JP6938145B2 - Image forming device - Google Patents

Description

The present invention relates to an image forming apparatus.

Conventionally, an intermediate transfer type image forming apparatus has been known in which a toner image formed on a photosensitive drum is primarily transferred to an intermediate transfer belt, and a toner image formed on the intermediate transfer belt by the primary transfer is secondarily transferred to a recording material. ing. In this image forming apparatus, a transfer voltage is applied to a secondary transfer inner roller that abuts on a secondary transfer outer roller and forms a secondary transfer portion (secondary transfer nip portion) with an intermediate transfer belt sandwiched between them. The next transcription is performed.

In the above-mentioned secondary transfer outer roller, an elastic layer is provided on the peripheral surface of the conductive shaft portion, and a conductive agent such as an ionic conductive agent is dispersed in the elastic layer to impart conductivity. However, in that case, as the transfer voltage is applied for a lapse of time, the ions in the ionic conductive agent are polarized so as to be biased toward either the roller surface side or the shaft portion side, and the electric resistance tends to increase. When the electric resistance increases, even if a transfer voltage having the same voltage value as before the increase is applied, it becomes difficult to pass the same transfer current as before the increase to the secondary transfer unit. Therefore, in order to suppress the increase in electrical resistance due to polarization, for example, a current is supplied to the secondary transfer outer roller from the power supply roller that is in contact with the surface of the secondary transfer outer roller (referred to as external power supply). , A device for transferring a toner image from an intermediate transfer belt to a recording material has been proposed (see Patent Document 1).

Japanese Unexamined Patent Publication No. 2005-316200

The image forming apparatus described in Patent Document 1 described above needs to be added to the secondary transfer unit in a state where the feeding roller is in contact with the secondary transfer outer roller, and the secondary transfer unit is larger than the conventional one. obtain. In order to reduce the size of the device, it is better that the secondary transfer unit is also small. In order to reduce the size of the secondary transfer unit, the length (longitudinal) in the width direction (rotation axis direction, longitudinal direction) of the feeding roller is as small as possible. Width) can be shortened. However, the longitudinal width of the feeding roller is related to the contact range with the secondary transfer outer roller, and depending on the longitudinal width of the feeding roller with respect to the secondary transfer outer roller, it may affect the suppression of the increase in electrical resistance due to the above-mentioned polarization. No. Therefore, in the case of external power supply, a device capable of suppressing an increase in electrical resistance due to polarization and reducing the size of the device, particularly the secondary transfer unit, has been conventionally desired. Such a device has not yet been proposed.

The present invention has been made in view of the above-mentioned problems, and in the case of external power feeding in which a current is supplied to a power feeding roller in contact with the secondary transfer outer roller, suppression of an increase in electrical resistance due to polarization and an apparatus It is an object of the present invention to provide an image forming apparatus capable of achieving both miniaturization and miniaturization.

One aspect of the present invention is a belt member that is movably provided and carries a toner image, a toner image forming unit that forms a toner image on the belt member, a metal shaft, and a metal shaft formed around the metal shaft. A first transfer roller having an elastic layer containing an ionic conductive agent, contacting the outer surface of the belt member, and transferring a toner image formed on the belt member to a recording material. The first transfer roller is provided with a conductive roller that comes into contact with the outer peripheral surface of the elastic layer of the first transfer roller, a metal shaft, and an elastic layer provided around the metal shaft, and sandwiches the belt member. A second transfer roller provided at a position facing the transfer roller, a conductive path for electrically connecting the second transfer roller to the ground potential, and the conductive roller having a polarity opposite to the charging polarity of the toner. A power supply unit for applying a voltage to pass a transfer current to the second transfer roller via the shaft of the first transfer roller is provided, and the conductive roller is provided at a position corresponding to an image forming region. With respect to the width direction intersecting the moving direction of the belt member, which has a large-diameter portion provided and a small-diameter portion having a diameter smaller than that of the large-diameter portion which is arranged at both ends of the conductive roller and is pivotally supported. The width of the large-diameter portion of the conductive roller is equal to or greater than the width of the maximum image forming region and narrower than the width of the elastic layer of the first transfer roller, and intersects the moving direction of the belt member. Regarding the width direction, the width of the elastic layer of the first transfer roller is narrower than the width of the belt member, and the width of the elastic layer of the second transfer roller is wider than the width of the belt member. It is a characteristic image forming apparatus.

According to the present invention, in the case of external power feeding in which a current is supplied by a power feeding roller to a transfer roller forming a transfer portion for transferring a toner image to a recording material, an increase in electrical resistance due to polarization in the transfer roller is suppressed. It is possible to achieve both miniaturization of the device and a simple configuration.

The schematic diagram which shows the structure of the image forming apparatus of this embodiment. The schematic diagram which shows the structure of the secondary transfer unit of 1st Embodiment. The schematic diagram explaining the feeding roller. It is a figure which shows the structure of the experimental apparatus, (a) is a schematic view, (b) is a schematic view. It is a schematic diagram explaining the electric resistance caused by polarization, (a) is the case where the side away from a power source is insulated, and (b) is the case where the side close to a power source is insulated. The schematic which shows the structure of the secondary transfer unit of 2nd Embodiment. The schematic which shows the structure of the secondary transfer unit of 3rd Embodiment. The schematic which shows the structure of the secondary transfer unit of 4th Embodiment. The schematic which shows the structure of the secondary transfer unit of 5th Embodiment. The schematic which shows the structure of the secondary transfer unit of 6th Embodiment.

<First Embodiment>
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. First, the image forming apparatus of this embodiment will be described with reference to FIG. FIG. 1 is a schematic view showing the configuration of an image forming apparatus. The image forming apparatus 100 shown in FIG. 1 is a tandem type intermediate transfer type full-color printer in which yellow, magenta, cyan, and black image forming portions UY, UM, UC, and UK are arranged along an intermediate transfer belt 12.

[Image forming device]
In the image forming unit UY, a yellow toner image is formed on the photosensitive drum 1Y and transferred to the intermediate transfer belt 12 as an image carrier. That is, the intermediate transfer belt 12 is a belt member that supports a movably provided toner image, and in the image forming unit UM, a magenta toner image is formed on the photosensitive drum 1M and transferred to the intermediate transfer belt 12. In the image forming units UC and UK, a cyan toner image and a black toner image are formed on the photosensitive drums 1C and 1K, respectively, and transferred to the intermediate transfer belt 12. The four-color toner image transferred to the intermediate transfer belt 12 is conveyed to the secondary transfer unit 20 (more specifically, the secondary transfer nip portion T2) and transferred to the recording material P (paper, sheet material such as OHP sheet, etc.). Batch secondary transfer.

