Electrophotography – Image formation – Development
Reexamination Certificate
1999-02-16
2001-10-02
Pendegrass, Joan (Department: 2852)
Electrophotography
Image formation
Development
Reexamination Certificate
active
06298210
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a device and a method for developing an electrostatic latent image, more particularly, one that is located on a movable image carrier, the latent image being developed by the use of a single or one-component toner.
Electrographic printing with high quality and at high speed is possible according to the present state of the art by two-component toners or one-component toners. A type of development of a latent image by a one-component toner has become known heretofore from the published German Non-prosecuted Patent Application (DE-OS) 197 28 309.
A main difficulty with one-component systems is that a uniform layer of toner particles which are charged as uniformly as possible have to be generated on a developing roller, also termed an ink applicator roller. Some commercially used systems employ a rejuvenator roller formed of a material resembling foam plastic, for transporting toner particles from a toner reservoir to the developing roller. Friction produced in this process charges the toner particles electrically, due to which they adhere to the electrically conductive developing roller in a more-or-less thick layer. In order to homogenize this layer, use has been made of fixed doctor blades which scrape off excess toner from the developing roller. There are systems with a hard developing roller, for example, formed of aluminum or steel, and a rubber lip as a doctor blade, as well as systems also with a hard doctor blade and a developing roller made of rubber material.
The hereinaforementioned German Non-prosecuted Patent Application (DE-OS) 197 28 309 describes an electrostatic developing device for one-component toners which, by comparison with the prior art of one-component toners, permits a clearly higher speed. In this case, the toner particles are initially applied to the developing roller via a so-called rejuvenator roller formed of plastic material. The friction produced during this process charges the toner particles electrostatically, and they adhere via an electrostatic imaging force to the surface of the metallic developing roller. In a subsequent step, a doctor roller serves to reduce the thickness of the toner layer to a desired thickness and to homogenize it or make it uniform. Speeds of at least 50 cm/sec. are possible with such a developing device. It is regarded as disadvantageous, with regard to the developing stations known in the prior state of the art, that the toner layers cannot yet be made ready on the developing roller with adequate uniformity, for the purpose of high-quality printing. With regard to relatively high speeds, in particular, in the doctor blade processes known from the prior state of the art, slight stripes are formed during the respective doctor blade process, and become quite distractingly apparent in the developed image.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a device and a method for developing an electrostatic latent image which ensure adequate homogenization or uniform distribution of the toner layer on the developing roller, in order thereby to produce high-quality printing.
With the foregoing and other objects in view, there is provided, in accordance with one aspect of the invention, a device for developing an electrostatic latent image located on a movable image carrier, the developing device including a toner feeder for feeding toner particles from a toner reservoir and for electrically charging the particles, and a rotatably mounted developing roller having a surface for accepting the charged toner particles, comprising a device spaced from the surface of the developing roller for generating an alternating electric field actable upon the toner particles.
In accordance with another feature of the invention, the field-generating device has an electrode structure.
In accordance with a further feature of the invention, the developing device includes a doctor roller having a surface separated from the surface of the developing roller by a gap wider than a mean diameter of the toner particles.
In accordance with an added feature of the invention, the field-generating device has a strip structure with radial structures.
In accordance with an additional feature of the invention, the field-generating device has a strip structure with axial electrodes.
In accordance with yet another feature of the invention, the electrode structure of the field-generating device has electrodes disposed in a mutual grid spacing corresponding to the spacing between the surface of the developing roller and the electrode structure.
In accordance with yet a further feature of the invention, the field-generating device has a strip structure disposed diagonally relative to a direction of movement thereof.
In accordance with yet an added feature of the invention, a continuous electrode is arranged downline of the field-generating device.
In accordance with yet an additional feature of the invention, the field-generating device is formed of wires.
In accordance with still another feature of the invention, the field-generating device has a conductor track structure.
In accordance with still a further feature of the invention, the surface of the developing roller has a defined surface roughness.
In accordance with still an added feature of the invention, the surface of the developing roller has a varied roughness.
In accordance with still an additional feature of the invention, the surface of the developing roller has a coating for promoting triboelectric interactions.
In accordance with another feature of the invention, the surface of the developing roller has a coating for promoting wear resistance of the surface.
In accordance with a further feature of the invention, the field-generating device is formed of metal.
In accordance with an added feature of the invention, the field-generating device is coated with an insulating material.
In accordance with an additional feature of the invention, a gap formed with varied size is located between the surface of the developing roller and the field-generating device.
In accordance with yet another feature of the invention, the gap is smaller at an input of the field-generating device than at an output of the field-generating device.
In accordance with an alternative feature of the invention, the gap is larger at an input of the field-generating device than at an output of the field-generating device.
In accordance with yet a further feature of the invention, an alternating voltage from an alternating voltage source is applicable to the field-generating device.
In accordance with yet an added feature of the invention, a temporary constant electric field is superimposable on an alternating electric field generated by the field-generating device.
In accordance with another aspect of the invention, there is provided a method for developing an electrostatic latent image produced on a movable image carrier by feeding electrically charged toner particles onto a surface of a developing roller and causing the particles to adhere thereat electrostatically, which comprises disposing a device for generating an electric field at a spaced distance from the surface of the developing roller, applying an alternating voltage to the field-generating device so as to generate a continuous alternating electric field actable on the toner particles for causing the toner particles on the surface of the developing roller to move and jump, respectively.
In accordance with a concomitant mode, the method of the invention includes superimposing a temporary constant electric field on the continuous alternating electric field.
The gist of the invention resides in installing an extended electrode structure (downline of the doctor blade), an alternating voltage being applied to the electrode structure. This measure generates a continuously alternating electric field, as a result of which the toner particles jump back and forth between the developing roller and the electrode structure, and thereby act upon the toner particles so that spati
Greenberg Laurence A.
Heidelberger Druckmaschinen AG
Lerner Herbert L.
Pendegrass Joan
Stemer Werner H.
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