Image forming device

Abstract

To provide an image forming apparatus that can prevent the occurrence of leading end registration deviation caused by a change in the amount of offset.SOLUTION: An image forming apparatus 100 has: image forming means 10; a belt 21; a plurality of stretching rollers including an inner roller 26; an outer roller 41; a position changing mechanism 101 that can change the relative position of the inner roller 26 to the outer roller 41 with respect to a circumferential direction of the inner roller 26 to a first relative position and a second relative position different from the first relative position; a conveying member 13 that conveys a recording material to a transfer part N2; and a conveyance driving unit 114 that drives the conveying member 13. The image forming apparatus has a control unit 150 that, for image formation on one recording material P, can execute control of changing the time from timing for the image forming means 10 to write an image to timing for the conveying member 13 to start conveying the recording material P between when performing transfer with the relative position as the first relative position and when performing transfer with the relative position as the second relative position.SELECTED DRAWING: Figure 7

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 to a sheet-like recording material such as paper for secondary transfer.

[0003] In an image forming apparatus using an intermediate transfer belt, a toner image formed on a photoreceptor or the like in an image forming section is primarily transferred to the 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. A secondary transfer section (secondary transfer nip) which is a contact section between the intermediate transfer belt and the outer member is formed by an inner member (secondary transfer inner member) provided on the inner peripheral surface side of the intermediate transfer belt and an outer member (secondary transfer outer member) provided on the outer peripheral surface side of the intermediate transfer belt. As the inner member, an inner roller (secondary transfer inner roller), which is one of a plurality of tension rollers that tension the intermediate transfer belt, is used. As the outer member, an outer roller (secondary transfer outer roller) is used which is disposed at a position facing the inner roller across the intermediate transfer belt and is pressed against the inner roller. A voltage of the opposite polarity to the charge polarity of the toner is then applied to the outer roller (or a voltage of the same polarity as the charge polarity of the toner is applied to the inner roller), whereby the toner image on the intermediate transfer belt is secondarily transferred onto the recording material at the secondary transfer section. Generally, a transport guide for guiding the recording material to the secondary transfer section is provided upstream of the secondary transfer section in terms of the transport direction of the recording material. Note that with respect to the recording material, the "leading edge" and "trailing edge" refer to the leading edge and trailing edge, respectively, in terms of the transport direction of the recording material.

[0004] Here, depending on the stiffness of the recording material, the behavior of the recording material changes in the vicinity of the upstream or downstream of the secondary transfer portion in the conveying direction of the recording material, which may affect the image that is the result. 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 together in the vicinity of the downstream of the secondary transfer portion 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. On the other hand, when the recording material is "thick paper," which is an example of a recording material with high stiffness, the rear end (rear end, or an area close to the rear end) of the recording material may collide with the intermediate transfer belt when the rear end of the recording material passes through the conveying guide. This may disturb the posture of the intermediate transfer belt in the vicinity of the upstream of the secondary transfer portion in the conveying direction of the recording material, and image defects (such as streak-like image disturbances extending in a direction approximately perpendicular to the conveying direction of the recording material) may occur at the rear end of the recording material. In recent years, these issues often become apparent in the commercial printing market, where there is a demand to respond to diversifying recording materials.

[0005] In view of this, a configuration has been proposed in which the shape of the secondary transfer portion (the position of the secondary transfer portion) is changed depending on the type of recording material (Patent Document 1). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2014-134718 A Summary of the Invention [Problem to be solved by the invention]

[0007] In order to improve the separation of the recording material from the intermediate transfer belt and to suppress image defects at the trailing end of the recording material, it is effective to change the shape of the secondary transfer portion (the position of the secondary transfer portion) according to the type of recording material, as described in Patent Document 1. The shape of the secondary transfer portion (the position of the secondary transfer portion) can be changed by moving the inner roller or the outer roller to change the relative position between the inner roller and the outer roller in the circumferential direction of the inner roller (the "offset amount" described later).

[0008] However, when the offset amount is changed according to the type of recording material, the position of the tension roller of the intermediate transfer belt, typically the position of the tension roller that applies tension to the intermediate transfer belt between the primary transfer unit and the secondary transfer unit, may change at the same time. When the position of the tension roller changes, the length from the primary transfer unit to the secondary transfer unit in the rotation direction of the intermediate transfer belt also changes. As a result, the timing at which the image on the intermediate transfer belt is conveyed to the secondary transfer unit may shift, causing a phenomenon in which the leading edge position of the image formed on the recording material shifts from the desired (original) position (also referred to here as "leading edge misalignment"). When leading edge misalignment occurs, for example, the leading edge or trailing edge of the image may extend beyond the recording material, or when printing in a writing frame on a preprinted paper, the printing position may extend beyond the writing frame or overlap the frame line.

[0009] 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.

[0010] SUMMARY OF THE PRESENT EMBODIMENT An object of the present invention is to provide an image forming apparatus capable of suppressing the occurrence of leading edge registration deviation caused by a change in the offset amount. [Means for solving the problem]

[0011] The above object is achieved by an image forming apparatus according to the present invention. In summary, the present invention includes an image carrier that carries a toner image, an image forming section that forms a toner image on the image carrier, an endless belt onto which the toner image formed on the image carrier is transferred at a primary transfer section, a plurality of tension rollers including an inner roller that tension the belt, an outer roller that cooperates with the inner roller to form a secondary transfer section where the toner image is transferred from the belt onto a recording material, a position change mechanism that moves the inner roller to change the position of the secondary transfer section in the circumferential direction of the inner roller, the position change mechanism being capable of changing the position of the inner roller to a plurality of positions including a first position and a second position that is located upstream of the first position in the rotation direction of the belt, a conveying member that conveys the recording material to the secondary transfer section, a conveying drive section that drives the conveying member, and a control section that controls the start timing of conveying the recording material by the conveying member, and the control section controls the position of the inner roller to be set when the toner image is transferred onto the recording material. The above No. 1 of In the case of the position, the timing at which the conveying member starts conveying the recording material is controlled to a first timing, and the position of the inner roller set when the toner image is transferred onto the recording material is The above Second of In the case of location, In order to suppress the amount of image positional deviation caused by changing the position of the inner roller, The image forming apparatus is characterized in that a timing at which the conveying member starts conveying the recording material is controlled to a second timing different from the first timing. According to another aspect of the present invention, a recording medium storing image recording apparatus includes an image carrier that carries a toner image, an image forming unit that forms a toner image on the image carrier, an endless belt to which the toner image formed on the image carrier is transferred at a primary transfer unit, a plurality of tension rollers including an inner roller that tension the belt, an outer roller that cooperates with the inner roller to form a secondary transfer unit where the toner image is transferred from the belt onto a recording material, a position change mechanism that moves the inner roller to change a position of the secondary transfer unit in a circumferential direction of the inner roller, the position change mechanism being capable of changing the position of the inner roller to a plurality of positions including a first position and a second position that is located upstream of the first position in the rotation direction of the belt, a conveying member that conveys the recording material to the secondary transfer unit, a conveying drive unit that drives the conveying member, and a control unit that controls a time difference between an image writing start timing of the image forming unit and a conveying start timing of the recording material by the conveying member, and the control unit controls the position of the inner roller set at the time of transferring the toner image onto the recording material to be equal to or larger than the first position. of In the case of the transfer of the toner image onto the recording material, the time difference is set to the first time difference, and the position of the inner roller set at the time of the transfer of the toner image onto the recording material is set to the second time difference. of In the case of location, In order to suppress the amount of image positional deviation caused by changing the position of the inner roller, There is provided an image forming apparatus, characterized in that the transport drive unit and the image forming unit are controlled so as to set the time difference to a second time difference different from the first time difference. According to another aspect of the present invention, a recording medium storing apparatus includes an image carrier that carries a toner image, an image forming unit that forms a toner image on the image carrier, an endless belt to which the toner image formed on the image carrier is transferred at a primary transfer unit, a plurality of tension rollers including an inner roller that tension the belt, an outer roller that cooperates with the inner roller to form a secondary transfer unit where the toner image is transferred from the belt onto a recording material, a position change mechanism that moves the inner roller to change a position of the secondary transfer unit in a circumferential direction of the inner roller, the position change mechanism being capable of changing the position of the inner roller to a plurality of positions including a first position and a second position that is located upstream of the first position in the rotation direction of the belt, a conveying member that conveys the recording material to the secondary transfer unit, a conveying drive unit that drives the conveying member, and a control unit that controls the conveying drive unit, wherein the control unit is configured to control the position of the inner roller when the position of the inner roller set when the toner image is transferred onto the recording material to be the first position. of In the case of the second position, the conveying drive unit is controlled by the first control, and the position of the inner roller set when the toner image is transferred onto the recording material is the second control. of In the case of location, In order to suppress the amount of image positional deviation caused by changing the position of the inner roller, There is provided an image forming apparatus, characterized in that the transport drive unit is controlled so as to be controlled by a second control different from the first control. Effect of the Invention

[0012] According to the present invention, it is possible to suppress the occurrence of leading edge registration deviation caused by a change in the offset amount. [Brief description of the drawings]

[0013] [Figure 1] FIG. 1 is a schematic cross-sectional view of an image forming apparatus. [Diagram 2] 2 is a schematic block diagram showing a control mode of a main part of the image forming apparatus; FIG. [Diagram 3] 5 is a schematic cross-sectional view of the vicinity of a secondary transfer nip for explaining the conveying posture of a recording material. FIG. [Figure 4] FIG. 11 is a schematic side view for explaining an offset mechanism. [Diagram 5]FIG. 11 is a schematic side view for explaining an offset mechanism. [Figure 6] 5 is a schematic cross-sectional view for explaining an example of the relationship between the offset amount and the tension state of the intermediate transfer belt. FIG. [Figure 7] FIG. 4 is a flow chart of control in the first embodiment. [Figure 8] FIG. 11 is a flow chart of control in the second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

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

[0015] [Example 1] 1. Overall configuration and operation of the image forming apparatus 1 is a schematic cross-sectional view of an image forming apparatus 100 according to this embodiment. The image forming apparatus 100 according to this embodiment is a tandem 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, recording medium, media) P such as paper using an electrophotographic method in response to an image signal transmitted from an external device, for example.

