Image forming apparatus

Abstract

To provide an image forming apparatus that, when forming images on both sides of a sheet, can increase the number of sheets processed per unit time and extend the life of a developing roller.SOLUTION: An image forming unit 3 changes to a first state for forming a monochrome image and a second state for forming a color image. When images formed on both sides of N-th (N is a positive integer), N+1-th, and N+2-th sheets SH are all monochrome images or all color images, a control unit C1 causes the image forming unit 3 to convey the N+2-th sheet SH and form the images before forming the images on both sides of the N+1-th sheet SH, and when images formed on both sides of the N-th, N+1-th, and N+2-th sheets SH include both monochrome and color images, causes the image forming unit 3 to convey the N+2-th sheet SH and form the images after forming the images on both sides of the N+1-th sheet SH.SELECTED DRAWING: Figure 6

Description

Technical Field

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

Background Art

[0002] Patent Document 1 discloses an example of a conventional image forming apparatus. This image forming apparatus includes an image forming unit, a conveyance unit, and a control unit.

[0003] The image forming unit has a plurality of sets of a developing roller and a photosensitive drum, and forms an image on a sheet. The conveyance unit can reverse the sheet after conveying it to the image forming unit and re-convey it to the image forming unit. The control unit controls the image forming unit and the conveyance unit, and can form images on both sides of the conveyed sheet.

[0004] The control unit can simultaneously convey three or more sheets to the conveyance unit inside the image forming apparatus. Thereby, when forming images on both sides of a sheet, the control unit can execute so-called "2413" control, "2461" control, or "2416" control.

[0005] "2413" control, "2461" control, and "2416" control are controls that indicate the conveyance order and the image forming order of sheets when the image forming apparatus forms images on both sides of a plurality of consecutive sheets. In "2461" control or "2416" control, the number of sheets processed per unit time is improved compared to "2413" control.

[0006] As an example, the case where the image forming apparatus executes "2413" control, "2461" control, or "2416" control based on the following image forming data will be briefly described. In this case, the image forming data has the first side of the first sheet as page 1, the second side of the first sheet as page 2, the first side of the second sheet as page 3, the second side of the second sheet as page 4, the first side of the third sheet as page 5, the second side of the third sheet as page 6, the first side of the fourth sheet as page 7, and the second side of the fourth sheet as page 8, and so on.

[0007] When the image forming apparatus executes "2413" control based on the above image forming data, the control unit controls the image forming unit and the conveyance unit so as to form images in the order of page 2 on the second side of the first sheet, page 4 on the second side of the second sheet, page 1 on the first side of the first sheet, page 3 on the first side of the second sheet, page 6 on the second side of the third sheet, and so on. At this time, the control unit causes two sheets to be conveyed to the conveyance unit inside the image forming apparatus simultaneously.

[0008] When the image forming apparatus executes "2461" control based on the above image forming data, the control unit controls the image forming unit and the conveyance unit so as to form images in the order of page 2 on the second side of the first sheet, page 4 on the second side of the second sheet, page 6 on the second side of the third sheet, page 1 on the first side of the first sheet, page 8 on the second side of the fourth sheet, page 3 on the first side of the second sheet, and so on. At this time, the control unit causes four or more sheets to be conveyed to the conveyance unit inside the image forming apparatus simultaneously.

[0009] When the image forming apparatus executes "2416" control based on the above image formation data, the control unit controls the image forming unit and the conveyance unit so that images are formed in the following order: the second page on the second side of the first sheet, the fourth page on the second side of the second sheet, the first page on the first side of the first sheet, the sixth page on the second side of the third sheet, the third page on the first side of the second sheet, the eighth page on the second side of the fourth sheet, and so on. At this time, the control unit causes three sheets to be conveyed to the conveyance unit inside the image forming apparatus simultaneously.

[0010] Although not specified in Patent Document 1, in such an image forming apparatus, in each of a plurality of sets, the photosensitive drum and the developing roller can be brought into contact with and separated from each other. Further, the image forming unit switches between a first state in which one set of the developing roller and the photosensitive drum are in contact with each other while forming a monochrome image and the other sets of the developing roller and the photosensitive drum are separated, and a second state in which all sets of the developing roller and the photosensitive drum are in contact with each other while forming a color image. Thereby, the operation time of the developing roller for forming a color image is reduced, and the long life of the developing roller is achieved.

Prior Art Documents

Patent Documents

[0011]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0012] By the way, when forming images on both sides of a sheet, in the case of the conventional image forming apparatus, when attempting to switch to the first state or the second state while conveying three or more sheets simultaneously inside the image forming apparatus, the interval between the sheets being conveyed simultaneously is short, and it is difficult to change the interval to be longer. Therefore, it is difficult to secure the time required for the switching operation and the time for suppressing defects such as overlapping after the switching operation.

[0013] Therefore, for example, if priority is given to improving the number of sheets processed per unit time, and the image is formed in the second state from the beginning to the end regardless of whether the image is monochrome or color, it becomes difficult to achieve a longer life of the developing roller.

[0014] On the other hand, if priority is given to extending the life of the developing roller, and two or fewer sheets are simultaneously conveyed inside the image forming apparatus from the beginning to the end and switched to the first state or the second state according to whether the image is monochrome or color, it becomes difficult to improve the number of sheets processed per unit time.

[0015] The present invention has been made in view of the above-described conventional circumstances, and an object thereof is to provide an image forming apparatus capable of realizing an improvement in the number of sheets processed per unit time and a longer life of the developing roller when forming images on both sides of a sheet.

Means for Solving the Problems

[0016] The image forming apparatus of the present invention includes a plurality of sets of a developing roller and a photosensitive drum that can come into contact with and separate from the developing roller, and an image forming unit that forms an image on a sheet. A conveying unit that can convey the sheet to the image forming unit and then reverse it and convey it back to the image forming unit. A control unit that controls the image forming unit and the conveying unit and can form images on both sides of the conveyed sheet. The image forming unit switches between a first state in which one set of the developing roller and the photosensitive drum come into contact with each other to form a monochrome image while the other sets of the developing roller and the photosensitive drum are separated, and a second state in which all sets of the developing roller and the photosensitive drum come into contact with each other to form a color image. The control unit When the images formed on both sides of the Nth, (N + 1)th, and (N + 2)th sheets are all monochrome or all color, the (N + 2)th sheet is conveyed to the image forming unit to form an image before forming an image on both sides of the (N + 1)th sheet. When the images formed on both sides of each of the Nth, (N + 1)th, and (N + 2)th sheets are a mixture of the monochrome and the color, after forming the images on both sides of the (N + 1)th sheet, the (N + 2)th sheet is conveyed to the image forming unit to form an image, which is characterized in that.

[0017] When all the images formed on both sides of each of the Nth (N is a positive integer), (N + 1)th, and (N + 2)th sheets are all monochrome or all color, the control unit of the image forming apparatus of the present invention conveys the (N + 2)th sheet to the image forming unit to form an image before forming the images on both sides of the (N + 1)th sheet. In this case, three or more sheets are simultaneously conveyed inside the image forming apparatus, and the first state or the second state is switched according to whether all of those images are monochrome or color.

[0018] Also, when the images formed on both sides of each of the Nth, (N + 1)th, and (N + 2)th sheets are a mixture of monochrome and color, after forming the images on both sides of the (N + 1)th sheet, the (N + 2)th sheet is conveyed to the image forming unit to form an image. In this case, two or less sheets are simultaneously conveyed inside the image forming apparatus, and the first state or the second state is switched each time according to whether each image is monochrome or color.

[0019] That is, this image forming apparatus changes the page order of the images formed by the image forming unit and enables switching between the first state and the second state by selecting whether to convey the (N + 2)th sheet after or before forming the images on both sides of the (N + 1)th sheet. Specifically, this image forming apparatus sets the page order for image formation as the next page order after "page 2, page 4", such as "page 6, page 1" or "page 1, page 6", or "page 1, page 3".

[0020] As a result, this image forming apparatus prioritizes improving the number of sheets processed per unit time and forms an image in the second state from start to finish regardless of whether the image is monochrome or color, or prioritizes extending the life of the developing roller and conveys two or fewer sheets simultaneously inside the image forming apparatus from start to finish, and there is no need to switch between the first state and the second state according to whether the image is monochrome or color.

[0021] Therefore, the image forming apparatus of the present invention can achieve an improvement in the number of sheets processed per unit time and an extension of the life of the developing roller when forming images on both sides of a sheet.

Brief Description of the Drawings

[0022]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Embodiments for Carrying Out the Invention

[0023] Hereinafter, embodiments embodying the present invention will be described with reference to the drawings.

[0024] (Embodiment) As shown in FIGS. 1 to 4, the image forming apparatus 1 (1A, 1B) of the embodiment is an example of a specific aspect of the image forming apparatus of the present invention. The image forming apparatus 1 (1A, 1B) is a laser printer that forms an image on a sheet SH by an electrophotographic method.

[0025] The image forming apparatus 1 (1A) shown in FIGS. 1 and 2 is a "2461" model capable of executing "2461" control. As shown in FIG. 2, the image forming apparatus 1 (1A) can simultaneously convey four or more sheets SH inside the apparatus main body 2.

[0026] The image forming apparatus 1 (1B) shown in FIGS. 3 and 4 is a "2416" model capable of executing "2416" control. As shown in FIG. 4, the image forming apparatus 1 (1B) can simultaneously convey three sheets SH inside the apparatus main body 2.

