Patent Publication Number: US-9417582-B1

Title: Image forming apparatus including a controller that controls superposition of a correction toner image

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2015-046152 filed Mar. 9, 2015. 
     BACKGROUND 
     Technical Field 
     The present invention relates to an image forming apparatus. 
     SUMMARY 
     According to an aspect of the present invention, an image forming apparatus includes an image forming section and a controller. The image forming section transfers a toner image to a transfer target and includes plural developing units. The plural developing units include a first developing unit and at least one second developing unit. The plural developing units each include an image holding body and a developing roller. The image holding body has an outer circumferential surface and is rotated. The developing roller holds developer, faces the image holding body, and is rotated in an opposite direction to a rotating direction of the image holding body such that an outer circumferential speed of the developing roller is higher than an outer circumferential speed of the image holding body. The controller controls the image forming section so that, when a low-area-coverage portion having a low area coverage and a high-area-coverage portion having a higher area coverage than the low area coverage are disposed adjacent to each other and are formed by the first developing unit, a correction toner image is superposed on a boundary portion, where the low-area-coverage portion and the high-area-coverage portion are adjacent to each other, by the at least one second developing unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is a schematic view of the structure of an image forming apparatus according to an exemplary embodiment; 
         FIG. 2  is a schematic view of a toner image forming section according to the present exemplary embodiment; 
         FIG. 3  is a schematic view of one of developing devices of the toner image forming section; 
         FIG. 4  is an explanatory view illustrating transference of a correction toner image onto a recording medium so as to superpose the correction toner image on a boundary portion of a low-area-coverage portion that is a boundary with a high-area-coverage portion on an upstream side in a gold toner image; 
         FIG. 5  is a plan view of a low-density portion generated in the boundary portion of the low-area-coverage portion that is the boundary with the high-area-coverage portion on the upstream side in the gold toner image; 
         FIGS. 6A to 6C  are explanatory views sequentially illustrating in this order how the low-density portion in the boundary portion of the gold toner image is generated by a toner image forming unit for gold; 
         FIG. 7  is a table that summarizes results of a first experiment; 
         FIG. 8  is a table that summarizes results of a second experiment; 
         FIG. 9  is a graph that summarizes results of a third experiment; 
         FIGS. 10A and 10B  are respectively a sectional view and a plan view of a toner image schematically illustrating a position of flat pigment particles; and 
         FIGS. 11A and 11B  are a plan view and a side view schematically illustrating one of the flat pigment particles included in toner. 
     
    
    
     DETAILED DESCRIPTION 
     An example of an image forming apparatus according to an exemplary embodiment of the present invention is described. 
     The Structure of an Image Forming Apparatus 
       FIG. 1  is a schematic view of the structure of an image forming apparatus  10  seen in a rotational axis direction of photosensitive drums  21 (V),  21 (Y),  21 (M),  21 (C), and  21 (K) and an intermediate transfer belt  31 , which will be described later. An “axial direction”, which will be described later, refers to this rotational axis direction, and a width direction D of  FIG. 5  is also the same as this axial direction. As illustrated in  FIG. 1 , the image forming apparatus  10  includes an image forming section  12 , a transport device  50 , a transfer device  30 , a fixing device  40 , a controller  70 , and a power unit  80 . The image forming section  12  forms toner images by an electrophotographic method. The transport device  50  transports recording media P. The transfer device  30  transfers the toner images onto the recording media P. The fixing device  40  fixes the toner images onto the recording media P. The controller  70  controls operations of components of the image forming apparatus  10 . The power unit  80  supplies power to the components. 
     The Transport Device 
     As illustrated in  FIG. 1 , the transport device  50  includes a container unit  51  and plural transport rollers  52 . The container unit  51  contains the recording media P. The transport rollers  52  transport each of the recording media P from the container unit  51  to a second transfer position NT, which will be described later. The transport device  50  further includes plural transport belts  58  and a transport belt  54 . The transport belts  58  transport the recording medium P from the second transfer position NT to the fixing device  40 . The transport belt  54  transports the recording medium P from the fixing device  40  toward an output unit (not illustrated) for the recording medium P. 
     The Image Forming Section 
     The image forming section  12  includes plural toner image forming units  20 (V),  20 (Y),  20 (M),  20 (C), and  20 (K) that each form a toner image and transfer the toner image onto the intermediate transfer belt  31  through first transfer. 
