Patent Publication Number: US-6343201-B1

Title: Developing apparatus comprising developing roller that is rotated upward from below in developing area and image forming apparatus using the same

Description:
BACKGROUND OF THE INVENTION 
     This application is based on Japanese application 214584/1999 and 222158/1999, the contents of which are hereby incorporated by reference. 
     1. Field of the Invention 
     The present invention relates generally to an image forming apparatus such as a copying machine or a printer and a developing apparatus used for the image forming aparatus, and more particularly, to a developing apparatus using only toners for a developer and adapted to rotate, in introducing the toners into a developing area opposite to an image carrying member by a developing roller to perform development, the development roller to move upward from below in the developing area opposite to the image carrying member as well as to regulate by a regulating member the amount of the toners to be conveyed to the developing area by the developing roller under the developing roller, and an image forming apparatus using the developing apparatus. 
     2. Description of the Related Art 
     Conventionally known as a developing apparatus, used in an image forming apparatus such as copying machine or a printer, has been a developing apparatus using a mono-component developer containing no carriers and only tones in addition to a developing apparatus using a two-component developer containing toners and carriers. 
     An example of the developing apparatus using a mono-component developer containing only toners is one as shown in FIG.  1 . 
     In the developing apparatus, toners t contained in a containing section  1   a  in an apparatus body  1 , are fed to a toner supplying member  3 , in a roller shape by a toner feeding member  2 , and the toner supplying member  3  is rotated, to supply the toners t to a developing roller  4 . 
     The developing roller  4  is rotated, to convey the toners t supplied in the above-mentioned manner. Further, a regulating member  5  is pressed against the surface of the developing roller  4 , to regulate by the regulating member  5  the amount of the toners t to be conveyed by the developing roller  4  as well as to subject the toners t to triboelectric charging. The toners t thus regulated and charged are introduced into a developing area opposite to an image carrying member  6  by the developing roller  4 , to perform development. 
     In the conventional developing apparatus, the developing roller  4  and the image carrying member  6  are so rotated as to move downward from above in the developing area where the developing roller  4  and the image carrying member  6  are opposite to each other, as shown in FIG.  1 . 
     Depending on the type of the image forming apparatus, however, it is necessary to move the image carrying member upward from below in the developing area. For example, in a color image forming apparatus using a plurality of developing apparatuses respectively containing toners having different colors and so adapted as to form toner images respectively having the colors in an image carrying member by the plurality of developing apparatuses as well as to successively transfer the toner images to an intermediate transfer member from the image carrying member, the image carrying member must, in some cases, be moved upward from below in the developing area in order to make the color image forming apparatus compact. 
     Consider a case where the image carrying member is thus moved upward from below in the developing area. In a case where the developing roller is so rotated as to move downward from above in the developing area, when the developing roller is brought into contact with image carrying member to perform development, the toners stay in an upstream portion where the developing roller is in contact with the image carrying member. Accordingly, the toners spill outward, and a formed image is fogged. On the other hand, when the developing roller and the image carrying member are not brought into contact with each other to perform development, a leak is liable to occur between the developing roller and the image carrying member. 
     In recent years, developing apparatuses each adapted so as to rotate, when an image carrying member is moved upward from below in a developing area where a developing roller and the image carrying member are opposite to each other, the developing roller so as to move upward from below in the developing area have been proposed, as disclosed in JP-A-8-297411, JP-A-9-244406, and JP-A-1-171226. 
     When the developing roller is rotated to move upward below in the developing area a regulating member for regulating toners supplied to the developing roller must be provided under the developing roller. 
     When the regulating member is thus provided under the developing roller, however, many of the toners supplied to the developing roller are dropped upon being separated from the developing roller by its own weight before being introduced into the regulating member, so that the toners in a sufficient amount are not introduced into the developing area. Accordingly, the density of a formed image is decreased. 
     Therefore, a developing apparatus which is provided with a sealing member so as to be brought into contact with the surface of a developing roller on the upstream side in the direction of rotation of the developing roller from a regulating member for regulating the amount of the toners in the developing roller has been proposed, as disclosed in JP-A-10-171226. 
     Even when the sealing member is provided to be brought into contact with the developing roller on the ups side in the direction of rotation of the developing roller from the regulating member, the toners are not sufficiently held on the developing roller, and are dropped upon being separated from the developing roller before being introduced into the position of the regulating member. Accordingly, the density of a formed image is still decreased. Particularly when a solid image containing a high proportion of a block portion is formed, the density of the obtained image is decreased, and the image becomes blurred. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to prevent, in a developing apparatus for rotating, in introducing toners into a developing area opposite to an image carrying member by a rotating developing roller to perform development, the developing roller so as to move upward from below in the developing area opposite to the image carrying member as well as regulating by a regulating member the amount of the toners to be conveyed to the developing area by the developing roller under the developing roller, the toners supplied to the developing roller from being dropped upon being separated from the developing roller before being introduced into the regulating member, to introduce the toners in a sufficient amount into the developing area. 
     Another object of the present invention is to obtain, when an image is formed by an image forming apparatus using the above-mentioned developing apparatus, an image having a sufficient image density, and particularly to obtain, even when a solid image containing a large proportion of an image portion is formed, a good image having a sufficient image density without being decreased in density and blurred. 
     A first developing apparatus according to the present invention comprises a developing roller for conveying charged toners to a developing area opposite to an image carrying member with the toners held on its surface, the developing roller being rotated upward from below in the developing area; a regulating member for regulating the amount of the toners to be conveyed to the developing area under the developing roller; a plate-shaped charging member for charging the toners; and a voltage applying device for applying a voltage to the charging member. 
     A second developing apparatus according to the present invention comprises a developing roller for conveying charged toners to a developing area opposite to an image carrying member with the toners held on its surface, the developing roller being rotated upward from below in the developing area; a plate-shaped regulating member for regulating the amount of the toners to be conveyed to the developing area under the developing roller; a plate-shaped charging member for charging the toners, the charging member being positioned on the upstream side in the direction of rotation of the developing roller from the regulating member, an end, on the downstream side in the direction of rotation of the developing roller, of the charging member being brought into contact with the developing roller, and a distance d from a point at which the charging member starts to come into contact with the developing roller to a front end, on the downstream side in the direction of rotation of the developing roller, of the charging member being not less than 0.3 mm. 
     An image forming apparatus according to the present invention comprises the first or second developing apparatus, and an image carrying member. The image carrying member is moved upward from below in the developing area. 
     These and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings which illustrates specific embodiments of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of a conventional developing apparatus; 
     FIG. 2 is a schematic illustration of a developing apparatus according to embodiment 1 of the present invention; 
     FIG. 3 is schematic illustration of a developing apparatus according to embodiment 2 of the present invention; 
     FIG. 4 is a partial illustration of a state where a charging member and a regulating member are brought into contact with a developing roller in the developing apparatus according to tho embodiment 2; 
     FIG. 5 is a schematic illustration showing a modified example of developing apparatus according to th embodiment 2; 
     FIG. 6 is a schematic illustration of a first color image forming apparatus using developing apparatus according to the present invention; and 
     FIG. 7 is a schematic illustration of a second color image forming apparatus using developing apparatus according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Developing apparatuses according to embodiments of the present invention will be specifically described on the basis of accompanying drawings. 
     (Embodiment 1) 
     In a developing apparatus according to the present invention, a developing roller  21  is provided so as to be opposed to an image carrying member  10  through a slight space, and the developing roller  21  and the image carrying member  10  are rotated in opposite directions, to respectively move upward from below in a developing area where the developing roller  21  and the image carrying member  10  are opposite to each other, as shown in FIG.  2 . 
     In the developing apparatus, toners t are contained in a containing section  20   a  in an apparatus body  20 , and a toner feeding member  22  provided with blade members  22   a  is provided so as to be rotatable. 
     The toner feeding member  22  is rotated, to supply the toners t contained in the conting section  20   a  to a toner shying section  24  provided with a receiving member  23  by the blade members  22   a  in the toner feeding member  22 . 
     In the toner staying section  24 , a toner supplying member  25  in a roller shape is provided so as to be brought into contact with the developing roller  21 . The toner supplying member  25  is rotated in the same direction as the developing roller  21 , to move in an opposite direction to the developing roller  21  in a portion where the toner supplying member  25  and the developing roller  21  are in contact with each other, and supply the toners t to the developing roller  21  from the toner supplying member  25 . 
     One end of a charging member  26  formed in a plate shape of a conductive material is attached to the receiving member  23 , and the charging member  26  is projected toward the downstream side in the direction of rotation of the developing roller  21  below the toner supplying member  25 , to bring a front end of the charging member  26  into contact with the developing roller  21  at suitable line pressure along the axis of the developing roller  21  at a position on the downstream side in the direction of rotation of the developing roller  21  from the toner supplying member  25 . Further, a bias voltage for charging the toners t to a predetermined polarity is applied to the charging member  26  from a bias power supply  26   a.  In the developing apparatus according to the present embodiment, a space of not less than 1 mm is provided between both ends of the charging member  26  and both sidewalls of the apparatus body  20  so that the excess toners t are returned to the containing section  20   a  in the apparatus body  20  from the space. 
