Transfer unit and image forming apparatus including the same

Abstract

To provide a transfer unit that can prevent the occurrence of an image defect, and an image forming apparatus including the same.SOLUTION: A transfer unit comprises a transfer roller that is brought into pressure contact with an image carrier to form a transfer nip part, and transfers a toner image formed on the image carrier to a recording medium passing through the transfer nip part. The transfer unit has a first roller and a second roller as a transfer roller, a switching mechanism, and a control section. The switching mechanism arranges either one of the first roller or the second roller to a reference position where it is brought into pressure contact with the image carrier to form the transfer nip part. The control section controls the switching mechanism. The first roller contains an ion conductive agent, and the second roller contains an electronic conductive agent.SELECTED DRAWING: Figure 3

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 to an image forming apparatus equipped with the same. [Background technology]

[0002] A conventional transfer unit includes a transfer roller that is pressed against an image carrier to form a transfer nip, and transfers a toner image formed on the image carrier to a recording medium that passes through the transfer nip. The transfer unit includes a first roller and a second roller as transfer rollers, a switching mechanism, and a control unit. The first roller and the second roller have different axial widths. The control unit controls the switching mechanism to press either the first roller or the second roller against the image carrier depending on the paper width of the recording medium to form the transfer nip. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-337454 Summary of the Invention [Problem to be solved by the invention]

[0004] In the configuration of Patent Document 1, the resistance of the transfer roller changes with changes in the ambient temperature and humidity. For example, in a low-temperature, low-humidity environment, the resistance of the transfer roller increases, potentially reducing the transfer current. Furthermore, in a high-temperature, high-humidity environment, the resistance of the transfer roller decreases, causing some of the transfer current to leak outside the printing area. This can result in a reduction in the transfer current within the printing area, potentially resulting in poor image quality.

[0005] SUMMARY OF THE INVENTION In view of the above problems, an object of the present invention is to provide a transfer unit that can suppress the occurrence of image defects and an image forming apparatus including the same. [Means for solving the problem]

[0006] In order to achieve the above object, a first aspect of the present invention is a transfer unit that includes a transfer roller that is 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 that passes through the transfer nip. The transfer unit has a first roller and a second roller as transfer rollers, a switching mechanism, and a control unit. The switching mechanism presses either the first roller or the second roller against the image carrier to form the transfer nip. The control unit controls the switching mechanism. The first roller contains an ionic conductive agent, and the second roller contains an electronic conductive agent. [Effects of the Invention]

[0007] According to the first aspect of the present invention, it is possible to provide a transfer unit that can suppress the occurrence of image defects, and an image forming apparatus that includes the same. [Brief explanation of the drawings]

[0008] [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] 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 7] 1 is a side cross-sectional view including a switching cam 50 of the secondary transfer unit 9 of the present embodiment. [Figure 8] 1 is a plan view of a switching cam 50 of a secondary transfer unit 9 according to the present embodiment; [Figure 9]1 is a side cross-sectional view including a switching cam 50 of the secondary transfer unit 9 of the present embodiment. [Figure 10] 1 is a side cross-sectional view including a switching cam 50 of the secondary transfer unit 9 of the present embodiment. [Figure 11] 1 is a side cross-sectional view including a switching cam 50 of the secondary transfer unit 9 of the present embodiment. [Figure 12] 1 is a side cross-sectional view including a switching cam 50 of the secondary transfer unit 9 of the present embodiment. [Figure 13] 1 is a side cross-sectional view including a switching cam 50 of the secondary transfer unit 9 of the present embodiment. [Figure 14] 1 is a flowchart showing an example of an image forming operation of an image forming apparatus 100 equipped with a secondary transfer unit 9 according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0009] 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 (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.

[0010] 1 is a so-called tandem color multifunction peripheral. The image forming apparatus 100 includes image forming units Pa, Pb, Pc, and Pd, an intermediate transfer belt (image carrier) 8, a primary transfer roller 6a (primary transfer member), a secondary transfer unit (transfer unit) 9, and a fixing unit 13.

[0011] The four image forming units Pa, Pb, Pc, and Pd are arranged in order from the upstream side in the transport direction (left side in FIG. 1) inside the main body of the image forming apparatus 100. 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 processes of charging, exposing, developing, and transferring, respectively. In other words, the image forming units Pa to Pd form toner images of different colors.

