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| United States Patent |
5,697,300
|
|
Lewis
, et al.
|
December 16, 1997
|
Method and apparatus for laser imaging of lithographic printing members
by thermal non-ablative transfer
Abstract
Apparatus and methods for rapid, efficient production of durable
lithographic printing plates by a thermal-transfer process that does not
involve ablation. In response to an imaging pulse, a transfer material
reduces in viscosity to a flowable state. The material exhibits a higher
melt adhesion for a plate substrate than for the carrier sheet to which it
is initially bound, so that in a flowable state it transfers completely to
the substrate. Following transfer, the carrier sheet, along with
untransferred material, is removed from the substrate.
| Inventors:
|
Lewis; Thomas E. (E. Hampstead, NH);
Cassidy; Kenneth R. (Goffstown, NH)
|
| Assignee:
|
Presstek, Inc. (Hudson, NH)
|
| Appl. No.:
|
376766 |
| Filed:
|
January 23, 1995 |
| Intern'l Class: |
B41C 001/10 |
| Field of Search: |
101/453,454,457,462,463.1,465,466,467,401.1,456
|
References Cited
U.S. Patent Documents
| 3472162 | Oct., 1969 | Newman et al. | 101/463.
|
| 3580719 | May., 1971 | Brinckman | 101/470.
|
| 3736873 | Jun., 1973 | Newman | 101/467.
|
| 3745586 | Jul., 1973 | Braudy | 101/471.
|
| 3816659 | Jun., 1974 | Landsman | 358/491.
|
| 3945318 | Mar., 1976 | Landsman | 101/467.
|
| 3962513 | Jun., 1976 | Eames | 101/470.
|
| 3964389 | Jun., 1976 | Peterson | 101/467.
|
| 4245003 | Jan., 1981 | Oransky et al. | 101/401.
|
| 4588674 | May., 1986 | Stewart et al. | 430/273.
|
| 4711834 | Dec., 1987 | Butters et al. | 430/201.
|
| 4846065 | Jul., 1989 | Mayrhofer et al. | 101/453.
|
| 5129321 | Jul., 1992 | Fadner | 101/465.
|
| 5132723 | Jul., 1992 | Gelbart | 355/41.
|
| 5156938 | Oct., 1992 | Foley et al. | 430/200.
|
| 5171650 | Dec., 1992 | Ellis et al. | 430/201.
|
| 5238778 | Aug., 1993 | Hirai et al. | 101/467.
|
| 5308737 | May., 1994 | Bills et al. | 430/201.
|
| 5326619 | Jul., 1994 | Dower et al. | 428/164.
|
| 5340693 | Aug., 1994 | Uytterhoeven et al. | 430/253.
|
| 5342731 | Aug., 1994 | Kelly et al. | 430/253.
|
| 5360781 | Nov., 1994 | Leenders et al. | 430/200.
|
| 5382964 | Jan., 1995 | Schneider | 101/467.
|
| 5460918 | Oct., 1995 | Ali et al. | 101/453.
|
| Foreign Patent Documents |
| 108512 | Sep., 1978 | JP | 101/467.
|
| 126104 | Oct., 1979 | JP | 101/467.
|
| 13168 | Feb., 1981 | JP | 101/467.
|
| 2176018 | Dec., 1986 | GB.
| |
Other References
Levene, M.L. et al. Applied Optics 9:2260 (1970).
Braudy, Robert S., J. Appl. Phys. 45:3512 (1974).
Pinto, 38 J. Imaging Sci. and Tech. 565 (1994).
|
Primary Examiner: Funk; Stephen R.
Attorney, Agent or Firm: Cesari and McKenna, LLP
Claims
What is claimed is:
1. A method of producing a lithographic printing member using non-ablative
radiation-induced material transfer, the method comprising the steps of:
a. providing a donor blank comprising a layer of transfer material disposed
on a carrier layer, the carrier layer being substantially transparent to
imaging radiation and the transfer material becoming flowable but not
ablating in response to imaging radiation;
b. providing an acceptor substrate, the transfer material and the acceptor
substrate having different affinities for at least one printing liquid
selected from the group consisting of ink and an abhesive fluid for ink,
and the transfer material exhibiting, in its flowable state, preferential
adhesion for the acceptor substrate relative to the carrier layer;
c. causing intimate contact between the transfer layer and the acceptor
substrate;
d. imagewise irradiating the transfer layer through the carrier layer so as
to cause imagewise displacement of the transfer material to the acceptor
substrate;
e. removing the carrier layer and unirradiated transfer material from the
acceptor substrate; and
f. heating the displaced transfer material to enhance adhesion with the
acceptor substrate.
2. The method of claim 1 wherein the transfer material is oleophobic and
the acceptor substrate is oleophilic.
3. The method of claim 1 wherein the substrate has a texture and heating of
the displaced transfer material causes it to flow into the texture.
4. The method of claim 1 wherein the transfer material comprises at least
one cross-linkable component and heating of the displaced transfer
material causes the cross-linkable component to cross-link.
5. The method of claim 1 wherein the irradiation is accomplished using
near-IR radiation.
6. The method of claim 1 wherein the irradiation is accomplished using at
least one laser source.
7. The method of claim 1 wherein the irradiation is accomplished by light
valving.
8. The method of claim 1 wherein irradiation melts the transfer layer.
9. The method of claim 1 wherein the transfer material has a
glass-transition temperature and irradiation heats the transfer material
above said temperature.
10. The method of claim 1 wherein the transfer material is oleophilic and
the acceptor substrate is hydrophilic.
11. The method of claim 1 wherein the transfer material is hydrophilic and
the acceptor substrate is oleophilic.
12. The method of claim 1 wherein the transfer material is oleophobic and
the acceptor substrate is oleophilic.
