Patent Abstract:
An image-forming device includes 1st to N th  image bearing members, 1st to N th  developing units provided in one-to-one correspondence with the 1st to N th  image bearing members, and a transfer unit. N is an integer number equal to or greater than two. The 1st to N th  image members have 1st to N th  surfaces respectively. The 1st to N th  electrostatic latent images are formable on the 1st to N th  surface respectively. The 1st to N th  developing units have 1st to N th  monochromatic developers respectively. The 1st monochromatic developer is of monochromatic black and has a toner particle substantially spherical in shape. The 1st to N th  developing units develop the 1st to N th  electrostatic latent images with the 1st to N th  monochromatic developers respectively in order to form 1st to N th  developer images respectively. The transfer unit transfers sequentially the 1st to N th  developer images to a recipient in a superimposed manner in order of the 1st to N th  developer image.

Full Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an image-forming device to form a multi-color image. 
   2. Description of Related Art 
   A conventional image-forming device, such as Japanese Patent application publication No. 2002-31933, forms a multi-color image as follows. Firstly, a plurality of developing units forms developer images sequentially on a plurality of corresponding image bearing members (or on one image bearing member, in the four-cycle method) on which latent electrostatic images are formed. Then, those developer images are transferred sequentially to a transfer recipient such as a sheet of paper or an intermediate transfer body. 
   SUMMARY OF THE INVENTION 
   It has recently been established that reverse transfer occurs when such an image-forming device forms a multi-color image. 
   Part of the developer that has been transferred to the transfer recipient from one image bearing member are charged to a polarity opposite to the polarity to which the developing unit has charged. When the second or later image bearing members performs transferring, the developer charged to the opposite polarity is reverse-transferred to the second or later image bearing members due to the reverse transfer. 
   The reverse transfer is more likely to occur as the amount of developer (amount of toner) that has been transferred to the transfer recipient increases. With the tandem method, for example, the amount of developer involved in the reverse transfer generally increases with later image bearing members positioned on the downstream side in the direction in which the paper is conveyed. 
   A conventional image-forming device with the simultaneous development/cleaning method (also called the cleanerless method) is not provided with a cleaning device for recovering waste developer. Therefore, if the waste developer (reverse transfer toner) that has been reverse-transferred to the image bearing members is recovered into the developing unit, the developer for the original colors will be mixed with the waste developer. 
   When charge capability of the waste developer (reverse-transferred toner) that has been reverse-transferred to the image bearing members is higher than that of the developer for the original colors, the waste developer rather than the developer for the original colors will tend to be transferred to the transfer recipient in the development, causing color mixing. In addition, muddying can also occur easily due to difference in charge amount, causing poor image quality. 
   Furthermore, if the cleaning effect is not sufficiently pronounced even when a cleaning device is provided with a recovering waste developer, the color mixing and muddying can occur in a similar manner to those with the simultaneous development/cleaning method. 
   In view of the foregoing, it is an objective of the present invention to provide an image-forming device that can form images while suppressing the effects of reverse transfer. 
   In order to attain the above and other objects, the present invention provides an image-forming device including 1st to N th  image bearing members, 1st to N th  developing units provided in one-to-one correspondence with the 1st to N th  image bearing members, and a transfer unit. N is an integer number equal to or greater than two. The 1st to N th  image members have 1st to N th  surfaces respectively. The 1st to N th  electrostatic latent images are formable on the 1st to N th  surface respectively. The 1st to N th  developing units have 1st to N th  monochromatic developers respectively. The 1st monochromatic developer is of monochromatic black and has a toner particle substantially spherical in shape. The 1st to N th  developing units develop the 1st to N th  electrostatic latent images with the 1st to N th  monochromatic developers respectively in order to form 1st to N th  developer images respectively. The transfer unit transfers sequentially the 1st to N th  developer images to a recipient in a superimposed manner in order of the 1st to N th  developer image. 
