Image forming apparatus

Abstract

To prevent waving of a transfer belt on a surface for conveying a recording material, while preventing permanent deformation of a transfer roller.SOLUTION: An image forming apparatus 100 has: an image carrier 31; an endless belt 40 that carries and conveys a recording material S; a transfer roller 41; a downstream roller 42 that is arranged to be in contact with an inner peripheral surface of the belt 40 on the downstream side of the transfer roller 41 in a conveyance direction of the recording material S, and forms a surface S1 of the belt 40 for conveying the recording material S with the transfer roller 41; and a driving source 421 that drives to rotate the downstream roller 42. The number of rotations of the driving source 421 during image formation is set so that the surface speed V3 of the downstream roller 42 becomes faster than the inner peripheral speed V2 of the belt 40.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus such as a copying machine, printer, facsimile machine, or multifunction machine that uses an electrophotographic or electrostatic recording method, or that has a plurality of functions selected from these. [Background technology]

[0002] In image forming apparatuses using electrophotography or the like, a toner image formed on an image carrier such as a photosensitive drum or intermediate transfer belt is transferred to a recording material. The toner image on the image carrier is electrostatically transferred to the recording material by sandwiching the recording material between the image carrier and a transfer device in a transfer section and forming a transfer electric field in the transfer section. A belt-transfer transfer device, equipped with a transfer belt that carries and transports the recording material, is sometimes used as this transfer device because of its high performance in separating the recording material from the image carrier. A belt-transfer transfer device is configured with an endless transfer belt and multiple tension rollers for tensioning the transfer belt.

[0003] Patent document 1 proposes a configuration in which, downstream of the secondary transfer section, the secondary transfer belt, which is a transfer belt, is brought into contact with a roller that supports the intermediate transfer belt, which is an image carrier, so that the recording material is attracted to and separated from the secondary transfer belt. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-347517 Summary of the Invention [Problem to be solved by the invention]

[0005] For the purposes of reducing costs and saving space, transfer devices with a belt transfer configuration are sometimes configured with two tension rollers, the minimum number required to tension the transfer belt, located on the inner circumferential surface of the transfer belt. The first tension roller is a transfer roller that contacts the image carrier via the transfer belt and transfers the toner image on the image carrier onto the recording material. The second tension roller is a separation roller that, in addition to forming a belt surface that transports the recording material on the transfer belt together with the transfer roller, uses its curvature to separate the recording material from the belt surface and transport it downstream.

[0006] In an image forming apparatus, the impact of the recording material entering the transfer section can cause fluctuations in the speed of the image carrier, resulting in image degradation. To address this issue, the hardness of the elastic member that makes up the transfer roller can be reduced, giving the transfer roller the ability to absorb impact, thereby suppressing image degradation.

[0007] However, when a transfer roller with low hardness is used, a strong tension is applied to the low hardness member of the transfer roller at the portion where the transfer belt is wound, which can cause the low hardness member to elastically deform. If this condition is maintained for a long period of time, the low hardness member at the portion where the transfer belt is wound will be permanently deformed, which will cause periodic density differences to appear when an image is formed.

[0008] On the other hand, when the tension is weak, the transfer belt may slacken, causing significant periodic out-of-plane displacement (waving) of the belt surface along which the recording material is transported, which may result in discharge in the gap between the transfer belt and the recording material, causing unfixed toner to scatter from the recording material and resulting in an abnormal image.

[0009] For example, in a transfer device in which a transfer belt is stretched by two tension rollers, it is difficult to provide a mechanism for loosening the tension of the transfer belt from the standpoint of cost reduction and space saving.

[0010] Thus, for example, in a configuration in which a transfer roller with low hardness is used as the first tension roller in a transfer device in which a transfer belt is tensioned by two tension rollers, it is required to suppress undulation of the transfer belt.

[0011] SUMMARY OF THE INVENTION It is therefore an object of the present invention to suppress the waviness of the surface of the transfer belt that transports the recording material while suppressing the permanent deformation of the transfer roller. [Means for solving the problem]

[0012] The above object can be achieved by an image forming apparatus according to the present invention. In summary, according to one aspect of the present invention, there is provided an image forming apparatus comprising: a rotatable image carrier that carries a toner image; an endless belt that carries and transports a recording material; a transfer roller that is arranged in contact with the inner peripheral surface of the belt and forms a transfer section that transfers the toner image from the image carrier to the recording material, the transfer roller sandwiching the belt between itself and the image carrier and rotating the belt together with the image carrier; a downstream roller that is arranged in contact with the inner peripheral surface of the belt downstream of the transfer roller in the recording material transport direction and that forms a surface of the belt that transports the recording material between itself and the transfer roller; and a drive source that rotationally drives the downstream roller to rotate in the same direction as the belt, wherein the rotation speed of the drive source during image formation is set so that the surface speed of the downstream roller is faster than the inner peripheral surface speed of the belt. [Effects of the Invention]

[0013] According to the present invention, it is possible to suppress permanent deformation of the transfer roller and also suppress waviness of the surface of the transfer belt that transports the recording material. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a schematic cross-sectional view of an image forming apparatus. [Figure 2] FIG. 2 is a schematic cross-sectional view of the vicinity of a secondary transfer device. [Figure 3]FIG. 4 is a schematic cross-sectional view of the vicinity of a secondary transfer device for explaining a driving mode of a secondary transfer belt. [Figure 4] FIG. 10 is a graph showing the effect of suppressing waving of the secondary transfer belt. [Figure 5] 10A and 10B are a schematic cross-sectional view and a top view for explaining another example of the driving mode of the secondary transfer device. DETAILED DESCRIPTION OF THE INVENTION

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

[0016] [Example 1] <Configuration and Operation of 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 printer employing an intermediate transfer method, capable of forming a full-color image using an electrophotographic method. The image forming apparatus 100 can form an image on a recording material S in accordance with an image signal transmitted from an external device.

[0017] Regarding the image forming apparatus 100 and its elements, the front side of the page in FIG. 1 is referred to as the "front" side, and the back side of the page as the "rear" side. The front-to-rear direction connecting the front and rear sides is approximately parallel to the rotation axis direction of the photosensitive drum 11, the tension roller of the intermediate transfer belt 31, or the tension roller of the secondary transfer belt 40, which will be described later. Regarding the image forming apparatus 100 and its elements, the up-down direction refers to the up-down direction in the direction of gravity (vertical direction), but does not mean just directly above or directly below. It also includes the upper and lower sides of a horizontal plane passing through a position or element of interest. Furthermore, although the recording material S is sometimes referred to as "paper," the recording material S is not limited to paper. The recording material S may be made of a material other than paper or a material containing a material other than paper, such as synthetic paper or film made primarily of a synthetic resin, or special paper such as metal-deposited paper.

