Image forming apparatus

Abstract

To provide an image forming apparatus that can reduce a force necessary for causing a slide member to slide.SOLUTION: An image forming apparatus has a belt contact and separation mechanism, which is a contact and separation mechanism that brings an intermediate transfer belt, which is a transfer body, into contact with photoreceptors, which are image carriers, and separates the intermediate transfer belt from the photoreceptors by moving primary transfer rollers 11 (b-d), which are nip forming members, to retreat positions where they are retreated from nip forming positions where the intermediate transfer belt is brought into contact with the photoreceptors, with slide of a slide member 81 in a direction opposite to an urging direction of pressure springs 70 (b-d) that press holding members 52 (b-d) holding the primary transfer rollers 11 (b-d) to locate the primary transfer rollers at the nip forming positions, relative to the holding members 52 (b-d). One ends of the pressure springs 70 (b-d) are engaged with spring receiving parts 85 (b-d) of the slide member 81.SELECTED DRAWING: Figure 13

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus. [Background technology]

[0002] Conventionally, there has been known an image forming device that includes a nip forming member that abuts a transfer body, onto which an image is transferred from an image carrier, against the image carrier to form a transfer nip, a pressure spring that urges a holding member that holds the nip forming member to position the nip forming member at a nip forming position where the transfer body abuts against the image carrier, and a separation mechanism that moves the nip forming member from the nip forming position to a retracted position by sliding a sliding member in the direction opposite to the urging direction of the pressure spring against the holding member, thereby separating the transfer body from the image carrier.

[0003] Patent Document 1 describes an image forming apparatus in which a holding member that holds a primary transfer roller, which is a nip forming member, is rotatably supported on a frame member of the image forming apparatus, and a pressure spring is received at one end by the frame member and at the other end by the holding member. The slide member is provided with a protrusion that pushes the holding member in the opposite direction when the slide member is slid in the opposite direction. By pushing the holding member in the opposite direction with this protrusion, the holding member is rotated so that the primary transfer roller moves from the nip forming position to the retracted position. Summary of the Invention [Problem to be solved by the invention]

[0004] In the above Patent Document 1, when the slide member is moved and the holding member is rotated, the length of the pressure spring changes in the direction in which the biasing force increases, and a large force is required to slide the slide member, which is a problem. [Means for solving the problem]

[0005] In order to solve the above-mentioned problems, the present invention is an image forming apparatus comprising: a nip forming member that abuts a transfer body, onto which an image is transferred from an image carrier, against the image carrier to form a transfer nip; a pressure spring that presses a holding member that holds the nip forming member to position the nip forming member at a nip forming position where the transfer body abuts against the image carrier; and a separation mechanism that moves the nip forming member from the nip forming position to a retracted position by sliding a slide member in a direction opposite to the biasing direction of the pressure spring against the holding member, thereby separating the transfer body from the image carrier, wherein the end of the pressure spring opposite to the biasing side end that biases the holding member is engaged with the slide member. [Effects of the Invention]

[0006] According to the present invention, the force required to slide the slide member can be reduced. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 is a schematic configuration diagram showing a printer according to an embodiment. [Figure 2] Printer hardware configuration diagram. [Figure 3] 10 is a schematic diagram illustrating the configuration of the belt deviation correction mechanism of the intermediate transfer unit immediately after assembly, as viewed from the axial direction of the tension roller. [Figure 4] 10 is a schematic diagram of the belt deviation correction mechanism when belt deviation occurs, viewed from the axial direction of the tension roller 5. FIG. [Figure 5] Cross section AA in Figure 3. [Figure 6] AA cross section in FIG. 4. [Figure 7] Schematic diagram of a photosensitive member viewed from the left and right of the device. [Figure 8] FIG. 4 is a diagram for explaining the positioning of a photosensitive member on the front side of the apparatus. [Figure 9] FIG. 4 is a schematic diagram illustrating the positioning of the photosensitive member in the main body of the apparatus. [Figure 10]1A is a diagram illustrating contact and separation of the intermediate transfer belt in a full-color mode, and FIG. 1B is a diagram illustrating contact and separation of the intermediate transfer belt in a black monochromatic mode. [Figure 11] 6A and 6B are diagrams illustrating the positioning of the primary transfer roller on the front side of the printer by a positioning protrusion of a holding member arranged on the front side of the printer. [Figure 12] 6A and 6B are diagrams illustrating the positioning of the primary transfer roller on the rear side of the printer by a positioning protrusion of a holding member arranged on the rear side of the printer. [Figure 13] Schematic diagram of the belt contact / separation mechanism in full color mode [Figure 14] 5 is a schematic diagram of a belt contact / separation mechanism in a black monochrome mode. [Figure 15] 6A and 6B are diagrams illustrating fluctuations in the winding angle of the intermediate transfer belt around the backup roller due to tilt of the tension roller. [Figure 16] A modified example of the belt contact / separation mechanism is shown. DETAILED DESCRIPTION OF THE INVENTION

[0008] The best mode for carrying out the present invention will be described below with reference to the drawings. Note that a person skilled in the art can easily modify or alter the present invention within the scope of the claims to create other embodiments, and these modifications and alterations are included within the scope of the claims. The following description is an example of the best mode for carrying out the present invention and does not limit the scope of the claims.

[0009] Hereinafter, an embodiment of an electrophotographic color printer (hereinafter simply referred to as printer 100) will be described as an example of an image forming apparatus to which the present invention is applied. First, the basic configuration of the printer 100 will be described. 1 is a schematic diagram showing the configuration of a printer 100 according to an embodiment. The printer 100 is a tandem color printer, and includes first to fourth photosensitive members 1 (a to d) as image carriers arranged inside a main body housing 101. Above the four photosensitive members 1, an intermediate transfer unit 60 is provided that includes an intermediate transfer belt 3 as a transfer member, and the intermediate transfer unit 60 is detachable from the main body of the printer 100.

[0010] Toner images of different colors are formed on the four photoconductors 1(a-d). In this embodiment, a black toner image, a magenta toner image, a cyan toner image, and a yellow toner image are formed on the four photoconductors 1(a-d), respectively. Although the photoconductors 1(a-d) shown in FIG. 1 are formed in a drum shape, an endless belt-like photoconductor that is wound around multiple rollers and driven to rotate can also be used as the photoconductor.

[0011] In the intermediate transfer unit 60, an intermediate transfer belt 3 is disposed so as to face the first to fourth photoconductors 1 (a to d), and in the state shown in FIG. 1, the four photoconductors 1 (a to d) are in contact with the surface of the intermediate transfer belt 3. The intermediate transfer belt 3 shown in FIG. 1 is wound around a plurality of support rollers, such as a secondary transfer opposing roller 4, a tension roller 5, an entrance roller 7, and a backup roller 47. One of these support rollers, the secondary transfer opposing roller 4, is configured as a drive roller driven by a drive source, and the drive of this secondary transfer opposing roller 4 drives the intermediate transfer belt 3 to rotate in the direction of arrow "A" in FIG. 2.

[0012] The intermediate transfer belt 3 may have either a multi-layer structure or a single-layer structure, but if it has a multi-layer structure, it is preferable that the base layer is made of a material with little stretch, such as a fluororesin, PVDF sheet, or polyimide resin, and that the surface is covered with a smooth coating layer of fluororesin or the like.If it is a single layer, it is preferable to use materials such as PVDF, PC, or polyimide.

[0013] The formation of a toner image on the photoreceptor 1 and the transfer of each toner image to the intermediate transfer belt 3 are substantially the same for all four photoreceptors 1 (a to d), except for the color of the toner image formed. For this reason, only the formation of a black toner image on the black photoreceptor 1a, which is located on the most downstream side in the surface movement direction of the intermediate transfer belt 3, and the transfer of the black toner image to the intermediate transfer belt 3 will be described.

