Patent Publication Number: US-9897952-B2

Title: Image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of application Ser. No. 14/858,144 filed on Sep. 18, 2015, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD 
     Embodiments described herein relate generally to an image forming apparatus. 
     BACKGROUND 
     An image forming apparatus is known which conveys a sheet-shaped recording medium (hereinafter collectively referred to as ‘sheet’) while forming an image on the sheet. The image forming apparatus comprises an image forming unit for forming a visible image (toner image) with a toner and a fixer for fixing the toner image on a sheet with heat and pressure. The image forming apparatus comprises a paper discharging tray for overlapping and stacking a plurality of sheets on which a toner image is fixed. However, if the temperature or the stack pressure of the plurality of sheets stacked on the paper discharging tray is increased, then the state of the toner is changed, which may result in the poor quality of images. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram schematically exemplifying the whole structure of an image forming apparatus according to an embodiment; 
         FIG. 2  is an oblique view of an upper paper discharging tray of an image forming apparatus according to an embodiment; 
         FIG. 3  is a sectional view of an upper paper discharging unit of an image forming apparatus according to an embodiment; 
         FIG. 4  is a block diagram exemplifying the functional structure of an image forming apparatus according to an embodiment; 
         FIG. 5  is a flowchart illustrating the flow of a print processing carried out by an image forming apparatus according to an embodiment; 
         FIG. 6  is a flowchart illustrating the flow of a print processing carried out by an image forming apparatus according to an embodiment; 
         FIG. 7  is a diagram exemplifying the variation of the surface temperature of the sheets in an upper paper discharging tray of an image forming apparatus, the output of a paper discharging sensor and the heights of a discharged paper surface temperature sensor and stacked sheets according to an embodiment; 
         FIG. 8  is a sectional view exemplifying the state of a sheet discharged to an upper paper discharging tray of an image forming apparatus, the state of a paper discharging sensor actuator and the state of a discharged paper surface temperature sensor according to an embodiment and a diagram showing a state in which a sheet presses a second end of the paper discharging sensor actuator upward; 
         FIG. 9  is a sectional view exemplifying the state of a sheet discharged to an upper paper discharging tray of an image forming apparatus, the state of a paper discharging sensor actuator and the state of a discharged paper surface temperature sensor according to an embodiment and a diagram showing a state in which a sheet presses a second end of a stacked sheet number sensor actuator upward; 
         FIG. 10  is a sectional view exemplifying the state of a sheet discharged to an upper paper discharging tray of an image forming apparatus, the state of a paper discharging sensor actuator and the state of a discharged paper surface temperature sensor according to an embodiment and a diagram showing a state in which a sheet is separated from a stacked sheet number sensor actuator; 
         FIG. 11  is a sectional view exemplifying the state of a sheet discharged to an upper paper discharging tray of an image forming apparatus, the state of a paper discharging sensor actuator and the state of a discharged paper surface temperature sensor according to an embodiment and a diagram showing a state in which a sheet is stacked in the upper paper discharging tray; 
         FIG. 12  is a diagram exemplifying the variation of the surface temperature of the sheets in an upper paper discharging tray of an image forming apparatus, the output of a paper discharging sensor and the heights of a discharged paper surface temperature sensor and stacked sheets according to an embodiment; 
         FIG. 13  is a sectional view exemplifying the state of a sheet discharged to an upper paper discharging tray of an image forming apparatus, the state of a paper discharging sensor actuator and the state of a discharged paper surface temperature sensor according to an embodiment and a diagram showing a state in which a sheet presses a second end of the paper discharging sensor actuator upward; 
         FIG. 14  is a sectional view exemplifying the state of a sheet discharged to an upper paper discharging tray of an image forming apparatus, the state of a paper discharging sensor actuator and the state of a discharged paper surface temperature sensor according to an embodiment and a diagram showing a state in which a sheet presses a second end of a stacked sheet number sensor actuator upward; 
         FIG. 15  is a sectional view exemplifying the state of a sheet discharged to an upper paper discharging tray of an image forming apparatus, the state of a paper discharging sensor actuator and the state of a discharged paper surface temperature sensor according to an embodiment and a diagram showing a state in which a sheet is to be separated from the paper discharging sensor actuator; and 
         FIG. 16  is a sectional view exemplifying the state of a sheet discharged to an upper paper discharging tray of an image forming apparatus, the state of a paper discharging sensor actuator and the state of a discharged paper surface temperature sensor according to an embodiment and a diagram showing a state in which a sheet is stacked in the upper paper discharging tray. 
     
    
    
     DETAILED DESCRIPTION 
     In accordance with an embodiment, an image forming apparatus comprises a paper feeding unit, an image forming unit, a fixing unit, a stacking unit, a temperature sensor and a control unit. The paper feeding unit feeds a sheet-shaped recording medium. The image forming unit forms a toner image on the recording medium fed from the paper feeding unit. The fixing unit fixes the toner image formed by the image forming unit on the recording medium. The stacking unit stacks the recording medium on which the toner image is fixed by the fixing unit. The temperature sensor detects the surface temperature of the recording mediums stacked on the stacking unit. The control unit determines, according to the printing information of the recording medium, whether or not to feed a new recording medium from the paper feeding unit when the surface temperature of the recording mediums detected by the temperature sensor is above a threshold temperature. 
     An image forming apparatus  100  of the present embodiment is described below with reference to accompanying drawings in which the same parts are denoted by the same reference signs. 
       FIG. 1  is a diagram schematically exemplifying the whole structure of the image forming apparatus  100  according to an embodiment. 
     As shown in  FIG. 1 , the image forming apparatus  100  comprises a control panel  1 , a scanner unit  2 , a printer unit  3 , a sheet accommodation unit  4  and a conveyance unit  5 . 
     The image forming apparatus  100  forms an image on the surface of a sheet S with a developing agent such as a toner. The sheet S is a piece of sheet-shaped paper, for example, paper or label paper. The sheet S may be any object as long as an image can be formed on the surface of the sheet S by the image forming apparatus  100 . 
     The control panel  1  comprises a display section  11  and an operation section  12 . The display section  11  is a display such as a liquid crystal display or an Electro Luminescence (EL) display. The display section  11  displays various kinds of information related to the image forming apparatus  100 . The operation section  12  comprises a plurality of buttons. The operation unit  12  receives the operation of the user on the plurality of buttons. The control panel  1  outputs a signal corresponding to the operation of the user on the operation unit  12  to the under-mentioned control section  101  shown in  FIG. 4 . Further, the display section  11  and the operation section  12  may be integrated into a touch panel. 
     The scanner unit  2  reads the image information of a read object as light intensity. The scanner unit  2  generates image data based on the read image information and stores the image data in the memory section  102  shown in  FIG. 4  which is described later. The image data generated by the scanner unit  2  can be sent to another information processing apparatus via, for example, a network. The image data generated by the scanner unit  2  can be output to, for example, the printer unit  3 . 
