Patent Publication Number: US-8995882-B2

Title: Image forming apparatus with first and second print engines

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electrophotographic image forming apparatus that forms an image on a recording medium. 
     2. Description of the Related Art 
     A color electrophotographic printer is known which includes a plurality of image forming units, each unit including a photoconductive drum, a charging unit, an exposing unit, and a developing unit. One such apparatus is a tandem color printer disclosed in Japanese Patent Application No. 2011-39378. Black (K), yellow (Y), magenta (M), and cyan (C) image forming units are aligned along the transport path of a print medium. As the print medium passes through the image forming units in sequence, toner images of corresponding colors are transferred onto an intermediate transfer belt in registration. The toner images are then transferred onto print paper fed in timed relation with the formation of the respective toner images. 
     When a color image is formed on a recording medium having a color other than white, a white toner may be used to hide the color of the recording medium. However, a color toner transferred onto the white toner can be mixed with the white toner, impairing a desired image quality. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an image forming apparatus capable of offering a quality image. 
     An image forming apparatus includes a first print engine and a second print engine. The first print engine forms a first image formed of a first toner having a first average diameter. The first image is transferred onto a recording medium. The second print engine forms a second image formed of a second toner having a second average diameter larger than the first average diameter. The second image is transferred onto the first image in registration. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limiting the present invention, and wherein: 
         FIG. 1  is a schematic diagram illustrating an image forming apparatus according to a first embodiment; 
         FIG. 2  is a block diagram illustrating the respective functions of the image forming apparatus; 
         FIGS. 3A-3C  illustrate how a white toner and a cyan toner are transferred when the white toner has a larger average particle diameter than the cyan toner; 
         FIGS. 4A-4C  illustrate how the white toner and the cyan toner are transferred when the white toner has a smaller average particle diameter than the cyan toner; 
         FIG. 5  is a table that lists the experimental results, illustrating the relationship between the average particle diameter of the white toner and the changes in shade of color due to the mixture of the white toner and cyan toner; 
         FIG. 6  lists experimental changes in the shade of color when the cyan toner having an average particle diameter of 6.8 μm is mixed with the white toners having different average particle diameters; 
         FIGS. 7A and 7B  illustrate the distribution of the toner particle diameters; 
         FIGS. 8A-8D  illustrate how toners are transferred onto a transfer belt and paper; 
         FIG. 9  illustrates the outline of the configuration of an image forming apparatus that employs a transparent toner; and 
         FIG. 10  illustrates the outline of a direct transfer image forming apparatus. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will be described in detail by way of preferred embodiments with reference to the accompanying drawings. 
     First Embodiment 
     Configuration of Image Forming Apparatus 
       FIG. 1  is a schematic diagram illustrating an image forming apparatus  1  according to a first embodiment.  FIG. 2  is a block diagram illustrating the respective functions of the image forming apparatus  1 . The image forming apparatus  1  forms images by electrophotography, and takes the form of a printer that prints an image on a recording medium or print paper P in accordance with print data received from an external apparatus. The print data includes that of a white image as a background. 
     Referring to  FIG. 1 , the image forming apparatus  1  includes five independent process units, which print black (K), yellow (Y), magenta (M), cyan (C), and white (W) images, respectively. The process units include print engines  10 K,  10 Y,  10 M,  10 C, and  10 W, respectively. The print engines  10 K,  10 Y,  10 M,  10 C, and  10 W are aligned along an intermediate transfer belt  32 . 
     The black print engine  10 K includes a photoconductive drum  11 K as an image bearing body, a charging roller  12 K, an exposing unit  20 K, a developing unit  13 K, a neutralizing light source  14 K as a neutralizer, and a toner cartridge  15 K that holds a black toner therein. The photoconductive drum  11 K bears an electrostatic latent image formed thereon. The charging roller  12 K charges the surface of the photoconductive drum  11 K. The exposing unit  20 K, illuminates the charged surface of the photoconductive drum  11 K to form an electrostatic latent image. The developing unit  13 K supplies the black toner to the electrostatic latent image to develop the electrostatic latent image, formed on the surface of the photoconductive drum  11 K, with the black toner into a black toner image. The developing unit  13 K includes a developing roller  16 K, a developing blade  17 K, and a supply-roller  18 K. The developing roller  16 K supplies the black toner to the electrostatic latent image on the photoconductive drum  11 K. The developing blade  17 K forms a thin layer of the black toner on the developing roller  16 K. The supply roller  18 K supplies the black toner to the developing roller  16 K. The neutralizing light source  14 K illuminates the surface of the photoconductive drum  11 K after transferring the toner image onto the print paper P. The toner cartridge  15 K holds the black toner therein, and supplies the black toner into the developing unit  13 K. 
     An LED head  20 K as the exposing unit is disposed above the photoconductive drum  11 K and parallels the photoconductive drum  11 K. The LED head  20 K illuminates the charged surface of the photoconductive drum  11 K to form an electrostatic latent image in accordance with the print data. The LED head  20 K includes a printed circuit board on which LED arrays, driver ICs that drive the LED arrays, a shift register that holds image data, and a SELFOC lens array that focuses the light from the LED arrays on the charged surface of the photoconductive drum  11 K. 
