JP2020008834A - Image formation device - Google Patents

Abstract

To appropriately manage a position of a sheet when forming an image on both sides of the sheet.SOLUTION: First conveyance means (71) conveys a sheet received from a first conveyance path (59) to a second conveyance path (79). If conveyance of a subsequent sheet (3A) is stopped without image formation on a first surface of the subsequent sheet after an image is formed on a first surface of a preceding sheet, the first conveyance means is stopped without inverting the preceding sheet (2B) to a direction toward the second conveyance path. After that, conveyance of the subsequent sheet is started followed by conveyance of the preceding sheet to the second conveyance path by the first conveyance means.SELECTED DRAWING: Figure 10

Description

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

  2. Description of the Related Art In an image forming apparatus including a printer, a copying machine, and a multifunction peripheral, a configuration capable of forming an image on both sides of a sheet is widely adopted. For example, after an image is formed on one side of the sheet, the sheet is conveyed again to the image forming unit in a state where the sheet is switched back by a pair of reversing rollers that can be rotated forward and backward, so that the other side of the sheet is Is used to form an image.

  Patent Document 1 discloses a configuration in which three sheets can be simultaneously conveyed in a re-feeding path from a pair of reversing rollers to a pair of registration rollers via a plurality of pairs of re-feeding rollers. In this configuration, the pair of reversing rollers, the pair of re-feeding rollers on the upstream side, and the pair of re-feeding rollers on the downstream side are driven by separate motors, and the driving speed of each motor is independently controlled. . In Japanese Patent Application Laid-Open No. H11-157, a configuration in which a gap between sheets is prevented from opening as much as possible according to the length of a sheet circulation path between an image forming unit and a double-sided conveyance unit and a plurality of sheets are kept on standby to suppress a decrease in productivity. And

JP 2013-010601 A JP-A-2002-365882

  By the way, for example, in a case where it takes time to develop image data having a large data amount, image formation cannot be started at a desired timing, and it may be necessary to wait for a sheet before image formation. Here, after an image is formed on the first surface of the preceding sheet, if the image formation on the first surface of the succeeding sheet cannot be started and the succeeding sheet is to be on standby, the sheet is reversed and transported to the image forming unit again. The preceding sheet is made to stand by on the path (re-conveying path).

  In such a case, in the configuration of each of the above documents, the state in which the preceding sheet is conveyed as much as possible to the downstream of the re-conveyance path under conditions such as not colliding with a sheet already existing on the re-conveyance path Wait at. However, it has been found that inconvenience may occur when the preceding sheet is conveyed as far downstream of the re-conveying path as possible.

  Accordingly, an object of the present invention is to provide an image forming apparatus capable of appropriately managing the position of a sheet when forming images on both sides of the sheet.

  One embodiment of the present invention includes a first conveyance path, an image forming unit that forms an image on a sheet passing through the first conveyance path, and a first conveyance unit that receives a sheet from the first conveyance path and reversely conveys the sheet. A second conveyance path for guiding the sheet inverted by the first conveyance means to the first conveyance path, a first drive source for driving the first conveyance means, and a control means for controlling the first drive source Wherein the control means includes a first surface of a preceding sheet, a first surface of a succeeding sheet fed to the first conveyance path following the preceding sheet, and a second surface of the preceding sheet. In the case where an image is formed, after the image is formed on the first surface of the preceding sheet, when the creation of an image for forming on the first surface of the succeeding sheet is not permitted, the preceding sheet is transported to the second transporter. The first transport means without reversing in the direction toward the road; And then, based on the timing at which the image forming unit starts creating an image to be formed on the first surface of the succeeding sheet, the first conveying unit conveys the preceding sheet to the second conveying path. The image forming apparatus.

  According to another aspect of the present invention, there is provided a first transport path, an image forming unit that forms an image on a sheet passing through the first transport path, and a first transport that receives a sheet from the first transport path and reversely transports the sheet. Means, a second conveyance path for guiding the sheet inverted by the first conveyance means to the first conveyance path, and a sheet arranged on the second conveyance path, wherein the sheet inverted by the first conveyance means is transferred to the second conveyance path. A third transport unit that transports the first transport unit, a first drive source that drives the first transport unit, a second drive source that drives the third transport unit, the first drive source, and the third drive unit. Control means for controlling the two driving sources, the control means comprising: a first surface of a preceding sheet, a first surface of a succeeding sheet to be fed to the first conveying path following the preceding sheet, and When forming an image on the second surface of the preceding sheet, After the image is formed on the first side of the sheet, when the creation of an image to be formed on the first side of the subsequent sheet is not permitted, the leading end of the preceding sheet in the sheet conveying direction of the second conveying path is Before reaching the third conveying means, the first driving source is stopped to stop conveying the preceding sheet, and thereafter, the image forming means is formed on the first surface of the succeeding sheet. The image forming apparatus according to claim 1, wherein the driving of the first driving source is started based on a timing at which the creation of the image is started, and the preceding sheet is transferred to the third transport unit.

  ADVANTAGE OF THE INVENTION According to this invention, when forming an image on both surfaces of a sheet, the position of a sheet can be managed appropriately.

1 is a schematic diagram of an image forming apparatus according to a first embodiment. FIG. 2 is a block diagram illustrating a control configuration of the image forming apparatus according to the first embodiment. FIG. 2 is a schematic diagram illustrating a driving configuration of a sheet conveying system according to the first embodiment. In the image forming apparatus according to the first embodiment, two-sided printing is performed on small-sized sheets (a), two-sided printing is performed on large-sized sheets (b), and two-sided printing is performed on many small-sized sheets. FIG. 7 is a conceptual diagram showing an image forming order in case (c). FIG. 3 is a schematic diagram illustrating a standby position of a sheet in a case where duplex printing is performed on a small size sheet and a case where duplex printing is performed on a large size sheet (b) in the image forming apparatus according to the first exemplary embodiment. 4 is an overall flowchart illustrating a print job control method according to the first embodiment. 6 is a flowchart of conveyance control performed on each sheet in a print job according to the first embodiment. FIG. 4 is a schematic diagram illustrating a state (a, b) of a double-sided printing operation in a case where standby processing is not performed in the image forming apparatus according to the first embodiment. 5 is a timing chart illustrating a driving state of each motor when a standby process is not performed in the image forming apparatus according to the first embodiment. FIG. 3 is a schematic diagram illustrating a state of double-sided printing operation (a to d) in a case where a standby process for a first surface is performed in the image forming apparatus according to the first embodiment. 6 is a timing chart illustrating a driving state of each motor when performing a standby process on a first surface in the image forming apparatus according to the first exemplary embodiment. FIG. 3 is a schematic diagram illustrating double-sided printing operations (a to d) in a case where the second surface standby processing is performed in the image forming apparatus according to the first embodiment. 6 is a timing chart illustrating a driving state of each motor when performing a standby process on a second surface in the image forming apparatus according to the first exemplary embodiment. FIGS. 3A and 3B are a schematic view (a) and an enlarged view (b) of a state where a second sheet is abutted against a second pair of double-sided rollers in the image forming apparatus according to the first embodiment. 9 is a timing chart illustrating a driving state of each motor when performing a standby process on a first surface in the image forming apparatus according to the second embodiment.

  Hereinafter, an image forming apparatus according to the present disclosure will be described with reference to the drawings. The image forming apparatus includes a printer, a copier, a facsimile, and a multifunction peripheral, and forms an image on a sheet used as a recording medium based on image information input from an external PC or image information read from a document.

  The image forming apparatus 1 according to the present embodiment is an electrophotographic full-color laser printer that forms and outputs an image on a sheet P used as a recording medium. As the sheet P, paper such as plain paper and envelopes, glossy paper, plastic films such as overhead projector sheets, cloth, and the like can be used. An apparatus main body 100 of the image forming apparatus 1 includes an image forming engine including four image forming units PY, PM, PC, and PK for forming yellow, magenta, cyan, and black toner images, and an intermediate transfer belt 21. 10 are accommodated. Each of the image forming units PY to PK has a photosensitive drum 11 as an image carrier, and forms a toner image on the photosensitive drum 11. The toner image carried on the photosensitive drum 11 is transferred to a sheet P via an intermediate transfer belt 21 which is an intermediate transfer member.

