JP6439711B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an electrophotographic image forming apparatus.

  Generally, an electrophotographic image forming apparatus includes a transfer unit that transfers a toner image to a sheet, and a fixing unit that fixes the toner image to the sheet. In the fixing unit, the sheet passes through a nip portion between a heating roller and a pressure roller, and a heater heats the toner image on the sheet via the heating roller. The heating roller may be referred to as a fixing roller.

  The sheet may be curved so as to warp toward the heating roller due to heat received from the heating roller. This phenomenon is referred to as curling of the sheet.

  In addition, it is known that the image forming apparatus includes a correction roller that corrects the curvature of the sheet (see, for example, Patent Document 1).

JP-A-11-268856

  By the way, it may be difficult to provide the dedicated correction roller for correcting the curvature of the sheet due to restrictions such as space and cost in the image forming apparatus.

  An object of the present invention is to provide an image forming apparatus capable of correcting the curvature of a sheet that has passed through a fixing unit without providing a dedicated correction roller.

  An image forming apparatus according to one aspect of the present invention includes a transfer unit, a fixing unit, a discharge unit, a reverse conveyance unit, and a control unit. The transfer unit transfers the toner image onto a sheet conveyed along the main conveyance path. The fixing unit includes a first roller and a second roller that rotate while sandwiching the sheet conveyed along the main conveyance path after passing through the transfer unit, and a heater that heats the first roller. The discharge unit discharges the sheet that has passed through the fixing unit to the outside of the apparatus. The reversing conveyance unit reverses the front and back of the sheet that has reached the discharge unit, and conveys the sheet again to the merging position on the upstream side of the fixing unit in the main conveyance path in the sheet conveyance direction. When the preset reversal condition is satisfied when the preset printing mode is a single-side mode in which an image is formed only on one side of the sheet and the sheet is sent to the discharge unit. Execute curl correction control. The curl correction control is a control in which the sheet that has reached the discharge unit is transported to the joining position by the reversal transport unit, and then again passes through the fixing unit and is discharged out of the apparatus by the discharge unit. .

  According to the present invention, it is possible to provide an image forming apparatus that can correct the curvature of the sheet that has passed through the fixing unit without providing a dedicated correction roller.

FIG. 1 is a configuration diagram of an image forming apparatus according to the first embodiment. FIG. 2 is a block diagram of a control-related unit in the image forming apparatus according to the first embodiment. FIG. 3 is a flowchart illustrating an example of a print processing procedure in the image forming apparatus according to the first embodiment. FIG. 4 is a configuration diagram of an image forming apparatus according to the second embodiment.

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In addition, the following embodiment is an example which actualized this invention, Comprising: It does not have the character which limits the technical scope of this invention.

[First Embodiment]
First, the configuration of the image forming apparatus 10 according to the first embodiment will be described with reference to FIG. The image forming apparatus 10 is an electrophotographic image forming apparatus that forms an image on a sheet 9. The sheet 9 is a sheet-like image forming medium such as paper, an envelope, and an OHP sheet.

  The image forming apparatus 10 includes a sheet supply unit 2 at the bottom of the main body 100, a sheet conveying unit 3, an image forming unit 4, a fixing unit 5, a control unit 8, an operation display unit 80, and the like in the main body 100.

  In the sheet supply unit 2, the sheet delivery unit 22 sends out the sheets 9 one by one from the sheet cassette 21 that stores the sheets 9 to the first conveyance path 301.

  In the sheet conveyance unit 3, a plurality of rollers such as a first conveyance roller 31, a second conveyance roller 32, and a third conveyance roller 33 convey the sheet 9. The first conveyance roller 31 conveys the sheet 9 supplied from the sheet supply unit 2 from the first conveyance path 301 to the second conveyance path 302.

  The image forming unit 4 includes a photoconductor 41, a charging unit 42, a developing unit 43, a transfer unit 45, a cleaning unit 47, an optical scanning unit 48, and the like. The optical scanning unit 48 is a so-called LSU (Laser Scanning Unit). The photoconductor 41 and the transfer unit 45 are provided to face each other at a specific position in the first conveyance path 301.

