Image forming apparatus with intermediate transfer member

An electrostatic latent image is formed on a rotary image carrier. A developing roller is separatably abutted on the image carrier to supply toner onto the image carrier to make the latent image visible as a toner image. An intermediate transfer member is adapted to temporarily hold the toner image. A first transferer presses the intermediate transfer member against the image carrier to define a primary transfer position therebetween, so that the toner image on the image carrier is transferred to the intermediate transfer member. A second transferer is separatably abutted on the intermediate transfer member to transfer the toner image on the intermediate transfer member to a recording medium. An operation for forming the latent image is started after a predetermined time period elapses since a toner attached on at least one of a first region of the intermediate transfer member, on which a toner image to be transferred onto the recording medium is not transferred, and a second region on the image carrier corresponding to the first region has passed through the primary transfer position.

BACKGROUND OF THE INVENTION

The present invention relates to an image forming apparatus such as a copying machine, a printer, and a facsimile employing electrophotography and, more specifically, an image forming apparatus provided with an intermediate transfer member.

In the above-described image forming apparatus, a toner image primarily transferred from a latent image carrier such as a photosensitive drum to an intermediate transfer member such as an intermediate transfer belt is secondarily transferred to a recording medium such as paper. After then, toner remaining on the intermediate transfer member is removed by a cleaner such as a cleaning blade which comes into contact with the surface of the intermediate transfer member and scraping the residual toner from the intermediate transfer member.

FIG. 1Ashows a state in which a cleaning blade14comes into contact with an intermediate transfer member12suspended by a driving roller10and a follower roller11. As shown inFIG. 1C, toner T is accumulated on the extremity of the cleaning blade14by the amount corresponding to the thickness of the blade. From this state, as shown inFIG. 1B, when the cleaning blade14is separated from the intermediate transfer member12, a toner line (separation line)24is generated. As shown inFIG. 1D, the width L of the toner line24is substantially equal to the thickness W of the cleaning blade14.

Consequently, there arises a problem that the toner line24overlaps a toner image which is to be primarily transferred to the intermediate transfer member12in the subsequent image forming process. Japanese Patent Publication Nos. 2000-231276A and 2002-82533A teach that the separation timing of the cleaner is determined with reference to the position of the toner image to be primarily transferred to the intermediate transfer member12, so as to prevent the toner line generated by the cleaner from overlapping the image area.

However, as shown inFIG. 2A, the toner line24formed in the above-described non-image area may attached to a secondary transfer roller15when the secondary transfer roller15comes into contact with the intermediate transfer member12immediately before the secondary transfer operation. Then, when the secondary transfer operation to a recording medium S is completed, and the secondary transfer roller15is separated from the intermediate transfer member12as shown inFIG. 2B, the toner line24is again attached on the intermediate transfer member12.

FIG. 3Ashows a state that a position C1at which the toner line24is attached proceeds toward a primary transfer position T1which is defined by an image carrier (photosensitive drum)3and a primary transfer roller13.FIG. 3Bshows a state that the toner line24reaches the primary transfer position T1, and an electrostatic latent image LI is formed by an exposure operation (i.e., light beam irradiation as indicated by an arrow).FIG. 3Cshows a state that the latent image LI is developed by a developing roller6aas a visible toner image TI.

The rotation velocity of the image carrier3changes when the image writing (exposure operation) is performed while the toner line24is at the primary transfer position T1and the width L of the toner line24is no less than the nip width N of the primary transfer position as shown inFIG. 3D. Consequently, unevenness of density or color shifting due to the rotation velocity fluctuations of the image carrier3, that is, so-called banding stain occurs. It results from the fact that when there exists the toner line24between the intermediate transfer member12and the image carrier3, a friction force between them is lowered, so that the image carrier3slips and results in the rotation velocity fluctuations.

FIG. 4shows experimental data obtained by measuring rotation velocity fluctuations of the image carrier3. In this experiment, the circumferential velocity of the intermediate transfer member is set to a value faster than that of the circumferential velocity of the image carrier by 0.7%, and the toner images are transferred to the intermediate transfer member in the order of Bk (black), C (cyan), M(magenta), and Y(yellow).

