Patent Publication Number: US-8971741-B2

Title: Image forming device

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is related to, claims priority from and incorporates by reference Japanese Patent Application No. 2012-1702049, filed on Aug. 2, 2012. 
     TECHNICAL FIELD 
     The present invention relates to an image forming device and can be applied to an electrophotographic printer, copier, and the like. 
     BACKGROUND 
     Conventionally, in an electrophotographic printer, when performing image formation on a translucent print medium, there is a case where a white toner is used. The translucent print medium corresponds to, for example, a film used with an OHP (overhead projector) (referred to as an “OHP film” in the following). Conventionally, as an image forming device that performs image formation using a white toner on an OHP film, there is a technology described in JP2007-083634A. 
     In the image forming device described in JP2007-083634A, when printing a color image on an OHP film and the like, in order to improve printing quality (image printing quality), a corresponding image is printed using a white toner on a surface opposite to a surface on which a color image is printed. 
     However, in a conventional image forming device, when performing image formation using a white toner, printing quality may deteriorate. 
     Therefore, an image forming device that can suppress deterioration of image formation quality due to characteristics of a developer is desired. 
     SUMMARY 
     An image forming device of the present invention includes (1) one or a plurality of developing devices, and (2) a control part. Each of the developing devices includes an image carrying part carrying an electrostatic latent image; a developing part on which a developing voltage is applied and which attaches developer to the image carrying part to develop the electrostatic latent image; and a charging part charging the image carrying part. The control part controls for each of the developing devices a developing potential difference between the developing voltage and potential of the image carrying part in such a manner that the developing potential difference of a developing device using a white developer is smaller than the developing potential difference of a developing device using a developer of other colors other than the white developer. 
     According to the embodiments of the present invention, deterioration of image quality can be reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating a configuration (functional configuration) of a control system of an image forming device according to a first embodiment. 
         FIG. 2  is a schematic cross-sectional view of the image forming device according to the first embodiment. 
         FIG. 3  is a cross-sectional view of a developing roller used in a developing device according to the first embodiment. 
         FIG. 4  is a cross-sectional view of a supply roller used in the developing device according to the first embodiment. 
         FIG. 5  is an explanatory diagram illustrating an example of content of a first setting table used in the image forming device according to the first embodiment. 
         FIG. 6  is a flowchart illustrating an operation of the image forming device according to the first embodiment. 
         FIG. 7  is a graph illustrating a relation between a potential difference PD and a color difference ΔE in the image forming device according to the first embodiment. 
         FIG. 8  is an explanatory diagram illustrating a method for measuring a parameter used in calculating the color difference ΔE in the image forming device according to the first embodiment. 
         FIG. 9  is a block diagram illustrating a configuration (functional configuration) of a control system of an image forming device according to a second embodiment. 
         FIG. 10  is an explanatory diagram illustrating an example of content of a second setting table used in the image forming device according to the second embodiment. 
         FIG. 11  is a flowchart illustrating an operation of the image forming device according to the second embodiment. 
         FIG. 12  is a block diagram illustrating a configuration (functional configuration) of a control system of an image forming device according to a third embodiment. 
         FIG. 13  is a schematic cross-sectional view of the image forming device according to the third embodiment. 
         FIG. 14  is a flowchart illustrating an operation of the image forming device according to the third embodiment. 
         FIG. 15  is a schematic cross-sectional view of an image forming device according to modified embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     (A) First Embodiment 
     In the following, a first embodiment of an image forming device according to the present invention is explained with reference to the drawings. 
     (A-1) Configuration of First Embodiment 
     First, an overall configuration of an image forming device  1  of the first embodiment is explained. 
       FIG. 2  is a schematic cross-sectional view of the image forming device  1  of this embodiment. 
     The image forming device  1 , for example, is an electrophotographic color image forming device and is capable of image formation using four toner colors of cyan (hereafter, may be referred to as “c”), yellow (hereafter, may be referred to as “y”), magenta (hereafter, may be referred as “m”) and white (hereafter, may be referred to as “w”) on a print medium  23  as a medium. 
     The image forming device  1  performs image formation on the print medium  23  using toner T (Tc, Ty, Tm, Tw) as a developer of each toner color. In the first embodiment, as the print medium  23 , for example, an OHP film, a transfer paper, a colored plain paper, and the like, can be used. The transfer paper, for example, is what is used as a medium for transferring an image to a cloth such as a T-shirt. By applying a print side of the transfer paper, on which toner is transferred, to the cloth and by heating and pressurizing with an iron and the like from a back side (opposite to the print side) of the transfer paper, the image of the transfer paper can be transferred to the cloth. The colored plain paper is a plain paper colored with a color other than white (for example, a paper of a color of black, blue, red, and the like). 
     The image forming device  1  has a developing device  11  ( 11   c ,  11   y ,  11   m ,  11   w ) for each toner color, an LED head  14  ( 14   c ,  14   y ,  14   m ,  14   w ) as an exposure part for forming an electrostatic latent image of each toner color, and a toner tank  18  ( 18   c ,  18   y ,  18   m ,  18   w ) as a developer storage part of each toner color. 
     The developing device  11  ( 11   c ,  11   y ,  11   m ,  11   w ) of each toner color has a charging roller  13  ( 13   c ,  13   y ,  13   m ,  13   w ) as a charging part, a supply roller  17  ( 17   c ,  17   y ,  17   m ,  17   w ) as a supply part, a developing roller  15  ( 15   c ,  15   y ,  15   m ,  15   w ) as a developing part, a layer forming blade  16  ( 16   c ,  16   y ,  16   m ,  16   w ), and a photosensitive drum  12  ( 12   c ,  12   y ,  12   m ,  12   w ) as an image carrying part. 
     The developing device  11  of each toner color is uniformly charged by the charging roller  13  that is in contact with the photosensitive drum  12 . An electrostatic latent image is formed by exposure by each LED head  14  on each photosensitive drum  12  that is charged. Each supply roller  17  supplies toner as a developer to each developing roller  15 . It is configured that when each layer forming blade  16  uniformly forms a toner layer on a surface of each developing roller  15 , a toner image is developed on each photosensitive drum  12 . The toner tank  18  ( 18   c ,  18   y ,  18   m ,  18   w ) of each toner color stores each toner T (Tc, Ty, Tm, Tw) as a developer, is detachably attached in each developing device  11 , and is configured to supply the stored toner to each developing device  11 . 
     Below each developing device  11 , a transfer roller  19  ( 19   c ,  19   y ,  19   m ,  19   w ) as a transfer part of each toner color, a transfer belt driving roller  25   b , and a transfer belt driven roller  25   a  are provided. Each transfer roller  19  is arranged capable of applying a bias voltage from a back surface of a transfer belt  9  to a transfer position. The transfer belt driving roller  25   b  and the transfer belt driven roller  25   a  extend and support the transfer belt  9  in a tensioned state and are configured capable of conveying the print medium  23  by the driving of the transfer belt driving roller  25   b.    
     Below the transfer belt  9 , a paper cassette  20  is detachably attached. In the paper cassette  20 , the print media  23  are loaded. 
