Image forming device

Abstract

To provide an image forming apparatus that can improve transferability for each of a plurality of types of recording materials in mixed job, while preventing a reduction in productivity.SOLUTION: In an image forming apparatus 100, during execution of a job for forming images on a plurality of recording materials S and outputting the materials, when a control unit 150 changes the relative positions of an inner roller 32 and an outer member 41 with respect to a circumferential direction of the inner roller 32, in a period from when a preceding recording material S passes through a transfer unit N2 until when a subsequent recording material S reaches the transfer unit N2, the control unit controls a position changing mechanism 1 to change the relative positions in a state where the outer member 41 is in contact with a belt 31.SELECTED DRAWING: Figure 9

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus such as a copying machine, a printer, or a facsimile machine that uses an electrophotographic or electrostatic recording method. [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 intermediate transfer image forming apparatus, a toner image formed on a photosensitive element or the like in an image forming unit is primarily transferred to an 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 nip, which serves as a secondary transfer unit and is a contact point between the intermediate transfer belt and the outer member, is formed by an inner member (secondary transfer inner member) provided on the inner peripheral surface of the secondary transfer belt and an outer member (secondary transfer outer member) provided on the outer peripheral surface of the secondary transfer belt. The inner member is an inner roller, which is one of multiple tension rollers that tension the intermediate transfer belt. The outer member is often an outer roller, which is positioned opposite the inner roller across the intermediate transfer belt. For example, a secondary transfer voltage of a polarity opposite to the charge polarity of the toner is applied to the outer roller, thereby secondarily transferring the toner image on the intermediate transfer belt to the recording material in the secondary transfer nip. In general, a conveyance guide for guiding the recording material to the secondary transfer nip is provided upstream of the secondary transfer nip in the conveyance direction of the recording material.

[0004] The shape of the secondary transfer nip affects the behavior of the recording material upstream and downstream of the secondary transfer nip in the recording material's transport direction. In recent years, there has been a demand for compatibility with a variety of recording materials with different stiffness due to differences in thickness and surface properties. The stiffness of the recording material also affects the behavior of the recording material upstream and downstream of the secondary transfer nip in the recording material's transport direction. For example, if the recording material is thin paper, an example of a recording material with low stiffness, the intermediate transfer belt and the recording material may stick together downstream of the secondary transfer nip in the recording material's transport direction, resulting in poor separation of the recording material from the intermediate transfer belt and resulting in a paper jam. This phenomenon is more pronounced when the recording material has low stiffness, as its weak stiffness makes it more likely to stick to the intermediate transfer belt.

[0005] On the other hand, for example, if the recording material is "cardboard," an example of a recording material with high stiffness, when the trailing edge of the recording material in the transport direction passes through the transport guide, the trailing edge of the recording material in the transport direction may collide with the intermediate transfer belt. This may disturb the posture of the intermediate transfer belt near the upstream side of the secondary transfer nip in the transport direction of the recording material, resulting in image defects (such as streak-like image disturbances extending in a direction approximately perpendicular to the transport direction of the recording material) at the trailing edge of the recording material in the transport direction. This phenomenon is more pronounced when the recording material has high stiffness, because the stiffness of the recording material makes it more likely that the trailing edge of the recording material in the transport direction will collide with the intermediate transfer belt with great force.

[0006] To address this issue, a configuration has been proposed in which the width of the secondary transfer nip in the rotation direction of the intermediate transfer belt is changed depending on the type of recording material (Patent Document 1). [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-134718 Summary of the Invention [Problem to be solved by the invention]

[0008] As described above, in order to improve the separation of the recording material from the intermediate transfer belt and to prevent image defects caused by the collision of the trailing edge of the recording material with the intermediate transfer belt in the transport direction, it is effective to change the width of the secondary transfer nip (position of the secondary transfer nip) in the rotation direction of the intermediate transfer belt depending on the type of recording material. The width of the secondary transfer nip can be changed by moving the inner roller or outer roller in a direction intersecting the pressing direction of the secondary transfer nip, changing the relative position of the inner roller and outer roller in the circumferential direction of the inner roller, and thereby changing the position of the secondary transfer nip.

[0009] In an image forming apparatus using an electrophotographic method or the like, a job (referred to herein as a "mixed job") for forming images on multiple types of recording materials, such as for bookbinding printing, may be executed. In a mixed job, changing the relative positions of the inner roller and the outer roller during the job is considered effective in achieving good transferability for each of the multiple types of recording materials with different stiffness. However, performing such an operation during the job may result in a decrease in productivity, for example, by requiring an increase in the distance between sheets. Therefore, it is important to perform the above operation while minimizing the decrease in productivity. For example, if an operation is performed to release the pressure between the inner roller and the outer roller in order to move the inner roller or the outer roller, the additional time required for this operation can significantly reduce productivity.

[0010] Note that the above has described conventional issues using the secondary transfer section, which is the section where a toner image is transferred from an intermediate transfer belt to a recording material, as an example, but similar issues exist with regard to sections where a toner image is transferred from other belt-like image carriers, such as photosensitive bodies, to a recording material.

[0011] SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide an image forming apparatus that can improve transferability for each of a plurality of types of recording materials in a mixed job while suppressing a decrease in productivity. [Means for solving the problem]

[0012] The above object is achieved by the image forming apparatus according to the present invention. In summary, the present invention provides an image forming apparatus including a belt that carries a toner image, a transfer roller that contacts the belt to form a transfer section that transfers the toner image, a first position change mechanism that changes the position of the transfer section relative to the rotation direction of the belt, a second position change mechanism that changes the position of the transfer roller between a first position where the transfer roller presses the belt to form the transfer section and a second position where the transfer roller does not press the belt, and a control unit that controls the first position change mechanism and the second position change mechanism, the control unit controls the first position change mechanism to change the position of the transfer unit relative to the rotation direction of the belt in accordance with information about the type of recording material; During the execution of a continuous image forming job in which images are formed and output on multiple recording materials In the case where the type of recording material on which an image is formed is changed from a first recording material of a first type to a second recording material of a second type, the position of the transfer unit relative to the rotation direction of the belt is changed, The control unit (i) controls the first position change mechanism and the second position change mechanism so as to change the position of the transfer unit with respect to the rotation direction of the belt while the transfer roller is pressing the belt, and (ii) after the change of the position of the transfer unit is completed, controls the second position change mechanism. Image formed on recording material The image forming apparatus is characterized in that the image forming operation is controlled so that formation of the latent image is started. According to another aspect of the present invention, a printing apparatus includes a belt that carries a toner image, a transfer roller that contacts the belt to form a transfer section that transfers the toner image, a position change mechanism that changes the position of the transfer roller so as to change the position of the transfer section relative to the rotation direction of the belt, the position change mechanism changing the position of the transfer roller between a first position where the transfer roller presses the belt to form the transfer section and a second position where the transfer roller does not press the belt, and a control unit that controls the position change mechanism, the control unit controls the position changing mechanism to change the position of the transfer unit relative to the rotation direction of the belt in accordance with information about the type of recording material; During the execution of a continuous image forming job in which images are formed and output on multiple recording materials In the case where the type of recording material on which an image is formed is changed from a first recording material of a first type to a second recording material of a second type, the position of the transfer unit relative to the rotation direction of the belt is changed, The control unit (i) controls the position change mechanism to change the position of the transfer unit relative to the rotation direction of the belt while the transfer roller is pressing the belt, and (ii) after the change in the position of the transfer unit is completed, controls the second Image formed on recording material The image forming apparatus is characterized in that the image forming operation is controlled so that formation of the latent image is started. [Effects of the Invention]

[0013] According to the present invention, it is possible to improve the transferability for each of a plurality of types of recording material in a mixed job while suppressing a decrease in productivity. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a schematic cross-sectional view of an image forming apparatus. [Figure 2] FIG. 2 is a schematic perspective view of the periphery of an intermediate transfer belt for explaining deviation control. [Figure 3] FIG. 10 is a schematic cross-sectional view for explaining an offset amount. [Figure 4] FIG. 10 is a schematic side view showing the offset mechanism. [Figure 5] FIG. 10 is a schematic side view showing a part of the offset mechanism. [Figure 6] FIG. 10 is a schematic diagram for explaining the arrangement of a rotation shaft of an inner roller holder. [Figure 7] FIG. 2 is a schematic side view showing a contact / separation mechanism. [Figure 8] FIG. 2 is a schematic block diagram showing a control mode of a main part of the image forming apparatus. [Figure 9] FIG. 10 is a flowchart showing an outline of the procedure of a job operation. [Figure 10] FIG. 10 is a flowchart outlining another example of the procedure of the job operation. [Figure 11] FIG. 10 is a schematic side view showing another example of an offset mechanism. [Figure 12] FIG. 10 is a schematic side view showing another example of the outer member. DETAILED DESCRIPTION OF THE INVENTION

[0015] The image forming apparatus according to the present invention will be described in more detail below with reference to the drawings.

[0016] [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 machine (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) S such as paper using an electrophotographic method in response to an image signal transmitted from an external device, for example.

[0017] Image forming apparatus 100 has a plurality of image forming units (stations), namely, four image forming units 10Y, 10M, 10C, and 10K that form images of yellow (Y), magenta (M), cyan (C), and black (K), respectively. These image forming units 10Y, 10M, 10C, and 10K are arranged in series along the direction of movement of the image transfer surface of intermediate transfer belt 31, which is disposed substantially horizontally, as described below. Elements in each image forming unit 10Y, 10M, 10C, and 10K that have the same or corresponding functions or configurations may be generally described 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 10 is configured to include photosensitive drums 11 (11Y, 11M, 11C, 11K), chargers 12 (12Y, 12M, 12C, 12K), exposure devices 13 (13Y, 13M, 13C, 13K), developing devices 14 (14Y, 14M, 14C, 14K), primary transfer rollers 35 (35Y, 35M, 35C, 35K), cleaning devices 15 (15Y, 15M, 15C, 15K), etc., which will be described later.

[0018] The photosensitive drum 11, a rotatable drum-type (cylindrical) photosensitive member (electrophotographic photosensitive member) serving as a first image carrier for carrying a toner image, receives driving force from a drum drive motor 111 (FIG. 8) serving as a drive source and is driven to rotate in the direction of arrow R1 (counterclockwise) in the drawing. The surface of the rotating photosensitive drum 11 is uniformly charged to a predetermined potential of a predetermined polarity (negative in this embodiment) by a charger 12 serving as a charging means. During charging, a predetermined charging voltage is applied to the charger 12 by a charging power source (not shown). The charged surface of the photosensitive drum 11 is scanned and exposed by an exposure device 13 serving as an exposure means (electrostatic image forming means) in accordance with an image signal, forming an electrostatic image (electrostatic latent image) on the photosensitive drum 11. In this embodiment, the exposure device 13 is configured as a laser scanner device that irradiates the photosensitive drum 11 with laser light modulated in accordance with the image signal. The electrostatic image formed on the photosensitive drum 11 is developed (visualized) by the developing device 14, which serves as a developing means, by supplying toner as a developer, and a toner image (developer image) is formed on the photosensitive drum 11. In this embodiment, toner charged with the same polarity as the charge polarity of the photosensitive drum 11 (negative polarity in this embodiment) adheres to the exposed portion (image portion) of the photosensitive drum 11, which has been uniformly charged and then exposed to light to reduce the absolute value of the potential (reverse development). The developing device 14 has a developing roller, which is a rotatable developer carrier, that carries the developer and transports it to a development position facing the photosensitive drum 11. The developing roller is rotated by a driving force transmitted from a developing motor 113 (FIG. 8) serving as a drive source. During development, a predetermined development voltage is applied to the developing roller by a development power source (not shown).