[0016] The image forming apparatus 100 has, as a plurality of image forming means, four image forming units (stations) 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 a row along the moving direction of an image transfer surface that is arranged substantially horizontally of an intermediate transfer belt 21, which will be described later. Elements in each of 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 element is for one of the colors. In this embodiment, the image forming unit 10 is configured to include photosensitive drums 1 (1Y, 1M, 1C, 1K), chargers 2 (2Y, 2M, 2C, 2K), exposure devices 3 (3Y, 3M, 3C, 3K), developing devices 4 (4Y, 4M, 4C, 4K), primary transfer rollers 23 (23Y, 23M, 23C, 23K), cleaning devices 5 (5Y, 5M, 5C, 5K), etc., which will be described later.

[0017] The image forming unit 10 is provided with a photosensitive drum 1, which is a rotatable drum-type (cylindrical) photosensitive body (electrophotographic photosensitive body) serving as a first image carrier that carries a toner image. The photosensitive drum 1 is rotated in the direction of arrow R1 (counterclockwise) in FIG. 1 by a driving force transmitted from a drum driving unit 111 (FIG. 2) serving as a driving means having a drum driving motor 111a serving as a driving source. The surface of the rotating photosensitive drum 1 is uniformly charged to a predetermined potential of a predetermined polarity (negative polarity in this embodiment) by a charger (charging roller) 2 serving as a charging means. During charging, a predetermined charging voltage is applied to the charger 2 by a charging power source (not shown). The surface of the charged photosensitive drum 1 is scanned and exposed by an exposure device 3 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 1. In this embodiment, the exposure device 3 is configured as a laser scanner device that irradiates the photosensitive drum 1 with laser light modulated in accordance with the image signal. The electrostatic image formed on the photosensitive drum 1 is developed (visualized) by supplying toner as a developer by the developing device 4 as a developing means, and a toner image (developer image) is formed on the photosensitive drum 1. In this embodiment, toner charged with the same polarity as the charging polarity of the photosensitive drum 1 (negative polarity in this embodiment) adheres to the exposed portion (image portion) on the photosensitive drum 1, in which the absolute value of the potential is reduced by being exposed after being uniformly charged (reverse development). The developing device 4 has a developing roller (not shown) 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 1. The developing roller is rotated by, for example, transmitting a driving force from the drive system of the photosensitive drum 1. During development, a predetermined developing voltage is applied to the developing roller by a developing power source (not shown).

[0018] An intermediate transfer belt 21, 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 1Y, 1M, 1C, and 1K. The intermediate transfer belt 21 is stretched around a plurality of tension rollers (support rollers), including a drive roller 22, an upstream auxiliary roller 25a, a downstream auxiliary roller 25b, a tension roller 24, a secondary transfer pre-roller 29, and an inner roller 26, and is stretched with a predetermined tension. The drive roller 22 transmits a driving force to the intermediate transfer belt 21. The tension roller 24 is disposed downstream of a primary transfer nip N1 (described later) and upstream of a secondary transfer nip N2 (described later) in terms of the rotation direction (conveyance direction, movement direction, running direction) of the intermediate transfer belt 21, and applies a predetermined tension to the intermediate transfer belt 21. The secondary transfer pre-roller 29 forms the surface of the intermediate transfer belt 21 in the vicinity of the upstream of the secondary transfer nip N2 in the rotation direction of the intermediate transfer belt 21. The inner roller (secondary transfer inner roller, secondary transfer counter roller, inner member) 26 functions as a counter member (counter electrode) of the outer roller 41 (described later). The upstream auxiliary roller 25a and the downstream auxiliary roller 25b form an image transfer surface disposed substantially horizontally. The driving roller 22 is driven to rotate by a driving force transmitted from the intermediate transfer belt driving unit 113 (FIG. 2) as a driving means having a belt driving motor 113a as a driving source. As a result, the intermediate transfer belt 21 receives a driving force from the driving roller 22 and rotates (moves around) in the direction of the arrow R2 in the figure (clockwise). The multiple tension rollers other than the driving roller 22 are rotated in accordance with the rotation of the intermediate transfer belt 21. Primary transfer rollers 23Y, 23M, 23C, and 23K, which are roller-shaped primary transfer members serving as primary transfer means, are disposed on the inner peripheral surface side of the intermediate transfer belt 21 in correspondence with the respective photosensitive drums 1Y, 1M, 1C, and 1K. The primary transfer rollers 23 press the intermediate transfer belt 21 toward the photosensitive drums 1 to form a primary transfer nip N1, which is a contact portion between the photosensitive drums 1 and the intermediate transfer belt 21, as a primary transfer portion.

[0019] The toner image formed on the photosensitive drum 1 as described above is primarily transferred onto the rotating intermediate transfer belt 21 at the primary transfer nip N1 by the action of the primary transfer roller 23. 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) (positive polarity in this embodiment) is applied to the primary transfer roller 23 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 1 are primarily transferred in sequence so as to be superimposed on the same image forming area on the intermediate transfer belt 21. In this embodiment, the primary transfer nip N1 is an image forming position where a toner image is formed on the intermediate transfer belt 21. The intermediate transfer belt 21 is an example of a rotatable endless belt that transports the toner image carried at the image forming position.

[0020] On the outer peripheral surface side of the intermediate transfer belt 21, an outer roller (secondary transfer outer roller, secondary transfer roller, outer member) 41, which is a roller-shaped secondary transfer member (transfer rotating body) as a secondary transfer means, is disposed at a position facing the inner roller 26. The outer roller 41 is pressed toward the inner roller 26 via the intermediate transfer belt 21, forming a secondary transfer nip N2 as a secondary transfer portion which is a contact portion between the intermediate transfer belt 21 and the outer roller 41. As described above, the toner image formed on the intermediate transfer belt 21 is secondarily transferred onto the recording material P, which is sandwiched and conveyed between the intermediate transfer belt 21 and the outer roller 41, by the action of the outer roller 41 in the secondary transfer nip N2. In this embodiment, during the secondary transfer, a secondary transfer voltage, which is a DC voltage of a polarity opposite to the normal charging polarity of the toner (positive polarity in this embodiment), is applied to the outer roller 41 by a secondary transfer power source (not shown). In this embodiment, the inner roller 26 is electrically grounded (connected to ground). Alternatively, the inner roller 26 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.

[0021] The recording material P is conveyed to the secondary transfer nip N2 in synchronization with the toner image on the intermediate transfer belt 21. That is, the recording material P is stored in a recording material storage (cassette) 11 as a recording material storage section. The recording material P is sent out from the recording material storage 11 by a feeding section such as a feeding roller 19 provided in the recording material storage 11. After the recording material P has its posture adjusted by the registration adjustment section 12, it is conveyed toward the secondary transfer nip N2 by the registration adjustment section 12 at a predetermined timing ("registration ON timing" to be described later). Here, the registration adjustment section 12 includes a pair of registration rollers (registration roller pair) 13, which are roller-shaped conveying members serving as conveying means, and a registration drive section (conveyance drive section) 114 (FIG. 2) serving as a driving section for driving the registration rollers 13. The registration rollers 13 are driven to rotate by the registration drive section 114, so that the recording material P is conveyed at the contact section (nip section) of the pair of registration rollers 13. The registration drive unit 114 includes a registration drive motor 114a as a drive source, and drives at least one (or both) of the pair of registration rollers 13. In this embodiment, the control unit 150 (FIG. 2) functions as a registration ON timing change unit, and can change the registration ON timing, that is, the timing at which the registration roller 13 starts conveying the recording material P. The control unit 150 may also be capable of changing the conveying speed of the recording material P in the secondary transfer nip N2 by controlling the number of rotations (rotation speed) of the registration drive motor 114a of the registration drive unit 114 to control the number of rotations (rotation speed) of the registration roller 13. The recording material P fed from the recording material storage 11 is temporarily stopped by the registration roller 13. Then, the recording material P is fed into the secondary transfer nip N2 by starting (resuming) the rotation drive of the registration roller 13 so that the toner image on the intermediate transfer belt 21 coincides with the desired image forming area on the recording material P in the secondary transfer nip N2. In addition, near the downstream side of the registration roller 13 in terms of the transport direction of the recording material P, a registration sensor 18 is provided as a recording material detection means (recording material detection section) for detecting the recording material P, particularly, the leading edge of the recording material P in this embodiment.

[0022] In terms of the conveying direction of the recording material P, downstream of the registration roller 13 and upstream of the secondary transfer nip N2, a conveying guide 27 for guiding the recording material P to the secondary transfer nip N2 is provided. The conveying guide 27 is configured to have a first guide member 27a capable of contacting the front surface of the recording material P (the surface onto which the toner image is transferred immediately after passing through the conveying guide 27) and a second guide member 27b capable of contacting the back surface of the recording material P (the surface opposite to the front surface). The first guide member 27a and the second guide member 27b are disposed opposite to each other, and the recording material P passes between these two members. The first guide member 27a regulates the movement of the recording material P in a direction approaching the intermediate transfer belt 21. The second guide member 27b regulates the movement of the recording material P in a direction away from the intermediate transfer belt 21.

[0023] The recording material P onto which the toner image has been transferred is conveyed by a conveyor belt 14 to a fixing device 15 as a fixing means. The conveyor belt 14 is driven by a conveyor drive motor (not shown). A suction fan (not shown) is arranged on the inner peripheral surface side of the conveyor belt 14 to attract the recording material P, and the recording material P is attracted toward the conveyor belt 14. The fixing device 15 heats and pressurizes the recording material P carrying the unfixed toner image, thereby fixing (melting and bonding) the toner image to the surface of the recording material P. Thereafter, the recording material P onto which the toner image has been fixed is discharged (output) by a discharge device 16 to a discharge tray 17 provided outside (outside) the device main body 110 of the image forming apparatus 100.

