JP2015200875A - image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus configured to bring a transfer roller into pressure-contact with an inside surface of a transfer belt, while sufficiently exerting smooth-out effect at both ends or a rear end in a conveyance direction of a recording material.SOLUTION: A conveyance surface forming roller 21 forms a conveyance surface of a recording material by stretching a secondary transfer belt 12 in a position to be reached by a tip in a conveyance direction of the recording material conveyed in a secondary transfer section T2. A stretching roller 22 stretches the secondary belt 12 at the downstream of the conveyance surface forming roller 21. A drive roller 23 drives the secondary transfer belt 12 at the downstream of the stretching roller 22. The conveyance surface forming roller 21 has areas formed at both ends in a rotation axis direction, for stretching the secondary transfer belt 21, with a diameter smaller than a central part. The stretching roller 22 has areas formed at both ends in the rotation axis direction, for stretching a secondary transfer belt 12, with a diameter larger than the central part. The drive roller 23 has an area for stretching the secondary transfer belt 12 with the same diameter.

Description

  The present invention relates to an image forming apparatus that transfers a toner image carried on an image carrier onto a recording material carried on a transfer belt.

  Image formation in which a recording material is carried on a transfer belt and passed through a transfer portion, and a toner image carried on an intermediate transfer belt, which is an example of an image carrier, is transferred to a recording material carried on the transfer belt by the transfer portion The device is widely used (Patent Document 1).

  In an image forming apparatus using a transfer belt, wrinkles may occur in a recording material when a recording material with low rigidity such as thin paper or a recording material that has undergone undulation deformation is passed through a transfer unit and the toner image is transferred.

  In view of this, it has been proposed that a transfer roller having a reverse crown shape whose diameter gradually increases from the central portion toward both ends is pressed against the image carrier via a transfer belt. As shown in Patent Document 2, the transfer roller having an inverted crown shape on the outer peripheral surface has a conveyance speed at the end in the rotational axis direction larger than the conveyance speed at the center, and therefore the recording material that passes through the transfer section The rear end side is expanded in the direction of the rotation axis. As a result, the wrinkle stretching effect at both ends and the rear end in the recording material conveyance direction is exhibited, and the occurrence of wrinkles at both ends and the rear end in the recording material conveyance direction may be prevented or reduced. It is.

JP 2012-128228 A JP-A-7-225523 JP 2011-123254 A

  As will be described later, in an image forming apparatus in which the transfer roller is pressed against the inner surface of the transfer belt to form a toner image transfer portion, a reverse crown shape is applied to the transfer roller to confirm the effect of wrinkling the recording material. (See Table 2). Then, when the transfer roller was pressed against the inner surface of the transfer belt, it was found that even if a reverse crown shape was given to the transfer roller, the effect of wrinkling the recording material when passing through the transfer portion could not be obtained. .

  SUMMARY OF THE INVENTION An object of the present invention is to provide an image forming apparatus in which a transfer roller is pressed against an inner surface of a transfer belt, and a wrinkle-stretching effect for a recording material passing through a transfer portion can be sufficiently exerted. It is said.

  An image forming apparatus according to the present invention includes an image carrier that carries a toner image, an endless transfer belt that conveys a recording material, and press-contacts the image carrier via the transfer belt to form a toner image on the recording material. The image forming apparatus includes a transfer roller that forms a transfer portion, and a conveyance surface forming roller that stretches the transfer belt to form a conveyance surface of a recording material. The conveying surface forming roller has regions where the transfer belt is stretched at a diameter smaller than that of the central portion in the rotation axis direction at both ends in the rotation axis direction.

  In the image forming apparatus of the present invention, the diameter of both ends in the rotation axis direction of the transport surface forming roller is smaller than the central portion, so that the diameters at both ends and the central portion are equal or the diameters at both ends are larger than the central portion. Compared to the case where it is large, wrinkles generated in the recording material when passing through the transfer portion are reduced. This has been confirmed by experiments (see Table 1). Therefore, in the image forming apparatus in which the transfer roller is pressed against the inner surface of the transfer belt, the effect of stretching the wrinkles with respect to the recording material passing through the transfer portion can be sufficiently exhibited.

2 is an explanatory diagram of a configuration of an image forming apparatus according to Embodiment 1. FIG. It is a perspective view of a secondary transfer belt unit. It is explanatory drawing of a structure of a secondary transfer belt unit. It is explanatory drawing of the crown shape of a conveyance surface formation roller and a tension roller. FIG. 6 is an explanatory diagram of generation of wrinkles of a recording material in a thin paper duplex printing mode. It is a figure explaining the amount of spread. It is a graph which shows the relationship between the amount of spread, the difference in winding length, and the presence or absence of image defect occurrence. It is a figure explaining winding length. It is a graph which shows the relationship between the difference in winding length, and stress. 5 is an explanatory diagram of a configuration of an image forming apparatus according to a second embodiment. It is explanatory drawing of an example of other embodiment.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

<Embodiment 1>
(Image forming device)
FIG. 1 is an explanatory diagram of the configuration of the image forming apparatus. As shown in FIG. 1, the image forming apparatus 100 is a tandem intermediate transfer type full color printer in which image forming portions PY, PM, PC, and PK of a process cartridge are arranged along the upward surface of the intermediate transfer belt 40.

  In the image forming unit PY, a yellow toner image is formed on the photosensitive drum 1Y and transferred to the intermediate transfer belt 40. In the image forming unit PM, a magenta toner image is formed on the photosensitive drum 1M and transferred to the intermediate transfer belt 40. In the image forming units PC and PK, a cyan toner image and a black toner image are formed on the photosensitive drums 1 </ b> C and 1 </ b> K and transferred to the intermediate transfer belt 40.

  The four color toner images transferred to the intermediate transfer belt 40 are conveyed to the secondary transfer portion T2 and secondarily transferred to the recording material P. The recording material P is taken out from the recording material cassette 31, separated one by one by the separation roller 32, and sent to the registration roller 13. The registration roller 13 sends the recording material P to the secondary transfer portion T2 in synchronization with the toner image on the intermediate transfer belt 40.

The secondary transfer belt unit 36 is in contact with the intermediate transfer belt 40 whose inner surface is supported by the secondary transfer inner roller 42 to form a secondary transfer portion T2. By applying a voltage to the secondary transfer roller 10, the toner image on the intermediate transfer belt 40 is secondarily transferred to the recording material P conveyed through the secondary transfer portion T2. Here, each color toner image transferred to the recording material P has a maximum reflection density of about 1.5 to 1.7. The toner loading amount of the toner image at the maximum reflection density is about 0.4 to 0.6 mg / cm ² .

  The recording material P onto which the four-color toner images have been secondarily transferred is conveyed to the pre-fixing conveying device 61 and sent to the fixing device 60, where the fixing device 60 receives heat and pressure to fix the image on the surface. The fixing device 60 melts and fixes the toner image on the recording material P by applying predetermined pressure and heat at a nip formed by the fixing roller 60a provided with the heater 60c and the pressure roller 60b.