     The print unit  3  forms an image on the surface of the sheet S based on the image data generated by the scanner unit or the image data received from the another information processing apparatus via the network. The printer unit  3  forms an image (hereinafter referred to as toner image) with, for example, a toner. The printer unit  3  transfers the toner image onto the surface of the sheet S. The printer unit  3  heats and presses the toner image on the surface of the sheet S to fix the toner image on the sheet S. 
     The sheet accommodation unit  4  is a paper feeding unit for feeding a sheet S. The sheet feeding unit  4  feeds sheets S, one by one, to the printer unit  3  matching in time with the formation of a toner image by the printer unit  3 . The sheet accommodation unit  4  is provided with a plurality of paper cassettes  20 A,  20 B and  20 C. The paper cassettes  20 A,  20 B and  20 C accommodate sheets S of preset sizes and types, respectively. The paper cassettes  20 A,  20 B and  20 C are provided with pickup rollers  21 A,  21 B and  21 C, respectively. The pickup rollers  21 A,  21 B and  21 C pick up sheets S, one by one, from the paper cassettes  20 A,  20 B and  20 C. The pickup rollers  21 A,  21 B and  21 C convey the sheet S picked up to the conveyance unit  5 . 
     The conveyance unit  5  conveys the sheet S in the printer unit  3  and the sheet accommodation unit  4 . The conveyance unit  5  comprises a conveyance roller  23  and a register roller  24 . The conveyance unit  5  conveys the sheets S fed from the pickup rollers  21 A,  21 B and  21 C to the register roller  24 . The register roller  24  conveys the sheet S according to the time at which the under-mentioned transfer section  28  of the printer unit  3  transfers a toner image onto the surface of the sheet S. The conveyance roller  23  collides the front end of the conveyance direction of sheets S with the nip N of the register roller  24 . The conveyance roller  23  adjusts the position of the front end of the sheet S in the conveyance direction by curving the sheet S. After settling the front end of the sheet S conveyed from the conveyance roller  23  in the nip N, the register roller  24  conveys the sheet S to the side of the transfer section  28 . 
     Next, the detailed structure of the printer unit  3  is described below. 
     The printer unit  3  comprises a plurality of image forming sections  25 Y,  25 M,  25 C,  25 K and  25 D, an exposure section  26 , an intermediate transfer belt  27 , a transfer section  28  and a fixing section  29 . 
     The plurality of image forming sections  25 Y,  25 M,  25 C,  25 K and  25 D form toner images which are transferred onto the sheet S later, respectively. Each of the image forming sections  25 Y,  25 M,  25 C,  25 K and  25 D comprises a photoconductive drum (image carrier)  25   a . Each of the image forming sections  25 Y,  25 M,  25 C,  25 K and  25 D comprises a developer  25   b  for selectively feeding a toner to the surface of each photoconductive drum  25   a . The developer  25   b  accommodates non-decolorizable yellow, magenta, cyan and black toners and a decolorizable toner. The decolorizable toner is decolorized at a temperature higher than a specific decolorization temperature. 
     The exposure section  26  is opposite to the photoconductive drums  25   a  of the image forming sections  25 Y,  25 M,  25 C,  25 K and  25 D. The exposure section  2 E irradiates the surface of the photoconductive drum  25   a  of each of the image forming sections  25 Y,  25 M,  25 C,  25 K and  25 D with laser beams based on image data. Through the irradiation of the laser beam, the exposure section  26  forms an electrostatic latent image on the surface of the photoconductive drum  25   a  of each of the image forming sections  25 Y,  25 M,  25 C,  25 K and  25 D. Each developer  25   b  develops the electrostatic latent image on the surface of each photoconductive drum  25   a  by feeding a toner to the electrostatic latent image. Each developer  25   b  forms a toner image by sticking a charged toner to the electrostatic latent image on the surface of each photoconductive drum  25   a . The developer  25   b  of the image forming section  25 Y develops the electrostatic latent image on the surface of the photoconductive drum  25   a  with the yellow toner. The developer  25   b  of the image forming section  25 M develops the electrostatic latent image on the surface of the photoconductive drum  25   a  with the magenta toner. The developer  25   b  of the image forming section  25 C develops the electrostatic latent image on the surface of the photoconductive drum  25   a  with the cyan toner. The developer  25   b  of the image forming section  25 K develops the electrostatic latent image on the surface of the photoconductive drum  25   a  with the black toner. The developer  25   b  of the image forming section  25 D develops the electrostatic latent image on the surface of the photoconductive drum  25   a  with the decolorizable toner. 
     Each of the image forming sections  25 Y,  25 M,  25 C,  25 K and  25 D transfers (primarily transfers) the charged toner image on the surface of each photoconductive drum  25   a  onto the surface of the intermediate transfer belt  27 . The image forming sections  25 Y,  25 M,  25 C,  25 K and  25 D apply a transfer bias to the toner images of the photoconductive drums  25   a  at a primary transfer position, respectively. The image forming sections  25 Y,  25 M,  25 C and  25 K overlap the toner images of different colors on the surfaces of the photoconductive drums  25   a  and transfer the overlapped toner images onto the surface of the intermediate transfer belt  27 . The image forming sections  25 Y,  25 M,  25 C and  25 K overlap the toner images of different colors and transfer the overlapped toner images onto the surface of the intermediate transfer belt  27  to form a color toner image. The image forming section  25 D transfers a decolorizable toner image onto the intermediate transfer belt  27 . 
     The transfer section  28  comprises a support roller  28   a  and a secondary transfer roller  28   b  which clamp the intermediate transfer belt  27  and the sheet S from two sides in the thickness direction. The position where the support roller  28   a  is opposite to the transfer roller  28   b  is a secondary transfer position. The transfer section  28  applies a transfer bias corresponding to a transfer current to a secondary transfer position, thereby transferring the toner image on the surface of the intermediate transfer belt  27  onto the surface of the sheet S. 
     The fixing section  29  comprises a heat roller  29   b  equipped with an internal heating portion  29   a  and a press roller  29   c . The press roller  29   c  is contacted with the heat roller  29   b  heated by the heating portion  29   a  under a pressurized state. The press roller  29   c  and the heat roller  29   b  convey the sheet S while clamping the sheet S from two sides of the thickness direction of the sheet S. The press roller  29   c  and the heat roller  29   b  fix the toner image on the surface of the sheet S on the sheet S by applying heat and pressure to the sheet S. 
     The printer unit  3  comprises a reversing unit  30 . The reversing unit  30  reverses the sheet discharged from the fixing section  29  through a switchback route. The reversing unit  30  conveys the reversed sheet S to the front of the register roller  24  again. The reversing unit  30  reverses the sheet S so as to form an image on the back of the sheet S subjected to the fixation processing. 
