Image forming apparatus

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 as a second image carrier that conveys a toner image that has been primarily transferred from a photoreceptor as a first image carrier, for secondary transfer onto a sheet-like recording material such as paper. The following mainly describes an image forming apparatus that employs an intermediate transfer method having an intermediate transfer belt.

[0003] In an intermediate transfer type image forming apparatus, a toner image formed on a photoreceptor or the like in an image forming section is primarily transferred to an intermediate transfer belt in a primary transfer section. The toner image primarily transferred to the intermediate transfer belt is secondarily transferred to a recording material in a secondary transfer section. An inner member (secondary transfer inner member) provided on the inner peripheral surface side of the secondary transfer belt and an outer member (secondary transfer outer member) provided on the outer peripheral surface side of the secondary transfer belt form a secondary transfer nip as a secondary transfer section, which is a contact portion between the intermediate transfer belt and the outer member. An inner roller, which is one of a plurality of tension rollers that tension the intermediate transfer belt, is used as the inner member. An outer roller, which is disposed at a position facing the inner roller across the intermediate transfer belt, is often used as the outer member. Then, for example, a secondary transfer voltage of a polarity opposite to the charge polarity of the toner is applied to the outer roller, so that the toner image on the intermediate transfer belt is secondarily transferred to the recording material in the secondary transfer nip. In general, a transport guide for guiding the recording material to the secondary transfer nip is provided upstream of the secondary transfer nip in the transport direction of the recording material.

[0004] Here, the behavior of the recording material changes in the vicinity of the upstream and downstream of the secondary transfer nip in the conveying direction of the recording material depending on the shape of the secondary transfer nip. In addition, in recent years, there has been a demand for compatibility with various recording materials with different stiffness due to differences in thickness and surface properties, but the stiffness of the recording material also changes the behavior of the recording material in the vicinity of the upstream and downstream of the secondary transfer nip in the conveying direction of the recording material. For example, when the recording material is "thin paper," which is an example of a recording material with low stiffness, the intermediate transfer belt and the recording material may stick to each other in the vicinity of the downstream of the secondary transfer nip in the conveying direction of the recording material, and a jam (paper jam) may occur due to poor separation of the recording material from the intermediate transfer belt. This phenomenon is more noticeable when the stiffness of the recording material is low, because the recording material is more likely to stick to the intermediate transfer belt due to its weak stiffness.

[0005] On the other hand, for example, when the recording material is "thick paper," which is an example of a recording material with high stiffness, the trailing end of the recording material in the transport direction may collide with the intermediate transfer belt when the trailing end of the recording material in the transport direction passes through the transport guide. This may disturb the posture of the intermediate transfer belt in the vicinity of the upstream of the secondary transfer nip in the transport direction of the recording material, causing image defects (such as streaky image disturbances extending in a direction approximately perpendicular to the transport direction of the recording material) at the trailing end of the recording material in the transport direction. This phenomenon is more noticeable when the recording material has high stiffness, because the trailing end of the recording material in the transport direction is more likely to collide with the intermediate transfer belt with great force due to the stiffness of the recording material.

[0006] In response to such a problem, 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] JP 2014-134718 A 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 suppress image defects caused by the collision of the trailing end of the recording material in the transport direction with the intermediate transfer belt, 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 according to the type of recording material. The width of the secondary transfer nip can be changed by moving the inner roller or the outer roller in a direction intersecting the pressing direction of the secondary transfer nip, changing the relative position of the inner roller and the outer roller in the circumferential direction of the inner roller, and thereby changing the position of the secondary transfer nip.

[0009] Here, in an image forming apparatus using an electrophotographic method or the like, a job (herein referred to as a "mixed job") may be executed to form images on a plurality of types of recording materials for, for example, bookbinding printing. In a mixed job, in order to obtain good transferability for each of a plurality of types of recording materials having different stiffness, it is considered effective to change the relative positions of the inner roller and the outer roller during the job. However, performing such an operation during the job may lead to a decrease in productivity, for example, by requiring an extension of the paper interval. Therefore, it is important to perform the above operation while suppressing a decrease in productivity. For example, if an operation is performed to release the pressing state between the inner roller and the outer roller in order to move the inner roller or the outer roller, the time required for this operation is extra, which is a factor that significantly reduces productivity.

[0010] Note that, in the above, conventional issues have been explained using the secondary transfer section, which is the section where the toner image is transferred from the intermediate transfer belt to the recording material, as an example. However, similar issues exist with regard to sections where the toner image is transferred from other belt-like image carriers, such as photoconductors, to the recording material.

[0011] SUMMARY OF THE PRESENTLY PREFERRED EMBODIMENTS An object of the present invention is to provide an image forming apparatus capable of improving 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 can be achieved by an image forming apparatus according to the present invention. In summary, the present invention is an image forming apparatus having a rotatable endless belt that transports a toner image, a plurality of tension rollers that tension the belt, a drive device that rotates and drives the belt, an external member that is arranged opposite an inner roller of the plurality of tension rollers and abuts against the outer peripheral surface of the belt to form a transfer section that transfers a toner image from the belt to a recording material, a position change mechanism that can change the position of the transfer section by changing the position of at least one of the inner roller or the external member to change the relative position between the inner roller and the external member in the circumferential direction of the inner roller, a separation mechanism that separates and abuts the external member against the belt, and a control unit that controls the position change mechanism and the separation mechanism, wherein the control unit controls the position change mechanism so that the relative position is changed while the external member is in contact with the belt, when the relative position is changed during the period after a preceding recording material has passed the transfer section and before a succeeding recording material reaches the transfer section during the execution of a job to form images on a plurality of recording materials and output them. Effect 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 materials in a mixed job while suppressing a decrease in productivity. [Brief description of the drawings]

[0014] [Figure 1] FIG. 1 is a schematic cross-sectional view of an image forming apparatus. [Diagram 2] FIG. 4 is a schematic perspective view of the periphery of an intermediate transfer belt for explaining deviation control. [Diagram 3] FIG. 11 is a schematic cross-sectional view for explaining an offset amount. [Figure 4] FIG. 4 is a schematic side view showing an offset mechanism. [Diagram 5] FIG. 4 is a schematic side view showing a part of the offset mechanism. [Figure 6]FIG. 4 is a schematic diagram for explaining the arrangement of a rotation shaft of an inner roller holder. [Figure 7] FIG. 4 is a schematic side view showing a contact / separation mechanism. [Figure 8] 2 is a schematic block diagram showing a control mode of a main part of the image forming apparatus; FIG. [Figure 9] FIG. 4 is a flowchart showing an outline of a procedure of a job operation. [Figure 10] FIG. 11 is a flowchart outlining another example of a procedure of a job operation. [Figure 11] FIG. 13 is a schematic side view showing another example of an offset mechanism. [Figure 12] FIG. 11 is a schematic side view showing another example of the external member. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] Hereinafter, the image forming apparatus according to the present invention will be described in more detail 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 type 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] The image forming apparatus 100 has, as a plurality of image forming units (stations), 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 moving direction of an image transfer surface of an intermediate transfer belt 31, which is arranged substantially horizontally, as described below. Elements in the image forming units 10Y, 10M, 10C, and 10K that have the same or corresponding functions or configurations may be generally described by omitting the suffixes Y, M, C, and K that indicate that the elements are for any 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, which is a rotatable drum-type (cylindrical) photosensitive member (electrophotographic photosensitive member) serving as a first image carrier for carrying a toner image, receives a driving force from a drum drive motor 111 (FIG. 8) serving as a driving source, and is driven to rotate in the direction of arrow R1 in the figure (counterclockwise). The surface of the rotating photosensitive drum 11 is uniformly charged to a predetermined potential of a predetermined polarity (negative polarity 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 surface of the charged 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, and an electrostatic image (electrostatic latent image) is formed 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 supplying toner as a developer by the developing device 14 as a developing means, and a toner image (developer image) is formed on the photosensitive drum 11. In this embodiment, toner charged with the same polarity as the charging polarity of the photosensitive drum 11 (negative polarity in this embodiment) adheres to the exposed portion (image portion) on 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 developing position, which is the portion facing the photosensitive drum 11. The developing roller is rotated by a driving force transmitted from a developing motor 113 (FIG. 8) as a driving source. During development, a predetermined developing voltage is applied to the developing roller by a developing power source (not shown).

