JP2018136390A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve productivity of a sheet on both sides of which images are formed.SOLUTION: An image forming apparatus 100 comprises: a conveyance path 28 that re-conveys, to an image forming part 101, a sheet on a first face of which an image is formed by the image forming part 101, to form an image on a second face of the sheet; and a confluent conveyance path 31 for re-conveying, to the image forming part 101, the sheet on the first face of which the image is formed by the image forming part 101, to form an image on the second face of the sheet, in a shorter distance than that when re-conveying the sheet on the conveyance path 28. A CPU of a control part 300 selects whether to re-convey the sheet on the conveyance path 28 or re-convey the sheet on the confluent conveyance path 31.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an image forming apparatus that forms an image on a sheet.

  In general, an image forming apparatus that forms an image on both sides of a sheet is known. In the image forming apparatus described in Patent Document 1, an image is formed on the first surface (front surface) of a sheet by an image forming unit, and the sheet is temporarily stopped by a reversing unit and then reversed and reversely conveyed to reverse the conveyance path. The sheet is sent to the path, conveyed to the image forming unit again, and an image is formed on the second surface (back surface) of the sheet by the image forming unit. In this type of image forming apparatus, in order to improve the productivity of the sheet on which the image is formed, when the preceding sheet on which the image is formed on the first surface is conveyed by being reversed by the reversing unit, the succeeding sheet is formed by the image forming unit. Sheet image formation has started. Therefore, by increasing the conveyance path length from the image forming unit to the reversing unit, contact between the preceding sheet reversed by the reversing unit and fed back to the double-sided conveyance path and the subsequent sheet conveyed to the reversing unit is performed. It is preventing.

JP2012-153521A

  However, depending on the sheet conveyance conditions and the like, there is a case where contact between the preceding sheet and the subsequent sheet that is reversed does not occur, and in order to avoid contact with any sheet when forming an image on both sides. If a long distance is conveyed, the productivity of a sheet on which an image is formed is reduced.

  Therefore, an object of the present invention is to improve the productivity of sheets on which images are formed on both sides.

  The image forming apparatus of the present invention includes a first conveyance path that re-conveys to the image forming unit in order to form an image on the second surface of the sheet on which the image is formed on the first surface by the image forming unit, and the image In order to form an image on the second surface of the sheet on which the image is formed on the first surface by the forming unit, the sheet is re-entered on the image forming unit at a shorter distance than when the sheet is re-conveyed by the first conveying path. A second conveying path for conveying the sheet; and a selection unit that selects whether the sheet is re-conveyed by the first conveying path or the sheet is re-conveyed by the second conveying path. And

  According to the present invention, the productivity of sheets on which images are formed on both sides is improved.

It is explanatory drawing which shows the image forming apparatus which concerns on 1st Embodiment. 2 is a block diagram of a control system of the image forming apparatus according to the first embodiment. FIG. 3 is a flowchart illustrating a control operation in a control unit of the image forming apparatus according to the first embodiment. (A)-(e) is a schematic diagram for demonstrating the conveyance state of a sheet | seat. 10 is a flowchart illustrating a control operation in a control unit of an image forming apparatus according to a second embodiment.

Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the drawings.
[First Embodiment]
FIG. 1 is an explanatory diagram illustrating an image forming apparatus according to the first embodiment. The image forming apparatus 100 is an electrophotographic monochrome copying machine including an apparatus main body 10, a document reading unit 25 disposed on the upper portion of the apparatus main body 10, and a control unit 300 that controls the entire apparatus. The document reading unit 25 reads the image on the lower surface of the document G placed on the platen glass 25a by the reading head 25b that moves along the platen glass 25a to generate image information.

  The apparatus main body 10 includes an image forming unit 101 that forms an image on the sheet S. The image forming unit 101 includes an image forming unit 102 that forms a toner image that is a developer image on the sheet S, and a fixing unit 19 that is an example of a fixing unit and fixes the toner image on the sheet S. The image forming unit 102 includes a photosensitive drum 18 that is an example of a photosensitive member. Around the photosensitive drum 18, a charging roller 18d, an exposure device 11, a developing device 18a, a transfer roller 17 that is an example of a transfer unit, and A drum cleaning device 18c is arranged. A toner image is formed on the surface of the photosensitive drum 18, and the toner image is transferred to the sheet S that passes through the transfer nip T between the photosensitive drum 18 and the transfer roller 17.

