Patent Publication Number: US-10331066-B2

Title: Image forming device

Description:
The present application claims priority from Japanese Patent Application No. 2017-096406 filed on May 15, 2017, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND 
     The present disclosure relates to an image forming device including a developing device. 
     A known conventional image forming device which forms an image on a sheet includes an image carrier, a developing device, and a developer container. When a developer is supplied from the developing device to the image carrier, an electrostatic latent image formed on the image carrier appears as a developer image. The developer container includes a developer discharge port and supplies a replenishment developer to a developer replenishing port provided in the developing device. 
     A volume supply type developing device is also known. The developing device includes a developing housing, a developing roller, a developer transport member, and a layer thickness regulating member. The developer transport member is rotated in the developing housing to transport and stir a developer. Arrangement of the layer thickness regulating member so as to be opposed to the developing roller regulates an amount of a developer to be carried on the developing roller. 
     Additionally, in the developing housing, the above developer replenishing port is opened above the developer transport member. In the developer transport member, a transport capacity suppressing portion which partly suppresses a transport capacity is formed downstream of the developer replenishing port. As a result, a developer retention part is formed around the developer replenishing port. When the amount of the developer in the developing housing is increased, the retention part seals the developer replenishing port, so that an inflow of a replenishment developer from the developer container to the developing housing is regulated. On the other hand, when the amount of the developer in the developing housing is reduced, a gap is formed between the retention part and the developer replenishing port, so that the replenishment developer flows into the developing housing from the developer container. 
     SUMMARY 
     An image forming device according to one aspect of the present disclosure includes a developer container; a developing device; a photosensitive drum; a transfer portion; a residual amount sensing portion; a driving portion; and a drive control portion. The developer container houses a developer therein and has a developer discharge port capable of discharging the developer. The developing device has a developing roller configured to be rotatable and carrying a developer on a circumference surface of the developing roller, and receives the developer from the developer container. The photosensitive drum has a surface on which an electrostatic latent image is formed, and carries a developer image made to appear from the electrostatic latent image by the developer on the developing roller. The transfer portion transfers the developer image on the photosensitive drum to a sheet. The residual amount sensing portion senses that a residual amount of a developer in the developer container falls below a predetermined threshold value. The driving portion generates a driving force to cause the developing roller to rotate around a predetermined axis in a first rotation direction and a second rotation direction opposite to the first rotation direction. The drive control portion controls the driving portion to rotatably drive the developing roller. The developing device includes a developing housing; a developer transport path; a developer replenishing port; a first transport member; a second transport member; a transport capacity suppressing portion; and a layer thickness regulating member. The developing housing rotatably supports the developing roller. The developer transport path is formed in the developing housing and includes a first transport path which is arranged at an interval from the developing roller and in which the developer is transported in a first direction along an axis direction of the developing roller, and a second transport path which is arranged between the developing roller and the first transport path and in which the developer is transported in a second direction opposite to the first direction. The developer is circularly transported between the first transport path and the second transport path. The developer replenishing port is opened, in the housing, below the developer discharge port and above the first transport path, and receives the developer from the developer container onto the developer transport path. The first transport member is rotatably arranged on the first transport path, and is configured to transport the developer in the first direction. The second transport member is rotatably arranged on the second transport path, and is configured to transport the developer in the second direction, and to supply the developer to the developing roller. The transport capacity suppressing portion is provided downstream of the developer replenishing port in the first direction to partly suppress a transport capacity of the developer in the first direction of the first transport member, thereby forming a developer retention part at a position opposed to the developer replenishing port. The layer thickness regulating member is arranged to be opposed to the developing roller to regulate a layer thickness of the developer supplied from the second transport member to the developing roller. During developing operation in which the electrostatic latent image is made to appear by the developer on the developing roller, the drive control portion causes the developing roller to rotate in the first rotation direction and when the residual amount sensing portion senses that a residual amount of the developer in the developer container falls below the threshold value, causes the developing roller to rotate in the second rotation direction at predetermined timing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic sectional view showing an internal structure of an image forming device according to one embodiment of the present disclosure; 
         FIG. 2  is a schematic plan view showing an internal structure of a developing device according to one embodiment of the present disclosure; 
         FIG. 3  is a sectional view showing the internal structure of the developing device according to one embodiment of the present disclosure; 
         FIG. 4  is a schematic sectional view showing how the developing device according to one embodiment of the present disclosure is replenished with a developer; and 
         FIG. 5  is a graph showing a relationship between an amount of a residual developer in a developer container and an amount of a developer in the developing device. 
     
