Image forming apparatus

Abstract

This improves the accuracy of the toner image formation position on the sheet in the transfer section. [Solution] The system includes a fixing unit having a rotating body for heating a toner image transferred to a recording material, a reversing transport unit that transports the recording material on which the toner image has been fixed by the fixing unit in a first direction, and then in a second direction opposite to the first direction, a first path which constitutes a path through which the recording material is transported from the feeding unit to the transfer unit, a second path which constitutes a path from when the recording material is transported in the second direction by the reversing transport unit until it merges with the first path, a single drive source for driving the rotating body and the reversing transport unit, a detection unit for detecting recording material passing through the second path, and a control unit that, based on the detection result of the detection unit that detected the first recording material passing through the second path (S301), controls the drive source so that the reversing transport unit changes the transport speed at which the second recording material, which is transported in the second path following the first recording material, is transported in the second direction (S204).

Description

Technical Field

[0001] The present invention relates to an image forming apparatus that forms an image on a recording material.

Background Art

[0002] Conventionally, in image forming apparatuses such as printers and multifunction devices, a user may print on both sides of a sheet. For this reason, an image forming apparatus having a duplex printing function has been developed that automatically reverses and conveys the sheet by the apparatus without the user having to manually turn over and set the sheet, and forms images on both sides of the sheet. In such an image forming apparatus, in order to realize the duplex printing function, it has a reverse conveyance mechanism for reversing the sheet in the conveyance path. In this reverse conveyance mechanism, the sheet is conveyed to a double-sided reverse path, temporarily stopped in the double-sided reverse path, and then the pair of reverse rollers is rotated in the reverse direction to convey it in the reverse direction.

[0003] As such a reverse conveyance mechanism of an image forming apparatus, a configuration of a re-conveyance method is known in which the sheet is switched to a re-conveyance path different from the conveyed conveyance path and re-conveyed to the transfer unit (see Patent Document 1). In this image forming apparatus, by temporarily stopping the sheet at a temporary stop point for a predetermined time during duplex printing and then restarting the conveyance, conveyance control is executed to keep the sheet interval constant after sheet standby due to a delay in image formation during duplex printing.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, in the reversal transport mechanism of the image forming apparatus described in Patent Document 1, the rotation direction of the reversal roller pair is reversed, so there is a risk that the time from outputting the reversal command until the sheet transport direction is actually reversed may vary due to individual variations and aging. If the variation in transport timing in the re-transport path becomes large, there is a risk that the timing of transferring the toner image to the sheet in the transfer section during the formation of the second image will not match, and the position of the toner image on the sheet may be misaligned.

[0006] The present invention aims to provide an image forming apparatus that can improve the accuracy of the toner image formation position on the sheet in the transfer section. [Means for solving the problem]

[0007] One aspect of the present invention comprises a storage unit for storing recording material, a feeding unit for feeding the recording material stored in the storage unit, an image forming unit for forming a toner image, a transfer unit for transferring the toner image formed by the image forming unit onto the recording material fed by the feeding unit, a fixing unit having a rotating body for heating the toner image transferred onto the recording material and fixing the toner image onto the recording material, a reversing transport unit that transports the recording material on which the toner image has been fixed by the fixing unit in a first direction and then in a second direction opposite to the first direction, a re-transport unit that transports the reversing transported recording material back to the transfer unit, and the recording material is in front The image forming apparatus is characterized by comprising: a first path that constitutes a path for transporting the recording material from the recording material supply unit to the transfer unit; a second path that constitutes a path from when the recording material is transported in the second direction by the reversing transport unit until it merges with the first path; a single drive source that drives the rotating body and the reversing transport unit; a detection unit that detects the recording material passing through the second path; and a control unit that controls the drive source to change the transport speed at which the reversing transport unit transports the second recording material, which is transported in the second path following the first recording material, in the second direction, based on the detection result of the detection unit that has detected the first recording material passing through the second path.

[0008] Another aspect of the present invention includes a storage unit for storing recording material, a feeding unit for feeding the recording material stored in the storage unit, an image forming unit for forming a toner image, a transfer unit for transferring the toner image formed by the image forming unit onto the recording material fed by the feeding unit, a fixing unit having a rotating body for heating the toner image transferred onto the recording material and fixing the toner image onto the recording material, a reversing transport unit that transports the recording material on which the toner image has been fixed by the fixing unit in a first direction and then in a second direction opposite to the first direction, a re-transport unit that transports the reversing transported recording material back to the transfer unit, and the recording material is forward The image forming apparatus is characterized by comprising: a first path that constitutes a path for transporting the recording material from the recording material supply unit to the transfer unit; a second path that constitutes a path from when the recording material is transported in the second direction by the reversing transport unit until it merges with the first path; a single drive source that drives the rotating body and the reversing transport unit; a detection unit that detects the recording material passing through the second path; and a control unit that controls the drive source to change the number of reversals by which the reversing transport unit transports the second recording material, which is transported following the first recording material, in the first and second directions, based on the detection result of the detection unit that detected the first recording material passing through the second path. [Effects of the Invention]

[0009] According to the present invention, the accuracy of the toner image formation position on the sheet in the transfer section can be improved. [Brief explanation of the drawing]

[0010] [Figure 1] This is a cross-sectional view showing an image forming apparatus according to the first embodiment. [Figure 2] This is a control block diagram according to the first embodiment. [Figure 3] This is a flowchart showing the first part of the control procedure for a comparative example in the first embodiment. [Figure 4] This is a flowchart showing the second part of the control procedure for the comparative example in the first embodiment. [Figure 5]This is a flowchart showing the third part of the control procedure for the comparative example in the first embodiment. [Figure 6] This is a flowchart showing the acceleration / deceleration / pause control of a comparative example in the first embodiment. [Figure 7] (a) is a flowchart showing the fourth part of the control procedure of the comparative example in the first embodiment, and (b) is a flowchart showing the inverted acceleration / deceleration control of the comparative example in the first embodiment. [Figure 8] This is a flowchart showing the first part of the control procedure in the first embodiment. [Figure 9] This is a flowchart showing the second part of the control procedure in the first embodiment. [Figure 10] This is a flowchart showing the third part of the control procedure in the first embodiment. [Figure 11] This is a flowchart showing the fourth part of the control procedure in the first embodiment. [Figure 12] This is a flowchart showing the reverse acceleration / deceleration control in the first embodiment. [Figure 13] This is a flowchart showing the procedure for setting the correction amount in the first embodiment. [Figure 14] This is a flowchart showing the first part of the control procedure for the comparative example in the second embodiment. [Figure 15] This is a flowchart showing the second part of the control procedure for the comparative example in the second embodiment. [Figure 16] This is a flowchart showing the control procedure of a comparative example in the second embodiment, where (a) is the third part and (b) is the fourth part. [Figure 17] This is a flowchart showing the reversal reciprocating control of a comparative example in the second embodiment. [Figure 18] This is a flowchart showing the first part of the control procedure in the second embodiment. [Figure 19] This is a flowchart showing the second part of the control procedure in the second embodiment. [Figure 20]It is a flowchart showing the third part of the control procedure in the second embodiment. [Figure 21] It is a flowchart showing the fourth part of the control procedure in the second embodiment. [Figure 22] It is a flowchart showing the reverse reciprocating control in the second embodiment.

Mode for Carrying Out the Invention

[0011] <First Embodiment> Hereinafter, the first embodiment of the present invention will be described in detail with reference to FIGS. 1 to 13. First, the schematic configuration of the image forming apparatus 1 according to the present embodiment will be described using FIG. 1. The dimensions, materials, relative positions, etc. of the components of the image forming apparatus 1 are not intended to limit the scope of the present invention only to those unless specifically described.

[0012] [Image Forming Apparatus] FIG. 1 is a schematic view of the image forming apparatus 1 according to the present embodiment. In the present embodiment, the image forming apparatus 1 is an electrophotographic color laser beam printer that forms an image on a sheet S as a recording material. The image forming apparatus 1 includes an image forming unit 2, a transfer unit 3, a sheet feeding unit 4, a sheet conveying unit 5, a fixing unit 6, a reverse conveying unit 7, a refeeding unit 8, a control unit 9, etc. housed inside a housing 1a.

[0013] The sheet feeding unit 4 includes a sheet cassette 41 for stacking the sheet S as a recording material and a feeding roller 42. The sheet cassette 41 is an example of a storage unit for storing the sheet S. The feeding roller 42 is an example of a feeding unit that feeds the sheet S stored in the sheet cassette 41.

[0014] The image forming unit 2 has process cartridges PY, PM, PC, and PK. In this embodiment, the image forming unit 2 forms a toner image and transfers it to the transfer unit 3. Each process cartridge PY, PM, PC, and PK contains yellow (Y), magenta (M), cyan (C), and black (K) toner (developer), respectively. Since the process cartridges PY, PM, PC, and PK have the same configuration except for the color of the toner they contain, the yellow process cartridge PY will be described here. The process cartridge PY has a photosensitive drum 22 corresponding to the yellow toner, a charging roller 23, and a developing roller 24.

