Image forming apparatus

Abstract

To reduce buckling of a sheet and occurrence of paper jam, even when the timing at which a rear face image in double-sided printing reaches a transfer unit is expected to be earlier than the timing at which a rear face sheet reaches the transfer unit.SOLUTION: In a case where a sheet 2 is conveyed through a double-sided conveyance path 33, when the sheet 2 is expected to reach a transfer unit earlier than a toner image formed by an image forming unit 307 to be transferred to a rear face (YES in S3140), an engine control unit 301 performs reciprocation control of alternately switching between first control and second control at least one or more times (S3170-S3220).SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus, that is, to an image forming apparatus using an electrophotographic method or an electrostatic recording method, for example, an image forming apparatus having a switchback mechanism that reverses the paper transport direction for double-sided printing. [Background technology]

[0002] Conventionally, electrophotographic image forming apparatuses for forming images are known. In electrophotographic image forming apparatuses, a toner image formed on an image carrier or a transfer member is transferred to a recording medium (hereinafter referred to as a sheet) such as plain paper or glossy paper, and then the sheet to which the toner image has been transferred is heated to fix the toner image on the sheet. In recent years, in order to improve productivity and user operability, such image forming apparatuses are required to have a high sheet conveying speed and various sheet handling functions. One of them is a device having a double-sided printing function that automatically reverses and conveys a sheet to form an image on both sides of the sheet when a user prints on both sides of the sheet, without having to take the trouble of manually turning the sheet over and setting it. The double-sided printing function requires a reversing conveying mechanism for reversing the sheet in the conveying path. As the reversing conveying mechanism, the sheet is conveyed to a conveying path (hereinafter referred to as a double-sided reversing conveying path) as a reversing conveying section, and the sheet is temporarily stopped there and then conveyed in the opposite direction. At that time, there is a method in which the sheet is reversed and conveyed by switching to a double-sided conveying path in a direction different from the conveying path from which the sheet was conveyed. For example, Patent Document 1 discloses that in double-sided printing mode, a sheet is temporarily stopped at a pause point for a predetermined time and then transport is resumed. This allows transport control to maintain a constant interval between sheets after paper standby due to a delay in image formation in double-sided printing. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 06-271144 Summary of the Invention [Problem to be solved by the invention]

[0004] However, in a space-saving image forming apparatus, when double-sided printing is performed on a sheet longer than a certain length, there are cases where the leading edge and trailing edge of the same sheet are sandwiched between multiple conveying rollers driven by different drive sources at the temporary stop point after the sheet is reversed. Specifically, the same sheet is simultaneously sandwiched between a reversing roller driven by a fixing motor, which is also the drive source of the fixing unit, and a registration roller driven by a registration motor.

[0005] Furthermore, there is an image forming apparatus that has only a clutch mechanism that switches the rotation direction of the reversing roller between forward and reverse rotation to reduce the cost of the switchback mechanism, but does not have a mechanism for disconnecting the fixing motor drive to the reversing roller. In this image forming apparatus, it is necessary to stop the fixing motor drive to stop the rotation of the reversing roller, and it is difficult to stop the reversing roller during a printing operation from the viewpoint of fixability. For example, if the fixing motor is stopped to stop the conveyance of the sheet during double-sided printing, there is a problem that the uniformity of the temperature in the direction of rotation of the fixing roller is lost, and uniform image quality cannot be obtained. Conversely, if the registration roller is stopped without stopping the fixing motor, the sheet is pushed into the double-sided conveyance path by the reversing roller, which may cause problems such as buckling of the sheet or paper jams.

[0006] The present invention has been made under such circumstances, and aims to reduce the occurrence of sheet buckling and paper jams even when it is expected that the back sheet will reach the transfer section earlier than the back image of double-sided printing reaches the transfer section. [Means for solving the problem]

[0007] In order to solve the above-mentioned problems, the present invention has the following configuration.

[0008] (1) An image forming means for forming a toner image, a transfer means for transferring the toner image formed by the image forming means in a transfer section, a fixing means for performing a fixing process on an unfixed toner image on a recording material, a first rotating body for transporting the recording material to the image forming means in a transport direction that is a first direction, a second rotating body provided downstream of the fixing means in the first direction, a drive source for driving the fixing means and the second rotating body, a detection means provided between the first rotating body and the second rotating body and capable of detecting the leading end and the trailing end of the recording material, a first control for controlling the second rotating body to transport the recording material in the first direction, and a second control for controlling the second rotating body to transport the recording material in a second direction opposite to the first direction. and a control means for switching between a first control and a second control for controlling the second rotating body, wherein when performing double-sided printing in which an image is formed on a first side and a second side different from the first side of a recording material, the control means switches the second rotating body from the first control to the second control to transport the recording material to a double-sided transport path and again to the transfer section, wherein the control means performs reciprocating control that alternates between the first control and the second control at least once when it is predicted that the recording material will reach the transfer section earlier than a toner image to be transferred to the second side formed by the image forming means while the recording material is being transported along the double-sided transport path. Effect of the Invention

[0009] According to the present invention, even when it is expected that the back sheet will reach the transfer section earlier than the back image of double-sided printing reaches the transfer section, the occurrence of sheet buckling and paper jams can be reduced. [Brief description of the drawings]

[0010] [Figure 1] Schematic diagram of a laser printer engine according to the first embodiment [Diagram 2] Functional block diagram of the first embodiment [Diagram 3] Flowchart when the transport control of the first embodiment is not performed [Figure 4] 1 is a flowchart showing a case where the transport control according to the first embodiment is carried out. [Diagram 5] Schematic diagram of a laser printer engine according to a second embodiment of the present invention. [Figure 6] Functional block diagram of the second embodiment [Figure 7] 11 is a flowchart showing the transport control according to the second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLES

[0011] A first embodiment to which the present invention can be applied will be described below. The overall configuration of a laser printer engine (hereinafter, referred to as a printer) as an image forming apparatus will be outlined with reference to FIG.

[0012] (Outline of image forming apparatus) The printer 200 forms an electrostatic latent image by image light formed based on an image signal sent from a controller unit (not shown), develops the electrostatic latent image, and transfers a visible image in a superimposed manner to form a color visible image. The printer 200 transfers the color visible image to a sheet 2 as a recording material, and fixes the unfixed color visible image on the sheet 2 (on the recording material). The image forming unit is composed of photosensitive drums 5Y, 5M, 5C, and 5K, chargers 7Y, 7M, 7C, and 7K, developers 8Y, 8M, 8C, and 8K, and an intermediate transfer belt 12, which are arranged in parallel for each developing color. The photosensitive drums 5Y, 5M, 5C, and 5K, chargers 7Y, 7M, 7C, and 7K, and developers 8Y, 8M, 8C, and 8K are mounted in process cartridges 22Y, 22M, 22C, and 22K that are detachable from the printer 200 main body.

