Patent Publication Number: US-7904002-B2

Title: Developing device and image forming apparatus with a flow guide plate

Description:
This Nonprovisional application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2007-210115 filed in Japan on 10 Aug. 2007, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE TECHNOLOGY 
     1. Field of the Technology 
     The technology relates to a developing device for visualizing an electrostatic latent image formed on a photoreceptor etc. with a developer, in particular relating to an agitating mechanism for dual-component developer composed of toner and carrier. 
     2. Description of the Prior Art 
     The developing device using a dual-component developer in an image forming apparatus such as a digital multifunctional machine or the like incorporates a developing roller opposing a photoreceptor, an agitating roller and a toner supply and agitation roller, all being arranged rotatably in the developing hopper. Formed over the toner supply and agitation roller is a toner supply port, over which a toner supply container is attached. 
     An excessive amount of developer that was separated from the developing roller by the layer thickness-regulating member inside the developing hopper passes by a flow-guide plate arranged nearby and is returned to the upside of the agitating roller. This surplus developer is mixed and agitated with the supplied toner and then fed once again to the developing roller. Further, there are many cases that the flow-guide plate is formed with partitioning ribs in order to prevent occurrence of uneven distribution of the developer inside the developing hopper. 
     In the prior art, since partitioning ribs  347  formed on the flow-guide plate do not move as shown in  FIG. 1 , the surplus developer that was separated by an unillustrated layer thickness-regulating member will not move in the longitudinal direction of the developing roller, designated at  341  and is returned sliding over a flow-guide plate  346  to the upside of an unillustrated agitating roller. For this reason, agitation of the developer in the longitudinal direction of developing roller  341  relies on the rotation of the agitating roller alone, hence there occurred many cases where it takes long time for agitation or where a lack of agitation takes place. 
     As a countermeasure to deal with this problem, a patent document 1 (Japanese Patent Application Laid-open 2006-154235) discloses a technique in which the developer returned from the flow-guide plate and the toner supplied from above are made to pass through an AC magnetic field generator so as to improve agitating and mixing performance. 
     In recent high-speed digital multi-functional machines, there are cases that image unevenness occurs when printing of the same print pattern has been continuously performed. That is, in a dual-component developing device, it is assumed that the dual-component developer having a uniform toner concentration is supplied to the developing roller using the agitating roller and the like so as to make the toner concentration uniform along the longitudinal direction of the developing roller. However, in the real situation, the amount of toner consumed from the developing roller is different depending on the print pattern, so that the toner concentration on the developing roller surface after development becomes different from one place to another. In this situation, when the toner concentration of the developer supplied to the developing roller is not uniform in the longitudinal direction, the toner concentration of the developer contributing to development resultantly becomes different across the length of the developing roller. This problem is particularly prone to occur for the developing roller that rotates at a high speed. 
     For example, when print patterns having a solid area in the central part have been continuously printed, the toner concentration of the developer has become lowered in the center across the length of the developing roller, hence causing image unevenness such that the necessary print density cannot be obtained. Accordingly, there occurs a large difference in the toner concentration of the dual-component developer between before and after it is agitated and mixed in the agitating roller area, hence the toner concentration will become uneven across the length of the developing roller even when the developer is supplied to the developing roller. Accordingly, the excessive dual-component developer is also different in toner concentration along the length of the developing roller when viewed microscopically. 
     However, the developing device disclosed in the aforementioned patent document 1 is constructed to focus on crushing toner aggregations, but the toner concentration across the length of the developing roller is not taken into consideration. Further, since this configuration takes such a structure that the developer is dropped through one place into the developing hopper, there has been the problem that it takes long time to make the developer concentration uniform. 
     SUMMARY OF THE TECHNOLOGY 
     The technology has been devised in view of the above problems entailing the conventional developing devices, it is therefore an object to provide a novel and improved developing device and image forming apparatus with which a dual-component developer of uniform toner concentration across the length of the developing roller can be quickly supplied. 