[0019] An intermediate transfer belt 31, which is a rotatable intermediate transfer member formed of an endless belt and serves as a second image carrier that carries a toner image, is disposed facing the four photosensitive drums 11Y, 11M, 11C, and 11K. The intermediate transfer belt 31 is stretched around a plurality of support rollers (support rollers), including a drive roller 33, a tension roller 34, a pre-secondary transfer roller 37, and an inner roller (secondary transfer opposing roller, inner member) 32, and is stretched with a predetermined tension. The drive roller 33 transmits a driving force to the intermediate transfer belt 31. The tension roller 34 applies a predetermined tension to the intermediate transfer belt 31. The pre-secondary transfer roller 37 forms a surface of the intermediate transfer belt 31 near the upstream side of a secondary transfer nip N2 (described below) in the rotation direction (travel direction) of the intermediate transfer belt 31. The inner roller 32 functions as an opposing member (opposing electrode) of an outer roller 41 (described below). The intermediate transfer belt 31 rotates (circumferentially moves) in the direction of arrow R2 (clockwise) in the figure when a driving force is transmitted from a belt drive motor 112 (FIG. 8) serving as a driving source (drive device) to the drive roller 33, which is driven to rotate. In this embodiment, the intermediate transfer belt 31 is driven to rotate at a peripheral speed of 400 mm / sec, for example. Of the multiple support rollers, the support rollers other than the drive roller 33 are driven to rotate in accordance with the rotation of the intermediate transfer belt 31. Primary transfer rollers 35Y, 35M, 35C, and 35K, which are roller-shaped primary transfer members serving as primary transfer means, are disposed on the inner circumferential surface of the intermediate transfer belt 31 and correspond to the photosensitive drums 11Y, 11M, 11C, and 11K, respectively. The primary transfer rollers 35 press the intermediate transfer belt 31 against the photosensitive drums 11 to form a primary transfer nip N1, which serves as a primary transfer portion and is a contact portion between the photosensitive drums 11 and the intermediate transfer belt 31. In this embodiment, the tension roller 34 also serves as a steering roller. That is, in this embodiment, the tension roller 34 applies a predetermined tension to the intermediate transfer belt 31 and corrects deviation of the intermediate transfer belt 31 (deviation of the running position in the width direction substantially perpendicular to the moving direction of the surface of the intermediate transfer belt 31) by tilting.

[0020] As described above, the toner images formed on the photosensitive drums 11 are primarily transferred onto the rotating intermediate transfer belt 31 at the primary transfer nip N1 by the action of the primary transfer rollers 35. During the primary transfer, a primary transfer voltage, which is a DC voltage of a polarity opposite to the normal charging polarity of the toner (the charging polarity of the toner during development), is applied to the primary transfer rollers 35 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 the photosensitive drums 11 are sequentially primarily transferred onto the intermediate transfer belt 31 so as to be superimposed on the same image forming area. In this embodiment, the primary transfer nip N1 is the image forming position where the toner images are formed on the intermediate transfer belt 31. The intermediate transfer belt 31 is an example of a rotatable endless belt that transports the toner images carried at the image forming position.

[0021] An outer roller (secondary transfer roller, outer member) 41, which is a roller-shaped secondary transfer member serving as secondary transfer means, is disposed on the outer peripheral surface of the intermediate transfer belt 31, facing the inner roller 32. The outer roller 41 is pressed against the inner roller 32 via the intermediate transfer belt 31, forming a secondary transfer nip N2, which serves as a secondary transfer portion where the intermediate transfer belt 31 and the outer roller 41 contact each other. The toner image formed on the intermediate transfer belt 31 as described above is secondarily transferred onto the recording material S, which is being conveyed while being sandwiched between the intermediate transfer belt 31 and the outer roller 41, by the action of the outer roller 41 in the secondary transfer nip N2. In this embodiment, during secondary transfer, a secondary transfer voltage, which is a DC voltage of a polarity opposite to the normal charging polarity of the toner, is applied to the outer roller 41 by a secondary transfer power source (not shown). In this embodiment, the inner roller 32 is electrically grounded (connected to ground). Alternatively, the inner roller 32 may be used as a secondary transfer member, and a secondary transfer voltage having the same polarity as the normal charging polarity of the toner may be applied to it, while the outer roller 41 may be used as a counter electrode and electrically grounded.

[0022] The recording material S is conveyed to the secondary transfer nip N2 in synchronization with the toner image on the intermediate transfer belt 31. That is, the recording material S stored in the recording material cassettes 61, 62, and 63 is fed out by the rotation of one of the feed rollers 71, 72, and 73. The recording material S passes through a feed conveying path 81 and is conveyed to a registration roller (pair of registration rollers) 74, which is a conveying member serving as a conveying means, and is temporarily stopped. The registration roller 74 is then rotated so that the toner image on the intermediate transfer belt 31 and the desired image formation area on the recording material S coincide with each other at the secondary transfer nip N2, thereby feeding the recording material S into the secondary transfer nip N2. A conveying guide 83 that guides the recording material S to the secondary transfer nip N2 is provided downstream of the registration roller 74 and upstream of the secondary transfer nip N2 in the conveying direction of the recording material S. The conveying guide 83 is configured to have a first guide member 83a that can come into contact with the front surface of the recording material S (the surface onto which a toner image is transferred immediately after passing through the conveying guide 83), and a second guide member 83b that can come into contact with the back surface of the recording material S (the surface opposite to the front surface). The first guide member 83a and the second guide member 83b are arranged opposite each other, and the recording material S passes between these two members. The first guide member 83a restricts the movement of the recording material S in a direction approaching the intermediate transfer belt 31. The second guide member 83b restricts the movement of the recording material S in a direction away from the intermediate transfer belt 31.

[0023] The recording material S onto which the toner image has been transferred is conveyed by a conveyor belt 42 to a fixing device 50 serving as a fixing means. The fixing device 50 applies heat and pressure to the recording material S bearing the unfixed toner image, thereby fixing (melting and adhering) the toner image to the surface of the recording material S. Thereafter, the recording material S onto which the toner image has been fixed passes through a discharge conveyance path 82 and is discharged (output) to a discharge tray 64 provided outside the apparatus main body 100a of the image forming apparatus 100.

[0024] Meanwhile, the toner remaining on the photosensitive drum 11 after the primary transfer (primary transfer residual toner) is removed from the photosensitive drum 11 and collected by a cleaning device 15 serving as cleaning means. Also, the toner remaining on the intermediate transfer belt 31 after the secondary transfer (secondary transfer residual toner) and adhering matters such as paper dust adhering from the recording material S are removed from the intermediate transfer belt 31 and collected by a belt cleaning device 36 serving as intermediate transfer body cleaning means.

[0025] In this embodiment, the intermediate transfer belt unit 30 serving as a belt conveying device includes the intermediate transfer belt 31 stretched over a plurality of tension rollers, primary transfer rollers 35, a belt cleaning device 36, and a frame supporting these components. The intermediate transfer belt unit 30 is detachable from the device main body 100a for maintenance or replacement.

[0026] Here, a belt made of a resin material with a single layer or multilayer structure can be used as the intermediate transfer belt 31. The intermediate transfer belt 31 has a thickness of 40 μm or more, a Young's modulus of 1.0 GPa or more, and a surface resistivity of 1.0×10 9 ~5.0×10 13 Those with a resistance of Ω / □ can be preferably used.

[0027] In this embodiment, the inner roller 32 is configured by providing an elastic layer (rubber layer) made of a rubber material as an elastic material on the outer periphery of a metal core (substrate). This elastic layer can be made of, for example, EPDM rubber (which may contain a conductive agent). In this embodiment, the inner roller 32 is configured to have an outer diameter of 20 mm and a thickness of the elastic layer of 0.5 mm. In this embodiment, the hardness of the elastic layer of the inner roller 32 is set to, for example, 70° (JIS-A). The inner roller 32 may also be configured as a metal roller made of a metal material such as SUM or SUS. The pre-secondary transfer roller 37 can have a configuration similar to that of the inner roller 32.

[0028] In this embodiment, the outer roller 41 is configured by providing a conductive elastic layer (which may be a solid rubber layer or a sponge layer (elastic foam layer)) made of a conductive rubber material as a conductive elastic material on the outer periphery of a metal core (substrate). This elastic layer may be formed of, for example, NBR rubber or EPDM rubber containing a conductive agent such as a metal complex or carbon. In this embodiment, the outer roller 41 is formed so that the outer diameter of the core is 12 mm, the thickness of the elastic layer is 6 mm, and the outer diameter of the outer roller 41 is 24 mm. In this embodiment, the hardness of the elastic layer of the outer roller 41 is set to, for example, 28° (Asker C). In this embodiment, the outer roller 41 is urged by a pressure spring 44 (FIG. 4), which is an urging member (elastic member) serving as urging means, so as to abut against the inner roller 32 with a predetermined pressure across the intermediate transfer belt 31.

[0029] In this embodiment, the rotation axes of the tension rollers for the intermediate transfer belt 31, including the inner roller 32, and the outer roller 41 are substantially parallel to each other. The support structure for the inner roller 32 and the outer roller 41 will be described in more detail later.

[0030] 2. Controlling intermediate transfer belt deviation The intermediate transfer belt 31 may shift due to misalignment of the tension rollers (alignment) or imbalance in pressure force, etc. The shift of the intermediate transfer belt 31 can be controlled by using at least one of the tension rollers as a steering roller and tilting the rotation axis of the steering roller relative to the rotation axis of the other support rollers to change the running direction of the intermediate transfer belt.

[0031] In this embodiment, the image forming apparatus 100 has a steering mechanism as a deviation control means for controlling deviation of the intermediate transfer belt 31. In this embodiment, the steering mechanism uses a signal from a sensor provided at an end of the intermediate transfer belt 31 in the width direction, and controls deviation by changing the alignment of the tension roller (also serving as a steering roller) 34 so that the detection value of the sensor becomes approximately constant. This will be explained in more detail below.

[0032] FIG. 2 is a schematic perspective view illustrating the steering mechanism 90 in this embodiment. As described above, in this embodiment, the tension roller 34 also serves as a steering roller. In this embodiment, the tension roller 34 is disposed downstream of the primary transfer nip N1 (the most downstream primary transfer nip N1K) and upstream of the secondary transfer nip N2 in the rotation direction of the intermediate transfer belt 31. As shown in FIG. 2, the multiple tension rollers may further include other tension rollers, such as auxiliary rollers 54 and 55, which form an image transfer surface disposed substantially horizontally in this embodiment. In the example shown in FIG. 2, the downstream auxiliary roller 54 is disposed downstream of the primary transfer nip N1 (the most downstream primary transfer nip N1K) and upstream of the tension roller 34 in the rotation direction of the intermediate transfer belt 31. In addition, the upstream auxiliary roller 55 is disposed downstream of the drive roller 33 and upstream of the primary transfer nip N1 (the most upstream primary transfer nip N1Y) in the rotation direction of the intermediate transfer belt 31. These auxiliary rollers 54 and 55 can be provided to, for example, block changes in the inclination of the intermediate transfer belt 31 that accompany the tilting of the tension roller 34, thereby maintaining the image transfer surface approximately horizontal.

