Patent Publication Number: US-9411258-B1

Title: Image forming apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Japanese Priority Patent Application JP2015-057714 filed on Mar. 20, 2015, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     The present disclosure relates to an electrophotographic image forming apparatus. 
     In an electrophotographic image forming apparatus, after a photoconductive drum is charged to negative by a charging roller, a negatively-charged part of the photoconductive drum is applied with a light beam, which forms a latent image. The latent image is developed by a developer that is supplied from a developing roller and a feeding roller, and a developer image formed by the development is transferred on paper by a transfer roller. 
     In the case where the image forming apparatus is a printer forming a color image, it is necessary to strictly control an amount of the developer to be transferred on paper in order to faithfully reproduce the color image. For example, in Japanese Unexamined Patent Application Publication No. 2004-29681, an image forming apparatus is disclosed in which density of a developer of a patch pattern printed on a transfer belt is measured, and density correction of the developer is performed by controlling a process condition, based on density data obtained through the measurement. 
     SUMMARY 
     In the image forming apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2004-29681, it is necessary to interrupt normal printing operation in order to perform the density correction of the developer. In a case where printing in which the printing operation is difficult to be interrupted is performed over long time, however, the density correction of the developer may have to be performed during the printing operation. In such a case, density or color tone may be largely varied in a printed image. 
     It is desirable to provide an image forming apparatus that makes it possible to suppress large variation in density or color tone in a printed image. 
     An image forming apparatus according to an embodiment of the technology includes: an image supporting member having a first circumferential surface that includes a photoreceptive layer; an exposure section configured to perform exposure of the first circumferential surface and thereby form latent images; a developer supporting member having a second circumferential surface opposed to the first circumferential surface, and configured to develop the latent images with use of a developer; a feeding member having a third circumferential surface opposed to the second circumferential surface, and configured to feed the developer to the developer supporting member; and a control section configured to control, while controlling exposure operation of the exposure section to allow the latent images to be formed side by side at a predetermined interval on the first circumferential surface, varying timing of a development voltage or both of the development voltage and a supply voltage to allow a portion P 1  or both of the portion P 1  and a portion P 2  to be located within a gap between the latent images on the first circumferential surface, the portion P 1  being a portion, in the first circumferential surface, opposed to the developer supporting member upon varying of the development voltage, the portion P 2  being a portion, in the first circumferential surface, opposed to a portion P 3  of the developer supporting member, and the portion P 3  being a portion, in the second circumferential surface, opposed to the feeding member upon varying of the supply voltage. As used herein, the term “oppose” and its grammatical variants are intended to encompass not only a separated state but also a contact state between one member and the other member. 
     An image forming apparatus according to another embodiment of the technology includes: an image supporting member having a first circumferential surface that includes a photoreceptive layer; an exposure section configured to perform exposure of the first circumferential surface and thereby form latent images; a developer supporting member having a second circumferential surface opposed to the first circumferential surface, and configured to develop the latent images with use of a developer; a feeding member having a third circumferential surface opposed to the second circumferential surface, and configured to feed the developer to the developer supporting member; and a control section configured to, in a label printing mode in which printing is performed on rolled paper to which a plurality of labels are attached at a predetermined interval, control varying timing of a development voltage or both of the development voltage and a supply voltage to allow a portion P 1  or both of the portion P 1  and a portion P 2  to be opposed to a gap between the labels on the rolled paper or a portion to be opposed to the gap between the labels, the portion P 1  being a portion, in the first circumferential surface, opposed to the developer supporting member upon varying of the development voltage, the portion P 2  being a portion, in the first circumferential surface, opposed to a portion P 3  of the developer supporting member, and the portion P 3  being a portion, in the second circumferential surface, opposed to the feeding member upon varying of the supply voltage. As used herein, the term “oppose” and its grammatical variants are intended to encompass not only a separated state but also a contact state between one member and the other member. 
     According to the image forming apparatuses of the respective embodiments of the disclosure, it is possible to suppress large variation in density and color tone in a printed image. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the technology as claimed. Also, effects of the invention are not limited to those described above. Effects achieved by the invention may be those that are different from the above-described effects, or may include other effects in addition to those described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the disclosure, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments and, together with the specification, serve to explain the principles of the technology. 
         FIG. 1  is a schematic diagram illustrating an outline configuration example of an image forming apparatus according to an embodiment of the disclosure. 
         FIG. 2  is a diagram illustrating a medium illustrated in  FIG. 1 , where (A) is a schematic diagram illustrating a plan structure example of the medium, and (B) is a sectional diagram illustrating a sectional structure example taken along a line A-A of (A). 
         FIG. 3  is a schematic diagram illustrating an outline configuration example of an image forming unit in  FIG. 1 . 
         FIG. 4  is a schematic diagram illustrating an example of a control mechanism of the image forming apparatus in  FIG. 1 . 
         FIG. 5  is a graph illustrating an example of a voltage setting expression. 
         FIG. 6  is a diagram illustrating an example of a correction table. 
         FIG. 7  is a diagram illustrating an example of variation in image density by continuous printing. 
         FIG. 8A  is a diagram illustrating an example of operation of the image forming unit at time T=T 1 . 
         FIG. 8B  is a diagram illustrating an example of the operation of the image forming unit at time T=T 2 . 
         FIG. 8C  is a diagram illustrating an example of the operation of the image forming unit at time T=T 3 . 
         FIG. 9  is a diagram illustrating an example of varying of a development voltage and a supply voltage. 
         FIG. 10  is a diagram illustrating an example of the operation of the image forming unit at time T=T 4 . 
         FIG. 11  is a diagram illustrating an example of varying of a development voltage and a supply voltage. 
         FIG. 12  is a flowchart illustrating an example of operation procedure of the image forming apparatus in  FIG. 1 . 
         FIG. 13  is a schematic diagram illustrating a modification of the outline configuration of the image forming apparatus in  FIG. 1 . 
         FIG. 14  is a diagram illustrating a modification of an outline configuration of an image forming section  30  and a transfer section  40  in the image forming apparatus in  FIG. 1  and  FIG. 13 . 
         FIG. 15A  is a diagram illustrating an example of operation of the image forming unit at time T=T 1 . 
         FIG. 15B  is a diagram illustrating an example of the operation of the image forming unit at time T=T 2 . 
         FIG. 15C  is a diagram illustrating an example of the operation of the image forming unit at time T=T 3 . 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, some embodiments of the disclosure are described in detail with reference to drawings. The following description is merely a specific example of the disclosure, and the disclosure is not limited to the embodiment described below. Positions, sizes, and size ratios of respective components illustrated in the drawings of the disclosure are not limited to those illustrated. Note that description is given in the following order. 
     1. Embodiment 
     An example in which a voltage-varying timing is controlled with use of user input 
     2. Modifications 
     Modification 1: an example in which voltage-varying timing is controlled with use of a detection result 
     Modification 2: an example in which voltage-varying timing is controlled with use of an exposure control signal 
     Modification 3: an example in which printing is performed by direct transfer method 
     Modification 4: various modifications 
     1. Embodiment 
     [Configuration] 
       FIG. 1  schematically illustrates an outline configuration example of an image forming apparatus  1  according to an embodiment of the disclosure. The image forming apparatus  1  may be a printer that forms a color image on a medium P with use of an electrophotographic method. The medium P corresponds to a specific but non-limiting example of “medium” in the disclosure. (A) of  FIG. 2  illustrates an example of a plan structure of the medium P. (B) of  FIG. 2  illustrates an example of a sectional structure of the medium P taken along a line A-A in (A) of  FIG. 2 . 
     For example, the medium P may be rolled paper including a long rolled mount Pa and long label paper Pb that are overlaid with each other. The rolled mount Pa supports the label paper Pb. The label paper Pb may include an adhesive layer on a surface close to the rolled mount Pa. The label paper Pb may include a plurality of labels LB and a gap LG between labels that is formed around the labels LB. Each label LB may be cut and separated from the gap LG between labels. In the label paper Pb, a line where each label LB is cut and separated from the gap LG between labels is referred to as a cut line CL. The plurality of labels LB may be arranged in a longitudinal direction of the rolled mount Pa with an interval each having a label gap size GS. For example, the medium P may be rolled paper on which the plurality of labels LB are attached at a predetermined interval. The medium P may include, on a surface of the long rolled mount Pa, the plurality of labels LB that are arranged in the longitudinal direction of the rolled mount Pa with an interval each having a label gap size Gs. 
     The image forming apparatus  1  may include a medium container  10 , a medium conveyor (paper conveyor)  20 , an image forming section  30 , a transfer section  40 , a fixing section  50 , a discharge section  60 , and a density sensor  70 . The medium container  10 , the medium conveyor  20 , the image forming section  30 , the transfer section  40 , the fixing section  50 , the discharge section  60 , and the density sensor  70  may be provided inside a housing  100 . 
     As used herein, a path on which the medium P is conveyed is referred to as a conveying path. In the conveying path PW, a direction toward the medium container  10  as viewed from any component or a position closer to the medium container  10  is referred to as “upstream in the conveying path PW”. In the conveying path PW, a direction opposite to the direction toward the medium container  10  as viewed from any component or a position further apart from the medium container  10  is referred to as “downstream in the conveying path PW”. In the conveying path PW, a direction in which the medium P travels (namely, a direction from the upstream toward the downstream in the conveying path PW) is referred to as a conveying direction F 1 . 
     [Configuration of Medium Container  10 ] 
     The medium container  10  may contain the medium P. For example, the medium container  10  may include a holding shaft  11  that holds the medium P rotatably. 
     [Configuration of Medium Conveyor  20 ] 
     The medium conveyor  20  may deliver the medium P from the medium container  10  and may prevent the medium P from skewing, and may convey the medium P to the transfer section  40  along the conveying path PW. The medium conveyor  20  may be located downstream of the medium container  10  in the conveying path PW. For example, the medium conveyor  20  may include a delivering roller pair  21 , a conveying roller pair  22 , and a resist roller pair  23 . The delivering roller pair  21 , the conveying roller pair  22 , and the resist roller pair  23  may be disposed in this order along the conveying direction F 1 . 
     The delivering roller pair  21  may feed the medium P to the conveying path PW. The delivering roller pair  21  may rotate in a direction in which the medium P is delivered to the conveying path PW under the control of a process control section  300  described later. The conveying roller pair  22  may convey the medium P in the conveying direction F 1  along the conveying path PW. The conveying roller pair  22  may rotate in a direction in which the medium P is conveyed in the conveying direction F 1  under the control of the process control section  300 . The resist roller pair  23  may prevent the medium P from skewing. The resist roller pair  23  may rotate in a direction in which the medium P is conveyed in the conveying direction F 1  and may prevent the medium P from skewing under the control of the process control section  300 . 
