JP2010095341A - Image forming device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming device capable of miniaturizing a re-conveying unit. <P>SOLUTION: After a sheet with an image being formed therein is drawn in by a multiple conveying path 5, the sheet is inverted, drawn in a double-side inversion path 6 branched from the multiple conveying path 5, and the inverted sheet drawn in the double-side inversion path 6 is conveyed from a re-feed path 7 to an image forming unit 101. A downstream end in the sheet draw-in direction of the multiple conveying path 5 is merged with the re-feed path 7. When a toner image is superposed on a toner image on the sheet, the sheet is conveyed to the re-feed path 7 without inverting the sheet. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an image forming apparatus, and more particularly to a configuration of a re-conveying unit that conveys a sheet on which a toner image is formed to an image forming unit again.

  2. Description of the Related Art In recent years, image forming apparatuses using electrophotographic methods such as copying machines and printers have been used in various fields, and accordingly, the demand for image quality has become higher and higher. For example, an image forming apparatus capable of forming a color image may be used not only for normal color image printing but also for applications such as decoration and packaging.

  In this case, by forming a transparent toner image on the toner image of a sheet on which an image has already been formed, it is possible to form a uniform glossy image from a white background portion to a solid portion. In addition, by temporarily fixing a color toner image and partially forming a transparent toner image on the color toner image, it is possible to express a mark king such as a watermark, an eye catch, a security mark, or the like.

  By the way, in such an image forming apparatus that forms a transparent toner image on a toner image, the sheet on which the toner image is formed is conveyed again to the image forming unit so that multiple printing can be performed in addition to double-sided printing of the sheet. A re-conveying section is provided (see Patent Document 1).

  FIG. 6 is a diagram showing a schematic configuration of a conventional image forming apparatus provided with such a re-conveying unit. The image forming apparatus includes an image reading unit 50, an image forming unit 51 including a photosensitive drum 31, a primary charger 33, a cleaning device 34, and the like, and a sheet feeding unit that feeds sheets P from sheet cassettes C1 and C2. A portion 52 is provided. The image forming apparatus also includes a transfer conveyance unit 53 that conveys the sheet P to a transfer unit A constituted by the photosensitive drum 31 and the transfer roller 35, and a fixing conveyance that conveys the transferred sheet P to the fixing device 54. A portion 55 is provided.

  Further, this image forming apparatus includes a discharge processing unit 56 that performs sheet discharge switching at the time of out-of-machine discharge and double-sided / multiple transfer, and double-sided / multiple that performs sheet path switching for double-sided or multiple transfer and reverse of the sheet P. And a processing unit 57. The image forming apparatus also includes a switchback path 62 that pulls in the sheet P during double-sided transfer, and a double-sided / multiple-conveying path 58 that transports the sheet P to the transfer / conveying unit 53 again during double-sided or multiplex image formation.

  The re-conveying unit 60, when performing the double-side / multi-processing unit 57, the switchback path 62, the double-side / multi-conveying path 58, and the multi-printing, the multi-print dedicated path for conveying the sheet P to the double-side / multi-conveying path 58 63 or the like.

  When forming an image on a sheet, first, in the image forming unit 51, a laser exposure apparatus corresponding to an image forming signal generated based on image information transmitted from an external device (not shown) or the image reading unit 50. The laser beam L is irradiated from 59 to the photosensitive drum 31. As a result, an electrostatic latent image is formed on the photosensitive drum, and the electrostatic latent image is developed by the developing device 32 to form a toner image on the photosensitive drum.

  Next, the sheet P is conveyed by the registration roller pair 41 provided in the transfer conveyance unit 53 in synchronization with the toner image on the photosensitive drum 31. In the transfer portion A, a transfer bias is applied to a transfer roller 35 disposed opposite to the photosensitive drum 31, so that the toner image on the photosensitive drum 31 is conveyed onto the sheet P conveyed in time. The toner image is transferred.

  Next, the sheet P to which the toner image has been transferred is conveyed to the fixing device 54 by the fixing conveyance unit 55, and image fixing is performed. Then, the sheet P on which the image is fixed after passing through the fixing device 54 passes through the discharge processing unit 56 and is discharged to the outside of the apparatus or sent to the double-side / multi-processing unit 57 according to the operation sequence.

  Here, during double-sided or multiple transfer, the sheet P is route-changed by the first route switching device 42 of the discharge processing unit 56 and conveyed to the double-side / multi-processing unit 57. Further, by the second path switching device 43, at the time of multiple transfer, it is transported to the duplex / multiple transport path 58 via the multiple print dedicated path 63.

