JPH07239588A - Image forming device - Google Patents

Abstract

PURPOSE:To improve the cleaning effect of the transfer roller of a photoreceptor unit in an image forming device. CONSTITUTION:An electrostatic charger 23 having a positive polarity, an exposure part by a scanner unit, a developing device 10 forming a toner image having the positive polarity, and a transfer roller 8 transferring a toner image on a photoreceptor drum 7 to paper at a transfer part by impressing transfer bias voltage having a negative polarity are arranged in order on the outer periphery of the rotating photoreceptor drum 7. When the paper does not exist at the transfer part, a controller controls so that cleaning bias voltage having the positive polarity is impressed on the transfer roller 8 so as to transfer electrostatically charged toner to the drum 7, and the area of the drum 7 passing the transfer part is electrostatically charged to be in the positive polarity by the charger 23 and then the voltage of nearly the same potential is impressed on the electrostatically charged area by a developing device 10.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a laser beam printer, a copying machine and a facsimile machine, and more particularly to a toner carried on the surface of an image carrier such as a charged photosensitive drum. An image forming apparatus configured to form an image by transferring an image onto a recording medium such as a sheet that is nipped and conveyed at a contact portion (transfer portion) between a transfer unit such as a transfer roller and the image carrier. The present invention relates to an image forming apparatus capable of forming a suitable image by preventing unnecessary toner from adhering to a transfer roller.

[0002]

2. Description of the Related Art Conventionally, as an image forming apparatus, the surface of a rotating photosensitive drum is uniformly charged in advance by a charging means, and the charged surface is exposed according to image data to form an electrostatic latent image. Then, a charged toner is supplied to the latent image from the developing device to form a toner image, and then the paper is supplied to the pressing contact portion (transfer portion) between the transfer roller and the photoconductor drum, at which time it is transferred to the transfer roller. A toner image is transferred to the paper surface by applying a bias voltage, and then the toner image on the paper surface is fixed by the fixing device on the downstream side of the conveyance, and the toner that has not transferred to the paper is transferred to the photosensitive drum surface. An image forming apparatus has been proposed that is configured to be removed by a cleaner blade that is in sliding contact.

However, in this type of image forming apparatus, the toner adhering to the surface of the photosensitive drum is transferred to the transfer roller when the paper does not pass, and the toner is transferred to the back surface of the succeeding paper and becomes dirty. was there. In order to solve this problem, in Japanese Patent Laid-Open No. 3-166573,
At the time of cleaning the transfer roller during the pre-rotation and / or the post-rotation, it has been proposed to perform constant current control so as to apply a voltage having the same polarity as the charging polarity of the toner to the transfer roller. When the cleaning bias voltage is applied to the transfer roller, the voltage application to the charging device and the developing device is generally off.

According to this image forming apparatus, in the case of negative polarity toner, for example, by applying a negative polarity cleaning bias voltage, the (negative polarity) toner on the transfer roller surface is exposed to a potential of 0 by the action of an electric field. It is said to be returned (attracted) to the body drum surface. Further, in Japanese Patent Application Laid-Open No. 4-212187, the charging polarity of the surface of the photosensitive drum by the charger is opposite to the charging polarity of the toner during development (in other words, charging of the charger for charging the surface of the photosensitive drum). In the normal developing type image forming apparatus in which the polarity and the charging polarity of the transfer roller at the time of transfer are the same, the application of the cleaning bias voltage to the transfer roller is the same as the charging polarity of the toner at the time of non-sheet passing. It is disclosed that the voltage is applied with polarity.

According to this image forming apparatus, the surface of the photosensitive drum is uniformly negatively charged in advance by the charging device, and the toner which is positively charged by the developing device is supplied to the exposed electrostatic latent image portion. Then, a positively charged toner image is formed. When this toner image portion reaches the transfer portion, the toner image is transferred to the paper surface by supplying the paper between the surface of the photosensitive drum and the transfer roller and negatively charging the transfer roller.

On the other hand, when cleaning the transfer roller,
The transfer roller is positively charged, and the toner (positively charged) attached to the surface is returned to the negatively charged surface of the photosensitive drum by the action of an electric field.

