Image forming apparatus featuring first and second developer agent removing members

An image forming apparatus including an image bearing member which bears developing-agent images; a first removing member which removes a developing agent from the image bearing member, wherein the first removing member is contacted with the image bearing member while a bias is applied to the first removing member; a second removing member which removes the developing agent from the image bearing member, wherein the second removing member is contacted with a region of the image bearing member from which the developing agent has been removed by the first removing member; and a member which releases the second removing member from the image bearing member in such a manner as to prevent the region of the image bearing member which is contacted with the first removing member at the time of interruption of the application of the bias to the first removing member from coming into contact with the second removing member.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electrophotographic type image forming apparatus such as a copying machine and a printer and, more particularly, to removing members for removing developing agents on an image bearing member.

2. Description of the Related Art

In recent years, in view of improvement of the quality of printed images, there has been a need for stabilization of the ability of cleaning members to remove developing agents from image bearing members.

Therefore, there have been used electrostatic cleaning members which capture and remove developing agents by being biased, as described in Japanese Patent Application Laid-open No. 2002-207403. Such electrostatic cleaning members are less prone to degrade their removing abilities due to wear of the cleaning members themselves and thus have stable removing abilities. In this case, some developing agents on the image bearing member bear smaller amount of electrical charges. Such developing agents are difficult to sufficiently remove with electrostatic cleaning members. Therefore, in order to compensate the removing ability of such electrostatic cleaning members, a contact-type cleaning member for removing developing agents from the image bearing member by contacting therewith is provided downstream side of the electrostatic cleaning members in the direction of the rotation of the image bearing member.

However, if the biases applied to the electrostatic cleaning members are interrupted, the developing agents captured by the electrostatic cleaning members will lose the constraint forces. Consequently, the developing agents captured by the electrostatic cleaning members will be adhered to the image bearing member, again. The developing agents moved from the electrostatic cleaning members to the image bearing member will be captured by the contact-type cleaning means and, at this time, the amount of the developing agents will be excessive for the contact-type cleaning member. This will cause the contact-type cleaning member to rub the developing agents on the image bearing member, contrary to the inherent effect of the contact-type cleaning member. It is difficult to properly form images on the regions having developing agents rubbed thereon, which may cause image failures.

SUMMARY OF THE INVENTION

It is an object of the present invention to prevent the occurrence of image failures due to developing agents rubbed on an image bearing member by a contact-type cleaning member.

It is another object to provide an image forming apparatus including: an image bearing member which bears developing-agent images;

a first removing member which removes a developing agent from the image bearing member, wherein the first removing member is contacted with the image bearing member while a bias is applied to the first removing member;

a second removing member which removes the developing agent from the image bearing member, wherein the second removing member is contacted with the region of the image bearing member from which the developing agent has been removed by the first removing means; and

means which releases the second removing member from the image bearing member in such a manner as to prevent the region of the image bearing member which is contacted with the first removing member at the time of interruption of the application of the bias to the first removing member from coming into contact with the second removing member.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention can prevent the region of an image bearing member which has been contacted with electrostatic cleaning members at the time of interruption of the application of biases to the electrostatic cleaning members (the first removing members) from coming into contact with a contact-type cleaning member (a second removing member), thereby preventing the occurrence of image failures.

Hereinafter, examples of the present invention will be described in detail.

Next, with reference to the drawings, there will be described an image forming apparatus according to an embodiment of the present invention.

First Embodiment

With reference toFIGS. 1 to 3, there will be described an image forming apparatus according to a first embodiment.

(The Entire Structure of the Image Forming Apparatus)

First, the entire structure of the image forming apparatus will be described. As illustrated inFIG. 1, the image forming apparatus according to the present embodiment is a tandem-type image forming apparatus including four image formation devices Pa, Pb, Pc and Pd arranged along an intermediate transfer belt (image bearing member)181which is an intermediate transfer member to be adapted to rotate in the direction of an arrow X. Namely, the four image formation devices Pa, Pb, Pc and Pd form toner images of yellow (Y), magenta (M), cyan (C), black (K), respectively, through electrophotography, and also primarily transfer the toner images to the intermediate transfer belt181by superimposing them thereon. Further, the toner images are collectively secondarily transferred to a sheet as a recording medium being transferred thereto to complete the image formation.

