Abstract:
A developer unit for a wet-type color image forming apparatus. The developer unit includes a developer bath filled with a liquid developing agent to a predetermined depth, and a developing roller that develops an electrostatic latent image formed on a photoreceptor to correspond to an original image by supplying the liquid developing agent while being partially immersed in the liquid developing agent. The developer unit further includes a cleaning roller that cleans the surface of the developing roller, a depositing roller that deposits the liquid developing agent onto the cleaned surface of the developing roller, and a metering roller that adjusts the thickness and concentration of the liquid developing agent deposited onto the surface of the developing roller by the depositing roller to suitable levels to develop the electrostatic latent image. The developer unit also includes a developer cartridge that supplies the liquid developing agent into the developer bath.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Korean Application No. 2002-7026, filed Feb. 7, 2002, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus, and more particularly, to a developer unit for a wet-type color image forming apparatus, having a metering roller and using a high-concentration liquid developing agent. 
     2. Description of the Related Art 
     An image forming apparatus, such as a printer, photocopier, or scanner, includes an electrostatic latent image formation unit, a developer unit to develop the electrostatic latent image, and a transfer unit to transfer the developed image to a printing paper. 
     A conventional wet-type color image forming apparatus having the above structure is shown in FIG.  1 . In FIG. 1, reference numerals  100 ,  102 ,  104 , and  106  denote first through fourth photoreceptors in which electrostatic latent images to be developed into color images of cyan (C), magenta (M), yellow (Y), and black (K) are formed, respectively. Reference numerals  110 ,  112 ,  114 , and  116  denote first through fourth cleaning blades that respectively clean the first through fourth photoreceptors  100 ,  102 ,  104 , and  106 . Reference numeral  130  denotes a transfer belt to which the developed images are sequentially transferred from the first through fourth photoreceptors  100 ,  102 ,  104 , and  106 , thereby forming an overlapped color image to be transferred to a paper  90 . A plurality of rollers  132 ,  134 ,  138 ,  140 ,  142 , and  144 , which have different functions, are arranged inside the transfer belt  130  and support the transfer belt  130  in a predetermined tensioned state. In particular, reference numeral  132  denotes a driver roller that rotates the transfer belt  130 , reference numeral  134  denotes a backup roller that supports the transfer belt  130  against a transfer-to-paper roller  136  while the developed color image is transferred to a printing paper  90  and to which a voltage required for color image transfer is applied. Reference numerals  138 ,  140 ,  142 , and  144  denote first through fourth transfer rollers that contact the respective first through fourth photoreceptors  100 ,  102 ,  104 , and  106 , with the transfer belt  130  therebetween. The transfer rollers  138 ,  140 ,  142  and  144  transfer the developed images from the respective first through fourth photoreceptors  100 ,  102 ,  104 , and  106  to the transfer belt  130 . Since the developed images on the first through fourth photoreceptors  100 ,  102 ,  104 , and  106  are charged, the first through fourth transfer rollers  138 ,  140 ,  142 , and  144  may be charged to have a polarity opposite to a polarity of the first through fourth photoreceptors  100 ,  102 ,  104 , and  106 , respectively. 
     Although not illustrated in FIG. 1, the first through fourth transfer rollers  138 ,  140 ,  142 , and  144  are connected to separate power sources. Once a color image is transferred to the transfer belt  130 , subsequently the color image is transferred to the printing paper  90 . A voltage having a polarity opposite to the polarity of the voltage that is applied to the first through fourth transfer rollers  138 ,  140 ,  142 , and  144  must be applied to the backup roller  134 . Although not illustrated in FIG. 1, the backup roller  134  is also connected to a power source. 
     Reference numerals  120 ,  122 ,  124 , and  126  denote first through fourth developers, respectively, which constitute a single developer unit to develop the electrostatic latent images. The first through fourth developers  120 ,  122   124 , and  126  supply ink of cyan, magenta, yellow, and black to the first through fourth photoreceptors  100 ,  102 ,  104 , and  106 , respectively, in order to develop the electrostatic latent images on the respective first through fourth photoreceptors  100 ,  102 ,  104 , and  106 . The first through fourth developers  120 ,  122 ,  124 , and  126  have the same internal structure. The ink of different colors is supplied to the first through fourth developers  120 ,  122 ,  124 , and  126  by respective first through fourth developing rollers  120   a ,  122   a ,  124   a , and  126   a , which are attached to the respective first through fourth developers  120 ,  122 ,  124 , and  126 . 
