Patent Publication Number: US-2009238607-A1

Title: Development Device and Image Forming Apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2008-072920, filed Mar. 21, 2008 and Japanese Patent Application No. 2008-259537, filed Oct. 6, 2008, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     1. Technical Field 
     The present invention relates to a development device for developing an electrostatic latent image formed on an image carrier by means of a liquid developer containing toner and carrier liquid and an image forming apparatus for forming an image by transferring a developer image developed by the development device and fixing it. 
     2. Related Art 
     There have been proposed various wet type image forming apparatus for developing and visualizing an electrostatic latent image by means of a highly viscous liquid developer prepared by dispersing solid toner in a liquid solvent. Developers that can be used for such wet image forming apparatus are prepared by suspending a solid ingredient (toner particles) in a highly viscous and electrically insulating organic solvent (carrier liquid) typically made of silicon oil, mineral oil or edible oil. The suspended toner particles are very fine and have a particle size of about 1 μm so that wet type image forming apparatus can produce high quality images if compared with conventional dry type image forming apparatus that use toner particles having a particle size of about 7 μm. 
     JP-A-2002-278296 (to be referred to as Patent Document 1 hereinafter) describes an image forming apparatus designed to use a liquid developer and an application roller having helically cut grooves on the surface thereof to prevent foreign objects from entering between the application roller and a quantity of application limiting member for limiting the quantity of liquid developer to be applied to the surface of the application roller. JP-A-2006-019282 (to be referred to as Patent Document 2 hereinafter) describes an image forming apparatus designed to use a supply roller around which a wire is wound in order to supply a liquid developer efficiently to a developer carrying member. 
     The application roller (supply roller) described in the Patent Document 1 sucks up the liquid developer that contacts a lower part of the roller and feeds it to a development roller arranged above it. It has been found that, as the application roller on the surface of which helical grooves are formed is driven to rotate on the liquid surface of the developer, the application roller exerts conveyance force for conveying the liquid developer to consequently give rise to an uneven liquid surface as indicated by a broken line in  FIG. 5 . Since the liquid developer is highly viscous, there arise non-contact regions between the surface of the development roller and the liquid surface of the developer as shown  FIG. 8  particularly when the application roller is driven to rotate at high speed because the liquid developer comes to show an uneven surface. Additionally, it has been found that such non-contact regions occur as the duration of rotation of the application roller increases even if the application roller is not driven at high speed. Such non-contact regions by turn produce unapplied regions on the supply roller where the liquid developer is not applied to consequently give rise to a serious defect on the part of the image that is formed and output. Even if the liquid surface shows unevenness of liquid level only to a small extent, and no unapplied regions are produced, the thickness of the layer of the liquid developer formed on the surface of the application roller becomes uneven to consequently degrade the output image. The supply roller of the Patent Document 2 is highly probably accompanied by a problem same as that of the supply roller of the Patent Document 1 because a wire is wound around it so that helical grooves are formed on the surface of the supply roller. 
     SUMMARY 
     In view of the above-identified problem, it is therefore an object of the present invention to provide a development device and an image forming apparatus that can dissolve the problem of uneven liquid surface of the liquid developer on a lower part of the surface of the supply roller where helical grooves are formed so as to make the supply roller free from unapplied regions of liquid developer and an uneven thickness of the layer of liquid developer formed on the surface thereof in order to secure a good image quality. 
     According to the present invention, the above object is achieved by providing a development device including: a developer reservoir that stores a liquid developer containing toner and carrier liquid; a supply port that is arranged at the developer reservoir to supply the liquid developer to the developer reservoir; a supply roller that has helical grooves and supplies the liquid developer stored in the developer reservoir; and a developer carrier that is supplied with the liquid developer by the supply roller and carries the supplied liquid developer, and the supply port being arranged in a direction opposite to a direction of conveyance by the helical grooves of liquid developer stored in the developer reservoir relative to an axial center of the supply roller. 
     In a development device as defined above, the supply port may be arranged vertically below the supply roller. 
     Preferably, in a development device as defined above, the supply port is arranged such that the liquid developer is supplied in a direction perpendicular relative to an axial direction of the supply roller. 
     Preferably, in a development device as defined above, the supply port is arranged such that the liquid developer is supplied in a direction opposite to a direction of conveyance by the helical grooves of liquid developer. 
     Preferably, a development device as defined above further includes: a collected liquid reservoir that stores a collected liquid developer; a wall section that is arranged between the developer reservoir and the collected liquid reservoir; and a collection port that is arranged at the wall section to make the liquid developer flow from the developer reservoir to the collected liquid reservoir. 
     Preferably, in a development device as defined above, the collection port is formed by notching part of the wall section to regulate a liquid level of the liquid developer stored in the developer reservoir. 
     Preferably, in a development device as defined above, the collection port is arranged at or near an axial end of the supply roller. 
     Preferably, in a development device as defined above, the collection port is formed by a first collection port arranged in a direction of conveyance by the helical grooves of the supply roller of liquid developer and a second collection port arranged in a direction opposite to a direction of conveyance by the helical grooves of the supply roller of liquid developer at a position vertically higher than the first collection port. 
     Preferably, in a development device as defined above, the supply port is arranged at a position different from a position of the collection port relative to an axial direction of the supply roller. 
     Preferably, in a development device as defined above, the collected liquid reservoir is provided with a discharge port that discharges the liquid developer from the collected liquid reservoir. 
     Preferably, in a development device as defined above, the discharge port is arranged at a side of the first collection port. 
     Preferably, in a development device as defined above, the developer reservoir is provided with a conveyance screw that conveys the liquid developer from the developer reservoir to the supply roller in an axial direction thereof. 
     Preferably, in a development device as defined above, a rate of conveyance of liquid developer to a first axial end side of the supply roller differs from a rate of conveyance of liquid developer to a second axial end side of the supply roller in an opposite direction. 
     In another aspect of the present invention, there is provided an image forming apparatus including: a liquid developer concentration control section that controls toner concentration of a liquid developer containing toner and carrier liquid; a supply section that supplies the liquid developer showing the toner concentration controlled by the liquid developer concentration control section; a developer reservoir that has a supply port connected to the supply section and stores the liquid developer; a supply roller that has helical grooves and supplies the liquid developer from the developer reservoir; a development section that has a developer carrier adapted to be supplied with the liquid developer from the supply roller and carry the liquid developer; and a latent image carrier that carries a latent image to be developed by the development section, and the supply port being arranged in a direction opposite to a direction of conveyance by the helical grooves of liquid developer stored in the developer reservoir relative to an axial center of the supply roller. 
