Transfer unit and image forming apparatus having the same

Abstract

The present application provides a transfer unit and an image forming apparatus provided with the transfer unit. The transfer unit includes 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 disposed at a downstream side of the first roller in a conveying direction of a recording medium. The roller switching mechanism selectively disposes the first roller or the second roller at a reference position forming a transfer nip. The upstream side guide and the downstream side guide are respectively disposed at an upstream side and a downstream side of the transfer nip in the conveying direction of the recording medium. When the first roller is moved to the reference position by the roller switching mechanism, the second roller is moved to a first retreat position at a back surface of the downstream side guide, and when the second roller is moved to the reference position by the roller switching mechanism, the first roller is moved to a second retreat position at a back surface of the upstream side guide.

Description

TECHNICAL FIELD

[0001] The present application relates to a transfer unit that transfers a toner image formed on an image carrier such as a photosensitive drum and an intermediate transfer belt to a recording medium, and an image forming apparatus provided with the transfer unit, and particularly, to a mechanism that switches the arrangement of a plurality of transfer members. BACKGROUND

[0002] Conventionally, an image forming apparatus of an intermediate transfer method is known, which is provided with a loop-shaped intermediate transfer belt that rotates in a predetermined direction, and a plurality of image forming sections that are arranged along the intermediate transfer belt, and after sequentially superimposing toner images of each color on the intermediate transfer belt by each image forming section to perform primary transfer, transfers the toner images to a recording medium such as paper by a secondary transfer roller.

[0003] In this image forming apparatus of the intermediate transfer method, due to durable printing, the adhesion of toner to the surface of the secondary transfer roller is intensified. In particular, in order to improve colorability and color reproducibility, it is necessary to perform calibration that corrects image density and color misregistration at a predetermined timing, but when the calibration is performed, a patch image formed on the intermediate transfer belt is not transferred to the paper but is removed by a belt cleaning device. Therefore, when the patch image passes through the secondary transfer roller, a part of the toner transferred to the intermediate transfer belt adheres to the secondary transfer roller.

[0004] Conventionally, a method of cleaning the secondary transfer roller by applying a transfer reverse voltage (a voltage of the same polarity as the toner) to the secondary transfer roller at a time other than image formation to return the toner adhering to the secondary transfer roller to the intermediate transfer belt is performed. However, in this method, the cleaning of the secondary transfer roller takes time, and thus there is a problem that the printing waiting time becomes long. SUMMARY

[0005] An object of the present application is to provide an image forming apparatus and a transfer unit that can accommodate a non-used roller in a space-saving manner when switching two transfer rollers that are selectively pressed against an image carrier.

[0006] The transfer unit of the first structure of the present application is characterized in that,

[0007] The transfer unit includes a transfer roller that has a core shaft and an elastic layer laminated on an outer circumferential surface of the core shaft, forms a transfer roller nip by pressing the elastic layer against an image carrier, and transfers a toner image formed on the image carrier to a recording medium that passes through the transfer roller nip,

[0008] The transfer unit includes:

[0009] as the first roller and the second roller of the transfer roller, the second roller is disposed on a downstream side of the first roller in a conveying direction of the recording medium, and an axial length of the elastic layer is different from that of the first roller;

[0010] a roller switching mechanism that selects and disposes the first roller or the second roller at a reference position at which the image carrier is pressed to form a transfer nip;

[0011] an upstream side guide and a downstream side guide that are disposed on an upstream side and a downstream side, respectively, of the transfer nip in a conveying direction of the recording medium,

[0012] when the first roller is moved to the reference position by the roller switching mechanism, the second roller is moved to a first retreat position on a back surface of the downstream side guide, and when the second roller is moved to the reference position by the roller switching mechanism, the first roller is moved to a second retreat position on a back surface of the upstream side guide.

[0013] Further, the present application provides an image forming apparatus including:

[0014] a plurality of the image forming portions that form the toner images of different colors;

[0015] an endless intermediate transfer belt that is moved along the image forming portions as the image carrier;

[0016] a plurality of primary transfer members that are disposed opposite to photosensitive drums disposed at the image forming portions with the intermediate transfer belt interposed therebetween, and that primary transfer the toner images formed on the photosensitive drums onto the intermediate transfer belt; and

[0017] a secondary transfer unit of the transfer unit described above that secondary transfers the toner images primary transferred onto the intermediate transfer belt onto the recording medium.

[0018] According to the first configuration of the present application, when the first roller is disposed at the reference position, the second roller is disposed at a first retreat position on a back surface of the downstream side guide, and when the second roller is disposed at the reference position, the first roller is disposed at a second retreat position on a back surface of the upstream side guide, and thus, the transfer roller that is not used is not exposed to a conveying passage of the recording medium. Therefore, paper jam of the recording medium can be suppressed. Further, since a cover member that covers the first roller and the second roller, and a retreat space of the first roller and the second roller that are not used are not required, reduction in the number of components and space saving are facilitated.

[0019] Further, according to the second configuration of the present application, a secondary transfer unit is provided as the transfer unit, the secondary transfer unit secondary-transferring the toner image once-transferred to the intermediate transfer belt onto a recording medium, whereby it is possible to suppress paper jam of the recording medium in the intermediate transfer type image forming apparatus and realize space saving. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 is a diagram showing an outline of an internal structure of an image forming apparatus 100 provided with the secondary transfer unit 9 of the present application.

[0021] Figure 2 is Figure 1 is an enlarged view of the vicinity of an image forming section Pa in the image forming apparatus 100 of

[0022] Figure 3 is a side sectional view of an intermediate transfer unit 30 mounted on the image forming apparatus 100.

[0023] Figure 4 is a perspective view of the secondary transfer unit 9 of one embodiment of the present application mounted on the image forming apparatus 100.

[0024] Figure 5 is an enlarged perspective view showing the structure of one end side of the secondary transfer unit 9 of the present embodiment.

[0025] Figure 6 is a perspective view of the vicinity of a roller support 47 of the secondary transfer unit 9 viewed from the inner side of the axis.

[0026] Figure 7 is a perspective view showing a drive mechanism of the secondary transfer unit 9 of the present embodiment.

[0027] Figure 8 is a block diagram showing an example of a control path of the image forming apparatus 100 loaded with the secondary transfer unit 9 of the present embodiment.