[0013] The photosensitive drums 5Y, 5M, 5C, and 5K are configured by coating the outer periphery of an aluminum cylinder with an organic photoconductive layer, and are rotated by the driving force of a drive motor (not shown). The drive motor rotates the photosensitive drums 5Y, 5M, 5C, and 5K, for example, in a clockwise direction in response to an image forming operation. Exposure light to the photosensitive drums 5Y, 5M, 5C, and 5K is sent from scanner units 10Y, 10M, 10C, and 10K, and is configured to selectively expose the surfaces of the photosensitive drums 5Y, 5M, 5C, and 5K to form electrostatic latent images.

[0014] Each station as an image forming means is provided with four chargers 7Y, 7M, 7C, and 7K as primary charging means for charging the photosensitive drums 5Y, 5M, 5C, and 5K of yellow (Y), magenta (M), cyan (C), and black (K), respectively. The chargers 7Y, 7M, 7C, and 7K are provided with sleeves 7YS, 7MS, 7CS, and 7KS, respectively.

[0015] To visualize the electrostatic latent image, each station is provided with four developing units 8Y, 8M, 8C, and 8K as developing means for developing yellow (Y), magenta (M), cyan (C), and black (K). Each of the developing units 8Y, 8M, 8C, and 8K is provided with a sleeve 8YS, 8MS, 8CS, and 8KS. Each of the developing units 8Y, 8M, 8C, and 8K is detachably attached to the printer 200 body.

[0016] The intermediate transfer belt 12 as an intermediate transfer body is in contact with the photosensitive drums 5Y, 5M, 5C, and 5K, and rotates, for example, in a counterclockwise direction during color image formation, rotating with the rotation of the photosensitive drums 5Y, 5M, 5C, and 5K. The intermediate transfer belt 12 receives a visible image transferred thereto by a primary transfer voltage applied to the primary transfer rollers 4Y, 4M, 4C, and 4K. The intermediate transfer belt 12 transfers and superimposes a color visible image onto the sheet 2 by sandwiching and conveying the sheet 2 at the position (transfer portion) of the secondary transfer roller 9 as a transfer means.

[0017] The fixing device 13 as a fixing means fixes the transferred color visible image while conveying the sheet 2. The fixing device 13 includes a fixing roller 14 for heating the sheet 2 and a pressure roller 15 for pressing the sheet 2 against the fixing roller 14. The fixing roller 14 and the pressure roller 15 are formed in a hollow shape, and a heater is built in the fixing roller 14. That is, the sheet 2 holding the color visible image is conveyed by the fixing roller 14 and the pressure roller 15, and the toner is fixed to the surface by applying heat and pressure. After the visible image is fixed, the sheet 2 is conveyed to the discharge conveying path 26 and discharged to the discharge section by the discharge roller 31, and the image forming operation is completed. The full-load sensor 39 is a sensor that detects that the multiple sheets 2 stacked on the discharge tray 27 are fully loaded. In addition, in the conveying path of the sheet 2 shown by the dashed line in FIG. 1, a registration sensor 19, a fixing discharge sensor 20, and a double-sided conveying sensor 28 that can detect the leading and trailing ends of the conveyed sheet 2 are arranged. The roller in front of the registration sensor 19 in the transport direction is the registration roller 3 as a first rotating body. Also, the printer 200 is not limited to the configuration shown in FIG.

[0018] (Double-sided transport control) Next, a description will be given of the double-sided conveying control when printing on the front side (first side) and back side (second side) of the sheet 2. After the sheet 2 has been printed on the front side and passed through the fixing unit 13, the sheet 2 is conveyed to the double-sided reversing path 29 by switching the rotation direction of the reversing roller 30 as the second rotating body and the state (solid line) of the double-sided flapper 32 by the reversing clutch 103 (see FIG. 2). When the rear end of the sheet 2 reaches the double-sided reversing path 29, the rotation direction of the reversing roller 30 and the state (dashed line) of the double-sided flapper 32 are switched again by the reversing clutch 103 to convey the sheet 2 to the double-sided conveying path 33 as the double-sided conveying path. The sheet 2 whose conveying direction has been reversed is conveyed to the double-sided conveying path 33 by the double-sided conveying roller 37 and the double-sided re-feed roller 35.

[0019] The conveying direction of the sheet 2 toward the discharge conveying path 26 or the double-sided reversing path 29 corresponds to the first direction, and the conveying direction of the sheet 2 toward the double-sided conveying path 33 (the direction opposite to the first direction) corresponds to the second direction. An engine control unit 301 (see FIG. 2), which will be described later, functions as a control unit that switches between a first control that controls the reversing rollers 30 to convey the sheet 2 in the first direction, and a second control that controls the reversing rollers 30 to convey the sheet 2 in the second direction. When performing double-sided printing in which images are formed on both the front and back sides of the sheet 2, the engine control unit 301 switches the reversing rollers 30 from the first control to the second control, and conveys the sheet 2 to the double-sided conveying path 33, so that the sheet 2 is conveyed to the secondary transfer rollers 9 again.

[0020] When the conveying direction of the sheet 2 in the double-sided reversing path 29 is switched, the fixing motor 102 (see FIG. 2) as the driving source remains driven and the drive to the reversing roller 30 cannot be cut off. Therefore, except for the short time required for changing the driving gear train, the conveying of the sheet 2 in the reversing roller 30 does not stop. In this way, the sheet 2 enters the print conveying path 25 again in an inverted state, where the toner image is transferred to the back side, the fixing process is performed, and the sheet is discharged to the discharge section.

[0021] (Function block diagram) 2 is a functional block diagram showing the functional configuration of the first embodiment. The engine control unit 301 performs image formation by the image forming unit 307. The image forming unit 307 determines the timing of image formation according to the image interval determined by the image formation interval control unit 320. The paper feed unit 303 issues a rotation command to the paper feed double-sided motor 101 to rotate the paper feed roller 40 and feed the sheet 2 from the cassette 1. An image formed by the image forming unit 307 is transferred onto the fed sheet 2. The paper transport unit 304 issues a rotation command to the fixing motor 102 to rotate the fixing roller 14, the pressure roller 15, and the discharge roller 31, and transports the sheet 2 to the discharge tray 27.

[0022] Next, a description will be given of each function of the duplex conveying control when printing on the front and back sides of the sheet 2. By turning on the reversing clutch 103, the position of the duplex flapper 32 is switched to convey the sheet 2 toward the duplex reversing path 29. The detection unit 306 detects the leading edge of the sheet 2 by the registration sensor 19 as the first detection means (detection means). From the time difference between the image formation interval and the image formation timing determined by the control unit 320, the paper feed unit 303 determines whether or not to change the speed of the paper feed duplex motor 101 and the timing of the speed change.

