Image forming apparatus

Abstract

To provide an image forming apparatus that can improve a printing speed when continuously printing a plurality of jobs on both sides of sheets.SOLUTION: A control unit 9 of an image forming apparatus 1, in executing continuous double-sided printing in which the control unit continuously prints a five-page preceding job and a five-page subsequent job on both sides of sheets, feeds a preceding sheet BS1-2 being the first sheet corresponding to the subsequent job from a sheet cassette 31 in the middle of printing of the preceding job, prints an image corresponding to the subsequent job on a first face of the preceding sheet BS1-2, and with a preceding sheet BS1-1 having an image printed on a first face being located on a paper feed path P0 or a re-conveyance path P3, ejects, onto a paper ejection tray 23a, a subsequent subsequent sheet AS3-2 being a last sheet corresponding to the preceding job.SELECTED DRAWING: Figure 9

Description

Technical Field

[0001] The present invention relates to an image forming apparatus.

Background Art

[0002] Conventionally, in an image forming apparatus capable of performing double-sided printing on a sheet, for example, a sheet on which printing on one side has been completed is reversed and re-fed, and double-sided printing is realized by performing printing on both sides.

[0003] When performing double-sided printing in an image forming apparatus, for example, as disclosed in Patent Document 1, in order to shorten the interval between conveyed sheets and improve the printing speed, control such as 2416 conveyance control and 2461 conveyance control is performed to set the printing order of a plurality of pages in a printing job to an efficient printing order. Here, for example, 2416 conveyance control is control in which printing is performed in the order of one side of the first sheet (second page) → one side of the second sheet (fourth page) → both sides of the first sheet (first page) → one side of the third sheet (sixth page).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] As described above, by performing control such as 2416 conveyance control and 2461 conveyance control when performing double-sided printing, it is possible to set the printing order to an efficient printing order and improve the printing speed. However, when continuously printing a plurality of printing jobs, there is room for further improvement in the printing speed by considering the printing order between the plurality of printing jobs.

[0006] Therefore, the present invention provides an image forming apparatus that can further improve the printing speed when printing multiple jobs consecutively on both sides, by feeding a sheet corresponding to a subsequent job in the middle of printing a preceding job. [Means for solving the problem]

[0007] The image forming apparatus that solves the above problems has the following features. That is, the image forming apparatus comprises an image forming unit that forms an image on a sheet, a sheet tray that supports the sheet fed to the image forming unit, a paper output tray from which the sheet on which the image formed by the image forming unit is discharged, and a control unit which comprises a device body having the image forming unit and further having a feeding path for feeding sheets from the sheet tray to the image forming unit, and a re-transport path for re-transporting a sheet on which an image has been formed on one side to the feeding path, and the control unit, when performing continuous double-sided printing in which at least three pages of preceding jobs and at least two pages of succeeding jobs are printed on both sides in succession, feeds a first sheet, which is the first sheet corresponding to the succeeding jobs, from the sheet tray in the middle of printing the preceding jobs, prints an image corresponding to the succeeding jobs on one side of the first sheet, and discharges a second sheet, which is the last sheet corresponding to the preceding jobs, to the paper output tray while the first sheet on which an image has been printed on one side is located in the feeding path or the re-transport path.

[0008] This allows for a reduction in the transport interval between sheets being printed for preceding and succeeding jobs, thereby improving printing speed.

[0009] Furthermore, for a job with 5 pages, the printing order is as follows: print page 2 on one side of sheet 1, print page 4 on one side of sheet 2, print page 1 on the second side of sheet 1, print page 3 on the second side of sheet 2, and print page 5 on one side of sheet 3.

[0010] This allows for improved printing speed when printing a 5-page job on both sides.

[0011] Furthermore, when performing double-sided printing for a job with an odd number of pages, the control unit ejects the sheet with the image corresponding to the last page printed on it to the output tray without transporting it to the re-transport path.

[0012] This allows for improved printing speed when the number of pages in a job is odd.

[0013] Furthermore, when performing continuous double-sided printing when the number of pages of the preceding job is odd, the control unit feeds the second sheet from the sheet tray while at least a portion of the first sheet, which has an image printed on one side, is located within the re-transport path, prints an image corresponding to the last page of the subsequent job on one side of the fed second sheet, and ejects the second sheet, which has an image printed on one side, into the output tray without transporting it back to the re-transport path.

[0014] This allows the spacing between the first and second sheets to be set to an appropriate interval when the number of pages in the preceding job is odd, thereby improving printing speed.

[0015] Furthermore, the re-transport path is located below the sheet tray.

[0016] This ensures sufficient route length for the re-transportation path and increases the flexibility in how sheets are stored along the re-transportation path.

[0017] Furthermore, the control unit feeds the second sheet from the sheet tray while the first sheet, on which an image is printed on one side, is stopped within the re-transport path.

[0018] This allows the spacing between the first and second sheets to be set to an appropriate interval, thereby improving the printing speed.

[0019] Furthermore, the device body includes a re-transport roller positioned in the re-transport path and transporting the sheet toward the supply path, a first sheet sensor provided in the re-transport path and detecting the sheet, and a clutch that switches between a transmission state that transmits driving force to the re-transport roller and a disconnection state that disconnects the transmission of driving force to the re-transport roller. The control unit switches the clutch from the transmission state to the disconnection state when the first sheet sensor detects the first sheet, thereby stopping the first sheet within the re-transport path.

[0020] This allows the first sheet to be stopped within the re-transport path while continuing to drive other drive components when the first sheet is positioned within the re-transport path.

[0021] Furthermore, the clutch is an electromagnetic clutch.

[0022] This allows switching between the transmission and disconnection of driving force to the re-transport rollers without using complex mechanisms.

[0023] The device body further includes a feeding roller for feeding a sheet supported by the sheet tray to the feeding path, a first conveying roller located downstream of the feeding roller in the sheet conveying direction in the feeding path, a second conveying roller located between the feeding roller and the first conveying roller in the feeding path, and downstream of the confluence point of the feeding path and the re-conveying path in the sheet conveying direction, and a second sheet sensor located between the first conveying roller and the second conveying roller in the feeding path for detecting a sheet. The control unit starts feeding the second sheet from the sheet tray to the feeding path on the condition that the second sheet sensor detects the rear end of the third sheet, which is fed from the sheet tray before the second sheet and has an image corresponding to the preceding job printed on one side of the third sheet.

[0024] As a result, the conveyance interval between the second sheet and the third sheet can be reduced, and the printing speed can be improved.

[0025] The apparatus main body further includes a feeding roller that feeds a sheet supported by the sheet tray to the feeding path, a first conveying roller positioned downstream of the feeding roller in the sheet conveying direction in the feeding path, and a third sheet sensor that is positioned between the feeding roller and the first conveying roller in the feeding path and detects the sheet. The control unit starts the re-conveyance of the first sheet stopped in the re-conveyance path to the feeding path on the condition that the rear end of the sheet fed from the sheet tray after the second sheet is detected by the third sheet sensor.

[0026] As a result, the conveyance interval between the sheet fed from the sheet tray after the second sheet and the first sheet stopped in the re-conveyance path can be reduced, and the printing speed can be improved.

[0027] The image forming unit includes a photosensitive drum on which a toner image is formed, a transfer roller that is disposed to face the photosensitive drum and to which a voltage is applied, and a fixing device that fixes the toner image transferred to the sheet. When performing printing on the second sheet, the control unit changes at least one of the transfer current flowing when applying a voltage to the transfer roller and the fixing temperature when fixing the toner image to the sheet by the fixing device, compared with when performing printing on a sheet on which images are printed on both sides and one side.

[0028] As a result, the moisture content of the sheet, which has passed through the fuser and been heated once, decreases and its resistance increases. Therefore, in single-sided printing mode, where printing is performed on a second sheet, it is possible to keep the transfer current value higher than in double-sided printing mode, where printing is performed on a sheet where images are formed on both sides, thereby keeping the voltage value of the transfer roller constant and achieving uniform print quality in both single-sided and double-sided printing modes. In addition, since the temperature of the sheet becomes higher due to heating after passing through the fuser once, it is possible to keep the fixing temperature constant in single-sided printing mode by setting the fixing temperature higher than in double-sided printing mode, thereby achieving uniform print quality in both single-sided and double-sided printing modes.

[0029] Furthermore, when printing on the second sheet, the control unit performs at least one of the following: a control that increases the transfer current value compared to when printing on a sheet in which an image is printed on both sides; and a control that increases the fixing temperature compared to when printing on a sheet in which an image is formed on both sides.