[0027] As shown in FIG. 1, the image forming apparatus 1 (1A) includes a reverse path P4 and a switchback roller pair 18, and a third motor M3 drives the re-conveying roller pair 11 and the switchback roller pair 18. Further, the image forming apparatus 1 (1A) includes a flapper 80A and a solenoid S1.

[0028] On the other hand, as shown in FIG. 3, the image forming apparatus 1(1B) does not include the inversion path P4 and the switchback roller pair 18, and the third motor M3 drives only the re-conveying roller pair 11. Further, the image forming apparatus 1(1B) does not include the flapper 80A and the solenoid S1, and instead includes the flapper 80B and a spring (not shown).

[0029] The other configurations of the image forming apparatus 1(1A) and the other configurations of the image forming apparatus 1(1B) are the same. Therefore, in the following description, the configuration of the image forming apparatus 1(1A) will be described in detail, and the image forming apparatus 1(1B) will be described focusing on the differences from the image forming apparatus 1(1A).

[0030] <Schematic Configuration of Image Forming Apparatus> As shown in FIG. 1, the image forming apparatus 1(1A) includes an apparatus main body 2, an image forming unit 3, a conveyance unit 4, and a control unit C1.

[0031] The apparatus main body 2 includes a sheet tray 2C. The sheet tray 2C is located at the lower part of the apparatus main body 2. The sheet tray 2C is a substantially box-shaped body with an open upper part, and stores a plurality of sheets SH in a stacked state. The sheets SH are postcards, envelopes, A4-sized plain paper, A3-sized plain paper, glossy paper, etc.

[0032] The apparatus main body 2 includes a discharge tray 2T. The discharge tray 2T is located on the upper surface of the apparatus main body 2. The discharge tray 2T supports the sheet SH that has completed image formation.

[0033] <Conveyance Path, Discharge Path, Re-conveyance Path, and Inversion Path> The apparatus main body 2 includes a conveyance path P1, a discharge path P2, a re-conveyance path P3, and an inversion path P4.

[0034] The conveyance path P1 extends upward from the front end of the sheet tray 2C in a U-shaped curve, then extends substantially horizontally backward, and further changes its direction upward on the rear side of the apparatus main body 2 to reach the branch point PB.

[0035] The discharge path P2 extends upward from the branch point PB and then changes its direction forward to reach the discharge tray 2T. Note that the discharge path P2 also serves as an inversion path for inverting the sheet SH to be re-conveyed and guiding it to the re-conveyance path P3.

[0036] The re-conveyance path P3 extends downward from the branch point PB, then changes its direction forward below the sheet tray 2C, extends substantially horizontally at the bottom of the apparatus main body 2, and further changes its direction upward on the front side of the apparatus main body 2 to merge with the conveyance path P1.

[0037] The inversion path P4 extends upward and backward slanting from the branch point PB. The inversion path P4 is a path for inverting the sheet SH to be re-conveyed and guiding it to the re-conveyance path P3.

[0038] That is, the image forming apparatus 1(1A) includes two inversion paths, namely the discharge path P2 and the inversion path P4. On the other hand, as shown in FIG. 3, the image forming apparatus 1(1B) includes one inversion path, namely the discharge path P2.

[0039] <Control unit> The control unit C1 includes an arithmetic unit mainly composed of a CPU, ROM, and RAM (not shown) and hardware for controlling a semiconductor laser, a motor, etc.

[0040] The ROM stores programs for the CPU to control various operations of the image forming apparatus 1 and programs for executing identification processing, for example, the double-sided printing program shown in FIG. 6.

[0041] The RAM has a storage area for temporarily recording data and signals used when the CPU executes the above programs or a work area for data processing. The control unit C1 controls the entire image forming apparatus 1 including the image forming unit 3 and the conveyance unit 4.

[0042] <Conveyance unit> As shown in FIG. 1, the conveyance unit 4 of the image forming apparatus 1 (1A) includes a supply unit 20, a post-fixing conveyance roller pair 25, a discharge unit 26, a switchback roller pair 18, and a re-conveyance unit 10. Further, the conveyance unit 4 of the image forming apparatus 1 (1A) includes a flapper 80A and a solenoid S1 located near the branch point PB.

[0043] On the other hand, as shown in FIG. 3, the conveyance unit 4 of the image forming apparatus 1 (1B) includes a supply unit 20, a post-fixing conveyance roller pair 25, a discharge unit 26, and a re-conveyance unit 10. Further, the conveyance unit 4 of the image forming apparatus 1 (1B) includes a flapper 80B and a spring (not shown) located near the branch point PB.

[0044] As shown in FIG. 1, the supply unit 20 is located at the front part of the apparatus main body 2. The supply unit 20 sends out the sheet SH accommodated in the sheet tray 2C to the conveyance path P1 by the supply roller 21. Then, the supply unit 20 conveys the sheet SH toward the image forming unit 3 by the conveyance roller pair 23 and the registration roller pair 24 located in the section where the conveyance path P1 makes a U-turn in front of the apparatus main body 2.

[0045] The post-fixing conveyance roller pair 25 is located between the branch part PB in the conveyance path P1 and the fixing unit 6 of the image forming unit 3. The post-fixing conveyance roller pair 25 conveys the sheet SH that has passed through the fixing unit 6 toward the branch part PB.

[0046] In the image forming apparatus 1 (1A), the flapper 80A is driven by the solenoid S1 controlled by the control unit C1 to swing, thereby switching the conveyance destination of the sheet SH conveyed along the conveyance path P1 and reaching the branch part PB to the discharge path P2 or the reverse path P4.

[0047] Note that when the re-conveyed sheet SH is reversed in the discharge path P2 or the reverse path P4 and guided to the re-conveyance path P3, the flapper 80A is driven by the solenoid S1 and swings to a position where it does not obstruct the re-conveyed sheet SH.

[0048] As shown in FIG. 3, in the image forming apparatus 1 (1B), the flapper 80B is biased by a spring (not shown), intersects the conveyance path P1, and is held at a position spaced apart from the re-conveyance path P3. As shown in FIG. 4, the flapper 80B is pushed by the sheet SH conveyed along the conveyance path P1, swings against the biasing force of the spring, and guides the sheet SH that has reached the branch portion PB to the discharge path P2.

[0049] Note that the flapper 80B is held at a position where it is biased by a spring and does not obstruct the re-conveyed sheet SH when the re-conveyed sheet SH is reversed in the discharge path P2 and guided to the re-conveyance path P3.

[0050] As shown in FIG. 1, the discharge unit 26 has a pre-discharge conveyance roller pair 28 and a discharge roller pair 29 located at the upper part on the rear surface side of the apparatus main body 2.

[0051] The pre-discharge conveyance roller pair 28 is located at a position close to the branch point PB in the discharge path P2. The discharge roller pair 29 is located at a position close to the discharge tray 2T in the discharge path P2.

[0052] The discharge unit 26 nips the sheet SH guided to the discharge path P2 after passing through the branch point PB with the pre-discharge conveyance roller pair 28 and conveys it toward the discharge roller pair 29, and further nips the sheet SH with the discharge roller pair 29 and discharges it to the discharge tray 2T.

[0053] Also, when forming images on both sides of the sheet SH, the discharge unit 26 nips the sheet SH with the pre-discharge conveyance roller pair 28 and the discharge roller pair 29 and discharges it halfway. After the rear end of the sheet SH has passed through the branch point PB, the discharge unit 26 reverses the sheet SH by switching the rotation directions of the pre-discharge conveyance roller pair 28 and the discharge roller pair 29 to the opposite direction of that during discharge, and re-conveys it to the re-conveyance path P3.

[0054] In the image forming apparatus 1 (1A), the switchback roller pair 18 is located behind the pre-discharge conveyance roller pair 28 on the rear surface side of the apparatus main body 2.

[0055] The switchback roller pair 18 nips the sheet SH guided to the reversal path P4 after passing through the branch point PB and conveys it along the reversal path P4. Then, after the trailing edge of the sheet SH has passed through the branch point PB, the switchback roller pair 18 switches the rotation direction to the direction opposite to that during conveyance, thereby inverting the sheet SH and re-conveying it to the re-conveyance path P3.

[0056] The re-conveyance unit 10 includes a re-conveyance roller pair 11 located at the lower part on the rear side of the apparatus main body 2, an inclined conveyance roller pair 12 and a return roller pair 13 located at the bottom of the apparatus main body 2.

[0057] The re-conveyance roller pair 11 is located in the middle of the portion extending downward from the branch portion PB in the re-conveyance path P3. The inclined conveyance roller pair 12 is located in the portion extending substantially horizontally on the rear side of the apparatus main body 2 in the re-conveyance path P3. The return roller pair 13 is located in the portion extending substantially horizontally in the middle part in the front-rear direction of the apparatus main body 2 in the re-conveyance path P3.

[0058] The re-conveyance unit 10 sequentially nips and conveys the sheet SH guided to the re-conveyance path P3 after passing through the branch point PB by the re-conveyance roller pair 11, the inclined conveyance roller pair 12 and the return roller pair 13, and merges it between the supply roller 21 and the conveyance roller pair 23 in the conveyance path P1.

[0059] <First to Third Motors, First and Second Electromagnetic Clutches, and Plurality of Sheet Sensors> The conveyance unit 4 includes a first motor M1, a second motor M2, a third motor M3, a first electromagnetic clutch CL1 and a second electromagnetic clutch CL2, and a plurality of sheet sensors SS1 to SS5.