[0028] Figure 9 is a side sectional view including a switching cam 50 of the secondary transfer unit 9 of the present embodiment, and is a view of a state in which the first roller 40 is disposed at a reference position at which a secondary transfer nip N is formed.

[0029] Figure 10 is a view showing Figure 9 the positional relationship between the first roller 40 and the second roller 41 and the upstream side guide 65 and the downstream side guide 67 in the state of

[0030] Figure 11 is Figure 10 is an enlarged view of the vicinity of the second roller 41 and the downstream side guide 67 in the image forming apparatus 100 of

[0031] Figure 12 is a plan view of the switching cam 50.

[0032] Figure 13 is a view showing the first roller 40 in the first separation state in which the switching cam 50 is rotated by a predetermined angle in the clockwise direction from the state of Figure 9 .

[0033] Figure 14 is a view showing the second roller 41 in the first separation state in which the switching cam 50 is further rotated by a predetermined angle in the clockwise direction from the state of Figure 13 .

[0034] Figure 15 is a view showing the state in which the shaft 51 is rotated in the counterclockwise direction from the state of Figure 14 , so that the second roller 41 is opposed to the drive roller 10.

[0035] Figure 16 is a view showing the state in which the switching cam 50 is rotated by a predetermined angle in the counterclockwise direction from the state of Figure 15 , so that the second roller 41 is disposed at a reference position at which the secondary transfer nip portion N is formed.

[0036] Figure 17 is a view showing the positional relationship between the first roller 40 and the second roller 41 and the upstream-side guide 65 and the downstream-side guide 67 in the state of Figure 16 .

[0037] Figure 18 is an enlarged view of the first roller 40 and the upstream-side guide 65 in the vicinity thereof in Figure 17 .

[0038] Figure 19 is a view showing the second roller 41 in the first separation state in which the switching cam 50 is further rotated by a predetermined angle in the counterclockwise direction from the state of Figure 16 .

[0039] Figure 20 is a view showing the second roller 41 in the second separation state in which the switching cam 50 is further rotated by a predetermined angle in the counterclockwise direction from the state of Figure 19 .

[0040] Figure 21 is a view showing the state in which the switching cam 50 is rotated by a predetermined angle in the clockwise direction from the state of Figure 20 , so that the first roller 40 is opposed to the drive roller 10. DETAILED DESCRIPTION

[0041] Hereinafter, an embodiment of the present application will be described with reference to the drawings. Figure 1 is a view showing the structure of an image forming apparatus 100 provided with the secondary transfer unit 9 of the present application,Figure 2 is Figure 1 An enlarged view of the vicinity of the image forming section Pa in

[0042] Figure 1 The image forming apparatus 100 shown in FIG. 1 is a so-called tandem type color printer, and has the following structure. Four image forming sections Pa, Pb, Pc, and Pd are arranged in this 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 in correspondence with four different colors (magenta, cyan, yellow, and black), and sequentially form images of magenta, cyan, yellow, and black by respective processes of charging, exposure, development, and transfer. Figure 1

[0043] In these image forming sections Pa to Pd, photosensitive drums la, lb, lc, and Id that carry visual images (toner images) of respective colors are arranged. Further, an intermediate transfer belt 8 that rotates in the counterclockwise direction in FIG. 1 is arranged adjacent to each of the image forming sections Pa to Pd. Toner images formed on these photosensitive drums la to Id are sequentially transferred to the intermediate transfer belt 8 that moves while abutting against each of the photosensitive drums la to Id, and then transferred once to a sheet S that is an example of a recording medium in a secondary transfer unit 9. Further, after being fixed to the sheet S in a fixing section 13, the sheet S is discharged from the main body of the image forming apparatus 100. While the photosensitive drums la to Id are rotated in the clockwise direction in FIG. 1, image forming processing is performed on each of the photosensitive drums la to Id. Figure 1 Figure 1

[0044] The sheet S on which a toner image is to be transferred is housed in a sheet housing cassette 16 in the lower portion 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. The intermediate transfer belt 8 mainly uses a belt having no joint (seamless).

[0045] Next, the image forming sections Pa to Pd will be described. Hereinafter, the image forming section Pa will be described in detail, but since the image forming sections Pb to Pd are basically the same structure, the description will be omitted. As shown in FIG. 2, in the vicinity of the photosensitive drum la, a charging device 2a, a developing device 3a, and a cleaning device 7a are arranged in this order in the drum rotation direction (the clockwise direction in FIG. 2), and a primary transfer roller 6a is arranged across the intermediate transfer belt 8. Further, with respect to the photosensitive drum la, a belt cleaning unit 19 is arranged on the upstream side in the rotation direction of the intermediate transfer belt 8, and the belt cleaning unit 19 opposes a tension roller 11 across the intermediate transfer belt 8. Figure 2 Figure 2

[0046] ​​​​​Next, the image forming step of the image forming apparatus 100 will be described. When the user inputs an image forming start, first, the rotation of the photosensitive drums la to Id is started by the main motor 60 (refer to Figure 8 ), and the surfaces of the photosensitive drums la to Id are uniformly charged by the charging rollers 20 of the charging devices 2a to 2d. Next, the surfaces of the photosensitive drums la to Id are subjected to light irradiation by a light beam (laser light) emitted from the exposure device 5, and electrostatic latent images corresponding to the image signals are formed on the respective photosensitive drums la to Id.

[0047] In the developing devices 3a to 3d, a prescribed amount of toner of each of the colors of magenta, cyan, yellow, and black is filled. In addition, in a case where the proportion of the toner in the two-component developer filled in each of the developing devices 3a to 3d due to the formation of the toner images described later is lower than a prescribed value, the toner is replenished from the toner containers 4a to 4d to each of the developing devices 3a to 3d. The toner in the developer is supplied to the photosensitive drums la to Id by the developing rollers 21 of the developing devices 3a to 3d, and is electrostatically attached to the photosensitive drums la to Id. Thus, toner images corresponding to the electrostatic latent images formed by the exposure from the exposure device 5 are formed.

