Transfer unit and image forming apparatus including the same

Abstract

To provide a transfer unit that can bring a plurality of transfer rollers into contact with a transfer belt and separate the transfer rollers from the transfer belt in the vertical direction, and can offset the transfer rollers in the horizontal direction, and an image forming apparatus including the same.SOLUTION: A transfer unit comprises: an endless transfer belt; a plurality of transfer rollers that is arranged opposite to image carriers arranged in a plurality of image forming units with the transfer belt sandwiched therebetween; a plurality of pairs of support members that rotatably supports both ends of rotation shafts of the transfer rollers, and is capable of reciprocating in directions to contact and separate from the transfer belt; a pair of sliders that is supported parallel to the direction of travel of the transfer belt in a reciprocatable manner, and supports the support members in the contact and separation directions relative to the transfer belt in a reciprocatable manner; and a drive transmission mechanism that transmits driving fore to the sliders. The amount of movement of the sliders can adjust the amount of offset of the plurality of transfer rollers relative to the plurality of image carriers.SELECTED DRAWING: Figure 14

Description

Technical Field

[0001] The present invention relates to a transfer unit having an endless transfer belt and a plurality of transfer rollers for sequentially laminating and transferring toner images of respective colors onto the transfer belt or onto a recording medium held and conveyed by the transfer belt, and an image forming apparatus including the same.

Background Art

[0002] Conventionally, various image forming apparatuses have been proposed, among which there is a tandem type color image forming apparatus that forms a full-color image by sequentially overlapping toner images of respective colors by a plurality of image forming units. Further, in the tandem type color image forming apparatus, there are a direct transfer method of transferring a toner image formed by each image forming unit onto a recording medium conveyed by an endless transfer belt, and an intermediate transfer method of sequentially overlapping (primarily transferring) toner images onto an endless intermediate transfer belt by a plurality of image forming units and then transferring (secondarily transferring) them onto the recording medium at once.

[0003] In such a tandem type color image forming apparatus, in a monochrome mode for outputting a monochrome image, image formation is performed using only black toner. If the image carriers of the image forming units for yellow, cyan, and magenta other than black are kept in contact with the transfer belt in this monochrome mode, problems such as soiling of the transfer belt and the recording medium due to contact with the image carriers of yellow, cyan, and magenta, and an unnecessarily large driving torque of the transfer belt occur. Therefore, an image forming apparatus is known that can switch between a contact mode in which a part or all of the transfer rollers are brought into contact with the yellow, cyan, magenta, and black image carriers with an intermediate transfer belt interposed therebetween, and a standby mode in which all the transfer rollers are separated.

[0004] For example, Patent Document 1 discloses an image forming apparatus including a contact and separation mechanism that moves an intermediate transfer belt in a direction away from a photoreceptor drum during non-image formation, a secondary transfer roller that transfers a toner image formed on the surface of the intermediate transfer belt onto a sheet, and a pressing portion that presses the secondary transfer roller in the direction of the intermediate transfer belt.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] By the way, there are cases where it is desired to offset the horizontal position of the primary transfer roller with respect to the image carrier. For example, when it is desired to adjust the pressing force of the primary transfer roller against the image carrier or when it is desired to adjust the current flowing through the belt. Also, in the same unit, there is a case where the image carrier facing the primary transfer roller changes from an a-Si photoreceptor drum to an organic photoreceptor (OPC) drum.

[0007] In the configuration of Patent Document 1, the contact and separation mechanism slides in the running direction of the intermediate transfer belt, so that at least one of the primary transfer rollers presses the intermediate transfer belt to bring the intermediate transfer belt into contact with each image carrier, and the primary transfer roller is vertically moved between a contact state where it is separated from the intermediate transfer belt and a separation state where it is separated from the intermediate transfer belt. However, in the configuration of Patent Document 1, it was not possible to separate the primary transfer roller in the vertical direction and offset it in the horizontal direction.

[0008] In view of the above problems, an object of the present invention is to provide a transfer unit in which a plurality of transfer rollers can be brought into contact with or separated from a transfer belt in the vertical direction and can also be offset in the horizontal direction, and an image forming apparatus including the same.