The respective image formation devices Pa, Pb, Pc and Pd are arranged such that the yellow image formation device Pa, the magenta image formation device Pb, the cyan image formation device Pc and the black image formation device Pd are arranged in the mentioned order from upstream side to downstream side in the direction of the rotation of the intermediate transfer belt181. These image formation devices have the same structure, except that the color of toner images to be formed.

In the respective image formation devices Pa, Pb, Pc and Pd, processing means are placed around drum-shaped electrophotographic photosensitive members (hereinafter, referred to as “photosensitive drums”)101a,101b,101cand101d, which are image bearing members rotatably placed. Namely, there are placed electrification rollers122a,122b,122cand122d, exposing means111a,111b,111cand111d, developing means123a,123b,123cand123d, and drum cleaning means112a,112b,112cand112d. Further, primary transfer rollers124a,124b,124cand124das primary transfer means are provided, at the positions where the respective image bearing members are in contact with the intermediate transfer belt181.

Now, there will be described the image forming operation by exemplifying the yellow image formation device Pa. A bias is applied to the electrification roller122ato uniformly electrify the surface of the photosensitive drum101abeing rotated, and light is directed thereto from the exposing means111a, according to image signals, to form an electrostatic latent image. The latent image is developed with a yellow toner by the developing means123ato form a visible image. The toner is electrified to have the negative polarity by developing means123a. Further, a bias with the polarity opposite to that of the toner image (positive polarity) is applied to the primary transfer roller124ato primarily transfer the toner image to the intermediate transfer belt181, at the primary transfer portion T1where the photosensitive drum101ais in contact with the intermediate transfer belt181. After the transferring of the toner image, the drum cleaning means112aremoves residual toner on the photosensitive drum101a.

The other image formation devices Pb, Pc and Pd similarly perform the aforementioned transferring of toner images to perform transferring and formation of a full-color toner image. Further, the intermediate transfer belt181is wound around a driving roller125, a tension roller126and a backup roller129, which are supporting members, and is rotated at a predetermined speed in the direction of the arrow X during image formation.

On the other hand, in synchronization with the aforementioned image formation, a sheet P fed from a sheet cassette160mounted at a lower portion of the apparatus is conveyed to a secondary transfer portion T2which is the portion where the intermediate transfer belt181is in contact with a secondary transfer roller140which is a secondary transfer means. Then, a bias with the polarity opposite to that of the toner image (positive polarity) is applied to the secondary transfer roller140to transfer the toner image on the intermediate transfer belt181to the conveyed sheet P. Further, the sheet P is conveyed to a fixing means150which fixes the toner thereto and then is discharged to a discharge tray151.

Further, intermediate-transfer-member cleaning means115and130remove toner (secondary transfer residual toner) which has not transferred from the intermediate transfer belt181to the sheet P at the secondary transfer portion T2.

The intermediate transfer belt181is an endless belt which is configured to run in the direction of the arrow X at a predetermined speed during image formation.

Further, the intermediate transfer belt181according to the present embodiment is configured to be an elastic belt having elasticity at its surface portion. More specifically, as illustrated inFIG. 2, the intermediate transfer belt181is an elastic belt having a three-layer configuration constituted by a resin layer181a, an elastic layer181band a surface layer181c.

The resin material constituting the resin layer181amay be, for example, polycarbonate, fluororesin (ETFE, PVDF), polystyrene or the like. The elastic material (elastic rubber, elastomer) constituting the elastic layer181bmay be butyl rubber, fluoro rubber, acrylic rubber or the like. There is no limitation on the material of the surface layer181c, but the material is required to reduce in adhesion force of toner to the surface of the intermediate transfer belt181for enhancing the secondary transferability. For example, the material of the surface layer181cmay be a resin material such as a polyurethane resin, a polyester resin, and an epoxy resin. However, it is not limited to these materials.

The intermediate transfer belt181having the elastic layer181bat its surface portion as described above enables formation of high-quality images without dropout of figures, increase of the transfer efficiency, reduction of the amount of transfer residual toner and increase of the transferability to thick sheets and rough paper.

Next, there will be described the structure for cleaning off adherents residual on the intermediate transfer belt181after the secondary transfer, such as transfer residual toners.

