Patent Publication Number: US-2007098464-A1

Title: Image transfer unit, electrophotographic image forming apparatus including the same, and electrophotographic image forming method

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS  
      This application claims the benefit under 35 U.S.C. § 119(a) of Korean Patent Application No. 10-2005-0104931, filed on Nov. 3, 2005, in the Korean Intellectual Property Office, the entire disclosure of which is hereby incorporated by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to an electrophotographic image forming apparatus. More particularly, the present invention relates to an image transfer unit substantially preventing defective color registration, an electrophotographic image forming apparatus including the image transfer unit, and an electrophotographic image forming method.  
      2. Description of the Related Art  
      Generally, an electrophotographic image forming apparatus, such as a laser printers or digital copying machine, forms an electrostatic latent image on an outer surface of a photosensitive medium by scanning light onto the photosensitive medium that is charged to a predetermined electric potential. The electrostatic latent image is developed into a visible image by using a developing agent, such as toner. The visible image is transferred and fused onto a printing medium.  
       FIG. 1  is an elevational view in cross section of an image transfer unit in an electrophotographic image forming apparatus according to the conventional art.  
      Referring to  FIG. 1 , the image transfer unit  10  includes a first roller  11  and a second roller  12  arranged in parallel to each other on upper and lower portions of the image transfer unit  10 , and a transfer belt  15  circulating around the first and second rollers  11  and  12 . Transfer rollers  20 Y,  20 M,  20 C, and  20 K are disposed between the first roller  11  and the second roller  12 . Photosensitive media  27 Y,  27 M,  27 C, and  27 K, which are included in developers  25 Y,  25 M,  25 C, and  25 K, face the transfer rollers  20 Y,  20 M,  20 C, and  20 K with the transfer belt  15  being disposed therebetween. The four developers  25 Y,  25 M,  25 C, and  25 K respectively store yellow (Y), magenta (M), cyan (C), and black (K) toners for printing a color image. Each of the developers  25 Y,  25 M,  25 C, and  25 K includes one of four photosensitive media  27 Y,  27 M,  27 C, and  27 K, on which four color toner images are formed, respectively. Additionally, four transfer nips N 1 , N 2 , N 3 , and N 4  are formed by the four photosensitive media  27 Y,  27 M,  27 C, and  27 K contacting the transfer belt  15 . A printing medium drawing roller  30  is disposed on an opposite side of the second roller  12 , with the transfer belt  15  being disposed therebetween. When a predetermined voltage is applied to the printing medium drawing roller  30 , static electricity is induced to a printing medium P, and thus, the printing medium P is drawn to the transfer belt  15  and is conveyed upwardly.  
      During the image transfer process of the image transfer unit  10 , linear velocities of the outer circumferences of the four photosensitive media  27 Y,  27 M,  27 C, and  27 K are the same as a linear velocity of the transfer belt  15 . However, even if the linear velocities are designed to be the same, the linear velocities of the outer circumferences of the photosensitive media  27 Y,  27 M,  27 C, and  27 K and the linear velocity of the transfer belt  15  may be a bit different from each other due to a tolerance of the first roller  11  driving the transfer belt  15  or a tolerance of a unit supplying driving power to the photosensitive media  27 Y,  27 M,  27 C, and  27 K or to the first roller  11 .  
      The difference between the linear velocities may cause a color registration defect of the toner image that is transferred to the transfer belt  15 , thereby degrading the printing quality. For example, if it is assumed that the linear velocities of the yellow photosensitive medium  27 Y, on which the Y toner image is formed, and the cyan photosensitive medium  27 C, on which the C toner image is formed, are slower than the linear velocity of the transfer belt  15 , and the linear velocities of the magenta photosensitive medium  27 M, on which the M toner image is formed, and the black photosensitive medium  27 K, on which the K toner image is formed, are faster than the linear velocity of the transfer belt  15 , a part of the printing medium P and the transfer belt  15  around the first and third transfer nips N 1  and N 3  are pressed downwardly by the yellow and cyan photosensitive media  27 Y and  27 C, and a part of the printing medium P and the transfer belt  15  around the second and fourth transfer nips N 2  and N 4  are pressed upwardly by the magenta and black photosensitive media  27 M and  27 K. Therefore, sections of the printing medium P and the transfer belt  15  between the first transfer nip N 1  and the second transfer nip N 2  and the sections between the third transfer nip N 3  and the fourth transfer nip N 4  are tightened. Therefore, the printing medium P and the transfer belt  15  may slip at some of the four transfer nips N 1 , N 2 , N 3 , and N 4 , and thus, color registration defects may occur.  