[0048] Then, the toner images of the colors of magenta, cyan, yellow, and black on the photosensitive drums la to Id are primary-transferred to the intermediate transfer belt 8 by the primary transfer rollers 6a to 6d with a prescribed transfer voltage applied between the primary transfer rollers 6a to 6d and the photosensitive drums la to Id. In order to form a prescribed color image, these 4-color images are formed in a predetermined prescribed positional relationship. After that, in order to prepare for the formation of a new electrostatic latent image to be performed next, the toner remaining on the surfaces of the photosensitive drums la to Id is removed by the cleaning blades 22 and the sliding friction rollers 23 of the cleaning devices 7a to 7d.

[0049] When the intermediate transfer belt 8 starts to rotate in the counterclockwise direction in conjunction with the rotation of the driving roller 10 by the belt driving motor 61 (refer to Figure 8 ), the paper sheet S is fed from the registration roller pair 12b to the secondary transfer unit 9 disposed adjacent to the intermediate transfer belt 8 at a prescribed timing, and the color image is transferred. The paper sheet S on which the toner image is transferred is fed to the fixing section 13. The toner remaining on the surface of the intermediate transfer belt 8 is removed by the belt cleaning unit 19.

[0050] The paper sheet S fed to the fixing section 13 is heated and pressurized by the fixing roller pair 13a to fix the toner image on the surface of the paper sheet S, and a prescribed color image is formed. The paper sheet S on which the color image is formed is distributed in the feeding direction by the branch section 14 branched in multiple directions, and is discharged to the paper discharge tray 17 directly (or after being fed to the double-sided feeding path 18 to perform double-sided printing) by the discharge roller pair 15.

[0051] An image density sensor 25 is disposed on the downstream side of the image forming section Pd at a position opposite the intermediate transfer belt 8. As the image density sensor 25, an optical sensor having a light emitting element composed of an LED or the like and a light receiving element composed of a photodiode or the like is generally used. In measuring the toner adhesion amount on the intermediate transfer belt 8, when measuring light is irradiated from the light emitting element to each patch image (reference image) formed on the intermediate transfer belt 8, the measuring light is incident to the light receiving element as light reflected by the toner and light reflected by the belt surface.

[0052] The reflected light from the toner and the belt surface includes specular reflected light and diffuse reflected light. The specular reflected light and the diffuse reflected light are respectively incident to different light receiving elements after being separated by a polarization beam splitter. Each light receiving element photoelectrically converts the received specular reflected light and diffuse reflected light and outputs an output signal to the control section 90 (refer to FIG. 1). Figure 8 ) outputs an output signal.

[0053] Then, the image density (toner amount) of the patch image and the image position are detected from the characteristic change of the output signals of the specular reflected light and the diffuse reflected light, and compared with a predetermined reference density and reference position, and the characteristic value of the developing voltage, the exposure start position and timing of the exposure device 5, and the like are adjusted, whereby density correction and color registration correction (calibration) are performed for each color.

[0054] Figure 3 is a side sectional view of the intermediate transfer unit 30 mounted on the image forming apparatus 100. As shown in Figure 3 , the intermediate transfer unit 30 has the intermediate transfer belt 8 spanning the driving roller 10 on the downstream side and the tension roller 11 on the upstream side, the primary transfer rollers 6a to 6d in contact with the photosensitive drums la to Id via the intermediate transfer belt 8, and the press switching roller 34.

[0055] A belt cleaning unit 19 for removing toner remaining on the surface of the intermediate transfer belt 8 is disposed at a position opposite the tension roller 11. The secondary transfer unit 9 is disposed via the intermediate transfer belt 8 at the driving roller 11, and a secondary transfer roller nip N is formed. The detailed structure of the secondary transfer unit 9 will be described later.

[0056] The intermediate transfer unit 30 is provided with a roller clutch mechanism 35 having a pair of support members (not shown) that rotatably support both end portions of the shafts of the primary transfer rollers 6a to 6d and the press switching roller 34 so as to be perpendicular to the advancing direction of the intermediate transfer belt 8. Figure 3vertical direction) and a driving unit (not shown) that moves the primary transfer rollers 6a to 6d and the press switching roller 34 back and forth along the vertical direction. The roller clutch mechanism 35 can be switched to the following modes: a color mode in which the four primary transfer rollers 6a to 6d are respectively pressed against the photosensitive drums la to Id via the intermediate transfer belt 8 (refer to Figure 1 ) ; a monochrome mode in which only the primary transfer roller 6d is pressed against the photosensitive drum Id via the intermediate transfer belt 8; and a retreat mode in which the four primary transfer rollers 6a to 6d are all separated from the photosensitive drums la to Id.

[0057] Figure 4 is a perspective view of a secondary transfer unit 9 according to an embodiment of the present application mounted on an image forming apparatus 100. Figure 5 is an enlarged perspective view showing a structure of one end side of the secondary transfer unit 9 according to the present embodiment. Figure 6 is a perspective view of a roller support 47 of the secondary transfer unit 9 according to the present embodiment, as viewed from the inner side in the axial direction. Figure 7 is a perspective view showing a driving mechanism of the secondary transfer unit 9 according to the present embodiment. In addition, Figure 4 and Figure 7 the description of the unit frame 9a is omitted in Figure 5 , and the unit frame 9a is shown in a see-through state in Figure 5 , Figure 6 the description of the switching cam 50 is omitted in

[0058] As shown in Figure 4 to Figure 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 support 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 support 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 (refer to Figure 8 ) that switches the arrangement of the first roller 40 and the second roller 41.

[0059] The first roller 40 and the second roller 41 are elastic rollers in which an elastic layer 40b, 41b having electrical conductivity is layered on the outer peripheral surface of a mandrel 40a, 41a. As the material of the elastic layer 40b, 41b, an ionically conductive rubber such as ECO (Epichlorohydrin Rubber) is used, for example.

[0060] The axial length of the elastic layer 40b of the first roller 40 is 311 mm, corresponding to a paper size of A3. The axial length of the elastic layer 41b of the second roller 41 is greater 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 a paper size of 13 inches.

[0061] A pair of first bearing components 43 are arranged at both ends of the first roller 40 along the axial direction to support the mandrel 40a so that it can rotate. A pair of second bearing components 45 are arranged at both ends of the second roller 41 along the axial direction to support the mandrel 41a so that it can rotate.