       FIG. 2  is an oblique view of an upper paper discharging tray of an image forming apparatus observed from above according to an embodiment.  FIG. 3  is a sectional view of an upper paper discharging unit of an image forming apparatus according to an embodiment. 
     As shown in  FIG. 2  and  FIG. 3 , the image forming apparatus  100  comprises an upper paper discharging section having an upper paper discharging opening  51  and a lower paper discharging section  57  having a lower paper discharging opening  55 . 
     The upper paper discharging section  53  is a stacking section for stacking sheets S. The upper paper discharging section  53  comprises an upper paper discharging roller  61  and an upper paper discharging tray  63 . The upper paper discharging roller  61  conveys a sheet S to the upper paper discharging opening  51 . The upper paper discharging tray stacks the sheet S discharged from the upper paper discharging opening  51 . 
     The upper paper discharging section  53  comprises a stacked sheet number sensor actuator  65 , a discharged paper surface temperature sensor  67 , a paper discharging sensor actuator  69  and a discharged paper cooling fan  71 . The stacked sheet number sensor actuator  65  is formed into the shape of a level which is inclined downward and protruded with respect to the direction from which a sheet S is discharged from the upper paper discharging opening  51 . A first end  65   a  of the stacked sheet number sensor actuator  65  is rotationally supported by a rotation shaft  65   b . The first end  65   a  is formed into a level shape. A stacked sheet number sensor  66  is a transmissive photoelectric sensor. The transmissive photoelectric sensor comprises a light emitting unit and a light receiving unit. If the stacked sheet number sensor actuator  65  rotates around the axis of the rotation shaft  65   b , then the first end  65   a  shields the light between the light emitting unit and the light receiving unit of the stacked sheet number sensor  66 . The stacked sheet number sensor  66  switches between ‘output on’ and ‘output off’ by shielding the light between the light emitting unit and the light receiving unit. The stacked sheet number sensor actuator  65  is limited to rotate within a specific range. The lowest position of a second end  65   c  is located above the upper paper discharging tray  63  and spaced from the upper paper discharging tray  63  by a specific distance. The specific distance refers to the thickness of the most sheets S assumed to be allowed to be stacked with toner images not stuck under specific conditions that are most conductive to the sticking of toner images. The specific conditions refer to an assumed maximum temperature, an assumed maximum print density, double-side printing and the type of the sheet that is most likely to be stuck. The second end  65   c  is located at the lowest position for the sake of the dead weight of the stacked sheet number sensor actuator  65  when the stacked sheet number sensor actuator  65  is not contacted with a sheet S. The second end  65   c  is contacted with the top sheet S in the upper paper discharging tray  63  when the whole thickness of the sheets stacked in the upper paper discharging tray  63  is beyond a specific distance. If the second end  65   c  is lifted up by the top sheet S in the upper paper discharging tray  63 , then the stacked sheet number sensor actuator  65  rotates upwards around the axis of the rotation shaft  65   b . The stacked sheet number sensor actuator  65  is intersected with the direction in which a sheet S is discharged from the upper paper discharging opening  51  and is inclined downwards. If the stacked sheet number sensor actuator  65  collides with the sheet S from the upper paper discharging opening  51 , then the stacked sheet number sensor actuator  65  rotates towards the direction in which the second end  65   c  is lifted up. 
     The discharged paper surface temperature sensor  67  is, for example, a thermistor. The discharged paper surface temperature sensor  67  is arranged on the second end  65   c  of the stacked sheet number sensor actuator  65 . When contacted with the top sheet S in the upper paper discharging tray  63 , the discharged paper surface temperature sensor  67  detects the surface temperature of the sheet S. When not contacted with a sheet S, the discharged paper surface temperature sensor  67  detects atmosphere temperature. 
     The paper discharging sensor actuator  69  is formed into a level shape. The paper discharging sensor actuator  69  is configured to block the conveyance path of sheets S. A first end  69   a  of the paper discharging sensor actuator  69  is rotationally supported by the rotation shaft  69   b . A paper discharging sensor  70  is a transmissive photoelectric sensor. The paper discharging sensor  70  switches between ‘output on’ and ‘output off’ through the shielding of the light of the paper discharging sensor  70  by the first end  69   a  according to the rotation of the paper discharging sensor actuator  69  around the rotation shaft  69   b . When not contacted with a sheet S, the second end  69   c  of the paper discharging sensor actuator  69  is intersected with the conveyance path of sheets S and inclined downwards. If a sheet S being conveyed collides with the paper discharging sensor actuator  69 , then the second end  69   c  is pressed upwards and rotated around the rotation shaft  69   b . If the second end  69   c  of the paper discharging sensor actuator  69  is pressed upwards, then the output of the paper discharging sensor  70  is ‘ON’. After the sheet S passes, the second end  69   c  is rotated downwards for the sake of the dead weight of the paper discharging sensor actuator  69 . 
     The discharged paper cooling fan  71  is arranged above the upper paper discharging opening  51  to cool the sheet S discharged from the upper paper discharging opening  51  with the wind supplied from the outside. 
     The lower paper discharging section  57  comprises a lower paper discharging roller  81  and a lower paper discharging tray  83 . The lower paper discharging roller  81  conveys the sheet S to the lower paper discharging opening  55 . The lower paper discharging tray  83  stacks sheets S discharged from the lower paper discharging opening  55 . 
     The functional structure of the image forming apparatus  100  is described below with reference to  FIG. 4  which is a block diagram exemplifying the functional structure of the image forming apparatus  100 . 
     The control panel  1 , the scanner unit  2  and the printer unit  3  are connected with the control section  101 . The control section  101  uniformly controls the whole action of the image forming apparatus  100 . The control section  101  controls the CPU of each of the control panel  1 , the scanner unit  2  and the printer unit  3 . The control section  101  comprises a CPU, a ROM and a RAM. The ROM stores a control program for controlling the whole action of the image forming apparatus  100 . The RAM temporarily stores the various data that are used by the CPU to carry out various processing. The control section  101  is connected with the memory section  102 , the stacked sheet number sensor  66 , the discharged paper surface temperature sensor  67  and the paper discharging sensor  70 . Particularly, the ROM stores the various thresholds that are used by the CPU to carry out various determination processing. 
     The memory section  102  stores the image data generated by the scanner unit  2  or the image data received from the another information processing apparatus via the network. The memory section  102  is, for example, a hard disk, a semiconductor memory or the like 
     The control panel  1  comprises a display section  11 , an operation section  12  and a panel control section  111 . The panel control section  111  controls the actions of the display section  11  and the operation section  12 . The panel control section  111  comprises a CPU, a ROM and a RAM. The ROM stores a control program for controlling the actions of the display section  11  and the operation section  12 . The RAM temporarily stores the various data that are used by the CPU to carry out various processing. 
