Patent Publication Number: US-2019171153-A1

Title: Image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of International Patent Application No. PCT/JP2017/026772, filed Jul. 25, 2017, which claims the benefit of Japanese Patent Application Nos. 2016-149388, 2016-150420, and 2016-150421 filed Jul. 29, 2016, all of which are hereby incorporated by reference herein in their entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an electrophotographic image forming apparatus, such as a copier or a laser beam printer, which has the capability of forming images on recording materials, and particularly relates to an image forming apparatus which also has the capability of reading documents. 
     BACKGROUND ART 
     An image forming apparatus, such as a copier or a multifunction printer (MFP), includes a document reading unit in the upper part thereof. Exemplary types of document reading include reading a document, such as a book, placed on a transparent document table, and reading a sheet-like document which is being conveyed with respect to a fixed reading sensor. Some document reading units support only one of the types described above, and others support both of the types described above. 
     MFPs often have a configuration in which a paper output tray is on the upper surface of a printer main body. Such an MFP has space left above the paper output tray to allow the user to easily access a recording material discharged onto the paper output tray. The document reading unit is disposed above this space. This increases the overall height of the apparatus including the printer main body and the document reading unit. 
     As a document reading unit, an image forming apparatus described in PTL 1 includes an automatic document feeder (ADF) for conveying a document, but does not include a document table. An image sensor that reads a document is disposed inside a printer main body. This image forming apparatus has no capability of reading a document, such as a book, because of the absence of a document table, but is advantageous in that it has lower overall height. 
     CITATION LIST 
     Patent Literature 
     PTL 1 Japanese Patent Laid-Open No. 2014-215431 
     In the image forming apparatus described in PTL 1, the image sensor is disposed adjacent to a duplex conveying path in the printer main body. In the printer, the duplex conveying path, which also serves as a conveying path for feeding a document and conveying it to the image sensor, is disposed inside the image sensor, and another conveying path for guiding a read document to a document discharge portion is disposed outside the image sensor. As a result, even a single function printer (SFP) with no capability of reading a document requires many conveying paths, and this leads to an increased size of the apparatus. 
     An object of the present invention is to provide an image forming apparatus that is compact but includes, in a printer main body, a sensor unit capable of reading an image on a document. 
     SUMMARY OF INVENTION 
     To solve the problem described above, the present invention provides an image forming apparatus that includes an image forming unit configured to form an unfixed image on a recording material; a fixing unit including a first rotary member configured to come into contact with the unfixed image on the recording material and a second rotary member configured to form a fixing nip with the first rotary member to convey the recording material therebetween, the fixing unit being configured to fix the unfixed image formed by the image forming unit onto the recording material; a reversing roller configured to reverse the recording material in such a manner that instead of a leading edge, a trailing edge of the recording material that has passed through the fixing unit is conveyed; a duplex conveying path configured to convey the reversed recording material to the image forming unit again; a discharge portion configured to discharge the recording material out of the image forming apparatus; and a sensor unit including a sensor that reads an image on a document. In the image forming apparatus, the sensor unit is at least partially disposed in a region below an upper end of the reversing roller, above a lower end of the second rotary member, and outside the duplex conveying path. 
     The present invention also provides an image forming apparatus that includes an image forming unit configured to form an unfixed image on a recording material; a fixing unit including a first rotary member configured to come into contact with the unfixed image on the recording material and a second rotary member configured to form a fixing nip with the first rotary member, the fixing unit being configured to fix the unfixed image on the recording material; a reversing portion configured to reverse the recording material in such a manner that instead of a leading edge, a trailing edge of the recording material that has passed through the fixing unit is conveyed; a duplex conveying path configured to convey the recording material reversed by the reversing portion to the image forming unit again; a document feeding portion configured to feed a document; a document discharge portion configured to discharge the document; a document conveying path configured to connect the document feeding portion to the document discharge portion; and a reading unit including a sensor that reads an image. In the image forming apparatus, the duplex conveying path is partially a common conveying path shared with the document conveying path; the reading unit is disposed on a side of the common conveying path facing one of two surfaces of the recording material reversed by the reversing portion, the one being a surface that has more recently come into contact with the first rotary member; and the reading unit has a capability of reading a fixed image on the recording material and an image on the document. 
     The present invention also provides an image forming apparatus that includes an image forming unit configured to form an unfixed image on a recording material; a fixing unit configured to fix the unfixed image on the recording material; a reversing portion configured to reverse the recording material in such a manner that instead of a leading edge, a trailing edge of the recording material that has passed through the fixing unit is conveyed; a duplex conveying path configured to convey the recording material reversed by the reversing portion to the image forming unit again; a document feeding portion configured to feed a document; a document discharge portion configured to discharge the document; a document conveying path configured to connect the document feeding portion to the document discharge portion; a reading unit disposed in the document conveying path and including a sensor that reads an image on the document; and two conveying paths, one being configured to connect the reversing portion to the document conveying path and the other being configured to connect the document conveying path to the duplex conveying path. In the image forming apparatus, the reading unit has a capability of reading a fixed image on the recording material and an image on the document. 
     The present invention also provides an image forming apparatus that includes an image forming unit configured to form an unfixed image on a recording material; a fixing unit configured to fix the unfixed image on the recording material; a reversing portion configured to reverse the recording material in such a manner that instead of a leading edge, a trailing edge of the recording material that has passed through the fixing unit is conveyed; a duplex conveying path configured to convey the recording material reversed by the reversing portion to the image forming unit again; a document feeding portion configured to feed a document; a document discharge portion configured to discharge the document; a document conveying path configured to connect the document feeding portion to the document discharge portion; a reading unit disposed in the document conveying path and including a sensor that reads an image on the document; and a conveying path configured to guide the recording material that has passed through the fixing unit to the document discharge portion. In the image forming apparatus, the reading unit has a capability of reading a fixed image on the recording material and an image on the document. 
     The present invention also provides an image forming apparatus that includes an image forming unit configured to form an unfixed image on a recording material; a fixing unit configured to fix the unfixed image on the recording material; a discharge portion configured to discharge the recording material having the fixed image thereon out of the image forming apparatus; a discharge conveying path configured to guide the recording material from the fixing unit to the discharge portion; a reversing portion configured to reverse the recording material in such a manner that instead of a leading edge, a trailing edge of the recording material that has passed through the fixing unit is conveyed; a duplex conveying path configured to guide the recording material reversed by the reversing portion to the image forming unit again; a first conveying path configured to guide the recording material from a branch point in the discharge conveying path to the reversing portion; a second conveying path configured to guide the recording material reversed by the reversing portion to the duplex conveying path; a reading unit configured to read an image formed on the recording material; a third conveying path configured to guide the recording material reversed by the reversing portion to the reading unit; and a fourth conveying path configured to guide the recording material that has passed through the reading unit to the duplex conveying path. In the image forming apparatus, a front and back orientation of the recording material coming from the reversing portion and passing through the third conveying path, the reading unit, the fourth conveying path, and the duplex conveying path is the same as a front and back orientation of the recording material coming from the reversing portion and passing through the second conveying path and the duplex conveying path, the front and back orientation being an orientation of a front surface on which an image is formed first and a back surface opposite the front surface; and the second conveying path is disposed between the reading unit and the fixing unit. 
