Patent Abstract:
An optical scanning device including: a light source part that is provided in a resin-molded casing and emits a laser beam; a deflector that is arranged in the casing and deflects and scans the laser beam, the deflector including, a rotary polygon mirror that reflects the laser beam, a driving source that rotates the rotary polygon mirror, and a substrate member; and the casing including, a fixed wall that extends in a direction perpendicular to a mirror surface of the rotary polygon mirror, a first fixing part that is provided to the fixed wall and fixes the light source part, a second fixing part that is provided to the fixed wall and fixes the substrate member, and a reinforcing part that is provided to the fixed wall and extends toward the emission direction so as to continuously connect the first fixing part and the second fixing part.

Full Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from Japanese Patent Application No. 2011-069546 filed on Mar. 28, 2011, the entire contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     Aspects of the invention relate to an optical scanning device that is used in an image forming apparatus such as laser printer. 
     BACKGROUND 
     As a printer of an electrophotographic type, there is known a printer including a photosensitive member on which an electrostatic latent image is formed and an exposure device that forms an electrostatic latent image on a surface of the photosensitive member by scanning laser beam based on image data to the photosensitive member. 
     As the exposure device provided to the printer, related-art shows an optical scanning device which has a light source unit that emits a light beam, a polygon scanner that deflects and scans the light beam emitted from the light source unit and an optical box that accommodates therein the light source unit and the polygon scanner. 
     In the related-art optical scanning device, both the light source unit and the polygon scanner are fixed to a bottom face part of the optical box. Also, the bottom face part is provided with ribs that surround the polygon scanner. 
     SUMMARY 
     In the related-art optical scanning device, when the polygon scanner rotates at high speed, a resonance, which changes a relative position between the light source unit and the polygon scanner, may be generated by vibrations caused due to the rotation of the polygon scanner. 
     When the relative position between the light source unit and the polygon scanner is changed by the resonance, the precision of the light beam emitted onto a surface of the photosensitive member is lowered and an image quality of a printed image may be deteriorated. 
     Accordingly, an object of the invention is to provide an optical scanning device capable of suppressing an image quality of a printed image from being deteriorated. 
     According to an aspect of the invention, there is provided an optical scanning device including: a light source part that is provided in a resin-molded casing and emits a laser beam; a deflector that is arranged in the casing downstream of the light source part with respect to an emission direction of the laser beam and deflects and scans the laser beam, the deflector including, a rotary polygon mirror that reflects the laser beam, a driving source that rotates the rotary polygon mirror, and a substrate member that supports the rotary polygon mirror and the driving source and is fixed to the casing; and the casing including, a fixed wall that extends in a direction perpendicular to a mirror surface of the rotary polygon mirror, a first fixing part that is provided to the fixed wall and fixes the light source part, a second fixing part that is provided to the fixed wall and fixes the substrate member, and a reinforcing part that is provided to the fixed wall and extends toward the emission direction so as to continuously connect the first fixing part and the second fixing part. 
     According to the invention, the light source part that emits the laser beam is fixed to the first fixing part provided to the fixed wall of the casing, the substrate member of the deflector that deflects and scans the laser beam is fixed to the second fixing part provided to the fixed wall of the casing and the reinforcing part that extends in the emission direction of the laser beam is provided to the fixed wall of the casing so as to continuously connect the first fixing part and the second fixing part. 
     Accordingly, the fixed wall between the light source part and the deflector is continuously connected and thus reinforced by the reinforcing part extending in the emission direction of the laser beam. 
     As a result, it is possible to suppress the resonance that changes a relative position between the light source part and the deflector, which is caused due to the rotation of the rotary polygon mirror, and further to suppress an image quality of a printed image from being deteriorated. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a central sectional view of a laser printer; 
         FIG. 2  is a sectional view showing a scanner unit that is a first illustrative embodiment of an optical scanning device of the invention; 
         FIG. 3  is an A-A sectional view of the scanner unit shown in  FIG. 2 ; 
         FIG. 4  illustrates a casing of a scanner unit of each illustrative embodiment, in which, 
         FIG. 4A  shows a second illustrative embodiment, 
         FIG. 4B  shows a third illustrative embodiment, 
         FIG. 4C  shows a fourth illustrative embodiment, 
         FIG. 4D  shows a fifth illustrative embodiment, 
         FIG. 4E  shows a sixth illustrative embodiment, 
         FIG. 4F  shows a seventh illustrative embodiment, and 
         FIG. 4G  shows an eighth illustrative embodiment; 
         FIG. 5  is a bottom view of a scanner unit that is a ninth illustrative embodiment of the optical scanning device of the invention; 
         FIG. 6  is a side view of the scanner unit shown in  FIG. 5 ; and 
         FIG. 7  illustrates a casing of a scanner unit of each comparative example, in which, 
         FIG. 7A  shows a first comparative example, 
         FIG. 7B  shows a second comparative example, and 
         FIG. 7C  shows a third comparative example. 
     
