Image forming device

Abstract

To prevent scattering of toner to a conveyance guide in a configuration having a pressing member that presses a belt from the back side.SOLUTION: An image forming apparatus 100 has: a belt 6; a plurality of tension rollers including an inner roller 21 and an upstream roller 24; an external member 9 that forms a transfer part N2; a guide member 11 that guides a recording material P to the transfer part N2; a sheet-like pressing member 26 that can be in contact with an inner peripheral surface of the belt 6; and a coating member 29 that is provided on a surface facing the belt 6 of the pressing member 26 such that it does not contact the belt 6. The surface resistivity of the coating member 29 is lower than the surface resistivity of the pressing member 26, and the coating member 29 is electrically grounded.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus such as a copying machine, printer, plotter, facsimile machine, or multifunction machine having a plurality of functions among these, which uses an electrophotographic system or an electrostatic recording system. [Background technology]

[0002] Conventionally, some image forming apparatuses using electrophotography or the like have an endless belt (hereinafter simply referred to as a "belt") as an image carrier that carries a toner image. For example, such a belt is an intermediate transfer belt that serves as a second image carrier and transports a toner image that has been primarily transferred from a photosensitive member or the like as a first image carrier onto a sheet-like recording material such as paper for secondary transfer. The following description mainly focuses on an image forming apparatus that employs an intermediate transfer method and has an intermediate transfer belt.

[0003] In an image forming apparatus using an intermediate transfer belt, a toner image formed on a photosensitive element or the like in an image forming unit is primarily transferred to the intermediate transfer belt in a primary transfer unit. The toner image primarily transferred to the intermediate transfer belt is then secondarily transferred to a recording material in a secondary transfer unit. A secondary transfer unit (secondary transfer nip) is formed as a contact area between the intermediate transfer belt and the outer member by an inner member (secondary transfer inner member) provided on the inner peripheral surface of the intermediate transfer belt and an outer member (secondary transfer outer member) provided on the outer peripheral surface of the intermediate transfer belt. The inner member is an inner roller (secondary transfer inner roller), which is one of multiple tension rollers that tension the intermediate transfer belt. The outer member is often an outer roller (secondary transfer outer roller), which is positioned opposite the inner roller across the intermediate transfer belt and pressed against the inner roller. Then, by applying a voltage of the opposite polarity to the charge polarity of the toner to the outer roller (or applying a voltage of the same polarity as the charge polarity of the toner to the inner roller), the toner image on the intermediate transfer belt is secondarily transferred onto the recording material at the secondary transfer section. In addition, a conveyance guide is generally provided upstream of the secondary transfer section in the conveyance direction of the recording material to guide the recording material to the secondary transfer section.

[0004] In recent years, with the diversification of recording materials in the commercial printing market, there has been a demand for satisfying image quality specifications under a variety of conditions, from thin paper with low stiffness to thick paper with high stiffness. Here, the stiffness of the recording material changes the behavior of the recording material upstream of the secondary transfer unit in the recording material's transport direction, which can affect the resulting image. For example, depending on the recording material's stiffness, when the leading or trailing edge of the recording material in the recording material's transport direction enters the secondary transfer unit, vibration of the intermediate transfer belt upstream of the secondary transfer unit can easily occur, resulting in image defects ("shock images" at the leading and trailing edges of the recording material).

[0005] Therefore, a configuration is known in which the shape of the tension surface of the intermediate transfer belt near the entrance of the secondary transfer unit is changed. For example, Patent Document 1 discloses a configuration in which a pressing member is provided to press the intermediate transfer belt from the back side, and the amount of penetration of this pressing member into the intermediate transfer belt is changed depending on the thickness of the recording material. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent No. 4680721 Summary of the Invention [Problem to be solved by the invention]

[0007] However, the pressing member that presses the intermediate transfer belt from the backside can become charged due to friction with the moving intermediate transfer belt, etc. In particular, near the secondary transfer section, the amount of charge on the pressing member can become relatively large due to the influence of high pressure applied to the transfer members (inner roller and outer roller).

[0008] When the pressing member becomes charged, an electric field is formed between the pressing member and a transport guide located upstream of the secondary transfer unit in the recording material transport direction. This electric field can cause toner to scatter from the intermediate transfer belt onto the transport guide, contaminating the guide. If the transport guide becomes contaminated with toner, the toner can unintentionally adhere to the recording material, causing toner contamination on the recording material. This requires the user or a service technician to clean the transport guide. Therefore, it is desirable to suppress toner scattering onto the transport guide, thereby reducing the risk of toner contamination on the recording material and reducing the service load (cleaning work).

[0009] SUMMARY OF THE INVENTION It is therefore an object of the present invention to suppress toner scattering onto a conveyance guide in a configuration having a pressing member that presses the belt from the rear surface. [Means for solving the problem]

[0010] The above object is achieved by an image forming apparatus according to the present invention. an image forming unit that forms a toner image; Toner image is transcribed a rotatable endless belt; and a plurality of tension rollers for tensioning the belt, the plurality of tension rollers including an inner roller and an upstream roller disposed adjacent to the inner roller and upstream of the inner roller in the rotation direction of the belt; Through the belt an outer member disposed opposite the inner roller and contacting the outer peripheral surface of the belt to form a transfer section for transferring a toner image from the belt to a recording material; a guide member for guiding the recording material to the transfer section; and a guide member disposed upstream of the inner roller in the direction of rotation of the belt. side and downstream of the upstream roller side a sheet-like pressing member that can contact the inner circumferential surface of the belt, and a surface of the pressing member that faces the belt Set in Vignettes , covering a part of the surface of the pressing member a covering member, the covering member is provided on the pressing member so as not to come into contact with the belt, and is electrically grounded or electrically conducted to the guide member; The covering member teeth, surface resistivity but the pressing portion Material Lower It is made of a high-quality material The image forming apparatus is characterized by the above. [Effects of the Invention]

[0012] According to the present invention, in a configuration having a pressing member that presses the belt from the rear surface, it is possible to suppress toner scattering onto the transport guide. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a schematic cross-sectional view of an image forming apparatus. [Figure 2] FIG. 4 is a schematic side view showing a pressing mechanism. [Figure 3] 10 is a graph showing the relationship between the rotation angle of the cam and the penetration amount of the pressing member. FIG. [Figure 4] FIG. 2 is a schematic block diagram showing a control mode of a main part of the image forming apparatus. [Figure 5] 1 is a schematic cross-sectional view of the vicinity of a pressing member showing the configuration of Experimental Example 1 (Example). [Figure 6] FIG. 10 is a schematic cross-sectional view of the vicinity of a pressing member showing the configuration of Experimental Example 2 (Example). [Figure 7] FIG. 10 is a schematic cross-sectional view of the vicinity of a pressing member showing the configuration of Experimental Example 3 (Example). [Figure 8] FIG. 10 is a schematic cross-sectional view of an image forming apparatus showing the configuration of Experimental Example 4 (Example). [Figure 9] FIG. 10 is a schematic cross-sectional view of the vicinity of a pressing member showing the configuration of Experimental Example 7 (Example). [Figure 10] FIG. 10 is a schematic cross-sectional view of the vicinity of a pressing member, showing the configuration of Experimental Example 8 (Comparative Example). [Figure 11] 10 is a table showing the results of an evaluation experiment. DETAILED DESCRIPTION OF THE INVENTION

[0014] The image forming apparatus according to the present invention will be described in more detail below with reference to the drawings.