    
    
     DETAILED DESCRIPTION 
     In the following, an embodiment of the present disclosure will be detailed with reference to the accompanying drawings.  FIG. 1  is a sectional side view showing an internal structure of an image forming device  1  according to one embodiment of the present disclosure. Although a monochrome printer is illustrated herein as the image forming device  1 , the image forming device may be a copying machine, a facsimile machine, or a multifunctional machine equipped with these functions, or an image forming device which forms a color image. 
     The image forming device  1  includes a main body housing  10  having a generally rectangular solid casing structure, a sheet feeding portion  16 , an image forming portion  30 , a fixing portion  40 , and a toner container  50  (a developer container) all housed in the main body housing  10 . 
     The main body housing  10  is provided with a front cover  11  on a front side thereof and a back cover  12  on a back side thereof. The toner container  50  is exposed to the front side by opening the front cover  11 . This allows a user to take out the toner container  50  from the front side of the main body housing  10  when a toner runs out. The back cover  12  is a cover opened at the time of sheet jam or maintenance. Each unit of the image forming portion  30  and the fixing portion  40  is allowed to be taken out from the back side of the main body housing  10  when the back cover  12  is opened. Also on an upper face of the main body housing  10 , a sheet ejection portion  13  is provided to which a sheet on which an image has been formed is ejected. In an inner space S of the main body housing  10 , various devices for executing image formation are installed. 
     The sheet feeding portion  16  includes a sheet feeding cassette  17  which houses a sheet to be subjected to image formation processing. The sheet feeding cassette  17  is provided with a sheet housing space in which a bundle of the sheets is housed, a lift plate which lifts up the bundle of sheets for sheet feeding, and the like. Above a back end side of the sheet feeding cassette  17 , a sheet feeding roller  18  is arranged for dispensing an uppermost sheet of a bundle of sheets in the sheet feeding cassette  17  one by one. 
     The image forming portion  30  conducts the image formation processing of forming a toner image on a sheet to be sent out from the sheet feeding portion  16 . The image forming portion  30  includes a photosensitive drum  31  (image carrier), a charging device  32 , an exposure device (not shown in  FIG. 1 ), a developing device  20 , a transfer roller  34  (transfer portion), and a cleaning device  35 , which are arranged around the photosensitive drum  31 . 
     The photosensitive drum  31  includes a rotation shaft (not shown), and a tubular surface (circumference surface) rotating around the rotation shaft. On the tubular surface, an electrostatic latent image is formed, and a toner image (developer image) caused to appear from the electrostatic latent image by a toner (developer) on a developing roller  21  is carried by the tubular surface. As the photosensitive drum  31 , a photosensitive drum using an amorphous silicon (a-Si) based material can be used. 
     The charging device  32  uniformly charges a surface of the photosensitive drum  31  and includes a charging roller which comes into contact with the photosensitive drum  31 . 
     The cleaning device  35  has a cleaning blade (not shown) to clean a toner attached to the tubular surface of the photosensitive drum  31  after toner image transfer, as well as transporting the toner to a collection device (not shown). 
     The exposure device, which has a laser light source and an optical apparatus such as a mirror, a lens or the like, radiates, to a circumference surface of the photosensitive drum  31 , light modulated on the basis of image data applied from an external device such as a personal computer, to form an electrostatic latent image. 
     The developing device  20  supplies a toner to the circumference surface of the photosensitive drum  31  for developing the electrostatic latent image on the photosensitive drum  31  to form a toner image. The developing device  20  includes the developing roller  21  which carries a toner on a circumference surface thereof to supply a toner to the photosensitive drum  31 , a first stirring screw  23  (a first transport member) which circularly transports a developer while stirring the same, and a second stirring screw  24  (a second transport member). Also, the developing device  20  receives a replenishment toner from the toner container  50 . The developing device  20  will be detailed later. 
     The transfer roller  34  is a roller for transferring a toner image formed on the tubular surface of the photosensitive drum  31  to a sheet. The transfer roller  34  is in contact with the tubular surface of the photosensitive drum  31  to form a transfer nip portion. The transfer roller  34  is given a transfer bias of a polarity opposite to that of a toner. 
     The fixing portion  40  executes fixing processing of fixing a transferred toner image on a sheet. The fixing portion  40  includes a fixing roller  41  having a heating source therein, and a pressuring roller  42  which is brought into contact with the fixing roller  41  by pressure to form a fixing nip portion with the fixing roller  41 . When a sheet to which a toner image is transferred is passed through the fixing nip portion, the toner image is fixed on the sheet by heating by the fixing roller  41  and pressing by the pressuring roller  42 . 
     The toner container  50  houses therein a toner (developer) to be replenished to the developing device  20 . The toner container  50  includes a container main body  51  as a main part for storing a toner, a cylindrical portion  52  provided to project from a lower part on one side surface of the container main body  51 , a rotation member  54  which transports a toner housed in the container, and a toner sensor  50 S (a residual amount sensing portion). As a result of rotatable driving of the rotation member  54 , the toner stored in the toner container  50  is supplied into the developing device  20  from a toner discharge port  521  (a developer discharge port) capable of discharging a toner and provided below a front end of the cylindrical portion  52 . The rotation member  54  is rotatably driven in a predetermined cycle. As a result, the toner in the container main body  51  is continuously sent to the front end side of the cylindrical portion  52 . 
     The toner sensor  50 S senses a residual amount of a toner in the container main body  51  (a residual amount of a toner falling below a predetermined threshold value). In detail, the toner sensor  50 S is formed with a piezoelectric element or a magnetic permeability sensor (magnetic sensor) to output a signal of +5 V (with toner) or a signal of 0 V (without toner) according to presence/absence of a toner in a region opposed to the toner sensor  50 S. An output of the toner sensor  50 S is referred to by a control portion  90  to be described later to sense that a residual amount of a toner in the toner container  50  is reduced. 
     In the main body housing  10 , there are provided a main transport path SP and a reverse transport path DP for transporting a sheet. The main transport path SP extends from the sheet feeding portion  16  via the image forming portion  30  and the fixing portion  40  to a sheet ejection port  14  provided to be opposed to the sheet ejection portion  13  on the upper face of the main body housing  10 . The reverse transport path DP is a transport path for returning a single-side printed sheet to an upstream side, on the main transport path SP, of the image forming portion  30  when double-side printing is conducted with respect to the sheet. 
     The main transport path SP is provided to extend so as to upwardly pass the transfer nip portion formed with the photosensitive drum  31  and the transfer roller  34  from below. In the main transport path SP, on the side upstream of the transfer nip portion, a pair of resist rollers  19  are arranged. A sheet is once stopped by the pair of resist rollers  19  and after being subjected to skew correction, is sent to the transfer nip portion at predetermined timing for image transfer. At appropriate positions on the main transport path SP and the reverse transport path DP, a plurality of transport rollers for transporting a sheet are arranged, and for example, in proximity to the sheet ejection port  14 , a pair of sheet ejection rollers  15  are arranged. The reverse transport path DP is formed at an inner side of the back cover  12  of the main body housing  10 . 
     &lt;As to Developing Device&gt; 
       FIG. 2  is a plan view showing an internal structure of the developing device  20 .  FIG. 3  is a sectional view of the developing device  20 , which is a sectional view orthogonal to an axis direction of the developing roller  21 . The developing device  20  includes a developing housing  210  having a box-shape long in one direction (the axis direction of the developing roller  21 , a right and left direction). The developing housing  210  has a storage space  220 . The developing housing  210  also has a toner replenishing port  25  opened. The developing device  20  further includes the developing roller  21 , the first stirring screw  23  and the second stirring screw  24 , and a regulation blade  26  (a layer thickness regulating member) which are arranged in the storage space  220 . In the present embodiment, a magnetic one-component developing system is applied, in which the storage space  220  is filled with a one-component magnetic toner as a developer. 
     The developing roller  21  with a tubular shape is provided to extend in a longitudinal direction of the developing housing  210 . The developing roller  21  includes a fixed magnet  21 A and a sleeve  21 B ( FIG. 3 ). The fixed magnet  21 A is a tubular magnet fixed to the developing housing  210 . The sleeve  21 B is supported in the developing housing  210  so as to be rotatable around the fixed magnet  21 A. The fixed magnet  21 A includes four magnetic poles along a rotation direction (circumferential direction) of the sleeve  21 B. Table 1 shows an example showing magnetic forces (a magnetic flux density at a peak position) and positions of the four magnetic poles of the fixed magnet  21 A. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Magnetic 
                 Angle (counterclockwise) from 
               
