Abstract:
A developing device includes a developer carrier and a developer regulating member including a developer regulating part opposing a surface of the developer carrier to regulate the developer carried and conveyed by the developer carrier. The developer regulating member is formed from a single metallic member and includes a space that faces an inner surface of the metallic member. The space extends in a direction perpendicular to a moving direction of the surface of the developer carrier. The developing device can include a cooling device that cools the developer regulating member from an inner surface side of the metallic member facing the space.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present application claims priority to Japanese Patent Application No. 2001-359098 filed in the Japanese Patent Office on Nov. 26, 2001, the disclosure of which is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a developing device and an electrophotographic image forming apparatus such as a copying machine, a printer, a facsimile machine, or other similar image forming apparatus including the developing devices, and more particularly relates to a developing device using a developer including toner and carrier. 
   2. Discussion of the Background 
   In an electrophotographic image forming method, an electrostatic latent image formed on a latent image carrier is developed with a developer containing a toner. The toner needs to be appropriately charged in the developer to develop the latent image. Generally, there are two methods of developing an electrostatic latent image: (1) a method of developing an electrostatic latent image with a two-component developer including a mixture of toner and carrier, and (2) a method of developing an electrostatic latent image with a one-component developer including toner as a main component. 
   The developing method using the one-component developer has a disadvantage such as unstable charging property of toner. In the developing method using the two-component developer, a relatively stable good quality image can be obtained. However, deterioration of carrier and variations of the mixing ratio of toner and carrier may tend to occur. When repeatedly developing electrostatic latent images with a two-component developer, a toner density (i.e., a weight ratio of toner to the developer) varies due to consumption of toner in the two-component developer. Therefore, the toner density needs to be controlled by supplying toner to the developer in order to obtain a stable good quality image. 
   In order to control the toner density, a toner supply control method has been proposed in which a toner supplying device controls the toner supply based on data of a toner density in a developing device. The density is detected by a toner density detecting device using a transmission sensor, a fluidity sensor, an image density sensor, a bulk density sensor, etc. As a recent trend, the image density sensor or a combination of the image density sensor and a magnetic permeability sensor (a kind of the bulk density sensor) is widely used. 
   In the toner supply control method using the image density sensor, an image pattern formed on a latent image carrier is developed with a two-component developer and exposed to light. A toner supply amount is controlled by detecting the image density of the developed image pattern based on the light reflected from the developed image pattern. In the toner supply control method using the combination of the image density sensor and the magnetic permeability sensor, a toner supply amount is controlled by changing a target value of the magnetic permeability sensor according to the image density of the developed image pattern. 
   The carrier in the two-component developer includes a core material covered with a resin coating layer. The resin coating layer is used for various purposes such as prevention of toner from forming films on the core material, provision of a uniform, non-abrasive surface, prevention of surface oxidation, prevention of moisture absorption, extension of useful lifetime, protection of a latent image carrier from damages or abrasion by carrier, control of charging polarity, and control of a charging amount. For example, a carrier core material may be coated with a resin material (for example, described in the published Japanese patent application No. 58-108548), or a resin coating layer to which various additives are added (for example, described in the published Japanese patent application Nos. 54-155048, 57-40267, 58-108549, 59-166968, 6-202381, and in the Japanese patent publication Nos. 1-19584, 3-628). Further, additives may be adhered onto a carrier surface (for example, described in the published Japanese patent application No. 5-273789), or a carrier core material may be covered with a resin coating layer containing a conductive powder in which the average particle diameter of the conductive powder is equal to the thickness of the resin coating layer or greater (for example, described in the published Japanese patent application No. 9-160304). Moreover, a carrier coating material may include benzoguanamines-n-butyl alcohol-formaldehyde copolymers as a main component (for example, described in the published Japanese patent application No. 8-6307), or a melamine resin crosslinked with an acrylic resin (for example, described in the Japanese Patent No. 2683624). 