[0018] The image forming apparatus 100 has four image forming units 1Y, 1M, 1C, and 1K that form images in the colors yellow (Y), magenta (M), cyan (C), and black (K), respectively. The image forming units 1Y, 1M, 1C, and 1K are arranged in series along the direction of movement of the image transfer surface of an intermediate transfer belt 31, which is disposed substantially horizontally and will be described later. Elements having the same or corresponding functions or configurations for each color may be generally described by omitting the Y, M, C, or K suffixes to designate elements for a specific color. In this embodiment, the image forming unit 1 includes a photosensitive drum 11, a charger 12, an exposure device 13, a developing device 14, a drum cleaning device 15, and other components, which will be described later.

[0019] The photosensitive drum 11, a rotatable drum-type (cylindrical) photosensitive member (electrophotographic photosensitive member) serving as a first image carrier, is driven to rotate in the direction of arrow R1 (counterclockwise) in the figure. The surface of the rotating photosensitive drum 11 is uniformly charged to a predetermined potential of a predetermined polarity (negative in this embodiment) by a charger 12 serving as charging means. The charged surface of the photosensitive drum 11 is scanned and exposed by an exposure device (laser scanner) 13 serving as exposure means, which irradiates the surface with image light corresponding to image information, thereby forming an electrostatic latent image (electrostatic image) on the photosensitive drum 11. The electrostatic latent image formed on the photosensitive drum 11 is developed (visualized) by a developing device 14 serving as developing means, which supplies toner as a developer, thereby forming a toner image on the photosensitive drum 11. In this embodiment, toner charged with the same polarity (negative in this embodiment) as the charge polarity of the photosensitive drum 11 adheres to the exposed portion (image portion) on the photosensitive drum 11, which has been uniformly charged and then exposed according to image information to reduce the absolute value of the potential (reverse development method). In this embodiment, the normal charge polarity of the toner, which is the main charge polarity of the toner during development, is negative.

[0020] An intermediate transfer belt 31, which is an intermediate transfer body formed of a rotatable endless belt serving as a second image carrier, is disposed opposite the four photosensitive drums 11. The intermediate transfer belt 31 is stretched by a plurality of tension rollers (support rollers), including a drive roller 33, a tension roller 34, and a secondary transfer opposing roller 32. In this embodiment, the intermediate transfer belt 31 has a base layer formed of a resin material such as polyimide or polycarbonate, and a surface layer formed of a rubber material. A motor 331, which serves as a drive source and constitutes a drive device (not shown) serving as a drive means for rotating the drive roller 33, is connected to an end of the drive roller 33 in the direction of its rotation axis. The drive device for the drive roller 33 is configured to include, in addition to the motor 331, drive transmission members, and the like. The drive device for the drive roller 33 operates under the control of a control unit (not shown) provided in the image forming apparatus 100. The intermediate transfer belt 31 rotates (circulates) in the direction of arrow R2 (clockwise) in the figure by transmitting a driving force as the drive roller 33 is driven to rotate. A tension roller 34 applies a predetermined tension to the intermediate transfer belt 31 in the transport direction (rotation direction, surface movement direction, running direction). A secondary transfer opposing roller (secondary transfer inner roller) 32, together with a secondary transfer roller (secondary transfer outer roller) 41 (described later), forms a secondary transfer section N2 (described later). Primary transfer rollers 35Y, 35M, 35C, and 35K, which are roller-type primary transfer members serving as primary transfer means, are disposed on the inner circumferential surface of the intermediate transfer belt 31, corresponding to the photosensitive drums 11Y, 11M, 11C, and 11K, respectively. In this embodiment, the primary transfer rollers 35Y, 35M, 35C, and 35K are disposed in positions facing the photosensitive drums 11Y, 11M, 11C, and 11K, respectively. The primary transfer rollers 35 are pressed against the photosensitive drums 11 and come into contact with the photosensitive drums 11 via the intermediate transfer belt 31, forming a primary transfer portion (primary transfer nip) N1, which is an area where the photosensitive drums 11 and the intermediate transfer belt 31 come into contact. The tension rollers of the intermediate transfer belt 31 other than the drive roller 33 and each primary transfer roller 35 are rotated in accordance with the rotation of the intermediate transfer belt 31.

[0021] The toner image formed on the photosensitive drum 11 is transferred (primary transfer) onto the rotating intermediate transfer belt 31 at the primary transfer portion N1. During the primary transfer process, a primary transfer voltage (primary transfer bias), which is a DC voltage of opposite polarity (positive polarity in this embodiment) to the normal charging polarity of the toner, is applied to the primary transfer roller 35. For example, when a full-color image is formed, toner images of yellow, magenta, cyan, and black formed on the photosensitive drums 11Y, 11M, 11C, and 11K are transferred sequentially so as to be superimposed on the same image position on the intermediate transfer belt 31.

[0022] A secondary transfer device 4 is disposed on the outer peripheral surface side of the intermediate transfer belt 31, facing a secondary transfer opposing roller 32 serving as an opposing member. The secondary transfer device 4 includes a secondary transfer belt 40 formed as an endless belt, and a secondary transfer roller 41 disposed on the inner peripheral surface side of the secondary transfer belt 40, facing the secondary transfer opposing roller 32. The secondary transfer roller 41 is pressed toward the secondary transfer opposing roller 32 and contacts the secondary transfer opposing roller 32 via the secondary transfer belt 40 and the intermediate transfer belt 31. This allows the secondary transfer roller 41 to form a secondary transfer portion (secondary transfer nip) N2, which is a region where the intermediate transfer belt 31 and the secondary transfer belt 40 contact each other. The toner image formed on the intermediate transfer belt 31 is transferred (secondary transferred) onto the recording material S, which is being conveyed while being sandwiched between the intermediate transfer belt 31 and the secondary transfer belt 40, at the secondary transfer portion N2. During the secondary transfer process, a secondary transfer voltage (secondary transfer bias), which is a DC voltage of the same polarity (negative in this embodiment) as the normal charging polarity of the toner, is applied to the secondary transfer opposing roller 32. The secondary transfer roller 41 is electrically grounded (connected to a ground potential). Alternatively, the secondary transfer opposing roller 32 may be electrically grounded, and a secondary transfer voltage of the opposite polarity to the normal charging polarity of the toner may be applied to the secondary transfer roller 41. The secondary transfer device 4 will be described in more detail later.

