Abstract:
According to an embodiment, an image shift can be decreased by a photoconductor belt which has a photosensitive layer formed on the surface of a cylindrically formed belt, a pair of rollers which is placed to apply tension to a photoconductor belt, and rotates the photoconductor belt in a specified direction, a charger which is placed opposite to the surface of the photoconductor belt placed between the pair of rollers, and charges the photosensitive layer, a light source which forms a latent image on the charged photosensitive layer, a developing unit which supplies a developing solution to the photosensitive layer having a latent image, develop the latent image, and forms a toner image, and an absorbing roller which is placed on the backside of the photoconductor belt placed between a pair of rollers, and absorbs the photoconductor belt.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a divisional application of Ser. No. 11/387,975, filed Mar. 24, 2006, which is based upon and claims benefit of priority from the prior Japanese Patent Applications No. 2005-172616, filed on Jun. 13, 2005; the entire contents of both of which are incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an electrophotographic apparatus, which uses a developing solution containing a liquid carrier formed by dispersing toner particles in a solvent. 
   2. Description of the Related Art 
   An electrophotographic apparatus using a liquid developing agent or a developing solution containing toner dispersed in a liquid carrier has been recently reevaluated in light of features unrealized by a dry-type electrophotographic recorder, such as high image quality equivalent to offset printing realized by using submicron size toner particles, sufficient image density obtained by a small amount of toner particles, reduced copy cost, and fixing of toner particles to a recording paper sheet at a relatively low temperature, and saving of energy. 
   In an electrophotographic apparatus using a developing solution, a developing unit having a development roller is generally used. A part of a development roller is usually immersed in a developing solution holder. When a developing solution held in the developing solution holder is stuck to and carried by a rotating development roller, fresh toner liquid is supplied to the surface of a photoconductor. Jpn. Pat. Appln. KOKAI No. 2001-228716 proposes providing a microgap between a development roller and the surface of a photoconductor, and forming a meniscus of developing solution in the gap, as a method of supplying a developing solution from a development roller to the surface of a photoconductor. 
   However, in the method described in the above publication, it is very difficult to control the size of the gap formed on photoconductor when using a photoconductor belt (belt-like photoconductor). The photoconductor belt is easily moved in a direction along the developing unit (side) direction, and therefore the gap of the meniscus of the photoconductor belt is small in many cases. 
   BRIEF SUMMARY OF THE INVENTION 
   According to an embodiment of the invention there is provided an electrophotographic apparatus comprising: 
   a photoconductor belt which has a photosensitive layer on the surface, and is configured to hold an electrostatic latent image; 
   a belt driving mechanism which includes at least a driving roller and a tension roller, applies tension to the photoconductor belt, and circulates the photoconductor belt in a specified direction; 
   a charger which is placed opposite to the surface of the photoconductor belt, and gives a specified potential to the photosensitive layer; 
   an exposure device which forms an electrostatic image on the charged photosensitive layer; 
   a developing unit which supplies a developing solution to the photosensitive layer having the latent image, develops the latent image, and forms a toner image; and 
   an absorbing roller which is provided at a specified position on the backside of the photoconductor belt, and prevents distortion of the toner image formed on the photoconductor belt by absorbing the photoconductive belt from the backside of the photoconductive belt. 
   Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the invention. 
       FIG. 1  is a schematic diagram of an electrophotographic apparatus according to an embodiment of the present invention; 
       FIG. 2  is a magnified view of a part in the vicinity of a developing unit of the electrophotographic apparatus shown in  FIG. 1 ; 
       FIG. 3  is a schematic diagram explaining the slack in a photoconductor belt in the vicinity of the developing unit of the electrophotographic apparatus shown in  FIG. 1 ; 
       FIG. 4  is a schematic diagram of an example of an absorbing roller in the developing unit shown in  FIG. 2 ; 
       FIG. 5  is a schematic diagram of another embodiment of an absorbing roller in the developing unit shown in  FIG. 2 ; 
       FIGS. 6A and 6B  are schematic diagrams showing an embodiment to control the speed of a photoconductor belt of the electrophotographic apparatus shown in  FIG. 1 ; and 
       FIG. 7  is a schematic diagram of still another embodiment of an absorbing roller in the developing unit shown in  FIG. 2 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Hereinafter, preferable embodiments of the present invention will be explained with reference to the accompanying drawings. The invention is not limited to the following embodiments, and can be applied in various ways. 
     FIG. 1  shows an embodiment of a liquid developing type electrophotographic apparatus using a liquid developing agent or a developing solution formed by dispersing toner particles in a liquid carrier. 
