Sheet conveyance device and image formation apparatus

Abstract

To evade the turning of a thin sheet and the darkening of a thick sheet, and to prevent a sheet skew correction shortage.SOLUTION: A sheet conveyance device comprises a first conveyance roller and a second conveyance roller which is provided at an upstream in a conveyance direction from the first conveyance roller and holds and conveys a sheet, in which the tip of the sheet is abutted against a nip part of the first conveyance roller to correct the skew of the sheet. The sheet conveyance device comprises: a conveyance unit having a third conveyance roller which is provided between the first conveyance roller and the second conveyance roller and conveys the sheet and a support member which supports the third conveyance roller and is rotatably provided; and a control unit which switches the position of the conveyance unit in accordance with the basis weight of the sheet. The control unit executes control in such a manner that, for a first sheet with less than a specified basis weight, the conveyance unit is made standby at a first position where the third conveyance roller is abutted against an opposite roller and the tip of the first sheet is received by the conveyance unit, and, for a second sheet with the specified basis weight or more, the conveyance unit is moved to a second position where the third conveyance roller is separated from the opposite roller, and the tip of the second sheet is received by the conveyance unit moved to the second position.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a sheet conveying device and an image forming apparatus including the same. [Background technology]

[0002] Conventionally, in a sheet conveying device used in an image forming apparatus that forms an image on a sheet, a technique is known in which the leading edge of the sheet is abutted against an abutment roller to correct the skew of the sheet (Patent Document 1). Also, a technique is common in which an upstream roller or an upstream guide arranged upstream in the conveying direction of the abutment roller is separated to secure a loop space for the sheet to correct the skew. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2015-78029 A Summary of the Invention [Problem to be solved by the invention]

[0004] However, in a method of correcting skew by abutting the leading edge of a sheet against an abutting roller, if the abutting force is too weak, the sheet skew correction is insufficient, while if the abutting force is too strong, problems such as edge curling, in which the abutted portion of the leading edge of the sheet curls up, and slip marks due to the conveying force of the upstream roller being defeated by the reaction force of the sheet, occur.

[0005] In addition, there is a demand these days for the printer to be able to handle a variety of sheets, from thin paper weighing around 45 to 52 g to thick paper weighing around 500 g.

[0006] When the leading edge of a thin sheet is abutted against an abutment roller to correct the skew of the sheet, the leading edge of the sheet is likely to curl up, and in order to avoid this, a guide shape like a jump pad is often formed in the upstream guide. When creating a jump pad in this way in the upstream guide, the gap between the guides facing each other across the sheet becomes narrow in the area where the jump pad is formed. For this reason, the jump pad is generally formed partially, rather than over the entire width of the sheet, which is perpendicular to the conveying direction.

[0007] On the other hand, when the leading edge of the cardboard is abutted against the abutment roller to correct the skew of the sheet, the contact pressure of the sheet against the upstream guide is higher at the jump pad portion than at other portions. Therefore, the sheet is rubbed strongly against the jump pad of the upstream guide (galvanized steel sheet), which may cause the plating to adhere to the sheet (black stains).

[0008] Another possible method to avoid edge curling and black spots is to separate the upstream rollers and guides for both thin and thick paper without forming a jump pad on the conveying guide. However, if the upstream rollers and guides are separated to receive thin paper, the loop space of the sheet becomes wider, and the abutting force of the leading edge of the sheet becomes weaker. In this case, there is a risk of insufficient correction of skew of the sheet.

[0009] SUMMARY OF THE PRESENTLY PREFERRED EMBODIMENT An object of the present invention is to prevent curling of thin paper and black stains on thick paper, and to prevent insufficient correction of skew of sheets. [Means for solving the problem]

[0010] A representative configuration of the present invention for achieving the above object is a sheet conveying device including a first conveying roller that pinches and conveys a sheet, and a second conveying roller that is provided upstream of the first conveying roller in a conveying direction and pinches and conveys the sheet, and corrects skew of the sheet by abutting a leading edge of the sheet against a nip portion of the first conveying roller, the sheet conveying device including a third conveying roller that is provided between the first conveying roller and the second conveying roller and conveys the sheet, and a support member that supports the third conveying roller and is rotatably provided, the sheet conveying device including a conveying unit that can switch the position of the third conveying roller to a position where the third conveying roller is in contact with or separated from an opposing roller, and a unit drive motor that switches the position of the conveying unit. the control unit controls the conveying unit to wait at a first position where the third conveying roller abuts against the opposing roller when a first sheet has a basis weight less than a predetermined basis weight, and to receive the leading edge of the first sheet by the conveying unit waited at the first position, and to move the conveying unit to a second position where the third conveying roller is separated from the opposing roller when a second sheet has a basis weight equal to or greater than the predetermined basis weight, and to receive the leading edge of the second sheet by the conveying unit moved to the second position. Effect of the Invention

