Abstract:
A liquid electrophotographic printing apparatus includes a photosensitive belt circulating around a predetermined track. A transfer roller contacts the photosensitive belt, and receives an image therefrom. A fuser roller presses against the transfer roller so that the toner image transferred to the transfer roller is transferred to a sheet of paper passing between the transfer roller and the fuser roller. A fuser roller cleaning device selectively contacts the fuser roller and maintains a peeling force of the fuser roller within a predetermined range. A peeling force adjusting device selectively contacts the transfer roller and maintains a peeling force of the transfer roller within a predetermined range. The surface energies SE T , SE P , and SE F  of the transfer roller, the paper sheet, and the fuser roller, respectively, satisfy the following inequality: SE T &lt;SE P &lt;SE F .

Description:
RELATED APPLICATIONS  
         [0001]    This application claims priority to an application entitled “Liquid Electrophotographic Printing Apparatus” filed in the Republic of Korea on May 15, 2000, and assigned Application No. 00-25768, the contents of which are hereby incorporated by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a liquid electrophotographic printing apparatus, and more particularly, to a liquid electrophotographic printing apparatus having an improved structure so that differences in peeling forces of a photosensitive belt, a transfer roller, and a fuser roller which occur during image transfer, can be controlled to be within a predetermined range.  
           [0004]    2. Description of the Related Art  
           [0005]    In general, in a liquid electrophotographic printing apparatus, a laser beam scans a photosensitive medium to form an electrostatic latent image on the photosensitive medium, a liquid developer having a predetermined color is applied to the electrostatic latent image area of the photosensitive medium to develop the electrostatic latent image, and then the developed image is printed on a sheet of paper when the image is transferred to the paper via a transfer unit.  
           [0006]    Referring to FIG. 1, in a conventional liquid electrophotographic printing apparatus, electrostatic latent images formed on a photosensitive belt  10  by laser scanning units are developed at developing units  20 , sequentially and respectively, and then carrier included in the developed image is dried by a drying unit  30 , and the image is transferred to a paper sheet (P) via a transfer unit  40 .  
           [0007]    The transfer unit  40  comprises a transfer roller  41  which is installed to face a transfer backup roller  12  and to which the image (I) developed on the photosensitive belt  10  is transferred, and a fuser roller  43  disposed to face the transfer roller  41 . Here, the image transfer from the photosensitive belt  10  to the transfer roller  41  is referred to as transfer T 1 , and the image transfer from the transfer roller  41  to the paper sheet (P) is referred to as transfer T 2 .  
           [0008]    In the liquid electrophotographic printing apparatus configured as described above, whether or not the developed image is transferred from one device to another is determined by differences in surface energies of the photosensitive belt  10 , the transfer roller  41 , the paper sheet (P), and the fuser roller  43 . The surface energy varies with not only materials of components but also heat and pressure developed during the transfer operation. That is, since the toner constituting an image is transferred from a component having a smaller surface energy to another having a larger surface energy, the materials of individual components and pressing forces between the components are decided by considering those factors. Here, the surface energy acts as a factor deciding the surface adhering force of the toner particle, and the surface energies of the components are relatively decided by measuring the peeling forces (g/inch) of the respective components which can be measured. Peeling force is the force required to peel off a measuring tape attached to a component such as the photosensitive belt  10 , the transfer roller  41 , the fuser roller  43 , and the like, and is a relative value which depends on the type of adhesive tape for measuring a peeling force, the pressing force applied when the adhesive tape is attached to an objective component, the speed of a peeling force measuring apparatus, the ambient temperature of the measuring apparatus, and the like.  
           [0009]    Since the peeling force measuring apparatus is described in U.S. application Ser No. 09/666,805 entitled “Electrophotographic Printing Apparatus and Image Transferring Method for an Electrophotographic Printing Apparatus”, filed by the applicant of this application, a detailed description thereof will be omitted. The contents of U.S. application Ser. No. 09/666,805 are hereby incorporated by reference.  
           [0010]    When the peeling forces required at the photosensitive belt  10 , the transfer roller  41 , and the fixing roller  43  were measured with the same conditions by the above measuring method according to the number of printed paper sheets, it was found that the peeling force increased as the number of printed sheets of paper increased in a general liquid electrophotographic printing apparatus, as shown by the graph in FIG. 2.  