[0023] The fixing discharge sensor 20 as the second detecting means (detecting means) causes the detecting unit 306 to detect the rear end of the sheet 2, the reversing timing control unit 310 determines the reversing timing, and the reversing unit 305 turns off the reversing clutch 103. As a result, the sheet 2 conveyed toward the double-sided reversing path 29 is conveyed to the double-sided conveying path 33. The reversing rollers 30 are driven by the fixing motor 102, and when the reversing clutch 103 is turned off, the reversing rollers 30 rotate toward the double-sided conveying path 33, and when the reversing clutch 103 is turned on, the reversing rollers 30 rotate toward the double-sided reversing path 29. That is, when the reversing clutch 103 is turned on, the reversing rollers 30 convey the sheet 2 in the same direction as the previous conveying direction, and when the reversing clutch 103 is turned off, the reversing rollers 30 convey the sheet 2 in the opposite direction to the previous conveying direction.

[0024] When the reversing clutch 103 is OFF, the double-sided flapper 32 switches the transport path to the discharge tray 27 direction (dashed line in FIG. 1), and when the reversing clutch 103 is ON, the transport path switches to the double-sided reversing path 29 direction (solid line in FIG. 1). The double-sided transport unit 302 rotates the double-sided transport roller 37 by issuing a rotation command to the paper feed double-sided motor 101, and transports the sheet 2 in an inverted state to the print transport path 25 again.

[0025] (Flowchart: When the control of the first embodiment is not performed) First, an example of the operation of the engine control unit 301 during double-sided printing when the control of the first embodiment is not performed will be described with reference to the flowchart of FIG. 3(a). In step 1010 (hereinafter, S1010), the engine control unit 301 judges whether or not a double-sided printing instruction has been received. If the engine control unit 301 judges in S1010 that a print instruction has not been received, the process returns to S1010, and if the engine control unit 301 judges that a print instruction has been received, the process proceeds to S1020. In S1020, the engine control unit 301 drives the fixing motor 102 and the paper feed double-sided motor 101 as a preparation operation for sheet conveyance. In S1030, the engine control unit 301 starts an image formation preparation operation by the image forming unit 307. When the image formation preparation is completed, the engine control unit 301 judges in S1035 that the preparation of image data for the first side of double-sided printing has been completed. If the engine control unit 301 determines in S1035 that the preparation of the image data is not complete, the process returns to S1035, and if the engine control unit 301 determines that the preparation of the image data is complete, the process proceeds to S1040. In S1040, the engine control unit 301 starts image formation for the first side.

[0026] In S1050, the engine control unit 301 determines whether or not a time X has elapsed until the start of the paper feed operation for the first side of the double-sided print, based on the image formation start time in S1040. At the same time, in S1051, the engine control unit 301 determines whether or not a time Y has elapsed until the start of the image formation for the second side of the double-sided print. Note that since the engine control unit 301 monitors S1050 and S1051 in parallel, the process after S1040 is branched as shown in FIG. 3(a).

[0027] Here, the time X in the first embodiment is determined in advance by the following calculation formula. Time Xi: Ideal time until the toner image formed by the image forming unit 307 reaches the secondary transfer roller 9 Time Xp: Ideal time until the sheet 2 fed from the paper feed unit 303 reaches the secondary transfer roller 9 Time X=Xi-Xp In addition, since the printer 200 in the first embodiment has a relationship of Xi>Xp, after the start of image formation, the feeding operation of the corresponding sheet 2 is started.

[0028] Moreover, the time Y in the first embodiment is determined in advance by the following calculation formula. Time Yp1: Ideal time for the leading edge of the first side of the sheet to reach the reversal position from the secondary transfer roller 9 Time Yp2: Ideal transport time for the reserved sheet length when printing Time Yp3: Ideal time for the leading edge of the second side sheet to reach the secondary transfer roller 9 from the inversion position Time Y=Yp1+Yp2+Yp3

[0029] Returning to the description of the flowchart, in S1050, if the engine control unit 301 determines that the time X has not elapsed, the process returns to S1050, and if it has elapsed, the process proceeds to S1060. In S1060, the engine control unit 301 starts a paper feed operation for the first side of the double-sided print. In S1051, if the engine control unit 301 determines that the time Y has not elapsed, the process returns to S1051, and if it has elapsed, the process proceeds to S1052. In S1052, the engine control unit 301 determines whether or not the preparation of the image data for the second side of the double-sided print is complete. In S1052, if the engine control unit 301 determines that the preparation has not been completed, the process returns to S1052, and if it determines that the preparation has been completed, the process proceeds to S1053. In S1053, the engine control unit 301 starts image formation for the second side, and the process proceeds to S1140. In this manner, image formation on the second side of the double-sided print is started in parallel with the transport operation for the first side of the double-sided print, which is being carried out as a result of the start of the paper feed operation.

[0030] In S1070, the engine control unit 301 determines whether or not the leading edge of the sheet on the first side (front side) of double-sided printing has reached the registration sensor 19. If the engine control unit 301 determines in S1070 that the leading edge of the sheet has not reached the registration sensor 19, the process returns to S1070, and if it determines that the leading edge has reached the registration sensor, the process proceeds to S1080. In S1080, the engine control unit 301 performs acceleration / deceleration / pause control.

[0031] (Acceleration / Deceleration / Pause Control) 3B, the acceleration / deceleration / pause control in S1080 will be described. In the acceleration / deceleration / pause control, the sheet 2 is transported a predetermined distance from the registration roller 3 to the secondary transfer roller 9 in the same time as the remaining time until the toner image reaches the secondary transfer roller 9. For this reason, the engine control unit 301 accelerates, decelerates, or stops the paper feed double-sided motor 101, and returns the rotation speed of the paper feed double-sided motor 101 so that the sheet 2 and the toner image are transported at the same speed when the leading edge of the sheet 2 reaches the secondary transfer roller 9.

[0032] In S2010, the engine control unit 301 calculates the amount of deviation between the image leading edge position and the sheet leading edge position from the difference between the time when image formation is started and the time when the leading edge of the sheet reaches the registration sensor 19. In S2020, the engine control unit 301 judges whether the amount of deviation can be eliminated within the range of the acceleration / deceleration performance of the paper feed duplex motor 101. If the engine control unit 301 judges in S2020 that the amount of deviation can be eliminated, the process proceeds to S2030. In S2030, the engine control unit 301 calculates the target speed of the paper feed duplex motor 101 and the maintenance time Ta for maintaining that speed. In S2040, the engine control unit 301 changes the speed of the paper feed duplex motor 101.