(Duplex printing mode)
In the single-sided printing mode, the recording material P that has passed through the fixing device 60 is discharged out of the apparatus as it is through the discharge roller 33. On the other hand, in the double-sided printing mode, the second side (back side) of the recording material once fixed is fed to the secondary transfer portion T2 again so that the image is formed on both sides of the recording material P. In the double-sided printing mode, it is possible to suppress consumption of the recording material by forming an image on both sides of the recording material.

  In the double-sided printing mode, the recording material P that has passed through the fixing device 60 is sent to the reverse conveyance path 34 and is conveyed to the double-sided conveyance path 35 by switching the leading and trailing ends by performing a switchback operation in the reverse conveyance path 34. The double-sided conveyance path 35 joins the recording material P to the registration roller 13 and conveys it again to the secondary transfer portion T2. The recording material P on which the four-color toner image is secondarily transferred to the back surface (second surface) and the image is fixed is discharged to the outside of the machine through the discharge roller 33. In the double-sided printing mode, as will be described later, the recording material is likely to wrinkle during the second transfer of the toner image.

(Image forming part)
The image forming units PY, PM, PC, and PK are substantially the same except that the color of toner used in the developing devices 5Y, 5M, 5C, and 5K is different from yellow, magenta, cyan, and black. In the following, the image forming unit PY will be described, and redundant description regarding the other image forming units PM, PC, and PK will be omitted.

  The image forming unit PY surrounds the photosensitive drum 1Y and includes a charging device 3Y, an exposure device 4Y, a developing device 5Y, a primary transfer roller 6Y, and a drum cleaning device 7Y. The photosensitive drum 1Y has a photosensitive layer formed on the outer peripheral surface of an aluminum cylinder, and rotates in the direction of arrow R1 at a predetermined process speed.

  The charging device 3Y charges the photosensitive drum 1Y to a uniform negative potential. The exposure device 4Y writes an electrostatic image of an image on the surface of the photosensitive drum 1Y by scanning with a rotating mirror a laser beam generated based on an image signal obtained by developing image data on a scanning line. The developing device 5Y transfers the toner to the photosensitive drum 1Y and develops the electrostatic image into a toner image. A developer replenishing section (not shown) replenishes the developing device 5Y with the toner that has been taken out from the developing device 5Y as the image is formed.

  The primary transfer roller 6Y presses the intermediate transfer belt 40 to form a primary transfer portion between the photosensitive drum 1Y and the intermediate transfer belt 40. When a positive DC voltage is applied to the primary transfer roller 6Y, a negative toner image carried on the photosensitive drum 1Y is transferred to the intermediate transfer belt 40. The drum cleaning device 7Y collects residual toner adhered to the surface of the photosensitive drum 1Y by sliding the cleaning blade on the photosensitive drum 1Y.

  As described above, the image forming units PY, PM, PC, and PK form toner images and carry them on the intermediate transfer belt 40 that is an example of an image carrier. The secondary transfer roller 10, which is an example of a transfer roller, is pressed against the intermediate transfer belt 40 via the secondary transfer belt 12, which is an example of a transfer belt, and forms a toner image transfer portion for the recording material.

(Intermediate transfer belt)
The intermediate transfer belt 40 is stretched around the drive roller 43, the tension roller 41, and the secondary transfer inner roller 42, and is driven by the drive roller 43 to rotate in the direction of arrow R2 at 250 to 300 [mm / sec]. The tension roller 41 is urged outward by pressure springs (not shown) at both ends to control the tension of the intermediate transfer belt 40 to be substantially constant. The secondary transfer inner roller 42 supports the inner surface of the intermediate transfer belt 40 that passes through the secondary transfer portion T2. The belt cleaning device 44 rubs the intermediate transfer belt 40 with a cleaning blade and collects transfer residual toner on the surface of the intermediate transfer belt 40.

The intermediate transfer belt 40 is adjusted to have a volume resistivity of 1 × 10 ⁹ to 1 × 10 ¹⁴ [Ω · cm] by containing an appropriate amount of carbon black as an antistatic agent in resins such as polyimide and polycarbonate, or various rubbers. It is. The thickness of the intermediate transfer belt 40 is 0.07 to 0.1 mm.

(Upstream guide)
The secondary transfer upstream upper guide 14 and the secondary transfer upstream lower guide 15 regulate the conveyance path until the recording material P is conveyed from the registration roller 13 to the secondary transfer portion T2.

  The secondary transfer upstream upper guide 14 regulates the behavior of the recording material P approaching the surface of the intermediate transfer belt 40. The secondary transfer upstream upper guide 14 guides the recording material P upstream of the secondary transfer portion T2, and superimposes the recording material on a predetermined position on the surface of the intermediate transfer belt 40.

  The secondary transfer upstream lower guide 15 regulates the behavior of the recording material P moving away from the surface of the intermediate transfer belt 40. The secondary transfer upstream lower guide 15 guides the recording material P upstream of the secondary transfer portion T2, and superimposes the recording material on a predetermined position on the surface of the intermediate transfer belt 40.

(Secondary transfer belt unit)
FIG. 2 is a perspective view of the secondary transfer belt unit. FIG. 3 is an explanatory diagram of the configuration of the secondary transfer belt unit. As shown in FIG. 1, the use of the secondary transfer belt 12 facilitates the separation of the recording material P from the intermediate transfer belt 40 after the transfer of the toner image in the secondary transfer portion T2, thereby stabilizing the recording material P. Then, it can be conveyed to the fixing device 60.

  As shown in FIG. 2, the secondary transfer belt unit 36 includes four tension rollers, that is, the secondary transfer belt 10, the transfer surface forming roller 21, the tension roller 22, and the driving roller 23. 12 is supported around. A transport surface forming roller 21 is disposed downstream of the secondary transfer roller 10 in the rotational direction of the secondary transfer belt 12. A stretching roller 22 is disposed downstream of the conveyance surface forming roller 21. A drive roller 23 is disposed downstream of the tension roller 22. The secondary transfer roller 10 is disposed downstream of the drive roller 23.

As shown in FIG. 3, the secondary transfer belt 12 is an endless belt member having a layer formed of a resin material or a metal material. The secondary transfer belt 12 is a resin material in which a volume resistivity is adjusted to 1 × 10 ⁹ to 1 × 10 ¹⁴ [Ω · cm] by containing an appropriate amount of carbon black as an antistatic agent in a resin such as polyimide or polycarbonate. Is formed. The secondary transfer belt 12 has a single layer structure and a thickness of 0.07 to 0.1 mm. The secondary transfer belt 12 has a Young's modulus value measured by a tensile test method (JIS K 6301) of 100 MPa or more and less than 10 GPa.

(Secondary transfer roller)
As shown in FIG. 3, the secondary transfer roller 10 is formed to have an outer diameter of 24 mm by disposing an elastic layer 10b of an ion conductive foamed rubber (NBR rubber) on the outer periphery of a stainless steel bar 10a. The secondary transfer roller 10 has a surface roughness Rz of the elastic layer 10b of 6.0 to 12.0 [μm]. The resistance value measured by applying 2 kV in a normal temperature and normal humidity environment (N / N: 23 ° C., 50% RH) is 1 × 10 ⁵ to 1 × 10 ⁷ [Ω]. The Asker-C hardness of the elastic layer 10b is about 30-40.

