Note: Descriptions are shown in the official language in which they were submitted.
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~o ~1/04154 1'Cr/US~0/0;392
1--
~ETHOD AND lt~:ANS FC)R CONTROLLING OVERBU~I IN
SPARX-IMAGED LITIIOGRP~PHY PI~TES
This application is a continuation-in-part of Serial No.
07/234,475.
This invention relates to offset lithography. It relates
more specifically to improved lithography plates and method and
apparatus ~or imaging these plates.
BACKG~OUND OF '~IE INVENTION
There are a variety of known ways to print hard copy in
black and white and in color~ The traditional techniques
include letterpress printing, rotogravure printing and offset
printing. These conventional printing processes produce high
quality copies. However, when only a limited number of copies
are required, the copies are relatively expensive. In the case
of letterpress and gravure printing, the maior expe~se results
from the fact that the image has to be cut or etched into the
plate using expensive photographic masking and chemical etching
techniques. Plates are also required in offset lithography.
However, the plates are in the form of mats or films which are
relatively inexpensive to make. The image is present on the
plate or mat as hydrophilic and hydrophobic (and ink-receptive)
surface areas. In wet lithography, water and then ink are
applied to the surface of the plate. Water tends to adhere to
the hydrophilic or water-receptive areas of the plate creating
a thin film of water there which does not accept ink. The ink
does adhere to the hydrophobic areas of the plate and those
inked areas, usually corresponding to the printed areas of the
original document, are transferred to a relatively soft blanket
cylinder and, from there, to the paper or other recording
medium brought into contact with the surface of the blanket
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WO91/04154 PCI/US()~)/()53~)2
--2--
cylinder by an impression cylinder.
Most conventional offset plates are also produced
photographically. In a typical negative-working, subtractive
process, the original document is photographed to produce a
photographic negative. The negative is placed on an aluminum
plate haviny a water-receptive oxide surface that is coated
with a photopolymer. Upon being exposed to light through the
negative, the areas of the coating that received light
(corresponding to the dar~ or prin-ted areas o~ the original)
cure to ~ durable oleophilic or ink-receptive state. The plate
is then subjected to a developing process which removes the
noncured areas of the coating that did not receive light
(corresponding to the light or background areas of the
original). The resultant plate now ~arries a positive or
direct image of the original document.
If a press is to print in more than one color, a separate
printing plate corresponding to each color is required, each of
which is usually madè photographically as aforesaid. In
addition to preparing the appropriate plates for the different
colors, the plates must be mounted properly on the print
cylinders in the press and the angular positions of the
cylinders coordinated so that the color components printed by
the different cylinders will be in register on the printed
copies~
The development of lasers has simplified the production of
lithographic plates to some extent. Instead of applying the
origina- image photographically to the pho~oresist-coated
printing plate as above, an original document or picture is
scanned line-by-line by an optical scanner which develops
strings of picture signals, one for each color~ These signals
are then used to control a laser plotter that writes on and
thus exposes the photoresist coating on the lithographic plate
to cure the coating in those areas which receive lights. That
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~'091/04154 PCI`/U~9~ 3()~ 2 ~ 3
plate is then developed in the usual way by removing the
unexposed areas of the coating to create a direct image on the
plate for that color. Thus, it is st:ill necessary to
chemically etch each plate in order t:o create an image on that
plate.
There have been some attempts tc) use more powerful lasers
to write images on lithographic plates by volatilizing the
surface coating so as to avoid the need for subsequent
developing. However, the use of such lasers ~or this purpose
has not been entirely satisfactory because the coating on the
plate must be compatible with the particular laser which limits
the choice of coating materials. Also, the pulsing frequencies
of some lasers used ~or this purpose are so low that the time
required to produce a hal~tone im~ge on the plate is
unacceptably long.
There have also been some attempts to use scanning E-beam
apparatus to etch away the surface coatings on plates used for
printing. However, such machines are very expensive. In
addition, they require the workpiece, i.e. the plate, be
maintained in a complete vacuum, making such apparatus
impractical for day-to-day use in a printing facility.
An image has also been applied to a lithographic plate by
electro-erosion. The type of plate suitable for imaging in
this fashion and disclosed in U.S. Patent 4,596,733, has an
oleophilic plastic substrate, e.g. Mylar brand plastic film,
having a thin coating of aluminum metal with an overcoating
containing conductive graphite which acts as a lubricant and
protects the aluminum coating against scratching. A stylus
electrode in contact with the graphite containing surface
coating is caused to move across the surface of the plate and
is pulsed in accordance with incoming picture signals. The
resultant current flow between the electrode and the thin metal
coating is by design large enough to erode away the thin metal
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WO91/Oql54 1~Cr/U~9020~3J"4~ ~ 3 3
coating and the overlying conductive graphite containing
surface coating thereby exposing the underlying ink receptive
plastic substrate on the areas of the plate corresponding to
the printed portions of the oriyinal document. This method of
making lithographic plates is disadvantaged in that the
described electro-erosion process on:Ly works on plates whose
conduc~ive surface coatings are very thin and the stylus
electrode which contacts the surface of the plate sometimes
scratches the plate. This degrades the image being written
onto the plate because the scratches constitute inadvertent or
unwanted image areas on the plate which print unwanted marks on
the copies.
Finally, we are aware of a press system, only recently
developed, which images a lithographic plate while the plate i.s
actually mounted on the print cylinder in the press. The
cylindrical surface of the plate, treated to render it either
oleophilic or hydrophilic, is written on by an ink jetter
arranged to scan over the surface of the plate. The ink jetter
is controlled 50 as to deposit on the plate surface a
thermoplastic image-forming resin or material which has a
desired affinity for the printing ink being used to print the
copies. For example, the image-forming material may be
attractive to the printing ink so that the ink adheres to the
plate în the areas t~ereof where the image-forming material is
present and phobic to the "wash" used in the press to prevent
inking of the background areas of the image on the plate.
While that prior system may be satisfactory for some
applications, it is not always possible to provide
thermoplastic image-forming material that is suitable for
jetting and also has the desired affinity (phyilic or phobic)
for all of the inks common~y used for making lithographic
copies. Al~o, ink jet printers are generally unable to produce
small enough ink dots to allow the production of smooth
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WO91/041~4 1'CI'/US(~0/U5392
continuous tones on the printed copies, i.e. the resolution is
not high enough.
Thus, although there have been all the aforesaid efforts
to improve different aspects of lithc,graphic plate produc~ion
and offset printing, these efforts have not reached full
fruition primarily because of the limited number of di~ferent
plate constructions available and the limited number of
different techniques for practically and economically imaging
those known plates. Accordingly, it would be highly desira~le
if new and different lithographic plates became available which
could be imaged by writing apparatus able to respond to
incoming digital data so as to apply a positive or negative
image directly to the plate in such a way as to avoid the need
of subsequent processing of the plate to develop or fix that
image.
SUMMARY OF TH~ INVENTION
Accordingly, the present invention aims to provide various
lithographic plata constructions which can be imaged or written
on to form a positive or negative image therein.
