Patent Publication Number: US-2006008721-A1

Title: Color image forming developer and color image forming apparatus

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a color image forming developer and a color image forming apparatus.  
      2. Description of the Related Art  
      In general, an image forming apparatus using the electrophotographic process is composed of a charging unit, an exposing unit, a developing unit, a transfer unit, a fixing unit and a cleaning unit, which are arranged around a photoconductor. The charging unit uniformly charges a surface of the photoconductor. The exposing unit exposes the charged photoconductor to light according to image information, and forms an electrostatic latent image on the surface of the photoconductor. The developing unit charges a developer by agitating, and forms a visible image by making the developer adhere to the electrostatic latent image formed on the surface of the photoconductor by electrostatic force. The transfer unit transfers a toner image to a recording material by giving the recording material an electric charge having the opposite polarity to that of toner. The fixing unit fixes the toner image transferred to the recording material by means of heating, pressuring or the like. The cleaning unit collects toner remaining on the surface of the photoconductor without being transferred to the recording material. The image forming apparatus by the electrophotographic process thus composed forms a desired image on a recording material.  
      In recent years, as documents are converted to color ones and display units of computers and the like are converted to color ones, color image forming apparatuses are being developed. One of the methods for forming a color image is to color-separate color image information into the respective hues of yellow, magenta and cyan, develop by the use of a color image forming developer responsive to the color-separated image information, and form a full-color image by utilizing the subtractive color mixture action.  
      In the case of superposing color developers of yellow, magenta and cyan with the intent of expressing black by this method, it is possible to artificially express black, but it does not become perfect black, and favorable black chromaticity cannot be obtained. Therefore, a color image forming apparatus having a black developer as well as color developers of yellow, magenta and cyan is generally used.  
      Some image forming apparatuses are equipped with image forming portions each including a photoconductor, a charging unit, a developing unit, a transfer unit and a cleaning unit for each of color developers of yellow, magenta and cyan and a black developer. Such a color image forming apparatus is widely used, because it can simultaneously form visible images of the respective colors on surfaces of the respective photoconductors, and therefore perform high speed full-color image formation.  
      A developer used in a color image forming apparatus contains a binding resin and a coloring agent, and further, a charge control agent, a wax, an additive and the like which are added as necessary. The binding resin is a resin for dispersing and molding a coloring agent, a charge control agent, a wax and the like which cannot be molded alone. As the coloring agents contained in color developers of yellow, magenta and cyan are used color materials such as organic pigments or dyes, and as a coloring agent contained in a black developer, carbon black is used heretofore, because it is low-cost and capable of expressing black alone.  
      Content of a coloring agent to a developer, however, is generally 5 to 10% by weight and is higher than the other additives. Therefore, when the black developer using carbon black of low electrical resistance as a coloring agent is used in a color image forming apparatus, a difference in charge property is generated between the black developer and the color developers using dye or pigments of high electrical resistance. Since there is a difference in charge property between the black developer and the color developers in this manner, the black developer adheres to the photoconductor by stronger force than the color developers in the case where the photoconductors for the color developers and the black developer are charged and the developers are agitated for charging in a developing unit under the same conditions. As a result, such a problem occurs that only black is printed in high density, the color tones of an image do not match, and the quality of the image deteriorates.  
      Accordingly, it is proposed recently to collect powdered color developers which are generated at the time of production of color developers of yellow, magenta and cyan and cannot be used as the color developers, reuse the collected color developers as part or all of the raw material for producing the black developer, and melt, knead and crush them to obtain a black developer (refer to Japanese Unexamined Patent Publication JP-A 4-39672 (1992), for example). The black developer thus obtained is considered to show the same action as the color developers of yellow, magenta and cyan with regard to a charge property at the time of development.  
      Another related art proposes a black developer which is produced by the emulsion polymerization method by the use of a coloring agent obtained by mixing the respective coloring agents of yellow, magenta and cyan at a predetermined ratio (refer to Japanese Unexamined Patent Publication JP-A 2002-182431, for example). Since this black developer has high black chromaticity and an excellent charge property, it is considered to realize excellent black-color fixation.  
      In the case of using the abovementioned developers such that the charge properties of the color developers of yellow, magenta and cyan and the charge property of the black developer are similar in a color image forming apparatus, it is certainly possible to set the conditions of agitating the developers, the conditions of applying development bias and so on to the same ones, and make the adhesion states of the developers to photoconductors the same in the developing process. However, even if developers showing the same actions with regard to charge properties are obtained, there is no guarantee that the value of the volume resistivity of the black developer and the value of the volume resistivities of the color developers are the same. In the case where there is a difference between the volume resistivity of the black developer and the volume resistivities of the color developers of yellow, magenta and cyan, the charge properties of the respective developers change through the transfer process and the cleaning process after the developing process, and differences in transfer efficiency and cleaning efficiency are generated between the color developers of yellow, magenta and cyan and the black developer. As a result, an image obtained in the transfer process is of low image quality, because the harmony of colors is not maintained. Moreover, there is a difference of the amounts of toner remaining after being transferred between the black developer and the color developers, so that there is a possibility that when cleaning is performed under the same conditions, some toner remains without being cleaned and causes the mixture of colors in the following printing process.  
      Accordingly, in the transfer process, in order to match the color tints of the black developer and the color developers and increase the image quality, there is a need to set a voltage applied for transferring the developer to a recording material, for each of the black developer and the color developers of yellow, magenta and cyan. Moreover, in the cleaning process, there is a need to perform cleaning depending on the remaining toner of each, and the contact pressure of a cleaning blade which collects toner remaining on the surface of the photoconductor, with respect to the photoconductor must be set for each, so that the operation conditions of the image forming apparatus should become complicated.  