      Accordingly, a need exists for an improved image transfer unit that substantially eliminates defective color registration.  
     SUMMARY OF THE INVENTION  
      The present invention provides an image transfer unit having an improved structure that reduces color registration defects, and an electrophotographic image forming apparatus including the image transfer unit.  
      The present invention also provides an electrophotographic image forming method substantially preventing the occurrence of color registration defects.  
      According to an aspect of the present invention, an image transfer unit includes at least one photosensitive medium on which an electrostatic latent image is formed by light scanning and a toner image formed by transferring toners onto the electrostatic latent image. A transfer belt is wound on at least a pair of rollers and circulates around the rollers and forms a transfer nip by contacting the photosensitive medium. A linear velocity of the transfer belt is set to be faster than the linear velocity of the outer circumferential surface of the photosensitive medium contacting the transfer belt. An electrophotographic image forming apparatus includes at least one light scanner scanning laser beam corresponding to an image to be printed, and the above image transfer unit.  
      The transfer belt may convey a printing medium by attaching the printing medium on a surface of the transfer belt. The toner image is transferred to the printing medium from the photosensitive medium.  
      The apparatus may include a plurality of photosensitive media to form a plurality of toner images of different colors on the plurality of photosensitive media. The transfer belt may contact the plurality of photosensitive media to form a plurality of transfer nips. The linear velocity of the transfer belt may be set to be faster than the linear velocities of the outer circumferential surfaces of all the photosensitive media.  
      The linear velocity of the transfer belt may be set to be at most 1.004 times faster than the linear velocity of the outer circumferential surface of the fastest photosensitive medium of the plurality of photosensitive media.  
      The transfer belt may be elastically adhered to the photosensitive medium.  
      A driving force for rotating the photosensitive medium may be larger than a driving force for circulating the transfer belt.  
      According to another aspect of the present invention, an electrophotographic image forming method includes forming an electrostatic latent image on an outer circumferential surface of at least one photosensitive medium by scanning a laser beam corresponding to an image to be printed onto the rotating photosensitive medium. A toner image is formed on the outer circumferential surface of the photosensitive medium by transferring toners on the electrostatic latent image. The toner image is transferred toward a transfer belt, which is wound on at least a pair of rollers and circulates around the rollers and forms a transfer nip by contacting the photosensitive medium. A linear velocity of the transfer belt is set to be faster than the linear velocity of an outer circumferential surface of the photosensitive medium contacting the transfer belt.  
      The transfer belt may convey a printing medium by attaching the printing medium on a surface of the transfer belt. The toner image may be transferred to the printing medium from the photosensitive medium in the transferring of the toner image.  
      A plurality of photosensitive media may be provided to form a plurality of toner images of different colors on the plurality of photosensitive media in the forming of the toner image. The toner images of different colors may be transferred from the photosensitive media to the transfer belt sequentially in the transferring of the toner image. The linear velocity of the transfer belt may be faster than the linear velocities of the outer circumferential surfaces of all photosensitive media.  
      The linear velocity of the transfer belt may be at most 1.004 times faster than the linear velocity of the outer circumferential surface of the fastest photosensitive medium of the plurality of photosensitive media.  
      Other objects, advantages and salient features of the invention will become apparent from the following detailed description, which, taken in conjunction with the annexed drawings, discloses exemplary embodiments of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings, in which:  
       FIG. 1  is an elevational view in cross section of an image transfer unit of an electrophotographic image forming apparatus according to the conventional art;  
       FIG. 2  is an elevational view in cross section of an electrophotographic image forming apparatus according to an exemplary embodiment of the present invention; and  
       FIG. 3  is an elevational view in cross section of an image transfer unit of the apparatus of  FIG. 2 . 
    
    
      Throughout the drawings, like reference numerals will be understood to refer to like parts, components and structures.  
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       FIG. 2  is an elevational view in cross section of an electrophotographic image forming apparatus according to an exemplary embodiment of the present invention.  FIG. 3  is an elevational view in cross section of an image transfer unit in the electrophotographic image forming apparatus of  FIG. 2 .  