Another object is to provide such plates which can be used
in a wet or dry press with a variety of different printing
inks .
Another object is to provide low cost lithographic plates
which can be imaged electrically.
A further object is to provide an improved method for
imaging lithographic printing plates.
Another object of the invention is to provide a method of
imaging lithographic plates which can be practiced while the
plate is mounted in a press.
Still another object of the invention is to provide a
method for wri~ing both positive and negative or background
images on lithographic plates.
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--6--
Still another object of the invention is to provide such a
method which can be used to apply imagas to a variety o~
different kinds of lithographic plates.
A ~urther object of the invention is to provide a method
of producing on lithographic plates half tone images with
variable dot sizes.
A ~urther object of the invention is to provide improved
apparatus for imaging lithographic plates.
Another object of the invention is to provide apparatus of
this type which applies the images to the plates ef~iciently
and with a minimum consumption of power.
Still another obiect o~ the invention is to provide such
apparatus which lends itself to control by incoming digital
data representing an original document or picture.
Other objects will, in part, be obvious and will, in part,
appear hereinafter. The invention accordingly comprises an
article of manufacture possessing the features and properties
exemplified in the constructions described herein a~d the
several steps and the relation of one or more o~ such steps
with respect to the others and the apparatus embodying the
features of construction, combination of elements and the
arrangement of parts which are adapted to effect such steps,
all as exemplified in the following detailed description, and
the scope of the invention will be indicated in the claims.
In accordance with the present invention, images are
applied to a lithographic printing plate by altering the plate
surface characteristics at selected points or areas o~ the
plate using a non-contacting writing head which scans over the
surface of the plate and is controlled by incoming picture
signals corresponding to the original document or picture being
copied. The writing head utilizes a precisely positioned high
voltage spark discharge electrode to create on the surface of
the plate an intense-heat spark zone as well as a corona zone
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WO91/04154 PCr/US')0/0~3~ 3
in a circular region surrounding the sp~rk zone. I~ response
to the incoming picture signals and ancillary data Keyed in by
the operator such as dot size, screen angle, screen mesh, etc.
and merged with the picture signals, high voltage pulses haviny
precisely controlled voltage and current pro~iles are applied
to the electrode to produce precisely positioned and defined
spark/corona discharges to the plate which etch, erode or
otherwise transform selected points or areas of the plate
surface to ren~er them either recept:ive or non-receptive to the
printing ink that will be applied to the plate to ma}ce the
printed copies
Lithographic plates are made inX receptive or oleophilic
initially by providing them with sur~ace areas consis~ing of
unoxidized metals or plastic materials to which oil and rubber
based inks adhere readily. On the other hand, plates are made
water receptive or hydrophilic inikially in one of three ways.
One plate embodiment is provided with a plated metal surface,
e.g. of chrome, whose topogr~phy or character i~ ~uch that it
is wetted by surface tension. A second plate h~s a surface
consisting of a metal oxide, e.g. aluminum oxide, which
hydrates with water. The third plate construction is provided
with a polar plastic surface which is also roughened to render
it hydrophilic. As will be seen later, certain ones of these~
plate embodiments are suitable for wet printing, others are
better suited for dry printing. Also, dif~erent ones of these
plate constructions are preferred for direct writing; others
are preferred for indirect or background writing.
The present apparatus can write images on all of these
different lithographic plates having either ink receptive or
water receptive surfaces. In other words, i~ the plate sur~ace
is hydrophilic initially, our apparatus will write a positive
or direct image on the plate by rendering oleophilic the points
or areas of the plate surface corresponding to the printed
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~'09l/041~4 rcr/us9n/~ )2
portion of the original document. On the other hand, i~ the
plate sur~ace is oleophilic initially, the apparatus will apply
a background or negative image to the plate surface by
rendering hydrophilic or oleophobic the points or areas of that
sur~ace corresponding to the background or non-printed portion
of the original document. Direct or positive writiny is
usually pre~erred since the amount of plate surface area that
has to be written on or converted is less because most
documents have less printed areas than non-printed areas.
The plate im~ging app~ratu6 incorpora~ing our invcntion is
preferably implemented as a scanner or plotter whose writiny
head consists of one or mo~e spark discharge electrodes. The
electrode (or electrodes) is positioned over the working
sur~ace of the lithographic plate and moved relative to the
plate so as to collectively scan the plate surface. Each
electrode is controlled by an incoming stream of picture
signals which is an electronic representation of an original
document or picture. The signals can originate from any
suitable source such as an optical scanner, a disk or tape
reader, a computer, etc. These signals are formatted so that
the apparatus' spark discharge electrode or electrodes write a
positive or negative image onto the surface of the lithographic
plate that corresponds to the original document.
If the lithoqraphic plates being imaged by our apparatus
are flat, then the spark discharge electrode or slectrodes may
be incorporated into a flat bed scanner or plotter. Usually,
however, such plates are designed to be mounted to a print
cylinder. Accordingly, for most applications, the spark
discharge writing head is incorporated into a so-called drum
scanner or plotter with the lithographic plate being mounted
~- to the cylindrical surface of the drum. Actually, as we shall
see, our invention can be practiced on a lithographic plate
already mounted in a press to apply an image to that plate in
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~0~1/04154 rcr/us~Jo/l~3(~2
situ. In this application, then, the print cylinder itself
constitutes the drum component of the scanner or plotter.
To achieve the requisite relative motion between the spark
discharge writing head and the cylindrical plate, the plate can
be rotated about its axis and the head moved parallel to the
rotation axis so that the plate is scanned circumferentially
with the image on the plate "growing" in the axial direction.
Alternatively, the writing head can move parallel to the drum
axis and after each pass o~ the head, the drum can be
incremented angularly so that the imaye on the plate grows
circumferentially. In both cases, after a complete scan by the
head, an image corresponding to the original document or
picture will have been applied to the surface of the printiny
plate.
As each electrode traverses the plate, it is supported on
a cushion of air so that it is maintained at a very small fixed
distance above the plate surface and cannot scratch that
surface. In response to the incoming picture signals, which
usually represent a half tone or screened image, each electrode
is pulsed ~r not pulsed at selected points in ~he scan
depending upon whether, according to the incoming data, the
electrode is to write or not write at these locations. Each
time the electrode is pulsed, a high voltaqe spark discharge
occurs between the electrode tip and the particular point on
the plate opposite the tip. The heat from that spark discharge
and the accompanying corona field surrounding the spark etches
or otherwise transforms the surface of the plate in a
controllable fashion to produce an image-forming spot or dot on
the plate surface which is precisely defined in terms of shape
and depth of penetration into the plate.
Preferably the tip of each electrode is pointed to obtain
close control over the definition of the spot on the plate that
is affected by the spark discharge from that electrode.
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WO91/04154 Pcr/us(Jo/o~392
--10--
Indeed, the pulse duration, current or voltage controlling the
discharge may be varied to produce ia variable dot on the plate.