     The present invention also provides an image forming apparatus that includes an image forming unit configured to form an unfixed image on a recording material; a fixing unit configured to fix the unfixed image on the recording material; a discharge reversing portion configured to be able to selectively discharge the recording material having the fixed image thereon out of the image forming apparatus and reverse the recording material in such a manner that instead of a leading edge, a trailing edge of the recording material that has passed through the fixing unit is conveyed; a duplex conveying path configured to guide the recording material reversed by the discharge reversing portion to the image forming unit again; a first conveying path configured to guide the recording material from the fixing unit to the discharge reversing portion; a second conveying path configured to guide the recording material reversed by the discharge reversing portion to the duplex conveying path; a reading unit configured to read an image formed on the recording material; a third conveying path configured to guide the recording material reversed by the discharge reversing portion to the reading unit; and a fourth conveying path configured to guide the recording material that has passed through the reading unit to the duplex conveying path. In the image forming apparatus, a front and back orientation of the recording material coming from the discharge reversing portion and passing through the third conveying path, the reading unit, the fourth conveying path, and the duplex conveying path is the same as a front and back orientation of the recording material coming from the discharge reversing portion and passing through the second conveying path and the duplex conveying path, the front and back orientation being an orientation of a front surface on which an image is formed first and a back surface opposite the front surface; and the second conveying path is disposed between the reading unit and the fixing unit. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a cross-sectional view of an image forming apparatus. 
         FIG. 2A  is a partial enlarged view of  FIG. 1 . 
         FIG. 2B  is a partial enlarged view of  FIG. 2A . 
         FIG. 3  is a cross-sectional view of a sensor unit. 
         FIG. 4  is a perspective view of the sensor unit. 
         FIG. 5  is a partial enlarged view of an image forming apparatus according to a second embodiment. 
         FIG. 6  is a partial enlarged view of an image forming apparatus according to a third embodiment. 
         FIG. 7  is a cross-sectional view of a sensor unit mounted in the image forming apparatus according to the third embodiment. 
         FIG. 8  is a cross-sectional view of an image forming apparatus according to a fourth embodiment. 
         FIG. 9  is a cross-sectional view of an image reading unit  100  according to the fourth embodiment. 
         FIG. 10  illustrates how a recording material flows during printing. 
         FIG. 11  illustrates how a document flows during reading of an image on the document. 
         FIG. 12  illustrates how a recording material flows during duplex printing and reading of a printed image. 
         FIG. 13  is an enlarged cross-sectional view of the image reading unit  100  according to a fifth embodiment. 
         FIG. 14  is an enlarged cross-sectional view of the image reading unit  100  according to a sixth embodiment. 
         FIG. 15  is an enlarged cross-sectional view of the image reading unit  100  according to a seventh embodiment. 
         FIG. 16  is an enlarged cross-sectional view of the image reading unit  100  according to an eighth embodiment. 
         FIG. 17  is an enlarged cross-sectional view of the image reading unit  100  according to a ninth embodiment. 
         FIG. 18  is an enlarged cross-sectional view of the image reading unit  100  according to a tenth embodiment. 
         FIG. 19  is a cross-sectional view of an image forming apparatus according to an eleventh embodiment. 
         FIG. 20  is an enlarged view of an upper right portion of  FIG. 19 . 
         FIG. 21  is a cross-sectional view of an image forming apparatus according to a twelfth embodiment. 
         FIG. 22  is an enlarged view of an upper right portion of  FIG. 21 . 
         FIG. 23  is a cross-sectional view of an image forming apparatus according to a thirteen embodiment. 
         FIG. 24  is an enlarged view of an upper right portion of  FIG. 23 . 
         FIG. 25  illustrates an operation of a conveyance sensor. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
     An image forming apparatus according to a first embodiment of the present invention will now be described using  FIGS. 1 to 4 .  FIG. 2A  is a partial enlarged view of  FIG. 1 .  FIG. 2B  is an enlarged view of  FIG. 2A .  FIGS. 3 and 4  are explanatory diagrams of a sensor unit. The image forming apparatus according to the first embodiment and second and third embodiments (described below) is a full-color laser beam printer including a plurality of photosensitive drums. The present invention is also applicable to a monochrome image forming apparatus including only one photosensitive drum. 
     (Feeding Unit) 
       FIG. 1  is a cross-sectional view of an image forming apparatus  1 . A cassette  2  that holds recording materials therein is mounted at the bottom of the image forming apparatus  1  in such a manner that it can be pulled out. A cassette feeding portion  3  is disposed near an end portion of the cassette  2 . A manual feeding portion  4  is disposed on the right-hand side of the image forming apparatus  1 . The cassette feeding portion  3  and the manual feeding portion  4  are both configured to feed recording materials one by one to registration rollers  5 . A conveyance sensor  200   a  that detects passage of a recording material is disposed downstream of the registration rollers  5  in a recording-material conveying direction. 
     (Image Forming Unit and Fixing Unit) 
     The image forming apparatus  1  includes an image forming unit  6 . The image forming unit  6  of the present embodiment includes four image forming stations  6 Y,  6 M,  6 C, and  6 K of yellow, magenta, cyan, and black. The image forming unit  6  also includes photosensitive members  7 Y,  7 M,  7 C, and  7 K each serving as an image bearing member, and chargers  8 Y,  8 M,  8 C, and  8 K each configured to uniformly charge the surface of a corresponding one of the photosensitive members  7 Y,  7 M,  7 C, and  7 K. The image forming unit  6  further includes a scanner unit  9  that emits laser beams on the basis of image information to form electrostatic latent images on the photosensitive members  7 Y,  7 M,  7 C, and  7 K, and developing units  10 Y,  10 M,  10 C, and  10 K that cause toners to adhere to the electrostatic latent images to develop the electrostatic latent images. The image forming unit  6  further includes primary transfer portions  12 Y,  12 M,  12 C, and  12 K that transfer the toner images on the photosensitive members  7 Y,  7 M,  7 C, and  7 K to an electrostatic transfer belt  11 . The toner images on the electrostatic transfer belt  11  are transferred onto a recording material at a secondary transfer portion  13 . The resulting toner image on the recording material is fixed by a fixing unit  14 . The fixing unit  14  includes a fixing roller (first rotary member)  14   a  that has an internal heat source therein, and a pressure roller (second rotary member)  14   b  that forms a fixing nip with the fixing roller  14   a  to convey the recording material therebetween. The recording material is heated at the fixing nip while being conveyed. An unfixed image on the recording material is thus fixed to the recording material. 
     (Discharge Portion and Reversing Portion) 
     A conveyance sensor  200   b  is disposed downstream of the fixing unit  14 . When the conveyance sensor  200   b  is pushed up by the recording material to the position indicated by dotted line in the drawing, the conveyance sensor  200   b  determines that the recording material is being properly conveyed. Then, if a flapper  15  is at the position indicated by solid line in the drawing, the recording material is guided to a discharge roller pair (discharge portion)  16  disposed to the upper left of the fixing unit  14 . If the flapper  15  is at the position indicated by broken line in the drawing, the recording material is guided to a switchback roller pair (reversing rollers, reversing portion)  17  disposed above the discharge roller pair  16 . The switchback roller pair  17  is a roller pair that reverses the recording material in such a manner that instead of the leading edge, the trailing edge of the recording material that has passed through the fixing unit  14  is conveyed. The recording material conveyed to the discharge roller pair  16  is discharged out of the apparatus, or more specifically, onto a recording-material supporting portion (recording-material discharge tray)  18 . 