     [Configuration of Image Forming Section  30 ] 
     The image forming section  30  may form an image (a toner image) on a circumferential surface  31 A of a photoconductive drum  31  described later. The image forming section  30  may include, for example, four image forming units. For example, as illustrated in  FIG. 1 , the four image forming units may be image forming units  30 Y,  30 M,  30 C, and  30 K. 
       FIG. 3  schematically illustrates an outline configuration example of any of the image forming units  30 Y,  30 M,  30 C, and  30 K. Each of the image forming units  30 Y,  30 M,  30 C, and  30 K may develop an electrostatic latent image Ia on the circumferential surface  31 A of the photoconductive drum  31  and may form a toner image Ib of corresponding color, with use of toner  37  of corresponding color. The toner  37  may include a yellow toner, a magenta toner, a cyan toner, and a black toner corresponding to the image forming units  30 Y,  30 M,  30 C, and  30 K, respectively. The toner  37  corresponds to a specific but non-limiting example of “developer” in the disclosure. The electrostatic latent image Ia corresponds to a specific but non-limiting example of “latent image” in the disclosure. The image forming units  30 Y,  30 M,  30 C, and  30 K may be disposed in this order, for example, toward a rotation direction F 2  of a transfer belt  41  described later. The image forming units  30 Y,  30 M,  30 C, and  30 K may include components identical to one another. 
     Each of the image forming units  30 Y,  30 M,  30 C, and  30 K may include, for example, the photoconductive drum  31 , a charging roller  32 , a light emitting diode (LED) head  33 , a developing roller  34 , a feeding roller  35 , a cartridge  36 , a regulation blade  38 , and a cleaning blade  39 . The cartridge  36  may be filled with the toner  37 . The photoconductive drum  31  corresponds to a specific but non-limiting example of “image supporting member” in the disclosure. The LED head  33  corresponds to a specific but non-limiting example of “exposure section” in the disclosure. The developing roller  34  corresponds to a specific but non-limiting example of “developer supporting member” in the disclosure. The feeding roller  35  corresponds to a specific but non-limiting example of “feeding member” in the disclosure. 
     The photoconductive drum  31  includes the circumferential surface  31 A that includes a photoreceptive layer (for example, an organic photoreceptor), and may be a columnar member adapted to support the electrostatic latent image Ia on the circumferential surface  31 A. Specifically, the photoconductive drum  31  may include an electrically-conductive support and a photoconductive layer that covers an outer periphery (a surface) thereof. The conductive support may be formed of, for example, a metal pipe made of aluminum. The photoconductive layer may include a structure in which, for example, a charge generation layer and a charge transport layer are stacked in order. The photoconductive drum  31  may rotate in a direction in which the transfer belt  41  rotates in the rotation direction F 2  at a predetermined circumferential velocity under the control of the process control section  300 . 
     The charging roller  32  may be a member (charging member) charging the circumferential surface  31 A of the photoconductive drum  31 . The charging roller  32  may be so disposed as to be opposed to the circumferential surface  31 A of the photoconductive drum  31 , and may be disposed to face the circumferential surface  31 A. The charging roller  32  may include, for example, a metal shaft made of stainless steel and a semiconductive elastic layer (for example, a semiconductive epichlorohydrin rubber layer) that covers an outer periphery (a surface) thereof. The charging roller  32  may rotate in a direction opposite to the rotation direction of the photoconductive drum  31  by, for example, drive transmission from the photoconductive drum  31 . The charging member of the charging roller  32  may be applied with a charged voltage from the process control section  300 . 
     The LED head  33  exposes a charged region of the circumferential surface  31 A that has been charged by the charging roller  32  under the control of the process control section  300 , thereby forming the electrostatic latent image Ia in the charged region of the circumferential surface  31 A. The LED head  33  may be disposed to face the circumferential surface  31 A at a position downstream of the charging roller  32  in the rotation direction of the photoconductive drum  31 . The LED head  33  may include a plurality of LED emitting sections that are arranged in a width direction of the photoconductive drum  31 . Each of the LED emitting sections may include, for example, a light source emitting irradiation light, such as a light emitting diode, and a lens array that causes the irradiation light to be collected on the surface of the photoconductive drum  31 . 
     The developing roller  34  may be a member that supports the toner  37  on the surface thereof, and develops the electrostatic latent image Ia with use of the toner  37  to form a toner image Ib. The developing roller  34  includes a circumferential surface  34 A opposed to the circumferential surface  31 A of the photoconductive drum  31 , and is disposed to face the circumferential surface  31 A at a position downstream of the LED head  33  in the rotation direction of the photoconductive drum  31 . The circumferential surface  34 A corresponds to a specific but non-limiting example of “second circumferential surface” in the disclosure. The developing roller  34  may include, for example, a metal shaft made of stainless steel, and a semiconductive elastic layer (for example, a semiconductive urethane rubber layer) covering an outer periphery (a surface) thereof. The developing roller  34  may rotate in a direction opposite to the rotation direction of the photoconductive drum  31  by, for example, drive transmission from the photoconductive drum  31 . The surface of the developing roller  34  may be applied with a development voltage V 34  from the process control section  300 . 
     The feeding roller  35  is a member (a feeding member) feeding the toner  37  to the developing roller  34 , and includes a circumferential surface  35 A opposed to the circumferential surface  34 A of the developing roller  34 . The circumferential surface  35 A corresponds to a specific but non-limiting example of “third circumferential surface” in the disclosure. The feeding roller  35  may include, for example, a metal shaft made of stainless steel and a foamed elastic layer (for example, a silicone rubber layer) covering an outer periphery (a surface) thereof. The feeding roller  35  may rotate in a direction opposite to the rotation direction of the developing roller  34  by, for example, drive transmission from the developing roller  34 . The surface of the feeding roller  35  may be applied with a supply voltage V 35  from the process control section  300 . The feeding roller  35  may generate an electric field between the feeding roller  35  and the developing roller  34  with use of the supply voltage V 35  applied on the surface of the feeding roller  35 , and may feed the toner  37  from the feeding roller  35  to the developing roller  34  through the function of the electric field. 
     The cartridge  36  may be a container in which the above-described toner  37  of corresponding one of the colors is contained. The yellow toner  37  may be contained in the cartridge  36  of the image forming unit  30 Y. The magenta toner  37  may be contained in the cartridge  36  of the image forming unit  30 M. The cyan toner  37  may be contained in the cartridge  36  of the image forming unit  30 C. The black toner  37  may be contained in the cartridge  36  of the image forming unit  30 K. The toner  37  may be, for example, a non-magnetic one-component developer. 
     The regulation blade  38  may regulate a layer thickness of the toner  37  supported on the surface of the developing roller  34 . The regulation blade  38  may be formed of, for example, a steel use stainless (SUS) thin plate. The regulation blade  38  may be disposed to allow a tip thereof to be pressed against the developing roller  34 . The regulation blade  38  may frictionally charge the toner  37  on the surface of the developing roller  34  and may regulate the layer thickness of the toner  37 . The cleaning blade  39  may scrape the toner  37  remained on the surface of the photoconductive drum  31 . The cleaning blade  39  may be formed of, for example, a flexible rubber material or a flexible plastic material. 
     [Configuration of Transfer Section  40 ] 
     The transfer section  40  may electrostatically transfer the toner image Ib that has been formed on the circumferential surface  31 A of the photoconductive drum  31 , on the medium P conveyed from the medium conveyor  20 . The transfer section  40  may include, for example, the transfer belt  41 , a driving roller  42 , a tension roller  43 , a plurality of primary transfer rollers  44 , a counter roller  45 , a secondary transfer roller  46 , and a cleaning member  47 . The driving roller  42  may drive the transfer belt  41 , and the tension roller  43  may serve as a driven roller. The transfer section  40  may be a mechanism sequentially transferring the toner images Ib formed by the respective image forming units  30 Y,  30 M,  30 C, and  30 K on the surface of the transfer belt  41 , and then transferring the toner images Ib formed on the transfer belt  41  on the medium P conveyed from the medium conveyor  20 . 
     The transfer belt  41  may be an endless elastic belt formed of a resin material such as a polyimide resin. The transfer belt  41  may be stretched and rotatably supported by the driving roller  42 , the tension roller  43 , and the counter roller  45 . The driving roller  42  may circularly rotate the transfer belt  41  in the rotation direction F 2  under the control of the process control section  300 . The tension roller  43  may adjust tension to be applied to the transfer belt  41  with use of biasing force by a biasing member. The tension roller  43  may rotate in the direction same as the rotation direction of the driving roller  42 . 
     The primary transfer rollers  44  may be assigned with the respective image forming units  30 Y,  30 M,  30 C, and  30 K. Each of the primary transfer rollers  44  may electrostatically transfer, on the transfer belt  41 , an image formed on the circumferential surface  31 A of the photoconductive drum  31 . Each of the primary transfer rollers  44  may be opposed to an inner circumferential surface of the transfer belt  41 , and may be disposed to face the corresponding photoconductive drum  31 . Each of the primary transfer rollers  44  may be formed of, for example, a metal shaft covered with an electrically-conductive elastic material. The surface of each of the primary transfer rollers  44  may be applied with a primary transfer voltage from the process control section  300 . 
     The counter roller  45  and the secondary transfer roller  46  may be disposed to face each other with the transfer belt  41  in between. The secondary transfer roller  46  may electrostatically transfer the toner image Ib having been formed on the transfer belt  41  on the medium P conveyed through the conveying path PW. The secondary transfer roller  46  may include, for example, a metal core and an elastic layer such as a foamed rubber layer that is so formed as to be wound around the outer circumferential surface of the core. The counter roller  45  may rotate in a direction in which the transfer belt  41  rotates in the rotation direction F 2  under the control of the process control section  300 . The surface of the secondary transfer roller  46  may be applied with a secondary transfer voltage from the process control section  300 . 
     The cleaning member  47  may be disposed downstream of the secondary transfer roller  46  and upstream of an uppermost image forming unit (the image forming unit  30 Y) in the rotation direction F 2  of the transfer belt  41 . The cleaning member  47  may scrape the toner  37  remained on the surface of the transfer belt  41 . The cleaning member  47  may be formed of, for example, a flexible rubber material or a flexible plastic material. 
     [Configuration of Fixing Section  50 ] 
     The fixing section  50  may fixe the toner image Ib on the medium P at predetermined temperature. The fixing section  50  may apply heat and pressure to the toner image Ib transferred on the medium P that has passed through the transfer section  40 , thereby fixing the toner image Ib on the medium P. The fixing section  50  may be disposed downstream of the transfer section  40  in the conveying path PW. The fixing section  50  may include, for example, an upper roller  51  and a lower roller  52 . 