InFIG. 4, the vertical axis represents rotation velocity fluctuations, the lateral axis represents time, and the rotation velocity fluctuation is obtained by subtracting the rotation velocity of the image carrier from the circulation velocity of the intermediate transfer member, and the result is then divided by the rotation velocity of the image carrier. Finally, the result is multiplied by 100 to obtain a percentage value. Accordingly, the “plus” value implies that the image carrier is slower than the intermediate transfer member, and the “minus” value implies that the image carrier is faster than the intermediate transfer member. The significant rotation velocity fluctuations appeared in the initial period are turbulence of an encoder signal occurred at the position corresponding to a seam of the intermediate transfer member (belt). The turbulences may be ignored because they are not actually the rotation velocity fluctuations. When the position of the toner line24on the intermediate transfer member12reaches the primary transfer position T1, distinctive rotation velocity fluctuations can be observed as shown by arrows A.

FIG. 5Ashows experimental data in a case where the toner line formed on the intermediate transfer member is not removed.FIG. 5Bshows experimental data in a case where the toner line formed on the intermediate transfer member is removed before it reaches the primary transfer position. From both results, it is apparent that the rotation velocity fluctuation pointed by an arrow is derived from the existence of the toner line.

The banding stain problem described above also occurs when the toner mark is attached to the area on the image carrier3corresponding to the non-image area on the intermediate transfer member12. As shown inFIG. 6A, when the developing roller6acomes into contact with a position C2on the image carrier3, a toner line (contact line)24is attached thereto due to the impact of the developing roller6a. As shown inFIG. 6B, the toner line24then reaches the primary transfer position T1. As shown inFIG. 6C, the toner line24is partially transferred to the non-image area on the intermediate transfer member12and passed through the primary transfer position T1, while the electrostatic latent image LI is developed by the developing roller6a.

Similarly, the rotation velocity of the image carrier3changes when the image writing (exposure operation) is performed while the toner line24is at the primary transfer position T1. Consequently, unevenness of density or color shifting due to the rotation velocity fluctuations of the image carrier3, that is, so-called banding stain occurs. It results from the fact that when there exists the toner line (contact line)24between the intermediate transfer member12and the image carrier3, a friction force between them is lowered, so that the image carrier3slips and results in the rotation velocity fluctuations.

In the experimental data shown inFIG. 7, the pressure fluctuations due to the contact line24can be observed at positions pointed by arrows B.

In such an image forming apparatus that an AC-superimposed bias is applied to a developing roller to develop an electrostatic latent image as a visible toner image, toner may locally attach to an image carrier by splashing or fogging of toner since the high voltage level of the bias cannot be stabilized at the initial stage of the application of the developing bias. In view of the above, Japanese Patent Publication No. 3-64073B teaches that an AC-superimposed bias is applied before the latent image on the image carrier reaches the developing position (that is, applied at a position corresponding to a non-image area) for stabilizing the bias before development, so that splashing or fogging of toner is prevented.

However, as shown inFIGS. 8A and 8B, when the AC-superimposed bias is applied to the position corresponding to the non-image area, toner in an area Y splashes on the image carrier3at the initial stage of bias application. Since the developing roller6aand the image carrier3rotate in the direction indicated by arrows and a gap between the image carrier3and the developing roller6ais increased, thereby weakening an electric field, the splashed toner cannot return to the developing roller6a. Also, since the power source may become unstable at the initial stage of bias application, so-called overshoot may occur, and hence toner may splash to the image carrier3. The toner line (bias application line)24is thus formed in the non-image area of the image carrier3.

As shown inFIG. 9A, when the developing bias generated from a bias power source25is applied at a position C3on the image carrier3via the developing roller6a, the toner line24is attached due to the impact of the image carrier3. As shown inFIG. 9B, the toner line24then reaches the primary transfer position T1. As shown inFIG. 9C, the toner line24is partially transferred to the non-image area on the intermediate transfer member12and passed through the primary transfer position T1, while the electrostatic latent image LI is developed by the developing roller6a.

The rotation velocity of the image carrier3changes when the image writing (exposure operation) is performed while the toner line24on the position C3is at the primary transfer position T1. Consequently, unevenness of density or color shifting due to the rotation velocity fluctuations of the image carrier3, that is, so-called banding stain occurs. It results from the fact that when there exists the toner line (bias application line)24between the intermediate transfer member12and the image carrier3, a friction force between them is lowered, so that the image carrier3slips and results in the rotation velocity fluctuations.