     Preferably, an image forming apparatus as defined above further includes: a collected liquid reservoir that stores the collected liquid developer; a wall section that is arranged between the developer reservoir and the collected liquid reservoir; a collection port that is arranged at the wall section to make the liquid developer flow from the developer reservoir to the collected liquid reservoir; and a collected liquid conveyance section that collects the liquid developer stored in the collected liquid reservoir and conveys the collected liquid developer to the liquid developer concentration control section. 
     The above-described arrangement dissolves the problem of uneven liquid surface of the liquid developer on a lower part of the surface of the supply roller where helical grooves are formed so as to make the supply roller free from unapplied regions of liquid developer and an uneven thickness of the layer of liquid developer formed on the surface thereof in order to secure a good image quality. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       This invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
         FIG. 1  is a schematic cross-sectional view of an image forming apparatus according to an embodiment of the present invention, showing principal components thereof; 
         FIG. 2  is a schematic cross-sectional view of a development device according to the present invention, showing principal components thereof; 
         FIG. 3  is a schematic perspective view of a supply roller that can be used for the purpose of the present invention; 
         FIGS. 4A and 4B  are schematic cross-sectional views of a development device according to the present invention; 
         FIGS. 5A through 5C  are schematic cross-sectional views of a development device according to the present invention; 
         FIG. 6  is a schematic perspective view of a development device according to the present invention, showing how it externally appears; 
         FIGS. 7A through 7C  are schematic illustrations of conveyance screws that can be employed of an image forming apparatus according to another embodiment of the present invention; 
         FIG. 8  is a schematic illustration of the conventional art; 
         FIG. 9  is a schematic perspective view of a development device according to an embodiment of the present invention, showing how it externally appears; 
         FIG. 10  is a schematic cross-sectional view of a development device according to an embodiment of the present invention; 
         FIG. 11  is a schematic cross-sectional view of a development device according to another embodiment of the present invention; 
         FIG. 12  is a schematic external view of the supply roller according to the embodiment of the present invention; 
         FIG. 13  is a schematic cross-sectional view of a development device according to another embodiment of the present invention; and 
         FIG. 14  is a schematic cross-sectional view of a development device according to still another embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Now, preferred embodiments of the present invention will be described in greater detail by referring to the accompanying drawings.  FIG. 1  is a schematic cross-sectional view of an image forming apparatus according to an embodiment of the present invention, showing principal components thereof. Image forming sections are arranged at the center of the image forming apparatus and four development devices  30 Y,  30 M,  30 C and  30 K are arranged respectively under the image forming sections while an intermediate transfer body  40  and a secondary transfer section (secondary transfer unit  60 ) are arranged on and above the image forming sections. In the following, only one of the image forming sections and one of the development devices  30 Y,  30 M,  30 C and  30 K will be described and the suffixes representing respective colors will be omitted because they are structurally the same. 
     The image forming section includes an image carrier  10 , a corona charger  11  and an exposure unit  12 . The exposure unit  12  has an optical system including a semiconductor laser, a polygon mirror and an F-θ lens (not shown). The image carrier  10  is uniformly electrically charged by the corona charger  11  and an electrostatic latent image is formed on the image carrier  10  by irradiating a laser beam that is modulated according to the input image signal to the electrically charged image carrier  10  from the exposure unit  12 . 
     The development device  30  includes a developer container  31  that stores a liquid developer of the corresponding color and a supply roller  32  for applying the liquid developer to the development roller  20  from the developer container  31  and develops the electrostatic latent image formed on the image carrier  10  by means of a liquid developer of corresponding color. The intermediate transfer body  40  is typically an endless belt that is wound around a drive roller  41  and a tension roller  42  and driven to rotate by the drive roller  41 , contacting the image carrier  10  at corresponding primary transfer section  50 . The primary transfer section  50  includes a primary transfer roller  51  that is arranged vis-a-vis the image carrier  10  with the intermediate transfer body  40  interposed between them. The contact position of the image carrier  10  and the primary transfer roller  51  operates as a transfer position. Thus, the developed toner images of the different colors on all the image carriers  10  are sequentially transferred onto the intermediate transfer body  40  to form a full color toner image. 
     The secondary transfer section  60  includes a secondary transfer roller  61  that is arranged vis-a-vis the drive roller  41  with the intermediate transfer body  40  interposed between them and a cleaning device having a secondary transfer roller cleaning blade  62 . The monochromatic or full color toner image formed on the intermediate transfer body  40  is transferred onto a recording medium, which may typically be a sheet of paper, film or cloth, being conveyed along a sheet member conveyance route L at the transfer position where the secondary transfer roller  61  is arranged. 
     A fixing unit (not shown) is arranged at a downstream position of the sheet member conveyance route L and the monochromatic or full color toner image that is transferred onto the recording medium such as a sheet of paper is fusion-bonded to the recording medium and fixed there. 
     The tension roller  42  bears the intermediate transfer body  40  wound around it with the belt drive roller  41  and a cleaning device having the intermediate transfer body cleaning blade  46  is held in contact with intermediate transfer body  40  at the position where the intermediate transfer body  40  is wound around the tension roller  42 . 
     Now the image forming sections and the development devices according to the embodiment of the present invention will be described below.  FIG. 2  is a schematic cross-sectional view of one of the image forming sections and one of the development devices  50 , showing principal components thereof. In the following, only the yellow (Y) image forming section and the yellow development device will be described and the suffix representing the yellow color will be omitted because all the image forming sections are structurally the same and so are all the development devices. 
     In the image forming section, an image carrier cleaning roller  16 , an image carrier cleaning blade  18 , a corona charger  11 , an exposure unit  12 , the development roller  20  of the development device  30 , an image carrier squeezing roller  13  and another image carrier squeezing roller  13 ′ are arranged along the outer periphery of the image carrier  10  in the mentioned order in the sense of rotation of the image carrier  10 . Reference symbol  17  denotes an image carrier cleaning roller cleaning blade that cleans the image carrier cleaning roller  16 . The image carrier squeezing rollers  13  and  13 ′ are provided respectively with cleaning devices having image carrier squeezing roller cleaning blades  14  and  14 ′ as auxiliary components. 