The image forming units UY, UM, UC, and UK are configured to be substantially the same except that the toner colors used in the developing devices 4Y, 4M, 4C, and 4K are different from those of yellow, magenta, cyan, and black. In the following, the configuration and operation of the image forming unit U will be described by assigning to the constituent members a code in which Y, M, C, and K at the end of the code indicating the distinction between the image forming unit UY, UM, UC, and UK are omitted. ..

The image forming unit U surrounds the photosensitive drum 1 and arranges a charging roller 2, an exposure device 3, a developing device 4, a primary transfer roller 5, and a drum cleaning device 6. The photosensitive drum 1 has a photosensitive layer formed on the peripheral surface of an aluminum cylinder, and rotates at a predetermined process speed in the direction of arrow R1 in the drawing. The length of the photosensitive drum 1 in the width direction (rotational axis direction) is, for example, 357 mm.

When a charging voltage is applied to the charging roller 2 and it comes into contact with the photosensitive drum 1, the photosensitive drum 1 is charged to a uniform negative electrode dark potential. The exposure apparatus 3 generates a laser beam obtained by ON-OFF modulation of scanning line image data obtained by developing separated color images of each color from a laser emitting element, scans the laser beam with a rotating mirror, and charges the surface of the photosensitive drum 1. Write an electrostatic image of the image. The developing device 4 supplies toner to the photosensitive drum 1 to develop an electrostatic image into a toner image. In the photosensitive drum 1, the maximum width in the width direction in which the toner image can be developed, which is determined in advance corresponding to the recording material having the maximum size that can form an image, is called the maximum image forming width.

The primary transfer roller 5 as a toner image forming unit is arranged to face the photosensitive drum 1 with the intermediate transfer belt 12 interposed therebetween, and forms a toner image primary transfer nip portion T1 between the photosensitive drum 1 and the intermediate transfer belt 12. .. In the primary transfer nip portion T1, for example, when a primary transfer voltage is applied to the primary transfer roller 5 by a high-voltage power supply (not shown), the toner image is primarily transferred from the photosensitive drum 1 to the intermediate transfer belt 12. That is, when a primary transfer voltage having a polarity opposite to the charging polarity of the toner is applied to the primary transfer roller 5, the toner image on the photosensitive drum 1 is electrostatically attracted to the intermediate transfer belt 12 to perform transfer.

The drum cleaning device 6 rubs the cleaning blade against the photosensitive drum 1 to collect a small amount of residual primary transfer toner remaining on the photosensitive drum 1 after the primary transfer.

The intermediate transfer belt 12 is supported by being hung on rollers such as a drive roller 22, a tension roller 23, and a secondary transfer inner roller 24, and is driven by the drive roller 22 to rotate (move) in the direction of arrow R2 in the drawing. Further, a force for pushing the intermediate transfer belt 12 from the back surface to the front surface is applied to the tension roller 23 by an elastic member such as a spring (not shown), and the intermediate transfer belt 12 is stretched with a tension of about 3 to 12 kgf. In the present embodiment, when the drive roller 22 is rotationally driven by a motor or the like (not shown), the intermediate transfer belt 12 is rotated in a state of being in contact with the photosensitive drum 1. The intermediate transfer belt 12 is rotationally driven by the drive roller 22 at a constant peripheral speed of, for example, 200 mm / s.

The secondary transfer nip portion T2 as the transfer portion is formed by abutting the secondary transfer outer roller 25 on the intermediate transfer belt 12 stretched from the inner peripheral surface on the secondary transfer inner roller 24 as the opposing roller. This is a toner image transfer nip portion to the recording material P. In the secondary transfer nip portion T2, the toner image is secondarily transferred from the intermediate transfer belt 12 to the recording material P which is sandwiched and conveyed from the intermediate transfer belt 12 to the secondary transfer nip portion T2 as the secondary transfer voltage is applied by the secondary transfer unit 20. NS. The secondary transfer residual toner that remains attached to the intermediate transfer belt 12 after the secondary transfer is removed by the belt cleaning device 11 rubbing the intermediate transfer belt 12.

The recording material P to which the four-color toner image is secondarily transferred by the secondary transfer unit 20 is conveyed to the fixing device 30. In the fixing device 30, the fixing rollers 31 and 32 come into contact with each other to form the fixing nip portion T3, and the toner image is fixed to the recording material P while the recording material P is conveyed by the fixing nip portion T3. In the fixing device 30, the fixing roller 32 is pressed against the fixing roller 31 that is heated from the inside by a lamp heater or the like (not shown) by an urging mechanism (not shown) to form a fixing nip portion T3. The recording material P is heated / pressurized by being sandwiched and conveyed by the fixing nip portion T3, and the toner image is fixed to the recording material P. The recording material P on which the toner image is fixed by the fixing device 30 is discharged to the outside of the machine body.

[Primary transfer roller]
The primary transfer roller 5 is a metal roller formed in a roller shape having a shaft portion using a metal such as SUM or SUS as the material. Alternatively, similarly to the secondary transfer outer roller 25 described later, the roller may have an elastic layer containing a conductive agent on the outer peripheral portion. The primary transfer roller 5 is formed in a straight shape in the direction of the rotation axis, and its diameter (roller diameter) is, for example, about 6 to 10 mm.

[Intermediate transfer belt]
The intermediate transfer belt 12 is a main body (belt) formed in an endless manner by using a resin such as polyimide or polyamide or an alloy thereof, or various rubber materials containing an appropriate amount of an antistatic agent such as carbon black. Part). The main body of the intermediate transfer belt 12 has a surface resistivity of 1.0 × 10 ^{9 to} 5.0 × 10 ¹³ (Ω / □) and a thickness of, for example, 0.04 to 0.5 mm. It is formed to be a degree.

[Secondary transfer unit]
The secondary transfer unit 20 of the first embodiment will be described with reference to FIGS. 2 to 5. As shown in FIG. 2, the secondary transfer unit 20 includes a secondary transfer inner roller 24 and a secondary transfer outer roller 25 arranged so as to face each other with the intermediate transfer belt 12 interposed therebetween, and a feeding roller 26. The secondary transfer inner roller 24, the secondary transfer outer roller 25, and the power supply roller 26 are substantially aligned with each rotation axis in the direction of the rotation axis of the intermediate transfer belt 12 (the width direction intersecting (orthogonal) in the moving direction). They are arranged in parallel. The secondary transfer inner roller 24 is driven to rotate according to the rotation of the intermediate transfer belt 12 which is rotationally driven by the drive roller 22. The secondary transfer outer roller 25 is rotationally driven at a peripheral speed of, for example, 200 mm / s by a motor or the like (not shown). In the secondary transfer unit 20, the secondary transfer nip portion T2 is formed by sandwiching the intermediate transfer belt 12 between the secondary transfer inner roller 24 and the secondary transfer outer roller 25. The power feeding roller 26 is rotatably provided on the outer periphery of the secondary transfer outer roller 25 and is provided in contact with the outer circumference of the secondary transfer outer roller 25 in the width direction, and is driven to rotate according to the rotation of the secondary transfer outer roller 25.

