Image forming apparatus

Upon occurrence of jamming in an apparatus, an electrification unit 4 ceases electrification of a surface of a photosensitive drum 3 and, simultaneously, an exposure unit 5 starts to expose the entire surface in a width direction to light. Further, a development unit 6 forms a toner adhesion region T on the photosensitive drum 3 and, also, stops a drum motor and a belt motor in such a manner that the toner adhesion region T stops at a transfer nip section N. Thus, it is possible to alleviate a rub between the photosensitive drum 3 and a conveyance belt 8 at the time when jamming is cleared, and to suppress generation of scratches on the photosensitive drum 3 and a surface of the conveyance belt 8.

BACKGROUND OF THE INVENTION

This application is based on Japanese Patent Application No. 2004-345223 filed on Nov. 30, 2004.

FIELD OF THE INVENTION

The present invention relates to an image forming apparatus such as a copier, a facsimile or a printer. In particular, the present invention relates to a method for preventing a photosensitive drum and a surface of a belt from being damaged upon occurrence of jamming in an image formation section.

DESCRIPTION OF THE PRIOR ART

In an image forming apparatus using electrophotography, such as a copier, a printer or a facsimile, a powdery development agent (hereinafter, referred to as “toner”) is mainly used, and the following process is generally performed. That is, a powdery development agent (hereinafter, referred to as “toner”) is mainly used, and an electrostatic latent image formed on an image carrier such as a photosensitive drum is visualized by the toner. This toner image is transferred onto a recording medium and, thereafter, is fixated.

FIG. 5is a sectional view showing an overall configuration of a conventional image forming apparatus. In an image forming apparatus1, an image formation section P is disposed above a conveyance belt8. The image formation section P forms a predetermined image by steps of electrification, exposure, development and transfer. Above the image formation section P, there is placed an image read section2for reading an original image by scanning.

The image formation section P includes a photosensitive drum3for carrying a visible image (toner image). The toner image formed on the photosensitive drum3is transferred onto a sheet (recording medium)7carried and conveyed by the conveyance belt8moving adjacent to the image formation section P and, further, is fixated on the sheet7at a fixation section9. Thereafter, the sheet7is ejected from an apparatus main body. An image forming process for the photosensitive drum3is performed while the photosensitive drum3is rotated in a clockwise direction inFIG. 5.

Next, specific description will be given of the image forming process in the image formation section P. Around and above the photosensitive drum3disposed rotatably, there are provided an electrification unit4for electrifying the photosensitive drum3, an exposure unit (laser scanning unit or the like)5for exposing the photosensitive drum3to image information, a development unit6for forming a toner image on the photosensitive drum3, and a cleaning section10for removing a development agent (toner) remaining on the photosensitive drum3.

First, the electrification unit4evenly electrifies a surface of the photosensitive drum3. Then, an electrostatic image according to an image signal is formed on the photosensitive drum3by a laser beam from the exposure unit5based on original image data read by the image read section2. The development unit6is filled with a predetermined amount of toner by a toner container11. This toner is supplied onto the photosensitive drum3by the development unit6and electrostatically adheres to the photosensitive drum3, so that a toner image is formed according to the electrostatic latent image formed by exposure from the exposure unit5.

The sheet7on which the toner image is transferred is accommodated in plural feed cassettes12a,12band12cfor accommodating sheets and a stack bypass (manual feed tray)12dprovided thereabove, is supplied onto the conveyance belt8through feed rollers13and a resist roller14, and is conveyed to a position of the photosensitive drum3. The conveyance belt8is formed from a dielectric resin sheet, and examples thereof include an endless belt in which both ends thereof are superposed on each other and bonded to each other, and a seamless belt having no seam.

The conveyance belt8extends between a conveyance roller15on an upstream side and a conveyance roller16on a downstream side. When the conveyance belt8starts to rotate in a counterclockwise direction, the sheet7is conveyed from the resist roller14onto the conveyance belt8. At this time, an image writing signal is turned on; thus, an image is formed on the photosensitive drum3at a predetermined timing. Then, an electric field is given to a lower portion of the photosensitive drum3by a transfer roller17to which a predetermined transfer voltage is applied, so that the toner image on the photosensitive drum3is transferred onto the sheet7. This sheet7is held on the conveyance belt8by an electrostatic suction force. The conveyance belt8, the conveyance rollers15and16and the transfer roller17configure a transfer unit R for transferring a toner image onto the sheet7.