     In order to achieve the above object, the aspect of the technology resides in a developing device for developing an electrostatic latent image formed on an image bearer with a developer that has been triboelectrically-charged by mixing and agitation of two components, toner and magnetic carrier, comprising: a developing hopper for storing the developer; an agitation and conveying member that is rotated inside the developing hopper for conveying the developer while agitating; a developer support which supports the developer that has been mixed and agitated inside the developing hopper and supplies the developer whilst rotating to a developing area located opposing the image bearer; a layer thickness-regulating member for regulating the layer thickness of the developer being conveyed by the developer support; a flow-guide plate member for flowing down the surplus developer that was rejected by the layer thickness-regulating member to a place located away from the layer thickness-regulating member; a plurality of diffusing elements arranged upright on the flow-guide plate member for flowing down the surplus developer in a predetermined direction whilst diffusing the developer with respect to the longitudinal direction of the developer support; and a downflow direction turning mechanism controlled so as to periodically turn the downflow direction of the surplus developer regulated by the diffusing elements. 
     With the above configuration, the downflow direction of the surplus developer that is collected by the flow-guide plate and flows down over it is periodically turned by the diffusing elements, i.e., the partitioning ribs formed on the flow-guide plate, by means of the downflow direction turning mechanism. Accordingly, the surplus developer flowing down can be uniformly dispersed with respect to the longitudinal direction of the developing roller as the developer support and the mixing and agitation performance in the developing hopper can be improved. As a result, image unevenness originating from the developing device can be prevented by making toner concentration uniform across the length of the developing roller. 
     Further, in the above configuration, the downflow direction turning mechanism may be driven and controlled so as to change the cycle of turning the downflow direction which is regulated by the diffusing elements, in accordance with the amount of developer consumed from the developer support. 
     As a usual case, if unbalanced print patterns continue as a print job, the developer concentration becomes locally uneven along the longitudinal direction of the developing roller. With the above configuration, it is possible for the downflow direction turning mechanism to make the toner concentration uniform with a higher precision, by turning the downflow direction of the developer regulated by the partitioning ribs, in accordance with the status of the print patterns, or in accordance with the amount of developer to be consumed in the developing roller, based on the area separation analysis on the print information, for instance. 
     Also, in the above configuration, the downflow direction turning mechanism may be driven and controlled so as to change the cycle of turning the downflow direction which is regulated by the diffusing elements, in accordance with the rotational speed of the developer support. 
     It is usual that the rotational speed of the developing roller is changed with the change of the processing speed of print paper in image forming. Though the mixing and agitation performance of the dual-component developer lowers with the increase of the processing speed, it is possible to make the developer concentration uniform with a higher precision by shortening the cycle in which the downflow direction turning mechanism turns the downflow direction. 
     Further, in the above configuration, the diffusing element may be formed such that the height from the surface of the flow-guide plate member on which the diffusing element is set becomes smaller as it goes in the downflow direction. 
     It is usual that in the rear half portion of the partitioning ribs that is away from the layer thickness-regulating member, the surplus developer has been rather mixed and made substantially even within each passage between the partitioning ribs. Accordingly, such formation of the partitioning ribs that their height becomes lower in the rear half enables the surplus developer in adjacent passages between partitioning ribs to mix up, hence it is possible to make the developer concentration uniform with a higher precision. 
     In the above configuration, the downflow direction turning mechanism may be additionally driven and controlled every time a predetermined period of time has elapsed while the developer support has been continuously driven. 
     Though it is usual that the fluidity and mixing and agitation performance of the dual-component developer become lowered as use of the developer becomes longer, it is possible to make the toner concentration uniform with a higher precision when the downflow direction turning mechanism for turning the downflow direction by the partitioning ribs is additionally driven and controlled in accordance with the continuous drive time of the developing roller. 
     Further, the above configuration may includes a toner concentration detector that is arranged in the proximity to the area where the developer is supplied from the agitating and conveying member to the developer support, to detect the toner concentration of the developer. 
     Since arrangement of the toner concentration sensor immediately before bringing up the developer to the developing roller enables detection of the toner concentration directly before development, it is possible to realize high image quality over a long period of time. 
     Additionally, in order to solve the above problems, another aspect of the technology resides in an image forming apparatus including an image bearer for supporting an electrostatic latent image and any one of the above-described developing devices for visualizing the electrostatic latent image on the image bearer with toner. 
     Since the above configuration makes it possible to improve the mixing and agitation performance in the developing hopper by uniformly dispersing the surplus developer collected over the flow-guide plate, it is possible to prevent image unevenness originating from the developing device by making toner concentration uniform across the length of the developing roller. 
