Patent Publication Number: US-7593662-B2

Title: Image-forming device having cooling mechanism for cooling a reading unit without overcooling a fixing unit

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   The present application claims priority from Japanese patent application No. 2006-231042, which was filed on Aug. 28, 2006, the disclosure of which is herein incorporated by reference in its entirety. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an electrophotographic image-forming device including a laser printer or a copying machine and, more particularly to a direct tandem-type image-forming device. 
   2. Description of the Related Art 
   In a direct-type image-forming device, typically, a developer image formed on a photosensitive drum is transferred to a recording sheet which has been conveyed by a conveying belt. After that, the developer image is heated by a fixing unit so as to be fixed on the recording sheet, thereby forming an image on the recording sheet. 
   Hence, a large amount of heat is generated in the fixing unit of the electrophotographic image-forming device. In order to cool the heat of the fixing unit, Japanese Patent Application Publication No. H10-268735 discloses that an image-forming device is formed with an air inlet port and is provided with an exhaust fan. The air inlet port is formed on the casing, is positioned below the fixing unit in the casing and takes cooling air into the casing. The exhaust fan is provided above the fixing unit and transfers the cooling air, which has cooled inner units including the fixing unit, out of the casing. 
   SUMMARY 
   In an image-forming device, a correction patch image having a predetermined pattern is periodically formed on a conveying belt. The patch image is read by a sensor such as an optical sensor, so that the operational control over various units is corrected based on the result read by the sensor. This prevents the quality of the image formed on a recording sheet from deteriorating. 
   The sensor is disposed near a driving roller so that the patch image is read on the driving roller, where the behavior of the conveying belt is stable. The driving roller is disposed near the fixing unit, which means that the sensor disposed near the driving roller is under the strong influence of the heat generated from the fixing unit. This causes the sensor to malfunction. In order to reduce the influence of the heat, some sort of cooler for cooling the sensor is required. 
   In response to this situation, Japanese Patent Application Publication No. 10-268735 proposes that the inlet port for taking in cooling air is formed below the fixing unit so as to cool the sensor. However, in the case where the fixing unit and the sensor are arranged to be close to each other, it is impossible to cool only the sensor; the fixing unit is unavoidably cooled together. 
   Further, a larger amount of cooling air is taken in so as to cool the sensor adequately, and the fixing unit is then cooled excessively. This causes insufficient heat to be provided for fixing the developer image on a recording sheet. 
   In view of the foregoing, it is an object of the invention to provide an image-forming device capable of cooling a reading unit such as an optical sensor adequately without cooling a fixing unit excessively. 
   To achieve the above and other objects, one aspect of the invention provides an image-forming device including a casing, a driving roller, a conveying belt, a photosensitive unit, a fixing unit, a reading unit, a fan, and a first partition wall. The casing has a wall formed with an air inlet port and defines an inner space. The air inlet port has a top end portion and a bottom end portion. The driving roller is provided in the casing and defines an axial direction. The conveying belt is circularly movably supported on the driving roller and is configured to convey a recording sheet in a sheet conveying direction perpendicular to the axial direction. The photosensitive unit is provided above the conveying belt and has at least one photosensitive drum configured to form a developer image to be transferred onto the recording sheet on the conveying belt. The photosensitive unit is configured to form a predetermined pattern on the conveying belt. The fixing unit is disposed to receive the recording sheet conveyed by the conveying belt and is configured to fix the developer image on the recording sheet by heat. The fixing unit has a top edge portion and a bottom edge portion. The reading unit is provided adjacent to the driving roller and is capable of reading the predetermined pattern formed on the conveying belt. The fan is provided on an upper section of the inner space separated from a lower section of the inner space by an upper part of the conveying belt moving toward the fixing unit. The fan transfers air from inside the casing to outside thereof. The first partition wall is located in the upper section of the inner space. The fan is provided on one side of the first partition wall and the fixing unit on another side thereof. The first partition wall has a bottom end. The bottom end of the first partition wall is positioned below the top edge portion of the fixing unit and above the upper part of the conveying belt. The air inlet port is vertically in a lower position than the fan. 