The sheet7on which the toner image is transferred is separated from the conveyance belt8and, then, is conveyed toward the fixation section9. After completion of the transfer of the toner image, in order to form a subsequent new electrostatic latent image, the cleaning section10removes toner remaining on the surface of the photosensitive drum3. The sheet7conveyed from the conveyance belt8to the fixation section9is heated and pressurized by a fixation roller9aand the toner image is fixated on the surface of the sheet7, so that a predetermined image is formed. Thereafter, the sheet7having the image formed thereon is ejected to an ejection tray19by an ejection roller18.

In the aforementioned image forming apparatus, the sheet7is sometimes jammed (hereinafter, referred to as “jamming”) during the image forming process due to nonuniform rotation of the photosensitive drum3and conveyance belt8and deviation in conveyance timing of the sheet7. In this case, the image forming process is automatically ceased and the image forming apparatus notifies a user of occurrence of jamming and a site where the jamming occurs. The user needs to remove the jammed sheet so as to restore the apparatus. For example, in the case that the sheet7is jammed between the photosensitive drum3and the conveyance belt8, the transfer unit R is separated from the photosensitive drum3to form a space between the conveyance belt8and the photosensitive drum3; thus, the jamming is cleared.

However, if foreign matters such as sand grains or chips (shavings) are attached onto the conveyance belt8, at the time when the transfer unit R is separated from the transfer belt3so as to remove the jammed sheet7and, then, is returned to its original position, the photosensitive drum3and the conveyance belt8are rubbed against each other, so that fine scratches is possibly made on the surface of the photosensitive drum3. The scratches on the surface of the photosensitive drum3appear as black points (color points) or fine lines in a white portion or a halftone of an image to be formed, and exert an adverse influence such as deterioration of image quality. This phenomenon is especially conspicuous in the case that the photosensitive drum3is made of amorphous silicon and the conveyance belt8is made of hard resin.

In order to solve the aforementioned problem, as disclosed in JP-A 11-219044 (1999), a conventional image forming apparatus has the following configuration. That is, a transfer roller for pressing a conveyance belt against a photosensitive drum is supported by an elastic member such as a spring, so that an impact generated when a transfer unit is retuned to its original position after jamming is cleared is alleviated. Further, JP-A 2000-250326 discloses an image forming apparatus having the following configuration. That is, a voltage having the same polarity as that of toner is applied to a conveyance belt during a period of driving of a conveyance belt other than a period of image formation, contaminated toner in the case that a sheet conveyance timing is delayed due to occurrence of jamming and a toner image is directly transferred on the conveyance belt can be removed quickly.

According to the configuration of JP-A 11-219044 (1999) in which the transfer roller is supported by the elastic member, a rub between the surface of the photosensitive drum and the conveyance belt at the time when the transfer unit is returned to its original position can be suppressed. However, since the conveyance belt is always pressed against the photosensitive drum, the rub therebetween cannot be avoided completely. In addition, according to the method of JP-A 2000-250326, contamination of the conveyance belt due to toner can be prevented; however, a rub between the surface of the photosensitive drum and the conveyance belt cannot be effectively reduced.

SUMMARY OF THE INVENTION

In view of the aforementioned problems, an object of the present invention is to provide an image forming apparatus capable of smoothly clearing jamming without damaging a photosensitive drum or a surface of a belt upon occurrence of the jamming in an image formation section.

In order to achieve the aforementioned object, the present invention provides an image forming apparatus comprising: an image formation section including a photosensitive drum, an electrification unit for evenly electrifying a surface of the photosensitive drum, an exposure unit for writing an electrostatic latent image onto the surface of the photosensitive drum, and a development unit for adhering toner to the surface of the photosensitive drum to form a toner image according to the electrostatic latent image; a transfer unit having a conveyance belt for conveying a recording medium and a transfer roller arranged so as to oppose the photosensitive drum with the conveyance belt interposed therebetween, and for transferring the toner image formed in the image formation section onto the recording medium in contact with the photosensitive drum; detection means for detecting occurrence of jamming in the apparatus; and control means for controlling operations of the image formation section and the transfer unit, wherein the transfer unit or the photosensitive drum is manually separated to thereby clear jamming. Herein, upon detection of jamming by the detection means, the control means adheres the toner to a predetermined region on the photosensitive drum and, also, ceases rotation of the photosensitive drum and driving of the conveyance belt in such a manner that the toner adhesion region stops at a contact position between the photosensitive drum and the conveyance belt.