     Likewise, the print engines  10 Y,  10 M,  10 C, and  10 W include photoconductive drums  11 Y,  11 M,  11 C, and  11 W, charging rollers  12 Y,  12 M,  12 C, and  12 W, developing units  13 Y,  13 M,  13 C, and  13 W, neutralizing light sources  14 Y,  14 M,  14 C, and  14 W, toner cartridges  15 Y,  15 M,  15 C, and  15 W, respectively. The developing units  13 Y,  13 M,  13 C, and  13 W include developing rollers  16 Y,  16 M,  16 C, and  16 W, developing blades  17 Y,  17 M,  17 C, and  17 W, and supply-rollers  18 Y,  18 M,  18 C, and  18 W, respectively. LED heads  20 Y,  20 ,  20 C, and  20 W are disposed over the print engines  10 Y,  10 M,  10 C, and  10 W, respectively. The LED heads  20 Y,  20 M,  20 C, and  20 W receive yellow, magenta, cyan, and white image signals, respectively, and illuminate the photoconductive drums  11 Y,  11 M,  11 C, and  11 W in accordance with the yellow, magenta, cyan, and white image signals, respectively, thereby forming electrostatic latent images of the respective colors. The term “color” refers to a chromatic color other than black and white. 
     The respective color toners contain polyester resin, internal additives, and an external additive. Polyester resin serves as a binder resin. The internal additives are a charge control agent, a toner release agent, and a colorant. The external additive is, for example, silica. The toners according to the embodiment are pulverized toners. Instead, the toners may be polymerized toners. 
     Primary transfer rollers  31 K,  31 Y,  31 M,  31 C, and  31 W are disposed under the print engines  10 K,  10 Y,  10 M,  10 C, and  10 W, and parallel the photoconductive drums  11 K,  11 Y,  11 M,  11 C, and  11 W, respectively, so that the intermediate transfer belt  32  is sandwiched between the photoconductive drums  11 K,  11 Y,  11 M,  11 C, and  11 W and the corresponding primary transfer rollers  31 K,  31 Y,  31 M,  31 C, and  31 W. The intermediate transfer belt  32  takes the form of an endless belt formed of, for example, a semiconductive plastic film having a smooth, flat surface, and serves as an image bearing body on which the toner images are carried. The intermediate transfer belt  32  is disposed about a drive roller  33 , a driven roller  34 , and a tension roller  36  under a predetermined tension. A belt motor  113  ( FIG. 2 ) drives the drive roller  33  in rotation, so that the intermediate transfer belt  32  runs in a direction shown by arrow E. The upper half of the intermediate transfer belt  32  is sandwiched between the photoconductive drums  11 K,  11 Y,  11 M,  11 C, and  11 W and the corresponding primary transfer rollers  31 K,  31 Y,  31 M,  31 C, and  31 W. The primary transfer rollers  31 K,  31 Y,  31 M,  31 C, and  31 W receive a dc voltage from a primary transfer voltage generator  124  ( FIG. 2 ), thereby transferring the toner images formed on the photoconductive drums  11 K,  11 Y,  11 M,  11 C, and  11 W onto the intermediate transfer belt  32 . 
     A paper feeding mechanism  50  is disposed at a lower portion of the image forming apparatus  1 , and feeds the paper P into a transport path  40  (enclosed by dotted lines in  FIG. 1 ). The paper feeding mechanism  50  includes a paper cassette  51 , a registry roller  52 , a pinch roller  53 , a hopping roller  54 , a guide  55 , and a paper sensor  56 . The hopping roller  52  advances the paper P from the paper cassette  51 . The pinch roller  53  cooperates with the registry roller  54  to correct the skew of the paper P. The registry roller  54  receives the paper P from the hopping roller  52 , and then feeds the paper P to a contact area between the tension roller  36  and a secondary transfer roller  35 . The guide  55  guides the paper P to the tension roller  36 . The paper sensor  56  senses the paper P when the paper P arrives at the nip area formed between the pinch roller  53  and the registry roller  54 . 
     The secondary transfer roller  35  is located downstream of the paper feeding mechanism  50 . The secondary transfer roller  35  faces the tension roller  36  so that the intermediate transfer belt  32  is sandwiched between the tension roller  36  and the secondary transfer roller  35 . The tension roller  36  pushes the intermediate transfer belt  32  against the secondary transfer roller  35 , thereby defining a secondary transfer point between the intermediate transfer belt  32  and the secondary transfer roller  35 . When the secondary transfer roller  35  is driven in rotation by a secondary transfer motor  115  ( FIG. 2 ), the tension roller  36  is driven in rotation due to the friction between the tension roller  36  and the intermediate transfer belt  32 . The secondary transfer roller  35  receives a predetermined dc voltage from the secondary transfer voltage generator  125  ( FIG. 2 ), thereby transferring the toner image on the intermediate transfer belt  32  onto the paper P. A cleaning blade  37  is formed of a flexible rubber material or a plastic material so that the cleaning blade  37  scrapes the residual toner from the secondary transfer roller  35  into a waste toner tank  38 . 
     A sensor  41 , a guide  42 , and a fixing mechanism  60  are located downstream of the secondary transfer roller  35 . The sensor  41  watches for wrapping of the paper P around the secondary transfer roller  35  and failure of the paper P to leave the intermediate transfer belt  32 . The guide  42  guides the paper P passing through the secondary transfer point, defined between the intermediate transfer belt  32  and the secondary transfer roller  35 , to the fixing mechanism  60 . 
     The fixing mechanism  60  includes a heat roller  61  and a pressure roller  62  that presses the heat roller  61 , and fixes the toner image on the paper P. The heat roller  61  is driven in rotation by a heater motor  116  ( FIG. 2 ) while the pressure roller  62  follows the heat roller  61  due to the friction between the heat roller  61  and the pressure roller  62 . The heat roller  61  incorporates a heater  63  in the form of a halogen lamp. A thermistor  64  is disposed in the vicinity of the heat roller  61 , and monitors the surface temperature of the heat roller  61 . 