   Another aspect of the present invention provides an image-forming device including a plurality of image bearing members, a plurality of developing units and a transfer unit, The plurality of image bearing members includes 1st to 4th image bearing members. The 1st to 4th image members have 1st to 4th surfaces respectively. 1st to 4th electrostatic latent images are formable on the 1st to 4th surface respectively. The plurality of developing units includes 1st to 4th developing units provided in one-to-one correspondence with the 1st to 4th image bearing members. The 1st to 4th developing units have 1st to 4th monochromatic developers respectively. The 1st monochromatic developer is of monochromatic black and has a toner particle substantially spherical in shape. The 1st to 4th developing units develop the 1st to 4th electrostatic latent images with the 1st to 4th monochromatic developers respectively in order to form 1st to 4th developer images respectively. A total amount of the 2nd monochromatic developer on the 2nd surface and the 3rd monochromatic developer on the 3rd surface is less than an amount of the 1st monochromatic developer on the 1st surface. The transfer unit transfers the 1st to 4th developer images to a recipient in a superimposed manner in order of the 1st to 4th developer image in order to form a black image. 
   Another aspect of the present invention provides an image-forming method including steps (a) to (d). The step (a) forms 1st to N th  electrostatic latent images on a surface formed on an image bearing members, N being an integer equal to or greater than two. The step (b) develops the 1st to N th  electrostatic latent images with 1st to N th  monochromatic developers respectively in order to form 1st to N th  developer images respectively, wherein the N−1 th  monochromatic developer being of monochromatic yellow. The (c) transfers sequentially the 1st to N th  developer images to a recipient in a superimposed manner in order of the 1st to N th  developer image. The step (d) removes residual developer that adheres to each image bearing member while developing each electrostatic latent image. 
   Another aspect of the present invention provides an image-forming device including at least one image bearing member, a plurality of developing units and a transfer unit. A plurality of electrostatic latent images is formable on at least one image bearing member. The plurality of developing units include 1st to N th  developing units. the 1st to N th  developing units have 1st to N th  monochromatic developers respectively. The 1st monochromatic developer is of monochromatic black and has a toner particle substantially spherical in shape. The 1st to N th  developing units develop the plurality of electrostatic latent images with the 1st to N th  monochromatic developers respectively in order to form 1st to N th  developer images respectively. The transfer unit transfers the 1st to N th  developer images to a recipient in a superimposed manner in order of the 1st to N th  developer image. 
   Another aspect of the present invention provides an image-forming device including at least one image bearing member, a plurality of developing units and a transfer unit. 1st to 4th electrostatic latent images are formable on at least one image bearing member. The plurality of developing units includes 1st to 4th developing units provided in one-to-one correspondence with the 1st to 4th image bearing members. The 1st to 4th developing units have 1st to 4th monochromatic developers respectively. The 1st monochromatic developer is of monochromatic black. The 1st to 4th developing units develop the 1st to 4th electrostatic latent images with the 1st to 4th monochromatic developers respectively in order to form 1st to 4th developer images respectively. A total amount of the 2nd monochromatic developer and the 3rd monochromatic developer on the image bearing member is less than an amount of the 1st monochromatic developer on the image bearing member. The transfer unit transfers the 1st to 4th developer images to a recipient in a superimposed manner in order of the 1st to 4th developer image in order to form a black image. 
   Another aspect of the present invention provides an image-forming device including a plurality of image bearing members, a plurality of developing units, a transfer unit and a cleaning member. The plurality of image bearing members include 1st to N th  image bearing members. N is an integer number equal to or greater than two. The 1st to N th  image members have 1st to N th  surfaces respectively. 1st to N th  electrostatic latent images are formable on the 1st to N th  surface respectively. The plurality of developing units include 1st to N th  developing units provided in one-to-one correspondence with the 1st to N th  image bearing members. The 1st to N th  developing units have 1st to N th  monochromatic developers respectively. The N−1 th  monochromatic developer is of monochromatic yellow. The 1st to N th  developing units develop the 1st to N th  electrostatic latent images with the 1st to N th  monochromatic developers respectively in order to form 1st to N th  developer images respectively. The transfer unit transfers sequentially the 1st to N th  developer images to a recipient in a superimposed manner in order of the 1st to N th  developer image. The cleaning member removes residual developer that adheres to each image bearing member after each developer image is transferred to the recipient. Each developing unit develops each electrostatic latent image while removing the residual developer with the cleaning member. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the invention will become more apparent from reading the following description of the preferred embodiments taken in connection with the accompanying drawings in which: 
       FIG. 1  is a section taken through the side of essential components of a color laser printer in a first embodiment; 
       FIG. 2  shows the configuration in the vicinity of a photosensitive drum in the first embodiment; 
       FIG. 3  is illustrative of the cause of reverse-charging; 
       FIG. 4  is illustrative of the sequence in which developer images are formed and the ease of reverse transfer; and. 