The belt cleaning device according to the present embodiment includes a first cleaning device115and a second cleaning device130. These devices are placed downstream side of the aforementioned secondary transfer portion T2, but upstream side of the primary transfer portion T1of the yellow image formation device Pa, in the conveyance direction of the intermediate transfer belt181.

The first cleaning device115is an adsorption-type cleaning means which applies, to cleaning members, a bias with the polarity opposite to that of the residual toner on the intermediate transfer belt for causing the cleaning members to adsorb to and remove the toner. In the present embodiment, electrically-conductive fur brushes are employed as the cleaning members, the fur brushes are rotated, and a bias is applied to the fur brushes for performing cleaning.

The second cleaning device130is a contact-type cleaning device which brings a cleaning member into contact with the intermediate transfer belt181and causes it to slide against the intermediate transfer belt181for removing residual toner on the belt or foreign materials thereon such as addition agents. In the present embodiment, a web member is employed as the cleaning member for wiping out toner. The second cleaning device is placed downstream side (hereinafter, referred to as “the downstream side”) of the aforementioned first cleaning device in the direction of the rotation of the intermediate transfer belt181, in order to remove toner and adherents on the belt which have been passed through the first cleaning device115.

Further, in the present example, electrostatic cleaning members (first removing members)118aand118b, which are the cleaning members of the first cleaning device115, are kept in contact with the intermediate transfer belt181. However, in order to prevent the deformation of the electrostatic cleaning members118aand118bwhich are fur brushes, they may be made contactable with and removable from the intermediate transfer belt181. In this case, the position at which the first toner cleaning device cleans off secondary-transfer residual toner on the intermediate transfer belt181is referred to as a first cleaning position. The position at which the second toner cleaning device cleans off secondary-transfer residual toner on the intermediate transfer belt181is referred to as a second cleaning position.

The first cleaning device115cleans off secondary-transfer residual toner on the intermediate transfer belt181, in the following manner. That is, during the time interval starting with the passage of the leading edge of the region of the intermediate transfer belt181on which the toner image is formed, reaching to the first cleaning position, and ending with the passage of the trailing edge of the region through the first cleaning position, the electrostatic cleaning members118aand118bare kept in contact with the intermediate transfer belt181and a bias is applied to the electrostatic cleaning members118aand118b.

The second cleaning device further cleans the region of the intermediate transfer belt181which has been cleaned by the first cleaning device115.

In the present example, a contact-type cleaning member131(a second removing member), which is the cleaning member of the second cleaning device130, is made contactable with and removable from the intermediate transfer belt181.

The second cleaning device130performs cleaning, in the following manner. That is, during the time interval starting with the passage of the leading edge of the region of the intermediate transfer belt181which has been cleaned by the first cleaning means, reaching to the second cleaning position, and ending with the passage of the trailing edge of the region through the second cleaning position, the contact-type cleaning member131of the second cleaning device is kept in contact with the intermediate transfer belt181.

Next, there will be described, in detail, the structures of the first cleaning device115and the second cleaning device130according to the present embodiment.

As illustrated inFIG. 3A, a device housing117is placed near the intermediate transfer belt181. The first cleaning device115is structured to include an upstream-side cleaning portion116aand a downstream-side cleaning portion116bprovided along the direction of the rotation of the intermediate transfer belt, within the aforementioned device housing117. The upstream-side cleaning portion116aand the downstream-side cleaning portion116binclude electrostatic cleaning members118aand118bwith electric conductivity, metal rollers119aand119b, and cleaning blades120aand120b. Further, there are provided electrode rollers113aand113bwhich are grounded and contacted with the intermediate transfer belt181while being driven to be rotated thereby, at positions faced to the electrostatic cleaning members118aand118bacross the intermediate transfer belt181.

The electrostatic cleaning members118aand118bare formed from carbon-dispersed nylon fibers implanted on the metal rollers with an implanting density of 500,000/inch2, wherein the carbon-dispersed nylon fibers have a resistance of 10 MΩ and a fiber thickness of 6 deniers. The metal rollers119aand119bare formed from electrically-conductive metal rollers made of aluminum which have been subjected at their surfaces to hard alumilite treatment, and the cleaning blades120aand120bare configured to be in contact with these metal rollers119aand119b.

The electrostatic cleaning members118aand118bare placed to slidably contact with the intermediate transfer belt181while being intruded to the intermediate transfer belt181by about 1.0 mm. Further, the electrostatic cleaning members118aand118bare configured to be rotated by a driven motor, not illustrated, in the direction of arrows inFIG. 3A, at a speed of 50 mm/sec.