[0019] An intermediate transfer belt 31, which is a rotatable intermediate transfer body formed of an endless belt as a second image carrier that carries a toner image, is disposed so as to face the four photosensitive drums 11Y, 11M, 11C, and 11K. The intermediate transfer belt 31 is stretched around a plurality of tension rollers (support rollers), including a drive roller 33, a tension roller 34, a secondary transfer pre-roller 37, and an inner roller (secondary transfer counter roller, inner member) 32, and 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 secondary transfer pre-roller 37 forms a surface of the intermediate transfer belt 31 in the vicinity of the upstream of a secondary transfer nip N2 (described later) with respect to the rotation direction (travel direction) of the intermediate transfer belt 31. The inner roller 32 functions as a counter member (counter electrode) of an outer roller 41 (described later). The intermediate transfer belt 31 rotates (circulates) in the direction of arrow R2 (clockwise) in the figure by the drive roller 33 being driven to rotate by the drive force transmitted from a belt drive motor 112 (FIG. 8) serving as a drive source (drive device). In this embodiment, the intermediate transfer belt 31 is driven to rotate at a peripheral speed of 400 mm / sec, for example. The support rollers other than the drive roller 33 among the multiple support rollers 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 arranged on the inner peripheral surface side of the intermediate transfer belt 31 in correspondence with the photosensitive drums 11Y, 11M, 11C, and 11K. The primary transfer roller 35 presses the intermediate transfer belt 31 toward the photosensitive drum 11 to form a primary transfer nip N1 serving as a primary transfer portion, which is a contact portion between the photosensitive drum 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 the deviation of the intermediate transfer belt 31 (deviation of the running position in the width direction approximately perpendicular to the moving direction of the surface of the intermediate transfer belt 31) by tilting.

[0020] As described above, the toner image formed on the photosensitive drum 11 is primarily transferred onto the rotating intermediate transfer belt 31 in the primary transfer nip N1 by the action of the primary transfer roller 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 roller 35 by a primary transfer power source (not shown). For example, when a full-color image is formed, the toner images of each color of yellow, magenta, cyan, and black formed on each photosensitive drum 11 are primarily transferred in sequence so as to be superimposed on the same image forming area on the intermediate transfer belt 31. In this embodiment, the primary transfer nip N1 is an image forming position where a toner image is formed on the intermediate transfer belt 31. The intermediate transfer belt 31 is an example of a rotatable endless belt that transports the toner image 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 a secondary transfer means, is disposed at a position facing the inner roller 32 on the outer peripheral surface side of the intermediate transfer belt 31. The outer roller 41 is pressed toward the inner roller 32 via the intermediate transfer belt 31, forming a secondary transfer nip N2 as a secondary transfer portion which is a contact portion between the intermediate transfer belt 31 and the outer roller 41. The toner image formed on the intermediate transfer belt 31 as described above is secondarily transferred onto the recording material S, which is sandwiched and conveyed 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, to which a secondary transfer voltage having the same polarity as the normal charging polarity of the toner is applied, and the outer roller 41 may be used as a counter electrode that is 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 sent out by the rotation of one of the feeding rollers 71, 72, and 73. The recording material S passes through a feeding conveying path 81 and is conveyed to a registration roller (a pair of registration rollers) 74, which is a conveying member as a conveying means, and is stopped once. Then, the recording material S is sent to the secondary transfer nip N2 by the registration roller 74 being rotated so that the toner image on the intermediate transfer belt 31 and a desired image forming area on the recording material S coincide with each other at the secondary transfer nip N2. In terms of the conveying direction of the recording material S, a conveying guide 83 is provided downstream of the registration roller 74 and upstream of the secondary transfer nip N2 to guide the recording material S to the secondary transfer nip N2. The conveying guide 83 is configured to include a first guide member 83a capable of contacting the front surface of the recording material S (the surface onto which the toner image is transferred immediately after passing through the conveying guide 83), and a second guide member 83b capable of contacting 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 disposed opposite each other, and the recording material S passes between these two members. The first guide member 83a regulates the movement of the recording material S in a direction approaching the intermediate transfer belt 31. The second guide member 83b regulates 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 carrying the unfixed toner image, thereby fixing (melting and bonding) the toner image to the surface of the recording material S. Thereafter, the recording material S onto which the toner image has been fixed is passed through a discharge conveyance path 82 and discharged (output) to a discharge tray 64 provided outside the device 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 a 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 powder adhering from the recording material S are removed from the intermediate transfer belt 31 and collected by a belt cleaning device 36 serving as an intermediate transfer body cleaning means.

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

[0026] Here, the intermediate transfer belt 31 may be made of a resin material having a single layer or multi-layer structure. 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 One having 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 (base material). 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 formed so that its outer diameter is 20 mm and the thickness of the elastic layer is 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 be configured as a metal roller made of a metal material such as SUM or SUS. The pre-secondary transfer roller 37 can be configured in the same manner as 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 (foamed elastic layer)) made of a conductive rubber material as a conductive elastic material on the outer periphery of a metal core (base material). This elastic layer can be made of, for example, NBR rubber or EPDM rubber containing a conductive agent such as 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) as an 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 rotational 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 the position (alignment) of the tension rollers, imbalance in pressure, 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 called steering roller) 34 so that the detection value of the sensor becomes approximately constant. This will be described in more detail below.