[0023] Recording material (transfer material, recording medium, sheet) S is stored in paper feed cassettes 61, 62, and 63 serving as recording material storage units. This recording material S is sent to a paper feed conveyance path 67 serving as a recording material conveyance path by the rotation of one of paper feed rollers 64, 65, and 66 serving as a feeding member, and is conveyed to registration rollers 21 serving as a conveyance member. This recording material S is conveyed by registration rollers 21 to secondary transfer unit N2 in synchronization with the toner image on intermediate transfer belt 31.

[0024] The recording material S onto which the toner image has been transferred is transported to a fixing device 5 as a fixing means by a secondary transfer belt 40 and a transport belt 71 as a transport member. The fixing device 5 applies heat and pressure to the recording material S bearing the unfixed toner image, thereby fixing (melting and adhering) the toner image to the surface of the recording material S. The recording material S onto which the toner image has been fixed passes through a paper discharge transport path 68 as a recording material transport path, and is discharged (output) to a paper discharge tray 69 as a discharge section.

[0025] Furthermore, the toner remaining on the photosensitive drum 11 after the primary transfer process (primary transfer residual toner) is removed from the photosensitive drum 11 and collected by a drum cleaning device 15 serving as cleaning means. Furthermore, the toner remaining on the intermediate transfer belt 31 after the secondary transfer process (secondary transfer residual toner) and other deposits are removed from the intermediate transfer belt 31 and collected by a belt cleaning device 36 serving as cleaning means.

[0026] In each image forming unit 1, the photosensitive drum 11, the charger 12, the developing unit 14, and the drum cleaning device 15 may be individually or integrally formed into a cartridge that is detachable from the main body 2 of the image forming apparatus 100. The intermediate transfer belt 31, the tension roller of the intermediate transfer belt 31, each primary transfer roller 35, and the belt cleaning device 36 constitute an intermediate transfer belt unit 3. The intermediate transfer belt unit 3 may be detachable from the main body 2 of the image forming apparatus 100.

[0027] In addition, in this embodiment, the image forming apparatus 100 is configured so that the fed recording material S is transported from right to left at the secondary transfer section N2 as shown in Figure 1, but this is not limited to this direction and the recording material S may also be transported from left to right.

[0028] <Configuration of the secondary transfer device> Next, the configuration of the secondary transfer device 4 in this embodiment will be further described. Fig. 2 is a schematic cross-sectional view showing the vicinity of the secondary transfer device 4 in this embodiment (showing a cross section approximately perpendicular to the rotation axis direction of the secondary transfer roller 41 and separation roller 42). Note that with respect to the secondary transfer belt 40 and the tension rollers of the secondary transfer belt 40, "upstream" and "downstream" refer to "upstream" and "downstream," respectively, in the transport direction of the recording material S transported through the secondary transfer belt 40 or the secondary transfer portion N2.

[0029] The secondary transfer device 4 has a secondary transfer belt (transfer belt) 40, which is an endless belt serving as a recording material carrier. The secondary transfer belt 40 is tensioned (supported) by a plurality of tension rollers (support rollers). The secondary transfer device 4 has the following two rollers arranged on the inner circumferential surface of the secondary transfer belt 40 as tension rollers for the secondary transfer belt 40: a secondary transfer roller 41, which is a transfer roller (transfer member) serving as a first tension roller (first support roller) that forms the secondary transfer portion N2, and a separation roller 42, which is a second tension roller (second support roller). The secondary transfer roller 41 and the separation roller 42 each contact the inner circumferential surface of the secondary transfer belt 40. That is, of the two rollers tensioning the secondary transfer belt 40 from the inner circumferential side, the secondary transfer roller 41 presses against the secondary transfer opposing roller 32 via the secondary transfer belt 40 and the intermediate transfer belt 31, thereby forming the secondary transfer portion N2. Each tension roller of the secondary transfer belt 40 is rotatably supported (held) at both ends in the direction of its rotation axis by a frame (not shown) that constitutes the secondary transfer device 4. The directions of the rotation axes of the secondary transfer roller 41 and the separation roller 42 are approximately parallel to each other.

[0030] The secondary transfer belt 40 is formed using a resin such as polyimide or polycarbonate. The material constituting the secondary transfer belt 40 contains an appropriate amount of a conductive material such as carbon black or an antistatic agent to adjust the electrical resistance (for example, a volume resistivity of 1×10 9 ~1×10 14 The secondary transfer belt 40 may have a resistivity of Ω·cm. The secondary transfer belt 40 may have a single-layer structure or a multi-layer structure. The material of the secondary transfer belt 40 is not limited to resin, and it may be made of metal. The peripheral length of the secondary transfer belt 40 is, for example, about 100 to 300 mm.

[0031] The secondary transfer roller 41 is an elastic roller with a multi-layer structure. In this embodiment, the secondary transfer roller 41 is configured by providing a foamed rubber layer (elastic layer) 41b, which is made of foamed rubber as a dielectric material, on the outer periphery of a core metal 41a, which is the lowest layer. In this embodiment, the foamed rubber constituting the secondary transfer roller 41 has an Asker-C hardness of 25° or more and 50° or less. As a result, the secondary transfer roller 41 has a lower hardness than the secondary transfer opposing roller 32.

[0032] The secondary transfer roller 41, together with the secondary transfer belt 40, is brought into contact (pressed) by a pressure mechanism (not shown) with the portion of the secondary transfer opposing roller 32 around which the intermediate transfer belt 31 is wound. In this way, the secondary transfer roller 41 is pressed against the secondary transfer opposing roller 32 via the secondary transfer belt 40 and the intermediate transfer belt 31. At the contact portion between the secondary transfer opposing roller 32 around which the intermediate transfer belt 31 is wound and the secondary transfer roller 41 around which the secondary transfer belt 40 is wound, the contact force elastically deforms the foamed rubber layer (low hardness member) 41b of the low hardness secondary transfer roller 41. This forms a secondary transfer portion N2, which is a contact area between the intermediate transfer belt 31 and the secondary transfer belt 40.

[0033] Incidentally, it is known that, for example, when a recording material S (such as cardboard) with high rigidity enters the secondary transfer portion N2, the impact can cause a speed fluctuation of the intermediate transfer belt 31, resulting in a misaligned toner image being transferred at the primary transfer portion N1 and resulting in image degradation. However, by making the secondary transfer roller 41 sufficiently low hardness as described above, the secondary transfer roller 41 can absorb the impact when the recording material S enters. This reduces the speed fluctuation of the intermediate transfer belt 31 and suppresses image degradation. In this embodiment, the hardness of the secondary transfer roller 41 is set lower than the hardness of the secondary transfer opposing roller 32 as described above.

