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.
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 A2, A1 and A0 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 (xe2x80x9cnip widthxe2x80x9d) 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 xcexcm, 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 A4 or A3 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 A0-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 A0-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 image is developed. As a result, stable, uniform images cannot be obtained consistently.
Furthermore, leakage of toner from both ends of the developing roller to the exterior of the developing apparatus is one problem with a contact-type developing apparatus that uses non-magnetic single-component toner. That is, because of the non-magnetic nature of the toner, the toner cannot be gathered together by magnetic force as in the manner of the conventional magnetic-developer system. Several alternative proposals for preventing such leakage have been made.
Most of these proposals place a lubricating seal between both ends of the developing roller and the side plates of the developing apparatus, thereby attempting to prevent leakage of the toner. However, in an arrangement in which such seals are placed, the seals wear out or deteriorate owing to long-term use and a satisfactory sealing effect cannot be maintained.
The present invention has been devised in view of the above-mentioned circumstances and seeks to provide a developing apparatus in which excellent development is possible at all times even when developing large-size images of size A0 and A1. A further object of the present invention is to provide a developing apparatus in which amount of bite of a resilient developing roller into an image carrier and width of a development nip zone can be made suitable values for the sake of achieving the excellent development mentioned above.
A further object of the present invention is to provide a developing apparatus in which it is possible to prevent toner leakage through a simple arrangement by utilizing a layer-thickness regulating roller for forming a thin layer on a developing roller.
According to the present invention, the foregoing objects are attained by providing a developing apparatus for forming a thin layer of toner, which comprises a non-magnetic single-component toner, on a resilient developing roller, bringing the roller into abutting contact with the surface of an image carrier, whereby toner on the resilient developing roller is supplied to an electrostatic latent image that has been formed on the surface of the image carrier, thereby developing the electrostatic latent image, and moving the image carrier and the resilient developing roller in a forward direction in such a manner that traveling speed of the image carrier and peripheral speed of the resilient developing roller become substantially identical, wherein width of a development nip zone that extends from a point at which the resilient developing roller starts to contact the image carrier to a point at which the resilient developing roller breaks contact with the image carrier is equal to or greater than 4 mm, preferably 5 to 10 mm, rubber hardness of the resilient roller is 20 to 40xc2x0 and diameter of the resilient developing roller falls within the range 40 to 100 mm, and amount of bite by which the surface of the image carrier bites into the resilient developing roller is set so as to be equal to or greater than {fraction (1/100)} of the radius of the resilient developing roller, characterized in that the amount of bite and the width of the development nip zone are set in such a manner that the resilient developing roller breaks contact with the image carrier while traveling speed of a local portion of the resilient developing roller in contact with the surface of the image carrier gradually decreases from the point at which contact starts and thenceforth gradually returns to the original speed owing to resilience of the resilient developing roller per se.
Further, the amount of bite of the resilient developing roller into the surface of the image carrier is set so as to be equal to or greater than {fraction (1/40)} of the radius of the developing roller.
Further, the depth of bite of the resilient developing roller into the surface of the image carrier is set to be equal to or greater than {fraction (1/100)} of the radius of the developing roller. In particular, the depth of bite is set to 0.2 to 3 mm. Further, it is preferred that the thin layer of toner formed on the resilient developing roller be a uniform layer of one to three layers of toner.
Furthermore, rubber hardness of the resilient roller is 20 to 40xc2x0, as measured according to the JIS K 6253 (Type A) standard.
Means for forming the thin toner layer on the resilient developing roller comprises a layer-thickness regulating roller placed in opposition to the developing roller.
The layer-thickness regulating roller has a central portion along the axial direction thereof and end portions that are electrically insulated from the central portion, and a bias voltage for preventing adhesion of toner to the end portions of the resilient developing roller is applied to the end portions.
Further, the apparatus is equipped with a scraping blade provided in pressured contact with a central portion of the layer-thickness regulating roller with respect to the axial direction thereof and with end portions of the roller, the scraping blade being so adapted as to scrape off toner that has adhered to the layer-thickness regulating roller.
Further, the developing roller is provided so as to be capable of contacting and separating from the surface of the image carrier in order to assure a suitable positional relationship between the developing roller and image carrier, both ends of the developing roller are provided with contact rollers, and the contact rollers are brought into abutting contact with both ends of the image carrier to regulate the width of the development nip zone.
Furthermore, the image carrier and developing roller are moved so as to mesh a gear provided on a flange of the image carrier with a gear provided on an end of the developing roller, whereby drive from the image carrier is transmitted to the developing apparatus.
In particular, the developing apparatus is characterized by further having separation means for causing the developing roller to separate from the image carrier, wherein the gear provided on the image carrier and the gear provided on the developing roller are made to mesh slightly when the image carrier and the developing roller are in a separated state.
Further, the present invention is characterized by having a cam in abutting contact with a portion of the developing apparatus for being turned at introduction of power to thereby move the developing apparatus in such a manner that the developing roller is pressed against the image carrier, and by provision of a capacitor charged when power is being introduced, wherein the capacitor is switched over to act as a power source at cut-off of power, thereby rotating the cam and moving the developing apparatus in such a manner that the developing roller moves in a direction in which it separates from the image carrier.
Furthermore, in accordance with the present invention, there is provided a developing apparatus for forming a thin layer of toner, which comprises a non-magnetic single-component toner, on a resilient developing roller, and bringing the roller into abutting contact with the surface of a drum-shaped image carrier, whereby toner on the resilient developing roller is supplied to an electrostatic latent image that has been formed on the surface of the image carrier, thereby developing the electrostatic latent image, characterized by comprising: means for rotating the image carrier and the resilient developing roller in a forward direction in such a manner that peripheral speed of the image carrier and peripheral speed of the resilient developing roller become substantially identical; and means for regulating depth of bite of the resilient developing roller into the surface of the image carrier and width of a development nip zone that extends from a point at which the resilient developing roller starts to contact the image carrier to a point at which the resilient developing roller breaks contact with the image carrier.
Furthermore, the invention is characterized in that the moving means comprises mutually meshing gears formed on ends of respective ones of the image carrier and developing roller, and in that the regulating means comprises rollers provided on both ends of the developing roller and the peripheral surfaces of which are brought into abutting contact with both ends of the image carrier.