Patent Publication Number: US-7593671-B2

Title: Developing device, a developing method, a process cartridge and an image forming apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application is claiming foreign priority of Japanese patent application No. 2005-350580 and Japanese patent application No. 2006-277122 whose entire disclosure is incorporated by reference herein. 
   FIELD OF THE INVENTION 
   This invention relates to a developing device for developing latent electrostatic images to toner images with developer. The developing device is used in an image forming apparatus such as a copying machine, a printer, fax machine or the like. 
   DESCRIPTION OF THE RELATED ART 
   An conventional electrostatic image forming machine, such as a copying machine, typically forms toner images by charging a surface of a latent image carrier, exposing the charged surface of the latent image carrier to form latent images, developing the latent images to toner images, transferring the toner images to recording media such as paper sheets and fixing the toner images to the recording media with heat. 
   In the developing device using a two-component developer which comprises toner particles and carrier particles, the toner particles in the developer are consumed during the development process. So, after the development process, new toner particles are supplied to the developer and stirred with the developer so that the developer can be used for the development process again. In this type of developing device, it is required to maintain a toner density in the developer and a charge quantity of the toner particles within predetermined ranges in order to stabilize the quality of the toner images. The toner density depends on the distribution of consumed toner particles and the distribution of newly supplied toner particles. The charge quantity of the toner particles depends on the condition of the friction between the carrier particles and the toner particles stirred together. In the developing device, the developer is agitated in order to adequately uniformly distribute toner particles and in order to electrically charge the toner particles enough for stabilizing the quality of the toner image. 
   A conventional developing device with two developer conveyers is shown in  FIG. 23 . In  FIG. 23 , a developer supplying conveyer  104  as the first developer conveyer and a developer agitating conveyer  101  as the second developer conveyer are horizontally disposed below the developer bearing member  103 . The developer bearing member  103  bears the developer in order to supply the developer to the development area in which the developer bearing member  103  faces the latent image carrier. The developer supplying conveyer  104  supplies the developer to the developer bearing member  103  and receives the developer from the developer bearing member  103  while conveying the developer, and the developer agitating conveyer  101  receives the developer from the downstream of the developer supplying conveyer  104  and supplies the developer to the upstream of the developer supplying conveyer  104  while conveying the developer. However, in this conventional developing device, it is difficult to suppress the deterioration of the developer. 
   Japanese Laid-Open Patent Publication No. 11-167260 and No. 2001-290369 disclose developing devices with three conveyers. As shown in  FIG. 20 , Japanese Laid-Open Patent Publication No. 11-167260 discloses a developer receiving conveyer  120  as the third developer conveyer. The developer receiving conveyer  120  receives the developer having passed a development area from the developer bearing member  123  while conveying the developer and supplies the developer back to the developer agitating conveyer  122 . The developer receiving conveyer  120  is described to be useful for keeping a toner density in the developer within the predetermined range when the developer is supplied to the developer supplying conveyer  117 . As the result, the unevenness of the density in a toner image can be suppressed. 
   Laid-Open Patent Publication No. 2001-290369 discloses a developing device, as shown  FIG. 21 , which comprises an rotating developer bearing member  441  disposed nearby an latent image carrier  1 , plural magnets  442  disposed within the developer bearing member  441  for generating a magnetic field to keep a developer on the surface of the developer bearing member  441 , a developer ripping member for ripping the developer from the developer bearing member  441 , a developer supplying conveyer  444  for supplying the developer to the developer bearing member  441 , a developer receiving conveyer  445  disposed parallel to and above the developer supplying conveyer  444  for receiving the ripped developer from the developer bearing member  441 , a developer agitating conveyer  446  for receiving the developer from the downstream of the developer supplying conveyer  444  so as to agitate the developer and send the developer to the upstream of the developer supplying conveyer  444 , a developer supplying conveyer-containing space  401  for containing the developer supplying conveyer  444 , a developer receiving conveyer-containing space  402  for containing the developer receiving conveyer  445 , a developer agitating conveyer-containing space  403  for containing the developer agitating conveyer  446 , a partitioning member for partitioning the developer supplying conveyer-containing space  401  from the developer receiving conveyer-containing space  402 , an opening of the partitioning member for sending the developer from the developer receiving conveyer-containing space  402  to the developer supplying conveyer-containing space  401  at the downstream in the developer conveying direction. 
   The developer ripped by the ripping member is sent to the developer receiving conveyer-containing space  402  in order to be agitated and conveyed by the developer receiving conveyer  445 . It is then sent to the downstream of the developer supplying conveyer-containing space  401  through the opening in order to be agitated and conveyed by the developer supplying conveyer  444 , sent to the developer agitating conveyer-containing space  403  in order to be agitated and conveyed by the developer agitating conveyer  446 , sent to the upstream of the developer supplying conveyer-containing space  401  and sent to the developer bearing member  441  for further development. 
   The developing device is described to be useful for keeping the toner density in the developer within the predetermined range when the developer is supplied to the developer supplying conveyer. As the result, the unevenness of the dense in toner images can be suppressed. 
   The developing device with three conveyers described above, in which the developer at the downstream of the development area is sent to the developer receiving conveyer instead of sent back directly to the developer supplying conveyer, can prevent the decline of the toner density at the downstream of the developer supplying conveyer which causes the unevenness of the toner density on the developer supplying conveyer in the widthwise direction. However, the developing device with three conveyers causes new problems to be solved. First, the amount of the developer on the developer supplying conveyer decreases in the downstream direction, resulting in the shortage of the developer. Second, the amount of the developer on the developer receiving conveyer becomes too much to be received at the downstream direction, resulting in the packing of the developer or adhesion of the developer to the developer bearing member. 
   To increase the rotating speed of the developer supplying conveyer or to increase the diameter of the developer supplying conveyer can be a solution to these new problems, but those solutions have only limited effect because of the endurance of a bearing supporting the developer supplying conveyer or because of the available space, especially when applied to an image forming apparatus with high image forming speed or long widthwise length. 
     FIG. 22  shows the simplified flow of the developer in the conventional developing device shown in  FIG. 20  or  FIG. 21 . 
   In  FIG. 22 , the developer bearing member is indicated with number  123 ,  442  indicating the index  123  in  FIG. 20  or the index  442  in  FIG. 21 . The developer supplying conveyer is indicated with number  117 ,  444  indicating the index  117  in  FIG. 20  or the index  444  in  FIG. 21 , the developer receiving conveyer is indicated with number  120 ,  402  indicating the index  120  in  FIG. 20  or the index  402  in  FIG. 21 , and the developer agitating conveyer is indicated with number  122 ,  446  indicating the index  122  in  FIG. 20  or the index  446  in  FIG. 21 . 
   The white arrow indicates the flow of the developer and the dotted area indicates the amount of the developer. To simplify, the widthwise length of the developer supplying conveyer, the developer receiving conveyer and the developer agitating conveyer are set to be the same. 
   The weight of the developer per one unit of the length at the downstream end of the developer receiving conveyer “Mr”, and the weight of the developer per one unit of the length at the downstream end of the developer supplying conveyer “ms” can be calculated as follows:
 
