Abstract:
In order to provide an image forming apparatus capable of maintaining the image quality by making smooth flowability of a surplus developer, which is discharged according to supply of a new developer (toner and carrier), toward a collection box, there are provided a stirring screw that electrify toner by stirring the toner together with a carrier; a casing that supports the stirring screw and stores a developer in which the toner and the carrier are mixed; a inlet used to supply the developer to the casing; a guide that guides the surplus developer discharged from the casing downward; and a reciprocating member that moves on a flow surface of the guide along which the surplus developer flows.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from U.S. provisional application 61/078,931, filed on Jul. 8, 2008, the entire contents of each of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an image forming apparatus that forms an image by transferring electrified toner onto a transferred body and in particular, to a discharge technology of a deteriorated developer. 
     BACKGROUND 
     In an image forming apparatus that adopts a developing system of transferring electrified toner onto a transferred body, a developer in which toner and carrier are mixed is stirred so that the toner electrified by friction. 
     In addition, only electrified toner of the developer is carried on a surface of a photoconductive drum and is transferred onto the transferred body, and the carrier remains in the developing device. 
     Accordingly, the toner concentration of the developer stored in the developing device is reduced by repetition of processing of developing an image. 
     For this reason, a developer is supplied to the developing device from an external toner tank when the concentration of toner becomes smaller than a threshold value. 
     This supply is performed including not only toner but also carrier because a coat agent of a carrier is pealed off or dropped or an additive of toner adheres to the carrier surface due to stirring in developing device. 
     In addition, an old surplus developer (developer with reduced toner concentration) generated by supply of the new developer is discharged from the developing device and is then collected (for example, JP-A-9-185177). 
     Meanwhile, in a transport path along which the surplus developer is discharged from an outlet of the developing device and reaches a collection box, there exists a place where the developer flows by gravity. 
     However, the flowability of the developer in the transport path may be stopped by a decrease in fluidity of the developer caused by abrasion and deterioration of the carrier. 
     Thus, when the smooth flow of the surplus developer to the collection box is obstructed, the outlet is clogged and the amount of developer stored in the developing device increases up to a value larger than a defined value. As a result, a problem that the quality of a formed image is lowered occurs. 
     SUMMARY 
     The invention was made in view of such a situation, and it is an object of the invention to provide an image forming apparatus, developing device and a developer discharge mechanism capable of maintaining the image quality by making smooth flow of a surplus developer, which is discharged according to supply of a new developer (toner and carrier), toward a collection box. 
     According to an aspect of the invention, an image forming apparatus includes: a developing device that electrify toner by stirring together with a carrier, and make toner image carried on a photoconductive drum; a supply portion that supplies a developer, in which the toner and the carrier are mixed, to the developing device; a collection portion that collects the surplus developer discharged from the developing device; a guide that guides the surplus developer from the developing device to the collection portion; and a reciprocating member that moves on a flow surface of the guide along which the surplus developer flows. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a view illustrating the configuration of an embodiment of an image forming apparatus of the invention; 
         FIG. 1B  is an enlarged view illustrating a developing unit; 
         FIG. 2A  is a horizontal sectional view illustrating an embodiment of a developer discharge mechanism of the invention; 
         FIG. 2B  is a side view illustrating a developer discharge mechanism; 
         FIGS. 3A to 3E  are views explaining an operation of a reciprocating member when a screw is made to rotate ¼ in the image forming apparatus according to the embodiment; 
         FIG. 4A  is a view illustrating an internal state of a guide which flow a surplus developer, which is discharged from the developer discharge mechanism in the embodiment, to a collection portion; 
         FIGS. 4B and 4C  are views illustrating internal states of guides in comparative examples where there is no reciprocating member; 
         FIG. 5A  is a table which summarizes an experimental result in the developer discharge mechanism according to the embodiment in order to prove the effects of the invention; and 
         FIG. 5B  is a table which summarizes an experimental result in a comparative example where there is no reciprocating member. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, embodiments of the invention will be described on the basis of the accompanying drawings. 
