JPH0915939A - Image forming device - Google Patents

Abstract

PURPOSE: To prevent the potential of an end part from being lowered in comparison with a central part and to prevent the density of an image from becoming high only at the end part by making the electrostatic charge width of an auxiliary electrostatic charge means wider than that of an electrostatic charge means. CONSTITUTION: The auxiliary electrostatic charger 15 and an electric discharge lamp 25 are provided at an identical place on the surface of a photoreceptor drum 4 by being superposed above and below in order to electrostatically charge the drum 4 and to expose the whole surface thereof at the same time. Before the drum 4 is primarily electrostatically charged, it is electrostatically charged by the charger 15 so that the polarity of the memory effect area thereof becomes identical to that of an electrostatic latent image formed thereon, that means, it becomes the negative polarity. Besides, the drum 4 is discharged and exposed by the lamp 25 at the same time. In such a case, the density of the end part of the image becomes high because the charger 15 is provided in order to eliminate the memory effect area. However, it is prevented from becoming high by making the aperture width of the charger 15 in a longitudinal direction wider than that of a primary electrostatic charger 6.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrophotographic image forming apparatus such as a copying machine or a printer.

[0002]

2. Description of the Related Art FIG. 15 shows a cross section of a multicolor electrophotographic copying machine.

As shown in FIG. 15, the multicolor electrophotographic copying apparatus has an image carrier, that is, a photosensitive drum 104, which is rotatably supported and rotates in the direction of an arrow, and the photosensitive drum 104 is provided on the outer peripheral portion thereof. Primary charger 106 for uniformly charging the surface
And an exposure means such as a laser beam exposure device for irradiating the photosensitive drum 104 with an optical image obtained by color-separating a color image or an optical image corresponding to the color image to form an electrostatic latent image of the image on the photosensitive drum 104. 107 and a rotary developing device 109 that develops the electrostatic latent image on the photosensitive drum 104 to visualize it as a toner image.

In the rotary developing device 109, a yellow developing agent, a magenta developing agent, a cyan developing agent, and a black developing agent are specially stored around a substantially cylindrical casing 104a which is rotatably supported. Individual developing device 109Y, 10
It holds 9M, 109C and 109K. In the rotary developing device 109, when the housing 104a rotates, a developing device that stores a developer of a color corresponding to the electrostatic latent image formed on the photosensitive drum 104 is developed to face the outer peripheral surface of the photosensitive drum 104. The electrostatic latent image on the photosensitive drum 104 is conveyed to a position, is developed with the developer of that color, is visualized as a toner image, and this is repeated for other colors, and full color development for four colors is possible. Has been done.

The toner image formed on the photosensitive drum 104 is transferred onto the transfer material P carried by the transfer device 105A. In this example, the transfer device 105A is a drum type including a transfer drum 105 that is rotatably supported, and as will be understood with reference to FIG. 16, the transfer drum 105 includes a pair of cylinders arranged at both ends. 105
A transfer material carrying sheet 501 is stretched as a transfer material carrying member on the outer peripheral surfaces of the air spaces a and 105a. A transfer material gripper 105c that holds the transfer material P fed from a sheet feeding device (not shown) is provided in a non-tensioned portion of the transfer material carrying sheet 501 on the outer peripheral surface of the transfer drum 105.
A charging device 105b that constitutes a transfer unit is provided in 05. Further, inside and outside the transfer drum 105, as shown in FIG. 15, an inner charge eliminator 105d and an outer charge eliminator 105e, which constitute a charge eliminator, are provided.

The transfer material carrying sheet 501 is usually provided with
Films such as polyethylene terephthalate resin and polyvinylidene fluoride resin are used.

The process of forming a full-color image by the multicolor electrophotographic copying apparatus having the above-mentioned structure will be briefly described as follows.

First, the primary charger 106 and the exposure means 10
By operating 7, a blue color-separated electrostatic latent image is formed on the outer peripheral surface of the photosensitive drum 104, and this electrostatic latent image is developed with a yellow developer by the developing device 109Y of the rotary developing device 109. It On the other hand, the transfer material P fed to the transfer device 105A is transferred to the gripper 105 on the transfer drum 105.
It is gripped by c and comes into contact with the toner image formed on the photosensitive drum 104 as the transfer drum 105 rotates. This toner image is transferred onto the transfer material P by the operation of the transfer charger 105b, and at the same time, the transfer material P is transferred onto the transfer material carrying sheet 501.
Adsorbed and retained by.