[0032] FIG. 2 is a schematic perspective view for explaining 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 that 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, 55 can be provided, for example, to prevent the change in inclination of the intermediate transfer belt 31 caused by 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 in the direction of its rotation axis. The bearing members at both ends in the direction of the rotation axis of the tension roller 34 are supported so as to be slidable in the direction from the inner circumferential surface side to the outer circumferential surface side of the intermediate transfer belt 31 and in the reverse direction. In addition, the bearing members at both ends are pressurized (biased) in the direction from the inner circumferential surface side to the outer circumferential surface side of the intermediate transfer belt 31 by the biasing force of a compression spring or the like, which is a biasing member (elastic member) as a biasing means. As a result, the tension roller 34 applies a predetermined tension to the intermediate transfer belt 31. In addition, the bearing member at one end in the direction of the rotation axis of the tension roller 34 (the rear side of the paper in FIG. 2) is rotatable about a rotation axis that is approximately perpendicular to the direction of the rotation axis of the tension roller 34. In addition, a bearing member at the other end of the tension roller 34 in the rotation axis direction (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 approximately parallel to the rotation axis direction of the tension roller 34. This allows the tension roller 34 to rotate so that the front end in FIG. 2 moves in the up and down direction in FIG. 2. By rotating the tension roller 34 in this manner, it is possible to tilt the tension roller 34 so that the rotation axis of the tension roller 34 is inclined with respect to the rotation axes of other support rollers such as the drive roller 33.

[0034] When the intermediate transfer belt 31 deviates to the front side or the back side in FIG. 2, the edge of the intermediate transfer belt 31 in the width direction moves the deviation detection sensor 93 in the direction of the arrow IF or the arrow IR in FIG. 2. A signal indicating the detection result of the deviation detection sensor 93 is input to the control unit 150 (FIG. 8) described later. The control unit 150 drives the deviation correction motor 91 as a drive source according to the running position in the width direction of the intermediate transfer belt 31 detected by the deviation detection sensor 93. When the deviation correction motor 91 is driven, the deviation correction cam 95 rotates and swings the deviation correction arm 94. As a result, the front end of the tension roller 34 in FIG. 2 moves up and down (in the direction of the arrow SF or the arrow SR), and the tension roller 34 tilts. By tilting the tension roller 34 in this way, the intermediate transfer belt 31 moves in the direction of the arrow IF or the 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 provided coaxially with the rotation axis of a deviation correction cam 95. The deviation detection sensor 93 is configured to have, for example, a flag that contacts the end 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 deviation detection sensor 93. By detecting this amount of light received, it is possible to grasp the running position of the intermediate transfer belt 31 in the width direction.

[0036] In this embodiment, a steering mechanism 90 is configured by including a deviation correction motor 91, an HP sensor 92, a deviation detection sensor 93, a deviation correction arm 94, a deviation 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 method of automatic alignment that uses frictional force without using a sensor to automatically control the deviation.

[0038] 3. Offset 3 is a schematic cross-sectional view (a cross-section substantially perpendicular to the rotation 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 addition, in FIG. 3, elements having the same or corresponding functions or configurations as those of 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 conveying 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, when 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 noticeable 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] That is, in the cross section shown in FIG. 3, a line showing the tension surface of the intermediate transfer belt 31 formed by tensioning the inner roller 32 and the pre-secondary transfer roller 37 is defined as a pre-nip tension line T. The pre-secondary transfer roller 37 is an example of an upstream roller arranged adjacent to the inner roller 32 upstream of the inner roller 32 in the rotation direction of the intermediate transfer belt 31 among the multiple tension rollers. In addition, in the same cross section, a straight line passing through the rotation center of the inner roller 32 and the rotation center of the outer roller 41 is defined as a nip center line Lc. In addition, in the same cross section, a line approximately perpendicular to the nip center line Lc is defined as a nip line Ln. Note that FIG. 3 shows a state in which the rotation center of the outer roller 41 is offset upstream in the rotation direction of the intermediate transfer belt 31 from the rotation center of the inner roller 32 in the direction along the pre-nip tension line T.

[0041] At this time, the recording material S tends to maintain its position substantially along the nip line Ln when it is sandwiched between the inner roller 32 and the outer roller 41 in the secondary transfer nip N2. 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 line 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 end of the recording material S in the conveying direction is in a position such that it is discharged near 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 the rotation center of the outer roller 41 is located upstream of the rotation center of the inner roller 32 in the direction along the pre-nip tension line T in the rotation direction of the intermediate transfer belt 31, 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 recording material S is discharged from the secondary transfer nip N2, the leading edge in the transport direction of the recording material S is oriented such that the recording material S is 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, when the recording material S is "thick paper," which is an example of a recording material S with high stiffness, the trailing end of the recording material S in the transport direction may collide with the intermediate transfer belt when the trailing end of the recording material S in the transport direction passes through the transport guide 83. This may result in image defects at the trailing end of the recording material S in the transport direction. This phenomenon becomes more noticeable when the stiffness of the recording material S is high, because the trailing end of the recording material S in the transport direction is more likely to collide with the intermediate transfer belt 31 with great force due to the stiffness of the recording material S.

[0043] That is, as described above, in the cross section shown in FIG. 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 posture substantially along the nip line Ln. Therefore, generally speaking, the more the center of rotation of the outer roller 41 is disposed upstream of the center of rotation of the inner roller 32 in the direction along the pre-nip tension wire T, the more the nip line Ln bites into the pre-nip tension wire T. As a result, when the trailing end of the recording material S in the transport direction leaves the transport guide 83, the trailing end of the recording material S in the transport direction collides with the intermediate transfer belt 31 as shown by the dashed line B in FIG. 3, and image defects are likely to occur at the trailing end of the recording material S in the transport direction. On the other hand, generally, by bringing the rotation center of the inner roller 32 and the rotation center of the outer roller 41 closer to each other in the direction along the pre-nip tension wire T, the rear end of the recording material S in the transport direction is prevented from colliding with the intermediate transfer belt 31 when it leaves the transport guide 83. This makes it difficult for image defects to occur at the rear end of the recording material S in the transport direction.

[0044] To address such a problem, it is effective to change the relative position between 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) according to the type of recording material S. The definition of the relative position between 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 line between 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. In addition, in the same cross section, a straight 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. In the same cross section, a straight 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. At this time, 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 (however, 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 is a positive value). The offset amount X can be a negative value, 0, or a positive value. By increasing the offset amount X, the width of the secondary transfer nip N2 in the rotation direction of the intermediate transfer belt 31 is expanded to the upstream side 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, the position of at least one of the inner roller 32 or the outer roller 41 is changed to change the relative position between the inner roller 32 and the outer roller 41 in the circumferential direction of the inner roller 32, thereby changing the position of the secondary transfer nip (transfer portion) N2.

[0045] In FIG. 3, the outer roller 41 is virtually shown as being in contact with the reference line L1 (pre-nip tension line T) without being deformed. However, the material of the outermost layer of the outer roller 41 is an elastic body such as rubber or sponge, and in reality, it is deformed by being pressed toward the inner roller 32 by the pressure spring 44. When the outer roller 41 is offset to the upstream side of the rotation direction of the intermediate transfer belt 31 with respect to the inner roller 32 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. Then, the posture of the recording material S guided by the conveying guide 83 and conveyed is also determined according to 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, in the case where the recording material S is "thin paper", by increasing the offset amount X, the separation property of the recording material S from the intermediate transfer belt 31 after passing through the secondary transfer nip N2 can be improved. However, as described above, if the offset amount X is large, for example, when the recording material S is "thick paper," the trailing end of the recording material S in the transport direction collides with the intermediate transfer belt 31 when the trailing end of the recording material S in the transport direction passes through the transport guide 83. This causes a decrease in image quality at the trailing end of the recording material S in the transport direction. Therefore, in this case, the offset amount X should be reduced.