With this configuration, when the image forming process is ceased due to occurrence of jamming, the toner layer is always present in the contact position between the photosensitive drum and the conveyance belt. In the case that the transfer unit is separated from the photosensitive drum to clear the jamming and, then, is returned to its original position, it is possible to alleviate a rub between the photosensitive drum and the conveyance belt and to suppress generation of scratches on the photosensitive drum and the surface of the conveyance belt.

According to the present invention, in the image forming apparatus having the aforementioned configuration, the toner adhesion region is formed by exposure of the predetermined region of the surface of the photosensitive drum by the exposure unit and/or cessation of electrification by the electrification unit.

With this configuration, the toner adhesion region is formed by the exposure of the predetermined region of the surface of the photosensitive drum by the exposure unit and/or the cessation of electrification by the electrification unit, so that it is possible to adhere a predetermined amount of toner to the predetermined region of the surface of the photosensitive drum in accordance with procedures similar to a case of normal toner image formation.

According to the present invention, in the image forming apparatus having the aforementioned configuration, the transfer roller is applied with a voltage having the same polarity as that of toner during a period from detection of jamming by the detection means until stoppage of the photosensitive drum.

With this configuration, the voltage having the same polarity as that of toner is applied to transfer roller during the period from detection of jamming by the detection means until stoppage of the photosensitive drum. As a result, in the case that the photosensitive drum and the conveyance belt are stopped with a sheet jammed therebetween, it is possible to prevent the toner adhering to the surface of the photosensitive drum from moving toward the conveyance belt and, even after removal of the sheet, to effectively prevent generation of scratches because of the presence of the toner layer at the contact position. In addition, since unfixated toner does not adhere onto a sheet or the conveyance belt, there is no fear that the interior of the apparatus and the hands and fingers of a user are contaminated when user clears jamming.

According to the present invention, in the image forming apparatus having the aforementioned configuration, a timing at which the driving of the conveyance belt is ceased is in synchronization with a timing at which the rotation of the photosensitive drum is ceased.

With this configuration, the timing at which the driving of the conveyance belt is ceased is made synchronous with the timing at which the rotation of the photosensitive drum is ceased, so that it is possible to eliminate the peripheral speed difference between the conveyance belt and the photosensitive drum and to effectively reduce friction at a contact position between the conveyance belt and the photosensitive drum.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, description will be given of embodiments of the present invention with reference to the drawings.FIG. 1is a block diagram showing a configuration of an image forming apparatus according to the present invention. Common components with the conventional example inFIG. 5are denoted by the same reference numerals. An image forming apparatus1includes an image read section2, an image formation section P, a transfer unit R, a control section32, a storage section33, an operation panel34and a jamming detection sensor35.

The image read section2includes a scanning optical system equipped with a scanner lamp for illuminating an original upon copying and a mirror for changing an optical path of light reflected from the original, a condenser lens for condensing the light reflected from the original to form an image, a CCD for converting formed image light into an electric signal, and the like. An image signal read by the image read section2is sent to the control section32, is appropriately subjected to image processing such as gradation processing, and is converted into image data.

The image formation section P includes a photosensitive drum3, an electrification unit4, an exposure unit5, a development unit6and the like, and forms an electrostatic latent image on the photosensitive drum3on the basis of the image data converted in the control section32. A drum motor36is provided for rotationally driving the photosensitive drum3. The control section32transmits a control signal to the drum motor36to control rotation and stoppage of the photosensitive drum3. The control section32also controls the respective components in the image forming apparatus, such as the image read section2, the electrification unit4, the exposure unit5and the development unit6, in accordance with a preset program.

