Transfer unit and image forming apparatus equipped with same

Abstract

To provide a transfer unit that grounds two transfer rollers selectively brought into pressure contact with an image carrier, and thereby can stably form high-quality images, and an image forming apparatus including the same.SOLUTION: A transfer unit comprises: a first roller and a second roller that are different from each other in one of the axial length of an elastic layer, volume resistivity, or hardness; a first bearing member and a second bearing member that rotatably support a core grid of the first roller and a core grid of the second roller; a roller holder; and a switching mechanism. The roller holder has a first bearing holding part and a second bearing holding part that slidably hold the first bearing member and the second bearing member. The switching mechanism drives to rotate the roller holder to arrange one of the first roller or the second roller to a reference position where the roller is brought into pressure contact with an image carrier to form a transfer nip part. The first bearing member and the second bearing member have ground members that ground the first roller and the second roller, respectively.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] The present invention relates to a transfer unit that transfers a toner image formed on an image carrier such as a photosensitive drum or an intermediate transfer belt onto a recording medium, and an image forming apparatus equipped with the same, and in particular to a mechanism for grounding the transfer member. [Background technology]

[0002] Conventionally, an intermediate transfer type image forming device has been known which includes an endless intermediate transfer belt that rotates in a predetermined direction and a plurality of image forming units arranged along the intermediate transfer belt, in which each image forming unit sequentially superimposes a toner image of each color onto the intermediate transfer belt to perform primary transfer, and then a secondary transfer roller performs secondary transfer of the toner image onto a recording medium such as paper.

[0003] In such intermediate transfer image forming devices, toner adhesion to the surface of the secondary transfer roller progresses due to long-term printing. To improve color development and color reproducibility, calibration must be performed at predetermined intervals to correct image density and color misalignment. However, patch images formed on the intermediate transfer belt during calibration are not transferred to paper but are instead removed by a belt cleaning device. As a result, when the patch images pass through the secondary transfer roller, some of the toner transferred onto the intermediate transfer belt adheres to the secondary transfer roller.

[0004] Conventionally, the secondary transfer roller has been cleaned by applying a reverse transfer voltage (a voltage with the same polarity as the toner) to the secondary transfer roller when no image is being formed, returning the toner adhering to the secondary transfer roller to the intermediate transfer belt. However, this method has the problem of taking time to clean the secondary transfer roller, which increases the waiting time for printing.

[0005] Therefore, a method has been proposed for improving productivity by making it possible to switch the secondary transfer roller to a size suitable for the recording medium. For example, Patent Document 1 discloses an image forming apparatus that includes a plurality of secondary transfer rollers each having a different axial length, a rotating body that rotatably supports the plurality of secondary transfer rollers and has a support part that can rotate around an axis parallel to the axial direction, and a control unit that selects one roller from the plurality of secondary transfer rollers according to the width of the recording medium and rotates the support part to cause the one roller to face an intermediate transfer belt. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-156653 Summary of the Invention [Problem to be solved by the invention]

[0007] In order to obtain a good transferred image by applying an appropriate transfer electric field between the transfer roller and the recording medium, it is necessary to ground the transfer roller and remove residual charge. Patent Document 1 does not describe a mechanism for grounding the secondary transfer roller, and there is a risk of transfer defects occurring due to residual charge on the secondary transfer roller.

[0008] In view of the above problems, the present invention aims to provide a transfer unit and an image forming apparatus equipped with the same that can stably form high-quality images by grounding two transfer rollers that are selectively pressed against an image carrier. [Means for solving the problem]

[0009] To achieve the above object, a first aspect of the present invention is a transfer unit that includes a transfer roller having a core and an elastic layer laminated on the outer peripheral surface of the core, with the elastic layer pressed against an image carrier to form a transfer nip, and that transfers a toner image formed on the image carrier to a recording medium passing through the transfer nip. The transfer unit includes first and second rollers as transfer rollers, a first bearing member, a second bearing member, a roller holder, and a switching mechanism. The first and second rollers differ in the axial length, volume resistivity, or hardness of the elastic layers. The first bearing member rotatably supports the core of the first roller. The second bearing member rotatably supports the core of the second roller. The roller holder has a first bearing holder and a second bearing holder that slidably hold the first and second bearing members, respectively, in a direction toward or away from the image carrier. The switching mechanism rotates the roller holder to position either the first roller or the second roller at a reference position where it is pressed against the image carrier to form a transfer nip. The first bearing member and the second bearing member have grounding members that ground the first roller and the second roller, respectively. [Effects of the Invention]