[0011] According to the present invention, curling of a sheet having a basis weight less than a predetermined value and black spots on a sheet having a basis weight equal to or greater than a predetermined value can be avoided, and insufficient correction of skew of a sheet can be prevented. [Brief description of the drawings]

[0012] [Figure 1] Schematic diagram of an image forming apparatus [Diagram 2] Cross-section of the cash register unit [Diagram 3] (a)(b)(c) Illustration of the contact and separation operations of the transport unit [Figure 4] (a)(b) Diagram of the pre-registration guide jump platform [Diagram 5] (a)(b) Illustrative diagram of skew correction for plain paper [Figure 6] (a)(b) Diagram of thick paper skew correction [Figure 7] Diagram of thick paper skew correction [Figure 8] Block diagram of an image forming apparatus [Figure 9] FIG. 11 is a flow chart showing the flow of sheet transport during image formation. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. However, the dimensions, materials, shapes, and relative positions of the components described in the following embodiments may be changed as appropriate depending on the configuration of the device to which the present invention is applied and various conditions, and are not intended to limit the scope of the present invention to these alone.

[0014] Fig. 1 is a schematic diagram of an image forming apparatus 100. First, the configuration of the image forming apparatus 100 will be described with reference to Fig. 1. The image forming apparatus 100 includes a feeding section 110 that feeds a sheet, and an image forming section 920 that forms a toner image to be fixed on the sheet fed by the feeding section 110. The image forming apparatus 100 further includes a belt conveying unit 904 that conveys the sheet onto which the toner image has been transferred to the fixing section 50, and a post-conveying section 903 that conveys the sheet onto which the toner image has been fixed by the fixing section 50.

[0015] The feeding section 110 includes a sheet cassette 111 for storing sheets, a pickup roller 112 for picking up a sheet from the sheet cassette 111, and a separation device 113 for separating and feeding the sheet picked up by the pickup roller 112.

[0016] Furthermore, the feeding section 110 includes a registration unit 300 as a sheet conveying device. The registration unit 300 includes a registration roller 301 as a first conveying roller that pinches and conveys a sheet, and a vertical path roller 306 as a second conveying roller that is provided upstream of the registration roller 301 in the conveying direction and pinches and conveys the sheet. The registration unit 300 conveys the sheet by the vertical path roller 306 in a conveying path 901 along which the sheet separated and fed by the separating device 113 is conveyed, and corrects skew of the sheet by abutting the leading edge of the sheet against a nip portion of the registration roller 301.

[0017] The image forming unit 920 is a so-called tandem type image forming means in which electrophotographic image forming stations 200Y, 200M, 200C, and 200K that form toner images of Y (yellow), M (magenta), C (cyan), and K (black) are arranged in series. The configurations of the image forming stations 200Y, 200M, 200C, and 200K are common except for the different toner colors. Therefore, the configuration of the image forming station 200Y will be described as an example, and the configurations of the image forming stations 200M, 200C, and 200K will be omitted. In FIG. 1, the configuration of the image forming station 200Y is suffixed with "Y", the configuration of the image forming station 200M with "M", the configuration of the image forming station 200C with "C", and the configuration of the image forming station 200K with "K". The image forming station 200Y includes a photosensitive drum 120Y, a primary charging device 121Y, an exposure device 122Y, and a developing device 123Y.

[0018] The image forming unit 920 includes an intermediate transfer belt 125 as an example of an image carrier on which the toner images visualized by the image forming stations 200Y, 200M, 200C, and 200K are carried. The intermediate transfer belt 125 is supported in a state of being stretched across a drive roller 126, a tension roller 127, and an inner transfer roller 128, and rotates in the direction of arrow R2 in FIG. 1 by being driven by the drive roller 126.

