U.S. patent number RE35,512 [Application Number 08/530,526] was granted by the patent office on 1997-05-20 for lithographic printing members for use with laser-discharge imaging.
This patent grant is currently assigned to Presstek, Inc.. Invention is credited to Thomas E. Lewis, Michael T. Nowak.
United States Patent |
RE35,512 |
Nowak , et al. |
* May 20, 1997 |
Lithographic printing members for use with laser-discharge
imaging
Abstract
Lithographic printing plates suitable for imaging by means of
laser devices that emit in the near-infrared region. Laser output
either ablates one or more plate layers or physically transforms a
surface layer, in either case resulting in an imagewise pattern of
features on the plate. The image features exhibit an affinity for
ink or an ink-abhesive fluid that differs from that of unexposed
areas.
Inventors: |
Nowak; Michael T. (Leominster,
MA), Lewis; Thomas E. (Hampstead, NH) |
Assignee: |
Presstek, Inc. (Hudson,
NH)
|
[*] Notice: |
The portion of the term of this patent
subsequent to August 23, 2011 has been disclaimed. |
Family
ID: |
27370287 |
Appl.
No.: |
08/530,526 |
Filed: |
September 19, 1995 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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62431 |
May 13, 1993 |
5339737 |
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917481 |
Jul 20, 1992 |
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Reissue of: |
247016 |
May 20, 1994 |
05379698 |
Jan 10, 1995 |
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Current U.S.
Class: |
101/454;
101/457 |
Current CPC
Class: |
B41C
1/1033 (20130101); B41N 1/14 (20130101); B41C
1/1008 (20130101); B41P 2227/70 (20130101); B41C
1/1016 (20130101); B41C 2201/04 (20130101); B41C
2201/10 (20130101); B41C 2210/02 (20130101); B41C
2210/04 (20130101); B41C 2210/08 (20130101); B41C
2210/14 (20130101); B41C 2210/22 (20130101); B41C
2210/26 (20130101) |
Current International
Class: |
B41C
1/10 (20060101); B41C 1/10 (20060101); B41N
1/12 (20060101); B41N 1/12 (20060101); B41N
1/14 (20060101); B41N 1/14 (20060101); B41N
001/08 () |
Field of
Search: |
;101/453,454,457-459,463.1,465-467,401.1 ;430/302-305 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1050805 |
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Mar 1979 |
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CA |
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0573091 |
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Dec 1993 |
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EP |
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0573092 |
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Dec 1993 |
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EP |
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3714157 |
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Nov 1988 |
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DE |
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235789 |
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Sep 1990 |
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JP |
|
3197190 |
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Aug 1991 |
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JP |
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3197191 |
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Aug 1991 |
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JP |
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3197192 |
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Aug 1991 |
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JP |
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5-8367 |
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Jan 1993 |
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JP |
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WO9207716 |
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May 1992 |
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WO |
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WO9401280 |
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Jan 1994 |
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WO |
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Other References
Molecular And Dynamic Studies On Lase Abalation Of Doped Polymer
Systems, 17 Polymer News (1991). .
E.B. Cargill et al., A Report On Polaroid's New Dry Imaging
Technology For Generating 8.times.10 Radiographic Films (Jan.
1993). .
E.B. Cargill et al., A Report On The Image Quality Characteristics
Of The Polaroid Helios Laser System (Oct. 1992). .
Leenders & Peeters, "Method and material for the production of
a dry planographic printing plate," Research Disclosure, Apr. 1980,
at p. 131. .
Nechiporenko et al., "Direct Method of Producing Waterless Offset
Plates By Controlled Laser Beam"..
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Primary Examiner: Funk; Stephen R.
Attorney, Agent or Firm: Cesari and McKenna
Parent Case Text
RELATED APPLICATION
This is a continuation-in-part of Ser. No. 08/062,431, filed on May
13, 1993, now U.S. Pat. No. 5,339,737 which is itself a
continuation-in-part of Ser. No. 07/917,481, filed on Jul. 20,
1992, now abandoned.
Claims
What is claimed is:
1. A lithographic printing member directly imageable by laser
discharge, the member comprising:
a. a topmost first layer which is polymeric; and
b. a thin metal layer underlying the first layer; and
c. a substrate underlying the metal layer; wherein
d. the metal layer is formed of a material which is subject to
ablative absorption of imaging infrared radiation and the first
layer is not; and
e. the first layer .Iadd.exhibits an affinity for an abhesive fluid
for ink .Iaddend.and the substrate .[.exhibit different affinities
for at least one printing liquid selected from the group consisting
of ink and an abhesive fluid for ink.]. .Iadd.exhibits an affinity
for ink.Iaddend..
2. The member of claim 1 wherein the metal is tintanium.
3. The member of claim 1 wherein the metal is selected from the
group consisting of alloys of titanium, aluminum, alloys of
aluminum, nickel, iron and chromium.
4. The member of claim 1 wherein the metal is deposited to a
thickness of less than 200 .ANG..
5. The member of claim 1 wherein the metal is deposited to an
optical density ranging from 0.2 to 1.0.
6. The member of claim 1 wherein the metal is deposited to an
optical density of 2.5 or less.
7. The member of claim .[.1.]. .Iadd.22 .Iaddend.wherein the first
layer is oleophobic. .[.8. The member of claim 7 wherein the first
layer is a
silicone chemical species..].9. The member of claim .[.8.].
.Iadd.22 .Iaddend.wherein the silicone is anchored to the metal
layer by means of a silane adhesion promoter. .[.10. The member of
claim 1 wherein the first
layer is hydrophobic..].11. The member of claim .[.10.]. .Iadd.1
.Iaddend.wherein the first layer is a polyvinyl alcohol chemical
species.
2. The member of claim 11 wherein the polyvinyl alcohol is anchored
to
the metal layer by means of a titanate adhesion promoter. 13. The
member of claim 1 wherein the first layer is anchored to the metal
layer by means
of an adhesion promoter. 14. The member of claim 1 wherein the
substrate comprises first and second surfaces, at least one of
which has means to
improve adhesion. 15. The member of claim 1 wherein the substrate
comprises first and second surfaces, at least one of which has
means to
reduce static buildup. 16. The member of claim 1 wherein the
substrate comprises first and second surfaces, the first has means
to improve
adhesion and the second surface has means to reduce static buildup.
17. The member of claim 1 further comprising an antireflective
layer
between the metal and first layers. 18. The member of claim 1
wherein the
substrate is laminated to a metal support. .Iadd.19. The member of
claim 1 wherein the substrate comprises a material that reflects
imaging radiation. .Iaddend..Iadd.20. The member of claim 19
wherein the material is IR-reflective barium sulfate.
.Iaddend..Iadd.21. The member of claim 20 wherein the substrate
comprises a polyester polymer within which the barium sulfate is
dispersed. .Iaddend..Iadd.22. A lithographic printing member
directly imageable by laser discharge, the member comprising:
a. a topmost first layer which is formed of an addition-cured
silicone;
b. a thin metal layer underlying the first layer and formed of
titanium or an alloy thereof; and
c. a substrate underlying the metal layer; wherein
d. the metal layer is subject to ablative absorption of imaging
infrared radiation and the first layer is not; and
e. the first layer and the substrate exhibit different affinities
for at least one printing liquid selected from the group consisting
of ink and an abhesive fluid for ink. .Iaddend..Iadd.23. The member
of claim 22 wherein the metal is deposited to a thickness of less
than 200 .ANG.. .Iaddend..Iadd.24. The member of claim 22 wherein
the metal is deposited to an optical density ranging from 0.2 to
1.0. .Iaddend..Iadd.25. The member of claim 22 wherein the metal is
deposited to an optical density of 2.5 or less. .Iaddend..Iadd.26.
The member of claim 22 wherein the substrate comprises first and
second surfaces, at least one of which has means to improve
adhesion. .Iaddend..Iadd.27. The member of claim 22 wherein the
substrate comprises first and second surfaces, at least one of
which has means to reduce static buildup. .Iaddend..Iadd.28. The
member of claim 22 wherein the substrate comprises first and second
surfaces, the first has means to improve adhesion and the second
surface has means to reduce static buildup. .Iaddend..Iadd.29. The
member of claim 22 further comprising an antireflective layer
between the metal and first layers. .Iaddend..Iadd.30. The member
of claim 22 wherein the substrate is laminated to a metal support.
.Iaddend..Iadd.31. The member of claim 22 wherein the substrate
comprises a material that reflects imaging radiation.
.Iaddend..Iadd.32. The member of claim 31 wherein the material is
IR-reflective barium sulfate. .Iaddend..Iadd.33. The member of
claim 32 wherein the substrate comprises a polyester polymer within
which the barium sulfate is dispersed. .Iaddend.
Description
BACKGROUND OF THE INVENTION
A. Field of the Invention
The present invention relates to digital printing apparatus and
methods, and more particularly to a system for imaging lithographic
printing plates on- or off-press using digitally controlled laser
output.