[0010] According to the first aspect of the present invention, the first roller and the second roller are always grounded (earthed) by the grounding member, so that no transfer electric field remains in the first roller and the second roller after transfer, making it possible to always apply an appropriate transfer electric field to the transfer nip area, and to stably obtain a good transferred image. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a schematic diagram showing the internal configuration of an image forming apparatus 100 equipped with a secondary transfer unit 9 of the present invention. [Figure 2] Enlarged view of the image forming section Pa in FIG. [Figure 3] A side cross-sectional view of an intermediate transfer unit 30 mounted in an image forming apparatus 100. [Figure 4]1 is a perspective view of a secondary transfer unit 9 according to an embodiment of the present invention, which is mounted in an image forming apparatus 100. [Figure 5] FIG. 1 is an enlarged perspective view showing the configuration of one end side of a secondary transfer unit 9 according to an embodiment of the present invention. [Figure 6] 1 is a perspective view of the periphery of a roller holder 47 of a secondary transfer unit 9 according to the present embodiment, viewed from the outside in the axial direction. [Figure 7] FIG. 10 is a perspective view of the roller holder 47 and its periphery of the secondary transfer unit 9 as seen from the front side, illustrating the contact state between the shaft 51 and the main body frame 101. [Figure 8] FIG. 10 is an enlarged perspective view of the periphery of a first bearing member 43 and a second bearing member 45 of the secondary transfer unit 9, as viewed from the outside in the axial direction. [Figure 9] FIG. 1 is a perspective view showing a drive mechanism of a secondary transfer unit 9 according to an embodiment of the present invention. [Figure 10] Circuit diagram showing the flow of secondary transfer current in the secondary transfer nip N [Figure 11] FIG. 1 is a block diagram showing an example of a control path of an image forming apparatus 100 equipped with a secondary transfer unit 9 according to an embodiment of the present invention. [Figure 12] FIG. 1 is a side cross-sectional view including a switching cam 50 of the secondary transfer unit 9 of the present embodiment, showing a state in which the first roller 40 is disposed at a reference position for forming the secondary transfer nip portion N. [Figure 13] 1 is a plan view of the switching cam 50 as seen from the inside in the axial direction. [Figure 14] 13 is a diagram showing the first roller 40 in a separated state after the switching cam 50 is rotated clockwise by a predetermined angle from the state shown in FIG. 12. [Figure 15] 15 is a diagram showing a state in which the shaft 51 is rotated counterclockwise from the state of FIG. 14 to bring the second roller 41 into opposition to the drive roller 10. FIG. [Figure 16] FIG. 16 shows a state in which the switching cam 50 is rotated counterclockwise by a predetermined angle from the state of FIG. 15, and the second roller 41 is positioned at a reference position for forming the secondary transfer nip portion N. [Figure 17] 17 is a diagram showing the second roller 41 in a separated state after the switching cam 50 is further rotated counterclockwise by a predetermined angle from the state shown in FIG. 16. [Figure 18] 18 is a diagram showing a state in which the switching cam 50 is rotated clockwise by a predetermined angle from the state of FIG. 17 so that the first roller 40 faces the drive roller 10. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Fig. 1 is a schematic diagram showing the configuration of an image forming apparatus 100 equipped with a secondary transfer unit 9 of the present invention, and Fig. 2 is an enlarged view of the vicinity of an image forming section Pa in Fig. 1.

[0013] The image forming apparatus 100 shown in Fig. 1 is a so-called tandem color multifunction peripheral, and has the following configuration: Four image forming units Pa, Pb, Pc, and Pd are arranged in this order within the main body of the image forming apparatus 100, from the upstream side in the transport direction (the left side in Fig. 1). These image forming units Pa to Pd are provided corresponding to images of four different colors (magenta, cyan, yellow, and black), and sequentially form magenta, cyan, yellow, and black images through the respective processes of charging, exposure, development, and transfer.

[0014] Each of the image forming stations Pa to Pd is provided with photosensitive drums 1a, 1b, 1c, and 1d, each carrying a visible image (toner image) of each color. An intermediate transfer belt 8, which rotates counterclockwise in FIG. 1, is provided adjacent to each of the image forming stations Pa to Pd. The toner images formed on the photosensitive drums 1a to 1d are sequentially transferred onto the intermediate transfer belt 8, which moves while contacting the photosensitive drums 1a to 1d, and then transferred all at once onto a sheet of paper S, an example of a recording medium, in a secondary transfer unit 9. The images are then fixed onto the sheet of paper S in a fixing unit 13, and then ejected from the image forming apparatus 100. While the photosensitive drums 1a to 1d are rotating clockwise in FIG. 1, an image formation process is performed on each of the photosensitive drums 1a to 1d.

[0015] The paper S onto which the toner image is transferred is stored in a paper cassette 16 at the bottom of the main body of the image forming apparatus 100, and is transported to the secondary transfer unit 9 via a paper feed roller 12a and a pair of registration rollers 12b. A seamless belt is usually used as the intermediate transfer belt 8.

[0016] Next, the image forming units Pa to Pd will be described. Image forming unit Pa will be described in detail below, but the image forming units Pb to Pd will not be described because they have basically the same configuration. As shown in FIG. 2, a charging device 2a, a developing device 3a, and a cleaning device 7a are arranged around the photosensitive drum 1a in the drum rotation direction (clockwise in FIG. 2), and a primary transfer roller 6a is arranged across the intermediate transfer belt 8. In addition, a belt cleaning unit 19 is arranged upstream of the photosensitive drum 1a in the rotation direction of the intermediate transfer belt 8, facing a tension roller 11 with the intermediate transfer belt 8 between them.

[0017] Next, an image formation procedure in the image forming apparatus 100 will be described. When a user inputs a command to start image formation, first, the main motor 60 (see FIG. 11) starts rotating the photosensitive drums 1a-1d, and the surfaces of the photosensitive drums 1a-1d are uniformly charged by the charging rollers 25 of the charging devices 2a-2d. Next, the surfaces of the photosensitive drums 1a-1d are irradiated with beam light (laser light) emitted from the exposure device 5, and an electrostatic latent image corresponding to an image signal is formed on each of the photosensitive drums 1a-1d.

[0018] The developing devices 3a-3d are filled with a predetermined amount of magenta, cyan, yellow, and black toner, respectively. When the toner content in the two-component developer filled in each of the developing devices 3a-3d falls below a predetermined value due to the formation of a toner image (described later), toner is replenished from the toner containers 4a-4d to each of the developing devices 3a-3d. The toner in the developer is supplied to the photosensitive drums 1a-1d by the developing rollers 22 of the developing devices 3a-3d and electrostatically adheres to them. This forms a toner image corresponding to the electrostatic latent image formed by exposure from the exposure device 5.

[0019] Then, primary transfer rollers 6a-6d apply an electric field at a predetermined transfer voltage between the primary transfer rollers 6a-6d and the photosensitive drums 1a-1d, and the magenta, cyan, yellow, and black toner images on the photosensitive drums 1a-1d are primarily transferred onto the intermediate transfer belt 8. These four color images are formed in a predetermined positional relationship for forming a predetermined full-color image. Thereafter, in preparation for the subsequent formation of a new electrostatic latent image, any toner remaining on the surfaces of the photosensitive drums 1a-1d is removed by cleaning blades 23 and rubbing rollers 24 of cleaning devices 7a-7d.

