Apparatus for non-interactive electrophotographic development

Abstract

An apparatus for non-interactive, dry powder development of electrostatic Images composed of solid areas and fine lines areas on an imageable surface including a housing containing developer material; a magnetic member, spaced a predefined distance from the image, for transporting the developer material from the housing to develop solid areas of the image, the magnetic roll including an magnetic core and a cylindrical sleeve enclosing and rotating about the magnetic core; and a donor member, adjacent to the magnetic roll and spaced from the image receiving member and adapted to transport marking particles to a development zone adjacent the image receiving member; an electrode positioned in the development zone between the image receiving member and the donor member; a voltage supply for electrically biasing the electrode during a developing operation with an alternating current to detach marking particles from said donor member, forming a cloud of marking particles in the development zone, and developing fine line areas of the image from the cloud.

Description

The invention relates generally to an electrophotographic printing machine and, more particularly, to the non-interactive development of electrostatic images.

BACKGROUND OF THE INVENTION

Generally, an electrophotographic printing machine includes a photoconductive member which is charged to a substantially uniform potential to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to an optical light pattern representing the document being produced. This records an electrostatic image on the photoconductive member corresponding to the informational areas contained within the document. After the electrostatic image is formed on the photoconductive member, the image is developed by bringing a developer material into effective contact therewith. Typically, the developer material comprises toner particles bearing electrostatic charges chosen to cause them to move toward and adhere to the desired portions of the electrostatic image. The resulting physical image is subsequently transferred to a copy sheet. Finally, the copy sheet is heated or otherwise processed to permanently affix the powder image thereto in the desired image-wise configuration.

Development may be interactive or non-interactive depending on whether toner already on the image may or may not be disturbed or removed by subsequent development procedures. Sometimes the terms scavenging and non-scavenging are used interchangeably with the terms interactive and non-interactive. Non-interactive development is most useful in color systems when a given color toner must be deposited on an electrostatic image without disturbing previously applied toner deposits of a different color, or cross-contaminating the color toner supplies. This invention relates to such image-on-image, non-interactive development.

U.S. Pat. No. 4,868,600 to Hays et al. discloses a non-interactive development system wherein toner is first developed from a two-component developer onto a metal-cored donor roll and thereafter disturbed into a powder cloud in the narrow gap between the donor roll and an electrostatic latent image existing on a photoreceptor surface. Development fields created between the donor roll core and the electrostatic latent image harvest some of the toner from the cloud onto the electrostatic image, thus developing it without physically disturbing any previously deposited toner layers. In this method the powder cloud generation is accomplished by thin, AC biased wires strung across the process direction and within the development gap. The wires ride on the toner layer and are biased relative to the donor roll core.

U.S. Pat. No. 4,557,992 to Haneda et al. describes a non-interactive magnetic brush development method wherein a two component developer consisting of magnetically soft carrier materials is carried into close proximity to an electrostatic image and caused to generate a powder cloud by the developer motion due, in part, by the inclusion of an AC voltage applied across the gap between the developer sleeve and the ground plane of the electrostatic image. Cloud generation directly from the surfaces of a two component developer avoids many of the problems created by wires. However, in practice such methods have been speed limited by their low toner cloud generation rate.

U.S. Pat. No. 5,409,791 to Kaukeinen et al. describes a non-interactive magnetic brush development method employing permanently magnetized carrier beads operating with a rotating multipole magnet within a conductive and nonmagnetic sleeve. Magnetic field lines form arches in the space above the sleeve surface creating chains of carrier beads which follow these magnetic field lines. The carrier chains are held in contact with the sleeve and spacing between the developer sleeve and a photoreceptor surface is sufficiently large to maintain the carrier bead chains out of direct contact with the photoreceptor surface. As the core rotates in one direction relative to the sleeve, the magnetic field lines beyond the sleeve surface rotate in the opposite sense, moving chains in a tumbling action, which transports developer material along the sleeve surface. The strong mechanical agitation very effectively dislodges toner particles generating a rich powder cloud, which can be developed to the adjacent photoreceptor surface under the influence of development fields between the sleeve and the electrostatic image. U.S. Pat. No. 5,409,791 is hereby incorporated by reference.