   In another aspect of the invention, there is provided an image-forming device including a casing, a driving roller, a conveying belt, a photosensitive unit, a fixing unit, a reading unit, a fan, and an air flow channel. The casing has a wall formed with an air inlet port and defines an inner space. The conveying belt is circularly movably supported on the driving roller and is configured to convey a recording sheet in a sheet conveying direction perpendicular to the axial direction. The photosensitive unit is provided above the conveying belt and has at least one photosensitive drum configured to form a developer image to be transferred onto the recording sheet on the conveying belt. The photosensitive unit is configured to form a predetermined pattern on the conveying belt. The fixing unit is disposed to receive the recording sheet conveyed by the conveying belt and is configured to fix the developer image on the recording sheet by heat. The reading unit is provided adjacent to the driving roller and is configured to read the predetermined pattern formed on the conveying belt. The air flow channel is formed between the air inlet port and the fan. A major part of air introduced into the inner space through the air inlet port is transferred outside the casing by the fan. The reading unit is disposed in the air flow channel. The fixing unit is disposed in a position offset from the air flow channel. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side cross-sectional view showing a main part of a laser printer according to embodiments of the present invention; 
       FIG. 2  is a schematic diagram showing a rear side of the laser printer according to a first embodiment of the present invention; 
       FIG. 3  is a schematic diagram showing a top side of the laser printer according to the first embodiment of the present invention; 
       FIGS. 4A-4C  are perspective views showing an exhaust duct of the laser printer according to the first embodiment of the present invention; 
       FIG. 5  is a cross-sectional view showing an inlet port of the laser printer according to the embodiments of the present invention; 
       FIG. 6  is a schematic diagram showing a rear side of a laser printer according a second embodiment of the present invention; and 
       FIG. 7  is a schematic diagram showing a rear side of a laser printer according to a third embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   An electrophotographic image-forming device according to a first embodiment of the present invention will be described while referring to the accompanying drawings. 
   In this embodiment, the electrophotographic image-forming device of the present invention is applied to a laser printer  1  shown in  FIG. 1 . Note that in the following description, the expressions “front,” “rear,” “left,” “right,” “above,” and “below” are used to define the various parts when the laser printer  1  is disposed in an orientation in which it is intended to be used. 
   First Embodiment 
   1. External Configuration of Laser Printer 
     FIG. 1  is a side cross-sectional view of the laser printer  1 . As shown in  FIG. 1 , the laser printer  1  includes a substantially box-shaped (cubic) casing  3  defining an inner space  3   a . A discharge tray  5  is formed on the top surface of the casing  3  for receiving a recording sheet, such as paper sheet or OHP sheet, discharged out of the casing  3  after images have been printed thereon. 
   In this embodiment, a frame member (not shown) formed of metal, resin, or the like, is provided inside the casing  3 . A process cartridge  70  and a fixing unit  80  described later are detachably provided to the frame member. 
   2. Internal Configuration of Laser Printer 
   The laser printer  1  further includes, within the inner space  3   a  of the casing  3 , an image forming unit  10  that forms an image on a recording sheet, a feeder unit  20  that feeds the recording sheet, a belt unit  30  that conveys the recording sheet fed from the feeder unit  20  to four process cartridges  70 K,  70 Y,  70 M, and  70 C. 
   2.1. Feeder Unit 
   The feeder unit  20  includes a sheet feed tray  21  disposed in the bottommost section of the casing  3 , a sheet feed roller  22  disposed above the front end of the sheet feed tray  21  for feeding recoding sheets stacked on the sheet feed tray  21  to the image forming unit  10 , and a separation pad  23  for separating recording sheets fed by the feed roller  22  one at a time by applying a predetermined feeding resistance to the recording sheets. A recording sheet fed from the sheet feed tray  21  is changed in a sheet conveying direction by approximately 90 degrees to the upward direction on the front side of the sheet feed tray  21  and then by approximately 90 degrees to the horizontally rearward direction, thereby supplied to the image forming unit  10 . 
   2.2. Belt Unit 
   The belt unit  30  includes a driving roller  31  that rotates in conjunction with the operation of the image forming unit  10 , a driven roller  32  rotatably disposed distant from the driving roller  31 , and a conveying belt  33  wound about the driving roller  31  and the driven roller  32 . 
   The driving roller  31  is rotatably supported, in a state that the position of its shaft is fixed, on a frame (not shown) of a belt unit  30 . The driven roller  32  is rotatably supported on the frame in a state that the position of its shaft is movable in the front-to-rear direction. 
   The driven roller  32  is directly or indirectly pressed by an elastic means (not shown) such as a spring, in a direction far from the driving roller  31 , so that a predetermined tension is given to the conveying belt  33 . 