[0030] This makes it possible to achieve uniform print quality between single-sided and double-sided printing modes. [Effects of the Invention]

[0031] According to the present invention, the transport interval between sheets on which preceding and succeeding jobs are printed can be reduced, thereby improving the printing speed. [Brief explanation of the drawing]

[0032] [Figure 1] This is a central cross-sectional view showing an image forming apparatus. [Figure 2] This is a block diagram of an image forming apparatus. [Figure 3] This diagram shows the sheet printing order in the 2416 transport control system. [Figure 4] This is a flowchart illustrating the transport control process. [Figure 5] This is a central cross-sectional view showing the image forming apparatus with the preceding sheet AS1-2 fed into the feeding path. [Figure 6] This is a central cross-sectional view showing the image forming apparatus in the state where the next sheet AS2-2 has been fed into the feeding path. [Figure 7] This is a central cross-sectional view showing an image forming apparatus in which the preceding sheet AS1-1 is being transported along the re-transport path and the next sheet AS2-2 is being transported to the discharge path. [Figure 8] This is a central cross-sectional view showing the image forming apparatus in which the preceding sheet AS1-1 has been fed into the feeding path. [Figure 9] This is a central cross-sectional view showing the image forming apparatus in which the preceding sheet BS1-2 has been fed into the feeding path. [Figure 10] This is a central cross-sectional view showing the image forming apparatus in which the next sheet AS2-1 has been fed into the feeding path. [Figure 11] This is a central cross-sectional view showing an image forming apparatus in which the preceding sheet BS1-1 is located in the re-transport path, and subsequent sheets are being fed to the AS3-2 feeding path. [Figure 12] This is a central cross-sectional view showing the image forming apparatus in a state where the next sheet BS2-2 has been fed into the feeding path. [Figure 13] This is a central cross-sectional view showing the image forming apparatus in a state where the preceding sheet BS1-1, which had been stopped in the re-transport path, has been fed into the feed path. [Figure 14] This is a central cross-sectional view of an image forming apparatus showing the state in which the next sheet BS2-1 is being transported along the re-transport path and the preceding sheet BS1-1 has been transported to the image forming section. [Figure 15] This is a central cross-sectional view showing an image forming apparatus in a state where the next sheet BS2-1 has been fed into the feeding path, and the following sheet BS3-2 has been fed into the feeding path. [Figure 16] This diagram shows the printing order of sheets in a 2416 control system when the preceding job is 6 pages and the succeeding job is 6 pages. [Figure 17]This diagram shows the sheet printing order in the 2461 transport control system. [Modes for carrying out the invention]

[0033] Next, embodiments for carrying out the present invention will be described with reference to the attached drawings.

[0034] [Image forming apparatus] The image forming apparatus 1 shown in Figure 1 is one embodiment of the image forming apparatus according to the present invention and is configured as a color laser printer that forms an image on a sheet S such as paper or an OHP sheet by superimposing images of multiple colors of toner using an electrophotographic method. However, the image forming apparatus 1 can also be configured as a monochrome laser printer that forms an image on a sheet using a single color of toner. Furthermore, the image forming apparatus 1 can also be configured as an inkjet printer.

[0035] In the following explanation, "left side," "right side," "back of the page," and "front of the page" in Figure 1 are defined as the "front," "rear," "left side," and "right side" of the image forming apparatus 1, respectively. Also, "upper side" and "lower side" in Figure 1 are defined as the "upper side" and "lower side" of the image forming apparatus 1, respectively.

[0036] The image forming apparatus 1 comprises an apparatus body 2, a paper feeding unit 3 for supplying sheets S, an image forming unit 5 for forming images on the conveyed sheets S, and a conveying unit 7 for conveying the sheets S from the image forming unit 5. The apparatus body 2 is a box-shaped structure formed in a substantially rectangular parallelepiped shape and houses the paper feeding unit 3, the image forming unit 5, and the conveying unit 7.

[0037] The upper surface 23 of the main body 2 of the device has a paper output tray 23a that is recessed and slopes downward from the front to the rear. The sheet S, which has an image formed by the image forming unit 5, is ejected into the paper output tray 23a. Thus, the main body 2 of the device has an image forming unit 5 and a paper output tray 23a.

[0038] The paper feeding unit 3 includes a sheet cassette 31, a first feeding roller 32, a transport roller 34a and a roller 34b positioned opposite the transport roller 34a, and a register roller 35a and a register roller 35b positioned opposite the register roller 35a. Inside the main body 2 of the device, a feeding path P0 is configured for feeding sheets S from the sheet cassette 31 to the image forming unit 5. The first feeding roller 32 is an example of a feeding roller that feeds sheets supported by a sheet tray into the feeding path. The register roller 35a is an example of a first transport roller, and the transport roller 34a is an example of a second transport roller.

[0039] The sheet cassette 31 supports multiple sheets S in a stacked state. The sheet cassette 31 is an example of a sheet tray that supports sheets fed to the image forming unit. The sheets S supported by the sheet cassette 31 are fed one by one to the feeding path P0 by the first feeding roller 32. The sheets S fed to the feeding path P0 are transported toward the image forming unit 5 by the transport roller 34a and the resist roller 35a. The resist roller 35a is located downstream of the first feeding roller 32 in the sheet transport direction in the feeding path P0. The transport roller 34a is located between the first feeding roller 32 and the resist roller 35a in the feeding path P0.

[0040] Furthermore, the device body 2 includes an opening 2A that opens to the front and a front cover 29 that can open and close the opening 2A. The opening 2A is open when the front cover 29 is open and closed when the front cover 29 is closed.

[0041] The paper feeding unit 3 is equipped with a second feeding roller 38. When the front cover 29 is open, the sheet S placed on the front cover 29 is fed to the feeding path P0 by the second feeding roller 38. The sheet S fed to the feeding path P0 is then transported toward the image forming unit 5 by the registration roller 35a.

[0042] The image forming unit 5 comprises four drum units 51 arranged in a row in the front-to-back direction. Each drum unit 51 corresponds to the respective colors: black, yellow, magenta, and cyan. Each drum unit 51 includes a photosensitive drum 51a and a developing roller 51b.

[0043] The image forming unit 5 also includes a scanner unit 52 and a fuser 6. The scanner unit 52 is located at the top of the main body 2 of the device, and laser light based on image data is irradiated at high speed onto the surface of the photosensitive drum 51a corresponding to each color, passing through polygon mirrors, lenses, reflectors, etc. The fuser 6 is located further downstream than the photosensitive drum 51a furthest downstream in the sheet transport direction.

[0044] A transfer belt 40 is positioned below the drum unit 51 in the image forming unit 5. The transfer belt 40 is stretched between a drive roller 41a and a driven roller 41b positioned in front of the drive roller 41a. Transfer rollers 42 of the image forming unit 5 are positioned opposite each photosensitive drum 51a, with the transfer belt 40 in between.

[0045] In the image forming unit 5, each photosensitive drum 51a, uniformly charged by a charger (not shown), is selectively exposed by the scanner unit 52. This exposure selectively removes charge from the surface of the photosensitive drum 51a, forming an electrostatic latent image on the surface of the photosensitive drum 51a.

[0046] A development bias is applied to the developing roller 51b. When the electrostatic latent image formed on the photosensitive drum 51a faces the developing roller 51b, toner is supplied from the developing roller 51b to the electrostatic latent image due to the potential difference between the electrostatic latent image and the developing roller 51b. As a result, a toner image is formed on the surface of the photosensitive drum 51a.

[0047] When the sheet S, which has been transported toward the image forming unit 5, is transported onto the transfer belt 40, it is carried by the transfer belt 40 and passes sequentially between the transfer belt 40 and each photosensitive drum 51a. Then, when the toner image on the surface of the photosensitive drum 51a faces the sheet S, it is transferred to the sheet S by the transfer bias, which is a voltage applied to the transfer roller 42.

[0048] The sheet S onto which the toner image has been transferred is transported to the fuser unit 6. The fuser unit 6 is equipped with a heating roller 61 that heats the sheet S and a pressure roller 62 positioned opposite the heating roller 61, thereby fixing the toner image transferred to the sheet S. The sheet S transported to the fuser unit 6 passes between the heating roller 61 and the pressure roller 62, which are pressed against each other, and the toner image is heat-fixed.

[0049] The sheet S on which the toner image has been heat-fixed is transported downstream of the image forming unit 5 by the transport unit 7. The transport unit 7 has a transport path P1, an discharge path P2, and a re-transport path P3. The transport path P1 is the path on which the sheet S transported from the image forming unit 5 is transported. The discharge path P2 branches off from the transport path P1 at the branching point Ps and is the path on which the sheet S is transported to the output tray 23a. The re-transport path P3 branches off from the transport path P1 and the discharge path P2 at the branching point Ps and is the path on which the sheet S, on which an image has been formed on one side by the image forming unit 5, is re-transported to the supply path P0.

[0050] The discharge path P2 extends upward from the branching point Ps, then bends and extends forward. The re-transport path P3 extends downward from the branching point Ps, then bends and extends forward below the sheet cassette 31, then bends upward in front of the sheet cassette 31, and merges with the supply path P0 at the merging point Pj. By positioning the re-transport path P3 below the sheet cassette 31, the path length can be secured, and the degree of freedom when storing the sheets S in the re-transport path P3 can be increased.

[0051] The conveying roller 34a is located between the first feeding roller 32 and the resist roller 35a in the feeding path P0, and is located downstream of the confluence point Pj in the sheet conveying direction.

[0052] Downstream of the sheet transport unit 6 in the transport path P1, there is a pair of post-fixing rollers 71 for transporting the sheet. At the downstream end of the sheet transport direction in the discharge path P2, there is a pair of discharge rollers 73 for transporting the sheet S, and upstream of the discharge roller pair 73 in the sheet transport direction in the discharge path P2, there is a pair of intermediate discharge rollers 72 for transporting the sheet S.

[0053] An intermediate retransport roller pair 75 for transporting the sheet S is located at the end of the retransport path P3 on the side of the branching point Ps. Downstream from the intermediate retransport roller pair 75 in the sheet transport direction in the retransport path P3, there is a first retransport roller 76a and a driven roller 76b opposite the first retransport roller 76a, and a second retransport roller 77a and a driven roller 77b opposite the second retransport roller 77a. The second retransport roller 77a and the driven roller 77b are located downstream from the first retransport roller 76a and the driven roller 76b in the sheet transport direction. The first retransport roller 76a and the second retransport roller 77a transport the sheet S toward the supply path P0.