[0033] The tension roller 34 is rotatably supported by the intermediate transfer belt unit 30 via bearing members (not shown) at both ends of the tension roller 34 in the direction of its rotation axis. The bearing members at both ends of the tension roller 34 in the direction of its rotation axis are supported so as to be slidable in a direction from the inner circumferential surface of the intermediate transfer belt 31 toward the outer circumferential surface thereof and in the reverse direction. The bearing members at both ends are pressed (biased) in a direction from the inner circumferential surface of the intermediate transfer belt 31 toward the outer circumferential surface thereof by a biasing force of a compression spring or other biasing member (elastic member) serving as a biasing means. This allows the tension roller 34 to apply a predetermined tension to the intermediate transfer belt 31. The bearing member at one end of the tension roller 34 in the direction of its rotation axis (the far side of the paper in FIG. 2 ) is rotatable about a rotation axis that is substantially perpendicular to the direction of the rotation axis of the tension roller 34. Furthermore, a bearing member at the other end of the tension roller 34 in the direction of its rotation axis (the front side of the paper in FIG. 2) is supported by the frame of the intermediate transfer belt unit 30 via a misalignment correction arm 94. This misalignment correction arm 94 is rotatable (swingable) about a rotation axis that is substantially parallel to the direction of the rotation axis of the tension roller 34. This allows the tension roller 34 to rotate so that the front end in FIG. 2 moves up and down in FIG. 2. By rotating the tension roller 34 in this manner, the tension roller 34 can be tilted so that the rotation axis of the tension roller 34 is inclined relative to the rotation axes of other support rollers such as the drive roller 33.

[0034] When the intermediate transfer belt 31 deviates toward the front or rear in FIG. 2, the widthwise edge of the intermediate transfer belt 31 causes the deviation detection sensor 93 to move in the direction of arrow IF or arrow IR in FIG. 2. A signal indicating the detection result of the deviation detection sensor 93 is input to a control unit 150 (FIG. 8), which will be described later. The control unit 150 drives a deviation correction motor 91 as a drive source in accordance with the widthwise running position of the intermediate transfer belt 31 detected by the deviation detection sensor 93. When the deviation correction motor 91 is driven, a deviation correction cam 95 rotates, causing a deviation correction arm 94 to swing. This causes the front end of the tension roller 34 in FIG. 2 to move up and down (in the direction of arrow SF or arrow SR), tilting the tension roller 34. This tilting of the tension roller 34 causes the intermediate transfer belt 31 to move in the direction of arrow IF or arrow IR in FIG. 2. By continuing these operations, the deviation of the intermediate transfer belt 31 is corrected.

[0035] The tilt position of the tension roller 34 is detected by an HP (home position) sensor 92, which is provided coaxially with the rotation axis of a misalignment correction cam 95. The misalignment detection sensor 93 is configured, for example, to include a flag that contacts the edge of the intermediate transfer belt 31 in the width direction, an LED as a light-emitting unit, and two photodiodes as light-receiving units. The amount of light received by the two photodiodes changes depending on the position of the flag of the misalignment detection sensor 93. By detecting this amount of received light, the running position of the intermediate transfer belt 31 in the width direction can be determined.

[0036] In this embodiment, the steering mechanism 90 includes a shift correction motor 91, an HP sensor 92, a shift detection sensor 93, a shift correction arm 94, a shift correction cam 95, and the like.

[0037] The configuration for controlling the deviation of the intermediate transfer belt 31 is not limited to that of this embodiment, and any known configuration may be used as appropriate. For example, there is a configuration that uses an automatic centering method that uses frictional force to automatically control the deviation without using a sensor.

[0038] 3. Offset 3 is a schematic cross-sectional view (a cross-section substantially perpendicular to the rotational axis direction of the inner roller 32) for explaining the behavior of the recording material S in the vicinity of the secondary transfer nip N2. In FIG. 3, elements having the same or corresponding functions or configurations as those in the image forming apparatus 100 of this embodiment are denoted by the same reference numerals.

[0039] As described above, the behavior of the recording material S in the upstream and downstream vicinity of the secondary transfer nip N2 in the conveyance direction of the recording material S changes depending on the shape of the secondary transfer nip N2 (the position of the secondary transfer nip N2) and the stiffness of the recording material S. For example, if the recording material S is "thin paper," which is an example of a recording material S with low stiffness, a jam (paper jam) may occur due to poor separation of the recording material S from the intermediate transfer belt 31. This phenomenon becomes more pronounced when the stiffness of the recording material S is low, because the recording material S is prone to sticking to the intermediate transfer belt 31 due to its weak stiffness.

[0040] 3, a line indicating the tension surface of the intermediate transfer belt 31 formed by tensioning the inner roller 32 and the pre-secondary transfer roller 37 is referred to as a pre-nip tension line T. The pre-secondary transfer roller 37 is an example of an upstream roller among the multiple tension rollers, disposed adjacent to the inner roller 32 and upstream of the inner roller 32 in the rotation direction of the intermediate transfer belt 31. Also, in the same cross section, a line passing through the rotation centers of the inner roller 32 and the outer roller 41 is referred to as a nip center line Lc. Also, in the same cross section, a line substantially perpendicular to the nip center line Lc is referred to as a nip line Ln. Note that FIG. 3 illustrates a state in which the rotation center of the outer roller 41 is offset upstream of the rotation center of the inner roller 32 in the rotation direction of the intermediate transfer belt 31 in the direction along the pre-nip tension line T.

[0041] At this time, when the recording material S is sandwiched between the inner roller 32 and the outer roller 41 at the secondary transfer nip N2, it tends to maintain its orientation substantially along the nip line Ln. Therefore, generally, when the rotation center of the inner roller 32 and the rotation center of the outer roller 41 are close to each other in the direction along the pre-nip tension wire T, the discharge angle θ of the recording material S becomes small, as shown by the dashed line A in FIG. 3 . In other words, the leading edge of the recording material S in the conveyance direction is oriented so as to be discharged close to the intermediate transfer belt 31 when it is discharged from the secondary transfer nip N2. This makes it easier for the recording material S to stick to the intermediate transfer belt 31. On the other hand, generally, the more upstream the rotation center of the outer roller 41 is in the rotation direction of the intermediate transfer belt 31 in the direction along the pre-nip tension wire T, the larger the discharge angle θ of the recording material S becomes, as shown by the solid line in FIG. 3 . That is, when the leading edge of the recording material S in the conveying direction is discharged from the secondary transfer nip N2, it is oriented so as to be discharged in a direction away from the intermediate transfer belt 31. This makes it difficult for the recording material S to stick to the intermediate transfer belt 31.

[0042] On the other hand, as described above, for example, if the recording material S is "cardboard," which is an example of a recording material S with high stiffness, the trailing end of the recording material S in the conveyance direction may collide with the intermediate transfer belt when the trailing end of the recording material S in the conveyance direction passes through the conveyance guide 83. This may result in image defects at the trailing end of the recording material S in the conveyance direction. This phenomenon becomes more pronounced when the stiffness of the recording material S is high, because the stiffness of the recording material S makes it more likely that the trailing end of the recording material S in the conveyance direction will collide with the intermediate transfer belt 31 with great force.

[0043] 3, when the recording material S is sandwiched between the inner roller 32 and the outer roller 41 at the secondary transfer nip N2, it tends to maintain its position substantially along the nip line Ln. Therefore, generally, the more upstream the center of rotation of the outer roller 41 is positioned in the direction of rotation of the intermediate transfer belt 31 relative to the center of rotation of the inner roller 32 along the pre-nip tension wire T, the more the nip line Ln will bite into the pre-nip tension wire T. As a result, when the trailing edge of the recording material S in the transport direction leaves the transport guide 83, the trailing edge of the recording material S in the transport direction will collide with the intermediate transfer belt 31, as indicated by the dashed line B in FIG. 3, and image defects will be more likely to occur at the trailing edge of the recording material S in the transport direction. In contrast to this, generally, if the rotation center of the inner roller 32 and the rotation center of the outer roller 41 are brought closer to each other in the direction along the pre-nip tension wire T, the rear end of the recording material S in the conveyance direction is prevented from colliding with the intermediate transfer belt 31 when it leaves the conveyance guide 83. This makes it less likely that image defects will occur at the rear end of the recording material S in the conveyance direction.

[0044] To address this issue, it is effective to change the relative position of the inner roller 32 and the outer roller 41 in the circumferential direction of the inner roller 32 (the rotation direction of the intermediate transfer belt 31) depending on the type of recording material S. The definition of the relative position of the inner roller 32 and the outer roller 41 will be described with reference to FIG. 3. In the cross section shown in FIG. 3, a common tangent to the inner roller 32 on the side around which the intermediate transfer belt 31 is wound and the pre-secondary transfer roller 37 is defined as a reference line L1. The reference line L1 corresponds to the pre-nip tension line T. Furthermore, in the same cross section, a line passing through the rotation center of the inner roller 32 and substantially perpendicular to the reference line L1 is defined as an inner roller center line L2. Furthermore, in the same cross section, a line passing through the rotation center of the outer roller 41 and substantially perpendicular to the reference line L1 is defined as an outer roller center line L3. In this case, the distance (vertical distance) between the center line L2 of the inner roller and the center line L3 of the outer roller is defined as the offset amount X (wherein the offset amount X is a positive value when the center line L3 of the outer roller is located upstream of the center line L2 of the inner roller in the rotation direction of the intermediate transfer belt 31). The offset amount X can be negative, zero, or positive. By increasing the offset amount X, the width of the secondary transfer nip N2 relative to the rotation direction of the intermediate transfer belt 31 increases upstream in the rotation direction of the intermediate transfer belt 31. In other words, the upstream end of the contact area between the outer roller 41 and the intermediate transfer belt 31 in the rotation direction of the intermediate transfer belt 31 is positioned more upstream than the upstream end of the contact area between the inner roller 32 and the intermediate transfer belt 31 in the rotation direction of the intermediate transfer belt 31. In this way, by changing the position of at least one of the inner roller 32 or the outer roller 41, the relative positions of the inner roller 32 and the outer roller 41 in the circumferential direction of the inner roller 32 are changed, and the position of the secondary transfer nip (transfer portion) N2 can be changed.

