Patent Publication Number: US-6704530-B2

Title: Method for forming image in electrophotographic printer

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for forming an image in an electrophotographic printer, and more particularly, to a method for forming an image in an image forming system in which a developing unit is attached and detached from a photosensitive medium and developing proceeds. The present application is based on Korean Application No. 2002-1380, filed Jan. 10, 2002, which is incorporated herein by reference. 
     2. Description of the Related Art 
     In general, an electrophotographic printer such as a color laser printer, has an image forming system in which an electrostatic latent image is formed on a photosensitive medium and then, the electrostatic latent image is developed with a toner, and a developed image passes through a predetermined transferring medium and is transferred onto a paper, and then, the developed image is heated and pressed, and the heated and pressed image is finally fixed on the paper. 
     FIG. 1 illustrates an example of an image forming system of an electrophotographic printer. 
     The image forming system includes a photosensitive drum  10  as a photosensitive medium, a charger  11  for charging the photosensitive drum  10 , a laser scanning unit  12  as an exposure unit, for scanning light on the electrified photosensitive drum  10  and forming an electrostatic latent image, a developing unit  13  for developing the electrostatic latent image with a toner having four colors such as yellow (Y), magenta (M), cyan (C), and black (K), a transferring belt  14  for sequentially overlapping the four color images which are developed on the photosensitive drum  10 , a first transferring roller  14   a  for transferring the image which is developed on the photosensitive drum  10  to the transferring belt  14 , a second transferring roller  14   b  for transferring an image which overlaps four colors on the transferring belt  14  onto the paper, and a fixing unit  15  for heating and pressing the print and fixing the transferred image. Four-color developers  13 -Y,  13 -M,  13 -C, and  13 -K, which are included in the developing unit  13 , are elastically biased by a spring (not shown) in a direction where they are separated from the photosensitive drum  10 . The four color developers selectively move toward the photosensitive drum  10  through the rotation of a cam  13   b , so that developing rollers  13   a  which are provided at the front end of the color developers contact the photosensitive drum  10 . Reference numerals  16 ,  17 ,  18 , and  19  denote a paper cassette, a blade for cleaning a photosensitive drum, an anti-electrostatic unit, and a transfer path through which the paper is discharged, respectively. 
     In the above structure, an image forming process is performed as described below. First, in a case where the photosensitive drum  10  is charged by the charger  11 , the laser scanning unit (LSU)  12  scans light and thus forms the electrostatic latent image of an image which is to be developed as a first color. For example, in a case where a yellow color is developed as the first color, as shown in FIG. 2, the developer  13 -Y with a yellow color is moved to the photosensitive drum  10  by the action of a corresponding cam  13   b -Y and thus the electrostatic latent image formed on the photosensitive drum  10  is developed as a yellow toner. 
     FIG. 3 illustrates the state where the photosensitive drum  10  is in contact with a developing roller  13   a -Y of the developer  13 -Y. In FIG. 3, a toner T which is stained at the outside circumferential side of the developing roller  13   a -Y is moved onto the portion of the electrostatic latent image of the photosensitive drum  10  contacting the toner T. In such a case, the developing roller  13   a -Y rotates at a linear velocity which is faster than the linear velocity of the photosensitive drum  10  so as to increase the amount of a toner per area with which the photosensitive drum  10  is coated. The yellow image developed thereby is firstly transferred to the transferring belt  14  through a first transferring nip N 1 . 
     Next, an electrostatic latent image for a second color is formed through charging and exposure of the photosensitive drum  10 . In a case where the second color is magenta, as shown in FIG. 4, the developer  13 -M is moved to the photosensitive drum  10  and thus the electrostatic latent image is formed. The magenta image developed thereby is overlapped and transferred to the transferring belt  14  to which the yellow image is transferred. 
     A cyan image for a third color and a black image for a fourth color are developed and transferred in the same way, and thereby an image having a desired color is finally made on the transferring belt  14 . 
     After that, the color image made thereby is transferred onto the paper which is supplied to a second transferring nip N 2  between the transferring belt  14  and the second transferring roller  14   b . The transferred image passes through the fixing unit  15 , where it is heated and pressed and thus is completely fixed on the paper. 
