Transfer unit and image forming apparatus including the same

Abstract

The present invention provides a transfer unit and an image forming apparatus including the transfer unit. The transfer unit of the present invention includes: a first roller and a second roller, any one of the axial length, volume resistivity, and hardness of the elastic layer being different; a first bearing member and a second bearing member that rotatably support the mandrels of the first roller and the second roller; a roller bracket; and a switching mechanism. The roller bracket has 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. The switching mechanism rotationally drives the roller bracket to dispose either the first roller or the second roller at a reference position where it is pressed against an image carrier to form a transfer nip portion. The first bearing member and the second bearing member each have a grounding member that grounds the first roller and the second roller, respectively.

Description

Technical Field

[0001] The present invention relates to a transfer unit that transfers a toner image formed on an image carrier such as a photosensitive drum or an intermediate transfer belt to a recording medium, and an image forming apparatus including the transfer unit, and particularly relates to a mechanism for grounding a transfer member. Background Art

[0002] Conventionally, there has been known an image forming apparatus of an intermediate transfer type, which includes: an annular intermediate transfer belt that rotates in a predetermined direction; and a plurality of image forming units that are provided along the intermediate transfer belt. After each color toner image is sequentially overlapped on the intermediate transfer belt by passing through each image forming unit for primary transfer, the toner image is secondarily transferred to a recording medium such as paper by a secondary transfer roller.

[0003] In such an image forming apparatus of the intermediate transfer type, with continuous printing, the adhesion of toner to the surface of the secondary transfer roller progresses. In particular, in order to improve colorability and color reproducibility, it is necessary to perform calibration for correcting image density and color misregistration at a predetermined timing. At this time, when performing calibration, the patch image formed on the intermediate transfer belt is not transferred to the paper and is removed by a belt cleaning device. Therefore, when the patch image passes through the secondary transfer roller, a part of the toner transferred on the intermediate transfer belt adheres to the secondary transfer roller.

[0004] Conventionally, a method of cleaning the secondary transfer roller has been used in which a transfer reverse voltage (a voltage of 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. However, since it takes a long time to clean the secondary transfer roller, this method has a problem that the printing waiting time becomes long. Summary of the Invention

[0005] An object of the present invention is to provide a transfer unit that can stably form a high-quality image by grounding two transfer rollers that selectively press against an image carrier, and an image forming apparatus including the transfer unit.

[0006] The transfer unit according to the first configuration of the present invention is characterized in that

[0007] the transfer unit includes a transfer roller having a core shaft and an elastic layer laminated on an outer peripheral surface of the core shaft, and forms a transfer nip portion by pressing the elastic layer against an image carrier, and the transfer unit transfers a toner image formed on the image carrier to a recording medium passing through the transfer nip portion.

[0008] The transfer unit includes:

[0009] As the first roller and the second roller of the transfer roller, any one of the axial length, volume resistivity, and hardness of the elastic layers of the first roller and the second roller is different;

[0010] A first bearing member that supports the core shaft of the first roller so as to be rotatable;

[0011] A second bearing member that supports the core shaft of the second roller so as to be rotatable;

[0012] A roller bracket having a first bearing holder and a second bearing holder, the first bearing holder and the second bearing holder respectively holding the first bearing member and the second bearing member so as to be slidable in a direction approaching or separating from the image carrier; and

[0013] A switching mechanism that rotationally drives the roller bracket and arranges either the first roller or the second roller at a reference position where it is pressed against the image carrier to form the transfer nip portion;

[0014] The first bearing member and the second bearing member each have a grounding member that grounds the first roller and the second roller.

[0015] In addition, the present invention provides an image forming apparatus, including:

[0016] A plurality of image forming units that form the toner images of different colors;

[0017] A ring-shaped intermediate transfer belt as the image carrier that moves along the image forming units;

[0018] A plurality of primary transfer members that are disposed opposite to the photosensitive drums arranged in the respective image forming units with the intermediate transfer belt therebetween, and primarily transfer the toner images formed on the photosensitive drums to the intermediate transfer belt; and

[0019] A secondary transfer unit as the transfer unit having the above-described configuration that secondarily transfers the toner images primarily transferred to the intermediate transfer belt to the recording medium.

[0020] According to the first configuration of the present invention, the first roller and the second roller are always grounded through the grounding members. Thus, since no transfer electric field remains on the first roller and the second roller after transfer, an appropriate transfer electric field can always be applied to the transfer nip portion, and a good transfer image can be stably obtained.

[0021] In addition, according to the second configuration of the present invention, the first roller or the second roller disposed at the reference position is switched according to the size in the width direction of the image data and the size in the width direction of the recording medium, so that an appropriate transfer roller corresponding to the image width and the width of the recording medium can be used, and transfer defects and the occurrence of back surface contamination of the recording medium caused by toner adhesion to the transfer roller can be effectively suppressed. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 FIG. is a schematic diagram showing the internal configuration of the image forming apparatus 100 including the secondary transfer unit 9 of the present invention.

[0023] Figure 2 is Figure 1 an enlarged view of the vicinity of the image forming section Pa in FIG.

[0024] Figure 3 is a side cross-sectional view of the intermediate transfer unit 30 mounted on the image forming apparatus 100.

[0025] Figure 4 is a perspective view of the secondary transfer unit 9 according to an embodiment of the present invention mounted on the image forming apparatus 100.

[0026] Figure 5 is an enlarged perspective view showing the configuration of one end side of the secondary transfer unit 9 of the present embodiment.