 Mr=ρvL/u 3
 
 ms=Ms−ρvL/u 1
 
   wherein L (m) is the widthwise length on the developer bearing member on which the developer is borne, wherein L can be equal to or longer than a widthwise length of the development area on which development process is executed, ρ(kg/m 2 ) is the amount of the developer on the developer bearing member per one unit of the area, v (m/sec) is the speed of the surface of the developer bearing member in the rotating direction, Ms (kg/m) is the weight of the developer per one unit of the length at the upstream end of the supplying member, u 3  (m/sec) is the speed of the developer conveyed by the developer receiving conveyer and u 1  (m/sec) is the speed of the developer conveyed by the developer supplying conveyer. 
   These equations indicate that, if ρ, v and L are fixed, u 3  and u 1  should be increased in order to decrease Mr or in order to increase ms. 
   Japanese Laid-Open Patent Publication No. 11-24403 and Japanese Patent No. 2981812 disclose a developing device with two developer agitating conveyers and one developer supplying/receiving conveyer, as shown in  FIGS. 24 and 25 . This developing device has an opening ( 483  in  FIG. 24  or  818  in  FIG. 25 ) disposed at the center of a partitioning member for partitioning the developer supplying/receiving conveyer from two developer agitating conveyers. The opening is set up in order to prolong the length of the developer agitating path in order to suppress the imbalance of a toner density in a widthwise direction. 
   However, the above-mentioned new problems caused in the developing device with three developer conveyers have not been solved. 
   SUMMARY OF THE INVENTION 
   One aspect of the present invention includes a developer bearing member, a developer supplying conveyer, a developer receiving conveyer and a developer agitating conveyer. 
   The developer bearing member carries a developer to an development area so that the developer on the developer bearing member faces to a latent image carrier for development process. 
   The developer supplying conveyer supplies the developer to the developer bearing member while conveying the developer in a widthwise direction, The developer receiving conveyer receives the developer from the developer bearing member after development while conveying the developer in the widthwise direction, The developer agitating conveyer receives the developer from the developer receiving conveyer and the developer supplying conveyer and supplies the developer to the developer supplying conveyer while agitating and conveying the developer in the widthwise direction. 
   The developer supplying conveyer or the developer receiving conveyer has at least one dividing position. The dividing position is a position at which the conveying direction of the developer reverses, and a position which is arranged so that if the dividing position is projected to the developer bearing member along a plane which is perpendicular to the widthwise direction of the developer bearing member, the projected position on the developer bearing member is within an area in which the developer is borne on the developer bearing member. 
   Accordingly, a first object of this invention is to provide a new developing device in which the shortage of the developer at the downstream of the developer supplying conveyer is improved, and in which the overflow of the developer at the downstream of the developer receiving conveyer is sufficiently suppressed. A second object of this invention is to improve the imbalance with regard to the amount of the developer in the widthwise direction on the developer supplying conveyer and the developer receiving conveyer. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  shows a developing device in which each screw is divided into two areas at a “dividing position”. 
       FIG. 1B  shows a simplified flow of the developer and a widthwise distribution of the amount of the developer in a developing device in which each screw is divided into two areas at a “dividing position”. The ratio of each area in widthwise length is “α” and “(1−α)”. The winding direction of the screw in one area is opposite to the winding direction of the screw in a second area. 
       FIG. 2A  shows an example of developing device in which the developer supplying conveyer has a “dividing position,” but the developer receiving conveyer doesn&#39;t have any “dividing position”. 
       FIG. 2B  shows a simplified flow of the developer and a widthwise distribution. 
       FIG. 3A  shows an example of developing device in which the developer receiving conveyer has a “dividing position,” but the developer supplying conveyer doesn&#39;t have any “dividing position”. 
       FIG. 3B  shows a simplified flow of the developer and a widthwise distribution. 
       FIG. 4A  shows an example of developing device in which the developer receiving conveyer and the developer supplying conveyer each has at least one “dividing position,” but the number of the “dividing positions” is different from each other. 
       FIG. 4B  shows a simplified flow of the developer and a widthwise distribution. 
       FIG. 5A  shows an example of developing device in which the developer receiving conveyer and the developer supplying conveyer each has “dividing positions” and the number of the “dividing positions” is the same. 
       FIG. 5B  shows a simplified flow of the developer and a widthwise distribution. 
       FIG. 6  shows an example of developing device in which “dividing positions” divide the area having the widthwise length L into three sub-areas each having the same widthwise length (=⅓ L) and the speed of the developer is set to be the same in each sub-area. 
       FIG. 7A  shows an example of the developing device in which there is only one “dividing position”. 
       FIG. 7B  shows a simplified flow of the developer and a widthwise distribution. 
       FIG. 8A  shows an example of the developing device in which there is only one “dividing position”. 
       FIG. 8B  shows a simplified flow of the developer and a widthwise distribution. 
       FIG. 9A  shows a relationship between the number of toner replenishing points and the amount of toner circulation in case the number of toner replenishing points is smaller than the number of the sub-areas. 
       FIG. 9B  shows a relationship between the number of toner replenishing points and the amount of toner circulation in case the number of toner replenishing points is equal to the number of the sub-areas. 
       FIG. 10A  shows the flow of the newly replenished toner particles in case the toner particles are replenished to each circulation point at the downstream of the developer receiving conveyer at a moment. 
       FIG. 10B  shows the flow of the newly replenished toner particles in case the toner particles are replenished to each circulation point at the downstream of the developer receiving conveyer at a moment which is half of a replenishing period later than  FIG. 10A . 
       FIG. 10C  shows the flow of the newly replenished toner particles in case the toner particles are replenished to each circulation point at the downstream of the developer receiving conveyer at a moment which is half of a replenishing period later than  FIG. 10B . 
       FIG. 10D  shows the flow of the newly replenished toner particles in case the toner particles are replenished to each circulation point at the downstream of the developer receiving conveyer at a moment which is half of a replenishing period later than  FIG. 10C . 
       FIG. 10E  shows the flow of the newly replenished toner particles in case the toner particles are replenished to each circulation point at the downstream of the developer receiving conveyer at a moment which is half of a replenishing period later than  FIG. 10D . 
       FIG. 10F  shows the flow of the newly replenished toner particles in case the toner particles are replenished to each circulation point at the downstream of the developer receiving conveyer at a moment which is half of a replenishing period later than  FIG. 10E . 
       FIG. 11  shows one embodiment of a screw with the winding pitch gradually changing near “A DIVIDING POSITION”. 
       FIG. 12A  shows a partitioning member which partitions the development area physically. 
       FIG. 12B  is a cross section of the developing device seen from the direction of arrow A in  FIG. 12A . 
       FIG. 12C  is a cross section of the developing device seen from the direction of arrow B in  FIG. 12A . 
       FIG. 13  shows a cross section of an image forming apparatus to which the present invention can be applied. 