     However, in the above configuration, since the four-color developers  13 -Y,  13 -M,  13 -C, and  13 -K of the developing unit  13  forms images while being alternately attached and detached from the photosensitive drum  10 , the photosensitive drum  10  may be shocked whenever each of the developing rollers  13   a  of each of the developers  13 -Y,  13 -M,  13 -C, and  13 -K collides with the photosensitive drum. In such a case, jitter errors caused by image tremble during collision may occur. However, a problem arises in that each of the developing rollers  13   a  of each of the developers  13 -Y,  13 -M,  13 -C, and  13 -K rotates at a linear velocity which is faster than that of the photosensitive drum  10  as mentioned above, and thus shock caused by a difference in the linear velocity occurs during contact, and thereby the shock applied to the photosensitive drum  10  is further increased. 
     That is, a photosensitive layer is usually coated on an aluminum material to form the photosensitive drum  10 , and the developing rollers  13   a  have a rubber material of predetermined hardness. Thus, in a case where the hardness of the rubber material is lowered, shock during contact may be slightly alleviated. However, the difference in the linear velocity described above is set so that the toner is sufficiently supplied to the photosensitive drum  10 . In a case where the developing roller  13   a  and the photosensitive drum  10 , which have different linear velocities, contact each other, the developers  13 -Y,  13 -M,  13 -C, and  13 -K are instantaneously pushed during contact, and thus registration defects in which the overlapping position between colors crosses each other, as well as the jitter errors occur, and thereby an image is not clear and abnormally developed. 
     In addition, due to shock during contact, the toner scatters and thus contaminates peripheral devices. The problem caused by the difference in linear velocities may occur during break off as well as during contact. In a case where the velocity of the developing roller is reduced continuously, the supply of the toner is insufficient during development, and thus image quality may be lowered. 
     Thus, for developing of a clear image, a method is needed in which the amount of the toner supplied to the photosensitive drum  10  during development is not reduced, and the above causes for the occurrence of defective images are eliminated. 
     SUMMARY OF THE INVENTION 
     To solve the above problems, it is an object of the present invention to provide a method for forming an image in an electrophotographic printer which is improved so that shock is alleviated when a photosensitive medium and a developer contact each other. 
     Accordingly, to achieve the above object, there is provided a method for forming an image in an electrophotographic printer, which is capable of removing a developing roller of a developer from a photosensitive medium on which an electrostatic latent image is formed, and of developing the electrostatic latent image with a predetermined color. The method includes the steps of rotating a developing roller at the same linear velocity as that of a photosensitive medium in a state where the developing roller is separated from the photosensitive medium, moving a developer so that the developing roller is in contact with the photosensitive medium, and changing the linear velocity of the developing roller to be faster than that of the photosensitive medium after contact and then proceeding with the developing process. 
     Here, in order to alleviate shock caused when the developing roller is detached, it is preferable that the method further includes the steps of rotating the developing roller at the same linear velocity as that of the photosensitive medium after completing developing, and moving the developer so that the developing roller is separated from the photosensitive medium. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above object 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: 
     FIG. 1 illustrates an image forming system of a conventional electrophotographic printer; 
     FIGS. 2 through 4 illustrate the attaching and detaching operation of a developing unit of the image forming system shown in FIG. 1; and 
     FIGS. 5A and 5B constitute a flow chart illustrating a method for forming an image in an electrophotographic printer according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 5A and 5B constitute a flow chart illustrating a method for forming an image in an electrophotographic printer according to the present invention. Hereinafter, the method for forming an image in an electrophotographic printer according to the present invention will be described with reference to the image forming system shown in FIG.  1  and the flow chart of FIGS. 5A and 5B. 
     In step S1, the charger  11  charges the photosensitive drum  10  as a photosensitive medium, to a predetermined electric potential. In step S2, the LSU  12  scans light to the charged photosensitive drum  10  and thus forms an electrostatic latent image corresponding to an image for a first color (for example, yellow). In step S3, the electrostatic latent image is developed first by one of the four developers of the developing unit  13 . For example, in step S4-1, in a case where the electrostatic latent image is developed by the yellow developer  13 -Y, a printer controller (not shown) for controlling an image forming process rotates the developing roller  13   a -Y of the yellow developer  13 -Y at the same linear velocity as that of the photosensitive drum  10 . This is a measure to alleviate the subsequent shock during contact with the photosensitive drum  10 . 