   Even though a resin coating layer is provided with a core material of carrier, the following problem may arise. When an original document having a low image area (e.g., an occupation ratio of an image on the original document is 3% or less) which subjects a two-component developer to much stresses, is repeatedly printed or copied, the charging amount of carrier increases due to the frictional charging of toner and carrier. As a result, a phenomenon in which a bulk density of the developer decreases due to the repulsive force between carrier particles, may occur. This phenomenon is accelerated when the external agents of toner become embedded in the toner due to rubbing against the toner between the carrier particles, and the fluidity of the entire developer decreases. 
   The above-described magnetic permeability sensor detects a distance between the magnetic carrier and the sensor. The detected value of the magnetic permeability sensor decreases as the carrier is away from the sensor and as the carrier becomes sparse in the developer. Therefore, when the carrier is away from the sensor and is sparse in the developer due to the decrease of the bulk density of the developer, the detected value of the magnetic permeability sensor decreases, and therefore the sensor erroneously detects that the toner density has increased, although the toner density has not varied. Because the toner supplied to the developer is decreased based on the above detection output of the sensor, the toner density in the developer decreases, thereby deteriorating developing performance. As described above, when the two-component developer is used in a high-stress condition, the bulk density of the developer varies, thereby causing the toner density to be unstably controlled. 
   SUMMARY OF THE INVENTION 
   According to an aspect of the present invention, a developing device includes a developer including toner having a coloring agent dispersed in a binder resin, and carrier having a core material, and a coating layer covering the core material and containing a binder resin and a powder, a toner density detecting device configured to detect a toner density of the developer by use of a bulk density sensor, and a control device configured to control the toner density based on a detection result of the toner density detecting device. The toner density is controlled such that a ratio (D/h) of an average particle diameter (D) of the powder to a thickness of the coating layer is greater than 1 and less than 10. 
   Objects, features, and advantages of the present invention will become apparent from the following detailed description when read in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the present invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
       FIG. 1  is a schematic view of a laser printer according to an embodiment of the present invention; 
       FIG. 2  is a schematic enlarged view of a construction of an image forming device that forms a magenta toner image in the laser printer of  FIG. 1 ; 
       FIG. 3  is a table showing results of running tests performed in Examples 1 through 5 and Comparative examples 1 and 2; 
       FIG. 4  is a table showing results of variations in bulk specific gravity of developer during a running test of 900 copies in Examples 1 through 5 and Comparative examples 1 and 2; 
       FIG. 5  is a graph showing a relationship between the output voltage of a magnetic permeability sensor and the number of copies in a running test performed in Example 1 and Comparative example 1; and 
       FIG. 6  is a graph showing a relationship between bulk specific gravity of a developer and the number of copies in a running test performed in Example 1 and Comparative example 1. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Preferred embodiments of the present invention are described in detail referring to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views. 
   In the preferred embodiment, the present invention is applied to an electrophotographic color laser printer (hereafter referred to as a laser printer) as an example of an image forming apparatus.  FIG. 1  is a schematic view of a laser printer according to an embodiment of the present invention. The laser printer of  FIG. 1  includes four image forming devices  1 M,  1 C,  1 Y, and  1 BK for respectively forming a magenta (hereafter abbreviated as “M”), cyan (“C”), yellow (“Y”), and black (“BK”) toner images, arranged in the above order from an upstream side in a moving direction of a transfer sheet  100  (illustrated in  FIG. 2 ) as a transfer material indicated by arrow (A) in FIG.  1 . The image forming devices  1 M,  1 C,  1 Y, and  1 BK respectively include photoreceptor units each including photoconductive drums  11 M,  11 C,  11 Y, and  11 BK serving as image carriers, and developing devices. The image forming devices  1 M,  1 C,  1 Y, and  1 BK are arranged such that rotation shafts of the photoconductive drums  11 M,  11 C,  11 Y, and  11 BK are parallel to each other at a predetermined pitch in the moving direction of the transfer sheet  100 . 
   The laser printer of  FIG. 1  further includes a laser writing unit  2  as a latent image forming device, sheet feeding cassettes  3  and  4 , and a transfer unit  6  including a transfer belt  60  serving as a transfer material conveying belt that conveys the transfer sheet  100  toward transfer sections each facing the photoconductive drums  11 M,  11 C,  11 Y, and  11 BK. The laser printer further includes a pair of registration rollers  5  that feed the transfer sheet  100  to the transfer belt  60 , a fixing unit  7  using a fixing belt, a sheet discharging tray  8 , and a sheet reversing unit  9 . Although not shown, the laser printer of  FIG. 1  further includes a manual sheet feeding tray, a toner supply container, a waste-toner bottle, a power supply unit, and other features of a laser printer known by one of ordinary skill in the art. 