   Resist sensors  34  are provided near the driving roller  31  so as to read a correction patch image formed on the conveying belt  33 . Each of the resist sensors  34  is an optical sensor which irradiates light such as infrared light on the conveying belt  33  and reads its reflected light. 
   As shown in  FIG. 2 , the resist sensors  34  are arranged along the left-to-right direction (an axial direction of the driving roller  31 ). In this embodiment, each resist sensor  34  is provided at each end of the driving roller  31  in the left-to-right direction. The resist sensors  34  irradiate lights on a portion, which the conveying belt  33  comes in contact with the driving roller  31 , of the conveying belt  33  so as to read the correction patch image. 
   The correction patch image is a test pattern for detecting a color shift amount among a plurality of colors. A control unit (not shown) of the laser printer  1  corrects the light-exposure timing by the scanner unit  60  described later based on the color shift amount read by the resist sensors  34 . 
   2.3. Image Forming Unit 
   The image forming unit  10  includes the scanner unit  60 , the process cartridge  70 , and the fixing unit  80 . The image forming unit  10  is disposed in an upper section  3   b  of the inner space  3   a . The upper section  3   b  of the inner space  3   a  is separated from a lower section (not labeled) of the inner space  3   a  by an upper part of the conveying belt  33  moving toward the fixing unit  80 . 
   The image forming unit  10  employs the direct tandem system and is capable of printing color images. The developer cartridges  70 K,  70 Y,  70 M, and  70 C correspond to developer (toner) of respective colors black, yellow, magenta, and cyan, are arranged along the sheet conveying direction of recording sheets and transfer each developer images on a recording sheet. 
   The four process cartridges  70 K,  70 Y,  70 M, and  70 C have the same configuration except colors of developer. Therefore, the process cartridges  70 K,  70 Y,  70 M, and  70 C are collectively referred to as the process cartridge  70 . 
   2.3.1. Scanner Unit 
   The scanner unit  60  is disposed in the upper section  3   b  of the casing  3  for forming an electrostatic latent image on the surface of each of photosensitive drums  71 . Although not shown in the drawings, the scanner unit  60  includes laser emitting sections, polygon mirrors, fθ lenses, reflecting mirrors, and the like. 
   Each laser emitting section emits a laser beam based on desired image data. The laser beam is reflected by the polygon mirror, passes through the fθ lens, is reflected by the reflecting mirror, and is reflected downward by the reflecting mirror so as to irradiate the surface of the photosensitive drum  71 , thereby forming an electrostatic latent image thereon. 
   2.3.2. Process Cartridge 
   Hereinafter, the structure of the process cartridge  70  will be described by referring to the process cartridge  70 C as an example. The process cartridge  70 C is detachably provided below the scanner unit  60  in the casing  3 . The process cartridge  70  includes the photosensitive drum  71 , a charging unit  72 , and a developer accommodating section  74 . 
   The photosensitive drum  71  is for bearing developer images to be transferred onto a recording sheet. The photosensitive drum  71  has a cylindrical shape and its outermost layer is a positively charged photosensitive layer formed of polycarbonate or the like. 
   The charging unit  72  is disposed diagonally above and rear of the photosensitive drum  71  and is spaced away from the photosensitive drum  71  by a predetermined space so as to avoid direct contact with the photosensitive drum  71 . 
   The charging unit  72  is a Scorotron type charge unit for generating a corona discharge from a tungsten charge wire, for example, to uniformly charge the surface of the photosensitive drum  71  with a positive polarity. 
   The developer accommodating section  74  is formed with a developer accommodating chamber  74 A for accommodating developer. Disposed inside the developer accommodating chamber  74 A are a developer supply roller  74 B, a developing roller  74 C, and a thickness-regulating blade  74 D. 
   The developer accommodated in the developer accommodating chamber  74 A is supplied to the developing roller  74 C by the rotation of the developer supply roller  74 B and is carried on the surface of the developing roller  74 C. The thickness-regulating blade  74 D regulates the thickness of the layer of the developer on the surface of the developing roller  74 C, forming a thin layer of developer having a uniform thickness on the developing roller  27 . Then, the developer is supplied to the surface of the photosensitive drum  71  which has been exposed by a laser beam from the scanner unit  60  as described above. 
   Transfer rollers  73  are rotatably disposed in opposition to the corresponding photosensitive drums  71  via the conveying belt  33 . Each of the transfer rollers  73  rotates in association with the rotation of the conveying belt  33 . 
