Donor For Touchdown Development

Abstract

Apparatus for developing a latent electrostatic image carried on an image retaining member comprising A member including an electrically conductive layer and a dielectric layer having a generally continuous surface for carrying toner particles for the development of the latent image, A plurality of stylii supported for relative movement with the donor member spaced from each other and positioned to contact the donor member enabling electrostatic charge to be deposited onto separate, discrete areas of the toner carrying surface of the donor member corresponding in size to the surface area of the tips of the stylii creating an electrostatic charge pattern comprising a plurality of lines on the donor member upon coupling the donor member conductive layer and stylii to an electrical energy source and upon moving the stylii and donor member relative to one another and Means to move the donor member relative to a supply of toner particles for loading toner particles onto the donor member in areas associated with the charge pattern of lines created by the plurality of stylii and to move the toner loaded donor member relative to the image retaining member for the development of the latent electrostatic image.

Description

BACKGROUND OF THE INVENTION

This method and apparatus relates to electrostatic image development and more particularly to an improved xerographic method and apparatus for the development of electrostatic images by which a toner layer is presented to the latent image for development thereof.

In the reproduction process of xerography, a photoconductive surface is charged and then exposed to a light pattern of the information to be reproduced, thereby forming an electrostatic latent image on the photoconductive surface. Toner particles, which may be finely divided, pigmented, resinous material are presented to the latent image where they are attracted to the photoconductive surface. The toner image can be fixed and made permanent on the photoconductive surface or it can be transferred to another surface where it is fixed.

One known method of developing latent electrostatic images is by a process called transfer development. Transfer development broadly involves bringing a layer of toner to an imaged photoconductor where toner particles will be transferred from the layer to the imaged areas. In one transfer development technique, the layer of toner particles is applied to a donor member which is capable of retaining the particles on its surface and then the donor member is brought into close proximity to the surface of the photoconductor. In the closely spaced position, particles of toner in the toner layer on the donor member, are attracted to the photoconductor by the electrostatic charge on the photoconductor so that development takes place. In this technique the toner particles must traverse an air gap to reach the imaged regions of the photoconductor. In two other transfer techniques the toner-laden donor actually contacts the imaged photoreceptor and no air gap is involved. In one such technique, the toner-laden donor is rolled in non-slip relationship into and out of contact with the electrostatic latent image to develop the image in a single rapid step. In another such technique, the toner-laden donor is skidded across the xerographic surface. Skidding the toner by as much as the width of the thinnest line will double the amount of toner available for development of a line which is perpendicular to the skid direction and the amount of skidding can be increased to achieve greater density or greater area coverage.

It is to be noted, therefore, that the term "transfer development" is generic to development techniques where (1) the toner layer is out of contact with the imaged photoconductor and the toner particles must traverse an air gap to effect development, (2) the toner layer is brought into rolling contact with the imaged photoconductor to effect development, and (3) the toner layer is brought into contact with the imaged photoconductor and skidded across the imaged surface to effect development. Transfer development has also come to be known as "touchdown development."

In a typical transfer development system, a cylindrical or endless donor member is rotated so that its surface can be presented to the moving surface of a photoconductive drum bearing an electrostatic latent image thereon. Positioned about the periphery of the donor member are a number of processing stations including, a donor loading station, at which toner is retained on the donor member surface; an agglomerate removal station at which toner agglomerates are removed from the toner layer retained on the surface of the donor member; a charging station at which a uniform charge is placed on the particles of toner retained on the donor surface; a clean up station at which the toner layer is converted into one of uniform thickness and at which any toner agglomerates not removed by the agglomerate removal station are removed; a development station at which the toner particles carried by said donor member are presented to the imaged photoconductor for image development; and a cleaning station at which a neutralizing charge is placed upon the residual toner particles and that which a cleaning member removes residual toner from the peripheral surface of the toner. In this manner, a continuous development process is carried out.

Among the donor members employed in the prior art are those embodying the principles described in U.S. Pat. No. 3,203,394. Such a donor includes, an electrically conductive support member in the form of a cylinder, a thin electrically insulating layer overlying support member, and a continuous, electrically conductive screen pattern overlying the insulating layer. The screen pattern is provided with an electrical connection to slip ring so that its potential may be varied between ground potential and a charge potential at different stages of process. A multitude of high fringe fields or microfields are created at the surface of this type of donor member. When this type of donor member is brought into contact with toner particles, it is in this manner loaded with toner.

A donor member of this type is quite expensive to manufacture, it is quite fragile in the screen regions and is subject to being electrically disabled, e.g., through shorting of the screen to the conductive substrate, unless considerable care is taken during its manufacture and use.

The art of xerographic development, and in particular transfer development, would be significantly advanced if a simpler and more reliable donor member were available.

Accordingly, it is an object of the invention to improve donor member apparatus for developing latent electrostatic images.

It is a further object of the invention to simplify the construction of a donor member apparatus for developing latent electrostatic images.

