Abstract:
A developing device has a first circulation tank and a second circulation tank. A developer is conveyed from the first circulation tank to the second circulation tank through a first opening, while the developer is partly discharged through an outlet. The developer conveyed to the second circulation tank is conveyed back in the first circulation tank via a second opening. A first screw provided in the first circulation tank has a first vane, which exerts a force in a conveying direction of the first circulation path, in a first portion upstream from the first opening with respect to the conveying direction. The first screw has no vanes in a second portion near the first opening. The first screw has a second vane, which exerts a force in a direction opposite to the conveying direction, in a third portion adjacently downstream from the second portion. The outlet is located at a point of the first circulation tank, downstream from the second vane. A portion of the first circulation tank downstream from the second vane is not connected to the second circulation tank in a direction perpendicular to the conveying direction.

Description:
This application claims benefit of priority to Japanese Patent Application No. 2014-179790 filed Sep. 4, 2014, the content of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a developing device and an image forming apparatus, and more particularly to a developing device that uses a two-component developer and that is resupplied with and discharges the developer as needed, and an image forming apparatus comprising the developing device. 
     A developing device that develops an electrostatic latent image formed on a photoreceptor drum with a two-component developer containing carrier and toner is well known as an example of developing devices to be employed in electrophotographic image forming apparatuses. In recent years, a trickle type developing device has been suggested. The trickle type developing device resupplies a developer, which contains toner and a tiny amount of carrier, to a developer tank in proportion as the toner consumption and discharges a fixed amount of developer from the developer tank. 
     In the trickle type developing device, it is important to keep the amount of developer in the developer tank at a constant level for maintenance of high quality of developed images. For this purpose, a screw provided in the developer tank to convey a developer has a vane for applying pressure to the developer in the conveying direction and a vane for applying pressure to the developer in the opposite direction. The pressure balancing achieved by these vanes allows adjustments of the amount of developer circulating in the developer tank and the amount of developer discharged from the developer tank so as to keep the amount of developer in the developer tank at a constant level. However, there is a risk of loosing the pressure balance due to an increase in the rotation speed of the developer conveying screw with an increase in the printing speed, thereby causing problems, such as discharge of too much developer, etc. In order to prevent the problems, Japanese Patent Laid-Open Publication No. 10-186829 discloses a developing device comprising a shutter system near an outlet for the developer so as to adjust the amount of discharged developer. In the following, this developing device will be referred to as a conventional developing device. 
     However, in a developing device provided with a shutter system as in the conventional developing device, there is a risk of causing convection of the developer near the shutter system, which causes trouble, such as closure of the outlet with the developer. Also, the shutter system has a complicate structure, and providing the shutter system results in an increase in the size of the developing device and an increase in the cost. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a developing device that is capable of keeping an appropriate amount of developer in the developer tank with a simple system as compared to a conventional developing device, and an image forming apparatus comprising the developing device. 
     A developing device according to a first aspect of the present invention is configured to develop an electrostatic latent image formed on an image supporting member with a developer containing toner and carrier, to be resupplied with a developer in proportion as toner consumption and to discharge a used developer, the developing device comprising: 
     a first circulation tank configured to discharge a part of the developer through an outlet while conveying the developer; 
     a second circulation tank divided from the first circulation tank by a partition wall, the second circulation tank configured to convey the developer conveyed thereto from the first circulation tank through a first opening made in the partition wall and the developer resupplied thereto through a resupply port to the first circulation tank through a second opening made in the partition wall such that the first circulation tank and the second circulation tank form a circulation pathway; 
     a first screw provided in the first circulation tank so as to convey the developer; and 
     a second screw provided in the second circulation tank so as to convey the developer, wherein: 
     the first screw has a first vane in a first portion upstream from the first opening with respect to a conveying direction of the first screw, the first vane exerting a force in the conveying direction on the developer; 
     the first screw has no vanes in a second portion near the first opening; 
     the first screw has a second vane in a third portion adjacently downstream from the second portion with respect to the conveying direction, the second vane exerting a force in a direction opposite to the conveying direction on the developer; 
     the outlet is located at a point of the first circulation tank, downstream from the second vane with respect to the conveying direction; and 
     a portion of the first circulation tank downstream from the second vane with respect to the conveying direction is not connected to the second circulation tank in a direction perpendicular to the conveying direction. 