[0015] [Example 1] 1. Overall configuration and operation of the image forming apparatus 1 is a schematic cross-sectional view of an image forming apparatus 100 according to this embodiment. The image forming apparatus 100 according to this embodiment is a tandem multifunction peripheral (having the functions of a copier, printer, and facsimile machine) that employs an intermediate transfer method. The image forming apparatus 100 can form a full-color image on a sheet-like recording material (transfer material, sheet material, recording medium, media) P such as paper using an electrophotographic method in response to an image signal transmitted from an external device such as a personal computer.

[0016] Image forming apparatus 100 has four image forming units (stations) SY, SM, SC, and SK that form images of yellow (Y), magenta (M), cyan (C), and black (K), respectively. These image forming units SY, SM, SC, and SK are arranged in a row along the direction of movement of the image transfer surface of intermediate transfer belt 6, which is arranged substantially horizontally, as described below. Elements in each image forming unit SY, SM, SC, and SK that have the same or corresponding functions or configurations may be described collectively by omitting the Y, M, C, or K suffix to the reference numeral indicating that the element is for one of the colors. In this embodiment, the image forming unit S is configured to include photosensitive drums 1 (1Y, 1M, 1C, 1K), charging devices 2 (2Y, 2M, 2C, 2K), exposure devices 3 (3Y, 3M, 3C, 3K), developing devices 4 (4Y, 4M, 4C, 4K), primary transfer rollers 5 (5Y, 5M, 5C, 5K), drum cleaning devices 7 (7Y, 7M, 7C, 7K), etc., which will be described later.

[0017] The photosensitive drum 1, a rotatable drum-type (cylindrical) photosensitive member (electrophotographic photosensitive member) serving as a first image carrier for carrying a toner image, receives a driving force from a drum drive motor (not shown) serving as a drive source and is driven to rotate in the direction of arrow R1 (counterclockwise) in FIG. 1 . The surface of the rotating photosensitive drum 1 is uniformly charged to a predetermined potential of a predetermined polarity (negative in this embodiment) by a charging device 2 serving as a charging means. The charged surface of the photosensitive drum 1 is scanned and exposed by an exposure device 3 serving as an exposure means in accordance with image information (image signals), forming an electrostatic latent image (electrostatic image) on the photosensitive drum 1. In this embodiment, the exposure device 3 is a laser scanner device that irradiates the photosensitive drum 1 with laser light modulated in accordance with the image information (image signal). The electrostatic latent image formed on the photosensitive drum 1 is developed (visualized) by a developing device 4 serving as a developing means, which supplies toner as a developer, thereby forming a toner image (toner image, developer image) on the photosensitive drum 1. In this embodiment, toner charged with the same polarity (negative in this embodiment) as the charge polarity of the photosensitive drum 1 adheres to the exposed area (image area) on the photosensitive drum 1, where the absolute value of the potential has been reduced by exposure after being uniformly charged (reverse development method). In this embodiment, the normal charge polarity of the toner, which is the charge polarity of the toner during development, is negative.

[0018] An intermediate transfer belt 6, a rotatable intermediate transfer body formed of an endless belt, is disposed facing the four photosensitive drums 1Y, 1M, 1C, and 1K and serves as a second image carrier for carrying a toner image. The intermediate transfer belt 6 is stretched around a plurality of support rollers (support rollers), including a drive roller 22, an upstream auxiliary roller 23, a downstream auxiliary roller 25, a tension roller 20, a pre-secondary transfer roller 24, and an inner roller 21, and is stretched under a predetermined tension. The drive roller 22 transmits a driving force to the intermediate transfer belt 6. The tension roller 20 applies a predetermined tension to the intermediate transfer belt 6, controlling the tension of the intermediate transfer belt 6 to a constant level. The pre-secondary transfer roller 24 forms the surface of the intermediate transfer belt 6 upstream of a secondary transfer section N2 (described later) in terms of the rotation direction (surface movement direction, running direction, transport direction) of the intermediate transfer belt 6. The inner roller (secondary transfer opposing roller, secondary transfer inner roller, inner member) 21 functions as an opposing member (opposite electrode) of the outer roller 9, which will be described later. The upstream auxiliary roller 23 and downstream auxiliary roller 25 form an image transfer surface that is disposed substantially horizontally. The drive roller 22 is driven to rotate by a driving force transmitted from a belt drive motor (not shown) serving as a drive source. As a result, the intermediate transfer belt 6 receives driving force from the drive roller 22 and rotates (circumferentially moves) in the direction of arrow R2 (clockwise) in FIG. 1. In this embodiment, the intermediate transfer belt 6 is driven to rotate at a peripheral speed of 150 to 470 mm / sec. The tension rollers other than the drive roller 22 among the multiple tension rollers are rotated in accordance with the rotation of the intermediate transfer belt 6. Primary transfer rollers 5Y, 5M, 5C, and 5K, which are roller-shaped primary transfer members serving as primary transfer means, are arranged on the inner circumferential surface of the intermediate transfer belt 6, corresponding to the photosensitive drums 1Y, 1M, 1C, and 1K, respectively. The primary transfer roller 5 presses the intermediate transfer belt 6 toward the photosensitive drum 1, forming a primary transfer portion (primary transfer nip portion) N1 as a primary transfer portion that is a contact portion between the photosensitive drum 1 and the intermediate transfer belt 6. Furthermore, on the inner circumferential surface side of the intermediate transfer belt 6, a pressing member 26 is provided upstream of the inner roller 21 and downstream of the pre-secondary transfer roller 24 in the rotation direction of the intermediate transfer belt 6.The pressing member 26 can contact the inner peripheral surface of the intermediate transfer belt 6 and press the intermediate transfer belt 6 from the inner peripheral surface side to the outer peripheral surface side. As a result, the pressing member 26 can cause the tension surface T (FIG. 2) of the intermediate transfer belt 6 formed between the inner roller 21 and the pre-secondary transfer roller 24 to protrude from the inner peripheral surface side to the outer peripheral surface side of the intermediate transfer belt 6. The pressing member 26 and the pressing mechanism 16 (FIG. 2) that changes the position of the pressing member 26 will be described in further detail below.

[0019] The toner images formed on the photosensitive drums 1 as described above are primarily transferred onto the rotating intermediate transfer belt 6 at the primary transfer portion N1 by the action of the primary transfer rollers 5. During the primary transfer, a primary transfer voltage (primary transfer bias), which is a constant-voltage controlled DC voltage of a polarity opposite to the normal charging polarity of the toner (positive polarity in this embodiment) is applied to the primary transfer rollers 5 by a primary transfer power supply (not shown). For example, when forming a full-color image, toner images of yellow, magenta, cyan, and black formed on each photosensitive drum 1 are sequentially primarily transferred onto the intermediate transfer belt 6 so as to be superimposed on the same image forming area. In this embodiment, the primary transfer portion N1 is the image forming position where the toner images are formed on the intermediate transfer belt 6. The intermediate transfer belt 6 is an example of a rotatable endless belt that transports the toner images carried at the image forming position.

[0020] On the outer peripheral surface of the intermediate transfer belt 6, an outer roller (secondary transfer outer roller, secondary transfer roller, outer member) 9, which is a roller-shaped secondary transfer member (transfer rotating body) serving as secondary transfer means, is disposed at a position facing the inner roller 21. The outer roller 9 is pressed against the inner roller 21 via the intermediate transfer belt 6, forming a secondary transfer portion (secondary transfer nip portion) N2, which serves as a secondary transfer portion, at the contact portion between the intermediate transfer belt 6 and the outer roller 9. The toner image formed on the intermediate transfer belt 6 as described above is secondarily transferred onto the recording material P being conveyed between the intermediate transfer belt 6 and the outer roller 9 at the secondary transfer portion N2 by the action of the outer roller 9. In this embodiment, during secondary transfer, a secondary transfer voltage (secondary transfer bias), which is a constant-voltage controlled DC voltage of a polarity opposite to the normal charging polarity of the toner (positive polarity in this embodiment), is applied to the outer roller 9 by a secondary transfer power source (high-voltage application means) 10. In this embodiment, for example, a secondary transfer voltage of +1 to +7 kV is applied and a secondary transfer current of +40 to +120 μA is passed, thereby secondarily transferring the toner image on the intermediate transfer belt 6 onto the recording material P. In this embodiment, the inner roller 21 is electrically grounded (connected to ground). Alternatively, the inner roller 21 may be used as a secondary transfer member and a secondary transfer voltage of the same polarity as the normal charging polarity of the toner may be applied thereto, and the outer roller 9 may be used as an opposing electrode and electrically grounded.