       FIG. 5  is a section taken through the side of essential components of a color laser printer in a modification of the first embodiment; 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   An image-forming device according to preferred embodiments of the present invention will be described while referring to the accompanying drawings wherein like parts and components are designated by the same reference numerals to avoid duplicating description. 
   A first embodiment of the present invention will be described with reference to  FIGS. 1 to 4 .  FIG. 1  is a side sectional view of a color laser printer  1  according to the first embodiment. As shown in  FIG. 1 , the color laser printer  1  has a visible image formation portion  4 , a paper conveyor belt  6 , a fixing portion  8 , a paper supply portion  9 , a stacker  12 , a control portion  10 , and a bias supply unit  11 . The color laser printer  1  forms a multi-color image by sequentially overlaying four color toner images on paper P, where the four colors corresponds to image data that is input from the exterior. 
   The visible image formation portion  4  has four developing units  51 BK,  51 M,  51 Y, and  51 C; four photosensitive drums  3 BK,  3 M,  3 Y, and  3 C being one-to-one correspondence with the developing units  51 BK,  51 M,  51 Y, and  51 C; four chargers  31 ,  32 ,  33 , and  34  being one-to-one correspondence with the developing units  51 BK,  51 M,  51 Y, and  51 C; and four exposure devices  41 ,  42 ,  43 , and  44  being one-to-one correspondence with the developing units  51 BK,  51 M,  51 Y, and  51 C. The developing units  51 BK,  51 M,  51 Y, and  51 C accommodates black (BK), magenta (M), yellow (Y), and cyan (C) toner respectively. The capital letters used as suffixes for the developing units in  FIG. 1  refer to the color of the toner housed in the corresponding developing units. 
   There are two methods of forming a black-colored image: one in which only a monochromatic black developer is used and another in which developers of other colors (such as yellow, cyan and magenta, or red, green, and blue) are overlaid on black-colored developer (mixed-color black development). Mixed-color black development produces blacks of a much higher image quality than monochromatic black development. Therefore, the mixed-color black development is used for forming a black-colored image in the present embodiment. 
   The configuration of each of the structural components will be described in detail as follows. The four photosensitive drums  3 BK,  3 M,  3 Y, and  3 C that are formed of a member of a substantially circular cylindrical form are disposed rotatably, spaced substantially equidistantly along a line in the horizontal direction (the widthwise direction in the plane of the paper in  FIG. 1 ). The substantially circular cylindrical material of each of the photosensitive drums  3 BK,  3 M,  3 Y, and  3 C is an aluminum base member on which a positively-charged photosensitive layer is formed, for example. The aluminum base member is grounded to the ground line of the color laser printer  1 . 
   Each of the four chargers  31  to  34  is a scorotron type of charger.  FIG. 2  shows the detailed configuration of the charger  31  that charges the photosensitive drum  3 BK for forming the black toner image. The charger  31  has a charge wire  36  and a shielding case  37 . The charge wire  36  extends to the axis direction of the photosensitive drum  3 BK (the direction into the paper in  FIG. 2 ) facing the surface of the photosensitive drum  3 BK. The shielding case  37  houses the charge wire  36  and is open towards the photosensitive drum  3 BK side. The shielding case  37  is provided with a grid  38  over the open portion. 
   The surface of the photosensitive drum  3 BK is charged to a positive polarity (such as +700 V) when a high voltage is applied to the charge wire  36 . The charge on the surface of the photosensitive drum  3 BK and the voltage of the grid are kept at substantially the same potential by applying a constant voltage to this grid  38 . The chargers  32 ,  33 , and  34  that are provided to correspond to the other photosensitive drums  3 M,  3 Y, and  3 C have the same structure as the charger  31 . 