               
                 Magnetic pole 
                 force 
                 position opposed to regulation blade 
               
               
                   
               
             
            
               
                 S1 (Regulation pole) 
                 80 mT 
                  4° 
               
               
                 N1 (Drawing-up pole) 
                 40 mT 
                  96° 
               
               
                 S2 (Transport pole) 
                 84 mT 
                 180° 
               
               
                 N2 (Main pole) 
                 88 mT 
                 255° 
               
               
                   
               
            
           
         
       
     
     An S 1  pole is arranged to be opposed to the regulation blade  26  and functions as a regulation pole which regulates an amount of a toner (an amount of a developer) to be carried on the developing roller  21  (the sleeve  21 B). An N 1  pole is arranged to be opposed to the second stirring screw  24  and functions as a drawing-up pole which receives a toner drawn up by the second stirring screw  24 . An S 2  pole functions as a transport pole which transports a toner for collecting a toner into the developing housing  210 , the toner having passed through a developing position at which the developing roller  21  and the photosensitive drum  31  are opposed to each other. An N 2  pole is arranged to be opposed to the photosensitive drum  31  and functions as a main pole which forms a magnetic brush at the developing position for supplying a toner to the photosensitive drum  31 . 
     The storage space  220  of the developing housing  210  is covered by a top (not shown) and is also sectioned, into a first transport path  221  and a second transport path  222  which are long in the right and left direction, by a partition plate  22  extending in the right and left direction. The first transport path  221  is a transport path which is arranged at an interval from the developing roller  21  and in which a toner is transported in a first direction (D 1 ) along the axis direction of the developing roller  21 . On the other hand, the second transport path  222  is a transport path which is arranged between the developing roller  21  and the first transport path  221  and in which a toner is transported in a second direction (D 2 ) opposite to the first direction. The partition plate  22  is shorter than a width of the developing housing  210  in the right and left direction and has a first communication path  223  and a second communication path  224  provided at left end and right end of the partition plate  22  for respectively making the first transport path  221  and the second transport path  222  communicate. In this manner, a circular path is formed in the storage space  220  from the first transport path  221  through the second communication path  224  and the second transport path  222  to reach the first communication path  223 . The toner is transported counterclockwise in the circular path in  FIG. 2 . 
     The toner replenishing port  25  (a developer replenishing port) is an opening portion which is opened in the top of the developing housing  210 , and arranged below the toner discharge port  521  of the toner container  50  ( FIG. 2 ) and above a left end side of the first transport path  221 . The toner replenishing port  25  is arranged to be opposed to the above circular path and has a function of receiving a replenishment toner (replenishment developer) replenished from the toner discharge port  521  into the storage space  220 . 
     The first stirring screw  23  is rotatably disposed in the first transport path  221 . The first stirring screw  23  includes a first rotation shaft  23   a , and a first spiral vane  23   b  provided to project in a spiral manner on a circumference of the first rotation shaft  23   a . The first stirring screw  23  transports a toner in a direction of an arrow D 1  in  FIG. 2  as a result of being rotatably driven around the first rotation shaft  23   a  (an arrow R 2 ). On the downstream side in the toner transport direction (D 1  direction) of the first stirring screw  23 , a first paddle  23   c  is disposed. The first paddle  23   c  transmits a toner from the first transport path  221  to the second transport path  222  toward a direction of an arrow D 4  in  FIG. 2 . The developing roller  21  is located obliquely above the second stirring screw  24 . The photosensitive drum  31 , the developing roller  21 , and the second stirring screw  24  are arranged on a generally straight line as shown in  FIG. 3 . 
     The second stirring screw  24  is rotatably disposed in the second transport path  222  so as to be opposed to the first stirring screw  23  in a horizontal direction. The second stirring screw  24  includes a second rotation shaft  24   a , and a second spiral vane  24   b  provided to project in a spiral manner on a circumference of the second rotation shaft  24   a . The second stirring screw  24  supplies a toner to the developing roller  21  while transporting a toner in a direction of an arrow D 2  in  FIG. 2  as a result of being rotatably driven around the second rotation shaft  24   a  (an arrow R 1 ). On the downstream side in the toner transport direction (D 2  direction) of the second stirring screw  24 , a second paddle  24   c  is disposed. The second paddle  24   c  transmits a toner from the second transport path  222  to the first transport path  221  toward a direction of an arrow D 3  in  FIG. 2 . 
     The regulation blade  26  is supported in the developing housing  210  so as to extend toward the circumference surface (the sleeve  21 B) of the developing roller  21 , as well as being opposed to the developing roller  21  as shown in  FIG. 3 . The regulation blade  26  regulates a layer thickness of a toner supplied from the second stirring screw  24  to the developing roller  21 . A predetermined gap (e.g. 0.2 to 0.4 mm) is formed between a front end of the regulation blade  26  and the sleeve  21 B. In the present embodiment, the regulation blade  26  slants to a forward direction toward the developing roller  21  with respect to a horizontal plane. Also, the regulation blade  26  extends along the axis direction such that a toner carrying region is included on the sleeve  21 B of the developing roller  21 . The regulation blade  26  is configured by fixing a magnetic member to a plate made of a nonmagnetic metal material. 