     An image forming apparatus  10  shown in  FIG. 1A  is configured to include: a transfer belt (transferred body)  14  which rotates an orbit, which is defined by a driving wheel  15  and a driven wheel  16 , at a predetermined speed; a developing unit  20  ( 20 K,  20 Y,  20 M,  20 C) to which each color material is supplied from a toner tank  13  ( 13 K,  13 Y,  13 M,  13 C) and which transfers a toner image of each color to the transfer belt  14 ; a collection portion  50  which collects a surplus developer from the developing unit  20  through a guide  70 ; a paper cassette  11  in which the paper is placed in a bundle; a paper feed roller  41  which supplies the paper from the paper cassette  11  one by one; a secondary transfer roller  42  which transfers onto the supplied paper a toner image transferred onto the transfer belt  14 ; a fixing roller  43  which fixes the toner image, which is transferred on the supplied paper, onto a surface of the paper; and a paper roller  44  which guides the paper, on which the toner image is fixed, to a paper tray  12 . 
     The image forming apparatus  10  configured as described above outputs a color image obtained by printing four colors of black K, yellow Y, magenta M, and cyan C on the paper so as to overlap on the basis of image data read by a scanner (not shown) or transmitted from a terminal device. 
     The toner tank  13  ( 13 K,  13 Y,  13 M,  13 C) contain a developer which is mixed toner and carrier. The toner is color materials of black K and the three primary colors (yellow Y, magenta M, cyan C) and particle diameter of about 10 μm. And a carrier, which is a magnetic particle that is iron powder or ferrite powder and which has a particle diameter of about 50 to 150 μm. 
     Moreover, toner tank  13  is replaced with the new one when the developer contained is consumed by image output of the image forming apparatus  10 . 
     The transfer belt  14  is an endless (seamless) belt which has a width almost equal to a length of photoconductive drum  21  in a direction (depth direction of the drawing) perpendicular to the carrying direction. 
     In addition, the transfer belt  14  is stretched over several driven wheels  16  and the driving wheel  15  rotating at a predetermined speed, and the developing unit  20  ( 20 K,  20 Y,  20 M,  20 C) of each color is arrayed in order. 
     In addition, the transfer belt  14  is a transferred body which moves at the same speed as the circumferential speed of the photoconductive drum  21  and in the same direction and onto which a toner image of each color (K, Y, M, C) carried on each photoconductive drum  21  is transferred in order by overlap printing. 
     The paper cassette  11  is provided at a lower side of the image forming apparatus  10  and receives the paper therein. Moreover, the paper feed roller  41  picks up the paper from the paper cassette  11  one by one and sends the paper in a direction shown by a broken line in the drawing. 
     The secondary transfer roller  42  serves to transfer a multicolor toner image, which was transferred from the developing unit  20  ( 20 K,  20 Y,  20 M,  20 C) onto the transfer belt  14  by overlap printing, onto paper. 
     The secondary transfer roller  42  collectively transfers the multicolor toner image from the transfer belt  14  onto the paper by an electric field formed when a predetermined bias voltage is applied between opposite rollers provided with the transfer belt  14  interposed therebetween. 
     The fixing roller  43  melts toner by applying heat and pressure to the paper, on which the multicolor toner image is transferred, so that the toner can be entangled with the fiber of the paper to be then fixed. The paper on which formed the color image is going upward to the paper tray  12  by the paper roller  44 . 
     The collection portion  50  is configured to include a carrying tube  51 , a collection screw  52 , and a collection box  53  and serves to collect a surplus developer, which is discharged from each developing unit  20  ( 20 K,  20 Y,  20 M,  20 C), through the guide  70 . 
     The carrying tube  51  is connected to a developing device  30  of each developing unit  20  through the guide  70  and serves to transport a surplus developer, which drops due to the gravity, to the collection box  53  by the collection screw  52  rotating within the carrying tube  51 . 
     The collection box  53  is configured to be detachable from the image forming apparatus  10  and is also configured such that a surplus developer collected and deposited can be discarded. 
