Patent Publication Number: US-8126356-B2

Title: Image forming apparatus having a lock mechanism releasable by lifting an upper cover

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2007-335634, filed on Dec. 27, 2007, the entire subject matter of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     Aspects of the present invention relates to an image forming apparatus having an upper cover that opens and closes an opening of a main housing by vertical pivoting action. 
     BACKGROUND 
     In an image forming apparatus, LED heads that generate electrostatic latent images on respective photosensitive drums are held on a pivotable upper cover by way of holding members, and the LED heads move, along with pivoting action of the upper cover between exposure positions where the photosensitive drums are exposed and retracted positions separated from the photosensitive drums. For example, JP-A-11-153893 describes such image forming apparatus and a lock mechanism (an open-close mechanism) for an upper cover. In the image forming apparatus, the mechanism releases a lock member as a result of pressing of an unlock button provided on an upper surface of a main body of the apparatus so that the mechanism lifts the upper cover to a predetermined height by means of a strong spring. And then, the upper cover is manually pivoted. 
     However, with the lock mechanism as mentioned above, the user has to perform two operations, that is, operation on the unlock button and pivoting movement of the upper cover. Further, since the strong spring for use in lifting the upper cover to the predetermined height is provided, the main housing of the apparatus and the upper cover have to be reinforced in order to maintain a locked state (a state where the upper cover is closed). 
     SUMMARY 
     Exemplary embodiments of the present invention address the above disadvantages and other disadvantages not described above. However, the present invention is not required to overcome the disadvantages described above, and thus, an exemplary embodiment of the present invention may not overcome any of the problems described above. 
     Accordingly, it is an aspect of the present invention to provide an image forming apparatus that enables opening of an upper cover in one operation. 
     According to an exemplary embodiment of the present invention, there is provided an image forming apparatus including: a housing including a photosensitive member and having an opening at an upper portion of the housing; an upper cover connected to the housing at one end of the upper cover via a hinge and configured to move between an opened state in which the upper cover does not cover the opening and a closed state in which the upper cover covers the opening; an exposure unit supported by the upper cover and configured to face the photosensitive member when the upper cover is in the closed state; and a lock mechanism. The lock mechanism is releasable by lifting the upper cover so that the upper cover becomes to the opened state. 
     According to another exemplary embodiment of the present invention, there is provided an image forming apparatus including: a housing including a photosensitive member and having an opening at an upper portion of the housing; an upper cover connected to the housing at one end of the upper cover via a hinge and configured to move between an opened state in which the upper cover does not close the opening and a closed state in which the upper cover closes the opening; an exposure unit supported by the upper cover and configured to face the photosensitive member when the upper cover is in the closed state; and a lock mechanism. The lock mechanism includes a first lock member which is provided to the upper cover; a second lock member which is provided to the housing; and an urging member which urges the first lock member to the second lock member so that the first lock member and the second lock member are engaged with each other when the upper cover is in the closed state. When the upper cover is lifted from the closed state to the opened state, the first lock member is slidably contact with the second lock member so that the engagement between the first lock member and the second lock member is released. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects of the present invention will become more apparent and more readily appreciated from the following description of exemplary embodiments of the present invention taken in conjunction with the attached drawings, in which: 
         FIG. 1  is a cross-sectional view showing the overall configuration of a color printer according to an exemplary embodiment; 
         FIG. 2  is a cross-sectional view showing the color printer in which an upper cover is opened; 
         FIG. 3  is a perspective view of the upper cover; 
         FIG. 4  is a cross-sectional view showing the configuration of a process cartridge and the configuration of an LED unit; 
         FIG. 5  is an enlarged view showing the configuration of a lock mechanism according to an exemplary embodiment; 
         FIG. 6  is a view showing force acting on an engagement point when the upper cover is being opened; 
         FIG. 7A  is a view showing the upper cover before the upper cover is closed; 
         FIG. 7B  is a view showing the upper cover in the course of being closed; and 
         FIG. 8  is a view showing force acting on the upper cover in a closed state. 
