Transcription Unit and Image Forming Apparatus Having the Same

Abstract

To provide a transfer unit that, when switching between two transfer rollers that are selectively used, can store the roller not to be used in a reduced space, and an image forming apparatus including the same.SOLUTION: A transfer unit comprises: a first roller and a second roller; a roller switching mechanism; and an upstream-side guide and a downstream-side guide. The second roller is arranged on the downstream side of the first roller with respect to a conveyance direction of a recording medium. The roller switching mechanism selectively arranges the first roller or the second roller at a reference position at which a transfer nip part is formed. The upstream-side guide and the downstream-side guide are arranged on the upstream side and the downstream side, respectively, of the transfer nip part with respect to the conveyance direction of the recording medium. When the first roller is moved to the reference position by the roller switching mechanism, the second roller moves to a first retreat position on the back of the downstream-side guide, and when the second roller is moved to the reference position, the first roller moves to a second retreat position on the back of the upstream-side guide.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 photoreceptor drum or an intermediate transfer belt to a recording medium, and an image forming apparatus including the same, and more particularly to a mechanism for switching the arrangement of a plurality of transfer members.

Background Art

[0002] Conventionally, there has been known an intermediate transfer type image forming apparatus including an endless intermediate transfer belt that rotates in a predetermined direction and a plurality of image forming units provided along the intermediate transfer belt. After sequentially superimposing and primarily transferring toner images of respective colors on the intermediate transfer belt by each image forming unit, the toner image is secondarily transferred onto a recording medium such as paper by a secondary transfer roller.

[0003] In such an intermediate transfer type image forming apparatus, toner adhesion to the surface of the secondary transfer roller progresses due to durable printing. In particular, in order to improve color developability and color reproducibility, it is necessary to perform calibration for correcting image density and color shift at a predetermined timing. However, the patch image formed on the intermediate transfer belt during the execution of calibration is removed by a belt cleaning device without being transferred to the paper. Therefore, a part of the toner transferred onto the intermediate transfer belt adheres to the secondary transfer roller when the patch image passes through the secondary transfer roller.

[0004] Conventionally, a method has been used in which a transfer reverse voltage (a voltage having the same polarity as the toner) is applied to the secondary transfer roller during non-image formation to return the toner adhering to the secondary transfer roller to the intermediate transfer belt, thereby cleaning the secondary transfer roller. However, this method has a problem in that it takes time to clean the secondary transfer roller, resulting in a long printing waiting time.

[0005] Therefore, a method for improving productivity has been proposed by making the secondary transfer roller switchable to a size suitable for the recording medium. For example, in Patent Document 1, there are a plurality of secondary transfer rollers having different axial lengths from each other, a rotating body that rotatably supports the plurality of secondary transfer rollers and has a support portion rotatable around an axis parallel to the axial direction, and according to the width of the recording medium, one roller is selected from the plurality of secondary transfer rollers, and an image forming apparatus is disclosed that includes a control unit that rotates the support portion to oppose the one roller to the intermediate transfer belt.

Prior Art Document

Patent Document

[0006]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0007] In the case of a configuration in which a plurality of secondary transfer rollers are selectively used, if a secondary transfer roller that is not used, as in Patent Document 1, is exposed in the conveyance path, there is a risk that the conveyed paper will come into contact and cause a jam (paper clogging). Therefore, a member for covering the unused roller or an arrangement space is required, which leads to an increase in the number of members and an increase in the size of the apparatus.

[0008] In view of the above problems, an object of the present invention is to provide a transfer unit capable of storing an unused roller in a space-saving manner when switching between two transfer rollers selectively pressed against an image carrier, and an image forming apparatus including the same.

Means for Solving the Problems

[0009] To achieve the above object, a first configuration of the present invention includes a core bar and an elastic layer laminated on the outer peripheral surface of the core bar, and a transfer roller that forms a transfer nip portion by pressing the elastic layer against an image carrier, and transfers a toner image formed on the image carrier to a recording medium passing through the transfer nip portion. The transfer unit includes a first roller and a second roller as transfer rollers, a roller switching mechanism, an upstream guide, and a downstream guide. The second roller is disposed downstream of the first roller in the conveyance direction of the recording medium, and the axial length of the elastic layer is different from that of the first roller. The roller switching mechanism selectively disposes the first roller or the second roller at a reference position where it is pressed against the image carrier to form a transfer nip portion. The upstream guide and the downstream guide are disposed upstream and downstream of the transfer nip portion with respect to the conveyance direction of the recording medium, respectively. When the first roller is moved to the reference position by the roller switching mechanism, the second roller moves to a first retracted position behind the downstream guide, and when the second roller is moved to the reference position, the first roller moves to a second retracted position behind the upstream guide.

Advantages of the Invention

[0010] According to the first configuration of the present invention, when the first roller is disposed at the reference position, the second roller is disposed at the first retracted position behind the downstream guide, and when the second roller is disposed at the reference position, the first roller is disposed at the second retracted position behind the upstream guide. Therefore, the unused transfer roller is not exposed to the conveyance path of the recording medium. Accordingly, the occurrence of jams in the recording medium can be suppressed. In addition, since there is no need to separately provide a cover member that covers the first roller and the second roller, or a retraction space for the unused first roller and second roller, it also contributes to reducing the number of components and saving space.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

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Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13

Figure 14

Figure 15

Figure 16

Figure 17

Figure 18

Figure 19

Figure 20

Figure 21

Embodiments for Carrying Out the Invention

[0012] Hereinafter, embodiments 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 including a secondary transfer unit 9 of the present invention, and FIG. 2 is an enlarged view of the vicinity of the image forming section Pa in FIG. 1

