Abstract:
An image forming method and apparatus includes a development device in which two rotary shafts enclosed in respective separate chambers for spirally circulating the developer between the two chambers to supply the developer to a development roller. The development device is provided with an extended circulation enclosure having a fill opening communicating with the two chambers.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a method and apparatus for image forming, and more particularly to a method and apparatus for image forming that is capable of performing a stable image development. 
   2. Discussion of the Background 
   Conventional image forming apparatuses form an electrostatic latent image and develop it into a toner image using a developer including toner. In the development process, the developer needs to be transported from a container to a surface of a development roller which supplies the developer to the electrostatic latent image, thereby forming a toner image. For the transportation of the developer to the development roller, two rotary transportation members are typically used. One of the members transports the developer along the surface of the development roller from one end to the other end of the surface of the development roller in an axis direction of the development roller. Consequently, the developer is primarily supplied to the surface of the development roller and a great part of the developer amount is transported to the other end of the surface of the development roller. The other rotary transportation member transports the developer in the reverse direction along the surface of the development roller so that the developer is sufficiently circulated. 
   In the above-described development mechanism, however, a part of the transported developer tends to stay and to build up at a place out of the end of the development roller without being used for development. 
   SUMMARY OF THE INVENTION 
   This patent specification describes a novel image forming apparatus which improves an image quality by reducing an occurrence of uneven toner density. In one example, a novel image forming apparatus includes a photoconductive member and a development device. The photoconductive member is configured to carry an electrostatic latent image thereon. The development device is configured to develop the electrostatic latent image formed on the photoconductive member into a visual image with a developer. The development device includes a main housing, an extended housing, a development roller, and first and second rotary shafts. The main housing has first and second side frames facing each other and a first partition for longitudinally dividing an inside space into first and second chambers communicating with each other through first and second openings formed in the first partition near the first and second side frames, respectively. The extended housing is connected to the first side frame of the main housing and has an extension side frame distantly facing the first side frame of the main housing and a second partition continuously formed in line with the first partition for making the first and second chambers of the main housing extended with an extension opening in the second partition near the extension side frame for communicating to the first and second chambers. The development roller has first and second ends held by the first and second side frames, respectively. The first rotary shaft has an extended length held by the first and second side frames and the extension side frame, is arranged in parallel to and immediately next to the development roller, and is configured to rotate to transport the developer along a longitudinal surface of the development roller in a first direction from the second end to the first end of the development roller and to the second chamber through the first opening and the extension opening. The second rotary shaft has an extended length held by the first and second side frames and the extension side frame, is arranged in parallel to and next to the development roller via the first rotary shaft, and is configured to rotate to transport the developer in a second direction opposite to the first direction and to the first chamber through the second opening. 
   The first opening may be smaller than the extension opening. 
   The extended housing may include a fill opening arranged at an extended position extended outside from the first opening of the main housing and is configured to communicate with the first and second chambers and to fill the developer. 
   Each of the first and second rotary shafts may include a spiral wing for transporting the developer. 
   This patent specification further describes a novel image developing apparatus which develops an electrostatic latent image formed on a photoconductive member into a visual image with a developer in an image forming apparatus. The development device includes a main housing, an extended housing, a development roller, and first and second rotary shafts. The main housing has first and second side frames facing each other and a first partition for longitudinally dividing an inside space into first and second chambers communicating with each other through first and second openings formed in the first partition near the first and second side frames, respectively. The extended housing is connected to the first side frame of the main housing and has an extension side frame distantly facing the first side frame of the main housing and a second partition continuously formed in line with the first partition for making the first and second chambers of the main housing extended with an extension opening in the second partition near the extension side frame for communicating to the first and second chambers. The development roller has first and second ends held by the first and second side frames, respectively. The first rotary shaft has an extended length held by the first and second side frames and the extension side frame, is arranged in parallel to and immediately next to the development roller, and is configured to rotate to transport the developer along a longitudinal surface of the development roller in a first direction from the second end to the first end of the development roller and to the second chamber through the first opening and the extension opening. The second rotary shaft has an extended length held by the first and second side frames and the extension side frame, is arranged in parallel to and next to the development roller via the first rotary shaft, and is configured to rotate to transport the developer in a second direction opposite to the first direction and to the first chamber through the second opening. 