[0045] In FIG. 3, the outer roller 41 is virtually depicted as being in contact with the reference line L1 (pre-nip tension wire T) without deformation. However, the outermost layer of the outer roller 41 is made of an elastic material such as rubber or sponge, and is actually deformed by being pressed toward the inner roller 32 by the pressure spring 44. When the outer roller 41 is offset upstream of the inner roller 32 in the rotation direction of the intermediate transfer belt 31 and pressed by the pressure spring 44 so as to sandwich the intermediate transfer belt 31 between the outer roller 41 and the inner roller 32, a substantially S-shaped secondary transfer nip N2 is formed. The posture of the recording material S guided by the conveyance guide 83 is also determined based on the shape of the secondary transfer nip N2. The larger the offset amount X, the more the recording material S is bent. Therefore, as described above, for example, if the recording material S is "thin paper," increasing the offset amount X can improve the separation of the recording material S from the intermediate transfer belt 31 after passing through the secondary transfer nip N2. However, if the offset amount X is large, as described above, for example, if the recording material S is "thick paper," the trailing edge of the recording material S in the conveying direction will collide with the intermediate transfer belt 31 when the trailing edge of the recording material S in the conveying direction passes through the conveying guide 83. This causes a decrease in image quality at the trailing edge of the recording material S in the conveying direction. Therefore, in this case, it is sufficient to reduce the offset amount X.

[0046] In this embodiment, the image forming apparatus 100 changes the offset amount X by changing the position of at least one of the inner roller 32 or the outer roller 41. Particularly, in this embodiment, the image forming apparatus 100 changes the position of the inner roller 32 to change the offset amount X. Furthermore, in this embodiment, the image forming apparatus 100 changes the offset amount X based on information about the type of recording material S related to the stiffness of the recording material S. For example, if the recording material S is "thick paper," the inner roller 32 is positioned at a first inner roller position where the offset amount X is a first offset amount X1. Then, if the recording material S is "thin paper," the inner roller 32 is positioned at a second inner roller position where the offset amount X is a second offset amount X2 that is larger than the first offset amount X1. The first offset amount X1 may be a positive value, 0, or a negative value, and the second offset amount X2 is typically a positive value.

[0047] 4. Secondary transfer configuration The configuration relating to the secondary transfer in this embodiment will be described in more detail. Here, for simplicity, an example will be described in which information on the basis weight of paper as the recording material S is used as information relating to the type of recording material S that is mainly related to the stiffness of the recording material S. "Thin paper" will be used as an example of a recording material S with low stiffness, and "cardboard" will be used as an example of a recording material S with high stiffness. However, as will be described later, the information relating to the type of recording material S that is related to the stiffness of the recording material S is not limited to information on the basis weight of the recording material S.

[0048] 4(a) and 4(b) are schematic side views of the main part of the vicinity of the secondary transfer nip N2 in this embodiment, viewed from one end side of the rotational axis direction of the inner roller 32 (the front side of the paper in FIG. 1) in the direction substantially parallel to the rotational axis direction. FIG. 4(a) shows the state in the case of "thick paper," and FIG. 4(b) shows the state in the case of "thin paper." Note that, for example, when the recording material S is "thin paper" and "thick paper," more specifically, when "thin paper" and "thick paper," respectively, are passed through the secondary transfer nip N2.

[0049] 4-1.Offset mechanism 4(a) and 4(b), in this embodiment, the image forming apparatus 100 has an offset mechanism (offset amount changing means) 1 as a position changing mechanism that changes the relative position of the inner roller 32 with respect to the outer roller 41 to change the offset amount X. While Figures 4(a) and 4(b) show the configuration of one end of the inner roller 32 in the rotational axis direction, the configuration of the other end is similar (substantially symmetrical with respect to the center of the inner roller 32 in the rotational axis direction).

[0050] Both ends of the inner roller 32 in the direction of its rotation axis are rotatably supported by inner roller holders 38 serving as support members. The inner roller holder 38 is supported by the frame of the intermediate transfer belt unit 30 or the like so as to be rotatable about a rotation shaft 38a. In this way, by rotating the inner roller holder 38 about the rotation shaft 38a and rotating the inner roller 32 about the rotation shaft 38a, the relative position of the inner roller 32 with respect to the outer roller 41 can be changed, thereby changing the offset amount X.

[0051] The inner roller holder 38 is configured to rotate by the action of an offset cam 39 serving as an actuating member. The offset cam 39 is supported on a frame or the like of the intermediate transfer belt unit 30 so as to be rotatable about an offset cam rotation shaft 39a. The offset cam 39 is rotatable about the offset cam rotation shaft 39a by being driven by an offset motor 110 serving as a drive source. The offset cam 39 is in contact with an offset cam follower (arm portion) 38c provided on the inner roller holder 38. In this embodiment, the inner roller holder 38 is biased by the tension of the intermediate transfer belt 31, as described below, so that the offset cam follower 38c rotates in a direction that engages with the offset cam 39. However, this is not limited to this, and the inner roller holder 38 may be biased by a biasing member (elastic member) such as a spring serving as a biasing means so that the offset cam follower 38c rotates in a direction that engages with the offset cam 39.

[0052] As described above, in this embodiment, the offset mechanism 1 is configured with the inner roller holder 38, the offset cam 39, the offset motor 110, and the like.

[0053] As shown in FIG. 4(a), in the case of "cardboard," the offset cam 39 is driven by the offset motor 110 to rotate, for example, clockwise. This causes the inner roller holder 38 to rotate counterclockwise about the rotation shaft 38a, and the relative position of the inner roller 32 with respect to the outer roller 41 is determined. This places the inner roller 32 in the first inner roller position, where the offset amount X is the first offset amount X1, which is relatively small. As a result, as described above, it is possible to suppress degradation in image quality at the trailing end of the "cardboard" in the conveyance direction.

[0054] 4(b), in the case of "thin paper," the offset cam 39 is driven by the offset motor 110 to rotate, for example, counterclockwise. This causes the inner roller holder 38 to rotate clockwise about the rotation shaft 38a, and the relative position of the inner roller 32 with respect to the outer roller 41 is determined. This places the inner roller 32 in the second inner roller position, which is the second offset amount X2, where the offset amount X is relatively large. As a result, the separation of the "thin paper" from the intermediate transfer belt 31 after passing through the secondary transfer nip N2 is improved, as described above.

[0055] FIG. 5 is a schematic side view of the vicinity of the inner roller holder 38, viewed from one end side of the rotational axis of the inner roller 32 (the front side of the paper in FIG. 1) in the direction substantially parallel to the rotational axis. As described above, in the case of "thick paper," the inner roller holder 38 rotates counterclockwise about the rotation axis 38a (solid line). Then, a cylindrical abutment portion 38b, which is provided on the inner roller holder 38 and is coaxial with the inner roller 32, abuts against the first positioning portion 40a. This positions the inner roller 32 at the first inner roller position (first offset amount X1). Also, as described above, in the case of "thin paper," the inner roller holder 38 rotates clockwise about the rotation axis 38a (two-dot chain line). Then, the abutment portion 38b, which is provided on the inner roller holder 38, abuts against the second positioning portion 40b. As a result, the inner roller 32 is positioned at the second inner roller position (second offset amount X2). The first and second positioning portions 40a and 40b are provided on the frame of the intermediate transfer belt unit 30 or the like.

[0056] In this embodiment, the offset amount X (X1, X2) is set to have, for example, the following two patterns based on the basis weight M of the recording material S. Note that gsm stands for g / m 2 means. (a) M≧52gsm: X1=1.0mm (b) M<52gsm: X2=2.5mm

[0057] In this embodiment, the position of the inner roller 32 in the setting (a) shown in Fig. 4(a) is the home position. Here, the home position refers to the position when the image forming apparatus 100 is in a sleep state (described later) or when the main power is turned off. However, this is not limited to this, and the position of the inner roller 32 in the setting (b) shown in Fig. 4(b) may also be the home position.

[0058] The offset amount X and the type of recording material S (here, the basis weight of the recording material S) assigned to each offset amount X are not limited to the specific example described above. These can be appropriately set through experiments, etc., from the viewpoint of improving the separation of the recording material S from the intermediate transfer belt 31 and suppressing image defects that occur near the secondary transfer nip N2, as described above. Although not limited to this, the offset amount X is preferably about -3 mm to +3 mm. Such a setting can achieve good transferability.

[0059] Furthermore, the number of patterns of the offset amount X is not limited to two, and three or more patterns may be set. Then, according to this embodiment, an appropriate setting can be selected from three or more patterns based on information about the type of recording material S related to the stiffness of the recording material S.

[0060] In this embodiment, in the cross section shown in FIG. 4, a counterclockwise moment is constantly applied to the inner roller holder 38 about the rotation shaft 38a due to the tension of the intermediate transfer belt 31. That is, in this embodiment, a moment is constantly applied to the inner roller holder 38 due to the tension of the intermediate transfer belt 31 in a direction that rotates the offset cam follower 38c so that the offset cam follower 38c engages with the offset cam 39. Also, in this embodiment, in the cross section shown in FIG. 4, the rotation shaft 38a is located downstream in the conveying direction of the recording material S with respect to a line (nip center line) Lc connecting the rotation centers of the inner roller 32 and the outer roller 41. As a result, when the outer roller 41 is in contact with the inner roller 32 via the intermediate transfer belt 31, the reaction force that the inner roller holder 38 receives from the outer roller 41 also becomes a counterclockwise moment in FIG. 4. With this configuration, a cam mechanism can be configured without using a separate biasing member such as a spring.

[0061] Furthermore, it is desirable to position the inner roller holder 38 inside the tension surface of the intermediate transfer belt 31 so as not to impede the ease of installing or removing the intermediate transfer belt 31 from the intermediate transfer belt unit 30 for replacement or other purposes. Therefore, in the cross section shown in FIG. 4, it is desirable to position the rotation shaft 38a in an area A between the straight line (nip center line) Lc and the post-nip tension line U. Here, the post-nip tension line U is a tension line that indicates the tension surface of the intermediate transfer belt 31 formed by tensioning the intermediate transfer belt 31 between the inner roller 32 and the drive roller 33 (see FIG. 1) in the cross section shown in FIG. 4. The drive roller 33 is an example of a downstream roller that is positioned adjacent to the inner roller 32 and downstream of the inner roller 32 in the rotation direction of the intermediate transfer belt 31 among the multiple tension rollers.

[0062] The reason why it is desirable to place the rotation shaft 38a in region A will be explained in more detail with reference to Fig. 6. Figs. 6(a) and 6(b) are schematic cross-sectional views (cross-sections substantially perpendicular to the rotation axis direction of the inner roller 32) near the secondary transfer nip N2 for explaining the effects of different placements of the rotation shaft 38a. In Figs. 6(a) and 6(b), the direction of the reaction force received from the tension of the intermediate transfer belt 31 is represented by a line Lp, and the direction of the reaction force received from the outer roller 41 is represented by a line Lc.

[0063] As shown in Figure 6(a), in this embodiment, the rotation shaft 38a is located in an area A between the post-nip tension wire U and the straight line Lc. As the position of the inner roller 32 is changed along the locus a, the tension angle of the pre-nip tension wire T is also changed as shown by the two-dot chain line T'. Here, in the cross section shown in Figure 6, the tension angle of the pre-nip tension wire T can be represented by the angle that the pre-nip tension surface T makes with a reference straight line (for example, the direction of gravity) with respect to the contact position between the pre-secondary transfer roller 37 and the intermediate transfer belt 31.