     An end of a regulating member  27  in a plate shape, which is projected toward the upstream side in the direction of rotation of the developing roller  21  under the developing roller  21  an the downstream side in the direction of rotation of the developing roller  21  from the front end of the charging member  26  is brought into contact with the developing roller  21  so as to have a suitable space from the bent end of the charging member  26 . 
     When the charging member  26  formed in a plate shape of a conductive material is provided so as to be brought into contact with the developing roller  21 , and the bias voltage for charging the toners t to a predetermined polarity is applied to the charging member  26 , as in the developing apparatus according to the present embodiment, the toners t supplied to the developing roller  21  are suitably charged to a predetermined polarity by the contact with the charging member  26 . 
     The toners t thus charged are held on the developing roller  21  by an electrostatic force. Before the toners t are introduced into the regulating member  27  by the developing roller  21 , the toners t are prevented from being dropped upon being separated from the developing roller  21 , so that the toners t in a sufficient amount are introduced into the position of the regulating member  27  by the developing roller  21 . 
     In a case where the charging member  26  is composed of a material positioned in a triboelectric series for charging the toners t to a predetermined polarity, and negatively charged toners t are used, as in the developing apparatus according to the present embodiment, when the charging member  26  is composed of a material whose work function is not more than the work function of the toners t, the toners t supplied to the developing roller  21  are further suitably charged to a predetermined polarity by the contact with the charging member  26 , the toners t in a sufficient amount are introduced into the position of the regulating member  27  by the developing roller  21 . 
     Furthermore, in bringing the front end of the charging member  26  into contact with the developing roller  21 , when a surface, at the front end in contact with the developing roller  21 , of the charging member  26  is too smooth, the toners t slide on the surface of the charging member  26 , so that the toners t are not suitably charged and are not sufficiently held on the developing roller  21 . On the other hand, when the surface, at the front end in contact with the developing roller  21 , of the charging member  26  is too rough, the toners t held on the developing roller  21 , and conveyed are non-uniform in amount by the contact with the charging member  26 . Therefore, it is preferable that ten-point average roughness Rz on a surface, in contact with the developing roller  21 , of the charging member  26  satisfies 0.1 μm≦Rz≦2.0 μm. 
     Furthermore, in bringing the front end of the charging member  26  into contact with the developing roller  21 , as described above, when contact pressure of the charging member  26  which is brought into contact with the developing roller  21  becomes low, the toners t are not sufficiently charged by the charging member  26 , so that the toners t are not sufficiently held on the developing roller  21 . On the other hand, when contact pressure of the charging member  26  which is brought into contact with the developing roller  21  becomes too high, the toners having a large particle diameter do not pass through the charging member  26 . Accordingly, only the toners t having small particle diameter are conveyed by the developing roller  21 , and the toners t having a large particle diameter remain. Consequently, the image quality of a formed image is gradually changed. In bringing the front end of the charging member  26  into contact with the developing roller  21 , it is preferable that the line pressure of the charging member  26  on the developing roller  21  is in a range of 0.1 to 4.0 gf/mm. 
     When the toners t are introduced into the position of the regulating member  27  by the developing roller  21 , as described above, the excess toners t are separated from the developing roller  21  by the regulating member  27 . Accordingly, the amount of the toners t to be conveyed to the developing area is regulated, and the toners t are charged. The toners t introduced into the position of the regulating member  27  by the developing roller  21  are suitably charged to a predetermined polarity by the charging member  26 , as described above. Accordingly, the toners t are also prevented from adhering to the regulating member  27 . 
     The excess toners t separated from the developing roller  21  by the regulating member  27 , as described above, are introduced downward through the space, and are returned to the containing section  20   a  by a return roller  28  provided below the developing roller  21 . 
     Furthermore, the toners t regulated by the regulating member  27  and charged, as described above, are conveyed to the developing area opposite to the image carrying member  10  by the developing roller  21 . 
     In the developing area, the developing roller  21  and the image carrying member  10  are respectively moved upward from below, as described above, and a developing bias voltage which is an overlapping of an AC voltage with a DC voltage is applied to the developing roller  21  from a developing bias power supply  29 , to exert a vibrating electric field between the developing roller  21  and the image carrying member  10 , and the toners t are supplied to a part of an electrostatic latent image formed on the image carrying member  10  from the developing roller  21  to perform development. Although in the present embodiment, the developing roller  21  and the image carrying member  10  are opposite to each other through a slight space in the developing area to perform development, as described above, the developing roller  21  and the image carrying member  10  can be brought into contact with each other in the developing area to perform development. 
     After the electrostatic latent image formed on the image carrying member  10  is developed in the above-mentioned manner, the toners t remaining on the developing roller  21  are returned to the apparatus body  20 . At a position where the toners t remaining on the developing roller  21  are returned to the apparatus body  20 , a sealing member  32  is pressed against the developing roller  21  by a cushioning member  31 , and a voltage for eliminating charges on the toners t is applied to the sealing member  32  from a charge eliminating power supply  33 , to eliminate the charges on the toners t remaining on the developing roller  21 . 
     After the charges on the toners t remaining on the developing roller  21  are thus eliminated, the toners t remaining on the developing roller  21  are separated from the developing roller  21  by the toner supplying member  25  which is moved in an opposite direction to the developing roller  21  in its portion in contact with the developing roller  21 , as described above, to mix the toners t with the toners t in the toner staying section  24 , and new toners t are supplied to the developing roller  21  from the toner supplying member  25 . 
     When the toner supplying member  25  is moved in an opposite direction to the developing roller  21  in the potion in contact with the developing roller  21 , as described above, the possibility that only the toners t having a large particle diameter are separated from the developing roller  21 , and the toners having a small particle diameter remain, as in a case where the toner supplying member  25  is moved in the same direction as the developing roller  21 , is eliminated. Accordingly, the toners t having a small particle diameter on the developing roller  21  are also reliably separated from the developing roller  21  by the toner supplying member  25 . Therefore, the possibility that many of the toners t having a small particle diameter are previously used for development, and the proportion of the toners t having a large particle diameter in the containing section  20   a  is gradually increased, so that the image quality of a formed image is changed is also eliminated. 
     An experiment in which in bringing the front end of the charging member  26  into contact with the developing roller  21  to charge the toners t in the developing apparatus shown in FIG. 2, the bias voltage to be applied to the charging member  26  from the bias power supply  26   a  is changed is then conducted, to clarify that the developing apparatus in the embodiment of the present invention is superior by taking comparative examples. 
     EXPERIMENT 1—1 
     In this experiment, a photosensitive drum having a diameter of 30 mm was used as the image carrying member  10 , while a developing roller having a diameter of 16 mm was used as the developing roller  21 . The developing roller  21  and the image carrying member  10  were provided so as to be opposed to each other through a space of 200 μm. The developing roller  21  and the image carrying member  10  were rotated in opposite directions, to move upward from below in the developing area where the developing roller  21  and the image carrying member  10  are opposite to each other. The peripheral speed v 1  of the image carrying member  10  was set to 160 mm/s, and the ratio θ (v 2 /v 1 ) of the peripheral speed v 2  of the developing roller  21  to the peripheral speed v 1  of the image carrying member  10  was set to 1.5. 
     Used as the charging member  26  was one composed of a plate material made of SUS and having ten-point average roughness, on its surface in contact with the developing roller  21 , of 0.5 μm. Used as the regulating member  27  was a plate material made of SUS having a plate thickness of 0.3 mm. Further, used as the toners t were negatively charged toners having a weight average particle diameter of 8.0 μm. 
     When a bias voltage Va is applied to the charging member  26  from the bias power supply  26   a  in bringing the front end of the charging member  26  into contact with the developing roller  21  to charge the toners t supplied to the developing roller  21 , the same bias voltage Va (Va=Vb) as a developing bias voltage Vb to be applied to the developing roller  21  from the developing bias power supply  29  was applied in an example 1—1, and a bias voltage Va (Va=Vb−100 V) shifted by 100 V toward the side of minus that is a polarity to which the toners t are charged from the developing bias voltage Vb to be applied to the developing roller  21  from the developing bias power supply  29  was applied in an example  1 - 2 , while no bias voltage Va was applied to the charging member  26  in a comparative example 1—1. 
     After the toners t supplied to the developing roller  21  were thus charged, the toners t conveyed by the developing roller  21  were then regulated by the regulating member  27 , and the toners t were subjected to triboelectric charging, to introduce the toners t into a developing area opposite to the image carrying member  10 . 
     A solid electrostatic latent image having a surface potential of −50 V was formed on the image carrying member  10 , while a developing bias voltage Vb which is an overlapping of an AC voltage having a peak-to-peak value Vpp of 1600 V and having a frequency of 2 kHz with a DC voltage of −400 V was applied to the developing roller  21  from the developing bias power supply  29 . The toners t were supplied to a part of the electrostatic latent image formed on the image carrying member  10  from the developing roller  21 , to respectively form solid images. 
     Each of the solid images thus formed was evaluated. A case where the solid image having a constant image density was obtained and a case where the image density at a rear end of the solid image was greatly decreased were respectively indicated by ◯ and × in Table 1: 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                 comparative 
               