   Each of the transfer rollers  73  is applied with a transfer bias with respect to the photosensitive drum  71  so as to transfer developer carried on the surface of the photosensitive drum  71  onto a printing surface of the recording sheet as the recording sheet passes by the photosensitive drum  71 . 
   2.3.3. Fixing Unit 
   As shown in  FIG. 1 , the fixing unit  80  is disposed on the downstream side of and near the driving roller  31  (the belt unit  30 ) with respect to the sheet conveying direction. The fixing unit  80  is for thermally fixing the developer transferred onto the recording sheet, and is detachably attached to the frame member (not shown). 
   Specifically, the fixing unit  80  includes a heat roller  81  and a pressure roller  82 . The heat roller  81  is disposed on the printing surface side of the recording sheet and applies conveying force to the recording sheet while heating developer clinging on the recording sheet. The pressure roller  82  is disposed in confrontation with the heat roller  81  and presses a recording sheet interposed between the pressure roller  82  and the heat roller  81  against the heat roller  81 . 
   The heat roller  81  rotates in synchronization with a developing roller  74 C, the conveying belt  33 , and the like. The pressure roller  82  is driven by the heat roller  81  through the rotation of the heat roller  81  via a sheet in contact therewith. 
   2.4. Exhaust Duct 
   As shown in  FIG. 1 , an exhaust duct  100  is provided on the upper section  3   b  of the casing  3  (near a discharge slit  7  or the discharge tray  5  in this embodiment). The exhaust duct  100  guides the air inside the casing  3  to an exhaust fan  120  so that the exhaust fan  120  exhausts the air from the casing  3 . In this embodiment, an axial fan (see JIS B 0132 No. 1012) is employed as the exhaust fan  120 , where air passes through the fan in its axis direction. 
   The exhaust duct  100  includes a first partition wall  101  and a second partition wall  102 . The first partition wall  101  divides the space inside the casing  3  into an exhaust fan side space  101 A where the exhaust fan  120  is provided and a fixing unit side space  101 B where the fixing unit  80  is provided. The second partition wall  102  is interposed between the first partition wall  101  and the process cartridge  70  and covers the exhaust fan  120  from above side. The first partition wall  101  and the second partition wall  102  define a ductal exhaust path extending from the lower side to the upper side (exhaust fan  120  side) in the exhaust fan side space  101 A. As shown in  FIG. 2 , the first partition wall  101  has one end portion  101 C and another end portion  101 D in the left-to-right direction. 
   As shown in  FIGS. 1 and 3 , the first partition wall  101  and the second partition wall  102  extend in a direction parallel to the axial direction of the driving roller  31  (the left-to-right direction of the laser printer  1 ). The first partition wall  101  and the second partition wall  102  divide the space on the upper side of the belt unit  30  within the casing  3  into a space on the fixing unit  80  side and a space on the belt unit  30  side. 
   As shown in  FIG. 1 , a lower portion (resist sensor  34  side) of the exhaust duct  100  has an opening and is in communication with the space inside the casing  3 . Bottom ends  100 A of the first partition wall  101  and the second partition wall  102  (exhaust duct  100 ) are positioned above a sheet feed path P extending from the conveying belt  33  to the fixing unit  80  and below a top edge portion  80 B of the fixing unit  80 . 
   As shown in  FIGS. 3 and 4A , the exhaust fan  120  is provided at the right end in an upper space within the exhaust duct  100  (the exhaust fan side space  101 A) in the left-to-right direction (at one end side in the axial direction of the driving roller  31 ). Hence, the exhaust fan  120  is positioned on the one end portion  101 C of the partition wall  101 . 
   As shown in  FIGS. 4A and 4C , in a lower portion of the first partition wall  101 , at the left side in the left-to-right direction (at the other end in the axial direction of the driving roller  31 ), a first connecting hole  103  is formed so as to be in communication with the fixing unit side space  101 B and the exhaust fan side space  101 A. In other words, the first connecting hole is formed on the lower portion of the another end portion  101 D of the first partition wall  101 . As shown in  FIG. 4B , second connecting holes  105  are formed in an upper portion of the second partition wall  102  so as to be in communication with the exhaust fan side space  101 A and the space where the belt unit  30  and the scanner unit  60  are disposed. 
   In the present embodiment, the first connecting hole  103  is one opening. On the other hand, the second connecting holes  105  are a plurality of openings arranged in a line in the left-to-right direction (a direction parallel to the axial direction of the driving roller  31 ). 