[0007]

By the way, in addition to toner having a predetermined polarity (hereinafter referred to as forward polarity toner), toner having a reverse polarity (hereinafter referred to as reverse polarity toner) is used as the toner in the developing device. May be mixed in a small amount. Further, most of the toner in the developing device is already charged to a predetermined polarity (forward polarity) by the developing sleeve or the like inside the developing device, and the surface of the developing sleeve (where the charged toner is carried) Is usually arranged close to the surface of the photosensitive drum.

Further, the potential of the surface of the photosensitive drum after cleaning may shift to the opposite polarity to the charging polarity of the charger, as described in the latter prior art. Therefore, when the potential of the photosensitive drum surface is lower than the potential of the charged toner (including the forward polarity toner and the reverse polarity toner) such that the potential of the photosensitive drum surface is 0,
When the charged toner on the developing device side (including the forward polarity toner and the reverse polarity toner) and the charging polarity on the surface of the photosensitive drum in the vicinity thereof are opposite in polarity, the toner is charged from the developing device portion during the cleaning. The toner is transferred to the surface of the rotating photoconductor drum, and the charged toner physically adheres to the surface of the transfer roller at the portion where the surface of the transfer roller is in close contact with the surface of the photoconductor drum (the transfer portion). It becomes a state where it is replenished, like both the above-mentioned prior art,
By applying a predetermined cleaning bias voltage, there is a problem in that the cleaning effect of returning the chargeable toner that has already adhered to the transfer roller surface to the photosensitive drum is offset.

Further, in the latter prior art, for example, even if a cleaning bias voltage for a positive band electrode is applied to the transfer roller in order to remove most of the positively charged toner having a positive polarity from the surface of the transfer roller, the negative polarity is applied. The reverse polarity toner of negative polarity, on the contrary, is electrostatically charged and strongly adheres to the surface of the transfer roller, and the problem remains that cleaning of the negative polarity reverse polarity toner becomes impossible.

The present invention has been made to solve these problems, and an object thereof is to provide an image forming apparatus having an improved cleaning effect.

[0011]

In order to achieve the above object, the image forming apparatus according to the present invention comprises a charging means for charging the image carrier on the outer periphery of the rotatable image carrier. Exposure means for forming a latent image on the image carrier charged by the charging means, developing means for forming a toner image on the image carrier, and toner on the image carrier by applying a transfer bias voltage. An image forming apparatus in which a transfer unit for transferring an image to a recording medium at a transfer site is sequentially arranged, and a cleaning bias voltage is applied to the transfer unit when the recording medium does not exist at the transfer site. Prevent the toner from adhering to the transfer means,
Further, a control means is provided for charging the area of the image carrier passing through the transfer portion by the charging means and applying a voltage to the charged area by the developing means.

The invention described in claim 2 is the same as claim 1.
In the image forming apparatus described in (1), the control unit charges the area of the image carrier that passes through the transfer portion by the charging unit, and applies the voltage applied to the developing unit to the image carrier by the charging unit. It is controlled so as to be equal or substantially equal to the body's electrification potential. further,
According to a third aspect of the present invention, in the image forming apparatus according to the second aspect, the transfer unit is formed by a transfer roller that is in contact with the outer periphery of the image carrier, and the contact portion constitutes the transfer portion. The recording medium is configured to be conveyed between the image carrier and the transfer roller.

[0013]

EXAMPLES Next, examples embodying the present invention will be described. FIG. 1 is a schematic side sectional view of a printer 1 as an image forming apparatus, and FIG. 2 is a side sectional view of a main part of a photoconductor unit. In the printer 1 of the present embodiment, a paper feed cassette 3 in which a large number of papers 42 as recording media are stacked is detachably mounted on one upper side of a main body case 2, and by a paper feed roller 4 and a separation pad body 5. Paper 42 as a recording medium stacked in the paper feed cassette 3 is separated into sheets one by one, and a photosensitive drum 7 as an image carrier and a transfer roller 8 as a transfer unit are provided via a pair of transport rollers 6. Charger 2 as charging means
It is conveyed to the photoconductor unit 9 composed of 3 and the like. A developing device 10 as a developing means for replenishing toner from a toner cartridge 26 is arranged adjacent to the photoconductor unit 9 on the side close to the paper feeding cassette 3, and on the opposite side, a heating roller 11 and a pressing roller 12 are provided. Fixing unit 13 consisting of
Are arranged.