     The Toner Image Forming Units 
     The plural toner image forming units  20 (V),  20 (Y),  20 (M),  20 (C), and  20 (K) are provided so that each of the toner image forming units  20 (V),  20 (Y),  20 (M),  20 (C), and  20 (K) forms the toner image of a corresponding one of colors and transfers the toner image onto the intermediate transfer belt  31 . According to the present exemplary embodiment, the toner image forming units  20 (V),  20 (Y),  20 (M),  20 (C), and  20 (K) are provided for a total of five colors, that is, a special color (V), yellow (Y), magenta (M), cyan (C), and black (K). Signs (V), (Y), (M), (C), and (K) indicate components corresponding to the above-described respective colors. These signs may be described only by characters V, Y, M, C, and K with the parentheses of (V), (Y), (M), (C), and (K) omitted in the description herein. Furthermore, in the description where the colors are not distinguished, V, Y, M, C, and K are appropriately omitted. 
     The toner image forming units  20  for these colors, that is, the special color (V), yellow (Y), magenta (M), cyan (C), and black (K) are arranged in this order from an upstream side toward a downstream side in a transport direction of the intermediate transfer belt  31 , which will be described later. In the present exemplary embodiment, a “gold toner” is used for the special color (V). 
     The structures of the toner image forming units  20  for the colors are generally similar to or the same as one another except for the toner used therein. Specifically, each of the toner image forming units  20  for the colors includes, as illustrated in  FIG. 2 , a corresponding one of the photosensitive drums  21  and a charger  22 . The photosensitive drum  21  serves as an example of an image holding body and is rotated clockwise in  FIG. 2 . The charger  22  charges the photosensitive drum  21 . 
     The toner image forming unit  20  for the color further includes a light exposure device  23 , a developing device  24 , a photosensitive body cleaner  25 , and a static eliminator  26 . The light exposure device  23  causes the photosensitive drum  21  charged by the charger  22  to be exposed to light so as to form an electrostatic latent image on the photosensitive drum  21 . The developing device  24  develops the electrostatic latent image formed on the photosensitive drum  21  by the light exposure device  23  so as to form the toner image. 
     The Developing Device 
     As illustrated in  FIG. 2 , the developing device  24  includes a container  241  and a developing roller  242 . Developer G is contained in the container  241 . Due to a potential difference generated between the developing roller  242  and the photosensitive drum  21  by applying a developing bias voltage to the developing roller  242 , the electrostatic latent image formed on an outer circumferential surface of the photosensitive drum  21  becomes visible as a toner image. The developing device  24  will be described later. 
     The Photosensitive Body Cleaner 
     The photosensitive body cleaner  25  includes a blade. The toner remaining on the surface of the photosensitive drum  21  after the first transfer of the toner image onto the intermediate transfer belt  31  has been performed is scraped off from the surface of the photosensitive drum  21  by the blade. 
     The Transfer Device 
     The transfer device  30  transfers the toner images from the photosensitive drums  21  for the respective colors onto the intermediate transfer belt  31  (an example of an intermediate transfer body) through the first transfer such that the toner images are superposed on one another. The transfer device  30  transfers the superposed toner images onto the recording medium P at the second transfer position NT through second transfer. 
     Intermediate Transfer Belt 
     The volume resistivity of the intermediate transfer belt  31  according to the present exemplary embodiment is set to 10 10  Ωcm or more. As illustrated in  FIG. 1 , the intermediate transfer belt  31  is an endless belt and looped over plural rollers  32 ,  32 B,  32 D, and  32 T. The roller  32 D functions as a drive roller that rotates the intermediate transfer belt  31  in an arrow A direction by using power from a motor (not illustrated). 
     By rotating the intermediate transfer belt  31  in the arrow A direction, the toner images for the colors transferred from the photosensitive drums  21  for the colors at respective first transfer positions T through the first transfer are superposed on one another, and the superposed toner images are transported to the second transfer position NT. The toner images having been transferred to the second transfer position NT are transferred to the recording medium P by a second transfer device  38  through the second transfer. 
     The roller  32 T functions as a tension applying roller that applies tension to the intermediate transfer belt  31 . The roller  32 B functions as a facing roller  32 B that faces a second transfer roller  34 , which will be described later. 
     A cleaner  35  that cleans the intermediate transfer belt  31  is disposed at a position that is downstream of the second transfer position NT and upstream of the first transfer position T (V) in a rotational direction (arrow A direction) of the intermediate transfer belt  31 . 
     The First Transfer Rollers 
     First transfer rollers  33 , which are disposed inside the intermediate transfer belt  31 , transfer the toner images on the respective photosensitive drums  21  onto the intermediate transfer belt  31 . Each of the first transfer rollers  33  faces a corresponding one of the photosensitive drums  21  for the colors with the intermediate transfer belt  31  interposed therebetween. By applying a first transfer voltage, the polarity of which is opposite to the polarity of the toner, to each of the first transfer rollers  33 , the toner image formed on each of the photosensitive drums  21  is transferred onto the intermediate transfer belt  31  at a corresponding one of the first transfer positions T. 