In the experimental data shown inFIG. 10, the pressure fluctuations due to the bias application line24can be observed at positions pointed by arrows C.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide an image forming apparatus capable of preventing the appearance of banding stain even if the toner line is attached on the non-image area of the intermediate transfer member or a portion of the image carrier corresponding to the non-image area.

In order to achieve the above object, according to the invention, there is provided an image forming apparatus, comprising:

a rotary image carrier, on which an electrostatic latent image is formed;

a developer, comprising at least one developing roller which is separatably abutted on the image carrier to supply toner onto the image carrier to make the latent image visible as a toner image;

an intermediate transfer member, adapted to temporarily hold the toner image;

a first transferee, which presses the intermediate transfer member against the image carrier to define a primary transfer position therebetween, so that the toner image on the image carrier is transferred to the intermediate transfer member; and

a second transferee, separatably abutted on the intermediate transfer member to transfer the toner image on the intermediate transfer member to a recording medium;

wherein an operation for forming the latent image is started after a predetermined time period elapses since a toner attached on at least one of a first region of the intermediate transfer member, on which a toner image to be transferred onto the recording medium is not transferred, and a second region on the image carrier corresponding to the first region has passed through the primary transfer position.

A circumferential velocity of the image carrier and a circumferential velocity of the intermediate transfer member may be different at the primary transfer position. Here, the image carrier and the intermediate transfer member may be driven by a common drive source.

The toner may be attached on the first region at least one of when the secondary transferer comes in contact with the intermediate transfer member and when the secondary transferer separates from the intermediate transfer member.

The toner may be attached on the second region at least one of when the developing roller comes in contact with the image carrier and when the developing roller separates from the image carrier.

In a case where a cleaner is separatably abutted on the intermediate transfer member to remove toner remaining thereon, the toner may be attached on the first region at least one of when the cleaner comes in contact with the intermediate transfer member and when the cleaner separates from the intermediate transfer member.

In a case where a cleaner is separatably abutted on the image carrier to remove toner remaining thereon, the toner may be attached on the second region at least one of when the cleaner comes in contact with the image carrier and when the cleaner separates from the image carrier.

In a case where a charger is separatably abutted on the image carrier to uniformly charge a surface of the image carrier before the latent image is formed, the toner may be attached on the second region at least one of when the charger comes in contact with the image carrier and when the charger separates from the image carrier.

With the above configurations, since the latent image formation is started after a toner line (separation toner line or contact toner line) passes through the primary transfer position, the image formation is not affected by the rotation velocity fluctuations of the image carrier due to the toner line, thereby avoiding the occurrence of the banding stain problem.

In a case where the cleaner is a blade member, and the first transferer applies a bias voltage to the intermediate transfer member, it is preferable that a nip width formed between the intermediate transfer member and the image carrier at the primary transfer position is larger than a thickness of the blade member.

It is further preferable that the nip width is two to fifth times of the thickness.

In a case where the cleaner is a brush member, and the first transferer applies a bias voltage to the intermediate transfer member, it is preferable that a nip width formed between the intermediate transfer member and the image carrier at the primary transfer position is larger than a circumferential length of a contact area between the brush member and the intermediate transfer member.

It is further preferable that the nip width is two to fifth times of the circumferential length.

With the above configurations, the rotation velocity fluctuations of the image carrier due to the toner line can be suppressed.

According to the invention, there is also an image forming apparatus, comprising:

a rotary image carrier, on which an electrostatic latent image is formed;

a developer, comprising at least one developing roller, through which a bias voltage is applied to supply toner onto the image carrier to make the latent image visible as a toner image;

an intermediate transfer member, adapted to temporarily hold the toner image; and

a transferer, which presses the intermediate transfer member against the image carrier to define a primary transfer position therebetween, so that the toner image on the image carrier is transferred to the intermediate transfer member,

wherein an operation for forming the latent image is started after a predetermined time period elapses since a portion on the image carrier to which the bias voltage is initially applied has passed through the primary transfer position.

A circumferential velocity of the image carrier and a circumferential velocity of the intermediate transfer member may be different at the primary transfer position. Here, the image carrier and the intermediate transfer member may be driven by a common drive source.