    
    
     DETAILED DESCRIPTION 
     1. Overall Configuration of Laser Printer 
     As shown in  FIG. 1 , a laser printer  1  has a feeder unit  3  and an image forming unit  4  in a body casing  2 . 
     A front cover  5  for attaching and detaching a developing unit  10  is provided on one sidewall of the body casing  2 . The front cover  5  is configured to be opened and closed freely. 
     Meanwhile, in the descriptions hereinafter, the side (right side of  FIG. 1 ) at which the cover  5  is provided is referred to as the front side and the opposite side (left side of  FIG. 1 ) thereto is referred to as the rear side. Also, the left and right sides are defined when seeing the printer  1  from the front side. That is, the front side in a direction perpendicular to the sheet of  FIG. 1  is the left side and the back side in a direction perpendicular to the sheet of  FIG. 1  is the right side. 
     (1) Feeder Unit 
     The feeder unit  3  has a sheet feeding tray  6  that stacks and accommodates sheets P. The sheet feeding tray  6  is detachably attached to a bottom part in the body casing  2 . A sheet feeding roller  7  is arranged upper to the front end of the sheet feeding tray  6  and register rollers  8  are arranged at the rear side of the sheet feeding roller  7 . 
     The sheets P accommodated in the sheet feeding tray  6  are delivered one by one toward the register rollers  8  by rotation of the sheet feeding roller  7 . The sheet is then delivered toward the image forming unit  4  (between a photosensitive drum and a transfer roller  16 , which will be described later) by rotations of the register rollers  8  at a predetermined timing. 
     (2) Image Forming Unit 
     The image forming unit  4  has a scanner unit  9 , which is an example of the optical scanning device, a developing unit  10  and a fixing unit  11 . 
     (2-1) Scanner Unit 
     The scanner unit  9  is arranged at an upper part in the body casing  2 . As shown with a dashed line, the scanner unit  9  emits a laser beam L based on image data toward the photosensitive drum  14  (which will be described later) of the developing unit  10  and moves the laser beam L at high speed in one side in left-right direction (main scanning direction), thereby scanning a surface of the photosensitive drum  14 . 
     (2-2) Developing Unit 
     The developing unit  10  is arranged lower to the scanner unit  9 . The developing unit  10  has a drum cartridge  12  and a developing cartridge  13  that is detachably mounted to the drum cartridge  12 . 
     The photosensitive drum  14  that extends in the left-right direction and has a substantially cylindrical shape is rotatably provided in the drum cartridge  12 . Also, a scorotron-type charger  15  and a transfer roller  16  are arranged around the photosensitive drum  14  in the drum cartridge  12 . 
     The developing cartridge  13  is arranged at the front side of the photosensitive drum  14  and has a developing roller  17 . 
     The developing roller  17  is rotatably supported to a rear end portion of the developing cartridge  13  so that it is exposed from the back side. The developing roller faces and contacts a front side of the photosensitive drum  14  so as to press the photosensitive drum  14  from the front side. 
     Also, the developing cartridge  13  accommodates therein toner corresponding to respective colors in a front space of the developing roller  17 . 
     (2-3) Development/Transfer Operations 
     The toner in the developing cartridge  13  is carried on a surface of the developing roller  17  as the developing roller  17  is rotated. 
     In the meantime, as the photosensitive drum  14  is rotated, the surface of the photosensitive drum  14  is uniformly charged by the scorotron-type charger  15  and then exposed by the high-speed scanning of the laser beam L (refer to the dashed line in  FIG. 1 ) emitted from the scanner unit  9 . Thereby, an electrostatic latent image, which corresponds to an image to be formed on the sheet P, is formed on the surface of the photosensitive drum  14 . 
     When the photosensitive drum  14  is further rotated, the toner carried on the surface of the developing roller  17  is supplied to the electrostatic latent image formed on the surface of the photosensitive drum  14 . Thereby, the electrostatic latent image of the photosensitive drum  14  becomes a visible image and a toner image by reversal developing is carried on the surface of the photosensitive drum  14 . 
     The toner image is transferred onto the sheet P that is conveyed (to a transfer position) between the photosensitive drum  14  and the transfer roller  16 . 
     (2-4) Fixing Unit 
     The fixing unit  11  is provided at the rear of the developing unit  10  and has a heating roller  18  and a pressing roller  19  that is pressure-contacted to the heating roller  18 . 
     The toner image transferred onto the sheet P is heated and pressed and thus heat-fixed on the sheet P while the sheet P passes between the heating roller  18  and the pressing roller  19 . 
     (3) Sheet Discharge 
     The sheet P having the toner image fixed thereon is conveyed toward sheet discharge rollers  20  and is discharged onto a sheet discharge tray  21 , which is formed on an upper surface of the body casing  2 , by the sheet discharge rollers  20 . 
     2. Details of Scanner Unit 
     (1) Configuration of Scanner Unit 
     As shown in  FIGS. 2 and 3 , the scanner unit  9  has, in a casing  31  made of resin, a light source  32 , which is an example of the light source part, a first cylindrical lens  33 , a deflector  34 , an fθ lens  35 , a mirror  36  and a second cylindrical lens  37 . 
     As specifically described later, the casing  31  has a substantially flat box shape that is thin in the upper-lower direction. Specifically, the casing  31  has a lower wall  40  that extends from front to rear and from left to right and is an example of the fixed wall, a sidewall  39  that extends upward from a periphery of the bottom wall  40  and an upper wall (not shown) that is opposed to the lower wall  40  in the upper-lower direction and is connected with the sidewall  39  at a periphery thereof. The lower wall  40  of the casing  31  is formed with a penetrated emission port  44  for emitting the laser beam L toward the photosensitive drum  14 . 