[0021] The recording material P is conveyed to the secondary transfer portion N2 in synchronization with the toner image on the intermediate transfer belt 6. That is, the recording material P stored in a recording material cassette 63 serving as a recording material storage portion is conveyed to a registration roller (pair of registration rollers) 8 serving as a conveying member serving as a conveying means by a feeding roller or the like serving as a feeding member serving as a feeding means, and is then stopped temporarily. The registration roller 8 is then rotated so that the toner image on the intermediate transfer belt 6 and the desired image formation area on the recording material P coincide with each other at the secondary transfer portion N2, and the recording material P is sent to the secondary transfer portion N2.

[0022] A conveyance guide 11 for guiding the recording material P to the secondary transfer portion N2 is provided downstream of the registration rollers 8 and upstream of the secondary transfer portion N2 in the conveyance direction of the recording material P. The conveyance guide 11 includes a first guide member (upper guide member) 11a that can contact the front surface of the recording material P (the surface onto which the toner image is transferred immediately after passing through the conveyance guide 11) and a second guide member (lower guide member) 11b that can contact the back surface of the recording material P (the surface opposite the front surface). The first guide member 11a and the second guide member 11b are disposed opposite each other, and the recording material P passes between these two members. The first guide member 11a restricts the movement of the recording material P in a direction toward the intermediate transfer belt 6. The second guide member 11b restricts the movement of the recording material P in a direction away from the intermediate transfer belt 6. The conveying guide 11 (first and second guide members 11a, 11b) is provided to improve the conveying accuracy when supplying the recording material P to the secondary transfer portion N2. In this embodiment, the conveying guide 11 (first and second guide members 11a, 11b) is formed from a metal plate made of a metal (conductive material) such as SUS (stainless steel). Also, in this embodiment, the length of the conveying guide 11 (first and second guide members 11a, 11b) in the longitudinal direction (width direction of the intermediate transfer belt 6) is equal to the length of the intermediate transfer belt 6 in the width direction.

[0023] The recording material P with the transferred toner image is conveyed by a pre-fixing conveying device 41 to a fixing device 40, which serves as a fixing unit. The pre-fixing conveying device 41 has a rotatable belt member, which is 100 to 110 mm wide and 1 to 3 mm thick and made of a rubber material such as EPDM, at its center in a direction substantially perpendicular to the conveyance direction of the recording material P. The pre-fixing conveying device 41 conveys the recording material P on the belt member. The belt member has holes with a diameter of 3 to 7 mm, which allow air to be drawn in from the inner peripheral surface, thereby increasing the holding force of the recording material P and stabilizing the conveyance of the recording material P. The fixing device 40 applies heat and pressure to the recording material P carrying the unfixed toner image while it is being sandwiched between a pair of fixing rotors and conveyed, thereby fixing (melting and adhering) the toner image to the surface of the recording material P. Thereafter, the recording material P on which the toner image has been fixed is discharged (output) to a discharge tray 15 provided outside (outside the machine) of the device main body 110 of the image forming device 100 by a discharge roller or other discharge member serving as a discharge means.

[0024] Meanwhile, toner remaining on the photosensitive drum 1 after the primary transfer (primary transfer residual toner) is removed from the photosensitive drum 1 and collected by a drum cleaning device 6 serving as a photosensitive body cleaning means. Also, toner remaining on the intermediate transfer belt 6 after the secondary transfer (secondary transfer residual toner) and deposits such as paper dust adhering from the recording material P are removed from the intermediate transfer belt 6 and collected by a belt cleaning device 12 serving as an intermediate transfer body cleaning means. In this embodiment, the belt cleaning device 12 electrostatically collects and cleans deposits such as secondary transfer residual toner on the intermediate transfer belt 6.

[0025] In this embodiment, an intermediate transfer belt unit 17 serving as a belt conveying device is configured by having the intermediate transfer belt 6 stretched over a plurality of tension rollers, the primary transfer rollers 5, the belt cleaning device 12, a frame supporting these, etc. The intermediate transfer belt unit 17 is detachable from the device main body 110 for maintenance or replacement.

[0026] The intermediate transfer belt 6 may be made of a single-layer or multi-layer resin material, or may be made of a multi-layer elastic material having an elastic layer.

[0027] In this embodiment, the primary transfer roller 5 is configured by providing an elastic layer made of ion-conductive foam rubber on the outer periphery of a metal core (core material). In this embodiment, the primary transfer roller 5 has an outer diameter of 15 to 20 mm, and an electrical resistance of 1×10 when measured by applying a voltage of 2 kV in an environment of 23°C and 50% RH. 5 ~1×10 8 It is Ω.

[0028] In this embodiment, the outer roller 9 is configured by providing an elastic layer of ion-conductive foamed rubber on the outer periphery of a metal core (core material). In this embodiment, the outer roller 9 has an outer diameter of 20 to 25 mm, and its electrical resistance is 1×10 when measured by applying a voltage of 2 kV in an environment of 23°C and 50% RH. 5 ~1×10 8 Ω. In this embodiment, both ends of the outer roller 9 in the direction of its rotation axis are rotatably supported by bearings. These bearings are slidable toward and away from the inner roller 21, and are pressed toward the inner roller 21 by a pressure spring (not shown) that is a compression spring, which is a biasing member (elastic member) serving as biasing means. As a result, the outer roller 9 comes into contact with the inner roller 21 with a predetermined pressure across the intermediate transfer belt 6, forming the secondary transfer portion N2.

[0029] In this embodiment, the inner roller 21 is configured by providing an elastic layer of electronically conductive rubber on the outer periphery of a metal core (core material). In this embodiment, the inner roller 21 has an outer diameter of 20 to 22 mm, and its electrical resistance is 1×10 when measured by applying a voltage of 50 V in an environment of 23°C and 50% RH. 5 ~1×10 8 The secondary transfer pre-roller 24 may have the same configuration as the inner roller 21, for example.

[0030] In this embodiment, the rotation axes of the tension rollers for the intermediate transfer belt 6, including the inner roller 21, and the outer roller 9 are substantially parallel to each other.

[0031] 2. Pressing member, pressing mechanism Next, the pressing member 26 in this embodiment and the pressing mechanism (position changing mechanism, variable mechanism) 16 that changes the position of the pressing member 26 will be described. Figures 2(a) and 2(b) are schematic side views of the vicinity of the secondary transfer portion N2 in this embodiment, viewed from one end side of the inner roller 21 in the rotational axis direction (the front side of the paper in Figure 1) in a direction substantially parallel to the rotational axis direction. Figure 2(a) shows a state in which the pressing member 26 presses the intermediate transfer belt 6 with a predetermined pressing force, and Figure 2(b) shows a state in which the pressing member 26 is separated from the intermediate transfer belt 6. Figures 2(a) and 2(b) show the configuration of one end side of the inner roller 21 in the rotational axis direction, but the configuration of the other end side is similar (substantially symmetrical with respect to the center of the inner roller 21 in the rotational axis direction).