   The laser writing unit  2  includes a power supply, a polygonal mirror, an f-θ lens, and reflection mirrors. The laser writing unit  2  irradiates the surfaces of the photoconductive drums  11 M,  11 C,  11 Y, and  11 BK with a laser beam based on image data of original documents. 
   Referring to  FIG. 1 , a conveyance path of the transfer sheet  100  is indicated by the dot-and-dash lines. The transfer sheet  100  fed from the sheet feeding cassettes  3  or  4  is conveyed by sheet conveying rollers while being guided by sheet guiding members (not shown) and is further conveyed to the registration rollers  5 . The registration rollers  5  feed out the transfer sheet  100  to the transfer belt  60  at an appropriate timing. Subsequently, the transfer sheet  100  is conveyed by the transfer belt  60  such that the transfer sheet  100  passes through transfer sections each facing the photoconductive drums  11 M,  11 C,  11 Y, and  11 BK. 
   With the above-described construction and operation of the laser printer of  FIG. 1 , toner images of respective colors formed on the photoconductive drums  11 M,  11 C,  11 Y, and  11 BK by the image forming devices  1 M,  1 C,  1 Y, and  1 BK are sequentially transferred onto the transfer sheet  100  while being superimposed upon each other. As a result, a superimposed color toner image is formed on the transfer sheet  100 . The transferred color toner image is fixed onto the transfer sheet  100  in the fixing unit  7 . Subsequently, the transfer sheet  100  having a fixed image is discharged onto the sheet discharging tray  8 . 
     FIG. 2  is a schematic enlarged view of a construction of the image forming device  1 M that forms a magenta toner image. The configurations of the image forming devices  1 M,  1 C,  1 Y, and  1 BK are substantially the same except for the color of their toner. For this reason, only the configuration of the image forming device  1 M will be described hereinafter. 
   Referring to  FIG. 2 , the image forming device  1 M includes a photoreceptor unit  10 M and a developing device  20 M. The photoreceptor unit  10 M includes the photoconductive drum  11 M, a cleaning blade  13 M that swings to remove residual toner remaining on the surface of the photoconductive drum  11 M, and a non-contact type charging roller  15 M that uniformly charges the surface of the photoconductive drum  11 M. The image forming device  1 M further includes a lubricant applying/discharging brush roller  12 M that applies a lubricant onto the surface of the photoconductive drum  11 M and also discharges the surface of the photoconductive drum  11 M. The lubricant applying/discharging brush roller  12 M includes a brush portion formed from conductive fibers and a core metal portion. A power supply (not shown) is connected to the core metal portion so as to apply a discharging bias to the core metal portion. 
   In the photoreceptor unit  10 M, the charging roller  15 M, to which a voltage is applied, uniformly charges the surface of the photoconductive drum  11 M. Subsequently, the surface of the photoconductive drum  11 M is exposed to a laser beam modulated and deflected in the laser writing unit  2 , and thereby an electrostatic latent image is formed on the surface of the photoconductive drum  11 M. The electrostatic latent image formed on the photoconductive drum  11 M is developed with magenta toner by the developing device  20 M and formed into a magenta toner image. At a transfer section (Pt) where the transfer sheet  100  carried on the transfer belt  60  passes through, the magenta toner image on the photoconductive drum  11 M is transferred onto the transfer sheet  100 . After the magenta toner image is transferred from the photoconductive drum  11 M onto the transfer sheet  100 , the lubricant applying/discharging brush roller  12 M applies a predetermined amount of lubricant onto the surface of the photoconductive drum  11 M, and discharges the surface of the photoconductive drum  11 M. The residual toner remaining on the surface of the photoconductive drum  11 M is removed by the cleaning blade  13 M. As a result, the surface of the photoconductive drum  11 M is prepared for a next image forming operation. 
   The developing device  20 M uses a two-component developer  28 M (hereafter simply referred to as a “developer”) including magnetic carrier and negatively charged magenta toner to develop an electrostatic latent image formed on the photoconductive drum  11 M. The developing device  20 M includes a case  21 M, a developing sleeve  22 M serving as a developer carrier formed from a non-magnetic material, and a magnet roller (not shown) serving as a magnetic field generating device fixed inside of the developing sleeve  22 M. The developing sleeve  22 M is arranged such that a part of the developing sleeve  22 M is exposed to outside through an opening of the case  21 M to face the photoconductive drum  11 M. The developing device  20 M further includes developer conveying screws  23 M and  24 M, a doctor blade  25 M, a magnetic permeability sensor  26 M serving as a toner density detecting device that detects the magnetic permeability of the developer  28 M, a toner cartridge  29 M that contains magenta toner, and a powder pump  27 M. A developing bias voltage, in which an alternating current (AC) voltage is superimposed on a negative direct current (DC) voltage, is applied from a developing bias power supply (not shown), serving as a developing electric field generating device, to the developing sleeve  22 M. Thereby, the developing sleeve  22 M is biased with a predetermined voltage relative to a substrate layer of the photoconductive drum  11 M. 