  A secondary transfer power supply 11 having a variable output current is connected to the secondary transfer roller 10. For example, the secondary transfer power supply 11 automatically adjusts the output voltage so that a transfer current of +40 to 60 μA flows. The secondary transfer power source 11 applies a voltage to the secondary transfer roller 10 to form a transfer electric field between the intermediate transfer belt 40 and the secondary transfer belt 12, and the toner image carried on the intermediate transfer belt 40. Is secondarily transferred to the recording material P carried on the secondary transfer belt 12. The recording material P is attracted to the secondary transfer belt 12 by the electrostatic force supplied from the secondary transfer power supply 11 along with the secondary transfer of the toner image.

  The secondary transfer belt 12 rotates in the direction of arrow B to convey the recording material P adsorbed on the surface of the secondary transfer belt 12 along with the secondary transfer of the toner image from the secondary transfer portion T2 to the downstream side. To do.

  On the peripheral surface of the secondary transfer roller 10, a regular crown shape of about 200 to 300 [μm] is formed. The reason for forming the regular crown shape on the secondary transfer roller 10 is to cancel the deflection of the secondary transfer roller 10 and prevent the central portion of the secondary transfer portion T2 in the rotation axis direction from being depressurized. is there.

  Since the secondary transfer belt 12 and the intermediate transfer belt 40 are supported by the secondary transfer inner roller 42, when the secondary transfer roller 10 is brought into pressure contact with the secondary transfer inner roller 42 as shown in FIG. The transfer portion T2 has a flat shape in the rotation axis direction. Even if a crown of about 200 to 300 [μm] is attached to the surface of the secondary transfer roller 10, the secondary transfer roller 10 is pressed against the secondary transfer belt 12 and the intermediate transfer belt 40. It is bent downward by about 200 to 300 [μm].

(Drive roller)
As shown in FIG. 3, the drive roller 23 is driven by the motor M3 to rotate the secondary transfer belt 12 in the direction of the arrow R3. The drive system for the secondary transfer belt 12 is provided independently of the drive system for the intermediate transfer belt 40 so that the speed difference between the secondary transfer belt 12 and the intermediate transfer belt 40 can be adjusted. The drive roller 23 has a thin rubber layer 23b fixed to the peripheral surface of the metal roller 23a to ensure a frictional force with the secondary transfer belt 12, and between the secondary transfer belt 12 and the drive roller 23 during driving. There is no slippage between them.

  The drive roller 23 is formed in a straight shape having an outer diameter of 20 to 24 mm, and rotationally drives the secondary transfer belt 12. By pulling the secondary transfer belt 12 with the straight drive roller 23, the secondary transfer belt 12 can be brought into close contact with the peripheral surfaces of the transport surface forming roller 21 and the stretching roller 22. By making the driving roller 23 have a straight shape, transfer unevenness caused by wrinkles of the secondary transfer belt 12 due to the reverse crown shape provided on the stretching roller 22 in the secondary transfer portion T2 positioned downstream of the driving roller 23, etc. Can be prevented.

  As described above, the drive roller 23, which is an example of the drive roller, drives the secondary transfer belt 12 on the downstream side of the stretching roller 22 in the rotation direction of the secondary transfer belt 12. The drive roller 23 has the same diameter in the entire region where the secondary transfer belt 12 is stretched in the rotation axis direction.

(Conveying surface forming roller)
FIG. 4 is an explanatory view of the crown shape of the conveying surface forming roller and the stretching roller. The conveyance surface forming roller 21 has a regular crown shape on the peripheral surface in order to prevent wrinkles generated in the recording material at the secondary transfer portion T2. The crown amount of the regular crown formed on the conveyance surface forming roller 21 is larger than the crown amount formed on the secondary transfer roller 10.

  As shown in FIG. 1, the transport surface forming roller 21 is a stretching roller for the secondary transfer belt 12 disposed downstream of the secondary transfer roller 10. The recording material P adsorbed on the surface of the secondary transfer belt 12 is separated from the surface of the secondary transfer belt 12 by the curvature on the curved surface of the secondary transfer belt 12 along the conveyance surface forming roller 21, and is conveyed before fixing. It is delivered to the device 61.

As shown in FIG. 4 (a), the conveyance surface forming roller 21 is cut out from a stainless steel round bar, which is an example of a metal material, so that the central portion of the peripheral surface in the rotational axis direction is larger than both ends. It is formed in a crown shape. In the rotation axis direction, the outer diameter of the portion with the largest outer diameter of the conveyance surface forming roller 21 is R1max, and the outer diameter of the portion with the smallest outer diameter is R1min. The difference in outer diameter between the portion with the largest outer diameter and the portion with the smallest outer diameter is defined as a positive crown amount ΔR1.
ΔR1 = R1max−R1min

  The experiment confirmed that the larger the positive crown amount ΔR1, the greater the effect of suppressing the wrinkle of the recording material at the secondary transfer portion T2.

  In the first embodiment, as shown in FIG. 3, the distance L from the secondary transfer portion T2 to the transport surface forming roller 21 is set to 20 to 30 mm, and the positive crown amount ΔR1 is set to 1 to 3 mm. Further, the outer diameter R1min of the portion having the smallest outer diameter of the transport surface forming roller 21 is set to 10 to 16 mm.

  The reason why the wrinkle generated in the recording material P that passes through the secondary transfer portion T2 is reduced as the conveying surface forming roller 21 having a larger positive crown amount ΔR1 is considered as follows.

  In the contour along the rotation axis of the transport surface forming roller 21, the central portion in the rotation axis direction is more convex than both ends. For this reason, the secondary transfer belt 12 on the downstream side of the secondary transfer portion T2 guided by the contour of the transport surface forming roller 21 is the width direction (direction perpendicular to the rotation direction) of the recording material carried. The center part of this is deformed so as to be more convex than both end parts. Along with such deformation of the secondary transfer belt 12, the recording material P that has passed through the secondary transfer portion T2 integrally with the secondary transfer belt 12 is subjected to secondary transfer on the upstream side of the secondary transfer portion T2. A force that lowers the lifting of the recording material P from the belt 12 acts. With such deformation of the secondary transfer belt 12, both ends of the recording material in the width direction (direction perpendicular to the transport direction) are moved toward the center downstream of the secondary transfer portion T2. On the upstream side of the next transfer portion T2, a force in the direction of expanding the rear end side of the recording material outward in the width direction acts. These forces realize the wrinkle-stretching effect at the center and the rear end in the width direction of the recording material.