     Between a front end of the paper cassette  20  (an end on a downstream side of medium conveyance) and the transfer belt driving roller  25   b , a paper feed roller  21 , a paper guide  21   a , and a conveying roller unit  22  (having a pair of conveying rollers  22   a ,  22   b ) that pulls out the print medium  23  from the paper guide  21   a  are arranged. Above the transfer belt  9 , an adsorption roller  27  for adsorbing the print medium  23  on the transfer belt  9  (for bringing the print medium  23  into contact with the transfer belt  9 ) is arranged at a position opposing the transfer belt driven roller  25   a.    
     The paper feed roller  21  separates and takes out the print medium  23  one by one from the paper cassette  20  and feeds the print medium  23  to the paper guide  21   a . The fed print medium  23  is conveyed along the paper guide  21   a  and is pulled out by the conveying roller unit  22  (the pair of the conveying rollers  22   a ,  22   b ). The print medium  23  that is pulled out by the conveying roller unit  22  is supplied to a transfer belt  9  side. The print medium  23  that is supplied from the conveying roller unit  22  to the transfer belt  9  is sandwiched on the transfer belt  9  between the adsorption roller  27  and the transfer belt driven roller  25   a . Then, the print medium  23  is adsorbed on the transfer belt  9 . 
     On a downstream side of medium conveyance of the transfer belt  9 , a fixing unit  24  is provided. The fixing unit  24  has a heating roller  28  inside which a heating body  28   a  (such as a halogen lamp) is arranged, and a pressing roller  29 . In the fixing unit  24 , the print medium  23  is sandwiched between the heating roller  28 , which is heated by the heating body  28   a , and the pressing roller  29 , and is conveyed to a downstream side of medium conveyance while being heated and pressed. 
     On a further downstream side of the fixing unit  24 , a paper guide  26   c , a eject roller unit  26  (a pair of eject rollers  26   a ,  26   b ), and an output tray  2  are arranged. The print medium  23  that is supplied from the fixing unit  24  to the paper guide  26   c  is ejected by the eject roller unit  26  (the pair of eject rollers  26   a ,  26   b ) and is placed on the output tray  2 . 
     Next, a material of the toner T (Tc, Ty, Tm, Tw) stored in each toner tank  18  ( 18   c ,  18   y ,  18   m ,  18   w ) is explained. 
     In the first embodiment, the toner T (Tc, Ty, Tm, Tw) of each toner color is composed of a polyester resin, a colorant, a charge control agent and a release agent, has an external additive (hydrophobic silica) added thereto, and uses a developer of a ground shape having an average particle size of 8 μm that is obtained by using a grinding method. As the toner T (Tc, Ty, Tm, Tw), a toner obtained by other commonly known method such as a polymerization method may also be used. 
     Further, in the first embodiment, as the colorant of the white toner Tw (white developer), titanium dioxide is used. As the colorant used for the toner Tw, a material such that an opaque toner image is obtained is desirable. In this respect, titanium dioxide is a preferred material. As the colorant of the white toner Tw, metal oxide other than titanium dioxide (such as aluminum oxide, barium sulfate, and zinc oxide) may also be used. 
     Further, as the colorants for the toners Tc, Ty, Tm of colors other than white (developers of other colors, or non-white developers), commonly known colorants (pigments) such as pigment cyan, pigment magenta and pigment yellow are respectively used. Further, for the toners Tc, Ty, Tm of colors other than white, pigments of colors that are transparent to some extent may also be used as colorants. 
     As described above, in the first embodiment, only the white toner Tw has different electrical characteristics such as charging characteristics. For example, an experiment is performed in which the white toner Tw used in the first embodiment and the toners Tc, Ty, Tm of colors other than white are charged under the same condition and the white toner Tw has a less charge amount. Specifically, the white toner Tw and the toners Tc, Ty, Tm of colors other than white are attached to a roller to which a same bias voltage is applied, and charge amounts of the toners are measured using a charge amount measuring device (Suction Type Small Charge Amount Measuring Device Model 212HS, by TREK JAPAN KK). As a result, the charge amount of the white toner Tw is −6 μC/g and the charge amounts of the toners Tc, Ty, Tm of colors other than white are −30 μC/g. Therefore, the charge amount of the white toner Tw used in this embodiment is found to have a higher conductivity and a property of being difficult to be charged than the toners Tc, Ty, Tm of colors other than white. 
     Next, a configuration of each developing roller  15  ( 15   c ,  15   y ,  15   m ,  15   w ) is explained.  FIG. 3  is cross-sectional view of each developing roller  15  ( 15   c ,  15   y ,  15   m ,  15   w ). 
     As illustrated in  FIG. 3 , each developing roller  15  ( 15   c ,  15   y ,  15   m ,  15   w ) of the first embodiment has an elastic layer  15   b  formed around a metallic shaft  15   a , the elastic layer  15   b  being made of an elastic body. As the elastic layer  15   b , for example, a semiconductive urethane rubber of a rubber hardness of 700 (Asker C hardness) can be used. 
     Next, a configuration of each supply roller  17  ( 17   c ,  17   y ,  17   m ,  17   w ) is explained.  FIG. 4  is a cross-sectional view of each supply roller  17  ( 17   c ,  17   y ,  17   m ,  17   w ). 
     As illustrated in  FIG. 4 , each supply roller  17  ( 17   c ,  17   y ,  17   m ,  17   w ) of the first embodiment has a foam layer  17   b  formed around a metallic shaft  17   a , the foam layer  17   b  being made of foam. As the foam layer  17   b , for example, a silicon foam of a hardness of 500 (Asker F hardness) can be used. 
     Next, a configuration of a control system of the image forming device  1  is explained using  FIG. 1 . 
     As illustrated in  FIG. 1 , the image forming device  1  has, as a configuration for controlling and driving the configuration elements illustrated in  FIG. 2 , a print control part  30  (control means) that performs central control processing. An interface part  32 , an operation input part  33 , a memory  34 , a CPU  37 , a sensor  38 , a process control part  40 , a developing voltage control part  41 , two layer formation and supply voltage control parts  42 ,  43 , two charging voltage control parts  44 ,  45 , exposure control parts  46  ( 46   c ,  46   y ,  46   m ,  46   w ) for performing exposure control of the LED heads  14  ( 14   c ,  14   y ,  14   m ,  14   w ) of the toner colors, a transfer control part  47 , and a motor control part  48  are connected to the print control part  30 . 
     The interface part  32  functions as an interface with a higher-level device  31  (for example, a supplier of print data, such as a PC) as an information input means. For example, when print data described in PDL (Page Description Language) and the like is supplied from the higher-level device  31 , the interface part  32  transfers the print data to the print control part  30 . 
     The memory  34  includes a RAM  36  (volatile memory) that is used as a working memory and the like and a ROM  35  (non-volatile memory) that stores various setting data (such as parameters used for controlling the configuration elements) and programs (for example, programs that are executed by the CPU  37  and the like). The ROM  35  may be as a data-rewritable EEPROM. 
     A setting table  351  is stored in the ROM  35 . The setting table  351  is used to define parameters that are used when controlling the configuration elements of the print control part  30 . Details of the setting table  351  will be described later. 