Also, the polarity of the voltage a;pplied to the electrode may
be made positive or negative depending upon the nature of the
plate surface to be a~fected by the writing, i.e. depending
upon whether ions need to be pulled from or repelled to the
surface of the plate at each image point in order to transform
the surface at that point to distinguish it imagewise from the
remainder of the plate surface, e.g. to render it oleophilic in
the case of direct writing on a plate whose surface is
hydrophilic. In this way, image spots can be written onto the
plate surface that have diameters in the order of 0.005 inch
all the way down to O.OOOl inch.
After a complete scan of the plate, then, the apparatus
will have applied a complete screened image to the plate in the
form of a multiplicity of surface spots or dots which are
different in their affinity for ink from the portions of the
plate surface not exposed to the sparX discharges from the
scanning electrode.
Thus, using our method and apparatus, high yuality images
can be applied to our special lithographic plates which have a
variety of different plate surfaces suitable for either dry or
wet offset printing. In all cases, the image is applied to the
plate relativ ly quickly and efficiently and in a precisely
controlled manner so that the image on the plate is an accurate
representation of the printing on the original document.
Actually usin~ our technique, a lithographic plate can be
imaged while it is mounted in its press thereby reducing set up
time considerably. An even greater reduction in set up time
results if the invention is practiced on plates mounted in a
multi-color press because correct color registration between
the plates on the various print cylinders can be accomplished
electronically rather than manually by controlling the timings
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. ~VO91/04154 l'Cr/US')0/0~3')2
of the input data applied to the electrodes that control the
writing of the images on the corresponding plates. As ~
consequence of the forgoing c~mbinatLon of features, our method
and apparatus for applying images to lithographic plates and
the plates themselves should receive wide acceptance in the
printing industry.
BRIEF DESCPcIPTION OF I~IE DR~WINGS
For a fuller understanding of the nature and ohjects o~
the invention, reference should be had to the following
detailed description taken in connection with the accompanying
drawings, in which:
FIG. l is a diagrammatic view of an offset press
incorporating a lithographic printing plate made in accordance
with this invention;
FIG. 2 is an isometric view on a larger scale showing in
greater detail the print cylinder portion of the FIG. l press;
FIG. 3 is a sectional view taken along line 3-3 of FIG. 2
on a larger scale showing the writing head that applies an
image to the surface of the FIG. 2 print cylinder, with the
: associated electrical components being represented in a block
~: diagram; and
FIGS. 4A to 4F are enlarged sectional views showing imaged
lithographic plates incorporating our invention;
~: FIG. 5A depicts the tendency of non-overlapping image
points to leave exposed surface area there between;
:~; : FIG. 5B depicts the effect of overlapping image points to
expose the interstitial surface area; and
FIG. 5C illustrates the manner in whi~h overlapping image
pints can produce adverse image effects~
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WO ~1/04154 1'Cr/US90/05392
--12--
DESCRIPTION OF THE PREFERRED hrMBODIM~:NTS
. .... .. _ .
Refer first to FIG. 1 of the drawings which shows a more
or less conventional offset press shown generally at lo which
can print copies using lithographic plates made in accordance
with ~his invention.
Press 10 includes a print cylinder or drum 12 around which
is wrapped a lithographic plate 13 whose opposite edge margins
are secured to the plate by a conventional clamping mechanism
12a incorporated into cylinder 12. Cylinder 12, or more
precisely the plate 13 thereon, contacts the surface of a
blanket cylinder 14 which, in turn, rotates in contact with a
large diameter impression cylinder 16. The paper sheet P to be
printed on is mounted to the surface of cylinder 16 so that it
passes through the nip between cylinders 14 and 16 before being
discharged to the exit end of the press 10. Ink for inking
plate 13 is delivered by an ink train 22, the lowermost roll
22a of which is in rolling engagement with plate 13 when press
10 is priilting. As is customary in presses of this type, the
various cylinders are all geared together so that they are
driven in unison by a single drive motor.
The illustrated press 10 is capable of wet as well as dry
printing. Accordingly, it includes a conventional dampening or
water fountain assembly 24 which is movable toward and away
from drum 12 in the directions indicated by arrow A ln FIG. 1
between active and inactive positions. Assembly 24 includes a
conventional water train shown generally at 26 which conveys
water from a tray 26a to a roller 26b which, when the dampening
assembly is active, is in rolling engagement with plate 13 and
the intermediat~ roller 22b of ink train 22 as shown in phantom
in FIG. 1.
When press 10 is operating in its dry printing mode, the
; dampening assembly 24 is inactive so that roller 26b is
~:
` WO91/04154 pcr/us9o/o~23(J~ 2
13~
retracted from roller 22b and the plate as shown in solid lines
in FIG. l and no water is applied to the plate. -The
lithographic plate on cylinder 12 in this case is designed for
such dry printing. See for example plate 13~ in FIG. 4D. It
has a surface which is oleophobic or non-receptive to ink
except in those areas that have been written on or imaged to
make them oleophilic or receptive to ink. As the cylinder 12
rotates, the plate is contacted by the ink- ~oated roller 22a
of ink train 2Z. The areas of the plate surface that have been
written on and thus made oleophilic pick up ink from roller
22a. Those areas of the plate surface not written on receive
no ink. Thus, after one revolution of cylinder 12, the image
written on the plate will have been inked or developed. That
image is then transferred to the blanket cylinder 14 and
finally, to the paper sheet P which is pressed into contact
with the blanket cylinder.
When press l0 i5 operating in its wet printing mode, the
dampening assembly 24 is active so that the water roller 26b
contacts ink roller 22b and the surface of the plate 13 as
shown in phantom in FIG. l. Pl~ke 13, which is described in
more detail in connection with FIG. 4A, is intended for wet
printing. It has a surface which is hydrophilic except in the
areas thereof which have been written on to make them
oleophilic. Those areas, which correspond to the printed areas
of the original document, shun water. In this mode of
operation, as the cylinder 12 xotates (clockwise in FIG. l),
water and ink are presented to the surface of plate 13 by the
; ~ rolls 26b and 22a, respectivel~. The water adheres to the
hydrophilic areas of that surface corresponding to the
background of the original document and those areas, being
coated with water, do not pick up ink from roller 22a. On the
other hand, the oleophilic areas of the plate surface which
hav- not been wetted by roller 26, pick up ink from roller 22a,
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~'09l/04154 l'CI/US90/0~392
again forming an inked image on the surface of the plate. As
before, that image is transferred via blanket roller 14 to the
paper sheet P on cylinder 16.
While the image to be applied to the lithographic plate 13
can be written onto the plate while the plate is "o~f press",
our invention lends itself to imaging the plate when the plate
is mounted on the print cylinder 12 and the apparatus for
accomplishing this will now be described with reference to FIG.
2. As shown in FIG. 2, the print cylinder 12 is rotatively
supported by the press frame lOa and rotated by a standard
electric motor 34 or other conventional means. The angular
position of cylinder 12 is monitored by conventional means such
as a shaft encoder 36 that rotates with the motor armature and
associated detector 36a If higher resolution is needed, the
angular position of the large diameter impression cylinder 16
may be monitored by a suitable magnetic detector that detects
the teeth of the circumferential drive gear on that cylinder
which gear meshes with a similar gear on the print cylinder to
rotate that cylinder.