  The first conveyance roller 31 present at the closest position on the upstream side in the sheet conveyance direction with respect to the transfer unit 45 in the first conveyance path 301 is a registration roller 31a. In addition, a sheet detection sensor 61 that detects the arrival of the sheet 9 is provided at a position upstream of the registration rollers 31 a in the first conveyance path 301 in the sheet conveyance direction.

  The sheet 9 reaches the registration roller 31a after elapse of a predetermined time after being detected by the sheet detection sensor 61. The registration roller 31a adjusts the timing at which the sheet 9 is sent to the transfer unit 45 by starting rotation after temporarily stopping at the timing at which the sheet 9 reaches the registration roller 31a.

  In the image forming unit 4, the drum-shaped photoconductor 41 rotates, and the charging unit 42 uniformly charges the surface of the photoconductor 41. The optical scanning unit 48 writes an electrostatic latent image on the surface of the photoconductor 41 by scanning with laser light. The developing unit 43 develops the electrostatic latent image with toner. As a result, a toner image is formed on the surface of the photoreceptor 41.

  The transfer unit 45 transfers the toner image on the surface of the photoconductor 41 to the sheet 9 conveyed along the first conveyance path 301. The cleaning unit 47 removes the toner remaining on the surface of the photoreceptor 41. The first transport path 301 is an example of a main transport path.

  The fixing unit 5 is provided at a position downstream of the transfer unit 45 in the first conveyance path 301 in the sheet conveyance direction. The fixing unit 5 fixes the toner image on the sheet 9 by heating the toner image on the surface of the sheet 9. The fixing unit 5 includes a heating roller 51, a pressure roller 52, a heater 53, a fixing temperature sensor 54, and the like.

  The heating roller 51 and the pressure roller 52 rotate while sandwiching the sheet 9 conveyed along the first conveyance path 301 after passing through the transfer unit 45. The heating roller 51 is a roller whose outer layer is harder than the pressure roller 52. The pressure roller 52 is a roller whose outer layer is softer than the heating roller 51.

  For example, the heating roller 51 includes a cylindrical metal member and a resin coating film mainly composed of silicon or the like formed thinly on the outer peripheral surface of the metal member. The pressure roller 52 includes a metal rod-shaped core member and an elastic outer layer mainly composed of silicon or the like formed thick around the core member.

  The pressure roller 52 is pressed against the heating roller 51 by a spring (not shown). Thereby, the part where the heating roller 51 and the pressure roller 52 come into contact forms a nip that is a part where the heating roller 51 bites into the elastic outer layer of the pressure roller 52. The sheet 9 passes through the nip.

  The heating roller 51 and the pressure roller 52 are examples of a hard first roller and a soft second roller, respectively.

  The heater 53 heats the heating roller 51. For example, the heater 53 is included in a cylindrical heating roller 51.

  The fixing temperature sensor 54 is a sensor that detects the temperature of the heating roller 51. When the fixing process of the toner image on the sheet 9 is performed, the power supplied to the heater 53 is adjusted by feedback control of the deviation between the detected temperature of the fixing temperature sensor 54 and a preset target temperature.

  The second conveyance roller 32 conveys the sheet 9 along the second conveyance path 302 by rotating forward, and further discharges the sheet 9 to the discharge tray 101 outside the apparatus. The second transport roller 32 located at the exit of the second transport path 302 is a discharge roller 32 a that discharges the sheet 9 to the discharge tray 101 by rotating forward. The second transport roller 32 that rotates forward is an example of a discharge unit.

  The second conveyance roller 32 rotates in the reverse direction to change the conveyance direction of the sheet 9 and conveys the sheet 9 from the second conveyance path 302 to the third conveyance path 303.

  When the double-sided printing process is executed, the sheet 9 having an image formed on one side is conveyed along the third conveyance path 303. The double-sided printing process is a process for forming images on both sides of the sheet 9.

  The third transport path 303 is formed in parallel with the first transport path 301 and joins the joining position P1 in the first transport path 301. The merge position P1 is a position on the upstream side in the sheet conveyance direction from the positions of the transfer unit 45 and the registration roller 31a in the first conveyance path 301.

  Needless to say, the merge position P1 is a position on the upstream side in the sheet conveyance direction with respect to the position of the fixing unit 5 in the first conveyance path 301.