     (Duplex Conveying Unit) 
     The recording material conveyed to the switchback roller pair  17  is reversed by the switchback roller pair  17  and conveyed to a duplex conveying path (discharge conveying path)  19 . The duplex conveying path  19  first extends from the switchback roller pair  17  toward the lower right in the drawing. Next, the duplex conveying path  19  is curved to avoid the fixing unit  14 , and then extends toward the bottom of the image forming apparatus  1 . As illustrated in  FIG. 2A , an opening and closing member  20  that allows the duplex conveying path  19  to be exposed is disposed on the right-hand side of the image forming apparatus  1 . The opening and closing member  20  pivots about a hinge  20   a  of the image forming apparatus  1 . When the opening and closing member  20  is closed as indicated by solid line, the inner surface of the opening and closing member  20  also serves as the discharge conveying path  19 . To allow the duplex conveying path  19  to be exposed, the opening and closing member  20  is opened to the position indicated by dotted line. A conveyance sensor  200   c  (see  FIG. 1 ) is disposed in the duplex conveying path  19 . When the conveyance sensor  200   c  is pushed down by the recording material to the position indicated by dotted line, the conveyance sensor  200   c  determines that the recording material is being properly conveyed. The recording material that has passed through the duplex conveying path  19  is guided to the registration rollers  5  again, passed through the secondary transfer portion  13  and the fixing unit  14 , conveyed to the discharge roller pair  16 , and discharged onto the recording-material supporting portion  18 . 
     (Image Reading Unit) 
     An image reading unit  100  that reads an image on a document will now be described. The image reading unit  100  is disposed inside the image forming apparatus  1  and located above the fixing unit  14 . The image reading unit  100  includes a sensor unit (image sensor)  101  that reads an image on a document, a document feeding portion  102 , registration rollers  103 , and a document discharge roller pair (document discharge portion)  104 . Documents placed at the document feeding portion  102  are conveyed one by one to the registration rollers  103 . While the document is being conveyed from the registration rollers  103 , an image on the document is read by the sensor unit  101 . After the image is read, the document is discharged by the document discharge roller pair  104  to a document supporting portion  105 . Image data read by the sensor unit  101  is either stored as electronic data in a computer on a network, or converted to print data by a control unit (not shown). By using this image data, the image forming unit  6  forms an image on the recording material. 
       FIGS. 3 and 4  are a cross-sectional view and a perspective view, respectively, of the sensor unit  101 . As illustrated in  FIG. 3 , the sensor unit  101  includes a reading surface  201  made of a transparent material, such as glass, a holder  202 , a housing  203 , a light-emitting diode (LED) array  204  with LEDs arranged in a sheet width direction, a photodetector  205 , and a rod lens  206 . The holder  202  is a resin component that holds the LED array  204 , the photodetector  205 , and the rod lens  206 . The housing  203  is made of aluminum. The document is conveyed with the read surface thereof being in contact with the reading surface  201 . As illustrated in  FIG. 4 , the reading surface  201  is longer than a width Ws of a maximum-size document that can be conveyed by the image reading unit  100  (W&gt;Ws). Light emitted from the LED array  204  is reflected off the document surface and converged by the rod lens  206  onto the photodetector  205 . By converting the received light into voltage, the photodetector  205  converts document surface information into an electric signal to generate image data. 
     (Layout of Sensor Unit  101 ) 
     The layout of the sensor unit  101  in the image forming apparatus  1  will now be described in detail. As described above, the duplex conveying path  19  is formed in such a manner as to avoid the fixing unit  14 , particularly the pressure roller  14   b . Therefore, an area which serves no function in an SFP is created in a region below a horizontal plane (P) defined by the upper end of the reversing rollers  17 , above a horizontal plane (Q) defined by the lower end of the pressure roller  14   b , and outside the duplex conveying path  19 . Specifically, this is the region diagonally shaded in  FIG. 2B . This diagonally shaded region in  FIG. 2B  is also surrounded by a vertical plane (R) defined by the hinge  20   a  (about which the opening and closing member  20  pivots). In the image forming apparatus  1  of the present embodiment, the image reading unit  100  is positioned in such a manner that the sensor unit  101  is at least partially disposed in this diagonally shaded region. 
     With this layout, an image forming apparatus can be provided, which is compact even though it includes the image reading unit  100  therein. Moreover, since all reading functions are gathered in the space above the image forming unit  6 , the functional separation achieved as a result can provide good usability. 
     Second Embodiment 
     A second embodiment will now be described using  FIG. 5 . The description of the same components as those of the first embodiment will be omitted.  FIG. 5  is an enlarged view of an image reading unit  110  according to the present embodiment. A sensor unit  111  has the same configuration as the sensor unit  101  of the first embodiment. In the present embodiment, a document feeding portion  112  is disposed closer to the reversing rollers  17  than the sensor unit  111  is. This means that a document is conveyed from left to right in  FIG. 5 . In addition to the sensor unit  111  and the document feeding portion  112 , the image reading unit  110  includes registration rollers  113  and a document discharge roller pair  114 . 
     As in the first embodiment, the sensor unit  111  of the present embodiment is at least partially disposed in the region below the horizontal plane (P) defined by the upper end of the reversing rollers  17 , above the horizontal plane (Q) defined by the lower end of the pressure roller  14   b , and outside the duplex conveying path  19 . 
     In the present embodiment, the recording-material discharge tray  18  onto which a recording material with an image formed thereon by the image forming unit  6  is discharged and a document supporting portion (document discharge tray)  115  onto which a document read by the image reading unit  110  is discharged are disposed at different positions in the horizontal direction. Additionally, a direction in which a recording material with an image formed thereon by the image forming unit  6  is discharged from the discharge roller pair  16  is opposite a direction in which a document is discharged from the document discharge roller pair  114 . This can prevent the user from confusing the document with the recording material and provide better usability. 
     Third Embodiment 
     A third embodiment will now be described using  FIGS. 6 and 7 . The description of the same components as those of the first and second embodiments will be omitted.  FIG. 6  is an enlarged view of an image reading unit  120  according to the present embodiment.  FIG. 7  is an enlarged view of sensor units  121   a  and  121   b.    
     A document fed from a document feeding portion  122  to document registration rollers  123  is conveyed between the sensor units  121   a  and  121   b  disposed to face the respective surfaces of the document, and images on both surfaces of the document are read. Although the image reading unit  120  of the present embodiment includes the two sensor units  121   a  and  121   b  for reading images on both surfaces of a document, a compact apparatus size can be achieved as long as at least one of the two sensor units  121   a  and  121   b  is at least partially disposed in the region specified in the first embodiment. 
     An image forming apparatus which offers high productivity in reading images on recording materials and documents will now be described. 
     Fourth Embodiment 
     An image forming apparatus according to a fourth embodiment of the present invention will now be described using  FIGS. 8 and 9 .  FIG. 9  is a partial enlarged view of  FIG. 8 . Although the image forming apparatus according to the fourth or any subsequent embodiment is a full-color laser beam printer including a plurality of photosensitive drums, it may be a monochrome image forming apparatus including only one photosensitive drum. 
     (Feeding Unit) 
       FIG. 8  is a cross-sectional view of the image forming apparatus  1 . The cassette  2  that holds recording materials therein is mounted at the bottom of the image forming apparatus  1  in such a manner that it can be pulled out. The cassette feeding portion  3  is disposed near an end portion of the cassette  2 . The manual feeding portion  4  is disposed on the right-hand side of the image forming apparatus  1 . The cassette feeding portion  3  and the manual feeding portion  4  are both configured to feed recording materials one by one to the registration rollers  5 . The conveyance sensor  200   a  that detects passage of a recording material is disposed downstream of the registration rollers  5  in the recording-material conveying direction. 