   An electrophotographic apparatus  1  includes a photoconductor unit  10 , first to fourth image forming units  20 ,  30 ,  40 ,  50 , a dryer  60 , a transfer/fixing unit  70 , a control unit  80  to control the operations of the component units of the electrophotographic apparatus  1  or the operation of the apparatus, and an image data controller  90  to process image data to be output as an image. 
   The photoconductor unit  10  includes a photoconductor belt (electrostatic latent image holder)  11  having a conductive layer that is a thin metal layer of aluminum (Al) or copper (Cu) on the surface of an endless belt made of resin such as polyethylene naphtalate (PEN) or polyethylene telephtalate (PET), and a photosensitive layer that is an organic or non-organic semiconductor laid on the conductive layer. The photoconductor belt  11  is given a specified tension (extended tightly) by a driving roller  12  and a tension roller  13 , and circulated in the space between the rollers (electrostatic latent image holder circulating unit)  12  and  13  by the rotation of the driving roller  12 . Namely, when the driving roller  12  is rotated in the direction of the arrow, the surface of the photoconductor belt  11  is moved in the direction from the driving roller  12  to the tension roller  13 , and vice versa. The photoconductor belt  11  is given a specified tension by the rollers  12  and  13 , so that the surface becomes substantially planar in the direction from the driving roller  12  to the tension roller  13 . 
   The first to fourth image forming units  20 ,  30 ,  40  and  50  are arranged in order in the direction of circulating the photoconductor belt  11 , along the belt surface moving from the driving roller  12  to the tension roller  13 . 
   The image forming units are provided with first to fourth chargers (electric charge supply units)  21 ,  31 ,  41  and  51 , first to fourth LEDS (exposure devices, i.e., image information recording unit)  22 ,  32 ,  42  and  52 , and first to fourth developing units (latent image visualizing units)  23 ,  33 ,  43  and  53 , respectively. The first to fourth chargers  21 ,  31 ,  41  and  51  are solid-state chargers such as a known scorotron or an ion generator, for example, and uniformly charge the surface of the photoconductor belt  11 . The first to fourth exposure devices  22 ,  32 ,  42  and  52  are LED exposure devices, which involve scanning in a main scanning direction like a laser exposure device, and can perform selective exposure with respect to the photoconductor belt  11 , and forms an electrostatic latent image. 
   The first charger  21 , first LED  22  and first developing unit  23  form a first color C (cyan), for example. The second charger  31 , second LED  32  and second developing unit  33  form a second color M (magenta), for example. The third charger  41 , third LED  42  and third developing unit  43  form a third color Y (yellow), for example. The fourth charger  51 , fourth LED  52  and fourth developing unit  53  form a fourth color BK (black), for example. 
   The first to fourth image forming units  20 ,  30 ,  40  and  50  are arranged substantially horizontally, so that they are opposed to the direction of the photoconductor belt  11  and regarded substantially as a straight line when viewed from the direction orthogonal to the thickness of the photoconductor belt  11  (hereinafter, referred to as a horizontal direction). This structure (arrangement) makes it easy to make the circumference of the photoconductor belt  11  long, and makes it possible to expand a span between the driving roller  12  and tension roller  13 , as a result. If a span is expanded, another image forming unit for forming another color image can be easily added to the first to fourth image forming units. 
   The dryer  60  is provided in the side that an optional position of the photoconductor belt  11  is moved from the tension roller  13  to the driving roller  12 , and used to dry and eliminate a liquid carrier in a developing solution. The dryer  60  includes an air blower, for example, and to provide an airflow of a specified velocity along the surface of the photoconductor belt  11 . 
   The transfer/fixing unit  70  has an intermediate transfer roller  71  and a pressing roller  72 , and transfers a toner image carried by the movement or circulation of an optional position of the photoconductor belt  11  (after the carrier liquid is eliminated and dried) to the intermediate transfer roller  71 , and fixes the toner image to a transfer medium guided to a transfer position, for example, a paper sheet P used for image output as hard copy. 
   In the electrophotographic apparatus  1  shown in  FIG. 1 , the photoconductor belt  11  is charged to a specified potential by corona discharging or scorotron discharging from the first charger  21 . On a photosensitive layer of the charged photoconductor belt, an electrostatic latent image is formed according to an image light from the first LED  22  corresponding to an image modulation signal generated by the control unit  80  and image data controller  90 . 
   The electrostatic latent image formed on the photosensitive layer is developed (visualized) by electrostatic adhesion of toner to the electrostatic latent image when a developing solution (liquid developer) is supplied from the first developing unit  23 . 
   It is well known that an LED (exposure device) forms an image-forming area (exposed part) and an image non-forming area (unexposed part) by selectively exposing an image on a photosensitive layer, or the uniformly charged surface of the photoconductor belt  11 . 