     As shown in  FIG. 1B , the developing unit  20  has an electric charger  23 , an exposure device (not shown) which outputs exposure light  24 , the developing device  30  which supports a developing roller  31 , a primary transfer roller  25  located with the transfer belt  14  interposed between the photoconductive drum  21  and the primary transfer roller  25 , and a cleaner  22  which are provided around the photoconductive drum  21  along the rotation direction. 
     Moreover, the guide  70  which guides a surplus developer discharged from the developing device  30  to the collection portion  50  is connected to the developing unit  20 . 
     The photoconductive drum  21  rotates not to slide on the developing roller  31  and the transfer belt  14  and has a cylindrical shape with a diameter of about 30 mm. The photoconductive drum  21  is a photoconductor in which only a portion that receives the exposure light  24  is changed from an insulator to a conductor. 
     The electric charger  23  applies a voltage difference of 1 kV to 2 kV between the electric charger  23  and the photoconductive drum  21  to continuously cause corona discharge, so that the surface of the photoconductive drum  21  is uniformly charged to about −600 V with static electricity. 
     The exposure light  24  is output from an exposure device (not shown) and is irradiated to a surface of the photoconductive drum  21  charged uniformly according to an image to be formed to thereby form an electrostatic latent image. 
     That is, a light beam is irradiated on and off from the exposure device to thereby form a static electricity pattern image according to conductive and insulating regions formed on the surface of the photoconductive drum  21 . 
     A magnet is disposed inside the developing roller  31 . When the developing roller  31  rotates, a carrier contained in the developing device  30  adheres to the surface of the developing roller  31  together with toner. 
     In addition, a negative bias voltage of −380 V is applied to the developing roller  31 , such that an electric field is formed between the developing roller  31  and the photoconductive drum  21 . That is, in a portion with no static electricity by the exposure light  24 , of the surface of the photoconductive drum  21  on which the static electricity pattern is formed, an electric field is formed in a direction in which the negatively electrified toner moves. On the other hand, in a portion with static electricity of the surface of the photoconductive drum  21 , the direction of the electric field is inverted. Accordingly, the toner cannot move. 
     Thus the toner image of each color ingredient of a picture is carried on each photoconductive dram  21  in the developing unit  20  ( 20 K,  20 Y,  20 M,  20 C). 
     A bias voltage of about +1 kV is applied to the primary transfer roller  25  by a voltage application unit (not shown), such that an electric field is formed between the primary transfer roller  25  and the photoconductive drum  21 . Then, by this electric field, the toner image carried on the photoconductive drum  21  is transferred onto the transfer belt  14 . 
     Then, toner images corresponding to other colors can be made to overlap the toner image, which was already transferred, by increasing the bias voltage applied to the primary transfer roller  25  in each developing unit  20  ( 20 K,  20 Y,  20 M,  20 C) to +1.0 V, +1.2 kV, +1.4 kV, and +1.6 kV gradually in this order. 
     The cleaner  22  serves to remove the toner which remains on the photoconductive drum  21  without being transferred onto the transfer belt  14 . Specifically, the cleaner  22  is configured to include a brush being in contact with the surface of the photoconductive drum  21  and a blade made of rubber. 
     As shown in  FIG. 2A , which is a horizontal sectional view taken along the line IIA-IIA of  FIG. 1B , and  FIG. 2B  which is the side view. The developing device  30  is configured to include the developing roller  31 , a stirring screw  32  ( 32 A,  32 B), a casing  33 , a dividing plate  34 , a concentration sensor  35 , a supply portion  36 , a reciprocating member  60 , and the guide  70 . 
     The developing device  30  configured in this way contains a developer corresponding to one of the colors (black K, yellow Y, magenta M, cyan C) and electrify a toner by stirring together with a carrier. 
     The stirring screw  32  is configured to include the going-path stirring screw  32 A and the returning-path stirring screw  32 B, and spiral flights  32   b  and  32   c  are formed on a shaft  32   a  which performs axial rotation. 
     The casing  33  supports the going-path stirring screw  32 A and the returning-path stirring screw  32 B in parallel separated from each other by the dividing plate  34  and contain a developer thereinside. 
     In addition, by rotating the two stirring screws  32  simultaneously, the developer contained can be circulated in the casing  33  in a direction indicated by the arrow in the drawing. 
