Patent Document

CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority under 35 U.S.C. §119(a) of Korean Patent Application No. 2008-0080495, filed on Aug. 18, 2008 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
       BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The present general inventive concept relates to an image forming apparatus, and more particularly, to an image forming apparatus, which can prevent inferior image quality due to current leakage by an optimized installation of an eraser. 
         [0004]    2. Description of the Related Art 
         [0005]      FIG. 1  schematically illustrates a conventional tandem-type electro-photographic image forming apparatus. 
         [0006]    Referring to  FIG. 1 , a conventional image forming apparatus includes a plurality of image bearing members  10 , on which an image is formed by a developing unit (not illustrated); a transfer unit  20  which transfers a visible image formed on each image bearing member  10  onto a printing medium M; and a fixing unit  30  which fixes an image on the printing medium M by heat and pressure. 
         [0007]    The transfer unit  20  includes a plurality of transfer rollers  21  disposed opposite to their respective image bearing member  10 , and a transfer belt  25  feeding the printing medium M while passing between the image bearing member  10  and the transfer rollers  21 . The transfer unit  20  directly transfers a toner on each image bearing member  10  to the printing medium M by a transfer voltage applied to the transfer roller and having a polarity opposite to that of a voltage applied to the image bearing members  10 . When the transfer voltage is applied to the transfer roller  21 , the transfer voltage is also applied to the printing medium M. 
         [0008]    When the printing medium M passes through a transfer nip formed between each image bearing member  10  and each transfer roller  21 , gap discharge (that is, printing medium detachment discharge) is generated in the front or the rear of the transfer nip, particularly, in an area A in front of the transfer nip as illustrated in  FIG. 1 , due to electric potential difference between the image bearing member  10  and the printing medium M, thereby causing scattering in an image formed by a toner. 
         [0009]    In order to prevent the image scattering due to the gap discharge in the area A, a conventional image forming apparatus includes an eraser  40 , as illustrated in  FIG. 2 . The eraser  40  erases a voltage applied to the printing medium M to reduce electric potential difference between the image bearing member  10  and the printing medium M, thereby restraining the gap discharge. 
         [0010]    Generally, current flowing between an eraser and the ground is less than 0.1-1.0 μA. Thus, when the eraser is disposed near the transfer roller  21 , a transfer current may not flow from the transfer roller  21  toward the printing medium M due to high resistance of the printing medium M under a certain condition, and thus, current leakage may occur toward the eraser  40  via the transfer belt  25 , thereby causing inferior image quality. 
         [0011]    Hence, an eraser should be installed in such a position as to prevent current leakage toward the eraser. 
         [0012]    Further, in a conventional electro-photographic image forming apparatus, when a printing medium of high resistance is used under a low-humidity environment or during duplex printing, resistance of a transfer belt increases. Thus, the amount of electric charges accumulated in the transfer belt is rapidly increased. As a result, it is difficult to form an electric field only by applying a voltage to a pressing roller ( 35  in  FIG. 1 ) of the fixing unit  30 , thereby causing an electrostatic offset. Thus, when a printing medium with toner being transferred is fixed in the electrified state, inferior image quality is likely to occur due to the electrostatic offset. 
       SUMMARY 
       [0013]    The present general inventive concept provides an image forming apparatus which can prevent inferior image quality due to current leakage toward an eraser and due to an electrostatic offset. 
         [0014]    Additional features and utilities of the present general inventive concept 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 present general inventive concept. 