     The Second Transfer Device 
     The second transfer device  38  transfers the toner images superposed on one another on the intermediate transfer belt  31  onto the recording medium P. The second transfer device  38  includes a second transfer belt  37 . The second transfer belt  37  is an endless belt and looped over the second transfer roller  34  and a driven roller  36 . 
     The second transfer roller  34  is disposed such that the intermediate transfer belt  31  and the second transfer belt  37  are interposed between the second transfer roller  34  and the aforementioned facing roller  32 B. The second transfer belt  37  and the intermediate transfer belt  31  are in contact with each other at a predetermined load. A nip between the second transfer belt  37  and the intermediate transfer belt  31  that are in contact with each other as described above serves as the second transfer position NT. 
     The recording medium P is supplied from the container unit  51  to this second transfer position NT at appropriate timing. The second transfer belt  37  is rotated by rotating the second transfer roller  34 . 
     According to the present exemplary embodiment, when transferring the toner images from the intermediate transfer belt  31  onto the recording medium P, a negative voltage is applied to the facing roller  32 B by the power unit  80 . This generates a potential difference between the facing roller  32 B and the second transfer roller  34 . That is, by applying the negative voltage to the facing roller  32 B, a second transfer voltage (positive voltage), the polarity of which is opposite to the polarity of the toner, is indirectly applied to the second transfer roller  34 , which serves as a counter electrode of the facing roller  32 B. This causes the toner images to be transferred from the intermediate transfer belt  31  to the recording medium P passing through the second transfer position NT. 
     The Fixing Device 
     The fixing device  40  fixes the toner images onto the recording medium P onto which the toner images have been transferred. Specifically, the fixing device  40  heats and applies pressure to the toner images in a fixing nip NF formed between a heating roller  41  and a pressure roller  42  so as to fix the toner images onto the recording medium P. 
     Image Forming Operation 
     Next, an outline of image forming steps performed on the recording medium P by the image forming apparatus  10  is described. 
     In response to an image forming instruction, the controller  70  causes the toner image forming units  20 , the second transfer device  38 , and the fixing device  40  to operate in the image forming apparatus  10  illustrated in  FIG. 1 . The controller  70  also causes the transport device  50  and the like to operate in synchronization with the operations of the toner image forming units  20 , the second transfer device  38 , and the fixing device  40 . 
     The photosensitive drums  21  for the colors are charged by the respective chargers  22  while being rotated. Furthermore, the controller  70  causes image data having undergone image processing performed by an image signal processing unit to be transmitted to the exposure devices  23 . Each of the exposure devices  23  radiates exposure light L (see  FIG. 2 ) in accordance with the image data so as to cause a corresponding one of the charged photosensitive drums  21  to be exposed to the exposure light L. Thus, an electrostatic latent image is formed on the outer circumferential surface of each of the photosensitive drums  21 . The electrostatic latent images formed on the photosensitive drums  21  are developed by the respective developing devices  24 . Thus, the toner images of the special color (V), yellow (Y), magenta (M), cyan (C), and black (K) are formed on the photosensitive drums  21  for the respective colors. 
     The toner images of the colors formed on the photosensitive drums  21  for the respective colors are sequentially transferred onto the rotating intermediate transfer belt  31  by the first transfer rollers  33  for the respective colors at the respective first transfer positions T through the first transfer. Thus, superposed toner images formed by superposing the toner images on one another are formed on the intermediate transfer belt  31 . These superposed toner images are transported to the second transfer position NT by rotating the intermediate transfer belt  31 . The recording medium P is fed to this second transfer position NT by the transport rollers  52  at timing adjusted to transportation of the superposed toner images. The superposed toner images are transferred from the intermediate transfer belt  31  onto the recording medium P at this second transfer position NT through the second transfer. 
     The recording medium P onto which the toner images have been transferred through the second transfer is transported toward the fixing device  40  by the transport belts  58  while being sucked to the transport belts  58  by a negative pressure. The fixing device  40  applies heat and pressure to the recording medium P passing through the fixing nip NF. Thus, the toner images having been transferred onto the recording medium P are fixed onto the recording medium P. 
     The recording medium P onto which the toner images have been fixed by the fixing device  40  is transported by the transport belt  54  and output to the output unit (not illustrated). 
     Meanwhile, residual toner that has not been transferred through the second transfer and remains on the intermediate transfer belt  31  is removed by the cleaner  35 . 
     The Structures of the Components 
     Next, the structures of the components according to the present exemplary embodiment are described. 