[0015] The photosensitive drum 22 is constructed by coating an organic light-conducting layer on the outer circumference of an aluminum cylinder and rotates when the driving force of a drive motor (not shown) is transmitted to it. The drive motor rotates the photosensitive drum 22 clockwise in Figure 1 according to the image formation operation. The photosensitive drum 22 is an example of an image carrier and carries toner images corresponding to each toner color. Exposure light to the photosensitive drum 22 is sent from the scanner unit 21 and is configured to selectively expose the surface of the photosensitive drum 22 to form an electrostatic latent image. The charging roller 23 is an example of a charging unit and charges the surface of the photosensitive drum 22 as a primary charging means. The scanner unit 21 is an example of an exposure unit and exposes the surface of the photosensitive drum 22, which has been charged by the charging roller 23, to form an electrostatic latent image. The developing roller 24 is an example of a developing unit and develops the electrostatic latent image with a developer to form a toner image and make it visible. The developing roller 24 is detachably attached to the process cartridge. In this embodiment, the image forming unit 2 is a concept that includes the photosensitive drum 22, the charging roller 23, the scanner unit 21, and the developing roller 24.

[0016] The transfer unit 3 includes an intermediate transfer belt 31, a primary transfer roller 32, a drive roller 33 for driving the intermediate transfer belt 31, a tension roller 34 for applying tension to the intermediate transfer belt 31, and a secondary transfer roller 35. The intermediate transfer belt 31 is an example of a transfer member, and is in contact with the photosensitive drum 22. During color image formation, it rotates in the counterclockwise direction in Figure 1, rotating in conjunction with the rotation of the photosensitive drum 22, and receives the transfer of the toner image by the primary transfer bias applied to the primary transfer roller 32. In this embodiment, image formation by the image forming unit 2 means the following operation: the scanner unit 21 starts exposure to the photosensitive drum 22 whose surface has been charged by the charging roller 23, forming an electrostatic latent image on the surface of the photosensitive drum 22, and the developing roller 24 visualizes the electrostatic latent image and transfers it to the intermediate transfer belt 31. Furthermore, in this specification, "after image formation is initiated by the image forming unit 2" or "image formation start timing" means the timing at which the scanner unit 21 begins surface exposure of the photosensitive drum 22. However, "after image formation is initiated by the image forming unit 2" or "image formation start timing" is not limited to the timing at which the scanner unit 21 begins surface exposure of the photosensitive drum 22. "After image formation is initiated by the image forming unit 2" or "image formation start timing" is used as a starting point for controlling various timings related to subsequent sheet transport, so it can be any fixed timing during the image formation operation. For example, it may be the timing at which the yellow developing roller 24 visualizes the electrostatic latent image and makes a primary transfer to the intermediate transfer belt 31.

[0017] The intermediate transfer belt 31 superimposes and transfers a color toner image onto the sheet S by gripping and conveying the sheet S at the position of the secondary transfer roller 35. That is, the secondary transfer roller 35 grips the intermediate transfer belt 31 and the sheet S between itself and the drive roller 33, forming a secondary transfer section 36. The secondary transfer section 36 is an example of a transfer section, and it transfers the toner image formed by the image forming section 2 onto the sheet S fed by the feed roller 42.

[0018] The fixing unit 6 fixes the transferred toner image while transporting the sheet S, and includes a fixing roller 61 that heats the sheet S and a pressure roller 62 that presses the sheet S against the fixing roller 61. The fixing roller 61 and the pressure roller 62 are formed in a hollow shape, and a heater is built into the inside of the fixing roller 61. The fixing roller 61 is an example of a rotating body and heats the toner image transferred to the sheet S while rotating it. The sheet S holding the toner image is transported by the fixing roller 61 and the pressure roller 62, and the toner image is fixed to the surface of the sheet S by the application of heat and pressure. After fixing, the sheet S is transported to the discharge path 53 and discharged to the discharge tray 11 by the discharge roller pair 54, and the image forming operation ends.

[0019] The sheet transport unit 5 has a transport path for transporting the sheet S from the feed roller 42 to the discharge tray 11, and includes a continuous printing transport path 51, a fixing transport path 52, and a discharge path 53, starting from the feed roller 42 side. The printing transport path 51 is an example of the first path and constitutes the path through which the sheet S is transported from the feed roller 42 to the secondary transfer unit 36. The fixing transport path 52 is a transport path for transporting the sheet S from the secondary transfer unit 36 ​​to the switching unit 72, which will be described later. The discharge path 53 is a transport path for transporting the sheet S from the switching unit 72 to the discharge tray 11. The sheet transport unit 5 also includes a pair of registration rollers 50 provided in the printing transport path 51 and a pair of discharge rollers 54 provided in the discharge path 53. The pair of registration rollers 50 is positioned between the feed roller 42 and the secondary transfer unit 36. The registration roller pair 50 transports the sheet S fed by the feed roller 42 to the secondary transfer section 36, and adjusts the position at which the image is formed on the sheet S by adjusting the speed at which the sheet S is transported to the secondary transfer section 36.

[0020] The sheet transport unit 5 is equipped with a resist sensor S1, a post-fixing sensor S2, and a full load sensor S3. The resist sensor S1 is located downstream of the registration roller pair 50 in the print transport path 51 and detects the leading and trailing ends of the transported sheet S. The post-fixing sensor S2 is located downstream of the fixing unit 6 in the fixing transport path 52 and detects the leading and trailing ends of the transported sheet S. The full load sensor S3 is located downstream of the discharge path 53 and detects when the discharge tray 11 is full of sheets.

[0021] The reversal transport unit 7 includes a pair of reversal rollers 70, a reversal path 71, and a switching section 72. The section where the fixing transport path 52 and the discharge path 53 connect is a branching section, and the reversal path 71 is connected to the branching section. A switching section 72 is provided at this branching section to switch the sheet transport path by rotation. That is, a sheet S transported from the fixing transport path 52 toward the branching section is transported to either the discharge path 53 or the reversal path 71 depending on the position of the switching section 72. Specifically, when the switching section 72 is positioned on the upper side in Figure 1, the sheet S is transported to the discharge path 53, and when it is positioned on the lower side, the sheet S is transported to the reversal path 71. The pair of reversal rollers 70 is an example of a reversal transport section provided on the reversal path 71, and after transporting the sheet S on which the toner image has been fixed by the fixing unit 6 in a first direction D1, it performs reversal transport to transport it in a second direction D2 opposite to the first direction D1. Here, the first direction D1 is the direction from the fixing unit 6 toward the pair of reversal rollers 70.

[0022] The reversing conveying unit 7 is detachably, i.e., replaceable, from the housing 1a. The housing 1a is equipped with a fixing motor 102 (see Figure 2) and a reversing clutch 103 (see Figure 2) that transmits the power of the fixing motor 102 to the reversing roller pair 70 and the switching unit 72. The reversing clutch 103 is an example of a reversing unit and is composed of a cutting clutch and multiple gears. The reversing clutch 103 is interposed in the power transmission path from the fixing motor 102 to the reversing roller pair 70 and the switching unit 72, and switches the rotational force of the fixing motor 102 in one direction to either forward rotation or reverse rotation and transmits it to the reversing roller pair 70 and the switching unit 72.

[0023] The retransport section 8 includes a retransport path 81, a pair of double-sided transport rollers 82, and a pair of re-feeding rollers 83. The retransport path 81 is connected to a branching section. Therefore, a sheet S located in the reversal path 71 is transported to the retransport path 81 when the switching section 72 is located on the lower side in Figure 1 and the reversal roller pair 70 reverses. A double-sided sensor S4 capable of detecting the leading and trailing ends of the sheet S passing through the retransport path 81 is positioned near the downstream side of the pair of double-sided transport rollers 82 in the retransport path 81. The downstream end of the retransport path 81 in the sheet transport direction merges with the feed roller 42 and registration roller pair 50 in the printing transport path 51. That is, the retransport path 81 is an example of a second path and constitutes the path from when the sheet S is transported in the second direction D2 by the reversal roller pair 70 until it merges with the printing transport path 51. As a result, the re-transport unit 8 transports the sheet S, which has been inverted and transported by the inverting roller pair 70, towards the secondary transfer unit 36.

[0024] Next, we will explain the double-sided transport control when printing on the first side (front) and second side (back) of the sheet S. After printing on the first side and passing through the fuser unit 6, the sheet S is transported to the reversal path 71 by the reversal clutch 103 (described later) which switches the rotation direction of the reversal roller pair 70 and the switching unit 72. When the rear end of the sheet S reaches the reversal path 71, the rotation direction of the reversal roller pair 70 and the switching unit 72 are switched again by the reversal clutch 103, and the sheet S is transported to the re-transport path 81. The sheet S, whose transport direction has been reversed from the first direction D1 to the second direction D2, is transported along the re-transport path 81 by the double-sided transport roller pair 82 and the re-feeding roller pair 83. Note that when the sheet transport direction is switched in the reversal path 71, the fuser motor 102 remains driven, so the drive to the reversal roller pair 70 is not disconnected. Therefore, except for the short time required for the drive gear train to switch, the sheet S is always transported without stopping at the reversal roller pair 70. In this way, the sheet S enters the print transport path 51 again in an inverted state, where the toner image is transferred and fixed to the second side, and then discharged into the output tray 11.

[0025] [Control System] Figure 2 is a control block diagram showing the control system and functional configuration of the first embodiment. The control unit 9 controls the image forming unit 2 by the image forming control unit 90 to perform image formation. The image forming control unit 90 determines the timing for image formation according to the image interval determined by the image forming interval control unit 91. The feeding control unit 92 rotates the feeding roller 42 by giving a rotation instruction to the feeding motor 101 to feed the sheet S. The image formed by the image forming unit 2 is transferred to the fed sheet S. The transport control unit 93 rotates the fixing roller 61, pressure roller 62, discharge roller pair 54, and reversing roller pair 70 by giving a rotation instruction to the fixing motor 102 to transport the sheet S to the discharge tray 27. The fixing motor 102 is an example of a drive source, and only one is provided.

