Image-forming device

Abstract

To solve the problem of occurrence of a strike-like image defect in a sheet after fixation.SOLUTION: An image-forming device includes: an image carrier; a transfer section; a fixing section; a pair of inversion rollers movable in a width direction of a sheet; first detection means for detecting the position of the sheet; a pair of conveyance rollers for performing conveyance toward the transfer section; and a control section for controlling the image writing position to the image carrier and movement in the width direction of the pair of inversion rollers. The control section performs control such that a first image writing position and a second image writing position are formed in different positions in a width direction of a sheet, a first travel amount in the width direction of the sheet is determined in the pair of inversion conveyance rollers on the basis of the detection result of the first detection means and the second image writing position, and a side end of a first surface of a sheet having an image formed in the first image writing position and a side end of a second surface of a sheet having an image formed in the second image writing position in the fixing section pass different positions in the width direction of the sheet.SELECTED DRAWING: Figure 10

Description

Technical Field

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

Background Art

[0002] Generally, in an image forming apparatus, a sheet may be laterally displaced in the width direction of the sheet, which is orthogonal to the sheet conveyance direction, while being conveyed. When an image is formed on the sheet in a state where the sheet is laterally displaced, the image is printed shifted from the center in the width direction of the sheet, so that the printing accuracy of the image with respect to the sheet deteriorates. For this reason, a shift roller mechanism for correcting lateral displacement in the width direction of the sheet is known.

[0003] It is desirable that the shift amount of the shift roller mechanism provided immediately before the transfer unit be minimized. This is because, for downsizing of the apparatus, the sheet is also sandwiched by the upstream roller pair during the shift operation, and if the shift amount becomes large, the conveyed sheet will skew due to "twisting" of the sheet.

[0004] Patent Document 1 discloses an image forming apparatus provided with a second shift roller mechanism in a re-conveying unit that reverses the front and back and conveys the sheet to the image forming unit again in order to minimize the shift amount of the shift roller mechanism immediately before the transfer unit.

[0005] Further, Patent Document 1 discloses a fixing device that fixes a toner image on a sheet by applying heat and pressure to the sheet on which an unfixed toner image is formed. In particular, the fixing member (roller or belt) of the fixing device is formed of a resin having relatively good releasability and flexibility, such as PFA or PTFE, in order to prevent toner adhesion from the sheet.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0007] However, in the image forming apparatus disclosed in Patent Document 1, although it is possible to perform high-precision position correction in the width direction corresponding to any sheet, as a result, there is a possibility that the end portion of the sheet in the width direction is repeatedly conveyed to the same position downstream of the shift roller mechanism.

[0008] In this case, when the sheet passes through the fixing nip portion, the surface of the fixing member of the fixing device may be damaged in the rotational direction (circumferential direction) by the paper edge generated at the cut end of the sheet end during cutting of the sheet. This damage tends to become deeper and larger as the end portion of the sheet repeatedly passes through the same position with respect to the fixing member. As a result, when a deeper and larger damage is generated on the surface of the fixing member, when a sheet having the maximum size width with respect to the width direction of the sheet passes through the fixing nip, the position of the damage overlaps the printing area of the sheet, and streak-like image defects occur on the sheet after fixing.

[0009] Therefore, an object of the present invention is to provide an image forming apparatus that prevents enlargement of damage on the surface of a fixing member that comes into contact with a conveyed sheet.

Means for Solving the Problems

[0010] One aspect of the invention for solving the above problems includes an image carrier that carries an image, an exposure unit that exposes the image writing position of the image carrier, a transfer unit that transfers the image formed on the image carrier to a sheet, a fixing unit that fixes the image transferred by the transfer unit to the sheet, a pair of reversing rollers that sandwich the sheet fixed by the fixing unit to reverse the front and back of the sheet and are movable in the width direction of the sheet perpendicular to the sheet conveyance direction, a first detection means provided in the vicinity of the pair of reversing rollers for detecting the position of the sheet in the width direction of the sheet, a pair of conveyance rollers that convey the sheet reversed by the pair of reversing rollers toward the transfer unit, and a control unit that controls the image writing position on the image carrier and the movement of the pair of reversing rollers in the width direction. The control unit controls the first image writing position for forming an image on the first surface of the sheet on the image carrier and the second image writing position for forming an image on the second surface of the sheet on the opposite side of the first surface of the sheet on the image carrier to be formed at different positions in the width direction of the sheet. Based on the detection result of the first detection means and the second image writing position, the first movement amount of the pair of reversing rollers in the width direction of the sheet is determined, and the side end of the first surface of the sheet having the image formed at the first image writing position and the side end of the second surface of the sheet having the image formed at the second image writing position in the fixing unit pass through different positions in the width direction of the sheet.

Effect of the Invention

[0011] According to the present invention, it is possible to prevent the enlargement of scratches on the surface of the fixing member that comes into contact with the conveyed sheet.

