     The activating section  63   b   1  is disposed in the vicinity of the development roller  62  (the toner holding surface  62   a ). Specifically, in the embodiment, the activating section  63   b   1  extends from the upper end of the vertical transfer board  63   b  to a portion thereof lower than the toner carrying position TCP. Namely, an upper end of the activating section  63   b   1  is disposed in a position as high as the upper end of the vertical transfer board  63   b . Further, a lower end of the activating section  63   b   1  is disposed in a position substantially as high as a lower end of the development roller  62 . 
     The always-transferring section  63   b   2  is disposed to be adjacent to the activating section  63   b   1 , beneath the activating section  63   b   1 . Specifically, an upper end of the always-transferring section  63   b   2  is disposed to be adjacent to the lower end of the activating section  63   b   1 . Further, a lower end of the always-transferring section  63   b   2  is disposed in a position as high as the lower end of the vertical transfer board  63   b  that faces the toner storage section  61   a.    
     The activating section  63   b   1  is configured to make the toner T leave the toner transfer path TTP and drop toward the toner storage section  61   a  in an activating process to be performed in an initial stage of an operation of the vertical transfer board  63   b  to transfer (or resume transferring) the toner T. Further, the activating section  63   b   1  is configured to, after the activating process, transfer the toner T in a toner transfer direction TTD heading for the toner carrying position TCP from the toner storage section  61   a . Meanwhile, the always-transferring section  63   b   2  is configured to always transfer the toner T in the toner transfer direction TTD heading for the toner carrying position TCP from the toner storage section  61   a  while the vertical transfer board  63   b  is conveying the toner T thereon. 
     The retrieving transfer board  63   c  is disposed to face the development roller  62  on the opposite side of the upper end of the vertical transfer board  63   b  across the development roller  62 . Namely, the retrieving transfer board  63   c  is disposed downstream relative to the opening  61   b  of the toner box  61  in the toner transfer direction TTD. In the embodiment, a downstream end of the retrieving transfer board  63   c  in the toner transfer direction TTD is placed in a position substantially as high as the lower end of the development roller  62 . 
     The retrieving transfer board  63   c  is configured to retrieve the toner T, which remains on the toner holding surface  62   a  without being consumed in the development position DP, from the development roller  62  and to convey the retrieved toner T down toward the toner storage section  61   a . Specifically, an upper portion of the retrieving transfer board  63   c  is bent in a shape of a concave curved surface shape so as to face the development roller  62  across a predetermined distance of gap. Further, a lower portion of the retrieving transfer board  63   c  is configured to transfer the toner T vertically downward. 
     &lt;&lt;&lt;Power Supply Circuit&gt;&gt;&gt; 
     The bottom transfer board  63   a  and the always-transferring section  63   b   2  of the vertical transfer board  63   b  are electrically connected with an always-transferring power supply circuit  64   a . The activating section  63   b   1  of the vertical transfer board  63   b  is electrically connected with an activating power supply circuit  64   b . The retrieving transfer board  63   c  is electrically connected with a retrieving power supply circuit  65 . The development roller  62  is electrically connected with a development bias power supply circuit  66 . 
     The always-transferring power supply circuit  64   a  is configured to generate a traveling-wave transfer electric field to transfer the toner T in the toner transfer direction TTD on the bottom transfer board  63   a  and the always-transferring section  63   b   2  when a transfer bias voltage containing a multi-phase alternating-current (AC) voltage component is applied to a plurality of transfer electrodes  631  (see  FIG. 3 ) of the bottom transfer board  63   a  and the always-transferring section  63   b   2  of the vertical transfer board  63   b.    
     The activating power supply circuit  64   b  is configured to, when applying a predetermined activating voltage to a plurality of transfer electrodes  631  (see  FIG. 3 ) of the activating section  63   b   1  of the vertical transfer board  63   b  in the activating process, generate on the activating section  63   b   1  such a transfer electric field as to make the toner T leave the toner transfer path TTP and drop toward the toner storage section  61   a . Further, the activating power supply circuit  64   b  is configured to, when applying a transfer bias voltage (see  FIG. 4A ) to the plurality of transfer electrodes  631  of the activating section  63   b   1  after the activating process, generate the aforementioned transfer electric field on the activating section  63   b   1 . Specifically, in the embodiment, the activating power supply circuit  64   b  outputs a reverse transfer bias voltage (see  FIG. 4B ) for generating such a traveling-wave reverse transfer electric field as to transfer the toner T in a direction opposite to the toner transfer direction TTD, in the activating process. 