     The Developing Devices 
     Initially, the developing devices  24  are described. It is noted that, in  FIG. 3 , carrier GA, toner GB, and a magnetic brush GC included in the developer G, which will be described later, are illustrated on an enlarged scale. Furthermore, the developing devices  24  according to the present exemplary embodiment each have the structure that is the same as or similar to that of a known developing device of a two-component development type. 
     As illustrated in  FIG. 3 , each of the developing devices  24  includes the developing roller  242 , and the developer G is contained in the container  241 . 
     The developing roller  242  is a magnetic roller in which a magnetic body (not illustrated) is disposed in a roller body (not illustrated). Furthermore, the developing roller  242  faces a corresponding one of the photosensitive drums  21  that is rotated, and the roller body is rotated in an arrow R 2  direction opposite to a rotational direction R 1  of the photosensitive drum  21 . The circumferential speed of the roller body of the developing roller  242  is higher than that of the photosensitive drum  21 . The magnetic body (not illustrated) in the roller body is rotated in a direction opposite to that of the roller body. 
     The developer G is a so-called two-component developer including the carrier GA having magnetic properties and the toner GB colored in a corresponding one of the colors. The carrier GA is charged to the positive polarity and the toner GB is charged to the negative polarity in the developer G according to the present exemplary embodiment. 
     Since the developing roller  242  is the magnetic roller in which the magnetic body (not illustrated) is disposed in the roller body as described above, the carrier GA to which the toner GB is attracted by an electrostatic force is held on an outer circumferential surface of the roller body of the developing roller  242  by a magnetic force. 
     In the developing device  24  having such a structure, the developer G held by the roller body of the developing roller  242  forms a magnetic brush GC. Also, the developer G (magnetic brush GC) is moved in the same direction as that of an outer circumferential surface of the photosensitive drum  21  at a speed higher than that of the outer circumferential surface of the photosensitive drum  21  in a nip (portion in contact with the photosensitive drum  21 ) between the magnetic brush GC (developer G) and the photosensitive drum  21  (also see  FIGS. 6A to 6C ). 
     The electrostatic latent image on the photosensitive drum  21  is developed by a so-called reversal development method in the nip (see  FIGS. 6A to 6C ) of the developing device  24  so that the electrostatic latent image becomes visible as the toner image. The developing bias is applied to the developing roller  242  by the power unit  80  (see  FIG. 1 ). 
     Toner 
     As illustrated in  FIGS. 10A and 10B , the gold toner used for the special color (V) includes metal pigment particles  110  that each serve as an example of a flat pigment, a yellow (Y) pigment (not illustrated), and binder resin  111 . The gold toner is used to give a metallically glossy appearance to images. 
     As illustrated in  FIG. 10A , the gold toner is arranged in a direction along a sheet surface PA with reflective surfaces  110 A of the metal pigment particles  110  facing in a direction perpendicular to the sheet surface PA. That is, the reflective surfaces  110 A of the metal pigment particles  110  assume positions in which directions of the reflective surfaces  110 A of the metal pigment particles  110  follow the direction of the sheet surface PA of the recording medium P. Thus, a direction of reflected light reflected by an image  200  closely follows the direction perpendicular to the sheet surface PA of the recording medium P. 
     This may improve the flop index (FI), which is an index that represents the metallically glossy appearance (that is, the metallically glossy appearance may be improved). 
     The images to which the metallically glossy appearance is given include images formed only of the gold toner and images formed of the gold toner and the toners other than the gold toner. 
     The metal pigment particles  110  according to the present exemplary embodiment are made of aluminum. Furthermore, as illustrated in  FIG. 11B , when one of the metal pigment particles  110  is placed on a flat surface and seen from a side, the dimensions of the metal pigment particle  110  are longer in the left-right direction than in the up-down direction in  FIG. 11B . 
     Furthermore, when the metal pigment particle  110  is seen from above in  FIG. 11B , the shape of the metal pigment particle  110  is enlarged as illustrated in  FIG. 11A  compared to that seen from the side. In a state in which the metal pigment particle  110  is placed on a flat surface (see  FIG. 11B ), the metal pigment particle  110  has a pair of reflective surfaces  110 A (flat surfaces), one and the other of which respectively face upward and downward. As has been described, the metal pigment particles  110  have a flat shape. 
     Although it is not illustrated, the toners of the colors other than gold (toners of the other colors), that is, the toners of yellow (Y), magenta (M), cyan (C), and black (K) include binder resin and pigment particles (for example, organic pigment particles or inorganic pigment particles) other than flat pigment particles. 