[0054] The main body of the device 2 has a first motor 81, a second motor 82, and a third motor 83. The first motor 81 drives the first feed roller 32, the fixed roller pair 71, the first re-transport roller 76a, and the second re-transport roller 77a.

[0055] A first electromagnetic clutch 84 is interposed between the first motor 81 and the first feed roller 32. By turning the first electromagnetic clutch 84 on or off, the transmission and interruption of driving force from the first motor 81 to the first feed roller 32 can be switched on and off.

[0056] A second electromagnetic clutch 85 is interposed between the first motor 81 and the first re-transport roller 76a and the second re-transport roller 77a. By turning the second electromagnetic clutch 85 on or off, the transmission and interruption of driving force from the first motor 81 to the first re-transport roller 76a and the second re-transport roller 77a can be switched.

[0057] Specifically, the second electromagnetic clutch 85 switches to a transmission state when turned ON, which transmits driving force from the first motor 81 to the first re-transport roller 76a and the second re-transport roller 77a, and switches to a disconnection state when turned OFF, which disconnects the transmission of driving force from the first motor 81 to the first re-transport roller 76a and the second re-transport roller 77a. The second electromagnetic clutch 85 is an example of a clutch that switches between a transmission state in which driving force is transmitted to the re-transport rollers and a disconnection state in which the transmission of driving force to the re-transport rollers is disconnected.

[0058] As a clutch that switches between transmitting and disconnecting the driving force from the first motor 81, a clutch mechanism that switches between transmitting and disconnecting the driving force by a mechanical structure can be used. However, by using electromagnetic clutches such as the first electromagnetic clutch 84 and the second electromagnetic clutch 85, it is possible to switch between the state of driving force transmission and disconnection without using a complex mechanism.

[0059] The second motor 82 drives the discharge roller pair 73 and the intermediate discharge roller pair 72. The discharge roller pair 73 and the intermediate discharge roller pair 72 are configured so that their rotation direction can be switched between forward and reverse rotation by the second motor 82. When the discharge roller pair 73 and the intermediate discharge roller pair 72 are rotating in the forward direction, they can transport the sheet S toward the paper output tray 23a, and when they are rotating in the reverse direction, they can transport the sheet S toward the re-transport path P3. As a result, the sheet S is configured so that its transport direction in the discharge path P2 can be reversed.

[0060] The third motor 83 drives the intermediate re-transport roller pair 75. The intermediate re-transport roller pair 75 transports the sheet S, which has been transported toward the re-transport path P3 by the intermediate discharge roller pair 72, downstream in the sheet transport direction along the re-transport path P3. The sheet S transported by the intermediate re-transport roller pair 75 is further transported downstream in the sheet transport direction along the re-transport path P3 by the first re-transport roller 76a and driven roller 76b, and the second re-transport roller 77a and driven roller 77b, until it reaches the supply path P0.

[0061] The image forming apparatus 1 is equipped with a first sheet sensor 91, a second sheet sensor 92, a third sheet sensor 93, a fourth sheet sensor 94, a fifth sheet sensor 95, and a sixth sheet sensor 96 for detecting sheets.

[0062] The first sheet sensor 91 is located between the first re-transport roller 76a and the second re-transport roller 77a in the re-transport path P3. The second sheet sensor 92 is located between the transport roller 34a and the resist roller 35a in the feed path P0. The third sheet sensor 93 is located between the first feed roller 32 and the resist roller 35a in the feed path P0, more specifically between the first feed roller 32 and the merging point Pj in the feed path P0.

[0063] The fourth sheet sensor 94 is located between the resist roller 35a and the image forming unit 5 in the feeding path P0. The fifth sheet sensor 95 is located between the fuser 6 and the fixed roller pair 71 in the transport path P1. The sixth sheet sensor 96 is located between the intermediate retransport roller pair 75 and the first retransport roller 76a in the retransport path P3.

[0064] As shown in Figure 2, the image forming apparatus 1 includes a control unit 9, to which a first motor 81, a second motor 82, a third motor 83, a first electromagnetic clutch 84, and a second electromagnetic clutch 85 are connected. The control unit 9 includes, for example, a CPU, ROM, RAM, EEPROM, and an ASIC. The ROM stores programs for controlling the operations performed by the CPU. The RAM is used as a storage area for temporarily recording data and signals used by the CPU when executing the above programs, or as a work area for data processing. The EEPROM stores settings and flags that should be retained even after the power is turned off. The first motor 81, second motor 82, third motor 83, first electromagnetic clutch 84, and second electromagnetic clutch 85 are connected to the ASIC. The ASIC generates drive signals to drive the first motor 81, second motor 82, third motor 83, first electromagnetic clutch 84, and second electromagnetic clutch 85, and controls each motor and electromagnetic clutch based on these drive signals. Furthermore, the control unit 9 is connected to the first sheet sensor 91, the second sheet sensor 92, the third sheet sensor 93, the fourth sheet sensor 94, the fifth sheet sensor 95, and the sixth sheet sensor 96.

[0065] The control unit 9 can switch the rotation and stopping of the fixed roller pair 71 by controlling the first motor 81. The control unit 9 can switch the rotation and stopping of the first feed roller 32 by controlling the first motor 81 and the first electromagnetic clutch 84. The control unit 9 can switch the rotation and stopping of the first re-transport roller 76a and the second re-transport roller 77a by controlling the first motor 81 and the second electromagnetic clutch 85.

[0066] The control unit 9 can switch the rotation direction of the discharge roller pair 73 and the intermediate discharge roller pair 72, as well as switch between rotation and stopping, by controlling the second motor 82. The control unit 9 can also switch between rotation and stopping of the intermediate re-transport roller pair 75 by controlling the third motor 83.

[0067] In the image forming apparatus 1 configured in this way, when performing double-sided printing to form images on both sides of a sheet S, the printing speed can be improved by having the control unit 9 perform high-speed transport control, which prints on the sheets S in a printing order that shortens the spacing between the transported sheets S.

[0068] For example, the control unit 9 can execute a 2416 transport control as a high-speed transport control, in which printing is performed in the following order: one side of the first sheet (page 2) → one side of the second sheet (page 4) → the second side of the first sheet (page 1) → one side of the third sheet (page 6). Alternatively, the control unit 9 can execute a 2461 transport control as a high-speed transport control, in which printing is performed in the following order: one side of the first sheet (page 2) → one side of the second sheet (page 4) → one side of the third sheet (page 6) → the second side of the first sheet (page 1).

[0069] In this way, by performing high-speed transport control when printing a job onto sheet S and performing double-sided printing, it is possible to set the printing order to an efficient order and improve the printing speed. A job is a print command input to the image forming apparatus 1, and a job includes, for example, image data to be printed on sheet S, the number of pages to be printed, print mode information such as color printing, monochrome printing, or single-sided printing, double-sided printing. A job can be input to the image forming apparatus 1 via a network from an external device such as a PC or mobile terminal connected to the image forming apparatus 1, or it can be generated by the image forming apparatus 1 using a user interface such as a touch panel provided on the image forming apparatus 1. Specifically, printing a job onto sheet S means printing the image data corresponding to each page included in the job onto each side of sheet S based on settings such as the print mode information included in the job, and by printing onto sheet S, images corresponding to each page of the job are formed on each side of sheet S.

[0070] As mentioned above, printing speed can be improved by performing high-speed printing control when printing a job on both sides of sheet S. However, when printing multiple jobs consecutively, there is room for further improvement in printing speed by considering the printing order between the multiple jobs.

[0071] Accordingly, the control unit 9 is configured to perform continuous double-sided printing, which prints at least three pages of preceding jobs and at least two pages of succeeding jobs in succession, by feeding a first sheet S, which is the first sheet corresponding to the succeeding jobs, from the sheet cassette 31 during the printing of the preceding jobs, printing an image corresponding to the succeeding jobs on one side of the first sheet, and then, with the first sheet having an image printed on one side, located in the feeding path P0 or the re-transport path P3, execute transport control to eject the second sheet S, which is the last sheet corresponding to the preceding jobs, into the output tray 23a.

[0072] In this case, the total number of pages in the preceding job can be odd or even, as long as it is 3 or more pages, and the total number of pages in the succeeding job can be odd or even, as long as it is 2 or more pages.

[0073] By implementing this type of transport control, the transport interval between the preceding and succeeding jobs on the sheet S can be reduced, thereby improving the printing speed.

[0074] [Conveyor control] Next, we will describe the transport control of the sheets S during double-sided printing performed by the control unit 9, particularly the 2416 transport control, which, when double-sided printing at least three pages of preceding jobs and at least two pages of succeeding jobs in succession, feeds the first sheet S corresponding to the succeeding jobs from the sheet cassette 31 during the printing of the preceding jobs, prints the image corresponding to the succeeding jobs on one side of the first sheet, and then, with the first sheet having the image printed on one side in the feed path P0 or re-transport path P3, ejects the second sheet, the last sheet S corresponding to the preceding jobs, into the output tray 23a.

[0075] The following describes the 2416 transport control when the preceding job has 5 pages and the succeeding job has 5 pages, and the preceding and succeeding jobs are printed consecutively.

[0076] A preceding job is the first job in a sequence of jobs that are printed consecutively, while a succeeding job is the job that follows the preceding job and begins printing after the preceding job has started.

