Abstract:
A developing device includes a supplying chamber, an agitating member, a toner carrier, a retaining part and a detecting part. The supplying chamber accommodates a toner replenished from a toner container. The agitating member agitates the toner in the supplying chamber. The toner carrier supplies an electrostatic latent image formed on an image carrier with the toner from the supplying chamber. The retaining part recovers and retains the toner dispersed in the vicinity of the image carrier and the toner carrier. The detecting part detects an amount of the toner in the retaining part. The retaining part is moreover configured to replenish the recovered toner from the retaining part to the supplying chamber when the detecting part detects an amount of the toner in the retaining part more than a predetermined amount in a state where the toner is needed to be replenished to the supplying chamber.

Description:
INCORPORATION BY REFERENCE 
     This application is based on and claims the benefit of priority from Japanese Patent application No. 2015-046744 filed on Mar. 10, 2015, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     The present disclosure relates to a developing device configured to supply a developer to an image carrier and an image forming apparatus including the developing device. 
     In the electrographic image forming apparatus, an electrostatic latent image formed on the surface of an image carrier (a photosensitive drum) is developed as a toner image in the developing device, and the developed toner image is transferred from the image carrier to a sheet in a transferring unit. A developer which is dispersed in the vicinity of the developing device or the transferring unit or a developer remaining on the image carrier after transferring of the toner image is often recovered, replenished into the developing device and reused. 
     A technique in which the recovered developer is replenished into the developing device and reused has been in a wide spread. However, the recovered developer has a low amount of charge, is charged in a reverse polarity or often has a small particle diameter. Therefore, there is a tendency that an image failure such as a fogging due to a poor charge in reuse occurs. In addition, when the recovered developer is mixed with a developer supplied from the toner container, a replenishment fogging due to a difference of a charging amount also is generated. Such an image failure is accelerated by aged deterioration of the developer, a decrease of the charging amount due to environmental change or the like. 
     With respect to the problems, there is a developing device configured to reproduce a developer by applying an external additive into the recovered developer. 
     SUMMARY 
     In accordance with an embodiment of the present disclosure, a developing device includes a supplying chamber, an agitating member, a toner carrier, a retaining part and a detecting part. The supplying chamber accommodates a toner replenished from a toner container. The agitating member agitates the toner in the supplying chamber. The toner carrier supplies an electrostatic latent image formed on an image carrier with the toner from the supplying chamber. The retaining part recovers and retains the toner dispersed in the vicinity of the image carrier and the toner carrier. The detecting part detects an amount of the toner in the retaining part. The retaining part is moreover configured to replenish the recovered toner from the retaining part to the supplying chamber when the detecting part detects an amount of the toner in the retaining part more than a predetermined amount in a state where the toner is needed to be replenished to the supplying chamber. 
     In accordance with an embodiment of the present disclosure, an image forming apparatus includes the developing device described above. 
     The above and other objects, features, and advantages of the present disclosure will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present disclosure is shown by way of illustrative example. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view schematically illustrating an internal structure of a color printer according to an embodiment of the present disclosure. 
         FIG. 2  is a sectional view illustrating an internal structure of a developing device according to the embodiment of the present disclosure. 
         FIG. 3  is a flow chart useful for explaining a procedure of replenishing a toner in a developing device according to the embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     In the following, a developing device and an image forming apparatus according to an embodiment of the present disclosure will be described with reference to the drawings. 
     First, with reference to  FIG. 1 , the entire structure of a color printer  1  (an image forming apparatus) will be described.  FIG. 1  is a sectional view schematically illustrating an internal structure of the color printer according to the embodiment. Hereinafter, it will be described so that the front side of the color printer is positioned at the near side of  FIG. 1 , and left and right directions are defined on the basis of a direction in which the color printer is seen from the front side. 
     The printer  1  includes a box-formed printer main body  2 . In a lower part of the printer main body  2 , a sheet feeder  4  configured to feed sheets from a sheet feeding cartridge  3  configured to store the sheets (not shown) is arranged. On an upper part of the printer main body  2 , an ejected sheet tray  5  is arranged. 