[0012] The image forming stations Pa to Pd are provided with photosensitive drums 1a, 1b, 1c, and 1d that carry visible images (toner images) of each color. Furthermore, an intermediate transfer belt (image carrier) 8 that rotates counterclockwise in FIG. 1 is provided adjacent to each of the image forming stations Pa to Pd. The toner images formed on these photosensitive drums 1a to 1d are sequentially transferred onto the intermediate transfer belt 8, which moves while contacting each of the photosensitive drums 1a to 1d, and then transferred all at once onto paper S, an example of a recording medium, in a secondary transfer unit 9.

[0013] Furthermore, after the image is fixed on the sheet S in the fixing unit 13, the sheet S is discharged from the main body of the image forming apparatus 100. While the photosensitive drums 1a to 1d are rotated clockwise in FIG. 1, an image forming process is carried out on each of the photosensitive drums 1a to 1d.

[0014] 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 mainly used as the intermediate transfer belt 8.

[0015] 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.

[0016] 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. 6) 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 beams of light (laser light) emitted from the exposure device 5, and electrostatic latent images corresponding to image signals are formed on the photosensitive drums 1a-1d.

[0017] 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.

[0018] An electric field is applied between the primary transfer rollers 6a-6d and the photosensitive drums 1a-1d at a predetermined transfer voltage. As a result, 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, the toner remaining on the surfaces of the photosensitive drums 1a-1d is removed by the cleaning blades 23 and rubbing rollers 24 of the cleaning devices 7a-7d.

[0019] 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. 6), 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.

[0020] 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.

[0021] An image density sensor 25 is disposed at a position facing the drive roller 10 across the intermediate transfer belt 8. The image density sensor 25 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.

[0022] 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 outputs an output signal to the control unit 90 (see FIG. 6).

[0023] 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.

[0024] 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.

[0025] 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.

[0026] 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 (up and down 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 up and down 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.

[0027] Fig. 4 is a perspective view of a secondary transfer unit (transfer unit) 9 according to one embodiment of the present invention, which is mounted in the image forming apparatus 100. Fig. 5 is an enlarged perspective view showing the configuration of one end side of the secondary transfer unit 9 of this embodiment. Note that Fig. 4 omits the illustration of the unit frame 9a. Fig. 5 also shows the unit frame 9a in a see-through state.

[0028] The secondary transfer unit (transfer unit) 9 includes a first roller 40 and a second roller 41 as secondary transfer rollers (transfer rollers), a switching mechanism 110, and a temperature and humidity detection sensor 26. The switching mechanism 110 includes a first bearing member 43, a second bearing member 45, a roller holder 47, a switching cam 50, and a roller switching motor 55.

[0029] The first roller 40 and the second roller 41 are elastic rollers in which conductive elastic layers 40b and 41b are laminated on the outer circumferential surfaces of cores 40a and 41a, respectively. The elastic layers 40b and 41b are made of, for example, rubber. Examples of rubber include polyurethane elastomer, hydrin rubber (specifically, epichlorohydrin rubber), styrene-butadiene rubber (SBR), polynorbornene rubber, ethylene-propylene-diene rubber (EPDM), acrylonitrile-butadiene rubber (NBR), hydrogenated acrylonitrile-butadiene rubber (H-NBR), butadiene rubber (BR), isoprene rubber (IR), natural rubber (NR), and silicone rubber. One of these rubbers may be used alone, or two or more may be used in combination. Epichlorohydrin rubber is preferred as the rubber constituting the elastic layers 40b and 41b.

[0030] Furthermore, the elastic layer 40b of the first roller 40 contains an ionic conductive agent, and the elastic layer 41b of the second roller 41 contains an electronic conductive agent. That is, the first roller 40 contains an ionic conductive agent, and the second roller 41 contains an electronic conductive agent.

[0031] The ionic conductive agent is blended into the rubber of the elastic layer 41b. By blending the ionic conductive agent, the elastic layer 41b has ionic conductivity. Examples of the ionic conductive agent include quaternary ammonium salts and borates.