               
                   
                 example 1.1 
                 example 1.2 
                 example 1.1 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Va (V) 
                 Vb 
                 Vb-100 
                 not applied 
               
               
                   
                 evaluation of 
                 ∘ 
                 ∘ 
                 x 
               
               
                   
                 solid image 
               
               
                   
                   
               
            
           
         
       
     
     As apparent from the results, in preliminary charging the toners t upon bringing the front end of the charging member  26  into contact with the developing roller  21 , the obtained solid image was a good solid image whose density was not decreased in each of the developing apparatuses in examples 1.1 and 1.2 in which the bias voltage Va for charging the tones t on the developing roller  21  to a negative polarity from the bias power supply  26   a  was applied to the charging member  26 . Contrary to this, in the developing apparatus in the comparative example 1.1 in which no bias voltage Va was applied to the charging member  26 , the toners t on the developing roller  21  were not suitably charged, and were dropped upon being separated from the developing roller  21  before being introduced into the regulating member  27 , so that the amount of the toners introduced the developing area was decreased. Accordingly, the image density at the rear end of the solid image was greatly decreased. 
     EXPERIMENT 1-2 
     In this experiment, the material of a surface, in contact with the developing roller  21 , of the above-mentioned erging member  26  used in the experiment 1—1 was changed, to use four types of charging members  26  respectively having work functions Wf, on their surfaces in contact with the developing roller  21 , of 4.2 eV, 4.6 eV, 5.0 eV, and 5.2 eV, as shown in Table 2. 
     In finding the work function Wf on the surface, in contact with the developing roller  21 , of each of the charging members  26 , a contact potentiometer (SSVII-10 manufactured by Kawaguchi Denki K.K.) was used. Au having a work function Wf of 4.9 eV was used as a basis, and a contact potential difference from Au was measured, to find the work function Wf on the surface. The work function Wf of the negatively charged toners was 5.0 eV. 
     In bringing each of the charging members  26  respectively having the above-mentioned work functions Wf on the surfaces into contract with the developing roller  21 , and cha the toners t supplied to the developing roller  21 , a bias voltage Va (=Vb−100V) shifted by 100 V toward the side of minus that is a polarity to which the toners t are charged from a developing bias voltage Vb to be applied to the developing roller  21  from the developing bias power suply  29  was applied to the charging member  26  from tlhe bias power supply  26   a,  to charge the toners t held on the developing roller  21  by the charging member  26 , and the toners t thus charged were introduced into the regulating member  27  by the developing roller  21 , to regulate by the regulating member  27  the amount of the toners t to be conveyed by the developing roller  21 . 
     The adhering state of the toners t on the regulating member  27  which had thus regulated the amount of the toners t to be conveyed by the developing roller  21  was examined. A case where the toners did not adhere to the regulating member and a case where the toners adhered to the regulating member were respectively indicated by ◯ and × in Table 2: 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
             