  The photosensitive drum 18 has a photosensitive layer formed on the peripheral surface of a metal cylinder, and rotates by driving a motor (not shown). The charging roller 18d is applied with an oscillating voltage obtained by superimposing an AC voltage on a DC voltage, and charges the photosensitive drum 18 to a uniform potential. The exposure device 11 drives a semiconductor laser with an image signal obtained by on-off modulation of a scanning line signal obtained by developing image data, generates a laser beam, scans the laser beam with a rotating mirror, and scans the image on the photosensitive drum 18. An electrostatic latent image is formed.

  The developing roller 18b of the developing device 18a carries toner and applies toner to the electrostatic latent image on the photosensitive drum 18 to form a toner image. The toner bottle unit 16 supplies toner to the developing device 18a. The transfer roller 17 is applied with a DC voltage having a polarity opposite to the charging polarity of the toner, and transfers the toner image on the photosensitive drum 18 to the sheet S passing through the transfer nip T.

  A feeding unit 12 is disposed at the lower part of the apparatus main body 10. The feeding unit 12 has a cassette 12a that can be attached to and detached from the apparatus main body 10, and the sheets S are stacked on the middle plate 12b of the cassette 12a. The intermediate plate 12b is urged upward by the pressure spring 12c to bring the sheet S into contact with the pickup roller 12f. The pickup roller 12f feeds the sheet S from the cassette 12a and feeds it to the separation unit 12e. The separation unit 12 e separates the multi-feed sheet and sends only the uppermost sheet S to the feeding path 14 and feeds it to the registration roller pair 15. The skew of the sheet S is corrected when the leading edge of the sheet S hits the nip of the stopped pair of registration rollers 15 and a loop is formed on the sheet S. The sheet S is fed to the transfer nip T by rotating the registration roller pair 15 in synchronization with the toner image on the photosensitive drum 18.

  The sheet S on which the toner image has been transferred at the transfer nip T is conveyed to the fixing unit 19, and is heated and pressurized in the process of being conveyed at the nip N1, and the toner image is fixed on the surface. The fixing unit 19 includes a heat generating member 19a as a heating unit, and a pressure roller 19b disposed to face the heat generating member 19a. Then, the unfixed toner on the sheet S is melted and the toner image is fixed on the sheet S by the fixing action by pressure and heating at the nip portion N1 of the heat generating member 19a and the pressure roller 19b.

  On the downstream side of the fixing unit 19 in the sheet conveyance direction, an upper conveyance roller pair 61 as an example of a first reversing unit and a lower conveyance roller pair 62 as an example of a second reversing unit are arranged. A tray 23 on which discharged sheets are stacked is disposed inside the body of the apparatus main body 10. When forming images on both sides of the sheet S, the upper conveying roller pair 61 can perform a reversing operation for reversing and conveying the sheet S on which the image is formed on one side. The lower conveying roller pair 62 reverses and conveys the sheet S formed on one side when discharging an image formed sheet S onto the tray 23 and forming an image on both sides of the sheet S. Inversion operation can be performed.

  The upper conveying roller pair 61 includes a driving roller 61a that is driven to be transmitted by a driving unit that will be described later, and a driven roller 61b that is rotated by the rotation of the driving roller 61a. The drive roller 61a is rotationally driven in forward and reverse directions, and can transport the sheet S together with the driven roller 61b in the sheet discharge direction or in the direction opposite to the sheet discharge direction. The lower conveying roller pair 62 as the second reversing means includes a driving roller 62a that is driven to be transmitted by a driving unit that will be described later, and a driven roller 62b that rotates following the rotation of the driving roller 62a. The driving roller 62a is rotationally driven in forward and reverse directions, and can transport the sheet S together with the driven roller 62b in the sheet discharging direction or in the direction opposite to the sheet discharging direction.