   As shown in  FIG. 1 , an inlet port  104  is formed on a rear side wall  3 C of the casing  3  so as to be in communication with the fixing unit side space  101 B and the outside of the casing  3 . A bottom end portion  104 A of the inlet port  104  is positioned below a bottom edge portion  80 A of the fixing unit  80 . A top end portion  104 B of the inlet port  104  is also positioned below the bottom edge portion  80 A of fixing unit  80 . The inlet port  104  is vertically in a lower position than the exhaust fan  102 . 
   As shown in  FIG. 2 , the size Y of the inlet port  104  in the left-to-right direction is longer than the distance X between two of the resist sensors  34  in the left-to-right direction. Further, the width of the first partition wall  101  in the left-to-right direction is longer than the size Y. 
   As shown in  FIGS. 2 and 3 , two resist sensors  34  are disposed between a left-end reference line L 1  which passes the left end of the inlet port  104 , and a right-end reference line L 2  which passes the right end of the inlet port  104  in the left-to-right direction. 
   As shown in  FIG. 5 , the inlet port  104  includes a plurality of strip louvers  104 C, like a louver window. Each of the louvers  104 C is inclined relative to the horizontal direction (the front-to-rear direction) so that its rear end is lower than the front end. 
   3. Feature of the Laser Printer  1  of this Embodiment 
   In this embodiment, the bottom ends  101 A of the first partition wall  101  (exhaust duct  100 ) are positioned above the sheet feed path P which extends from the conveying belt  33  to the fixing unit  80  and below the top edge portion  80 B of the fixing unit  80 . Furthermore, the bottom end portion  104 A of the inlet port  104  is positioned below the bottom edge portion  80 A of the fixing unit  80 . This structure allows most of the cooling air taken in the fixing unit side space  101 B through the inlet port  104 , to flow toward the bottom end  100 A of the first partition wall  101 , and then to be exhausted from the casing  3  by the exhaust fan  120  after reaching the exhaust fan side space  101 A. 
   Therefore, most of the cooling air, which flows through the inlet port  104  toward the bottom end  100 A of the first partition wall  101 , passes below the fixing unit  80  to cool the resist sensors  34 , without passing through the fixing unit  80 , and then reaches the exhaust fan side space  101 A. That is, an air flow channel F is formed between the inlet port  104  and the exhaust fan  102 , and a major part of the cooling air introduced into the inner space  3   a  through the inlet port  104  is transferred outside the casing  3  by the exhaust fan  102 . The resist sensors  34  are disposed in the air flow channel F, and the fixing unit  80  is disposed in a position offset from the air flow channel F. This prevents the fixing unit  80  from being cooled excessively while the resist sensor  34  is cooled adequately. 
   Since the top end portion  104 B of the inlet port  104  is positioned below the bottom end  80 A of the fixing unit  80 , the cooling air taken through the inlet port  104  is properly guided below the fixing unit  80 . 
   The air heated by the fixing unit  80  is accumulated above the fixing unit  80 . Since the cooling air taken through the inlet port  104  is guided below the fixing unit  80 , the heated air, which has been accumulated above the fixing unit  80 , is prevented from mixing with the cooling air taken through the inlet port  104 . 
   Accordingly, the cooling air can be guided to the resist sensors  34  while the temperature increase of the cooling air taken through the inlet port  104  is prevented, thereby cooling the resist sensors  34  adequately. 
   Furthermore, the distance X between the two resist sensors  34  ( 34 A and  34 B) is shorter than the size Y of the inlet port  104  in the left-to-right direction. Hence, the resist sensors  34  are positioned within the flow path of the cooling air taken through the inlet port  104 , thereby cooling the resist sensor  34  adequately. 
   In the case where the fixing unit  80  is cooled excessively, it becomes impossible to adequately heat the developer image transferred to the recording sheet. On the other hand, in the case where the temperature of the fixing unit  80  rises too high, the fixing unit  80  is subject to heat damage. 
   In response, the first connecting hole  103  is formed in the first partition wall  101  so as to be in communication with the fixing unit side space  101 B and the exhaust fan side space  101 A. Hence, a part of the heated air which has been accumulated in the fixing unit side space  101 B can be exhausted from the casing  3 . 
   Hence, the fixing unit  80  is cooled adequately, thereby preventing the temperature of the fixing unit  80  from rising too high. 
   Furthermore, the second connecting holes  105  are formed in the second partition wall  102 . The air which has been accumulated above the process cartridge  70  can be efficiently exhausted from the casing  3  through the second connecting holes  105 . Also, the bottom end  100 A of the second partition wall  102  is positioned above the sheet feed path P and below the top edge portion  80 B of the fixing unit  80 . The air which has been accumulated near the belt unit  30  can be exhausted from the casing  3  efficiently. 