B. Description of the Related Art
Traditional techniques of introducing a printed image onto a
recording material include letterpress printing, gravure printing
and offset lithography. All of these printing methods require a
plate, usually loaded onto a plate cylinder of a rotary press for
efficiency, to transfer ink in the pattern of the image. In
letterpress printing, the image pattern is represented on the plate
in the form of raised areas that accept ink and transfer it onto
the recording medium by impression. Gravure printing cylinders, in
contrast, contain series of wells or indentations that accept ink
for deposit onto the recording medium; excess ink must be removed
from the cylinder by a doctor blade or similar device prior to
contact between the cylinder and the recording medium.
In the case of offset lithography, the image is present on a plate
or mat as a pattern of ink-accepting (oleophilic) and ink-repellent
(oleophobic) surface areas. In a dry printing system, the plate is
simply inked and the image transferred onto a recording material;
the plate first makes contact with a compliant intermediate surface
called a blanket cylinder which, in turn, applies the image to the
paper or other recording medium. In typical sheet-fed press
systems, the recording medium is pinned to an impression cylinder,
which brings it into contact with the blanket cylinder.
In a wet lithographic system, the non-image areas are hydrophilic,
and the necessary ink-repellency is provided by an initial
application of a dampening (or "fountain") solution to the plate
prior to inking. The ink-abhesive fountain solution prevents ink
from adhering to the non-image areas, but does not affect the
oleophilic character of the image areas.
If a press is to print in more than one color, a separate printing
plate corresponding to each color is required, each such plate
usually being made photographically as described below. In addition
to preparing the appropriate plates for the different colors, the
operator must mount the plates properly on the plate cylinders of
the press, and coordinate the positions of the cylinders so that
the color components printed by the different cylinders will be in
register on the printed copies. Each set of cylinders associated
with a particular color on a press is usually referred to as a
printing station.
In most conventional presses, the printing stations are arranged in
a straight or "in-line" configuration. Each such station typically
includes an impression cylinder, a blanket cylinder, a plate
cylinder and the necessary ink (and, in wet systems, dampening)
assemblies. The recording material is transferred among the print
stations sequentially, each station applying a different ink color
to the material to produce a composite multi-color image. Another
configuration, described in U.S. Pat. No. 4,936,211 (co-owned with
the present application and hereby incorporated by reference),
relies on a central impression cylinder that carries a sheet of
recording material past each print station, eliminating the need
for mechanical transfer of the medium to each print station.
With either type of press, the recording medium can be supplied to
the print stations in the form of cut sheets continuous "web" of
material. The number of print stations on a press depends on the
type of document to be printed. For mass copying of test or simple
monochrome line-art, a single print station may suffice. To achieve
full tonal rendition of more complex monochrome images, it is
customary to employ a "duotone" approach, in which two stations
apply different densities of the same color or shade. Full-color
presses apply ink according to a selected color model, the most
common being based on cyan, magenta, yellow and black (the "CMYK"
model). Accordingly, the CMYK model requires a minimum of four
print stations; more may be required if a particular color is to be
emphasized. The press may contain another station to apply spot
lacquer to various portions of the printed document, and may also
feature one or more "perfecting" assemblies that invert the
recording medium to obtain two-sided printing.
The plates for an offset press are usually produced
photographically. To prepare a wet plate using a typical
negative-working subtractive process, the original document is
photographed to produce a photographic negative. This negative is
placed on an aluminum plate having a water-receptive oxides surface
coated with a photopolymer. Upon exposure to light or other
radiation through the negative, the areas of the coating that
received radiation (corresponding to the dark or printed areas of
the original) cure to a durable oleophilic state. The plate is then
subjected to a developing process that removes the uncured areas of
the coating (i.e., those which did not receive radiation,
corresponding to the non-image or background areas of the
original), exposing the hydrophilic surface of the aluminum
plate.
A similar photographic process is used to create dry plates, which
typically include an ink-abhesive (e.g., silicon) surface layer
coated onto a photosensitive layer, which is itself coated onto a
substrate of suitable stability (e.g., an aluminum sheet). Upon
exposure to actinic radiation, the photosensitive layer cures to a
state that destroys its bonding to the surface layer. After
exposure, a treatment is applied to deactivate the photoresponse of
the photosensitive layer in unexposed areas and to further improve
anchorage of the surface layer to these areas. Immersion of the
exposed plate in developer results in dissolution and removal of
the surface layer at those portions of the plate surface that have
received radiation, thereby exposing the ink-receptive, cured
photosensitive layer.
Photographic platemaking processes tend to be time-consuming and
require facilities and equipment adequate to support the necessary
chemistry. To circumvent these shortcomings, practitioners have
developed a number of electronic alternatives to plate imaging,
some of which can be utilized on-press. With these systems,
digitally controlled devices alter the ink-receptivity of blank
plates in a pattern representative of the image to be printed. Such
imaging devices include sources of electromagnetic-radiation
pulses, produced by one or more laser or non-laser sources, that
create chemical changes on plate blanks (thereby eliminating the
need for a photographic negative); ink-jet equipment that directly
deposits ink-repellent or ink-accepting spots on plate blanks; and
spark-discharge equipment, in which an electrode in contact with or
spaced close to a plate blank produces electrical sparks to
physically alter the topology of the plate blank, thereby producing
"dots" which collectively form a desired image (see, e.g., U.S.
Pat. No. 4,911,075, co-owned with the present application and
hereby incorporated by reference).
Because of the ready availability of laser equipment and their
amenability to digital control, significant effort has been devoted
to the development of laser-based imaging systems. Early examples
utilized lasers to etch away material from a plate blank to form an
intaglio or letterpress pattern. See, e.g., U.S. Pat. Nos.
3,506,779; 4,347,785. This approach was later extended to
production of lithographic plates, e.g., by removal of a
hydrophilic surface to reveal an oleophilic underlayer. See, e.g.,
U.S. Pat. No. 4,054,094. These systems generally require high-power
lasers, which are expensive and slow.
A second approach to laser imaging involves the use of
thermal-transfer materials. See, e.g., U.S. Pat. Nos. 3,945,318;
3,962,513; 3,964,389; and 4,395,946. With these systems, a polymer
sheet transparent to the radiation emitted by the laser is coated
with a transferable material. During operation the transfer side of
this construction is brought into contact with an acceptor sheet,
and the transfer material is selectively irradiated through the
transparent layer. Irradiation causes the transfer material to
adhere preferentially to the acceptor sheet. The transfer and
acceptor materials exhibit different affinities for fountain
solution and/or ink, so that removal of the transparent layer
together with unirradiated transfer material leaves a suitably
imaged, finished plate. Typically, the transfer material is
oleophilic and the acceptor material hydrophilic. Plates produced
with transfer-type systems tend to exhibit short useful lifetimes
due to the limited amount of material that can effectively be
transferred. In addition, because the transfer process involves
melting and resolidification of material, image quality tends to be
visibly poorer than that obtainable with other methods.
Finally, lasers can be used to expose a photosensitive blank for
traditional chemical processing. See, e.g., U.S. Pat. Nos.
3,506,779; 4,020,762. In an alternative to this approach, a laser
has been employed to selectively remove, in an imagewise pattern,
an opaque coating that overlies a photosensitive plate blank. The
plate is then exposed to a source of radiation, with the unremoved
material acting as a mask that prevents radiation from reaching
underlying portions of the plate. See, e.g., U.S. Pat. No.
4,132,168. Either of these imaging techniques requires the
cumbersome chemical processing associated with traditional,
non-digital platemaking.
DESCRIPTION OF THE INVENTION
Brief Summary of the Invention
The present invention enables rapid, efficient production of
lithographic printing plates using relatively inexpensive laser
equipment that operates at low to moderate power levels. The
imaging techniques described herein can be used in conjunction with
a variety of plate-blank constructions, enabling production of
"wet" plates that utilize fountain solution during printing or
"dry" plates to which ink is applied directly. As used herein, the
term "plate" refers to any type of printing member or surface
capable of recording an image defined by regions exhibiting
differential affinities for ink and/or fountain solution; suitable
configurations include the traditional planar or curved
lithographic plates that are mounted on the plate cylinder of a
printing press, but can also include seamless cylinders (e.g., the
roll surface of a plate cylinder), an endless belt, or other
arrangement.
A key aspect of the present invention lies in use of materials that
enhance the ablative efficiency of the laser beam. Substances that
do not heat rapidly or absorb significant amounts of radiation will
not ablate unless they are irradiated for relatively long intervals
and/or receive high-power pulses; such physical limitations are
commonly associated with lithographic-plate materials, and account
for the prevalence of high-power lasers in the prior art.
In one embodiment of our invention, a suitable plate construction
includes a first layer and a substrate underlying the first layer,
the substrate being characterized by efficient absorption of
infrared ("IR") radiation, and the first layer and substrate having
different affinities for ink (in a dry-plate construction) or an
abhesive fluid for ink (in a wet-plate construction). Laser
radiation is absorbed by the substrate, and ablates the substrate
surface in contact with the first layer; this action disrupts the
anchorage of the substrate to the overlying first layer, which is
then easily removed at the points of exposure. The result of
removal is an image spot whose affinity for the ink or ink-abhesive
fluid differs from that of the unexposed first layer.
In a variation of this embodiment, the first layer, rather than the
substrate, absorbs IR radiation. In this case the substrate serves
a support function and provides contrasting affinity
characteristics.