     The retrieving power supply circuit  65  is configured to apply, to a plurality of transfer electrodes  631  (see  FIG. 3 ) of the retrieving transfer board  63   c , such a retrieving bias voltage containing a multi-phase AC voltage component as to retrieve the toner T from the development roller  62  (the toner holding surface  62   a ) on the retrieving transfer board  63   c  and transfer the retrieved toner T vertically down toward the toner storage section  61   a  under a traveling-wave retrieving transfer electric field. The retrieving bias voltage is set to provide an average electric potential lower than an electric potential of an exposed region, of the electrostatic latent image holding surface LS, to be supplied with the toner T. 
     The development bias power supply circuit  66  is configured to apply to between the vertical transfer board  63   b  and the development roller  62  such a development bias voltage as to generate an electric field for transferring the positively charged toner T from the vertical transfer board  63   b  toward the development roller  62 . 
     Additionally, the always-transferring power supply circuit  64   a , the activating power supply circuit  64   b , the retrieving power supply circuit  65 , and the development bias power supply circuit  66  are configured to output respective voltages required for circulating the toner T in the toner transfer direction TTD along the toner transfer path TTP (i.e., for conveying the toner T stored in the toner storage section  61   a  to the toner carrying position TCP on the vertical transfer board  63   b , transferring the toner T onto the development roller  62  around the toner carrying position TCP, once having the toner holding surface  62   a  carry the toner T thereon, retrieving from the development roller  62  the toner T left there without being consumed in the development position DP, and conveying the retrieved toner T back down to the toner storage section  61   a ), in a normal electric-field transferring operation of transferring the toner T after the activating process. 
     Specifically, the always-transferring power supply circuit  64   a  is configured to output a rectangular four-phase AC voltage of +400 V to +1000 V (amplitude: 300 V, and DC offset: +700 V) with a frequency of 300 Hz (see  FIG. 4A ). Meanwhile, the activating power supply circuit  64   b  is configured to, in the activating process, output a four-phase AC voltage (see  FIG. 4B ) that has the same waveform as that of the voltage output from the always-transferring power supply circuit  64   a  and a phase shift direction opposite to that of the voltage output from the always-transferring power supply circuit  64   a . In addition, the activating power supply circuit  64   b  is configured to, after the activating process, output the same voltage as that output from the always-transferring power supply circuit  64   a.    
     Further, the retrieving power supply circuit  65  is configured to output a rectangular AC voltage of −300 V to +300 V (amplitude: 300 V, and DC offset: 0 V) with a frequency of 300 Hz (see  FIG. 4A ). Moreover, the development bias power supply circuit  66  is configured to output a direct-current (DC) voltage with a DC offset of +400 V. 
     &lt;&lt;&lt;Internal Configuration of Transfer Board&gt;&gt;&gt; 
     Referring to  FIG. 3 , the transfer board  63  is a thin plate member configured in the same manner as a flexible printed-circuit board. Specifically, the transfer board  63  includes the transfer electrodes  631 , a transfer electrode supporting film  632 , a transfer electrode coating layer  633 , and a transfer electrode overcoating layer  634 . 
     Each transfer electrode  631  is a linear wiring pattern having a longitudinal direction parallel to the main scanning direction (i.e., perpendicular to the auxiliary scanning direction), and made of a copper thin film. The transfer electrodes  631  are arranged parallel to each other along the toner transfer path TTP. 