[0032] The electronic conductive agent is compounded into the rubber of the elastic layer 41b. Compared to ionic conductive agents, electronic conductive agents tend to have smaller fluctuations in electrical resistance with temperature changes. Therefore, by including the electronic conductive agent in the elastic layer 41b, fluctuations in the resistance of the second roller 41 in high-temperature, high-humidity environments and in low-temperature, low-humidity environments can be suppressed. Examples of electronic conductive agents include carbon black, graphite, potassium titanate particles, iron oxide particles, titanium oxide particles, zinc oxide particles, and tin oxide particles. Tin oxide particles are preferred as the electronic conductive agent. The average particle size of the electronic conductive agent is preferably 5 nm or more and 200 nm or less.

[0033] 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.

[0034] The temperature and humidity detection sensor 26 is disposed, for example, opposite the first roller 40 and the second transfer roller 41, and detects the temperature and humidity around the first roller 40 and the second transfer roller 41.

[0035] 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 52 is rotatably inserted through the insertion hole 47c. The roller holder 47 is made of an insulating material such as synthetic resin.

[0036] 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 52 (a direction in which they are pressed against the drive roller 10).

[0037] A first light-shielding plate 52a is attached to the shaft 52 (see FIG. 4), and by blocking light from reaching the detection portion of the first position detection sensor S1 (see FIG. 7), it is possible to detect the rotation angle of the shaft 52. 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 the second position detection sensor S2 arranged on the unit frame 9a.

[0038] The first light-shielding plate 52a 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 52), thereby making it possible to detect the positions of the first roller 40 and the second roller 41 supported by the roller holder 47. The position detection control of the first roller 40 and the second roller 41 will be described later.

[0039] A pair of switching cams 50 are arranged inside the roller holder 47 at both axial ends of the first roller 40 and the second roller 41. The switching cam 50 has a partially cut-out fan shape in side view, and the main part of the fan shape (the apex where the two radii intersect) is fixed to a shaft 52. A roller switching motor 55 is connected to the shaft 52 via a gear (not shown). The positions of the first roller 40 and the second roller 41 are switched by rotating the switching cam 50 together with the shaft 52. The control of switching between the first roller 40 and the second roller 41 will be described later.

[0040] FIG. 6 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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. Figures 7 and 8 to 13 are side cross-sectional views including a switching cam 50 of the secondary transfer unit 9 of this embodiment, and are views seen from the axial inside of the state where the first roller 40 is positioned to form the secondary transfer nip portion N. Figure 8 is a plan view of the switching cam 50.

[0048] The switching cam 50 is included in the roller holder 47, and has a sector shape in plan view with a corner cut out on one side in the rotation direction (the second bearing member 45 side). A first guide hole 63 and a second guide hole 65 are formed in the switching cam 50. The first guide hole 63 and the second guide hole 65 are formed in a substantially arc shape at positions that are different distances from the rotation center (shaft 52) ​​of the switching cam 50.

[0049] A first recess 64 is formed on the radially outer peripheral edge of the first guide hole 63. A second recess 66 is formed on the radially outer peripheral edge of the second guide hole 65. A first engaging portion 43a that engages with the first guide hole 63 is formed on the first bearing member 43. A second engaging portion 45a that engages with the second guide hole 65 is formed on the second bearing member 45.

[0050] The first recess 64 of the switching cam 50 has a bottom portion 64a that is recessed to the outermost position in the radial direction and an inclined portion 64b that inclines radially inward from the bottom portion 64a. By rotating the switching cam 50, the first engagement portion 43a (see FIG. 7) of the first bearing member 43 engages with the bottom portion 64a or the inclined portion 64b of the first recess 64, or moves away from the first recess 64, thereby switching the contact state of the first roller 40 with the intermediate transfer belt 8, as will be described later.

[0051] The second recess 66 of the switching cam 50 has a bottom portion 66a that is recessed to the outermost position in the radial direction and an inclined portion 66b that inclines radially inward from the bottom portion 66a. By rotating the switching cam 50, the second engagement portion 45a (see FIG. 7) of the second bearing member 45 engages with the bottom portion 66a or the inclined portion 66b of the second recess 66, or moves away from the second recess 66, thereby switching the contact state of the second roller 41 with the intermediate transfer belt 8, as will be described later.