     The upper roller  51  may include a heat source, and may function as a heating roller applying heat to the toner image Ib on the medium P. The heat source may be a heater such as a halogen lamp inside the upper roller  51 . The upper roller  51  may rotate in a direction in which the medium P is conveyed in the conveying direction F 1  under the control of the process control section  300 . The heat source in the upper roller  51  may control the temperature of the surface of the upper roller  51  under the control of the process control section  300 . The lower roller  52  may be disposed to face the upper roller  51  such that a pressure contact part is formed between the lower roller  52  and the upper roller  51 , and may function as a pressure roller applying pressure to the toner image Ib on the medium P. The lower roller  52  may include a surface layer formed of an elastic material. 
     [Configuration of Discharge Section  60 ] 
     The discharge section  60  may discharge the medium P on which the toner image Ib has been fixed by the fixing section  50 , to the outside. The discharge section  60  may be disposed downstream of the fixing section  50  in the conveying path PW. The discharge section  60  may include, for example, a conveying roller pair  61 . The conveying roller pair  61  may discharge the medium P to the outside through the conveying path PW, and may stock the medium P in an outside stacker, for example. The conveying roller pair  61  may rotate in a direction in which the medium P is conveyed in the conveying direction F 1  under the control of the process control section  300 . 
     [Configuration of Density Sensor  70 ] 
     The density sensor  70  may detect density of a non-printing-use toner image Ib on the transfer belt  41 . “Non-printing-use” refers to the toner image Ib that is not intended to be printed on the medium P. The density sensor  70  may detect density of the non-printing-use toner image Ib on the transfer belt  41  before printing start under the control of the controller  200 . “Printing start” refers to time at which printing of a printing-use toner image that is formed through development by the developing roller  34  on the medium P is started. “Printing-use” refers to the toner image Ib that is intended to be printed on the medium P. 
     The density sensor  70  may include, for example, a light emitting diode (LED) and a photoreceptor diode. The light emitting diode may apply light to the non-printing-use toner image Ib on the transfer belt  41 . The photoreceptor diode may receive light (reflected light) that has been reflected by the non-printing-use toner image Ib on the transfer belt  41 , out of the light emitted from the light emitting diode. A detection signal outputted from the photoreceptor diode may relate to intensity of the reflected light that is correlated with the density of the non-printing-use toner image Ib. The density sensor  70  may drive the light emitting diode and the photoreceptor diode, for example, based on a control signal received from the controller  200 . The density sensor  70  may include a drive circuit providing the controller  200  with the detection signal outputted from the photoreceptor diode. The density sensor  70  may process the detection signal outputted from the photoreceptor diode to generate density data of the non-printing-use toner image Ib, thereby outputting the generated density data. The density sensor  70  may be disposed at a position facing the transfer belt  41 . For example, the density sensor  70  may be disposed downstream of the primary transfer roller  44  and upstream of the primary transfer roller  46  in the rotation direction F 2  of the transfer belt  41 . 
     [Control Mechanism] 
     A control mechanism of the image forming apparatus  1  is described with reference to  FIG. 4  in addition to  FIG. 1 .  FIG. 4  is a block diagram illustrating an example of the control mechanism of the image forming apparatus  1 . 
     The image forming apparatus  1  may include, for example, the controller  200  and the process control section  300  as the control mechanism. The controller  200  may control the medium container  10 , the medium conveyor  20 , the image forming section  30 , the transfer section  40 , the fixing section  50 , and the discharge section  60  through the process control section  300 , based on, for example, print data Dp received from an information processor  400 . The process control section  300  may control the medium container  10 , the medium conveyor  20 , the image forming section  30 , the transfer section  40 , the fixing section  50 , and the discharge section  60 , based on the control signal received from the controller  200 . 
     The print data Dp may include at least image data Di. The print data Dp may include a label size LS and the label gap size GS in addition to the image data Di. The image data Di corresponds to a specific but non-limiting example of “image data” in the disclosure. The label size LS corresponds to a specific but non-limiting example of “label size” in the disclosure. The label gap size GS corresponds to a specific but non-limiting example of “label gap size” in the disclosure. 
     The controller  200  may include, for example, a CPU  201 , a ROM  202 , a RAM  203 , and a non-volatile memory  204 . The ROM  202  may be a memory holding a control program used to operate the image forming apparatus  1 . For example, the CPU  201  may control various components in the image forming apparatus  1  through an internal bus  211 . The CPU  201  may control printing operation of the image forming apparatus  1 , based on, for example, the control program read from the ROM  202  and the print data D P  received from the outside. The RAM  203  may be a memory holding work necessary for operation of the image forming apparatus  1 . The non-volatile memory  204  may hold, for example, a voltage setting expression  220 , a target value Dg, a setting value V 34S , and a setting value V 35S . 
     The voltage setting expression  220  is described.  FIG. 5  is a graph illustrating an example of the voltage setting expression  220 . The voltage setting expression  220  may show an example of relationship between the development voltage V 34  and the image density D 1 . In  FIG. 5 , a potential difference between the development voltage V 34  and the supply voltage V 35  may be fixed. The image density D I  may indicate intensity of reflected light of the toner image Ib on the transfer belt  41  with use of OD value that is an index of optical density. As illustrated in  FIG. 5 , the development voltage V 34  may be substantially proportional to the image density D I  within a limited range. Thus, adjusting the development voltage V 34  makes it possible to adjust the image density D I  to the target value Dg. For example, in the example of  FIG. 5 , when the target value Dg of the image density D I  is adjusted to the OD value of 1.5, −170 V is set to the setting value V 34S  of the development voltage V 34 . At this time, for example, −270 V may be set to the setting value V 35S  of the supply voltage V 35 . Note that specific derivation and utilization of the voltage setting expression  220  are described in detail later. 
     The non-volatile memory  204  may hold, for example, a correction table  230  or a plurality of thresholds Nc_th that are different from one another.  FIG. 6  illustrates an example of the correction table  230 . The correction table  230  may include correction values of the development voltage V 34  set for each range of the development voltage V 34  at the printing start. In the correction table  230 , the range of continuous printing count Nc may be divided into a plurality of ranges R 1  by the plurality of thresholds Nc_th. For example, the range of the continuous printing count Nc may be divided into six ranges R 1  by five thresholds Nc_th. The six ranges R 1  may be, for example, “a range from 500 counts or more to less than 1000 counts (a range R 1 ( 1 ))”, “a range from 1000 counts or more to less than 1500 counts (a range R 1 ( 2 ))”, “a range from 1500 counts or more to less than 2000 counts (a range R 1 ( 3 ))”, “a range from 2000 counts or more to less than 2500 counts (a range R 1 ( 4 ))”, “a range from 2500 counts or more to less than 3000 counts (a range R 1 ( 5 ))”, and “a range of 3000 counts or more (a range R 1 ( 6 ))”. 
     In the correction table  230 , the set range of the development voltage V 34  may be further divided into a plurality of ranges R 2 . For example, the set range of the development voltage V 34  may be divided into three ranges R 2 . The three ranges R 2  may be, for example, “a range where |V 34 | is lower than 180 V (a range R 2 ( 1 ))”, “a range where |V 34 | is equal to or higher than 180 V and lower than 230 V (a range R 2 ( 2 ))”, and “a range where |V 34 | is equal to or higher than 230 V (a range R 2 ( 3 ))”. 
     In the correction table  230 , the correction value of the development voltage V 34  may be assigned to each of the divided ranges R 1 . For example, in the range R 2 ( 1 ), +17 V may be assigned to the range R 1 ( 1 ) as the correction value of the development voltage V 34 . For example, in the range R 2 ( 1 ), +34 V may be assigned to the range R 1 ( 2 ) as the correction value of the development voltage V 34 . For example, in the range R 2 ( 1 ), +51 V may be assigned to the range R 1 ( 3 ) as the correction value of the development voltage V 34 . For example, in the range R 2 ( 1 ), +68 V may be assigned to the range R 1 ( 4 ) as the correction value of the development voltage V 34 . For example, in the range R 2 ( 1 ), +85 V may be assigned to the range R 1 ( 5 ) as the correction value of the development voltage V 34 . For example, in the range R 2 ( 1 ), +102 V may be assigned to the range R 1 ( 6 ) as the correction value of the development voltage V 34 . 
     In the correction table  230 , the correction value of the development voltage V 34  may be further assigned to each of the divided ranges R 2 . For example, in the range R 1 ( 1 ), +17 V may be assigned to the range R 2 ( 1 ) as the correction value of the development voltage V 34 . For example, in the range R 1 ( 1 ), +12 V may be assigned to the range R 2 ( 2 ) as the correction value of the development voltage V 34 . For example, in the range R 1 ( 1 ), +8 V may be assigned to the range R 2 ( 3 ) as the correction value of the development voltage V 34 . 
     In each range R 1  of the correction table  230 , the correction value of the development voltage V 34  may be varied depending on the range R 2 . Further, in each range R 1  of the correction table  230 , an absolute value of the correction value of the development voltage V 34  may be increased as the range R 2  becomes lower. For example, in the range R 1 ( 1 ), +8 V may be assigned to the range R 2 ( 3 ) as the correction value of the development voltage V 34 . For example, in the range R 1 ( 1 ), +12 V (&gt;+8 V) may be assigned to the range R 2 ( 2 ) as the correction value of the development voltage V 34 . For example, in the range R 1 ( 1 ), +17 V (&gt;+12 V) may be assigned to the range R 2 ( 1 ) as the correction value of the development voltage V 34 . 
     One significance of the correction table  230  is described.  FIG. 7  illustrates an example of variation in the image density D I  by the continuous printing. As can be seen from  FIG. 7  that the image density D I  is increased with an increase in the continuous printing count Nc. For example, in a case where the continuous printing count Nc is increased from 0 count to 1600 counts as a result of the continuous printing, the OD value is increased from 1.50 to 1.62. For example, when 1000 pieces of the labels with the same image pattern are printed while the continuous printing count Nc is increased from 0 count to 1600 counts, the color of the image is gradually varied, which results in remarkable color tone difference between a first label and 1000th label. Accordingly, a method may be contemplated to adjust the process condition (such as the development voltage V 34 ) even during the continuous printing to minimize variation in the image density D I . The tolerance of difference in color tone depends on, for example, user and the purpose of use. However, to avoid difference in color tone remarkable visually, the difference of the image density D I  may be preferably within 0.05 in OD value. 
     To adjust the image density D I  to the target value, a method may be contemplated to adjust the development voltage V 34  with use of the above-described voltage setting expression  220 . As will be described later, this, on the other hand, requires interruption of the continuous printing in order to use the above-described voltage setting expression  220 . However, when the continuous printing count NC during the continuous printing is within each range R 1  of the correction table  230 , it is possible to adjust the development voltage V 34  with use of the correction table  230  instead of the above-described voltage setting expression  220 . In other words, using the correction table  230  makes it possible to adjust the development voltage V 34  without interrupting the continuous printing. A specific method of utilizing the correction table  230  is described in detail later. 