[0013] The image forming apparatus 100 shown in FIG. 1 is a so-called tandem type color printer and has the following configuration. Four image forming sections Pa, Pb, Pc, and Pd are arranged in order from the upstream side in the conveyance direction (the left side in FIG. 1) in the main body of the image forming apparatus 100. These image forming sections Pa to Pd are provided corresponding to images of four different colors (magenta, cyan, yellow, and black), and form magenta, cyan, yellow, and black images sequentially through the steps of charging, exposure, development, and transfer

[0014] In these image forming units Pa to Pd, photosensitive drums 1a, 1b, 1c, and 1d that carry visible images (toner images) of respective colors are disposed. Further, an intermediate transfer belt 8 that rotates in the counterclockwise direction in FIG. 1 is provided adjacent to each of the image forming units Pa to Pd. The toner images formed on these photosensitive drums 1a to 1d are sequentially transferred onto the intermediate transfer belt 8 that moves while contacting the respective photosensitive drums 1a to 1d, and then are transferred onto a sheet S as an example of a recording medium at once in the secondary transfer unit 9. Further, after being fixed on the sheet S in the fixing unit 13, they are discharged from the main body of the image forming apparatus 100. While rotating the photosensitive drums 1a to 1d in the clockwise direction in FIG. 1, an image forming process for each of the photosensitive drums 1a to 1d is executed.

[0015] The sheet S onto which the toner image is transferred is stored in a sheet cassette 16 at the lower part of the main body of the image forming apparatus 100, and is conveyed to the secondary transfer unit 9 via a paper feed roller 12a and a registration roller pair 12b. A belt having no seam (seamless) is mainly used for the intermediate transfer belt 8.

[0016] Next, the image forming units Pa to Pd will be described. Hereinafter, the image forming unit Pa will be described in detail. However, since the image forming units Pb to Pd basically have the same configuration, the description thereof will be omitted. As shown in FIG. 2, around the photosensitive drum 1a, a charging device 2a, a developing device 3a, and a cleaning device 7a are disposed along the drum rotation direction (clockwise direction in FIG. 2), and a primary transfer roller 6a is disposed with the intermediate transfer belt 8 interposed therebetween. Further, on the upstream side in the rotation direction of the intermediate transfer belt 8 with respect to the photosensitive drum 1a, a belt cleaning unit 19 that faces a tension roller 11 with the intermediate transfer belt 8 interposed therebetween is disposed.

[0017] Next, the image forming procedure in the image forming apparatus 100 will be described. When the user inputs the start of image formation, first, the rotation of the photoreceptor drums 1a to 1d is started by the main motor 60 (see FIG. 8), and the surfaces of the photoreceptor drums 1a to 1d are uniformly charged by the charging rollers 20 of the charging devices 2a to 2d. Next, the surfaces of the photoreceptor drums 1a to 1d are irradiated with light by the beam light (laser light) emitted from the exposure device 5, and an electrostatic latent image corresponding to the image signal is formed on each of the photoreceptor drums 1a to 1d.

[0018] The developing devices 3a to 3d are each filled with a predetermined amount of toner of each color: magenta, cyan, yellow, and black. In addition, when the ratio of the toner in the two-component developer filled in each of the developing devices 3a to 3d falls below a specified value due to the formation of the toner image described later, toner is replenished from the toner containers 4a to 4d to each of the developing devices 3a to 3d. The toner in this developer is supplied onto the photoreceptor drums 1a to 1d by the developing rollers 21 of the developing devices 3a to 3d and adheres electrostatically. Thereby, a toner image corresponding to the electrostatic latent image formed by the exposure from the exposure device 5 is formed.

[0019] Then, an electric field is applied at a predetermined transfer voltage between the primary transfer rollers 6a to 6d and the photoreceptor drums 1a to 1d by the primary transfer rollers 6a to 6d, and the magenta, cyan, yellow, and black toner images on the photoreceptor drums 1a to 1d are primarily transferred onto the intermediate transfer belt 8. These four-color images are formed with a predetermined positional relationship determined in advance for predetermined full-color image formation. Thereafter, in preparation for the formation of a new electrostatic latent image that will be continued, the toner remaining on the surfaces of the photoreceptor drums 1a to 1d is removed by the cleaning blades 22 and the rubbing rollers 23 of the cleaning devices 7a to 7d.

[0020] When the intermediate transfer belt 8 starts to rotate counterclockwise as the drive roller 10 rotates by the belt drive motor 61 (see FIG. 8), the sheet S is conveyed from the registration roller pair 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. The sheet S onto which the toner image has been transferred is conveyed to the fixing unit 13. The toner remaining on the surface of the intermediate transfer belt 8 is removed by the belt cleaning unit 19.

[0021] The sheet S conveyed to the fixing unit 13 is heated and pressurized by the fixing roller pair 13a, and the toner image is fixed on the surface of the sheet S, forming a predetermined full-color image. The sheet S on which the full-color image has been formed has its conveyance direction diverted by the branching unit 14 that branches in multiple directions, and is then discharged to the discharge tray 17 by the discharge roller pair 15 (either as it is or after being sent to the duplex conveyance path 18 for duplex printing).

[0022] An image density sensor 25 is disposed at a position facing the intermediate transfer belt 8 on the downstream side of the image forming unit Pd. As the image density sensor 25, an optical sensor including a light emitting element generally composed of an LED or the like and a light receiving element composed of a photodiode or the like is used. When measuring the toner adhesion amount on the intermediate transfer belt 8, when the measuring light is irradiated from the light emitting element onto each patch image (reference image) formed on the intermediate transfer belt 8, the measuring light enters the light receiving element as the light reflected by the toner and the light reflected by the belt surface.

