Patent Publication Number: US-10775719-B2

Title: Duct mechanism and image forming apparatus including the same

Description:
BACKGROUND 
     1. Field 
     The present disclosure relates to a duct mechanism used in an image forming apparatus and an image forming apparatus including the same. Specifically, the present disclosure relates to a duct mechanism that restrains heat of a fixing device included in an image forming apparatus from being transferred to the inside of the apparatus, and an image forming apparatus including the same. 
     2. Description of the Related Art 
     Japanese Unexamined Patent Application Publication No. 2012-141645 discloses an example of the image forming apparatus in the related art. An image forming apparatus in the related art includes a fixing device, a driving roller disposed in the vicinity of the fixing device, and an exhaust duct provided above the fixing device. The exhaust duct constitutes an axially extending tubular air flow passage such as a heating roller and a driving roller of the fixing device. Further, the exhaust duct communicates with a first space provided between the fixing device and the driving roller, and an exhaust fan is provided in the exhaust duct for discharging the air in the first space to the outside of the image forming apparatus. 
     In the image forming apparatus in the related art, since a direction in which air is suctioned into the exhaust duct and a direction in which air flows in the exhaust duct are different from each other, the flow of air in the exhaust duct is not uniform and unevenness occurs. Therefore, when heat of the fixing portion is thermally insulated by the exhaust duct, there is a problem in that the heat is not uniformly insulated, and thereby unevenness occurs in the heat insulating effect of the exhaust duct. 
     Therefore, it is desirable to provide a new duct mechanism. 
     It is also desirable to provide a duct mechanism which can effectively insulate the heat of the fixing unit without unevenness, in the duct mechanism used in the image forming apparatus. 
     SUMMARY 
     According to a first aspect of the disclosure, there is provided a duct mechanism used in an image forming apparatus including an apparatus body; a fixing unit that is provided in the apparatus body, and heats and fixes a toner image transferred to a recording medium. The duct mechanism includes a first duct disposed at a position adjacent to the fixing unit and an exhaust fan for discharging air of the first duct to the outside of the apparatus body. The inside of the first duct is divided into a plurality of air flow paths. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view illustrating a schematic configuration of an image forming apparatus which is a first embodiment of the disclosure in a case of being viewed from a front surface; 
         FIG. 2  is a schematic sectional view illustrating a structure of an exhaust portion provided in the image forming apparatus illustrated in  FIG. 1 ; 
         FIG. 3  is a schematic sectional view illustrating the structure of the exhaust unit in a state where a fixing unit is separated; 
         FIG. 4  is a schematic sectional view illustrating a flow of air in a fine particle collecting duct and a first duct; 
         FIG. 5  is a schematic sectional view illustrating the flow of air in the first duct; 
         FIG. 6  is a schematic sectional view illustrating structures of an air supply unit and an exhaust unit of a second embodiment; 
         FIG. 7  is a schematic view illustrating the flow of air in a case where the first duct and the second duct of the second embodiment are not connected to each other; 
         FIG. 8  is a schematic view illustrating the flow of air in a case where the first duct and the second duct of the second embodiment are connected to each other; 
         FIG. 9  is a schematic perspective view illustrating a structure of the second duct of a third embodiment; 
         FIG. 10  is a schematic sectional view illustrating structures of the first duct and the second duct before the process unit is inserted, in the third embodiment; 
         FIG. 11A  is a schematic view illustrating the structures of the first duct and the second duct before the process unit is inserted, in the third embodiment.  FIG. 11B  is a schematic view illustrating the structures of the first duct and the second duct after the process unit is inserted, in the third embodiment; 
         FIG. 12  is a schematic sectional view illustrating the structures of the first duct and the second duct after the process unit is inserted, in the third embodiment; 
         FIG. 13  is a schematic perspective view illustrating a structure of process unit of a fourth embodiment; and 
         FIG. 14  is a schematic perspective view illustrating a structure of the second duct of the fourth embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     First Embodiment 
       FIG. 1  is a schematic view illustrating a schematic configuration of an image forming apparatus  100  which is a first embodiment of the disclosure. The image forming apparatus  100  illustrated in  FIG. 1  is a multifunction printer having a copying function, a printer function, a scanner function, and a facsimile function, and forms a monochrome image on a recording medium by electrophotography. The recording medium can be a sheet or an overhead projector sheet, but the following description explains the use of the sheet. 
     In this specification, out of the horizontal direction in a case where the image forming apparatus  100  is viewed from the front surface, the left side is defined as a left direction and the right side is defined as a right direction. The front surface side of the image forming apparatus  100  is defined as a forward direction (front surface direction) in the backward direction in a case where the image forming apparatus  100  is viewed from above (below), and the rear surface side of the image forming apparatus  100  is defined as a backward direction (rear surface direction). 
     First, a configuration of the image forming apparatus  100  will be schematically described. As illustrated in  FIG. 1 , the image forming apparatus  100  includes an apparatus body  12  provided with an image forming unit  30 , and an image reading device  14  disposed above the apparatus body. 
     The image reading device  14  includes a document placing table  16  formed of a transparent material. Above the document placing table  16 , a document pressing cover  18  is openably and closably attached via a hinge or the like. A document feeding tray  20  is provided on an upper surface of the document pressing cover  18 , and an automatic document feeder (ADF) is provided therein. The ADF automatically feeds the document placed on the document feeding tray  20  one by one to the image reading position  22  and ejects the document to the document discharging tray  24 . 
     The image reading unit  26  incorporated in the image reading device  14  includes a light source, a plurality of mirrors, an imaging lens, and a line sensor. The image reading unit  26  exposes the surface of the document by a light source and guides the reflected light reflected from the surface of the document to the imaging lens by the plurality of mirrors. Then, the reflected light is imaged on the light-receiving element of the line sensor by the imaging lens. In the line sensor, brightness and chromaticity of the reflected light formed on the light-receiving element are detected, and image data based on the image of the surface of the document is generated. As a line sensor, a charge coupled device (CCD), a contact image sensor (CIS), or the like can be used. 
     On the front surface side of the image reading device  14 , there is provided an operation panel (not shown) for receiving an input operation such as a print instruction by a user. The operation panel has a display with a touch panel and a plurality of operation buttons. 
     In addition, a control unit (not shown) including a CPU, a memory, and the like is provided in the apparatus body  12 . The control unit transmits control signals to various parts of the image forming apparatus  100  and performs various operations on the image forming apparatus  100  in accordance with an input operation to the operation panel and the like. 