     (Image Forming Unit and Fixing Unit) 
     The image forming apparatus  1  includes the image forming unit  6 . The image forming unit  6  of the present embodiment includes the four image forming stations  6 Y,  6 M,  6 C, and  6 K of yellow, magenta, cyan, and black. The image forming unit  6  also includes the photosensitive members  7 Y,  7 M,  7 C, and  7 K each serving as an image bearing member, and the chargers  8 Y,  8 M,  8 C, and  8 K each configured to uniformly charge the surface of a corresponding one of the photosensitive members  7 Y,  7 M,  7 C, and  7 K. The image forming unit  6  further includes the scanner unit  9  that emits laser beams on the basis of image information to form electrostatic latent images on the photosensitive members  7 Y,  7 M,  7 C, and  7 K, and the developing units  10 Y,  10 M,  10 C, and  10 K that cause toners to adhere to the electrostatic latent images to develop the electrostatic latent images. The image forming unit  6  further includes the primary transfer portions  12 Y,  12 M,  12 C, and  12 K that transfer the toner images on the photosensitive members  7 Y,  7 M,  7 C, and  7 K to the electrostatic transfer belt  11 . The toner images on the electrostatic transfer belt  11  are transferred onto a front surface (first surface) of a recording material at the secondary transfer portion  13 . The resulting toner image on the recording material is fixed by the fixing unit  14 . The fixing unit  14  includes the fixing roller (first rotary member)  14   a  that has an internal heat source therein, and the pressure roller (second rotary member)  14   b  that forms a fixing nip with the fixing roller  14   a  to convey the recording material therebetween. The recording material is heated at the fixing nip while being conveyed. An unfixed image on the recording material is thus fixed to the recording material. Of the fixing roller (first rotary member)  14   a  and the pressure roller (second rotary member)  14   b , only the fixing roller (first rotary member)  14   a  comes into contact with the unfixed image. 
     (Discharge Portion and Reversing Portion) 
     As illustrated in  FIG. 9 , the conveyance sensor  200   b  is disposed downstream of the fixing unit  14 . When the conveyance sensor  200   b  is pushed up by the recording material to the position indicated by dotted line in the drawing, the conveyance sensor  200   b  determines that the recording material is being properly conveyed. Then, if a first flapper  15   a  is at the position indicated by solid line in the drawing, the recording material is guided to the discharge roller pair (discharge portion)  16  disposed to the upper left of the fixing unit  14  (see  FIG. 10 ). If the first flapper  15   a  is at the position indicated by broken line in the drawing, the recording material is guided to the switchback roller pair (reversing portion)  17  disposed above the discharge roller pair  16 . The switchback roller pair  17  is a roller pair that reverses the recording material in such a manner that instead of the leading edge, the trailing edge of the recording material that has passed through the fixing unit  14  is conveyed. The discharge roller pair  16  is a roller pair that discharges a recording material having a fixed image thereon to the outside of the apparatus. The recording material conveyed to the discharge roller pair  16  is discharged onto the recording-material supporting portion (recording-material discharge tray)  18 . The discharge conveying path  19  guides the recording material from the fixing unit  14  to the discharge portion  16 . 
     (Image Reading Unit) 
     The image reading unit  100  that reads an image on a document will now be described. The image reading unit  100  is disposed inside the image forming apparatus  1  and located above the fixing unit  14 . The image reading unit  100  includes sensor units (image sensors)  101   a  and  101   b  that read an image on a document, a document feeding tray  1102  that holds documents thereon, and a document feeding portion  1103  that feeds and conveys documents. The image reading unit  100  further includes document registration rollers  1104  that convey the document toward the sensor units  101   a  and  101   b , the document discharge roller pair (document discharge portion)  104  that discharges the document after image reading, and a document discharge tray  106  that holds documents after image reading. The configuration of the sensor units  101   a  and  101   b  is not described, as it is the same as that illustrated in  FIG. 3 . 
     (Document Feeding and Discharge Operation, Document Reading Operation) 
     A document feeding and discharge operation, and an operation of reading an image on a document, will now be described. A second flapper  15   b , a third flapper  15   c , and the image reading unit  100  are disposed in the upper right part of the image forming apparatus  1 . 
     For reading an image on a document, the second flapper  15   b  and the third flapper  15   c  are each moved to the position indicated by solid line in the drawing to form a document conveying path  107  which conveys the document from the document registration rollers  1104  to the document discharge roller pair  104  (see  FIG. 11 ). Then, the document feeding portion  1103  conveys documents one by one from the document feeding tray  1102  to the document registration rollers  1104 . The document is further conveyed by the document registration rollers  1104  to the sensor unit  101   a , which reads an image on the document. 
     The sensor units  101   a  and  101   b  are disposed inside the image forming apparatus  1  in such a manner that their surfaces where image reading takes place (see the reading surface  201  in  FIG. 3 ) face each other, with the document conveying path  107  interposed therebetween. The sensor units  101   a  and  101   b  may each be a contact image sensor (CIS), such as that illustrated in  FIG. 3 . With the two sensor units  101   a  and  101   b  disposed to face each other, images on both surfaces of the document, or images printed on both surfaces of a recording material (described below), conveyed between the sensor units  101   a  and  101   b  can be read in a single pass. When there is no need to read images on both surfaces of the document or recording material at the same time, only one of the sensor units  101   a  and  101   b  may be provided. In this case, the sensor unit  101   a  is removed, with the sensor unit  101   b  left in place. 
     After image reading, the document is conveyed by the document discharge roller pair  104  and discharged onto the document discharge tray  106 . Image data read by the sensor unit  101   a  is either stored as electronic data in a computer on a network, or converted to print data by the control unit (not shown) and printed on a recording material. 
     The document feeding tray  1102  is disposed above the manual feeding portion  4  (see  FIG. 8 ), and the document discharge tray  106  is disposed above the recording-material supporting portion  18 . The direction in which the user accesses the apparatus to load (or place) documents or recording materials is the same as the direction in which the user accesses the apparatus to collect discharged documents or recording materials. This makes it easier for the user to use the apparatus. 
     (Printing on Back Surface of Recording Material) 
     For printing on the back surface (second surface) of the recording material, the first flapper  15   a  is moved to the position indicated by broken line, and the second flapper  15   b  is moved to the position indicated by solid line. This forms a first conveying path  21  that conveys (guides) the recording material from a first branch point  40  in the discharge conveying path  19  to the switchback roller pair  17 . The recording material is thus conveyed from the fixing unit  14  to the switchback roller pair  17 . Then, by rotating the switchback roller pair  17  in such a manner that the the recording material is conveyed leftward in  FIG. 9 , the recording material is conveyed until the trailing edge of the recording material passes through the tip of the second flapper  15   b . By then moving the second flapper  15   b  to the position indicated by broken line, a second conveying path  22  is formed, which conveys (guides) the recording material from the switchback roller pair  17  to the sensor units  101   a  and  101   b . The third flapper  15   c  is then moved to the position indicated by broken line. The switchback roller pair  17  is stopped and rotated in such a manner that the recording material is conveyed rightward in  FIG. 9 . The recording material is thus conveyed through the second conveying path  22  and the sensor units  101   a  and  101   b  toward a duplex conveying path  27 . The second flapper  15   b  and the third flapper  15   c  are moved when a predetermined time period elapses after the trailing edge of the recording material passes through the conveyance sensor  200   b . The conveyance sensor  200   c  is disposed in the duplex conveying path  27  (see  FIG. 8 ). With the same mechanism as the conveyance sensor  200   a , the conveyance sensor  200   c  determines whether the recording material is being properly conveyed.  FIG. 12  illustrates how a recording material flows during printing on the back surface. 
     (Refeeding) 
     The recording material conveyed to the duplex conveying path  27  passes through a duplex conveying roller pair  25  and a roller pair  26  (see  FIG. 8 ) and is conveyed to the registration rollers  5  again. The recording material is then further conveyed through the secondary transfer portion  13 , the fixing unit  14 , and the discharge conveying path  19  (see  FIG. 12 ) to the discharge roller pair  16  and discharged onto the recording-material supporting portion  18 . 