Below the photoconductor unit 9, a scanner unit 17 as an exposing means composed of a laser emitting section 14, a lens 15, a reflecting mirror 16 and the like, control boards 18 and 19 as a controlling means, a power supply unit 20 and the like are arranged. The cover member 21 is provided with an operation member including a plurality of operation buttons 22. Charger 23 as charging means
The surface of the photoconductor drum 7 which has been charged in advance is irradiated with light (laser beam) emitted from the scanner unit 17 according to image data transmitted from an external device such as a host computer (not shown), After performing the developing process of forming an electrostatic latent image on the surface of the body drum 7 and then visualizing the latent image with the charged powdery toner supplied and charged by the developing device 10, The toner image is transferred onto the sheet (recording medium) 42 supplied between the photoconductor drum 7 and the transfer roller 8, and then the fixing unit 13 applies heat and pressure to the transferred image. Then, the toner image is fixed on the paper 42, and the paper is ejected from the paper ejection roller pair 24 to the paper ejection tray 25 of the printer 1.

Next, the structure of the developing device 10 as the developing means will be described with reference to FIGS. 1 and 2.
A toner cartridge 26 for replenishing the non-magnetic toner or the magnetic toner is detachably attached to the stirring chamber 30 in the above, and the supplied toner is supplied to the second stirring member 31 composed of rotating blades in the stirring chamber 30. Be stirred. A first stirring member 3 including a developing roller 32 arranged in the vicinity of the surface of the photosensitive drum 7 and a rotating blade below the developing roller 32.
The developing chamber 34 having 3 and the stirring chamber 30 are separated by a partition member 35, and an opening 36 is opened in the partition member 35 so as to face one side of the peripheral surface of the developing roller 32. .

Then, as shown in FIG.
In the vicinity of 6, a restriction member 37 having elasticity such as a magnetically permeable PET (polyethylene terephthalate) film
Of the regulating member 37 extends downward in the developing chamber 34 to face the peripheral surface of the developing roller 32, and the regulating member 37 has its free end as the first stirring member. The first agitating member 3 is arranged so as to be close to the outer side of the rotation locus of 33 and the toner is supplied from the lower side of the regulating member 37.
By being taken into the developing chamber 34 at 3, the toner is replenished from the agitating chamber 30 by the amount of the gap space formed between the opening and the base end side of the regulating member 37.

As shown in FIG. 2, the second stirring member 31 rotates in the direction of arrow A, that is, so as to scrape up the toner in the stirring chamber 30 from the bottom toward the opening 36. The first stirring member 33 rotates in the direction of the arrow B, and while stirring the toner supplied from the opening 36 and the magnetic carrier previously stored in the developing chamber 34, the toner is once developed via the bottom of the developing chamber 34. The roller 32 is rotated so as to flip up toward the lower surface of the roller 32. The developing roller 32 rotates in the direction of arrow C, while the magnet roller 38 (in which the N poles and the S poles are radially arranged alternately) in the inner diameter portion thereof rotates in the direction of arrow D.

By the rotation of the developing roller 32 and the magnet roller 38, the magnetic carrier and the toner are frictionally charged with each other, the toner is carried around the magnetic carrier, and the toner in this state is attached to the peripheral surface of the rotating developing roller 32. It is carried as a magnetic brush made of a developer having a predetermined height (layer thickness), and as described later, it becomes possible to develop an electrostatic latent image on the peripheral surface of the photosensitive drum 7.

In the developing chamber 34, the photosensitive drum 7
A trimmer blade 39 for adjusting the layer thickness (developer layer thickness) of the toner carried on the magnetic carrier is provided on the peripheral surface of the developing roller 32 at a position close to one side of the peripheral surface of the developing roller 32. . Further, the toner not transferred to the paper 42 at the transfer portion is removed by the cleaner 41 having the cleaning blade 40 that is in sliding contact with the other peripheral surface of the photosensitive drum 7 that rotates in the arrow E direction.