      Referring to  FIG. 2 , the electrophotographic image forming apparatus  100  is a direct transfer type color image forming apparatus in which visible toner images of different colors are sequentially transferred onto a printing medium to form a color image directly on the printing medium P. The electrophotographic image forming apparatus  100  includes four developers  110 Y,  110 M,  110 C, and  110 K, four light scanners  125 Y,  125 M,  125 C, and  125 K, an image transfer unit  130 , and a fuser  150 , all of which are accommodated in a case  101 . Additionally, the image forming apparatus  100  further includes a paper cassette  127  loading printing media P, a pickup roller  128  picking a printing medium P from the paper cassette  127  one by one, a conveying roller  129  conveying the picked printing medium P, and a discharge roller  153  discharging out of the case  101  the printing medium P on which an image is printed.  
      The developers  110 Y,  110 M,  110 C, and  110 K are of a cartridge type, and may be replaced when toner, that is, a developing agent, contained therein is exhausted. In  FIG. 2 , the four developers include toners of different colors, for example, yellow (Y), cyan (C), magenta (M), and black (K) colors, respectively.  
      When a door  102  on a side of the case  101  is opened, a transfer belt  135  is disposed in a lateral direction in communication with the opening of the door  102 , and thus, the developers  110 Y,  110 M,  110 C, or  110 K, the toner of which is exhausted, may be replaced.  
      According to an exemplary embodiment of the present embodiment, four light scanners  125 Y,  125 M,  125 C, and  125 K are formed to correspond to the four developers  110 Y,  110 M,  110 C, and  110 K. Each of the light scanners  125 Y,  125 M,  125 C, and  125 K scans a laser beam corresponding to image information of Y, M, C, and K to photosensitive media  145 Y,  145 M,  145 C, and  145 K installed in the developers  110 Y,  110 M,  110 C, and  110 K, respectively. Alternatively, laser scanning units (LSUs) using a laser diode as a light source may be used as the light scanner  125 Y,  125 M,  125 C, and  125 K.  
      The developers  110 Y,  110 M,  110 C, and  110 K respectively include the photosensitive media  145 Y,  145 M,  145 C, and  145 K and developing rollers  115 Y,  115 M,  115 C, and  115 K. The outer circumferential surfaces of the photosensitive media  145 Y,  145 M,  145 C, and  145 K contact the transfer belt  135  to transfer toner images. Additionally, the developers  110 Y,  110 M,  110 C, and  110 K respectively include charging rollers  119 Y,  119 M,  119 C, and  119 K. Charging biase voltages are applied to the charging rollers  119 Y,  119 M,  119 C, and  119 K to charge the outer circumferential surfaces of the photosensitive media  145 Y,  145 M,  145 C, and  145 K to a constant electric potential.  
      The toners are attached to the outer circumferential surfaces of the developing rollers  115 Y,  115 M,  115 C, and  115 K, and then, supplied to the photosensitive media  145 Y,  145 M,  145 C, and  145 K. Developing bias voltages are applied to the developing rollers  115 Y,  115 M,  115 C, and  115 K to supply toners to the photosensitive media  145 Y,  145 M,  145 C, and  145 K. Additionally, although not shown in the drawings, each of the developers  110 Y,  110 M,  110 C, and  110 K includes a supplying roller for supplying the toner to the developing roller  115 Y,  115 M,  115 C, or  115 K, a doctor blade for controlling an amount of the toner attached on the developing roller  115 Y,  115 M,  115 C, or  115 K, and an agitator for agitating the toner respectively received in the developers  110 Y,  110 M,  110 C, or  110 K and conveying the toner to the supplying roller.  
      The image transfer unit  130  includes the four photosensitive media  145 Y,  145 M,  145 C, and  145 K. Additionally, the image transfer unit  130  includes a first roller  131 , that is, a driving roller, and a second roller  132 , that is, a slave roller, disposed under the first roller  131  in parallel to the first roller  131 . The transfer belt  135  is wound on the first and second rollers  131  and  132  to circulate thereon. Four transfer rollers  140 Y,  140 M,  140 C, and  140 K are disposed between the first roller  131  and the second roller  132 . Additionally, the image transfer roller  130  also includes auxiliary supporting rollers  133  and  134  supporting the transfer belt  135 . The four transfer rollers  140 Y,  140 M,  140 C, and  140 K are disposed on opposite sides of the four photosensitive media  145 Y,  145 M,  145 M, and  145 K with the transfer belt  135  being disposed therebetween. A transfer bias voltage is applied to the transfer rollers  140 Y,  140 M,  140 C, and  140 K.  