[0033] In S2050, the engine control unit 301 determines whether or not the maintenance time Ta has elapsed since the speed change of the paper feed double-sided motor 101. If the engine control unit 301 determines in S2050 that the maintenance time Ta has not elapsed, the process returns to S2050, and if it determines that the maintenance time Ta has elapsed, the process proceeds to S2060. In S2060, the engine control unit 301 changes the speed of the paper feed double-sided motor 101 to the same speed as the toner image, and ends the acceleration / deceleration / pause control. When the processes of S2030 to S2060 are performed, the timing at which the toner image and the leading edge of the sheet reach the secondary transfer roller 9 can be matched.

[0034] On the other hand, if the engine control unit 301 determines in S2020 that the amount of misalignment cannot be eliminated, the process proceeds to S2031. In S2031, the engine control unit 301 determines that a misprint has occurred because the timing at which the toner image and the leading edge of the sheet reach the secondary transfer roller 9 cannot be matched with each other under the acceleration / deceleration / pause control. In this case, the engine control unit 301 ends the process without changing the speed of the paper feed duplex motor 101.

[0035] Returning to the flowchart of FIG. 3A, when the acceleration / deceleration / pause control is ended in S1080 and the sheet transport is continued, the leading edge of the sheet S onto which the toner image has been transferred approaches the sheet inversion position downstream of the fixing unit 13. The engine control unit 301 performs the following process in S1090 to transport the sheet 2 of the first side of the double-sided print to the inversion section until the leading edge of the sheet reaches the transport path branch point by the double-sided flapper 32. In S1090, the engine control unit 301 judges whether or not a predetermined time has elapsed since the acceleration / deceleration / pause control was ended. If the engine control unit 301 judges in S1090 that the predetermined time has not elapsed, the process returns to S1090, and if it judges that the predetermined time has elapsed, the process proceeds to S1100.

[0036] In S1100, the engine control unit 301 operates the reversing clutch 103, specifically, turns on the reversing clutch 103 to switch the position of the duplex flapper 32 and the rotation direction of the reversing rollers 30. As a result, the duplex flapper 32 switches the conveying path to the duplex reversing path 29 direction (solid line in FIG. 1), and the reversing rollers 30 convey the sheet 2 in the same direction as the previous conveying direction. In S1110, the engine control unit 301 determines whether or not the rear end of the sheet 2 has reached the fixing discharge sensor 20 after the leading edge of the sheet has entered the reversing unit. If the engine control unit 301 determines in S1110 that the trailing edge of the sheet has not reached the fixing discharge sensor 20, the process returns to S1110, and if it determines that the trailing edge has reached the fixing discharge sensor 20, the process proceeds to S1120.

[0037] In order to reverse the conveying direction of the first side of the double-sided print and start pulling it into the double-sided conveying path 33, in S1120 the engine control unit 301 determines whether or not time B has elapsed since the rear end of the sheet S reached the fixing discharge sensor 20. If the engine control unit 301 determines in S1120 that time B has not elapsed, the process returns to S1120, and if it determines that time B has elapsed, the process proceeds to S1130.

[0038] In S1130, the engine control unit 301 operates the reversing clutch 103, specifically, turns off the reversing clutch 103 and switches the position of the duplex flapper 32 and the rotation direction of the reversing roller 30. As a result, the duplex flapper 32 switches the conveying path to the duplex conveying path 33 direction (broken line in FIG. 1), and the reversing roller 30 conveys the sheet 2 in the opposite direction to the previous conveying direction. Note that time B of S1120 in the first embodiment is preset so as to reverse the conveying direction of the sheet 2 at the timing when the rear end position of the first side of the sheet 2 in the duplex print reaches a position, for example, 15 mm upstream of the reversing roller 30.

[0039] When the conveying direction of the sheet 2 is switched, the rear end of the sheet of the first side of the double-sided print becomes the leading edge of the sheet of the second side of the double-sided print and passes through the double-sided conveying path 33. In S1140, the engine control unit 301 judges whether or not the leading edge of the sheet of the second side (reverse side) of the double-sided print has reached the registration sensor 19. If the engine control unit 301 judges in S1140 that the leading edge of the sheet has not reached the registration sensor 19, the process returns to S1140, and if it judges that the leading edge has reached the registration sensor 19, the process proceeds to S1150. In S1150, the engine control unit 301 again performs the acceleration / deceleration / pause control described in FIG. 3B. After the acceleration / deceleration / pause control is completed, the engine control unit 301 judges in S1160 whether or not the trailing edge of the sheet of the second side of the double-sided print has reached the fixing / discharging sensor 20. If the engine control unit 301 judges in S1160 that the trailing edge of the sheet has not reached the fixing / discharging sensor 20, the process returns to S1160, and if it judges that the trailing edge has reached the fixing / discharging sensor 20, the process proceeds to S1170.

[0040] In S1170, engine control unit 301 determines whether a predetermined time sufficient for discharging the second side of a double-sided print to discharge tray 27 has elapsed. If engine control unit 301 determines in S1170 that the predetermined time has not elapsed, it returns the process to S1170, and if it determines that the predetermined time has elapsed, it advances the process to S1180. In S1180, engine control unit 301 stops the operation of paper feed double-sided motor 101, fixing motor 102, and image forming unit 307, and ends the printing operation. In this way, the fixing motor 102 is maintained in a driven state from the time it is started to be driven in S1020 until it is stopped in S1180.

[0041] (Control issue in Fig. 3(a)) Here, a problem that occurs in the operation example of FIG. 3(a) will be described. The printer 200 may perform a print operation in a universal size mode in which the user does not specify a sheet size. In the universal size mode, the printer 200 operates normally with all sizes of sheets from the minimum size to the maximum size that can be printed by the printer 200, so the printer 200 operates at a timing assuming the maximum size. However, even when the printer 200 performs a print operation in the universal size mode, various sizes of sheets different from the maximum size are actually used. Furthermore, the user of the printer 200 may install and use a sheet of a size different from the print instruction to the device due to an operation error or mistake. Furthermore, there may be a sheet of a size different from the specified size due to variations in sheet manufacturing.

[0042] For example, printer 200 in the first embodiment supports ledger size sheets (279.4 mm wide x 431.8 mm long) as the maximum sheet size during double-sided printing. Assume that when double-sided printing is performed with printer 200, the universal size mode is specified in S1010 described above, and a legal size sheet (215.9 mm wide x 355.6 mm long) that is shorter than the maximum sheet size, ledger size, is set and used. In this case, the trailing edge of the first side of the sheet reaches fixing discharge sensor 20 earlier in S1110, and reversing roller 30 continues to rotate almost without stopping during switchback. For this reason, the leading edge of the second side of the double-sided print after the transport direction is reversed also reaches registration sensor 19 earlier.