     Inside the printer main body  2 , an intermediate transferring unit  6  is installed at the upper side and an exposure device  7  composed of a laser scanning unit (LSU) is installed at the lower side. The intermediate transferring unit  6  has an intermediate transferring belt  8  bridged between a plurality of rollers and four image forming parts  9  are placed for respective colors of toners along a lower part of the intermediate transferring belt  8 . 
     In each of the image forming parts  9 , a photosensitive drum  10  is rotatably arranged. Around the photosensitive drum  10 , a charger  11 , a developing device  12 , a primary transferring unit  13 , a cleaning device  14  and a static eliminator  15  are placed in order of a primary transfer process. Four toner containers  16  corresponding to the respective image forming parts  9  are installed for respective colors of toners (Y,M,C and K) and each toner container  16  is placed above each developing device  12 . In each developing device  12 , a two-component developer obtained by mixing a toner having each color with a carrier is filled in a predetermined amount. 
     Inside the printer main body  2 , at a right end of the intermediate transferring unit  6 , a transfer roller  18  constituting a secondary transferring unit  17  is arranged across the intermediate transferring belt  8 . Above the secondary transferring unit  17 , a fixing device  19  is installed, and, above the fixing device  19 , a sheet ejecting unit  20  is arranged. In addition, inside the printer main body  2 , a sheet conveying path  21  configured to travel from the sheet feeder  4  through the secondary transferring unit  17  and the fixing device  19  to the sheet ejecting unit  20  is arranged at one side (the right side in  FIG. 1 ). 
     Next, an image forming operation of the color printer  1  having the configuration described above will be described. After charging of the surface of the photosensitive drum  10  by the charger  11 , exposure is carried out to the photosensitive drum  10  by laser light (see arrow P) from an exposure unit  7  so as to correspond to image data, and thereby, an electrostatic latent image is formed on the surface of the photosensitive drum  10 . The electrostatic latent image is developed as a toner image having correspondent color by the developing device  12 , and the toner image is primary-transferred on the surface of the intermediate transferring belt  8  in the primary transferring unit  13 . The above-mentioned operation is sequentially repeated by the respective image forming parts  9 , and thereby, a full-color toner image is formed on the intermediate transferring belt  8 . It is to be noted that the toner and charge remaining on the photosensitive drum  10  is removed by the cleaning device  14  and the static eliminator  15 . 
     On the other hand, a sheet fed from the sheet feeding cartridge  3  by the sheet feeder  4  or from a manual bypass tray (not shown) is conveyed to the secondary transferring unit  17  in a suitable timing for the above-mentioned image forming operation, and then, the toner image having the full-color toner image on the intermediate transferring belt  8  is transferred onto the sheet in the secondary transferring unit  17 . The sheet with the secondary-transferred toner image is conveyed to a downstream in the conveying path  21  to go forward to the fixing device  19 , and then, the toner image is fixed on the sheet in the fixing device  19 . The sheet with the fixed toner image is ejected from the sheet ejecting unit  20  onto the ejected sheet tray  5 . 
     Next, the developing device  12  will be described with reference to  FIG. 2 .  FIG. 2  is a sectional view schematically showing an internal structure of the developing device. 
     The developing device  12  includes a casing  31  storing the two-component developer. The casing  31  has a hollow part elongated in front and rear directions, and is formed with an opening  32  facing to the photosensitive drum.  10  in an upper part of a right side face thereof. 
     In a lower part of the casing  31 , a lower partition wall  33  extending in the front and rear directions is vertically arranged, and divides the lower part of the casing  31  into a supplying chamber  34  at the left side and a conveyance chamber  35  at the right side. With the supplying chamber  34 , a toner replenishment path (not shown) connected to the toner container  16  is communicated, and the toner is replenished from the toner container  16  to the supplying chamber  34 . In addition, the supplying chamber  34  is provided with a toner density sensor (not shown), and in a case in which the toner density detected by the toner density sensor is lower than a predetermined value, the toner is replenished from the toner container  16  to the supplying chamber  34 . In addition, in front and rear end parts of the lower partition wall  33 , communicating ports communicating the supplying chamber  34  and the conveyance chamber  35  are formed. 