  Further, during double-side transfer, the sheet is sent to the switchback path 62 by the second path switching device 43 and the reverse roller 44. After that, by reverse rotation of the reverse roller 44 and switching of the second path switching device 43, the leading edge and the trailing edge are reversed and the sheet is conveyed to the double-sided / multiple conveying path 58 with the front and back reversed. Then, the sheet P whose front end and rear end are reversed and turned upside down is sent again from the double-side / multiple conveyance path 58 to the transfer conveyance unit 53, and the next image transfer is performed.

  Note that, when the both sides are reversed, the switchback path 62 into which the sheet enters becomes long to accommodate a large size sheet. However, when the switchback path 62 is lengthened in this way, the apparatus becomes large. For this reason, for example, there is a configuration in which the switchback path 62 and the duplex / multiple conveyance path 58 are made continuous so that the leading edge of the sheet conveyed when the both sides are reversed can enter the duplex / multiple conveyance path 58. (See Patent Document 2).

JP 2004-045476 A Japanese Patent No. 3242054

  However, in such a conventional image forming apparatus, the re-conveying unit 60 includes a multiple print dedicated path 53 used when performing multiple printing and a switchback path 62 used when performing double-sided printing. The size is increased. When the re-conveying unit 60 is increased in size, the image forming apparatus main body is also increased in size.

  Accordingly, the present invention has been made in view of such a current situation, and an object of the present invention is to provide an image forming apparatus capable of downsizing a re-conveying unit.

  The present invention provides an image forming apparatus comprising: an image forming unit; and a reconveying unit that conveys a sheet on which a toner image has been formed by the image forming unit to the image forming unit. A first conveyance path for reversing and conveying the sheet on which the sheet is formed, and a second conveyance path for drawing the sheet branched from the first conveyance path and reversed by the first conveyance path A reversing sheet drawn into the second conveying path to the image forming unit, and a third conveying path where the first conveying path merges, and superimposes the toner image on the toner image on the sheet When forming the sheet, the sheet drawn into the first conveyance path is conveyed from the first conveyance path to the third conveyance path without being inverted.

  As in the present invention, when a toner image is superimposed on an image-formed sheet, the re-conveying unit can be reduced in size by conveying the sheet to the third conveying path without inverting it. it can.

  Hereinafter, the best mode for carrying out the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a diagram showing a schematic configuration of a color image forming apparatus which is an example of an image forming apparatus according to an embodiment of the present invention.

  In FIG. 1, reference numeral 100 denotes a color image forming apparatus, and 100A denotes a color image forming apparatus main body (hereinafter referred to as apparatus main body). The apparatus main body 100 </ b> A includes an image forming unit 101, a sheet feeding unit 102 that feeds a sheet P, and a sheet conveyance device 104 that conveys the sheet P from the sheet feeding unit 102. The apparatus main body 100 </ b> A includes a secondary transfer unit 105 that secondarily transfers the toner image formed by the image forming unit 101 to the sheet P conveyed by the sheet conveying device 104.

  Here, the image forming unit 101 includes an image forming unit 200 (200Y, 200M, 200) that is five image forming units of yellow (Y), magenta (M), cyan (C), black (Bk), and transparent (T). 200C, 200K, 200T). Each of these image forming units 200 includes a photosensitive drum 120 (120Y, 120M, 120C, 120K, 120T) and a primary charger 121 (121Y, 121M, 121C, 121K, 121T). Each image forming unit 200 includes a developing device 123 (123Y, 123M, 123C, 123K, 123T) and the like.

  That is, the color image forming apparatus 100 according to the present embodiment is of an intermediate transfer tandem system in which transparent and four-color image forming units 200 are arranged side by side on an intermediate transfer belt described later as the image forming unit 101. Note that the colors formed by each image forming unit 200 are not limited to these four colors, and the color arrangement order is not limited to this.

  The sheet feeding unit 102 includes sheet feeding cassettes C1 and C2 and a sheet feeding roller 102a that feeds the sheets P stored in the sheet feeding cassettes C1 and C2. The secondary transfer unit 105 is stretched by rollers such as a drive roller 106, a tension roller 107, and a secondary transfer inner roller 108, and is driven by an intermediate transfer belt 125 that is driven in the direction of an arrow and a secondary transfer outer roller 126. It is formed.

  In FIG. 1, reference numeral 2 denotes a reversing discharge refeeding unit that is a refeeding unit for performing reversal discharging, refeeding, etc. of the sheet P to the image forming unit 101. Reference numeral 122 (122Y, 122M, 122C, 122K, 122T) denotes an exposure apparatus, and 124 (124Y, 124M, 124C, 124K, 124T) denotes a primary transfer apparatus. Reference numeral 150 denotes a control unit which controls an image forming operation in each mode of the color image forming apparatus 100 described later.

