Abstract:
An image forming apparatus includes a development mechanism using a two-component development agent and which includes a development roller and first and second transfer screws. The first transfer screw, arranged at a position approximately horizontal and in parallel to the development roller, includes a first number of screw spirals for transferring the development agent from a first end to a second end of the first transfer screw and conveying the development agent to the development roller. The second transfer screw, arranged approximately horizontal and in parallel to the first transfer screw, includes a second number of screw spirals for transferring the development agent from a first end to a second end of the second transfer screw and conveying the development agent from the second end of the second transfer screw to the first end of the first transfer screw. The first and second numbers of the screw spirals are different from each other.

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 can perform an effective mixing of development agent. 
     2. Discussion of the Background 
     In a recent background image forming apparatus shown in FIG. 1, a development mechanism using a two-component development agent typically includes a development roller  5 , and transfer screws  6  and  7  for transferring the development agent inside a housing (not shown) of the development mechanism arranged adjacent to an image carrying member  12 . In this type of the two-component development mechanism, the transfer screws  6  and  7  are arranged in parallel to each other and one of the two is placed close to and in parallel to the development roller  5  in the width direction thereof. The other of the two is placed farther from, and in parallel to, the development roller  5  in the width direction thereof. The development agent is circulated using the thus-configured two transfer screws  6  and  7 . 
     The transfer screws  6  and  7  perform mixture and transfer of the development agent in a superior manner in a direction X in parallel to a length direction of the development roller  5 . However, the transfer of the development agent in a direction Y orthogonal to the length direction of the development roller  5  is not performed in a manner as good as that performed in the direction X. 
     Accordingly, the transfer screw  6  arranged closer to the development roller  5  is prone to cause an uneven density in a form of a screw pitch when a great amount of toner is being supplied. Particularly, in a color copying machine, the uneven density appears as an unevenness of color and therefore it greatly reduces image quality. 
     SUMMARY OF THE INVENTION 
     The present invention provides a novel image forming apparatus. In one example, a novel image forming apparatus includes a development mechanism using a two component development agent and which includes a development roller and first and second transfer screws. The first transfer screw includes a first number of screw spirals for transferring the development agent from a first end of the first transfer screw to a second end of the first transfer screw and conveying the development agent to the development roller. The first transfer screw is arranged approximately horizontal and in parallel to the development roller. The second transfer screw includes a second number of screw spirals for transferring the development agent from a first end of the second transfer screw to a second end of the second transfer screw and conveying the development agent from the second end of the second transfer screw to the first end of the first transfer screw. The second transfer screw is arranged approximately horizontal and in parallel to the first transfer screw. In this development mechanism, the first and second numbers of the screw spirals are different from each other. 
     The first number of the screw spirals may be greater than the second number of screw spirals. 
     The first transfer screw may have a first spiral pitch which is greater than a second spiral pitch of the second transfer screw. 
     In the above-mentioned image forming apparatus, when the first and second transfer screws have spiral diameters equal to each other and shaft diameters equal to each other, the first and second transfer screws may satisfy relationships of: 
     
       
           P   1 ≧ 1 . 5 × P   2 ; and 
       
     
     