     A sensor  43  is disposed downstream of the fixing mechanism  60  with respect to the paper transport path. The sensor  43  watches for paper jam and wrapping of the paper P around the heat roller  61 . Guides  45  are disposed downstream of the sensor  43  and guide the paper P to a stacker  44  located at the upper portion of the image forming apparatus  1 , the printed paper P being discharged onto the stacker  44 . 
     A cleaning blade  71  contacts the surface of the intermediate transfer belt  32 , and removes the toner that failed to be transferred and remains on the intermediate transfer belt  32 . The cleaning blade  71  is disposed so that the intermediate transfer belt  32  is sandwiched between the cleaning blade  71  and a roller  72 . The cleaning blade  71  is formed of a flexible rubber material or a plastic material, and scrapes the residual toner off the intermediate transfer belt  32  into a waste toner tank  73 . 
     The configuration of the control circuit of the image forming apparatus  1  according to the first embodiment will be described. Referring to  FIG. 2 , the image forming apparatus  1  includes a host interface  101 , a command/image processing section  102 , an LED head interface  103 , a mechanism controller  104 , and a high voltage controller  120 . 
     The host interface  101  performs a physical hierarchical interface with a host computer (not shown), and includes a connector and a communication chip. 
     The command/image processing section  102  parses the commands received from the host computer, and interprets the image data, i.e., renders the image data into bit map data. The command/image processing section  102  includes a microprocessor, a random access memory (RAM) and hardware specially designed for rendering the image data into the bit map data, and performs the overall control of the image forming apparatus  1 . 
     The LED head interface  103  includes a semi customized large scale integrated circuit (LSI) and a RAM, and processes the bit map data received from the command/image processing section  102  so that the LED heads  20 K,  20 Y,  20 M,  20 C, and  20 W can work with the bit map data. 
     The mechanism controller  104  performs the control of the respective portions of the print engines  10 K,  10 Y,  10 M,  10 C and  10 W of the image forming apparatus  1 . In accordance with the commands from the command/image processing section  102  and the outputs of the paper sensor  56 , sensor  41 , and sensor  43 , the mechanism controller  104  controls a hopping motor  111 , registry motor  112 , belt motor  113 , drum motor  114 , secondary transfer motor  115 , heater motor  116 , heater  63 , and high voltage controller  120 , thereby controlling the mechanism of the print engines and the high voltage power supply. 
     The hopping motor  111 , registry motor  112 , belt motor  113 , and secondary transfer motor  115  drive the hopping roller  52 , registry roller  54 , drive roller  33 , and secondary transfer roller  35  in rotation. The drum motor  114  drives the print engines  10 K,  10 Y,  10 M,  10 C, and  10 W to operate. The heater motor  116  drives the heat roller  61 . Each motor is driven by a corresponding driver. The heater  63  incorporates a halogen lamp therein. The thermistor  64  is disposed in the vicinity of the surface of the heat roller  61 . The mechanism controller  104  performs the temperature control of the heat roller  61  in accordance with the output of the thermistor  64 . 
     The high voltage controller  120  is in the form of a microprocessor or customized LSI, and controls a charging voltage generator  121 , a supply roller voltage generator  122 , a developing roller voltage generator  123 , the primary transfer voltage generator  124 , and the secondary transfer voltage generator  125 . 
     The charging voltage generator  121  generates or does not generate the charging voltages that should be supplied to the charging rollers  12 K,  12 Y,  12 M,  12 C and  12 W in accordance with the command from the high voltage controller  120 . 
     In response to the command received from the high voltage controller  120 , the supply roller voltage generator  122  generates the supply roller voltage that should be supplied to the supply-rollers  18 K,  18 Y,  18 M,  18 C, and  18 W. 
     In response to the command received from the high voltage controller  120 , the developing roller voltage generator  123  generates the developing voltages that should be supplied to the developing rollers  16 K,  16 Y,  16 M,  16 C, and  16 W, respectively. 
     In response to the command received from the high voltage controller  120 , the primary transfer voltage generator  124  generates primary transfer voltages that should be supplied to the primary transfer rollers  31 K,  31 Y,  31 M,  31 C, and  31 W, respectively. 
     In response to the command received from the high voltage controller  120 , the secondary transfer voltage generator  125  generates the secondary transfer voltage that should be supplied to the secondary transfer roller  35 . 
     {Operation of Image Forming Apparatus} 
     A description will be given of the operation of the image forming apparatus  1 . Upon reception of the image data from the host computer via the host interface  101 , the command/image processing section  102  commands to initiate warming up of the fixing mechanism  60  of the mechanism controller  104 , and renders the image data into the bit map data on a page-by-page basis for each color. Upon reception of a warm-up command from the command/image processing section  102 , the heater motor  116  drives the heat roller  61 . The mechanism controller  104  then adjusts the fixing temperature by turning on and off the heater  63  in accordance with the output of the thermistor  64 . The command/image processing section  102  starts a printing operation when the fixing temperature reaches a preset temperature high enough for fixing the toner image on the paper P. 
     The command/image processing section  102  controls the mechanism controller  104 , which in turn controls the belt motor  113 , drum motor  114 , and secondary transfer motor  115 , thereby driving the drive roller  33 , various rollers of print engines  10 K,  10 Y,  10 M,  10 C, and  10 W, and secondary transfer roller  35 . 