     The non-volatile memory  204  may hold, for example, a threshold Nt_th. The threshold Nt_th may be larger than the threshold Nc_th. For example, a result detected by a drum counter  205  described later, the continuous printing count Nc, and an accumulated count Nt described later may be held by the non-volatile memory  204 . The result detected by the drum counter  205  may include, for example, the number of rotations of the photoconductive drum  31 , or physical quantity correlated with the number of rotations of the photoconductive drum  31 . The threshold Nt_th, the number of rotations of the photoconductive drum  31 , the physical quantity correlated with the number of rotations of the photoconductive drum  31 , the continuous printing count Nc, and the accumulated count Nt are described in detail later. 
     Next, other configurations in the controller  200  are described. The controller  200  may further include, for example, the drum counter  205 , an operation panel  206 , a host I/F  207 , an external I/F  208 , a voltage setting section  209 , and a voltage correction section  210 . 
     The drum counter  205  may detect the number of rotations of the photoconductive drum  31  or the physical quantity correlated with the number of rotations of the photoconductive drum  31 . The drum counter  205  may perform counting of the continuous printing count Nc and the accumulated count Nt during a predetermined period. The drum counter  205  may store the continuous printing count Nc and the accumulated count Nt that are obtained by the counting, in the non-volatile memory  204 . The initial values of the continuous printing count Nc and the accumulated count Nt may be, for example, zero. The drum counter  205  may reset the continuous printing count NC stored in the non-volatile memory  204  to the initial value at the time when the printing is stopped or started. The drum counter  205  may reset the accumulated count Nt stored in the non-volatile memory  204  to the initial value at the time when density correction described later is performed. 
     Here, the predetermined period refers to a period from a time point when the setting value V 34S  set by a high-voltage control section  303  described later is applied as the development voltage V 34  to the developing roller  34  to a time point when the printing is stopped. The continuous printing count Nc and the accumulated count Nt each refer to, for example, the number of pulses of a drive pulse signal outputted to a motor from a motor control section  302  when the motor control section  302  pulse-controls the motor that rotates the photoconductive drum  31 . At this time, the continuous printing count Nc and the accumulated count Nt may be specific but non-limiting examples of the physical quantity correlated with the number of rotations of the photoconductive drum  31 . Further, at this time, the drum counter  205  may count the number of pulses of the above-described drive pulse signal. Note that the continuous printing count Nc and the accumulated count Nt may be different from the number of pulses of the above-described drive pulse signal as long as being the number of rotations of the photoconductive drum  31  or the physical quantities correlated with the rotation number of the photoconductive drum  31 . 
     The continuous printing count Nc and the accumulated count Nt each may be incremented by one, for example, every time the photoconductive drum  31  rotates once. At this time, the continuous printing count Nc and the accumulated count Nt each may be equal to the number of rotations of the photoconductive drum  31 . At this time, for example, the drum counter  205  may detect, once, a marker provided at a predetermined position of the photoconductive drum  31  every time the photoconductive drum  31  rotates once, and may increment each of the continuous printing count Nc and the accumulated count Nt by one every time detecting the marker. 
     Note that the continuous printing count Nc illustrated in the drawings may be incremented by one every time the photoconductive drum  31  rotates once. In the case where the continuous printing count Nc is incremented by one every time the photoconductive drum  31  rotates once, one count corresponds to an image formation length of 94.2 mm for one rotation where the diameter of the photoconductive drum  31  is 30 mm. When vertical feed amount of A 6  size is 148 mm and the gap between the labels is 3 mm, the continuous printing count Nc may be incremented by 1.6 every time one label is printed. Therefore, when the 1000 pieces of A 6  labels are printed, the continuous printing count Nc may become 1600 counts. 
     As described above, the continuous printing count Nc may be the number of rotations of the photoconductive drum  31  or the physical quantity correlated with the number of rotations of the photoconductive drum  31 . The drum counter  205  may thus count the continuous printing counts Nc as the number of rotations of the photoconductive drum  31  or the physical quantity correlated with the number of rotations of the photoconductive drum  31 . The drum counter  205  may measure the number of rotations of the photoconductive drum  31  or the physical quantity correlated with the number of rotations of the photoconductive drum  31  by a method other than the method described above. Note that “number of rotations of photoconductive drum  31  or physical quantity correlated with number of rotations of photoconductive drum  31 ” is referred to as “result counted by drum counter  205 ” in the following description. 
     The operation panel  206  may display a state of the image forming apparatus  1  or display information to prompt a user to action. The operation panel  206  may display a plurality of kinds of printing paper and a plurality of kinds of printing modes to make the user select printing paper and a printing mode. The operation panel  206  may transfer the printing mode selected by the user to the CPU  201 . Examples of the printing modes may include free layout printing and label printing. The free layout printing may be printing according to a layout specified by the print data Dp. The label printing may be printing on rolled paper attached with a plurality of labels LB at a predetermined interval. 
     In the case where the printing mode selected by the user is the label printing, the operation panel  206  may allow the user to input the label size LS and the label gap size GS. At this time, the operation panel  206  may transfer the label size LS and the label gap size GS inputted (externally) by the user, to the CPU  201 . In the case where the printing mode selected by the user is the label printing, the controller  200  may extract the label size LS and the label gap size GS from the print data Dp. In this case, it is necessary for the print data Dp to include the label size LS and the label interval size GS, in addition to the image data Di. 
     The host I/F  207  may acquire the print data Dp that is transmitted from the external information processor  400  coupled to the image forming apparatus  1 , and may transfer the print data Dp to the CPU  201 . The external I/F  208  may transfer the control signal transmitted from the CPU  201 , to the process control section  300 , and may transfer data (such as density data) transmitted from the process control section  300 , to the CPU  201 . 
     The voltage setting section  209  may set the development voltage V 34  to be applied to the developing roller  34 , based on the density of the non-printing-use toner image Ib detected by the density sensor  70 . The voltage setting section  209  may set the development voltage V 34  to be applied to the developing roller  34 , based on the detected signal outputted from the density sensor  70 . The voltage setting section  209  may set the development voltage V 34  to be applied to the developing roller  34  for each of the image forming units  30 Y,  30 M,  30 C, and  30 K. The voltage setting section  209  may store, as the setting value V 34S , the development voltage V 34  set for each of the image forming units  30 Y,  30 M,  30 C, and  30 K in the non-volatile memory  204 . Note that the voltage setting section  209  may set the development voltage V 34  to be applied to the developing roller  34  by a method common to the image forming units  30 Y,  30 M,  30 C, and  30 K. Thus, a method of setting the development voltage V 34  to be applied to the developing roller  34  of the image forming unit  30 Y is described below as a representative of the image forming units  30 Y,  30 M,  30 C, and  30 K. 
     The voltage setting section  209  may set the development voltage V 34  to be applied to the developing roller  34  of the image forming unit  30 Y in the following manner, for example. The voltage setting section  209  may first derive the voltage setting expression  220  while varying the development voltage V 34  to be applied to the developing roller  34  of the image forming unit  41 , based on the detection signals obtained from respective three non-printing-use toner images Ib formed on the transfer belt  41 . 
     For example, it is assumed that the detection signal obtained when the development voltage V 34  is set to −140 V is a signal corresponding to the OD value of 1.45. Also, for example, it is assumed that the detection signal obtained when the development voltage V 34  is set to −200 V is a signal corresponding to the OD value of 1.55. Also, for example, it is assumed that the detection signal obtained when the development voltage V 34  is set to −260 V is a signal corresponding to the OD value of 1.65. The voltage setting section  209  may derive an approximate straight line from the three setting values V 34S  of the development voltage V 34  and the three measured OD values. The approximate straight line may be represented by, for example, the voltage setting expression  220  in  FIG. 5 . Subsequently, the voltage setting section  209  may derive the development voltage V 34  corresponding to the target value Dg of the OD value set by the user, with use of the derived approximate straight line. For example, when the target value Dg of the image density D I  is adjusted to the OD value of 1.5, the voltage setting section  209  may set, for example, −170 V to the setting value V 34S  of the development voltage V 34  corresponding to the target value Dg, with use of the voltage setting expression  220  in  FIG. 5 . 
     The voltage setting section  209  may further set the supply voltage V 35 , based on the value of the development voltage V 34 . Specifically, the voltage setting section  209  may set the supply voltage V 35  to allow a potential difference between the development voltage V 34  and the supply voltage V 35  to be a fixed value. When the voltage setting section  209  sets, for example, −170 V to the setting value V 34S , the voltage setting section  209  may set −270 V to the setting value V 35S  of the supply voltage V 35 . 
     The voltage correction section  210  may correct the development voltage V 34  with use of, for example, the correction table  230 . The voltage correction section  210  may read, for example, a correction value assigned to the range R 1  including the result counted by the drum counter  205 , from the correction table  230  in the non-volatile memory  204 , and may correct the development voltage V 34  with use of the read correction value. The voltage correction section  210  may further read, for example, the correction value assigned to the range R 2  including the result counted by the drum counter  205 , from the correction table  230  in the non-volatile memory  204 , and may correct the development voltage V 34  with use of the read correction value. The voltage correction section  210  may correct the supply voltage V 35  to allow the potential difference between the development voltage V 34  and the supply voltage V 35  to be a fixed value, with use of the corrected development voltage V 34 . 
     Next, the process control section  300  is described. The process control section  300  may include, for example, a fixation control section  301 , the motor control section  302 , the high-voltage control section  303 , and an exposure control section  304 . The fixation control section  301  may control the heat source in the upper roller  51  to allow the temperature of the upper roller  51  to be the set fixation temperature, under the control of the controller  200 . The motor control section  302  may control motors rotating the photoconductive drum  31  and other various rollers, under the control of the controller  200 . 
     The exposure control section  304  may control the exposure operation of the LED head  33  under the control of the controller  200 . The exposure control section  304  may convert the print data Dp received from the controller  200  into exposure data, and may provide the exposure data to the LED heads  33  of the respective image forming units  30 Y,  30 M,  30 C, and  30 K. 
