Image forming apparatus

Abstract

This approach suppresses the occurrence of image defects while also preventing the device from becoming larger and increasing costs. [Solution] Of the outer circumferential surface of the intermediate transfer belt 8, the tensioned surface 8a stretched from the drive roller 9 to the secondary transfer opposing roller 11 in the direction of arrow R3 is positioned to intersect with the common tangent L1 of the register roller pair 21 in the conveying nip N4. The shutter member 24 has a rear end contact portion 24c that can contact the rear end of the sheet S that is moving toward the tensioned surface 8a after passing through the intermediate contact portion 24b of the shutter member 24 where the rear end of the sheet S conveyed by the register roller pair 21 is in a retracted position.

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus for forming an image on a sheet. [Background technology]

[0002] Generally, in an image forming apparatus, the toner image formed on the intermediate transfer belt or photosensitive drum in the transfer section is transferred to a sheet, and then the toner image is fixed to the sheet by heat and pressure. Upstream of the transfer section in the sheet transport direction, a pair of registration rollers (hereinafter referred to as the registration roller pair) is provided, and the sheet fed from the paper tray is sent to the intermediate transfer belt by the registration roller pair.

[0003] The sheet, fed onto the intermediate transfer belt by the register roller pair, is conveyed at a predetermined angle with respect to the tensioned surface formed by multiple tension rollers that extend beyond the intermediate transfer belt. If this predetermined angle is too small, the intermediate transfer belt and the sheet may come into contact upstream of the transfer section in the image forming apparatus, potentially causing the toner image on the intermediate transfer belt to rub and resulting in an image defect called "image rubbing." Conversely, if the sheet is conveyed at a sufficient angle with respect to the tensioned surface of the intermediate transfer belt, the impact of the rear end of the sheet contacting the intermediate transfer belt may distort the toner image on the intermediate transfer belt, potentially causing an image defect called "rear end blur" near the rear end of the image. Therefore, a system has been proposed that suppresses the occurrence of various image defects by providing a guide member between the register roller pair and the transfer section to regulate the sheet's conveyance path (see Patent Document 1). [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2016-38526 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] In the image forming apparatus described in Patent Document 1, in order to suppress the occurrence of various image defects, it is necessary to restrict the sheet transport path with a guide member up to the vicinity of a virtual line extending from the tensioned surface of the intermediate transfer belt. This necessitates space between the register roller pair and the intermediate transfer belt, which leads to the problem of increasing the size of the apparatus. Furthermore, the need for a guide member to restrict the transport path increases the number of parts and thus increases costs.

[0006] Therefore, the present invention aims to provide an image forming apparatus that can suppress the occurrence of image defects while suppressing the increase in size and cost of the apparatus. [Means for solving the problem]

[0007] One aspect of the present invention is a first conveyor roller pair having a first conveyor nip for gripping and conveying a sheet in the sheet conveying direction, and a second conveyor roller pair forming a second conveyor nip for gripping and conveying a sheet located at the second conveyor nip, the second conveyor roller pair being arranged downstream of the first conveyor roller pair in the sheet conveying direction, and having a front contact portion and a surface contact portion, the front contact portion moving between a first contact position where it can contact the front end of the sheet and a second contact position where it can contact the surface of the sheet. A possible contact member, wherein the tip contact portion is pressed by the tip of the sheet being conveyed by the first conveying nip, causing the contact member to rotate in a first direction about a pivot axis from a first contact position to a second contact position; an image carrier that carries a toner image; a belt on which the toner image is transferred from the image carrier while rotating in a predetermined rotational direction; an outer roller that contacts the outer circumferential surface of the belt; and a member positioned opposite the outer roller with the belt in between, contacting the inner circumferential surface of the belt and the outer roller. The image forming apparatus comprises an inner roller that forms a transfer nip for transferring a toner image on the belt to a sheet, and a tension roller that contacts the inner circumferential surface of the belt and is positioned upstream of the inner roller in the rotational direction, wherein the front contact portion is positioned upstream of the second transport nip in the sheet transport direction when the contact member is in the first contact position, and the surface contact portion is positioned downstream of the second transport nip in the sheet transport direction when the contact member is in the second contact position, and the portion of the outer circumferential surface stretched from the tension roller to the inner roller in the rotational direction is positioned to intersect with the common tangent of the second transport roller pair in the second transport nip, and the contact member has a rear end contact portion that can contact the rear end of the sheet that moves in a direction approaching the portion of the outer circumferential surface after the rear end of the sheet transported by the second transport roller pair has passed the surface contact portion of the contact member located in the second contact position. [Effects of the Invention]

[0008] According to the present invention, it is possible to suppress the occurrence of image defects while suppressing the increase in size and cost of the device.

Brief Description of the Drawings

[0009] [Figure 1] It is a diagram showing a schematic configuration of an image forming apparatus according to a first embodiment. [Figure 2] It is a block diagram showing a control system of an image forming apparatus according to a first embodiment. [Figure 3] It is a perspective view of the resist roller side of a registration unit according to a first embodiment. [Figure 4] It is a cross-sectional view of a registration unit according to a first embodiment. [Figure 5] (a) is a diagram for explaining a state where a shutter member according to a first embodiment is located at a standby position. (b) is a diagram for explaining a state where the shutter member is located at a retracted position. (c) is a diagram for explaining a state where the shutter member is rotated about the roller axis from the retracted position. [Figure 6] It is a diagram for explaining the behavior of a rear end abutting portion according to a first embodiment. [Figure 7] (a) is a diagram for explaining a case where a shutter member according to a first embodiment is located at a standby position, and (b) is a diagram showing a state where the shutter member is rotated from the standby position to a maximum rotation position downstream of the retracted position. [Figure 8] It is a cross-sectional view of a registration unit according to a second embodiment. [Figure 9] It is a cross-sectional view of a registration unit according to another embodiment. ]> [Figure 10] It is a cross-sectional view of a registration unit according to another embodiment. [[ID=3^{6}]]

Mode for Carrying Out the Invention

[0010] [[ID=4^{0}]] [First Embodiment] - Hereinafter, a first embodiment of the present invention will be described in detail with reference to the drawings. FIG. 1 is a schematic cross-sectional view showing a schematic configuration of an image forming apparatus 100 according to the first embodiment.

[0011] <Schematic Configuration of Image Forming Apparatus> As shown in FIG. 1, the image forming apparatus 100 of the first embodiment is a tandem type laser beam printer that employs an intermediate transfer method capable of forming full-color images on A3-sized paper using an electrophotographic method.

[0012] The image forming apparatus 100 has four image forming units 110Y, 110M, 110C, and 110K arranged in a row. Each of the image forming units 110Y, 110M, 110C, and 110K has a process cartridge 7Y, 7M, 7C, or 7K for forming images of each color of yellow (Y), magenta (M), cyan (C), and black (K). In the first embodiment, the configurations and operations of the image forming units 110Y, 110M, 110C, and 110K are substantially the same except that the colors of the toners used are different. Therefore, hereinafter, when no distinction is required, the suffixes Y, M, C, and K of the symbols indicating elements for any color will be omitted for description.

[0013] The process cartridge 7 has a photosensitive drum 1, a charging roller 2, a developing device 4, and a drum cleaning device 6, and is configured to be detachably attached to the apparatus main body of the image forming apparatus 100 integrally. The photosensitive drum 1 is a drum-type (cylindrical) image carrier that carries a toner image and is rotationally driven in the direction of arrow R1. Around the photosensitive drum 1, in order along the rotation direction of the photosensitive drum 1, a charging roller 2 as charging means, an exposure device 3 as exposure means, a developing device 4 as developing means, and a drum cleaning device 6 as cleaning means are arranged. Also, a primary transfer roller 5, which is a primary transfer member as primary transfer means, is arranged.

[0014] The charging roller 2 is a roller-shaped charging member and is configured as a process means that acts on the photosensitive drum 1. The exposure apparatus 3 consists of a scanner unit that scans laser light with a multifaceted mirror and irradiates the photosensitive drum 1 with a scanning beam modulated based on the image signal. The developing apparatus 4 contains a non-magnetic single-component toner as a developer and has a developing sleeve 41 as a developer carrier, etc. The primary transfer roller 5 comes into contact with the intermediate transfer belt 8 at a position facing the photosensitive drum 1. The drum cleaning apparatus 6 removes any remaining toner from the photosensitive drum 1.