[Roller in secondary transfer]
The secondary transfer inner roller 24 has, for example, a shaft portion (24a) of an aluminum cylinder and a main body portion 24b of an elastic layer made of EPDM rubber or the like arranged on the peripheral surface of the shaft portion 24a. The elastic layer is imparted with conductivity by containing a conductive agent such as a carbon filler, which is an electronically conductive conductive agent. Then, the hardness is set to, for example, 70 ° (Asker C). The hardness of the elastic layer of the secondary transfer inner roller 24 is preferably 50 to 90 ° (Asker C). In other words, the secondary transfer inner roller 24 includes a metal shaft 24a and an elastic layer 24b provided around the metal shaft 24a, and the first transfer roller 24 sandwiches the belt member 12. It can be said that the second transfer roller is provided at a position facing the secondary transfer outer roller 25 and forms a transfer portion.

[Secondary transfer outer roller]
The secondary transfer outer roller 25 (secondary transfer roller) as the transfer roller has an elastic layer containing a conductive agent on the outer peripheral portion. Specifically, for example, a main body of a sponge-like elastic layer 25b made of NBR rubber, EPDM rubber, or the like is arranged on the peripheral surface of a conductive shaft portion (conductive portion) 25a of a stainless steel cylinder. The elastic layer 25b is imparted with conductivity by containing a conductive agent such as a metal complex or an ionic conductive agent, which is an ionic conductive conductive agent. On top of that, the hardness is set to, for example, 30 ° (Asker C), which is lower than the elastic layer of the secondary transfer inner roller 24. The hardness of the elastic layer 25b of the secondary transfer outer roller 25 is preferably 20 to 45 ° (Asker C). The diameters (roller diameters) of the secondary transfer inner roller 24 and the secondary transfer outer roller 25 are formed to be, for example, 20 mm and 24 mm, respectively. That is, the secondary transfer outer roller 25 has a conductive inner portion (a roller portion forming portion of the shaft portion 25a excluding the shaft support portion) and an outer portion containing a conductive agent formed on the outer periphery of the inner portion (the outer portion). It is a transfer roller having an elastic layer 25b) and forming a transfer portion for transferring a toner image formed on an intermediate transfer belt 12 as an image carrier to a recording material. In other words, the secondary transfer outer roller 25 has a metal shaft 25a and an elastic layer 25b containing a conductive agent formed around the metal shaft 25a, and serves as an image carrier. It can also be said to be a transfer roller that comes into contact with the outer surface of the intermediate transfer belt 12 to form a transfer portion that transfers the toner image formed on the image carrier to the recording material. Further, the secondary transfer outer roller 25 has a metal shaft 25a and an elastic layer 25b containing a conductive agent formed around the metal shaft 25a, and comes into contact with the outer surface of the belt member 12 to contact the belt member. It can also be said to be a first transfer roller that forms a transfer portion that transfers the toner image formed on the 12 to the recording material.

[Power supply roller]
The power feeding roller 26 as the power feeding rotating body is a metal roller formed in a roller shape having a shaft portion 26a using a metal such as SUM or SUS as the material. The main body of the power feeding roller 26, excluding the shaft portion 26a, is formed in a straight shape in the width direction (rotational axis direction) and has a diameter (roller diameter) of, for example, about 10 mm. That is, the power feeding roller 26 is arranged at both ends of the large diameter portion 26b provided at a position corresponding to the image forming region and both ends of the power feeding roller 26, and has a smaller diameter than the large diameter portion 26b on which the feeding roller 26 is pivotally supported. It has a small diameter portion 26a and.

[Secondary transfer power supply and external power supply]
As shown in FIG. 2, the secondary transfer unit 20 has a secondary transfer power supply 40 and an external power supply 50. In the present embodiment, as the secondary transfer power supply 40 connected to the secondary transfer inner roller 24, for example, a constant voltage source having an applied voltage of up to −6000 (V) is used as the external power supply 50 connected to the power supply roller 26. A constant current source up to +6500 (V) was used. Further, the secondary transfer unit 20 is electrically connected between the shaft portion 25a of the secondary transfer outer roller 25 and the ground potential, and is transferred from the secondary transfer outer roller 25 (specifically, the shaft portion 25a) to the ground potential. It has a conductive path 60 as a current path through which an electric current flows. The conductive path 60 is a conductive member having conductivity.

The secondary transfer power source 40 as the primary power source is a power source for applying a transfer bias for transferring the toner image from the image carrier to the recording material, and is used with the inner surface of the elastic layer 25b to form the transfer bias. It is configured to allow a transfer current to flow between the outer surfaces. More specifically, the secondary transfer power supply 40 is connected to the secondary transfer inner roller 24, and can apply a voltage having the same polarity as the toner charging polarity (negative electrode property in this embodiment) to the secondary transfer inner roller 24. be. In other words, in the secondary transfer power supply 40 as the power supply unit, the side of the secondary transfer outer roller 25 is positive between the shaft core of the secondary transfer inner roller 24 and the shaft portion 25a of the secondary transfer outer roller 25. Form a potential gradient. When a secondary transfer voltage is applied, the secondary transfer current that forms a secondary transfer electric field in the direction in which the toner image on the intermediate transfer belt 12 is electrostatically attracted to the recording material P (see FIG. 1) is secondary transfer. It flows to the nip portion T2. As the secondary transfer current, for example, a current maintained at 50 μA flows. In response to this, the toner image is transferred from the intermediate transfer belt 12 to the recording material P. At this time, on the secondary transfer nip portion T2 side of the secondary transfer outer roller 25, positive ions in the elastic layer 25b are on the outer peripheral side and negative ions are on the inner circumference in the radial direction of the secondary transfer outer roller 25. It tends to be biased and polarized toward the side (shaft portion 25a side).