  Further, when the recording material P is deformed so as to protrude upward downstream of the secondary transfer portion T2, the recording material P protrudes upward downstream of the secondary transfer portion T2 sandwiching the recording material P straight. A force that causes the recording material P to be deformed downward is exerted by the reaction of the bent portion. Then, when a force that causes the recording material P to be convexly deformed downward acts on the upstream side of the secondary transfer portion T2, it causes wrinkles in the secondary transfer portion T2 and causes the upward convex bulge and undulation to rise. Will be reduced. For this reason, the conveyance surface forming roller 21 has a shape that can be deformed so that the central portion in the rotation axis direction of the secondary transfer belt 12 protrudes to the surface side from both ends, so that the secondary transfer portion T2 Wrinkles that occur can be reduced. An image defect due to wrinkling in the secondary transfer portion T2 due to the waviness or lifting of the recording material due to the wrinkling and lifting of the recording material that works efficiently on the upstream side across the secondary transfer portion T2. Can be prevented.

  In addition, the shape which deform | transforms the center part of the rotation surface direction of the conveyance surface formation roller 21 so that the secondary transfer belt 12 may become convex on the surface side rather than an edge part is not restricted to a regular crown shape. The secondary transfer belt 12 from the secondary transfer portion T2 to the conveyance surface forming roller 21 may be replaced with another shape having a contour line along the rotation axis that swells outward from both ends at the center portion in the width direction. it can.

  Further, when forming a regular crown shape on the transport surface forming roller 21, the regular crown shape is not limited to a contour line whose diameter changes in a parabolic function along the rotation axis. The shape of the contour line along the rotation axis may be a hyperbolic shape, a prefectural perpendicular shape, an arc shape, an elliptical shape, or the like.

  As described above, the conveyance surface forming roller 21 which is an example of the conveyance surface forming roller holds the secondary transfer belt 12 at a position where the leading end in the conveyance direction of the recording material conveyed through the secondary transfer portion T2 can reach. The recording material conveyance surface is formed by stretching. The conveyance surface forming roller 21 has regions where the secondary transfer belt 12 is stretched at a diameter smaller than that of the central portion at both ends in the rotation axis direction.

(Tension roller)
The effect that the conveyance surface forming roller 21 suppresses wrinkles of the recording material P is that the secondary transfer belt 12 that conveys the recording material P is tightly attached to the shape of the circumferential surface of the regular crown shape of the conveyance surface forming roller 21. Decreases if not mounted. Since the secondary transfer belt 12 is formed of a resin material and has a hard property, it is not easy to make the secondary transfer belt 12 closely contact with the shape of the circumferential surface of the regular crown shape of the transport surface forming roller 21.

  Therefore, in the first embodiment, the stretching roller 22 having a reverse crown-shaped peripheral surface is provided further downstream of the transport surface forming roller 21. The circumferential surface of the reverse roller shape of the tension roller 22 pulls the secondary transfer belt 12 along the normal crown shape of the conveyance surface forming roller 21 without excess and shortage, so that the normal crown shape of the conveyance surface forming roller 21 is formed. The secondary transfer belt 12 can be brought into close contact with the peripheral surface.

  The central portion of the secondary transfer belt 12 in the width direction of the secondary transfer belt 12 is projected to the front surface side in the secondary transfer belt 12 without generating a gap between both ends of the regular crown shape of the secondary transfer belt 21 and the secondary transfer belt 12. Can be transformed to Even if the secondary transfer belt 12 is formed of a hard resin material, the secondary transfer belt 12 is brought into close contact with both ends of the regular crown shape of the conveyance surface forming roller 21 to form the conveyance surface of the secondary transfer belt 12. The roller 21 can be deformed along the contour of the regular crown shape. Thereby, it is possible to sufficiently exhibit the effect of suppressing the generation of wrinkles when the recording material P on the secondary transfer belt 12 passes through the secondary transfer portion T2.

As shown in FIG. 4B, the stretching roller 22 is cut out from a stainless steel round bar, which is an example of a metal material, to form an inverted crown shape in which both end portions in the rotation axis direction swell more than the center portion. Has been. The outer diameter of the portion with the largest outer diameter of the stretching roller 22 in the rotation axis direction is R2max, and the outer diameter of the portion with the smallest outer diameter is R2min. The difference in outer diameter between the portion with the largest outer diameter and the portion with the smallest outer diameter is defined as the reverse crown amount ΔR2.
ΔR2 = R2max−R2min

  In the first embodiment, the outer diameter R2min of the portion having the smallest outer diameter can be set to 16 to 22 mm, and ΔR2 can be set to about 1 to 3 mm.

However, the tension roller 22 is preferably formed so that the reverse crown amount ΔR2 is within a range of ± 0.2 mm from a predetermined reference value. The reference value ΔR2a is a reverse crown amount by which the transfer belt 12 can be stretched so that the peripheral length of the end portion of the transfer belt 12 is equal to the peripheral length of the center portion. In the present embodiment, the stretching roller 22 is formed so that the difference between the reverse crown amount ΔR2 and the reference value ΔR2a is within ± 0.2 mm.
ΔR2−ΔR2a ≦ | 0.2 | (absolute value)
That is, if the tension roller 22 is formed within the difference between the reverse crown amount ΔR2 and the reference value ΔR2a within ± 0.2 mm, the peripheral length of the end portion of the transfer belt 12 is equal to the peripheral length of the central portion. Thus, the transfer belt 12 is stretched.

  As described above, the stretching roller 22 which is an example of the stretching roller stretches the secondary transfer belt 12 on the downstream side of the conveyance surface forming roller 21 in the rotation direction of the secondary transfer belt 12. The stretching roller 22 has regions for stretching the secondary transfer belt 12 at both ends in the rotation axis direction with a diameter larger than that of the central portion in the rotation axis direction. The diameters of the end portion and the central portion of the stretching roller 22 in the rotational axis direction are set so that the peripheral length of the end portion of the secondary transfer belt 12 is equal to the peripheral length of the central portion.

(Close contact conditions)
As shown in FIG. 2, the central surface and both end portions of the secondary transfer belt 12 by the regular crown-shaped peripheral surface of the transport surface forming roller 21 are disposed on the peripheral surface of the stretching roller 22 downstream of the transport surface forming roller 21. The reverse crown shape is formed so as to cancel the circumferential length difference of the rotation trajectory.

As shown in FIG. 3, the winding angle of the secondary transfer belt 12 with respect to the central portion in the rotational axis direction of the conveying surface forming roller 21 is θ1, and the winding roller 22 of the secondary transfer belt 12 is in the central portion of the secondary transfer belt 12 in the rotational axis direction. The winding angle is θ2. As described above, the forward crown amount of the conveying surface forming roller 21 is ΔR1, and the reverse crown amount of the stretching roller 22 is ΔR2. At this time, in order to keep the secondary transfer belt 12 stretched around the regular crown-shaped peripheral surface of the transport surface forming roller 21, the following relational expression must be satisfied between ΔR1, ΔR2, θ1, and θ2. Is desirable.
ΔR1 × θ1 ≦ ΔR2 × θ2

  That is, when the difference between the maximum outer diameter and the minimum outer diameter of the conveying surface forming roller 21 is ΔR1, the winding angle is θ1, the difference between the maximum outer diameter and the minimum outer diameter of the stretching roller 22 is ΔR2, and the winding angle is θ2. , ΔR1 × θ1 ≦ ΔR2 × θ2 is preferably satisfied. In other words, the amount of deformation of the end portion of the secondary transfer belt 12 from the center by the reverse-crown-shaped stretching roller 22 is increased, and the center of the secondary transfer belt 12 is protruded by the transport surface forming roller 21 having the shape of a normal crown. The amount of deformation is equal to or greater than that. As a result, even when the secondary transfer belt 12 is made of a hard material such as a resin, the secondary transfer belt 12 is moved to the end of the conveyance surface forming roller 21 by the force of the stretching roller 22 pulling the secondary transfer belt 12 in the end direction. It is possible to follow along the entire longitudinal direction. Therefore, it is desirable that the above magnitude relationship is established.