         [0015]    Embodiments of the present general inventive concept can be achieved by providing an image forming apparatus including a plurality of image bearing members spaced apart from each other and including a first image bearing member and a second image bearing member to be disposed adjacent to the first image bearing member, an exposure unit to form a latent image on each of the image bearing members, a developing unit to form a visible image on each of the image bearing member, a transfer unit transferring the visible image onto the printing medium including a plurality of transfer rollers which may be respectively disposed opposite to the plurality of image bearing members and form a transfer nip between each transfer roller and each image bearing member and a plurality of erasers which may be respectively disposed between the plurality of transfer rollers and erase a transfer voltage in a printing medium, and a fixing unit to fix the visible image on the printing medium. The plurality of transfer rollers may include a first transfer roller and a second transfer roller which may be disposed opposite to the first image bearing member and the second image bearing member, respectively. At least one of the plurality of erasers may be disposed between the first transfer roller and the second transfer roller in a zone Z 1  satisfying the following formula: 
         [0000]        D 1 ≦Z 1 ≦D−D 3 
         [0000]      D1=4.5 mm 
         [0000]      D3=4.5 mm 
         [0000]      9.0 mm&lt;D 
         [0016]    Here, D 1  may be an allowable minimum distance between an end part of the eraser and a center of a first transfer nip formed between the first image bearing member and the first transfer roller, D 3  may be an allowable minimum distance between the end part of the eraser and a center of a second transfer nip formed between the second image bearing member and the second transfer roller, and D may be a distance between the center of the first transfer nip and the center of the second transfer nip. 
         [0017]    Each of the plurality of erasers may include a first erasing plate disposed substantially perpendicular to a traveling path of the printing medium. 
         [0018]    At least one of the first erasing plates may be disposed in a zone Z 2  satisfying the following formula: 
         [0000]        R 1 +D 2 ≦Z 2 ≦D −( R 2+ D 4) 
         [0000]      D2=1.7 mm 
         [0000]      D4=1.7 mm 
         [0019]    Here, R 1  may be a radius of the first transfer roller, R 2  may be a radius of the second transfer roller, D 2  may be an allowable minimum distance between the first erasing plate of the eraser and the nearest periphery of the first transfer roller, and D 4  may be an allowable minimum distance between the first erasing plate of the eraser and the nearest periphery of the second transfer roller. 
         [0020]    Each of the plurality of erasers may include a second erasing plate disposed in parallel with a traveling path of the printing medium. 
         [0021]    The transfer unit may further include a transfer belt passing between the plurality of image bearing members and the plurality of transfer rollers and feeding the printing medium. 
         [0022]    The transfer unit may further include an auxiliary eraser disposed between the transfer roller in the most downstream position along the printing medium traveling path and the fixing unit and may erase a transfer voltage in the printing medium. 
         [0023]    An end part of the auxiliary eraser may be disposed in a zone Z 3  satisfying the following formula: 
         [0000]      D5≦Z3≦DF 
         [0000]      D5=4.5 mm 
         [0024]    Here, D 5  may bean allowable minimum distance along the printing medium traveling path between a center of a last transfer nip formed between the image bearing member adjacent to the fixing unit and the corresponding transfer roller and the end part of the auxiliary eraser, and DF may be a distance along the printing medium traveling path between the center of the last transfer nip and a center of a fixing nip of the fixing unit. 
         [0025]    The auxiliary eraser may include an auxiliary erasing plate disposed substantially perpendicular to the printing medium traveling path, the auxiliary erasing plate being disposed in a zone Z 4  satisfying the following formula: 
         [0000]        R 3+ D 6≦ Z 4≦ DF−R 4 
         [0000]        D 6=1.7 mm≦ D 6&lt; DF    
         [0026]    Here, R 3  may be a radius of the transfer roller adjacent to the fixing unit, R 4  may be a radius of a pressing roller of the fixing unit, and D 6  may be an allowable minimum distance between the auxiliary erasing plate of the auxiliary eraser and the nearest periphery the transfer roller adjacent to the fixing unit. 
         [0027]    The auxiliary erasing plate of the auxiliary eraser may be disposed in parallel with the printing medium traveling path. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    These and/or other features and utilities of the present general inventive concept will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings, in which: 