Also supported on frame lOa adjacent to cylinder 12 is a
~ writing head assembly shown generally at 42. This assembly
:~ ~ ; comprises a lead screw 42a whose opposite ends are rotativçly
~: supported in the press frame lOa, which frame also supports the
opposite ends of a guide bar 42b spaced parallel to lead screw
42a. Mounted for movement along the lead screw and guide bar
: is a carriage 44. When the lead screw is rotated by a step
motor 46, caxriage 44 is moved axially with respect to print
cylinder 12.
The cylinder driye motor 34 and step motor 46 are opexated
:. in synchronism by a controller 50 ~FIG. 3), which also receives
~signals from detector 36a, so that as the drum rotates, the
~ carriage 4~ moves axially along the drum with the controller
; "knowing" the instantaneous relative position of the carriage
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~'09~/04l5~ r~cr/uss()/~s
and cylinder at any given moment. The control circuitry
required to accomplish this is already very well known in the
scanner and plotter art.
Refer now to FIG. 3 which depicts an illustrative
embodiment of carriage 44. It includes a block 52 having a
threaded opening 52a for threadedly receiving the lead screw
42a and a second parallel opening 52b for slidably receiving
the guide rod 42b. A bore or recess 54 extends in from the
underside of block 52 for slidably receiving a discoid writing
head 56 made of a suitable rigid electrical insulating
material. An axial passage 57 extends through head 56 for
snugly receiving a wire electrode 58 wh,ose diameter has been
exaggerated for clarity. The upper end 58a o~ the wire
electrode is received and anchored in a socket 62 mounted to
the top of head 56 and the lower end 58b of the electrode 58 is
preferably pointed as shown in FIG. 3. Electrode 58 is made of
an electrically conductive metal, such as thoriated tungsten,
capable of withstanding very high temperatures. An insulated
conductor 64 connects socket 62 to a terminal 64a at the top of
block 52. I~ the carriage 44 has more than one electrode 58,
similar connections are made to those electrodes so that a
plurality of points on the plate 13 can be imaged
simultaneously by assembly ~2.
Also formed in hsad 56 are a plurality of small air
passages 66. These passages are dlstributed around electrode
58 and the upper.ends of the passages are connected by way of
~lexible tubes or hoses 68 to a corresponding plurality of
vertical passages 72. These passages extend from the inner
wall of block bore 54 to an air manifold 74 inside the block
which has an inlet passage 76 extending to the top of the
block. Passage 76 is connected by a pipe 78 to a source of
pressurizad air. In the line from the air source is an
adjustable valve 82 and a flow restrictor 84. Also, a branch
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~'091/04154 1'Cr/US90/053')2
-16-
li~e 78a leading from pipe 78 downst:ream from restrictor 84
connects to a pressure sensor so which produces an output for
controlling the setting of valve ~2
When the carriage 44 is positioned opposite plate 13 as
shown in FIG. 3 and air is supplied to its manifold 74, the air
issues from the lower ends of passages 66 with sufficient force
to support the head above the plate surface. The bac~ pressure
in passages 66 and manifold 74 varies directly with the spacing
of head 56 from the surface of plate 13 and this back pressure
is sensed by pressure sensor 90. The s~nsor controls valve 82
to adjust the air flow to head 5~ so that the tip 58b of the
needle electrode 58 is maintained at a precisely controlled
very small spacing, e.g. O.OOOl inch, above the surface of
plate 13 as the carriage 44 scans along the surface of the
plate.
Still referring ~o FIG. 3, the writing head 56, and
particularly the pulsing of its electrode 58, is controlled by
a pulse circuit 96. One suitable circuit comprises a
transformer 98 whose secondary winding 98a is connected at one
end by way of a variable resistor 102 to terminal 64a which, as
noted previously, is connected electrically to electrode 58.
The opposite end of winding 98a is connecked to electrical
ground. The transformer primary winding 9Rb is connected to a
DC voltage source 104 that supplies a voltage in the order of
lO00 volts. The transformer primary circuit include~ ~ large
capacitor 106 and a resistor 107 in series. The capacitor is
maintained at full voltage by the resistor 107. An electronic
switch 108 is connected in shunt with winding 98b and the
capacitor. This switch is controlled by switching signals
received from controller 50~
It should be understood that circuit 96 specifically
illustrated is only one of many known circuits that can be used
to provide variable high voltage pulses of short duration to
~091/0415~ l~CI/U~9~1/()5~ 3
-17-
electrode 58. For example, a high voltage switch and a
capacitor-regenerating resistor may be used to avoid the need
for transformer 98. Also, a bias voltage may be applied to the
electrode 58 to provide higher voltage output pulses to the
electrode without requiring a high voltage rating on the
switch.
When an image is being written on plate 13, the press 10
is operated in a non-print or imaging mode with both the ink
and water rollers 22a and 26b being disengaged ~rom cylinder
12. The imaging of plate 13 in press 10 is con~rolled by
controller 50 which, as noted previously, also controls the
rotation of cylinder 12 and the scanning of the plate by
carriage assembly 42. The signals for imaging plate 13 are
applied to controller 50 by a convenkional source o~ picture
signals such as a disk reader llq. l'he controller 50
synchronizes the image data from disk reader ~14 with the
control signals that control rotation of cylinder 12 and
movement of carriage 44 50 that when the electrode 58 is
positioned over uniformly spaced image points on the plate 13,
switch 108 is either closed or not closed depending upon
whether that particular point i~ to be written on or not
written on.
If that point is not to be writ~en on, i.e. it corresponds
to a location in the backyround o~ the original document, the
electrode is not pulsed and proceeds to the next image point.
On the other hand, if that point in the plate does correspond
to a location in the printed area of the original document,
switch 108 is closed. The closing of that switch discharges
capacitor 106 so that a pxecisely shaped, i.e. squarewave, high
voltage pulse, i.e. 1000 volts, of only about one microsecond
duration is applied to trans*ormer 98. The transformer applies
a stepped up pulse. of about 3000 volts to electrode 58 causing
a spark discharge S between the electrode tip 58b and plate 13.
.
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WO9l/04154 PCr/(JS90/05392
-18-
That sparks and the accompanying corona field S' surroundiny
the spark zone etches or transforms the surface of the plate at
the point thereon directly opposite the electrode tip 58b to
render that point either receptive or non-receptive to ink,
depending upon the type of surface on the plate.