In both of these two-ply plate types, a single layer serves two
separate functions, namely, absorption of IR radiation and
interaction with ink or ink-abhesive fluid. In a second embodiment,
which represents the primary subject of the present application,
these functions are performed by two separate layers. The first,
topmost layer is chosen for its affinity for (or repulsion of) ink
or an ink-abhesive fluid. Underlying the first layer is a thin
metal layer that absorbs IR radiation. A strong, stable substrate
underlies the metal layer, and is characterized by an affinity for
(or repulsion of) ink or an ink-abhesive fluid opposite to that of
the first layer. Exposure of the plate to a laser pulse ablates the
absorbing second layer, weakening the topmost layer as well. As a
result of ablation of the second layer, the weakened surface layer
is no longer anchored to an underlying layer, and is easily
removed. The disrupted topmost layer (and any debris remaining from
destruction of the absorptive second layer) is removed in a
post-imaging cleaning step. This, once again, creates an image spot
having a different affinity for the ink or ink-abhesive fluid than
the unexposed first layer.
Post-imaging cleaning can be accomplished using a contact cleaning
device such as a rotating brush (or other suitable means as
described in U.S. Pat. No. 5,148,746, commonly owned with the
present application and hereby incorporated by reference). Although
post-imaging cleaning represents an additional processing step, the
persistence of the topmost layer during imaging can actually prove
beneficial. Ablation of the absorbing layer creates debris that can
interfere with transmission of the laser beam (e.g., by depositing
on a focusing lens or as an aerosol (or mist) of fine particles
that partially blocks transmission). The disrupted but unremoved
topmost layer prevents escape of this debris.
The printing members of the present invention are preferably
manufactured for convenient bulk use on automatic plate-material
dispensing equipment, such as that described in U.S. Pat. No.
5,355,795. Because in such arrangements rolled plate material is
stored on a small-diameter core from which it is drawn tightly
around the plate cylinder, it is important to utilize materials
that are flexible and have low dynamic friction coefficients to
accommodate free movement, but which also exhibit the durability
required of a lithographic printing member.
The imaging apparatus of the present invention includes at least
one laser device that emits in the IR, and preferably near-IR
region; as used herein, "near-IR" means imaging radiation whose
lambda.sub.max lies between 700 and 1500 nm. An important feature
of the present invention is the use of solid-state lasers (commonly
termed semiconductor lasers and typically based on gallium aluminum
arsenide compounds) as sources; these are distinctly economical and
convenient, and may be used in conjunction with a variety of
imaging devices. The use of near-IR radiation facilitates use of a
wide range of organic and inorganic absorption compounds and, in
particular, semiconductive and conductive types.
Laser output can be provided directly to the plate surface via
lenses or other beam-guilding components, or transmitted to the
surface of a blank printing plate from a remotely sited laser using
a fiber-optic cable. A controller and associated positioning
hardware maintains the beam output at a precise orientation with
respect to the plate surface, scans the output over the surface,
and activates the laser at positions adjacent selected points or
areas of the plate. The controller responds to incoming image
signals corresponding to the original document or picture being
copied onto the plate to produce a precise negative or positive
image of that original. The image signals are stored as a bitmap
data file on a computer. Such files may be generated by a raster
image processor (RIP) or other suitable means. For example, a RIP
can accept input data in page-description language, which defines
all of the features required to be transferred onto the printing
plate, or as a combination of page-description language and one or
more image data files. The bitmaps are constructed to define the
hue of the color as well as screen frequencies and angles.
The imaging apparatus can operate on its own, functioning solely as
a platemaker,, or can be incorporated directly into a lithographic
printing press. In the latter case, printing may commence
immediately after application of the image to a blank plate,
thereby reducing press set-up time considerably. The imaging
apparatus can be configured as a flatbed recorder or as a drum
recorder, with the lithographic plate blank mounted to the interior
or exterior cylindrical surface of the drum. Obviously, the
exterior drum design is more appropriate to use in situ, on a
lithographic press, in which case the print cylinder itself
constitutes the drum component of the recorder or plotter.
In the drum configuration, the requisite relative motion between
the laser beam and the plate is achieved by rotating the drum (and
the plate mounted thereon) about its axis and moving the beam
parallel to the rotation axis, thereby scanning the plate
circumferentially so the image "grows" in the axial direction.
Alternatively, the beam can move parallel to the drum axis and,
after each pass across the plate, increment angularly so that the
image on the plate "grows" circumferentially. In both cases, after
a complete scan by the beam, an image corresponding (positively or
negatively) to the original document or picture will have been
applied to the surface of the plate.
In the flatbed configuration, the beam is drawn across either axis
of the plate, and is indexed along the other axis after each pass.
Of course, the requisite relative motion between the beam and the
plate may be produced by movement of the plate rather than (or in
addition to) movement of the beam.
Regardless of the manner in which the beam is scanned, it is
generally preferable (for reasons of speed) to employ a plurality
of lasers and guide their outputs to a single writing array. The
writing array is then indexed, after completion of each pass across
or along the plate, a distance determined by the number of beams
emanating from the array, and by the desired resolution (i.e., the
number of image points per unit length).
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing discussion will be understood more readily from the
following detailed description of the invention, when taken in
conjunction with the accompanying drawings, in which:
FIG. 1 is an isometric view of the cylindrical embodiment of an
imaging apparatus in accordance with the present invention, and
which operates in conjunction with a diagonal-array writing
array;
FIG. 2 is a schematic depiction of the embodiment shown in FIG. 1,
and which illustrates in greater detail its mechanism of
operation;
FIG. 3 is a front-end view of a writing array for imaging in
accordance with the present invention, and in which imaging
elements are arranged in a diagonal array;
FIG. 4 is an isometric view of the cylindrical embodiment of an
imaging apparatus in accordance with the present invention, and
which operates in conjunction with a linear-array writing
array;
FIG. 5 is an isometric view of the front of a writing array for
imaging in accordance with the present invention, and in which
imaging elements are arranged in a linear array;
FIG. 6 is a side view of the writing array depicted in FIG. 5;
FIG. 7 is an isometric view of the flatbed embodiment of an imaging
apparatus having a linear lens array;
FIG. 8 is an isometric view of the interior-drum embodiment of an
imaging apparatus having a linear lens array;
FIG. 9 is a cutaway view of a remote laser and beam-guiding
system;
FIG. 10 is an enlarged, partial cutaway view of a lens element for
focusing a laser beam from an optical fiber onto the surface of a
printing plate;
FIG. 11 is an enlarged, cutaway view of a lens element having an
integral laser;
FIG. 12 is a schematic circuit diagram of a laser-driver circuit
suitable for use with the present invention; and
FIGS. 13A-13I are enlarged sectional views showing lithographic
plates imageable in accordance with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
1. Imaging Apparatus
a. Exterior-Drum Recording
Refer first to FIG. 1 of the drawings, which illustrates the
exterior drum embodiment of our imaging system. The assembly
includes a cylinder 50 around which is wrapped a lithographic plate
blank 55. Cylinder 50 includes a void segment 60, within which the
outside margins of plate 55 are secured by conventional clamping
means (not shown). We note that the size of the void segment can
vary greatly depending on the environment in which cylinder 50 is
employed.
If desired, cylinder 50 is straightforwardly incorporated into the
design of a conventional lithographic press, and serves as the
plate cylinder of the press. In a typical press construction, plate
55 receives ink from an ink train, whose terminal cylinder is in
rolling engagement with cylinder 50. The latter cylinder also
rotates in contact with a blanket cylinder, which transfers ink to
the recording medium. The press may have more than one such
printing assembly arranged in a linear array. Alternatively, a
plurality of assemblies may be arranged about a large central
impression cylinder in rolling engagement with all of the blanket
cylinders.
The recording medium is mounted to the surface of the impression
cylinder, and passes through the nip between that cylinder and each
of the blanket cylinders. Suitable central-impression and in-line
press configurations are described in U.S. Pat. No. 5,163,368
(commonly owned with the present application and hereby
incorporated by reference) and the '075 patent.
Cylinder 50 is supported in a frame and rotated by a standard
electric motor or other conventional means (illustrated
schematically in FIG. 2). The angular position of cylinder 50 is
monitored by a shaft encoder (see FIG. 4). A writing array 65,
mounted for movement on a lead screw 67 and a guide bar 69,
traverses plate 55 as it rotates. Axial movement of writing array
65 results from rotation of a stepper motor 72, which turns lead
screw 67 and thereby shifts the axial position of writing array 55.
Stepper motor 72 is activated during the time writing array 65 is
positioned over void 60, after writing array 65 has passed over the
entire surface of plate 55. The rotation of stepper motor 72 shifts
writing array 65 to the appropriate axial location to begin the
next imaging pass.
The axial index distance between successive imaging passes is
determined by the number of imaging elements in writing array 65
and their configuration therein, as well as by the desired
resolution. As shown in FIG. 2, a series of laser sources L.sub.1,
L.sub.2, L.sub.3 . . . L.sub.n, driven by suitable laser drivers
collectively designated by reference numeral 75 (and discussed in
greater detail below), each provide output to a fiber-optic cable.