[0043] On the other hand, the engine control unit 301 starts forming a toner image on the second side when the time Y determined from the sheet length of the ledger size, which is the maximum sheet size, has elapsed in response to a print command in the universal size mode (S1051 YES). Therefore, when the leading edge of the sheet for the second side of the double-sided print reaches the registration sensor 19, the remaining time until the toner image for the second side of the double-sided print reaches the secondary transfer roller 9 becomes longer. The shorter the length of the sheet size actually used is compared with the sheet size assumed when the print command was issued, the larger the deviation between the toner image and the sheet calculated in the acceleration / deceleration control in S1150 (S2010). Therefore, a pause is selected in the acceleration / deceleration / pause control for the second side of the double-sided print.

[0044] Next, the distance relationship between the roller pair and the sensor of the printer 200 in the embodiment 1 will be described. The transport distance from the registration roller 3 to the registration sensor 19 is 1 mm, and the transport distance from the reversing roller 30 to the registration roller 3 is 350 mm. Therefore, when printing on both sides of a legal size sheet, when the leading edge of the sheet on the second side reaches the registration sensor 19, the trailing edge of the sheet on the second side is still held by the reversing roller 30.

[0045] In other words, when the registration rollers 3 are stopped or decelerated by the acceleration / deceleration / pause control for the second side of double-sided printing, the rear end side of the sheet for the second side is pushed into the double-sided conveying path 33 due to the speed difference with the reversing rollers 30 driven by the fixing motor 102. This increases the possibility of problems occurring, such as buckling of the sheet 2 during conveyance or paper jams.

[0046] Furthermore, when stopping or decelerating the reversing roller 30 in accordance with the acceleration / deceleration / pause control for the second side to avoid buckling or paper jams of the sheet 2, it is necessary to stop or decelerate the fixing motor 102. In that case, the speed fluctuation of the fixing roller 14 may cause a loss of heat uniformity in the rotation direction of the fixing roller 14, which may result in a decrease in the fixability of the toner image on the second side. For this reason, stopping or decelerating the fixing motor 102 is not preferable.

[0047] (Flowchart: When the control of the first embodiment is performed) Next, the operation of the engine control unit 301 during double-sided printing when the control of the first embodiment is implemented will be described with reference to the flowchart of Fig. 4. The processes from S1010 to S1130 and S1051 to S1053 in the flowchart of Fig. 4 are the same as the processes from S1010 to S1130 and S1051 to S1053 in the flowchart of Fig. 3, and therefore will not be described.

[0048] The engine control unit 301 calculates the expected timing Tp (predicted timing) at which the rear end of the sheet reaches the fixing / discharge sensor 20 based on the image formation start time using the following calculation formula. Time Xi: Ideal time until the toner image formed by the image forming unit 307 reaches the secondary transfer roller 9 Time Xf: Ideal time for the leading edge of the sheet onto which the toner image has been transferred to reach the fixing discharge sensor 20 from the secondary transfer roller 9 Time Yp2: Ideal transport time for the sheet length assumed when printing is instructed Scheduled timing Tp=Xi+Xf+Yp2 Here, the time Xf is an ideal time based on the distance between the secondary transfer roller 9 and the fixing discharge sensor 20, the conveying speed of the sheet 2, and the length of the sheet 2 in the conveying direction obtained in S1010.

[0049] In S3140, the engine control unit 301 determines whether or not the sheet trailing end has been detected earlier than the scheduled timing Tp described above, based on the actual timing Tr at S1110 when the sheet trailing end actually reached the fixing discharge sensor 20. The engine control unit 301 predicts whether or not the sheet 2 will reach the secondary transfer roller 9 earlier, based on the difference between the scheduled timing Tp and the actual timing Tr. If the engine control unit 301 determines in S3140 that the sheet trailing end has been detected earlier than scheduled, the process proceeds to S3150.

[0050] In S3150, the engine control unit 301 calculates the time E, the time F, and the number of reciprocating movements G, and sets a reciprocating counter that counts the reciprocating operation to 0. Note that the reciprocating operation refers to alternately repeating the first control and the second control for the sheet 2 being transported along the double-sided transport path 33, thereby moving the sheet 2 reciprocally within the double-sided transport path 33.

[0051] Here, the time E is calculated by the following formula: Time E is the time it takes for the leading edge of the sheet 2, which has started to be inverted, to reach the front side of the double-sided conveying rollers 37. Distance L1: Distance from the reversing roller 30 to the double-sided conveying roller 37 (135 mm) Distance L2: From the reversing roller 30 to the start position of the reversing of the leading edge of the second side of the double-sided print sheet (15 mm) Distance L3: Margin to prevent intrusion into the double-sided conveying roller 37 (10 mm) Time E = (L1 - (L2 + L3)) ÷ conveying speed

[0052] Moreover, to calculate the time F and the number of reciprocating motions G, the delay time Td (which is also the difference time) for delaying the transport of the second side of a double-sided print is calculated using the following formula. The delay time Td is the time for delaying the timing at which the leading edge of the sheet 2 on the second side arrives at the registration rollers 3. Moreover, the time F is the time for performing the first control and the second control, and the number of reciprocating motions G is such that one set of the first control and the second control is counted as one. Delay time Td = Scheduled timing Tp - Actual timing Tr (However, the minimum value of Td is 0.) Here, if the above-mentioned time Xf is the ideal time Ttp and the actual timing Tr is the detected time Ttd, the delay time Td can also be said to be the difference between the ideal time Ttp and the detected time Ttd.

[0053] Furthermore, in order to prevent the sheet 2 from falling (for example, the sheet 2 is returned too far and falls onto the discharge tray 27), the number of reciprocations is calculated using the following formula. Number of round trips G = (Td÷2)÷E (where G is a positive integer rounded up to the nearest whole number) Time F = Td ÷ (G × 2)

[0054] In this way, the time F (time T1) for performing the first control in the reciprocating control, the time F (time T2) for performing the second control, and the number of reciprocating operations G are determined according to the delay time Td. Note that, in the first embodiment, the first control and the second control are performed for the same time F, but they may be different times (T1 ≠ T2). Also, the time (time T3) from the start of the control (second control) in S1130 to the start of the reciprocating control in S3180 is at least longer than half the delay time Td (T3 > Td / 2).

[0055] In S3160, engine control unit 301 determines whether or not time E has elapsed based on the timing at which reverse clutch 103 was turned off in S1130. If engine control unit 301 determines in S3160 that time E has not elapsed, it returns the process to S3160, and if it determines that time E has elapsed, it advances the process to S3170.