Means for Solving the Problems

[0009] To achieve the above object, a first configuration of the present invention is a transfer unit including a transfer belt, a plurality of transfer rollers, a plurality of pairs of support members, a pair of sliders, and a drive transmission mechanism. The transfer belt is endless and moves along a plurality of image forming units. The plurality of transfer rollers are disposed opposite to the image carriers disposed at each image forming unit with the transfer belt interposed therebetween, and transfer the toner image formed on the image carrier onto the transfer belt or onto a recording medium held on the transfer belt. The plurality of pairs of support members rotatably support both end portions of the rotation axis of the transfer roller, and are reciprocally movable in a direction of approaching and separating from the transfer belt. The pair of sliders are supported so as to be reciprocally movable parallel to the traveling direction of the transfer belt, and support the support members so as to be reciprocally movable in a direction of approaching and separating from the transfer belt. The drive transmission mechanism transmits a driving force to the slider. The offset amount of the plurality of transfer rollers with respect to the plurality of image carriers can be adjusted according to the movement amount of the slider.

Advantages of the Invention

[0010] According to the first configuration of the present invention, the offset amount of the transfer roller with respect to the image carrier can be adjusted by using the slider that brings the transfer roller into contact with and separates it from the transfer belt. Therefore, there is no need to separately provide a mechanism for adjusting the offset amount of the transfer roller, the configuration of the transfer unit is simplified, and the number of components can be reduced.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13

Figure 14

DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic cross-sectional view of an image forming apparatus equipped with an intermediate transfer unit 30 of the present invention, and here, a tandem type color printer 100 is shown. Inside the color printer 100 main body, four image forming units Pa, Pb, Pc, and Pd are arranged in order from the upstream side in the conveyance direction (left side in FIG. 1). These image forming units Pa to Pd are provided corresponding to images of four different colors (yellow, cyan, magenta, and black), and yellow, cyan, magenta, and black images are sequentially formed by the respective steps of charging, exposure, development, and transfer.

[0013] In these image forming units Pa to Pd, photosensitive drums 1a, 1b, 1c, and 1d for carrying visible images (toner images) of respective colors are arranged, and further, an intermediate transfer belt 8 that rotates counterclockwise in FIG. 1 by a driving device (not shown) is provided adjacent to each of the image forming units Pa to Pd.

[0014] When image data is input from a host device such as a personal computer, first, the surfaces of the photosensitive drums 1a to 1d are uniformly charged by chargers 2a to 2d. Next, light irradiation is performed according to the image data by an exposure device 5 to form an electrostatic latent image corresponding to the image data on each of the photosensitive drums 1a to 1d. The developing devices 3a to 3d are filled with a predetermined amount of a two-component developer (hereinafter, also simply referred to as a developer) containing toners of respective colors of cyan, magenta, yellow, and black by toner containers (not shown), and the toners in the developer are supplied onto the photosensitive drums 1a to 1d by the developing devices 3a to 3d and electrostatically adhere. Thereby, a toner image corresponding to the electrostatic latent image formed by the exposure from the exposure device 5 is formed.

[0015] Then, a transfer electric field is applied at a predetermined transfer voltage between the primary transfer rollers 6a to 6d and the photoreceptor drums 1a to 1d by the primary transfer rollers 6a to 6d, and the cyan, magenta, yellow, and black toner images on the photoreceptor drums 1a to 1d are primarily transferred onto the intermediate transfer belt 8. Toner and the like remaining on the surfaces of the photoreceptor drums 1a to 1d after the primary transfer are removed by the cleaning devices 7a to 7d.

[0016] The transfer paper P onto which the toner image is to be transferred is housed in the paper cassette 16 at the lower part of the color printer 100, and the transfer paper P is conveyed to the nip portion (secondary transfer nip portion) between the secondary transfer roller 9 provided adjacent to the intermediate transfer belt 8 and the intermediate transfer belt 8 at a predetermined timing via the paper feed roller 12a and the registration roller pair 12b. The transfer paper P onto which the toner image has been secondarily transferred is conveyed to the fixing unit 13. Toner and the like remaining on the surface of the intermediate transfer belt 8 after the secondary transfer are removed by the belt cleaning device 19.

[0017] The transfer paper P conveyed to the fixing unit 13 is heated and pressurized so that the toner image is fixed on the surface of the transfer paper P, and a predetermined full-color image is formed. The transfer paper P on which the full-color image has been formed is discharged to the discharge tray 17 by the discharge roller pair 15 (either as it is or after being diverted to the reverse conveyance path 18 by the branch portion 14 and having images formed on both sides).