     Referring to FIG. 2, the first developer  120  includes a developer bath  120   c  filled with ink  120   b  to a predetermined height, the first developing roller  120   a , which is partially immersed in the ink  120   b  and contacts the first photoreceptor  100 , and a cleaning roller  120   d , which is immersed in the ink  120   b  and removes the unnecessary residual ink from the surface of the first developing roller  120   a . The first developer  120  further includes an ink depositing roller  120   e , which is immersed in the ink  120   b  and electrically deposits the ink  120   b  onto the surface of the first developing roller  120   a  from which the unnecessary residual ink has been removed, and a metering blade  120   f , which is separated by a predetermined distance above the ink  120   b  and appropriately controls the thickness and concentration of the ink layer deposited on the first developing roller  120   a  by the ink depositing roller  120   e . The ink depositing roller  120   e  is connected to a power source S to be able to electrically deposit the ink  120   b  on the first developing roller  120   a . As a predetermined voltage is applied to the ink depositing roller  120   e  from the power source S, the ink  120   b  is electrically charged to be deposited onto the first developing roller  120   a.    
     As described above, since the developer unit of the conventional wet-type color image forming apparatus includes the metering blade installed around the developing roller, a high-concentration ink of about 3-18% or a constant amount of ink can be supplied to the transfer roller regardless of changes in the concentration of the ink. This is done so that images can be uniformly developed. 
     However, when toner particles are back-plated onto the surface of the developing roller, uniform development cannot be achieved. In other words, when toner particles are stuck in the space between the metering blade and the developing roller, when toner particles that have lost magneticity form clusters, when impurities are generated, or when the metering blade has a defect at its edge, the ink may be applied partially to the developing roller or in a low concentration. The non-uniform ink layer on the developing roller is transferred to the photoreceptor. As a result, a uniform, perfect color image, as shown in FIG. 3, cannot be achieved. Instead, the final color image may have a stripe pattern P, as shown in FIG.  4 . 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an aspect of the present invention to provide a developer unit for a wet-type color image forming apparatus, capable of preventing image quality degradation due to toner impurities adhered to a developing roller or due to a defect in a metering roller. 
     Additional aspects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention. 
     The foregoing and/or other aspects of the present invention may be achieved by providing a developer unit for a wet-type color image forming apparatus to develop an electrostatic latent image formed on a photoreceptor, including a developer bath filled with a liquid developing agent; a developing roller to develop the electrostatic latent image to correspond to an original image by supplying the liquid developing agent while being partially immersed in the liquid developing agent; a cleaning roller to clean a surface of the developing roller; a depositing roller to deposit the liquid developing agent onto the cleaned surface of the developing roller; a metering roller to adjust a thickness and a concentration of the liquid developing agent deposited onto the surface of the developing roller to suitable levels; and a developer cartridge to supply the liquid developing agent into the developer bath. 
     An initial concentration of the liquid developing agent may be 2% or more, and more specifically, may be in the range of 2-40%. In the developer unit, the metering roller may be driven by the developing roller or a separate driving source. A power source may be connected to the metering roller. 
     When the developer unit for the wet color image forming apparatus according to the embodiment of the present invention is used, the metering roller continuously rotates rather than being fixed, and attracts back-plated toner particles or impurities so that no toner particles or impurities are stuck to the space between the developing roller and the metering roller. As a result, a developing agent layer deposited on the developing roller can be protected from being scratched by the back-plated toner particles or impurities, and thus no unwanted stripe pattern appears on the final image. 
     The foregoing and/or other aspects are achieved by providing an apparatus, including a photoreceptor having an electrostatic latent image thereon; a developing roller to develop the electrostatic latent image with a developer comprised of particles; and a metering roller to continuously rotate to adjust a thickness of the developer on the developing roller. The developing roller may be partially immersed in the developer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings of which: 
     FIG. 1 is a sectional view showing the structure of a conventional wet color image forming apparatus; 
     FIG. 2 is a sectional view partially showing the internal structure of the developer unit of FIG. 1; 
     FIG. 3 shows a uniform full solid image; 
     FIG. 4 shows a non-uniform full solid image having a stripe pattern, according to the conventional apparatus; and 
     FIG. 5 is a sectional view showing the structure of a developer unit with a metering roller for a wet-type color image forming apparatus according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Reference will now be made in detail to the present preferred embodiment of the present invention, an example of which is illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. 