[0020] When the intermediate transfer belt 8 starts to rotate counterclockwise in accordance with the rotation of the drive roller 10 by the belt drive motor 61 (see FIG. 11), the paper S is transported from the pair of registration rollers 12b to the secondary transfer unit 9 provided adjacent to the intermediate transfer belt 8 at a predetermined timing, and a full-color image is transferred onto the paper S. The paper S onto which the toner image has been transferred is transported to the fixing unit 13. Any toner remaining on the surface of the intermediate transfer belt 8 is removed by the belt cleaning unit 19.

[0021] The paper S transported to the fixing unit 13 is heated and pressurized by the fixing roller pair 13a, and the toner image is fixed to the surface of the paper S, forming a predetermined full-color image. The paper S on which the full-color image has been formed has its transport direction diverted by the branching unit 14, which branches in multiple directions, and is discharged directly (or after being sent to the double-sided transport path 18 and printed on both sides) by the discharge roller pair 15 onto the discharge tray 17.

[0022] An image density sensor 28 is disposed at a position facing the drive roller 10 across the intermediate transfer belt 8. The image density sensor 28 is generally an optical sensor equipped with a light-emitting element such as an LED and a light-receiving element such as a photodiode. When measuring the amount of toner adhesion on the intermediate transfer belt 8, the light-emitting element irradiates each patch image (reference image) formed on the intermediate transfer belt 8 with measurement light, and the measurement light is reflected by the toner and the belt surface and enters the light-receiving element.

[0023] The light reflected from the toner and belt surface includes specularly reflected light and diffusely reflected light. This specularly reflected light and diffusely reflected light are separated by a polarizing separation prism and then incident on separate light receiving elements. Each light receiving element photoelectrically converts the received specularly reflected light and diffusely reflected light and sends an output signal to the control unit 90 (see FIG. 11).

[0024] Then, the image density (toner amount) and image position of the patch image are detected from the characteristic changes of the output signals of the specularly reflected light and the diffusely reflected light, and density correction and color misregistration correction (calibration) are performed for each color by comparing them with a predetermined reference density and reference position and adjusting the characteristic value of the development voltage, the exposure start position and timing of the exposure device 5, etc.

[0025] 3 is a side cross-sectional view of an intermediate transfer unit 30 mounted in the image forming apparatus 100. As shown in Fig. 3, the intermediate transfer unit 30 includes an intermediate transfer belt 8 stretched between a downstream drive roller 10 and an upstream tension roller 11, primary transfer rollers 6a to 6d that contact the photosensitive drums 1a to 1d via the intermediate transfer belt 8, and a pressure switching roller 34.

[0026] A belt cleaning unit 19 is disposed opposite the tension roller 11 to remove toner remaining on the surface of the intermediate transfer belt 8. A secondary transfer unit 9 is disposed and pressed against the drive roller 10 via the intermediate transfer belt 8, forming a secondary transfer nip N. The detailed configuration of the secondary transfer unit 9 will be described later.

[0027] The intermediate transfer unit 30 is equipped with a roller contact / separation mechanism 35 having a pair of support members (not shown) that rotatably support both ends of the rotation shafts of the primary transfer rollers 6a-6d and the pressure switching roller 34 and move perpendicularly to the direction of travel of the intermediate transfer belt 8 (vertical direction in FIG. 3), and a drive means (not shown) that reciprocates the primary transfer rollers 6a-6d and the pressure switching roller 34 in the vertical direction. The roller contact / separation mechanism 35 is switchable between a color mode in which the four primary transfer rollers 6a-6d are pressed against the photosensitive drums 1a-1d (see FIG. 1) via the intermediate transfer belt 8, a monochrome mode in which only the primary transfer roller 6d is pressed against the photosensitive drum 1d via the intermediate transfer belt 8, and a retraction mode in which all four primary transfer rollers 6a-6d are separated from the photosensitive drums 1a-1d.

[0028] FIG. 4 is a perspective view of a secondary transfer unit 9 according to an embodiment of the present invention, which is installed in an image forming apparatus 100. FIG. 5 is an enlarged perspective view showing the configuration of one end of the secondary transfer unit 9 according to this embodiment. FIG. 6 is a perspective view of the periphery of the roller holder 47 of the secondary transfer unit 9 according to this embodiment, as viewed from the axial outside. FIG. 7 is a view showing the contact state between the shaft 51 and the main body frame 101. FIG. 8 is an enlarged perspective view of the periphery of the first bearing member 43 and the second bearing member 45 of the secondary transfer unit 9, as viewed from the axial outside. FIG. 9 is a perspective view showing the drive mechanism of the secondary transfer unit 9 according to this embodiment. Note that the unit frame 9a is not shown in FIGS. 4 and 9. Also, the unit frame 9a is shown in a see-through state in FIG. 5. The first bearing member 43 is not shown in FIG. 6, and the switching cam 50 is not shown in FIGS. 6 and 7.

[0029] As shown in Figures 4 to 9, the secondary transfer unit 9 includes a first roller 40 and a second roller 41 as secondary transfer rollers, a first bearing member 43, a second bearing member 45, a roller holder 47, a switching cam 50, and a roller switching motor 55.

[0030] The first roller 40 and the second roller 41 are elastic rollers in which conductive elastic layers 40b, 41b are laminated on the outer circumferential surfaces of core metals 40a, 41a, respectively. The elastic layers 40b, 41b are made of an ion-conductive rubber such as ECO (epichlorohydrin rubber).

[0031] The axial length of the elastic layer 40b of the first roller 40 is 311 mm, which is compatible with A3 size paper. The axial length of the elastic layer 41b of the second roller 41 is longer than that of the elastic layer 40b of the first roller 40. More specifically, the axial length of the elastic layer 41b is 325 mm, which is compatible with 13-inch size paper.