     The sensor  38  is used to detect the print medium  23  at a predetermined position on the medium conveyance path illustrated in  FIG. 2 . When the print medium  23  is detected, the sensor  38  supplies a signal indicating the detection to the print control part  30 . 
     Based on the control of the print control part  30 , the process control part  40  performs processing such as management of voltages of rollers illustrated in  FIG. 1 . 
     The developing voltage control part  41  as a developing bias application means functions to apply a bias voltage (developing bias voltage) to the charging roller  13  ( 13   c ,  13   y ,  13   m ,  13   w ) of each toner color. In the following, the bias voltage applied to the charging roller  13  may also be referred to as the “developing voltage DB.” 
     The layer formation and supply voltage control part  42  as a supply bias application means functions to apply bias voltages (supply bias voltages) to the supply rollers  17   c ,  17   y ,  17   m , and the layer forming blades  16   c ,  16   y ,  16   m . Further, the layer formation and supply voltage control part  43  functions to apply bias voltages to the supply roller  17   w  and the layer forming blade  16   w . In the following, the bias voltages applied to the supply roller  17  and layer forming blade  16  may also be referred to as the “supply voltages SB.” 
     The charging voltage control part  44  as a charging bias application means functions to apply bias voltages (charging bias voltages) to the charging rollers  13   c ,  13   y ,  13   m . In the following, the bias voltage applied to the charging roller  13  may also be referred to as the “charging voltage CH.” 
     The exposure control part  46  ( 46   c ,  46   y ,  46   m ,  46   w ) of each toner color functions to perform exposure control (light emission control) of the LED head  14  ( 14   c ,  14   y ,  14   m ,  14   w ). 
     The transfer control part  47  as a transfer bias application means functions to apply a bias voltage (transfer bias voltage) to the transfer roller  19  ( 19   c ,  19   y ,  19   m ,  19   w ) of each toner color. In the following, the bias voltage applied to the charging roller  13  may also be referred to as the “transfer voltage TR.” 
     The motor control part  48  functions to control and rotationally drive each motor in the image forming device  1 . Specifically, the motor control part  48  controls and rotationally drives motors of the photosensitive drums  12  ( 12   c ,  12   y ,  12   m ,  12   w ), the paper feed roller  21 , the conveying roller unit  22 , the driving rollers  25   a ,  25   b  for the transfer belt  9 , the adsorption roller  24 , the heating roller  28 , the pressing roller  29 , the eject roller unit  26  and the like. 
     Each photosensitive drum  12  ( 12   c ,  12   y ,  12   m ,  12   w ) is rotationally driven in a direction of an arrow a 1  (see  FIG. 2 ) by a photosensitive drum motor (not illustrated in the drawing). Further, each photosensitive drum  12  ( 12   c ,  12   y ,  12   m ,  12   w ) has a gear (not illustrated in the drawing) arranged at one end portion of the corresponding developing roller  15  ( 15   c ,  15   y ,  15   m ,  15   w ) and supply roller  17  ( 17   c ,  17   y ,  17   m ,  17   w ). The developing rollers  15   c ,  15   y ,  15   m ,  15   w  and the supply rollers  17   c ,  17   y ,  17   m ,  17   w  are rotationally driven by respectively engaging the gears of the photosensitive drums  12   c ,  12   y ,  12   m ,  12   w . In the present invention, one of the photosensitive drum  12   w  for which the white toner is used is a white developing device. The other photosensitive drums ( 12   c ,  12   y ,  12   m ) for which the non-white toners are used are non-white developing devices. 
     Next, the setting table  351  is explained in detail. In the first embodiment, content of the setting table  351  is defined as illustrated in  FIG. 5 . 
     In the setting table  351 , corresponding parameters are defined for each toner color. In the setting table  351  illustrated in  FIG. 5 , the parameters related to the toner of one color are defined in one column. In the setting table  351  illustrated in  FIG. 5 , for each toner color, parameters including the developing voltage DB, the charging voltage CH, the supply voltage SB, the transfer voltage TR, a light emission time TL, and drum surface potentials DS 1 , DS 2  are set. 
     In the following, by that a voltage, potential or potential difference is reduced (decreased), it means that the absolute value (output amount) of the voltage, potential or potential difference is made small. Conversely, by that a voltage, potential or potential difference is made high (is increased), it means that the absolute value (output amount) of the voltage, potential or potential difference is made large. 
     Items of the “developing voltage DB” respectively illustrate bias voltages applied to the charging rollers  13  ( 13   c ,  13   y ,  13   m ,  13   w ). The items of the developing voltage DB in the setting table  351  illustrated in  FIG. 5  are set to be −200 V for all of the toner colors (c, m, y, w). 
     Items of the “charging voltage CH” respectively illustrate bias voltages applied to the charging rollers  13  ( 13   c ,  13   y ,  13   m ,  13   w ). The items of the charging voltage CH in the setting table  351  illustrated in  FIG. 5  are set to be −1200 V for the toner colors (c, m, y) other than white and −1000 V for the white (w) toner color. In the image forming device  1 , in order to apply the charging voltage CH that is different only for the charging roller  13   w  corresponding to the white (w) color, the charging voltage control part  45  is provided separately from the charging voltage control part  44 . 
     Items of the “supply voltage SB” respectively illustrate bias voltages applied to the supply rollers  17  ( 17   c ,  17   y ,  17   m ,  17   w ) and the layer forming blades  16  ( 16   c ,  16   y ,  16   m ,  16   w ). The items of the supply voltage SB in the setting table  351  illustrated in  FIG. 5  are set to be −300 V for the toner colors (c, m, y) other than white and −400 V for the white (w) toner color. In the image forming device  1 , in order to apply the supply voltage SB that is different only for the supply roller  17   w  and layer forming blade  16   w  corresponding to the white (w) color, the layer formation and supply voltage control part  43  is provided separately from the layer formation and supply voltage control part  42 . 
     Items of the “transfer voltage TR” respectively illustrate bias voltages applied to the transfer rollers  19  ( 19   c ,  19   y ,  19   m ,  19   w ). The items of the TR in the setting table  351  illustrated in  FIG. 5  are set to be +4000 V for all of the toner colors (c, m, y, w). 
     Items of the “light emission time TL” respectively illustrate light emission times of light emissions (exposures) from the LED heads  14  ( 14   c ,  14   y ,  14   m ,  14   w ) to the photosensitive drums  12  ( 12   c ,  12   y ,  12   m ,  12   w ). In  FIG. 5 , items of LED are illustrated as ratios (%) of increase of decrease with respect to a predetermined reference time. For example, when the above-described reference time is T 1  [μS], a parameter value to which the item of the LED is set is k [%], and the light emission time indicated by this parameter is T 2  [μs], T 2  can be expressed by the following formula (1).
 
 T 2 =T 1×( k/ 100)  (1)
 
In  FIG. 5 , the light emission times TL of toner colors (c, m, y) other than white are 0% and the light emission time of the white (w) color is −40%. Therefore, in  FIG. 5 , the light emission time of the white (w) color is set to be a light emission time that is 40% shorter as compared other toner colors (reference time T 1 ).
