Image forming apparatus having a charge member with a foamed layer

An image forming apparatus includes an image bearing member that bears a toner image, and a rotatable and endless intermediate transfer member, wherein a toner image is primarily transferred from the image bearing member to the intermediate transfer member in a first primary transfer part, and a toner image is secondarily transferred from the intermediate transfer member to the image bearing member in a secondary primary transfer part. In addition, a charge member is provided upstream of the first primary transfer part and downstream of the secondary primary transfer part in a rotation direction of the intermediate transfer member to charge residual toner remaining on the intermediate transfer member and not being transferred onto the transfer material in the secondary primary transfer part. The charge member includes a conductive roller whose surface layer is a foamed layer, and in a contact area in which the foamed layer contacts the intermediate transfer member, a space is formed between a part of a surface of the foamed layer and the intermediate transfer member, wherein a size of the space is larger than an average particle size of the residual toner.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to image forming apparatuses, such as copying machines and laser printers, that adopt an intermediate transfer system of an electrophotographic system or an electrostatic recording system for transferring a toner image formed on an image bearing member onto an intermediate transfer member and thereafter transferring the toner image onto a transfer material.

2. Description of the Related Art

In an image forming apparatus, part of toner may fail to be transferred from an intermediate transfer member to a transfer material, and this residual toner needs to be cleaned from the intermediate transfer member for the next printing. A proposed method for cleaning the residual toner uses a charge member for charging the residual toner and applies a voltage to the charge member to charge the residual toner to a predetermined polarity. In this method, a primary transfer member then moves the residual toner from the intermediate transfer member to a photosensitive drum.

Japanese Patent Application Laid-Open No. H10-49023 discloses that a roller with electrical conductivity is used as a charge member and provided with a release layer on a front layer thereof by coating or the like, thus suppressing adhesion of residual toner to the conductive roller.

However, in the configuration in Japanese Patent Application Laid-Open No. H10-49023, when a voltage is applied to the conductive roller, a discharge current may be locally generated to break part of the release layer of the conductive roller. Then, an excessive current may flow through the broken part. As a result, the excessive current may flow from the charge member into the intermediate transfer member. Furthermore, if the conductive roller includes the release layer, the conductive roller charges the residual toner in very small spaces located upstream and downstream of a charging nip formed by the conductive roller and intermediate transfer member contacted with each other. Thus, if the amount of the residual toner is large, the residual toner may fail to be sufficiently charged.

SUMMARY OF THE INVENTION

A purpose of the present invention is to provide an image forming apparatus allowing toner to be sufficiently charged while preventing an excessive current from flowing from a charge member into an intermediate transfer member.

Another purpose of the present invention is to provide an image forming apparatus including an image bearing member that bears a toner image, a rotatable and endless intermediate transfer member, a primary transfer member that primarily transfers the toner image from the image bearing member to said intermediate transfer member in a primary transfer part, a secondary transfer member that secondarily transfers the toner image from said intermediate transfer member to a transfer material in a secondary transfer member, and a charge member provided upstream of the primary transfer member and downstream of the secondary transfer member in a rotation direction of the intermediate transfer member to charge residual toner remaining on the intermediate transfer member and not being transferred onto the transfer material in the secondary transfer member, wherein after the charge member charges the residual toner, the primary transfer member moves the residual toner from the intermediate transfer member to the image bearing member, wherein the charge member is a conductive roller that has electrical conductivity and rotates according to rotation of the intermediate transfer member, and the conductive roller includes a foamed front layer and charges the residual toner upstream and downstream of a contact area in which the conductive roller contacts the intermediate transfer member and charges the residual toner in voids formed on the formed layer in the contact area.

DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present invention will be described in detail by way of example with reference to the drawings. The sizes, materials, forms, and relative configuration of components described in the following embodiments may be changed as appropriate depending on the configuration and conditions of an apparatus that incorporates the present invention.

First Embodiment

FIG. 1is a schematic diagram illustrating an exemplary embodiment of an image forming apparatus according to the present invention.

The image forming apparatus according to the present exemplary embodiment is of an electrophotographic type using an intermediate transfer scheme. In the image forming apparatus, toner images in a plurality of colors are sequentially superimposed on one another on an intermediate transfer member making a plurality of rotations. Then, a secondary transfer member transfers the toner images onto a transfer material all at once.

