1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus.
2. Description of the Related Art
Conventionally, as an electrophotographic image forming apparatus, there are an electrophotographic copying machine, an electrophotographic printer (such as an LED printer and a laser beam printer), an electrophotographic facsimile machine, and the like.
In image forming apparatuses of these types, a surface of an electrophotographic photosensitive member (hereinafter, referred to as a photosensitive drum or a drum) is evenly charged by a primary charger and the charged photosensitive drum surface is exposed by an exposure device to form an electrostatic latent image. Then, the electrostatic latent image is developed by a developing device to form a developer image (hereinafter, referred to as a toner image). The toner image is transferred by a transfer device onto a recording material such as a sheet. After that, the toner image is fixed by a fixing device onto the recording material as a fixed image to be output. Residual toner remaining on the photosensitive drum surface after the toner image is transferred is cleaned by a cleaning device to be ready for the next image forming operation.
In recent years, a charging device according to a contact charging method is mounted in many image forming apparatuses, and such a charging device is the dominating charging device. In most charging devices according to the contact charging method, a conductive roller is adopted as a contact charging member, and voltage is applied with the conductive roller being in contact with a photosensitive drum, which is referred to as roller charging. The contact charging method is broken down into a direct current method where only a direct current voltage is applied to the contact charging member to charge the photosensitive drum surface and an alternating current superimposition method where direct current voltage with alternating current voltage superimposed thereon is applied to charge the photosensitive drum surface.
In the alternating current superimposition method, there is an advantage that the photosensitive drum surface can be evenly charged. However, since discharge occurs in a minute gap, the photosensitive drum surface is damaged, the amount of wear of the photosensitive drum increases, and the lifetime of the photosensitive drum is shortened.
On the other hand, in the direct current method, compared with the case in the alternating current superimposition method, the energy of discharge which occurs in a minute gap is smaller, so the photosensitive drum is less damaged and the lifetime of the photosensitive drum can be made longer. Therefore, from the viewpoint of increasing the lifetime of the photosensitive drum, it is preferred to charge the photosensitive drum using the direct current method.
However, particularly when the photosensitive drum is charged using the direct current method, there arise the following problems.
The potential of the photosensitive drum surface after an image is formed thereon becomes uneven according to the formed image. Even if the next charge is carried out in this state, charging may not be carried out evenly depending on the image formed last time. As a result, the potential of the photosensitive drum surface exposed by an exposure device, such as a laser, also becomes uneven. In other words, a so-called ghost image may occur. More specifically, when a halftone image is formed after a high-contrast pattern is formed, the so-called ghost image that the previous image pattern emerges in the halftone image may occur.
Therefore, conventionally, a device for exposure before charging having a light source, such as an LED, evenly exposes the photosensitive drum surface to light. This makes even the potential (light section potential) of the image on the photosensitive drum surface when the photosensitive drum surface is exposed by an exposure device and the potential (dark section potential) of the remaining non-image, and thus, an even charge for the next time is made possible and a ghost image is prevented from occurring.
On the other hand, when an image is formed repeatedly for a long time, exposure before charging evenly accumulates charge on the photosensitive drum surface, and a predetermined photosensitive drum light section potential may not be obtained. In other words, the sensitivity of the photosensitive drum may become dull. When the sensitivity of the photosensitive drum becomes dull, there arise problems that a formed image becomes faint and that gradations of an image are lost.
Such deterioration in sensitivity of the photosensitive drum becomes more apparent when the exposure before charging is carried out continuously for a longer time or when the light amount of the exposure before charging is larger.
In Japanese Patent Application Laid-Open No. 11-174755, it is reported that use of a device for exposure before charging for a long time decreases the light amount of the device for exposure before charging. If the light amount of the device for exposure before charging is decreased, the history of a previously-formed image cannot be completely erased, and a ghost image occurs.
Therefore, in order to suppress occurrence of a ghost image and deterioration in sensitivity of the photosensitive drum and to increase the lifetime of the device for exposure before charging, it is necessary to adopt an optimum pre-exposure light amount according to the state of use of the image forming apparatus and variations in manufacture of the pre-exposure light source, such as an LED, and of the sensitivity of the photosensitive drum.
Conventionally, as described in Japanese Patent Application Laid-Open No. 11-174755, control of the exposure light amount in the exposure before charging is carried out according to the relationship between detection by a photosensitive drum surface potential detecting device and electric current applied to the charging device.
However, such a photosensitive drum surface potential detecting device is expensive. Therefore, an image forming apparatus which is less expensive and which can suppress a ghost image and deterioration in sensitivity of the photosensitive drum is desired. In order to attain this, it is necessary to adopt an optimum light amount of the exposure before charging according to the state of use of the image forming apparatus and variations in manufacture of the light source for exposure before charging, such as an LED, and the sensitivity of the photosensitive drum without using a photosensitive drum surface potential detecting device.