With the above configurations, since the latent image formation is started after a toner line (bias application toner line) passes through the primary transfer position, the image formation is not affected by the rotation velocity fluctuations of the image carrier due to the toner line, thereby avoiding the occurrence of the banding stain problem.

In a case where the cleaner is a blade member, and the transferer applies a bias voltage to the intermediate transfer member, it is preferable that a nip width formed between the intermediate transfer member and the image carrier at the primary transfer position is larger than a thickness of the blade member.

It is further preferable that the nip width is two to fifth times of the thickness.

In a case where the cleaner is a brush member, and the transferer applies a bias voltage to the intermediate transfer member, it is preferable that a nip width formed between the intermediate transfer member and the image carrier at the primary transfer position is larger than a circumferential length of a contact area between the brush member and the intermediate transfer member.

It is further preferable that the nip width is two to fifth times of the circumferential length.

With the above configurations, the rotation velocity fluctuations of the image carrier due to the toner line can be suppressed.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention will be described below in detail with reference to the accompanying drawings.

As shown inFIG. 11, an image forming apparatus according to one embodiment of the invention comprises a body casing2provided with a sheet discharging tray21formed at the top portion thereof and a front door cover2a. In the body casing2, there are disposed a rotary development unit8in which plural toner cartridges6C,6M,6Y,6K are detachably mounted, a photosensitive drum3on which an electrostatic latent image is formed and a toner image is developed, an intermediate transfer unit onto which the toner image on the photosensitive drum3is transferred, a control unit to control respective driving motors and bias voltages, a power supply16, a sheet feeding tray17containing recording media (e.g., sheets of paper), a fuser20to fix a toner image on a recording medium, etc. Also, inside the front cover2ais provided a medium transporter22to transport a recording medium from the sheet feeding tray17to the fuser20through a secondary transfer roller15. In addition, each unit is detachably provided in the main body, so that each unit is independently repaired or replaced during a maintenance work.

The photosensitive drum3, serving as an image carrier, includes a conductive base material of a thin cylindrical shape, and a photosensitive layer formed on the surface thereof. Around the periphery of the photosensitive drum3are provided a charger4to uniformly charge the outer circumferential surface of the photosensitive drum3, an exposer (or an image writer)5to form an electrostatic latent image on the photosensitive drum3, the rotary development unit8to develop the electrostatic latent image, an intermediate transfer belt12onto which the toner image on the photosensitive drum3is primarily transferred, the intermediate transfer unit to perform primary transfer for the toner image to be transferred onto the intermediate transfer belt12, a cleaner7to clean the surface of the photosensitive drum3after the primary transfer is performed.

The intermediate transfer unit comprises: a driving roller10; a follower roller11; the intermediate transfer belt12, which is an endless belt stretched by these rollers10and11and is circulated in a direction indicated by an arrow inFIG. 11; a primary transfer roller13, provided oppositely to the photosensitive drum3on the back side of the intermediate transfer belt12, to perform primary transfer for the toner image on the photosensitive drum3to be transferred onto the intermediate belt12; a belt cleaner (cleaning blade)14to remove residual toner on the intermediate transfer belt12; and the secondary transfer roller15, provided oppositely to the driving roller10, to perform secondary transfer for a toner image formed on the intermediate transfer belt12to be transferred onto a recording medium (e.g., a sheet of paper).

The power supply16is provided below the exposer5, and the sheet feeding tray17is provided at the bottom of the body casing2. A recording medium in the sheet feeding tray17is transported to the sheet discharge tray21by way of a pick-up roller18, a transportation path19, the secondary transfer roller15, and the fuser20. A holder17bis attached to the sheet feeding tray17so as to be pulled out forward from the apparatus, and in order to handle a sheet of paper of a larger size, an auxiliary tray17a, protruding from behind the apparatus, is attached so as to be pulled out from the apparatus.

With the image forming apparatus1configured as described above, when an image forming signal is inputted into the exposer5, the photosensitive drum3, a developing roller6aprovided with each toner cartridge in the rotary development unit8, and the intermediate transfer belt12are driven under the control of the control unit, and the outer circumferential surface of the photosensitive drum3is uniformly charged first by the charger4. Subsequently, the surface of the photosensitive drum3is selectively exposed by the exposer5according to image information to form an electrostatic latent image.