     
    
    
     DETAILED DESCRIPTION 
     An exemplary embodiment of the present invention will now be described in detail with reference to the drawings. In the drawings,  FIG. 1  is a cross-sectional view showing the overall configuration of a color printer;  FIG. 2  is a cross-sectional view showing the color printer in which an upper cover is opened;  FIG. 3  is a perspective view of the upper cover; and  FIG. 4  is a cross-sectional view showing the configuration of a process cartridge and an LED unit. 
     In the following, the description will be made with reference to user&#39;s directions in use of the printer. Specifically, in  FIG. 1 , the left side of the sheet is taken as “front”; the right side of the sheet is taken as “rear”; a direction away from the viewer in the sheet is taken as “left”; and a direction toward the viewer in the sheet is taken as “right.” The vertical direction of the sheet is taken as the “upper and lower direction.” 
     As shown in  FIG. 1 , a color printer  1  has, within a main housing  10 , a sheet feeding section  20  for feeding a sheet P; an image forming section  30  for forming an image on the fed sheet P; and a sheet discharging section  90  that discharges the sheet P on which an image is formed. The main housing  10  has an opening  10 A at an upper portion thereof. As shown in  FIG. 2 , an upper cover  11  for opening and closing the opening  10 A of the main housing  10  is provided. The upper cover  11  is connected to the main housing  10  at a rear end of the upper cover  11  via a hinge. The hinge is configured by the rear end of the upper cover  11  and a rotary shaft  12  provided at a rear side of the main housing  10 . The upper cover  11  is configured to pivot about the rotary shaft  12  in the upper and lower direction to move between an opened state in which the opening  10 A is opened and a closed state in which the upper cover  11  closes the opening  10 A. 
     As shown in  FIG. 1 , the upper surface of an upper cover  11  constitutes a sheet discharging tray  13  on which the sheets P discharged from the main housing  10  are stacked. A lower surface of the upper cover  11  is provided with a plurality of holding members  14  that hold (support) LED units  40 , respectively. A control circuit (control substrate)  15  and a shield plate (sheet-metal member)  16  facing the control circuit  15  are provided inside of the upper cover  11 . Further, as shown in  FIG. 3 , a recessed grip  17 , which is gripped mainly when the upper cover  11  is opened, is provided at the center of a front edge of the upper cover  11 . 
     As shown in  FIG. 1 , a lock mechanism  100  is provided at a front upper portion of the main housing  10  and a front end of the upper cover  11 , respectively. The upper cover  11  is maintained in the closed state when the lock mechanism  100  is in a locked state. The lock of lock mechanism  100  is released into a released state by gripping the grip  17  and lifting of the upper cover  11  so that the upper cover  11  becomes into the opened state. A detailed configuration of the lock mechanism  100  will be described later. 
     By a related-art technique, the control circuit  15  outputs signals to respective LEDs of LED heads  41 , to be described later, on the basis of data pertaining to an image to be generated, to thus control light emission of the LEDs. 
     The shield plate  16  is a plate material made of metal and shields the control circuit  15  from noise arising outside of the control circuit  15 . As shown in  FIG. 1 , the shield plate  16  includes an upper shield plate  16 A disposed in the front side of the upper cover  11  and that opposes an upper surface of the control circuit  15 ; and a lower shield plate  16 B that opposes a lower surface of the control circuit  15 . The shield plate  16  acts as a reinforcement member and contributes to enhancement of strength of the upper cover  11 . 
     As shown in  FIG. 2 , the control circuit  15  and the shield plate  16  are disposed at the front interior side of the upper cover  11  so that the centroid G of the upper cover  11  is positioned at more front than the center C located at an equidistance L from the front end and the rear end of the upper cover  11 . In other words, the centroid G of the upper cover  11  is positioned between the front end thereof and the center C thereof. The centroid of the control circuit  15  and the centroid of the shield plate  16  are also positioned at more front than the center C of the upper cover  11  shown in  FIG. 2 . Further, the control circuit  15  is arranged at a position closer to the grip  17  than to the hinge. 
     As shown in  FIG. 1 , the sheet feeding section  20  includes a sheet feeding tray  21  that is provided in a lower inner portion of the main housing  10  and that is removably attached to the main housing  10 ; and a sheet feeding mechanism  22  that conveys the sheets P from the sheet feeding tray  21  to the image forming section  30 . The sheet feeding mechanism  22  is provided on the right side of the sheet feeding tray  21  and includes a feed roller  23 , a separation roller  24 , and a separation pad  25 . 