     Concurrently with the control of the belt motor  113 , drum motor  114 , and secondary transfer motor  115 , the mechanism controller  104  sends a command to the high voltage controller  120 , which in turn drives the charging voltage generator  121 , supply roller voltage generator  122 , and developing roller voltage generator  123  to supply high bias voltages to the print engines  10 K,  10 Y,  10 M,  10 C, and  10 W, respectively. 
     A description will be given of the operation of the print engines  10 K,  10 Y,  10 M,  10 C, and  10 W. Each of the print engines  10 K,  10 Y,  10 M,  10 C, and  10 W may be substantially identical; for simplicity, only the print engine  10 K will be described, it being understood the remaining print engines  10 Y,  10 M,  10 C, and  10 W may work in a similar fashion. 
     The high voltage controller  120  supplies a charging voltage of −1100 V to the charging roller  12 K, thereby charging the surface of the photoconductive drum  11 K to −600 V. The high voltage controller  120  supplies voltages of −200 V and −250 V to the developing roller  16 K and supply roller  18 K, respectively, so that an electric field is developed in the vicinity of the nip area formed between the developing roller  16 K and supply roller  18 K. The black toner supplied from the toner cartridge  15 K is triboelectrically charged due to the friction between the developing roller  16 K and the supply roller  18 K and the polarity of voltages applied to the developing roller  16 K and the supply roller  18 K. In the present embodiment, the toner is negatively charged. The negatively charged toner is deposited to the developing roller  16 K by the Coulomb force due to the electric field in the direction from the developing roller  16 K to the supply roller  18 K. As the developing roller  16 K rotates, the toner on the developing roller  16 K is brought into contact with the developing blade  17 K, which in turn forms a thin toner layer having a uniform thickness on the developing roller  16 K. As the developing roller  16 K further rotates, the thin toner layer is brought into contact with the electrostatic latent image formed on the photoconductive drum  11 K. 
     In the mean time, the command/image processing section  102  sends the bit map data to the LED head interface  103  on a page-by-page basis. The LED head interface  103  drives the LEDs of the LED head  20 K to be energized in accordance with the bit map data received from the command/image processing section  102 , thereby forming an electrostatic latent image on the photoconductive drum  11 K. The charges in illuminated areas have been dissipated so that the illuminated areas have a potential of about −50V. 
     As the photoconductive drum  11 K rotates, the electrostatic latent image moves into contact with the thin toner layer formed on the developing roller  16 K. Since the toner on the developing roller  16 K has been negatively charged, the toner is attracted only to the areas illuminated by the LED head  20 K. Thus, the electrostatic latent image is developed with the black toner. 
     Next, a description will be given of a primary transfer operation in which the toner images formed on the photoconductive drums  11 K,  11 Y,  11 M,  11 C and  11 W are transferred onto the intermediate transfer belt  32 , and a secondary transfer operation in which the toner image on the intermediate transfer belt  32  is transferred onto the paper P. 
     As the photoconductive drums  11 K,  11 Y,  11 M,  11 C, and  11 W rotate, the toner images on the photoconductive drums  11 K,  11 Y,  11 M,  11 C, and  11 W arrive at corresponding transfer points defined between the intermediate transfer belt  32  and the photoconductive drums  11 K,  11 Y,  11 M,  11 C, and  11 W. The mechanism controller  104  then sends a command to the high voltage controller  120 , commanding to generate the primary transfer voltages in timed relation with the arrival of the respective toner images at the transfer points. In response to the command, the high voltage controller  120  drives the primary transfer voltage generator  124  to supply the primary transfer voltages to the primary transfer rollers  31 K,  31 Y,  31 M,  31 C, and  31 W. The primary transfer voltage according to the present embodiment is selected to be +3000 V. The primary transfer voltages applied to the transfer rollers  31 K,  31 Y,  31 M,  31 C, and  31 W develop electric fields in the direction from the transfer rollers  31 K,  31 Y,  31 M,  31 C, and  31 W to the corresponding photoconductive drums  11 K,  11 Y,  11 M,  11 C, and  11 W, so that the negatively charged toner images of the corresponding colors are transferred one over the other onto the intermediate transfer belt  32  in sequence. 
     Before the toner image on the intermediate transfer belt  32  arrives at the secondary transfer nip formed between the secondary transfer roller  35  and the tension roller  36 , the mechanism controller  104  causes the hopping motor  111  to drive the hopping roller  52  into rotation, thereby feeding a sheet of the paper P from the paper cassette  51  into the nip between the pinch roller  53  and registry roller  54 . The mechanism controller  104  monitors the output of the paper sensor  56  to detect when the leading edge of the paper P arrives at the nip between the pinch roller  53  and the registry roller  54 . Once the leading edge of the paper P is detected, the mechanism controller  104  stops the hopping motor  111 . 
     The mechanism controller  104  causes the registry motor  112  to drive the pinch roller  53  and the registry roller  54  into rotation when the toner image on the intermediate transfer belt  32  arrives at the nip formed between the secondary transfer roller  35  and the tension roller  36 . The guide  55  guides the paper P to the nip where the secondary transfer takes place. 
     The mechanism controller  104  sends a command to the high voltage controller  120 , commanding to generate the secondary transfer voltage when the toner image on the intermediate transfer belt  32  arrives at the secondary transfer nip. In response to the command, the high voltage controller  120  drives the secondary transfer voltage generator  125  to supply the secondary transfer voltage to the secondary transfer roller  35 . In the present embodiment, the secondary transfer voltage is selected to be +2500 V. Since the toner on the intermediate transfer belt  32  has been negatively charged, the toner image is attracted to the paper P due to the electric field developed across the secondary transfer roller  35  and the tension roller  36 . 