       FIG. 14  shows a main portion of the image formation part to which the present invention can be applied. 
       FIG. 15  shows an image forming apparatus to which the present invention can be applied. 
       FIG. 16  shows a schematic diagram for explaining the form factor SF- 1 . 
       FIG. 17  shows a schematic diagram for explaining the form factor SF- 2 . 
       FIG. 18A  shows the toner particles replenishing device and the carrier particles replenishing device. 
       FIG. 18B  shows the developer replenishing device. 
       FIG. 19  shows a developing device to which the present invention can be applied. 
       FIG. 20  shows a conventional developing device including a developer supplying conveyer, a developer receiving conveyer and a developer agitating conveyer. 
       FIG. 21  shows a conventional developing device including a developer supplying conveyer, a developer receiving conveyer and a developer agitating conveyer. 
       FIG. 22  shows the flow of the developer in a conventional developing device including a developer supplying conveyer, a developer receiving conveyer and a developer agitating conveyer. 
       FIG. 23  shows a conventional developing device including a developer supplying/agitating member and a developer agitating conveyer. 
       FIG. 24  shows a conventional developing device with two developer agitating conveyers and one developer supplying/receiving conveyer. 
       FIG. 25  shows a conventional developing device with two developer agitating conveyers and one developer supplying/receiving conveyer. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention will be described in more detail below with reference to the accompanying drawings illustrating preferred embodiments. Although various modifications will be possible for those skilled in the art after receiving the present disclosure, the embodiments described below are only the preferred embodiments and the present invention is not limited to the embodiments. 
   In following embodiments, a developer supplying conveyer, a developer receiving conveyer and a developer agitating conveyer each has the shape of screw and may be also described as a “developer supplying screw”, a “developer receiving screw”, or a “developer receiving screw”. Any of these conveyers may be described as just “screws”. 
   The shape of the developer supplying conveyer, the developer receiving conveyer and the developer agitating conveyer is not restricted to be a screw type and various shapes are applicable to the present invention as long as it conveys the developer. 
   In following descriptions, a developing device develops latent images to toner images with a two-component developer. The two-component developer includes toner particles and magnetic carrier particles. The developer bearing member includes a sleeve on which the developer is carried and magnets inside the sleeve configured to attract the developer on the sleeve and configured to make the magnetic field along which the carrier particles form chain-like shapes called “magnetic brushes”. The sleeve can rotate while the magnets are fixed. The developer is attracted and borne on the sleeve by the magnetic force from the magnets and is carried to a “development area” in response to the rotation of the sleeve. In the “development area”, the developer forms “magnetic brushes” configured to contact with a photoconductor as a latent image carrier. The toner particles are transported to the photoconductor in response to an electric bias between the sleeve and the photoconductor. It is also possible that a “magnetic brush” does not touch the surface of the photoconductor. 
   The present invention is suitable to this type of developing device. However, it is possible to apply the present invention to a known developing device using a one-component developer which includes toner particles but does not include carrier particles. 
   The developing device described in Japanese Laid-Open Patent Publication No. 11-24403 ( FIG. 24 ) and Japanese Patent No. 2981812 ( FIG. 25 ) is essentially different from the present invention. The present invention provides an improvement for the developing device which includes a developer receiving conveyer, a developer supplying conveyer and a developer agitating conveyer so that the developer at the downstream of the development area is not sent back to the developer supplying conveyer directly. The improvement is not for the developing device which includes two developer agitating conveyers and one developer supplying/receiving conveyer as shown in the related arts. The functional difference of each conveyer results in a difference of the purpose and the function. 
   The purpose of the related arts is to improve the efficiency of agitation and to suppress the toner density fluctuation in the widthwise direction. The toner density fluctuation happens because two functions (supplying and receiving the developer) are given to one conveyer (the developer supplying/receiving conveyer). 
   On the other hand, the purpose of the present invention is to improve an imbalance with regard to the amount of the developer in the widthwise direction. The imbalance happens because the developer at the downstream of the development area is not sent back directly to the developer supplying conveyer. The developing device described in Japanese Laid-Open Patent Publication No. 11-24403 and Japanese Patent No. 2981812 does not have this new problem of the imbalance with regard to the amount of the developer in the widthwise direction. 
   In this invention, the shortage of the developer at the downstream of the developer supplying conveyer can be improved by reversing a conveying direction of the developer on the developer supplying conveyer in order to suppress the fluctuation of the amount of the developer. The overflow of the developer at the downstream of the developer receiving conveyer can be improved by reversing the conveying direction of the developer on the developer receiving conveyer in order to suppress the fluctuation of the developer amount. 
   It is preferable to solve the shortage and the overflow of the developer together in order to use the developing device with the developer receiving conveyer, the developer supplying conveyer and the developer agitating conveyer. This invention can solve those two problems together by reversing the conveying direction of the developer on the developer supplying conveyer and the developer receiving conveyer. 
   Embodiment 
   To better understand the present invention, the amount of the developer in a widthwise direction in the developing device of the present invention will be described in  FIG. 1B  in comparison to  FIG. 22 . 
   A developing device of this embodiment is shown in  FIG. 19 . The developing device includes a developer bearing member  205 , a developer supplying conveyer  208 , a developer receiving conveyer  206  and a developer agitating conveyer  211 . The developer bearing member and the three screws are disposed approximately horizontally. 
   The developer bearing member includes a rotating sleeve and magnets fixed inside the sleeve. The developer on the developer bearing member is attached to the developer bearing member because of the magnetic field generated by magnets inside the sleeve and conveyed by the rotation of the sleeve. 
     FIG. 1A  shows a developing device in which each screw is divided into two areas at a “dividing position”. 
     FIG. 1B  shows a simplified flow of the developer and a widthwise distribution of the amount of the developer in a developing device shown in  FIG. 1A , in comparison to  FIG. 22 . 
   The winding direction of the developer supplying screw, the developer receiving screw and the developer agitating screw is reversed at a point within the development area at which the flow of the developing bearing member is divided into two areas each having the widthwise length αL and the widthwise length (1−α)L wherein 0&lt;α&lt;1. 
   Hereinafter, a position at which the conveying direction of the developer reverses may be described as a “dividing position”. The “dividing position” is arranged so that if the position is projected to the developer bearing member along the plane which is perpendicular to the widthwise direction of the developer bearing member, the projected position on the developer bearing member is within the area in which the development process is executed. 
   The weight of the developer per one unit of the length at the downstream end of the screw can be calculated as follows:
 