[0034] In this embodiment, a potential of the same polarity as the normal charging polarity of the toner is applied to the secondary transfer opposing roller 32 from an external power supply device (not shown) as a voltage application means, and the secondary transfer roller 41 is electrically grounded, thereby forming a secondary transfer electric field at the secondary transfer portion N2. However, the mode of power supply to the secondary transfer portion N2 is not limited to this, and the secondary transfer opposing roller 32 may be electrically grounded, and a potential of the opposite polarity to the normal charging polarity of the toner may be applied to the secondary transfer roller 41.

[0035] The separation roller (downstream roller) 42 is disposed downstream of the secondary transfer roller 41. The separation roller 42 and the secondary transfer roller 41 form a recording material conveying surface (here, also referred to as the "upper surface") S1, which is the belt surface of the secondary transfer belt 40 that supports and conveys the recording material S. The recording material S passes through the secondary transfer portion N2, is electrostatically attracted to the outer circumferential surface (surface) of the upper surface S1 of the secondary transfer belt 40, and is conveyed by the rotation of the secondary transfer belt 40, and is peeled off from the secondary transfer belt 40 by utilizing the curvature of the separation roller 42. As a result, the recording material S is handed over from the secondary transfer belt 40 to the conveying belt 71.

[0036] As mentioned above, when a low-hardness transfer roller is used, if a strong tension is applied to the low-hardness member of the transfer roller at the portion where the transfer belt is wrapped around it for a long period of time, the low-hardness member may permanently deform in that portion. Permanent deformation of the low-hardness member of the transfer roller can cause periodic density differences when images are formed. In particular, a configuration in which the transfer belt is tensioned by two tension rollers, such as the secondary transfer device 4 of this embodiment, is prone to permanent deformation of the low-hardness member because the transfer belt wraps around the transfer roller to a large extent, which tends to increase the tension applied to the wrapped portion. Furthermore, in a transfer device with such a configuration, it is difficult to provide a mechanism for loosening the tension of the transfer belt due to factors such as cost and space conservation.

[0037] From the viewpoint of reducing the cost and space required for the secondary transfer device 4, it is desirable to have only two tension rollers, the secondary transfer roller 41 and the separation roller 42, as in this embodiment. However, the present invention is not limited to this configuration. For example, the secondary transfer device 4 may be provided with additional rollers such as a drive roller, a tension roller, a cleaning counter roller, and a steering roller. These additional rollers may be provided so as to contact the inner circumferential surface of the secondary transfer belt 40 other than between the secondary transfer roller 41 and the separation roller 42, and one or more of these additional rollers may be provided. The drive roller is a roller that transports the secondary transfer belt 40 in the circumferential direction. The tension roller is a roller that applies tension to the secondary transfer belt 40. If the secondary transfer device 4 is provided with a cleaning member for collecting toner adhering to the secondary transfer belt 40, the cleaning counter roller is a roller that is positioned opposite the cleaning member and against which the cleaning member abuts via the secondary transfer belt 40. The steering roller is a roller that controls the movement of the secondary transfer belt 40 in the width direction (the direction substantially perpendicular to the moving direction of the surface) during transport.

[0038] Next, a method for setting the inter-axial distance, which is defined as the distance between the axial centers of the secondary transfer roller 41 and the separation roller 42 in this embodiment, will be described with reference to Fig. 3. Fig. 3 is a schematic cross-sectional view showing the vicinity of the secondary transfer device 4 in this embodiment to explain this inter-axial distance (showing a cross section approximately perpendicular to the rotational axis direction of the secondary transfer roller 41 and the separation roller 42).

[0039] First, in this embodiment, the secondary transfer belt 40 manufactured by molding has a circular cross section as a single product. A tension roller is disposed on the inner peripheral side of this secondary transfer belt 40, and the secondary transfer belt 40 is forcibly deformed into an elliptical shape, thereby forming the secondary transfer device 4.

[0040] In this embodiment, the placement locations of the tension rollers, i.e., the inter-axial distance, are set under conditions that maintain the relationships described below. Here, the length of the upper surface S1 of the secondary transfer belt 40 formed by the two tension rollers (secondary transfer roller 41, separation roller 42) is defined as L1. The length of the lower surface S2 of the secondary transfer belt 40 is defined as L2. The length of the first winding portion S3 of the secondary transfer belt 40 that is wound around the secondary transfer roller 41 is defined as L3. Furthermore, the length of the second winding portion S4 of the secondary transfer belt 40 that is wound around the separation roller 42 is defined as L4. More specifically, the upper surface S1, the lower surface S2, the first winding portion S3, the second winding portion S4, and the lengths L1, L2, L3, and L4 are defined as follows:

[0041] The secondary transfer roller 41 and the separation roller 42 are arranged at an arbitrary center distance and viewed approximately parallel to the rotational axis of the secondary transfer roller 41. Then, a straight line is drawn tangent to each outer diameter circle of the secondary transfer roller 41 and the separation roller 42, and the intersections of the straight line and each circle are defined as the endpoints of the line segments. In this case, the upper surface S1 is the portion corresponding to the line segment tangent to the upper surface of each roller. The lower surface S2 is the portion corresponding to the line segment tangent to the lower surface of each roller. The first wrapping portion S3 is the portion wrapped around the secondary transfer roller 41 to connect the upper surface S1 and the lower surface S2. The second wrapping portion S4 is the portion wrapped around the separation roller 42 to connect the upper surface S1 and the lower surface S2. Therefore, on the inner peripheral surface side of the secondary transfer belt 40, the length L1 of the upper surface S1, the length L2 of the lower surface S2, the length L3 of the first winding portion S3, and the length L4 of the second winding portion S4 are as follows. L1: Line distance tangent to the top surface of each roller L2: Line distance tangent to the bottom surface of each roller L3: the length of the arc connecting the end points of the line segments corresponding to the upper surface S1 and the lower surface S2 on the secondary transfer roller 41 side along the outer diameter circle of the secondary transfer roller 41 L4: The length of the arc connecting the end points of the line segments corresponding to the upper surface S1 and the lower surface S2 on the separation roller 42 side along the outer diameter circle of the separation roller 42

[0042] In addition, the inner peripheral length of the secondary transfer belt 40=S1+S2+S3+S4 The center distance in this case is defined as L. In other words, L is the center distance in an ideal state where the secondary transfer belt 40 is stretched without sagging and is free from deformation.