In this instance, the rotary development unit8is rotated such that the developing roller6aof the toner cartridge is brought into contact with the photosensitive drum3. The electrostatic latent image is made visible as a toner image formed on the photosensitive drum3. The toner image is transferred onto the intermediate transfer belt12by the primary transfer roller13to which a primary transfer voltage of a polarity opposite to a toner charging polarity is being applied. Toner remaining on the photosensitive drum3is then removed by the cleaner7.

In a full-color image forming apparatus, toner cartridges6Y,6M,6C, and6K, respectively corresponding to yellow Y, magenta M, cyan C, and black K, are detachably mounted to the rotary development unit8. During an image forming operation, the surface of the photosensitive drum3is selectively exposed by the exposer5according to image information of a first color, for example, yellow Y, to form an electrostatic latent image of yellow Y. In this instance, the rotary development unit8moves by rotation in such a manner that the developing roller6aof the toner cartridge6Y of yellow Y abuts on the photosensitive drum3for a toner image of the electrostatic latent image of yellow Y to be formed thereon. The toner image is subsequently transferred onto the intermediate transfer belt12by the primary transfer roller13to which the primary transfer voltage of a polarity opposite to a toner charting polarity is being applied.

During the foregoing operations, the belt cleaner14and the secondary transfer roller15are kept spaced apart from the intermediate transfer belt12. By repetitively performing a series of these operations for image forming signals of a second color, a third color, and a fourth color, toner images of yellow Y, magenta M, cyan C, and black K corresponding to the contents of the respective image forming signals are transferred from the photosensitive drum3to be superposed sequentially on the intermediate transfer belt12, and as a result, a full-color image of four colors is formed thereon. The color order of the development is arbitrary.

At a timing at which a full-color image in which the respective colors of toner images are superposed reaches the secondary transfer roller15, a recording medium in the feed tray17is transferred from the pick-up roller18to the secondary transfer roller15via the transportation path19, and the secondary transfer roller15is pressed against the intermediate transfer belt12while being applied with a secondary transfer voltage. The full-color toner image on the intermediate transfer belt12is thereby transferred onto the recording medium by the secondary transfer roller15. When the recording medium, onto which the full-color toner image has been transferred in this manner, is transferred to the fuser20via the medium transporter22, the toner image on the recording medium is heated and pressurized by the fuser20to be fixed thereon. Toner remaining on the intermediate transfer belt12is then removed by the belt cleaner14.

In the case of double-sided printing, a recording medium coming out from the fuser20is switched back so that the rear end comes to the forefront, and is fed to the secondary transfer roller15again by way a double-sided printing transportation path in the medium transporter22. A full-color toner image on the intermediate transfer belt12is then transferred onto the other side of the recording medium, and is fixed thereon through heating and pressuring by the fuser20again, after which the recording medium is discharged onto the sheet discharge tray21.

In this embodiment, four toner cartridges6Y,6M,6C, and6K are mounted to the rotary development unit8to constitute a full-color image forming apparatus of four colors. However, the toner cartridge6K for the toner of black K alone may be mounted to constitute a monochrome image forming apparatus, in which the toner cartridge6K stands by at the stand-by position (home position), and when an image is formed, the toner cartridge6K of black K moves by rotation from the stand-by position to the developing position to develop an electrostatic latent image on the photosensitive drum3into a toner image. This allows the use of the rotary development unit8of the same design specifications for both full-color and monochrome images. By using the common specifications for full-color and monochrome images, it is possible to remarkably save the maintenance, design, and manufacturing costs in comparison with a case where an image forming apparatus is designed separately for a full-color image and a monochrome image.

A control sequence according to a first embodiment of the invention will be described with reference toFIGS. 12 to 14.

The surface of the image carrier3is uniformly charged by the charger4, the image signal is turned on synchronously with the vertical synchronizing (vsync) signal, selective exposure according to image information of a first color is performed on the surface of the image carrier3to form an electrostatic latent image. At this time, the rotary developing unit8rotates so that the developing roller6afor the first color comes into contact with the image carrier3, a toner image of the first color is formed on the image carrier3and transferred to the intermediate transfer member12by the primary transfer roller13on which a primary transfer voltage is applied.