By repeating the above-described image forming and transferring operations for each color of magenta, cyan and black, an image in which the toner images of four colors are superposed and transferred onto the transfer material P can be obtained. The transfer material P on which the toner images of four colors have been transferred is the inner charging device 105d and the outer charging device 10.
After the charge is removed by 5e, it is separated from the transfer drum 105 by the separation claw 108a, the image is fixed by the heat fixing roller 126, and then discharged to the outside of the copying machine.

On the other hand, the toner remaining on the photosensitive drum 104 is removed by the cleaner 113 and is again subjected to the image forming process.

As described above, when transfer is performed by transfer charging having a polarity opposite to that of the primary charging, as shown in FIG. 17, when the transfer material P separates from the photosensitive drum 104, peeling discharge is generated, which causes air in the air. Negative charge of the transfer material P is attracted by the positive charge of the transfer material P and moves onto the transfer material P. However, the positive charge in the air is negatively charged on the photosensitive drum 104.
It moves upward and damages the photosensitive drum 104, that is, a transfer memory effect is produced.

In the transfer memory, the primary charging amount on the photosensitive drum 104 by the primary charger 106 is stored in the photosensitive drum 10.
The width of the photosensitive drum 104 is reduced in the axial direction of 4, and as a result, it is impossible to uniformly charge the photosensitive drum 104, which causes a remarkable image defect.

The transfer memory as described above is generated mainly in response to the operation of the transfer charger 105b. However, as shown in FIG. 17, especially at the end portion Pa of the transfer material P, the positive charge is generated. Are more likely to be accumulated, and as a result, the memory effect is more strongly generated, and strong streak-shaped image unevenness is likely to occur along the axial direction of the photosensitive drum 104.

The transfer memory includes the transfer charger 10.
It is effective to provide a separate charger (not shown) in the subsequent stage of 5b and remove the transfer memory of the photosensitive drum 104 after the transfer process by this charger.

[0015]

However, when the charging having the same polarity as that of the primary charging is performed between the transfer and the primary charging, a new problem arises that the image density at the end portion becomes higher than that at the central portion.

[0016]

SUMMARY OF THE INVENTION According to the present invention for solving the above problems, a photoreceptor, a charging means for charging the photoreceptor, and a photoreceptor charged by the charging means are imagewise exposed to form an electrostatic image. Image exposing means, developing means for developing an electrostatic image with toner charged to the same polarity as that of the photoreceptor, transfer means for forming a transfer electric field for transferring the toner onto the transfer material, and moving direction of the photoreceptor. In an image forming apparatus having an auxiliary charging unit that charges the photoconductor to the same polarity as the charging unit on the downstream side of the transfer unit and on the upstream side of the charging unit, the charging width of the auxiliary charging unit is wider than the charging width of the charging unit. It is characterized by that.

According to the present invention, it is possible to prevent the potential of the end portion from lowering as compared with the central portion due to the influence of the auxiliary charging means, and the image density from increasing only in the end portion.

[0018]

Embodiment 1 FIG. 1 is a schematic configuration diagram showing an embodiment of an image forming apparatus of the present invention. The image forming apparatus of this embodiment is applied to a multicolor electrophotographic copying machine having four image units I to IV.

In the present multicolor electrophotographic copying machine, each of the image forming units I to IV has photosensitive drums 4a to 4d,
Around it, primary chargers 6a to 6d with grit, developing devices 9a to 9d, transfer chargers 10a to 10d, cleaner 13
a to 13d are arranged, and further image forming units I to IV
An endless transfer material carrying belt 41 is arranged below the photosensitive drums 4a to 4d as a conveying means in a manner of penetrating the transfer material P, and transfers the transfer material P fed by a paper feed roller to the transfer charger 10a.
Image forming units I to IV at positions where 10 to 10d are arranged.
The photosensitive drums 4a to 4d are configured so as to contact the photosensitive drums 4a to 4d for conveyance.

Further, in order to charge the photosensitive drums 4a to 4d and to expose the entire surface at the same time, auxiliary charging devices 15a to 15d are provided.
And static elimination lamps 25a to 25d (see FIG. 3) are provided at the same position on the surface of the photosensitive drums 4a to 4d so as to be vertically stacked.