The developing method used in this embodiment is 95 to 98% (weight ratio) for the magnetic carrier and 2 for the non-magnetic toner.
A so-called two-component developing system composed of ˜5% (weight ratio) and a magnetic carrier of 30 to 80% (weight ratio) and a so-called 1.5 of magnetic toner composed of 20 to 70% (weight ratio).
Any of the component developing methods may be used. Further, in this embodiment, most of the magnetic toners (forward polarity toners) are of positive polarity.

Although one embodiment of the charger 23 as the charging means uses a corona charger such as a scorotron charger or a corotron charger, it may be a charging roller. The transfer roller 8 is a conductive sponge roller, and a portion where the surface of the roller is pressed against the surface of the photosensitive drum 7 is a transfer portion. FIG. 3 is a functional block diagram of control means for controlling the image forming operation on the paper 42 and the cleaning operation of the transfer roller 8 by the photoconductor unit 9. The controller 44 includes a drive motor 43 that rotates the photosensitive drum 7 in the direction of arrow E,
A charger 23 for pre-charging the surface of the photosensitive drum 7,
A scanner unit 17 that exposes the surface of the photosensitive drum 7 according to image data, a developing device 10, and a transfer roller 8
And a command signal to each of them to control their operation, and a voltage is applied to the charger 23 from a high voltage unit 45 having a high voltage of about 4 KV to 7 KV. The scanner unit 17 is operation-controlled via a video controller 46 and a scanner controller 47 which are controlled by image data from an external device such as a host computer.

Next, an image forming mode in the printer 1 thus constructed will be described with reference to FIGS. 3 to 5 and FIG. 6 showing a time chart. 3 and 4, Y1 to Y with respect to the circumferential surface of the photosensitive drum 7
Position 4 is as follows. That is, the point Y1 is the charged portion by the charger 23, and the point Y2 is the scanner unit 17
, The exposure area by Y3, the development area by the developing device 10,
Y4 indicates a transfer portion by the transfer roller 8. In addition, on the circumferential surface of the photosensitive drum 7, the transfer unit Y4 to the charger 2
A cleaner 41 for removing the toner remaining on the circumferential surface of the photosensitive drum 7 is provided between the charging portion 3 and the charging portion Y1.

As shown in FIG. 4, when the photosensitive drum 7 rotates in the direction of arrow E at a constant rotation speed, t _CD is a required time corresponding to the distance from the charging portion Y1 to the developing portion Y3, t _ED Is a required time corresponding to the distance from the exposed portion Y2 to the developing portion Y3, t _DT is a required time corresponding to the distance from the developing portion Y3 to the transfer portion Y4, and t _ET is from the exposed portion Y2 to the transfer portion Y4. The time required for each distance is shown. The time required for one rotation of the transfer roller 8 is to. In this embodiment, the diameter of the photosensitive drum 7 is 30 mm and the peripheral speed is 35 mm / sec. Therefore, the time required for one rotation of the photosensitive drum 7 is 2.693 seconds in the above embodiment. Moreover, since the diameter of the transfer roller 8 is 16 mm, the time to is 1.436 seconds.

First, an embodiment for forming an image on one sheet of paper 42 will be described with reference to the time chart shown in FIG. When the drive motor 43 is driven (ON state) to start the image forming operation, the photosensitive drum 7 rotates in the direction of arrow E in FIG. 3 at a constant speed. Next, with a slight time delay, the charger 23 is attached to the charging portion Y1 on the circumferential surface of the photosensitive drum 7.
Then, a voltage of + Vc is applied to positively and uniformly charge (+800 V in the example). In this case, the developing portion Y
3, when the positively charged potential on the surface of the photoconductor drum 7 is smaller than the positive potential in the developing device 10, the electric field force due to the potential difference causes the toner to fly to the surface of the photoconductor drum 7, and the toner is transferred. The sheet 42 transferred to the transfer roller 8 of the part Y4 and then sent to the transfer part Y4.
The toner on the surface of the transfer roller 8 is pressed against the back surface of the paper 4
2 It will stain the back side. In order to prevent this phenomenon, prior to image formation, which will be described later, the transfer roller cleaning is performed in advance before the toner flies to the circumferential surface of the photosensitive drum 7 and the toner on the circumferential surface of the transfer roller 8 is removed. There is a need to. Therefore, prior to the start of exposure, charging is advanced at least for a period of (to + t _CD −t _ED ) or more.