[0026] Next, the functions of the double-sided transport control when printing on the first and second sides of sheet S will be explained. By turning on the reversal clutch 103, the position of the switching unit 72 is switched and sheet S is transported in the direction of the reversal path 71. The sensor unit 94 detects the leading edge of sheet S with the resist sensor S1, and the feed control unit 92 determines whether or not to change the speed of the feed motor 101 and the timing of the speed change based on the time difference with the image formation timing determined by the image formation interval control unit 91. After fixing, the sensor unit 94 detects the trailing edge of sheet S with the post-fixing sensor S2, the reversal timing control unit 96 determines the reversal timing, and the reversal control unit 95 turns off the reversal clutch 103, so that sheet S, which has been transported in the direction of the reversal path 71, is transported to the re-transport path 81.

[0027] Subsequently, the sensor unit 94 detects the leading edge of the sheet S transported to the re-transport path 81 using a double-sided sensor S4, which is an example of a detection unit. The reversing roller pair 70 is driven by the fixing motor 102, and when the reversing clutch 103 is OFF, it rotates to transport the sheet S in a second direction D2 toward the re-transport path 81. On the other hand, when the reversing clutch 103 is ON, it rotates to transport the sheet S in a first direction D1 toward the reversing path 71. When the reversing clutch 103 is OFF, the switching unit 72 is located on the upper side in Figure 1, connecting the fixing transport path 52 and the discharge path 53, and switching the transport path toward the discharge tray 27. When the reversing clutch 103 is ON, the switching unit 72 is located on the lower side in Figure 1, connecting the fixing transport path 52 and the reversing path 71. The double-sided transport unit 97 rotates the double-sided transport roller pair 82 by issuing a rotation command to the feed motor 101, and transports the sheet S, which is inverted, back to the print transport path 51.

[0028] (Comparative Example 1) Here, an example of the operation of the control unit 9 during double-sided printing in a comparative example will be explained using the flowcharts shown in Figures 3, 4, 5, 6, and 7(a)(b). The control unit 9 determines whether or not it has received a print command (S1), and if it determines that it has received one (S1:YES), it drives the fuser motor 102 and the feed motor 101 as a preparatory operation for sheet transport (S2). The control unit 9 starts the image formation preparation operation of the image forming unit 2 (S3). Once the image formation preparation is complete, the control unit 9 determines whether or not the image formation data for the first side of the double-sided print is ready (S4). If the control unit 9 determines that it is ready (S4:YES), it starts forming the toner image for the first side (S5). Furthermore, using the start time of toner image formation as a starting point, it simultaneously and in parallel starts monitoring the time X until the feed operation for the first side of the double-sided print is started, and the time Y until the image formation for the second side of the double-sided print is started (S6, S23).

[0029] In this embodiment, time X is predetermined by the following formula. It should be assumed that each ideal time is pre-set and recorded in the memory unit 98 or the like. Time X=Xi-Xp However, time Xi: the ideal time until the toner image formed in the image forming unit 2 reaches the secondary transfer roller 35. Time Xp: Ideal time for the sheet S fed from the feed roller 42 to reach the secondary transfer roller 35. In this embodiment, since the relationship Xi > Xp holds true in the image forming apparatus 1, the feeding operation of the corresponding sheet S is started after image formation is initiated.

[0030] Furthermore, the time Y in this embodiment is predetermined by the following calculation formula. Time Y=Yp1+Yp2+Yp3 However, time Yp1: the ideal time until the leading edge of the first sheet reaches the inversion roller 70 from the secondary transfer roller 35. Time Yp2: Ideal transport time equal to the sheet length Time Yp3: Ideal time until the leading edge of the second sheet reaches the secondary transfer roller 35 from the inversion roller pair 70.

[0031] The control unit 9 determines whether time X has elapsed (S6), and if it determines that time X has elapsed (S6:YES), it starts the feeding operation for the first side of the double-sided print (S7). Meanwhile, the control unit 9 determines whether time Y has elapsed (S23), and if it determines that time Y has elapsed (S23:YES), it determines whether the preparation of the image forming data for the second side of the double-sided print is complete (S24). If the control unit 9 determines that the preparation of the image forming data for the second side of the double-sided print is complete (S24:YES), it starts the feeding operation for the second side of the double-sided print (S25). Image formation for 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 progressing as a result of the start of the feeding operation.

[0032] The control unit 9 determines whether the leading edge of the first side of the double-sided print sheet has reached the resist sensor S1 (S8). If it determines that it has reached it (S8:YES), it executes acceleration / deceleration / pause control at the moment it reaches the resist sensor S1 (S9).

[0033] Here, the acceleration / deceleration / pause control in this embodiment will be explained using the flowchart shown in Figure 6. In acceleration / deceleration / pause control, the control unit 9 controls the feed motor 101 to transport the sheet S a predetermined distance from the registration roller pair 50 to the secondary transfer roller 35 for the same amount of time remaining until the toner image reaches the secondary transfer roller 35. The control of the feed motor 101 here is acceleration, deceleration, or stopping. Furthermore, the control unit 9 returns the rotational speed of the feed motor 101 to its original speed so that the sheet S and the toner image are transporting at the same speed when the leading edge of the sheet S reaches the secondary transfer roller 35.

[0034] The control unit 9 calculates the amount of misalignment between the image leading edge position and the sheet leading edge position from the difference between the time when image formation started and the time when the leading edge of the sheet reached the resist sensor S1 (S101). The control unit 9 determines whether the amount of misalignment can be eliminated within the acceleration / deceleration performance range of the feed motor 101 (S102). If the control unit 9 determines that the amount of misalignment can be eliminated (S102: YES), it calculates the target speed of the feed motor 101 and the time Ta to maintain that speed (S103), and changes the speed of the feed motor 101 (S104). Furthermore, the control unit 9 determines whether time Ta has elapsed since the speed change of the feed motor 101 (S105). If the control unit 9 determines that time Ta has elapsed (S105: YES), it changes the speed of the feed motor 101 to the same speed as the toner image (S106), and terminates the acceleration / deceleration / pause control. In this way, by executing S103 to S106, the timing at which the toner image and the leading edge of the sheet reach the secondary transfer roller 35 can be synchronized.

[0035] On the other hand, if the control unit 9 determines in S102 that the amount of misalignment cannot be eliminated (S102: NO), it cannot synchronize the timing at which the toner image and the leading edge of the sheet reach the secondary transfer roller 35. For this reason, the control unit 9 determines that it is a misprint (S107) and terminates without changing the speed of the feed motor 101.

[0036] After the acceleration / deceleration / pause control (S9) is completed and sheet transport continues, the leading edge of the sheet S on which the toner image has been transferred approaches the reversal transport unit 7 downstream of the fuser unit 6. The control unit 9 transports the first sheet S for double-sided printing to the reversal roller pair 70 before the leading edge of the sheet reaches the branching point of the transport path by the switching unit 72. That is, the control unit 9 determines whether a predetermined time has elapsed (S10), and if it determines that the time has elapsed (S10:YES), it operates the reversal clutch 103 to switch the position of the switching unit 72 and the rotation direction of the reversal roller pair 70 to ON (S11). After the leading edge of the sheet enters the reversal path 71, the control unit 9 determines whether the trailing edge of the sheet has been detected (ON) by the post-fusing sensor S2 (S12).

[0037] If the control unit 9 determines that the rear end of the sheet has been detected by the post-fixing sensor S2 (S12:YES), it determines whether a predetermined time B has elapsed (S13). That is, in order to reverse the transport direction of the first side of the double-sided print and start pulling it into the re-transport path 81, the control unit 9 operates the reversal clutch 103 after a predetermined time B has elapsed from the moment the rear end of the sheet S reaches the post-fixing sensor S2. If the control unit 9 determines that the predetermined time B has elapsed (S13:YES), it switches the position of the switching unit 72 and the rotation direction of the reversal roller pair 70 to OFF (S14). In this embodiment, the predetermined time B in S13 is preset to reverse the sheet transport direction at the moment when the rear end of the first side of the double-sided print sheet is 15 mm upstream of the reversal roller pair 70. However, it is of course not limited to this time.

[0038] When the sheet transport direction is reversed, the control unit 9 transports the sheet to the re-transport path 81 with the trailing edge of the first double-sided printed sheet as the leading edge of the second double-sided printed sheet. After reversing the sheet transport direction, the control unit 9 performs reverse acceleration and deceleration control until it reaches the double-sided transport roller pair 82 (S15).

[0039] Here, the reverse acceleration / deceleration control in this embodiment will be explained using Figure 7(b). The control unit 9 calculates the deviation from the ideal transport timing from the timing when the trailing edge of the first sheet of the double-sided print reaches the fixing sensor S2, so that the leading edge of the second sheet of the double-sided print reaches the ideal transport timing. Then, the control unit 9 reverses the sheet transport direction and performs reverse acceleration / deceleration control from there to the double-sided transport roller pair 82.