     A cleaning blade  21 , a supply roller  32  and a toner compression corona generator  22  are arranged along the outer periphery of the development roller  20 . The supply roller  32  is held in contact with a limiting blade  33  for regulating the quantity of liquid developer to be supplied to the development roller  20 . The liquid developer container  31  has a developer reservoir  312  and a collected liquid reservoir  315  formed therein and a conveyance screw  34  and a collection screw  35  are respectively contained in the developer reservoir  312  and the collected liquid reservoir  315 . 
     Primary transfer roller  51  of the primary transfer section  50  is arranged along the intermediate transfer body  40  at a position located vis-a-vis the image carrier  10  and an intermediate transfer body squeezing device  52  that includes a squeezing roller  53 , a backup roller  54  and an intermediate transfer body squeezing roller cleaning blade  55  is arranged at the downstream side of the primary transfer roller  51  as viewed in the moving direction of the intermediate transfer body  40 . 
     The image carrier  10  is a cylindrical photosensitive drum having a width larger than the development roller  20  and provided with a photosensitive layer formed on the outer peripheral surface thereof. It is typically driven to rotate clockwise as shown in  FIG. 2 . The photosensitive layer of the image carrier  10  is an organic image carrier, an amorphous silicon image carrier or the like. The corona charger  11  is arranged at the upstream side of the image carrier  10  relative to the nip section of the image carrier  10  and the development roller  20  and a voltage is applied to it from a power supply unit (not shown) to corona-charge the image carrier  10 . The exposure unit  12  is arranged so as to irradiate a laser beam to the image carrier  10 , which is electrically charged by the corona charger  11 , at the downstream side relative to the corona charger  11  in the sense of rotation of the image carrier  10  to form an electrostatic latent image on the image carrier  10 . 
     The development device  30  includes a toner compression corona generator  22  for exerting a compaction effect and a developer container  31  that stores a liquid developer in a state where toner is dispersed in carrier liquid to show a weight ratio of about 20%. The developer container  31  is provided with a collection screw  35  for collecting the liquid developer not supplied to the supply roller  32 . 
     The developer container  31  has a supply port  311  at a lower part thereof. A developer supply route  71  is linked to the supply port  311  so that the liquid developer pumped up from the developer supply section  73  by a pump  74  is supplied to the supply port  311 . The developer supply section  73  operates as a liquid developer concentration control section that controls the ratio of toner relative to carrier liquid (toner concentration). The developer supply route  71  and the pump  74  operate as supply section for supplying the liquid developer whose toner concentration is controlled to the developer reservoir  312 . 
     Thus, the development device includes the development roller  20  for carrying the liquid developer, the supply roller  32  for applying the liquid developer to the development roller  20 , the limiting blade  33  for limiting the quantity of liquid developer to be applied to the development roller  20 , the conveyance screw  34  for conveying the liquid developer and supplying it to the supply roller  32 , the toner compression corona generator  22  for bringing the liquid developer carried by the development roller  20  into a compacted state and the development roller cleaning blade  21  for cleaning the development roller  20 . 
     The liquid developer contained in the developer container  31  is not a conventional popular volatile liquid developer that is a low concentration (1 to 2 wt %) and low viscosity liquid developer that is volatile at room temperature and formed by using Isopar (trademark: available from Exxon) as carrier but a high concentration and high viscosity (about 30 to 10,000 mPa·s) liquid developer showing toner solid concentration of about 20% and formed by adding a solid component prepared by dispersing a coloring agent such as a pigment into resin that is nonvolatile at room temperature into a liquid solvent such as organic solvent, silicon oil, mineral oil or edible oil with a dispersant. 
     The supply roller  32  has a function of supplying a liquid developer to the development roller  20 . The supply roller  32  is a cylindrical member, or a roller, having projections and recesses on the surface thereof that are formed by uniformly cutting fine spiral grooves in order to make it carry the liquid developer on its surface with ease. Thus, the liquid developer is supplied from the developer container  31  to the development roller  20  by the supply roller  32 . When the apparatus is in operation, the conveyance screw  34  is driven to rotate clockwise as shown in  FIG. 2  to supply the liquid developer to the supply roller  32 , which supply roller  32  is driven to rotate counterclockwise to apply the liquid developer to the development roller  20 . Note that the conveyance screw  34  is not necessarily required to dissolve the unevenness, if any, of the level of liquid developer and hence may be omitted for the purpose of the present invention. 
     The limiting blade  33  is an elastic blade formed by coating an elastic member on the surface. It includes a rubber section to be held in contact with the surface of the supply roller  32 , which rubber section is typically made of urethane rubber, and a metal plate for supporting the rubber section. It controls the film thickness and the quantity of the liquid developer that is carried and conveyed by the supply roller  32  so as to supply the liquid developer to the development roller  20  by a regulated quantity. 
     The development roller  20  is a cylindrical member that is driven to rotate counterclockwise around an axis of rotation as shown in  FIG. 2 . The development roller  20  is formed by arranging an elastic layer of polyurethane rubber, silicon rubber of NBR on the outer peripheral section of an inner core that is made of metal such as iron. The development roller cleaning blade  21  is typically made of rubber and adapted to be held in contact with the surface of the development roller  20 . It is arranged at the downstream side relative to the development nip section where the development roller  20  contacts the image carrier  10  in the sense of rotation of the development roller  20  so as to scrape off and remove the liquid developer remaining on the development roller  20 . 
     The toner compression corona generator  22  is an electric field application means for boosting the charged bias of the surface of the development roller  20 . The liquid developer conveyed by the development roller  20  is subjected to toner compression as an electric field is applied to it at a position located close to the toner compression corona generator  22 . The toner compression corona generator, or the corona discharger for discharging corona shown in  FIG. 2 , that operates as the electric field application means may be replaced by a compaction roller. A compaction roller is a cylindrical member having a metal roller base member and an electrically conductive resin layer or rubber layer arranged on the metal roller base member as surface layer. Such a compaction roller is driven to rotate clockwise or in the sense of rotation opposite to that of the development roller  20 . 