[0064] As shown in FIG. 6B, if the pivot shaft 38a were located in region C between the straight line Lp and the pre-nip tension wire T (solid line), both the tension of the intermediate transfer belt 31 and the moment due to the reaction force from the outer roller 41 would be applied clockwise. In this case, by changing the location of the offset cam 39, a cam mechanism can be configured without adding a separate biasing member. However, as the position of the inner roller 32 changes along the locus c, the tension angle of the pre-nip tension wire T also changes as shown by the two-dot chain line T', and the amount of change is greater than when the pivot shaft 38a is located in region A. The tension angle of the pre-nip tension wire T must be appropriately set to prevent image quality degradation due to discharge between the pre-nip tension wire T and the recording material S before it enters the secondary transfer nip N2. Therefore, it is desirable that the tension angle of the pre-nip tension wire T not change significantly when the offset amount X is changed. Therefore, it is preferable to locate the pivot shaft 38a in region A rather than region C.

[0065] 6(b), consider the case where the rotation shaft 38a is located in region B between lines Lc and Lp (dotted line). In this case, the reaction force due to the tension of the intermediate transfer belt 31 generates a counterclockwise moment, while the reaction force due to the outer roller 41 generates a clockwise moment. Therefore, in order to stably apply a moment to one side to configure a cam mechanism, it is necessary to add a separate biasing member such as a spring.

[0066] Therefore, in this embodiment, the rotation shaft 38a is disposed in the area A.

[0067] 4-2. Disconnection mechanism The contact and separation mechanism 2 of the outer roller 41 in this embodiment will be described. Fig. 7 is a schematic diagram showing the general configuration of the contact and separation mechanism 2. Fig. 7 shows the configuration of one end of the inner roller 32 in the rotational axis direction, but the configuration of the other end is similar (substantially symmetrical with respect to the center of the inner roller 32 in the rotational axis direction).

[0068] Both ends of the outer roller 41 in the direction of its rotation axis are rotatably supported by bearings 43. The bearings 43 are supported on a frame of the apparatus main body 100a or the like so as to be slidable in a predetermined direction (for example, a direction substantially perpendicular to the aforementioned reference line L1) toward the inner roller 32 and in the opposite direction. The bearings 43 are pressed toward the inner roller 32 by a pressure spring 44, which is a biasing member (elastic member) serving as a biasing means and is made of a compression spring. As a result, the outer roller 41 comes into contact with the inner roller 32 with the intermediate transfer belt 31 sandwiched therebetween, forming the secondary transfer nip N2.

[0069] In this embodiment, the image forming apparatus 100 has a separation / compression mechanism (separation means) 2 for separating and contacting the outer roller 41 with respect to the intermediate transfer belt 31. As shown in FIG. 7 , the separation / compression mechanism 2 is configured to include a separation / compression arm 122, a separation / compression cam 121, a separation / compression motor 123, and the like. The separation / compression arm 122 is supported on the frame of the apparatus main body 100a or the like so as to be rotatable about a rotation shaft 122a, and is engaged with a bearing 43. The separation / compression arm 122 is configured to rotate by the action of the separation / compression cam 121 serving as an actuating member. The separation / compression cam 121 is supported on the frame of the apparatus main body 100a or the like so as to be rotatable about a separation / compression cam rotation shaft 120. The separation / compression cam 121 is rotatable about the separation / compression cam rotation shaft 120 by being driven by the separation / compression motor 123 as a drive source. The separation / compression cam 121 is in contact with a separation / compression cam follower 122b provided on the separation / compression arm 122. Further, the separation arm 122 is biased by the pressure spring 44 so as to rotate in a direction in which the separation cam follower 122b engages with the separation cam 121.

[0070] The separation mechanism 2 moves the outer roller 41 in a direction away from and toward the inner roller 32. As shown by the solid line in Fig. 7, when separating the outer roller 41 from the intermediate transfer belt 31, the separation cam 121 is driven by the separation motor 123 to rotate, for example, counterclockwise, causing the separation arm 122 to rotate clockwise. As a result, the separation arm 122 moves the bearing 43 in a direction away from the inner roller 32 (downward) against the biasing force of the pressure spring 44, separating the outer roller 41 from the intermediate transfer belt 31. On the other hand, as shown by the two-dot chain line in Fig. 7, when bringing the outer roller 41 into contact with the intermediate transfer belt 31, the separation cam 121 is driven by the separation motor 123 to rotate, for example, clockwise, causing the separation arm 122 to rotate counterclockwise due to the biasing force of the pressure spring 44. As a result, the contact arm 122 moves the bearing member 43 in a direction (upward) approaching the inner roller 32, and the outer roller 41 comes into contact with the intermediate transfer belt 31.

[0071] In this embodiment, the separation / contact mechanism 2 separates the outer roller 41 from the intermediate transfer belt 31 to prevent toner that is not transferred to the recording material S, such as a test image (patch) for image density correction or color misregistration correction, formed on the intermediate transfer belt 31 from adhering to the surface of the outer roller 41. The separation / contact mechanism 2 also separates the outer roller 41 from the intermediate transfer belt 31 when a paper jam is being cleared. If the outer roller 41 continues to be pressed against the inner roller 32 after a job (described later) is completed, the inner roller 32 and the outer roller 41 may become deformed. Therefore, in this embodiment, the separation / contact mechanism 2 separates the outer roller 41 from the intermediate transfer belt 31 when the job is completed and the image forming apparatus 100 enters a standby state to wait for the next job. The outer roller 41 remains separated from the intermediate transfer belt 31 even when the image forming apparatus 100 is in a sleep state or the main power is turned off.

[0072] The offset mechanism 1 may be configured to change the offset amount X either when the outer roller 41 is in contact with the intermediate transfer belt 31 or when the outer roller 41 is separated from the intermediate transfer belt 31. However, as will be described in detail later, in this embodiment, when the offset amount X is changed during execution of a mixed job, the outer roller 41 is in contact with the intermediate transfer belt 31 when the inner roller 32 moves. The offset mechanism 1 may be configured to change the offset amount X either when the intermediate transfer belt 31 is stopped or when the intermediate transfer belt 31 is rotating. However, as will be described in detail later, in this embodiment, when the offset amount X is changed during execution of a mixed job, the intermediate transfer belt 31 is rotating at the drive speed (circumferential speed) during normal image formation when the inner roller 32 moves.

[0073] 5. Problems and Overview of the Configuration of the Present Embodiment As described above, the image forming apparatus 100 may execute a job (a "mixed job") for forming images on multiple types of recording material S, for example, for bookbinding printing. In a mixed job, in order to obtain good transferability for each of multiple types of recording material S with different stiffness, such as "thin paper" and "thick paper," it is effective to change the offset amount X during the job. However, in this case, if an operation is performed to release the pressure between the inner roller 32 and the outer roller 41 in order to move the inner roller 32 or the outer roller 41, the extra time required for this operation becomes a factor that significantly reduces productivity.

[0074] Therefore, in this embodiment, when the offset amount X is changed during execution of a mixed job, the offset mechanism 1 performs an operation (herein also referred to as an "offset operation" or "position change operation") to change the position of at least one of the inner roller 32 or the outer roller 41 (particularly the inner roller 32 in this embodiment) while the outer roller 41 is in contact with the intermediate transfer belt 31 (i.e., while forming the secondary transfer nip N2).

[0075] If the normal sheet interval in a job of continuous image formation on the same type of recording material S is insufficient for the offset operation, the sheet interval is extended to a length sufficient for the offset operation. Here, the sheet interval refers to the period from when the preceding recording material S passes through the secondary transfer nip N2 until the succeeding recording material S reaches the secondary transfer nip N2.

[0076] 6. Control mode 8 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 (controller) 150 as a control means is configured with a CPU 151 as an arithmetic control means that is a central element for performing arithmetic processing, a memory (storage medium) 152 such as a ROM or RAM as a storage means, and an interface unit 153. The RAM, which is a rewritable memory, stores information input to the control unit 150, detected information, arithmetic results, etc., while the ROM stores control programs, pre-determined data tables, etc. The CPU 151 and memory 152 can transfer and read data to and from each other. The interface unit 153 controls the input and output (communication) of signals between the control unit 150 and devices connected thereto.

[0077] The control unit 150 is connected to various components of the image forming apparatus 100 (such as the image forming unit 10, the drive devices for the intermediate transfer belt 31 and the components related to the conveyance of the recording material S, and various power sources). In relation to the present embodiment, the control unit 150 is particularly connected to an offset motor 110, which is the drive source for the offset mechanism 1, and a separation motor 123, which is the drive source for the separation mechanism 2. The control unit 150 is also connected to a drum drive motor 111, a belt drive motor 112, a development motor 113, a steering mechanism 90, and various high-voltage power sources (charging voltage, development voltage, primary transfer voltage, secondary transfer voltage). The control unit 150 is also connected to an operation unit (operation panel) 160 provided in the image forming apparatus 100. The operation unit 160 includes a display unit that displays information under the control of the control unit 150, and an input unit that inputs information to the control unit 150 through operation by an operator such as a user or a service representative. The operation unit 160 may be configured with a touch panel that functions as both a display unit and an input unit. In addition, the control unit 150 may be connected to an external device 200 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.

[0078] The control unit 150 controls each unit of the image forming apparatus 100 based on job information to form an image. The job information includes a start instruction (start signal) input from the operation unit 160 or the external device 200, and information (command signal) regarding image formation conditions such as the type of recording material S. The job information also includes image information (image signals) input from the image reading device or the external device 200. Note that information regarding the type of recording material (also simply referred to as "information regarding the recording material") includes any information that can distinguish the recording material, such as attributes based on general characteristics (such as plain paper, fine paper, glossy paper, coated paper, embossed paper, thick paper, and thin paper) (so-called paper type categories), numerical values ​​or numerical ranges for basis weight, thickness, size, and brand (including manufacturer, product number, etc.). In this embodiment, the information regarding the type of recording material S includes information regarding the type of recording material S related to the stiffness of the recording material S, particularly, as an example, information regarding the basis weight of the recording material S.

[0079] Here, the image forming apparatus 100 executes a job, which is a series of operations that starts with a single start command and forms and outputs an image on one or multiple recording materials S. A job generally includes an image formation process (printing operation, image forming operation), a pre-rotation process, a sheet-to-sheet process when forming images on multiple recording materials S, and a post-rotation process. The image formation process is a period during which electrostatic image formation, toner image formation, primary transfer of the toner image, and secondary transfer of the toner image are performed for the image that is actually formed and output on the recording materials S, and this period is referred to as the image formation period. More specifically, the timing of the image formation differs depending on the positions where the electrostatic image formation, toner image formation, primary transfer of the toner image, and secondary transfer are performed. The pre-rotation process is a period during which preparatory operations are performed before the image formation process, from when a start command is input until the actual start of image formation. The sheet-to-sheet process is a period corresponding to the interval between recording materials S when image formation is performed continuously on multiple recording materials S (continuous image formation). The post-rotation process is a period during which a rearrangement operation (preparatory operation) is performed after the image formation process. Non-image formation time (non-image formation period) is a period other than image formation time, and includes the pre-rotation process, the inter-sheet process, the post-rotation process, and the pre-multi-rotation process, which is a preparatory operation when the image forming apparatus 100 is turned on or when the image forming apparatus 100 returns from a sleep state. Note that the sleep state (pause state) is a state in which the supply of power to each unit of the image forming apparatus 100 other than the control unit 150 (or a part thereof) is stopped, and power consumption is reduced compared to the standby state. In this embodiment, a case in which the aforementioned "offset operation" is performed during non-image formation time, particularly during the inter-sheet process, will be described.