In this embodiment, the photosensitive drums 4a to 4d have a diameter of 60 mm, and are set to rotate in the arrow direction at 135 mm / sec. The surface of the photosensitive drums 4a to 4d is -300V to -900V by the primary chargers 6a to 6d.
Are charged to the surface, and the surface potential thereof is sensors 14a to 14d.
(See FIG. 3), and the proper photosensitive drum 4a is monitored.
A surface potential of ~ 4d is calculated. A laser beam exposure device was used as the exposure means 2.

The developing devices 9a to 9d of the respective colors have negatively charged toners of the respective colors, and the toners are carried on the respective developing sleeves and conveyed to the developing areas in the vicinity of the photosensitive drums 4a to 4d. The voltage (developing bias) applied to each developing sleeve and the photosensitive drums 4a-4
By a developing electric field formed by the surface potential of d, the toner is attached to the electrostatic latent images on the photosensitive drums 4a to 4d by reversal development and visualized as a toner image.

After the residual toner on the surface of each of the photosensitive drums 4a to 4d is removed by the cleaners 13a to 13d, the auxiliary chargers 15a to 15d are used to remove the residual toner.
a to 4d are charged with the same polarity as that of the electrostatic latent image formed thereon, that is, are negatively charged, and at the same time, are uniformly exposed by the charge eliminating lamp, and both the memory effect area and the normal area have the surface potential. The charge is removed to about 0V. Thereafter, by operating the primary chargers 6a to 6d and the laser beam exposure device 2 of the image exposure means, color-separated electrostatic latent images corresponding to the image exposure patterns are formed on the photosensitive drums 4a to 4d, and the developing device 9a. .About.9d, the electrostatic latent images are developed into visible images developed by yellow, magenta, cyan, and black toners, respectively, and these visible images are changed by the transfer chargers 10a to 10d. Carrying belt 3
1 is sequentially transferred onto the transfer material P carried on the transfer material P.
A full color is formed on top.

In this embodiment, a blade-shaped contact type transfer charger was used, but the optimum transfer current is determined by the environment. In this embodiment, it is set to 18 μA when the temperature is 25 ° C. and the humidity is 5%. .

FIG. 2 is a diagram showing the transition of the surface potential when the electrostatic latent image forming area of the photosensitive drum having the memory effect area is subjected to the primary charging as in the conventional case. As is apparent from FIG. 2, the potential of the photosensitive drum memory effect region (about +100 to +700 V) remains as it is even after the photosensitive drum is neutralized by the neutralization exposure, and the surface potential of the photosensitive drum is -700 V by performing the primary charging. The potential of the memory effect area is -300V to -650V even when charged
It has become. Therefore, the potential of the memory effect area is -30
A developing electric field corresponding to a potential difference (shown by a dotted line in the drawing) which is a difference between 0V to -650V and a developing bias voltage of -550V is formed, that is, even in a non-image area where an electrostatic latent image is not originally formed, a memory effect is caused. An electrostatic latent image has been formed, and this is developed to form an unnecessary toner image. As a result, this unnecessary toner image is transferred onto the transfer material P, and image unevenness occurs.

Alternatively, since the memory effect area is formed in the image area where the electrostatic latent image is formed, even if the electrostatic latent image is formed in the image area, the electrostatic latent image has no memory effect. The potential becomes lower than the potential of the electrostatic latent image in the normal image area, and the electrostatic latent image in the memory effect area is deeply developed, resulting in image unevenness.

That is, since the memory effect area of the photosensitive drum is charged with a potential having a polarity opposite to that of the electrostatic latent image formed on the photosensitive drum, that is, positive polarity in this example, the discharge lamp 9 is charged. The following primary charging step is performed without being discharged by the discharging exposure by. Therefore, in order to charge the memory area to a desired potential, more charge is applied to the normal area which has been subjected to static elimination. As a result, the memory effect area is normally charged even after the primary charge. That is, the electric potential becomes lower than that of the region charged to.

In particular, in the memory effect region of the portion corresponding to the edge of the transfer material P, the memory effect is remarkable, so that the surface potential difference from the normal region after primary charging also remains, resulting in a large image density difference. Will appear as.

Therefore, in the present invention, in order to solve the above-mentioned problems, the entire surface of the photosensitive drum is exposed while being charged. Therefore, in this embodiment, as shown in FIG. 3 which shows the schematic configuration of the latent image forming portion of the image forming apparatus of FIG. 1, the auxiliary charger 15 and the discharge lamp 25 are vertically stacked at the same position on the surface of the photosensitive drum. The image defect due to the memory effect described above can be sufficiently reduced.

