The present invention relates to lithographic printing plates directly
imageable by laser discharge.
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. Patent No. 4,936,211 (co-owned with the present application
and herein referred to,) 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 or a 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 text 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 oxide 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
A similar photographic process is used to,create dry plates, which
typically include an ink-abhesive (e.g., silicone) 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. Patent No.
4,911,075, co-owned with the present application and herein referred to).
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. Patent
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. Patent 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. Patent 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. Patent 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. Patent No. 4,132,168. Either of these imaging
techniques requires the cumbersome chemical processing associated with traditional,
According to the present invention, there is provided a Lithographic
printing plate as defined in claim 1 below.
According to the present invention, there is further provided a Lithographic
printing plate as defined in claim 8 below.
Lithographic printing plates embodying the present invention can be
made 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
In embodiments of the present invention materials that enhance the
ablative efficiency of the laser beam are used. 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, 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 second layer, which absorbs
IR radiation. A strong, stable substrate underlies the second 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 US Patent
5,148,746). 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 performance of either of the foregoing embodiments is modified
for more efficient performance by addition, beneath the absorbing layer, of an
additional layer that reflects IR radiation. This additional layer reflects any
radiation that penetrates the absorbing layer back through that layer, so that
the effective flux through the absorbing layer is significantly increased. The
increase in effective flux improves imaging performance, reducing the power (that
is, energy of the laser beam multiplied by its exposure time) necessary to ablate
the absorbing layer. Of course, the reflective layer must either be removed along
with the absorbing layer by action of the laser pulse, or instead serve as a printing
surface instead of the substrate.
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 lambdamax 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-guiding 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
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).
The foregoing discussion will be understood more readily from the
following detailed description of embodiments of the invention, when taken in conjunction
with the accompanying drawings, in which:
1. Imaging Apparatus
a. Exterior-Drum Recording
- FIG. 1 is an isometric view of an imaging apparatus with which embodiments
of the present invention may be used, and which operates in conjunction with a
diagonal-array writing array;
- FIG. 2 is a schematic depiction of the arrangement 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 embodiments of
the present invention, and in which imaging elements are arranged in a diagonal
- FIG. 4 is an isometric view of the imaging apparatus;
- FIG. 5 is an isometric view of the front of a writing array for imaging, 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 a flatbed imaging apparatus having a linear
- FIG. 8 is an isometric view of an interior-drum 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 embodiments of the present invention; and
- FIGS. 13A-13I are enlarged sectional views showing lithographic plates embodying
the present invention.
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
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 US Patents 5,163,368 and 4,911,075.
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
L1, L2, L3 ... Ln, 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/6.54cm2 and preferably at least
0.6 megawatt/6.45cm2. 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
12.3 µm to 49 µm (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 FIGS 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 µsec (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 US Patent 5,174,205.
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 US Patent 5,174,205. 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 airay 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 face 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 the 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
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° 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
µsec 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 µsec) 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 product sold by E.I. duPont de Nemours Co., Wilmington, DE,
or, alternatively, the Melinex product sold by ICI Films, Wilmington, DE) furnish
useful examples. A preferred polyester-film thickness is 0 18mm, 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. Patent No. 5,188,032. 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
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
REFERENCE EXAMPLES 1-7
These examples describe preparation of positive-working dry plates
that include silicone coating layers and polyester substrates, which are coated
with nitrocellulose materials to form the absorbing layers. The nitrocellulose
coating layers include thermoset-cure capability and are produced as follows:
2-Butanone (methyl ethyl ketone)
The nitrocellulose utilized was the 30% isopropanol wet 5-6 Sec RS
Nitrocellulose supplied by Aqualon Co., Wilmington, DE. 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:
Heliogen Green L
Nigrosine Base NG-1
NaCure 2530, supplied by King Industries, Norwalk, CT, is an amine-blocked
p-toluenesulfonic acid solution in an isopropanol/methanol blend. Vulcan XC-72
is a conductive carbon black pigment supplied by the Special Blacks Division of
Cabot Corp., Waltham, MA. The titanium carbide used in Example 2 was the Cerex
submicron TiC powder supplied by Baikowski International Corp., Charlotte, NC.