[0080] 7. Control Procedure FIG. 9 is a flowchart outlining an example of a job control procedure in this embodiment. Here, a mixed-load job in which "thin paper" and "thick paper" are used as the recording material S will be described as an example. More specifically, a case will be described in which the job is started from the home position, printing is performed on "thick paper" first, and the recording material S is switched from "thick paper" to "thin paper" during the job. Even when switching from "thin paper" to "thick paper" during the job, the positions of the inner roller 32 before and after the offset operation are different, but the procedure is the same as the procedure described below. Here, a case will be described in which an operator causes the image forming apparatus 100 to execute a job from the external device 200. Note that FIG. 9 outlines the control procedure focusing on the offset operation, and omits many other operations that are normally required to execute the job and output an image.

[0081] First, job information (image information, information on image forming conditions, and a start instruction) is input to the control unit 150 from the external device 200 (S101). When the job information is input, the control unit 150 acquires information on the type of recording material S for each page included in the job information. In this embodiment, the information on the type of recording material S includes at least information on the basis weight of the recording material S. The control unit 150 can acquire information on the type of recording material S that is input directly (including selection from multiple options) from the external device 200 (or the operation unit 160) by an operator's operation. The control unit 150 can also acquire information on the type of recording material S based on information on the cassettes 61, 62, and 63 from which the recording material S for the job is sent, which information is input from the external device 200 (or the operation unit 160) by an operator's operation. In this case, the control unit 150 can acquire information about the type of recording material S from information about the type of recording material S stored in each of the cassettes 61, 62, 63, which is stored in advance in memory 152 in association with each of the cassettes 61, 62, 63. Here, when registering information about the type of recording material S, the control unit 150 may select a corresponding type from a list of types of recording material S stored in advance in memory 152 or in a storage device connected to the control unit 150 via a network.

[0082] Next, the control unit 150 sends a control signal to the separation / contact mechanism 2 (more specifically, the separation motor 123) to bring the outer roller 41 into contact with the intermediate transfer belt 31, thereby preparing for the printing operation (S102). Next, the control unit 150 sends an image formation signal to each image forming unit 10, etc., based on the job information, to cause the printing operation to be performed (S103). The control unit 150 determines whether the job will be continued for each page (S104). If the control unit 150 determines in S104 that the job will not be continued, it ends the job. On the other hand, if the control unit 150 determines in S104 that the job will be continued, it determines whether the type of recording material S will be changed from the printing operation of the previous page in the printing operation of the next page (S105). If the control unit 150 determines in S105 that the type of recording material S will not be changed, it proceeds to the processing of S103 and causes the printing operation of the next page to be performed. On the other hand, if the control unit 150 determines in S105 that the type of recording material S has changed, it determines whether the position of the inner roller 32 needs to be changed (S106). That is, the control unit 150 determines whether the position of the inner roller 32 needs to be changed based on the current position of the inner roller 32 and the position of the inner roller 32 corresponding to the new type of recording material S. Here, an example is taken of a case where a job is started from the home position corresponding to "thick paper," a printing operation is first performed on "thick paper," and the recording material S is switched from "thick paper" to "thin paper" during the job. Therefore, if the recording material S for the next page is "thin paper," it is determined that the position of the inner roller 32 needs to be changed. The control unit 150 can obtain information about the current position of the inner roller 32, for example, from information indicating the position of the inner roller 32 stored in the memory 152 each time the position of the inner roller 32 is changed, or from information about whether the device has entered a sleep state. More specifically, the control unit 150 may determine the position of the inner roller 32 for each page as follows. That is, the predetermined threshold value of the basis weight of the recording material S (for example, the above-mentioned 52 g / m 2) is stored in memory 152. Then, when printing on a recording material S whose basis weight is equal to or greater than the threshold, the control unit 150 determines the first inner roller position to be the first offset amount X1, where the offset amount X is relatively small. Also, when printing on a recording material S whose basis weight is less than the threshold, the control unit 150 determines the second inner roller position to be the second offset amount X2, where the offset amount X is relatively large. Note that, as described above, when three or more patterns of positions of the inner roller 32 are set, multiple threshold information may be set to define basis weight ranges corresponding to each pattern.

[0083] If the control unit 150 determines in S106 that a position change is not necessary, it proceeds to the process of S103 and performs the printing operation for the next page. On the other hand, if the control unit 150 determines in S106 that a position change is necessary, it changes the position of the inner roller 32 in the sheet gap process between the previous page and the next page to change the offset amount X. That is, the control unit 150 sends a control signal to the offset mechanism 1 (more specifically, the offset motor 110) to change the position of the inner roller 32 (S107). At this time, the change in the position of the inner roller 32 must be completed between the time when the preceding recording material S ("thin paper") leaves the secondary transfer nip N2 and the time when the succeeding recording material S ("thick paper") reaches the secondary transfer nip N2. If it is not possible to complete this operation with the normal sheet gap, the control unit 150 widens the sheet gap. Specifically, the control unit 150 can adjust the sheet gap by controlling the conveyance timing of the succeeding recording material S and the start timing of image formation. Next, if the change in recording material S requires a change in image forming conditions such as high pressure conditions, the control unit 150 changes the image forming conditions (S108). As a result, an image can be formed, and the control unit 150 returns to the process of S103 and causes the printing operation of the next page to be performed.

[0084] Here, the printing operation for the next page includes the operation of forming a latent image on the photosensitive drum 11 in order to form the image of the next page. In this embodiment, if it is necessary to change the position of the inner roller 32 in the inter-page process between the previous page and the next page (S107), the printing operation for the next page (forming a latent image on the photosensitive drum 11 for forming the image of the next page) is started after the position change of the inner roller is completed. This is because there is a risk that the movement of the inner roller 32 will disrupt the surface speed of the intermediate transfer belt 31, which may result in distortion of the image at the primary transfer nip N1.

[0085] However, if the change in position of the inner roller 32 has little effect on the surface speed of the intermediate transfer belt 31, latent image formation on the photosensitive drum 11 for forming the image of the next page may be started while the position of the inner roller 32 is being changed. However, from the viewpoint of image quality, it is preferable to configure the inner roller 32 so that the position of the inner roller 32 is not changed at least during the period when the image of the next page is being primarily transferred at the primary transfer nip N1.

[0086] In this embodiment, when a job is completed and the image forming apparatus 100 enters a standby state to wait for the next job, the control unit 150 sends a control signal to the separation mechanism 2 to separate the outer roller 41 from the intermediate transfer belt 31. More specifically, the separation mechanism 2 starts an operation (separation operation) to separate the outer roller 41 from the intermediate transfer belt 31 after the last recording material S of the job has passed through the secondary transfer nip N2.

[0087] 8.Effects As described above, in this embodiment, the offset amount X is changed in the sheet interval process during execution of a mixed job. That is, in this embodiment, during execution of a job in which images are formed on and output from a plurality of recording materials S, the relative positions of the inner roller 32 and the outer roller 41 in the circumferential direction of the inner roller 32 are changed during the period (sheet interval) from when the preceding recording material S passes through the secondary transfer nip N2 until the succeeding recording material S reaches the secondary transfer nip N2. This changes the shape of the secondary transfer nip N2 (the position of the secondary transfer nip N2). In this embodiment, the outer roller 41 is in contact with the intermediate transfer belt 31 when the inner roller 32 moves (i.e., the secondary transfer nip N2 is formed). In this way, the position of the inner roller 32 is changed while the inner roller 32 and the outer roller 41 remain in a pressing state substantially the same as during image formation. This eliminates the need for additional time other than the time required to change the position of the inner roller 32, thereby suppressing a decrease in productivity. Therefore, according to this embodiment, it is possible to prevent downtime (periods during which images cannot be output) and thereby suppress declines in productivity, while improving transferability for each of multiple types of recording material S in mixed jobs.

[0088] [Example 2] Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of the image forming apparatus of embodiment 1. Therefore, in the image forming apparatus of this embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of embodiment 1 are assigned the same reference numerals as those of embodiment 1, and detailed descriptions thereof will be omitted.

[0089] In the first embodiment, when the offset amount X is changed during the execution of a mixed job, the intermediate transfer belt 31 rotates at the driving speed (circumferential speed) during normal image formation when the inner roller 32 moves.

[0090] However, because the inner roller 32, which is moved during the offset operation, is one of the rollers tensioning the intermediate transfer belt 31, the movement of the inner roller 32 may affect the running of the intermediate transfer belt 31. For example, if the movement of the inner roller 32 disrupts the surface speed of the intermediate transfer belt 31, image distortion may occur at the primary transfer nip N1. Furthermore, when performing the aforementioned misalignment control of the intermediate transfer belt 31, the movement of the inner roller 32 may increase the change in the amount of misalignment. If image formation is performed while the change in the amount of misalignment (waveform) is unstable, image defects such as color registration may occur. Therefore, when changing the offset amount X during a mixed job, it may be desirable for the intermediate transfer belt 31 to decelerate to a driving speed (second speed) lower than the driving speed (first speed) during normal image formation, or to stop when the inner roller 32 moves.

[0091] The fluctuations in the surface speed and their impact on misalignment control are proportional to the travel distance of the intermediate transfer belt 31. Therefore, by slowing down the drive speed of the intermediate transfer belt 31, the travel distance per unit time can be shortened, thereby reducing the fluctuations in the surface speed and their impact on misalignment control that occur when the inner roller 32 is moved while the intermediate transfer belt 31 is running. The drive speed of the intermediate transfer belt 31 after deceleration can be appropriately set depending on the drive control characteristics of the intermediate transfer belt 31, the time required to change the position of the inner roller 32, and the fluctuations in the surface speed and their impact on misalignment control. While not limited to this, from the perspective of suppressing the fluctuations in the surface speed and their impact on misalignment control, the drive speed of the intermediate transfer belt 31 after deceleration is preferably about half or less of the drive speed during normal image formation, and the intermediate transfer belt 31 may be stopped. However, from the perspective of reducing the time required to return the drive speed of the intermediate transfer belt 31 to its normal speed, the drive speed of the intermediate transfer belt 31 after deceleration is preferably about one-fifth or more of the drive speed during normal image formation. For example, in this embodiment, the driving speed (first speed) of the intermediate transfer belt 31 during normal image formation is 400 mm / sec. When the offset amount X is changed during execution of a mixed job, for example, the driving speed (second speed) of the intermediate transfer belt 31 when the inner roller 32 moves can be reduced to half that speed, 200 mm / sec, or the intermediate transfer belt 31 can be stopped.

[0092] Fig. 10 is a flowchart showing an example of a job control procedure in this embodiment. As with the procedure in Fig. 9 described in the first embodiment, here, a case will be described in which a job is started from the home position, a printing operation is first performed on "thick paper", and the recording material S is switched from "thick paper" to "thin paper" during the job. Descriptions of processes similar to the procedure in Fig. 9 described in the first embodiment will be omitted as appropriate.