      A driving force for rotating the photosensitive media  145 Y,  145 M,  145 C, and  145 K in the electrophotographic image forming apparatus  100  is larger than a driving force for circulating the transfer belt  135 . A driving gear (not shown) supplying the driving force is connected to each of the photosensitive media  145 Y,  145 M,  145 M, and  145 K. However, the transfer belt  135  is circulated only by the driving force of the first roller  131 . Additionally, the other rollers  132 ,  133 ,  134 ,  141 Y,  141 M,  141 C, and  141 K are the slave rollers driven by the circulation of the transfer belt  135 , and thus, the driving force of the transfer belt  135  is smaller than the rotational driving force of the photosensitive media  145 Y,  145 M,  145 C, and  145 K.  
      Referring to  FIG. 3 , shafts  141 Y,  141 M,  141 C, and  141 K of the transfer rollers  140 Y,  140 M,  140 C, and  140 K are elastically pressed toward the photosensitive media  145 Y,  145 M,  145 C, and  145 K by springs  143 Y,  143 M,  143 C, and  143 K. The transfer belt  135  is elastically adhered to the photosensitive media  145 Y,  145 M,  145 C, and  145 K by the elastic force, and thus, transfer nips N 1 , N 2 , N 3 , and N 4  may be stably maintained. Additionally, the image transfer unit  130  includes a printing medium drawing roller  148  disposed on an opposite portion of the second roller  132  with the transfer belt  135  disposed therebetween. The printing medium drawing roller  148  charges the printing medium P that is picked from the paper cassette  127  and moved upwardly by using static electricity so that the printing medium P may be adhered onto the surface of the transfer belt  135 .  
      When the toner images are transferred toward the transfer belt  135  from the photosensitive media  145 Y,  145 M,  145 C, and  145 K, the linear velocity of the transfer belt  135  is slightly faster than the linear velocities of the outer circumferential surfaces of the photosensitive media  145 Y,  145 M,  145 C, and  145 K in the image transfer unit  130  to substantially prevent color registration defects from being generated. In an exemplary embodiment, the linear velocity of the transfer belt  135  is faster than the linear velocities of the outer circumferential surfaces of the photosensitive media  145 Y,  145 M,  145 C, and  145 K. Additionally, the linear velocity of the transfer belt  135  may be set to be approximately 1.004 times faster than the linear velocity of the fastest photosensitive medium among the photosensitive media  145 Y,  145 M,  145 C, and  145 K. When the linear velocity of the transfer belt  135  is excessively faster than the linear velocities of the outer circumferential surfaces of the photosensitive media  145 Y,  145 M,  145 C, and  145 K, the transfer belt  135  and the printing medium P adhered onto the transfer belt  135  may slip continuously with respect to the photosensitive media  145 Y,  145 M,  145 C,  145 K at the transfer nip sections N 1 , N 2 , N 3 , and N 4 . Therefore, defective printing or jam of the printing medium P may be generated.  
      Angular velocities of the photosensitive media  145 Y,  145 M,  145 C, and  145 K or an angular velocity of the first roller  131 , that is, the driving roller, of the transfer belt  135  may be changed to set the linear velocity of the transfer belt  135  to be higher than the linear velocities of the outer circumferential surfaces of the photosensitive media  145 Y,  145 M,  145 C, and  145 K. However, this is not easy because the differences between the linear velocities of photosensitive media  145 Y,  145 M,  145 C, and  145 K and the linear velocity of the transfer belt  135  are small in the exemplary embodiments of the present invention. Therefore, the angular velocities of the photosensitive media  145 Y,  145 M,  145 C, and  145 K and the angular velocity of the first: roller  131  may be set as in the conventional art, and diameters of the photosensitive media  145 Y,  145 M,  145 C, and  145 K or a diameter of the first roller  131  may be set to be different from those of the conventional art to set the linear velocity of the transfer belt  135  to be faster than the outer circumferential linear velocities of the photosensitive media  145 Y,  145 M,  145 C, and  145 K. Otherwise, the diameters of the photosensitive media  145 Y,  145 M,  145 C, and  145 K are substantially the same as in the conventional art and the diameter of the first roller  131  is slightly larger than in the conventional art to set the linear velocity of the transfer belt  135  faster than the outer circumferential linear velocities of the photosensitive media  145 Y,  145 M,  145 C, and  145 K. Through the above processes, the image transfer unit  130  of the exemplary embodiments of the present invention may be manufactured easily though it is produced through the manufacturing management system with the same tolerances as in the conventional art.  
      Hereinafter, printing processes of the electrophotographic image forming apparatus  100  are described with reference to  FIGS. 2 and 3 .  