     As has been described, the surplus developer separated by the layer thickness-regulating member can be uniformly distributed across the length of the developing roller by periodically changing the downflow direction of the surplus developer using the movable partitioning ribs on the guide-flow plate. Accordingly, it is possible to improve the agitation and mixture of the surplus developer and supplied toner and hence suppress image unevenness originating from the developing device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top view partially showing a developing device having conventional partitioning ribs; 
         FIG. 2  is a view showing a configuration of an image forming apparatus as the first embodiment; 
         FIG. 3  is an enlarged view schematically showing a configuration around a developing device provided for the image forming apparatus shown in  FIG. 2 ; 
         FIG. 4  is a schematic top view showing a developing device of the same embodiment with its top cover removed; 
         FIG. 5  is a sectional view of a flow-guide plate shown in  FIG. 4 , cut along a plane A 1 -A 2 ; 
         FIG. 6  is a schematic top view showing a variational example of a developing device of the same embodiment with its top cover removed; 
         FIG. 7  shows print patterns for evaluating examples to determine suitable set conditions for a printing operation using the developing device of the same embodiment; and, 
         FIG. 8  is a table showing the set conditions in examples and comparative examples and evaluation on image quality to determine suitable set conditions for a printing operation using the developing device of the same embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the technology will hereinafter be described in detail with reference to the accompanying drawings. In the specification and drawings, the constituents having essentially the same functional configurations will be allotted with the same reference numerals to omit repeated description. 
     To begin with, the overall configuration of an image forming apparatus to which the first embodiment of the developing device is applied will be outlined with reference to  FIG. 2 . 
     An image forming apparatus  100  of the present embodiment is to form on a recording paper a monochrome image represented by the image data that was obtained, for example by scanning a document or that was received from without, and is essentially comprised of a document feeder (ADF)  101 , an image reader  102 , a printing portion  103 , a recording paper conveyor  104  and a paper feeder  105 . 
     In document feeder  101 , when, at least, one document is set on a document set tray  11  and the documents are pulled out from document set tray  11 , sheet by sheet, the document is conducted to and passed over a document reading window  102   a  of paper reader  102  and discharged to a document output tray  12 . 
     A CIS (contact image sensor)  13  is arranged over document reading window  102   a . This CIS  13  repeatedly reads the image on the rear side of the document in the main scan direction while the document is passing over document reading window  102   a , to thereby output the image data that represents the image on the rear side of the document. 
     Further, image reader  102  illuminates the document surface with light from the lamp of a first scan unit  15  when the document passes over document reading window  102   a  and the reflected light from the document surface is lead to an image focusing lens  17  by way of the mirrors of first and second scan units  15  and  16 , so that the image on the document surface is focused by image focusing lens  17  onto the a CCD (charge coupled device)  18 . CCD  18  repeatedly reads the image of the document surface in the main scan direction to thereby output image data that represents the image on the document surface. 
     On the other hand, when the document is placed on the platen glass on the top of image reader  102 , first and second scan units  15  and  16  are moved keeping a predetermined speed relationship relative to each other while the document surface on the platen glass is illuminated by first scan unit  15 , and the light reflected off the document surface is lead to image focusing lens  17  by means of first and second scan units  15  and  16  so that the image on the document surface is focused by image focusing lens  17  onto CCD  18 . 
     The image data output from CIS  13  or CCD  18  is subjected to various kinds of image processes by a control circuit such as a microcomputer etc. and then output to printing portion  103 . 
     Printing portion  103  is to record the document images represented by image data on sheets of paper and includes a photoreceptor drum  21 , a charger  22 , an exposure unit  23 , a developing device  24 , a transfer device  25 , a cleaning unit  26 , a fixing unit  27  and the like. 
     While photoreceptor drum  21  rotates in one direction, its surface is cleaned by cleaning unit  26  and then charged uniformly by charger  22 . Charger  22  may be either a corona discharge type or a roller or brush type that contacts with photoreceptor drum  21 . 
     Exposure unit  23  is a laser scanning unit (LSU) including two laser emitters  28   a  and  28   b  and two mirror groups  29   a  and  29   b . This exposure unit  23  receives image data and emits laser beams from laser emitters  28   a  and  28   b  in accordance with the image data. These laser beams are radiated on photoreceptor drum  21  by way of respective mirror groups  29   a  and  29   b  to thereby illuminate the photoreceptor drum  21  surface that has been uniformly electrified, forming an electrostatic latent image on the photoreceptor drum  21  surface. 