[0014] The black photoconductor 1a is rotated clockwise in FIG. 1, as indicated by arrow "C" in FIG. 1, and at this time, light from a static eliminator is irradiated onto the surface of the black photoconductor 1a, initializing the surface potential of the black photoconductor 1a. The initialized surface of the black photoconductor 1a is uniformly charged to a predetermined polarity (negative polarity in this embodiment) by the black charging device 8a. An optically modulated laser beam L emitted from the exposure device 9 is irradiated onto the thus-charged surface of the black photoconductor 1a, and an electrostatic latent image corresponding to the written information is formed on the surface of the black photoconductor 1a. The printer 100 shown in FIG. 1 uses the exposure device 9, which is a laser writing device that emits a laser beam, but it is also possible to use an exposure device having an LED array and imaging means.

[0015] The electrostatic latent image formed on the black photoconductor 1a is visualized as a black toner image when it passes through an area facing the black developing device 10a. Meanwhile, a black primary transfer roller 11a, which is a nip forming member and a primary transfer member, is disposed inside the intermediate transfer belt 3 at a position facing the black photoconductor 1a across the intermediate transfer belt 3. This black primary transfer roller 11a abuts against the back surface of the intermediate transfer belt 3, and an appropriate primary transfer nip is formed between the black photoconductor 1a and the intermediate transfer belt 3.

[0016] A transfer voltage of a polarity opposite to the toner charge polarity of the toner image formed on the black photoreceptor 1a (positive polarity in this embodiment) is applied to the black primary transfer roller 11a. This forms a transfer electric field between the black photoreceptor 1a and the intermediate transfer belt 3, and the black toner image on the black photoreceptor 1a is electrostatically transferred onto the intermediate transfer belt 3, which is rotated in synchronization with the black photoreceptor 1a. Any residual toner remaining on the surface of the black photoreceptor 1a after the black toner image has been transferred to the intermediate transfer belt 3 is removed by the black cleaning device 12a, and the surface of the black photoreceptor 1a is cleaned.

[0017] Similarly, a magenta toner image, a cyan toner image, and a yellow toner image are formed on the other three photoconductors 1 (b, c, d), respectively. The toner images of each color are electrostatically transferred onto the intermediate transfer belt 3 in the order of yellow toner image, cyan toner image, magenta toner image, and black toner image, superimposed one on top of the other.

[0018] As shown in Fig. 1, a paper feeder 14 is disposed below the main body casing 101 of the printer 100. In the paper feeder 14, recording paper P, which is a recording medium, is fed in the direction of arrow "B" in Fig. 2 by the rotation of a paper feed roller 15. The fed recording paper P hits a pair of registration rollers 16 and temporarily stops.

[0019] The portion of the intermediate transfer belt 3 that is wound around the secondary transfer opposing roller 4 comes into contact with a secondary transfer roller 17, which is a secondary transfer member disposed opposite the secondary transfer belt 3, forming a secondary transfer nip. The recording paper P that strikes the pair of registration rollers 16 is transported toward the secondary transfer nip at a predetermined timing. At this time, a predetermined transfer voltage is applied to the secondary transfer roller 17, which causes the toner images that have been transferred onto the intermediate transfer belt 3 in layers to be secondarily transferred onto the recording paper P.

[0020] The recording paper P onto which the toner image has been secondarily transferred is transported further upward within the main body casing 101 and passes through the fixing device 18. At this time, the toner image on the recording paper P is fixed by the action of heat and pressure in the fixing device 18. After passing through the fixing device 18, the recording paper P is discharged outside the printer 100 by a pair of paper discharge rollers 19 provided in the paper discharge section.

[0021] Residual toner remaining on the surface of the intermediate transfer belt 3 after the toner image has been transferred to the recording paper P is removed from the intermediate transfer belt 3 by a belt cleaning device 20. The belt cleaning device 20 in this embodiment uses a blade-shaped cleaning blade 21 made of urethane or the like, and the cleaning blade 21 is brought into contact in the counter direction with respect to the direction of movement of the surface of the intermediate transfer belt 3. Various types of belt cleaning devices can be used as appropriate, and for example, the belt cleaning device 20 may be an electrostatic type.

[0022] The residual toner removed from the intermediate transfer belt 3 by the cleaning blade 21 is sent to the rear longitudinal side by the waste toner coil inside the cleaning case, and is transported to a waste toner container through a waste toner path provided in the main body of the printer 100.

[0023] FIG. 2 is a diagram showing the hardware configuration of the printer 100. As shown in FIG. As shown in FIG. 2, the printer 100 includes a controller 910, a short-range communication circuit 920, an engine control unit 930, an operation panel 940, and a network I / F 950.

[0024] The controller 910 controls the entire printer 100, for example, controlling drawing, communication, input from the operation panel 940, and the like. The controller 910 has a CPU 901, which is the main part of the computer, a system memory (MEM-P) 902, a north bridge (NB) 903, a south bridge (SB) 904, an ASIC (Application Specific Integrated Circuit) 906, a local memory (MEM-C) 907, which is a storage unit, an HDD controller 908, and an HD 909, which is also a storage unit. The NB 903 and the ASIC 906 are connected by an AGP (Accelerated Graphics Port) bus 921.

[0025] The CPU 901 is a control unit that performs overall control of the printer 100. The NB 903 is a bridge that connects the CPU 901 with the MEM-P 902, the SB 904, and the AGP bus 921, and includes a memory controller that controls reading and writing to the MEM-P 902, a PCI (Peripheral Component Interconnect) master, and an AGP target.

[0026] The MEM-P 902 comprises a ROM 902a, which is memory for storing programs and data that realize the functions of the controller 910, and a RAM 902b, which is used for expanding the programs and data and as a drawing memory during memory printing. The programs stored in the RAM 902b may be provided by being recorded in an installable or executable file format on a computer-readable recording medium such as a CD-ROM, CD-R, or DVD.

[0027] The SB 904 is a bridge for connecting the NB 903 with PCI devices and peripheral devices. The ASIC 906 is an integrated circuit (IC) for image processing applications that has hardware elements for image processing and functions as a bridge connecting the AGP bus 921, PCI bus 922, HDD controller 908, and MEM-C 907. The ASIC 906 includes a PCI target and AGP master, an arbiter (ARB) that forms the core of the ASIC 906, a memory controller that controls the MEM-C 907, multiple direct memory access controllers (DMACs) that perform image data rotation using hardware logic, and a PCI unit that transfers data between the printer unit 932 and the ASIC 906 via the PCI bus 922. A Universal Serial Bus (USB) interface or an Institute of Electrical and Electronics Engineers 1394 (IEEE 1394) interface may also be connected to the ASIC 906.

[0028] The MEM-C907 is a local memory used as an image buffer for copying and a code buffer. The HD909 is a storage for storing image data, font data used during printing, and forms. The HD909 controls the reading and writing of data from and to the HD909 under the control of the CPU901. The AGP bus 921 is a bus interface for a graphics accelerator card proposed to speed up graphics processing. By directly accessing the MEM-P902 at high throughput, the graphics accelerator card can be made faster. Further, the short-distance communication circuit 920 includes a short-distance communication circuit 920a. The short-distance communication circuit 920 is a communication circuit such as NFC or Bluetooth (registered trademark).

[0029] The engine control unit 930 controls the image forming operation by controlling the printer unit 932. The printer unit 932 includes various devices for forming an image on the recording paper P, such as a drive unit that rotates the photosensitive member, a drive unit that rotates the intermediate transfer belt, and the developing device 10. The printer unit 932 also includes an image processing unit that performs error diffusion, gamma conversion, etc.