     Every fourth one of the transfer electrodes  631 , arranged along the toner transfer path TTP, is connected with a specific one of four power supply circuits VA, VB, VC, and VD. In other words, the transfer electrodes  631  are arranged along the toner transfer path TTP in the following order: a transfer electrode  631  connected with the power supply circuit VA, a transfer electrode  631  connected with the power supply circuit VB, a transfer electrode  631  connected with the power supply circuit VC, a transfer electrode  631  connected with the power supply circuit VD, a transfer electrode  631  connected with the power supply circuit VA, a transfer electrode  631  connected with the power supply circuit VB, a transfer electrode  631  connected with the power supply circuit VC, a transfer electrode  631  connected with the power supply circuit VD, . . . (note: the power supply circuits VA, VB, VC, and VD are included in each power supply circuit shown in  FIG. 2  such as the always-transferring power supply circuit  64   a  and the activating power supply circuit  64   b ). 
       FIGS. 4A and 4B  exemplify waveforms of the voltages output from the power supply circuits VA, VB, VC, and VD shown in  FIG. 3 . In the embodiment, as illustrated in  FIGS. 4A and 4B , the power supply circuits VA, VB, VC, and VD are configured to generate respective AC driving voltages having substantially the same waveform. Further, the power supply circuits VA, VB, VC, and VD are configured to generate the respective AC driving voltages with a phase difference of 90 degrees between any adjacent two of the power supply circuits VA, VB, VC, and VD in the aforementioned order. 
     As depicted in  FIG. 4A , the always-transferring power supply circuit  64   a  is configured such that the power supply circuits VA, VB, VC, and VD thereof always output the respective AC driving voltages each of which is delayed by a phase of 90 degrees behind the voltage output from a precedent adjacent one of the power supply circuits VA, VB, VC, and VD in the aforementioned order. Meanwhile, as depicted in  FIG. 4B , the activating power supply circuit  64   b  is configured such that in the activating process, the power supply circuits VA, VB, VC, and VD thereof output the respective AC driving voltages each of which is advanced by a phase of 90 degrees ahead of the voltage output from a precedent adjacent one of the power supply circuits VA, VB, VC, and VD in the aforementioned order. Further, as depicted in  FIG. 4A , the activating power supply circuit  64   b  is configured such that after the activating process, the power supply circuits VA, VB, VC, and VD thereof output the respective AC driving voltages each of which is delayed by a phase of 90 degrees behind the voltage output from a precedent adjacent one of the power supply circuits VA, VB, VC, and VD in the aforementioned order. 
     The transfer electrodes  631  are formed on a surface of the transfer electrode supporting film  632 . The transfer electrode supporting film  632  is a flexible film made of electrically insulated synthetic resin such as polyimide resin. 
     The transfer electrode coating layer  633  is made of electrically insulated synthetic resin. The transfer electrode coating layer  633  is provided to coat the transfer electrodes  631  and a surface of the transfer electrode supporting film  632  on which surface the transfer electrodes  631  are formed. 
     On the transfer electrode coating layer  633 , the transfer electrode overcoating layer  634  is provided. Namely, the transfer electrode coating layer  633  is formed between the transfer electrode overcoating layer  634  and the transfer electrodes  631 . The surface of the transfer electrode overcoating layer  634  is formed as a smooth surface with a very low level of irregularity, so as to smoothly convey the toner T. 
     &lt;General Overview of Operations of Laser Printer&gt; 
     Subsequently, a general overview will be provided of operations of the laser printer  1  configured as above with reference to the relevant drawings. 
     &lt;&lt;Sheet Feeding Operation&gt;&gt; 
     Referring to  FIG. 1 , firstly, a leading end of a sheet P placed on the feed tray (not shown) is fed to the registration rollers  21 . The registration rollers  21  perform skew correction for the sheet P, and adjust a moment when the sheet P is to be fed forward. After that, the sheet P is fed to the transfer position TP. 
     &lt;&lt;Formation of Toner Image on Electrostatic Latent Image Holding Surface&gt;&gt; 
     While the sheet P is being conveyed to the transfer position TP as described above, an image of the toner T (hereinafter referred to as a toner image) is formed on the electrostatic latent image holding surface LS that is the outer circumferential surface of the photoconductive drum  3 , as will be mentioned below. 