     Control 
     As illustrated in  FIGS. 4 and 5 , in some cases a gold toner image  200  that has a high-area-coverage portion  220  having a higher area coverage than that of a low-area-coverage portion  210  having a low area coverage is formed adjacent to the low-area-coverage portion  210  upstream of the low-area-coverage portion  210  by the toner image forming unit  20 V for the special color (V), that is, gold on the upstream side. When such a gold toner image  200  is formed, the controller  70  (see  FIG. 1 ) causes the toner image forming units  20 Y,  20 M,  20 C, and  20 K on the downstream side to transfer a correction toner image  250  so as to superpose the correction toner image  250  on a boundary portion  212  of the low-area-coverage portion  210  that is a boundary with the high-area-coverage portion  220 . The details of this control will be described later. 
     Operation 
     Next, operation according to the present exemplary embodiment is described. 
     Initially, a low-density portion  214  generated in the boundary portion  212  of the low-area-coverage portion  210  that is the boundary with the high-area-coverage portion  220  is described in the case where the gold toner image  200  that has the high-area-coverage portion  220  having a higher area coverage than that of the low-area-coverage portion  210  having a low area coverage is formed adjacent to the low-area-coverage portion  210  on the upstream of the low-area-coverage portion  210  by the toner image forming unit  20 V for gold. The boundary portion  212  and the low-density portion  214  illustrated in  FIGS. 4 and 5  are the same region. 
     This low-density portion  214  (boundary portion  212 ) is an edge defect (void) characteristic of the electrophotographic method caused at an image boundary where the area coverage is steeply increased. This defect is thought to be caused by a pickup effect of the counter charge of the carrier GA. 
     Accordingly, the pickup effect of the counter charge of the carrier GA is described next with reference to  FIGS. 6A to 6C . 
     For clarity of understanding, the outer circumferential surface of the photosensitive drum  21 V, which actually has an arc shape in sectional view, is illustrated as a line in  FIGS. 6A to 6C . Also in  FIGS. 6A to 6C , an arrow R 1  indicates rotation of the photosensitive drum  21 V, an arrow R 2  indicates rotation of the developing roller  242 V, and an arrow E indicates an electric field. 
     A latent image  201  formed on the photosensitive drum  21 V corresponds to the gold toner image  200 , a latent image  211  corresponds to the low-area-coverage portion  210 , a latent image  221  corresponds to the high-area-coverage portion  220 , and a latent image  213  corresponds to the boundary portion  212 . 
     As illustrated in  FIGS. 6A and 6B , when the latent image  221  corresponding to a high area coverage is developed with the developer G, a charge opposite to that of the toner GB, that is, a so-called counter charge remains in the carrier GA. 
     As described above, the developer G (magnetic brush GC) is moved in the same direction as that of the outer circumferential surface of the photosensitive drum  21  at a higher speed than that of the outer circumferential surface in the nip (portion in contact with the photosensitive drum  21 ) (see  FIG. 3 ). 
     Accordingly, referring to  FIG. 6C , the developing roller  242 V holds the developer G having been used to develop the latent image  221  corresponding to the high-area-coverage portion  220  on the upstream side. This developer G passes the high-area-coverage portion  220  and approaches the low-area-coverage portion  210 . When the developer G approaches the low-area-coverage portion  210  on the downstream side as described above, the carrier GA in which the counter charge remains attracts the toner GB in the boundary portion  212  of the previously developed low-area-coverage portion  210  of the photosensitive drum  21 V on the upstream side. This moves back the toner GB from the photosensitive drum  21  to the developing roller  242 V. 
     Thus, the low-density portion (void)  214  is generated in the boundary portion  212  of the low-area-coverage portion  210  that is the boundary with the high-area-coverage portion  220 . 
     Thus, according to the present exemplary embodiment, the toner image forming units  20 Y,  20 M,  20 C, and  20 K on the downstream side transfer the correction toner image  250  so as to superpose the correction toner image  250  on the boundary portion  212  of the low-area-coverage portion  210  that is the boundary with the high-area-coverage portion  220 , that is, the low-density portion  214  of the gold toner image  200  formed by the gold toner image forming unit  20 V on the upstream side as illustrated in  FIG. 4 . 
     Thus, after the transfer onto the recording medium P has been performed, the correction toner image  250  is provided between the boundary portion  212 , that is, the low-density portion  214  and the recording medium P. This may make the image density of the low-density portion  214  (boundary portion  212 ) appropriate (improve the image density), and accordingly, may reduce the likelihood of the low-density portion  214  (reduction of the density in the boundary portion  212 ) being visually recognizable (noticeable). Accordingly, compared to the case where the correction toner image  250  is not transferred, the image quality may be improved. 