[0018] FIG. 2 is an external perspective view of the intermediate transfer unit 30 according to an embodiment of the present invention mounted on the color printer 100 shown in FIG. 1 as viewed from below, and FIG. 3 is a perspective view showing the internal structure of the intermediate transfer unit 30 of this embodiment. Parts common to FIG. 1 are denoted by the same reference numerals and the description thereof is omitted.

[0019] The intermediate transfer unit 30 includes a unit main body 35 composed of two side frames 31 and 32 and an upper surface frame (not shown), primary transfer rollers 6a to 6d, a tension roller 10, a drive roller 11, a backup roller 33, a guide roller 34 supported between the side frames 31 and 32, and an endless intermediate transfer belt 8 stretched around these rollers.

[0020] Sliders 37a and 37b are supported on the side frames 31 and 32 so as to be slidable in the horizontal direction. Both ends of the rotating shafts of the primary transfer rollers 6a to 6d and the backup roller 33 are rotatably supported by bearing holders 38a to 38e, and the bearing holders 38a to 38e are supported by the sliders 37a and 37b so as to be movable in the vertical direction. Further, the tension roller 10, the drive roller 11, and the guide roller 34 are rotatably supported by the side frames 31 and 32.

[0021] A shaft 46 is disposed inside the intermediate transfer unit 30, and pinion gears 47a and 47b are fixed to both ends of the shaft 46. The shaft 46 penetrates the side frame 32 (see FIG. 2) and protrudes outside the intermediate transfer unit 30, and a drive input gear 70 is fixed near the tip end. A rotational driving force is transmitted from a drive motor (not shown) provided on the main body side of the color printer 100 to the drive input gear 70 via a gear train (not shown).

[0022] The pinion gears 47a and 47b are disposed at positions meshing with racks 48 formed on the lower surfaces of the sliders 37a and 37b, respectively. By rotating the pinion gears 47a and 47b forward and backward, the sliders 37a and 37b can be reciprocally moved in the horizontal direction. The shaft 46 and the pinion gears 47a and 47b constitute a slider drive mechanism for reciprocally moving the sliders 37a and 37b.

[0023] FIG. 4 is a side view of the slider 37a used in the intermediate transfer unit 30 of the present embodiment, as viewed from the inside of the intermediate transfer unit 30. Note that the slider 37b has the same configuration as the slider 37a except that it is symmetric left and right, so the description thereof is omitted. Four step ribs 50 to 53 including a first step rib 50, a second step rib 51, a third step rib 52, and a fourth step rib 53 are formed on the inner surface of the slider 37a. Each of the step ribs 50 to 53 is composed of one lower step portion 50a to 53a, one upper step portion 50b to 53b, and one inclined portion 50c to 53c that connects the corresponding lower step portion 50a to 53a and upper step portion 50b to 53b.

[0024] FIGS. 5 and 6 are partial enlarged views of the bearing holder 38a of the primary transfer roller 6a and the bearing holder 38e of the backup roller 33, as viewed from the inside of the intermediate transfer unit 30. Note that FIG. 5 shows a state in which the primary transfer roller 6a and the backup roller 33 are in contact with the intermediate transfer belt 8, and FIG. 6 shows a state in which the primary transfer roller 6a and the backup roller 33 are separated from the intermediate transfer belt 8. Also, the bearing holders 38b to 38d have the same configuration as the bearing holder 38a, so the description thereof is omitted.

[0025] The bearing holder 38a is composed of a holder body 55, a bearing portion 57 that is supported so as to be slidable up and down with respect to the holder body 55, and a coil spring 60 disposed between the holder body 55 and the bearing portion 57. The bearing portion 57 is biased in a direction (downward) away from the holder body 55 by the biasing force of the coil spring 60. The bearing holder 38e is configured such that the bearing portion 57 is fixed to the lower end position with respect to the holder body 55, and the coil spring 60 is not provided.

[0026] Further, at the upper end of the holder body 55, a clamping portion 55a is formed by arranging a pair of V-shaped ribs such that their respective vertices face each other. By arranging the bearing holders 38a and 38e so that this clamping portion 55a clamps the first-step ribs 50 of the sliders 37a and 37b from above and below, the bearing holders 38a and 38e are slidably supported by the first-step ribs 50 of the sliders 37a and 37b.