  The third conveyance roller 33 takes over the conveyance of the sheet 9 conveyed from the second conveyance path 302 to the third conveyance path 303, and further conveys the sheet 9 from the third conveyance path 303 to the first conveyance path 301. .

  The sheet 9 transported from the third transport path 303 to the first transport path 301 is reversed with respect to the sheet 9 transported from the sheet supply unit 2 to the first transport path 301.

  That is, the second transport roller 32 and the third transport roller 33 when rotating in the reverse direction transport the sheet 9 that has passed through the fixing unit 5 to the merging position P <b> 1 in the first transport path 301 again with its front and back reversed. The third conveyance path 303 is a reverse conveyance path through which the reversed sheet 9 is conveyed.

  Normally, when the preset printing mode is the single-sided mode, the sheet conveyance unit 3 conveys the sheet 9 from the first conveyance path 301 to the second conveyance path 302 and discharges it without passing through the third conveyance path 303. It is discharged to the tray 101. The single-side mode is an operation mode in which processing for forming an image on only one side of the sheet 9 is performed, and the sheet 9 is sent to the discharge roller 32 a via the fixing unit 5.

  On the other hand, when the preset printing mode is the duplex mode, the sheet transport unit 3 transports the sheet 9 from the first transport path 301 to the second transport path 302, and then from the second transport path 302 to the second transport path 302. The sheet is conveyed again to the first conveyance path 301 via the three conveyance paths 303 and then discharged to the discharge tray 101. The double-sided mode is an operation mode when processing for forming images on both sides of the sheet 9 is performed and the sheet 9 is sent to the discharge roller 32a.

  The control unit 8 controls an electrical device provided in the image forming apparatus 10. The operation display unit 80 is a user interface including an operation unit that receives information input and a display unit that displays information. For example, it is conceivable that the display unit includes a touch panel, and the display unit includes a display panel such as a liquid crystal panel.

  The print mode is set by the control unit 8 in accordance with a predetermined setting operation on the operation display unit 80.

  The sheet 9 may be bent so as to warp toward the heating roller 51 due to heat received from the heating roller 51. This phenomenon is referred to as curling of the sheet 9 or the like.

  By the way, it may be difficult to provide a dedicated correction roller for correcting the curvature of the sheet 9 due to restrictions such as space and cost in the image forming apparatus 10.

  The image forming apparatus 10 has a configuration capable of correcting the curvature of the sheet 9 that has passed through the fixing unit 5 without providing a dedicated correction roller. Hereinafter, the configuration will be described.

[Configuration of control-related parts]
As shown in FIG. 2, the control unit 8 includes a conveyance control unit 81, an image formation control unit 82, a heater control unit 83, a UI (User Interface) control unit 84, a communication unit 85, and the like.

  The conveyance control unit 81 controls the sheet supply unit 2 and the sheet conveyance unit 3. For example, the conveyance control unit 81 controls operations of a motor and a solenoid that drive the sheet feeding unit 22. Further, the conveyance control unit 81 controls the operation of a motor that drives the plurality of sheet conveyance rollers in the sheet supply unit 2.

  The image forming control unit 82 controls the image forming unit 4. For example, the image forming control unit 82 controls the operation of a motor that drives the photoconductor 41 and the developing unit 43, the blinking of the laser light source in the optical scanning unit 48, the voltage of the charging unit 42, the bias voltage of the developing unit 43, and the like. . The heating roller 51 of the fixing unit 5 rotates in conjunction with the photoconductor 41 and the like.

  The heater control unit 83 controls the power supplied to the heater 53 of the fixing unit 5. For example, the heater control unit 83 adjusts the power supplied to the heater 53 by feedback control of the deviation between the detected temperature of the fixing temperature sensor 54 and the target temperature.

  The UI control unit 84 controls the operation display unit 80. For example, the UI control unit 84 outputs the print mode selection menu screen to the display unit of the operation display unit 80. Furthermore, the UI control unit 84 sets one of the single-side mode and the double-side mode as the print mode in accordance with a selection operation on the operation unit.