[0030] The operation panel 940, which serves as an operation input unit, includes a panel display unit 940a and an operation unit 940b. The panel display unit 940a displays current setting values, selection screens, etc., and is configured with a touch panel or the like that receives input from the operator. The operation unit 940b includes a numeric keypad that receives setting values ​​for image formation conditions such as density setting conditions, and a start key that receives a copy start instruction.

[0031] The printer 100 can be switched between the document box function, printer function, and facsimile function in sequence using the application switching key on the operation panel 940. When the document box function is selected, the printer enters document box mode, when the printer function is selected, the printer mode, and when the facsimile mode is selected, the facsimile mode.

[0032] The network I / F 950 is an interface for performing data communication using a communication network. The short-range communication circuit 920 and the network I / F 950 are electrically connected to the ASIC 906 via a PCI bus 922.

[0033] The intermediate transfer unit 60 of this embodiment includes a belt deviation correction mechanism that corrects deviation of the intermediate transfer belt 3. Fig. 3 is a schematic diagram of the configuration of the belt deviation correction mechanism 29 of the intermediate transfer unit 60 immediately after assembly, as viewed from the axial direction of the tension roller 5. Fig. 4 is a schematic diagram of the belt deviation correction mechanism 29 in a state where belt deviation has occurred, as viewed from the axial direction of the tension roller 5. Fig. 5 is a cross-sectional view taken along line AA in Fig. 3. Fig. 6 is a cross-sectional view taken along line AA in Fig. 4.

[0034] A tension roller shaft 5a is provided on the outside of the axial end of the tension roller 5, and is coaxial with the rotation axis of the tension roller 5. The tension roller shaft 5a is cylindrical with a smaller diameter than the tension roller 5, and is joined to the tension roller 5. Both ends of the tension roller shaft 5a are supported by the belt deviation correction mechanism 29.

[0035] 3 and 4, the belt deviation correction mechanism 29 includes a tension roller bearing member 33 that supports and tensions the tension roller 5, and a tension spring 32 that applies tension to the tension roller bearing member 33. It also includes a roller shaft support member 34 that is rotatably supported by a rotation shaft 36 on an intermediate transfer frame 37. It also includes a support spring 40 that supports the roller shaft support member 34, and the support spring 40 biases the roller shaft support member 34 in the clockwise direction in FIGS.

[0036] 5 and 6, the belt deviation correction mechanism 29 includes, in axial order from the inside along the tension roller shaft 5a, a belt deviation detection member 30, a shaft tilting member 31, an intermediate transfer frame 37, and a roller shaft support member 34. The tension roller shaft 5a passes through these members. Both axial ends of the tension roller shaft 5a are supported by the roller shaft support member 34 via tension roller bearing members 33.

[0037] The belt deviation detection member 30 and the shaft inclination member 31 are supported on the tension roller shaft 5a so as to be freely movable in the axial direction. The belt deviation detection member 30 has a flange portion 30a, the outer diameter of which is larger than that of the tension roller 5, on the axially outer side of a cylindrical portion 30b, the outer diameter of which is smaller than that of the tension roller 5. When the intermediate transfer belt 3 deviates from the belt, the widthwise end of the intermediate transfer belt 3 abuts against the axially inner surface of the flange portion 30a.

[0038] The shaft tilting member 31 has an upper inclined surface 31f that is inclined relative to the tension roller shaft 5a. A shaft guide portion 35 provided on the intermediate transfer frame 37 of the intermediate transfer unit 60 abuts against this inclined surface 31f from the axial outer side (the right side in FIG. 5) of the tension roller shaft 5a. The contact between the shaft tilting member 31 and the shaft guide portion 35 maintains the rotational position of the roller shaft support member 34 against the biasing force of the support spring 40.

[0039] In this embodiment, the inclination angle of the inclined surface 31f relative to the tension roller shaft 5a (angle "α" in Figure 4) is 30°, and the material of the shaft inclination member 31 is POM (polyacetal), but this is not limited to this.

[0040] 3 and 4, the rotation shaft 36 that supports the roller shaft support member 34 is disposed on the opposite side of the bisector L of the angle formed by the intermediate transfer belt 3 from the contact portion between the shaft tilting member 31 and the shaft guide portion 35. In this way, a force acts on the shaft tilting member 31 shown in FIGS. 5 and 6 to move it toward the shaft guide portion 35, and the shaft tilting member 31 and the shaft guide portion 35 can be brought into contact with each other.

[0041] Next, the operation of the belt deviation correction mechanism 29 will be described. When the secondary transfer opposing roller 4, which is a drive roller, starts to rotate, the tension roller 5, which is a driven roller around which the intermediate transfer belt 3 is wound, also starts to rotate. At this time, if the end of the intermediate transfer belt 3 or the vicinity of the end is in contact with the belt deviation detection member 30, the belt deviation detection member 30 also starts to rotate.

[0042] In this state, if the intermediate transfer belt 3 shifts to the right in FIG. 5 due to factors such as the parallelism between the components, the right end of the intermediate transfer belt 3 in the width direction will come into contact with the flange portion 30a of the belt shift detection member 30. This contact force causes the belt shift detection member 30 to move axially outward (to the right in FIG. 5). When the belt shift detection member 30 moves axially outward, the shaft tilting member 31 is pressed axially outward by the belt shift detection member 30. As a result, the shaft tilting member 31 also moves axially outward.

[0043] Due to the movement of the shaft tilting member 31 outward in the axial direction, the shaft guide portion 35 in contact with the inclined surface 31f of the shaft tilting member 31 moves relatively along the inclined surface 31f (see FIG. 6). As a result, the contact position between the inclined surface 31f and the shaft guide portion 35 tends to be displaced upward on the inclined surface 31f.

[0044] Because the shaft guide 35 is part of the intermediate transfer frame 37 fixed to the main body housing 101 of the printer 100, it does not displace upward, and the shaft tilting member 31, which has the inclined surface 31f, is displaced downward by the reaction force received from the shaft guide 35. As a result, the end of the tension roller shaft 5a in the direction in which the intermediate transfer belt 3 moves due to the belt bias is pushed down against the upward force exerted by the biasing force of the support spring 40 (see FIG. 4). This causes the tension roller shaft 5a to tilt.

[0045] In this way, as the tension roller shaft 5a tilts, the speed at which the intermediate transfer belt 3 moves in the belt width direction gradually slows, and eventually the intermediate transfer belt 3 starts to move in the opposite direction in the belt width direction. As a result, the widthwise position of the intermediate transfer belt 3 is gradually returned, the deviation of the intermediate transfer belt 3 is corrected, and the intermediate transfer belt 3 can run stably at the same position in the width direction. This is also true when the belt deviation of the intermediate transfer belt 3 occurs in the opposite direction (toward the left in FIG. 5).

[0046] Next, the positioning of the photosensitive members 1 (a to d) within the printer body will be described. Figures 7 to 9 are schematic diagrams illustrating the positioning of the photosensitive member in the device body. Figure 7 is a schematic diagram of the photosensitive member 1d as seen from the left and right direction (X direction) of the device, Figure 8 is a diagram illustrating the positioning of the photosensitive member on the front side of the device, and Figure 9 is a diagram illustrating the positioning of the photosensitive member on the back side of the device.

[0047] The photosensitive drums 1(a-d) are rotatably held by photosensitive drum frames 49(a-d). Positioning abutments 43(a-d) and 44(a-d) are provided on both ends of the shaft of the photosensitive drum 1 to position the photosensitive drum in the printer body. As shown in Figures 8 and 9, each of the positioning abutments 43(a-d) and 44(a-d) has a semicircular shape with approximately the same diameter as the photosensitive drum. The front and back sides of the photosensitive drum frames 49(a-d) are biased toward the intermediate transfer unit 60 by springs 42(a-d) which serve as biasing members.