            
               
                   
                 Wf (eV) on surface of 
                 4.2 
                 4.6 
                 5.0 
                 5.2 
               
               
                   
                 charging member 
               
               
                   
                 adhesion of toners to 
                 ∘ 
                 ∘ 
                 ∘ 
                 x 
               
               
                   
                 regulating member 
               
               
                   
                   
               
            
           
         
       
     
     As apparent from the results, when the negatively charged toners were used, a portion where the charging member  26  is in contact with the developing roller  21  was composed of a material whose work function is not more than the work function of the toners t, the toners t were negatively charged by the contact with the charging member  26 , so that the toners t did not adhere to the regulating member  27 . 
     Contrary to this, when the portion where the charging member  26  is in contact with the developing roller  21  was composed of a material whose work function is higher than the work function of the toners t, toners t positively charged were produced by the contact with the charging member  26 . The toners t thus positively charged adhered to the regulating member  27 , so that the toners t to be conveyed by the developing roller  21  are non-uniform in a stripe shape, and stripe-shaped noise occurred in a formed image. 
     EXPERIMENT 1-3 
     In this experiment, used as the charging member  26  was ones composed of a plate material made of SUS, as in the above-mentioned experiment 1, while having ten-point average roughness Rz, on their surfaces in contact with the developing roller  21 , changed in a range of 0.05 to 2.5 μm, as shown in the following Table 3. 
     In bringing each of the charging members  26  into contact with the developing roller  21 , to charge toners t supplied to the developing roller  21 , a bias voltage Va(=Vb−100V) shifted by 100 V toward the side of minus than is a polarity to which the toners t are charged from a developing bias voltage Vb to be applied to the developing roller  21  from the developing bias power supply  29  was applied to the charging member  26  from the bias power supply  26   a,  to charge the toners t held on the developing roller  21  by the charging member  26 , as in the abovementioned examples  1 - 2 . 
     The toners t charged by each of the charging members  26  as described above were then introduced into the regulating member  27  by the developing roller  21 , to regulate by the regulating member  27  the amount of the toners t held on the developing roller  21  as well as to subject the toners t to triboelectric charging, to introduce the toners t to a developing am opposite to the image carrying member  10 . 
     Non-uniformity in conveyance of the toners t on the developing roller  21  thus introduced into the developing area opposite to the image carrying member  10  was visually observed. A case where there was no non-uniformity in conveyance and a case where there was non-uniformity in conveyance were respectively indicated by ◯ and × in Table 3. 
     A solid electrostatic latent image having a surface potential of −50 V was formed on the image carrying member  10 , while a developing bias voltage Vb which is an overlapping of an AC voltage having a peak-to-peak value Vpp of 1600 V and having a frequency of 2 kHz with a DC voltage of −400 V was applied to the developing roller  21  from the developing bias power supply  29 . The toners t were supplied to a part of the electrostatic latent image formed on the image carrying member  10  from the developing roller  21 , to respectively form solid images, as in the above-mentioned experiment 1—1. 
     Each of the solid images thus formed was evaluated. A case where the solid image having a constant image density was obtained and a case where the image density at a rear end of the solid image was greatly decreased were respectively indicated by ◯ and × in Table 3: 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
             
            
               
                   
                 Rz (μm) 
                 0.05 
                 0.1 
                 2.0 
                 2.5 
               
               
                   
                 non-uniformity 
                 ∘ 
                 ∘ 
                 ∘ 
                 x 
               
               
                   
                 in conveyance 
               
               
                   
                 evaluation of 
                 x 
                 ∘ 
                 ∘ 
                 ∘ 
               
               
                   
                 solid image 
               
               
                   
                   
               
            
           
         
       