     The emission port  44  has a substantially rectangular shape extending in the left-right direction at a rear end portion of the casing  31 , when seen in a plan view. 
     The light source  32  is disposed at a substantial center of a right end portion of the casing  31  in the front-rear direction. Also, the light source  32  has a light source holder  53 , a semiconductor laser  51  and a coupling lens  52 . 
     The light source holder  53  has a substantially rectangular flat plate shape extending from front to rear and from left to right, when seen in a plan view, and is formed at both end portions thereof in the front-rear direction with an insertion penetration hole (not shown) into which a light source fixing screw  54  (which will be described later) is inserted, respectively. 
     The semiconductor laser  51  is supported at a substantial center of a rear end portion of the light source holder  53  in the front-rear direction. The semiconductor laser  51  emits the laser beam L toward the left side (specifically, toward the left-front side). 
     The coupling lens  52  is supported at a substantial center of a left end portion of the light source holder  53  in the front-rear direction so that it is opposed to the semiconductor laser  51 . The coupling lens  52  converts the laser beam L, which is emitted from the semiconductor laser  51 , into a parallel light flux. 
     The first cylindrical lens  33  has a substantially flat plate shape extending in the front-rear direction (specifically, in a direction connecting the right-front side and the left-rear side) and is arranged with an interval at the left side of the light source  32  so that it is opposed to the coupling lens  52 . The first cylindrical lens  33  has refractive power only in a sub-scanning direction (direction perpendicular to both the traveling direction of the laser beam L and the scanning direction of the laser beam L). 
     The deflector  34  is disposed at the left-front side of the first cylindrical lens  33 , in the left-front end portion of the casing  31 . The deflector  34  has a motor base plate  63  that is an example of the substrate member, a motor  62  that is an example of the driving source and a polygon mirror  61  that is an example of the rotary polygon mirror. 
     The motor base plate  63  has a substantially rectangular flat plate shape extending from front to rear and from left to right, when seen in a plan view, and supports the motor  62 . Each of four corners of the motor base plate  63  is formed with an insertion penetration hole (not shown) into which a deflector fixing screw  66  (which will be described later) is inserted, respectively. 
     The motor  62  has a substantially cylindrical shape extending in the upper-lower direction and is fixed on a lower surface of the motor base plate  63 . The motor  62  has a driving shaft  65  that extends in the upper-lower direction and can be rotated. 
     The polygon mirror  61  has a substantially regular hexagonal flat plate shape when seen in a plan view and has a thickness in the upper-lower direction. Each side of the polygon mirror  61  is formed with a mirror surface  64  extending in the upper-lower direction. The polygon mirror  61  is supported, at a substantial center thereof when seen in a plan view, to a lower end portion of the driving shaft  65  of the motor  62  so that it cannot be relatively rotated. Also, the polygon mirror  61  is arranged to face the lower wall  40  of the casing  31  with an interval therebetween in the upper-lower direction. 
     The fθ lens  35  is a lens having an fθ characteristic, has a substantially flat plate shape extending in the left-right direction. The fθ lens  35  is arranged at a substantial center of the casing  31  in the front-rear direction and at the rear side of the deflector  34  to face the polygon mirror  61 . A rear end surface of the fθ lens  35  has a substantially circular arc shape having a predetermined curvature so that a substantial center thereof in the left-right direction protrudes rearward. A front end surface of the fθ lens  35  has a substantially circular arc shape having a curvature smaller than that of the rear end surface so that a substantial center thereof in the left-right direction is concave rearward. 
     The mirror  36  has a substantially flat plate shape extending in the left-right direction and is disposed at a rear side periphery of the emission port  44  to face the fθ lens  35 , in the rear end portion of the casing  31 . Also, the mirror  36  has a front face that is a mirror surface and is inclined such that it is directed downward as it is directed toward the rear side, so that the front face is opposed to the emission port  44 . 
     The second cylindrical lens  37  has a substantially flat plate shape extending in the left-right direction and is arranged to face the mirror  36  in the emission port  44 . The second cylindrical lens  37  has refractive power only in the sub-scanning direction. 
     (2) Details of Casing 
     (2-1) Configuration Regarding Fixing of Light Source, First Cylindrical Lens and Deflector 
     In the casing  31 , two light source fixing parts  41  for fixing the light source  32 , which are an example of the first fixing part, a first cylindrical lens fixing part  42  for fixing the first cylindrical lens  33  and four deflector fixing parts  43  for fixing the deflector  34 , which are an example of the second fixing part, are provided. 
     The respective light source fixing parts  41  are arranged with an interval in the front-rear direction at a substantial center of the right end portion of the casing  31  in the front-rear direction so that the respective light source fixing parts correspond to the respective insertion penetration holes (not shown) of the light source holder  53 . Each of the light source fixing parts  41  has a substantially cylindrical shape (refer to  FIG. 3 ) extending and protruding vertically from the lower wall  40  of the casing  31  and has a screw hole (not shown) at a substantially diametrical center thereof, which is formed downward from the upper end surface. 