     After passing through the secondary transfer roller  35 , the paper P leaves the intermediate transfer belt  32 , being guided by the guide  42  to the fixing mechanism  60 . When the paper P is being guided, the mechanism controller  104  monitors the output of the sensor  41  to detect whether the paper P has wrapped around the secondary transfer roller  35  and whether the paper P has successfully left the intermediate transfer belt  32 . 
     When the paper P arrives at the fixing mechanism  60 , the paper P is pulled in between the heat roller  61  and the pressure roller  62  which have reached a predetermined temperature, so that the toner image on the paper P is fused by heat and pressure into the paper P. 
     After fixing, the paper P is guided by the guides  45 , and is discharged by discharging rollers (not shown) onto the stacker  44 . When the paper P is being guided, the mechanism controller  104  monitors the output of the sensor  43  to detect whether the paper P has become jammed or has wrapped around the heat roller  61 . 
     Concurrently with the fixing operation, the cleaning blade  71  scrapes the residual toner from the intermediate transfer belt  32  into the waste toner tank  73 . 
     After completion of all processes, the mechanism controller  104  causes the belt motor  113 , drum motor  114 , and secondary transfer motor  115  to stop, and sends a command to the high voltage controller  120 , commanding the charging voltage generator  121 , supplying roller voltage generator  122 , and developing roller voltage generator  123  to stop supplying the high bias voltages to the rollers of the print engines  10 K,  10 Y,  10 M,  10 C, and  10 W. The mechanism controller  104  causes the heater motor  116  and heater  63  to stop, thereby completing the printing operation. 
     {Toners According to Invention} 
     The toner according to the present invention will be described. A white solid toner image is formed as a background on the entire surface of the print paper P, and at least one of black, yellow, magenta, and cyan images is formed on the white toner image. The solid white toner image serves to cover the color of the paper P other than white. 
     If a color toner image other than a white toner image is mixed with the white toner image, a desired shade of color is not obtained. 
       FIGS. 3A-3C  illustrate how the white toner and the cyan toner are transferred when the white toner has a larger average particle diameter than the cyan toner. 
     With reference to  FIGS. 3A-3C , a description will be given of a case in which the white toner has a larger particle diameter than the color toner, e.g., cyan, other than white toner. 
     Referring to  FIG. 3A , the cyan toner Tc is transferred onto the relatively smooth surface of the intermediate transfer belt  32  having small surface relief heights (i.e., ridges and furrows). Subsequently, the white toner Tw is transferred onto the layer of the cyan toner Tc, as shown in  FIG. 3B . 
     Referring to  FIG. 3C , the cyan toner Tc and the white toner Tw formed on the intermediate transfer belt  32  are transferred onto the paper P. As a result, the white toner layer is first transferred onto the paper P and then the cyan toner layer is transferred onto the white toner layer, the white toner layer serving to cover the color of the paper P. 
     Due to manufacturing errors and fibers of the material, the paper P has relatively large surface relief heights. For example, the surface relief heights of the paper P are larger than those of the intermediate transfer belt  32 . When the toner images are transferred from the intermediate transfer belt  32  onto the paper P, if the average particle diameter of the white toner Tw is not sufficiently small as compared to the surface relief heights of the paper P, the particles of the white toner Tw cannot sufficiently fill the furrows in the paper P, failing to provide a sufficiently smooth surface of the layer of the white toner Tw. Since the cyan toner Tc has a smaller average particle diameter than the white toner Tw, the particles of the cyan toner Tc tend to enter the gaps among the particles of the white toner Tw. The larger the average particle diameter of the white toner Tw, the larger the gaps among the white toner particles, so that more of the cyan toner particles enter an area A, enclosed in dotted line in  FIG. 3C . As a result, some of the cyan toner particles get under the white toner particles, so that some of the particles of the cyan toner are mixed with those of the white toner Tw and are therefore difficult to be deposited on the ridges of the layer of the white toner Tw, causing the white toner particles to become exposed as shown by arrows B in  FIG. 3C . 
     As described above, if the cyan toner Tc has a smaller average particle diameter than the white toner Tw, the cyan toner particles tend to enter the gaps among the white toner particles, so that the cyan toner particles are covered with the white toner particles. As a result, the cyan toner image has a lighter shade of color than it should have. The white toner having a large particle diameter fails to provide a white toner layer having a smooth surface, preventing the cyan toner Tc from being transferred uniformly onto the white toner layer. This causes the change in the shade of color. 
     For the aforementioned reasons, the white toner Tw has a smaller average particle diameter than the cyan toner Tc. The average particle diameter according to the present embodiment is a median diameter in a distribution of particle size expressed in terms of a projected area diameter and measured by microscopy. 
       FIGS. 4A-4C  illustrate how the toner particles are transferred when the white toner Tw has a smaller average particle diameter than the cyan toner Tc. 
     The image of the cyan toner Tc is transferred onto the intermediate transfer belt  32  as shown in  FIG. 4A , and then a solid image of the white toner Tw is transferred onto the cyan toner Tc as shown in  FIG. 4B . Subsequently, the solid image of the white toner Tw and the image of the cyan toner Tc are transferred onto the paper P as shown in  FIG. 4C . 
     The white toner Tw having a smaller average particle diameter than the cyan toner Tc reduces the chance of the particles of the cyan toner Tc entering the gaps among the particles of the white toner Tw when the toners Tw and Tc are transferred onto the paper P, which reduces the chance of the particles of the cyan toner Tc being mixed with the particles of the white toner Tw. The white toner Tw with a relatively small average particle diameter is advantageous in filling the furrows in the surface of the paper P, providing a relatively smooth surface of the layer of the white toner Tw and hence relatively uniform transfer of the cyan toner particles. 