 Mr=ρv (1−α) L/u 3
 
 ms=Ms−ρv (1−α) L/u 1
 
 Mr′=ρvαL/u 3′
 
 ms′=Ms−ρvαL/u 1′
 
Wherein:
 
   Mr: the weight of the developer per one unit of the length at the downstream end of the developer receiving conveyer in the area having the widthwise length (1−α)L 
   ms: the weight of the developer per one unit of the length at the downstream end of the developer supplying conveyer in the area having the widthwise length (1−α)L 
   Mr′: the weight of the developer per one unit of the length at the downstream end of the developer receiving conveyer in the area having the widthwise length αL 
   ms′: the weight of the developer per one unit of the length at the downstream end of the developer supplying conveyer in the area having the widthwise length αL 
   u3: the speed of the developer conveyed by the developer receiving conveyer in the area having the widthwise length (1−α)L 
   u1: the speed of the developer conveyed by the developer supplying conveyer in the area having the widthwise length (1−α)L 
   u3′: the speed of the developer conveyed by the developer receiving conveyer in the area having the widthwise length αL 
   u1′: the speed of the developer conveyed by the developer supplying conveyer in the area having the widthwise length αL 
   These equations indicate that reversing the winding direction of the screw (i.e. reversing the conveying direction of the developer) at a point within the development area has the same effect as shortening the value L. Thus, reversing the winding direction of the screw is effective to keep Mr, ms, Mr′ and ms′ small without enlarging u 1 , u 3 , u 1 ′ and u 3 ′. 
   A “dividing position” divides the development area into plural sub-areas. The flow of the developer in each sub-area is approximately separated as if there was a plane between sub-areas. Hereinafter, this imaginary plane will be expressed as Sn (n is a index indicating each imaginary plane). There is only one imaginary plane “S 1 ” in the developing device shown in  FIG. 1A  and  FIG. 1B . 
   It is preferable that the developer supplying conveyer has the same number of “dividing positions” as that of the developer receiving conveyer. 
   Some variations of the “dividing position” will be described in  FIGS. 2A ,  2 B,  3 A,  3 B,  4 A,  4 B,  5 A and  5 B. In  FIGS. 3A-5B , widthwise lengths of(1−α−β)L, βL, and αL are depicted and are discussed with regard to their “dividing positions” below. 
   (1)  FIG. 2A  shows an example of developing device in which the developer supplying conveyer has a “dividing position,” but the developer receiving conveyer does not have any “dividing position”. 
     FIG. 2B  shows a simplified flow of the developer and a widthwise distribution of the amount of the developer in a developing device shown in  FIG. 2A . 
   (2)  FIG. 3A  shows an example of developing device in which the developer receiving conveyer has two “dividing positions,” but the developer supplying conveyer does not have any “dividing position”. 
     FIG. 3B  shows a simplified flow of the developer and a widthwise distribution of the amount of the developer in a developing device shown in  FIG. 3A . 
   (3)  FIG. 4A  shows an example of developing device in which the developer receiving conveyer and the developer supplying conveyer each has at least one “dividing position,” but the number of the “dividing position” is different from each other.  FIG. 4B  shows a simplified flow of the developer and a widthwise distribution of the amount of the developer in a developing device shown in  FIG. 4A . 
   As shown in  FIGS. 2A ,  2 B,  3 A,  3 B,  4 A and  4 B, if the number of the “dividing position” is different between the developer supplying conveyer and the developer receiving conveyer, the weight of the developer on the developer agitating conveyer per one unit of the length varies immensely in the widthwise direction. The reason for this variation is that the developer agitating conveyer conveys the developer coming from only the downstream of the developer supplying conveyer in one area, and conveys the developer coming from the downstream of both of the developer supplying conveyer and the developer receiving conveyer in another area. 
   As a result, the space around the developer agitating conveyer is not used efficiently. 
   (4)  FIG. 5A  shows an example of the developing device in which the developer receiving conveyer and the developer supplying conveyer each has “dividing positions” and the number of the “dividing positions” is the same. 
     FIG. 5B  shows a simplified flow of the developer and a widthwise distribution of the amount of the developer in a developing device shown in  FIG. 5A . 
   Compared with above-mentioned cases (1), (2), and (3), if the number of the “dividing positions” in the developer supplying conveyer and the developer receiving conveyer is the same like case (4), the developer agitating conveyer conveys the developer coming from the downstream of both of the developer supplying conveyer and the developer receiving conveyer in every widthwise point. As a result, the space around the developer agitating conveyer is used efficiently. 
     FIG. 6  shows an example of developing device in which two “dividing positions” divide the area having the widthwise length L into three sub-areas, each having the same widthwise length (=⅓ L). The speed of the developer is set to be the same in each sub-area. In other words, u 1 =u 1 ′=u 1 ″, u 2 =u 2 ′=u 2 ″ and u 3 =u 3 ′=u 3 ″ wherein u 1 , u 1 ′, u 1 ″ is the speed of the developer on the developer supplying conveyer in each sub-area, u 2 , u 2 ′, u 2 ″ is the speed of the developer on the developer agitating conveyer in each sub-area and u 3 , u 3 ′, u 3 ″ is the speed of the developer on the developer receiving conveyer in each sub-area as described in  FIG. 6 . There are two imaginary planes S 1 , S 2  in  FIG. 6 . 
   As a result, the weight of the developer on the developer agitating conveyer per one unit of the length becomes approximately the same at every widthwise point. The maximum and minimum weight of the developer on the developer supplying conveyer per one unit of the length becomes approximately the same at every widthwise point and the maximum weight of the developer on the developer receiving conveyer per one unit of the length becomes approximately the same at every widthwise point. 
   Thus, the space around each screw can be used with the maximum efficiency. 
   (5)  FIG. 7A  and  FIG. 8A  show special cases of the developing device explained in (4), in which there is only one “dividing position”. 
     FIG. 7B  shows a simplified flow of the developer and a widthwise distribution of the amount of the developer in a developing device shown in  FIG. 7A . 
     FIG. 8B  shows a simplified flow of the developer and a widthwise distribution of the amount of the developer in a developing device shown in  FIG. 8A . 
   This is not only the simplest structure of the developing device explained in (4), but also the most effective structure to make the toner particles spread in the developer since the developer conveying path is the longest. 
   There is an opening at the center in the widthwise direction of a partitioning board which partitions the developer supplying conveyer from the developer agitating conveyer. The developer moves through the opening. 
   The conveyance of the developer between the screws can be achieved by a known system such as paddles. 
   A toner particle replenishment system suitable to the present invention will be described. In the conventional developing device, the toner particles are replenished to one predetermined point on the developer receiving conveyer  206  or the developer agitating conveyer  211 . However, it is a problem to have only one toner replenishing point in the developing device in which the circulation of the developer is divided by reversing the winding direction of the screws. If there is only one toner replenishing point, it is difficult to replenish toner particles to a circulation point apart from the toner replenishing point, and there may be fluctuation of the toner density in the widthwise direction on the developer bearing member  205 . 
   An example of this problem is shown in  FIG. 9A . In  FIG. 9A and 9B , the flow of the replenished toner particles is shown by arrows with unfilled triangles and thin lines. Also, the replenishing position is described as a circle filled with a line. 
   In  FIG. 9A , the circulation of the developer is divided into three sub-areas by reversing the winding direction of the screws at two points, but there is only one point for the toner replenishment. Therefore, in the left sub-area placed at the furthest point from the toner replenishing point, the developer after the development with low toner density (shown by arrows with filled triangles and thin lines) reaches the developer bearing member without being mixed well enough with the newly replenished toner particles. 
   By replenishing toner particles to every sub-area, the newly replenished toner particles can be mixed with the developer well enough in every circulation area as shown in  FIG. 9B . It is recommended to replenish the toner particles at functionally similar positions in every circulation area, such as “the downstream of the developer receiving conveyer  206 ” or “the upstream of the developer agitating conveyer  211 ”, because that will make it easy to control or design the flow of the developer. 
   By arranging each two positions to which the toner particles are replenished to be disposed symmetrically with respect to a imaginary plane which is disposed between two replenishing positions, the toner particles can be replenished at functionally similar positions. 
     FIG. 10A  through  FIG. 10F  show the flow of the newly replenished toner particles (shown by arrows with filled triangles and thin lines) in case the toner particles are replenished to each circulation area at the downstream of the developer receiving conveyer  206 . The timing for replenishing the toner particles to each one of two replenishing positions is shifted by half of a replenishing period so that the toner particles are replenished alternatively to one of two replenishing positions at a time. 
   Time passes from  FIG. 10A  to  FIG. 10F  so that each figure shows the condition of circulation half of a period after a previous figure.  FIG. 10B  shows the condition of circulation half of the replenishing period after  FIG. 10A .  FIG. 10C  shows the condition of circulation half of the replenishing period after  FIG. 10B .  FIG. 10D  shows the condition of circulation half of the replenishing period after  FIG. 10C .  FIG. 10E  shows the condition of circulation half of the replenishing period after  FIG. 10D .  FIG. 10F  shows the condition of circulation half of the replenishing period after  FIG. 10E . The replenishing position at each timing is shown as “R” in each figure. This control makes the same situation as if the toner particles are continuously replenished to an area surrounded by the dotted lines shown in  FIG. 10D . Thus, the fluctuation of the toner density on the developer bearing member  205  can be suppressed efficiently. 