     The image forming unit  30  includes an exposure unit (optical scanning unit)  32 , a developing unit  34 , a photosensitive drum  36 , a cleaner unit (cleaning unit)  38 , a charging unit  40 , a transfer unit  42 , a fixing unit  44 , and a toner supply device  46 , and forms an image on a sheet transported from the sheet feeding cassette  48  or the like, and the ejects the sheet on which the image formed to an sheet ejecting tray  50 . As image data for forming an image on the sheet, image data read by the image reading unit  26  or image data transmitted from an external computer or the like is used. 
     The photosensitive drum  36  is an image holding member having a photosensitive layer formed on the surface of a conductive cylindrical base and is configured to be rotated about axis by a rotary driving source (not shown) such as a motor. The charging unit  40  charges the surface of the photosensitive drum  36  to a predetermined potential. The exposure unit  32  is configured as a laser scanning unit (LSU) including a laser emitting unit and a reflecting mirror, and exposes the surface of the charged photosensitive drum  36  to form an electrostatic latent image corresponding to image data on the surface of the photosensitive drum  36 . The developing unit  34  includes a developer tank (developing housing) for containing toner, supplies toner to the surface of the photosensitive drum  36 , visualizes the electrostatic latent image formed on the surface of the photosensitive drum  36  with toner (a toner image is formed). A toner concentration detection sensor for detecting the toner concentration is provided in the developer tank. When the toner concentration detected by this toner concentration detection sensor becomes lower than a predetermined value, toner is supplied from the toner supply device  46  to the developer tank. The cleaner unit (cleaning unit)  38  includes a cleaning blade  382  (refer to  FIG. 12 ) that abuts against the surface of the photosensitive drum  36  and a transport screw, removes toner remaining on the surface of the photosensitive drum  36  after development and image transfer, and then transports the removed toner to a waste toner box (not shown). However, in the image forming apparatus  100  of the first embodiment, the photosensitive drum  36 , the charging unit  40 , and the cleaner unit  38  are unitized, and are detachably provided as a process unit  64  including these units in the apparatus body  12 . 
     The transfer unit  42  is a unit for transferring a toner image formed on the surface of the photosensitive drum  36  onto a sheet, and includes a transfer roller  42   a  provided so as to press the photosensitive drum  36 , and the like. When an image is formed, a predetermined voltage is applied to the transfer roller  42   a , and thereby a transfer electric field is formed between the photosensitive drum  36  and the transfer roller  42   a . With this action of the transfer electric field, while the sheet passes through a transfer nip portion between the photosensitive drum  36  and the transfer roller  42   a , the toner image formed on the outer peripheral surface of the photosensitive drum  36  is transferred onto the sheet. 
     The fixing unit  44  includes a heat roller (fixing roller)  44   a  and a pressure roller  44   b , and is disposed above the transfer unit  42  (the downstream side in the sheet transport direction). Further, the heat roller  44   a  is disposed on the sheet ejecting tray  50  side (left side) with respect to the pressure roller  44   b . Further, the heat roller  44   a  is supported by a first support member  442 , and the pressure roller  44   b  is supported by a second support member  444 . Further, the first support member  442  is configured to surround three sides of the upper surface (top surface), the left side surface (one side surface), and the lower surface (bottom surface) of the heat roller  44   a . The second support member  444  is configured to surround three sides of an upper surface (top surface), a right side surface, and a lower surface (bottom surface) of the pressure roller  44   b.    
     The heat roller  44   a  is set to be at a predetermined fixing temperature (for example, 160° C.), and when the sheet passes through the fixing nip portion between the heat roller  44   a  and the pressure roller  44   b , the toner image transferred to the sheet is melted, mixed, and pressed, and thus is thermally fixed (heated and fixed) on the sheet. 
     In such an apparatus body  12 , the first sheet transport path L 1 , the second sheet transport path L 2 , and a third sheet transport path L 3 , to which a sheet is transported, are formed. The first sheet transport path L 1  is provided to send the sheet transported from the sheet feeding cassette  48  and the like to the register roller  56 , the transfer unit  42 , and the fixing unit  44 . The second sheet transport path L 2  is provided to send the sheet after thermal fixing by the fixing unit  44  to the sheet ejecting tray  50 , following the first sheet transport path L 1 . The third sheet transport path L 3  is a path for transporting the sheet after single-sided printing and passing through the fixing unit  44 , from the second sheet transport path L 2  to the first sheet transport path L 1  on the upstream side in the sheet transport direction of the transfer roller  42   a  (the transfer nip portion). Here, the image forming apparatus  100  of the first embodiment is a so-called vertical transport type image forming apparatus. Therefore, in the first sheet transport path L 1  and the second sheet transport path L 2 , the sheet is transported from the lower side to the upper side. On the other hand, in the third sheet transport path L 3 , the sheet is transported from the upper side to the lower side. Hereinafter, the term “sheet transport direction” simply means the sheet transport direction (direction from the lower side to the upper side) in the first sheet transport path L 1  and the second sheet transport path L 2 . 
     The sheet feeding cassette  48  is provided with the sheet feeding tray for storing sheets, a pick-up roller  52  for picking up sheets stored in the sheet feeding tray one by one and supplying them to the first sheet transport path L 1 , and a separation roller  54 . The second sheet transport path L 2  is provided with a transport roller  58  for imparting a propelling force to the sheet, and an ejecting roller  60  for ejecting the sheet to the sheet ejecting tray  50 . Further, on the third sheet transport path L 3 , a transport roller  62  for applying the propelling force to the sheet is appropriately provided. 
     When single-sided printing is performed in the apparatus body  12 , the sheet is guided one by one from the sheet feeding cassette  48  to the first sheet transport path L 1  and transported to the register roller  56 . Then, the register roller  56  transports the sheet to the transfer nip portion at a timing when the leading edge of the sheet and the leading edge of image information (toner image) on the photosensitive drum  36  are aligned, and the toner image is transferred onto the sheet. Thereafter, by passing through the fixing unit  44  (fixing nip portion), an unfixed toner on the sheet is thermally fixed. Thermal fixed sheet is transported to the second sheet transport path L 2  by the transport roller  58  and the ejecting roller  60 , and is ejected to the sheet ejecting tray  50 . 
     On the other hand, at the time of performing dual-sided printing, when the printing on the front side is finished and a trailing end portion of the sheet having passed through the fixing unit  44  reaches the ejecting roller  60 , the ejecting roller  60  and the transport roller  58  are reversely rotated, and thereby the sheet reversely travels and is guided from the second sheet transport path L 2  to the third sheet transport path L 3 . The sheet guided to the third sheet transport path L 3  is transported to the third sheet transport path L 3  by the transport roller  62  and guided to the first sheet transport path L 1  of the register roller  56 . Since the front and back of the sheet are reversed at this point, thereafter, the sheet passes through the transfer nip portion and the fixing nip portion, and thereby the printing is performed on the rear surface side of the sheet. 