The transfer unit R includes a conveyance belt8, a transfer roller17and the like. When a high-voltage power supply circuit (not shown) supplies a transfer bias voltage to the transfer roller17, the transfer unit R transfers, onto a sheet, a toner image developed on the photosensitive drum3. A belt motor37is provided for rotationally driving the conveyance belt8. The control section32transmits a control signal to the belt motor37to control rotation and stoppage of the conveyance belt8.

The storage section33stores control programs for the respective components in the image forming apparatus, used by the control section32. As will be described later, the storage section33also stores an execution time of a step of adhering toner onto the photosensitive drum3and stoppage timings of the drum motor36and belt motor37at the time when jamming is detected by the jamming detection sensor35. The operation panel34includes an operation key (not shown) by which a user performs setting of the functions of the apparatus, printing conditions and the like, and a display part (not shown) for displaying setting conditions, the status of the apparatus, and the like.

The jamming detection sensors35are provided at plural portions on a sheet conveyance path in the image forming apparatus1. Each jamming detection sensor35detects passage of a sheet to determine whether or not jamming occurs in the apparatus and, also, transmits the detection result to the control section32. The control section32controls the electrification unit4, the exposure unit5, the development unit6and the transfer roller17on the basis of the detection result of the jamming detection sensor35, forms a toner adhesion region at a predetermined region on the photosensitive drum3, and stops the photosensitive drum3and the conveyance belt8at a predetermined timing.

As the jamming detection sensor35, various sensors capable of detecting passage of a sheet can be used, and examples thereof include an arm-type sensor for detecting a sheet in such a manner that the sheet directly abuts against the sensor, a reflection-type sensor having a light emitting part for emitting light onto a sheet and a light receiving part for detecting light reflected from the sheet, and the like. The detection result of the jamming detection sensor35is displayed on the display part of the operation panel34, and the user receives notification about occurrence of jamming and a site where the jamming occurs.

The present invention has a feature in that, if jamming occurs in the apparatus, a region where toner adheres to a predetermined position on the photosensitive drum3(hereinafter, referred to as “toner adhesion region”) is formed and, also, the toner adhesion region is stopped at a contact position (transfer nip portion) between the photosensitive drum3and the conveyance belt8. With this feature, when the image forming process is ceased after occurrence of jamming, a toner layer is always present in the transfer nip portion. In the case that the transfer unit R is separated from the photosensitive drum3to clear the jamming and the transfer unit R is again brought into contact with the photosensitive drum3, it is possible to alleviate a rub between the photosensitive drum3and the conveyance belt8and to suppress generation of scratches on the photosensitive drum3and the surface of the conveyance belt8.

The user is not always necessarily to accurately grasp the position where the jamming occurs. It is considered that even when the jamming occurs at a position other than the image formation section P, the transfer unit R is separated from the photosensitive drum3. Therefore, it is preferred that the toner adhesion region is formed irrespective of the position where the jamming occurs and is stopped at the contact position between the photosensitive drum3and the conveyance belt8.

FIGS. 2A to 2Care schematic views each showing states of the image formation section and transfer unit during a period from detection of jamming until cessation of a job. For the sake of simplification of description, the cleaning section10and the conveyance rollers15and16will not described herein. With reference toFIG. 1andFIGS. 2A to 2C, specific description will be given of formation of a toner adhesion region in the image forming apparatus according to the present invention.

FIG. 2Ashows a state of the image formation section P immediately after detection of jamming. InFIG. 2A, the photosensitive drum3rotates in the direction of arrow A and the conveyance belt8is driven in the direction of arrow B. When the jamming detection sensor35detects occurrence of jamming and the control section32receives the detection result, the control section32controls the electrification unit4so as to cease electrification of the surface of the photosensitive drum3and, simultaneously, controls the exposure unit5so as to expose, to light, the entire surface of the photosensitive drum3in a width direction.

More specifically, in the surface of the photosensitive drum3, a region (region C inFIG. 2A) passing the exposure unit5after detection of jamming is not electrified. From this state, the photosensitive drum3further rotates in the direction of arrow A and toner adheres to the region C by the development unit6, so that a toner adhesion region is formed. In addition, the transfer roller17is applied with a positive bias voltage having the same polarity as that of the toner, and the toner adhering to the photosensitive drum3does not move onto the conveyance belt8.