[0048] 7 and 8, the positioning abutment portions 43 (a to d) on the front side of the device fit into concave positioning recesses 56 (a to d) provided at the lower end of the front side intermediate transfer frame 37. Furthermore, they abut against the positioning recesses 56 (a to d) and are positioned by the biasing force of the springs 42 (a to d).

[0049] 7 and 9, the positioning abutment portions 44(a-d) at the rear side of the device are fitted into positioning recesses 57(a-d) provided in a positioning holder 48d attached to the main body structure 45 at the rear side of the device. Furthermore, they are positioned by abutting against positioning recesses 56(a-d) by the biasing force of springs 42(a-d).

[0050] 9 indicates the positions of the positioning recesses 56(a-d) provided on the intermediate transfer frame 37 on the front side of the device, and the positioning recesses 57(a-d) of the positioning holder 48d and the positioning recesses 56(a-d) of the intermediate transfer frame are provided in the same positions. This prevents the photosensitive member from tilting up and down and left and right, and allows it to be accurately positioned in the printer body.

[0051] The printer 100 has two drive modes: a full-color mode in which four colors of toner are used, and a black monochrome mode in which only black toner is used. In the full-color mode, as shown in Figure 10(a), the intermediate transfer belt 3 and the four photoconductors 1 (a-d) come into contact with each other, and a four-color toner image is transferred onto the intermediate transfer belt 3. On the other hand, in the black monochrome mode, only the black photoconductor 1a comes into contact with the intermediate transfer belt 3, as shown in Figure 10(b), and only black toner is transferred onto the intermediate transfer belt 3.

[0052] Both axial ends of each primary transfer roller 11 (a to d) are rotatably supported by holding members 52 (a to d), and each holding member 52 (a to d) is rotatably supported by a support shaft 39 (a to d) provided on the intermediate transfer frame 37.

[0053] In this embodiment, a backup roller 47 is disposed between the tension roller 5 and the primary transfer roller 11d for yellow, which is disposed at the most upstream side in the direction of movement of the intermediate transfer belt 3. The backup roller 47 is used to form the desired shape of the primary transfer nip (primary transfer nip for yellow) at the most upstream side in the direction of movement of the intermediate transfer belt 3. The backup roller 47 is also rotatably supported by a yellow holding member 52d that holds the primary transfer roller 11d for yellow.

[0054] In the black monochrome mode, the belt contact / separation mechanism 80 (described later) rotates the magenta, cyan, and yellow holding members 52 (b-d), moving the three primary transfer rollers 11 (b-d) and the backup roller 47 from the nip formation position to the retracted position, and separating the intermediate transfer belt 3 from the photosensitive members 1 (b-d).

[0055] In the black monochrome mode, by also moving the backup roller 47 to the retracted position, it is possible to prevent the intermediate transfer belt 3 from coming into contact with the yellow photosensitive member. In this embodiment, the backup roller 47 is held by a holding member 52d that holds the yellow primary transfer roller 11d. This reduces the number of parts compared to a case in which the backup roller 47 is held by a separate holding member like the other primary transfer rollers 11(a-c), and allows for a smaller, less expensive, and simpler image forming apparatus.

[0056] Each of the holding members 52(a-d) is provided with a pressed portion 55(a-d) and a positioning protrusion 53(a-d). The positioning protrusion 53(a-d) of each of the holding members 52(a-d) positions the primary transfer roller 11(a-d) at a nip forming position where the intermediate transfer belt 3 is brought into contact with the photosensitive member 1(a-d) to form a primary transfer nip.

[0057] The pressed portion 55a of the black holding member 52a is directly biased by the pressure spring 70a, and is biased so that the black primary transfer roller 11a is positioned at the nip formation position. Meanwhile, the pressed portions 55(b-d) of the magenta, cyan, and yellow holding members 52(b-d) extend outward in the axial direction. The pressed portions 55a receive the biasing force (pressing force) of the pressure springs 70(b-d) via pressure relay members 83(b-d) of the belt contact / separation mechanism 80 (described later) (see FIGS. 13 and 14), and this biasing force positions the primary transfer rollers 11(b-d) at the nip formation position.

[0058] Fig. 11 is a diagram illustrating the positioning of the primary transfer rollers 11(a-d) on the front side of the printer 100 by the positioning protrusions 53(a-d) of the holding members 52(a-d) arranged on the front side of the printer 100. Fig. 12 is a diagram illustrating the positioning of the primary transfer rollers 11(a-d) on the rear side of the printer 100 by the positioning protrusions 53(a-d) of the holding members 52(a-d) arranged on the rear side of the printer 100.

[0059] As shown in Figure 11, the intermediate transfer frame 37 on the front side of the device is provided with a positioning portion 63 extending in the axial direction from the intermediate transfer frame 37. The positioning protrusions 53(a-d) of each holding member 52(a-d) on the front side of the device abut against the positioning portion 63 due to the biasing force of the pressure springs 70(a-d) (see Figures 13 and 14 for the pressure springs 70(b-d)). As a result, the front side of the device of the primary transfer rollers 11(a-d) is positioned at the target nip formation position where the intermediate transfer belt 3 is brought into contact with the photosensitive members 1(a-d) to form a primary transfer nip.

[0060] As described above, the front side of the photosensitive drums 1(a-d) is positioned by the intermediate transfer frame 37. Therefore, by positioning the positioning protrusions 53(a-d) of each holding member 52(a-d) on the front side of the device by abutting them against the positioning portions 63 of the intermediate transfer frame 37, the front side of the primary transfer rollers 11(a-d) is positioned by the member that positions the photosensitive drums. This prevents the accumulation of assembly errors and dimensional errors, and allows the front side of the primary transfer rollers 11(a-d) to be positioned with high precision relative to the photosensitive drums 1(a-d).

[0061] As shown in FIG. 12, the positioning protrusions 53(a-d) of the holding members 52(a-d) at the rear of the printer 100 abut against the photoconductor frames 49(a-d) that rotatably hold the photoconductors 1(a-d) by the biasing force of the pressure springs 70(a-d) (see FIGS. 13 and 14 for the pressure springs 70(b-d)). This positions the rear of the primary transfer rollers 11(a-d) at the nip formation position. In this way, the rear of the primary transfer rollers 11(a-d) is positioned by the photoconductor frames 49(a-d) that hold the photoconductors 1(a-d), so that the rear of the primary transfer rollers 11(a-d) can be positioned with high precision relative to the photoconductors 1(a-d).

[0062] In addition, the positioning protrusions 53 (a-d) of the holding members 52 (a-d) at the rear side of the device may be abutted against a positioning holder 48d that positions the photosensitive member 1 within the printer body, thereby positioning the rear side of the device of the primary transfer rollers 11 (a-d) at the nip formation position.

[0063] In this way, each primary transfer roller 11 (a-d) is accurately positioned at the front and back of the device relative to the photosensitive member at the nip formation position, allowing the primary transfer nip to be positioned as desired in the belt width direction. This reduces variations in image density in the belt width direction, resulting in a high-quality image.

[0064] Next, we will explain the belt contact / separation mechanism, which is a characteristic feature of this embodiment, that moves the primary transfer rollers 11 (b-d) for magenta, cyan, and yellow and the backup roller 47 to a retracted position and separates the intermediate transfer belt 3 from the photosensitive member.

[0065] 13 and 14 are schematic diagrams of the belt contact / separation mechanism 80. FIG. 13 is a schematic diagram of the belt contact / separation mechanism 80 in the full-color mode (the primary transfer rollers 11 (b-d) for magenta, cyan, and yellow and the backup roller 47 are located at the nip forming position). FIG. 14 is a schematic diagram of the belt contact / separation mechanism 80 in the black single-color mode (the primary transfer rollers 11 (b-d) for magenta, cyan, and yellow and the backup roller 47 are located at the retracted position). FIGS. 13(a) and 14(a) are schematic diagrams of the belt contact / separation mechanism 80 as viewed from the axial direction (Y direction), and FIGS. 13(b) and 14(b) are cross-sectional views taken along line AA in FIGS. 13(a) and 14(a).