     The display section  11  displays various kinds of information that are related to the actions of the image forming apparatus  100  and specified by the user. The display section  11  displays the information input by the user through operating the operation section  12 . The operation section  12  accepts the operation of the user, for example, for the input of information related to the processing separately implemented by the scanner unit  2  and the printer unit  3 . The information related to the processing implemented by the scanner unit  2  is, for example, a resolution, a compression ratio and color information. The information related to the processing implemented by the printer unit  3  is, for example, the number of the sheets S to be printed, a print condition, the size of a sheet S and the type of a sheet S. The print condition is, for example, the specification on black-and-white printing or color printing, the specification on single-side printing or double-side printing, the specification on print density and the orientation of a sheet S in a conveyance direction. The operation section outputs the various kinds of information specified by the user to the panel control section  111 . The panel control section  111  outputs the information from the operation section  12  to the control section  101 . The control section  101  outputs the information from the panel control section  111  to the scanner unit  2  or the printer unit  3 . 
     The scanner unit  2  comprises a scanner control section  121 . The scanner control section  121  controls the reading of image information by a reading section  122 . The scanner control section  121  comprises a CPU, a ROM and a RAM. The ROM stores a control program for controlling the action of the reading section  122 . The RAM temporarily stores the various data that are used by the CPU to carry out various processing. The reading section  122  reads the image information of a read object as light intensity and generates image data according to the image information. The printer unit  3  comprises a printer control section  131 . The printer control section  131  controls the printing of an image on the sheet S by the printer unit  3 . The printer control section  131  comprises a CPU, a ROM and a RAM. The ROM stores control programs for controlling the actions of the plurality of image forming sections  25 Y,  25 M,  25 C,  25 K and  25 D, the exposure section  26 , the transfer section  28  and the fixing section  29 . The RAM temporarily stores the various data that are used by the CPU to carry out various processing. 
     The flow of a print processing carried out by the image forming apparatus  100  is described below with reference to  FIG. 5  and  FIG. 6 . 
       FIG. 5  and  FIG. 6  are flowcharts illustrating a print processing carried out by the image forming apparatus  100  according to an embodiment. 
     First, the user inputs information indicating the execution of a print processing from the operation section  12  of the control panel  1 . The panel control section  111  accepts the processing represented by the information input from the operation section  12  as a print job. The control section  101  writes the print job into an internal RAM while acquiring the print job from the panel control section  111  (Act  01 ). The print job includes information related to the processing separately carried out by the scanner unit  2  and the printer unit  3 . 
     Next, the control section  101  reads information on a print condition for the print job from the internal RAM and determines whether or not the information on the print condition includes the specification on double-side printing (Act  02 ). 
     If the result of the determination is ‘No’ (Act  02 : No), the control section  101  makes the flow proceed to Act  03 . 
     On the other hand, if the result of the determination is ‘Yes’ (Act  02 : Yes), the control section  101  makes the flow proceed to Act  04 . 
     Then, the control section  101  executes a basic print processing (Act  03 ). The basic print processing refers to other print processing, excluding a double-side printing processing, implemented on a sheet S. 
     Sequentially, the control section  101  separately instructs the scanner control section  121  and the printer control section  131  to implement a processing corresponding to the print job (Act  04 ). The control section  101  outputs information related to the processing separately carried out by the scanner control section  121  and the printer control section  131 . The information output to the scanner unit  2  is, for example, a resolution, a compression ratio and color information. The information output to the printer control section  131  is, for example, the number of the sheets S to be printed, a print condition, the size of a sheet S and the type of a sheet S. The print condition includes: the specification on double-side printing, the specification on print density, the orientation of a sheet S in a conveyance direction and other information. The scanner control section  121  and the printer control section  131  separately write the information output from the control section  101  into the internal RAM. 
     The scanner control section  121  reads the information related to the processing instructed to be implemented by the control section  101  into the internal RAM. The scanner control section  121  causes the reading section  122  to read the image information of a read object according to the information acquired from the RAM. The reading section  122  reads a plurality of pieces of image information based on the information related to double-side printing included in the print job. The reading section  122  reads, for example, the image information on two sides of the read object or the image information of a plurality of read objects. The information related to double-side printing is information specifying which side of the sheet S is to be printed with the image information of the read object. The scanner control section  121  acquires the image data generated by the reading section  122  based on the image information (Act  05 ). The scanner control section  121  outputs the image data to the control section  101 . The control section  101  associates the image data from the scanner control section  121  with the first side and the second side of the sheet S based on the information related to double-side printing included in the print job. The control section  101  distinguishes the image data to be printed on the one of the two sides of the sheet S which is printed first (a first side) from that to be printed on a second side, that is, the back side of the first side, and stores the distinguished image data in the memory section  102 . The control section  101  outputs the image data to be printed on the first side and the image data to be printed on the second side during a double-side printing process to the printer control section  131 . The printer control section  131  writes the image data to be printed on the first side and the image data to be printed on the second side received from the control section  101  into the internal RAM. 
     Sequentially, the control section  101  instructs the sheet accommodation unit  4  and the conveyance unit  5  to start the conveyance of a sheet S to the printer unit  3  (Act  06 ). The control section  101  specifies one of the paper cassettes  20 A,  20 B and  20 C based on the sheet size and the sheet type information included in the print job. The sheet accommodation unit  4  picks up sheets S one by one from the specified one of the paper cassettes  20 A,  20 B and  20 C according to an instruction from the control section  101  and feeds the sheets S to the conveyance unit  5 . The conveyance unit  5  conveys the sheet S from the sheet accommodation unit  4  to the transfer section  28  matching in time with the transfer of the toner image on the intermediate transfer belt  27  to the sheet S by the transfer section  28 . 
     Then, the printer control section  131  instructs the plurality of image forming sections  25 Y,  25 M,  25 C,  25 K and  25 D, the exposure section  26  and the transfer section  28  to implement the action on the first side (Act  07 ). First, the exposure section  26  reads the image data corresponding to the first side from the RAM and forms an electrostatic latent image on the surface of the photoconductive drum  25   a  with the image data corresponding to the first side. Then, at least one of the image forming sections  25 Y,  25 M,  25 C,  25 K and  25 D develops the electrostatic latent image on the photoconductive drum  25   a  with a toner to form a toner image. Sequentially, at least one of the image forming sections  25 Y,  25 M,  25 C,  25 K and  25 D transfers the toner image on the photoconductive drum  25   a  onto the surface of the intermediate transfer belt  27 . Afterwards, the transfer section  28  transfers the toner image on the surface of the intermediate transfer belt  27  onto the sheet S conveyed from the conveyance section  5 . 