[0062] A pair of roller supports 47 are disposed at both ends of the first roller 40 and the second roller 41 along their axial direction. The roller supports 47 are generally V-shaped in side view and have a first bearing retainer 47a, a second bearing retainer 47b, and a through hole 47c. The first bearing retainer 47a and the second bearing retainer 47b respectively hold the first bearing component 43 and the second bearing component 45 in a sliding manner. The through hole 47c is formed at the apex of the V-shape, and the shaft 51 is rotatably inserted into the through hole 47c. The roller supports 47 are formed of an insulating material such as synthetic resin.

[0063] like Figure 5 As shown, a first helical spring 48 is disposed between the first bearing retainer 47a and the first bearing component 43. A second helical spring 49 is disposed between the second bearing retainer 47b and the second bearing component 45. The first roller 40 is subjected to force by the first helical spring 48 in a direction away from the shaft 51 (the direction of pressing against the drive roller 10), and the second roller 41 is subjected to force by the second helical spring 49 in a direction away from the shaft 51 (the direction of pressing against the drive roller 10).

[0064] like Figure 4 As shown, a first light-shielding plate 51a is attached to shaft 51 to block the first position detection sensor S1 (see reference). Figure 9 The detection unit of the shaft 51 can detect the rotation angle of the shaft 51. Furthermore, as... Figure 6 As shown, a second light-shielding plate 47d is formed on one side of the roller support 47 in the rotation direction. The second light-shielding plate 47d is formed at a position that can block the detection part of the second position detection sensor S2 disposed on the unit frame 9a.

[0065] The first light-shielding plate 51a and the second light-shielding plate 47d switch the first position detection sensor S1 and the second position detection sensor S2 on or off according to the rotation angle of the roller support 47 (shaft 51), thereby enabling the detection of the positions of the first roller 40 and the second roller 41 supported on the roller support 47. The position detection control of the first roller 40 and the second roller 41 will be described later.

[0066] At both ends of the first roller 40 and the second roller 41 along the axial direction, a pair of switching cams 50 are arranged inside the roller support 47. The switching cams 50 are viewed from the side as fan-shaped, and the main part of the fan shape (the vertex part where the two radii intersect) is fixed to the shaft 51.

[0067] like Figure 7As shown, the roller switching motor 55 is coupled to the shaft 51 via the gears 52, 53. The configuration of the first roller 40 and the second roller 41 is switched by rotating the switching cam 50 together with the shaft 51. The switching control of the first roller 40 and the second roller 41 will be described later.

[0068] Figure 8 is a block diagram showing an example of a control path of the image forming apparatus 100 of the present embodiment in which the secondary transfer unit 9 is mounted. In addition, various controls of the respective parts of the apparatus are performed on the basis of the use of the image forming apparatus 100, and thus the control path of the entire image forming apparatus 100 becomes complicated. Here, the parts required for the implementation of the present application in the control path are mainly described.

[0069] The control section 90 has at least a CPU (Central Processing Unit) 91 as a central arithmetic processing device, a ROM (Read Only Memory) 92 as a read-only storage section, a RAM (Random Access Memory) 93 as a readable and writable storage section, a temporary storage section 94 that temporarily stores image data and the like, a counter 95, and a plurality of (two in this case) I / F (interface) 96 that transmit control signals to the respective devices in the image forming apparatus 100 or receive input signals from the operation section 80. In addition, the control section 90 can be disposed at any place inside the main body of the image forming apparatus 100.

[0070] The control program of the image forming apparatus 100 and data and the like that are required for the control and do not change in the use of the image forming apparatus 100 are stored in the ROM 92. Necessary data generated in the middle of the control of the image forming apparatus 100 and data that temporarily become necessary for the control of the image forming apparatus 100 are stored in the RAM 93. In addition, a density correction table for calibration, the relationship between the on / off state of the first position detection sensor S1 and the second position detection sensor S2 and the rotation angle of the first roller 40 and the second roller 41 for the roller switching control described later, and the like are stored in the RAM 93 (or the ROM 92). The counter 95 accumulates and counts the number of prints.

[0071] In addition, the control section 90 transmits control signals to the respective parts and devices in the image forming apparatus 100 from the CPU 91 through the I / F 96. In addition, signals indicating the states thereof and input signals are transmitted from the respective parts and devices to the CPU 91 through the I / F 96. The respective parts and devices controlled by the control section 90 are, for example, the image forming sections Pa to Pd, the exposure device 5, the primary transfer rollers 6a to 6d, the secondary transfer unit 9, the roller clutch mechanism 35, the main motor 60, the belt driving motor 61, the voltage control circuit 71, the operation section 80, and the like.

[0072] The image input section 70 is a reception section that receives image data transmitted from a host device such as a personal computer to the image forming apparatus 100. The image signal input by the image input section 70 is sent to the temporary storage section 94 after being converted into a digital signal.

[0073] 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 each of the power supplies by an output signal from the control section 90. Each of the power supplies operates in accordance with a control signal from the voltage control circuit 71, the charging voltage power supply 72 applies a prescribed voltage to the charging rollers 20 in the charging devices 2a to 2d, the developing voltage power supply 73 applies a prescribed voltage to the developing rollers 21 in the developing devices 3a to 3d, and the transfer voltage power supply 74 applies a prescribed voltage to the first roller 40 and the second roller 41 in the primary transfer rollers 6a to 6d and the secondary transfer unit 9.

[0074] The operation section 80 is provided with a liquid crystal display section 81 and LEDs 82 that indicate various states, and a stop / clear button that is operated by a user to suspend image formation, and a reset button that sets various settings of the image forming apparatus 100 to default states. The liquid crystal display section 81 displays the states of the image forming apparatus 100 or displays the image forming conditions and the number of printouts. Various settings of the image forming apparatus 100 are made from a printer driver of a personal computer.

[0075] Next, the switching control and the 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. Figure 9 is a side sectional view including the switching cam 50 of the secondary transfer unit 9 of the present embodiment, and is a view showing a state in which the first roller 40 is disposed at a position at which the secondary transfer nip N is formed.

[0076] As shown in Figure 9 , the switching cam 50 is formed with a guide hole 63 in the shape of a circular arc. A recess 64 is formed in the center of the peripheral portion on the radially outer side of the guide hole 63. The first bearing member 43 and the second bearing member 45 are each formed with a first engagement portion 43a and a second engagement portion 45a that engage with the guide hole 63.