[0077] Furthermore, the first sheet S on which the preceding job is printed, which is first fed from the sheet cassette 31, is referred to as the preceding sheet AS1, the second sheet S fed from the sheet cassette 31 after the preceding sheet AS1 is referred to as the next sheet AS2, and the third sheet S fed from the sheet cassette 31 after the next sheet AS2 is referred to as the next-next sheet AS3. The next-next sheet AS3 is an example of the second sheet, which is the last sheet corresponding to the preceding job. The next sheet AS2 is an example of the third sheet fed from the sheet tray before the second sheet.

[0078] Specifically, the first sheet S on which the image corresponding to the second page of the preceding job is printed on one side is referred to as the preceding sheet AS1-2, the first sheet S on which the image corresponding to the first page of the preceding job is printed on two sides is referred to as the preceding sheet AS1-1, the second sheet S on which the image corresponding to the fourth page of the preceding job is printed on one side is referred to as the next sheet AS2-2, the second sheet S on which the image corresponding to the third page of the preceding job is printed on two sides is referred to as the next sheet AS2-1, and the third sheet on which the image corresponding to the fifth page, which is the final page of the preceding job, is printed on one side is referred to as the next-next sheet AS3-2.

[0079] Furthermore, the first sheet S on which subsequent jobs are printed, which is initially fed from the sheet cassette 31, is referred to as the preceding sheet BS1; the second sheet S fed from the sheet cassette 31 after the preceding sheet BS1 is referred to as the next sheet BS2; and the third sheet S fed from the sheet cassette 31 after the next sheet BS2 is referred to as the subsequent sheet BS3. The preceding sheet BS1 is an example of the first sheet, which is the first sheet corresponding to the subsequent job.

[0080] Specifically, the first sheet S on which the image corresponding to the second page of the subsequent job is printed on one side is referred to as the preceding sheet BS1-2, the first sheet S on which the image corresponding to the first page of the subsequent job is printed on two sides is referred to as the preceding sheet BS1-1, the second sheet S on which the image corresponding to the fourth page of the subsequent job is printed on one side is referred to as the next sheet BS2-2, the second sheet S on which the image corresponding to the third page of the subsequent job is printed on two sides is referred to as the next sheet BS2-1, and the third sheet on which the image corresponding to the fifth page, which is the final page of the subsequent job, is printed on one side is referred to as the sheet after that BS3-2.

[0081] The preceding sheet AS1-2 switches to the preceding sheet AS1-1 when the transport direction is reversed in the discharge path P2, and the next sheet AS2-2 switches to the next sheet AS2-1 when the transport direction is reversed in the discharge path P2. Similarly, the preceding sheet BS1-2 switches to the preceding sheet BS1-1 when the transport direction is reversed in the discharge path P2, and the next sheet BS2-2 switches to the next sheet BS2-1 when the transport direction is reversed in the discharge path P2.

[0082] As shown in Figure 3, in the transport control by the control unit 9 in this embodiment, printing is performed on the sheets S in the following order: preceding sheet AS1-2 (second page of the preceding job) → next sheet AS2-2 (fourth page of the preceding job) → preceding sheet AS1-1 (first page of the preceding job) → preceding sheet BS1-2 (second page of the following job) → next sheet AS2-1 (third page of the preceding job) → next sheet AS3-2 (fifth page of the preceding job) → next sheet BS2-2 (fourth page of the following job) → preceding sheet BS1-1 (first page of the following job) → next sheet BS2-1 (third page of the following job) → next sheet BS3-2 (fifth page of the following job).

[0083] In this case, the printing order for a preceding job with 5 pages is as follows: print the image corresponding to the second page of the preceding job on the preceding sheet AS1-2, which will be the first side of the first sheet → print the image corresponding to the fourth page of the preceding job on the next sheet AS2-2, which will be the first side of the second sheet → print the image corresponding to the first page of the preceding job on the preceding sheet AS1-1, which will be the second side of the first sheet → print the image corresponding to the third page of the preceding job on the next sheet AS2-1, which will be the second side of the second sheet → print the image corresponding to the fifth page of the preceding job on the sheet after that AS3-2, which will be the first side of the third sheet.

[0084] Furthermore, for a subsequent job with 5 pages, the printing order is as follows: print the image corresponding to the second page of the subsequent job on the preceding sheet BS1-2, which is the first side of the first sheet → print the image corresponding to the fourth page of the subsequent job on the next sheet BS2-2, which is the first side of the second sheet → print the image corresponding to the first page of the subsequent job on the preceding sheet BS1-1, which is the second side of the first sheet → print the image corresponding to the third page of the subsequent job on the next sheet BS2-1, which is the second side of the second sheet → print the image corresponding to the fifth page of the subsequent job on the sheet BS3-2, which is the first side of the third sheet.

[0085] Figure 4 shows a flowchart of the transport control. Transport control is initiated when a print instruction is input to the control unit 9. As shown in Figure 5, when transport control is initiated, the control unit 9 causes the lead sheet AS1-2, supported by the sheet cassette 31, to be fed to the feed path P0 (step S01). In this case, the control unit 9 drives the first motor 81 and turns on the first electromagnetic clutch 84 to transmit the driving force from the first motor 81 to the first feed roller 32, thereby rotating the first feed roller 32.

[0086] The lead sheet AS1-2, supported by the sheet cassette 31, is fed to the feeding path P0 by the first feeding roller 32. The lead sheet AS1-2 fed to the feeding path P0 is then transported to the image forming unit 5 by the transport roller 34a and the register roller 35a. During the process of transporting the lead sheet AS1-2 to the image forming unit 5, when the leading edge of the lead sheet AS1-2 reaches the position of the fourth sheet sensor 94, the fourth sheet sensor 94 switches from the off state to the on state and transmits a detection signal to the control unit 9. The control unit 9 detects the lead sheet AS1-2 based on the detection signal received from the fourth sheet sensor 94. When the rear end of the lead sheet AS1-2 passes the fourth sheet sensor 94, the fourth sheet sensor 94 switches from the on state to the off state and transmits a detection signal to the control unit 9. The control unit 9 detects that the preceding sheet AS1-2 has passed the fourth sheet sensor 94 based on the detection signal received from the fourth sheet sensor 94.

[0087] The lead sheet AS1-2 is transported to the transport path P1 after the image corresponding to the second page of the lead job is printed on its entire surface in the image forming unit 5. When the leading edge of the lead sheet AS1-2 being transported to the transport path P1 reaches the position of the fifth sheet sensor 95, the fifth sheet sensor 95 switches from the off state to the on state and transmits a detection signal to the control unit 9. The control unit 9 detects the lead sheet AS1-2 based on the detection signal received from the fifth sheet sensor 95. The lead sheet AS1-2 being transported along the transport path P1 is further transported to the discharge path P2 through the branching point Ps. The lead sheet AS1-2 is transported toward the paper output tray 23a by the intermediate discharge roller pair 72 and the discharge roller pair 73, which rotate in the forward rotation direction. When the rear end of the lead sheet AS1-2 being transported along the discharge path P2 passes the fifth sheet sensor 95, the fifth sheet sensor 95 switches from the on state to the off state and transmits a detection signal to the control unit 9. The control unit 9 detects that the preceding sheet AS1-2 has passed the fifth sheet sensor 95 based on the detection signal received from the fifth sheet sensor 95.

[0088] As shown in Figure 6, after feeding the preceding sheet AS1-2 in step S01, the control unit 9 causes the next sheet AS2-2, supported by the sheet cassette 31, to be fed into the feeding path P0 by the first feeding roller 32 (step S02). In this case, the control unit 9 can start feeding the next sheet AS2-2 at a predetermined time after detecting, for example, that the rear end of the preceding sheet AS1-2 has passed the fifth sheet sensor 95.

[0089] Furthermore, after a predetermined time has elapsed since the control unit 9 detected that the rear end of the leading sheet AS1-2 had passed the fifth sheet sensor 95, it switches the rotation direction of the second motor 82 from the forward rotation direction to the reverse rotation direction. As a result, the rotation direction of the intermediate discharge roller pair 72 and the discharge roller pair 73 is switched from the forward rotation direction to the reverse rotation direction, and the leading sheet AS1-2 is transported toward the re-transport path P3 with its transport direction reversed. The leading sheet AS1-2 is then switched to leading sheet AS1-1 due to the reversal of its transport direction.

[0090] As shown in Figure 7, the lead sheet AS1-1 is transported to the retransport path P3 via the branching point Ps. The lead sheet AS1-1 is transported along the retransport path P3 by the intermediate retransport roller pair 75, and then further transported along the retransport path P3 by the first retransport roller 76a and the second retransport roller 77a.

[0091] In this case, the control unit 9 can start driving the third motor 83 at the timing when the rotation direction of the second motor 82 switches from forward rotation to reverse rotation, thereby rotating the intermediate re-transport roller pair 75. Also, when the leading edge of the lead sheet AS1-1 reaches the sixth sheet sensor 96, the sixth sheet sensor 96 switches from the off state to the on state, and the sixth sheet sensor 96 transmits a detection signal to the control unit 9. Based on the detection signal received from the sixth sheet sensor 96, the control unit 9 detects that the leading edge of the lead sheet AS1-1 has reached the sixth sheet sensor 96. For example, after a predetermined time has elapsed since detecting that the leading edge of the lead sheet AS1-1 has reached the sixth sheet sensor 96, the control unit 9 can turn on the second electromagnetic clutch 85 and rotate the first re-transport roller 76a and the second re-transport roller 77a.