     The light source fixing screws  54  inserted into the respective insertion penetration holes (not shown) of the light source holder  53  are screwed into the respective light source fixing parts  41 , so that the light source holder  53  of the light source  32  is fixed. 
     The first cylindrical lens fixing part  42  is arranged at a left side of the front light source fixing part  41  and has a substantially rectangular frame shape extending in the front-rear direction, when seen in a plan view. In the meantime, left and right sidewalls of the first cylindrical lens fixing part  42  are notched at parts through which the laser beam L passes. 
     The first cylindrical lens  33  is fitted and fixed in the first cylindrical lens fixing part  42 . 
     The respective deflector fixing parts  43  are arranged at the left-front end portion of the casing  31  in two lines of left and right, which are parallel, in the front-rear direction with an interval therebetween, so that they correspond to the respective insertion penetration holes (not shown) of the motor base plate  63 . Each of the deflector fixing parts  43  has a substantially cylindrical shape extending and protruding vertically from the lower wall  40  of the casing  31  (refer to  FIG. 3 ) and has a screw hole (not shown) at a substantially diametrical center thereof, which is formed downward from the upper end surface. 
     The deflector fixing screws  66  inserted into the respective insertion penetration holes (not shown) of the motor base plate  63  are screwed into the respective deflector fixing parts  43 , so that the motor base plate  63  of the deflector  34  is fixed. 
     (2-2) Configuration about Reinforcement of Casing 
     In the casing  31 , a first rib  45  and a second rib  46 , which are an example of the reinforcing part, are provided. 
     The first rib  45  is a protrusion that protrudes upward from the lower wall  40  of the casing  31  and extends in the left-right direction while being curved. The first rib  45  is arranged between the polygon mirror  61  and the fθ lens  35  at the rear side of the light path of the laser beam L so that it follows the laser beam L heading for the polygon mirror  61  from the light source  32 . 
     Specifically, the first rib  45  integrally has a first part  45 A, a second part  45 B and a third part  45 C. 
     The first part  45 A continuously connects the rear light source fixing part  41  and a rear end portion of the first cylindrical lens fixing part  42 . Specifically, the first part  45 A extends from the rear light source fixing part  41  to the left-front side, is bent leftward at the left side of the coupling lens  52 , further extends in the left-front direction and is then connected to the rear end portion of the first cylindrical lens fixing part  42 . 
     The second part  45 B continuously connects the rear end portion of the first cylindrical lens fixing part  42  and one of the deflector fixing part  43  which is positioned at the left-rear side. Specifically, the second part  45 B has a substantially linear shape extending in a direction of connecting the right-rear side and the left-front side so that it is inclined at an angle smaller than 90 degrees with respect to the laser beam L passing a center of the scanning range of the laser beam L in the left-right direction. Also, the second part  45 B is notched downward from the upper end at a left half thereof facing the polygon mirror  61  in the front-rear direction so that it does not interfere with the laser beam L. 
     The third part  45 C has a substantially linear shape extending in the left-right direction so that it continuously connects one of the deflector fixing parts  43 , which is located at the left-rear side, and the left sidewall  39  of the casing  31 . 
     The second rib  46  is a protrusion that protrudes upward from the lower wall  40  of the casing  31  and extends in the left-right direction while being curved. The second rib  46  is arranged at the front side of the light path of the laser beam L with an interval between the first rib  45  so that it follows the laser beam L heading for the polygon mirror  61  from the light source  32 . That is, when projected in the upper-lower direction, the first rib  45  and the second rib  46  are arranged to sandwich the laser beam L, which is heading for the polygon mirror  61  from the first cylindrical lens  33 , in the front-rear direction. 
     Specifically, the second rib  46  integrally has a first part  46 A, a second part  46 B and a third part  46 C. 
     The first part  46 A continuously connects the front light source fixing part  41  and the front end portion of the first cylindrical lens fixing part  42 . Specifically, the first part  46 A extends from the front light source fixing part  41  to the left-rear side, is bent leftward at the left-front side of the coupling lens  52 , further extends in the left-lower direction and is then connected to the front end portion of the first cylindrical lens fixing part  42 . 
     The second part  46 B continuously connects the front end portion of the first cylindrical lens fixing part  42  and one of the deflector fixing part  43  which is positioned at the left-front side (i.e., the deflector fixing part  43  located at the most distant position from the first cylindrical lens fixing part  42 ). Specifically, the second part  46 B of the second rib  46  has a substantially linear shape extending in a direction of connecting the right-rear side and the left-front side so that it is inclined at an angle smaller than 90 degrees with respect to the laser beam L passing a center of the scanning range of the laser beam L in the left-right direction. Also, when projected in the upper-lower direction, a left half of the second part  46 B of the second rib  46  extends to cross the front end portion of the polygon mirror  61  in the left-right direction, and is notched downward from the upper end thereof so that it does not interfere with the polygon mirror  61  (refer to  FIG. 3 ). 
     The third part  46 C has a substantially linear shape extending in the left-right direction so that it continuously connects one of the deflector fixing parts  43 , which is located at the left-front side, and the left sidewall  39  of the casing  31 . 
