Patent Publication Number: US-7593663-B2

Title: Image forming apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims the foreign priority benefit under Title 35, United States Code, §119(a)-(d) of Japanese Patent Application No. 2006-293089 filed on Oct. 27, 2006 in the Japan Patent Office, the disclosure of which is herein incorporated by reference in its entirety. 
   BACKGROUND OF THE INVENTION 
   The present invention relates to an image forming apparatus equipped with a charger for electrically charging a photosensitive element and an exhaust fan for ventilating the image forming apparatus. 
   Generally, a known image forming apparatus electrically charges a photosensitive drum and emits laser light on this electrically charged photosensitive drum so that the electric potential of the exposed area (i.e., area on which the laser light is emitted) lowers to form an electrostatic latent image on the photosensitive drum. The image forming apparatus then supplies developer on this latent image to form a developer image, which is transferred on a paper so that a predetermined image is formed on the paper. 
   As one example of such an image forming apparatus, Japanese Laid-open Patent Application No. 2005-292356, which corresponds to US 2005/0220479A1, discloses an image forming apparatus equipped with a scanner unit for scanning laser light and a process unit positioned below the scanner unit with a predetermined gap interposed therebetween. This image forming apparatus mainly includes in the process unit a photosensitive drum for carrying developer images, a charger for electrically charging the photosensitive drum, and an exposure window through which laser light from the scanner unit is emitted on to the photosensitive drum. The charger and the exposure window are arranged adjacent to each other. 
   This kind of image forming apparatus is usually provided with an exhaust fan for mainly discharging heat from the inside of the apparatus. Air between the scanner unit and the process unit is drawn by the exhaust fan from the exposure window toward the charger so as to perform a heat discharging operation, etc. 
   However, when the exhaust fan draws air between the scanner unit and the process unit, there may be a possibility that air flows into the process unit through the exposure window, etc. and then flows out from the process unit through the charger. If this happens, contrary to ion flow generated during the electric charge by the charger (i.e., airflow generated by the movement of ions upon electric charge) and directed from the charger to the photosensitive drum, the operation of the exhaust fan generates airflow which flows in the counter direction of the ion flow. As a result, efficiency of electric charge may be decreased by this counter flow. 
   In view of the foregoing drawback of the conventional image forming apparatus, the present invention seeks to provide an image forming apparatus which can prevent a decrease in efficiency of electric charge. 
   SUMMARY OF THE INVENTION 
   According to the present invention, there is provided an image forming apparatus comprising: a photosensitive element; a charger for electrically charging the photosensitive element; an exposure device for scanning laser light across the photosensitive element that has been electrically charged by the charger; an exposure window positioned away from the exposure device for a predetermined distance and through which the laser light passes; and an exhaust fan for drawing air between the exposure device and the exposure window toward the charger and discharging the air from the image forming apparatus. The exposure window is defined by a plurality of walls, in which a first wall that is the farthest wall from the charger and a second wall that is the nearest wall from the charger satisfy a formula: A&lt;B, where A is a distance between the first wall and the exposure device, and B is a distance between the second wall and the exposure device. 
   With this configuration of the image forming apparatus, the exposure window is defined by a plurality of walls including a first wall that is the farthest wall from the charger and a second wall that is the nearest wall from the charger, and the distance A between the first wall and the exposure device and the distance B between the second wall and the exposure device satisfy the formula: A&lt;B. Therefore, the flow velocity of the airflow flowing through a space between the first wall and the exposure device is higher than the flow velocity of the airflow flowing through a space between the second wall and the exposure device. This makes it possible to restrict the amount of air flowing into the process unit through the exposure window, so that an occurrence of the counter airflow flowing in the opposite direction of the ion flow that is generated upon electric charge by the charger can be restricted. As a result, it is possible to restrict a decrease in the efficiency of electric charge. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and aspects of the present invention will become more apparent by describing in detail illustrative, non-limiting embodiments thereof with reference to the accompanying drawings, in which: 
       FIG. 1  is a side sectional view illustrating a laser printer as one example of an image forming apparatus according to the present invention; 
       FIG. 2  is an enlarged sectional view showing the structure around a longitudinal airway defined in the laser printer; and 