It is still an object of this invention to improve upon known transfer development processes.

SUMMARY OF THE INVENTION

The invention is directed to an apparatus for developing a latent electrostatic image formed on the surface of an image retaining surface. The apparatus includes a means for developing a latent image and the means includes: (a) a donor member adapted to transport toner particles to the latent image comprising, an endless electrically conductive support member carrying on the outer surface thereof a continuous dielectric layer; (b) means to maintain the conductive support member at a reference potential; (c) means to cause electrical discharge at least at the surface of defined regions of said dielectric layer to collect an electrical charge pattern on the surface of said dielectric layer; (d) means to continuously advance said donor member past a plurality of treating stations, said treating stations including; (1) a toner loading station including a supply of toner particles at which said toner particles are contacted by said donor member and toner particles retained on said donor member in response to the force of said charge pattern; (2) a developing station at which the retained toner particles are presented in developing relation to a latent electrostatic image on an image retaining element.

In a preferred embodiment, the means to cause electrical discharge at the surface of the defined regions of the dielectric layer is at least one electrically conductive member, e.g., a stylus, in a small area-contact with the surface of the dielectric layer and a voltage source in electrical communication with the conductive member. It is also preferred that this means includes a plurality of uniformly spaced electrically conductive members each in small area-contact with the surface of the dielectric layer. The same or different voltages may be applied to the conductive members.

The present invention is also directed to a latent electrostatic image developing method. The latent image is formed on the surface of an image retaining member, e.g., a xerographic plate. A layer of toner particles is retained on the surface of an endless donor member, said donor comprising an electrically conductive support member carrying on the outer surface thereof a continuous dielectric layer, by (1) maintaining said conductive support member at a reference potential (2) bringing at least one electrically conductive member into small area-contact with the surface of the dielectric layer, (3) causing relative motion between the conductive member and the surface of the dielectric layer, (4) causing electrical discharge between the surface of the dielectric layer and the contact surface of the conductive member so as to collect an electrical charge in the regions defined by the small area-contact, and (5) bringing the so-charged surface of the donor member into contact with the supply of toner particles to permit toner particles to adhere to the surface of the donor member; and presenting the adhered toner particles to the latent image to effect development thereof.

Other treating stations and processing steps may be added to the apparatus and process. For example, a toner agglomerate removal station may follow the toner loading station, a uniform charging station may follow the toner agglomerate removal station, a clean-up station may follow the uniform charging station and a residual or ghost image removal station may follow the developing station.

BRIEF DESCRIPTION OF THE DRAWING

For a better understanding of the invention as well as other objects and further features thereof, reference is made to the accompanying drawing, wherein:

FIG. 1 is a sectional view of xerographic apparatus in accordance with the present invention; and

FIG. 2 is an isometric view of a preferred donor member in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is a transfer development system and method in which toner particles are applied to an electrostatic latent image on the photoconductive plate to develop the image. Although the system and method is described herein as part of the xerographic copier, it can be utilized in conjunction with any reproduction system wherein a latent image is to be developed by applying toner thereto, e.g., a latent image in an electrographic system.

Referring to FIG. 1, there is shown a xerographic reproduction system utilizing the concept of the present invention. In this apparatus a xerographic plate is in the form of a drum 10 which passes through stations A-E in the direction shown by the arrow. The drum has a suitable photosensitive surface, such as one including selenium overlying a layer of conductive material, on which a latent electrostatic image can be formed. The various stations about the periphery of the drum which carry out the reproduction process are: charging station A, exposing station B, developing station C, transfer station D, and cleaning station E. Stations A, B, D, and E represent a conventional means for carrying out their respective functions. Apart from their association with the novel arrangement to be described with respect to station C they form no part of the present invention.

At station A, a suitable charging means 12, e.g., a corotron, places a uniform electrostatic charge on the photoconductive material. As the drum rotates, a light pattern, via a suitable exposing apparatus 14, e.g., a projector, is exposed onto the charged surface of drum 10. The latent image thereby formed on the surface of the drum is developed or made visible by the application of a finely divided pigmented, resinous powder called toner at developing station C, which is described in greater detail below. After the drum is developed at station C, it passes through transfer station D, comprising copy sheet 16, corona charging device 18 and fuser device 20. Following transfer and fixing of developed image to the copy sheet, the drum rotates through cleaning station E, comprising cleaning device 22, e.g., a rotating brush.

At developing station C, the apparatus includes a donor member 24 (more particularly described below) rotatably mounted adjacent a toner reservoir 26 containing a supply of toner 28. The donor roll 24 is positioned so that a portion of its periphery comes into contact with toner 28. The donor roll is also located so as to provide a small gap between the surface of drum 10 and the outer surface of a toner layer carried by donor roll 24. This gap can be approximately 1 mil.