[0060] The first motor M1 is switched between stop, normal rotation, and reverse rotation under the control of the control unit C1. The second motor M2 and the third motor M3 are each switched between stop, normal rotation, and reverse rotation under the control of the control unit C1.

[0061] The first electromagnetic clutch CL1 and the second electromagnetic clutch CL2 are each switched between a connected state in which driving force is transmitted and a disconnected state in which driving force is not transmitted by being controlled by the control unit C1.

[0062] The first electromagnetic clutch CL1 is interposed between the first motor M1 and the supply roller 21. The second electromagnetic clutch CL2 is interposed between the first motor M1 and the diagonal feed roller pair 12 and the return roller pair 13.

[0063] The sheet sensor SS1 detects the sheet SH passing between the supply roller 21 and the conveyance roller pair 23 in the conveyance path P1. The sheet sensor SS2 detects the sheet SH passing between the registration roller pair 24 and the image forming unit 3 in the conveyance path P1. The sheet sensor SS3 detects the sheet SH passing between the fixing unit 6 and the post-fixing conveyance roller pair 25 in the conveyance path P1.

[0064] The sheet sensor SS4 detects the sheet SH passing between the re-conveyance roller pair 11 and the diagonal feed roller pair 12 in the re-conveyance path P3 in the vicinity of the re-conveyance roller pair 11. The sheet sensor SS5 detects the sheet SH passing between the diagonal feed roller pair 12 and the return roller pair 13 in the re-conveyance path P3 in the vicinity of the return roller pair 13.

[0065] The sheet sensors SS1 to SS5 each transmit the detection result to the control unit C1. When transporting the sheet SH to form an image, the control unit C1 controls the first to third motors M1 to M3 and the first and second electromagnetic clutches CL1 and CL2 based on the detection results of the sheet sensors SS1 to SS5 and the like.

[0066] When the first motor M1 switches from stop to forward rotation, driving force is transmitted to the first electromagnetic clutch CL1 and the second electromagnetic clutch CL1, and driving force is transmitted to the post-fixing conveyance roller pair 25, causing the post-fixing conveyance roller pair 25 to rotate in the direction of transporting the sheet SH.

[0067] In this state, when the first electromagnetic clutch CL1 switches from the disengaged state to the engaged state, the supply roller 21 rotates in the direction of conveying the sheet SH. Also, in this state, when the second electromagnetic clutch CL2 switches from the disengaged state to the engaged state, the skew feed roller pair 12 and the return roller pair 13 rotate in the direction of re-conveying the sheet SH.

[0068] When the second motor M2 switches from stopping to forward rotation, the pre-discharge conveying roller pair 28 and the discharge roller pair 29 rotate in the direction of discharging the sheet SH on the discharge path P2 to the discharge tray 2T. When the second motor M2 switches from forward rotation to reverse rotation, the pre-discharge conveying roller pair 28 and the discharge roller pair 29 rotate in the direction opposite to the discharging direction, that is, in the direction of re-conveying the sheet SH on the discharge path P2.

[0069] In the image forming apparatus 1(1A), when the third motor M3 switches from stopping to forward rotation, the switchback roller pair 18 rotates in the direction of conveying the sheet SH on the reverse path P4. At this time, the re-conveying roller pair 11 also rotates in the direction opposite to the direction of re-conveying the sheet SH, but does not contact the sheet SH. When the third motor M3 switches from forward rotation to reverse rotation, the switchback roller pair 18 rotates in the direction opposite to the conveying direction, that is, in the direction of re-conveying the sheet SH on the reverse path P4, and at the same time, the re-conveying roller pair 11 rotates in the direction of re-conveying the sheet SH.

[0070] As shown in FIG. 3, in the image forming apparatus 1(1B), when the third motor M3 switches from stopping to forward rotation, the re-conveying roller pair 11 rotates in the direction of re-conveying the sheet SH. Note that in the image forming apparatus 1(1B), it is not necessary for the third motor M3 to rotate reversely.

[0071] Although illustration is omitted, the conveying unit 4 has a motor and a driving force transmission mechanism for driving the registration roller pair 24, the transfer belt 3B described later, and the fixing unit 6. The control unit C1 controls a motor and a driving force transmission mechanism (not shown) based on the detection results of the sheet sensors SS1 to SS5 and the like when conveying the sheet SH to form an image.

[0072] As shown in an example in FIG. 2, in the image forming apparatus 1 (1A), based on the detection results of the sheet sensors SS1 to SS5 and the like, the control unit C1 controls the first to third motors M1 to M3, the first and second electromagnetic clutches CL1 and CL2, a motor (not shown), a driving force transmission mechanism, etc., so that it is possible to simultaneously convey four or more sheets SH to the conveying unit 4 inside the apparatus main body 2.

[0073] As shown in an example in FIG. 4, in the image forming apparatus 1 (1B), based on the detection results of the sheet sensors SS1 to SS5 and the like, the control unit C1 controls the first to third motors M1 to M3, the first and second electromagnetic clutches CL1 and CL2, a motor (not shown), a driving force transmission mechanism, etc., so that it is possible to simultaneously convey three sheets SH to the conveying unit 4 inside the apparatus main body 2.

[0074] <Image forming unit> As shown in FIG. 1, the image forming unit 3 is located above the sheet tray 2C inside the apparatus main body 2. The sheet SH conveyed toward the image forming unit 3 by the supply unit 20 passes through the image forming unit 3 at a substantially horizontal extending portion of the conveyance path P1.

[0075] The image forming unit 3 is a direct transfer type color electrophotographic system. The image forming unit 3 has four sets of a process cartridge 7 having a developing roller 7A and a photosensitive drum 5. Further, the image forming unit 3 has a switching mechanism 3C, a transfer belt 3B, a scanner unit 8, a fixing unit 6, etc.

[0076] Each photosensitive drum 5 corresponds to four colors of toner: black, yellow, magenta, and cyan, and is arranged in series along a substantially horizontal portion of the conveyance path P1. Each photosensitive drum 5 is a cylindrical rotating body. A positively charged photosensitive layer is formed on the surface of each photosensitive drum 5. There is a charger (not shown) near each photosensitive drum 5.

[0077] The switching mechanism 3C supports each process cartridge 7. Each process cartridge 7 is located above and in front of the paired photosensitive drums 5. Each process cartridge 7 rotatably supports a developing roller 7A at its lower end. Each process cartridge 7 has a toner storage portion inside.

[0078] As shown in FIG. 5, the switching mechanism 3C is controlled by the control unit C1 and can move each process cartridge 7 in the vertical direction. The developing roller 7A and the photosensitive drum 5 can be brought into contact with and separated from each other by the vertical movement of the process cartridge 7.

[0079] Although not shown in the figure, the switching mechanism 3C has a rail for guiding the process cartridge 7, a cam for pushing the process cartridge 7 in one direction in the vertical direction, a biasing spring for biasing the process cartridge 7 in the other direction in the vertical direction, and the like.

[0080] When the switching mechanism 3C operates, the image forming unit 3 switches between a first state shown in FIG. 5(a) and a second state shown in FIG. 5(b).

[0081] In the first state shown in FIG. 5(a), one set of developing rollers 7A corresponding to black toner and the photosensitive drum 5 (5K) for forming a monochrome image are in contact with each other, while the other three sets of developing rollers 7A corresponding to yellow, magenta, and cyan toners and the photosensitive drums 5 (5C, 5M, 5Y) are separated.

[0082] In the first state, the operating time of the three developing rollers for forming a color image can be reduced, and the long life of these developing rollers can be realized.

[0083] In the second state shown in FIG. 5(b), all sets of developing rollers 7A and the photosensitive drums 5 (5C, 5M, 5Y, 5K) for forming a color image are in contact with each other.

[0084] As shown in FIG. 1, the transfer belt 3B is positioned below the substantially horizontal portion of the conveyance path P1 and faces each photosensitive drum 5 from below. The transfer belt 3B circulates while sandwiching the conveyed sheet SH together with each photosensitive drum 5.

[0085] The scanner unit 8 is positioned above each process cartridge 7. The scanner unit 8 has a laser light source, a polygon mirror, an fθ lens, a reflecting mirror, and the like. The scanner unit 8 irradiates each photosensitive drum 5 with a laser beam from above.

[0086] The fixing unit 6 is positioned behind each photosensitive drum 5 and the transfer belt 3B. The fixing unit 6 has a heating roller 6A and a pressure roller 6B. The fixing unit 6 heats and presses the sheet SH that has passed below each process cartridge 7 by sandwiching it between the heating roller 6A and the pressure roller 6B.

[0087] The image forming unit 3 forms an image on the sheet SH conveyed along the conveyance path P1 as follows. At this time, when forming a color image on the sheet SH, the image forming unit 3 switches to the second state shown in FIG. 5(b).

[0088] The surface of each photosensitive drum 5 (5C, 5M, 5Y, 5K) is uniformly positively charged by a charger as it rotates, and then exposed by the high-speed scanning of the laser beam irradiated from the scanner unit 8. As a result, an electrostatic latent image corresponding to the image to be formed on the sheet SH is formed on the surface of each photosensitive drum 5. Next, toner is supplied from the toner storage unit to the surface of each photosensitive drum 5 corresponding to the electrostatic latent image. Then, when the sheet SH is conveyed along the conveyance path P1 and passes through the image forming unit 3, the toner carried on the surface of each photosensitive drum 5 is transferred to one surface of the sheet SH. Then, in the fixing unit 6, the image is thermally fixed on the sheet SH by heating and pressing the sheet SH.