The transformations thak do occur with our dif~erent
lithographic plate constructions will be described in more
detail later. Suffice it to say at this point, that resistor
102 is adjusted ~or the different plate embodiments ~o produce
a spark discharge that writes a clearly defined image spot on
the plate surface which is in the order of 0.005 to 0.0001 inch
in diameter. That resistor 102 may be varied manually or
automatically via controller 50 to produce dots of variable
size. Dot size may also be varied by varying the voltage
and/or duration of the pulses that produce the spark
discharges. Means for doing this are quite well known in the
art. Likewise, dot size may be varied by repeated pulsing of
the electrode at each image point, the number of pulses
determining the dot size (pulse count modulation). If the
electrode has a pointed end 58b as shown and the gap between
tip 58b and the plate is made very small, i.e. 0.001 inch, the
spark discharge is focused so that image spots as small as
0.0001 inch or even less can be formed while keeping voltage
requirements to a minimum. The polarity of the voltage applied
to the electrode may be positive or negative although
preferably, the polarity is selected according to whether ions
need to be pulled from or repelled to the plate surface to
effect the desired surface transformations on the various
plates to be described.
As the electrode 58 is scanned across the plate surface,
it can be pulsed at a maximum rate of about 500;000 pulses/sec.
However, a more typical rate is 25,000 pulses/sec. Thus, a
broad range of dot densities can be achieved, e.g. 2,000
V09l/04154 Pcr/us~)o/o~29~ ~ 2 ~ ~ ~
-19-
dots/inch to 50 dots/inch. Th~ dots can be printed side-by-
side or they may be made to overlap so that substantially loo~
of the surface area of the plate can be imaged. Thus, in
response to the incoming data, an image corresponding to the
original document hu~lds up on th~ plate ~ur~ce conGtituted by
the points or spots on the plate surface that have been etched
or transPormed-by the spark discharg~e S, as compared with the
areas of the plate surface that have not been so a~ected by
the spark discharge.
In the case of axial scanning, then, after one revolution
of print cylinder 12, a complete image will have been applied
to plate 13. The press 10 can then be operated in its printing
mode by moving the ink roller ~2a to its inking position shown
in solid lines in FIG. 1, and, in the case of wet printing, by
also shifting the water fountain roller 26b to its dotted line
position shown in FIG. 1. As the plate rotates, ink will
adhere only to the image points written onto the plate that
correspond to the printed portion of the original document.
That ink image will then be trans~erred in the usual way via
blanket cylinder 14 to the paper sheet P mounted to cylinder
16.
Forming the image on the plate 13 while the plate is on
the cylinder 12 provides a number of advantages, the most
important of which is the significant decrease in the
preparation and set up time, particularly if the invention is
incorporated into a multi-color press. Such a press includes a
plurality of sections similar to press 10 described herein, one
for each color being printed. Whereas normally the print
cylinders in the different press sections after the first are
adjusted axially and in phase so that the different color
images printed by the lithographic plates in the various press
sections will appear in register on the printed copies, it is
apparent fxom the foregoing that, since the images are applied
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WO9l/0~154 I~CI/US~0/0~3')2
-20--
to the plates 13 while they are mounted in the press sections,
such print registration can be accomplished electronically in
the present case.
More particularly, in a multicolor press, incorporating a
plurality of press sections similar to press 10, the controller
50 would adjust the timings of the picture signals controlling
the writing of the images at the second and subsequent printing
sections to write the image on the lithographic plate 13 in
each such station with an axial and/or angular of fset that
compensates for any misregistration with respect to the image
on the first plate 13 in the press. In other words, instead of
achieving such registration by repositioning the print
cylinders or plates, the registration errors are accounted for
when writing the images on the plates. Thus once imaged, the
plates will automatically print in perfect register on paper
sheet P.
Refer now to FIGS. 4A to 4F which illustrate various
lithographic plate embodiments which are capable of being
imaged by the apparatus depicted in FIGS. 1 to 3. In FIG. ~A,
the plate 13 mounted to the print cylinder 12 comprises a steel
base or substrate layex ~3a having a flash coating 13b of
copper metal which is, in turn, plated over by a thin layer 13c
of chrome metal. As described in detail in U.S. Patent
4,596,760, the plating process produces a surface topography or
texture which is hydrophilic. Therefore, plate 13 is a
preferred one for use in a dampening~type offset press.
During a writillg operation on plate 13 as described above,
voltage pulses are applied to electrode 58 50 that spark
discharges S occur between the electrode tip 58b and the
surface layer 13c of plate 13. Each spark discharge, coupled
with the accompanying corona field S' surrounding the sparX
zone, melts the surface of layer 13c at ths imaging point I on
that surface directly opposite tip 5~b. Such melting suffices
.. . .. . . .. . ... . . 1.
~O91/041S4 l'Cr/US90/053
--21-
to modify the surface structure or topography at that point on
the surface so that water no longer tends to adhere to that
surface area. Accordingly, when plate 13 is imaged in this
fashion, a multiplicity of non-water-receptive spots or dots I
are formed on the otherwise hydrophi].ic plate surface, which
spots or dots represent the printed portion of the oriyinal
document being copied.
When press 10 is operated in its wet prinkiny mode, i.e.
with dampening assembly 24 in its position shown in phantom in
FIG. 1, the water from the dampening roll 26b adheres only to
the surPace areas of plate 13 that were not subjected to the
spark discharges from electrode 58 during the imaging
operation. on the other hand, the ink from the ink roll 22a
does adhere to those plate surface areas written on, but does
not adhere to the sur~ace areas of the plate where the water or
wash solution is present. When printing, the ink adhering to
the plate, which forms a direct image of the original document,
is transferred via the blanket cylinder 14 to the paper sheet P
on cylinder 16. While the polarity of the voltage applied to
electrode 58 during the imaging proaess described above can be
positive or negative, we have found that ~or imaging a plate
with a bare chrome surface such as the one in FIG. 4A, a
positive polarity is preferred because it enables better
control over the formation of the spots or dots on the surface
of the plate.
FIG. 4B ill~strates another plate embodiment which is
written on directly and used in a dampening-type press. This
plate, shown generally at 122 in FIG. 4B, has a substrate 124
made of a metal such as aluminum which has a structured oxide
~; surface layer 126. This surface layer may be produced;by any
one of a number of known chemical treatments, in some cases
assisted by the use of fine abrasives to roughen ~he plate
~; surface. The controlled oxidation of the plate surface is
.
: ~ :
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~'O~l/04154 1'1/lJS~)~/0~3~2
commonly called anodizing while the surface structure o~ the
plate is referred to as grain or graininy. As part of the
chemical treatment, modifiers such as silicates, phosphates,
etc. are used to stabilize the hydrophilic character of the
plate surface and to promote both adhesion and the stability of
the photosensitive layer(s) that are coated on the plates.
The aluminum oxide on the surface of the plate is not the
crystalline structure associated with corundum or a laser ruby
(both are aluminum oxide crystals), and shows considerable
interaction with water to form hydrates of the form Al2O3~2O.
This interaction with contributions ~rom silicate, phosphake,
etc. modifiers is the source of the hydrophilic nature of the
plate surface. Formation of hydrates is also a problem when
the process proceeds uncheckedO Eventually a solid hydrate
mass forms that effectively plugs and eliminates the structure
of the plate surface. Ability to effectively hold a thin film
of water required to produce nonimage areas is thus lost which
renders the plate useless. Most plates are supplied with
photosensitive layers in place that protect the plate s~r~aces
until the time the plates are exposed and developed. At this
point, the plates are either immediately used or stored for u~e
at a latter time. If the plates are stored, they are coated
with a water soluble polymer to protect hydrophilic surfaces.