A problem with non-interactive development methods is achieving good solid region development while maintaining good fine line development and vice versa. Many non-interactive development methods function by generating a powder cloud in the gap between a photoreceptor and another member which serves as a development electrode. It is generally observed that this gap should be as small as possible, on the order of 0.010 inches or less. Generally, the larger the gap, the larger become certain image defects in the development of fine lines and edges. As examples of these defects: lines do not develop to the correct width, lines near solid areas are distorted, and the edges of solids are softened, especially at corners. It is understood that these defects are the result of lateral components of the electric field lines occurring due to the charge patterns existing on the imagewise discharged photoreceptor. Electrostatic field lines emanating from the photoreceptor reach up from the latent electrostatic image patterns of lines and at the edges of solid areas and arch back toward the adjacent photoreceptor regions. These lateral components of the electric field lines result in displacement from the intended pathway of the charged toner particles and in incomplete development of the latent electrostatic images. Defects due to the electrostatic field arches are less serious in interactive two component development subsystems because toner particles can be delivered through these field arches by carrier particles. Nor are they an issue in interactive single component development because a strong, cross-gap AC field is superposed which impart sufficient toner particle velocity toward the photoreceptor to overcome the aforementioned field arch patterns.

SUMMARY OF THE INVENTION

The present invention obviates the problems noted with achieving good solid region development while maintaining good fine line development, by providing an apparatus for non-interactive, dry powder development of electrostatic images composed of solid areas and fine line areas on an imageable surface including a housing containing developer material; a magnetic member, spaced a predefined distance from said image, for transporting said developer material from said housing to develop solid areas of said image, said magnetic member including a magnetic core and a cylindrical sleeve enclosing and rotating about said magnetic core; and a donor member, adjacent to said magnetic member and spaced from the imageable surface and adapted to transport developer material to a development zone adjacent the imageable surface; an electrode positioned in the development zone between the imageable surface and the donor member; a voltage supply for electrically biasing said electrode during a developing operation with an alternating current to detach developer material from said donor member, forming a cloud of developer material in the development zone, and developing fine line areas of said image from the cloud.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view, in section, of a four color xerographic reproduction machine incorporating the non-interactive developer of the present invention.

FIG. 2 is an enlarged side view of the developer unit of the present invention.

FIG. 3 is an enlarged view of the developer roll shown in FIG. 2 .

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1 of the drawings, there is shown a xerographic type reproduction machine 8 incorporating an embodiment of the non-interactive development system of the present invention, designated generally by the numeral 80 . Machine 8 has a suitable frame (not shown) on which the machine xerographic components are operatively supported. As will be familiar to those skilled in the art, the machines xerographic components include a recording member, shown here in the form of a translatable photoreceptor 12 . In the exemplary arrangement shown, photoreceptor 12 comprises a belt having a photoconductive surface 14 . The belt is driven by means of a motorized linkage along a path defined by rollers 16 , 18 and 20 , and those of transfer assembly 30 , the direction of movement being counter-clockwise as viewed in FIG. 1 and indicated by the arrow marked P. Operatively disposed about the periphery of photoreceptor 12 are charge corotrons 22 for placing a uniform charge on the photoconductive surface 14 of photoreceptor 12 ; exposure stations 24 where the uniformly charged photoconductive surface 14 constrained by positioning shoes 50 is exposed in patterns representing the various color separations of the document being generated; development stations 28 where the electrostatic image created on photoconductive surface 14 is developed by toners of the appropriate color; and transfer and detack corotrons (not shown) for assisting transfer of the developed image to a suitable copy substrate material such as a copy sheet 32 brought forward in timed relation with the developed image on photoconductive surface 14 at transfer assembly 30 . In preparation for the next imaging cycle, unwanted residual toner is removed from the belt surface at a cleaning station (not shown).