[0089] When forming a monochrome image on the sheet SH, the image forming unit 3 switches to the first state shown in Fig. 5(a). Then, a pair of developing rollers 7A corresponding to black toner and the photosensitive drum 5(5K) operate as described above.

[0090] In the case of single-sided printing where images are not formed on both sides of the sheet SH, the sheet SH that has passed through the fixing unit 6 is guided to the discharge path P2 and discharged to the discharge tray 2T by the discharge unit 26.

[0091] In the case of double-sided printing where images are formed on both sides of the sheet SH, the control unit C1 executes the double-sided printing program shown in Fig. 6. In this program, variables N, Fin, M, and mode are used.

[0092] The variable N is a positive integer. The control unit C1 uses the variable N to identify each of a plurality of sheets SH, such as the Nth sheet SH, the (N + 1)th sheet SH, the (N + 2)th sheet SH, and so on.

[0093] The variable Fin is a flag for determining the end of printing. When printing has not ended, the variable Fin is 0. When printing has ended, the variable Fin is 1.

[0094] The variable M is a positive integer. The control unit C1 uses the variable M to identify the sheet SH when the conveyance of the sheet SH scheduled for conveyance is not performed.

[0095] The control unit C1 uses the variable mode to identify any one of the first to third modes. In the case of the first mode, the variable mode is 1. In the case of the second mode, the variable mode is 2. In the case of the third mode, the variable mode is 3.

[0096] Also, when the control unit C1 performs double-sided printing, the even page (2N) is printed before the odd page (2N - 1) so that the surface facing down of the Nth sheet SH discharged to the discharge tray 2T is the odd page (2N - 1) and the surface facing up of that sheet SH is the even page (2N).

[0097] First, in step S11, the control unit C1 assigns 1 to the variable N and assigns 0 to the variable Fin.

[0098] Next, the control unit C1 moves to step S12 and starts the "printing up to 2(N + 1)" subroutine shown in FIG. 7.

[0099] When the control unit C1 moves to step S41, if the image to be formed on the even page of the Nth sheet SH, that is, the 2Nth page, is in color, the switching mechanism 3C is controlled to switch the image forming unit 3 to the second state. On the other hand, if the image to be formed on the 2Nth page is monochrome, the switching mechanism 3C is controlled to switch the image forming unit 3 to the first state.

[0100] Note that when the state of the image forming unit 3 is already in the state to be switched, the control unit C1 does not perform the switching.

[0101] Next, the control unit C1 moves to step S42 and prints the 2Nth page.

[0102] Next, the control unit C1 moves to step S43 and determines whether there is data for the even page of the (N + 1)th sheet SH, that is, the 2(N + 1)th page. If "Yes" in step S43, the control unit C1 moves to step S51. The processing after step S51 will be described later. On the other hand, if "No" in step S43, the control unit C1 moves to step S44.

[0103] When the control unit C1 moves to step S44, it determines whether a timeout has occurred. If the waiting time has not elapsed, it becomes "No" in step S44, so the control unit C1 returns to step S43. On the other hand, if the waiting time has elapsed, it becomes "Yes" in step S44, so the control unit C1 moves to step S45.

[0104] In this embodiment, the control unit C1 executes the processes of steps S43 and S44 triggered by the following conditions. Specifically, the control unit C1 starts the determination in step S43 from the timing when the rear end of the Nth sheet SH on which 2N pages are printed in step S42 passes through the sheet sensor SS2, and determines in step S44 whether or not a predetermined waiting time has elapsed.

[0105] The control unit C1 also executes the processes of steps S54 and S55, steps S73 and S74, steps S106 and S121, and steps S206 and S221 described below triggered by the same conditions as the processes of steps S43 and S44. Note that the control unit C1 may execute the processes such as steps S43 and S44 triggered by conditions different from the above.

[0106] The fact that it becomes "Yes" in step S44 means that only one sheet SH to be double-sided printed during the current execution of this subroutine.

[0107] When the control unit C1 proceeds to step S45, if the image to be formed on the odd page of the Nth sheet SH, that is, the 2N - 1th page, is in color, the switching mechanism 3C is controlled to switch the image forming unit 3 to the second state. On the other hand, if the image to be formed on the 2N - 1th page is monochrome, the switching mechanism 3C is controlled to switch the image forming unit 3 to the first state.

[0108] Next, the control unit C1 proceeds to step S46 and prints the 2N - 1th page.

[0109] Next, the control unit C1 proceeds to step S47 and substitutes 1 into the variable Fin. After that, the control unit C1 ends the "printing up to 2(N + 1)" subroutine and proceeds to step S13 shown in FIG. 6, and determines whether or not the variable Fin = 1. When proceeding from step S47 to step S13, it becomes "Yes", so the double-sided printing program ends.

[0110] Returning to the "Printing up to 2(N + 1)" subroutine shown in Fig. 7 and shifting from step S43 to step S51, if the image to be formed on the even page of the (N + 1)-th sheet SH, that is, the 2(N + 1)-th page, is in color, the control unit C1 controls the switching mechanism 3C to switch the image forming unit 3 to the second state. On the other hand, if the image to be formed on the 2(N + 1)-th page is monochrome, the control unit C1 controls the switching mechanism 3C to switch the image forming unit 3 to the first state.

[0111] Next, the control unit C1 shifts to step S52 and prints the 2(N + 1)-th page.

[0112] Next, the control unit C1 shifts to step S53 and determines whether it is the "2461" model. If it is not the image forming apparatus 1(1A), it becomes "No" in step S53 and shifts to step S71 shown in Fig. 8. On the other hand, if it is the image forming apparatus 1(1A), it becomes "Yes" in step S53 shown in Fig. 7 and shifts to step S54.

[0113] When the control unit C1 shifts to step S54, it determines whether there is data for the even page of the (N + 2)-th sheet SH, that is, the 2(N + 2)-th page. If it is "Yes" in step S54, the control unit C1 ends the "Printing up to 2(N + 1)" subroutine and shifts to step S13 shown in Fig. 6 to determine whether the variable Fin = 1. Since it becomes "No" when shifting from step S54 to step S13, the control unit C1 shifts to step S14. The processing after step S14 will be described later. On the other hand, if it is "No" in step S54 shown in Fig. 7, the control unit C1 shifts to step S55.

[0114] When the control unit C1 shifts to step S55, it determines whether a timeout has occurred. If the waiting time has not elapsed, it becomes "No" in step S55, so the control unit C1 returns to step S54. On the other hand, if the waiting time has elapsed, it becomes "Yes" in step S55, so the control unit C1 shifts to step S61 shown in Fig. 8.

[0115] In step S55 shown in FIG. 7, "Yes" means that only two sheets SH to be double-sided printed during the current execution of this subroutine, that is, only the N-th sheet and the (N + 1)-th sheet.

[0116] When the control unit C1 shifts to step S61 shown in FIG. 8, if the image to be formed on the odd page of the N-th sheet SH, that is, the (2N - 1)-th page, is in color, the switching mechanism 3C is controlled to switch the image forming unit 3 to the second state. On the other hand, if the image to be formed on the (2N - 1)-th page is monochrome, the switching mechanism 3C is controlled to switch the image forming unit 3 to the first state.

[0117] Next, the control unit C1 shifts to step S62 and prints the (2N - 1)-th page.

[0118] Next, the control unit C1 shifts to step S63. If the image to be formed on the odd page of the (N + 1)-th sheet SH, that is, the (2(N + 1) - 1)-th page, is in color, the switching mechanism 3C is controlled to switch the image forming unit 3 to the second state. On the other hand, if the image to be formed on the (2(N + 1) - 1)-th page is monochrome, the switching mechanism 3C is controlled to switch the image forming unit 3 to the first state. If the state of the image forming unit 3 is already in the state to be switched, the control unit C1 does not perform the switching.

[0119] Next, the control unit C1 shifts to step S64 and prints the (2(N + 1) - 1)-th page.

[0120] Next, the control unit C1 shifts to step S65 and assigns 1 to the variable Fin. After that, as shown in FIG. 7, the control unit C1 ends the "printing up to 2(N + 1)" subroutine and shifts to step S13 shown in FIG. 6 to determine whether the variable Fin = 1. When shifting from step S65 to step S13, it becomes "Yes", so the double-sided printing program ends.

[0121] Returning to the "Printing up to 2(N + 1)" subroutine shown in FIG. 7 and shifting from step S53 to step S71 shown in FIG. 8, when the control unit C1 moves to the odd page of the Nth sheet SH, that is, if the image to be formed on page 2N - 1 is in color, the control unit C1 controls the switching mechanism 3C to switch the image forming unit 3 to the second state. On the other hand, if the image to be formed on page 2N - 1 is in monochrome, the control unit C1 controls the switching mechanism 3C to switch the image forming unit 3 to the first state. If the state of the image forming unit 3 is already in the state to be switched, the control unit C1 does not perform the switching.

[0122] Next, the control unit C1 moves to step S72 and prints page 2N - 1.