     Also, two light source reinforcement ribs  48 , which respectively connect the respective light source fixing parts  41  and the right sidewall  39  of the casing  31 , are provided in the casing  31 . 
     Specifically, a rear-light source part side plate  49 , which is arranged with an interval at the rear side of the light source  32  and extends leftward continuously from the right sidewall  39  of the casing  31 , and a front-light source part side plate  50 , which is arranged with an interval at the front side of the light source  32  and extends leftward continuously from the right sidewall  39  of the casing  31 , are formed in the casing  31 . 
     The front light source reinforcement rib  48  has a substantially linear shape extending in the front-rear direction so that it continuously connects the front light source fixing part  41  and the front-light source part side plate  50 . That is, the front light source reinforcement rib  48  is connected to the right sidewall  39  of the casing  31  via the front-light source part side plate  50 . 
     Also, the rear light source reinforcement rib  48  has a substantially linear shape extending in the front-rear direction so that it continuously connects the rear light source fixing part  41  and the rear-light source part side plate  49 . That is, the rear light source reinforcement rib  48  is connected to the right sidewall  39  of the casing  31  via the rear-light source part side plate  49 . 
     (3) Operations of Scanner Unit 
     When the scanner unit  9  is operated, the motor  62  of the deflector  34  is first driven and then the polygon mirror  61  is rotated at high speed. 
     Then, the laser beam L is emitted from the light source  32  toward the polygon mirror  61  that is being rotated. 
     When the laser beam L emitted from the light source  32  passes the first cylindrical lens  33 , the laser beam is converged in the sub-scanning direction and then enters onto the polygon mirror  61  that is being rotated. 
     Then, as the laser beam L is reflected from the mirror surface  64  of the polygon mirror  61 , the laser beam is deflected to perform equiangular movement and is scanned in the main scanning direction. 
     The laser beam L scanned by the polygon mirror  61  is converted into a constant speed scanning when passing through the fθ lens  35 . Then, the laser beam L is reflected from the mirror  36 . After that, the laser beam L passes through the second cylindrical lens  37  and is then illuminated on the surface of the photosensitive drum  14 . 
     3. Operational Effects 
     (1) According to the scanner unit  9 , as shown in  FIG. 2 , the first rib  45  and second rib  46  extending toward the emission direction (the left-front side) of the laser beam L are provided on the lower wall  40  of the casing  31  so as to continuously connect the light source fixing parts  41  provided on the lower wall  40  of the casing  31  and the deflector fixing parts  43  provided on the lower wall  40  of the casing  31 . 
     Therefore, it is possible to continuously connect and reinforce the lower wall  40  between the light source  32  and the deflector  34  by the first rib  45  and second rib  46  extending toward the emission direction of the laser beam L. 
     As a result, it is possible to suppress the resonance that changes the relative position between the light source  32  and the polygon mirror  61  of the deflector  34 , which is due to the vibrations caused due to the rotation of the polygon mirror  61  of the deflector  34 . Accordingly, it is possible to suppress the image quality of a printed image from being deteriorated. 
     (2) According to the scanner unit  9 , as shown in  FIG. 2 , the first rib  45  and second rib  46  extend so that the ribs are inclined at the angle smaller than 90 degrees with respect to the laser beam L passing to a center of the scanning range of the laser beam L in the left-right direction. 
     Therefore, it is possible to enable the first rib  45  and second rib  46  to follow the laser beam L, which is emitted from the light source part (light source  32  and first cylindrical lens  33 ) while being inclined in the left-front direction. 
     As a result, it is possible to further reinforce the lower wall  40  of the casing  31  with respect to the emission direction of the laser beam L. 
     (3) According to the scanner unit  9 , as shown in  FIG. 2 , the second rib  46  continuously connects the deflector fixing part  43  of the left-front side, which is located at the most distant position from the light source fixing part  41 , and the light source fixing part  41 . 
     Therefore, it is possible to make the second rib  46  long in the left-right direction, so that it is possible to further reinforce the lower wall  40  of the casing  31 . 
     (4) According to the scanner unit  9 , as shown in  FIG. 2 , the first rib  45  and the second rib  46  continuously connect the deflector fixing parts  43  and the left sidewall  39  of the casing  31 . 
     Therefore, it is possible to connect the lower wall  40  and the left sidewall  39  of the casing  31  by the first rib  45  and second rib  46 . 
     As a result, it is possible to suppress the resonance that changes the relative position between the light source  32  and the polygon mirror  61  of the deflector  34 , which is due to the vibrations caused due to the rotation of the polygon mirror  61  of the deflector  34 . 
     (5) According to the scanner unit  9 , as shown in  FIG. 2 , when projected in the upper-lower direction, the second rib  46  extends to cross the polygon mirror  61  in the left-right direction. 
     Therefore, it is possible to reinforce the lower wall  40  of the casing  31  at a position at which the second rib overlaps with the polygon mirror  61 , when projected in the upper-lower direction. 
     As a result, it is possible to further suppress the resonance that changes the relative position between the polygon mirror  61  and the light source  32 . 
     (6) According to the scanner unit  9 , as shown in  FIG. 2 , when projected in the upper-lower direction, the first rib  45  and second rib  46  continuously connect the light source fixing parts  41  and the deflector fixing parts  43  of the left-rear and left-front sides, respectively, so as to sandwich the laser beam L heading for the polygon mirror  61  from the light source  32  in the front-rear direction. 