[0052] For example, in the state shown in FIG. 7, the first engagement portion 43a engages with the bottom 64a of the first recess 64. The second engagement portion 45a is spaced from the second recess 66. 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 N, and the first roller 40 rotates in response to the drive roller 10. A transfer voltage of opposite polarity (negative polarity in this case) to the toner is applied to the first roller 40 by the transfer voltage power supply 74 (see FIG. 6). Specifically, when the first roller 40 is positioned as shown in FIG. 7, the transfer voltage is applied via the first bearing member 43, which is electrically connected to the transfer voltage power supply 74.

[0053] Additionally, the first light-shielding plate 52a of the shaft 52 (see FIG. 4) 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.

[0054] FIG. 9 shows the state in which the switching cam 50 has been rotated clockwise by a predetermined angle (here, 10.6° from the reference position in FIG. 9 ) from the state shown in FIG. 7 . When the shaft 52 is rotated clockwise, the switching cam 50 also rotates along with the shaft 52. 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 moves from the bottom 64a of the first recess 64 to the inclined portion 64b, and the first bearing member 43 moves toward the shaft 52 against the biasing force of the first coil spring 48 (see FIG. 5 ). This places the first roller 40 in a state (first separated state) in which it is slightly spaced (2 mm) from the intermediate transfer belt 8. Note that, as the switching cam 50 rotates, the second engaging portion 45a also moves within the second guide hole 65, but the second engaging portion 45a remains spaced from the second recess 66.

[0055] If the first roller 40 is kept in pressure contact with the drive roller 10 for a long period of time, the first roller 40 may bend and deform in the axial direction. Therefore, after the job is completed, the first roller 40 needs to be separated from the intermediate transfer belt 8 (drive roller 10). At this time, the first roller 40 is in the first separated state shown in FIG.

[0056] Additionally, the first light-shielding plate 52a of the shaft 52 is retracted (OFF) from the detection portion of the first position detection sensor S1, and the second light-shielding plate 47d of the roller holder 47 continues to shield (ON) the detection portion of the second position detection sensor S2. That is, when the detection state shifts from Fig. 7 (S1 / S2 ON) to the detection state of Fig. 9 (S1 OFF / S2 ON), it is possible to detect the movement of the first roller 40 from the reference position to the first separated state.

[0057] FIG. 10 shows the state in which the switching cam 50 has been further rotated clockwise by a predetermined angle (here, 46.4° from the reference position in FIG. 7) from the state shown in FIG. 9. Further rotating the shaft 52 clockwise causes the switching cam 50 to further rotate clockwise along with the shaft 52. 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 moves out of the first recess 64, and the first bearing member 43 moves further toward the shaft 52 against the biasing force of the first coil spring 48 (see FIG. 5). This causes the first roller 40 to enter a state (second separated state) in which it is completely separated (6.5 mm) from the intermediate transfer belt 8. This second separated state is used only when switching from the first roller 40 to the second roller 41.

[0058] 10 is the same as the first separation state (S1 off / S2 on) shown in FIG. 9. Therefore, when the S1 off / S2 on state is detected at the time of startup of image forming apparatus 100, roller holder 47 is rotated toward the main body of image forming apparatus 100 (counterclockwise) for a certain period of time to distinguish between the first separation state and the second separation state. If the S1 / S2 on state is detected, the state is determined to be the first separation state, and if the S1 / S2 on state is not detected, the state is determined to be the second separation state.

[0059] Furthermore, when returning the first roller 40 from the second separated state to the reference position, the roller holder 47 and the switching cam 50 must first be rotated counterclockwise to switch to the reference position of the second roller 41 (see Figure 11), and then returned to the reference position of the first roller 40 (see Figure 7).

[0060] Next, a procedure for switching the roller that forms the secondary transfer nip portion N from the first roller 40 to the second roller 41 will be described. When the shaft 52 is rotated counterclockwise from the second separated state shown in FIG. 10, the switching cam 50 also rotates counterclockwise together with the shaft 52. Furthermore, the first bearing member 43 is biased in a direction away from the shaft 52 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 52 by the biasing force of the second coil spring 49 (see FIG. 5). Therefore, the first engaging portion 43a is pressed against the radially outer peripheral portion of the first guide hole 63 of the switching cam 50. Furthermore, the second engaging portion 45a is pressed against the radially outer peripheral portion of the second guide hole 65 of the switching cam 50. As a result, the roller holder 47 also rotates counterclockwise together with the switching cam 50.

