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Description  |
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CROSS-REFERENCE TO RELATED APPLICATION
This application is related to co-pending U.S. patent application Ser. No.
621,691, now abandoned, filed in the name of DeBoer et al. concurrently
herewith.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to an imaging system, and specifically to
an improved migration imaging system utilizing an imaging member having a
thermoplastic imaging surface layer.
2. Description of the Prior Art
Within the art of electrophotography are imaging processes or systems which
involve the migration of particles in a liquid or softenable medium to
achieve an imagewise pattern. Particle migration to provide a latent image
has been disclosed, for example, in processes based upon electrophoretic
and photoelectrophoretic imaging of photoconductive particles dispersed in
liquids. In solid mediums that are nominally not permeable, particle
migration is typically facilitated by the softening of the medium by the
application of heat or solvents.
Most conventional migration imaging systems will arrange the marking
particles in an imagewise pattern on the softenable member before any
migration is accomplished. Thus, some means must be provided for composing
the particles in an image-wise pattern, and another means may be necessary
to transfer the pattern to a softenable layer. Then, a further means is
used to soften the layer, and another means is used to migrate the
particles into the softened layer. The system is complicated and the
process is time-consuming. A simpler and more efficient system is desired.
Some migration imaging systems utilize a solid migration imaging member
which typically comprises a substrate, a layer of softenable material, and
a layer of photosensitive marking material deposited on the softenable
layer. A latent image is formed by electrically charging the member and
then exposing the member to an imagewise pattern of light to discharge
selected portions of the marking material layer. The entire softenable
layer is then made permeable by dissolving, swelling, melting, or
softening it by application of heat or a solvent, or both. Portions of the
marking material that retain a differential residual charge due to the
light exposure will migrate into the softened layer by electrostatic
force. One example of such an imaging process is disclosed in U.S. Pat.
No. 4,883,731, issued to Tam et al.
An imagewise pattern may also be composed in a solid imaging member by
establishing a differential in the density of colorant particles in imaged
vs. non-imaged areas. In other words, the colorant particles are uniformly
dispersed and then selectively migrated such that they are further
dispersed to a greater or lesser extent. The differential density
determines the image. The overall quantity of particles on the substrate
is unchanged. Alternatively, the particles are migrated such that certain
particles agglomerate or coalesce, thus achieving a differential density.
Or, in what is known as a heat development method, a solid imaging member
will include colloidal pigment particles dispersed in a heat-softenable
resin film on a transparent conductive substrate. An electrostatic image
is transferred to the film, which is then softened by heating. The charged
colloidal particles migrate to the oppositely charged image. Image areas
are thereby increased in particle density while the background areas are
less dense. Heat development is described by Schaffert, R. M., in
Electrophotography, (Second Edition, Focal Press, 1980) at pp. 44-47 and,
in particular, in U.S. Pat. No. 3,254,997.
However, the images formed in the solid imaging members processed according
to the foregoing approaches have been found to lack the image contrast,
gray scale accuracy, and sharp resolution required in high-resolution
image reproduction. A simpler and more efficient imaging system would be
desirable.
In another imaging process known generally as adhesive transfer, a solid,
multilayered donor-acceptor imaging member is used to produce image
copies. The donor layer includes a uniform fracturable layer of marking
particles, a marking particle release layer, and a supporting carrier or
sheet. An adhesive-coated acceptor layer overlies the marking particle
layer. Areas of the marking particles are softened by localized heating in
an imagewise pattern such that their attraction to, or retention by, the
donor portion is less than the attraction of particles to non-heated
areas. The acceptor layer may then be stripped from the member, taking the
imaged pattern of marking particles from the release layer.
The aforementioned adhesive-transfer systems operate on a frangible
dispersion of marking particles under a separable adhesive layer. Such
systems typically cannot offer high resolution image reproductions because
of an inherent compromise between the frangibility of the particles in
non-imaged areas vs. the cohesiveness of particles in an imaged area. For
example, in a peel-away system, any imaged area of the particulate layer
must be cohesive enough to be carried with the peel-away layer. However,
the imaged area must break cleanly at a border with a non-imaged area.
Serifs, fine lines, dot images, and the like can receive an undesirably
ragged edge during such a process.