[0032] In this embodiment, the image forming apparatus 100 has a sheet-like (plate-like) pressing member (backup sheet, backup member) 26. The pressing member 26 presses the inner circumferential surface of the intermediate transfer belt 6 near the entrance of the secondary transfer portion N2, causing the intermediate transfer belt 6 to bulge outward. The pressing member 26 is disposed so as to be able to contact the inner circumferential surface of the intermediate transfer belt 6 upstream of the inner roller 21 and downstream of the pre-secondary transfer roller 24 in the rotation direction of the intermediate transfer belt 6. In particular, in this embodiment, the pressing member 26 is disposed so as to be able to contact the inner circumferential surface of the intermediate transfer belt 6 upstream of the inner roller 21 and downstream of the downstream tip of the conveyance guide 11 (first and second guide members 11a and 11b) in the conveyance direction of the recording material P (FIG. 5).

[0033] In this embodiment, the pressing member 26 is a sheet-like (plate-like) member that is generally rectangular in plan view and has a predetermined length in both a longitudinal direction (which is generally parallel to the width direction of the intermediate transfer belt 6) and a lateral direction (which is generally perpendicular to the longitudinal direction) and a predetermined thickness. The width direction of the intermediate transfer belt 6 is generally perpendicular to the direction of movement of the surface of the intermediate transfer belt 6. The longitudinal length of the pressing member 26 is equal to the width length of the intermediate transfer belt 6. As long as the pressing member 26 can perform the function of pressing the intermediate transfer belt 6, the longitudinal length of the pressing member 26 may be generally the same as the width length of the intermediate transfer belt 6, or may be shorter or longer than the width length. The pressing member 26 has a free end, which is one end in the lateral direction (the end downstream in the rotation direction of the intermediate transfer belt 6), that can contact the inner circumferential surface of the intermediate transfer belt 6 over substantially the entire width of the intermediate transfer belt 6 and press the intermediate transfer belt 6. The end of the pressing member 26 on the free end side in the lateral direction is also referred to as the tip 26a, and the end on the fixed end side is also referred to as the base end 26b.

[0034] In this embodiment, the image forming apparatus 100 has a pressing mechanism (position changing mechanism, movement mechanism) 16. The pressing mechanism 16 changes the position of a pressing member 26 to change at least one (in this embodiment, both) of the penetration amount (pressing amount) of the pressing member 26 into the intermediate transfer belt 6 and the state of contact or separation of the pressing member 26 with respect to the intermediate transfer belt 6. Note that, for simplicity, changing the penetration amount (pressing amount) of the pressing member 26 into the intermediate transfer belt 6 may be described as including changing the state of contact or separation of the pressing member 26 with respect to the intermediate transfer belt 6. Also, for simplicity, changing (adjusting) the position of the pressing member 26 may be described as simply changing (adjusting) the penetration amount (pressing amount).

[0035] The pressing member 26 is rotatably held by a holding member (pressing member holder) 28. One end of the pressing member 26 in the short direction (the end on the upstream side in the rotation direction of the intermediate transfer belt 6), which is a fixed end, is fixed to the holding member 28 over substantially the entire longitudinal width. The holding member 28 is supported by a frame of the intermediate transfer belt unit 17 or the like so as to be rotatable about a rotation axis (pressing member rotation axis) 28a. In this manner, the position of the pressing member 26 can be changed by rotating the holding member 28 about the rotation axis 28a and rotating the pressing member 26 about the rotation axis 28a. This makes it possible to change the amount of penetration (pressure) of the pressing member 26 into the intermediate transfer belt 6 and at least one (in this embodiment, both) of the state of contact or separation of the pressing member 26 with respect to the intermediate transfer belt 6.

[0036] The holding member 28 is configured to rotate by the action of a cam (pressure cam) 27 serving as an actuating member. The cam 27 is supported on a frame of the intermediate transfer belt unit 17 or the like so as to be rotatable about a cam rotation shaft 27a. The cam 27 rotates about the cam rotation shaft 27a upon receiving drive from a cam drive motor 211 (FIG. 4) serving as a drive source. The cam 27 also contacts a cam follower 28b provided on the holding member 28. In this embodiment, the holding member 28 is biased by a holding member biasing means (not shown) formed of a tension spring or the like, which is a biasing member (elastic member), so that the cam follower 28b rotates in a direction engaging with the cam 27. In this embodiment, the image forming apparatus 100 is also provided with a cam position sensor (cam HP sensor) 212 (FIG. 4) as position detection means for detecting the position of the cam 27 in the rotation direction, particularly, in this embodiment, the home position (HP) in the rotation direction. The cam position sensor 212 can be configured to include, for example, the cam 27 or a flag provided coaxially with the cam 27 as an indicator, and a photointerrupter as a detector.

[0037] As described above, in this embodiment, the pressing mechanism 16 is configured to include the holding member 28, the cam 27, the cam drive motor 211, the cam position sensor 212, and a holding member biasing means (not shown).

[0038] 2(a), when the pressing member 26 presses the intermediate transfer belt 6, the cam 27 is driven by the cam drive motor 211 to rotate clockwise. This causes the holding member 28 to rotate counterclockwise around the rotation shaft 28a, and the pressing member 26 is positioned at a position where the pressing member 26 penetrates the intermediate transfer belt 6 by a predetermined amount. At this time, the tip 26a of the pressing member 26 comes into contact with the inner circumferential surface of the intermediate transfer belt 6 near the entrance of the secondary transfer unit N2, causing the intermediate transfer belt 6 to protrude toward the outer circumferential surface.

[0039] 2(b), when the pressing member 26 is separated from the intermediate transfer belt 6, the cam 27 is driven by the cam drive motor 211 to rotate counterclockwise. As a result, the holding member 28 rotates clockwise around the rotation shaft 28a, and the pressing member 26 is placed in a position where it is separated from the intermediate transfer belt 6.

[0040] 2(a), when the pressing member 26 comes into contact with the intermediate transfer belt 6 and presses the intermediate transfer belt 6 with a predetermined pressing force, the tension T of the intermediate transfer belt 6 changes, and the tension in the vicinity of the secondary transfer portion N2 increases. This makes it possible to suppress vibration of the intermediate transfer belt 6 and reduce "shock images" at the leading and trailing ends of the recording material P.

[0041] In this embodiment, the cam 27 is shaped so that the amount of penetration (pressure) of the pressing member 26 into the intermediate transfer belt 6 changes depending on the rotation angle. As a result, in this embodiment, the amount of penetration (pressure) of the pressing member 26 into the intermediate transfer belt 6 can be adjusted by controlling the rotation angle of the cam 27. In this embodiment, a control unit 200 (FIG. 4), which will be described later, controls a cam drive motor 211 to control the pressing member 26 so that it presses the intermediate transfer belt 6 with a predetermined pressure or so that the pressing member 26 moves away from the intermediate transfer belt 6. FIG. 3 is a graph showing the relationship between the rotation angle of the cam 27 and the amount of penetration (pressure) of the pressing member 26 into the intermediate transfer belt 6 in this embodiment.

[0042] In this embodiment, the initial setting value (predetermined pressing force) of the pressing member 26 for the amount of penetration (pressure) into the intermediate transfer belt 6 is set to 1.0 to 3.0 mm. In this embodiment, the pressing member 26 can be positioned at a distance from the intermediate transfer belt 6, at a position where the pressing member 26 comes into contact with the intermediate transfer belt 6 and the amount of penetration (pressure) is 0 to 3.0 mm. Although not limited to this, the amount of penetration (pressure) is preferably about 3.5 mm or less. If the amount of penetration (pressure) is greater than this, the load on the contact surface between the pressing member 26 and the intermediate transfer belt 6 increases, which may make it difficult for the intermediate transfer belt 6 to rotate smoothly.