[0027] Figure 6 is a perspective view of the periphery of the roller bracket 47 of the secondary transfer unit 9 of the present embodiment as viewed from the outer side in the axial direction.

[0028] Figure 7 is a perspective view of the periphery of the roller bracket 47 of the secondary transfer unit 9 as viewed from the front side and is a view showing the contact state between the shaft 51 and the main body frame 101.

[0029] Figure 8 is an enlarged perspective view of the periphery of the first bearing member 43 and the second bearing member 45 of the secondary transfer unit 9 as viewed from the outer side in the axial direction.

[0030] Figure 9 is a perspective view showing the drive mechanism of the secondary transfer unit 9 of the present embodiment.

[0031] Figure 10 is a circuit diagram showing the flow of the secondary transfer current in the secondary transfer nip portion N.

[0032] Figure 11 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.

[0033] Figure 12It is a side sectional view of the switching cam 50 including the secondary transfer unit 9 of the present embodiment and shows a state in which the first roller 40 is disposed at a reference position forming the secondary transfer nip portion N.

[0034] Figure 13 It is a top view of the switching cam 50 as viewed from the inner side in the axial direction.

[0035] Figure 14 It shows from Figure 12 The state in which the switching cam 50 is rotated clockwise by a predetermined angle and the separated state of the first roller 40.

[0036] Figure 15 It shows from Figure 14 The state in which the shaft 51 is rotated counterclockwise and the second roller 41 is opposed to the driving roller 10.

[0037] Figure 16 It shows from Figure 15 The state in which the switching cam 50 is rotated counterclockwise by a predetermined angle and the second roller 41 is disposed at a reference position forming the secondary transfer nip portion N.

[0038] Figure 17 It shows from Figure 16 The separated state of the second roller 41 when the switching cam 50 is further rotated counterclockwise by a predetermined angle from the state.

[0039] Figure 18 It shows from Figure 17 The state in which the switching cam 50 is rotated clockwise by a predetermined angle and the first roller 40 is opposed to the driving roller 10. Detailed Embodiment

[0040] Hereinafter, embodiments of the present invention will be described with reference to the drawings. Figure 1 It is a schematic diagram showing the configuration of the image forming apparatus 100 including the secondary transfer unit 9 of the present invention, Figure 2 It is Figure 1 An enlarged view near the image forming section Pa in

[0041] Figure 1 The image forming apparatus 100 shown is a so-called tandem type color digital multi-function peripheral and has the following configuration. Inside the main body of the image forming apparatus 100, from the upstream in the conveyance direction ( Figure 1Starting from the center left side (center left), four image forming units Pa, Pb, Pc, and Pd are arranged in sequence. These image forming units Pa-Pd are provided to correspond to images of four different colors (magenta, cyan, yellow, and black). Through the charging, exposure, development, and transfer processes, they form magenta, cyan, yellow, and black images in sequence. The image reading unit 20 is arranged above the main body of the image forming apparatus 100. An automatic document feeder is provided in the image reading unit 20.

[0042] The image forming units Pa to Pd are provided with photosensitive drums 1a, 1b, 1c, and 1d that carry visible images (toner images) of respective colors. Figure 1 In the image forming apparatus 100, the intermediate transfer belt 8 rotating in the counterclockwise direction is arranged adjacent to each image forming unit Pa~Pd. The toner images formed on these photosensitive drums 1a~1d are sequentially transferred to the intermediate transfer belt 8 which moves while contacting each photosensitive drum 1a~1d, and then are transferred to paper S as an example of a recording medium by the secondary transfer unit 9. In addition, after being fixed to the paper S by the fixing unit 13, the toner images are discharged from the main body of the image forming apparatus 100. Figure 1 The image forming process is executed on each of the photosensitive drums 1 a to 1 d while rotating in the clockwise direction.

[0043] Paper S to be transferred with a toner image is stored in a paper cassette 16 at the bottom of the image forming apparatus 100 and is conveyed to the secondary transfer unit 9 by a paper feed roller 12a and a resist roller pair 12b. A seamless belt is mainly used as the intermediate transfer belt 8.

[0044] Next, the image forming units Pa to Pd will be described. The image forming unit Pa will be described in detail below. As for the image forming units Pb to Pd, since they have basically the same structure, their description will be omitted. Figure 2 As shown, around the photosensitive drum 1a, along the drum rotation direction ( Figure 2 A charging device 2a, a developing device 3a, and a cleaning device 7a are arranged in the clockwise direction of the photosensitive drum 1a, and a primary transfer roller 6a is arranged across the intermediate transfer belt 8. Furthermore, a belt cleaning unit 19 is arranged upstream of the intermediate transfer belt 8 in the rotational direction of the photosensitive drum 1a, facing the tension roller 11 across the intermediate transfer belt 8.

[0045] Next, the image forming process of the image forming apparatus 100 will be described. When the user inputs the start of image formation, first, the main motor 60 (see FIG. 10 ) is driven. Figure 11) Rotate the photosensitive drums 1a to 1d, and uniformly charge the surfaces of the photosensitive drums 1a to 1d by the charging rollers 25 of the charging devices 2a to 2d. Next, irradiate the surfaces of the photosensitive drums 1a to 1d with light beams (lasers) emitted from the exposure device 5. Thereby, an electrostatic latent image is formed on each of the photosensitive drums 1a to 1d in accordance with the image signal. The image signal is obtained by reading the original image by the image reading unit 20, or is sent from an external device such as a computer through the image input unit 70 (refer to Figure 11 )

[0046] Each of the developing devices 3a to 3d is filled with a predetermined amount of toner of each color of magenta, cyan, yellow, and black. In addition, when the proportion of the toner in the two-component developer filled in each of the developing devices 3a to 3d becomes lower than the specified value due to the formation of the toner image described later, toner is supplied from the toner containers 4a to 4d to each of the developing devices 3a to 3d. The toner in the developer is supplied by the developing rollers 22 of the developing devices 3a to 3d and electrostatically attached to the photosensitive drums 1a to 1d. Thereby, a toner image corresponding to the electrostatic latent image formed by the exposure from the exposure device 5 is formed.