   Referring to  FIG. 2 , the developer  28 M contained in the case  21 M is charged by friction while being agitated and conveyed by the developer conveying screws  23 M and  24 M. A part of the developer  28 M is carried on the surface of the developing sleeve  22 M, and a thickness of the developer  28 M is regulated by the doctor blade  25 M. Subsequently, the developer  28 M is conveyed to a development position opposite to the photoconductive drum  11 M. At the development position, an electrostatic latent image on the photoconductive drum  11 M is developed with charged magenta toner in the developer  28 M carried on the developing sleeve  22 M. 
   Because the density of magenta toner in the developer  28 M contained in the case  21 M decreases due to the consumption of the developer in the image forming operation, the magenta toner is supplied from the toner cartridge  29 M into the case  21 M through the powder pump  27 M according to an image area and a detected value (Vt) of the magnetic permeability sensor  26 M. Thereby, the density of magenta toner is maintained at a predetermined value. The developing device  20 M includes a control device  30 M including a central processing unit (CPU), a read-only memory (ROM), a random-access memory (RAM), and an input/output (I/O) interface, so as to control the toner density. 
   Specifically, the control device  30 M calculates a difference (ΔT) between a target value (Vref) of toner density and the detected value (Vt) of the magnetic permeability sensor  26 M. When the difference (ΔT) is positive, the control device  30 M judges that the toner density is sufficiently high and controls the toner cartridge  29 M to reduce the supply of magenta toner sent into the case  21 M. When the difference (ΔT) is negative, the control device  30 M judges that the toner density is too low and controls the toner cartridge  29 M to increase the supply of magenta toner sent into the case  21 M relative to greater the absolute value of the difference (ΔT). The amount of toner supplied into the case  21 M is controlled to increase such that the detected value (Vt) of the magnetic permeability sensor  26 M approaches the target value (Vref). The target value (Vref), the charging potential, and the laser amount are preferably set by a process control performed one time for every 10 copies (about 5 to 200 copies depending on a copying speed). For example, each toner density of a plurality of halftone and solid filled pattern images formed on the photoconductive drum  11 M is detected by a reflection toner density sensor, and an adhesion amount of toner is calculated. Then, the target value (Vref), the charging potential, and the laser amount are set such that a target adhesion amount of toner can be obtained. 
   In the laser printer of  FIG. 1 , one of the four photoconductive drums  11 M,  11 C,  11 Y,  11 BK located at the most downstream side in the moving direction of the transfer sheet  100  (i.e., the photoconductive drum  11 BK in  FIG. 1 ) is in constant contact with the transfer belt  60 . The photoconductive drums  11 M,  11 C, and  11 Y are configured to be brought into contact with and separated from the transfer belt  60 . 
   In a multi-color image formation mode, the four photoconductive drums  11 M,  11 C,  11 Y, and  11 BK are brought in contact with the transfer belt  60 . An adsorbing bias applying roller  61  applies an electric charge having a polarity equal to that of the toner to the transfer sheet  100  to adsorb the transfer sheet  100  to the transfer belt  60 . The transfer sheet  100  is conveyed while being adsorbed to the transfer belt  60 . The magenta, cyan, and yellow toner images respectively formed on the photoconductive drums  11 M,  11 C, and  11 Y are sequentially transferred onto the transfer sheet  100  while being superimposed upon each other. Lastly, the black toner image formed on the photoconductive drum  11 BK is transferred onto the superimposed color toner image on the transfer sheet  100 . Subsequently, the transferred multi-color toner image on the transfer sheet  100  is fixed thereonto in the fixing unit  7 . 
   In a single color image formation mode in which a black image is formed on the transfer sheet  100 , the photoconductive drums  11 M,  11 C, and  11 Y are separated from the transfer belt  60  and only the photoconductive drum  11 BK is brought in contact with the transfer belt  60 . The transfer sheet  100  is conveyed to a transfer section formed between the photoconductive drum  11 BK and the transfer belt  60 , and the black toner image formed on the photoconductive drum  11 BK is transferred onto the transfer sheet  100 . The transferred black toner image is fixed onto the transfer sheet  100  in the fixing unit  7 . 
   Having generally described this invention, further understanding can be obtained by reference to certain specific examples which are provided herein for the purpose of illustration only and are not intended to be limiting. In each of the examples and comparative examples described below, the mechanical conditions and toner conditions are maintained as shown in Table 1, while the carrier conditions are changed among the examples. Parts and percentages are determined by weight. 
   