     In order to achieve a high-speed printing operation, this exposure unit  23  employs a two-beam system including two laser emitters  28   a  and  28   b  to thereby reduce the burden entailing the high frequency of irradiation. 
     Here, as the exposure unit  23 , an array of light emitting elements, e.g., an EL writing head or LED writing head may be used instead of the laser scanning unit. 
     Developing device  24  supplies toner to the photoreceptor drum  21  surface to develop the electrostatic latent image into a toner image on the photoreceptor drum  21  surface. Transfer unit  25  transfers the toner image on the photoreceptor drum  21  surface to the recording paper that is conveyed by paper conveyor  104 . Fixing unit  27  heats and presses the recording paper to fix the toner image onto the recording paper. Thereafter, the recording paper is further conveyed by paper conveyor  104  and discharged to a paper output tray  47 . In this while, cleaning unit  26  removes and collects the toner left over on the photoreceptor drum  21  surface after transfer. 
     Here, transfer unit  25  includes a transfer belt  31 , drive roller  32 , driven roller  33 , elastic conductive roller  34  and the like, and rotates transfer belt  31  by supporting and tensioning the belt on the aforementioned rollers  32  to  34  and other rollers. Transfer belt  31  has a predetermined resistivity (e.g., 1×10 9  to 1×10 13  Ω·cm) and conveys the recording paper placed on its surface. Elastic conductive roller  34  is pressed against the photoreceptor drum  21  surface with transfer belt  31  in between, so as to press the recording paper on transfer belt  31  against the photoreceptor drum  21  surface. Applied to this elastic conductive roller  34  is an electric field that has a polarity opposite to the charge of the toner image on the photoreceptor drum  21  surface. This electric field of the opposite polarity causes the toner image on the photoreceptor drum  21  surface to transfer to the recording paper on transfer belt  31 . For example, when the toner image bears negative (−) charge, the polarity of the electric field applied to elastic conductive roller  34  is set to be positive (+). 
     Fixing unit  27  includes a heat roller  35  and pressing roller  36 . A heater is arranged inside heat roller  35  in order to set the heat roller  35  surface at a predetermined temperature (fixing temperature: approximately 160 to 200 deg. C.). A pair of unillustrated pressing members are arranged at both ends of pressing roller  36  so that pressing roller  36  comes into pressing contact with heat roller  35  with a predetermined pressure. As the recording paper reaches the pressing contact portion called as the fixing nip portion between heat roller  35  and pressing roller  36 , the unfixed toner image on the recording paper is fused and pressed while it is being conveyed by the rollers  35  and  36 , so that the toner image is fixed to the recording paper. 
     Paper conveyor  104  includes a plurality of conveying rollers  41  for conveying recording paper, a pair of registration rollers  42 , a conveyance path  43 , an inversion/conveyance path  44 , a plurality of branch claws  45 , a pair of paper discharge rollers  46  and the like. 
     Conveyance path  43  receives recording paper delivered from paper feeder  105  and conveys the recording paper until its leading end reaches registration rollers  42 . Since registration rollers  42  are temporarily stopping at that timing, the leading end of the recording paper reaches and abuts registration rollers  42  so that the recording paper bends. The resiliency of this bent recording paper makes the front edge of the recording paper substantially parallel to registration rollers  42 . Thereafter, registration rollers  42  start rotating so as to convey the recording paper to transfer unit  25  of printing portion  103  and then further conveyed by paper discharge rollers  46  to paper output tray  47 . 
     Stoppage and rotation of registration rollers  42  can be controlled by switching on or off the clutch between registration roller  42  and its drive shaft or by turning on or off the motor as the drive source of registration rollers  42 . 
     When another image is recorded on the rear side of the recording paper, a plurality of branch claws  45  are turned to switch the paper path from conveyance path  43  to inversion/conveyance path  44  so that the recording paper is turned upside down and returned through inversion/conveyance path  44  to registration roller  42  in conveyance path  43 . Thus, another image is recorded on the rear side of the recording paper. 
     Arranged at the necessary positions along conveyance path  43  and inversion/conveyance path  44  are several sensors for detecting the recording paper position etc., and based on the position of the recording paper detected at each sensor, the drives of the conveying rollers and registration rollers are controlled so as to convey and position the recording paper. 