The metal rollers119aand119bare placed to be kept intruded to the electrostatic cleaning members118aand118bby about 1.0 mm and also are placed to be rotated in the direction of the arrows inFIG. 3Aat a speed equivalent to that of the electrostatic cleaning members118aand118b. The cleaning blades120aand120bwhich are in contact with the metal rollers119aand119bare made of a polyurethane rubber and are placed to be kept intruded to the metal rollers by 1.0 mm.

Further, a direct current constant voltage of −700 V (relative to the ground, the same will apply hereinafter) from a direct-current power supply121ais applied to the metal roller119ain the upstream-side cleaning portion116apositioned at the upstream side in the direction of the rotation of the intermediate transfer belt. Further, a direct-current voltage of −700V, which is controlled to be a constant voltage, is applied to the electrostatic cleaning member118athrough the metal roller119afrom the direct-current power supply121a. At this time, the direct-current power supply121acontrols the bias applied to the electrostatic cleaning member118ato a constant voltage. On the other hand, a direct-current constant voltage of +700 V with the polarity opposite from that of the upstream-side cleaning portion116a, from a direct-current power supply121b, is applied to the metal roller119bin the downstream side cleaning portion116bpositioned at the downstream side in the direction of the rotation of the intermediate transfer belt. Further, a direct-current voltage of +700 V, which is controlled to be a constant voltage, from the direct-current power supply121b, is applied to the electrostatic cleaning member118bthrough the metal roller119b. At this time, the direct-current power supply121bcontrols the bias applied to the electrostatic cleaning member118bto a constant voltage.

As described above, the respective voltages from the power supplies121aand121bare applied to the metal rollers119aand119b. This causes an electric potential difference between the electrostatic cleaning members118aand118b, which causes (+) toner in the transfer residual toner on the intermediate transfer belt181to be adsorbed to and transferred to the electrostatic cleaning member118a. The adsorbed and removed toner is transferred from the electrostatic cleaning member118ato the metal roller119adue to the electric potential difference and is swept away by the cleaning blade120a.

Although the upstream-side cleaning portion116acleans off the transfer residual toner on the intermediate transfer belt181, there is still left toner having no polarity or having the negative polarity on the intermediate transfer belt181. Such toner is electrified to have the negative polarity, with the (−) bias applied to the electrostatic cleaning member118ain the upstream-side cleaning portion116a. It is deemed that the electrification is caused by injection of electric charge or by discharge.

Further, such toner can be removed by applying a (+) bias voltage to the downstream-side cleaning portion116bplaced downstream side of the upstream-side cleaning portion116afor performing cleaning. Further, the removed toner is transferred from the electrostatic cleaning member118bto the metal rollers119bdue to the electric potential difference and then is swept away by the cleaning blade120b, so that all the transfer residual toner on the intermediate transfer belt181can be removed.

As described above, the first cleaning device115is configured to realize a cleaning system using the electrostatic cleaning members118, which can reduce the burden on the intermediate transfer belt181and thus is advantageous, in particular, for cleaning the elastic intermediate transfer belt.

As illustrated inFIG. 3A, the second cleaning device130brings the contact-type cleaning member (the second removing member)131into contact with the intermediate transfer belt181and causes it to slide against the intermediate transfer belt181for cleaning it. In the present embodiment, the contact-type cleaning member131is wound around a supply roll132aand a wind-up roll132band also is brought into contact with the intermediate transfer belt181at a predetermined pressure through a contacting roll133. In the present embodiment, the contact-type cleaning member131is brought into contact with the intermediate transfer belt181at a total pressure of 2.0 kg.

The contact-type cleaning member131may be made of one or more types of materials selected out of nonwoven fabric, polyester, acrylic, vinylon, water soluble vinylon, rayon, nylon, polypropylene, cotton and the like. However, it is not limited to the aforementioned materials.

At portions where the intermediate transfer belt181is pressurized, such as the transfer portions, addition agents liberated from toner are rubbed on and adhered to the surface of the intermediate transfer belt181. Such additional agents can not be collected by the first cleaning device115and, therefore, are mechanically collected by the contact-type cleaning member131.