   Next, the suitable shape of the screw to the present invention will be discussed. Reversing the winding direction of a screw may cause a collision of the flows of developer conveyed in opposite directions to each other, such as the developer agitating screw  211  in  FIG. 7A , the developer supplying screw  208  in  FIG. 8A  and the developer receiving screw  206  in  FIG. 8A . These collisions may increase the stress on the developer. In order to suppress this stress, it is preferable to change the winding pitch of the screw gradually near the “dividing position” so that the winding pitch of the screw at a closer position to the “dividing position” is longer than the winding pitch of the screw at a further position from the “dividing position”, as shown in  FIG. 11 . 
   If the conveyer does not have a screw form, it is preferable to change the conveying speed of the developer along the conveyer gradually near the “dividing position” so that the conveying speed of the developer at a closer position to the “dividing position” is slower than the conveying speed of the developer at a further position from the “dividing position”. 
   This gradual change of the winding pitch of the screw has another favorable feature when applied to the developer supplying screw  208 . It increases the amount of the developer at the downstream of the developer supplying screw. 
   Next, a partitioning member which partitions above-mentioned development area (having the length L) into sub-areas will be discussed. This partitioning member physically divides the development area instead of dividing the development area by imaginary planes. In the present invention, the partitioning member can be used as well as the imaginary plane. As an example, the partitioning member has the shape of the planar board as shown in  FIG. 12A . The  FIG. 12B  is a cross section of the developing device seen from the direction of arrow A in  FIG. 12A . The  FIG. 12C  is a cross section of the developing device seen from the direction of arrow B in  FIG. 12A . 
   The partitioning member comprises an upper part  11  and a lower part  12  as shown in  FIGS. 12B and 12C , and screws are put between those two parts. This structure makes it easy to construct the partitioning member in the developing device. The holes in the partitioning member through which screws go have a diameter slightly greater than the diameter of screws to suppress the friction between the partitioning member and screws. Sponges are put between the partitioning member and screws. 
   Next, the developing device to which the present invention applies will be discussed. The present invention can be applied not only to the developing device in this embodiment as shown  FIG. 19 , but also to the conventional developing device as shown in  FIG. 20  or  FIG. 21 . 
     FIG. 19  shows the developing device in which the developer receiving conveyer, the developer supplying conveyer and the developer agitating conveyer are arranged so that the three conveyers lines approximately horizontally. 
     FIG. 20  shows the developing device in which the center axis of the developer agitating conveyer is disposed lower than the center axis of the developer receiving conveyer, and the center axis of the developer receiving conveyer is disposed lower than the center axis of the developer bearing member. 
     FIG. 21  shows the developing device in which the center axis of the developer supplying conveyer is disposed lower than the center axis of the developer bearing member, the center axis of the developer supplying conveyer is disposed approximately as high as the center axis of the developer agitating conveyer and the center axis of the developer receiving conveyer is disposed higher than the center axis of the developer agitating conveyer. 
   The image forming apparatus in this embodiment is illustrated in  FIG. 13 . Each element of this image forming apparatus will be discussed first, and the motion of this image forming apparatus will be discussed next. 
     FIG. 13  shows the cross section of the image forming apparatus  100  to which the present invention is applied. 
     FIG. 13  shows the configuration of a full color printer capable of double-faced printing by electrophotography, with the image formation apparatus related to the present embodiment labeled as  100 . 
   As shown in the figure, the primary image formation part  20  is positioned above, and the second image formation part  30  is positioned below, the recording medium feed path  43 A within the main body  100  of this image formation apparatus. The primary image formation part  20  is provided with a first intermediate transfer belt  21  moving endlessly in the direction of the arrow, and the second image formation part  30  is provided with a second intermediate transfer belt  31  moving endlessly in the direction of the arrow. Four first image formation units  80 Y,  80 C,  80 M, and  80 K are positioned on the upper tensioned face of the first intermediate transfer belt  21 . On the other hand, four second image formation units  81 Y,  81 C,  81 M, and  81 K are positioned on the upper tensioned face of the second intermediate transfer belt  31 . The designations Y, C, M, and K are associated with the numbers of these primary and second image formation units corresponding to the colors of toner handled, Y corresponding to yellow, C to cyan, M to magenta, and K to black. The same Y, C, M, and K are applied to photoconductors (latent image bearing members)  1  which are provided in the first or second image formation units and rotate together with the first intermediate transfer belt  21  or second intermediate transfer belt  31 . The photoconductors  1 Y through  1 K are positioned equidistantly within the image formation parts  20  and  30 , and in contact with at least part of the upper tensioned face of the intermediate transfer belts  21  and  31  respectively during image formation. 
   The main portion of the image formation part is shown in  FIG. 14 . 
   In  FIG. 14 , the cylindrical photoconductor  1  is driven by a drive device (not shown) to rotate in the direction of the arrow when the printer part  100  is operated. Image formation devices, such as a scorotron charger (a charging device)  3 , an optical writing device  4 , a developing device  5 , a cleaning device  2 , a discharger Q, etc., and an electric potential sensor S 1  and an image sensor S 2  are arranged around the photoconductor  1 . 
   The photoconductor  1  includes an aluminum cylinder whose diameter may be from 30 mm to 120 mm, the surface of which is covered with a layer of photoconductive material, such as an organic photoconductive (OPC) layer. The first photoconductor  1  may be an aluminum cylinder covered with an amorphous silicon (a-Si) layer. Further, the first photoconductor  1  may be formed as a belt. 
   The cleaning device  2  includes a cleaning brush  2   a,  a cleaning blade  2   b,  a collecting member  2   c,  etc., and is configured to remove and to collect residual toner remaining on the surface of the photoconductor  1 . 
   The optical writing device  4  radiates light beams on the electrically charged surface of the photoconductor according to the image data of each color in order to discharge the electrical charge and form the electrical latent image. 
   In the shown example, the optical writing device  4  is formed of a light emitting diode (LED) array and a focusing element. A known laser scan system using a laser light source, a polygon mirror, and the like can be also used as the optical writing device  4 . 
   Instead of the scorotron charger  3 , another type of charging device can be used. For example, a charging roller in contact with the surface of the photoconductor  1  can be used. 
   The developing device  5  develops latent images to toner images by developing discharged areas of latent images. A two-component developer including toner particles and carrier particles is used. The detail of the developing device  5  has been discussed in  FIG. 19 . 
   The photoconductor  1  is uniformly charged to a negative polarity by the scorotron charger  3 . The area on the photoconductor  1  to be developed is discharged by beams from the optical writing device  4  and developed by the developing device  5  with the toner particles with negative polarity. 
   Next, description will be made of the intermediate transfer belt. 
   As the primary intermediate transfer body, the first intermediate transfer belt  21  is supported by a plurality of rollers  23 ,  24 ,  25 ,  26  (two),  27 ,  28 , and  29  running in the direction of the arrow, and provided at the bottom of the photoconductors  1 Y,  1 C,  1 M, and  1 K in the first image formation units  80 Y through  80 K. This first intermediate transfer belt  21  is endless, and is tensioned and positioned so that it is in contact with part of each photoconductor after the developing process. 
   Furthermore, the primary transfer rollers  22  are provided on the inner periphery of the first intermediate transfer belt  21  opposite the photoconductors  1 Y,  1 C,  1 M, and  1 K. The cleaning apparatus  20 A is provided at a position opposite to the roller  23  on the outer periphery of the first intermediate transfer belt  21 . This cleaning apparatus  20 A wipes and removes excess toner and recording medium dust and the like remaining on the surface of the first intermediate transfer belt  21 . 
   The first intermediate transfer belt  21 , the first image formation units  80 Y,  80 C,  80 M, and  80 K, and the cleaning apparatus  20 A are integrated to comprise the first image formation unit  20  being removable from the image formation apparatus  100 . 
   On the other hand, the second intermediate transfer belt  31  corresponding to a second intermediate transfer body is supported by a plurality of rollers  33 ,  34 ,  35 ,  36  (two),  37 , and  38  running in the direction of the arrow. This second intermediate transfer belt  31  is endless, and is tensioned and positioned so that it is in contact with the photoconductors  1 Y,  1 C,  1 M, and  1 K in the second image formation units  81 Y through  81 K. 
   This second intermediate transfer belt  31  is endless, and is tensioned and positioned so that it is in contact with part of each photoconductor after the developing process. The primary transfer rollers  32  are provided on the inner periphery of the second intermediate transfer belt  31  opposite the photoconductors  1 Y,  1 C,  1 M, and  1 K. 
   The cleaning apparatus  30 A is provided at a position opposite to the roller  33  on the outer periphery of the second intermediate transfer belt  31 . This cleaning apparatus  30 A wipes and removes excess toner and recording medium dust and the like remaining on the surface of the intermediate transfer belt  31 . 
   The second intermediate transfer belt  31 , the second image formation units  81 Y,  81 C,  81 M, and  81 K, and the cleaning apparatus  30 A are integrated to comprise the second image unit  30  being removable from the image formation apparatus  100 . 
   A transfer roller  46  is arranged at outer periphery of the first intermediate transfer belt  21  and close to the supporting roller  28 . Toner images on the first intermediate transfer belt  21  are transferred a recording medium P by an electric bias applied to the roller  46  while the recording medium P passes between the first intermediate transfer belt  21  and the transfer roller  46 . 
   A transfer charger  47  is arranged at outer periphery of the second intermediate transfer belt  31  and close to the supporting roller  34 . The transfer charger  47  may be of a known type in which a discharge electrode of a thin tungsten or gold wire is held within a casing and a transfer bias is applied to the discharge electrode by the electric source (not shown). 