     Since a charge amount of the gold toner including the flat metal pigment particles is lower than that of toner including the pigment particles other than metal pigment particles, the carrier GA having a high electrical resistance needs to be used. When the electrical resistance of the carrier GA is high, however, the counter charge is large. This increases the amount of the toner GB moved back to the developing roller  242 V (see  FIG. 6C ), and accordingly, the reduction of the density is increased. Thus, it may be effective to transfer the correction toner image  250  onto the low-density portion  214  of the gold toner image  200  having metallic luster so as to superpose the correction toner image  250  on the low-density portion  214 , thereby improving the image quality of the gold toner image  200  having metallic luster. 
     The area coverages of yellow, magenta, cyan, and black correction toner image components respectively formed by the toner image forming units  20 Y,  20 M,  20 C, and  20 K are adjusted so that the color of the correction toner image  250  becomes close to the color of the boundary portion  212 , that is, the low-density portion  214  of the low-area-coverage portion  210  with the high-area-coverage portion  220  on the upstream side in the gold toner image  200 . 
     Specifically, when the area coverage of the low-area-coverage portion  210  of the gold toner image  200  is M 1 , the area coverage of the yellow correction toner image component is M 2 , the area coverage of the magenta correction toner image component is M 3 , the area coverage of the cyan correction toner image component is M 4 , and the area coverage of the black correction toner image component is M 5 , M 2  is set to from M 1 −15% to M 1 +5%, that is, (M 1 −15%)≦M 2 ≦(M 1 +5%), M 3  is set to from 13.3 to 25% of M 2 , that is, (M 2 ×13.3%)≦M 3 ≦(M 2 ×25%), and M 4  and M 5  are set to 0%. The upper limit values of M 2  and M 3  are 100%, and the lower limit values of M 2  and M 3  are 0%. 
     Furthermore, the width of the correction toner image  250  in a transport direction (arrow A direction) is set to 0.5 to 1.0 mm. 
     According to the present exemplary embodiment, M 2  is set to M 1 −5% (M 2 =M 1 −5%), M 3  is set to ⅙ (1.67%) of M 2 , that is, (M 3 =M 2 /6), and the width of the correction toner image  250  in the transport direction (arrow A direction) is set to 1.0 mm. 
     Furthermore, the degree of reduction of the density of the boundary portion  212  (low-density portion  214 ) varies depending on the difference in the area coverage between the low-area-coverage portion  210  and the high-area-coverage portion  220  of the gold toner image  200  and the area coverage values of the low-area-coverage portion  210  and the high-area-coverage portion  220  of the gold toner image  200 . Thus, M 2  and M 3  of the correction toner image  250  may be appropriately adjusted in accordance with the area coverages of the low-area-coverage portion  210  and the high-area-coverage portion  220  of the gold toner image  200 . Such adjustment may be performed by any method. For example, M 2  and M 3  corresponding to the area coverages of the low-area-coverage portion  210  and the high-area-coverage portion  220  of the gold toner image  200  may be obtained in advance and stored in the controller  70 . 
     Here, outer end portions of the developing roller  242  in the axial direction extend further to the outside than end portions of the electrostatic latent image on the photosensitive drum  21  in the axial direction. Thus, the toner GB is moved to and attracted to end portions of the electrostatic latent image in the axial direction also from the outer end portions of the developing roller  242 . Accordingly, the amount of toner tends to increase at the end portions than in a central portion of the electrostatic latent image in the axial direction. Thus, the reduction of the density (void) is less likely to occur in end portions than in a central portion of the boundary portion  212  of the gold toner image  200  in the axial direction (end portions in the width direction D (see  FIG. 5 )). In other words, the degree of reduction of the density is smaller in the end portions than in the central portions of the boundary portion  212  of the gold toner image  200  in the axial direction. 
     Accordingly, the correction toner image  250  may be set such that the area coverage of the correction toner image  250  is lower near end portions (end portions in the width direction (see  FIG. 5 )) than in a central portion of the correction toner image  250  in the axial direction. With such a setting, the image density of the end portions of the boundary portion  212  of the gold toner image  200  in the axial direction may be more appropriately corrected, and accordingly, the image quality may be improved. 
     It is noted that the case of “the area coverage . . . is lower” may be a case of 0% area coverage, that is, a case where parts of the correction toner image at (near) end portions in the axial direction are not transferred. In other words, the length of the correction toner image  250  in the axial direction (width direction D (see  FIG. 5 )) may be shorter than the length of the boundary portion  212 . 