  Since the image forming units PY to PK have the same configuration except that the color of the toner used for development is different, the configuration of the image forming unit and the toner image forming process (image forming operation) will be described using the yellow image forming unit PY as an example. Will be described. The image forming unit PY includes, in addition to the photosensitive drum 11, a charging roller 12 as a charging unit, an exposing device 13Y as an exposing unit, a developing device 14 as a developing unit, and a drum cleaner. The photosensitive drum 11 is a drum-shaped photosensitive member having a photosensitive layer on an outer peripheral portion, and rotates in a direction along the rotation direction R1 of the intermediate transfer belt 21. The charging roller 12 uniformly charges the surface of the photosensitive drum 11, and the exposing device 13Y irradiates the photosensitive drum 11 with a laser beam modulated in accordance with image information, thereby forming an electrostatic latent image on the surface of the photosensitive drum 11. Is performed. The developing device 14 stores a developer containing toner and supplies the toner to the photosensitive drum 11 to develop the electrostatic latent image into a toner image. The toner image formed on the photosensitive drum 11 is primarily transferred to the intermediate transfer belt 21 by a primary transfer roller 25 as a primary transfer device. The residual toner remaining on the photosensitive drum 11 after the transfer is removed by a drum cleaner.

  The intermediate transfer belt 21 is wound around a secondary transfer inner roller 22, a stretching roller 23, a tension roller 24, and a primary transfer roller 25, and is driven to rotate counterclockwise (R1) in the figure. The above-described image forming operation proceeds in parallel in each of the image forming units PY to PK, and the full-color toner image is formed on the intermediate transfer belt 21 by the multiple transfer of the four color toner images so as to overlap each other. . This toner image is carried on the intermediate transfer belt 21 and is conveyed to a transfer section (secondary transfer section) configured as a nip between the secondary transfer roller 43 and the secondary transfer inner roller 22. When a bias voltage having a polarity opposite to the charging polarity of the toner is applied to the secondary transfer roller 43 as a transfer unit, the toner image carried on the intermediate transfer belt 21 is secondarily transferred onto the sheet P. The residual toner remaining on the intermediate transfer belt 21 after the transfer is removed by the belt cleaner 26.

  The sheet P to which the toner image has been transferred is transferred to the fixing device 50. The fixing device 50 has a fixing roller pair 51 for nipping and transporting the sheet P, and a heat source such as a halogen heater, and applies pressure and heat to the toner image carried on the sheet P. Thereby, the toner particles are melted and fixed, and a fixed image fixed on the sheet P is obtained.

  Next, the operation of conveying the sheet P by the image forming apparatus 1 will be described. Feeding cassettes 31, 32, which are examples of sheet storage means, store sheets P and are detachably attached to the apparatus main body 100. The sheets P stored in the feed cassettes 31 and 32 are fed one by one by the feed unit 40. The feed unit 40 includes a pickup roller 40a that sends out the sheet P from the feed cassettes 31 and 32, and a feed roller 40b that receives and transports the sheet P from the pickup roller 40a. Further, the feeding unit 40 includes a separation roller 40c that separates the sheet P conveyed by the feeding roller 40b from another sheet P. The feeding unit 40 is an example of a feeding unit that feeds the sheet P, and is a belt type in which the sheet P is attracted and conveyed to a belt member by a suction fan, or a friction separating type using a pad. May be used. Further, the user can directly set the sheet P on the manual feed tray 33 provided on the side of the apparatus main body 100, and the sheet P set on the manual feed tray 33 is fed by the feeding unit.

  The sheet P sent from the feeding unit 40 is conveyed to a registration roller pair (hereinafter, referred to as a registration roller pair) 42 via a pre-registration roller pair (hereinafter, referred to as a pre-registration roller pair) 41. The registration roller pair 42, which is an example of a registration unit, corrects the skew of the sheet P by contacting the front end of the sheet P, that is, the downstream end in the sheet conveyance direction. Thereafter, the registration roller pair 42 feeds the sheet P to the secondary transfer unit at a timing corresponding to the degree of progress of the image forming operation by the image forming units PY to PK. The sheet P on which the toner image has been transferred in the secondary transfer section and the image has been fixed by the fixing device 50 is transferred to the sheet discharge section 60, and the conveyance path of the sheet P can be switched by the post-fixing roller pair 61. It is conveyed toward the member 64.

  When the image formation on the sheet P has been completed, the sheet P having the image formed on the first surface (front surface) is discharged to the discharge tray 80 by the discharge roller pair 62. When an image is formed on the second surface (back surface) of the sheet P, the sheet P is transferred by the switching member 64 to the reverse conveyance unit 70 via the conveyance roller pair 63. The reversing conveyance section 70 includes a pair of reversing rollers 71 for reversing and conveying (switching back) the sheet P, and a two-sided path 79 for guiding the sheet P switched back by the pair of reversing rollers 71 to the pair of registration rollers 42. The reversing roller pair 71 conveys the sheet P to the discharge space above the discharge tray 80 by a predetermined distance and then conveys the sheet P in the reverse direction, thereby sending the sheet P to the double-sided path 79. In the two-sided path 79, a plurality of transport roller pairs (72, 73, 74) are arranged as described later in detail, and transport the sheet P again to the registration roller pair 42. Then, the sheet P having the image formed on the second surface by passing through the secondary transfer unit and the fixing device 50 is discharged to the discharge tray 80 by the discharge roller pair 62.

  The image forming engine 10 is an example of an image forming unit, and may be, for example, a direct transfer system in which a toner image formed on a photoconductor is directly transferred to a sheet in a transfer unit. Further, the configuration of the image forming unit may be an ink jet system or an offset printing system.

  Hereinafter, a control configuration for controlling the functions of the image forming apparatus 1 will be described with reference to the block diagram of FIG. The image forming apparatus 1 includes a control unit 200 having respective functional units such as a CPU (Central Processing Unit) 201, a memory 202, an operation unit 203, an image formation control unit 205, and a sheet conveyance control unit 206. The CPU 201 reads out and executes a control program stored in the memory 202 to realize various processes performed by the image forming apparatus 1. The memory 202 includes, for example, a RAM (Random Access Memory) and a ROM (Read Only Memory), and stores various programs and various data in a predetermined storage area. The operation unit 203 includes devices such as a liquid crystal panel and buttons, and inputs various information (information such as size, basis weight, surface properties, etc.) relating to a sheet used for printing, and executes printing and instructs interruption of the printing. Various operations performed by the user are received.

  The image forming control unit 205 controls the image forming operation by issuing an instruction to each unit of the image forming engine 10, including the exposure writing unit 13 that causes the exposure devices 13Y, 13M, 13C, and 13K to execute the image writing operation. The sheet conveyance control unit 206 issues an instruction to the feed motor 110 and the discharge motor 160 for driving various conveyance rollers, and controls the conveyance of the sheet P. The sensor control unit 207 controls the start and stop of detection by the registration sensor 44 and the double-sided sensor 76 described below, and receives the detection results of these sensors. Further, the control unit 200 can receive various types of information regarding sheets used for printing from a computer 204 connected to the image forming apparatus 1 via a network.

(Sheet transport system)
Next, a sheet conveyance system controlled by the sheet conveyance control unit 206 and its driving configuration will be described with reference to FIG. The sheet conveying system provided in the image forming apparatus 1 includes a feeding path 49, an image forming path 59, and a two-sided path 79. The sheet conveyance space in these passes is constituted by a guide member supported by the apparatus main body 100. In the following description, the “sheet conveyance direction” indicates a main conveyance direction of a sheet in a conveyance path in which a target member is arranged.

  The feeding path 49 is a conveying path for feeding the sheet P, and includes the feeding unit 40 and the pre-registration roller pair 41. The feed unit 40 is driven by a feed motor 110, and the pre-registration roller pair 41 is driven by a pre-registration motor 120.

  The image forming path 59 is a conveyance path for forming an image while conveying the sheet P, and includes the registration roller pair 42, the secondary transfer roller 43 and the secondary transfer inner roller 22, and the fixing roller pair 51. The registration roller pair 42 is driven by a registration motor 130, the secondary transfer inner roller 22 is driven by an ITB (Intermediate transfer belt) motor 140, and the fixing roller pair 51 is driven by a fixing motor 150.

  The duplex path 79 is a transport path for transporting the sheet P sent from the image forming path 59 again to the image forming path 59 when performing duplex printing, and includes a first duplex roller pair 72 and a second duplex roller. A pair 73 and a third double-sided roller pair 74 are arranged. The reversing roller pair 71 receives and reverses the sheet P from the image forming path 59, and sends the sheet P to the two-sided path 79. The first to third double-sided roller pairs 72 to 74 are arranged in this order along the direction from the reversing roller pair 71 to the registration roller pair 42 in the double-sided path 79.

  The reversing roller pair 71 and the first double-sided roller pair 72 are driven by a reversing motor 170, and the second double-sided roller pair 73 and the third double-sided roller pair 74 are driven by a double-sided motor 180. The reversing motor 170 is a motor rotatable in a first direction (forward rotation) and a reverse second direction (reverse rotation). The first double-sided roller pair 72 is connected to the reversing motor 170 via a one-way clutch 75. . A two-sided sensor 76 is disposed between the first pair of two-sided rollers 72 and the second pair of two-sided rollers 73 in the sheet conveying direction as a detecting means capable of detecting a sheet. In addition, a registration sensor 44 is disposed at a position near the upstream side of the registration roller pair 42 as another detection unit. As these sensors, for example, photoelectric sensors capable of detecting that light is shielded by a sheet can be used.