[0092] When the leading edge of the lead sheet AS1-1 being transported along the re-transport path P3 reaches the first sheet sensor 91, the first sheet sensor 91 switches from the off state to the on state and transmits a detection signal to the control unit 9. Based on the detection signal received from the first sheet sensor 91, the control unit 9 detects that the leading edge of the lead sheet AS1-1 has reached the first sheet sensor 91. Then, when the trailing edge of the lead sheet AS1-1 being transported along the re-transport path P3 passes the first sheet sensor 91, the first sheet sensor 91 switches from the on state to the off state and transmits a detection signal to the control unit 9. Based on the detection signal received from the first sheet sensor 91, the control unit 9 detects that the lead sheet AS1-1 has passed the first sheet sensor 91.

[0093] In step S02, the next sheet AS2-2, which was fed into the feeding path P0, is transported to the image forming unit 5. In the image forming unit 5, an image corresponding to the fourth page of the preceding job is printed on one side of the sheet, and then it is transported to the transport path P1. The next sheet AS2-2 is further transported to the discharge path P2 via the branching point Ps. The transport of the next sheet AS2-2 from the feeding path P0 to the image forming unit 5, and from the transport path P1 to the discharge path P2, is carried out in the same manner as for the preceding sheet AS1-2. The same applies to subsequent sheets S.

[0094] As shown in Figure 8, the preceding sheet AS1-1, which is transported along the retransportation path P3, is retransported from the retransportation path P3 to the supply path P0 via the merging point Pj (step S03). The preceding sheet AS1-1 is transported to the supply path P0 after the next sheet AS2-2 is supplied to the supply path P0 in step S02.

[0095] In this case, if the control unit 9 detects that there is a risk of interference between the preceding sheet AS1-1 and the next sheet AS2-2 located downstream in the sheet transport direction when the preceding sheet AS1-1 is retransported to the feed path P0, it may decelerate the transport of the preceding sheet AS1-1 along the retransport path P3 or stop the preceding sheet AS1-1 within the retransport path P3 for a predetermined time. The situation in which there is a risk of interference between the preceding sheet AS1-1 and the next sheet AS2-2 is, for example, when the feeding timing of the next sheet AS2-2 from the sheet cassette 31 is delayed.

[0096] Furthermore, the next sheet AS2-2, which is transported along the discharge path P2, is transported towards the re-transport path P3 with the transport direction reversed, just as with the preceding sheet AS1-2. The same applies to subsequent sheets S. The next sheet AS2-2 switches to the next sheet AS2-1 when the transport direction is reversed.

[0097] The next sheet AS2-1, which has been switched over from the previous sheet AS2-2 due to the reversal of the conveying direction, is conveyed along the re-conveying path P3 by the intermediate re-conveying roller pair 75, just as in the case of the preceding sheet AS1-1, and is further conveyed along the re-conveying path P3 by the first re-conveying roller 76a and the second re-conveying roller 77a. The same applies to subsequent sheets S.

[0098] As shown in Figure 9, in step S03, after the lead sheet AS1-1 has been re-transported to the feed path P0, the control unit 9 feeds the lead sheet BS1-2 from the sheet cassette 31 to the feed path P0 (step S04). In this case, the control unit 9 can start feeding the lead sheet BS1-2 after a predetermined time has elapsed since detecting, for example, that the rear end of the lead sheet AS1-1 has passed the second sheet sensor 92. The method of detecting the sheet by the second sheet sensor 92 is the same as for the other sheet sensors. The control unit 9 can also start feeding the lead sheet BS1-2 after a predetermined time has elapsed since detecting that the rear end of the lead sheet AS1-1 has passed the first sheet sensor 91.

[0099] The preceding sheet AS1-1, which was being transported to the feed path P0, is then transported to the image forming unit 5, where the image corresponding to the first page of the preceding job is printed on both sides. When the preceding sheet BS1-2 is being fed to the feed path P0, the next sheet AS2-1 is located on the re-transport path P3.

[0100] As shown in Figure 10, the next sheet AS2-1, which was being transported along the retransportation path P3, is retransported from the retransportation path P3 to the supply path P0 via the merging point Pj in step S04, after the preceding sheet BS1-2 has been supplied to the supply path P0 (step S05). In this case, if there is a risk of interference between the next sheet AS2-1 and the preceding sheet BS1-2, the control unit 9 can, similar to the case of the preceding sheet AS1-1, decelerate the transport of the next sheet AS2-1 along the retransportation path P3, or stop the next sheet AS2-1 within the retransportation path P3 for a predetermined time.

[0101] The preceding sheet BS1-2, which was fed to the supply path P0, is transported to the image forming unit 5, where the image corresponding to the second page of the subsequent job is printed on one side, and then transported to the transport path P1. In other words, the preceding sheet BS1-2, which is the first sheet S corresponding to the subsequent job, is fed from the sheet cassette 31 during the printing of the preceding job, and the image corresponding to the subsequent job is printed on one side of the preceding sheet BS1-2. The preceding sheet BS1-2 is then transported to the discharge path P2 via the branching point Ps.

[0102] Furthermore, the lead sheet AS1-1, on which the image corresponding to the first page of the lead job is printed on both sides, is transported along the transport path P1 and the discharge path P2 and discharged into the output tray 23a. In this case, the lead sheet AS1-1 is fed from the sheet cassette 31, passes through the image forming unit 5, has its transport direction reversed in the discharge path P2, passes through the image forming unit 5 again via the re-transport path P3, and is then discharged into the output tray 23a. As a result, the lead sheet AS1-1 discharged into the output tray 23a has both sides facing downwards. In this way, the control unit 9 performs double-sided printing on the lead sheet AS1, printing on both one side and both sides.

[0103] As shown in Figure 11, the lead sheet BS1-2, which was being transported along the discharge path P2, reverses its transport direction and is transported towards the re-transport path P3. The lead sheet BS1-2 is switched to lead sheet BS1-1 by the re-transport roller pair 75, and is further transported along the re-transport path P3 by the first re-transport roller 76a and the second re-transport roller 77a.

[0104] The control unit 9 determines whether the first sheet sensor 91 has detected the leading edge of the leading sheet BS1-1 when the leading edge of the leading sheet BS1-1 is being transported along the re-transport path P3, that is, whether the first sheet sensor 91 has switched from the off state to the on state because the leading edge of the leading sheet BS1-1 has reached the first sheet sensor 91 (step S06). If the control unit 9 determines in step S06 that the first sheet sensor 91 has not detected the leading edge of the leading sheet BS1-1 (step S06; N), it repeats step S06 until it determines that the first sheet sensor 91 has detected the leading edge of the leading sheet BS1-1.

[0105] In step S06, when the control unit 9 determines that the first sheet sensor 91 has detected the leading edge of the preceding sheet BS1-1 (step S06; Y), after a predetermined time has elapsed since the first sheet sensor 91 detected the leading edge of the preceding sheet BS1-1, it switches the second electromagnetic clutch 85 from the transmission state to the disconnection state to disconnect the transmission of driving force from the first motor 81 to the first re-transport roller 76a and the second re-transport roller 77a, thereby stopping the preceding sheet BS1-1 within the re-transport path P3 (step S07).

[0106] In other words, the control unit 9 switches the second electromagnetic clutch 85 from a transmission state to a disconnection state when the first sheet sensor 91 detects the leading edge of the preceding sheet BS1-1, which is the first sheet S corresponding to the subsequent job, and stops the preceding sheet BS1-1, which has the image corresponding to the second page of the subsequent job printed on its entire surface, within the re-transport path P3. In this case, by switching the second electromagnetic clutch 85 to the disconnection state and stopping the preceding sheet BS1-1 within the re-transport path P3, it is possible to stop the preceding sheet BS1-1, which has the subsequent job printed on its entire surface, within the re-transport path P3 while continuing to drive other drive parts.

[0107] In step S05, after the next sheet AS2-1 is re-transported to the feeding path P0, the control unit 9 determines whether the second sheet sensor 92 has detected the rear end of the next sheet AS2-1, that is, whether the second sheet sensor 92 has switched from the ON state to the OFF state as the rear end of the next sheet AS2-1 has passed through the second sheet sensor 92 (step S08). In step S08, if the control unit 9 determines that the second sheet sensor 92 has not detected the rear end of the next sheet AS2-1 (step S08; N), it repeatedly executes step S08 until it determines that the second sheet sensor 92 has detected the rear end of the next sheet AS2-1.

[0108] In step S08, when the control unit 9 determines that the second sheet sensor 92 has detected the rear end of the next sheet AS2-1 (step S08; Y), it feeds the next-next sheet AS3-2, which is the last sheet S corresponding to the preceding job, from the sheet cassette 31 to the feeding path P0 after a predetermined time has elapsed since the second sheet sensor 92 detected the rear end of the next sheet AS2-1 (step S09).

[0109] In other words, the control unit 9 starts feeding the next sheet AS3-2 to the feeding path P0, provided that the second sheet sensor 92 detects the rear end of the next sheet AS2-1, which is the next sheet AS2 that was fed from the sheet cassette 31 before the next sheet AS3-2 and has an image corresponding to the preceding job printed on its entire surface.

[0110] The next sheet AS2-1, which was being transported to the supply path P0, is then transported to the image forming unit 5. In the image forming unit 5, an image corresponding to the third page of the preceding job is printed on both sides, and then the sheet is transported to the transport path P1.