  In the color image forming apparatus 100 having such a configuration, when an image is formed, first, the surface of the photosensitive drum 120 is uniformly charged by the primary charger 121 in advance. Thereafter, the exposure device 122 emits a laser beam L based on an image information signal sent from an external device (not shown), and the photosensitive drum 120 is irradiated with this laser beam L, whereby the surface of the photosensitive drum is irradiated. A latent image is formed.

  Next, toner development is performed on the electrostatic latent image formed on the photoconductive drum 120 by the developing device 123 to form a toner image on the photoconductive drum. In a normal image forming operation, an image is formed by four image forming units 200 (200Y, 200M, 200C, and 200K) of yellow (Y), magenta (M), cyan (C), and black (Bk). Is done. Thereafter, a predetermined pressure and electrostatic load bias are applied by the primary transfer device 124, and the toner image is transferred onto the intermediate transfer belt 125.

  Note that image formation by each image forming unit 200 is performed at the timing of superimposing on the upstream toner image primarily transferred onto the intermediate transfer belt. As a result, a full-color toner image is finally formed on the intermediate transfer belt 125.

  Further, the sheet P is fed from the sheet feeding cassettes C1 and C2 by the sheet feeding roller 102a, and then conveyed to the registration roller pair 112 by the conveying roller 111 of the sheet conveying apparatus 104. At this time, the rotation of the registration roller pair 112 is stopped, and the skew of the sheet P is corrected by abutting the sheet P against the nip of the registration roller pair 112 in the stopped state. Thereafter, the sheet P is conveyed to the secondary transfer unit 105 by rotating the registration roller pair 112 in synchronization with the rotation of the intermediate transfer belt 125.

  Next, by applying a predetermined pressure and an electrostatic load bias to the secondary transfer outer roller 126, the full-color toner image on the intermediate transfer belt 125 is secondarily transferred onto the sheet P in the secondary transfer unit 105. The Thereafter, the sheet P on which the toner image is secondarily transferred is conveyed to the fixing device 127 by the conveyance belt 113. In the fixing device 127, the toner is melted and fixed on the sheet P by being pressurized and heated, and then the sheet P having the fixed image passes through the discharge conveyance path 131 and is discharged by the discharge roller pair 132. It is discharged to the tray 133.

  In addition, when the image is formed on both sides of the sheet P, the image is further superimposed on the image of the sheet, or the sheet is discharged with the image side down, the discharge switching member 130 is switched. Yes. Then, the sheet P on which the toner image is fixed by the switching of the discharge switching member 130 is conveyed to the reverse discharge refeed unit 2.

  The reversing discharge refeeding unit 2 as a reconveying unit includes a reconveying path 3, a reversing discharging path 4, a multiple conveying path 5, a double-side reversing path 6, and a refeeding path 7. Here, the re-conveying path 3 is a path branched from the discharge conveying path 131, and the reverse discharging path 4 is branched from the re-conveying path 3, and the sheet P is discharged onto the discharge tray with the image surface facing down. It is a path to do.

  For example, when forming an image on both sides of the sheet P or discharging the image P with the image side down, the multiple conveyance path 5 pulls the sheet P that has passed through the re-conveyance path 3 and then reverses and conveys the sheet P. The 1st conveyance path for this is comprised.

  The double-side reversing path 6 branches from the multiple conveyance path 5 and constitutes a second conveyance path for drawing the sheet reversed by the multiple conveyance path 5 when images are formed on both sides of the sheet P. The refeed path 7 transports the sheet to the secondary transfer unit 105 (image forming unit 101) again when forming an image on both sides of the sheet P or when forming an image on the image. It constitutes the third transport path.

  A reversing roller pair 9 is provided at the boundary between the re-conveying path 3 and the multiple conveying path 5 to convey the sheet that has passed through the re-conveying path 3 to the multiple conveying path 5 or the reverse discharge path 4. The reversing roller pair 9 includes a reversing roller 9a and a reversing roller 9b pressed against the reversing roller 9a by pressing means (not shown). The reversing roller 9a is rotated forward and backward by a motor (not shown). It is possible. The reversing roller 9 a rotates (reverses) clockwise when the sheet P is conveyed to the reversal discharge path 4, and rotates (forwards) counterclockwise when it is conveyed to the multiple conveyance path 5.

  A reverse switching member 10 for switching the transport path is provided at a branch point of the transport path 3 with the reverse discharge path 4. The reversal switching member 10 is configured so that the sheet P can pass from the multiple conveyance path 5 side to the reverse discharge path 4 side, but cannot pass from the multiple conveyance path 5 side to the reconveyance path 3 side. It is a directional valve system.