     SUMMARY OF THE INVENTION  
      An object of the invention is to provide a color image forming black developer which has a volume resistivity equal to those of the respective color developers of yellow, magenta and cyan and makes it possible to obtain an image of high image quality, and provide a color image forming apparatus which is equipped with the developer and has a simple configuration.  
      The invention provides a color image forming developer for use in visualizing a latent image which is formed on the basis of image information separated into color components of yellow, magenta, cyan and black, the color image forming developer comprising: 
          a yellow developer which develops a latent image formed on the basis of image information relating to the color component of yellow;     a magenta developer which develops a latent image formed on the basis of image information relating to the color component of magenta;     a cyan developer which develops a latent image formed on the basis of image information relating to the color component of cyan; and     a black developer which develops a latent image formed on the basis of image information relating to the color component of black,     wherein a volume resistivity of the black developer is substantially equal to a volume resistivity of the yellow developer, a volume resistivity of the magenta developer, and a volume resistivity of the cyan developer.        

      Further, in the invention, it is preferable that each of the developers contains at least a binding resin and a coloring agent, and 
          the black developer contains a yellow coloring agent, a magenta coloring agent and a cyan coloring agent which are used for the yellow developer, the magenta developer and the cyan developer, respectively, at a predetermined ratio.        

      Furthermore, in the invention, it is preferable that the predetermined ratio is such that weights of the yellow coloring agent, the magenta coloring agent and the cyan coloring agent are in substantially equal proportions.  
      Furthermore, in the invention, it is preferable that the predetermined ratio is 1:1:1 by weight.  
      Furthermore, in the invention, it is preferable that the predetermined ratio is (1.2-1.5):1:1 by weight.  
      Still further, in the invention, it is preferable that a ratio of the sum of the weights of the yellow coloring agent, the magenta coloring agent and the cyan coloring agent in the black developer to the weight of the black developer is substantially equal to a ratio of the weight of the yellow coloring agent in the yellow developer to the weight of the yellow developer, a ratio of the weight of the magenta coloring agent in the magenta developer to the weight of the magenta developer, and a ratio of the weight of the cyan coloring agent in the cyan developer to the weight of the cyan developer.  
      Still further, the invention provides a color image forming apparatus in which a latent image which is formed on the basis of image information separated into color components of yellow, magenta, cyan and black is visualized, the color image forming apparatus comprising: 
          a photoconductor on which a latent image is formed by being exposed to light according to image information;     a charging unit which charges the photoconductor before being exposed;     a developing unit which is equipped with the color image forming developer mentioned above, agitates and charges the color image forming developer, and supplies the color image forming developer to the surface of the photoconductor to visualize the latent image;     a transfer unit which applies electric potential of an opposite polarity to a polarity of the color image forming developer to a to-be-transferred material and transfers the visualized image; and     a cleaning unit which removes the color image forming developer that remains on the surface of the photoconductor after transfer,     wherein at the time of image formation, the operation conditions of the charging unit, the developing unit, the transfer unit and the cleaning unit are set to same ones for the respective color developers of yellow, magenta and cyan and the black developer.        

      Still further, in the invention, it is preferable that the operation conditions include at least charge electric potential at which the charging unit charges the photoconductor, an application voltage of a developer tank which contains the developer and a condition of agitating the developer in the developing unit, transfer electric potential at which the transfer unit applies to the to-be-transferred material, and a contact pressure of the cleaning unit to the surface of the photoconductor.  
      According to the invention, the volume resistivity of the black developer is substantially equal to the volume resistivities of the color developers of yellow, magenta and cyan (hereinafter, referred to as color developers) so that a difference is not generated between the charge property of the black developer and the charge properties of the color developers in the respective processes for forming a color image. Therefore, it is possible to obtain an image of high image quality in which the harmony of colors is maintained, and it is possible to set the charge electric potential at which the charging unit charges the photoconductor, the application voltage of the developer tank that contains the developer and the condition of agitating the developer in the developing unit, the transfer electric potential at which the transfer unit applies to the to-be-transferred material, and the contact pressure of the cleaning unit to the surface of the photoconductor to the same ones, for the black developer and the color developers.  
      Further, according to the invention, a coloring agent used for the black developer contains a yellow coloring agent, a magenta coloring agent and a cyan coloring agent which are used for the yellow developer, the magenta developer and the cyan developer, respectively, at a predetermined ratio, so that it is possible to obtain a color image forming black developer which has a volume resistivity equal to those of the color developers.  
      Furthermore, according to the invention, the predetermined ratio of the coloring agents used for the black developer is such that the weights of the coloring agents of the respective colors, that is, the weight of the yellow coloring agent, the weight of the magenta coloring agent and the weight of the cyan coloring agent are in substantially equal proportions, and the coloring agents of the respective colors are mixed at a favorable ratio, so that it is possible to obtain a color image forming black developer which has high black chromaticity and is excellent in image quality.  
      Still further, according to the invention, a ratio of the sum of the weights of the yellow coloring agent, the magenta coloring agent and the cyan coloring agent in the black developer to the weight of the black developer is substantially equal to a ratio of the weight of the yellow coloring agent in the yellow developer to the weight of the yellow developer, a ratio of the weight of the magenta coloring agent in the magenta developer to the weight of the magenta developer, and a ratio of the weight of the cyan coloring agent in the cyan developer to the weight of the cyan developer, so that the relation between the densities of images formed by the color developers and the density of an image formed by the black developer becomes favorable, and it is possible to form an image of excellent image quality as a whole color image.  
      Still further, according to the invention, in a color image forming apparatus equipped with the aforementioned color image forming developer, the volume resistivities of the color developers and the volume resistivity of the color image forming black developer are substantially equal, so that at the time of image formation, the operation conditions of the charging unit, the developing unit, the transfer unit and the cleaning unit can be set to the same ones, for the color developers and the black developer. Therefore, it is not necessary to regulate the settings of the operation conditions between the color developers and the black developer, so that it is possible to obtain a color image forming apparatus that has a simpler configuration.  
