Abstract:
An electrophotographic image forming apparatus for enhancing color registration is provided. The electrophotographic image forming apparatus includes a developing unit including a photosensitive medium, an image transferring device, and a paper feeding device. The image transferring device includes a pair of conveying rollers, a conveying belt, and a plurality of transfer rollers. The conveying belt is supported by the conveying rollers and rotates around the conveying rollers and conveys a recording medium by attaching a recording medium to a surface thereof. The plurality of transfer rollers are disposed inside the conveying belt and form a transfer nip between the recording medium and the photosensitive medium. The paper feeding device provides the recording medium to the image transferring device. The force of the image transferring device that maintains the position of the recording medium with respect to the conveying belt during image transferring is equal to or greater than the maximum force applied to the recording medium by the paper feeding device.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS  
       [0001]     This application claims the benefit under 35 U.S.C. § 119(a) of Korean Patent Application No. 10-2005-0089505, filed on Sep. 26, 2005 in the Korean Intellectual Property Office, the entire disclosure of which is hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to an electrophotographic image forming apparatus. More particularly, the present invention relates to an electrophotographic image forming apparatus which enhances the transfer quality of a toner image transferred from a photosensitive medium to a sheet of paper.  
         [0004]     2. Description of the Related Art  
         [0005]     An electrophotographic image forming apparatus produces an image by scanning light corresponding to a desired image onto a photosensitive medium charged to a uniform electrostatic potential, thereby forming an electrostatic latent image. The electrostatic latent image is developed into a visible image by applying a developer, such as toner, onto the electrostatic latent image. The developed image is then transferred from the photosensitive medium onto a sheet of paper, and the transferred image is fused onto the paper.  
         [0006]     An image forming apparatus which produces a color image develops yellow, magenta, cyan, and black images using developer cartridges containing yellow, magenta, cyan, and black developers, respectively. The developed color images are superimposed upon each other to form a full-color image. A color image forming apparatus may include a plurality of developer cartridges that share a single photosensitive medium or a plurality of developer cartridges that include individual photosensitive media.  
         [0007]     In a conventional electrophotographic image forming apparatus, the recording medium is supplied by a paper feeding device. While an image is being transferred onto a leading portion of a recording medium, a trailing portion of the recording medium is affected by the paper feeding device. This may cause slip between the photosensitive media and the recording medium.  
         [0008]     When a developed image is transferred onto the recording medium, the recording medium is firmly pressed against each photosensitive medium by transfer rollers which correspond to the developer cartridges. In this case, when the first developed color is transferred (typically, the yellow developer), the friction force between the conveying device, the photosensitive media and the recording medium is relatively small. Because there is little friction force, the force applied by the paper feeding device to the recording medium may cause an increased color registration error.  
         [0009]     Accordingly, there is a need for an improved image forming apparatus which minimizes color registration errors caused by the paper feeding device.  
       SUMMARY OF THE INVENTION  
       [0010]     The present invention relates to a color image forming apparatus including a plurality of developer cartridges which each include a photosensitive media. The color image forming apparatus includes a conveying device which is installed opposite to each of the photosensitive media included in the developer cartridges, and forms a color image by transferring toner from the developer cartridges onto a desired image on a recording medium, for example, a sheet of paper, in a superimposed manner while the conveying device is conveying the recording medium. A paper feeding device is installed in front of the conveying device to feed the recording medium to the conveying device.  
         [0011]     When a color image is formed, the developers of four colors should overlap with each other at a precise location, that is, there should be precise color registration. Therefore, the recording medium conveying speed and the times that developer is transferred from each developer cartridge are accurately controlled. Furthermore, slip is minimized while a recording medium is conveyed by a plurality of developer cartridges.  
         [0012]     An aspect of the present invention is to provide an electrophotographic image forming apparatus which provides accurate color registration of a print image by preventing a recording medium from being moved by a paper feeding device while a color image is transferred to the recording medium.  