[0093] The processes of S201 to S206 in FIG. 10 are the same as the processes of S101 to S106 in FIG. 9. Next, in preparation for the offset operation, the control unit 150 first sends control signals to various high-voltage power supplies (charging voltage, developing voltage, primary transfer voltage, secondary transfer voltage) of the image formation system, such as each image forming unit 10, to turn off all high voltages input to the image formation system (S207). Next, the control unit 150 sends a control signal to the development motor 113 to stop driving the development roller of the developer 14 (S208). Next, the control unit 150 sends control signals to the belt drive motor 112 and the drum drive motor 111 to reduce the drive speed of the intermediate transfer belt 31 and the photosensitive drum 11 to half the speed during normal image formation, or to stop driving the intermediate transfer belt 31 and the photosensitive drum 11 (S209). Next, after the driving speed of the intermediate transfer belt 31 and the photosensitive drum 11 has slowed down to the half speed, or after the rotation of the intermediate transfer belt 31 or the photosensitive drum 11 has stopped, the control unit 150 sends a control signal to the offset mechanism 1 (more specifically, the offset motor 110) to change the position of the inner roller 32 (S210).

[0094] After changing the position of the inner roller 32, the printing operation is resumed by reversing the procedure from before the change. That is, the control unit 150 sends control signals to the drum drive motor 111 and the belt drive motor 112 to increase the drive speeds of the photosensitive drum 11 and the intermediate transfer belt 31 to the drive speeds during normal image formation (S211). At this time, if the drive of the photosensitive drum 11 and the intermediate transfer belt 31 was stopped in S209, the drive of the photosensitive drum 11 and the intermediate transfer belt 31 is started, and the drive speeds of the photosensitive drum 11 and the intermediate transfer belt 31 are increased to the drive speeds during normal image formation. Next, the control unit 150 sends a control signal to the development motor 113 to start driving the development roller of the developer 14 (S212). Next, the control unit 150 sends control signals to various high-voltage power supplies (charging voltage, development voltage, primary transfer voltage, secondary transfer voltage) of the image formation system, such as each image forming unit 10, to increase the high voltages input to the image formation system (S213). At this time, if the change in the recording material S requires a change in the image forming conditions, such as a high-pressure condition, the control unit 150 changes the image forming conditions. As a result, the image can be formed, and the control unit 150 returns to the process of S103 and causes the printing operation of the next page to be performed.

[0095] As described above, in this embodiment, when the offset amount X is changed during execution of a mixed job, the belt drive motor 112 changes the drive speed of the intermediate transfer belt 31 from the first speed used during transfer to a second speed that is slower than the first speed, and then the offset mechanism 1 performs the offset operation. Then, after the offset mechanism 1 performs the offset operation, the belt drive motor 112 changes the drive speed of the intermediate transfer belt 31 from the second speed to the first speed. Alternatively, in this embodiment, when the offset amount X is changed during execution of a mixed job, the belt drive motor 112 stops driving the intermediate transfer belt 31, and then the offset mechanism 1 performs the offset operation. Then, after the offset mechanism 1 performs the offset operation, the belt drive motor 112 starts driving the intermediate transfer belt 31.

[0096] Here, performing the offset operation after changing the drive speed of the intermediate transfer belt 31 means, more specifically, that the offset mechanism 1 starts the offset operation after the drive speed of the intermediate transfer belt 31 reaches the second speed (the constant speed after the change). Typically, the offset operation starts after the drive speed of the intermediate transfer belt 31 reaches the second speed, but the reaching of the second speed and the start of the offset operation may occur approximately simultaneously. The timing of reaching the second speed can be determined based on the timing when the drive speed of the intermediate transfer belt 31 actually reaches the second speed, as well as the timing when the drive signal input from the control unit 150 to the belt drive device 112 changes. The timing of starting the offset operation can be determined based on the timing when the movement of the inner roller 32 or the outer roller 41 actually starts, the timing when the control unit 150 starts inputting a drive signal to the offset mechanism 1 (more specifically, the offset motor 110), the timing when the control unit 150 inputs a drive start signal to the offset mechanism 1, and the like.

[0097] Furthermore, "changing the drive speed of the intermediate transfer belt 31 after the offset operation" means, more specifically, that the belt drive motor 112 starts changing the drive speed of the intermediate transfer belt 31 from the second speed to the first speed after the offset mechanism 1 completes the offset operation. Typically, the change in drive speed starts after the completion of the offset operation, but the completion of the offset operation and the start of the change in drive speed may occur approximately simultaneously. The timing at which the offset operation is completed can be determined based on the timing at which the movement of the inner roller 32 or the outer roller 41 actually ends, the timing at which the input of the drive signal from the control unit 150 to the offset mechanism 1 stops, the timing at which the drive stop signal is input from the control unit 150 to the offset mechanism 1, and the like. The timing at which the change in drive speed starts can be determined based on the timing at which the drive speed of the intermediate transfer belt 31 actually starts to change, the timing at which the drive signal input from the control unit 150 to the belt drive device 112 changes, and the like.

[0098] Similarly, "performing the offset operation after stopping the drive of the intermediate transfer belt 31" means, more specifically, that the offset mechanism 1 starts the offset operation after the rotation of the intermediate transfer belt 31 stops. Typically, the offset operation starts after the rotation of the intermediate transfer belt 31 stops, but the rotation of the intermediate transfer belt 31 and the start of the offset operation may occur approximately simultaneously. The timing at which the rotation of the intermediate transfer belt 31 stops can be determined based on the timing at which the intermediate transfer belt 31 actually stops, the timing at which the input of the drive signal from the control unit 150 to the belt drive device 112 stops, the timing at which the drive stop signal is input from the control unit 150 to the belt drive motor 112, and the like. The timing at which the offset operation starts can be determined as described above.

[0099] Furthermore, "starting the drive of the intermediate transfer belt 31 after the offset operation" means, more specifically, that the rotation of the intermediate transfer belt 31 starts after the offset mechanism 1 completes the offset operation. Typically, the rotation of the intermediate transfer belt 31 starts after the completion of the offset operation, but the completion of the offset operation and the start of rotation of the intermediate transfer belt 31 may be approximately simultaneous. The timing at which the offset operation is completed can be determined as described above. The timing at which the rotation of the intermediate transfer belt 31 starts can be determined based on the timing at which the control unit 150 starts inputting a drive signal to the belt drive device 112, the timing at which the control unit 150 inputs a drive start signal to the belt drive motor 112, etc., in addition to the timing at which the intermediate transfer belt 31 actually starts rotating.

[0100] As described above, according to this embodiment, it is possible to prevent the change in the position of the inner roller 32 from disturbing the surface speed of the intermediate transfer belt 31 or increasing the change in the amount of misalignment. Furthermore, according to this embodiment, it takes more time to change the drive speed of the intermediate transfer belt 31 compared to embodiment 1, but it is possible to prevent a decrease in productivity compared to when the offset operation is performed after separating the outer roller 41 from the intermediate transfer belt 41. Therefore, according to this embodiment, like embodiment 1, it is possible to improve the transferability for each of the multiple types of recording materials S in a mixed job while preventing a decrease in productivity.

[0101] [Example 3] Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of the image forming apparatus of embodiment 1. Therefore, in the image forming apparatus of this embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of embodiment 1 are assigned the same reference numerals as those of embodiment 1, and detailed descriptions thereof will be omitted.

[0102] In the first embodiment, the offset amount X is changed by changing the position of the inner roller 32. In the present embodiment, the offset amount X is changed by changing the position of the outer roller 41. In the first embodiment, the inner roller 32 is moved upstream in the rotation direction of the intermediate transfer belt 31 relative to the outer roller 41 for "thick paper." Correspondingly, the outer roller 41 can be moved downstream in the rotation direction of the intermediate transfer belt 31 relative to the inner roller 32. Similarly, in the first embodiment, the inner roller 32 is moved downstream in the rotation direction of the intermediate transfer belt 31 relative to the outer roller 41 for "thin paper." Correspondingly, the outer roller 41 can be moved upstream in the rotation direction of the intermediate transfer belt 31 relative to the inner roller 32. Since the shape of the secondary transfer nip N2 (the position of the secondary transfer nip N2) formed by the outer roller 41 and the inner roller 32 is the same, the same effect as that described in the first embodiment can be obtained.

[0103] FIG. 11 is a schematic side view of the main part of the vicinity of the secondary transfer nip N2 in this embodiment, viewed from one end of the inner roller 32 in the rotational axis direction (the front side of the paper in FIG. 1 ) in the direction substantially parallel to the rotational axis direction. While FIG. 11 shows the configuration of one end of the inner roller 32 in the rotational axis direction, the configuration of the other end is similar (substantially symmetrical with respect to the center of the inner roller 32 in the rotational axis direction). In this embodiment, the outer roller 41 is slidably moved in a predetermined first direction (e.g., a direction substantially perpendicular to the reference line L1) toward the inner roller 32 and in the opposite direction (the direction indicated by the white arrow in FIG. 11 ), as in the first embodiment. Furthermore, in this embodiment, the outer roller 41 is slidably moved in a predetermined second direction (e.g., a direction substantially parallel to the reference line L1) that intersects with the first direction, independently of the first direction, toward the downstream side of the conveyance direction of the recording material S and in the opposite direction (the direction indicated by the black arrow in FIG. 11 ).

[0104] In this embodiment, a support member 132, which supports the bearing 43 of the outer roller 41 so as to be slidable along the first direction, is supported on the frame of the device main body 100a or the like so as to be slidable in the second direction. The support member 132 is configured to slide by the action of an offset cam 131 serving as an actuating member. The offset cam 131 is supported on the frame of the device main body 100a or the like so as to be rotatable about an offset cam rotation shaft 130. The offset cam 131 is rotatable about the offset cam rotation shaft 130 by being driven by an offset motor 133 serving as a drive source. The offset cam 131 is in contact with an offset cam follower 132a provided on the support member 132. The support member 132 is biased by an offset spring 134, which is a biasing member (elastic member) such as a compression spring, so that the offset cam follower 132a slides in a direction to engage with the offset cam 131. As described above, in this embodiment, the offset mechanism 1 is configured to include the support member 134, the offset cam 131, the offset motor 133, the offset spring 134, and the like.

[0105] In the case of "thick paper," the offset cam 131 is driven by the offset motor 133 to rotate counterclockwise, for example. Then, the support member 132 slides toward the downstream side in the conveyance direction of the recording material S due to the biasing force of the offset spring 134, and the relative position of the outer roller 41 with respect to the inner roller 32 is determined. As a result, the outer roller 41 is disposed at the first outer roller position, which is the first offset amount X1, where the offset amount X is relatively small. As a result, as described in the first embodiment, it is possible to suppress deterioration in image quality at the trailing end of "thick paper" in the conveyance direction. In the case of "thin paper," the offset cam 131 is driven by the offset motor 133 to rotate clockwise, for example. Then, the support member 132 slides toward the upstream side in the conveyance direction of the recording material S against the biasing force of the offset spring 134, and the relative position of the outer roller 41 with respect to the inner roller 32 is determined. As a result, the outer roller 41 is disposed at the second outer roller position, which is the second offset amount X2, where the offset amount X is relatively large. As a result, as described in the first embodiment, the separation of the "thin paper" from the intermediate transfer belt 31 after passing through the secondary transfer nip N2 is improved.