  The image forming path 59 corresponds to a first conveying path on which the image forming unit is arranged, and the two-sided path 79 is a second conveying path that guides the sheet inverted by the pair of reversing rollers 71 and the like toward the first conveying path. Is equivalent to The reversing roller pair 71 corresponds to a first transport unit that reversely transports a sheet from the first transport path. The first pair of double-sided rollers 72, the second pair of double-sided rollers 73, and the third pair of double-sided rollers 74 include a second conveying unit, a third conveying unit, and a third conveying unit that are sequentially arranged in the second conveying path along the sheet conveying direction. This corresponds to four transport means. Further, the reversing motor 170 corresponds to a first drive source for driving the first transport unit and the second transport unit, and the double-sided motor 180 corresponds to a second drive source for driving the third transport unit and the fourth transport unit. The registration motor 130 corresponds to a third drive source that drives the registration unit.

  In addition, a discharge motor 160 that drives the pair of transport rollers (61 to 63) that constitute the sheet discharge unit 60 is provided. The sheet conveying operation described below is also applicable to a configuration in which the pair of reversing rollers 71 also serves as a discharging unit for discharging the sheet P (a configuration in which the sheet discharging unit is omitted).

(Standby state)
Next, a case will be described in which a sheet is made to stand by upstream of a transfer unit (secondary transfer unit in this embodiment) during execution of double-sided printing. When performing double-sided printing on a plurality of sheets, image transfer to the sheets is usually performed at a fixed interval (image forming interval) according to a printing order described later, and this interval is the number of output sheets per unit time, that is, Defines the productivity of the image forming apparatus. However, for various reasons as described below, there is a case where the image formation is temporarily stopped while the productivity is allowed to be lowered, and the image formation interval becomes longer.

  For example, when the temperature of the fixing device rises excessively and exceeds the allowable temperature limit of the fixing roller, the conveyance of the sheet to the transfer unit and the fixing device may be stopped to secure the cooling time. Further, when a sheet processing device connected to the downstream side of the discharge roller and performing a process such as a binding process on a sheet is mounted, image forming is stopped to wait for a processing operation by the sheet processing device. is there. Further, a time (image development time) for the control unit of the image forming apparatus to develop the image data transferred from the computer into a data format supported by the image forming unit (a format that can be transferred to the exposure writing unit 13) is provided. There are cases where it is necessary.

  When image formation is stopped, writing of a new image in the image forming units PY to PK is stopped, while the sheet P is made to wait on the upstream side of the secondary transfer unit. When the image formation is delayed for the above-described reason, the control unit 200 may determine the image formation delay immediately before the start of the image writing operation by the exposure writing unit 13 depending on the timing at which the reason occurs. For this reason, the sheet P may have any of the first surface (front surface) and the second surface (back surface). In addition, from the viewpoint of improving the productivity of the image forming apparatus 1, the time lag from when the control unit 200 determines that the image writing can be performed to when the exposure apparatuses 13Y to 13K actually start the image writing operation is as short as possible. It is configured to be.

  On the other hand, the time lag from the start of the image writing operation to the time when the image is transferred to the sheet P depends on the hardware configuration for conveying the toner image and the sheet P to the secondary transfer unit. For example, the rotational speed (process speed) of the photosensitive drum 11 and the intermediate transfer belt 21 is high, and the latent image written on the photosensitive drum 11 by the exposure devices 13Y to 13K is developed and finally moves to the secondary transfer portion. The shorter the moving distance to perform, the smaller the time lag. Similarly, the shorter the conveyance speed of the sheet P and the shorter the conveyance distance from the registration roller pair 42, which is the standby position of the sheet P, to the secondary transfer portion, the smaller the time lag.

  Also in the present embodiment, in order to meet the demand for improving the productivity and reducing the size of the image forming apparatus 1, from the most downstream (that is, black) primary transfer roller 25 to the secondary transfer roller 43 in the rotation direction of the intermediate transfer belt 21. Is configured as short as possible. Further, the distance from the registration roller pair 42 to the secondary transfer roller 43 is set as short as possible, and is set to the same length as the distance from the most downstream primary transfer roller 25 to the secondary transfer roller 43.

  Under the above configuration, when the start of the image writing operation in the image forming units PY to PK is not permitted, the sheet P is caused to wait by the registration roller pair 42. When the start of the image writing operation is permitted, the writing of the electrostatic latent image is started immediately thereafter, and at the same time, the conveyance of the sheet P is started by the registration roller pair 42, and then the toner image is transferred to the sheet P in the secondary transfer unit. Is transferred to Naturally, the start timing of the image writing operation and the timing of restarting the conveyance of the sheet P are controlled so that the image transferred to the sheet P in the secondary transfer unit does not shift.

(Circulation operation)
Next, a description will be given of a sheet circulating operation when performing double-sided printing. FIG. 4 is a conceptual diagram showing the order in which the image forming operation is performed, and shows that the image is transferred through the secondary transfer section (broken line) in order from the sheet on the left side of the figure as time elapses. FIG. 5 is a schematic diagram showing a state in which the maximum number of sheets are on standby during duplex printing. 4A and 5A show the case where a small-sized sheet having a relatively small length in the sheet conveying direction is printed on both sides, and FIG. 4B shows the large size having a relatively large length in the sheet feeding direction. This is a case where the sheet is printed on both sides. Examples of small size are A4 (210 mm) and letter (215.9 mm), and examples of large size are A3 (420 mm) and leisure (431.8 mm). FIG. 4C will be described later.

  In addition, reference numerals such as “3A” and “1B” indicate a state in which a numeral specifies a sheet, and an alphabet indicates a state before the sheet is inverted (A) or after the sheet is inverted (B). For example, “3A” represents a state in which the third sheet fed is before reversal, and “1B” represents a state in which the first sheet fed is reversed.

  As shown in FIG. 4A, in the case of a small-sized sheet, after images are sequentially formed on the front surfaces (1A to 3A) of three sheets, the images are formed on the back surface (1B) of the first sheet. It is formed. After that, image formation on the front and back sides is performed alternately as (1B → 4A → 2B → 5A...), And images are continuously formed on the back sides (3B to 5B) of the last three sheets. The five-sided double-sided printing is completed. If the number of double-sided printing is six or more, the period during which the front and back sides are printed alternately is extended.

  Here, for example, when the standby process is performed on the back surface (1B) of the first sheet, that is, the start of the image writing operation for the image to be printed on the back surface of the first sheet is not permitted, and the registration roller pair 42 Let the sheet (1B) be on standby. That is, the image forming interval between the sheet (3A) and the sheet (1B) is long. In this case, as shown in FIG. 5A, the first sheet (1B) comes into contact with the registration roller pair 42. Wait in state. When the standby time of the first sheet (1B) is long, the subsequent second and third sheets (2B, 3B) are also made to wait at any position on the transport path.

  That is, in the case of small-sized sheets, it is necessary to make a maximum of three sheets wait. In accordance with this, the reversing conveyance section 70, which is a re-conveying path from the reversing roller pair 71 to the registration roller pair 42 via the double-sided path 79, has a length capable of holding three sheets. In addition, referring to FIG. 4A, for example, when waiting for image formation on the front surface (5A) of the fifth sheet, the third sheet and the fourth sheet ( 3B, 4B). This is because the first and second sheets (1B, 2B) are discharged from the image forming apparatus 1.

  As described above, the image writing permission may be determined just before the sheet P reaches the registration roller pair 42 in some cases. When the sheet P is the second side, first, the first sheet (1B) of the two-sided pass 79 is stopped, then the second sheet (2B) is stopped, and then the third sheet (3B) is stopped. ) Is stopped, and it is necessary to independently control the conveyance of the three sheets. That is, in order to make the three sheets wait in the reversing roller pair 71 and the two-sided path 79 having a limited length of the conveyance path, the conveyance of the preceding sheet is stopped after the conveyance of the preceding sheet is stopped. It is necessary to reduce the gap between sheets. Further, by making the sheet interval in the standby state as small as possible, it is possible to improve the productivity after restarting the conveyance. The third sheet (3B) is preferably placed on standby at a position sufficiently separated from the image forming path 59 in order to avoid inconveniences such as deterioration of an image and sticking of sheets due to heat of the fixing device. In the embodiment, the image forming apparatus is made to stand by while being held by the reversing roller pair 71.

  In order to be able to independently control the conveyance of the three sheets, it is easy to provide three driving sources such as motors. That is, one driving source corresponds to one sheet, and when a certain sheet reaches a predetermined sheet position (standby position), driving of the corresponding driving source is stopped. However, disposing a large number of drive sources leads to an increase in cost. In general, the cost of the motor makes a relatively large contribution to the entire product. Therefore, if the number of drive sources can be reduced while maintaining the function, a significant cost reduction can be achieved.