   Toner images on the second intermediate transfer belt  31  are transferred to a recording medium P by the transfer current is applied to the discharge electrode while the recording medium P passes between the second intermediate transfer belt  31  and the transfer charger  47 . 
   The polarity of the transfer bias applied to the transfer roller  46  and transfer charger  47  is positive, opposite to that of the toner. 
   The recording medium supply apparatus  40  enclosing a supply of recording media is positioned at the right of the image formation apparatus  100  and feeds recording media to the recording medium path  43 B and  43 A. One sheet is fed at a time by a plurality of pairs of feed rollers  42 B. 
   A recording medium transport device  50  is provided to feed a recording medium having passed through the second transfer position on the extension of the recording medium feed path  43 A up to the fixing nip in the fixing apparatus  60  provided downstream in the recording medium feed direction while maintaining it in a flat condition. The recording medium transport device  50  has rollers  52 ,  53 ,  54 ,  55  and  56  supporting the endless feed belt  51  transporting the recording medium in the direction of the arrow. 
   A cleaning apparatus  50 A is provided opposite to the roller  55 , a suction charger  57  to grip the recording medium P is provided opposite the roller  56 , and a discharging/separation charger  58  are provided opposite the roller  54 , on the outside of the feed belt  51 . 
   The feed belt  51 , contacting an unfixed toner image and moving with the recording medium P, is electrically charged by the suction charger  57  with the same negative polarity as the toner particles. The feed belt  51  can be metal belt, polyimide belt or polyamide belt as long as the resistivity value is suitable to be charged. The feed belt  51  is configured to release the toner images. The moving speed of the feed belt  51  is set to be the same speed as the speed of a recording medium passing through the fixing apparatus  60 . 
   The fixing apparatus  60  having a heating device is provided downstream in the direction of the recording medium transport device  50 . Possible heating devices include a heater provided within a roller, a belt fixing apparatus running a heated belt, or a fixing apparatus wherein induction heating is employed as the heating method. Material, hardness, and a surface nature of the fixing rollers and fixing belts is made the same top and bottom to ensure the same hue and glossiness of the images on both faces of the recording medium. Furthermore, fixing conditions are controlled according to an image forming condition, such as full color or monochrome images, single or double-faced operation, or according to recording medium type, by a control device (not shown) to ensure that fixing conditions are optimized. A pair of cooling rollers  70  having a cooling function are provided in the feed path after fixing in order to cool the recording medium for which fixing is complete, and to stabilize unstable toner as soon as possible. Rollers of a heat-pipe construction having a heat spreader can be employed as this pair of cooling rollers  70 . The cooled recording medium is discharged from the image formation apparatus  100  to the recording medium stack tray  75  by the pair of ejecting rollers  71 . 
   The recording medium stack tray  75  employs a mechanism in which a receiving member is moved by an elevator mechanism (not shown) upward and downward according to the height of stacked recording mediums. A separate recording medium processing apparatus may be arranged so that the recording medium P is conveyed thereto passing the recording medium stack tray  75  to the recording medium processing apparatus. As the recording medium processing apparatus, a bookbinding apparatus performing punching, cutting, folding, binding, etc. may be provided. 
   The toner bottles  86 Y,  86 C,  86 M, and  86 K, containing unused toner particles of respective colors and carrier particles, are detachably accommodated in the bottle accommodation part  85 . The toner particles are supplied as necessary to each development device by a toner supply mechanism. 
   In this embodiment, each of the toner bottles  86 ,  86 C,  86 M, and  86 K supplies toner to respective development devices of the first image formation part  80  and the second image formation part  81 , using the same toner. However, separate toner bottles may be provided for supplying toner of respective colors to the development devices of the first image formation part and the second image formation part. Further, the toner bottle  86 K containing frequently consumed black toner may be configured to contain a large volume of toner. 
   The bottle accommodation part  85  is arranged at the depth side of the printer part  100 , and a flat surface part in front of the bottle accommodation part  85  and at the upper surface of the printer  100  is provided to serve as a working table. 
   Single-faced recording operation wherein a full color image is formed on one face of the recording medium P in the image formation apparatus  100  will be described below. 
   The single-faced recording method is basically of two types, either of which may be selected. One of the two types is a method whereby the image carried by the first intermediate transfer belt  21  is transferred directly to upper face of the recording medium, and the other is a method whereby the image carried by the second intermediate transfer belt  31  is transferred directly to lower face of the recording medium. 
   When there are plural pages of image data to be formed, it is preferable to control the order of pages so that recording mediums are discharged on the recording medium stack tray  75  with correct order of pages. 
   The method whereby the image is carried by the first intermediate transfer belt  21  and transferred to the recording medium will be described below. The larger-numbered page is formed earlier than the smaller-numbered page so as to the order of the page is controlled appropriately. 
   When the image formation apparatus  100  is operated, the first intermediate transfer belt  21 , and the photoconductors  1 Y,  1 C,  1 M, and  1 K in the first image formation units  80 Y through  80 K, rotate. The second intermediate transfer belt  31  rotates simultaneously. However, the photoconductors  1 Y,  1  C,  1 M, and  1  K in the second image formation units  81 Y through  81 K are separated from the second intermediate transfer belt  31  and do not rotate. 
   First, operation begins with image formation with the image formation unit  80 Y. A Y color toner image is formed on the photoconductor  1 Y by the following process. The photoconductor  1 Y is uniformly charged with a negative polarity by the scorotron charger  3 . The area on the photoconductor  1 Y to be developed is discharged by beams from the optical writing device  4 , according to the image data for yellow color, and an electrical latent image is formed on the photoconductor  1 Y. Then, the latent image is developed to a toner image by the developing device  5  with the toner particles having negative polarity. This Y color toner image formed on the photoconductor is primary-transferred to the first intermediate transfer belt  21  moving synchronously with the photoconductor  1 Y by the transfer action of the primary transfer rollers  22 . In the same manner, primary transfer operation is also conducted in sequence with the appropriate timing for the photoconductors  1 C,  1 M, and  1 K. 
   Thus, a full color toner image wherein the yellow, cyan, magenta, and black toner images are overlapped in sequence is carried on the primary intermediate transfer belt  21 . This full color toner image is moved with the primary intermediate transfer belt  21  in the direction of the arrow in the figure. 
   Simultaneously, the recording medium P used for recording is fed from the recording medium supply tray  40   a  or a recording medium cassette  40   b,    40   c,  and  40   d  in the recording medium supply apparatus  40  by one of the recording medium supply and separation devices  41 A,  41 B,  41 C, and  41 D. The recording medium is then fed to the recording medium feed path  43 C by the pair of feed rollers  42 B and  42 C. Prior to the leading edge of the recording medium being gripped by the pair of registration rollers  45 , the horizontal registration compensation mechanism  44  is slid so that it is pressed against the reference guide horizontal in relation to the recording medium feed direction in order to align the recording medium in the horizontal direction. The recording medium is temporarily halted by the pair of registration rollers  45  and again fed to the transfer area with the appropriate timing to ensure that the recording medium is in the correct position in relation to the image on the primary intermediate transfer belt  21 . 
   The full color toner image on the primary intermediate transfer belt  21  is transferred by the transfer action of the first secondary transfer roller  46  to the upper surface of the recording medium P fed synchronously with the primary intermediate transfer belt  21 . The bias provided to the first secondary transfer roller  46  is positive (opposite of toner charging polarity). Following transfer, the surface of the primary intermediate transfer belt  21  is cleaned with the belt cleaning apparatus  20 A. Furthermore, foreign matter such as toner and the like remaining on the surface of the photoconductors  1 Y,  1 C,  1 M, and  1 K in the first image formation units  80 Y through  80 K for which primary transfer is complete is removed with the cleaning brush  2   a  and the cleaning blade  2   b  in the cleaning apparatus  2 . 
   The surface of each photoconductor is discharged by the discharger Q for the next image formation. Removed matter such as toner and the like is sent to the gathering box  87  by collecting member  2   c.  The electric potential sensor S 1  and the image sensor S 2  sense electric potential on the photoconductor after exposure and toner density on the photoconductor after development, respectively, and send those sensed data to the controller (not shown) for setting and controlling image forming conditions appropriately. 
   The recording medium P whereon the full color toner image on the primary intermediate transfer belt  21  has been transferred is transported towards the fixing apparatus  60  by the feed belt  51  of the recording medium transport device  50 . The surface of the feed belt  51  is charged by the recording medium suction charger  58  beforehand to ensure that the recording medium P can be reliably fed on the feed belt  51 . The destaticizer and separation charger  57  then operates to ensure that the recording medium P is separated from the feed belt  51  and fed reliably to the fixing apparatus  60 . 
   The full color toner image on the recording medium P is fixed by the heat of the fixing apparatus  60  and melted, and colors mixed, to form a complete full color image. Since toner is present only on one face (the top surface) of the recording medium, the heat energy required for fixing is low compared to that for double-faced recording with toner images on both surfaces. The control device (not shown) controls the electric power used by the fixing apparatus to the optimum in response to the image. 