13. Printing apparatus comprising:
a. at least one print station including:
i. a plate cylinder;
ii. a printing member comprising (A) a donor blank comprising a layer of
transfer material disposed on a carrier layer, the carrier layer being
substantially transparent to imaging radiation and the transfer material
becoming flowable, but not ablating, in response to imaging radiation, and
(B) an acceptor substrate, the transfer material and the acceptor
substrate having different affinities for at least one printing liquid
selected from the group consisting of ink and an abhesive fluid for ink,
and the transfer material exhibiting, in its flowable state, preferential
adhesion for the acceptor substrate relative to the carrier layer;
iii. means for causing intimate contact between the donor blank and the
acceptor substrate;
iv. means for supporting the printing member;
v. at least one source of imaging radiation focused on the transfer
material;
vi. means for causing relative movement between the radiation source and
the support means to imagewise expose the transfer material to the imaging
radiation, thereby causing imagewise displacement of the transfer material
to the acceptor substrate; and
vii. means for heating the displaced transfer material to enhance adhesion
with the acceptor substrate; and
b. means for transferring a recording medium to the print station.
14. A method of printing with a printing press that includes a plate
cylinder, the method comprising the steps of:
a. providing a donor blank comprising a layer of transfer material disposed
on a carrier layer, the carrier layer being substantially transparent to
imaging radiation and the transfer material becoming flowable but not
ablating in response to imaging radiation;
b. providing an acceptor substrate, the transfer material and the acceptor
substrate having different affinities for at least one printing liquid
selected from the group consisting of ink and an abhesive fluid for ink,
and the transfer material exhibiting, in its flowable state, preferential
adhesion for the acceptor substrate relative to the carrier layer;
c. causing intimate contact between the transfer layer and the acceptor
substrate;
d. mounting the donor blank on the plate cylinder;
e. imagewise irradiating the transfer layer through the carrier layer so as
to cause imagewise displacement of the transfer material to the acceptor
substrate;
f. removing the carrier layer and unirradiated transfer material from the
acceptor substrate;
g. heating the displaced transfer material to enhance adhesion with the
acceptor substrate and thereby create a working printing member;
h. applying ink to the printing member; and
i. transferring the ink to a recording medium.
15. Printing apparatus comprising:
a. a printing member comprising:
i. a donor blank comprising a layer of transfer material disposed on a
carrier layer, the carrier layer being substantially transparent to
imaging radiation and the transfer material becoming flowable, but not
ablating, in response to imaging radiation; and
ii. an acceptor substrate, the transfer material and the acceptor substrate
having different affinities for at least one printing liquid selected from
the group consisting of ink and an abhesive fluid for ink, and the
transfer material exhibiting, in its flowable state, preferential adhesion
for the acceptor substrate relative to the carrier layer;
b. means for causing intimate contact between the donor blank and the
acceptor substrate;
c. means for supporting the printing member;
d. at least one source of imaging radiation focused on the transfer
material;
e. means for causing relative movement between the radiation source and the
support means to imagewise expose the transfer material to the imaging
radiation, thereby causing imagewise displacement of the transfer material
to the acceptor substrate; and
f. means for heating the displaced transfer material to enhance adhesion
with the acceptor substrate.
16. The apparatus of claim 15 wherein the acceptor substrate is hydrophilic
and the transfer material is oleophilic.
17. The apparatus of claim 16 wherein the acceptor substrate is a textured
metal.
18. The apparatus of claim 17 wherein the metal is grain-anodized aluminum.
19. The apparatus of claim 17 wherein the metal is chromium.
20. The apparatus of claim 16 wherein the acceptor substrate is a polyvinyl
alcohol chemical species.
21. The apparatus of claim 15 wherein the acceptor substrate is is
oleophilic and the transfer material is hydrophilic.
22. The apparatus of claim 15 wherein the acceptor substrate is oleophilic
and the transfer material is oleophobic.
23. The apparatus of claim 22 wherein the acceptor substrate is polyester.
24. The apparatus of claim 15 wherein the acceptor substrate is oleophobic
and the transfer material is oleophilic.
25. The apparatus of claim 15 wherein the transfer material comprises at
least one absorber of imaging radiation.
26. The apparatus of claim 25 wherein the absorber is a limited-stability
absorber that ceases absorbing before the transfer layer can ablate.
27. The apparatus of claim 25 wherein the absorber comprises at least one
pigment.
28. The apparatus of claim 25 wherein the absorber comprises at least one
chromophore chemically integrated within the transfer layer.
29. The apparatus of claim 25 wherein the absorber comprises at least one
dye.
30. The apparatus of claim 29 wherein the transfer material further
comprises a pigment that does not significantly absorb imaging radiation.
31. The apparatus of claim 15 wherein the absorber is present at in an
amount that prevents absorption of sufficient imaging radiation to cause
the transfer layer to ablate.
32. The apparatus of claim 15 wherein the source of imaging radiation is at
least one laser.
33. The apparatus of claim 15 wherein the source of imaging radiation is a
light-valving assembly.
Description
BACKGROUND OF THE INVENTION
1. 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.
2. 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, as described, for example, in U.S.
Pat. No. 5,163,368 (co-owned with the present application and hereby
incorporated by reference). Each printing 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 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 have traditionally been 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 plate.
A similar photographic process is usually employed to create dry plates as
well. These ordinarily 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. 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 system generally require high-power lasers,
which are expensive and slow.
A second approach to laser imaging involves the use of transfer materials.
See, e.g., U.S. Pat. Nos. 3,945,318; 3,962,513; 3,964,389; 4,245,003;
4,395,946; 4,588,674; and 4,711,834. 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.
Typically, the transfer material exhibits a high degree of absorbence for
imaging laser radiation, and ablates--that is, virtually explodes into a
cloud of gas and charred debris--in response to a laser pulse. This
action, which may be further enhanced by self-oxidation (as in the case,
for example, of nitrocellulose materials), ensures complete removal of the
transfer material from its carrier. Material that survives ablation
adheres to the acceptor sheet.
Alternatively, instead of laser activation, transfer of the thermal
material can be accomplished through direct contact. U.S. Pat. No.
4,846,065, for example, describes the use of a digitally controlled
pressing head to transfer oleophilic material to an image carrier.
Regardless of the actual transfer mechanism, the transfer and acceptor
materials ordinarily 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. Unfortunately, plates produced with transfer-type systems
tend to exhibit performance limitations associated with uneven material
transfer. This contributes, for example, to the short useful lifetimes
exhibited by transfer-type plates (although this problem probably derives
primarily from transfer of degraded, partially ablated materials).