On the other hand, the developing device 10 includes the charger 23.
The same potential V as the positively charged potential on the surface of the photosensitive drum 7 by
d (+800 V in the example) is applied. This timing may be started when the portion of the photosensitive drum 7 charged by the charger 23 moves to the portion of the developing device 10 (developing portion Y3). Therefore, it is assumed that the time t _{CD is} delayed from the start of charging by the charger 23. As a result, in the developing portion Y3, the developing device 10 and the photoconductor drum 7
Since the surface potential is positively charged and the same potential, the force of the electric field does not act between the two, and the positive polarity toner in the developing device 10 and the minute amount mixed in the developing device 10 are positively charged. The electric field force of attracting the opposite polarity toner (toner having a negative charging polarity) to at least the surface side of the photosensitive drum 7 does not act.

The transfer roller 8 is delayed by a time t _DT (the time required for the positively charged portion of the developing portion Y3 to reach the transferring portion Y4) from the start of the voltage application to the developing device 10. A cleaning bias voltage + Vo of positive potential (about 1.5 KV by constant voltage control in the embodiment) is applied. The application time of the transfer roller 8 is equal to or longer than the time to make one rotation of the transfer roller 8.

As a result, the positive polarity toner, which is attached to the surface of the transfer roller 8 and is positively charged, can be returned to the surface of the photosensitive drum 7 at the transfer portion Y4, so that one sheet of paper is enough. It is possible to prevent the back surface of the paper 42 fed to the transfer portion Y4 and fed to the transfer portion Y4 at the exposure start timing corresponding to the data for image formation from being contaminated with the toner, and the returned toner is collected by the cleaner 41. The cleaning by the blade 40 is completed and the cleaning of the photosensitive drum 7 is completed. In this case, the end of the application of the cleaning bias voltage + Vo to the transfer roller 8 is delayed by the time t _ET (the required time corresponding to the distance from the exposed portion Y2 to the transfer portion Y4) from the start of exposure described later, and At the end of the application of the voltage + Vd to the developing device 10, time t
By delaying by _ED (the required time corresponding to the distance from the exposed portion Y2 to the developing portion Y3), the surface of the photosensitive drum 7 before the portion where the developing toner adheres to the surface of the photosensitive drum 7 along with the image data is delayed. Adhesion of toner and adhesion of toner to the transfer roller 8 can be completely prevented, and contamination of the conveyed paper 42 on the front side of the conveyance side can be completely prevented.

Next, when the voltage of + V _c is applied to the charger 23 and the time to or more has elapsed, the scanner unit 17 scans and irradiates the exposure unit Y2 in accordance with the image data in accordance with the print command. The surface of the photosensitive drum 7 is exposed. In the bright part of the exposure,
It is neutralized so that the charge potential on the surface of the photosensitive drum 7 is lowered to about + 100V. In the unexposed dark area, the charge potential on the surface of the photosensitive drum 7 is naturally maintained at the initial charge potential + 800V. In this way, an electrostatic latent image is formed.

At the same time, that is, from the start of exposure to the time t _ED (from the exposed portion Y2 to the developing portion Y)
By applying a voltage of + V _d0 (development voltage, which is a portion for which the voltage has been changed for toner density adjustment) at a portion (developing portion Y3) of the developing device 10 after delaying by a required time corresponding to the distance up to 3. The positive polarity toner charged to the positive polarity is transferred to the surface of the photosensitive drum 7. In this case, the potential at the developing portion Y3 is applied to a voltage of + 600V to + 300V by adjusting the density (cotrast) of the image or the like. Therefore, in the bright portion of the electrostatic latent image exposed, the developing device 10 is used.
The forward polarity toner is transferred to the surface of the photoconductor drum 7 having a lower potential than the side potential, and in the dark portion of the exposure, the charging potential on the surface side of the photoconductor drum 7 is higher, so No transfer takes place. In this way, the manifested toner image is formed.