This is the process usually referred to as gumming in the
trade. Plates that are supplied without photosensitive layers
are usually treated in a similar manner.
The loss of hydrophilia character during storage or
extended interruptions while the plate is being used is
generally referred to as oxidation in the trade. Depending on
the amount of structuring and chemical modifiers used, there is
a considerable variation in plate sensitivity to excessive
hydration.
When the plate 122 is subjected to the spark discharge
: :: : :, : ~.
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~2~3~
~\'091/04154 PCr/US~0/0~392
-23-
from electrode 58, the heat from the spark S and associated
corona S' around the spark zone renders oleophilic or ink
recepkive a precisely defined image point I opposite the
electrode tip 58b.
The behavior of the imaged aluminum plate suggests that
the image points I are ths result of combined partial
processes. It is believed that dehydration, some formation of
fused aluminum oxide, a~d the melting and transport to the
surface of aluminum metal occur. The comhined effects of the
three processes, we suppose, reduce the hydrophilic character
of the plate surface at the image point. Aluminum is
chemically reac~ive with the result that the metal i5 alway~
found with a thin oxide coating regardless of how smooth or
bright the metal appears. This oxide coating does not exhibit
a hydrophilic character, which agrees with our observation that
an imaged aluminum-based plate can be stored in air more than
24 hours without the loss of an image. In water, aluminum can
react rapidly under both basic and acidic conditions including
several electrochemical reactions. The mildly acidic fountain
solutions used in presses are believed to have this effect on
the thin films of aluminum exposed during imaging resulting in
their removal.
Because of the above-mentioned ability of the imaged
surface areas of the plate to react with water, protection of
the just~imaged plate 122 requires that the plate surface be
shielded from contact with water or water-based material
This may be done by applying ink to the plate without the use
of a dampening or fountain solution, i.e. with water roll 26b
disengaged in FIG. l. This results in the entire plate surface
being coated with a layer of ink. Dampening water i5 then
applied (i.e. the water roll 2Sb is engaged) to the plate.
Those areas of the plate that were not imaged acquire a thin
film of water that dislodges the overlying ink allowing its
.
. . . ~ . . . ,:
. . .
,~ :
2 ~ 3 3 ~
WO~1/04154 l~cr/us9o/o~3()2
_ Z ~
removal from the plate. The plate areas thak were imaged do
not acquire a thin film of water with the result that the ink
remains in place.
The images generated on a chrome plate show a similar
sensitivity to water contact preceding ink contact. However,
after the ink application step, the images on a chrome plate
are more stable and the plate can be run without additional
steps to preserve the image.
The ink remaining on the image points I is c~uite fragile
and must be left to dry or set so that the ink becomes more
durable. Alternatively, a standard ink which cures or sets in
response to ultraviolet light or heat may be used with plate
122. In this event, a standard ultraviolet lamp 126 may be
mounted adjacent to print cylinder 12 as depicted in FIGS. 1
and 2 to cure the particular ink. The lamp 126 should extend
the full length of cylinder 12 and be supported by ~rame
members 10_ close to the surface of cylinder 12 or, more
particularly, the lithographic plate thereon.
We havs found that imaging a plate such as plate 122 based
on aluminum is optimized if a negative voltage is applied to
the imaging electrode 58. This is because positive aluminum
ions produced at each image point migrate well in the hiyh
intensity current flow of the spark discharge and will move
toward the negative electrode.
FIG. 4C shows a plate embodiment 130 suitable for direct
imaging in a press without dampening. Plate 130 comprises a
substrate 132 made of a conductive metal such as aluminum or
steel. The substrate carries a thin coating 134 of a highly
oleophobic material such as a fluoropolymer or silicone. One
suitable coating material is an addition-cured relsase coating
marketed by Dow Corning under its designation SYL-OFF 7044~
Plate 130 is written on or imaged by decomposing the surface of
coating 134 using spark discharges from electrode 58. The heat
-: . . . . . , ,.............. , . ;-. .
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2~ 3
~'09l/04154 ~'CI/US90/()~3')2
-25-
from the spark and associated corona decompose the silicone
coating into silicon dioxida, carbon dioxide, and water.
Hydrocarbon fragments in trace amounts are also possible
depending on the chemistry of the silicone polymers used.
Silicone resins do not have carbon in their backbones which
means various polar structures such as C-OH are not ~ormed.
Silanols, which are Si-oH structures are po~sible structures,
but these are reactive which means they react to form other,
stable structures.
Such decomposition coupled with surface roughening of
coating 134 due to the spark discharge renders that surface
oleophilic at each image point I directly opposite the tip of
electrode 58. Preferably tha~ coating is made quite thin, e.g.
0.0003 inch to minimize the voltage required to break down the
material to render it ink receptive. Resultantly, when plate
130 i~ inked by roller 22a in press 10, ink adheres only to
those transformed image points I on the plate surface. Areas
of the plate not so imaged, corresponding to the background
area of the original document to be printed, do not pick up ink
from roll 22a. The inked image on the plate is then
transferred by blanket cylinder 14 to the paper sheet P as in
any conventional offset press.
FIG. 4D illustrates a lithographic plate 152 suitable for
indirect imaging and for wet printing. The plate 152 comprises
a substrate 154 made of a suitable conductive metal such as
aluminum or copper. Applied to the surface of substrate 154 is
a layer 156 of phenolic resin, parylene, diazo-resin or other
such material to which oil and rubber-based inks adhere
readily. Suitable positive working, subtractive plates of this
type are available from the Enco Division of American Hoechst
Co. under that company's designation P-800.
When the coating 156 is subjected to a spark discharge
from electrode 58, the image point I on ~he surface of layer
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:
2~2~3~
-26-
156 opposite the electrode tip 58b decomposes under the heat
and becomes etched so that it readily accepts water. Actually,
if layer 156 is thick enough, substrate 154 may simply be a
separate flat electrode member disposed opposite the electrode
58. Accordi~gly, when the plate 152 is coated with water and
ink by the rolls 26_ ancl 22a, respectively, of press 10, water
adheres to the image points I on plate 152 Eormed by the spark
discharges from electrode 58. Ink, on the other hand, shuns
those water-coated surface points on the plate corresponding to
the background or non-printed areas of the original document
and adheres only to the non-imaged areas of plate 152.
Another offset plate suitable for indirect writing and for
use in a wet press is depicted in FIG. 4E. This plate, indi-
cated at 162 in that figure, consists simply of a metal plate,
for example, copper, zinc or stainless steel, having a clean
and polished surfa~e 162a. Metal surfaces such as this are
normaIly oleophilic or ink-receptive due to surface tension.