[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 and 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. Also, in this embodiment, the image forming apparatus 100 changes the offset amount X based on information on the type of the recording material S related to the stiffness of the recording material S. For example, when the recording material S is "thick paper", the inner roller 32 is disposed at the first inner roller position where the offset amount X is the first offset amount X1. And, for example, when the recording material S is "thin paper", the inner roller 32 is disposed at the second inner roller position where the offset amount X is the 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 related to the secondary transfer in this embodiment will be described in more detail. For simplicity, the following will be described by taking as an example a case where information on the basis weight of paper as the recording material S is used as information on the type of recording material S related to the stiffness of the recording material S. In addition, "thin paper" is used as an example of a recording material S with low stiffness, and "thick paper" is used as an example of a recording material S with high stiffness. However, as will be described later, the information on the type of recording material S 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 (b) are schematic side views of the main parts in the vicinity of the secondary transfer nip N2 in this embodiment, viewed from one end side in 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, "thin paper" and "thick paper", respectively, refer to the cases when "thin paper" and "thick paper" are passed through the secondary transfer nip N2.

[0049] 4-1.Offset mechanism 4(a) and (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. Although Figs. 4(a) and (b) show the configuration of one end of the inner roller 32 in the rotation axis direction, the configuration of the other end is similar (substantially symmetrical with respect to the center of the inner roller 32 in the rotation axis direction).

[0050] Both ends of the inner roller 32 in the direction of the 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 manner, 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, and the offset amount X can be changed.

[0051] The inner roller holder 38 is configured to rotate by the action of an offset cam 39 serving as an operating member. The offset cam 39 is supported by a frame of the intermediate transfer belt unit 30 or the like so as to be rotatable about an offset cam rotation shaft 39a. The offset cam 39 is driven by an offset motor 110 serving as a drive source and is rotatable about the offset cam rotation shaft 39a. 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 later, so that the offset cam follower 38c rotates in a direction in which it engages with the offset cam 39. However, the present invention 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 in which it engages with the offset cam 39.

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

[0053] As shown in FIG. 4(a), in the case of "thick paper", 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 results in the inner roller 32 being disposed at the first inner roller position, which is the first offset amount X1, where the offset amount X is relatively small. As a result, as described above, it is possible to suppress deterioration in image quality at the rear end of the "thick paper" in the conveying 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 rotary shaft 38a, and the relative position of the inner roller 32 with respect to the outer roller 41 is determined. This results in the inner roller 32 being disposed at the second inner roller position, which is the second offset amount X2, in which 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, seen from one end side of the rotation axis direction of the inner roller 32 (the front side of the paper surface in FIG. 1) in the direction substantially parallel to the rotation axis direction. As described above, in the case of "thick paper", the inner roller holder 38 rotates counterclockwise around the rotation axis 38a (solid line). Then, the abutting portion 38b, which is cylindrical and coaxial with the inner roller 32 and is provided on the inner roller holder 38, abuts against the first positioning portion 40a. As a result, the position of the inner roller 32 is positioned 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 around the rotation axis 38a (two-dot chain line). Then, the abutting portion 38b 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 above 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 above setting (b) shown in Fig. 4(b) may 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 as described above and suppressing image defects that occur near the secondary transfer nip N2. Although not limited to this, the offset amount X is preferably about -3 mm to +3 mm. With such a setting, good transferability can be obtained.

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

[0060] Here, in this embodiment, in the cross section shown in FIG. 4, a counterclockwise moment is always applied to the inner roller holder 38 around the rotation shaft 38a due to the tension of the intermediate transfer belt 31. That is, in this embodiment, a moment in a direction in which the offset cam follower 38c rotates to engage with the offset cam 39 is always applied to the inner roller holder 38 due to the tension of the intermediate transfer belt 31. Also, in this embodiment, in the cross section shown in FIG. 4, the rotation shaft 38a is disposed downstream in the conveying direction of the recording material S with respect to a straight line (nip center line) Lc connecting the rotation center of the inner roller 32 and the rotation center of the outer roller 41. As a result, when the outer roller 41 is abutted against 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, it is possible to configure a cam mechanism without using a separate biasing member such as a spring.

[0061] In addition, in order not to impede the workability of the operation of attaching or detaching the intermediate transfer belt 31 to or from the intermediate transfer belt unit 30 for replacing the intermediate transfer belt 31, it is desirable to arrange the inner roller holder 38 inside the tension surface of the intermediate transfer belt 31. Therefore, in the cross section shown in FIG. 4, it is desirable to arrange the rotation shaft 38a in the 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 indicating the tension surface of the intermediate transfer belt 31 formed by tensioning 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 arranged adjacent to the inner roller 32 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 rotating shaft 38a in the region A will be described in more detail with reference to Fig. 6. Figs. 6(a) and (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 effect of different positions of the rotating shaft 38a. In Figs. 6(a) and (b), the direction of the reaction force from the tension of the intermediate transfer belt 31 is represented by a straight line Lp, and the direction of the reaction force from the outer roller 41 is represented by a straight line Lc.

[0063] As shown in Fig. 6(a), in this embodiment, the pivot shaft 38a is disposed 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 trajectory 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 Fig. 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 (e.g., 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 rotating shaft 38a is arranged in the 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 are received in a clockwise direction. In this case, if the arrangement of the offset cam 39 is changed, a cam mechanism can be configured without the need to add a separate biasing member. However, as the position of the inner roller 32 is changed along the trajectory c, the tension angle of the pre-nip tension wire T is also changed as shown by the two-dot chain line T', and the amount of change is larger than when the rotating shaft 38a is arranged in the region A. The tension angle of the pre-nip tension wire T needs to be set appropriately to prevent deterioration of image quality caused by discharge between the recording material S before entering the secondary transfer nip N2. Therefore, it is desirable that the tension angle of the pre-nip tension wire T does not change significantly by changing the offset amount X. Therefore, it is preferable to arrange the rotating shaft 38a in the region A rather than the region C.

[0065] 6(b), consider the case where the rotation shaft 38a is disposed in the area B between the straight 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 region 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 a schematic configuration of the contact and separation mechanism 2. Fig. 7 shows the configuration of one end of the rotation axis direction of the inner roller 32, but the configuration of the other end is the same (substantially symmetrical with respect to the center of the rotation axis direction of the inner roller 32).