[0032] A pair of first bearing members 43 are arranged at both axial ends of the first roller 40, and rotatably support the core metal 40a. A pair of second bearing members 45 are arranged at both axial ends of the second roller 41, and rotatably support the core metal 41a.

[0033] A pair of roller holders 47 are arranged at both axial ends of the first roller 40 and the second roller 41. The roller holder 47 is generally V-shaped in side view, and has a first bearing holder 47a, a second bearing holder 47b, and an insertion hole 47c. The first bearing holder 47a and the second bearing holder 47b slidably hold the first bearing member 43 and the second bearing member 45, respectively. The insertion hole 47c is formed at the apex of the V-shape, and the shaft 51 is rotatably inserted through the insertion hole 47c. The roller holder 47 is made of an insulating material such as synthetic resin.

[0034] 5, a first coil spring 48 is disposed between the first bearing holder 47a and the first bearing member 43. A second coil spring 49 is disposed between the second bearing holder 47b and the second bearing member 45. The first roller 40 is biased by the first coil spring 48, and the second roller 41 is biased by the second coil spring 49, in a direction away from the shaft 51 (a direction in which they are pressed against the drive roller 10).

[0035] As shown in Figures 6 and 8, first ground contact members 56a and 56b are disposed on the first bearing member 43 and the second bearing member 45. The first ground contact members 56a and 56b are formed by bending a metal plate into a predetermined shape. One end of the first ground contact member 56a contacts a metal bearing 40c attached to the core metal 40a of the first roller 40, and the other end contacts the upper end of the first coil spring 48. One end of the first ground contact member 56b contacts a metal bearing 41c attached to the core metal 41a of the second roller 41, and the other end contacts the upper end of the second coil spring 49.

[0036] As shown in Figures 6 and 7, a second grounding member 57 is disposed on the roller holder 47. The second grounding member 57 is formed by bending a metal plate into a predetermined shape. The second grounding member 57 is bent so that its upper end overlaps the bottom surfaces of the first bearing holder 47a and the second bearing holder 47b, and is in contact with the lower ends of the first coil spring 48 and the second coil spring 49. A conduction hole 57a through which the shaft 51 passes is formed in the lower end of the second grounding member 57. The inner diameter of the conduction hole 57a is the same as the outer diameter of the shaft 51, and the inner peripheral edge of the conduction hole 57a is in contact with the outer peripheral surface of the shaft 51.

[0037] As shown in FIG. 7, the outer circumferential surface of the shaft 51 is in contact with a contact piece 101a formed on the main body frame 101 of the image forming apparatus 100, and the shaft 51 is grounded via the main body frame 101.

[0038] With the above configuration, the first roller 40 is grounded (earthed) via the first grounding member 56a, the first coil spring 48, the second grounding member 57, the shaft 51, and the main body frame 101. In addition, the second roller 41 is grounded (earthed) via the first grounding member 56b, the second coil spring 49, the second grounding member 57, the shaft 51, and the main body frame 101.

[0039] As shown in Fig. 4, a first light-shielding plate 51a is attached to the shaft 51, and by blocking light from reaching the detection portion of a first position detection sensor S1 (see Fig. 11), it is possible to detect the rotation angle of the shaft 51. Also, as shown in Fig. 7, a second light-shielding plate 47d is formed on one side surface of the roller holder 47 in the rotation direction. The second light-shielding plate 47d is formed in a position where it can block light from reaching the detection portion of a second position detection sensor S2 (see Fig. 11) disposed on the unit frame 9a.

[0040] The first light-shielding plate 51a and the second light-shielding plate 47d turn on or off the first position detection sensor S1 and the second position detection sensor S2 depending on the rotation angle of the roller holder 47 (shaft 51), thereby making it possible to detect the positions of the first roller 40 and the second roller 41 supported by the roller holder 47. Position detection control of the first roller 40 and the second roller 41 will be described later.

[0041] 8, the first bearing member 43 and the second bearing member 45 have arc-shaped first bearing portions 43b and second bearing portions 45b that hold the metal cores 40a and 41a of the first roller 40 and the second roller 41, respectively. Restriction members 58 that restrict upward movement of the metal cores 40a and 41a are attached to the tops of the first bearing portions 43b and the second bearing portions 45b, and caps 59 are attached to the tips of the metal cores 40a and 41a. The caps 59 prevent the metal cores 40a and 41a from coming off the first bearing portion 43b and the second bearing portion 45b.

[0042] The first bearing member 43 and the second bearing member 45 are respectively held by the first bearing holding portion 47a and the second bearing holding portion 47b of the roller holder 47 with a predetermined tolerance (play) in the left-right direction (the rotation direction of the roller holder 47).

[0043] A pair of switching cams 50 are arranged outside the roller holder 47 at both axial ends of the first roller 40 and the second roller 41. The switching cam 50 is fan-shaped when viewed from the side, and the main part of the fan shape (the vertex where the two radii intersect) is fixed to the shaft 51. A parallel pin 51b extending in the radial direction is fixed to the shaft 51. A pin insertion portion 65 into which the parallel pin 51b is inserted is formed in the switching cam 50. The parallel pin 51b is inserted into the pin insertion portion 65 without any gap in the circumferential direction of the shaft 51.

[0044] An arc-shaped guide hole 63 (see FIG. 12) is formed on the axially inner side of the switching cam 50. A recess 64 (see FIG. 12) is formed in the center of the radially outer peripheral edge of the guide hole 63. A first engaging portion 43a and a second engaging portion 45a that engage with the guide hole 63 are formed on the first bearing member 43 and the second bearing member 45, respectively.