           [0029]      FIG. 1  schematically illustrates a conventional tandem-type electro-photographic image forming apparatus; 
           [0030]      FIG. 2  partially illustrates a conventional image forming apparatus having an eraser; 
           [0031]      FIG. 3  schematically illustrates an image forming apparatus according to an exemplary embodiment of the present general inventive concept; 
           [0032]      FIG. 4  schematically illustrates a main part of the image forming apparatus in  FIG. 3 ; 
           [0033]      FIG. 5  schematically illustrates another main part of the image forming apparatus in  FIG. 3 ; 
           [0034]      FIG. 6  schematically illustrates an image forming apparatus according to another exemplary embodiment of the present general inventive concept; and 
           [0035]      FIG. 7  schematically illustrates a main part of the image forming apparatus in  FIG. 6 . 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0036]    Reference will now be made in detail to the embodiments of the present general inventive concept, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present general inventive concept by referring to the figures. Repetitive description to like elements of different embodiments may be omitted as necessary 
         [0037]      FIG. 3  schematically illustrates an image forming apparatus  300  according to an exemplary embodiment of the present general inventive concept. 
         [0038]    Referring to  FIG. 3 , the image forming apparatus  300  according to an exemplary embodiment of the present general inventive concept is illustrated in  FIG. 3  as a tandem-type electro-photographic image forming apparatus in which images of a plurality of colors may be transferred onto a printing medium in a single path. This image forming apparatus may include a plurality of color image bearing members  110  disposed spacedly apart from each other in a printing path, a plurality of exposure units  123  to form a latent image on the respective image bearing members  110 , a plurality of developing units  125  to form a visible image on the respective image bearing members  110 , a transfer unit  130 ; and a fixing unit  150 . The image forming apparatus may further include an electrifier  121  to electrify each image bearing member at a predetermined electric potential. 
         [0039]    The exposure unit  123  may expose the image bearing member  110  electrified by the electrifier  121  to light and to form a latent image thereon, and may include a light scan unit (LSU) to scan light corresponding to an image to be printed on a printing medium. 
         [0040]    The developing units  125  may be provided with a plurality of colors, and may provide toner to the image bearing members  110  on which latent images may be formed to form visible images. 
         [0041]    The transfer unit  130  may transfer the visible image onto a printing medium M supplied along a printing medium traveling path, and may include a plurality of transfer rollers  131  and a plurality of erasers  140  respectively disposed between the adjacent transfer rollers  131  to erase a transfer voltage useable with the printing medium M. Each transfer roller  131  may be disposed opposite to each image bearing member  110  and may form a transfer nip between the transfer roller  131  and the image bearing member  110 . As illustrated in  FIG. 3 , the plurality of image bearing members  110  may include a first to a fourth image bearing members  110 Y,  110 M,  110 C, and  110 K. The plurality of transfer rollers  131  may include a first to a fourth transfer rollers  131 Y,  131  M,  131 C, and  131 K corresponding to the first to fourth image bearing members  110 Y,  110 M,  110 C, and  110 K, respectively. Further, the transfer unit  130  may include a transfer belt  135  which may pass between the plurality of image bearing members  110  and the plurality of transfer rollers  131  and may feed the printing medium M. 
         [0042]    The plurality of erasers  140  may prevent image scattering due to gap discharge generated in a front area of the transfer nip due to electric potential difference between the image bearing member and the printing medium M when the printing medium M passes through the transfer nip formed between the image bearing member  110  and the transfer roller  131 . To this end, the plurality of erasers  140  may include a first eraser  141 Y disposed between the first transfer roller  131 Y and the second transfer roller  131 M; a second eraser  141 M disposed between the second transfer roller  131 M and the third transfer roller  131 C; and a third eraser  141 C disposed between the third transfer roller  131 C and the fourth transfer roller  131 K. 