  When the reverse roller pair 9 (the reverse roller 9a) is reversed by the reverse switching member 10 as described above, the sheet P conveyed to the multiple conveyance path 5 by the normal rotation of the reverse roller pair 9 is the reverse discharge path 4. Enter. Thereafter, the paper is discharged to the discharge tray 133 by the discharge roller pair 132.

  The multiple conveyance path 5 is formed so that the downstream end in the sheet drawing direction joins the refeed path 7. Further, a double-sided reversing roller pair 11 is provided between a branch point between the multiple conveyance path 5 and the double-sided reversing path 6 and a junction point between the multiple conveying path 5 and the refeed path 7.

  The double-sided reversing roller pair 11 includes a double-sided reversing roller 11a and a double-sided reversing roller 11b pressed against the double-sided reversing roller 11a by pressing means (not shown). The double-sided reversing roller 11a is not shown. The motor can rotate forward and backward. The double-side reversing roller 11a rotates (forwardly) counterclockwise as indicated by an arrow A to reverse the sheet when the sheet P conveyed to the multiple conveyance path 5 is conveyed to the refeed path 7. When fed into the double-sided reversing path 6, it is rotated (reversed) clockwise.

  Here, a double-sided reversing switching member 12 for switching the transporting path is provided at a branch point between the multiple transporting path 5 and the double-sided reversing path 6. The double-sided reversing switching member 12 allows the sheet P to pass from the multiple transport path 5 side to the double-sided reverse path 6 side, but not from the multiple transport path 5 side to the re-transport path 3 side. It is a one-way valve system.

  Then, the sheet P conveyed to the multiple conveyance path 5 is reversed and conveyed to the double-side reversal path 6 by the forward / reverse rotation of the double-side reversal switching member 12 and the double-side reversal roller pair 11 or the refeed path. 7 is selected. That is, when the double-side reversing roller pair 11 (the double-side reversing roller 11a) reverses, the sheet P is guided by the double-side reversing switching member 12 and conveyed to the double-side reversing path 6. When the double-side reversing roller pair 11 (the double-side reversing roller 11a) rotates in the forward direction, the sheet P is conveyed to the re-feeding path 7 via the multiple conveyance path 5.

  Here, the double-sided reversing path 6 is formed continuously with the re-feeding path 7 so as to transport the sheet (reversed sheet) P received from the multiple transport path 5 to the re-feeding path 7. A duplex conveying roller pair 13 for conveying to the feeding path 7 is provided. The pair of double-sided conveyance rollers 13 includes a double-sided conveyance roller 13a and a double-sided conveyance roller 13b pressed against the double-sided conveyance roller 13a by a pressing unit (not shown). The double-sided conveyance roller 13a can be rotated clockwise by a motor (not shown).

  In FIG. 1, reference numeral 14 denotes a first refeed roller pair that is provided on the downstream side of the merging portion of the multiplex transport path 5 and the refeed path and is a sheet transport section that transports the sheet. Then, by the first refeed roller pair 14, the sheet P is received from the multiple conveyance path 5 or the double-sided reversal path 6 to the refeed path 7, and is transferred to the second refeed roller pair 15 of the refeed path 7. Transport.

  Here, in the present embodiment, the length between the branch point of the multiplex conveyance path 5 and the double-side reversal path 6 and the junction between the multiplex conveyance path 5 and the refeed path 7 is defined as the sheet conveyance direction of the sheet. It is shorter than the length. For this reason, when the sheet P is drawn into the multiple conveyance path 5, the leading end of the drawn sheet P reaches the first refeed roller pair 14.

  The second refeed roller pair 15 includes a second refeed roller 15a and a second refeed roller 15b that is pressed against the second refeed roller 15a by pressing means (not shown). Has been. The second refeed roller 15a can be rotated counterclockwise by a motor (not shown).

  The first refeed roller pair 14 includes a first refeed roller 14a and a first refeed roller 14b that is pressed against the first refeed roller 14a by pressing means (not shown). Yes. A clutch gear 16 that is a drive transmission unit is provided at an end of the first re-feed roller 14 a and is connected to a motor 17 that is a drive unit via the clutch gear 16.

  When the sheet P is conveyed to the second refeed roller pair 15 by the first refeed roller pair 14, the first refeed roller 14 a is rotated in the arrow direction A together with the clutch gear 16 by driving the motor 17. To do. Conversely, when the sheet P is conveyed from the multiple conveyance path 5 to the double-side reversal path 6 by the double-side reversing roller pair 11, the clutch gear 16 that selectively transmits the driving of the motor 17 is idled and the driving of the motor 17 is restarted. Transmission to the feeding roller 14a is interrupted.