           [0011]    Here, the peeling force required at the sheet of paper varies depending on the manufacturer and the use of the printed sheet of paper, and has a constant value in the range of about 600 to 800 g/inch.  
           [0012]    Referring to FIGS. 1 and 2, in the transfer T 1 , since the difference in the peeling forces of the photosensitive belt  10  and the transfer roller  41  is not large at an initial stage (area S 1  of FIG. 2), the toner image (I) developed on the photosensitive belt  10  is not completely transferred to the transfer roller  41 . The peeling force of the photosensitive belt  10  is increased by the toner of the remaining toner image on the photosensitive belt  10 . Accordingly, the efficiency of image transfer from the photosensitive belt  10  to the transfer roller decreases.  
           [0013]    Also, in the transfer T 2 , as the difference in the peeling forces of the transfer roller  41  and the sheet of paper (P) decreases gradually in area S 2  as the number of printed sheets increases, the toner image is not completely transferred from the transfer roller  41  to the sheet of paper (P), and remains on the transfer roller  41 . Accordingly, as the toner continues to accumulate on the transfer roller  41 , the peeling force of the transfer roller  41  increases. In addition, since the fuser roller  43  continues to have a peeling force smaller than that of the transfer roller  41 , the toner accumulated on the transfer roller  41  cannot easily move to the fuser roller  43 , and the toner continues to accumulate on the transfer roller  41  resulting in poor transfer.  
           [0014]    In addition, the peeling force of the transfer roller  41  increases beyond 500 g/inch when the number of printed sheets is greater than 2000 in area S 3 , and therefore the difference in the peeling forces of the sheet of paper (P) and the transfer roller  41  begins to decrease. In addition, since the peeling force of the employed toner itself is about 600 g/inch, there is a problem in that the sheet of paper (P) wraps around the transfer roller  41 , and cannot be discharged to the outside to result in a paper jam in the printer.  
         SUMMARY OF THE INVENTION  
         [0015]    To solve the above problems, it is an objective of the present invention to provide an electrophotographic printing apparatus adapted to set up the relationships of the peeling forces required at a photosensitive belt, a transfer roller, and a fuser roller, and to adjust the peeling force of the transfer roller to decrease the possibility of a bad image transfer.  
           [0016]    Accordingly, to achieve the above objective, there is provided a liquid electrophotographic printing apparatus including: a photosensitive belt circulating around a predetermined track; a transfer roller to which an image developed by developing units on the photosensitive belt is transferred while the transfer roller contacts and is rotated by the photosensitive belt; a fuser roller providing a pressing force for the transfer roller so that the toner image transferred to the transfer roller can be transferred to a sheet of paper passing through between the transfer roller and the fuser roller; a fuser roller cleaning device contacting the fuser roller continuously or intermittently and maintaining the peeling force of the fuser roller so that the value of the peeling force of the fuser roller can be controlled to within a predetermined range; and a peeling force adjusting device for maintaining the peeling force of the transfer roller so that the value of the peeling force of the transfer roller can be controlled to within a predetermined range, wherein the surface energies SE T , SE P , and SE F  of the transfer roller, the paper sheet, and the fuser roller, respectively, satisfy the following inequality  
           SE T &lt;SE P &lt;SE F .  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    The above objective and advantages of the present invention will become more apparent by describing in detail preferred embodiments thereof with reference to the attached drawings in which:  
         [0018]    [0018]FIG. 1 is a schematic diagram illustrating a conventional electrophotographic printing apparatus;  
         [0019]    [0019]FIG. 2 is a graph illustrating the relationships of the peeling forces of a photosensitive belt, a transfer roller, and a fuser roller of the printing apparatus shown in FIG. 1 according to the number of printed sheets of paper;  
         [0020]    [0020]FIG. 3 is a diagram illustrating an electrophotographic printing apparatus according to an embodiment of the present invention; and  
         [0021]    [0021]FIG. 4 is a graph illustrating the relationships of the peeling forces of a photosensitive belt, a transfer roller, and a fuser roller of the printing apparatus shown in FIG. 3 according to the number of printed sheets of paper. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]    Referring to FIG. 3, a liquid electrophotographic printing apparatus according to a preferred embodiment of the present invention comprises a photosensitive belt  110  circulating around a predetermined track, laser scanning units (LSU) for sequentially scanning the photosensitive belt  110  with respective beams to form respective electrostatic images, developing units  120  for sequentially developing the respective electrostatic latent images on the photosensitive belt  110 , a drying unit  130  for drying the carrier remaining in the toner image formed on the photosensitive belt  110 , a transfer unit  140  for transferring the image (I) transported by the photosensitive belt  110  to a sheet of paper (P), a fuser roller cleaning device  150 , and a peeling force adjusting device  160 .  