 
     Items of the “drum surface potential DS 1 ” respectively illustrate values of surface potentials [V] of portions of the photosensitive drums  12  ( 12   c ,  12   y ,  12   m ,  12   w ) that are not exposed (non-exposure parts OPC). The items of the drum surface potential DS 1  in the setting table  351  illustrated in  FIG. 5  are set to be −600 V for the toner colors (c, m, y) other than white and −400 V for the white (w) toner color. 
     Items of the “drum surface potential DS 2 ” respectively illustrate values of potentials [V] of portions of the photosensitive drums  12  ( 12   c ,  12   y ,  12   m ,  12   w ) that are exposed (exposure parts OPC). The items of the exposure part OPC and the drum surface potential DS 2  in the setting table  351  illustrated in  FIG. 5  are set to be −50 V for all of the toner colors (c, m, y, w). 
     In this embodiment, the drum surface potential DS 1  is a value based on the charging voltage CH. For example, when the charging voltage CH increases, the charge amount of the charging roller  13  increases and the drum surface potential DS 1  also rises. Further, in this embodiment, the drum surface potential DS 2  is a value based on the drum surface potential DS 1  and the light emission time TL. For example, when the light emission time TL is the same, the higher the drum surface potential DS 1  is, the higher the drum surface potential DS 2  will be. Further, when the drum surface potential DS 1  is the same, the longer the light emission time TL is, the lower the drum surface potential DS 2  will be. 
     Therefore, in the setting table  351  illustrated in  FIG. 5 , in order to simplify the explanation, the items of the drum surface potentials DS 1 , DS 2  are provided. However, in fact, when the developing voltage DB, the charging voltage CH and the light emission time TL are set, the items of the drum surface potentials DS 1 , DS 2  may be omitted. In other words, in the setting table  351  illustrated in  FIG. 5 , for each toner color, as parameters for realizing targeted drum surface potentials DS 1 , DS 2 , the developing voltage DB, the charging voltage CH and the light emission time TL are set. 
     In the first embodiment, as a parameter for adjusting an exposure energy amount with respect to the photosensitive drum  12  (exposure part OPC), the light emission time TL is used. However, it also possible to use an output amount of illumination intensity of light emitted from the LED head  14  to adjust the exposure energy amount with respect to the photosensitive drum  12  (exposure part OPC). The larger the exposure energy amount with respect to the photosensitive drum  12  (exposure part OPC) is, the larger the potential difference between the non-exposure part OPC and the exposure part OPC (reduction amount of the potential due to the exposure) will be. Therefore, by adjusting the light emission time and/or output amount of the LED head  14 , the potential difference between the non-exposure part OPC and the exposure part OPC can be adjusted. 
     As described above, in the setting table  351 , the parameters of the white (w) toner color and the parameters of the toner colors (c, M, y) other than white are set to different values. As described above, in the first embodiment, only the white (w) toner color has characteristics different from other toner colors, and a risk for “fogging” to occur for the white (w) toner color is high as compared to the other toner colors. Therefore, in the image forming device  1  of the first embodiment, for the white (w) toner color, the setting table  351  as illustrated in  FIG. 5  is defined in order to suppress “fogging (or blushing).” 
     In the present invention, “fogging” refers to a phenomenon in which, for example, excessive toner is attached to an area where image formation is not performed (area where image formation is not planed). 
     (A-2) Operation of First Embodiment 
     Next, operation of the image forming device  1  of the first embodiment having the configuration as described above is explained using a flowchart of  FIG. 6 . 
     Regarding operations of the developing devices  11   c ,  11   y ,  11   m  (developing devices of cyan, yellow and magenta; developing devices of toner colors other than white), the same operations are performed. Therefore, in the following, for some steps, only operations of the developing device  11   c  and the developing device  11   w  are explained. The operations of the other developing devices  11   y ,  11   m  are the same as that of the developing device  11   c  and thus detailed explanation thereof is omitted. 
     First, image forming device  1  is powered on (S 101 ) and started. Along with the power-on, in the image forming device  1 , initialization of the configuration elements, status confirmation and the like are performed by the print control part  30 , and the image forming device  1  transitions to a state (online state) capable of receiving print data. 
     Thereafter, image data for printing is supplied from the higher-level device  31  to the print control part  30  via the interface part  32  (S 102 ). In this case, the print control part  30  temporarily stores the supplied image data in the RAM  36 . 
     Next, the print control part  30  instructs the motor control part  48  to start the motors of the rollers (driving starts), and the rollers begin to rotate (S 103 ). Here, as an example, the motor control part  48  controls the driving motors  25   a ,  25   b  to drive the transfer belt  9  to move at a speed of 130 mm/s along an a 2  direction (see  FIG. 2 ), assuming a print speed of 30 PPM. Parameters for such operation of the motor control part  48  may be stored, for example, in the ROM  35 . 
     Next, the print control part  30  reads in the setting table  351  from the ROM  35  (S 104 ). 
     Next, the print control part  30  controls the voltage control parts (the developing voltage control parts  41 ,  42 , the layer formation and supply voltage control part  43 , the charging voltage control parts  44 ,  45 ) to apply bias voltages of values according to the setting table  351  (S 105 ). 
     Next, the print control part  30  controls the configuration elements to develop a toner image based on the image data temporarily stored in the RAM  36  (S 106 ). 
     As described above, the photosensitive drums  12   c ,  12   w  rotate at a speed of 130 mm/s. Along with the rotation of the photosensitive drums  12   c ,  12   w , the developing roller  15   c ,  15   w  and the supply roller  17   c ,  17   w  that are engaged with the photosensitive drums  12   c ,  12   w  via gears also rotate. 
     Further, the supply voltages SB are applied to the layer forming blades  16   c ,  16   w  and the supply roller  17   c ,  17   w  by the layer formation and supply voltage control parts  42 ,  43 . Further, the developing voltages DB according to the setting table  351  are respectively applied to the developing roller  15   c ,  15   w . Next, the toners Tc, Tw of the toner tanks  18   c ,  18   w  are attached to surfaces of the supply rollers  17   c ,  17   w  to which the supply voltages SB are applied. Next, along with the rotation of the supply rollers  17   c ,  17   w  and the developing rollers  15   c ,  15   w , the toners attached to the supply rollers  17   c ,  17   w  are supplied to the surfaces of the developing rollers  15   c ,  15   w  due to potential differences between the rollers. Next, along with the rotation of the developing rollers  15   c ,  15   w , toner layers on surfaces of the developing roller  15   c ,  15   w  are uniformly regulated by shearing forces of the layer forming blade  16   c ,  16   w , when passing through positions of the layer forming blades  16   c ,  16   w , and become uniform. 
     Further, the charging voltages CH are applied to the charging rollers  13   c ,  13   w  by the charging voltage control parts  44 ,  45  and the charging rollers  13   c ,  13   w  become charged. Next, when the photosensitive drums  12   c ,  12   w  rotate, the surfaces of the photosensitive drums  12   c ,  12   w  are charged by the opposing charging rollers  13   c ,  13   w.    