   The exposure device  41  will be described referring to  FIG. 2 . The exposure device  41  exposes the photosensitive drum  3 BK for forming a latent electrostatic image on the surface of the photosensitive drum  3 BK. As shown in  FIG. 2 , the exposure device  41  is disposed on the downstream side of the charger  31  with respect to the direction of rotation of the photosensitive drum  3 BK (clockwise in this figure). A light source of the exposure device  41  outputs a laser beam corresponding to one color component of image data (in this case, black) that is input from the exterior. The laser beam is scanned by the mirrored surfaces of a polygon mirror (not shown) that is driven to rotate by a polygon motor (also not shown), to illuminate the surface of the photosensitive drum  3 BK. Note that large portions of the exposure devices  41  to  44  are omitted from  FIGS. 1 and 2 ; only the portions that emit the laser beam are shown therein. 
   When the surface of the photosensitive drum  3 BK is illuminated by the laser beam, the surface potential of the illuminated portions drops (to +150 V, by way of example) to form a latent electrostatic image on the surface of the photosensitive drum  3 BK. The other exposure devices  42 ,  43 , and  44  that are disposed facing the corresponding photosensitive drums  3 M,  3 Y, and  3 C have the same configuration as that of the above-described exposure device  41 , and each outputs a laser beam for the corresponding color, based on image data that is input from the exterior. 
   The first developing unit  51 BK, which develops the latent electrostatic image formed by black toner, will be described referring to  FIG. 2 . As shown in  FIG. 2 , the developing unit  51 BK has a toner hopper  54  to house the toner, a supply roller  55  to supply the toner, and a developing roller  52  to bear the toner, within a developing unit case  53 . 
   The toner hopper  54  is an interior space in the developing unit case  53  and accommodates black toner. An agitator  56  is provided at one end portion within the toner hopper  54 . In the present embodiment, the toner housed in the toner hopper  54  is positively charged, non-magnetic, single-component developer that is formed from a suspended polymer or emulsified polymer. The particles of the toner are substantially spherical to have excellent fluidity. 
   The supply roller  55  has a roller shaft and an electrically conductive sponge material coated around the metal roller shaft. The supply roller  55  is disposed at the bottom part within the toner hopper  54 . The supply roller  55  is supported rotatably in the same direction as the developing roller  52  (in the counterclockwise direction in  FIG. 2 ), facing the developing roller  52 . 
   The developing roller  52  is disposed rotatably at a position at which the developing roller  52  is in mutual contact with the supply roller  55 . The developing roller  52  is configured of a circular cylindrical member that is made of electrically conductive silicone rubber or the like as a base member. The surface of the developing roller  52  is formed with a coating of a rubber material or a resin comprising fluoride. 
   The developing roller  52  is disposed in contact with the photosensitive drum  3 BK on the downstream side of the exposure device  41  in the direction of rotation of the photosensitive drum  3 BK. The developing unit  51 BK supplies the toner charged to a positive polarity for the developing roller  52  as a uniform thin layer. Inverted developing method is used to form a toner image while providing the latent electrostatic image of a positive polarity that has been formed on the photosensitive drum  3 BK with the positively-charged toner, at the contact portion between the developing roller  52  and the photosensitive drum  3 BK. 
   The other developing units  51 M,  51 Y, and  51 C each have a configuration that is similar to that of developing unit  51 BK shown in  FIG. 2 , except that the colors of the toner accommodated therein are different (these developing units hold magenta, yellow, and cyan toner, respectively). 
   The paper supply portion  9  is provided in the lowermost portion of the color laser printer  1  and is configured of an accommodation tray  91  to accommodate the paper P and a pick-up roller  92  to transmit the paper. The paper P that is accommodated in the accommodation tray  91  is taken out one sheet at a time by the pick-up roller  92  and is transmitted to the paper conveyor belt  6  via conveyor rollers  99  or the like. 
   The paper conveyor belt  6  is formed in a loop and suspended between a drive roller  62  and a driven roller  63 . The paper conveyor belt  6  can run integrally with the paper P supported on the upper surface of the paper conveyor belt  6 . The width of the paper conveyor belt  6  is narrower than the width of the photosensitive drums  3 BK,  3 M,  3 Y, and  3 C. Four transfer rollers  66 ,  67 ,  68 , and  69  are provided at positions where the four transfer rollers  66 ,  67 ,  68 , and  69  face the corresponding photosensitive drums  3 BK,  3 M,  3 Y, and  3 C via the paper conveyor belt  6  respectively. 