[0023] The reflected light from the toner and the belt surface includes specularly reflected light and diffusely reflected light. After being separated by a polarization separation prism, the specularly reflected light and the diffusely reflected light enter 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. 8).

[0024] Then, the image density (toner amount) and image position of the patch image are detected from the characteristic changes in the output signals of the specularly reflected light and the diffusely reflected light, and compared with a predetermined reference density and reference position to adjust the characteristic value of the developing voltage, the exposure start position and timing of the exposure device 5, etc., thereby performing density correction and color shift correction (calibration) for each color.

[0025] FIG. 3 is a side cross-sectional view of the intermediate transfer unit 30 mounted on the image forming apparatus 100. As shown in FIG. 3, the intermediate transfer unit 30 includes an intermediate transfer belt 8 spanned between a downstream driving 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 pressing switching roller 34.

[0026] A belt cleaning unit 19 for removing toner remaining on the surface of the intermediate transfer belt 8 is disposed at a position facing the tension roller 11. A secondary transfer unit 9 is disposed on the driving roller 10 via the intermediate transfer belt 8, forming a secondary transfer nip portion N. The detailed configuration of the secondary transfer unit 9 will be described later.

[0027] The intermediate transfer unit 30 includes a pair of support members (not shown) that rotatably support both ends of the rotation axes of the primary transfer rollers 6a to 6d and the pressing switching roller 34 and are movable in a direction perpendicular to the traveling direction of the intermediate transfer belt 8 (the vertical direction in FIG. 3), and a driving means (not shown) that reciprocally moves the primary transfer rollers 6a to 6d and the pressing switching roller 34 in the vertical direction, and is provided with a roller contact / separation mechanism 35. The roller contact / separation mechanism 35 can be switched between a color mode in which the four primary transfer rollers 6a to 6d are each pressed against the photosensitive drums 1a to 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 of the four primary transfer rollers 6a to 6d are separated from the photosensitive drums 1a to 1d.

[0028] FIG. 4 is a perspective view of a secondary transfer unit 9 according to an embodiment of the present invention mounted on an image forming apparatus 100. FIG. 5 is an enlarged perspective view showing a configuration on one end side of the secondary transfer unit 9 of the present embodiment. FIG. 6 is a perspective view of the periphery of a roller holder 47 of the secondary transfer unit 9 viewed from the inner side in the axial direction. FIG. 7 is a perspective view showing a drive mechanism of the secondary transfer unit 9 of the present embodiment. In FIGS. 4 and 7, the description of the unit frame 9a is omitted, and in FIG. 5, the unit frame 9a is shown in a transparent state. Also, in FIGS. 5 and 6, the description of the switching cam 50 is omitted.

[0029] As shown in FIGS. 4 to 7, 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 first coil spring 48, a second coil spring 49, a switching cam 50, and a roller switching motor 55. The first bearing member 43, the second bearing member 45, the roller holder 47, the first coil spring 48, the second coil spring 49, the switching cam 50, and the roller switching motor 55 constitute a roller switching mechanism 57 for switching the arrangement of the first roller 40 and the second roller 41.

[0030] The first roller 40 and the second roller 41 are elastic rollers in which elastic layers 40b and 41b having conductivity are laminated on the outer peripheral surfaces of the mandrels 40a and 41a, respectively. As the material of the elastic layers 40b and 41b, for example, an ion-conductive rubber such as ECO (epichlorohydrin rubber) is used.

[0031] The first roller 40 has an axial length of the elastic layer 40b of 311 mm, corresponding to A3-size paper. The second roller 41 has an axial length of the elastic layer 41b larger 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, corresponding to 13-inch-size paper.

[0032] The first bearing member 43 is arranged in a pair at both axial ends of the first roller 40 and rotatably supports the mandrel 40a. The second bearing member 45 is arranged in a pair at both axial ends of the second roller 41 and rotatably supports the mandrel 41a.

[0033] The roller holders 47 are arranged in a pair at both axial ends of the first roller 40 and the second roller 41. The roller holder 47 is substantially V-shaped in side view and has a first bearing holding portion 47a, a second bearing holding portion 47b, and an insertion hole 47c. The first bearing holding portion 47a and the second bearing holding portion 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 therethrough. The roller holder 47 is formed of an insulating material such as synthetic resin.

[0034] As shown in FIG. 5, a first coil spring 48 is arranged between the first bearing holding portion 47a and the first bearing member 43. A second coil spring 49 is arranged between the second bearing holding portion 47b and the second bearing member 45. The first roller 40 is urged by the first coil spring 48 and the second roller 41 is urged by the second coil spring 49 in a direction away from the shaft 51 (the direction of being pressed against the drive roller 10).

[0035] As shown in FIG. 4, a first light shielding plate 51a is attached to the shaft 51, and the rotation angle of the shaft 51 can be detected by shielding the detection portion of the first position detection sensor S1 (see FIG. 9). Further, as shown in FIG. 6, a second light shielding plate 47d is formed on one side surface in the rotation direction of the roller holder 47. The second light shielding plate 47d is formed at a position where it can shield the detection portion of the second position detection sensor S2 arranged on the unit frame 9a.

[0036] According to the rotation angle of the roller holder 47 (shaft 51), 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, thereby enabling detection of 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.

[0037] 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 sector shape in side view, and the key portion of the sector (the vertex portion where two radii intersect) is fixed to the shaft 51.

[0038] As shown in FIG. 7, a roller switching motor 55 is connected to the shaft 51 via gears 52 and 53. By rotating the switching cam 50 together with the shaft 51, the arrangement of the first roller 40 and the second roller 41 is switched. The switching control of the first roller 40 and the second roller 41 will be described later.