[0043] Here, it is desirable that the pressing member 26, more specifically, the tip 26a of the pressing member 26, be as close as possible to the inner roller 21, but not come into contact with the inner roller 21. The pressing member 26 is positioned so that the tip 26a of the pressing member 26 comes into contact with the inner circumferential surface of the intermediate transfer belt 6 at a position, for example, about 2 mm or more, typically about 10 mm or more, upstream in the rotation direction of the intermediate transfer belt 6 from the position where the inner roller 21 and the intermediate transfer belt 6 contact each other. The pressing member 26 is also positioned so that the tip 26a of the pressing member 26 comes into contact with the inner circumferential surface of the intermediate transfer belt 6 at a position, for example, about 40 mm or less, typically about 25 mm or less, upstream in the rotation direction of the intermediate transfer belt 6 from the position where the inner roller 21 and the intermediate transfer belt 6 contact each other. This allows the shape of the tensioned surface of the intermediate transfer belt 6 near the entrance of the secondary transfer unit N2 to be sufficiently changed. In other words, typically, the pressing member 26 is positioned so that the tip 26a of the pressing member 26 contacts the back surface of the intermediate transfer belt 6 at a position 10 to 40 mm upstream from the position where the inner roller 21 and the intermediate transfer belt 6 contact each other, so as not to come into contact with the inner roller 21.

[0044] Furthermore, it is sufficient that the amount of penetration (pressure) of the pressing member 26 into the intermediate transfer belt 6 is a desired value when the recording material P is near the entrance of the secondary transfer portion N2 and when it is passing through the secondary transfer portion N2. More specifically, the vicinity of the entrance of the secondary transfer portion N2 is a region corresponding to a region of the intermediate transfer belt 6 between the position where the pressing member 26 contacts the intermediate transfer belt 6 and the secondary transfer portion N2 in the conveyance direction of the recording material P.

[0045] Furthermore, if the image forming apparatus 100 is left with the pressing member 26 positioned to press the intermediate transfer belt 6, this may cause deformation of the pressing member 26 over time. Therefore, for example, when the image forming apparatus 100 is in a power-off state or sleep state, the pressing member 26 can be positioned away from the intermediate transfer belt 6, as shown in FIG. 2(b).

[0046] The amount of pressure applied by the pressing member 26 to the intermediate transfer belt 6 can be expressed, for example, by the amount of penetration of the pressing member 26 into the intermediate transfer belt 6. This penetration amount is roughly the amount by which the pressing member 26 causes the intermediate transfer belt 6 to protrude outward from a tension surface (tension surface) T of the intermediate transfer belt 6 formed by tensioning the intermediate transfer belt 6 between the inner roller 21 or the outer roller 9 and the secondary pre-transfer roller 24. The secondary pre-transfer roller 24 is an example of an upstream roller that is disposed adjacent to the inner roller 21 and upstream of the inner roller 21 in the rotation direction of the intermediate transfer belt 6 among the multiple tension rollers.

[0047] 3. Control mode 4 is a schematic block diagram showing the control mode of the main parts of the image forming apparatus 100 of this embodiment. The control unit 200 as a control means is configured with a CPU as an arithmetic control means which is a central element for performing arithmetic processing, memories (storage media) such as ROM and RAM as storage means, and an interface unit (input / output circuit). The RAM, which is a rewritable memory, stores information input to the control unit 200, detected information, arithmetic results, etc., while the ROM stores control programs, pre-determined data tables, etc. The CPU and memory can transfer and read data to and from each other. The interface unit controls the input and output (communication) of signals between the control unit 200 and devices connected thereto.

[0048] The control unit 200 is connected to each unit of the image forming apparatus 100 (such as the image forming unit S, the drive devices for the members related to the conveyance of the intermediate transfer belt 6 and the recording material P, and various power supplies). In this embodiment, the control unit 200 has, as its functional blocks, a calculation unit 201, a drive control unit 210, and a memory unit 220. In this embodiment, the calculation unit 201 and the drive control unit 210 are realized by the CPU operating in accordance with a predetermined program. In this embodiment, the memory unit 220 is realized by the memory. The drive control unit 210 is connected to drive units of each unit of the image forming apparatus 100, such as a cam drive motor 211, a drum drive motor, and a belt drive motor. The drive control unit 210 operates the drive units of each unit of the image forming apparatus 100, such as the cam drive motor 211, in response to commands from the calculation unit 201.

[0049] An operation unit (operation panel) 120 provided in the image forming apparatus 100 is also connected to the control unit 200. The operation unit 120 has a display unit (display means) that displays information under the control of the control unit 200, and an input unit (input means) that inputs information to the control unit 200 through operation by an operator such as a user or a service representative (here, the user will be described as a representative). The operation unit 120 may be configured with a touch panel that has the functions of the display means and the input means. The control unit 200 may also be connected to an external device (not shown) such as an image reading device (not shown) provided in or connected to the image forming apparatus 100, or a personal computer connected to the image forming apparatus 100.

[0050] The control unit 200 controls each unit of the image forming apparatus 100 to perform image forming operations based on information about a job (a series of operations that starts with one start instruction and forms and outputs an image on one or more recording materials P). The job information includes a start instruction (start signal) input from the operation unit 120 or an external device, and information (command signal) about image forming conditions such as the type of recording material P. The job information also includes image information (image signal) input from the image reading device, the external device, or the operation unit 120.

[0051] 4. Adjusting the position of the pressing member Next, a method for adjusting the amount of penetration (the position of the pressing member 26) in this embodiment will be specifically described.

[0052] 4, in this embodiment, the storage unit 220 stores cam shaft position information 222 acquired from a cam position sensor 212 that detects the home position (HP) of the cam 27. In this embodiment, the storage unit 220 also stores a pressure amount conversion table 223 for rotationally driving the cam 27 to a predetermined position in order to obtain an optimal image.

[0053] 3 shows the relationship between the rotation angle of cam 27 and the amount of penetration. Based on this pressure amount conversion table 223 and cam shaft position information 222, calculation unit 201 determines the rotation angle of cam 27 required to set the amount of penetration to a predetermined amount. Then, in accordance with the result, drive control unit 210 operates cam drive motor 211 by the required control amount to rotate cam 27, thereby setting the amount of penetration to the predetermined amount.

[0054] In this embodiment, the user instructs the control unit 200 to adjust the amount of penetration via the input unit of the operation unit 101. Then, the calculation unit 201 of the control unit 200 reflects information on the amount of penetration specified by the user via the input unit of the operation unit 101 in the operation of the cam drive motor 211. Note that, although in this embodiment, the adjustment of the amount of penetration is instructed via the operation unit 101, it may also be possible to instruct it from an external device communicatively connected to the image forming apparatus 100. In this case, the interface unit (input / output circuit) and the like described above function as the input unit.

[0055] 5. Detailed configuration of pressing member Next, the detailed configuration of the pressing member 26 in this embodiment will be described. Figure 5 is a schematic cross-sectional view of the pressing member 26 and its vicinity in this embodiment, viewed in a direction substantially parallel to the rotation axis of the inner roller 21.