  The print mode setting result is transmitted from the UI control unit 84 to the image forming control unit 82. The initial state of the print mode is the single-side mode.

  The communication unit 85 performs data communication with other devices such as an information terminal. For example, the communication unit 85 receives print job data from the other apparatus and transmits the received data to the image formation control unit 82. The image forming control unit 82 causes the image forming unit 4 to execute processing for converting the print job data into image data and transferring the toner image corresponding to the image data to the sheet 9.

  In addition, the image forming apparatus 10 includes a curvature detection sensor 62 that detects the degree of curvature of the sheet 9 at a position downstream of the fixing unit 5 in the first conveyance path 301 in the sheet conveyance direction. The curvature detection sensor 62 is an example of a curvature detection unit.

  For example, the curvature detection sensor 62 is a distance sensor that is fixed to a sheet guide that forms the first conveyance path 301 and detects the distance from the sensor to the leading end of the sheet 9 that is passing through the fixing unit 5 in a non-contact manner. It is possible. The distance sensor is, for example, an infrared reflection type distance sensor or an ultrasonic sensor.

  When the distance sensor is fixed to the sheet guide on the heating roller 51 side in the first conveyance path 301, the higher the degree of curvature of the sheet 9, the closer the distance between the curvature detection sensor 62 and the leading end of the sheet 9. Therefore, the distance sensor can be employed as the curvature detection sensor 62.

  The curvature detection sensor 62 also functions as a sensor that detects whether or not the sheet 9 is passing the attachment position of the curvature detection sensor 62.

[Print processing]
Next, an example of a print processing procedure executed by the control unit 8 will be described with reference to the flowchart shown in FIG. The printing process is executed when the communication unit 85 receives the print job data. In the following description, S1, S2,... Represent identification codes of processes executed by the control unit 8.

<Process S1>
First, the image forming control unit 82 and the heater control unit 83 cause the image forming unit 4 to execute print preparation processing. The print preparation process includes a process in which the image forming control unit 82 converts the print job data into image data. Further, the print preparation process includes a process in which the heater control unit 83 raises the temperature of the heating roller 51 to the target temperature by operating the heater 53.

<Process S2>
Next, the conveyance control unit 81 controls the sheet conveyance unit 3 to start the process of conveying the sheet 9 from the sheet cassette 21 to the second conveyance path 302 via the first conveyance path 301.

<Process S3>
Further, the image forming control unit 82 controls the image forming unit 4 to cause the transfer unit 45 to execute a process of transferring the toner image onto the sheet 9 moving on the first conveyance path 301. As a result, the transfer unit 45 transfers the toner image on the photoreceptor 41 to the sheet 9, and the fixing unit 5 fixes the toner image on the sheet 9.

<Step S4>
Furthermore, the conveyance control unit 81 determines the degree of curvature of the sheet 9 passing through the fixing unit 5 from the detection result of the curvature detection sensor 62. For example, the conveyance control unit 81 determines which of a plurality of previously divided ranges the detection distance to the sheet 9 obtained by the curvature detection sensor 62 belongs to. Then, the conveyance control unit 81 determines the rank corresponding to the range to which the detection distance belongs as the degree of curvature of the sheet 9.

<Step S5>
Next, the conveyance control unit 81 determines whether or not the sheet 9 after image formation has passed through the fixing unit 5. For example, the conveyance control unit 81 fixes the sheet 9 when a predetermined time elapses after the rotation of the registration roller 31a temporarily stopped according to the detection of the sheet 9 by the sheet detection sensor 61 is started. It determines with having passed the part 5.

  Further, the conveyance control unit 81 determines that the sheet 9 moves the fixing unit 5 when the detection result of the curvature detection sensor 62 changes from the state in which the sheet 9 is passing the attachment position of the curvature detection sensor 62 to the state after the passage. It can also be determined that it has passed.

  When the conveyance control unit 81 determines that the sheet 9 has passed through the fixing unit 5, whether the passage is the first passage for one sheet 9 or the second passage. Also judge.

<Step S6>
When it is determined that the sheet 9 has passed through the fixing unit 5 for the first time, the image forming control unit 82 determines whether the preset print mode is the single-side mode or the double-side mode. To do.