      Still further, according to the invention, the operation conditions include at least the charge electric potential at which the charging unit charges the photoconductor, the application voltage of the developer tank that contains the developer and the condition of agitating the developer in the developing unit, the transfer electric potential at which the transfer unit applies to the to-be-transferred material, and the contact pressure of the cleaning unit to the surface of the photoconductor, and it is possible to set the configurations and so on of voltage applying means which applies a voltage, developer agitating means in the developer tank and the cleaning unit to the same ones, so that the need for individually setting the operation conditions for the color developers and the black developer is eliminated. Moreover, since the components can be made to be common, it is possible to obtain a color image forming apparatus that has a simpler configuration and is miniaturized. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Other and further objects, features, and advantages of the invention will be more explicit from the following detailed description taken with reference to the drawings wherein:  
       FIG. 1  is a schematic cross section view showing, in a simplified manner, the structure of toner of a black developer according to one embodiment of the color image forming developer of the invention;  
       FIG. 2  is a schematic cross section view showing, in a simplified manner, the structures of toners of color developers of yellow, magenta and cyan;  
       FIG. 3  is a cross section view showing, in a simplified manner, the entire configuration of a color image forming apparatus according to another embodiment of the invention;  
       FIG. 4  is a magnified view of the essential part of the color image forming apparatus shown in  FIG. 3 ;  
       FIG. 5  is a magnified view of the essential part of a color image forming apparatus according to still another embodiment of the invention;  
       FIG. 6  is a schematic cross section view showing, in a simplified manner, the structure of toner of the black developer using carbon black as a coloring agent according to a comparative example to the invention; and  
       FIG. 7  shows the result of measurement of electrical resistance in the developer of the invention and in the black developer of the comparative example. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Now referring to the drawings, preferred embodiments of the invention are described below.  
      The color image forming developer of the invention is characterized in that a volume resistivity of a black developer is substantially equal to a volume resistivity of a yellow developer, a volume resistivity of a magenta developer, and a volume resistivity of a cyan developer.  
      In producing such a developer, for example, it is preferable that the black developer contains a yellow coloring agent, a magenta coloring agent and a cyan coloring agent which are used for the yellow developer, the magenta developer and the cyan developer, respectively, at a predetermined ratio as a coloring agent. Hereinafter, a method for producing the black developer will be described.  
       FIG. 1  is a schematic cross section view showing, in a simplified manner, the structure of toner  1   b  of a black developer according to one embodiment of the color image forming developer of the invention, and  FIG. 2  is a schematic cross section view showing, in a simplified manner, the structures of toners  1   y ,  1   m ,  1   c  of color developers of yellow, magenta and cyan. In the following description, alphabetical letters y, m, c, b attached to the respective Arabic numerals correspond to the colors of yellow, magenta, cyan and black, respectively. Hereinafter, except the case of a description by specifying a specific color, the alphabetical letters y, m, c, b will be omitted in the description.  
      The toner  1  of the color image forming developer of the invention contains at least a binding resin  2  and a coloring agent  3 .  
      The binding resin  2  is, for example, polystyrene, styrene-acrylic ester-based styrene resins, vinyl chloride resins, phenol resins, epoxy resins, polyester resins or the like. A vinyl-based monomer which can be polymerized with styrene used for styrene-acrylic ester-based styrene resins is: acrylic esters having ethylenic double bond, such as acrylic acid, methyl acrylate, ethyl acrylate, butyl acrylate, dodecyl acrylate, octyl acrylate, 2-ethylhexyl acrylate, phenyl acrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylate, octyl methacrylate, acrylonitrile, methacrylonitrile and acrylamide; maleic acid half esters and diesthers such as maleic acid and butyl maleate; vinyl esters such as vinyl acetate, vinyl chloride, vinyl methyl ether, vinyl ethyl ether, vinyl propyl ether and vinyl butyl ether; and vinyl ketones such as vinyl methyl ketone, vinyl ethyl ketone and vinyl hexyl ketone.  
      A yellow coloring agent  3   y  is used as the coloring agent of the toner  1   y  of the yellow developer, a magenta coloring agent  3   m  is used as the coloring agent of the toner  1   m  of the magenta developer, and a cyan coloring agent  3   c  is used as the coloring agent of the toner  1   c  of the cyan developer. Moreover, as the coloring agent of the toner  1   b  of the black developer, the yellow coloring agent  3   y , the magenta coloring agent  3   m  and the cyan coloring agent  3   c  used for the respective toners of the yellow developer, the magenta developer and the cyan developer are used.  
      The yellow coloring agent  3   y  is, for example, disazo pigments such as C.I. pigment yellow 17, monoazo pigments such as C.I. pigment yellow 74 and C.I. pigment yellow 97, condensed azo pigments such as C.I. pigment yellow 93 and C.I. pigment yellow 128, benzimidazolone pigments such as C.I. pigment yellow 180 and C.I. pigment yellow 194, or the like.  
      The magenta coloring agent  3   m  is, for example, quinacridone pigments such as C.I. pigment red 122 and C.I. pigment red 202, lake azo pigments such as C.I. pigment red 57:1, perylene pigments such as C.I. pigment red 149, C.I. pigment red 190 and C.I. pigment red 224, naphthol-benzimidazolone pigments such as C.I. pigment red 184 and C.I. pigment red 185, or the like.  
      The cyan coloring agent  3   c  is, for example, phthalocyanine pigments such as C.I. pigment blue 15, C.I pigment blue 15:3 and C.I. pigment blue 15:4.  