     
     As apparent from the results, when the charging member  26  having ten-point average roughness Rz, at its front end in contact with the developing roller  21 , in a range of 0.1 to 2.0 μm was used, the toners t to be conveyed to the developing area by the developing roller  21  were not non-uniform in amount. Accordingly, the obtained solid image was a good solid image which was not non-uniform in density and whose density was not decreased. 
     Contrary to this, when the charging member  26  having ten-point average roughness Rz, at its front end in contact with the developing roller  21 , of less than 0.1 μm was used, the loners t on the developing roller  21  were not suitably charged, so that the toners t were not sufficiently held on the developing roller  21 . The toners t were dropped upon being separated form the developing roller  21  before being introduced into the regulating member  27 , and the amount of the toners t introduced into the developing area was decreased, so that to density of the solid image was decreased. On the other hand, when the charging member  26  having ten-point average roughness Rz, at its front end in contact with the developing roller  21 , exceeding 2.0 μm was used, the toes t held in the developing roller  21  and conveyed were non-uniform in amount by the contact with the charging member  26 . 
     (Embodiment 2) 
     Also in a developing apparatus according to the present invention, a developing roller  21  is provided so as to be opposed to an image carrying member  10  through a slight space, and the developing roller  21  and the image carrying member  10  are rotated in opposite directions, to respectively move upward from below in a developing area where the developing roller  21  and the image carrying member  10  are opposite to each other, as in the developing apparatus according to embodiment 1, as shown in FIG.  3 . 
     Toners t are contained in a containing section  20   a  in an apparatus body  20 , and a toner feeding member  22  provided with blade members  22   a  is provided so as to be rotatable. 
     The toner feeding member  22  is rotated, to supply the toners t contained in the containing section  20   a  to a toner staying section  24  provided with a receiving member  23  by the blade members  22   a  in the toner feeding member  22 . 
     In the toner staying section  24 , a toner supplying member  25  in a roller shape is provided so as to be brought into contact with the developing roller  21 . The toner supplying member  25  is rotated in the same direction as the developing roller  21 , to move in an opposite direction to the developing roller  21  in a portion where the toner supplying member  25  and tie developing roller  21  are in contact with each other, and the toners t are supplied to the developing roller  21  from the toner supplying member  25 . 
     One end of a charging member  26  formed in a plate shape is attached to the receiving member  23 , and the charging member  26  is projected toward the downstream side in the direction of rotation of the developing roller  21  below the toner supplying member  25 , to bring a front end of the charging member  26  into contact with the developing roller  21  at suitable line pressure at a position on the downstream side in the direction of rotation of the developing roller  21  from the toner supplying member  25 . 
     In the developing apparatus according to the present embodiment, in bringing the front end of the charging member  26  into contact with the developing roller  21 , as shown in FIG. 4, a distance d from a point at which the charging member  26  starts to come into contact with the developing roller  21  to the front end of the charging member  26  is set to not less than 0.3 mm, to charge the toners t supplied to the developing roller  21  by the charging member  26 . 
     Under the developing roller  21  on the downstream side in the direction of rotation of the developing roller  21  from the front end of the charging member  26 , an end of a regulating member  27  in a plate shape which is projected toward the upstream side in the direction of rotation of the developing roller  21  is brought into contact with the developing roller  21  so as to have a suitable space s from the front end of the charging member  26 . 
     When the charging member  26  is brought into contact with the developing roller  21  in such a manner that the distance d from to point at which the charging member  26  starts to come into contact with the developing roller  21  to the front end of the developing roller  21  is not less than 0.3 mm, as described above, the toners t supplied to the developing roller  21  are suitably charged by the contact with the charging member  26 . Accordingly, the toners t thus charged are held on the developing roller by an electrostatic force. The toners t are prevented from being dropped upon being separated from the developing roller  21  before being introduced into the regulating member  27  by the developing roller  21 . Accordingly, the toners t in a sufficient amount are introduced into the position of the regulating member  27  by the developing roller  21 . 
     In the developing apparatus according to the present embodiment, in bringing the front end of the regulating member  27  into contact with the developing roller  21  so as to have a suitable space s from the front end of the charging member  26 , as described above, when the space s between the charging member  26  and the regulating member  27  is narrowed, the space between the charging member  26  and the regulating member  27  is clogged with the toners t conveyed by the developing roller  21 . Accordingly, the toners t cannot be suitably regulated by the regulating member  27 , and the toners t may be non-uniform in amount. Therefore, it is preferable that the space s between the front end, on the downstream side in the direction of rotation of the developing roller  21 , of the charging member  26  and the end, on the side of the charging member  26 , of the regulating member  27  is not less than 0.2 mm. 
     Also in the developing apparatus according to he present embodiment, in bringing the front end of the charging member  26  into contact with the developing roller  21 , as in the developing apparatus according to embodiment 1, when the surface, at the front end in contact with the developing roller  21 , of the charging member  26  is too smooth, the toners t slide on the surface of the charging member  26 , so that the toners t are not suitably charged and are not sufficiently held on the developing roller  21 . On the other hand, when the surface, at the front end in contact with the developing roller  21 , of the charging member  26  is too rough, the toners t held on the developing roller  21  and conveyed are non-uniform in amount by the contact with to charging member  26 . Therefore, it is preferable that ten-point average roughness Rz on the surface, in contact with the developing roller  21 , of the charging member  26  satisfies 0.1 μm Rz 2.0 μm. 
     Furthermore, in bringing the front end of the charging member  26  into contact with the developing roller  21 , as described above, when contact pressure of the charging member  26  which is brought into contact with the developing roller  21  becomes low, the toners t are not sufficiently charged by the charging member  26 . 
     Accordingly, the toners t are not sufficiently held on the developing roller  21 . On the other hand, when contact pressure of the charging member  26  which is brought into contact with the developing roller  21  becomes too high, the toners having a large particle diameter do not pass through the charging member  26 . Accordingly, only the toners t having a small particle diameter are conveyed by the developing roller  21 , and the toners t having a large particle diameter remain. Consequently, the image quality of a formed image is gradually changed. In bringing the front end of the charging member  26  into contact with to developing roller  21 , therefore, it is preferable that line pressure of the charging member  26  on the developing roller  21  is in a range of 0.1 to 4.0 gf/mm. 
     When the toners t are introduced into the position of the regulating member  27  by the developing roller  21 , as described above, the excess toners t are separated from the developing roller  21  by the regulating member  27 . Accordingly, the amount of the toners t to be conveyed to the developing area is regulated, and the toners t are charged. The toners t introduced into the position of the regulating member  27  by the developing roller  21  are suitably charged to a predetermined polarity by the charging member  26 , as described above. Accordingly, the toners t are also prevented from adhering to the regulating member  27 . 
     In the developing apparatus according to the present embodiment, a toner discharge section  20   b  is provided below the receiving member  23 , and a return roller  28  is provided in the toner discharge section  20   b.  A scraping member  28   a  is brought into contact with the return roller  28  on the side of the containing section  20   a.    