[0024] Meanwhile, the toner remaining on the photosensitive drum 1 after the primary transfer (primary transfer residual toner) is removed from the photosensitive drum 1 and collected by a cleaning device 5 as a cleaning means. Also, the toner remaining on the intermediate transfer belt 21 after the secondary transfer (secondary transfer residual toner) and adhering matters such as paper powder adhering from the recording material P are removed from the intermediate transfer belt 21 and collected by a belt cleaning device 28 as an intermediate transfer body cleaning means.

[0025] In this embodiment, the intermediate transfer belt unit 20 as a belt conveying device includes the intermediate transfer belt 21 stretched around a plurality of tension rollers, the primary transfer rollers 23, the belt cleaning device 28, and a frame supporting these. The intermediate transfer belt unit 20 is detachable from the device main body 110 for maintenance or replacement.

[0026] 2. Offset 3 is a schematic cross-sectional view (a cross-section substantially perpendicular to the rotation axis direction of the inner roller 26) for explaining the behavior of the recording material P in the vicinity of the secondary transfer nip N2. 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.

[0027] In this embodiment, both ends of the outer roller 41 in the direction of its rotation axis are rotatably supported by bearings 43. The bearings 43 are slidable in the direction toward the inner roller 26 and in the opposite direction. The bearings 43 are supported by the frame of the device main body 110 or the like. The bearings 43 are pressed toward the inner roller 26 by a pressure spring 44, which is a biasing member (elastic member) serving as a biasing means, and is constituted by a compression spring. As a result, the outer roller 41 abuts against the inner roller 26 with a predetermined pressure across the intermediate transfer belt 21, forming the secondary transfer nip N2. In this embodiment, the outer roller 41 is rotated in response to the rotation of the intermediate transfer belt 21. Here, the directions of the rotation axes of the tension rollers of the intermediate transfer belt 21, including the inner roller 26, and the outer roller 41 are approximately parallel to each other.

[0028] As described above, the behavior of the recording material P in the upstream and downstream vicinity of the secondary transfer nip N2 in the conveying direction of the recording material P 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 P. For example, when the recording material P is "thin paper," which is an example of a recording material P with low stiffness, a jam (paper jam) may occur due to poor separation of the recording material P from the intermediate transfer belt 21. This phenomenon becomes more noticeable when the stiffness of the recording material P is low, because the recording material P is more likely to stick to the intermediate transfer belt 21 due to its weak stiffness.

[0029] That is, in the cross section shown in FIG. 3, a line showing the tension surface of the intermediate transfer belt 21 formed by tensioning the inner roller 26 and the pre-secondary transfer roller 29 is defined as a pre-nip tension line T. The pre-secondary transfer roller 29 is an example of an upstream roller arranged adjacent to the inner roller 26 upstream of the inner roller 26 in the rotation direction of the intermediate transfer belt 21 among the multiple tension rollers. In addition, in the same cross section, a straight line passing through the rotation center of the inner roller 26 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 21 from the rotation center of the inner roller 26 in the direction along the pre-nip tension line T.

[0030] At this time, the recording material P tends to maintain its position substantially along the nip line Ln when it is sandwiched between the inner roller 26 and the outer roller 41 in the secondary transfer nip N2. Therefore, generally, when the rotation center of the inner roller 26 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 θa of the recording material P becomes small as shown by the dashed line A in FIG. 3. That is, the leading end of the recording material P is in a position such that it is discharged near the intermediate transfer belt 21 when it is discharged from the secondary transfer nip N2. This makes the recording material P more likely to stick to the intermediate transfer belt 21. On the other hand, the more the rotation center of the outer roller 41 is disposed upstream of the rotation center of the inner roller 26 in the direction along the pre-nip tension line T in the rotation direction of the intermediate transfer belt 21, the larger the discharge angle θb of the recording material P becomes as shown by the solid line in FIG. 3. That is, when the leading edge of the recording material P is discharged from the secondary transfer nip N2, the leading edge of the recording material P is in a position such that the recording material P is discharged in a direction away from the intermediate transfer belt 21. This makes it difficult for the recording material P to stick to the intermediate transfer belt 21.

[0031] On the other hand, as described above, for example, when the recording material P is "thick paper," which is an example of a recording material P with high stiffness, the rear end of the recording material P may collide with the intermediate transfer belt 21 when the rear end of the recording material P passes through the conveying guide 27. This may cause image defects at the rear end of the recording material P. This phenomenon becomes more noticeable when the stiffness of the recording material P is high, because the stiffness of the recording material P makes it easier for the rear end of the recording material P to collide with the intermediate transfer belt 21 with great force.

[0032] That is, as described above, in the cross section shown in FIG. 3, when the recording material P is sandwiched between the inner roller 26 and the outer roller 41 at the secondary transfer nip N2, it tends to maintain its posture substantially along the nip line Ln. Therefore, the more the center of rotation of the outer roller 41 is located upstream of the center of rotation of the inner roller 26 in the direction along the pre-nip tension line T in the rotation direction of the intermediate transfer belt 21, the more the nip line Ln bites into the pre-nip tension line T. As a result, when the trailing end of the recording material P leaves the conveying guide 27, as shown by the dashed line B in FIG. 3, the trailing end of the recording material P collides with the intermediate transfer belt 21, and image defects are likely to occur at the trailing end of the recording material P. In response to this, if the center of rotation of the inner roller 26 and the center of rotation of the outer roller 41 are brought closer to each other in the direction along the pre-nip tension line T, the trailing end of the recording material P is prevented from colliding with the intermediate transfer belt 21 when it leaves the conveying guide 27. As a result, image defects at the trailing edge of the recording material P are less likely to occur.

[0033] Therefore, in order to improve the separation of the recording material P from the intermediate transfer belt 21 and to suppress image defects at the trailing end of the recording material P, it is effective to do the following: Change the relative position of the inner roller 26 and the outer roller 41 in the circumferential direction of the inner roller 26 (the rotation direction of the intermediate transfer belt 21) and change the shape of the secondary transfer nip N2 (the position of the secondary transfer nip N2) according to the type of recording material P.

[0034] With reference to FIG. 3, the definition of the offset amount X indicating the relative position between the inner roller 26 and the outer roller 41 will be described. In the cross section shown in FIG. 3, a common tangent line between the inner roller 26 and the pre-secondary transfer roller 29 on the side around which the intermediate transfer belt 21 is wound 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 26 and substantially perpendicular to the reference line L1 is defined as an inner roller center line L2. In addition, 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 (perpendicular distance) between the inner roller center line L2 and the outer roller center line L3 is defined as the offset amount X (a positive value is defined when L3 is located upstream of L2 in the rotation direction of the intermediate transfer belt 21). 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 21 increases toward the upstream side in the rotation direction of the intermediate transfer belt 21. In other words, the upstream end of the contact area between the outer roller 41 and the intermediate transfer belt 21 in the rotation direction of the intermediate transfer belt 21 is positioned more upstream than the upstream end of the contact area between the inner roller 26 and the intermediate transfer belt 21 in the rotation direction of the intermediate transfer belt 21. In this way, by changing the position of at least one of the inner roller 26 or the outer roller 41 to change the relative position between the inner roller 26 and the outer roller 41 in the circumferential direction of the inner roller 26, the position of the secondary transfer nip (transfer portion) N2 can be changed.

[0035] Here, 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 26 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 21 with respect to the inner roller 26 and pressed by the pressure spring 44 so as to sandwich the intermediate transfer belt 21 between the outer roller 41 and the inner roller 26, a substantially S-shaped secondary transfer nip N2 is formed. Then, the posture of the recording material P guided by the conveying guide 27 and conveyed is also determined according to the shape of the secondary transfer nip N2. The larger the offset amount X, the greater the amount of bending of the recording material P. Therefore, for example, when the recording material P is "thin paper", by increasing the offset amount X, the separation property of the recording material P from the intermediate transfer belt 21 after passing through the secondary transfer nip N2 can be improved. However, if the offset amount X is large, the bending amount of the recording material P is large, and for example, if the recording material P is "thick paper," the rear end of the recording material P is likely to collide with the intermediate transfer belt 21 when the rear end of the recording material P leaves the conveying guide 27. This causes a decrease in image quality at the rear end of the recording material P, but in this case, the offset amount X should be reduced.

[0036] 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 26 or the outer roller 41. Particularly, in this embodiment, the image forming apparatus 100 changes the position of the inner roller 26 to change the offset amount X. Also, in this embodiment, the image forming apparatus 100 changes the offset amount X based on the basis weight information of the recording material (paper) P as information on the type of the recording material P related to the stiffness of the recording material P. For example, when the recording material P is "thin paper", the inner roller 26 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 P is "thick paper", the inner roller 26 is disposed at the second inner roller position where the offset amount X is the second offset amount X2 smaller than the first offset amount X1. The first offset amount X1 is typically a positive value, and the second offset amount X2 may be a positive value, 0, or a negative value. In this embodiment, the relative position between the inner roller 26 and the outer roller 41 when the offset amount X is the first offset amount X1 is the first relative position, and the relative position between the inner roller 26 and the outer roller 41 when the offset amount X is the second offset amount X2 is the second relative position. That is, in this embodiment, the position of the secondary transfer nip N2 when the offset amount X is the first offset amount X1 is the first position of the transfer portion, and the position of the secondary transfer nip N2 when the offset amount X is the second offset amount X2 is the second position of the transfer portion.

[0037] 3.Offset mechanism Next, the offset mechanism 101 in this embodiment will be described. Here, "thin paper" is used as an example of a recording material P with low stiffness, and "thick paper" is used as an example of a recording material P with high stiffness. Figures 4(a) and (b) are schematic side views of the main part of the vicinity of the secondary transfer nip N2 in this embodiment, viewed from one end side in the rotational axis direction of the inner roller 26 (the front side of the paper in Figure 1) in the direction substantially parallel to the rotational axis direction. Figure 4(a) shows the state when the condition of the recording material P passing through the secondary transfer nip N2 is "thin paper", and Figure 4(b) shows the state when it is "thick paper".