[0027] Similarly, the bearing holder 38b of the primary transfer roller 6b is slidably supported by the second-step rib 51, the bearing holder 38c of the primary transfer roller 6c is slidably supported by the third-step rib 52, and the bearing holder 38d of the primary transfer roller 6d is slidably supported by the fourth-step rib 53.

[0028] FIG. 7 is an enlarged perspective view around the drive input gear 70 in the intermediate transfer unit 30 of the present embodiment. FIG. 8 is a perspective view of the drive input gear 70 and the gear position detection sensor 80 as viewed from the side frame 32 side (the back side of the paper surface in FIG. 7), and FIG. 9 is an enlarged perspective view of the vicinity of the gear position detection sensor 80 in FIG. 8.

[0029] By rotating the drive motor provided on the main body side of the color printer 100 in the forward and reverse directions by a predetermined amount, the drive input gear 70 also rotates in the forward and reverse directions by a predetermined amount. As the drive input gear 70 rotates, the pinion gears 47a and 47b (see FIG. 3) fixed to the shaft 46 also rotate in the forward and reverse directions by a predetermined amount. Therefore, the sliders 37a and 37b (see FIG. 3) having the racks 48 meshing with the pinion gears 47a and 47b also move in the left-right direction in FIG. 3. As a result, as described above, the arrangement of the primary transfer rollers 6a to 6d is switched to the standby mode, the monochrome mode (the first mode), or the color mode (the second mode).

[0030] As shown in FIG. 8, a light shielding plate 71 is integrally formed on the drive input gear 70. The light shielding plate 71 projects annularly in the direction of the side frame 32 from a position closer to the radially inner side than the outer peripheral edge of the drive input gear 70. The light shielding plate 71 has a pulse portion 73 in which a plurality of slits 73a are formed at equal intervals, and light shielding portions 75 and light transmitting portions 77 disposed adjacent to both sides of the pulse portion 73.

[0031] Further, in the vicinity of the drive input gear 70, a gear position detection sensor 80 for detecting the home position and gear position (rotation angle) of the drive input gear 70 is disposed. The gear position detection sensor 80 is a PI (photo interrupter) sensor, and is disposed so as to sandwich the light shielding plate 71 from the front and back directions by a U-shaped detection portion 81 provided with a light emitting portion 81a and a light receiving portion 81b.

[0032] With the rotation of the drive input gear 70, the light shielding plate 71 rotates in the same direction as the drive input gear 70. Then, when the slit 73a of the pulse portion 73 passes through the detection portion 81, the light reception signal level of the detection portion 81 is switched between LOW (OFF state) and HIGH (ON state) at a constant timing. By detecting the timing at which this light reception signal level switches, the rotation angle of the light shielding plate 71 can be detected, and based on this, the position (rotation angle) of the drive input gear 70 can be detected.

[0033] Next, the operations of the primary transfer rollers 6a to 6d and the backup roller 33 in the intermediate transfer unit 30 of the present embodiment will be described. FIG. 10 is a side view showing the arrangement of the slider 37a, the bearing holders 38a to 38e, the primary transfer rollers 6a to 6d, and the backup roller 33 in the color mode. Here, the tension roller 10, the drive roller 11, the guide roller 34, and the slider 37b are not described, but since the arrangement of each roller and the slider 37b in the color mode is shown in FIG. 3, reference will be made to FIG. 3 as necessary for the description.

[0034] In the color mode for outputting a color image, since image formation is performed by the four image forming units Pa to Pd, the four primary transfer rollers 6a to 6d are pressed against the photosensitive drums 1a to 1d via the intermediate transfer belt 8, respectively. Further, the backup roller 33 needs to be disposed at a position where it presses the intermediate transfer belt 8 in the direction of the photosensitive drums 1a to 1d.

[0035] Therefore, by rotating the pinion gears 47a and 47b by a predetermined angle in a predetermined direction (clockwise direction in FIG. 3) to slide the sliders 37a and 37b in the right direction, the bearing holders 38a to 38e are disposed at the lower portions 50a to 53a of the respective stepped ribs 50 to 53 as shown in FIG. 10.