     Paper feeder  105  includes a plurality of paper feed trays  51 . Each paper feed tray  51  is a tray for holding a stack of recording sheets and is arranged under image forming apparatus  100 . Also, each paper feed tray  51  includes a pickup roller or the like for pulling out recording paper sheet by sheet so as to deliver the picked up sheet to conveyance path  43  of recording paper conveyor  104 . 
     Since image forming apparatus  100  of the present embodiment is aimed at high speed printing processing, each paper feed tray  51  has a volume capable of stacking 500 to 1500 sheets of recording paper of a regular size. 
     Arranged on the flank of image forming apparatus  100  are a large capacity paper cassette (LCC)  52  for accommodating large amounts of a plurality of types of recording paper and a manual feed tray  53  for essentially supplying recording paper of irregular sizes. 
     Paper output tray  47  is arranged on the side opposite from manual feed tray  53 . It is also possible to optionally provide an output paper finisher (for stapling, punching, etc.) or a multi-bin paper output tray, in place of the paper output tray  47 . 
     Referring next to  FIG. 3 , a developing device will be outlined. Developing device  24  of this embodiment has the function of developing the electrostatic latent image that has been formed on the surface of the image bearer, i.e., photoreceptor  21  by exposure unit  23  ( FIG. 2 ), to form a visual image with toner. As shown in  FIG. 3 , developing device  24  includes a toner supply portion  40 , a developing hopper  240 , a developing roller  241 , a layer thickness-regulating member  242 , agitating rollers  243  ( 243   a ,  243   b ), toner agitation rollers  244  ( 244   a ,  244   b ), a toner concentration sensor  245 , a flow-guide plate  246  and partitioning ribs  247 . 
     Developing hopper  240  is a container formed of, for example, a hard synthetic resin, rotatably supporting developing roller  241 , agitating rollers  243   a  and  243   b  and toner agitation roller  244   a  and  244   b  and holding toner supplied from toner supply portion  40 . In the present embodiment, toner concentration sensor  245  is arranged at a position near agitating roller  243   a  that is located close to developing roller  241  in developing hopper  240  in order to detect the toner concentration of the developer immediately before development that will directly contribute to the development. 
     In order to obtain the exact toner concentration of the dual-component developer that actually contributes to development by detecting the toner concentration immediately before supply to developing roller  241 , toner concentration sensor  245  is disposed in proximity to agitating roller  243   a . As an example of toner concentration sensor  245 , a high-precision magnetic permeability sensor, e.g., TS-L, TS-A and TS-K (trade names of products of TDK Corporation) may be used. The measurement of the toner concentration by toner concentration sensor  245  is output to an unillustrated controller provided for image forming apparatus  100  ( FIG. 2 ). Since the above arrangement of toner concentration sensor  245  at a position immediately before bringing up the developer to developing roller  241  enables detection of the toner concentration immediately before development, it is possible to keep the toner images with high image quality over a long period of time. 
     Developing roller  241  is arranged opposing photoreceptor  21  and serving as a developer support for supporting the developer that was mixed and agitated inside developing hopper  240  and supplies toner to the developing area where the electrostatic latent image on photoreceptor  21  surface resides as it is rotating in the direction of the arrow shown in FIG.  3  (in the counterclockwise direction in  FIG. 3 ). 
     Agitating rollers  243   a  and  243   b  are the agitating and conveying members which convey the developer toward developing roller  241  while agitating the dual-component developer of electrostatic toner and magnetic carrier by their rotational drive in developing hopper  240  and which have the developer in developing hopper  240  carried on developing roller  241 . The agitating rollers also mix up the developer with the surplus developer that flows down from flow-guide plate  246  and convey the mixture toward developing roller  241 . 
     Toner agitation rollers  244   a  and  244   b  are the rollers that agitate mainly the toner that has fallen from toner supply rollers  401   a  and  401   b  of toner supply portion  40  located over and above developing hopper  240  and convey the toner in developing roller  240 . 
     Layer thickness-regulating member  242  regulates the amount of the developer carried on developing roller  241  to a predetermined thickness while forming spikes of developer from the surplus developer that was rejected by the layer thickness-regulating member  242  and moves the spikes toward flow-guide plate  246  which is located on the right side in  FIG. 3 . 