For example, International Patent Application WO 88/04237, filed Dec. 7,
1987 by Polaroid Corporation, discloses a thermal imaging medium which
includes a support sheet having a surface of a heat-liquifiable material
and a layer of a particulate or porous image-forming substance. A
pressure-sensitive adhesive layer overlies the particulate layer. The
liquifiable material is imagewise exposed to heat to cause it to flow by
capillary action into the image-forming substance. With cooling, the
imaged areas of the substance are thereby retained by the material on the
support sheet. The adhesive layer is then peeled away, causing the
unexposed areas of the particulate layer to break from the exposed areas
and be carried with the adhesive layer. The support sheet retains the
exposed pattern.
However, the fracturing between exposed and unexposed areas can be uneven
or irregular. Moreover, the heat-softened material is expected to flow
only into a certain volume of the colorant, but the flow is not
restricted. The softened material can flow laterally into a volume that is
adjacent the heated area and which is not part of the image to be
reproduced. The perimeter of an image component (a dot, for example) would
then be greater than intended. As a result, image quality can be degraded.
In general, adhesive transfer and migration imaging systems are also
materials-intensive and thus are costly to operate. This is especially so
in systems which consume materials that are not provided in a simple,
easy-to-use, and inexpensive form.
Significant waste products are generated in many of the above-described
systems. Solvent-based systems generate a solvent effluent that is
hazardous, expensive to discard, and cumbersome. Adhesive transfer systems
generate discarded peel-away films which are usually not reusable. Proper
disposal of such waste is inconvenient and increases operating costs.
Migration imaging and adhesive transfer processes have, therefore, not been
favored for image reproduction in a number of applications, especially in
high-resolution or high-speed printing.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an improved imaging
system for the production of high-quality, high-resolution image
reproductions without the disadvantages found in the prior art imaging
systems.
It is a further object of the present invention to provide a versatile
imaging system usable for generating image reproductions in the form of
monochromatic or multicolor prints, transparencies, xeroprinting masters,
exposure masks, graphic printing plates, or color printing proofs.
It is another object of the present invention to provide image
reproductions in a simple and efficient apparatus using image data from a
rasterized image data source.
It is another object of the present invention to provide image
reproductions by use of simple consumable materials such as toner, which
may be reused if not consumed.
These and other objects are met by a novel migration imaging system using a
thermoplastic imaging member. The imaging member comprises a supporting
section and a thermoplastic imaging surface layer.
In the practice of the invention, a charged layer of marking particles,
such as toner, is deposited on the imaging surface layer. The marking
particles are thereby subject to an electrostatic attraction to the
supporting section. The imaging member is selectively exposed to
heat-inducing energy, such as a scanning infrared beam, in an imagewise
pattern. The applied energy transforms selected portions of the imaging
surface layer to a permeable state.
The charged marking particles that superpose the transformed portions then
migrate into the imaging surface layer so as to be retained by the surface
layer. In some applications, the addressed particles are also tacked
together due to the applied energy. Unaddressed marking particles are
cleaned away.
The imaging member may then be used simply as a hard copy image in the form
of a reflection copy, a transparency, or as an image master.
Alternatively, the imaging member may be transferred and attached at its
imaging surface layer to a receiver means, such as a web or transfer drum,
or to a receiver sheet, such as a film sheet or paper sheet. In another
embodiment, the imaging surface layer is separable from the imaging member
and attachable to a receiver means or to one or more receiver sheets.
A set of color separation images of good contrast ratio, high resolution,
and high image quality may be written on one imaging member. The images
may be written in series, and a set of hard copy color separations may be
generated for use as, for example, color separation proofs. Alternatively,
the color separations may be transferred in superposition to a single
receiver to generate a composite color print.
An imaging system according to the invention is envisioned for use in
direct digital color proofing, wherein near-photographic quality prints
may be generated at higher speed and lower cost than by conventional
methods such as thermal dye transfer. Pigments or ink particles to be used
in the lithographic printing run may be used as the marking particles in
generating a color proof. The resulting color proof has better color
accuracy and therefore is more valuable than those provided by
conventional processes.
The contemplated imaging member is formed of simple materials that are
inexpensive and easy to handle. No solvents are required and virtually no
waste is generated in the imaging process. In fact, the unaddressed
marking particles may be reserved for subsequent imaging.