     At a left upper corner of the casing  31  and above the supplying chamber  34 , by an upper partition wall  37  extending in the front and rear directions, a toner retaining part  38  (a retaining part) having a predetermined volume is formed. The toner retaining part  38  is formed such that the sectional area at the lower part thereof is smaller than the upper part thereof. The upper partition wall  37  is formed with an opening  39  extending in the front and rear directions, and the toner retaining part  38  is communicated with the inside of the casing  31 , particularly the upper part of the inside of the casing  31  including an opening part  32  through the opening  39 . In addition, in the toner retaining part  38 , a suction fan  42  is provided together with a toner filter  41 , and operation of the suction fan  42  makes the pressure applied to the toner retaining part  38  smaller than the pressure inside the casing  31 . It is to be noted that the suction fan  42  and the toner filter  41  may be placed around the center in the front and rear directions in the toner retaining part  38 , particularly may be placed in respective front and rear end parts in which the toner is likely to be dispersed. 
     Further, the toner retaining part  38  is provided with a toner amount sensor  44  (a detecting part) configured to detect the amount of the toner. As a toner amount sensor  44 , a weight sensor, a level sensor, a piezoelectric sensor or the like may be used. It is to be noted that in a case in which the toner amount sensor  44  is the weight sensor or piezoelectric sensor, the toner amount indicates a weight, and in a case in which the toner amount sensor  44  is the level sensor, the toner amount indicates a volume. 
     In addition, the toner retaining part  38  is communicated with the supplying chamber  34  through the conveying path  46 . The conveying path  46  is arranged between a bottom part of the toner retaining part  38  and an upper part of the supplying chamber  34  so as to extend in upper and lower directions. In addition, the conveying path  46  has a shutter  48  in an end part at a side of the toner retaining part  38 , and when the shutter  48  is opened, the toner retaining part  38  is communicated with the supplying chamber  34  through the conveying path  46 . 
     Inside the casing  31 , an agitating screw  51  (an agitating member) placed in the supplying chamber  34 , a conveyance screw  52  placed in the conveyance chamber  35 , a magnetic roller  53  placed above the conveyance screw  52 , a developing roller  54  (a developer carrier, a toner carrier) placed so as to face to the magnetic roller  53  obliquely upward at the right side of the magnetic roller  53 , and an ear-breaking blade  55  placed so as to face to the magnetic roller  53  are provided. 
     The agitating screw  51  and the conveyance screw  52 , respectively, are composed of spindles  51   a ,  52   a  and screw blades  52   b ,  52   b  spirally arranged on outer circumference faces of the spindles  51   a ,  52   a . Each of the spindles  51   a ,  52   a  is supported by the supplying chamber  34  and the conveyance chamber  35  so as to rotate around an axis direction. According to this, the toner supplied from the toner container  16  (see  FIG. 1 ) is mixed with a carrier and the mixture is agitated and electrically charged in a predetermined charging amount. The agitating screw  51  is designed so as to change rotating speed thereof, and is configured to rotate at higher speed than the rotating speed when the toner replenished from the toner container  16  is agitated. 
     The magnetic roller  53  is configured with a spindle  53   a  unrotatably supported, a magnetic pole member  53   b  fixed on the spindle  53   a  and having a fan shape in a sectional view, a non-magnetic rotating sleeve  53   c  involving the spindle  53   a  and the magnetic pole member  53   b . The both end parts of the rotating sleeve  53   c  in an axis direction is pivotably supported by the casing  31  so as to rotate, and the rotating sleeve  53   c  is rotated in a counterclockwise direction as shown by an outlined arrow in  FIG. 2 . 
     In addition, the magnetic roller  53  is connected to a development bias power source via a bias control circuit (both not shown), and a direct current voltage and an alternate current voltage are applied from the development bias power source to the magnetic roller  53 . 