However, if the same surface of the contact-type cleaning member131is used for a long time, the amount of adherents that can be collected by the contact-type cleaning member131will be exceeded, which may cause adherents to be contrarily adhered to the surface of the intermediate transfer belt181. This may change the electrical resistance of the intermediate transfer belt181, thus resulting in image failures and the like. Accordingly, the contact-type cleaning member131is wound up around the windup roll132bby a certain amount, after the elapse of a certain time period, to provide a new surface to be brought into contact with the intermediate transfer belt181.

In the present embodiment, the timing and the amount at and by which the contact-type cleaning member131is wound up are such that the contact-type cleaning member131is wound up by 5 mm every time one hundred A4 sheets have been printed. This enables successfully removing adherents adhered to the surface of the intermediate transfer belt181.

While, in the present embodiment, there has been exemplified the second cleaning device130employing a web, the second cleaning device130may have any configuration having the function of capturing, into its inside, adherents on the surface of the intermediate transfer belt by contacting with the intermediate transfer belt181. For example, the second cleaning device130may have a configuration which brings a roller including an non-woven cloth wound around its surface into contact with the intermediate transfer belt181and drives and rotates the roller.

Further, as shown inFIG. 3Bthe second cleaning device130is supported rotatably about a rotation shaft134, and the contact-type cleaning member131is made contactable with and removable from the intermediate transfer belt181through a pressurization/release means135constituted by a solenoid or the like. When the pressurization/release means135is off, the contact-type cleaning member131is kept in contact with the intermediate transfer belt181. On the other hand, if the pressurization/release means135is turned on, this will cause the second cleaning device130to rotate about the rotation shaft134, thereby causing the contact-type cleaning member131to be released from the intermediate transfer belt181.

(Control for Removing the Second Cleaning Device)

The image forming apparatus according to the present embodiment is configured such that the contact-type cleaning member131in the aforementioned second cleaning device130is released from the intermediate transfer belt181, at the time of interruption of the application of the bias to the first cleaning device. This is for preventing an excessive amount of toner from being adhered to the contact-type cleaning member131. Next, there will be described the configuration for controlling the release of the contact-type cleaning member131.

In the image forming apparatus according to the present embodiment, the contact portion where the downstream-side electrostatic cleaning member118bin the first cleaning device115is contacted with the intermediate transfer belt181(hereinafter, referred to as “a first contact portion”) has a width of 8 mm. Further, the contact portion where the contact-type cleaning member131in the second cleaning device130is contacted with the intermediate transfer belt181(hereinafter, referred to as “a second contact portion”) has a width of 4 mm. Further, in the direction of the rotation of the intermediate transfer belt181, the distance between the center of the contact region of the aforementioned first contact portion and the center of the contact region of the second contact portion is set to 50 mm. Further, the processing speed of the image forming apparatus according to the present embodiment, namely the peripheral speed of the rotation of the intermediate transfer belt181, is set to 300 mm/sec.

In this case, when the application of the biases to the first cleaning device115, namely the electrostatic cleaning members118, are interrupted at the end of a job, the toner which has been constrained to the electrostatic cleaning members118aand118bloses the electrostatic constraint force. Consequently, the toner may be adhered to the intermediate transfer belt181, again.

On the other hand, in the image forming apparatus according to the present embodiment, the time interval from the starting of interruption of the application of the biases to the first cleaning device115to the completion of the interruption of the biases (hereinafter, referred to as “a bias-off time interval”) is 100 msec.

In this case, the time of the interruption of the application of the biases to the first cleaning device115, namely the electrostatic cleaning members118aand118b, refers to the time as follows. Namely, in the case where the biases applied to the electrostatic cleaning members118aand118bare controlled to predetermined constant voltages, the aforementioned time refers to the time when the biases start changing from the predetermined voltages toward 0 V.

Namely, in the present example, the bias applied to the electrostatic cleaning member118ais controlled to a constant voltage of −700 V, by the direct-current power supply121a. As illustrated inFIG. 4A, when the direct-current power supply121ais turned off, the bias applied to the electrostatic cleaning member118ais gradually changed from −700 V to 0 V for 100 msec. In this case, the time of the interruption of the application of the bias to the electrostatic cleaning member118arefers to the moment when the bias starts changing from −700 V toward 0 V (time Ta0). In this case, time Ta1refers to the time when the bias reaches 0 V.