     When the photoconductive drum  31 , the developing roller  34 , and the feeding roller  35  rotate after the printing start in the label printing mode, the exposure control section  304  may control the exposure operation of the LED head  33  to allow a plurality of electrostatic latent images Ia to be formed side by side at a predetermined interval on the circumferential surface  31 A. In the label printing mode, the exposure control section  304  may control the exposure operation, based on the image data Di, the label size LS, and the label gap size GS that are provided from outside. Specifically, the exposure control section  304  may derive the exposure start timing and the exposure end timing of each electrostatic latent image Ia to be formed, to allow each toner image Ib to be transferred on the surface of the label LB, based on the image data Di, the label size LS, and the label gap size GS that are provided from the outside. The exposure control section  304  may provide an exposure control signal  304 A to the LED heads  33  of the respective image forming units  30 Y,  30 M,  30 C, and  30 K. The exposure control signal  304 A may include the derived exposure start timing, the derived exposure end timing, and exposure data. The exposure control signal  304 A may be a signal used to control the LED head  33 . The exposure control signal  304 A corresponds to a specific but non-limiting example of “exposure control signal” in the disclosure. The exposure control section  304  may generate the exposure control signal  304 A, based on the image data Di, the label size LS, and the label gap size GS that are provided from the outside. 
     The high-voltage control section  303  may output a charged voltage V 32 , the development voltage V 34 , the supply voltage V 35 , the primary transfer voltage, and the secondary transfer voltage under the control of the controller  200 . The high-voltage control section  303  may apply the charged voltage V 32  to the charging roller  32 . The high-voltage control section  303  may further apply the development voltage V 34  to the developing roller  34 , and may apply the supply voltage V 35  to the feeding roller  35 . 
     The high-voltage control section  303  may apply the development voltage V 34  set by the voltage setting section  209 , to the developing roller  34  at a predetermined timing. When the development voltage V 34  is corrected by the voltage correction section  210 , the high-voltage control section  303  may apply the corrected development voltage V 34  to the developing roller  34  during the continuous printing. The high-voltage control section  303  may vary, at a predetermined timing during the continuous printing, the development voltage V 34  to be applied to the developing roller  34  from the development voltage V 34  before the correction to the corrected development voltage V 34 . Specifically, the controller  200  may vary the development voltage V 34  from the development voltage V 34  before the correction to the corrected development voltage V 34  at a predetermined timing during the continuous printing without stopping the printing. The high-voltage control section  303  may apply the latest development voltage V 34  (i.e., the development voltage V 34  before the next correction) to the developing roller  34  until the next correction is performed by the voltage correction section  210 . 
     The high-voltage control section  303  may apply the supply voltage V 35  set by the voltage setting section  209 , to the feeding roller  35  at a predetermined timing. When the supply voltage V 35  is corrected by the voltage correction section  210 , the high-voltage control section  303  may apply the corrected supply voltage V 35  to the feeding roller  35  during the continuous printing. The high-voltage control section  303  may vary, at a predetermined timing during the continuous printing, the supply voltage V 35  to be applied to the feeding roller  35  from the supply voltage V 35  before the correction to the corrected supply voltage V 35 . Specifically, the controller  200  may vary the supply voltage V 35  from the supply voltage V 35  before the correction to the corrected supply voltage V 35  at a predetermined timing during the continuous printing without stopping the printing. The high-voltage control section  303  may apply the latest supply voltage V 35  (i.e., the supply voltage V 35  before next correction) to the feeding roller  35  until the next correction is performed by the voltage correction section  210 . 
       FIG. 8A  illustrates an example of the operation of the image forming unit  30 Y when the supply voltage V 35  is varied (time T=T 1 ) immediately before the development voltage V 34  is varied.  FIG. 8B  illustrates an example of the operation of the image forming unit  30 Y when the development voltage V 34  is varied (time T=T 2 ) immediately after the supply voltage V 35  is varied.  FIG. 8C  illustrates an example of the operation of the image forming unit  30 Y immediately after the development voltage V 34  is varied (time T=T 3 ).  FIG. 9  illustrates an example of the voltage varying of the development voltage V 34  and the supply voltage V 35 . 
     Note that time difference T 2 −T 1  may correspond to a time necessary for a portion P 3  described later to travel from a position at which the portion P 3  is opposed to the circumferential surface  35 A of the feeding roller  35  to a position at which the portion P 3  is opposed to the circumferential surface  31 A of the photoconductive drum  31 , in the circumferential surface  34 A of the developing roller  34  while the photoconductive drum  31 , the developing roller  34 , and the feeding roller  35  rotate. Also, time difference T 3 −T 2  may correspond to a time necessary for a portion P 1  described later to travel from a position at which the portion P 1  is opposed to the circumferential surface  34 A of the developing roller  34  to a position at which the portion P 1  is opposed to the surface of the transfer belt  41 , in the circumferential surface  31 A of the photoconductive drum  31 , while the photoconductive drum  31 , the developing roller  34 , and the feeding roller  35  rotate. 
     The high-voltage control section  303  may control the varying timing of the development voltage V 34  resulting from the correction of the development voltage V 34 . In the label printing mode, the high-voltage control section  303  may control the varying timing of the development voltage V 34 , based on the label size LS and the label gap size GS that are provided from the outside. The high-voltage control section  303  may perform this control while the exposure control section  304  controls, based on the image data Di, the label size LS, and the label gap size GS that are provided from the outside, the exposure operation. 
     Specifically, in the label printing mode, the high-voltage control section  303  may control the varying timing of the development voltage V 34  to allow a portion P 1  to be located within a gap G 1  between the electrostatic latent images of the circumferential surface  31 A as illustrated in  FIG. 8B  and  FIG. 9 . The high-voltage control section  303  may perform the control while the exposure control section  304  controls the exposure operation of the LED head  33  to allow the plurality of electrostatic latent images Ia to be formed side by side at a predetermined interval on the circumferential surface  31 A of the photoconductive drum  31 . The portion P 1  may be a portion, in the circumferential surface  31 A, opposed to the developing roller  34  upon varying of the development voltage V 34 . The gap G 1  between the electrostatic latent images may be a gap between the electrostatic latent images Ia adjacent to each other. The gap G 1  between the electrostatic latent images corresponds to a specific but non-limiting example of “gap between latent images”. 
     In the label printing mode, the high-voltage control section  303  may control the varying timing of the development voltage V 34  to allow the portion P 1  to be opposed to the portion  41 A that is opposed to the gap LG between labels in the transfer belt  41  as illustrated in  FIG. 8C  and  FIG. 9 . The high-voltage control section  303  may perform the control while the exposure control section  34  controls the exposure operation of the LED head  33  to allow the plurality of electrostatic latent images Ia to be formed side by side at a predetermined interval on the circumferential surface  31 A. 
     The high-voltage control section  303  may control the varying timing of the supply voltage V 35  resulting from correction of the supply voltage V 35  that is associated with the correction of the development voltage V 34 . In the label printing mode, the high-voltage control section  303  may correct the development voltage V 34  after correcting the supply voltage V 35 . 
     In the label printing mode, the high-voltage control section  303  may control the varying timing of the supply voltage V 35 , based on the label size LS and the label gap size GS that are provided from the outside. The high-voltage control section  303  may perform the control while the exposure control section  304  controls, based on the image data Di, the label size LS, and the label gap size GS that are provided from the outside, the exposure operation. 
     Specifically, in the label printing mode, the high-voltage control section  303  may preferably control the varying timing of the supply voltage V 35  to allow a portion P 2  to be located within the gap G 1  between the electrostatic latent images of the circumferential surface  31 A as illustrated in  FIG. 8A ,  FIG. 8B , and  FIG. 9 . The high-voltage control section  303  may perform the control while the exposure control section  304  controls the exposure operation of the LED head  33  to allow the plurality of electrostatic latent images Ia to be formed side by side at a predetermined interval on the circumferential surface  31 A of the photoconductive drum  31 . The portion P 2  may be a portion, in the circumferential surface  31 A, opposed to a portion P 3  of the developing roller  34 . The portion P 3  may be a portion, in the circumferential surface  34 A of the developing roller  34 , opposed to the feeding roller  35  upon varying of the voltage V 34 . 
     In the label printing mode, the high-voltage control section  303  may control the varying timing of the supply voltage V 35  to allow the portion P 2  to be opposed to the portion  41 A that is opposed to the gap LG between labels, in the transfer belt  41  as illustrated in  FIG. 8C  and  FIG. 9 . The high-voltage control section  303  may perform the control while the exposure control section  304  controls the exposure operation of the LED head  33  to allow the plurality of electrostatic latent images Ia to be formed side by side at a predetermined interval on the circumferential surface  31 A. 
     In  FIG. 8B  and  FIG. 8C , the high-voltage control section  303  may control the varying timing of both of the development voltage V 34  and the supply voltage V 35  to allow both of the portions P 1  and P 2  to be coincident with each other. Note that the high-voltage control section  303  may control the varying timing of both of the development voltage V 34  and the supply voltage V 35  to allow the portion P 2  to be located upstream of the portion P 1  in the rotation direction of the circumferential surface  31 A of the photoconductive drum  31  as illustrated in  FIG. 10  and  FIG. 11 . Note that  FIG. 10  illustrates an example of the operation of the image forming unit  30 Y when the portion P 2  is opposed to the portion P 3  (time T=T 4 ) immediately after the supply voltage V 35  is varied.  FIG. 11  illustrates an example of the voltage varying of the development voltage V 34  and the supply voltage V 35 . 
     In  FIGS. 8A to 11 , variation in thickness and charged amount of the toner  37  attached on the circumferential surface  34 A of the developing roller  34  resulting from the varying of the supply voltage V 35  may reach a portion, in the circumferential surface  34 A of the developing roller  34 , opposed to the circumferential surface  31 A of the photoconductive drum  31  upon or before varying of the development voltage V 34 . As a result, it is possible to suppress large variation in the image density D I  before and after varying of the development voltage V 34 , as compared with the case where variation in the thickness and the charged amount of the toner  37  attached on the circumferential surface  34 A of the developing roller  34  resulting from varying of the supply voltage V 35  reaches a portion, in the circumferential surface  34 A of the developing roller  34 , opposed to the circumferential surface  31 A of the photoconductive drum  31  after varying of the development voltage V 34 . Note that the variation in the image density D I  resulting from the varying of the supply voltage V 35  may be smaller than the variation in the image density D I  resulting from the varying of the development voltage V 34 . Therefore, in the label printing mode, there may be a case where the portion P 2  is accepted to be located at a position outside the gap G 1  between the electrostatic latent images of the circumferential surface  31 A. 
     The controller  200  may stop printing every time the result counted by the drum counter  205  exceeds the threshold Nt_th. The controller  200  may further control the image forming section  30  and the transfer section  40  to allow the plurality of non-printing-use toner images Ib with different development voltages from one another to be formed on the transfer belt  41  while the printing is stopped. At this time, the voltage correction section  210  may reset the result counted by the drum counter  205  stored in the non-volatile memory  204  every time the printing is stopped. The density sensor  70  may detect the density of the non-printing-use toner image Ib on the transfer belt  41  while the printing is stopped. The voltage setting section  209  may set the development voltage V 34  to be applied to the developing roller  34 , based on the density of the toner image Ib detected by the density sensor  70 , every time the detection by the density sensor  70  is performed. The controller  200  may start printing after the development voltage V 34  is set by the voltage setting section  209 . The high-voltage control section  303  may apply the reset development voltage V 34  to the developing roller  34  every time the development voltage V 34  is reset by the voltage setting section  209 . 