     Furthermore, since the movement of the toner GB back to the developing roller  242 V (see  FIG. 6C ) increases toward the high-area-coverage portion  220  side (upstream side) in the low-density portion  214  of the gold toner image  200 , the reduction of the density increases toward the high-area-coverage portion  220 . Accordingly, the correction toner image  250  may be set such that the area coverage of the correction toner image  250  increases from the low-area-coverage portion  210  side (downstream side) toward the high-area-coverage portion  220  side (upstream side). With such a setting, the image density of the low-density portion  214  (boundary portion  212 ) of the gold toner image  200  may be appropriately corrected, and accordingly, the image quality may be improved. 
     Experiments 
     Next, experiments are described. The experiments are performed with the image forming apparatus  10  according to the present exemplary embodiment so as to obtain the area coverages M 2  and M 3  of the yellow and magenta correction toner image components of the correction toner image and the length of the correction toner image in the transport direction. Each of the experiments is performed in the environment where the temperature is 21° C. and the humidity is 10% RH. 
     First Experiment 
     The image quality of the low-density portion  214  (boundary portion  212 ) is visually checked while the area coverage M 2  of the yellow correction toner image component of the correction toner image  250  is varied. The area coverages of the high-area-coverage portion  220  and the low-area-coverage portion (halftone)  210  of the gold toner image  200  are respectively set to 100% and 40%. 
     A table of  FIG. 7  lists evaluation results when the area coverage M 2  of the yellow correction toner image component of the correction toner image  250  is values obtained by adding −20%, −15%, −10%, −5%, 0%, +5%, and +10% to the area coverage M 1  of the low-area-coverage portion  210  of the gold toner image  200 . In the table, the evaluation result “A” indicates that the low-density portion  214  (boundary portion  212 ) is almost visually unrecognized or visually unrecognized, and the evaluation result “B” indicates that the low-density portion  214  (boundary portion  212 ) is visually recognized and noticeable. 
     The area coverage M 3  of the magenta correction toner image component is set to ⅙ (1.67%) of M 2 . The length of the correction toner image  250  in the transport direction is set to 1.0 mm. 
     As illustrated in this table of  FIG. 7 , when M 2  is the values obtained by adding −15% to +5% to M 1 , the density of the low-density portion (void)  214  is increased. This may improve the image quality. 
     Second Experiment 
     The image quality of the low-density portion  214  (boundary portion  212 ) is visually checked while the length of the correction toner image  250  in the transport direction is varied. The area coverages of the high-area-coverage portion  220  and the low-area-coverage portion (halftone)  210  of the gold toner image  200  are respectively set to 100% and 40%. 
     A table of  FIG. 8  illustrates evaluation results when the length of the correction toner image  250  in the transport direction is set to 0.5 mm, 1.0 mm, and 1.5 mm. In the table, the evaluation result “A” indicates that the low-density portion  214  (boundary portion  212 ) is almost visually unrecognized or visually unrecognized, and the evaluation result “B” indicates that the low-density portion  214  (boundary  212 ) or the correction toner image  250  is visually recognized and noticeable. 
     The area coverage M 2  of the yellow correction toner image component is set to 30% (M 1 −10%), and M 3  is set to ⅙(1.67%) of M 2 . 
     As illustrated in this table of  FIG. 8 , when the length is 0.5 mm or 1.0 mm, the image density of the low-density portion (void)  214  is increased. This may improve the image quality. When the length is 1.5 mm, the image density of the low-density portion (void)  214  is increased. However, the correction toner image  250  extends toward the downstream side beyond the low-density portion  214 . Thus, the correction toner image  250  may become noticeable and the image quality may be degraded. 
     Third Experiment 
     When the area coverage of the high-area-coverage portion  220  of the gold toner image  200  is 100%, the color of the low-density portion  214  (boundary portion  212 ) of the low-area-coverage portion (halftone)  210  and the color of the correction toner image  250  are visually compared. Although the colors are visually compared according to the present exemplary embodiment, the colors may be measured by a measuring device such as a colorimeter and compared. 
     The area coverage M 1  of the low-area-coverage portion  210  of the gold toner image  200  is set to 10%, 20%, 30%, and 40%. 
     The area coverage M 2  of the yellow correction toner image component is set to the same values as those of M 1 , that is, 10%, 20%, 30%, and 40%. The correction toner image  250  is created by superposing the magenta correction toner image component the area coverage of which is 1%, 2%, 3%, 4%, 5%, 6%, 8%, and 10% on the yellow correction toner image component. The color of the low-density portion  214  of the gold toner image  200  is visually compared. The experiment is not performed on some of the combinations of the area coverages. 
     As illustrated in a graph of  FIG. 9 , when the area coverage M 3  of the magenta correction toner image component is higher than 25% of the area coverage M 2  of the yellow correction toner image component, magenta (M) is noticeable, and when M 3  is lower than 13.3% of the area coverage M 2 , yellow is noticeable. Accordingly, when the area coverage M 3  of the magenta correction toner image component is from 13.3 to 25% of M 2 , a color close to that of the low-density portion  214  of the gold toner image  200  is obtained. 