         [0013]     According to an aspect of the present invention, an electrophotographic image forming apparatus comprises a developing unit including a photosensitive medium, an image transferring device and a paper feeding device. The image transferring device comprises a pair of conveying rollers, a conveying belt, and a plurality of transfer rollers. The conveying belt is supported by the conveying rollers, rotates around the conveying rollers, and conveys a recording medium by attaching the recording medium to a surface thereof. The plurality of transfer rollers are disposed inside the conveying belt, and form transfer nips with the photosensitive medium. The paper feeding device provides the recording medium to the image transferring device. The force of the image transferring device that maintains a position of the recording medium with respect to the conveying belt during image transferring is equal to or greater than the maximum force applied to the recording medium by the paper feeding device.  
         [0014]     The force of the image transferring device that maintains the position of the recording medium with respect to the conveying belt and the force applied to the recording medium from the paper feeding roller are friction forces. The direction of the friction force is opposite to a sliding direction of the printing medium, and the maximum static friction force is calculated by multiplying a normal force and a friction coefficient. Thus, the maximum static friction can be easily controlled by controlling the normal forces of rollers pressing the recording medium, such as a paper charging roller, transfer rollers, and feeding rollers.  
         [0015]     The feeding device generally picks up a recording medium which is stacked in a feeding cassette, that is, a sheet of paper, and feeds the paper to the image transferring device. According to an embodiment of the present invention, the feeding device is feeding rollers that convey the paper to the image transferring device. The feeding rollers may affect the printing medium during image transferring. The present invention is not limited to feeding rollers, however, and encompasses other paper feeding devices that feed a printing medium and affect the recording medium during image transfer.  
         [0016]     When the recording medium is provided to the image transferring device, the conveying belt and the paper charging roller receive the recording medium. A toner image starts to be transferred to the recording medium at about the time when a leading end of the recording medium, which is attached to the conveying belt by electrostatic forces, arrives at a transfer nip formed by a photosensitive medium and the conveying belt. At this time, a trailing end of the printing medium is affected by the feeding device.  
         [0017]     In the electrophotographic image forming apparatus, the force applied to the printing medium from the feeding device during image transferring is smaller than a force conveying the printing medium while attaching the printing medium to the conveying belt. In other words, the force that maintains the position of the printing medium with respect to the conveying belt is equal to or larger than the force applied to the printing medium from the feeding device during image transferring. Thus, color registration is improved. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     The above and other objects, features, and advantages of certain exemplary embodiments of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings, in which:  
         [0019]      FIG. 1  is a cross-sectional view of an electrophotographic image forming apparatus according to an exemplary embodiment of the present invention;  
         [0020]      FIG. 2  illustrates an image transferring device and a paper feeding device according to an exemplary embodiment of the present invention;  
         [0021]      FIG. 3  is a detailed view of a recording medium entering portion of an image transferring device according to an exemplary embodiment of the present invention;  
         [0022]      FIG. 4  illustrates friction forces applied to a recording medium when a feeding roller travels at low speed;  
         [0023]      FIG. 5  illustrates friction forces applied to the recording medium when the feeding roller travels at high speed;  
         [0024]      FIG. 6  illustrates color registration errors in a conventional electrophotographic image forming apparatus; and  
         [0025]      FIG. 7  illustrates color registration errors in an electrophotographic image forming apparatus according to an exemplary embodiment of the present invention. 
     
    
       [0026]     Throughout the drawings, the same reference numerals will be understood to refer to the same elements, features, and structures.  
       DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0027]     The matters defined in the description such as a detailed construction and elements are provided to assist in a comprehensive understanding of the exemplary embodiments of the invention and are merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the exemplary embodiments described herein can be made without departing from the scope and spirit of the invention. Also, descriptions of well-known functions and constructions are omitted for clarity and conciseness.  