     The toner container  50  ( FIG. 1 ) is arranged above the toner replenishing port  25  of the developing housing  210 . The toner container  50  includes the above-described toner discharge port  521  ( FIG. 2 ). The toner discharge port  521  is disposed in the cylindrical portion  52  of the toner container  50  so as to correspond to the toner replenishing port  25  of the developing device  20  ( FIG. 1 ). The toner falling down from the toner discharge port  521  is replenished from the toner replenishing port  25  to the developing device  20 . 
     Further, the developing device  20  includes a driving portion M and the control portion  90  ( FIG. 2 ). The driving portion M is configured with a motor and a group of gears, which are not shown. In the present embodiment, the motor of the driving portion M is rotatable in forward and backward directions. As a result, the driving portion M generates a driving force which enables the developing roller  21 , the first stirring screw  23 , and the second stirring screw  24  to rotate around a predetermined axis in a first rotation direction and a second rotation direction opposite to the first rotation direction. Additionally, the group of gears of the driving portion M is rotatably supported in the developing housing  210  to transmit a rotation driving force generated by the motor to the developing roller  21 , the first stirring screw  23 , and the second stirring screw  24  in synchronization with each other. At the time of image formation, in which an image is formed on a sheet in the image forming device  1 , in other words, at the time of developing operation in which an electrostatic latent image on the photosensitive drum  31  is made to appear by the developing device  20 , the photosensitive drum  31 , the developing roller  21 , the first stirring screw  23 , and the second stirring screw  24  are respectively rotated in the arrows shown in  FIG. 3 . Specifically, the first stirring screw  23  upwardly rotates in a region opposed to the partition plate  22  from below, and the second stirring screw  24  downwardly rotates in a region opposed to the partition plate  22  from above. The developing roller  21  and the second stirring screw  24  rotate in regions opposed to each other in the same direction. The regulation blade  26  is located above a region in which the developing roller  21  and the second stirring screw  24  are opposed to each other. 
     The control portion  90  controls the driving portion M at predetermined timing at the time of image formation, thereby rotatably driving the developing roller  21 , the first stirring screw  23 , and the second stirring screw  24 . At the time of reverse rotation control to be described later, the control portion  90  causes the developing roller  21 , the first stirring screw  23 , and the second stirring screw  24  to rotate in a direction opposite to the arrows in  FIG. 3  by reversely rotating the motor of the driving portion M. The control portion  90  also controls operation and rotation of other members in the image forming device  1  including the photosensitive drum  31 . 
     &lt;As to Replenishment of Toner&gt; 
     Next, description will be made of a flow of a toner to be newly replenished from the toner replenishing port  25 .  FIG. 4  is a sectional view in proximity to the toner replenishing port  25  disposed in the developing device  20  and the toner discharge port  521  disposed in the toner container  50 . 
     A replenishment toner T 2  supplied from the toner discharge port  521  of the toner container  50  falls down to the first transport path  221  to be mixed with an existing toner T 1  and transported by the first stirring screw  23  in the arrow D 1  direction. At this time, the toners T 1  and T 2  are stirred to be charged. 
     The first stirring screw  23  includes, at a downstream side of the toner replenishing port  25  in the toner transport direction (the first direction), a suppression paddle  28  (a transport capacity suppressing portion) which partly suppresses a developer transportation performance. In the present embodiment, the suppression paddle  28  is a plate-like member arranged between the adjacent first spiral vanes  23   b  of the first stirring screw  23 . Rotation of the suppression paddle  28  around the first rotation shaft  23   a  causes a toner transported from the side upstream of the suppression paddle  28  to start retention. Then, the retention of the toner is accumulated to a position which is immediately upstream of the suppression paddle  28  and at which the toner replenishing port  25  is opposed to the first transport path  221 . As a result, near an entrance of the toner replenishing port  25 , a retention part  29  of a developer (a developer retention part) is formed. In other embodiment, the transport capacity suppressing portion may be formed by a region obtained as a partly missing part of the first spiral vane  23   b  of the first stirring screw  23 , in which region the first rotation shaft  23   a  is partly exposed along the axis direction. Also in such a configuration, a transport capacity of the first stirring screw  23  is partly suppressed, resulting in forming a developer retention part. 
     When the replenishment toner T 2  is replenished from the toner replenishing port  25  to increase an amount of a toner in the storage space  220 , retention of the toner in the retention part  29  blocks the toner replenishing port  25  (seal) to suppress further replenishment of a toner. Thereafter, when the toner in the storage space  220  is consumed from the developing roller  21  to reduce toner retention in the retention part  29 , the amount of the toner blocking the toner replenishing port  25  is reduced to generate a gap between the retention part  29  and the toner replenishing port  25 . As a result, the replenishment toner T 2  again flows into the storage space  220  from the toner replenishing port  25 . Thus, the present embodiment adopts a volume supply type toner replenishment mode in which an amount of a replenishment toner to be received is adjusted as the toner retention in the retention part  29  is reduced. 