     In the sheet feeding section  20  configured as mentioned above, the sheets P mounted in the sheet feeding tray  21  are separated one at a time and fed upwardly. After sheet powder is removed during the course of the sheet passing between a sheet powder removal roller  26  and a pinch roller  27 , the sheet passes through a conveyance path  28 , to thus be turned back and fed to the image forming section  30 . 
     The image forming section  30  includes the four LED units  40 ; four process cartridges  50 ; a transfer unit  70 ; and a fixing unit  80 . 
     The LED units  40  are disposed above photosensitive drums  53 , respectively. As shown in  FIG. 4 , each of the LED units  40  includes an LED head  41 ; a frame  42 ; a coil spring  43 ; and a guide roller  44 . The LED heads  41  are configured to be disposed opposite (facing) the photosensitive drums  53 , respectively. 
     A plurality of light-emitting diodes (LEDs, not shown) is arranged in a horizontal direction on the surface of the LED head  41  while opposing (facing) the photosensitive drum  53 . Upon receipt of a signal from the control circuit  15 , each of the LEDs illuminates based on the data pertaining to an image to be formed, thereby exposing the surface of the photosensitive drum  53 . 
     The frame  42  covers the LED head  41 . A lower portion of a back plate is formed in a concave. An upper end of the LED head  41  is inserted into the concave. Thus, the LED head  41  is slidable in the vertical direction with respect to the frame  42 . The coil spring  43  is interposed between the frame  42  and the LED head  41 . The frame  42  is pivotally supported by the upper cover  11  through a holding member  14 . As a result, the LED unit  40  (the LED head  41 ) is movable between a retracted position and an exposure position where the LED unit  40  opposes the photosensitive drum  53  by upwardly pivoting the upper cover  11  (see  FIG. 2 ). 
     When the LED head  41  is positioned at the exposure position, the guide rollers  44  roll over the surface of the photosensitive drum  53  while contacting therewith, thereby regulating an interval between the LED head  41  and the photosensitive drum  53 . The guide rollers  44  are provided at both horizontal ends of a tip portion of the LED head  41  and located outside of a range of the surface of the photosensitive drum  53  where an electrostatic latent image is to be formed (see  FIG. 3 ). 
     When the LED head  41  is positioned at the exposure position, namely, in a state where the upper cover  11  is closed, the guide rollers  44  are brought into contact with the surface of the photosensitive drum  53 , whereby coil springs  43  are compressed between the LED head  41  and the frame  42 , so that the LED head  41  is urged toward the photosensitive drum  53 . 
     The process cartridges  50  are aligned in a longitudinal direction between the upper cover  11  and the sheet feeding section  20  (see  FIG. 1 ). Each of the process cartridges  50  has a drum unit  51  and a developing unit  61  removably attached to the drum unit  51 . The process cartridges  50  can be replaced through the opening  10 A of the main housing  10  after the upper cover  11  is pivoted upwardly (see  FIG. 2 ). The process cartridges  50  differ from each other only in the color of toner (a developing agent) housed in a toner housing chamber  66  of a developing unit  61  and are identical with each other in a structure. 
     Each of the drum units  51  includes a drum case  52 ; a photosensitive drum  53  rotatably supported by the drum case  52 ; and an electrifier  54 . 
     As a result of the developing unit  61  being attached to the drum case  52 , an exposure space  55  (see  FIG. 4 ) through which the photosensitive drum  53  is viewed from the outside is defined. The LED unit  40  (the LED head  41 ) is inserted into the exposure space  55  so as to oppose an upper area of the surface of the photosensitive drum  53 . 
     The developing unit  61  has a case  62 ; a developing roller  63  and a supply roller  64  that are rotatably supported by the case  62 ; and a blade assembly  65 . Further, the developing unit  61  has the toner housing chamber  66  that houses toner. 
     As shown in  FIG. 1 , the transfer unit  70  is interposed between the sheet feeding section  20  and the respective process cartridges  50 . The transfer unit  70  includes a drive roller  71 , a driven roller  72 , a conveyance belt  73 , transfer rollers  74 , and a cleaning section  75 . 