[0047] Then, an electric field is applied between the primary transfer rollers 6a to 6d and the photosensitive drums 1a to 1d with a predetermined transfer voltage by the primary transfer rollers 6a to 6d, and the magenta, cyan, yellow, and black toner images on the photosensitive drums 1a to 1d are primarily transferred to the intermediate transfer belt 8. These four-color images are formed with a predetermined positional relationship for forming a predetermined full-color image. After that, preparation for the formation of a new electrostatic latent image is made, and the toner remaining on the surfaces of the photosensitive drums 1a to 1d is removed by the cleaning blades 23 and the sliding friction rollers 24 of the cleaning devices 7a to 7a.

[0048] Along with the rotation of the drive roller 10 using the belt drive motor 61 (refer to Figure 11 ), the intermediate transfer belt 8 starts to rotate counterclockwise. Next, the paper S is conveyed from the alignment roller pair 12b to the secondary transfer unit 9 adjacent to the intermediate transfer belt 8 at a predetermined timing, and the full-color image is transferred. The paper S on which the toner image is 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.

[0049] The paper S conveyed to the fixing unit 13 is heated and pressed by the fixing roller pair 13a, and the toner image is fixed on the surface of the paper S, forming a predetermined full-color image. The paper S on which the full-color image is formed is distributed to the conveyance direction by the branch unit 14 that branches in multiple directions, and is discharged to the discharge tray 17 as it is (or after being sent to the duplex conveyance path 18 for duplex printing).

[0050] The image density sensor 28 is disposed at a position facing the driving roller 10 with the intermediate transfer belt 8 therebetween. As the image density sensor 28, an optical sensor including 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. When measuring the toner adhesion amount on the intermediate transfer belt 8, measurement light is irradiated from the light emitting element to each patch image (reference image) formed on the intermediate transfer belt 8. The measurement light is incident on the light receiving element as light reflected by the toner and light reflected by the belt surface.

[0051] The reflected light from the toner and the belt surface includes specular reflection light and diffuse reflection light. After the specular reflection light and the diffuse reflection light are separated by a polarization beam splitter, they are respectively incident on different light receiving elements. Each light receiving element performs photoelectric conversion on the received specular reflection light and diffuse reflection light and outputs a signal to the control unit 90 (refer to Figure 11 ).

[0052] Moreover, based on the characteristic changes of the output signals of the specular reflection light and the diffuse reflection light, the image density (toner amount) and image position of the patch image are detected, 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, etc. are adjusted. Thereby, density correction and color registration correction (calibration) are performed for each color.

[0053] Figure 3 It is a side cross-sectional view of the intermediate transfer unit 30 mounted on the image forming apparatus 100. As Figure 3 shown, the intermediate transfer unit 30 includes: an intermediate transfer belt 8 stretched between the downstream driving roller 10 and the upstream tension roller 11; primary transfer rollers 6a to 6d that contact the photosensitive drums 1a to 1d through the intermediate transfer belt 8; and a pressing switching roller 34.

[0054] 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. The secondary transfer unit 9 is disposed in pressure contact with the driving roller 10 through the intermediate transfer belt 8 to form a secondary transfer nip portion N. The detailed configuration of the secondary transfer unit 9 will be described later.

[0055] The intermediate transfer unit 30 is provided with a roller clutch mechanism 35, and the roller clutch mechanism 35 has: a pair of support members (not shown), which are rotatable and perpendicular to the advancing direction of the intermediate transfer belt 8 ( Figure 3Moves the upper and lower ends of the rotary shafts of the primary transfer rollers 6a to 6d and the pressing and switching roller 34 in the vertical direction; and a driving device (not shown) that moves the primary transfer rollers 6a to 6d and the pressing and switching roller 34 back and forth in the vertical direction. The roller clutch mechanism 35 can switch the four primary transfer rollers 6a to 6d to a color mode in which they are respectively pressed against the photosensitive drums 1a to 1d (refer to Figure 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 retracted mode in which all four primary transfer rollers 6a to 6d are separated from the photosensitive drums 1a to 1d.

[0056] Figure 4 is a perspective view of the secondary transfer unit 9 according to an embodiment of the present invention mounted on the image forming apparatus 100. Figure 5 is an enlarged perspective view showing the configuration of one end side of the secondary transfer unit 9 of the present embodiment. Figure 6 is a perspective view of the periphery of the roller bracket 47 of the secondary transfer unit 9 of the present embodiment as viewed from the outer side in the axial direction. Figure 7 is a view showing the contact state between the shaft 51 and the main body frame 101. Figure 8 is an enlarged perspective view of the periphery of the first bearing member 43 and the second bearing member 45 of the secondary transfer unit 9 as viewed from the outer side in the axial direction. Figure 9 is a perspective view showing the drive mechanism of the secondary transfer unit 9 of the present embodiment. In addition, in Figure 4 and Figure 9 , the description of the unit frame 9a is omitted. In addition, in Figure 5 , the unit frame 9a is shown in a transparent state. In Figure 6 , the description of the first bearing member 43 is omitted, and in Figure 6 and Figure 7 , the description of the switching cam 50 is omitted.