[0038] As shown in Figures 4(a) and (b), in this embodiment, the image forming apparatus 100 has an offset amount changing mechanism (hereinafter, also simply referred to as "offset mechanism") 101 as an offset amount changing means. In this embodiment, the offset mechanism 101 functions as a position changing means (position changing mechanism) and changes the relative position of the inner roller 26 with respect to the outer roller 41 to change the offset amount X. Figures 4(a) and (b) show the configuration of one end of the inner roller 26 in the rotation axis direction, but the configuration of the other end is similar (substantially symmetrical with respect to the center of the inner roller 26 in the rotation axis direction).

[0039] Both ends of the inner roller 26 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 20 or the like so as to be rotatable about an inner roller rotation shaft 38a. In this manner, by rotating the inner roller holder 38 about the inner roller rotation shaft 38a to rotate the inner roller 26 about the inner roller rotation shaft 38a, the relative position of the inner roller 26 with respect to the outer roller 41 can be changed, and the offset amount X can be changed.

[0040] 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 20 or the like so as to be rotatable about an offset cam rotation shaft 39a. The offset cam 39 is driven by an offset cam drive motor 101a 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. As will be described later, the inner roller holder 38 is biased by the tension of the intermediate transfer belt 21 so that the offset cam follower 38c rotates in a direction in which it contacts the offset cam 39. However, this is not limited to this, and the inner roller holder 38 may be biased by a spring or the like, which is a biasing member (elastic member) serving as a biasing means, so that the offset cam follower 38c rotates in a direction in which it contacts the offset cam 39.

[0041] In this embodiment, the image forming apparatus 100 is provided with an offset cam position sensor 37 that detects the position in the rotation direction of the offset cam 39 as a detection unit that detects the relative position between the inner roller 26 and the outer roller 41 (the position of the inner roller 26 in this embodiment). The offset cam position sensor 37 is configured to have, for example, a flag as an indicator provided on the offset cam 39 or coaxially with the offset cam 39, a photointerrupter as a detector, and the like.

[0042] As described above, in this embodiment, the offset mechanism 101 is configured by having the inner roller holder 38, the offset cam 39, the offset cam drive motor 101a, the offset cam position sensor 37, and the like.

[0043] As shown in FIG. 4A, in the case of "thin paper", the offset cam 39 is driven by the offset cam drive motor 101a to rotate, for example, counterclockwise. This causes the inner roller holder 38 to rotate clockwise around the inner roller rotation shaft 38a, and the relative position of the inner roller 26 with respect to the outer roller 41 is determined. As a result, the inner roller 26 is placed at the first inner roller position, which is the first offset amount X1, in which the offset amount X is relatively large. In this state, the recording material P is easily bent in the secondary transfer nip N2, and therefore, as described above, the separation of the "thin paper" from the intermediate transfer belt 21 after passing through the secondary transfer nip N2 is improved.

[0044] As shown in FIG. 4B, in the case of "thick paper", the offset cam 39 is driven by the offset cam drive motor 101a to rotate, for example, clockwise. This causes the inner roller holder 38 to rotate counterclockwise around the inner roller rotation shaft 38a, and the relative position of the inner roller 26 with respect to the outer roller 41 is determined. This results in a state in which the inner roller 26 is disposed at the second inner roller position, which is the second offset amount X2, in which the offset amount X is relatively small. In this state, the bending of the recording material P in the secondary transfer nip N2 can be reduced, and therefore, as described above, the deterioration of image quality at the rear end of the "thick paper" can be suppressed.

[0045] 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 (gsm) of the recording material P. Here, gsm is g / m 2 means. (a) M≦300gsm: X1=+2.5mm (b) M>300gsm: X2=-1.0mm

[0046] In this embodiment, the position of the inner roller 26 (the relative position between the inner roller 26 and the outer roller 41) in the above setting (a) shown in Fig. 4(a) is the home position of the inner roller 26 (the relative position between the inner roller 26 and the outer roller 41). Here, the home position refers to the position when the image forming apparatus 100 is in a sleep state (described later) or in a state where the main power is turned off. However, this is not limited to this, and the position of the inner roller 26 in the above setting (b) shown in Fig. 4(b) may also be the home position of the inner roller 26.

[0047] Furthermore, the offset amount X and the type of recording material P (here, the basis weight of the recording material P) assigned to each offset amount X are not limited to the above specific example. These can be appropriately set through experiments, etc., from the viewpoint of improving the separation of the recording material P from the intermediate transfer belt 21 and suppressing image defects occurring in the vicinity of the secondary transfer nip N2 as described above. For example, in the configuration of this embodiment, the offset amount X is preferably about -3 mm to +3 mm. Furthermore, the pattern of the offset amount X is not limited to two patterns, and three or more patterns may be set. Then, according to this embodiment, an appropriate setting can be selected from three or more patterns based on the basis weight information of the recording material P as information related to the stiffness of the recording material P.

[0048] 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 inner roller rotation shaft 38a due to the tension of the intermediate transfer belt 21. That is, in this embodiment, a moment is always applied to the inner roller holder 38 in a direction in which the offset cam follower 38c rotates so as to engage with the offset cam 39 due to the tension of the intermediate transfer belt 21. Also, in the cross section shown in FIG. 4, the inner roller rotation shaft 38a is disposed downstream in the conveying direction of the recording material P with respect to a straight line (nip center line) Lc connecting the rotation center of the inner roller 26 and the rotation center of the outer roller 41. As a result, when the outer roller 41 is abutted against the inner roller 26 via the intermediate transfer belt 21, 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.

[0049] In addition, in order not to impede the workability of the operation of mounting or removing the intermediate transfer belt 21 from the intermediate transfer belt unit 20 for replacing the intermediate transfer belt 21, it is desirable to arrange the inner roller holder 38 on the inside of the tension surface of the intermediate transfer belt 21. Therefore, in the cross section shown in FIG. 4, it is desirable to arrange the inner roller rotation shaft 38a in the region 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 line indicating the tension surface of the intermediate transfer belt 21 formed by tensioning it between the inner roller 26 and the drive roller 22 (see FIG. 1) in the cross section shown in FIG. 4. The drive roller 22 is an example of a downstream roller arranged adjacent to the inner roller 26 downstream of the inner roller 26 in the rotation direction of the intermediate transfer belt 21 among the multiple tension rollers.

[0050] 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 26 (the front side of the paper in FIG. 1) in the direction substantially parallel to the rotation axis direction. The state shown by the two-dot chain line in FIG. 5 is the state in which the inner roller 26 is in the position for "thick paper". In this state, the inner roller holder 38 receives a counterclockwise moment about the inner roller rotation shaft 38a due to the tension of the intermediate transfer belt 21 and the reaction force received from the outer roller 41. In addition, the abutting portion 38b, which is cylindrical and coaxial with the inner roller 26 and is provided on the inner roller holder 38, abuts against the second positioning portion 40b. As a result, the inner roller 26 is positioned at the position of the second offset amount X2 (=-1.0 mm). In addition, the state shown by the solid line in FIG. 5 is the state in which the inner roller 26 is in the position for "thin paper" corresponding to the home position. As the offset cam 39 rotates, it comes into contact with and presses the arm portion 38c of the inner roller holder 38, causing the inner roller holder 38 to rotate clockwise around the inner roller rotation shaft 38a. Then, the abutment portion 38b abuts against the first positioning portion 40a. As a result, the inner roller 26 is positioned at the position of the first offset amount X1 (=+2.5 mm). The first and second positioning portions 40a and 40b are provided on the frame of the intermediate transfer belt unit 20, etc.

[0051] 4. Change in offset amount X and tip registration deviation Next, the above-mentioned leading edge registration deviation caused by changing the offset amount X will be further described. Fig. 6 is a schematic cross-sectional view (a cross-section substantially perpendicular to the rotation axis direction of the inner roller 26) of the intermediate transfer belt unit 20 showing the tension state of the intermediate transfer belt 21 when the offset amount X is different in this embodiment. The solid line in Fig. 6 shows the tension state when the inner roller 26 is at the position of the first offset amount X1, and the two-dot chain line shows the tension state when the inner roller 26 is at the position of the second offset amount X2.

[0052] In this embodiment, the tension roller 24 receives a biasing force from a tension spring 24a, which is a compression spring that is a biasing member (elastic member) serving as a biasing means (tension applying member). In this embodiment, the tension roller 24 is supported so as to be movable in a direction along the biasing direction of the tension spring 24a (the direction of the arrow S in FIG. 6). As a result, the tension roller 24 presses the intermediate transfer belt 21 from its inner circumferential surface side toward its outer circumferential surface side, thereby applying a predetermined tension to the intermediate transfer belt 21. Therefore, in this embodiment, the position of the tension roller 24 is determined in the direction of the arrow S at a position where the biasing force of the tension spring 24a balances with the reaction force received from the intermediate transfer belt 21.

[0053] The structure for movably supporting the tension roller 24 is provided on the frame of the intermediate transfer belt unit 20 or the like. In this embodiment, both ends of the tension roller 24 in the direction of the rotation axis are rotatably supported by bearing members (not shown). Each bearing member is held by the frame of the intermediate transfer belt unit 20 so as to be slidable in a direction along the biasing direction of the tension spring 24a (the direction of the arrow S in FIG. 6). The tension roller 24 is biased from the inner peripheral surface side to the outer peripheral surface side of the intermediate transfer belt 21 via the bearing member by the tension spring 24a attached in a compressed state between the bearing member and the frame of the intermediate transfer belt 20.

[0054] When the position of the inner roller 26 is moved due to the change in the offset amount X, as shown in FIG. 6, the tension form of the intermediate transfer belt 21 between the inner roller 26 and the tension rollers arranged upstream and downstream of the inner roller 26 in the rotation direction of the intermediate transfer belt 21, and the circumferential length of the intermediate transfer belt 21 between the tension rollers change. For example, the circumferential length of the intermediate transfer belt 21 between the inner roller 26 and the driving roller 22, which is the tension roller one downstream in the rotation direction of the intermediate transfer belt 21 as seen from the inner roller 26, is Ld. Then, Ld when the inner roller 26 is at the position of the first offset amount X1 (solid line in FIG. 6) is Ld1, and Ld when the inner roller 26 is at the position of the second offset amount X2 (two-dot chain line in FIG. 6) is Ld2. At this time, in the configuration of this embodiment, Ld1 is longer than Ld2 (Ld1>Ld2). This is because, in the configuration of this embodiment, in the case of the first offset amount X1, the inner roller 26 is positioned at a position moved to the left in FIG. 6 (toward the outer peripheral surface of the intermediate transfer belt 21) compared to the case of the second offset amount X2, and Ld becomes longer.