[0061] 11 is a side cross-sectional view including the switching cam 50 of the secondary transfer unit 9 of this embodiment, showing a state in which the second roller 41 is positioned at a reference position for forming the secondary transfer nip portion N. When the roller holder 47 rotates from the state shown in FIG. 10 until it abuts against the restricting rib 9c (see FIG. 5), the second roller 41 is positioned facing the drive roller 10. After the roller holder 47 abuts against the restricting rib 9c, when the switching cam 50 is further rotated counterclockwise together with the shaft 52, the second engagement portion 45a of the second bearing member 45 moves to the bottom 66a of the second recess 66, and the second bearing member 45 moves in a direction away from the shaft 52 due to the biasing force of the second coil spring 49 (see FIG. 5).

[0062] As a result, the second roller 41 is pressed against the drive roller 10 via the intermediate transfer belt 8, forming a secondary transfer nip N, and the second roller 41 rotates in response to the drive roller 10. A transfer voltage of opposite polarity (negative in this case) to the toner is applied to the second roller 41 by a transfer voltage power supply 74 (see FIG. 6). Specifically, when the second roller 41 is positioned as shown in FIG. 11, the transfer voltage is applied via a second bearing member 45 electrically connected to the transfer voltage power supply 74. As the switching cam 50 rotates, the first engagement portion 43a of the first bearing member 43 also moves within the first guide hole 63, but the first engagement portion 43a remains spaced apart from the first recess 64.

[0063] Additionally, the first light-shielding plate 52a of the shaft 52 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 shifts from that of FIG. 12 (S1 OFF / S2 ON) to that of FIG. 13 (S1 ON / S2 OFF), 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.

[0064] FIG. 12 shows a state in which the switching cam 50 has been rotated counterclockwise by a predetermined angle (here, 10.6° from the reference position in FIG. 11) from the state shown in FIG. 11. When the shaft 52 is rotated counterclockwise, the switching cam 50 rotates counterclockwise along with the shaft 52. Meanwhile, the roller holder 47 is restricted from rotating counterclockwise by the restricting rib 9c (see FIG. 5). As a result, the second engaging portion 45a of the second bearing member 45 moves from the bottom 66a of the second recess 66 to the inclined portion 66b, and the second bearing member 45 moves toward the shaft 52 against the biasing force of the second coil spring 49 (see FIG. 5). This places the second roller 41 in a state (first separation state) in which it is slightly spaced (2 mm) from the intermediate transfer belt 8.

[0065] If the second roller 41 is kept in pressure contact with the drive roller 10 for a long period of time, the second roller 41 may bend and deform in the axial direction. For this reason, the second roller 41 needs to be separated from the intermediate transfer belt 8 (drive roller 10) after the job is completed. At this time, the second roller 41 is placed in the first separated state shown in FIG. 12. Furthermore, when calibration is performed while the second roller 41 is in use, the second roller 41 is placed in the first separated state so that the reference image formed on the intermediate transfer belt 8 does not adhere to the second roller 41. Note that when calibration is performed with the second roller 41 in the first separated state, the reference image can also be formed in the center of the intermediate transfer belt 8 in the width direction.

[0066] Furthermore, the first light-shielding plate 52a of the shaft 52 is retracted (off) from the detection portion of the first position detection sensor S1, and the second light-shielding plate 47d of the roller holder 47 continues to be retracted (off) from the detection portion of the second position detection sensor S2. That is, when the detection state shifts from Fig. 11 (S1 on / S2 off) to the detection state of Fig. 12 (S1 / S2 off), it is possible to detect the movement of the second roller 41 from the reference position to the first separated state.

[0067] FIG. 13 shows the state in which the switching cam 50 has been further rotated counterclockwise by a predetermined angle (here, 46.4° from the reference position in FIG. 13) from the state in FIG. 12. Further rotation of the shaft 52 counterclockwise causes the switching cam 50 to further rotate counterclockwise along with the shaft 52. 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 moves out of the second recess 66, and the second bearing member 45 moves further toward the shaft 52 against the biasing force of the second coil spring 49 (see FIG. 5). This causes the second roller 41 to be completely spaced (6.5 mm) from the intermediate transfer belt 8 (a second spaced state). This second spaced state is used only when switching from the second roller 41 to the first roller 40.