A detailed description will be given below with reference to FIG. FIG. 4 shows changes in surface potential when the present invention is applied to a photosensitive drum having a memory effect area.

In the present embodiment, after the residual toner on the surface of the photosensitive drum 4 is removed by the cleaner 13, the photosensitive drum assists the same polarity as the electrostatic latent image formed on the photosensitive drum 4, that is, the negative polarity. It is charged by the charger 15 (first charging), and at the same time, the static elimination lamp 25 uniformly exposes the entire surface of the photosensitive drum. In this case, if only the charging by the auxiliary charger 15 is performed and the exposure by the discharge lamp 25 is not performed
The memory effect area and the normal area of the photosensitive drum 4 are shown in FIG.
As shown therein, they will be charged to about 0V to -700V and about -800V to -850V, respectively.

On the other hand, as described above, the static elimination lamp 25
Is operated simultaneously with the auxiliary charger 15 and sufficient exposure is performed at the same time as charging, the charge charged by the auxiliary charger 15 is immediately conducted through the photoconductive layer of the photosensitive drum 4 and attenuated. As a result, the normal area of the photosensitive drum 4 is discharged to almost 0 V without being charged to a significantly high potential by the auxiliary charger 15, and the memory effect area of the photosensitive drum 4 is charged to the opposite polarity (positive polarity). (Charging) is removed and the memory effect disappears, and it is almost 0 as in the normal area.
The charge is removed to V. After that, the photosensitive drum 4 becomes the primary charger 6
Charging is performed (second charging), that is, the primary charging charges the surface potential of −700V.

The developing bias voltage for forming the developing electric field is set to -550V. The difference between the charging potential of the photosensitive drum of -700V and the developing bias voltage of -550V, 150V, is the fog removal voltage. Due to the electric field formed by this potential difference, the toner of the developing device is not always attracted to the developing sleeve of the developing device and does not adhere to the photosensitive drum 4, and therefore the fog does not occur in the white portion of the image.

On the other hand, since the portion of the photosensitive drum 4 corresponding to the image pattern is irradiated with the laser beam with the intensity corresponding to the image density, the potential of the portion exposed by the laser beam is sufficiently lowered, and FIG. As shown, the developing bias voltage becomes -350V, which is lower than -550V, and the toner on the developing sleeve causes the toner on the developing drum to develop due to the developing electric field formed by the developing bias voltage and the surface potential of the exposed portion of the photosensitive drum. 4 and a toner image is formed on the photosensitive drum 4.

As described above, according to this embodiment, prior to the primary charging of the photosensitive drum 4, the memory effect area of the photosensitive drum 4 has the same polarity as the electrostatic latent image formed on the photosensitive drum 4, that is, Since the auxiliary charging device 15 charges the photosensitive drum 4 (first charging) so as to have a negative polarity, and at the same time, the photosensitive drum 4 is discharged by the discharging lamp 25, so that the photosensitive drum 4 is charged before the primary charging. It is possible to remove the memory effect region and to eliminate the charge to a substantially uniform potential without charging the normal region not subjected to the memory effect to a remarkably high potential. Therefore, the subsequent secondary charging, that is, the primary charging, the image exposure, and the development are normally performed, and a toner image is formed in the memory effect area as in the conventional case, and the toner image is transferred to the transfer material P. Disturbance did not occur.

However, as compared with the case of performing only the primary charging in the related art, the following problems occur by performing the auxiliary charging. That is, in the auxiliary charging, the entire surface is exposed at the same time to remove the electric charge. However, by operating the auxiliary charging, there is a residual potential, and a potential difference occurs between the end portion of the charger and the insufficient charging portion. As a result, a phenomenon occurs in which the image density at both ends of the image becomes high.

A detailed description will be given below.

FIG. 8 schematically shows the surface potential of the photosensitive drum when the image density is increased at the edge (hereinafter, edge density is increased). V _A0 pre-exposure ON auxiliary charge OFF primary charge OFF V _A500 Pre-exposure ON Auxiliary charge OFF Primary charge ON (Grid bias potential -500V) V _B0 Pre-exposure ON Auxiliary charge ON Primary charge OFF V _B500 Pre-exposure ON Auxiliary charge ON Primary charge ON (Grid bias potential -500V) . The screen grid of the primary charger uses the etching grid of SUS304 as shown in FIG. The opening width X in the longitudinal direction of the etching grid is 3
16 mm. On the other hand, the opening width of the auxiliary charging 15 in the longitudinal direction is also 316 mm.