         [0043]    Each image bearing member contains a rotating axis. For example in  FIG. 4 , the rotating axis of the first image bearing member  110 Y is illustrated and labeled as  110 C. Also, each transfer roller contains a rotating axis. For example in  FIG. 4 , the rotating axis of the first transfer roller  131  is illustrated and labeled as  131 C. 
         [0044]    Each of the first to third erasers  141 Y,  141 M, and  141 C may include an erasing plate  141  disposed substantially perpendicular to the printing medium traveling path. 
         [0045]    The erasers may be divided into two sections. For example in  FIG. 4 , a section  141 B of first eraser  141 Y is parallel to a line connecting the rotating axis of the first transfer roller  131   c  with the rotating axis of the first image bearing member  110 C. Also, a section  141 C of the first eraser is bent from  141 B toward the rotating axis of the first image bearing member  110 C. In other words, the section  141 C of the first eraser is bent to minimize the distance between the first eraser  141 Y and the first transfer roller  131 Y. 
         [0046]    Further, the end part  141 a of the eraser  141  may be disposed between perimeters of the image bearing member  110 Y and the transfer roller  131 Y. Also, the end part  141 a of the eraser  141  may be disposed between a line passing the rotation axes of the image bearing member and transfer roller and a line connecting circumference surfaces of the image bearing member and the transfer roller. 
         [0047]    Also, the end part  141 a of the eraser  141 , as illustrated in  FIG. 4 , may be disposed adjacent to a corresponding transfer roller and between a first line connecting rotation axes of the image bearing member  110 Y and transfer roller  131 Y and a second line parallel to the first line and tangent to a circumference surface of the image bearing member  110 Y. 
         [0048]    The fixing unit  150  may fix a visible image which may be transferred onto the printing medium M by the transfer unit  130  on the printing medium M, and may include a heat source  151 , a fixing roller  153 , and a pressing roller  155  to press the printing medium M against the fixing roller  153 . 
         [0049]    According to the present embodiment, the erasers  140  may be arranged such that gap discharge with respect to the adjacent transfer rollers  131  can be effectively prevented and at the same time current leakage toward the erasers  141  can be effectively prevented. 
         [0050]    Referring to  FIG. 4 , which schematically illustrates a main part of the image forming apparatus according to the present embodiment, the first eraser  141 Y disposed between the first transfer roller  131 Y and the second transfer roller  131 M may be disposed in a zone Z 1  satisfying the following formula 1: 
         [0000]        D 1 ≦Z 1 ≦D−D 3 
         [0000]      D1=4.5 mm 
         [0000]      D3=4.5 mm 
         [0000]      9.0 mm&lt;D 
         [0051]    Here, D 1  may be an allowable minimum distance between the end part  141   a  of the eraser  141 Y and a center N 1  of a first transfer nip, D 3  may be an allowable minimum distance between the end part  141   a  of the eraser  141 Y and a center N 2  of a second transfer nip, and D may be a distance between the center N 1  and the center N 2 . In this respect, the first transfer nip may be formed between the first image bearing member  110 Y and the first transfer roller  131 Y, and the second transfer nip may be formed between the second image bearing member  110 M and the second transfer roller  131 M. In the formula 1, the center N 1  of the first transfer nip may be considered as an origin. 
         [0052]    The distance T 1  between the first transfer nip N 1  and the end part  141   a  of the eraser  141 Y may be greater than or equal to the allowable minimum distance D 1  between the end part  141   a  of the eraser  141 Y and a center N 1  of a first transfer nip N 1 . 
         [0053]    The conditions of formula  1  may be set in consideration of an erasing current depending on the distance between the center of the transfer nip and the end part of the eraser and a transfer state. 
         [0054]    Table 1 represents erasing currents depending on change in the distance T 1  between the center N 1  of the first transfer nip to the end part  141   a  of the eraser  141 Y when a transfer current of 10 μA is used under a low-temperature and low-humidity environment 
         [0000]    
       