     Therefore, it is possible to reinforce the lower wall  40  of the casing  31  at both the front and rear sides of the light path of the laser beam L. 
     As a result, it is possible to further reinforce the lower wall  40  of the casing  31  with respect to the emission direction of the laser beam L. 
     (7) According to the scanner unit  9 , as shown in  FIG. 3 , the polygon mirror  61  is provided below the motor base plate  63 . That is, the motor base plate  63  is provided above the lower wall  40  of the casing  31  with the polygon mirror  61  being interposed therebetween. 
     Therefore, it is possible to arrange the deflector  34  based on the lengths of the first rib  45  and second rib  46  in the upper-lower direction. 
     As a result, when the first rib  45  and second rib  46  are formed, it is possible to suppress the scanner unit  9  from becoming larger. 
     (8) According to the scanner unit  9 , as shown in  FIG. 2 , the light source  32  of the light source part (light source  32  and first cylindrical lens  33 ) has the semiconductor laser  51  that emits the laser beam L and the coupling lens  52  that converts the laser beam L from the semiconductor laser  51  into the parallel light flux. 
     Therefore, the first rib  45  and second rib  46  can reinforce the lower wall  40  of the casing  31  between the semiconductor laser  51  and coupling lens  52  and the deflector  34 . 
     As a result, it is possible to suppress the resonance that changes the relative position between the light source  32  of the polygon mirror  61  of the deflector  34 , which is due to the vibrations caused due to the rotation of the polygon mirror  61  of the deflector  34 . 
     (9) According to the scanner unit  9 , as shown in  FIG. 3 , the first rib  45  and second rib  46  are protrusions that extend upward from the lower wall  40  of the casing  31 . 
     Therefore, it is possible to reinforce the lower wall  40  of the casing  31  by a simple configuration. 
     (10) According to the scanner unit  9 , as shown in  FIG. 3 , the respective deflector fixing parts  43  extend vertically from the lower wall  40  of the casing  31  and the motor base plate  63  is screwed to the respective deflector fixing parts  43 . 
     Therefore, it is possible to securely fix the motor base plate  63  to the respective deflector fixing parts  43  with a simple configuration. 
     (11) According to the scanner unit  9 , as shown in  FIG. 2 , the first part  45 A of the first rib  45  and the first part  46 A of the second rib  46 , which reinforce the lower wall  40  between the light source  32  and the first cylindrical lens  33 , are provided in the casing  31 . 
     Therefore, it is possible to further suppress the resonance that changes the relative position between the light source  32  and the polygon mirror  61  of the deflector  34 , which is due to the vibrations caused due to the rotation of the polygon mirror  61  of the deflector  34 . 
     As a result, it is possible to further suppress the image quality of the printed image from being deteriorated. 
     4. Respective Illustrative Embodiments 
     (1) Second to Eighth Illustrative Embodiments 
     Second to eighth illustrative embodiments are described with reference to  FIG. 4 . In the meantime,  FIG. 4  shows only the main parts of  FIG. 2 . In the second to eighth illustrative embodiments, the same members as those of the first illustrative embodiment are indicated with the same reference numerals and the descriptions thereof are omitted. 
     In the above-described first illustrative embodiment, the casing  31  is provided with the first rib  45  that connects the light source fixing part  41  and the deflector fixing part  43  of the left-rear side and the second rib  46  that connects the light source fixing part  41  and the deflector fixing part  43  of the left-front side, and the first rib  45  and the second rib  46  are connected to the sidewall  39 . 
     In the second illustrative embodiment, as shown in  FIG. 4A , the first rib  45  of the first illustrative embodiment is formed of the first part  45 A and the second part  45 B and the second rib  46  is formed of the first part  46 A and the second part  46 B. That is, in the second illustrative embodiment, the first rib  45  and the second rib  46  are not connected to the sidewall  39 , differently from the first illustrative embodiment. 
     Further, in the third illustrative embodiment, as shown in  FIG. 4B , a third rib  81  that is an example of the reinforcing part continuously connecting the deflector fixing part  43  of the left-rear side and the deflector fixing part  43  of the left-front side is additionally provided to the configuration of the second illustrative embodiment. The third rib  81  is a protrusion having a substantially linear shape that protrudes upward from the lower wall  40  of the casing  31 , when seen in a plan view. 
     Further, in the fourth illustrative embodiment, as shown in  FIG. 4C , only the second rib  46  is provided in comparison to the configuration of the second illustrative embodiment. 
     Further, in the fifth illustrative embodiment, as shown in  FIG. 4D , the second rib  46  is configured to connect the light source fixing part  41  and the deflector fixing part  43  of the right-rear side and the third ribs  81  are configured to connect the deflector fixing parts  43  of the left-rear and left-front sides, the deflector fixing parts  43  of the left-front and right-front sides, the deflector fixing parts  43  of the right-front and right-rear sides and the deflector fixing parts  43  of the right-rear and left-rear sides, respectively. 
     Further, in the sixth illustrative embodiment, as shown in  FIG. 4E , the third ribs  81  of the fifth illustrative embodiment are configured to connect the deflector fixing parts  43  of the left-rear and left-front sides and the deflector fixing parts  43  of the right-rear and left-rear sides, respectively. 
     Further, in the seventh illustrative embodiment, as shown in  FIG. 4F , the first rib  81  of the fifth illustrative embodiment is configured to connect the deflector fixing parts  43  of the right-rear and left-rear sides. 