     On the other hand, the liquid developer that is carried by the development roller  20  and subjected to toner compression is then subjected to a development process at the development nip section where the development roller  20  contacts the image carrier  10  so as to develop the electrostatic latent image on the image carrier  10  as a desired electric field is applied to it. The developer that is left after the development process is scraped off by the development roller cleaning blade  21  and dropped into the collection section in the developer container  31  so as to be recycled and reused. 
     The image carrier squeezing device is arranged at the upstream side relative to the primary transfer section and vis-a-vis the image carrier  10  at the downstream side relative to the development roller  20  to collect the surplus developer left on the image carrier  10  after the development of the toner image. It includes the image carrier squeezing rollers  13  and  13 ′ that are elastic roller members having an elastic surface coat and held in contact with the image carrier  10  so as to be driven to slide on the image carrier  10  and rotate and cleaning blades  14  and  14 ′ pressed respectively against the image carrier squeezing rollers  13  and  13 ′ so as to driven to slide thereon and clean their surfaces. It has a function of collecting excessive carrier liquid from the developed developer on the image carrier  10  and raising the content ratio of toner particles in the visible image. While a plurality of image carrier squeezing rollers  13  and  13 ′ are provided in the image carrier squeezing device so as to operate prior to the primary transfer operation in this embodiment, they may be replaced by a single image carrier squeezing roller. Still alternatively, one of the image carrier squeezing rollers  13  and  13 ′ may be removably held in contact with the image carrier  10  depending on the conditions of the liquid developer on the image carrier  10 . 
     The developer image developed on the image carrier  10  is transferred onto the intermediate transfer body  40  by means of the primary transfer roller  51  in the primary transfer section  50 . The image carrier  10  and the intermediate transfer body  40  are driven to move at the same speed to alleviate the load of driving them to rotate and move and suppress the effect of disturbances on the visible toner image on the image carrier  10 . 
     The image carrier cleaning device arranged at the downstream side of the primary transfer section cleans the image carrier  10  and removes the liquid developer left on the image carrier  10  after the primary transfer operation as it is arranged vis-a-vis the image carrier at a position downstream relative to the primary transfer section  50 . A bias voltage is applied to the image carrier cleaning roller  16  in order to attract toner particles in the liquid developer on the image carrier  10 . Thus, the liquid developer collected by the image carrier cleaning roller  16  contains toner particles to a large extent. Thus, the solid-rich liquid developer collected by the image carrier cleaning roller  16  in this way is then scraped off by the image carrier cleaning roller cleaning blade  17  that is held in contact with the image carrier cleaning roller  16  and dropped vertically downward. 
     The intermediate transfer body squeezing device  52  includes the intermediate transfer body squeezing roller  53  that is an elastic roller member having an elastic surface coat and held in contact with the intermediate transfer body  40  so as to slide on the intermediate transfer body  40 , the backup roller  54  arranged vis-a-vis the intermediate transfer body squeezing roller  53  with the intermediate transfer body  40  interposed between them and the intermediate transfer body squeezing roller cleaning blade  55  pressed against the squeezing roller  53  so as to slide and clean the surface thereof. It has a function of collecting the surplus carrier liquid from the developer transferred onto the intermediate transfer body  40  for primary transfer. 
     Now, a supply roller  32  that is employed for a development device and an image forming apparatus according to the present invention will be described below.  FIG. 3  is a schematic perspective view of a supply roller that can be used for the purpose of the present invention and an enlarged partial view thereof. The supply roller  32  to be used for the purpose of the present invention has helical grooves  321  as indicated by oblique lines in  FIG. 3  that are formed on the surface thereof for the purpose of efficiently supplying a liquid developer. As described above by referring to  FIG. 2 , as the supply roller  32  is driven to rotate, it sucks up the liquid developer that contacts a lower part thereof and feeds it to the developer carrier  20 . When the supply roller  32  is provided with the helical grooves  321  formed on its surface as shown in  FIG. 3 , conveyance force is generated on the surface of the stored liquid developer by the helical grooves  321  to convey the liquid developer in the direction indicated by a leftward arrow in  FIG. 8 . While the conveyance force depends on the rotational speed of the supply roller  32  and the viscosity of the liquid developer, unevenness is produced to the liquid level of liquid developer in an axial direction as indicated by a broken line in  FIG. 8 . When the unevenness is remarkable, there may consequently arise unapplied regions where the liquid level of liquid developer does not get to the supply roller  32 . Then, when such unapplied regions are formed, developer is not sufficiently supplied from the supply roller  32  to the developer carrier  20  to by turn produce defects in the formed image. Even if such unapplied regions are not formed, the supply roller  32  sucks up the liquid developer ununiformly in an axial direction when the liquid level is not even. Such a situation is by no means desirable. 
     The present invention employs an arrangement as described below in order to dissolve the problem of unevenness of the liquid level caused by the liquid developer conveyed in an axial direction by revolutions of the supply roller  32 . Now, principal components of a development device according to the present invention will be described below by referring to  FIGS. 4A and 4B . 
       FIG. 4A  is a cross-sectional view of a development device according to the present invention as shown in  FIG. 2  taken at the supply port  311  (along line B-B′ in  FIG. 4B ).  FIG. 4B  is a cross-sectional view of the development device taken along line A-A′ in  FIG. 4A . 
     Referring to  FIG. 4A , a developer reservoir  312  is formed in the developer container  31  to store a developer. The developer container  31  is provided at a lower part thereof with a supply port  311  and the liquid developer is supplied from the supply port  311  to the developer reservoir  312 . The developer container  31  is also provided with a partition section  313  that is a wall section and the partition section  313  forms the developer reservoir  312  and a collected liquid reservoir  315 , which will be described hereinafter, as separate chambers. 
     The partition section  313  has notches near the opposite ends thereof as indicated by a broken line in  FIG. 4  to reduce the height thereof at parts located near the opposite ends. The two notched parts of the partition section  313  operate as collection ports  314  for collecting the liquid developer from the developer reservoir  312  to the collected liquid reservoir  315  (the left notched part and the right notched part in  FIG. 4  being referred to respectively as first collection port and second collection port). 
     The conveyance screw  34  is arranged in the developer reservoir  312 . The conveyance fins of the conveyance screw  34  convey the liquid developer fed in from the supply port  311  toward the collection ports  314  arranged respectively at the left side and at the right side. While the conveyance screw  34  is not an indispensable component for the purpose of the present invention as pointed out earlier, it is preferably provided to efficiently circulate the liquid developer. 