      The photosensitive media  145 Y,  145 M,  145 C, and  145 K are charged with a constant electric potential by the charging bias voltages applied to the charging rollers  119 Y,  119 M,  119 C, and  119 K. The four light scanners  125 Y,  125 M,  125 C, and  125 K scan laser beams corresponding to Y, M, C, and K image information to the photosensitive media  145 Y,  145 M,  145 C, and  145 K. Then, Y, M, C, and K electrostatic latent images are formed on the outer circumferential surfaces of the photosensitive media  145 Y,  145 M,  145 C, and  145 K. Developing bias voltages are applied to the developing rollers  115 Y,  115 M,  115 C, and  115 K. Then, the toners are moved from the developing rollers  115 Y,  115 M,  115 C, and  115 K to the outer circumferential surfaces of the photosensitive media  145 Y,  145 M,  145 C, and  145 K. Thus, Y, M, C, and K visible toner images are formed on the outer circumferential surfaces of the photosensitive media  145 Y,  145 M,  145 C, and  145 K.  
      The printing medium P is picked by the pickup roller  128  from the paper cassette  127 , and is fed by the conveying roller  129 . When a predetermined voltage is applied to the printing medium drawing roller  148 , the printing medium P fed upwardly is charged by static electricity and adhered onto the surface of the transfer belt  135 , and is conveyed at the same velocity as the linear velocity of the transfer belt  135 .  
      A front edge of the printing medium P that is adhered onto the transfer belt  135  to be conveyed reaches the first transfer nip N 1  at the time when a front edge of the Y toner image formed on the outer circumferential surface of the lowermost photosensitive medium  145 Y reaches the first transfer nip N 1  that corresponds to the transfer belt  135 . At this time, when the transferring bias is applied to the transfer roller  140 Y, the Y toner image formed on the photosensitive medium  145 Y is transferred onto the printing medium P. Additionally, as the printing medium P is conveyed, the M, C, and K toner images formed on the other photosensitive media  145 M,  145 C, and  145 K are transferred onto the printing medium P sequentially and overlap each other. Thus, a color toner image is formed on the printing medium P. The fuser  150  applies heat and pressure onto the printing medium P to fuse the color toner image on the printing medium P. The printing medium P on which the toner image is completely fused is discharged out of the case  101  by the discharge roller  153 .  
      As described above, the linear velocity of the transfer belt  135  is slightly faster than the outer circumferential linear velocities of the photosensitive media  145 Y,  145 M,  145 C, and  145 K during the printing processes. However, the photosensitive media  145 Y,  145 M,  145 C, and  145 K and the transfer belt  135  are adhered to form the transfer nips N 1 , N 2 , N 3 , and N 4 , and the rotational driving forces of the photosensitive media  145 Y,  145 M,  145 C, and  145 K are larger than the driving force of the transfer belt  135 . Therefore, the transfer belt  135  and the printing medium P attached on the transfer belt  135  by the static electricity do not slip with respect to the photosensitive media  145 Y,  145 M,  145 C, and  145 K at the transfer nips N 1 , N 2 , N 3 , and N 4 . Instead, the transfer belt  135  and the printing medium P travel at substantially the same velocity as the outer circumferential linear velocities of the photosensitive media  145 Y,  145 M,  145 C, and  145 K at the transfer nips N 1 , N 2 , N 3 , and N 4 , and portions of the transfer belt  135  and the printing medium P under the transfer nips N 1 , N 2 , N 3 , and N 4  are pressed downwardly as shown in  FIG. 3 . Because the transfer belt  135  and the printing medium P are pressed downwardly around the transfer nips N 1 , N 2 , N 3 , and N 4 , the possibility of slips of the transfer belt  135  and the printing medium P is substantially reduced. Additionally, because the transfer belt  135  and the printing medium P travel at substantially the same velocity as that of the outer circumferential surfaces of the photosensitive media  145 Y,  145 M,  145 C, and  145 K at the transfer nips N 1 , N 2 , N 3 , and N 4 , the color registration defects of the four (YMCK) toner images that are transferred to the printing medium P may be reduced.  
      According to exemplary embodiments of the present invention, the linear velocity of the transfer belt and the outer circumferential linear velocity of the photosensitive media are substantially the same at the transfer nips. Thus, the printing medium and the transfer belt do not slip at the transfer nips and the color registration defects may be prevented during the transfer of the toner images.  
      While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims. For example, technical features of the present invention may be applied to an electrophotographic image forming apparatus of intermediate transfer type, that is, a toner image is transferred onto a surface of a transfer belt from a photosensitive medium, and then the toner image is transferred onto a printing medium.