Uneven material transfer is explained, at least in part, by the formation
of gas pockets during the ablation process. This effect is illustrated in
FIGS. 1A-1C. A representative donor transfer blank, indicated generally by
reference numeral 30, includes an aluminum plate substrate 32 and a
transfer sheet held in intimate contact with substrate 32. The transfer
sheet comprises a carrier film layer 34 that is substantially transparent
to imaging radiation and, bonded to carrier layer 34, a transfer layer 36
that responds to imaging radiation. As shown in FIG. 1A, an imaging pulse
38 from a laser source strikes transfer blank 30 and spans a diameter
indicated by boundaries A and B. The intense heating of layer 36 caused by
the laser beam at least partially ablates layer 36 within the imaging zone
A-B, resulting in production of gases that gather into a pocket 40 (see
FIG. 1B) and lift the transfer blank away from substrate 32. The beam also
results in transfer to substrate 32 of a slug 42 of transfer material; the
transfer is incomplete, however, partly as a result of interference by gas
pocket 40.
The gases in pocket 40 can continue to spread well beyond the imaging zone
A-B, as shown in FIG. 1C, lifting even more of the transfer blank away
from substrate 32 across a region that now spans boundaries A to C. The
disruption of the contact between the donor transfer blank (layers 34, 36)
and substrate 32 further degrades imaging capability in the
as-yet-unexposed region B-C. In other words, laser-induced transfer of
material at one site--incomplete in itself as a result of gas-pocket
formation--causes adjacent regions to become even less responsive to
subsequent laser exposure. The overall result is partial and inconsistent
transfer of material across the blank. This behavior manifests itself in
final plate images of varying quality, durability and adhesion which, when
employed in commercial printing environments requiring 50,000 or more
impresssions, remain vulnerable to degradation. Indeed, image degradation
through the course of plate usage represents a common problem with
virtually all transfer-type processes, since the transfer material remains
bound to the substrate by relatively weak adhesion forces.
DESCRIPTION OF THE INVENTION
Brief Summary of the Invention
The present invention facilitates rapid, efficient production of durable
lithographic printing plates by a radiation-induced thermal-transfer
process. Unlike well-known prior-art systems, however, the invention
deliberately avoids ablation as a transfer mechanism. Instead, in response
to an imaging radiation pulse, our transfer material reduces in viscosity
to a flowable state. The material is formulated to exhibit a higher melt
adhesion for a plate substrate than for the carrier sheet to which it is
initially bound, so that in a flowable state it transfers completely to
the substrate. Following transfer, the carrier sheet, along with
untransferred material, is removed from the substrate.
The transferred material is then subjected to a fusing step. Unlike the
prior art, which relies on a short exposure to both transfer and fix the
donor material onto the acceptor sheet, the fusing step chemically and/or
physically anchors our transfer material onto the substrate, resulting in
enhanced adhesion properties. Moreover, since the constructions may be
imaged while on-press, the fusing step imposes little additional
processing burden or mechanical requirements.
The present invention preferably employs, as imaging devices, relatively
inexpensive laser equipment that operates at low to moderate power levels.
However, other digitally controllable approaches to delivering imaging
radiation (e.g., light valving, as described, for example, in U.S. Pat.
Nos. 4,577,932; 4,743,091; 5,049,901; and 5,132,723, the entire
disclosures of which are hereby incorporated by reference) can be used
instead, and may in fact prove preferable for off-press applications. In
one implementation, the invention employs imaging apparatus including 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. The present invention can
employ 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 that facilitate imaging and, in particular, semiconductive and
conductive compounds.
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" or "member"
refers to any type of printing medium 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, as exemplified in U.S. Pat. No. 5,440,987,
entitled LASER IMAGED SEAMLESS OFFSET LITHOGRAPHIC PRINTING MEMBERS AND
METHOD OF MAKING, the entire disclosure of which is hereby incorporated by
reference), an endless belt, or other arrangement.
In one embodiment, the substrate is a textured hydrophilic metal (e.g.,
chromium or grain-anodized aluminum, as described in U.S. Pat. Nos.
4,911,075 and 4,958,563, the disclosures of which are hereby incorporated
by reference), and the transfer material is an oleophilic, hydrophobic
polymer that becomes flowable in response to imaging radiation. Upon
exposure, the transfer material decreases in viscosity and develops
adhesion with the substrate surface; at this point, as with conventional
processes, contact between the transfer material and the substrate is
largely limited to elevated texture peaks. Following complete imagewise
exposure of the plate, the untransferred material is removed, and the
transferred material is thermally fused into the substrate texture.
Specifically, the imaged construction is heated to raise the temperature
of the transferred polymer (e.g., above the glass-transition point
T.sub.g) so that it re-enters a flowable state; the heated polymer soaks
into the porosity of the substrate, becoming firmly bound therein. When
the finished plate cools and the polymer solidifies, its mechanical and
chemical adhesion to the plate surface will be substantial and the plate
will exhibit commensurate durability. Moreover, because the polymer has
become integrated within the substrate texture, the plate will continue to
function even if the layer of polymer overlying the plate surface wears
away: interstitial material, which remains virtually impervious to
extraction from the surface within which it is bound, will continue to
defeat the natural hydrophilicity of that surface.
In a second embodiment, the fusing mechanism is chemical in addition to or
instead of thermal. Although the approach of the first embodiment can be
applied to non-metal surfaces, intimate bonding to weakly textured
hydrophilic materials (such as films based on polyvinyl alcohol species)
may be accomplished chemically more readily than physically. In these
circumstances, instead of using heat fusion, the transfer material
includes some form of delayed chemical reactivity that may be selectively
triggered following deposition on the substrate, and which serves to
anchor the material to that substrate. At the same time, chemical bonding
can also be used to advantage in connection with textured metal
substrates, either in lieu of or in addition to the mechanical fusing
discussed above. Suitable chemical species, which desirably are chemically
integrated into the polymer backbone of the transfer material, include
carboxyl-functional groups (which adhere well to metal surfaces),
condensation-cure and addition-cure functional groups, and
radiation-curable groups.
The approach of the present invention can also be used to produce dry
plates. In this case, the transfer material is oleophobic and the
substrate oleophilic, or vice versa.