[0066] The belt contact / separation mechanisms 80 are provided on both the front and rear sides of the device. Since the two front and rear belt contact / separation mechanisms 80 are nearly symmetrical, only the belt contact / separation mechanism 80 on the rear side of the device will be described below.

[0067] The belt contact / separation mechanism 80 has a slide member 81, a cam member 82, and three pressing relay members 83 (b to d) for magenta, cyan, and yellow. The slide member 81 is held by the intermediate transfer frame 37 so as to be slidable in the left-right direction of the device (left-right direction in the drawing).

[0068] The slide member 81 is biased toward the cam member (to the right in the drawing) by a contact / separation spring 84, and a cam contact portion 86 provided on the cam-side end of the slide member 81 contacts the cam member 82. The cam member 82 is fixed to a cam shaft 82a rotatably supported on the device body, and is rotated by a driving force transmitted from a drive motor serving as a drive source provided on the device body via a drive transmission member such as a gear. The slide member 81 is also provided with spring receiving portions 85 (b-d) that receive one end (the right end in the drawing) of the pressure springs 70 (b-d). Each spring receiving portion 85 (b-d) extends from the main body of the slide member toward the rear of the device and is located within a pressure relay member 83 (b-d) arranged outside the slide member 81.

[0069] The slide member 81 also has through holes 90(b-d) through which the pressed portions 55(b-d) extending toward the rear of the holding members 52(b-d) pass. The pressed portions 55(b-d) of the holding members 52(b-d) pass through the through holes 90(b-d) of the slide member 81, and their tip ends are located inside the pressing relay members 83(b-d).

[0070] The three pressure relay members 83(b-d) are held by the slide member 81 or the intermediate transfer frame 37 so as to be slidable in the left-right direction of the device (left-right direction in the drawing). Each pressure spring 70(b-d) that biases each holding member 52(b-d) is disposed within the pressure relay member 83. One end (the right end in the drawing) of each pressure spring 70(b-d) is received by a spring receiving portion 85(b-d) provided on the slide member 81. The other end (the left end in the drawing) of each pressure spring 70(b-d) is received by a side wall 89(b-d) at the left end in the drawing of the pressure relay member 83(b-d). In a compressed state, each pressure spring 70(b-d) is received by the spring receiving portion 85(b-d) of the slide member 81 and the side wall 89(b-d) at the left end in the drawing of the pressure relay member 83(b-d).

[0071] The pressing relay members 83(b-d) have first pressing portions 87(b-d) and second pressing portions 88(b-d). The first pressing portions 87(b-d) are side walls at the right ends of the pressing relay members 83(b-d) in the drawings. The second pressing portions 88(b-d) are located between the spring receiving portions 85(b-d) of the sliding member 81 and the pressed portions 55(b-d) of the holding members 52(b-d), and are provided so as to separate the spring receiving portions 85(b-d) from the pressed portions 55(b-d).

[0072] As shown in FIG. 13, in the full-color mode, the spring receiving portions 85(b-d) and the second pressing portions 88(b-d) of the sliding member 81 are spaced apart. At this time, the pressing relay members 83(b-d) are biased leftward in the figure by the biasing force of the pressure springs 70(b-d), and the first pressing portions 87(b-d) bias the pressed portions 55(b-d) of the holding members 52(b-d) leftward in the figure. The pressed portions 55(b-d) of the holding members 52(b-d) are biased by the biasing force of the pressure springs 70(b-d) via the first pressing portions 87(b-d), causing each holding member 52(b-d) to rotate counterclockwise as shown in FIG. 10(a) around the support shafts 39(b-d). As a result, as shown in FIG. 11, the positioning protrusions 53(b-d) of the holding members 52(b-d) abut against the positioning portions 63, and the primary transfer rollers 11(b-d) and the backup roller 47 are positioned at the target nip formation positions.

[0073] When switching from full-color mode to black monochrome mode, the drive motor is driven to rotate cam member 82. When cam member 82 rotates from the state shown in Fig. 13, sliding member 81 slides to the right in the figure (the opposite direction to the biasing direction of the pressure spring against the holding member via the pressure relay member) due to the biasing force of contact / separation spring 84.

[0074] The slide member 81 slides to the right in the figure relative to the pressure relay member 83(b-d). This sliding of the slide member 81 causes the spring receiving portions 85(b-d) to move to the right in the figure by the distance X in FIG. 13, increasing the distance between the spring receiving portions 85(b-d) of the slide member 81 and the left side wall 89(b-d) of the pressure relay member in the figure. As a result, the compressed pressure springs 70b-70d expand, and the biasing forces of the pressure springs 70(b-d) are reduced.

[0075] As the cam member 82 further rotates and the slide member 81 slides to the right in the figure, the spring receiving portions 85 (b-d) come into contact with the second pressing portions 88 (b-d). As the cam member 82 further rotates from this state and the slide member 81 slides to the right in the figure, the spring receiving portions 85 (b-d) press the second pressing portions 88 (b-d) to the right in the figure. This causes the pressing relay members 83 (b-d) to slide to the right together with the slide member 81. As a result, the first pressing portions 87 (b-d) move away from the pressed portions 55 (b-d) of the holding member, and the second pressing portions 88 (b-d) come into contact with the pressed portions 55 (b-d) as shown in FIG. 14. As the slide member 81 further slides to the right in the figure, the pressed portions 55 (b-d) are pressed to the right in the figure by the second pressing portion 88b. This pressure causes each holding member 52 (b-d) to rotate clockwise in Figure 10(a) around the support shaft 39 (b-d), and the primary transfer roller 11 (b-d) and backup roller 47 move from the nip formation position shown in Figure 10(a) to the retracted position.

[0076] 14, when the cam member 82 rotates half a turn, the primary transfer rollers 11(b to d) and the backup roller 47 are positioned at the retracted positions shown in FIG. 10(b), and the rotation of the cam member 82 stops.

[0077] When the drive motor is driven to rotate the cam member 82 from the state shown in Fig. 14 to switch from the black monochrome mode to the color mode, the slide member 81 slides to the left in Fig. 14 against the biasing force of the contact / separation spring 84. The pressing relay members 83(b-d) slide to the left in Fig. 14 together with the slide member 82 until the pressed portions 55(b-d) of the holding members 52(b-d) abut against the first pressing portions 87(b-d). When the pressed portions 55(b-d) abut against the first pressing portions 87(b-d), only the slide member 81 slides to the left in Fig. 14 until the biasing force biasing the left side wall 89(b-d) of the pressing relay members 83(b-d) at the other ends of the pressure springs 70(b-d) becomes equal to or greater than the force required to rotate the holding members 52(b-d). As a result, the spring receiving portions 85(b-d) move away from the second pressing portions 88(b-d), and the pressure springs 70(b-d) are compressed. When the biasing force of the pressure springs 70(b-d) becomes equal to or greater than the force required to rotate the holding member 52, the pressure relay members 83(b-d) move again to the left in the figure together with the slide member 81. When the positioning protrusions 53(b-d) of the holding member 52 abut against the positioning portions 63 and the primary transfer roller 11(b-d) and the backup roller 47 are positioned at the intended nip formation positions, the movement of the pressure relay members 83(b-d) to the left in the figure stops, and only the slide member 81 moves to the left in the figure. This further compresses the pressure springs 70(b-d), and the biasing force of the pressure springs 70(b-d) increases. Then, when the cam member 82 rotates half a turn and reaches the state shown in Figure 13, the biasing force of the pressure spring 70 (b-d) becomes the targeted biasing force, and the holding member 52 (b-d) can be biased with the desired biasing force via the pressing relay member 83 (b-d).