[0077] In Figure 9 the state, the first engagement portion 43a of the first bearing member 43 engages with the recess 64. Thus, the first roller 40 is pressed against the driving roller 10 via the intermediate transfer belt 8 by the force of the first coil spring 48 (see Figure 5 ) to form the secondary transfer nip N, and the first roller 40 is driven to rotate with the driving roller 10. The first roller 40 is applied with a transfer voltage of opposite polarity (in this case, negative polarity) to the toner by the transfer voltage power supply 74 (see Figure 8 ). Specifically, when the first roller 40 is disposed at the position at which the secondary transfer nip N is formed, the first roller 40 is applied with a transfer voltage of negative polarity by the transfer voltage power supply 74.Figure 9 The transfer voltage is applied via the first bearing member 43 electrically connected to the transfer voltage power source 74 when the first roller 40 is positioned at the first position.

[0078] Further, the first light shielding plate 51a (refer to Figure 4 ) of the shaft 51 shields (turns on) the detection portion of the first position detection sensor S1, and the first light shielding plate 47d (refer to Figure 6 ) of the roller holder 47 shields (turns on) the detection portion of the second position detection sensor S2. This state (S1 / S2 on) is set as the reference position (start position) of the first roller 40. The configuration and separation state of the first roller 40 are controlled based on the rotation time of the switching cam 50 from the reference position to limit the rotation angle of the switching cam 50.

[0079] Figure 10 is a view 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 Figure 9 . Figure 11 is an enlarged view of the second roller 41 and the surroundings of the downstream guide 65 in Figure 10 . The secondary transfer unit 9 is provided with the downstream guide 65 and the upstream guide 67.

[0080] The downstream guide 65 guides the paper S after passing through the secondary transfer nip N to the fixing portion 13 (refer to Figure 1 ). The upstream guide 67 guides the paper S conveyed from the paper storage cassette 16 via the registration roller pair 12b (both refer to Figure 1 ) to the secondary transfer nip N. The downstream guide 65 and the upstream guide 67 constitute a part of the conveyance passage from the paper feed roller 12a (paper feed portion) to the discharge roller pair 15 (paper discharge portion).

[0081] As shown in Figure 10 and Figure 11 , the second roller 41 is retracted to the space on the back surface of the downstream guide 65 (first retraction position) when the first roller 40 is configured at the reference position. First roller guide portions 66 are formed at both end portions in the width direction of the downstream guide 65 (direction perpendicular to the paper surface of Figure 11 ). The shaft 41a of the second roller 41 is pressed against the first roller guide portions 66 by the force of the second coil spring 49 (refer to Figure 5 ). When the roller holder 47 is rotated in the clockwise direction, the shaft 41a moves along the first roller guide portions 66 to guide the second roller 41 to the first retraction position.

[0082] Figure 12This is a top view of the switching cam 50. The recess 64 of the switching cam 50 is generally trapezoidal in top view, having a bottom 64a corresponding to the upper side of the trapezoid and an inclined portion 64b corresponding to the hypotenuse of the trapezoid. By rotating 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 or disengage from the bottom 64a and inclined portion 64b of the recess 64, thereby enabling the switching of the contact state of the first roller 40 and the second roller 41 relative to the intermediate transfer belt 8 as described later.

[0083] Figure 13 This indicates that the switching cam 50 is switched from... Figure 9 The state rotates clockwise by a specified angle (from here on). Figure 9 The diagram shows the state at a reference position of 10.6°. When shaft 51 is rotated clockwise, switching cam 50 also rotates with shaft 51. On the other hand, roller support 47 is constrained by limiting rib 9b (see reference). Figure 5 This restricts clockwise rotation. As a result, the first engaging portion 43a of the first bearing component 43 moves from the bottom 64a of the recess 64 towards the inclined portion 64b, and the first bearing component 43 overcomes the first coil spring 48 (see reference). Figure 5 The force of the roller moves it towards the shaft 51. As a result, the first roller 40 is in a state that is slightly (2 mm) separated from the intermediate transfer belt 8 (first separation state).

[0084] If the first roller 40 is continuously pressed against the drive roller 10 for an extended period, the first roller 40 may deform axially. Therefore, it is necessary to separate the first roller 40 from the intermediate transfer belt 8 (drive roller 10) after the operation is completed. At this time, it becomes... Figure 13 The first separation state is shown.

[0085] Furthermore, the first light-shielding plate 51a of shaft 51 retracts (disconnects) from the detection section of the first position detection sensor S1, while the second light-shielding plate 47d of roller bracket 47 continues to shield (connects) the detection section of the second position detection sensor S2. That is, when from Figure 9 The detection status (S1 / S2 on) to Figure 13 When the detection state (S1 off / S2 on) changes, it can detect the movement from the reference position of the first roller 40 to the first separation state.

[0086] Figure 14 This indicates that the switching cam 50 is switched from... Figure 13 The state is further rotated clockwise by a specified angle (from here on). Figure 9The diagram shows the state at a reference position of 46.4°. When the shaft 51 is rotated further clockwise, the switching cam 50 also rotates further clockwise along with the shaft 51. On the other hand, the roller support 47 is restrained by rib 9b (see reference). Figure 5 This restricts clockwise rotation. As a result, the first engaging portion 43a of the first bearing component 43 moves from the recess 64, and the first bearing component 43 overcomes the first coil spring 48 (see reference). Figure 5 The force of the transfer belt moves the roller 40 further towards the axis 51. As a result, the first roller 40 is completely separated (6.5 mm) from the intermediate transfer belt 8 (second separation state). This second separation state is only used when switching from the first roller 40 to the second roller 41.

[0087] in addition, Figure 14 The detection states of the first position detection sensor S1 and the second position detection sensor S2 in the middle are related to Figure 13 The first separation state shown is the same (S1 off / S2 on). Therefore, when the image forming apparatus 100 is in the S1 off / S2 on state when it is started, in order to distinguish between the first separation state and the second separation state, the roller support 47 is rotated towards the main body of the image forming apparatus 100 (counterclockwise) for a predetermined time. Then, if it is in the S1 / S2 on state, it is determined to be in the first separation state; if it is not in the S1 / S2 on state, it is determined to be in the second separation state.