     Referring to FIG. 5, a wet color image forming apparatus according to an embodiment of the invention includes a transfer unit T, a photoreceptor unit O, and a developer unit D. The transfer unit T includes a transfer belt  200  to which multiple toner images of different colors developed in the photoreceptor unit O are transferred to overlap one another to form a predetermined color image. The apparatus further includes a plurality of rollers  202 ,  204 ,  206 ,  208 ,  210 ,  212 , and  214  displaced inside the transfer belt  200 , and first through fifth power sources S 1 , S 2 , S 3 , S 4 , and S 5  to supply power to the rollers  204 ,  208 ,  210 ,  212  and  214 . The apparatus further includes a transfer-to-paper roller  216  to transfer the color image to a printing paper  218 , which is grounded. The roller  204  is a backup roller, which is involved in transferring the color image to the printing paper  218  together with the transfer-to-paper roller  216 , and is connected to the first power source S 1 . The first power source S 1  charges the backup roller  204  to a same polarity as a polarity of the color image so as to apply a repulsive electrostatic force to the charged color image on the transfer belt  200 . The rollers  208 ,  210 ,  212 , and  214  are transfer rollers to transfer the developed images from all or some of the first through fourth photoreceptors  300 ,  302 ,  304 , and  306  of the photoreceptor unit O while the developed images overlap one another. The rollers  202  and  206  support the transfer belt  200  together with the first through fourth transfer rollers  208 ,  210 ,  212 , and  214  in a predetermined tensioned state. The roller  202  rotates the transfer belt  200  as a driver roller at an appropriate speed to transfer images. 
     Electrostatic latent images are formed in a region of photoreceptors  300 ,  302 ,  304 , and  306 . The images corresponding to original images of cyan (C), magenta (M), yellow (Y), and black (K) are formed in a predetermined region, respectively. First through fourth cleaning blades  300   a ,  302   a ,  304   a , and  306   a  are disposed on the respective first through fourth photoreceptors  300 ,  302 ,  304 , and  306  so as to remove the unnecessary ink remaining on the surfaces of the first through fourth photoreceptors  300 ,  302 ,  304 , and  306  after the images have been transferred to the transfer belt  200 . The first through fourth cleaning blades  300   a ,  302   a ,  304   a , and  306   a  may have a same shape, or, alternately, may have different shapes. The photoreceptor unit O further includes first through fourth dischargers  300   b ,  302   b ,  304   b , and  306   b  to neutralize the surface charge of the respective first through fourth photoreceptors  300 ,  302 ,  304 , and  306  after the ink residue has been removed. The photoreceptor unit O further includes first through fourth chargers  300   c ,  302   c ,  304   c , and  306   c  to charge, for example, positively, the neutralized surface of the respective first through fourth photoreceptors  300 ,  302 ,  304 , and  306 . 
     The first through fourth dischargers  300   b ,  302   b ,  304   b , and  306   b  may have a same configuration. Alternately, the first through fourth dischargers  300   b ,  302   b ,  304   b , and  306   b  can have different configurations as long as they can provide the discharging function. The same principle can be applied to the first through fourth chargers  300   c ,  302   c ,  304   c , and  306   c . First through fourth laser scanning units (LSUs)  300   d ,  302   d ,  304   d , and  306   d  form the electrostatic images corresponding to the original images of C, M, Y, and K by scanning a predetermined charge region of the respective first through fourth photoreceptors  300 ,  302 ,  304 , and  306 . The LSUs  300   d ,  302   d ,  304   d  and  306   d  are disposed between the developer unit D and the respective first through fourth chargers  300   c ,  302   c ,  304   c , and  306   c.    
     The developer unit D includes first through fourth developers  400 ,  402 ,  404 , and  406 , which respectively correspond to the first through fourth photoreceptors  300 ,  302 ,  304 , and  306 , and develop the respective electrostatic latent images. The first through fourth developers  400 ,  402 ,  404 , and  406  respectively include first through fourth developer baths  400   a ,  402   a ,  404   a , and  406   a  and first through fourth developer cartridges  400   b ,  402   b ,  404   b , and  406   b . The first through fourth developer cartridges  400   b ,  402   b ,  404   b , and  406   b  supply at least 2%, for example, 2-40% of a high-concentration developing agent  400   c ,  402   c ,  404   c  and  406   c . The developing agent may be, for example, ink of C, M, Y, and K, to the respective first through fourth developer baths  400   a ,  402   a ,  404   a , and  406   a . The first through fourth developer baths  400   a ,  402   a ,  404   a , and  406   a  are filled to a predetermined height with the respective first through fourth developing agents  400   c ,  402   c ,  404   c , and  406   c.    
     The first through fourth developers  400 ,  402 ,  404 , and  406  include respective first through fourth developing rollers  400   d ,  402   d ,  404   d , and  406   d  that are partially immersed in and respectively apply the first through fourth developing agents  400   c ,  402   c ,  404   c , and  406   c  to the respective first through fourth photoreceptors  300 ,  302 ,  304 , and  306 . The developing rollers  400   d ,  402   d ,  404   d , and  406   d  rotate in contact with the respective first through fourth photoreceptors  300 ,  302 ,  304 , and  306 . First through fourth cleaning rollers  400   e ,  402   e ,  404   e , and  406   e  remove the toner layer remaining on the surface of the respective first through fourth developing rollers  400   d ,  402   d ,  404   d , and  406   d  while rotating in contact with the respective first through fourth developing rollers  400   d ,  402   d ,  404   d , and  406   d . First through fourth depositing rollers  400   f ,  402   f ,  404   f , and  406   f  are disposed in contact with the respective first through fourth developing rollers  400   d ,  402   d ,  404   d , and  406   d  while being immersed in the respective first through fourth developing agents  400   c ,  402   c ,  404   c , and  406   c  so as to deposit a sufficient amount of the first through fourth developing agents  400   c ,  402   c ,  404   c , and  406   c  onto the respective first through fourth developing rollers  400   d ,  402   d ,  404   d , and  406   d.    