[0123] Next, the control unit C1 moves to step S73 and determines whether there is data for the even page of the (N + 2)th sheet SH, that is, page 2(N + 2). In step S73, if "Yes", as shown in FIG. 7, the control unit C1 ends the "Printing up to 2(N + 1)" subroutine and moves to step S13 shown in FIG. 6 to determine whether the variable Fin = 1. Since it is "No" when moving from step S73 to step S13, the control unit C1 moves to step S14. The processing after step S14 will be described later. On the other hand, in step S73 shown in FIG. 8, if "No", the control unit C1 moves to step S74.

[0124] When the control unit C1 moves to step S74, it determines whether a timeout has occurred. If the waiting time has not elapsed, it is "No" in step S74, so the control unit C1 returns to step S73. On the other hand, if the waiting time has elapsed, it is "Yes" in step S74, so the control unit C1 moves to step S75.

[0125] The fact that it is "Yes" in step S74 means that during this execution of this subroutine, only 2 sheets of the sheet SH to be double-sided printed, that is, only the Nth sheet and the (N + 1)th sheet.

[0126] When the control unit C1 proceeds to step S75, if the image to be formed on the odd page of the (N + 1)-th sheet SH, that is, the page 2(N + 1) - 1, is in color, the switching mechanism 3C is controlled to switch the image forming unit 3 to the second state. On the other hand, if the image to be formed on the page 2(N + 1) - 1 is in monochrome, the switching mechanism 3C is controlled to switch the image forming unit 3 to the first state. Note that if the state of the image forming unit 3 is already in the state to be switched, the control unit C1 does not perform the switching.

[0127] Next, the control unit C1 proceeds to step S76 and prints the page 2(N + 1) - 1.

[0128] Next, the control unit C1 proceeds to step S77 and assigns 1 to the variable Fin. Then, as shown in FIG. 7, the control unit C1 ends the "printing up to 2(N + 1)" subroutine and proceeds to step S13 shown in FIG. 6, and determines whether the variable Fin = 1. When proceeding from step S77 to step S13, it becomes "Yes", so the duplex printing program ends.

[0129] Returning to the duplex printing program shown in FIG. 6, when proceeding from step S13 to step S14, the control unit C1 starts the "mode determination process" subroutine shown in FIG. 9.

[0130] When the control unit C1 proceeds to step S81, it determines whether all the images to be formed on both sides of the N-th, (N + 1)-th, and (N + 2)-th sheets SH, that is, whether all the images to be formed on the pages 2N, 2(N + 1), 2(N + 2), 2N - 1, 2(N + 1) - 1, and 2(N + 2) - 1 are in monochrome.

[0131] If it is "Yes" in step S81, the control unit C1 proceeds to step S82, assigns 1 to the variable mode, then ends the "mode determination process" subroutine and proceeds to step S15 shown in FIG. 6. The processing after step S15 will be described later. On the other hand, if it is "No" in step S81 shown in FIG. 9, the control unit C1 proceeds to step S83.

[0132] When the control unit C1 shifts to step S83, it determines whether the images of the even pages of each of the Nth and (N + 1)th sheets SH are both in color, that is, whether the images to be formed on pages 2N and 2(N + 1) are both in color.

[0133] In the case of "No" in step S83, the control unit C1 shifts to step S89, substitutes 3 into the variable mode, then ends the "mode determination process" subroutine and shifts to step S15 shown in FIG. 6. On the other hand, in the case of "Yes" in step S83 shown in FIG. 9, the control unit C1 shifts to step S84.

[0134] When the control unit C1 shifts to step S84, it determines whether it is the "2461" model. In the case of the image forming apparatus 1 (1A), it becomes "Yes" in step S84 and shifts to step S85. On the other hand, in the case of not being the image forming apparatus 1 (1A), it becomes "No" in step S84 and shifts to step S87.

[0135] When the control unit C1 shifts to step S85, it determines whether the image of the even page of the (N + 2)th sheet SH and the image of the odd page of the Nth sheet SH are both in monochrome, that is, whether the images to be formed on pages 2(N + 2) and 2N - 1 are both in monochrome.

[0136] If at least one of the image of the even page of the (N + 2)th sheet SH and the image of the odd page of the Nth sheet SH is not in monochrome, it becomes "No" in step S85, so the control unit C1 shifts to step S88.

[0137] On the other hand, in the case of "Yes" in step S85, the control unit C1 shifts to step S86, substitutes 3 into the variable mode, then ends the "mode determination process" subroutine and shifts to step S15 shown in FIG. 6.

[0138] When the process proceeds from step S84 to step S87 shown in FIG. 9, the control unit C1 determines whether the images on the odd-numbered pages of the Nth sheet SH and the images on the even-numbered pages of the N+2th sheet SH are both in color, i.e., whether the images to be formed on pages 2N-1 and 2(N+2) are both in color.

[0139] If the image on the odd-numbered page of the Nth sheet SH and the image on the even-numbered page of the (N+2)th sheet SH are both in color, the result in step S87 is "Yes," and the control section C1 proceeds to step S88.

[0140] On the other hand, if the answer is "No" in step S87, the control unit C1 proceeds to step S89, where it assigns 3 to the variable mode, and then ends the "mode determination process" subroutine and proceeds to step S15 shown in FIG.

[0141] When the process proceeds from step S85 or step S87 shown in FIG. 9 to step S88, the control unit C1 assigns 2 to the variable mode, then ends the "mode determination process" subroutine and proceeds to step S15 shown in FIG.

[0142] That is, in the "mode determination process" subroutine, the control unit C1 selects the first mode when the images to be formed on both sides of the Nth, N+1th, and N+2th sheets SH are all monochrome. The control unit C1 selects the second mode when the images to be formed on both sides of the Nth, N+1th, and N+2th sheets SH are all color. Furthermore, when the images to be formed on both sides of the Nth, N+1st, and N+2th sheets SH are a mixture of monochrome and color, the control unit C1 selects the third mode with some exceptions, and selects the second mode with some exceptions.

[0143] Returning to the duplex printing program shown in FIG. 6 and transitioning from step S14 to step S15, the control unit C1 determines whether the variable mode = 2. If "Yes" in step S15, the control unit C1 proceeds to step S16 and starts the "Second Mode: Color Printing Process" subroutine shown in FIG. 11.

[0144] On the other hand, if "No" in step S15 shown in FIG. 6, the control unit C1 proceeds to step S17 and determines whether the variable mode = 1. If "Yes" in step S17, the control unit C1 proceeds to step S18 and starts the "First Mode: Monochrome Printing Process" subroutine shown in FIG. 10.

[0145] On the other hand, if "No" in step S17 shown in FIG. 6, the control unit C1 proceeds to step S21 and starts the "Third Mode: Odd - numbered Page Printing" subroutine shown in FIG. 12.

[0146] When the control unit C1 starts the "First Mode: Monochrome Printing Process" subroutine shown in FIG. 10, it controls the switching mechanism 3C to switch the image forming unit 3 to the first state. If the image forming unit 3 is already in the first state, the control unit C1 does not perform the switching.

[0147] Then, the control unit C1 proceeds to step S101 and prints 2(N + 2) pages.

[0148] The printing of 2(N + 2) pages in step S101 corresponds to "When all the images formed on both sides of each of the N - th, (N + 1) - th, and (N + 2) - th sheets are monochrome... before forming the images on both sides of the (N + 1) - th sheet, the (N + 2) - th sheet is conveyed to the image forming unit to form the image," according to claim 1 of the present invention.

[0149] Next, the control unit C1 proceeds to step S102 and determines whether it is the "2461" model.

[0150] In the case of the image forming apparatus 1 (1A), it becomes "Yes" in step S102 and proceeds to step S103. Then, after printing 2N - 1 pages, the control unit C1 proceeds to step S105.

[0151] On the other hand, in the case of not being the image forming apparatus 1 (1A), it becomes "No" in step S102 and proceeds to step S104. Then, after printing 2(N + 1) - 1 pages, the control unit C1 proceeds to step S105.

[0152] When shifting from step S103 or step S104 to step S105, the control unit C1 substitutes N + 1 for the variable N.

[0153] Next, the control unit C1 proceeds to step S106 and determines whether there is data for the even page of the (N + 2)-th sheet SH, that is, page 2(N + 2). The (N + 2)-th sheet SH in step S106 corresponds to the "sheet scheduled to be conveyed after the (N + 2)-th sheet" according to claim 3 of the present invention by substituting N + 1 for the variable N each time step S105 is executed.

[0154] If it is "Yes" in step S106, the control unit C1 proceeds to step S111. The processing after step S111 will be described later. On the other hand, if it is "No" in step S106, the control unit C1 proceeds to step S121.

[0155] When the control unit C1 proceeds to step S121, it determines whether a timeout has occurred. If the waiting time has not elapsed, it becomes "No" in step S121, so the control unit C1 returns to step S106. On the other hand, if the waiting time has elapsed, it becomes "Yes" in step S121, so the control unit C1 proceeds to step S122.

[0156] When the control unit C1 proceeds to step S122, it substitutes 1 for the variable Fin and proceeds to step S131. The processing after step S131 will be described later.

[0157] When shifting from step S106 to step S111, the control unit C1 determines whether the images formed on page 2(N + 2) and page 2(N + 2) - 1 are both monochrome. If "Yes" in step S111, the control unit C1 returns to step S101. On the other hand, if "No" in step S111, the control unit C1 shifts to step S112.