      Here, the toner  1   b  of the black developer in the color image forming developer of the invention contains the yellow coloring agent  3   y , the magenta coloring agent  3   m  and the cyan coloring agent  3   c  at a predetermined ratio. Here, the predetermined ratio is such that weights of the yellow coloring agent  3   y , the magenta coloring agent  3   m  and the cyan coloring agent  3   c  are in substantially equal proportions.  
      By equally mixing the coloring agents of the respective colors of yellow, magenta and cyan at a ratio of 1:1:1 by weight, it is possible to obtain a coloring agent of black color. However, even if the densities are equal, they may not look like equal by human vision in the case of different hues, and especially, the density of yellow is often visually felt to be low as compared with the other hues. Therefore, there is a case where an obtained coloring agent does not look like true black color even when the respective coloring agents of yellow, magenta and cyan are equally mixed, so that it is preferable to mix yellow, magenta and cyan at a ratio of, for example, (1.2-1.5):1:1 by weight, which is a ratio such that a ratio of the mixed yellow coloring agent  3   y  is slightly higher. This is made in consideration of the amounts of the developers of the respective hues in order to obtain the same densities in the same areas, and is determined from the ratio of the amounts of the developers of the respective hues in order to obtain ID density of 1.4 (saturation density). Therefore, the weights being in substantially equal proportions includes a case where a ratio of yellow is slightly higher than a ratio of magenta and a ratio of cyan and a case where a ratio of yellow, a ratio of magenta and a ratio of cyan are the same.  
      It is preferable that 5-10% by weight of the coloring agents  3   y ,  3   m ,  3   c  in the yellow, magenta, cyan and black developers are contained in the toners  1 . In the case of less than 5% by weight of the coloring agents, an image having sufficient image density cannot be obtained. In the case of over 10% by weight, the dispersiveness of the coloring agents in the binding resin becomes bad.  
      Further, it is preferable that a ratio of the sum of the weights of the yellow coloring agent  3   y , the magenta coloring agent  3   m  and the cyan coloring agent  3   c  in the black developer to the weight of the toner  1   b  of the black developer is substantially equal to a ratio of the weight of the yellow coloring agent  3   y  in the yellow developer to the weight of the toner ly of the yellow developer, a ratio of the weight of the magenta coloring agent  3   m  in the magenta developer to the weight of the toner  1   m  of the magenta developer, and a ratio of the weight of the cyan coloring agent  3   c  in the cyan developer to the weight of the toner  1   c  of the cyan developer. Here, the language “substantially equal” means that a proportion of the density of the coloring agent  3  in the toner  1   b  of the black developer to the density of the coloring agents  3  in the toners  1   y - c  of the yellow, magenta and cyan developers is 0.9-1.1.  
      The toner  1  may contain, in addition to the binding resin  2  and the coloring agent  3 , an additive such as a charge control agent in the range that a preferable property is not impaired. By adding the charge control agent, it is possible to make the friction charge amount of the toner  1  favorable. As the charge control agent, it is possible to use a well-known one, which is, for example, a colorless positive charge control agent represented by metal salt of quaternary ammonium salt, and a colorless negative charge control agent represented by metal salt of alkyl salicyl. It is preferable that 2% by weight of the charge control agent is contained in the toner  1  in general.  
      Further, in addition to the binding resin  2  and the coloring agent  3 , a release agent such as wax may be contained in the range that a favorable property is not impaired. As the wax, it is possible to use a well-known one, which is, for example, silicone varnish and silicone oil. It is preferable that 3% by weight of the wax is contained in the toner  1  in general.  
      The toner  1  of the color image forming developer of the invention can be obtained by a well-known method. For example, a method to mix the charge control agent, the wax and so on as necessary into principal ingredients such as the binding resin  2  and the coloring agent  3  or into a so-called masterbatch composition in which the coloring agent  3  is preliminary-dispersed in the binding resin  2  in advance, by a mixing machine, and thereafter, melt-knead, uniformly disperse, crush, and classify is used.  
      As the mixing machine, a Henschel-type mixing apparatus such as a Henschel mixer (produced by MITSUI MINING Co., LTD.), a super-mixer (produced by KAWATA MFG Co., Ltd.) or a MECHANOMILL (produced by Okada Seiko Co., Ltd.), or an apparatus such as an Angmill (produced by Hosokawamicron Corporation), a HYBRIDIZATION SYSTEM (produced by Nara Machinery Co., Ltd.) or a COSMOSYSTEM (produced by Kawasaki Heavy Industries, Ltd.) can be used, and as a kneading machine, a uniaxial or biaxial extruder such as TEM-100B (produced by Toshiba Machine) or PCM-65/87 (produced by Ikegai), or an open-roll-type machine such as a Kneadex (produced by MITSUI MINING Co., LTD.) can be used. In specific, in the melting-kneading process, in order to efficiently disperse the additive, it is desirable to knead by high shearing force at low temperatures so that the resin viscosity at the time of melting does not decrease too much, in particular, it is preferable to use an open-roll-type kneading machine in specific.  
      To crush toner particles, a collision-type air flow crusher using jet air flow, a mechanical crusher or the like can be used, and the toner particles are subjected to classification by wind force and regulated to a predetermined particle size.  
      Further, the toner  1  of the color image forming developer of the invention can also be obtained by a so-called polymerization method such as the suspension method, the emulsion aggregation method or the liquid dry method to generate particles in an aqueous solution or a solvent.  
      It is preferable that the toner particles thus produced have a volume average particle diameter of 3 to 10 μm. In the case where the volume average particle diameter of the toner is less than 3 μm, the toner particle diameter becomes too small, and the toner is not stably supplied to the photoconductor, whereby there are fears that image fog occurs. On the other hand, in the case where the volume average particle diameter of the toner exceeds 10 μm, the particle diameter of the toner is large, so that an image of high image quality cannot be obtained.  