[0111] As shown in Figure 12, the next sheet AS2-1, on which the image corresponding to the third page of the preceding job is printed on both sides and transported along the transport path P1, is further transported along the discharge path P2 before being discharged into the output tray 23a. In this case, the next sheet AS2-1 is fed from the sheet cassette 31, passes through the image forming unit 5, has its transport direction reversed in the discharge path P2, passes through the image forming unit 5 again via the re-transport path P3, and is then discharged into the output tray 23a. As a result, the next sheet AS2-1 discharged into the output tray 23a has both sides facing downwards. In this way, the control unit 9 performs double-sided printing on the next sheet AS2, printing on both sides.

[0112] Furthermore, the next sheet AS3-2, which was fed to the feed path P0 in step S09, is transported to the image forming unit 5. After the image forming unit 5 prints an image corresponding to the 5th page of the preceding job on one surface, it is transported to the transport path P1.

[0113] In step S09, after the next sheet AS3-2 has been fed to the feeding path P0, the control unit 9 feeds the next sheet BS2-2 from the sheet cassette 31 to the feeding path P0 (step S10). In this case, the control unit 9 can start feeding the next sheet BS2-2 after a predetermined time has elapsed since detecting that the rear end of the next sheet AS3-2 has passed the third sheet sensor 93. Alternatively, the control unit 9 can start feeding the next sheet BS2-2 after a predetermined time has elapsed since detecting that the rear end of the next sheet AS3-2 has passed the second sheet sensor 92. In Figure 12, the preceding sheet BS1-1 is stopped in the re-transport path P3.

[0114] As shown in Figure 13, the next sheet AS3-2, which has the image corresponding to the 5th page of the preceding job printed on one side and is transported along the transport path P1, is further transported along the discharge path P2 and then discharged into the output tray 23a without being transported again along the re-transport path P3. In this case, the next sheet AS3-2 is fed from the sheet cassette 31, passes through the image forming unit 5, and then discharged into the output tray 23a without the transport direction being reversed in the discharge path P2. Therefore, one side of the next sheet AS3-2 discharged into the output tray 23a faces downwards.

[0115] In this way, the control unit 9 performs printing on the next sheet AS3-2 in single-sided printing mode, printing only on one side of the next sheet AS3-2. In other words, when printing the preceding job on sheet S, the control unit 9 prints on the preceding sheet AS1 and the next sheet AS2 in double-sided printing mode, then switches the printing mode from double-sided printing mode to single-sided printing mode, and performs printing on the next sheet AS3-2 in single-sided printing mode.

[0116] Furthermore, the control unit 9 stops the preceding sheet BS1-1, which has the image corresponding to the second page of the subsequent job printed on its surface, within the re-transport path P3, and then feeds the next sheet AS3-2 from the sheet cassette 31 to the image forming unit 5. After printing the image corresponding to the fifth page, which is the final page of the preceding job, on one side of the fed next sheet AS3-2, the next sheet AS3-2, which has the image corresponding to the fifth page of the preceding job printed on its surface, is ejected to the output tray 23a without being re-transported to the re-transport path P3.

[0117] This allows for setting an appropriate transport interval between the next sheet AS3-2, which has the final page of the preceding job printed on an odd-numbered page, and the preceding sheet BS1-1, on which the image of the subsequent job is formed, thereby improving the printing speed. It also allows for improving the printing speed of the preceding job, which has an odd number of pages.

[0118] In this embodiment, the control unit 9 stops the preceding sheet BS1-1, which has an image printed on one side, within the re-transport path P3, and then feeds the next sheet AS3-2 from the sheet cassette 31 to the image forming unit 5. The next sheet AS3-2, which has an image printed on one side, is then ejected to the output tray 23a without being re-transported back to the re-transport path P3.

[0119] However, the control unit 9 can also, when at least a portion of the preceding sheet BS1-1, which has an image printed on one side, is located within the re-transport path P3, feed the next sheet AS3-2 from the sheet cassette 31 to the image forming unit 5, print an image corresponding to the final page of the subsequent job on one side of the fed next sheet AS3-2, and discharge the next sheet AS3-2, which has an image printed on one side, into the output tray 23a without re-transporting it to the re-transport path P3.

[0120] Furthermore, when the control unit 9 prints on the successive sheet AS3-2 in single-sided printing mode, it controls at least one of the following: the transfer current that flows when voltage is applied to the transfer roller 42, and the fixing temperature when the fuser 6 fixes the toner image to the sheet S, compared to when printing on the sheet S in double-sided printing mode.

[0121] For example, when the control unit 9 prints on the next sheet AS3-2 in single-sided printing mode, it performs at least one of the following: a control that increases the transfer current value compared to when printing in double-sided printing mode, and a control that increases the fixing temperature compared to when printing in double-sided printing mode.

[0122] Since the moisture content of the sheet S, which has been heated once after passing through the fuser 6, decreases and its resistance increases, in single-sided printing mode, when printing on the next sheet AS3-2 that is ejected after the last page of the preceding job has been printed on one side, it is possible to keep the voltage value of the transfer roller 42 constant by setting the transfer current value higher than in double-sided printing mode, when printing on a sheet S on which images are formed on both sides. This makes it possible to make the print quality uniform in single-sided printing mode and double-sided printing mode.

[0123] Furthermore, since the sheet S, once it has passed through the fuser unit 6, becomes hotter due to heating, it is possible to keep the fixing temperature constant by setting a higher fixing temperature in single-sided printing mode than in double-sided printing mode, thereby making it possible to achieve uniform print quality in both single-sided and double-sided printing modes.

[0124] In step S10, the next sheet BS2-2, which was fed to the supply path P0, is transported to the image forming unit 5. In the image forming unit 5, an image corresponding to the fourth page of the subsequent job is printed on one side of the sheet, and then it is transported to the transport path P1. The next sheet BS2-2 is further transported to the discharge path P2 via the branching point Ps.

[0125] In step S10, after the next sheet BS2-2 is fed to the feeding path P0, the control unit 9 determines whether the third sheet sensor 93 has detected the rear end of the next sheet BS2-2, that is, whether the third sheet sensor 93 has switched from the ON state to the OFF state as the rear end of the next sheet BS2-2 has passed through the third sheet sensor 93 (step S11). If the control unit 9 determines in step S11 that the third sheet sensor 93 has not detected the rear end of the next sheet BS2-2 (step S11; N), it repeatedly executes step S11 until it determines that the third sheet sensor 93 has detected the rear end of the next sheet BS2-2.

[0126] In step S11, when the control unit 9 determines that the third sheet sensor 93 has detected the rear end of the next sheet BS2-2 (step S11; Y), after a predetermined time has elapsed since the third sheet sensor 93 detected the rear end of the next-next sheet BS2-2, it turns on the second electromagnetic clutch 85 to rotate the first re-transport roller 76a and the second re-transport roller 77a, and resumes transporting the preceding sheet BS1-1 that has stopped in the re-transport path P3. The preceding sheet BS1-1, whose transport has resumed, is re-transported from the re-transport path P3 through the merging point Pj to the supply path P0 (step S12).

[0127] In other words, the control unit 9, on the condition that the third sheet sensor 93 detects the trailing end of the next sheet BS2-2 which is fed from the sheet cassette 31 after the next sheet AS3-2, starts the re-transportation of the preceding sheet BS1-1, which is stopped in the re-transportation path P3, to the feeding path P0.

[0128] Furthermore, the control unit 9 can, when it is advantageous to reduce the transport interval of each sheet S being transported, initiate the re-transportation of the preceding sheet BS1-1 that was stopped in the re-transportation path P3, and, with the preceding sheet BS1-1 positioned in the supply path P0, eject the next sheet AS3-2, which has an image printed on one side, into the output tray 23a.

[0129] As shown in Figure 14, the next sheet BS2-2, which is transported along the discharge path P2, reverses its transport direction and is transported towards the re-transport path P3. The next sheet BS2-2 is switched to the next sheet BS2-1 by the re-transport roller pair 75, and is further transported along the re-transport path P3 by the first re-transport roller 76a and the second re-transport roller 77a.

[0130] In step S12, transport from the retransport path P3 is resumed, and the preceding sheet BS1-1, which has been retransported to the feed path P0, is transported to the image forming unit 5, where the image corresponding to the first page of the subsequent job is printed on both sides of the image forming unit 5.

[0131] As shown in Figure 15, the lead sheet BS1-1, on which the image corresponding to the first page of the subsequent job is printed on both sides, is transported along the transport path P1 and the discharge path P2 and discharged into the output tray 23a. In this case, the lead sheet BS1-1 is fed from the sheet cassette 31, passes through the image forming unit 5, has its transport direction reversed in the discharge path P2, passes through the image forming unit 5 again via the re-transport path P3, and is then discharged into the output tray 23a. As a result, the lead sheet BS1-1 discharged into the output tray 23a has both sides facing downwards. In this way, the control unit 9 performs double-sided printing on the lead sheet BS1, printing on both sides.

[0132] In step S12, after the preceding sheet BS1-1 has been retransported to the supply path P0, the control unit 9 causes the next sheet BS2-1, which is being transported along the retransport path P3, to be retransported from the retransport path P3 to the supply path P0 via the merging point Pj (step S13). In this case, the control unit 9 can suppress interference between the preceding sheet BS1-1 and the next sheet BS2-1 by, for example, detecting that the rear end of the preceding sheet BS1-1 has passed the second sheet sensor 92 and then waiting for a predetermined amount of time to elapse before retransporting the next sheet BS2-1 to the supply path P0.