     Flow-guide plate  246  is a flow-guide plate member that flows down the surplus developer that was rejected by layer thickness-regulating member  242  toward and between agitating roller  243   b  and toner agitation roller  244   a , which are located away from layer thickness-regulating member  242 . The surplus developer having transferred over flow-guide plate  246  slides down over the slope of flow-guide plate  246  and flows down toward and between agitating roller  243   b  and toner agitation roller  244   a.    
     Partitioning rib  247  is a diffusing element that flows the surplus developer downward in the predetermined direction whilst diffusing the developer with respect to the longitudinal direction of developing roller  241  in order to prevent the surplus developer from going too far to one side and make the carried amount of developer uniform across the length of developing roller  241 . There are a plurality of partitioning ribs  247  arranged upright on flow-guide plate  246 . In the present embodiment, the drive of partitioning ribs  247  is controlled so that the direction in which the surplus developer flows down is made to change periodically. The configuration of the downflow direction turning mechanism and the switching drive control for periodically turning the downflow direction of the surplus developer by partitioning ribs  247  will be described later. 
     Next, the configuration and operation of the downflow direction turning mechanism provided for the developing device in the present embodiment will be specifically described.  FIG. 4  is a schematic top view showing the developing device of the present embodiment with its top cover removed, and  FIG. 5  is a sectional view of  FIG. 4 , cut along a plane A 1 -A 2 . 
     As described above, in this embodiment, a plurality of (six in the example shown in  FIG. 4 ) partitioning ribs  247  are provided approximately parallel to each other. These partitioning ribs  247  are driven and controlled by a downflow direction turning mechanism  248  so that the downflow direction of the surplus developer will change periodically. 
     Each partitioning rib  247  is pivotally supported at both ends  247   a   1 ,  247   a   2  on their proximal sides by a pair of supporting rod members  251  and  252  which are arranged on the side (underside) of flow-guide plate  246  opposite from the side where the ribs are projected upright. These supporting rod members  251  and  252  are pivotally supported at both ends thereof, designated at  251   a   1  and  251   a   2 , and at  252   a   1  and  252   a   2 , respectively, by a pair of connection rod members  253  and  254 , forming a set of linkage mechanism  249  made up of a pair of supporting rod members  251  and  252  and a pair of connecting rod members  253  and  254 . 
     Connecting rod members  253  and  254  are rotationally supported at their approximately centers by fulcrums  255  and  256 , respectively, so as to reciprocate supporting rod members  251  and  252  in the longitudinal direction of developing roller  241  (the X-direction shown in  FIG. 4 ) pivoting on the two fulcrums  255  and  256 . 
     Further, in the present embodiment, a reciprocation drive mechanism  257  for reciprocatingly moving supporting rod member  251  in the longitudinal direction (the X-direction shown in  FIG. 4 ) of developing roller  241  is disposed outside developing hopper  240 . This reciprocation drive mechanism  257  is comprised of a power transmitter  258  such as a plunger etc. for reciprocatingly moving supporting rod member  251  in the X-direction and a gear  260  having a tapered element  259  that abuts the power transmitter  258  and an unillustrated drive motor that rotates gear  260  in the direction of A 1  shown in  FIG. 4 . That is, supporting rod members  251  and  252  of linkage mechanism  249  are reciprocated in the X-direction, pivoting on two fulcrums  255  and  256 , by rotational drive of the gear  260 . With the thus constructed reciprocation drive mechanism  257 , six partitioning ribs  247  are pivotally supported by supporting rod member  251  that is supported at both ends  251   a   1  and  251   a   2  and supporting rod member  252  that is supported at both ends  252   a   1  and  252   a   2 , so that both ends  247   a   1  and  247   a   2  of each partitioning rib  247  can reciprocate in the X-direction. Thus, it is possible to turn the regulating direction of partitioning ribs  247  for regulating the downflow direction of surplus developer flowing down over guide-plate  246 . 
     In the above way, in the present embodiment, downflow direction turning mechanism  248  is constructed so as to cause supporting rod members  251  and  252  of linkage mechanism  249  to reciprocate in the X-direction, pivoting on two fulcrums  255  and  256 , hence it is possible to change the direction that is regulated by partitioning ribs  247  which are rotatably supported at their ends on supporting rod members  251  and  252 . 