<Step S7>
When it is determined that the printing mode is the duplex mode, the conveyance control unit 81 causes the sheet conveyance unit 3 to pass the sheet 9 that has passed the fixing unit 5 from the second conveyance path 302 to the third conveyance path 303. Then, the process of transporting again to the merge position P1 of the first transport path 301 is executed.

  More specifically, the conveyance control unit 81 rotates the third conveyance roller 33 while rotating the second conveyance roller 32 in the reverse direction. That is, the second conveyance roller 32, the third conveyance roller 33, and the conveyance control unit 81 that controls them, the sheet 9 that has passed through the fixing unit 5 when the preset printing mode is the duplex mode, The front and back sides are reversed and conveyed again to the merging position P1 of the first conveying path 301. As a result, the sheet 9 that has been turned upside down is conveyed along the first conveyance path 301.

  After step S7, the process from step S3 is executed again. As a result, images are formed on both sides of the sheet 9.

<Step S8>
On the other hand, when it is determined in step S5 that the printing mode is the single-sided mode, the conveyance control unit 81 determines whether or not a predetermined reversal condition is satisfied. The inversion condition is a precondition when the sheet 9 is conveyed in reverse in the single-side mode.

  For example, the inversion condition may include a first condition and a second condition shown below. The first condition is a condition that an operation for designating a curvature correction mode has been performed on the operation unit of the operation display unit 80 in advance.

  The second condition is a condition that the degree of curvature detected by the curvature detection sensor 62 for the sheet 9 that has passed the fixing unit 5 for the first time exceeds a predetermined allowable range. The degree of curvature referred to in the determination of the second condition is determined in step S4. The conveyance control unit 81 that determines the second condition determines whether or not the degree of curvature of the sheet 9 exceeds the allowable range based on the detection result of the curvature detection unit. It is an example of the control part which determines success or failure.

  The conveyance control unit 81 determines that the reversal condition is satisfied when both the first condition and the second condition are satisfied, and determines that the reversal condition is not satisfied in other cases.

<Step S9>
If it is determined that the reversal condition is not satisfied, the conveyance control unit 81 continues the normal rotation of the second conveyance roller 32. As a result, the sheet conveyance unit 3 discharges the sheet 9 from the second conveyance path 302 to the discharge tray 101 without passing through the third conveyance path 303.

  Also, in step S <b> 5, even when it is determined that the sheet 9 has passed the fixing unit 5 for the second time, the conveyance control unit 81 continues the normal rotation of the second conveyance roller 32. As a result, the sheet transport unit 3 discharges the sheet 9 that has passed through the third transport path 303 to the discharge tray 101.

<Step S10>
On the other hand, when it is determined that the reversal condition is satisfied under the situation where the printing mode is the single-sided mode, the conveyance control unit 81 causes the sheet conveyance unit 3 to transfer the sheet 9 that has passed the fixing unit 5 to the second state. A process of conveying from the conveyance path 302 to the merging position P1 of the first conveyance path 301 through the third conveyance path 303 is executed again.

  More specifically, the conveyance control unit 81 rotates the third conveyance roller 33 while rotating the second conveyance roller 32 in the reverse direction.

  That is, the second conveyance roller 32, the third conveyance roller 33, and the conveyance control unit 81 that controls them are fixed when the reversal condition is satisfied under the situation where the preset printing mode is the one-side mode. The sheet 9 that has passed through the section 5 is conveyed to the merging position P1 of the first conveyance path 301 again with its front and back reversed.

  In addition, the 2nd conveyance roller 32 and the 3rd conveyance roller 33 which reversely rotate, and the conveyance control part 81 which controls them are examples of a reverse conveyance part.

<Step S11>
Further, the heater when the heater 9 passes the fixing unit 5 for the second time according to the curvature detected by the curvature detection sensor 62 for the sheet 9 that has passed the fixing unit 5 for the first time. The power supplied to 53 is changed.

  The degree of curvature detected by the curvature detection sensor 62 is the degree of curvature determined in step S4. Further, the process of step S11 is executed under the situation where the printing mode is the single-sided mode.