   Thereafter, latent images of the colors to be developed by the corresponding developing units according to the image lights of the second to fourth colors are sequentially formed on the photosensitive layer of the photoconductor belt  11 , and the latent images are developed by the corresponding developing units, whereby a full-color toner image is formed on the photoconductor belt  11 . 
   After a developing process using a liquid developing agent (developing solution) of all colors, the toner image on the surface of the photoconductor belt  11  is dried by the dryer  60  and transferred to the peripheral surface of the intermediate transfer roller  71 . 
     FIG. 2  shows a magnified view of the first developing unit  23 . The second to fourth developing units  33 ,  43  and  53  have the same structure. 
   Each developing unit  23  ( 33 ,  43 ,  53 ) has a development roller  111  and a squeeze roller  112 . 
   The development roller  111  and squeeze roller  112  are arranged opposite to the photoconductor belt  11  through a microgap. For example, a gap between the development roller  111  and the surface of the photoconductor belt  11  is 100 μm, and a preferable range of the gap is 50-150 μm. A gap between the squeeze roller  112  and the surface of the photoconductor belt  11  is 50 μm, and a preferable range of the gap is 40-70 μm. 
   The development roller  111  is rotated at the position opposite to the photosensitive layer of the photoconductor belt  11  so that the rotating direction of the peripheral surface becomes the same direction as the belt circulating direction (the direction of moving an optional position of the belt), and supplies a developing solution to the gap (development gap) between the photoconductor belt  11  and development roller  111 . When an appropriate potential is apphed to the surface of the development roller  111  in this state, an electric field (development electric field) is generated in the development gap. By this electric field, the toner particles contained in the developing solution is moved and made to adhere to the electrostatic latent image part formed on the photosensitive layer of the photoconductor belt  11 . In other words, the toner particles moved and made to adhere to the electrostatic latent image, named electrophoresis, in the expression that the toner moves, and electrostatic absorption in the expression that the toner is absorbed by the electric field. As a result, a toner image is formed on the photosensitive layer of the photoconductor belt  11 . 
   The squeeze roller  112  is rotated with the peripheral surface rotating direction reversed to (opposite to) the belt circulating direction (the direction of moving an optional position of the belt), and removes unnecessary developing solution from the belt surface on which the toner image is formed. 
   The gap (squeeze gap) between the squeeze roller  112  and the surface of the photoconductor belt  11  is very small as described above. Therefore, it should be predicated that an absorbing force (a force in the direction of absorbing the belt  11 , regarding the squeeze roller  112  as an absorbing side) is generated between the squeeze roller  112  and photoconductor belt  11 , depending on the viscosity of the developing solution. 
   When an absorbing force is generated between the squeeze roller  112  and belt  11 , the toner image formed on the surface of the photoconductor belt  11  is removed as shown in  FIG. 3 , and an image error may occur as a result. 
   Further, as already explained in  FIG. 1 , when using a belt-like photoconductor made large in the part regarded as a straight line when viewed from the direction orthogonal (horizontal) to the thickness of the belt, it should be predicted that jitter (uneven speed) occurs while the belt is moving, and as a result positions of images may be slightly shifted when stacking the toner images formed by the developing units. This image position shift occurring when stacking the toner images formed by developing units is known as color shift. 
     FIG. 4  shows an example of an absorbing roller in the developing unit shown in  FIG. 2 . 
   The example of  FIG. 4  is characterized in that an absorbing roller (image distortion correction unit)  101  is provided on the backside of the photoconductor belt  11  so as to be opposed to the developing unit  23  ( 33 ,  43  and  53 ). 
   The surface of the absorbing roller  101  is processed to absorb the photoconductor belt  11 . The absorbing roller  101  prevents slacking of the photoconductor belt  11  (by the above-mentioned absorbing force generated by the squeeze roller  112 ). 
   By placing the absorbing roller  101  at the position opposite to the development roller  111  or squeeze roller  112 , the absorbing gap and squeeze gap can be kept constant. 
   If the absorbing gap and squeeze gap are kept constant, distortion and uneven density of image are of course not generated in an electrophotographic apparatus. 
   As a surface treatment of the absorbing roller  101 , it is sufficient that a sufficient force to absorb the photoconductor belt  11  can be obtained. Making the surface rough (matting) is one of the surface treatments. As an example of making the roller surface rough, a center average roughness, called Ra, is preferably 0.4 μm or less, and a surface roughness (maximum surface roughness) is preferably 1.6 μm or less. 
   A surface treatment of the absorbing roller  101  may be mirror finish. For example, a silicon rubber layer controlled in the surface roughness may be formed on the surface of the roller  101 . Namely, the absorbing force of the surface of the roller  101  can be increased by increasing the thickness of a rubber layer or by using a soft rubber material. The reason why the mirror finish can increase the absorbing force is that the mirror-finished surface of the roller  101  becomes easy to deform. As a material of rubber, at least one of silicon resin, urethane resin and butyl resin is used as a main component. 