     Then, the printer control section  131  instructs the fixing section  29  to perform a fixation processing on the first side of the sheet S (Act  08 ). The press roller  29   c  is contacted with the heat roller  29   b  heated by the heating portion  29   a  under a pressurized state, thereby forming a nip. The press roller  29   c  rotates to drive the heat roller  29   b  to rotate. The press roller  29   c  and the heat roller  29   b  fix the toner image by heating and pressurizing the sheet S from the transfer section  28  in the nip. 
     Then, the printer control section  131  instructs the reversing section  30  to reverse and convey the sheet S (Act  09 ). The reversing unit  30  reverses the sheet S discharged from the fixing section  29  and conveys the reversed sheet S to the front of the register roller  24  again. 
     Next, the printer control section  131  instructs the plurality of image forming sections  25 Y,  25 M,  25 C,  25 K and  25 D, the exposure section  26  and the transfer section  28  to implement the action on the second side (Act  10 ). First, the exposure section  26  reads the image data corresponding to the second side from the RAM and forms an electrostatic latent image on the surface of the photoconductive drum  25   a  with the image data corresponding to the second side. Then, at least one of the image forming sections  25 Y,  25 M,  25 C,  25 K and  25 D develops the electrostatic latent image on the photoconductive drum  25   a  with a toner to form a toner image. Sequentially, at least one of the image forming sections  25 Y,  25 M,  25 C,  25 K and  25 D transfers the electrostatic latent image on the photoconductive drum  25   a  onto the surface of the intermediate transfer belt  27 . Afterwards, the transfer section  28  transfers the toner image on the surface of the intermediate transfer belt  27  onto the sheet S conveyed from the conveyance section  5 . 
     Then, the printer control section  131  instructs the fixing section  29  to perform a fixation processing on the second side of the sheet S (Act  11 ). The press roller  29   c  is contacted with the heat roller  29   b  heated by the heating portion  29   a  under a pressurized state, thereby forming a nip. The press roller  46  rotates to drive the heat roller  29   b  to rotate. The press roller  29   c  and the heat roller  29   b  fix the toner image by heating and pressurizing the sheet S from the transfer section  28  in the nip. The printer control section  131  counts, based on the increment of a counter, the number of the accumulated sheets the first side and the second side of which are subjected to the fixation processing. The printer control section  131  writes the count value of the counter into the internal RAM. 
     Sequentially, the control section  101  acquires an ‘on’ or ‘off’ signal output from the paper discharging sensor  70 . The control section  101  writes the signal received from the paper discharging sensor  70  into the internal RAM. The control section  101  determines whether or not the signal from the paper discharging sensor  70  is an ‘on’ signal representing the contact of a sheet S with the paper discharging sensor actuator  69  (Act  12 ). 
     If the result of the determination is ‘No’ (Act  12 : No), the control section  101  implements the determination processing of Act  12  repeatedly. 
     On the other hand, if the result of the determination is ‘Yes’ (Act  12 : Yes), the control section  101  makes the flow proceed to Act  13 . 
     Next, the control section  101  acquires the temperature detected by the discharged paper surface temperature sensor  67  at this moment. The control section  101  writes the temperature information acquired by the discharged paper surface temperature sensor  67  into the internal RAM. When contacted with the top sheet S stacked in the upper paper discharging tray  63 , the discharged paper surface temperature sensor  67  detects the surface temperature of the sheet S. When not contacted with a sheet S, the discharged paper surface temperature sensor  67  detects atmosphere temperature (Act  13 ). 
     Then, the control section  101  acquires, from the internal ROM, a specific temperature threshold corresponding to the temperature detected by the discharged paper surface temperature sensor  67 . The internal ROM of the control section  101  stores the specific temperature threshold corresponding to the temperature detected by the discharged paper surface temperature sensor  67 . The specific temperature threshold is a minimal temperature threshold at which the sheet S stacked in the upper paper discharging tray  63  is assumed to suffer the sticking of toner images. The specific temperature threshold for estimating the temperature difference between the surface temperature of the top sheet S and stacked overlapped sheets has a specific surplus. The specific temperature threshold is surplus for continuing the printing of the sheet S being printed and stacking the sheet S being printed in the upper paper discharging tray  63  even if a new print operation is determined to be stopped. The specific temperature threshold is, for example, 60 degrees centigrade. The control section  101  determines whether or not the temperature detected by the discharged paper surface temperature sensor  67  is above the specific temperature threshold (Act  14 ). 
     If the result of the determination is ‘No’ (Act  14 : No), the control section  101  makes the flow proceed to Act  24 . 
     On the other hand, if the result of the determination is ‘Yes’ (Act  14 : Yes), the control section  101  makes the flow proceed to Act  15 . 
     Then, the control section  101  reads information on a print condition for the print job from the internal RAM and determines whether or not the information on the print condition includes the specification on color printing (Act  15 ). The information specifying color printing included in the print condition is printing information of the sheet S which is used to determine whether or not to feed a sheet S from the sheet accommodation unit  4  newly. If the result of the determination is ‘No’ (Act  15 : No), the control section  101  makes the flow proceed to Act  16 . 
     On the other hand, if the result of the determination is ‘Yes’ (Act  15 : Yes), the control section  101  makes the flow proceed to Act  17 . 
     Then, the control section  101  acquires specific print density information from the internal ROM, wherein the specific print density information is a threshold corresponding to the print density information included in the print condition for the print job. The internal ROM of the control section  101  stores a specific print, density corresponding to the print density included in the print condition. The specific print density is a minimal print density at which the sheet S stacked in the upper paper discharging tray  63  suffers the sticking of toner images. The control section  101  determines whether or not the print density included in the print condition for the print job is above the specific print density (Act  16 ). The print density information included in the print condition is printing information of the sheet S which is used to determine whether or not to feed a sheet from the sheet accommodation unit  4  newly. 
     If the result of the determination is ‘No’ (Act  16 : No), the control section  101  ends the processing and stops the printer unit  3 . 
     On the other hand, if the result of the determination is ‘Yes’ (Act  16 : Yes), the control section  101  makes the flow proceed to Act  17 . 
     Then, the control section  101  determines whether or not a print job is being implemented, whether or not a print job is being implemented means that whether or not one of an image formation operation, a transfer operation, a fixation operation and a paper feed operation is being implemented (Act  17 ). 
     If the result of the determination is ‘No’ (Act  17 : No), the control section  101  makes the flow proceed to Act  18 . 
     On the other hand, if the result of the determination is ‘Yes’ (Act  17 : Yes), the control section  101  makes the flow proceed to Act  19 . 
     Then, the control section  101  stops the feed of a new sheet S from the sheet accommodation unit  4  to the printer unit  3  and the new print operation of the printer unit  3  (Act  18 ). 