[0133] The next sheet BS2-1, which has been re-transported to the feed path P0, is transported to the image forming unit 5, where the image corresponding to the third page of the subsequent job is printed on both sides, and then transported to the transport path P1. The next sheet BS2-1, which has had the image corresponding to the third page of the subsequent job printed on both sides and has been transported to the transport path P1, is further transported along the discharge path P2 and then discharged into the paper output tray 23a.

[0134] In this case, the next sheet BS2-1 is fed from the sheet cassette 31, passes through the image forming unit 5, has its transport direction reversed in the discharge path P2, passes through the image forming unit 5 again via the re-transport path P3, and is then discharged into the paper output tray 23a. As a result, the next sheet BS2-1 discharged into the paper output tray 23a has two sides facing downwards. In this way, the control unit 9 performs double-sided printing on the next sheet BS2, printing on both sides.

[0135] In step S13, after the next sheet BS2-1 has been re-transported to the feeding path P0, the control unit 9 feeds the next sheet BS3-2 from the sheet cassette 31 to the feeding path P0 (step S14). In this case, the control unit 9 can feed the next sheet BS3-2 from the sheet cassette 31 to the feeding path P0 after a predetermined time has elapsed since the second sheet sensor 92 detected the rear end of the next sheet BS2-1.

[0136] In step S14, the next sheet BS3-2, which was fed to the feed path P0, is transported to the image forming unit 5. After the image corresponding to the 5th page of the subsequent job is printed on one side in the image forming unit 5, it is transported to the transport path P1. The next sheet BS3-2 is then transported along the discharge path P2 and, without being transported again to the re-transport path P3, is discharged into the output tray 23a. In this case, since the next sheet BS3-2 is fed from the sheet cassette 31, passes through the image forming unit 5, and is then discharged into the output tray 23a without the transport direction being reversed in the discharge path P2, one side of the next sheet BS3-2 discharged into the output tray 23a faces downwards.

[0137] In this way, the control unit 9 performs printing on the next sheet BS3-2 in single-sided printing mode, printing only on one side of the next sheet BS3-2. In other words, when printing a subsequent job on sheet S, the control unit 9 prints on the preceding sheet BS1 and the next sheet BS2 in double-sided printing mode, then switches the printing mode from double-sided printing mode to single-sided printing mode, and prints on the next sheet BS3-2 in single-sided printing mode.

[0138] When printing on the next sheet BS3-2 in single-sided printing mode, the control unit 9 can perform control to change at least one of the transfer current that flows when voltage is applied to the transfer roller 42 and the fixing temperature when the fuser 6 fixes the toner image to the sheet S, similar to when printing on the next sheet AS3-2 in single-sided printing mode, compared to when printing on a sheet S that is printed in double-sided printing mode.

[0139] As each sheet BS3-2 is ejected into the output tray 23a, the control unit 9 terminates its control of sheet S transport.

[0140] As described above, in the transport control by the control unit 9 in this embodiment, the first sheet S corresponding to the subsequent job, which is the preceding sheet BS1-1, is fed from the sheet cassette 31 during the printing of the preceding job. When the preceding sheet BS1-1, which has an image printed on one side, is located in the transport path P0 or the re-transport path P3, the last sheet S corresponding to the preceding job, which is the next sheet AS3-2, is ejected to the output tray 23a. This makes it possible to reduce the transport interval between the sheets S on which the preceding and subsequent jobs are printed, thereby improving the printing speed.

[0141] In particular, the control unit 9 starts feeding the next sheet AS3-2 from the sheet cassette 31 to the feeding path P0 only when the second sheet sensor 92 detects the rear end of the next sheet AS2-1, which is fed from the sheet cassette 31 before the next sheet AS3-2 and has an image corresponding to the preceding job printed on its entire surface. This allows the transport interval between the next sheet AS2-1 and the next sheet AS3-2 to be reduced, thereby improving the printing speed.

[0142] Furthermore, the control unit 9, on the condition that the third sheet sensor 93 detects the trailing end of the next sheet BS2-2 which is fed from the sheet cassette 31 after the next sheet AS3-2, starts the re-transport of the preceding sheet BS1-1, which is stopped in the re-transport path P3, to the feeding path P0. This makes it possible to reduce the transport interval between the next sheet BS2-2 fed from the sheet cassette 31 and the preceding sheet BS1-1 stopped in the re-transport path P3, thereby improving the printing speed.

[0143] Furthermore, in the transport control by the control unit 9, the printing order for a preceding job with 5 pages is as follows: print the 2nd page on the preceding sheet AS1-2 → print the 4th page on the next sheet AS2-2 → print the 1st page on the preceding sheet AS1-1 → print the 3rd page on the next sheet AS2-1 → print the 5th page of the preceding job on the sheet after that AS3-2. Similarly, the printing order for a subsequent job with 5 pages is as follows: print the 2nd page on the preceding sheet BS1-2 → print the 4th page on the next sheet BS2-2 → print the 1st page on the preceding sheet BS1-1 → print the 3rd page on the next sheet BS2-1 → print the 5th page on the sheet after that BS3-2.

[0144] This makes it possible to improve printing speed when printing two jobs, one preceding and one succeeding, both with 5 pages, on both sides.

[0145] [First modified example of transport control] In this embodiment, the 2416 transport control was described for the case where a preceding job with 5 pages and a succeeding job with 5 pages are printed consecutively. However, it is also possible to print a preceding job with 6 pages and a succeeding job with 6 pages consecutively using the 2416 transport control.

[0146] As shown in Figure 16, in the 2416 transport control that prints a preceding job with 6 pages and a succeeding job with 6 pages consecutively, printing on sheet S is performed in the following order: preceding sheet AS1-2 (page 2 of the preceding job) → next sheet AS2-2 (page 4 of the preceding job) → preceding sheet AS1-1 (page 1 of the preceding job) → next sheet AS3-2 (page 6 of the preceding job) → next sheet AS2-1 (page 3 of the preceding job) → preceding sheet BS1-2 (page 2 of the succeeding job) → next sheet AS3-1 (page 5 of the preceding job) → next sheet BS2-2 (page 4 of the succeeding job) → preceding sheet BS1-1 (page 1 of the succeeding job) → next sheet BS3-2 (page 6 of the succeeding job) → next sheet BS2-1 (page 3 of the succeeding job) → next sheet BS3-1 (page 5 of the succeeding job).

[0147] In this case, the first sheet S on which the image corresponding to the second page of the preceding job is printed on one side is described as the preceding sheet AS1-2, the first sheet S on which the image corresponding to the first page of the preceding job is printed on two sides is described as the preceding sheet AS1-1, the second sheet S on which the image corresponding to the fourth page of the preceding job is printed on one side is described as the next sheet AS2-2, the second sheet S on which the image corresponding to the third page of the preceding job is printed on two sides is described as the next sheet AS2-1, the third sheet on which the image corresponding to the sixth page of the preceding job is printed on one side is described as the next-next sheet AS3-2, and the third sheet on which the image corresponding to the fifth page of the preceding job is printed on two sides is described as the next-next sheet AS3-1.

[0148] Furthermore, the first sheet S on which the image corresponding to the second page of the subsequent job is printed on one side is referred to as the preceding sheet BS1-2, the first sheet S on which the image corresponding to the first page of the subsequent job is printed on two sides is referred to as the preceding sheet BS1-1, the second sheet S on which the image corresponding to the fourth page of the subsequent job is printed on one side is referred to as the next sheet BS2-2, the second sheet S on which the image corresponding to the third page of the subsequent job is printed on two sides is referred to as the next sheet BS2-1, the third sheet on which the image corresponding to the sixth page of the subsequent job is printed on one side is referred to as the next-next sheet BS3-2, and the third sheet on which the image corresponding to the fifth page of the subsequent job is printed on two sides is referred to as the next-next sheet BS3-1.

[0149] When the control unit 9 prints a preceding job with 6 pages on sheet S, it prints the preceding sheet AS1, the next sheet AS2, and the sheet after that AS3 in duplex printing mode. When the control unit 9 prints a subsequent job with 6 pages on sheet S, it prints the preceding sheet BS1, the next sheet BS2, and the sheet after that BS3 in duplex printing mode.

[0150] Thus, even when performing continuous double-sided printing, which involves double-sided printing of a preceding job with six even-numbered pages and a succeeding job with six even-numbered pages, the transport control can be applied in the following manner: "While the preceding job is printing, the first sheet S corresponding to the succeeding job is fed from the sheet cassette 31, the image corresponding to the succeeding job is printed on one side of the first sheet, and with the first sheet, on which the image has been printed, located in the feed path P0 or re-transport path P3, the second sheet, which is the last sheet S corresponding to the preceding job, is ejected into the output tray 23a."

[0151] Furthermore, if the preceding job has 3 or more pages and the succeeding job has 2 or more pages, this transport control can also be applied when the preceding job has an odd number of pages and the succeeding job has an even number of pages, and when the preceding job has an even number of pages and the succeeding job has an odd number of pages.

[0152] Furthermore, although this embodiment describes the case of double-sided printing of two consecutive jobs, the 2416 transport control described in this embodiment may also be applied when printing a single job. For example, if a job includes information on the number of copies to print, and the nth and n+1th copies of the job are to be printed consecutively on both sides, this transport control using the 2416 transport control can also be applied.