[0040] The control unit 9 obtains the difference between the time Td when the trailing edge of the first sheet of the double-sided print reaches the fixing sensor S2 and the ideal time Te from the start of toner image formation until the trailing edge of the toner image reaches the double-sided sensor S4 via the secondary transfer roller 35. Based on this difference, the control unit 9 calculates the amount of deviation La between the trailing edge position of the sheet and the ideal trailing edge position (S201). Time Td: The time when the rear end of the sheet reaches sensor S2 after fixing. Ideal time Te: The ideal time from the start of image formation until the trailing edge of the toner image reaches the duplex sensor S4 via the secondary transfer roller 35. Deviation La = (Ideal time Te - Actual arrival time Td) ÷ Transport speed

[0041] Next, the control unit 9 determines the speed difference ΔV, where Vref is the original speed of the fixing motor 102 and Vchg is the accelerated / decelerated speed (S202). ΔV = Vref - Vchg Here, if the displacement amount La is a positive value and the sheet transport timing is early, the fixing motor 102 is decelerated, so ΔV becomes a positive value. If the displacement amount La is a negative value and the sheet transport timing is late, the fixing motor 102 is accelerated, so ΔV becomes a negative value.

[0042] Furthermore, Lu is the distance traveled while switching from the original speed Vref to the accelerated / decelerated speed Vchg, and Ld is the distance traveled while returning from the accelerated / decelerated speed Vchg to the original speed Vref. The control unit 9 calculates the acceleration / deceleration time Tb by subtracting Lu and Ld from the displacement amount La and dividing the distance by the speed difference ΔV. Tb = (La - (Lu + Ld)) ÷ ΔV Furthermore, the accelerated / decelerated speed Vchg can be fixed as, for example, +105% or -105% of the original speed, or the speed can be determined according to the amount of deviation La; there is no limitation on the accelerated / decelerated speed.

[0043] The control unit 9 determines the time Tb for acceleration and deceleration, reverses the sheet transport direction, and then changes the speed of the fixing motor 102 to change the transport speed of the reversing roller pair 70 (S204). The control unit 9 determines whether or not time Tb has elapsed (S205), and if it determines that it has elapsed (S205:YES), it returns the speed of the fixing motor 102 to 100% before the leading edge of the second sheet of the double-sided print reaches the double-sided transport roller pair 82 (S206). In other words, in reversing acceleration and deceleration control, the control unit 9 calculates the deviation (mm) from the ideal value based on the detection time by the post-fixing sensor S2, and changes the speed of the reversing roller pair 70 to recover that deviation.

[0044] After performing reverse acceleration / deceleration control (S15), the control unit 9 determines whether the leading edge of the second sheet of the double-sided print has reached the double-sided sensor S4 (S16). If the control unit 9 determines that the leading edge of the second sheet of the double-sided print has reached the double-sided sensor S4 (S16:YES), it then determines whether it has reached the resist sensor S1 (S17). If the control unit 9 determines that the leading edge of the second sheet of the double-sided print has reached the resist sensor S1 (S17:YES), it performs acceleration / deceleration / pause control again (S18). Once the acceleration / deceleration / pause control is complete, the control unit 9 determines whether the trailing edge of the second sheet of the double-sided print has reached the post-fixing sensor S2 (S19). If the control unit 9 determines that the trailing edge of the second sheet of the double-sided print has reached the post-fixing sensor S2 (S19:YES), it determines whether a predetermined time has elapsed (S20). If the control unit 9 determines that a predetermined time has elapsed (S20: YES), it has waited long enough to discharge the printed sheet S into the discharge tray 27.

[0045] The control unit 9 determines whether or not there is a next print command (S21). If it determines that there is (S21:YES), it returns to waiting for the first image data to be ready (S4). If the control unit 9 determines that it has not received a next print command (S21:NO), it stops the operation of the feed motor 101, the fuser motor 102, and the image forming unit 2 (S22), and terminates the printing operation.

[0046] (Issues of Comparative Example 1) Here, we will explain the problems that occur in Comparative Example 1 described above. In Comparative Example 1, the reversal clutch 103 is operated to reverse the sheet transport direction of the first side of the double-sided print and switch the rotation direction of the reversal roller pair 70 in order to start pulling it into the re-transport path 81. However, due to individual variations in the reversal transport mechanism and aging, the time from operating the reversal clutch 103 until the rotation direction of the reversal roller pair 70 is switched may vary. In other words, the arrival of the leading edge of the second side of the double-sided print at the resist sensor S1 after the transport direction is reversed will also vary. If this variation is large, the amount of discrepancy between the toner image timing and the sheet transport timing will increase, and the possibility of misprints will increase.

[0047] Furthermore, we will consider the distance relationship between the roller pair and the sensor in the image forming apparatus 1 in this embodiment. In this embodiment, the transport distance from the registration roller pair 50 to the resist sensor S1 is 1 mm, and the transport path distance from the reversing roller pair 70 to the registration roller pair 50 is 350 mm. Therefore, when printing on both sides of a legal-sized sheet, when the leading edge of the second sheet reaches the resist sensor S1, the trailing edge of the second sheet is still being held by the reversing roller pair 70. That is, the path length of the sheet transport path from the reversing roller pair 70 through the re-transport section 8 to the registration roller pair 50 is shorter than, for example, the maximum length of a legal-sized sheet S in the sheet transport direction, thus achieving miniaturization. In other words, when the registration roller pair 50 is stopped or decelerated by the acceleration / deceleration / pause control of the second side of double-sided printing, the trailing edge of the second sheet is pushed into the re-transport path 81 due to the speed difference with the reversing roller pair 70 driven by the fixing motor 102. This increases the likelihood of problems such as buckling of the sheet S during transport or paper jams occurring.

[0048] In response to this, one could consider stopping or slowing down the reversing roller pair 70 in accordance with the acceleration / deceleration / pause control of the second side to avoid buckling or paper jams. However, in this case, since the fuser motor 102 needs to be stopped or slowed down, the uniformity of heat in the rotation direction of the fuser roller 61 may be lost due to the speed fluctuations of the fuser roller 61, which could worsen the fixing performance of the toner image on the second side.

[0049] In this embodiment, the length of the sheet transport path from the reversing roller pair 70 through the retransport section 8 to the registration roller pair 50 is described as being shorter than, for example, the maximum length of a legal-sized sheet S in the sheet transport direction, but it is not limited to this. The length of the sheet transport path from the reversing roller pair 70 through the retransport section 8 to the registration roller pair 50 may be any other length.

[0050] (Procedure for double-sided printing according to this embodiment) Next, the operation procedure of the control unit 9 during double-sided printing when this embodiment is applied will be explained using the flowcharts shown in Figures 8, 9, 10, 11, 12, and 13. In this flowchart, steps S1 to S14 are the same as in Comparative Example 1, so their explanation will be omitted.

[0051] The control unit 9 switches the position of the switching unit 72 and the rotation direction of the reversing roller pair 70 to OFF (S14), and then performs reverse acceleration / deceleration control (S30). The reverse acceleration / deceleration control will be explained below using the flowchart shown in Figure 12. The control unit 9 calculates the amount of deviation La between the rear end position of the sheet and the ideal rear end position (S301). The calculation of the deviation amount La is the same as in Comparative Example 1, so the explanation will be omitted. In this embodiment, if the correction amount Lx, which will be described later, has been determined for the preceding sheet, the reverse acceleration / deceleration control for the subsequent sheet is performed including the correction amount Lx. This makes it possible to bring the ideal transport timing, in which the leading edge of the second double-sided printed sheet reaches the double-sided transport roller pair 82, closer than in Comparative Example 1.

[0052] When performing reverse acceleration / deceleration control, if the correction amount Lx is not 0, the control unit 9 adds the correction amount Lx to the deviation amount La between the rear end position of the seat and the ideal rear end position to obtain the corrected deviation amount La0 (S302). Corrected deviation amount La0 = Deviation amount La + Correction amount Lx

[0053] Next, the control unit 9 calculates the difference ΔV in acceleration and deceleration speeds of the fixing motor 102 (S303), and calculates the acceleration / deceleration time Tb from the corrected displacement amount La0 (S304). Tb = (La' - (Lu + Ld)) ÷ ΔV Note that the methods for determining ΔV, Lu, and Lb are the same as those explained in Comparative Example 1, so the explanation is omitted. Also, since S204 to S206 are the same as in Comparative Example 1, the explanation is omitted.

[0054] After performing reverse acceleration / deceleration control (S30), the control unit 9 determines whether the leading edge of the second sheet of the double-sided print has reached the double-sided sensor S4 (S16). If the control unit 9 determines that the leading edge of the second sheet of the double-sided print has reached the double-sided sensor S4 (S16: YES), it calculates the reference timing Tref (S31). Here, the control unit 9 calculates the reference timing Tref, when the leading edge of the second sheet of the double-sided print reaches the double-sided sensor S4, from the start timing of image formation on the second sheet, using the following formula. Tref=Xi+Xd However, time Xi: the ideal time until the leading edge of the toner image formed in the image forming unit 2 reaches the secondary transfer roller 35. Time Xd: Ideal time for the leading edge of sheet S to reach the double-sided sensor S4 from the secondary transfer roller 35.

[0055] The control unit 9 acquires the actual timing Tdtct from the start of image formation until the leading edge of the second sheet of the double-sided print reaches the double-sided sensor S4 (S32). Furthermore, the control unit 9 compares the reference timing Tref with the actual timing Tdtct to determine the correction amount Lx to be used for timing correction in the reversal acceleration / deceleration control of the subsequent sheet (S33).