The transfer material is ordinarily disposed on a carrier sheet transparent
to the imaging radiation; the carrier sheet is held in intimate contact
with the substrate during imaging. In order to render the transfer
material responsive to imaging radiation at relatively low power levels,
the transfer material preferably contains a radiation-sensitive compound
having an absorption peak at or near the imaging wavelength. The
absorptive material may be a pigment or dye dispersed or dissolved in the
polymer matrix, or a chromophore (such as phthalocyanine or
naphthalocyanine, as described in U.S. Pat. No. 5,310,869 and the
references cited therein) chemically integrated therewith.
Laser output is either 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 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:
FIGS. 1A-1C are elevational sections of prior-art, ablation-type plate
blanks, showing their behavior in response to imaging radiation and the
formation of gas pockets.
FIG. 2 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. 3 is a schematic depiction of the embodiment shown in FIG. 2, and
which illustrates in greater detail its mechanism of operation;
FIG. 4 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. 5 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. 6 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. 7 is a side view of the writing array depicted in FIG. 6;
FIG. 8 is an isometric view of the flatbed embodiment of an imaging
apparatus having a linear lens array;
FIG. 9 is an isometric view of the interior-drum embodiment of an imaging
apparatus having a linear lens array;
FIG. 10 is a cutaway view of a remote laser and beam-guiding system;
FIG. 11 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. 12 is an enlarged, cutaway view of a lens element having an integral
laser;
FIG. 13A is an isometric view of a typical laser diode;
FIG. 13B is a plan view of the diode shown in FIG. 13A, showing the
dispersion of radiation exiting therefrom along one dimension;
FIG. 13C is an elevation of the diode shown in FIG. 13A, showing the
dispersion of radiation exiting therefrom along the other dimension;
FIG. 14 illustrates a divergence-reduction lens for use in conjuncion with
the laser diode shown in FIGS. 13A-13C;
FIG. 15 schematically depicts a focusing arrangement that provides an
alternative to the apparatus shown in FIG. 10;
FIGS. 16A and 16B are side and end elevations of a chisel-edge end face of
a fiber-optic cable;
FIGS. 17A and 17B are side and end elevations of a hemispherical end face
of a fiber-optic cable;
FIG. 18 is a side elevation of an optical-coupling arrangement that employs
a cylindrical lens;
FIGS. 19A and 19B are schematic circuit diagrams of laser-driver circuits
suitable for use with the present invention;
FIGS. 20A-20D are enlarged sectional views showing the manner in which
suitable lithographic plate constructions are imaged in accordance with
the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
1. Imaging Apparatus
a. Exterior-Drum Recording
Refer first to FIG. 2 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 allowed application 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. 3).
The angular position of cylinder 50 is monitored by a shaft encoder (see
FIG. 5). 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 65.
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. 3, 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 (but not excessive) power and duration to effect material
transfer as described 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 density preferably falls in the range of 0.2
megawatt/in.sup.2 to 0.6 megawatt/in.sup.2.
Because preferred feature sizes are ordinarily quite small--on the order of
0.2 to 1.4 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. 3, 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 1-15.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 87 (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 appropriate o 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. 4. 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. Alternatively, a single lens placed in front the output-cable
termini (staggered as shown in FIG. 4) can be used to focus them all onto
the surface of plate 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. 5, which also shows the
detector 85 mounted to the cylinder 50. Preferred detector designs are
described in the '205 patent. 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. 8. 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 186 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. 9. 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. 10-12. Refer first to FIG. 10, 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 one 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. 11.
The exemplary double-lens system shown in FIG. 10, while adequate in many
arrangements, can be improved to accommodate the characteristics of
typical laser diodes. FIG. 13A shows a common type of laser diode, in
which radiation is emitted through a slit 502 in the diode face 504. The
dimensions of slit 502 are specified along two axes, a long axis 502l and
a short axis 502s. Radiation disperses as it exits slit 502, diverging at
the slit edges. This is shown in FIGS. 13B and 13C. The dispersion around
the short edges (i.e., along long axis 502l), as depicted in FIG. 13B
(where diode 500 is viewed in plan), is defined by an angle a; the
dispersion around the long edges (i.e., along short axis 502s), as
depicted in FIG. 13C (where diode 500 is viewed in elevation), is defined
by an angle .beta.. The numerical aperture (NA) of slit 502 along either
axis is defined as the sine of the dispersion angle .alpha. or .beta..
For optimum performance, .alpha.=.beta. and the unitary NA is less than
0.3, and preferably less than 0.2. Small NA values correspond to large
depths-of-focus, and therefore provide working tolerances that facilitate
convenient focus of the radiation onto the end face of a fiber-optic
cable. Without correction, however, these desirable conditions are usually
impossible, even with special mask structures that have recently been
applied to the multi-stripe and single-stripe semiconductor lasers useful
in the present invention; laser diode 500 typically does not radiate at a
constant angle, with divergence around the long edges exceeding that
around the short edges, so .beta.>.alpha..
Assuming that the NA along long axis 502l falls within acceptable limits,
the NA along the short axis 502s can be made to approach the long-axis NA
by controlling dispersion around the long edges. This is achieved using a
divergence-reduction lens. Suitable configurations for such a lens include
a cylinder (essentially a glass rod segment of proper diameter), a
planoconvex bar, and the concave-convex trough shown in FIG. 15. The
divergence-reduction lens is positioned adjacent slit 502 with its length
following long axis 502l, and with its convex face adjacent the slit.
If the NA along long axis 502l also exceeds acceptable limits, the
dispersion around the short edges can be diminished using a suitable
condensing lens. In this case the optical characteristics of
divergence-reduction lens 520 are chosen such that the NA along short axis
502s approaches that along long axis 502l after correction.
Advantageous use of a divergence-reduction lens is not limited to slit-type
emission apertures. Such lenses can be usefully applied to any
asymmetrical emission aperture in order to ensure even dispersion around
its perimeter.