[0056] In this embodiment, a covering member (shielding member, conductive member) 29 is provided on the surface of the pressing member 26 facing the intermediate transfer belt 6 so as not to come into contact with the intermediate transfer belt 6. In this embodiment, the covering member 29 is attached to the surface of the pressing member 26 facing the intermediate transfer belt 6. The covering member 29 does not come into contact with the intermediate transfer belt 6 regardless of the position of the pressing member 26 within its movable range. In this embodiment, as described above, the pressing member 26 is made of a sheet-like member that is approximately rectangular in plan view. In this embodiment, the covering member 29 is also made of a sheet-like member that is approximately rectangular in plan view, having a predetermined length and a predetermined thickness in both the longitudinal direction, which is disposed approximately parallel to the width direction of the intermediate transfer belt 6, and the lateral direction, which is approximately perpendicular to the longitudinal direction.

[0057] The pressing member 26 is formed using a material such as a polyester resin, such as PPS (polyphenylene sulfide) or PET (polyethylene terephthalate), or a resin, such as PEEK (polyether ether ketone), that generally has no electrical conductivity. In this embodiment, the pressing member 26 is formed using PPS. The thickness of the pressing member 26 is approximately 0.4 to 1.5 mm, typically approximately 0.5 to 1.0 mm. The pressing member 26 formed using such a resin material can elastically bias the intermediate transfer belt 6 by utilizing its flexural elasticity. Meanwhile, the covering member 29 is formed using a material that generally has electrical conductivity, such as a metal, such as copper foil tape or SUS sheet metal, or a conductive resin, such as a resin sheet containing a conductive material as a filler. In this embodiment, the covering member 29 is formed using a polyethylene sheet containing carbon black. In this embodiment, the covering member 29 is attached to the surface of the pressing member 26 facing the intermediate transfer belt 6 using conductive double-sided tape. The covering member 29 can be fixed to the pressing member 26 by any fixing means such as adhesion, adhesion, fusion, fitting (locking), etc. In this embodiment, the covering member 29 is electrically grounded (connected to the main body earth).

[0058] The surface resistivity of the covering member 29 (the surface exposed on the intermediate transfer belt 6 side) is lower than the surface resistivity of the pressing member 26 (the surface exposed on the intermediate transfer belt 6 side). The surface resistivity of the polyethylene sheet containing carbon black used as the covering member 29 in this example was measured using an ASP probe of Loresta GP (Mitsubishi Chemical) and was found to be 1.0 × 10 8 The surface resistivity of the covering member 29 (the surface exposed on the intermediate transfer belt 6 side) was set to 1.0×10 8 On the other hand, the surface resistivity of the pressing member 26 (the surface exposed on the intermediate transfer belt 6 side) is typically 10 13 Ω / □ or more (generally 10 18 Ω / □ or less).

[0059] In this embodiment, the covering member 29 is provided on the surface of the pressing member 26 facing the intermediate transfer belt 6 from downstream of a portion 6a of the conveyance guide 11 facing the intermediate transfer belt 6 to upstream of the portion 6a in the conveyance direction of the intermediate transfer belt 6, so as to cover the pressing member 26. That is, when viewed along the rotational axis direction of the inner roller 21 (the width direction of the intermediate transfer belt 6), the covering member 29 is provided on the surface of the pressing member 26 facing the intermediate transfer belt 6 from downstream of the position to upstream of the position in the movement direction of the intermediate transfer belt 6, so as to include a position intersecting a line substantially perpendicular to the surface of the intermediate transfer belt 6 that passes through the portion of the conveyance guide 11 closest to the intermediate transfer belt 6. In this embodiment, the downstream leading edge of the first guide member 11a of the conveyance guide 11 in the conveyance direction of the recording material P is closest to the intermediate transfer belt 6. However, the part closest to the intermediate transfer belt 6 may be a part other than the tip of the first guide member 11a, or the tip of the second guide member 11b or another part may be closest to the intermediate transfer belt 6. In this embodiment, the closest distance between the conveying guide 11 and the intermediate transfer belt 6 is about 2 to 3 mm.

[0060] More specifically, in this embodiment, the covering member 29 is provided continuously on the surface of the pressing member 26 facing the intermediate transfer belt 6 in a predetermined region from the base end 26b to the tip end 26a in the lateral direction, including the region where the conveying guide 11 faces the intermediate transfer belt 6 at the point 6a where it is closest to the intermediate transfer belt 6. Moreover, in this embodiment, the covering member 29 is not provided on the surface of the pressing member 26 facing the intermediate transfer belt 6 in the remaining region from the tip end 26a to the base end 26b in the lateral direction. Moreover, in this embodiment, the covering member 29 is provided continuously so as to cover substantially the entire region of the pressing member 26 in the longitudinal direction.

[0061] In addition, the pressing member can also be considered to be composed of a member having at least two layers: a main portion (non-conductive portion, pressing portion, first sheet portion) and a covering portion (conductive portion, shielding portion, second sheet portion).

[0062] 6. Evaluation Experiment The results of evaluation experiments conducted on several configurations including the configuration of this embodiment will be described.

[0063] 6-1. Experimental example <Experimental Example 1> This example has the configuration of the present embodiment described above.

[0064] <Experimental Example 2> The configuration of this example is the same as that of Example 1 (Experimental Example 1) except for the detailed configuration of the pressing member 26. Figure 6 is a schematic cross-sectional view of the vicinity of the pressing member 26 in this example, viewed approximately parallel to the rotation axis direction of the inner roller 21.

[0065] In this example, the covering member 29 is electrically connected to the transport guide 11 (first and second guide members 11a and 11b). In this example, the covering member 29 and the transport guide 11 (first and second guide members 11a and 11b) are electrically floating. Other detailed configurations of the pressing member 26 are the same as those in Example 1 (Experimental Example 1).

[0066] <Experimental Example 3> The configuration of this example is the same as that of Example 1 (Experimental Example 1) except for the detailed configuration of the pressing member 26. Figure 7 is a schematic cross-sectional view of the vicinity of the pressing member 26 in this example, viewed approximately parallel to the rotation axis direction of the inner roller 21.

[0067] In this example, a covering member 29 made of a coating layer obtained by coating the surface of the pressing member 26 with a conductive material was provided in an area corresponding to the area where the covering member 29 was provided in the pressing member 26 of Example 1 (Experimental Example 1). Specifically, the pressing member 26 was masked with masking tape except for the area where the covering member 29 was to be provided, and platinum was vapor-deposited by sputtering to provide the covering member 29. The other detailed configuration of the pressing member 26 was the same as that of Example 1 (Experimental Example 1).

[0068] <Experimental Example 4> This example has the same configuration as that of Example 1 (Experimental Example 1) except for the configuration for applying the secondary transfer voltage at the secondary transfer portion N2. Figure 8 is a schematic cross-sectional view of the image forming apparatus 100 of this example.

[0069] In this example, a secondary transfer voltage, which is a constant voltage controlled DC voltage of the same polarity as the normal charging polarity of the toner, is applied to the inner roller 21 from the secondary transfer power supply 10. For example, a secondary transfer voltage of -1 to -7 kV is applied and a secondary transfer current of -40 to -120 μA is passed, thereby secondarily transferring the toner image on the intermediate transfer belt 6 onto the recording material P. The rest of the configuration of the image forming apparatus 100 is the same as that of Example 1 (Experimental Example 1).

[0070] <Experimental Example 5> In this example, the covering member 29 is not provided, and a secondary transfer voltage, which is a constant voltage controlled DC voltage of the same polarity as the normal charging polarity of the toner, is applied to the inner roller 21 from the secondary transfer power source 10. The other configurations of the image forming apparatus 100 are the same as those of Example 1 (Experimental Example 1).

[0071] <Experimental Example 6> In this example, there is no covering member 29. The other configurations of the image forming apparatus 100 are the same as those in Example 1 (Experimental Example 1).