[0068] Both ends of the outer roller 41 in the direction of the rotation axis are rotatably supported by bearings 43. The bearings 43 are supported by a frame of the device main body 100a or the like so as to be slidable in a direction toward the inner roller 32 and in the opposite direction along a predetermined direction (for example, a direction substantially perpendicular to the above-mentioned reference line L1). The bearings 43 are pressed toward the inner roller 32 by a pressure spring 44 constituted by a compression spring that is a biasing member (elastic member) serving as a biasing means. 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 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 mechanism 2 is configured to have a separation arm 122, a separation cam 121, a separation motor 123, and the like. The separation arm 122 is supported by a frame of the apparatus main body 100a or the like so as to be rotatable around a rotation shaft 122a, and is engaged with a bearing 43. The separation arm 122 is configured to rotate by the action of the separation cam 121 as an operating member. The separation cam 121 is supported by a frame of the apparatus main body 100a or the like so as to be rotatable around a separation cam rotation shaft 120. The separation cam 121 is rotatable around the separation cam rotation shaft 120 by receiving drive from the separation motor 123 as a drive source. The separation cam 121 is in contact with a separation cam follower 122b provided on the separation 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 122 b 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 the outer roller 41 is separated from the intermediate transfer belt 31, the separation cam 121 is driven by the separation motor 123 to rotate, for example, counterclockwise, and the separation arm 122 rotates 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, and the outer roller 41 is separated from the intermediate transfer belt 31. On the other hand, as shown by the two-dot chain line in Fig. 7, when the outer roller 41 is brought into contact with the intermediate transfer belt 31, the separation cam 121 is driven by the separation motor 123 to rotate, for example, clockwise, and the separation arm 122 rotates counterclockwise by 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 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 shift correction formed on the intermediate transfer belt 31, from adhering to the surface of the outer roller 41. The separation mechanism 2 also separates the outer roller 41 from the intermediate transfer belt 31 when a jam (paper jam) is being processed. 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 be deformed. Therefore, in this embodiment, the separation mechanism 2 separates the outer roller 41 from the intermediate transfer belt 31 when the job is completed and the image forming apparatus 100 is in a standby state waiting for the next job. The outer roller 41 is maintained in a state separated from the intermediate transfer belt 31 even when the image forming apparatus 100 is in a sleep state or a state in which 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 the execution of a mixed job, the outer roller 41 is in contact with the intermediate transfer belt 31 when the inner roller 32 moves. Also, 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 the execution of a mixed job, the intermediate transfer belt 31 is rotating at the driving 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 ("mixed job") for forming images on a plurality of types of recording materials S, for example, for bookbinding printing. In a mixed job, in order to obtain good transferability for each of a plurality of types of recording materials S having 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 pressing state 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 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 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 the secondary transfer nip N2 is being formed) (here, this is also referred to as an "offset operation" or "position change operation").

[0075] In addition, when the normal paper interval in a job of continuous image formation on the same type of recording material S is insufficient for the offset operation, the paper interval is extended to an extent sufficient for the offset operation. Here, the paper 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 which 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., and the ROM stores a control program, a data table obtained in advance, etc. The CPU 151 and the memory 152 are capable of transferring and reading 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 each part of the image forming apparatus 100 (image forming unit 10, intermediate transfer belt 31, driving devices for members related to the conveyance of the recording material S, various power sources, etc.). In relation to this embodiment, the control unit 150 is particularly connected to an offset motor 110 which is a driving source of the offset mechanism 1, a separation motor 123 which is a driving source of the separation mechanism 2, etc. Also, the control unit 150 is connected to a drum drive motor 111, a belt drive motor 112, a development motor 113, a steering mechanism 90, various high voltage power sources (charging voltage, development voltage, primary transfer voltage, secondary transfer voltage), etc. Also, the control unit 150 is connected to an operation unit (operation panel) 160 provided in the image forming apparatus 100. The operation unit 160 has a display unit as a display means for displaying information under the control of the control unit 150, and an input unit as an input means for inputting information to the control unit 150 by operation by an operator such as a user or a service representative. The operation unit 160 may be configured to have a touch panel having the functions of 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 the image forming apparatus 100 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 the 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) on image forming conditions such as the type of recording material S. The job information also includes image information (image signal) input from the image reading device or the external device 200. Note that the information on the type of recording material (also simply referred to as "information on 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, thin paper, etc. (so-called paper type category), numerical values ​​or numerical ranges of basis weight, thickness, size, or brand (including manufacturer, product number, etc.). In this embodiment, the information on the type of recording material S includes information on the type of recording material S related to the stiffness of the recording material S, particularly, as an example, information on the basis weight of the recording material S.

[0079] Here, the image forming apparatus 100 executes a job, which is a series of operations for forming and outputting an image on one or multiple recording materials S, which is started by one start instruction. A job generally includes an image forming process (printing operation, image forming operation), a pre-rotation process, a paper-interval process in the case of forming an image on multiple recording materials S, and a post-rotation process. The image forming process is a period in which electrostatic image formation, toner image formation, primary transfer of the toner image, and secondary transfer of the toner image are performed for an image that is actually formed on the recording material S and output, and the image formation time (image formation period) refers to this period. More specifically, the timing of the image formation time differs depending on the position where each of the electrostatic image formation, toner image formation, primary transfer of the toner image, and secondary transfer is performed. The pre-rotation process is a period in which a preparatory operation is performed before the image forming process from when a start instruction is input until the image formation actually starts. The paper-interval process is a period corresponding to the period 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 in which a sorting operation (preparatory operation) is performed after the image forming process. Non-image formation time (non-image formation period) is a period other than image formation time, and includes the above-mentioned pre-rotation process, inter-paper process, post-rotation process, and further the pre-multiple-rotation process which is a preparatory operation when the image forming apparatus 100 is turned on or when returning from a sleep state. Note that the sleep state (pause state) is a state in which the supply of power to each part 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 above-mentioned "offset operation" is performed during non-image formation time, particularly during the inter-paper process, will be described.

[0080] 7. Control Procedure FIG. 9 is a flow chart showing an outline of an example of a job control procedure in this embodiment. Here, a mixed 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 a job is started from the home position state, a printing operation on "thick paper" is performed first, and the recording material S is switched from "thick paper" to "thin paper" during the job. However, even when the recording material S is switched from "thin paper" to "thick paper" during the job, for example, 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. Also, here, a case in which an operator causes the image forming apparatus 100 to execute a job from the external device 200 will be described as an example. Note that FIG. 9 shows an outline of the control procedure focusing on the offset operation, and many other operations that are usually required to execute the job and output an image are omitted.