Brief Description of the Drawings

[0012]

Figure 1

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Figure 10

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Figure 13

Mode for Carrying Out the Invention

[0013] [Embodiment 1] Hereinafter, an image forming apparatus according to an embodiment of the present invention will be described with reference to the drawings. The image forming apparatus according to the present embodiment is an intermediate transfer system in which a toner image is primarily transferred to an intermediate transfer belt and then secondarily transferred to a sheet, such as a copying machine, a printer, a facsimile machine, and these multifunction devices. In the following embodiments, an intermediate transfer type image forming apparatus in which four-color image forming units are arranged on an intermediate transfer belt will be used for explanation.

[0014] [Configuration of the Entire Image Forming Apparatus] Printer 1 as an image forming apparatus is a full-color laser beam printer using the electrophotographic method. As shown in FIG. 1, Printer 1 is configured to be divided into a housing 1a and a housing 1b and connected. The housing 1a has paper feed units 10a and 10b for feeding sheets and an image forming unit 90 for forming images. Further, the housing 1a includes extraction units 20a and 20b for transporting the sheets fed by the paper feed unit, and a registration unit 30 for skewing the sheets and transporting them to the image forming unit 90 at the right timing with respect to the sheets. And the housing 1a has a first duplex transport unit 60 for transporting the sheets to transport the sheets on the back surface of the sheets. On the other hand, the housing 1b has a fixing unit 100 for fixing the image, which is a fixing unit, and a cooling unit 110 for cooling the sheets. Further, the housing 1b includes a branch transport unit 120 for branching the transport direction of the sheets, a reverse transport unit 130 for reversing and transporting the sheets, a second duplex transport unit 150 for transporting the sheets, and a decurl unit 170 for correcting the curl of the sheets.

[0015] The image forming unit 90 includes four process cartridges 99Y, 99M, 99C, and 99K for forming toner images of four colors: yellow (Y), magenta (M), cyan (C), and black (K). Further, the housing 1a includes exposure devices 93, 96, 97, and 98, which are exposure units. Note that the four process cartridges 99Y, 99M, 99C, and 99K have the same configuration except that the colors of the images formed are different. For this reason, only the configuration of the process cartridge 99Y and the image forming process will be described, and the description of the process cartridges 99M, 99C, and 99K will be omitted.

[0016] The process cartridge 99Y includes a photosensitive drum 91 as an image carrier, a charging roller (not shown), a developing device 92, and a cleaner 95. The photosensitive drum 91 is configured by applying an organic photoconductive layer to the outer periphery of an aluminum cylinder and rotates by a drive motor (not shown). Further, the image forming unit 90 includes a transfer unit, and an intermediate transfer belt 50 that rotates in the direction of arrow T by a drive roller 52 is provided. The intermediate transfer belt 50 is wound around a tension roller 51, a drive roller 52, and a secondary transfer inner roller 53. Primary transfer rollers 55Y, 55M, 55C, and 55K are provided inside the intermediate transfer belt 50, and a secondary transfer outer roller 54 is provided outside the intermediate transfer belt 50 so as to face the secondary transfer inner roller 53, forming a secondary transfer nip T2 described later.

[0017] The feeding unit 10a includes a lift plate 11a that moves up and down while loading the sheet S, a pickup roller 12a that feeds the sheet S loaded on the lift plate 11a, and a separation roller pair 13a that separates the fed sheets one by one. The feeding unit 10b is provided at a position close to the feeding unit 10a. Similar to the feeding unit 10a, the feeding unit 10b includes a lift plate 11b that moves up and down while loading the sheet S, a pickup roller 12b that feeds the sheet S loaded on the lift plate 11b, and a separation roller pair 13b that separates the fed sheets one by one.

[0018] The extraction units 20a and 20b have extraction roller pairs 20a and 20b for conveying the sheet.

[0019] The registration unit 30 includes a pre-registration roller pair 31 for conveying the sheet and a registration roller pair 32 for correcting the skew of the sheet and conveying it in synchronization with the secondary transfer nip T2 described later. Further, the registration unit 30 includes a registration sensor 33 for detecting the position of the sheet S in the conveying direction and a CIS 34 for detecting the position of the sheet in the width direction.

[0020] The fixing unit 100 forms a fixing nip with a belt member as a fixing member on the upper side and a pressure roller as a roller member facing it on the lower side in the figure, and the roller member is held in a state of being pressed toward the belt member. As an example, the pressure roller has a silicone rubber layer as an elastic layer provided with a uniform thickness on a metal core bar having a positive crown shape with a diameter of 20 mm at the center in the width direction (rotation axis direction) intersecting the sheet conveyance direction and a diameter of 19 mm at the end. A fluororesin (for example, perfluoroalkoxy fluororesin - PFA) with a thickness of 30 μm is provided as a release layer on the surface of the elastic layer. The fixing belt has an elastic layer, which is a heat-resistant rubber layer, formed on the outer periphery of a base layer made of an iron alloy, a nickel alloy, copper, silver, or the like. A release layer is formed on the outer periphery of this elastic layer to enhance the releasability with respect to the toner. The release layer is a fluororesin layer such as PFA or PTFE, and its thickness is formed to be, for example, 30 μm. In the present embodiment, the fixing member is a belt member, but it may also be a roller member.