         [0028]      FIG. 1  is a cross-sectional view of an electrophotographic image forming apparatus  100  according to an exemplary embodiment of the present invention. Referring to  FIG. 1 , the electrophotographic image forming apparatus  100  is a tandem type image forming apparatus in which visible images of different colors are sequentially transferred to a sheet of paper P and superimposed upon each other to form a full-color image on the paper P. The electrophotographic image forming apparatus  100  includes a case  101  in which an image transferring device  140 , a paper feeding device for feeding the paper P to the image transferring device  140 , a fuser  130 , and discharge rollers  132  for discharging the paper P on which an image has been printed to the outside of the case  101  are included. The image transferring device  140  includes four developing units  110 Y,  110 M,  110 C, and  110 K, four light scanners  125 Y,  125 M,  125 C, and  125 K, and a conveying belt  141 . The paper feeding device includes a paper feeding cassette  127  for holding sheets of paper P, a pickup roller  128  for picking up a sheet of paper P from the paper feeding cassette  127 , and feeding rollers  129  for conveying the picked up paper P.  
         [0029]     The developing units  110 Y,  110 M,  110 C, and  110 K are cartridge type developing units which may be replaced with new ones when developer, such as toner, stored therein is exhausted. In the present exemplary embodiment, the four developing units  110 Y,  110 M,  110 C, and  110 K contain yellow, magenta, cyan, and black toners, respectively.  
         [0030]     The image transferring device  140  is laterally arranged with respect to the door  102  and the developing units  110 Y,  110 M,  110 C, and  110 K may be replaced by opening a door  102  formed on a side of the case  101 .  
         [0031]     In the present exemplary embodiment, the four light scanners  125 Y,  125 M,  125 C, and  125 K correspond to the four developing units  110 Y,  110 M,  110 C, and  110 K, respectively. Each of the light scanners  125 Y,  125 M,  125 C, and  125 K scans light corresponding to image information of yellow, magenta, cyan, and black colors to photosensitive media  114 Y,  114 M,  114 C, and  114 K which are included in housings  111 Y,  111 M,  111 C, and  111 K, respectively. The light scanners  125 Y,  125 M,  125 C, and  125 K may be laser scanning units (LSUs) that use a laser diode as a light source.  
         [0032]     The developing units  110 Y,  110 M,  110 C, and  110 K include housings  111 Y,  111 M,  111 C, and  111 K, respectively. The housings  111 Y,  111 M,  111 C, and  111 K include photosensitive media  114 Y,  114 M,  114 C, and  114 K and developing rollers  115 Y,  115 M,  115 C, and  115 K, respectively. The portion of each of the photosensitive media  114 Y,  114 M,  114 C, and  114 K that is located opposite to the conveying belt  141  while printing an image is exposed to the outside of the housings  111 Y,  111 M,  111 C, and  111 K so that the image may be transferred. Furthermore, the developing units  110 Y,  110 M,  110 C, and  110 K include discharging rollers  119 Y,  119 M,  119 C, and  119 K, respectively. A charge bias voltage is applied to the discharge rollers  119 Y,  119 M,  119 C, and  119 K to charge the outer circumferential surface of each of the photosensitive media  114 Y,  114 M,  114 C, and  114 K to a uniform electrostatic potential.  
         [0033]     Toner is attached to the outer circumferential surfaces of the developing rollers  115 Y,  115 M,  115 C, and  115 K and then provided to the photosensitive media  114 Y,  114 M,  114 C, and  114 K. A developing bias voltage is applied to the developing rollers  115 Y,  115 M,  115 C, and  115 K to supply the toner to the photosensitive media  114 Y,  114 M,  114 C, and  114 K. The housings  111 Y,  111 M,  111 C, and  111 K contain supply rollers (not illustrated) that provide toner to the developing rollers  115 Y,  115 M,  115 C, and  115 K, doctor blades (not illustrated) that regulate the amount of toner attached to the developing rollers  115 Y,  115 M,  115 C, and  115 K, and agitators (not illustrated) that agitate the toner contained in the housings  111 Y,  111 M,  111 C, and  111 K for supplying toner to the supply roller and preventing the toner from hardening. In the present exemplary embodiment, each of the developing units  110 Y,  110 M,  110 C, and  110 K respectively includes an opening  112 Y,  112 M,  112 C, and  112 K that forms a path for allowing light to be scanned from the light scanning units  125 Y,  125 M,  125 C, and  125 K onto the photosensitive media  114 Y,  114 M,  114 C, and  114 K.  