The same applies to the electrostatic cleaning member118b, to which a bias controlled to a constant current of +700 V is applied. Namely, as illustrated inFIG. 4B, the moment when the bias starts changing from +700 V towards 0 V (time Tb0) since the direct-current power supply121bis turned off is defined as the time of the interruption of the application of the bias to the electrostatic cleaning member118b. In this case, time Tb1refers to the time when the bias reaches 0 V.

The time of the interruption of the application of the biases to the first cleaning means115, namely the electrostatic cleaning members118aand118bis defined as described above for the following reason. That is, when the direct-current power supplies121aand121bare turned off, the ratio of change of the biases applied to the electrostatic cleaning members118aand118b(the change of the biases per unit time) becomes largest at the time when the biases start changing from the predetermined electric potentials toward 0 V. Accordingly, the amount of toner that is released from the electrostatic cleaning members118aand118band is re-adhered to the intermediate transfer belt181becomes greatest at the time when the biases start changing from the predetermined electric potentials toward 0 V.

In the present embodiment, the processing speed (the speed of the travel of the intermediate transfer belt181) is 300 mm/sec. Therefore, even if the biases applied to the first cleaning means115are interrupted concurrently with the completion of a job, toner will be re-adhered to the intermediate transfer belt181and will be moved by at least 30 mm in the downstream direction, during the bias-off time interval. Further, even if the driving motor for the intermediate transfer belt181is turned off concurrently therewith, the intermediate transfer belt181will be moved by about 30 mm, due to the inertia. Accordingly, the toner re-adhered to the intermediate transfer belt181will be moved by a total of at least 60 mm, in the downward direction.

In this case, the smallest distance between the first contact portion190and the second contact portion191is the distance between the most downstream position190aof the contact region with a width of 8 mm of the first contact portion190(downstream from the center of the contact region by 4 mm) and the most upstream position191aof the contact region with a width of 4 mm of the second contact portion191(upstream from the center of the contact region by 2 mm). Namely, in the image forming apparatus according to the present embodiment, the aforementioned smallest distance is 44 mm.

Accordingly, if the toner t re-adhered to the intermediate transfer belt181is moved by 60 mm downwardly from the most downstream position190aof the contact region of the first contact portion190, the toner t will intrude into the most upstream position191aof the contact region of the second contact portion191. Namely, if the contact-type cleaning member131in the second cleaning device130is kept in contact with the intermediate transfer belt181, the toner t re-adhered to the intermediate transfer belt181will be captured by the contact-type cleaning member131.

Therefore, the image forming apparatus according to the present embodiment is configured such that the pressurization/release means135is turned on to release the contact-type cleaning member131in the second cleaning device130from the intermediate transfer belt181, concurrently with the interruption of the application of the biases to the first cleaning member115.

Accordingly, in the image forming apparatus according to the present embodiment, the timing of the interruption of the biases applied to the electrostatic cleaning members118aand118band the timing of the release of the contact-type cleaning member131from the intermediate transfer belt181are set as follows. Namely, before the toner released from the electrostatic cleaning members118aand118band re-adhered to the intermediate transfer belt181, due to the interruption of the application of the biases, reaches the contact-type cleaning member131, the contact-type cleaning member131is released from the intermediate transfer belt181. Namely, the contact-type cleaning member131is released from the intermediate transfer belt181, in such a manner as to prevent the region of the intermediate transfer belt181which is contacted with the electrostatic cleaning members118aand118bat the time of the interruption of the application of the biases to the electrostatic cleaning members118aand118bfrom coming into contact with the contact-type cleaning member131.

This can prevent the toner and the like which is re-adhered to the intermediate transfer belt181at the time of the interruption of the biases applied to the electrostatic cleaning members118aand118bfrom being adhered to the contact-type cleaning member131. This can prevent an excessive amount of toner from being deposited on the contact-type cleaning member131and can prevent toner from being rubbed on the intermediate transfer belt.

Further, even in cases where the biases applied to the first cleaning device115are controlled to constant electric currents, the same effects can be provided by interrupting the application of the biases to the first cleaning device and releasing the second cleaning device in the aforementioned manner. In this case, biases which are controlled to constant electric currents of −15 μA and +15 μA are applied to the electrostatic cleaning members118aand118b. The power supplies121aand121bcontrol the respective biases to constant electric currents.