When the surface 162a is subjected to a spark discharge from
electrode 58, the spark and ancillary corona field etch that
surface creatiny small capillaries or fissures in the surface
; at the image point I opposite the electrode tip 58k which tend
to be receptive to or wick up water. Therefore, during print-
ing the image points I on plate 162, corresponding to the back-
ground or non-printed areas of the original document, receive
water from roll 26_ of press 10 and shun ink from the ink roll
22a. Thus ink adher s only to the areas of plate 162 that were
not subjected to spark discharges from electrode 58 as de-
scribed above and which correspond to the printed portions of
the original document.
Refer now to FIG. 4F which illustrates still another plate
embodiment 172 suita~le for direct imaging and for use in an
offset press without dampening. We have found that this novel
plate 172 actually produces the best results of all of the
: `~
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\~'09l/04l54 ~l/U~ 5~ QJ 3
-27-
plates described herein in terms of the quality and use~ul life
of the image impressed on the plate.
Plate 172 comprises a base or substrate 174, a base coat
or layer 176 containing pigment or particles 177, a thin
conductive metal layer 178, an ink repellent silicone top or
surface layer 184, and, if necessary, a primer layer 186
between layers 178 and 184.
1. Substrate 174
The material of substrate 174 should have mechanical
strength, lack of extension (stretch) and heat resistance.
Polyester film meets all these requirements well and is readily
available. Dupont's Mylar and ICI's Melinex are two
commercially available films. Other films that can be used for
substrate 174 are those based on polyimides (Dupont's Kapton)
and polycarbonates (GE's Lexan). A preferred thickness is
0.005 inch, but thinner and thicker versions can be used
effectively.
There is no reyuirement for an optically clear film or a
smooth film surface (within reason). The use of pigmented
films including ~ilms pigmented to the point of opacity are
feasible for the substrate, providing mechanical properties are
not lost.
2. Base Coat 176
An important feature of this l~yer is that it is strongly
textured. In this case, "texturedi' means that the surface
topology has numerous peaks and valleys. When this surface is
coated with the thin metal layer 178, the projecting peaks
create a surface that can be described as containing numerous
tiny electrode tips (point source electrodes) to which the
spark from the imaging electrode 58 can jump. This texture is
conveniently created by the filler particles 177 included in
the base coat, as will be described in detail hereina~ter under
the section entitled Filler Particles 177. Other requirements
~.
2 ~ 3
WO91/04154 P~r/US'30/053
-2~-
of base coat 176 include:
a) adhesion to the substrate 174;
b) metallizable using typical processes such as vapor
deposition or sputtering and providing a surface to
which the metal(s) will adhere strongly;
c) resistance to the components of of~set printing inks
and to the cleaning materials used with these inks;
d) heat resistance; and
e~ flexibility equivalent to the substrate.
The chemistry of the base coat that can be used is wide
ranging. Application can be from solvents or from water.
Alternatively, 100% solids coatings such as characterize
conventional UV and EB curable coating can be used. A number
of curing methods (chemical reactions that create crosslinking
o~ coating components) can be used to establish the performance
properties desired of the coatings. Some of these are:
a) Thermoset Typi~al thermoset reactions are those as an
aminoplast resin with hydroxyl sites of the primary
coating resi~. These reactions are greatly
accelerated by creation of an acid environment and the
use o~ heat.
b) Isocyanate Based One typical approach are two part
urethanes in which an isocynate component reacts with
~;~hydroxyl sites on one or more "backbone" resins often
referred to as the "polyol" component. Typical
polyols include polyethers, polyesters, an acrylics
having two or more hydroxyl functional sites.
Important modifying resins include hydroxyl ~unctional
vinyl resins and celluIose ester resins. The
isocya~ate component will have two or more isocyanate
groups and is either monomeric or oligomeric. The
reactions will proceed at ambient temperatures, but
can be accelerated using heat and selected catalysts
:: ~
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W091/0~154 rcr/US9~/o73~2~ 3
-29-
which include tin compounds and tertiary amines. The
normal technique is to mix the isocynate functional
component(s) with the polyol component(s) just prior
to use. The reactions beg:in, but are slow enough at
ambient temperatures to al:Low a "potlife" during which
the coating can be applied.
In another approach, the isocyanate is used in a
"blocked'l form in which the isocyanate component has
been reacted with another component ~uch as a phenol
or a ketoxime to produce an inactive, metastable
compound. This compound is designed for decomposition
at elevated temperatures to liberate the active
iSOGyanate component which then reacts to cure the
coating, the reaction being accelerated by
incorporation of appropriate catalysts in the coating
formulation.
c) Aziridines The typical use is the crosslinking of
.
waterborne coatings based on carboxyl functional
resins. The carboxyl groups are incorporated into the
resins to provide sites that form salts with water
soluble amines, a reaction integral to the
solubilizing or dispersing of the resin in water. The
; reaction proceeds at ambient temperatures a~ter the
water and solubilizing amine(s) have been evaporated
upon deposition of the coating. The aziridines are
added to the coating at the time of use and have a
potlife governed by their rate of hydrolysis in water
to produce inert by-products.
d) Epoxy Reactions The elevated temperatures cure of
boron trifluoride complex catalyzed resins can be
used, particularly for resins based on cycloaliphatic
epoxy functional groups. Another reaction is based on
W exposure generated cationic catalysts for the
'
:
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~'O~I/04154 I~C~ S~ 3~ 3 3
-30-
reaction. Union Carbide's cyracure system is a
commercially available version.
e) Radiation Cures are usually free radical
polymerizations of mixtures of monomeric and
oligomeric acrylates and methacrylates. Free radicals
to initiate the reaction are created by exposure of
the coating to an electron beam or by a
photoinitiation system incorporated into a coating to
be cured by W exposure. The choice of chemistry to
be used will depend on the type of coating equipment
to be used and environmental concerns rather than a
limitation by required performance properties. A
crosslinking reaction i5 also not an ab~olute
requirement. For example, there are resins soluble in
a limited range of solvents not including those
typical of offset inks and their cleaners that can be
used.
3. Filler Particles l?7
The filler particles 177 used to create the important
surface structure are chosen based on the following
considerations:
a) the ability of a particle 177 of a given size to
contribute to the surface structure of the base coat
176. This is dependent on the thickness of the
coating to be deposited. This is illustrated for a 5
micron thick (.0002 inch) coat 176 pigmented with
particles 177 of spherical geometry ~hat remain well
dispersed throughout deposition and curing of the
coat. Particles with diameters of 5 microns and less
would not be expected to contribute greatly to the
surface structure because they could be contained
within the thickness of the coating. Larger
, ~ . . ;, . .. ~ ~, i: ;
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~'09l/0~154 PCr/US90/()~39
-31-
particles, e.g. 10 microns ln diameter, would make
significant contributions because they could project 5
microns above the base coat 176 sur~ace, creating high
points that are twice the average thickness o~ that
coat.
b) the geometry of the particles 177 is important.
Equidimensional particles ~3uch as the spherical
particles described above and depicted in FIG. 4F will
contribute the same degree regardless of particle
orientation within the base coat and are therefore
preferred. Particles with one dimen~ion much greater
than the others, acicular types being one example, are
not usually desirabl~. These particles will tend to
orient themselves with their long dimensions parallel
~to the surface of the coating, creating low rounded
ridges rather than the desirable distinct peaks.