  The lower conveyance roller pair 62 is disposed in the conveyance path of the sheet S between the fixing unit 19 and the upper conveyance roller pair 61. That is, the lower conveyance roller pair 62 is disposed on the downstream side of the fixing unit 19 in the sheet conveyance direction and on the upstream side of the upper conveyance roller pair 61 in the sheet conveyance direction. Accordingly, the conveyance distance from the fixing unit 19 to the upper conveyance roller pair 61 is longer than the conveyance distance from the fixing unit 19 to the lower conveyance roller pair 62.

  On the downstream side of the fixing unit 19 in the sheet conveyance direction, a conveyance path 27 through which the sheet conveyed to the lower conveyance roller pair 62 passes, and a second conveyance path through which the sheet reversed by the lower conveyance roller pair 62 is conveyed. And a conveyance path 28 constituting a first conveyance path through which a sheet branched from the conveyance path 27 and conveyed to the pair of upper conveyance rollers 61 passes. A lower flapper 29a and a middle flapper 29b are arranged at the branching points of the transport paths as transport path switching members. The middle flapper 29b is switched between a position where the sheet S conveyed from the fixing unit 19 is guided to the upper conveying roller pair 61 and a position where the sheet S is guided to the lower conveying roller pair 62 by a lower switching solenoid 29c described later. The lower flapper 29a is a self-weight flapper that does not require a driving means, and allows the leading edge of the sheet S conveyed from the fixing unit 19 to push up and rotate the lower flapper 29a to enter the conveying path 27, and to lower the lower conveying roller pair. The sheet S reversed and fed backward from 62 is not guided to the upstream fixing unit 19 but is disposed at a position where it is guided to the merging conveyance path 31.

  A double-sided conveyance path 33 for re-conveying the sheet S to the image forming unit 101 is branched from the conveyance path 28, and an upper flapper 30 as a conveyance path switching member is disposed at this branch point. . The upper flapper 30 performs both-side conveyance of the sheet S conveyed by the upper conveyance roller pair 61 and the position where the sheet S conveyed on the conveyance path 28 is guided to the upper conveyance roller pair 61 by an upper switching solenoid 30c described later. The position is switched to the position where the road 33 is guided. In the middle of the double-sided conveyance path 33, a merging conveyance path 31 that joins the sheet reversed by the lower conveyance roller pair 62 to the double-sided conveyance path 33 is connected. A plurality of conveyance roller pairs 26 a and 26 b are arranged in the double-side conveyance path 33 along the sheet conveyance direction, and the image forming unit 101 is again in a state where the sheet S conveyed to the double-side conveyance path 33 is reversed. Can be conveyed.

  In the case of a job for forming an image on one side of a sheet, an image is formed on the first side of the sheet S fed from the feeding unit 12 to the image forming unit 101. The sheet S that has passed through the image forming unit 101 and is formed on the first surface is conveyed to the lower conveying roller pair 62 and is discharged to the tray 23 by the lower conveying roller pair 62.

  In the case of a job for forming images on both sides of a sheet, first, an image is formed on the first surface (front surface) of the sheet S fed from the feeding unit 12 to the image forming unit 101. The sheet S on which the image is formed on the first surface after passing through the image forming unit 101 is conveyed to the lower conveyance roller pair 61 and the lower conveyance depending on whether one or a plurality of sheets form an image on both sides. It is conveyed to one of the roller pairs 62. In other words, depending on the presence / absence of a succeeding sheet conveyed subsequent to the image forming object sheet on which an image is to be formed on the second surface (back surface), the image forming object is placed on either the upper conveying roller pair 61 or the lower conveying roller pair 62 Sheet is conveyed. Then, the sheet S reversed and conveyed by the upper conveyance roller pair 61 or the lower conveyance roller pair 62 passes through the double-side conveyance path 33 and is conveyed again to the image forming unit 101 in a state where the front and back are reversed. The sheet S that has passed through the image forming unit 101 and is image-formed on the second surface opposite to the first surface is conveyed to the lower conveying roller pair 62 and is discharged to the tray 23 by the lower conveying roller pair 62.