[0019] The secondary transfer roller 131 is pressed against the intermediate transfer belt 125, which is supported from the inside of the intermediate transfer belt 125 by the inner transfer roller 128, to form a secondary transfer nip portion N2 between the intermediate transfer belt 125 and the secondary transfer roller 131. The secondary transfer roller 131, the intermediate transfer belt 125, and the inner transfer roller 128 constitute a secondary transfer portion 130 as a transfer means in this embodiment.

[0020] The cleaning device 129 rubs a cleaning web against the intermediate transfer belt 125 to remove residual toner, paper powder, and the like remaining on the surface of the intermediate transfer belt 125 after it has passed through the secondary transfer nip portion N2.

[0021] The fixing unit 50, which is disposed downstream of the secondary transfer unit 130 in the sheet conveying direction FD, is a fixing unit that fixes a toner image to a sheet by heat and pressure. The fixing unit 50 includes a heating roller 52 having an internal heater, and a pressure roller 53 that is disposed so as to be able to come into contact with the heating roller 52 and forms a fixing nip portion N together with the heating roller 52. The fixing unit 50 also includes a heating roller temperature sensor 70 that detects the surface temperature of the heating roller 52, and a pressure roller temperature sensor 71 that detects the surface temperature of the pressure roller 53. The heating roller temperature sensor 70 and the pressure roller temperature sensor 71 are provided to maintain the surface temperatures of the heating roller 52 and the pressure roller 53 at appropriate temperatures, respectively.

[0022] In the sheet transport direction FD, a belt transport unit 904 is disposed between the secondary transfer unit 130 and the fixing unit 50. The belt transport unit 904 is composed of a first belt transport unit 10 disposed on the upstream side of the sheet transport direction FD, and a second belt transport unit 20 disposed on the downstream side.

[0023] The rear conveying unit 903 includes discharge rollers 911 that discharge the sheet discharged from the fixing unit 50 to the outside of the image forming apparatus 100. The rear conveying unit 903 further includes reversing rollers 912 that reverse and convey the sheet, and a double-sided conveying path 913 that conveys the sheet reversed by the reversing rollers 912 and merges it with the conveying path 901.

[0024] Next, a series of steps for forming an image on a sheet in the image forming apparatus 100 will be described. Based on an image forming job input to the image forming apparatus 100, first, the exposure device 122Y exposes the photosensitive drum 120Y to light to form an electrostatic latent image on the surface of the photosensitive drum 120Y. The electrostatic latent image on the photosensitive drum 120Y is developed by the development device 123Y and visualized as a toner image. The toner images carried on the surface of the photosensitive drum 120Y are primarily transferred by the primary transfer device 124Y in a sequentially overlapping manner onto the intermediate transfer belt 125. The toner images primarily transferred onto the intermediate transfer belt 125 are secondarily transferred to the sheet S fed from the feeding unit 110 at the secondary transfer nip portion N2.

[0025] The registration rollers 301 receive the sheet S while stopping rotation and keep it waiting, and then send out the sheet S towards the secondary transfer nip N2 in time with the toner image on the intermediate transfer belt 125. The sheet S carrying the toner image transferred at the secondary transfer nip N2 is transported from the secondary transfer nip N2 to the fixing section 50 by the belt transport unit 904. In the fixing section 50, the sheet S is sandwiched at the fixing nip N and heat and pressure are applied to the unfixed toner image to fix the toner image to the sheet. The sheet S sent out from the fixing section 50 is discharged outside the image forming apparatus 100 by the discharge rollers 911.

[0026] When forming images on both sides of a sheet, the sheet sent out from the fixing unit 50 is conveyed to reversing rollers 912, and after being reversed by the reversing rollers 912, is conveyed toward a double-sided conveying path 913. Then, the sheet is conveyed again to the conveying path 901 via the double-sided conveying path 913, and a toner image is formed on the second side (back side) of the sheet in the same manner as on the first side (front side).

[0027] Next, the configuration and basic operation of the register unit 300 will be described with reference to Fig. 2. Fig. 2 is a cross-sectional view of the register unit 600.