[0088] Furthermore, when returning the first roller 40 from the second separation state to the reference position, it is necessary to temporarily rotate the roller support 47 and the switching cam 50 counterclockwise to switch to the reference position of the second roller 41 (see reference). Figure 16 After that, it returns to the reference position of the first roller 40 (refer to...). Figure 9 ).

[0089] Next, the step of switching the roller that forms the secondary transfer roller gap N from the first roller 40 to the second roller 41 will be described. When the shaft 51 is moved from... Figure 14 When the second separation state shown rotates counterclockwise, the switching cam 50 also rotates counterclockwise along with the shaft 51. Furthermore, the first bearing component 43 is connected by the first helical spring 48 (see reference...). Figure 5 The force exerted by the second bearing component 45 is directed away from the shaft 51 by the second helical spring 49 (see reference). Figure 5 The force exerted by the cam 43a is directed away from the shaft 51. Therefore, the first engaging portion 43a and the second engaging portion 45a are pressed against the radially outer periphery of the guide hole 63 of the switching cam 50. Consequently, the roller support 47 also rotates counterclockwise along with the switching cam 50.

[0090] Then, when the roller support 47 rotates to the position of the limiting rib 9c (see reference)Figure 5 ) abuts, as shown in Figure 15 , the second roller 41 is disposed at a position opposite the driving roller 10. In Figure 15 , the first light shield 51a of the shaft 51 retreats (turns off) from the detection portion of the first position detection sensor S1, and the second light shield 47d of the roller holder 47 retreats (turns off) from the detection portion of the second position detection sensor S2. That is, when shifting from the detection state (S1 off / S2 on) of Figure 14 to the detection state (S1 / S2 off) of Figure 15 , the movement of the second roller 41 to the position opposite the driving roller 10 can be detected.

[0091] Figure 16 is a view showing the state in which the switching cam 50 is rotated by a predetermined angle in the counterclockwise direction from the state of Figure 15 . When the shaft 51 is rotated in the counterclockwise direction, the switching cam 50 also rotates together with the shaft 51. On the other hand, the roller holder 47 is restricted from rotating in the counterclockwise direction by the restriction rib 9c (refer to Figure 5 ). As a result, the second engagement portion 45a of the second bearing member 45 moves to the bottom portion 64a of the recess 64, and the second bearing member 45 moves in a direction away from the shaft 51 by the force of the second coil spring 49 (refer to Figure 5 ).

[0092] Thus, the second roller 41 is pressed against the driving roller 10 via the intermediate transfer belt 8 to form a secondary transfer nip N, and the second roller 41 rotates in a driven manner with the driving roller 10. A transfer voltage of opposite polarity (here, negative polarity) to the toner is applied to the second roller 41 by the transfer voltage power source 74 (refer to Figure 8 ). Specifically, when the second roller 41 is disposed at the position of Figure 16 , the transfer voltage is applied via the second bearing member 45 that is electrically connected to the transfer voltage power source 74.

[0093] Further, the first light shield 51a of the shaft 51 shields (turns on) the detection portion of the first position detection sensor S1, and the second light shield 47d of the roller holder 47 retreats (turns off) from the detection portion of the second position detection sensor S2. This state (S1 on / S2 off) is set as the reference position (initial position) of the second roller 41. That is, when shifting from the detection state (S1 / S2 off) of Figure 15 to the detection state (S1 on / S2 off) of Figure 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 limited, and the disposition and separation state of the second roller 41 is controlled.

[0094] Figure 17 is a view showing Figure 16A 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 current state. Figure 18 yes Figure 17 Enlarged view of the first roller 40 and the surrounding area of ​​the upstream guide 67.

[0095] like Figure 17 and Figure 18 As shown, when the second roller 41 is positioned in the reference position, the first roller 40 retracts into the space behind the upstream guide 67 (second retracted position). In the width direction of the upstream guide 67 (and... Figure 18 The two ends of the paper (in the direction perpendicular to the paper surface) are provided with support frames 67a that connect the upstream guide 67 to the unit frame 9a. A second roller guide portion 68 is formed in the support frame 67a.

[0096] The second roller guide portion 68 is U-shaped when viewed from the side, and the distance between the opposing outer side surface 68a and inner side surface 68b is slightly larger than the outer diameter of the mandrel 40a. The mandrel 40a of the first roller 40 utilizes the first helical spring 48 (see reference). Figure 5 The force of the action is pressed against the outer surface 68a of the second roller guide 68. When the roller support 47 rotates counterclockwise, the spindle 40a moves along the second roller guide 68, and the first roller 40 is guided to the second retracted position.

[0097] Figure 19 This indicates that the switching cam 50 is rotated further counterclockwise by a specified angle from the state shown in Figure 16 (here from...). Figure 16 The diagram shows the state at a reference position of 10.6°. When the shaft 51 is rotated further counterclockwise, the switching cam 50 also rotates further counterclockwise along with the shaft 51. On the other hand, the roller support 47 is restrained by rib 9c (see reference). Figure 5 This restricts counterclockwise rotation. As a result, the second engaging portion 45a of the second bearing component 45 moves from the bottom 64a of the recess 64 to the inclined portion 64b, and the second bearing component 45 overcomes the second coil spring 49 (see reference). Figure 5 The force of the roller moves it towards the shaft 51. As a result, the second roller 41 is slightly separated (2 mm) from the intermediate transfer belt 8 (first separation state).

[0098] When the second roller 41 is continuously pressed against the drive roller 10 for an extended period, the second roller 41 may deform axially. Therefore, it is necessary to separate the second roller 41 from the intermediate transfer belt 8 (drive roller 10) after the operation is completed. At this time, it is set as follows: Figure 19the first separation state. Further, when the calibration is performed in the use of the second roller 41, the second roller 41 is set to the first separation state so that the reference image formed on the intermediate transfer belt 8 does not adhere to the second roller 41. In addition, when the calibration is performed with the second roller 41 set to the first separation state, the reference image can be formed in the width direction central portion of the intermediate transfer belt 8.