  For this reason, when the sheet P is conveyed by the double-side reversing roller pair 11, the refeed roller 14a is driven to rotate in the direction of arrow B by the sheet P as shown in FIG.

  That is, in the present embodiment, when the sheet P is reversed and conveyed by the double-sided reversing roller pair 11, the sheet P is conveyed to the double-sided reversing path 6 while the first refeed roller 14a is driven to rotate. Note that after the sheet P is conveyed to the duplex reversing path 6 in this way, the sheet P is conveyed to the refeed path side by the duplex conveyance roller pair 13 and reaches the first refeed roller pair 14, but the leading end is the first re-feed. Before reaching the feed roller pair 14, the trailing edge of the sheet P passes through the first refeed roller pair 14.

  For this reason, when the sheet P reaches the first refeed roller pair 14, the sheet P is sent to the refeed path 7 by the first refeed roller pair 14 rotating by the drive of the motor 17, and thereafter Then, the sheet is re-fed to the secondary transfer unit 105 by the second re-feed roller pair 15 and the conveying roller 111. In this embodiment, the clutch gear 16 is used as the drive transmission unit, but a one-way clutch or the like may be used.

  By the way, the color image forming apparatus 100 forms an image on one side, forms the image on one side, forms the image on the one side, and places the image side on the lower side. A back side discharge mode is provided for discharging the sheet P onto the discharge tray. In addition, a multiple print mode is provided in which after a toner image is formed on one side of the sheet P, a transparent toner image is formed on the surface on which the image is formed, and a partial gloss difference is imparted to the image after fixing.

  Further, a double-sided print mode for forming images on both sides of the sheet P is provided. In addition, a double-sided multiplex print in which a toner image is fixed on the surface of the sheet P, a transparent toner image is formed on the surface on which the image is formed, a toner image is further fixed on the back surface, and then a transparent toner image is formed on the back surface image. Has a mode.

  Next, a sheet conveying operation and a sheet discharging operation in each mode of the color image forming apparatus 100 will be described. Each mode can be set by an operation unit (not shown).

  First, the front paper discharge mode will be described.

  When the front paper discharge mode is selected, the discharge switching member 130 is switched to the position shown in FIG. As a result, the sheet P on which the color toner image is fixed on one side passes through the discharge conveyance path 131 and is discharged to the discharge tray 133 by the discharge roller pair 132.

  Next, the back side paper discharge mode will be described.

  When the back side paper discharge mode is selected, first, the discharge switching member 130 is switched to the position shown in FIG. As a result, the leading edge of the sheet P on which the color toner image is fixed on one side is conveyed to the reverse paper discharge re-feeding unit 2.

  Next, when the post-fixing sensor 128 detects the trailing edge of the sheet P, a second sheet conveyance speed (2 in the present embodiment at the time of image formation) that is faster than the first sheet conveyance speed that is the sheet conveyance speed at the time of image formation. The reverse roller pair 9 is rotated in the direction of arrow A at a double speed). Further, the double-side reversing roller pair 11 of the multiple conveyance path 5 is rotated in the arrow A direction at the second sheet conveyance speed. Note that productivity is improved by increasing the sheet conveyance speed of the post-fixing sheet P in this way.

  Then, the leading edge of the sheet P is drawn into the multiple conveyance path 5 by the reversing roller pair 9 and the double-sided reversing roller pair 11 that rotate in the direction of arrow A at the second sheet conveying speed. Depending on the size of the sheet P, the leading end of the sheet P reaches the refeed path 7. Thereafter, the re-feed sensor 8 provided in the re-feed path 3 detects the rear end of the sheet P, and thereafter, after a predetermined time has passed, the rear end of the sheet P is changed between the transport path 3 and the reverse discharge path 4. After passing through the branch point, the reverse roller pair 9 and the double-sided reverse roller pair 11 are temporarily stopped.

  Next, as shown in FIG. 4, the reverse roller pair 9 and the double-sided reverse roller pair 11 are rotated in the arrow B direction at the second sheet conveyance speed. As a result, the sheet P is guided to the reverse discharge path 4 from the reverse switching member 10 having the above-described configuration, passes through the reverse discharge path 4, and is discharged to the discharge tray 133 by the discharge roller pair 132 with the image surface facing downward. The Note that the sheet P can be output without changing the page order by discharging the sheet P with the image surface facing down.

  Next, the multiple print mode will be described.