[0158] When starting the "First Mode: Monochrome Printing Process" subroutine and getting "No" in step S111, it corresponds to the case where "the images formed on both sides of each of the Nth, (N + 1)th, and (N + 2)th sheets are all monochrome, but at least one of the images on both sides formed on the sheets scheduled to be conveyed after the (N + 2)th sheet is not monochrome" according to claim 3 of the present invention.

[0159] When the control unit C1 shifts to step S112, it substitutes N + 2 into the variable M and then shifts to step S131. Here, for the (N + 2)th sheet SH when getting "No" in step S111, although it was scheduled to be conveyed, it will not be conveyed during the current execution of the "First Mode: Monochrome Printing Process" subroutine.

[0160] When shifting from step S112 or step S122 to step S131, the control unit C1 determines whether it is the "2461" model.

[0161] If it is the image forming apparatus 1(1A), it gets "Yes" in step S131 and shifts to step S132. Then, after the control unit C1 prints page 2N - 1, it shifts to step S133.

[0162] On the other hand, if it is not the image forming apparatus 1(1A), it gets "No" in step S131 and shifts to step S133.

[0163] When shifting from step S131 or step S132 to step S133, after the control unit C1 prints 2(N + 1) - 1 pages, it ends the "First Mode: Monochrome Printing Process" subroutine and shifts to step S19 shown in FIG. 6. The processing after step S19 will be described later.

[0164] The 2N - 1 page in step S132 and the 2(N + 1) - 1 page in step S133 are examples of the "odd-numbered pages of the sheet during conveyance" according to claim 3 of the present invention.

[0165] When the control unit C1 starts the "Second Mode: Color Printing Process" subroutine shown in FIG. 11, it controls the switching mechanism 3C to switch the image forming unit 3 to the second state. If the image forming unit 3 is already in the second state, the control unit C1 does not perform the switching.

[0166] Then, the control unit C1 shifts to step S201 and prints 2(N + 2) pages.

[0167] The printing of the 2(N + 2) page in step S201 corresponds to "when the images formed on both sides of each of the Nth, (N + 1)th, and (N + 2)th sheets are all in color, the (N + 2)th sheet is conveyed to the image forming unit to form an image before forming the images on both sides of the (N + 1)th sheet," according to claim 1 of the present invention.

[0168] Next, the control unit C1 shifts to step S202 and determines whether it is the "2461" model.

[0169] In the case of the image forming apparatus 1(1A), it becomes "Yes" in step S202 and shifts to step S203. Then, after the control unit C1 prints 2N - 1 pages, it shifts to step S205.

[0170] On the other hand, if it is not the image forming apparatus 1 (1A), it becomes "No" in step S202 and proceeds to step S204. Then, after printing 2(N + 1) - 1 pages, the control unit C1 proceeds to step S205.

[0171] When shifting from step S203 or step S204 to step S205, the control unit C1 substitutes N + 1 for the variable N.

[0172] Next, the control unit C1 proceeds to step S206 and determines whether there is data for the even page of the (N + 2)-th sheet SH, that is, page 2(N + 2). The (N + 2)-th sheet SH in step S206 corresponds to the "sheet scheduled to be conveyed after the (N + 2)-th sheet" according to claims 5 and 8 of the present invention by substituting N + 1 for the variable N each time step S205 is executed.

[0173] If it is "Yes" in step S206, the control unit C1 proceeds to step S211. The processing after step S211 will be described later. On the other hand, if it is "No" in step S206, the control unit C1 proceeds to step S221.

[0174] When the control unit C1 proceeds to step S221, it determines whether a timeout has occurred. If the waiting time has not elapsed, it becomes "No" in step S221, so the control unit C1 returns to step S206. On the other hand, if the waiting time has elapsed, it becomes "Yes" in step S221, so the control unit C1 proceeds to step S222.

[0175] When the control unit C1 proceeds to step S222, it substitutes 1 for the variable Fin and proceeds to step S231. The processing after step S231 will be described later.

[0176] When transitioning from step S206 to step S211, the control unit C1 determines whether the images formed on the 2(N + 2)-th page and the (2N - 1)-th page are both monochrome. If the result in step S211 is "Yes", the control unit C1 proceeds to step S214. On the other hand, if the result in step S211 is "No", the control unit C1 proceeds to step S212.

[0177] When the control unit C1 proceeds to step S212, it determines whether it is the "2461" model.

[0178] In the case of the image forming apparatus 1(1A), the result in step S212 is "Yes", and it returns to step S201. On the other hand, in the case of not being the image forming apparatus 1(1A), the result in step S212 is "No", and it proceeds to step S213.

[0179] When the control unit C1 proceeds to step S213, it determines whether the image formed on the 2(N + 2)-th page is monochrome. If the result in step S213 is "No", the control unit C1 returns to step S201. On the other hand, if the result in step S213 is "Yes", the control unit C1 proceeds to step S214.

[0180] When starting the "Second Mode: Color Printing Process" subroutine in the image forming apparatus 1(1A) and the result in step S211 is "Yes", it corresponds to the case where "the images of the even pages formed on the sheets scheduled to be conveyed after the (N + 2)-th sheet and the images of the odd pages formed on the sheet two sheets before the sheet scheduled to be conveyed are both monochrome" according to claim 5 of the present invention.

[0181] When starting the "Second Mode: Color Printing Process" subroutine in the image forming apparatus 1(1B) and the result in step S211 is "Yes", or when the result in step S213 is "Yes", it corresponds to the case where "the image of the even page formed on the sheet scheduled to be conveyed next to the (N + 2)-th sheet is monochrome" according to claim 8 of the present invention.

[0182] When the control unit C1 proceeds to step S214, it substitutes N + 2 into the variable M and proceeds to step S231. Here, for the (N + 2)-th sheet SH in the case of "Yes" in step S211 and the case of "Yes" in step S213, although it was scheduled for conveyance, it will not be conveyed during the current execution of the "Second Mode: Color Printing Process" subroutine.

[0183] When proceeding from step S214 or step S222 to step S231, the control unit C1 determines whether it is the "2461" model.

[0184] If it is the image forming apparatus 1(1A), it becomes "Yes" in step S231 and proceeds to step S232. Then, after printing 2N - 1 pages in the second state, the control unit C1 proceeds to step S233.

[0185] On the other hand, if it is not the image forming apparatus 1(1A), it becomes "No" in step S231 and proceeds to step S233.

[0186] When proceeding from step S231 or step S232 to step S233, the control unit C1 prints 2(N + 1) - 1 pages in the second state and then ends the "Second Mode: Color Printing Process" subroutine and proceeds to step S19 shown in FIG. 6.

[0187] The 2N - 1 pages in step S232 are an example of the "odd-numbered pages of the sheet in conveyance" according to claim 5 of the present invention. The 2(N + 1) - 1 pages in step S233 are an example of the "odd-numbered pages of the sheet in conveyance" according to claims 5 and 8 of the present invention.

[0188] Returning to the duplex printing program shown in FIG. 6 and proceeding from step S16 or step S18 to step S19, the control unit C1 determines whether the variable Fin = 1. When proceeding to step S19 via step S122 or step S222, it becomes "Yes", so the duplex printing program ends.

[0189] On the other hand, if the answer is "No" in step S19, the control unit C1 proceeds to step S20, substitutes M into the variable N, and then returns to step S12. Substituting M into the variable N corresponds to "changing the sequence number of the sheet scheduled for conveyance to the Nth sheet" according to claims 3, 5, and 8 of the present invention.

[0190] When the control unit C1 starts the "third mode: printing on odd pages" subroutine shown in FIG. 12, it proceeds to step S301. If the image to be formed on the odd page of the Nth sheet SH, that is, the (2N - 1)th page, is in color, the switching mechanism 3C is controlled to switch the image forming unit 3 to the second state. On the other hand, if the image to be formed on the (2N - 1)th page is monochrome, the switching mechanism 3C is controlled to switch the image forming unit 3 to the first state. If the state of the image forming unit 3 is already the state to be switched, the control unit C1 does not perform the switching.

[0191] Next, the control unit C1 proceeds to step S302 and prints the (2N - 1)th page.

[0192] Next, the control unit C1 proceeds to step S303. If the image to be formed on the odd page of the (N + 1)th sheet SH, that is, the (2(N + 1) - 1)th page, is in color, the switching mechanism 3C is controlled to switch the image forming unit 3 to the second state. On the other hand, if the image to be formed on the (2(N + 1) - 1)th page is monochrome, the switching mechanism 3C is controlled to switch the image forming unit 3 to the first state. If the state of the image forming unit 3 is already the state to be switched, the control unit C1 does not perform the switching.

[0193] Next, the control unit C1 proceeds to step S304, prints the (2(N + 1) - 1)th page, and then ends the "third mode: printing on odd pages" subroutine and proceeds to step S22 shown in FIG. 6.

[0194] When the control unit C1 proceeds to step S22, it substitutes N + 2 into the variable N and then returns to step S12.

[0195] The processes of step S21 and step S22 shown in FIG. 6 correspond to "when the images formed on both sides of each of the Nth, (N + 1)th, and (N + 2)th sheets are a mixture of monochrome and color, after forming images on both sides of the (N + 1)th sheet, the (N + 2)th sheet is conveyed to the image forming unit to form an image", which is according to claim 1 of the present invention.

[0196] Substituting N + 2 into the variable N in step S22 corresponds to "changing the sequence number of the sheet to be conveyed to the Nth sheet", which is according to claim 2 of the present invention.