      Furthermore, it is preferable that the particle size distribution of the toner particles thus produced is narrow, but it is possible to use ones having a particle size which can be obtained by a normal crushing method. In specific, it is preferable to regulate so that with respect to a volume average particle diameter D50, particles having a particle diameter of 0.5×D50 are 20% by number or less and particles having a particle diameter of 2×D50 or more are 2% by volume or less.  
      Into the toner  1  thus obtained, an external additive which has functions of, for example, increase of the powder fluidity, increase of a friction charge property, improvement of a heat-resistance property and a long-term preservation property, improvement of a cleaning property, control of the abrasion property of the surface of the photoconductor, and the like may be mixed. The external additive is, for example, silica fine powder, titanium oxide fine powder, alumina fine powder and the like. An addition amount of the external additive favorable in practical use is 1 part by weight or less with respect to 100 parts by weight of toner particles in consideration of application of a necessary charge amount for the toner, an influence on the photoconductor, environmental characteristics of the toner and so on.  
      In the color image forming developer of the invention containing the toner  1  thus produced, the volume resistivity of the black developer is substantially equal to the volume resistivity of the yellow developer, the volume resistivity of the magenta developer, and the volume resistivity of the cyan developer. Therefore, a difference is not generated between the charge property of the black developer and the charge properties of the color developers of yellow, magenta and cyan in the respective processes such as the developing process and the transfer process, so that it is possible to obtain an image of high image quality in which the harmony of colors is maintained.  
      The color image forming developer of the invention can also be a two-component developer made by forming in the above manner and mixing a carrier into the toner  1  with an external additive added as necessary. As the carrier used for the two-component developer, it is possible to use a well-known one, which is, for example, a ferrite carrier and an iron powder carrier. In a preferable mixture ratio on this occasion, 3 to 8% by weight of the toner  1  is contained in the two-component developer.  
      Further, in the case of using the two-component developer as the color image forming developer of the invention, it is preferable that the volume resistivity of the toner  1   b  of the black developer is substantially equal to the volume resistivity of the toner  1   y  of the yellow developer, the volume resistivity of the toner  1   m  of the magenta developer, and the volume resistivity of the toner  1   c  of the cyan developer. In the case of the two-component developer, by making the volume resistivities of the respective toner substantially equal, it is possible to prevent generation of a difference in charge property not only in the developing process but also in the transfer process and the cleaning process.  
      Hereinafter, a color image forming apparatus according to another embodiment of the invention in which the color image forming developer of the invention is used in a favorable manner will be described.  
       FIG. 3  is a cross section view showing, in a simplified manner, the entire configuration of a color image forming apparatus  101  according to another embodiment of the invention, and  FIG. 4  is a magnified view of the essential part of the color image forming apparatus  101  shown in  FIG. 3 . In the color image forming apparatus  101 , a yellow developer  51   y , a magenta developer  51   m , a cyan developer  51   c  and a black developer  51   b  each containing the toner  1  of the color image forming developer described above are used for development.  
      The color image forming apparatus  101  is basically composed of an image forming portion  108  and a paper feeding portion  109 , and forms a polychrome image or a monochrome image on a sheet of paper on the basis of a printing job from an information processing apparatus (not shown) such as a personal computer connected externally.  
      Hereinafter, the configuration and operation of the image forming portion  108  will be described. In the image forming portion  108 , alphabetical letters y, m, c, b attached to the respective Arabic numerals correspond to the colors of yellow, magenta, cyan and black, respectively. Hereinafter, except a case of a description by specifying a member associated with a specific color, the alphabetical letters y, m, c, b on members disposed for the respective colors will be omitted in the description. Moreover, “y-c” and “y-b” refer to “y, m, c” and “y, m, c, b,” respectively.  
      The image forming portion  108  comprises: photoconductors  21   y ,  21   m ,  21   c ,  21   b  disposed for the respective colors of yellow, magenta, cyan and black in order to form a polychrome image by the use of the respective colors; charging units  22   y ,  22   m ,  22   c ,  22   b  each of which is arranged around the photoconductor and charges the photoconductor  21 ; developing units  23   y ,  23   m ,  23   c ,  23   b  each of which supplies each of the aforementioned developers to the surface of the photoconductor  21  and forms a toner image; cleaning units  24   y ,  24   m ,  24   c ,  24   b  each of which is arranged around the photoconductor and collects toner remaining on the surface of the photoconductor  21 ; an exposing unit  10  that forms an electrostatic latent image on the surface of the photoconductor  21  with light responsive to image information; a transfer belt unit  30  that includes a transfer belt  31  to which the toner images on the surfaces of the photoconductors  21  are temporarily transferred; and a transfer roller  36  that transfers a polychrome toner image transferred to the transfer belt  31 , to a sheet of paper. A transfer unit includes the transfer belt unit  30  and the transfer roller  36 .  
      The charging units  22  uniformly charge the surfaces of the photoconductors  21  at a predetermined electric potential, respectively. The charging units  22  are disposed so as to come in contact with the photoconductors  21 , and provided with charge voltage applying means  61 ,  62  for uniformly charging the photoconductors  21  at a predetermined electric potential, respectively. The charging unit  22  of the color image forming apparatus  101  of the invention is a roller-type charging unit, but may be a charging unit of a brush type or a charger type, instead of the roller-type. As the charge voltage applying means  61 ,  62 , for example, a rectifying unit having a transformer and a rectifier, a converter or the like is used, and the charge voltage applying means  61  applies voltages to the charging units  22   y - c  and charges the photoconductors  21   y - c , whereas the charge voltage applying means  62  applies a voltage to the charging unit  22   b  and charges the photoconductor  21   b . In the color image forming apparatus of the present embodiment, a single charge voltage applying means is used as the charge voltage applying means for yellow, magenta and cyan, so that it is possible to seek reduction of the component count and miniaturization of the apparatus.  