     The drive roller  71  and the driven roller  72  are provided in parallel while being spaced apart from each other in the longitudinal direction. The conveyance belt  73  formed from an endless belt is wound around the drive roller  71  and the driven roller  72 . An external surface of the conveyance belt  73  is in contact with the respective photosensitive drums  53 . Four transfer rollers  74  that nip the conveyance belt  73  in conjunction with the respective photosensitive drums  53  are disposed inside of the conveyance belt  73  so as to oppose the respective photosensitive drums  53 . A transfer bias voltage is applied to the transfer rollers  74  by constant current control operation performed during transfer. 
     The cleaning section  75  is disposed below the conveyance belt  73  and configured so as to remove the toner adhering to the conveyance belt  73  and cause the thus-removed toner to fall into a toner reservoir section  76  disposed below the cleaning section  75 . 
     The fixing unit  80  is disposed at the rear of the respective process cartridges  50  and the transfer unit  70  and includes a heating roller  81  and a pressing roller  82  that is disposed opposite the heating roller  81  and presses the heating roller  81 . 
     In the image forming section  30  configured as mentioned above, surfaces of the respective photosensitive drums  53  are uniformly charged by the electrifiers  54  and subsequently exposed by LED light emitted from the respective LED heads  41 . Thereby, the electric potential of exposed areas becomes lower, and electrostatic latent images based on image data are formed on the respective photosensitive drums  53 . 
     The toner in the toner housing chamber  66  is supplied to the developing roller  63  by rotation of the supply roller  64 , and the thus-supplied toner enters a space between the developing roller  63  and the blade assembly  65  by rotation of the developing roller  63 , whereupon the toner is held on the developing roller  63  as a thin layer of specific thickness. 
     The toner held on the developing roller  63  is supplied to the electrostatic latent image formed on the photosensitive drum  53  when the developing roller  63  contacts the photosensitive drum  53  in an opposing manner. Thereby, the toner is selectively held on the photosensitive drum  53 , so that the electrostatic latent image is visualized and that a toner image is formed by this reversal development. 
     In the course of the sheet P fed on the conveyance belt  73  passing between the respective photosensitive drums  53  and the respective transfer rollers  74  disposed inside of the conveyance belt  73 , the toner images formed on the respective photosensitive drums  53  are sequentially transferred to the sheet P. When the sheet P passes between the heating roller  81  and the pressing roller  82 , the toner images transferred onto the sheet P are thermally fixed. 
     The sheet discharging section  90  includes a sheet discharging path  91  that is formed so as to upwardly extend from an exit of the fixing unit  80  and turn to the right side and a plurality of conveyance roller pairs  92  for conveying the sheet P. The sheet P on which the toner images are transferred and thermally fixed is conveyed along the discharging path  91  by the conveyance rollers  92 , discharged to the outside of the main housing  10 , and stacked on the sheet discharging tray  13 . 
       FIG. 5  is an enlarged view showing the configuration of the lock mechanism  100 . 
     As shown in  FIG. 5 , the lock mechanism  100  has an engagement protrusion  110  provided at a front end of the upper cover  11 ; a latch claw  120  provided at a front upper portion of the main housing  10 ; and a coil spring  130 . 
     The engagement protrusion  110  is provided at each end of the grip  17  at the front end of the upper cover  11  (see  FIG. 3 ). Each of the engagement protrusions  110  includes a shaft engagement section  111 , an arm  112 , a pivotal movement regulation section  113 , and a protrusion section  114 , all of which are formed integrally. 
     The shaft engagement section  111  is formed into a substantially C-shape when viewed from the side and engages with a substantially cylindrical pivotal shaft  11 A provided to the upper cover  11 . Accordingly, the engagement protrusion  110  is supported by the upper cover  11  pivotably in a direction in which the engagement protrusion  110  approaches or separates from the latch claw  120 , that is, in the longitudinal direction. 
     The arm  112  extends downwardly from the shaft engagement section  111  and couples the shaft engagement section  111  to the pivotal movement regulation section  113  and the protrusion section  114 . A support wall  11 B extending from the upper cover  11  is provided at the rear of the arm  112 , and the coil spring  130  is interposed between the support wall  11 B and the arm  112 . 