       FIG. 3  is a graph explaining relations between ratio A/B of distances A and B and flow rate at predetermined points. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   With reference to  FIG. 1 , brief description will be given on the overall structure of a laser printer as one example of an image forming apparatus according to the present invention. 
   Overall Structure of Laser Printer 
   As seen in  FIG. 1 , a laser printer  1  includes a main body casing  2 , and provided in the main body casing  2  are a feeder unit  4  for feeding papers  3 , an image forming unit  5  for forming an image on a paper  3  to be fed from the feeder unit  4 , etc. In the following description, unless otherwise stated, the right side of  FIG. 1  is referred to as a “front or near side” and the left side of  FIG. 1  is referred to as a “back or far side”, based on the directions where the user stands in front of the laser printer  1  and looks at the same upon using the laser printer  1 . 
   Structure of Feeder Unit 
   The feeder unit  4  includes a paper feed tray  6  slidable into and detachable from a bottom space provided in the main body casing  2 , and a paper lifting plate  7  provided in the paper feed tray  6 . The feeder unit  4  further includes a pick-up roller  11  provided above the paper feed tray  6  at one end side thereof, a feed roller  8  positioned downstream of the pick-up roller  11  as seen in the conveying direction of papers  3 , a paper feed pad  9 , a pinch roller  10 , and a cleaning roller  50  for removing paper dust. Further, a paper stop roller  12  is provided in the feeder unit  4  at a position downstream of the cleaning roller  50 . 
   According to this feeder unit  4 , the paper lifting plate  7  collects papers  3  in the paper feed tray  6  toward the pick-up roller  11 . The pick-up roller  11  then picks up and feeds out a paper  3  between the feed roller  8  and the paper feed pad  9  at which the paper  3  is fed out on a one-by-one basis. The paper  3  then passes the rollers  10 ,  50 ,  12  and is conveyed to the image forming unit  5 . 
   Structure of Image Forming Unit 
   The image forming unit  5  includes a scanner unit  16  as one example of an exposure device, a process cartridge  17 , a fixing unit  18 , etc. 
   Structure of Scanner Unit 
   The scanner unit  16  is provided at an upper part of the main body casing  2 . The scanner unit  16  includes a laser light emitting device (not shown), a rotatable polygon mirror  19 , a plurality of lenses  20 ,  21 , and reflecting mirrors  22 ,  23 . A laser beam is emitted from a laser light emitting device based on image data. As shown by the chain line of  FIG. 1 , the laser beam emitted from the laser light emitting device passes in order through or is reflected at the polygon mirror  19 , the lens  20 , the reflecting mirror  22 , the lens  21 , and the reflecting mirror  23 , and thereafter it is irradiated onto the photosensitive drum  27  of the process cartridge  17  by high speed scanning. 
   Structure of Process Cartridge 
   The process cartridge  17  is arranged below the scanner unit  16  with a predetermined gap interposed therebetween. The process cartridge  17  is attached to and detachable from the main body casing  2 . The process cartridge  17  mainly consists of a developer cartridge  28  and a drum unit  51 . 
   The developer cartridge  28  is attached to and detachable from the drum unit  51 , and includes a development roller  31 , a doctor blade  32 , a supply roller  33 , and a toner hopper  34 . Toner stored in the toner hopper  34  is supplied to the development roller  31  by the supply roller  33 , during which the toner charges positively due to frictional contact between the supply roller  33  and the development roller  31 . When the development roller  31  rotates, the toner deposited on the development roller  31  passes through a space between the doctor blade  32  and the development roller  31 , so that the toner is carried on the development roller  31  as a thin layered toner having a constant thickness. 
   The drum unit  51  mainly includes a photosensitive drum  27  as one example of the photosensitive element, a Scorotron charger  29  as one example of the charger, and a transfer roller  30 . 
   The photosensitive drum  27  is rotatably supported in the housing of the drum unit  51 . The photosensitive drum  27  has a drum main body to be earthed, and the surface of the photosensitive drum  27  is made of a photosensitive layer to be charged positively. An exposure window  51   a  is arranged above the photosensitive drum  27 . The exposure window  51   a  is provided as an opening formed in the housing of the drum unit  51 . 
   The Scorotron charger  29  is positioned away from the photosensitive drum  27  with a predetermined gap interposed therebetween. To be more specific, the Scorotron charger  29  is positioned diagonally upward from the photosensitive drum  27  (i.e., backward and upward from the photosensitive drum  27 ) facing to the photosensitive drum  27 . The Scorotron charger  29  (hereinafter referred to as the “charger  29 ”) is a Scorotron-type charger having a charging wire made of tungsten or the like for producing corona discharge and configured to electrically charge the surface of the photosensitive drum  27  uniformly in the plus polarity. Ion flow is generated in the charger  29  due to corona discharge. The ion flow flows in the direction from the charger  29  to the photosensitive drum  27 . 