[0045] When the first engagement portion 43a engages with the recess 64, the first bearing member 43 is pressed by the first coil spring 48 and moves in a direction away from the shaft 51. As a result, the first roller 40 is pressed against the drive roller 11 via the intermediate transfer belt 8. Furthermore, when the second engagement portion 45a engages with the recess 64, the second bearing member 45 is pressed by the second coil spring 49 and moves in a direction away from the shaft 51. As a result, the second roller 41 is pressed against the drive roller 11 via the intermediate transfer belt 8. In other words, the positions of the first bearing member 43 and the second bearing member 45 in the rotation direction of the roller holder 47 are determined.

[0046] As described above, the first bearing member 43 and the second bearing member 45 are respectively held with a predetermined tolerance (backlash) by the first bearing holder 47a and the second bearing holder 47b of the roller holder 47. Here, the positional relationship of the switching cam 50 with the shaft 51 is maintained by the parallel pin 51b. Furthermore, the positional relationship between the switching cam 50 and the first roller 40 and the second roller 41 is maintained by the engagement between the first engagement portion 43a and the second engagement portion 45b and the recessed portion 64 of the switching cam 50.

[0047] 13, which will be described later, the pin insertion portion 65 of the switching cam 50 is formed along a straight line that passes through the shaft 51 and the recess 64. That is, when the first engagement portion 43a and the second engagement portion 45b engage with the recess 64, the first roller 40 and the second roller 41 are positioned in the extending direction of the parallel pin 51b.

[0048] 9, a roller switching motor 55 is connected to the shaft 51 via gears 52 and 53. The arrangement of the first roller 40 and the second roller 41 is switched by rotating the switching cam 50 together with the shaft 51. The control of switching between the first roller 40 and the second roller 41 will be described later.

[0049] 10 is a circuit diagram showing the flow of secondary transfer current in the secondary transfer nip portion N. As shown in Fig. 10, the drive roller 10 is electrically connected to the positive terminal of the transfer voltage power supply 74. The first roller 40 and the second roller 41 are electrically connected to the positive terminal of the transfer voltage power supply 74 while being disposed opposite the drive roller 10.

[0050] When a secondary transfer voltage of the same polarity as the toner (positive polarity in this case) is applied to the drive roller 10 from the transfer voltage power supply 74, a secondary transfer current flows from the drive roller 10 to the first roller 40 or the second roller 41 via the intermediate transfer belt 8. This generates a predetermined secondary transfer electric field in the secondary transfer nip N, and the toner image that has been primarily transferred onto the intermediate transfer belt 8 is secondarily transferred onto the paper S that passes through the secondary transfer nip N.

[0051] As described above, the first roller 40 and the second roller 41 are always grounded (earthed) by the first grounding members 56a, 56b and the second grounding member 57. Therefore, part of the secondary transfer current flowing through the first roller 40 or the second roller 41 flows to the main body frame 101. However, because not all of the secondary transfer current flows to the main body frame 101, there is no risk of the secondary transfer performance being affected even if the first roller 40 and the second roller 41 are always grounded.

[0052] Furthermore, in this embodiment, a secondary transfer voltage of the same polarity (positive polarity) as that of the toner is applied to the drive roller 10 to generate a secondary transfer electric field in the secondary transfer nip N, but a secondary transfer voltage of the opposite polarity (negative polarity) to that of the toner can also be applied to the first roller 40 and the second roller 41 to generate a secondary transfer electric field in the secondary transfer nip N. Even in this case, the secondary transfer current flows in the same direction as in Figure 10, so part of the secondary transfer current flows into the main body frame 101, but there is no risk of this affecting secondary transfer properties.

[0053] 11 is a block diagram showing an example of a control path of an image forming apparatus 100 equipped with the secondary transfer unit 9 of this embodiment. Note that, since various controls are performed on each part of the image forming apparatus 100 when the image forming apparatus 100 is used, the control path of the entire image forming apparatus 100 becomes complex. Therefore, the following description will focus on the parts of the control path that are necessary for implementing the present invention.

[0054] The control unit 90 includes at least a CPU (Central Processing Unit) 91 as a central processing unit, a ROM (Read Only Memory) 92 as a read-only memory, a RAM (Random Access Memory) 93 as a readable and writable memory, a temporary memory 94 that temporarily stores image data and the like, a counter 95, and a plurality of (here, two) I / Fs (interfaces) 96 that transmit control signals to each device in the image forming apparatus 100 and receive input signals from the operation unit 80. The control unit 90 can be placed anywhere inside the main body of the image forming apparatus 100.

[0055] ROM 92 stores data such as a control program for image forming apparatus 100, numerical values ​​necessary for control, and other data that will not be changed while image forming apparatus 100 is in use. RAM 93 stores necessary data generated during the control of image forming apparatus 100, and data temporarily required for controlling image forming apparatus 100. In addition, RAM 93 (or ROM 92) also stores density correction tables used for calibration. Counter 95 accumulates and counts the number of printed sheets.

[0056] Furthermore, the control unit 90 transmits control signals from the CPU 91 to each part and device in the image forming apparatus 100 via the I / F 96. Furthermore, signals indicating the state of each part and device and input signals are transmitted from each part and device to the CPU 91 via the I / F 96. Examples of each part and device controlled by the control unit 90 include image forming units Pa to Pd, exposure device 5, primary transfer rollers 6a to 6d, secondary transfer unit 9, roller contact / separation mechanism 35, main motor 60, belt drive motor 61, voltage control circuit 71, and operation unit 80.

[0057] The image input unit 70 is a receiving unit that receives image data transmitted from a host device such as a personal computer to the image forming apparatus 100. The image signal input from the image input unit 70 is converted into a digital signal and then sent to the temporary storage unit 94.