     Further, in the eighth illustrative embodiment, as shown in  FIG. 4G , only the second rib  46  of the fifth illustrative embodiment is provided. 
     According to the third and fifth to seventh illustrative embodiments, as shown in  FIGS. 4B and 4D  to  4 F, the third rib  81  continuously connects at least two deflector fixing parts  43 . 
     Accordingly, it is possible to suppress the vibrations of the deflector fixing parts  43  connected to each other and to thus suppress the vibration of the deflector  34  itself. 
     As a result, it is possible to suppress the resonance that changes the relative position between the light source  32  and the polygon mirror  61  of the deflector  34 , which is due to the vibrations caused due to the rotation of the polygon mirror  61  of the deflector  34 . 
     According to the fifth to eighth illustrative embodiments, as shown in  FIGS. 4D to 4G , the second rib  46  continuously connects the deflector fixing part  43  of the right-rear side (the deflector fixing part  43  positioned to be closest to the mirror surface  64  of the polygon mirror  61 , from which the laser beam L is reflected) and the light source fixing part  41 . 
     Therefore, it is possible to reinforce the lower wall  40  of the casing  31  between the mirror surface  64  of the polygon mirror  61 , from which the laser beam L is reflected, and the light source  32 . 
     As a result, it is possible to further suppress the resonance that changes the relative position between the light source  32  and the polygon mirror  61  of the deflector  34 , which is due to the vibrations caused due to the rotation of the polygon mirror  61  of the deflector  34 . 
     In addition, in the above respective illustrative embodiments, the same operational effects as those of the first illustrative embodiment can be realized. 
     (2) Ninth Illustrative Embodiment 
     A ninth illustrative embodiment is described with reference to  FIGS. 5 and 6 . Meanwhile, in the ninth illustrative embodiment, the same members as those of the first illustrative embodiment are indicated with the same reference numerals and the descriptions thereof are omitted. 
     In the first illustrative embodiment, the first rib  45  and second rib  46  are provided to protrude upward from the lower wall  40  of the casing  31 . However, in the ninth illustrative embodiment, as shown in  FIGS. 5 and 6 , a first rib  71  and a second rib  72  are provided to protrude downward from the lower wall  40  of the casing  31 . 
     The first rib  71  is a protrusion having a substantially linear shape that protrudes downward from the lower wall  40  of the casing  31  and extends in the left-right direction, when seen in a plan view, and integrally has a first left rib  71 A and a first right rib  71 B. 
     The first left rib  71 A continuously connects the lower wall  40  below the first cylindrical lens fixing part  42  and a part of the deflector fixing part  43  of the left-rear side, which protrudes downward. 
     The first right rib  71 B continuously connects the lower wall  40  below the first cylindrical lens fixing part  42  and a part of the rear light source fixing part  41 , which protrudes downward. 
     The second rib  72  is a protrusion having a substantially linear shape that protrudes downward from the lower wall  40  of the casing  31  and extends in the left-right direction, when seen in a plan view, and integrally has a second left rib  72 A and a second right rib  72 B. 
     The second left rib  72 A continuously connects the lower wall  40  below the first cylindrical lens fixing part  42  and a part of the deflector fixing part  43  of the left-front side, which protrudes downward. 
     The second right rib  72 B continuously connects the lower wall  40  below the first cylindrical lens fixing part  42  and a part of the front light source fixing part  41 , which protrudes downward. 
     According to the ninth illustrative embodiment, it is possible to provide the first rib  71  and second rib  72  on an opposite surface (lower surface) to an upper surface of the lower wall  40  to which the deflector  34  is fixed so that the ribs protrude downward from the lower wall  40  of the casing  31 . 
     Accordingly, it is possible to reinforce the lower wall  40  of the casing  31  while simplifying the configuration of the inside of the casing  31 , to which the deflector  34  is fixed, and further freely designing the layout thereof. 
     Additionally, in the ninth illustrative embodiment, the same operational effects as the first illustrative embodiment can be realized. 
     EXAMPLES 
     For the configurations described in the above illustrative embodiments and configurations of comparative examples which are described below, vibration analysis is performed. 
     1. Comparative Examples 
     (1) Comparative Example 1 
     As shown in  FIG. 7A , a rib  91  that is not continuous to any of the respective light source fixing parts  41  and respective deflector fixing parts  43  is provided at a substantial center of the casing  31 , when seen in a plan view. 
     (2) Comparative Example 2 
     As shown in  FIG. 7B , only the third ribs  81  of the fifth illustrative embodiment were provided. 
     (3) Comparative Example 3 
     As shown in  FIG. 7C , a rib was not provided within a range of an angle smaller than 90 degrees with respect to the laser beam L passing to a center of the scanning range of the laser beam L in the left-right direction (that is, a rib that reinforces the casing  31  was not substantially provided). 
     2. Vibration Analysis 
     (1) Analysis Solver 
     LS-DYNA R4.2.1 
     (2) Method 
     Implicit method, eigenvalue analysis 
     (3) Material Properties of Casing 
     Young&#39;s modulus: 4380 MPa 
     Density: 1.0 g/cm 3    
     (4) With the above-described conditions, natural frequencies were analyzed in resonance modes (resonance mode (1) and resonance mode (2)) in which the relative position between the light source  32  and the deflector  34  changes. The analysis results are shown in table 1. Note that, the higher the natural frequency, the higher the rigidity of the casing. 
     