  Therefore, in the present embodiment, a configuration in which a predetermined number (for example, three) of sheets can be made to wait at a desired position with a smaller number (for example, two) of driving sources than the predetermined number is adopted. . The specific configuration and operation will be described later in detail.

  Further, in this embodiment, the standby position of the sheet in the reverse conveyance unit 70 is changed depending on whether the sheet P waiting by the registration roller pair 42 is the first surface or the second surface. This also explains that the position of the sheet can be appropriately managed so as to minimize the possibility of occurrence of inconvenience such as defective conveyance of the sheet.

(Sheet conveyance operation)
Hereinafter, a method of transporting sheets while waiting three sheets as necessary by the registration motor 130, the reversing motor 170, and the double-sided motor 180 will be described with reference to FIGS. FIG. 6 is a flowchart of the entire conveying operation, and FIG. 7 is a flowchart showing the control contents performed for each sheet.

  6 and 7 are achieved when the CPU 201 of the control unit 200 (see FIG. 2) controls each unit of the image forming apparatus 1 via each functional block (205, 206, 207). You. This flowchart is executed when the user instructs execution of a print job from the operation unit 203 of the image forming apparatus 1 or from the computer 204 connected directly to the image forming apparatus 1 or via a network. The user can specify the number of pages and the number of copies for each copy printed by a print job, and sheet information of sheets to be used for printing (attributes such as size and basis weight, and a cassette of a supply source).

  When the control unit 200 receives a print job and starts executing the job (S101), sheet feeding is started in order from the first sheet (N = 1) (S102, S103). The transport operation for each sheet is controlled with reference to the state of the first sheet preceding the second sheet and the state of the third sheet following the second sheet as a reference in accordance with the flowchart of FIG. 7 (S104). It is not necessary to mount the sheet as a parallel processing program as long as the conveyance operation is realized for each sheet according to the flowchart of FIG. When the number of sheets necessary for the execution of the print job is fed and the image formation on those sheets is completed (S105: Yes), the print job ends (S106).

  The control method of the transport operation for each sheet will be described with reference to the flowchart of FIG. Here, the conveyance operation for the second sheet will be described, but the sheet fed from the feeding path 49 prior to the target sheet will be described as the first sheet, and the sheet following the target sheet will be described. The sheet fed from the feeding path 49 can be replaced with a third sheet. In other words, the “first sheet”, “second sheet”, and “third sheet” represent the relative order when each sheet is fed from the feeding path 49. Further, as a precondition, it is assumed that the first sheet and the second sheet are image-formed on the first side, and the conveyance control described here is started from a state where the first sheet is on standby on both sides.

  When the second sheet arrives at the reversing position of the pair of reversing rollers 71, it is determined whether or not an image can be written on the first surface of the third sheet (S111). The reversing position of the reversing roller pair 71 means that the reversing roller pair 71 conveys the second sheet a predetermined distance in the normal rotation direction (first direction) and rotates the second sheet in the reverse rotation direction (second direction). This indicates a position where the sheet can be conveyed to the double-sided path 79. If it is determined in S111 that writing is possible, image formation on the first surface of the third sheet is performed without causing the registration roller pair 42 to wait for the third sheet (S112). That is, after the registration roller pair 42 performs skew correction of the third sheet, the driving of the registration motor 130 is started so that the third sheet is conveyed at a predetermined interval with respect to the second sheet. The predetermined interval corresponds to the interval between “2A” and “3A” when the productivity of the image forming apparatus is the maximum in FIG.

  If writing is not permitted in S111, the third sheet is made to wait by the registration roller pair 42. That is, the driving of the registration motor 130 is not started even if the second sheet moves away more than the predetermined interval. At this time, the second sheet waits in the reversing unit (S113), and continues waiting until image writing permission for the third sheet (S114) is issued. The reversing unit refers to a portion of the reversing conveyance unit 70 used for reversing the sheet (conveying means, conveyance guide, and the like), and in the present embodiment, the reversing roller pair 71 constitutes the reversing unit. When the image writing permission of the third sheet is issued, the image formation of the first surface of the third sheet is executed (S112).

  After the start of image formation of the third sheet, when the leading edge of the third sheet reaches a predetermined position (the secondary transfer unit in the present embodiment), conveyance of the second sheet toward the two-sided path 79 is started (S115). The second sheet is carried into the double-sided path 79. Then, at the timing when the second sheet reaches the second double-sided roller pair 73, the subsequent operation is switched depending on whether or not the preceding first sheet is waiting at the registration roller pair 42 (S116).

  If the first sheet is not on standby by the registration roller pair 42 (S116: No), a mode (second mode) in which the standby processing of the second sheet is not performed is selected. In this case, the target sheet enters the second pair of double-sided rollers 73 in the driven state, is pulled out by the second pair of double-sided rollers 73, and is continuously conveyed toward the pair of registration rollers 42 (S117). The second sheet that has reached the registration roller pair 42 is conveyed to the secondary transfer unit after image writing on the second sheet is permitted (S120, S121), and an image is formed on the second surface (S122). .

  On the other hand, when the first sheet is waiting at the registration roller pair 42, that is, when step S121 is being performed on the first sheet, a standby process (S118) of waiting the second sheet to be transported to the predetermined position is performed. The mode (first mode) to be performed is selected. In this case, the second sheet is conveyed by the first double-sided roller pair 72 to a position where the second sheet is abutted against the second double-sided roller pair 73 in the stopped state. Thereafter, when the conveyance of the preceding sheet is restarted by the double-sided motor 180, the conveyance of the target sheet is restarted by the second double-sided roller pair 73 driven by the double-sided motor 180 (S119). Then, the second sheet that has reached the registration roller pair 42 is conveyed to the secondary transfer unit after image writing on the second sheet is permitted (S120, S121), and an image is formed on the second surface (S120, S121). S122).

  Hereinafter, with reference to FIGS. 8 to 13, sheet behavior and drive control of each motor when the standby process is not performed, when the standby process on the first surface is performed, and when the standby process on the second surface is performed. explain in detail. In the following description, the first to third sheets are common to the flowchart of FIG.

(1. When standby processing is not performed)
FIG. 8 illustrates a transport operation when neither the first surface standby process nor the second surface standby process is performed. In this case, since the image writing on the first surface of the third sheet (3A) can be performed before the second sheet (2B) reaches the reversing roller pair 71, the standby processing on the first surface is not performed ( S111: No). After the third sheet (3A) is temporarily stopped by the pair of registration rollers 42, as shown in FIG. 8A, the conveyance of the third sheet (3A) is started at a predetermined interval from the rear end of the preceding sheet (here, the second sheet). . The second sheet (2B) is conveyed toward the first pair of double-sided rollers 72 by receiving a stop signal for a predetermined time (settling time 100 ms) after the reversing motor 170 rotates forward by a predetermined amount and then reversely rotating. You.

  Further, in this case, the image writing on the first sheet (1B) can be performed before the front end of the second sheet (2B) after the reversal reaches the second double-sided roller pair 73. Not performed (S120: No). After temporarily stopping at the registration roller pair 42, the first sheet (1B) is started to be conveyed at a predetermined interval from the rear end of the preceding third sheet (3A) as shown in FIG. 8B. The second sheet (2B) is conveyed toward the third pair of double-sided rollers 74 so as to follow the first sheet (1B) in the double-sided path 79.

  FIG. 9 shows a timing chart of each motor regarding the conveyance control of the first sheet and the second sheet in the two-sided pass 79. After the first sheet (1B) is temporarily stopped upstream of the registration roller pair 42, after the start of image formation on the third sheet (3A), the two-sided motor 180 is driven at a speed (peripheral speed) of 300 mm / s by the two-sided roller pair 73 and The third pair of double-sided rollers 73 is driven. Thus, the first sheet (1B) is conveyed toward the registration roller pair 42 while maintaining a predetermined interval with respect to the third sheet (3A) on which the image is formed (see FIG. 8A )).

  When the registration sensor 44 detects the first sheet (1B), the third double-sided roller pair 73 is reduced to the transport speed V1 for forming a loop on the sheet. The first sheet (1B) collides with the registration roller pair 42 in a stopped state, forms a loop, and is corrected for skew. In the present embodiment, V1 = 220 mm / s, and the timing at which the drive speed of the double-sided motor 180 is reduced is substantially equal to the timing at which the image writing determination on the second surface of the first sheet (1B) is performed. When the determination result of the image writing is “OK”, the driving of the registration motor 130 is started at a time Ta when a predetermined amount of loop is formed on the first sheet (1B), and the registration roller pair 42 conveys the first sheet (1B). To start.