[0055] On the other hand, the circumferential length of the intermediate transfer belt 21 is constant, and a constant tension is applied to the intermediate transfer belt 21 by the tension roller 24 as described above. Therefore, when Ld changes, the position where the tension is balanced changes, and the tension roller 24 moves. That is, the position of the tension roller 24 changes with the change in the offset amount X. In the configuration of this embodiment, in the case of the first offset amount X1 (solid line in FIG. 6), the position of the tension roller 24 is determined to be a position moved to the left in FIG. 6 (toward the inner peripheral surface of the intermediate transfer belt 21) compared with the case of the second offset amount X2 (two-dot chain line in FIG. 6). This is because the circumferential length of the intermediate transfer belt 21 between the inner roller 26 and the roller upstream of the inner roller 26 in the rotation direction of the intermediate transfer belt 21 changes by the amount of change in Ld under each offset amount X condition as described above. In the configuration of this embodiment, in the case of the first offset amount X1 (solid line in FIG. 6), the length of the intermediate transfer belt 21 between the upstream roller and the inner roller 26 in the circumferential direction is shorter than in the case of the second offset amount X2 (two-dot chain line in FIG. 6). That is, the length of the intermediate transfer belt 21 along the circumferential direction from the primary transfer nip N1K for black to the secondary transfer nip N2 is defined as Lt. Then, Lt when the inner roller 26 is at the position of the first offset amount X1 (solid line in FIG. 6) is defined as Lt1, and Lt when the inner roller 26 is at the position of the second offset amount X2 (two-dot chain line in FIG. 6) is defined as Lt2. In this case, in the configuration of this embodiment, Lt1 is shorter than Lt2.

[0056] If the length of Lt is different, the time required until the toner image primarily transferred onto the intermediate transfer belt 21 by the primary transfer nip N1K for black is secondarily transferred onto the recording material P by the secondary transfer nip N2 also differs. That is, as a result of changing the offset amount X, the leading edge position of the toner image formed on the recording material P in the conveyance direction of the recording material P (here, also referred to as the "leading registration position") deviates from the desired (original) position. Note that, more specifically, the leading registration position is represented by the leading edge position of the image formation area (area where a toner image can be formed) on the recording material P. Thus, a phenomenon (leading registration deviation) occurs in which the leading edge position of the image formed on the recording material deviates from the desired (original) position.

[0057] As described above, in the configuration of this embodiment, Lt1 < Lt2. Therefore, the leading registration position shifts downstream in the conveyance direction of the recording material P in the case of the first offset amount X1 compared to the case of the second offset amount X2. That is, let the shift amount of the leading registration position according to the offset amount X (here, also referred to as the "leading registration shift amount") be ΔR(X) (unit: mm). And let the leading registration shift amount in the case of the first offset amount X1 be ΔR(X1), and the leading registration shift amount in the case of the second offset amount X2 be ΔR(X2). In the configuration of this embodiment, based on experimental data, the leading registration shift amount ΔR(X) according to the offset amount X is ΔR(X1)=0 ΔR(X2)=1.54 is as follows.

[0058] Here, ΔR(X1)=0 because in the configuration of this embodiment, the position of the inner roller 26 in the case of the first offset amount X1 is the home position of the inner roller 21, and the amount of leading edge registration deviation ΔR(X) is defined based on the leading edge registration position under this condition. In addition, the leading edge registration deviation amount ΔR(X) is defined as a positive direction in the upstream direction with respect to the conveying direction of the recording material P. Unlike the configuration of this embodiment, if the position of the inner roller 26 in the case of the second offset amount X2 is set as the home position of the inner roller 26, the leading edge registration deviation amount ΔR(X) will have a relatively different value from the above, and will be ΔR(X1)=-1.54 and ΔR(X2)=0. For simplicity, the leading edge registration deviation amount has been described using the toner image primarily transferred at the primary transfer nip N1K for black, i.e., a black image, as an example. The amount of leading edge registration deviation caused by changing the offset amount X is the same for the toner images primarily transferred at the primary transfer nips N1 (N1Y, N1M, N1C) for the other colors, so a duplicated description will be omitted.

[0059] As described above, the front registration position differs depending on the offset amount X. This is because the circumferential length of the intermediate transfer belt 21 from the primary transfer nip N1 to the secondary transfer nip N2 differs depending on the offset amount X, and the front registration position of the image secondarily transferred to the recording material P is shifted.

[0060] Therefore, the image forming apparatus 100 of this embodiment changes the timing at which the registration rollers 13 start conveying the recording material P to the secondary transfer nip N2 (also referred to here as "registration ON timing") in accordance with the offset amount X. In this way, the image forming apparatus 100 of this embodiment suppresses pre-registration deviation in the secondary transfer nip N2 caused by changes in the offset amount X. Note that in this embodiment, the conveying speed of the recording material P by the registration rollers 13 (represented by the circumferential speed of the registration rollers 13) is constant regardless of the offset amount X.

[0061] In the configuration of this embodiment, in the case of the second offset amount X2, the leading registration position on the recording material P is shifted upstream in the conveying direction of the recording material P by ΔR(X2)-ΔR(X1) compared to the case of the first offset amount X1. Here, the registration ON timing according to the offset amount X is Rt(X) (unit: s). The registration ON timing in the case of the first offset amount X1 is Rt(X1), and the registration ON timing in the case of the second offset amount X2 is Rt(X2). The conveying speed of the intermediate transfer belt 21 is VI (unit: mm / s). In this embodiment, the conveying speed (represented by the circumferential speed of the driving roller 22) VI of the intermediate transfer belt 21 corresponds to the process speed of the image forming apparatus 100. At this time, in this embodiment, when the offset amount X is switched from the first offset amount X1 to the second offset amount X2, the registration ON timing is delayed by (Rt(X2)-Rt(X1)) / VI. This makes it possible to suppress leading-edge registration deviation. In other words, it is possible to synchronize the timing at which the leading edge of the image forming area on the intermediate transfer belt 21 reaches the secondary transfer nip N2 with the timing at which the leading edge of the image forming area on the recording material P reaches the secondary transfer nip N2. In this embodiment, VI is 430 (mm / s). Therefore, in this embodiment, when the offset amount X is switched from the first offset amount X1 to the second offset amount X2, the registration ON timing is set to (Rt(X2)-Rt(X1)) / VI=3.54×10 -3 Just slow it down by (s).

[0062] 5. Control Mode 2 is a schematic block diagram showing the control mode of the main parts of the image forming apparatus 100 of this embodiment. The image forming apparatus 100 has a control unit 150 as a control means. The control unit 150 is configured to have 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 (I / F) 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 can transfer and read data to each other. The interface unit 153 controls the input and output (communication) of signals between the control unit 150 and devices connected thereto.

[0063] The control unit 150 is connected to each part of the image forming apparatus 100 (image forming unit 10, intermediate transfer belt 21, driving devices for members related to the conveyance of the recording material P, various power sources, etc.). For example, the control unit 150 is connected to a drum driving unit 111, an exposure device (laser scanner device) 3, an intermediate transfer belt driving unit 113, a resist driving unit 114, an offset mechanism 101, various high voltage power sources (charging voltage, developing voltage, primary transfer voltage, secondary transfer voltage), etc. Furthermore, signals (output values) indicating the detection results of various sensors such as an offset cam position sensor 37 are input to the control unit 150. The output value of the offset cam position sensor 37, i.e., information regarding the position of the inner roller 26 (the relative position between the inner roller 26 and the outer roller 41), is stored in a memory 152. Furthermore, an operation unit (operation panel) 160 provided in the image forming apparatus 100 is connected to the control unit 150. The operation unit 160 has a display means for displaying information under the control of the control unit 150, and an input means for inputting information to the control unit 150 through operation by an operator such as a user or a service representative. The operation unit 160 may be configured to have a touch panel having the functions of the display means and the input means. 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.

[0064] 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) and information (command signal) on printing operation conditions such as the type of recording material P input from the operation unit 160 or the external device 200. 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 P includes information on the type of recording material P related to the stiffness of the recording material P, particularly, information on the basis weight of the recording material P as an example. When information on the printing operation conditions is input from the operation unit 160, the operation unit 160 functions as an input unit that inputs information on the basis weight of the recording material P to the control unit 150 to which the toner image is transferred. In addition, when information regarding printing operation conditions is input from an external device 200 such as a personal computer, the interface unit 153 functions as an input unit that inputs information regarding the basis weight of the recording material P to which the toner image is transferred to the control unit 150.

[0065] In this embodiment, more specifically, job information is input to the control unit 150 from the external device 200 or the like via a controller (video controller) (not shown) provided in the image forming apparatus 100. The controller analyzes the job information and inputs information on print operating conditions and image information (video signal) expanded into bitmap data to the control unit 150. The control unit 150 controls each unit of the image forming apparatus 100 based on the information on print operating conditions and image information input from the controller. In this embodiment, when the control unit 150 receives a job start instruction (print start command) from the controller, it outputs a / TOP signal and a / BD signal that are reference timings for outputting a video signal to the controller. The / TOP signal is a signal that is a reference in the sub-scanning direction when outputting a video signal, and the / BD signal is a signal that is a reference in the main scanning direction when outputting a video signal. That is, every time the / TOP signal is input, output of a video signal for printing on a new page is started. Also, every time the / BD signal is input, output of a video signal for one line in the main scanning direction is started. In this way, the / TOP signal corresponds to a synchronization signal in the sub-scanning direction when forming an image, and image formation by image control unit 10 is started in response to this / TOP signal.