     Thus, since carrier and toner rub on each other within the developing device  30 , the carrier is electrified to plus (+) and the toner is electrified to minus (−). Accordingly, the toner is attracted to the carrier. 
     The supply portion  36  is provided at a inlet  36   a  opened to the casing  33  and serves to supply a developer, in which the toner and the carrier are mixed, from the toner tank  13  ( FIG. 1A ) to the developing device  30 . 
     That is, when a toner image is developed on the transferred body, the toner concentration in the developer is reduced since only the toner in the casing  33  is consumed. Then, the concentration sensor  35  detects it, and a developer is supplied into the casing  33  by operating a supply portion  36  so that the toner concentration is maintained constant. 
     In addition, a outlet  73  is provided on a side surface of the casing  33  so that a surplus developer overflows to be discharged from the outlet  73  as much as a developer supplied from the supply portion  36 . Accordingly, the amount of developer in the developing device  30  is maintained constant and an old deteriorated carrier is replaced with a new carrier little by little. 
     In addition, the flight  32   c  of a portion of the stirring screw  32 B located near the outlet  73  has a pitch set smaller than the flight  32   b  of the other portions. Accordingly, the flow of a developer slow down in the portion of the flight  32   b , a developer level is rise, and it becomes easily discharged from the outlet  73 . 
     Moreover, a through hole serving as a rotation center  61  of the reciprocating member  60  is provided on a side surface of the casing  33  near the outlet  73 . 
     The guide  70  is configured to include a slope  71 , which is inclined with respect to a horizontal plane at a lower side of the outlet  73  in the vertical direction, and a hole  72  which continues from the slope  71  and is connected to the carrying tube  51  of the collection portion  50 . In addition, the reciprocating member  60 , which will be described later, is disposed at the slope  71 . 
     The guide  70  configured as described above serves to make a surplus developer overflowed from the outlet  73  flow onto the slope  71  and to guide the surplus developer to the collection portion  50  through the hole  72 . 
     Here, if an angle of inclination of the slope  71  is small, the gravity component in the inclination direction is decreased. As a result, a developer dropped from the outlet  73  may be deposited without flowing. On the other hand, if the angle of inclination of the slope  71  is large, a distance between the outlet  73  and the slope surface is reduced. As a result, a developer which cannot flow may be deposited to clog the outlet  73 . 
     For this reason, an optimal range of the angle of inclination of the slope  71  exists. However, it largely depends on an angle of repose whether or not a discharged developer is deposited on the slope. 
     Here, the angle of repose indicates an angle of a slope at which the powder of a developer maintains a stable state without flowing and is a property value measured by a physical property measuring method defined in JISR9301-2-2. 
     However, it is known that an angle of repose of an initial developer is about 35° but the angle of repose gradually increases because a coating agent of a carrier is peeled off or dropped or an additive of toner adheres to the carrier surface due to stirring in the developing device  30 . Moreover, the angle of repose is also influenced by an environment or the toner concentration of the developer. 
     Accordingly, it is difficult to solve the problem that a developer is deposited on the slope  71  to clog the outlet  73  only by optimizing the angle of inclination. 
     For this reason, the reciprocating member  60  which moves on the slope  71  of the guide  70  is provided. In this case, since a developer is not deposited on the slope  71 , the developer can be made to flow smoothly by the gravity. 
     As shown in  FIGS. 3A to 3E , the reciprocating member  60  is configured to include: an operation portion  62  which moves back and forth along a fan-shaped sweep range R; a rotating shaft  63  which is rotatably supported on a rotation center  61 ; and a slide contact portion  64  which moves in conjunction with a rotational movement of the stirring screw  32 . 
     In this case, the slide contact portion  64  located at one end of the reciprocating member  60  comes in slide contact with a surface of the stirring screw  32 , and the operation portion  62  located at the other end wipes the sweep range R of the flow surface of the slope  71 . 
     In the configuration described above, a driving unit that interlocks the reciprocating member  60  does not need to be provided separately and the reciprocating member  60  can be made to have a simple configuration. 