     The excess toner t separated from the developing roller  21  by the regulating member  27 , as described above, are introduced into the toner discharge section  20   b,  and the return roller  28  is rotated, to introduce the toners t which have been introduced into the toner discharge section  20   b  into the containing section  20   a  through a space between the return roller  28  and the receiving member  23 . The toners t are separated from the return roller  28  by the scraping member  28   a,  and are returned to the containing section  20   a.  In introducing the toners t into the containing section  20   a  through the space between the return roller  28  and the receiving member  23  by the return roller  28 , as described above, when the space between the return roller  28  and the receiving member  23  is narrowed, the space between the return roller  28  and the receiving member  23  is clogged with the toners t Accordingly, the toners t are not easily introduced into the containing section  20   a.  On the other hand, when the space between he return roller  28  and the receiving member  23  is widened because the diameter of the return roller  28  is decreased, the amount of the toners t remaining in the toner discharge section  20   b  is increased. Accordingly, it is preferable that the minimum space between the return roller  28  and the receiving member  23  is in a range of 1 mm to 3 mm. 
     Although in the developing apparatus according to the present embodiment, the toners t which have been introduced into the toner discharge section  20   b  are introduced into the containing section  20   a  through the space between the return roller  28  and the receiving member  23  by the return roller  28 , as described above, it is also possible to rotate the return roller  28  in an opposite direction to that in the above-mentioned case, introduce the toners t which have been introduced into the toner discharge section  20   b  into the containing section  20   a  through a space between the return roller  28  and the bottom of the apparatus body  10  by the return roller  28 , and separate the toners t from the return roller  28  by the scraping member  28   a  to return the tones t to the containing section  20   a,  as shown in FIG.  5 . In this case, it is preferable that mimimum space between the return roller  28  and the bottom of the apparatus body  20  is in a range of 1 mm to 3 mm. 
     The toners t regulated by the regulating member  27  and charged, as described above, are conveyed to the developing area opposite to the image carrying member  10  by the developing roller  21 . 
     In the developing area, the developing roller  21  and the image carrying member  10  are respectively moved upward from below, as described above, and a developing bias voltage which is an overlapping of an AC voltage with a DC voltage is applied to the developing roller  21  from a developing bias power supply  29 , to exert a vibrating electric field between the developing roller  21  and the image carrying member  10 , and the toners t are supplied to a part of the electrostatic latent image formed on the image carrying member  10  from the developing roller  21  to perform development. Although in the present embodiment, the developing roller  21  and the image carrying member  10  are opposite to each other through a slight space in the developing area to perform development, as described above, the developing roller  21  and the image carrying member  10  can be also brought into contact with each other in the developing area to perform development. 
     After an electrostatic latent image formed on the image carrying member  10  is developed in the above-mentioned manner, the toners t remaining on the developing roller  21  are returned to the apparatus body  20 . At the position where the toners t remaining on the developing roller  21  are returned to the apparatus body  20 , a sealing member  32  is pressed against the developing roller  21  by a cushioning member  31 , and a voltage for eliminating charges on the toners t is applied to the sealing member  32  from a charge eliminating power supply  33 , to eliminate the charges on the toners t remaining on the developing roller  21 . 
     After the charges on the toners t remaining on the developing roller  21  are thus eliminated, the toners t remaining on the developing roller  21  are separated from the developing roller  21  by the toner supplying member  25  which is moved in an opposite direction to the developing roller  21  in its portion in contact with the developing roller  21 , as described above, to mix the toners t with the toners t in the toner staying section  24 , and new toners t are supplied to the developing roller  21  from the toner supplying member  25 , as described above. 
     When the toner supplying member  25  is moved in an opposite direction to the developing roller  21  in the portion in contact with the developing roller  21 , as described above, the possibility that only the toners t having a large particle diameter are separated from the developing roller  21 , and the toners t having a small particle diameter remain, as in a case where the toner supplying member  25  is moved in the same direction as the developing roller  21 , is eliminated. Accordingly, the toners t having a small particle diameter on the developing roller  21  are also reliably separated from the developing roller  21  by the toner supplying member  25 . Therefore, the possibility that many of the toners t having a small particle diameter are previously used for development, and the proportion of the toners t having a large particle diameter in the containing section  20   a  is gradually increased, so that the image quality of a formed image is changed is also eliminated. 
     An experiment in which in bringing the front end of the charging member  26  into contact with the developing roller  21  to charge the toners t in the developing apparatus shown in FIG. 3, the distance d from the point at which the charging member  26  starts to come into contact with the developing roller  21  to the front end of the charging member  26  is changed is then conducted, to clarify that the developing apparatus in the present embodiment satisfying the conditions of the present invention is superior by taking comparative examples. 
     EXPERIMENT 1—1 
     In this experiment, a photosensitive drum having a diameter of 30 mm was used as the image carrying member  10 , while a developing roller having a diameter of 16 mm was used as the developing roller  21 . The developing roller  21  and the image carrying member  10  were provided so as to be opposed to each other through a space of 200 μm. The developing roller  21  and the image carrying member  10  were rotated in opposite directions, to move upward from below in the developing area where the developing roller  21  and the image carrying member  10  are opposite to each other. The peripheral speed v 1  of the image carrying member  10  was set to 160 mm/s, and the ratio θ (v 2 /v 1 ) of the peripheral speed v 2  of the developing roller  21  to the peripheral speed v 1  of the image carrying member was set to 1.5. 
     Used as the charging member  26  was one composed of a plate material made of SUS having a plate thickness of 0.3 mm and having ten-point average roughness, on its surface in contact with the developing roller  21 , of 0.5 μm. Used as the regulating member  27  was a plate material made of SUS having a plate thickness of 0.08 mm. A space s between a front end, on the downstream side in the direction of rotation of the developing roller  21 , of the charging member  26  and an end, on the side of the charging member  26 , of the regulating member  27  was set to 2.8 mm. Further, used as the toners t were negatively charged toners having a weight average particle diameter of 8.0 μm. 
     In bringing the front end of the charging member  26  into contact with developing roller  21 , line pressure of the charging member  26  on the developing roller  21  was set to 1.0 gf/mm, and a distance d from a point at which the charging member  26  starts to come into contact with the developing roller  21  to the front end of the developing roller  21  was changed in a range 0.1 mm to 2.0 mm, as shown in Table 4. 
     The toner t held on the developing roller  21  were charged by each of the charging members  26 , the amount of the toners t to be conveyed by the developing roller  21  was regulated by the regulating member  27 , and the amount of the toners (mg/cm 2 ) introduced into the developing area opposite to the the carrying member  10  by the developing roller  21  was measured. The results were shown in Table 4. In introducing the toners t into the developing area opposite to the image carrying member  10  by the developing roller  21  to perform development, the amount of the toners introduced into the developing area must be not less than 0.6 mg/cm 2  in order to obtain an image having a sufficient image density. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 4 
               