     Sequentially, the control section  101  stops the feed of a new sheet S from the sheet accommodation unit  4  to the printer unit  3  (Act  19 ). 
     Then, the control section  101  instructs the printer control section  131  to continue to print on the sheet S which is already fed from the sheet accommodation unit  4  to the printer unit  3  (Act  20 ). The printer control section  131  continues to perform an image formation processing, a transfer processing, a fixation processing and other processing on the sheet S according to an instruction from the control section  101 . 
     Next, the control section  101  discharges the printed sheet S discharged from the fixing section  29  to the upper paper discharging tray  63  through the upper paper discharging roller  61  (Act  21 ). 
     Afterwards, the control section  101  acquires the temperature detected by the discharged paper surface temperature sensor  67  at this moment. The control section  101  writes the temperature information acquired by the discharged paper surface temperature sensor  67  into the internal RAM. The control section  101  acquires, from the internal ROM, the specific temperature threshold with respect to the temperature detected by the discharged paper surface temperature sensor  67 . The control section  101  determines whether or not the temperature detected by the discharged paper surface temperature sensor  67  is below the specific temperature threshold (Act  22 ). 
     If the result of the determination is ‘No’ (Act  22 : No), the control section  101  repeats the determination processing of Act  22 . 
     On the other hand, if the result of the determination is ‘Yes’ (Act  22 : Yes), the control section  101  makes the flow proceed to Act  23 . 
     Then, the control section  101  instructs the sheet accommodation unit  4  to feed a new sheet S to the printer unit  3  and the printer unit  3  to start a new print operation again (Act  23 ). 
     Next, the printer control section  131  reads ‘number of copies’ of sheets S from the internal RAM and the count value of the counter. The printer control section  131  determines whether or not the count value of the counter written in the internal RAM in Act  11  reaches the ‘number of copies’ (Act  24 ). 
     If the result of the determination is ‘No’ (Act  24 : No), the printer control section  131  makes the flow return to Act  07 . 
     On the other hand, if the result of the determination is ‘Yes’ (Act  24 : Yes), the printer control section  131  ends the processing and stops the printer unit  3 . 
     The actions carried out by the image forming apparatus  100  in a case where a small number of sheets S are stacked in the upper paper discharging tray  63  and the discharged paper surface temperature sensor  67  is not contacted with a sheet S. 
       FIG. 7  is a diagram exemplifying the variation of the surface temperature of the sheets S in the upper paper discharging tray  63 , the output of the paper discharging sensor  70  and the heights of the discharged paper surface temperature sensor  67  and stacked sheets.  FIG. 8  is a sectional view exemplifying the state of a sheet S discharged to the upper paper discharging tray of an image forming apparatus, the state of the paper discharging sensor actuator  69  and the state of the discharged paper surface temperature sensor  67  according to an embodiment and a diagram showing a state in which a sheet S presses the second end  69   c  of the paper discharging sensor actuator  69  upward.  FIG. 9  is a sectional view exemplifying the state of a sheet S discharged to the upper paper discharging tray of an image forming apparatus, the state of the paper discharging sensor actuator  69  and the state of the discharged paper surface temperature sensor  67  according to an embodiment and a diagram showing a state in which a sheet S presses the second end  65   c  of the stacked sheet number sensor actuator upward.  FIG. 10  is a sectional view exemplifying the state of a sheet S discharged to the upper paper discharging tray of an image forming apparatus, the state of the paper discharging sensor actuator  69  and the state of the discharged paper surface temperature sensor  67  according to an embodiment and a diagram showing a state in which a sheet S is separated from the stacked sheet number sensor actuator  65 .  FIG. 11  is a sectional view exemplifying the state of a sheet S discharged to the upper paper discharging tray of an image forming apparatus, the state of the paper discharging sensor actuator  69  and the state of the discharged paper surface temperature sensor  67  according to an embodiment and a diagram showing a state in which a sheet S is stacked in the upper paper discharging tray  63 . 
     The paper discharging sensor  70  carries out no detection for a sheet S prior to the moment t 1  shown in  FIG. 7 . The discharged paper surface temperature sensor  67  detects atmosphere temperature. The height Ha of the sheets S stacked in the upper paper discharging tray  63  is smaller than the height Hb of the discharged paper surface temperature sensor  67 . 
     The sheet S which is just fixed by and discharged from the fixing section  29  is discharged from the upper paper discharging opening  51  at the moment t 1  shown in  FIG. 7 , as shown in  FIG. 8 . The sheet S presses the second end  69   c  of the paper discharging sensor actuator  69  upwards. The paper discharging sensor  70  outputs an ‘on’ signal representing the existence of a sheet. The discharged paper cooling fan  71  supplies air for the sheet S discharged from the upper paper discharging opening  51 . The temperature of the sheet S just fixed is, for example, about 80 degrees centigrade. As the sheet S from the upper paper discharging opening  51  is not contacted with the stacked sheet number sensor actuator  65 , no external force is applied to the stacked sheet number sensor actuator  65 . The second end  65   c  of the stacked sheet number sensor actuator  65  descends for the sake of the dead weight thereof and stops at the lowest position in an action range. A proper gap is kept between the discharged paper surface temperature sensor  67  and the sheets S stacked in the upper paper discharging tray  63 . The discharged paper surface temperature sensor  67  is not contacted with the top sheet S in the upper paper discharging tray  63 . The discharged paper surface temperature sensor  67  detects atmosphere temperature Ta. The atmosphere temperature Ta is, for example, about 35 degrees centigrade. As the temperature detected by the discharged paper surface temperature sensor  67  is below the specific temperature threshold, the control section  101  continues to feed a new sheet S to the printer unit  3  and continues the action of the printer unit  3 . 
     The sheet S from the upper paper discharging opening  51  presses the stacked sheet number sensor actuator  65  upwards at the moment t 2  shown in  FIG. 7 , as shown in  FIG. 9 . The sheet S pressing the stacked sheet number sensor actuator  65  upwards is contacted with the discharged paper surface temperature sensor  67 . The height He of the discharged paper surface temperature sensor  67  is increased after being contacted with the sheet S when compared with the height Hb. Because of the cooling of the discharged paper cooling fan  71 , the temperature of the sheet S contacted with the discharged paper surface temperature sensor  67  drops from the temperature of the sheet just fixed. The temperature of the sheet S contacted with the discharged paper surface temperature sensor  67  is, for example, about 70 degrees centigrade. If the sheet S is contacted with the discharged paper surface temperature sensor  67 , then the temperature detected by the discharged paper surface temperature sensor  67  is gradually increased from the atmosphere temperature Ta to the temperature Tb of the sheet S. 