         [0023]    The photosensitive belt  110  circulates around a driving roller  111 , a transfer backup roller  112 , and a steering roller  113 . In the vicinity of the photosensitive belt  110 , a discharging device  103  for lowering the potential formed on the photosensitive belt  110  to a constant level by illuminating the photosensitive belt  110 , and a charging device  105  for charging the photosensitive belt  110  discharged by the discharging device  103  to a predetermined potential level, are installed.  
         [0024]    In addition, the drying unit  130  comprises a drying roller  131  which contacts the surface of the photosensitive belt  110  on which an image is formed for absorbing the remaining carrier, and a regeneration roller  133  for heating the surface of the drying roller  131  and evaporating the absorbed carrier thereof.  
         [0025]    The transfer unit  140  comprises a transfer roller  141  and a fuser roller  143 . The transfer roller  140  is disposed to face the transfer backup roller  112  with the photosensitive belt  110  disposed therebetween so that the image (I) developed on the photosensitive belt  110  can be transferred thereto. The fuser roller  143  is disposed to face the transfer roller  141  with the sheet of paper (P) passing therebetween for fusing the image on the sheet of paper (P) so that the image can be transferred to the paper sheet (P).  
         [0026]    Here, the surface energies SE T , SE P , and SE F  of the transfer roller  141 , the paper sheet (P), and the fuser roller  143 , respectively, satisfy the inequality in Equation (1) below.  
         SE T &lt;SE P &lt;SE F   (1)  
         [0027]    Preferably, the peeling forces PF O , PF T , and PF F  of the photosensitive belt  110 , the transfer roller  141 , and the fuser roller  143 , respectively, satisfy the inequalities in Equation (2) below, even when the number of printed sheets of paper increases.  
         5≦PF O ≦200 (g/inch)  
         200≦PF T ≦500 (g/inch)  
         900≦PF F ≦1200 (g/inch)  (2)  
         [0028]    Here, the peeling force PF P  of the sheet of paper (P) is a fixed value in the range of 600˜800 g/inch depending on the material of the sheet of paper (P).  
         [0029]    It is preferable that the fuser roller  143  is made of a metal such as aluminum (Al) and aluminum alloys whose peeling force is greater than that of the transfer roller so that the inequality of Equation (1) can be satisfied. As a result of measurement, in general, the peeling force of aluminum is about 900 g/inch. In an embodiment of the present invention, aluminum heat-treated to have a peeling force of 1200 g/inch is employed. Therefore, since the toner and other foreign materials remaining on the transfer roller  141  are easily moved to the fuser roller  143 , and the fuser roller  143  serves to clean the transfer roller  141 , an increase in the peeling force of the transfer roller  141  is prevented.  
         [0030]    In addition, since the peeling force of the toner itself is about 600 g/inch, and is lower than that of aluminum, when the toner and other foreign materials continue to accumulate on the fuser roller  143 , the peeling force of the fuser roller  143  decreases gradually due to the accumulated toner. As described above, when the peeling force of the fuser roller  143  is lowered, the back surface of the paper sheet (P) may become contaminated, or a picking event in which an image is partially picked off may occur when both sides of the paper sheet are printed.  
         [0031]    The fuser roller cleaning device  150  is intended to restrain the peeling force of the fuser roller from being lowered as described above. The fuser roller cleaning device  150  comprises a cleaning belt  151  contacting the fuser roller  143 , a supply reel  152  around which the cleaning belt  151  is wound, a take-up reel  153  for winding the cleaning belt  151 , and a pressing device for causing the cleaning belt  151  to selectively contact the fuser roller  143 .  