     [Operation] 
     The operation of the image forming apparatus  1  is described. In the image forming apparatus  1 , the toner image Ib may be formed with respect to the medium P in the following way. When the printing job is supplied to the CPU  201  from the image processor  400  coupled to the image forming apparatus  1 , the CPU  201  may perform the printing processing to allow each component in the image forming apparatus  1  to perform the following operation, based on the printing job. 
     First, heating of the upper roller  51  by the heater may be started. When the temperature of the upper roller  51  reaches the predetermined temperature, the medium P contained in the medium container  10  may be taken out by the delivering roller pair  21 , and the medium P may be then delivered to the conveying path PW. The medium P delivered to the conveying path PW may be then conveyed through the conveying path PW by the conveying roller pair  22  in the conveying direction F 1 , and then skewing of the medium P may be corrected by the resist roller pair  23 . The operation of both of the image forming section  30  and the transfer section  40  may be started at respective predetermined timings, and the medium P may be conveyed to the transfer section  40 , and the toner image formed by the image forming section  30  in the following manner may be transferred on the medium P. The image may be printed on the medium P in the foregoing way. 
     In the image forming section  30 , the toner image Ib may be formed by the following electrophotographic process. When the charged voltage V 32  is applied from the high-voltage control section  303  to the charging roller  32 , the surface (the surface layer) of the charging roller  32  may be uniformly charged, and the portion of the circumferential surface  31 A of the photoconductive drum  31  opposed to the charging roller  32  may be accordingly charged to the predetermined voltage (for example, −600 V). Then, when the illumination light is applied from the LED head  33  toward the charged region of the circumferential surface  31 A of the photoconductive drum  31  and the circumferential surface  31 A of the photoconductive drum  31  is thereby exposed, the electrostatic latent image Ia corresponding to the printing pattern that is specified by the above-described printing job may be formed on the circumferential surface  31 A. At this time, the voltage of the portion of the circumferential surface  31 A of the photoconductive drum  31  corresponding to the electrostatic latent image Ia may be, for example, about 0 V. 
     When the supply voltage V 35  is applied from the high-voltage control section  303  to the feeding roller  35 , the surface (the surface layer) of the feeding roller  35  may be charged to the predetermined voltage (for example, −300 V). Likewise, when the development voltage V 34  is applied from the high-voltage control section  303  to the developing roller  34 , the surface (the surface layer) of the developing roller  34  may be charged to the predetermined voltage (for example, −205 V). At this time, the feeding roller  35  may be opposed to the developing roller  34 , and the feeding roller  35  and the developing roller  34  may rotate at respective predetermined circumferential velocities. This allows the toner  37  charged to negative to be attracted by the developing roller  34  due to potential difference between the voltage V 35  of the feeding roller  35  and the voltage V 34  of the developing roller  34 . As a result, the toner  37  may be supplied from the surface of the feeding roller  35  to the surface of the developing roller  34 . Subsequently, the toner  37  on the developing roller  34  may be charged by, for example, friction by the regulation blade  38  in contact with the developing roller  34 . Here, the thickness of the toner  37  on the developing roller  34  may be defined by, for example, the development voltage V 34  of the developing roller  34 , the supply voltage V 35  of the feeding roller  35 , and the pressing pressure of the regulation blade  38 . The developing roller  34  may be opposed to the photoconductive drum  31 , and the developing roller  34  and the photoconductive drum  31  may rotate at respective predetermined circumferential velocities. Therefore, the negatively-charged toner  37  may be attracted to the photoconductive drum  31  by the potential difference between the development voltage V 34  of the developing roller  34  and the voltage at the portion, in the circumferential surface  31 A of the photoconductive drum  31 , corresponding to the electrostatic latent image Ia. As a result, the toner  37  may be attached to the electrostatic latent image Ia on the photoconductive drum  31 , and the toner image Ib may be accordingly formed. Note that, since the voltage of the portion, in the circumferential surface  31 A of the photoconductive drum  31 , corresponding to the charged region is lower than the development voltage V 34  of the developing roller  34 , the negatively-charged toner  37  may not be attracted to the charged region. 
     Thereafter, the toner image Ib on the photoconductive drum  31  may be transferred to the transfer belt  41  by means of an electric field between the photoconductive drum  31  and the primary transfer roller  44 . Note that the toner  37  remained on the surface of the photoconductive drum  31  may be removed by being scraped by the cleaning blade  39 . Subsequently, the toner image Ib on the transfer belt  41  may be transferred on the medium P by an electric field between the counter roller  45  and the primary transfer roller  46 . The toner  37  remained on the surface of the transfer belt  41  may be removed by being scraped by the cleaning blade  39 . The toner image may be then fixed on the medium P by being applied with heat and pressure by the fixing section  50 . 
     An operation of the image forming apparatus  1  is described in detail. The operation of the image forming apparatus  1  at the time of setting or correcting the development voltage V 34  and the supply voltage V 35  is specifically described in detail below. 
       FIG. 12  illustrates an example of operation procedure of the image forming apparatus  1 . The printing job may be supplied, to the CPU  201  through communication network, from an image transfer apparatus coupled to the image forming apparatus  1 . The CPU  201  may then perform the printing processing to allow each component in the image forming apparatus  1  to perform the following operation, based on the printing job. 
     The CPU  201  may determine whether the result counted by the drum counter  205  (the continuous printing count Nc) exceeds the threshold Nt_th (step S 101 ). When the continuous printing count Nc exceeds the threshold Nt_th, the CPU  201  may perform density correction. 
     Specifically, the CPU  201  may first instruct each of the image forming units  30 Y,  30 M,  30 C, and  30 K of the image forming section  30  to form three non-printing-use toner images Ib with development voltages V 34  different from one another. Then, the three non-printing-use toner images with the development voltages V 34  different from one another may be formed on the circumferential surface  31 A of the photoconductive drum  31  of each of the image forming units  30 Y,  30 M,  30 C, and  30 K. The CPU  201  may also instruct the image forming section  30  and the transfer section  40  to transfer the non-printing-use toner images Ib formed by the image forming section, on the transfer belt  41 . This causes the non-printing use toner images Ib formed on the circumferential surface  31 A to be transferred on the transfer belt  41 . The non-printing-use toner images Ib may be formed on the transfer belt  41  in this way (step S 102 ). 
     The CPU  201  may then instruct the density sensor  70  to perform density measurement. Thus, light may be applied from the density sensor  70  to each of the non-printing-use toner images Ib on the transfer belt  41 , and light reflected by each of the non-printing-use toner images Ib may be detected by the density sensor  70 . As a result, a detection signal relating to the intensity I R  of light reflected by each of the non-printing-use toner images Ib may be outputted from the density sensor  70 . The density of each of the non-printing-use toner images Ib may be detected in this way (step S 103 ). 
     The CPU  201  may then instruct the voltage setting section  209  to derive the voltage setting expression  220 . The voltage setting section  209  may then derive the voltage setting expression  220  for each of the image forming units  30 Y,  30 M,  30 C, and  30 K, based on the detection signals outputted from the density sensor  70  and the development voltage V 34  applied to each of the image forming units  30 Y,  30 M,  30 C, and  30 K. The voltage setting section  209  may further derive the setting value V 34S  of the development voltage V 34  corresponding to the target value Dg and the setting value V 35S  of the supply voltage V 35  corresponding to the setting value V 34S  for each of the image forming units  30 Y,  30 M,  30 C, and  30 K, with use of the derived voltage setting expression  220 . The voltage setting section  209  may store the derived setting value V 34S  and the derived setting value V 35S  in the non-volatile memory  106 . The density correction value may be set in this way (step S 104 ). 
     The CPU  201  may then initialize the continuous printing count Nc, and may then start printing with use of the derived setting value V 34S  and the derived setting value V 35S  (steps S 106  and S 107 ). Also in the case where the accumulated count Nt is smaller than the threshold Nt_th, the CPU  201  may initialize the continuous printing count Nc, and may then start printing with use of the derived setting value V 34S  (steps S 106  and S 107 ). In the case where the continuous printing count Nt is smaller than the threshold Nt_th, however, the last density correction value may be set (step S 108 ). In printing, the CPU  201  may instruct the high-voltage control section  303  to output the development voltage V 34  of the derived setting value V 34S  and the supply voltage V 35  of the derived setting value V 35S . The development voltage V 34  of the derived setting value V 34S  may be thus applied to the development roller  34 , and the supply voltage V 35  of the derived setting value V 35  may be thus provided to the feeding roller  35 . 
     The CPU  201  may then instruct the voltage correction section  210  to correct the development voltage V 34 . The voltage correction section  210  may then correct the development voltage V 34 , based on the continuous printing count Nc counted by the drum counter  205 . Specifically, the voltage correction section  210  may determine whether the continuous printing count Nc counted by the drum counter  205  exceeds the threshold Nc_th (step S 109 ). When the continuous printing count Nc exceeds the threshold Nc_th, the voltage correction section  210  may correct the development voltage V 34 . Specifically, the voltage correction section  210  may read out, from the correction table  230 , a correction value that is assigned to a range Ac 1  including the continuous printing count Nc and is assigned to a range Ac 2  including the setting value V 34S  of the development voltage V 34 . The voltage correction section  210  may then correct the development voltage V 34  with use of the read correction value (step S 110 ). For example, the voltage correction section  210  may add the correction value read out from the correction table  230  to the development voltage V 34 . The voltage correction section  210  may further correct the supply voltage V 35 . Specifically, the voltage correction section  210  may correct the supply voltage V 35  to allow the potential difference between the development voltage V 34  and the supply voltage V 35  to be fixed. When the continuous printing count Nc is smaller than the threshold value Nc_th, the voltage correction section  210  may not correct the development voltage V 34  and the supply voltage V 35 . 
     The CPU  201  may then instruct the high-voltage control section  203  to output the corrected development voltage V 34  and the corrected supply voltage V 35 . Specifically, the CPU  201  may instruct the high-voltage control section  203  to change the voltage to be outputted to the developing roller  34  from the development voltage V 34  before the correction to the corrected development voltage V 34 . Thus, the voltage to be outputted to the developing roller  34  may be changed from the development voltage V 34  before the correction to the corrected development voltage V 34 . The CPU  201  may further instruct the high-voltage control section  203  to change the voltage to be outputted to the feeding roller  35  from the supply voltage V 35  before the correction to the corrected supply voltage V 35 . Thus, the voltage to be outputted to the feeding roller  35  may be changed from the supply voltage V 35  before the correction to the corrected supply voltage V. In each of the image forming units  30 Y,  30 M,  30 C, and  30 K, the corrected development voltage V 34  and the corrected supply voltage V 35  may be applied, for example, at respective timings illustrated in  FIG. 8A  to  FIG. 11 . 