       FIG. 5  is a table that lists the experimental results, illustrating the relationship between the average particle diameter of the white toner Tw and the change in shade of color due to the mixture of the white toner Tw and cyan toner Tc. 
     In this experiment, using a cyan toner having an average particle diameter of 7.0 μm and white toners having average particle diameters of 6.0, 6.1, 6.3, 6.5, 6.7, 6.9, 8.9, and 11.2 μm, the shades of color caused by the mixture of the white toner and cyan toner were measured. The shades of color are expressed in terms of a color difference ΔE. The color differences ΔE were measured for white toners having these eight different average particle diameters. A rectangular solid cyan image of 30×25 mm was printed directly on white paper that serves as a reference, and then the Lab value of the solid cyan image, a first Lab value, was measured using a spectrophotometer, MODEL CM-2600d available from KONIA MINOLTA. Rectangular solid white images of 30×25 mm were printed on the white paper and then the solid cyan image was printed on each of the white solid images in registration, and then the Lab values of the solid cyan images, second Lab values, were measured. The first Lab value is compared with the second Lab values. The smaller the ΔE, the smaller the change in the shade of color. In other words, a small ΔE indicates that only small portions of the white toner and cyan toner are mixed. The color differences ΔE were measured for eight different white toners, and were then evaluated. Specifically, the color differences ΔE were rated on a scale of five levels: ΔE&gt;10, 5≦ΔE≦10, 3&lt;ΔE≦5, 1&lt;ΔE≦3, and ΔE≦3. The color differences in the range of ΔE&gt;10 indicate “very poor.” The color differences in the range of 5≦ΔE≦10 indicate “poor.” The color differences in the range of 3≦ΔE≦5 indicate “slightly poor.” The color differences in the range of 1≦ΔE≦3 indicate “good.” The color differences in the range of ΔE≦1 indicate “very good.” Referring to  FIG. 5 , symbol “XX” denotes “very poor” and symbol “X” denotes “poor.” Symbol “Δ” denotes “slightly poor” and symbol “◯” denotes “good.” The symbol “⊚” denotes “very good.” The symbols “◯” and “⊚” are color differences which users are unable to detect. The symbol “XX” and “X” are color differences which are unsatisfactory to the users by inspection. The “Δ” is a color difference which is difficult to detect by inspection but is still not acceptable. 
     The experimental results listed in  FIG. 5  show that white toners having smaller average particle diameters cause smaller changes in the shade of color if the cyan toner has a fixed average particle diameter of 7.0 μm. The white toner having an average particle diameter of 6.5 μm or less resulted in “good” or better color differences. The ratio of the average particle diameter of 6.5 μm of the white toner to that of 7 μm of the cyan toner is 6.5/7.0=0.93≈0.95. 
     The color toner according to the present invention has an average particle diameter of 6.9 μm, more specifically, in the range of 6.8 to 7.0 μm due to the manufacturing errors. 
     In the first embodiment, the cyan toner has a minimum average particle diameter of 6.8 μm.  FIG. 6  lists experimental changes in the shade of color when the cyan toner having an average particle diameter of 6.8 μm is mixed with the white toner having eight different average particle diameters. The experiments were conducted under the same condition except the average particle diameter of 6.8 μm of the cyan toner. As is clear from  FIG. 6 , the change in the shade of color (ΔE) was “good” for the white toner having an average particle diameter of 6.3 μm or smaller. The ratio of the average particle diameter of 6.3 μm of the white toner to that of 6.8 μm of the cyan toner is 6.3/6.8=0.93≈0.95. 
     Similar experiments were conducted for black, yellow, and magenta toners, and the results were quite similar to those described above. 
     The above described experimental results show that the ratio of the average particle diameter of the white toner to that of the color toner not larger than 0.95 is effective in reducing the unwanted mixture of the white toner and color toner, thus implementing a desired shade of color. If a color toner has an average particle diameter of 6.9+0.1 μm, the white toner may have an average particle diameter equal to or smaller than 6.7 μm, preferably equal to or smaller than 6.5 μm, so that the unwanted mixture of the white toner and the color toner may be reduced, implementing a desired shade of color. If a color toner has an average particle diameter of 6.9-0.1 μm, the white toner may have an average particle diameter equal to 6.5 μm or smaller, preferably 6.3 μm or smaller, thereby implementing a desired shade of color. Manufacturing the toner having an average particle diameter smaller than 6.0 μm is difficult or at least not economical. Thus, the average particle diameter of the white toner is preferably equal to or larger than 6.0 μm. 
     In order to fill the furrows in the paper P, the white toner preferably has an average particle diameter smaller than the furrows. The ridges and furrows in the paper P are expressed in terms of ten-point height of irregularities Rz defined by JIS B0601:1944. The thickness of the white toner image is preferably larger than that of the color toner image, and is preferably larger than the ridges and furrows in the paper P. 
     {Effects} 
     The first embodiment provides the following advantages. The first toner image (e.g., white toner image) and the second toner image (e.g., color image) are transferred onto the paper P in this order. The first toner has a smaller average particle diameter than the second toner. Therefore, when the first and second toners are transferred onto the recording medium in this order, there is less chance of the second toner entering the gaps among the first toner particles, which provides a good image quality. 
