Patent Publication Number: US-2005129426-A1

Title: Photosensitive drum having a spacer and electrophotograhic image forming apparatus having the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application claims the benefit of Korean Patent Application No. 2003-91172, filed on Dec. 15, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a photosensitive drum and an electrophotographic image forming apparatus having the same, and more particularly, to a photosensitive drum with a fixed photosensitive sheet thereon and an electrophotographic image forming apparatus having the same.  
      2. Description of the Related Art  
      In general, electrophotographic image forming apparatuses print images on a sheet of paper by radiating light onto a photosensitive medium that is charged with a uniform electrostatic potential to form an electrostatic latent image, supplying a developing material to the electrostatic latent image to develop the electrostatic latent image, and transferring the developed image onto the paper to print the image.  
      A charger that charges the photoreceptor may be a non-contact type corona discharger, such as corotron or scorotron, or a contact type charger, such as an electrostatic charging roller. The disadvantages of the corona discharger include a high operating voltage and requiring an expensive high voltage power supply, and producing many substances such as ozone and nitrogen oxide. Consequently, electrostatic charging rollers that produce less harmful substances and stable charging efficiency are more commonly used.  
      For the photoreceptor, a photosensitive belt, or a photosensitive drum, may be used. The photosensitive drum is produced by either coating photosensitive materials on the outer circumference of a cylindrical body or by fixing a photosensitive sheet on the outer circumference of the cylindrical body. Recently, to obtain a photosensitive drum with a large diameter at a relatively low price, a photosensitive drum with a fixed photosensitive sheet on the outer circumference of a cylindrical body is used.  
      A seam is formed when the photosensitive sheet is fixed on the cylindrical body. When the electrostatic charging roller charges the photosensitive drum with the fixed photosensitive sheet on the cylindrical body, the electrostatic charging roller comes in contact with the seam and gets damaged by the impact. In addition, the electrophotographic image forming apparatus has a developing roller, which supplies the developing material to the electrostatic latent image, in a state of contacting or not contacting the photosensitive drum. The developing roller is also in danger of getting damaged because of the seam.  
     SUMMARY OF THE INVENTION  
      Accordingly, it is an aspect of the present invention to provide an improved photosensitive drum that prevents damage caused by a seam of a photosensitive sheet to an electrostatic charging roller and other components that access the photosensitive drum during an image forming process, and an electrophotographic image forming apparatus including the same.  
      Additional aspects and/or advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.  
      The foregoing and/or other aspects of the present invention are achieved by providing a photosensitive drum comprising: a cylindrical body; a fixed photosensitive sheet to define an image forming area on an outer circumference of the cylindrical body and including a seam; a spacer disposed on an end of an outer portion of the image forming area on the cylindrical body to push an electrostatic charging roller or a developing roller, which access the photosensitive sheet during an image forming process, when the seam of the photosensitive sheet approaches the electrostatic charging roller or the developing roller, to thereby make distant the seam from the electrostatic charging roller or the developing roller.  
      The foregoing and/or other aspects of the present invention are achieved by providing an electrophotographic image forming apparatus comprising: a photosensitive drum, comprising: a cylindrical body, and a fixed photosensitive sheet defining an image forming area on an outer circumference of the cylindrical body and comprising a seam; and an electrostatic charging roller that rotates in contact with the photosensitive sheet to charge uniform electrostatic potential on a surface thereof, wherein the photosensitive drum further comprises a spacer disposed on an end of an outer portion of the image forming area on the cylindrical body to, thereby distance the electrostatic charging roller from the seam of the photosensitive sheet by pushing the electrostatic charging roller when the seam of the photosensitive sheet approaches the electrostatic charging roller.  
      Additionally, the electrophotographic image forming apparatus of the embodiment of the present invention further comprising a developing roller that supplies a developing material to an electrostatic latent image formed on the photosensitive sheet, wherein the spacer pushes the developing roller as the seam of the photosensitive sheet approaches the developing roller to distance the developing roller from the seam of the photosensitive sheet.  