     The pivotal movement regulation section  113  extends from the arm  112  in a rearward direction at a position below the coil spring  130  and loosely fits into a through hole  11 C opened in the support wall  11 B of the upper cover  11 . Thereby, the engagement protrusion  110  longitudinally pivots without deviating in the left and right direction. 
     Each of the protrusion sections  114  protrudes from a front lower portion of the arm  112  to the front so as to have a tapered shape when viewed from the side. Specifically, each protrusion section  114  includes an upper slope  114 A that tilts upwardly from the front to the rear and a lower slope  114 B that tilts downwardly from the front to the back. An upper end of the upper slope  114 A acts as an engagement point K that engages with the latch claw  120  when the upper cover  11  is closed. 
     Two of the latch claws  120  are provided at a front upper portion of the main housing  10  correspondingly to the engagement protrusions  110  of the upper cover  11 . Each latch claw  120  includes a slope  121  that downwardly tilts from the front to the rear. A lower end of the slope  121  acts as the engagement point K that engages with the engagement protrusion  110  when the upper cover  11  is closed. In  FIG. 5 , the latch claws  120  are provided as members separate from the main housing  10  but may also be formed integrally with the main housing  10 . 
     The coil spring  130  is interposed between the arm  112  of the engagement protrusion  110  and the support wall  11 B of the upper cover  11  as mentioned above. The coil spring  130  is compressed by the arm  112  in the state shown in  FIG. 5 ; namely, the state where the upper cover  11  is closed. Thereby, the engagement protrusion  110  is urged in a direction (forward direction) in which the protrusion approaches the latch claw  120 . 
     The operations of the color printer  1  configured as mentioned above will now be described.  FIG. 6  is a view showing force acting on the engagement point K when the upper cover  11  is being opened.  FIG. 7A  is a view showing the upper cover  11  before being closed; and  FIG. 7B  is a view showing the upper cover  11  in the course of being closed.  FIG. 8  is a view showing force acting on the closed upper cover  11 . 
     First, the force acting on the lock mechanism  100  when the upper cover  11  is being opened and operation of the lock mechanism  100  performed when the upper cover  11  is being opened will be described. 
     When the grip  17  (see  FIG. 3 ) of the upper cover  11  is gripped and the front side of the upper cover  11  is pulled up, upward force F 1  for lifting the color printer  1  acts on the engagement point K of the upper cover  11  as shown in  FIG. 6 . Further, downward force F 2  acts on the engagement point K. The downward force F 2  is generated due to gravity force of the color printer  1  as counterforce to the force F 1  so as to maintain the color printer  1  at the current position. 
     The maximum value of the downward force F 2  for maintaining the color printer  1  at the current position is determined by the weight W of the color printer  1  except the upper cover  11  and components attached to the upper cover  11 . Specifically, the maximum value corresponds to force achieved if a coil spring which does not elastically deform would be arranged in place of the coil spring  130  and if the upper cover  11  would be pulled up and the color printer  1  would be pivotably lifted about fulcrum “f” as a pivotal center. 
     In the meantime, the maximum value of the upward force F 1  for lifting the color printer  1  is determined by engagement force between the engagement protrusions  110  and the latch claws  120 . The engagement force occurring between the engagement protrusions  110  and the latch claws  120  corresponds to vertical force that acts on engagement points between the engagement protrusions  110  and the latch claws  120  as a result of lifting of the upper cover  11 . Specifically, the engagement force is force that upwardly acts on the lower ends of the slopes  121  of the latch claws  120  provided on the main housing  10  from the upper ends of upper slopes  114 A of the engagement protrusions  110  provided on the upper cover  11  when the upper cover  11  is lifted. The force F 1  is generated as a result of the coil springs  130  urging the engagement protrusions  110  in the direction in which the protrusions approach the latch claws  120 . Hence, the maximum value of the force F 1  is determined by the urging force of the coil springs  130 . 