The opening width of the primary charger in the circumferential direction is 21.
Although it is mm, that of the auxiliary charger is 12 mm due to the installation space problem. Normally, the chargeability of the photosensitive drum at the end portion varies depending on the opening width in the circumferential direction, and the smaller the opening width, the more likely the sagging occurs at the end portion. Further, while the total discharge current of the primary charger is −600 μA, the auxiliary charger is set to −30 smaller than the primary charger due to the problem of 0 _{3 and so on.}
It is set to 0 μA. As a result, the sagging of the electric potential at the ends is further increased.

On the other hand, as the transfer charger in this embodiment, a blade type contact type charger is used. The maximum image width in this embodiment is 297 mm. Transfer this image onto the transfer paper. The width of the charger must be longer than this.
6 mm. Further, if the width of the primary charger is shorter than this, the photosensitive drum outside the primary charging width is always positively charged, and from the viewpoint of the developing bias potential, the toner is in the state of being developed on the drum, The required toner is developed to form a band outside the image area. this is,
The transfer belt, etc. may become dirty and cause various image defects. Therefore, the primary charging width needs to be wider than the transfer charging width.

By the way, the surface potential of the photosensitive drum is shown in FIG.
As shown in, the potential difference between when the auxiliary charging is operating and when the auxiliary charging is not operating is about 15 to 20V. The developing electric field formed between the surface potential of the electrostatic latent image and the developing bias applied to the developing sleeve causes toner, which is charged particles, to adhere to the photosensitive drum to form a toner image. Since the image forming apparatus indicated by (1) performs development by reversal development, the negative potential is lower with respect to the development bias (for example, the development bias is -350V, the surface potential is -150V to -100V. The lower the toner is, the more easily toner adheres to the electrostatic latent image.

When the opening widths in the longitudinal direction of the primary charger 6 and the auxiliary charger 15 are the same, a potential sag occurs at both ends like V _B0 and V _{B500 in} FIG. The potential difference in the sag was about 15 to 20V. It can be seen that this occurs because the amount of charge at the end of the auxiliary charging is weak, and the density increases at both ends on the image, which correspond to both ends in the longitudinal direction of the charger, on the image. Therefore, in the image forming apparatus of the present embodiment, by making the opening width in the longitudinal direction of the primary charger, that is, the opening width in the longitudinal direction of the etching grid longer than the opening width in the auxiliary charger, the above-mentioned problem occurs. It was possible to solve. Specifically, by setting the opening width of the auxiliary charger in the longitudinal direction from 316 mm to 330 mm, the potential sag does not occur within the primary charging width, and the density increase at the edge of the image does not occur. confirmed.

As described above, in order to increase the density of the image edge portion caused by providing the auxiliary charging for the purpose of removing the memory effect area, the opening width in the longitudinal direction of the auxiliary charging device is set to the opening width of the primary charging device (main In the example, it was possible to prevent the generation by making it longer than the opening width in the longitudinal direction of the etching grid.

Furthermore, by making the primary charging width wider than the image exposure width and the transfer charging width, and more particularly, making the transfer charging width wider than the image exposure width, it is possible to achieve even density distribution in the longitudinal direction. It was

Embodiment 2 The apparatus configuration of this embodiment is the same as that of Embodiment 1 except for the auxiliary charger. That is, the auxiliary charger 15 used in Example 1
Was a corotron type charger without a grid, which was configured to include a charging wire 42a connected to a high voltage power source 42a, as shown in FIG. On the other hand, the auxiliary charger 15 used in this embodiment is a scorotron type charger having a grid 42c as shown in FIG. According to this charger, the amount of discharge emitted from the charging line 42a due to the application of the high voltage from the high voltage power source 42a is determined by the grid 42 from the grid bias power source 42d.
It can be controlled by applying a control voltage to c.

In this example, the same effect as in Example 1 was obtained by increasing the longitudinal opening width of the grid of the auxiliary charger and the longitudinal opening width of the primary charger.

Further, as in Example 1, it is preferable that the image exposure width <transfer charging width <primary charging width.