     The supply roller  32  is arranged at a position where it contacts the liquid surface of liquid developer when the development device is driven to operate. Additionally, the supply roller  32  is arranged at a position where it contacts the developer carrier  20  so that the layer of liquid developer whose thickness is limited by the limiting member  33  can be applied to the developer carrier  20 . 
     The liquid developer that is fed to and near the center of the developer reservoir  312  from the supply port  311  is then conveyed toward the two collection ports  314  arranged respectively at the left side and at the right side. Then, the liquid developer overflows from the collection ports  314  defined by the low heights of the corresponding parts of the partition section  313  so as to be fed into the collected liquid reservoir  315 . Thus, the liquid developer stored in the developer reservoir  312  shows a uniform height in an axial direction as long as the development device is not driven to operate but the level of the liquid surface of the liquid developer changes as the supply roller  32  where the helical grooves  321  are formed according to the present invention is driven to rotate. More specifically, in the instance illustrated in  FIG. 4 , the liquid developer is urged lopsidedly toward the left end to raise the liquid level at the left side in an axial direction to produce an uneven liquid level when the supply roller  32  is driven to rotate. 
     According to the present invention, this problem of uneven liquid level is dissolved by arranging the supply port  311  at a position that characterizes the present invention. More specifically, the supply port  311  is arranged at a position lopsided to the right relative to the center line of the supply roller  32  indicated by double dot chain line in  FIG. 4A  that is perpendicular to an axial line thereof or in the direction opposite to the direction in which the liquid developer is urged by the supply roller  32 . As the supply port  311  is arranged at such a position, the pressure of liquid developer is higher at the supply port  311  than at any other position to dissolve the problem of uneven liquid level of liquid developer. 
     When the collection ports  314  are formed by notching the corresponding parts of the partition section  313  and hence the collection ports  314  formed at the partition section  313  operate to regulate the liquid level of the stored liquid developer, the position of the supply port  311  is preferably other than a position below either of the collection ports  314 . When the supply port  311  is arranged at a position below either of the collection ports  314 , the upwardly directed pressure that is applied to the liquid developer from the supply port  311  escapes toward the corresponding collection port  314  so that the effect of the present invention that is provided by the positions of the collection ports  314  will become insufficient. 
     Liquid developer is supplied vertically from below from the supply port  311  into the developer reservoir  312  in the instance of  FIG. 4A . However, it is not necessary that the liquid developer is supplied in such a direction and an appropriate angle of supply may be selected for supplying the liquid developer into the developer reservoir  312  in order to dissolve the problem of uneven liquid level for the purpose of the present invention. More specifically, the liquid level is raised at the left side in  FIG. 4A  by the revolutions of the supply roller  32  so that the liquid level can be made even by selecting an angle of supply that is inclined to the right side for supplying the liquid developer into the developer reservoir  312 , or in the opposite direction relative to the raised liquid level. With such an arrangement, not only the position of arrangement of the supply port  311  but also the position at which the pressure of the liquid developer rises can be selected in a desired manner. Additionally, with such an arrangement, the degree of freedom of selection of the position of the supply port  311  in the developer container  31  is raised to provide an auxiliary advantage from the viewpoint of designing the image forming apparatus. 
     A color image forming apparatus includes a plurality of development devices  30  (Y, M, C and K) as shown in  FIG. 1  and liquid developers of different colors are employed in the development devices  30 . Then, the liquid developers employed in the respective development devices  30  (Y, M, C and K) may show respective viscosities that are different from each other. Then, the position of arrangement of the supply port  311  may be made vary among the plurality of development devices  30  (Y, M, C and K) so that liquid developers may be controlled according to their colors. 
     As described above, the liquid level can be held even by appropriately adjusting the position of the supply port  311  and the angle of supply of liquid developer. Therefore, differences in the viscosity of liquid developer can be flexibly accommodated when the position of the supply port  311  and the angle of supply of liquid developer from the supply port  311  into the developer reservoir  312  can be adjusted mechanically. 
     Additionally, the pressure of the liquid developer that is supplied from the supply port  311  is utilized for the purpose of the present invention. Therefore, the problem of uneven liquid level can be dissolved further by adjusting the pressure of the pump  74  for pumping up the liquid developer from the developer supply section  73  into the developer reservoir  312 . 
     Furthermore, since the viscosity of liquid developer varies as a function of the temperature thereof, the pressure of the pump  74  may be adjusted according to the temperature of liquid developer. More specifically, a sensor for detecting the temperature of liquid developer may be provided and the pressure of the pump  74  may be adjusted under control by feeding back the detected temperature to maintain the liquid level even. Alternatively, a sensor for detecting the liquid level of liquid developer may be provided instead of a temperature sensor so as to adjust the pressure of the pump  74  according to the liquid level detected by the sensor. With such an arrangement, the liquid level of liquid developer can be adaptively maintained even if the viscosity of liquid developer changes as a function of the temperature thereof. 
     Now, collection and discharge of liquid developer for the purpose of the present invention will be described below by referring to  FIGS. 5A through 5C .  FIG. 5A  is a cross-sectional view of the development device taken near the partition section  313  of  FIG. 2  (and along line D-D′ in  FIG. 5B ).  FIG. 5B  is a cross-sectional view of the development device taken near the collection port  314  (and along line C-C′) in  FIG. 5A .  FIG. 5C  is a top view of the development device of  FIG. 5A . 
     As shown in  FIG. 5A , the liquid developer supplied from the supply port  311  into the developer reservoir  312  is then conveyed both in the left direction and in the right direction by the conveyance screw  34  and overflows from the collection ports  314  arranged at the left and right ends to the side of the collected liquid reservoir  315 . The liquid developer that overflows is conveyed in one direction by the collection screw  35  arranged in the collected liquid reservoir  315  and discharged into a developer collection route  72 Y from the discharge port  316 . Arrows shown in  FIGS. 5A and 5C  indicate the conveyance route of liquid developer. The developer collection route  72 Y is connected to the developer supply section  73  that operates as a liquid developer concentration control section so that it may be referred to as a collected liquid conveyance section for reusing the collected liquid developer. 