(Comparative experiment)
A secondary transfer belt unit 36 was manufactured by making the peripheral surface shapes of the secondary transfer roller 10, the conveyance surface forming roller 21, the stretching roller 22, and the driving roller 23 shown in FIG. Then, the prototype secondary transfer belt unit 36 was mounted on the image forming apparatus 100, and the wrinkle generation state of the recording material in the double-sided printing mode of thin paper was compared. This is because, as will be described later, in the thin paper double-sided printing mode, wrinkles of the recording material are often generated. Note that the secondary transfer roller 10 used in the experiment is formed in a slight regular crown shape as described above, but is described as straight in a substantial sense.

  As shown in Table 1, in Examples 1 and 2 in which the peripheral surface of the conveying surface forming roller 21 was a regular crown, an effect was obtained in preventing wrinkling of the recording material in the thin paper double-sided printing mode. In particular, in Example 1 in which the circumferential surface of the tension roller 22 was formed in an inverted crown shape, a great effect was obtained regarding prevention of wrinkling of the recording material.

  The comparative example 1 corresponds to the conventional configuration when the secondary transfer belt 12 is not provided, and when the secondary transfer belt 12 is not provided, a great effect can be obtained regarding prevention of wrinkling of the recording material. However, the effect of preventing wrinkles disappeared when the secondary transfer belt 12 was stretched.

  Comparative Example 2 is not effective in preventing wrinkle generation. In Comparative Examples 3, 4, and 5, the wrinkles were remarkable due to the adverse effect.

  Therefore, the effect of preventing the occurrence of wrinkles can be obtained by making the peripheral surface of the conveying surface forming roller 21 into a normal crown shape, and the effect of preventing the occurrence of wrinkles can be enhanced by making the stretching roller 22 into a reverse crown shape.

(Supplementary explanation of Comparative Example 1)
In the image forming apparatus described in Patent Document 3, the recording material after fixing is rapidly cooled in order to reduce waviness generated in the fixing device. However, if a system for rapidly cooling the recording material is provided, a large-scale cooling device is required, which increases manufacturing and operating costs, increases the size of the image forming apparatus, and increases power consumption. Come out.

  In the image forming apparatus disclosed in Patent Document 2, a conveyance speed distribution in which an inverted crown shape is formed on the peripheral surface of a transfer roller that forms a toner image transfer portion, and an end portion in the width direction of the recording material extends outward. Is generated in the secondary transfer portion T2. As a result, the wrinkles of the recording material can be extended outward at the secondary transfer portion or the nip portion of the fixing device.

  Therefore, if the reverse crown shape is formed on the peripheral surface of the secondary transfer roller 10 as in Comparative Example 1 described above, even the recording material P that has been wavy in the fixing device 60 passes through the secondary transfer portion T2. It is expected that no wrinkles will occur. However, as described above, Comparative Example 1 does not have such an effect. The reason is considered as follows.

  When the secondary transfer belt 12 is tightly wound around the surface of the secondary transfer roller 10 in order to stably convey the recording material P, the secondary transfer belt 12 is opposed to the tension to be stretched. The inner roller 42 is deformed flat. For this reason, it is impossible to form a velocity distribution that stretches the wrinkles of the recording material.

  That is, when a secondary transfer belt is stretched around a secondary transfer roller having a reverse crown shape on the peripheral surface, the wrinkle of the paper cannot be sufficiently stretched due to the reverse crown shape of the secondary transfer roller. Due to the waviness generated in the fixing device 60, uneven transfer due to wrinkles easily occurs in the secondary transfer portion T2.

  On the other hand, in the first embodiment, even when the wrinkles of the recording material cannot be sufficiently stretched due to the shape of the peripheral surface of the secondary transfer roller 10 in the secondary transfer portion T2, the wrinkles generated in the secondary transfer portion T2. Can output high-quality deliverables.

(Thin paper duplex printing mode)
FIG. 5 is an explanatory diagram of the occurrence of wrinkles in the recording material in the thin paper duplex printing mode. As shown in FIG. 1, in the double-sided printing mode, a recording material containing a certain amount of water when heat and pressure are applied by the fixing device 60 to fix the first toner image on the recording material P. Moisture is lost from P, and thereafter the recording material P rapidly absorbs moisture from the surroundings. When the moisture content of the recording material P changes suddenly before and after passing through the fixing device 60, the paper fibers of the recording material P may partially expand and contract, and the recording material may be wavy. The change in the moisture content is more noticeable at the edge than at the center of the recording material, so both ends of the recording material are longer than at the center, causing the phenomenon that the edges are wavy and wavy. It tends to occur in the recording material P that has passed 60.

  When the recording material P is conveyed again to the secondary transfer portion T2 in order to transfer the toner image to the back surface of the recording material P where the waviness has occurred, the length of the wavy portion at the end of the recording material P is the length. The wrinkles are changed when passing through the secondary transfer portion T2 in accordance with the center so as to match. When a significant wrinkle is generated at the secondary transfer portion T2, if the recording material is extremely brittle, the recording material itself is folded at the crease. Even if the recording material is not bent, the toner image transfer unevenness occurs between the wrinkled portion and the non-wrinkled portion, so that the quality of the output image is surely impaired. The undulation of the recording material generated in the fixing device 60 becomes worse as the thin paper has a lower rigidity, and the transfer unevenness due to wrinkles in the secondary transfer portion T2 due to the undulation is more likely to occur in the thin paper.

  As shown in FIG. 5A, the recording material P in which undulation is generated has a length in the transport direction at the center portion longer than the length in the transport direction at both end portions in the width direction. As shown in FIG. 5B, when the recording material P in which undulation is generated is stuck on the surface of the flat secondary transfer belt 12, due to the difference in length in the transport direction between the both ends and the center, A central portion in the width direction of the recording material bulges upward so as to be away from the secondary transfer belt 12. When the recording material P is conveyed to the nip of the secondary transfer portion T2 while the central portion in the width direction is swollen, the swollenness of the central portion in the width direction is drawn toward the upstream side, which is the opposite side of the conveying direction B. Go. As shown in FIG. 5C, when the bulge in the central portion in the width direction cannot withstand the nip pressure of the secondary transfer portion T2, the bulge in the central portion in the width direction is crushed by the nip pressure to cause wrinkles. Become.