       
           S   1 ≧ 4 × S   2 , 
       
     
     wherein P 1  and S 1  represent the first spiral pitch and the first screw spirals, respectively, of the first transfer screw and P 2  and S 2  represent the second spiral pitch and the second screw spirals, respectively, of the second transfer screw. 
     The first and second transfer screws may be rotated in directions different from each other. 
     The present invention further provides a novel method of transferring development agent. In one example, a novel method includes the steps of providing, placing, rotating, and reverse rotating. The providing step provides a first transfer screw at a position approximately horizontal and in parallel to a development roller. The first transfer screw has a first number of screw spirals. The placing step places a second transfer screw at a position approximately horizontal and in parallel to the first transfer screw. The second transfer screw has a second number of screw spirals in parallel to the first transfer screw. In this case, the second number of screw spirals is different from the first number of screw spirals. The rotating step rotates the first transfer screw. The reverse rotating step rotates the second transfer screw in a reverse direction relative to the rotation of the first transfer screw to transfer the development agent in a direction from a first end of the second transfer screw to a second end of the second transfer screw and in a direction from the second transfer screw to the first transfer screw so that the development agent is consequently transferred by the first transfer screw in a direction from a first end of the first transfer screw to a second end of the first transfer screw and in a direction from the first transfer screw to the development roller. 
     The first number of the screw spirals may be greater than the second number of the screw spirals. 
     The first transfer screw may be a first spiral pitch which is greater than a second spiral pitch of the second transfer screw. 
     In the above-mentioned method, when the first and second transfer screws have spiral diameters equal to each other and shaft diameters equal to each other, the first and second transfer screws satisfy relationships of: 
     
       
           P   1 ≧ 1 . 5 × P   2 ; and 
       
     
     