A sheet of paper 42 fed from the paper feed cassette 3 in accordance with the print command is timed with the exposure by a detection signal from a paper leading edge detector (not shown).
The transfer roller 8 is conveyed to the transfer portion Y4, which is a pressing portion between the transfer roller 8 and the photosensitive drum 7, and the transfer roller 8 is transferred from the back surface of the paper 42.
The transfer bias voltage −Vt is applied in the state of being pressed by. The transfer bias voltage is about -1 KV to -4 KV, and constant current control of 1.5 μA to 5 μA is executed.
At the transfer site Y4, the negatively charged transfer roller 8 causes the positively charged forward polarity toner to fly and be transferred to the front surface side of the paper 42 by the force of the electric field. Then, the fixing unit 13 applies heat and pressure to the transferred image to fix the toner image on the paper 42, and the paper is ejected from the paper ejection roller pair 24 to the paper ejection tray 25 of the printer 1.

Photosensitive drum 7 corresponding to the outside of the sheet width
The charged toner remaining on the peripheral surface is scraped off and removed by the blade 40 of the cleaner 41. Next, a sheet of paper 4
The cleaning mode of the transfer roller 8 and the photoconductor drum 7 after the image formation of No. 2 will be described. In the timing chart of FIG. 6, when the exposure corresponding to the image data for one sheet is completed, + Vd is applied to the developing device 10 after a delay of time t _ED (the required time corresponding to the distance from the exposed portion Y2 to the developing portion Y3). Start applying the voltage. Also, the voltage of cleaning bias voltage + Vo is started to be applied to the transfer roller 8 with a delay of time t _ET (the required time corresponding to the distance from the exposed portion Y2 to the transfer portion Y4) from the end of the exposure. In other words, when the surface of the photosensitive drum 7 corresponding to the end portion of the exposure passes through the developing portion Y3 facing the developing device 10, a voltage of + Vd is started to be applied to the developing device 10, and the exposure When the surface of the photosensitive drum 7 corresponding to the end portion passes the transfer portion Y4, application of the cleaning bias voltage to the transfer roller 8 is started.

The application time of the cleaning bias voltage is at least 3 to the period during which the transfer roller 8 makes three revolutions, and the timing is matched with that, that is,
The charging of the positive potential + Vc by the charger 23 is ended before the end of the cleaning bias voltage at the transfer roller 8 by the time t _DT + t _CD , and the developing device 10 is preceded by the time t _DT before the end of the cleaning bias voltage.
The application of the voltage of + Vd to is terminated.

As described above, in order to clean the transfer roller 8 during the period after one sheet 42 has passed, a cleaning bias voltage + Vo, for example, a voltage of about +1.5 KV is applied to the transfer roller 8. By positively charging, the positive polarity toner that is attached to the surface of the transfer roller 8 and is positively charged can be returned to the surface of the photosensitive drum 7 at the transfer portion Y4.

At approximately the same time, the charging device 23 receives the voltage + Vc.
Is applied to positively charge the surface of the photoconductor drum 7 to about 800 V, and a voltage + Vd of about +800 V is also applied to the developing device 10, so that the developing device 10 and the photoconductor are located at the developing portion Y3. Since the electric potential of the surface of the drum 7 is positively charged and the same electric potential, the force of the electric field does not act between them, and the positive polarity toner in the developing device 10 and the positive polarity toner are mixed in the developing device 10. The force of the electric field that a minute amount of the opposite polarity toner (toner having a negative charging polarity) is attracted at least to the surface side of the photosensitive drum 7 does not act.

The drive motor 43 for rotating the photosensitive drum 7 is turned off and stopped after an appropriate time has elapsed after the completion of the post-cleaning work. Further, the state of the surface of the photosensitive drum 7 immediately after the trailing end of the preceding sheet has passed the transfer portion Y4 will be further considered with reference to FIG. 5A, the area Z on the surface of the photoconductor drum 7 that passes through the transfer portion Y4 is located on the upstream side of the charger 23 in the rotation direction of the photoconductor drum 7 in the initial state when the paper is not passed. 5B shows a state after the charging area Zo as a portion charged with the same charging polarity by the charger 23 and the developing device 10 has passed the portion of the developing device 10.

As described above, the area Z on the surface of the photosensitive drum 7 (see the shaded portion in FIG. 5) passing through the transfer portion Y4 when the paper is not passed is positively charged by the charger 23, and When a positive voltage is applied to the developing device 10 with respect to the charging area Zo, if both charging voltages are made equal, the toner does not fly or adhere to the charging area Zo from the developing device 10 side, and the transfer roller 8 The cleaning effect at the transfer portion Y4 is not reduced or cancelled. The section from the developing portion Y3 to the transferring portion Y4 is a portion of the developing device 10 (developing portion Y
Since it has already passed 3), there is no problem with the transfer of the charged toner from the developing device 10 to the photosensitive drum 7.