Example 3 In this example, the present invention is applied to a conventional image forming apparatus. FIG. 7 shows the vicinity of the image forming unit, and an auxiliary charger 15 is provided. A static elimination lamp 25 is installed on the upper portion of the auxiliary charger, and the auxiliary charger 15 discharges electricity while charging. Also in this embodiment, by making the longitudinal opening width of the auxiliary charger 15 longer than the longitudinal opening width of the primary charger, the memory effect area is effectively removed and the auxiliary charging such as the edge density increase is performed. By doing so, it is possible to suppress the occurrence of image defects.

[0049]

As described above, according to the present invention, it is possible to remove the potential memory due to peeling discharge during transfer and prevent the increase of the density at the end.

[Brief description of the drawings]

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

FIG. 2 is a diagram showing a transition of a surface potential of a photosensitive drum having a memory effect area in a conventional image forming apparatus.

FIG. 3 is a schematic configuration diagram illustrating a latent image forming unit of the image forming apparatus according to the first exemplary embodiment.

FIG. 4 is a diagram showing a transition of a surface potential of a photosensitive drum having a memory effect area.

FIG. 5 is a diagram showing a relationship between a transfer current value and a surface potential of a photosensitive drum having a memory effect area.

FIG. 6 is a diagram showing a relationship between a surface potential of a memory effect region and a necessary charging current of an auxiliary charger.

FIG. 7 is a schematic configuration diagram showing a third embodiment of the image forming apparatus of the invention.

FIG. 8 is a diagram schematically showing a potential in a longitudinal direction of a photosensitive drum.

FIG. 9 is a schematic configuration diagram showing a latent image forming unit of a conventional image forming apparatus.

FIG. 10 is a diagram showing an etching grid.

FIG. 11 is a cross-sectional view showing a fuse lamp array as a static elimination lamp that can be used in the present invention.

FIG. 12 is a cross-sectional view showing an LED lamp array as a static elimination lamp that can be used in the present invention.

FIG. 13 is a cross-sectional view showing a scorotron type charger as an auxiliary charger used in the present invention.

FIG. 14 is a sectional view showing a corotron type charger as an auxiliary charger used in the present invention.

FIG. 15 is a schematic configuration diagram showing an example of a conventional image forming apparatus.

16 is a perspective view showing a transfer drum of a transfer device used in the image forming apparatus of FIG.

FIG. 17 is an explanatory diagram showing peeling discharge at the end of the transfer material.

[Explanation of symbols]

 4, 4a to 4d Photosensitive drum 6, 6a to 6d Primary charging device 9, 9a to 9d Developing device 10, 10a to 10d Transfer charging device 13, 13a to 13d Cleaner 15, 15a to 15d Auxiliary charging device 25, 25a to 25d lamp

Claims (7)

[Claims] 1. A photosensitive member, a primary charging unit for charging the photosensitive member, an image exposing unit for exposing the photosensitive member charged by the primary charging unit to form an electrostatic image, and an electrostatic image for the photosensitive member. Developing means for developing with toner charged to the same polarity as the charging polarity, transfer means for forming a transfer electric field for transferring the toner to the transfer material, and downstream of the transfer means in the moving direction of the photoreceptor.
In an image forming apparatus having an auxiliary charging unit that charges the photoconductor on the upstream side of the charging unit with the same polarity as the charging by the primary charging unit, the charging width of the auxiliary charging unit is wider than the charging width of the primary charging unit. A characteristic image forming apparatus. 2. The rising of the charging potential at the charging width end is
The image forming apparatus according to claim 1, wherein the primary charging unit is sharper than the auxiliary charging unit. 3. The image forming apparatus according to claim 1, wherein the primary charging means has a scorotron, and the auxiliary charging means has a corotron. 4. The image forming apparatus according to claim 1, further comprising an entire surface exposing means for attenuating a charging potential of the auxiliary charging means, wherein the primary charging means charges the photosensitive member which is entirely exposed. 5. A cleaning means for cleaning the residual toner on the surface of the photoconductor after transfer is provided, and the auxiliary charging means is provided downstream of the cleaning means in the moving direction of the photoconductor. Image forming apparatuses 1 to 4. 6. The image forming apparatus according to claim 1, wherein the charging width of the primary charging means is wider than the charging width of the transfer means and the exposure width of the image exposing means. 7. The image forming apparatus according to claim 6, wherein the charging width of the transfer means is wider than the exposure width of the image exposure means.