[0099] Further, the first light shield 51a of the shaft 51 retreats (disengages) from the detection portion of the first position detection sensor S1, and the second light shield 47d of the roller holder 47 continues to retreat (disengage) from the detection portion of the second position detection sensor S2. That is, when the detection state of the first position detection sensor S1 and the second position detection sensor S2 is shifted from the state of S1 on / S2 off to the state of S1 off / S2 on, the movement of the second roller 41 from the reference position to the first separation state can be detected. Figure 16 Figure 19 Further, when the detection state of the first position detection sensor S1 and the second position detection sensor S2 is shifted from the state of S1 on / S2 off to the state of S1 off / S2 on, the movement of the second roller 41 from the reference position to the first separation state can be detected.

[0100] Figure 20 is a view showing the state in which the switching cam 50 is further rotated in the counterclockwise direction from the state of Figure 19 by a predetermined angle (here, 46.4° from the reference position of Figure 16 ). When the shaft 51 is further rotated in the counterclockwise direction, the switching cam 50 is also further rotated in the counterclockwise direction together with the shaft 51. On the other hand, the roller holder 47 is restricted from rotating in the counterclockwise direction by the restriction rib 9c (see Figure 5 ). As a result, the second engagement portion 45a of the second bearing member 45 moves from the recess 64, and the second bearing member 45 moves in a direction further approaching the shaft 51 against the force of the second coil spring 49 (see Figure 5 ). Thus, the second roller 41 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 second roller 41 to the first roller 40.

[0101] Further, the detection state of the first position detection sensor S1 and the second position detection sensor S2 in Figure 20 is the same as the first separation state shown in Figure 19 (S1 / S2 off). Therefore, when the state of S1 / S2 off 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 clockwise direction to the double-sided conveyance passage 18 side for a predetermined time. Then, if the state of S1 on / S2 off is present, it is determined to be the first separation state, and if the state of S1 on / S2 off is present, it is determined to be the second separation state.

[0102] ​Further, in the case of returning the second roller 41 from the second separation state to the reference position, the roller holder 47 and the switching cam 50 need to return to the reference position of the second roller 41 (refer to Figure 9 ) after temporarily rotating in the clockwise direction to switch to the reference position of the first roller 40 (refer to Figure 16 ).

[0103] In the case of switching the roller forming the secondary transfer nip N from the second roller 41 to the first roller 40, the switching cam 50 is rotated by a predetermined angle in the clockwise direction from the second separation state shown in Figure 20 . By this, the switching cam 50 and the roller holder 47 are also rotated by a predetermined angle in the clockwise direction, and when the roller holder 47 is rotated to abut against the restriction rib 9b, the state of Figure 21 in which the first roller 40 opposes the drive roller 10 is obtained. When the switching cam 50 is further rotated by a predetermined angle in the clockwise direction from the state of Figure 21 , the state of Figure 9 in which the first roller 40 is disposed at the reference position is obtained. Hereinafter, switching between the first roller 40 and the second roller 41 is performed by repeating the above steps.

[0104] According to the configuration of the present embodiment, by using the simple structure of the roller holder 47 and the switching cam 50, either one of the first roller 40 and the second roller 41 can be disposed to oppose the drive roller 10, and the first roller 40 or the second roller 41 disposed to oppose the drive roller 10 can be selectively disposed at the reference position forming the secondary transfer nip N and the separation position separated from the intermediate transfer belt 8.

[0105] For example, in the case where the paper S is smaller than a predetermined size (here, A3 size), the first roller 40 having the elastic layer 40b with a smaller axial length is disposed at the reference position. By this, in the case where calibration is performed in the image formation by forming a reference image outside the image area in the width direction of the intermediate transfer belt 8 (axial outside of the first roller 40), the reference image formed on the intermediate transfer belt 8 does not contact the first roller 40. Therefore, calibration can be performed in the image formation, and the image quality can be improved without reducing the image processing efficiency (productivity).

[0106] Further, backside smearing of the paper S due to toner adhering to the first roller 40 adhering to the paper S can be effectively suppressed. Furthermore, since a cleaning operation of returning toner adhering to the first roller 40 to the intermediate transfer belt 8 does not need to be performed, it is also possible to shorten the printing waiting time.

[0107] On the other hand, in the case where the paper S is larger than a predetermined size (here, 13 inches size), the second roller 41 having the elastic layer 41b with a larger axial length is disposed at the reference position. By this, it is possible to reliably secondary transfer toner images to both ends in the width direction of the paper S of a large size.

[0108] Further, when the first roller 40 is disposed at the reference position, the second roller 41 is disposed at the first retreat position on the back surface of the downstream guide 65, and when the second roller 41 is disposed at the reference position, the first roller 40 is disposed at the second retreat position on the back surface of the upstream guide 67, so the non-used transfer roller is not exposed to the paper S transport passage. Therefore, it is possible to suppress paper jam of the paper S. Further, since it is not necessary to separately provide a cover member covering the first roller 40 and the second roller 41, and a retreat space for the non-used first roller 40 and the second roller 41, it is also advantageous for reduction in the number of components, space saving of the secondary transfer unit 9, and the image forming apparatus 100.

[0109] Further, in the present embodiment, it is possible to switch the separation position of the first roller 40 and the second roller 41 to a first separation state in which the separation distance from the intermediate transfer belt 8 is small, and a second separation state in which the separation distance is large. Thereby, in a case where the first roller 40 and the second roller 41 are separated from the drive roller 10 at the end of the job for preventing deformation of the first roller 40 and the second roller 41, by setting the first roller 40 and the second roller 41 to the first separation state in a case where calibration is performed using the second roller 41, it is possible to shorten the time until the reference position at which the secondary transfer nip N is formed is reached. Therefore, it is possible to minimize the reduction in image processing efficiency (productivity) accompanying movement of the first roller 40 and the second roller 41.

[0110] Further, in the present embodiment, it is possible to drive the roller support 47 and the switching cam 50 using one roller switching motor 55. Thereby, compared to a case where the roller support 47 and the switching cam 50 are driven using different motors, it is possible to simplify the drive mechanism and the drive control, and contribute to low cost and compactness of the image forming apparatus 100.

[0111] Further, the present application is not limited to the above-described embodiments, and various modifications can be made within the scope of the present application. For example, in the above-described embodiments, the axial length of the elastic layer 41b of the second roller 41 is made larger than the axial length of the elastic layer 40b of the first roller 40, but the axial length of the elastic layer 41b of the second roller 41 can be made smaller than the axial length of the elastic layer 40b of the first roller 40. Further, the shape, size, and the like of the roller support 47, the switching cam 50, the upstream guide 65, the downstream guide 67, and the like constituting the secondary transfer unit 9 are only examples, and can be arbitrarily changed within a range that does not hinder the effects of the present application.