[0015] The image forming apparatus 100 includes a secondary transfer unit 13 that contacts the photosensitive drums 1Y, 1M, 1C, and 1K of the image forming sections 110Y, 110M, 110C, and 110K, and to which the toner images of each color formed on the image forming sections 110Y, 110M, 110C, and 110K are first transferred. The secondary transfer unit 13 includes an intermediate transfer belt 8 to which the toner images of each color are first transferred, three rollers as support members for the intermediate transfer belt 8 (a drive roller 9, a tension roller 10, and a secondary transfer opposing roller 11), and a belt cleaning device 12. The secondary transfer unit 13 is configured to be detachably attached to the main body of the image forming apparatus 100.

[0016] The intermediate transfer belt 8 is an endless belt and a movable intermediate transfer body. The intermediate transfer belt 8 is stretched at a predetermined tension by the drive roller 9, tension roller 10, and secondary transfer opposing roller 11. The intermediate transfer belt 8 is also biased toward the primary transfer roller 5 by the photosensitive drum 1 with a predetermined pressure, forming a primary transfer nip N1 (primary transfer section) where the intermediate transfer belt 8 and the photosensitive drum 1 come into contact. At the primary transfer nip N1, the toner image supported on the photosensitive drum 1 is transferred to the intermediate transfer belt 8.

[0017] The drive roller 9 is connected to the intermediate transfer motor M1 (see Figure 2) and is driven to rotate by the intermediate transfer motor M1 in response to instructions from the control unit 70 (see Figure 2). The intermediate transfer belt 8 rotates in the direction of arrow R2, which is a predetermined direction of rotation, in accordance with the rotation of the drive roller 9. In the image forming apparatus 100, an outer roller that contacts the intermediate transfer belt 8 and a secondary transfer roller 17 that acts as a transfer member are arranged on the outer circumferential surface side of the intermediate transfer belt 8. In addition, in the image forming apparatus 100, an inner roller that contacts the intermediate transfer belt 8 and a secondary transfer opposing roller 11 that acts as an opposing member are arranged on the inner circumferential surface side of the intermediate transfer belt 8, opposite the secondary transfer roller 17. The drive roller 9 also functions as a tension roller that contacts the inner circumferential surface of the intermediate transfer belt 8 and is positioned upstream of the secondary transfer opposing roller 11 in the direction of arrow R2, which is the direction of rotation. In the following description, the portion of the outer surface of the intermediate transfer belt 8 that is stretched from the drive roller 9 to the secondary transfer opposing roller 11 in the direction of arrow R2 will also be referred to as the tensioned surface 8a.

[0018] In the image forming apparatus 100, the intersection point of the perpendicular line extending from the axis center of the secondary transfer roller 17 to the tension surface 8a is located upstream in the sheet transport direction, compared with the intersection point of the perpendicular line extending from the axis center of the secondary transfer opposing roller 11 to the tension surface 8a. In other words, the secondary transfer opposing roller 11 and the secondary transfer roller 17 are positioned such that the point at which the intermediate transfer belt 8 begins to contact the secondary transfer roller 17 is located upstream in the sheet transport direction, compared with the point at which the intermediate transfer belt 8 begins to contact the secondary transfer opposing roller 11. Hereafter, we will define the secondary transfer roller 17 as being offset upstream relative to the secondary transfer opposing roller 11 for the purpose of this explanation.

[0019] The secondary transfer roller 17 is biased by a predetermined pressure against the secondary transfer opposing roller 11 via the intermediate transfer belt 8, contacting the intermediate transfer belt 8 and forming a secondary transfer nip N2 (secondary transfer section) that transfers the toner image on the intermediate transfer belt 8 to the sheet S. The secondary transfer roller 17 rotates either directly with the rotation of the intermediate transfer belt 8 or driven by a sheet such as recording paper (secondary transfer driven method). In the image forming apparatus 100, the secondary transfer roller 17, a support member for the secondary transfer roller 17, a transport guide member for the sheet S, etc. are combined and unitized as a secondary transfer roller unit 18.

[0020] In the image forming apparatus 100, a belt cleaning device 12 is positioned downstream of the secondary transfer nip N2 in the direction of rotation of the intermediate transfer belt 8, and opposite the secondary transfer opposing roller 11, as an intermediate transfer body cleaning means. The belt cleaning device 12 has a rubber cleaning blade 12a, and the cleaning blade 12a is pressed against the intermediate transfer belt 8 at a predetermined pressure to collect unwanted toner.

[0021] In the image forming apparatus 100, when the image forming operation starts, each photosensitive drum 1 and intermediate transfer belt 8 begin to rotate in the directions of arrows R1 and R2, respectively, at a predetermined process speed. In the first embodiment, the peripheral speed of the intermediate transfer belt 8 is 150 mm / sec as the first image forming process speed and 90 mm / sec as the second speed. The photosensitive drum 1 rotates at a peripheral speed 0.5% faster than each of the respective process speeds.

[0022] The image forming apparatus 100 allows for the use of a first and second image forming process speed depending on the thickness of the sheet to ensure sufficient fixation. For plain paper, printing at a first image forming process speed of 150 mm / sec is recommended, while for sheets with a basis weight of 100 g / m2 or more, printing at a second image forming process speed of 90 mm / sec is recommended.

[0023] The surface of the rotating photosensitive drum 1 is charged substantially uniformly to a predetermined polarity (negative polarity in the first embodiment) by the charging roller 2. At this time, a predetermined charging bias is applied to the charging roller 2 from a charging bias application means (not shown). Next, the charged surface of the photosensitive drum 1 is exposed by the exposure device 3 according to the image information corresponding to each image forming unit 110. As a result, an electrostatic latent image according to the image information is formed on the surface of the photosensitive drum 1. Meanwhile, the toner in the developing device 4 is negatively charged by a developer application blade (not shown) and applied to the developing sleeve 41. A predetermined developing bias is also applied to the developing sleeve 41 from a developing bias application means (not shown). When the latent image formed on the photosensitive drum 1 reaches the opposing part (developing unit) between the photosensitive drum 1 and the developing sleeve 41, the latent image on the photosensitive drum 1 is made visible by the negatively charged toner, and a toner image is formed on the photosensitive drum 1.

[0024] Next, the toner image formed on the photosensitive drum 1 is transferred (primary transfer) to the rotating intermediate transfer belt 8 at the primary transfer nip N1 by the action of the primary transfer roller 5. At this time, the primary transfer roller 5 is subjected to a primary transfer bias from a primary transfer power supply E1 (not shown) which is a primary transfer bias application means, with a primary transfer bias of the opposite polarity (positive polarity in the first embodiment) to the charge polarity of the toner during development. For example, when forming a full-color image, a latent image is formed on the photosensitive drum 1 for each color station, and this is developed to become a toner image. The toner images of each color formed on each photosensitive drum 1 of each image forming unit 110 are sequentially transferred to the intermediate transfer belt 8 at each primary transfer nip N1Y, N1M, N1C, N1K, so as to be superimposed, and four full-color toner images of each color are formed on the intermediate transfer belt 8.

[0025] The intermediate transfer belt 8 of the first embodiment is an endless belt member consisting of two layers, a base layer and a surface layer, with a circumference of 800 mm. The surface layer is a layer that carries the toner transferred from the photosensitive drum 1, and is, for example, a layer with a thickness of about 2 μm in which zinc oxide is dispersed as an electrical resistance modifier in an acrylic resin as a base material. The base layer is, for example, a layer with a thickness of about 70 μm in which carbon black is dispersed as an electrical resistance modifier in a polyethylene naphthalate resin.

[0026] Materials that can be used for the base layer include polyethylene naphthalate resin and thermoplastic resins. Examples of thermoplastic resins include polycarbonate, polyvinylidene fluoride (PVDF), polyethylene, polypropylene, polystyrene, polyamide, polyarylate polyethylene naphthalate, polybutylene naphthalate, and thermoplastic polyimide. Two or more of these can also be used in combination.