  For example, the heater control unit 83 may stop power supply to the heater 53 when the curvature is less than a predetermined threshold level. In addition, when the degree of curvature is equal to or higher than the threshold level, the heater control unit 83 may set the target temperature of the fixing unit 5 to a lower temperature than when the printing process is performed. As a result, the power supplied to the heater 53 is lower than when the printing process is performed.

  That is, when the sheet 9 passes through the fixing unit 5 for the second time, the heater control unit 83 lowers the supply power to the heater 53 or stops the power supply compared to when the sheet 9 passes through the fixing unit 5 for the first time. (S11).

  After the process of step S11, the process from step S5 is executed again. In this case, when the sheet 9 is transported along the first transport path 301 for the second time, the process of step S3 is not executed. Accordingly, when the sheet 9 passes through the transfer unit 45 for the second time under the situation where the printing mode is the single-side mode, the transfer unit 45 does not transfer the toner image to the sheet 9.

  Then, when the sheet 9 passes through the fixing unit 5 for the second time, the curvature of the sheet 9 is corrected by the heating roller 51 and the pressure roller 52. The sheet 9 whose curvature is corrected, that is, the sheet 9 that has passed through the fixing unit 5 twice is discharged to the discharge tray 101 by the process of step S9 (from step S5 to step S9).

  As described above, when it is determined that the reversal condition is satisfied under the situation where the printing mode is the single-sided mode, the conveyance control unit 81 includes a curl including the process of step S10 and the process of step S9. Execute straightening control. In the output curl discharge control, the sheet 9 that has passed through the fixing unit 5 and has reached the discharge roller 32a is transported to the joining position P1 by the reverse transport units 32 and 33, and then again along the first transport path 301. In this control, the fixing unit 5 is passed and discharged to the discharge tray 101 by the discharge roller 32a.

  Further, the image forming control unit 4 does not cause the transfer unit 45 to transfer the toner image onto the sheet 9 when the conveyance control unit 81 executes the curl correction control. For example, the image forming control unit 4 performs control to stop the application of the transfer voltage in the transfer unit 45 or to separate the transfer unit 45 from the photoconductor 41. Thereby, the transfer process of the toner image by the transfer unit 45 is not executed.

  If the image forming apparatus 10 is employed, the curvature of the sheet 9 that has passed through the fixing unit 5 can be corrected without providing a dedicated correction roller.

  When the sheet 9 passes through the transfer portion 45 for the second time, the sheet 9 is in a relatively high temperature state. Therefore, when the sheet 9 passes the fixing unit 5 for the second time, the curvature of the sheet 9 is appropriately corrected when the temperature of the heating roller 51 is relatively low.

  Furthermore, the curvature of the sheet 9 is appropriately corrected by changing the power supplied to the heater 53 in accordance with the degree of curvature.

  In the present embodiment, correction of the curvature of the sheet 9 by the fixing unit 5 is performed on the condition that the degree of curvature detected by the curvature detection sensor 62 exceeds the allowable range. Thereby, it is possible to prevent the correction of the curvature of the sheet 9 from being executed even when it is unnecessary.

[Second Embodiment]
Next, an image forming apparatus 10A according to the second embodiment will be described with reference to FIG. The image forming apparatus 10A has a configuration in which a fourth transport path 304 and a path switching unit 34 are added to the configuration of the image forming apparatus 10 shown in FIGS.

  The sheet conveying unit 3A of the image forming apparatus 10A is a mechanism in which a path switching unit 34 is added to the sheet conveying unit 3 of the image forming apparatus 10.

  4, the same components as those shown in FIGS. 1 and 2 are given the same reference numerals. Hereinafter, differences between the image forming apparatus 10A and the image forming apparatus 10 will be described.

  In the following description, the joining position P1 of the first conveyance path 301 in the image forming apparatus 10A is referred to as a first joining position P1.

  The fourth conveyance path 304 is a sheet conveyance path that branches from the middle of the third conveyance path 303 and joins the second merge position P <b> 2 between the transfer unit 45 and the fixing unit 5 in the first conveyance path 301. The fourth conveyance path 304 is a curved conveyance path, and is a conveyance path that reverses from the sheet conveyance direction in the third conveyance path 303 to the sheet conveyance direction in the first conveyance path 301. The fourth transport path 304 is provided with a fourth transport roller 35 that transports the sheet 9 toward the second joining position P2.