  When the multiple print mode is selected, the discharge switching member 130 is first switched to the position shown in FIG. As a result, the leading edge of the sheet P on which the color toner image is fixed on one side is conveyed to the reverse paper discharge re-feeding unit 2. Next, when the post-fixing sensor 128 detects the trailing edge of the sheet P, the reverse roller pair 9 is rotated in the arrow A direction at the second sheet conveyance speed. Further, the double-side reversing roller pair 11 of the multi-pass 5 and the first re-feed roller pair 14 and the second re-feed roller pair 15 of the re-feed path 7 are rotated in the arrow A direction at the second sheet transport speed.

  Then, the reversing roller pair 9, the double-sided reversing roller pair 11, and the first refeed roller pair 14 that rotate in the direction of arrow A at the second sheet conveying speed cause the leading edge of the sheet P to pass through the multi-conveying path 5 and the refeed path 7 is sent to the re-feed standby position. Thereafter, the reverse roller pair 9, the double-sided reverse roller pair 11, the first refeed roller pair 14, and the second refeed roller pair 15 are temporarily stopped, and the rotation is resumed at a predetermined timing. As a result, the sheet P is conveyed by the reverse roller pair 9, the double-side reverse roller pair 11, the first refeed roller pair 14, and the second refeed roller pair 15.

  Next, the sheet P is transported to the registration roller pair 112 by the transport roller 111, and the skew is corrected by hitting the nip of the registration roller pair 112, and then transported to the secondary transfer unit 105 at the first sheet transport speed. Is done. Thereafter, the secondary transfer unit 105 transfers the transparent toner image formed by the image forming unit 200T to the same surface as the surface on which the color toner image is fixed.

  Next, the sheet P on which the transparent toner image has been transferred is conveyed to the fixing device 127, and is heated and pressed by the fixing device 127, whereby the unfixed toner image is melted and fixed. The fixed sheet P is discharged by the discharge switching member 130 to the discharge tray 133 by the discharge roller pair 132 through the discharge conveyance path 131. Then, after fixing the color toner image on one side of the sheet P in this way, a transparent toner image is formed on the surface on which the image is formed, whereby a partial gloss difference can be imparted to the image after fixing.

  Next, the double-sided print mode will be described. In the double-sided printing mode, images are formed on both sides of the sheet P. In the present embodiment, a full transparent toner image is formed and fixed on a color toner image, and a white background portion is thereby formed. A uniform glossy double-sided image can be formed up to a solid portion.

  When the duplex printing mode is selected, the discharge switching member 130 is first switched to the position shown in FIG. As a result, the leading edge of the sheet P on which the color toner image is fixed on one side is conveyed to the reverse paper discharge re-feeding unit 2.

  Next, when the post-fixing sensor 128 detects the trailing edge of the sheet P, the reversing roller pair 9, the duplex reversing roller pair 11 in the multiple conveying path 5, and the first refeeding in the refeeding path 7 at the second sheet conveying speed. The roller pair 14 is rotated in the direction of arrow A. As a result, the sheet P is fed into the multiple conveyance path 5. Thereafter, the re-feed sensor 8 detects the rear end of the sheet P, and when a predetermined time elapses and the rear end of the sheet P passes through the branch point between the double-side reverse path 6 and the multiple conveyance path 5, the pair of double-side reverse rollers 11 and the first refeed roller pair 14 are temporarily stopped.

  Next, as shown in FIG. 2, the duplex reversing roller pair 11 and the duplex conveying roller pair 13 are rotated in the arrow B direction at the second sheet conveying speed. As a result, the sheet P is guided to the double-side reversal path 6 from the double-side reversal switching member 12 having the above-described configuration.

  At this time, the first re-feed roller pair 14 is dragged by the sheet P and rotated in the direction of arrow B. Here, when the first refeed roller pair 14 is conveyed while being dragged in this way, the sheet P is in a state where a tension corresponding to the idling torque of the first refeed roller is applied. Then, the sheet P is conveyed in a state where a tension corresponding to the idling torque is applied, so that the curling of the sheet P caused by the contraction of the toner can be corrected.

  Next, when the trailing edge of the sheet P conveyed to the double-side reversing path 6 passes through the first refeed roller pair 14, the clutch gear 16 of the first refeed roller 14a is operated, and the first refeed roller The pair 14 rotates in the arrow A direction at the second sheet conveyance speed. Accordingly, when the sheet P conveyed by the double-side reversing roller pair 11 and the first refeed roller pair 14 thereafter reaches the first refeed roller pair 14, the sheet P becomes the first refeed roller pair 14. Thus, the sheet is conveyed toward the second refeed roller pair 15.