[0081] First, job information (image information, information on image forming conditions, and start instruction) is input from the external device 200 to the control unit 150 (S101). When the job information is input, the control unit 150 acquires information on the type of recording material S of 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 directly input (including selection from a plurality of options) from the external device 200 (or the operation unit 160) by the operation of the operator. 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 that send out the recording material S in the job, which is input from the external device 200 (or the operation unit 160) by the operation of the operator. In this case, the control unit 150 can obtain information regarding the type of recording material S from information regarding the type of recording material S stored in each of the cassettes 61, 62, 63, which is previously associated with each of the cassettes 61, 62, 63 and stored in the memory 152. Here, when registering information regarding the type of recording material S, a corresponding one may be selected from a list of types of recording material S previously stored in the 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 contact mechanism 2 (more specifically, the contact motor 123) to bring the outer roller 41 into contact with the intermediate transfer belt 31, and prepares for the printing operation (S102). Next, the control unit 150 sends an image formation signal to each image forming unit 10 based on the job information, and causes the image forming unit 10 to perform the printing operation (S103). The control unit 150 judges whether the job is to be continued for each page (S104). If the control unit 150 judges in S104 that the job is not to be continued, the control unit 150 ends the job. On the other hand, if the control unit 150 judges in S104 that the job is to be continued, the control unit 150 judges whether the type of the recording material S is changed from the printing operation of the previous page in the printing operation of the next page (S105). If the control unit 150 judges in S105 that the type of the recording material S is not changed, the control unit 150 proceeds to the process of S103 and causes the printing operation of the next page to be performed. On the other hand, when the control unit 150 determines in S105 that the type of the recording material S has been changed, it determines whether or not the position of the inner roller 32 needs to be changed (S106). That is, the control unit 150 determines whether or not 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 type of the recording material S after the change. Here, a case is taken as an example in which a job is started from a state of the home position corresponding to "thick paper", a printing operation is performed on "thick paper" first, and the recording material S is switched from "thick paper" to "thin paper" during the job. Therefore, if the recording material S of 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 on the current position of the inner roller 32 from, for example, 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 information on whether or not the inner roller 32 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 the memory 152. The control unit 150 determines the first inner roller position where the offset amount X is a relatively small first offset amount X1 when printing on a recording material S whose basis weight is equal to or greater than the threshold value. The control unit 150 determines the second inner roller position where the offset amount X is a relatively large second offset amount X2 when printing on a recording material S whose basis weight is less than the threshold value. 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 the position change is not necessary, it proceeds to the process 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 S106 that the position change is necessary, it changes the position of the inner roller 32 in the sheet interval 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, it is necessary to complete the change in the position of the inner roller 32 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 interval, the control unit 150 widens the sheet interval. Specifically, the control unit 150 can adjust the sheet interval by controlling the conveying timing of the succeeding recording material S and the image formation start timing. Next, if the image forming conditions, such as high pressure conditions, need to be changed due to the change in the recording material S, the control unit 150 changes the image forming conditions (S108). As a result, an image can be formed, so the control unit 150 returns to the process of S103 and causes the print operation of the next page to be performed.

[0084] Here, the printing operation of the next page includes an operation of forming a latent image on the photosensitive drum 11 in order to form an image of the next page. In this embodiment, when 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 of the next page (forming a latent image on the photosensitive drum 11 for forming an 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 disturb the surface speed of the intermediate transfer belt 31, which may result in a disturbance 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 image formation 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 so that the position of the inner roller 32 is not changed at least during the period during which the image of the next page is primarily transferred at the primary transfer nip N1.

[0086] In this embodiment, when a job ends and image forming apparatus 100 enters a standby state waiting for the next job, control unit 150 sends a control signal to separation mechanism 2 to separate outer roller 41 from intermediate transfer belt 31. More specifically, at this time, separation mechanism 2 starts an operation (separation operation) to separate outer roller 41 from intermediate transfer belt 31 after the last recording material S of the job has finished passing through 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 the execution of a mixed job. That is, in this embodiment, during the execution of a job in which images are formed on a plurality of recording materials S and output, the relative position between the inner roller 32 and the outer roller 41 in the circumferential direction of the inner roller 32 is changed during the period (sheet interval) after the preceding recording material S has passed the secondary transfer nip N2 and before 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, in this case, 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 are in substantially the same pressing state as during image formation. This makes it possible to suppress a decrease in productivity because no extra time is required other than the time required for changing the position of the inner roller 32. 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 the transferability of each of multiple types of recording materials S in a mixed job.

[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 given the same reference numerals as those of embodiment 1, and detailed explanations are omitted.

[0089] In the first embodiment, when the offset amount X is changed during 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, since the inner roller 32 that is moved in the offset operation is one of the rollers that stretch 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 disturbs the surface speed of the intermediate transfer belt 31, the image may be disturbed at the primary transfer nip N1. In addition, when the shift control of the intermediate transfer belt 31 is performed as described above, the movement of the inner roller 32 may cause a large change in the shift amount. If image formation is performed in a state where the shift amount change (waveform) is not stable, image defects such as color shift may occur. Therefore, when changing the offset amount X during the execution of a mixed job, it may be desirable for the intermediate transfer belt 31 to be decelerated to a driving speed (second speed) lower than the driving speed (first speed) during normal image formation, or to be stopped when the inner roller 32 moves.

[0091] The disturbance of the surface speed and the influence on the deviation control are proportional to the travel distance of the intermediate transfer belt 31. Therefore, by slowing down the driving speed of the intermediate transfer belt 31, the travel distance per unit time can be shortened, and the disturbance of the surface speed and the influence on the deviation control when the inner roller 32 is moved while the intermediate transfer belt 31 is running can be blunted. The driving speed of the intermediate transfer belt 31 after deceleration can be appropriately set according to the driving control characteristics of the intermediate transfer belt 31, the time required to change the position of the inner roller 32, or the disturbance of the surface speed and the influence on the deviation control. Although not limited to this, from the viewpoint of suppressing the disturbance of the surface speed and the influence on the deviation control, the driving speed of the intermediate transfer belt 31 after deceleration is preferably about 1 / 2 or less of that during normal image formation, and the intermediate transfer belt 31 may be stopped. However, from the viewpoint of reducing the time required to return the driving speed of the intermediate transfer belt 31 to its original speed, the driving speed of the intermediate transfer belt 31 after deceleration is preferably about 1 / 5 or more of the driving speed of the intermediate transfer belt 31 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. Then, when changing the offset amount X 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 decelerated to half that speed, 200 mm / sec, or stopped.

[0092] Fig. 10 is a flow chart showing an outline of 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 state, a printing operation is performed on "thick paper" first, 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 forming system such as each image forming unit 10, and turns off all high voltages input to the image forming system (S207). Next, the control unit 150 sends a control signal to the developing motor 113 to stop driving the developing roller of the developing unit 14 (S208). Next, the control unit 150 sends control signals to the belt driving motor 112 and the drum driving motor 111 to reduce the driving 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 been decelerated to the above-mentioned 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 in the reverse order to that 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 speed of the photosensitive drum 11 and the intermediate transfer belt 31 to the drive speed during normal image formation (S211). At this time, if the drive of the photosensitive drum 11 and the intermediate transfer belt 31 is stopped in S209, the drive of the photosensitive drum 11 and the intermediate transfer belt 31 is started to increase the drive speed of the photosensitive drum 11 and the intermediate transfer belt 31 to the drive speed 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 sources (charging voltage, development voltage, primary transfer voltage, secondary transfer voltage) of the image formation system such as each image forming unit 10 to raise the high voltage input to the image formation system (S213). At this time, if it is necessary to change the image forming conditions such as high pressure conditions due to the change in the recording material S, the control unit 150 changes the image forming conditions. As a result, an image can be formed, so the control unit 150 returns to the process of S103 and causes the print operation of the next page to be performed.

[0095] In this manner, in this embodiment, when the offset amount X is changed during the execution of a mixed job, the belt drive motor 112 changes the drive speed of the intermediate transfer belt 31 from the first speed during transfer to a second speed that is smaller 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 the 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 driving speed of the intermediate transfer belt 31 means, more specifically, that the offset mechanism 1 starts the offset operation after the driving speed of the intermediate transfer belt 31 reaches the second speed (the constant speed after the change). Typically, the offset operation starts after the driving speed reaches the second speed, but the second speed and the offset operation may start substantially simultaneously. The timing of reaching the second speed can be determined based on the timing when the driving speed of the intermediate transfer belt 31 actually reaches the second speed, as well as the timing when the driving signal input from the control unit 150 to the belt driving 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, as well as the timing when the input of the driving signal from the control unit 150 to the offset mechanism 1 (more specifically, the offset motor 110) starts, the timing when the drive start signal is input from the control unit 150 to the offset mechanism 1, and the like.