   The transfer roller  30  is rotatably supported in the housing of the drum unit  51  at a position under the photosensitive drum  27  and opposed to and in contact with the photosensitive drum  27 . The transfer roller  30  has a metallic roller shaft covered with a conductive rubber material. During the transfer process, a transfer bias is applied to the transfer roller  30  under constant current control. 
   In operation, the surface of the photosensitive drum  27  is uniformly charged to the plus polarity by the charger  29 , and thereafter exposed to the laser beam from the scanner unit  16  by high speed scanning. This exposure process lowers the potential of the exposed surface on the photosensitive drum  27 , thereby forming an electrostatic latent image based on the image data. The term “electrostatic latent image” indicates an invisible image produced on the uniformly positively charged surface of the photosensitive drum  27  with the exposed area made lower in potential by exposure to the laser beam. Next, as the development roller  31  rotates, toner deposited on the development roller  31  comes into contact with the opposed photosensitive drum  27  so that the toner is supplied to the electrostatic latent image formed on the photosensitive drum  27 . The toner is selectively retained on the photosensitive drum  27  solely in the area corresponding to the electrostatic latent image, so as to visualize the latent image. This reversal development forms a toner image on the photosensitive drum  27 . 
   Thereafter, the photosensitive drum  27  and the transfer roller  30  rotate so that the paper  3  is held and fed forward between the rollers  27 ,  30 , during which the toner image formed on the surface of the photosensitive drum  27  is transferred to the paper  3 . 
   Structure of Fixing Unit 
   The fixing unit  18  is positioned downstream of the process cartridge  17 , and includes a heating roller  41 , and a pressure roller  42  opposed to and pressing the heating roller  41 . In the fixing unit  18  as constructed above, the toner image transferred onto the paper  3  is fixed by heating and fusing the toner while the paper  3  passes between the heating roller  41  and the pressure roller  42 . The paper  3  that has been fixed by heating and fusing at the fixing unit  18  is conveyed to a paper output roller  45  positioned downstream of the fixing unit  18 , and thereafter discharged from the paper output roller  45  onto a paper output tray  46 . 
   In order to exhaust heat in the laser printer  1 , an exhaust fan  60  is provided between the fixing unit  18  and the scanner unit  16 . Provided below the exhaust fan  60  and between the fixing unit  18  and the process cartridge  17  is a duct  61  for rectifying and regulating a flow of air. 
   By this arrangement of the exhaust fan  60  and the duct  61 , when the exhaust fan  60  is operated, the exhaust fan  60  sucks air between the drum unit  51  and the fixing unit  18  directly or through the duct  61  and discharges the air to the outside. The exhaust fan  60  also sucks air from the front upper side of the laser printer  1  through a passage formed between the scanner unit  16  and the process cartridge  17 , that is an airway  62  extending in the longitudinal direction of the laser printer  1 , and then discharges the air to the outside. Structure around the airway  62  will be described below in detail. 
   Structure Around Airway 
   As seen in  FIG. 2 , the scanner unit  16  by which the upper wall of the airway  62  is defined is arranged in a position not across (i.e., offset from) the extension L 1  extending from the line connecting the center of the photosensitive drum  27  and the center of the charger  29 . Further, as shown in  FIG. 1 , the photosensitive drum  27 , the charger  29 , and the exhaust fan  60  are arranged in line. Furthermore, the duct  61  is provided at a position opposed to the outlet of the airway  62 . By this arrangement of the parts, the flow direction of the airflow coming from the exposure window  51   a  and flowing toward the charger  29  is bent around the charger  29  in a direction away from the photosensitive drum  27 . 
   Back side of the airway  62  comprises a scanner plate  16   a  by which the lower wall of the scanner unit  16  is defined, and a first wall  63  and a second wall  64  which form the exposure window  51   a . The first wall  63  is a wall that is positioned farthest from the charger  29 , and the second wall  64  is a wall that is positioned nearest from the charger  29 . The charger  29  is adjacent to the second wall  64  and positioned behind the second wall  64 , i.e., downstream of the second wall  64  as seen in the flow direction of air. The second wall  64  has a plurality of openings  64   b  with a predetermined pitch in the right and left direction (i.e., direction orthogonal to the drawing plane). Further, the first wall  63  and the second wall  64  are arranged such that the distance between the first and second walls  63 ,  64  is in the range from 5 to 30 mm. 
   The first wall  63  is provided with a projection  63   a  which protrudes upward relative to the upper surface  64   a  of the second wall  64 , so that the airway  62  is restricted at the projection  63   a . To be more specific, the distance A between the first wall  63  and the scanner plate  16   a  in the direction orthogonal to the scanner plate  16   a  and the distance B between the second wall  64  and the scanner plate  16   a  satisfy the following formulae:
 