[0027] The secondary transfer roller 17 is a 22mm outer diameter roller in which a 5mm thick foamed sponge body, whose volume resistivity is adjusted to 1×10⁶ to 1×10⁹ Ω·cm, is coated as an elastic layer around a 12mm outer diameter nickel-plated steel rod. Examples of foamed sponge bodies include NBR (nitrile butadiene rubber), SBR (styrene butadiene rubber), EPDM (ethylene propylene rubber), and silicone rubber.

[0028] The secondary transfer opposing roller 11 is made of aluminum and has an outer diameter of 25 mm. The secondary transfer opposing roller 11 may also have its surface coated with conductive rubber. The secondary transfer roller 17 is in contact with the intermediate transfer belt 8 with a pressure of 50 N and rotates in conjunction with the rotation of the intermediate transfer belt 8.

[0029] In the image forming apparatus 100, the secondary transfer roller 17 is offset upstream to grip and transport the sheet S. Therefore, the sheet S is always subjected to a force that presses it in the direction of the intermediate transfer belt 8 upstream of the secondary transfer nip N2 in the sheet transport direction while being transported.

[0030] In the image forming apparatus 100, if the upstream offset amount is small, the distance between the sheet S and the intermediate transfer belt 8 upstream of the secondary transfer nip N2 in the sheet transport direction tends to increase, making image defects due to abnormal discharge more likely to occur in low-temperature, low-humidity environments. On the other hand, in the image forming apparatus 100, if the upstream offset amount is too large, the sheet S is subjected to force in the direction of the intermediate transfer belt 8 by the secondary transfer nip N2, causing the distance between the sheet S and the intermediate transfer belt 8 to decrease, worsening image friction. For this reason, in the image forming apparatus 100 of the first embodiment, the upstream offset amount of the secondary transfer roller 17 relative to the secondary transfer opposing roller 11 is set to 4.0 mm.

[0031] The sheets S loaded in the sheet storage cassette 14 are transported by a pair of feed rollers 15, which serve as a first transport roller pair, to a registration unit 25 located downstream in the transport direction of the sheets S. The feed roller pair 15 includes a drive roller 15a that is rotationally driven by a driving force transmitted from a feed motor M2 (see Figure 2), which serves as a second motor, via a drive train (not shown), and a driven roller 15b that is rotated by the drive roller 15a. The drive roller 15a and the driven roller 15b form a transport nip N3, which serves as a first transport nip for transporting the sheets S.

[0032] The registration unit 25 has a registration roller pair (hereinafter simply referred to as the registration roller pair) 21 as a second transport roller pair. The registration roller pair 21 has registration rollers (hereinafter simply referred to as registration rollers) 22 that are rotationally driven by a driving force transmitted from a feed motor M2 (see Figure 2) via a drive train (not shown). The registration roller pair 21 also has registration rollers (hereinafter simply referred to as registration rollers) 23 that are driven to rotate by the registration rollers 22. The registration rollers 22 as the first transport rotating body and the registration rollers 23 as the second transport rotating body form a transport nip N4 that grips and transports the sheet S.

[0033] The registration unit 25 has a contact member that corrects the skewness of the sheet S when the leading edge of the sheet S abuts against it, and a shutter member 24 that acts as a skew correction member. After the skewness of the sheet S is corrected by the registration unit 25, the sheet S is transported at a predetermined transport timing that matches the transfer timing in which the toner image on the intermediate transfer belt 8 is transferred by the secondary transfer nip N2. Then, the four-color multi-layer toner image supported on the intermediate transfer belt 8 is transferred to the sheet S all at once by the action of the secondary transfer roller 17 at the secondary transfer nip N2. At this time, a secondary transfer bias is applied to the secondary transfer roller 17 from a secondary transfer power supply (not shown) which acts as a secondary transfer bias application means, with a polarity opposite to the charge polarity of the toner during development (positive polarity in the first embodiment).

[0034] The sheet S onto which the toner image has been transferred is transported to a fixing device 19, which serves as a fixing means. The sheet S is pressed and heated during the transport process while being held between the fixing roller and pressure roller of the fixing device 19, thereby fixing the toner image on the sheet S. The sheet S with the fixed toner image is discharged to the outside of the main body of the image forming apparatus 100. Any toner remaining on the intermediate transfer belt 8 after the toner image transfer to the sheet S is removed from the intermediate transfer belt 8 by the cleaning blade 12a of the belt cleaning device 12.

[0035] <Control Block> Figure 2 is a control block diagram of the image forming apparatus 100 according to the first embodiment. As shown in Figure 2, the image forming apparatus 100 has a control unit 70. The control unit 70 has a CPU 70a, a ROM 70b, and a RAM 70c. The CPU 70a reads various programs stored in the ROM 70b and controls each part of the image forming apparatus 100. The RAM 70c is used as a workspace for the CPU 70a.

[0036] The control unit 70 is connected to a video controller 71, an intermediate transfer motor M1, and a feed motor M2. When the video controller 71 (see Figure 2) receives image data from an external device (not shown), such as a host computer, it sends a print signal to the control unit 70 and converts the received image data into bitmap data for image formation. The control unit 70 scans the laser beam according to the image signal derived from the bitmap data converted for image formation.

[0037] The intermediate transfer motor M1 drives the drive roller 9 for rotating the intermediate transfer belt 8. The feed motor M2 drives the drive roller 15a and the resist roller 22 via a drive train (not shown). The control unit 70 is configured to control the driving force of the intermediate transfer motor M1 and the feed motor M2 according to the basis weight of the sheet S on which the image is formed. More details will be described later.

[0038] <Registration Unit> Next, the configuration of the registration unit 25 will be explained using Figures 3 and 4. Figure 3 is a perspective view of the registration unit 25 on the registration cluster 23 side, and Figure 4 is a cross-sectional view of the registration unit 25.

[0039] As described above, the registration unit 25 includes a pair of registration rollers 21 on which a transport nip N4 is formed for transporting the sheet S by the registration rollers 22 and registration rollers 23, and a shutter member 24 that contacts the leading edge of the sheet S. The registration rollers 22 have an outer diameter of 7 mm and are made of rubber such as EPDM. Multiple pairs of registration rollers 21 are arranged in a row in the rotation axis direction (longitudinal direction) of the registration rollers 22.

[0040] The resist roller 23 is rotatably supported with respect to the roller shaft 28 so as to follow the rotation of the resist roller 22. The shutter members 24 are positioned between two adjacent pairs of resist rollers 21 in the longitudinal direction and are fixed to a connecting member 26. The connecting member 26 is made of a 1.0 mm thick steel plate and is rotatably supported around the roller shaft 28, which serves as the pivot axis. The connecting member 26 is configured to rotate in the direction of arrow R3, which is the first direction, when the front contact portion 24a of the shutter member 24 is pressed by the conveyed sheet S.

[0041] In other words, the pivot axis of the connecting member 26 having the shutter member 24 and the rotation axis of the resist roller 23 are composed of the same roller shaft 28, and the pivot axis of the connecting member 26 having the shutter member 24 is located coaxially with the rotation axis of the resist roller 23.

[0042] Thus, in the image forming apparatus 100 of the first embodiment, the axis that serves as the pivot center for rotating the shutter member 24 and the axis that serves as the rotation center for the resist roller 23 are located coaxially. As a result, the image forming apparatus 100 can achieve skew correction of the sheet S by the shutter member 24 with a simple mechanism.

[0043] Furthermore, the connecting member 26 is provided with a shutter spring 27 that biases in the direction of arrow R4, which is a second direction opposite to the direction of arrow R3.

[0044] By providing the shutter spring 27, the image forming apparatus 100 of the first embodiment can flexibly adjust the load required when rotating the shutter member 24 using the biasing force of the shutter spring 27, thereby improving the design freedom of the shutter member 24.

[0045] <Shutter components> Figure 5(a) illustrates the state in which the shutter member 24 is in the standby position, which is the first contact position, and Figure 5(b) illustrates the state in which the shutter member 24 is in the retracted position, which is the second contact position. Furthermore, Figure 5(c) illustrates the state in which the shutter member 24 is located downstream of the retracted position in the direction of arrow R3.