     
       
             
           
             
             
             
           
             
             
           
         
             
               TABLE 1 
             
             
                 
             
           
           
             
               &lt;mechanical conditions&gt; 
             
           
        
         
             
               Gap between developing sleeve and 
               0.5 
               mm 
             
             
               photoconductive drum: 
             
             
               Gap between developing sleeve and doctor 
               0.75 
               mm 
             
             
               blade: 
             
             
               Diameter of developing sleeve: 
               18 
               mm 
             
             
               Linear velocity of photoconductive drum: 
               125 
               mm/sec 
             
             
               Ratio of linear velocity of developing roller 
               1.5 
             
             
               relative to linear velocity of photoconductive 
             
             
               drum: 
             
           
        
         
             
               Toner density sensor: 
               Magnetic 
             
             
                 
               permeability 
             
             
                 
               sensor 
             
             
               &lt;Toner conditions&gt; 
             
             
               Polyol resins 
             
             
               Weight average particle diameter: 
               6 μm to 7 μm 
             
             
               External additives: 
               1.85 parts by 
             
             
                 
               weight per 100 
             
             
                 
               parts by weight 
             
             
                 
               of toner 
             
             
                 
             
           
        
       
     
   
   EXAMPLE 1 
   The carrier conditions for example 1 were as follows: 
   
     
       
             
           
             
             
             
             
           
         
             
                 
             
             
               &lt;Carrier conditions&gt; 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
                 
               Acrylic resin solution: 
               56 
               parts 
             
             
                 
               (solid content: 50%) 
             
             
                 
               Guanamine solution: 
               15.6 
               parts 
             
             
                 
               (solid content: 77%) 
             
             
                 
               Alumina particles: 
               160 
               parts 
             
             
                 
               (average particle diameter: 0.3 μm, 
             
             
                 
               resistivity: 10 14  Ω-cm) 
             
             
                 
               Toluene: 
               900 
               parts 
             
             
                 
               Butyl cellosolve: 
               900 
               parts 
             
             
                 
                 
             
           
        
       
     
   
   The above-described components of carrier were mixed with a homomixer for 10 minutes to prepare a resin layer coating liquid. The resin layer coating liquid was applied to ferrite particles as a carrier core material by SPIRA COTA (manufactured by Okada Seiko K.K.) and dried to form a resin coating layer of 0.15 μm in thickness. The coated particles were then calcined at 150° C. for one hour in an electric oven and the resulting bulk of the ferrite particles were crushed and sieved with a sieve having a sieve opening of 100 μm to obtain a carrier. The thickness of the resin coating layer of the carrier was found by measurement of cross-sections of the carrier with a transmission electron microscope, and was defined by the mean value of the measured carrier. The carrier core material preferably has an average particle diameter of at least about 20 μm to prevent the carrier from adhering onto the photoconductive drum as the image carrier, and preferably has an average particle diameter of not greater than about 100 μm to prevent image deterioration caused by, for example, carrier streak. Specific examples of the core material include materials known as electrophotographic two-component carrier such as ferrite, magnetite, iron, nickel, and the like. 
   The thus obtained carrier was subjected to a running test in which 900 copies were continuously produced using a digital full color copier (Ipsio Color 8000 manufactured by Ricoh Company, Ltd.) using a single black color toner. Specifically, 900 copies of an original document having no image were continuously produced to subject a two-component developer to extreme stresses. The results are shown in  FIGS. 3 and 4 . Further, the measurement result of variations in output voltage (Vt) of the magnetic permeability sensor in the running test is shown in  FIG. 5 , and the measurement result of variations in bulk specific gravity of the developer in the running test is shown in FIG.  6 . 
   EXAMPLE 2 
   The carrier conditions for Example 2 were as follows: 
   
     
       
             
           
             
             
             
             
           
         
             
                 
             
             
               &lt;Carrier conditions&gt; 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
                 
               Silicone resin solution: 
               227 
               parts 
             
             
                 
               (SR2411 manufactured by Dow 
             
             
                 
               Corning-Toray Silicone Co., Ltd., 
             
             
                 
               solid content: 15%) 
             
             
                 
               γ-(2-Aminoethyl) aminopropyl 
               6 
               parts 
             
             
                 
               trimethoxysilane: 
             
             
                 
               Alumina particles: 
               160 
               parts 
             
             
                 
               (average particle diameter: 0.3 μm, 
             
             
                 
               resistivity: 10 14  Ω-cm) 
             
             
                 
               Toluene: 
               900 
               parts 
             
             
                 
               Butyl cellosolve: 
               900 
               parts 
             
             
                 
                 
             
           
        
       
     
   
   The above-described components of carrier were mixed with a homomixer for 10 minutes to prepare a resin layer coating liquid. The resin layer coating liquid was applied to ferrite particles as a carrier core material by SPIRA COTA (manufactured by Okada Seiko K.K.) and dried to form a resin coating layer of 0.15 μm in thickness. The coated particles were then calcined at 300° C. for two hours in an electric oven and the resulting bulk of the ferrite particles were crushed and sieved with a sieve having a sieve opening of 100 μm to obtain a carrier. The thus obtained carrier was subjected to a running test in the same manner as that in Example 1. The results are shown in  FIGS. 3 and 4 . 
   EXAMPLE 3 
   The carrier conditions for Example 3 were as follows: 
   