     According to the present invention, the supply port  311  is arranged at a position lopsided relative to the center of an axial line of the supply roller  32  in the direction opposite to the direction in which the liquid developer is conveyed by the helical grooves  321  in order to make the liquid level even in the developer reservoir  312 . However, in reality, it is not possible to make the liquid level sufficiently even in the developer reservoir  312  and it has been found that a phenomenon that a liquid developer is urged to a side by the supply roller  32  or the pressure of the liquid developer being supplied from the supply port  311  becomes dominant. Then, as a result, the quantity of overflowing liquid developer is greater at one of the two collection ports  314  than at the other collection port  314 . Due to this phenomenon, the discharge port  316  is arranged at the side of the collection port  314  where the quantity of overflowing liquid developer is greater. 
     In the embodiment illustrated in  FIGS. 5A through 5C , a liquid developer is predominantly urged in one of the opposite directions of the developer reservoir  312  by the supply roller  32  so that the discharge port  316  is arranged at the side of the collection port  314  (the first collection port) to which the liquid developer is urged by the supply roller  32  and the liquid developer is conveyed by the collection screw  35  in the direction directed to the side of the discharge port  316 . With such an arrangement, the route from the collection port  314  showing a greater quantity of overflowing liquid developer to the discharge port  316  can be made short to improve the efficiency of collection of the liquid developer. 
       FIG. 6  is a schematic perspective view of a development device according to the present invention, showing how it externally appears. This view shows well the route by way of which the liquid developer is collected and discharged as described above by referring to  FIGS. 5A through 5C . The liquid developer that is supplied from the developer supply route  71  overflows from the collection ports  314  arranged near the opposite ends of the partition section  313  and moves to the side of the collected liquid reservoir  315  where the collection screw  35  is arranged. The collection screw  35  has a helical profile as illustrated in  FIG. 6  and conveys the liquid developer that overflows in a single direction toward the discharge port  316  as it is driven to rotate. The discharge port  316  is connected to the developer collection route  72  to collect the liquid developer. 
     The conveyance screw  34  may be provided with a functional feature that makes the liquid level of liquid developer more even. Now, such conveyance screws  34  will be described below by referring to  FIGS. 7A through 7C . 
       FIGS. 7A through 7C  show variations of conveyance screw  34  that characterizes the present invention. As described earlier by referring to  FIG. 2 , the conveyance screw  34  is a member for conveying a liquid developer from the developer reservoir  312 . The variations shown in  FIGS. 7A through 7C  are different from each other in terms of arrangement of the first conveyance fin  344  and the second conveyance fin  345 . Firstly, the conveyance screw  34  of  FIG. 7A  will be described below particularly in terms of major components thereof. 
     The conveyance screw  34  shown in  FIG. 7A  has a shaft at the center thereof which is rotatably supported by the developer container  31  at the first shaft end section  341  and the second shaft end section  342 . A plurality of conveyance fins  344  and  345  are formed on the shaft. Thus, as the conveyance screw  34  is driven to rotate, the conveyance fins  344  and  345  convey the liquid developer in an axial directions. 
     The two different conveyance fins including the first conveyance fin  344  and the second conveyance fin  345  are arranged so as to extend in the opposite directions from a changeover section  343  on the conveyance screw  34 . The first conveyance fin  344  and the second conveyance fin  345  have respective profiles that are adapted to convey the liquid developer in opposite directions. Thus, as the conveyance screw  34  is driven to rotate in a predetermined sense of rotation, the first conveyance fin  344  is provided with conveyance force for conveying the liquid developer from the changeover section  343  toward the first shaft end section  341 , whereas the second conveyance fin  345  is provided with conveyance force for conveying the liquid developer from the changeover section  343  toward the second shaft end section  342  so that the liquid developer supplied from the supply port  311  that is arranged near the changeover section  343  is conveyed in the opposite directions. 
     The first conveyance fin  344  and the second conveyance fin  345  shown in  FIG. 7A  have respective pitches of fin arrangement that are different from each other, an axial pitch of the first conveyance fin  344  being longer than an axial pitch of the second conveyance fin  345 . With this arrangement, it is possible to make the rate of conveyance of liquid developer at the left side is differentiated from the rate of conveyance of liquid developer at the right side at the changeover section  343 . By making an axial pitch of the first conveyance fin  344  longer than an axial pitch of the second conveyance fin  345 , the rate at which the liquid developer is conveyed by the second conveyance fin  345  is greater than the rate at which the liquid developer is conveyed by the first conveyance fin  344 . 
     In this embodiment, the unevenness of the liquid level of liquid developer that is caused by the helical grooves  321  of the supply roller  32  can be dissolved further by making use of the difference of rate of conveyance of liquid developer between the first conveyance fin  344  and the second conveyance fin  345  of the conveyance screw  34 . The conveyance screw  34  having the first conveyance fin  344  and the second conveyance fin  345  that are described above is arranged in a manner as illustrated in  FIG. 7A  relative to the direction in which the liquid developer is urged by the supply roller  32 , or the direction in which the liquid level of liquid developer is gradually raised, as indicated by a leftward arrow in  FIG. 7A . As the conveyance screw  34  is arranged in this way, the urge given to the liquid developer by the supply roller  32  can be offset by the difference of rate of conveyance of liquid developer between the opposite axial directions of the conveyance screw  34  so as to make the liquid level of liquid developer more even. 
     Additionally, since the unevenness of the liquid level of liquid developer is principally dissolved by the positional arrangement of the supply port  311  for the purpose of the present invention, variations of the conveyance screw  34  are selectively used for auxiliary adjustment. In other words, the arrangement of the conveyance fins is not limited to the above-described one and, conversely, an axial pitch of the first conveyance fin  344  may be made shorter than an axial pitch of the second conveyance fin  345 . 
     In each of the variations of the conveyance screw  34  shown in  FIGS. 7B and 7C , the rate of conveyance of liquid developer of the first conveyance fin  344  is differentiated from the rate of conveyance of liquid developer of the second conveyance fin  345  at the changeover section  343 . In both of the variations shown in  FIGS. 7B and 7C , the rate of conveyance of liquid developer of the second conveyance fin  345  is made greater than the rate of conveyance of liquid developer of the first conveyance fin  344 . 