In the present exemplary embodiment, the image forming apparatus includes a photosensitive drum1serving as an image bearing member and borne so as to be rotatable. A charging device2serving as an image bearing charging roller, an exposure unit3, and a developing unit4are arranged around the photosensitive drum1. Moreover, an intermediate transfer belt9, a primary transfer roller10, and a cleaning part15are arranged around the photosensitive drum1; the intermediate transfer belt9is a belt-like intermediate transfer member, and the primary transfer roller10is a primary transfer member. Furthermore, a secondary transfer roller11serving as a secondary transfer member is arranged around the intermediate transfer belt9so as to be able to come into contact with and detach from the intermediate transfer belt9. In the present exemplary embodiment, the developing unit4includes developing devices5,6,7and8and a rotary4A serving as a rotatable support member supporting the developing devices5,6,7and8.

Moreover, the image forming apparatus includes a DC voltage power supply part16configured to apply DC voltages of positive polarity and negative polarity to the primary transfer roller10and a DC voltage power supply part17configured to apply DC voltages of positive polarity and negative polarity to the secondary transfer roller11. Additionally, the image forming apparatus includes a charge member arranged therein and which can come into contact with and detach from the intermediate transfer belt9. The charge member is arranged downstream of the secondary transfer roller11and upstream of the primary transfer roller in the direction of rotation of the intermediate transfer belt9. The charge member is a conductive roller22shaped like a roller and includes a DC voltage power supply part18configured to apply DC voltages of positive polarity and negative polarity to the conductive roller22.

The DC voltage power supply part16can apply voltages within the range of −2000 V to +2500 V. The DC voltage power supply parts17and18can apply voltages within the range of −2000 V to +4000 V.

Furthermore, in coming into contact with the intermediate transfer belt9, the secondary transfer roller11and the conductive roller22rotate in conjunction with driving by the intermediate transfer belt9and in the same direction as that in which the intermediate transfer belt9rotates.

The photosensitive drum1is driven in the direction of arrow R1by a driving part (not shown in the drawings) and evenly charged to a negative potential by the charging roller2.

Then, the photosensitive drum1is irradiated, by the exposure unit3, with laser light L based on image information to form a latent image thereon. The latent image is developed by any one of the developing devices5,6,7and8to form a single-color toner image of negative polarity. The single-color toner image on the photosensitive drum1is transferred to the intermediate transfer belt9. Single-color toner images formed as described above are superimposed on one another on the intermediate transfer belt to form a multicolor toner image. The multicolor toner image is transferred to a transfer material P all at once.

As described above, in the present exemplary embodiment, the developing unit4configured to visualize the latent images on the photosensitive drum1includes the four developing devices5,6,7and8configured to develop yellow Y, magenta M, cyan C and black K, respectively. The developing devices5,6,7and8are mounted in the rotary4A. Rotating the rotary4A in the direction of arrow R0allows the developing devices5,6,7and8to move sequentially to a position where the developing device comes into contact with the photosensitive drum1. Then, the yellow Y, magenta M, cyan C and black K are developed in this order.

The intermediate transfer belt9serving as an intermediate transfer member is tensioned around tension rollers12and13and can be rotated in the direction of arrow R3. The intermediate transfer belt9includes an endless belt formed of resin and having a surface resistance of 5.0×1010Ω/□, a volume resistance of 2.0×1011Ωm, a relative permittivity of 3, and a thickness of 100 μm. The intermediate transfer belt9is in contact with the photosensitive drum1and is rotated by a driving motor (not shown in the drawings) in the direction of R3at the same peripheral speed as that of the photosensitive drum1. The surface resistance of the intermediate transfer belt9was measured by Hirester MP-CHT450 manufactured by Mitsubishi Chemical Analytech Co., Ltd.

The primary transfer roller10serving as a primary transfer member is arranged opposite the photosensitive drum1across the intermediate transfer belt9, that is, the primary transfer roller10is arranged at a primary transfer member N1. A voltage of a positive polarity is applied to the primary transfer roller10to primarily transfer toner images formed on the photosensitive drum1onto the intermediate transfer belt9.