[0036] Specifically, as shown in FIG. 9, when the passage of the edge 75a of the light-shielding portion 75 formed on the light-shielding plate 71 is detected by the detection portion 81 of the gear position detection sensor 80 and the light reception signal level of the detection portion 81 is switched from HIGH (ON state) to LOW (OFF state), the light reception signal level of the detection portion 81 is maintained at LOW. Therefore, when the light reception signal level of the detection portion 81 continues to be LOW for a predetermined time, the drive motor is stopped, whereby the bearing holders 38a to 38e are disposed at the lower portions 50a to 53a of the first stepped rib 50 to the fourth stepped rib 53. Note that the rotational position (rotation angle) of the drive input gear 70 at this time is taken as a reference position.

[0037] At this time, the primary transfer rollers 6a to 6d and the backup roller 33 are in contact with the intermediate transfer belt 8. However, as shown in FIG. 5, since the compression length of the coil springs 60 in the bearing holders 38a to 38d changes due to the resistance force from the intermediate transfer belt 8, a gap is formed between the holder body 55 and the bearing portion 57, and the primary transfer rollers 6a to 6d are pressed against the photosensitive drums 1a to 1d via the intermediate transfer belt 8 with a predetermined pressure. Further, the backup roller 33 presses the intermediate transfer belt 8 downward at a predetermined position. Thereby, the bearing holders 38a to 38e move downward, and a color mode is established in which the four primary transfer rollers 6a to 6d and the backup roller 33 are pressed against the intermediate transfer belt 8.

[0038] FIG. 11 is a side view showing the arrangement of the sliders 37a, the bearing holders 38a to 38e, the primary transfer rollers 6a to 6d, and the backup roller 33 in the monochrome mode. In the monochrome mode for outputting a monochrome image, since image formation is performed using only the black image forming unit Pd, it is necessary to press only the primary transfer roller 6d against the photosensitive drum 1d via the intermediate transfer belt 8. Therefore, the pinion gears 47a and 47b are rotated in the reverse direction (counterclockwise direction in FIG. 3) by a predetermined angle, and the sliders 37a and 37b are slid leftward by a predetermined amount from the state shown in FIG. 10.

[0039] When the pinion gears 47a and 47b are rotated in the reverse direction with the drive input gear 70 in the reference position, the drive input gear 70 fixed to the shaft 46 rotates in the clockwise direction in FIG. 8. Then, the number of times the light reception signal level is switched from LOW to HIGH by the passage of the slit 73a of the pulse portion 73 is counted. For example, the drive of the drive motor is continued until the light reception signal level of the detection unit 81 is switched from LOW to HIGH five times and the passage of the edge 73b is detected.

[0040] As a result, the pinion gears 47a and 47b are rotated in the reverse direction by a predetermined angle, and the bearing holders 38a to 38e move from the lower portions 50a to 52a of the first to third step ribs 50 to 52 through the inclined portions 50c to 52c to the upper portions 50b to 52b. On the other hand, the bearing holder 38d remains at the lower portion 53a of the fourth step rib 53. Thereby, the bearing holders 38a to 38c move upward, and the mode is switched to the monochrome mode in which only the primary transfer roller 6d is pressed against the intermediate transfer belt 8.

[0041] In the monochrome mode, since the primary transfer rollers 6a to 6c and the backup roller 33 are separated from the intermediate transfer belt 8, it is possible to prevent the intermediate transfer belt 8 from being soiled by the residual toner on the surfaces of the photosensitive drums 1a to 1c. Also, there is no possibility that the drive torque of the drive roller 11 that rotates the intermediate transfer belt 8 increases unnecessarily.

[0042] Figure 12 is a side view showing the arrangement of the slider 37a, the bearing holders 38a to 38e, the primary transfer rollers 6a to 6d, and the backup roller 33 in the standby mode. In the standby mode where no image is output, the pinion gears 47a and 47b are further rotated in the reverse direction to slide the sliders 37a and 37b leftward by a predetermined amount from the state shown in FIG. 11.

[0043] By further rotating the drive input gear 70 from the state shown in FIG. 11, the light-transmitting portion 77 adjacent to the pulse portion 73 passes through the detection portion 81. At this time, the light reception signal level of the detection portion 81 is maintained at HIGH (ON state). By stopping the drive of the drive motor at this position, the bearing holders 38d and 38e move to the upper portion 53a of the third-step rib 53. That is, by stopping the drive motor when the light reception signal level of the detection portion 81 continues to be HIGH for a predetermined time, as shown in FIG. 12, all the bearing holders 38a to 38e move to the upper portions 50b to 53b, and the four primary transfer rollers 6a to 6d and the backup roller 33 are switched to the standby mode in which they are separated from the intermediate transfer belt 8.