     
       
             
           
             
             
             
             
           
         
             
                 
             
             
               &lt;Carrier conditions&gt; 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
                 
               Acrylic resin solution: 
               56 
               parts 
             
             
                 
               (solid content: 50%) 
             
             
                 
               Guanamine solution: 
               15.6 
               parts 
             
             
                 
               (solid content: 77%) 
             
             
                 
               Silica particles: 
               160 
               parts 
             
             
                 
               (average particle diameter: 0.2 μm, 
             
             
                 
               resistivity: 10 13  Ω-cm) 
             
             
                 
               Toluene: 
               900 
               parts 
             
             
                 
               Butyl cellosolve: 
               900 
               parts 
             
             
                 
                 
             
           
        
       
     
   
   The above-described components of carrier were mixed with a homomixer for 10 minutes to prepare a resin layer coating liquid. The resin layer coating liquid was applied to ferrite particles as a carrier core material by SPIRA COTA (manufactured by Okada Seiko K.K.) and dried to form a resin coating layer of 0.10 μm in thickness. The coated particles were then calcined at 150° C. for one hour in an electric oven and the resulting bulk of the ferrite particles were crushed and sieved with a sieve having a sieve opening of 100 μm to obtain a carrier. The thus obtained carrier was subjected to a running test in the same manner as that in Example 1. The results are shown in  FIGS. 3 and 4 . 
   EXAMPLE 4 
   The carrier conditions for Example 4 were as follows: 
   
     
       
             
           
             
             
             
             
           
         
             
                 
             
             
               &lt;Carrier conditions&gt; 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
                 
               Acrylic resin solution: 
               30 
               parts 
             
             
                 
               (solid content: 50%) 
             
             
                 
               Guanamine solution: 
               8.3 
               parts 
             
             
                 
               (solid content: 77%) 
             
             
                 
               Silica particles: 
               160 
               parts 
             
             
                 
               (average particle diameter: 0.2 μm, 
             
             
                 
               resistivity: 10 13  Ω-cm) 
             
             
                 
               Toluene: 
               900 
               parts 
             
             
                 
               Butyl cellosolve: 
               900 
               parts 
             
             
                 
                 
             
           
        
       
     
   
   The above-described components of carrier were mixed with a homomixer for 10 minutes to prepare a resin layer coating liquid. The resin layer coating liquid was applied to ferrite particles as a carrier core material by SPWRA COTA (manufactured by Okada Seiko K.K.) and dried to form a resin coating layer of 0.08 μm in thickness. The coated particles were then calcined at 150° C. for one hour in an electric oven and the resulting bulk of the ferrite particles were crushed and sieved with a sieve having a sieve opening of 100 μm to obtain a carrier. The thus obtained carrier was subjected to a running test in the same manner as that in Example 1. The results are shown in  FIGS. 3 and 4 . 
   EXAMPLE 5 
   The carrier conditions for Example 5 were as follows: 
   
     
       
             
           
             
             
             
             
           
         
             
                 
             
             
               &lt;Carrier conditions&gt; 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
                 
               Acrylic resin solution: 
               30 
               parts 
             
             
                 
               (solid content: 50%) 
             
             
                 
               Guanamine solution: 
               8.3 
               parts 
             
             
                 
               (solid content: 77%) 
             
             
                 
               Silica particles: 
               160 
               parts 
             
             
                 
               (average particle diameter: 0.2 μm, 
             
             
                 
               resistivity: 10 13  Ω-cm) 
             
             
                 
               Toluene: 
               900 
               parts 
             
             
                 
               Butyl cellosolve: 
               900 
               parts 
             
             
                 
                 
             
           
        
       
     
   
   The above-described components of carrier were mixed with a homomixer for 10 minutes to prepare a resin layer coating liquid. The resin layer coating liquid was applied to ferrite particles as a carrier core material by SPIRA COTA (manufactured by Okada Seiko K. K.) and dried to form a resin coating layer of 0.03 μm in thickness. The coated particles were then calcined at 150° C. for one hour in an electric oven and the resulting bulk of the ferrite particles were crushed and sieved with a sieve having a sieve opening of 100 μm to obtain a carrier. The thus obtained carrier was subjected to a running test in the same manner as that in Example 1. The results are shown in  FIGS. 3 and 4 . 
   COMPARATIVE EXAMPLE 1 
   The carrier conditions for comparative Example 1 were as follows: 
   
     
       
             
           
             
             
             
             
           
         
             
                 
             
             
               &lt;Carrier conditions&gt; 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
                 
               Acrylic resin solution: 
               56 
               parts 
             
             
                 
               (solid content: 50%) 
             
             
                 