[0072] <Experimental Example 7> The configuration of this example is the same as that of Example 1 (Experimental Example 1) except for the detailed configuration of the pressing member 26. Figure 9 is a schematic cross-sectional view of the vicinity of the pressing member 26 in this example, viewed approximately parallel to the rotation axis direction of the inner roller 21.

[0073] In this example, the covering member 29 is attached to the pressing member 26 in a narrowed area only on the upstream side in the conveyance direction of the intermediate transfer belt 6 so as not to include the area facing the intermediate transfer belt 6 at the point 6a where the conveyance guide 11 is closest to the intermediate transfer belt 6. However, even in this example, the covering member 29 is provided on the surface of the pressing member 26 facing the intermediate transfer belt 6 so that at least a portion of the covering member 29 and the conveyance guide 11 overlap with each other in the conveyance direction of the intermediate transfer belt 6. More specifically, in this example, the covering member 29 is provided continuously on the surface of the pressing member 26 facing the intermediate transfer belt 6 in a predetermined area from the base end 26b to the tip end 26a in the lateral direction of the pressing member 26, in an area closer to the base end 26b than the area where the conveyance guide 11 faces the intermediate transfer belt 6 at the point 6a where the conveyance guide 11 is closest to the intermediate transfer belt 6. Furthermore, in this example, the covering member 29 is not provided on the surface of the pressing member 26 facing the intermediate transfer belt 6 in the remaining area from the tip end 26a to the base end 26b in the lateral direction of the pressing member 26. Other details of the configuration of the pressing member 26 are the same as those in Example 1 (Experimental Example 1).

[0074] <Experimental Example 8> The configuration of this example is the same as that of Example 1 (Experimental Example 1) except for the detailed configuration of the pressing member 26. Figure 10 is a schematic cross-sectional view of the vicinity of the pressing member 26 in this example, viewed approximately parallel to the rotation axis direction of the inner roller 21.

[0075] In this example, the covering member 29 is attached to the pressing member 26 with an area expanded so as to come into contact with the intermediate transfer belt 6. More specifically, in this example, the covering member 29 is provided continuously from the base end 26b to the tip 26a in the short direction of the pressing member 26 so as to include the area facing the intermediate transfer belt 6 at the point 6a where the conveying guide 11 is closest to the intermediate transfer belt 6. Other detailed configurations of the pressing member 26 are the same as those in Example 1 (Experimental Example 1).

[0076] 6-2. Experimental methods and results A paper feed durability test was conducted for the configurations of Experimental Examples 1 to 8 using an image forming apparatus (copier) 100 configured according to this embodiment. Specifically, a durability test was conducted in a low-humidity environment (temperature 23°C, relative humidity 5%) using an imagePRESS C910 (manufactured by Canon Inc.) in which 100,000 sheets of GF-C081 A4 paper were fed, and the contamination of the conveying guide 11 was observed. The results were evaluated as follows: substantially no contamination was observed, "good," slight contamination, "fair," potentially problematic contamination, "poor," and significant contamination, "xx" (significantly poor). Secondary color full-surface solid images of magenta and cyan were also output, and the presence or absence of transfer defects was observed. The results were evaluated as "good," no transfer defects, and "poor," respectively. The evaluation results for Experimental Examples 1 to 8 are shown in FIG. 11 . The surface potentials of the intermediate transfer belt 6 and the conveying guide 11 were measured using a commercially available surface potential meter. For convenience, the measurement result of the surface potential of the intermediate transfer belt 6 is shown, but it is known that the surface potential of the pressing member 26 is approximately the same as the surface potential of the intermediate transfer belt 6.

[0077] First, significant contamination was observed in Experimental Example 5 (Comparative Example). The surface potential of the intermediate transfer belt 6 was measured at the point 6a where the transport guide 11 was closest to the intermediate transfer belt 6, and it was found to be charged to -2 kV. Furthermore, the surface potential of the transport guide 11 was measured while the paper was passing through it, and it was found to be charged to only about -0.2 kV. In other words, a potential difference of -1.8 kV occurred between the intermediate transfer belt 6 and the transport guide 11, and it is believed that this potential difference caused toner, which has a predominantly negative charge polarity, to scatter onto the transport guide 11.

[0078] Next, in Experimental Example 6 (Comparative Example), problematic staining was observed. The surface potentials of the intermediate transfer belt 6 and the transport guide 11 were measured in the same manner as above, and were found to be +1.0 kV and -0.2 kV, respectively. A potential difference of +1.2 kV occurred between the intermediate transfer belt 6 and the transport guide 11. However, unlike Experimental Example 5, the intermediate transfer belt 6 had a higher potential in the positive polarity direction than the transport guide 11, which is why toner with a polarity opposite to the primary charge polarity (positive polarity) is thought to have scattered onto the transport guide 11. Because the amount of toner with a polarity opposite to the primary charge polarity (positive polarity) was small, it is thought that the staining was lighter than in Experimental Example 5.

[0079] Next, slight staining was observed in Experimental Example 7 (Example). The reason for the slight staining in Experimental Example 7 is as follows. That is, the covering member 29 was not attached to the area facing the intermediate transfer belt 6 at the point 6a where the conveying guide 11 is closest to the intermediate transfer belt 6. Therefore, the surface potential of the intermediate transfer belt 6 was relatively high at +1.0 kV, and the effect of reducing the potential difference between the intermediate transfer belt 6 and the conveying guide 11 was thought to be reduced compared to Experimental Example 1, which will be described later.

[0080] Next, in Experimental Example 8 (Comparative Example), although no contamination was observed on the conveying guide 11, transfer defects were observed. This is thought to be because the conductive covering member 29 was in contact with the intermediate transfer belt 6, causing the transfer current that should have flowed at the secondary transfer portion N2 to leak into the covering member 29.

[0081] On the other hand, in Experimental Examples 1 to 4 (Examples), no staining of the transport guide 11 or transfer defects were observed, and good results were obtained. In Experimental Examples 1, 3, and 4, the conductive covering member 29 was electrically grounded, thereby keeping the surface potential of the intermediate transfer belt 6 low. As a result, the potential difference between the intermediate transfer belt 6 and the transport guide 11 was small. In Experimental Example 2, although the surface potential of the intermediate transfer belt 6 was high at +1.0 kV, the conductive covering member 29 was electrically connected to the conductive transport guide 11, thereby reducing the potential difference between the intermediate transfer belt 6 and the transport guide 11. For these reasons, it is believed that good results were obtained in Experimental Examples 1 to 4.

[0082] 7.Effects As described above, the image forming apparatus 100 according to the embodiment of the present invention includes a rotatable endless belt 6 carrying a toner image, a plurality of tension rollers around which the belt 6 is tensioned, including an inner roller 21 and an upstream roller 24 disposed adjacent to the inner roller 21 and upstream of the inner roller 21 in the direction of rotation of the belt 6, an outer member 9 disposed opposite the inner roller 21 and contacting the outer surface of the belt 6 to form a transfer portion N2 where the toner image is transferred from the belt 6 to a recording material P, a guide member 11 that guides the recording material P to the transfer portion N2, a sheet-like pressing member 26 that is capable of contacting the inner surface of the belt 6 upstream of the inner roller 21 and downstream of the upstream roller 24 in the direction of rotation of the belt 6, and a covering member 29 provided on the belt-side surface of the pressing member 26 so as not to come into contact with the belt 6, the surface resistivity of the covering member 29 being lower than that of the pressing member 26, and the covering member 29 being electrically grounded. The covering member 29 may also be electrically connected to the guide member 11. When a plurality of guide members 11 are provided, it is preferable that the covering member 29 be electrically connected to at least the guide member 11 closest to the belt 6 .