     (Reading of Image (Printed Image) on Recording Material) 
     For reading a toner image fixed on a recording material by the fixing unit  14 , the recording material is conveyed exactly in the same process as in the case of printing on the back surface of the recording material (see  FIG. 12 ). The image on the recording material is read by the sensor unit  101   b  as the recording material passes through the sensor unit  101   b . The read image information is subjected to processing, such as recording or feature extraction, by an image information processing unit (not shown). By using information obtained by such processing, evaluation of a printed image or adjustment of image forming conditions can be made. The read image information may be stored in an electronic recording unit, such as an internal memory (not shown) of the apparatus or a computer (not shown) on a network. 
     In the image forming apparatus  1  of the present embodiment, the duplex conveying path  27  is partially a common conveying path  27 P (see  FIG. 9 ) shared with the document conveying path  107 . The sensor unit (reading unit)  101   b  is disposed on a side of the common conveying path  27 P facing one of two surfaces of the recording material reversed by the reversing portion  17 , the one being a surface that has more recently come into contact with the first rotary member  14   a . The sensor unit  101   b  has the capability of reading a fixed image on the recording material and reading an image on a document. An image forming apparatus can thus be provided, which offers high productivity in reading images on recording materials and documents. 
     In the image forming apparatus  1  of the present embodiment, the document conveying direction (leftward in  FIG. 9 ) in which a document fed from the document feeding portion  1103  is conveyed for reading an image on the document, is opposite the recording-material conveying direction (rightward in  FIG. 9 ) in which a recording material is conveyed for reading a fixed image on the recording material. Images on the document and the recording material conveyed in the opposite directions are read by the sensor units  101   a  and  101   b . Thus, the feeding areas for feeding the recording material and the document are both on the right-hand side in  FIG. 8 , and the discharge areas for discharging the recording material and the document are both on the left-hand side in  FIG. 8 . With this configuration, it is possible to achieve both good user accessibility and high productivity in reading images on recording materials and the documents. 
     Fifth Embodiment 
     A printer of a fifth embodiment is the same as that of the fourth embodiment, except the process of printing on the back surface of the recording material and the process of reading a printed image on the recording material. The description of the same components as those of the fourth embodiment will be omitted. On the basis of  FIG. 13 , characteristic differences between the fourth and fifth embodiments will be described in terms of the process of printing on the back surface of the recording material and the process of reading a printed image on the recording material. The same components as those of the fourth embodiment are denoted by the same reference numerals. 
     (Printing on Back Surface of Recording Material) 
     For printing on the back surface of the recording material, the first flapper  15   a  is moved to the position indicated by broken line in the drawing, and the second flapper  15   b  is moved to the position indicated by solid line in the drawing, so that the recording material is conveyed from the fixing unit  14  to the switchback roller pair  17 . Then, by rotating the switchback roller pair  17  in such a manner that the recording material is conveyed leftward in  FIG. 13 , the recording material is conveyed until the trailing edge of the recording material passes through a conveying guide end  28 . By then moving the first flapper  15   a  to the position indicated by solid line, with the second flapper  15   b  kept at the position indicated by solid line, a conveying path (duplex conveying path)  23  is formed, which conveys the recording material from the switchback roller pair  17  to the duplex conveying path  27 . The conveying path  23  is disposed inside the image forming apparatus  1  in such a manner that it is located between the fixing unit  14  and the sensor units  101   a  and  101   b  and joins the duplex conveying path  27 . Then, by stopping the switchback roller pair  17  and rotating it to convey the recording material rightward in  FIG. 13 , the recording material is conveyed through the conveying path  23  toward the duplex conveying path  27 . 
     (Reading of Image (Printed Image) on Recording Material) 
     For reading a toner image fixed on a recording material by the fixing unit  14 , the first flapper  15   a  is moved to the position indicated by broken line and the second flapper  15   b  is moved to the position indicated by solid line, so that the recording material is conveyed from the fixing unit  14  to the switchback roller pair  17 . Then, by rotating the switchback roller pair  17  to move the recording material leftward in  FIG. 13 , the recording material is conveyed until the trailing edge of the recording material passes through the tip of the second flapper  15   b . The second flapper  15   b  and the third flapper  15   c  each are then further moved to the position indicated by broken line. This forms the conveying path  22  that conveys the recording material from the switchback roller pair  17  to the sensor units  101   a  and  101   b , and a conveying path  24  that conveys the recording material from the sensor units  101   a  and  101   b  to the duplex conveying path  27 . The second flapper  15   b  and the third flapper  15   c  are moved with the same timing as in the first embodiment. Then, by stopping the switchback roller pair  17  and rotating it to move the recording material rightward in  FIG. 13 , the recording material is conveyed through the conveying path  22 , the sensor units  101   a  and  101   b , and the conveying path  24  toward the duplex conveying path  27 . The sensor unit  101   b  reads an image printed on the recording material during the conveyance. The operation that follows will not be described, as it is the same as that described in the first embodiment. 
     In the present embodiment, the sensor unit (reading unit)  101   b  is disposed in the document conveying path  107 . The conveying path  22  that connects the switchback roller pair (reversing portion)  17  to the document conveying path  107  and the conveying path  24  that connects the document conveying path  107  to the duplex conveying path  27  are provided, and the sensor unit  101   b  has the capability of reading a fixed image on the recording material and an image on the document. An image forming apparatus can thus be provided, which offers high productivity in reading images on recording materials and documents. Since the document conveying path  107  and the duplex conveying path  23  are separate, reading of an image on the document and conveyance of the recording material along the duplex conveying path  23  for printing on the second surface of the recording material can be carried out at the same time. 
     Sixth Embodiment 
     For reading an image on a recording material in a printer of a sixth embodiment illustrated in  FIG. 14 , a document discharge roller pair  30  serves as a reversing portion (i.e., switchback roller pair  17  in the fifth embodiment) that revers the recording material. For forming an image on the second surface of the recording material, the switchback roller pair  17  reverses the recording material. The recording material that has passed through the fixing unit  14  is guided through a conveying path  22 E to the document conveying path  107  (or to the document discharge roller pair  30 ). The other operations are the same as those in the fifth embodiment and their description will be omitted. With this configuration, an image forming apparatus can be provided, which offers high productivity in reading images on recording materials and documents. Again, since the document conveying path  107  and the duplex conveying path  23  are separate, reading of an image on the document and conveyance of the recording material along the duplex conveying path  23  for printing on the second surface of the recording material can be carried out at the same time. 
     The duplex conveying path  23  is formed in such a manner as to avoid the fixing unit  14 , particularly the pressure roller  14   b . Therefore, an area which serves no function in an SFP is created in the region below the horizontal plane (P) defined by the upper end of the reversing rollers  17 , above the horizontal plane (Q) defined by the lower end of the pressure roller  14   b , and outside the duplex conveying path  23 . Specifically, this is the region diagonally shaded in  FIG. 14 . This diagonally shaded region is also surrounded by the vertical plane (R) defined by the hinge  20   a  (about which the opening and closing member  20  pivots). In the image forming apparatus  1  of the present embodiment, as in the first to third embodiments, the image reading unit  100  is positioned in such a manner that the sensor unit  101  is at least partially disposed in this diagonally shaded region. This layout of the sensor unit  101  is adopted in other embodiments, as well as in the first to third embodiments and the sixth embodiment. 
     Seventh Embodiment 
     In a printer of a seventh embodiment illustrated in  FIG. 15 , a discharge roller pair (switchback roller pair, discharge reversing portion)  31  that discharges a recording material onto the recording-material discharge tray  18  serves as a switchback roller pair. For reading an image on the recording material, the discharge roller pair  31  serves as a reversing portion (i.e., switchback roller pair  17  in the fourth embodiment) that reverses the recording material. For forming an image on the second surface of the recording material, the switchback roller pair  31  also reverses the recording material. A conveying path  22 F conveys the recording material from the discharge roller pair (switchback roller pair)  31  to the sensor units  101   a  and  101   b  (or to the document conveying path  107 ). The other operations are the same as those in the fourth embodiment and their description will be omitted. With this configuration, an image forming apparatus can be provided, which offers high productivity in reading images on recording materials and documents. The present embodiment requires a smaller number of conveying roller pairs and thus can simplify the configuration of conveying paths. 