[0028] As shown in FIG. 2, the registration unit 300 as a sheet conveying device has the registration roller 301 as described above. The registration roller 301 is a roller pair consisting of an upper roller 301U and a lower roller 301L facing the upper roller 301U. The registration roller 301 is a first conveying roller that conveys a sheet by pinching it between the upper roller 301U and the lower roller 301L, and is driven by a registration roller driving motor M3 (FIG. 8). The registration roller driving motor M3 is driven and controlled by the control unit 170 (FIGS. 1 and 8). The registration roller 301 is provided with registration guides 302U and 302L that form a conveying path for guiding the sheet. Upstream of the registration guide 302 in the conveying direction, a pre-registration upper guide 303 and a pre-registration lower guide 304 that form a conveying path for guiding the sheet are provided. A pre-registration sensor S2 that detects the sheet is provided in the pre-registration upper guide 303. The pre-registration sensor S2 is connected to the control unit 170 (FIGS. 1 and 8), and a detection signal is sent to the control unit 170 (FIGS. 1 and 8). The pre-registration lower guide 304 is a guide member that forms a curved conveying path for guiding a sheet together with a pre-registration guide 322 (described later), and is disposed on the inside of the curved conveying path.

[0029] The registration unit 300 has a conveying guide 305 and a vertical path roller 306. The conveying guide 305 is disposed between the conveying unit 320 described later and the vertical path roller 306. The vertical path roller 306 is a roller pair consisting of a right roller 306R and a left roller 306L facing the right roller 306R. The vertical path roller 306 is disposed downstream in the conveying direction from the registration roller 301. The vertical path roller 306 is disposed upstream in the conveying direction from the pre-registration roller described later. The vertical path roller 306 is a second conveying roller that conveys a sheet by pinching it between the right roller 306R and the left roller 306L, and is driven by a vertical path roller driving motor M1 (FIG. 8). The vertical path roller driving motor M1 is driven and controlled by the control unit 170 (FIGS. 1 and 8). The vertical path roller 306 is provided with a right vertical path guide 307 and a left vertical path guide 308 that form a conveying path for guiding the sheet. A feeding path guide 309 that guides a sheet and a double-sided junction guide 310 are arranged upstream of the vertical path roller 306 in the conveying direction. A vertical path sensor S1 that detects a sheet is arranged in the double-sided junction guide 310. The vertical path sensor S1 is connected to the control unit 170 (FIGS. 1 and 8), and a detection signal from the vertical path sensor S1 is sent to the control unit 170 (FIGS. 1 and 8).

[0030] The registration unit 300 has a transport unit 320 that can be switched to a position where the upper pre-registration roller 321U abuts against or is separated from the lower pre-registration roller 321L. The transport unit 320 is provided between the registration roller 301 and the vertical path roller 306. The transport unit 320 has an upper pre-registration roller 321U and a pre-registration guide 322.

[0031] The pre-registration rollers 321 form a roller pair that sandwiches and conveys a sheet between the upper pre-registration roller 321U and the lower pre-registration roller 321L, and are driven by a pre-registration roller drive motor M2 (FIG. 8). The pre-registration roller drive motor M2 is driven and controlled by the control unit 170 (FIGS. 1 and 8). The upper pre-registration roller 321U is provided between the registration roller 301 and the vertical path roller 306, and is a third conveying roller that conveys a sheet. The lower pre-registration roller 321L is an opposing roller that is disposed opposite the upper pre-registration roller 321U, and is disposed in the pre-registration lower guide 304. The pre-registration roller 321 is disposed upstream of the registration roller 301 in the conveying direction and downstream of the vertical path roller 306 in the conveying direction.

[0032] The pre-registration guide 322 is a support member that supports the upper pre-registration roller 321U and is provided rotatably around a rotation center 320C. The pre-registration guide 322 is a guide member that forms a curved conveying path that guides a sheet together with the pre-registration lower guide 304 in the conveying unit 320, and is disposed outside the curved conveying path.

[0033] The pre-registration guide 322 can rotate around a rotation center 320C as a transport unit 320 integrally with the upper pre-registration roller 321U. The pre-registration guide 322 is driven by a unit drive motor M4 (FIG. 8). The upper pre-registration roller 321U and the pre-registration guide 322 can rotate together as a transport unit 320 about a rotation center 320C, and the upper pre-registration roller 321U can come into contact with or separate from the lower pre-registration roller 321L.