In the case where the biases applied to the first cleaning means are controlled to predetermined constant electric current values, the time of the interruption of the application of the biases to the first cleaning device115refers to the time when the biases start changing from the predetermined electric current values toward 0 μA.

Namely, in the present example, the bias applied to the electrostatic cleaning member118ais controlled to a constant electric current of −15 μA, through the direct-current power supply121a. If the direct-current power supply121ais turned off, the bias applied to the electrostatic cleaning member118ais gradually changed from −15 μA to 0 μA for 100 msec. In this case, the time of the interruption of the application of the bias to the electrostatic cleaning member118arefers to the moment when the bias starts changing from −15 μA to 0 μA.

The same applies to the electrostatic cleaning member118b, to which a bias controlled to a constant electric current of +15 μA is applied.

Namely, the moment when the bias starts changing from +15 μA toward 0 μA since the direct-current power supply121bis turned off is defined as the time of interruption of the application of the bias to the electrostatic cleaning member118b.

Second Embodiment

In the aforementioned embodiment, the contact-type cleaning member131is released from the intermediate transfer belt181concurrently with the interruption of the application of the biases to the electrostatic cleaning members118aand118b. However, the contact-type cleaning member131may be released from the intermediate transfer belt181, before the interruption of the application of the biases to the electrostatic cleaning members118aand118b(100 msec in advance of the interruption of the application of the biases, in the present embodiment).

In this case, similarly to the aforementioned first embodiment, the contact-type cleaning member131is released from the intermediate transfer belt181, before the toner and the like released from the electrostatic cleaning members118aand118band re-adhered to the intermediate transfer belt181at the time of the interruption of the biases reaches the contact-type cleaning member131. This can provide effects similar to those of the first embodiment.

In the present example, similarly, the biases applied to the electrostatic cleaning members118aand118bcan be controlled to predetermined constant voltages or predetermined constant electric currents.

Third Embodiment

In the present embodiment, the contact-type cleaning member131is released from the intermediate transfer belt181, after the interruption of the application of the biases to the electrostatic cleaning members118aand118b(after 100 msec following the interruption of the application of the biases, in the present embodiment). As described above, the smallest distance between the first contact portion190and the second contact portion191is the distance between the most downstream position190aof the contact region with a width of 8 mm of the first contact portion190(downstream from the center of the contact region by 4 mm) and the most upstream position of the contact region with a width of 4 mm of the second contact portion191(upstream from the center of the contact region by 2 mm). Namely, in the image forming apparatus according to the present embodiment, the aforementioned smallest distance is 44 mm.

In the image forming apparatus according to the present example, the speed of the travel of the intermediate transfer belt181is 300 mm/sec.

Therefore, the intermediate transfer belt181requires a time period of 146 msec to travel from the most downstream position190aof the contact region of the first contact portion190to the most upstream position191aof the contact region of the second contact portion191.

Accordingly, the second cleaning device130is released from the intermediate transfer belt181, before the toner adhered to the intermediate transfer belt181due to the interruption of the application of the biases to the electrostatic cleaning members118aand118breaches the second cleaning device130.

In the present example, similarly, the biases applied to the electrostatic cleaning members118aand118bcan be controlled to predetermined constant voltages or predetermined constant electric currents.

As described in the aforementioned first to third embodiments, when the region of the intermediate transfer belt181which is contacted with the first cleaning device115at the time of the interruption of the application of the biases to the first cleaning device115reaches the second contact portion, the second cleaning device is released from the intermediate transfer belt181.

This can prevent the second cleaning means130from rubbing the toner which has been adhered to the intermediate transfer belt181, due to the interruption of the application of the biases to the first cleaning means115, on the intermediate transfer belt181.

In the aforementioned embodiments, there has been exemplified the first cleaning device115employing the two electrostatic cleaning members118aand118b. However, in the first to third embodiments, the first cleaning means may be configured to include only a single electrostatic cleaning member For example, in cases of an image forming apparatus for forming toner images with the negative polarity, most of secondary-transfer residual toner has the positive polarity and, therefore, a bias with the negative polarity may be applied to a single electrostatic cleaning member to remove the toner on the intermediate transfer belt.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of priority from the prior Japanese Patent Application No. 2005-262542 filed on Sep. 9, 2005 the entire contents of which are incorporated by reference herein.