Preferably, the divergence-reduction lens has an antireflection coating to
prevent radiation from rebounding and interfering with operation of diode
500 (for example, by causing the condition known as "mode hopping"). A
practical manufacturing approach utilizes a facet coater to place an
antireflection coating on the glass rod intended to serve as a cylindrical
divergence-reduction lens. The coating, preferably a multilayer broad-band
coating such as magnesium fluoride over titanium, is applied first along
one half of the circumference and then along the other half. Overlap of
the two applications is preferable to an uncoated gap. Therefore, to
prevent transmission losses, the coated lens is oriented with respect to
slit 502s such that radiation passes through lens regions have not been
doubly coated; the opposed, doubly coated arc segments are positioned
above and below the path of radiation emitted from diode 500. This
positioning is straightforwardly obtained using known techniques of
microscopic mechanical manipulation.
With the radiation emitted through slit 502 fully corrected as described
above, it can be straightforwardly focused onto the end face of a
fiber-optic cable by a suitable optical arrangement, such as that
illustrated in FIG. 15. The depicted optical arrangement utilizes a
planoconvex bar as a divergence-reduction lens 520, which is oriented with
respect to diode 500 as described above; a collimating lens 525, which
draws the corrected but still divergent radiation into parallel rays; and
an aspheric focusing lens 530, which focuses the parallel rays onto the
end face 265f of fiber-optic cable 265. In some cases it is possible to
replace lenses 525 and 530 with a custom aspheric lens 535 as shown.
The face 265f of fiber-optic cable 265 can also be shaped to contribute to
optical coupling or even to replace the collimating and focusing lenses
entirely. For example, face 265f can be tapered by grinding into a flat
chisel edge 550 that accepts beam radiation along a sufficiently narrow
edge to avoid back reflection and consequent modal instability, as shown
in FIGS. 16A and 16B. So long as the divergence of radiation emitted from
slit 502 has been adequately reduced or controlled, the arrangement shown
in FIGS. 16A and 16B will perform comparably to the separate lens
configuration shown in FIG. 10. In another embodiment, illustrated in
FIGS. 17A and 17B, the face of fiber-optic cable 265 is rounded into a
hemisphere 552, again functioning to accept incoming radiation without
mode hopping.
Another approach to optical coupling, which utilizes a cylindrical lens
560, appears in FIG. 18. As shown in the figure, cylindrical lens 560,
which has received an antireflection coating, is interposed directly
between slit 502 and a flat fiber face 265f, preferably in intimate
contact with the fiber face and spaced slightly from the diode 500. Lens
560 reduces divergence around edges 502l, as discussed above, and focuses
the laser beam onto face 265f.
In some arrangements, it may prove necessary or desirable to utilize a
fiber with a flat face 265f that is smaller in diameter than the length of
diode's large axis. Unless the the radiation emitted along the long axis
is concentrated optically, the loss of radiation that fails to impinge on
end face 265f must either be accepted or the end face distorted (e.g.,
into an ellipse) to more closely match the dimensions of slit 502f.
Refer now to FIG. 11, which illustrates an exemplary output assembly to
guide radiation from fiber-optic cable 265 to the imaging surface. As
shown in the 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 at a diameter optimal for imaging. This distance can be
altered to vary the size of an image feature and to avoid astigmatism and
aberration.
The diameter of an image feature is given by the ratio of the distance A to
the distance between lens 280a and the surface of plate 55, multiplied by
the diameter of the emitting fiber face. To increase depth-of-focus, it
may prove desirable to restrict the passage even of collimated/radiation
to a minimal radial extent from the central propagated ray (although the
power represented by the blocked radiation will thereby be lost). In
practice, the minimum necessary depth-of-focus is based on mechanical
adjustment and accuracy limitations; with this quantity and the necessary
degree of beam demagnification effectively fixed, the optimal beam
restriction is determined primarily by the NA value of the radiation
emitted at the fiber face, which is itself governed by the numerical
aperture of radiation coupled into the fiber at its proximal end face
265f. In an exemplary embodiment, an aperture diameter of 0.109 inch
provides effective results in conjunction with an NA value of 0.095. To
implement this aspect of the invention, an annular wall having a
selected-size orifice therethrough is interposed between lenses 280a,
280b.
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. 11, 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. 12, 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; a heat sink 307; and the annnular wall mentioned
previously, shown at reference numeral 310.
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. 12.
e. Driver Circuitry
A suitable circuit for driving a diode-type (e.g., gallium arsenide) laser
is illustrated schematically in FIG. 19A. Operation of the circuit is
governed by controller 80, which generates a fixed-pulse-width signal
(preferably 1 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.
An alternative arrangement, which utilizes feedback, appears in FIG. 19B.
In this case, a fixed current-limiting resistor 350 is used instead of a
variable resistor, and the input terminals of an amplifier 352 are
connected across this resistor. The output of amplifier 352 is connected
to a first functional input terminal of a comparator 354. A second
functional input terminal of comparator 354 is connected to the output of
a digital-to-analog (D/A) converter 356. D/A converter 356 includes an
internal latch capable of storing a digital value (provided by controller
80) corresponding to a desired diode current; the converter transforms
this value into the analog output provided to comparator 354. Controller
80 directly controls the operation of comparator 354, actuating it only
when diode 330 overlies plate locations at which image points are to be
written.
The operation of this circuit is as follows. The voltage across resistor
350, which determines the output of amplifier 352, is proportional to the
current into diode 330. When comparator 354 is operative, the circuit will
supply to diode 330 that amount of current necessary to equalize the
voltage at the two comparator input terminals; accordingly, the latched
value dictates the maximum diode current, and the circuit prevents
overshoot of this current (which might easily damage diode 330).
2. Lithographic Printing Members and Imaging Methods
Refer now to FIGS. 20A-20C, which illustrate constructions imageable to
produce lithographic printing plates, and the manner in which these
constructions are imaged in accordance with the present invention. As
shown in FIG. 20A, an imageable construction 400 includes a plate
substrate 410 and a transfer sheet held in intimate contact therewith. The
transfer sheet comprises a carrier film layer 412 that is transparent to
imaging radiation and, bonded thereto, a transfer layer 414 that responds
to imaging radiation in the manner described below. An imaging pulse 38
from a laser or other suitable source strikes construction 400,
illuminating an area indicated by boundaries A and B.