A&lt;B  (1)
 
 A/B&lt; 0.8  (2)
 
   Next, the influence of the projection  63   a  provided in the airway  62  will be described. 
   As shown in  FIG. 1 , when the exhaust fan  60  is operated, the exhaust fan  60  sucks air from the front upper side of the laser printer  1  through the longitudinal airway  62  so that air flows in the airway  62  toward the back side of the laser printer  1 . As shown in  FIG. 2 , when the air passes through the space defined by the projection  63   a  of the first wall  63  and the scanner plate  16   a , the flow velocity of the air is increased because the sectional area of the airway  62  is restricted at the projection  63   a . As a result, most of the air flows in the airway  62  without flowing into the process cartridge  17  through the exposure window  51   a , and then flows upward away from the charger  29 . 
   In other words, the airflow passing along the projection  63   a  of the first wall  63  has a higher flow velocity than the airflow passing along the second wall  64 , so that pressure is reduced at the downstream side of the first wall  63  (i.e., upper space of the exposure window  51   a ) than at the downstream side of the second wall  64  (i.e., upper space of the charger  29 ). Therefore, air at the upper side of the charger  29  is drawn and flows into the charger  29  and the process cartridge  17  and flows out from the process cartridge  17  through the exposure window  51   a . This makes it possible to restrict an occurrence of the counter airflow flowing from the inside to the outside of the process cartridge  17  through the charger  29 . Thereafter, the flow of air is bent around the charger  29  in a diagonally upward direction toward the exhaust fan  60  and away from the photosensitive drum  27 , and then flows out to the outside from the exhaust fan  60 . 
   According to the laser printer  1  as constructed above, the following advantages can be obtained. 
   (1) The flow velocity increases when airflow passes through the restriction defined by the projection  63   a , so that the amount of air flowing into the process cartridge  17  through the exposure window  51   a  can be decreased. It is possible to restrict an occurrence of the counter airflow flowing in the opposite direction of the ion flow that is generated upon electric charge by the charger  29 , thereby restricting a decrease in the efficiency of electric charge. 
   (2) The distance A and the distance B have a relation to satisfy the formula (2) defined by A/B&lt;0.8, so that airflow flowing through the space between the second wall  64  and the photosensitive drum  27  flows in the direction away from the charger  29  (i.e., direction toward the front side of the laser printer  1 ). It is possible to reliably restrict a decrease in the efficiency of electric charge. The flow direction of the airflow between the second wall  64  and the photosensitive drum  27  was proved by experiments carried out by the inventor. See Example to be described later. 
   (3) Since the distance from the first wall  63  to the second wall  64  is in the range from 5 to 30 mm, the opening area of the exposure window  51   a  becomes relatively small. This makes it possible to restrict air from flowing into the process cartridge  17  through the exposure window  51   a  while reliably guiding laser light from the scanner unit  16  to the photosensitive drum  27 . 
   The present invention is not limited to the above specific embodiment, and various changes and modifications may be made without departing from the scope of the attached claims. 
   According to the above preferred embodiment, the projection  63   a  is provided on the first wall  63 . However, a projection may be provided on the bottom wall of the scanner plate  16   a  opposed to the first wall  63 , or a projection may be provided both on the scanner plate  16   a  and the first wall  63 . 
   Although the present invention has been described as being applied to a laser printer, the present invention is applicable to other image forming apparatus such as a copying machine and a multifunction device. 
   Example 
   One example for the above preferred embodiment will be described below. More specifically, experimental results (simulation results) are shown for relations between ratio A/B of the distance A to the distance B and flow rate of air at predetermined points. 
   In the experiments (simulations) according to this example, the exhaust fan  60 , the charger  29 , the exposure window  51   a , the first wall  63 , the second wall  64 , etc. are arranged as described above in the preferred embodiment. To be more specific, arrangements of the parts are the same as those of HL-2040 model laser printer manufactured by Brother Industries, Ltd. except for the height of the projection  63   a  provided on the first wall  63 . The flow rate of air discharged by the exhaust fan  60  is 4.25×10 −6  m 3 /s. While maintaining these conditions, the height of the projection  63   a  on the first wall  63  is changed in accordance with the TABLE 1 below, and measurements are made for the flow rate of air flowing through the openings  64   b  formed in the second wall  64  and the flow rate of air flowing through a space between the second wall  64  and the photosensitive drum  27 . Accordingly, the experimental results as shown in TABLE 1 and  FIG. 3  were obtained. Plus-minus signs (plus sign is omitted) found in TABLE 1 at columns indicating flow rate are determined such that plus sign is used for the flow rate of air flowing through the openings  64   b  of the second wall  64  in the direction from the outside to the inside of the process cartridge  17  and that plus sign is used for the flow rate of air flowing through the space between the second wall  64  and the photosensitive drum  27  in the direction from the charger  29  to the exposure window  51   a . 
   