     In the image forming apparatus  100  as described above, a manual sheet feeding tray is provided, or an external sheet feeding unit is mounted in some cases. In such a case, in place of the sheet feeding cassette  48 , a sheet may be fed from the manual sheet feeding tray or the sheet feeding unit to the first sheet transport path L 1 . 
     Further, the image forming apparatus  100  of the first embodiment includes an exhaust unit (exhaust device)  10  that discharges the air in the apparatus body  12  to the outside of the apparatus body  12 . Hereinafter, the structure of the exhaust unit  10  will be described with reference to the drawings.  FIG. 2  is a schematic sectional view illustrating a structure of the exhaust unit  10  provided in the image forming apparatus  100  illustrated in  FIG. 1 .  FIG. 3  is a schematic sectional view illustrating the structure of the exhaust unit  10  in a state where the fixing unit  44  is separated.  FIG. 4  is a schematic sectional view illustrating a flow of air in a fine particle collecting duct  70  and a first duct  90 .  FIG. 5  is a schematic sectional view illustrating the flow of air in the first duct  90 . 
     As illustrated in  FIGS. 2 and 3 , the exhaust unit  10  includes the fine particle collecting duct  70  and the first duct  90 . Each of the fine particle collecting duct  70  and the first duct  90  are ducts for guiding the air inside the apparatus body  12  to the outside of the apparatus body  12 , are formed in a substantially cylindrical shape extending in the front-rear direction, and are arranged in parallel with each other. Each of the fine particle collecting duct  70  and first duct  90  is connected to an exhaust port (not shown) on the rear surface side of the apparatus body  12 , and communicates with the outside of the apparatus body  12  via the exhaust port of the apparatus body  12 . Further, although the details will be described later, the exhaust direction of the fine particle collecting duct  70  and the first duct  90  is set on the back surface side. Therefore, in the fine particle collecting duct  70  and the first duct  90 , the front surface side is the upstream side of the flow of the air (air flow) and the back surface side is the downstream side of the air flow. 
     First, a configuration of the fine particle collecting duct  70  will be described. The fine particle collecting duct  70  is disposed above the fixing unit  44 . Specifically, the fine particle collecting duct  70  is disposed above the first support member  442  supporting the heat roller  44   a  and the heat roller  44   a.    
     The fine particle collecting duct  70  includes a fine particle collecting duct A portion  702  constituting the lower side of the fine particle collecting duct  70 , a fine particle collecting duct B portion  704  constituting the upper side of the fine particle collecting duct  70 , and the second sheet transport path L 2  (sheet transport space after heat fixing) formed to be sandwiched between the fine particle collecting duct A portion  702  and the fine particle collecting duct B portion  704 . 
     The fine particle collecting duct A portion  702  is partitioned by a first duct A forming member  72  and a separating member  80 . The fine particle collecting duct A forming member  72  has a U-shaped cross section opened downward and is a member extending in the front-rear direction. The separating member  80  is a plate-like member extending in a substantially horizontal direction of the front and rear, and seals the lower side of the fine particle collecting duct A forming member  72 . That is, the bottom surface of the fine particle collecting duct  70  is sealed by the separating member  80 . Here, the separating member  80  is bent and has roughness formed therein. 
     The fine particle collecting duct B portion  704  is partitioned by a fine particle collecting duct B forming member  74  and a fine particle collecting duct B wall member  76 . The fine particle collecting duct B forming member  74  is disposed above the fine particle collecting duct A forming member  72  with a predetermined space therebetween and has a U-shaped cross section opened upward and is a member extending in the front-rear direction. The fine particle collecting duct B wall member  76  is a plate-like member extending in a substantially horizontal direction of the front and rear, and seals the upper side of the fine particle collecting duct B forming member  74 . That is, the top surface of the fine particle collecting duct  70  is sealed by the fine particle collecting duct B wall member  76 . 
     Further, the above-described second sheet transport path L 2  is configured to traverse the fine particle collecting duct  70  in the left and right direction. Specifically, the second sheet transport path L 2  at the portion crossing the fine particle collecting duct  70  is formed of the top surface (top wall) of the fine particle collecting duct A forming member  72  and the bottom surface (bottom wall) of the fine particle collecting duct B forming member  74  disposed above the fine particle collecting duct A forming member  72 . 
     As illustrated in  FIG. 4 , a plurality of communication ports  72   a  are formed on the top wall of the fine particle collecting duct A forming member  72 , and a plurality of communication ports  74   a  are formed on the bottom wall of the fine particle collecting duct B forming member  74 . Each of the plurality of communication ports  72   a  and each of the plurality of communication ports  74   a  are formed to line up in the front-rear direction along the air flow of the fine particle collecting duct  70 . The fine particle collecting duct A portion  702 , the second sheet transport path L 2 , and the fine particle collecting duct B portion  704  communicate with each other by the plurality of communication ports  72   a  and the plurality of communication ports  74   a , and a series of spaces (ventilation path) is formed in the fine particle collecting duct  70 . 
     However, as described above, the top surface and the bottom surface of the fine particle collecting duct  70  are sealed by the fine particle collecting duct B wall member  76  and the separating member  80 . Therefore, the second sheet transport path L 2  is separated from the internal space of the apparatus body  12  other than the fine particle collecting duct  70  except for the entrance and the exit thereof by the fine particle collecting duct  70 . 
     Further, the separating member  80  is made of a material having high thermal conductivity. For example, the separating member  80  is formed of metallic materials. As the metallic material constituting the separating member  80 , a cold rolled steel plate such as aluminum, an aluminum alloy, or SPCC, an electrogalvanized steel sheet such as SECC, a hot-dip galvanized steel sheet such as SGCC, and stainless steel such as SUS can be used. 
     Next, a configuration of the first duct  90  will be described. As illustrated in  FIGS. 2 and 3 , the first duct  90  is provided along a portion of the side surface (left side surface) of the top surface, and the bottom surface of the fixing unit  44  of the sheet ejecting tray  50 . That is, the first duct  90  is provided so as to surround three sides of the fixing unit  44 . Specifically, the first duct  90  is provided along a portion of the left side surface, the top surface, and the bottom surface of the heat roller  44   a  and the first support member  442  that supports the heat roller  44   a.    
     The first duct  90  includes a first duct A portion (fixing side surface duct portion)  902  that covers the left side surface and the bottom surface of the fixing unit  44  (the first support member  442 ) and a first duct B portion (fixing top surface duct portion)  904  that covers the top surface of the fixing unit  44  (the first support member  442 ). 