   Conversely, it is desirable to increase the hardness of the surface of the absorbing roller to increase the precision of the development gap and squeeze gap. This is caused by a material with high hardness providing high processing accuracy in many cases. 
   It is also useful to place a gap ring  151  at both ends of the absorbing roller  101  as shown in  FIG. 4  in order to control (keep constant) the size of the development gap and squeeze gap. Thus, when forming the absorbing roller  101  with a rubber-based material, it is preferable to form a resin layer on the surface of the roller. The resin layer thickness is preferably 0.1-2 mm. The resin layer hardness is preferably 60-90 degrees (in JIS K6257 same as ISO 7619). 
   When the absorbing roller  101  contacts the backside of the photoconductor belt  11 , the clearance between the absorbing roller  101  and the backside of the photoconductor belt  11  becomes substantially a vacuum (vacuum lower than atmospheric pressure), and adhesive force can be obtained. 
   The above absorption (adhesion) is also generated by rotation of the absorbing roller  101  when the photoconductor belt  11  is circulated. Therefore, the size of the development gap and/or squeeze gap is kept substantially constant. According to experiment, a demanded absorbing force is 30 gf per cm in the width direction (the direction orthogonal to the belt thickness) of the photoconductor belt  11  and in the axial direction of the rotation axis (the driving roller  12  or the tension roller  13 ). 
     FIG. 5  shows an example the speed of a photoconductor belt of the electrophotographic apparatus shown in  FIG. 1 . 
     FIG. 5  shows an example in which a timing belt (synchronous belt/toothed belt)  203  is laid under the absorbing roller  101 , and the moving speed of the peripheral surface (by rotation) is made identical to that of the photoconductor belt  11 . As shown in  FIG. 5 , the driving roller  12  and absorbing roller  101  are provided integrally with timing pulleys  201  and  202 , and each connected by the timing belt  203 . In  FIG. 5 , the speed ratio of the pulleys  201  and  202  is set so that the speed of rotating the absorbing roller  101  by the timing belt  203  becomes the same as the speed of driving the photoconductor belt  11  by the driving roller  12 . 
   In the example of  FIG. 5 , when the photoconductor belt  11  is circulated (driven), a local fluctuation of tension caused by the extension and/or contraction of the belt does not occur at the position where the photoconductor belt  11  is opposed to the absorbing roller  101  (the absorbing force is generated). In the configuration of  FIG. 5 , the magnitude (degree) of the jitter in the speed generated in the photoconductor belt  11  can be decreased. 
   Now, speed jitter will be explained in detail with reference to  FIGS. 6A and 6B . 
   As shown in  FIG. 6A , a jitter in the photoconductor belt  11  is an uneven speed generated in the moving (circulating) direction of the photoconductor belt  11 . The amplitude (of the jitter) is increased proportional to the length of a free running part (a part not held by a roller) of the photoconductor belt  11 . 
   As shown in  FIG. 6B , by placing the absorbing roller  101  (two or more) on the backside of the photoconductor belt  11  and causing the roller to absorb the belt  11 , the free run length of the photoconductor belt  11  can be reduced. As a result, the amplitude of speed jitter can be decreased. 
   According to an experiment, when an absorbing roller is not provided, a developing position of each color is shifted by 100 μm maximum by jitter in a developing part. By placing the absorbing roller  101  as described above, the shift can be decreased to 20 μm maximum. 
     FIG. 7  shows an example of generating an absorbing force by positively using a vacuum pressure in an absorbing roller. 
   As shown in  FIG. 7 , an absorbing roller has a fixed cylinder  301  including a chamber  300  opened in one end in the width direction, and a porous cylindrical roller  302  rotatable around the fixed cylinder. Though not described in detail, a negative pressure (lower than atmospheric pressure) is applied to the chamber  300  by using a vacuum pump or a compressor, and an absorbing force toward the photoconductor belt  11  can be obtained through the holes formed on the surface of the cylindrical roller  302 . 
   The example of  FIG. 7  is advantageous in ease of controlling the absorbing force by the suction force of a vacuum pump or a compressor. The magnitude of the absorbing force may be changed corresponding to the free length of a belt. 
   The example of  FIG. 7  can control the absorbing force even if the surface of the absorbing roller is stained with dust, for example. The example is easy to replace or clean a belt, and advantageous in maintenance. 
   As explained alone, in a liquid developing type image forming apparatus using a developing solution according to the present invention, it is possible to prevent distortion and uneven density of an image. 
   The embodiment of the invention is not limited to the aforementioned embodiments. The invention may be embodied in other specific forms or modified without departing from its spirit or essential characteristics. Each embodiment may be appropriately combined as far as possible. Effects by combination will be obtained.