Heliogen Green L 8730 is a green pigment supplied by BASF Corp., Chemicals Division,
Holland, MI. Nigrosine Base NG-1 is supplied as a powder by N H Laboratories,
Inc., Harrisburg, PA. The tungsten oxide (WO2.9) and vanadium oxide
(V6O13) used above are supplied as powders by Cerac Inc.,
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 148°C in a lab convection oven performed
both functions), the coatings were deposited at 1 g/m2.
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.
(These components are described in greater detail, and their sources
indicated, in the US Patents 5,118,032; US 5,212,048, US 5,310,869. These applications
describe numerous other silicone formulations useful as the material of an oleophobic
We applied the mixture using a wire-wound rod, then dried and cured
it to produce a uniform coating deposited at 2 g/m2. The plates are
then ready to be imaged.
REFERENCE EXAMPLES 8-9
The following examples describe preparation of a plate using an aluminum
Ucar Vinyl VMCH
Ucar Vinyl VMCH is a carboxy-functional vinyl terpolymer supplied
by Union Carbide Chemicals & Plastics Co., Danbury, CT.
In both examples, we coated a 0.13mm (5mil) 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 148°C in a lab convection
oven to produce application weights of 1.0 g/m2 for Reference Example
8 and 0.5 g/m2 for Reference Example 9.
For Reference Example 8, we overcoated the dried sheet with the silicone
coating described in the previous examples to produce a dry plate.
For Reference 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 Reference 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.
REFERENCE EXAMPLE 10
Another aluminum plate is prepared by coating an aluminum 0,178 mm
(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:
NeoRez R-960, supplied by ICI Resins US, Wilmington, MA, 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 148 °C to produce an application
weight of 1.0 g/m2. Over this coating, which serves as a primer, we
applied the absorbing layer described in Reference Example 1 and dried it to produce
an application weight of 1.0 g/m2. We then coated this absorbing layer
with the silicone coating described in the previous examples to produce a useful
Although it is possible to avoid the use of a priming layer, as was
done in Reference 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.
REFERENCE 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.
5% ICP-117 in Ethyl Acetate
5-6 Sec RS Nitrocellulose
Americhem Green #34384-C3
The ICP-117 is a proprietary polypyrrole-based conductive polymer
supplied by Polaroid Corp. Commercial Chemicals, Assonet, MA. Americhem Green #34384-C3
is a proprietary polyaniline-based conductive coating supplied by Americhem, Inc.,
Cuyahoga Falls, OH.
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/m2.
REFERENCE 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 Reference
Example 5 is utilized here in solubilized form.
5-6 Sec RS Nitrocellulose
Projet 900 NP
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,
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/m2. A silicone
layer was applied thereto to produce a working plate.
Substitutions may be made in all of the foregoing Reference 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.
REFERENCE 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
Ucar Vinyl VAGH
Nigrosine Base NG-1
Ucar Vinyl VAGH is a hydroxy-functional vinyl terpolymer supplied
by Union Carbide Chemicals & Plastics Co., Danbury, CT. Saran F-310 is a vinylidenedichloride-acrylonitrile
copolymer supplied by Dow Chemical Co., Midland, MI.
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/m2. A silicone
layer was applied thereto to produce a working dry plate.
To produce a wet plate, the polyvinylidenedichloride-based polymer
of Reference Example 16 is used as a primer and coated onto the coating of Reference
Example 1 as follows:
The primer is prepared by combining the foregoing ingredients and
is applied to the coating of Reference Example 1 using a wire-wound rod. The primed
coating is dried for 1 min at 148°C in a lab convection oven for an application
weight of 0.1 g/m2.