[0056] In S3170, the engine control unit 301 determines whether the reciprocation counter is less than the number of reciprocations G. If it is determined in S3170 that the reciprocation counter is less than the number of reciprocations G, the process proceeds to S3180. In S3180, the engine control unit 301 turns on the reversing clutch 103 again. This causes the reversing rollers 30 to convey the sheet 2 in the direction of the duplex reversing path 29, that is, the sheet 2 runs in the reverse direction within the duplex conveying path 33. In addition, the engine control unit 301 starts the reciprocation control while performing the second control.

[0057] In S3190, the engine control unit 301 determines whether or not time F has elapsed based on the timing when the reversing clutch 103 was turned ON in S3180. If the engine control unit 301 determines in S3190 that time F has not elapsed, the process returns to S3190, and if it determines that time F has elapsed, the process proceeds to S3200. In S3200, the engine control unit 301 turns off the reversing clutch 103. As a result, the reversing rollers 30 transport the sheet 2 in the direction of the duplex transport path 33, that is, the sheet 2 is transported through the duplex transport path 33 toward the registration rollers 3.

[0058] In S3210, the engine control unit 301 determines whether or not a further time F has elapsed. If the engine control unit 301 determines in S3210 that the further time F has not elapsed, the process returns to S3210, and if it determines that the time F has elapsed, the process proceeds to S3220. Through the processes from S3180 to this point, the sheet 2 has made one round trip in the reverse and forward directions within the duplex conveying path 33. In S3220, the engine control unit 301 counts up (+1) the round trip counter, and the process returns to S3170.

[0059] If the engine control unit 301 determines in S3170 that the reciprocation counter is equal to or greater than the number of reciprocations G, the engine control unit advances the process to S1140. In S1140, the engine control unit waits until the leading edge of the sheet on the second side of the double-sided printing reaches the registration sensor 19. That is, in the first embodiment, the engine control unit 301 performs the processes of S3140 to S3220. As a result, the rotation direction of the reversing rollers 30 is switched twice (S3180, S3200) until the leading edge of the sheet on the second side of the double-sided printing reaches the double-sided conveying rollers 37, and the reciprocating operation of moving the sheet 2 back and forth is performed at least once.

[0060] If the detection time of the sheet rear end in S3140 is later than expected, the engine control unit 301 advances the process to S1140 and waits for the sheet front end of the second side to reach the registration sensor 19. The subsequent processes from S1140 to S1180 are the same as the processes from S1140 to S1180 in FIG. 3, and therefore the description will be omitted. Note that the reciprocating operation is performed before the acceleration / deceleration / pause control in S1150. In this way, the engine control unit 301 performs the reciprocating control at least once when it is predicted that the sheet 2 will reach the secondary transfer roller 9 earlier than the toner image to be transferred to the back side formed while the sheet 2 is being transported along the double-sided transport path 33.

[0061] As described above, in the first embodiment, when the length of the sheet actually conveyed is shorter than the sheet length assumed at the start of printing, the reversing clutch is operated during sheet conveyance in the duplex conveying path to reciprocate the sheet. This makes it possible to delay the timing at which the leading edge of the sheet on the second side of duplex printing reaches the registration roller. In other words, it is possible to prevent deceleration and pause control from being selected during acceleration / deceleration / pause control for the second side of duplex printing. This makes it possible to prevent buckling or paper jams caused by the trailing edge of the sheet on the second side of duplex printing being pushed into the duplex conveying path. There is no need to change the speed / stop the fixing motor in synchronization with the acceleration / deceleration / pause control for the second side of duplex printing, so duplex printing can be performed with stable image quality.

[0062] (Modification) In the first embodiment, for the sake of simplicity, the explanation is given using the times E, F, and G of reciprocating motions calculated while ignoring the hardware operation time and its variation time required for switching the conveying direction by turning the reversing clutch ON / OFF. However, the times E, F, and G of reciprocating motions may be calculated taking into account the operation time and variation time obtained experimentally. In addition, if the transport time in the double-sided transport path changes due to differences in sheet type, environmental conditions, durability deterioration, etc., the time E, time F, etc. (times T1, T2, T3, etc.) may be corrected using an offset value or a correction coefficient based on the difference in transport time in the double-sided transport path obtained experimentally. In the first embodiment, the method of calculating the time E, the time F, and the number of reciprocating movements G using the difference between the ideal time when the fixing discharge sensor 20 detects the sheet rear end and the actual detection time has been described. However, the time E, the time F, and the number of reciprocating movements G may be calculated from the difference between the sheet length Lp reserved and assumed at the start of printing and the actual sheet length Lr calculated using the sheet detection timing by the registration sensor 19. For example, Difference Ld = Lp - Lr (where the minimum value of Ld is 0) Delay time Td2 = Ld ÷ conveying speed Time E (as stated above) Number of round trips G = (Td2 ÷ 2) ÷ E (where G is a positive integer rounded up to the nearest whole number) Time F = Td2 ÷ (G × 2) It is also possible to use the following.

[0063] In this way, for example, the registration sensor 19 is used as a length detection unit that detects the length in the transport direction of the sheet 2. The engine control unit 301 may obtain each value based on the difference between the length (Lp) in the transport direction of the sheet 2 reserved at the start of printing and the length (Lr) in the transport direction detected by the registration sensor 19. In other words, the number of reciprocations G, the first time for performing the first control, and the second time for performing the second control may be obtained.

[0064] As described above, according to the first embodiment, even when it is expected that the back sheet will reach the transfer section earlier than the back image of double-sided printing reaches the transfer section, the occurrence of sheet buckling and paper jams can be reduced. EXAMPLES

[0065] Next, a second embodiment to which the present invention can be applied will be described. In Example 1, the actual timing Tr at which the sheet trailing edge actually reaches the fixing and discharging sensor 20 is compared with the scheduled timing Tp at which the sheet trailing edge reaches the fixing and discharging sensor 20, based on the image formation start time. Then, the times E and F and the number of reciprocating movements G are calculated, and the timing at which the sheet on the second side reaches the registration sensor is delayed by reciprocating the sheet on the double-sided conveying path. In Example 2, a case in which the image formation on the second side cannot be started at the scheduled time due to reasons such as a delay in the generation of image data on the second side. In such a case, in Example 2, a form corresponding to the case in which the time F and the number of reciprocating movements G cannot be calculated is described by reciprocating the sheet conveyance using the detection result of the inversion sensor.