   When the drive roller  62  rotates, the paper conveyor belt  6  in a loop also rotates as shown in  FIG. 1 . The paper P that has been transmitted by the conveyor rollers  99  or the like is conveyed sequentially between each of the photosensitive drums  3 BK,  3 M,  3 Y, and  3 C and the surface of the paper conveyor belt  6 , then on to the fixing portion  8 . 
   A suitable transfer bias that is controlled at −10 to −15 μA, by way of example, is applied between each of the transfer rollers  66  to  69  and the corresponding photosensitive drums  3 BK,  3 M,  3 Y, and  3 C in order to electrostatically transfer the toner image that is formed on each photosensitive drum in sequence to the paper P that is conveyed by the paper conveyor belt  6 . Specifically, a voltage having a polarity (in the present embodiment, negative polarity) opposite to that (in the present embodiment, positive polarity) of the charge on each of the corresponding photosensitive drums  3 BK,  3 M,  3 Y, and  3 C is applied to each of the four transfer rollers  66 ,  67 ,  68 , and  69 . 
   Taking the toner image formed by black toner as an example, if the transfer bias of a high voltage of a negative polarity is applied to the transfer roller  66 , the toner image on the photosensitive drum  3 BK is transferred to the paper P at the position at which the photosensitive drum  3 BK faces the transfer roller  66 , in other words, at a transfer nip portion TP at which the paper P is in contact with the photosensitive drum  3 BK. 
   In other words, the application of the transfer bias generates an electric field from the photosensitive drum  3 BK to the transfer roller  66 . The toner image of a positive polarity on the photosensitive drum  3 BK transfers to the paper P electrostatically due to the electric field transfers. The transfer of the toner images on the other photosensitive drums  3 M,  3 Y, and  3 C is done in the same way. 
   Thus, the toner images of the corresponding colors are transferred sequentially in order of black, magenta, yellow, and cyan by the application of the transfer bias to the corresponding transfer rollers  67 ,  68 , and  69 . In other word, the desired multi-color image is created by overlaying toner images sequentially in order of black, magenta, yellow, and cyan onto the paper P. Note that the use of constant-current control over the transfer bias is cited merely as an example, and thus another control method could be used, such as constant-voltage control. 
   A cleaning brush  105  is disposed at the downstream of the drive roller  62 , facing the surface of the paper conveyor belt  6 . The cleaning brush  105  has a brush provided around the periphery of a substantially circular cylindrical member whose axis extends across the width of the paper conveyor belt  6 . The cleaning brush  105  rotates in contact with the paper conveyor belt  6 . A bias voltage is applied between the cleaning brush  105  and an electrode roller  104  that is provided at a position on the other side of the paper conveyor belt  6  and faces to the cleaning brush  105 . 
   A recovery roller  106  and a collection box  107  are provided in the vicinity of the cleaning brush  105 . The recovery roller  106  removes toner that adheres to the cleaning brush  105 . The collection box  107  accumulates the toner removed from the cleaning brush  105  by the recovery roller  106 . 
   The fixing portion  8  is configured of a heating roller  81 , a pressure roller  82  and a fixing sheet  83 . The paper P, on which a multi-color image formed of toner images in four colors is born, is conveyed between the heating roller  81  and the pressure roller  82  via the fixing sheet  83 . The heating roller  81  heats and the pressure roller  82  press the paper P to fix the multi-color image to the paper P. 
   The stacker  12  is provided on the upper surface of the color laser printer  1  and on the paper discharge side of the fixing portion  8 . The stacker  12  holds the paper P that is discharged from the fixing portion  8 . 
   The control portion  10  is provided with a well-known CPU to control all the operations of the color laser printer  1 . The control portion  10  also controls the bias supply unit  11  to apply the transfer bias to each of the transfer rollers  66 ,  67 ,  68 , and  69 ; the cleaning bias between the electrode roller  104  and the cleaning brush  105 , and the voltage to each of the chargers  31  to  34 . 