[0034] Next, the contact and separation operation of the transport unit 320 will be described with reference to Figures 3(a), 3(b), and 3(c). Figures 3(a), 3(b), and 3(c) are explanatory diagrams of the contact and separation operation of the transport unit 320. Figure 3(a) shows the position where the rollers of the transport unit 320 are in contact. Figures 3(b) and 3(c) show the position where the rollers of the transport unit 320 are separated.

[0035] The registration unit 300 has a unit drive motor M4 that switches the position of the transport unit 320, and a position detection sensor S3 as a detection member that detects the position of the transport unit 320. The unit drive motor M4 moves the transport unit 320 (pre-registration guide 322) and switches the position where the upper pre-registration roller 321U abuts against or is separated from the lower pre-registration roller 321L. In addition, a detection flag (not shown) that detects the amount of rotation of the transport unit 320 and the position detection sensor S3 are exemplified here as the detection member, but are not limited thereto.

[0036] As described above, the upper pre-registration roller 321U and the pre-registration guide 322 of the transport unit 320 rotate integrally around the rotation center 320C. The transport unit 320 rotates by being driven by the unit drive motor M4 (FIG. 8), and its position is detected by a detection flag (not shown) that detects the amount of rotation and a position detection sensor S3. Therefore, the control unit 170 described later can freely control the position of the transport unit 320 based on the amount of rotation detected by the position detection sensor S3.

[0037] Here, the conveying unit 320 can be switched by the control unit 170 to a first position shown in FIG. 3(a), a second position shown in FIG. 3(b), and a third position shown in FIG. 3(c) based on the amount of rotation detected by the position detection sensor S3. The first position shown in FIG. 3(a) is a position where the upper pre-registration roller 321U abuts against the lower pre-registration roller 321L. The second position shown in FIG. 3(b) is a position where the upper pre-registration roller 321U is separated from the lower pre-registration roller 321L. The third position shown in FIG. 3(c) is a position where the upper pre-registration roller 321U is further separated from the lower pre-registration roller 321L to open the gap between the upper pre-registration roller 321U and the lower pre-registration roller 321L wider than the second position.

[0038] In addition, the control unit 170, which will be described later, can rotate the transport unit 320 in accordance with the timing at which the vertical path sensor S1 and the pre-registration sensor S2 detect the sheet. This allows the upper pre-registration roller 321U to be brought into contact with or separated from the lower pre-registration roller 321L sequentially even during sheet transport.

[0039] Next, the jump platform 322J of the pre-registration guide 322 will be described with reference to Fig. 4(a) and Fig. 4(b). Fig. 4(a) and Fig. 4(b) are explanatory diagrams of the jump platform 322J of the pre-registration guide 322. Fig. 4(a) is a cross-sectional view of the jump platform 322J of the pre-registration guide 322, and Fig. 4(b) is a perspective view of the jump platform 322J of the pre-registration guide 322.

[0040] The pre-registration guide 322 has a jump platform 322J as a protrusion that guides the leading edge of the sheet to the upper pre-registration roller 321U. The pre-registration guide 322 has a jump platform 322J as a measure against edge curling that occurs when the leading edge of the sheet abuts against the upper pre-registration roller 321U.

[0041] The jump platform 322J is provided so as to protrude from the upper pre-registration roller 321U side toward the opposing lower pre-registration roller 321L side, and has an inclination approaching the lower pre-registration roller 321L side from the upstream to the downstream in the conveying direction. This reduces the contact angle of the sheet leading edge with the upper pre-registration roller 321U (lays flat), making it possible to prevent the edge of the sheet leading edge from curling up.

[0042] The dotted line 322d shown in Fig. 4(a) is a guide surface without a jump pad, but the jump pad 322J protrudes from this guide surface (dotted line 332d) toward the conveying surface side. Also, as shown in Fig. 4(b), the jump pad 322J is arranged partially, not over the entire area, in the width direction perpendicular to the sheet conveying direction in the pre-registration guide 322.

[0043] If super-thick paper of 500 g or the like is passed through the transport unit 320 in a closed state, the contact pressure with the jump platform 322J may become high in some places, which may cause scratches during transport or black spots.

[0044] Next, the basic operation of forming a loop in a sheet and correcting skew in the registration unit 300 will be described with reference to Figures 5 to 7. The operation of correcting skew of a sheet with low stiffness, such as plain paper, will be described with reference to Figures 5(a) and 5(b). The operation of correcting skew of a sheet with high stiffness, such as cardboard, will be described with reference to Figures 6(a), 6(b), and 7.