[0078] Unlike the present embodiment, for example, if a spring receiving portion for receiving one end of the pressure spring 70(b-d) is provided on the intermediate transfer frame, the following problem occurs. Specifically, when the pressure relay member 83(b-d) moves toward the cam member together with the slide member during switching from full-color mode to black monochrome mode, the distance between the spring receiving portion provided on the intermediate transfer frame 37 and the left side wall 89(b-d) of the pressure relay member 83(b-d) in the figure decreases. As a result, the compressed pressure springs 70(b-d) are further compressed, increasing their biasing force. This requires the slide member to move to the right in the figure against the increased biasing force of the pressure springs, increasing the force required to slide the slide member. To move the slide member to the right in the figure against the biasing force of the pressure springs 70(b-d), the biasing force of the contact / separation spring 84 must be increased. The tension spring 84 compresses and its biasing force decreases as the slide member 81 moves to the right in the figure (toward the cam member). Therefore, even in this reduced state, the biasing force of the spring 84 must be greater than the biasing force of the pressure springs 70 (b-d). As a result, when switching from black monochrome mode to full-color mode, the torque required to rotate the cam member 82 increases when moving the slide member 71 to the left in the figure against the biasing force of the spring 84. Furthermore, as the biasing force of the spring 84 increases, the stress applied to the slide member 81 in the state of FIG. 13 (full-color mode) increases, which could damage the slide member 81.

[0079] On the other hand, in this embodiment, a spring receiving portion that receives one end of the pressure spring 70(b-d) is provided on the slide member 81. As a result, when the spring receiving portion 85(b-d) abuts against the second pressing portion 88b and the pressure relay member 83(b-d) slides to the right in FIG. 13 together with the slide member 81 against the biasing force of the pressure spring, the spring receiving portion 85b of the slide member 81 also slides to the right in FIG. 13 by the same amount. As a result, the distance between the spring receiving portion 85b and the left side wall 89(b-d) in the figure becomes approximately constant, and the pressure springs 70(b-d) maintain an approximately constant length. Therefore, the biasing force of the pressure springs 70(b-d) is kept approximately constant. This reduces the increase in force required to slide the slide member to the right in FIG. 13 compared to when one end of the pressure springs 70(b-d) is supported by the intermediate transfer frame. As a result, even if the biasing force of the contact / separation spring 84 is weak, the biasing force of the contact / separation spring 84 can slide the slide member 81 to the right in Fig. 13. This makes it possible to suppress an increase in the torque required to rotate the cam member 82 when moving the slide member 81 from the state in Fig. 14 to the left in the figure against the biasing force of the contact / separation spring 84 (when switching from monochrome mode to full color mode).

[0080] In this embodiment, as shown in Fig. 13, the holding member 52d that holds the yellow primary transfer roller 11d and the backup roller 47 has a long distance L from the support shaft 39d, which is the fulcrum for rotation of the holding member 52d, to the backup roller 47. Therefore, a large force is required to bring the backup roller 47 into contact with the intermediate transfer belt 3 with the desired contact pressure. Therefore, the biasing force of the pressure spring 70d must be stronger than the biasing forces of the other pressure springs 70b and 70c.

[0081] Furthermore, in this embodiment, the belt misalignment is corrected by tilting the tension roller 5 using the belt misalignment correction mechanism 29. As shown in FIG. 15, the tilt of the tension roller 5 changes the winding angle of the intermediate transfer belt 3 wound around the backup roller 47. As a result, the force that the holding member 52d that holds the backup roller 47 receives from the intermediate transfer belt 3 also changes. If the biasing force (pressing force) of the pressure spring 70d that positions the yellow primary transfer roller 11d at the nip formation position is weak, the holding member 52d may rotate clockwise in FIG. 15 depending on the force received by the backup roller 47. As a result, the position of the yellow primary transfer roller 11d becomes unstable. Therefore, the biasing force of the pressure spring 70d must be set to a value that prevents the holding member 52d from rotating regardless of the orientation of the tension roller 5. By setting such a biasing force, the position of the yellow primary transfer roller 11d held by the holding member 52d is always the same. This allows for the production of high-quality images with minimal density deviation. However, the biasing force of the pressure spring 70d becomes larger than the biasing forces of the other pressure springs 70b and 70c.

[0082] In this embodiment, as described above, the biasing force of the pressure springs 70 (b-d) does not increase when switching from the color mode to the black monochrome mode, so the increase in force required to slide the slide member when switching from the color mode to the black monochrome mode is suppressed. As a result, even if a spring with a strong biasing force is used for pressure spring 70d, the weak biasing force of the contact / separation spring 84 can slide slide member 81 toward the cam member. Therefore, the increase in torque required to rotate cam member 82 when switching from the black monochrome mode to the full-color mode (when sliding the slide member against the biasing force of the contact / separation spring) can be suppressed.

[0083] 13, when the primary transfer rollers 11(b-d) and the backup roller 47 are positioned at the nip forming positions, the spring receiving portions 85(b-d) are separated from the second pressing portions 88(b-d). As a result, the compressed pressure springs 70(b-d) expand and the biasing force of the pressure springs 70(b-d) can be weakened until the spring receiving portions 85(b-d) contact the second pressing portions 88 and the pressure relay members 83(b-d) slide together with the slide member 81. This further weakens the biasing force of the contact / separation springs 84, thereby suppressing an increase in the torque required to rotate the cam member 82 when switching from the black monochrome mode to the color mode (when the slide member 81 slides against the biasing force of the contact / separation springs 84).

[0084] 13, when each of the primary transfer rollers 11(b-d) and the backup roller 47 is positioned at the nip forming position, the second pressing portions 88(b-d) are spaced apart from the pressed portions 55(b-d). With this configuration, the holding members 52(b-d) are able to move the primary transfer rollers from the nip forming position to the retracted position by the amount of the gap between the second pressing portions 88(b-d) and the pressed portions 55(b-d).

[0085] In this embodiment, the movement amount from the nip formation position to the retracted position of the yellow primary transfer roller 11d is 4 mm, that of the magenta primary transfer roller 11c is 3 mm, and that of the cyan primary transfer roller 11b is 2 mm, so that the movement amounts are different from one another. In this embodiment, the movement amounts from the nip formation position to the retracted position of the yellow, magenta, and cyan primary transfer rollers 11c are made different from one another by adjusting the timing at which the second pressing portion 88 abuts against the pressed portion of the holding member and the distance between the support shaft 39 that rotatably supports the holding member 52 and the primary transfer rollers.

[0086] In this embodiment, the spring receiving portions 85(b-d) in the state shown in FIG. 13 are separated by a distance X from the second pressing portions 88(b-d). In this embodiment, the distance X is 1.5 mm, ensuring a distance of 0 or greater even when the tolerances of the finished parts are accumulated. The longer the distance X, the greater the amount of extension of the compressed pressure springs 70(b-d). This reduces the biasing force of the pressure springs 70(b-d) when switching from the color mode to the black monochrome mode, thereby reducing the biasing force of the contact / separation spring 84. However, this increases the sliding distance of the slide member 81, which may result in an increase in the size of the intermediate transfer unit 60 in the left-right direction of the device. Furthermore, this reduces the amount of movement (the amount of rotation of the holding member) of the primary transfer rollers 11(b-d) between the nip formation position and the retracted position. Therefore, in this embodiment, the distance X between the spring receiving portion 85 (b to d) and the second pressing portion 88 (b to d) is set to 1.5 mm, which is shorter than the movement amount (movement distance) from the nip formation position of the primary transfer roller to the retracted position.

[0087] FIG. 16 shows a modified example of a belt contact / separation mechanism 80'. FIG. 16(a) is a schematic diagram in the full-color mode, and FIG. 16(b) is a schematic diagram in the black monochrome mode. As shown in Fig. 16, in this modified example, the pressing relay member is eliminated. The other ends of the pressure springs 70(b-d) are attached to the pressed portions 55(b-d) of the holding members 52(b-d). In this modified example, each pressure spring 70(b-d) is supported in a compressed state by the pressed portions 55(b-d) and spring support portions 85(b-d) provided on the slide member.