     &lt;&lt;Formation of Electrostatic Latent Image&gt;&gt; 
     Firstly, the electrostatic latent image holding surface LS of the photoconductive drum  3  is charged evenly and positively by the electrification device  4 . The electrostatic latent image holding surface LS, charged by the electrification device  4 , is moved along the auxiliary scanning direction to the scanned position SP to face the scanning unit  5 , when the photoconductive drum  3  rotates in the direction indicated by arrows in  FIG. 1 . 
     In the scanned position SP, the electrostatic latent image holding surface LS is exposed to the laser beam LB modulated based on the image data. Namely, while being scanned along the main scanning direction, the laser beam LB is rendered incident onto the electrostatic latent image holding surface LS. In accordance with the modulation of the laser beam LB, areas with no positive charge remaining thereon are generated on the electrostatic latent image holding surface LS. Thereby, an electrostatic latent image is formed with a positive charge pattern (positive charges distributed in the shape of an image) on the electrostatic latent image holding surface LS. 
     The electrostatic latent image, formed on the electrostatic latent image holding surface LS, is transferred to the development position DP to face the toner supply device  6  when the photoconductive drum  3  rotates in the direction indicated by the arrows in  FIG. 1 . 
     &lt;&lt;Transfer and Supply of Charged Toner&gt;&gt; 
     Referring to  FIGS. 2 and 3 , the toner T stored in the toner box  61  is charged due to contact or friction with the transfer electrode overcoating layer  634  on the bottom transfer board  63   a . The charged toner T, which is in contact with or proximity to the transfer electrode overcoating layer  634  on the bottom transfer board  63   a , is conveyed in the toner transfer direction TTD by the traveling-wave electric field generated when the aforementioned transfer bias voltage is applied to the transfer electrodes  631  of the bottom transfer board  63   a . Thereafter, the charged toner T is transferred to the vertical transfer board  63   b.    
     In the embodiment, a downstream end of the bottom transfer board  63   a  in the toner transfer direction TTD, i.e., a joint portion of the bottom transfer board  63   a  with the vertical transfer board  63   b  is formed as a curved surface. Thereby, the toner T is smoothly transferred from the bottom transfer board  63   a  to the lower end of the vertical transfer board  63   b.    
     The vertical transfer board  63   b  conveys the toner T, received at the lower end thereof from the bottom transfer board  63   a , vertically up toward the toner carrying position TCP. During the time period, the toner T is further charged up due to contact or friction with the transfer electrode overcoating layer  634  of the vertical transfer board  63   b.    
     The toner T transferred from the bottom transfer board  63   a  to the vertical transfer board  63   b  contains uncharged toner for some reasons (e.g., the uncharged toner contained may be caught up by an airflow). Nonetheless, in the embodiment, the uncharged toner leaves the toner transfer path TTP and drops from the vertical transfer board  63   b  by the action of the gravity. Then, the toner T, which has dropped from the vertical transfer board  63   b , returns into the toner storage section  61   a.    
     The toner T being conveyed on the vertical transfer board  63   b  reaches the vicinity of the toner carrying position TCP, in a state further charged up in the aforementioned manner to a predetermined charged level. In the vicinity of the toner carrying position TCP, the positively charged toner T is transferred onto the toner holding surface  62   a  by the action of the development bias voltage. 
     The toner holding surface  62   a , onto which the toner T has been transferred in the vicinity of the toner carrying position TCP, moves in the direction perpendicular to the main scanning direction when the development roller  62  is driven to rotate, and then reaches the vicinity of the development position DP. In the vicinity of the development position DP, the toner T carried on the toner holding surface  62   a  is supplied to the photoconductive drum  3 . Namely, the electrostatic latent image formed on the electrostatic latent image holding surface LS is developed with the toner T. Specifically, the toner T adheres onto the areas where there is no positive charge remaining on the electrostatic latent image holding surface LS. Thereby, the image of the toner T (i.e., the toner image) is held and carried on the electrostatic latent image holding surface LS. 