[0046] The shutter member 24 has a front contact portion 24a, an intermediate contact portion 24b, and a rear contact portion 24c formed in a continuous manner. The front contact portion 24a contacts the front end of the sheet S that is gripped and conveyed by the conveying nip N3 (see Figure 1) of the feed roller pair 15 (see Figure 1) when the shutter member 24 is in the standby position shown in Figure 5(a). The intermediate contact portion 24b is located at the conveying nip N4 and contacts the side of the sheet S on the resist roller 23 side that is gripped and conveyed by the resist roller pair 21 when the shutter member 24 is in the retracted position shown in Figure 5(b). The rear contact portion 24c can contact the rear end of the sheet S that is gripped and conveyed by the intermediate transfer belt 8 and the secondary transfer roller 17 at the secondary transfer nip N2 when the shutter member 24 is located downstream in the direction of arrow R3 from the retracted position shown in Figure 5(c).

[0047] The sheet S's skewness is corrected when it comes into contact with the front contact portions 24a of multiple shutter members 24 arranged in the longitudinal direction. The standby position of the shutter members 24 is set upstream of and near the transport nip N4 in the transport direction of the sheet S, so that the sheet S enters the transport nip N4 with the sheet S in contact with the front contact portions 24a and its skewness corrected. In other words, the front contact portions 24a of the shutter members 24 are located upstream of the transport nip N4 in the transport direction of the sheet S when the shutter members 24 are in the standby position.

[0048] The shutter member 24 is configured to be maintained in a standby position until the load (tip load) at which the tip of the sheet S presses against the tip contact portion 24a exceeds a predetermined load, due to the biasing force of the shutter spring 27 and gravity acting on the connecting member 26. When the tip load at which the tip of the sheet S presses against the tip contact portion 24a exceeds a predetermined load, the shutter member 24 rotates from the standby position to the retracted position around the roller shaft 28 in the direction of arrow R3.

[0049] In the image forming apparatus 100, if the predetermined load required for the shutter member 24 to start rotating after receiving the front load is too small, for example, 0.2N or less, the performance in correcting the skewness of the sheet S will be reduced. Also, in the image forming apparatus 100, if the predetermined load required for the shutter member 24 to start rotating after receiving the front load is too large, for example, 0.35N or more, there is a risk that the shutter member 24 may not be able to rotate when transporting sheets with low rigidity, such as thin paper. For these reasons, in the image forming apparatus 100 of the first embodiment, the biasing force of the shutter spring 27 is set so that the predetermined load required for the shutter member 24 to start rotating after receiving the front load is approximately 0.3N.

[0050] The shutter member 24 is positioned in the retracted position shown in Figure 5(b) when the sheet S enters the transport nip N4 and the sheet S is being transported by the resist roller pair 21. When the shutter member 24 is in the retracted position, the intermediate contact portion 24b, which acts as both the sheet contact portion and the surface contact portion, contacts the surface of the sheet S on the side facing the resist roller 23. When the shutter member 24 is in the retracted position, it brings the intermediate contact portion 24b, which is located downstream of the transport nip N4 in the transport direction of the sheet S, into contact with the sheet S, thereby maintaining the orientation of the sheet S being transported.

[0051] As shown in Figure 5(b), the tensioning surface 8a is positioned to intersect with the common tangent L1 between the resist roller 22 and the resist roller 23 at the transport nip N4. In this way, the image forming apparatus 100 guides the leading edge of the sheet S, which is transported by the pair of resist rollers 21, toward the secondary transfer nip N2 using the tensioning surface 8a.

[0052] As described above, the sheet S is transported at the secondary transfer nip N2 while being pressed in the direction of the intermediate transfer belt 8. Therefore, in the image forming apparatus 100, when the rear end of the sheet S passes the transport nip N4, the rear end of the sheet S moves in the direction of the intermediate transfer belt 8 with an acceleration corresponding to the rigidity of the sheet S and the distance from the transport nip N4 to the secondary transfer nip N2. For this reason, in the image forming apparatus 100, if the speed of the rear end of the sheet S when it contacts the intermediate transfer belt 8 is high, the shock upon contact with the intermediate transfer belt 8 will be large, and there is a risk of rear end wobble occurring.

[0053] The shutter member 24 of the first embodiment has a rear end contact portion 24c that can contact the rear end of the sheet S that is moving toward the tension surface 8a after the rear end of the sheet S conveyed by the register roller pair 21 has passed the intermediate contact portion 24b. As described above, the shutter member 24 has a front contact portion 24a, an intermediate contact portion 24b, and a rear end contact portion 24c formed in a continuous manner.

[0054] With this configuration, the image forming apparatus 100 of the first embodiment can keep the shutter member 24 in contact with the sheet S from the time the sheet S comes into contact with the front contact portion 24a until the rear end of the sheet S comes into contact with the rear contact portion 24c. Therefore, the image forming apparatus 100 can suppress the shutter member 24 from temporarily separating from the sheet S after the sheet S comes into contact with the front contact portion 24a, and from colliding with the sheet S again.

[0055] As shown in Figure 5(a), the rear end contact portion 24c is positioned on the opposite side of the roller shaft 28 with respect to the common tangent L1 when the shutter member 24 is in the standby position. Similarly, in the shutter member 24, the intermediate contact portion 24b is also positioned on the opposite side of the roller shaft 28 with respect to the common tangent L1 when the shutter member 24 is in the standby position.

[0056] Thus, in the first embodiment of the image forming apparatus 100, when the shutter member 24 is in the standby position, an intermediate contact portion 24b and a rear end contact portion 24c are arranged on the opposite side of the roller shaft 28 with respect to the common tangent L1 of the register roller pair 21. With this configuration, the image forming apparatus 100 can prevent the rear end of the sheet S from moving far away from the shutter member 24 between passing the intermediate contact portion 24b and contacting the rear end contact portion 24c, thereby preventing the rear end of the sheet S from strongly colliding with the rear end contact portion 24c.

[0057] As shown in Figure 5(b), the rear end contact portion 24c is positioned between the common tangent line L1 and the imaginary line L2 extending along the tensioning surface 8a when the shutter member 24 is in the retracted position.

[0058] With this configuration, the image forming apparatus 100 of the first embodiment can prevent the rear end contact portion 24c from interfering with the sheet S being conveyed by the register roller pair 21. Furthermore, the image forming apparatus 100 can bring the rear end of the sheet S into contact with the rear end contact portion 24c after the rear end of the sheet S being conveyed by the register roller pair 21 has passed the intermediate contact portion 24b.

[0059] Here, the trailing edge runout is 120 g / m² 2 This tends to occur with thicker sheets than those specified above. In other words, with a basis weight of 120 g / m². 2 When transporting regular paper or thin paper with a basis weight of less than 1000, the low basis weight and low rigidity tend to reduce trailing edge wobble.

[0060] Therefore, the control unit 70 of the first embodiment determines that the basis weight of the sheet S is 120 g / m². 2If the value is less than 150 mm / sec, a first mode is executed in which the sheet S is transported at a first image forming process speed of 150 mm / sec. When transported at 150 mm / sec, the rear end of the sheet S contacts the intermediate transfer belt 8 without contacting the rear end contact portion 24c.

[0061] In other words, of the basis weight of sheet S, 120 g / m² 2 A basis weight of less than 10

[0062] Furthermore, the control unit 70 determines that the basis weight of the sheet S is 120 g / m². 2 If the above conditions are met, a second mode is executed in which the sheet is transported at a second transport speed slower than the first transport speed and a second image forming process speed of 90 mm / sec. When transported at 90 mm / sec, the movement trajectory t1 of the rear end of the sheet S intersects with the rear end contact portion 24c of the shutter member 24 which is located in the retracted position.

[0063] In other words, of the basis weight of sheet S, 120 g / m² 2 The above basis weight constitutes a second basis weight in which rear end wobble may occur if the rear end of the sheet S is not in contact with the rear end contact portion 24c. Furthermore, a sheet S transport speed of 90 mm / sec constitutes a second transport speed in which the movement trajectory of the rear end of the sheet S intersects with the rear end contact portion 24c of the shutter member 24, which is located in the retracted position.

[0064] Thus, the control unit 70 is configured to perform a first mode in which the sheet S is transported by the pair of register rollers 21 at a speed of 150 mm / sec, and a second mode in which the sheet S is transported by the pair of register rollers 21 at a speed of 90 mm / sec.