[0058] Voltage control circuit 71 is connected to charging voltage power supply 72, developing voltage power supply 73, transfer voltage power supply 74, and cleaning voltage power supply 75, and operates each of these power supplies in response to an output signal from control unit 90. In response to a control signal from voltage control circuit 71, each of these power supplies causes charging voltage power supply 72 to apply a predetermined charging voltage to charging roller 25 in charging devices 2a-2d, developing voltage power supply 73 to apply a predetermined developing voltage to developing roller 22 in developing devices 3a-3d, and transfer voltage power supply 74 to apply a predetermined primary transfer voltage to primary transfer rollers 6a-6d. In addition, transfer voltage power supply 74 applies a predetermined secondary transfer voltage to drive roller 10.

[0059] The operation unit 80 is provided with a liquid crystal display unit 81 and LEDs 82 that indicate various states, and the user operates the stop / clear button on the operation unit 80 to stop image formation, and operates the reset button to reset various settings of the image forming apparatus 100 to their default states. The liquid crystal display unit 81 indicates the state of the image forming apparatus 100, as well as the image formation status and the number of copies to be printed. Various settings of the image forming apparatus 100 are made using a printer driver on a personal computer.

[0060] Next, switching control and position detection control of the first roller 40 and the second roller 41 in the secondary transfer unit 9 of this embodiment will be described. Figure 12 is a side cross-sectional view including a switching cam 50 of the secondary transfer unit 9 of this embodiment, showing a state in which the first roller 40 is positioned to form the secondary transfer nip portion N. Figure 13 is a plan view of the switching cam 50 as seen from the inside in the axial direction.

[0061] 13, the recess 64 of the switching cam 50 is generally trapezoidal in plan view, and has a bottom 64a that corresponds to the upper side of the trapezoid and an inclined portion 64b that corresponds to the oblique side of the trapezoid. By rotating the switching cam 50, the first engagement portion 43a of the first bearing member 43 and the second engagement portion 45a of the second bearing member 45 engage with the bottom 64a or the inclined portion 64b of the recess 64, or move away from the recess 64, thereby switching the contact state of the first roller 40 and the second roller 41 with the intermediate transfer belt 8, as will be described later.

[0062] In the state shown in FIG. 12, the first engagement portion 43a of the first bearing member 43 is engaged with the bottom portion 64a of the recess 64. As a result, the first roller 40 is pressed against the drive roller 10 via the intermediate transfer belt 8 by the biasing force of the first coil spring 48 (see FIG. 5), forming a secondary transfer nip portion N, and the first roller 40 rotates in response to the drive roller 10. A predetermined secondary transfer current is passed through the first roller 40 by the transfer voltage power supply 74 (see FIG. 11). Specifically, when the first roller 40 is positioned as shown in FIG. 12, a transfer voltage of the same polarity as the toner (positive polarity in this case) is applied to the drive roller 10, which is electrically connected to the transfer voltage power supply 74, and a secondary transfer current passes through the first roller 40 via the intermediate transfer belt 8.

[0063] Additionally, the first light-shielding plate 51a (see FIG. 4) of the shaft 51 shields (turns on) the detection portion of the first position detection sensor S1, and the first light-shielding plate 47d of the roller holder 47 shields (turns on) the detection portion of the second position detection sensor S2. This state (S1 / S2 on) is the reference position (home position) of the first roller 40. The rotation angle of the switching cam 50 is regulated based on the rotation time of the switching cam 50 from this reference position, and the position and separation state of the first roller 40 are controlled.

[0064] FIG. 14 shows the state in which the switching cam 50 has been rotated clockwise by a predetermined angle (here, 46.4° from the reference position in FIG. 12) from the state shown in FIG. 12. Further rotation of the shaft 51 in the clockwise direction causes the switching cam 50 to further rotate clockwise along with the shaft 51. Meanwhile, the roller holder 47 is restricted from clockwise rotation by the restricting rib 9b (see FIG. 5). As a result, the first engaging portion 43a of the first bearing member 43 moves out of the recess 64, and the first bearing member 43 moves toward the shaft 51 against the biasing force of the first coil spring 48 (see FIG. 5). This causes the first roller 40 to be spaced from the intermediate transfer belt 8 (a spaced state). The detection states of the first position detection sensor S1 and the second position detection sensor S2 in FIG. 14 are S1-off and S2-on.

[0065] 14, when the shaft 51 is rotated counterclockwise, the switching cam 50 also rotates counterclockwise together with the shaft 51. Furthermore, the first bearing member 43 is biased in a direction away from the shaft 51 by the biasing force of the first coil spring 48 (see FIG. 5), and the second bearing member 45 is biased in a direction away from the shaft 51 by the biasing force of the second coil spring 49 (see FIG. 5). Therefore, the first engaging portion 43a and the second engaging portion 45a are pressed against the radially outer peripheral edge portion of the guide hole 63 of the switching cam 50. As a result, the roller holder 47 also rotates counterclockwise together with the switching cam 50.

[0066] Then, when roller holder 47 rotates until it abuts against restricting rib 9c (see FIG. 5), second roller 41 is positioned opposite drive roller 10, as shown in FIG. 15. In the state of FIG. 15, first light-shielding plate 51a of shaft 51 is retracted (off) from the detection portion of first position detection sensor S1, and second light-shielding plate 47d of roller holder 47 is retracted (off) from the detection portion of second position detection sensor S2. In other words, when the detection state shifts from FIG. 14 (S1 off / S2 on) to FIG. 15 (S1 / S2 off), it is possible to detect the movement of second roller 41 to a position opposite drive roller 10.

[0067] Figure 16 is a diagram showing a state in which the switching cam 50 has been rotated counterclockwise by a predetermined angle from the state shown in Figure 15. When the shaft 51 is rotated counterclockwise, the switching cam 50 also rotates together with the shaft 51. Meanwhile, the roller holder 47 is restricted from rotating counterclockwise by the restricting rib 9c (see Figure 5). As a result, the second engaging portion 45a of the second bearing member 45 moves to the bottom portion 64a of the recess 64, and the second bearing member 45 moves in a direction away from the shaft 51 due to the biasing force of the second coil spring 49 (see Figure 5).