The lasers are preferably gallium-arsenide models, although any
high-speed lasers that emit in the near infrared region can be
utilized advantageously.
The size of an image feature (i.e., a dot, spot or area) and image
resolution can be varied in a number of ways. The laser pulse must
be of sufficient power and duration to produce useful ablation for
imaging; however, there exists an upper limit in power levels and
exposure times above which further useful, increased ablation is
not achieved. Unlike the lower threshold, this upper limit depends
strongly on the type of plate to be imaged.
Variation within the range defined by the minimum and upper
parameter values can be used to control and select the size of
image features. In addition, so long as power levels and exposure
times exceed the minimum, feature size can be changed simply by
altering the focusing apparatus (as discussed below). The final
resolution or print density obtainable with a given-sized feature
can be enhanced by overlapping image features (e.g., by advancing
the writing array an axial distance smaller than the diameter of an
image feature). Image-feature overlap expands the number of gray
scales achievable with a particular feature.
The final plates should be capable of delivering at least 1,000,
and preferably at least 50,000 printing impressions. This requires
fabrication from durable material, and imposes certain minimum
power requirements on the laser sources. For a laser to be capable
of imaging the plates described below, its power output should be
at least 0.2 megawatt/in.sup.2 and preferably at least 0.6
megawatt/in.sup.2. Significant ablation ordinarily does not occur
below these power levels, even if the laser beam is applied for an
extended time.
Because feature sizes are ordinarily quite small--on the order of
0.5 to 2.0 mils--the necessary power intensities are readily
achieved even with lasers having moderate output levels (on the
order of about 1 watt); a focusing apparatus, as discussed below,
concentrates the entire laser output onto the small feature,
resulting in high effective energy densities.
The cables that carry laser output are collected into a bundle 77
and emerge separately into writing array 65. It may prove
desirable, in order to conserve power, to maintain the bundle in a
configuration that does not require bending above the fiber's
critical angle of refraction (thereby maintaining total internal
reflection); however, we have not found this necessary for good
performance.
Also as shown in FIG. 2, a controller 80 actuates laser drivers 75
when the associated lasers reach appropriate points opposite plate
55, and in addition operates stepper motor 72 and the cylinder
drive motor 82. Laser drivers 75 should be capable of operating at
high speed to facilitate imaging at commercially practical rates.
The drivers preferably include a pulse circuit capable of
generating at least 40,000 laser-driving pulses/second, with each
pulse being relatively short, i.e., on the order of 10-15 .mu.sec
(although pulses of both shorter and longer durations have been
used with success). A suitable design is described below.
Controller 80 receives data from two sources. The angular position
of cylinder 50 with respect to writing array 65 is constantly
monitored by a detector 85 (described in greater detail below),
which provides signals indicative of that position to controller
80. In addition, an image data source (e.g., a computer) also
provides data signals to controller 80. The image data define
points on plate 55 where image spots are to be written. Controller
80, therefore, correlates the instantaneous relative positions of
writing array 65 and plate 55 (as reported by detector 85) with the
image data to actuate the appropriate laser drivers at the
appropriate times during scan of plate 55. The control circuitry
required to implement this scheme is well-known in the scanner and
plotter art; a suitable design is described in U.S. Pat. No.
5,174,205, commonly owned with the present application and hereby
incorporated by reference.
The laser output cables terminate in lens assemblies, mounted
within writing array 65, that precisely focus the beams onto the
surface of plate 55. A suitable lens-assembly design is described
below; for purposes of the present discussion, these assemblies are
generically indicated by reference numeral 96. The manner in which
the lens assemblies are distributed within writing array 65, as
well as the design of the writing array, require careful design
considerations. One suitable configuration is illustrated in FIG.
3. In this arrangement, lens assemblies 96 are staggered across the
face of body 65. The design preferably includes an air manifold
130, connected to a source of pressurized air and containing a
series of outlet ports aligned with lens assemblies 96.
Introduction of air into the manifold and its discharge through the
outlet ports cleans the lenses of debris during operation, and also
purges fine-particle aerosols and mists from the region between
lens assemblies 96 and plate surface 55.
The staggered lens design facilitates use of a greater number of
lens assemblies in a single head than would be possible with a
linear arrangement. And since imaging time depends directly on the
number of lens elements, a staggered design offers the possibility
of faster overall imaging. Another advantage of this configuration
stems from the fact that the diameter of the beam emerging from
each lens assembly is ordinarily much smaller than that of the
focusing lens itself. Therefore, a linear array requires a
relatively significant minimum distance between beams, and that
distance may well exceed the desired printing density. This results
in the need for a fine stepping pitch. By staggering the lens
assemblies, we obtain tighter spacing between the laser beams and,
assuming the spacing is equivalent to the desired print density,
can therefore index across the entire axial width of the array.
Controller 80 either receives image data already arranged into
vertical columns, each corresponding to a different lens assembly,
or can progressively sample, in columnar fashion, the contents of a
memory buffer containing a complete bitmap representation of the
image to be transferred. In either case, controller 80 recognizes
the different relative positions of the lens assemblies with
respect to plate 55 and actuates the appropriate laser only when
its associated lens assembly is positioned over a point to be
imaged.
An alternative array design is illustrated in FIG. 4, which also
shows the detector 85 mounted to the cylinder 50. Preferred
detector designs are described in the '199 application. In this
case the writing array, designated by reference numeral 150,
comprises a long linear body fed by fiber-optic cables drawn from
bundle 77. The interior of writing array 150, or some portion
thereof, contains threads that engage lead screw 67, rotation of
which advances writing array 150 along plate 55 as discussed
previously. Individual lens assemblies 96 are evenly spaced a
distance B from one another. Distance B corresponds to the
difference between the axial length of plate 55 and the distance
between the first and last lens assembly; it represents the total
axial distance traversed by writing array 150 during the course of
a complete scan. Each time writing array 150 encounters void 60,
stepper motor 72 rotates to advance writing array 150 an axial
distance equal to the desired distance between imaging passes
(i.e., the print density). This distance is smaller by a factor of
n than the distance indexed by the previously described embodiment
(writing array 65), where n is the number of lens assemblies
included in writing array 65.
Writing array 150 includes an internal air manifold 155 and a
series of outlet ports 160 aligned with lens assemblies 96. Once
again, these function to remove debris from the lens assemblies and
imaging region during operation.
b. Flatbed Recording
The imaging apparatus can also take the form of a flatbed recorder,
as depicted in FIG. 7. In the illustrated embodiment, the flatbed
apparatus includes a stationary support 175, to which the outer
margins of plate 55 are mounted by conventional clamps or the like.
A writing array 180 receives fiber-optic cables from bundle 77, and
includes a series of lens assemblies as described above. These are
oriented toward plate 55.
A first stepper motor 182 advances writing array 180 across plate
55 by means of a lead screw 184, but now writing array 180 is
stabilized by a bracket 186 instead of a guide bar. Bracket 180 is
indexed along the opposite axis of support 175 by a second stepper
motor 188 after each traverse of plate 55 by writing array 180
(along lead screw 184). The index distance is equal to the width of
the image swath produced by imagewise activation of the lasers
during the pass of writing array 180 across plate 55. After bracket
186 has been indexed, stepper motor 182 reverses direction and
imaging proceeds back across plate 55 to produce a new image swath
just ahead of the previous swath.
It should be noted that relative movement between writing array 180
and plate 155 does not require movement of writing array 180 in two
directions. Instead, if desired, support 175 can be moved along
either or both directions. It is also possible to move support 175
and writing array 180 simultaneously in one or both directions.
Furthermore, although the illustrated writing array 180 includes a
linear arrangement of lens assemblies, a staggered design is also
feasible.
c. Interior-Arc Recording
Instead of a flatbed, the plate blank can be supported on an
arcuate surface as illustrated in FIG. 8. This configuration
permits rotative, rather than linear movement of the writing array
and/or the plate.
The interior-arc scanning assembly includes an arcuate plate
support 200, to which a blank plate 55 is clamped or otherwise
mounted. An L-shaped writing array 205 includes a bottom portion,
which accepts a support bar 207, and a front portion containing
channels to admit the lens assemblies. In the preferred embodiment,
writing array 205 and support bar 207 remain fixed with respect to
one another, and writing array 205 is advanced axially across plate
55 by linear movement of a rack 210 mounted to the end of support
bar 207. Rack 210 is moved by rotation of a stepper motor 212,
which is coupled to a gear 214 that engages the teeth of rack 210.
After each axial traverse, writing array 205 is indexed
circumferentially by rotation of a gear 220 through which support
bar 207 passes and to which it is fixedly engaged. Rotation is
imparted by a stepper motor 222, which engages the teeth of gear
220 by means of a second gear 224. Stepper motor 222 remains in
fixed alignment with rack 210.