     The sheet S is conveyed and separated from the paper discharging sensor actuator  69  from the moment t 3  to the moment t 4  shown in  FIG. 7 , as shown in  FIG. 10 . The second end  69   c  of the paper discharging sensor actuator  69  is rotated downwards for the sake of the dead weight thereof. The paper discharging sensor  70  outputs an ‘off’ signal representing the absence of a sheet. If the rear end of the sheet S is discharged from the upper paper discharging opening  51 , then the sheet S falls towards the upper paper discharging tray  63 . The second end  65   c  of the stacked sheet number sensor actuator  65  descends for the sake of the dead weight thereof and stops at the lowest position in an action range. The temperature detected by the discharged paper surface temperature sensor  67  is gradually reduced from the temperature Tb of the sheet S to the atmosphere temperature Ta. 
     The sheet S from the upper paper discharging opening  51  is stacked in the upper paper discharging tray  63  from the moment t 4  to the moment t 5  shown in  FIG. 7 , as shown in  FIG. 11 . The second end  69   c  of the paper discharging sensor actuator  69  stops at the lowest position in the action range. The paper discharging sensor  70  outputs an ‘off’ signal representing the absence of a sheet. The second end  65   c  of the stacked sheet number sensor actuator stops at the lowest position in an action range. A proper gap is kept between the discharged paper surface temperature sensor  67  and the sheets S stacked in the upper paper discharging tray  63 . The discharged paper surface temperature sensor  67  is not contacted with the top sheet S in the upper paper discharging tray  63 . The discharged paper surface temperature sensor  67  detects atmosphere temperature Ta. Through the cooling of the discharged paper cooling fan  71 , the temperature of the sheets S stacked in the upper paper discharging tray  63  is gradually reduced. 
     The sheet S which is just fixed by the fixing section  29  is discharged from the upper paper discharging opening  51  at the moment t 5  shown in  FIG. 7 , as shown in  FIG. 8 . The sheet S presses the second end  69   c  of the paper discharging sensor actuator  69  upwards. The paper discharging sensor  70  outputs an ‘on’ signal representing the existence of a sheet. A proper gap is kept between the discharged paper surface temperature sensor  67  and the sheets S stacked in the upper paper discharging tray  63 . The discharged paper surface temperature sensor  67  is not contacted with the top sheet S in the upper paper discharging tray  63 . The discharged paper surface temperature sensor  67  detects atmosphere temperature Ta. The atmosphere temperature is, for example, about 35 degrees centigrade. As the temperature detected by the discharged paper surface temperature sensor  67  is below the specific temperature threshold, the control section  101  continues to feed a new sheet S to the printer unit  3  and continues the action of the printer unit  3 . 
     The actions carried out by the image forming apparatus  100  in a case where a great number of sheets S are stacked in the upper paper discharging tray  63  and the discharged paper surface temperature sensor  67  is contacted with a sheet S. 
       FIG. 12  is a diagram exemplifying the variation of the surface temperature of the sheets in the upper paper discharging tray  63 , the output of the paper discharging sensor  70  and the heights of the discharged paper surface temperature sensor  67  and stacked sheets S.  FIG. 13  is a sectional view exemplifying the state of a sheet S discharged to the upper paper discharging tray of an image forming apparatus, the state of the paper discharging sensor actuator  69  and the state of the discharged paper surface temperature sensor  67  according to an embodiment and a diagram showing a state in which a sheet S presses the second end  69   c  of the paper discharging sensor actuator  69  upward.  FIG. 14  is a sectional view exemplifying the state of a sheet S discharged to the upper paper discharging tray of an image forming apparatus, the state of the paper discharging sensor actuator  69  and the state of the discharged paper surface temperature sensor  67  according to an embodiment and a diagram showing a state in which a sheet S presses the second end  65   c  of the stacked sheet number sensor actuator upward.  FIG. 15  is a sectional view exemplifying the state of a sheet S discharged to the upper paper discharging tray of an image forming apparatus, the state of the paper discharging sensor actuator  69  and the state of the discharged paper surface temperature sensor  67  according to an embodiment and a diagram showing a state in which a sheet S is to be separated from the paper discharging sensor actuator  69 .  FIG. 16  is a sectional view exemplifying the state of a sheet S discharged to the upper paper discharging tray of an image forming apparatus, the state of the paper discharging sensor actuator  69  and the state of the discharged paper surface temperature sensor  67  according to an embodiment and a diagram showing a state in which a sheet S is stacked in the upper paper discharging tray  63 . 
     The paper discharging sensor  70  carries out no detection for a sheet S prior to the moment t 1  shown in  FIG. 12 . The height Hd of sheets S stacked in the upper paper discharging tray  63  is higher than the lowest position of the second end  65   c  of the stacked sheet number sensor actuator  65 . The second end  65   c  of the stacked sheet number sensor actuator  65  is lifted upwards by the sheet S in the upper paper discharging tray  63 . For the upper paper discharging tray  63 , the height Hd of the discharged paper surface temperature sensor  67  is equal to the height Hd of the stacked sheets S. The discharged paper surface temperature sensor  67  is contacted with the top sheet S in the upper paper discharging tray  63 . The discharged paper surface temperature sensor  67  detects the surface temperature Tc of the top sheet S in the upper paper discharging tray  63 . The surface temperature of the top sheet S in the upper paper discharging tray  63  is gradually reduced through natural heat dissipation. The surface temperature of the sheet S detected by the discharged paper surface temperature sensor  67  is higher than the atmosphere temperature Ta. The surface temperature of the sheets S is, for example, about 59 degrees centigrade. 
     The sheet S which is just fixed by the fixing section  29  is discharged from the upper paper discharging opening  51  at the moment t 1  shown in  FIG. 12 , as shown in  FIG. 13 . The sheet S presses the second end  69   c  of the paper discharging sensor actuator  69  upwards. The paper discharging sensor  70  outputs an ‘on’ signal representing the existence of a sheet. The discharged paper cooling fan  71  supplies air for the sheet S discharged from the upper paper discharging opening  51 . The temperature of the sheet S just fixed is, for example, about 80 degrees centigrade. The sheet S from the upper paper discharging opening  51  is not contacted with the stacked sheet number sensor actuator  65 . The discharged paper surface temperature sensor  67  detects the surface temperature of the top sheet S in the upper paper discharging tray  63 . As the temperature detected by the discharged paper surface temperature sensor  67  is below the specific temperature threshold, the control section  101  continues to feed a new sheet S to the printer unit  3  and continues the action of the printer unit  3 . 