[0056] Next, the method for determining the correction amount Lx will be explained using the flowchart shown in Figure 13. The control unit 9 determines whether Tref > Tdtct (S401), and if it determines that Tref > Tdtct (S401: YES), it increments the fast arrival counter Ce by 1 (S402). The control unit 9 prints multiple sheets and determines whether the fast arrival counter Ce has reached 3 or more (S403). If the control unit 9 determines that the fast arrival counter Ce has reached 3 or more (S403: YES), it adds 1 mm to the correction amount Lx (S404) and resets the fast arrival counter Ce to 0 (S405).

[0057] On the other hand, when the control unit 9 determines that Tref > Tdtct (S401: NO), it determines whether Tref < Tdtct (S406). When the control unit 9 determines that Tref < Tdtct (S406: YES), it decrements the early arrival counter Ce by 1 (S407). The control unit 9 performs multiple sheet printing and determines whether the early arrival counter Ce has become -3 or less (S408). When the control unit 9 determines that the early arrival counter Ce has become -3 or less, it subtracts 1 mm from the correction amount Lx (S409) and clears the early arrival counter Ce to 0 (S405).

[0058] Incidentally, when the control unit 9 determines in S406 that Tref < Tdtct is not satisfied (S406: NO), since Tref = Tdtct, it ends the process without changing the correction amount. The subsequent flow S17 to S22 after determining the correction amount Lx is the same as that in Comparative Example 1, so the description thereof is omitted. Also, in the present embodiment, the control unit 9 corrects the deviation amount based on the calculated correction amount (distance) and adjusts the sheet conveyance speed. Therefore, the correction amount here has a correlation with the sheet conveyance speed, and calculating the correction amount to correct the deviation amount is considered to have a correlation with adjusting the sheet conveyance speed.

[0059] Here, the control unit 9 has a storage unit 98 such as a RAM or an EPROM that stores information. The control unit 9 determines whether there is a next print instruction (S21). When it determines that there is a next print instruction (S21: YES), it continues the process. At this time, the correction amount may be calculated again, or the already calculated correction amount may be stored in the storage unit 98, and printing may be performed for a predetermined number of sheets using the correction amount stored in the storage unit 98 from the next sheet.

[0060] Furthermore, the control unit 9 determines whether or not there is a next print instruction (S21), and if it determines that no next print instruction has been received (S21: NO), it stops the operation of the feed motor 101, the fuser motor 102, and the image forming unit 2 (S22), and terminates the printing operation. At this time, the correction amount set in the print job immediately before termination may be stored in the storage unit 98, and that correction amount may be applied to the first sheet of the next print job. That is, the control unit 9 stores the correction amount in the storage unit 98 in the first image forming job. As mentioned above, there is a correlation between the correction amount and the sheet transport speed, so storing the correction amount may be equivalent to storing the sheet transport speed. Then, in the second image forming job that follows the first image forming job, the control unit 9 retrieves the correction amount from the storage unit 98 and applies the correction amount to at least the first sheet S transported in the second image forming job to adjust the sheet transport speed. This makes it possible to apply the correction amount to the first sheet of the image forming job as well, thereby optimizing the print position.

[0061] Furthermore, in this embodiment, the image forming apparatus 1 has a temperature sensor 99 (see Figure 2). The temperature sensor 99 is an example of an environmental detection unit that detects environmental information, and in this case, it detects temperature as environmental information. The control unit 9 can initialize the correction amount stored in the storage unit 98 when the detection result of the temperature sensor 99 exceeds a threshold. In addition, the inversion transport unit 7 is detachable from the housing 1a and can be replaced. The control unit 9 can initialize the correction amount stored in the storage unit 98 when the inversion transport unit 7 is replaced.

[0062] In this embodiment, the threshold value for the early arrival counter Ce is described in section 3, but this is not limited to a specific value, and other values ​​may be used. Also, the increase or decrease in the correction amount when the early arrival counter Ce exceeds the threshold is set to ±1 mm, but this is not limited to a specific value, and other values ​​may be used. Furthermore, the method of determining the correction amount Lx using the early arrival counter Ce is merely one example; there are other methods, such as determining the correction amount Lx using the difference between the reference timing Tref and the actual timing Tdtct, and the method of determining the correction amount Lx is not limited. For example, in this embodiment, the deviation amount and correction amount Lx are calculated as distance, but this is not limited to a specific method, and the correction amount may be calculated as time obtained by dividing the correction amount by the speed.

[0063] Thus, in this embodiment, the control unit 9 acquires the detection result of the double-sided sensor S4 that detects the preceding sheet (first recording material) that has been conveyed in the second direction D2 by the reversing roller pair 70. Based on this detection result, the control unit 9 controls the fixing motor 102 to change the conveying speed of the subsequent sheet (second recording material) that follows the preceding sheet in the second direction D2 by the reversing roller pair 70. That is, based on the detection result of the double-sided sensor S4 that detects the preceding sheet passing through the re-conveying path 81, the control unit 9 controls the fixing motor 102 to change the conveying speed at which the reversing roller pair 70 conveys the subsequent sheet being conveyed in the re-conveying path 81 in the second direction D2.

[0064] Furthermore, the control unit 9, for example, causes the preceding sheet to be transported in the second direction D2 at a first transport speed by the reversing roller pair 70, and measures the detection time from when image formation is started by the image forming unit 2 until the preceding sheet is detected by the double-sided sensor S4. Then, if the detection time at that time is a second time, which is shorter than a predetermined first time (ideal time), the control unit 9 controls the second transport speed when the following sheet is transported in the second direction D2 to be slower than the first transport speed. On the other hand, if the detection time at that time is a third time, which is longer than the first time, the control unit 9 controls the second transport speed when the following sheet is transported in the second direction D2 to be faster than the first transport speed.

[0065] Furthermore, in this embodiment, by using an early arrival counter, the transport speed is corrected if multiple preceding sheets are reversed with an imbalance of either being earlier or later. Specifically, the control unit 9 changes the second transport speed based on multiple detection results obtained by the double-sided sensor S4 of multiple sheets S that have been transported in the re-transport path 81 prior to the subsequent sheets.

[0066] As described above, in this embodiment, the control unit 9 changes the transport speed of the subsequent sheet in the second direction D2 by the reversing roller pair 70 based on the detection result of the double-sided sensor S4, which detects the preceding sheet that has been transported in the second direction D2 by the reversing roller pair 70. This makes it possible to position the image formation position on the sheet S with high precision. Furthermore, even with a configuration in which the fixing roller 61 and the reversing roller pair 70 are driven by a single fixing motor 102, high-precision positioning becomes possible without stopping the fixing motor 102.

[0067] In other words, by adding a correction amount Lx to the misalignment amount La and performing reverse acceleration / deceleration control, it is possible to take into account not only the misalignment amount at the timing when the sheet reaches the sensor S2 after fixing, but also a correction amount for the variation in the rotation direction of the reverse roller pair 70. Therefore, it is possible to approach the ideal transport timing at which the leading edge of the second sheet of the double-sided print reaches the double-sided transport roller pair 82. As a result, by suppressing variations in transport timing in the double-sided transport path, it becomes possible to align the position of the toner image on the sheet S in the secondary transfer section 36 with high precision.

[0068] In the above-described embodiment, the case where there is only one correction amount Lx was explained, but this is not the only case. For example, the correction amount Lx may be determined separately for each of the following: the transport speed of the sheet S at the transfer position of the secondary transfer roller 35, the type of sheet (basis weight and surface properties), and the size of the sheet.

[0069] Furthermore, although the above-described embodiment explained the case in which the correction amount is calculated using the timing at which the preceding sheet is detected by the double-sided sensor S4, it is not limited to this. That is, the detection unit used to calculate the correction amount only needs to detect the sheet passing through the re-transport path 81 and does not necessarily need to be located in the re-transport path 81. For example, a sensor such as the resist sensor S1 provided outside the re-transport path 81 may be used, which is located downstream of the reversing roller pair 70 in the sheet transport direction and upstream of the secondary transfer unit 36. However, since there is less disturbance when the transport distance of the sheet S is short, a more accurate correction amount can be calculated by using the double-sided sensor S4.

[0070] Furthermore, in the above-described embodiment, the detection time for calculating the correction amount was explained as starting from the image formation start timing by the image forming unit 2, but this is not limited to this. For example, the timing detected by the resist sensor S1 or the timing detected by the post-fixing sensor S2 may be used.

[0071] <Second Embodiment> Next, a second embodiment of the present invention will be described in detail with reference to Figures 14, 15, 16, 17, 18, 19, 20, 21, and 22. This embodiment differs from the first embodiment in that it performs reversing reciprocating control rather than reversing acceleration / deceleration control using the reversing roller pair 70. However, the other components are the same as in the first embodiment, so the same reference numerals are used and detailed descriptions are omitted. In this embodiment, variations in the operation of the reversing roller pair 70 are compensated for by reciprocating the sheet S after reversing in the transport direction, thereby adjusting the timing between the sheet S being transported after reversing and the toner image, and suppressing misalignment of the toner image relative to the sheet S.