[0057] As shown in Figures 4 to 9 , the secondary transfer unit 9 includes a first roller 40 and a second roller 41 as secondary transfer rollers, a first bearing member 43, a second bearing member 45, a roller bracket 47, a switching cam 50, and a roller switching motor 55.

[0058] The first roller 40 and the second roller 41 are elastic rollers having an elastic layer 40b and 41b with conductivity laminated on the outer circumferences of the mandrels 40a and 41a, respectively. As the material of the elastic layers 40b and 41b, for example, a conductive rubber such as ECO (epichlorohydrin rubber) is used.

[0059] The axial length of the elastic layer 40b of the first roller 40 is 311 mm, corresponding to A3-sized paper. The axial length of the elastic layer 41b of the second roller 41 is 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-sized paper.

[0060] A pair of first bearing members 43 are arranged at both axial ends of the first roller 40 to support the core shaft 40a for rotation. A pair of second bearing members 45 are arranged at both axial ends of the second roller 41 to support the core shaft 41a for rotation.

[0061] A pair of roller brackets 47 are arranged at both axial ends of the first roller 40 and the second roller 41. The roller bracket 47 is substantially V-shaped when viewed from the side and has a first bearing holding portion 47a, a second bearing holding portion 47b, and an insertion through 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 through hole 47c is formed at the vertex portion of the V shape, and the shaft 51 is rotatably inserted into the insertion through hole 47c. The roller bracket 47 is formed of an insulating material such as synthetic resin.

[0062] As Figure 5 shown, 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 in a direction away from the shaft 51 (the direction of pressing against the driving roller 10), and the second roller 41 is urged by the second coil spring 49 in a direction away from the shaft 51 (the direction of pressing against the driving roller 10).

[0063] As Figure 6 and Figure 8 shown, first grounding members 56a and 56b are arranged on the first bearing member 43 and the second bearing member 45. The first grounding members 56a and 56b are formed by bending a metal plate into a specified shape. One end of the first grounding member 56a contacts the metal bearing 40c mounted on the core shaft 40a of the first roller 40, and the other end contacts the upper end of the first coil spring 48. One end of the first grounding member 56b contacts the metal bearing 41c mounted on the core shaft 41a of the second roller 41, and the other end contacts the upper end of the second coil spring 49.

[0064] Figure 6 and Figure 7 ​As shown, the second grounding member 57 is disposed on the roller bracket 47. The second grounding member 57 is formed by bending a metal plate into a prescribed shape. The upper end portion of the second grounding member 57 is bent so as to overlap the bottom surfaces of the first bearing holding portion 47a and the second bearing holding portion 47b, and contacts the lower end portions of the first coil spring 48 and the second coil spring 49. A guide through-hole 57a through which the shaft 51 passes is formed in the lower end portion of the second grounding member 57. The inner diameter of the guide through-hole 57a is the same as the outer diameter of the shaft 51, and the inner peripheral surface of the guide through-hole 57a contacts the outer peripheral surface of the shaft 51.

[0065] As Figure 7 shown, the outer peripheral surface of the shaft 51 contacts the contact piece 101a formed on the main body frame 101 of the image forming apparatus 100, and the shaft 51 is grounded via the main body frame 101.

[0066] With the above configuration, the first roller 40 is grounded via the first grounding member 56a, the first coil spring 48, the second grounding member 57, the shaft 51, and the main body frame 101. Further, the second roller 41 is grounded via the first grounding member 56b, the second coil spring 49, the second grounding member 57, the shaft 51, and the main body frame 101.

[0067] As Figure 4 shown, a first light shielding plate 51a is provided on the shaft 51, and by shielding the detection portion of the first position detection sensor S1 (refer to Figure 11 ), the rotation angle of the shaft 51 can be detected. Further, as Figure 7 shown, a second light shielding plate 47d is formed on one side surface in the rotation direction of the roller bracket 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 (refer to Figure 11 ) disposed in the unit frame 9a.

[0068] According to the rotation angle of the roller bracket 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, whereby the positions of the first roller 40 and the second roller 41 supported by the roller bracket 47 can be detected. The position detection control of the first roller 40 and the second roller 41 will be described later.

[0069] As Figure 8As shown, the first bearing member 43 and the second bearing member 45 respectively have arcuate first bearing portions 43b and second bearing portions 45b that hold the mandrels 40a and 41a of the first roller 40 and the second roller 41. A restricting member 58 that restricts upward movement of the mandrels 40a and 41a is mounted above the first bearing portion 43b and the second bearing portion 45b, and caps 59 are mounted at the front ends of the mandrels 40a and 41a. The caps 59 prevent the mandrels 40a and 41a from coming off the first bearing portion 43b and the second bearing portion 45b.

[0070] The first bearing member 43 and the second bearing member 45 are respectively held with a prescribed margin (extra clearance) in the left - right direction (the rotation direction of the roller support 47) with respect to the first bearing holding portion 47a and the second bearing holding portion 47b of the roller support 47.

[0071] A pair of switching cams 50 are arranged outside the roller support 47 at both axial ends of the first roller 40 and the second roller 41. When viewed from the side, the switching cam 50 has a fan - shaped shape, and the main part of the fan (the vertex part where two radii intersect) is fixed to the shaft 51. A parallel pin 51b extending radially is fixed to the shaft 51. A pin insertion portion 65 into which the parallel pin 51b is inserted is formed in the switching cam 50. The parallel pin 51b is inserted into the pin insertion portion 65 without clearance in the circumferential direction of the shaft 51.