Incidentally, the belt cleaner14and the secondary transfer roller15are separated from the intermediate transfer member12. The image of four full colors is formed by the toner images according to the contents of the respective image forming signals being transferred and overlapped from the image carrier3to the intermediate transfer member12in sequence by performing the series of procedures repeatedly for a second color, a third color, and a fourth color of the image forming signal. Then, at a timing when the image formed by superimposing the toner images in the respective colors reaches the secondary transfer roller15, the recording medium is carried to the secondary transfer roller15, the secondary transfer roller15is pressed against the intermediate transfer belt12, and the secondary transfer voltage is applied thereon so that the toner image on the intermediate transfer belt12is transferred to the recording medium by the secondary transfer roller15.

When the secondary transfer operation is completed, the secondary transfer roller15is separated from the intermediate transfer member12and, at this moment, a toner line is attached to the intermediate transfer member12. However, in this embodiment, as shown in detail inFIG. 12, the image signal for forming an electrostatic latent image is turned on after a predetermined time period t has elapsed after the position C1on the intermediate transfer member12, from which the secondary transfer roller15is separated, reaches the primary transfer position T1. Since the image is written (an arrow WR inFIG. 13) after the toner line24has passed through the primary transfer position T1(an arrow A inFIG. 13), occurrence of the banding stain may be prevented without being affected by the rotation velocity fluctuations of the image carrier.

A control sequence according to a second embodiment of the invention will be described with reference toFIGS. 15 to 17.

As shown inFIG. 15, the surface of the image carrier3is uniformly charged by the charger4, the image signal is turned on synchronously with the vsync signal, selective exposure according to image information of the first color is performed on the surface of the image carrier3to form an electrostatic latent image. Incidentally, the rotary developing unit8rotates so that the developing roller6afor the first color comes into contact with the image carrier3, and the toner image of the first color is formed on the image carrier3.

When the developing roller6acomes in contact with the position C2on the image carrier3corresponding to the non-image area on the intermediate transfer member12, the toner line24is attached to the image carrier3due to the impact of the developing roller6a. However, in this embodiment, as shown in detail inFIG. 16, the image signal for forming the electrostatic latent image is turned on after a predetermined time period t has elapsed after the position C2where the developing roller6acomes into contact with the image carrier3reaches the primary transfer portion T1. Since the image is written (an arrow WR inFIG. 17) after the toner line24has passed through the primary transfer position T1(an arrow B inFIG. 17), occurrence of the banding stain may be prevented without being affected by the rotation velocity fluctuation of the image carrier.

A control sequence according to a third embodiment of the invention will be described with reference toFIGS. 18 to 20.

As shown inFIG. 18, the surface of the image carrier3is uniformly charged by the charger4, the image signal is turned on synchronously with the vertical synchronizing (vsync) signal, selective exposure according to image information of a first color is performed on the surface of the image carrier3to form an electrostatic latent image. At this time, the rotary developing unit8rotates so that the developing roller6afor the first color comes into contact with the image carrier3, a toner image of the first color is formed on the image carrier3and transferred to the intermediate transfer member12by the primary transfer roller13on which a primary transfer voltage is applied.

Incidentally, the belt cleaner14and the secondary transfer roller15are separated from the intermediate transfer member12. The image of four full colors is formed by the toner images according to the contents of the respective image forming signals being transferred and overlapped from the image carrier3to the intermediate transfer member12in sequence by performing the series of procedures repeatedly for a second color, a third color, and a fourth color of the image forming signal. Then, at a timing when the image formed by superimposing the toner images in the respective colors reaches the secondary transfer roller15, the recording medium is carried to the secondary transfer roller15, the secondary transfer roller15is pressed against the intermediate transfer belt12, and the secondary transfer voltage is applied thereon so that the toner image on the intermediate transfer belt12is transferred to the recording medium by the secondary transfer roller15.

Subsequently, toner remaining on the intermediate transfer belt12is removed by the belt cleaner14. The belt cleaner14is separated from the intermediate transfer member12before the trailing edge of the image in the third color comes into contact with the intermediate transfer member12after having passed the contact position of the belt cleaner14, and the leading edge of the image which corresponds to first color of the next image formed in the subsequent process reaches the contact position of the belt cleaner14.