  As described above, in the present embodiment, even if “waving” occurs, the conveyance surface forming roller 21 is formed in a regular crown shape and the stretching roller 22 is formed so that the recording material P is not wrinkled. It is formed in an inverted crown shape. In this case, as the difference between the winding length of the secondary transfer belt 12 at the center portion in the rotation axis direction of the transport surface forming roller 21 and the winding length of the secondary transfer belt 12 at the end portion is larger as a result of experiments described later, It was confirmed that wrinkles are less likely to occur. Table 3 shows the relationship between the amount of spread obtained from the experiment and the presence or absence of image defects. In Table 3, a case where an image defect occurs is indicated by “x”, and a case where no image defect occurs is indicated by “◯”. The spread amount is the deformation amount of the recording material P when the recording material P is deformed by being pushed outward in the width direction (direction orthogonal to the transport direction) on the upstream side of the secondary transfer portion T2 at the time of transfer. As can be understood from Table 3, no image defect occurs when the spread amount is 3 mm or more.

  The spread amount will be described with reference to FIG. 6 shows the state of the recording material P before passing through the secondary transfer portion T2 (before paper passing), and the right diagram in FIG. 6 shows the recording after passing through the secondary transfer portion T2 (after paper passing). The state of the material P is shown. As the recording material P, as shown in the left diagram of FIG. 6, A3 size paper whose rear end side was cut into a strip shape along the transport direction was used. Here, the number of strips is five, the gap between the strips is 20 mm, and the strip length is 200 mm. When the sheet passes through the secondary transfer portion T2, the rear end side of the sheet spreads outward in the width direction as shown in the right diagram of FIG. Therefore, the width of the paper before and after passing through the secondary transfer portion T2 is measured, respectively, and the trailing edge width G of the paper before passing through the secondary transfer portion T2 and the back of the paper after passing through the secondary transfer portion T2. The difference from the end width H was defined as the spread amount.

  Then, the sheet was passed through the secondary transfer portion T2 while changing the difference in the winding length of the secondary transfer belt 12, and the amount of spread at that time and the occurrence of image defects were examined. FIG. 7 shows the experimental results. FIG. 7 shows the relationship between the difference in winding length of the secondary transfer belt 12 and the amount of spread obtained by experiments. It also shows the relationship between the occurrence of image defects. As shown in FIG. 7, when the spread amount is larger than a predetermined value (here, 3 mm), no image defect occurred. The spread amount becomes larger than a predetermined value (3 mm) when the difference in winding length is larger than 0.8 mm. From this, it can be understood that when the difference in winding length is 0.8 mm or less, an image defect is likely to occur, and when it is greater than 0.8 mm, the image defect is difficult to occur. In other words, it can be said that the wrinkle-stretching effect on the recording material P is easily obtained when the difference in winding length is made larger than 0.8 mm.

  The winding length of the secondary transfer belt 12 will be described with reference to FIG. As shown in FIG. 8, in the present embodiment, the outer diameter of the portion with the largest outer diameter of the transport surface forming roller 21 in the rotation axis direction is R1max, and the outer diameter of the portion with the smallest outer diameter is R1min. The winding angle of the secondary transfer belt 12 with respect to the central portion of the conveyance surface forming roller 21 in the rotation axis direction is θ1, and the winding angle of the secondary transfer belt 12 with respect to the end portion of the conveyance surface forming roller 21 in the rotation axis direction is θ3. And In this case, the winding lengths at the central portion and the end portion are represented by “(R1max / 2) × θ1” and “(R1min / 2) × θ3”, respectively.

As described above, if the difference between the winding lengths is larger than 0.8 mm, the wrinkle stretching effect on the recording material P can be obtained. That is, in order to suppress the generation of wrinkles, R1max, R1min, θ1, and θ3 may satisfy the following relational expression.
0.8 <{(R1max / 2) × θ1− (R1min / 2) × θ3}

The wrinkle-stretching effect on the recording material P is greater as the difference in the winding length is larger. Therefore, it can be considered that the secondary transfer belt 12 is largely wound around the transport surface forming roller 21 to further increase the difference in winding length. However, when the secondary transfer belt 12 is largely wound around the conveyance surface forming roller 21 having a regular crown shape, the rotation of the secondary transfer belt 12 may be hindered or the secondary transfer belt 12 may be broken. Therefore, in consideration of the stress applied to the secondary transfer belt 12, R1max, R1min, θ1, and θ3 must further satisfy the following relational expressions in this embodiment so that a larger wrinkle stretching effect can be obtained.
E / F> 16.3 × {(R1max / 2) × θ1− (R1min / 2) × θ3} −2.4

  Here, E (MPa) is the yield strength of the secondary transfer belt 12, and F (kg) is the tension applied to the secondary transfer belt 12.

This relational expression is obtained by numerical analysis of the experimental result shown in FIG. FIG. 9 shows the relationship between the difference in the winding length of the secondary transfer belt 12 and the maximum stress applied to the secondary transfer belt 12 obtained by experiments. The experimental results shown in FIG. 9 were analyzed to obtain an approximate curve representing the relationship between the maximum stress Em (MPa) and the difference in winding length (mm). An approximate expression representing this approximate curve is shown.
Em = 16.3 × {(R1max / 2) × θ1− (R1min / 2) × θ3} −2.4

It can be seen that the tension F (kg) and the maximum stress Em (MPa) are in a proportional relationship. Therefore, when the above approximate expression is divided by the tension F (kg), the following expression is derived.
Em / F = 16.3 × {(R1max / 2) × θ1− (R1min / 2) × θ3} −2.4

Furthermore, the yield strength E (MPa) of the secondary transfer belt 12 must be greater than the maximum stress Em (MPa). From the above, the above-described relational expression is obtained. This relational expression indicates the upper limit of the difference in winding length of the secondary transfer belt 12.
E / F> 16.3 × {(R1max / 2) × θ1− (R1min / 2) × θ3} −2.4

  In this embodiment, the yield strength E (MPa) of the secondary transfer belt 12 is 100 to 150 (MPa). The tension F (kg) is 1.5 to 4 (kg).

  In the present embodiment, the difference in the winding length is preferably about 1.2 mm, for example. Since this value is larger than 0.8 (mm), the wrinkle stretching effect on the recording material P can be sufficiently obtained. In this case, the stress applied to the secondary transfer belt 12 is smaller than the yield strength of the secondary transfer belt 12. Therefore, the rotation of the secondary transfer belt 12 is not hindered and the secondary transfer belt 12 is not broken.

  The transport surface forming roller 21 also serves as a separation roller that forms a curved surface on the secondary transfer belt 12 for separating the curvature of the recording material P from the secondary transfer belt 12. When the conveyance surface forming roller 21 is formed in a regular crown shape, the thin paper having low rigidity is conveyed while being adsorbed to the secondary transfer belt 12 along the secondary transfer belt 12. However, the thick paper having high rigidity does not follow the secondary transfer belt 12 and is separated from the secondary transfer belt 12, so that the thick paper is stably conveyed by the secondary transfer belt 12 downstream from the conveyance surface forming roller 21. Cannot be transported. Considering that the recording material P is stably transported from the thin paper to the thick paper to the fixing device 60, the transport surface forming roller 21 also serves as a separation roller, and thereafter is transported to the fixing device 60 by the pre-fixing transport device 61. It is desirable.