   The color laser printer  1  of the present embodiment uses a method simultaneous development/cleaning method by which residual toner that has not been transferred, and thus remains on the photosensitive drum surfaces after the transfer of the toner images from the photosensitive drums  3 BK,  3 M,  3 Y, and  3 C onto the paper P, is recovered into the toner hopper  54  via the developing roller  52  and the supply roller  55  while developing being performed. 
   Although the precise mechanism that results in the reverse transfer is still not clear, the cause of the reverse transfer, more specifically, the cause of reverse-charging of toner, is deduced from the results of inspection. When a strong electrical field is generated between the toner and the paper, the discharge occurs within the toner layer that has been transferred onto the paper P. When the discharge occurs, the toner is charged to opposite polarity. When toner of different colors is transferred sequentially, the later toner is overlaid onto the toner that has been already transferred on the paper P. The overall potential is increased due to the charge possessed by the toner layer itself and the electrostatic capacitance generated by the toner layer, causing generating a discharge within the toner layer to charge the upper layer to a negative polarity. 
   More specifically, as shown in  FIG. 3(   a ), a toner image (of positive polarity)  71  on each of the photosensitive drums  3 BK,  3 M,  3 Y, and  3 C is transferred onto a toner image  70  of a positive polarity onto the paper (not shown), at corresponding transfer nip portion TP which is the position at which the photosensitive drums  3 BK,  3 M,  3 Y, and  3 C face the transfer rollers  66 ,  67 ,  68 , and  69  respectively, as the paper is conveyed to the left in the figures by the paper conveyor belt  6 . Thus, a layered toner image  72  as shown in  FIG. 3(   b ) is formed. 
   In transferring, a discharge (separation discharge) occurs within the toner image  72 , due to the charges possessed by the toners. As a result, a reverse-charged toner image  73  whose upper layer portion is charged to the polarity (negative polarity) opposite to the regular charge polarity (positive polarity), is created as shown in  FIG. 3(   a ). Even if the reverse-charging does not occur after the paper has passed the transfer nip portion TP, it is possible that reverse-charging could occur at the next transfer position when the next color is transferred to the paper. In other words, the amount of charge (potential) of the toner image is increased since the charge is imparted to the toner from the photosensitive drum  3  during the transfer. The reverse-charging occurs easily, especially when the transfer bias is applied, as the amount of charge on the toner image increases. This consideration can help explain the results of experiments. 
   Further, when the four developing units  51 BK,  51 M,  51 Y, and  51 C corresponding to four colors performs development sequentially, the magenta toner from the second developing unit  51 M, which has been overlaid on the black toner from the first developing unit  51 BK, is reverse-transferred to the third developing unit  51 Y, as shown in  FIG. 4 . Similarly, the magenta toner and yellow toner from the second and third developing units  51 M and  51 Y, which have been overlaid on the black toner from the first developing unit  51 BK, are reverse-transferred to the fourth developing unit  51 C. It is determined that the second toner (magenta) and third toners (yellow) that are overlaid on the first black toner is reverse-transferred to the fourth developing unit  51 C much larger than the first toner (black). Therefore, the first toner (black) has little adverse effect concerning reverse transfer to the second and subsequent developing units  51 M,  51 Y, and  51 C. 
   By the way, muddying that is generated when the black toner is reverse-transferred has much effect on the image quality than muddying that is generated when the other colors (magenta, yellow, and cyan) are reverse-transferred. On the other hand, as is clear from  FIG. 4 , the toner that is reverse-transferred most easily is not the toner in the lowermost layer on the paper P but the toner in the second and subsequent layers that are overlaid thereon. 
   In the present embodiment, the black toner from the first developing unit  51 BK is transferred to the paper firstly, as described previously. Since the black toner that has much effect on the muddying is transferred firstly, the muddying caused by the black toner is suppressed, causing the image quality to be improved. 
   Substantially spherical particles that have a high fluidity and good transferability are used as the toner in the present embodiment. If the black toner that has above-described features is transferred onto the paper at the end of mixed-color black development, the black toner that has adhered to the uppermost layer is repulsed by the electrical field that is generated by the toner in the lower layers, due to the extremely high fluidity of the toner. As a result, the colors of the other toners are exposed, making it impossible to form a high-quality black image. However, the image-forming device according to the first embodiment can prevent this problem since the black toner is transferred onto the paper firstly. 