When the belt cleaner14is separated from the intermediate transfer member12, the toner line24is generated. However, in this embodiment, as shown in detail inFIG. 19, the image signal for forming an electrostatic latent image is turned on after a predetermined time period t has elapsed after the position C1on the intermediate transfer member12, form which the belt cleaner14is separated, reaches the first transfer position T1. Since the image is written (an arrow WR inFIG. 20) after the toner line24has passed through the primary transfer position T1(an arrow A inFIG. 20), occurrence of the banding stain may be prevented without being affected by the rotation velocity fluctuations of the image carrier3.

The present invention is not limited to the aforementioned embodiments, and various modifications may be made. For example, although examples of separation of the second transfer roller15, contact of the developing roller6aand separation of the belt cleaner14have been described in the aforementioned embodiments, since the toner mark is generated by the contact of the secondary transfer roller15or the belt cleaner14and separation of the developing roller6a, the invention may be applied to such cases. Furthermore, since attachment of toner mark to the image carrier3may be generated either in the case where the cleaner7comes into the image carrier3or separates therefrom, and in the case where a brush member serving as the charger4starts or stops driving, the present invention may be applicable to such cases as well.

In short, the image forming apparatus having a member which comes into and away from contact or a member which is driven or stopped in the non-image area on the intermediate transfer member12or in the area of the image carrier3corresponding thereto is characterized in that latent image is formed by the exposer5after the position corresponding to the downstream side of the position on the intermediate transfer member12which performs any one of the actions of separation, contact, drive or stop has passed through the primary transfer position T1.

A control sequence according to a fourth embodiment of the invention will be described with reference toFIGS. 21A to 22.

As shown inFIG. 21A, the surface of the image carrier3is uniformly charged by the charger4, the image signal is turned on synchronously with the vertical synchronizing (vsync) signal, selective exposure according to image information of a first color is performed on the surface of the image carrier3to form an electrostatic latent image. At this time, the rotary developing unit8rotates so that the developing roller6afor the first color comes into contact with the image carrier3. The AC-superimposed bias is applied to the developing roller6aso that a toner image of the first color is formed on the image carrier3and transferred to the intermediate transfer member12by the primary transfer roller13on which a primary transfer voltage is applied.

When the developing bias is applied to the image carrier3through the developing roller6a, the toner line24is generated. However, in this embodiment, as shown in detail inFIG. 21B, the image signal for forming an electrostatic latent image is turned on after a predetermined time period t has elapsed after the position C3on the image carrier, to which the developing bias is applied, reaches the first transfer position T1. Since the image is written (an arrow WR inFIG. 22) after the toner line24has passed through the primary transfer position T1(an arrow C inFIG. 22), occurrence of the banding stain may be prevented without being affected by the rotation velocity fluctuations of the image carrier3.

Next, a fifth embodiment of the invention will be described.FIG. 23shows a drive system of the image carrier3and the intermediate transfer member12. A drive gear3ais connected to one end of the image carrier3, and the drive gear3ais connected to an output gear23aof a drive motor23via transmission gears3b,3c. A drive gear10ais connected to one end of the driving roller10for circulating the intermediate transfer member12, and is connected to the output gear of the drive motor23via a transmission gear10b.

As shown inFIG. 24A, the primary transfer roller13is connected to a primary transfer power source26. As shown inFIG. 24B, the primary transfer roller13may be substituted by a blade member13a.

In this embodiment, as shown inFIG. 25, it is configured that the width L of the toner line24(the thickness W of the belt cleaner14) is smaller than the width N of the primary transfer position T1. In a case where the toner line24exists between the intermediate transfer member12and the image carrier3, the friction force between them suddenly reduces. However, in this embodiment, since there is a portion where no toner exists within the nip width N, the friction force can be maintained in the primary transfer position T1. On the other hand, as regards the portion having no toner, since the image carrier3and the intermediate transfer member12are in direct contact with each other, an electrostatic adsorptive force due to primary transfer bias increases. Therefore, the rotation velocity fluctuations of the image carrier3caused by slippage of the image carrier3can be reduced.