       
           S   1 ≧ 4 × S   2 , 
       
     
     wherein P 1  and S 1  represent the first spiral pitch and the first screw spirals, respectively, of the first transfer screw and P 2  and S 2  represent the second spiral pitch and the second screw spirals, respectively, of the second transfer screw. 
     The first and second transfer screws may be rotated in directions different from each other. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the present application 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 perspective schematic view of a background development mechanism used in an image forming apparatus; 
     FIG. 2 is a schematic cross-sectional side view of a color image forming apparatus according to an embodiment of the present invention; 
     FIG. 3 is a schematic cross-sectional side view showing a multi-color development section of the color image forming apparatus of FIG. 1; 
     FIG. 4 is a schematic cross-sectional side view showing one of development units included in the multi-color development station of FIG. 2; 
     FIG. 5 is a schematic side view showing a first transfer screw included in each of the development units of FIG. 3; and 
     FIG. 6 is a schematic side view showing a second transfer screw included in each of the development units of FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In describing preferred embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the invention 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 which 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. 2, a color image forming apparatus  10  according to an embodiment of the present invention is described. In FIG. 2, the color image forming apparatus  10  is provided with a drum-shaped image carrying member  12  arranged at a position slightly right from the center inside an exterior housing  11 . A charger  13  is arranged above the image carrying member  12 . From the charger  13 , a revolving development station  14 , an intermediate transfer unit  15 , and a cleaning unit  16  are arranged around the image carrying member  12  in this order in a counterclockwise direction. A laser writing unit  18  is arranged above the charger  13 , the revolving development station  14 , and the cleaning unit  16 . 
     The revolving development station  14  includes, as shown in FIG. 3, development units  22   a ,  22   b ,  22   c , and  22   d  that develop yellow, magenta, cyan, and black toner images, respectively, and toner cartridges  23   a ,  23   b ,  23   c , and  23   d  that contain yellow, magenta, cyan, and black toner, respectively. 
     Each of the development units  22   a - 22   d  includes a development case  24  and, inside the development case  24 , a development roller  25 , a first transfer screw  26 , a second transfer screw  27 , and a development doctor  28 . The toner in a chamber of the second transfer screw  27  is transferred by the second transfer screw  27  into a chamber of the first transfer screw  26  and is then transferred by the first transfer screw  26  to the development roller  25 . The development doctor  28  regulates a thick toner layer deposited on the surface of the development roller  25  into a thin uniform toner layer. 
     Each of the toner cartridges  23   a - 23   d  is detachably mounted to a cartridge guide (not shown) to which a toner supply screw  30  is provided so that the toner supply screw  30  sends the toner in the corresponding toner cartridge into the development case  24 . 
     The thus-arranged revolving development station  14  is rotated about a center axis  31  provided at a center of a development supporting member (not shown) so that the development rollers of the development units  22   a - 22   d  selectively face the circumferential surface of the image carrying member  12 . 
     As shown in FIG. 2, the intermediate transfer unit  15  includes an endless-belt-shaped intermediate transfer member  34  extended around a plurality of rollers  33 . A part of the intermediate transfer member  34  is in contact with the image carrying member  12 . Around the intermediate transfer member  34  are provided a transfer unit  35  and a belt cleaning unit  36 . The belt cleaning unit  36  is movably mounted so that it can be attached and detached. 
     The laser writing unit  18  generates laser light L and uses it to write an image in accordance with data obtained with an optical reading unit  38 . As a result, an electrostatic latent image is formed on the surface of the image carrying member  12 . The optical reading unit  38  reads an original placed on a contact glass  39  provided at a top portion of the color image forming apparatus  10 . 
     As shown in FIG. 2, the color image forming apparatus  10  further includes a plurality of feed rollers  41 , a pair of registration rollers  42 , a feed belt  43 , a fixing unit  44 , a pair of ejection rollers  45 , a pair of ejection rollers  45 , a recording media cassette  47 , and a manual insertion tray  48 . The plurality of feed rollers  41  and the pair of registration rollers  42  are arranged upstream from the intermediate transfer unit  15  and the transfer unit  35  in a sheet direction along a transfer path (not shown) for transferring a recording media. The feed belt  43 , the fixing unit  44 , and the pair of ejection rollers  45  are arranged downstream from the intermediate transfer unit  15  and the transfer unit  35  in the sheet direction along the transfer path. 
     To reproduce a color image on the above-described color image forming apparatus  10 , a start switch (not shown) provided thereto is pressed after an original is first placed on the contact glass  39 , and a recording media sent from the recording media cassette  47  is guided to the above-mentioned transfer path via an automatic sheet feeding unit (not shown) or a recording media placed in the manual insertion tray  48  is sent to the transfer path via a manual feed path (not shown). Then, the recording media is transferred to the registration rollers  42  by the transfer rollers  41 . 
     During the above-mentioned process, the optical reading unit  38  is activated to scan the surface of the original placed on the contact glass  39  so as to read the image of the original. Also, as the image carrying member  12  is rotated in the counterclockwise direction in FIG. 2, the intermediate transfer member  34  is rotated in the clockwise direction with the rotation of the plurality of rollers  33 . During the rotation of the image carrying member  34 , the surface thereof is evenly charged with the charger  13  and is exposed to the laser light L emitted in accordance with the information read by the optical reading unit  38  so that an electrostatic latent image representing the first color is formed on the image carrying member  12 . 
     Then, the above first color latent image is visualized with toner by the development unit  22   a  that contains the first color toner so that the first color image is formed on the image carrying member  12 . This mono-color toner image is transferred onto the intermediate transfer member  34  of the intermediate transfer unit  15 . After the transfer process, the surface of the image carrying member  12  is discharged with a precleaning charger (not shown) for increasing the cleaning effect and the residual first color toner is cleaned by the cleaning unit  16 . After that, the surface of the image carrying member  12  is again discharged with a pre-charging discharger (not shown). 
     The image carrying member  12  continues to be rotated for the second rotation, and the surface thereof is evenly charged with the charger  13  for the second time. The second color electrostatic latent image is written on the surface of the image carrying member  12  and the second color latent image is visualized with toner by the development unit  22   b  that contains the second color toner so that the second color image is formed on the image carrying member  12 . This mono-color toner image is transferred onto the intermediate transfer member  34  of the intermediate transfer unit  15  such that the first mono-color toner image is overlaid by the second mono-color toner image. After such transfer process, the surface of the image carrying member  12  is cleaned after the pre-cleaning discharging and is again discharged. 