[0039] FIG. 8 is a block diagram showing an example of the control path of the image forming apparatus 100 equipped with the secondary transfer unit 9 of the present embodiment. In using the image forming apparatus 100, various controls are performed for each part of the apparatus, so the control path of the entire image forming apparatus 100 is complicated. Therefore, here, the parts necessary for the implementation of the present invention in the control path will be mainly described.

[0040] The control unit 90 includes at least a CPU (Central Processing Unit) 91 as a central processing unit, a ROM (Read Only Memory) 92 which is a read-only storage unit, a RAM (Random Access Memory) 93 which is a readable and writable storage unit, a temporary storage unit 94 for temporarily storing image data and the like, a counter 95, and a plurality (here, two) of I / F (interfaces) 96 for transmitting control signals to each device within the image forming apparatus 100 and receiving input signals from the operation unit 80. Further, the control unit 90 can be arranged at any location inside the main body of the image forming apparatus 100.

[0041] The ROM 92 stores data such as a control program for the image forming apparatus 100, numerical values necessary for control, and data that should not be changed during the use of the image forming apparatus 100. The RAM 93 stores necessary data generated during the control of the image forming apparatus 100 and data that is temporarily required for the control of the image forming apparatus 100. Also, in the RAM 93 (or ROM 92), a density correction table used for calibration, the on / off states of the first position detection sensor S1 and the second position detection sensor S2 used for the roller switching control described later, and the relationship between the rotation angles of the first roller 40 and the second roller 41 are also stored. The counter 95 accumulates and counts the number of printed sheets.

[0042] Further, the control unit 90 transmits control signals to each part and device in the image forming apparatus 100 from the CPU 91 through the I / F 96. Also, signals indicating their states and input signals from each part and device are transmitted to the CPU 91 through the I / F 96. Examples of the parts and devices controlled by the control unit 90 include, for example, the image forming units Pa to Pd, the exposure device 5, the primary transfer rollers 6a to 6d, the secondary transfer unit 9, the roller contact / separation mechanism 35, the main motor 60, the belt drive motor 61, the voltage control circuit 71, the operation unit 80, and the like.

[0043] 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.

[0044] The voltage control circuit 71 is connected to the charging voltage power supply 72, the developing voltage power supply 73, and the transfer voltage power supply 74, and operates these power supplies according to the output signal from the control unit 90. These power supplies, according to the control signal from the voltage control circuit 71, apply a predetermined voltage to the charging roller 20 in the charging devices 2a to 2d by the charging voltage power supply 72, to the developing roller 21 in the developing devices 3a to 3d by the developing voltage power supply 73, and to the primary transfer rollers 6a to 6d and the first roller 40 and the second roller 41 in the secondary transfer unit 9 by the transfer voltage power supply 74, respectively.

[0045] The operation unit 80 is provided with a liquid crystal display unit 81 and LEDs 82 indicating various states. The user operates the stop / clear button of the operation unit 80 to cancel image formation, and operates the reset button to return various settings of the image forming apparatus 100 to the default state. The liquid crystal display unit 81 shows the state of the image forming apparatus 100, and also displays the image formation status and the number of printed sheets. Various settings of the image forming apparatus 100 are made from the printer driver of the personal computer.

[0046] Next, the switching control and position detection control of the first roller 40 and the second roller 41 in the secondary transfer unit 9 of the present embodiment will be described. FIG. 9 is a side cross-sectional view including the 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 position forming the secondary transfer nip portion N.

[0047] As shown in FIG. 9, an arc-shaped guide hole 63 is formed in the switching cam 50. A recess 64 is formed at the center of the peripheral portion on the outer side in the radial direction of the guide hole 63. First engaging portions 43a and second engaging portions 45a that engage with the guide hole 63 are formed in the first bearing member 43 and the second bearing member 45, respectively.

[0048] In the state of Fig. 9, the first engaging portion 43a of the first bearing member 43 is engaged with the concave portion 64. As a result, the first roller 40 is pressed against the driving roller 10 via the intermediate transfer belt 8 by the biasing force of the first coil spring 48 (see Fig. 5) to form the secondary transfer nip portion N, and the first roller 40 rotates following the driving roller 10. A transfer voltage having a polarity opposite to that of the toner (here, a negative polarity) is applied to the first roller 40 by a transfer voltage power source 74 (see Fig. 8). Specifically, when the first roller 40 is disposed at the position of Fig. 9, the transfer voltage is applied via the first bearing member 43 electrically connected to the transfer voltage power source 74.

[0049] Also, the first light shielding plate 51a (see Fig. 4) of the shaft 51 shields the detection portion of the first position detection sensor S1, and the first light shielding plate 47d (see Fig. 6) of the roller holder 47 shields the detection portion of the second position detection sensor S2. This state (S1 / S2 on) is defined as 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 arrangement and separation state of the first roller 40 are controlled.

[0050] Fig. 10 is a diagram showing the positional relationship between the first roller 40 and the second roller 41 and the downstream guide 65 and the upstream guide 67 in the state of Fig. 9. Fig. 11 is an enlarged view around the second roller 41 and the downstream guide 65 in Fig. 10. The secondary transfer unit 9 includes a downstream guide 65 and an upstream guide 67.

[0051] The downstream guide 65 guides the sheet S that has passed through the secondary transfer nip portion N to the fixing unit 13 (see Fig. 1). The upstream guide 67 guides the sheet S conveyed from the paper storage cassette 16 via the registration roller pair 12b (both see Fig. 1) to the secondary transfer nip portion N. The downstream guide 65 and the upstream guide 67 constitute a part of the conveyance path from the paper feed roller 12a (paper feed unit) to the discharge roller pair 15 (paper discharge unit).

