Color electrophotographic apparauts having image registration

Details Color electrophotographic apparauts having image registration Color electrophotographic apparauts having image registration

1997-07-03

2001-02-20

Beatty, Robert (Department: 2852)

Incremental printing of symbolic information

Electric marking apparatus or processes

Electrostatic

C347S232000

Reexamination Certificate

active

06191801

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a color electrophotographic apparatus. More particularly, the present invention relates to a color electrophotographic apparatus which can provide multi-color image reproduction with one-pass developing.
2. Description of the Related Art
Most computer systems, and most particularly small or “personal” computer systems, have been making increasing use of colors in the display of information. The increasing use of color in personal computers reflects both the more ready availability of low price color displays and the more powerful and faster microprocessors that are used in personal computers. The technology for providing printed color output, i.e., “hard copy” output such as color printouts on paper, plastic or other materials, has not kept pace with the technology for the color display of information. Presently, the options available for color hard copy output either do not present sufficiently high quality output or are undesirably expensive for home or small office use. Examples of this conventional color output technology include ink jet printing, whether using liquid or solid inks, as well as a few different implementations of color electrophotographic printing. Ink jet printing is comparatively inexpensive, at least when using liquid inks, but tends to be slow and it is difficult to obtain acceptably high quality output using liquid ink jet printing technology.
Color electrophotographic printing is commercially available, but tends to be expensive and slow. For example, multipass color electrophotographic printing is a process by which multiple photoconductor exposures and multiple developing processes are used to create a multicolor image on the surface of a paper sheet. In essence, conventional multipass color electrophotographic printing consists of repeated application of single color or monochrome electrophotographic printing processes using different colors for each successive application or pass through the multipass printer. Traditional monochrome (black and white) electrophotographic printing forms an image on the optically active surface of a photoconductor by exposing the photoconductor using a laser or an equivalent high intensity light source. Before exposure, a uniform charge distribution is provided over the surface of the photoconductor and, after exposure, a charge pattern corresponding to the exposure image to be printed is present on the surface of the photoconductor. The latent image corresponding to the charge pattern on the surface of the photoconductor is converted to a physical image by a developer which adheres charged toner particles to the charge pattern on the photoconductor in a pattern corresponding to the latent image. The toner image is transferred onto a paper sheet using an electrostatic transfer process and then fusing is performed to fix the toner image on the paper sheet. In a multipass color laser printer or other similar electrophotographic apparatus, this printing process would be repeated several times.
FIG. 1
illustrates a conventional, multipass color electrophotographic apparatus which is assumed to be a laser printer for the purposes of this discussion. The multipass laser printer of
FIG. 1
includes a photoconductive drum
10
, a charger
14
for creating a uniform charge distribution on the surface of the photoconductive drum
10
, a laser beam scanning unit
16
for exposing the surface of the photoconductive drum with an optical image, a developing device
20
including a plurality of single color developing units
22
,
24
,
26
,
28
for developing a latent image, a transfer charger
30
for applying a transfer electrical field, and a fuser
40
for fixing an image onto a recording medium
32
such as a paper sheet. The photoconductive drum
10
is generally a metal cylinder covered by an optically active material
12
known as the photoconductor. The photoconductor generally is highly insulative in the dark while developing a substantial level of conductivity under illumination. Thus, the photoconductor
12
can hold a charge on its surface in the dark, but charge on the surface of the photoconductor is discharged under illumination.
In operation, a uniform charge is applied to the surface of the photoconductor
12
at the beginning of each pass of the multipass color printing process. Charging of the photoconductor surface is accomplished with charger
14
, which typically uses corona charging or a similar technique to provide charge to the surface of the photoconductor
12
. After the charging operation, the photoconductor
12
has on its surface a uniform charge distribution corresponding to a voltage of ±600~±800 V. When the photoconductor
12
is exposed by the laser beam scanning unit
16
, a laser beam
18
directed by the scanning unit
16
illuminates a specified area of the photoconductor
12
in accordance with an image modulation pattern generated by a controller (not shown). The voltage on the portions of the photoconductor
12
illuminated by the laser beam
18
is discharged to approximately 0~±150 V.
Multipass color laser printing is accomplished by successively forming on the surface of the photoconductor
12
successive monochrome images so that, when all of the monochrome images are combined together on the photoconductor
12
, the combined image provides an acceptable color image. Typical multipass color electrophotographic strategies use four printing passes, with each successive pass applying a different optical image to the photoconductor corresponding to a different monochrome image component. Each successive image is developed after the exposure portion of the pass with a developer having the appropriate color of toner corresponding to that monochrome portion of the image. To effect this strategy, it is necessary to provide four different developing units
22
-
28
as shown in
FIG. 1
having four distinct colors of toner to be applied in successive ones of four different passes. Thus, four developing units
22
-
28
corresponding to yellow (Y) toner, magenta (M) toner, cyan (C) toner and black (K) toner respectively are provided for the
FIG. 1
printers. The reproduced image is therefore made up of a plurality of colors applied in varying concentrations to achieve various gray levels.
In a first pass of the multicolor printing process of
FIG. 1
, the laser beam scanning unit
16
exposes the surface of the photoconductor with modulated laser light
18
to create a first latent image component corresponding to the first monochrome component of the image to be printed. After the photoconductor
12
is exposed with the first latent image component, the first component of the image pattern is developed using a first developer
22
, described in greater detail below, to provide a first color of toner to the surface of the photoconductor. After the first monochrome component of the color image to be printed has been provided on the surface of the photoconductor, a second pass is performed to provide a second monochrome component of the color image to be printed. The photoconductor
12
on the drum is charged to provide a new uniform charge distribution on the photoconductor. The laser beam scanning unit
16
then scans the laser beam
18
over the surface of the photoconductor to expose the photoconductor
12
with a second latent image component. A second color of toner is applied by the second developing unit
24
so that it adheres to the photoconductor
12
in a pattern corresponding to the second latent image component and overlies the first toner image. This process is repeated for the third and fourth components of the image, using the third and fourth developing units
26
and
28
, respectively, to provide four different overlaid monochrome toner images on the photoconductor. The four color toner image is then transferred onto the surface of a recording medium
32
such as a paper sheet at the transfer charger
30
and the toner image is fused to the recording medium
32
at fuse

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