     The developing roller  54  is configured with a spindle  54   a  unrotatably supported, a developing magnetic pole member  54   b  arranged at a position facing to the magnetic roller  53 , and a developing sleeve  54   c  formed in a cylindrical shape with non-magnetic metal material. The developing sleeve  54   c  is rotatably supported in the spindle  54   a , and is rotated in the counterclockwise direction as shown by an outlined arrow in  FIG. 2 . The developing roller  54  is placed so as to face to the magnetic roller  53  with a predetermined gap and is exposed from the opening  32  of the casing  31  so as to face to the photosensitive drum  10  (see  FIG. 1 ). 
     The developing roller  54  is connected to a developing bias power source via a bias control circuit (both not shown), and a direct current voltage and an alternate current voltage is applied from the development bias power source to the developing roller  54 . 
     The ear-breaking blade  55  is attached to the casing  31  at an upstream side in the rotation direction of the magnetic roller  53  relative to a position in which the developing roller  54  faces to the magnetic roller  53 , in a state that a tip thereof is inclined to a lower left direction. The tip part of the ear-breaking blade  55  is placed along an axis direction of the magnetic roller  53  with a slight gap from the surface of the magnetic roller  53 . 
     In the developing device  12  having the configuration described above, when the agitating screw  51  and the conveyance screw  52  are rotated, the developer is conveyed in the axis direction while being agitated in the supplying chamber  34  and the conveyance chamber  35 , and circulated between the supplying chamber  34  and the conveyance chamber  35  through a communicating route arranged in both end parts of the lower partition wall  33 . 
     The agitated and circulated developer is electrically charged and is conveyed to the magnetic roller  53  by the conveyance screw  52 , and forms a magnetic brush on the magnetic roller  53 . The magnetic brush on the magnetic roller  53  is subjected to layer thickness regulation by the ear-breaking blade  55 , and then, is conveyed to a part in which the magnetic roller  53  faces to the developing roller  54 , and makes a toner thin layer on the developing roller  54  by a potential difference between the voltage applied to the magnetic roller  53  and the voltage applied to the developing roller  54  and a magnetic field. 
     In the toner thin layer made on the developing roller  54 , when the developing roller  54  is rotated to a position facing to the photosensitive drum  10 , the toner flies due to the potential difference between the developing roller  54  and the photosensitive drum  10  to be supplied to the electrostatic latent image on the photosensitive drum  10 , and thereby, the electrostatic latent image is developed as the toner image. 
     The toner remaining without being used for the development is conveyed again to a position in which the developing roller  54  faces to the magnetic roller  53 , and is recovered by the magnetic brush on the magnetic roller  53 . The magnetic brush is peeled from the magnetic roller  53  in a homopolar part of the pole member  53   b , and then falls into the conveyance chamber  35 . 
     In the supplying chamber  34 , a predetermined amount of toner is replenished from the toner container  16  based on result detected by the toner density sensor, and the developer is electrically charged homogenously again with an appropriate toner density while circulating between the conveyance chamber  35  and the supplying chamber  34 . 
     During developing operation, the toner having a lower charging amount or the toner having a smaller particle diameter in the developer is dispersed around the developing roller  54  or the photosensitive drum  10  in the casing  31  without being supplied to the electrostatic latent image or being recovered by the magnetic brush. The dispersed toner is drawn toward the toner retaining part  38  having a negative pressure due to the suction fan  42  (see a two-dot chain line in  FIG. 2 ), and passes through the opening  29  of the upper partition wall  27  to enter into the toner retaining part  38  and to be recovered by the toner retaining part  38 . The amount of the recovered toner is detected by the toner amount sensor  44 . 
     Next, with reference to  FIG. 3 , a procedure in which the recovered toner is replenished to the supplying chamber  34  will be described. Decision in the procedure described hereinafter is executed by a controller of the developing device  12 . As described above, the toner density is detected by the toner density sensor in the supplying chamber  34 . At step S 1 , when it is decided that the toner density detected by the toner density sensor is lower than a defined density (x %), the procedure proceeds to step S 2  to decide whether or not the amount of the recovered toner detected by the toner amount sensor  44  provided in the toner retaining part  38  is larger than a predetermined amount (for example, y g). 