       FIG. 5  is a graph showing a relationship between an amount of a residual developer in the toner container  50  and an amount of a developer in the developing device  20 . As one example, a new toner container  50  is filled with 500 g of toner. In this state, when image formation operation is executed in the image forming device  1 , as the developing device  20  consumes a toner, a toner is replenished from the toner container  50  to the developing device  20  according to the above volume supply type toner replenishment mode. During reduction in an amount of a toner in the toner container  50  from 500 g to 100 g, the amount of the developer in the developing device  20  becomes around 160 g. In this case, a sufficient toner is drawn up from the second stirring screw  24  to the developing roller  21 . The toner supplied to the developing roller  21  is transported toward the regulation blade  26  as the sleeve  21 B rotates. Then, the toner having passed through the gap between the regulation blade  26  and the sleeve  21 B is transported toward the developing position and is supplied to the photosensitive drum  31  as required. 
     On the other hand, the toner regulated by the regulation blade  26  is retained in a lower surface portion (a back surface portion) of the regulation blade  26  by a magnetic field formed by the S 1  pole ( FIG. 3 ) and the regulation blade  26  (a regulated retention part TS). Here, since the magnetic field formed by the S 1  pole is limited, a toner overflowing from the regulated retention part TS in due time falls downward as indicated by the arrow in  FIG. 3 . Accordingly, since replacement of a toner is stably conducted in the regulated retention part TS, toner aggregate is less liable to be generated. 
     With reference to  FIG. 5 , when an amount of a toner in the toner container  50  falls below 100 g, approximately in proportion to an amount of residual toner in the toner container  50 , the amount of a developer in the developing device  20  is reduced. This is because even though a gap is formed between the retention part  29  ( FIG. 4 ) and the toner replenishing port  25  in the developing device  20 , a sufficient toner does not flow into the developing device  20 . In this case, the amount of a toner drawn up from the second stirring screw  24  into the developing roller  21  is also reduced. As a result, replacement of a toner in the regulated retention part TS in  FIG. 3  is not sufficiently conducted, so that the toner is liable to be aggregated in the regulated retention part TS. In particular, since a pressure of the toner to be upwardly transported from below by the sleeve  21 B is applied to the lower surface portion of the regulation blade  26 , a large stress is applied to the toner around the lower surface portion to cause toner aggregation. Such an aggregate of a toner is drawn up onto the developing roller  21  by the second stirring screw  24  after falling down from the regulated retention part TS due to vibration caused at the time of spin-up of rotation of the developing roller  21  and the like. However, since the aggregate is not allowed to pass through a gap between the regulation blade  26  and the sleeve  21 B, the aggregate stays in the gap. As a result, in a toner layer having passed the regulation blade  26 , a part with a small layer thickness is formed corresponding to a position of the aggregate. This leads to generation of a low density streak (white streak) in an image. 
     In order to solve such a problem, in the volume supply type developing device  20 , the control portion  90  (drive control portion) causes the developing roller  21  to reversely rotate for a predetermined time according to an output of the toner container  50 S in the present embodiment. In other words, at the time of the developing operation in which an electrostatic latent image on the photosensitive drum  31  is made to appear by a toner on the developing roller  21 , the control portion  90  causes the developing roller  21  to rotate in the first rotation direction, and when the toner sensor  50 S senses that a residual amount of a toner in the toner container  50  falls below a predetermined threshold value, causes the developing roller  21  to rotate in the second rotation direction at predetermined timing different from that of the developing operation. 
     As a result, the toner in the regulated retention part TS falls down upon receiving rotary power of the sleeve  21 B, or moves within the regulated retention part TS. Therefore, when the control portion  90  next causes the sleeve  21 B to rotate in a forward direction, the toner in the regulated retention part TS is replaced. As a result, generation of a toner aggregate is suppressed in the regulated retention part TS. This prevents the aggregate between the regulation blade  26  and the developing roller  21  from clogging to suppress generation of streaked image failure. 
     Further, also in a period before the toner sensor  50 S senses a residual amount of a toner in the toner container  50  falling below the threshold value, the control portion  90  may cause the developing roller  21  to rotate in the second rotation direction at predetermined timing. In this case, since the developing roller  21  is reversely rotated even when a residual amount of a toner in the toner container  50  is sufficient, replacement in the regulated retention part TS is always accelerated to further suppress generation of a streaked image failure. 
     Additionally, the control portion  90  preferably causes the developing roller  21  to rotate in the second rotation direction at the time of non-image-formation operation different from the image formation operation in which a toner image is transferred to a sheet. In this case, the developing roller  21  can be rotated in the second rotation direction without affecting the image formation operation. Further, as non-image-formation operation, the developing roller  21  is preferably rotated in the second rotation direction in periods before and after the image formation operation, or a period corresponding to an interval between sheets when images are successively formed on a plurality of sheets. In this case, the developing roller  21  can be rotated in the second rotation direction without considerably reducing productivity in the image formation operation. 
     Additionally, in the reverse rotation control of the developing roller  21  according to the present embodiment, the sleeve  21 B is desirably rotated reversely within a range of one rotation or less (rotation angle). In this case, a toner retained on the back surface (regulated retention part TS) of the regulation blade  26  can be moved without considerably losing a toner layer on the developing roller  21 . 
     EXAMPLES 
     Next, effects of the present disclosure will be described in further detail through Examples. The present disclosure is not limited to the following Examples. Experiment 1 and Experiment 2 to be described later were conducted under the following common experiment conditions and experiment method. 
     &lt;Common Experiment Conditions&gt; 
     