         [0034]     The image transferring device  140  includes a first conveying roller  143 , a second conveying roller  145 , a conveying belt  141 , four transfer rollers  150 Y,  150 M,  150 C, and  150 K and auxiliary supporting rollers  147 . The first conveying roller  143  is a driving roller, and the second conveying roller  145  is an idle roller. The second conveying roller is disposed under the first conveying roller  143 . The conveying belt  141  is supported by the first and second conveying rollers  143  and  145  and rotates around the first and second conveying rollers  143  and  145 . Auxiliary supporting rollers  147  also support the conveying belt  141 . The four transfer rollers  150 Y,  150 M,  150 C, and  150 K are arranged between the first and second conveying rollers  143  and  145 , and are arranged opposite to the four photosensitive media  114 Y,  114 M,  114 C, and  114 K, respectively, so that the conveying belt  141  is interposed therebetween. A transfer bias voltage is applied to the transfer rollers  150 Y,  150 M,  150 C, and  150 K.  
         [0035]     When the four transfer rollers  150 Y,  150 M,  150 C, and  150 K are elastically pressed to the conveying belt  141 , a transfer nip is formed between the transfer rollers  150 Y,  150 M,  150 C, and  150 K and the photosensitive medium. Also, a normal force that produces friction between the paper P, the conveying belt  141  and the photosensitive medium is generated. The normal force may be generated by an elastic member (not shown) that presses a shaft of the transfer roller. Preferably, the normal force is the same intensity for the four transfer rollers  150 Y,  150 M,  150 C, and  150 K.  
         [0036]     The image transferring device  140  includes a paper charging roller  152  disposed opposite to the second conveying roller  145 . The conveying belt  141  is interposed between the paper charging roller  152  and the second conveying roller  145 . The paper charging roller  152  charges the paper P, which is picked up from the paper feeding cassette  127  and upwardly conveyed, by electrostatic induction so that the paper P is attached to a surface of the conveying belt  141 . When the paper charging roller  152  is elastically pressed to the conveying belt  141 , the force of the paper charging roller  152  pressing the paper P acts as a normal force that causes friction, thereby maintaining the position of the paper P with respect to the conveying belt  141 . Therefore, the maximum static fiction force between the paper P and the paper charging roller  152  can be easily controlled by controlling the elastic force of the elastic member that presses the paper charging roller  152  toward the conveying belt  141 .  
         [0037]     An example of a process of forming a color image with the electrophotographic image forming apparatus described above will now be described.  
         [0038]     A charge bias voltage is applied to each of the photosensitive media  114 Y,  114 M,  114 C, and  114 K to charge the photosensitive media  114 Y,  114 M,  114 C, and  114 K to a uniform electrostatic potential. The light scanners  125 Y,  125 M,  125 C, and  125 K scan light corresponding to yellow, magenta, cyan, and black images to the photosensitive media  114 Y,  114 M,  114 C, and  114 K through the openings  112 Y,  112 M,  112 C, and  112 K, respectively. Thus, an electrostatic latent image is formed on the outer circumferential surface of each of the photosensitive media  114 Y,  114 M,  114 C, and  114 K. When a developing bias voltage is applied to the developing rollers  115 Y,  115 M,  115 C, and  115 K, the toner is transferred from the developing rollers  115 Y,  115 M,  115 C, and  115 K onto the outer circumferential surfaces of the photosensitive media  114 Y,  114 M,  114 C, and  114 K so that visible yellow, magenta, cyan, and black toner images are formed on the outer circumferential surfaces of the photosensitive media  114 Y,  114 M,  114 C, and  114 K, respectively.  