[0088] 16(a), in the full-color mode, the pressing portions 92(b-d) provided on the sliding member are separated from the pressed portions 55(b-d), and the pressed portions are pressed to the left in the figure by the biasing force of the compressed pressure springs 70(b-d). As a result, the holding member rotates counterclockwise in the figure, and the primary transfer rollers 11(b-d) are positioned at the nip forming position.

[0089] When switching from full-color mode to black monochrome mode, the cam member 82 is rotated, as described above, to move the slide member to the right in the figure. Only the spring receiving portions 85(b-d) move to the right in the figure until the pressing portions 92(b-d) of the slide member abut against the pressed portions 55(b-d) of the holding member 52(b-d). This causes the compressed pressure springs 70(b-d) to expand, reducing the biasing force of the pressure springs 70(b-d). The slide member 81 then moves to the right in the figure, and the pressing portions 92(b-d) of the slide member 81 abut against the pressed portions 55(b-d), pressing the pressed portions 55(b-d) in the opposite direction (to the right in the figure) to the pressing direction of the pressure springs 70(b-d). As a result, the pressed portion 55 (b-d) moves to the right in the figure against the biasing force of the pressure spring 70 (b-d), the holding member 52 (b-d) rotates clockwise in the figure, and the primary transfer roller 11 (b-d) moves from the nip formation position to the retracted position shown in Figure 16 (b).

[0090] In this modification, when switching from full-color mode to black monochrome mode, the spring receiving portions 85 (b-d) move rightward in the figure by approximately the same amount as the amount of pressure applied by the pressed portions 55 (b-d) of the pressing portions 92 (b-d), and the length of the pressure springs 70 (b-d) is maintained approximately constant. This prevents the biasing force of the pressure springs 70 (b-d) from increasing when the primary transfer roller 11 (b-d) is moved from the nip formation position to the retracted position, thereby suppressing the biasing force of the contact / separation spring 84. This prevents the rotational torque of the cam member 82 from increasing when switching from black monochrome mode to full-color mode, in which the slide member 81 slides against the biasing force of the contact / separation spring 84.

[0091] The belt contact / separation mechanism 80' of this modified example does not have a pressing relay member, so the number of parts can be reduced, and the size in the front-rear direction of the device and the cost can be reduced compared to the embodiment.

[0092] On the other hand, the belt contact / separation mechanism 80 of the embodiment provided with the pressure relay members 83(b-d) has the following advantage over the belt contact / separation mechanism of the modified example shown in FIG. 16 . Specifically, the rotation of the holding members 52(b-d) moves the primary transfer rollers 11(b-d) from the nip formation position to the retracted position, and this rotation also displaces the pressed portions 55(b-d) in the vertical direction (Z direction) of the device. In a configuration in which the other ends of the pressure springs 70(b-d) are directly attached to the pressed portions 55(b-d) as in the modified example, the vertical displacement of the pressed portions 55(b-d) bends the pressure springs 70(b-d), applying excessive force to the pressed portions 55(b-d). As a result, there is a risk of an increase in the rotational torque of the cam member 82 or the pressure springs 70(b-d) coming off the pressed portions 55(b-d).

[0093] In contrast to this, by providing pressing relay members 83 (b-d), it is possible to always press the pressed portions 55 (b-d) in the horizontal direction (the left-right direction of the device (X direction in the figure)). This has the advantage that no unnecessary force is applied in the vertical direction to the pressed portions 55 (b-d), allowing the holding members 52 (b-d) to rotate smoothly and effectively suppressing an increase in the rotational torque of the cam member 82.

[0094] In this embodiment, the cam member is located downstream (on the right side in FIGS. 13 and 14) in the sliding direction of the sliding member when switching from full-color mode to black monochrome mode. However, the cam member may also be located upstream (on the left side in FIGS. 13 and 14) in the sliding direction of the sliding member when switching from full-color mode to black monochrome mode. In such a configuration, the torque required to rotate the cam member when switching from full-color mode to black monochrome mode increases due to the increase in the biasing force of the pressure spring when switching from full-color mode to black monochrome mode. However, by providing spring receiving portion 85 on the sliding member to prevent the increase in the biasing force of the pressure spring when switching from full-color mode to black monochrome mode, the increase in the torque required to rotate the cam member can be suppressed.

[0095] Although the preferred embodiments of the present invention have been described above, the present invention is not limited to such specific embodiments, and unless otherwise specifically limited in the above description, various modifications and variations are possible within the spirit and scope of the present invention as set forth in the claims.

[0096] The above description is merely an example, and each of the following aspects provides unique effects. (Aspect 1) An image forming apparatus including a nip forming member such as a primary transfer roller 11 that forms a transfer nip by bringing a transfer body such as an intermediate transfer belt 3 onto which an image is transferred from an image carrier such as a photosensitive member 1 into contact with the image carrier, a pressure spring 70 that presses a holding member 52 that holds the nip forming member so that the nip forming member is positioned at a nip forming position where the transfer body is brought into contact with the image carrier, and a separation mechanism such as a belt contact / separation mechanism 80 that moves the nip forming member from the nip forming position to a retracted position by sliding a slide member 81 in a direction opposite to the biasing direction of the pressure spring 70 against the holding member 52, thereby separating the transfer body from the image carrier, The end (one end of the pressure spring) opposite to the biasing end (the other end of the pressure spring) of the pressure spring that biases the holding member was engaged with the slide member 81. Generally, when the slide member is in a standby position (a nip formation position where the biasing force of the pressure spring positions the nip formation member at the nip formation position), a means for sliding the slide member is used to keep the slide member in the standby position so that the slide member does not slide freely in the direction opposite to the pressing direction of the pressure spring against the holding member and move the nip formation member to the retracted position. For example, if the means for sliding the slide member includes a cam member and the slide member is slid by rotation of the cam member, the slide member is biased by a spring to abut against the cam member, and the spring's biasing force keeps the slide member in the standby position even if a sliding force is applied to the slide member. If the means for sliding the slide member includes a rack and pinion mechanism, the reduction ratio of a gear unit that transmits the driving force of the drive source to the rack and pinion mechanism is increased, and the slide member is kept in the standby position by the torque of the drive source. By engaging the end of the pressure spring opposite to the end that biases the holding member with the slide member, the slide member is subjected to a biasing force from the pressure spring that slides in the opposite direction, but as described above, the means for sliding the slide member is configured to hold the slide member in the standby position, so the slide member waits in the standby position without sliding due to the biasing force of the pressure spring. Therefore, when the slide member is in the standby position, the pressure spring biases the holding member with the desired biasing force. When the slide member slides in the opposite direction to the pressing direction of the pressure spring against the holding member to move the nip forming member from the nip forming position to the retracted position, the opposite end of the pressure spring engaged with the slide member moves in the opposite direction along with the slide member. When the slide member slides to move the nip forming member from the nip forming position to the retracted position, the holding member presses the biasing side end of the pressure spring in the opposite direction, causing the biasing side end of the pressure spring to also move in the opposite direction. Because both ends of the pressure spring move in the opposite direction, the entire pressure spring moves in the opposite direction, thereby minimizing change in the length of the pressure spring. Therefore, compared to Patent Document 1, in which the opposite end of the pressure spring engages with a non-sliding frame member of the device body, the increase in the biasing force of the pressure spring against the holding member when the slide member slides is suppressed. This reduces the force required to slide the slide member when separating the transfer body from the image carrier, compared to Patent Document 1.