      The electrophotographic image forming apparatus of the embodiment of the present invention further comprising: a rotatable gap controlling that is disposed on opposite ends of the developing roller and in contact with ends of an outer portion of the image forming area on the cylindrical body to a developing gap between the surface of the photosensitive sheet and an outer circumference of the developing roller, wherein the spacer pushes the gap controlling roller to distance the developing roller from the seam of the photosensitive sheet. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the embodiment, taken in conjunction with the accompanying drawings of which:  
       FIG. 1  is a structural diagram illustrating an electrophotographic image forming apparatus according to an embodiment of the present invention;  
       FIG. 2  is a perspective view of a photosensitive drum according to the embodiment of the present invention;  
       FIGS. 3 and 4  are cross-sectional views of a fixed photosensitive sheet of  FIGS. 1 and 2 ;  
       FIGS. 5 and 6  are cross-sectional views taken along lines I-I′ and II-II′ of  FIG. 2 , respectively;  
       FIG. 7  is a perspective view illustrating a relationship between the photosensitive drum and an electrostatic charging roller of  FIG. 1 ;  
       FIGS. 8 and 9  are cross-sectional views of the lower portion of  FIG. 7 ;  
       FIG. 10  is a plan view illustrating a relationship between the photosensitive drum and a developing roller of  FIG. 1 ;  
       FIG. 11  is a cross-sectional view of the lower portion of  FIG. 10 ; and  
       FIG. 12  is another plan view illustrating a relationship between the photosensitive drum and the developing roller. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Reference will now be made in detail to the embodiment of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below to explain the present invention by referring to the figures.  
       FIG. 1  is a sectional view illustrating an electrophotographic image forming apparatus according to an embodiment of the present invention. Referring to  FIG. 1 , a photosensitive drum  101 , an electrostatic charging roller  102 , an exposure unit  103 , a developer  104 , and a transfer belt  105  are illustrated. The electrostatic charging roller  102  rotates in contact with the photosensitive drum  101  and charges the surface of the photosensitive drum  101  with a uniform electrostatic potential. The exposure unit  103  radiates light corresponding to image information to the photosensitive drum  101  that is charged with uniform electrostatic potential to form an electrostatic latent image. For the exposure unit  103 , a laser scanning unit (LSU), with laser diodes as the light source, is usually used. A developing material (hereinafter referred to as a toner) is housed inside the developer  104 . A developing roller  106  supplies the toner to the electrostatic latent image formed on the photosensitive drum  101 . The transfer belt  105  is an example of an intermediate transfer medium that receives a toner image from the photosensitive drum  101  and transfers the toner image to paper P.  
      The exposure unit  103  radiates light signals corresponding to the image information onto the photosensitive drum  101  that is charged with the uniform electrostatic potential. Then, the resistance of the part where the light is radiated decreases, and the electric charge attached to the outer circumference of the photosensitive drum  101  is discharged. As a result, a potential difference occurs between where the light is radiated and not radiated and thus, an electrostatic latent image forms on the outer circumference of the photosensitive drum  101 . As the photosensitive drum  101  rotates and the electrostatic latent image approaches the developer  104 , the toner housed in the developer  104  is attached to the electrostatic latent image and forms the toner image. When the toner image approaches the transfer belt  105  by the rotation of the photosensitive drum  101 , the toner image transfers to the transfer belt  105  due to the potential difference and/or contact pressure with respect to the transfer belt  105 . The toner image is transferred to paper P that passes between the transfer belt  105  and a transfer roller  107 . A fixing unit (not shown) fixes the transferred toner image on paper P by heat and pressure.  