Therefore, in the present embodiment, the power feeding roller 26 is brought into contact with the secondary transfer outer roller 25 and arranged to face each other, and a current is supplied from the power supply roller 26 to the secondary transfer outer roller 25 by the external power source 50 as the secondary power source. Yes (external power supply). The power supply roller 26 can supply a constant current having the same magnitude as the secondary transfer current supplied by the secondary transfer power supply 40 in response to the application of a voltage having the opposite polarity to that of the secondary transfer power supply 40 by the external power supply 50. Is. Specifically, in the radial direction of the secondary transfer outer roller 25, the current that cancels the state in which positive ions are biased toward the outer peripheral side and negative ions are biased toward the inner peripheral side (shaft portion 25a side) is the feeding roller. It was made to flow from 26 to the secondary transfer outer roller 25. That is, the external power supply 50 applies a voltage having a polarity opposite to the charging polarity of the toner image (positive electrode property in this embodiment) to the feeding roller 26. In other words, the external power supply 50 forms a potential gradient between the shaft portion 25a of the secondary transfer outer roller 25 and the feeding roller 26 so that the secondary transfer outer roller 25 has a negative electrode.

In such a case, an electric field in the direction of moving the positive ions in the elastic layer 25b to the inner peripheral side and the negative ions to the outer peripheral side is applied between the feeding roller 26 and the secondary transfer outer roller 25. Therefore, on the secondary transfer nip portion T2 side of the secondary transfer outer roller 25, positive ions biased toward the outer peripheral side move to the inner peripheral side, and negative ions biased toward the inner peripheral side move to the outer peripheral side. In this way, in the elastic layer 25b of the secondary transfer outer roller 25, the ions in the ion conductive agent are generated on the outer peripheral side and the inner peripheral side of the secondary transfer outer roller 25 every half rotation of the secondary transfer outer roller 25. Since it moves alternately to the shaft portion 25a side), it is not biased to one side. Therefore, in the present embodiment, the polarization of the ionic conductive agent is less likely to occur, so that an effect that an increase in electrical resistance due to polarization can be suppressed can be obtained. That is, the power feeding roller 26 abuts on the outer peripheral surface of the secondary transfer outer roller 25 as a transfer roller to supply a current to the secondary transfer outer roller 25, and at the time of transfer, between the inner surface and the outer surface of the elastic layer 25b. Supply current to. When the direction from the inner surface to the outer surface of the elastic layer 25b is positive, the direction of the current supplied by the feeding roller 26 and the direction of the transfer current are opposite to each other with respect to the radial direction of the secondary transfer outer roller 25. There is.

As described above, in the case of external power feeding in which the current is supplied to the power feeding roller 26 in contact with the secondary transfer outer roller 25, an increase in electrical resistance due to polarization can be suppressed. However, in the case of external power supply, since the power supply roller 26 is arranged substantially in parallel with the secondary transfer outer roller 25 and brought into contact with the secondary transfer outer roller 25, the secondary transfer unit 20 tends to be larger than in the case of no external power supply. Therefore, in the present embodiment, the length (longitudinal width) of the power feeding roller 26 in the width direction is adjusted in order to avoid increasing the size of the secondary transfer unit 20. This point will be described with reference to FIGS. 3 to 5. The longitudinal width referred to here is the length in the width direction (longitudinal direction) of the main body portion such as the roller portion and the belt portion excluding the shaft portion and the like.

In the present embodiment, the power feeding roller 26 is formed to have a longitudinal width equal to or larger than the maximum image forming width of the intermediate transfer belt 12 and shorter than the secondary transfer outer roller 25. As shown in FIG. 3, the secondary transfer inner roller 24, the secondary transfer outer roller 25, and the power feeding roller 26 are arranged so that the rotation axes of the rollers 24, 25, and 26 face the same direction and are substantially parallel to each other. Further, these rollers 24, 25, 26 and the intermediate transfer belt 12 are arranged so as to coincide with each other at the center position N in the width direction. Then, for example, the secondary transfer inner roller 24 is formed to have a longitudinal width of 352 mm, the secondary transfer outer roller 25 is formed to have a longitudinal width of 330 mm, and the feeding roller 26 is formed to have a longitudinal width of 326 mm. On the other hand, the intermediate transfer belt 12 is formed to have a longitudinal width of, for example, 344 mm.

The intermediate transfer belt 12 has a region (a region between the two straight lines M in FIG. 3) corresponding to the maximum image formation width H of the photosensitive drum 1, and the toner image transferred from the photosensitive drum 1 can be supported in the region. .. The maximum image formation width H is shorter than the longitudinal widths of the secondary transfer inner roller 24, the secondary transfer outer roller 25, and the power feeding roller 26, and is secured to be, for example, 305 mm width with reference to the central position N of the intermediate transfer belt 12. .. In this way, the feeding roller 26 is formed to have a longitudinal width (326 mm) shorter than the longitudinal width (330 mm) of the secondary transfer outer roller 25 at the maximum image forming width H (305 mm) or more. In other words, in the power feeding roller 26, the axial length of the contact region where the power feeding roller 26 comes into contact with the secondary transfer outer roller 25 is larger than the axial length of the secondary transfer outer roller 25. It can be said that it is configured to be shorter.

In the present embodiment, the secondary transfer outer roller 25 has the end portion (position of the straight line M in FIG. 3) of the region where both end portions 251 correspond to the maximum image formation width H in the width direction and the end portion 121 of the intermediate transfer belt 12. It is arranged so that it is between. In other words, in the secondary transfer outer roller 25, the first end portion 251X on one side in the width direction has the end portion M1 on one side in the width direction of the region corresponding to the maximum image formation width H and the width direction of the intermediate transfer belt 12. The second end 251Y on the other side in the width direction is located between the side end 121X and the width of the other end M2 in the width direction of the region corresponding to the maximum image formation width H and the intermediate transfer belt 12. It is arranged so as to be located between the end portion 121Y on the other side of the direction. On the other hand, in the above-mentioned longitudinal width feeding roller 26, both ends correspond to the maximum image forming width H in the width direction at the end (position of the straight line M in FIG. 3) and the end of the secondary transfer outer roller 25. It is arranged so as to be between the portion 251 and the portion 251 (range X in FIG. 3). That is, the first end portion 261X of the power feeding roller 26 on one side in the width direction is the end portion M1 on one side in the width direction of the region corresponding to the maximum image formation width H and the end on one side in the width direction of the secondary transfer outer roller 25. The second end portion 261Y of the power feeding roller 26 on the other side in the width direction, which is located between the portion 25X and the portion 25X (range X1 in FIG. 3), is the end portion on the other side in the width direction of the region corresponding to the maximum image formation width H. It is located between M2 and the end 25Y on the other side in the width direction of the secondary transfer outer roller 25 (range X2 in FIG. 3). For example, if the longitudinal width of the secondary transfer outer roller 25 is 330 mm and the longitudinal width of the feed roller 26 is 326 mm, both ends 261 of the feed roller 26 are centered by 2 mm from the end 251 of the secondary transfer outer roller 25. It is located on the side closer to N.