     The toner T, which remains on the toner holding surface  62   a  (without being consumed in the development position DP) even after passing through the development position DP, is transferred onto the retrieving transfer board  63   c  by the action of a retrieving electric field generated between the development roller  62  (the toner holding surface  62   a ) and the retrieving transfer board  63   c  when the aforementioned retrieving bias voltage is applied to the retrieving transfer board  63   c . Namely, the toner T is retrieved from the toner holding surface  62   a  by the retrieving transfer board  63   c.    
     In the embodiment, the retrieving bias voltage containing the multi-phase AC voltage component is applied to the retrieving transfer board  63   c  while the DC voltage is applied to the development roller  62 . Therefore, by the action of the AC voltage component of the retrieving bias voltage, the toner T carried on the toner holding surface  62   a  is vibrated in the vicinity of the retrieving transfer board  63   c . Owing to the vibration of the toner T, the toner T floating above the toner holding surface  62   a  collides against the toner T adhering onto the toner holding surface  62   a . Thereby, the toner T carried on the toner holding surface  62   a  becomes more likely to float apart from the toner holding surface  62   a.    
     Further, in the embodiment, the average electric potential (0 V) of the retrieving bias voltage is set lower than the electric potential (240 V) of the exposed region, of the electrostatic latent image holding surface LS, to be supplied with the toner T. By the action of the retrieving bias voltage, the toner T, which remains on the toner holding surface  62   a  without being consumed even after passing through the development position DP, is removed in a preferable manner and transferred onto the retrieving transfer board  63   c . Thus, it is possible to prevent generation of a ghost on the formed image as effectively as practicable. 
     The toner T, transferred onto the retrieving transfer board  63   c  from the toner holding surface  62   a , is conveyed down toward the toner storage section  61   a  by the action of the electric field generated when the retrieving bias voltage is applied to the transfer electrodes  631  of the retrieving transfer board  63   c . At the lower end of the retrieving transfer board  63   c , the toner T is conveyed vertically downward. At this time, an inertia force acts on the toner T in the same direction as the gravity. Then, in a region below the lower end of the retrieving transfer board  63   c , the toner T drops into the toner storage section  61   a  by the action of the gravity and the inertia force in the same direction as the gravity. Hence, even though the retrieving transfer board  63   c  is not provided up to the toner storage section  61   a , the retrieved toner T is returned into the toner storage section  61   a  in a preferable manner. 
     &lt;&lt;Transfer of Toner Image from Electrostatic Latent Image Holding Surface onto Sheet&gt;&gt; 
     Referring to  FIG. 1 , the toner image, which is held and carried on the electrostatic latent image holding surface LS of the photoconductive drum  3  as described above, is conveyed to the transfer position TP when the electrostatic latent image holding surface LS moves in the auxiliary scanning direction in response to the photoconductive drum  3  rotating in the direction as indicated by the arrows in  FIG. 1  (i.e., counterclockwise in  FIG. 1 ). Then, in the transfer position TP, the toner image is transferred from the electrostatic latent image holding surface LS onto the sheet P. After that, the toner image is fixed onto the sheet P by a fixing unit (not shown). Thereby, the toner image is formed on the sheet P. 
     &lt;Operations and Effects&gt; 
     When a toner transferring operation of transferring the toner T on the transfer board  63  is suspended, the toner T may remain on the toner transfer path TTP (i.e., adhere onto the transfer board  63 ). 
     Such residual toner (especially, toner remaining on the vertical transfer board  63   b ) is not adequately charged at the time to reach the vicinity of the toner holding position TCP, as a transfer distance for the residual toner to the toner holding position TCP is not sufficiently ensured. Hence, in the case where the residual toner is conveyed as is toward the development roller  62  at the time when the toner transferring operation is resumed, inadequately charged residual toner might be held on the development roller  62  and exert a negative influence on the formed image. 
     In the embodiment, the activating process is carried out at the initial stage of the toner transferring operation (including an initial stage after resuming the toner transferring operation. For example, the laser printer  1  performs the activating process at the time when starting a first image forming operation after being powered on, an image forming operation after waiting for job data, or an image forming operation after maintenance such as re-supply of sheets and settlement of a paper jam. 