After writing array 205 has been indexed circumferentially, stepper
motor 212 reverses direction and imaging proceeds back across plate
55 to produce a new image swath just ahead of the previous
swath.
d. Output Guide and Lens Assembly
Suitable means for guiding laser output to the surface of a plate
blank are illustrated in FIGS. 9-11. Refer first to FIG. 9, which
shows a remote laser assembly that utilizes a fiber-optic cable to
transmit laser pulses to the plate. In this arrangement a laser
source 250 receives power via an electrical cable 252. Laser 250 is
seated within the rear segment of a housing 255. Mounted within the
forepart of housing are two or more focusing lenses 260a, 260b,
which focus radiation emanating from laser 250 onto the end of a
fiber-optic cable 265, which is preferably (although not
necessarily) secured within housing 255 by a removable retaining
cap 267. Cable 265 conducts the output of laser 250 to an output
assembly 270, which is illustrated in greater detail in FIG.
10.
With reference to that figure, fiber-optic cable 265 enters the
assembly 270 through a retaining cap 274 (which is preferably
removable). Retaining cap 274 fits over a generally tubular body
276, which contains a series of threads 278. Mounted within the
forepart of body 276 are two or more focusing lenses 280a, 280b.
Cable 265 is carried partway through body 276 by a sleeve 280. Body
276 defines a hollow channel between inner lens 280b and the
terminus of sleeve 280, so the end face of cable 265 lies a
selected distance A from inner lens 280b. The distance A and the
focal lengths of lenses 280a, 280b are chosen so that at normal
working distance from plate 55, the beam emanating from cable 265
will be precisely focused on the plate surface. This distance can
be altered to vary the size of an image feature.
Body 276 can be secured to writing array 65 in any suitable manner.
In the illustrated embodiment, a nut 282 engages threads 278 and
secures an outer flange 284 of body 276 against the outer face of
writing array 65. The flange may, optionally, contain a transparent
window 290 to protect the lenses from possible damage.
Alternatively, the lens assembly may be mounted within the writing
array on a pivot that permits rotation in the axial direction
(i.e., with reference to FIG. 10, through the plane of the paper)
to facilitate fine axial positioning adjustment. We have found that
if the angle of rotation is kept to 4.degree. or less, the
circumferential error produced by the rotation can be corrected
electronically by shifting the image data before it is transmitted
to controller 80.
Refer now to FIG. 11, which illustrates an alternative design in
which the laser source irradiates the plate surface directly,
without transmission through fiber-optic cabling. As shown in the
figure, laser source 250 is seated within the rear segment of an
open housing 300. Mounted within the forepart of housing 300 are
two or more focusing lenses 302a, 302b, which focus radiation
emanating from laser 250 onto the surface of plate 55. The housing
may, optionally, include a transparent window 305 mounted flush
with the open end, and a heat sink 307.
It should be understood that while the preceding discussion of
imaging configurations and the accompanying figures have assumed
the use of optical fibers, in each case the fibers can be
eliminated through use of the embodiment shown in FIG. 11.
e. Driver Circuitry
A suitable circuit for driving a diode-type (e.g., gallium
arsenide) laser is illustrated schematically in FIG. 12. Operation
of the circuit is governed by controller 80, which generates a
fixed-pulse-width signal (preferably 5 to 20 .mu.sec in duration)
to a high-speed, high-current MOSFET driver 325. The output
terminal of driver 325 is connected to the gate of a MOSFET 327.
Because driver 325 is capable of supplying a high output current to
quickly charge the MOSFET gate capacitance, the turn-on and
turn-off times for MOSFET 327 are very short (preferably within 0.5
.mu.sec) in spite of the capacitive load. The source terminal of
MOSFET 327 is connected to ground potential. When MOSFET 327 is
placed in a conducting state, current flows through and thereby
activates a laser diode 330. A variable current-limiting resistor
332 is interposed between MOSFET 327 and laser diode 330 to allow
adjustment of diode output. Such adjustment is useful, for example,
to correct for different diode efficiencies and produce identical
outputs in all lasers in the system, or to vary laser output as a
means of controlling image size.
A capacitor 334 is placed across the terminals of laser diode 330
to prevent damaging current overshoots, e.g., as a result of wire
inductance combined with low laser-diode inter-electrode
capacitance.
2. Lithographic Printing Plates
Refer now to FIGS. 13A-13I, which illustrate various lithographic
plate embodiments that can be imaged using the equipment heretofore
described. The plate illustrated in FIG. 13A includes a substrate
400, a layer 404 capable of absorbing infrared radiation, and a
surface coating layer 408.
Substrate 400 is preferably strong, stable and flexible, and may be
a polymer film, or a paper or metal sheet. Polyester films (in the
preferred embodiment, the MYLAR film product sold by E. I. dupont
de Nemours Co. Wilmington, Del. or, alternatively, the MELINEX film
product sold by ICI Films, Wilmington, Del.) furnish useful
examples. A preferred polyester-film thickness is 0.007 inch, but
thinner and thicker versions can be used effectively. Aluminum is a
preferred metal substrate. Paper substrates are typically
"saturated" with polymerics to impart water resistance, dimensional
stability and strength.
For additional strength, it is possible to utilize the approach
described in U.S. Pat. No. 5,188,032 (the entire disclosure of
which is hereby incorporated by reference). As discussed in that
application, a metal sheet can be laminated either to the substrate
materials described above, or instead can be utilized directly as a
substrate and laminated to absorbing layer 404. Suitable metals,
laminating procedures and preferred dimensions and operating
conditions are all described in the '032 patent, and can be
straightforwardly applied to the present context without undue
experimentation.
The absorbing layer can consist of a polymeric system that
intrinsically absorbs in the near-IR region, or a polymeric coating
into which near-IR-absorbing components have been dispersed or
dissolved.
Layers 400 and 408 exhibit opposite affinities for ink or an
ink-abhesive fluid. In one version of this plate, surface layer 408
is a silicone polymer that repels ink, while substrate 400 is an
oleophilic polyester or aluminum material; the result is a dry
plate. In a second, wet-plate version, surface layer 408 is a
hydrophilic material such as a polyvinyl alcohol (e.g., the Airvol
125 material supplied by Air Products, Allentown, Pa.), while
substrate 400 is both oleophilic and hydrophobic.
Exposure of the foregoing construction to the output of one of our
lasers at surface layer 408 weakens that layer and ablates
absorbing layer 404 in the region of exposure. As noted previously,
the weakened surface coating (and any debris remaining from
destruction of the absorbing second layer) is removed in a
post-imaging cleaning step.
Alternatively, the constructions can be imaged from the reverse
side, i.e., through substrate 400. So long as that layer is
transparent to laser radiation, the beam will continue to perform
the functions of ablating absorbing layer 404 and weakening surface
layer 408. Although this "reverse imaging" approach does not
require significant additional laser power (energy losses through a
substantially transparent substrate 400 are minimal), it does
affect the manner in which the laser beam is focused for imaging.
Ordinarily, with surface layer 408 adjacent the laser output, its
beam is focused onto the plane of surface layer 408. In the
reverse-imaging case, by contrast, the beam must project through
the medium of substrate 400 before encountering absorbing layer
404. Therefore, not only must the beam be focused on the surface of
an inner layer (i.e., absorbing layer 404) rather than the outer
surface of the construction, but that focus must also accommodate
refraction of the beam caused by its transmission through substrate
400.
Because the plate layer that faces the laser output remains intact
during reverse imaging, this approach prevents debris generated by
ablation from accumulating in the region between the plate and the
laser output. Another advantage of reverse imaging is elimination
of the requirement that surface layer 408 efficiently transmit
laser radiation. Surface layer 408 can, in fact, be completely
opaque to such radiation so long as it remains vulnerable to
degradation and subsequent removal.
EXAMPLES 1-7
These examples describe preparation of positive-working dry plates
that include silicone coating layers and polyester substrates,
which are coated with nitro-cellulose materials to form the
absorbing layers. The nitrocellulose coating layers include
thermoset-cure capability and are produced as follows:
______________________________________ Component Parts
______________________________________ Nitrocellulose 14 Cymel 303
2 2-Butanone (methyl ethyl ketone) 236
______________________________________
The nitrocellulose utilized was the 30% isopropanol wet 5-6 Sec RS
Nitrocellulose supplied by Aqualon Co., Wilmington, Del. Cymel 303
is hexamethoxymethylmelamine, supplied by American Cyanamid
Corp.
An IR-absorbing compound is added to this base composition and
dispersed therein. Use of the following seven compounds in the
proportions that follow resulted in production of useful absorbing
layers:
______________________________________ Example 1 2 3 4 5 6 7
Component Parts ______________________________________ Base
Composition 252 252 252 252 252 252 252 NaCure 2530 4 4 4 4 4 4 4
Vulcan XC-72 4 -- -- -- -- -- -- Titanium Carbide -- 4 -- -- -- --
-- Silicon -- -- 6 -- -- -- -- Heliogen Green -- -- -- 8 -- -- -- L
8730 Nigrosine Base -- -- -- -- 8 -- -- NG-1 Tungsten Oxide -- --
-- -- -- 20 -- Vanadium Oxide -- -- -- -- -- -- 10
______________________________________
NaCure 2530, supplied by King Industries, Norwalk, Conn., is an
amine-blocked p-toluenesulfonic acid solution in an
isopropanel/methanol blend. Vuclan XC-72 is a conductive carbon
black pigment supplied by the Special Blacks Division of Cabot
Corp., Waltham, Mass. The titanium carbide used in Example 2 was
the Cerex submicron TiC powder supplied by Baikowski International
Corp., Charlotte, N.C. Heliogen Green L 8730 is a green pigment
supplied by BASF Corp., Chemicals Division, Holland, Mich.