[0083] Preferably, the covering member 29 is provided so that at least a portion of the guide member 11 overlaps with the covering member 29 in the moving direction of the belt 6. More preferably, the covering member 29 is provided from the downstream side of the region facing the belt 6 at the point 6a where the guide member 11 is closest to the belt 6 to the upstream side of the region in the moving direction of the belt 6. The pressing member 26 is disposed such that its longitudinal direction is substantially parallel to the width direction of the belt 6, its upstream end in the moving direction of the belt 6 in the lateral direction is held by the holding member 28, and its downstream end in the moving direction of the belt 6 in the lateral direction is capable of contacting the inner circumferential surface of the belt 6. Preferably, the surface resistivity of the covering member 29 is 1×10 8 The surface resistivity of the pressing member 26 is typically 1×10 13 The resistance is Ω / □ or more. During transfer, a voltage of the opposite polarity to the normal charging polarity of the toner is applied to the outer member 9, or a voltage of the same polarity as the normal charging polarity of the toner is applied to the inner roller 21. In this embodiment, the belt 6 is an intermediate transfer member that transports the toner image that has been primarily transferred from the image carrier to the recording material P for secondarily transferring at the transfer section N2.

[0084] The length of the covering member 29 in the longitudinal direction of the pressing member 26 is typically approximately the same as the longitudinal length of the pressing member 26. However, it may be shorter than the longitudinal length of the pressing member 26 as long as toner scattering onto the transport guide 11 is sufficiently reduced. Furthermore, the covering member 29 may be discontinuous and divided in at least one of the longitudinal and lateral directions of the pressing member 26 as long as toner scattering onto the transport guide 11 is sufficiently reduced. However, from the perspective of reducing toner scattering onto the guide member 11, it is more preferable that the covering member 29 be provided continuously so as to cover approximately the entire longitudinal area of ​​the pressing member 26 that faces the guide member 11. Furthermore, the pressing member 26 may be attached to a mounting portion such as a sheet metal made of a metal (conductive material) such as SUS, and this mounting portion may be fixed to the holding member with screws or the like. In this case, the covering member may be electrically grounded or electrically connected to the transport guide 11 via the conductive mounting portion.

[0085] As described above, according to the configuration of the embodiment of the present invention, by providing an area covered with a covering member (shielding member, conductive member) 29 on the surface of the pressing member 26, it is possible to reduce toner scattering onto the conveying guide 11. In other words, according to the embodiment of the present invention, in a configuration having a pressing member that presses the belt from the backside, it is possible to suppress toner scattering onto the conveying guide. This reduces the risk of toner contamination of the recording material P and also reduces the service load (cleaning work).

[0086] [others] Although the present invention has been described above with reference to specific embodiments, the present invention is not limited to the above-described embodiments.

[0087] In the above-described embodiment, an outer roller that directly contacts the outer peripheral surface of the intermediate transfer belt is used as the outer member that forms the secondary transfer nip together with the inner roller serving as the inner member. Alternatively, the outer roller and the secondary transfer belt that is stretched between the outer roller and another roller may be used as the outer member. That is, the image forming apparatus may have, as the outer member, a stretching roller, an outer roller, and a secondary transfer belt that is stretched between these rollers. The outer roller may then contact the outer peripheral surface of the intermediate transfer belt via the secondary transfer belt. In such a configuration, the intermediate transfer belt and the secondary transfer belt are sandwiched between the inner roller that contacts the inner peripheral surface of the intermediate transfer belt and the outer roller that contacts the inner peripheral surface of the secondary transfer belt, thereby forming the secondary transfer nip. In this case, the contact portion between the intermediate transfer belt and the secondary transfer belt is the secondary transfer nip, serving as the secondary transfer portion.

[0088] In the above-described embodiment, an actuator that operates the movable part using a cam is used as the pressing mechanism, but the present invention is not limited to this. The pressing mechanism may be any mechanism that can achieve the same operation as in the above-described embodiment, and may be, for example, an actuator that operates the movable part using a solenoid.

[0089] Furthermore, the pressing member is not limited to being movable, and may be fixed so that the pressing member penetrates the belt by a predetermined amount.

[0090] In the above embodiment, the belt-shaped image carrier is an intermediate transfer belt, but the present invention can be applied to any image carrier that is an endless belt that transports a toner image carried at an image forming position. Examples of such a belt-shaped image carrier include the intermediate transfer belt in the above embodiment, a photosensitive belt, and an electrostatic recording dielectric belt.

[0091] The present invention can also be implemented in other embodiments in which some or all of the configurations of the above-described embodiments are replaced with alternative configurations. Therefore, as long as the image forming apparatus uses a belt-shaped image carrier, it can be implemented regardless of whether it is a tandem type or a single-drum type, a charging method, an electrostatic image forming method, a developing method, a transfer method, or a fixing method. While the above-described embodiment has focused on the main components related to the formation / transfer of a toner image, the present invention can be implemented in a variety of applications, such as printers, various printing machines, copiers, fax machines, and multifunction machines, by adding the necessary devices, equipment, and housing structures. [Explanation of symbols]

[0092] 1 Photosensitive drum 5 Primary transfer roller 6 Intermediate transfer belt 9 Outer roller 10 Secondary transfer power supply 11 Transport guide 21 Inner roller 26 Pressing member 29 Covering materials

Claims

1. an image forming unit that forms a toner image; a rotatable endless belt onto which a toner image is transferred; a plurality of tension rollers for tensioning the belt, the plurality of tension rollers including an inner roller and an upstream roller disposed adjacent to the inner roller and upstream of the inner roller in the rotation direction of the belt; an outer member that is disposed opposite the inner roller with the belt interposed therebetween and that forms a transfer section that contacts the outer peripheral surface of the belt to transfer a toner image from the belt to a recording material; a guide member that guides the recording material to the transfer section; a sheet-like pressing member that is capable of contacting an inner circumferential surface of the belt upstream of the inner roller and downstream of the upstream roller in the rotation direction of the belt; a covering member provided on a surface of the pressing member facing the belt, the covering member covering a part of the surface of the pressing member; and an image forming apparatus characterized in that the covering member is provided on the pressing member so as not to come into contact with the belt, and is electrically grounded or electrically conductive with the guide member, and the covering member is formed of a material having a surface resistivity lower than that of the pressing member.

2. 2. The image forming apparatus according to claim 1, wherein the covering member is provided so that at least a portion of the covering member and at least a portion of the guide member are located at the same position in the moving direction of the belt.

3. 2. The image forming apparatus according to claim 1, wherein the covering member is provided to include an area facing the belt at a position where the guide member is closest to the belt, and is defined from the downstream side of the area to the upstream side of the area in the direction of movement of the belt.

4. 2. The image forming apparatus according to claim 1, wherein the pressing member is arranged along the width direction of the belt so that the upstream end of the pressing member in the direction of movement of the belt is held by a holding member and the downstream end of the pressing member in the direction of movement of the belt can contact the inner surface of the belt.

5. The covering member is 1×10 ８ 2. The image forming apparatus according to claim 1, wherein the surface resistivity is Ω / □ or less.

6. The pressing member is 1×10 １３ 2. The image forming apparatus according to claim 1, wherein the surface resistivity is Ω / □ or more.

7. 2. The image forming apparatus according to claim 1, wherein a voltage having a polarity opposite to a normal charging polarity of the toner is applied to the external member during the transfer.

8. 2. The image forming apparatus according to claim 1, wherein a voltage having the same polarity as the normal charging polarity of the toner is applied to the inner roller during the transfer.

9. 2. The image forming apparatus according to claim 1, wherein the belt is an intermediate transfer member that conveys a toner image that is primarily transferred from an image carrier and then secondarily transferred onto a recording material at the transfer section.

Images