[0043] In this embodiment, the secondary transfer roller 41 and the separation roller 42 are arranged so that the center distance between them is L±5%, preferably L±1.5%. Typically, the secondary transfer roller 41 and the separation roller 42 are arranged so that the total length of L1, L2, L3, and L4 is equal to or less than the inner peripheral length of the secondary transfer belt 40 (i.e., the total length is the same as or shorter than the inner peripheral length).

[0044] When the secondary transfer roller 41 and the separation roller 42 are arranged with the center distance set by the above-described method, the forcibly deformed secondary transfer belt 40 tries to return to its original circular shape, and typically slack occurs in the secondary transfer belt 40. Therefore, the first winding portion S3 wound around the secondary transfer roller 41 is essentially only given a weak initial tension force, which is a restoring force that causes the deformed secondary transfer belt 40 to return to its original shape. This makes it possible to reduce the risk of permanent deformation of the foamed rubber layer 41b of the secondary transfer roller 41.

[0045] The method for setting the center distance as described above is not limited to fixing the positions of the two tension rollers at a predetermined position. The center distance may also be set by fixing the position of one of the tension rollers and applying a tensioning force to the other tension roller in a direction that increases the center distance using a spring or other biasing member. However, it is necessary to select a spring strong enough to prevent the foamed rubber layer 41b from being permanently deformed by the tensioning force (as long as the risk of permanent deformation of the foamed rubber layer 41b is reduced to the same extent as when the center distance is set by fixing the positions of the two tension rollers). The initial tensioning force required to prevent permanent deformation of the foamed rubber layer 41b varies depending on the hardness of the secondary transfer roller 41 and the Young's modulus of the secondary transfer belt 40, so it is desirable to set the center distance and spring force appropriately.

[0046] <Drive mode of the tension roller of the secondary transfer belt> Next, the driving mode of the tension roller of the secondary transfer belt 40 in this embodiment will be described with reference to FIG.

[0047] The intermediate transfer belt 31 is transported in the direction of arrow R2 (clockwise) by the rotation of the drive roller 33, and also rotates the secondary transfer belt 40 and secondary transfer roller 41, which are in contact with the intermediate transfer belt 31 at the secondary transfer portion N2. As a result, the secondary transfer belt 40 rotates (circulates) in the direction of arrow R3 (counterclockwise) at the secondary transfer portion (contact portion) N2. If the coefficient of friction between the intermediate transfer belt 31 and the secondary transfer belt 40 is sufficiently high and the load in the normal direction within the secondary transfer portion N2 is sufficiently large, the intermediate transfer belt 31 and the secondary transfer belt 40 tightly grip each other. Therefore, the transport speed V1 of the intermediate transfer belt 31 and the transport speed V2 of the secondary transfer belt 40 are the same.

[0048] The conveying speed V1 of the intermediate transfer belt 31 is approximately equal to the surface speed (outer peripheral speed, outer peripheral movement speed, circumferential speed) of the intermediate transfer belt 31. The conveying speed V2 of the secondary transfer belt 40 is approximately equal to the inner peripheral speed (inner peripheral movement speed) of the secondary transfer belt 40. The surface speed V1 of the intermediate transfer belt 31 and the inner peripheral speed V2 of the secondary transfer belt 40 can be calculated based on the rotation speed of the motor 331 (FIG. 1) that drives the drive roller 33 that rotates the intermediate transfer belt 31, and the configurations of the drive transmission member, drive roller 33, intermediate transfer belt 31, and secondary transfer belt 40. However, the surface speed V1 of the intermediate transfer belt 31 and the inner peripheral speed V2 of the secondary transfer belt 40 may also be measured directly.

[0049] A motor 421, which is a drive source, is connected to an end of the separation roller 42 in the direction of its rotation axis, constituting a drive device (not shown) as a drive means for rotating the separation roller 42. The drive device for the separation roller 42 is configured to include, in addition to the motor 421, a drive transmission member and the like. The drive device for the separation roller 42 operates under the control of a control unit (not shown) provided in the image forming apparatus 100. The separation roller 42 receives a drive force from the motor 421 and rotates in the direction of arrow R4 in the figure (counterclockwise), i.e., in the same direction as the secondary transfer belt 40.

[0050] In a configuration of the secondary transfer device 4 having tension rollers (secondary transfer roller 41, separation roller 42) arranged using the above-mentioned inter-axis distance setting method, the secondary transfer belt 40 typically slackens, which may result in noticeable waviness.

[0051] Therefore, in this embodiment, the rotation speed (rpm) of the motor 421 is set so that the surface speed (outer peripheral surface speed, outer peripheral surface movement speed, circumferential speed) V3 of the separation roller 42 is faster than the inner peripheral surface speed V2 of the secondary transfer belt 40. As a result, the surface (outer peripheral surface) of the separation roller 42 is maintained in a slip state relative to the inner peripheral surface (rear surface) of the secondary transfer belt 40 in the conveying state, and a dynamic friction force is steadily applied to the inner peripheral surface of the secondary transfer belt 40 in the conveying state. This dynamic friction force is applied to the secondary transfer belt 40 as a force pulling the upper surface S1 of the secondary transfer belt 40, and slackens the secondary transfer belt 40 at the lower surface S2 of the secondary transfer belt 40. In other words, by essentially applying an additional tension force only to the upper surface S1 of the secondary transfer belt 40 that conveys the recording material S, the upper surface S1 is stabilized and the amount of waviness can be reduced. In addition, since slack occurs on the underside S2 of the secondary transfer belt 40, the tension force applied to the first winding portion S3 of the secondary transfer belt 40 wound around the secondary transfer roller 41 is small, and permanent deformation of the foamed rubber layer 41b of the secondary transfer roller 41 can be suppressed.

[0052] It is sufficient that this speed relationship is satisfied at least during image formation, more specifically, at least while the recording material S supplied for image formation passes over the secondary transfer portion N2 and the upper surface S1 of the secondary transfer belt 40. The surface speed V3 of the separation roller 42 can be determined based on the rotation speed of the motor 421 that drives the separation roller 42, the drive transmission member, the configuration of the separation roller 42, etc. However, the surface speed V3 of the separation roller 42 may also be measured directly.