[0097] Moreover, changing the driving speed of the intermediate transfer belt 31 after the offset operation is performed means, more specifically, that the belt driving motor 112 starts changing the driving 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 the driving speed starts after the completion of the offset operation, but the completion of the offset operation and the start of the change in the driving speed may be approximately simultaneous. 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 driving 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 the driving speed starts can be determined based on the timing at which the driving speed of the intermediate transfer belt 31 actually starts to change, the timing at which the driving signal input from the control unit 150 to the belt driving 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 stops and the offset operation starts substantially 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] Moreover, 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 the rotation of the intermediate transfer belt 31 may be substantially 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 intermediate transfer belt 31 actually starts to rotate, 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, and the like.

[0100] As described above, according to this embodiment, it is possible to suppress 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 deviation amount. Furthermore, according to this embodiment, it takes more time to change the driving speed of the intermediate transfer belt 31 than in the first embodiment, but it is possible to suppress the decrease in productivity compared to the case where the outer roller 41 is separated from the intermediate transfer belt 41 and then the offset operation is performed. Therefore, according to this embodiment, like the first embodiment, it is possible to improve the transferability for each of the multiple types of recording materials S in a mixed job while suppressing the 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 given the same reference numerals as those of embodiment 1, and detailed explanations are 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 to the upstream side of the rotation direction of the intermediate transfer belt 31 relative to the outer roller 41 in the case of "thick paper." In response to this, the outer roller 41 may be moved to the downstream side of 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 to the downstream side of the rotation direction of the intermediate transfer belt 31 relative to the outer roller 41 in the case of "thin paper." In response to this, the outer roller 41 may be moved to the upstream side of 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, seen from one end side of the rotation axis direction of the inner roller 32 (the front side of the paper in FIG. 1) in the direction substantially parallel to the rotation axis direction. FIG. 11 shows the configuration of one end of the rotation axis direction of the inner roller 32, but the configuration of the other end is similar (substantially symmetrical with respect to the center of the rotation axis direction of the inner roller 32). In this embodiment, the outer roller 41 is slidable in a direction toward the inner roller 32 along a predetermined first direction (for example, a direction substantially perpendicular to the reference line L1 described above) and in the opposite direction (the white arrow direction in FIG. 11) as in the first embodiment. In addition, in this embodiment, the outer roller 41 is slidable in a direction toward the downstream side of the conveying direction of the recording material S along a predetermined second direction (for example, a direction substantially parallel to the reference line L1 described above) independent of the first direction and in the opposite direction (the black arrow direction in FIG. 11) to the direction toward the downstream side of the conveying direction of the recording material S.

[0104] In this embodiment, the support member 132, which supports the bearing 43 of the outer roller 41 so as to be slidable along the first direction, is supported by 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 as an operating member. The offset cam 131 is supported by the frame of the device main body 100a or the like so as to be rotatable around an offset cam rotation shaft 130. The offset cam 131 is driven by an offset motor 133 as a drive source and is rotatable around the offset cam rotation shaft 130. The offset cam 131 is in contact with an offset cam follower 132a provided on the support member 132. The support member 132 is urged by an offset spring 134, which is an urging member (elastic member) such as a compression spring, as an urging means, so that the offset cam follower 132a slides in a direction in which it engages 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, for example, counterclockwise. Then, the support member 132 slides in a direction toward the downstream side in the conveying 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 in the first outer roller position, which is the first offset amount X1, in which the offset amount X is relatively small. As a result, as described in the first embodiment, it is possible to suppress the deterioration of the image quality of the trailing end of the "thick paper" in the conveying direction. In addition, in the case of "thin paper", the offset cam 131 is driven by the offset motor 133 to rotate, for example, clockwise. Then, the support member 132 slides in a direction toward the upstream side in the conveying 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 in the second outer roller position, which is the second offset amount X2, in which 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 / detachment 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 provide the same effects as those of embodiments 1 and 2. However, since the outer roller 41 needs to be movable in two directions in this embodiment, it can be said that the configuration of embodiment 1 is more advantageous in simplifying the device configuration and making it smaller than the configuration of this embodiment.

[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 given the same reference numerals as those of embodiment 1, and detailed explanations are omitted.

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

[0110] 12 is a schematic side view of the main part of the vicinity of the secondary transfer nip N2 in this embodiment, seen from one end side of the rotation axis direction of the inner roller 32 (the front side of the paper in FIG. 1) in the rotation axis direction. In this embodiment, the image forming apparatus 100 has, 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 contacting the inner peripheral surface of the intermediate transfer belt 31 and the outer roller 41 contacting the inner peripheral surface of the secondary transfer belt 45 sandwich the intermediate transfer belt 31 and the secondary transfer belt 45 to form 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 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. In this embodiment, the separation mechanism 2 has the same configuration as in the first embodiment. In this embodiment, the separation 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 embodiment or the second embodiment. In addition, even when the outer roller and the secondary transfer belt stretched between 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 provide the same effects as those of embodiments 1 and 2. Furthermore, in this embodiment, it is possible to improve the conveyance 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-mentioned embodiments.

[0114] In the above embodiment, information on the basis weight of the recording material is used as information on the type of recording material related to the stiffness of the recording material, but this is not limited to this. When the paper type category (for example, a paper type category based on the surface property such as plain paper or coated paper) or brand (including manufacturer, product number, etc.) is the same, the basis weight and thickness of the recording material are often in a substantially proportional relationship (the thicker the thickness, the larger the basis weight). Also, when the paper type category or brand is the same, the stiffness of the recording material and the basis weight or thickness of the recording material are often in a substantially proportional relationship (the larger the basis weight or thickness, the larger 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. Then, the control unit can operate the offset mechanism so as to obtain an offset amount according to 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. Moreover, the information on the type of recording material is not limited to quantitative information such as the basis weight, thickness, or stiffness of the recording material. For example, only qualitative information such as the paper type category, brand, or a combination of the paper type category and brand may be used as the information on the type of recording material. For example, the offset amount may be set according to the paper type category, brand, or a combination of the paper type category and brand, and the control unit may determine the offset amount according to the paper type category, brand, and other information 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], and can be measured by a commercially available Gurley stiffness tester. For example, the basis weight threshold value of 52 g / m in the above embodiment may be set to 100 g / m. 2 The Gurley stiffness (MD) of an example of "thin paper" as a recording material less than 0.3 mN 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 one example is about 2mN, and the "cardboard" (basis weight 200g / m 2 An example of the Gurley stiffness (MD) may be around 20 mN.

