Radiation imagery chemistry: process – composition – or product th – Electric or magnetic imagery – e.g. – xerography,... – Post imaging process – finishing – or perfecting composition...
Reexamination Certificate
1995-06-21
2001-11-13
Dote, Janis L. (Department: 1753)
Radiation imagery chemistry: process, composition, or product th
Electric or magnetic imagery, e.g., xerography,...
Post imaging process, finishing, or perfecting composition...
C430S108600, C430S111330, C430S111400, C430S111410
Reexamination Certificate
active
06316156
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a carrier which composes a two component type developer used to develop an electrostatic latent image or magnetic latent image in electrophotography or electrostatic printing, and a two component type developer making use of the carrier. More particularly, it relates to a carrier which composes a two component type developer remarkably improved in running performance and environmental properties, and a two component type developer making use of the carrier. The present invention also relates to an image forming method carried out using the two component type developer.
2. Related Background Art
As developing methods for electrophotography, there are a number of methods such as cascade development and magnetic brush development. Developers used in these developing methods include one-component type developers and two component type developers.
Carriers that composes such two component type developers can be roughly grouped into a conductive carrier and an insulative carrier. The conductive carrier is usually comprised of oxidized or unoxidized iron powder. Two component type developers comprised of this iron powder carrier, however, have the problem that the triboelectric chargeability to a toner is unstable and hence fog tends to occur on visible images formed. More specifically, as the two component type developer is used, toner particles adhere to and accumulate on the surfaces of the iron powder carrier particles (i.e., toner is spent), so that the electrical resistance of carrier particles increases to lower bias currents, and also to make the triboelectric chargeability unstable, resulting in a decrease in the image density of visible images formed and an increase of fog. Thus, when copies are continuously taken by an electrophotographic copying machine by the use of the two component type developer containing the iron powder carrier, the two component type developer may deteriorate upon copying on a small number of copy sheets and hence it becomes necessary to change the two component type developer at an early stage, resulting in a high cost after all.
Accordingly, in recent years, as a measure for eliminating disadvantages of such iron powder carrier, it is proposed to use ferrite particles as the carrier (Japanese Patent Applications Laid-open No. 52-565367, No. 58-202456, etc.).
Ferrite carriers hitherto put into practical use can exhibit excellent performance not achievable by iron powder carriers, when used in specific toners and electrophotographic equipments. However, taking account of various required performances, there is still room for further improvement, and no perfect ferrite carriers are known at present. For example, even if a carrier with a proper electrical resistance can be obtained by selecting ferrite composition and firing temperature, no desired magnetic properties can be obtained, or even if the electrical resistance and magnetic properties can be kept within proper ranges, the charge quantity can not be well controlled. Such difficulties have been involved.
The insulative carrier is commonly typified by a carrier comprising carrier core particles comprised of a ferromagnetic material such as iron, nickel or ferrite whose surfaces are uniformly coated with an insulating resin. Two component type developers that employ this carrier may little cause the melt-adhesion of toner particles to the carrier surfaces, compared with the case of the conductive carrier, and at the same time the triboelectric chargeability of carriers on toners can be controlled with ease. Hence, there is the advantage that it is suitable particularly for high-speed electrophotographic copying machines in view of its superior durability and long lifetime.
There are various performances required for the insulative carrier. Particularly important performances can be set out as proper chargeability, impact resistance, wear resistance, a good adhesion between cores and coating materials, and uniformity in charge distribution.
Moreover, in recent years, there is an increasing commercial demand for making copying machines achieve a higher minuteness and making images have a higher quality. In the present technical field, it is attempted to make toner particle diameter smaller so that color images can be formed in a high image quality. Making smaller the particle diameters of toner particles, however, results in an increase in the surface area per unit weight, tending not only to bring about an excessively large quantity of triboelectricity of the toner but also a decrease in charging speed. This is accompanied with a possibility of the insufficiency of image density or the deterioration of durability or running performance such as fog and toner scatter.
That is, in the development of electrostatic latent images held on an electrostatic latent image bearing member, toner is blended with a carrier formed of relatively large particles and is used as a two component type developer for electrophotography. The composition of both the toner and the carrier is selected so that as a result of their mutual contact friction the toner can have, e.g., a polarity reverse to the charges present on the photoconductive layer. As a result of contact friction between the both, the carrier further electrostatically attracts the toner to its particle surfaces to transport the toner as a developer through a developing assembly and also feed the toner onto the photoconductive layer of the electrostatic latent image bearing member.
When, however, copies are continuously taken on many copy sheets by an electrophotographic copying apparatus using such a two component type developer, although sharp images with a good image quality can be obtained at the initial stage, edge effect with much fog may seriously occur after copies have been taken on several tens of thousands of sheets, resulting in images having poor gradation and sharpness.
In color copying carried out using toners with chromatic colors, continuous gradation is an important factor that influences image quality, and the edge effect that stresses only margins of images, occurring after copies have been taken on a large number of copy sheets, greatly damages the gradation of images. For example, quasi-contours due to the edge effect are formed in the vicinity of actual contours, resulting in a loss of copying reproducibility including color reproducibility in color copying.
Image area used in conventional black and white copying is 10% or less and images are almost held by line images as in letters, documents, reports and so forth. On the other hand, in the case of color copying, image area is 20% at least, and images are held by gradational solid images at a reasonable frequency or occupancy as in photographs, catalogues, maps, pictures and so forth.
When copies are continuously taken using such originals having a large image area, reproductions with a high image density can be obtained at the initial stage in usual instances, but the feeding of toner to the two component type developer may become insufficient with time to cause a decrease in density, or the toner being fed and the carrier may mix in the state of charge insufficiency to cause fog or cause a local increase or decrease in toner concentration (which indicates toner-carrier mixing ratio) on the developing sleeve, tending to result in blurred images or non-uniform image density. This tendency becomes more remarkable when the toner has a smaller particle diameter.
Such under-development and fog are presumed to be caused by an excessively low toner content (i.e., toner concentration) in the two component type developer or a poor rise for rapid triboelectric charging between the toner being fed and the carrier contained in the two component type developer, where any uncontrollable, insufficiently charged toner thereby produced participates in development.
It is essential for color developers to have the ability to always output images with a good image quality in the continuous copying of originals havi
Ida Tetsuya
Okado Kenji
Takiguchi Tsuyoshi
Canon Kabushiki Kaisha
Dote Janis L.
Fitzpatrick ,Cella, Harper & Scinto
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