In the above-described steps, the four colors, the yellow Y, magenta M, cyan C and black K are primarily transferred onto the intermediate transfer belt9so as to be sequentially superimposed on one another. Then, a toner image in the plurality of colors is formed on the intermediate transfer belt9. While the yellow Y, magenta M and cyan C are being primarily transferred, the secondary transfer roller11arranged at a secondary transfer member N2is detached from the intermediate transfer belt9and thus prevented from coming into contact with the toner image on the intermediate transfer belt9to disturb the image. Similarly, the conductive roller22is detached from the intermediate transfer belt9and thus prevented from coming into contact with the toner image on the intermediate transfer belt9to disturb the image; the conductive roller22is arranged downstream of the secondary transfer member N2and upstream of the primary transfer member N1in the direction of movement of the intermediate transfer belt9. After the cyan is primarily transferred and the trailing end of the image passes through the conductive roller22, the secondary transfer roller11and the conductive roller22are brought into contact with the intermediate transfer belt9. A DC voltage with the positive polarity is then applied to the secondary transfer roller11and the conductive roller22.

After the secondary transfer roller11comes into contact with the intermediate transfer belt9, the transfer material P is conveyed by a sheet feeding roller. The transfer material P is fed to a secondary transfer member where the secondary transfer roller11comes into contact with the intermediate transfer belt9, that is, the secondary transfer nip portion N2, at a predetermined timing. A DC voltage of positive polarity is applied to the secondary transfer roller11to allow a multicolor toner image to be secondarily transferred from the intermediate transfer belt9to the transfer material P. After the transfer material P passes through the secondary transfer nip portion N2, the DC voltage applied to the secondary transfer roller11and the conductive roller22is interrupted. After the interruption, the secondary transfer roller11and the conductive roller22are detached from the intermediate transfer belt9.

The transfer material P having passed through the secondary transfer nip portion N2is conveyed to a fixation part (not shown in the drawings). The fixation part fixes the toner image on the transfer material P, which is then discharged and conveyed as an image formed article (print or copy).

If images are consecutively formed, then after the primary transfer of the black is finished, the yellow in the right next image is primarily transferred. Thus, the above-described image forming process is repeated.

Residual toner remaining on the intermediate transfer belt9instead of being secondarily transferred to the transfer material P is charged to positive polarity by the conductive roller22. Then, simultaneously with the primary transfer of the yellow in the next image, the residual toner is moved to the photosensitive drum1. Finally, the residual toner is collected by the cleaning part15on the photosensitive drum1. The residual toner need not be moved simultaneously with the primary transfer but may be moved at any other timing. Alternatively, the residual toner may be exclusively moved.

Now, a procedure for collecting the residual toner on the intermediate transfer belt9will be described.

In the present embodiment, the regular polarity of toner housed in the developing devices5,6,7and8is negative. Thus, the toner remaining on the intermediate transfer belt9after the secondary transfer mostly has negative polarity. Since the DC voltage of positive polarity for the primary transfer is applied to the primary transfer roller11, the residual toner maintaining negative polarity cannot be collected on the photosensitive drum1.

Thus, a DC voltage of about 2500 V and of positive polarity is applied to the conductive roller22to charge the residual toner to positive polarity.

As illustrated inFIG. 2, the conductive roller22is a rubber roller formed using NBR and hydrin as main compounds. A cored bar22bis passed through a central portion of the rubber roller22a. The rubber roller22ais 9.5 mm in diameter, and the core bar22bis 5 mm in diameter. Furthermore, as illustrated inFIG. 2, voids Ga are formed in a front layer of the rubber roller and inside the rubber roller as a result of foaming. The shape of the voids Ga varies but is adjusted such that the voids Ga have circular cross sections and are about 100 μm in diameter. The width of the void is hereinafter defined as the length from the end to end of the cross section of the void. The rubber roller has a resistance value of 3.15×107Ω and a hardness of 53 degrees (Asker C hardness).

FIG. 3is a diagram illustrating the configuration of a circuit configured to measure the resistance of the conductive roller22. The resistance value was calculated by measuring the potentials V of points located upstream and downstream of a resistor R inFIG. 3which potentials were obtained when the conductive roller22with a voltage applied thereto was brought into contact with a metal roller22mbeing rotationally driven.