     In a way similar to the above, the third mono-color toner image is formed on the surface of the image carrying member  12  and is transferred onto the intermediate transfer member  34 . If a black toner image is required, it is also formed on the surface of the image carrying member  12  and is transferred onto the intermediate transfer member  34 . In this way, a combined color image is consequently formed in accordance with the information read from the original on the intermediate transfer member  34 . 
     After that, the registration rollers  42  are rotated in synchronism with the rotation of the intermediate transfer member  34  so that the recording media which is stopped by the registration rollers  42  is advanced to the intermediate transfer unit  15  and the transfer unit  35 . The color image conveyed on the intermediate transfer member  34  is transferred onto the recording media. The recording media having the color image thereon is moved to the fixing unit  44  by the feed belt  43  and the color image is fixed on the recording media with heat and pressure by the fixing unit  44 . After the fixing process, the recording media is ejected by the ejection rollers  45  to an ejection tray (not shown) in which the recording media is stacked one after another. 
     After the transfer process, the intermediate transfer member  34  is cleaned by the belt cleaning unit  36  so that the residual toner deposited on the surface of the intermediate transfer member  34  is removed and the surface of the intermediate transfer member  34  is discharged by a discharging unit (not shown). 
     FIG. 3 shows a multi-color development section  50  including the above-described revolving development station  14 . The multi-color development section  50  is movably mounted in the color image forming apparatus  10  so that the revolving development station  14  can be pulled out from the exterior housing  11 . That is, when the multi-color development section  50  is pulled out from the exterior housing  11 , the revolving development station  14  can be moved at an accessible position at which each of the development units  22   a - 22   d  can be removed. 
     As described above, the development roller  25 , the first and second transfer screws  26  and  27 , and the development doctor  28  are included in each of the development units  22   a - 22   d . The development roller  25  includes a magnet roller and a development sleeve, both which are not shown. The development agent including a magnetic carrier is conveyed by a rotation of the development sleeve of the development roller  25 . 
     The magnet roller of the development roller  25  internally includes magnets and is fixed. Magnetic poles of the above magnets of the magnet roller include poles P 1 -P 5  arranged as shown in FIG.  4 . The magnetic pole P 1  is referred to as a development pole positioned closest to the image carrying member  12 . The magnetic poles P 2  and P 3  have the same polarity so that the development agent is released from the development roller  25  at an area between the magnetic poles P 2  and P 3  but is again attracted to the development roller  25  by the magnetic pole P 3 . The magnetic pole P 4  is arranged directly under the doctor blade  28 , and a convection space for receiving the development agent is provided at a position upstream from the development doctor  28  in a direction of a flow of the development agent. In the above convection space, the development agent receives more charges by the action of friction so that the development agent rises on the surface of the magnet roller  25  when it is attracted thereto. 
     Since the first and second transfer screws  26  and  27  are configured to move the development agent in directions different from each other while transferring it toward the development roller  25 , the development agent is sufficiently mixed when it reaches the above-mentioned convection space. The development agent thus mixed is moved in a direction A by the magnetic force of the development roller  25  and is regulated by the doctor blade  28  so that an amount of the development agent necessary for the development process is passed through a doctor gap formed between the doctor blade  28  and the surface of the development roller  25 . The excess amount of the development agent rejected by doctor blade  28  is returned in a direction B. The development agent passing through the above doctor gap is subjected to the development process at a development position C; that is, the toner included in the development agent is used. The development agent from which the toner is consumed is removed from the development roller  25  in a direction D by forces of magnetic repulsion and gravity. Thus, the development agent, minus the used toner is returned into and is again mixed by the first transfer screw  26  with the fresh development agent which has not yet been used. 
     If a relatively great amount of toner is used at the development position C by, for example, a solid-colored image, the development agent mainly including the carrier is returned to the fresh development agent and is mixed with it. In this case, if mixing is not made in a sufficient manner, the concentration of toner in the development agent is uneven. If such development agent is transferred by the first transfer screw  26  and is raised in the direction A, the development process performed at the development position C uses an unevenly mixed development agent. This results in an uneven density in an image, typically in a direction relative to a spiral pitch of the first screw  26 . 
     To avoid this phenomenon, in the example being explained, the mixture of the development agent around the development roller  25  is improved. More specifically, the first and second screws  26  and  27  are improved in the mixing performance, particularly, in a transverse (vertical) direction (i.e., a direction Y in FIG. 4) relative to a direction (i.e., a direction X in FIG. 4) in which the development agent is transferred. Such a mixing is referred to as a vertical mixing. In contrast to vertical mixing, the mixing in the direction in which the development agent is transferred is referred to as a horizontal mixing. 
     Each of the first and second transfer screws  26  and  27  has superior horizontal mixing capabilities but inferior vertical mixing capabilities, and therefore needs to be improved in its vertical mixing ability. For example, the spiral pitch of the transfer screw may be made greater and an angle of the spiral relative to a plane including the shaft of the screw is made smaller. Thereby, the mixing force in the vertical direction (i.e., the direction Y) is increased, relative to the mixing force in the transferring (horizontal) direction (i.e., the direction X). However, such an arrangement causes an extreme reduction of the horizontal mixing. As a result, an image has an uneven density in the direction X in parallel to the length direction of the development roller  25 . 
     In the example being explained, as shown in FIG. 5, the first transfer screw  26  is configured to have a plurality of spirals, such as five spirals, for example. With this configuration, the horizontal mixing weakened by having the greater spiral pitch is reinforced. At the same time, each spiral is made to have a smaller angle relative to the plane including screw shaft so that the vertical mixing can also be strengthened relative to horizontal mixing in the direction in which the development agent is transferred. 
     More specifically, the first transfer screw  26  has five screws each having a spiral pitch of 50 mm, as shown in FIG. 5, and the second transfer screw  27  has a single spiral of 25 mm, as shown in FIG.  6 . The first and second transfer screws  26  and  27  have the same spiral diameter and the same shaft diameter. In addition, the first and second transfer screws  26  and  27  are rotated to transfer the development agent in directions different from each other. 
     The first and second transfer screws  26  and  27  have the same spiral diameter and the same shaft diameter, and a screw pitch P 1  (see FIG. 5) and a number S 1  of spirals of the first transfer screw  26  and a screw pitch (see FIG. 6) and a number S 2  of spirals of the second transfer screw  27  satisfy formulas, 
     