[0021] The cooling unit 110 has an upper cooling belt 111a that rotates in the direction of arrow T by an upper cooling drive roller 112a. Similarly, it has a lower cooling belt 111b that rotates in the direction of arrow T by a lower cooling drive roller 112b. It also has a heat sink 113 for cooling the sheet.

[0022] Next, the image forming process and the sheet conveying process of the printer 1 configured as described above will be explained. First, the image forming process will be explained. When an image signal is input from a personal computer or the like (not shown) to the exposure device 93, laser light corresponding to the image signal is irradiated from the exposure device 93 onto the photosensitive drum 91 of the process cartridge 99Y. At this time, the surface of the photosensitive drum 91 is uniformly charged in advance to a predetermined polarity and potential by the charging roller, and an electrostatic latent image is formed on the surface when the laser light is irradiated from the exposure device 93 via the mirror 94. The electrostatic latent image formed on the photosensitive drum 91 is developed by the developing device 92, and a yellow (Y) toner image is formed on the photosensitive drum 91. Similarly, laser light is irradiated from the exposure devices 96, 97, and 98 onto the photosensitive drums of the process cartridges 99M, 99C, and 99K, and magenta (M), cyan (C), and black (K) toner images are formed on the respective photosensitive drums. The toner images of each color formed on each photosensitive drum are transferred to the intermediate transfer belt 50 by the primary transfer rollers 55Y, 55M, 55C, and 55K. Then, the full-color toner image is conveyed by the intermediate transfer belt 50 rotated by the drive roller 52 to the secondary transfer nip T2 formed by the secondary transfer inner roller 53 and the secondary transfer outer roller 54. The toner remaining on the photosensitive drum 91 is recovered by the cleaner 95. Note that the image forming process for each color is performed at the timing of overlapping with the upstream toner image primarily transferred onto the intermediate transfer belt 50.

[0023] In parallel with the above-described image forming process, a sheet conveyance process is performed. A sheet S is fed from either of the feeding units 10a and 10b, and the sheet S is conveyed to the registration unit 30 by either of the pulling-out units 20a and 20b. In the registration unit 30, the pre-registration roller pair 31 abuts the leading edge of the sheet S against the registration nip portion of the registration roller pair 32. Thereby, the skew of the leading edge of the sheet S is corrected. Then, it is conveyed to the secondary transfer nip T2 at a predetermined conveyance timing. A full-color toner image on the intermediate transfer belt 50 is transferred onto the first sheet surface (front surface) of the sheet S by the secondary transfer bias applied to the secondary transfer outer roller 54. Also, the residual toner remaining on the intermediate transfer belt 50 is collected by the belt cleaner 56.

[0024] The sheet S onto which the toner image has been transferred is conveyed to the fixing unit 100 by the pre-fixing conveyance unit 57. Then, the sheet S is guided to the fixing nip, and a predetermined heat and pressure are applied to melt and fix (fix) the toner. The sheet S that has passed through the fixing unit 100 is nipped by the upper cooling belt 111a, which is an endless belt, and the lower cooling belt 111b in the cooling unit 110, and the sheet S is conveyed by the rotation of the upper cooling drive roller 112a and the lower cooling drive roller 112b. Then, the sheet S is brought into contact with the heat sink 113 via the belt, and the heat is transferred to the heat sink 113 to cool the sheet S.

[0025] Subsequently, the branch conveyance unit 120 selects a path to convey the sheet to the decal unit 170 to discharge the sheet outside the image forming apparatus or to the reverse conveyance unit 130. After the sheet S is conveyed to the reverse conveyance unit 130, the sheet S can be reversed so that the first sheet surface on which the image was formed at the secondary transfer nip T2 is on the lower side, and the sheet S can be conveyed to the decal unit 170.

[0026] When forming an image on only one side of the sheet S, the sheet S is conveyed from the branch conveyance unit 120 to the decal unit 170, and the curl of the sheet is corrected by a small-diameter hard roller and a large-diameter soft roller. Subsequently, the sheet S that has passed through the decal unit 170 is discharged outside the machine or delivered to a paper discharge option device (not shown).

[0027] When forming an image on both sides of the sheet S, the sheet S is conveyed by the branch conveyance unit 120 to the reverse conveyance unit 130 and is switched back in the reverse conveyance unit 130. The switched-back sheet S is conveyed from the reverse conveyance unit 130 to the second duplex conveyance unit 150 and the first duplex conveyance unit 60 and is guided again to the registration unit 30. Thereafter, an image is formed on the second sheet surface (back surface) of the sheet S at the secondary transfer nip T2, and the sheet is discharged outside the machine via the branch conveyance unit 120 and the decal unit 170. In the image forming apparatus according to the present embodiment, as an example, a central reference sheet conveyance method is adopted in which the center in the width direction of the sheet orthogonal to the sheet conveyance direction in the sheet conveyance path is made to coincide with the center in the width direction of the sheet to convey the sheet, and the description will proceed based on this assumption.