[0065] Therefore, the image forming apparatus 100 of the first embodiment has a sheet S with a basis weight of 120 g / m².2 In the above case, the rear end of the sheet S that has passed through the intermediate contact portion 24b can be brought into contact with the rear end contact portion 24c of the shutter member 24 which is located in the retracted position. As a result, the image forming apparatus 100 can achieve a basis weight of 120 g / m². 2 When forming an image on the sheet S described above, the speed at which the rear end of the sheet S moves in the direction of the intermediate transfer belt 8 after passing through the intermediate contact portion 24b can be suppressed.

[0066] Furthermore, after the rear end of the sheet S passes the intermediate contact portion 24b, it moves from the position where it was in contact with the intermediate contact portion 24b to a position closer to the intermediate transfer belt 8, while in contact with the rear end contact portion 24c, and then comes into contact with the intermediate transfer belt 8.

[0067] As shown in Figure 5(c), the shutter member 24 is configured to rotate in the direction of arrow R3 from a retracted position around the roller shaft 28 when the rear end contact portion 24c is pressed by the rear end of the sheet S.

[0068] Details of the rear end contact portion 24c will be explained using Figure 4. As shown in Figure 4, the rear end contact portion 24c is configured such that the distance from the center of the roller shaft 28, which is the pivot center of the shutter member 24, increases from upstream to downstream in the direction of arrow R3 when viewed in the direction of the roller shaft 28. Specifically, the rear end contact portion 24c is formed from a linear slope-shaped plane 24d, where the distance from the center of the roller shaft 28 increases from upstream to downstream in the direction of arrow R3, with a rate of change of 0.08 mm / deg.

[0069] The plane 24d has a linear slope shape from end 24da, which is the first part and the upstream end in the direction of arrow R3, to end 24db, which is the second part and the downstream end in the direction of arrow R3. In the shutter member 24, the distance D2 from the center of the roller shaft 28 to end 24db is longer than the distance D1 from the center of the roller shaft 28 to end 24da.

[0070] With this configuration, the image forming apparatus 100 of the first embodiment can keep the rear end of the sheet S, which has passed through the intermediate contact portion 24b and is moving toward the intermediate transfer belt 8, in contact with the rear end contact portion 24c. As a result, the image forming apparatus 100 can continuously suppress the speed at which the rear end of the sheet S moves toward the intermediate transfer belt 8 while it is in contact with the rear end contact portion 24c.

[0071] Furthermore, in order to prevent the rear end contact portion 24c of the shutter member 24 from interfering with the intermediate transfer belt 8, the image forming apparatus 100 is provided with a clearance of 2.0 mm when the shutter member 24 is closest to the intermediate transfer belt 8. As a result, the height of the image forming apparatus 100 is 2.5 mm higher than when the rear end contact portion 24c of the shutter member 24 is not provided.

[0072] Figure 6 illustrates the behavior of the rear end contact portion 24c. Figure 7(a) illustrates the case when the shutter member 24 is in the standby position, and Figure 7(b) illustrates the state when the shutter member 24 has rotated from the standby position in the direction of arrow R3 to the maximum rotation position, which is the third contact position downstream of the retracted position. As shown in Figure 6, in the image forming apparatus 100, when the sheet S is transported to the secondary transfer nip N2, the rear end of the sheet S moves toward the intermediate transfer belt 8 while contacting the rear end contact portion 24c. At this time, the shutter member 24 rotates in the direction of arrow R3 around the roller shaft 28 as the rear end contact portion 24c is pressed by the rear end of the sheet S.

[0073] A load (rear end load) acts on the shutter member 24, in a direction opposite to the direction in which the biasing force of the shutter spring 27 and gravity act on the connecting member 26, causing the rear end of the sheet S to press against the rear end contact portion 24c. If the rear end load is greater than the forces acting on the connecting member 26 by the biasing force of the shutter spring 27 and gravity, the shutter member 24 rotates in the direction of arrow R3.

[0074] The sheet S experiences a decrease in acceleration as it moves in the direction of the intermediate transfer belt 8 due to a force acting on it corresponding to the angle θ between its rear end and the rear end contact portion 24c. The larger the angle θ, the smaller the effect of reducing the acceleration of the sheet S as it moves in the direction of the intermediate transfer belt 8.

[0075] The maximum rotation position shown in Figure 7(b) is the downstream end in the direction of arrow R3 within the range in which the connecting member 26 can rotate in the direction of arrow R3, and is located downstream in the direction of arrow R3 from the retracted position. In other words, the shutter member 24 is configured to be rotatable around the roller shaft 28 within the range between the standby position and the maximum rotation position. The sheet S presses the rear end contact portion 24c of the shutter member 24 with its rear end, rotates to the maximum rotation position, and then moves away from the shutter member 24, causing its rear end to begin moving toward the intermediate transfer belt 8, after which the rear end comes into contact with the intermediate transfer belt 8.

[0076] With this configuration, the image forming apparatus 100 of the first embodiment can move the rear end of the sheet S from the position where it was in contact with the intermediate contact portion 24b to a position closer to the intermediate transfer belt 8, while bringing it into contact with the rear end contact portion 24c, and then bring it into contact with the intermediate transfer belt 8. This allows the image forming apparatus 100 to reduce the acceleration of the rear end of the sheet S when it moves away from the rear end contact portion 24c and towards the intermediate transfer belt 8.

[0077] In the first embodiment, the shutter member 24 is configured such that the angle α (maximum rotation angle) when rotated from the standby position to the maximum rotation position is approximately 60°. Furthermore, when the shutter member 24 is rotated to the maximum rotation position, the angle θ between the rear end of the sheet S and the rear end contact portion 24c is approximately 70°.

[0078] When the shutter member 24 is rotated to its maximum rotational position, the downstream end 24db of the plane 24d that forms the rear end contact portion 24c intersects with the imaginary line L2 when the tensioning surface 8a is extended in the direction of the shutter member 24.

[0079] With such a configuration, the image forming apparatus 100 of the first embodiment can prevent the rear end of the sheet S from reaching the end portion 24db of the rear end contact portion 24c until the shutter member 24 rotates to the maximum rotation position. As a result, the image forming apparatus 100 can move the position of the rear end of the sheet S closer to the intermediate transfer belt 8 when the rear end of the sheet S starts to move toward the intermediate transfer belt 8, and can reduce the acceleration of the rear end of the sheet S when moving toward the intermediate transfer belt 8.

[0080] Here, if the predetermined load required to rotate the shutter member 24 to the maximum rotation position is too small, for example, less than 0.2 N, the shutter member 24 may rotate beyond the preset maximum rotation position, and the position of the shutter member 24 may become unstable. Also, if the predetermined load required to rotate the shutter member 24 to the maximum rotation position is too large, for example, 0.3 N or more, it may be difficult for the shutter member 24 to rotate depending on the rigidity of the sheet S, and the effect of improving the rear end blur may be reduced. From these, in the image forming apparatus 100 of the first embodiment, the biasing force of the shutter spring 27 is set to be about 0.25 N as the predetermined load required to rotate the shutter member 24 to the maximum rotation position.

[0081] <Effect of the rear end contact portion> Next, the effect of reducing the rear end blur by providing the rear end contact portion 24c on the shutter member 24 will be described using an image forming apparatus of a comparative example in which the rear end contact portion is not provided on the shutter member. Table 1 is a table summarizing the presence or absence of the occurrence of rear end blur when an image is formed on sheets of each basis weight in each of the comparative example image forming apparatus and the image forming apparatus of the first embodiment.

[0082] Note that the comparative example image forming apparatus has the same configuration as the image forming apparatus of the first embodiment, except that the rear end contact portion is not provided on the shutter member. Also, in the comparative example image forming apparatus, when the basis weight is 120 g / m 2When forming an image on a sheet with a basis weight of less than 120 g / m², the system is controlled in a first mode, where the sheet is transported at a first image formation process speed of 150 mm / sec. In the comparative example image forming apparatus, the basis weight is 120 g / m². 2 When forming an image on the above sheet, the system is controlled in a second mode, where the sheet is transported at a second image formation process speed of 90 mm / sec.