     The CPU  201  may then determine whether the print data Dp remains (step S 111 ). When no print data Dp remains, the CPU  201  may complete the printing. When the print data Dp remains, the CPU  201  may continue printing to execute the step S 107 . 
     [Effects] 
     Some effects of the image forming apparatus  1  of the present embodiment are described. In general, in an electrophotographic image forming apparatus, toner amount to be transferred on paper is strictly controlled in order to faithfully reproduce a color image. For example, toner density of a patch pattern printed on the transfer belt may be measured, and a process condition may be controlled based on the density data obtained through the measurement. To measure the toner density of the patch pattern, it is necessary to interrupt normal printing. This makes it difficult to measure the toner density of the patch pattern in printing. Accordingly, in the case where the continuous printing time is long under the process condition set once, printed image density may be varied between at the beginning of the printing and after the longtime printing. 
     In contrast, in the image forming apparatus  1  according to the present embodiment, the development voltage V 34  set before the printing start may be corrected based on the result counted by the drum counter  205 . Further, the supply voltage V 35  set before the printing start may be corrected based on the corrected development voltage V 34 . This makes it possible to perform correction based on the number of rotations of the photoconductive drum  31  on the development voltage V 34  and the supply voltage V 35  that are set before the printing start, without stopping the printing during the continuous printing. As a result, it is possible to stabilize the printed image density in longtime printing. 
     Incidentally, in general, in the case where printing in which printing operation is difficult to be interrupted is performed over long time, density correction of the developer may have to be performed during the printing operation. In such a case, the density or color tone may be largely varied in a printed image. For example, in the case where the label printing is performed on the rolled paper, the gap LG between labels may be extremely small, for example, about 3 mm and fixed. Therefore, the density correction of the toner  37  may have to be performed during development of the image (the toner image Ib) to be printed on the label LB. In such a case, however, the density or the color tone may be largely varied in the developed toner image Ib. 
     In contrast, in the image forming apparatus  1  according to the present embodiment, in the label printing mode, the varying timing of the development voltage V 34  or the varying timing of both of the development voltage V 34  and the supply voltage V 35  may be controlled to allow the portion P 1  to be located within the gap G 1  between the electrostatic latent images on the circumferential surface  31 A as illustrated in  FIG. 8B  and  FIG. 9  while exposure operation of the LED head  33  is controlled to allow the plurality of electrostatic latent images Ia to be formed side by side at a predetermined interval on the circumferential surface  31 A. In the present embodiment, the varying timing of the development voltage V 34  that may cause large variation in the density of the toner image Ib is not included in the period in which the electrostatic latent image Ia is developed. This makes it possible to suppress large variation in density or color tone within the developed toner image Ib. 
     2. Modifications 
     Some modifications of the image forming apparatus  1  according to the above-described embodiment are described below. Note that, in the following description, like numerals are used to designate components common to those in the above-described embodiment. Description is mainly given of components different from those in the above-described embodiment, and the description of the components common to those in the above-described embodiment will not be described in detail. 
     [Modification 1] 
     In the above-described embodiment, the image forming apparatus  1  may further include a detection section  80 , for example, as illustrated in  FIG. 13 .  FIG. 13  schematically illustrates an example of an outline configuration of the image forming apparatus  1  according to the modification 1. The detection section  80  corresponds to a specific but non-limiting example of “detection section” of the disclosure. 
     In the label printing mode, the detection section  80  may detect the labels LB on the medium P (rolled paper) and may derive the label size LS and the label gap size GS. The detection section  80  may include a light emitting diode (LED) and a photoreceptor diode. The light emitting diode may apply light to the medium P to be conveyed in a segment between the resist roller pair  23  and the transfer section  40  of the conveying path PW, for example. The photoreceptor diode may detect light (reflected light) reflected by the surface of the medium P to be conveyed, out of the light emitted from the light emitting diode. A detection signal outputted from the photoreceptor diode may relate to intensity of the reflected light correlated with irregularity of the surface of the medium P. The detection section  80  may drive the light emitting diode and the photoreceptor diode, for example, based on the control signal provided from the controller  200 . The detection section  80  may include a drive circuit that derives the label size LS and the label gap size GS, based on the detection signal provided from the photoreceptor diode and provides the derived label size LS and the derived label gap size GS to the controller  200 . 
     In the modification 1, in the label printing mode, the exposure control section  304  may control the exposure operation, based on the image data Di provided from the outside and the label size LS and the label gap size GS both obtained by the detection section  80 . Specifically, in the label printing mode, the exposure control section  304  may derive the exposure start timing and the exposure end timing of each electrostatic latent image Ia to be formed, to allow each toner image Ib to be transferred on the surface of the label LB, based on the image data Di provided from the outside and the label size LS and the label gap size GS both obtained by the detection section  80 . The exposure control section  304  may generate the exposure control signal  304 A, based on the image data Di provided from the outside and the label size LS and the label gap size GS both obtained by the detection section  80 . 
     The high-voltage control section  303  may control the varying timing of the development voltage V 34  resulting from the correction of the development voltage V 34 . In the label printing mode, the high-voltage control section  303  may control the varying timing of the development voltage V 34 , based on the label size LS and the label gap size GS obtained by the detection section  80 . The high-voltage control section  303  may perform the control while the exposure control section  304  controls the exposure operation, based on the image data Di provided from the outside and the label size LS and the label gap size GS both obtained by the detection section  80 . 
     In the label printing mode, the high-voltage control section  303  may control the varying timing of the supply voltage V 35 , based on the label size LS and the label gap size GS both obtained by the detection section  80 . The high-voltage control section  303  may perform the control while the exposure control section  304  controls the exposure operation, based on the image data Di provided from the outside and the label size LS and the label gap size GS both obtained by the detection section  80 . 
     The image forming apparatus  1  according to the modification 1 is different from the image forming apparatus  1  according to the above-described embodiment in that the control based on the label size LS and the label gap size GS obtained by the detection section  80  may be performed. Otherwise, the image forming apparatus  1  according to the modification 1 may include the configuration similar to that of the image forming apparatus  1  according to the above-described embodiment. Therefore, also in the modification 1, effects similar to those in the above-described embodiment are obtainable. 
     [Modification 2] 
     In the above-described embodiment and the above-described modification 1, in the label printing mode, the high-voltage control section  303  may control the varying timing of the development voltage V 34  or both of the development voltage V 34  and the supply voltage V 35 , based on the exposure control signal  304 A generated by the exposure control section  304 . 
     In the modification 2, specifically, the high-voltage control section  303  may extract the exposure start timing and the exposure end timing from the exposure control signal  304 A generated by the exposure control section  304 . The high-voltage control section  303  may predict the timing at which the gap G 1  between the electrostatic latent images is formed, based on the extracted exposure start timing and the extracted exposure end timing, and may control the varying timing of the development voltage V 34  or both of the development voltage V 34  and the supply voltage V 35 , based on the predicted timing. 
     The image forming apparatus  1  according to the modification 2 is different from the image forming apparatus  1  according to the above-described embodiment and the image forming apparatus  1  according to the modification 1 in that the varying timing of the development voltage V 34  or both of the development voltage V 34  and the supply voltage V 35  may be controlled based on the exposure control signal  304 A. Otherwise, the image forming apparatus  1  according to the modification 2 may include a configuration similar to that of the above-described embodiment and that of the modification 1. Accordingly, also in the modification 2, effects similar to those in the above-described embodiment and the modification 1 are obtainable. 
     [Modification 3] 
     The indirect image transfer is employed in the above-described embodiment and the above-described modifications 1 and 2; however, direct image transfer may be employed.  FIG. 14  illustrates a modification of an outline configuration of the image forming section  30  and the transfer section  40  in the image forming apparatus  1  according to any of the above-described embodiment and modifications 1 and 2. The image forming apparatus  1  according to the modification 3 is configured by omitting the transfer belt  41 , the driving roller  42 , the tension roller  43 , the counter roller  45 , the secondary transfer roller  46 , and the cleaning member from the image forming apparatus  1  according any of the above-described embodiment and modifications 1 and 2, and providing a plurality of primary transfer rollers  44 , for example, in a segment between the resist roller pair  23  and the fixing section  50  of the conveying path PW. In the modification 3, image forming units  30 CL,  30 Y,  30 M,  30 C, and  30 K may be disposed in this order along the conveying direction F 1 . 
     In the modification 3, in the label printing mode, the high-voltage control section  303  may control the varying timing of the development voltage V 34  or both of the development voltage V 34  and the supply voltage V 35  to allow the portion P 1  or both of the portions P 1  and P 2  to be opposed to the gap LG between labels of the medium P (rolled paper). 
       FIG. 15A  illustrates an example of the operation of the image forming unit  30 Y at the time when the supply voltage V 35  is varied (time T=T 1 ) immediately before the development voltage V 34  is varied.  FIG. 15B  illustrates an example of the operation of the image forming unit  30 Y at the time when the development voltage V 34  is varied (time T=T 2 ) immediately after the supply voltage V 35  is varied.  FIG. 15C  illustrates an example of the operation of the image forming unit  30 Y immediately after the development voltage V 34  is varied (time T=T 3 ). Note that, in the modification 3, time difference T 3 −T 2  may correspond to a time necessary for the portion P 1  to travel from a position at which the portion P 1  is opposed to the circumferential surface  34 A of the developing roller  34  to a position at which the portion P 1  is opposed to the surface of the medium P, in the circumferential surface  31 A of the photoconductive drum  31  while the photoconductive drum  31 , the developing roller  34 , and the feeding roller  35  rotate. 
     The high-voltage control section  303  may control the varying timing of the development voltage V 34  resulting from the correction of the development voltage V 34 . In the label printing mode, the high-voltage control section  303  may control the varying timing of the development voltage V 34 , based on the label size LS and the label gap size GS that are provided from the outside (or obtained by the detection section  80 ). Specifically, in the label printing mode, the high-voltage control section  303  may control the varying timing of the development voltage V 34  to allow the portion P 1  to be opposed to the gap LG between labels of the medium P (rolled paper) as illustrated in  FIG. 15B ,  FIG. 15C , and  FIG. 9 . The portion P 1  may be a portion, in the circumferential surface  31 A, opposed to the developing roller  34  upon varying of the development voltage V 34 . 