  FIG. 2 is a block diagram of a control system of the image forming apparatus according to the first embodiment. As illustrated in FIG. 2, the control unit 300 includes a CPU 301, a ROM 302, a RAM 303, and a counter 304 that is a time measuring unit. The CPU 301 comprehensively controls the entire apparatus by holding data and programs recorded in the ROM 302 in the RAM 303 and executing necessary calculations and processing. The CPU 301 receives and holds image information sent from an external computer (not shown) and image information of a document captured by the document reading unit 25. The CPU 301 of the control unit 300 performs image processing on the image information to generate an exposure signal, and transmits the exposure signal to the image forming unit 101 for image formation.

  The CPU 301 of the control unit 300 is connected to a lower switching solenoid 29c that is an example of a driving unit that drives the middle flapper 29b and an upper switching solenoid 30c that is an example of a driving unit that drives the upper flapper 30. The CPU 301 controls the positions of the middle flapper 29b and the upper flapper 30 by controlling the driving of the switching solenoids 29c and 30c.

  Sensors 36, 38, and 39 that detect the presence or absence of the sheet S passing on the conveyance path are connected to the CPU 301. The registration sensor 36 is arranged upstream of the registration roller pair 15, the upper conveyance sensor 38 is arranged upstream of the upper conveyance roller pair 61, and the lower conveyance sensor 39 is arranged upstream of the lower conveyance roller pair 62. ing.

  In addition, the CPU 301 rotates the upper transport motor 61c, which is an example of a drive unit that rotationally drives the drive roller 61a of the upper transport roller pair 61 in the forward and reverse directions, and the drive roller 62a of the lower transport roller pair 62 in the forward and reverse directions. The lower conveyance motor 62c which is an example of the drive part to drive is connected, respectively. The CPU 301 controls the conveyance of the sheet S by the conveyance roller pair 61 and 62 by controlling the driving of the conveyance motors 61c and 62c.

  Hereinafter, the main operation of the control by the CPU 301 of the control unit 300 in the first embodiment will be specifically described. The CPU 301 receives a job for forming an image on a sheet from an external computer or an operation panel (not shown) operated by an operator. The CPU 301 determines whether the input job is a job for forming an image only on one side of the sheet or a job for forming an image on both sides of the sheet.

  In the case of a job for forming an image only on one side of the sheet, the CPU 301 switches and controls the middle flapper 29 b to a position where the sheet is guided to the lower conveyance roller pair 62. As a result, the sheet S on which the image is formed on the first surface by the image forming unit 101 is guided to the lower conveyance roller pair 62. Then, the sheet S guided by the lower conveyance roller pair 62 is discharged onto the tray 23 by the lower conveyance roller pair 62.

  FIG. 3 is a flowchart illustrating a control operation in the control unit of the image forming apparatus according to the first embodiment. FIG. 3 illustrates a control operation in the case of a job for forming an image on both sides of a sheet. In the case of a job for forming images on both sides of a sheet, the CPU 301 determines whether the number of sheets on which images are to be formed on both sides is only one or plural (S101).

  The CPU 301 as the selection unit, when the number of sheets on which images are formed on both sides is only one (S101: Yes), that is, subsequent to the image forming target sheet on which the images are formed on the second side. When there is no sheet, it is selected that the sheet is conveyed again by the merging conveyance path 31 constituting the second conveyance path, and the middle flapper 29 b is switched to a position where the sheet is guided to the lower conveyance roller pair 62. Specifically, the CPU 301 switches on and controls the middle flapper 29b so that the sheet is guided to the lower conveying roller pair 62 by turning on the lower switching solenoid 29c (S102). As a result, the image forming target sheet S on the second surface of the image forming unit 101 is guided to the lower conveying roller pair 62. In addition, the CPU 301 rotates the lower transport motor 62c forward (S103). As a result, the driving roller 62a of the lower conveyance roller pair 62 rotates forward, and the sheet S is conveyed in the sheet discharge direction by the lower conveyance roller pair 62.