[0153] [Second variation of transport control] Furthermore, although this embodiment describes an example where the control unit 9 performs transport control 2416, the transport control described here, "when performing continuous double-sided printing in which at least three pages of preceding jobs and at least two pages of succeeding jobs are printed on both sides in succession, the first sheet S corresponding to the succeeding jobs is fed from the sheet cassette 31 during the printing of the preceding jobs, the image corresponding to the succeeding jobs is printed on one side of the first sheet, and the second sheet, which is the last sheet S corresponding to the preceding jobs, is ejected to the output tray 23a while the first sheet with the image printed on one side is located in the feed path P0 or the re-transport path P3," can also be applied when performing transport control 2461.

[0154] As shown in Figure 17, for example, in the 2461 transport control when the preceding job has 5 pages and the following job has 5 pages, printing on sheet S is performed in the following order: preceding sheet AS1-2 (2nd page of the preceding job) → next sheet AS2-2 (4th page of the preceding job) → preceding sheet BS1-2 (2nd page of the following job) → preceding sheet AS1-1 (1st page of the preceding job) → next sheet BS2-2 (4th page of the following job) → next sheet AS2-1 (3rd page of the preceding job) → next sheet AS3-2 (5th page of the preceding job) → preceding sheet BS1-1 (1st page of the following job) → next sheet BS2-1 (3rd page of the following job) → next sheet BS3-2 (5th page of the following job).

[0155] In this case, the first sheet S on which the image corresponding to the second page of the preceding job is printed is described as the preceding sheet AS1-2, the first sheet S on which the image corresponding to the first page of the preceding job is printed on two sides is described as the preceding sheet AS1-1, the second sheet S on which the image corresponding to the fourth page of the preceding job is printed on one side is described as the next sheet AS2-2, the second sheet S on which the image corresponding to the third page of the preceding job is printed on two sides is described as the next sheet AS2-1, and the third sheet on which the image corresponding to the fifth page, which is the final page of the preceding job, is printed on one side is described as the sheet after that AS3-2.

[0156] Furthermore, the first sheet S on which the image corresponding to the second page of the subsequent job is printed is described as the preceding sheet BS1-2, the first sheet S on which the image corresponding to the first page of the subsequent job is printed on two sides is described as the preceding sheet BS1-1, the second sheet S on which the image corresponding to the fourth page of the subsequent job is printed on one side is described as the next sheet BS2-2, the second sheet S on which the image corresponding to the third page of the subsequent job is printed on two sides is described as the next sheet BS2-1, and the third sheet on which the image corresponding to the fifth page, which is the final page of the subsequent job, is printed on one side is described as the sheet after that BS3-2.

[0157] When the control unit 9 performs double-sided printing of a preceding job with 5 pages and a succeeding job with 5 pages using 2461 transport control, when printing the preceding job on sheet S, it prints the preceding sheet AS1 and the next sheet AS2 in double-sided printing mode, then switches the printing mode from double-sided printing mode to single-sided printing mode and prints the next sheet AS3-2 in single-sided printing mode. Similarly, when printing the succeeding job on sheet S, the control unit 9 prints the preceding sheet BS1 and the next sheet BS2 in double-sided printing mode, then switches the printing mode from double-sided printing mode to single-sided printing mode and prints the next sheet BS3-2 in single-sided printing mode.

[0158] Thus, this transport control can also be applied when printing two consecutive double-sided jobs, one with an odd number of pages (5 pages) and the other with an odd number of pages (5 pages), using the 2461 transport control.

[0159] Furthermore, if the preceding job has 3 or more pages and the succeeding job has 2 or more pages, this transport control using 2461 transport control can also be applied when the preceding job has an even number of pages and the succeeding job has an even number of pages, when the preceding job has an odd number of pages and the succeeding job has an even number of pages, and when the preceding job has an even number of pages and the succeeding job has an odd number of pages. [Explanation of Symbols]

[0160] 1. Image forming apparatus 2. Main unit of the device 5 Image forming unit 6. Fuser 9. Control Unit 23a Paper output tray 31 Sheet Cassette 32 First feeding roller 34a Conveyor roller 35a Resist Roller 42 Transfer Roller 51a Photosensitive drum 76a First re-conveying roller 77a Second re-conveying roller 85. Second electromagnetic clutch 91 First sheet sensor 92 Second seat sensor 93 Third Sheet Sensor P0 Supply Route P3 Retransport Route S Seat AS1, AS1-1, AS1-2 (preceding sheets for preceding jobs) AS2, AS2-1, AS2-2 (Next sheet of preceding job) AS3, AS3-1, AS3-2 (preceding job) successive sheets BS1, BS1-1, BS1-2 (preceding seats for subsequent jobs) BS2, BS2-1, BS2-2 (Next sheet for subsequent jobs) BS3, BS3-1, BS3-2 (successor job)

Claims

1. An image forming unit that forms an image on a sheet, A sheet tray that supports the sheet supplied to the image forming unit, A paper output tray from which sheets with images formed by the image forming unit are ejected, An apparatus body having the image forming unit, further comprising: a feeding path for feeding a sheet from the sheet tray to the image forming unit; and a re-transport path for re-transporting a sheet on which an image has been formed to the feeding path. It comprises a control unit and, The control unit, When performing continuous double-sided printing, which involves printing two preceding jobs of at least three pages and two succeeding jobs of at least two pages consecutively on both sides, During the printing of the preceding job, a first sheet, which is the first sheet corresponding to the subsequent job, is fed from the sheet tray, and an image corresponding to the subsequent job is printed on one side of the first sheet. An image forming apparatus characterized in that, while the first sheet on which an image is printed is located in the feeding path or the re-transport path, the second sheet, which is the last sheet corresponding to the preceding job, is discharged into the output tray.

2. The print order for a job with 5 pages is: The image forming apparatus according to claim 1, characterized in that the order is to print the second page on one side of the first sheet, print the fourth page on one side of the second sheet, print the first page on the second side of the first sheet, print the third page on the second side of the second sheet, and print the fifth page on one side of the third sheet.

3. The control unit, When performing double-sided printing for a job with an odd number of pages, The image forming apparatus according to claim 1 or claim 2, characterized in that it discharges a sheet on which an image corresponding to the last page is printed into the output tray without transporting it to the re-transport path.

4. The control unit, When performing continuous double-sided printing when the number of pages in the preceding job is odd, With at least a portion of the first sheet, on which an image is printed, located within the re-transport path, the second sheet is fed from the sheet tray. The image corresponding to the final page of the subsequent job is printed on one side of the second sheet that was fed, The image forming apparatus according to claim 3, characterized in that the second sheet, on which an image is printed on one side, is discharged into the paper output tray without being transported to the re-transport path.

5. The image forming apparatus according to claim 4, characterized in that the retransport path is located below the sheet tray.

6. The control unit, The image forming apparatus according to claim 5, characterized in that the first sheet, on which an image is printed on one side, is stopped in the re-transport path, and the second sheet is fed from the sheet tray.

7. The main body of the aforementioned device is A re-transport roller is positioned in the aforementioned re-transport path and transports the sheet toward the aforementioned feeding path, A first sheet sensor is provided in the aforementioned re-transport path to detect the sheet, The system further includes a clutch that switches between a transmission state in which driving force is transmitted to the re-transport roller and a disconnection state in which the transmission of driving force to the re-transport roller is interrupted. The control unit, The image forming apparatus according to claim 6, characterized in that, on the condition that the first sheet sensor detects the first sheet, the clutch is switched from the transmission state to the disconnection state, thereby stopping the first sheet within the re-transport path.

8. The image forming apparatus according to claim 7, characterized in that the clutch is an electromagnetic clutch.

9. The main body of the aforementioned device is A feeding roller that feeds the sheet supported by the sheet tray to the feeding path, A first conveying roller located downstream of the feeding roller in the sheet conveying direction in the feeding path, A second conveyor roller is located between the feeding roller and the first conveyor roller in the feeding path, and is situated downstream in the sheet conveying direction from the point where the feeding path and the re-conveying path merge. The system further includes a second sheet sensor located between the first and second conveying rollers in the feeding path, which detects the sheet. The control unit, The image forming apparatus according to claim 5, characterized in that the feeding of the second sheet from the sheet tray to the feeding path is started on the condition that the second sheet sensor detects the rear end of the third sheet, which is fed from the sheet tray before the second sheet, and which has an image corresponding to the preceding job printed on one side of it.

10. The main body of the aforementioned device is A feeding roller that feeds the sheet supported by the sheet tray to the feeding path, A first conveying roller located downstream of the feeding roller in the sheet conveying direction in the feeding path, The system further includes a third sheet sensor located between the feeding roller and the first conveying roller in the feeding path, which detects the sheet. The control unit, The image forming apparatus according to claim 6, characterized in that, on the condition that the third sheet sensor detects the trailing end of the sheet being fed from the sheet tray after the second sheet, the re-transportation of the first sheet, which is stopped in the re-transportation path, to the feeding path is started.

11. The image forming unit is A photosensitive drum in which a toner image is formed, A transfer roller is positioned opposite the photosensitive drum and to which voltage is applied, It has a fuser that fixes the toner image transferred to the sheet, The control unit, The image forming apparatus according to claim 4, characterized in that when printing on the second sheet, at least one of the transfer current that flows when voltage is applied to the transfer roller and the fixing temperature when the toner image is fixed to the sheet by the fuser is changed compared to when printing on a sheet on which an image is printed on both sides.

12. The control unit, The image forming apparatus according to claim 11, characterized in that when printing on the second sheet, at least one of the following is performed: a control that increases the transfer current value compared to when printing on a sheet on which an image is printed on both sides, and a control that increases the fixing temperature compared to when printing on a sheet on which an image is formed on both sides.

Images