      The exposing unit  10  is formed by a laser scanning unit (LSU) equipped with a laser irradiating portion  11 , and a polygon mirror  12  and reflection mirrors  13   y ,  13   m ,  13   c ,  13   b ,  14   y ,  14   m ,  14   c  are disposed in positions such that they can apply a laser beam emitted from the laser irradiating portion  11  to the surfaces of the photoconductors  21 . Instead of the laser irradiating portion  11 , a writing head in which light emitting elements such as ELs (electroluminescences) or LEDs (light emitting diodes) are arranged into an array may be used. The exposing unit  10  emits a laser beam from the laser irradiating portion  11  in response to image information, and exposes the laser light to the surfaces of the photoconductors  21  uniformly charged by the charging units  22  via the polygon mirror  12  and the reflection mirrors  13 ,  14 , thereby forming electrostatic latent images composed of exposed portions and unexposed portions with different charging amounts on the surfaces of the photoconductors  21 .  
      The developing units  23  are arranged so as to face the photoconductors  21 , contain developers of the respective colors of the yellow developer  51   y , the magenta developer  51   m , the cyan developer  51   c  and the black developer  51   b  as the color image forming developer of the invention in developer containing vessels (not shown), and include developing rollers  52   y ,  52   m ,  52   c ,  52   b  that supply the developers to the surfaces of the photoconductors  21 , agitating rollers  53   y ,  53   m ,  53   c ,  53   b  that agitate the developers  51 , and developing bias applying means  63 ,  64  serving as developing bias applying means.  
      In the developing unit  23 , the developing roller  52  has a cylindrical shape and is rotatably supported within the developing unit via a rotation shaft (not shown), and driven to rotate in a fixed direction by a motor (not shown).  
      The agitating roller  53  is arranged within the developing unit  23  so as to locate on the opposite side to the photoconductor  21  about the developing roller  52 , and rotatably supported in a fixed direction about the axial line thereof. The agitating roller  53  agitates the developer  51  and supplies the developer  51  to the developing roller  52 . The agitation property of the agitating roller  53  determines the charge amount of the developer  51 , but in the color image forming apparatus  101  of the invention, the volume resistivity of the black developer  51   b  is substantially equal to the volume resistivities of the color developers  51   y - c  of yellow, magenta and cyan, so that a difference is not generated in charge amounts of the respective developers even if the agitating rollers  53   y ,  53   m ,  53   c ,  53   b  of the same shapes and materials are used. Therefore, it is possible to simplify the setting of the conditions of agitating the developers, and it is possible to seek reduction of the kinds of components, so that it is possible to realize cost reduction.  
      As the developing bias applying means  63 ,  64 , for example, a rectifying unit provided with a transformer and a rectifier, a converter or the like is used. The developing bias applying means  63  charges the developing rollers  52   y - c , whereas the developing bias applying means  64  charges the developing roller  52   b . In the color image forming apparatus of the present embodiment, a single developing bias applying means is used as the developing bias applying means for yellow, magenta and cyan, so that it is possible to seek reduction of the component count and miniaturization of the apparatus.  
      In the developing unit  23 , the developer  51  supplied to the developing roller  52  while being agitated by the agitating roller  53 , is supplied to the electrostatic latent image which is formed on the surface of the photoconductor  21  by the exposing unit  10 , to form a toner image on the surface of the photoconductor  21 , by rotation of the developing roller  52  and developing bias applied to the developing roller  52 .  
      Further, the color image forming apparatus  101  of the invention does not employ a method of transferring a toner image formed on the photoconductor  21  directly to a sheet of paper, but employs a method of performing a first transfer process of once transferring by laminating toner images formed on the surfaces of the photoconductors  21 , to the transfer belt  31  in the order of yellow, magenta, cyan and black, and thereafter, performing a second transfer process of transferring onto a sheet of paper fed from the paper feeding portion  109 .  
      As a first transfer unit, the transfer belt unit  30  serving as the transfer unit relating to the invention is mounted above the photoconductors  21 . The transfer belt unit  30  has the transfer belt  31 , a transfer belt driving roller  32 , a transfer belt driven roller  33 , a transfer belt tension mechanism  34 , and intermediate transfer rollers  35   y ,  35   m ,  35   c ,  35   b.    
      The transfer belt driving roller  32  and the transfer belt driven roller  33  stretch out the transfer belt  31  to be belted, and rotate the transfer belt  31  in the direction of an arrow  81  when the driving force of the transfer belt driving roller  32  driven to rotate by the motor is transmitted to the transfer belt  31 . The transfer belt  31  is formed into an endless shape by the use of, for example, a film having a thickness of 75 to 120 μm, and the photoconductors  21  come in contact with the surface thereof. By transferring toner images of the respective colors formed on the surfaces of the photoconductors  21  in the order of yellow, magenta, cyan and black so as to be superposed, a color toner image (polychrome toner image) is formed on the transfer belt  31 .  
      Transfer of a polychrome toner image from the photoconductors  21  to the transfer belt  31  is performed by the intermediate transfer rollers  35  that come in contact with a back side of the transfer belt  31 . The intermediate transfer roller  35  is formed so that the surface of a metal shaft body having a diameter of 8 to 10 mm made of stainless steel or the like is covered with an elastic material such as EPDM with an electrically conductive filler added or urethane foam, for example. With the elastic material having electrical conductivity, the intermediate transfer rollers  35  apply voltages uniformly to the transfer belt  31 , and transfer a polychrome toner image to the transfer belt  31 . As means for applying voltages to the intermediate transfer rollers  35  in the first transfer process, the first transfer voltage applying means  65 ,  66  are used. The first transfer voltage applying means  65 ,  66  apply voltages of the opposite polarity to the charge polarity of toner, to the intermediate transfer rollers  35 , and as the first transfer voltage applying means, for example, a rectifying unit provided with a transformer and a rectifier, a converter or the like is used. The first transfer voltage applying means  65  charges the intermediate transfer rollers  35   y - c , and the first transfer voltage applying means  66  charges the intermediate transfer roller  35   b . In the color image forming apparatus of the present embodiment, a single first transfer voltage applying means is used as the first transfer voltage applying means for yellow, magenta and cyan, so that it is possible to seek reduction of the component count and miniaturization of the apparatus.  