     The first toner (e.g., white toner) is used to form a background and the second toner (e.g., color toner) is used to form an image on the first toner. The use of the white toner (first toner) having an average particle diameter not larger than that of the color toner (second toner) reduces the change in the shade of color that would otherwise be caused. 
     In one embodiment, the average particle diameter of the first toner (e.g., white toner) is equal to or smaller than 0.95 times that of the second toner (e.g., color toner). This ratio of the diameters is effective in reducing unwanted mixture of the first and second toners. 
     In one embodiment, the first toner has an average particle diameter not smaller than 6.0 μm and not larger than 6.7 μm, preferably not larger than 6.5 μm. When the second toner has an average particle diameter not smaller than 7.0 μm, the unwanted mixture of the first and second toners may be minimized. 
     In one embodiment, the first toner has an average particle diameter not smaller than 6.0 μm and not larger than 6.5 μm, preferably not larger than 6.3 μm. When the second toner has an average particle diameter not smaller than 6.8 μm, the unwanted mixture of the first and second toners may be minimized. 
     In one embodiment, the toner image formed of the first toner (e.g., white toner) has a larger thickness than that formed of the second toner (e.g., color toner). Therefore, the first toner serves to smooth out the surface relief heights of the recording medium (e.g., paper P), and then the second toner is transferred onto the surface of the layer of the first toner which has been a relatively smooth surface. 
     In one embodiment, the first toner (e.g., white toner) has an average particle diameter smaller than the furrows in the surface of the recording medium, which advantageously fills the furrows to create a smoothed, flat surface which allows the second toner to be transferred uniformly onto the layer of the first toner. 
     In one embodiment, the image forming apparatus includes an image bearing body (e.g., intermediate transfer belt), a first image forming section (e.g., print engines  10 K,  10 Y,  10 M,  10 C, and  10 W) that forms a first toner image (e.g., cyan toner image) on the image bearing body, a second image forming section (e.g., print engine  10 W) that forms a second toner image (white toner image) in registration with the first toner image, and a transfer section (e.g., secondary transfer roller). The surface of the image bearing body has furrows larger than the average particle diameter of the second toner (e.g., white toner). The flatness (ridges and furrows) of the surface of the image bearing body is expressed in terms of ten point height of irregularities Rz determined by JIS B061:1994. This embodiment minimizes the mixture of the first and second toners on the image bearing body, and provides good images. 
     The surface of the image bearing body (specifically an intermediate transfer belt) may have furrows smaller than the average particle diameter of the first toner (e.g., white toner). 
     Second Embodiment 
     An image forming apparatus according to a second embodiment will be described. The image forming apparatus according to the second embodiment differs from that according to the first embodiment in the toner used in the print engine  10 W. The second embodiment will be described with respect to portions different from those of the first embodiment. Like elements have been given like reference numerals and a detailed description thereof is omitted. 
       FIGS. 7A and 7B  illustrate the distribution of the toner particle diameters. A description will be given of the white toner used in the second embodiment. 
       FIG. 7A  shows a distribution Dw1 of the particle diameters for the white toner and a distribution Dc of the particle diameters for a color toner, according to the first embodiment. The distribution Dw has a peak at a particle diameter Pw1 and the distribution Dc has a peak at a particle diameter Pc, the Pw1 being smaller than the Pc. For example, Pw1 is 6.5 μm and Pc is 6.9 μm. The profile of the distributions Dw1 and Dc is substantially identical. In order for the white toner to have a smaller average particle diameter than the color toner, the distribution Dw1 is selected so that the Pw1 is much smaller than the Pc. Thus, most of the white toner particles have smaller particle diameters than the color toner particles. 
       FIG. 7B  shows the distribution Dw2 of particle diameters for the white toner according to the second embodiment and a distribution Dc of particles diameter for the color toner, according to the second embodiment. In the second embodiment, the profile of the distribution Dw2 of the white toner has a first peak Pw21 and a second peak Pw22, the first peak Pw21 having substantially the same average particle diameter as the average particle diameter of the color toner and the second peak Pw22 having a smaller particle diameter than the first peak Pw21. In other words, a large, significant proportion of the white toner is distributed in the vicinity of the second peak Pw22 smaller than the first peak Pw21 so that the average particle diameter of the white toner is smaller than that of the color toner. For example, Pw21=Pc=6.9 μm and 6.0 μm&lt;Pw22&lt;6.5 μm. The white toner according to the second embodiment may be obtained by mixing a white toner having substantially the same distribution of particle diameters as the color toner with a fine white toner having an average diameter in the vicinity of the smallest diameters of the color toner, as shown in  FIG. 7B . 
     From a point of view of filling the furrows in the paper P as a recording medium, the particle diameter of the second peak Pw22 is preferably smaller than the furrows in the paper P. For example, the white toner preferably includes fine toner particles having smaller diameters than the paper P. The furrows in the paper P are expressed in terms of ten point height irregularities Rz determined under JIS B0601:1944. The height irregularity Rz of ordinary paper is in the range of 14 to 20 μm. 
       FIGS. 8A-8D  illustrate how toners are transferred onto an intermediate transfer belt  32 . Referring to  FIGS. 8A and 8B , a cyan toner Tc is first transferred onto the intermediate transfer belt  32  and then a white toner Tw is transferred onto the cyan toner Tc in registration. The cyan toner Tc and the white toner Tw are then transferred onto paper P as shown in  FIGS. 8C and 8D . The paper P has surface relief which is in a variety of shapes depending on the material and manufacturing method thereof, some paper having relatively large surface relief heights (i.e., ridges and furrows) and some other paper having relatively small surface relief heights.  FIG. 8C  illustrates how the toner is transferred onto the paper P having relatively small furrows.  FIG. 8D  illustrates how the toner is transferred onto the paper P having relatively large surface relief heights. 