   This patent specification further describes a novel image forming method used in an image forming apparatus which develops an electrostatic latent image formed on a photoconductive member into a visual image with a developer in an image forming apparatus. In one example, a novel method includes the steps of rotating, circulating, bypassing, and adding. The rotating step rotates a development roller having first and second longitudinal ends. The circulating step circulates a developer with first and second rotary wing shafts through a main circulating loop having a first passage directly facing a circumferential surface of the development roller and a second passage isolated from the first passage by a first partition. The first passage has a first upstream area arranged in a vicinity of the first end of the development roller and a first downstream area arranged in a vicinity of the second end of the development roller. The second passage has a second upstream area and a second downstream area. The first and second passages communicate with each other between the first upstream area and the second downstream area and between the first downstream area and the second upstream area. The circulating step comprising sub-steps of transporting and returning. The transporting step transports the developer with the first rotary wing shaft from the first upstream area to the first downstream area through the first passage. The returning step returns the developer with the second rotary wing shaft from the second upstream area to the second downstream area through the second passage. The bypassing step bypasses the developer through a bypass loop with extended portions of the first and second rotary wing shafts extended respectively in line in an outward direction from the second end of the development roller from the first downstream area to the second upstream via a returning area of the bypass loop. The adding-step adds the developer into the bypass loop through the returning area. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
       FIG. 1  is a schematic diagram of an image forming apparatus according to an embodiment of the present invention; 
       FIG. 2  is a diagrammatic perspective view of a development unit included in the image forming apparatus of  FIG. 1 ; 
       FIG. 3  is another diagrammatic perspective view of the development unit with a housing partly in cross section; 
       FIG. 4  is a diagrammatic perspective view of the housing of  FIG. 3 ; and 
       FIG. 5  is an illustration for explaining a flow of the developer in the development unit of  FIG. 2 . 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   In describing preferred embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner. Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, particularly to  FIG. 1 , an image forming apparatus according to a preferred embodiment of the present invention is described. The image forming apparatus illustrated in  FIG. 1  is a color copying machine using a tandem type development system, as one example. The image forming apparatus forms a color image on a recording sheet with black, yellow, magenta, and cyan color toners, and applies two-component type developer including toner and carrier to an image development. As shown in  FIG. 1 , the image forming apparatus includes an image forming unit  100 , a sheet cassette table  200 , a scanner  300 , and an automatic document feeder (ADF)  400 . 
   The image forming unit  100  includes an intermediate transfer member  10  which is formed in an endless belt shape and is provided at an approximately center of the image forming unit  100 . The intermediate transfer member  10  includes a base layer made of materials including a less-extensible fluorocarbon resin or a less-extensible muslin with extensible rubber material, for example. The intermediate transfer member  10  further includes an elastic layer which overlays the base layer. The elastic layer includes a fluorocarbon rubber or acrylonitrile-butadiene copolymer rubber, for example. The surface of the elastic layer is coated with a fluorocarbon resin, for example, thereby forming a smooth-surface coat layer. 
   The intermediate transfer member  10 , as illustrated in  FIG. 1 , is extended among rollers  14 ,  15 , and  16  for rotation in a clockwise direction indicated by an arrow. 
   The image forming unit  100  is provided with a cleaning device  17  for removing the residual developer after the development process from the surface of the intermediate transfer member  10 . As one example, the cleaning device  17  is disposed at a place next to the roller  15 . 
   The image forming unit  100  is further provided with four image forming mechanisms  18  for yellow, cyan, magenta, and black color toners, laid in line on a straight portion of the intermediate transfer member  10  extended between the rollers  14  and  15  in a moving direction of the intermediate transfer member  10 . This arrangement of the four image forming mechanisms  18  is referred to as a tandem layout, and reference numeral  20  of  FIG. 1  denotes a tandem image forming unit. Over such a tandem image forming unit  20 , an optical unit  21  is installed. 
   Each one of the image forming mechanisms  18  includes a photoconductor  40  and various associated components such as, a charging device (not shown), a development device  41 , a primary transfer device  62 , a photoconductor cleaning device (not shown), and a discharging device (not shown), for example. These components are disposed around the photoconductor  40 . 
   The image forming unit  100  is further provided with a secondary transfer device  22  disposed opposite to the tandem image forming unit  20  relative to the intermediate transfer member  10 . As shown in  FIG. 1 , the secondary transfer device  22  includes a belt  24  extended between two rollers  23 . One of the rollers  23  is disposed close to the roller  16 . The secondary transfer device  22  is caused to operate such that the belt  24  pushes the roller  16  via the intermediate transfer member  10 , thereby transferring a toner image carried by the intermediate transfer member  10  to a recording sheet. 
   The image forming unit  100  is further provided with a fixing device  25  disposed next to the secondary transfer device  22  for fixing a transferred image held on a recording sheet transported from the sheet cassette table  200 . The fixing device  25  has a structure in which a pressure roller  27  is held in contact with an endless rotary fixing belt  26  under pressure. 