The imaging member is especially compatible with a conventional laser
scanner because the aforementioned selective exposure to heat-inducing
energy may be provided by a scanning laser beam modulated by a rasterized
data stream. Image information may be provided to the scanner and recorded
in the thermoplastic imaging surface layer at a high data rate. The
contemplated imaging member also may be thermally biased so as to be
exposable by a scanning beam moving at an especially high scan rate, which
further enhances the speed and efficiency of the imaging process.
The imaging surface layer may be attached to papers that normally do not
retain a toned image. Alternatively, the supporting section may be paper
whereby no transfer of the processed imaging surface layer is needed.
Thus, hard copy reproductions may be produced on, or transferred to, a
variety of papers or films that are not usable in the typical copier due
to their weight, moisture content, surface layer texture or irregularity,
electrical resistance, or other characteristics. The imaging surface
layer, when transferred, also provides a more uniform gloss to the
receiver.
One preferred application of the imaging member is in the production of
high-quality hard copy images for the graphics arts industry and for
diagnostic imaging equipment, such as ultrasonic, radiographic, and
nuclear medical imaging devices. Such equipment is increasingly
incorporated in large-scale digital picture-archiving and communication
systems used in medical and other scientific research institutions.
In another preferred embodiment, the supporting section of the imaging
member comprises a film base having photoconductive constituents. The
imaging surface layer, after having an imagewise pattern of marking
particles migrated therein, may be illuminated. Light not obscured by the
marking particles will then discharge the film base in an imagewise
pattern. The resulting latent image may then be developed and transferred
to a receiver according to known xeroprinting methods.
The invention, and its objects and advantages, will become more apparent in
the detailed description of the preferred embodiments presented below.
BRIEF DESCRIPTION OF THE DRAWINGS
In the detailed description of the preferred embodiments of the invention
presented below, reference is made to the accompanying drawings.
FIG. 1 is a side schematic view of a migration imaging system using a novel
imaging member constructed according to the present invention. The imaging
member is illustrated during the step of deposition of marking particles
on the imaging member.
FIGS. 2 and 3 are side schematic views of the imaging system of FIG. 1
during the steps of imagewise exposure and cleaning, respectively, of the
thermoplastic imaging surface layer on the imaging member.
FIG. 4A is a side schematic view of the imaging member of FIG. 3 during
transfer of the imaging member to a receiver means.
FIGS. 4B and 4C are a side schematic views of the imaging member of FIG. 3
during transfer of the thermoplastic imaging surface layer from the image
member to receiver means or a receiver sheet, respectively.
FIG. 4D is a side schematic view of the imaging member of FIG. 3 during
transfer of the imaging member to a receiver sheet.
FIG. 5 is a side sectional view of the imaging member of FIGS. 1-4 on a
support.
FIG. 6 is a side sectional view of an alternative embodiment of the imaging
member of FIG. 5.
FIG. 7 is a side sectional view, in greater detail, of the exposed portion
of the imaging member of FIG. 2.
FIGS. 8 and 9 are side sectional views of the exposed portion of the
imaging member of FIG. 7 after exposure and cleaning, respectively.
FIGS. 10 and 11 are side sectional views of another exposed portion of the
imaging member of FIG. 7 after exposure and cleaning, respectively.
FIG. 12 is a side schematic view of an embodiment of an imaging system
usable with the imaging member of FIGS. 5 or 6.
FIG. 13 is a side schematic view of an embodiment of a xeroprinting system
usable with the imaging member of FIGS. 5 or 6.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
As illustrated in FIGS. 1-4, a novel thermoplastic imaging member 10 is
processed in a migration imaging system 12 constructed according to the
invention. As shown in FIG. 1, a thermoplastic imaging surface layer 14
receives a marking particle layer 24 deposited by a particle deposition
means 20A such as a biased magnetic brush connected to a bias voltage
supply 22. The particle deposition means 20A is equipped with a quantity
of marking particles which are then deposited on the imaging surface layer
14 as the means 20A passes over the imaging surface 14.
A supporting section 15 of the imaging member 10 is connected to one
potential of the bias voltage supply 22 such that an electrostatic field
is established between the marking particle layer 24 and the supporting
section 15 according to known bias development techniques. The marking
particle layer 24 is attracted to the imaging surface layer 14 by virtue
of the electrostatic attraction of the individual particles 24A to the
imaging member 10. Alternatively, the marking particles may be first
uniformly deposited and then charged by known techniques to cause them to
be attracted to the imaging surface layer 14.