Following transfer, the sheet 32 is carried forward to a fusing station (not shown) where the toner image is fixed by pressure or thermal fusing methods familiar to those practicing the electrophotographic art. After fusing, the copy sheet 32 is discharged to an output tray.

At each exposure station 24 , photoreceptor 12 is guided over a positioning shoe 50 so that the photoconductive surface 14 is constrained to coincide with the plane of optimum exposure. A laser diode raster output scanner (ROS) 56 generates a closely spaced raster of scan lines on photoconductive surface 14 as photoreceptor 12 advances at a constant velocity over shoe 50 . A ROS includes a laser source controlled by a data source, a rotating polygon mirror, and optical elements associated therewith. At each exposure station 24 , a ROS 56 exposes the charged photoconductive surface 14 point by point to generate the electrostatic image associated with the color separation to be generated. It will be understood by those familiar with the art that alternative exposure systems for generating the electrostatic images, such as print bars based on liquid crystal light valves and light emitting diodes (LEDs), and other equivalent optical arrangements could be used in place of the ROS systems such that the charged surface may be imagewise discharged to form an electrostatic image of the appropriate color separation at each exposure station.

A suitable controller is provided for operating the various components of machine 8 in predetermined relation with one another to produce full color images.

Referring now to FIGS. 2 and 3 in greater detail, developing station 26 includes a developer housing 44 defining a chamber 76 for storing a supply of developer material therein. A toner dispensing cartridge (not shown) dispenses toner particles downward into a sump area occupied by the auger. The auger loads toner onto developing member 41 .

Continuing with the description of operation at each developing station 26 26 , developing members 41 and 42 are disposed in predetermined operative relation to the photoconductive surface 14 of photoreceptor 12 , the length of developing members being equal to or slightly greater than the width of photoconductive surface 14 , with the functional axis of the developing members parallel to the photoconductive surface and oriented at a right angle with respect to the path of the photoreceptor 12 . Advancement of each developing member carries the developer blanket into the development zone in proximal relation with the photoconductive surface 14 of photoreceptor 12 to develop the electrostatic imagethereon.

Donor member 41 comprises an interior rotatable harmonic multipole magnetic assembly 43 and an outer sleeve 45 . The sleeve can be rotated in either the “with” or “against” direction relative to the direction of motion of the photoreceptor belt 12 . Similarly, the magnetic assembly can be rotated in either the “with” or “against” direction relative to the direction of motion of the sleeve 45 . Blade 38 is placed in near contact with the rotating donor member 41 to trim the height of the developer bed. A cleaning blade (not shown) is placed in contact with the rotating donor member 41 to continuously remove developer from the donor member 41 for return to the developer chamber 76 . Donor member 41 has a DC power source 203 and an AC power source 204 electrically attached thereto.

The primary function of donor member 41 is to develop solid areas of the latent image. Donor member 41 is spaced between 0.020″ and 0.050″ from the photoreceptor. A DC voltage by supply 203 is applied to insure background regions of the latent electrostatic image are not developed. For example, in Discharge Area Development (DAD), the DC voltage is set to 100 to 500 volts in accordance to photoreceptor charge and discharge voltages. For Charge Area Development (CAD), the voltage is set just above the residual voltage of the photoreceptor about 50 to 200 volts. Interactivity is reduced by using low momentum toner i.e. minimizing the applied AC voltage; and by maintaining a relatively large spacing between donor member 41 and photoreceptor 12 . For example, the development system of the present invention can be setup as follows. For donor member 41 , it is desired to have a toner bed height between 0.015″ to 0.045″, this can be accomplished by configuring the pole spacing of the magnetic assembly to give the desired bed height or trim blade 38 could be employed to give the desired bed height. The AC frequency for supply is selected to provide maximum development below interactively which is 1 Khz to 4 Khz.