[0052] As shown in FIGS. 10 and 11, when the first roller 40 is disposed at the reference position, the second roller 41 is retracted to the space (the first retracted position) at the back of the downstream guide 65. First roller guide portions 66 are formed at both ends in the width direction of the downstream guide 65 (the direction perpendicular to the plane of FIG. 11). The shaft 41a of the second roller 41 is pressed against the first roller guide portion 66 by the biasing force of the second coil spring 49 (see FIG. 5). When the roller holder 47 rotates in the clockwise direction, the shaft 41a moves along the first roller guide portion 66 and the second roller 41 is guided to the first retracted position.

[0053] FIG. 12 is a plan view of the switching cam 50. The recess 64 of the switching cam 50 is substantially trapezoidal in plan view, and has a bottom portion 64a corresponding to the upper side of the trapezoid and an inclined portion 64b corresponding to the inclined side of the trapezoid. By the rotation of the switching cam 50, the first engaging portion 43a of the first bearing member 43 and the second engaging portion 45a of the second bearing member 45 engage with the bottom portion 64a or the inclined portion 64b of the recess 64, or are separated from the recess 64, so that the contact states of the first roller 40 and the second roller 41 with respect to the intermediate transfer belt 8 can be switched as described later.

[0054] FIG. 13 is a view showing a state in which the switching cam 50 is rotated clockwise by a predetermined angle (here, 10.6° from the reference position of FIG. 9) from the state of FIG. 9. When the shaft 51 is rotated clockwise, the switching cam 50 also rotates together with the shaft 51. On the other hand, the rotation of the roller holder 47 in the clockwise direction is restricted by the restricting rib 9b (see FIG. 5). As a result, the first engaging portion 43a of the first bearing member 43 moves from the bottom portion 64a of the recess 64 to the inclined portion 64b, and the first bearing member 43 moves in a direction approaching the shaft 51 against the biasing force of the first coil spring 48 (see FIG. 5). Thereby, the first roller 40 becomes a state slightly (2 mm) separated from the intermediate transfer belt 8 (the first separated state).

[0055] If the first roller 40 is continuously pressed against the driving roller 10 for a long time, the first roller 40 may bend and deform in the axial direction. Therefore, it is necessary to separate the first roller 40 from the intermediate transfer belt 8 (driving roller 10) after the job is completed. At this time, the first separation state shown in FIG. 13 is adopted.

[0056] Also, the first light shielding plate 51a of the shaft 51 retreats (turns off) from the detection part of the first position detection sensor S1, and the second light shielding plate 47d of the roller holder 47 continuously shields (turns on) the detection part of the second position detection sensor S2. That is, when shifting from the detection state (S1 / S2 on) in FIG. 9 to the detection state (S1 off / S2 on) in FIG. 13, it is possible to detect the movement of the first roller 40 from the reference position to the first separation state.

[0057] FIG. 14 is a diagram showing a state in which the switching cam 50 is further rotated clockwise by a predetermined angle (here, 46.4° from the reference position in FIG. 9) from the state in FIG. 13. When the shaft 51 is further rotated clockwise, the switching cam 50 also rotates further clockwise together with the shaft 51. On the other hand, the rotation of the roller holder 47 in the clockwise direction is restricted 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 concave portion 64, and the first bearing member 43 moves further in the direction approaching the shaft 51 against the biasing force of the first coil spring 48 (see FIG. 5). Thereby, the first roller 40 becomes a state of being completely separated (6.5 mm) from the intermediate transfer belt 8 (second separation state). This second separation state is used only when switching from the first roller 40 to the second roller 41.

[0058] Note that the detection states of the first position detection sensor S1 and the second position detection sensor S2 in FIG. 14 are the same as the first separation state shown in FIG. 13 (S1 off / S2 on). Therefore, when the state is S1 off / S2 on at the startup of the image forming apparatus 100, in order to distinguish between the first separation state and the second separation state, the roller holder 47 is rotated in the counterclockwise direction (toward the main body side of the image forming apparatus 100) for a certain period of time. Then, if it becomes the S1 / S2 on state, it is determined as the first separation state, and if it does not become the S1 / S2 on state, it is determined as the second separation state.

[0059] Also, when returning the first roller 40 from the second separation state to the reference position, it is necessary to first rotate the roller holder 47 and the switching cam 50 in the counterclockwise direction to switch to the reference position of the second roller 41 (see FIG. 16), and then return to the reference position of the first roller 40 (see FIG. 9).

[0060] Next, a procedure for switching the roller forming the secondary transfer nip portion N from the first roller 40 to the second roller 41 will be described. When the shaft 51 is rotated in the counterclockwise direction from the second separation state shown in FIG. 14, the switching cam 50 also rotates in the counterclockwise direction together with the shaft 51. Further, 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 peripheral portion on the radially outer side of the guide hole 63 of the switching cam 50. As a result, the roller holder 47 also rotates in the counterclockwise direction together with the switching cam 50.

[0061] Then, when the roller holder 47 rotates until it abuts against the regulating rib 9c (see FIG. 5), as shown in FIG. 15, the second roller 41 is disposed at a position facing the driving roller 10. In the state of FIG. 15, the first light-shielding plate 51a of the shaft 51 has retreated (turned off) from the detection part of the first position detection sensor S1, and the second light-shielding plate 47d of the roller holder 47 has retreated (turned off) from the detection part of the second position detection sensor S2. That is, when shifting from the detection state (S1 off / S2 on) of FIG. 14 to the detection state (S1 / S2 off) of FIG. 15, it is possible to detect the movement of the second roller 41 to the position facing the driving roller 10.