     Further, under the control of the print control part  30 , the exposure control parts  46   c ,  46   w  instruct the LED heads  14   c ,  14   w  to perform exposure based on the image data. Next, based on the instructions of the exposure control parts  46   c ,  46   w , the LED heads  14   c ,  14   w  perform exposure (exposure of patterns based on the image data) on the charged surfaces of the photosensitive drums  12   c ,  12   w , and electrostatic latent images are formed on the surfaces of the photosensitive drums  12   c ,  12   w . In this case, the print control part  30  adjusts the light emission time of each of the LED heads  14  according to the content of the setting table  351  (the items of the light emission time TL). 
     Further, when the surfaces of the developing rollers  15   c ,  15   w , on which toner layers are formed on the surfaces, and the surfaces of the photosensitive drums  12   c ,  12   w  are in contact with each other, toners of the toner layers are attached to the photosensitive drums  12   c ,  12   w  and electrostatic latent images are developed, and toner images are formed. 
     As described above, in the image forming device  1 , based on the control of the print control part  30 , toner images based on the electrostatic latent images are developed on the photosensitive drums  12   c ,  12   w.    
     Next, under the control of the print control part  30  (such as control with respect to the motor control part  48 ), the print medium  23  is fed out from the paper cassette  20  conveyed to a position of the transfer belt  9  (S 107 ). 
     Next, the transfer voltages TR according to the setting table  351  are applied to the transfer rollers  19   c ,  19   w  by the transfer control part  47 , and the toner images on the surfaces of the photosensitive drums  12   c ,  12   w  are transferred to the print medium  23  that is conveyed on the transfer belt  9  (S 108 ). 
     Next, the print medium  23  on which the toner images have been transferred is conveyed to the fixing unit  24 . Then, the toner image of the print medium  23  is subjected to a fixing treatment (heating treatment and pressure treatment) by the fixing unit  24  and is fixed (S 109 ). 
     In the fixing unit  24 , when the fixing treatment is performed, the heating roller  28  is heated by the heating body  28   a  to a predetermined temperature (for example, 60° C.). When the print medium  23  is conveyed in a state being sandwiched between the heating roller  28  that is heated to the predetermined temperature and the pressing roller  29 , the toner image on the print medium  23  is heated and pressed and is fixed on the print medium  23 . 
     Next, the print medium  23  that has been subjected to the fixing treatment by the fixing unit  24  is ejected to the output tray  2  by the eject unit  26  (S  110 ) and one printing process with respect to the print medium  23  is completed. 
     (A-3) Effects of First Embodiment According to the first embodiment, the following effects can be achieved. 
     (A-3-1) First, when the charging voltage CH of the charging roller  13  is lowered, the degree of “fogging” that appears as dirt in an area where image formation is not performed on the print medium  23 . This is explained using a graph of experimental results illustrated in  FIG. 7 . 
     A potential difference PD as a developing potential difference that is indicated on a horizontal axis of the graph of  FIG. 7  represents a difference between an absolute value of the developing voltage DB and an absolute value of the drum surface potential DS 1 . 
     Next, color difference ΔE indicated on a vertical axis of the graph of  FIG. 7  is explained using  FIG. 8 . 
     The color difference ΔE in  FIG. 7  represents a difference between results measured by using a spectrophotometer with respect to a sample area A 1  (area where image is not formed) of a print medium  23 A on which image formation is performed by the image forming device  1  and a sample area A 2  (area corresponding to the sample area A 1 ) of a print medium  23 B (a print medium on which image formation is not performed) that serves as a reference. In the experiment illustrated in  FIG. 7 , as the print mediums  23 A,  23 B, an OHP film CG3700 by Sumitomo 3M is used. Further, on the print medium  23 A, the image formation is performed using one developing device  11  (of any one of the toner colors c, m, y, w). Then, values (L*, a*, b*) of an L*a*b* color system are measured using a spectrophotometer CM-2600d by Konica Minolta, Inc., respectively with respect to the sample areas A 1 , A 2  of the print media  23 A,  23 B. In this case, with respect to a print medium  23 A on which an image is formed with the white (w) toner color, the measurement is performed using the spectrophotometer by using a black paper as an underlay. Further, with respect to a print medium  23 A on which an image is formed with a toner color (c, m, y) other than white, the measurement is performed using the spectrophotometer by using a white paper as an underlay. Then, the color difference ΔE is obtained by using the following formula (2). In the following formula (2), measurement results of the sample area A 1  of the print medium  23 A are represented as “L* 1 , a* 1 , b* 1 .” Measurement results of the sample area A 2  of the print medium  23 B are represented as “L* 2 , a* 2 , b* 2 .”
 
Color Difference Δ E =(( L* 1 −L* 2)^2+( a* 1 −a* 2)^2+( b* 1 −b* 2)^2)^0.5  (2)
 
Therefore, a larger value of the color difference ΔE indicates a larger degree of the “fogging” with respect to the print medium  23 A (large degree of poor printing quality).
 
     As described above, the graph of  FIG. 7  illustrates the color difference ΔE (vertical axis) when the potential difference PD (horizontal axis) is varied between 100-600 V. In other words, the graph of  FIG. 7  illustrates a relation between the potential difference PD and the color difference ΔE for each toner color.  FIG. 7  illustrates the color difference ΔE for the case where the image is formed with the white (w) toner color and the color difference ΔE for the case where the image is formed with a toner color (c, y, m) other than white. For toner colors (c, y, m) other than white, the results are the same and thus are represented with the same sample values in  FIG. 7 . 
     In the setting table  351  illustrated in  FIG. 5 , the potential difference PD for toner colors (c, y, m) other than white is 400 V. Therefore, in this case, the color difference ΔE=0.6 from the graph of  FIG. 7 . Further, in the setting table  351  illustrated in  FIG. 5F , the potential difference PD for the white (w) toner color is 200 V, and thus the color difference ΔE=1.2. 
     Assume a case where the values of the charging voltage CH and the drum surface potential DS 1  of the developing device  11   w  of the white (w) toner color are the same as those of the developing devices  11   c ,  11   y ,  11   m  of the other toner colors, that is, a case where the charging voltage CH is set to be −1200 V (the drum surface potential DS 1  is set to be −600 V). In this case, since the potential difference PD is 400 V, when this is fitted to the graph of  FIG. 7 , it gives that the color difference ΔE=1.7. That is, as compared to the state set according to the setting table  351  of  FIG. 5 , the degree of “fogging” is deteriorated about 40%. As illustrated in  FIG. 7 , there is a tendency that the smaller the value of the potential difference PD is, the smaller the value of the color difference ΔE will be. In other words, it is clear that, in the image forming device  1 , the smaller the value of the potential difference PD is, the smaller the degree of the “fogging” will be. 
     In general, when the color difference ΔE is equal to or smaller than 1.6, it is a level at which a viewer can slightly feel the difference of the color, and it can be considered as a good printing quality for a printer and the like. Therefore, as illustrated in  FIG. 7 , in the first embodiment, by lowering the potential difference PD for the white (w) toner color from 400 V to 200 V, the color difference ΔE is improved from 1.7 to 1.2, and a good printing quality can be realized. 