The image forming apparatus according to the present exemplary embodiment includes pressurization mechanisms (not shown in the drawings) provided at the respective opposite ends of the conductive roller22. When the conductive roller22comes into contact with the intermediate transfer belt9, the conductive roller22is pressurized at the opposite ends thereof under a predetermined force. When coming into contact with the intermediate transfer belt9, the conductive roller22may be pressurized to the degree that the conductive roller22is rotated in conjunction with rotation of the intermediate transfer belt9. That is, the conductive roller22is rotated in the same direction as that in which the intermediate transfer belt rotates. Furthermore, a pressure contact portion N3between the conductive roller22and the intermediate transfer belt9may have a width of about 0.5 mm to 2.0 mm so that a discharge current can be efficiently generated in the voids formed upstream and downstream of the pressure contact portion N3. To meet these conditions, the conductive roller22may have a hardness of about 45° to 60° (Asker C hardness) and may be subjected to a total pressurization force of about 3.0 N to 10 N.

In the present exemplary embodiment, the conductive roller22has a hardness of 53 degrees and is pressurized at the opposite ends thereof under a force of about 1.5 N. Thus, the pressure contact portion N3is about 0.5 mm in width.

Moreover, to charge the residual toner, the DC voltage power supply part18applies a voltage of about 2500 V to the conductive roller22.

Now, a method for manufacturing the conductive roller22will be described.

In the steps of manufacturing the conductive roller22, first, a rubber component containing a mixture of a main rubber component, a foaming agent and a vulcanizing agent is shaped like a hollow cylinder by an extrusion molding machine and cut into a specified size.

Then, the rubber component cut into the specified size is placed in a pressurization furnace, in which the rubber component is foamed and vulcanized. Controlling the temperature, pressure and pressurization time of the pressurization furnaces allows adjustment of the size of the voids formed in and on the front layer of rubber component and inside the rubber component as a result of foaming.

In Exemplary Embodiment 1, the voids were adjusted to have a width of about 100 μm. After the rubber component is placed in the pressurization furnace, the treated rubber component is further placed in an electric furnace for vulcanization. Thus, a part of the rubber component which has failed to be vulcanized is completely vulcanized.

Then, the rubber component shaped like a hollow cylinder (rubber roller)22aproduced as described above is press-fitted over the core bar22bwith an adhesive applied thereto. After the press-fitting, the rubber component22ais heated in the electric furnace to melt the adhesive.

Finally, the rubber is cut at the opposite ends thereof to a specified size. Moreover, the surface of the rubber component (rubber roller)22ais polished to a specified outer diameter. Thus, the conductive roller22is completed.

Now, charging of the residual toner will be described.

As shown inFIGS. 4A and 4B, the residual toner is charged by passage, through the residual toner, of a discharge current generated between the conductive roller22and the intermediate transfer belt9when a DC voltage is applied to the conductive roller22. Thus, the conductive roller22needs to be configured to efficiently generate a discharge current when the DC voltage is applied to the conductive roller22.

The discharge current is generated when a very small space has a potential difference of at least a given value. Since the conductive roller22is shaped like a roller, very small spaces are formed between the conductive roller22and the intermediate transfer belt9and upstream and downstream of the pressure contact portion N3, the area in which the conductive roller22and the intermediate transfer belt9contact each other. As shown inFIG. 4B, discharge occurs in the very small spaces located upstream and downstream of the pressure contact portion N3to charge the residual toner on the intermediate transfer belt9.

Furthermore, the conductive roller22involves voids Ga of about 100 μm formed in the area of the pressure contact portion N3as a result of foaming. Thus, a discharge current is also generated in the area of the pressure contact portion N3. The reason is as follows. The toner used in the present exemplary embodiment is about 5 μm in particle size and is sufficiently small compared to the void, which is 100 μm in width. Hence, in the pressure contact portion N3, the residual toner is contained in the voids as shown inFIG. 5and charged by a discharge current generated in the voids Ga.

Thus, if voids Ga are formed in the front layer of the conductive roller22, a discharge current is generated not only in the spaces located upstream and downstream of the pressure contact portion N3but also in the pressure contact portion N3. Hence, the toner can be efficiently charged.

Now, a comparative example will be described in which the conductive roller22includes a release layer provided in the front layer of the non-foamed conductive roller and configured to prevent the toner from adhering to the conductive roller.