         [0032]    The pressing device is intended to cause the cleaning belt  151  which is normally spaced from the fuser roller  143  to contact the fuser roller  143  after a predetermined number of sheets of paper have been printed. The pressing device includes a pressing member  155  disposed to face the fuser roller  143  with the cleaning belt  151  disposed therebetween, an elastic member  156  for elastically biasing the pressing member  155  so that the pressing member  155  can cause the cleaning belt  151  to contact the fuser roller  143 , and a cylinder  157  for separating the pressing member  155  from the cleaning belt  151 . In addition, it is preferable that a carrier supplying device  159  is installed at the fuser roller cleaning device  150  for supplying a liquid carrier to the contacting portion between the cleaning belt  151  and the fuser roller  143  when the cleaning belt  151  contacts the fuser roller  143 .  
         [0033]    The peeling force adjusting device  160  adjusts the peeling forces so that the peeling force of the transfer roller  141  can be greater than that of the photosensitive belt  110 , and can be smaller than that of the sheet of paper (P) in a printing mode. The peeling force adjusting device  160  comprises a friction member  161  for removing foreign materials remaining on the photosensitive belt  110  and the transfer roller  141 , and a pressing device for causing the friction member  161  to intermittently contact or be separated from the photosensitive belt  110  and the transfer roller  141 . The friction member  161  contacts the photosensitive belt  110  and the transfer roller  141  simultaneously.  
         [0034]    In addition, it is preferable that the peeling force adjusting device  160  further includes a carrier supply nozzle  165  so that when the friction member  161  contacts the photosensitive belt  110  and the transfer roller  141 , the liquid carrier can be supplied to the friction member  161 . The pressing member includes a spring member  163  for pressing the friction member  161  toward the photosensitive belt  110  and the transfer roller  141 , and a cylinder  164  for separating the spring member  163  from the photosensitive belt  110  and the transfer roller  141 . The friction member  161  intermittently contacts the photosensitive belt  110  and the transfer roller  141 , and removes foreign materials such as toner remaining on the photosensitive belt  110  and the transfer roller  141 . Therefore, this restrains the peeling forces of the photosensitive belt  110  and the transfer roller  141  from increasing.  
         [0035]    Now, the operation of the liquid electrophotographic printing apparatus configured as described according to an embodiment of the present invention will be described in detail with reference to FIGS. 3 and 4.  
         [0036]    At the initial stage of printing, the carrier is supplied to the friction member  161  while the friction member  161  is caused to contact the surfaces of the photosensitive belt  110  and the transfer roller  141 . When the carrier is supplied to the photosensitive belt  110  and the transfer roller  141 , the peeling force of the transfer roller  141  increases faster than that of the photosensitive belt  110 , as shown in hatched area S 5  of FIG. 4, and the difference in the peeling forces of the photosensitive belt  110  and the transfer roller  141  becomes larger. Therefore, the efficiency of image transfer from the photosensitive belt  110  to the transfer roller  141  can be enhanced.  
         [0037]    Thereafter, when the peeling force of the transfer roller  141  becomes greater than 400 g/inch as indicated by the dotted line A of FIG. 4 as the number of printed sheets of paper surpasses  1000 , bad transfer T 2  is caused. The peeling force adjusting device  160  restrains the peeling force from increasing as indicated by dotted line A. That is, when about 1000 sheets of paper are printed, the printing operation is stopped, and the toner remaining on the transfer roller  141  is removed by causing the friction member  161  to contact the transfer roller  141 . In addition, while the toner remaining on the transfer roller  141  is removed, the friction member  161  is also caused to contact the photosensitive belt  110 , and the peeling force of the photosensitive belt  110  is caused to decrease at the same time. Preferably, such a peeling force adjusting operation is performed at a predetermined intervals, for example, at every  100  printed sheets considering that the peeling force of the transfer roller increases in a predetermined pattern.  
         [0038]    In addition, it is preferable that the peeling forces are lowered after a printing operation by cleaning the photosensitive belt  110  and the transfer roller  141  so that foreign materials such as the toner attached to the photosensitive belt  110  and the transfer roller  141  may not affect the subsequent printing operation.  