     The first through fourth depositing rollers  400   f ,  402   f ,  404   f , and  406   f  deposit the respective first through fourth developing agents  400   c ,  402   c ,  404   c , and  406   c  onto the respective first through fourth developing rollers  400   d ,  402   d ,  404   d , and  406   d  using electrostatic force. To this end, the first through fourth depositing rollers  400   f ,  402   f ,  404   f , and  406   f  are connected to sixth through ninth power sources S 6 , S 7 , S 8 , and S 9 , respectively. When the first through fourth developing agents  400   c ,  402   c ,  404   c , and  406   c  are positively charged, a positive voltage is applied to the first through fourth depositing rollers  400   f ,  402   f ,  404   f , and  406   f  from the respective sixth through ninth power sources S 6 , S 7 , S 8 , and S 9 . 
     Metering rollers  400   g ,  402   g ,  404   g , and  406   g  are separated by a predetermined distance above the surface of the first through fourth developing agents  400   c ,  402   c ,  404   c , and  406   c . The first through fourth developing agents  400   c ,  402   c ,  404   c , and  406   c  can be deposited onto the surface of the respective first through fourth developing rollers  400   d ,  402   d ,  404   d , and  406   d  to an appropriate thickness and concentration to develop the electrostatic latent images formed on the first through fourth photoreceptors  300 ,  302 ,  304 , and  306 . The first through fourth metering rollers  400   g ,  402   g ,  404   g , and  406   g  may rotate in a direction opposite to a direction in which the first through fourth developing rollers  400   d ,  402   d ,  404   d , and  406   d  rotate. The metering rollers  400   g ,  402   g ,  404   g  and  406   g  may rotate due to the first through fourth developing rollers  400   d ,  402   d ,  404   d , and  406   d , or due to separate driving units. 
     In an alternative embodiment of the present invention, in order to adjust the thickness and concentration of the developing agents  400   c ,  402   c ,  404   c  and  406   c  deposited onto the first through fourth developing rollers  400   d ,  402   d ,  404   d , and  406   d  to optimal levels, the tenth through thirteenth power sources S 10 , S 11 , S 12 , and S 13  direct electrostatic force from the first through fourth metering rollers  400   g ,  402   g ,  404   g , and  406   g  to the first through fourth developing rollers  400   d ,  402   d ,  404   d , and  406   d.    
     According to experiment, the developer unit having the metering rollers according to the embodiment of the present invention and a conventional developer unit having metering blades were both applied, for example, to a printer. As a result, a pull-down failure in a dot area occurred when the conventional developer unit was applied, whereas no pull-down failure in the dot area occurred when the developer unit having the metering rollers according to the embodiment of the present invention was applied. 
     While the present invention has been particularly described in the above with reference to embodiments thereof, the above embodiments of the present invention are for illustrative purposes and are not intended to limit the scope of the present invention. For example, it will be understood by those skilled in the art that the metering roller according to the present invention can be applied to any color image forming apparatus using a low-concentration liquid developing agent or using a solid developing agent. Alternatively, a power source can be connected to the conventional metering blade as in the present invention. Therefore, the spirit and scope of the invention should be defined by the appended claims rather than by the above-described embodiments. 
     As described above, a developer unit for a wet-type color image forming apparatus according to the embodiment of the present invention includes a cleaning roller, a depositing roller, and a metering roller. The metering roller adjusts the thickness and concentration of a developing agent layer deposited on the developing roller to suitable levels to develop an electrostatic latent image formed on a photoreceptor while rotating in a direction opposite to a direction in which the developing roller rotates. Since the metering roller according to the present invention rotates rather than being fixed, as in the conventional metering blade, back-plated toner particles or impurities deposited between the developing roller and the metering roller can be easily separated. This differs from the conventional developer units in which the back-plated toner particles or impurities are stuck in the space between the developing roller and the fixed metering blade. 
     The present metering roller continuously rotates in contact with the developing roller and attracts the back-plated toner particles or impurities so that no toner particle or impurity is stuck to the space between the developing roller and the metering roller. As a result, the developing agent layer deposited on the developing roller can be protected from being scratched by the back-plated toner particles or impurities, and thus no unwanted stripe pattern appears on the final image. 
     Although a preferred embodiment of the present invention has been shown and described, it will be appreciated by those skilled in the art that changes may be made in this embodiment without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.