     An image forming apparatus according to a second aspect of the present invention comprises the developing device above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view illustrating an internal structure of an image forming apparatus comprising a developing device according to an embodiment of the present invention. 
         FIG. 2  is a sectional view of the developing device according to the embodiment, around a developing roller, when viewed from a direction parallel to a central axis of a supply screw of the developing device. 
         FIG. 3  is a sectional view of a developer tank of the developing device according to the embodiment, when viewed from a direction orthogonal to the central axis of the supply screw of the developing device. 
         FIG. 4  is a sectional view of a developer tank of a developing device according to a comparative example, when viewed from a direction orthogonal to a central axis of a supply screw of the developing device. 
         FIG. 5  is a schematic sectional view of the developer tank of the developing device according to the comparative example, when viewed from the direction orthogonal to the central axis of the supply screw of the developing device, illustrating the flow of a developer during high-speed rotation of the supply screw. 
         FIG. 6  is a graph indicating the relation between the rotation speed of the supply screw of the developing device according to the comparative example and the amount of developer circulating in the developing device. 
         FIG. 7  is a schematic sectional view of the developer tank of the developing device according to the comparative example, when viewed from the direction orthogonal to the central axis of the supply screw of the developing device, illustrating the flow of a developer during low-speed rotation of the supply screw. 
         FIG. 8  is a schematic sectional view of the developer tank of the developing device according to the embodiment, when viewed from the direction orthogonal to the central axis of the supply screw of the developing device, illustrating the flow of a developer during high-speed rotation of the supply screw. 
         FIG. 9  is a graph indicating the relation between the rotation speed of the supply screw of the developing device according to the embodiment and the amount of developer circulating in the developing device. 
         FIG. 10  is a schematic sectional view of the developer tank of the developing device according to the embodiment, when viewed from the direction orthogonal to the central axis of the supply screw of the developing device, illustrating the flow of a developer during low-speed rotation of the supply screw. 
         FIG. 11  is a sectional view of the developing device according to a modification, around a developing roller, when viewed from a direction parallel to a central axis of a supply screw of the developing device. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     General Structure of Image Forming Apparatus; See FIG.  1   
     A developing device according to an embodiment of the present invention and an image forming apparatus comprising the developing device are described with reference to the drawings. In the drawings, the same members and the same parts are provided with the same reference symbols, and the same descriptions are not repeated. 
     An image forming apparatus  1  illustrated in  FIG. 1  is an electrophotographic color printer of a tandem type. The printer generally comprises imaging units  10  ( 10   y ,  10   m ,  10   c ,  10   k ) configured to form toner images in colors of Y (yellow), M (magenta), C (cyan) and K (black), respectively, and an intermediate transfer unit  20 . In the printer, further, hoppers  15  ( 15   y ,  15   m ,  15   c ,  15   k ) configured to resupply toner to the imaging units  10 , respectively, are provided above the intermediate transfer unit  20 . 
     Each of the imaging units  10  includes a charger  12 , a developing device  40 , etc. arranged around a photoreceptor drum  11 . In each of the imaging units  10 , the photoreceptor drum  11  is irradiated with a laser emitted from a laser scanning unit  14 , and thereby, an electrostatic latent image is written on the photoreceptor drum  11 . Then, the electrostatic latent image is developed by the corresponding developing device  40  into a toner image. Accordingly, toner images in the respective colors are formed on the photoreceptor drums  11 . The intermediate transfer unit  20  includes an endless intermediate transfer belt  21  to be driven to rotate in a direction indicated by arrow W. By electric fields applied from first transfer rollers  22  opposed to the respective photoreceptor drums  11 , the toner images formed on the photoreceptor drums  11  are transferred to the intermediate transfer belt  21  so as to be combined to form a composite image (first transfer). Such an electrophotographic process is well known, and a detailed description thereof is omitted. 