   The secondary transfer device  22  has, in addition to the above-described image transfer function, a transportation function for transporting the recording sheet to the fixing device  25 . Although the image transfer function can be achieved by an alternative mechanism such as a transfer roller, a non-contact type charger, or the like, it would be difficult for such an alternative mechanism to carry out also the transportation function. 
   The image forming unit  100  is further provided with a sheet turnover device  28  placed under the secondary transfer device  22  and the fixing device  25  and in parallel with the tandem image forming unit  20 . The sheet turnover device  28  turns the recording sheet to allow a duplex print for print on both sides of the recording sheet. 
   To make a copy of an original with the above-described image forming apparatus, the original is placed on an input tray  30  of the ADF  400 . As an alternative way, the original can be set directly on a contact glass  32  of the scanner  300  by lifting up the ADF  400  to a preparatory position and is held by lowering the ADF  400  to a set position. 
   Upon a depression of a start key (not shown), the original placed on the input tray  30  is transported to the contact glass  32 . Then, the scanner  300  is started to move first and second sliding members  33  and  34 . 
   When the original is set directly on the contact glass  32 , the scanner  300  is immediately started to move the first and second sliding members  33  and  34  upon a depression of the start key. 
   The first sliding member  33  irradiates light from a light source (not shown), receives with a mirror (not shown) the light reflected by the original and further reflects the light towards a mirror (not shown) of the second sliding member  34 . The light is then reflected by the second sliding member  34  towards a sensor  36  through an imaging lens  35 . Thus, the original is read and image data is obtained. 
   In parallel to the above-described operations, upon the depression of the start key, one of the rollers  14 - 16 , which is a drive roller, is started to be driven to rotate the intermediate transfer member  10  with the remaining two of the rollers  14 - 16 , which are free rollers. At the same time, the photoconductive members  40  of the image forming devices  18  are started to be rotated in a counterclockwise direction as indicated in  FIG. 1 . 
   The image data read from the original is, after execution of various signal processing, converted via an image modulation into laser light beams representing black, yellow, magenta, and cyan color image signals. The laser light beams are emitted to the photoconductive members  40  to form respective electrostatic latent images in black, yellow, magenta, and cyan colors. Then, the development devices  41  which contain the two-component type developer develop the respective electrostatic latent images with the black, yellow, magenta, and cyan color toners into black, yellow, magenta, and cyan color toner images, respectively. These color toner images are sequentially transferred onto the intermediate transfer member  10  so as to overlay one on another to form them into a single synthetic color image, as the intermediate transfer member  10  rotates. 
   Further, in parallel to the above-described operations, upon the depression of the start key, one of pickup rollers  42  of the sheet cassette table  200  is selected and is driven so as to move an uppermost recording sheet towards a separation roller  45  in a corresponding one of sheet cassettes  44  contained in a sheet bank  43 . At this time, a few recording sheets may be moved together with the uppermost recording sheet due to a friction force. The separation roller  45  separates the uppermost recording sheet from the rest and feeds it into a sheet transportation passage  46 . The recording sheet guided into the sheet transportation passage  46  is further forwarded by a plurality of feed rollers  47  into a sheet transportation passage  48  disposed in the image forming unit  100 . Through the sheet transportation passage  48 , the recording sheet is moved to a registration roller  49  and is then temporarily stopped by the registration roller  49 . 
   Alternatively, a recording sheet can be inserted into the image forming unit  100  manually from an auxiliary input tray  51 . In this case, upon the depression of the start key, a feed roller  50  is started to be driven to transfer the recording sheet placed on the auxiliary input tray  51  towards a separation roller  52 . As in the case of feeding from the sheet cassette, a few recording sheets may be moved together with an uppermost recording sheet when a number of sheets are placed. Then, the separation roll  52  separates and feeds further the uppermost recording sheet to the registration roller  49 . 
   The registration roller  49  which is stopping the recording sheet is started to be driven to restart the transportation of the recording sheet in synchronism with a movement of the synthetic color toner image carried on the intermediate transfer member  10 . The recording sheet is inserted between the intermediate transfer member  10  and the secondary transfer device  22 . As the recording sheet is advanced in contact with the intermediate transfer member  10 , the synthetic color toner image is transferred from the intermediate transfer member  10  onto the recording sheet by an action of the secondary transfer device  22 . 