      The cleaning units  24  remove and collect toner remaining on the surfaces of the photoconductors  21  after toner images are transferred to the transfer belt  31 . The cleaning units  24  are respectively composed of cleaning blades  54   y ,  54   m ,  54   c ,  54   b  made of an elastic material or the like, and so on.  
      In the case of using the color image forming developer of the invention, a difference is not generated between the charge property of the black developer  51   b  and the charge properties of the color developers  51   y - c , so that it is possible to make the adhesion forces of toner remaining on the surfaces of the photoconductors  21  approximately the same. Therefore, it is possible to use the same shape and material of cleaning blades  54  for all the developers, so that it is possible to seek cost reduction by reduction of the kinds of members, and it is possible to realize simplification of the setting of the operation conditions. Moreover, since the volume resistivities of the respective developers  51  are substantially equal, it becomes relatively easy to set the respective voltages so that the amount of remaining toner becomes small, and it is also possible to miniaturize the vessel for collecting removed toner.  
      In the second transfer process, a polychrome toner image formed by laminating on the intermediate transfer belt  31  is transferred onto a sheet of paper by rotation of the transfer belt  31  and rotation of the transfer roller  36  arranged in the position where the sheet of paper and the transfer belt  31  come in contact with each other. On this occasion, the transfer roller  36  and the transfer belt  31  are pressed against each other, and a high voltage of the opposite polarity to the charge polarity of toner is applied to the transfer roller  36  by second transfer voltage applying means  67  in order to transfer the polychrome toner image to the sheet of paper. As the second transfer voltage applying means  67 , for example, a rectifying unit provided with a transformer and a rectifier, a converter or the like is used. Moreover, in order to make the contact pressure between the transfer belt  31  and the transfer roller  36  constant, one of the transfer belt driving roller  32  and the transfer roller  36  is made of a hard material such as metal, and the other is made of a soft material such as elastic rubber or a foamed resin.  
      In the second transfer process, the need for changing the setting of the transfer conditions between the case of transferring a polychrome toner layer and the case of transferring a monochrome toner layer is eliminated, because the volume resistivities of all the developers are substantially equal values. Therefore, in the second transfer voltage applying means, the voltage can be set in consideration of only the kind of a conveyed sheet of paper, so that it is possible to make the apparatus have a simpler configuration.  
      Toner remaining on the transfer belt  31  without being transferred onto a sheet of paper by the transfer roller  36  is removed and collected by a transfer belt cleaning unit  37  disposed near the transfer belt driven roller  33 , because it causes a mixture of colors of toner in the following image forming process. The transfer belt cleaning unit  37  is equipped with a cleaning blade  37   a  arranged so as to abut against the transfer belt  31 .  
      In the case of using the color image forming developer of the invention, the volume resistivities of the developers are substantially equal, and the adhesion forces of toners remaining on the transfer belt  31  becomes the same, so that it becomes relatively easy to set the respective voltages so that the amount of remaining toner becomes small. Therefore, it is possible to miniaturize the vessel for collecting removed toner, thereby achieving space saving as the whole apparatus, and it is possible to decrease load to the cleaning blade  37   a.    
      Thus, the image forming portion  108  included in the image forming apparatus  101  of the invention forms a desired image on a sheet of paper. Hereinafter, the configurations and operations of the paper feeding portion  109  and a fixing unit  27  will be described.  
      The paper feeding portion  109  includes a manual-bypass tray  41  and a paper feeding cassette  42  that contain sheets of paper used for image formation. The manual-bypass tray  41  is attached externally to the casing of the image forming apparatus  101 , and a small number of desired sheets of paper are placed thereon and taken into the image forming apparatus  101 . On the other hand, the paper feeding cassette  42  is disposed below the image forming portion  108  and the exposing unit  10 , and contains a large number of sheets of paper of a size prescribed by the apparatus or a predetermined size. The sheets of paper placed on the manual-bypass tray  41  are taken into the apparatus by a pickup roller  41   a  at a timing directed through an operation panel (not shown), and conveyed to the image forming portion  108  by conveying rollers  41   b ,  41   c ,  41   d  disposed along a sheet of paper conveying path S 1 . On the other hand, the sheets of paper contained in the paper feeding cassettes  42  are separately fed one by one by a pickup roller  42   a , and conveyed to the image forming portion  108  through a conveying roller  42   b  disposed along a sheet of paper conveying path S 2 .  
      A registration rollers  26  are disposed below the transfer roller  36  and the transfer belt driving roller  32 , and the registration rollers  26  convey a sheet of paper conveyed from the paper feeding portion  109  to the transfer roller  36  at such timing that a toner image on the transfer belt  31  is transferred to a predetermined position of the sheet of paper, whereby the toner image on the transfer belt  31  is transferred onto the sheet of paper.  