     Referring to  FIGS. 8C and 8D , the white toner having a large proportion of fine, smaller diameter particles effectively fills a variety of furrows of different sizes in the surface of the paper P, so that the cyan toner may be uniformly transferred onto a layer of the white toner. 
     The second embodiment provides the following effects in addition to those obtained by the first embodiment. 
     The profile of a distribution of first toner (e.g., white toner) has a first peak and a second peak. The first peak is located at substantially the same particle diameter as the peak of a second toner (e.g., color toner), the second peak being located at a smaller particle diameter than the first peak. This profile of distribution decreases the chance of the first toner (white toner) being mixed with the second toner (e.g., color toner) when the toners are transferred onto the paper having surface relief heights, thereby providing a good image quality. 
     In one embodiment, the peak of the profile of distribution is located at a particle diameter smaller than the furrows in the recording medium. The second embodiment enables the furrows in a variety of recording media to be filled. For example, fine toner particles smaller than the furrows in the recording medium may be advantageously used for a recording medium having smaller furrows. 
     The smaller the particle diameter of toner, the higher the manufacturing cost. Thus, the profile of distribution of toner particles shown in  FIG. 7B  may be more advantageous in terms of manufacturing cost than that shown in  FIG. 7A . 
     The smaller the toner particle diameter is, the larger the amount of charge, i.e., the absolute value of Q/M (Q: charge, M: weight of toner particles) on the toner particles is. Thus, a larger amount of charge on the toner requires a higher transfer voltage during a transfer process. For this reason, the profile of the distribution of particle diameters shown in  FIG. 7B  is more advantageous than that shown in  FIG. 7A . 
     While the first and second embodiments have been described with respect to the combination of the white toner and color toner, the combination is not limited to this. A variety of combinations may be possible as long as use of two toners may cause unwanted mixing of the toners that deteriorates image quality. For example, the invention may be applied to a case in which a first toner forms a first toner image and a second toner covers the first toner image. The first toner may be a color toner and the second toner may be a transparent toner, in which case, the first toner may have a smaller average particle diameter than the second toner so that unwanted mixing of the first and second toners may be minimized or prevented. For example, the transparent toner may be used to provide the image with a gloss finish. 
       FIG. 9  illustrates the outline of the configuration of an image forming apparatus  2  that employs a transparent toner Tt. The image forming apparatus  2  has substantially the same configuration as the image forming apparatus  1 , but includes print engines  10 T,  10 K,  10 Y,  10 M, and  10 C that form a transparent image, a black image, a yellow image, a magenta image, and a cyan image, respectively. The print engine  10 T is disposed upstream of the print engines  10 K,  10 Y,  10 M, and  10 C with respect to the direction of travel of the intermediate transfer belt  32 . 
     The first and second embodiments have been described in terms of an intermediate transfer image forming apparatus, but are not limited to this. Instead, the present invention may be applied to a direct transfer image forming apparatus. 
     A direct transfer image forming apparatus includes at least two print engines: a first print engine has a first image forming section that forms a first toner image (e.g., white toner image) on a first image bearing body (e.g., photoconductive drum) and a second print engine has a second image forming section that forms a second toner image (e.g., color toner image) on a second image bearing body (e.g., photoconductive drum). Each print engine includes a charging unit that charges the surface of the image bearing body, an exposing unit that illuminates the charged surface of the image bearing body to form an electrostatic latent image, a developing unit that supplies the toner to the electrostatic latent image to develop the electrostatic latent image with the toner into a toner image, and a transfer unit that transfers the toner image directly onto a recording medium.  FIG. 10  illustrates the outline of a direct transfer image forming apparatus  3 . Elements similar to those shown in  FIG. 1  have been given similar reference numerals and a description thereof is omitted. The photoconductive drums  11 W,  11 K,  11 Y,  11 M, and  11 C are aligned along a transport belt  90  in a direction of travel of the paper P, and form toner images of the respective colors. As opposed to an intermediate transfer image forming apparatus, the toner images formed by the print engines  10 K,  10 Y,  10 M,  10 C, and  10 W are not transferred onto the transport belt  90  but directly onto the paper P. Since the white toner image formed in the print engine  10 W is transferred onto the paper P before the toner images of the respective colors, i.e., black, yellow, magenta, and cyan, are transferred onto the paper P, the print engine  10 W is located upstream of the print engines  10 K,  10 Y,  10 M, and  10 C. When the transport belt  90  advances through the print engines  10 W,  10 K,  10 Y,  10 M, and  10 C, the transport belt  90  receives the paper P from the paper feeding mechanism  50 , and transports the paper P in a direction shown by arrow E. The transfer rollers  31 W,  31 K,  31 Y,  31 M and  31 C transfer the toner images, formed on the photoconductive drums  11 W,  11 K,  11 Y,  11 M, and  11 C, onto the paper P. The paper P is then fed to a fixing mechanism  60  where the toner images on the paper P are fixed by heat and pressure. After fixing, the paper P is discharged through guides  45  onto a stacker  44 . 
     The first and second embodiments have been described in terms of a configuration in which four color toners, i.e., black (K), yellow (Y), magenta (M), and cyan (C), are used. The number of colors is not limited to four. The image forming apparatus may be a color printer that uses a single color (e.g., black). 
     The invention is not limited to the first and second embodiments and may be modified in a variety of ways within the scope of the invention.