[0072] An arcuate guide hole 63 (see Figure 12 ) is formed inside the switching cam 50 in the axial direction. A recess 64 (see Figure 12 ) is formed at the center of the peripheral portion on the radially outer side of the guide hole 63. First engaging portions 43a and second engaging portions 45a that engage with the guide hole 63 are respectively formed in the first bearing member 43 and the second bearing member 45.

[0073] By the first engaging portion 43a engaging with the recess 64, the first bearing member 43 is pressed by the first spiral spring 48 and moves in a direction separating from the shaft 51. As a result, the first roller 40 is pressed against the driving roller 11 via the intermediate transfer belt 8. Further, by the second engaging portion 45a engaging with the recess 64, the second bearing member 45 is pressed by the second spiral spring 49 and moves in a direction separating from the shaft 51. As a result, the second roller 41 is pressed against the driving roller 11 via the intermediate transfer belt 8. That is, the positions of the first bearing member 43 and the second bearing member 45 in the rotation direction of the roller support 47 are determined.

[0074] As described above, the first bearing member 43 and the second bearing member 45 are held with a prescribed margin (extra clearance) with respect to the first bearing holding portion 47a and the second bearing holding portion 47b of the roller support 47, respectively. Here, the switching cam 50 maintains its positional relationship with the shaft 51 via the parallel pin 51b. In addition, the positional relationship between the switching cam 50 and the first roller 40 and the second roller 41 is maintained by the engagement of the first engaging portion 43a and the second engaging portion 45b with the concave portion 64 of the switching cam 50.

[0075] In addition, as will be described later Figure 13 as shown, the pin insertion portion 65 of the switching cam 50 is formed along a straight line passing through the shaft 51 and the concave portion 64. That is, when the first engaging portion 43a and the second engaging portion 45b are engaged with the concave portion 64, the first roller 40 and the second roller 41 are positioned in the extending direction of the parallel pin 51b.

[0076] As Figure 9 shown, the roller switching motor 55 is connected to the shaft 51 via the gears 52 and 53. By rotating the switching cam 50 together with the shaft 51, the configurations of the first roller 40 and the second roller 41 are switched. The switching control of the first roller 40 and the second roller 41 will be described later.

[0077] Figure 10 is a circuit diagram showing the flow of the secondary transfer current at the secondary transfer nip portion N. As Figure 10 shown, the driving roller 10 is electrically connected to the positive terminal of the transfer voltage power source 74. The first roller 40 and the second roller 41 are electrically connected to the positive terminal of the transfer voltage power source 74 in a state of being disposed opposite to the driving roller 10.

[0078] If a secondary transfer voltage having the same polarity as the toner (here, positive polarity) is applied from the transfer voltage power source 74 to the driving roller 10, the secondary transfer current flows from the driving roller 10 via the intermediate transfer belt 8 to the first roller 40 or the second roller 41. Thereby, a prescribed secondary transfer electric field is generated at the secondary transfer nip portion N, and the toner image transferred onto the intermediate transfer belt 8 for the first time is secondarily transferred onto the paper S passing through the secondary transfer nip portion N.

[0079] As described above, the first roller 40 and the second roller 41 are always grounded via the first grounding members 56a, 56b and the second grounding member 57. Therefore, a part of the secondary transfer current flowing to the first roller 40 or the second roller 41 flows to the main body frame 101. However, since not all of the secondary transfer current flows to the main body frame 101, even if the first roller 40 and the second roller 41 are always grounded, it does not affect the secondary transfer property.

[0080] In addition, in the present embodiment, a secondary transfer voltage having the same polarity (positive polarity) as the toner is applied to the driving roller 10 to generate a secondary transfer electric field at the secondary transfer nip portion N. However, a secondary transfer voltage having a polarity opposite to that of the toner (negative polarity) may be applied to the first roller 40 and the second roller 41 to generate a secondary transfer electric field at the secondary transfer nip portion N. Even in this case, since the secondary transfer current also flows in the same direction as Figure 10 the same direction, a part of the secondary transfer current flows into the main body frame 101, and it does not affect the secondary transfer property.

[0081] Figure 11 FIG. is a block diagram showing an example of a control path of the image forming apparatus 100 equipped with the secondary transfer unit 9 of the present embodiment. In addition, since various controls of each part of the apparatus are performed on the basis of using the image forming apparatus 100, the control path of the entire image forming apparatus 100 becomes a complicated path. Therefore, the parts necessary for the implementation of the present invention in the control path are mainly described herein.

[0082] The control unit 90 includes 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 unit, a RAM (Random Access Memory) 93 as a rewritable storage unit, a temporary storage unit 94 for temporarily storing image data, etc., a counter 95, and a plurality (two in this case) of I / Fs (Interfaces) 96 for sending control signals to each device in the image forming apparatus 100 or receiving input signals from the operation unit 80. In addition, the control unit 90 may be disposed at any position inside the main body of the image forming apparatus 100.

[0083] In the ROM 92, control programs for the image forming apparatus 100, data such as values required for control, which do not change during the use of the image forming apparatus 100, etc. are stored. In the RAM 93, data required during the control of the image forming apparatus 100, data that temporarily becomes required during the control of the image forming apparatus 100, etc. are stored. In addition, a density correction table for calibration, etc. is also stored in the RAM 93 (or ROM 92). The counter 95 accumulates and counts the number of printed sheets.