FIG. 6is a diagram illustrating how discharging occurs in the conductive roller in the comparative example. As illustrated inFIG. 6, without foaming, no voids are formed in the front layer of the conductive roller22A. Thus, no discharge current is generated in the pressure contact portion N3. The residual toner is charged and discharged only in the spaces located upstream and downstream of the pressure contact portion N3.

As a result, the toner cannot be charged in the pressure contact portion N3but only in voids located close to the pressure contact portion N3. Thus, in the comparative example, a sufficient discharge current to charge the toner failed to be obtained even by applying a 2500 VDC to the conductive roller22as is the case with Exemplary Embodiment 1.

Thus, in the comparative example, an increased voltage needs to be applied to the conductive roller22A. A voltage of 3000 V needed to be applied in order to sufficiently charge the toner. However, if a voltage of as high as 3000 V is applied, the toner is charged by a discharge current generated in the voids located close to the pressure contact portion N3but a large amount of discharge current locally flows. Hence, the intermediate transfer belt9and the release layer of the conductive roller22A may be damaged. Accordingly, the voltage applied to the conductive roller22A needed to be less than 3000 V.

Furthermore, local flow of a discharge current damaged not only the intermediate transfer belt9but also the release layer of the conductive roller22A.

In contrast, in the configuration of the conductive roller22according to Exemplary Embodiment 1, a discharge current is also generated in the pressure contact portion N3to allow the toner to be charged by application of 2500 V. Thus, the intermediate transfer belt9is protected from damage. Additionally, since the release layer is not provided, there is no possibility of damaging the release layer.

For Exemplary Embodiment 1 and the Comparative Example Table 1 illustrated below indicates charging of the toner and damage to the intermediate transfer belt observed when voltages are applied to the conductive roller.

TABLE 12000 V2500 V3000 V3500 VVoltage applied toTonerBeltTonerBeltTonerBeltTonerBeltconductive rollerchargingdamagechargingdamagechargingdamagechargingdamageExemplary Embodiment 1FAIRNo damagePASSNo damagePASSNo damagePASSBelt(voids in front layer)damagedComparative exampleFAILNo damageFAIRNo damagePASSBeltPASSBelt(no void + release layer)damageddamagedPASS . . . Residual toner was sufficiently positively charged and appropriately collected on the photosensitive drum.FAIR . . . Residual toner was positively charged and mostly collected on the photosensitive drum, but a small amount of toner failed to be collected.FAIL . . . Only part of the residual toner was successfully positively charged, and much of the residual toner failed to be collected on the photosensitive drum.

In the comparative example, when a voltage of at least 3000 V was applied to the conductive roller22A, the toner was sufficiently charged, whereas the intermediate transfer belt9was damaged.

In contrast, the conductive roller22according to Exemplary Embodiment 1 can be sufficiently charged to the toner by application of a voltage of at least 2500 V. Furthermore, areas where a discharge current is generated are dispersed, and the intermediate transfer belt9is more unlikely to be damaged than in the conventional art.

The potential of the surface of the conductive roller22was calculated using such a model of discharge in voids as shown inFIG. 7, in order to verify that a discharge current was generated in the pressure contact portion N3. The surface potential of the conductive roller22is determined by:
Vs=(∈G×V)/(∈G+D)  (1)

In Expression (1), the voltage applied to the conductive roller22is denoted by V, and the dielectric constant of the intermediate transfer belt9is denoted by ∈. The size of the voids (the gap inFIG. 7) is represented by G, and the thickness of the intermediate transfer belt9is denoted by D. Expression (1) is used to determine the potential of the front layer of the conductive roller22obtained when the size of the voids is varied at each applied voltage since the dielectric constant and thickness of the intermediate transfer belt9are constant.

In the present exemplary embodiment, the intermediate transfer belt9has a dielectric constant ∈ of 3 and a thickness D of 100 μm. A discharge current is generated when the potential in Expression (1) is higher than a discharge threshold potential.

FIG. 8illustrates the difference between the potential of the surface of the conductive roller22determined by Expression (1) and the discharge start potential determined based on the Paschen's Law when the voltage applied to the conductive roller22is 2500 V. The amount of discharge current increases consistently with the value of the difference.

FIG. 8indicates that no discharge current is generated when the voids are 0 μm in size and that the amount of discharge current increases consistently with the size of the voids, with the amount of discharge current generated reaching a peak when the voids are about 80 μm in size. When the voids are at least 80 μm in size, the amount of discharge current generated decreases gradually with increasing void size.