Although the marking particle layer 24 is illustrated for clarity as being
a single layer of positively charged particles 24A, in practice, the layer
is several particles deep. The polarities of the marking particle layer 24
and the supporting section 15 may in the alternative be reversed,
depending upon the application.
Preferably, the marking particles 24A are dry pigmented toner particles.
Suitable toner formulations are disclosed in U.S. Pat. No. 4,546,060,
issued to Miskinis et al. on Oct. 8, 1985, the content of which is
incorporated herein by reference. Preferably, a matrix of thermoplastic
pigmented particles are mixed with hard magnetic carrier particles to form
a two-component developer usable by a magnetic brush.
Common magnetic brush systems include one system consisting of a fixed
magnetic core with a rotating nonmagnetic shell. Another common system is
a rotating magnetic core with a rotating or nonrotating shell. The
magnetic core is constructed of similar strength magnets that are arranged
in an alternate pole fashion.
In the fixed magnetic core system, material is pulled tightly to the
surface of the shell. As the shell rotates, it pushes material from one
magnetic region to another. The material lines up in long chains
perpendicular to the shell surface and flips very quickly at the pole
transitions. The alignment of particles at any given location around the
core axis remains constant and dependent on the local magnetic field
configuration. In the rotating magnetic core system, chains of material
are in a state of constant flipping action as they traverse around the
surface of the shell. This motion delivers a large amount of marking
particles to the image member.
Suitable carrier formulations and magnetic brush development means are
disclosed in U.S. Pat. No. 4,546,060, issued to Miskinis et al. on Oct. 8,
1985; U.S. Pat. No. 4,473,029, issued to Fritz et al. on Sep. 25, 1984;
and U.S. Pat. No. 4,531,832, issued to Kroll et al. on Jul. 30, 1985, the
contents of which are incorporated herein by reference.
It is contemplated that other electrostatically-chargeable marking
particles, such as dye particles, single-component developers, pigmented
graphics art inks, or liquid toners may be uniformly deposited by other
appropriate deposition means known in the art.
As shown in FIG. 2, the imaging member 10 is exposed to imagewise-modulated
heat-inducing energy. Preferably, the exposure is accomplished by a
modulated scanning light beam 42 provided by an beam scanner 40. The
scanning beam 42, which in a particularly preferred embodiment is an
infrared laser beam, may be directed from scanner 40 through either side
of the imaging member 10 to one of several components of the imaging
member 10. For example, the beam 42 may be focussed through the supporting
section backside 16 to heat the supporting section, or may be focussed
deeper, at the thermoplastic layer 14. Alternatively, the beam 42 may be
directed onto the marking particle layer 24 whereupon the exposed
particles absorb the incident radiation and are heated, and whereupon the
heat so generated is conducted to the underlying thermoplastic layer 14.
Finally, the beam 42 may be directed through the marking particle layer 24
to heat the thermoplastic layer 14 if the marking particles in layer 24
are substantially non-absorptive of the scanning beam.
Those skilled in the art will recognize that the selection of the beam
focal point is determined according to several factors such as the
wavelength of the incident beam and the materials that constitute the
imaging member 10 and the particle layer 24. Many formulations of
non-carbon toner, for example, are non-absorptive at infrared wavelengths.
Whether the focal point is selected as being in the supporting section 15,
the imaging surface layer 14, or the marking particle layer 24, the object
of the exposure is to establish (by direct radiation or by conduction) a
selectively-intensive amount of heat within a minute volume, or pixel 50,
of the imaging surface layer 14.
The beam 42, in addition to being modulated according to the image data to
be recorded, is also line-scanned across the imaging member. The
contemplated exposure to heat-inducing energy heats a succession of pixels
50 in the imaging member 10. At each exposed pixel there is a respective
localized state change, or transformation, of the imaging surface layer
14. That is, the imaging surface layer becomes selectively permeable by
the superposed marking particles 54, according to the amount and location
of the heat that it receives. The exposure of pixels in the imaging
surface layer to effect the desired transformation is characterized as
addressing.
The marking particles 54 that superpose a transformed pixel (such particles
hereinafter characterized as addresse | | |