   In this embodiment, since the inlet port  104  is provided in a lower portion of the casing  3 , external light is likely to enter through the inlet port  104  from above relative to the horizontal direction. In response to this situation, the inlet port  104  is formed of a louver window. Further, each of the louvers  104 C is inclined relative to the horizontal direction (the front-to-rear direction) so that its rear end is positioned lower than the front end. Accordingly, the louvers  104 C can cut off incoming light properly and the resist sensor  34  can read the correction patch image consistently. 
   Second Embodiment 
   Next, a second embodiment of the present invention will be described while referring to  FIG. 6 .  FIG. 6  is a schematic diagram showing the rear side of a laser printer  201 . In the first embodiment, two resist sensors  34  are provided next to both ends of the driving roller  31  in its axial direction (the left-to-light direction), respectively. In this embodiment, however, two resist sensors  234  are provided on the right end side of the driving roller  31  in the left-to-right direction. 
   In this embodiment, the size Y of the inlet port  204  in the left-to-right direction is longer than the distance X between the two resist sensors  234  ( 234 A and  234 B) in the left-to-right direction. 
   As shown in  FIG. 6 , in this embodiment, the two resist sensors  234  are positioned on one side in the left-to-right direction of the driving roller  31 . The size Y of the inlet port  204 , therefore, is shorter than the size of the belt unit  30  in the left-to-right direction. However, the present invention is not limited to this case. The size Y of the inlet port  204  may be longer than the size of the belt unit  30  in the left-to-right direction. 
   Third Embodiment 
   Next, a third embodiment of the present invention will be described while referring to  FIG. 7 .  FIG. 7  is a schematic diagram showing the rear side of a laser printer  301 . Although the above embodiments employ two resist sensors  34  and  234 , respectively, three resist sensors  334  are employed in this embodiment. 
   As shown in  FIG. 7 , the size Y of the inlet port  304  in the left-to-right direction is longer than the distance X between a rightmost resist sensor  334 A and a leftmost resist sensor  334 B of the three resist sensors  334 . 
   While the invention has been described in detail with reference to specific embodiments thereof, it would be apparent to those skilled in the art that many modifications and variations may be made therein without departing from the spirit of the invention, the scope of which is defined by the attached claims. 
   While the exhaust duct  100  includes the second partition wall  102  as well as the first partition wall  101  in the preferred embodiments described above, the present invention is not limited to this construction. The second partition wall  102  may be omitted. 
   While the first connecting hole  103  and the second connecting holes  105  are formed in the first partition wall  101  and the second partition wall  102 , respectively, in the preferred embodiments described above, the present invention is not limited to this construction. The first connecting hole  103  and the second connecting holes  105  may be omitted. 
   While the exhaust fan  120  is provided only at one end in the upper space within the exhaust duct  100  in the left-to-right direction (the axial direction of the driving roller  31 ), the present invention is not limited to this construction. Two exhaust fans  120  may be provided at both ends in the upper space in the left-to-right direction. 
   While an axial fan is employed as the exhaust fan  120  in the preferred embodiments described above, the present invention is not limited to this construction. For example, a centrifugal multiblade fan (see JIS B 0132 No. 1004) such as a turbo fan or a silocco fan; or a cross flow fan (see JIS B 0132 No. 1017) may be employed. 
   While the resist sensor  34  having an optical sensor is employed as a reading unit in the preferred embodiments described above, the present invention is not limited to this construction. For example, a density sensor, which detects the density of the correction patch image formed on the conveying belt  33 , may be employed as a reading sensor. 
   While the louvers  104 C are provided in the inlet port  104  in the preferred embodiments described above, the present invention is not limited to this construction. For example, a net-like member having a multitude of holes may be provided in the inlet port  104 . 
   The inclination and longitudinal directions of the louvers  104 C are not limited to the directions shown in  FIG. 5 . In the case where a protrusion is provided on the top side or bottom side of the outer surface of the fixing unit casing which contains the heat roller  81  and the pressure roller  82  of the fixing unit  80 , the top edge portion  80 B of the fixing unit  80  indicates a portion near the root of the protrusion provided on the top side of the outer surface. Similarly, the bottom edge portion  80 A of the fixing unit  80  indicates a portion near the root of the protrusion provided on the bottom side of the outer surface.