     
       
         
             
             
             
           
             
               TABLE 1 
             
             
                 
             
             
                 
                 
               Flow rate of air flowing 
             
             
                 
               Flow rate of air flowing 
               through a space between 
             
             
                 
               through openings of second 
               second wall and 
             
             
               A/B 
               wall (m 3 /s) 
               photosensitive drum (m 3 /s) 
             
             
                 
             
           
          
             
               1.07 
               −3.54 × 10 −7     
               −3.43 × 10 −7     
             
             
               0.93 
               −1.98 × 10 −7     
               −1.25 × 10 −7     
             
             
               0.80 
               1.57 × 10 −8   
               3.26 × 10 −8   
             
             
               0.66 
               3.28 × 10 −7   
               2.08 × 10 −7   
             
             
               0.53 
               6.05 × 10 −7   
               3.91 × 10 −7   
             
             
               0.40 
               9.49 × 10 −7   
               6.18 × 10 −7   
             
             
               0.27 
               1.66 × 10 −6   
               1.06 × 10 −6   
             
             
               0.13 
               2.88 × 10 −6   
               1.44 × 10 −6   
             
             
                 
             
          
         
       
     
   
   According to this example, when the ratio of the distance A to the distance B is set to satisfy A/B&lt;0.8, it is proved that air flowing through the space between the second wall  64  and the photosensitive drum  27  is directed to the direction from the charger  29  to the exposure window  51   a . Also, when the condition A/B&lt;0.8 is satisfied, it is proved that air flowing through the openings  64   b  of the second wall  64  is directed to the direction from the outside to the inside of the process cartridge  17  and that a counter flow of air does not occur, which flows into the process cartridge  17  through the exposure window  51   a  and flows out from the openings  64   b  of the second wall  64 . As described above, when the condition A/B&lt;0.8 is satisfied, it is proved that ion flow generated upon electric charge by the charger  29  is hardly interrupted by other airflow and a smooth and favorable flow of the ion flow can be obtained. 
   On the contrary, when A/B is 0.93 or 1.07, it is proved that air flowing through the space between the second wall  64  and the photosensitive drum  27  is directed to the direction from the exposure window  51   a  to the charger  29  and that air flowing through the openings  64   b  of the second wall  64  is directed to the direction from the inside to the outside of the process cartridge  17 . However, when compared with the results between the conditions of A/B is 0.93 and A/B is 1.07, it is proved that the flow rate of air is smaller when A/B is 0.93. Accordingly, it is proved that the smaller is the ratio A/B, the more can be restricted for the flow of air interrupting the electric charge by the charge  29 . Therefore, when the condition A&lt;B is satisfied, it is proved that ion flow generated upon electric charge by the charger  29  is not likely to be interrupted by other airflow.