[0028] <Detailed description of the registration unit> Hereinafter, the specific configuration of the registration unit 30 will be described in detail. The registration unit 30 shown in FIG. 2 includes a registration roller pair 32 and a pre-registration roller pair 31. Further, the registration unit 30 has a registration sensor 33 for detecting a sheet and a CIS 34 (Contact Image Sensor) for detecting the side edge in the width direction of the sheet.

[0029] The registration roller pair 32 and the pre-registration roller pair 31 are rotationally driven by the registration drive motor 36 and the pre-registration drive motor 35 shown in FIG. 8. Further, the registration roller pair 32 is movable in the width direction of the sheet while sandwiching the sheet, and is moved by the registration shift motor 37 shown in FIG. 8. That is, the sheet S sandwiched by the registration roller pair 32 is moved in the direction indicated by the arrow C in FIG. 2.

[0030] Upstream of the registration roller pair 32 in the sheet conveyance direction, a CIS 34 for detecting the side edge position of the sheet S in the width direction is arranged. The CIS 34 is arranged at a position offset from the center of the width direction of the sheet S with respect to the conveyance direction indicated by the arrow A in FIG. 2. This is because in the position correction of the sheet S, it is only necessary to detect the side edge position of only one side of the sheet S.

[0031] Further, the CIS 34 is arranged so as to extend in the width direction of the sheet so that it can detect the side edge positions of the sheet S1 with the smallest sheet width and the sheet S2 with the largest sheet width among the sheet sizes permitted to be used in the image forming apparatus. In order not to reduce the detection accuracy of the CIS 34, the position where the CIS 34 is arranged is made as close as possible to the registration roller pair 32 in the sheet conveyance direction. Further, the conveyance guide gap (not shown) of the CIS 34 is made uniform, and it is desirable to provide a space that can accommodate the deflection generated in the sheet between the CIS 34 and the pre-registration roller pair 31. This is because, as described above, deflection is formed between the pre-registration roller pair 31 and the registration roller pair 32 for performing skew correction.

[0032] FIG. 3 is a flowchart showing the processing procedure of the shift processing of the sheet S in the registration unit 30. FIG. 4 is a diagram for explaining the skew correction operation and the width direction position correction operation (horizontal registration correction operation) of the sheet S in the registration unit 30. The shift processing for the sheet S will be described with reference to these figures.

[0033] First, upon receiving a print execution instruction from the user via a computer, a print job is started (S101). Note that the user instructs the number of printed copies and the like, and also specifies the type of sheet to be used for printing. Thereafter, feeding of the sheet S is started (S102), and it is determined whether it is the printing of the first side (the front surface of the sheet) in the print job (S103). If it is determined that it is the printing of the first side (S103: first side), an image is written at the image writing position g1, which is a position determined in advance (S104). Here, g1 is a value based on the result of writing position adjustment performed at the time of factory shipment or the like. On the other hand, the sheet S is conveyed to the pre-registration roller pair 31. Here, it is assumed that the conveyed sheet S is in a state of rotating counterclockwise and skewing with respect to the sheet conveyance direction A as shown in FIG. 4(a). Note that the dotted lines X1 and X2 shown in FIG. 4(a) schematically show both end positions of the sheet S that has been conveyed without skewing and horizontal registration deviation, and at this time, the left side is defined as the positive direction with the side end position in the sheet width direction as the zero point.

[0034] The leading end of the sheet S conveyed by the pre-registration roller pair 31 is abutted against the nip portion of the registration roller pair 32. Thereafter, based on the detection result of the registration sensor 33 that has detected the sheet, conveyance by the pre-registration roller pair 31 is performed by a set feed amount, so that a predetermined amount of deflection is formed in the sheet S. Then, the leading end of the sheet S comes to follow the nip portion of the registration roller pair 32 (S106, FIGS. 4(b)(c)). In this way, skew correction of the sheet S is performed, and when the rotational drive of the registration roller pair 32 is started at a predetermined timing, the sheet S becomes the state shown in FIG. 4(d). (S107) Then, with respect to the sheet S after skew correction of the sheet as shown in FIG. 4(d), the side end position on the leading end side in the conveyance direction is detected by the CIS 34 (S108), and the correction amount in the sheet width direction is determined based on this detection result (L1). The sheet position correction amount in this case can be obtained by subtracting the image writing position (g1) from the detection result (L1) of the CIS 34 (L1 - g1).

[0035] The registration roller pair 32 during conveyance of the sheet S is shifted by the sheet position correction amount (L1 - g1) (S109). The state where the registration roller pair 32 is shifted by the determined sheet position correction amount is the state shown in FIG. 4(e). Thereafter, the conveyance by the registration roller pair 32 is further continued, and an image (toner image) is transferred onto the sheet S in the secondary transfer unit T2 (S110).