[0066] An outline of the overall configuration of a printer 200 as an image forming apparatus is shown in Fig. 5. The printer 200 of the second embodiment further includes a reversal sensor 34 as a third detection means between the reversal roller 30 and the double-sided conveying roller 37, capable of detecting the leading and trailing ends of the sheet 2 after reversal. The engine control unit 301 predicts that the sheet 2 will reach the secondary transfer roller 9 earlier based on the detection result by the reversal sensor 34. As the rest is similar to the first embodiment, a description thereof will be omitted. Note that the image forming apparatus is not limited to the configuration shown in Fig. 5 as long as double-sided printing is possible.

[0067] (Function block diagram) 6 is a functional block diagram showing a functional configuration of the embodiment 2. The detection unit 306 detects the leading and trailing ends of the sheet 2 by the inversion sensor 34. The rest is the same as in the embodiment 1, so the description will be omitted.

[0068] (Flowchart: When the control of the second embodiment is performed) Next, the operation of the engine control unit 301 during double-sided printing when the control of the second embodiment is implemented will be described with reference to the flowchart of Fig. 7. The processes from S1010 to S1130 and S1051 to S1053 in the flowchart of Fig. 7 are the same as the processes from S1010S to S1130 and S1051 to S1053 in the flowchart of Fig. 3, and therefore will not be described.

[0069] After the leading edge of the sheet for the second side of double-sided printing enters the double-sided conveying path 33 by turning off the reversal clutch 103 in S1130, the engine control unit 301 determines in S4140 whether the leading edge of the sheet for the second side has been detected by the reversal sensor 34 (no paper -> paper present). If the engine control unit 301 determines in S4140 that the leading edge of the sheet has not been detected by the reversal sensor 34, it returns the process to S4140, and if it determines that the leading edge has been detected, it proceeds to S4150.

[0070] In S4150, engine control unit 301 determines whether the time elapsed from the start of image formation on the second side is less than a pre-calculated time H. If engine control unit 301 determines in S4150 that the time elapsed from the start of image formation on the second side is less than time H, it advances the process to S4160. In S4160, engine control unit 301 turns on reversing clutch 103 again. This causes reversing roller 30 to convey sheet 2 in the direction of duplex reversing path 29, that is, sheet 2 runs in reverse in duplex conveying path 33. Engine control unit 301 repeatedly performs reciprocating control from the start of image formation of the toner image to be formed on the back side until a predetermined time (time H) (third time) has elapsed.

[0071] The time H is calculated using the following formula. Time Xi: Ideal time until the toner image formed by the image forming unit 307 reaches the secondary transfer roller 9 Time Xf: Ideal time for the leading edge of the sheet onto which the toner image has been transferred to reach the fixing discharge sensor 20 from the secondary transfer roller 9 Time Xb: Time B shown in Example 1 Time Yp2: Ideal transport time for the sheet length assumed when printing is instructed Time Ys: Ideal transport time from a position 15 mm downstream of the reversing roller 30 to the reversing sensor 34 Time H=Xi+Xf+Xb+Yp2+Ys

[0072] While the reversing clutch 103 is turned on and the second-side sheet flows backward on the duplex conveying path 33, the engine control unit 301 performs the following process. In S4170, the engine control unit 301 judges whether the reversing sensor 34 has changed from paper-present to paper-absent in order to wait for the timing when the leading edge of the second-side sheet reaches the upstream of the reversing sensor 34. If the engine control unit 301 judges in S4170 that the reversing sensor 34 has detected a sheet, the process returns to S4170, and if the engine control unit 301 judges that the reversing sensor 34 has changed from paper-present to paper-absent, the process proceeds to S4180. In S4180, the engine control unit 301 turns off the reversing clutch 103 to switch the second-side sheet 2 to the pull-in direction to the duplex conveying path 33, returns the process to S4140, and waits again until the leading edge of the second-side sheet reaches the position of the reversing sensor 34.

[0073] If the engine control unit 301 determines in S4150 that the time H has elapsed since the start of image formation on the second side, the process proceeds to S1140, and waits for the leading edge of the sheet on the second side to reach the registration sensor 19. The subsequent processes from S1140 to S1180 are the same as the processes from S1140 to S1180 in the flowchart of FIG. 3, and therefore description thereof will be omitted.

[0074] As described above, when the leading edge of the sheet for the second side of the double-sided printing reaches the position of the reversing sensor 34, if the image formation for the second side has not yet started, the reversing clutch 103 is repeatedly turned on and off based on the reversing sensor 34. Even if the timing for starting the image formation for the second side is unknown, the reversing clutch 103 is repeatedly turned on and off based on the reversing sensor 34 provided in the double-sided conveying path. This makes it possible to delay the timing when the leading edge of the sheet for the second side of the double-sided printing reaches the registration roller 3. In other words, it is possible to prevent the deceleration and pause control from being selected during the acceleration / deceleration / pause control for the second side of the double-sided printing. This makes it possible to prevent buckling or paper jams caused by the rear edge of the sheet for the second side of the double-sided printing being pushed into the double-sided conveying path. Since there is no need to change or stop the fixing motor 102 in synchronization with the acceleration / deceleration / pause control for the second side of the double-sided printing, it is possible to perform double-sided printing with stable image quality.

[0075] In the second embodiment, for the sake of simplicity, the method of reciprocating the sheet in the duplex conveying path by turning the reversing clutch 103 ON / OFF at the timing of switching ON / OFF of the reversing sensor 34 has been described. However, there may be an audible problem (influence of operation sound) due to an increase in the number of operations of the clutch mechanism, or an influence such as wear of the clutch mechanism's life. In such a case, for example, a waiting time of a predetermined time or more may be provided between ON and OFF of the reversing clutch 103 or between OFF and ON of the reversing clutch 103 to extend the ON / OFF cycle of the reversing clutch 103. In addition, when the conveying time in the duplex conveying path changes due to differences in sheet type, environmental conditions, durability deterioration, etc., the predetermined time for extending the ON / OFF cycle may be corrected by an offset value or a correction coefficient based on the difference in conveying time of the duplex conveying path obtained experimentally.

[0076] As described above, according to the second embodiment, even when it is expected that the back sheet will reach the transfer section earlier than the back image of double-sided printing reaches the transfer section, the occurrence of sheet buckling and paper jams can be reduced.