[0066] In this embodiment, at the timing when a job start instruction is input from the controller to the control unit 150 (the timing when the controller outputs a job start instruction to the control unit 150), the recording material P stored in the recording material storage 11 is fed by the feed roller 19 and the like. This recording material P is further conveyed toward the secondary transfer nip N2 by the registration roller 13. Then, when the leading edge of this recording material P is detected by the registration sensor 18, the registration roller 13 is temporarily stopped, and the recording material P is in a standby state. Also, the control unit 150 outputs a / TOP signal to the controller in synchronization with a detection signal of the leading edge of the recording material P output by the registration sensor 18. The controller outputs image information to the control unit 150 in synchronization with the / TOP signal. Then, the control unit 150 starts exposure by the exposure device 3 in accordance with this image information. Also, the control unit 150 starts (resumes) the rotational drive of the registration roller 13 at the timing when a predetermined time has elapsed from the / TOP signal, and conveys the recording material P in a standby state to the secondary transfer nip N2. In this embodiment, the / TOP signal is generated based on a detection signal from the registration sensor 18, but is not limited to this and may be generated as long as it can be used as a reference timing signal for starting the image forming process. That is, in this embodiment, the generation timing of the / TOP signal can be regarded as the start timing of the image forming process (image writing start timing of the image forming means), more specifically, the exposure start timing of the exposure device 3. This exposure start timing of the exposure device 3 corresponds to the writing start timing of the leading edge of the image when forming a full solid image (an image in which the entire image forming area has the highest density level) on one sheet of recording material P.

[0067] Here, the image forming apparatus 100 executes a job (print job, print job) which is a series of operations for forming and outputting an image on one or multiple recording materials P, which is started by one start instruction. A job generally includes an image forming process (printing operation, print operation, image forming operation), a pre-rotation process, a paper interval process in the case of forming an image on multiple recording materials P, and a post-rotation process. The image forming process is a period in which an electrostatic image of an image to be actually formed on the recording material P and outputted, a toner image, a primary transfer of the toner image, and a secondary transfer are performed, and the image formation time (image formation period) refers to this period. 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 actually starts to be formed. The paper interval process is a period corresponding to the period between recording materials P when image formation is performed continuously on multiple recording materials P (continuous image formation). The post-rotation process is a period in which an arrangement operation (preparatory operation) is performed after the image forming process. The non-image forming time (non-image forming period) is a period other than the image forming time, and includes the above-mentioned pre-rotation process, paper interval 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. 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 the amount of power consumption is reduced compared to the standby state. In this embodiment, during the non-image forming time, the offset mechanism 101 performs an operation (also referred to as an "offset operation" here) of changing the position of at least one of the inner roller 26 and the outer roller 41 (particularly, the inner roller 26 in this embodiment) to change the offset amount.

[0068] 6. Control Procedure Fig. 7 is a flow chart showing an outline of an example of a job control procedure in this embodiment. Here, a case will be described in which one job is executed to form an image on one sheet of recording material P from a state in which the inner roller 26 is at the home position and the offset amount X is the first offset amount X1. Also, here, a case in which an operator causes the image forming apparatus 100 to execute a job from the operation unit 160 will be taken as an example. Note that Fig. 7 shows an outline of the control procedure focusing on the offset operation and the change of the registration ON timing, and many other operations that are usually required to execute a job and output an image are omitted.

[0069] First, when an operator operates the operation unit 160 to set a job, the information is notified to the control unit 150. The control unit 150 starts the job by sending commands to each unit of the image forming apparatus 100 based on the information (S101). The job information sent to the control unit 150 includes information on the type of recording material P. In this embodiment, the information on the type of recording material P includes at least information on the basis weight of the recording material P. The information on the recording material P may include information on the surface properties of the recording material P, information on the electrical resistance value of the recording material P, and other information in addition to the basis weight of the recording material P. The control unit 150 can obtain information on the type of recording material P that is directly input (including selection from a plurality of options) from the operation unit 160 (or the external device 200) by the operator's operation. The control unit 150 can also obtain information on the type of recording material P based on information on the recording material storage 11 that sends out the recording material P in the job, which is input from the operation unit 160 (or the external device 200) by the operator's operation. In this case, the control unit 150 can obtain information regarding the type of recording material P from information regarding the type of recording material P that is previously associated with each of the multiple recording material storage cabinets 11 and stored in the memory 152. Here, when registering information regarding the type of recording material P, a corresponding one may be selected from a list of types of recording material P that is previously stored in the memory 152 or in a storage device connected to the control unit 150 via a network.

[0070] When the control unit 150 acquires information regarding the type of recording material P used in a job, it sets the printing operating conditions of the job to printing operating conditions that are predetermined for each type of recording material P. Table 1 shows an example of the settings of the offset amount X and the registration ON timing that are predetermined in accordance with the basis weight of the recording material P as the printing operating conditions in this embodiment. Information on the printing operating conditions as shown in Table 1 is stored in advance in the memory 152.

[0071] The registration-ON timing is represented by the time required from the start timing of the image forming process, that is, the image writing timing (exposure start timing) of the exposure device 3 to the timing when the registration roller 13 starts (resumes) conveying the recording material P in order to perform the second transfer of the toner image to a predetermined position on the recording material P. In this embodiment, more specifically, the registration-ON timing is represented by the time that elapses from the generation of the / TOP signal by the control unit 150 to the start (resumes) of the rotational drive of the registration roller 13. Here, the above-mentioned time under the condition where the offset amount X is the first offset amount X1 is defined as Rt1(s) (first time, first timing), and this is defined as the registration-ON timing for the first offset amount X1. The registration-ON timing (second time, second timing) for the second offset amount X2 is expressed based on the above-mentioned Rt1.

[0072] [Table 1]

[0073] When the control unit 150 acquires the job information in S101, it determines whether the basis weight of the recording material P used in the job is 300 gsm or less (S102). If the control unit 150 determines in S102 that the basis weight is 300 gsm or less, it starts the printing operation without changing the offset amount X (S103). In this case, the offset amount X may remain at +2.5 mm (first offset amount X1), which is the default value corresponding to the home position of the inner roller 26. Then, the control unit 150 ends the printing operation after the printing operation for the predetermined number of prints set by the operator is completed (S104), and ends the job (S105).

[0074] On the other hand, if the control unit 150 determines in S102 that the basis weight is greater than 300 gsm, it performs the following: That is, it sends a command to the offset mechanism 101 (more specifically, the offset cam drive motor 101a) to turn on the drive of the offset mechanism 101 and change the offset amount X to -1.0 mm (the second offset amount X2) (S106). Thereafter, the control unit 150 sends a command to the offset mechanism 101 to turn off the drive of the offset mechanism 101 (S107). In addition, the control unit 150 sets the registration ON timing to Rt1+3.54×10 -3 The control unit 150 then changes the setting in the memory 152 to change the offset amount X to +2.5 mm (first offset amount X1) (S108). Thereafter, the control unit 150 starts the printing operation (S109), and ends the printing operation after the printing operation for the predetermined number of prints set by the operator is completed (S110). After the printing operation is completed, the control unit 150 sends a command to the offset mechanism 101 to turn on the drive of the offset mechanism 101 and change the offset amount X to the default value of +2.5 mm (first offset amount X1) (S111). Thereafter, the control unit 150 sends a command to the offset mechanism 101 to turn off the drive of the offset mechanism 101 (S112). The control unit 150 also returns the setting of the registration ON timing in the memory 152 to the default value Rt1 (S113), and ends the job (S105).

[0075] 7 shows an example of a job in which an image is formed on one sheet of recording material P. In a continuous image forming job in which images are continuously formed on a plurality of recording materials P, if the type of recording material P changes during the job and it is necessary to change the offset amount X, the following can be done. That is, in the sheet spacing process, the offset amount X is changed, and the registration ON timing is changed according to the changed offset amount X.

[0076] Here, the offset amount X may be a desired offset amount X when the recording material P passes through the secondary transfer nip N2 (during secondary transfer). In other words, the change of the offset amount X is completed before the recording material P on which an image is formed with the changed offset amount X reaches the secondary transfer nip N2. Typically, the change of the offset amount X is performed so as to be completed before the recording material P is conveyed by the registration rollers 13 or before the feeding of the recording material P from the recording material storage 11 is started. Also, the registration ON timing may be a desired value when the recording material P starts to be conveyed from the registration rollers 13. In other words, the change of the registration ON timing is completed before the recording material P conveyed at the changed registration ON timing reaches the registration rollers 13. Typically, the change of the registration ON timing (change of the setting) is performed so as to be completed before the feeding of the recording material P from the recording material storage 11 is started.

[0077] Also, here, a case where the registration-on timing is the default value when the offset amount X is the first offset amount X1 is described as an example. Therefore, when the offset amount X is the second offset amount X2, the registration-on timing is made later than the default value. As a result, the time from the exposure start timing of the exposure device 3 to the registration-on timing is longer when the offset amount X is the second offset amount X2 than when the offset amount X is the first offset amount X1. On the other hand, the registration-on timing when the offset amount X is the second offset amount X2 may be set to the default value. In that case, when the offset amount X is the first offset amount X1, the registration-on timing is made earlier than the default value.

[0078] In this embodiment, the conveying speed of the recording material P by the registration rollers 13 is constant, but the image forming apparatus 100 may be capable of performing image formation (secondary transfer) in a plurality of modes in which the conveying speed of the recording material P by the registration rollers 13 is different. In this case, too, when the conveying speed of the recording material P by the registration rollers 13 is substantially the same, the registration ON timing can be changed according to the offset amount X.