FIG. 2Bshows a state that, after lapse of a predetermined time from the state ofFIG. 2A, the drum motor36for driving the photosensitive drum3and the belt motor37for driving the conveyance belt8are stopped. Toner successively adheres to the region C passing the development unit6; thus, a toner adhesion region T is formed. The photosensitive drum3and the conveyance belt8are not stopped immediately after stoppage of the corresponding motors36and37, but are stopped after moving in the corresponding directions of arrows A and B by a predetermined distance due to inertia.

Therefore, in consideration of the movement distance due to inertia, it is necessary to stop the drum motor36and the belt motor37before the toner adhesion region T arrives at the transfer nip portion N. At this time, it is more preferred that the photosensitive drum3and the conveyance belt8are controlled in such a manner that the respective stoppage timings are in synchronization with each other. Thus, it is possible to eliminate the peripheral speed difference between the photosensitive drum3and the conveyance belt8in stoppage and to further reduce the friction in the transfer nip portion N.

FIG. 2Cshows a state that, after further lapse of a predetermined time from the state inFIG. 2Bthat the drum motor36and the belt motor37are stopped, the photosensitive drum3and the conveyance belt8are stopped in such a manner that the toner adhesion region T is present in the transfer nip portion N. Simultaneously, exposure by the exposure unit5, application of the development bias to the development unit6, and application of the bias voltage to the transfer roller17are ceased. When the user manually separates the transfer unit R to clear the jamming and, then, returns the transfer unit R to its original position in this state, the toner layer alleviates the rub between the photosensitive drum3and the conveyance belt8. Herein, the toner on the surface of the photosensitive drum3is recovered by the cleaning section10(seeFIG. 5) at the restart of the image forming process after completion of clearing the jamming.

Herein, the following method is also considered. That is, a negative bias voltage having the polarity reverse to that of the toner is applied to the transfer roller17, the jamming is cleared in the state that the toner adheres to the conveyance belt8, and then the toner is recovered by belt cleaning means (not shown). However, in the case that the photosensitive drum3and the conveyance belt8are stopped in the state that jamming occurs therebetween, the toner is placed on the sheet. Therefore, the toner layer is not present at the time when the conveyance belt8is brought into contact with the photosensitive drum3after removal of the sheet; thus, there is a fear that scratches are made on the photosensitive drum3or the conveyance belt8. Since the unfixated toner is placed on the sheet or the conveyance belt8, the interior of the apparatus and the hands and fingers of the user are contaminated in the case that the user clears the jamming. In order to avoid this disadvantage, as shown inFIGS. 2A to 2C, it is preferred to apply the positive bias voltage having the same polarity as that of the toner.

Next, description will be given of the operations of the image forming apparatus according to this embodiment.FIG. 3is a flow chart showing operations from detection of jamming until cessation of a job, performed by the image forming apparatus according to the present invention.FIG. 4is a timing chart showing operations of the respective components in the image forming apparatus during a period from detection of jamming until cessation of a job. With reference toFIGS. 1 and 4, description will be given of control procedures of the image forming apparatus upon detection of jamming, in accordance with steps ofFIG. 3.

When the jamming detection sensor35detects jamming during the image forming process (step S1) and the detection result is transmitted to the control section32, the control section32transmits control signals to the respective components in the image forming apparatus and changes a control from an image forming mode to a toner placing mode. More specifically, the exposure upon formation of a latent image is changed to exposure, to light, of the entire region in the sub-scanning direction upon formation of a solid image in the exposure unit5(arrow A inFIG. 4), application of a voltage to the electrification unit4is ceased (arrow B inFIG. 4), and electrification to the surface of the photosensitive drum3is ceased (step S2). On the other hand, application of the bias voltage to the development unit6(arrow C inFIG. 4) is continuously performed even after detection of jamming (step S3). In addition, simultaneously with detection of jamming, application of a bias voltage (transfer bias) having a polarity reverse to that of the toner to the transfer roller17is changed to application of the bias voltage having the same polarity as that of the toner (arrow D inFIG. 4) (step S4).