     In a lower section of the printer body, an automatic sheet feeder unit  30  is provided. The sheet feeder unit  30  is configured to feed recording sheets (which will be hereinafter referred to as sheets) one by one. A sheet is fed from a feed roller (not illustrated in the drawings) to a pair of timing rollers  31 , and further fed to a nip portion between the intermediate transfer belt  21  and a secondary transfer roller  25 . In the nip portion, the toner image (composite color image) is transferred to the sheet by an electric field applied from the secondary transfer roller  25  (second transfer). Thereafter, the sheet is fed to a fixing unit  35 , where the sheet undergoes a heating treatment for fixation of the toner image thereon. Then, the sheet is ejected to a printed-sheet tray  5  provided on an upper surface of the printer body. In this embodiment, the present invention is applied to the developing device  40  of the black imaging unit  10   k . However, the present invention is applicable also to the developing devices  40  of the other imaging units  10   y ,  10   m  and  10   c.    
     Structure of Developing Device; See FIGS.  2  and  3   
     The developing device  40  according to this embodiment uses a two-component developer containing carrier and toner. The developing device  40  receives a developer supplied from the hopper  15  while discharging excess developer. As illustrated in  FIG. 2 , the developing device  40  includes a developing roller  44 , a supply screw  46  and a stirring screw  48  arranged in parallel to one another in a casing  42 . In the following, the direction in which the developing roller  44 , the supply screw  46  and the stirring screw  48  extend is referred to as x-direction. The direction orthogonal to the x-direction and to the vertical direction is referred to as y-direction. 
     The developing roller  44  is a sleeve including a plurality of built-in magnets (not illustrated in the drawings) having different magnetic poles, and is driven to rotate keeping a slight space from the photoreceptor drum  11 . Underneath the developing roller  44 , a developer tank  50  is provided. As illustrated in  FIG. 3 , the developer tank  50  is a container elongated in the x-direction, and the developer tank  50  is divided into two circulation tanks  52  and  54  by a partition wall  56  extending in the x-direction. The partition wall  56  is arranged such that an opening  58  is formed at a negative end in the x-direction of the partition wall  56  and such that an opening  60  is formed at a positive end in the x-direction of the partition wall  56 . 
     The circulation tank  52  is a space from which a developer is supplied to the developing roller  44  and in which the used developer after development is collected. Therefore, at the positive end in the x-direction of the developer tank  52 , an outlet  66  for the used developer after development is provided. 
     The circulation tank  54  extends along the circulation tank  52  with the partition wall  56  in between. The circulation tank  54  is a space in which the developer circulating in the tank  50  and the developer resupplied thereto through a resupply port  68  formed therein are stirred and mixed. 
     The supply screw  46  is provided in the circulation tank  52 , and the stirring screw  48  is provided in the circulation tank  54 . Each of the supply screw  46  and the stirring screw  48  has a cylindrical rotary shaft extending in the x-direction, and vanes are provided on the outer circumference of the cylindrical rotary shaft so as to be arranged spirally. The supply screw  46  and the stirring screw  48  serve to convey the developer. 
     The supply screw  46  is divided into three portions. In a portion  46   a  of the supply screw  46 , which is from the negative end in the x-direction of the supply screw  46  to a point near the positive end in the x-direction of the supply screw  46  and farther in the negative x-direction than a middle point c 1  of the opening  60  with respect to the x-direction, vanes W 1  for conveying the developer in the positive x-direction are provided. The supply screw  46  also has a portion  46   b  next to the portion  46   a  and near the opening  60 . In the portion  46   b , no vanes are provided. The supply screw  46  still has a portion  46   c  next to the portion  46   b  and at the positive end in the x-direction of the supply screw  46 . In the portion  46   c , vanes W 2  are arranged spirally in the direction opposite to the spiral arrangement of the vanes W 1 . The opening  60  stretches from a point overlapping with the positive end in the x-direction of the portion  46   a  provided with the vanes W 1  to the positive end in the x-direction of the portion  46   c  provided with the vanes W 2 . The screw diameter d 1  of the portion  46   a  provided with the vanes W 1  is equal to the screw diameter d 2  of the portion  46   c  provided with the vanes W 2 . The pitch P 1  of the vanes W 1  is greater than the pitch P 2  of the vanes W 2 . The length L 1  (the size in the x-direction) of the portion  46   h  provided with no vanes is smaller than the pitch P 1  of the vanes W 1  and greater than a half of the pitch P 2  of the vanes W 2 . 