     Further, flow-guide plate  246  is formed with cutouts  246   h  having a shape like the section of a sandglass as shown in  FIG. 4 , in the areas corresponding to the reciprocating motion of partitioning ribs  247 , so that partitioning ribs  247  will not be hindered in their reciprocating motion. 
     Also, as shown in  FIG. 5 , a surplus developer receiver  270  for preventing the surplus developer flowing over flow-guide plate  246 , from falling is provided under cutouts  246   h  that are formed in flow-guide plate  246 . 
     Partitioning rib  247  is formed so that its height H 1  is greater (6 mm as an example) on the side closer to layer thickness-regulating member  242  (on the left side in  FIG. 5 ) and its height H 2  smaller (3 mm as an example) on the side more distant from layer thickness-regulating member  242  (on the right side in  FIG. 5 ). That is, since a certain amount of surplus developer is returned from developing roller  241  in the area close to the layer thickness-regulating member (on the left side in  FIG. 5 ), each partitioning rib  247  of the present embodiment needs to be equal to or higher than a certain height so as not to allow the developer to converge to a particular partitioning rib  247  and so as to make the amount of the developer in each passage between partitioning ribs  247  uniform. On the other hand, in the rear half of the flowing movement of the surplus developer over flow-guide plate  246 , the height of the collection of developer has been made substantially even by the reciprocating movement of partitioning ribs  247 , so that each partitioning rib  247  is formed to be low so as to allow the developer in each passage between partitioning ribs  247  to mix with that in adjacent one. The formation of partitioning ribs  247  so that their height from the flow-guide plate  246  surface becomes lower in the above way as it goes downstream, makes it possible to positively distribute the surplus developer on the entrance side of flow-guide plate  246  and make the surplus developer in adjacent passages between partitioning ribs  247  mix up by lowering partitioning ribs  247  in the rear half thereof. As a result, it is possible to make the developer concentration uniform with high precision. 
     As another example of the reciprocation drive mechanism for driving linkage mechanism  249 , a reciprocation drive mechanism  262  using a solenoid  261  as shown in  FIG. 6  may be used. Illustratively, a plunger  263  of solenoid  261  expands or contacts to cause a supporting rod member  252 ′ of linkage mechanism  249  to reciprocate in the X-direction by means of a drive transmitter  264  with which plunger  263  is engaged, whereby the ends  247   a   1  and  247   a   2  of each partitioning rib  247  reciprocate with respect to fulcrums  255  and  256 . In other words, supporting rod member  252 ′ of linkage mechanism  249  moves reciprocatingly in the X-direction so as to change the regulating direction of partitioning ribs  247  and regulate the surplus developer that flows down over flow-guide plate  246 . 
     Next, examples and comparative examples for determining suitable set conditions for an image forming (printing) operation with the developing device of the present embodiment will be described.  FIG. 7  shows print patterns for evaluating the examples to determine suitable set conditions for the printing operation with the developing device of the present embodiment.  FIG. 8  is a table showing the set conditions in the examples and comparative examples and evaluation on the image quality to determine suitable set conditions for the printing operation of the developing device of the same embodiment. 
     Examples 1 to 3 and Comparative Examples 1 to 2 
     In examples 1 to 3, A4-sized originals having 5%, 10% and 30% solid strip printing patterns as shown in  FIG. 7  were used to perform continuous printing tests of 500 sheets using the above-described developing device  24  of the first embodiment with a dual-component developer containing 6% toner. After continuous printing of 500 sheets, the developer was sampled from the upper part of the conveyor roller so that the toner concentration was measured. In measuring the toner concentration, a solvent method was used and the developer was sampled from three points, namely, front side (F), center (C) and rear side (R) in the developing hopper. The test was done with developing roller  241  driven at a rotational speed of 864 mm/sec. Evaluation on the image quality after 500 printouts was done based on the variation of the printed density at the central part using a reflective densitometer (RD918: a product of MACBETH) for evaluation. As comparative examples 1 and 2, similar evaluation tests were carried out with the partitioning ribs inactive. 
     Example 4 and Comparative Example 3 
     In example 4, a continuous printing test of 500 sheets was performed using the A4-sized original having a 10% solid strip printing pattern in the same manner as in example 1 except that the rotational speed of developing roller  241  was set at 432 mm/sec. After continuous printing of 500 sheets, the developer was sampled from the upper part of the conveyor roller so that the toner concentration was measured in the same manner. Evaluation on the image quality after 500 printouts was done similarly based on the variation of the printed density at the central part. Further, as comparative example 3, a similar evaluation test was carried out with the partitioning ribs inactive. 