               
                   
                   
               
             
            
               
                   
                 d (mm) 
                 0.1 
                 0.2 
                 0.3 
                 0.4 
                 0.5 ˜ 2.0 
               
               
                   
                 amount of toners 
                 0.1 
                 0.4 
                 0.6 
                 0.7 
                 0.7 
               
               
                   
                 (mg/cm 2 ) introduced 
               
               
                   
                 into developing area 
               
               
                   
                   
               
            
           
         
       
     
     As apparent from the results, in bringing the front end of the charging member  26  into contact with the developing roller  21 , when the distance d from the point at which the charging member  26  starts to come into contact with the developing roller  21  to the front end of the charging member  26  was set to not less than 0.3 mm satisfying the conditions of the present invention, the amount of the toners introduced into the developing area was not less than 0.6 mg/cm 2 . Accordingly, an image having a sufficient image density was obtained. Contrary to this, when the distance d from the point at which the charging member  26  starts to come into contact with the developing roller  21  to the front end of the charging member  26  was less than 0.3 mm, the amount of the toners introduced into the developing area was less than 0.6 mg/cm 2 . Accordingly, an image having a sufficient image density was not obtained. When the distance d from the point at which the charging member  26  starts to come into contact with the developing roller  21  to the front end of the charging member  26  was not less than 0.4 mm, the amount of the toners t conveyed by the developing roller  21  was regulated by the regulating member  27 , as described above, so that the amount of the toners introduced into the developing area was 0.7 mg/cm 2  in either case. 
     EXPERIMENT 2—2 
     In this experiment the lengths of the charging member  26  and the regulating member  27  which were used in the experiment 2-1 were changed, to change the distance d from the point at which the charging member  26  starts to come into contact with the developing roller  21  to the front and of the charging member  26  and the space s between the front end, on the downstream side in the direction or rotation of the developing roller  21 , of to charging member  26  and the end, on the side of the charging member  26 , of the regulating member  27  were respectively changed, as shown in Table 5. 
     After toners t held on the developing roller  21  as described above were charged by to charging member  26 , the amount of the toners t to be conveyed by the developing roller  21  was regulated by the regulating member  27 , and the toners t were subjected to triboelectric charging, to introduce the toners t into a developing area opposite to the image carrying member  10  by the developing roller  21 . 
     A solid electrostatic latent image having a surface potential of −50 V was then formed on the image carrying member  10 , while a developing bias voltage which is an overlapping of an AC voltage having a peak-to-peak value Vpp of 1600 V and having a frequency of 2 kHz with a DC voltage of −400 V was applied to the developing roller  21  from the developing bias power supply  29 . The toners t were supplied to a part of the electrostatic latent image formed on the image carrying member  10  from the developing roller  21  to perform development, to respectively form solid images. 
     With respect to each of the solid images formed by changing the distance d from the point at which the charging member  26  starts to come into contact with the developing roller  21  and the front end of the charging member  26  and the space s between the front end, on the downstream side in the direction of rotation of the developing roller  21 , of the charging member  26  and the end, on the side of the charging member  26 , of the regulating member  27 , the image density and image noise were evaluated. The results thereof were shown in Table 5. 
     In evaluating the image density, a case where the image density in the solid image was not decreased to the end and a case where the image density was decreased were respectively indicated by ◯ and ×. Further, in evaluating the image noise, the presence or absence of white stripe noise in the solid image was examined. A case where no white stripe noise existed and a case where white stripe noise existed were respectively indicated by ◯ and ×. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE 5 
               
               
                   
               
             
            
               
                 d (mm) 
                 3.2 
                 3.1 
                 3.0 
                 2.9 
                 2.8˜0.3 
                 0.2 
                 0.1 
               
               
                 S (mm) 
                 0.1 
                 0.2 
                 0.3 
                 0.4 
                 0.5˜3.0 
                 3.1 
                 3.2 
               
               
                 image density 
                 ◯ 
                 ◯ 
                 ◯ 
                 ◯ 
                 ◯ 
                 X 
                 X 
               
               
                 image noise 
                 X 
                 ◯ 
                 ◯ 
                 ◯ 
                 ◯ 
                 ◯ 
                 ◯ 
               
               
                   
               
            
           
         
       
     
     As apparent from the results, in bringing the front end of the charging member  26  into contact with the developing roller  21 , when the distance d from the point at which the charging member  26  starts to come into contact with the developing roller  21  to the front end of the charging member  26  was not less than 0.3 mm and the space s between the front end, on the downstream side in the direction of rotation of the developing roller  21 , of the charging member  26  and the end, on the side of the charging member  26 , of the regulating member  27  was not less than 0.2 mm, the solid image whose density was not decreased was obtained, and no white stripe noise occurred in the solid image. 
     Contrary to this, when the distance d from the point at which the charging member  26  starts to come into contact with the developing roller  21  to the front end of the charging member  26  was less than 0.3 mm, the amount of the toners introduced into the developing area was decreased, as described above, so that the density of the solid image was decreased. Further, the space s between the front end, on the downstream side in the direction of rotation of the developing roller  21 , of the charging member  26  and the end, on the side of the charging member  26 , of the regulating member  27  was less than 0.2 mm, the space between the charging member  26  and the regulating member  27  was partially clogged with the toners t conveyed by the developing roller  21 , and the toners t were not conveyed by the developing roller  21  in the space. Consequently, white stripe noise occurred in the solid image. 
     EXPERIMENT 2-3 
     In this experiment, solid images were formed in the same manner as that in the above-mentioned experiment 2-2 except that ten-point average roughness Rz at the front end, in contact with the developing roller  21 , of the charging member  26  was changed in a range of 0.05 to 2.2 μm, as shown in Table 6, in the above-mentioned experiments 2-1 and 2-2, while the distance d from the point at which the charging member  26  starts to come into contact with the developing roller  21  to the front end of the charging member  26  was set to 0.5 mm, and the space s between the front end, on the downstream side in the direction of rotation of the developing roller  21 , of the charging member  26  and the end, on the side of the the charging member  26 , of the regulating member  27  was set to 2.8 mm. 
     With respect to each of the formed solid images, the image density and the non-uniformity in density were evaluated. The results thereof were shown in the following Table 6. 
     In evaluating the image density, a case where the image density in the solid image was not decreased to the end and a case where the image density was decreased were respectively indicted by ◯ and ×, as in the above-mentioned experiment 2-2. Further, in evaluating the non-uniformity in density, the presence or absence of non-uniformity in tone in the solid image was examined. A case where no non-uniformity in tone occurred and a case where non-uniformity in tone occurred were respectively indicated by ◯ and ×. 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 6 
               
               
                   
               
             
            
               
                 RZ (μm) 
                 0.05 
                 0.1 
                 0.2 
                 0.3 
                 0.4 
                 0.5˜2.0 
                 2.1 
                 2.2 
               
               
                 image density 
                 X 
                 ◯ 
                 ◯ 
                 ◯ 
                 ◯ 
                 ◯ 
                 ◯ 
                 ◯ 
               
               
                 non-uniformity of image 
                 ◯ 
                 ◯ 
                 ◯ 
                 ◯ 
                 ◯ 
                 ◯ 
                 X 
                 X 
               
               
                   
               
            
           
         
       
     