         
         diameter of the photosensitive drum  31 : 30 mm 
         linear speed (process speed) of the photosensitive drum  31 : 140 mm/sec 
         diameter of the developing roller  21 : 20 mm 
         linear speed (peripheral speed) of the developing roller  21 : 224.72 mm/sec 
         the regulation blade  26 : product of SUS430 
         peak magnetic flux density of the regulation pole (the S 1  pole): 80 mT 
         interval (blade gap) between the regulation blade  26  and the developing roller  21 : 300 μm 
         diameters of the first stirring screw  23  and the second stirring screw  24 : 22.5 mm 
         the number of rotations of the first stirring screw  23  and the second stirring screw  24 : 23 rpm 
         rotation angle of the developing roller  21  at the time of the reverse rotation control: 30 degrees 
         toner: magnetic one-component toner, volume-average particle size 6.8 μm, made of polyester resin 
         experiment environment: 28° C., 80% RH 
         print pattern: image density 4%, character chart 
         the residual toner amount sensing portion: a magnetic permeability sensor (the toner sensor  50 S)
 
&lt;Common Experiment Method&gt;
 
       
    
     Image printing was conducted until a residual amount of a toner of 500 g in the toner container  50  becomes 80 g to obtain a state where an amount of a toner in the developing device  20  is reduced along with a residual amount of a toner in the toner container  50 . In this state, a half tone image for visual check of a toner layer on the sleeve  21 B and image evaluation was output for level check of a toner aggregate streak. Determination criteria of a toner aggregate streak are as follows, in which 3 or above was evaluated as a passing level. 
     5: a toner layer on the sleeve  21 B is uniform and no streak is generated in an image. 
     4: a level on which a slight vertical streak is generated on the sleeve  21 B but does not appear on an image. 
     3: a level on which a vertical streak is generated on a sleeve, the streak being slightly recognized in an image, the level with no problem in practical use. 
     2: a vertical streak was generated on the sleeve  21 B, the vertical streak being generated of a level on which a streak can be recognized in an image as well. 
     1: a state where a vertical streak is generated on the sleeve  21 B, and a toner aggregate is clogged between the regulation blade  26  and the developing roller  21  (blade gap). An image also has a white streak formed. 
     &lt;Experiment 1&gt; 
     Each condition of Experiment 1 and evaluation results are shown in Table 2. In the reverse rotation control of the developing roller  21 , “normal” represents execution irrespective of a residual amount of a toner in the toner container  50 , “after reduction in residual amount of toner” represents execution when the toner sensor  50 S determines that a residual amount of a toner in the toner container  50  is small (80 g or less). This is also the case with Experiment 2. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                   
                 Toner aggregate 
               