     In the present exemplary embodiment, the urging force of the coil springs  130  is set so that the maximum value of the force F 1  is smaller than the maximum value of the force F 2 . As a result, when the force F 1  exceeds the maximum value, the upper slopes  114 A of the engagement protrusions  110  shown in  FIG. 5  start upwardly sliding along the lower ends of the slopes  121  of the latch claws  120 , whereupon the engagement protrusions  110  start moving upwardly. At this time, the coil springs  130  are compressed by the arms  112  of the engagement protrusions  110 . In a state where the lower ends (the apexes of the protrusion sections  114 ) of the upper slopes  114 A have passed by the lower ends of the slopes  121 , the engagement (lock) of the engagement protrusions  110  with the latch claws  120  is released. Hence, the upper cover  11  is capable of pivoting in an upward direction as shown in  FIG. 2 . 
     In other words, in the present exemplary embodiment, the force F 1  for lifting the color printer  1  acts on the engagement point K of the upper cover  11  when the grip  17  is lifted up. This force F 1  has a component for rotating the color printer  1  in the clockwise direction around the fulcrum f. The gravity force due to the weight W of the color printer  1  except the upper cover  11  and components attached to the upper cover  11  acts on the centroid G of the color printer  1 . The gravity force due to the weight W of the color printer  1  has a component for rotating the color printer  1  in the counterclockwise direction. In order to release the engagement (lock) between the engagement protrusions  110  and the latch claws  120  before the color printer  1  is lifted up, it is necessary to set the force F 1  so that a component of the force F 1  for rotating the color printer  1  in the clockwise direction is smaller than a component of the gravity force due to the weight W of the color printer  1  for rotating the color printer  1  in the counterclockwise direction 
     According to the exemplary present embodiment, the engagement (lock) between the engagement protrusions  110  and the latch claws  120  is released by lifting the upper cover  11  as mentioned above, and hence the upper cover  11  can be opened in single operation; namely, lifting of the upper cover  11 . Additionally, the force required at the time of opening and closing the upper cover  11  can be readily adjusted by changing the urging force of the coil springs  130 . In the exemplary present embodiment, since the grip  17  (see  FIG. 3 ) is provided at the center of the front edge remote from the rotary shaft  12 , the upper cover  11  can be lifted by comparatively smaller force than that required when the grip  17  is provided at a position closer to the rotary shaft  12 . 
     Further, in the present exemplary embodiment, even when the force F 1  exceeds the maximum value, the force F 2  that is counterforce to the force F 1  does not reach the maximum value. Therefore, the upper cover  11  can be opened while the color printer  1  maintains its current position; namely, while the main housing  10  is not lifted and maintains its stationary state. 
     In other words, when the upper cover  11  is opened, clockwise moment generated around the fulcrum “f” by the force F 1  becomes smaller than counterclockwise moment generated around the fulcrum “f” by the weight W of the color printer  1  except the upper cover  11  and members attached thereto. Therefore, the main housing  10  is not lifted and can maintain its stationary state. 
     In the case where the maximum value of the force F 1  is greater than the maximum value of the force F 2 , the coil springs  130  are not sufficiently compressed even when the force F 2  exceeds the maximum value. Therefore, the engagement (lock) between the engagement protrusions  110  and the latch claws  120  is not released, and the force F 1  becomes greater in accordance with the force used for lifting the upper cover  11 . Therefore, the color printer  1  is lifted about the fulcrum “f” as the pivotal center. 
     The force acting on the lock mechanism  100  when the upper cover  11  is being closed and operation of the lock mechanism  100  performed when the upper cover  11  is being closed will now be described. 
     In the stationary state shown in  FIG. 7A , downward force F 3  due to weight of the upper cover  11  acts on the upper cover  11 . At this time, the protrusion sections  114  of the engagement protrusions  110  (the lower slopes  114 B) come to standstill while remaining in contact with the upper ends of the slopes  121  of the latch claws  120 . Specifically, the urging force of the coil springs  130  is set so that the coil springs  130  are not sufficiently compressed when only the downward force F 3  due to the weight of the upper cover  11  acts on the upper cover  11 . 
     When the downward force F 4  for closing the upper cover  11  additionally acts on the upper cover  11  in this state, the lower slopes  114 B start downwardly sliding over the upper ends of the slopes  121 , whereupon the engagement protrusions  110  start moving downwardly. At this time, the coil springs  130  are further compressed by the arms  112  of the engagement protrusions  110 . Moreover, as shown in  FIG. 7B , the upper ends (the apexes of the protrusion section  114 ) of the lower slopes  114 B slide over the slopes  121 , whereby the engagement protrusions  110  move further downwardly. In association with such downward movement, the coils springs  130  are further compressed by the arms  112  of the engagement protrusions  110 . 