     As mentioned above, the stirring screw  48  is provided in the circulation path  54 . The stirring screw  48  has vanes throughout the full length thereof. The stirring screw  48  serves to circulate the developer in the developer tank  50  and to stir and mix the developer resupplied thereto through the resupply port  68  made in the circulation tank  54 . In this embodiment, the stirring screw  48  conveys the developer in the negative x-direction. 
     In the developer tank  50  of the developing device  40  having the structure above, the developer is conveyed in the positive x-direction in the circulation tank  52  by the supply screw  46  and comes into the stirring tank  54  through the opening  60 . The developer that has come into the stirring tank  54  is conveyed in the negative x-direction by the stirring screw  48  and returns to the circulation tank  52  through the opening  58 . Thus, the developer circulates in the developer tank  50  by flowing in the circulation tank  52  from the opening  58  to the opening  60  and flowing in the circulation tank  54  from the opening  60  to the opening  58 . 
     When the pressure of the developer in the circulation tank  52  exceeds a predetermined value, a part of the developer conveyed in the circulation tank  52  flows to the positive end in the x-direction of the circulation tank  52  across the vanes W 2  of the supply screw  46  without coming into the circulation tank  54  through the opening  60 . The part of the developer coming to the positive end in the x-direction of the circulation tank  52  is discharged to the outside of the developing device  40  through the outlet  66 . The portion of the circulation tank  52  having the outlet  66 , that is, the positive end portion in the x-direction of the circulation tank  52  that is farther in the positive x-direction than the vanes W 2  of the supply screw  46  is not connected to the circulation tank  54  in the y-direction. 
     Advantageous Effects 
     The developing device  40  according to the embodiment and the image forming apparatus  1  comprising the developing device  40  are capable of maintaining the amount of developer in the developer tank at an appropriate level. In the following, this is described by comparison to a comparative example, which is a developing device  100  fabricated by removing the shutter system from the conventional developing device.  FIGS. 6 and 9  are graphs indicating the relation between the rotation speed of the supply screw  46  and the amount of developer circulating in the developing device. In  FIGS. 6 and 9 , the vertical axis indicates the amount of developer circulating in the developing device, and the horizontal axis indicates the rotation speed of the supply screw  46 . In  FIGS. 6 and 9 , the solid line indicates the relation between the rotation speed of the supply screw  46  and the amount of developer circulating in the developing device in a case where the developer tank  50  is tilted in such a direction as to make it difficult to discharge the developer, and the broken line indicates the relation between the rotation speed of the supply screw  46  and the amount of developer circulating in the developing device in a case where the developer tank  50  is tilted in such a direction as to make it easy to discharge the developer. 