               
                   
                 Reverse rotation 
                 streak level 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 After reduction 
                   
                 After reduction 
               
               
                   
                   
                 in residual 
                   
                 in residual 
               
               
                   
                 Normal 
                 amount of toner 
                 Normal 
                 amount of toner 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                 Example 1 
                 None 
                 Once in 
                 4 
                 5 
               
               
                   
                   
                 30 seconds 
               
               
                 Example 2 
                 None 
                 Once in 
                 4 
                 4 
               
               
                   
                   
                 1 minute 
               
               
                 Example 3 
                 None 
                 Once in 
                 4 
                 3 
               
               
                   
                   
                 3 minutes 
               
               
                 Compar- 
                 None 
                 None 
                 4 
                 1 
               
               
                 ative 
               
               
                 Example 1 
               
               
                   
               
            
           
         
       
     
     In Comparative Example 1, since the reverse rotation control of the developing roller  21  was not conducted, toner replacement was not sufficiently conducted on the back of the regulation blade  26 , so that a vertical streak was generated due to an aggregate of a toner on the sleeve  21 B, and a vertical streak was generated on a level that can be recognized in an evaluation image. In Example 1, as a result of printing while reversely rotating the sleeve  21 B by a rotation angle of 30 degrees in a direction reverse to that of the image formation at the timing of once in 30 seconds of the developing driving time, the toner layer on the sleeve  21 B was uniform and no streak was generated by an aggregate of a toner on the evaluation image. In Example 2, since frequency of reverse rotation was reduced as compared with Example 1, a vertical streak of a slight level was confirmed on the sleeve  21 B, on which level, no streak appeared on the evaluation image at all. In Example 3, since the frequency of reverse rotation was further reduced, while a vertical streak was confirmed on the sleeve  21 B, which was of a level where a streak was slightly recognized in an evaluation image, the level having no problem in practical use. 
     As described in the foregoing, when a residual amount of a toner in the toner container  50  became smaller than a predetermined threshold value, by executing the reverse rotation control of the developing roller  21  at a predetermined interval, generation of a toner aggregate streak, generated immediately before replacement of the toner container  50 , could be suppressed. By limiting execution of the reverse rotation control to immediately before replacement of the toner container  50 , reduction in productivity of the image forming device  1  due to interruption of the reverse rotation control can be suppressed to be minimum. 
     &lt;Experiment 2&gt; 
     Each condition of Experiment 2 and evaluation results are shown in Table 3. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 3 
               
             
            
               
                   
                   
               
               
                   
                   
                 Toner aggregate 
               
               
                   
                 Reverse rotation 
                 streak level 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 After reduction 
                   
                 After reduction 
               
               
                   
                   
                 in residual 
                   
                 in residual 
               
               
                   
                 Normal 
                 amount of toner 
                 Normal 
                 amount of toner 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                 Example 4 
                 Once in 
                 Once in 
                 5 
                 5 
               
               
                   
                 3 minutes 
                 30 seconds 
               
               
                 Example 5 
                 Once in 
                 Once in 
                 5 
                 4 
               
               
                   
                 3 minutes 
                 1 minute 
               
               
                 Example 6 
                 Once in 
                 Once in 
                 5 
                 3 
               
               
                   
                 3 minutes 
                 3 minutes 
               
               
                 Example 7 
                 Once in 
                 Once in 
                 5 
                 5 
               
               
                   
                 30 seconds 
                 30 seconds 
               
               
                 Compar- 
                 Once in 
                 None 
                 5 
                 1 
               
               
                 ative 
                 3 minutes 
               
               
                 Example 2 
               
               
                   
               
            
           
         
       
     