               Guanamine solution: 
               15.6 
               parts 
             
             
                 
               (solid content: 77%) 
             
             
                 
               Toluene: 
               900 
               parts 
             
             
                 
               Butyl cellosolve: 
               900 
               parts 
             
             
                 
                 
             
           
        
       
     
   
   The above-described components of carrier were mixed with a homomixer for 10 minutes to prepare a resin layer coating liquid. The resin layer coating liquid was applied to ferrite particles as a carrier core material by SPIRA COTA (manufactured by Okada Seiko K. K.) and dried to form a resin coating layer of 0.15 μm in thickness. The coated particles were then calcined at 150° C. for one hour in an electric oven and the resulting bulk of the ferrite particles were crushed and sieved with a sieve having a sieve opening of 100 μm to obtain a carrier. The thus obtained carrier was subjected to a running test in the same manner as that in Example 1. The results are shown in  FIGS. 3 and 4 . Further, the measurement result of variations in output voltage (Vt) of the magnetic permeability sensor in the running test is shown in  FIG. 5 , and the measurement result of variations in bulk specific gravity of the developer in the running test is shown in FIG.  6 . 
   COMPARATIVE EXAMPLE 2 
   The carrier conditions for comparative Example 2 were as follows: 
   
     
       
             
           
             
             
             
             
           
         
             
                 
             
             
               &lt;Carrier conditions&gt; 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
                 
               Acrylic resin solution: 
               56 
               parts 
             
             
                 
               (solid content: 50%) 
             
             
                 
               Guanamine solution: 
               15.6 
               parts 
             
             
                 
               (solid content: 77%) 
             
             
                 
               Titanium oxide particles: 
               26.7 
               parts 
             
             
                 
               (average particle diameter: 0.02 μm, 
             
             
                 
               resistivity: 10 7  Ω-cm) 
             
             
                 
               Toluene: 
               900 
               parts 
             
             
                 
               Butyl cellosolve: 
               900 
               parts 
             
             
                 
                 
             
           
        
       
     