     The first duct A portion  902  is partitioned by a first duct A forming member  92 . The first duct A forming member  92  includes a vertically elongated portion which forms a space extending in the vertical direction along the left side surface of the fixing unit  44  (the first support member  442 ), a lower end portion which is connected to the lower end of the vertically elongated portion and forms a space extending to the fixing unit  44  side (the first sheet transport path L 1  side) along the bottom surface of the fixing unit  44  (first support member  442 ). A space (ventilation path) having a substantially L-shaped cross section which is partitioned by the vertically elongated portion and the lower end portion of the first duct A forming member  92  is formed in the first duct A portion  902 . 
     Further, a process unit  64  is disposed below the first duct A portion  902  (the lower end portion of the first duct A forming member  92 ). That is, a portion of the first duct A portion  902  (the lower end portion of the first duct A forming member  92 ) is provided so as to enter the gap between the fixing unit  44  and the process unit  64 . 
     The first duct B portion  904  is partitioned by the first duct B forming member  94  and the separating member  80 . The first duct B forming member  94  is a member which is provided adjacent to the upper side of the first duct A forming member  92 , has a U-shaped cross section opened toward the upper side, and extends to in the front-rear direction along the top surface of the fixing unit  44  (the first support member  442 ). However, in a case of being viewed from the front-rear direction, the first duct B forming member  94  is provided so as to have a flat shape in which the vertical direction is short and the horizontal direction is long, enter the gap between the bottom surface of the fine particle collecting duct  70  and the fixing unit  44 , and cover the top surface of the fixing unit  44  (top wall of the first support member  442 ). Further, the upper side of the first duct B forming member  94  is sealed by the separating member  80 . That is, the top surface of the first duct  90  is sealed by the separating member  80 . 
     As described above, the separating member  80  seals the lower surface of the fine particle collecting duct  70  and seals the top surface of the first duct  90 . That is, the fine particle collecting duct  70  and the first duct  90  are provided so as to be adjacent to each other with the separating member  80  interposed therebetween. Also, it can be said that the first duct B portion  904  is formed between the fine particle collecting duct  70  and the fixing unit  44 . 
     Here, the lower surface (a side wall on a fixing unit side of first duct B portion  904 ) of the first duct B forming member  94  is formed of the material with heat resistance. To be heat resistant means that the heat resistant temperature exceeds 100 degrees. Further, the lower surface of the first duct B forming member  94  may have the heat resistance equivalent to or more than that of the fixing temperature. For example, as the material constituting the lower surface of the first duct B forming member  94 , in addition to a general heat-resistant resin such as polyether sulfone (PES), polyphenylene sulfide (PPS), liquid crystal polymer (LCP), polyether nitrile (PEN), polyimide (PI), polyamide imide (PAI), polyether ether ketone (PEEK) and polyethylene terephthalate (PET), composite materials formed of these resins and glass fiber, metal, ceramics, and the like can be used. Note that the entirety of the first duct B forming member  94  may be made of a material with heat resistance. 
     In addition, as illustrated in  FIGS. 4 and 5 , the first communication port  96  and the second communication port  98  communicating with the first duct A portion  902  and the first duct B portion  904  are formed in the first duct  90 . Each of the first communication port  96  and the second communication port  98  is formed by a communication hole formed in a portion of the lower surface of the first duct A forming member  92  and a portion of the lower surface of the first duct B forming member  94 . The first communication port  96  is positioned on the upstream side (front surface side) of the air flow in the first duct  90 . Further, the second communication port  98  is positioned on the downstream side (rear surface side) of the air flow in the first duct  90 . The first communication port  96  and the second communication port  98  are formed at positions separated from each other (in the front-rear direction) along the air flow, and a separation wall  906  for separating the first duct A portion  902  and the first duct B portion  904  is formed between the first communication port  96  and the second communication port  98 . That is, an internal flow path of the first duct  90  is divided into a plurality of air flow paths. 
     Further, as illustrated in  FIGS. 2 to 5 , a plurality of intake ports  92   a  through which the air in the internal space of the apparatus body  12  other than the fine particle collecting duct  70  passes are formed in the first duct  90 . The plurality of intake ports  92   a  are formed in the bottom wall of the first duct A forming member  92 . As illustrated in  FIGS. 2 and 3 , the plurality of intake ports  92   a  are formed at the end portion on the right side (the fixing unit  44  side or the first sheet transport path L 1  side) of the bottom wall of the first duct A forming member  92 . That is, the plurality of intake ports  92   a  are formed in a part where the first duct  90  covers the lower side of the fixing unit  44 . Further, a plurality of intake ports  92   a  are formed on the upstream side from the fixing unit  44  in the sheet transport direction. That is, the plurality of intake ports  92   a  are formed below the fixing unit  44 . 
     The plurality of intake ports  92   a  are formed in the vicinity of the top surface of the process unit  64  and open toward the process unit  64 . Therefore, the plurality of intake ports  92   a  are provided so as to suck the air at the side surface portion of the process unit  64  on the side of the fixing unit  44 . The lower end portion of the right side wall of the first duct  90  and the top wall of the process unit  64  are disposed without any gap therebetween so that the air in the space on the first sheet transport path L 1  side is not suctioned into the plurality of intake ports  92   a.    
     As illustrated in  FIGS. 4 and 5 , the plurality of intake ports  92   a  are arranged at a predetermined interval in the front-rear direction along the air flow of the first duct  90 . Here, at least one of the plurality of air intake ports  92   a  is positioned on the upstream side (front surface side) of the air flow from the end portion on the upstream side (front surface side) of the air flow of the separation wall  906 . Further, the plurality of intake ports  92   a  may be formed by arranging a plurality of ribs opposed to one opening. 
     As described above, the fine particle collecting duct  70  and the first duct  90  are formed. In addition, as illustrated in  FIG. 4 , the fine particle collecting duct  70  is provided with a fine particle collecting duct exhaust fan  82  and a filter  84 . The fine particle collecting duct exhaust fan  82  is disposed at the end portion on the rear surface side (downstream side of the air flow) of the fine particle collecting duct  70 , and the filter  84  is disposed on the rear surface side (downstream side of the air flow) of the fine particle collecting duct exhaust fan  82 . As illustrated in  FIGS. 4 and 5 , a first exhaust fan (exhaust fan)  86  is provided in the first duct  90 . The first exhaust fan  86  is disposed at the end portion on the rear surface side (downstream side of the air flow) of the first duct  90 . 
     The fine particle collecting duct exhaust fan  82  and the first exhaust fan  86  are axial flow fans, for example, propeller fans. The fine particle collecting duct exhaust fan  82  and the exhaust direction of the first exhaust fan  86  are set on the rear surface side. Therefore, the fine particle collecting duct exhaust fan  82  suctions the air inside the fine particle collecting duct  70  and sends the suctioned air to the rear surface side (the outside of the apparatus body  12 ). Further, the first exhaust fan  86  suctions the air inside the first duct  90  and sends the suctioned air to the outside of the apparatus body  12 . The fine particle collecting duct exhaust fan  82  and the first exhaust fan  86  are controlled by the control unit of the image forming apparatus  100  and are operated and stopped in accordance with instructions from the control unit. 