[0083] [Table 1]

[0084] As described above, in the image forming apparatus 100 of the first embodiment, the basis weight is 120 g / m². 2 When forming an image on a sheet smaller than 68 g / m², the sheet S is transported at 150 mm / sec, so the rear end of the sheet S does not come into contact with the rear end contact portion 24c. Therefore, as shown in Table 1, the basis weight is 68 g / m². 2 The results regarding the occurrence of trailing edge blur when forming an image on the sheet were the same for both the comparative image forming apparatus and the image forming apparatus 100 of the first embodiment (no blur occurred).

[0085] As shown in Table 1, in the comparative example image forming apparatus in which the shutter member does not have a rear end contact portion, the basis weight is 128 g / m². 2 When an image was formed on the sheet, slight trailing edge blur occurred. In addition, the image forming apparatus in the comparative example had a basis weight of 157 g / m². 2 When an image is formed on this sheet, the basis weight is 128 g / m². 2 More trailing edge blur occurred compared to when the image was formed on the sheet.

[0086] In contrast, as shown in Table 1, in the image forming apparatus 100 of the first embodiment having a rear end contact portion 24c on the shutter member 24, the basis weight is 128 g / m². 2 When an image was formed on the sheet, trailing edge blur was eliminated. Furthermore, in the image forming apparatus 100, the basis weight was 157 g / m². 2When an image was formed on the sheet, the occurrence of trailing edge blur was kept to a minimum, and was reduced compared to the trailing edge blur that occurred in the comparative image forming apparatus.

[0087] Thus, the image forming apparatus 100 of the first embodiment has a sheet S with a basis weight of 120 g / m². 2 When the above conditions are met, the shutter member 24 can be brought into contact with the rear end contact portion 24c by being controlled in the second mode. As a result, the image forming apparatus 100 can use a basis weight of 120 g / m². 2 When forming an image on the sheet S described above, the speed at which the rear end of the sheet S moves in the direction of the intermediate transfer belt 8 after passing through the intermediate contact portion 24b can be suppressed, thereby suppressing the occurrence of rear end wobble.

[0088] <Summary of the First Embodiment> As described above, in the first embodiment of the image forming apparatus 100, the rear end of the sheet S, after passing through the intermediate contact portion 24b, contacts the rear end contact portion 24c of the shutter member 24 before it contacts the intermediate transfer belt 8. The shutter member 24 rotates in the direction of arrow R3 around the roller shaft 28 as the rear end contact portion 24c is pressed against the rear end of the sheet S, suppressing the speed at which the rear end of the sheet S approaches the intermediate transfer belt 8, and bringing the rear end of the sheet S closer to the intermediate transfer belt 8.

[0089] As a result, the image forming apparatus 100 can reduce the acceleration of the rear end of the sheet S in the direction of the intermediate transfer belt 8 by bringing the rear end of the sheet S into contact with the intermediate transfer belt 8 after the rear end of the sheet S approaches the intermediate transfer belt 8. Furthermore, by reducing the acceleration of the rear end of the sheet S in the direction of the intermediate transfer belt 8, the image forming apparatus 100 can reduce the shock when the rear end of the sheet S comes into contact with the intermediate transfer belt 8, thereby suppressing the occurrence of rear end vibration.

[0090] Furthermore, the image forming apparatus 100 can suppress rear-end vibration by using the rear-end contact portion 24c provided on the shutter member 24, without requiring a separate transport guide for the shutter member 24. As a result, the image forming apparatus 100 can suppress cost increases due to an increase in the size of the apparatus body and the number of parts.

[0091] [Second Embodiment] Next, the image forming apparatus 100 of the second embodiment will be described. In the image forming apparatus 100 of the second embodiment, the distance from the center of the roller shaft 28 to the rear end contact portion of the shutter member 24 is configured to increase quadratically from upstream to downstream in the direction of arrow R3. In this respect, the image forming apparatus 100 of the second embodiment differs from the first embodiment described above. The other configurations are the same as those of the first embodiment, so components common to the first embodiment are denoted by the same reference numerals, and control processes common to the first embodiment are denoted by the same step numbers and their descriptions are omitted.

[0092] <Shutter components> Figure 8 is a cross-sectional view of the registration unit 25 in the second embodiment. As shown in Figure 8, in the shutter member 24 of the second embodiment, the rear end contact portion 24c is configured such that the distance from the center of the roller shaft 28, which is the rotation center of the shutter member 24, increases from upstream to downstream in the direction of arrow R3. Specifically, the rear end contact portion 24c is formed from a curved surface 24f with a quadratic slope shape, where the distance from the center of the roller shaft 28 increases from upstream to downstream in the direction of arrow R3.

[0093] Because the rear end contact portion 24c is formed from a curved surface 24f, the image forming apparatus 100 can make the angle θ between the rear end of the sheet S and the rear end contact portion 24c smaller than when the rear end contact portion 24c is formed from a straight plane. Furthermore, the image forming apparatus 100 is configured such that the angle θ between the rear end of the sheet S and the rear end contact portion 24c decreases as the sheet S is transported downstream in the sheet S transport direction.

[0094] With this configuration, in the second embodiment, as the sheet S is transported downstream in the transport direction, the force with which the rear end of the sheet S presses against the rear end contact portion 24c increases. Therefore, the shutter member 24 can reduce the acceleration of the sheet S when it moves away from the rear end contact portion 24c and toward the intermediate transfer belt 8. In addition, the shutter member 24 can rotate in the direction of arrow R3 by an angle of more than approximately 60°, and the position of the rear end of the sheet S when it begins to move toward the intermediate transfer belt 8 can be brought closer to the intermediate transfer belt 8.

[0095] In the second embodiment, the rear end contact portion 24c is longer in the case where the distance from the center of the roller shaft 28 at the downstream end in the direction of arrow R3 is greater than in the case where it is a straight, planar shape. When the clearance between the shutter member 24 and the intermediate transfer belt 8 is the same as in the first embodiment (2.0 mm), the height of the main body of the image forming apparatus 100 in the second embodiment is about 3 mm higher than the height of the main body of the image forming apparatus 100 in the first embodiment. Also, the height of the main body of the image forming apparatus 100 in the second embodiment is about 5.5 mm higher than the height of the main body of an image forming apparatus without a rear end contact portion.

[0096] <Effect of the rear end contact part> Next, the effect of reducing rear-end blur by the shutter member 24 having the rear-end contact portion 24c of the second embodiment will be described. Table 2 is a table summarizing whether or not rear-end blur occurs when images are formed on sheets of each basis weight in the image forming apparatus of the comparative example described above and the image forming apparatus of the first and second embodiments.

[0097] [Table 2]

[0098] As shown in Table 2, in the image forming apparatus 100 of the second embodiment, which has a rear end contact portion 24c with a quadratic curve on the shutter member 24, the basis weight is 157 g / m². 2Even when an image was formed on the sheet, trailing edge blur was eliminated.

[0099] <Summary of the second embodiment> As described above, in the image forming apparatus 100 of the second embodiment, the rear end contact portion 24c of the shutter member 24 contacts the rear end contact portion 24c of the shutter member 24 before the rear end of the sheet S, which has passed through the intermediate contact portion 24b, contacts the intermediate transfer belt 8. The shutter member 24 rotates in the direction of arrow R3 around the roller shaft 28 as the rear end contact portion 24c is pressed against the rear end of the sheet S, suppressing the speed at which the rear end of the sheet S approaches the intermediate transfer belt 8, while bringing the rear end of the sheet S closer to the intermediate transfer belt 8. Furthermore, since the rear end contact portion 24c of the shutter member 24 is composed of a curved surface 24f with a quadratic slope shape, the force acting on the rear end of the sheet S can be increased even when the shutter member 24 rotates in the direction of arrow R3 around the roller shaft 28.

[0100] As a result, the image forming apparatus 100 can bring the rear end of the sheet S closer to the intermediate transfer belt 8 than when the rear end contact portion 24c is formed from a straight slope shape, and can reduce the acceleration of the rear end of the sheet S in the direction of the intermediate transfer belt 8. Furthermore, the image forming apparatus 100 can reduce the acceleration of the rear end of the sheet S in the direction of the intermediate transfer belt 8 than when the rear end contact portion 24c is formed from a straight slope shape. Therefore, the image forming apparatus 100 can reduce the shock when the rear end of the sheet S contacts the intermediate transfer belt 8 and suppress the occurrence of rear end vibration.