[0062] FIG. 16 is a view showing a state in which the switching cam 50 is rotated counterclockwise by a predetermined angle from the state of FIG. 15. When the shaft 51 is rotated counterclockwise, the switching cam 50 also rotates together with the shaft 51. On the other hand, the rotation of the roller holder 47 in the counterclockwise direction is restricted by the regulating rib 9c (see FIG. 5). As a result, the second engaging portion 45a of the second bearing member 45 moves to the bottom portion 64a of the concave portion 64, and the second bearing member 45 moves in a direction away from the shaft 51 by the biasing force of the second coil spring 49 (see FIG. 5).

[0063] Thereby, the second roller 41 is pressed against the driving roller 10 via the intermediate transfer belt 8 to form the secondary transfer nip portion N, and the second roller 41 rotates following the driving roller 10. A transfer voltage having a polarity opposite to that of the toner (here, a negative polarity) is applied to the second roller 41 by a transfer voltage power source 74 (see FIG. 8). Specifically, when the second roller 41 is disposed at the position of FIG. 16, the transfer voltage is applied via the second bearing member 45 electrically connected to the transfer voltage power source 74.

[0064] Further, the first light-shielding plate 51a of the shaft 51 shields (turns on) the detection part of the first position detection sensor S1, and the second light-shielding plate 47d of the roller holder 47 retracts (turns off) from the detection part of the second position detection sensor S2. This state (S1 on / S2 off) is defined as the reference position (home position) of the second roller 41. That is, when shifting from the detection state (S1 / S2 off) in FIG. 15 to the detection state (S1 on / S2 off) in FIG. 16, the movement of the second roller 41 to the reference position can be detected. Based on the rotation time of the switching cam 50 from this reference position, the rotation angle of the switching cam 50 is regulated, and the arrangement and separation state of the second roller 41 are controlled.

[0065] FIG. 17 is a diagram showing the positional relationship between the first roller 40 and the second roller 41 and the downstream guide 65 and the upstream guide 67 in the state of FIG. 16. FIG. 18 is an enlarged view around the first roller 40 and the upstream guide 67 in FIG. 17.

[0066] As shown in FIGS. 17 and 18, when the second roller 41 is arranged at the reference position, the first roller 40 is retracted to the space (second retracted position) on the back surface of the upstream guide 67. Support frames 67a for connecting the upstream guide 67 to the unit frame 9a are attached to both ends in the width direction (direction perpendicular to the paper surface of FIG. 18) of the upstream guide 67. A second roller guide portion 68 is formed on the support frame 67a.

[0067] The second roller guide portion 68 is U-shaped in side view, and the distance between the opposing outer surface 68a and the inner surface 68b is slightly larger than the outer diameter of the mandrel 40a. The mandrel 40a of the first roller 40 is pressed against the outer surface 68a of the second roller guide portion 68 by the biasing force of the first coil spring 48 (see FIG. 5). When the roller holder 47 rotates counterclockwise, the mandrel 40a moves along the second roller guide portion 68 and the first roller 40 is guided to the second retracted position.

[0068] FIG. 19 is a view showing a state in which the switching cam 50 is further rotated counterclockwise by a predetermined angle (here, 10.6° from the reference position in FIG. 16) from the state in FIG. 16. When the shaft 51 is further rotated counterclockwise, the switching cam 50 also rotates further counterclockwise together with the shaft 51. On the other hand, the rotation of the roller holder 47 in the counterclockwise direction is restricted 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 portion 64a of the concave portion 64 to the inclined portion 64b, and the second bearing member 45 moves in a direction approaching the shaft 51 against the biasing force of the second coil spring 49 (see FIG. 5). Thereby, the second roller 41 becomes a state slightly (2 mm) separated from the intermediate transfer belt 8 (first separated state).

[0069] If the second roller 41 is continuously pressed against the driving roller 10 for a long time, the second roller 41 may be bent and deformed in the axial direction. Therefore, it is necessary to separate the second roller 41 from the intermediate transfer belt 8 (driving roller 10) after the job is completed. At this time, the first separated state shown in FIG. 19 is set. Further, when calibration is executed during use of the second roller 41, the second roller 41 is set 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 executed with the second roller 41 in the first separated state, a reference image can also be formed at the center in the width direction of the intermediate transfer belt 8.

[0070] Also, the first light shielding plate 51a of the shaft 51 has retreated (turned off) from the detection portion of the first position detection sensor S1, and the second light shielding plate 47d of the roller holder 47 has continuously retreated (turned off) from the detection portion of the second position detection sensor S2. That is, when shifting from the detection state in FIG. 16 (S1 on / S2 off) to the detection state in FIG. 19 (S1 / S2 off), it is possible to detect the movement of the second roller 41 from the reference position to the first separated state.

[0071] FIG. 20 shows a state in which the switching cam 50 is further rotated counterclockwise by a predetermined angle (here, 46.4° from the reference position in FIG. 16) from the state in FIG. 19. When the shaft 51 is further rotated counterclockwise, the switching cam 50 also rotates further counterclockwise together with the shaft 51. On the other hand, the rotation of the roller holder 47 counterclockwise is restricted 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 concave portion 64, and the second bearing member 45 moves further in the direction approaching the shaft 51 against the biasing force of the second coil spring 49 (see FIG. 5). Thereby, the second roller 41 is in a state of being completely separated (6.5 mm) from the intermediate transfer belt 8 (second separation state). This second separation state is used only when switching from the second roller 41 to the first roller 40.