[0045] In the case of a sheet with low rigidity, as shown in FIG. 5(a), the conveying unit 320 receives the leading edge of the sheet at a first position where the upper pre-registration roller 321U abuts against the lower pre-registration roller 321L and nip it. Then, as shown in FIG. 5(b), the pre-registration roller 321 feeds the sheet P by a predetermined amount while the registration roller 301 is stopped, forming a loop PL on the sheet P. When the loop PL is formed, a force F1 is generated by the rigidity of the sheet P to strike the registration roller 301, and the leading edge of the sheet strikes the nip of the registration roller 301. That is, the sheet P conveyed in a skewed manner follows the nip of the registration roller 301, and the skew of the sheet is corrected. In order to correctly strike the leading edge of the sheet against the registration roller 301 and correctly correct the skew, the force F1 that strikes the leading edge of the sheet is important. If the force F1 is small, the leading edge of the sheet does not strike completely, and the skew remains. Conversely, if the force F1 is too large, damage such as a crease occurs on the sheet.

[0046] Similarly, in the case of a sheet with high stiffness, as shown in Fig. 6(a), the transport unit 320 receives the leading edge of the sheet at a second position where the upper pre-registration roller 321U is separated from the lower pre-registration roller 321L. Then, as shown in Fig. 6(b), before the leading edge of the sheet reaches the registration roller 301, the transport unit 320 moves the upper pre-registration roller 321U further away from the lower pre-registration roller 321L to a third position where the distance between the rollers 321U and 321L is wider than that at the second position. Then, as shown in Fig. 7, the leading edge of the sheet is abutted against the registration roller 301 to generate an abutment force F2 to form a loop PL in the sheet, thereby correcting the skew of the sheet.

[0047] Since plain paper has a lower sheet stiffness than thick paper, it is preferable to form the loop PL with the transport unit 320 closed in order to generate a predetermined abutting force F1. On the other hand, if thick paper, which has a higher stiffness than plain paper, is used, and the transport unit 320 is closed, the roller pressure will be defeated by the reaction force of the thick paper, resulting in problems such as slip marks. Therefore, it is preferable to move the transport unit 320 to the third position and form a loop on the sheet in a state in which the loop formation space (the space between the rollers 321U and 321L) is secured to be wider than that in the second position. Furthermore, by receiving the leading edge of the sheet with the transport unit 320 moved to the second position, it is possible to avoid black spots caused by the contact pressure against the jump stand 322J being partially high.

[0048] In this way, by controlling the position of the transport unit 320 according to the sheet stiffness (basis weight of the sheet), it is possible to simultaneously correct skew of plain paper, correct skew of thick paper, and avoid slip marks and black stains.

[0049] Next, the flow of image formation and sheet transport will be described with reference to the block diagram of Fig. 8 and the flow chart of Fig. 9. Fig. 8 is a block diagram of an image forming apparatus. Fig. 9 is a flow chart showing the flow of sheet transport during image formation.

[0050] First, a brief explanation will be given of Fig. 8. Image forming apparatus 100 has a control unit 170 that controls the entire image forming apparatus. Control unit 170 is composed of a CPU 171, a memory 172, an I / O port 173, a communication interface 174, and a timer 175. CPU 171 comprehensively controls image processing and printing based on programs and the like stored in memory 172. Memory 172 stores programs and image data required when CPU 171 performs various processes, and operates as a work area.

[0051] The control unit 170 receives signals from the vertical path sensor S1, the pre-registration sensor S2, and the position detection sensor S3. The control unit 170 also receives image formation jobs from external devices. The control unit 170 also receives various setting inputs and operation instructions from the user through the operation unit 210. In this embodiment, the control unit 170 switches the position of the transport unit 320 depending on the basis weight of the sheet included in the information from the operation unit 210.

[0052] Based on the received signals, the control unit 170 controls the operations of the feeding unit 110, the image forming unit 920, the vertical path roller drive motor M1, the pre-registration roller drive motor M2, the registration roller drive motor M3, and the unit drive motor M4.

[0053] The flow of sheet conveyance during image formation will be described with reference to Figures 8 and 9. A user inputs information such as the size and basis weight of a sheet from the operation unit 210, and a job is started (S801). When the job is started, sheet feeding from the feeding unit 110 begins (S802).