(Effect of Embodiment 1)
In the first embodiment, the circumferential surface of the conveyance surface forming roller 21 downstream of the secondary transfer portion T2 is formed into a regular crown shape, and the central portion in the width direction of the secondary transfer belt 12 is more convex on the surface side than both ends. Therefore, the recording material is less likely to be wrinkled. Even in a thin paper recording material, even in the double-sided printing mode, the recording material is less likely to wrinkle. By making the peripheral surface of the conveyance surface forming roller 21 into a regular crown shape, the recording material P on the secondary transfer belt 12 is deformed so that the surface side is convex on the downstream side of the secondary transfer portion T2. When a rigid body such as paper is nipped at the nip portion of the secondary transfer portion T2, a force that protrudes downward in the vicinity of the secondary transfer portion T2 acts by reaction. This force reduces wrinkles that are convex on the recording material that causes wrinkles on the upstream side of the secondary transfer portion T2, so that wrinkles that occur in the secondary transfer portion T2 can be suppressed.

  In the first embodiment, since the circumferential surface of the tension roller 22 is formed in an inverted crown shape and the central portion in the width direction of the secondary transfer belt 12 is deformed so as to be concave on the surface side from both ends, secondary transfer is performed. The belt 12 contacts the transport surface forming roller 21 without a gap. Since the circumferential surface of the stretching roller 22 downstream of the conveyance surface forming roller 21 has an inverted crown shape, even if the secondary transfer belt 12 is a hard resin belt, the secondary transfer belt 12 is not connected to the conveyance surface forming roller 21. It is hung around in a state of being properly stretched in the shape of a regular crown on the peripheral surface. In this state, the recording material P downstream of the secondary transfer portion T2 is transported along a shape in which the center is swollen. For this reason, the effect of making the peripheral surface of the conveyance surface forming roller 21 into a regular crown shape is enhanced, and the effect of extending the wrinkles of the recording material P by the conveyance surface forming roller 21 becomes remarkable. Image defects due to wrinkles are also reduced.

  In the first embodiment, the conveyance surface forming roller 21 also serves as a separation roller for separating the curvature of the recording material P from the secondary transfer belt 12, and therefore an independent separation roller is disposed on the downstream side of the conveyance surface forming roller 21. There is no need. For this reason, the number of parts of the secondary transfer belt unit 36 is reduced, and downsizing is achieved.

  From the viewpoint of reducing the number of parts, as shown in Patent Document 3, it is conceivable that the secondary transfer portion T2 to the fixing device 60 are configured by one transfer belt unit. In this configuration, a transport surface forming roller is disposed downstream of the transport surface forming roller 21 and an independent separation roller is disposed downstream of the transport surface forming roller. At this time, even if the peripheral surface of the transport surface forming roller has a regular crown shape, the thin paper having low rigidity is transported while being adsorbed to the secondary transfer belt 12 along the secondary transfer belt 12. However, the thick paper having high rigidity does not follow the secondary transfer belt 12 and is separated from the secondary transfer belt 12, so that the thick paper cannot be stably conveyed downstream from the conveyance surface forming roller.

  Accordingly, when the recording material P is to be stably conveyed from the thin paper to the thick paper and from the secondary transfer portion T2 to the fixing device 60, the conveyance surface forming roller 21 also serves as the separation roller as shown in FIG. It is desirable to arrange at a position close to the roller 10. The space from the conveyance surface forming roller 21 to the fixing device 60 is desirably conveyed by a pre-fixing conveyance device 61 which is another conveyance means.

  According to the first embodiment, even if the recording material P supplied to the secondary transfer portion T2 is once undulated through the fixing device 60, the secondary transfer portion T2 caused by undulations. It is possible to reliably reduce image defects due to wrinkles.

<Embodiment 2>
FIG. 10 is an explanatory diagram of a configuration of the image forming apparatus according to the second embodiment. As shown in FIG. 1, in the first embodiment, the secondary transfer belt 12 is stretched in a trapezoidal shape by four stretch rollers. On the other hand, in the second embodiment, the secondary transfer belt 12 is stretched in a triangle by three stretch rollers. Since the configuration other than the stretch shape of the secondary transfer belt 12 and the number of stretch rollers is the same as that of the first embodiment, the same reference numerals as those in FIG. A duplicate description is omitted.

  As shown in FIG. 10, the secondary transfer belt 12 of the secondary transfer belt unit 36 </ b> B is stretched by the conveyance surface forming roller 21, the stretching roller 22, and the secondary transfer roller 10. A transport surface forming roller 21 that also serves as a separation roller for the secondary transfer belt 12 is disposed downstream of the secondary transfer roller 10. A stretching roller 22 is disposed downstream of the conveyance surface forming roller 21. The secondary transfer roller 10 is disposed downstream of the stretching roller 22.

  In the second embodiment, the driving roller (23) and the motor (M3) for driving the driving roller are not provided as in the first embodiment, and the secondary transfer belt 12 is in contact with the intermediate transfer belt 40 and driven to rotate. To do.

As shown in FIG. 4A, the conveying surface forming roller 21 has a regular crown shape on the peripheral surface. The outer diameter of the conveyance surface forming roller 21 having the largest outer diameter is R1max, the outer diameter of the smallest outer diameter is R1min, and the positive crown amount ΔR1 is defined as follows.
ΔR1 = R1max−R1min

  At this time, in Embodiment 2, R1min is about 10 to 16 mm, and ΔR1 is about 1 to 3 mm.

As shown in FIG. 4B, the tension roller 22 has a reverse crown shape on the peripheral surface. The outer diameter of the portion with the largest outer diameter of the tension roller 22 is R2max, the outer diameter of the portion with the smallest outer diameter is R2min, and the reverse crown amount ΔR2 is defined as follows.
ΔR2 = R2max−R2min

  At this time, in Embodiment 2, R2min is 16 to 22 mm, and ΔR2 is about 1 to 3 mm.

  The transport surface forming roller 21 also serves as a separation roller that forms a curved surface on the secondary transfer belt 12 for separating the curvature of the recording material P from the secondary transfer belt 12. When the circumferential surface of the conveyance surface forming roller 21 has a regular crown shape, a thin paper with low rigidity is conveyed while being adsorbed to the secondary transfer belt 12 along the secondary transfer belt 12. However, the thick paper having high rigidity does not follow the secondary transfer belt 12 and is separated from the secondary transfer belt 12, so that the thick paper is stably conveyed by the secondary transfer belt 12 downstream from the conveyance surface forming roller 21. Cannot be transported. Considering that the recording material P is stably transported from the thin paper to the thick paper to the fixing device 60, the transport surface forming roller 21 also serves as a separation roller, and thereafter is transported to the fixing device 60 by the pre-fixing transport device 61. It is desirable.