[0084] In addition, the control unit 90 sends control signals from the CPU 91 to each part and device of the image forming apparatus 100 through the I / F 96. In addition, signals indicating the states of each part and device and input signals are sent from each part and device to the CPU 91 through the I / F 96. As each part and device controlled by the control unit 90, 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 clutch mechanism 35, the main motor 60, the belt drive motor 61, the voltage control circuit 71, the operation unit 80, etc. can be cited.

[0085] The image input unit 70 is a receiving unit that receives image data transmitted from an upper device such as a computer to the image forming apparatus 100. After the image signal input through the image input unit 70 is converted into a digital signal, it is sent to the temporary storage unit 94.

[0086] The voltage control circuit 71 is connected to the charging voltage power supply 72, the developing voltage power supply 73, the transfer voltage power supply 74, and the cleaning voltage power supply 75, and operates these power supplies according to the output signal from the control unit 90. According to the control signal from the voltage control circuit 71, these power supplies cause the charging voltage power supply 72 to apply a prescribed charging voltage to the charging roller 25 in the charging devices 2a to 2d, the developing voltage power supply 73 to apply a prescribed developing voltage to the developing roller 22 in the developing devices 3a to 3d, and the transfer voltage power supply 74 to apply a prescribed primary transfer voltage to the primary transfer rollers 6a to 6d. In addition, the transfer voltage power supply 74 applies a prescribed secondary transfer voltage to the driving roller 10.

[0087] The operation unit 80 is provided with a liquid crystal display unit 81 and LEDs 82 indicating various states. When the user operates the stop / cancel button of the operation unit 80, the image formation is aborted, and when the user operates the reset button, various settings of the image forming apparatus 100 are set to the default state. The liquid crystal display unit 81 displays the state of the image forming apparatus 100, or displays the image formation status and the number of printed copies. Various settings of the image forming apparatus 100 are made from the printer driver of the computer.

[0088] Next, the switching control and the position detection control of the first roller 40 and the second roller 41 of the secondary transfer unit 9 of the present embodiment will be described. Figure 12 FIG. is a side cross-sectional view of the switching cam 50 including the secondary transfer unit 9 of the present embodiment, and is a view showing a state where the first roller 40 is disposed at a position forming the secondary transfer nip portion N. Figure 13 FIG. is a plan view of the switching cam 50 as viewed from the inner side in the axial direction.

[0089] As Figure 13 shown, the concave portion 64 of the switching cam 50 has a substantially trapezoidal shape 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 oblique 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 are engaged with or separated from the bottom portion 64a or the inclined portion 64b of the concave portion 64, whereby the contact state of the first roller 40 and the second roller 41 with respect to the intermediate transfer belt 8 can be switched as described later.

[0090] In Figure 12In the state where the first engaging portion 43a of the first bearing member 43 engages with the bottom portion 64a of the concave portion 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 ), and a secondary transfer nip portion N is formed. The first roller 40 rotates following the driving roller 10. A prescribed secondary transfer current flows through the first roller 40 by the transfer voltage power source 74 (see Figure 11 ). Specifically, when the first roller 40 is disposed at the position of Figure 12 , a transfer voltage of the same polarity as the toner (here, the positive polarity) is applied to the driving roller 10 electrically connected to the transfer voltage power source 74, and the secondary transfer current flows through the first roller 40 via the intermediate transfer belt 8.

[0091] In addition, the first light shielding plate 51a of the shaft 51 (see Figure 4 ) shields (turns on) the detection portion of the first position detection sensor S1, and the second light shielding plate 47d of the roller bracket 47 shields (turns on) the detection portion of the second position detection sensor S2. This state (S1 / S2 turned on) is set as the reference position (starting position) of the first roller 40. According to the rotation time of the switching cam 50 rotating from this reference position, the rotation angle of the switching cam 50 is restricted, and the arrangement and separation state of the first roller 40 are controlled.

[0092] Figure 14 It is a diagram showing a state where the switching cam 50 is rotated by a prescribed angle in the clockwise direction from the state of Figure 12 (here, 46.4° from the reference position of Figure 12 ). If the shaft 51 is further rotated in the clockwise direction, the switching cam 50 also rotates further in the clockwise direction together with the shaft 51. On the other hand, the rotation of the roller bracket 47 in the clockwise direction is restricted by the restricting rib 9b (see Figure 5 ). As a result, the first engaging portion 43a of the first bearing member 43 moves away from the concave portion 64, and the first bearing member 43 moves in a direction approaching the shaft 51 against the force of the first coil spring 48 (see Figure 5 ). Thus, the first roller 40 becomes a state of being separated from the intermediate transfer belt 8 (separation state). Figure 14 The detection states of the first position detection sensor S1 and the second position detection sensor S2 in

[0093] are S1 off / S2 on. Figure 14 If the shaft 51 is rotated in the counterclockwise direction from the state of Figure 5 , the switching cam 50 also rotates in the counterclockwise direction together with the shaft 51. In addition, the first bearing member 43 is under the action of the first coil spring 48 (see Figure 5) are respectively applied with forces in directions separating from the shaft 51. Therefore, the first engaging portion 43a and the second engaging portion 45a press the peripheral portion on the radially outer side of the guide hole 63 of the switching cam 50. Thereby, the roller bracket 47 also rotates counterclockwise together with the switching cam 50.