       
           P   1 ≧ 1 . 5 × P   2 , and 
       
     
     
       
           S   1 ≧ 4 × S   2 . 
       
     
     Accordingly, the first transfer screw  26  thus configured to have the five screws produces the above-described positive effects. However, if the second transfer screw  27  is configured to have a plurality of screws, it becomes difficult to balance the development agent in the direction X in parallel to the length direction of the development roller  25 . This would cause a problem such as an uneven density or overflow of the development agent, for example. This is due to interference caused at an area where the development agent is transferred from the second transfer screw  27  to the first transfer screw  26 . 
     That is, the transfer of the development agent from the second transfer screw  27  to the first transfer screw  26  is performed at an end portion thereof in the direction X in parallel to the width direction of the revolving development unit  14 , in a manner similar to that of the background art explained with reference to FIG.  6 . If each of the first and second transfer screws  26  and  27  is provided with more than one screw, the horizontal mixing is strengthened and therefore the development agent elements are pushed against each other at the above end portion where the transfer occurs. As a result, the transfer is not smoothly performed. 
     Therefore, in this example, the second transfer screw  27  is provided with a single screw. Such a single screw configuration achieves a sufficient conveyance of the development agent from the second transfer screw  27  to the transfer screw  26  at the leading end portion of the second transfer screw  27  and a sufficient receipt of the development agent at the trailing edge portion of the second transfer screw  27 . Thereby, the development units  22   a - 22   d  can effectively perform the circulation of the development agent while improving the vertical mixing at the side of the development roller  25 . 
     Numerous additional modifications and variations of the present application are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the present application may be practiced otherwise than as specifically described herein. 
     This application claims priority to Japanese patent application No. JPAP2000-067513 filed on Mar. 10, 2000 in the Japanese Patent Office, the entire contents of which are hereby incorporated by reference.