[0072] (Comparative Example 2) First, an example of the operation of the control unit 9 during double-sided printing according to Comparative Example 2 of this embodiment will be explained using the flowcharts in Figures 14 to 17. Steps S1 to S14 are the same as in the first embodiment, so their explanation will be omitted. After the rear end of the sheet reaches the fixed sensor S2, the control unit 9 switches the position of the switching unit 72 and the rotation direction of the reversing roller pair 70 using the reversing clutch 103 (S14), and performs reversing reciprocating control (S40).

[0073] Here, the reversal reciprocation control in this embodiment will be explained using the flowchart shown in Figure 17. Reversal reciprocation control is performed to adjust the amount of deviation in transport timing that occurs when the actual transported sheet length is shorter than the sheet length assumed at the start of printing. In reversal reciprocation control, the control unit 9 calculates the deviation from the ideal transport timing from the timing when the trailing end of the sheet on the first side of the double-sided print reaches the fixing sensor S2, so that the leading edge of the second side of the double-sided print reaches the ideal transport timing. Then, the control unit 9 operates the reversal clutch 103 during transport in the double-sided transport path to reciprocate the reversed sheet in the transport direction. In reversal reciprocation control, adjustment is only possible to delay the leading edge of the second side of the double-sided print so that it reaches the ideal transport timing. For this reason, it is preferable to set the toner image formation start timing corresponding to the second side of the double-sided print to be delayed in advance to account for variations in the operation of the reversal transport mechanism.

[0074] The control unit 9 obtains the time Td when the trailing edge of the first sheet of the double-sided print reaches the fixing sensor S2, and the ideal time Te from the start of toner image formation until the trailing edge of the toner image reaches the fixing paper discharge sensor via the secondary transfer roller 35. Then, from the difference between these, it calculates the delay time Th and the amount of deviation Lb between the trailing edge position of the sheet and the ideal trailing edge position (S501).

[0075] The delay time Th and the amount of deviation Lb between the rear end position of the sheet and the ideal rear end position are calculated using the following formulas. Delay time Th = Ideal time Te - Actual time reached Td However, the minimum value of Th is set to 0. Displacement Lb = Delay time Th × Transport speed

[0076] The control unit 9 determines whether the time Td at which the rear end of the sheet actually reached the fixed sensor S2 was detected earlier than planned (deviation amount Lb > 0) relative to the ideal time Te (S502). If the control unit 9 determines that it was detected earlier than planned (deviation amount Lb > 0) (S502: YES), it calculates predetermined times E and F and the number of round trips G, and sets the round trip counter that counts the round trip operation to 0 (S503).

[0077] The specified time E is calculated using the following formula. Time E = (L1 - (L2 + L3)) ÷ Conveyor Speed However, distance L1: distance from the reversing roller pair 70 to the double-sided conveying roller pair 82 (135 mm) Distance L2: Reversal roller 70 to the reversal start position of the leading edge of the second sheet of double-sided printing (15mm) Distance L3: Double-sided conveyor rollers have a margin (10mm) to prevent them from entering the 82.

[0078] The predetermined time F and the number of round trips G are calculated using a formula based on the time Th required to delay the transport of the second side of the double-sided print. To prevent the sheet from falling, the number of round trips is calculated using the following formula. Number of round trips G = (Th ÷ 2) ÷ E However, G is a positive integer rounded up to the nearest whole number. The specified time F = Th ÷ (G × 2)

[0079] The control unit 9 determines whether a predetermined time E has elapsed based on the timing when the reversing clutch 103 was turned OFF (S504). If the control unit 9 determines that the predetermined time E has elapsed (S504:YES), it determines whether the reciprocating counter is less than the number of reciprocations G (S505). If the control unit 9 determines that the reciprocating counter is less than the number of reciprocations G (S505:YES), it turns the reversing clutch 103 ON again (S506). The control unit 9 determines whether a predetermined time F has elapsed based on the timing when the reversing clutch 103 was turned ON again (S507), and if it determines that the time has elapsed (S507:YES), it turns the reversing clutch 103 OFF again (S508). The control unit 9 determines whether a predetermined time F has elapsed based on the timing when the reversing clutch 103 was turned OFF again (S509), and if it determines that the time has elapsed (S509:YES), it increments the reciprocating counter by 1 (S510) and then returns to S505.

[0080] After executing the reversal reciprocation control (S40), the control unit 9 determines whether the leading edge of the second sheet of the double-sided print has reached the double-sided sensor S4 when the reciprocation counter reaches the number of reciprocations G or more (S16). In other words, by executing S506 to S510, the control unit 9 performs a reciprocating operation at least once, switching the rotation direction of the reversal roller pair 70 twice to move the sheet back and forth before the leading edge of the second sheet of the double-sided print reaches the double-sided transport roller pair 82.

[0081] If the control unit 9 determines in S502 that the sheet trailing edge detection time was later than planned (S502: NO), it determines whether the leading edge of the second sheet has reached the double-sided sensor S4 (S16). The following steps S16 to S25 are the same as in the first embodiment, so their explanation is omitted. With the above control, if the actual sheet length being transported is shorter than the sheet length assumed at the start of printing, the reversal clutch 103 can be operated during transport in the double-sided transport path to perform reversal reciprocation control. This makes it possible to delay the timing at which the leading edge of the second sheet of the double-sided print reaches the registration roller pair 50.

[0082] (Issues of Comparative Example 2) In Comparative Example 2 described above, the same problems as in Comparative Example 1 may occur. Specifically, due to individual variations in the reversal transport mechanism and aging, the time from operating the reversal clutch 103 until the rotation direction of the reversal roller pair 70 changes may vary. In other words, the arrival of the leading edge of the second side of the double-sided print at the resist sensor S1 after the transport direction is reversed will also vary. If this variation is large, the amount of discrepancy between the toner image timing and the sheet transport timing will increase, increasing the likelihood of misprints.

[0083] (Procedure for double-sided printing according to this embodiment) Next, the operation procedure of the control unit 9 during double-sided printing when this embodiment is applied will be explained using the flowcharts shown in Figures 18 to 22. In this flowchart, steps S1 to S14 are the same as in the first embodiment, so their explanation will be omitted. After the rear end of the sheet reaches the fixed sensor S2, the control unit 9 switches the position of the switching unit 72 and the rotation direction of the reversing roller pair 70 using the reversing clutch 103 (S14), and performs reversing reciprocating control (S50).

[0084] Next, the reversal reciprocal control will be explained using the flowchart shown in Figure 22. The control unit 9 obtains the time Td when the trailing edge of the first sheet of the double-sided print reaches the fixing sensor S2, and the ideal time Te from the start of toner image formation until the trailing edge of the toner image reaches the fixing paper discharge sensor via the secondary transfer roller 35. From the difference between these, the control unit 9 calculates the delay time Th and the amount of deviation Lb between the trailing edge position of the sheet and the ideal trailing edge position (S601).

[0085] When performing reversing reciprocating control, the control unit 9 adds a correction amount Lx to the deviation amount Lb between the rear end position of the sheet and the ideal rear end position to determine the corrected deviation amount Lb0 and the delay time Th0 (S601). The corrected deviation amount Lb0 and the delay time Th0 are calculated using the following formulas. Corrected deviation Lb0 = Deviation Lb + Correction amount Lx Corrected delay time Th0 = Corrected deviation Lb0 ÷ Transport speed However, the minimum value of Th0 is set to 0. The method for determining the correction amount Lx is the same as in the first embodiment, so the explanation will be omitted. In this embodiment, the correction amount is correlated with the number of round trips G controlled by the control unit 9, and calculating the correction amount and correcting the deviation is correlated with adjusting the number of round trips G.

[0086] The control unit 9 determines whether the corrected deviation amount Lb0 is greater than 0 (S602). If the control unit 9 determines that the corrected deviation amount Lb0 is greater than 0 and that reversal reciprocal control is necessary (S602: YES), it calculates predetermined times E and F, and the number of reciprocations (number of reversals) G, and sets the reciprocation counter that counts the reciprocating operation to 0 (S603). Here, the calculation of the predetermined time E is the same as in Comparative Example 2, so the explanation is omitted. The predetermined time F and the number of reciprocations G are calculated using formulas based on the corrected delay time Th0. Number of round trips G = (Th0 ÷ 2) ÷ E However, G is a positive integer rounded up to the nearest whole number. The given time F = Th0 ÷ (G × 2)

[0087] Specifically, the control unit 9 controls the fixing motor 102 to change the number of reciprocating cycles G by which the subsequent sheet is reversed and conveyed by the reciprocating roller pair 70, based on the detection result of the double-sided sensor S4 that detects the preceding sheet conveyed in the second direction D2 by the reciprocating roller pair 70. In this case, the control unit 9 controls the number of reciprocating cycles G to be the first cycle if the detection time from when image formation is started by the image forming unit 2 until the preceding sheet is detected by the double-sided sensor S4 is shorter than a predetermined first cycle (second cycle). Also, the control unit 9 controls the number of reciprocating cycles G to be the second cycle, which is less than the first cycle, if the detection time is longer than a predetermined first cycle (third cycle). In this embodiment, the control unit 9 changes the number of reciprocating cycles G based on multiple detection results of multiple sheets S conveyed in the re-conveyance path 81 prior to the subsequent sheet, detected by the double-sided sensor S4.

[0088] Steps S504 to S510 are the same as in Comparative Example 2, so their explanation is omitted. If the control unit 9 determines in S602 that the corrected displacement amount Lb' is 0 or less and that reversal reciprocal control is unnecessary (S602: NO), it determines whether the leading edge of the second sheet has reached the double-sided sensor S4 (S16). Steps S16 to S25 are the same as in Comparative Example 2, so their explanation is omitted.