     Variations 
     Exemplary embodiments of the present invention are not limited to the aforementioned exemplary embodiment. 
     For example, although the toner GB of the toner image forming unit  20 V for the special color (V) as an example of the toner image forming unit on the upstream side is the gold toner, this is not limiting. This toner GB of the toner image forming unit  20 V may be silver toner that includes metal pigment particles that each serve as an example of the flat pigment. Alternatively, the toner GB of the toner image forming unit  20 V may be, for example, orange, violet, green, light cyan, light magenta, white, or transparent toner that includes pigment particles other than flat pigment particles. 
     As is the case with the gold toner, reduction of the density in the boundary portion is increased when the silver toner that includes the flat metal pigment particles is used. Accordingly, it may also be effective to transfer the correction toner image onto the boundary portion (low-density portion) so as to superpose the correction toner image on the boundary portion, thereby improving the image quality. 
     A toner image of the special color (V) of the toner image forming unit  20 V on the upstream side other than gold may also have a similar low-density portion. For a correction toner image in this case, the area coverages of the correction toner image components to be transferred by the toner image forming units  20 Y,  20 M,  20 C, and  20 K may be appropriately adjusted so that the color of the correction toner image becomes close to the color of the boundary portion (low-density portion) in a low-area-coverage portion of the toner image formed of the toner for the special color (V) and transferred onto the intermediate transfer belt  31 . 
     Furthermore, when a toner image formed by the toner image forming units  20 Y,  20 M, or  20 C has a similar low-density portion, correction toner may be transferred by the toner image forming unit  20  or the toner image forming units  20  on the downstream side. Also in this case, the area coverage of the correction toner image component or the area coverages of the correction toner image components to be transferred by the toner image forming unit  20  or the toner image forming units  20  on the downstream side may be appropriately adjusted so that the color of the correction toner image becomes close to the color of a boundary portion (low-density portion) in a low-area-coverage portion of the toner image. 
     In short, it is sufficient that, when the high-area-coverage portion which has a higher area coverage than that of the low-area-coverage portion having a low area coverage is formed adjacent to the low-area-coverage portion on the upstream side in a toner image transferred onto the intermediate transfer body by the toner image forming unit on the upstream side, control be performed so as to transfer the correction toner image by the toner image forming unit or the toner image forming units disposed downstream of the toner image forming unit on the upstream side so that the correction toner image is superposed on the boundary portion of the low-area-coverage portion that is the boundary with the high-area-coverage portion. 
     Furthermore, the structure of the image forming apparatus is not limited to that of the aforementioned exemplary embodiment. The image forming apparatus may have any one of a variety of structures. For example, instead of the intermediate transfer belt, any one of other intermediate transfer bodies such as an intermediate transfer roller may be used. Instead of transferring an image onto the intermediate transfer body through the first transfer and transferring onto the recording medium P through the second transfer, the image may be directly transferred onto the recording medium P without using the intermediate transfer body. 
     Alternatively, the image forming apparatus may be a so-called rotary developing-type image forming apparatus that includes a rotary developing mechanism in which the developing devices are disposed along a circumference. The developing devices are sequentially changed over by rotating the rotary developing mechanism, thereby causing each of the developing devices to face a corresponding one of photo sensitive bodies (image holding bodies) on which an electrostatic latent image for a corresponding one of colors is formed and to develop the electrostatic latent image with the toner of the corresponding one of the colors. In this case, when a toner image to be corrected is formed by developing the low-area-coverage portion having a low area coverage and then developing the high-area-coverage portion having a higher area coverage than that of the low-area-coverage portion disposed adjacent to the low-area-coverage portion by using a particular one of the developing devices, control may be performed so that the correction toner image is transferred by the other developing device or the other developing devices so as to be superposed on the boundary portion of the low-area-coverage portion that is the boundary with the high-area-coverage portion in the toner image to be corrected. 
     When this is described from another viewpoint, with respect to the order of development, when the toner image to be corrected is formed by developing the low-area-coverage portion having a low area coverage before and then developing the high-area-coverage portion having a higher area coverage than that of the low-area-coverage portion disposed adjacent to the low-area-coverage portion, the control may be performed so that the correction toner image is transferred so as to be superposed on the boundary portion of the low-area-coverage portion that is the boundary with the high-area-coverage portion in the toner image to be corrected after the toner image to be corrected has been transferred onto a transfer target. 
     The foregoing description of the exemplary embodiment of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiment was chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.