[0115] In the above embodiment, the control unit has been described as acquiring information on the type of recording material based on input from an operation unit operated by an operator or an external device, but the information may be acquired based on input of a detection result from a detection unit that detects information on 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. As a basis weight sensor, for example, a basis weight sensor that uses ultrasonic attenuation is known. This basis weight sensor has an ultrasonic generator and an ultrasonic receiver arranged to sandwich the conveyance path of the recording material. The basis weight sensor receives ultrasonic waves generated by the ultrasonic generator and attenuated by passing through the recording material with the ultrasonic 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 unit that can detect an index value correlated with the basis weight of the recording material, and is not limited to one that uses ultrasonic waves, and may be one that uses light, for example. The index value correlated with the basis weight of the recording material is not limited to the basis weight itself, and may be a thickness corresponding to the basis weight. In addition, a surface property sensor can be used as a smoothness detection means for detecting an index value correlating with the smoothness of the surface of the recording material that can be used to detect the paper type category. A specular reflection light sensor is known as a surface property sensor, which irradiates light onto the recording material and reads the intensity of the specular reflection light and the diffuse reflection light with a light quantity sensor. If the surface of the recording material is smooth, the specular reflection light is strong, and if the surface of the recording material is rough, the diffuse reflection light is strong. Therefore, the surface property sensor can detect an index value correlating with the smoothness of the surface of the recording material by measuring the amount of specular reflection light and the amount of diffuse reflection light. Note that the smoothness detection means is not limited to the above-mentioned light quantity sensor as long as it can detect an index value correlating with the smoothness of the surface of the recording material, and may be, for example, an imaging element. The index value correlating with the smoothness of the surface of the recording material is not limited to a value converted into a value according to a predetermined standard such as Beck smoothness, and may be a value that has a correlation with the smoothness of the surface of the recording material. For example, these detection means can be disposed adjacent to the recording material conveying path upstream of the registration roller in the conveying direction of the recording material. 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 embodiment, the offset mechanism and the separation mechanism are actuators that operate the movable parts with cams, but the present invention is not limited to this. The offset mechanism and the separation mechanism may be any mechanism that can realize the operations according to the above embodiment, and may be, for example, an actuator that operates the movable parts with a solenoid.

[0117] In the above embodiment, the configuration in which either the inner roller or the outer roller is moved has been described, but the offset amount may be changed 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 conveys 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 photoconductor belt, and an electrostatic recording dielectric belt.

[0119] The present invention can also be embodied in other embodiments in which some or all of the configurations of the above-mentioned embodiments are replaced with alternative configurations. Therefore, as long as the image forming apparatus uses a belt-shaped image carrier, it can be embodied regardless of whether it is a tandem type / one-drum type, a charging method, an electrostatic image forming method, a developing method, a transfer method, or a fixing method. In the above-mentioned embodiment, the main parts related to the formation / transfer of the toner image are mainly described, but the present invention can be embodied in various applications such as printers, various printing machines, copiers, FAX machines, and multifunction machines by adding 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 rotatable endless belt for transporting a toner image; A plurality of tension rollers for tensioning the belt; A drive device that rotates and drives the belt; an outer member that is disposed opposite an inner roller of the plurality of tension rollers and contacts an outer peripheral surface of the belt to form a transfer section that transfers a toner image from the belt to a recording material; a position changing mechanism that changes a position of at least one of the inner roller and the outer member to change a relative position between the inner roller and the outer member in a circumferential direction of the inner roller, thereby changing a position of the transfer unit; a separation mechanism for separating and contacting the external member with respect to the belt; A control unit that controls the position change mechanism and the contact mechanism; In an image forming apparatus having The control unit controls the position change mechanism so that the relative position is changed while the external member is in contact with the belt when the relative position is changed during a job of forming and outputting images on multiple recording materials during a period after a preceding recording material has passed the transfer section and before a succeeding recording material reaches the transfer section.

2. 2 . The image forming apparatus according to claim 1 , wherein the control unit controls the contact / separation mechanism so as to separate the external member from the belt when the job is completed.

3. The image forming apparatus according to claim 1 or 2, characterized in that, when the control unit changes the relative position during the period, it controls the position changing mechanism so that the change in the relative position is not performed during the period in which a toner image is transferred from an image carrier to the belt.

4. 3. The image forming apparatus according to claim 1, wherein the control unit controls the image forming operation so that, when the relative position is changed during the period, a print operation for the subsequent recording material is started after the change in the relative position is completed.

5. The image forming apparatus according to any one of claims 1 to 4, characterized in that, when changing the relative position during the period, the control unit controls the position changing mechanism so that the driving device changes the driving speed of the belt from a first speed when performing the transfer to a second speed lower than the first speed, and then changes the relative position.

6. 6. The image forming apparatus according to claim 5, wherein the control unit controls the position changing mechanism so as to start changing the relative position after the driving speed of the belt reaches the second speed.

7. The image forming apparatus according to claim 5 or 6, characterized in that, when the control unit changes the relative position during the period, the control unit controls the drive device so as to change the drive speed of the belt from the second speed to the first speed after changing the relative position.

8. 8. The image forming apparatus according to claim 7, wherein the control unit controls the drive device to start changing the drive speed of the belt from the second speed to the first speed after the change in the relative position is completed.

9. 5. The image forming apparatus according to claim 1, wherein the control unit controls the position changing mechanism so that, when changing the relative position during the period, the relative position is changed after the driving device stops driving the belt.

10. 10. The image forming apparatus according to claim 9, wherein the control unit controls the position changing mechanism so as to start changing the relative position after the rotation of the belt has stopped.

11. 11. The image forming apparatus according to claim 9, wherein, when the control unit changes the relative position during the period, the control unit controls the driving device so as to start driving the belt after the relative position is changed.

12. 12. The image forming apparatus according to claim 11, wherein the control unit controls the driving device so that the belt starts to rotate after the change in the relative position is completed.

13. the outer member is configured to have an outer roller that directly contacts the outer peripheral surface of the belt, or an endless separate belt and an outer roller that contacts the outer peripheral surface of the belt via the separate belt, the plurality of tension rollers include an upstream roller disposed adjacent to the inner roller upstream of the inner roller in a rotation direction of the belt, In a cross section substantially perpendicular to the direction of the rotation axis of the inner roller, when a common tangent to the inner roller and the upstream roller on which the belt is wound is defined as a reference line L1, a straight line passing through the rotation center of the inner roller and substantially perpendicular to the reference line L1 is defined as an inner roller center line L2, a straight line passing through the rotation center of the outer roller and substantially perpendicular to the reference line L1 is defined as an outer roller center line L3, and the distance between the inner roller center line L2 and the outer roller center line L3 is defined as an offset amount X (which is a positive value when the outer roller center line L3 is located upstream of the inner roller center line L2 in the rotation direction of the belt), 13. The image forming apparatus according to claim 1, wherein the position changing mechanism changes the offset amount X by changing the position of at least one of the inner roller and the outer roller.

14. 14. The image forming apparatus according to claim 1, wherein the position changing mechanism changes a position of the inner roller.

15. 15. The image forming apparatus according to claim 1, further comprising a guide member for guiding the recording material to the transfer portion, the guide member being disposed upstream of the transfer portion in a conveying direction of the recording material.

16. 16. The image forming apparatus according to claim 1, wherein the belt is an intermediate transfer member that conveys a toner image that has been primarily transferred from an image carrier to a recording material in the transfer portion for secondary transfer.

Images