  Since the tension roller 22 disposed downstream of the regular crown-shaped conveyance surface forming roller 21 has a reverse crown shape, even if the secondary transfer belt 12 is a hard resin belt, the secondary transfer belt 12 is properly tensioned on the conveyance surface forming roller 21. It is hung in the state where it was done.

  In the second embodiment, the number of parts of the secondary transfer belt unit 36B is smaller than that in the first embodiment, and the manufacturing cost can be reduced. Although it is a simple system in which the downstream of the stretching roller 22 is the secondary transfer roller 10, the generation of wrinkles in the secondary transfer portion T2 can be suppressed as in the first embodiment. Even when the recording material P supplied to the secondary transfer portion T2 is wavy through the fixing device 60, image defects due to wrinkles in the secondary transfer portion T2 caused by the waviness can be reduced. it can. Therefore, two stretching rollers other than the secondary transfer roller 10 may be used to prevent wrinkles caused by undulations in the secondary transfer portion T2.

<Other embodiments>
FIG. 11 is an explanatory diagram of an example of another embodiment. The present invention is not limited to the configuration, control, material, design, and dimensions described in the first and second embodiments. The secondary transfer belt unit 36 shown in FIG. 1 may include four or more stretching rollers excluding the secondary transfer roller 10. The conveyance surface forming roller 21 does not need to serve as a separation roller. The configuration of the first embodiment is not effective only in the second transfer of the toner image in the duplex printing mode.

  As shown in FIG. 11A, the conveying surface forming roller 21 is not limited to one having a diameter that continuously changes in the direction of the rotation axis, or one having a regular crown shape on the peripheral surface. The contour shape of the transport surface forming roller 21 that can deform the secondary transfer belt 12 so that the center portion in the width direction is convex toward the surface side from both end portions is not limited to the regular crown shape. The resin rollers 21b and 21d may be fixed to the end of the stainless steel central shaft 21a, and the resin roller 21c may be fixed to the center.

  As shown in FIG. 11B, the stretching roller 22 is not limited to one having a diameter that continuously changes in the direction of the rotation axis, or one having a reverse crown shape on the peripheral surface. The contour shape of the tension roller 22 that can be deformed so that the central portion in the width direction protrudes to the inner peripheral side from both ends is not limited to the reverse crown shape. The resin rollers 22b and 22d may be fixed to the end portion of the stainless steel central shaft 22a, and the resin roller 22c may be fixed to the center portion.

  The image carrier that carries the toner image and transfers the toner image to the recording material P carried on the secondary transfer belt 12 is not limited to the intermediate transfer belt 40 which is merely an example of the image carrier. A toner image may be transferred from the photosensitive drum to the transfer belt by bringing a photosensitive drum as another example of the image carrier into contact with the transfer belt corresponding to the secondary transfer belt 12.

DESCRIPTION OF SYMBOLS 1 Photosensitive drum 3 Charging device 4 Exposure device 5 Development device 6 Primary transfer roller 7 Drum cleaning device 10 Secondary transfer roller 11 Secondary transfer power source 12 Secondary transfer belt 13 Registration roller 14 Secondary transfer Upper upstream guide 15 Secondary transfer upstream lower guide, 21 Conveying surface forming roller 22 Stretch roller, 23 Drive roller, 40 Intermediate transfer belt 41 Tension roller, 42 Secondary transfer inner roller, 43 Drive roller 44 Belt cleaning device, 60 Fixing 61, Conveyor P before fixing P Recording material, T2 Secondary transfer section

Claims (10)

An image carrier for carrying a toner image;
An endless transfer belt for conveying the recording material;
A transfer roller that presses against the image carrier via the transfer belt to form a toner image transfer portion on a recording material;
A transfer surface forming roller that stretches the transfer belt to form a transfer surface of the recording material,
The image forming apparatus, wherein the conveyance surface forming roller has regions where the transfer belt is stretched at a diameter smaller than a central portion in the rotation axis direction at both ends in the rotation axis direction. A tension roller that stretches the transfer belt on the downstream side of the transport surface forming roller in the rotation direction of the transfer belt;
2. The image forming apparatus according to claim 1, wherein the stretching roller has regions where the transfer belt is stretched at a diameter larger than a central portion in the rotation axis direction at both ends in the rotation axis direction. A drive roller for driving the transfer belt on the downstream side of the stretching roller in the rotation direction of the transfer belt;
The image forming apparatus according to claim 2, wherein the drive roller has the same diameter in the entire region where the transfer belt is stretched in the rotation axis direction.   With respect to the rotation axis direction of the transport surface forming roller, the diameter of the end portion of the stretching roller in the rotation axis direction and the center portion thereof are set such that the peripheral length of the end portion of the transfer belt is equal to the peripheral length of the center portion. The image forming apparatus according to claim 2, wherein a diameter is set. The difference between the maximum outer diameter and the minimum outer diameter of the conveying surface forming roller is ΔR1, the winding angle of the transfer belt with respect to the central portion in the rotation axis direction of the conveying surface forming roller is θ1, and the maximum outer diameter of the stretching roller And the minimum outer diameter difference is ΔR2, and the winding angle of the transfer belt with respect to the central portion in the rotation axis direction of the stretching roller is θ2,
ΔR1 × θ1 ≦ ΔR2 × θ2
The image forming apparatus according to claim 2, wherein the relational expression is satisfied. The maximum outer diameter of the conveying surface forming roller is R1max, the minimum outer diameter is R1min, the winding angle of the transfer belt with respect to the central portion in the rotation axis direction of the conveying surface forming roller is θ1, and the conveying surface forming roller of the transfer belt is When the winding angle with respect to the end in the rotation axis direction is θ3,
0.8 <{(R1max / 2) × θ1− (R1min / 2) × θ3}
The image forming apparatus according to claim 2, wherein the relational expression is satisfied. When the yield strength of the transfer belt is E and the tension applied to the transfer belt is F,
E / F> 16.3 × {(R1max / 2) × θ1− (R1min / 2) × θ3} −2.4
The image forming apparatus according to claim 6, wherein the relational expression is satisfied. The transport surface forming roller has a regular crown-shaped peripheral surface whose diameter continuously decreases from the central portion in the rotation axis direction toward both end portions,
The said tension roller has the surrounding surface of the reverse crown shape from which the diameter becomes large continuously toward the both ends from the center part of a rotating shaft direction, The one of the Claims 2 thru | or 7 characterized by the above-mentioned. Image forming apparatus. The transfer roller has a regular crown-shaped peripheral surface whose outer diameter continuously decreases from the central portion in the rotation axis direction toward both end portions,
The transport surface forming roller is configured such that the difference between the maximum outer diameter and the minimum outer diameter of the transport surface forming roller is larger than the difference between the maximum outer diameter and the minimum outer diameter of the transfer roller.
The image forming apparatus according to claim 7, wherein the image forming apparatus is an image forming apparatus.   The image forming apparatus according to claim 1, wherein the transfer belt includes a layer formed of a resin material or a metal material.

Images