The discharge current is generated as follows. Accidental electrons in the air are accelerated by an electric field generated between electrodes. The electrons collide against air molecules and are repeatedly ionized and excited. Then, the number of the electrons increases in geometric progression. Thus, if almost no void is generated, the probability that electrons collide against air molecules is low, making a discharge current unlikely to be generated. Furthermore, excessively large voids contribute to reducing the magnitude of the electric field generated between the electrodes. This prevents the electrons from being accelerated, with no discharge current generated.

Therefore, with optimum voids, a discharge current can be efficiently generated.

The graph inFIG. 8varies depending on the voltage applied to the conductive roller22, the dielectric constant ∈ of the intermediate transfer belt9, and the thickness D. Thus, a variation in any of these values varies the void size with which the amount of dielectric current generated reaches a peak. However, the actual dielectric constant ∈ of the intermediate transfer belt9is within the range of about 3 to 9. The thickness D is within the range of about 50 μm to 150 μm in terms of the handling ability during assembly and flexibility. These ranges of the dielectric constant ∈ and the thickness D allow a sufficient discharge current to be generated when the voids are about 50 μm to 200 μm in size and are thus advantageous for charging of the toner.

To check whether or not a discharge current was actually generated, measurement was carried out to determine how the amount of current flowing from the conductive roller22changed when the size of the voids in the conductive roller22was changed.

The current was measured using a circuit similar to that illustrated inFIG. 3. The current value was calculated by measuring the potential across the resistor R illustrated inFIG. 3.

InFIG. 9, the axis of abscissas indicates the voltage applied to the conductive roller22. The axis of ordinate indicates the current. Comparison was made among the cases where the voids formed between the conductive roller22and the metal roller22mby a recess and protrusion structure in the front layer of the conductive roller22were 0 μm, 20 μm, 40 μm, 50 μm and 100 μm, respectively, in size.

FIG. 9indicates that when the size was 50 μm and 100 μm, a discharge current was efficiently generated in the pressure contact portion N3. Thus, when the size of the voids is one of 50 μm and 100 μm, more current is generated with the same applied voltage than when the size of the voids is 30 μm. Thus, the voids in the front layer of the conductive roller22are desirably set to at least 50 μm in size in order to efficiently generate a discharge current in the pressure contact portion N3.

On the other hand, an increase in void size increases the intervals among the areas where a discharge current is generated, resulting in uneven charging of the toner.

Table 2 indicates the results of checks obtained by examining how the residual toner was collected on the photosensitive drum when a variation was made in the size of the voids and in the voltage applied to the conductive roller.

When the voids are at most 300 μm in size, the residual toner can be more appropriately collected by increasing the voltage applied to the conductive roller22. In contrast, when the voids are 350 μm in size, the residual toner is unevenly charged in association with the pattern of the voids in the front layer of the conductive roller22. Thus, the toner fails to be appropriately collected regardless of the applied voltage. The reason is as follows: even with an increase in applied voltage and thus in the amount of discharge current generated, the excessively large voids contribute to increasing the intervals among the areas where a discharge current is generated, causing the residual toner to be unevenly charged. Discharging allows part of the toner to be sufficiently charged. However, another part of the toner is completely uncharged. As a result, the charged residual toner is collected on the photosensitive drum1, whereas the uncharged toner fails to be collected on the photosensitive drum1and remains on the intermediate transfer belt9. Thus, the voids are desirably at most 300 μm in size.

That is, as understood from the above description, the size of the voids Ga may be at least 50 μm and at most 300 μm.

As described above, in Exemplary Embodiment 1, the voids Ga were formed in the front layer of the conductive roller22to allow a discharge current to be generated in the pressure contact portion N3between the conductive roller22and the intermediate transfer belt9. This enabled the toner to be also charged in the pressure contact portion N3. As a result, the toner was successfully charged by applying, to the conductive roller22, a voltage of 2500 V, which is lower than that in the conventional art. This enabled an excessive current to be prevented from flowing from the conductive roller22into the intermediate transfer belt.

In Exemplary Embodiment 2, a method for more effectively charging the residual toner using the conductive roller22will be described. The present exemplary embodiment is particularly effective on a case where the residual toner includes at least two layers.