     In the activating process, the rotating of the development roller  62  is stopped. Further, a reverse transfer bias voltage is applied as an activating voltage to the transfer electrodes  631  of the activating section  63   b   1  that is disposed near the development roller  62  and blow the toner holding position TCP. Meanwhile, even in the activating process, the normal transfer bias voltage is applied to the always-transferring section  63   b   2  disposed across a range of a position facing the toner storage section  61   a  to a position lower than and adjacent to the activating section  63   b   1 . 
     Thereby, residual toner on the activating section  63   b   1  is conveyed downward (to get away from the toner carrying position TCP) from the toner carrying position TCP by the action of the activating voltage. Then, at a boundary between the lower end of the activating section  63   b   1  and the upper end of the always-transferring section  63   b   2  (i.e., at a portion where a reverse transfer electric field collides against the transfer electric field), the residual toner leaves the toner transfer path TTP and drops into the toner storage section  61   a.    
     Meanwhile, residual toner on the always-transferring section  63   b   2  is pushed out by the action of the transfer electric field or by newly transferred toner T to reach the boundary between the lower end of the activating section  63   b   1  and the upper end of the always-transferring section  63   b   2 . The residual toner stays once at the boundary as transferring of the residual toner in the toner transfer direction TTD is blocked there. Nonetheless, the residual toner soon leaves the toner transfer path TTP and drops into the toner storage section  61   a  due to its own weight (therefore, a time period during which the residual toner is staying at the boundary is actually short). 
     Thus, the residual toner remaining on the toner transfer path TTP (i.e., adhering onto the transfer board  63 ) leaves the toner transfer path TTP and drops into the toner storage section  61   a  without being transferred onto the toner holding surface  62   a  in the toner carrying position TCP. 
     After the residual toner is removed from the toner transfer path TTP in a preferable manner, the activating process is terminated. Thereafter, the normal transfer bias voltage is applied to the activating section  63   b   1  such that a normal toner transferring operation is performed on the transfer board  63 . 
     Therefore, in the embodiment, adequately charged toner T is always conveyed to (the vicinity of) the toner carrying position TCP on the transfer board  63  and transferred onto the toner holding surface  62   a  in (the vicinity of) the toner carrying position TCP. Thus, according to the embodiment, it is possible to always supply the adequately charged toner T to the photoconductive drum  3  (the electrostatic latent image holding surface LS). 
     Hereinabove, the embodiment according to aspects of the present invention has been described. The present invention can be practiced by employing conventional materials, methodology and equipment. Accordingly, the details of such materials, equipment and methodology are not set forth herein in detail. In the previous descriptions, numerous specific details are set forth, such as specific materials, structures, chemicals, processes, etc., in order to provide a thorough understanding of the present invention. However, it should be recognized that the present invention can be practiced without reapportioning to the details specifically set forth. In other instances, well known processing structures have not been described in detail, in order not to unnecessarily obscure the present invention. 
     Only an exemplary embodiment of the present invention and but a few examples of their versatility are shown and described in the present disclosure. It is to be understood that the present invention is capable of use in various other combinations and environments and is capable of changes or modifications within the scope of the inventive concept as expressed herein. For example, the following modifications are feasible. 
     &lt;Modifications&gt; 
     Aspects of the present invention may be applied to electrophotographic image forming apparatuses such as color laser printers, and monochrome and color copy machines, as well as the single-color laser printer as exemplified in the aforementioned embodiment. Further, the photoconductive body is not limited to the drum-shaped one as exemplified in the aforementioned embodiment. For instance, the photoconductive body may be formed in the shape of a plate or an endless belt. 
     Additionally, light sources (e.g., LEDs, electroluminescence devices, and fluorescent substances) other than a laser scanner may be employed as light sources for exposure. In such cases, the “main scanning direction” may be parallel to a direction in which light emitting elements such as LEDs are arranged. 
     Further, for example, aspects of the present invention may be applied to negatively chargeable development agent or a negatively chargeable photoconductive body. 
     Alternatively, aspects of the present invention may be applied to image forming apparatuses employing methods (e.g., a toner-jet method using no photoconductive body, an ion flow method, and a multi-stylus electrode method) other than the aforementioned electrophotographic method. 