  Here, in order to improve the productivity of the image forming apparatus 1, the transport speed (400 mm / s) of the registration roller pair 42 before the sheet reaches the secondary transfer unit is set to the transport speed (process speed, 300 mm / s). After accelerating to a speed corresponding to 400 mm / s, the registration motor 130 is decelerated to 300 mm / s before the sheet reaches the secondary transfer unit.

  Here, since it is considered that the image writing on the first sheet (1B) is permitted before the time Tb when the second sheet (2B) reaches the second double-sided roller pair 73, the double-side motor 180 is not stopped. Then, the accelerometer 130 is accelerated again in accordance with the drive start time Ta. That is, the double-sided motor 180 is re-accelerated so that the third double-sided roller pair 74 rotates at a peripheral speed of 400 mm / s, which is the same as the target speed of the registration roller pair 42, and further decelerated in accordance with the deceleration of the registration motor 130. Thus, the conveyance speed of the first sheet (1B) by the pair of registration rollers 42 and the pair of third double-sided rollers 72 is controlled.

  On the other hand, the second sheet (2B) is conveyed at a predetermined speed (300 mm / s) following the first sheet (1B) by the first double-sided roller pair 72 by the reverse rotation drive of the reversing motor 170 (FIG. 8A). )reference). After the double-side sensor 76 detects the front end of the second sheet (2B), the reversing motor 170 controls the conveyance speed of the first double-sided roller pair 72 when the second sheet (2B) enters the second double-sided roller pair 73. The speed is reduced so that V2 = 200 mm / s.

  Here, the second double-sided roller pair 73 is driven at a transport speed (300 mm / s) higher than the first double-sided roller pair 72, and a one-way clutch is provided between the reversing motor 170 and the first double-sided roller pair 72. 75 are provided. For this reason, when the second sheet (2B) enters the second double-sided roller pair 73, the one-way clutch 75 slides and the second sheet (2B) is pulled out from the first double-sided roller pair 72 (see FIG. 8B). . Further, even if the sheet interval is shorter than the timing chart of FIG. 9 and the second sheet (2B) enters the second double-sided roller pair 73 during the formation of the loop of the first sheet (1B), V2> V1 Therefore, the subsequent sheet (2B) is smoothly pulled out. That is, even when the duplex motor 180 is driven at a higher speed than the reversing motor 170, the transfer of the second sheet (2B) is performed smoothly, and the first sheet (1B) and the second sheet (2B) are It is in a state of being transported by the driving force.

  Even if the reversing motor 170 rotates forward after the second sheet (2B) is transferred to the second double-sided roller pair 73 by the action of the one-way clutch 75, the driving force is transmitted to the second double-sided roller pair 73. Not done. For this reason, after the second sheet (2B) has entered the second pair of double-sided rollers 73, pulling out the second sheet (2B) is not hindered even if the reversing motor 170 is rotated forward. Therefore, as shown in FIG. 8B, the reversing roller pair 71 rotates the third sheet (3A) by rotating the reversing motor 170 forward before the next third sheet (3A) reaches the reversing roller pair 71. ) Can be started to start reverse conveyance.

(2. When standby processing of the first surface is performed)
Next, an operation when the standby processing of the first surface is performed will be described. FIGS. 10A to 10D show the operation when the result of S111 in FIG. 7 is Yes, that is, when the second sheet (2B) waits in the reversing unit. FIG. 11 shows a timing chart of each motor regarding the conveyance control of the first to third sheets. However, in FIG. 11, the sheet conveying direction in the two-sided path 79 with respect to the reversing motor 170 is represented by a positive conveying speed, and the conveying direction before the reversing roller pair 71 reverses the sheet is represented by a negative conveying speed.

  In this case, even if the second sheet (2B) reaches the reversing roller pair 71, the image writing on the first side of the third sheet (3A) is not permitted, and the standby processing of the first side is performed ( S111: Yes). As shown in FIG. 11, after the first sheet (1B) reaches the standby position of the two-sided pass 79 and the driving of the two-sided motor 180 is stopped (T1), the image writing on the first side of the third sheet (3A) is performed. Is not permitted and enters a standby state (T2). That is, the pre-registration motor 120 is stopped at the time (T2) when the third sheet (3A) is abutted against the stopped registration roller pair 42 by the pre-registration roller pair 41 (see FIG. 3) to form a loop (T2), and the stopped state of the registration motor 130 is changed. Will be maintained.

  When the driving of the pre-registration motor 120 is stopped, the second sheet is being conveyed by the pair of reversing rollers 71, but the conveyance is stopped after the rear end of the sheet reaches the branch from the image forming path 59 to the two-sided path 79 ( T3). In other words, the driving of the reversing motor 170 is stopped in a state where the rear end of the second sheet has approached the nip position of the reversing roller pair 71 by a predetermined distance (30 mm) (T3). As a result, as shown in FIG. 10A, the first to third sheets are stopped (corresponding to T4 in FIG. 11).

  Thereafter, when the image writing on the first surface of the third sheet (3A) is permitted, first, the driving of the pre-registration motor 120 and the registration motor 130 is started (T5), and the third sheet (3A) is sent to the secondary transfer unit. (FIG. 10B). When the third sheet (3A) is conveyed by a predetermined distance (for example, when it enters the secondary transfer section), the reverse rotation of the reversing motor 170 is started (T6), and the second sheet (2B) is carried into the double-sided path 79. (FIG. 10 (c)). Further, the drive of the double-sided motor 180 is started so as to leave a predetermined interval from the rear end of the third sheet (3A) (T7), and the first sheet (1B) is conveyed toward the registration roller pair 42 (T7). FIG. 10 (d)).

  After the driving of the double-sided motor 180 is started, the second sheet (2B) enters the second double-sided roller pair 73, and the second sheet (2B) is continuously transported by the second double-sided roller pair 73. In other words, the drive restart timing (T6) of the reversing motor 170 is set to start driving the registration motor 130 such that the second sheet (2B) is delivered to the second pair of double-sided rollers 73 while the double-sided motor 180 is rotating. It is set later than the timing (T5). Since the state shown in FIG. 10D is the same as “1. When the standby process is not performed”, the description is omitted. Similarly to “1. When the standby process is not performed”, the transport speed of the first double-sided roller pair 72 when the second sheet (2B) enters the second double-sided roller pair 73 is set to the second double-sided roller. The reversing motor 170 may be decelerated so as to be slower than the transport speed of the pair 73.

(3. When standby processing of the second surface is performed)
Next, an operation when the standby processing of the second surface is performed will be described. FIGS. 12A to 12D show the case where the result of step S116 in FIG. 7 is Yes, that is, the state in which the front end of the second sheet (2B) is in contact with the second double-sided roller pair 73 and the first sheet ( 1B) shows the operation in the case of waiting for the image writing permission on the second surface of 1B).

  FIG. 12A shows the same state as FIG. 8A. The first sheet (1B) is conveyed by the third pair of double-sided rollers 74 toward the registration roller pair 42 at a predetermined speed (300 mm / s). ing. The second sheet (2B) is conveyed at a predetermined speed (300 mm / s) by the first pair of double-sided rollers 72 by the reverse rotation of the reversing motor 170.

  Here, as shown in FIG. 13, at the timing when the second sheet (2B) reaches the second double-sided roller pair 73 (Tb in FIG. 13), the determination result of the image writing on the first sheet (1B) is “No”. ". That is, unlike “1. When the standby process is not performed”, the timing at which the image writing permission for the first sheet (1B) is issued is such that the second sheet (2B) reaches the second pair of double-sided rollers 73. It is later than the timing (Tb <Ta). In this case, in a state where the first sheet (1B) is sandwiched between the third pair of double-sided rollers 74 and the front end of the first sheet (1B) contacts the registration roller pair 42 to form a predetermined amount of loop, the double-sided motor 180 Is stopped (FIG. 12B).

  After the double-side sensor 76 detects the front end of the second sheet (2B), the reversing motor 170 adjusts the conveyance speed of the first double-sided roller pair 72 when the second sheet (2B) enters the second double-sided roller pair 73. The speed is reduced so that V2 = 200 mm / s. Since the driving of the second double-sided roller pair 73 by the double-sided motor 180 is stopped, the front end of the second sheet (2B) is abutted against the second double-sided roller pair 73. The reversing motor 170 is stopped in a state where the second sheet (2B) contacts the second pair of double-sided rollers 73 to form a loop (FIG. 12B).

  Here, since the one-way clutch 75 is disposed between the reversing motor 170 and the first double-sided roller pair 72, the next sheet third sheet (3A) is conveyed while the second sheet (2B) is on standby. It is possible to start (FIG. 12B). That is, after the second sheet (2B) comes into contact with the second double-sided roller pair 73, the second double-sided roller pair 73 holds the second sheet (2B) even if the reversing motor 170 is stopped or switched to normal rotation. Can be pulled out. Therefore, by rotating the reversing motor 170 forward after the second sheet (2B) is conveyed to the standby position (see FIG. 13), the reversing roller pair 71 can receive the third sheet (3A). Note that the positive and negative definitions of the transport speed of the reversing motor in FIG. 13 are the same as in FIG. The reversing motor 170 stops the reversing roller pair 71 with the third sheet (3A) conveyed to a predetermined standby position, that is, a position where the sheet is reversed (see FIG. 12C).