[0053] 4 is a graph showing the results of verifying the effect of suppressing the amount of waviness on the upper surface of the secondary transfer belt 40 using the secondary transfer device 4 in this embodiment. The specifications of the elements related to the secondary transfer unit N2 used in this verification example are as follows: Intermediate transfer belt 31 Surface material: Chloroprene rubber Base material: Polyimide Thickness: 355 μm Conveying speed: 435 mm / s Secondary transfer opposing roller 32 Surface material: Conductive EPDM Outer diameter: 16mm Rubber thickness: 0.5mm Hardness: JIS-A70° Secondary transfer belt 40 Material: Polyimide Thickness: 85 μm Secondary transfer roller 41 Material: Conductive sponge rubber Outer diameter: 24mm Rubber thickness: 6mm Hardness: Asker-C28° Separation roller 42 Material: SUS Outer diameter: 13mm

[0054] The vertical axis in Fig. 4 indicates the amount of waviness, which is the ratio (%) of the maximum amplitude when the amount of displacement of the upper surface S1 of the secondary transfer belt 40 in the out-of-plane direction is measured using a laser displacement meter. This ratio was defined as 100% when the amount of waviness was 100% when the separation roller 42 was not driven and was rotated in a driven manner relative to the secondary transfer belt 40. The horizontal axis in Fig. 4 indicates the speed ratio (%) of the surface speed V3 of the separation roller 42 to the inner surface speed V2 of the secondary transfer belt 40, which is expressed by the following equation: {(V3-V2) / V2}×100 (here, this is also simply referred to as the "speed difference").

[0055] By setting the speed difference to 5% or more, it is possible to achieve a certain degree of effect in suppressing the amount of waviness. Furthermore, by setting the speed difference to 10% or more, the effect of suppressing the amount of waviness becomes even greater. Furthermore, by setting the speed difference to 15% or more, even better results were obtained, with the amount of waviness being suppressed by up to 60%. It was also found that even with a further increase in the speed difference, the amount of waviness suppression does not change significantly. This indicates that in the region where the speed difference is 15% or more, a stable dynamic friction force is applied to the upper surface S1 of the secondary transfer belt 40, and that the dynamic friction force is not dependent on the surface speed V3 of the separation roller 42.

[0056] On the other hand, in the configuration of this test example, abnormal noise sometimes occurred near the separation roller 42 in areas where the speed difference was less than 15%. This noise was found to be due to chattering of the secondary transfer belt 40. In other words, areas with small speed differences are stick-slip areas, where the surface of the separation roller 42 alternates between slipping and gripping with respect to the inner surface of the transported secondary transfer belt 40. Furthermore, the experiment revealed that vibrations caused by stick-slip can vibrate the secondary transfer belt 40 and increase the amount of waviness. As mentioned above, the horizontal axis of FIG. 4 represents the amount of waviness when the separation roller 42 is rotated relative to the secondary transfer belt 40 as 100%. In this test example, the surface speed of the separation roller 42 is slightly slower than the inner surface speed of the secondary transfer belt 40. The amount of waviness then increases slightly from that state until the speed difference becomes 0%, resulting in the amount of waviness being greater than 100% when the speed difference is 0%.

[0057] To enhance the waving suppression effect, it is desirable to set the rotation speed of the motor 421 so that dynamic friction force can be continuously applied. Based on the results of the above-described verification, the speed difference between the surface speed V3 of the separation roller 42 and the inner peripheral surface speed V2 of the secondary transfer belt 40 (= {(V3-V2) / V2} × 100) should be at least 5%, preferably 10% or more, and more preferably 15% or more. However, as described above, even if this speed difference is increased beyond 15%, the waving suppression effect may not change significantly. Furthermore, for example, if this speed difference is made too large, it is considered that the life of the secondary transfer device 4 will be shortened due to wear on the inner peripheral surface of the secondary transfer roller 41 and wear on the drive system of the separation roller 42. Therefore, this speed difference of 50% or less is often sufficient, preferably 40% or less, and more preferably 30% or less.

[0058] However, it is expected that the boundary between the dynamic friction region and the stick-slip region will fluctuate due to fluctuations in the coefficient of friction between the secondary transfer belt 40 and the separation roller 42, variations in the center distance, and deflection in the width direction of the secondary transfer belt 40 as the operation amount of the secondary transfer device 4 increases. Therefore, it is desirable to set the rotation speed of the motor 421 appropriately, taking into consideration variations and fluctuations in factors related to frictional force.

[0059] Here, in order to achieve the above-mentioned speed difference, it is necessary for the surface of the separation roller 42 to slide on the inner circumferential surface of the secondary transfer belt 40. The requirements for this (frictional force relationship, normal force, friction coefficient) will be described.

[0060] In the configuration of this embodiment, since the secondary transfer belt 40 rotates, (1) the driving force of the intermediate transfer belt 31 and (2) the driving force of the separation roller 42 are transmitted to the secondary transfer belt 40.

[0061] The static friction force F1 applied to the secondary transfer belt 40 by the above (1) = the coefficient of friction between the surface of the intermediate transfer belt 31 and the surface of the secondary transfer belt 40 x the pressure force (normal force) applied when the secondary transfer opposing roller 32 abuts against the secondary transfer roller 41.

[0062] Furthermore, the static friction force F2 applied to the secondary transfer belt 40 by (2) above = the friction coefficient between the surface of the separation roller 42 and the inner surface of the secondary transfer belt 40 x the restoring force (normal force) acting in the axial direction between the secondary transfer roller 41 and the separation roller 42 as the elliptical secondary transfer belt 40 returns to a circular shape.

[0063] When F1>F2, the secondary transfer belt 40 slips relative to the separation roller 42 (a state in which a dynamic friction force is acting). Under the above conditions, by setting the rotation speed of the motor 421 that drives the separation roller 42 so that the surface speed of the separation roller 42 is greater than the inner peripheral surface speed of the secondary transfer belt 40, a force is applied in the conveyance direction to pull the upper surface S1 side of the secondary transfer belt 40 and loosen the lower surface S2 side (the direction of the dynamic friction force that is actually applied is reversed).

[0064] In this embodiment, the separation roller 42 is configured as a metal roller made of SUS (stainless steel), which is a metal, but the separation roller 42 is not limited to a metal roller. As long as the above-mentioned conditions for the surface of the separation roller 42 and the inner circumferential surface of the secondary transfer belt 40 to slide on each other are satisfied, the separation roller 42 may be, for example, a roller with a solid rubber roller portion, a plastic roller, or the like.