     When upper ends (the apexes of the protrusion section  114 ) of the lower slopes  114 B pass by the lower ends of the slopes  121 , the upper slopes  114 A downwardly slide over the lower ends of the slopes  121 , and the engagement protrusions  110  move further downwardly. At this time, the engagement protrusions  110  are pressed by the coil springs  130 , to be urged toward the latch claws  120  (to the front). As shown in  FIG. 5 , the upper ends of the upper slopes  114 A come into contact with the lower ends of the slopes  121 , whereby the engagement protrusions  110  engage with the latch claws  120 , whereby the upper cover  11  is maintained in a closed state. 
     As mentioned above, in the present exemplary embodiment, the downward force F 4  is exerted on the upper cover  11 ; namely, when downward force (F 3 +F 4 ) exceeding the downward force F 3  due to the weight of the upper cover  11  is exerted on the upper cover  11 , the coil springs  130  are compressed, and the upper cover  11  can be finally closed. In other words, when the downward force F 4  is not exerted on the upper cover  11 ; namely, when only the downward force F 3  due to the weight of the upper cover  11  is exerted on the upper cover  11 , the upper cover  11  is not closed. Therefore, the user&#39;s finger can be prevented from being pinched between the main housing  10  and the upper cover  11 . 
     Next, the force acting on the closed upper cover  11  will be described. 
     As shown in  FIG. 8 , when the guide rollers  44  come into contact with the surfaces of the photosensitive drums  53 , the coil springs  43  located between the LED heads  41  and the frame  42  are compressed, thereby generating upward force F 5 . The force F 5  acts on the closed upper cover  11 . The force F 5  acts in a direction in which the upper cover  11  is opened. The force F 5  is a sum of four upward forces generated as a result of compression of the four coil springs  43 . 
     The force resistant to the force F 5 , that is, the force acting in the direction in which the upper cover  11  is closed includes downward force F 6  due to the weight of the upper cover  11  and engagement force F 7  that is generated by the engagement protrusions  110  and the latch claws  120  and that downwardly acts in response to the upward force (mainly a difference between the force F 5  and the force F 6  (F 5 −F 6 )). The engagement force occurs between the engagement protrusions  110  and the latch claws  120  in this case are forces that downwardly act on the upper ends of the upper slopes  114 A of the engagement protrusions  110  provided on the upper cover  11  from the lower ends of the slopes  121  of the latch claws  120  provided on the main housing  10 . The engagement force F 7  is a force that arises as a result of the coil springs  130  urging the engagement protrusions  110  in the direction approaching the latch claws  120 . 
     In the present exemplary embodiment, a sum of the maximum value of the engagement force F 7  occurring between the engagement protrusions  110  and the latch claws  120  and the downward force F 6  due to the weight of the upper cover  11  is set so as to be greater than the maximum value of the force F 5 . As a result, in the state where the upper cover  11  is closed, the force acting in the direction in which the upper cover  11  is opened (that is, the working force F 5 , and in this case the maximum value is achieved because the coil springs  43  are compressed to the maximum) becomes equal to the force acting in the direction in which the upper cover  11  is closed (the force F 6 +the engagement force F 7 ), and hence the upper cover  11  can be maintained in the closed state. 
     In other words, the sum of the counterclockwise moment generated around the rotary shaft  12  by the force F 6  and the counterclockwise moment generated around the rotary shaft  12  by the engagement force F 7  becomes equal to the clockwise moment generated around the rotary shaft  12  by the force F 5 , and therefore, the upper cover  11  can be maintained in the closed state. 
     Even when upward force is exerted on the upper cover  11 , a sum of the maximum value of the engagement force F 7  and the force F 6  is greater than the force F 5  (since the coil springs  43  slightly expand in this case, the sum might become a value that is smaller than the maximum value), the upper cover  11  can be maintained in the closed state. Specifically, when the upper cover  11  is lifted, the upper cover  11  is maintained in the closed state unless the engagement force F 7  reaches the maximum value. As the force for lifting the upper cover  11  is increased, the coil springs  130  are compressed, whereupon the engagement force F 7  increases. When the engagement force F 7  reaches the maximum value, the engagement (lock) between the engagement protrusions  110  and the latch claws  120  are released, so that the upper cover  11  can be opened. 