Layers 410 and 414 (or a surface thereof) exhibit opposite affinities for
ink and/or an ink-abhesive fluid. In one embodiment, directed toward
production of direct-write wet plates, substrate 410 is a hydrophilic,
surface-textured metal such as aluminum or chromium, and layer 414 is an
oleophilic, hydrophobic, polymeric material. In related version, substrate
410 is a hydrophilic polymer, such as a polyvinyl alcohol species. In an
indirect-write counterpart to this embodiment, layer 414 is a polyvinyl
alcohol species, and layer 410 is an oleophilic, hydrophobic material such
as a polyester primed with a vinylidene dichloride-based polymer; a useful
example of such a material is Saran F-310, a vinylidene
dichloride-acrylonitrile copolymer supplied by Dow Chemical Co., Midland,
Mich.
In another embodiment, directed toward production of direct-write dry
plates, substrate 410 is an oleophilic polymer, such as polyester, and
layer 414 is an oleophobic polymer. One useful version of this embodiment
includes a titanium-metallized polyester layer 410 (where the titanium is
deposited to a thickness of approximately 200 .ANG.) in conjunction with a
B-staged (i.e., partially cured but still reactive) silicone donor layer
414. Titanium in its native and naturally oxidized states provides a
catalytic surface that promotes further cure of the silicone during the
fusing step. In an indirect-write counterpart to this embodiment,
polymeric substrate 410 is oleophobic and layer 414 is the oleophilic
polymer. A useful combination for this purpose is an acrylate-functional
silicone (as described in U.S. Pat. Nos. 5,212,048 and 5,310,869, the
entire disclosures of which are hereby incorporated by reference),
employed as layer 410, and an acrylate-functional acrylate donor layer
414. Following transfer, the imaged construction is exposed to radiation,
cross-linking the substrate and the transferred material.
In any case, layer 414 is formulated to interact in a controlled fashion
with imaging radiation. In particular, the constructions of the present
invention do not rely on creation of a gas or plasma pressure to effect
the transfer of material from donor to acceptor. Instead, an imaging pulse
heats the exposed portion of layer 414 to a flowable state (e.g., by
melting layer 414 or raising its temperature above the glass-transition
point T.sub.g). In its flowable state, layer 414 exhibits a higher melt
adhesion for substrate 410 than for carrier film 412, and the exposed
portion of layer 414 therefore preferentially adheres to substrate 410.
Accordingly, a key feature of layer 414 is its absorption of sufficient
energy from imaging pulse 38 to reach a flowable state, but not so much as
to ablate. Compatibility between the absorption characteristics of layer
414 and the wavelength and power of the imaging radiation is therefore
critical. Such compatibility is conveniently attained for a range of power
levels by including, in layer 414, radiation absorbers that exhibit
limited stability in the presence of intense imaging radiation.
Alternatively, stable radiation absorbers can be employed at loading
levels that render them only partially effective at absorbing imaging
radiation; in this case, formulation of suitable compositions requires
more detailed knowledge of the power levels likely to be applied.
Limited stability in a radiation absorber can result from vulnerability to
chemical breakdown (i.e., photo-cleavage into molecular fragments having
little or no absorption capacity) or thermal breakdown, or to a
combination of both. Either way, the intentional self-induced failure acts
as a fuse, imposing a ceiling on the temperature the transfer layer may
reach in response to an imaging pulse so as to avoid unwanted ablation.
Thus, as shown in FIG. 20A, imaging pulse 38 renders flowable the material
of layer 414 across a region approximating the area A-B. As a practical
matter, however, the effect is not that precise, since the temperature
does not decay suddenly at the boundaries. Instead, a thermal gradient,
indicated at A', B', will extend into the unheated area adjacent region
A-B as a result of heat conduction. Somewhere within this thermal gradient
lies a viscosity transition where the layer 414 material will cease to
flow. Inside this transition boundary, as shown in FIG. 20B, the material
will adhere to substate 410.
The location of the separation boundary within the thermal gradient depends
on the degree of internal cohesion within layer 414 and the amount of
melt-adhesion preference of this layer for substrate 410 over carrier film
412. These behaviors can be altered by loading layer 414 with additives
such as pigments or dyes (the latter affecting behavior to a lesser
degree). Desirable additives reduce cohesion within the thermal gradient,
reduce adhesion to carrier film 412 and increase adhesion to substrate
410. Typically, the mechanism by which a useful additive exerts its
effects comprises interaction between pigment surfaces (or dye molecules)
and the flowable polymer(s) of layer 412 that alters the binding between
polymer chains and between the surfaces in contact with the polymer(s) and
the polymer chains. Further effects arise from intense local heating of
polymer(s) adjacent to the surface of radiation-absorptive pigment
particles.
Following imagewise transfer of material from layer 414 onto substrate 410
and removal of carrier film 412 (along with untransferred material),
substrate 410 (and the array of image spots 420 thereon) is subjected to a
fusing step that anchors, by mechanical and/or chemical means, image spots
420 more firmly to substrate 410 (using, for example, a heating source 425
that melts image spot 420).
EXAMPLES 1-11
These examples describe preparation of positive-working wet plates in
accordance with the present invention and, for comparative purposes, in
accordance with prior-art techniques. The below formulations were coated
on a "print-treated" polyester film, substantially transparent to imaging
IR radiation, to form a transfer sheet. The print or coatability treatment
promotes adhesion, and is furnished with various suitable polyester films
(e.g., the J films marketed by E.I. dupont de Nemours Co., Wilmington,
Del., and the MELINEX 453 film sold by ICI Films, Wilmington, Del.).
Coatings were deposited using wire-wound rods and dried in a convection
oven to yield final coating weights of 2 g/m.sup.2.
The prepared transfer sheets were brought into intimate contact with
aluminum substrates, each 0.006 inch in thickness and having grained,
anodized and silicated surfaces, and mechanically clamped together at the
edges. (It should be understood that many alternative approaches, e.g.,
vacuum and electrostatic binding, are available and well known to those
skilled in the art.) The resulting constructions were imaged in accordance
with the techniques hereinbefore described to transfer the material,
following which they were fused by heating at 300.degree. F. for 1 min.
(equivalent results can be obtained by heating at 400.degree. F. for 0.5
min.).