[0101] [Other embodiments] In the first and second embodiments, the rear end contact portion 24c is configured such that the distance from the center of the roller shaft 28 increases as you move from upstream to downstream in the direction of arrow R3, but it is not limited to this configuration. The rear end contact portion 24c may be formed in a stepped shape, for example, having a convex portion that extends in the direction that increases the distance from the center of the roller shaft 28.

[0102] Figure 9 shows a shutter member 24 having a rear end contact portion 24c with a convex portion formed thereon. As shown in Figure 9, the rear end contact portion 24c has a plane 24g with a linear slope shape, the distance from the center of the roller shaft 28 increasing from distance D3 to distance D4 as you move from upstream to downstream in the direction of arrow R3. The rear end contact portion 24c also has a convex portion 24h that extends from the plane 24g to a position where the distance from the center of the roller shaft 28 is longer than distance D4, at a distance D5. The rear end contact portion 24c is formed in a stepped shape by the plane 24g and the convex portion 24h.

[0103] Because the rear end contact portion 24c is formed in a stepped shape, the shutter member 24 can bring the rear end of the sheet S into contact with the protrusion 24h when the sheet S contacts the rear end contact portion 24c. As a result, the shutter member 24 can prevent the distance from the center of the roller shaft 28 required to rotate to the maximum rotation position from becoming excessively long, and can set the maximum rotation position downstream in the direction of arrow R3 while suppressing an increase in the height of the main body of the image forming apparatus 100.

[0104] In this configuration, the image forming apparatus 100 may experience a shock when the rear end of the sheet S contacts the protrusion 24h after passing the intermediate contact portion 24b. To suppress the occurrence of a shock when contacting the protrusion 24h, the rear end contact portion 24c may be configured to have a plurality of protrusions 24i to 24k, each at a different distance from the center of the roller axis 28, as shown in Figure 10.

[0105] In this configuration, the rear end contact portion 24c has a protrusion 24i that extends from the plane 24g to a position where the distance from the center of the roller shaft 28 is a distance D6 which is longer than the distance D4. The rear end contact portion 24c also has a protrusion 24j that extends from the protrusion 24i to a position where the distance from the center of the roller shaft 28 is a distance D7 which is longer than the distance D6. The rear end contact portion 24c also has a protrusion 24k that extends from the protrusion 24j to a position where the distance from the center of the roller shaft 28 is a distance D8 which is longer than the distance D7.

[0106] By providing multiple protrusions 24i to 24k, the distance of each protrusion from the center of the roller axis 28 can be made shorter than the distance from the center of the roller axis 28 when a single protrusion 24h extends from the plane 24g. As a result, in the image forming apparatus 100, the shock when the rear end of the sheet S contacts each protrusion after passing through the intermediate contact portion 24b can be suppressed.

[0107] Furthermore, the shape of the rear end contact portion 24c may be selected and adjusted according to the characteristics of the apparatus, such as the type of sheet S used to form the image, the toner, etc., and the target range of basis weight of the sheet S used in the image forming apparatus 100. The shape of the rear end contact portion 24c should be formed so as to intersect with the movement trajectory of the rear end of the sheet S after it has passed the intermediate contact portion 24b.

[0108] Furthermore, although the first and second embodiments described the second image forming process speed as 90 mm / sec as an example, the invention is not limited to this. In the image forming apparatus 100, if the image forming process speed is such that the rear end of the sheet S contacts the rear end contact portion 24c, the effect of reducing rear end vibration caused by the provision of the rear end contact portion 24c can be achieved.

[0109] Furthermore, in the first embodiment, the maximum rotation angle is approximately 60°, but it is not limited to this. As described above, in the image forming apparatus 100, the larger the maximum rotation angle of the shutter member 24, the more likely the sheet S and the intermediate transfer belt 8 are to come into contact, and the more likely image abrasion is to occur. For this reason, it is preferable that the shutter member 24 be configured so that the maximum rotation angle can be adjusted according to the type of sheet on which the image is formed, the amount of rear-end blur that occurs, and whether or not image abrasion occurs.

[0110] Furthermore, in the first and second embodiments, a shutter spring 27 is provided on the connecting member 26, but the invention is not limited thereto. The registration unit 25 may have a force acting in the direction of arrow R4, opposite to the direction of arrow R3, due to the weight of the connecting member 26 and the shutter member 24, and does not necessarily have a biasing member.

[0111] Furthermore, in the first and second embodiments, the pivot axis of the shutter member 24 is configured to be coaxial with the roller axis 28 of the resist roller 23, but the shutter member 24 is not limited to this. The shutter member 24 may be configured to be rotatable around an axis other than the roller axis 28.

[0112] Furthermore, in the first and second embodiments, the shutter member 24 is configured to rotate around the roller shaft 28 from the retracted position downstream in the direction of arrow R3 when the rear end contact portion 24c is pressed against the rear end of the sheet S, but is not limited to this. The shutter member 24 may be configured to be maintained in the retracted position even when the rear end contact portion 24c is pressed against the rear end of the sheet S. In this configuration, it is preferable that the shape of the rear end contact portion 24c of the image forming apparatus 100 is set such that the rear end of the sheet S moves toward the intermediate transfer belt 8 while in contact with the rear end contact portion 24c of the shutter member 24 which is maintained in the retracted position.

[0113] Summary of this disclosure This disclosure includes at least the following: (Composition 1) A first conveying roller pair having a first conveying nip that grips the sheet and conveys it in the sheet conveying direction, A second pair of conveying rollers that forms a second conveying nip and grips and conveys a sheet located at the second conveying nip, the second pair of conveying rollers being positioned downstream of the first pair of conveying rollers in the sheet conveying direction, A contact member having a tip contact portion and a surface contact portion, wherein the tip contact portion is movable between a first contact position in which it can contact the tip of the sheet and a second contact position in which it can contact the surface of the sheet, and the contact member rotates in a first direction about a pivot axis from the first contact position to the second contact position when the tip contact portion is pressed by the tip of the sheet being conveyed by the first conveying nip, An image carrier that holds the toner image, A belt on which a toner image is transferred from the image carrier while rotating in a predetermined rotational direction, An outer roller that contacts the outer surface of the belt, An inner roller is positioned opposite the outer roller, sandwiching the belt, and contacts the inner circumferential surface of the belt, forming a transfer nip together with the outer roller that transfers the toner image on the belt to the sheet. The belt comprises a tension roller that contacts the inner circumferential surface of the belt and is positioned upstream of the inner roller in the rotational direction, The tip contact portion is located upstream of the second conveying nip in the sheet conveying direction when the contact member is in the first contact position. The surface contact portion is located downstream of the second conveying nip in the sheet conveying direction when the contact member is in the second contact position. Of the outer circumferential surface, the portion stretched from the tension roller to the inner roller in the rotational direction is arranged to intersect with the common tangent of the second conveyor roller pair in the second conveyor nip. The contact member has a rear end contact portion that can contact the rear end of the sheet that moves toward the portion of the outer circumferential surface after the rear end of the sheet, which is being conveyed by the second pair of conveying rollers, has passed the surface contact portion of the contact member where it is located at the second contact position. An image forming apparatus characterized by the following features. (Configuration 2) The rear end contact portion is positioned between the common tangent line and a virtual line extending along the portion of the outer circumferential surface when the contact member is in the second contact position. The image forming apparatus according to configuration 1, characterized in that it is a picture forming apparatus. (Composition 3) The rear end contact portion is provided such that, when the contact member is in the second contact position, it intersects with the movement trajectory of the rear end of the sheet that has passed through the surface contact portion. An image forming apparatus according to configuration 1 or 2, characterized by the above. (Composition 4) The contact member rotates about the pivot axis downstream in the first direction from the second contact position when the rear end contact portion is pressed against the rear end of the sheet being conveyed by the second conveyor roller pair. An image forming apparatus according to any one of configurations 1 to 3, characterized by the above. (Composition 5) The aforementioned rear end contact portion is A first portion whose distance from the pivot axis is a first distance, It has a second portion whose distance from the pivot axis is a second distance which is longer than the first distance, The second part is located downstream of the first part in the first direction. An image forming apparatus according to any one of configurations 1 to 4, characterized by the above. (Composition 6) The rear end contact portion is configured such that the distance from the pivot axis increases as you move from upstream to downstream in the first direction. The image forming apparatus according to configuration 5, characterized by the features described herein. (Composition 7) The contact member is rotatable about the pivot axis within the range between the first contact position and a third contact position downstream of the second contact position in the first direction. The rear end contact portion is formed such that, when viewed in the direction of the pivot axis, the distance from the pivot axis increases as it moves downstream in the first direction. The downstream end of the rear end contact portion in the first direction intersects with a virtual line extending along the portion of the outer circumferential surface when the contact member is positioned at the third contact position. An image forming apparatus according to any one of configurations 1 to 6 characterized by the above. (Composition 8) The contact member is formed such that the front contact portion, the surface contact portion, and the rear end contact portion are formed in a continuous manner. An image forming apparatus according to any one of configurations 1 to 7, characterized by the above. (Composition 9) The rear end contact portion is positioned on the opposite side of the pivot axis to the common tangent of the second conveyor roller pair when the contact member is in the first contact position. An image forming apparatus according to any one of configurations 1 to 8, characterized by the above. (Composition 10) The second conveyor roller pair comprises a first conveyor rotating body that is rotationally driven, and a second conveyor rotating body that is driven to rotate by the first conveyor rotating body. The pivot axis of the contact member is located coaxially with the rotation center of the second conveying rotating body. An image forming apparatus according to any one of configurations 1 to 9, characterized by the above. (Composition 11) The system further comprises a biasing member that biases the contact member toward a second direction opposite to the first direction. An image forming apparatus according to any one of configurations 1 to 10, characterized by the above. (Composition 12) The system further includes a control unit capable of performing a first mode in which the sheet is transported by the second pair of transport rollers at a first transport speed, and a second mode in which the sheet is transported by the second pair of transport rollers at a second transport speed slower than the first transport speed. The rear end contact portion contacts the rear end of the sheet being conveyed by the second conveyor roller pair when the control unit executes the second mode. An image forming apparatus according to any one of configurations 1 to 11, characterized by the above. (Composition 13) The control unit executes the first mode when transporting a first sheet having a first basis weight, and executes the second mode when transporting a second sheet having a larger basis weight than the first sheet. The image forming apparatus according to configuration 12, characterized in that... [Explanation of symbols]