[0197] The 2N - 1 page in step S302 and the 2(N + 1) - 1 page in step S304 shown in FIG. 12 are examples of "odd - numbered pages of the sheet being conveyed", which is according to claim 2 of the present invention.

[0198] When the double - sided printing program ends, if there is data for the odd - numbered page of the (N + 1)th sheet SH, that is, the 2(N + 1) - 1 page, printing is performed on one side, that is, without using the re - conveyance path P3, and the 2(N + 1) - 1 page is printed.

[0199] <Advantages and effects> When the image forming apparatus 1 (1A, 1B) of the embodiment forms images on both sides of the sheet SH, when attempting to switch to the first state or the second state while simultaneously conveying three or more sheets SH inside the apparatus main body 2 of the image forming apparatus 1 (1A, 1B), it may be difficult to ensure the time required for the switching operation performed by the switching mechanism 3C and the time for suppressing problems such as overlapping after the switching operation.

[0200] In this regard, the control unit C1 of this image forming apparatus 1 (1A, 1B) executes the double - sided printing program shown in FIG. 6. And when the images formed on both sides of each of the Nth, (N + 1)th, and (N + 2)th sheets SH are all monochrome or all color, before forming images on both sides of the (N + 1)th sheet SH, the (N + 2)th sheet SH is conveyed to the image forming unit 3 to form an image.

[0201] Specifically, in the "mode determination process" subroutine shown in FIG. 9, the control unit C1 selects the first mode when all the images formed on both sides of each of the Nth, (N + 1)th, and (N + 2)th sheets SH are monochrome. Then, the control unit C1 proceeds to step S101 shown in FIG. 10 and prints 2(N + 2) pages. Also, in the "mode determination process" subroutine, when all the images formed on both sides of each of the Nth, (N + 1)th, and (N + 2)th sheets SH are color, the control unit C1 selects the second mode. Then, the control unit C1 proceeds to step S201 shown in FIG. 11 and prints 2(N + 2) pages.

[0202] In this case, as shown in FIGS. 2 and 4, three or more sheets SH are simultaneously conveyed inside the apparatus main body 2 of the image forming apparatus 1 (1A, 1B), and the first state or the second state is switched according to whether all of those images are monochrome or color.

[0203] Further, when the images formed on both sides of each of the Nth, (N + 1)th, and (N + 2)th sheets SH are a mixture of monochrome and color, the control unit C1 causes the (N + 2)th sheet SH to be conveyed to the image forming unit 3 to form an image after forming images on both sides of the (N + 1)th sheet SH.

[0204] Specifically, in the "mode determination process" subroutine, when the images formed on both sides of each of the Nth, (N + 1)th, and (N + 2)th sheets SH are a mixture of monochrome and color, the control unit C1 selects the third mode except for some exceptions. Then, the control unit C1 executes steps S21 and S22 shown in FIG. 6.

[0205] In this case, two or fewer sheets SH are simultaneously conveyed inside the apparatus main body 2 of the image forming apparatus 1 (1A, 1B), and the first state or the second state is switched each time according to whether each image is monochrome or color.

[0206] That is, this image forming apparatus 1 (1A, 1B) can change the page order of the images formed by the image forming unit 3 and switch between the first state and the second state by selecting the timing for transporting the (N + 2)-th sheet SH either after or before forming images on both sides of the (N + 1)-th sheet SH. Specifically, this image forming apparatus 1 (1A, 1B) can set the page order of image formation as, for example, the page order after "the second page and the fourth page" to be "the sixth page, the first page" or "the first page, the sixth page", or "the first page, the third page".

[0207] As a result, this image forming apparatus 1 (1A, 1B) can prioritize improving the number of sheets SH processed per unit time and form images in the second state from the beginning to the end regardless of whether the images are monochrome or color, or can prioritize extending the service life of the developing roller 7A and transport two or fewer sheets SH simultaneously inside the apparatus main body 2 of the image forming apparatus 1 (1A, 1B) from the beginning to the end, and switch between the first state and the second state according to whether the images are monochrome or color.

[0208] Therefore, the image forming apparatus 1 (1A, 1B) of the embodiment can achieve an improvement in the number of sheets SH processed per unit time and an extension of the service life of the developing roller 7A when forming images on both sides of the sheet SH.

[0209] Also, the control unit C1 of this image forming apparatus 1 (1A, 1B) selects the third mode in the "mode determination process" subroutine shown in FIG. 9 and executes the "third mode: odd-page printing" subroutine shown in FIG. 12 to form images in the order of the even pages of the N-th sheet SH, the even pages of the (N + 1)-th sheet SH, the odd pages of the N-th sheet SH, and the odd pages of the (N + 1)-th sheet SH. Thereafter, the control unit C1 proceeds to step S22 shown in FIG. 6, changes the serial number of the (N + 2)-th sheet SH to the N-th sheet, and transports the N-th sheet SH.

[0210] With this configuration, by selecting the timing to convey the (N + 2)-th sheet SH after forming images on both sides of the (N + 1)-th sheet SH, it is possible to switch between the first state and the second state. As a result, the page order of the images formed by the image forming unit 3 is changed so as to be "2413" control. Then, by changing the sequence number of the sheet SH after the execution, images can be continuously formed for the remaining sheets SH as well.

[0211] Furthermore, the control unit C1 of this image forming apparatus 1 (1A, 1B) selects the first mode in the "mode determination process" subroutine shown in FIG. 9, and when it becomes "Yes" in step S111 during the execution of the "first mode: monochrome printing process" subroutine shown in FIG. 10, steps S112 to S133 are executed, and further step S20 shown in FIG. 6 is executed. Thereby, without conveying the sheet SH scheduled to be conveyed, after forming an image on the odd-numbered pages of the sheet SH being conveyed, the sequence number of the sheet SH scheduled to be conveyed is changed to the N-th sheet, and the N-th sheet SH is conveyed.

[0212] With this configuration, when the continuity of monochrome images is interrupted, it is possible to switch between the first state and the second state by changing the order of forming images for the sheet SH scheduled to be conveyed and the sheet SH being conveyed.

[0213] Also, the control unit C1 of the image forming apparatus 1 (1A) of the "2461" model becomes "Yes" in step S83 of the "mode determination process" subroutine shown in FIG. 9, becomes "Yes" in step S84, and becomes "No" in step S85, thereby selecting the second mode and executing the "second mode: color printing process" subroutine shown in FIG. 11. Thereby, with the image forming unit 3 remaining in the second state, after forming images in order on the even-numbered pages of the N-th sheet SH, the even-numbered pages of the (N + 1)-th sheet SH, the (N + 2)-th sheet SH is conveyed to form an image, and then an image is formed on the odd-numbered pages of the N-th sheet SH.

[0214] With this configuration, regarding the timing of transporting the (N + 2)-th sheet SH, before forming images on both sides of the (N + 1)-th sheet SH, it is selected, and while remaining in the second state, the image forming unit 3 forms an image by "2461" control, so that the number of sheets SH processed per unit time can be improved. Note that for monochrome pages as well, the image forming unit 3 forms an image while remaining in the second state.

[0215] Furthermore, when the control unit C1 of the image forming apparatus 1(1A) becomes "Yes" in step S211 during the execution of the "Second Mode: Color Printing Process" subroutine shown in FIG. 11, it executes steps S214 to S233 and further executes step S20 shown in FIG. 6. Thereby, without transporting the sheet SH scheduled to be transported, after forming an image on the odd-numbered pages of the sheet SH being transported, the sequence number of the sheet SH scheduled to be transported is changed to the N-th sheet, and the N-th sheet SH is transported.

[0216] With this configuration, when monochrome is mixed with color images continuously under predetermined conditions, by changing the order of forming images for the sheet SH scheduled to be transported and the sheet SH being transported, it is possible to switch between the first state and the second state. Note that even when the odd-numbered page images formed on the sheet SH being transported are monochrome, the image forming unit 3 forms an image while remaining in the second state.

[0217] Also, the control unit C1 of the image forming apparatus 1(1B) of the "2416" model becomes "Yes" in step S83 of the "Mode Judgment Process" subroutine shown in FIG. 9, becomes "No" in step S84, and becomes "Yes" in step S87 to select the second mode, and executes the "Second Mode: Color Printing Process" subroutine shown in FIG. 11. Thereby, while the image forming unit 3 remains in the second state, after forming images in the order of the even-numbered pages of the N-th sheet, the even-numbered pages of the (N + 1)-th sheet, and the odd-numbered pages of the N-th sheet, the (N + 2)-th sheet is transported to form an image.

[0218] With this configuration, for the timing of conveying the (N + 2)-th sheet SH, it is selected before forming images on both sides of the (N + 1)-th sheet SH. By having the image forming unit 3 form an image under the "2416" control while remaining in the second state, the number of sheets SH processed per unit time can be improved. Note that for monochrome pages as well, the image is formed by the image forming unit 3 while remaining in the second state.

[0219] Furthermore, when the control unit C1 of the image forming apparatus 1(1B) becomes "Yes" in step S83 of the "mode determination process" subroutine shown in FIG. 9, becomes "No" in step S84, and becomes "Yes" in step S87, it includes the case where the odd-numbered pages of the (N + 1)-th sheet SH are monochrome. Even in this case, the control unit C1 has the image forming unit 3 remain in the second state, forms an image on the (N + 2)-th sheet SH, and then forms an image on the odd-numbered pages of the (N + 1)-th sheet SH.