  The CPU 301 determines whether or not the sheet has passed based on the detection result of the lower conveyance sensor 39, that is, whether or not the sheet has been detected (S104). When the sheet absence is detected by the lower conveyance sensor 39 (S104: Yes), the CPU 301 detects that the sheet S is not separated from the lower conveyance roller pair 62, that is, after the absence of the sheet is detected by the lower conveyance sensor 39. The time is counted by the counter 304, and when the predetermined time T1 has elapsed (S105: Yes), the drive roller 62a of the lower conveying roller pair 62 is temporarily stopped and then controlled to rotate in reverse (S106). That is, the CPU 301 reverses the sheet S to the lower conveyance roller pair 62. As a result, the sheet S is switched back by the lower conveyance roller pair 62 and is reversely conveyed in the middle of the double-sided conveyance path 33 through the merging conveyance path 31 constituting the second conveyance path. An image is formed on two sides. The sheet S on which the image is formed on the second surface by the image forming unit 101 is guided to the lower conveyance roller pair 62 and is discharged onto the tray 23 by the lower conveyance roller pair 62.

  As described above, when the number of sheets forming images on both sides is only one, the CPU 301 as the selection unit selects the merging conveyance path 31 constituting the second conveyance path. Further, the sheet is reversely conveyed by the lower conveyance roller pair 62 closer to the fixing unit 19. Therefore, the sheet is shortcutted in the middle of the double-sided conveyance path 33 via the merging conveyance path 31, and the time required for image formation, that is, the time from when the sheet is fed from the feeding unit 12 until it is discharged to the tray 23 is shortened. can do. Therefore, the productivity of sheets on which images are formed on both sides can be improved.

  When there are a plurality of sheets on which an image is formed (S101: No), the CPU 301 as the selection unit, that is, a subsequent sheet to be conveyed following an image formation target sheet on which an image is to be formed on the second surface. In some cases, it is selected that the sheet is conveyed again by the conveyance path 28 constituting the first conveyance path, and the middle flapper 29b is switched to a position where the sheet is guided to the upper conveyance roller pair 61. Specifically, the CPU 301 switches and controls the middle flapper 29b so that the sheet is guided to the pair of upper conveying rollers 61 by turning off the lower switching solenoid 29c (S107). As a result, the sheet S on which the image is formed on the second surface is guided to the upper conveying roller pair 61. Further, the CPU 301 rotates the upper transport motor 61c in the normal direction (S108). As a result, the driving roller 61a of the upper conveyance roller pair 61 rotates forward, and the sheet S is conveyed in the sheet discharge direction by the upper conveyance roller pair 61.

  The CPU 301 determines whether or not the sheet has passed based on the detection result of the upper conveyance sensor 38, that is, whether or not the sheet has been detected (S109). When the upper conveyance sensor 38 detects the absence of the sheet (S109: Yes), the CPU 301 determines that the sheet S has not been separated from the upper conveyance roller pair 61, that is, after the upper conveyance sensor 38 detects the absence of the sheet. The time is counted by the counter 304, and when the predetermined time T2 has passed (S110: Yes), the drive roller 61a of the upper transport roller pair 61 is temporarily stopped and then controlled to rotate in reverse (S111). That is, the CPU 301 reverses the sheet S to the upper conveyance roller pair 61 and conveys the sheet S to the duplex conveyance path 33. The sheet S conveyed to the duplex conveyance path 33 is image-formed on the second surface by the image forming unit 101. The sheet S on which the image is formed on the second surface by the image forming unit 101 is guided to the lower conveyance roller pair 62 and is discharged onto the tray 23 by the lower conveyance roller pair 62.

  FIG. 4A to FIG. 4E are schematic views for explaining the sheet conveyance state. First, in FIG. 4A, the registration roller pair 15 starts to rotate after a predetermined time after the leading edge of one sheet S conveyed from the feeding unit 12 passes the registration sensor 36. The sheet S passes through the transfer roller 17, the photosensitive drum 18, and the fixing unit 19, forms an image on the first surface, and is conveyed to the conveyance path 27.

  Before the leading edge of the sheet S1 reaches the middle flapper 29b, the middle flapper 29b is switched so that the sheet is guided to the lower conveying roller pair 62. As shown in FIG. It is conveyed to the conveyance roller pair 62. After the trailing edge of the sheet S passes through the lower conveyance sensor 39, the sheet S is temporarily stopped by the lower conveyance roller pair 62 and switchback is started.