Nigrosine Base NG-1 is supplied as a powder by N H Laboratories,
Inc., Harrisburg, Pa. The tungsten oxide (WO.sub.2.9) and vanadium
oxide (V.sub.6 O.sub.13) used above are supplied as powders by
Cerac Inc., Milwaukee, Wis.
Following addition of the IR absorber and dispersion thereof in the
base composition, the blocked PTSA catalyst was added, and the
resulting mixtures applied to the polyester substrate using a
wire-wound rod. After drying to remove the volatile solvent(s) and
curing (1 min at 300.degree. F. in a lab convection oven performed
both functions), the coatings were deposited at 1 g/m.sup.2.
The nitrocellulose thermoset mechanism performs two functions,
namely, anchorage of the coating to the polyester substrate and
enhanced solvent resistance (of particular concern in a pressroom
environment).
The following silicone coating was applied to each of the anchored
IR-absorbing layers produced in accordance with the seven examples
described above.
______________________________________ Component Parts
______________________________________ PS-445 22.56 .[.PC-072
.70.]. .Iadd.PC-072 .04.Iaddend. VM&P Naphtha 76.70 .[.Syl-Off
7367 .04.]. .Iadd.Syl-Off 7367 .70.Iaddend.
______________________________________
(These components are described in greater detail, and their
sources indicated, in the '032 patent and also in U.S. Pat. Nos.
5,212,048 and 5,310,869, both commonly owned with the present
invention and hereby incorporated by reference; these patents
describe numerous other silicone formulations useful as the
material of an oleophobic layer 408.)
We applied the mixture using a wire-wound rod, then dried and cured
it to produce a uniform coating deposited at 2 g/m.sup.2. The
plates are then ready to be imaged.
EXAMPLES 8-9
The following examples describe preparation of a plate using an
aluminum substrate.
______________________________________ Example 8 9 Component Parts
______________________________________ Ucar Vinyl VMCH 10 10 Vulcan
XC-72 4 -- Cymel 303 -- 1 NaCure 2530 -- 4 2-Butanone 190 190
______________________________________
Ucar Vinyl VMCH is a carboxy-functional vinyl terpolymer supplied
by Union Carbide Chemicals & Plastics Co., Danbury, Conn.
In both examples, we coated a 5-mil aluminum sheet (which had been
cleaned and degreased) with one of the above coating mixtures using
a wire-wound rod, and then dried the sheets for 1 min at
300.degree. F. in a lab convection oven to produce application
weights of 1.0 g/m.sup.2 for Example 8 and 0.5 g/m.sup.2 for
Example 9.
For Example 8, we overcoated the dried sheet with the silicone
coating described in the previous examples to produce a dry
plate.
For Example 9, the coating described above served as a primer
(shown as layer 410 in FIG. 13B). Over this coating we applied the
absorbing layer described in Example 1, and we then coated this
absorbing layer with the silicone coating described in the previous
examples. The result, once again, is a useful dry plate with the
structure illustrate in FIG. 13B.
EXAMPLE 10
Another aluminum plate is prepared by coating an aluminum 7-mil
"full hard" 3003 alloy (supplied by All-Foils, Brooklyn Heights,
Ohio) substrate with the following formulation (based on an aqueous
urethane polymer dispersion) using a wire-wound rod:
______________________________________ Component Parts
______________________________________ NeoRez R-960 65 Water 28
Ethanol 5 Cymel 385 2 ______________________________________
NeoRez R-960, supplied by ICI Resins US, Wilmington, Mass., is an
aqueous urethane polymer dispersion. Cymel 385 is a
high-methylol-content hexamethoxymethylmelamine, supplied by
American Cyanamid Corp.
The applied coating is dried for 1 min at 300.degree. F. to produce
an application weight of 1.0 g/m.sup.2. Over this coating, which
serves as a primer, we applied the absorbing layer described in
Example 1 and dried it to produce an application weight of 1.0
g/m.sup.2. We then coated this absorbing layer with the silicone
coating described in the previous examples to produce a useful dry
plate.
Although it is possible to avoid the use of a priming layer, as was
done in Example 8, the use of primers has achieved wide commercial
acceptance. Photosensitive dry plates are usually produced by
priming an aluminum layer, and then coating the primed layer with a
photosensitive layer and then a silicone layer. We expect that
priming approaches used in conventional lithographic plates would
also serve in the present context.
EXAMPLES 11-12
In the following examples, we prepared absorbing layers from
conductive polymer dispersions known to absorb in the near-IR
region. Once again, these layers were formulated to adhere to a
polyester film substrate, and were overcoated with a silicone
coating to produce positive-working, dry printing plates.
______________________________________ Example 11 12 Component
Parts ______________________________________ 5% ICP-117 in Ethyl
Acetate 200 -- 5-6 Sec RS Nitrocellulose 8 -- Americhem Green
#34384-C3 -- 100 2-Butanone -- 100
______________________________________
The ICP-117 is a proprietary polypyrrole-based conductive polymer
supplied by Polaroid Corp. Commercial Chemicals, Assonet, Mass.
Americhem Green #34384-C3 is a proprietary polyaniline-based
conductive coating supplied by Americhem, Inc., Cuyahoga Falls,
Ohio.
The mixtures were each applied to a polyester film using a
wire-wound rod and dried to produce a uniform coating deposited at
2 g/m.sup.2.
EXAMPLES 13-14
These examples illustrate use of absorbing layers containing
IR-absorbing dyes rather than pigments. Thus, the nigrosine
compound present as a solid in Example 5 is utilized here in
solubilized form.
______________________________________ Example 13 14 Component
Parts ______________________________________ 5-6 Sec RS
Nitrocellulose 14 14 Cymel 303 2 2 2-Butanone 236 236 Project 900
NP 4 -- Nigrosine Oleate -- 8 Nacure 2530 4 4
______________________________________
Projet 900 NP is a proprietary IR absorber marketed by ICI Colours
& Fine Chemicals, Manchester, United Kingdom. Nigrosine oleate
refers to a 33% nigrosine solution in oleic acid supplied by N H
Laboratories, Inc., Harrisburg, Pa.
The mixtures were each applied to a polyester film using a
wire-wound rod and dried to produce a uniform coating deposited at
1 g/m.sup.2. A silicone layer was applied thereto to produce a
working plate.
Substitutions may be made in all of the foregoing Examples 1-14.
For instance, the melamine-formaldehyde crosslinker (Cymel 303) can
be replaced with any of a variety of isocyanate-functional
compounds, blocked or otherwise, that impart comparable solvent
resistance and adhesion properties; useful substitute compounds
include the Desmodur blocked polyisocyanate compounds supplied by
Mobay Chemical Corp., Pittsburgh, Pa. Grades of nitrocellulose
other than the one used in the foregoing examples can also be
advantageously employed, the range of acceptable grades depending
primarily on coating method.
EXAMPLES 15-16
These examples provide coatings based on polymers other than
nitrocellulose, but which adhere to polyester film and can be
overcoated with silicone to produce dry plates.
______________________________________ Example 15 16 Component
Parts ______________________________________ Ucar Vinyl VAGH 10 --
Saran F-310 -- 10 Vulcan XC-72 4 -- Nigrosine Base NG-1 -- 4
2-Butanone 190 190 ______________________________________
Ucar Vinyl VAGH is a hydroxy-functional vinyl terpolymer supplied
by Union Carbide Chemicals & Plastics Co., Danbury, Conn. Saran
F-310 is a vinylidenedichloride-acrylonitrile copolymer supplied by
Dow Chemical Co., Midland, Mich.
The mixtures were each applied to a polyester film using a
wire-wound rod and dried to produce a uniform coating deposited at
1 g/m.sup.2. A silicone layer was applied thereto to produce a
working dry plate.
To produce a wet plate, the polyvinylidenedichloride-based polymer
of Example 16 is used as a primer and coated onto the coating of
Example 1 as follows:
______________________________________ Component Parts
______________________________________ Saran F-310 5 2-Butanone 95
______________________________________
The primer is prepared by combining the foregoing ingredients and
is applied to the coating of Example 1 using a wire-wound rod. The
primed coating is dried for 1 min at 300.degree. F. in a lab
convection oven for an application weight of 0.1 g/m.sup.2.
A hydrophilic plate surface coating is then created using the
following polyvinyl alcohol solution:
______________________________________ Component Parts
______________________________________ Airvol 125 5 Water 95
______________________________________
Airvol 125 is a highly hydrolyzed polyvinyl alcohol supplied by Air
Products, Allentown, Pa.
This coating solution is applied with a wire-wound rod to the
primed, coated substrate, with is dried for 1 min at 300.degree. F.
in a lab convection oven. An application weight of 1 g/m.sup.2
yields a wet printing plate capable of approximately 10,000
impressions.