Similarly to the third embodiment, the image signal for forming an electrostatic latent image is turned on after a predetermined time period t has elapsed after the position C1on the intermediate transfer member12, form which the belt cleaner14is separated, reaches the first transfer position T1. Incidentally, since the nip width N at the primary transfer position T1is made larger than the width L of the toner line24(the thickness W of the belt cleaner14) and the primary transfer bias from the power source26is always applied, even when the toner line24passes through the primary transfer position T1, the friction force between the image carrier3and the intermediate transfer member12does not suddenly change, and hence the rotation velocity fluctuations of the image carrier3can be reduced.

If the rotation velocity fluctuations of the image carrier3can be sufficiently reduced, the image signal may be turned on while the toner line24passes the primary transfer position T1.

FIG. 26shows experimental data that the rotation velocity fluctuations of the image carrier3are observed under a condition that the nip width N at the primary transfer position T1is made larger than twice of the thickness W of the belt cleaner14. Arrows designate timings at which the toner line24passes through the primary transfer position T1. It is apparent that the rotation velocity fluctuation is suppressed at those timings.

FIG. 27shows experimental data in which the lateral axis represents values of N/W and the vertical axis represents the peak values of rotation velocity fluctuations of the image carrier3. The peak value is an average value of five peak values during the image forming operation for magenta. It is apparent that the rotation velocity fluctuation can be reduced in a rage of N/W between 2 and 5.

Next, a sixth embodiment of the invention will be described with reference toFIG. 28. In this embodiment, a cleaning brush14ais used as the belt cleaner14. In this case, the width L of the toner line24is coincident with the contact width W′ of the cleaning brush14a. The contact width W′ is defined as a width of an area from a position that the tip ends of the cleaning brush14acome in contact with the surface of the image carrier3to a position that the tip ends separate from the surface. Also in this case, the rotation velocity fluctuation can be reduced in a rage of N/W′ between 2 and 5.

Next, a seventh embodiment of the invention will be described. As is explained with reference toFIG. 23, the image carrier3and the intermediate transfer member12are driven by the common drive motor23via the gear trains respectively. With this structure, in this embodiment, the circumferential velocities of the image carrier3and the intermediate transfer member12are substantially equalized at the primary transfer position T1. However, the image carrier3and the intermediate transfer member12are driven by individual motors only if the circumferential velocities of the image carrier3and the intermediate transfer member12are substantially equalized.

Similarly to the third embodiment, the image signal for forming an electrostatic latent image is turned on after a predetermined time period t has elapsed after the position C1on the intermediate transfer member12, form which the belt cleaner14is separated, reaches the first transfer position T1. Incidentally, since the nip width N at the primary transfer position T1is made larger than the width L of the toner line24(the thickness W of the belt cleaner14) and the circumferential velocities of the image carrier3and the intermediate transfer member12are substantially equalized at the primary transfer position T1, even when the toner line24passes through the primary transfer position T1, the friction force between the image carrier3and the intermediate transfer member12does not suddenly change, and hence the rotation velocity fluctuations of the image carrier3can be reduced.

If the rotation velocity fluctuations of the image carrier3can be sufficiently reduced, the image signal may be turned on while the toner line24passes the primary transfer position T1.

FIG. 29shows experimental data that the rotation velocity fluctuations of the image carrier3are observed under a condition that the circumferential velocities of the image carrier3and the intermediate transfer member12are substantially equalized at the primary transfer position T1. Arrows designate timings at which the toner line24passes through the primary transfer position T1. It is apparent that the rotation velocity fluctuation is eliminated at those timings.

FIG. 30shows experimental data in which the lateral axis represents values of N/W and the vertical axis represents the peak values of rotation velocity fluctuations of the image carrier3. The peak value is an average value of five peak values during the image forming operation for magenta. As to the material of the intermediate transfer member12, the material A is PET coated with conductive material and fluorine contained resin, and the material B includes conductive material and polycarbonate. It is apparent that the rotation velocity fluctuations can be reduced within the range of difference in circumferential velocities of ±0.3%.

Although the electrostatic latent image is formed on the image carrier3by the exposer5in the above described embodiments, it is also possible to form an electrostatic latent image by a charge injection device. Although the intermediate transfer belt has been described in the above described embodiments, it is also possible to apply it to an intermediate transfer drum, which is defined as the intermediate transfer member in the present invention.