[0094] Then, if the roller bracket 47 rotates until it abuts against the restricting rib 9c (refer to Figure 5 ), as shown in Figure 15 , the second roller 41 is disposed at a position facing the driving roller 10. In the state of Figure 15 , the first light shielding plate 51a of the shaft 51 retracts (disconnects) from the detection portion of the first position detection sensor S1, and the second light shielding plate 47d of the roller bracket 47 retracts (disconnects) from the detection portion of the second position detection sensor S2. That is, when transferring from the detection state of Figure 14 (S1 disconnected / S2 connected) to the detection state of Figure 15 (S1 / S2 disconnected), it is possible to detect the movement of the second roller 41 to the position facing the driving roller 10.

[0095] Figure 16 is a view showing a state in which the switching cam 50 has been rotated counterclockwise by a predetermined angle from the state of Figure 15 . If 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 bracket 47 in the counterclockwise direction is restricted by the restricting rib 9c (refer to Figure 5 ). As a result, the second engaging portion 45a of the second bearing member 45 moves toward the bottom 64a of the concave portion 64, and the second bearing member 45 moves in a direction separating from the shaft 51 by the action of the second coil spring 49 (refer to Figure 5 ).

[0096] Thereby, the second roller 41 is pressed against the driving roller 10 via the intermediate transfer belt 8 to form a secondary transfer nip portion N, and the second roller 41 rotates following the driving roller 10. A predetermined secondary transfer current flows through the second roller 41 from a transfer voltage power source 74 (refer to Figure 11 ). Specifically, when the second roller 41 is disposed at the position of Figure 16 , a transfer voltage of the same polarity as the toner (here, positive polarity) is applied to the driving roller 10 electrically connected to the transfer voltage power source 74, and a secondary transfer current flows through the second roller 41 via the intermediate transfer belt 8.

[0097] In addition, the first light shielding plate 51a of the shaft 51 shields (connects) the detection portion of the first position detection sensor S1, and the second light shielding plate 47d of the roller bracket 47 retracts (disconnects) from the detection portion of the second position detection sensor S2. This state (S1 connected / S2 disconnected) is set as the reference position (starting position) of the second roller 41. That is, when from Figure 15The detection status (S1 / S2 disconnected) transfers to Figure 16 When the detection status is (S1 connected / S2 disconnected), the movement of the second roller 41 toward the reference position can be detected. Based on the rotation time of the switching cam 50 rotating from this reference position, the rotation angle of the switching cam 50 is restricted, and the configuration and separation state of the second roller 41 are controlled.

[0098] Figure 17 It represents from Figure 16 The state where the switching cam 50 is rotated counterclockwise by a predetermined angle from the state of (here, rotated 46.4° from the reference position of Figure 16 ). If 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 bracket 47 in the counterclockwise direction is restricted by the restricting rib 9c (refer to Figure 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 acting force of the second coil spring 49 (refer to Figure 5 ). Thereby, the second roller 41 becomes a state of being separated from the intermediate transfer belt 8 (separation state). Figure 17 The detection status of the first position detection sensor S1 and the second position detection sensor S2 in

[0099] 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 state of Figure 17 . Thereby, the switching cam 50 and the roller bracket 47 also rotate clockwise by a predetermined angle. When the roller bracket 47 rotates until it abuts against the restricting rib 9b, it becomes a state where the first roller 40 faces the driving roller 10 in Figure 18 . If the switching cam 50 is further rotated clockwise by a predetermined angle from the state of Figure 18 , it becomes a state where the first roller 40 is arranged at the reference position in Figure 12 . Hereinafter, by repeating the above steps, the switching between the first roller 40 and the second roller 41 is performed.

[0100] According to the configuration of the present embodiment, with a simple configuration using the roller bracket 47 and the switching cam 50, either the first roller 40 or the second roller 41 can be arranged to face the driving roller 10, and the first roller 40 or the second roller 41 arranged to face the driving roller 10 can be selectively arranged at the reference position forming the secondary transfer nip portion N and the separation position separated from the intermediate transfer belt 8.

[0101] For example, when the paper S is smaller than a specified size (here, A3 size), the first roller 40 having an elastic layer 40b with a small axial length is arranged at the reference position. Thus, 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) for calibration during image formation, the reference image formed on the intermediate transfer belt 8 does not contact the first roller 40. Therefore, calibration can be performed during image formation, and the image quality can be improved without reducing the image processing efficiency (productivity).

[0102] In addition, soiling of the back surface of the paper S caused by toner adhering to the first roller 40 and then to the paper S can be effectively suppressed. Further, since there is no need to perform a cleaning operation to return the toner adhering to the first roller 40 to the intermediate transfer belt 8, the printing waiting time can also be shortened.

[0103] On the other hand, when the paper S is larger than the specified size (here, 13-inch size), the second roller 41 having an elastic layer 41b with a large axial length is arranged at the reference position. Thus, reliable secondary transfer of the toner image to both end portions in the width direction of the large-size paper S can be performed.

[0104] In addition, the first roller 40 and the second roller 41 are always grounded through the first grounding members 56a, 56b and the second grounding member 57. Thus, after secondary transfer, no transfer electric field remains on the first roller 40 and the second roller 41. Therefore, an appropriate transfer electric field can always be applied to the secondary transfer nip portion N, and a good transfer image can be stably obtained.

[0105] In addition, the first bearing member 43 and the second bearing member 45 are held with a specified margin (extra clearance) with respect to the first bearing holding portion 47a and the second bearing holding portion 47b of the roller bracket 47, respectively. Thus, the installation of the first bearing member 43 and the second bearing member 45 to the roller bracket 47 becomes easy, and the assembly workability of the secondary transfer unit 9 can be improved.