It should be noted that polyvinyl alcohols are typically produced
by hydrolysis of polyvinyl acetate polymers. The degree of
hydrolysis affects a number of physical properties, including water
resistance and durability. Thus, to assure adequate plate
durability, the polyvinyl alcohols used in the present invention
reflect a high degree of hydrolysis as well as high molecular
weight. Effective hydrophilic coatings are sufficiently crosslinked
to prevent redissolution as a result of exposure to fountain
solution, but also contain fillers to produce surface textures that
promote wetting. Selection of an optimal mix of characteristics for
a particular application is well within the skill of practitioners
in the art.
EXAMPLE 17
The polyvinyl-alcohol surface-coating mixture described immediately
above is applied directly to the anchored coating described in
Example 16 using a wire-wound rod, and is then dried for 1 min at
300.degree. F. in a lab convection oven. An application weight of 1
g/m.sup.2 yields a wet printing plate capable of approximately
10,000 impressions.
Various other plates can be fabricated by replacing the Nigrosine
Base NG-1 of Example 16 with carbon black (Vulcan XC-72) or
Heliogen Green L 8730.
EXAMPLE 18
A layer of titanium oxide (TiO) was sputtered onto a polyester film
to a thickness of 600 .ANG. and coated with silicone. The result
was a nearly transparent, imageable dry plate.
Refer now to FIG. 13C, which illustrates a two-layer plate
embodiment including a substrate 400 and a surface layer 416. In
this case, surface layer 416 absorbs infrared radiation. Our
preferred dry-plate variation of this embodiment includes a
silicone surface layer 416 that contains a dispersion of
IR-absorbing pigment or dye. We have found that many of the surface
layers described in U.S. Pat. Nos. 5,109,771, 5,165,345, and
5,249,525 (all commonly owned with the present application and all
of which are hereby incorporated by reference), which contain
filler particles that assist the spark-imaging process, can also
serve as an IR-absorbing surface layer. In fact, the only filler
pigments totally unsuitable as IR absorbers are those whose surface
morphologies result in highly reflective surfaces. Thus, white
particles such as TiO.sub.2 and ZnO, and off-white compounds such
as SnO.sub.2, owe their light shadings to efficient reflection of
incident light, and prove unsuitable for use.
Among the particles suitable as IR absorbers, direct correlation
does not exist between performance in the present environment and
the degree of usefulness as a spark-discharge plate filler. Indeed,
a number of compounds of limited advantage to spark-discharge
imaging absorb IR radiation quite well. Semiconductive compounds
appear to exhibit, as a class, the best performance characteristics
for the present invention. Without being bound to any particular
theory or mechanism, we believe that electrons energetically
located in and adjacent to conducting bands are readily promoted
into and within the band by absurbing IR radiation, a mechanism in
agreement with the known tendency of semiconductors to exhibit
increased conductivity upon heating due to thermal promotion of
electrons into conducting bands.
Currently, it appears that metal borides, carbides, nitrides,
carbonitrides, bronze-structured oxides, and oxides structurally
related to the bronze family but lacking the A component (e.g.,
WO.sub.2.9) perform best.
IR absorption can be further improved by adding an IR-reflective
surface below the IR-absorbing layer (which may be layer 404 or
layer 416). This approach provides maximum improvement to
embodiments in which the absorbing layer is partially transmissive,
and therefore fails to absorb a sufficient proportion of incident
energy. FIG. 13D illustrates introduction of a reflective layer 418
between layers 416 and 400. To produce a dry plate having this
layer, a thin layer of reflective metal, preferably aluminum of
thickness ranging from 200 to 700 .ANG. or thicker, is deposited by
vacuum evaporation or sputtering directly onto substrate 400;
suitable means of deposition, as well as alternative materials, are
described in connection with layer 178 of FIG. 4F in the '075
patent mentioned earlier. The silicone coating is then applied to
layer 418 in the same manner described above. Exposure to the laser
beam results in ablation of layer 418. In a similar fashion, a than
metal layer can be interposed between layers 404 and 400 of the
plate illustrated in FIG. 13A.
Because this layer is not ablated, its proper thickness is
determined primarily by transmission characteristics and the need
to function as a printing surface. Layer 418 should reflect almost
all radiation incident thereon. To support dry printing, the metal
layer (which is exposed at image points where the overlying
IR-absorbing layer is removed) accepts ink; to support wet
printing, the metal layer exhibits sufficiently low affinity for
fountain solution that ink will displace it when applied. Aluminum,
we have found, provides both of these properties, and can therefore
be used in wet-plate and dry-plate constructions. Those skilled in
the art will appreciate the usefulness of a wide variety of metals
and alloys as alternatives to aluminum; such alternatives include
nickel and copper.
In a highly advantageous variation of this embodiment, illustrated
in FIG. 13I, the metal layer is transformed into an ablation layer
by the addition thereover of a thin layer of an IR-absorptive metal
oxide. A preferred construction of this type includes a substrate
400 (e.g., 7-mil Mylar D film or a metal sheet); a layer 418 of
metal deposited thereon; a metal-oxide layer 425 deposited onto
metal layer 418; and a surface layer 408, which may be receptive to
fountain solution (e.g., polyvinyl alcohol) or ink-repellent (e.g.,
silicone). Metal layer 418 is preferably aluminum, approximately
700 .ANG. thick and exhibiting conductivity in the range of 1.5-1.7
mhos. Metal-oxide layer 425 is preferably titanium oxide (TiO),
although other IR-absorptive materials (e.g., oxides of vanadium,
manganese, iron or cobalt) can instead be used. Layer 425 is
deposited (e.g., by sputtering) to a thickness of 100-600 .ANG.,
with preferred thicknesses ranging from 200-400 .ANG..
In operation, metal-oxide layer 425 becomes sufficiently hot upon
exposure to IR radiation to ignite metal layer 418, which ablates
along with layer 425. We have found that the resulting thermal
discharge is intense enough to weaken the overlying surface layer
408, thereby easing the removal of that layer following
imaging.
In a second variation of the construction shown in FIG. 13D, the
reflecting layer is itself the substrate, resulting once again in
the construction illustrated in FIG. 13C. A preferred construction
of this sort includes an IR-absorbing layer 416 coated directly
onto a polished aluminum substrate having a thickness from 0.004 to
0.02 inch. Once again, pure aluminum can be replaced with an
aluminum alloy or a different metal (or alloy) entirely, so long as
the criteria of sturdiness, reflectivity and suitability as a
printing surface are maintained. Furthermore, instead of directly
coating layer 416 onto substrate 400, the two layers can be
laminated together as described in the '032 patent (so long as the
laminating adhesive can be removed by laser ablation).
one can also employ, as an alternative to a metal reflecting layer,
a layer containing a pigment that reflects IR radiation. Once
again, such a layer can underlie layer 408 or 416, or may serve as
substrate 400. A material suitable for use as an IR-reflective
substrate is the white 329 film supplied by ICI Films, Wilmington,
Del., which utilizes IR-reflective barium sulfate as the white
pigment.
Silicone coating formulations particularly suitable for deposition
onto an aluminum layer are described in the '032 patent and the
'048 patent. In particular, commercially prepared pigment/gum
dispersions can be advantageously utilized in conjunction with a
second, lower-molecular-weight second component.
EXAMPLES 19-21
In the following coating examples, the pigment/gum mixtures, all
based on carbon-black pigment, are obtained from Wacker Silicones
Corp., Adrian, Mich. In separate procedures, coatings are prepared
using PS-445 and dispersions marketed under the designations C-968,
C-1022 and C-1190 following the procedures outlined in the '032 and
'048 patents. The following formulations are utilized to prepare
stock coatings:
______________________________________ Order of Addition Component
Weight Percent ______________________________________ 1 VM&P
Naphtha 74.8 2 PS-445 15.0 3 Pigment/Gum Dispersion 10.0 4 Methyl
Pentynol 0.1 5 PC-072 0.1
______________________________________
Coating batches are then prepared as described in the '032 and '048
patents using the following proportions:
______________________________________ Component Parts
______________________________________ Stock Coating 100 VM&P
Naphtha 100 PS-120 (Part B) 0.6
______________________________________
The coatings are straightforwardly applied to aluminum layers, and
contain useful IR-absorbing material.
We have also found that a metal layer disposed as illustrated in
FIG. 13D can, if made thin enough, support imaging by absorbing,
rather than reflecting, IR radiation. This approach is valuable
both where layer 416 absorbs IR radiation (as contemplated in FIG.
13D) or is transparent to such radiation. In the former case, the
very thin metal layer provides additional absorptive capability
(instead of reflecting radiation back into layer 416); in the later
case, this layer functions as does layer 404 in FIG. 13A.
Furthermore, this type of construction exhibits substantial
flexibility, and is therefore well-suited to plate-winding
arrangements. Appropriate metal layers are appreciably thinner than
the 200.gtoreq.700 .ANG. thickness useful in a fully reflective
layer.