  Then, the sheet P that has reached the second refeed roller pair 15 in this way is conveyed by the second refeed roller pair 15 to the refeed standby position of the refeed path 7. Note that after the sheet P is conveyed to the re-feed standby position, the double-side reversing roller pair 11, the first re-feed roller pair 14, and the second re-feed roller pair 15 are temporarily stopped.

  Next, the first refeed roller pair 14 and the second refeed roller pair 15 are rotated at a predetermined timing. As a result, the sheet P is conveyed to the registration roller pair 112 by the conveyance roller 111, abuts against the nip of the registration roller pair 112 and corrects skew, and is conveyed to the secondary transfer unit 105 at the first sheet conveyance speed. The Thereafter, the secondary transfer unit 105 transfers the toner image to the surface opposite to the surface on which the color toner image is fixed.

  Next, the sheet P carrying the toner image in the secondary transfer unit 105 is conveyed to the fixing device 127 and heated and pressed by the fixing device 127 to melt and fix the unfixed toner image. Thereafter, the fixed sheet P is discharged by the discharge switching member 130 to the discharge tray 133 through the discharge conveyance path 131 and the discharge roller pair 132.

  Next, the duplex printing mode will be described. In the double-sided multiple print mode, first, a transparent toner image is transferred onto the same side as the surface on which the color toner image is fixed in the same manner as the multiple print mode described above. Next, an image is formed on the surface opposite to the surface on which the color toner image is fixed in the same manner as in the double-sided print mode described above, and then the transparent toner image is converted into a color toner image in the same manner as in the multiple print mode. Transfer to the same surface as the fixed surface.

  As described above, in the duplex printing mode, after fixing the color toner image on the surface of the sheet P, the transparent toner image is formed on the surface on which the image is formed, and the color toner image is fixed on the back surface on which the transparent toner image is formed. Thereafter, a transparent toner image is formed on the surface on which the back image is formed. Thereby, a partial gloss difference can be given to the image after fixing on both the front surface and the back surface. That is, in the double-sided multiple print mode, after the multiple printing of the front surface, the sheet P is reversed by the double-sided reversal path 6 and the back side is printed in the multiple print mode, thereby minimizing the number of double-sided reversals of the sheet P, We try to keep productivity as low as possible.

  Next, the control of the control unit 150 when forming a transparent toner image will be described with reference to the flowchart shown in FIG.

  First, the control unit 150 checks whether there is an instruction for the double-sided printing mode, the double-sided multiple printing mode, or the multiple printing mode from the operation unit (S801), and if there is an instruction (Y in S801), the transparent toner image data Is created (S802). Next, it is confirmed whether there is a transparent toner image (S803). Here, when there is a transparent toner image (Y in S803), a color toner image is formed, and the re-fed sheet is processed in the same manner as a normal image forming process to form a transparent toner image on the sheet. A transparent toner image forming process is performed (S810). Then, the transparent toner image is fixed by conveying the sheet P on which the transparent toner image is thus formed to the fixing device.

  Next, after the transparent toner image is fixed on the sheet, it is determined whether or not double-sided transparent toner image formation is performed on the sheet (S811). That is, it is determined whether the duplex printing mode is instructed. When double-sided transparent toner image formation is performed (Y in S811), as described above, the sheet on which the transparent toner image is multiply formed on one side is reversed from the double-sided reversal path 6 to the refeed path 7. Then, a double-sided conveyance process is performed (S812). Further, the transparent toner image data on the back surface is generated (S813).

  Next, the presence / absence of a transparent toner image on the back surface is determined (S814). If there is a transparent toner image on the back surface (Y in S814), the multiple conveyance for conveying the sheet from the multiple conveyance path 5 to the refeed path 7 is performed. Processing is performed (S815). Next, a transparent toner image forming process is performed on the back surface (S816), and thereafter, an external discharge process is performed (S807). If the transparent toner image is not formed on the back side, the outside discharge processing is performed as it is (S807).

  As described above, in the present embodiment, the multiple conveyance path 5 and the refeed path 7 are merged, and the sheet is reversed during multiple printing in which a toner image is formed on an image-formed sheet. It is made to convey to the refeed path | pass 7 without making it. That is, the multiplex transport path 5 is also used as a multiplex printing transport path, whereby the reverse discharge re-feeding unit 2 can be reduced in size. As a result, the entire image forming apparatus having the double-sided and multiple printing functions can be reduced in size.

Incidentally, as described above, in this embodiment, in order to improve productivity, the sheet conveyance speed of the sheet P is increased immediately after the sheet P passes through the fixing device 127. Here, when the multiple conveyance path 5 is narrow, particularly when a sheet such as thin paper of 80 g / m ² or less is conveyed, the leading end of the sheet is conveyed while receiving the sliding resistance of the multiple conveyance path 5. . As a result, sheet buckling and undulation are likely to occur in the multiple conveyance path. Furthermore, the longer the sheet length, the worse the sheet buckling and waviness.