[0097] (Aspect 2) In aspect 1, the holding member 52 is supported so as to be freely rotatable, and the separation mechanism such as the belt contact / separation mechanism 80 includes a pressing relay member 83 having a first pressing portion 87 that presses the holding member 52 with the biasing force of the pressure spring 70, and a second pressing portion 88b that moves together with the slide member 81 and presses the holding member 52 in the opposite direction. As a result, unlike the embodiment described above in which the pressure spring 70 directly presses the holding member 52, the pressure spring 70 does not bend when the holding member rotates, which prevents unnecessary force from being generated and prevents an increase in the sliding load of the slide member 81.

[0098] (Aspect 3) In aspect 2, the sliding distance of the slide member 81 until the slide member 81 abuts against the pressing relay member 83 and the pressing relay member 83 starts moving together with the slide member 81 (in this embodiment, the distance X between the spring receiving portion 85 and the second pressing portion 88) is less than the movement distance of a nip forming member such as the primary transfer roller 11 from the nip forming position to the retracted position. As a result, as described in the embodiment, the slide member slides a predetermined amount before abutting against the pressure relay member 83, which then moves together with the slide member 81, thereby reducing the biasing force of the pressure spring 70 and reducing the force required to slide the slide member. Furthermore, by making the sliding amount of the slide member 81 (in this embodiment, the distance X between the spring receiving portion 85 and the second pressing portion 88) smaller than the movement amount of a nip forming member such as the primary transfer roller 11 from the nip forming position to the retracted position, it is possible to suppress an increase in the sliding amount of the slide member, and to suppress an increase in the size of the image forming apparatus in the sliding direction of the slide member.

[0099] (Aspect 4) In any of aspects 1 to 3, the holding member 52 is brought into contact with an image carrier holding member such as the photosensitive member frame 49 that holds an image carrier such as the photosensitive member 1, or a photosensitive member positioning member such as the intermediate transfer frame 37 or positioning holder 48d that positions the image carrier within the device body, thereby positioning a nip forming member such as the primary transfer roller 11 at the nip forming position. As described in the embodiment, this allows accurate positioning relative to the image carrier such as the photosensitive member at the nip formation position, forming the desired transfer nip, which allows the image on the image carrier to be transferred satisfactorily to the transfer member such as the intermediate transfer belt 3, thereby obtaining a good image.

[0100] (Aspect 5) In any of aspects 1 to 4, there are multiple image carriers such as photosensitive members 1 and nip forming members such as primary transfer rollers 11, and a separation mechanism such as a belt contact / separation mechanism 80 moves the multiple nip forming members by sliding a slide member 81. According to this, one slide member 81 can move a plurality of nip forming members such as the primary transfer rollers 11 from the nip forming position to the retracted position.

[0101] (Aspect 6) In the fifth aspect, a backup roller 47 is provided which is arranged upstream of the multiple primary transfer rollers 11 and the nip forming member in the direction of movement of a transfer body such as the intermediate transfer belt 3, and which sets the transfer nip at the most upstream position in the direction of movement of the transfer body to a target state, and the backup roller 47 is held by a holding member 52d which holds the nip forming member arranged at the most upstream position in the direction of movement of the transfer body. This allows for a reduction in the number of parts compared to the case where a separate holding member for holding the backup roller 47 is provided, as explained in the embodiment, and allows for a reduction in the cost of the device.

[0102] (Aspect 7) In any of aspects 1 to 6, the transfer body is an intermediate transfer body such as an intermediate transfer belt 3, and the nip forming member is a primary transfer member such as a primary transfer roller 11 that performs primary transfer of an image from an image carrier such as a photoreceptor 1 to the intermediate transfer body.

[0103] (Aspect 8) In any of the first to seventh aspects, the transfer body such as the intermediate transfer belt 3 is a belt member, and is provided with a belt deviation correction mechanism 29 that corrects belt deviation by tilting a rotating member such as the tension roller 5 that supports the transfer body. This allows the transfer body to run stably in the belt width direction, as described in the embodiment. [Explanation of symbols]

[0104] 1: Photoreceptor 3: Intermediate transfer belt 4: Secondary transfer opposing roller 5: Tension roller 5a: Tension roller shaft 7: Entrance roller 11: Primary transfer roller 20: Belt cleaning device 21: Cleaning blade 29: Belt deviation correction mechanism 30: Belt deviation detection member 30a: Flange part 30b: Cylindrical part 31: Axis tilt member 31f: Inclined surface 32: Tension spring 33: Tension roller bearing member 34: Roller shaft support member 35: Axis guide part 36: Rotation axis 37: Intermediate transfer frame 39: Support shaft 40: Support spring 42: Spring 43: Positioning stopper on the front side of the device 44: Positioning stopper at the back of the device 45: Main structure 47: Backup roller 48d: Positioning holder 49: Photoconductor frame 52: Holding member 53: Positioning protrusion 55: Pressed part 56: Positioning recess on the front side of the device 57: Positioning recess on the back side of the device 60: Intermediate transfer unit 63: Positioning part on the front side of the device 64: Slider contact area 70: Pressure spring 80: Belt contact / separation mechanism 81: Slide member 82: Cam member 82a: Camshaft 83: Pressurized relay member 84: Contact spring 85: Spring support 86: Cam contact part 87: First pressing part 88: Second pressing part 89: Left side wall 90:Through hole 92: Pressing part 100: Printer [Prior art documents] [Patent documents]

[0105] [Patent Document 1] Patent No. 5862130

Claims

1. a nip forming member that forms a transfer nip by bringing a transfer body onto which an image is transferred from an image carrier into contact with the image carrier; a pressure spring that presses a holding member that holds the nip forming member to position the nip forming member at a nip forming position where the transfer body abuts on the image carrier; a separating mechanism that separates the transfer body from the image carrier by moving the nip forming member from the nip forming position to a retracted position by sliding a slide member in a direction opposite to a biasing direction of the pressure spring against the holding member, an end portion of the pressure spring opposite to an end portion of the pressure spring that biases the holding member, the end portion being engaged with the slide member;

2. 2. The image forming apparatus according to claim 1, The holding member is supported so as to be rotatable, The image forming apparatus is characterized in that the separation mechanism includes a pressure relay member having a first pressing portion that presses the holding member with the biasing force of the pressure spring, and a second pressing portion that moves together with the slide member and presses the holding member in the opposite direction.

3. 3. The image forming apparatus according to claim 2, an amount of sliding of the slide member until the slide member abuts against the pressure relay member and the pressure relay member starts to move together with the slide member is less than an amount of movement of the nip forming member from the nip forming position to the retracted position.

4. 2. The image forming apparatus according to claim 1, An image forming apparatus characterized in that the holding member is abutted against an image carrier holding member that holds the image carrier, or a photosensitive member positioning member that positions the image carrier within the apparatus main body, thereby positioning the nip forming member at the nip forming position.

5. 2. The image forming apparatus according to claim 1, a plurality of the image carriers and the nip forming members; The image forming apparatus is characterized in that the spacing mechanism moves a plurality of nip forming members by sliding the slide member.

6. 6. The image forming apparatus according to claim 5, a backup roller disposed upstream of the plurality of nip forming members in the direction of movement of the transfer body, for setting the most upstream transfer nip in the direction of movement of the transfer body in a target state; The image forming apparatus is characterized in that the backup roller is held by a holding member that holds a nip forming member disposed at the most upstream side in the direction of movement of the transfer member.

7. 2. The image forming apparatus according to claim 1, the transfer body is an intermediate transfer body, The image forming apparatus is characterized in that the nip forming member is a primary transfer member that primarily transfers the image from the image carrier to the intermediate transfer member.

8. 2. The image forming apparatus according to claim 1, the transfer body is a belt member, The image forming apparatus is characterized by comprising a belt deviation correction mechanism that corrects belt deviation by tilting a rotating member that supports the transfer body.

Images