[0072] Note that the detection states of the first position detection sensor S1 and the second position detection sensor S2 in FIG. 20 are the same as those in the first separation state shown in FIG. 19 (S1 / S2 off). Therefore, when the S1 / S2 off state is present at the start of the image forming apparatus 100, in order to distinguish between the first separation state and the second separation state, the roller holder 47 is rotated in the double-sided conveyance path 18 side (clockwise direction) for a certain period of time. Then, if it becomes the S1 on / S2 off state, it is determined as the first separation state, and if it does not become the S1 on / S2 off state, it is determined as the second separation state.

[0073] Also, when returning the second roller 41 from the second separation state to the reference position, it is necessary to rotate the roller holder 47 and the switching cam 50 once clockwise to switch to the reference position of the first roller 40 (see FIG. 9), and then return to the reference position of the second roller 41 (see FIG. 16).

[0074] When switching the roller forming the secondary transfer nip portion N from the second roller 41 to the first roller 40, the switching cam 50 is rotated clockwise by a predetermined angle from the second separated state shown in FIG. 20. As a result, the switching cam 50 and the roller holder 47 also rotate clockwise by a predetermined angle, and when the roller holder 47 rotates until it contacts the regulating rib 9b, the first roller 40 faces the driving roller 10 as shown in FIG. 21. When the switching cam 50 is further rotated clockwise by a predetermined angle from the state shown in FIG. 21, the state shown in FIG. 9 in which the first roller 40 is disposed at the reference position is obtained. Hereinafter, the switching between the first roller 40 and the second roller 41 is performed by repeating the above procedure.

[0075] According to the configuration of the present embodiment, with a simple configuration using the roller holder 47 and the switching cam 50, either one of the first roller 40 and the second roller 41 is disposed to face the driving roller 10, and the first roller 40 or the second roller 41 disposed to face the driving roller 10 can be selectively disposed at the reference position forming the secondary transfer nip portion N and the separated position separated from the intermediate transfer belt 8.

[0076] For example, when the sheet S is of a predetermined size (here, A3 size) or less, the first roller 40 having the elastic layer 40b with a small axial length is disposed at the reference position. Thereby, 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 for calibration, the reference image formed on the intermediate transfer belt 8 does not contact the first roller 40. Therefore, calibration can be executed during image formation, and the image quality can be improved without degrading the image processing efficiency (productivity).

[0077] In addition, it is possible to effectively suppress the back contamination of the sheet S caused by the toner adhering to the first roller 40 adhering to the sheet S. Further, since there is no need to perform a cleaning operation for returning the toner adhering to the first roller 40 onto the intermediate transfer belt 8, the printing waiting time can also be shortened.

[0078] On the other hand, when the paper S is larger than a predetermined size (here, 13 inches), the second roller 41 having the elastic layer 41b with a large axial length is arranged at the reference position. Thereby, the secondary transfer of the toner image to both end portions in the width direction of the large-sized paper S can be surely performed.

[0079] Further, when the first roller 40 is arranged at the reference position, the second roller 41 is arranged at the first retracted position on the back surface of the downstream guide 65, and when the second roller 41 is arranged at the reference position, the first roller 40 is arranged at the second retracted position on the back surface of the upstream guide 67. Therefore, the unused transfer roller is not exposed to the conveyance path of the paper S. Accordingly, jamming of the paper S can be suppressed. In addition, since there is no need to separately provide a cover member for covering the first roller 40 and the second roller 41 and a retraction space for the unused first roller 40 and second roller 41, it also contributes to reduction of the number of members and space saving of the secondary transfer unit 9 and the image forming apparatus 100.

[0080] In addition, in the present embodiment, the separation positions of the first roller 40 and the second roller 41 can be switched between a first separation state with a small separation distance from the intermediate transfer belt 8 and a second separation state with a large separation distance. Thereby, when separating from the driving roller 10 at the end of the job to prevent deformation of the first roller 40 and the second roller 41, and when performing calibration during use of the second roller 41, the first roller 40 and the second roller 41 are set to the first separation state, so that the time until being arranged at the reference position for forming the secondary transfer nip portion N can be shortened. Accordingly, a decrease in image processing efficiency (productivity) associated with the movement of the first roller 40 and the second roller 41 can be minimized.

[0081] Furthermore, in the present embodiment, one roller switching motor 55 can be used to drive the roller holder 47 and the switching cam 50. Thereby, compared with the case where the roller holder 47 and the switching cam 50 are driven using separate motors, the drive mechanism and drive control can be simplified, which contributes to cost reduction and compactification of the image forming apparatus 100.

[0082] In addition, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention. For example, in the above-described embodiment, the axial length of the elastic layer 41b of the second roller 41 is larger than the axial length of the elastic layer 40b of the first roller 40. However, the axial length of the elastic layer 41b of the second roller 41 may be smaller than the axial length of the elastic layer 40b of the first roller 40. Further, the shapes, dimensions, etc. of the roller holder 47, the switching cam 50, the upstream guide 65, the downstream guide 67, etc. constituting the secondary transfer unit 9 are merely examples, and can be arbitrarily changed without impairing the effects of the present invention.

[0083] Also, in the above-described embodiment, the arrangement of the first roller 40 and the second roller 41 is switched by a roller switching mechanism including the first bearing member 43, the second bearing member 45, the roller holder 47, the first coil spring 48, the second coil spring 49, the switching cam 50, and the roller switching motor 55. However, the arrangement of the first roller 40 and the second roller 41 may be switched by another switching mechanism.

[0084] Further, in the above-described embodiment, the intermediate transfer type image forming apparatus 100 including the secondary transfer unit 9 for secondarily transferring the toner image primarily transferred onto the intermediate transfer belt 8 to the paper S is exemplified. However, the present invention can be similarly applied to a transfer unit mounted on a direct transfer type image forming apparatus that directly transfers the toner image formed on the photosensitive drum to the paper.