[0068] As a result, second roller 41 is pressed against drive roller 10 via intermediate transfer belt 8, forming secondary transfer nip N, and second roller 41 rotates following drive roller 10. A predetermined secondary transfer current flows through second roller 41 from transfer voltage power supply 74 (see FIG. 11). Specifically, when second roller 41 is positioned as shown in FIG. 16, a transfer voltage of the same polarity as that of the toner (positive polarity in this case) is applied to drive roller 10, which is electrically connected to transfer voltage power supply 74, and a secondary transfer current flows through second roller 41 via intermediate transfer belt 8.

[0069] Additionally, the first light-shielding plate 51a of the shaft 51 shields (ON) the detection portion of the first position detection sensor S1, and the second light-shielding plate 47d of the roller holder 47 is retracted (OFF) from the detection portion of the second position detection sensor S2. This state (S1 ON / S2 OFF) is set as the reference position (home position) of the second roller 41. That is, when the detection state (S1 / S2 OFF) of FIG. 15 shifts to the detection state (S1 ON / S2 OFF) of FIG. 16, movement of the second roller 41 to the reference position can be detected. The rotation angle of the switching cam 50 is regulated based on the rotation time of the switching cam 50 from this reference position, and the position and separation state of the second roller 41 are controlled.

[0070] FIG. 17 shows the state in which the switching cam 50 has been rotated counterclockwise by a predetermined angle (here, 46.4° from the reference position in FIG. 16) from the state shown in FIG. 16. Further rotation of the shaft 51 counterclockwise causes the switching cam 50 to further rotate counterclockwise along with the shaft 51. Meanwhile, the roller holder 47 is restricted from counterclockwise rotation by the restricting rib 9c (see FIG. 5). As a result, the second engaging portion 45a of the second bearing member 45 moves out of the recess 64, and the second bearing member 45 moves further toward the shaft 51 against the biasing force of the second coil spring 49 (see FIG. 5). This causes the second roller 41 to be spaced apart from the intermediate transfer belt 8 (a spaced state). The detection states of the first position detection sensor S1 and the second position detection sensor S2 in FIG. 17 are S1 / S2 off.

[0071] When switching the roller that forms the secondary transfer nip N from the second roller 41 to the first roller 40, the switching cam 50 is rotated a predetermined angle clockwise from the state shown in FIG. 16. This causes the switching cam 50 and roller holder 47 to also rotate a predetermined angle clockwise, and when the roller holder 47 has rotated until it abuts against the restricting rib 9b, the state shown in FIG. 18 is reached in which the first roller 40 faces the drive roller 10. When the switching cam 50 is further rotated a predetermined angle clockwise from the state shown in FIG. 18, the state shown in FIG. 12 is reached in which the first roller 40 is positioned at the reference position. The above procedure is repeated to switch between the first roller 40 and the second roller 41.

[0072] According to the configuration of this embodiment, with a simple configuration using a roller holder 47 and a switching cam 50, either the first roller 40 or the second roller 41 can be positioned opposite the drive roller 10, and the first roller 40 or the second roller 41 positioned opposite the drive roller 10 can be selectively positioned at a reference position that forms the secondary transfer nip portion N or a spaced position spaced away from the intermediate transfer belt 8.

[0073] For example, when the paper S is equal to or smaller than a predetermined size (here, A3 size), the first roller 40 having the elastic layer 40b with a small axial length is placed at the reference position. As a result, when a reference image is formed outside the image area in the width direction of the intermediate transfer belt 8 (outside the axial direction of the first roller 40) during image formation and calibration is performed, the reference image formed on the intermediate transfer belt 8 does not come into contact with the first roller 40. Therefore, calibration can be performed during image formation, and image quality can be improved without reducing image processing efficiency (productivity).

[0074] It is also possible to effectively prevent the back side of the paper S from being soiled due to the toner adhering to the first roller 40 adhering to the paper S. Furthermore, since there is no need to perform a cleaning operation to return the toner adhering to the first roller 40 onto the intermediate transfer belt 8, the print waiting time can also be shortened.

[0075] On the other hand, when the paper S is larger than the predetermined size (here, 13 inches), the second roller 41 having the elastic layer 41b with a large axial length is positioned at the reference position. This ensures that the secondary transfer of the toner image can be performed to both widthwise ends of the large-sized paper S.

[0076] Furthermore, the first roller 40 and the second roller 41 are always grounded (earthed) by the first grounding members 56a, 56b and the second grounding member 57. As a result, no transfer electric field remains in the first roller 40 and the second roller 41 after the secondary transfer, so that an appropriate transfer electric field can always be applied to the secondary transfer nip N, and a good transferred image can be stably obtained.

[0077] Furthermore, the first bearing member 43 and the second bearing member 45 are held with a predetermined margin (backlash) by the first bearing holding portion 47a and the second bearing holding portion 47b of the roller holder 47, respectively. This makes it easy to attach the first bearing member 43 and the second bearing member 45 to the roller holder 47, improving the workability of assembling the secondary transfer unit 9.

[0078] When first engagement portion 43a and second engagement portion 45b engage with recess 64, first roller 40 and second roller 41 are positioned in the extending direction of parallel pin 51b. Therefore, even if first bearing member 43 and second bearing member 45 are not positioned by first bearing holder 47a and second bearing holder 47b, first roller 40 and second roller 41 are accurately positioned relative to drive roller 11, and secondary transfer nip N can be stably formed.

[0079] Furthermore, in this embodiment, roller holder 47 and switching cam 50 can be driven using one roller switching motor 55. This simplifies the drive mechanism and drive control compared to when roller holder 47 and switching cam 50 are driven using separate motors, contributing to lower costs and more compact image forming apparatus 100.

