Curved print head for charged particle generation

Details Curved print head for charged particle generation Curved print head for charged particle generation

2000-08-23

2002-05-14

Gordon, Raquel Yvette (Department: 2853)

Incremental printing of symbolic information

Ink jet

Ejector mechanism

Reexamination Certificate

active

06386684

ABSTRACT:

BACKGROUND OF THE INVENTION
The field of this invention is electrostatic printing, and more specifically a print head for charged particle generation.
Electrostatic printing, which is also referred to as ion deposition printing, charge deposition printing or electron beam imaging, has been used successfully for many years in a number of commercial embodiments. The apparatus and method disclosed in U.S. Pat. No. 4,155,093 to Fotland et. al., issued on May 15, 1979, form the basis of modern electrostatic printing technology. A flat print head is used in conjunction with a dielectric drum to create charge patterns on the drum, which attract toner particles. A piece of paper is then pressed into contact with the drum, acquiring the toner particles from the drum to receive a printed image.
Referring to
FIG. 1
, a typical flat print head
100
known in the art is shown. The flat print head typically includes two sets of selectively-controlled electrodes separated from one another by a high-strength first dielectric
108
. The first set of electrodes
102
, often referred to as driver electrodes
102
, extend along the longer dimension of the print head, typically spanning the width of a page or other paper to be printed upon. The second set of electrodes
104
, often referred to as finger electrodes
104
, crosses the first electrodes obliquely. The driver electrodes
102
and the finger electrodes
104
form a matrix of crossing junctions between them, referred to as discharge sites
116
. To create a charged particle discharge at a particular discharge site
116
, a radio frequency (RF) signal of several thousand volts is applied to the driver electrode
102
at that discharge site
116
. When a second charge is applied to the finger electrode
104
at that discharge site
116
, charged particles are discharged at that discharge site
116
as a low energy spark or electric discharge. The print head
100
may be constructed to discharge either positive or negative charges. The negative charge may contain ions, electrons or a combination of both. The charged particles from a discharge site
116
cross a gap and impact a drum
112
, where they are deposited on its dielectric surface
114
. The print head
100
is configured such that the charge deposited by each discharge site
116
forms a dot-like latent charge image on the drum. Images or text can be created as aggregations of such dots. Thus, by controlling the discharge of particles from the matrix of discharge sites
116
, and rotating the drum
112
, images larger than the matrix of discharge sites
116
can be created and transferred onto paper or another surface.
In print heads of this type, RF-driven driver electrodes
102
are typically line conductors extending along the length of the print head, spanning a number of finger electrodes
104
which typically cross the driver electrode
102
at an angle. In an exemplary commercial embodiment, sixteen parallel driver electrodes
102
extend the width of a printed page, and they are crossed obliquely by
160
finger electrodes
104
. A discharge site
116
is located at each point where a driver electrode
102
intersects a finger electrode
104
. Each finger electrode
104
crosses the driver electrode
102
sixteen times, and can project up to sixteen charge dots, one from each discharge site
116
arranged along its length. According to Gauss' Law, electric field lines originate perpendicular to a conducting surface. Theoretically, charge deposition follows those electric field lines, because electric force is exerted substantially along those field lines. In practice, charge eventually builds up on the drum
112
, creating an electric field opposing the existing field. As a result of the presence of the opposing electric field on the drum
112
, the charged particles will follow trajectories altered from the ideal trajectories perpendicular to the discharge surface, causing the charge to spread out. This is referred to as the blooming effect. The blooming effect becomes more severe as the distance between the print head
100
and the drum
112
increases, because the electric field generated by the print head
100
weakens with distance, subjecting the charged particles to increased influence from the opposing electric field exerted by the accumulated particles on the drum
112
.
Because the print head
100
is flat and the drum
112
is cylindrical, the gap distance between the discharge sites
116
and the drum
112
is not uniform across the width of the print head
100
. The electric field between the print head
100
and the drum
112
weakens as the distance between the discharge sites
116
and the longitudinal centerline of the print head
100
increases. In this document, the longitudinal direction is understood to be the direction of the axis of the drum
112
. Consequently, the charge deposited on the drum
112
from the discharge sites
116
is not uniform across the width of the print head
100
. As a result, the dots produced by the discharge sites
116
located further from the centerline of the print head
100
are weaker than those produced by discharge sites
116
at or near the centerline of the print head
100
. This varying charge dot intensity caused by nonuniform charge deposition creates artifacts such as but not limited to smearing and venetian blinding in the image laid down by the drum
112
. Venetian blinding is a defect well known to those skilled in the art, in which striations extending parallel to the direction of motion of the drum
112
appear in the image. These striations have different intensities of shading, directly correlating to the different charge intensities deposited on the drum
112
from the discharge sites
116
.
A number of different attempts have been made to fix the image artifacts caused by varying charge dot intensity across the print head
100
.
One category of attempts to solve the image artifact problem utilizes additional electrodes to better focus the charged particle beam. One or more additional sets of electrodes
106
, generally referred to as screen electrodes
106
, may be provided between the finger electrodes
104
and the drum
112
. The screen electrodes
106
are apertured, and separated from the finger electrodes
104
by a second dielectric
110
having a number of cavities corresponding to the discharge sites
116
and the apertures in the screen electrodes
106
. By applying a constant bias between the screen electrodes
106
and the drum
112
, and a switchable bias between the screen electrodes
106
and the finger electrodes
104
, the screen electrodes
106
act as lenses to improve image quality, and additionally act to prevent accidental erasure of deposited charges. The use of one or more sets of screen electrodes
106
in a print head
100
is described in, for example, U.S. Pat. No. 4,160,257; U.S. Pat. No. 4,675,703; U.S. Pat. No. 5,159,358; and U.S. Pat. No. 5,278,588. While the screen electrodes
106
can improve the quality of the printed image, the addition of one or more electrodes to the print head
100
increases the number of manufacturing steps required, and requires more parts which can malfunction or be damaged as the print head
100
is used. The added complexity of manufacturing also results in increased cost to the end user.
A second category of attempts to solve the image artifact problem modifies the discharge sites
116
or the dielectric material adjacent the discharge sites
116
to improve control over the charged particle stream emitted from the discharge site
116
. Such modifications to the discharge site include angling the walls of the dielectric cavity adjacent each discharge site (U.S. Pat. No. 4,691,213; U.S. Pat. No. 4,683,482), providing a number of separate apertures at each discharge site (U.S. Pat. No. 4,879,569), and inserting dielectric material into the second electrode (U.S. Pat. No. 4,891,656). While the modification of the shape and configuration of each individual discharge site
116
can improve the quality of the printed image, the creation of

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