       
         
               
               
               
             
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Natural frequencies (Hz) in 
                   
               
               
                   
                 respective resonance modes 
               
             
          
           
               
                   
                 resonance 
                 resonance 
               
               
                   
                 mode (1) 
                 mode (2) 
               
               
                   
                   
               
             
          
           
               
                   
                 First illustrative embodiment 
                 207.7 
                 461.8 
               
               
                   
                 Second illustrative embodiment 
                 179.4 
                 439.6 
               
               
                   
                 Third illustrative embodiment 
                 181.1 
                 439.7 
               
               
                   
                 Fourth illustrative embodiment 
                 175.9 
                 435.0 
               
               
                   
                 Fifth illustrative embodiment 
                 172.3 
                 439.0 
               
               
                   
                 Sixth illustrative embodiment 
                 170.0 
                 439.7 
               
               
                   
                 Seventh illustrative embodiment 
                 169.0 
                 439.4 
               
               
                   
                 Eighth illustrative embodiment 
                 167.0 
                 425.7 
               
               
                   
                 Comparative example 1 
                 170.3 
                 430.3 
               
               
                   
                 Comparative example 2 
                 168.7 
                 404.5 
               
               
                   
                 Comparative example 3 
                 166.0 
                 407.6

Technology Classification (CPC): 1