         [0039]     The paper P is picked up from the paper feeding cassette  127  by the pickup roller  128 , and fed by the feeding rollers  129 . When a predetermined voltage is applied to the paper charging roller  152 , the upwardly conveyed paper P is charged by electrostatic induction, then attached to a surface of the conveying belt  141 , and conveyed at the same speed as the speed of the conveying belt  141 . A leading end of the paper P attached to and conveyed by the conveying belt  141  arrives at a transfer nip formed between the opposing transfer roller  150 Y and conveying belt  141  at about the time when a leading end of a yellow visible toner image formed on the circumferential surface of the undermost photosensitive medium  114 Y arrives at the transfer nip. At this moment, when a transfer bias voltage is applied to the transfer roller  150 Y, the toner image formed on the photosensitive medium  114 Y is transferred to the paper P. As the paper P is continuously moved, magenta, cyan, and black toner images formed on the other photosensitive media  114 M,  114 C, and  114 K are sequentially transferred to the paper P so that they are superimposed upon each other to form a full-color image on the paper P. The fuser  130  fuses the image on the paper P by applying heat and pressure. The paper P with the completely fused image is discharged outside the case  101  by the discharging roller  132 .  
         [0040]     According to the exemplary embodiment of the present invention, the electrophotographic image forming apparatus  100  maintains the speed of the paper P so that it is the same as the speed of the conveying belt  141 . This prevents the paper P from being affected by the feeding rollers  129  which retain a trailing end of the paper P even when a leading end of the paper P is held in the first transfer nip. Therefore, a color registration error due to the feeding roller  129  can be avoided.  
         [0041]      FIG. 2  illustrates an image transferring device  140  and a paper feeding device according to an exemplary embodiment of the present invention. In the present exemplary embodiment, the four transfer rollers  150 Y,  150 M,  150 C, and  150 K are elastically pressed toward the four opposite photosensitive media  114 Y,  114 M,  114 C, and  114 K. The conveying belt  141  is interposed between the transfer rollers  150 Y,  150 M,  150 C, and  150 K and the photosensitive media  114 Y,  114 M,  114 C, and  114 K. A sheet of paper P is attached to an outer surface of the conveying belt  141  and is conveyed by the conveying belt  141 . The four transfer rollers  150 Y,  150 M,  150 C, and  150 K are preferably pressed with approximately the same force.  
         [0042]     The paper charging roller  152  is disposed in a portion of the image transferring device  140  where the paper P is fed into the image transferring device  140 . The paper charging roller  152  is disposed opposite to the second conveying roller  145  and the conveying belt  141  is interposed therebetween. The shaft of the paper charging roller  152  is elastically pressed toward the conveying belt  141 .  
         [0043]     A pair of feeding rollers  129  and  126  are installed in front of the image transferring device  140 . More specifically, the feeding roller  129  is a driving roller and the roller  126  is a pressing roller. The pressing roller  126  is elastically pressed toward the feeding driving roller  129 , and conveys the paper P while rotating in contact with the feeding driving roller  129 . The paper P is conveyed due to friction between the pressing roller  126 , feeding driving roller  129  and paper P, and the maximum force applied by the pair of the feeding rollers  129  and  126  is the maximum static friction force.  
         [0044]      FIG. 3  is a detailed view of a recording medium entering portion of an image transferring device according to an exemplary embodiment of the present invention. In  FIG. 3 , the sheet of paper P is fed by the feeding roller  129 , passes by the paper charging roller  152 , and then arrives at the first transfer nip. The first transfer nip is formed by the photosensitive medium  114 Y on which a yellow image is developed and the transfer roller  150 Y.  
         [0045]     Thus, when a leading end of the paper P arrives at the first transfer nip and the yellow image is transferred to the paper P, the paper is subjected to a force T o , which is perpendicular to the direction that the paper travels from the transfer roller  150 Y and photosensitive medium  114 Y, a force T P , which is perpendicular to the direction that the paper travels from the paper charging roller  152  and second conveying roller  145 , and a force T r , which is perpendicular to the direction that the paper travels from the feeding roller  129  and press roller  126 . The forces applied to the paper P from the opposing rollers are action and reaction forces that compensate each other. Hence, the same normal forces are applied to both sides of the paper P.  