     Eighth Embodiment 
     In a printer of an eighth embodiment illustrated in  FIG. 16 , the discharge roller pair  31  that discharges a recording material onto the recording-material discharge tray  18  also serves as a switchback roller pair. For reading an image on the recording material, the discharge roller pair  31  serves as a reversing portion (i.e., switchback roller pair  17  in the fifth embodiment) that reverses the recording material. For forming an image on the second surface of the recording material, the switchback roller pair  31  also reverses the recording material. A conveying path  22 G conveys the recording material from the discharge roller pair (switchback roller pair)  31  to the sensor units  101   a  and  101   b  (or to the document conveying path  107 ). The other operations are the same as those in the fifth embodiment and their description will be omitted. With this configuration, an image forming apparatus can be provided, which offers high productivity in reading images on recording materials and documents. The present embodiment requires a smaller number of conveying roller pairs and thus can simplify the configuration of conveying paths. Also, since the document conveying path  107  and the duplex conveying path  23  are separate, reading of an image on the document and conveyance of the recording material along the duplex conveying path  23  for printing on the second surface of the recording material can be carried out at the same time. 
     Ninth Embodiment 
     For reading an image on a recording material in a printer of a ninth embodiment illustrated in  FIG. 17 , a document discharge roller pair  32  serves as a reversing portion (i.e., switchback roller pair  17  in the sixth embodiment) that revers the recording material. For forming an image on the second surface of the recording material, the document discharge roller pair  32  also reverses the recording material. The recording material that has passed through the fixing unit  14  is guided through a conveying path  22 H to the document conveying path  107  (or to the document discharge roller pair  32 ). The other operations are the same as those in the sixth embodiment and their description will be omitted. With this configuration, an image forming apparatus can be provided, which offers high productivity in reading images on recording materials and documents. The present embodiment requires a smaller number of conveying roller pairs and thus can simplify the configuration of conveying paths. 
     Tenth Embodiment 
     A printer of a tenth embodiment illustrated in  FIG. 18  differs from the printer of the ninth embodiment in that it includes the duplex conveying path  23 . For forming an image on the second surface of the recording material, the document discharge roller pair  32  reverses the recording material and guides it to the duplex conveying path  23 . The recording material that has passed through the fixing unit  14  is guided through the conveying path  22 H to the document conveying path  107  (or to the document discharge roller pair  32 ). The other operations are the same as those in the ninth embodiment and their description will be omitted. With this configuration, an image forming apparatus can be provided, which offers high productivity in reading images on recording materials and documents. The present embodiment requires a smaller number of conveying roller pairs and thus can simplify the configuration of conveying paths. Also, since the document conveying path  107  and the duplex conveying path  23  are separate, reading of an image on the document and conveyance of the recording material along the duplex conveying path  23  for printing on the second surface of the recording material can be carried out at the same time. 
     Other exemplary image forming apparatuses will now be described, which can reduce transfer of heat from the fixing unit  14  to the image reading unit  100  and requires less time for image reading. 
     Eleventh Embodiment 
     An image forming apparatus according to an eleventh embodiment of the present invention will be described using  FIGS. 19 and 20 .  FIG. 20  is a partial enlarged view of  FIG. 19 . Although the image forming apparatus according to the eleventh or any subsequent embodiment is a full-color laser beam printer including a plurality of photosensitive drums, it may be a monochrome image forming apparatus including only one photosensitive drum. 
     (Feeding Unit) 
       FIG. 19  is a cross-sectional view of the image forming apparatus  1 . The cassette  2  that holds recording materials therein is mounted at the bottom of the image forming apparatus  1  in such a manner that it can be pulled out. The cassette feeding portion  3  is disposed near an end portion of the cassette  2 . The manual feeding portion  4  is disposed on the right-hand side of the image forming apparatus  1 . The cassette feeding portion  3  and the manual feeding portion  4  are both configured to feed recording materials one by one to the registration rollers  5 . The conveyance sensor  200   a  that detects passage of a recording material is disposed downstream of the registration rollers  5  in the recording-material conveying direction. 
     (Conveyance Sensor) 
     The conveyance sensor  200   a  will now be described using  FIG. 25 . The conveyance sensor  200   a  includes a sensor arm  221  and a detecting portion  224 . The sensor arm  221  is disposed to protrude across a conveying path  223  and pivotally supported about a shaft  222 . The detecting portion  224  is configured to detect the motion of the sensor arm  221 . The sensor arm  221  is retained by a biasing means (not shown), such as a spring, at the position indicated by solid line. When a recording material  225  moves in the direction of solid arrow in  FIG. 25  and reaches the conveyance sensor  200   a , the sensor arm  221  is pushed upward by the recording material  225  to the position indicated by broken line in  FIG. 25 . The detecting portion  224  detects the motion of the sensor arm  221  and transmits, to a control means (not shown), a signal indicating that the recording material  225  has reached the conveyance sensor  200   a . When the recording material  225  further moves and an edge  226  of the recording material  225  on the downstream side in the conveying direction leaves the sensor arm  221 , then the sensor arm  221  moves to the position indicated by solid line in  FIG. 25 . An edge of the recording material  225  facing the downstream side in the conveying direction is defined as a leading edge, and the other edge of the recording material  225  facing the upstream side in the conveying direction is defined as a trailing edge. The detecting portion  224  detects the motion of the sensor arm  221  and transmits, to the control means (not shown), a signal indicating that the recording material  225  has passed through the conveyance sensor  200   a . The conveyance sensors  200   b  and  200   c  (described below) have the same structure as the conveyance sensor  200   a.    
     (Image Forming Unit and Fixing Unit) 
     The image forming apparatus  1  includes the image forming unit  6 . The image forming unit  6  of the present embodiment includes the four image forming stations  6 Y,  6 M,  6 C, and  6 K of yellow, magenta, cyan, and black. The image forming unit  6  also includes the photosensitive members  7 Y,  7 M,  7 C, and  7 K each serving as an image bearing member, and the chargers  8 Y,  8 M,  8 C, and  8 K each configured to uniformly charge the surface of a corresponding one of the photosensitive members  7 Y,  7 M,  7 C, and  7 K. The image forming unit  6  further includes the scanner unit  9  that emits laser beams on the basis of image information to form electrostatic latent images on the photosensitive members  7 Y,  7 M,  7 C, and  7 K, and the developing units  10 Y,  10 M,  10 C, and  10 K that cause toners to adhere to the electrostatic latent images to develop the electrostatic latent images. The image forming unit  6  further includes the primary transfer portions  12 Y,  12 M,  12 C, and  12 K that transfer the toner images on the photosensitive members  7 Y,  7 M,  7 C, and  7 K to the electrostatic transfer belt  11 . The toner images on the electrostatic transfer belt  11  are transferred onto a front surface (first surface) of a recording material at the secondary transfer portion  13 . The resulting toner image on the recording material is fixed by the fixing unit  14 . The fixing unit  14  includes the fixing roller (first rotary member)  14   a  that has an internal heat source therein, and the pressure roller (second rotary member)  14   b  that forms a fixing nip with the fixing roller  14   a  to convey the recording material therebetween. The recording material is heated at the fixing nip while being conveyed. An unfixed image on the recording material is thus fixed to the recording material. Of the fixing roller (first rotary member)  14   a  and the pressure roller (second rotary member)  14   b , only the fixing roller (first rotary member)  14   a  comes into contact with the unfixed image. 