The following formulations were used to produce transfer layers:
__________________________________________________________________________
Example 1 2 3 4 5 6 7 8 9 10 11
__________________________________________________________________________
Component Weight %
__________________________________________________________________________
5-6" RS Nitrocellulose
12.5
12.5
12.5
12.5
-- -- -- 2.5
2.5
2.5
2.5
Acryloid B-44
-- -- -- -- 12.5
12.5
12.5
10.0
10.0
10.0
10.0
Vulcan XC-72
11.0
8.0
-- -- 8.0
-- -- -- -- 8.0
8.0
Heliogen Green L 8605
-- -- 8.0
-- -- -- 8.0
-- 8.0
-- --
Kodak IR-810
-- 3.0
3.0
3.0
-- 3.0
3.0
3.0
3.0
3.0
--
Methyl ethyl ketone
76.5
76.5
76.5
84.5
79.5
84.5
76.5
76.5
76.5
76.5
79.5
__________________________________________________________________________
Results
__________________________________________________________________________
Transfer Yes
Yes
Inc.
Yes Yes
Yes
Yes
Yes Yes Incomplete
Gas Pockets
Yes
Yes
Severe
Yes No No No Severe
No Yes
Adhesion to Substrate
Yes
Yes
Yes Yes No Yes
Yes
Yes Yes No
Film Split Yes
Yes
No Minor
-- No No No No --
Plate Life Test
-- -- -- -- -- Pass
Pass
Fail
Fail
--
__________________________________________________________________________
The nitrocellulose utilized was the 30% isopropanol wet 5-6 Sec RS
Nitrocellulose supplied by Aqualon Co., Wilmington, Del. Acryloid B-44 is
an acrylic resin supplied by Rohm & Haas, Philadelphia, Pa. Vulcan XC-72
is a conductive carbon black pigment supplied by the Special Blacks
Division of Cabot Corp., Waltham, Mass. Kodak IR-810 is an IR-absorbing
dye obtained from Eastman Fine Chemicals, Eastman Kodak Co., Rochester,
N.Y. Heliogen Green L 8730 is a green pigment supplied by BASF Corp.,
Chemicals Division, Holland, Mich.
In these examples, "transfer" indicates whether sufficient amounts of
material transferred to the substrate to facilitate imaging (the notation
"Inc." indicating incomplete transfer). "Gas pockets" refers to the
above-described condition resulting from accumulation of ablation-created
gas(es), and which produces uneven or missing transfer. None of the
examples exhibited substantial adhesion to the substrate prior to the
heat-fusion step. "Film split" measures the cohesive strength of the
transferred and heat-fused coatings. The film-split test is performed by
affixing adhesive tape to the finished plate and then withdrawing the
tape; deposition of material onto the tape indicates weak interior
adhesion. The plates were subjected to 50,000 impressions, and the results
of the plate life test indicate whether the plate remained usable after
this degree of wear.
Examples 1-4 are coatings formulated along lines known from the prior art.
All contain a self-oxidizing nitrocellulose binder; Examples 1 and 2
utilize carbon-black pigment. Example 1 utilizes the pigment alone,
Examples 2 and 3 an IR-absorptive dye in combination therewith, and
Example 4 an IR-absorptive dye alone. None of these formulations is useful
in the context of the present invention. Replacement of carbon black with
a different pigment (as in Example 3) and even its complete replacement by
an IR-absorptive dye (as in Example 4) fails to overcome problems arising
from gas pockets.
Example 5 eliminates the self-oxidizing nitrocellulose binder but
reintroduces carbon black; this formulation also exhibits gas pockets and
is likewise unsuitable. Example 6, which avoids both carbon black and
self-oxidizing binders, represents a coating formulation suitable for use
with the present invention.
Example 7 exemplifies a second category of useful formulation containing a
pigment that absorbs IR imaging radiation only weakly, if at all, and an
IR-absorptive dye. In Example 7, the green pigment is relatively
non-absorptive in the near-IR region but serves to beneficially modify
transfer properties.
Example 8 shows that formulations based on another traditional
ablation-transfer material, nitrocellulose, produce undesirable gas
pockets. However, when combined at low levels with a particulate filler
that suppresses formation of gas pockets (e.g., by adsorption or
absorption, or reaction with the gas), nitrocellulose can be employed to
advantage.
Once again, however, using carbon black as the particulate filler, as in
Examples 10 and 11, renders otherwise worthwhile material unusable.
__________________________________________________________________________
EXAMPLES 12-21
Example 12 13 14 15 16 17 18 19 20 21
__________________________________________________________________________
Component Weight %
__________________________________________________________________________
5-6" RS Nitrocellulose
12.5
-- -- -- -- -- -- -- -- --
Acryloid B-44
-- 12.5
12.5
12.5
12.5
12.5
12.5
-- -- --
Estane 5715
-- -- -- -- -- -- -- 12.5
12.5
--
Vitel PE-200
-- -- -- -- -- -- -- -- -- 12.5
Vulcan XC-72
1.0
1.0
1.0
-- -- -- -- -- -- --
Kodak IR-810
-- -- 1.0
3.0
3.0
3.0
-- 3.0
3.0
3.0
Titanyl phthalocyanine
-- -- -- -- -- -- 4.0
-- -- --
Heliogen Green L 8605
-- -- -- 4.0
-- -- -- -- 4.0
--
Hostaperm Blue A2R
-- -- -- -- 4.0
-- -- -- -- --
Orasol Black RLI
-- -- -- -- -- 4.0
-- -- -- --
Methyl ethyl ketone
87.5
87.5
87.5
80.5
80.5
80.5
83.5
84.5
84.5
84.5
__________________________________________________________________________
Results
__________________________________________________________________________
Transfer Yes
Yes
Yes
Yes
Yes
Yes
Yes
Marg.
Yes
Yes
Gas Pockets
No?