       FIG. 2  is a perspective view of a photosensitive drum  101  according to an embodiment of the present invention.  FIGS. 3 and 4  are cross-sectional views of a fixed photosensitive sheet  2  of the photosensitive drum  101 .  FIGS. 5 and 6  are portions of sectional views taken along lines I-I′ and II-II′ of  FIG. 2 , respectively.  
      Referring to  FIG. 2 , the photosensitive sheet  2  is fixed on the outer circumference of a cylindrical body  1 . The photosensitive sheet  2  defines an image forming area “A” where an electrostatic latent image to create an image is formed. That is, the photosensitive sheet  2  includes a photosensitive layer formed on top of a plastic film deposited with an electrically conductive metal layer. Typically, a polyethylene terephthalate (PET) is used for the plastic film. When the photosensitive sheet  2  is adhered to the cylindrical body  1  using a double-sided adhesive tape, the thickness of the photosensitive sheet  2  including that of the double-sided tape is about 105-225 μm. As illustrated in  FIG. 3 , both ends  4  and  5  of the photosensitive sheet  2  are fixed exactly in line or with a predetermined gap. Or as illustrated in  FIG. 4 , both ends  4  and  5  are fixed to overlap by a small amount. As in  FIG. 4 , when both ends  4  and  5  are fixed to overlap each other, a seam  3  is stepped up by about 210-450 μm from the outer circumference of the cylindrical body  1 . For example, the photosensitive sheet  2  in which a photosensitive layer of about 25 μm thickness is formed on the PET sheet where the thickness of the PET sheet including an electrically conductive metal layer is about 110 μm is adhered around the cylindrical body  1  with a 120 mm long diameter using a double-sided adhesive sheet that is about 50 μm thick. In this case, the surface of the photosensitive sheet  2  is stepped up by about 180 μm from the outer circumference of the cylindrical body  1 . Also, when the seam  3  is formed as illustrated in  FIG. 4 , the seam  3  will be stepped up by about 360 μm from the outer circumference of the cylindrical body  1 .  
      In order to lessen the impact applied to the transfer belt  105 , the seam  3  can be inclined with respect to the longitude of the photosensitive drum  101 , as illustrated in a solid line in  FIG. 2 . Since an effective image forming area may decrease if the seam  3  is inclined to greatly with respect to the longitude of the photosensitive drum  101 , a length difference L between both ends of the seam  3  should be about from {fraction (1/50)} to ¼ of the diameter of the photosensitive drum  101  and may be between {fraction (1/30)} and {fraction (1/15)}. The thickness of the photosensitive sheet  2  is very small as compared to the diameter of the cylindrical body  1 . Therefore, the diameter of the cylindrical body  1  can be the basis for setting the length difference L of the inclined seam  3 . In the present embodiment, the length difference L is {fraction (1/12)} of the diameter of the cylindrical body  1 , which is about 10 mm.  
      As illustrated in  FIGS. 2, 5 , and  6 , a spacer  20  is disposed respectively on both ends of the cylindrical body  1 . The spacer  20  is to distance the electrostatic charging roller  102  and/or the developing roller  106  from the seam  3  of the photosensitive sheet  2  so that the electrostatic charging roller  102  and/or the developing roller  106  does not come in contact with the seam  3  of the photosensitive sheet  2 . For example, the spacer  20  may be fixed to the cylindrical body  1  with a screw S. As the seam  3  approaches the electrostatic charging roller  102  and/or the developing roller  106 , the spacer  20  smoothly pushes back the electrostatic charging roller  102  and/or the developing roller  106  to create a distance from the seam  3 , and when the seam  3  passes the electrostatic charging roller  102  and/or the developing roller  106 , the electrostatic charging roller  102  and/or the developing roller  106  smoothly return to the original position. Therefore, as illustrated in  FIG. 5 , the spacer  20  may have a shape of a smooth curve or a mountain with a smooth slope. To sufficiently distance the electrostatic charging roller  102  and/or the developing roller  106  from the seam  3 , a height difference H of the surface of the photosensitive sheet  2  and a peak  21  of the spacer  20  should be more than 100 μm, and may be between 200 and 800 μm and not to exceed 1 mm. A width W of the spacer  20  should be wide enough to sufficiently prevent the seam  3  of the photosensitive sheet  2  from contacting the electrostatic charging roller  102  and/or the developing roller  106 . When considering these conditions, the width W of spacer  20  should be from 5 to 30 mm, for example, from 10 to 25 mm.  