     (Discharge Portion and Reversing Portion) 
     The conveyance sensor  200   b  is disposed downstream of the fixing unit  14 . When the conveyance sensor  200   b  is pushed up by the recording material to the position indicated by dotted line in the drawing, the conveyance sensor  200   b  determines that the recording material is being properly conveyed. Then, if the first flapper  15   a  is at the position indicated by solid line in the drawing, the recording material is guided to the discharge roller pair (discharge portion)  16  disposed to the upper left of the fixing unit  14 . If the first flapper  15   a  is at the position indicated by broken line in the drawing, the recording material is guided to the switchback roller pair (reversing portion)  17  disposed above the discharge roller pair  16 . The switchback roller pair  17  is a roller pair that reverses the recording material in such a manner that instead of the leading edge, the trailing edge of the recording material that has passed through the fixing unit  14  is conveyed. The discharge roller pair  16  is a roller pair that discharges a recording material having a fixed image thereon to the outside of the apparatus. The recording material conveyed to the discharge roller pair  16  is discharged onto the recording-material supporting portion (recording-material discharge tray)  18 . The discharge conveying path  19  guides the recording material from the fixing unit  14  to the discharge portion  16 . 
     (Image Reading Unit) 
     The image reading unit  100  that reads an image on a recording material will now be described. The image reading unit  100  is disposed inside the image forming apparatus  1  and located above the fixing unit  14 . The image reading unit  100  includes the sensor units (image sensors)  101   a  and  101   b  that read an image on a recording material. The configuration of the sensor units  101   a  and  101   b  is the same as that illustrated in  FIG. 3  and its description will be omitted. 
     (Printing on Back Surface of Recording Material) 
     For printing on the back surface (second surface) of the recording material, the first flapper  15   a  is moved to the position indicated by broken line, and the second flapper  15   b  is moved to the position indicated by solid line. This forms the first conveying path  21  that guides the recording material from the first branch point  40  in the discharge conveying path  19  to the switchback roller pair  17 . The recording material is thus conveyed from the fixing unit  14  to the switchback roller pair  17 . Then, by rotating the switchback roller pair  17  in such a manner that the recording material is conveyed leftward in  FIG. 20 , the recording material is conveyed until the trailing edge of the recording material passes through the conveying guide end  28  illustrated in  FIG. 20 . By then moving the first flapper  15   a  to the position indicated by solid line, the second conveying path  22  is formed, which guides the recording material from the switchback roller pair  17  to the duplex conveying path  27 . The second conveying path  22  is a conveying path that guides the recording material reversed by the switchback roller pair  17  to the duplex conveying path  27 . The first flapper  15   a  and the second flapper  15   b  are moved when a predetermined time period elapses after the trailing edge of the recording material passes through the conveyance sensor  200   b . By then stopping the switchback roller pair  17  and rotating it to convey the recording material rightward in  FIG. 20 , the recording material is conveyed through the second conveying path  22  toward the duplex conveying path  27 . The second conveying path  22  is formed to pass above the fixing unit  14 . The conveyance sensor  200   c  (see  FIG. 19 ) is disposed in the duplex conveying path  27 . With the same mechanism as the conveyance sensor  200   a , the conveyance sensor  200   c  determines whether the recording material is being properly conveyed. The recording material conveyed to the duplex conveying path  27  passes through the duplex conveying roller pair  25  and the roller pair  26  (see  FIG. 19 ) and is conveyed to the registration rollers  5  again. The recording material is then further conveyed through the secondary transfer portion  13 , the fixing unit  14 , and the discharge conveying path  19  to the discharge roller pair  16  and discharged onto the recording-material supporting portion  18 . 
     (Reading of Image (Printed Image) on Recording Material) 
     When the image reading unit  100  reads a toner image fixed on the recording material by the fixing unit  14 , the recording material is conveyed through the first conveying path  21  to the switchback roller pair  17 . Then, by rotating the switchback roller pair  17  in such a manner that the recording material is conveyed leftward in  FIG. 20 , the recording material is conveyed until the trailing edge of the recording material passes through a point  29  (see  FIG. 20 ) on the conveying guide. By then moving the first flapper  15   a  to the position indicated by solid line and moving the second flapper  15   b  to the position indicated by broken line, the third conveying path  23  is formed, which conveys the recording material from the switchback roller pair  17  to the image reading unit  100 . 
     The sensor units  101   a  and  101   b  of the image reading unit  100  are disposed inside the image forming apparatus  1  and above the second conveying path  22 , with their surfaces where image reading takes place (see the reading surface  201  in  FIG. 3 ) facing each other. The sensor units  101   a  and  101   b  may each be a contact image sensor (CIS). With the two sensor units  101   a  and  101   b  disposed to face each other, images printed on both surfaces of the recording material conveyed between the sensor units  101   a  and  101   b  can be read at the same time. When there is no need to read images on both surfaces of the recording material at the same time, only one of the sensor units  101   a  and  101   b  may be provided. In this case, the sensor unit  101   a  is removed, with the sensor unit  101   b  left in place. The sensor unit  101   b  is disposed to face one of two surfaces of the recording material reversed by the reversing portion  17 , the one being a surface that has more recently come into contact with the first rotary member  14   a . With at least the sensor unit  101   b  of the two sensor units  101   a  and  101   b , it is possible to reduce the time between when the recording material passes through the fixing unit  14  and when the sensor unit  101   b  reads an image. The time required for image reading can thus be reduced. 
     The first flapper  15   a  and the second flapper  15   b  are moved when a predetermined time period elapses after the trailing edge of the recording material passes through a conveyance sensor  200   d  disposed between the second flapper  15   b  and the switchback roller pair  17 . The conveyance sensor  200   d  has the same mechanism as the conveyance sensor  200   a  described above. Then, by stopping the switchback roller pair  17  and rotating it to move the recording material rightward in  FIG. 20 , the recording material is conveyed through the third conveying path  23 , the image reading unit  100 , and the fourth conveying path  24  to the duplex conveying path  27 . The fourth conveying path  24  extends above the second conveying path  22 . The image reading unit  100  reads an image on the recording material being conveyed. The read image information is subjected to processing, such as recording or feature extraction, by an image information processing unit (not shown). By using information obtained by such processing, evaluation of a printed image or adjustment of image forming conditions can be made. The read image information may be stored in an electronic recording unit, such as an internal memory (not shown) of the apparatus or a computer (not shown) on a network. 
     In the image forming apparatus  1  of the present embodiment, the second conveying path  22  is disposed between the fixing unit  14  and the image reading unit  100 . In other words, the second conveying path  22  is disposed between the fixing unit  14  and the third and fourth conveying paths  23  and  24 . With this configuration, the transfer of heat generated in the fixing unit  14  is partially blocked by air in the second conveying path  22 . This can reduce temperature rise in the image reading unit  100 . 
     Of the two surfaces of the recording material, a surface on which an image is first formed in duplex printing is defined as a front surface and the other surface opposite the front surface is defined as a back surface. In the image forming apparatus  1  of the present embodiment, the front and back orientation of the recording material coming from the reversing portion  17  and passing through the third conveying path  23 , the image reading unit  100 , the fourth conveying path  24 , and the duplex conveying path  27  is the same as the front and back orientation of the recording material coming from the reversing portion  17  and passing through the second conveying path  22  and the duplex conveying path  27 . Specifically, of the two surfaces of the recording material passing through the second conveying path  22 , a surface that has more recently come into contact with the first rotary member  14   a  is a surface facing downward in  FIG. 20 . Also, of the two surfaces of the recording material passing through the third and fourth conveying paths  23  and  24 , a surface that has more recently come into contact with the first rotary member  14   a  is a surface facing downward in  FIG. 20 . That is, the recording material coming from the reversing portion  17  and passing through the third conveying path  23 , the image reading unit  100 , the fourth conveying path  24 , and the duplex conveying path  27  does not need to be reversed before being conveyed to the image forming unit  6  (see  FIG. 19 ) again. Therefore, as in the case of the recording material conveyed through the second conveying path  22 , an image can be formed on the second surface promptly after image formation on the first surface and the time required for reading images on both surfaces can also be reduced. 