   After the image transfer process, the secondary transfer device  22  further transports the recording sheet to the fixing device  25 . The fixing device  25  fixes the color toner image onto the recording sheet with heat and pressure. Then, the recording sheet having the fixed color toner image thereon is forwarded to an ejection roller  56  via a switch pawl  55  and is ejected to an output tray  57  by the ejection roller  56 . When the switch pawl  55  is set to a duplex position, the recording sheet is guided to a different passage so as to be forwarded to the sheet turnover device  28 . The sheet turnover device  28  turns the recording sheet so that a blank surface of the recording sheet is to face the intermediate transfer member  10  during a second image forming process. Then, the reversed recording sheet is transported again to the registration roller  49  for the second image forming process. After the second image forming process, the recording sheet having images on the both sides is caused to pass through the fixing device  25  and is ejected via the switch pawl  55  and the ejection roller  56  to the output tray  57 . 
   During the secondary image transfer, it is possible that some toner particles are not transferred and remain on the intermediate transfer member  10  even after the secondary image transfer. The cleaning device  17  removes these residual toner particles from the surface of the intermediate transfer member  10 . Thus, the tandem image forming unit  20  becomes ready for the next image forming process. 
   The registration roller  49  may possibly be connected to a bias voltage to remove dust of the recording sheets, although the registration roller  49  is generally grounded. 
   Referring to  FIGS. 2-5 , the development device  41  is explained. As illustrated in  FIGS. 2 and 3 , the development device  41  includes a development roller  70 , a housing  71 , a first partition wall  72 , an extended portion  73 , a first rotary shaft  74 , and a second rotary shaft  75 . The development device  41  further includes a fill opening  76 , a pass-through opening  77 , a first circulation opening  78 , a second partition wall  79 , and a second circulation opening  80 . The housing  71  includes a first side portion  71   a  and a second side portion  71   b . The partition  72  is disposed between the first and second rotary shafts  74  and  75 . The extended portion  73  is extended outward from the first side portion  71   a . The first and second rotary shafts  74  and  75  have a length across the housing  71  and the extended portion  73  and are provided with a spirally-formed screw. The first rotary shaft  74  is arranged immediately next to the development roller  70 , and the second rotary shaft  75  is arranged opposite to the development roller  70  relative to the first rotary shaft  74 . The fill opening  76  is formed in the extended portion  73 . The pass-through opening  77  is formed between the second side portion  71   a  and one vertical side edge of the first partition  72 . The first circulation opening  78  is formed between the first side portion  71   b  and the other vertical side edge of the first partition  72 . The second partition  79  is provided inside the extended portion  73 . The second circulation opening  80  is formed inside the extended portion  73  at an area opposite from the first side portion  71   a  relative to a vertical edge of the second partition  79 . 
   As illustrated in  FIG. 4 , the first partition  72  is configured to stand on a bottom and in an approximate middle part of the housing  71  to divide an inside space of the housing  71  in a longitudinal direction. The second partition  79  is configured to continue in line with the first partition  72  so as to divide an inside space of the extended portion  73  in the longitudinal direction. 
   As illustrated in  FIG. 5 , the first side portion  71   a  of the housing  71  is provided with a first hole  81  which the first rotary shaft  74  passes through and a second hole  82  which the second rotary shaft  75  passes through. In this example, the second hole  82  has a diameter greater than that of the first hole  81 . In addition, the first circulation opening  78  is smaller than the second circulation opening  80 . 
   The first and second rotary shafts  74  and  75  are arranged facing each other in parallel with the first and second partition  72  and  79  in between. The second side portion  71   b  and the outermost side portion of the extended portion  73  have sets of bearings (not shown) for supporting the first and second rotary shafts  74  and  75 . 
   The development roller  70  is arranged next to and slightly above the first rotary shaft  74  and is supported by sets of bearings (not shown) provided to the first and second side portions  71   a  and  71   b . In addition, the development device  41  is provided with a mixing roller (not shown) under the development roller  70 . 
   The development roller  40 , the first and second rotary shafts  74  and  75 , and the mixing roller are engaged with each other to rotate together, via gears provided them outside the second side portion  71   b.    