       FIG. 7  is a perspective view illustrating a relationship between the photosensitive drum  101  and the electrostatic charging roller  102 .  FIGS. 8 and 9  are cross-sectional views of the lower portion of  FIG. 7 . Referring to  FIGS. 7 and 8 , a bearing unit  201  supports both ends of the electrostatic charging roller  102 . An elastic member  202  electrically biases the bearing unit  201  in the direction of the electrostatic charging roller  102  so that the bearing unit  201  contacts the photosensitive sheet  2 . The bearing unit  201  meets the cylindrical body  1  outside the photosensitive sheet  2 . As the photosensitive drum  101  rotates, the seam  3  of the photosensitive sheet  2  moves closer to the electrostatic charging roller  102 . Then, the spacer  20  comes in contact with the bearing unit  201  and pushes back the bearing unit  201  as illustrated in  FIG. 9 . Consequently, the electrostatic charging roller  102  is distanced from the photosensitive sheet  2 . As the photosensitive drum  101  continues to rotate and the seam  3  passes, the interference of the spacer  20  and the bearing unit  201  ends and the electrostatic charging roller  102  comes in contact with the photosensitive sheet  2  again.  
      The distance from the center of the electrostatic charging roller  102  to the surface of the electrostatic charging roller  102  (or the radius of the electrostatic charging roller  102 ) is “a”( FIG. 8 ) while the distance from the center of the electrostatic charging roller  102  to the edge of the bearing unit  201  is “b.” In the case where the thickness of the photosensitive sheet  2  is less than 150 μm and the length of “a” and “b” are identical, the electrostatic charging roller  102  cannot sufficiently contact the photosensitive sheet  2 . Therefore, “b” should be smaller than “a” by no more than 1 mm. When the thickness of the photosensitive sheet  2  is more than 150 μm, the electrostatic charging roller  102  can sufficiently contact the photosensitive sheet  2 . In some cases, however, “b” may be slightly smaller than “a.” A height difference between “b” and the distance from the surface of the photosensitive sheet  2  to the spacer  20  should be set so that the electrostatic charging roller  102  is distanced enough from the seam  3  and does not contact the seam  3 .  
       FIG. 10  is a plan view illustrating a relationship between the photosensitive drum  101  and the developing roller  106 .  FIG. 11  is cross-sectional view of the lower portion of  FIG. 10 .  FIG. 10  illustrates one example of a non-contact developing method where the developing roller  106  is distanced as much as a developing gap Dg from a photosensitive medium for a developing process. Referring to  FIG. 10 , the developer  104  elastically supports the developing roller  106  with a bearing unit  204  and an elastic member  205 . A rotatable gap controlling roller  203  is disposed on both ends of the developing roller  106 . The gap controlling roller  203  is to maintain the surface of the developing roller  106  and the surface of the photosensitive sheet  2  to be as distant as much as the developing gap Dg. The gap controlling roller  203  comes in contact with both ends of the cylindrical body  1 . The diameter of the gap controlling roller  203  is longer than the diameter of the developing roller  106  by as much as the sum of the thickness of the photosensitive sheet  2  and the developing gap Dg. Typically, the developing gap Dg is from 50 to 500 μm.  