   
   The above-described components of carrier were mixed with a homomixer for 10 minutes to prepare a resin layer coating liquid. The resin layer coating liquid was applied to ferrite particles as a carrier core material by SPIRA COTA (manufactured by Okada Seiko K. K.) and dried to form a resin coating layer of 0.15 μm in thickness. The coated particles were then calcined at 150° C. for one hour in an electric oven and the resulting bulk of the ferrite particles were crushed and sieved with a sieve having a sieve opening of 100 μm to obtain a carrier. The thus obtained carrier was subjected to a running test in the same manner as that in Example 1. The results are shown in  FIGS. 3 and 4 . 
   As seen from the results in  FIGS. 5 and 6 , the carrier of Example 1 containing an alumina powder having the resistivity of 10 14  Ω-cmm, the ratio (D/h) of 2.0, and the content ratio of 80 wt % gives good results in which the variations in the bulk specific gravity of the developer are relatively small and the variations in the output voltage of the magnetic permeability sensor are little. Although not shown in  FIGS. 5 and 6 , as similarly in Example 1, the carrier of Examples 2 to 5 containing alumina or silica powder having the resistivity of 10 12  Ω-cm or greater, the ratio (D/h) of greater than 1 and less than 10, and the content ratio from 50 to 95 wt % gives good results in which the variations in the bulk specific gravity of the developer are relatively small. 
   On the other hand, as seen from the results in  FIGS. 5 and 6 , the carrier of Comparative example 1 not containing a powder does not give good results because the variations in the bulk specific gravity of the developer are greater than that in Example 1 and the variations in the output voltage of the magnetic permeability sensor are relatively great. Although not shown in  FIGS. 5 and 6 , as similarly in Comparative example 1, the carrier of Comparative example 2 containing a titanium oxide powder, which does not satisfy the above-described conditions of the resistivity of 10 12  Ω-cm or greater, the ratio (D/h) of greater than 1 and less than 10, and the content ratio from 50 to 95 wt %, does not give good results because the variations in the bulk specific gravity of the developer are relatively great. 
   Thus, as a result of the investigations described above, the present inventors found that when the ratio (D/h) of an average particle diameter (D) of the powder in the coating layer of the carrier to a thickness (h) of the coating layer is greater than 1 and less than 10, preferably greater than 1 and less than 5, a good effect of suppressing the variations in the bulk density of the developer is obtained, even though the developer is subjected to much stresses. It is considered that because the powder protrudes through the surface of the coating layer of the carrier, a contact area of carrier particles while being agitated is reduced, thereby decreasing the charging amount of the carrier. Further, it is considered that because the protrusion of the powder from the surface of the coating layer provides space between carrier particles, the extent of rubbing against toner while being agitated is reduced, thereby preventing external agents of the toner from being embedded in the toner (hereinafter referred to as a space effect). 
   With the above-described conditions, when the toner density is constant, the phenomenon in which the bulk density of the developer decreases can be suppressed, thereby reducing the variations in the bulk density of the developer. Thus, in the image forming apparatus according to the present embodiment, variations in the bulk density of the developer due to causes other than the toner density can be suppressed, thereby preventing the detection error of the bulk density sensor. Therefore, the toner density can be stably controlled. 
   When the ratio (D/h) is 1 or less, the powder is buried within the coating layer, and the above-described good effect is hard to be obtained. When the ratio (D/h) is 10 or greater, the powder cannot be tightly secured by the coating layer because the contact area of the powder and the binder resin in the coating layer is small. As a result, the powder is easily detached from the coating layer. In order to prevent the powder from being detached from the coating layer, it is preferable that the ratio (D/h) is 5 or less. 
   In the above-described embodiment, the magnetic permeability sensor as a kind of the bulk density sensor is used as a toner density detecting device to control the toner density based on the detected value of the magnetic permeability sensor in the developing device. With use of the above-described carrier of the present invention in this developing device, a stable toner density control can be performed even though the developer is used in a high-stress giving condition. 
   Further, in the above-described embodiment, the resistivity of the powder of the carrier is 10 12  Ω-cm or greater. Because of the high resistivity, even when the powder secured to the core material by the binder is exposed on the surface of the carrier, leakage of charges does not occur. Thus, throughout its long service period, the carrier exhibits a satisfactory charging amount and a stable chargeability. When the resistivity of the powder is less than 10 12  Ω-cm, leakage of the charge on the carrier occurs through the powder. In the present embodiment, the powder is used not as a resistivity controlling agent, but as a protecting agent for the coating layer and as an agent for controlling the shape of the surface of the coating layer. Any powder may be used so long as the resistivity of the powder is at least 10 12  Ω-cm. 
   Further, in the above-described embodiment, the amount of the powder in the coating layer is preferably 50-95% by weight, more preferably 70-90% by weight. When the amount of the powder in the coating layer is less than 50% by weight, the sufficient stable bulk density of the developer cannot be obtained because the carrier does not provide the above-described effects such as the decrease of charging amount of the carrier and the space effect. Too large an amount of the powder, in excess of 95% by weight, causes reduction of chargeability of the carrier. In addition, as the amount of the carrier is much greater than that of the binder resin in the coating layer, the binder resin cannot securely hold the powder. Therefore, the powder tends to be detached from the coating layer, thereby decreasing the durability of the carrier. Any binder resin generally used for coating a core material of carrier may be employed in the present embodiment. 
   In the present invention, the powder may be alumina, silica, or a mixture of alumina and silica. In the case of using alumina powder, it is preferable that an average particle diameter of the alumina powder is 10 μm or less. Surface-treated or non-treated alumina powder may be used. The surface treatment may be to impart hydrophobicity to the alumina powder. Alternatively, surface-treated or non-treated silica powder may be used. The surface treatment may be to impart hydrophobicity to the silica powder. 
   The coating layer of the carrier may include one or more additives as a charging or resistivity controlling agent such as carbon black, an acid catalyst, and a combination of carbon black and acid catalyst. The carbon black may be one generally used for carrier and toner. The acid catalyst, which may be, for example, a compound having an alkyl group or a reactive group such as a methylol group, an imino group or both methylol and imino groups, serves to catalyze. The above-described examples of the acid catalyst are not limited thereto. 
   In the above-described image forming apparatus according to the embodiment of the present invention, even when the developer is used in a high-stress condition, for example, when an original document having a low image area (e.g., an occupation ratio of an image on the original document is 3% or less) is repeatedly printed or copied, variations in the bulk density of the developer can be suppressed and a toner density can be stably controlled. As a result, a high quality image can be obtained. 
   The present invention has been described with respect to the embodiments as illustrated in the figures. However, the present invention is not limited to the embodiment and may be practiced otherwise. For example, in the above-described embodiment, a stable toner density control can be performed by use of the bulk density sensor other than the magnetic permeability sensor. Moreover, the present invention has been described with respect to an electrophotographic color laser printer as an example of an image forming apparatus. However, the present invention may be applied to other image forming apparatuses such as a copying machine or a facsimile machine. 
   In the above-described color image forming apparatus, the order of forming images of respective colors and/or the arrangement of the image forming devices for respective colors are not limited to the ones described above and can be practiced otherwise. In addition, the above-described image forming apparatus may form single-color images instead of multi-color images. 
   Numerous additional modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the present invention may be practiced otherwise than as specifically described herein.