In the case of the present embodiment, since the longitudinal width of the feeding roller 26 is shorter than the longitudinal width of the secondary transfer outer roller 25, the non-contact region 252 that does not abut the secondary transfer outer roller 25 with the feeding roller 26 (FIG. 3). See the shaded area in). If the non-contact region 252 occurs in the secondary transfer outer roller 25 and the effect of suppressing the increase in electrical resistance due to polarization is reduced, the longitudinal width of the power feeding roller 26 may be adjusted to that of the secondary transfer outer roller 25. It is better not to make it shorter than the longitudinal width. That is, the longitudinal width of the feeding roller 26 is related to the contact range with the secondary transfer outer roller 25, and depending on the longitudinal width of the feeding roller 26 with respect to the secondary transfer outer roller 25, the increase in electrical resistance due to the above-mentioned polarization can be suppressed. It can have an impact.

Therefore, the inventors conducted the experiment described below in order to verify the suppression of the increase in electrical resistance when the non-contact region 252 occurs in the secondary transfer outer roller 25. The experiment will be described with reference to FIGS. 4 (a) to 5 (b). FIG. 4 shows the experimental device.

As shown in FIG. 4A, in the experimental apparatus, the metal roller 91 (corresponding to the secondary transfer inner roller) and the metal roller 92 (corresponding to the feeding roller) having a diameter of 30 mm are combined with a sponge roller 93 (corresponding to a feeding roller) having a diameter of 24 mm. It is rotatably contacted with the next transfer outer roller). The sponge roller 93 is provided with an elastic layer having a thickness of 4 mm on the peripheral surface of a conductive shaft portion having a diameter of 16 mm, and an ionic conductive agent is dispersed in the elastic layer to impart conductivity. The metal rollers 91 and 92 and the sponge roller 93 are arranged so that their rotation axes are aligned in the same direction and substantially parallel to each other. However, the metal roller 91 and the metal roller 92 connect a straight line connecting the rotation center T of the sponge roller 93 and the rotation center S of the metal roller 91 with the rotation center T of the sponge roller 93 and the rotation center W of the metal roller 92. It is arranged so that the angle formed by the straight line is 90 degrees. A negative voltage of 2 kV is applied to the metal roller 91 by a power source, and a positive voltage of 2 kV is applied to the metal roller 92 by a power source. The shaft of the sponge roller 93 is connected to the ground potential.

Further, as shown in FIG. 4B, the longitudinal widths of the metal roller 91, the metal roller 92, and the sponge roller 93 are all formed to have the same length as 330 mm. The metal roller 92 is provided with an insulating region 921 on which an insulating tape having a width of 50 mm is attached over one circumference at the end near the power supply (left side in the drawing), and the sponge roller 93 and the metal roller 92 are provided in the insulating region 921. No current flows between them. In this state, while rotating each of the rollers 91, 92, and 93 at a rotation speed of 300 mm / s, a voltage was applied to the metal roller 91 and the metal roller 92 for about 50 hours. By doing so, in the sponge roller 93, except for the insulating region 921, the ions in the ion conductive agent can move alternately to the outer peripheral side and the inner peripheral side in the elastic layer every half rotation of the sponge roller 93.

After the completion of energization for about 50 hours, as shown in FIG. 5A, the metal roller 91 was removed, and instead of the sponge roller 93, a contact portion 253 and a contact portion 254 made of a conductive elastic layer were provided. A two-point contact type sponge roller 93A was attached. The longitudinal widths of the contact portion 253 and the contact portion 254 were set to be the same width as the insulating region 921 of the metal roller 92, that is, a width of 50 mm. Then, an insulating tape having a width extending from the end portion on the side not connected to the power supply (right side in the drawing) to the central position N was attached to the metal roller 92 over one circumference to provide an insulating region 922. That is, in the insulating region 922, no current flows between the sponge roller 93A (specifically, the contact portion 254) and the metal roller 92. In this state, a positive voltage of 2 kV was applied to the metal roller 92, and the current flowing between the metal roller 92 and the sponge roller 93A was measured. Voltage obtained in this case - when determining the electrical resistance of the sponge roller 93A from current characteristic was 7.6 × 10 ⁸ Ω.

Subsequently, while peeling off the insulating tape forming the insulating region 922 from the metal roller 92, as shown in FIG. 5 (b), from the end of the side connected to the power supply (left side of the drawing) to the center position N. An insulating tape having a width over a wide range was attached over one circumference to provide an insulating region 923. That is, in the insulating region 923, no current flows between the sponge roller 93A (specifically, the contact portion 253) and the metal roller 92. In this state, a positive voltage of 2 kV was applied to the metal roller 92, and the current flowing between the metal roller 92 and the sponge roller 93A was measured. Voltage obtained in this case - when determining the electrical resistance of the sponge roller 93A from current characteristic was 2.9 × 10 ⁸ Ω.

Before the start of energization for about 50 hours, was examined the electric resistance value of the sponge roller 93 shown in FIG. 4 (b), it was 4.0 × 10 ⁷ Ω. If so, the electrical resistance of the contact portion 254 and the contact portion 253 formed on the 50mm width should be about 2.6 × 10 ⁸ Ω. However, as shown in FIG. 5A, when the current does not flow between the sponge roller 93A and the metal roller 92 in the insulating region 922, the electric resistance value is about 2.9 times as described above. It was. This is because, in the case of FIG. 4B described above, since no current flows in the insulating region 921, the increase in electrical resistance due to polarization is not suppressed in the range of 50 mm width corresponding to the contact portion 253. It represents that. On the other hand, as shown in FIG. 5B, when the current does not flow between the sponge roller 93A and the metal roller 92 in the insulating region 923, the electric resistance value is about 1.1 times as described above. It was. This means that in the case of FIG. 4B described above, the increase in electrical resistance due to polarization is suppressed by the current flowing in the range other than the insulating region 921.

From the above experiment, it was confirmed that the non-contact region 252 of the secondary transfer outer roller 25 shown in FIG. 3 may have less current flow than the contact region 255 in contact with the feeding roller 26. This is because the non-contact region 252 is a region that is not in contact with the feeding roller 26, and the non-contact region 252 tends to increase the electrical resistance due to polarization.