         
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
             
               
                   
                 T1 (mm) 
                 3.0 
                 3.5 
                 4.0 
                 4.5 
                 5.0 
               
               
                   
                 Erasing 
                 3.8 
                 2.2 
                 1.2 
                 0.8 
                 0.8 
               
               
                   
                 current (μA) 
               
               
                   
                 Transfer grade 
                 4 
                 3 
                 2 
                 1 
                 1 
               
               
                   
                   
               
             
          
         
       
     
         [0055]    In Table 1, decreasing erasing currents means that current leakage becomes smaller, that is, the transfer grade or an erasing characteristic becomes better. Further, it is illustrated that if the distance T 1  between the center N 1  of the first transfer nip and the end part  141   a  of the eraser  141 Y is smaller than 4.5 mm, the transfer grade is 2, 3, and 4, which means that current leakage toward the eraser may be relatively high. Thus, it may be preferable that the distance T 1  is 4.5 mm or above. 
         [0056]    The erasing characteristic in Table 1 may be applicable to the relationship between the eraser and the second transfer roller, and thus, a distance between the end part  141   a  of the eraser  141 Y and the center N 2  of the second transfer nip may be preferably 4.5 mm or above. 
         [0057]    Further, the erasing plate  141  of the first eraser  141 Y may be disposed in a zone Z 2  satisfying the following formula 2: 
         [0000]        R 1+ D 2≦ Z 2≦ D −( R 2+ D 4) 
         [0000]      D2=1.7 mm 
         [0000]      D4=1.7 mm 
         [0058]    Here, R 1  may be a radius of the first transfer roller, R 2  may be a radius of the second transfer roller, D 2  may be an allowable minimum distance between the erasing plate  141  of the eraser  141 Y and the nearest periphery of the first transfer roller  131 Y, and D 4  may be an allowable minimum distance between the erasing plate  141  of the eraser  141 Y and the nearest periphery of the second transfer roller. 
         [0059]    The conditions of formula 2 may be set in consideration of an erasing current depending on the distance between the transfer roller and the erasing plate of the eraser and a transfer state. 
         [0060]    Table 2 represents erasing currents according to change in the distance T 2  between the erasing plate  141  and the nearest periphery of the first transfer roller  131 Y when a transfer current of 10 μA is applied under a low-temperature and low-humidity environment 
         [0000]    
       
         
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
             
             
               
                   
                 T2 (mm) 
                 0.5 
                 0.9 
                 1.3 
                 1.7 
                 2.1 
               
               
                   
                 Erasing 
                 0.8 
                 0.6 
                 0.2 
                 0.0 
                 0.0 
               
               
                   
                 current (μA) 
               
               
                   
                 Transfer grade 
                 2 
                 2 
                 2 
                 1 
                 1 
               
               
                   
                   
               
             
          
         
       