No No No No No No No No No
Adhesion to Substrate
Yes
Yes
Yes
Yes
No Yes
Yes
Yes Yes
Yes
Film Split No No No No No No No No No No
Plate Life Test
Fail
Fail
Pass
Pass
Pass
Pass
Pass
-- Pass
Pass
__________________________________________________________________________
Estane 5715 is a polyurethane polymer obtained from The BF Goodrich Co.,
Cleveland, Ohio. Vitel PE-200 is a polyester polymer obtained from
Goodyear Tire & Rubber Co., Akron, Ohio. Hostaperm Blue A2R is a blue
pigment supplied by the Specialty Chemicals Group, Hoechst Celanese Corp.,
Charlotte, N.C. Orasol Black RLI is an IR-absorptive dye obtained from the
Pigments Division, Ciba-Geigy Corp., Newport, Del.
Examples 12 and 13 represent attempts to improve the unacceptable
performance of the coating of Example 1 by lowering the carbon-black
content and, in Example 13, replacing the potentially self-oxidizing
nitrocellulose with an acrylic polymer. While gas pockets and film split
are overcome, the transferred coatings lack the durability necessary for
commercially realistic printing runs. Thus transfer materials based solely
on carbon black, even at low concentrations and in the absence of
self-oxidizing binders, are unsuitable for the present invention. In
particular, Example 13 suggests that the localized "hot spots" produced by
irradiation of the highly stable carbon-black particles diminish
durability, either by local degradation by ablation of the immediately
surrounding polymer, non-uniform heating of the bulk transfer material, or
some combination of these mechanisms.
In Example 14, an IR-absorbing dye is added to the formulation of Example
13. The result is a plate that passes the 50,000-impression test. The
inclusion of a soluble dye, which absorbs at the molecular (as opposed to
particle) level and is evenly dispersed throughout the absorptive transfer
material, promotes highly even heating of that layer by laser pulses. It
appears, therefore, that uniform heating is important to production of
durable coatings with the present invention, and that the lack of this
response primarily accounts for the poor durability characteristics
exhibited by the Example 13 formulation.
Example 15 represents a variation of the Example 7 formulation, in which
the amount of pigment has been reduced. Taken together, the two examples
illustrate the ability to vary pigment loading fractions while maintaining
desired properties.
In Example 16, we substituted a blue pigment (also a weak IR absorber) for
the Heliogen Green pigment of Example 4. We anticipate that a range of
pigments that advantageously modify transfer properties will be usable in
the context of the present invention.
An IR-absorptive phthalocyanine pigment was used in Example 18. Unlike
carbon black, this pigment is thermally unstable. The success of this
formulation may also be due to use of the pigment in small enough amounts
to avoid overheating.
In Example 17, we replaced the Heliogen Green pigment of Example 4 with a
soluble dye. This approach is advantageous where the need for property
modification, as can be achieved using pigments, is not present:
dissolving a dye involves considerably less manufacturing inconvenience
than dispersing a pigment.
In Examples 19 and 20, we replaced the acrylic polymer of Example 4 with a
polyurethane polymer. Although the transfer properties of the resulting
material suffer using the IR-810 pigment, performance improves
substantially with the substitution of Heliogen Green. Once again, these
examples demonstrate the considerable variation in physical properties
that may be obtained using different types and amounts of pigments.
Example 21 represents another variation of the Example 4 formulation,
illustrating that advantageous results are obtainable with yet another
class of polymer base (in this case polyester).
EXAMPLES 22-25
The following examples illustrate cross-linking as a fusing mechanism
following transfer.
______________________________________
Example 22 23 24 25
______________________________________
Component Weight %
Acryloid B-44 12.5 12.5 -- --
Dianal BR-87 -- -- 12.5 --
Estane 5715 -- -- -- 12.5
Kodak IR-810 3.0 3.0 3.0 3.0
Heliogen Green L 8605
-- 4.0 -- --
Cymel 303 3.0 3.0 3.0 2.0
NaCure 2530 4.0 4.0 4.0 3.0
Methyl ethyl ketone
77.5 73.5 77.5 75.5
Results
Transfer Marg. Yes Yes Yes
Gas Pockets No No No No
Adhesion to Substrate
Yes Yes Yes Yes
Film Split No No No No
Plate Life Test -- Pass Pass Pass
______________________________________
NaCure 2530, supplied by King Industries, Norwalk, Conn., is an
Mine-blocked p-toluenesulfonic acid solution in an isopropanol/methanol
blend. Cymel 303 is hexamethoxymethylmelamine, supplied by American
Cyanamid Corp. Dianal BR-87 is an acrylic copolymer supplied by Dianal
America, Inc., Pasadena, Tex., in which the major component is methyl
methacrylate and the minor component is methacrylic acid.
To prepare the coatings, the various components, including the blocked PTSA
catalyst, were combined and the resulting mixtures applied to an aluminum
substrate using a wire-wound rod. The coatings were allowed to dry without
heating to yield final coating weights of 2 g/m.sup.2.
Following imagewise transfer of the material onto the aluminum substrates,
the substrates were cured by heating for 1 min. at 300.degree. F. in a
convection oven. In Examples 22-24, curing was by self-condensation of the
melamine resin. In Example 25, the melamine cross-linked with hydroxyl
groups present on the polyurethane polymer.
The addition of Cymel 303 and the catalyst lowered the T.sub.g and adhesion
characteristics otherwise associated with Acryloid-based formulations.
Accordingly, in Example 23, the Heliogen Green pigment was added to the
Example 22 formulation to beneficially modify physical characteristics and
thereby achieve better transfer properties. Example 24 illustrates use of
a polymer with carboxyl functional groups that promote adhesion with the
aluminum substrate, and which are not consumed by cross-linking reactions.
Still other cross-linking systems can also be utilized. For example, the
base polymer (e.g., Acryloid B-44) can include epoxy functional groups; in
this case, the formulation will include a BF.sub.3 -amine complex that may
be thermally activated following imaging. It is also possible to utilize
radiation-cure materials, although, if the post-transfer heating step is
omitted in connection with a textured substrate, the benefits of
mechanical locking will be lost. Suitable radiation-cure coatings will be
largely unreactive with imaging radiation; for example,
acrylate-functional materials are useful in conjunction with near-IR
imaging radiation; these may be cured directly by electron-beam exposure,
or may incorporate a photoinitiator for cure by exposure to ultraviolet
radiation.
It will therefore be seen that we have developed a highly versatile
approach to automated production of lithographic printing members by
non-ablative transfer. 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.
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