The primary function of donor member 42 primary function is to develop remaining fine lines and edges by reducing fringe field effects by employing a close photoreceptor to donor member spacing and a low toner bed height. Since large solid areas are developed by donor member 41 thereby neutralizing major portions of the charge areas of the latent image. This enables improved developability of the fine lines and edge details to be developed by donor member 42 .

Donor member 42 has a plurality of electrode wires 60 closely spaced from the toned donor member 42 in the development zone. Wires 60 are (i.e. 50 to 100 microns diameter) conductive wires which are lightly positioned against the toner on the donor member 42 . The distance between the wires and the donor member is self-spaced by the thickness of the toner layer, which is approximately 25 microns. The extremities of the wires are supported by end blocks (not shown) at points slightly above a tangent to the donor member surface. An AC voltage is applied to the wires to generate a toner cloud in the development zone. The donor member 42 generally consists of a conductive core covered with a thin (50 μm) semiconductive layer. The toner layer on the donor member 42 is then disturbed by electric fields from a wire or set of wires so as to produce and sustain an agitated cloud of toner particles. This cloud develops the remaining fine lines and edges of the latent image. Typical AC voltages of the wires relative to the donor member are 700-900 Vpp at frequencies of 5-15 kHz. These AC signals are often square waves, rather than pure sinusoidal waves.

Donor member 42 is loaded with toner by donor member 41 at reload zone 300 . The donor member 41 is held at an electrical potential difference relative to the donor member 42 core to produce the field necessary for toner development onto donor member 42 .

The invention has been described in detail with particular reference to a preferred embodiment thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention as described hereinabove and as defined in the appended claims.

Claims

1. Apparatus for non-interactive, dry powder development of electrostatic images composed of solid areas and fine line areas on an imageable surface with developer material comprising:a housing containing developer material; a magnetic member, spaced a predefined distance from said images, for transporting said developer material from said housing to develop solid areas of said images, said magnetic member including a magnetic core and a cylindrical sleeve enclosing and rotating about said magnetic core; a donor member, adjacent to said magnetic member and spaced from the imageable surface and adapted to transport developer material to a development zone adjacent the imageable surface said magnetic member loads said donor member with developer material; an electrode positioned in the development zone between the imageable surface and the donor member; and a voltage supply for electrically biasing said electrode during a developing operation with an alternating current to detach developer material from said donor member, forming a cloud of developer material in the development zone, and developing fine line areas of said images from the cloud.

2. Apparatus for non-interactive, dry powder development of electrostatic images composed of solid areas and fine line areas on an imageable surface with developer material comprising:a housing containing developer material; a magnetic member, spaced a predefined distance from said images, for transporting said developer material from said housing to develop solid areas of said images, said magnetic member including a magnetic core and a cylindrical sleeve enclosing and rotating about said magnetic core; means for biasing said magnetic member with a DC and AC bias; a donor member, adjacent to said magnetic member and spaced from the imageable surface and adapted to transport developer material to a development zone adjacent the imageable surface; an electrode positioned in the development zone between the imageable surface and the donor member; and a voltage supply for electrically biasing said electrode during a developing operation with an alternating current to detach developer material from said donor member, forming a cloud of developer material in the development zone, and developing fine line areas of said images from the cloud.

3. The apparatus according to claim 2, wherein said DC applied is to said magnetic member to insure background regions of the images are not developed.

4. The apparatus according to claim 2, wherein said predefined distance is between 0.020″ and 0.050″.

5. The apparatus of claim 2, wherein said magnetic member has a developer material bed height of 0.015″ and 0.045″.

6. The apparatus of claim 5, further comprising means for adjusting developer material bed height on said magnetic member.

7. The apparatus of claim 6, wherein said adjusting means includes a trim blade.

8. The apparatus of claim 2, wherein said biasing means for said magnetic member has a frequency between 1 Khz and 4 KHz.