     The filter  84  is a UFP collection filter for collecting ultrafine particles (UFP) by heating a sheet or toner by using the fixing unit  44 . 
     In addition to the UFP collection filter, the filter  84  may include a VOC collection filter for collecting volatile organic compounds (VOC) or ozone. 
     Next, the flow of the air in the exhaust unit  10  of the first embodiment will be described. First, the flow of the air in the fine particle collecting duct  70  will be described. As illustrated in  FIG. 4 , in the fine particle collecting duct  70 , when the fine particle collecting duct exhaust fan  82  operates, the air in the fine particle collecting duct A portion  702 , the second sheet transport path L 2 , and the fine particle collecting duct B portion  704  is suctioned into the fine particle collecting duct exhaust fan  82 . 
     In this way, in the fine particle collecting duct  70 , the air in a space (the second sheet transport path L 2 ) in which the sheet is transported and the air in a space (the fine particle collecting duct A portion  702  and the fine particle collecting duct B portion  704 ) on the upper and lower sides thereof passed through the filter  84  and guided to the outside of the apparatus body  12 . That is, the fine particle collecting duct  70  functions as a duct for collecting substances such as UFPs. 
     Next, the flow of the air in the first duct  90  will be described. As illustrated in  FIGS. 4 and 5 , in the first duct  90 , when the first exhaust fan  86  is operated, the air in the first duct A portion  902  is suctioned into the first exhaust fan  86 . Further, the air flows into the first duct A portion  902  from the plurality of intake ports  92   a . At this time, a part of the air flowing into the first duct A portion  902  from the intake port  92   a  positioned on the upstream side (front surface side) of the air flow from the separation wall  906  separating the first duct A portion  902  and the first duct B portion  904  moves upward through the first communication port  96  and flows into the first duct B portion  904 , flows through the first duct B portion  904  toward the rear surface side, passes through the second communication port  98 , and then flows into the first duct A portion  902  again. 
     In the first embodiment, the first duct  90  includes the first duct A portion  902  along a portion of the left side surface and the bottom surface of the fixing unit  44  and the first duct B portion  904  along the top surface of the fixing unit  44 . In this way, by dividing the duct for each surface facing the fixing unit  44 , it is possible to restrain unevenness in the flow of air in each of the first duct A portion  902  and the first duct B portion  904  so as to secure an air flow rate. Therefore, in the first duct  90 , it is possible to interrupt the heat of the fixing unit  44  directed to the left side of the lower side due to the air flowing through the first duct A portion  902 , and to interrupt the heat of the fixing unit  44  directed to the upper side due to the air flowing through the first duct B portion  904 . That is, it is possible to effectively interrupt the heat of the fixing unit  44  without unevenness. 
     In particular, in the image forming apparatus  100  having the above-described configuration, the top surface of the first support member  442  supporting the heat roller  44   a  is heated to a high temperature. In the first embodiment, since the heat of the fixing unit  44  directed to the upper side is interrupted due to the air flowing through the first duct B portion  904 , it is possible to restrain the fine particle collecting duct  70  (the second sheet transport path L 2 ) from being directly exposed to the heat of the fixing unit  44 . Therefore, the temperature rise inside the fine particle collecting duct  70  can be suppressed. 
     Further, in the first embodiment, the first duct  90  is provided with the separation wall  906  for separating the first duct A portion  902  and the first duct B portion  904 , so that the air flowing from the intake port  92   a  flows through the first duct B portion  904 . Therefore, it is possible to secure the flow rate of the air flowing through the first duct B portion  904 , thereby securing a heat insulating effect on the top surface side of the fixing unit  44 . 
     Furthermore, in the first example, the first duct B forming member  94  (the bottom wall of the first duct B portion  904 ) facing the top surface of the first support member  442  is formed of a material with heat resistance. Thus, the heat resistance of the first duct B portion  904  can be secured. 
     Further, if the heat of the fixing unit  44  is transferred to the process unit  64 , there is a problem that the temperature of the inside of the process unit  64  becomes higher, the toner between the cleaning blade of the cleaner unit  38  and the photosensitive drum  36  is melted, and thereby a cleaning failure occurs in which toner remains on the surface of the photosensitive drum  36 . In the first embodiment, a portion of the first duct A portion  902  is formed between the fixing unit  44  and the process unit  64 , and thus it is possible to interrupt the heat of the fixing unit  44  directed to the lower side by the first duct A portion  902 , and to restrain the process unit  64  from being directly exposed to the heat of the fixing unit  44 . 
     Furthermore, since the plurality of intake ports  92   a  are provided such that the air around the process unit  64  passes through, the top surface of the process unit  64  is cooled by the air suctioned into the plurality of intake ports  92   a . Therefore, it is possible to suppress the temperature rise in the process unit  64  and to restrain the above-described cleaning failure. 
     Further, the fine particle collecting duct  70  is provided with a high-density filter  84  for collecting the substances such as UFPs. Since this filter  84  has a large air flow resistance, the flow velocity of the air flow passing through the filter  84  is decreased, and the flow rate of the air discharged to the outside of the apparatus body  12  from the fine particle collecting duct  70  is decreased. That is, the fine particle collecting duct  70  has a capacity of collecting the substances such as UFPs, but there is a problem in that a cooling capacity is deteriorated, and the temperature of the inside of the fine particle collecting duct  70  becomes higher due to the heated and fixed sheet. On the other hand, since no filter is provided in the first duct  90 , the flow rate of the air discharged from the first duct  90  to the outside of the apparatus body  12  can be secured. Here, the fine particle collecting duct  70  and the first duct  90  are provided so as to be adjacent to each other with a separating member  80  formed of a material with high thermal conductivity. That is, the fine particle collecting duct  70  and the first duct  90  are indirectly thermally coupled (thermally coupled) via the separating member  80 , and the heat can be mutually transferred between the fine particle collecting duct  70  and the first duct  90 . Therefore, by transferring the heat inside the fine particle collecting duct  70  to the air flowing through the first duct  90  via the separating member  80  and discharging the air to the outside of the apparatus body  12 , it is possible to suppress the internal temperature of the fine particle collecting duct  70  from becoming higher. That is, the heat of the fine particle collecting duct  70  can be dissipated to the first duct  90  to compensate for lowering a cooling capacity of the fine particle collecting duct  70 . In addition, since the separating member  80  has roughness formed, a surface area of the separating member  80  is increased, and the heat radiation effect of the fine particle collecting duct  70  can be enhanced. 