Industrial Applicability

[0085] The present invention can be used in an image forming apparatus including 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 including the same that can switch two transfer rollers having different axial lengths with a simple configuration and can also suppress a decrease in image forming efficiency accompanying the switching of the transfer rollers.

Explanation of Reference Numerals

[0086] Photoreceptor drums (image carriers) Pa to Pd Photoreceptor drums (image carriers) 1a to 1d Primary transfer rollers 6a to 6d Intermediate transfer belt (image carrier) 8 Secondary transfer unit (transfer unit) 9 Unit frame 9a Image density sensor 25 Intermediate transfer unit 30 First roller (transfer roller) 40 Second roller (transfer roller) 41 First bearing member 43 First engaging portion 43a Second bearing member 45 Second engaging portion 45a Roller holder 47 First coil spring 48 Second coil spring 49 Switching cam 50 Shaft 51 Roller switching motor (drive mechanism) 55 Roller switching mechanism 57 Guide hole 63 Recess 64 Bottom portion 64a Inclined portion 64b Downstream guide 65 First roller guide portion 66 Upstream guide 67 Support frame 67a Second roller guide portion 68 Transfer voltage power supply 74 Control unit 90 Image forming apparatus 100 Secondary transfer nip portion N Paper (recording medium) S First position detection sensor S1 Second position detection sensor S2

Claims

1. A transfer unit having a core bar and an elastic layer laminated on the outer peripheral surface of the core bar, the transfer unit including a transfer roller that forms a transfer nip portion by pressing the elastic layer against an image carrier, and transferring a toner image formed on the image carrier to a recording medium passing through the transfer nip portion, a first roller as the transfer roller, and a second roller disposed downstream of the first roller in the conveyance direction of the recording medium, the axial length of the elastic layer being different from that of the first roller, a roller switching mechanism that selectively disposes the first roller or the second roller at a reference position where the first roller or the second roller is pressed against the image carrier to form a transfer nip portion, an upstream guide and a downstream guide respectively disposed on the upstream side and the downstream side of the transfer nip portion with respect to the conveyance direction of the recording medium, comprising: when the first roller moves to the reference position by the roller switching mechanism, the second roller moves to a first retracted position behind the downstream guide, and when the second roller moves to the reference position, the first roller moves to a second retracted position behind the upstream guide, the downstream guide has a first roller guide portion against which the core bar of the second roller abuts, and the upstream guide has a second roller guide portion against which the core bar of the first roller abuts, when the first roller moves to the reference position, the core bar of the second roller moves along the first roller guide portion and the second roller is guided to the first retracted position, and when the second roller moves to the reference position, the core bar of the first roller moves along the second roller guide portion and the first roller is guided to the second retracted position. A transfer unit characterized by this.

2. A transfer unit having a core bar and an elastic layer laminated on the outer peripheral surface of the core bar, the transfer unit including a transfer roller that forms a transfer nip portion by pressing the elastic layer against an image carrier, and transferring a toner image formed on the image carrier to a recording medium passing through the transfer nip portion, a first roller as the transfer roller, and a second roller disposed downstream of the first roller in the conveyance direction of the recording medium, the axial length of the elastic layer being different from that of the first roller, a roller switching mechanism that selectively disposes the first roller or the second roller at a reference position where the first roller or the second roller is pressed against the image carrier to form a transfer nip portion, An upstream guide and a downstream guide respectively arranged on the upstream side and the downstream side of the transfer nip portion with respect to the conveyance direction of the recording medium, comprising: When the first roller moves to the reference position by the roller switching mechanism, the second roller moves to a first retracted position on the back surface of the downstream guide, and when the second roller moves to the reference position, the first roller moves to a second retracted position on the back surface of the upstream guide, The roller switching mechanism is a first bearing member that rotatably supports the first roller, a second bearing member that rotatably supports the second roller, a roller holder having a first bearing holder and a second bearing holder that slidably hold the first bearing member and the second bearing member in directions approaching or separating from the image carrier respectively, a first biasing member 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 biasing member 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 drive mechanism for rotationally driving the roller holder and the switching cam, comprising: By rotating the roller holder, either the first roller or the second roller is disposed opposite to the image carrier, and By rotating the switching cam to change the engagement position of the first engaging portion and the second engaging portion in the guide hole, the first roller or the second roller disposed opposite to the image carrier is selectively arranged at a reference position where it is pressed against the image carrier to form a transfer nip portion and a separated position separated from the image carrier. A transfer unit characterized by this.

3. The transfer unit according to claim 2, wherein the switching cam has a recess formed in a peripheral portion on the outer side in the radial direction of the guide hole, and the first roller or the second roller disposed opposite to the image carrier is disposed at the reference position by engaging the first engaging portion or the second engaging portion with the recess.

4. The concave portion has a trapezoidal shape in plan view. By engaging the first engaging portion or the second engaging portion with the inclined portion of the concave portion, the first roller or the second roller is brought into a first separated state in which it is separated from the image carrier by a predetermined distance. By separating the first engaging portion or the second engaging portion from the concave portion, the first roller or the second roller is brought into a second separated state in which the separation distance is larger than that in the first separated state. The transfer unit according to claim 3, characterized in that.

5. 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 that are disposed opposite to the photosensitive drums disposed in the respective image forming units with the intermediate transfer belt interposed therebetween, and that primarily transfer the toner image formed on the photosensitive drum onto the intermediate transfer belt, An image forming apparatus comprising: a secondary transfer unit as the transfer unit according to any one of claims 1 to 4 that secondarily transfers the toner image primarily transferred onto the intermediate transfer belt onto the recording medium.

Images