When a predetermined time is elapsed from the detection of jamming and the photosensitive drum3rotates by a predetermined distance, a region exposed by the exposure unit5passes the development unit6. At this time, the toner adheres to the entire region of the surface of the photosensitive drum3, that has passed the exposure unit5after detection of jamming (step S5); thus, a toner adhesion region is formed. Next, in consideration of movement distances due to inertia of the photosensitive drum3and the conveyance belt8, the drum motor36and the belt motor37are stopped (arrow E inFIG. 4) at a timing that this toner adhesion region stops at a transfer nip portion (step S6).

Then, after lapse of a predetermined time, driving of the photosensitive drum3and that of the conveyance belt8are simultaneously ceased in the state that the toner adhesion region is present in the transfer nip portion (step S7). At the same time, application of the development bias to the development unit6and exposure by the exposure unit5are ceased (steps S8, S9), application of the bias voltage to the transfer roller17is ceased (step S10), and the toner placing mode is completed. Finally, occurrence of jamming and a site where the jamming occurs are displayed on the operation panel34in order to notify the user of the occurrence of jamming and the site where the jamming occurs (step S11).

By performing the control in accordance the aforementioned procedures, the apparatus is stopped in the state that the toner layer is always present in the transfer nip portion, upon occurrence of jamming. Therefore, it is possible to suppress generation of scratches on the photosensitive drum3and the surface of the conveyance belt8with reliability at the time when the transfer unit is detached to clear the jamming.

The present invention is not limited to the aforementioned embodiment, numerous modifications and variations can be devised without departing from the scope of the present invention. For example, in the aforementioned embodiment, jamming is cleared while the transfer unit R is separated from the photosensitive drum3. However, the photosensitive drum3may be made movable and may be separated from the transfer unit R. Also in the aforementioned embodiment, a toner adhesion region is formed by cessation of electrification by the electrification unit4and exposure by the exposure unit5. However, the toner adhesion region may be formed by either the cessation of electrification or the exposure. The present invention is applicable to various types of image forming apparatuses such as a digital copier using a conveyance belt in a transfer unit, a tandem color copier, an analog monochrome copier, a facsimile and a printer.

With reference toFIGS. 2A to 2C, specific description will given of controls from occurrence of jamming until cessation of a job in the image forming apparatus shown inFIG. 1. It is assumed herein that a image processing rate is 100 mm/sec, a distance from the electrification unit4to the exposure unit5on the outer peripheral face of the photosensitive drum3is 15 mm (150 msec), a distance from the exposure unit5to the development unit6is 5 mm (50 msec), a distance from the development unit6to the transfer nip portion N is 25 mm (250 msec), an inertial movement distance from stoppage of the drum motor36and belt motor37until stoppage of the photosensitive drum3and conveyance belt8is about 20 mm (about 400 msec).

First, the jamming detection sensor35detects jamming, simultaneously, no voltage is applied to the electrification unit4as shown inFIG. 2Aand exposure by the exposure unit5is changed to entire exposure. In addition, the development bias voltage is continuously applied to the development unit6and the polarity of the bias voltage to be applied to the transfer roller17is changed from a polarity reverse to that of the toner to the same polarity as that of the toner. Thus, the toner adhesion region T is formed on the surface of the photosensitive drum3.

Thereafter, the drum motor36and belt motor37are stopped in such a manner that the toner adhesion region T stops at the transfer nip portion N. In consideration of unevenness of the stoppage position of the photosensitive drum3, it is assumed that the photosensitive drum3is stopped at a position that the tip end of the toner adhesion region T passes the transfer nip portion N by 10 mm. Since the inertial movement distance of the photosensitive drum3is about 20 mm, it is sufficient that the drum motor36and the belt motor37are stopped at a time point that the tip end of the toner adhesion region T arrives at a position 10 mm before the transfer nip portion N.

The tip end of the toner adhesion region T is a portion passing the exposure unit5to be entirely exposed to light upon detection of jamming. Since the distance from the exposure unit5to the transfer nip portion N is expressed by the following equation: 5+25=30 mm, the distance from the exposure unit5to the position 10 mm before the transfer nip portion N is expressed by the following equation: 30−10=20 mm (200 msec); thus, each of the stoppage timings of the drum motor36and the belt motor37inFIG. 2Bis after 200 msec from the detection of jamming. Thereafter, the photosensitive drum3and the conveyance belt8rotate by 20 mm (about 400 msec) due to inertia and are stopped in the state shown inFIG. 2C.