     The developing device  100  according to the comparative example is different from the developing device according to the embodiment in the shape of the supply screw  46 . In the supply screw  46  of the developing device  100 , as seen in  FIG. 4 , the vanes W 1  for conveying a developer in the positive x-direction are also provided near the opening  60 , while in the supply screw  46  of the developing device  40 , no vanes are provided near the opening  60 . In the developing device  100  having such a structure, when the rotation speed of the supply screw  46  is high, as illustrated in  FIG. 5 , the supply screw  46  exerts an excessive pressure on the developer, and the amount of developer discharged from the developing tank  50  becomes larger than the amount of developer resupplied to the developer tank  50 . Consequently, as indicated in  FIG. 6 , the amount of developer circulating in the developer tank  50  decreases drastically. However, when the rotation speed of the supply screw  46  is low, the pressure exerted by the vanes W 1  of the supply screw  46  on the developer is balanced with the pressure in the opposite direction exerted by the vanes W 2  on the developer. Further, as illustrated in  FIG. 7 , the pressure exerted by the supply screw  46  on the developer in the direction from the circulation tank  52  to the circulation tank  54  counteracts the pressure exerted by the stirring screw  48  on the developer in the direction from the circulation tank  54  to the circulation tank  52 . Accordingly, only an appropriate amount of developer is discharged from the developer tank  50 , and a decrease in the amount of developer circulating in the developer tank  50  is prevented. In the developing device  40  according to the embodiment, on the other hand, when the rotation speed of the supply screw  46  is high, as illustrated in  FIG. 8 , it is unlikely that the supply screw  46  exerts an excessive pressure on the developer because the supply screw  46  has a portion provided with no vanes near the opening  60 . Accordingly, it is unlikely that a large amount of developer is discharged through the outlet  66 . Thus, as indicated by  FIG. 9 , even when the rotation speed of the supply screw  46  is high, a sharp decrease of the amount of developer circulating in the developer tank  50  is prevented. When the rotation speed of the supply screw  46  is low, as illustrated in  FIG. 10 , a part of the developer flowing toward the circulation tank  54  is returned to the circulation tank  52  and flows to the outlet  66  because the supply screw  46  has a portion provided with no vanes near the opening  60 . Accordingly, in the developing device  40  according to the embodiment, when the rotation speed of the developer supply screw  46  is low, a part of the developer flowing toward the circulation tank  54  is returned to the circulation tank  52  and goes to the outlet  66  although the vanes W 1  of the supply screw  46  of the developing device  40  has a weaker force to convey the developer than the vanes W 1  of the supply screw  46  of the developing device  100 . Therefore, an appropriate amount of developer is discharged through the outlet  66 . Thus, in the developing device  40  according to the embodiment, it is possible to maintain the amount of developer in the developer tank  50  at an appropriate level regardless of the rotation speed of the supply screw  46 . 
     The developing device  40  according to the embodiment has a structure that no vanes are provided in a portion of the supply screw  46 , and this structure permits the amount of developer in the developer tank  50  to be maintained at an appropriate level. This structure is very simple as compared to the conventional developing device having a shutter system in a developer tank. Further, in the simple structure, there is almost no risk of causing the problems that are possibly caused by the shutter system, such as closure of the outlet with the developer. 
     In the developing device  40  according to the embodiment, further, the portion of the circulation tank  52  having the outlet  66  is not connected to the circulation tank  54  in the y-direction. Accordingly, the developer coming toward the outlet  66  across the vanes W 2  of the supply screw  46  is prevented from flowing into the circulation tank  54 . 
     Modification; See FIG.  11   
     A developing device  40 A according to a modification is different from the developing device  40  according to the above-described embodiment in the size of the opening  60  made in the partition wall  56 . Specifically, as illustrated in  FIG. 11 , the opening  60  of the developing device  40 A according to the modification stretches from a point overlapping with the positive end in the x-direction of the portion  46   a  provided with the vanes W 1  to the negative end in the x-direction of the portion  46   c  provided with the vanes W 2 . 
     Thus, the opening  60  of the developing device  40 A is smaller than the opening  60  of the developing device  40  according to the above-described embodiment. In the developing device  40 A with the smaller opening  60 , the portion  46   c  provided with the vanes W 2  is separated from the circulation tank  54 . Accordingly, the pressure of the vanes W 2  to push back the developer in the negative x-direction is prevented from diverging toward the circulation tank  54 , and appropriate circulation of the developer in the developer tank  50  is ensured. 
     OTHER EMBODIMENTS 
     Developing devices and image forming apparatuses according to the present invention are not limited to the embodiment and the modification described above. The shapes of the openings and the details of the vanes of the screws may be arbitrarily designed. 
     Although the present invention has been described in connection with the preferred embodiments above, it is to be noted that various changes and modifications may be obvious to persons having ordinary skill in the art. Such changes and modifications are to be understood as being within the scope of the present invention.