[0036] Thereafter, the toner image is fixed via the fixing device 50 (S111). In the case of a single-sided job, the sheet S is discharged outside the machine via the branch unit 120, or delivered to a paper discharge option device (not shown) (S112), and the job ends. In the case of a double-sided job, the sheet S is subjected to an inversion process for forming an image on the second side opposite to the first side of the sheet. It is determined whether there is a subsequent print (S113). If it is determined that there is no subsequent sheet (S113: No), the print job is terminated (S114). If not (S113: Yes), the registration roller pair 32 is returned to the home position (central position) (S115). Thereafter, the process returns to step S103.

[0037] If it is determined that the print job is a second-side print (S103: second side), the sheet S is switchback inverted by the inversion conveyance unit 130. Then, the inverted sheet S is conveyed by the conveyance roller pairs provided in the second double-sided conveyance unit 150 and the first double-sided conveyance unit 70, and is conveyed again to the pre-registration roller pair 32.

[0038] The skew correction operation of the sheet by the registration roller pair 32 of the second side is the same as that of the first side (S116 to S121). After transferring the image of the second side to the sheet S (S122), the toner image is fixed in step S111 in the same manner as the first side.

[0039] <Detailed description of the inversion conveyance unit> Next, the configuration of the inversion conveyance unit 130 will be described. FIG. 5 is a perspective view of the inversion conveyance unit 130. The inversion conveyance unit 130 includes a conveyance roller pair 131 that conveys a sheet, an inversion sensor 138, an inversion unit CIS 139 that is a first detection means, and an inversion shift unit 132 including a first inversion shift roller pair 132a and a second inversion shift roller pair 132b that are inversion roller pairs.

[0040] The inversion conveyance unit 130 also includes a mechanism for moving the sheet in the width direction, and its configuration is the same as that of the registration unit 30. The first inversion shift roller pair 132a and the second inversion shift roller pair 132b are rotationally driven by the inversion drive motor 38 shown in FIG. 8. The inversion shift unit 132 is movable in the width direction of the sheet S, and the inversion shift motor 39 in FIG. 8 moves the sheet S sandwiched by the inversion shift unit 132 in the width direction indicated by the arrow C in FIG. 5. An inversion unit CIS 139 for detecting the side end position of the sheet S is provided upstream of the inversion shift unit 132 in the sheet conveyance direction. The inversion unit CIS 139 is provided at a position offset from the center in the width direction of the sheet S with respect to the conveyance direction indicated by the arrows A and B in FIG. 5. This is because in the position correction of the sheet S, it is only necessary to detect the side end position of only one side of the sheet S.

[0041] Here, the reason for shifting the sheet in the width direction in the reverse conveyance unit will be described. Since the second-sided printing of the sheet will involve conveying it over a long distance after correcting skew and horizontal registration with the registration unit 30 during the first-sided printing, skew and horizontal registration are often deteriorated more than during the first-sided printing due to variations in the components of each unit. Therefore, the shift amount, which is the second movement amount of the registration roller pair 32, increases. When the registration roller pair 32 shifts, the sheet S rubs against the conveyance guide member, and particularly when the size of the sheet S is large, there is a large resistance because it is nipped by other rollers. As a result, when the shift amount is large, when the registration roller pair 32 shifts due to these resistances, the sheet S may skew, the horizontal registration shift amount of the sheet S may become smaller than expected, or the sheet S may become wrinkled. To reduce such problems, in this image forming apparatus, the reverse conveyance unit 130 also performs horizontal registration shift of the sheet S.

[0042] Next, with reference to FIGS. 6 and 7, the reverse shift operation for moving the sheet S in the sheet width direction will be described. FIG. 6 is a flowchart showing the processing procedure of the reverse shift process of the sheet S in the image forming apparatus 1. FIG. 7 is a schematic cross-sectional view for explaining the switchback operation in the reverse conveyance unit 130.

[0043] When the print job is for double-sided printing, the sheet S on which the first image has been formed is sent by the branch unit 120 to the reverse conveyance unit 130. The reverse conveyance unit 130 has a reverse double-sided switching member 143, which is attached in a state positioned by a biasing member (not shown). The sheet S conveyed from the branch unit is sent to the pair of conveyance rollers 131 and is conveyed while pressing the reverse double-sided switching member 143 against the biasing force of the biasing member (Fig. 7(a)). Subsequently, the position of the sheet S is detected by the reverse sensor 138 (S201). Thereafter, the side edge position on the leading end side in the sheet conveyance direction is detected by the reverse unit CIS 139 (S202), and a correction amount with respect to the sheet width direction is determined based on this detection result (L3). Subsequently, based on the detection result of the reverse sensor 138, when the trailing end of the conveyed sheet S has advanced a predetermined distance from the reverse double-sided switching member 143, the driving of the reverse drive motor is stopped and the sheet S is stopped (S203). The state in which the sheet S has stopped is shown in Fig. 7(b).

[0044] After the sheet S stops, the reverse shift motor shifts the reverse shift unit 132 during the conveyance of the sheet S by the sheet position correction amount (L3 - g3) (S204). The control means of the present embodiment can execute an acquisition process for acquiring an offset amount of the target position g3 separately from the job for obtaining the printed product. In this flowchart, a measurement sheet that is not intended to form an image as the printed product is used.