[0044] In the standby mode, since the primary transfer rollers 6a to 6d and the backup roller 33 are separated from the intermediate transfer belt 8, the tension applied to the intermediate transfer belt 8 is relaxed, and deformation and elongation of the intermediate transfer belt 8 can be prevented.

[0045] When shifting from the standby mode to the color mode, the sliders 37a and 37b and the bearing holders 38a to 38e operate in the reverse manner to the above. Also, the procedure for detecting the rotation angle of the drive input gear 70 by the gear position detection sensor 80 is also reversed to the above.

[0046] According to the above configuration, by detecting the light shielding plate 71 integrally formed on the drive input gear 70 with the gear position detection sensor 80, the rotation amounts (rotation angles) of the pinion gears 47a and 47b fixed coaxially (shaft 46) with the drive input gear 70 can be detected. Thereby, the movement amounts and movement directions of the sliders 37a and 37b can also be accurately controlled.

[0047] Further, a pulse portion 73, a light shielding portion 75, and a light transmitting portion 77 are formed on the light shielding plate 71, and by setting the rotation position (rotation angle) of the drive input gear 70 when the edge 75a of the light shielding portion 75 passes through the detection portion 81 of the gear position detection sensor 80 as the reference position, the drive input gear 70 can be accurately stopped at the reference position. That is, both the reference position and the rotation angle of the drive input gear 70 can be detected by one gear position detection sensor 80. Therefore, it is not necessary to provide a plurality of expensive PI sensors, and an inexpensive DC brush motor can be used as the drive motor, which is also advantageous in terms of cost.

[0048] In the above embodiment, the rotation position (rotation angle) of the drive input gear 70 when the edge 75a of the light shielding portion 75 passes through the detection portion 81 is set as the reference position. However, the rotation position (rotation angle) of the drive input gear 70 when the edge 77a of the light transmitting portion 77 (see FIG. 8) passes through the detection portion 81 may be set as the reference position. Also, in the above embodiment, the color mode is set when the drive input gear 70 is at the reference position, but the standby mode may be set when the drive input gear 70 is at the reference position.

[0049] By the way, in the intermediate transfer type color printer 100 as shown in FIG. 1, when using the intermediate transfer belt 8 made of resin, since the intermediate transfer belt 8 is harder than the elastic belt, it is difficult to bring the concave portion of the paper into close contact with the intermediate transfer belt 8 in the secondary transfer. In the concave portion of the paper, the gap between the paper and the intermediate transfer belt 8 becomes large and the transfer electric field becomes weak, so the secondary transfer performance deteriorates. In order to improve the transferability of the toner even with a weak transfer electric field, it is necessary to weaken the non-electrostatic adhesion force between the intermediate transfer belt 8 and the toner and improve the releasability of the toner from the intermediate transfer belt 8.

[0050] As a means for weakening the non-electrostatic adhesion force between the intermediate transfer belt 8 and the toner, there is known a method of offsetting the positions of the primary transfer rollers 6a to 6d from directly above the photosensitive drums 1a to 1d to the downstream side in the advancing direction of the intermediate transfer belt 8, thereby weakening the concentration of the pressing force and weakening the non-electrostatic adhesion force of the toner.

[0051] In the present embodiment, by moving the sliders 37a and 37b, the positions of the primary transfer rollers 6a to 6d are offset from directly above the photosensitive drums 1a to 1d to the downstream side in the advancing direction of the intermediate transfer belt 8.

[0052] FIG. 13 is a side view showing the arrangement of the slider 37b and the bearing holder 38d in a state where the primary transfer roller 6d is disposed directly above the photosensitive drum 1d. Note that FIGS. 13 and 14 described later show the configuration on the side frame 31 side (slider 37b side) of the intermediate transfer unit 30, and the advancing direction of the intermediate transfer belt 8 is the reverse direction to that in FIG. 10 (from right to left in FIG. 13). Further, the configuration on the side frame 32 side (slider 37a side) is the same as that in FIGS. 13 and 14 except that it is symmetric about the left and right.