  The path switching unit 34 is configured to guide the sheet 9 as it is along the third conveyance path 303 at the branch position to the fourth conveyance path 304 in the third conveyance path 303 and the fourth conveyance path 304. The sheet guide member is displaceable to a second position for guiding to the second position. The path switching unit 34 is rotatably supported, and an actuator such as a solenoid actuator (not shown) displaces the path switching unit 34. In the image forming apparatus 10 </ b> A, the conveyance control unit 81 of the control unit 8 controls the path switching unit 34.

  Then, when the preset printing mode is the double-sided mode, the conveyance control unit 81 rotates the second conveyance roller 32 in the reverse direction and displaces the path switching unit 34 to the first position. The conveyance roller 33 is rotated. Accordingly, the second conveyance roller 32 and the third conveyance roller 33 convey the sheet that has passed through the fixing unit 5 for the first time to the first merge position P1 of the first conveyance path 301 by inverting the front and back of the sheet ( Step S7) in FIG. In this case, the second transport roller 32 that rotates in the reverse direction, the path switching unit 34 and the third transport roller 33 that are displaced to the first position are examples of the first reverse transport unit.

  In addition, the conveyance control unit 81 rotates the second conveyance roller 32 in the reverse direction when the reversing condition is satisfied under the condition that the preset printing mode is the single-sided mode, and the path switching unit 34 is The fourth transport roller 35 is rotated by being displaced to the second position. As a result, the second transport roller 32 and the fourth transport roller 33 transport the sheet 9 that has passed the fixing unit 5 for the first time to the second joining position P <b> 2 of the first transport path 301 with its front and back reversed. Step S10 in FIG. In this case, the second transport roller 32 that rotates in the reverse direction, the path switching unit 34 that is displaced to the second position, and the fourth transport roller 35 are examples of the second reverse transport unit.

  In the image forming apparatus 10 </ b> A, the second conveyance roller 32, the third conveyance roller 33, the path switching unit 34, and the conveyance control unit 81 that controls them reverse the front and back of the sheet 9 that has passed through the fixing unit 5. This is an example of a reversing transport unit that transports again to the first joining position P1 or the second joining position P2 of the first transport path 301.

  If the image forming apparatus 10A is employed, the same effects as those obtained when the image forming apparatus 10 is employed can be obtained. Furthermore, in the image forming apparatus 10A, the conveyance distance of the sheet 9 when the curvature of the sheet 9 is corrected in the single-side mode is shortened. Thereby, the fall of the printing speed in case the curvature of the sheet | seat 9 is corrected can be suppressed.

[Application example]
It is also conceivable that the image forming apparatuses 10 and 10A described above do not include the curvature detection sensor 62. In this case, it is conceivable that the reversal condition is only the first condition that the operation for designating the curvature correction mode is performed on the operation unit in advance.

  When the sheet 9 passes the fixing unit 5 for the second time under the condition that the printing mode is the single-sided mode, the heater control unit 83 determines a power supply state to the heater 53 in advance regardless of the degree of curvature. It is also possible to control the state.

  For example, the heater control unit 83 stops power supply to the heater 53 regardless of the degree of curvature, or sets the target temperature of the fixing unit 5 to a lower temperature than when the printing process is performed. And so on.

  It is also conceivable that the image forming apparatuses 10 and 10A are tandem color image forming apparatuses. The color image forming apparatus includes a plurality of transfer units 45, and the transfer units 45 are primary transfer units that transfer the toner image from the photoreceptor 41 to an intermediate transfer belt. Further, the color image forming apparatus includes a secondary transfer unit that transfers the toner image on the intermediate transfer belt to the sheet 9 conveyed along the first conveyance path 301. The fixing unit 5 fixes the toner image transferred to the sheet 9 by the secondary transfer unit on the sheet 9.

  In the image forming apparatus 10, it is conceivable that the curvature detection sensor 62 is a detection sensor whose output signal changes when the degree of curvature of the sheet 9 exceeds the allowable range. For example, the detection sensor may be a transmissive or reflective photosensor that changes its output signal when the leading end of the sheet 9 that has passed through the fixing unit 5 reaches a predetermined position.