     Here, the reciprocating member  60  is formed of a wire with a diameter of about 0.5 mm, and the entire reciprocating member  60  including the slide contact portion  64  may be formed of a piece of continuous wire. In addition to the plate shape shown in the drawing, the slide contact portion  64  may be applied only by bending the wire tip in the U shape. 
     However, the reciprocating member  60  in the invention is not limited to the embodiment, but any reciprocating member may be used as long as it moves on the slope  71  of the guide  70 . In addition, any one other than the stirring screw  32  may be adopted as a driving source of the reciprocating member  60 . 
     In such a configuration, when the flow surface of the slope  71  is a gentle slope, the fluidity of a developer discharged from the developing device  30  is reduced because the gravity component in a fall line direction of the slope is small. However, since the kinetic energy is given to the developer by the reciprocating member  60  which, the fluidity of the developer can be improved. Accordingly, the slope of the flow surface  71  can flow the developer along the fall line. 
     An operation of the reciprocating member  60  will be described with reference to  FIGS. 3A to 3E . Here,  FIGS. 3A to 3E  show cases where the stirring screw  32  is made to rotate ¼. A left column shows views seen from an X-Z plane, a middle column shows views seen from a Y-X plane, and a right column shows views seen from a Y-Z plane. 
     First, when the stirring screw  32  is made to rotate ¼ in an initial state of  FIG. 3A , the slide contact portion  64  is pushed by the flight  32   b  and moves as shown in  FIGS. 3B and 3C . Then, the twisting power is generated in the rotating shaft  63 , which causes torque having the rotation center  61  as a center in the operation portion  62 . The operation portion  62  for which the torque is caused is displaced to wipe the fan-shaped sweep range R. 
     Then, when the stirring screw  32  rotates, the slide contact portion  64  is willing to return to the original position over the flight  32   b  by the restoring force of the rotating shaft  63  which is twisted to deform, as shown in  FIG. 3D . At this time, the operation portion  62  is also displaced in the reverse direction in synchronization with the movement of the flight  32   b.    
     Then, as shown in  FIG. 3E , the reciprocating member  60  also returns to the initial state when the stirring screw  32  makes one rotation from the initial state ( FIG. 3A ). 
     Thus, the operation portion  62  moves back and forth along the sweep range R once whenever the stirring screw  32  makes one rotation, and the operation portion  62  wipes the sweep range R when the stirring screw  32  rotates continuously. 
     When the number of rotations of the stirring screw  32  is set to 358 rpm, for example, the reciprocating member  60  wipes the sweep range R while moving back and forth about six times for one second. 
     As shown in  FIG. 4A , a developer overflowed from the outlet  73  of the developing device  30  ( FIG. 1B ) drops through a space of the guide  70  and lands on the slope  71 , and then flows to be guided to the collection portion  50 . Even if an overflowed developer is deposited at a landing point of the slope  71  due to a decrease in the fluidity of the surplus developer, the surplus developer moves to the collection portion  50  without being deposited in the middle of the guide  70  by movement of the reciprocating member  60 . 
     On the other hand, when there is no reciprocating member  60  like comparative examples shown in  FIGS. 4B and 4C , a developer may be deposited on the slope  71  ( FIG. 4B ) depending on conditions. In a worst case, the developer may clog the outlet  73  ( FIG. 4C ). 
     Referring to  FIGS. 5A and 5B , examples of an experiment for proving the effects of the invention are shown. 
       FIG. 5A  is a table in which an experimental result in a developing device including the developer discharge mechanism according to the embodiment of the invention is summarized, and  FIG. 5B  is a table in which an experimental result in a comparative example where there is no reciprocating member is summarized. 
     Three kinds of developer samples with different angles of repose were prepared as developers. 
     The first sample was a developer A with a toner concentration of 8.5% made by using a carrier with a volume average particle diameter of 60 μm, which was obtained by performing silicon-based resin coating on a ferrite core, and black toner with a volume average particle diameter of 6 μm using a polyester resin as a main material. 
     When the angle of repose of the developer A related to the first sample was measured, it was about 35°. 