[0112] Further, in the above-described embodiment, the configuration in which the first roller 40 and the second roller 41 are switched by the roller switching mechanism including the first bearing member 43, the second bearing member 45, the roller support 47, the first coil spring 48, the second coil spring 49, the switching cam 50, and the roller switching motor 55 is configured, but the configuration in which the first roller 40 and the second roller 41 are switched by other switching mechanism can also be configured.

[0113] Further, in the above-described embodiment, the image forming apparatus 100 of the intermediate transfer type provided with the secondary transfer unit 9 that secondarily transfers the toner image once transferred to the intermediate transfer belt 8 onto the paper sheet S is exemplified, but the transfer unit mounted on an image forming apparatus of the direct transfer type that directly transfers the toner image formed on the photosensitive drum onto a paper sheet can also be applied similarly.

[0114] The present application can be applied to an image forming apparatus provided with a transfer unit that transfers a toner image formed on an image carrier onto a recording medium. By utilizing the present application, it is possible to provide a transfer unit in which two transfer rollers having different axial lengths can be switched with a simple configuration, and in which a decrease in image forming efficiency accompanying the switching of the transfer rollers can also be suppressed, and an image forming apparatus provided with the transfer unit.

Claims

1. A transfer unit characterized by comprising a transfer roller having a core shaft and an elastic layer laminated on an outer circumferential surface of the core shaft, and a transfer roller nip is formed by pressing the elastic layer against an image carrier, the transfer unit transfers a toner image formed on the image carrier onto a recording medium passing through the transfer roller nip, the transfer unit comprises: a first roller and a second roller as the transfer roller, the second roller is disposed on a downstream side of the first roller in a conveying direction of the recording medium, and an axial length of the elastic layer is 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 at which the transfer roller nip is formed by pressing the image carrier; an upstream side guide and a downstream side guide that are respectively disposed on an upstream side and a downstream side of the transfer roller nip in the conveying direction of the recording medium, when the first roller is moved to the reference position by the roller switching mechanism, the second roller is moved to a first retreat position on a back surface of the downstream side guide, and when the second roller is moved to the reference position by the roller switching mechanism, the first roller is moved to a second retreat position on a back surface of the upstream side guide, the downstream side guide has a first roller guide portion that abuts against the core shaft of the second roller, the upstream side guide has a second roller guide portion that abuts against the core shaft of the first roller, when the first roller is moved to the reference position, the core shaft of the second roller is moved along the first roller guide portion, and the second roller is guided to the first retreat position, and when the second roller is moved to the reference position, the core shaft of the first roller is moved along the second roller guide portion, and the first roller is guided to the second retreat position.

2. A transfer unit characterized by comprising a transfer roller having a core shaft and an elastic layer laminated on an outer circumferential surface of the core shaft, and a transfer roller nip is formed by pressing the elastic layer against an image carrier, the transfer unit transfers a toner image formed on the image carrier onto a recording medium passing through the transfer roller nip, the transfer unit comprises: a first roller and a second roller as the transfer roller, the second roller is disposed on a downstream side of the first roller in a conveying direction of the recording medium, and an axial length of the elastic layer is 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 at which the transfer roller nip is formed by pressing the image carrier; an upstream side guide and a downstream side guide that are respectively disposed on an upstream side and a downstream side of the transfer roller nip in the conveying direction of the recording medium, when the first roller is moved to the reference position by the roller switching mechanism, the second roller is moved to a first retreat position on a back surface of the downstream side guide, and when the second roller is moved to the reference position by the roller switching mechanism, the first roller is moved to a second retreat position on a back surface of the upstream side guide, the roller switching mechanism comprises: a first bearing member that rotatably supports the first roller; ​ ​ a second bearing member that supports the second roller so as to be rotatable; a roller holder having a first bearing holding portion and a second bearing holding portion that hold the first bearing member and the second bearing member so as to be slidable in a direction of approach to or separation from the image carrier, respectively; a first urging member disposed between the first bearing holding portion and the first bearing member that urges the first bearing member in a direction of approach to the image carrier; a second urging member disposed between the second bearing holding portion and the second bearing member that urges the second bearing member in a direction of approach to the image carrier; a switching cam having a guide hole in which a first engagement portion formed on the first bearing member and a second engagement portion formed on the second bearing member are engaged; and a drive mechanism that rotationally drives the roller holder and the switching cam, by rotating the roller holder, either one of the first roller and the second roller is disposed in opposition to the image carrier, and by changing the engagement positions of the first engagement portion and the second engagement portion in the guide hole of the switching cam by rotating the switching cam, the first roller or the second roller disposed in opposition to the image carrier is selectively disposed in a reference position in which the image carrier is pressed to form the transfer nip, or a separation position in which the image carrier is separated.

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

4. The transfer unit according to claim 3, wherein the recess is trapezoidal in plan view, and by engaging the first engagement portion or the second engagement portion with an inclined portion of the recess, the first roller or the second roller is set to a first separation state in which the image carrier is separated by a prescribed distance, and by separating the first engagement portion or the second engagement portion from the recess, the first roller or the second roller is set to a second separation state in which the image carrier is separated by a distance greater than in the first separation state.

5. The transfer unit according to claim 4, wherein when the toner image is not transferred to the recording medium, the first roller or the second roller disposed in the reference position is set to the first separation state.

6. The transfer unit according to claim 4, wherein when the first roller disposed in opposition to the image carrier is switched to the second roller, the first roller is set to the second separation state, and when the second roller disposed in opposition to the image carrier is switched to the first roller, the second roller is set to the second separation state.

7. An image forming apparatus characterized by comprising: including: a plurality of image forming portions that form toner images of different colors; an endless intermediate transfer belt that is an image carrier and is moved along the image forming portions; a plurality of primary transfer members disposed opposite the photosensitive drums of the image forming portions across the intermediate transfer belt to primary transfer the toner images formed on the photosensitive drums to the intermediate transfer belt; and a secondary transfer unit of the transfer unit according to any one of claims 1 to 6 to secondary transfer the toner images primary transferred to the intermediate transfer belt to the recording medium.

Images