Developing apparatus

Details Developing apparatus Developing apparatus

2002-01-14

2004-03-23

Beatty, Robert (Department: 2852)

Electrophotography

Image formation

Development

C399S234000, C399S284000

Reexamination Certificate

active

06711372

ABSTRACT:

TECHNICAL FIELD
This invention relates to a developing apparatus used in an electrophotographic apparatus and, more particularly, to a developing apparatus in which an electrostatic latent image formed on an image carrier is developed and visualized by a non-magnetic single-component developer.
BACKGROUND ART
Generally, in an electrophotographic apparatus such as a copier, printer or plotter that utilizes electrophotography, the electrostatic latent image of a desired image is formed on an image carrier such as a photosensitive drum and a developer is supplied by a developing apparatus to develop the electrostatic latent image so that a visible toner image is formed on the image carrier. A two-component developer comprising toner and carrier, and a magnetic single-component developer or non-magnetic single-component developer comprised of toner alone are known as developers. Various developing systems suited to these developers have been developed and proposed.
Non-magnetic single-component developers in particular have various advantages but utilization thereof in actual machines has been slow. In recent years, however, utilization in actual machines has spread rapidly with the development of new or improved developers, which are the result of performance enhancement, such as polymer toners that excel in image reproducibility and transfer.
A contact-type developing apparatus has been proposed as a developing apparatus that uses a non-magnetic single-component developer, in which a flexible developing roller exhibiting electroconductivity or an appropriate electrical resistance is used as a developer carrier for supplying a developer to an image carrier, a thin layer of the developer is formed on the surface of the roller and then the roller is brought into contact with the surface of the image carrier at a suitable pressure to develop the image. It is known that such a contact-type developing apparatus can be used preferably in development which does not require an edge enhancement effect and in which it is required that the developing characteristics of line drawings and pictorial images be identical, as in a digital printer in which an image is formed by monochrome bi-level values. This is known also as a cleanerless system because cleaning can be carried out at the same time as development.
In an early apparatus of this kind, the arrangement is such that a physical or mechanical load brought about by contact between the developing roller and the surface of the image carrier is mitigated by making the peripheral speed of the elastic developing roller, which rotates in the forward direction, and the peripheral speed of the image carrier approximately identical. However, difficulties arise in terms of image quality relating to image definition, texture smudging and fogging. An arrangement which provides a difference in speed between the peripheral speed of the image carrier and the peripheral speed of the developing roller has been proposed as an improvement (e.g., see the specifications of Japanese Patent Nos. 2598131 and 2803822).
In accordance with the proposed apparatus, the surface of the developing roller is brought into sufficient sliding frictional contact with the surface of the image carrier via a toner layer owing to the difference in the peripheral speeds between the developing roller and image carrier, whereby excellent development and cleaning are carried out simultaneously. In order to achieve such sliding frictional contact, the developing roller is set to as to rotate at a peripheral speed that is 1.5 to 4 times that of the image carrier. Further, it is disclosed that the contact width between the developing roller and surface of the image carrier, namely the development nip zone, should be equal to or more than 50 times but equal to or less than 500 times the volume average particle diameter of the developer particles.
In experiments, however, the Inventors have found that several problems still need to be solved in terms of structure and requirements in order to obtain fully satisfactory image quality, especially the fact that some points that do not give rise to problems in small type printers that develop small-size images do represent major problems when developing large-size images such as images of size A
2
, A
1
and A
0
by large type printers.
One problem is as follows: When the force with which the developing roller comes into pressured contact with the image carrier is comparatively large and the peripheral speed of the developing roller differs from that of the image carrier, the toner on the surface of the developing roller is pulverized by the pressure of sliding contact, resulting in rapid toner deterioration. Further, toner adheres to (or becomes fused to) the surface of a developer-layer regulating member, which regulates the thickness of the layer of developer that is formed on the developing roller, owing to the development of, say, several thousand meters, and the adherence of the toner prevents the formation of a uniform thin layer of the developer, thereby causing white stripes to appear on the image. An additional drawback is that the image carrier rotates unevenly owing to the action of pressing force applied to the image carrier by the developing roller rotating at a different peripheral speed. Furthermore, in a large-size electrophotographic apparatus for developing large-size images, the torque for driving the developing roller is fairly large in order to produce the aforementioned sliding contact. This is uneconomical.
Further, in the prior art described above, maintaining the width of the development nip zone (“nip width”) is a major factor in achieving good development and the nip width is to be made 50 to 500 times the average particle diameter of the toner. Accordingly, if the diameter of the toner used in such development is on the order of 8 &mgr;m, the nip width will be 0.4 to 4 mm, which is 50 to 500 times this diameter. In a case where a developing roller having a diameter of 40 mm is made to contact an image carrier having a diameter of 120 mm, for example, the positional dimensions between the developing roller and image carrier must be maintained in such a manner that the depth of bite of the developing roller into the image carrier will be 0.001 to 0.134 mm. Considerable dimensional precision and setting of position will be required of these members.
Even if this is a soluble problem in a small-size developing apparatus of size A
4
or A
3
having an image carrier or developing roller of comparatively small length, it is a problem of considerable difficulty in a large-size developing apparatus having a developing roller of large length. For example, finishing of the developing roller usually is performed by grinding. In an instance where an A
0
-size image is to be developed, a roller having a length of about 850 mm must be machined as the developing roller. Finishing the roller to a diametric error of tens of microns over its entire length so as to satisfy the above requirement is considerably difficult and results in costly machining. Further, in a case where the amount of wobble of an A
0
-size image carrier at rotation thereof and the amount of wobble of the developing roller are each 0.1 mm and, hence, there is an error in the diameter between these members, the depth of bite of the developing roller into the surface of the image carrier varies from area to area and, as a result, image density varies locally and gives rise to uneven development.
In addition, a developing roller made of a resilient material such as rubber exhibits a large coefficient of thermal expansion and therefore the diameter thereof tends to change with a change in ambient temperature. As a result, a problem which arises is that the nip width between the image carrier and developing roller varies with a change in temperature. This is a further cause of uneven development.
Thus, in the prior art as described above, satisfactory mechanical precision for coping with the environment of use is difficult to obtain in cases where a large-size imag

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