[0089] As described above, in this embodiment, the control unit 9 changes the transport speed of the subsequent sheet in the second direction D2 by the reversing roller pair 70 based on the detection result of the double-sided sensor S4, which detects the preceding sheet that has been transported in the second direction D2 by the reversing roller pair 70. This makes it possible to position the image formation position on the sheet S with high precision. Furthermore, even with a configuration in which the fixing roller 61 and the reversing roller pair 70 are driven by a single fixing motor 102, high-precision positioning becomes possible without stopping the fixing motor 102.

[0090] In the embodiments described above, inverse acceleration / deceleration control and reciprocating control were performed respectively, but the system is not limited to this, and both controls may be performed in parallel. For example, if the corrected deviation amount Lb0 is large, the transport timing may be adjusted using inverse reciprocating control, and then the remaining deviation amount that could not be adjusted by inverse reciprocating control may be fine-tuned using inverse acceleration / deceleration control.

[0091] Furthermore, the disclosure of this embodiment includes the following configuration examples and method examples. (Composition 1) A storage section for storing recording materials, A feeding unit for feeding the recording material stored in the storage unit, An image forming unit that forms a toner image, A transfer unit transfers the toner image formed by the image forming unit to the recording material supplied by the feeding unit, A fixing unit having a rotating body for heating the toner image transferred to the recording material, and fixing the toner image to the recording material, After the recording material on which the toner image has been fixed by the fixing unit is transported in a first direction, the reversing transport unit performs a reversing transport in a second direction opposite to the first direction, A retransport unit that transports the inverted recording material back to the transfer unit, A first path that constitutes a path for transporting the recording material from the feeding unit to the transfer unit, A second path which constitutes the path from when the recording material is transported in the second direction by the reversal transport unit until it merges with the first path, A single drive source that drives the rotating body and the reversing conveying unit, A detection unit for detecting the recording material passing through the second path, The system includes a control unit that controls the drive source to change the transport speed at which the reversing transport unit transports the second recording material, which is transported along the second path following the first recording material, in the second direction, based on the detection result of the detection unit that detects the first recording material passing through the second path, An image forming apparatus characterized by the following features. (Configuration 2) The control unit, The drive source is controlled by the reversing transport unit to transport the first recording material in the second direction at a first transport speed. If the detection time from when the image forming unit starts to when the first recording material is detected by the detection unit is a second time that is shorter than a predetermined first time, the second transport speed when the second recording material is transported in the second direction is controlled to be slower than the first transport speed. If the detection time is a third time which is longer than the first time, the second transport speed is controlled to be faster than the first transport speed. The image forming apparatus according to configuration 1, characterized in that it is a picture forming apparatus. (Composition 3) The image forming unit is Image carrier and, A charging part that charges the surface of the image carrier, An exposure unit that exposes the surface of the image carrier, which has been charged by the charging unit, to form an electrostatic latent image, It has a developing unit which develops the electrostatic latent image with a developer to form a toner image, The image carrier, whose surface has been charged by the charging unit, is exposed by the exposure unit to form an electrostatic latent image on the surface of the image carrier, and the developing unit visualizes the electrostatic latent image and transfers it to a transfer member, thereby performing image formation. The image forming apparatus according to configuration 2, characterized in that... (Composition 4) The control unit changes the second transport speed based on the detection results obtained when the detection unit detects a plurality of recording materials that have been transported along the second path prior to the second recording material. The image forming apparatus according to configuration 2 or 3, characterized by the above. (Composition 5) The control unit changes the second transport speed based on at least one of the type and size of the second recording material sheet and the detection time. An image forming apparatus according to any one of configurations 2 to 4, characterized by the above. (Composition 6) Equipped with a memory unit for storing information, The control unit, In the first image forming job, the second transport speed is stored in the storage unit. In a second image forming job following the first image forming job, the second transport speed is obtained from the storage unit, and the drive source is controlled so that at least the first recording material transported in the second image forming job is transported by the inversion transport unit at the second transport speed. An image forming apparatus according to any one of configurations 2 to 5, characterized by the above. (Composition 7) The aforementioned reversing transport unit is replaceable. The control unit initializes the second transport speed stored in the storage unit when the reversing transport unit is replaced. The image forming apparatus according to configuration 6, characterized by the features described therein. (Composition 8) Equipped with an environmental detection unit that detects environmental information, The control unit initializes the second transport speed stored in the storage unit when the detection result of the environmental detection unit exceeds a threshold. The image forming apparatus according to configuration 6 or 7, characterized by the above. (Composition 9) The detection unit is located in the second path. An image forming apparatus according to any one of configurations 1 to 8, characterized by the above. (Composition 10) The aforementioned reversing and conveying unit is A pair of reversing rollers that grip and transport the aforementioned recording material, The system includes a reversing unit interposed in the power transmission path from the drive source to the reversing roller pair, which switches the rotational force of the drive source in one direction to either forward or reverse and transmits it to the reversing roller pair. An image forming apparatus according to any one of configurations 1 to 9, characterized by the above. (Composition 11) The system includes a pair of registration rollers that transport the recording material supplied by the feeding unit to the transfer unit, and adjust the position at which an image is formed on the recording material by adjusting the speed at which the recording material is transported to the transfer unit. The length of the sheet transport path from the reversal transport unit through the re-transport unit to the pair of registration rollers is shorter than the length of the sheet transport direction of legal-sized recording material. An image forming apparatus according to any one of configurations 1 to 10, characterized by the above. (Composition 12) A storage section for storing recording materials, A feeding unit for feeding the recording material stored in the storage unit, An image forming unit that forms a toner image, A transfer unit transfers the toner image formed by the image forming unit to the recording material supplied by the feeding unit, A fixing unit having a rotating body for heating the toner image transferred to the recording material, and fixing the toner image to the recording material, After the recording material on which the toner image has been fixed by the fixing unit is transported in a first direction, the reversing transport unit performs a reversing transport in a second direction opposite to the first direction, A retransport unit that transports the inverted recording material back to the transfer unit, A first path that constitutes a path for transporting the recording material from the feeding unit to the transfer unit, A second path which constitutes the path from when the recording material is transported in the second direction by the reversal transport unit until it merges with the first path, A single drive source that drives the rotating body and the reversing conveying unit, A detection unit for detecting the recording material passing through the second path, The system includes a control unit that controls the drive source to change the number of times the reversal transport unit reverses the transport of the second recording material, which is transported following the first recording material, in the first direction and the second direction, based on the detection result of the detection unit that detects the first recording material passing through the second path, An image forming apparatus characterized by the following features. (Composition 13) The control unit, If the detection time from when the image forming unit starts to when the first recording material is detected by the detection unit is a second time, which is shorter than a predetermined first time, the number of inversions is controlled to be the first number. If the detection time is a third time which is longer than a predetermined first time, the number of reversals is controlled to be a second time which is less than the first time. The image forming apparatus according to configuration 12, characterized in that... (Composition 14) The image forming unit is Image carrier and, A charging part that charges the surface of the image carrier, An exposure unit that exposes the surface of the image carrier, which has been charged by the charging unit, to form an electrostatic latent image, It has a developing unit which develops the electrostatic latent image with a developer to form a toner image, The image carrier, whose surface has been charged by the charging unit, is exposed by the exposure unit to form an electrostatic latent image on the surface of the image carrier, and the developing unit visualizes the electrostatic latent image and transfers it to a transfer member, thereby performing image formation. The image forming apparatus according to configuration 13, characterized by the features described above. (Composition 15) The control unit changes the number of inversions based on the multiple detection results obtained when the multiple recording materials transported along the second path prior to the second recording material are detected by the detection unit. The image forming apparatus according to configuration 13 or 14, characterized by the above. (Composition 16) The control unit changes the number of inversions based on at least one of the type and size of the second recording material sheet and the detection time. An image forming apparatus according to any one of configurations 13 to 15, characterized by the above. (Composition 17) Equipped with a memory unit for storing information, The control unit, In the first image forming job, the number of inversions is stored in the memory unit. In a second image forming job following the first image forming job, the number of inversions is obtained from the storage unit, and the drive source is controlled so that at least the first recording material transported in the second image forming job is transported by the inversion transport unit in the same number of inversions as the number of inversions. An image forming apparatus according to any one of the configurations 13 to 16 characterized by the above. (Composition 18) The aforementioned reversing transport unit is replaceable. When the reversing transport unit is replaced, the control unit initializes the number of times the unit is reversed and transported, which is stored in the storage unit. The image forming apparatus according to configuration 17, characterized by the features described above. (Composition 19) Equipped with an environmental detection unit that detects environmental information, The control unit initializes the number of times the inverted transport is performed, which is stored in the storage unit, when the detection result of the environmental detection unit exceeds a threshold. The image forming apparatus according to configuration 17 or 18, characterized by the above. (Composition 20) The detection unit is located in the second path. An image forming apparatus according to any one of configurations 12 to 19, characterized by the features described herein. (Composition 21) The aforementioned reversing and conveying unit is A pair of reversing rollers that grip and transport the aforementioned recording material, The system includes a reversing unit interposed in the power transmission path from the drive source to the reversing roller pair, which switches the rotational force of the drive source in one direction to either forward or reverse and transmits it to the reversing roller pair. An image forming apparatus according to any one of the configurations 12 to 20, characterized by the features described herein. (Composition 22) The system includes a pair of registration rollers that transport the recording material supplied by the feeding unit to the transfer unit, and adjust the position at which an image is formed on the recording material by adjusting the speed at which the recording material is transported to the transfer unit. The length of the sheet transport path from the reversal transport unit through the re-transport unit to the pair of registration rollers is shorter than the length of the sheet transport direction of legal-sized recording material. An image forming apparatus according to any one of the configurations 12 to 21, characterized by the features described herein. [Explanation of Symbols]