[0077] The disclosure of this embodiment includes the following configuration. (Configuration 1) an image forming means for forming a toner image; a transfer unit that transfers the toner image formed by the image forming unit in a transfer section; A fixing unit for performing a fixing process on an unfixed toner image on a recording material; a first rotating body that conveys a recording material to the image forming means in a first direction; a second rotating body provided downstream of the fixing means in the first direction; A drive source that drives the fixing means and the second rotating body; a detection unit provided between the first rotating body and the second rotating body, the detection unit being capable of detecting the leading edge and the trailing edge of the recording material; a control unit that switches between a first control for controlling the second rotating body so as to transport the recording material in the first direction and a second control for controlling the second rotating body so as to transport the recording material in a second direction opposite to the first direction; Equipped with the control unit is an image forming apparatus that, when performing double-sided printing in which an image is formed on a first side and a second side different from the first side of a recording material, switches the second rotating body from the first control to the second control to transport the recording material to a double-sided transport path and transports the recording material to the transfer unit again, The image forming apparatus is characterized in that, when the control means is predicting that the recording material will reach the transfer section earlier than the toner image to be transferred to the second surface formed by the image forming means when the recording material is transported along the double-sided transport path, the control means performs reciprocating control that alternates between the first control and the second control at least once. (Configuration 2) The image forming apparatus according to configuration 1, wherein the control unit starts the reciprocating control while the second control is being performed after the fixing process on the first side is completed. (Configuration 3) The image forming apparatus according to configuration 1 or 2, characterized in that the detection means includes a first detection means provided upstream of the image forming means in the first direction, and a second detection means provided downstream of the fixing means in the first direction. (Configuration 4) The image forming apparatus according to any one of configurations 1 to 3, characterized in that the control means predicts whether the recording material will arrive at the transfer section earlier based on a difference between the predicted timing at which the second detection means detects the trailing end of the recording material and the actual timing at which the second detection means actually detects the trailing end of the recording material, the predicted timing being based on a distance between the image forming means and the second detection means, a conveying speed of the recording material, and a length of the recording material in the conveying direction. (Configuration 5) The image forming apparatus according to configuration 4, wherein the control means obtains, based on the difference, a number of times to perform the reciprocating control, a first time for performing the first control in the reciprocating control, and a second time for performing the second control in the reciprocating control. (Configuration 6) The image forming apparatus according to configuration 5, characterized in that a third time from switching from the first control to the second control to transport the recording material to the double-sided transport path to starting the reciprocating control is longer than half of the difference. (Configuration 7) 7. The image forming apparatus according to claim 6, wherein the control means corrects the first time, the second time, and the third time by a predetermined offset value or correction coefficient. (Configuration 8) a third detection unit provided in the double-sided conveying path and capable of detecting a leading edge and a trailing edge of the recording material being conveyed by the second control; The image forming apparatus according to any one of configurations 1 to 3, wherein the control unit predicts whether the recording material will arrive at the transfer unit earlier based on a detection result by the third detection unit. (Configuration 9) The image forming apparatus according to configuration 8, wherein the control means repeatedly performs the reciprocating control from the start of image formation by the image forming means of the toner image to be formed on the second surface until a predetermined specified time has elapsed. (Configuration 10) a length detection unit that detects the length of the recording material being conveyed in the conveying direction, The image forming apparatus according to any one of configurations 1 to 4, characterized in that the control means obtains the number of times to perform the reciprocating control, a first time for performing the first control in the reciprocating control, and a second time for performing the second control in the reciprocating control based on the difference between the length of the recording material in the transport direction reserved at the start of printing and the length in the transport direction detected by the length detection means. [Explanation of symbols]

[0078] 3 Registration roller 9 Secondary transfer roller 13 Fixing unit 19 Resist sensor 20 Fixing discharge sensor 30 Reversing roller 102 Fixing motor 200 Laser Printer Engine 301 Engine control unit

Claims

1. an image forming means for forming a toner image; a transfer unit that transfers the toner image formed by the image forming unit in a transfer section; A fixing unit for performing a fixing process on an unfixed toner image on a recording material; a first rotating body that conveys a recording material to the image forming means in a first direction; a second rotating body provided downstream of the fixing means in the first direction; a drive source that drives the fixing means and the second rotating body; a detection unit provided between the first rotating body and the second rotating body, the detection unit being capable of detecting a leading end and a trailing end of a recording material; a control unit that switches between a first control for controlling the second rotating body so as to transport the recording material in the first direction and a second control for controlling the second rotating body so as to transport the recording material in a second direction opposite to the first direction; Equipped with the control unit is an image forming apparatus that, when performing double-sided printing in which an image is formed on a first side of a recording material and a second side different from the first side, switches the second rotating body from the first control to the second control to transport the recording material to a double-sided transport path and transports the recording material to the transfer unit again, The image forming apparatus is characterized in that, when the control means is configured to perform reciprocating control that alternates between the first control and the second control at least once when it is predicted that the recording material will reach the transfer section earlier than the toner image to be transferred to the second surface formed by the image forming means when the recording material is transported along the double-sided transport path.

2. 2 . The image forming apparatus according to claim 1 , wherein the control unit starts the reciprocating control while the second control is being performed after the fixing process on the first side is completed.

3. 2. The image forming apparatus according to claim 1, wherein the detection means includes a first detection means provided upstream of the image forming means in the first direction, and a second detection means provided downstream of the fixing means in the first direction.

4. The image forming apparatus according to claim 3, characterized in that the control means predicts whether the recording material will reach the transfer section earlier based on a difference between the predicted timing at which the second detection means detects the trailing end of the recording material and the actual timing at which the second detection means actually detects the trailing end of the recording material, the predicted timing being based on the distance between the image forming means and the second detection means, the transport speed of the recording material, and the length of the recording material in the transport direction.

5. The image forming apparatus according to claim 4, characterized in that the control means obtains the number of times to perform the reciprocating control, a first time for performing the first control in the reciprocating control, and a second time for performing the second control in the reciprocating control based on the difference.

6. 6. An image forming apparatus according to claim 5, wherein a third time from switching from the first control to the second control to transport the recording material to the double-sided transport path to starting the reciprocating control is longer than half the difference.

7. 7. The image forming apparatus according to claim 6, wherein the control unit corrects the first time, the second time, and the third time with a predetermined offset value or correction coefficient.

8. a third detection unit provided in the double-sided conveying path and capable of detecting a leading edge and a trailing edge of the recording material being conveyed under the second control; 2. The image forming apparatus according to claim 1, wherein the control means predicts whether the recording material will reach the transfer portion earlier or not based on the detection result by the third detection means.

9. 9. The image forming apparatus according to claim 8, wherein the control means repeatedly performs the reciprocating control from the start of image formation by the image forming means of the toner image to be formed on the second surface until a predetermined specified time has elapsed.

10. a length detection unit that detects the length of the recording material being conveyed in the conveying direction, The image forming apparatus according to claim 1, characterized in that the control means obtains the number of times to perform the reciprocating control, a first time for performing the first control in the reciprocating control, and a second time for performing the second control in the reciprocating control based on the difference between the length of the recording material in the transport direction reserved at the start of printing and the length in the transport direction detected by the length detection means.

Images