         [0046]      FIG. 4  illustrates friction forces applied to a recording medium when a feeding roller travels at low speed. Since the same normal forces are applied to both sides of the paper P when the paper P is pressed, the maximum static friction force f o  applied to the paper P in the first transfer nip can be calculated by multiplying the friction coefficient μ o  and a normal force T o . The friction coefficient μ o  is obtained by adding the friction coefficient between the photosensitive medium and the paper to the friction coefficient between the paper and the conveying belt. In the same fashion, the maximum static friction force f p  generated by the paper charging roller is calculated by multiplying the friction coefficient μ p  and a normal force T p . The friction coefficient μ p  is obtained by adding the friction coefficient between the paper charging roller and the paper to the friction coefficient between the paper and the conveying belt. Moreover, the maximum static friction force f r  generated by the feeding roller is calculated by multiplying a friction coefficient μ r  and a normal force T r . The friction coefficient μ r  is obtained by adding the friction coefficient between the feeding roller and the paper and the friction coefficient between the press roller and the paper. The above friction coefficients are the maximum static friction coefficients.  
         [0047]     When the linear speed of the feeding roller is slower than the conveying speed of the conveying belt, the friction forces f o  and f p  generated by the transfer roller and the paper charging roller are applied to the paper in a paper traveling direction, while the friction force f r  generated by the feeding roller is applied to the paper in a direction opposite to the paper traveling direction. According to the exemplary embodiment of the present invention, in this case, the sum of the maximum static friction force f o  generated by the transfer roller and the maximum static friction force f p  generated by the paper charging roller is greater than the maximum static friction force f r  generated by the feeding roller. Hence, even when the linear speed of the feeding roller is slower than the speed of the conveying belt, slippage between the paper and the photosensitive medium in a transfer nip does not occur, and slippage between the feeding roller and the paper occurs earlier. That is, color registration is not affected.  
         [0048]      FIG. 5  illustrates a friction force applied to the recording medium when the feeding roller travels at high speed. When the linear speed of the feeding roller is faster than the speed of the conveying belt, the paper is curled between a nip at the feeding roller and a nip at the paper charging roller. When a restoring force corresponding to a variation of the paper is applied to both surfaces of the curled portion of the paper, a friction force is generated in the direction illustrated with an arrow in  FIG. 5 . Since a recording medium, such as paper, typically used for an electrophotographic image forming apparatus is thin and flexible, the restoring force generated by the curling is much smaller than the maximum static friction forces at the transfer roller, feeding roller, and paper charging roller, and a maximum static friction force between the conveying belt and the recording medium. Therefore, the curling of the paper does not need to be taken into consideration when the pressing forces of the above rollers are determined.  
         [0049]     Even when the recording medium has a high stiffness, slippage in the transfer nip can be prevented when the sum of the maximum static friction forces f o ′ and f p ′ is greater than the maximum static friction force f r ′ at the feeding roller.  
         [0050]      FIG. 6  illustrates color registration errors in a conventional electrophotographic image forming apparatus, and  FIG. 7  illustrates color registration errors in an electrophotographic image forming apparatus according to an exemplary embodiment of the present invention. The horizontal axis represents the position of an image on the paper, and the vertical axis represents the registration error of each color at the image position.  
         [0051]     In  FIGS. 6 and 7 , □ denotes yellow color registration error, Δ denotes magenta color registration error, ⋄ denotes cyan color registration error, and ◯ denotes black color registration error. Referring to  FIG. 6 , in a conventional image forming apparatus, yellow color registration error is large, and each of the other color registration errors become increasingly larger towards the trailing portion of the paper. On the other hand, referring to  FIG. 7 , in an electrophotographic image forming apparatus according to an exemplary embodiment of the present invention, the color registration errors of all four colors are not large, and the appearances of errors are similar to each other.  
         [0052]     According to the exemplary embodiment of the present invention, an electrophotographic image forming apparatus minimizes the possibility of a recording medium from being moved by a paper feeding device, and thus produces more precise color registration of a print image.  
         [0053]     While the invention has been shown and described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.