  As shown in FIG. 4C, the sheet S is conveyed to the duplex conveyance path 33 through the merging conveyance path 31. As shown in FIG. 4D, the sheet S passes through the transfer roller 17, the photosensitive drum 18, and the fixing unit 19 to form an image on the second surface. As shown in FIG. It is conveyed to the pair 62 and discharged to the tray 23.

  The CPU 301 switches off the energization to the switching solenoid 29c, thereby switching the middle flapper 29b to a position where the sheet that has passed through the fixing unit 19 is guided to the lower conveyance roller pair 62, and turns on the energization to the switching solenoid 29c. Thus, the middle flapper 29 b may be switched to a position where the sheet that has passed through the fixing unit 19 is guided to the upper conveying roller pair 61.

  As described above, when an image is formed on both surfaces of only one sheet, the sheet S is reversely conveyed by the lower conveyance roller pair 62 closer to the fixing unit 19 than the upper conveyance roller pair 61. Therefore, the sheet S is shortcutted in the middle of the double-sided conveyance path 33 through the merging conveyance path 31, and the time required for image formation, that is, the time from when the sheet S is fed from the feeding unit 12 until it is discharged to the tray 23. Can be shortened. For example, the sheet conveyance speed (process speed) in the image forming unit 101 is 250 [mm / sec]. The conveyance distance from the fixing unit 19 to the point where the merged conveyance path 31 merges with the double-sided conveyance path 33 via the conveyance path 27 and the lower conveyance roller pair 62 is 160 [mm]. A conveyance distance from the fixing unit 19 to the point where the merging conveyance path 31 merges in the duplex conveyance path 33 via the conveyance path 27, the conveyance path 28, and the upper conveyance roller pair 61 is 440 [mm]. For this reason, the conveyance length can be shortened by 440 [mm] −160 [mm] = 280 [mm] by reversely conveying the sheet S by the lower conveyance roller pair 62. Since the process speed is 250 [mm / sec], it can be shortened by 280 [mm] / 250 [mm / sec] = 1.12 [sec]. Therefore, the productivity of sheets on which images are formed on both sides can be improved.

[Second Embodiment]
Next, an image forming apparatus according to the second embodiment will be described. In the first embodiment, when a plurality of sheets form images on both sides, the sheet is conveyed to the upper conveyance roller pair 61 without using the merging conveyance path 31 constituting the second conveyance path. However, in the second embodiment, even when there are a plurality of sheets, the lower conveyance roller pair 62 may reversely convey the sheet and use the merging conveyance path 31 in some cases. FIG. 5 is a flowchart illustrating a control operation in the control unit of the image forming apparatus according to the second embodiment. The configuration of the image forming apparatus in the second embodiment is the same as that of the image forming apparatus 100 described in the first embodiment, and a description thereof is omitted. The CPU 301 of the control unit 300 executes the processes of steps S202 to S206 when there is one sheet for forming images on both sides (S201: Yes). Since the processing of steps S202 to S206 is the same as the processing of steps S102 to S106 described in the first embodiment, the description thereof is omitted.

  If there are a plurality of sheets on which images are to be formed on both sides (S201: No), that is, if there is a subsequent sheet to be conveyed following the image formation target sheet on which the image is to be formed on the second side, the CPU 301 Since the conveyance interval between the sheet and the subsequent sheet (paper interval) may be changed according to the image forming conditions and the like, the conveyance interval between the preceding sheet and the subsequent sheet in the image forming unit 101 is determined (S207).

  Here, as an example of control of the conveyance interval (between sheets) of the sheet conveyed to the image forming unit 101, in order to satisfy the fixability of a thick sheet that requires a higher fixing temperature than the thin sheet, the conveyance interval There is control to sufficiently heat the fixing unit 19 by widening. In this control, the conveyance interval is widened from the first interval D1 where the productivity of double-sided printing is set to 100 [%] to the second interval D2 wider than the first interval D1, and the productivity is set to 75 [%], for example. To.