Because such a thin metal layer may be discontinuous, it can be
useful to add an adhesion-promoting layer to better anchor the
surface layer to the other (non-metal) plate layers. Inclusion of
such a layer is illustrated in FIG. 13E. This construction contains
a substrate 400, the adhesion-promoting layer 420 thereon, a thin
metal layer 418, and a surface layer 408. Suitable
adhesion-promoting layers, sometimes termed print or coatability
treatments, are furnished with various polyester films that may be
used as substrates. For example, the J films marketed by E. I.
dupont de Nemours Co., Wilmington, Del., and Melinex 453 sold by
ICI Films, Wilmington, Del. serve adequately as layers 400 and 420.
Generally, layer 420 will be very thin (on the order of 1 micron or
less in thickness) and, in the context of a polyester substrate,
will be based on acrylic or polyvinylidene chloride systems.
In a particularly preferred construction of this type, at least one
very thin (preferably 200 .ANG. or less) layer of a metal,
preferably titanium, is deposited onto a polyester substrate 400
and coated with an oleophobic material (e.g., a fluoropolymer or,
preferably, silicone) or a hydrophilic material (e.g., a polyvinyl
alcohol). Once again, exposure of this construction to a laser
pulse ablates the thin metal layer and weakens the topmost layer
and destroys its anchorage, rendering it easily removed. The
detached topmost layer (and any debris remaining from destruction
of the absorptive second layer) is removed in a post-imaging
cleaning step.
Preferred polyester films for use in this embodiment have surfaces
to which the deposited metal adheres well, and exhibit substantial
flexibility to facilitate spooling and winding over the surface of
a plate cylinder. One useful class of preferred polyester material
is the unmodified film exemplified by the dupont MYLAR D and MYLAR
A products, and the 3930 film product marketed by Hoechst-Celanese,
Greer, S.C. Also advantageous, depending on the metal employed, are
polyester materials that have been modified to enhance surface
adhesion characteristics as described above. The modified surface
accepts metal with greater ease than an unmodified surface.
Suitable polyesters of this type include the ICI 453 product.
For traditional applications involving plates that are individually
mounted to the plate cylinder of a press, the adhesion-promoting
surface can also (or alternatively) be present on the side of the
polyester film in contact with the cylinder. Plate cylinders are
frequently fabricated from material with respect to which the
adhesion-promoting surface exhibits a high static coefficient of
friction, reducing the possibility of plate slippage during actual
printing. The ICI 561 product and the dupont MYLAR J102 film have
adhesion-promoting coatings applied to both surfaces, and are
therefore well-suited to this environment.
For applications involving automatic plate-material dispensing
apparatus, however, the ease of winding the material around the
cylinder represents an equally important consideration, and favors
the use of materials having a low dynamic coefficients of friction
with respect to the cylinder. Adhesion-promoting surfaces should
not be used on the exterior polyester surface if the result is
excessive resistance to movement. On the other hand, antistatic
treatments can impart a beneficial reduction of resistance to
movement with respect to many surfaces (compared with unmodified
polyester). This is particularly true for plate constructions
featuring semiconductive layers, which can accumulate static
charges that retard free travel along the plate cylinder. Examples
of antistatic polyester films include the dupont MYLAR JXM301 and
JMX502 products; the latter film includes an adhesion-promoting
treatment on its reverse side.
Ideally, and to the extent practicable, the cylinder and the
polyester surface in contact with it are matched to provide low
dynamic but high static coefficients of friction. For this reason,
it is important to consider both the dynamic and static behavior of
any surface treatment in conjunction with a particular type of
plate cylinder, and to evaluate this behavior against an unmodified
surface.
The metal layer 418 is preferably deposited to an optical density
ranging from 0.2 to 1.0, with a density of 0.6 being especially
preferred. However, thicker layers characterized by optical
densities as high as 2.5 can also be used to advantage. This range
of optical densities generally corresponds to a thickness of 200
.ANG. or less. While titanium is preferred as layer 418,
alternative metals include alloys of titanum, aluminum, alloys of
aluminum, nickel, iron, chromiun, and others exhibiting the
required optical densities and adequate radiation absorption.
Metals such as titanium may be conveniently applied by well-known
deposition techniques such as sputtering, electron-beam evaporation
and vacuum evaporation. Depending on the condition of the polyester
surface, sputtering can prove particularly advantageous in the
ready availability of coprocessing techniques (e.g., glow discharge
and back sputtering) that can be used to modify polyester prior to
deposition.
Depending on requirements relating to imaging speed and laser
power, it may prove advantageous to provide the metal layer with an
antireflective overlay to increase interaction with the imaging
pulses. The refractive index of the antireflective material, in
combination with that of the metal, creates interfacial conditions
that favor laser penetration over reflection. Suitable
antireflective materials are well-known in the art, and include a
variety of dielectrics (e.g., metal oxides and metal halides).
Materials amenable to application by sputtering can ease
manufacture considerably, since both the metal and the
antireflection coating can be applied in the same chamber by
multiple-target techniques.
The coating layer 416 is preferably a silicone composition, for
dry-plate constructions, or a polyvinyl alcohol composition in the
case of a wet plate. Our preferred silicone formulation is that
described earlier in connection with Examples 1-7, applied to
produce a uniform coating deposited at 2 g/m.sup.2. The anchorage
of coating layer 416 to metal layer 418 can be improved by the
addition of an adhesion promoter, such as a silane composition (for
silicone coatings) or a titanate composition for polyvinyl-alcohol
coatings.
Although the foregoing construction is well-suited to plate
material intended for automatic-dispensing apparatus, it can also
be utilized in composite laminated designs using relatively thin
(e.g., 0.001-0.0045 inch) polyester films adhered to a stable metal
support as described in the '032 patent.
With renewed reference to FIG. 13E, we note that it is also
possible to add a near-IR absorbing layer to that construction in
order to eliminate any need for IR-absorption capability in surface
layer 408, but where a very thin metal layer alone provides
insufficient absorptive capability. Refer now to FIG. 13F, which
shows such a construction. An IR-absorbing layer 404, as described
above, has been introduced below surface layer 408 and above very
thin metal layer 418. Layers 404 and 418, both of which are ablated
by laser radiation during imaging, cooperate to absorb and
concentrate that radiation, thereby ensuring their own efficient
ablation. For plates to be imaged in a reversed orientation, as
described above, the relative positions of layers 418 and 404 can
be reversed and layer 400 chosen so as to be transparent. Such an
alternative is illustrated in FIG. 13G.
Any of a variety of production sequences can be used advantageously
to prepare the plates shown in FIGS. 13A-13G. In one representative
sequence, substrate 400 (which may be, for example, polyester or a
conductive polycarbonate) is metallized to form reflective layer
418, and then coated with silicone or a fluoropolymer (either of
which may contain a dispersion of IR-absorptive pigment) to form
surface layer 408; these steps are carried out as described, for
example, in the '345 patent in connection with FIGS. 4F and 4G.
Alternatively, one can add a barrier sheet to surface layer 408 and
build up the remaining plate layers from that sheet. A barrier
sheet can serve a number of useful functions in the context of the
present invention. First, as described previously, those portions
of surface layer 408 that have been weakened by exposure to laser
radiation must be removed before the imaged plate can be used to
print. Using a reverse-imaging arrangement, exposure of surface
layer 408 to radiation can result in its molten deposition, or
decaling, onto the inner surface of the barrier sheet; subsequent
stripping of the barrier sheet then effects removal of superfluous
portions of surface layer 408. A barrier sheet is also useful if
the plates are to include metal bases (as described in the '032
patent), and are therefore created in bulk directly on a metal coil
and stored in roll form; in that case surface layer 408 can be
damaged by contact with the metal coil.
A representative construction that includes such a barrier layer,
shown at reference numeral 427, is depicted in FIG. 13H; it should
be understood, however, that barrier sheet 427 can be utilized in
conjunction with any of the plate embodiments discussed herein.
Barrier layer 427 is preferably smooth, only weakly adherant to
surface layer 408, strong enough to be feasibly stripped by hand at
the preferred thicknesses, and sufficiently heat-resistant to
tolerate the thermal processes associated with application of
surface layer 408. Primarily for economic reasons, preferred
thicknesses range from 0.00025 to 0.002 inch. Our preferred
material is polyester; however, polyolefins (such as polyethylene
or polypropylene) can also be used, although the typically lower
heat resistance and strength of such materials may require use of
thicker sheets.
Barrier sheet 427 can be applied after surface layer 408 has been
cured (in which case thermal tolerance is not important), or prior
to curing; for example, barrier sheet 427 can be placed over the
as-yet-uncured layer 408, and actinic radiation passed therethrough
to effect curing.
One way of producing the illustrated construction is to coat
barrier sheet 427 with a silicone material (which, as noted above,
can contain IR-absorptive pigments) to create layer 408. This layer
is then metallized, and the resulting metal layer coated or
otherwise adhered to substrate 400. This approach is particularly
useful to achieve smoothness of surface layers that contain high
concentrations of dispersants which would ordinarily impart
unwanted texture.
It will therefore be seen that we have developed a highly versatile
imaging system and a variety of plates for use therewith. The terms
and expressions employed herein are used as terms of description
and not of limitation, and there is no intention, in the use of
such terms and expressions, of excluding any equivalents of the
features shown and described or portions thereof, but it is
recognized that various modifications are possible within the scope
of the invention claimed.
* * * * *