[0045] Also, in parallel with the shift operation, the reverse drive motor 136 is rotated in the reverse direction, so that the conveyance by the first reverse shift roller pair 132a and the second reverse shift roller pair 132b is continued and a switchback operation is performed (S205). Then, the sheet S is sent to the second double-sided conveyance unit 150 and heads for the formation of the second-sided image (Fig. 7(c)).

[0046] After that, it is determined whether there is a subsequent print (S206). If it is determined that there is no subsequent print (S206: No), the operation of the reverse paper discharge unit 130 is terminated. If not (S206: Yes), the reverse shift unit 132 is returned to the home position (center position) (S207). After that, the process returns to the process of step S201. Here, the operation order of steps S204 and S205 may be either first.

[0047] As described above, by performing the horizontal registration shift operation of the sheet by the reverse conveyance unit 130, the horizontal registration shift amount of the sheet by the registration unit 30 can be reduced. By performing the shift operation with the reverse conveyance unit 130, the rollers other than the rollers that perform the shift operation do not nip the sheet S. That is, regardless of the size of the sheet S, it is not nipped by other rollers and does not become a resistance, and the shift operation can be performed.

[0048] Furthermore, in the present embodiment, the reverse guide 142, which is a guide member for the sheet S adjacent to the reverse shift unit 132, is provided only on one surface side of the sheet and is not arranged on the other surface. Since the guide member of the switchback portion can guide the sheet S even with only one surface side, the resistance due to the rubbing between the sheet S and the guide member can be reduced. In addition, in the present embodiment, the reverse shift unit 132 simultaneously shifts two roller pairs, the first reverse shift roller pair 132a and the second reverse shift roller pair 132b. By performing the shift operation while nipping the sheet S with the two roller pairs, it is possible to reduce the occurrence of slip between the sheet S and the rollers during the shift operation and cause the sheet to skew, and it becomes possible to perform a stable shift operation.

[0049] As described above, by performing the shift operation with the reverse conveyance unit 130, the resistance received during the shift operation can be reduced, and the shift operation can be performed stably.

[0050] <Control block diagram> FIG. 8 shows a control block diagram of the image forming apparatus 1. It is a control block diagram for executing the control of the aforementioned registration unit 30 and the reverse conveyance unit 130. When a print execution instruction from a user is received via a computer, the image forming control unit 205, the sheet conveyance control unit 206, the sensor control unit 207, and the shift control unit 208 included in the control unit 200 control each motor and sensor to execute a printing operation. Since the detailed operations by each control unit are as described above, the description thereof is omitted.

[0051] <Horizontal registration shift control by the registration unit and the reverse conveyance unit> The horizontal registration shift control by the registration unit 30 and the reverse conveyance unit 130 will be described. FIG. 9 shows a top view schematic diagram for explaining the correction positions of the horizontal registration correction in the registration unit 30 and the reverse conveyance unit 130. The position indicated by (0) in the figure is the sheet edge position of the name of the CIS in the reverse CIS 139 and the registration unit CIS 34, and the downward direction in the figure is the plus direction. The correction positions of the horizontal registration correction of the registration unit 30 are the image writing positions g1 (first side) and g2 (second side). In this schematic diagram, the image writing positions g1 and g2 are common.

[0052] The correction position of the horizontal registration correction of the reverse conveyance unit 130 is a value obtained by offsetting the amount Δd by which the sheet S is displaced in the width direction in the conveyance path from the reverse conveyance unit 130 to the registration unit 30 with respect to the name position (0) of the reverse CIS 139. Thereby, the sheet S is conveyed to the position of g2 in the registration unit CIS 34. That is, let g3 = -Δd.

[0053] In FIG. 9, d1 shows how the sheet S is conveyed to the registration unit CIS34 when the correction position of the lateral registration correction of the reverse conveyance unit 130 is set as the reference position (0) of the reverse CIS 139. d2 shows how the sheet S is conveyed to the registration unit CIS34 when the correction position of the lateral registration correction of the reverse conveyance unit 130 is set as g3 = -Δd. Δd is the difference between the widthwise position L2 of the sheet S conveyed to the registration unit CIS34 and the image writing position g2. While the sheet S is conveyed as shown by d1 in the figure, by setting the correction position g3 = -Δd, the sheet S after inversion is conveyed as shown by d2 in the figure, and the amount of lateral registration correction in the registration unit 30 can be reduced.

[0054] <Fixing damage due to image writing position and lateral registration shift of the sheet> FIG. 10 shows an explanatory diagram regarding the damage to the fixing device due to the image writing position and the lateral registration shift of the sheet. After passing through the CIS 34 of the registration unit, the sheet passing through the secondary transfer outer roller 54 is denoted as Sa. Then, it is conveyed in the conveyance direction A and denoted as Sb just before being nipped by the nip of the fixing unit 100. As described above, the sheet Sa is aligned toward the image writing position (g1) by the registration roller pair, and an image (toner image) is transferred onto the sheet S at the secondary transfer nip portion T2. Then, it is conveyed to the position of Sb, nipped by the nip of the fixing unit 100, and the toner image is fixed. Similarly for the second side, it is aligned toward the image writing position (g2) and conveyed to the fixing unit 100. Here, in this explanatory diagram, the image writing positions g1 and g2 are common.