     As described above, the first duct  90  has the heat insulating effect of insulating the heat of the fixing unit  44  so as not to be transferred to other components of the image forming apparatus  100 , and a cooling effect of suppressing an increase in the internal temperature of the image forming apparatus  100 . Here, since the intake port  92   a  of the first duct  90  is formed on the upstream side from the fixing unit  44  in the sheet transport direction, air having a relatively low temperature can be taken in the inside of the first duct  90 . Therefore, the above-described heat insulating effect and cooling effect can be efficiently obtained. Further, since the substances such as UFPs are not generated on the upstream side from the fixing unit  44  in the sheet transport direction, the substances such as UFP do not flow into the first duct  90  and are not discharged to the outside of the apparatus body  12 . 
     Second Embodiment 
     Since an image forming apparatus  100  of a second embodiment is the same as the image forming apparatus  100  of the first embodiment except that it further includes an air blowing unit  110  that sends auxiliary air to the fine particle collecting duct  70  and the first duct  90 , contents different from those of the first embodiment will be described, and redundant explanation will not be made. 
       FIG. 6  is a schematic sectional view illustrating structures of the air blowing unit  110  and the exhaust unit  10  of the second example.  FIG. 7  is a schematic view illustrating the flow of air in a case where the first duct  90  and the second duct  112  of the second embodiment are not connected to each other.  FIG. 8  is a schematic view illustrating the flow of air in a case where the first duct  90  and the second duct  112  of the second embodiment are connected to each other. 
     As illustrated in  FIG. 6 , the air blowing unit (air blowing device)  110  includes a second duct  112 . The second duct  112  is a duct which is formed of a second duct forming member  116  and guides air (fresh air) outside the apparatus body  12  to the fine particle collecting duct  70  and the first duct  90 . One end portion of the second duct  112  is connected to a ventilation portion (not shown) provided at the front surface side end portion on the left side surface of the apparatus body  12 , and communicates with the outside of the apparatus body  12  via the ventilation portion of the apparatus body  12 . 
     An intake fan  114  is provided on the downstream side of the ventilation portion of the apparatus body  12  in the second duct  112 . The intake fan  114  is an axial flow fan, for example, a propeller fan. Further, the exhaust direction of the intake fan  114  is set to the right side. Therefore, the air intake fan  114  suctions the air outside the apparatus body  12  from the ventilation portion and sends the suctioned air from to the inside of the second duct  112 . The intake fan  114  is controlled by the control unit of the image forming apparatus  100  and is operated and stopped in accordance with instructions from the control unit. 
     Further, as illustrated in  FIGS. 7 and 8 , the second duct  112  is branched (separated) to the second duct A portion (duct enlarged portion)  120  and the second duct B portion  130  by the separation wall  118  on the downstream side of the air flow from the intake fan  114 . An end portion of the second duct A portion  120  on the downstream side communicates with the first duct  90 , and an end portion of the second duct B portion  130  on the downstream side communicates with the fine particle collecting duct  70 . Therefore, the air sent to the inside of the second duct  112  by the intake fan  114  is sent to the first duct  90  through the second duct A portion  120 , and is sent (supplied) to the fine particle collecting duct  70  through the second duct B portion  130 . 
     The second duct B portion  130  communicates with the end portion on the front surface side of the fine particle collecting duct  70  by extending the right surface side to the front surface side in the apparatus body  12 . As illustrated in  FIG. 6 , an inflow port communicating with the second duct B portion  130  is formed at an end portion on the front surface side of the in the fine particle collecting duct  70 . Therefore, the air sent by the intake fan  114  flows into the fine particle collecting duct  70  from the inflow port. 
     The second duct A portion  120  is formed such that inside the apparatus body  12  (below the sheet ejecting tray  50 ) extends to the right side and the flow path expands toward the front-rear direction as going to the right side (communication portion with the first duct  90 ). The end portion of the second duct A portion  120  on the downstream side enters the lower side of the first duct  90  and communicates with the first duct  90  via the plurality of intake ports  92   a . Here, the end portion of the second duct A portion  120  on the downstream side is formed so as to include all of the intake ports  92   a  of the first duct  90  in the front-rear direction. Therefore, the air sent by the intake fan  114  flows into the first duct  90  from each of the intake ports  92   a . That is, a series of ducts (ventilation paths) constituted by the second duct  112  (the second duct A portion  120 ) and the first duct  90  has a push-pull structure in which the intake fan  114  and the first exhaust fan  86  are arranged in series. Thus, it is possible to sufficiently secure the flow rate of air passing through each of the intake ports  92   a.    
     In addition, the second duct A portion  120  is provided with a plurality of shunt flow rectifying ribs (rectifying plate)  122  and a shunt rib (shunt plate)  124 . The plurality of shunt flow rectifying ribs  122  are arranged at the end portion of the second duct A portion  120  on the downstream side, that is, connection portions of the plurality of first ducts  90 . Each of the plurality of shunt flow rectifying ribs  122  is a plate-shaped rib extending in the horizontal direction, and is provided substantially in parallel with a predetermined space therebetween. The air flowing through the second duct A portion  120  flows radially from the right side to the rear surface side on the upstream side from the plurality of flow dividing and rectifying ribs  122  as illustrated in  FIG. 6 , and the direction (direction perpendicular to the direction of the air flow in the first duct  90 ) is changed to the right by each of the plurality of shunt flow rectifying ribs  122 , and is guided to the plurality of intake ports  92   a.    
     Here, the shunt flow rectifying rib  122  includes a rectifying rib A (rectifying plate)  122   a  and a rectifying rib B (rectifying plate)  122   b . The rectifying rib B 122   b  is set to be long on the upstream side from the rectifying rib A 122   a . Thus, in the rectifying rib B 122   b , the air can flow more than that in the rectifying rib A 122   a . Therefore, by disposing the rectifying rib B 122   b  at a predetermined position, it is possible to increase the flow rate of the air passing through the intake port  92   a  far from the first exhaust fan  86 . For example, the second and fifth shunt flow rectifying ribs  122  from the front surface side (the side closer to the intake fan  114 ) are constituted by the rectifying rib B 122   b , and the other shunt flow rectifying rib  122  is constituted by the rectifying rib A 122   a.    
     Further, the shunt rib  124  is disposed at a position which is the upstream side of the air flow from the shunt flow rectifying rib  122  and in which the flow path of the second duct A portion  120  expands. This shunt rib  124  sends air to the shunt flow rectifying rib  122  disposed at a position far from the intake fan  114  among the plurality of shunt flow rectifying ribs  122  so that the air flows in a balanced manner to each of the plurality of intake ports  92   a.    
     As illustrated in  FIGS. 6 and 8 , the plurality of intake ports  92   a  are arranged such that distances to the first exhaust fan  86  are different from each other. In addition, the plurality of intake ports  92   a  are arranged such that distances to the intake fan  114  are different from each other. 