     In the variation of  FIG. 7B , the first conveyance fin  344  and the second conveyance fin  345  of the conveyance screw  34  are made of respective materials that are different from each other. For example, the first conveyance fin  344  may be an elastic member made of an elastic material while the second conveyance fin  345  may be a rigid member made of a rigid material. With this arrangement, a liquid developer is reliably conveyed by the second conveyance fin  345  that is a rigid member as the conveyance screw  34  is driven to rotate, whereas the first conveyance fin  344  that is an elastic member is partly elastically deformed due to the resistance of liquid developer to consequently relieve part of the liquid developer there from conveyance force so that the rate of conveyance of liquid developer of the first conveyance fin  344  may be made smaller than the rate of conveyance of liquid developer of the second conveyance fin  345 . While an elastic member and a rigid member are selected in this example, the selection of members is by no means limited thereto so long as the rate of conveyance of the first conveyance fin  344  is differentiated from the rate of conveyance of the second conveyance fin  345 . For example, elastic members may be selected for both of the conveyance fins so long as they show different moduli of elasticity. 
       FIG. 7C  shows a conveyance screw  34  where the first conveyance fin  344  and the second conveyance fin  345  have respective angles of inclination that are different from each other, the angle of inclination of the first conveyance fin  344  being smaller than the angle of inclination of the second conveyance fin  345 . With this arrangement, when the conveyance screw  34  is driven to rotate in the liquid developer, the resistance that the second conveyance fin  345  receives from the liquid developer is greater than the resistance that the first conveyance fin  344  receives so that the second conveyance fin  345  provides a greater rate of conveyance of liquid developer. 
     Again, since the unevenness of the liquid level of liquid developer is principally dissolved by the positional arrangement of the supply port  311  for the purpose of the present invention, the variations of the conveyance screw  34  shown in  FIGS. 7B and 7C  may be selectively used for auxiliary adjustment. In other words, the arrangement of the conveyance screw  34  is not limited to the above-described ones and, conversely, the configuration of the first conveyance fin  344  and that of the second conveyance fin  345  may be modified appropriately. 
     Variations of the conveyance screw  34  are described above by referring to  FIGS. 7A through 7C  to exploit the difference of rate of conveyance between the first conveyance fin  344  and the second conveyance fin  345 . However, the present invention is by no means limited thereto and the rate of conveyance of the first conveyance fin  344  may be differentiated from the rate of conveyance of the second conveyance fin  345  by means of an appropriate combination of pitch, material and angle of inclination or by using some other arrangement. For example, the size of the first conveyance fin  344  may be differentiated from that of the second conveyance fin  345 . 
     Since the major objective of the present invention to dissolve the uneven distribution of liquid developer that is produced by the supply roller  32  in an axial direction, there may be provided a plurality of first conveyance fins  344  or a plurality of second conveyance fins  345  that show different rates of conveyance. With such an arrangement, it is possible to finely adjust the rate of conveyance not only at the left side and at the right side of the changeover section  343  but also between the changeover section  343  and the first shaft end section  341  or between the changeover section  343  and the second shaft end section  342  so as to realize more even liquid surface. 
     Thus, as described above, according to the present invention, it is now possible to dissolve the problem of unevenness of the liquid developer under a lower part of the surface of the supply roller  32  where the helical grooves  321  are formed and hence unapplied regions of liquid developer and an uneven layer thickness of liquid developer are prevented from taking place to ensure a good image quality for the image forming apparatus. 
     Now, the configuration of a development device according to the present embodiment will be described below by referring to  FIGS. 9 and 10 .  FIG. 9  is a schematic perspective view of development device, showing how it externally appears.  FIG. 10  is a schematic cross-sectional view of the development device taken along plane E in  FIG. 9 . It will be seen from  FIG. 9  how development roller cleaning blade  21  is held in contact with development roller  20 . The development roller  20  and the development roller cleaning blade  21  are taken off from the development device of  FIG. 6 , which shows a perspective view of the development device, in order to illustrate the route of collection and discharge of liquid developer. On the other hand,  FIG. 9  shows the entire development device and how it externally appears. The operation of fitting the development device in place and various servicing operations can be conducted with ease by realizing the development device as a removable unit. 
       FIG. 10  is a schematic cross-sectional view of the development device taken along plane E in  FIG. 9  and shows a development roller  20 , a development roller cleaning blade  21 , a corona charger  11 , a supply roller  32 , a limiting blade  33 , a developer container  31 , a conveyance screw  34  and a collection screw  35  as components of the development device (or the development section of an image forming apparatus). As pointed out earlier, the conveyance screw  34  is not an indispensable component for the purpose of the present invention. 
     Now, the operation of supplying a liquid developer will be described below by referring to another embodiment shown in  FIG. 11 . In the embodiment of  FIG. 4 , a liquid developer is supplied vertically from below from the supply port  311  into the developer reservoir  312  and the liquid level is made even by utilizing the upwardly directed pressure being applied to the liquid developer from the supply port  311 . However, it is known that the liquid level can be made even simply by arranging the supply port  311  at a position of the side opposite to that of the end of the supply roller  32  toward which the liquid developer is urged relative to an axial center of the supply roller  32  without utilizing the upwardly directed pressure. This is because a difference of flow rate of liquid developer arises and the liquid developer flows in at a higher rate at the side where the supply port  311  is arranged in a lopsided manner. Therefore, the liquid level of liquid developer can be made even without arranging the supply port  311  vertically right under the developer container  31 . 
       FIG. 11  illustrates an embodiment where the supply port  311  is arranged not vertically right under the developer container  31 . In  FIG. 11 , the supply port  311  is arranged at a lateral side of the developer container  31  in such a way that the liquid developer flows into the developer reservoir  312  from that lateral side. Although not shown in  FIG. 11 , the supply port  311  is arranged at a lopsided position of the side opposite to that of the end of the supply roller  32  toward which the liquid developer is urged relative to an axial center of the supply roller  32  as in the case of  FIG. 4A . Thus, the liquid level of the liquid developer can be made even simply by arranging the supply port  311  at a lopsided position without utilizing the pressure of the liquid developer that is flowing in. With this arrangement, the position of arrangement of the supply port  311  is not limited to right under the developer container  31  and can be appropriately selected to increase the degree of freedom from the viewpoint of designing the image forming apparatus. 