[0065] The secondary transfer roller 41 is not limited to being driven by rotation, and may have a driving function. In other words, the image forming apparatus 100 may have a driving source that applies a driving force to the secondary transfer roller 41, independent of the driving source of the intermediate transfer belt 31 (and further, the driving source of the separation roller 42). This has the effect of suppressing slippage between the recording material S and the intermediate transfer belt 31, which occurs when the recording material S, which has a small surface friction coefficient, enters the secondary transfer portion N2 or when there is a large amount of toner. In other words, this slippage can cause image degradation, in which the length of the toner image in the transport direction changes before and after transfer. In contrast, by providing the secondary transfer roller 41 with a driving function, the secondary transfer belt 40 is transported independently of the intermediate transfer belt 31, and therefore the secondary transfer portion N2 can be prevented from slipping. can assist the conveyance of the recording material S. As a result, the conveyance speeds of the intermediate transfer belt 31 and the recording material S match, so that even when the friction coefficient of the surface of the recording material S is small or when the toner amount is large, it is easy to transfer the toner image from the intermediate transfer belt 31 to the recording material S while keeping the length of the toner image in the conveyance direction constant. In this case, the conveyance speed of the secondary transfer belt 40 is controlled by the rotation speed of the secondary transfer roller 41 by setting the center distance and tension so that the friction force between the surface of the separation roller 42 and the inner circumferential surface of the secondary transfer belt 40 is smaller than the friction force between the surface of the secondary transfer roller 41 and the inner circumferential surface of the secondary transfer belt 40 at the secondary transfer portion N2.

[0066] As described above, in this embodiment, the image forming apparatus 100 includes a rotatable image carrier (intermediate transfer belt) 31 that carries a toner image, an endless belt (secondary transfer belt) 40 that carries and transports a recording material S, a transfer roller (secondary transfer roller) 41 that is arranged to contact the inner peripheral surface of the belt 40 and forms a transfer section (secondary transfer section) N2 that transfers the toner image from the image carrier 31 to the recording material S, and that rotates the belt 40 together with the image carrier 31 by sandwiching the belt 40 between the transfer roller 41 and the image carrier 31, and the recording material S. The transfer roller 41 includes a downstream roller (separation roller) 42 disposed downstream of the transfer roller 41 in the conveyance direction of the recording material S so as to contact the inner circumferential surface of the belt 40. The downstream roller (separation roller) 42 forms a surface S1 of the belt 40 along which the recording material S is conveyed between the downstream roller 42 and the transfer roller 41. The drive source (motor) 421 drives the downstream roller 42 to rotate in the same direction as the belt 40. The rotation speed of the drive source 421 during image formation is set so that the surface speed V3 of the downstream roller 42 is faster than the inner circumferential speed V2 of the belt 40. In this embodiment, the transfer roller 41 has an elastic layer 41b having an Asker-C hardness of 25° or more and 50° or less. The rotation speed of the drive source 421 during image formation is set so that the difference between the surface speed V3 of the downstream roller 42 and the inner circumferential speed V2 of the belt 40 is 5% or more and 50% or less, preferably 10% or more and 50% or less, and more preferably 15% or more and 50% or less. In this embodiment, the transfer roller 41 rotates following the rotation of the image carrier 31. However, the transfer roller 41 may be rotated independently of the image carrier 31. In this embodiment, the transfer belt 40 is supported by two rollers: the transfer roller 41 and a downstream roller 42.In this embodiment, when viewed substantially parallel to the rotational axis of the transfer roller 41, two tangents circumscribing the outer diameter circles of the transfer roller 41 and the downstream roller 42 without intersecting each other are drawn. The lengths of the two line segments are L1 and L2, respectively, and the lengths of the two arcs along the outer diameter circles of the transfer roller 41 and the downstream roller 42, connecting the transfer roller end points and the downstream roller end points of the two line segments, are L3 and L4, respectively. When the total length of L1, L2, L3, and L4 is equal to the inner peripheral length of the belt, the center distance between the transfer roller and the downstream roller is L. The transfer roller 41 and the downstream roller 42 are positioned such that the center distance is L ± 5%. Typically, the total length of L1, L2, L3, and L4 is equal to or less than the inner peripheral length of the belt 40. In addition, in this embodiment, the image carrier 31 is an intermediate transfer body that transports a toner image transferred from another image carrier (photosensitive drum) 11 to transfer it to a recording material S at the transfer section N2, and the transfer roller 41 abuts against an opposing roller 32 that is arranged opposite the transfer roller 41 across the intermediate transfer body 40.

[0067] As described above, in this embodiment, in a configuration having two tension rollers, the low-hardness secondary transfer roller 41 and separation roller 42, the separation roller 42 is rotated so that the surface speed of the separation roller 42 is faster than the inner peripheral surface speed of the secondary transfer belt 40. This makes it possible to suppress permanent deformation of the low-hardness secondary transfer roller 41 while suppressing the amount of waviness of the upper surface (recording material conveying surface) S1, which is the belt surface of the secondary transfer belt 40 along which the recording material S is conveyed. Therefore, according to this embodiment, it is possible to suppress image degradation caused by permanent deformation of the secondary transfer roller 41 and waviness of the secondary transfer belt 40.

[0068] [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 the first embodiment. 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 the first embodiment are assigned the same reference numerals as those of the first embodiment, and detailed descriptions thereof will be omitted.

[0069] 5(a) is a schematic cross-sectional view showing the vicinity of the secondary transfer device 4 in this embodiment (showing a cross section substantially perpendicular to the rotation axis direction of the secondary transfer roller 41 and the separation roller 42). Also, FIG. 5(b) is a schematic top view of the secondary transfer device 4 in this embodiment.

[0070] In this embodiment, a torque limiter 422 is provided in a drive transmission section that serves as a drive transmission means for transmitting the drive force from the motor 421 to the separation roller 42. In other words, the torque limiter 422 is connected and disposed between the motor 421 and the separation roller 42.

[0071] By using a torque limiter, it is possible to protect the drive system by preventing excessive force from being applied to the drive source and other drive systems in the event of an abnormality. Torque limiters come in two types: one that completely cuts off the drive force from the drive source when a load exceeding the limited torque is applied, and another that transmits the drive force while maintaining it within the limited torque. In this embodiment, by using the latter type, the separation roller 42 is maintained in a state where it is driven with a predetermined torque. This makes it possible to apply additional tension to the secondary transfer belt 40.

[0072] In this embodiment, similarly to the first embodiment, the rotation speed of the motor 421 is set so that the surface speed V3 of the separation roller 42 is faster than the inner peripheral surface speed V2 of the secondary transfer belt 40.