     The development roller  62  may be disposed in contact with the photoconductive drum  3 . Further, the development roller  62  may be substitutable for a cylindrical development sleeve. 
     As long as the vertical transfer board  63   b  extends substantially in the vertical direction, the vertical transfer board  63   b  may be slightly tilted with respect to the vertical direction. In addition, the lower end of the retrieving transfer board  63   c  may be slightly tilted with respect to the vertical direction. Further, between the upper end (near the toner carrying position TCP) and the lower end of the vertical transfer board  63   b , there may be provided a curved portion having a down-facing transfer surface on which the toner T is transferred. 
     A central portion of the bottom transfer board  63   a  may be flat. Namely, only a joint portion of the bottom transfer board  63   a  with the lower end of the vertical transfer board  63   b  may be curved. 
     The downstream end of the retrieving transfer board  63   c  in the toner transfer direction TTD may be connected with the bottom transfer board  63   a.    
     The configuration of the transfer board  63  is not limited to that as exemplified in the aforementioned embodiment. For example, the transfer board  63  may be configured without the transfer electrode overcoating layer  634 . Alternatively, the transfer board  63  may be configured with the transfer electrodes  631  being implanted in the transfer electrode supporting film  632 . In this case, the transfer board  63  may be configured without the transfer electrode coating layer  633  or the transfer electrode overcoating layer  634 . 
     Referring to  FIG. 4A , the waveform of the voltage output from each of the power supply circuits VA, VB, VC, and VD may be a sinusoidal waveform or a triangle waveform, instead of the rectangular waveform as exemplified in the aforementioned embodiment. 
     In the aforementioned embodiment, the four power supply circuits VA, VB, VC, and VD are provided to generate respective voltages with a phase difference of 90 degrees between any adjacent two of the four power supply circuits VA, VB, VC, and VD in the aforementioned order. However, three power supply circuits may be provided to generate respective voltages with a phase difference of 120 degrees between any two of the three power supply circuits. 
     In the aforementioned embodiment, the activating section  63   b   1  and the always-transferring section  63   b   2  are connected with the respective different power supply circuits. However, the activating section  63   b   1  and the always-transferring section  63   b   2  may be connected with the same power supply circuit, which may be configured with an appropriately-designed wiring pattern for supplying an electric power to the transfer electrodes  631  of the activating section  63   b   1 . 
     The activating voltage applied to the transfer electrodes  631  of the activating section  63   b   1  may be a voltage containing only a DC voltage component, instead of the reverse transfer bias voltage as exemplified in the aforementioned embodiment. 
       FIG. 5  shows a simulation result for verifying an operation and an effect of a modification employing the activating voltage as illustrated in  FIG. 4B . It is noted that the state shown in  FIG. 5  is a mirror-reversed image of the state shown in  FIG. 2  owing to settings for the simulation, but it does not exert any negative influence on the simulation result. 
     In  FIG. 5 , each arrow represents both magnitude and direction of a force acting on toner, and each curved line shows a constant-electric-potential line. The activating voltage applied to the activating section  63   b   1  is +800 V. The development bias voltage applied to the development roller  62  is +800 V. The DC voltage component of the transfer bias voltage applied to the always-transferring section  63   b   2  is +500 V (the amplitude of the AC voltage component is 300 V). 
     Namely, in the modification (where the toner is charged positively), the following relational expression is established:
 
V2&lt;V1=V3,
 
where V 1 , V 2 , and V 3  represent the electric potential of the transfer electrodes  631  (see  FIG. 3 ) of the activating section  63   b   1 , the electric potential of the transfer electrodes  631  (see  FIG. 3 ) of the always-transferring section  63   b   2 , and the electric potential of the development roller  62 , respectively.
 
     Further,  FIGS. 6 and 7  show simulation results when the activating voltage contains only a DC voltage component in the same manner as the case shown in  FIG. 5  and the lower end of the activating section  63   b   1  is disposed in different positions from the position shown in  FIG. 5 . 