[0068] 13 is the same as the first separation state (S1 / S2 off) shown in FIG. 12. Therefore, when the S1 / S2 off state is detected at the time of startup of image forming apparatus 100, roller holder 47 is rotated toward duplex conveying path 18 (clockwise) for a certain period of time to distinguish between the first separation state and the second separation state. If the S1 on / S2 off state is detected, the state is determined to be the first separation state, and if the S1 on / S2 off state is not detected, the state is determined to be the second separation state.

[0069] Furthermore, when returning the second roller 41 from the second separated state to the reference position, the roller holder 47 and the switching cam 50 must first be rotated clockwise to switch to the reference position of the first roller 40 (see Figure 7), and then returned to the reference position of the second roller 41 (see Figure 11).

[0070] To switch the roller that forms the secondary transfer nip N from the second roller 41 to the first roller 40, the shaft 52 is rotated clockwise by a predetermined angle from the second separated state shown in FIG. 13. This causes the switching cam 50 and roller holder 47 to also rotate clockwise by a predetermined angle. When the roller holder 47 rotates until it abuts against the restricting rib 9b, the first roller 40 faces the drive roller 10. Furthermore, after the roller holder 47 abuts against the restricting rib 9c, if the switching cam 50 is further rotated clockwise together with the shaft 52, the first engagement portion 43a of the first bearing member 43 moves to the bottom 64a of the first recess 64, and the first roller 40 is positioned at the reference position, as shown in FIG. 7. By repeating the above procedure, the first roller 40 and the second roller 41 are switched.

[0071] This allows either the first roller 40 or the second roller 41 to be positioned opposite the drive roller 10 with a simple configuration using a roller holder 47 and a switching cam 50, and the first roller 40 or the second roller 41 positioned opposite the drive roller 10 can be selectively positioned at either a reference position that forms the secondary transfer nip portion N or a spaced position spaced away from the intermediate transfer belt 8.

[0072] The first roller 40 containing an ionic conductive agent can generate a stable electric field with little localized unevenness in resistance, enabling uniform image formation throughout the entire axial and circumferential directions. The second roller 41 containing an electronic conductive agent exhibits smaller fluctuations in resistance in response to environmental loads than the first roller 40 containing an ionic conductive agent.

[0073] On the other hand, the second roller 41 containing the electronic conductive agent has a higher pressure sensitivity than the first roller 40 containing the ionic conductive agent. Therefore, the resistance of the second roller 41 increases as the frequency of use increases.

[0074] In addition, the second roller 41 is pressed at both axial ends via the second coil spring 49. For this reason, as the frequency of use of the second roller 41 increases, the resistance at both axial ends tends to increase more than at the axial center. This may cause image unevenness in the axial direction of the second roller 41.

[0075] Furthermore, it is more difficult to uniformly disperse electronic conductive agents in rubber than ionic conductive agents. As a result, the second roller 41 containing electronic conductive agents is more likely to have uneven resistance across the entire axial and circumferential direction than the first roller 40 containing ionic conductive agents. This can lead to image defects such as white dots on the reverse side when printing a mixed-color image on both sides.

[0076] In contrast, in this embodiment, when the control unit 90 determines based on the detection results of the temperature and humidity detection sensor 26 that the first roller 40 and the second roller 41 are in a low-temperature, low-humidity environment or a high-humidity, high-humidity environment, the control unit 90 presses the second roller 41 against the intermediate transfer belt 8 to form the secondary transfer nip N. Note that a low-temperature, low-humidity environment is, for example, an environment of 10°C or lower and a relative humidity of 10% or lower. A high-temperature, high-humidity environment is, for example, an environment of 25°C or higher and a relative humidity of 60% or higher.

[0077] 14 is a flowchart showing an example of the execution of an image forming operation in image forming apparatus 100. When a print job is started, in step S1, control unit 90 detects the temperature and humidity around first roller 40 and second roller 41 based on the detection results of temperature and humidity detection sensor 26. Control unit 90 also determines whether the detection results indicate a low-temperature, low-humidity environment or a high-temperature, high-humidity environment.