It is considered that the energization between the secondary transfer outer roller 25 and the power feeding roller 26 is mainly performed in the contact region 255 of the secondary transfer outer roller 25. In the case of the present embodiment, even if the longitudinal width of the feeding roller 26 is shorter than the longitudinal width of the secondary transfer outer roller 25, the contact region 255 is secured with a width larger than the maximum image forming width H. Therefore, in the region corresponding to at least the maximum image formation width H of the secondary transfer outer roller 25, an increase in electrical resistance due to polarization can be suppressed, so that a sufficient transfer current is applied to the secondary transfer nip portion T2 during image formation. It can be flowed, and image defects are unlikely to occur.

As described above, the image forming apparatus 100 of the present embodiment employs an external power supply that supplies a current to the power supply roller 26 that is in contact with the secondary transfer outer roller 25. In this case, the longitudinal width of the feeding roller 26 is formed to be equal to or larger than the maximum image forming width of the intermediate transfer belt 12 and shorter than the secondary transfer outer roller 25. That is, in the width direction, the width of the feeding roller 26 is equal to or larger than the maximum image forming width H and shorter than the width of the secondary transfer outer roller 25 as the transfer roller. As a result, the secondary transfer unit 20 can be miniaturized. Further, the power feeding roller 26 having such a longitudinal width is arranged so that both ends thereof are located between the end portion of the region corresponding to the maximum image forming width and the end portion of the secondary transfer outer roller 25 in the width direction. As a result, even if the longitudinal width of the feeding roller 26 is made shorter than the longitudinal width of the secondary transfer outer roller 25, an increase in electrical resistance due to polarization can be suppressed. As described above, according to the present embodiment, in the case of external power supply, it is possible to achieve both suppression of an increase in electrical resistance due to polarization and miniaturization of the apparatus with a simple configuration. The width of the transfer roller and the width of the power feeding roller described above do not include the length of the shaft portion of each roller, but are the length of the roller portion. That is, the width of each roller is a portion excluding the axially supported small diameter portions at both ends of each roller, and is the width of the large diameter portion provided so as to correspond to the image forming region.

Further, when the longitudinal width of the feeding roller 26 is shorter than the longitudinal width of the secondary transfer outer roller 25 as in the present embodiment, the leakage current flowing from the feeding roller 26 to the ground potential can be reduced. The leak current is a current that flows from the feeding roller 26 to the ground potential through the surfaces of the end portion 261 and the end portion 251 without passing through the shaft portion 25a of the secondary transfer outer roller 25. The higher the voltage applied to the feeding roller 26, the easier it is for the leak current to flow. Therefore, if the leak current can be reduced, the upper limit of the voltage applied to the feeding roller 26 can be increased. By doing so, a high voltage can be applied to the feeding roller 26 according to the voltage value applied to the secondary transfer inner roller 24, so that a higher effect can be obtained with respect to suppressing an increase in electrical resistance due to polarization. can.

Specifically, when the longitudinal width of the feeding roller 26 is 326 mm with respect to the longitudinal width (330 mm) of the secondary transfer outer roller 25, and the elastic layer 25b of the secondary transfer outer roller 25 is 6 mm thick, the secondary is secondary. The creepage distance between the shaft portion 25a of the transfer outer roller 25 and the power feeding roller 26 is 8 mm. Generally, the creepage distance that insulation can maintain is about 1 mm for a potential difference of 1 kV. Therefore, if a creepage distance of about 8 mm can be secured, it is possible to prevent a leak current flowing between the secondary transfer outer roller 25 and the power feeding roller 26. can. The creepage distance referred to here is the shortest path of the current supplied from the feeding roller 26 to the ground potential along the surface between the conductive portions of the secondary transfer outer roller 25 and the feeding roller 26. ..

<Second Embodiment>
The secondary transfer unit of the second embodiment will be described with reference to FIG. The secondary transfer unit 20A of the second embodiment is different from the secondary transfer unit 20 of the first embodiment in that a Zener diode 61 as an electric member is interposed in the conductive path 60 (see FIG. 2). Other configurations are the same. The same components are designated by the same reference numerals as those in the first embodiment, and the description thereof will be omitted.

In the case of the configuration shown in FIG. 6, since the breakdown voltage of the Zener diode 61 is a value sufficiently smaller (for example, 50 V) than the applied voltage of the secondary transfer power supply 40 and the external power supply 50, the current applied by the power feeding roller 26 Most of them flow to the ground potential via the Zener diode 61. That is, the Zener diode 61 causes the current flowing through the shaft portion 25a to flow to the ground potential when the potential difference between the potential of the shaft portion 25a and the ground potential is equal to or greater than a predetermined value. Therefore, using Zener diodes 61 having different yield voltages, it is possible to adjust the potential of the shaft portion 25a of the secondary transfer outer roller 25 so that the current supplied by the feeding roller 26 flows to the ground potential. According to this, the amount of current flowing from the secondary transfer inner roller 24 to the secondary transfer outer roller 25 and the amount of current flowing from the power supply roller 26 to the secondary transfer outer roller 25 can be adjusted to be substantially the same, which is caused by polarization. It is possible to suppress an increase in electrical resistance. Even in this case, as described above, the feeding roller 26 having a longitudinal width shorter than that of the secondary transfer outer roller 25 has the end of the region where both ends correspond to the maximum image formation width in the width direction and the secondary transfer outer roller 25. It is placed so that it is between the edges of the. As a result, the same effect as that of the first embodiment described above can be obtained.

<Third Embodiment>
The secondary transfer unit of the third embodiment will be described with reference to FIG. The secondary transfer unit 20B of the third embodiment is different from the secondary transfer unit 20 of the first embodiment (see FIG. 2) in that a varistor 62 as an electric member is interposed in the conductive path 60. The composition of is the same. The same components are designated by the same reference numerals as those in the first embodiment, and the description thereof will be omitted.

As shown in FIG. 7, when the varistor 62 is used, it is the same as the Zener diode 61 shown in the second embodiment. That is, if the varistor voltage of the varistor 62 is a value sufficiently smaller than the applied voltages of the secondary transfer power supply 40 and the external power supply 50 (for example, 50V), most of the current supplied by the feeding roller 26 flows to the ground potential. .. According to this, the amount of current flowing from the secondary transfer inner roller 24 to the secondary transfer outer roller 25 and the amount of current flowing from the power supply roller 26 to the secondary transfer outer roller 25 can be adjusted to be substantially the same, which is caused by polarization. It is possible to suppress an increase in electrical resistance. Even in this case, as described above, the feeding roller 26 having a longitudinal width shorter than that of the secondary transfer outer roller 25 has the end of the region where both ends correspond to the maximum image formation width in the width direction and the secondary transfer outer roller 25. It is placed so that it is between the edges of the. As a result, the same effect as that of the first embodiment described above can be obtained.