   With the simultaneous development/cleaning method (otherwise known as the cleanerless method) used in this embodiment, which necessitates reliable recovery of waste toner in the developing units without using any special cleaner, the effects of reverse transfer are greater than in a configuration in which a dedicated cleaner for recovering waste toner is provided. The yellow toner is transferred onto the paper thirdly in the present embodiment, since the toner that is transferred onto the paper thirdly is most likely to be reverse-transferred to the fourth developing unit  51 C. Since the image quality with yellow toner is not as obvious as that with the other colors of toner (black, magenta, and cyan), the effects of reverse transfer is suppressed, even when the simultaneous development/cleaning method is used. 
   The description now turns to a second embodiment of this invention. Since the configuration of the image-forming device according to the second embodiment is basically the same as that of the first embodiment, further description of components that have the same reference numbers as those in the first embodiment is omitted and the description below concerns only differences from the first embodiment. In the second embodiment, similar to the first embodiment, the black toner is developed onto the paper P firstly, then the magenta, yellow, and cyan non-black toners are developed in the second to fourth places. 
   In the second embodiment, the total developer amount of the second and third toners (magenta and yellow) is less than the developer amount of the black toner in a mixed-color black development. More specifically, the amounts of each of the magenta, yellow, and cyan toners are equal and less than 50% with respect to 100% of black toner. The developer amount could be adjusted by giving the exposure devices  41  to  44  image data to form the latent electrostatic image of a density (dot spacing) corresponding to the developer amount (%) for each toner on the photosensitive drums  3 BK,  3 M,  3 Y, and  3 C. In other words, the developer amount is determined by a difference in density (dot area) of each color with respect to the region in which the black-colored image is formed on the paper P. 
   As described above, the amount of toner from the second developing unit  51 M and third developing unit  51 Y that is reverse-transferred to the fourth developing unit.  51 C is larger than the amount of toner from the fourth developing unit  51 C. However, the image-forming device according to the second embodiment restrains the total developer amount of the second and third toners (magenta and yellow) in mixed-color black development, thus preventing the effects of reverse transfer. 
   While the invention has been described in detail with reference to the specific embodiment thereof, it would be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention. 
   For example, the black-colored image may be formed by monochromatic black development with a single color of black toner. Such a case, the quality of the black-colored image formation is slightly degraded but deterioration of the quality due to reverse transfer can be prevented. 
   Only the black toner may be of a substantially spherical form, though all of the toners are of a substantially spherical form in the first embodiment. 
   The cyan toner is developed by the second developing unit and the magenta toner is developed by the fourth developing unit. Note that the toner of the fourth developing unit is preferably the toner that has the largest amount of toner adhering per unit area (M/A) of the corresponding photosensitive drum. This puts the largest reverse transfer to the fourth developing unit and can minimize the amount of reverse transfer of developer due to the second and third developing units. 
   The yellow toner may be developed by the second developing unit. It should be noted, however, that yellow toner is preferably used as the third developer, since the large amount of the toner from the third developer is reverse-transferred to the fourth developing unit. 
   In the above-described embodiments, an image-forming device that uses a “direct transfer method” is described, wherein a visible image (developer image) formed on each photosensitive drum  3  is directly transferred onto the paper P as the transfer recipient. However not limited thereto, an “intermediate transfer method” may be used for the image-forming device, wherein after the visible image formed on each photosensitive drum is transferred to an intermediate transfer body such as an intermediate transfer belt or an intermediate transfer drum as the transfer recipient (primary transfer), the image is transferred from the intermediate transfer body to paper (recording recipient). An OHP sheet may be used instead of the paper P. In addition, not limited to the tandem method, a four-cycle method in which each developing unit forms developer images on a common photosensitive drum can also be used. 
   A complex machine that is provided with a facsimile, a printing function, or scanner function may be used instead of the printer such as the color laser printer  1 . The laser printer  1  may be provided with cleaning rollers  111 - 114  to clean up the photosensitive drums  3 BK,  3 M,  3 Y, and  3 C, as shown in  FIG. 5 .

Technology Classification (CPC): 6