[0078] At this time, the determination of whether the environment is a low-temperature, low-humidity environment or a high-temperature, high-humidity environment is made based on, for example, an absolute humidity conversion table stored in advance in ROM. Based on the absolute humidity conversion table, the control unit 90 determines that the environment is a low-temperature, low-humidity environment when the detection result of the temperature and humidity detection sensor is below a predetermined threshold, and determines that the environment is a high-temperature, high-humidity environment when the detection result is above the predetermined threshold.

[0079] If the control unit 90 determines that the areas around the first roller 40 and the second roller 41 are not in a low-temperature, low-humidity environment or a high-temperature, high-humidity environment, the process proceeds to step S2. On the other hand, if the control unit 90 determines that the areas around the first roller 40 and the second roller 41 are in a low-temperature, low-humidity environment or a high-temperature, high-humidity environment, the process proceeds to step S3.

[0080] In step S2, the first roller 40, which is disposed opposite the drive roller 10, is placed in a reference position for forming the secondary transfer nip N, and a printing operation is performed. Meanwhile, in step S3, the second roller 41, which is disposed opposite the drive roller 10, is placed in a reference position for forming the secondary transfer nip N, and a printing operation is performed.

[0081] According to this embodiment, when the first roller 40 and the second roller 41 are surrounded by a low-temperature, low-humidity environment or a high-temperature, high-humidity environment, the second roller 41, which has little resistance fluctuation in response to environmental loads, is pressed against the intermediate transfer belt 8 to form the secondary transfer nip N. This makes it possible to prevent image defects from occurring in a low-temperature, low-humidity environment or a high-temperature, high-humidity environment.

[0082] Furthermore, when the areas around the first roller 40 and the second roller 41 are not in a low-temperature, low-humidity environment or a high-temperature, high-humidity environment, the first roller 40, which can form a stable electric field with little localized unevenness in resistance, is pressed against the intermediate transfer belt 8 to form the secondary transfer nip N. This reduces the occurrence of image defects when the area is not in a low-temperature, low-humidity environment or a high-temperature, high-humidity environment, and also reduces the frequency of use of the second roller 41. This suppresses wear on the second roller 41 and reduces the occurrence of image defects during use.

[0083] 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.

[0084] 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]

[0085] 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 using the present invention, it is possible to provide a transfer unit and an image forming apparatus equipped with the same that can switch between two transfer rollers with different axial lengths using a simple configuration and can suppress a decrease in image formation efficiency that accompanies switching between the transfer rollers. [Explanation of symbols]

[0086] Pa~Pd Image forming section 1a to 1d Photosensitive drum 6a~6d Primary transfer rollers 8 Intermediate transfer belt (image carrier) 9 Secondary transfer unit (transfer unit) 9a Unit Frame 25 Image density sensor 26 Temperature and humidity detection sensor 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 52 Shaft 55 Roller switching motor 63 First guide hole 64 First recess 65 Second guide hole 66 Second recess 64a, 66a bottom 64b, 66b Slope section 74 Transfer voltage power supply 90 Control Unit 100 Image forming device 110 Switching mechanism N Secondary transfer nip (transfer nip) S Paper (recording medium) S1 First position detection sensor S2 Second position detection sensor

Claims

1. a transfer unit including a transfer roller that is pressed against an image carrier to form a transfer nip, the transfer roller transferring a toner image formed on the image carrier to a recording medium that passes through the transfer nip; a first roller and a second roller as the transfer rollers; a switching mechanism that presses either the first roller or the second roller against the image carrier to form the transfer nip portion; a control unit that controls the switching mechanism, the first roller contains an ionic conductive agent; The second roller contains an electronically conductive agent.

2. a temperature and humidity detection sensor for detecting the temperature and humidity around the transfer roller; The transfer unit according to claim 1, wherein the control unit presses the second roller against the image carrier to form the transfer nip when it determines that the transfer roller is in a low-temperature, low-humidity environment or a high-temperature, high-humidity environment based on the detection results of the temperature and humidity detection sensor.

3. The first roller and the second roller each have a core metal and an elastic layer laminated on an outer circumferential surface of the core metal, the ionic conductive agent is contained in the elastic layer of the first roller, The transfer unit according to claim 1 or 2, wherein the electronic conductive agent is contained in the elastic layer of the second roller.

4. 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; 3. 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