Radiation imagery chemistry: process – composition – or product th – Electric or magnetic imagery – e.g. – xerography,... – Radiation-sensitive composition or product
Reexamination Certificate
2001-11-15
2003-11-11
Chapman, Mark A. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Electric or magnetic imagery, e.g., xerography,...
Radiation-sensitive composition or product
C430S124300, C399S159000
Reexamination Certificate
active
06645688
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an image-forming apparatus and an image-forming method which make use of an amorphous-silicon electrophotographic photosensitive member, a contact charging means and a spherical toner.
2. Related Background Art
Conventionally, it is common to use corona charging assemblies in charging units for photosensitive members used in, e.g., plain-paper copying machines, laser beam printers, LED printers and liquid-crystal shutter printers, and such corona charging assemblies are in wide use. The corona charging assemblies charge object members electrostatically by applying a high voltage of about 5 to 10 kV to a metal wire of about 50 to 100 &mgr;m in diameter to ionize the atmosphere.
For structural reasons, the corona charging assemblies have a disadvantage that generation of ozone in a large quantity accompanies corona discharging. With their repeated used, ozone and corona products may become deposited on the photosensitive member surface, under the influence of which the photosensitive member surface may become susceptible to humidity to tend to absorb moisture content. This may cause a lateral flow of electric charges on the photosensitive member surface in an environment of high temperature and high humidity to cause a lowering of image quality which is called smeared images. In particular, electrophotographic photosensitive members making use of amorphous silicon (hereinafter “a-Si photosensitive member”) have so high a surface hardness that, while they are durable to printing on a large number of sheets, their surfaces may abrade with difficulty. Hence, corona products having once adhered can be removed with difficulty to have a great influence.
The corona charging assemblies are also usually often used under constant-current control. In such a case, they tends to be affected by any uneven layer thickness and resistance distribution of the photosensitive member. This may cause unevenness in surface potential, and may consequently cause uneven density on images.
In order to solve such a problem on image quality, various charging units are proposed.
In a contact charging unit as disclosed in Japanese Patent Application Laid-open No. 63-208878, a charging member to which a voltage is kept applied is brought into contact with an object member to be charged (photosensitive member), which is called charging object member, to charge the photosensitive member surface to an intended potential. Compared with the corona charging assemblies, such a unit can achieve a low voltage in respect of the applied voltage necessary for providing the desired potential on the charging object member surface, and does not cause any smeared images due to the ozone products because the quantity of ozone occurring in the course of charging is zero or is very small. Also, in such contact charging, the surface of the photosensitive member is charged to have substantially a uniform potential in accordance with the applied voltage, and hence uneven image density may little occur. It has such advantages.
In the way of progress such that a series of contact charging members are improved in various manners, as disclosed in Japanese Patent Application Laid-open No. 8-6353, a mechanism is proposed in which a contact charging member making use of particles in the form of a magnetic brush comprised of a magnetic material and magnetic particles (or powder) is brought into contact with an electrophotographic photosensitive member to provide it with charge. Also proposed is, as disclosed in Japanese Patent Application Laid-open No. 10-307454, a new method of a mechanism in which a carrying member having conductivity and elasticity so constructed that charged particles are carried on the surface is brought into contact with a photosensitive member to provide it with charge.
Attempts to achieve much higher image quality are also made from improvements of toners. More specifically, polymerization toners are on studies in place of conventional pulverization toners.
The polymerization toners have superior fluidity because they have particles in substantially a uniform spherical shape and having less scattering in particle diameter. Also, they are advantageous to the achievement of high image quality because they do not let colorants come bare to particle surfaces and have uniform triboelectric chargeability. Still also, they can enclose wax in particles, and can attain good fixing performance and anti-offset properties. Hence, the polymerization toners are being gradually widely employed in high-image-quality machines. As a patent application which proposes a magnetic polymerization toner, EP1058157 A1 is accessible.
In recent years, what also attracts notice is to make image-forming apparatus small-sized. In image-forming apparatus, usually a latent image is developed with a toner to make it into a visible image, the toner image is transfer to a transfer medium such as paper, and thereafter toner particles having remained on a photosensitive member without being transferred onto the transfer medium are removed through a cleaning step. With regard to this cleaning step, blade cleaning, fur brush cleaning, roller cleaning and so forth have conventionally been used. However, from the viewpoint of apparatus, apparatus are necessarily set up in a large size because a unit for such cleaning must be provided. This has been a bottleneck in making apparatus compact.
In addition, from the viewpoint of ecology, the waste toner that comes from the cleaning step is undesirable. In the sense of effective utilization of toners, too, it has been sought to provide a system which does not send forth any waste toner.
As one means for meeting such demands, an image-forming apparatus employing the technique called cleaning-at-development or cleanerless. The cleanerless image-forming apparatus is an apparatus in which any conventional cleaning unit is not provided and the transfer residual toner having remained on the surface of an electrophotographic photosensitive member is collected at its developing means which performs development simultaneously. Employment of this technique makes it possible to save the space for the part of the cleaner, and can contribute towards making image-forming apparatus compact. Also, since any waste toner does not come out, such apparatus have the merit of being tender of environment and improving utilization efficiency of toners.
As stated above, attempts to achieve much higher image quality are being made by combining the formation of uniform latent images free of any unfocused or uneven images that is attributable to contact charging units with the formation of faithful visible images that is attributable to polymerization toners.
However, in the case when the voltage application type contact charging unit is utilized as a means for charging the electrophotographic photosensitive member, there are the following problems.
In such a contact charging unit, it has very good charge potential uniformity when viewed macroscopically as stated above. However, when viewed microscopically, for the reasons of its construction, marks of contact of the magnetic brush or charged particles with the photosensitive member (brush images) may appear. In such a case, it is necessary to make higher the relative speed between the charging unit and the photosensitive member to make them rub against each other in a greater extent so that the charging unit can be brought into uniform contact with the electrophotographic photosensitive member. However, because of such rubbing, the surface of the photosensitive member may abrade or wear, though slightly. Although such wear is at a small level, even microscopic abrasion may have a great influence when it lasts over a long period time, because the a-Si photosensitive member has a long lifetime originally. Accordingly, it is a subject how the contact performance be improved while the abrasion level of the photosensitive member surface is reduced.
As another problem other than such uneven charging, there is also
Hashizume Junichiro
Hosoi Kazuto
Okamura Ryuji
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