     As described above, in the first embodiment, the white toner Tw uses metal oxide such as titanium dioxide in the colorant. Therefore, as compared to toners Tc, Tm, Ty of other colors, the white toner Tw has a good conductivity and a property of being difficult to be charged. Usually, charge distribution of a toner spreads like a Gaussian distribution. Therefore, when the potential differences PD are the same, the white toner Tw has a characteristic that the degree of “fogging” becomes larger as compared to the toners Tc, Tm, Ty of other colors. Therefore, in the first embodiment, by reducing the charging voltage CH to reduce the drum surface potential DS 1  only for the developing device  11   w  of the white (w) toner color, the degree of “fogging” for the white (w) toner color is reduced. 
     (A-3-2) As described above, the white toner Tw has a charging ability lower than toners Tc, Tm, Ty of other colors. Therefore, in the image forming device  1 , by increasing the supply voltage SB for the white (w) toner color more than other toner colors (c, m, y), the amount of toner attached on the developing roller  15   w  is increased. As a result, in the developing device  11   w , a Coulomb force between the developing roller  15   w  and the supply roller  17   w  becomes strong, and thus the white toner Tw, even with a low charging ability, can be stably supplied to the developing roller  15   w . As a result, in the developing device  11   w , printing at a stable concentration becomes possible. 
     When the potential difference PD is reduced, there may be a case where it is disadvantageous (printing quality is degraded) in terms of gradation expression. However, the white (w) toner color, for example, as in JP2007-083634A, is often used to print a solid image, for which it is rare that fine gradation expression is required. On the other hand, when printing a solid image, dirt such as “fogging” is easily noticeable and thus significantly affects the printing quality. Therefore, in the image forming device  1 , with respect to the white (w) toner color, in order to suppress dirt such as “fogging”, even when gradation expression is sacrificed to some extent, its impact on overall quality of the image formation is insignificant. 
     Further, in the image forming device  1 , with respect to the white (w) toner color, by suppressing dirt such as “fogging”, consumption of the white toner Tw can also be suppressed. 
     (A-3-3) In the image forming device  1 , as illustrated in  FIG. 2 , from the upstream side of the medium conveyance, the developing devices  11   c ,  11   y ,  11   m ,  11   w  are sequentially arranged in this order. That is, the developing device  11   w  of the white (w) toner color is positioned at the most downstream side of the medium conveyance. In the image forming device  1 , there may be cases where the white toner Tw is superimposed on toners Tc, Tm, Ty of other colors and is transferred. As described above, the toner Tw of the white (w) toner color has the property of being difficult to be charged than toners Tc, Tm, Ty of other colors. Therefore, in the image forming device  1 , when the toner Tw of the white (w) toner color is superimposed on toners Tc, Tm, Ty of other colors and is transferred, as compared to a case where toners of the same charging characteristics are superimposed and transferred, an effect of being easily transferred can be achieved. 
     (B) Second Embodiment 
     In the following, a second embodiment of an image forming device according to the present invention is explained with reference to the drawings. 
     (B-1) Configuration of Second Embodiment 
     A schematic cross-sectional view of an image forming device  1 A of a second embodiment can also be illustrated using the above-described  FIG. 2 . 
       FIG. 9  is a block diagram illustrating a configuration of a control system of the image forming device  1 A of the second embodiment. In  FIG. 9 , a part that is the same as or corresponding to a part in the above-described  FIG. 1  is indicated using the same or corresponding reference numeral. 
     In the following, with respect to the second embodiment, differences as compared to the first embodiment are explained. 
     The image forming device  1 A of the second embodiment is different in that the one developing voltage control part in the first embodiment is separated into two developing voltage control parts  41 ,  49 . As illustrated in  FIG. 9 , the developing voltage control part  49  applies a developing voltage DB to the developing roller  15   w  of the white (w) toner color. The developing voltage control part  41  applies a developing voltage DB to the developing rollers  15   c ,  15   y ,  15   m  of toner colors (c, m, y) other than white. That is, the second embodiment has a configuration in which different developing voltages DB can be supplied to the developing roller  15   w  of the white (w) toner color and the developing rollers  15   c ,  15   y ,  15   m  of toner colors (c, m, y) other than white. 
     Further, the second embodiment is different from the first embodiment in that in the ROM  35  of the second embodiment, in addition to a first setting table  351 , a second setting table  352  is added. 
     The print control part  30  of the second embodiment selects a applicable setting table according to the kind of the print medium  23  (the print medium  23  that is supplied to the transfer belt  9 ) used for image formation. In this embodiment, the print control part  30  controls the voltage control parts using the first setting table  351  when the print medium  23  used for image formation is a plain paper and controls the voltage control parts using the second setting table  352  when the print medium  23  used for image formation is an OHP film. As the OHP film, for example, an OHP film CG3700 by Sumitomo 3M can be used. 
     A configuration in which the print control part  30  recognizes the kind of the print medium  23  (the print medium  23  that is supplied to the transfer belt  9 ) used for image formation is not limited. However, in this embodiment, the kind of the print medium  23  used for image formation is determined according to content of operation setting data  361  that is stored in the RAM  36 . The operation setting data  361  is flag information indicating the kind of the print medium  23  used for the next image formation. The print control part  30 , for example, recognizes that the kind of the print medium  23  used for the next image formation is a “plain paper” when a first value (for example, “0”) is set as the operation setting data  361 , and recognizes that the kind of the print medium  23  used for the next image formation is an “OHP film” when a second value (for example, “1”) is set as the operation setting data  361 . The value that is set as the operation setting data  361 , for example, may be modified according to a user&#39;s operation (for example, an operation with respect to the operation input part  33 ), and may be also modified based on an instruction from the higher-level device  31 . For the operation setting data  361 , any value may be set as a default value. 
     Further, in the image forming device  1 A, for example, the kind of the print medium  23  that is supplied to the transfer belt  9  may also be determined using an optical sensor (for example, based on a degree of light transmission). 
     When an OHP film having a resistance higher than a plain paper is used as the print medium  23 , in order to transfer a toner image on the photosensitive drum  12 , it is preferable that the transfer voltage TR applied to the transfer roller  19  be larger than that for the plain paper. On the other hand, when the transfer voltage TR is increased, potential of an area on the surface of the photosensitive drum  12  where the print medium does not pass through may decrease and dirt may occur on the print medium  23  (printing quality may deteriorate). Therefore, when the transfer voltage TR is increased, it is desirable that the charging voltage CH be also increased. However, when the charging voltage CH is increased, the potential difference PD increases and, as illustrated in the above-described graph of  FIG. 7 , the color difference ΔE increases. As a result, the degree of “fogging” also increases (printing quality deteriorates). In particular, printing quality due to the developing device  11   w  of the white (w) toner color that has low charging ability is significantly affected. 
     Therefore, in the image forming device  1 A of the second embodiment, content of the second setting table  352  that is applied when an OHP film is used as the print medium  23  is in  FIG. 10 . In the second setting table  352 , taking the above-described points into consideration, values suitable for the case where an OHP film is used as the print medium  23  are set as values of the parameters. 