     When it is decided at step S 2  that the amount of the toner recovered in the toner retaining part  38  is larger than the predetermined amount (y g), the procedure proceeds to step S 3  to supply the recovered toner to the supplying chamber  34 . That is, the toner is not replenished from the toner container  16  but the recovered toner is replenished. Specifically, the shutter  48  of the toner retaining part  38  is opened, and the recovered toner is allowed to fall from the toner retaining part  38  through the conveying path  46  into the supplying chamber  34  by its own weight. Thereby, it is possible to replenish the recovered toner from the toner retaining part  38  to the supplying chamber  34  only by opening the shutter  48 . 
     Next, the procedure proceeds to step S 4 , the rotation speed of the agitating screw  51  is set to be faster than the rotation speed when the toner replenished from the toner container  16  is agitated. Thereby, the toner is hardly agitated in the supplying chamber  34  to be likely to be subjected to frictional electrification, consequently the toner charging amount is increased. The toner having the increased charging amount is pumped from the magnetic roller  53  to the developing roller  54  while circulating between the conveyance chamber  35  and the magnetic roller  53  to develop electrostatic latent image on the photosensitive drum  10 . 
     On the other hand, when it is decided at step S 2  that the amount of the toner recovered into the toner retaining part  38  is not more than the predetermined amount (y g), the procedure proceeds to step S 5  to replenish the toner from the toner container  16 . 
     As described above, in the developing device  12  of the present disclosure, the dispersed toner having an unsatisfactory charging amount is recovered to store until the charging amount reaches the predetermined amount, and the toner is replenished to the supplying chamber  34  separately from a fresh toner replenished from the toner container  16 . Therefore, the toner having a satisfactory charging amount is scarcely mixed with the toner having the unsatisfactory charging amount in the supplying chamber  34 . Then, when the recovered toner having the unsatisfactory charging amount is replenished the agitating speed of the agitating screw  51  is increased to accelerate frictional electrification, and thus the charging amount of the recovered toner can be homogeneously increased. Therefore, even if the recovered toner is replenished, an image failure such as replenishment fogging resulted from the difference in the charging amount does not occur. 
     In such a manner, the reuse of the recovered toner enables reduction of the amount of the toner consumed as well as restraint of pollution of the toner inside and outside the developing device  12 . Incidentally, although the present embodiment has been described about a touchdown development type developing device  12 , the present disclosure can be also applied to other developing devices such as a jumping type developing device using a one-component developer and a magnetic brush type developing device using a two-component developer. It is to be noted that, if the one-component developer is used, a timing to replenish the toner is decided on the basis of detection of a weight or volume of the toner in the supplying chamber. 
     In the present embodiment, the developing device is configured so that the toner retaining part  38  is provided above the supplying chamber  34  and the recovered toner is allowed to fall from the toner retaining part  38  into the supplying chamber  34  by its own weight. Therefore, it is possible to replenish the recovered toner to the supplying chamber  34  with an easy mechanism. Alternatively, the developing device may be configured so that the bottom part of the toner retaining part  38  is configured so as to perform a seesaw movement and, when the toner having a predetermined weight is accumulated in the bottom part, the toner is allowed to fall from the bottom part by the seesaw movement. It is to be noted that in a case in which the toner retaining part  38  cannot be provided above the supplying chamber  34  or in a case in which the distance between the toner retaining part  38  and the supplying chamber  34  is long, a conveyance member forcedly conveying the toner in the conveying path  46  may be provided. 
     It is to be noted that the description of the embodiment of the present disclosure described above describes suitable embodiments of the developing device and the image forming apparatus in the present disclosure. Therefore, technically preferred various configurations have been illustrated. However, the technical scope of the present disclosure is not limited to the embodiments unless there is description particularly limiting the present disclosure. Further, components in the embodiments of the present disclosure described above may be appropriately replaced with existing components, and various variations including a combination with other existing components may be used. The description of the embodiment of the present disclosure described above does not limit the content of the disclosure described in claims.