     Therefore, as illustrated in  FIG. 7 , in the first duct  90 , among the plurality of intake ports  92   a , the intake port  92   a  having a short distance to the first exhaust fan  86  (close to the first exhaust fan  86 ) and the intake port  92   a  having a long distance to the first exhaust fan  86  (far from the first exhaust fan  86 ) have different duct resistance (pipe friction loss), and the air flow rate (intake air amount) suctioned by the first exhaust fan  86  becomes ununiform. For this reason, although the air flow rate can be secured at the intake port  92   a  on the rear surface side close to the first exhaust fan  86 , the air flow rate is decreased at the intake port  92   a  on the front surface side far from the first exhaust fan  86 . Particularly, when the air flow rate of the intake port  92   a  on the front surface side is decreased, the amount of air flowing into the first duct B portion  904  is decreased, and the heat insulating property of the first duct B portion  904  is deteriorated. 
     In addition, in the second duct  112 , since the distances to the intake fan  114  are different on the front surface side and the rear surface side at end portion (communication portion with the first duct  90 ) of the second duct  112  on the downstream side, a difference in the duct resistance occurs, so that the air flow rate (the amount of air guided to each of the intake ports  92   a ) sent by the intake fan  114  becomes ununiform. For this reason, although the flow rate of the air guided to the intake port  92   a  on the front surface side close to the intake fan  114  can be secured, the flow rate of the air guided to the intake port  92   a  on the rear surface side far from the intake fan  114  is decreased. 
     In this way, in the intake port  92   a  on the rear surface side, the flow rate of air sent by the intake fan  114  is small while the flow rate of air suctioned by the first exhaust fan  86  is large. On the other hand, in the intake port  92   a  on the front surface side, the flow rate of air sent by the intake fan  114  is large while the flow rate of air suctioned by the first exhaust fan  86  is small. 
     As illustrated in  FIG. 8 , by connecting the first duct  90  having such characteristics and the second duct  112  to each other, a total air flow rate of the flow rate of air suctioned by the first exhaust fan  86  and the flow rate sent by the intake fan  114  can be made uniform in each intake port  92   a . That is, it is possible to make the flow rate of air passing through each of the intake ports  92   a  uniform. 
     In addition, in the second embodiment, with the plurality of the shunt flow rectifying rib  122 , it is possible to increase the flow rate of the air passing through the intake port  92   a  far from the first exhaust fan  86 . Thus, it is possible to compensate for the decrease in the flow rate of air passing through the intake port  92   a  far from the first exhaust fan  86  due to the difference in duct resistance so as to make the flow rate of air passing through each intake port  92   a  uniform. 
     Third Embodiment 
     Since an image forming apparatus  100  of a third embodiment is the same as the image forming apparatus  100  of the second embodiment except that the first duct  90  and the second duct  112  are connected to each other by a portion of the process unit  64 , contents different from those of the second embodiment will be described, and redundant explanation will not be made. 
       FIG. 9  is a schematic perspective view illustrating a structure of the second duct  112  of the third embodiment.  FIG. 10  is a schematic sectional view illustrating structures of the first duct  90  and the second duct  112  before the process unit  64  is inserted, in the third embodiment.  FIG. 11A  is a schematic view illustrating the structures of the first duct  90  and the second duct  112  before the process unit  64  is inserted, in the third embodiment.  FIG. 11B  is a schematic view illustrating the structures of the first duct  90  and the second duct  112  after the process unit  64  is inserted, in the third embodiment.  FIG. 12  is a schematic sectional view illustrating the structures of the first duct  90  and the second duct  112  after the process unit  64  is inserted, in the third embodiment. Note that,  FIGS. 8 and 11  are illustrated as engaging on one plane in order to explain a state of engagement between the first duct and the second duct in an easy-to-understand manner; however, in actuality, the first duct is installed and engaged at an angle in a direction perpendicular to the sheet surface. 
     As illustrated in  FIGS. 9 and 10 , in the third embodiment, a gap (opening)  126  is formed in a space (communication portion) between the first duct  90  and the second duct  112 . The gap  126  is formed by an opening formed in the second duct forming member  116 . In addition, the gap  126  is formed over the entire front-rear direction at the end portion of the second duct A portion  120  on the downstream side. Further, as illustrated in  FIG. 10 , the gap  126  is formed below the first duct  90 , and faces the plurality of intake ports  92   a  and the bottom wall of the first duct  90  (first duct B portion  904 ). 
     In addition, as illustrated in  FIGS. 11A and 11B , the process unit  64  is provided so as to be removable in the front-rear direction. The process unit  64  is inserted to the rear surface side from the front surface side below the gap  126 , and is mounted on the apparatus body  12 . As illustrated in  FIG. 11A , the gap  126  is open before the process unit  64  is mounted on the main body  12 . On the other hand, as illustrated in  FIG. 11B , in a state where the process unit  64  is mounted on the apparatus body  12 , the process unit is disposed at a position adjacent to the communication portion, and a top wall (facing wall portion)  642  of the process unit  64  is configured to seal the gap  126 . That is, the top wall  642  of the process unit  64  is formed on the wall surface of the communication portion, and the first duct  90  and the second duct  112  are connected to each other by the top wall  642  of the process unit  64 . 
     Further, two ribs  648  extending in the horizontal direction are formed on the top wall  642  of the process unit  64 . One of the two ribs  648  is formed on the front surface side of the top wall  642  such that the front wall of the end portion of the first duct  90  (first duct A portion  902 ) on the upstream side, and the front wall of the end portion of the second duct  112  (second duct A portion  120 ) on the downstream side are connected to each other without any gap. The other one of the two ribs  648  is formed on the rear surface side of the top wall  642  such that the rear wall of the end portion of the first duct  90  on the upstream side and the rear wall of the end portion of the second duct  112  on the downstream side. With these two ribs  648 , the air is restrained from leaking to the front surface side and the rear surface side. The two ribs  648  may be wall-shaped. 
     In addition, as illustrated in  FIG. 12 , an engagement piece  644  is formed at one end portion (end portion on the downstream side (right side) of the air flow) of the top wall  642  of the process unit  64 , and an engagement piece  646  is formed at the other end portion (end portion on the upstream side (left side) of the air flow). Each of the engagement piece  644  and the engagement piece  646  is a portion of the top wall  642 , and the engagement piece  644  is formed into a plate shape extending toward the downstream side of the air flow, and the engagement piece  646  is formed into a plate shape extending toward the upstream side of the air flow. In addition, each of the engagement piece  644  and the engagement piece  646  is formed across at least two ribs  648  in the front-rear direction of the process unit  64 . 