<Fourth Embodiment>
The secondary transfer unit of the fourth embodiment will be described with reference to FIG. The secondary transfer unit 20C of the fourth embodiment is different from the secondary transfer unit 20 of the first embodiment in that a low-voltage power supply 63 as a third power source is arranged in the conductive path 60 (see FIG. 2). , Other configurations are the same. The same components are designated by the same reference numerals as those in the first embodiment, and the description thereof will be omitted.

As shown in FIG. 8, the case where the low-voltage power supply 63 is arranged in the conductive path 60 is the same as the case where the Zener diode 61 shown in the second embodiment is arranged. The low-voltage power supply 63 applies a voltage having an absolute value smaller than each voltage applied by the secondary transfer power supply 40 and the external power supply 50. If the voltage applied by the low-voltage power supply 63 is sufficiently smaller than the voltage applied by the secondary transfer power supply 40 and the external power supply 50 (for example, 20 V), most of the current supplied by the power supply roller 26 flows to the ground potential. .. That is, by changing the voltage applied by the low voltage power supply 63, it is possible to adjust the potential of the shaft portion 25a of the secondary transfer outer roller 25 so that the current supplied by the power feeding roller 26 flows to the ground potential. .. According to this, the amount of current flowing from the secondary transfer inner roller 24 to the secondary transfer outer roller 25 and the amount of current flowing from the power supply roller 26 to the secondary transfer outer roller 25 can be adjusted to be substantially the same, which is caused by polarization. It is possible to suppress an increase in electrical resistance. Even in this case, as described above, the feeding roller 26 having a longitudinal width shorter than that of the secondary transfer outer roller 25 has the end of the region where both ends correspond to the maximum image formation width in the width direction and the secondary transfer outer roller 25. It is placed so that it is between the edges of the. As a result, the same effect as that of the first embodiment described above can be obtained.

<Fifth Embodiment>
The secondary transfer unit of the fifth embodiment will be described with reference to FIG. Compared to the secondary transfer unit 20 of the first embodiment, the secondary transfer unit 20D of the fifth embodiment is electrically floating because the secondary transfer outer roller 25 is not connected to the ground potential. The difference is that it is said to be. Further, the secondary transfer power supply 40 is not connected to the secondary transfer inner roller 24, but instead is connected to the ground potential by a conductive path 60. Further, the external power source is different in that a constant voltage source (secondary transfer power source 70) is used instead of a constant current source. Other configurations are the same. The same components are designated by the same reference numerals as those in the first embodiment, and the description thereof will be omitted. In this embodiment, a secondary transfer power source 70 having an applied voltage of up to +6000 (V) was used.

In the case of the configuration shown in FIG. 9, a current flows from the feeding roller 26 to the secondary transfer outer roller 25 as the voltage is applied by the secondary transfer power source 70. The current supplied from the power feeding roller 26 flows to the shaft portion 25a of the secondary transfer outer roller 25, and also flows from the shaft portion 25a to the secondary transfer inner roller 24. In this case, the electric field in the direction in which the toner image on the intermediate transfer belt 12 is electrostatically attracted to the recording material P (see FIG. 1) by the current flowing from the shaft portion 25a to the secondary transfer inner roller 24 is the secondary transfer nip. It is formed in the portion T2. At this time, on the secondary transfer nip portion T2 side of the secondary transfer outer roller 25, positive ions in the elastic layer 25b are on the outer peripheral side and negative ions are on the inner circumference in the radial direction of the secondary transfer outer roller 25. It tends to be biased toward the side (shaft 25a side). However, on the side where the power feeding roller 26 and the secondary transfer outer roller 25 come into contact with each other, an electric field in the direction of moving the positive ions in the elastic layer 25b to the inner peripheral side and the negative ions to the outer peripheral side is the power feeding roller 26. It is applied between the secondary transfer outer roller 25 (specifically, the contact area). Therefore, even if polarization occurs on the secondary transfer nip portion T2 side, positive ions biased toward the outer peripheral side on the secondary transfer nip portion T2 side are on the inner peripheral side, and negative ions biased toward the inner peripheral side are on the outer circumference. Move to the side. In this way, the ions in the ion conductive agent in the elastic layer 25b alternately move to the outer peripheral side and the inner peripheral side of the secondary transfer outer roller 25 every half rotation of the secondary transfer outer roller 25 to one side. Since there is no bias, it is possible to suppress an increase in electrical resistance due to polarization. Even in this case, as described above, the feeding roller 26 having a longitudinal width shorter than that of the secondary transfer outer roller 25 has the end of the region where both ends correspond to the maximum image formation width in the width direction and the secondary transfer outer roller 25. It is placed so that it is between the edges of the. As a result, the same effect as that of the first embodiment described above can be obtained.

<Sixth Embodiment>
The secondary transfer unit of the sixth embodiment will be described with reference to FIG. The secondary transfer unit 20E of the sixth embodiment has a width of the secondary transfer outer roller 25 shorter than the width of the feeding roller 26 as compared with the secondary transfer unit 20 of the first embodiment (see FIG. 3). Is different. Further, in this embodiment, the width of the power feeding roller 26 is shorter than that of the secondary transfer inner roller 24 and the intermediate transfer belt 12.

By adopting such a configuration, it is possible to alleviate an increase in energization resistance over the entire length of the secondary transfer outer roller 25 while preventing the main body size of the power supply roller 26 from increasing in the longitudinal direction. Become. In the present embodiment, the width of the power feeding roller 26 is shorter than that of either the secondary transfer inner roller 24 or the intermediate transfer belt 12, but if it can be made shorter than either one, the size of the apparatus main body can be increased. Can be suppressed.

Further, according to the present embodiment, the width of the secondary transfer outer roller 25 is shorter than the width of the power feeding roller 26. Therefore, the following effects can also be obtained. That is, when the width of the secondary transfer outer roller 25 is longer than the width of the power feeding roller 26, the edges of both ends of the power feeding roller 26 are pressed against the outer surface of the secondary transfer outer roller 25. When the edges of both ends of the power feeding roller 26 are pressed against the outer surface of the secondary transfer outer roller 25, the portion of the secondary transfer outer roller 25 to which the edges of both ends of the power feeding roller 26 are in contact is locally worn. Will end up. Then, the scraped portion of the secondary transfer outer roller 25 generated by wear may be mixed in the transfer portion or the like and cause transfer failure. However, according to the configuration of this embodiment, since the width of the secondary transfer outer roller 25 is shorter than the width of the power supply roller 26, the edges at both ends of the power supply roller 26 are arranged outside the secondary transfer outer roller 25. NS. Therefore, local wear of the secondary transfer outer roller 25 described above can be suppressed.