      The sheet of paper onto which the toner image is transferred is conveyed substantially vertically, and reaches the fixing unit  27  disposed above the transfer roller  36 . The fixing unit  27  includes a heating roller  27   a  and a pressuring roller  27   b , keeps the heating roller  27   a  at a predetermined fixture temperature by controlling heating means such as a heater lamp on the basis of a detection value of a temperature detector (not shown), rotates the heating roller  27   a  and the pressuring roller  27   b  in the state where the sheet of paper onto which the toner image is transferred is sandwiched by the heating roller  27   a  and the pressuring roller  27   b , and thermally fixes the toner image onto the sheet of paper by heat of the heating roller  27   a . The sheet of paper on which the toner image is thermally fixed is discharged by a conveying roller  27   c  disposed near the outlet of the fixing unit  27 .  
      In the case of a single-side printing request, the sheet of paper passed through the fixing unit  27  is discharged onto a discharge tray  43  through a discharge roller  28  in the state where the image formed surface of the sheet of paper faces downward. On the other hand, in the case of a double-side printing request, the sheet of paper is temporarily retained by the discharge roller  28 , and the sheet of paper is guided to a double-side document conveying path S 3  by causing the discharge roller  28  to reversely rotate, and conveyed to the registration rollers  26  again by conveying rollers  29   a ,  29   b . Then, a toner image is transferred and thermally fixed on another side of the sheet of paper, and the sheet of paper is discharged onto the discharge tray  43  by the discharge roller  28 .  
       FIG. 5  is a magnified view of the essential part of a color image forming apparatus according to still another embodiment of the invention. The color image forming apparatus of the present embodiment is similar to the color image forming apparatus  101  of the aforementioned embodiment, so that corresponding portions will be denoted by the same reference numerals and the description thereof will be omitted, and a configuration view of the whole will be omitted.  
      What should be noted in the color image forming apparatus of the present embodiment is that a single charge voltage applying means  71 , a single developing bias applying means  72 , and a single first transfer voltage applying means  73  are disposed. As these voltage applying means, a rectifying unit provided with a transformer and a rectifier, a converter or the like can be used as in the aforementioned case.  
      By using the color image forming developer of the invention as a developer, the volume resistivity of the black developer  51   b  and the volume resistivities of the color developers  51   y - c  of yellow, magenta and cyan become substantially equal, so that a difference in charge property is not generated between the black developer  51   b  and the color developers  51   y - c  of yellow, magenta and cyan.  
      Accordingly, it is possible to charge the photoconductors  21   y - b  by the single charge voltage applying means  71  in the charging process, it is possible to develop by applying developing bias by the single developing bias applying means  72  in the developing process, and it is possible to charge the intermediate transfer rollers  35   y - b  by the single first transfer voltage applying means  73  and transfer a color image to the transfer belt  31  in the intermediate transfer process.  
      Since the volume resistivities of all the developers are substantially equal as described above, there is no need to provide voltage applying means for each developer in all the processes, and it is possible to decrease the component count, so that it is possible to realize space saving. Moreover, since it is possible to obtain a favorable image even when applying a voltage under the same conditions for each color, the need for changing the setting for each developer is eliminated, and it is possible to simplify the configuration of the apparatus.  
      The invention is not limited to the abovementioned embodiments, and it is needless to say that the abovementioned embodiments can be modified and changed within the scope of the invention.  
     EXAMPLE  
      Hereinafter, an example of the invention will be described.  
      The black developer of the invention was produced by the use of equal weights of C.I. pigment yellow 17 as the yellow coloring agent  3   y , C.I. pigment red 57:1 as the magenta coloring agent  3   m  and C.I pigment blue 15 as the cyan coloring agent  3   c  so that the density of the coloring agent in the developer became 7% by weight.  
      Further, the yellow developer was produced by the same method as the black developer except that only the aforementioned yellow coloring agent  3   y  was used as the coloring agent  3 .  
      The magenta developer was produced by the same method as the black developer except that only the aforementioned magenta coloring agent  3   m  was used as the coloring agent  3 .  
      The cyan developer was produced by the same method as the black developer except that only the aforementioned cyan coloring agent  3   c  was used as the coloring agent  3 .  
      On the other hand, as a comparative example, a black developer of the comparative example was produced by the same method as the black developer of the invention except that carbon black was used as a coloring agent.  FIG. 6  is a schematic cross section view showing, in a simplified manner, the structure of toner  4  of the black developer using carbon black  5  as a coloring agent according to the comparative example to the invention.  
      The yellow developer, the magenta developer, the cyan developer and the black developer produced in the above manner, and the black developer using the carbon black  5  as a coloring agent of the comparison example were pressurized so as to become pellets, and measured by a dielectric loss measuring apparatus (produced by Ando Electric Co., Ltd.). The result is shown in  FIG. 7 . The color developers of yellow, magenta and cyan exhibited volume resistivities of 1×10 11 ±0.5×10 11  Ω·cm. Moreover, the volume resistivity of the black developer of the invention was 1.8×10 11  Ω·cm, whereas the volume resistivity of the black developer using carbon black as a coloring agent of the comparison example was 9×10 9 ±2.0×10 9  Ω·cm.  
      It becomes clear from  FIG. 7  that the volume resistivity of the black developer of the invention was substantially equal to the volume resistivities of the color developers of the yellow developer, the magenta developer and the cyan developer. Therefore, the black developer of the invention does not cause a difference in charge property between the black developer itself and the color developers even if going through the respective processes for forming a color image. In the case of using this developer in a color image forming apparatus, it is possible to obtain an image of high image quality in which the harmony of colors is maintained, and it is possible to miniaturize the color image forming apparatus by making the charge electric potential that a charging unit charges a photoconductor, the application voltage of a developing tank containing the developer and the conditions of agitating the developer in the developing unit, the transfer electric potential that the transfer unit applies to a to-be-transferred material and the contact pressure of a cleaning unit to the surface of the photoconductor the same for the black developer and the color developers, and simplify the configuration of the apparatus.  
      The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and the range of equivalency of the claims are therefore intended to be embraced therein.