     
         [0061]    As illustrated in Table 2, if the distance T 2  between the erasing plate  141  and the nearest periphery of the first transfer roller is less than 1.7 mm, the transfer grades may be 2, which means that current leakage toward the eraser may be relatively high. Thus, it may be preferable that T 2  is 1.7 mm or above. 
         [0062]    The erasing characteristic of Table 2 may be applicable to the relationship between the eraser and the second transfer roller, and thus, it may be preferable that the distance between the erasing plate of the eraser and the nearest periphery of the second transfer roller is 1.7 mm or above. 
         [0063]    Formulas 1 and 2 useable with the first eraser  141 Y may be also applicable to the second and third erasers  141 M and  141 C. 
         [0064]    The transfer unit  130  may be disposed between the last fourth transfer roller  130 K and the fixing unit  150 , and may further include an auxiliary eraser  143  to erase a transfer voltage in the printing medium M after toner transferring is completed to the printing medium M, so as to prevent an electrostatic offset, and thus inferior image quality. 
         [0065]      FIG. 5  schematically illustrates a main part of the image forming apparatus in  FIG. 3 . Referring to  FIG. 5 , the auxiliary eraser  143  may be disposed between the fourth transfer roller  131 K and the pressing roller  155 , and may be arranged in a zone Z 3  satisfying the following formula 3: 
         [0000]        D 5≦ Z 3&lt; DF    
         [0000]      D5=4.5 mm 
         [0066]    Here, D 5  may be an allowable minimum distance along the printing medium traveling path between a center N 3  of a transfer nip formed between the fourth image bearing member  110 K and the fourth transfer roller  131 K and an end part  143   a  of the auxiliary eraser  143 ; and DF may be a distance along the printing medium traveling path between the center N 3  and a center NF of a fixing nip of the fixing unit  150 . 
         [0067]    By arranging the auxiliary eraser  143  in this manner, the first transfer grade can be maintained, as represented in Table 1. Thus, current leakage, which may be generated due to a close distance between the center N 3  of the transfer nip and the end part  143   a  of the auxiliary eraser  143 , can be prevented. 
         [0068]    Further, the auxiliary eraser  143  may include an erasing plate disposed substantially perpendicular to the printing medium traveling path, as illustrated in  FIGS. 3 and 6 . Preferably, the erasing plate of the auxiliary eraser  143  may be arranged in a zone Z 4  satisfying the following formula 4: 
         [0000]        R 3+ D 6≦ Z 4≦ DF−R 4 
         [0000]      D6=1.7 mm 
         [0069]    Here, R 3  may be a radius of the fourth transfer roller  131 K adjacent to the fixing unit  150 ; R 4  may be a radius of the pressing roller  155 ; and D 6  may be an allowable minimum distance between the erasing plate of the auxiliary eraser  143  and the nearest periphery of the fourth transfer roller  131 K. 
         [0070]    By arranging the auxiliary eraser  143  in this way, the first transfer grade can be maintained as illustrated in Table 2. Thus, current leakage, which may be generated due to a close distance between the fourth transfer roller  131 K and the end part  143   a  of the auxiliary eraser  143 , can be prevented. 
         [0071]      FIG. 6  schematically illustrates an image forming apparatus  600  according to another exemplary embodiment of the present general inventive concept. 
         [0072]    The image forming apparatus  600 , according to another exemplary embodiment of the present general inventive concept, may also be a tandem-type electro-photographic image forming apparatus, and is different from the image forming apparatus according to an exemplary embodiment of the present general inventive concept in that a configuration of a plurality of erasers  140  may be modified. 
         [0073]    Referring to  FIG. 6 , the plurality of erasers  140  may include first to third erasers  145 Y,  145 M, and  145 C, respectively disposed between first to fourth transfer rollers  131 Y,  131 M,  131 C, and  131 K. Each of the first to third erasers  145 Y,  145 M, and  145 C may have an erasing plate  145  disposed in parallel with a printing medium traveling path. 
         [0074]      FIG. 7  schematically illustrates a main part of the image forming apparatus in  FIG. 6 . Referring to  FIG. 7 , the first eraser  145 Y may be disposed between the first transfer roller  131 Y and the second transfer roller  131 M, in a zone Z 1  satisfying the above-described formula 1. 
         [0075]    By arranging each eraser  145  in the zone Z 1  in this way, the first transfer grade can be maintained as illustrated in Table 1. Thus, current leakage, which may be generated due to a close distance between a center of a transfer nip and an end part of the eraser  145 , can be prevented. 
         [0076]    Further, the image forming apparatus, according to another embodiment of the present general inventive concept may include an auxiliary eraser  147  disposed between the fourth transfer roller  131 K and the fixing unit  150 . The auxiliary eraser  147  may include an erasing plate disposed in parallel with the printing medium traveling path, like the first to third erasers  145 Y,  145 M and  145 C. The auxiliary eraser  147  may be arranged in a zone Z 3  satisfying the above-described formula 3. By arranging the auxiliary eraser  147  in the zone Z 3 , the first transfer grade can be maintained as illustrated in Table 1. 
         [0077]    Additionally, the first eraser  145  illustrated in  FIG. 7  may be defined as having a distal end  145 F disposed toward the transfer nip N 1  and a distal end opposite  145 F and disposed toward the transfer nip N 2 . Also, the first eraser  145  has a length defined between the distal ends disposed toward N 1  and N 2 , wherein the length of the first eraser  145  is larger than the thickness of the first eraser  145 . 
         [0078]    As described above, according to the present general inventive concept, the eraser may be disposed in such a zone as to satisfy formulas 1 to 2, thereby preventing current leakage toward the eraser, and thus inferior image quality. 
         [0079]    Further, the auxiliary eraser may be disposed in such a zone as to satisfy formulas 3 and 4, thereby preventing an electrostatic offset and thus inferior image quality, even in a low-humidity environment and a duplex printing environment. 
         [0080]    Although a few exemplary embodiments of the present general inventive concept have been illustrated and described, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the present general inventive concept, the scope of which is defined in the appended claims and their equivalents.

Technology Category: 3