[0073] As described above, in this embodiment, the torque limiter 422 is provided in the drive transmission section that transmits the drive force from the drive source 421 to the downstream roller 42. In this embodiment, the torque limiter 422 is configured to maintain a state in which the downstream roller 42 is driven with a torque within a predetermined limit torque so that the surface speed V3 of the downstream roller 42 is faster than the inner peripheral surface speed V2 of the belt 40 during image formation.

[0074] When the load on the torque limiter 422 is less than the limit torque, the driving force of the motor 421 is transmitted to the separation roller 42 at the same rate, so that the rotation speed of the motor 421 is the same as the rotation speed of the separation roller 42. In other words, the surface speed V3 of the separation roller 42 is faster than the conveying speed V2 of the secondary transfer belt 40, so that the same effect of suppressing ripples as in the first embodiment can be obtained.

[0075] On the other hand, if the load on the torque limiter 422 is equal to or greater than the limit torque, the driving force transmitted from the motor 421 to the separation roller 42 is limited. That is, if the rotation speed of the separation roller 42 falls below the rotation speed of the motor 421, the surface speed V3 of the separation roller 42 decreases and may become equal to, for example, the inner peripheral surface speed V2 of the secondary transfer belt 40. In this case, the additional tension applied from the surface of the separation roller 42 to the inner peripheral surface of the secondary transfer belt 40 decreases, and it is expected that a sufficient wave suppression effect will not be obtained. Therefore, it is desirable to appropriately set the limit torque of the torque limiter, taking into account the additional tension required to achieve the desired wave suppression effect. As a result, unless some abnormality occurs, the torque limiter 422 will not reduce the effect of applying additional tension by the separation roller 42.

[0076] [others] Although the present invention has been described above with reference to specific embodiments, the present invention is not limited to the above-described embodiments.

[0077] In the above-described embodiment, the intermediate transfer body is an intermediate transfer belt formed of an endless belt, but it may also be an intermediate transfer drum formed into a drum shape by attaching a sheet (film) to a frame. In this case, the secondary transfer roller contacts a secondary transfer opposing roller disposed opposite the intermediate transfer drum with the sheet sandwiched therebetween.

[0078] In the above-described embodiment, the present invention is applied to a secondary transfer device that transfers a toner image from an intermediate transfer body serving as an image carrier to a recording material, but the present invention is not limited to this embodiment. For example, in a monochrome image forming apparatus, the present invention may be applied to a transfer device that transfers a toner image from a photosensitive body serving as an image carrier to a recording material. In other words, the image carrier that the transfer belt contacts is not limited to an intermediate transfer belt and may be a photosensitive body. Furthermore, the image carrier may be an electrostatic recording dielectric (such as an electrostatic recording dielectric belt). [Explanation of symbols]

[0079] 3 Intermediate transfer belt unit 31 Intermediate transfer belt 32 Secondary transfer opposing roller 33 Drive roller 34 Tension roller 40 Secondary transfer belt 41 Secondary transfer roller 42 Separation roller 100 Image forming device 421 Motor N1 Primary transfer section N2 secondary transfer section S recording material

Claims

1. a rotatable image carrier that carries a toner image; an endless belt that carries and transports a recording material; a transfer roller that is disposed so as to contact the inner circumferential surface of the belt and forms a transfer section that transfers a toner image from the image carrier to a recording material, the transfer roller sandwiching the belt between itself and the image carrier and rotating the belt together with the image carrier; a downstream roller disposed downstream of the transfer roller in the recording material conveying direction so as to contact the inner circumferential surface of the belt, and forming a surface of the belt that conveys the recording material between the downstream roller and the transfer roller; a drive source that rotationally drives the downstream roller so as to rotate the downstream roller in the same direction as the belt; and 10. An image forming apparatus according to claim 9, wherein the rotation speed of said drive source during image formation is set so that the surface speed of said downstream roller is faster than the inner peripheral surface speed of said belt.

2. 2. The image forming apparatus according to claim 1, wherein the transfer roller has an elastic layer having an Asker-C hardness of 25 degrees or more and 50 degrees or less.

3. 2. The image forming apparatus according to claim 1, wherein the rotation speed of the drive source during image formation is set so that the speed difference between the surface speed of the downstream roller and the inner surface speed of the belt is 5% or more and 50% or less.

4. 2. The image forming apparatus according to claim 1, wherein the rotation speed of the drive source during image formation is set so that the speed difference between the surface speed of the downstream roller and the inner surface speed of the belt is 10% or more and 50% or less.

5. 2. The image forming apparatus according to claim 1, wherein the rotation speed of the drive source during image formation is set so that the speed difference between the surface speed of the downstream roller and the inner surface speed of the belt is 15% or more and 50% or less.

6. 2. The image forming apparatus according to claim 1, wherein the transfer roller rotates in accordance with the rotation of the image carrier.

7. 2. The image forming apparatus according to claim 1, wherein the transfer roller is rotated independently of the image carrier.

8. 2. The image forming apparatus according to claim 1, wherein a torque limiter is provided in a drive transmission section that transmits the drive force from the drive source to the downstream roller.

9. 9. The image forming apparatus according to claim 8, wherein the torque limiter is configured to maintain a state in which the downstream roller is driven with a torque within a predetermined limit torque so that the surface speed of the downstream roller is faster than the inner surface speed of the belt during image formation.

10. 2. The image forming apparatus according to claim 1, wherein the transfer belt is supported by two rollers, the transfer roller and the downstream roller.

11. When viewed substantially parallel to the rotation axis direction of the transfer roller, the lengths of two line segments whose endpoints are the intersections of two tangent lines circumscribing the outer diameter circles of the transfer roller and the downstream roller so as not to intersect with each other and the outer diameter circles of the transfer roller and the downstream roller are respectively L1 and L2; the lengths of two arcs along the outer diameter circles of the transfer roller and the downstream roller are defined as L3 and L4, respectively, so as to connect the end points of the two line segments on the transfer roller side and the end points of the two line segments on the downstream roller side; When the total length of L1, L2, L3, and L4 is equal to the inner peripheral length of the belt, the center distance between the transfer roller and the downstream roller is L.

11. The image forming apparatus according to claim 10, wherein the transfer roller and the downstream roller are disposed at positions where the distance between the axes is L±5%.

12. 12. The image forming apparatus according to claim 11, wherein the total length of L1, L2, L3, and L4 is equal to or less than the inner peripheral length of the belt.

13. 13. The image forming apparatus according to claim 1, wherein the image carrier is an intermediate transfer body that transports a toner image transferred from another image carrier to be transferred to a recording material at the transfer section, and the transfer roller abuts against an opposing roller that is arranged opposite the transfer roller across the intermediate transfer body.

Images