[0080] The present invention is not limited to the above-described embodiment, and various modifications are possible without departing from the spirit of the present invention. For example, the shapes, dimensions, etc. of the first roller 40, the second roller 41, the roller holder 47, the switching cam 50, etc. that constitute the secondary transfer unit 9 are merely examples, and can be modified as desired without impairing the effects of the present invention.

[0081] Furthermore, in the above embodiment, two secondary transfer rollers are provided, consisting of a first roller 40 and a second roller 41, whose elastic layers 40b, 41b have different axial lengths, and either the first roller 40 or the second roller 41 is positioned at a reference position depending on the size information of the paper S. However, two secondary transfer rollers may be provided, consisting of a first roller 40 and a second roller 41, whose elastic layers 40b, 41b have different volume resistivities or hardnesses, and either the first roller 40 or the second roller 41 may be positioned at a reference position depending on information regarding the physical properties of the paper S (resistivity, thickness, basis weight, surface smoothness, etc.).

[0082] In addition, in the above embodiment, an intermediate transfer type image forming device 100 is exemplified, which is equipped with a secondary transfer unit 9 that secondarily transfers the toner image that has been primarily transferred onto the intermediate transfer belt 8 onto the paper S, but the present invention can also be applied to a transfer unit mounted on a direct transfer type image forming device that directly transfers the toner image formed on the photosensitive drum onto the paper. [Industrial Applicability]

[0083] The present invention can be used in an image forming apparatus equipped with a transfer unit that transfers a toner image formed on an image carrier to a recording medium. By utilizing the present invention, it is possible to provide a transfer unit that can stably form high-quality images by grounding two transfer rollers that are selectively pressed against the image carrier, and an image forming apparatus equipped with the same. [Explanation of symbols]

[0084] Pa~Pd Image forming section 1a to 1d Photosensitive drum (image carrier) 6a~6d Primary transfer rollers 8 Intermediate transfer belt (image carrier) 9 Secondary transfer unit (transfer unit) 9a Unit Frame 30 Intermediate transfer unit 40 First roller (transfer roller) 41 Second roller (transfer roller) 43 First bearing member 43a first engagement portion 45 Second bearing member 45a Second engaging part 47 Roller holder 48 First coil spring 49 Second coil spring 50 Switching Cam 51 Shaft 51b Parallel pin 55 Roller switching motor 56a, 56b First grounding member 57 Second grounding member 63 Guide hole 64 recess 64a bottom 64b Slope 65 Pin insertion section 74 Transfer voltage power supply 90 Control Unit 100 Image forming device 101 Main frame N Secondary transfer nip S Paper (recording medium)

Claims

1. A transfer unit includes a transfer roller having a core metal and an elastic layer laminated on an outer peripheral surface of the core metal, the elastic layer being pressed against an image carrier to form a transfer nip portion, the transfer unit transferring a toner image formed on the image carrier to a recording medium passing through the transfer nip portion, The transfer unit comprises: a first roller and a second roller serving as the transfer rollers, each having an elastic layer different in any one of an axial length, a volume resistivity, and a hardness; a first bearing member that rotatably supports the core metal of the first roller; a second bearing member that rotatably supports the core metal of the second roller; a roller holder having a first bearing holding portion and a second bearing holding portion that hold the first bearing member and the second bearing member so that they can slide toward or away from the image carrier, respectively; a switching mechanism that rotates the roller holder to place either the first roller or the second roller in a reference position where the roller is pressed against the image carrier and forms the transfer nip portion; and The transfer unit is characterized in that the first bearing member and the second bearing member have grounding members that ground the first roller and the second roller, respectively.

2. The switching mechanism includes: a first coil spring disposed between the first bearing holder and the first bearing member, and biasing the first bearing member in a direction approaching the image carrier; a second coil spring disposed between the second bearing holder and the second bearing member, and biasing the second bearing member in a direction approaching the image carrier; a switching cam having a guide hole with which a first engaging portion formed on the first bearing member and a second engaging portion formed on the second bearing member engage; a shaft fixed to the rotation center of the switching cam and rotatably supporting the roller holder; a roller switching motor that rotates the shaft; Equipped with the shaft is grounded via a main body frame of an image forming apparatus on which the transfer unit is mounted, The grounding member is a pair of first grounding members electrically connecting the core metal and the first coil spring or the second coil spring; a second grounding member that electrically connects the first coil spring, the second coil spring, and the shaft; 2. The transfer unit according to claim 1, further comprising:

3. By rotating the roller holder, one of the first roller and the second roller is disposed opposite to the image carrier, and 3. The transfer unit according to claim 2, wherein the first roller or the second roller arranged opposite the image carrier is selectively positioned between the reference position and a spaced apart position spaced apart from the image carrier by rotating the switching cam to change the engagement positions of the first engagement portion and the second engagement portion in the guide hole.

4. The transfer unit according to claim 3, characterized in that the switching cam has a recess formed in the radially outer peripheral portion of the guide hole, and by engaging the first engagement portion or the second engagement portion with the recess, the first roller or the second roller arranged opposite the image carrier is positioned at the reference position.

5. 5. The transfer unit according to claim 4, wherein the first bearing member and the second bearing member are held by the first bearing holder and the second bearing holder, respectively, with a predetermined tolerance in the rotational direction of the roller holder.

6. 6. The transfer unit according to claim 5, wherein a parallel pin extending in a radial direction is fixed to the shaft, and a pin insertion portion into which the parallel pin is inserted is formed in the switching cam.

7. a plurality of image forming units that form the toner images of different colors; an endless intermediate transfer belt as the image carrier that moves along the image forming unit; a plurality of primary transfer members disposed opposite the photosensitive drums disposed in the image forming units across the intermediate transfer belt, and configured to primarily transfer the toner images formed on the photosensitive drums onto the intermediate transfer belt; 7. An image forming apparatus comprising: a secondary transfer unit as the transfer unit according to claim 1, which secondarily transfers the toner image, which has been primarily transferred onto the intermediate transfer belt, onto the recording medium.

Images