[0106] In this embodiment, the contact / separation mechanism 2 has the same configuration as in embodiment 1. The configuration of this embodiment can be applied to the operations described in either embodiment 1 or embodiment 2.

[0107] As described above, the configuration of this embodiment can also achieve the same effects as those of embodiments 1 and 2. However, in this embodiment, it is necessary to make the outer roller 41 movable in two directions, and therefore, compared to the configuration of this embodiment, the configuration of embodiment 1 can be said to be advantageous in terms of simplifying the configuration of the device and making it smaller.

[0108] [Example 4] Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of the image forming apparatus of embodiment 1. Therefore, in the image forming apparatus of this embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of embodiment 1 are assigned the same reference numerals as those of embodiment 1, and detailed descriptions thereof will be omitted.

[0109] In the first embodiment, the outer roller 41 that directly contacts the outer peripheral surface of the intermediate transfer belt 31 is used as the outer member that forms the secondary transfer nip N2 together with the inner roller 32 as the inner member. In contrast, in the present embodiment, the outer roller and the secondary transfer belt that is stretched between the outer roller and another roller are used as the outer members.

[0110] FIG. 12 is a schematic side view of the vicinity of the secondary transfer nip N2 in this embodiment, viewed from one end of the inner roller 32 in the rotational axis direction (the front side of the paper in FIG. 1 ), generally parallel to the rotational axis direction. In this embodiment, the image forming apparatus 100 includes, as external members, a tension roller 46, an outer roller 41, and a secondary transfer belt 45 stretched between these rollers. The outer roller 41 contacts the outer peripheral surface of the intermediate transfer belt 31 via the secondary transfer belt 45. That is, the inner roller 32 contacts the inner peripheral surface of the intermediate transfer belt 31, and the outer roller 41 contacts the inner peripheral surface of the secondary transfer belt 45, thereby sandwiching the intermediate transfer belt 31 and the secondary transfer belt 45, thereby forming the secondary transfer nip N2. In this embodiment, the contact portion between the intermediate transfer belt 31 and the secondary transfer belt 45 is the secondary transfer nip N2, which serves as the secondary transfer portion.

[0111] In this embodiment, the offset amount X is defined by the relative positions of the inner roller 32 and the outer roller 41, as in the first embodiment. Also in this embodiment, the contact / detachment mechanism 2 has the same configuration as in the first embodiment. In this embodiment, the contact / detachment mechanism 2 moves the outer roller 41 in a direction away from and toward the inner roller 32, as in the first embodiment, to separate and contact the secondary transfer belt 45 with the intermediate transfer belt 31. The configuration of this embodiment can be applied to the operations described in either the first or second embodiment. Even in the case where an outer roller and a secondary transfer belt stretched around the outer roller and another roller are used as the external member as in this embodiment, the offset amount X can be changed by changing the position of the external member with respect to the inner roller 32, as in the third embodiment.

[0112] As described above, the configuration of this embodiment can also achieve the same effects as those of Embodiments 1 and 2. Furthermore, in this embodiment, it is possible to improve the conveyance performance of the recording material S passing through the secondary transfer nip N2.

[0113] [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.

[0114] In the above-described embodiment, information on the basis weight of the recording material is used as information on the type of recording material related to its stiffness, but this is not limited to this. For the same paper type category (e.g., paper type categories based on surface properties such as plain paper and coated paper) or brand (including manufacturer and product number), the basis weight and thickness of the recording material are often approximately proportional (the thicker the thickness, the greater the basis weight). Furthermore, for the same paper type category or brand, the stiffness of the recording material is often approximately proportional (the greater the basis weight or thickness, the greater the stiffness). Therefore, for example, the offset amount can be set based on the basis weight, thickness, or stiffness of the recording material for each paper type category, brand, or combination of paper type category and brand. The control unit can then operate the offset mechanism to obtain an offset amount appropriate for the recording material based on information such as the paper type category and brand, and information such as the basis weight, thickness, and stiffness of the recording material, input from the operation unit or an external device. Furthermore, the information regarding the type of recording material is not limited to quantitative information such as the basis weight, thickness, or stiffness of the recording material. It is also possible to use only qualitative information such as the paper type category, brand, or a combination of the paper type category and brand as information regarding the type of recording material. For example, the offset amount can be set according to the paper type category, brand, or a combination of the paper type category and brand, and the control unit can determine the offset amount according to information such as the paper type category and brand input from the operation unit or an external device. In this case, the offset amount is also assigned based on the difference in stiffness of each recording material. The stiffness of the recording material can be represented by the Gurley stiffness (MD / longitudinal grain) [mN], which can be measured using a commercially available Gurley stiffness tester. For example, if the basis weight threshold in the above-mentioned embodiment is 52 g / m 2 The Gurley stiffness (MD) of an example of "thin paper" as a recording material less than 52 g / m2 may be about 0.3 mN. 2 As the recording material for the above, "plain paper" (basis weight 80 g / m 2The Gurley stiffness (MD) of an example of "cardboard" (basis weight 200 g / m) is about 2 mN. 2 An example of the Gurley stiffness (MD) may be around 20 mN.

[0115] In the above-described embodiment, the control unit acquires information about the type of recording material based on input from an operating unit operated by an operator or an external device. However, the information may also be acquired based on input from a detection unit that detects information about the type of recording material. For example, a basis weight sensor can be used as a basis weight detection unit that detects an index value correlated with the basis weight of the recording material. A known basis weight sensor is one that utilizes ultrasonic wave attenuation. This basis weight sensor has an ultrasonic wave generator and an ultrasonic wave receiver, disposed on opposite sides of the recording material transport path. The basis weight sensor receives ultrasonic waves generated by the ultrasonic wave generator and attenuated by passing through the recording material with the ultrasonic wave receiver, and detects an index value correlated with the basis weight of the recording material based on the attenuation of the ultrasonic waves. Note that the basis weight detection unit may be any device capable of detecting an index value correlated with the basis weight of the recording material, and is not limited to one that uses ultrasonic waves. For example, a device that uses light may be used. Furthermore, the index value correlated with the basis weight of the recording material is not limited to the basis weight itself, but may also be the thickness corresponding to the basis weight. Furthermore, a surface property sensor can be used as a smoothness detection device for detecting an index value correlating with the surface smoothness of a recording material, which can be used to detect the paper type category. A known surface property sensor is a specular reflection light sensor that irradiates light onto the recording material and reads the intensity of specularly reflected light and diffusely reflected light using a light intensity sensor. A smooth recording material surface has a stronger specular reflection light, while a rough recording material surface has a stronger diffuse reflection light. Therefore, the surface property sensor can detect an index value correlating with the surface smoothness of the recording material by measuring the amount of specularly reflected light and diffusely reflected light. The smoothness detection device is not limited to the above-described light intensity sensor, and may be, for example, a device using an image sensor. The index value correlating with the surface smoothness of the recording material is not limited to a value converted according to a predetermined standard, such as Beck smoothness, but may be any value correlating with the surface smoothness of the recording material. These detection devices can be located, for example, adjacent to the recording material transport path upstream of the registration rollers in the recording material transport direction. Furthermore, for example, a sensor (media sensor) in which the basis weight sensor, surface property sensor, etc. are configured as one unit may be used.

[0116] In the above-described embodiment, an actuator that operates a movable part using a cam is used as the offset mechanism and the separation mechanism, but the present invention is not limited to this. The offset mechanism and the separation mechanism may be any mechanism that can realize operations conforming to the above-described embodiment, and for example, an actuator that operates a movable part using a solenoid may be used.

[0117] Furthermore, in the above embodiment, the configuration in which either the inner roller or the outer roller is moved has been described, but it is also possible to change the offset amount by moving both the inner roller and the outer roller.

[0118] 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.

[0119] 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]

[0120] 1 Offset mechanism (position change mechanism) 2 Separation mechanism 31 Intermediate transfer belt 32 Inner roller 37 Secondary transfer front roller 41 Outer roller 44 Pressure spring 83 Transport guide S recording material X offset amount

Claims

1. a belt that carries a toner image; a transfer roller that contacts the belt and forms a transfer section that transfers a toner image; a first position changing mechanism that changes the position of the transfer unit in relation to the rotation direction of the belt; a second position change mechanism that changes the position of the transfer roller between a first position where the transfer roller presses the belt to form the transfer portion and a second position where the transfer roller does not press the belt; a control unit that controls the first position change mechanism and the second position change mechanism; and the control unit controls the first position change mechanism to change the position of the transfer unit relative to the rotation direction of the belt in accordance with information about the type of recording material; During execution of a continuous image forming job in which images are formed on a plurality of recording materials and output, when the type of recording material on which images are formed is changed from a first recording material of a first type to a second recording material of a second type, the position of the transfer unit in relation to the rotation direction of the belt is changed, the control unit: (i) controlling the first position change mechanism and the second position change mechanism so as to change the position of the transfer unit with respect to the rotation direction of the belt while the transfer roller presses the belt; (ii) controlling an image forming operation so that formation of a latent image of the image to be formed on the second recording material is started after the change in the position of the transfer unit is completed; An image forming apparatus characterized by:

2. 2. The image forming apparatus according to claim 1, wherein the control unit controls the second position changing mechanism so as to change the position of the transfer roller from the first position to the second position upon completion of the job.

3. 2. The image forming apparatus according to claim 1, wherein the control unit controls the first position change mechanism so as to change the position of the transfer unit to one of a first transfer position and a second transfer position based on the type of the recording material.

4. a belt that carries a toner image; a transfer roller that contacts the belt and forms a transfer section that transfers a toner image; a position change mechanism that changes the position of the transfer roller so as to change the position of the transfer portion relative to the rotation direction of the belt, the position change mechanism changing the position of the transfer roller between a first position where the transfer roller presses the belt to form the transfer portion and a second position where the transfer roller does not press the belt; a control unit that controls the position change mechanism; and the control unit controls the position changing mechanism to change the position of the transfer unit relative to the rotation direction of the belt in accordance with information about the type of recording material; During execution of a continuous image forming job in which images are formed on a plurality of recording materials and output, when the type of recording material on which images are formed is changed from a first recording material of a first type to a second recording material of a second type, the position of the transfer unit in relation to the rotation direction of the belt is changed, the control unit: (i) controlling the position change mechanism to change the position of the transfer unit relative to the rotation direction of the belt while the transfer roller is pressing the belt; (ii) controlling an image forming operation so that formation of a latent image of the image to be formed on the second recording material is started after the change in the position of the transfer unit is completed; An image forming apparatus characterized by:

5. The image forming apparatus described in Claim 4, characterized in that the control unit controls the position change mechanism so as to change the position of the transfer roller from the first position to the second position upon completion of the job.

6. 5. The image forming apparatus according to claim 4, wherein the control unit controls the position changing mechanism so as to change the position of the transfer unit to one of a first transfer position and a second transfer position based on the type of the recording material.

Images