     The high-voltage control section  303  may control the varying timing of the supply voltage V 35  resulting from the correction of the supply voltage V 35  that is performed in association with the correction of the development voltage V 34 . In the label printing mode, the high-voltage control section  303  may correct the development voltage V 34  after correcting the supply voltage V 35 . In the label printing mode, the high-voltage control section  303  may control the varying timing of the supply voltage V 35 , based on the label size LS and the label gap size GS that are provided from the outside (or obtained by the detection section  80 ). Specifically, in the label printing mode, the high-voltage control section  303  may preferably control the varying timing of the supply voltage V 35  to allow the portion P 2  to be opposed to the gap LG between labels of the medium P (rolled paper) as illustrated in  FIG. 15A ,  FIG. 15B ,  FIG. 15C , and  FIG. 9 . The portion P 2  may be a portion, in the circumferential surface  31 A, opposed to the portion P 3  of the developing roller  34 . The portion P 3  may be a portion, in the circumferential surface  34 A of the developing roller  34 , opposed to the feeding roller  35  upon varying of the supply voltage V 35 . 
     In  FIG. 15B  and  FIG. 15C , the high-voltage control section  303  may control the varying timing of both of the development voltage V 34  and the supply voltage V 35  to allow the portions P 1  and P 2  to be coincident with each other. Note that the high-voltage control section  303  may control the varying timing of both of the development voltage V 34  and the supply voltage V 35  to allow the portion P 2  to be located upstream of the portion P 1  in the rotation direction of the circumferential surface  31 A of the photoconductive drum  31  as illustrated in  FIG. 10  and  FIG. 11 . 
     In  FIGS. 15A to 15C  and  FIGS. 9 to 11 , variation in the thickness and the charged amount of the toner  37  attached on the circumferential surface  34 A of the developing roller  34  resulting from varying of the supply voltage V 35  may reach the portion, in the circumferential surface  34 A of the developing roller  34 , opposed to the circumferential surface  31 A of the photoconductive drum  31  upon or before varying of the development voltage V 34 . As a result, it is possible to suppress large variation in the image density D I  before and after varying of the development voltage V 34 , as compared with the case where the variation in the thickness and the charged amount of the toner  37  attached on the circumferential surface  34 A of the developing roller  34  resulting from varying of the supply voltage V 35  reaches the portion, in the circumferential surface  34 A of the developing roller  34 , opposed to the circumferential surface  31 A of the photoconductive drum  31  after varying of the development voltage V 34 . Note that the variation in the image density D I  resulting from the varying of the supply voltage V 35  may be smaller than variation in the image density D I  resulting from the varying of the development voltage V 34 . Therefore, in the label printing mode, there may be a case where the portion P 2  is accepted to be located at a position outside the gap G 1  between the electrostatic latent images of the circumferential surface  31 A. 
     In the modification 3, in the label printing mode, the high-voltage control section  303  may control the varying timing of the development voltage V 34  or both of the development voltage V 34  and the supply voltage V 35 , based on the exposure control signal  304 A generated by the exposure control section  304 . 
     [Modification 4] 
     In the above-described embodiment and modifications 1 to 3, the medium P is rolled paper to which the plurality of labels LB are attached at a predetermined interval. The medium P, however, may be rolled paper in which a long seal having a size same as that of a long rolled mount is attached on a surface of the long rolled mount. In this case, there may be no cut line CL described above in the long seal. 
     In the modification 4, the controller  200  may transfer, as the image data Di to the exposure control section  304 , the image data Di included in the print data Dp with image data of a cut line corresponding to the above-described cut line CL. The exposure control section  304  may perform data conversion of the print data including the image data Di with the image data of the cut line corresponding to the above-described cut line CL. Accordingly, in the modification 4, a cut line may be formed on outer periphery of each toner image Ib printed on the medium P. 
     The image forming apparatus  1  according to the modification 4 is different from the image forming apparatus  1  according to any of the above-described embodiment and modifications 1 to 3 in that the data of the cut line may be added to the image data Di. Otherwise, the image forming apparatus  1  according to the modification 4 may have a configuration similar to that of the image forming apparatus  1  according to any of the above-described embodiment and modifications 1 to 3. Therefore, also in the modification 4, effects similar to those in the above-described embodiment and modifications 1 to 3 are obtainable. 
     [Modification 5] 
     Some modifications are described below. 
     In the above-described embodiments, the development system using non-magnetic one-component developer is employed. In the above-described embodiment and the modifications thereof, two-component magnetic brush development system using two-component developer that includes magnetic carrier and non-magnetic toner, or one-component magnetic development system using magnetic toner may be employed. In the above-described embodiments, the image forming units  30 Y,  30 M,  30 C, and  30 K of four colors are used. Alternatively, in the above-described embodiment and the modifications thereof, for example, image forming units of three or less or five or more colors may be used. In the above-described embodiments, the LED head  33  is used. Alternatively, in the above-described embodiment and the modifications thereof, for example, a laser device may be used in place of or together with the LED head  33 . 
     The series of processes described in the above-described embodiment and the modifications thereof may be executed by hardware (circuits) or by software (programs). In the case where the series of processes is executed by software, the software is configured of a program group causing a computer to execute each function. For example, each program may be incorporated in the above-described computer in advance or may be installed from any network or a recording medium to the above-described computer and used. 
     In the above-described embodiment and the modifications thereof, some embodiments of the disclosure have been described by taking a color electrophotographic printer as an example. The embodiments of disclosure are not limited to application of a color machine and a printer, and are applicable to an image forming apparatus that forms an image on a medium to be conveyed. Embodiments of the disclosure may be applicable to, for example, a monochrome copy machine, a color copy machine, a monochrome MFP, and a color MFP. 
     In the above-described embodiment and the modifications thereof, the image forming apparatus having a printing function has been described as a specific but non-limiting example of “image forming apparatus” in the disclosure. However, embodiments of the disclosure are not limited to application of the image forming apparatus having the printing function, and are applicable to an image forming apparatus that functions as a complex machine having, for example, a scan function and a fax function. 
     It is possible to achieve at least the following configurations from the above-described example embodiments of the invention. 
     (1) An image forming apparatus, including: 
     an image supporting member having a first circumferential surface that includes a photoreceptive layer; 
     an exposure section configured to perform exposure of the first circumferential surface and thereby form latent images; 
     a developer supporting member having a second circumferential surface opposed to the first circumferential surface, and configured to develop the latent images with use of a developer; 
     a feeding member having a third circumferential surface opposed to the second circumferential surface, and configured to feed the developer to the developer supporting member; and 
     a control section configured to control, while controlling exposure operation of the exposure section to allow the latent images to be formed side by side at a predetermined interval on the first circumferential surface, varying timing of a development voltage or both of the development voltage and a supply voltage to allow a portion P 1  or both of the portion P 1  and a portion P 2  to be located within a gap between the latent images on the first circumferential surface, the portion P 1  being a portion, in the first circumferential surface, opposed to the developer supporting member upon varying of the development voltage, the portion P 2  being a portion, in the first circumferential surface, opposed to a portion P 3  of the developer supporting member, and the portion P 3  being a portion, in the second circumferential surface, opposed to the feeding member upon varying of the supply voltage. 
     (2) The image forming apparatus according to (1), wherein, while controlling the exposure operation, based on image data, a label size, and a label gap size that are provided from outside, the control section controls the varying timing, based on the label size and the label gap size that are provided from the outside. 
     (3) The image forming apparatus according to (1), further including a detection section configured to, in a label printing mode in which printing is performed on rolled paper to which a plurality of labels are attached at a predetermined interval, detect the labels on the rolled paper to derive a label size and a label gap size, wherein 
     the control section controls the exposure operation, based on image data provided from outside and the label size and the label gap size both obtained by the detection section, and 
     the control section controls the varying timing, based on the label size and the label gap size both obtained by the detection section. 
     (4) The image forming apparatus according to (1), wherein the control section controls the exposure operation and the varying timing, based on an exposure control signal adapted to control the exposure section. 
     (5) The image forming apparatus according to (4), wherein the control section generates the exposure control signal, based on image data, a label size, and a label gap size that are provided from outside. 
     (6) The image forming apparatus according to (4), further including a detection section configured to, in a label printing mode in which printing is performed on rolled paper to which a plurality of labels are attached at a predetermined interval, detect the labels on the rolled paper to derive a label size and a label gap size, wherein 
     the control section generates the exposure control signal, based on image data provided from outside and the label size and the label gap size both obtained by the detection section. 
     (7) An image forming apparatus, including: 
     an image supporting member having a first circumferential surface that includes a photoreceptive layer; 
     an exposure section configured to perform exposure of the first circumferential surface and thereby form latent images; 
     a developer supporting member having a second circumferential surface opposed to the first circumferential surface, and configured to develop the latent images with use of a developer; 
     a feeding member having a third circumferential surface opposed to the second circumferential surface, and configured to feed the developer to the developer supporting member; and 
     a control section configured to, in a label printing mode in which printing is performed on rolled paper to which a plurality of labels are attached at a predetermined interval, control varying timing of a development voltage or both of the development voltage and a supply voltage to allow a portion P 1  or both of the portion P 1  and a portion P 2  to be opposed to a gap between the labels on the rolled paper or a portion to be opposed to the gap between the labels, the portion P 1  being a portion, in the first circumferential surface, opposed to the developer supporting member upon varying of the development voltage, the portion P 2  being a portion, in the first circumferential surface, opposed to a portion P 3  of the developer supporting member, and the portion P 3  being a portion, in the second circumferential surface, opposed to the feeding member upon varying of the supply voltage. 
     (8) The image forming apparatus according to (7), wherein the control section controls the varying timing, based on a label size and a label gap size both provided from outside. 
     (9) The image forming apparatus according to (7), further including a detection section configured to, in the label printing mode, detect the labels on the rolled paper to derive a label size and a label gap size, wherein 
     the control section controls the varying timing, based on the label size and the label gap size both obtained by the detection section. 
     (10) The image forming apparatus according to (7), wherein the control section controls the varying timing, based on an exposure control signal adapted to control the exposure section. 
     (11) The image forming apparatus according to (10), wherein the control section generates the exposure control signal, based on image data, a label size, and a label gap size that are provided from outside. 
     (12) The image forming apparatus according to (10), further including a detection section configured to, in the label printing mode, detect the labels on the rolled paper to derive a label size and a label gap size, wherein 
     the control section generates the exposure control signal, based on image data provided from outside and the label size and the label gap size both obtained by the detection section. 
     As used herein, the term “oppose” and its grammatical variants are intended to encompass not only a separated state but also a contact state between one member and the other member. 
     It should be understood by those skilled in the art that various modifications, combinations, sub-combinations, and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.