That is, a time from detection of jamming until stoppage of the apparatus becomes about 600 msec, and the toner adhesion region T becomes a region (35 mm) obtained by subtracting the region (5 mm) from the exposure unit5to the development unit6from the region passing the development unit6during a period from detection of jamming until stoppage of the photosensitive drum3(movement distance until stoppage of motor: 20 mm+inertial movement distance: 20 mm=40 mm).

In the aforementioned example, the toner continuously adheres until the photosensitive drum3and the conveyance belt stop forming the toner adhesion region T. However, the size of the toner adhesion region T is not particularly limited as long as the toner adhesion region T stops at the transfer nip portion N with reliability. Therefore, application of the development bias to the development unit6may be ceased before stoppage of the photosensitive drum3and the conveyance belt8. In addition, the stoppage timings of the drum motor36and the belt motor37can be set freely according to motor characteristics, the throughput of the image forming apparatus, and the like. If the inertial movement distance of the photosensitive drum3is different from that of the conveyance belt8, it is sufficient that stoppage timings of the photosensitive drum3and the conveyance belt8are adjusted in such a manner that they are in synchronization with each other.

There was examined an effect of suppressing generation of scratches on the surface of the photosensitive drum3in the case that jamming is cleared through the separating/returning operation of the transfer unit R in the image forming apparatus according to the present invention. Herein, a test was conducted using an amorphous silicon drum with a diameter of 30 mm as the photosensitive drum3and a belt made of hard resin as the conveyance belt8. As a test method, a large number of grains of sandpaper (grain size: about 100 μm) were dispersed around the transfer nip portion of the conveyance belt8, an amount of toner adhering to the toner adhesion region was set to 0.5 mg/cm2, the separating/returning operation of the conveyance belt8was repeatedly performed five times. Thereafter, as for a case that the toner adhesion region is formed (the present invention) and a case that the toner adhesion region is not formed (comparative example), respectively, the surface of the photosensitive drum and a predetermined half image were visually observed and presence/absence of scratches was evaluated.

As a result of this test, in the present invention wherein the toner adhesion region was formed on the surface of the photosensitive drum and was stopped at the transfer nip portion, no scratches were generated on the surface of the photosensitive drum. On the other hand, in the comparative example wherein the toner adhesion region was not formed on the surface of the photosensitive drum, a large number of scratches of about 200 to 500 μm were confirmed on the transfer nip portion of the surface of the photosensitive drum.

According to the present invention, upon occurrence of jamming, the image forming process is stopped in the state that the toner layer is always present in the contact position between the photosensitive drum and the conveyance belt. Thus, it is possible to provide a simple image forming apparatus capable of alleviating a rub between a photosensitive drum and a conveyance belt and suppressing generation of scratches on the photosensitive drum and the surface of the conveyance belt due to foreign matters in the case that a transfer unit or the photosensitive drum is separated to clear jamming and, then, is returned to its original position.

In addition, the exposure unit exposes the predetermined region of the surface of the photosensitive drum to light or electrification by the electrification unit is ceased to form the toner adhesion region. Therefore, it is possible to adhere toner to the predetermined region of the surface of the photosensitive drum in accordance with procedures similar to a normal image forming mode, without providing toner adhesion means separately.

Further, the voltage having the same polarity as that of toner is applied to the transfer unit during a period from detection of jamming until stoppage of the photosensitive drum. Therefore, it is possible to prevent the toner adhering to the surface of the photosensitive drum from moving toward the conveyance belt. Further, even after removing a sheet jammed between the photosensitive drum and the conveyance belt, it is possible to effectively prevent generation of scratches because of presence of the toner layer at the contact position between the photosensitive drum and the conveyance drum. In addition, unfixated toner does not adhere onto the sheet or the conveyance belt. Therefore, it is possible to prevent the interior of the apparatus and the hands and fingers of the user from being contaminated, in the case of clearing jamming.

Moreover, the stoppage timing of the conveyance belt is made synchronous with that of the photosensitive drum, so that it is possible to eliminate the peripheral speed difference between the conveyance drum and the photosensitive drum to thereby further reduce the friction in the contact position.