     While the supply port  311  of the embodiment of  FIG. 11  is so arranged as to supply the liquid developer from a lateral side, the angle of supply of liquid developer may alternatively be appropriately modified without changing the position of arrangement of the supply port  311 . For example, if the liquid developer is made to be supplied upwardly toward the developer reservoir  312 , the liquid level can be made even by utilizing the upwardly directed pressure of the entering liquid developer. If the liquid developer is made to be supplied with an angle of inclination that makes the liquid developer to enter in a direction opposite to the direction in which the liquid developer is urged by the supply roller, the supplied liquid developer is subjected to force that is directed oppositely relative to the conveyance force exerted by the helical grooves of the supply roller to be of great advantage to an even liquid level. The use of an upwardly directed angle of supply and that of an angle of supply directed in the direction opposite to the urging direction of the supply roller may be combined appropriately. Additionally, differences in the viscosity of liquid developer can be flexibly accommodated when the position of the supply port  311  and the angle of supply of liquid developer from the supply port  311  into the developer reservoir  312  can be adjusted mechanically. 
     Now, other embodiments where the supply roller  32  is arranged at a different position relative to the developer container  31  will be described by referring to  FIGS. 12 and 13 .  FIG. 12  is a schematic external view of the supply roller and  FIG. 13  is a schematic cross-sectional view of another embodiment showing that the supply roller  32  and the development roller  20  are arranged in a lopsided manner relative to the developer container  31 . 
     Firstly, the supply roller  32  will be described in greater detail in terms of an axial center thereof by referring  FIG. 12 .  FIG. 12  is a schematic external view of the supply roller  32 . The supply roller  32  is a cylindrical member and the helical grooves  321  are formed on the cylindrical surface by cutting the surface to produce such fine and uniform helical grooves so as make it capable of carrying a liquid developer with ease. The center of the region where the helical grooves  321  are formed operates as an axial center of the supply roller because the liquid developer is urged toward one of the opposite axial ends of the supply roller mainly by the helical grooves  321 . More specifically, if the length of the region where the helical grooves  321  are formed is L, an axial center of the supply roller  32  is located at the position of L/2 (to be referred to as “reference center line” hereinafter). According to the present invention, the liquid level of liquid developer can be made even by arranging the supply port  311  at the side opposite to that of the end of the supply roller  32  toward which the liquid developer is urged by the supply roller  32  relative to the reference center line. 
     In the embodiment of  FIG. 4 , the center of the developer container  31  (developer reservoir  312 ) and the center of the supply roller  32  are substantially made to agree with each other in the longitudinal direction of the supply roller  32 . On the other hand, in  FIG. 13 , the supply roller  32  is arranged at a position lopsided relative to the developer container  31  (developer reservoir  312 ) in the direction in which the liquid developer is urged by the supply roller  32 . Further, the development roller  20  is also arranged at a lopsided position in accordance with the arrangement of the supply roller  32 . The supply port  311  is arranged at the right side in  FIG. 13  relative to the reference center line of the supply roller  32 , or at the side opposite to that of the end toward which the liquid developer is urged by the supply roller  32 . 
     With this arrangement, the developer reservoir  312  can be expanded at the right side of the supply roller  32  (at the side opposite to that of the end toward which the liquid developer is urged by the supply roller  32 ) so that a sufficient amount of liquid developer can be secured in the area of the developer reservoir  312  that tends to be short of the liquid developer due to the urging effect of the supply roller  32 . Particularly, when a conveyance screw  34  is incorporated, the sufficient amount of liquid developer can be mixed by it to sufficiently raise the liquid level of liquid developer at the right end part of the supply roller  32  and the liquid developer can be made to reliably adhere to the supply roller  32 . 
     Thus, as the supply roller  32  is lopsided relative to the developer container  31  (developer reservoir  312 ), a sufficient amount of liquid developer can be secured at the side opposite to that of the end toward which the liquid developer is urged by the supply roller  32  in the above-described manner. Now, another embodiment designed to secure a sufficient amount of liquid developer will be described below by referring to  FIG. 14 . This embodiment is realized by modifying the partition section  313  of the embodiment of  FIG. 13 . When the collection ports  314  formed at the left and right ends of the partition section  313  is arranged at the same level in  FIG. 13 , the level of the collection port  314  (the second collection port at the right side in  FIG. 14 ) located at the side opposite to that of the end toward which the liquid developer is urged by the supply roller  32  is made higher than the level of the other collection port  314  (the first collection port)+Thus, a sufficient amount of liquid developer can be secured by raising the level of the second collection port  314  so that the liquid developer can be made to reliably adhere to the supply roller  32 . 
     This will be described more specifically below. If the height of the partition section  313  at the center thereof is H1, the level of the collection port  314  (the second collection port at the right side in  FIG. 14 ) located at the side opposite to that of the end toward which the liquid developer is urged by the supply roller  32  is H2 and the level of the other collection port  314  (the first collection port) is H3, a relationship of H1&gt;H2&gt;H3 holds true. The relationship of H1&gt;H2 is defined because, if the liquid developer adheres to the shaft of the supply roller, it can flow to the end parts of the shaft and eventually leak out. 
     Thus, it is possible to store a sufficient amount of liquid developer and make it reliably adhere to the supply roller  32  by expanding the developer reservoir  312  not only longitudinally but also upwardly. The levels of the collection ports  314  of  FIG. 4  may be modified in a manner as shown in  FIG. 14  so that the developer reservoir  312  is expanded only upwardly. 
     Finally, the rate of supply of liquid developer from the supply port  311  will be described below by way of specific values. If the number of helical grooves  321  formed per inch on the supply roller  32  is 150 lpi, the film thickness of liquid developer formed on the development roller  20  is 6 μm and the printing speed is 40 ppm, the rate of consumption of liquid developer per unit time is 50 cc/min. Then, the liquid level of liquid developer can be made even and a sufficient amount of liquid developer can be made to adhere to the supply roller  32  by supplying the liquid developer to an excessive supply ratio of about 150%. The excessive supply ratio [%] is defined by formula of [developer supply rate]/[developer consumption rate]×100. With this formula, the rate at which the liquid developer is supplied from the supply port  311  will be 75 cc/min. Thus, the liquid level can be held even with ease by supplying the liquid developer excessively at a rate higher than the rate at which the liquid developer is sucked up by the supply roller  32 . The excessively supplied liquid developer then leaks out from the collection ports  314  so as to be collected in the collected liquid reservoir  315 .