[0054] When sheet feeding starts, the control unit 170 determines whether the basis weight of the sheet used in the job is equal to or greater than a predetermined basis weight (S803). If the sheet is a first sheet with a basis weight less than the predetermined basis weight, the process proceeds to S804. On the other hand, if the sheet is a second sheet with a basis weight equal to or greater than the predetermined basis weight, the process proceeds to S812. Here, cardboard of 151 gsm or more is exemplified as a second sheet with a basis weight equal to or greater than the predetermined basis weight. On the other hand, plain paper (or thin paper) of 150 gsm or less, which has a lower stiffness than cardboard, is exemplified as a first sheet with a basis weight less than the predetermined basis weight. Note that the type and basis weight of each sheet are merely examples and are not limited to these.

[0055] In S803, if the sheet has a predetermined basis weight or more (e.g., cardboard), the conveying unit 320 receives the leading edge of the sheet in an open state. That is, before the leading edge of the sheet reaches the nip of the pre-registration roller 321 of the conveying unit 320, the conveying unit 320 is moved to the second position shown in FIG. 6(a). Specifically, after the vertical path sensor S1 detects the leading edge of the sheet and the sheet passes a predetermined distance, the unit drive motor M4 is driven, and the conveying unit 320 is moved to the second position and waits (S804). This predetermined distance is calculated by the CPU 171 when the vertical path roller drive motor M1 rotates a predetermined number of pulses after the vertical path sensor S1 detects the leading edge of the sheet. All the following predetermined distances are determined by the same calculation process.

[0056] In addition, the timing of moving the transport unit 320 to the second position in S804 is not limited to the time when the transport unit 320 passes a predetermined distance after the vertical path sensor S1 detects the leading edge of the sheet. For example, the transport unit 320 may be configured to move to the second position immediately after the vertical path sensor S1 detects the leading edge of the sheet.

[0057] Then, after the leading edge of the sheet is delivered to the transport unit 320 at the second position, and before the leading edge of the sheet hits the registration roller 301, the unit drive motor M4 is driven again to move the transport unit 320 to the third position shown in Fig. 6(b) (S806). Specifically, when the leading edge of the sheet passes through the transport unit 320 by a predetermined distance and reaches the pre-registration upper guide 303 (S805), the unit drive motor M4 is driven again to move the transport unit 320 to the third position shown in Fig. 6(b) (S806). For example, when the vertical path sensor S1 detects the leading edge of the sheet and then sends the sheet by a predetermined amount, the transport unit 320 is moved to the third position.

[0058] By moving the conveying unit 320 to the third position, the space between the rollers 321U and 321L is opened wider than that at the second position. In this state, as shown in FIG. 7, the leading edge of the sheet P is abutted against the nip portion of the registration roller 301, a predetermined amount of loop is formed in the sheet P, and skew correction of the sheet P is performed (S807). Thereafter, the driving motors M1 to M3 are driven to resume conveyance of the sheet (S808), and the sheet is conveyed to the secondary transfer unit 130 and the fixing unit 50, where an image is formed, and then the sheet is conveyed to the post-conveying unit 903, where the sheet is discharged (S809). Similar operations are performed up to the last sheet of the job, and the job is completed (S810, S811).

[0059] On the other hand, in S803, if the sheet has a basis weight less than the predetermined value (for example, plain paper or thin paper), the conveying unit 320 receives the leading edge of the sheet in a closed state. That is, the conveying unit 320 waits at the first position shown in FIG. 5A, and receives the leading edge of the sheet in a nipped state with the pre-registration rollers 321 (S812). In this state, as shown in FIG. 5B, the leading edge of the sheet P is abutted against the nip portion of the registration rollers 301 to form a predetermined amount of loop in the sheet P, and skew correction of the sheet P is performed (S813). Thereafter, the sheet conveyance is resumed (S814), and image formation and discharge (S815) are performed up to the last sheet, and the job is completed (S810, S811).

[0060] As described above, by controlling the position of the transport unit 320 according to the basis weight of the sheet, it is possible to avoid curling of sheets with a basis weight less than a specified value and slip marks and black spots on sheets with a basis weight equal to or greater than a specified value, and to prevent insufficient correction of the skew of the sheet.

[0061] In the above-described embodiment, a color image forming apparatus having four image forming stations is exemplified, but this is not limited to this and may be, for example, an image forming apparatus having one image forming station.