[0092] 2: Image forming unit / 6: Fixing unit / 7: Reversal transport unit (reversal transport unit) / 8: Re-transport unit / 9: Control unit / 21: Scanner unit (exposure unit) / 22: Photosensitive drum (image carrier) / 23: Charging roller (charging unit) / 24: Developing roller (developing unit) / 31: Intermediate transfer belt (transfer material) / 36: Secondary transfer unit (transfer unit) / 41: Sheet cassette (storage unit) / 42: Feeding roller (feeding unit) / 50: Registration roller pair / 51: Print transport path (first path) / 61: Fixing roller (rotating body) / 70: Reversal roller pair (reversal transport unit) / 81: Re-transport path (second path) / 98: Memory unit / 99: Temperature sensor (environmental detection unit) / 102: Fixing motor (drive source) / 103: Reversal clutch (reversal unit) / S: Sheet (recording material) / S4: Double-sided sensor (detection unit)

Claims

1. A storage section for storing recording materials, A feeding unit for feeding the recording material stored in the storage unit, An image forming unit that forms a toner image, A transfer unit transfers the toner image formed by the image forming unit to the recording material supplied by the feeding unit, A fixing unit having a rotating body for heating the toner image transferred to the recording material, and fixing the toner image to the recording material, After the recording material on which the toner image has been fixed by the fixing unit is transported in a first direction, the reversing transport unit performs a reversing transport to transport the material in a second direction opposite to the first direction, A retransport unit that transports the inverted recording material back to the transfer unit, A first path that constitutes a path for transporting the recording material from the feeding unit to the transfer unit, A second path which constitutes the path from when the recording material is transported in the second direction by the reversal transport unit until it merges with the first path, A single drive source that drives the rotating body and the reversing conveying unit, A detection unit for detecting the recording material passing through the second path, The system includes a control unit that controls the drive source to change the transport speed at which the reversing transport unit transports the second recording material, which is transported in the second path following the first recording material, in the second direction, based on the detection result of the detection unit that detects the first recording material passing through the second path, An image forming apparatus characterized by the following features.

2. The control unit, The drive source is controlled by the reversing transport unit to transport the first recording material in the second direction at a first transport speed. If the detection time from when the image forming unit starts to when the first recording material is detected by the detection unit is a second time that is shorter than a predetermined first time, the second transport speed when the second recording material is transported in the second direction is controlled to be slower than the first transport speed. If the detection time is a third time which is longer than the first time, the second transport speed is controlled to be faster than the first transport speed. The image forming apparatus according to feature 1.

3. The image forming unit is Image carrier and, A charging part that charges the surface of the image carrier, An exposure unit that exposes the surface of the image carrier, which has been charged by the charging unit, to form an electrostatic latent image, It has a developing unit which develops the electrostatic latent image with a developer to form a toner image, The image carrier, whose surface has been charged by the charging unit, is exposed by the exposure unit to form an electrostatic latent image on the surface of the image carrier, and the developing unit visualizes the electrostatic latent image and transfers it to a transfer member, thereby performing image formation. The image forming apparatus according to feature 2.

4. The control unit changes the second transport speed based on the detection results obtained when the detection unit detects a plurality of recording materials that have been transported along the second path prior to the second recording material. The image forming apparatus according to feature 2.

5. The control unit changes the second transport speed based on at least one of the type and size of the second recording material sheet and the detection time. The image forming apparatus according to feature 2.

6. Equipped with a memory unit for storing information, The control unit, In the first image forming job, the second transport speed is stored in the storage unit. In a second image forming job following the first image forming job, the second transport speed is obtained from the storage unit, and the drive source is controlled so that at least the first recording material transported in the second image forming job is transported by the inversion transport unit at the second transport speed. The image forming apparatus according to feature 2.

7. The aforementioned reversing transport unit is replaceable. When the reversing transport unit is replaced, the control unit initializes the second transport speed stored in the storage unit. The image forming apparatus according to feature 6.

8. Equipped with an environmental detection unit that detects environmental information, The control unit initializes the second transport speed stored in the storage unit when the detection result of the environmental detection unit exceeds a threshold. The image forming apparatus according to feature 6.

9. The detection unit is located in the second path. The image forming apparatus according to feature 1.

10. The aforementioned reversing and conveying unit is A pair of reversing rollers that grip and transport the aforementioned recording material, The system includes a reversing unit interposed in the power transmission path from the drive source to the reversing roller pair, which switches the rotational force of the drive source in one direction to either forward or reverse and transmits it to the reversing roller pair. The image forming apparatus according to feature 1.

11. The system includes a pair of registration rollers that transport the recording material supplied by the feeding unit to the transfer unit, and adjust the position at which an image is formed on the recording material by adjusting the speed at which the recording material is transported to the transfer unit. The length of the sheet transport path from the reversal transport unit through the re-transport unit to the pair of registration rollers is shorter than the length of the sheet transport direction of legal-sized recording material. The image forming apparatus according to feature 1.

12. A storage section for storing recording materials, A feeding unit for feeding the recording material stored in the storage unit, An image forming unit that forms a toner image, A transfer unit transfers the toner image formed by the image forming unit to the recording material supplied by the feeding unit, A fixing unit having a rotating body for heating the toner image transferred to the recording material, and fixing the toner image to the recording material, After the recording material on which the toner image has been fixed by the fixing unit is transported in a first direction, the reversing transport unit performs a reversing transport to transport the material in a second direction opposite to the first direction, A retransport unit that transports the inverted recording material back to the transfer unit, A first path that constitutes a path for transporting the recording material from the feeding unit to the transfer unit, A second path which constitutes the path from when the recording material is transported in the second direction by the reversal transport unit until it merges with the first path, A single drive source that drives the rotating body and the reversing conveying unit, A detection unit for detecting the recording material passing through the second path, The system includes a control unit that controls the drive source to change the number of times the reversal transport unit reverses the transport of the second recording material, which is transported following the first recording material, in the first direction and the second direction, based on the detection result of the detection unit that detects the first recording material passing through the second path, An image forming apparatus characterized by the following features.

13. The control unit, If the detection time from when the image forming unit starts to when the first recording material is detected by the detection unit is a second time, which is shorter than a predetermined first time, the number of inversions is controlled to be the first number. If the detection time is a third time which is longer than a predetermined first time, the number of reversals is controlled to be a second time which is less than the first time. The image forming apparatus according to feature 12.

14. The image forming unit is Image carrier and, A charging part that charges the surface of the image carrier, An exposure unit that exposes the surface of the image carrier, which has been charged by the charging unit, to form an electrostatic latent image, It has a developing unit which develops the electrostatic latent image with a developer to form a toner image, The image carrier, whose surface has been charged by the charging unit, is exposed by the exposure unit to form an electrostatic latent image on the surface of the image carrier, and the developing unit visualizes the electrostatic latent image and transfers it to a transfer member, thereby performing image formation. The image forming apparatus according to feature 13.

15. The control unit changes the number of inversions based on the multiple detection results obtained when the multiple recording materials that were transported along the second path prior to the second recording material are detected by the detection unit. The image forming apparatus according to feature 13.

16. The control unit changes the number of inversions based on at least one of the type and size of the second recording material sheet and the detection time. The image forming apparatus according to feature 13.

17. Equipped with a memory unit for storing information, The control unit, In the first image forming job, the number of inversions is stored in the memory unit. In a second image forming job following the first image forming job, the number of inversions is obtained from the storage unit, and the drive source is controlled so that at least the first recording material transported in the second image forming job is transported by the inversion transport unit in the same number of inversions as the number of inversions. The image forming apparatus according to feature 13.

18. The aforementioned reversing transport unit is replaceable. When the reversing transport unit is replaced, the control unit initializes the number of times the unit is reversed and transported, which is stored in the storage unit. The image forming apparatus according to feature 17.

19. Equipped with an environmental detection unit that detects environmental information, The control unit initializes the number of times the inverted transport is performed, which is stored in the storage unit, when the detection result of the environmental detection unit exceeds a threshold. The image forming apparatus according to feature 17.

20. The detection unit is located in the second path. The image forming apparatus according to feature 12.

21. The aforementioned reversing and conveying unit is A pair of reversing rollers that grip and transport the aforementioned recording material, The system includes a reversing unit interposed in the power transmission path from the drive source to the reversing roller pair, which switches the rotational force of the drive source in one direction to either forward or reverse and transmits it to the reversing roller pair. The image forming apparatus according to feature 12.

22. The system includes a pair of registration rollers that transport the recording material supplied by the feeding unit to the transfer unit, and adjust the position at which an image is formed on the recording material by adjusting the speed at which the recording material is transported to the transfer unit. The length of the sheet transport path from the reversal transport unit through the re-transport unit to the pair of registration rollers is shorter than the length of the sheet transport direction of legal-sized recording material. The image forming apparatus according to feature 12.

Images