[0053] As shown in FIG. 13, the bearing holder 38d that holds the primary transfer roller 6d is in contact with a regulating member 90 provided on the side frame 31 of the intermediate transfer unit 30. The regulating member 90 is held on the side frame 31 via a regulating spring 91. The regulating spring 91 biases the regulating member 90 in a direction away from the side frame 31 (right direction in FIG. 13).

[0054] The bearing holder 38d is sandwiched between the pressing portions 37c of the sliders 37a and 37b and the regulating member 90 and is positioned in the horizontal direction. In this state, a perpendicular line O passing through the rotation center of the primary transfer roller 6d passes through the rotation center of the photosensitive drum 1d (see FIG. 1). That is, the primary transfer roller 6d is positioned directly above the photosensitive drum 1d.

[0055] FIG. 14 is a side view showing the arrangement of the slider 37b and the bearing holder 38d in a state where the primary transfer roller 6d is offset to the downstream side of the photosensitive drum 1d. By moving the slider 37b from the state of FIG. 13 to the downstream side (left direction in FIG. 13) in the traveling direction of the intermediate transfer belt 8, the bearing holder 38d is pressed by the pressing portion 37c of the slider 37b and moves to the downstream side. Along with the movement of the bearing holder 38d, the regulating member 90 also moves to the downstream side while compressing the regulating spring 91.

[0056] As a result, the perpendicular line O' passing through the rotation center of the primary transfer roller 6d is shifted to the downstream side from the perpendicular line O passing through the rotation center of the photosensitive drum 1d. The bearing holder 38d is positioned by the pressing portion 37c and the regulating member 90 biased by the regulating spring 91. That is, the primary transfer roller 6d is positioned in a state of being offset to the downstream side in the traveling direction of the intermediate transfer belt 8 with respect to the photosensitive drum 1d. The distance between the perpendicular lines O and O' becomes the offset amount d.

[0057] Although the mechanism for offsetting the primary transfer roller 6d with respect to the photosensitive drum 1d has been described with reference to FIGS. 13 and 14, the mechanism for offsetting the primary transfer rollers 6a to 6c with respect to the photosensitive drums 1a to 1c is exactly the same.

[0058] According to the above configuration, the offset amount of the primary transfer rollers 6a to 6d with respect to the photosensitive drums 1a to 1d can be adjusted by using the sliders 37a and 37b for bringing the primary transfer rollers 6a to 6d into contact with and separating them from the intermediate transfer belt 8. Therefore, it is not necessary to separately provide a mechanism for adjusting the offset amount of the primary transfer rollers 6a to 6d, the configuration of the intermediate transfer unit 30 is simplified, and the number of components can be reduced.

[0059] If the offset amounts of the primary transfer rollers 6a to 6d become too large, there is a risk of image defects occurring. More specifically, when the photosensitive drums 1a to 1d are amorphous silicon photosensitive drums, white spots where toner is reversely charged due to discharge and transfer failure occurs are generated. In the case of OPC photosensitive drums, the photosensitive layers of the photosensitive drums 1a to 1d are reversely charged due to discharge, and black streaks as transfer memory are generated because the charging devices 2a to 2d cannot completely erase the reverse charging history. The above image defects are more likely to occur as the surface resistance of the intermediate transfer belt 8 increases. Furthermore, there is also a risk of a decrease in the secondary transferability with respect to transfer paper P (rough paper) having relatively large surface irregularities.

[0060] On the other hand, if the surface resistance of the intermediate transfer belt 8 becomes too low, when foreign particles (carriers, toner aggregates, etc.) having a diameter larger than that of the toner adhere to the intermediate transfer belt 8, the primary transfer electric field weakens around the voids formed by the foreign particles, and white defects (white streaks) occur in the image. If a primary transfer voltage (primary transfer current) is applied above a certain level, the toner flies over the voids, so white streaks do not occur.

[0061] Therefore, it is preferable to set the offset amounts of the primary transfer rollers 6a to 6d based on the surface resistance of the intermediate transfer belt 8, the primary transfer current, the type of the transfer paper P, etc.

[0062] In addition, the present invention is not limited to the above-described embodiments, and various modifications are possible without departing from the spirit of the present invention. For example, the arrangement order of the image forming units Pa to Pd corresponding to each of yellow, cyan, magenta, and black can be arbitrarily set.