  As described above, when the waiting time for permitting the image writing on the first sheet (1B) is long, a maximum of three sheets (1B, 2B, 3B) are on standby in the reverse transport unit 70. . In other words, in a configuration in which the number of driving sources is smaller (for example, two) than the maximum standby number (for example, three), the sheets of the maximum standby number stand by at appropriate standby positions. Then, the drive of the double-sided motor 180 is started at the timing (Ta) when the determination of the image writing on the first sheet (1B) becomes “OK”, so that the second sheet (2B) together with the first sheet (1B). Is restarted (see FIGS. 12C and 12D).

  As described above, in the present embodiment, when the first sheet (1B) is on standby, the second sheet (2B) is transported automatically by the restart of the duplex motor 180 to the position where the conveyance of the second sheet (2B) is automatically restarted. (2B) is made to stand by in a transported state. In other words, in a state where the first sheet (1B) is held by the fourth conveying means (74) and the driving of the second driving source (180) is stopped, the second sheet (2B) is moved to the third conveying means ( A first operation (S118) for transporting the sheet to a position where the sheet comes into contact with (73) is performed. Therefore, after the first operation, the second driving source performs the second operation (S119) for starting the driving of the third conveying unit and the fourth conveying unit, so that the first and second sheets in the standby state are executed. Can be restarted.

  On the other hand, the transmission of the driving force of the first driving source (170) to the second sheet via the second conveying means (72) can be restricted by the restricting means (75). While the second sheet is on standby, the conveyance of the third sheet can be independently controlled. That is, by driving the first conveying means (71) by the first driving source in a state where the transmission of the force from the first driving source to the second sheet is restricted, the third conveying means (3A) is driven by the first conveying means. ). Thus, a configuration in which three sheets are made to wait and conveyance is restarted as necessary can be realized using two motors. That is, with the configuration of the present embodiment, the number of motors can be reduced without reducing the number of sheets that can be circulated in the duplex printing operation, and the cost of the product can be reduced.

  Note that FIGS. 11 and 12 have described the case where all three sheets are in the standby state. However, before the third sheet (3A) reaches the reversal standby position, image writing on the first sheet (1B) is performed. When permitted, the third sheet (3A) does not have to be turned over. That is, in this case, the third sheet (3A) is reversely conveyed by the reversing roller pair 71 and sent to the double-sided path 79 without stopping the driving of the reversing motor 170. In other words, according to the configuration of the present embodiment, during the execution of the second operation (that is, in a state where the second sheet is being pulled out from the second conveying unit), the first driving unit drives the first conveying unit. Thus, the conveyance of the third sheet can be started.

  Here, a state where the second sheet (2B) is abutted against the second pair of double-sided rollers 73 will be described with reference to FIG. FIG. 14A is a schematic diagram showing a state in which the succeeding sheet (2B) is abutted against the second pair of double-sided rollers 73 by the standby process and the driving of the reversing motor 170 is stopped, and FIG. It is an enlarged view of the area | region shown.

  As shown in FIG. 14B, the reversing motor 170 has a state in which the front end of the second sheet (2B) abuts on the nip portion N2 of the second pair of double-sided rollers 73 (a state in which a loop is formed). The first pair of double-sided rollers 72 is driven so as to be conveyed to a certain position. That is, after the double-sided sensor 76 detects the second sheet (2B), the reversing motor 170 controls the reversing motor 170 so that a predetermined amount of feeding is performed even after the front end of the second sheet (2B) reaches the second double-sided roller pair 73. Drive control. In the state where the loop is formed, the sheet P is nipped at the nip portion N1 of the first double-sided roller pair 72, so that the front end of the second sheet (2B) is pushed into the nip portion N2 of the second double-sided roller pair 73. Such force is generated. Therefore, when the driving of the second double-sided roller pair 73 by the double-sided motor 180 is started, the second sheet (2B) is securely held between the second double-sided roller pair 73, and is conveyed together with the first sheet (1B). Will be resumed.

  Here, as shown by the broken line in FIG. 14B, when the front end of the second sheet (2B) has an extremely large curl or bend that cannot be assumed normally, the second double-sided roller pair 73 is used. May not sufficiently enter the nip portion N2. In this case, the conveyance of the second sheet (2B) may not be started even when the driving of the two-sided motor 180 is started. Therefore, when the second sheet (2B) is put on standby in the re-conveying path, it is desirable to stop the conveyance of the second sheet (2B) at a position where the second sheet (2B) does not contact the second pair of double-sided rollers 73 when unnecessary. In the present embodiment, as described above, the second sheet (2B) is caused to stand by by the reversing roller pair 71 in “2. When the standby processing of the first surface is performed”. Risk can be avoided.

(4. Operation during continuous printing)
The behavior of sheets when a standby process is performed when performing double-sided printing on a large number of sheets continuously will be described. As shown in FIG. 4C, in the configuration of the present embodiment, when performing double-sided printing continuously on small-sized sheets, the Nth sheet forms an image on the first side (N: A). After the image is formed and before the image is formed on the second surface (N: B), images for four pages are formed.

  Among them, the behavior of the N-th sheet in the case of (ii) or (iii) is “2. When the standby processing of the first surface is performed” or “3. When the standby processing of the second surface is performed” This is the same as the second sheet (2B) described in (2). That is, in the case of (ii) where the standby process is performed on the first surface (N + 1: A) of the (N + 1) th sheet, the Nth sheet waits while being held by the pair of reversing rollers 71. In the case of (iii) where the standby process is performed on the second surface (N-1: B) of the (N-1) th sheet, the Nth sheet waits while being in contact with the second double-sided roller pair 73. I do.

  When the standby process is performed on the second surface (N-2: B) of the N-2 sheet, the behavior of the N-th sheet in the case of (i) is described in “3. This is the same as the case of the third sheet. That is, the N-th sheet is moved by the reversing roller pair 71 after the preceding (N-1) -th sheet comes into contact with the second double-sided roller pair 73 (the position of the second sheet 2B in FIG. 12B). The apparatus waits while being conveyed a predetermined distance in the normal rotation direction (the position of the third sheet 3B in FIG. 12C).

  In the case of (iv) where the standby process is performed on the first surface (N + 2: A) of the (N + 2) th sheet, the behavior of the Nth sheet is as described in “2. The case is the same as the first sheet. That is, the N-th sheet waits while being sandwiched by the third pair of double-sided rollers 74 (the position of the first sheet 1B in FIG. 10A). Thereafter, when the registration roller pair 42 starts conveying the (N + 2) th sheet, the Nth sheet is fed to the registration roller pair 42 with a predetermined interval at the rear end of the (N + 2) th sheet (FIG. 10 ( d)).

  In this manner, the position of each sheet when performing double-sided printing on a large number of sheets continuously can be appropriately managed.

(Effects of the present embodiment)
As described above, in this embodiment, the standby processing of the first surface of the succeeding sheet is performed in a configuration in which the preceding sheet can be made to stand by in a state of being in contact with the second pair of double-sided rollers 73 if necessary. In this case, the preceding sheet is made to wait without being turned over. That is, after the image formation on the first surface of the second sheet, when the conveyance of the third sheet (3A) by the registration roller pair 42 is stopped, the reversing roller pair 71 passes the second sheet (2B) through the two-sided path. The conveyance of the second sheet (2B) is stopped in a state where the sheet is not reversed to 79. Thereafter, the image writing permission for the third sheet (3A) is issued, and the image forming units PY to PK start the creation of the image to be formed on the first surface of the third sheet (3A). The roller pair 71 conveys the second sheet (2B) to the double-sided path 79. In other words, after the image is formed on the first surface of the preceding sheet, when the image formation is not performed on the first surface of the succeeding sheet and the conveyance of the succeeding sheet is stopped, the preceding sheet is moved to the second conveyance path. The first transporting means stops without being reversed in the direction. Thereafter, based on the timing when the image forming unit starts the image forming operation (S112) for forming an image on the first surface of the succeeding sheet, the operation of causing the first conveying unit to convey the preceding sheet to the second conveying path is performed. Be executed.

  This makes it possible to appropriately manage the sheet position when forming images on both sides of the sheet so as to minimize the risk of transport failure.