The section (region) from the charging portion Y1 to the developing portion Y3 is preceded in time by the charger 23 to a predetermined positive polarity in order to prepare for the image formation on the preceding sheet. Since it is still charged, by applying the voltage of + Vd to the developing device 10 in the charging area in this section, a large amount of the positive polarity toner charged in the positive polarity in the developing device 10 described above is generated. The electric field force that transfers to the surface side of the photosensitive drum 7 does not act. When the area Z extending from the charged portion Y1 in the direction of the cleaner 41 passes through the charged portion Y1, the charger 23 is charged to a predetermined positive polarity, and the developing device 10 is charged to the charged area Zo.
By carrying out the positive charging of the predetermined potential, the flying (transfer) of the charged toner from the developing device 10 is eliminated.

The transfer bias voltage and the cleaning bias voltage shown in the drawings and described in the embodiments show only the DC component. Then, during the cleaning operation, the voltage application + 700V in the developing device 10 may be set slightly lower than the charging voltage + 800V in the charger 23. In this case, the developing device 10
Since the electric field force such that a large amount of the positive polarity toner charged in the positive polarity in (3) is transferred to the surface side of the photoconductor drum 7 does not act, the charged toner does not adhere to the charging area Zo when the paper is not passed. It is possible to prevent the situation where the cleaning effect of the transfer roller 8 is canceled out.

When images are continuously formed on a plurality of sheets of paper 42 at short intervals (corresponding to the case where the rotation of the drive motor 43 is continuously executed), the process shown in FIG. The transfer roller 8 is not cleaned after passing the sheet of paper 42. That is, the transfer roller 8 holds the state in which the normal transfer bias voltage −Vt is applied. The transfer bias voltage is about -1 KV to -4 KV, and constant current control of 1.5 μA to 5 μA is executed.
Further, the charging voltage of + Vo is continuously applied to the charging portion Y1, the exposure corresponding to the image data for each sheet of paper is executed to the exposure portion Y2, and the toner density is applied to the developing device 10 at the developing portion Y3. A predetermined voltage of + V _d0 by adjustment is applied. In this way, when images are continuously formed on a plurality of sheets of paper, the transfer roller 8 in the transfer portion Y4 is
Even if the application of the cleaning bias voltage + Vo is omitted, since the distance between the end of the preceding paper 42 and the leading end of the following paper 42 (the distance between the non-sheet passing portions) is short, the non-sheet passing portion is the transfer portion Y4. Even if the toner passes through the transfer roller 8, the toner transfer to the surface of the transfer roller 8 and the stain on the back surface of the paper 42 are less likely to occur, but the timing control system associated with the voltage application of the cleaning bias voltage can be simplified. Play.

In the above embodiment, the positive polarity toner was positively charged, but when using the negatively charged toner, the charger 23 as the charging means and the developing device 10 as the developing means are used. Needless to say, the charging polarity of the transfer roller 8 as the transfer means may be opposite to that in each of the embodiments.

[0041]

As described above, claim 1
According to another aspect of the invention, there is provided an image forming apparatus comprising: a charging means for charging the image bearing body on the outer periphery of a rotatable image bearing body; and an exposing means for forming a latent image on the image bearing body charged by the charging means. An image in which a developing unit that forms a toner image on the image carrier and a transfer unit that transfers the toner image on the image carrier to a recording medium at a transfer site by applying a transfer bias voltage are sequentially arranged. In the forming apparatus, when the recording medium does not exist at the transfer portion, a cleaning bias voltage is applied to the transfer means to prevent toner from adhering to the transfer means, and the image passing through the transfer portion. The charging device is equipped with a control device for charging the region of the carrier by the charging device and applying a voltage to the charging region by the developing device.