   Referring to  FIG. 5 , a flow of the developer in the development device  41  is explained. For the explanation purpose, a space surrounding the first rotary shaft  74  is referred to as a first space and a space for the second rotary shaft  75  is as a second space. When the first rotary shaft  74  is driven, the developer in the first space is moved in an axial direction towards the first side portion  71   a  along the rotation of the first rotary shaft  74 . When the second rotary shaft  75  is driven, the developer in the second space is moved in an axial direction towards the second side portion  71   b  along the rotation of the second rotary shaft  75 . That is, the developer inside the first space is flown by the spiral movement of the rotary shaft  74  from the area of the second side portion  71   b  to the area of the first side portion  71   a  and further to the extended portion  73  through the first hole  81 . The developer transported inside the extended portion  73  is further moved towards the outermost side portion of the extended side portion  73  and is transported into the second space through the second circulating opening  80 . The developer entered into the second space is further transported by the spiral rotation of the second rotary shaft  75  to the beyond the first side portion  71   a  through the second hole  82 . The developer traveled through the second hole  82  is further advanced towards the second side portion  71   b  and is eventually returned to the starting point in the first space through the pass-though opening  77 . In addition, a part of the developer is moved from the first space to the second space through the first circulation opening  78 . In this way, the development device  41  continuously circulates the developer from the first space to the second space to pass along the surface of the development roller  70  in one direction. 
   The developer supplied through the fill opening  76  is mixed with the carrier and the toner remaining inside the development device  41 , in the second space in the extended portion  73 . The supplied developer is then transported towards the second side portion  71   b  along the spiral rotation of the second rotary shaft  75  and is entered into the first space through the pass-through opening  77 . Thus, the supplied developer is provided to the development roller  70 . The development roller  70  transports the developer to the development process and carries back unused portion of the developer to the first space. The returned developer is again circulated through the first and second spaces. 
   A portion of the development roller  70  held by the first side portion  71   a  is referred to as a downstream edge portion and an opposite portion held by the second side portion  71   b  is referred to as an upstream edge portion. As described above, the circulation passage including the first and second spaces is provided with the first circulation opening  78  in the vicinity of the downstream edge portion of the development roller  70  and the second circulation opening  80  in the vicinity of the edge portion of the first rotary shaft  74  which is beyond the downstream edge portion of the development roller  70 . The developer contained inside the development device  41  can be moved from the first space to the second space through the first and second circulation openings  78  and  80 . In particular, the developer which usually tends to stay around the downstream edge portion of the development roller  70  in the first space due to the presence of the first side portion  71   a  as a barrier can easily and smoothly escape to the second space through the first circulation opening  78 . Thereby, the amount of the developer staying around the downstream edge portion of the development roller  70  is reduced. 
   As a consequence, the developer is prevented from clogging around an area in the vicinity of the downstream edge portion of the development roller  70  and is smoothly circulated. Thereby, it becomes possible to stably supply the developer to the development roller  70 . As a result, an uneven toner density which sensitively appears on an image is prevented. 
   Extension of the space by the extended portion  73  increases the amount of the developer which lengthens the maintenance cycle of the image forming apparatus, resulting in a reduction of maintenance cost. 
   The balance of the first and second circulation openings  78  and  80  in size, that is, the first circulation opening  78  is smaller than the second circulation opening  80  is critical. If this balance is set other way around, the main circulation will be made between the first circulation opening  78  and the pass-through opening  77  without conducting a sufficient mixture with the newly supplied developer. As a result, a problem of an uneven toner density will be caused. 
   In addition, making the distance between the fill opening  76  and the pass-through opening  77  longer is also critical. This is because a sufficient mixture of the newly supplied developer with the existing developer needs a reasonable time. If an insufficiently mixed developer is given to the development roller  70 , a problem of an uneven toner density would be caused. Therefore, the development device  41  is provided with the extended portion  73  having the fill opening  76  to mix the newly supplied developer sufficiently with the existing developer. 
   Further, supplying the developer to the development roller  70  in the way of such a systematic circulation, as described above, is made primarily by dividing the inside space of the development device  41  into the first and second spaced with the first and second partitions  72  and  79 . With this structure, a sufficient mixture of the newly supplied developer with the existing developer is achieved. 
   Although the present invention applies to the image forming apparatus which forms a color image on a recording sheet with black, yellow, magenta, and cyan color toners, and applies two-component type developer including toner and carrier to an image development, it is possible to apply the present invention to other image forming apparatuses which form a black and white image and also those which use a single component type developer including toner. 
   The above-described embodiments are illustrative, and numerous additional modifications and variations are possible in light of the above teachings. For example, elements and/or features of different illustrative and exemplary embodiments herein may be combined with each other and/or substituted for each other within the scope of this disclosure and appended claims. It is therefore to be understood that within the scope of the appended claims, the disclosure of this patent specification may be practiced otherwise than as specifically described herein. 
   This patent specification is based on Japanese patent application, No. 2004-178308 filed on Jun. 16, 2004, in the Japan Patent Office, the entire contents of which are incorporated by reference herein.