   Until the fixed toner becomes fully hardened on the recording medium, toner images may be rubbed by the feed path guide members and the like, and image drop-out and disturbance may occur. To prevent this problem, a pair of cooling rollers  70  being a cooling device operates to cool the toner and recording medium. 
   The recording medium is ejected by the ejecting rollers  71  with the toner image on the upper side. The order of pages to be formed is controlled so that a smaller-numbered page is stacked on a larger-numbered page. As the recording medium stack tray  75  moves downward as the number of the stacked recording media increases, the recording media are stacked in order. Instead of stacked in the recording medium stack tray  75 , recording media may be transferred to the recording medium processing apparatus for punching, cutting, folding, binding, etc. 
   Another method whereby the image is carried by the second intermediate transfer belt  21  and transferred to the recording medium will be executed basically the same way, except the second image formation units  81 Y through  81 K form toner images instead of the first image formation units  80 Y through  80 K, and the smaller-numbered pages are formed earlier than the larger-numbered pages so as to control the order of the pages appropriately. 
   Operation during double-faced recording wherein an image is formed on both faces of the recording medium P will be described below. 
   When the start signal is input to the image formation apparatus, an image in each color is formed in sequence on the first image formation units  80 Y,  80 C,  80 M, and  80 K, and primary-transferred in sequence to the primary intermediate transfer belt  21 . Almost in parallel with the process of carrying this image as the first image, a process is conducted whereby the images of each color formed in sequence on the second image formation units  81 Y,  81 C,  81 M, and  81 K are primary-transferred in sequence to the second intermediate transfer belt  31  and carried as second images. Furthermore, since the recording medium is halted and fed again by the pair of registration rollers  45 , the recording medium is supplied in consideration of this time period, and aligned with the horizontal registration compensation mechanism  44 . The pair of registration rollers  45  feed the recording medium to the first transfer position comprising the first secondary transfer roller  46  and the first intermediate transfer belt  21  with the appropriate timing. A positive transfer current flows in the first secondary transfer roller  46 , and the image is transferred from the first intermediate transfer belt to upper face of the recording medium P. 
   The recording medium P having an image on one face in this manner is then fed to the second secondary transfer roller  47  at the second transfer position. By applying a positive transfer current to the second secondary transfer roller  47 , the full color second image already carried on the second intermediate transfer belt  31  is transferred to the lower face of the recording medium P in one action. 
   The recording medium P whereon full color toner images have been transferred to both faces in this manner is fed to the fixing apparatus  60  by the feed belt  51 . The surface of the feed belt  51  is charged with a negative charge (same polarity as the toner) by the suction charger  57 . Care is taken to ensure that toner on the lower face of the recording medium which is not yet fixed is not transferred to the belt. An alternating current is applied to the destaticizer and separation charger  58 , and the recording medium is separated from the belt  51  and transported to the fixing apparatus  60 . The toner images on both faces of the recording medium are fixed by the heat of the fixing apparatus  60  and melted so that colors mix. The recording medium is then passed through the pair of cooling rollers and ejected by the ejecting rollers  71  to the recording medium stack tray  75 . 
   When double-faced recording is executed on plural number of recording media, the control device controls recording so that smaller-numbered pages are formed on the lower face of the recording medium. With that control, when printed documents are taken out of the recording medium stack tray  75  and turned upside down, those documents are arranged in order so that a first page is on upper face of a first recording medium, a second page is on lower face of the first recording medium, a third page is on upper face of a second recording medium, a forth page is on lower face of the second recording medium and so on. 
   Although the motions of the image forming apparatus forming full color images have been shown in this embodiment, monochrome images can be also formed. 
   Another image forming apparatus to which the present invention can be applied will be illustrated in  FIG. 15 . This image forming apparatus is so called tandem type and forms a toner image only on one side of recording medium at a time. 
   In  FIG. 15 , process cartridges are arranged in a row. The process cartridge is defined here as a detachable cartridge including a latent image carrier and a developing device. The process cartridge is detachable from an image forming apparatus such as a copying machine, printer or the like. In  FIG. 15 , each process cartridge  10  forms toner images with each color. Each process cartridge  10  includes a photoconductor  1 , a charging device  3 , a developing device  5  and a cleaning device  2 . Also, there are other elements in the image forming apparatus such as an optical writing device  4 , an intermediate transfer device  6 , a transfer device  8 , a fixing device  9 , recording medium feeding member and so on. 
   The function of each element is the same as explained in  FIG. 13  except the second image formation part  30  is missing. The photoconductor  1 , the charging device  3 , the developing device  5  and the cleaning device  2  have the same function as elements with same index number in  FIGS. 13 and 14 . The intermediate transfer device  6  has the same function as intermediate transfer belt  21  in  FIG. 13  and the fixing device  9  has the same function as the fixing apparatus  60  in  FIG. 13 . 
   Next, preferable carrier particles for present invention will be discussed. 
   Preferably, a volume average diameter of the carrier particles is from 20 μm to 60 μm. By using carrier particles with the volume average diameter not greater than 60 μm, it is possible to reduce the amount of the developer on the developer bearing device  205  without damaging the ability of development. Reducing the amount of the developer in the developing device provides the following advantages. 
   (1) extending the lifetime of the carrier particles because of less stress to the carrier particles when the carrier particles pass through the regulating member which is configured to regulate the amount of the developer on the developer bearing member  205 ; 
   (2) reducing the inside volume of the developing device; and 
   (3) achieving high quality image because the magnetic brush has a higher density in the development area. 
   If carrier particles with volume average diameter greater than 60 μm are used, overflow of the carrier particles may happen during circulation. On the other hand, if carrier particles with volume average diameter smaller than 20 μm are used, carrier adhesion to the photoconductor or scattering of the carrier particles from the developing device may happen. 
   With regard to measuring the average particle diameter of carrier particle, an SRA-type microtrack particle size analyzer (manufactured by Nikkiso Co., Ltd.) is used with a range of from 0.7 to 125 μm. 
   It is preferable to use toner particles with an volume average diameter (D 4 ) of 3 μm to 8 μm. The toner particles with a small diameter and a sharp particle size distribution make the distance between the toner particles small and lead to the following effects. 
   (1) the required amount of toner particles can be reduced without damaging the reproduction of color. Thus, the fluctuation in density can be reduced. 
   (2) small dots in images with the resolution higher than 600 dpi can be formed more stably. Thus, stable images can be formed for longer time. 
   On the other hand, if toner particles with an volume average diameter (D 4 ) smaller than 3 μm are used, it tends to be difficult to transfer the toner particles efficiently or to clean the toner particles with a cleaning blade. If toner particles with a volume average diameter (D 4 ) larger than 8 μm are used, the height of toner images tends to be large and it tends to be difficult to suppress the scattering of the toner particles when a character image or line image is formed. 
   Further, it is preferable to use toner particles with a ratio of D 4 /D 1  from 1.00 to 1.30, where D 1  represents the number average diameter of the toner particles. The closer to 1.00 D 4 /D 1  becomes, the sharper the particle size distribution of the toner particles becomes. The toner particles with a smaller diameter and a sharp distribution like this are preferable to achieve the sharper distribution of the charging quantity of the toner particles, and higher image quality with less toner adhesion to the photoconductor and higher efficiency in transferring the toner particles electrically. 
   Specific examples of devices measuring particle size distribution of toner particles using the Coulter method include Coulter Counter TA-II and Coulter Multisizer II (both are manufactured by Beckman Coulter Inc.). The measuring method is described below. 
   (1) Add 0.1 to 5 ml of a surface active agent (preferably a salt of an alkyl benzene sulfide) as a dispersant to 100 to 150 ml of an electrolytic aqueous solution. The electrolytic aqueous solution is an about 1% NaCl aqueous solution prepared by using primary NaCl (e.g., ISOTON-II, manufactured by Beckman Coulter Inc.). 
   (2) Add 2 to 20 mg of a measuring sample to the electrolytic aqueous solution. 
   (3) Subject the electrolytic aqueous solution in which the measuring sample is suspended to a dispersion treatment for 1 to 3 minutes with a supersonic disperser. 
   (4) Measure the number distribution for each particle diameter channel described below while the aperture is set to 100 μm for the measuring device mentioned above. 
   (5) Calculate the weight average particle diameter (D 4 ) and the number average particle diameter (D 1 ) of the toner from the obtained distribution. The whole range is a particle diameter of from 2.00 to not greater than 40.30 μm and the number of the channels is 13. Each channel is: from 2.00 to not greater than 2.52 μm; from 2.52 to not greater than 3.17 μm; from 3.17 to not greater than 4.00 μm; from 4.00 to not greater than 5.04 μm; from 5.04 to not greater than 6.35 μm; from 6.35 to not greater than 8.00 μm; from 8.00 to not greater than 10.08 μm; from 10.08 to not greater than 12.70 μm; from 12.70 to not greater than 16.00 μm, from 16.00 to not greater than 20.20 μm; from 20.20 to not greater than 25.40 μm; from 25.40 to not greater than 32.00 μm; and from 32.00 to not greater than 40.30 μm. 
   In addition, the toner of the present invention preferably has a form factor SF- 1  of from 100 to 180 and a form factor of SF- 2  of from 100 to 180.  FIGS. 16 and 17  are schematic diagrams for explaining the form factors SF- 1  and SF- 2 . 
   The form factor SF- 1  represents the degree of roundness of a toner particle and is defined by the following relationship (1):
 