[0063] In addition, as an image forming apparatus on which the intermediate transfer unit 30 of the present invention is mounted, here a color printer 100 has been described as an example. However, the present invention is applicable to various image forming apparatuses using a transfer unit including an endless transfer belt and a plurality of transfer rollers, such as a color copier, a facsimile machine, etc. For example, it is also applicable to a transfer unit mounted on a direct transfer type color image forming apparatus that holds and conveys a sheet on an endless transfer belt and directly transfers toner images of respective colors formed by an image forming unit onto the sheet.

Industrial Applicability

[0064] The present invention can be used for a transfer unit including an endless transfer belt that moves along each image forming unit and a plurality of transfer rollers that come into contact with or separate from the transfer belt. By using the present invention, it is possible to provide a transfer unit and an image forming apparatus in which the arrangement of a plurality of transfer rollers with respect to the transfer belt can be accurately switched with a simple configuration.

Explanation of Reference Numerals

[0065] Pa~Pd Image forming unit 1a~1d Photoconductor drum (image carrier) 6a~6d Primary transfer roller (transfer roller) 8 Intermediate transfer belt (transfer belt) 9 Secondary transfer roller 10 Tension roller 11 Driving roller 30 Intermediate transfer unit (transfer unit) 31, 32 Side frames 33 Backup roller 34 Guide roller 37a, 37b Slider (moving member) 37c Pressing part 38a~38e Bearing holder (supporting member) 46 Shaft 47a, 47b Pinion gear (drive transmission mechanism) 50 First step rib 51 Second step rib 52 Third-step rib 53 Fourth-step rib 50a~53a Lower part 50b~53b Upper part 50c~53c Inclined part 55 Holder body 55a Clamping part 57 Bearing part 90 Restricting member 91 Restricting spring 100 Color printer

Claims

1. An endless transfer belt that moves along a plurality of image forming units, A plurality of transfer rollers that are disposed opposite to image carriers disposed in each of the image forming units with the transfer belt interposed therebetween, and transfer a toner image formed on the image carrier onto the transfer belt or onto a recording medium held on the transfer belt, A plurality of pairs of support members that rotatably support both ends of the rotation axis of the transfer roller and are reciprocally movable in a direction of approaching and separating from the transfer belt, A pair of sliders that are supported so as to be reciprocally movable parallel to the traveling direction of the transfer belt, and support the support members so as to be reciprocally movable in a direction of approaching and separating from the transfer belt, A drive transmission mechanism that transmits a driving force to the moving member, In a transfer unit provided with the above, A transfer unit, characterized in that the offset amount of the plurality of transfer rollers with respect to the plurality of image carriers can be adjusted by the moving amount of the slider.

2. The pair of sliders are each supported so as to be horizontally movable on a pair of side frames of the transfer unit, and have a pressing portion that contacts the support member from the upstream side in the offset direction, Each of the pair of side frames has a regulating member disposed on the downstream side of the support member with respect to the offset direction, and a regulating spring that biases the regulating member toward the upstream side in the offset direction, The transfer unit according to claim 1, wherein the plurality of pairs of support members are each sandwiched between the pressing portion and the regulating member and positioned horizontally.

3. The pair of sliders can be switched between a standby mode in which all the transfer rollers are separated from the transfer belt, a first mode in which only the transfer roller disposed opposite to the image carrier disposed in a specific image forming unit is pressed against the transfer belt, and a second mode in which all the transfer rollers are pressed against the transfer belt, The transfer unit according to claim 1, characterized in that the offset amount of the plurality of transfer rollers with respect to the plurality of image carriers in the second mode can be adjusted by the moving amount of the slider.

4. On the side surfaces of the pair of sliders, a plurality of step ribs are formed, each having a lower stage portion, an upper stage portion parallel to the lower stage portion, and an inclined portion connecting the lower stage portion and the upper stage portion, A clamping portion for slidably clamping the step rib is formed on each of the plurality of pairs of the support members. The transfer unit according to claim 3, wherein the arrangement of the transfer roller is switched to any one of the first mode, the second mode, and the standby mode by reciprocating the pair of sliders to arrange the support member at the lower stage portion or the upper stage portion.

5. The transfer unit according to claim 1, wherein the transfer belt is an intermediate transfer belt on which a toner image formed on the image carrier is primarily transferred.

6. An image forming apparatus including the transfer unit according to any one of claims 1 to 5.

Images