[0062] In the above-described embodiment, an image forming apparatus is illustrated in which an intermediate transfer belt is used, toner images of each color are transferred to the intermediate transfer belt in a sequentially overlapping manner, and the toner images carried on the intermediate transfer belt are transferred to a sheet all at once, but the present invention is not limited to this. An image forming apparatus may be used in which a sheet carrier is used, toner images of each color are transferred to a sheet carried on the sheet carrier in a sequentially overlapping manner, and the same effect can be obtained by applying the present invention to such an image forming apparatus.

[0063] In the above embodiment, the recording method is an electrophotographic method, but the recording method is not limited to this, and other recording methods such as an inkjet method may also be used. [Explanation of symbols]

[0064] M1, M2, M3, M4 ... Driving motors S1 … Vertical path sensor S2: Sensor in front of the register S3 … Position detection sensor 100 ... Image forming device 170 ...Control section 210...Operation unit 300 ... cash register units 301 ... Registration roller (first conveying roller) 306 ... Vertical path roller (second conveying roller) 320 ...Transport unit 320C …Pivot center 321 ... Preregistration Roller 321L ... Lower pre-registration roller (opposing roller) 321U ... Upper pre-registration roller (third conveyor roller) 322 ... Pre-registration guide (support member, guide member) 322J … Jump platform (protrusion) 920 ... Image forming section

Claims

1. A sheet conveying device including a first conveying roller that pinches and conveys a sheet, and a second conveying roller that is provided upstream of the first conveying roller in a conveying direction and pinches and conveys the sheet, and corrects skew of the sheet by abutting a leading edge of the sheet against a nip portion of the first conveying roller, a conveying unit including a third conveying roller provided between the first conveying roller and the second conveying roller and configured to convey a sheet, and a support member provided rotatably to support the third conveying roller, the conveying unit being capable of switching the third conveying roller to a position where it is in contact with or separated from a counter roller; A unit drive motor for switching the position of the transport unit; A detection member for detecting a position of the transport unit; a control unit that switches the position of the conveying unit according to the basis weight of the sheet, The control unit is When the first sheet has a basis weight less than a predetermined value, the conveying unit is placed on standby at a first position where the third conveying roller is in contact with the counter roller, and the conveying unit placed on standby at the first position receives a leading edge of the first sheet; A sheet conveying device characterized in that, when the second sheet has a basis weight equal to or greater than the specified basis weight, the conveying unit is moved to a second position where the third conveying roller is separated from the opposing roller, and the conveying unit moved to the second position is controlled to receive the leading edge of the second sheet.

2. The control unit is When the first sheet has a basis weight less than a predetermined value, the leading edge of the first sheet is received by the conveying unit that is waiting at the first position, and the third conveying roller that is in contact with the opposing roller strikes the sheet against a nip portion of the first conveying roller to correct a skew of the sheet; 2. The sheet conveying device according to claim 1, wherein, when the second sheet has a basis weight equal to or greater than the specified basis weight, the leading edge of the second sheet is received by the conveying unit moved to the second position, and before the leading edge of the second sheet abuts against the first conveying roller, the conveying unit is moved to a third position in which the third conveying roller is further separated from the opposing roller to open a gap between the third conveying roller and the opposing roller wider than at the second position, and the second conveying roller abuts the sheet against a nip portion of the first conveying roller to correct the skew of the sheet.

3. 2. The sheet transport device according to claim 1, wherein the support member is a guide member that forms a curved transport path for guiding the sheet in the transport unit, and is disposed outside the curved transport path.

4. 2. The sheet transport device according to claim 1, wherein the support member has a protrusion for guiding a leading edge of the sheet to the third transport roller.

5. 5. The sheet transport device according to claim 4, wherein the protrusions are arranged on the support member not over the entire area but over part of the width of the support member in a width direction perpendicular to the sheet transport direction.

6. The detection member detects the amount of rotation of the transport unit, 2. The sheet transport device according to claim 1, wherein the control section is capable of freely controlling a position of the transport unit based on an amount of rotation detected by the detection member.

7. An image forming apparatus including an image forming unit that forms an image on a sheet, and a sheet conveying device that conveys the sheet to the image forming unit, 7. An image forming apparatus comprising: the sheet transport device according to claim 1;

Images