In the present invention, the polarity of the charged toner charged by the developing means is the same as the polarity of charging the surface of the image carrier by the charging means, and the charging potential of the image carrier after exposure is on the developing means side. This is a mode in which the charged toner is transferred to the surface of the image carrier by lowering the potential. Then, when the recording medium does not exist at the transfer portion, by applying a cleaning bias voltage of the same polarity to the transfer means at a potential higher than the potential of the image carrier surface side, the transfer means is already applied to the transfer means. The charged toner that has adhered can be transferred to the image carrier side to clean the transfer means.

At this time, the area of the image carrier passing through the transfer portion is charged by the charging means by the control means, and a voltage is applied to the charged area by the developing means. If such control is performed, the chargeable toner on the developing unit side is less likely to transfer to the image carrier side, the toner is conveyed together with the image carrier, and the toner is transferred from the image carrier to the transfer unit during the cleaning operation. It is less likely to transfer to the side and offset the cleaning effect. Therefore, according to the present invention, it is possible to improve the cleaning effect in the image forming apparatus.

According to a second aspect of the present invention, in the image forming apparatus according to the first aspect, the control means charges the area of the image carrier passing through the transfer portion by the charging means, and the developing means. The voltage applied to the means is controlled so as to be equal or substantially equal to the charging potential of the image carrier by the charging means, whereby the difference between the potential of the developing means and the potential of the area of the image carrier is controlled. Will be zero or less, and there will be no or less force of the electric field between them, so there will be no or even less transfer of charged toner to the area. In particular, in addition to the majority of forward polarity toner in the developing means,
Reverse polarity toner that is mixed in a small amount can also be eliminated or even prevented from transferring to the area.
The cleaning effect of the image forming apparatus can be further improved.

According to a third aspect of the present invention, in the image forming apparatus according to the second aspect, the transfer means is formed by a transfer roller that abuts the outer periphery of the image carrier, and the abutting portion is the transfer portion. The recording medium is configured to be conveyed between an image carrier and a transfer roller,
If the charge is applied while rotating the transfer roller,
There is an effect that all the charged toner on the peripheral surface of the transfer roller can be transferred to the image carrier side for cleaning, and at the same time, the transfer of toner from the image carrier side to the transfer roller can be prevented.

[Brief description of drawings]

FIG. 1 is a schematic side sectional view of a laser printer.

FIG. 2 is a side sectional view of a main part of a photoconductor unit.

FIG. 3 is a functional block diagram of control means.

FIG. 4 is an explanatory diagram showing a time required between respective parts of the photosensitive drum.

5A is an explanatory diagram showing a state of the photosensitive drum immediately after passing a sheet, and FIG. 5B is an explanatory diagram showing each part in a cleaning operation state.

FIG. 6 is a timing chart of image forming and cleaning operations.

[Explanation of symbols]

 1 Printer 7 Photosensitive Drum 8 Transfer Roller 9 Photosensitive Unit 10 Developing Device 17 Scanner Unit 18 Control Board 23 Charging Device 32 Developing Roller 41 Cleaner 43 Drive Motor 44 Controller

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI technical display location G03G 15/16 103 21/10

Claims (3)

[Claims] 1. An outer peripheral surface of a rotatable image carrier, a charging unit for charging the image carrier, an exposing unit for forming a latent image on the image carrier charged by the charging unit, and an image. In an image forming apparatus in which a developing unit that forms a toner image on a carrier and a transfer unit that transfers a toner image on the image carrier to a recording medium at a transfer site by applying a transfer bias voltage are sequentially arranged. When the recording medium does not exist at the transfer portion, a cleaning bias voltage is applied to the transfer means to prevent the toner from adhering to the transfer means, and to prevent the toner from adhering to the area of the image carrier that passes through the transfer portion. On the other hand, the image forming apparatus is provided with a control unit that charges the charging unit and applies a voltage to the charging region by the developing unit. 2. The control means charges the area of the image carrier passing through the transfer portion by the charging means, and applies a voltage to the developing means to charge the image carrier by the charging means. The image forming apparatus according to claim 1, wherein the image forming apparatus is controlled so as to be equal to or substantially equal to the potential. 3. The transfer means is formed of a transfer roller that abuts on the outer periphery of the image carrier, and the abutting part constitutes the transfer part, and the recording medium is an image carrier and a transfer roller. The image forming apparatus according to claim 2, wherein the image forming apparatus is conveyed between the two.