     Further, an engaging portion  1262  engaging with the engagement piece  644  is formed at the end portion of an opening end constituting the gap  126  on the downstream side of the air flow, and an engaging portion  1264  engaging with the engagement piece  646  is formed at the end portion of an opening end constituting the gap  126  on the upstream side of the air flow. The engaging portion  1262  has a U-shaped cross section opened toward upstream of the air flow, and the engaging portion  1264  has a U-shaped cross section opened toward downstream of the air flow. Each of the engaging portion  1262  and the engaging portion  1264  extends in the front-rear direction. In the state where the process unit  64  is mounted on the apparatus body  12 , the engagement piece  644  and the engaging portion  1262  are engaged with each other, and the engagement piece  646  and the engaging portion  1264  are engaged with each other. That is, when the engagement piece  644  and the engaging portion  1262  are engaged with each other, and the engagement piece  646  and the engaging portion  1264  are engaged with each other, the air is restrained from leaking from the gap  126 . 
     Here, the engagement piece  644  is slidable with respect to the engaging portion  1262 , and the engagement piece  646  is slidable with respect to the engaging portion  1264 . Therefore, in the state where the engagement piece  644  and the engaging portion  1262  are engaged with each other, and the engagement piece  646  and the engaging portion  1264  are engaged with each other, the process unit  64  is slidable in the front-rear direction, and is inserted into the apparatus body  12 , or drawn from the apparatus body  12 . That is, each of the engagement piece  644 , the engagement piece  646 , the engaging portion  1262 , and the engaging portion  1264  serves as an insertion and removal guide portion (guide) of the process unit  64 . 
     In addition, when the process unit  64  is mounted (installed) on the apparatus body  12 , the engagement pieces  644  and  646  are closely attached to the engaging portions  1262  and  1264  formed in a U-shape by the weight of the process unit  64  respectively, and thus it is possible to restrain the air from leaking. 
     In addition, the process unit  64  is provided with a cleaning blade  382  of the cleaner unit  38  and a collected toner transporting member  384  on the inside of the top wall  642 . The top wall  642  is provided with inclined surfaces  6422  and  6423  inclined downward so as to be close to the cleaning blade  382  and the collected toner transporting member  384 . These inclined surfaces  6422  and  6423  are formed on the upstream side of the air flow from the plurality of intake ports  92   a  and substantially right under the bottom wall of the first duct  90  (the first duct A portion  902 ). Therefore, the air passing above the top wall  642  of the process unit  64  flows so as to curve downwardly along the inclined surfaces  6422  and  6423 , and thus the cleaning blade  382  disposed in the vicinity of the inclined surfaces  6422  and  6423  and the collected toner transporting member  384  can be effectively cooled. Therefore, it is possible to effectively restrain a cleaning defect caused by melting and fusing the toner collected by the cleaning device by the heat of the fixing portion, and a transport defect of the collected toner. 
     Note that, in a case where only one side of the cleaning blade  382  and the collected toner transporting member  384  is to be cooled, or a case where both are disposed relatively close to the top wall  642 , a recessed inclined portion is may be formed flat without being provided in the top wall  642 . 
     In addition, as illustrated in  FIGS. 10 and 12 , the gap  126  may be provided with a guide portion  128  protruding toward the process unit  64  side. Since the guide portion  128  is provided with an inclined surface inclined down toward the process unit  64  side, it is possible to cause the air flowing through the gap  126  to flow toward the process unit  64  side, and the top wall  642  of the process unit  64  can be effectively cooled. 
     In the third embodiment, since the gap  126  is formed between the first duct  90  and the second duct  112 , and top wall  642  of the process unit  64  seals the gap  126 , the air flowing through the gap  126  is in directly contact with the top wall  642  of the process unit  64 , thereby effectively suppressing temperature rise in the process unit  64 . 
     In the third embodiment, the inclined surfaces  6422  and  6423  are formed on the top wall  642  of the process unit  64 , so that the flow of air passing above the top wall  642  of the process unit  64  is curved (the inside of the process unit  64  is recessed) so as to approach the cleaner unit  38 . With this, it is possible to effectively cool the cleaner unit  38 , suppress the temperature rise in the cleaner unit  38 , and restrain a cleaning defect and a transport failure of the collected toner. 
     Fourth Embodiment 
     Since an image forming apparatus  100  of a fourth embodiment is the same as the image forming apparatus  100  of the third embodiment except that a plurality of rectifying ribs  6424  are formed on the top wall  642  of the process unit  64 , contents different from those of the third embodiment will be described, and redundant explanation will not be made. 
       FIG. 13  is a schematic perspective view illustrating a structure of process unit  64  of a fourth embodiment.  FIG. 14  is a schematic perspective view illustrating a structure of the second duct  112  of the fourth embodiment. 
     As illustrated in  FIG. 13 , in the fourth embodiment, the plurality of rectifying ribs  6424  are formed on the top wall  642  of the process unit  64 . Each of the plurality of rectifying ribs  6424  is a plate-shaped rib extending in the horizontal direction along the flow of air flowing through the communication portion between the first duct  90  and the second duct  112 , and are disposed substantially parallel to each other with a predetermined space therebetween on the inclined surface  6422  of the top wall  642 . That is, as illustrated in  FIG. 14 , each of the plurality of rectifying ribs  6424  is disposed in parallel to the plurality of shunt flow rectifying ribs  122 . In addition, each of the plurality of rectifying ribs  6424  is disposed at a position different from the shunt flow rectifying rib  122  in the front-rear direction. That is, the plurality of rectifying ribs  6424  and the shunt flow rectifying ribs  122  are alternately arranged in the front-rear direction. The plurality of rectifying ribs  6424  are formed so as to form a gap between the guide portion  128  provided in the gap  126 . 
     In this fourth embodiment, since the plurality of rectifying ribs  6424  are formed on the top wall  642  of the process unit  64 , the surface area of the top wall  642  of the process unit  64  is increased. Therefore, it is possible to effectively suppress the temperature rise in the process unit  64 . 
     In each of the above-described embodiments, the image forming apparatus  100  is configured as a multifunction printer; however, the image forming apparatus of the disclosure may be configured as a printer, a copying machine, or a facsimile machine. 
     Further, in each of the above-described embodiments, the image forming apparatus  100  is configured as a monochrome compound machine; however, the image forming apparatus of the disclosure may be configured as a color printing machine or a color multifunction printer. 
     Further, the specific shapes and the like exemplified in the above examples are merely examples, and can be appropriately changed according to actual products. 
     The present disclosure contains subject matter related to that disclosed in Japanese Priority Patent Application JP 2017-250780 filed in the Japan Patent Office on Dec. 27, 2017, the entire contents of which are hereby incorporated by reference. 
     It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.