     The sheet S from the upper paper discharging opening  51  presses the stacked sheet number sensor actuator  65  upwards at the moment t 2  shown in  FIG. 12 , as shown in  FIG. 14 . The sheet S pressing the stacked sheet number sensor actuator  65  upwards is contacted with the discharged paper surface temperature sensor  67 . The height He of the discharged paper surface temperature sensor  67  is increased after being contacted with the sheet S. Through the cooling of the discharged paper cooling fan  71 , the temperature of the sheet S contacted with the discharged paper surface temperature sensor  67  drops from the temperature of the sheet S just fixed. The temperature of the sheet S contacted with the discharged paper surface temperature sensor  67  is, for example, about 70 degrees centigrade. The temperature detected by the discharged paper surface temperature sensor  67  is gradually increased from the surface temperature of the sheets S in the upper paper discharging tray  63  to the temperature Tb of the sheet S from the upper paper discharging opening  51 . 
     The sheet S is conveyed and separated from the paper discharging sensor actuator  69  from the moment t 3  to the moment t 4  shown in  FIG. 12 , as shown in  FIG. 15 . The second end  69   c  of the paper discharging sensor actuator  69  is rotated downwards for the sake of the dead weight thereof. The paper discharging sensor  70  outputs an ‘off’ signal representing the absence of a sheet. After the rear end of a sheet S is discharged from the upper paper discharging opening  51 , then the sheet S falls towards the upper paper discharging tray  63 . The second end  65   c  of the stacked sheet number sensor actuator  65  stops at the lowest position in an action range for the sake of the dead weight thereof. Cooled by the discharged paper cooling fan  71  for the sheet S discharged from the upper paper discharging opening  51 , the temperature detected by the discharged paper surface temperature sensor  67  is gradually reduced from the temperature Tb. 
     The sheet S from the upper paper discharging opening  51  is stacked in the upper paper discharging tray  63  from the moment t 4  to the moment t 5  shown in  FIG. 12 , as shown in  FIG. 16 . The paper discharging sensor  70  outputs an ‘off’ signal representing the absence of a sheet. Before stopping at the lowest position in an action range, the second end  69   c  of the paper discharging sensor actuator  69  stops at the height Hd of the sheets stacked in the upper paper discharging tray  63 . For the upper paper discharging tray  63 , the height Hd of the discharged paper surface temperature sensor  67  is equal to the height Hd of the stacked sheets S. The discharged paper surface temperature sensor  67  is contacted with the top sheet S in the upper paper discharging tray  63 . The discharged paper surface temperature sensor  67  detects the surface temperature of the top sheet S in the upper paper discharging tray  63 . The surface temperature of the top sheet S in the upper paper discharging tray  63  is gradually reduced through the cooling of the discharged paper cooling fan  71 . 
     The sheet S which is just fixed by the fixing section  29  is discharged from the upper paper discharging opening  51  at the moment t 5  shown in  FIG. 12 , as shown in  FIG. 13 . The sheet S presses the second end  69   c  of the paper discharging sensor actuator  69  upwards. The paper discharging sensor  70  outputs an ‘on’ signal representing the existence of a sheet. The discharged paper surface temperature sensor  67  is contacted with the top sheet S in the upper paper discharging tray  63 . The discharged paper surface temperature sensor  67  detects the surface temperature of the top sheet S in the upper paper discharging tray  63 . Because of the heat of the newly stacked sheet S, the surface temperature of the sheet S is higher than the surface temperature presented at the moment t 1 . The surface temperature of the sheets S is, for example, about 61 degrees centigrade. As the temperature detected by the discharged paper surface temperature sensor  67  is above the specific temperature threshold, the control section  101  determines whether or not color printing is specified in the print job. If color printing is specified in the print job, the control section  101  stops the feed of a new sheet S to the printer unit  3  and the new printing operation of the printer unit  3 . If color printing is not specified in the print job, the control section  101  determines whether or not the print density included in the print condition for the print job is above the specific print density. If the print density included in the print condition for the print job is above the specific print density, the control section  101  stops the feed of a new sheet S to the printer unit  3  and the new printing operation of the printer unit  3 . 
     Provided with the control section  101  capable of determining whether or not to implement a new printing action and feed a new sheet, the image forming apparatus  100  described in the foregoing embodiments is capable of properly preventing the occurrence of sticking. The control section  101  determines whether or not to implement a new printing action and feed a new sheet according to the surface temperature of stacked sheets S, thus preventing a printing action and the feed of a sheet from being interrupted because the surface temperature is below a temperature threshold. The control section  101  determines whether or not to implement a new print action and feed a sheet according to the content of the print job, thus preventing the occurrence of sticking from being unpredictable and the print job from being interrupted. A new print job and the feed of a new sheet can be properly interrupted after the occurrence of sticking is predicted during a print job. In double-side printing involving the contact of toners of stacked sheets S, the occurrence of sticking can be prevented in the case of a color printing realized by overlapping toners of a plurality of toners or in a case where the print density is above a specific print density. 
     Because of the control section  101  which is capable of determining, for each sheet S, whether or not to implement a new print action and whether or not to feed the sheet S, the occurrence of sticking can be properly prevented. Even if heat is increased because of a newly stacked sheet S and heat dissipation is deteriorated as more sheets are stacked, the surface temperature of the sheets S is still below the temperature threshold. As the control section  101  can continue the current print action, the printing interruption caused by an inappropriate condition is avoided. 
     Because of the discharged paper surface temperature sensor  67  which detects the surface temperature of stacked sheets S every time the discharging of a sheet S is stacked, temperature can be detected at proper time. 
     Because of the discharged paper surface temperature sensor  67  arranged on existing stacked sheet number sensor actuator  65 , the apparatus is reduced in cost and simplified in structure. 
     Because of the control section  101  which releases an interrupted state when the surface temperature of the sheets S is below the temperature threshold after the implementation of a new print action and the feed of a sheet are interrupted, the interruption can be prevented from being surplus. 
     Variations of the foregoing embodiments are described below. 
     In the foregoing embodiments, the printer unit  3  implements double-side printing with the image data generated by the scanner unit  2 , however, the present invention is not limited to this. 
     In a variation of the present invention, the printer unit  3  may implement double-side printing with the image data received from another information processing apparatus via a network. 
     In the image forming apparatus  100  of the foregoing embodiment, the discharged paper surface temperature sensor  67  is a contact type temperature sensor; however, the present invention is not limited to this. 
     In an image forming apparatus  100  involved in a variation of the foregoing embodiment, the discharged paper surface temperature sensor  67  may be a non-contact type temperature sensor. 
     In accordance with at least one of the foregoing embodiments, the control section  101  is capable of determining whether or not to implement a new printing action and the feed of a new sheet, thus preventing the occurrence of sticking properly. As the control section  101  is capable of determining whether or not to implement a new printing action and the feed of a new sheet according to the surface temperature of stacked sheets S, the interruption of a printing action and the feed of a new sheet occurring when the surface temperature is below a temperature threshold is prevented. The control section  101  determines whether or not to implement a new print action and the feed of a new sheet according to the content of a print job, thus preventing the occurrence of sticking from being unpredictable and the print job from being interrupted. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the invention. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the invention. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the invention.