[0079] 7.Effects Thus, in this embodiment, the image forming apparatus 100 includes an image forming means 10 that forms a toner image, a rotatable endless belt 21 that transports the toner image formed by the image forming means 10 and carried at an image forming position N1, a plurality of tension rollers including an inner roller 26 that tension the belt 21, an outer roller 41 that is arranged opposite the inner roller 26 and abuts against the outer peripheral surface of the belt 21 to form a transfer section N2 that transfers the toner image from the belt 21 to a recording material P, a position change mechanism 101 that can change the position of at least one of the inner roller 26 or the outer roller 41 to change the relative position between the inner roller 26 and the outer roller 41 in the circumferential direction of the inner roller 26 between a first relative position and a second relative position different from the first relative position, a transport member 13 that transports the recording material P to the transfer section N2, and a transport drive section 114 that drives the transport member 13. In this embodiment, the image forming apparatus 100 further includes a control unit 150 capable of executing control for changing the time from the image writing timing of the image forming means 10 to the timing when the conveyance member 13 starts conveying the recording material P, in relation to image formation on one sheet of recording material P, between a case where transfer is performed with the above-mentioned relative position set as the first relative position and a case where transfer is performed with the above-mentioned relative position set as the second relative position. Note that the time from the image writing timing of the image forming means 10 to the timing when the conveyance member 13 starts conveying the recording material P can be typically obtained by comparing the time from the image writing timing of the image formed on the first sheet of recording material P to the timing when the conveyance member 13 starts conveying the recording material P in jobs executed with the above-mentioned relative position set as the first relative position or the second relative position.

[0080] Here, in the above control, the control unit 150 changes the above time so that the positional deviation between the image forming area on the belt at the transfer unit N2 and the image forming area on the recording material in the transport direction of the recording material P is smaller when the transfer is performed with the above relative position as the first relative position and when the transfer is performed with the above relative position as the second relative position than when the above time is not changed. In particular, in this embodiment, the multiple tension rollers are arranged downstream of the image forming position N1 and upstream of the inner roller 26 in terms of the rotation direction of the belt 21, and include a tension roller 24 that applies tension to the belt 21, the position change mechanism 101 changes the position of the inner roller 26 to change the relative position between a first relative position and a second relative position, the inner roller 26 being positioned downstream in the rotation direction of the belt 21 relative to the outer roller 41 in the case of the first relative position than in the case of the second relative position, and the control unit 150 changes the time in the above control so that the time when transfer is performed with the relative position as the second relative position is longer than the time when transfer is performed with the relative position as the first relative position. In this embodiment, the position change mechanism 101 changes the offset amount X to a first offset amount X1 in the case of the first relative position and a second offset amount X2 in the case of the second relative position, the first offset amount X1 being a positive value, and the second offset amount X2 being 0 or a negative value. In this embodiment, the control unit 150 changes the above-mentioned time by controlling the conveyance drive unit 114 so as to change the timing at which the conveyance member 13 starts conveying the recording material P in the above control. In this embodiment, the image writing timing of the image forming unit 10 is the exposure start timing of the exposure unit 3 for forming an electrostatic image provided in the image forming unit 10. In this embodiment, the belt 21 is an intermediate transfer body that conveys the toner image primarily transferred from the image carrier 1 provided in the image forming unit 10 to the recording material P in the transfer unit N2 for secondary transfer.

[0081] As described above, in this embodiment, the offset amount X is changed according to the basis weight of the recording material P as information on the type of the recording material P. In this embodiment, when the offset amount X is changed, the registration ON timing is changed according to the offset amount X. In other words, in this embodiment, the offset amount X is changed according to the basis weight of the recording material P as information on the type of the recording material P, and the registration ON timing is changed according to the basis weight of the recording material P as information on the type of the recording material P. This makes it possible to correct the deviation of the leading registration position caused by the change in the offset amount X and suppress the occurrence of leading registration deviation. Therefore, according to this embodiment, the offset amount X is changed to improve the transferability for each of the multiple types of recording materials P, and the occurrence of leading registration deviation caused by the change in the offset amount X can be suppressed. In other words, according to this embodiment, it is possible to suppress the leading registration deviation in the secondary transfer nip N2 while dealing with media with a wide range of stiffness.

[0082] [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.

[0083] 1. Overview of this embodiment In the first embodiment, the deviation of the pre-registration position caused by the change in the offset amount X is corrected by changing the timing at which the registration rollers 13 start conveying the recording material P (registration ON timing).

[0084] In contrast to this, in this embodiment, the image writing timing (exposure start timing) of the exposure device 3 is changed to change the image leading edge position, thereby correcting the deviation of the leading edge registration position due to the change in the offset amount X. In this embodiment, the control unit 150 functions as an image writing timing changing means, and changes the image writing timing of the exposure device 3. The change amount of the registration ON timing described in the first embodiment can be read as the change amount of the image writing timing in this embodiment.

[0085] In this embodiment, the image write timing when the offset amount X is the first offset amount X1 is the default value. Therefore, when the offset amount X is the second offset amount X2, the image write timing is earlier than the default value. As a result, the time from the exposure start timing of the exposure device 3 to the registration ON timing is longer when the offset amount X is the second offset amount X2 than when the offset amount X is the first offset amount X1. On the other hand, the image write timing when the offset amount X is the second offset amount X2 may be the default value. In that case, when the offset amount X is the first offset amount X1, the image write timing is later than the default value. By changing the image write timing of the exposure device 3 in this way, the time from the exposure start timing of the exposure device 3 to the registration ON timing can also be changed.

[0086] In this embodiment, the conveying speed of the recording material P by the registration rollers 13 is constant, but the image forming apparatus 100 may be capable of performing image formation (secondary transfer) in a plurality of modes in which the conveying speed of the recording material P by the registration rollers 13 is different. In this case, too, when the conveying speed of the recording material P by the registration rollers 13 is substantially the same, the image writing timing can be changed according to the offset amount X.

[0087] 2. Control Procedure Fig. 8 is a flow chart showing an outline of an example of a job control procedure in this embodiment. The processes of S201 to S213 in the control procedure in Fig. 8 are similar to the processes of S101 to S113 in the control procedure in Fig. 7 described in the first embodiment. However, in this embodiment, in S208, the control unit 150 changes the setting of the image write timing in the memory 152. Also, in this embodiment, in S213, the control unit 150 returns the setting of the image write timing in the memory 152 to the default value.

[0088] Table 2 shows an example of the settings of the offset amount and the image writing timing, which are determined in advance as the printing operation conditions in this embodiment according to the basis weight of the recording material P. Information on the printing operation conditions as shown in Table 2 is stored in the memory 152 in advance.

[0089] The image write timing is represented by the time required from the start timing of the job to the start timing of the image forming process (image write timing of the exposure device 3) in order to perform the second transfer of the toner image to a predetermined position on the recording material P. In this embodiment, more specifically, the write timing is represented by the time that elapses from when a job start instruction (print start command) is input to the control unit 150 until the control unit 150 generates a / TOP signal. Here, the above-mentioned time under the condition where the offset amount X is the first offset amount X1 is set as It1(s) (first time, first timing), and this is set as the write timing in the case of the first offset amount X1. And, the write timing in the case of the second offset amount X2 (second time, second timing) is expressed based on the above-mentioned It1. Here, the write timing is compared when the processing during the pre-rotation process executed from the job start instruction to the / TOP signal is substantially the same. For example, if the processing during the pre-rotation step, which is executed periodically or in response to an instruction from an operator, is different, the write timing based on the instruction to start the job may differ even under the same offset amount X conditions.

[0090] [Table 2]

[0091] The control procedure in Fig. 8 takes as an example a job in which an image is formed on one sheet of recording material P. In a continuous image formation job in which images are continuously formed on a plurality of recording materials P, if the type of recording material P changes midway through the job and it becomes necessary to change the offset amount X, the following can be done. That is, in the sheet spacing process, the offset amount X is changed, and the write start timing is changed according to the changed offset amount X.

[0092] 3.Effects In this manner, in this embodiment, in controlling image formation on one sheet of recording material P, the control unit 150 changes the time from the image writing timing of the image forming means 10 to the timing when the conveying member 13 starts conveying the recording material P between a case where transfer is performed with the relative position between the inner roller 26 and the outer roller 41 as a first relative position and a case where transfer is performed with the above relative position as a second relative position, by controlling the image forming means 10 to change the image writing timing of the image forming means 10. In this embodiment, the image writing timing of the image forming means 10 is the exposure start timing of the exposure means 3 for forming an electrostatic image provided in the image forming means 10.

[0093] As described above, in this embodiment, the offset amount X is changed according to the basis weight of the recording material P as information on the type of the recording material P. In this embodiment, when the offset amount X is changed, the writing timing of the exposure device 3 is changed according to the offset amount X. In other words, in this embodiment, the offset amount X is changed according to the basis weight of the recording material P as information on the type of the recording material P, and the writing timing of the exposure device 3 is changed according to the basis weight of the recording material P as information on the type of the recording material P. This makes it possible to correct the shift in the leading edge registration position caused by the change in the offset amount X, and to suppress the occurrence of leading edge registration shift. Therefore, according to this embodiment, the offset amount X is changed to improve the transferability for each of the multiple types of recording materials P, and the occurrence of leading edge registration shift caused by the change in the offset amount X can be suppressed.

[0094] [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.

[0095] In the above embodiment, the image forming apparatus is configured to change the offset amount by changing the position of the inner roller, but it may be configured to change the offset amount by changing the position of the outer roller. Also, it is not limited to a configuration in which either the inner roller or the outer roller is moved, and it is also possible to change the offset amount by moving both the inner roller and the outer roller. Here, for example, in a configuration in which the offset amount is changed by moving the outer roller, the aforementioned length Lt may be shorter when the outer roller is located relatively upstream of the inner roller in terms of the rotation direction of the intermediate transfer belt, as opposed to the above embodiment. In this case, the registration ON timing and the write start timing with respect to the relative positions of the inner roller and the outer roller may have an inverse relationship to those in the above embodiment (if the timing is delayed in the above embodiment, it is made earlier, and if the timing is made earlier, it is made later).

[0096] In the above embodiment, a configuration in which a tension roller is provided between the primary transfer nip and the secondary transfer nip in relation to the rotation direction of the intermediate transfer belt has been described as an example. In this configuration, the travel distance of the intermediate transfer belt from the tension roller to the secondary transfer nip is easily changed by changing the offset amount as described above, so that the effect of the present invention can be obtained particularly remarkably. However, even if such a configuration is not used, the position of the secondary transfer portion in relation to the rotation direction of the intermediate transfer belt may be changed by changing the offset amount, and the leading edge registration position may be shifted if control according to the present invention is not performed. Therefore, the present invention can be effectively applied even in such a configuration. In other words, in a configuration in which the length from the image forming position to the transfer portion in relation to the rotation direction of the belt changes by changing the offset amount, the same effect as the above embodiment can be obtained by applying the present invention.