[0220] With this configuration, even if the odd-numbered pages of the (N + 1)-th sheet SH are monochrome, since the "2416" control is executed in the second state, the number of sheets SH processed per unit time can be further improved.

[0221] Also, when the control unit C1 of the image forming apparatus 1(1B) becomes "Yes" in step S211 or becomes "Yes" in step S213 during the execution of the "second mode: color printing process" subroutine shown in FIG. 11, it executes steps S214 to S233 and further executes step S20 shown in FIG. 6. Thereby, without conveying the sheet SH scheduled for conveyance, after forming an image on the odd-numbered pages of the sheet SH being conveyed, it changes the sequence number of the sheet SH scheduled for conveyance to the N-th sheet and conveys the N-th sheet SH.

[0222] With this configuration, when monochrome is mixed in a continuous color image under predetermined conditions, it is possible to switch between the first state and the second state by changing the order of forming images for the sheet SH scheduled for conveyance and the sheet SH being conveyed. Note that even when the images of the odd-numbered pages formed on the sheet SH being conveyed are monochrome, the image forming unit 3 forms the images while remaining in the second state.

[0223] As described above, the present invention has been described with reference to the embodiments. However, the present invention is not limited to the above embodiments, and it goes without saying that the present invention can be appropriately modified and applied without departing from the spirit thereof.

[0224] For example, the determination as to whether it is the "2461" model in steps S53, S84, S102, S131, S202, and S231 may not be necessary. For example, for an apparatus in which it is determined in advance whether it is the 2461 model, a duplex printing program adapted to the apparatus may be set. That is, for an apparatus that is the "2461" model, from the duplex printing program of the embodiment, the process of step S53, the processes from step S71 to S77, the processes of step S84 and S87, the processes of step S102 and S104, the process of step S131, the processes of step S201 and S204, and the process of step S231 may be deleted. Similarly, for an apparatus that is the "2416 model", from the duplex printing program of the embodiment, the processes from step S53 to S55, the processes from step S61 to S65, the processes from step S84 to S86, the processes of step S102 and S103, the processes of step S131 and S132, the processes of step S202 and S203, and the processes of step S231 and S232 may be deleted. Alternatively, a duplex printing program dedicated to the "2461" model and a duplex printing program dedicated to the "2416" model may be stored in advance, and it may be determined which program to execute at the start of duplex printing.

[0225] Also, in this embodiment, the control unit C1 executed the mode determination process shown in FIG. 9 at the timing of step S14, that is, at the timing when printing up to 2(N + 1) pages was completed. However, for example, it may be determined at the start timing of double-sided printing, that is, before the operation of step S12. However, by executing the mode determination process of step S14 after printing up to 2N pages and 2(N + 1) pages is completed, even if there is no data for forming an image on the (N + 2)-th sheet SH, the images can be formed on 2N pages and 2(N + 1) pages first, so the time related to image formation can be shortened.

[0226] Also, the waiting times in steps S44, S55, S74, S121, and S221 may all be the same, may all be different, or some may be the same and some may be different.

Industrial Applicability

[0227] The present invention can be used, for example, in an image forming apparatus or a multifunction peripheral.

Explanation of Signs

[0228] 1... Image forming apparatus 1A... Image forming apparatus of "2461" model 1B... Image forming apparatus of "2416" model 7A... Developing roller 5... Photosensitive drum SH... Sheet 3... Image forming unit 4... Conveying unit C1... Control unit

Claims

1. An image forming unit having a plurality of sets of a developing roller and a photosensitive drum that can come into contact with and separate from the developing roller, and forming an image on a sheet; A conveying unit that can reverse the sheet after conveying it to the image forming unit and convey it back to the image forming unit; A control unit that controls the image forming unit and the conveying unit and can form images on both sides of the conveyed sheet; The image forming unit is in a first state in which one set of the developing roller and the photosensitive drum are in contact with each other to form a monochrome image, while the other sets of the developing roller and the photosensitive drum are separated, and a second state in which all sets of the developing roller and the photosensitive drum are in contact with each other to form a color image, and switches between the two states; The control unit: When the images formed on both sides of each of the Nth, (N + 1)th, and (N + 2)th sheets are all monochrome or all color, the (N + 2)th sheet is conveyed to the image forming unit to form an image before forming an image on both sides of the (N + 1)th sheet; When the images formed on both sides of each of the Nth, (N + 1)th, and (N + 2)th sheets are a mixture of monochrome and color, the (N + 2)th sheet is conveyed to the image forming unit to form an image after forming an image on both sides of the (N + 1)th sheet; The control unit, when the images formed on both sides of each of the Nth, (N + 1)th, and (N + 2)th sheets are all monochrome and at least one of the images on both sides to be formed on the sheet scheduled to be conveyed next to the (N + 2)th sheet is not monochrome, does not convey the sheet scheduled to be conveyed, forms an image on the odd pages of the sheet in the middle of conveyance, then changes the sequence number of the sheet scheduled to be conveyed to the Nth, and conveys the Nth sheet. An image forming apparatus characterized by this.

2. The control unit, when the images formed on both sides of each of the Nth, (N + 1)th, and (N + 2)th sheets are a mixture of monochrome and color, forms images in the order of the even pages of the Nth sheet, the even pages of the (N + 1)th sheet, the odd pages of the Nth sheet, and the odd pages of the (N + 1)th sheet, then changes the sequence number of the (N + 2)th sheet to the Nth and conveys the Nth sheet. The image forming apparatus according to Claim 1.

3. The control unit can simultaneously convey four or more sheets to the conveying unit inside the image forming apparatus; When the images of the even pages of the Nth and (N + 1)th sheets are both in the color, and at least one of the image of the even page of the (N + 2)th sheet and the image of the odd page of the Nth sheet is not in the monochrome, the image forming unit remains in the second state, forms images in the order of the even page of the Nth sheet and the even page of the (N + 1)th sheet, then conveys the (N + 2)th sheet to form an image, and then forms an image on the odd page of the Nth sheet. The image forming apparatus according to claim 1 or 2.

4. When the images of the even pages to be formed on the sheet scheduled to be conveyed next to the (N + 2)th sheet and the images of the odd pages to be formed on the sheet two sheets before the sheet scheduled to be conveyed are both in the monochrome, the conveyance of the sheet scheduled to be conveyed is not performed, the image forming unit remains in the second state, forms an image on the odd page of the sheet being conveyed, then changes the sequence number of the sheet scheduled to be conveyed to the Nth, and conveys the Nth sheet. The image forming apparatus according to claim 3.

5. The control unit can simultaneously convey three sheets to the conveyance unit inside the image forming apparatus. When the images of the even pages of the Nth and (N + 1)th sheets are both in the color, and the images of the odd page of the Nth sheet and the even page of the (N + 2)th sheet are both in the color, the image forming unit remains in the second state, forms images in the order of the even page of the Nth sheet, the even page of the (N + 1)th sheet, and the odd page of the Nth sheet, and then conveys the (N + 2)th sheet to form an image. The image forming apparatus according to claim 1 or 2.

6. When the odd page of the (N + 1)th sheet is in the monochrome, the image forming unit remains in the second state, forms an image on the (N + 2)th sheet, and then forms an image on the odd page of the (N + 1)th sheet. The image forming apparatus according to claim 5.

7. When the image of the even page to be formed on the sheet scheduled to be conveyed next to the (N + 2)th sheet is in the monochrome, the conveyance of the sheet scheduled to be conveyed is not performed, the image forming unit remains in the second state, forms an image on the odd page of the sheet being conveyed, then changes the sequence number of the sheet scheduled to be conveyed to the Nth, and conveys the Nth sheet. The image forming apparatus according to claim 5 or 6. Claim 8 An image forming unit having a plurality of sets of a developing roller and a photosensitive drum that can come into contact with and separate from the developing roller, and forming an image on a sheet; A conveying unit that can reverse the sheet after conveying it to the image forming unit and convey it back to the image forming unit; A control unit that controls the image forming unit and the conveying unit and can form images on both sides of the conveyed sheet; and The image forming unit switches between a first state in which one set of the developing roller and the photosensitive drum are in contact with each other to form a monochrome image while the other sets of the developing roller and the photosensitive drum are separated, and a second state in which all sets of the developing roller and the photosensitive drum are in contact with each other to form a color image; The control unit: When the images formed on both sides of each of the Nth, (N + 1)th, and (N + 2)th sheets are all monochrome or all color, the (N + 2)th sheet is conveyed to the image forming unit to form an image before forming an image on both sides of the (N + 1)th sheet; When the images formed on both sides of each of the Nth, (N + 1)th, and (N + 2)th sheets are a mixture of monochrome and color, the (N + 2)th sheet is conveyed to the image forming unit to form an image after forming an image on both sides of the (N + 1)th sheet; The control unit can simultaneously convey three sheets to the conveying unit inside the image forming apparatus; When the images on the even pages of the Nth and (N + 1)th sheets are both color, and the images on the odd page of the Nth sheet and the even page of the (N + 2)th sheet are both color, with the image forming unit remaining in the second state, images are formed in the order of the even page of the Nth sheet, the even page of the (N + 1)th sheet, and the odd page of the Nth sheet, and then the (N + 2)th sheet is conveyed to form an image; The control unit forms an image on the odd page of the (N + 1)th sheet after forming an image on the (N + 2)th sheet with the image forming unit remaining in the second state when the odd page of the (N + 1)th sheet is monochrome. An image forming apparatus characterized by this.

Images