[0106] In addition, when the first engaging portion 43a and the second engaging portion 45b are engaged with the recess 64, the first roller 40 and the second roller 41 are positioned in the extending direction of the parallel pin 51b. Therefore, even if the first bearing member 43 and the second bearing member 45 are not positioned in the first bearing holding portion 47a and the second bearing holding portion 47b, the first roller 40 and the second roller 41 can be positioned with high precision with respect to the driving roller 10, and the secondary transfer nip portion N can be stably formed.

[0107] In addition, in the present embodiment, it is possible to drive the roller support 47 and the switching cam 50 using a single roller switching motor 55. As a result, compared with the case of driving the roller support 47 and the switching cam 50 using different motors respectively, the drive mechanism and drive control can be simplified, which can contribute to cost reduction and compactification of the image forming apparatus 100.

[0108] Furthermore, the present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the gist of the present invention. For example, the shapes, dimensions, etc. of the first roller 40, the second roller 41, the roller support 47, the switching cam 50, etc. constituting the secondary transfer unit 9 are one example, and can be arbitrarily changed within the range not impairing the effects of the present invention.

[0109] In addition, in the above-described embodiment, there are provided two secondary transfer rollers constituted by the first roller 40 and the second roller 41 having different axial lengths of the elastic layers 40b and 41b, and either the first roller 40 or the second roller 41 is arranged at the reference position according to the size information of the paper S. However, there may also be provided two secondary transfer rollers constituted by the first roller 40 and the second roller 41 having different volume resistivities or hardnesses of the elastic layers 40b and 41b, and either the first roller 40 or the second roller 41 is arranged at the reference position according to the information related to the physical properties (resistance value, thickness, basis weight, surface smoothness, etc.) of the paper S.

[0110] In addition, in the above-described embodiment, an image forming apparatus 100 of an intermediate transfer system having a secondary transfer unit 9 for secondarily transferring the toner image once transferred onto the intermediate transfer belt 8 onto the paper S is illustrated. However, the present invention can be similarly applied to the transfer unit of an image forming apparatus of a direct transfer system that directly transfers the toner image formed on the photosensitive drum onto the paper.

[0111] The present invention can be used for an image forming apparatus having a transfer unit that transfers the toner image formed on the image carrier onto the recording medium. By using the present invention, it is possible to provide a transfer unit that can stably form a high-quality image by grounding two transfer rollers that selectively press against the image carrier, and an image forming apparatus having the transfer unit.

Claims

1. A transfer unit, characterized in that: The transfer unit includes a transfer roller having a core shaft and an elastic layer laminated on the outer peripheral surface of the core shaft. The elastic layer is pressed against an image carrier to form a transfer roller nip portion, and the transfer unit transfers a toner image formed on the image carrier to a recording medium passing through the transfer roller nip portion. The transfer unit includes: A first roller and a second roller as the transfer roller, wherein any one of the axial length, volume resistivity, and hardness of the elastic layers of the first roller and the second roller is different; A first bearing member that supports the core shaft of the first roller to be rotatable; A second bearing member that supports the core shaft of the second roller to be rotatable; A roller bracket having a first bearing holding portion and a second bearing holding portion, and the first bearing holding portion and the second bearing holding portion respectively hold the first bearing member and the second bearing member so as to be slidable in a direction approaching or separating from the image carrier; and A switching mechanism that rotationally drives the roller bracket to dispose either the first roller or the second roller at a reference position where it is pressed against the image carrier to form the transfer roller nip portion. The first bearing member and the second bearing member respectively have a grounding member that grounds the first roller and the second roller.

2. The transfer unit according to claim 1, characterized in that: The switching mechanism includes: A first helical spring disposed between the first bearing holding portion and the first bearing member, and applying a force to the first bearing member in a direction approaching the image carrier; A second helical spring disposed between the second bearing holding portion and the second bearing member, and applying a force to the second bearing member in a direction approaching the image carrier; A switching cam having a guide hole for engaging a first engaging portion formed on the first bearing member and a second engaging portion formed on the second bearing member; A metal shaft fixed at the rotation center of the switching cam and supporting the roller bracket to be rotatable; and A roller switching motor that rotates the shaft. The shaft is grounded via the main body frame of the image forming apparatus on which the transfer unit is mounted. The grounding member includes: A pair of first grounding members that electrically connect the core shaft to the first helical spring or the second helical spring; and A second grounding member that electrically connects the first helical spring and the second helical spring to the shaft.

3. The transfer unit according to claim 2, characterized in that: By rotating the roller bracket, 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 disposed at the reference position and a separation position separated from the image carrier.

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

5. The transfer unit according to claim 4, wherein: The first bearing member and the second bearing member are respectively held in the first bearing holding portion and the second bearing holding portion with a predetermined margin in the rotation direction of the roller bracket.

6. The transfer unit according to claim 5, wherein: Parallel pins extending in the radial direction are fixed to the shaft, and pin insertion portions into which the parallel pins are inserted are formed in the switching cam.

7. The transfer unit according to claim 6, wherein: The parallel pins are inserted into the pin insertion portions without clearance in the circumferential direction of the shaft.

8. An image forming apparatus, characterized in that, Comprising: A plurality of image forming units that form the toner images of different colors; A ring-shaped intermediate transfer belt serving as the image carrier that moves along the image forming units; 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 therebetween, and that primarily transfer the toner images formed on the photosensitive drums to the intermediate transfer belt; and A secondary transfer unit that is the transfer unit according to any one of claims 1 to 7, and that secondarily transfers the toner images primarily transferred to the intermediate transfer belt to the recording medium.

Images