[0055] FIG. 10(b) shows the relationship between the passing position (horizontal axis) of the sheet end portion on the fixing belt and the number of passing sheets (vertical axis). Note that the first side and the second side are superimposed and represented. When continuously feeding paper, it has a distribution with a width of ΔX (about 1 mm in this embodiment) centered on g1 and g2, and the number of sheets passing on the belt is represented. This is due to, for example, variations in the paper posture during conveyance in the pre-fixing conveyance unit 57 (not shown) or changes in the paper posture occurring at the transfer section between the pre-fixing conveyance unit 57 and the conveyance guides before and after it.

[0056] Here, scratches on the fixing tend to become deeper and larger when the sheet repeatedly passes through the same position relative to the fixing belt. When the scratch exceeds a certain threshold value, streak-like image defects at a visible level on the image will occur, and thus the fixing belt reaches the end of its life. In this explanatory diagram, since the image writing positions g1 and g2 are common, the first and second passes form a distribution centered on the same passing position on the belt and overlap with each other, so there is a problem that the maximum number of paper passes in terms of the belt life is reached early.

[0057] FIG. 10(c) shows another example of the relationship between the passing position (horizontal axis) of the sheet end on the fixing belt and the number of passes (vertical axis). In FIG. 10(c), the image writing positions g1 and g2 are arbitrarily set to be different (in the figure, g2 is set with an offset of 0.3 mm from g1). Compared with the example of FIG. 10(b), due to the shift in the positions of g1 and g2, the maximum value of the number of paper passes is slightly smaller, but it still exceeds the life of the fixing belt.

[0058] Therefore, in the present embodiment, as shown in FIG. 10(d), g2 is set to a position g2' that is offset by ΔL (moved by a predetermined amount) from g1. As a result, the overlap of the distributions of the passing positions of the first and second sheets disappears, so the passing position of the side edge of the sheet on the belt can be dispersed, and the life of the fixing belt can be extended. In the present embodiment, ΔL is set to a position where the passing positions of the first and second sheets do not overlap. For example, ΔL = 1.5 mm. Regarding the image writing position g2', it may be changed again after passing a predetermined number of sheets. For example, by setting g2' = g1 + 2·ΔL, the passing position of the second sheet can be shifted again to change the distribution of the passing positions.

[0059] Fig. 11 shows a control flowchart. In this embodiment, the position correction amount in the width direction of the inversion shift unit is determined based on the image writing position g2’ offset by ΔL from g1. After stopping the sheet S (S203), the control unit shifts the inversion shift unit during the conveyance of the sheet S by the sheet position correction amount (L3―g3) by the inversion shift motor (S204). Here, the correction position g3 of the horizontal registration of the inversion unit 130 is the difference between the width direction position L2 of the sheet S conveyed to the registration unit CIS and the image writing position g2’.

[0060] As a result, the inverted sheet can be moved in advance so as to reach the image writing position g2’. According to the present invention, since the passing position of the paper edge can be changed while reducing the shift amount at the registration pair 32, the life of the fixing belt can be improved without affecting the finished product.

[0061] [Embodiment 2] Fig. 12 shows an explanatory diagram in Embodiment 2 of the present invention. Different from Embodiment 1, in Embodiment 2, after passing through the CIS 34 of the registration unit, the sheet passing through the secondary transfer outer roller 54 is denoted as Sa. Thereafter, it is conveyed in the conveyance direction A, and the sheet passing through the fixing unit 100 is denoted as Sb. As described above, the sheet Sa is aligned toward the image writing position (g1) by the registration roller pair, and an image (toner image) is transferred onto the sheet S in the secondary transfer unit T2. Thereafter, it is conveyed to the position of the sheet Sb and is nipped by the nip of the fixing unit 100 to fix the toner image.

[0062] Here, the state where sheets having different lengths in the conveyance direction are nipped by the nip of the fixing unit 100 is denoted as Sc. The sheet Sc is detected by the inversion CIS 139 arranged on the downstream side at the end position g4 shifted by D from g1. In the case of a sheet having a long length in the conveyance direction of the sheet, due to the alignment deviation of the secondary transfer outer roller 54 and the alignment deviation of the connection between the housing 1a and the housing 2a, the time for the sheet to receive the turning force is different. For this reason, when the sheet reaches the fixing unit 100, the passing positions of the side ends of the sheet may be different.

[0063] Figure 12(b) shows the relationship between the passing position of the paper edge on the fixing belt and the number of passed sheets. The passing position of the sheet Sb after passing a predetermined amount of paper (single-sided passing) is represented by a distribution with a width of ΔX (about 1 mm in this embodiment) centered on g1. The passing position of the first side of the sheet Sc with different lengths in the conveying direction is represented by a distribution centered on the position g4 shifted by D from g1.