  Note that the image forming start timing in the present embodiment is set at the time when the exposure device writes an electrostatic latent image on the surface of the photosensitive drum in any of the image forming units PY to PK (normally, the yellow image forming unit PY). Indicates the timing when the write operation is first started. Further, in the example shown in FIG. 11, the conveyance of the succeeding sheet (3A) is started immediately after the issuance of the image writing permission for the succeeding sheet (3A) (T5), and thereafter the conveyance of the preceding sheet (2B) is restarted (T6). But not limited to this. If it is after the image forming start timing for the subsequent sheet (3A), the conveyance of the preceding sheet (2B) toward the second conveyance path may be started before the conveyance of the subsequent sheet is started.

  Starting the reversal of the preceding sheet based on the image forming start timing of the succeeding sheet also has the following advantages. Generally, when a sheet is conveyed, the conveyance timing is shifted due to various factors. For example, reduction in outer diameter due to outer diameter tolerance or wear of the transport roller, delay or early arrival due to slip of the transport roller, and the like. These transport deviations can be absorbed by, for example, abutting the sheet against the registration roller pair 42 and the second double-sided roller pair 73 and then starting driving the roller pair at a predetermined timing. In particular, when the transport path is long, the risk of colliding with the front and rear sheets is reduced by providing many locations for absorbing the transport deviation. On the other hand, the configuration becomes more complicated as more locations for absorbing the transport deviation are provided. In the present embodiment, this is equivalent to absorbing the above-described conveyance deviation in the reversing section, and contributes to achieving both the simplicity of the configuration and the conveyance stability.

  Therefore, starting the reversal of the preceding sheet based on the image formation start timing of the succeeding sheet is applicable not only to the configuration of the reversing conveyance unit 70 of the present embodiment. For example, a motor for driving the first double-sided roller pair 72 may be provided separately from the reversing motor 170 and the double-sided motor 180, and the standby position of three sheets may be controlled by three driving sources. In this case, the operation of abutting the second sheet (2B) against the second double-sided roller pair 73 and waiting for the second sheet (2B) described in “3. When the standby process for the second surface is performed” is not performed, and the second sheet (2B) is not performed. ) Stands by upstream of the second double-sided roller pair 73. Also in this configuration, by using the present technology, both the simplicity of the configuration and the stability of transport can be achieved. In addition, the present technology may be applied to a configuration in which four or more sheets are on standby in the reversing transport unit 70 or a configuration in which two sheets are on standby in the reversing transport unit 70.

  In the first embodiment, the configuration is such that the reversal of the preceding sheet is started based on the image formation start timing of the subsequent sheet. However, the present invention is not limited to this. From the viewpoint of avoiding abutment against the second double-sided roller pair 73 as much as possible, after the preceding sheet is reversed, the conveyance of the preceding sheet may be stopped in a state where the preceding sheet has not reached the second double-sided roller pair 73. . Hereinafter, this embodiment will be described with reference to FIG. However, components having the same configuration and operation as those of the first embodiment are denoted by the same reference numerals as those of the first embodiment, and description thereof is omitted.

  FIG. 15 is a timing chart of each motor corresponding to FIG. 11 in the above embodiment. The operation until the pair of reversing rollers 71 conveys the second sheet (2B) by a predetermined distance is the same as in “2. When the standby processing of the first surface is performed” in the first embodiment, T1 and T21, and T2 and T22. , T3 and T23 correspond to each other.

In this embodiment, after the reversing roller pair 71 conveys the second sheet (2B) in the normal direction for a predetermined distance and stops for a predetermined time (settling time 100 ms, T23 to T24), conveyance in the reverse direction starts. (T24). The conveyance in the reverse rotation direction is started regardless of whether the image writing permission for the third sheet (3A) has been issued.

  When the state where the image writing permission of the third sheet (3A) has not been issued continues, the driving of the reversing motor 170 is stopped in a state where the second sheet (2B) has been conveyed immediately before the second double-sided roller pair 73. (T25). The stop timing of the reversing motor 170 is set so that the second sheet (2B) stops at a position where the second sheet (2B) does not contact the second double-sided roller pair 73, for example, by referring to the detection result of the double-sided sensor 76.

  Thereafter, when the image writing permission for the first surface of the third sheet (3A) is issued, the conveyance of the third sheet (3A) starts (T26). Further, the conveyance of the first sheet (1B) and the conveyance of the second sheet (2B) are restarted based on the elapsed time from the start of conveyance of the third sheet (3A). However, since the standby position of the second sheet (2B) is set on the downstream side of the reversing conveyance unit 70 compared to the first embodiment, the conveyance restart of the second sheet (2B) is set later than the first embodiment. Is done. That is, after the conveyance of the first sheet (1B) is started at a predetermined interval from the rear end of the third sheet (3A) (T28), the reversing motor 170 starts to rotate in the reverse direction (T29). Accordingly, the second sheet (2B) is transferred from the first double-sided roller pair 72 to the second double-sided roller pair 73 in a state where the second double-sided roller pair 73 is rotating, and the risk of poor conveyance or the like is reduced. .

  As described above, when the standby processing is performed on the first surface of the third sheet, the state where the second sheet has not reached the second pair of double-sided rollers 73 (third transport unit) after the second sheet is inverted. Then, the conveyance of the second sheet is stopped. Thereafter, the conveyance of the second sheet is restarted based on the image formation start timing for the first surface of the third sheet, and the sheet is delivered to the third conveyance unit. As a result, similarly to the first embodiment, it is possible to appropriately manage the sheet position so as to suppress the risk of a conveyance failure or the like.

(Other embodiments)
In the first and second embodiments, the one-way clutch 75 is used as the restricting unit that can restrict the transmission of the driving force of the first driving source to the sheet via the second conveying unit. It may be replaced with another clutch mechanism that can be disconnected. Also, a mechanism for switching the second double-sided roller pair 73 between a contact state in which the sheet is nipped and a separated state in which the sheet is nipped (for example, a mechanism which is connected to at least one of the roller shafts of the roller pair and has a center distance between the roller pairs) May be used as the regulating means.

  In the above-described embodiment, the conveyance is stopped in a state where the front end of the sheet (2B) is abutted against the second pair of double-sided rollers 73 to form a loop. The conveyance may be stopped at the timing when the nip position of the two-sided roller pair 73 is reached. However, in order to cope with a variation in the sheet position due to the variation in the transport speed, the stability of the transport of the sheet can be ensured by forming a loop as in the above embodiment. The feeding amount for forming the loop was preferably, for example, 5 mm. In addition, the sheet guide can be reduced by curving the conveyance guide constituting the portion where the loop of the double-sided path 79 is formed to secure a loop space allowing the loop of the sheet. Instead of forming a loop in the sheet (2B), for example, a clutch mechanism is arranged in a drive transmission path from the double-sided motor 180 to the second double-sided roller pair 73, and the front end of the subsequent sheet is set to the nip of the double-sided roller pair 73 during standby. It may be configured to enter the section.

  In the above embodiment, the reversing motor 170 and the double-sided motor 180 each drive two pairs of rollers, but one or both motors may drive three or more pairs of rollers. For example, a roller pair may be added between the second double-sided roller pair 73 and the third double-sided roller pair 74, or a roller pair may be added between the reversing roller pair 71 and the first double-sided roller pair 72. Good. However, when the sheet is held in the standby state and a roller pair is added at a position where the sheet is pulled out by the second double-sided roller pair 73, a regulating means such as a one-way clutch is required.

  In addition, the speed control and the specific drive speed of each motor described with reference to FIGS. 9, 11, 13, and 15 can be appropriately changed. For example, in the above-described embodiment, the driving of the registration motor 130 is started without stopping the driving of the double-sided motor 180 when there is no need to wait the sheet by the registration roller pair 42 (see FIG. 8). However, instead of such a so-called non-stop registration, the driving of the double-sided motor 180 may be temporarily stopped after the loop is formed.

  In the above embodiment, a motor is used as a drive source, but an actuator other than a motor may be used as the first to third drive sources. For example, a path including one common motor and transmitting a driving force to the reversing roller pair 71 and the first double-sided roller pair 72 and a path transmitting the driving force to the second double-sided roller pair 73 and the third double-sided roller pair 74 May be provided. In this case, each of the drive transmission members configured to be able to independently control the drive state by an electromagnetic clutch or the like corresponds to a drive source.

  The present invention supplies a program for realizing one or more functions of the above-described embodiments to a system or an apparatus via a network or a storage medium, and one or more processors in a computer of the system or the apparatus read and execute the program. This processing can be realized. Further, it can be realized by a circuit (for example, an ASIC) that realizes one or more functions.