     Specifically, in the second setting table  352 , the transfer voltages TR for all toner colors are increased as compared to the case for a plain paper (the first setting table  352 ) and are set to be +6000 V. Further, in the second setting table  352 , in order to suppress deterioration of printing quality, the charging voltages CH for all toner colors are increased as compared to the case for a plain paper (the first setting table  352 ) and are set to be −1300 V. As a result, when the second setting table  352  is applied, the drum surface potential DS 1  of the photosensitive drum  12  for each toner color is lowered and increase in the degree of “fogging” can be suppressed. In the second setting table  352 , the charging voltages CH (drum surface potentials DS 1 ) for all the toner colors are set to be the same. 
     In the second setting table  352 , along with increasing the charging voltage CH, in order to adjust the potential difference PD for the white (w) toner color, only the developing voltage DB for the white (w) toner color is increased and is −400 V (the developing voltages DB for other toner colors remain as −200 V). 
     Further, in the second setting table  352 , the light emission times TL for all toner colors are increased by 20% as compared to the first setting table  351 . Thereby, the drum surface potentials DS 2  (drum surface potentials of non-exposure parts) are adjusted. That is, in the second setting table  352 , the potential difference PD for each toner color is the same as that of the first setting table  351 . Further, in the second setting table  352 , difference between the developing voltage DB and the supply voltage SB for each toner color is also adjusted to be the same as that of the case where the first setting table  351  is applied. Therefore, even in the case where the second setting table  352  is applied, the amount of toner supplied from the supply roller  17  to the developing roller  15  for each toner color is also the same as in the case where the first setting table  351  is applied. Therefore, even in the case where the second setting table  352  is applied, the concentration of the toner used in image formation for each toner color is about the same as in the case where the first setting table  351  is applied. 
     (B-2) Operation of Second Embodiment 
     Next, operation of the image forming device  1 A of the second embodiment having the configuration as described above is explained using a flowchart of  FIG. 11 . 
     First, image forming device  1 A is powered on (S 201 ) and started. 
     Thereafter, print data is supplied from the higher-level device  31  to the print control part  30  via the interface part  32  (S 202 ). In this case, the print control part  30  temporarily stores image data contained in the supplied print data in the RAM  36 . 
     Next, the print control part  30  reads in operation setting data  361  from the RAM  36  and obtains information (a value indicating either a plain paper or an OHP film) about the print medium  23  used for image formation (S 203 ). 
     Next, the print control part  30  instructs the motor control part  48  to start the motors of the rollers (driving start), and the rollers begin to rotate. 
     Next, based on the content of the obtained operation setting data  361 , the print control part  30  determines the print medium  23  used for image formation (determines whether it is an OHP film) (S 205 ). Next, when the print medium  23  used for image formation is an OHP film, the print control part  30  operates from the processing of S 206  (to be described later). On the other hand, when the print medium  23  used for image formation is a plain paper, the print control part  30  operates from the processing of S 213  (to be described later). 
     When the print medium  23  used for image formation is an OHP film, the print control part  30  obtains the second setting table  352  from the RAM  36  (S 206 ). 
     Next, the print control part  30  controls the voltage control parts to apply bias voltages of voltages according to the second setting table  352  (S 207 ). 
     Next, the print control part  30  control the configuration elements (for example, the LED head  14  of each toner color, and the like) according to the second setting table  352  to perform development of a toner image, paper-feeding of the print medium  23  (OHP film), transfer of the toner image to the print medium  23  (OHP film), fixing treatment of the toner image to the print medium  23  (OHP film), and eject processing of the print medium  23  (OHP film) (S 208 -S 212 ). Except that the configuration elements are controlled according to the second setting table  352 , S 208 -S 212  are the same as in the operation of the first embodiment (S 106 -S 110 ) and thus detailed explanation thereof is omitted. 
     On the other hand, when the print medium  23  used for image formation is a plain paper, the print control part  30  obtains the first setting table  351  from the RAM  36  (S 213 ). 
     Next, the print control part  30  controls the voltage control parts to apply bias voltages of voltages according to the first setting table  351  (S 214 ). 
     Next, the print control part  30  control the configuration elements according to the first setting table  351  to perform development of a toner image, paper-feeding of the print medium  23  (plain paper), transfer of the toner image to the print medium  23  (plain paper), fixing treatment of the toner image to the print medium  23  (plain paper), and eject processing of the print medium  23  (plain paper) (S 214 -S 217 , S 211 , S 212 ). S 214 -S 217 , S 211  and S 212  are the same as in the operation of the first embodiment (S 106 -S 110 ) and thus detailed explanation thereof is omitted. 
     (B-3) Effects of Second Embodiment 
     According to the second embodiment, in addition to the effects of the first embodiment, the following effects can be achieved. 
     In the image forming device  1 A, when a print medium having high electrical resistance such as an OHP film is used as the print medium  23 , by applying the second setting table  352 , deterioration of printing quality is suppressed. 
     Further, in the second setting table  352 , even when the transfer voltage TR is increased, the light emission time TL (exposure energy amount) is adjusted while maintaining the potential difference PD. Therefore, a toner image of a stable concentration having a low degree of “fogging” (good printing quality) can be realized. 
     (C) Third Embodiment 
     In the following, a third embodiment of an image forming device according to the present invention is explained with reference to the drawings. 
     (C-1) Configuration of Third Embodiment 
     A schematic cross-sectional view of an image forming device  1 B of a third embodiment can be illustrated using  FIG. 13 . In  FIG. 13 , a part that is the same as or corresponding to a part in the above-described  FIG. 1  is indicated using the same or corresponding reference numeral. 
       FIG. 12  is a block diagram illustrating a configuration of a control system of the image forming device  1 B of the third embodiment. In  FIG. 12 , a part that is the same as or corresponding to a part in the above-described  FIG. 1  is indicated using the same or corresponding reference numeral. In the following, with respect to the third embodiment, differences as compared to the second embodiment are explained. 
     In the image forming device  1 B, four housing parts SL 1 -SL 4  for housing four developing devices  11   c ,  11   y ,  11   m ,  11   w  are provided. The developing devices  11   c ,  11   y ,  11   m ,  11   w  of the toner colors are respectively detachably housed in the housing parts SL 1 -SL 4 . 
     The third embodiment is different from the second embodiment in that, in the image forming device  1 B of the third embodiment, the print control part  30 A is replaced with a print control part  30 B. 
     In third embodiment, the developing devices  11   c ,  11   y ,  11   m ,  11   w  are respectively arbitrarily housed in the housing parts SL 1 -SL 4  by a user. Therefore, in the print control part  30 B, it is necessary to obtain information (such as toner color) about the developing device  11  housed in each of the housing parts SL 1 -SL 4 . A configuration in which the print control part  30 B obtains information about the developing device  11  housed in each of the housing parts SL 1 -SL 4  is not limited. In this embodiment, as an example, wireless tags I (Ic, Iy, Im, Iw) are respectively attached to the developing devices  11  ( 11   c ,  11   y ,  11   m ,  11   w ). As the wireless tag I, for example, a wireless tag such as a RFID can be used. Each wireless tag I at least stores information about identification (such as a value of any one of c, y, n, w) indicating a toner color of the developing device  11  on which the wireless tag I is attached, and can transmit the identification information via wireless communication. In the image forming device  1 B, wireless tag communication parts  55 - 1 - 55 - 4  are respectively provided for the housing parts SL 1 -SL 4  and can communicate with the wireless tags I of the developing devices  11  housed in the housing parts to obtain the identification information and the like. The print control part  30 B can use the wireless tag communication parts  55 - 1 - 55 - 4  to recognize toner colors and the like of the developing devices  11  housed in the housing parts SL 1 -SL 4 . 