  The image forming apparatus according to the present invention can be freely combined with the above-described embodiments and application examples, or can be appropriately modified within the scope of the invention described in each claim. Alternatively, it may be configured by omitting a part.

2: Sheet feeding unit 3, 3A: Sheet conveying unit 4: Image forming unit 5: Fixing unit 8: Control unit 9: Sheet 10, 10A: Image forming apparatus 21: Sheet cassette 22: Sheet feeding unit 31: First conveying roller 31a: registration roller 32: second transport roller 32a: discharge roller 33: third transport roller 34: path switching unit 41: photoconductor 42: charging unit 43: developing unit 45: transfer unit 47: cleaning unit 48: optical scanning unit 51: Heating roller 52: Pressure roller 53: Heater 54: Fixing temperature sensor 61: Sheet detection sensor 62: Curvature detection sensor 80: Operation display unit 81: Conveyance control unit 82: Image forming control unit 83: Heater control unit 84: UI control unit 85: communication unit 100: main body 101: discharge tray 301: first transport path (main transport path)
302: 2nd conveyance path 303: 3rd conveyance path 304: 4th conveyance path P1: Merge position (1st merge position)
P2: Second joining position

Claims (3)

A first conveyance unit that conveys the sheet along the first conveyance path;
A transfer unit for transferring a toner image to a sheet conveyed along the first conveyance path;
A fixing unit having a hard first roller and a soft second roller that rotate while sandwiching the sheet conveyed along the first conveyance path after passing through the transfer unit, and a heater that heats the first roller; ,
By normal operation, the sheet that has passed through the fixing unit is sequentially conveyed along the second conveyance path subsequent to the first conveyance path and is discharged out of the apparatus from the outlet of the second conveyance path. By operating, the sheet that has passed through the fixing unit is reversely conveyed along the second conveyance path, and the transfer in the first conveyance path from the entrance connected to the first conveyance path in the second conveyance path. A second transport section that transports the first transport position toward the first transport position upstream from the position of the section in the sheet transport direction ;
A third conveying section for conveying the sheet toward the first joining position in the third conveying path;
In the fourth conveyance path, the sheet is branched from the middle of the third conveyance path to the first conveyance path side and folded and joined to the second merge position between the transfer unit and the fixing unit in the first conveyance path. A fourth transport unit that transports the second head toward the second joining position;
At a branch position of the third conveyance path to the fourth conveyance path, a first state in which the sheet is guided along the third conveyance path and a second state in which the sheet is guided to the fourth conveyance path. A path switching unit to be switched;
A curvature detection unit that detects a degree of curvature of the sheet at a position downstream of the fixing unit in the first conveyance path in the sheet conveyance direction;
A control unit for controlling power supplied to the second transport unit, the path switching unit, and the heater;
The third transport path is parallel to the first transport path on the side where the second roller is disposed,
The controller is
The degree of curvature of the sheet detected by the curvature detection unit when the preset printing mode is a one-sided mode in which an image is formed only on one side of the sheet and the sheet is sent to the second transport unit . When a reversal condition including a condition that exceeds a predetermined allowable range is established, the second transport unit is operated in reverse and the second transport unit is moved after the path switching unit is switched to the second state. Performing curl correction control for discharging the sheet that has passed through the fixing unit twice by performing a normal operation to the outside of the apparatus ;
When the printing mode is a double-sided mode in which images are formed on both sides of the sheet, the second conveying unit is operated reversely and the second conveying unit is switched after the path switching unit is switched to the first state. An image forming apparatus that discharges the sheet that has passed through the transfer unit and the fixing unit twice by performing a normal operation . The control unit, when executing the curl correction control, stops power supply to the heater when the degree of curvature of the sheet detected by the curvature detection unit is less than a predetermined threshold level. The image forming apparatus according to claim 1 , wherein when the degree of curvature is equal to or higher than the threshold level, the power supplied to the heater is lower than when the sheet passes through the fixing unit for the first time . It has an operation unit that accepts information input operations,
The image forming apparatus according to claim 1 , wherein the reversal condition includes that an operation for designating a curvature correction mode is performed on the operation unit in advance.

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