      As the photosensitive drum  101  rotates, the seam  3  of the photosensitive sheet  2  moves toward the developing roller  106 . When the spacer  20  comes in contact with the gap controlling roller  203 , the spacer  20  pushes back the gap controlling roller  203 . Then, the developing roller  106  is pushed back in the direction illustrated in  FIG. 11  and is distanced from the photosensitive sheet  2 . When the photosensitive drum  101  continues to rotate and the seam  3  passes, interference of the spacer  20  and the gap controlling roller  203  terminates, and the developing roller  106  returns to a position to maintain the developing gap Dg with the photosensitive sheet  2 . In the case where the developing gap Dg is sufficiently larger than the thickness of the photosensitive sheet  2 , the developing roller  106  and the seam  3  do not come in contact with each other and so the developing roller  106  does not necessarily need to be distanced from the seam  3 . Therefore, in this case, the gap controlling roller  203  and the spacer  20  should be in place so that the gap controlling roller  203  and the spacer  20  do not interfere with each other, as illustrated in  FIG. 12 .  
      According to the above-mentioned structure, the damage caused by repetitive contact with the seam  3  of the photosensitive sheet  2  to the electrostatic charging roller  102  and the developing roller  106  can be prevented. Consequently, the electrostatic charging roller  102  may be used to charge the photosensitive drum  101  with the photosensitive sheet  2  fixed on the cylindrical body  1 . Therefore, harmful substances produced during the charging process can be decreased and stable charging characteristics can increase the quality of the developed image. Also, by using the photosensitive drum  101  that is produced at a moderate price, the price of the image forming apparatus may be lowered. According to the result of continuously printing 10,000 pages using the developing gap Dg of 150 μm, there was hardly any damage to the electrostatic charging roller  102  and the developing roller  106 , and satisfactory image quality was obtained.  
      The photosensitive drum  101  with above-mentioned structure can also be used in an image forming apparatus with a contact type developing method where the developing roller  106  comes in contact with the photosensitive drum  101  to develop an image. For example, in  FIG. 10 , the developing roller  106  comes in contact with the surface of the photosensitive sheet  2  if the radius of the gap controlling roller  203  is either the same or smaller than the radius of the developing roller  106 . As the photosensitive drum  101  rotates, the seam  3  of the photosensitive sheet  2  comes closer to the developing roller  106 . Then, the spacer  20  comes in contact with the gap controlling roller  203  and pushes back the gap controlling roller  203 . As a result, the developing roller  106  is distanced from the photosensitive sheet  2 . As the photosensitive drum  101  continues to rotate and the seam  3  passes, then the interference by the spacer  20  and the gap controlling roller  203  terminates and the developing roller  106  re-contacts the photosensitive sheet  2 . According to the described structure, the damage to the electrostatic charging roller  102  and the developing roller  106  caused by contacting the seam  3  of the photosensitive sheet  2  can be prevented. The result of continuously printing 10,000 papers with the image forming apparatus with the described structure shows that there was little damage to the electrostatic charging roller  102  and the developing roller  106 , and satisfactory image quality was achieved.  
      In the described image developing apparatus, there is a need to make sure that the image does not form on the seam  3  of the photosensitive sheet  2 . A sensor (not shown) that detects the seam  3  of the photosensitive sheet  2  may be used.  
      The present invention is not limited to the described embodiment where a single color image forming apparatus having one developer  104  was described. The described embodiment can be applied to a color image developing apparatus that has four developers with toners for the colors: cyanogens, magenta, yellow, and black or combinations thereof.  
      According to the embodiment of the present invention, the following effects may be achieved when using a photosensitive drum with a fixed photosensitive sheet on a cylindrical body as a photosensitive medium, a spacer can prevent damage to an electrostatic charging roller and/or a developing roller. Also, the electrostatic charging roller can be used to charge the photosensitive drum. This decreases the amount of harmful substances produced during a charging process and stable charging characteristics help to improve the quality of the image. In addition, by using the photosensitive drum that is produced at a moderate price, the price of an image forming apparatus can be lowered.  
      Although an embodiment of the present invention has been shown and described, it would be appreciated by those skilled in the art that changes may be made in this embodiment without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.