     As described above, the image forming apparatus  1  of the present embodiment can reduce transfer of heat from the fixing unit  14  to the image reading unit  100  and requires less time for image reading. 
     Twelfth Embodiment 
     An image forming apparatus according to a twelfth embodiment will be described using  FIGS. 21 and 22 . The description of the same components as those of the eleventh embodiment will be omitted. 
     In the image forming apparatus  1  of the present embodiment, the discharge roller pair  16  and the switchback roller pair  17  described in the eleventh embodiment are replaced by the discharge roller pair (discharge reversing portion)  31 . 
     When a recording material having a toner image fixed thereon by the fixing unit  14  is discharged out of the apparatus, the first flapper  15   a  and the second flapper  15   b  are each moved to the position indicated by solid line in the drawing. This forms the first conveying path  21  that conveys the recording material from the fixing unit  14  to the discharge roller pair  31 . Then, by rotating the discharge roller pair  31  in such a manner that the recording material is conveyed leftward in  FIG. 22 , the recording material is discharged onto the recording-material supporting portion  18 . An additional conveying path may be created, which conveys the recording material discharged from the discharge roller pair  31  to a post-processing device (not shown) disposed near the image forming apparatus  1 , so that post-processing, such as separation, stapling, or sorting, can be performed on the recording material. 
     (Printing on Back Surface of Recording Material) 
     For printing on the back surface of the recording material, the recording material is conveyed from the fixing unit  14  through the first conveying path  21  to the discharge roller pair  31 . Then, by rotating the discharge roller pair  31  in such a manner that the recording material is conveyed leftward in  FIG. 22 , the recording material is conveyed until the trailing edge of the recording material passes through the conveying guide end  28  illustrated in  FIG. 22 . By then moving the first flapper  15   a  to the position indicated by broken line and moving the second flapper  15   b  to the position indicated by solid line, the second conveying path  22  is formed, which guides the recording material from the discharge roller pair  31  to the duplex conveying path  27 . The first flapper  15   a  and the second flapper  15   b  are moved with the same timing as in the eleventh embodiment. Then, by stopping the discharge roller pair  31  and rotating it to move the recording material rightward in  FIG. 22 , the recording material is conveyed through the second conveying path  22  to the duplex conveying path  27 . 
     (Reading of Image (Printed Image) on Recording Material) 
     For reading a toner image fixed on the recording material by the fixing unit  14 , the recording material is conveyed through the first conveying path  21  to the discharge roller pair  31 . Then, by rotating the discharge roller pair  31  in such a manner that the recording material is conveyed leftward in  FIG. 22 , the recording material is conveyed until the trailing edge of the recording material passes through the point  29  (see  FIG. 22 ) on the conveying guide. By then moving the first flapper  15   a  to the position indicated by solid line and moving the second flapper  15   b  to the position indicated by broken line, the third conveying path  23  is formed, which conveys the recording material from the discharge roller pair  31  to the image reading unit  100 . The first flapper  15   a  and the second flapper  15   b  are moved with the same timing as in the eleventh embodiment. Then, by stopping the discharge roller pair  31  and rotating it to move the recording material rightward in  FIG. 22 , the recording material is conveyed through the third conveying path  23 , the image reading unit  100 , and the fourth conveying path  24  to the duplex conveying path  27 . The image reading unit  100  reads an image on the recording material being conveyed. 
     In the image forming apparatus  1  of the present embodiment, as in the eleventh embodiment, the second conveying path  22  is disposed between the fixing unit  14  and the image reading unit  100 . In other words, the second conveying path  22  is disposed between the fixing unit  14  and the third and fourth conveying paths  23  and  24 . With this configuration, the transfer of heat generated in the fixing unit  14  is partially blocked by air in the second conveying path  22 . This can reduce temperature rise in the image reading unit  100 . 
     Of the two surfaces of the recording material, a surface on which an image is first formed in duplex printing is defined as a front surface and the other surface opposite the front surface is defined as a back surface. In the image forming apparatus  1  of the present embodiment, the front and back orientation of the recording material coming from the discharge reversing portion  31  and passing through the third conveying path  23 , the image reading unit  100 , the fourth conveying path  24 , and the duplex conveying path  27  is the same as the front and back orientation of the recording material coming from the discharge reversing portion  31  and passing through the second conveying path  22  and the duplex conveying path  27 . Therefore, as in the case of the recording material conveyed through the second conveying path  22 , an image can be formed on the second surface promptly after image formation on the first surface and the time required for reading images on both surfaces can also be reduced. 
     As described above, the image forming apparatus  1  of the present embodiment can reduce transfer of heat from the fixing unit  14  to the image reading unit  100  and requires less time for image reading. 
     Again, with at least the sensor unit  101   b  of the two sensor units  101   a  and  101   b , it is possible in the present embodiment to reduce the time between when the recording material passes through the fixing unit  14  and when the sensor unit  101   b  reads an image. 
     Thirteenth Embodiment 
     An image forming apparatus according to a thirteenth embodiment will be described using  FIGS. 23 and 24 . The description of the same components as those of the eleventh embodiment will be omitted. 
     The image forming apparatus  1  of the present embodiment is provided with a post-processing device  301  to the upper right thereof. The post-processing device  301  includes a recording-material retracting path  302 , a post-processing discharge roller pair  303 , and a post-processed recording-material supporting portion  304 . The recording-material retracting path  302  is connected to the switchback roller pair  17  of the image forming apparatus  1  and configured to temporarily store the recording material partially discharged from the switchback roller pair  17 . Although the post-processing device  301  of the present embodiment only has the capability of separating and discharging recording materials using the post-processed recording-material supporting portion  304 , the post-processing device  301  may have the capability of stapling or sorting the recording materials. 
     When the recording material with a toner image fixed thereon by the fixing unit  14  is conveyed to the post-processing discharge roller pair  303 , the first flapper  15   a  is moved to the position indicated by solid line in the drawing and a post-processing conveying flapper  44  is moved to the position indicated by broken line in the drawing. This forms a post-processing conveying path  42  (see  FIG. 24 ) which conveys the recording material from a second branch point  41  between the first branch point  40  in the discharge conveying path  19  and the discharge roller pair  16  to a post-processing conveying roller pair  43 . Then, the recording material is conveyed through a post-processing discharge conveying path  305  to the post-processing discharge roller pair  303  and discharged onto the post-processed recording-material supporting portion  304 . A conveyance sensor  200   e  is disposed between the post-processing conveying flapper  44  and the first flapper  15   a . With the same mechanism as the conveyance sensor  200   a  described in the eleventh embodiment, the conveyance sensor  200   e  determines whether the recording material is being properly conveyed. Although the post-processing conveying roller pair  43  is disposed inside the image forming apparatus  1  in the present embodiment, it may be disposed inside the post-processing device  301 . For conveying the recording material to a post-processing device including a plurality of conveying paths other than the recording-material retracting path  302 , there may be provided a plurality of post-processing conveying paths  42 , post-processing conveying roller pairs  43 , and post-processing conveying flappers  44 . 
     The image forming apparatus  1  of the present embodiment includes conveying portions for conveying printed recording materials to the post-processing device  301  which performs post-processing, such as stapling or sorting. With this configuration, it is still possible to reduce transfer of heat from the fixing unit  14  to the image reading unit  100  and reduce time required for image reading. 
     The present invention can provide an image forming apparatus that is compact but includes, in a printer main body, a sensor unit capable of reading an image on a document. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.