     In the present exemplary embodiment, the centroid G of the upper cover  11  is positioned at more front than the center C of the upper cover  11  shown in  FIG. 2 . Therefore, when compared with the case where the centroid G is positioned more rear than the center C, the force F 4  required at the time of closing of the upper cover  11  (see  FIG. 7B ) becomes smaller, and the coil springs  130  that generate smaller urging force can be employed. As a result of the force F 4  required at the time of closing of the upper cover  11  being made smaller, the upper cover  11  can be readily closed. Further, use of the coil springs  130  that generate smaller urging force results in a reduction in the force required at the time of opening of the upper cover  11 . Accordingly, stability of the color printer  1  can be enhanced. 
     Since the control circuit  15  and the shield plate  16  are arranged so that the centroid G of the upper cover  11  is positioned more front than the center C, the control circuit  15  and the shield plate  16  made of metal act as weights, thereby contributing to the upper cover  11  maintained in the closed state. Moreover, the control circuit  15  and the shield plate  16  act as weights, whereby the force F 4  required at the time of closing of the upper cover  11  can be made small. Accordingly, the upper cover  11  can be readily closed. 
     While the present 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. 
     Although the exemplary embodiment provides the case where the engagement protrusions  110  are provided on the upper cover  11  and where the latch claws  120  are provided on the main housing  10 , the inventive concept of the present invention is not limited to the exemplary embodiment. For example, the engagement protrusions  110  may be provided on the main housing  10  and in which the latch claws  120  may be provided on an upper cover  11 . 
     The exemplary embodiment provides the case where the LED heads  41  using LEDs are adopted for exposing the photosensitive drum  53 . However, the inventive concept of the present invention is not limited to the LEDs. For example, exposure portions using OLED (Organic Light Emitting Diode), fluorescent substances, or the like, may also be adopted in place of the LEDs. Moreover, there may also be adopted an exposure member that includes a plurality of optical shutters (e.g., liquid-crystal elements, PLZT elements, and the like) arranged for controlling light from a single or a plurality of light sources and that selectively controls an opening and closing time of the optical shutters on the basis of image data. 
     Although the exemplary embodiment provides the configuration in which the control circuit  15  and the shield plate  16  are arranged on the front side within the upper cover  11  in such a way that the centroid G of the upper cover  11  comes to the position closer to the front end than to the center C of the upper cover  11  shown in  FIG. 2 , the inventive concept of the present invention is not limited to the configuration. Specifically, the control substrate  15  and a sheet-metal member  16  may not be provided within the upper cover  11 , so long as the centroid G of the upper cover  11  is positioned at an opposite side to the center C of the upper cover  11  with respect to of the fulcrum f. Moreover, a sheet-metal member  16  may not be provided (only the control substrate  15  is arranged within the upper cover  11  in such a way that the centroid G of the upper cover  11  is positioned at an opposite side the center C of the upper cover  11  with respect to the fulcrum f). 
     In the exemplary embodiment, the color printer  1  having the four LED units  40  and the four process cartridges  50  is explained. However, the inventive concept of the present invention is not limited to this exemplary embodiment. Specifically, the exemplary embodiment of the present invention can be applied to an image forming apparatus having one photosensitive element and one exposure member (e.g., a monochrome printer). 
     In the above exemplary embodiment, the photosensitive drums  53 , the coil springs  130  and the coil springs  43  are explained. However, the inventive concept of the present invention is not limited thereto. For example, an endless-belt-shaped photosensitive element or a flat-surface-shaped photosensitive element may also be adopted in place of the photosensitive drums  53 . Further, a leaf spring, or the like, may also be adopted in place of the coil springs  130  or the coil springs  43 . 
     The shape of the lock mechanism  100  (the engagement protrusion  110  and the latch claw  120 ) is not limited to the shape described in the present exemplary embodiment. No specific limitations are imposed on the shape, so long as the lock mechanism  100  obtains an effect analogous to that obtained in the exemplary embodiment.