As illustrated inFIG. 10, if the residual toner includes at least two layers, when the configuration according to Exemplary Embodiment 1 is used to charge the residual toner, a discharge current generated by the conductive roller22charges only the front-layer residual toner. The front-layer residual toner charged to positive polarity is collected on the photosensitive drum1. However, the under-layer toner maintains negative polarity. The under-layer toner thus fails to be collected on the photosensitive drum1and remains on the intermediate transfer belt9.

The toner remaining on the intermediate transfer belt9is transferred from the surface of the intermediate transfer belt9to the transfer material P during the next image formation, resulting in an inappropriate image.

As described above, if the residual toner includes at least two layers, the evenness of charging of the residual toner varies between the front layer and the under layer. This may prevent part of the residual toner from being collected on the photosensitive drum1. To allow all of the residual toner to be collected on the photosensitive drum1, the residual toner needs to be evenly charged to positive polarity. To allow the residual toner to be evenly charged, the toner may be dispersed into one layer before passing through the conductive roller22.

Thus, in Exemplary Embodiment 2, to disperse the residual toner into one layer, the apparatus includes a conductive brush23arranged upstream of the conductive roller22and downstream of the secondary transfer roller11and serving as a slidable rubbing member configured to come into contact with and detach from the intermediate transfer belt9in conjunction with the conductive roller22, as illustrated inFIG. 11. The apparatus further includes a DC voltage power supply part19arranged therein to apply DC voltages of the positive and negative polarities to the conductive brush23. An image forming process according to the present exemplary embodiment is similar to that in Exemplary Embodiment 1.

The conductive brush23is formed of conductive nylon fibers with a fiber diameter about 20 μm and a density such that about 120 fibers are woven per 1 mm2.

Since the fibers with a fiber diameter similar to the particle size of the toner are densely woven, the residual toner formed of at least two layers is swept off and dispersed into one layer upon passing through the conductive brush23, as illustrated inFIG. 12. Furthermore, the conductive brush23with a DC voltage of positive polarity applied thereto enables the residual toner to be charged to positive polarity.

After passing through the conductive brush23, the residual toner passes through the conductive roller22. A DC voltage of positive polarity has been applied to the conductive roller22as is the case with the Exemplary Embodiment 1. Thus, the residual toner is charged to positive polarity by a discharge current generated in the voids located close to the pressure contact portion N3between the conductive roller22and the intermediate transfer belt9. The residual toner dispersed into one layer by the conductive brush23is evenly subjected to the discharge current and thus evenly charged to positive polarity.

Furthermore, the residual toner is charged to positive polarity by a discharge current generated in the voids Ga in the pressure contact portion N3which are located in the front layer of the conductive roller22. Also in this case, the residual toner, dispersed into one layer, is efficiently charged to positive polarity.

As is the case with Exemplary Embodiment 1, the residual toner charged to positive polarity upon passing through the conductive roller22is reversely transferred to the photosensitive drum1, while at the same time the yellow in the next image is primarily transferred. The residual toner is finally collected on the photosensitive drum1by the cleaning part15.

As described above, Exemplary Embodiment 2 uses the conductive brush23to disperse the residual toner formed of at least two layers into one layer so as to allow the residual toner to be efficiently charged to positive polarity by the conductive roller22. Then, the conductive brush23sweeps and disperses the residual toner into one layer, which is then charged by the conductive roller22.

Even if the residual toner is formed of at least two layers, the configuration according to the present exemplary embodiment allows the residual toner to be efficiently charged to positive polarity by a discharge current generated in the voids located close to the pressure contact portion N3between the conductive roller22and the intermediate transfer belt9and by a discharge current generated in the voids Ga in the pressure contact portion N3which are located in the front layer of the conductive roller22. Thus, the residual toner is reliably collected on the photosensitive drum1.

Furthermore, the conductive brush23is used to disperse the residual toner into one layer. However, applying a DC voltage of positive polarity to the conductive brush23not only enables the function to disperse the residual toner into one layer but also allows the residual toner to be partly charged to positive polarity. Therefore, the whole residual toner can be more efficiently charged to positive polarity.

This application claims the benefit of Japanese Patent Application No. 2010-104506, filed Apr. 28, 2011, which is hereby incorporated by reference herein in its entirety.