Located between the toner reservoir 26 and the developing region is a charging means, such as a corona charging device 36, which is adapted to place a uniform charge on the toner particles of a polarity opposite to the polarity of the latent image on the photoconductive drum in those instances where non-uniformly charged toner particles are present in the toner reservoir.

Referring now also to FIG. 2 of the drawing, there is shown a donor member of the type contemplated by the present invention. Donor member 24 comprises a conductive drum 42, for example a metal drum such as aluminum, copper, etc, over the surface of which is coated a dielectric layer 44 which can be for example an organic plastic, glass, ceramic, etc. As indicated above donor member 24 is positioned so that a portion of its periphery may be rotated into contact with a mass of toner particles 28 in a toner reservoir 26. The metallic drum 42 is maintained at a reference potential, e.g., at ground potential, via contact member 34. In contact with the surface of dielectric layer 44 are a plurality of conductive styli 46. These styli 46 are connected via conductive members 30 to a common electrical lead 48 which in turn is in electrical communication with a voltage source 32, e.g., a battery. It is to be understood that instead of a single voltage source, individual voltages may be applied to the conductive styli to facilitate gray scale rendition during development.

In operation, as donor member 24 rotates in the direction shown by the arrow in FIG. 1, and voltage source 32 is applied to the system, a discharge occurs between the tips of styli 46 and the surface of dielectric layer 44. This discharge causes an electrical charge to be collected on the surface of the dielectric 44. The charge is represented by dash lines 50 and are approximately as wide as the small contact area of the tips of the styli. As the drum continues to rotate into contact with the toner particles in reservoir 26, toner is attracted to the charged lines on the drum. The donor member then continues to rotate so as to present the adhered toner particles into developing relation with the imaged portion of drum 10.

It is to be understood that the individual styli can be connected to different voltage sources of the same polarity, so that a different magnitude of charge can be collected on the surface of the donor member. By this means, gray scale development can take place at the development region of the system. The styli may be conveniently separated by a spacing of from 1 to 5 mils. During rotation of the donor, a suitable oscillating means 52 can be employed to impart a lateral or oscillating motion to the conducting styli. This will give a zig-zag design to the charge pattern and permit a larger amount of a single polarity toner to be adhered to the donor surface.

Following development, the donor roll is prepared for toner reloading by discharging the surface of the drum at a point just beyond the developing region. The drum may be discharged by a number of techniques, e.g., a conductive rubber roller or an AC corona, for example, corona device 38, as shown in FIG. 1. After discharge of the surface of the donor member, residual toner may be removed by any cleaning means, for example brush 40, as shown in FIG. 1. Instead of the toner loading station shown in the drawing, other toner loading means may be employed, for example, fur brush, magnetic brush, etc.

In addition to the several stations described in the apparatus of the present invention it is to be understood that other stations may be strategically located about the periphery of the donor member, for instance, a toner agglomerate removal station may be located between toner reservoir 26 and charging means 36. This agglomerate removal station may conveniently be a vacuum means adapted to remove large toner agglomerates from the surface of the retained toner particles.

It is to be understood while for purposes of illustration the donor member has been described basically as a cylinder, it may be an an endless belt adapted to deliver toner from the toner source to the several stations.

Conventional drive means, e.g., motors, belts, etc. are employed to drive the several movable members all in a manner within the skill of the art.

Since many changes can be made in the above construction and many apparently widely different embodiments of this invention can be made without departing from the scope thereof it is intended that all matter contained in the drawing and specification should be interpreted illustratively and not in a limited sense.

Claims

What is claimed is:

1. Apparatus for developing a latent electrostatic image carried on an image retaining member comprising

a donor member including an electrically conductive layer and a dielectric layer having a generally continuous surface for carrying toner particles for the development of the latent image,

a plurality of stylii supported for relative movement with the donor member spaced from each other and positioned to contact the donor member enabling electrostatic charge to be deposited onto separate, discrete areas of the tonor carrying surface of the donor member corresponding in size to the surface area of the tips of the stylii creating an electrostatic charge pattern comprising a plurality of lines on the donor member upon coupling the donor member conductive layer and stylii to an electrical energy source and upon moving the stylii and donor member relative to one another and

means to move the donor member relative to a supply of tonor particles for loading tonor particles onto the donor member in areas associated with the charge pattern of lines created by the plurality of stylii and to move the tonor loaded donor member relative to the image retaining member for the development of the latent electrostatic image.

2. The apparatus of claim 1 including oscillating means for imparting relative movement between the stylii and donor member in a lateral coordinate generally transverse to the direction of movement between the donor member and the image retaining member.

3. The apparatus of claim 1 including an electrical energy source coupled to said stylii to maintain them at substantially the same potential.

4. The apparatus of claim 1 including an electrical energy source coupled to said stylii by means to maintain some stylii at different potentials from other stylii.

5. The apparatus of claim 1 wherein the dielectric layer of the donor member includes glass.

6. The apparatus of claim 1 wherein said donor member includes a metal drum having an outermost coating of glass comprising the dielectric layer.