     Here, the angle of repose was set as an angle obtained in a way of dropping the developer using a funnel from the position of about 50 mm above a circular base of about 80 mm in diameter and measuring the angle of the skirt of the mount when the mount of the developer deposited on the circular base was observed from the side surface. In addition, the diameter of a lower portion of the funnel is about 5 mm. 
     A developer B after printing about 60 k pieces of paper with the developer A at 6% of printing rate using the copying machine by Toshiba TEC Corp. is assumed to the second sample. 
     When the angle of repose of the developer B related to the second sample was measured, it was about 38°. 
     In addition, a developer C after printing about 140 k pieces of paper with the developer A at 6% of printing rate using the copying machine by Toshiba TEC Corp. is assumed to the third sample. 
     When the angle of repose of the developer C related to the third sample was measured, it was about 40°. 
     Then, three different kinds of guides  70  were prepared for the experiment, of which angles of inclination of the slope  71  were 40°, 55°, and 65° respectively. Moreover, the guide  70  in which the reciprocating member  60  was provided on the slope  71  having an angle of inclination and a guide in which the reciprocating member  60  was not provided on the slope  71  having an angle of inclination were prepared. 
     The amount of developer stored in the casing  33  was set to about 400 g. That is, if the developer enters more than 400 g, it is discharged through the outlet  73 . In this state, additional 50 g of the developer was supplied to confirm whether or not the discharged developer would be deposited on the slope. The result is shown in  FIGS. 5A and 5B . 
       FIG. 5A  shows a case where the reciprocating member  60  is provided on the slope  71 . 
     Regardless of the developer A (angle of repose of 35°), the developer B (angle of repose of 38°), and the developer C (angle of repose of 40°), the developers were not deposited on the slope. Also, regardless of the slope angles (40°, 55°, 65°), the developers were not deposited on the slope. 
       FIG. 5B  shows a case where the reciprocating member  60  is not provided on the slope  71 . 
     The developer A (angle of repose of 35°) was not deposited on the slope regardless of the slope angles. 
     The developer B (angle of repose of 38°) was not deposited on the slope at the slope angles of 55° and 65°. However, when the slope angle was 40°, the developer B was deposited on the slope. In this case, the developer B discharged as time went by was gradually deposited on the slope, and finally its level rose above the outlet. 
     The developer C (angle of repose of 40°) was not deposited on the slope at the slope angle of 65°. However, when the slope angle was 40° and 55°, the developer C was deposited on the slope. 
     As described above, disposing a reciprocating member on the flow surface of the guide  70  which guides the developer discharged from the developing device  30  is able to maintain stabilized image formation by preventing the outlet  73  of the developing device  30  from being clogged. 
     The invention is not limited to the above-described embodiment, but may be suitably modified within the common technical scope of the invention. 
     For example, a monochrome image forming apparatus may also be used, even though a color image forming apparatus was illustrated. In this case, the photoconductive drum  21  may directly transfer the toner to paper (transferred body) with no need of the transfer belt  14 . 
     In addition, the screw was illustrated as a driving source of the reciprocating member, but an exclusive driving source may be disposed. 
     Furthermore, the case where the reciprocating member was disposed on the slope surface because the developer was easy to be deposited on the slope surface was illuminated. However, this arrangement position is not limited, either. That is, a reciprocating member may be disposed on a flow surface that is a surface which forms a space where flowing developer would be easily clogged. 
     Moreover, it is also possible to adopt a method of attaching a piezoelectric element to the outer side of a sidewall of the slope as a reciprocating member, instead of the operation portion  62  which moves back and forth along the sweep range R ( FIG. 3 ), so that a portion on which the developer flows can be intensively vibrated. 
     Specifically, a piezoelectric element (vibration generating portion) which vibrates at the frequency of 5 kH was provided on a bottom surface of the slope  71  of the guide  70  under the same conditions as the experiment example executed in  FIGS. 5A and 5B . In this case, the same result as the table shown in  FIG. 5A  was obtained, and the effect was confirmed. 
     That is, a reciprocating member does not have the capability for self to convey a developer, what is necessary is to give vibration to the developer on a slope, and just to have the function to promote a flow by gravity.