     As shown in  FIG. 7 , when the lower end of the activating section  63   b   1  is too close to the toner carrying position TCP (i.e., when it is not necessarily certain that the activating section  63   b   1  is substantially disposed lower than the toner carrying position TCP), such an electric field is generated that the toner heads for the toner holding surface  62   a  in the vicinity of the toner carrying position TCP. Additionally, at the boundary between the lower end of the activating section  63   b   1  and the upper end of the always-transferring section  63   b   2 , the toner is pressed against the vertical transfer board  63   b  by the action of such a strong electric field as to head the toner for the vertical transfer board  63   b  from the development roller  62 . Namely, even though toner is once removed from the toner transfer path TTP (see  FIG. 2 ) by the activating section  63   b   1 , the toner is captured by the strong electric field and pressed against the vertical transfer board  63   b . Thus, in the configuration as shown in  FIG. 7 , it is impossible to make the residual toner drop into the toner storage section  61   a  (see  FIG. 2 ) in a preferable manner. 
     On the contrary, as shown in  FIGS. 5 and 6 , when the lower end of the activating section  63   b   1  is disposed far from the toner carrying position TCP (i.e., when the activating section  63   b   1  is substantially disposed lower than the toner carrying position TCP), the aforementioned problem is not caused, and the residual toner can be dropped without being carried on the toner holding surface  62   a.    
       FIG. 5  shows a configuration that the lower end of the activating section  63   b   1  is disposed substantially as high as the lower end of the development roller  62 . In addition  FIG. 6  shows a configuration that the lower end of the activating section  63   b   1  is disposed lower than the lower end of the development roller  62 . 
     According to the configuration shown in  FIG. 5 , such an electric field as to head the toner for the vertical transfer board  63   b  from the development roller  62  is weak, while such an electric field as to head the toner downward from the activating section  63   b   1  is generated in a favorable fashion. Thereby, when the toner conveyed upward by the always-transferring section  63   b   2  reaches the boundary between the lower end of the activating section  63   b   1  and the upper end of the always-transferring section  63   b   2 , the toner is prevented from being conveyed further upward and gradually accumulated at the boundary. Then, when it becomes difficult for the transfer board  62   b  to hold the toner, the toner drops in a favorable manner. 
     In order to increase a cleaning area on the vertical transfer board  63   b  where the residual toner is removed from the toner transfer path TTP in the activating process, the lower end of the activating section  63   b   1  is desired to be disposed as close to the toner carrying position TCP as possible within a range where it is possible to drop the toner in a favorable manner (e.g., as the configuration shown in  FIG. 5 ). Nonetheless, it is possible to apply the configuration shown in  FIG. 6  as well, which configuration makes it possible to generate such an electric field as to head the toner downward from the activating section  63   b   1  in a preferable fashion. Namely, the lower end of the activating section  63   b   1  is desired to be disposed around or lower than the lower end of the toner holding surface  62   a  (the lower end of the activating section  63   b   1  may be disposed in a position slightly higher than the lower end of the toner holding surface  62   a  as far as such a configuration makes it possible to drop the toner in a favorable manner). 
     It is noted that the aforementioned requirement (relation) “V 1 =V 3 ” may not necessarily be satisfied. The same effects may be provided under the following relation:
 
V2&lt;V1≦V3.
 
     The aforementioned relations among the electric potentials V 1 , V 2 , and V 3  may be applied to the aforementioned embodiment (where in the activating process, the reverse transfer bias voltage is applied as the activating voltage to the transfer electrodes  631  of the activating section  63   b   1 ). 
     Further, when the toner is charged negatively, the relation among the electric potentials V 1 , V 2 , and V 3  is desired as follows:
 
V3≦V1&lt;V2.
 
     The activating section  63   b   1  may be provided up to a position higher than the toner carrying position TCP, or provided only below the toner carrying position TCP. As illustrated in  FIG. 8 , the activating section  63   b   1  may be disposed only in an area around the lower end of the toner holding surface  62   a . In this case, the activating section  63   b   1  may be disposed within an area as large as one to several pieces of transfer electrodes (see  FIG. 3 ). 
     The development bias voltage applied to the development roller  62  may contain only a DC voltage component (including the level of ground). Further, the various bias voltages may be changed as needed.