A hydrophilic plate surface coating is then created using the following
polyvinyl alcohol solution:
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, which is dried for 1 min at 300 °F in a lab convection oven.
An application weight of 1 g/m2
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.
REFERENCE EXAMPLE 17
The polyvinyl-alcohol surface-coating mixture described immediately
above is applied directly to the anchored coating described in Reference Example
16 using a wire-wound rod, and is then dried for 1 min at 148 °C in a lab convection
oven. An application weight of 1 g/m2 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 Reference Example 16 with carbon black (Vulcan XC-72) or Heliogen
Greeen L 8730.
A layer of titanium oxide (TiO2) was sputtered onto a
polyester film to a thickness of 600 Å 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.
Patent Nos. 5,109,771; 5,165,345, and 5,249,525, 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 TiO2 and ZnO, and off-white compounds such as SnO2,
owe their light shadings to efficient reflection of incident light, and prove unsuitable
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 absorbing 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
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., WO2.9) perform best.
IR absorption is 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 420. To produce a dry plate having this layer, a thin layer of reflective
metal, preferably aluminum of thickness ranging from 20 to 70 nm (200 to 700 Å)
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 US patent 4,911,075 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 thin 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., 0,178 mm (7-mil) Mylar D film or a
metal sheet); a layer 418 of metal deposited thereon; a metaloxide 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 70 nm (700 Å) thick and exhibiting conductivity
in the range of 1.5-1.7 mhos. Metaloxide 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 10-60 nm (100-600 Å), with preferred thicknesses ranging from
20-40 nm (200-400 Å).
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
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 0.1 to 0.51 mm. 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 US patent 5,188,032 (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, DE, 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 US patents 5,188,032 and 5,212,048.
In particular, commercially prepared pigment/gum dispersions can be advantageously
utilized in conjunction with a second, lower-molecular-weight second component.
REFERENCE 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,
MI. 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 patent and '048 patent. The following formulations are utilized
to prepare stock coatings:
Order of Addition
Coating batches are then prepared as described in the '032 and '048
patents using the following proportions:
PS-120 (Part B)
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 latter case, this layer functions
as does layer 404 in FIG. 13A.
To perform an absorptive function, metal layer 418 should transmit
as much as 70% (and at least 5%) of the IR radiation incident thereon; if transmission
is insufficient, the layer will reflect radiation rather than absorbing it, while
excessive transmission levels appear to be associated with insufficient absorption.
Suitable aluminum layers are appreciably thinner than the 20-70 nm (200-700 Å)
thickness useful in a fully reflective layer. Alternative metals include titanium,
nickel, iron and chromium.
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, DE, and Melinex 453 sold by ICI Films, Wilmington,
DE 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.
REFERENCE EXAMPLE 22
A stock coating is prepared using PS-445 and the C-1190 dispersion
following the procedures outlined in the '032 and '048 patents according to the
Order of Addition
A coating batch is then prepared as described in the '032 and '048
patents using the following proportions:
PS-120 (Part B)
Plates suitable for coating are prepared by vacuum-evaporating, onto
a 0.178 mm (7 mil) print-treated polyester substrate, an aluminum layer to a thickness
that transmits 60% incident visible radiation. The silicone coating whose preparation
is set forth above is then applied to this aluminized substrate to produce a useful
REFERENCE EXAMPLE 23
A coating is prepared using WO2.9 as a selective near-IR
absorber following standard dispersion procedures and according to the following
Order of Addition
Syl-Off 7367 is supplied by Dow Corning Corp., Midland, MI.
A dry plate using this formulation and the base construction set
forth in Reference Example 22 is prepared by applying the mixture using a wire-wound
rod, then drying and curing it to produce a uniform coating deposited at 2 g/m2.
It is also possible to add a near-IR absorbing layer to the construction
shown in FIG. 13E 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 0.01 - 0.05 mm. 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.