     As apparent from the results, when the charging member  26  having the ten-point average roughness Rz, at the front end in contact with the developing roller  21 , in a range of 0.1 to 2.0 μm was used, the solid image whose density was not decreased was obtained, and no non-uniformity occurred in the solid image. 
     Contrary to this, when the charging member  26  having the ten-point average roughness Rz, at the front end in contact with the developing roller  21 , of less than 0.1 μm was used, the toners t in the developing roller  21  were not suitably changed, so that the toners t were not sufficiently held on the developing roller  21 . The toners t were dropped upon being separated from the developing roller  21  before being introduced into the regulating member  27 , and the amount of the toners t introduced into the developing area was decreased, so that e density of the solid image was decreased. On the other hand, when the charging member  26  having the ten-point average roughness Rz, at the front end in contact with the developing roller  21 , exceeding 2.0 μm was used, the toners t held on the developing roller  21  and conveyed were non-uniform in amount by the contact with the charging member  26 , so that non-uniformity in tone occurred in the solid image. 
     Two types of image forming apparatuses for forming a color image using the developing apparatuses described in the embodiments 1 and 2 will be specifically described on the basis of accompanying drawings. 
     (First Color Image Forming Apparatus) 
     In a first color image forming apparatus, four developing apparatuses A 1  to A 4  are used, toners respectively having different colors, i.e., yellow, magenta, cyan, and black colors are contained in the four developing apparatuses A 1  to A 4 , and the four developing apparatuses A 1  to A 4  are held in a rotating holder  40 , as shown in FIG.  6 . 
     The holder  40  is rotated to change the positions of the developing apparatuses A 1  to A 4 , to successively introduce a developing roller  21  in each of the developing apparatuses A 1  to A 4  into a position opposite to an image carrying member  10 , and respectively move the developing roller  21  and the image carrying member  10  upward from below in a developing area where the developing roller  21  and the image carrying member  10  are opposite to each other. 
     In forming a color image by the first color image forming apparatus, the developing roller  21  in the first developing apparatus A 1  containing the yellow toners are first positioned so as to be opposed to the image carrying member  10 , and the image carrying member  10  is rotated, to uniformly charge the surface of the image carrying member  10  by a charging device  41 . The image carrying member  10  thus charged is subjected to exposure conforming to an image signal by an exposing device  42 , to form an electrostatic latent image on the surface of the image carrying member  10 . 
     In a developing area where the image carrying member  10  having the electrostatic latent image thus formed thereon and the developing roller  21  in the first developing apparatus A 1  are opposite to each other, the developing roller  21  and the image carrying member  10  are then respectively moved upward from below, the yellow toners are supplied to a part of the electrostatic latent image formed on the image carrying member  10  from the developing roller  21  to form a yellow toner image corresponding to the electrostatic latent image on the image carrying member  10 . 
     The yellow toner image thus formed on the image carrying member  10  is transferred to an intermediate transfer member  43  in an endless belt shape which is stretched above the image carrying member  10 , while the yellow toners remaining in the image carrying member  10  after the transfer are removed from the image carrying member  10  by a cleaning device  44 . 
     Thereafter, the holder  40  is rotated, to then position the developing roller  21  in the second developing apparatus A 2  containing the magenta toners so as to be opposed to the image carrying member  10 . As in the first developing apparatus A 1 , a magenta toner image is formed on the surface of the image carrying member  10 , and the magenta toner image is transferred to the intermediate transfer member  43  to which the yellow toner image has been transferred, while the magenta toners remaining in the image carrying member  10  after the transfer are removed from the image carrying member  10  by the cleaning device  44 . 
     By performing similar operations, a cyan toner image is formed on the surface of the image carrying member  10  by the third developing apparatus A 3  containing the cyan toners, and the cyan toner image is transferred to the intermediate transfer member  43 . Further, a black toner image is formed on the surface of the image carrying member  10  by the fourth developing apparatus A 4  containing the black toners, and the black toner image is transferred to the intermediate transfer member  43 . The yellow, magenta, cyan, and black toner images are thus transferred to the intermediate transfer member  43 , to form a full-color toner image. 
     Recording sheets  46  are introduced from a paper cassette  45  provided in a lower part of the color image forming apparatus into a portion where the intermediate transfer member  43  and a transfer roller  48  are opposite to each other by feeding rollers  47 , and the full-color toner image formed on the intermediate transfer member  43  is transferred to the recording sheets  46 . The full-color toner image thus transferred to the recording sheet  46  is fixed to the recording sheets  46  by a fixing device  49  to discharge the recording sheets  46 , while the toners remaining on the intermediate transfer member  43  without being transferrd are removed from the intermediate transfer member  43  by a cleaning device  50 . 
     (Second Color Image Forming Apparatus) 
     In a second color image forming apparatus, four developing apparatuses A 1  to A 4  are used, toners respectively having different colors, i.e., yellow, magenta, cyan, and black colors are contained in the four developing apparatuses A 1  to A 4 , and the four developing apparatuses A 1  to A 4  are arranged in descending order in the color image forming apparatus, as shown in FIG.  7 . 
     An image carrying member  10  is provided so as to be opposed to a developing roller  21  in each of the developing apparatuses A 1  to A 4 , to move the developing roller  21  and the image carrying member  10  upward from below in a developing area where the developing roller  21  and the image carrying member  10  are opposite to each other. 
     An intermediate transfer member  43  in an endless belt shape which is stretched in the vertical direction is provided at a position on the opposite side, through the image carrying member  10 , to each of the developing apparatuses A 1  to A 4 , to move the intermediate transfer member  43  downward from above on the side of the contact with the image carrying member  10 . 
     In forming a color image by the second color image forming apparatus, consider a case of the first developing apparatus A 1  containing the yellow toners. The developing roller  21  and the image carrying member  10  opposite to the developing roller  21  are rotated, to uniformly charge the surface of the image carrying member  10  by a charging device  41 . The image carrying member  10  thus charged is subjected to exposure conforming to an image signal by an exposing device  42 , to form an electrostatic latent image on the surface of the image carrying member  10 . 
     In a developing area where the image carrying member  10  having the electrostatic latent image formed thereon and the developing roller  21  in the first developing apparatus A 1  are opposite to each other, the developing roller  21  and the image carrying member  10  are respectively moved upward from below, to supply the yellow toners to a part of the electrostatic latent image formed on the image carrying member  10  from the developing roller  21 , thereby forming a yellow toner image corresponding to the electrostatic latent image on the image carrying member  10 . 
     The yellow toner image thus formed on the image carrying member  10  is then transferred to the intermediate transfer member  43 , while the yellow toners remaining on the image carrying member  10  after the transfer are removed from the image carrying member  10  by a cleaning device  44 . 
     As in the above-mentioned first developing apparatus A 1 , magenta, cyan, and black toner images are successively formed on image carrying members  10  opposite to developing rollers  21  in the second to fourth developing apparatuses A 2  to A 4 , and the magenta, cyan, and black toner images formed on the image carrying members  10  are successively transferred to the intermediate transfer member  43  to which the yellow toner image has been transferred, as described above, to form a full-color toner image on the intermediate transfer member  43 . 
     Thereafter, as in the above-mentioned first color image forming apparatus, recording sheets  46  are introduced form a paper cassette  45  provided in a lower part of the color image foming apparatus into a portion where the intermediate transfer member  43  and a transfer roller  48  are opposite to each other by feeding rollers  47 , and the full-color toner image formed on the intermediate transfer member  43  is transferred to the recording sheets  46 . The full-color toner image thus transferred on the recording sheets  46  is fixed to the recording sheets  46  by a fixing device  49  to discharge the recording sheets  46 , while the toners remaining on the intermediate transfer member  43  without being transferred are removed from to intermediate transfer member  43  by a cleaning device  50 . 
     Although the present invention has been fully described by way of examples, it is to be noted that various changes and modifications will be apparent to those skilled in the art. 
     Therefore, unless otherwise such changes and modifications depart from the scope of the present invention, they should be construed as being included therein.