[0097] In the above-mentioned embodiment, an outer roller that directly contacts the outer peripheral surface of the intermediate transfer belt is used as an outer member that forms a secondary transfer nip together with an inner roller as an inner member. In contrast, an outer roller and a secondary transfer belt stretched between the outer roller and another roller may be used as an outer member. That is, the image forming apparatus may have a tension roller, an outer roller, and a secondary transfer belt stretched between these rollers as an outer member. The outer roller can be made to contact the outer peripheral surface of the intermediate transfer belt via the secondary transfer belt. In such a configuration, the intermediate transfer belt and the secondary transfer belt are sandwiched between the inner roller that contacts the inner peripheral surface of the intermediate transfer belt and the outer roller that contacts the inner peripheral surface of the secondary transfer belt to form a secondary transfer nip. In this case, the contact portion between the intermediate transfer belt and the secondary transfer belt is the secondary transfer nip as a secondary transfer portion. In this case, the offset amount X is also defined by the relative positions of the inner roller and the outer roller, as described above.

[0098] 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. Furthermore, 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. The information on the type of recording material may be qualitative information such as the paper type category, brand, or a combination of the paper type category and brand. 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.

[0099] 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.

[0100] In the above-mentioned embodiment, an actuator that operates the movable part by a cam is used as the offset mechanism, but the present invention is not limited to this. The offset mechanism may be any mechanism that can realize the operation conforming to the above-mentioned embodiment, and may be, for example, an actuator that operates the movable part by a solenoid.

[0101] 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.

[0102] 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]

[0103] 13 Registration roller 21 Intermediate transfer belt 26 Inner roller 41 Outer roller 100 Image forming device 101 Offset mechanism 150 Control section

Claims

1. an image carrier that carries a toner image; an image forming unit that forms a toner image on the image carrier; an endless belt onto which the toner image formed on the image carrier is transferred at a primary transfer portion; A plurality of tension rollers including an inner roller for tensioning the belt; an outer roller which cooperates with the inner roller to form a secondary transfer portion where a toner image is transferred from the belt onto a recording material; a position changing mechanism that changes a position of the secondary transfer portion in a circumferential direction of the inner roller by moving the inner roller, the position changing mechanism being capable of changing the position of the inner roller to a plurality of positions including a first position and a second position that is located upstream of the first position in a rotation direction of the belt; a conveying member that conveys a recording material to the secondary transfer portion; A transport drive unit that drives the transport member; a control unit for controlling a timing at which the conveying member starts conveying the recording material; having The control unit controls the timing at which the conveying member starts conveying the recording material to a first timing when the position of the inner roller set when the toner image is transferred onto the recording material is the first position, and controls the timing at which the conveying member starts conveying the recording material to a second timing different from the first timing when the position of the inner roller set when the toner image is transferred onto the recording material is the second position so as to suppress the amount of image misalignment that occurs when the position of the inner roller is changed.

2. 2. The image forming apparatus according to claim 1, wherein the control unit controls the position of the inner roller set when a toner image is transferred onto the recording material and the timing at which the conveying member starts conveying the recording material based on information regarding the type of recording material.

3. the plurality of tension rollers include a downstream roller provided adjacent to the inner roller downstream of the inner roller in a rotation direction of the belt, When the length of the belt stretched between the inner roller and the downstream roller in the circumferential direction is Ld, Ld is Ld1 when the inner roller is in the first position, and Ld2 when the inner roller is in the second position, Ld1 being longer than Ld2, The image forming apparatus according to claim 1 , wherein the control unit performs control such that the second timing is later than the first timing.

4. 4. The image forming apparatus according to claim 3, wherein the plurality of tension rollers include a tension roller that applies tension to the belt, the tension roller being disposed downstream of the primary transfer portion and upstream of the inner roller in relation to the rotation direction of the belt.

5. 2. The image forming apparatus according to claim 1, wherein, when the position of the inner roller set when a toner image is transferred onto a recording material is the first position, a time value from a predetermined reference timing to a timing at which the conveying member starts conveying the recording material, for a conveying speed of the belt during transfer, is a first predetermined value, and, when the position of the inner roller set when a toner image is transferred onto a recording material is the second position, a time value for a conveying speed of the belt during transfer is a second predetermined value different from the first predetermined value.

6. The image forming apparatus according to claim 5, characterized in that the control unit changes the value of the time from the specified reference timing to the timing when the conveying member starts conveying the recording material, relative to the conveying speed of the belt during transfer, depending on the position of the inner roller, based on the type of recording material.

7. the plurality of tension rollers include an upstream roller provided 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), the position change mechanism changes the offset amount X to a first offset amount X1 in the first position and a second offset amount X2 in the second position; 2. The image forming apparatus according to claim 1, wherein the first offset amount X1 is a positive value, and the second offset amount X2 is 0 or a negative value.

8. 2. The image forming apparatus according to claim 1, wherein the outer roller is in contact with an outer peripheral surface of the belt via another endless belt that is stretched between the outer roller and another roller.

9. 2. The image forming apparatus according to claim 1, further comprising a guide member that is provided upstream of the secondary transfer portion in a conveying direction of the recording material and that guides the recording material to the secondary transfer portion.

10. an image carrier that carries a toner image; an image forming unit that forms a toner image on the image carrier; an endless belt onto which the toner image formed on the image carrier is transferred at a primary transfer portion; A plurality of tension rollers including an inner roller for tensioning the belt; an outer roller which cooperates with the inner roller to form a secondary transfer portion where a toner image is transferred from the belt onto a recording material; a position changing mechanism that changes a position of the secondary transfer portion in a circumferential direction of the inner roller by moving the inner roller, the position changing mechanism being capable of changing the position of the inner roller to a plurality of positions including a first position and a second position that is located upstream of the first position in a rotation direction of the belt; a conveying member that conveys a recording material to the secondary transfer portion; A transport drive unit that drives the transport member; a control unit that controls a time difference between an image writing start timing of the image forming unit and a conveyance start timing of the conveyance member; having The control unit controls the conveying drive unit and the image forming unit so as to set the time difference to a first time difference when the position of the inner roller that is set when the toner image is transferred onto the recording material is the first position, and to set the time difference to a second time difference different from the first time difference when the position of the inner roller that is set when the toner image is transferred onto the recording material is the second position, so as to suppress the amount of image position shift that occurs when the position of the inner roller is changed.

11. 11. The image forming apparatus according to claim 10, wherein, when the position of the inner roller set when the toner image is transferred onto the recording material is the first position, the control unit controls the time difference so that the time difference becomes a first predetermined value, and, when the position of the inner roller set when the toner image is transferred onto the recording material is the second position, the control unit controls the time difference so that the time difference becomes a second predetermined value different from the first predetermined value.

12. the image forming unit has an exposure device that irradiates a surface of the image carrier with a laser beam modulated in accordance with an image signal; 11. The image forming apparatus according to claim 10, wherein the image writing start timing of the image forming unit is the image writing start timing of the exposure device.

13. The image forming apparatus according to claim 10, wherein the control unit controls the position of the inner roller set when a toner image is transferred onto the recording material and the timing at which the conveying member starts conveying the recording material, based on information regarding the type of recording material.

14. 13. The image forming apparatus according to claim 12, wherein the control unit controls the position of the inner roller and the image writing timing of the exposure device, which are set when a toner image is transferred onto a recording material, based on information regarding the type of recording material.

15. the plurality of tension rollers include a downstream roller provided adjacent to the inner roller downstream of the inner roller in a rotation direction of the belt, When the length of the belt stretched between the inner roller and the downstream roller in the circumferential direction is Ld, Ld is Ld1 when the inner roller is in the first position, and Ld2 when the inner roller is in the second position, Ld1 being longer than Ld2, The image forming apparatus according to claim 10 , wherein the control unit performs control such that the second time difference is longer than the first time difference.

16. 11. The image forming apparatus according to claim 10, wherein the plurality of tension rollers include a tension roller that applies tension to the belt and is provided downstream of the primary transfer unit and upstream of the inner roller in the rotation direction of the belt.

17. the plurality of tension rollers include an upstream roller provided 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), the position change mechanism changes the offset amount X to a first offset amount X1 in the first position and a second offset amount X2 in the second position; 11. The image forming apparatus according to claim 10, wherein the first offset amount X1 is a positive value, and the second offset amount X2 is 0 or a negative value.

18. 11. The image forming apparatus according to claim 10, wherein the outer roller is in contact with an outer peripheral surface of the belt via another endless belt that is stretched between the outer roller and another roller.

19. 11. The image forming apparatus according to claim 10, further comprising a guide member that is provided upstream of the secondary transfer portion in a conveying direction of the recording material and that guides the recording material to the secondary transfer portion.

20. 11. The image forming apparatus according to claim 10, wherein the transport member is a registration roller.

21. 11. The image forming apparatus according to claim 10, wherein the transport member is provided adjacent to the secondary transfer portion upstream of the secondary transfer portion.

22. an image carrier that carries a toner image; an image forming unit that forms a toner image on the image carrier; an endless belt onto which the toner image formed on the image carrier is transferred at a primary transfer portion; A plurality of tension rollers including an inner roller for tensioning the belt; an outer roller which cooperates with the inner roller to form a secondary transfer portion where a toner image is transferred from the belt onto a recording material; a position changing mechanism that changes a position of the secondary transfer portion in a circumferential direction of the inner roller by moving the inner roller, the position changing mechanism being capable of changing the position of the inner roller to a plurality of positions including a first position and a second position that is located upstream of the first position in a rotation direction of the belt; a conveying member that conveys a recording material to the secondary transfer portion; A transport drive unit that drives the transport member; A control unit that controls the transport drive unit; having The control unit controls the transport drive unit with a first control when the position of the inner roller set when transferring a toner image onto a recording material is the first position, and controls the transport drive unit with a second control different from the first control when the position of the inner roller set when transferring a toner image onto a recording material is the second position, so that the amount of image position shift that occurs due to a change in the position of the inner roller is suppressed.

Images