         [0039]    Further, the cleaning belt  151  is caused to contact the surface of the fuser roller  143  continuously or intermittently to remove foreign materials such as the toner remaining on the fuser roller  143 , and therefore the peeling force of the fuser roller  143  is restrained from decreasing as indicated by dotted line B of FIG. 4.  
         [0040]    The toner remaining on the fuser roller  143  is removed by the cleaning belt  151  which is caused to contact the fuser roller  143  by the pressing member  155 . At this time, a carrier is supplied to the contacting portion of the cleaning belt  151  and the fuser roller  143  so that the toner attached to the fuser roller  143  is easily moved to the cleaning belt  151 .  
         [0041]    In addition, the cleaning belt  151  is moved intermittently so that the cleaning performance thereof does not deteriorate due to the transferred toner. In an embodiment of the present invention, the take-up reel is rotated at every  100  printed sheets, and the cleaning belt  151  is moved about 2˜4 mm from the supply reel to the take-up reel each time.  
         [0042]    [0042]FIG. 4 shows a graph illustrating the variations of the peeling force of the fuser roller  143  when the cleaning belt  151  is caused to contact the fuser roller  143  at a period of 50 printed sheets. As a result of measuring the peeling force, the peeling force of the fuser roller  143  was about 1200 g/inch before the printing operation was performed, and the peeling force of the fuser roller  143  decreased to about 900 g/inch after about 50 sheets were printed. In a state in which the peeling force of the fuser roller  143  decreased as described above, the fuser roller  143  was contacted and cleaned by the cleaning belt  151 , and then it was found that the peeling force of the fuser roller  143  increased to about 1200 g/inch. Therefore, since the peeling force of the fuser roller  143  can be controlled to always be greater than that of the paper sheet (P), the toner remaining on the transfer roller  141  can be moved easily to the fuser roller  143 . In addition, since the toner remaining on the fuser roller  143  can be removed, contamination of the back side of the sheet of paper (P) can be prevented.  
         [0043]    In addition, it is preferable that, in the liquid electrophotographic printing apparatus according to an embodiment of the present invention, the pressing force between the transfer roller  141  and the fuser roller  143  is 10 kgf greater than that between the photosensitive belt  110  and the transfer roller  141 .  
         [0044]    For example, referring to FIG. 3, when, in transfer T 1 , the pressing force applied between the photosensitive belt  110  and the transfer roller  141  is F1, and in transfer T 2 , the pressing force applied between the transfer roller  141  and the fuser roller  143  is F2, an image formed on the transfer roller  141  is transferred to the sheet of paper (P) in a condition in which F1 is 60 kgf, and F2 is 70 kgf. When the value of F2 is set to be 10 kgf greater than the value of F1, the peeling force of the sheet of paper (P) in transfer T 2  is greater than the peeling force of the transfer roller  141  in transfer T 1 . Therefore, when the contacting condition between the transfer roller  141  and the fuser roller  143  (that is, the pressing condition of the sheet of paper (P) passing through between the transfer roller  141  and the fuser roller  143 ) is enhanced, the efficiency of image transfer to the sheet of paper (P) can be enhanced.  
         [0045]    Though the pressing force F1 between the photosensitive belt  110  and the transfer roller  141  is exemplified in an embodiment of the present invention, it is understood that the set value is not an absolute value, and may be changed in the process of assembling the components, and also the value of F2 can be changed according to the change of the value of F1.  
         [0046]    As described above, since, in the liquid electrophotographic printing apparatus according to the preferred embodiment of the present invention, the relationships of the peeling forces of the photosensitive belt, the transfer roller, and the fuser roller are set, and the peeling force adjusting device is provided for adjusting the peeling forces, the efficiency of image transfer from the photosensitive belt to the transfer roller, and the efficiency of image transfer from the transfer roller to the paper sheet can be enhanced. Therefore, occurrences of problems such as paper jams in which a sheet of paper wraps around the transfer roller, and contamination of the backside of a sheet of paper can reduced.  
         [0047]    Although particular embodiments of the invention have been described with reference to the accompanying drawings for the purposes of illustration, it should be understood that various modifications and equivalents may be made by those skilled in the art without departing from the spirit and scope of the invention.