     In Example 4 to Example 7, conducting the reverse rotation control of the developing roller  21  at a predetermined interval irrespective of a residual amount of a toner in the toner container  50  resulted in having a more excellent level of a toner aggregate streak in a period when the toner container  50  had a sufficient residual amount of a toner than in the previous Example 1 to Example 3. On the other hand, in Comparative Example 2, although the reverse rotation control of the developing roller  21  in normal execution was conducted, no execution of the reverse rotation control at the time when the residual amount of a toner becomes small causes insufficient replacement of a toner on the back of the regulation blade  26 , so that clogging of the aggregate toner was confirmed in a blade gap to generate a void in an image. By executing the reverse rotation control of the developing roller  21  irrespective of a residual amount of a toner in the toner container  50  in this manner, high image quality can be continuously maintained. On the other hand, since the image formation operation should be temporarily interrupted or an interval between sheets should be long for the reverse rotation control of the developing roller  21 , executing the reverse rotation control of the developing roller  21  only after a residual amount of a toner in the toner container  50  becomes small enables image quality to be maintained without greatly affecting productivity of the image forming device  1 . 
     In the foregoing, the image forming device  1  according to the embodiment of the present disclosure has been described. In the present embodiment, even when reduction in a residual amount of a toner in the toner container  50  in the volume supply type developing device  20  makes a toner retained on the back surface of the regulation blade  26  be liable to aggregate, the developing roller  21  is reversely rotated at predetermined timing. As a result, an aggregate is prevented from clogging between the regulation blade  26  and the developing roller  21 , thereby suppressing generation of a streaked image failure. On the other hand, the present disclosure is not limited thereto and a modified embodiment such as follows can be adopted. 
     (1) While in the above embodiment, the residual amount sensing portion according to the present disclosure has been described using the toner sensor  50 S provided in the toner container  50 , the present disclosure is not limited thereto. The control portion  90  may sense (estimate) a residual amount of a toner in the toner container  50  becoming small when a total driving time of the rotation member  54  after attachment of a new toner container  50  exceeds a predetermined threshold value, or when a total printing ratio in the image forming device  1  after attachment of a new toner container  50  exceeds a predetermined threshold value, or when an image density (patch density) sensed by a density sensor (not shown) falls below a predetermined threshold value. In this case, the control portion  90  desirably includes a residual developer amount estimate portion (not shown). 
     (2) Although the above embodiment has been described with respect to the configuration in which the driving portion M causes the developing roller  21 , the first stirring screw  23 , and the second stirring screw  24  to rotate in the forward rotation direction and the backward rotation direction in association with each other, the present disclosure is not limited thereto. The developing device  20  may be configured to include a first driving portion which rotatably drives the developing roller  21  and a second driving portion which rotatably drives the first stirring screw  23  and the second stirring screw  24 . Alternatively, the driving portion M may include a rotation direction regulation member such as a one-way clutch on a driving channel linked to the first stirring screw  23  and the second stirring screw  24 . In either case, at the time of the reverse rotation control of the developing roller  21 , reverse rotation of the first stirring screw  23  and the second stirring screw  24  may be regulated, i.e., rotation of the first stirring screw  23  and the second stirring screw  24  may be stopped. 
     As described above, the first stirring screw  23  includes the suppression paddle  28 , so that as the first stirring screw  23  rotates, the retention part  29  of a developer is formed. The retention part  29  functions to control replenishment of a toner from the toner container  50  to the developing device  20 . When the first stirring screw  23  is reversely rotated at the reverse rotation operation of the developing roller  21 , however, the retention part  29  formed around the toner replenishing port  25  breaks down, so that a toner flows in a direction opposite to the first direction D 1 . In this case, at the start of next printing operation, inflow of unnecessary replenishment toner might occur. Therefore, as described above, preventing reverse rotation of at least the first stirring screw  23  at the time of reverse rotation operation of the developing roller  21  suppresses the retention part  29  of a toner from breaking down. As a result, volume supply type toner replenishment can be stably realized, while suppressing a toner aggregate and generation of a streak image around the regulation blade  26 , which phenomena are caused as a residual amount of a toner in the toner container  50  is reduced. 
     Further, the above one-way clutch may function only with respect to the first stirring screw  23 , while at the time of the reverse rotation control of the developing roller  21 , the second stirring screw  24  may be rotated in the forward rotation direction (the same rotation direction as the rotation direction of the second stirring screw  24  when the developing roller  21  is rotated in the first rotation direction). Also in this case, the retention part  29  of a toner being formed by the suppression paddle  28  is suppressed from breaking down while the developing roller  21  is reversely rotated. 
     (3) As shown in  FIG. 3 , the developing roller  21  is located obliquely above the second stirring screw  24 , and the regulation blade  26  is arranged above the second stirring screw  24  so as to be opposed to the developing roller  21 . The second stirring screw  24  rotates so as to upwardly move in a region closer to the developing roller  21  in the horizontal direction from below at the time of the developing operation. In this state, when the developing roller  21  is rotated in a direction opposite to the arrow in  FIG. 3 , a part of the toner in the regulated retention part TS falls down. At this time, when the second stirring screw  24  is rotated in the rotation direction of the arrow (the forward rotation direction) in  FIG. 3 , the fallen toner is temporarily transported to the partition plate  22  side. In other words, the fallen toner is suppressed from again attaching to the developing roller  21 . As a result, a toner present in the regulated retention part TS and in short of a charged amount is suppressed from moving to the photosensitive drum  31  side and from causing an image failure such as toner overlapping or a color point. Such effect is exhibited even if rotation of the second stirring screw  24  is stopped at the time of reverse rotation of the developing roller  21 . In this case, the fallen toner is temporarily transported to the partition plate  22  side upon start of subsequent rotation of the second stirring screw  24 . 
     (4) The control portion  90  may increase frequency of rotation of the developing roller  21  in the second rotation direction as a residual amount of a toner in the toner container  50  becomes less. In this case, clogging between the regulation blade  26  and the developing roller  21  is prevented to further suppress generation of a streaked image failure. 
     (5) Additionally, although in the above embodiment, the transfer portion has been described using the transfer roller  34 , the transfer portion may include a well-known intermediate transfer unit, etc. 
     Although the present disclosure has been fully described by way of example with reference to the accompanying drawings, it is to be understood that various changes and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the present disclosure hereinafter defined, they should be construed as being included therein.