[0064] Figure 12(C) shows another example of the relationship between the passing position of the paper edge on the fixing belt and the number of passed sheets. This explanatory drawing shows the passing position of the second side of the sheet Sc with different sub-scanning lengths. According to the first embodiment of the present invention, when the image writing position of the second side is shifted to g2', the passing position on the fixing belt of the second side of the sheet Sc is represented by a distribution of a broken line centered on g2'. However, in reality, it is the same as in the case of the first side described above, and it passes through the position Sc' shifted by D. Since the sub-scanning lengths of the sheets are different, an overlap occurs between the distribution of the first side and the distribution of the second side, so the effect of Embodiment 1 becomes small.

[0065] Figure 12(d) shows the relationship between the passing position of the paper edge on the fixing belt and the number of passed sheets in Embodiment 2 of the present invention. In this embodiment, it is set at a position offset by the aforementioned shift amount D from the image writing position g2' in Embodiment 1. As a result, the overlap of the distributions of the passing positions of the first side and the second side of the paper shown in Figure 12(C) is eliminated, and the passing position of the paper on the belt can be dispersed. In this embodiment, the shift amount D is determined based on the detection result of the inversion CIS139 arranged downstream of the fixing unit 100 (g1 - g4). Thereby, based on the paper width detection result at a position close to the fixing unit, the image position of the second side can be determined, so that the passing position of the paper on the fixing belt can be accurately determined. Also, in the first embodiment, for example, ΔL which was set to 1.5 mm may also be determined based on the detection result of the inversion CIS139.

[0066] FIG. 13 shows a control flowchart in the second embodiment of the present invention. In this embodiment, based on the image writing position g2’’ based on the detection result of the reverse CIS, the position correction amount in the width direction of the reverse shift unit is determined.

[0067] After the control unit stops the sheet S (S203), the reverse shift unit during the conveyance of the sheet S is shifted by the sheet position correction amount (L3 - g3) via the reverse shift motor and the transmission gear (S204). Here, the correction position g3 of the lateral registration of the reverse unit 130 is the difference between the width direction position L2 of the sheet S conveyed to the registration unit CIS and g2’’.

[0068] Thereby, since the reversed sheet can be moved in advance to reach the image writing position g2’’, the passing position of the paper edge can be changed while reducing the shift amount at the registration roller pair 32.

Explanation of Reference Numerals

[0069] 91 Image carrier (drum) 93 Exposure unit (exposure device) T2 Transfer unit (secondary transfer unit) 100 Fixing unit (fixing unit) 132 Pair of reverse rollers 139 First detection means (CIS of the reverse unit) 150, 60 Pair of conveyance rollers (duplex conveyance unit) 200 Control unit 32 Pair of registration rollers 34 Second detection means (CIS of the registration unit)

Claims

1. An image carrier that carries an image, An exposure unit that exposes the image writing position of the image carrier, A transfer unit that transfers the image formed on the image carrier to a sheet, A fixing unit that fixes the image transferred by the transfer unit to the sheet, An inversion roller pair that sandwiches the sheet fixed by the fixing unit and inverts the front and back of the sheet, and is movable in the width direction of the sheet orthogonal to the sheet conveyance direction, A first detection means provided in the vicinity of the inversion roller pair for detecting the position of the sheet in the sheet width direction, A conveyance roller pair that conveys the sheet inverted by the inversion roller pair toward the transfer unit, A control unit that controls the image writing position on the image carrier and the movement of the inversion roller pair in the width direction, and has, The control unit controls so that a first image writing position for forming an image on the first surface of the sheet on the image carrier and a second image writing position for forming an image on the second surface of the sheet on the opposite side of the first surface of the sheet on the image carrier are formed at different positions in the width direction of the sheet. Based on the detection result of the first detection means and the second image writing position, the first movement amount of the inversion roller pair in the width direction of the sheet is determined, The side end portion of the first surface of the sheet having the image formed at the first image writing position and the side end portion of the second surface of the sheet having the image formed at the second image writing position in the fixing unit pass through different positions in the width direction of the sheet, An image forming apparatus characterized by this.

2. The control unit controls so as to move a predetermined amount so that the side end positions of the sheets transferred to the images formed at the first image writing position and the side end positions of the sheets transferred to the images formed at the second image writing position do not overlap, The image forming apparatus according to claim 1, characterized by this.

3. A registration roller pair provided upstream of the transfer unit, forming a nip, correcting the skew of the sheet by abutting the leading end of the sheet against the nip, and movable in the width direction of the sheet orthogonal to the sheet conveyance direction, A second detection means provided in the vicinity of the registration roller pair for detecting the position of the sheet in the sheet width direction, and further comprising, The control unit determines the second movement amount of the second surface of the sheet based on the detection result of the second detection means and the second image writing position, The image forming apparatus according to claim 1, characterized in that...

4. The control unit determines the first movement amount such that the second movement amount becomes smaller than a predetermined amount. The image forming apparatus according to claim 3, characterized in that...

5. The control unit changes the image writing position on the second surface of the sheet based on the number of sheets passed through the fixing unit. The image forming apparatus according to claim 1, characterized in that...

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