Here, a specific length relationship of the longitudinal width of each member related to the secondary transfer portion will be described. In this embodiment, the intermediate transfer belt 12 is 351 mm, the secondary transfer inner roller 24 is 331 mm, the secondary transfer outer roller 25 is 310 mm, the power feeding roller 26 is 310.6 mm, the maximum size paper width is 320 mm, and the maximum image-forming width is It is 305 mm. Further, in the present embodiment, the edge portions at both ends of the power feeding roller 26 have tapered portions whose diameters decrease toward the outside of the thrust width. This is provided to remove burrs and the like generated at both ends (edge portions) of the large diameter portion of the power feeding roller 26. Excluding the tapered portion, the thrust width of the power feeding roller 26 is 310.2 mm. In addition, it is arranged so that the center position in the longitudinal direction, the maximum paper passing width, and the center position of the maximum image width of each member related to the secondary transfer portion are all aligned, and these positional relationships are as shown in FIG. The longitudinal end is located outside the longitudinal end of the secondary transfer outer roller 25.

<Other Embodiments>
It should be noted that the power feeding roller 26 is located between the end of the region corresponding to the maximum paper passing width (that is, the maximum size width of the recording material) O and the end of the secondary transfer outer roller 25 in the width direction. May be placed in. As shown in FIG. 3, the maximum paper passing width O is secured with a width longer than the maximum image forming width H, for example, 320 mm width, with reference to the central position N of the intermediate transfer belt 12. The maximum paper passing width O is the width of the passing region through which the maximum size recording material P capable of secondary transfer of the toner image from the intermediate transfer belt 12 passes through the secondary transfer nip portion T2. Further, in this case, it is preferable that the power feeding roller 26 has a longitudinal width substantially the same as the longitudinal width of the recording material having the maximum size. According to this, it becomes possible to reduce the size of the apparatus as much as possible while securing a margin within a range in which an increase in electrical resistance due to polarization can be surely suppressed.

The secondary transfer inner roller 24, the secondary transfer outer roller 25, the power feeding roller 26, and the intermediate transfer belt 12 are shown to be arranged so that the center positions N coincide with each other, but the present invention is not limited to these. It does not have to match at the center position N.

In each of the above-described embodiments, a constant voltage source is used as the secondary transfer power supply 40, and a constant current source is used as the external power supply 50, but the present invention is not limited to this. For example, a constant current source may be used as the secondary transfer power supply 40, and a constant voltage source may be used as the external power supply 50. Alternatively, both the secondary transfer power source 40 and the external power source 50 may be a constant voltage source or a constant current source. In any combination, it is possible to suppress the increase in electrical resistance due to the above-mentioned polarization.

In each of the above-described embodiments, the intermediate transfer method of transferring the toner image formed on the photosensitive drum to the intermediate transfer belt has been described, but the direct transfer method of directly transferring the toner image formed on the photosensitive drum to the recording material is also available. Applicable. Further, the above-mentioned invention is applicable not only to a printer but also to other image forming apparatus such as a copying machine, a facsimile or a multifunction device. Further, each embodiment may be appropriately combined with other embodiments without departing from the idea of the present invention.

1Y to 1K ... Photosensitive drum, 5 ... Toner image forming unit (primary transfer roller), 12 ... Belt member (intermediate transfer belt), 20 (20A to 20D) ... Toner image forming unit (secondary transfer unit), 24 ... Opposing Roller (secondary transfer inner roller), 24a ... metal shaft (shaft), 24b ... elastic layer, 25 ... transfer roller (secondary transfer roller, secondary transfer outer roller), 25a ... metal shaft (shaft), 25b ... Elastic layer, 26 ... Power supply roller, 26a ... Small diameter part (shaft part), 26b ... Large diameter part, 40 ... First power supply (secondary transfer power supply), 50 ... Second power supply (external power supply), 60 ... Conductive path , 61 ... Electric member (Zener diode), 62 ... Electric member (varistor), 63 ... Third power supply (low voltage power supply), 70 ... Secondary transfer power supply, 100 ... Image forming device, P ... Recording material, T2 ... Transfer unit (Secondary transfer nip part)

Claims (4)

A belt member that is movable and supports a toner image,
A toner image forming unit that forms a toner image on the belt member,
A toner having a metal shaft and an elastic layer formed around the metal shaft and containing an ionic conductive agent, which comes into contact with the outer surface of the belt member and is formed on the belt member. The first transfer roller that transfers the image to the recording material,
A conductive roller that comes into contact with the outer peripheral surface of the elastic layer of the first transfer roller,
A second transfer roller provided with a metal shaft and an elastic layer provided around the metal shaft, and provided at a position facing the first transfer roller with the belt member interposed therebetween.
A conductive path that electrically connects the second transfer roller to the ground potential, and
A power supply unit is provided, which applies a voltage having a polarity opposite to the charging polarity of the toner to the conductive roller and causes a transfer current to flow to the second transfer roller via the shaft of the first transfer roller.
The conductive roller has a large-diameter portion provided at a position corresponding to an image forming region and a small-diameter portion having a diameter smaller than that of the large-diameter portion arranged at both ends of the conductive roller and supported by an axis. Have and
With respect to the width direction intersecting the moving direction of the belt member, the width of the large diameter portion of the conductive roller is equal to or larger than the width of the maximum image forming region and more than the width of the elastic layer of the first transfer roller. Is also narrow,
The width of the elastic layer of the first transfer roller is narrower than the width of the belt member with respect to the width direction intersecting the moving direction of the belt member, and the width of the elastic layer of the second transfer roller is the belt. Wider than the width of the member,
An image forming apparatus characterized in that. With respect to the width direction, the width of the large diameter portion of the conductive roller is narrower than the width of the maximum size recording material.
The image forming apparatus according to claim 1. The conductive roller is arranged so that the center in the width direction corresponds to the center in the width direction of the first transfer roller.
The image forming apparatus according to claim 1 or 2. The belt member is an intermediate transfer belt that supports a toner image that is primarily transferred from a photosensitive drum.
The first transfer roller is a secondary transfer roller that secondarily transfers a toner image from the intermediate transfer belt to the recording material.
The image forming apparatus according to any one of claims 1 to 3.

Images