       FIGS. 12 and 13  illustrate a state in which the developing devices  11   w ,  11   y ,  11   m ,  11   c  are respectively housed in the housing parts SL 1 -SL 4 . As compared ro the above-described  FIG. 2 , the position of the developing device  11   w  and the position of the developing device  11   c  are switched. 
     In the image forming device  1 B, LED heads  14 - 1 - 14 - 4  are respectively arranged in the housing parts SL 1 -SL 4  for exposing photosensitive drums  12  of the corresponding developing devices  11 . Further, in the image forming device  1 B, exposure control parts  46 - 1 - 46 - 4  perform exposure control of the LED heads  14 - 1 - 14 - 4 . Further, in the image forming device  1 B, transfer rollers  19 - 1 - 19 - 4  are respectively arranged below the housing parts SL 1 -SL 4 . The configuration of each of the LED head  14 , the exposure control part  46  and the transfer roller  19  is the same as in the second embodiment. However, in the third embodiment, each of the LED head  14 , the exposure control part  46  and the transfer roller  19  is not used for a particular toner color. Therefore, the reference numerals are changed. 
     The print control part  30 B recognizes the toner color and the like of the developing device  11  housed in each of the housing parts SL 1 -SL 4  and performs control processing in such a manner that each of the developing device  11 , the LED head  14  and the transfer roller  19  performs operation corresponding to the recognized toner color. 
     A developing voltage control part  51  used in the image forming device  1 B of the third embodiment can respectively apply individually different developing voltages DB to the developing rollers  15  of the developing devices  11  housed in the housing parts SL 1 -SL 4 . The developing voltage control part  51  applies a developing voltage DB based on an instruction from the print control part  30 B to each developing roller  15 . 
     A layer formation and supply voltage control part  52  used in the image forming device  1 B of the third embodiment can respectively apply individually different supply voltages SB to the supply rollers  17  and the layer forming blades  16  of the developing devices  11  housed in the housing parts SL 1 -SL 4 . The layer formation and supply voltage control part  52  applies the supply voltage SB based on an instruction from the print control part  30 B to each supply roller  17  and each layer forming blade  16 . 
     A charging voltage control part  53  used in the image forming device  1 B of the third embodiment can respectively apply individually different charging voltages CH to the charging rollers  13  of the developing devices  11  housed in the housing parts SL 1 -SL 4 . The charging voltage control part  53  applies a charging voltage CH based on an instruction from the print control part  30 B to each charging roller  13 . 
     A transfer control part  54  used in the image forming device  1 B of the third embodiment can respectively apply individually different transfer voltages TR to the transfer roller  19 - 1 - 19 - 4  that respectively correspond to the housing parts SL 1 -SL 4 . The transfer control part  54  applies a transfer voltage TR based on an instruction from the print control part  30 B to each transfer roller  19 . 
     The print control part  30 B recognizes identification information (toner color) of each of the developing devices  11  house in the housing parts SL 1 -SL 4  and, based on the recognition result, instructs the developing voltage control part  51 , the layer formation and supply voltage control part  52 , the charging voltage control part  53  and the transfer control part  54  regarding bias voltages (developing voltage DB, charging voltage CH, supply voltage SB, and transfer voltage TR) for each of the developing devices  11 . Further, based on the recognition result, the print control part  30  instructs the exposure control parts  46 - 1 - 46 - 4  regarding data of an electrostatic latent image used for image formation, light emission times TL and the like. 
     (C-2) Operation of Third Embodiment 
     Next, operation of the image forming device  1 B of the third embodiment having the configuration as described above is explained using a flowchart of  FIG. 14 . In  FIG. 14 , a step performing the same processing as in the above-described  FIG. 11  is indicated using the same reference numeral. 
     The third embodiment is different from the second embodiment in that operation of S 301  is inserted in the third embodiment. 
     At S 301 , the print control part  30 B uses the wireless tag communication parts  55 - 1 - 55 - 4  to recognize toner colors of the developing devices  11  housed in the housing parts SL 1 -SL 4 . 
     Processing of other steps is the same as in the second embodiment except that the print control part  30 B controls the developing voltage control part  51 , the layer formation and supply voltage control part  52 , the charging voltage control part  53 , the transfer control part  54  and the exposure control parts  46 - 1 - 46 - 4 , based on the recognition result of S 301 . Therefore, detailed explanation of the processing of the other steps is omitted. 
     (C-3) Effects of Third Embodiment 
     In the third embodiment, in addition to the effects of the second embodiment, the following effects can be achieved. 
     In the print control part  30 B of the third embodiment, even when a developing device  11  of any toner color is housed in any one of housing parts SL by a user, the same effect as the second embodiment can be achieved. For example, in a case where a plurality of toner colors are to be superimposed for printing (for example, a case where white toner is used as a base and toners of other toner colors are superimposed thereon), it can be realized by housing the developing device  11  of the toner color of a bottom layer in the housing part SL 1  and housing the developing device  11  of the toner color of a top layer in the housing part SL 4 . 
     (D) Other Embodiments 
     The present invention is not limited to the above-described embodiments, but can also include modified embodiments as exemplified in the following. 
     (D-1) In the third embodiment, it is described that a configuration, in which a developing device of any toner color can be housed at any position (housing part), is added to the image forming device of the second embodiment. However, the same configuration may also be added to the image forming device of the first embodiment. 
     Further, in the first and second embodiments, the housing position of each developing device, the number of housed developing devices and the types and combinations of the toner colors that are applied are not limited. 
     For example, as illustrated in  FIG. 15 , a developing device  11   k , an LED head  14   k  and a transfer roller  19   k  that correspond to a toner color of black (indicated as “k” in  FIG. 5 ) may also be added to the image forming device of the first embodiment. 
     Further, for example, the image forming device of the second embodiment may also be configured as an image forming device capable of housing only one developing device (for example, an image forming device capable of image formation using only one color among a plurality of toner colors including white). 
     (D-2) In the above-described embodiments, an example is explained in which the image forming device of the present invention is applied to an image forming device of a tandem system. However, the image forming device of the present invention may also be applied to an image forming device of a four-cycle system in which one photosensitive drum is shared by a plurality of toner colors. Further, in the above-described embodiments, an example is explained in which the image forming device of the present invention is applied to an image forming device of a direct-transfer system (a system in which a toner image is directly transferred from a photosensitive drum to a print medium). However, the image forming device of the present invention may also be applied to an image forming device of an intermediate belt transfer system (a system in which a toner image is transferred to a print medium via an intermediate transfer belt). 
     Further, in the above-described embodiments, an example is explained in which the image forming device of the present invention is applied to a printer. However, the device to which the image forming device of the present invention is applied is not limited to this. For example, the image forming device of the present invention may also be applied to various image forming devices such as a color copier and a facsimile apparatus.