 SF -1={( MXLNG ) 2 /(AREA)}×(100π/4)  (1)
 
wherein, MXLNG represents a diameter of the circle circumscribing the image of a toner particle obtained, for example, by observing the toner particle with a microscope, and AREA represents the area of the image.
 
   When a toner has a form factor SF- 1  close to 100, the toner has a form close to a true sphere. When the form factor SF- 1  is too high, the form is irregular. 
   The form factor SF- 2  represents the degree of concavity and convexity of a toner particle and is defined by the following relationship (2):
 
 SF -2={( PERI ) 2 /(AREA)}×(100/4π)  (2)
 
wherein, PERI represents the peripheral length, or perimeter, of the image of a toner particle observed, for example, by a microscope; and AREA represents the area of the image. When the form factor SF- 2  gets close to 100, the toner has a surface with less concavity and convexity. When the form factor SF- 2  is too large, the roughness of the surface is significant.
 
   The form factors SF- 2  are determined by the following method. Photographs of the toner particles are taken using a scanning electron microscope (S-800, manufactured by Hitachi Ltd.). The photographs are analyzed using an image analyzer (LUSEX 3 manufactured by Nireco Corp.) to calculate the form factors. 
   When a toner has a form factor SF- 1  close to 100, that is, the toner has a form close to a true sphere, the contact between the toner particles becomes a point to point contact. Thereby the adhesion force between the toner particles weakens and therefore, the toner has a good fluidity. Good fluidity of toner particles leads less stress and it becomes easier to stabilize the flow of the developer for a longer time. Also, if the toner has a form close to a true sphere, the contact between toner particles and the photoconductor becomes a point to point contact. Thereby the adhesion force between the toner particles and the photoconductor weakens and therefore, the efficiency in transferring the toner particles is improved and higher image quality is achieved. 
   On the other hand, if either of SF- 1  or SF- 2  becomes greater than 180, the fluidity of the developer becomes bad and it becomes difficult to flow the developer smoothly. Also, the efficiency in transferring the toner particles tends to decline. 
   In this embodiment, external additive agents having primary particle diameters from 50 nm to 500 nm and a bulk density greater than 0.3 mg/cm 3  are adhered to the toner particles. 
   Silica agents are often used as the external additive agents to increase the fluidity of the developer, but usually, its primary particle diameter is from 10 nm to 30 nm and its bulk density is from 0.1 mg/cm 3  to 0.2 mg/cm 3 . 
   In the present invention, external additive agents having an appropriate characteristic preferably exist on the surface of the toner particles to form a gap between the toner particles and objects such as photoconductors. As the external additive agents are uniformly contacted with the toner particles, the photoconductor and the charging member have a small contact area. Thus, the adherence of the toner to the photoconductor and charging member can be decreased, and the developing efficiency and the transfer efficiency of the toner can also be improved. Also, external additive agents increase the fluidity of the developer and therefore decrease stress on the developer. Accordingly, the developer can be used for a longer period of time. 
   In addition, the external additive agents plays a role as a roller bearing, so that the photoconductor is not abraded and damaged. Moreover, the external additive particle is hardly embedded into the toner particles even when a high stress is applied to the photoconductor by the cleaning blade. Even if the external additive agents are slightly embedded to the toner particles, the external additive agents can leave from the toner particles and the developer can recover. Therefore, a stable cleanability can be imparted to the toner particles for a long period. Furthermore, the external additive agents moderately leaves from the surface of the toner particles and are adhered to the edge of the cleaning blade, resulting in function of a dam. The dam has an effect on avoiding the phenomenon in that the toner passes through the cleaning blade. 
   The external additive agents mentioned above decrease the shear applied to the toner, and thereby formation of a film of the toner on the photoconductor, etc., which is caused by the low-rheological components included in the toner, in a high-speed fixation (low-energy fixation) is reduced. In addition, external additive agents having an average primary particle diameter of from 50 to 500 nm improve the cleaning property of the resultant toner without decreasing the fluidity of the resultant toner. The reason is not certain, but is considered as follows. When a surface-treated external additive agents are added to the toner particles, the deterioration level of the developer is low even if the external additive agents contaminate the carrier particles. Therefore, the deterioration of the fluidity and charging quantity is sufficiently suppressed for a longer period, the flow of the developer is stabilized and image quality is stabilized. 
   The external additive agents preferably have an average primary particle diameter of from 50 to 500 nm, and preferably from 100 to 400 nm. When the average primary particle diameter is less than 50 nm, the external additive agents tend to be buried in the concavity of the toner surface and deteriorate the role of the roller bearing. In contrast, when the average primary particle diameter is larger than 500 nm, the defective cleaning problem in that the toner passes through the blade occurs. This is because the external additive agents have a particle diameter on the order of that of the toner, and the toner particles passes through the gap formed between the cleaning blade and the photoconductor by the external additive agents. 
   The bulk density of the external additive agents is preferably not less than 0.3 mg/cm 3 . When the bulk density is too small, the fluidity of the toner improves, but the resultant toner and the external additive agents are easily scattered and the adherence thereof to the photoconductor, etc. is increased. Therefore, the dam effect deteriorates, resulting in occurrence of defective cleaning. 
   Specific examples of inorganic particles for use as the external additive agents include SiO 2 , TiO 2 , Al 2 O 3 , MgO, CuO, ZnO, SnO 2 , CeO 2 , Fe 2 O 3 , BaO, CaO, K 2 O, Na 2 O, ZrO 2 , CaO.SiO 2 , K 2 O(TiO 2 )n, Al 2 O 3 .2SiO 2 , CaCO 3 , MgCO 3 , BaSO 4 , MgSO 4 , SrTiO 3 , etc. Among these, SiO 2 , TiO 2  and Al 2 O 3  are preferably used. These inorganic compounds may be treated by a surface treatment agent such as coupling agents, hexamethyldisilazane, dimethyldichlorosilane, and octyltrimethoxysilane. 
   Specific examples of organic particles for use as the external additive agents include thermoplastic resins and thermosetting resins, such as vinyl resins, polyurethane resins, epoxy resins, polyester resins, polyamide resins, polyimide resins, silicone resins, phenol resins, melamine resins, urea resins, aniline resins, ionomer resins, polycarbonate resins, etc. These resins may be used in combination. In order to easily make a water dispersion of fine resin particles, vinyl resins, polyurethane resins, epoxy resins, polyester resins and these combinations are preferably used. 
   Specific examples of the vinyl resins for use as the external additive agents include polymers formed from a polymerization reaction or a copolymerization reaction of vinyl monomer such as styrene-methacrylate copolymers, styrene-butadiene copolymers, methacrylic acid-methacrylate copolymers, styrene-acrylonitrile copolymers, styrene-maleic anhydride copolymers, styrene-methacrylic acid copolymer, etc. 
   The bulk density of the external additive agents is measured as follows: 
   Putting the external additive agents gradually into a measuring cylinder with 100 mL volume without vibration till the amount of external additive agents becomes 100 mL. Then the weight of external additive agents (Wa) is obtained by subtracting the weight of the measuring cylinder without the external additive agents from the weight of the measuring cylinder with 100 mL of the external additive agents. 
   The bulk density of the external additive agents (Be) is obtained by following calculation.
 
 Be (g/cm 3 )= Wa (g/100 mL)/100
 
   In the present invention, the external additive agents are typically added to the toner by a method including; mechanically mixing mother toner particles and an external additive by a known mixing device; or a method including dispersing the mother toner particles and the external additive in a liquid using a surfactant to adhere to, and drying. 
   Next, developer replenishing devices applicable to the present invention will be discussed. 
   The developing device of this embodiment has an opening as a toner introduction part through which new toner particles and carrier particles are sent to the developing device. Also, the developing device of this embodiment has an opening as a toner discharge part which discharges the developer from the developing device. 
   The first example of the developer replenishing device is shown in  FIG. 18A . The developer replenishing device comprises a toner replenishing device which includes a toner container for containing toner particles, a carrier replenishing device which includes a carrier container for containing carrier particles, a toner replenishing controller for controlling the replenishment of the toner particles, a carrier replenishing controller for controlling the replenishment of the carrier particles and a developer conveyance path. The toner container joins with the carrier container at a point in the developer conveyance path, and the toner particles and the carrier particles are conveyed together to the opening in the developing device as the toner introduction part. 
   The amount of replenished toner particles is controlled by the toner replenishing controller and the amount of replenished carrier particles is controlled by the carrier replenishing controller. The toner replenishing controller and the carrier replenishing controller can control the amount of the replenished powder independently to each other. 
   The toner replenishing device and the carrier replenishing device essentially have the same structure. Either of those replenishing devices can rotate and has an opening with a shutter so that the shutter is opened or closed in accordance with the rotating motion of those replenishing devices and the amount of the replenished toner particles or replenished carrier particles is controlled according to the quantity of rotations, θ. 
   A sensor for sensing the toner density is disposed at the downstream of the developer agitating conveyer and the amount of replenished toner particles is controlled by the toner replenishing controller in response to the output of this sensor. The amount of replenished carrier particles is controlled by the carrier replenishing controller according to the deterioration of the carrier which can be estimated according to the driving time of the developing device or the like. 
   The positional restriction to dispose the toner container is relatively little when this type of the developer replenishing device is adapted. It increases freedom to allocate the space inside the image forming apparatus because the toner container and the carrier container are separate from the developing device. And since the toner particles are replenished from the toner container, it is not necessary for the developing device to have large space for containing the toner particles to be replenished. So the developing device can be downsized. 
   The second example of the developer replenishing device is shown in  FIG. 18B . The developer replenishing device includes a developer container for containing new toner particles and new carrier particles together, a developer replenishing controller for controlling the replenishment of the developer and a developer conveyance path along which the developer is conveys to the opening of the developing device. The weight percent of the toner particles in the new developer is approximately 15% by weight. The weight percent of the toner particles is not restricted to 15%. It is decided according to the developing device, the capacity of the developer container, the lifetime of the developer or the like. The developer can be conveyed by a screw pump which is shown in U.S. Pat. No. 6,785,496. 
   A toner density sensor is placed below the developer agitating conveyer and the developer is replenished according to output signals from this sensor. 
   The positional restriction to dispose the toner container is relatively little when this type of the developer replenishing device is adapted. It increases freedom to allocate the space inside the image forming apparatus because the developer container is separate from the developing device. And since the toner particles are replenished from the toner container, it is not necessary for the developing device to have large space for containing the toner particles to be replenished. So the developing device can be downsized. 
   Although the developer replenishing devices shown in  FIGS. 18A and 18B  replenish the carrier particles as well as toner particles, a developer replenishing device which replenishes toner particles only is also applicable to the present invention.