  On the other hand, when the entire surface is covered with a transparent toner and fixed to achieve uniform gloss from a white background portion to a solid portion, a transparent toner image is formed on the color toner image, so the amount of toner on the sheet increases. When the amount of toner on the sheet increases in this manner, the amount of curling of the sheet caused by toner contraction becomes larger than when only the color toner image is fixed.

  In order to suppress such curling of the sheet, it is desirable to correct the curl by making the space of the multiple conveyance path 5 as small as possible. That is, in order to correct the sheet curl, it is desirable to make the space of the multiple conveyance path 5 as narrow as possible. However, when the space of the multiplex conveyance path 5 is narrowed in this way, curling can be corrected and downsizing can be achieved, but as described above, sheet buckling and undulation are likely to occur.

  Therefore, in the present embodiment, the interval between the multiplex conveyance paths 5 is widened to prevent the occurrence of sheet buckling and undulation, and the first re-connection is formed at the junction between the multiplex conveyance path 5 and the refeed path 7. A feed roller pair 14 is provided. The first re-feed roller pair 14 conveys the sheet with the vicinity of the leading end of the sheet held therebetween, thereby reducing sheet buckling and undulation.

  Further, as described above, in the double-sided printing mode, the first refeed roller pair 14 is driven to rotate by being dragged by the sheet P. Therefore, the tension corresponding to the idling torque of the first refeed roller is applied to the sheet P. It is conveyed in the state where it was done. Therefore, it is possible to correct the toner curl generated by the contraction of the toner.

  In the above description, the full-color intermediate transfer type image forming apparatus has been described as an example of the image forming apparatus. However, the present invention is not limited to this, and may be a monochrome image forming apparatus.

1 is a diagram illustrating a schematic configuration of a color image forming apparatus which is an example of an image forming apparatus according to an embodiment of the present invention. FIG. 4 is a diagram illustrating sheet movement in the double-sided printing mode of the color image forming apparatus. FIG. 4 is a diagram illustrating sheet movement when the color image forming apparatus is in a front-surface discharge mode. FIG. 4 is a diagram illustrating sheet movement in a back side discharge mode of the color image forming apparatus. 6 is a flowchart for forming a transparent toner image of the color image forming apparatus. 1 is a diagram illustrating a schematic configuration of a conventional image forming apparatus.

Explanation of symbols

2 Reversing discharge refeeding section 3 Reconveying path 4 Reverse discharging path 5 Multiple conveying path 6 Double-side reversing path 7 Refeeding path 14 First refeed roller pair 16 Clutch gear 100 Color image forming apparatus 101 Image forming section 150 200 Image forming unit P sheet

Claims (5)

An image forming apparatus comprising: an image forming unit; and a re-conveying unit that conveys the sheet on which the toner image is formed by the image forming unit to the image forming unit again.
The re-transport unit is
A first transport path for transporting the sheet on which the image is formed after being reversed;
A second conveyance path for drawing the sheet branched from the first conveyance path and reversed by the first conveyance path;
A reversing sheet drawn into the second transport path is transported to the image forming unit, and a third transport path where the first transport path joins,
When a toner image is formed on a toner image on a sheet so as to overlap, the sheet drawn into the first conveyance path is conveyed from the first conveyance path to the third conveyance path without being inverted. Image forming apparatus.   The length between the branch point of the first conveyance path and the second conveyance path and the junction between the first conveyance path and the third conveyance path is made shorter than the length of the sheet in the sheet conveyance direction. The image forming apparatus according to claim 1. A sheet conveyance unit for conveying the sheet to the junction between the first conveyance path and the third conveyance path;
3. The sheet conveyance unit according to claim 2, wherein when the sheet drawn into the first conveyance path is reversed and conveyed to the second conveyance path, the sheet conveyance unit is driven to rotate by the reversely conveyed sheet. Image forming apparatus. A driving unit for driving the sheet conveying unit;
A drive transmission unit that selectively transmits the drive of the drive unit to the sheet conveying unit,
The drive transmission unit transmits the drive of the drive unit to the sheet conveyance unit when conveying a sheet, and when the sheet is reversed and conveyed, the sheet conveyance unit is driven by the reversely conveyed sheet. The image forming apparatus according to claim 3, wherein the image forming apparatus is configured to rotate.   The image forming apparatus according to claim 1, wherein the image forming unit includes an image forming unit that forms a transparent toner image.

Images