[0114] 1…Image carrier (photosensitive drum) / 8…Belt (intermediate transfer belt) / 9…Tension roller (drive roller) / 11…Inner roller (secondary transfer opposing roller) / 15…First transport roller pair (feeding roller pair) / 17…Outer roller (secondary transfer roller) / 21…Second transport roller pair (resist roller pair) / 22…First transport rotating body (resist roller) / 23…Second transport rotating body (resist roller) / 24…Contact member (shutter member) / 24a…Tip contact part / 24b…Surface Contact part (intermediate contact part) / 24c... Rear end contact part / 24da... First part (end) / 24db... Second part (end) / 27... Biasing member (shutter spring) / 28... Rotation axis (roller axis) / 70... Control unit / 100... Image forming apparatus / D1... First distance (distance) / D2... Second distance (distance) / L1... Common tangent line / L2... Imaginary line / N2... Transfer nip (secondary transfer nip) / N3... First transport nip (transport nip) / N4... Second transport nip (transport nip) / S... Sheet

Claims

1. A pair of first conveying rollers having a first conveying nip that grips the sheet and conveys it in the sheet conveying direction, A second pair of conveying rollers that forms a second conveying nip and grips and conveys a sheet located at the second conveying nip, the second pair of conveying rollers being arranged downstream of the first pair of conveying rollers in the sheet conveying direction, A contact member having a tip contact portion and a surface contact portion, wherein the tip contact portion is movable between a first contact position in which it can contact the tip of the sheet and a second contact position in which it can contact the surface of the sheet, and the contact member rotates in a first direction about a pivot axis from the first contact position to the second contact position when the tip contact portion is pressed by the tip of the sheet being conveyed by the first conveying nip, An image carrier that holds the toner image, A belt on which a toner image is transferred from the image carrier while rotating in a predetermined rotational direction, An outer roller that contacts the outer surface of the belt, An inner roller is positioned opposite the outer roller, sandwiching the belt, and contacts the inner circumferential surface of the belt, forming a transfer nip together with the outer roller that transfers the toner image on the belt to the sheet. The belt comprises a tension roller that contacts the inner circumferential surface of the belt and is positioned upstream of the inner roller in the rotational direction, The tip contact portion is located upstream of the second conveying nip in the sheet conveying direction when the contact member is in the first contact position. The surface contact portion is located downstream of the second conveying nip in the sheet conveying direction when the contact member is in the second contact position. Of the outer circumferential surface, the portion stretched from the tension roller to the inner roller in the rotational direction is arranged to intersect with the common tangent of the second conveyor roller pair in the second conveyor nip. The contact member has a rear end contact portion that can contact the rear end of the sheet that moves toward the portion of the outer circumferential surface after the rear end of the sheet, which is being conveyed by the second pair of conveying rollers, has passed the surface contact portion of the contact member where it is located at the second contact position. An image forming apparatus characterized by the following features.

2. The rear end contact portion is positioned between the common tangent line and a virtual line extending along the portion of the outer circumferential surface when the contact member is in the second contact position. The image forming apparatus according to feature 1.

3. The rear end contact portion is provided such that, when the contact member is positioned at the second contact position, it intersects with the movement trajectory of the rear end of the sheet that has passed through the surface contact portion. The image forming apparatus according to feature 1.

4. The contact member rotates about the pivot axis downstream in the first direction from the second contact position when the rear end contact portion is pressed against the rear end of the sheet being conveyed by the second conveyor roller pair. The image forming apparatus according to feature 1.

5. The aforementioned rear end contact portion is A first portion whose distance from the pivot axis is a first distance, It has a second portion whose distance from the pivot axis is a second distance which is longer than the first distance, The second part is located downstream of the first part in the first direction. The image forming apparatus according to feature 1.

6. The rear end contact portion is configured such that the distance from the pivot axis increases as you move from upstream to downstream in the first direction. The image forming apparatus according to feature 5.

7. The contact member is rotatable about the pivot axis within the range between the first contact position and the third contact position downstream of the second contact position in the first direction. The rear end contact portion is formed such that, when viewed in the direction of the pivot axis, the distance from the pivot axis increases as it moves downstream in the first direction. The downstream end of the rear end contact portion in the first direction intersects with a virtual line extending along the portion of the outer circumferential surface when the contact member is positioned at the third contact position. The image forming apparatus according to feature 1.

8. The contact member is formed such that the front contact portion, the surface contact portion, and the rear end contact portion are formed in a continuous manner. The image forming apparatus according to feature 1.

9. The rear end contact portion is positioned on the opposite side of the pivot axis to the common tangent of the second conveyor roller pair when the contact member is in the first contact position. The image forming apparatus according to feature 1.

10. The second conveyor roller pair comprises a first conveyor rotating body that is rotationally driven, and a second conveyor rotating body that is driven to rotate by the first conveyor rotating body. The pivot axis of the contact member is located coaxially with the rotation center of the second conveying rotating body. The image forming apparatus according to feature 1.

11. The system further comprises a biasing member that biases the contact member toward a second direction opposite to the first direction. The image forming apparatus according to feature 1.

12. The system further includes a control unit capable of performing a first mode in which the sheet is transported by the second pair of transport rollers at a first transport speed, and a second mode in which the sheet is transported by the second pair of transport rollers at a second transport speed slower than the first transport speed. The rear end contact portion contacts the rear end of the sheet being transported by the second transport roller pair when the control unit executes the second mode. The image forming apparatus according to feature 1.

13. The control unit executes the first mode when transporting a first sheet having a first basis weight, and executes the second mode when transporting a second sheet having a larger basis weight than the first sheet. The image forming apparatus according to feature 12.

Images