DESCRIPTION OF SYMBOLS 1 ... Image forming apparatus / 10 ... Image forming means (image forming engine) / 42 ... Registration means (pair of registration rollers) / 59 ... First conveyance path (image forming path) / 71 ... First conveyance means (reversing roller) Pair) / 72 ... Second conveying means (first double-sided roller pair) / 73 ... Third conveying means (second double-sided roller pair) / 75 ... Regulator (one-way clutch) / 79 ... Second conveying path (double-sided path) / 170: first drive source (reversing motor) / 180: second drive source (double-sided motor) / 200: control means (control unit)

Claims (15)

A first transport path;
Image forming means for forming an image on a sheet passing through the first conveyance path;
A first transport unit that receives a sheet from the first transport path and reversely transports the sheet;
A second conveyance path for guiding the sheet inverted by the first conveyance means to the first conveyance path,
A first drive source for driving the first transport unit;
Control means for controlling the first drive source,
The control means,
In the case where an image is formed on the first surface of the preceding sheet, the first surface of the succeeding sheet that is fed to the first conveyance path following the preceding sheet, and the second surface of the preceding sheet,
After the image is formed on the first surface of the preceding sheet, when creation of an image to be formed on the first surface of the succeeding sheet is not permitted, the preceding sheet is reversed in a direction toward the second transport path. Without stopping the first transport means,
Then, based on the timing at which the image forming unit starts creating an image to be formed on the first surface of the succeeding sheet, the first conveying unit conveys the preceding sheet to the second conveying path.
An image forming apparatus comprising: The first transport unit transports the sheet received from the first transport path in the first direction for a predetermined distance, and then transports the sheet in the second direction opposite to the first direction, thereby transporting the sheet to the second transport path. And
The control unit is configured to, when creation of an image for forming on the first surface of the succeeding sheet is not permitted before the first conveying unit conveys the preceding sheet in the first direction by the predetermined distance, Stopping the first transport unit without transporting the preceding sheet in the second direction;
The image forming apparatus according to claim 1, wherein: A registration unit disposed in the first conveyance path and conveying a sheet to the image forming unit;
A second transport unit that is disposed in the second transport path and is configured to be driven by the first drive source to transport the sheet inverted by the first transport unit;
A third transport unit disposed on the second transport path and transporting the sheet sent from the second transport unit toward the registration unit;
A second drive source for driving the third transport unit;
Regulating means capable of regulating drive transmission from the first drive source to the second transport means,
The control means, after stopping the first conveyance means without reversing the preceding sheet, when restarting the conveyance of the preceding sheet,
A standby state is made in a state where the preceding sheet is brought into contact with the third conveying means in a stopped state, and then, in a state in which the drive transmission to the second conveying means is regulated by the regulating means, the third conveying means A first mode for transporting the preceding sheet;
And a second mode in which the preceding sheet is transferred from the second conveying means to the driving third conveying means without bringing the preceding sheet into contact with the stopped third conveying means. is there,
The image forming apparatus according to claim 1, wherein: The restricting means is a one-way clutch,
The image forming apparatus according to claim 3, wherein: The control means,
The first surface of the first sheet, the first surface of the second sheet that is fed to the first transport path following the first sheet, and the first sheet that is fed to the first transport path after the second sheet When an image is formed on the first surface of the third sheet, the second surface of the first sheet, and the second surface of the second sheet,
After the image is formed on the first surface of the second sheet, if the creation of the image formed on the first surface of the third sheet is not permitted, the first conveyance without reversing the second sheet is performed. Stop the means,
After the image is formed on the first surface of the third sheet, if creation of the image formed on the second surface of the first sheet is not permitted, the second sheet is turned over and the third transport unit is turned over. Stopping the second transport means after transporting to a position where the second transport means contacts
The image forming apparatus according to claim 3, wherein: The control means,
When resuming conveyance of the preceding sheet after stopping the first conveyance unit without reversing the preceding sheet, an image is transferred to another sheet conveyed through the second conveyance path prior to the preceding sheet. The registration unit is configured to transfer the preceding sheet from the second conveyance unit to the third conveyance unit after the second driving source starts rotation of the third conveyance unit for forming. Delaying the restart of conveyance of the preceding sheet compared to the start of conveyance of the succeeding sheet,
The image forming apparatus according to claim 3, wherein: The control means,
In order to form the second conveyance path prior to the preceding sheet on another sheet to be conveyed before the preceding sheet reaches the third conveying means after the conveyance of the preceding sheet is restarted. Executing the first mode on the preceding sheet when the creation of the image is not permitted;
If the creation of an image to be formed on the other sheet is permitted between the time when the conveyance of the preceding sheet is restarted and the time when the preceding sheet reaches the third conveyance means, Performing the second mode on the other hand,
The image forming apparatus according to any one of claims 3 to 6, wherein: A first transport path;
Image forming means for forming an image on a sheet passing through the first conveyance path;
A first transport unit that receives a sheet from the first transport path and reversely transports the sheet;
A second conveyance path for guiding the sheet inverted by the first conveyance means to the first conveyance path,
A third transport unit disposed on the second transport path and transporting the sheet inverted by the first transport unit toward the first transport path;
A first drive source for driving the first transport unit;
A second drive source for driving the third transport unit;
Control means for controlling the first drive source and the second drive source,
The control means,
In the case where an image is formed on the first surface of the preceding sheet, the first surface of the succeeding sheet that is fed to the first conveyance path following the preceding sheet, and the second surface of the preceding sheet,
After the image is formed on the first surface of the preceding sheet, when creation of an image to be formed on the first surface of the succeeding sheet is not permitted, the leading end of the preceding sheet in the sheet conveying direction of the second conveying path In a state before reaching the third conveying means, the first driving source is stopped to stop the conveyance of the preceding sheet,
Thereafter, based on the timing at which the image forming unit starts creating an image to be formed on the first surface of the succeeding sheet, the driving of the first drive source is started to transfer the preceding sheet to the third conveying unit. Hand over to
An image forming apparatus comprising: The control unit is configured to, after an image is formed on the first surface of the preceding sheet, when the creation of an image to be formed on the first surface of the succeeding sheet is not permitted, transfer the subsequent sheet to the second conveyance path. Stopping the conveyance of the preceding sheet without reversing in the direction toward
The image forming apparatus according to claim 8, wherein: The control means, after an image is formed on the first surface of the preceding sheet, when the creation of an image for forming on the first surface of the succeeding sheet is not permitted, the first conveying means Inverting, and stopping the conveyance of the preceding sheet in a state where the preceding sheet has not reached the third conveying means,
The image forming apparatus according to claim 8, wherein: A registration unit disposed in the first conveyance path and conveying a sheet to the image forming unit;
A second transport unit that is disposed on the second transport path and is driven by the first drive source to transport the sheet inverted by the first transport unit toward the third transport unit;
Regulating means capable of regulating drive transmission from the first drive source to the second transport means,
The control means, after stopping the conveyance of the preceding sheet in a state where the preceding sheet has not reached the third conveyance means, when restarting the conveyance of the preceding sheet,
A standby state is made in a state where the preceding sheet is brought into contact with the third conveying means in a stopped state, and then, in a state in which the drive transmission to the second conveying means is regulated by the regulating means, the third conveying means A first mode for transporting the preceding sheet;
And a second mode in which the preceding sheet is transferred from the second conveying means to the driving third conveying means without bringing the preceding sheet into contact with the stopped third conveying means. is there,
The image forming apparatus according to claim 8, wherein: The restricting means is a one-way clutch,
The image forming apparatus according to claim 11, wherein: The control means,
The first surface of the first sheet, the first surface of the second sheet that is fed to the first transport path following the first sheet, and the first sheet that is fed to the first transport path after the second sheet When an image is formed on the first surface of the third sheet, the second surface of the first sheet, and the second surface of the second sheet,
After the image is formed on the first surface of the second sheet, if the creation of the image formed on the first surface of the third sheet is not permitted, the second sheet reaches the third transport unit. The first driving source is stopped in a state where the second sheet is not conveyed,
After the image is formed on the first surface of the third sheet, if the creation of the image formed on the second surface of the first sheet is not permitted, the second sheet is brought into contact with the third transport unit. Stopping the first driving source after being transported to the position,
The image forming apparatus according to claim 11, wherein: The control means,
When the first driving source is stopped and the conveyance of the preceding sheet is stopped in a state where the preceding sheet has not reached the third conveyance unit, the conveyance of the preceding sheet is restarted. The second driving source starts the rotation of the third conveying unit after the second driving source starts the rotation of the third conveying unit to form an image on the second surface of another sheet conveyed along the second conveying path. Delaying resumption of conveyance of the preceding sheet as compared with the start of conveyance of the succeeding sheet by the registration means, such that the preceding sheet is delivered to the third conveyance means;
The image forming apparatus according to claim 11, wherein: The control means,
In order to form the second conveyance path prior to the preceding sheet on another sheet to be conveyed before the preceding sheet reaches the third conveying means after the conveyance of the preceding sheet is restarted. Executing the first mode on the preceding sheet when the creation of the image is not permitted;
If the creation of an image to be formed on the other sheet is permitted between the time when the conveyance of the preceding sheet is restarted and the time when the preceding sheet reaches the third conveyance means, Performing the second mode on the other hand,
The image forming apparatus according to claim 11, wherein:

Images