     In step S4-2, once the developing roller  13   a -Y rotates at the same linear velocity as that of the photosensitive drum  10 , the cam  13   b -Y is driven, and thus the yellow developer  13 -Y is moved into the photosensitive drum  10 , and the developing roller  13   a -Y is in contact with the photosensitive drum  10 . In such a case, since two rollers such as the developing roller  13   a -Y and the photosensitive drum  10 , rotate at the same linear velocity, due to the difference in the linear velocity, one of the two rollers is not pushed; also, vibration of the two rollers does not occur. 
     In step S4-3, in this way, after the developing roller  13   a -Y is in contact with the photosensitive drum  10 , the printer controller increases the rotation speed of the developing roller  13   a -Y such that the toner stained in the developing roller  13   a -Y is sufficiently moved into the portion of the electrostatic latent image formed on the photosensitive drum  10 . In such a case, preferably, the ratio of the linear velocity of the photosensitive drum  10  to that of the developing roller  13   a -Y is set to be greater than 1:1 and less than 1:1.5 such that the adequate amount of the toner is supplied to the portion of the electrostatic latent image of the photosensitive drum  10 . 
     In step S4-4, when it is judged that developing of a corresponding color is completed, the process advances to step S4-5, where the linear velocity of the developing roller  13   a -Y is reduced to be the same as that of the photosensitive drum  10 . This is a measure to prevent shock which may occur when the two rollers  13   a -Y and  10  contact each other or when contact is broken off, and to avoid deleterious effects on an image developed on the photosensitive drum  10 . 
     In step S4-6, in a case where the linear velocity of the two rollers  13   a -Y and  10  is equalized, the cam  13   b  is driven, and thus the yellow developer  13 -Y is returned to its original separated position. 
     In such a case, since the linear velocity of the two rollers  13   a -Y and  10  are equalized whenever the developing roller  13   a  of the yellow developer  13 -Y and the photosensitive drum  10  contact each other or whenever contact is broken off, shock caused by the difference in the linear velocity during attachment and detachment is eliminated. Here, a stepping motor capable of easily controlling speed change is preferably used as a motor for driving the developing roller  13   a -Y. 
     In step S5, the yellow image developed thereby is transferred to the transferring belt  14  through the first transferring nip N 1 . 
     In step S6, in this way, after the first yellow image is transferred to the transferring belt  14  from the photosensitive drum  10 , it is checked whether developing for the next color proceeds or not. Thus, in a case where developing for the next color proceeds, cleaning and anti-electrostatic operations of the photosensitive drum  10  are performed, and then, the above developing is performed. As in the usual case of forming a color image, in a case where all of four colors are developed and overlap, the remaining magenta, cyan, and black developers are selected one by one (step 3), and simultaneously the above developing for a corresponding color is repeated. Then, four colors overlap on the transferring belt  14 , and thereby a final color image is completely formed. 
     Likewise, in a case where the color image is completed on the transferring belt  14 , in step S7, the paper is supplied from the paper cassette  16 , and the image on the transferring belt  14  is transferred to the paper from the second transferring nip N 2 . 
     In step S8, the paper to which the image is transferred passes through the fixing unit  15  and is heated and pressed. In step S9, the paper is discharged outside of the printer body along the transfer path. 
     Thus, in the above method for forming an image, as above, since the linear velocity of the photosensitive drum  10  should be equalized with that of the developing roller  13   a  whenever the developing roller  13   a  of the developer  13  which is attached and detached from the photosensitive drum  10  is attached and detached, the supply of the toner to the photosensitive drum  10  can be sufficient during development, and simultaneously shock caused by the attaching and detaching operation is alleviated, and thereby the possibility of defective images thereof can be greatly reduced. 
     As described above, the method for forming an image in an electrophotographic printer according to the present invention can equalize the linear velocity of the developing roller, which is attached and detached from the photosensitive drum, with the linear velocity of the photosensitive drum whenever attachment and detachment occur, and once attachment occurs, the linear velocity of the developing roller is changed to be faster than that of the photosensitive drum, and after developing the linear velocity of the developing roller is changed to be the same as that of the photosensitive drum and thereby the following effects can be obtained. 
     First, because shock caused by the attaching and detaching operation is alleviated, the toner during development is sufficiently supplied, and defective images resulting from conditions such as registration defects caused by the break off of contact between the developer and the drum and jitter errors caused by vibration, can be suppressed. 
     Second, because shock during attachment and detachment is alleviated, the toner of the developer is prevented from scattering to peripheral devices due to shock caused by attachment and detachment. 
     While this invention has been particularly shown and described with reference to preferred 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.