Electrophotography – Control of electrophotography process – Control of charging
Reexamination Certificate
2000-06-28
2001-11-27
Lee, Susan S.Y. (Department: 2852)
Electrophotography
Control of electrophotography process
Control of charging
C399S043000, C399S175000
Reexamination Certificate
active
06324357
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
The present invention relates to an image forming apparatus such as a copying machine, a printer, or the like, which employs an electrophotographic or electrostatic recording method. It also relates to a process cartridge removably installable in such an image forming apparatus.
In recent years, an image forming apparatus has been progressively reduced in size, and there has appeared a cleaner-less image forming apparatus, which lacks a cleaner as a cleaning means for removing residue, such as the toner which remains on the peripheral surface of the photosensitive drum after image transfer. In the case of a cleaner-less image forming apparatus, the cleaning is done by a developing apparatus at the same time and place as developing is done (development-recovery).
The term “development-cleaning” method fog-removal bias, during the following development processes. According to this method, the transfer residual toner is recovered to be used in the following image formation processes, and no waste toner is produced, making it possible to reduce the annoying time and effort required for maintenance. Further, it is advantageous in terms of space; in other words, elimination of a cleaner makes it possible to drastically reduce the size of an image forming apparatus.
Regarding a charging means for charging an object, for example, the aforementioned photosensitive drum, to predetermined polarity and potential level, a contact charging apparatus (contact charging device) has been put to practical use because of its advantage of being smaller in ozone production, power consumption, and the like. A contact charging apparatus is a charging apparatus which charges an object by placing a charging member (contact charging member), to which voltage is being applied, in contact with the object.
As a typical contact charging apparatus, a magnetic brush type contact charging apparatus (magnetic brush type charging device) comes to mind. A magnetic brush type contact charging apparatus employs a magnetic brush as a contact charging member. A magnetic brush is formed by magnetically confining electrically conductive magnetic particles directly on a magnet, or on a sleeve which contains a magnet. It is placed in contact with an object, while being held stationary or being rotated. The object begins to be charged as voltage is applied to the magnetic brush.
There are contact charging members other than a magnetic type contact charging member. For example, a brush formed by shaping a bundle of electrically conductive fiber into the form of a brush (fur brush), and an electrically conductive rubber roller formed by shaping electrically conductive rubber into the form of a roller (charge roller), are also used as a preferable contact charging member.
When a magnetic brush type contact charging apparatus is used to charge a photosensitive member formed by placing on an ordinary organic photosensitive member, a surface layer (charge injection layer) in which electrically conductive microscopic particles are dispersed, or a photosensitive member based on amorphous silicon, it is possible to give the photosensitive member a surface potential level approximately equal to the potential level of the DC component of the bias applied to the magnetic brush. This type of charging method is called “charge injection”. This charge injection based on a magnetic brush does not rely on electrical discharge on which a corona type charging device relies, to charge an object. Therefore, the usage of this magnetic based charge injection makes it possible to charge an object without producing any amount of ozone, and also to reduce the amount of power consumption. Thus, a magnetic brush based charge injection has recently been attracting attention.
Further using toner excelling in the mold releasing property, for example, toner manufactured by polymerization, is very effective to improve the efficiency of the development-cleaning process.
The aforementioned cleaner-less system does not require a waste toner container, and therefore, when it is employed by an image forming apparatus which employs a process cartridge system, it is possible to extend the service life of the image forming apparatus, although the actual length of the service life depends on the durability of the components other than those for a cleaning device. To summarize, incorporation of a cleaner-less system into an image forming apparatus makes it possible to realize an image forming apparatus which is smaller in size and running cost, and produces no ozone.
However, in the case of an image forming apparatus which employs a contact charging apparatus as a charging means for charging an image bearing member such as a photosensitive member, as image formation is repeated, toner particles as developer mix into the contact charging member from the surface of the image bearing member, thereby contaminating the contact charging member. As a result, the performance of the contact charging member is sometimes compromised in consistency.
This problem is particularly conspicuous in a cleaner-less system, such as a cleaner-less image forming apparatus which lacks a cleaning apparatus for removing the transfer residual toner from the image bearing member, and in which the transfer residual toner mixes into the contact charging member by a relatively large amount.
The transfer residual toner on the image forming apparatus preserves the pattern of the preceding image virtually as it was. Therefore, if the portion of the peripheral surface of the photosensitive drum, where the transfer residual toner is present, is passed through the charging portion (charging area), that is, the interface between the contact charging member and image bearing member, while the transfer residual toner remains undisturbed, problems occur. For example, the potential level of the photosensitive drum is reduced across the area covered with the transfer residual toner, or the transfer residual toner blocks the exposure light during the following image forming rotational cycles of the photosensitive drum. As a result, the following image formation step, that is, the development step, is affected; during the following image forming rotational cycles of the photosensitive drum, the image is formed lighter or darker, across the portions corresponding to where the transfer residual toner is. In other words, a ghost is created.
As a countermeasure for the formation of a ghost, it has been proposed to employ a leveling member in the form of a brush for evenly distributing the transfer residual toner which preserves the pattern of the preceding image, across the peripheral surface of the photosensitive drum, or to employ a simplified cleaning member which temporarily relies on the effects of bias.
In the case of the contact charging, there is a bias condition under which the toner, which has mixed into, or adhered to, a contact charging member, can be easily ejected out onto the photosensitive drum. Thus, the service life of a contact charging member can be extended with the provision of an operational mode in which toner is ejected from the contact charging member using this bias condition.
However, the aforementioned bias condition favorable for toner ejection does not necessarily coexist with a condition favorable for charging. Further, it is possible that regularly running a long sequence of forcefully ejecting toner may result in the consumption of an unnecessary amount of time. In addition, in the case of a simply structured image forming apparatus which does not have any means for detecting the state of a contact charging means in terms of the deterioration resulting from usage, it is difficult to eject toner effectively in a short time, while maintaining satisfactory image quality.
For example, even if a case in which a relatively small number of prints with a high image ratio are produced in a relatively short period, and a case in which a relatively large number of prints with a low image ratio are produced in a relatively long period, are equal in the
Gomi Fumiteru
Hashimoto Kouichi
Komiya Yoshiyuki
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Lee Susan S.Y.
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