  The CPU 301 causes the upper conveyance roller pair 61 or the lower conveyance roller pair 62 to invert the image formation target sheet S in accordance with the set conveyance interval. Specifically, when the conveyance interval is the first interval D1 (S207: D1), the CPU 301 reverses the image formation target sheet by the upper conveyance roller pair 61 (S208 to S212). The processing in steps S208 to S212 is the same as the processing in steps S107 to S111 described in the first embodiment, and thus description thereof is omitted. When the conveyance interval is the second interval D2 that is wider than the first interval D1 (S207: D2), the CPU 301 reversely conveys the preceding sheet that forms an image on the second surface with the lower conveyance roller pair 62, Since the sheet does not collide with the leading edge of the succeeding sheet conveyed from the image forming unit 101, the image forming target sheet is reversed by the lower conveying roller pair 62 (S202 to S206).

  The determination in step S207 can also be performed using a threshold value. That is, when the sheet conveyance interval is the first interval D1 that is smaller than the threshold value, the sheet may be reversed to the upper conveyance roller pair 61. When the sheet conveyance interval is the second interval D <b> 2 that is larger than the threshold value, the sheet may be reversed to the lower conveyance roller pair 62. Alternatively, whether to use the merged conveyance path 31 may be stored in the ROM 302 of the control unit 300 in a table format according to the set conveyance interval and the conveyed sheet size.

  As described above, when the sheet conveyance interval is widened, it is possible to avoid collision between the preceding sheet that is reversed and conveyed by the lower conveyance roller pair 62 and the leading edge of the subsequent sheet that is conveyed from the image forming unit 101. Therefore, the productivity of a sheet on which images are formed on both sides can be improved as compared with the case where the upper conveyance roller pair 61 is reversed. In addition, since the transport distance is shortened, it is possible to reduce a transport load due to sliding of the transported sheet with the transport guide, nipping by the transport roller, and the like.

  The present invention is not limited to the embodiment described above, and many modifications are possible within the technical idea of the present invention. In addition, the effects described in the embodiments are merely a list of the most preferable effects resulting from the present invention, and the effects of the present invention are not limited to those described in the embodiments. For example, in the second embodiment, the image forming condition for changing the conveyance interval (paper interval) between the preceding sheet and the succeeding sheet is the change in the fixing property due to the thickness of the sheet. An image forming condition may be a change in the sheet interval due to the edge temperature rise countermeasure by continuous fixing. Further, the selection unit selects the second conveyance path when the preceding sheet and the subsequent sheet do not collide, but this is not limited to the variation in the conveyance interval (inter-paper interval), and is not limited to the conveyance direction of the conveyed sheet. When the preceding sheet and the succeeding sheet do not collide according to the size, a configuration in which the second conveyance path is selected may be used.

  In the above-described embodiment, the case where the image forming apparatus is a copying machine has been described. However, the present invention is not limited to this, and the image forming apparatus may be a printer, a facsimile machine, a multifunction machine, or the like.

28 ... conveyance path (first conveyance path), 31 ... confluence conveyance path (second conveyance path), 100 ... image forming apparatus, 101 ... image forming section, 301 ... CPU (selection means)

Claims (6)

A first conveyance path for re-conveying to the image forming unit to form an image on the second surface of the sheet on which the image is formed on the first surface by the image forming unit;
In order to form an image on the second surface of the sheet on which the image is formed on the first surface by the image forming unit, the sheet is formed on the image forming unit at a shorter distance than when the sheet is conveyed again by the first conveying path. A second transport path for re-transporting,
An image forming apparatus comprising: a selecting unit that selects whether the sheet is re-conveyed by the first conveying path or the sheet is re-conveyed by the second conveying path.   The image forming apparatus according to claim 1, wherein the second conveyance path is a conveyance path that branches in the middle of a conveyance path that conveys a sheet from the image forming unit to the first conveyance path.   2. The selection unit according to claim 1, wherein the second conveyance path is selected when the preceding sheet and the subsequent sheet do not collide even when the sheet is re-conveyed by the second conveyance path. Or the image forming apparatus according to 2;   The image forming apparatus according to claim 3, wherein the selection unit selects the second conveyance path when there is no subsequent sheet. First reversing means for reversing the sheet re-conveyed by the first conveying path;
5. The apparatus according to claim 1, further comprising: a second reversing unit that is different from the first reversing unit for reversing the sheet re-conveyed by the second conveying path. 2. The image forming apparatus according to item 1.   The image forming apparatus according to claim 5, wherein the second reversing unit also discharges an image-formed sheet.

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