     Examples 5 to 7 
     In examples 5 to 7, continuous printing tests were performed using the A4-sized original having a 10% solid strip printing pattern in the same manner except that the rotational speed of the developing roller was set at 864 mm/sec, the developing roller was continuously driven for 30 min. or for 60 min, and ten reciprocating movements of the partitioning ribs were added every 15 minutes. After continuous printing, the developer was sampled from the upper part of the conveyor roller so that the toner concentration was measured in the same manner. Evaluation on the image quality at the end of printing was done based on the variation of the printed density at the central part. In example 7, no additional reciprocating motion of the partitioning ribs was done. 
     The results of the above examples and comparative examples were summarized in Table 1 shown in  FIG. 8 . As to evaluation of toner concentration, as long as the difference before and after printing fell within 0.1 wt % in absolute value or the difference in image density fell within 0.1, the result was regarded as a practically permissible level or OK level. 
     As in Table 1 in  FIG. 8 , it was found from the comparison between examples 1 to 3 and comparative examples 1 and 2 that the difference in toner concentration between before and after printing can be suppressed and sharp printed images can be obtained by performing reciprocating movement of the partitioning ribs provided on the flow-guide plate and also by increasing the number of reciprocations of the partitioning ribs with increase of printing pattern area (coverage ratio). That is, if, as a usual case, unbalanced print patterns continue as a print job, the developer concentration may become locally uneven across the length of the developing roller. However, it was found that even in such a case, if an unillustrated controller of the image forming apparatus is adapted to control or change the cycle in which the downflow direction turning mechanism turns the downflow direction regulated by the partitioning ribs, in accordance with the status of the print patterns, or in accordance with the amount of developer to be consumed in the developing roller, based on the area separation analysis on the print information, it is possible to make the toner concentration uniform with a higher precision. 
     Further, it was found from the comparison between examples 2, 4 and comparative example 3 that the difference in toner concentration between before and after printing can be suppressed and sharp printed images can be obtained by performing reciprocating movement of the partitioning ribs and also by increasing the number of reciprocations per minute with increase of the rotational speed of the developing roller. That is, it is usual that the rotational speed of the developing roller is changed with the change of the processing speed of print paper in image forming. However, as the processing speed is increased, the mixing and agitation performance of the dual-component developer lowers. To deal with this situation, it was found that if the controller of the image forming apparatus is adapted to control or change the cycle in which the downflow direction turning mechanism turns the downflow direction to be shorter, it is possible to make the toner concentration uniform with a higher precision. 
     Further, it was found from examples 5 and 6 that the difference in toner concentration between before and after printing can be suppressed and sharp printed images can be obtained by performing additional reciprocating movement of the partitioning ribs if the continuous drive time of the developing roller becomes long. That is, it is usual that the fluidity and the mixing and agitation performance of the dual-component developer become lowered as use of the developer becomes longer. To deal with this situation, it was found that if the controller of the image forming apparatus is adapted to additionally drive the downflow direction turning mechanism for turning the downflow direction by the partitioning ribs, in accordance with the continuous drive time of the developing roller, it is possible to make the toner concentration uniform with a higher precision. 
     From the above result of the examples and comparative examples, it was found that the surplus developer that flows down can be uniformly diffused with respect to the longitudinal direction of the developer support, i.e., the developing roller and the mixing and agitation performance in the developing hopper can be improved since the developing device is driven and controlled such that the surplus developer collected by the flow-guide plate flows down in a varying direction that is regulated and changed periodically by the partitioning ribs of the downflow direction turning mechanism. Accordingly, it was found that image unevenness originating from the developer can be prevented since toner concentration can be made uniform across the length of the developing roller. 
     Having described the preferred embodiment with reference to the attached drawings, it goes without saying that the technology should not be limited to the above-described examples, and it is obvious that various changes and modifications will occur to those skilled in the art within the scope of the appended claims. Such variations are therefore understood to be within the technical scope of the technology. 
     For example, in the above embodiment, the developing device is applied to a monochrome image forming apparatus having one toner cartridge mounted therein, but the developing device of the technology can also be applied to a color image forming apparatus.