Particles that are platelets are also undesirable.
; These particles tend to orient themselves with their
broad dimensions (faces) parallel to the coating
surface, thereby creating low, broad, rounded mounds
rather than desirable, distinct peaks.
c) the total particle content or density within the
coating is a function of the image density to be
encountered. For example, if the plate is to be
imaged at 400 dots per centimeter or 160,000 dots per
square centimeter, it would be desirable to have at
least that many peaks (particles) present a~d
positioned so that one occurs at each of the possible
positions at which a dot may be created. For a
coating 5 microns thick, with peaks produced by
; ~ individual particles 177, this would correspond to a
density of 3.2 x 108 particles/cubic centimeter (in
the dried, cured base coat 176~.
::
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W091/041S4 rCr/US90/0~39
-32-
Particle sizes, geometries, ancl densities are readily
available data for most filler particle candidates, but there
are two important complications. Particle sizes are averages
or mean valves that describe the distribution of sizes that are
characteristic of a given powder or pigment as supplied. This
means that both larger and smaller sizes than the average or
mean are present and are significanl; contributors to particle
size considerations. Also, there is always some degree of
particle association present when particles are dispersed into
a fluid medium, which usually increases during the application
and curing of a coating. Resultantly, peaks are produced by
groups of particles, as well as by individual particles.
Preferred filler particles 177 include the following:
a) amorphous silicas (via various commercial processes)
b) microcrystalline silicas
c) synthetic metal oxides (single and in multi-component
mixtures)
d) metal powders (single metals, mixtures and alloys)
e1 graphite (synthetic and natural)
f) carbon black (via various commercial processes)
Preferred particle sizes for the filler particles to be
used is highly dependent on the thickness oP the layer 176 to
be deposited. For a 5 micron thick layer (preferred
application), the preferred sizes fall into one of the
following two ranges:
;~ ; a) 10 ~/- 5 microns for particles 177 that act
predominantly as individuals to create surface
structure, and
b) 4 +/- 2 microns for particles that act as groups
(agglomeFates) to create surface structure.
.
t~
~'091/~41~4 PCI~/US9~)/05392
For both particle ranges, it should be under~tood that
larger and smaller sizes will be present as part of a size
distribution range, i.e. the values given are for the average
or mean particle size.
The method of coating base layer 176 with the particles
177 dispersed therein onto the substrate 174 may be by any o~
the currently available commercial coating processes.
A preferred application of the ba.se coat is as a layer 5
~/- 2 microns thick. In practice, it is expected that base
coats could range from as little as 2 microns to as much as lO
microns in thickness. Layers thicker than 10 microns are
possible, and may be required to produce plates of high
durability, but there would be considerable difficult~ in
texturing these thick coatings via the use of filler pigments.
Also, in some cases, the base coat 176 may not be required
if the substrate 174 has the proper, and in a sense equivalent,
properties. More particularly, the use for substrate 17~ of
films with surface textures ~structures) created by mechanical
means such as embossing rolls or by the use of filler pigments
may have an important advantage in some applications provided
they meet two conditions:
a) the films are metalizable with the deposited metal
~orming layer 178 having adequate adhesion; and
b) their film surface texture produces the important
feature of the hase coat described in detail above.
4. Thin Metal LaYer 178
This layer 178 is important to ~ormation of an image and
must be uniformly present if uniform imaging of the plate is to
occur. The image carrYing (i.e. inX receptive) areas of the
plate 172 are created when the spark discharge volatizes a
portion of the thin metal layer 178. The size of the feature
: ~:
:
~:
~2~3~
WO91/04154 ~'Cr/US')()/~53~)2
-3~-
formed by a spark discharge ~rom electrode tip s8b of a given
energy is a function of the amount of metal that i5 volatized.
This is, in turn, a function of the amount of metal present and
the energy required to volatize the metal used. An important
modifier is the energy available from oxidation of the
volatized metal ~i.e. that can contribute to the volatizing
process), an important partial process present when most metals
~re vaporized into~a routine or ambient atmosphere.
The metal preferred for layer 178 is aluminum, which can
be applied by the process of vacuum mctallization ~most
commonly used) or sputtering to create a uni~orm layer 300
lO0 Angstroms thick. Other suitable metals include chrome,
copper and zinc. In general, any metal or metal mixture,
including alloys, that can be deposited on base coat 176 can be
made to work, a consideration since the sputtering process can
then deposit mixtures, alloys, refractories, etc. Also, the
thickness of the deposit is a variable that can be expanded
outside the indicated range. That is, it is possible to image
a plate through a lO00 Angstrom layer o~ mct~ nd to im~gc
layers less than lO0 Angstroms thick. The use of thicker
layers reduces the size of the image formed, which is desirable
when resolution is to be improved by using smaller size images,
points or dots.
5. Primer 1~6 (when reouired~
The primer layer l36 anchors the ink repellent silicone
coating 184 to the thin metal layer 178. Effective primers
include the following:
a) silanes (monomers and polymeric forms)
b. titanates
c) polyvinyl alcohols
d) polyimides and polyamide-imides
Silanes and titanates are deposited from dilute solutions,
typically 1-3% solids, while polyvinyl alcohols, polyimides,
- ~., . . , - ; : . ;;:, : . ,: , ."., :. . : : .
,: ,. : : . : ~:
.,
2 ~ 3 ~
~'091/04154 rCI/US90/0~392
~35-
and pol~amides-imides are deposited as thin films, typically 3
+/- l microns. The techniques for the use of these materials
is well known in the art.
6. Ink RePellent Silicone Surface Layer l84
As pointed out in the background section of the
application, the use o~ a coatiny such as this is not a new
concept in offset printing plates. However, many of the
variations that have been proposed previously involve a
photosensitizing mechanism. The two general approaches have
been to incorporate the photoresponse into a silicone coating
formulation, or to coat silicone over a photosensitive layer.
When the latter is done, photoexposure either results in firm
anchorage o~ the silicone coating to the photosensitive layer
so that it will remain after the developing process removes the
unexposed silicone coating to create image areas (a positive
working, subtractive plate) or the exposure destroys anchorage
of the silicone coating to the photosensitive layer so that it
is removed by "developing" to create image areas leaving the
unexposed silicone coating in place (a negative working,
subtractive plate). Other approaches to the use o~ silicone
coatings can be described as modifications of xerographic
processes that result in an image-carrying material being
implanted on a silicone coating followed by curing to establish
durable adhesion of the particles.
The plates disclosed in the aforementioned U.S. Patent
4,596,733 use a silicone coating as a protective surface layer.
This coating is not formulated to release ink, but rather is
removable to allow the plates to be used with dampening water
applied.
The silicone coating here is preferably a mixture of two
or more components, one of which will usually be a linear
silicone polymer terminated at both ends with functional
(chemically reactive) groups. Alternatively, in place of a
;. . - .
\\'091/0~154 PCr/US~0/0~392
-3G-
linear difunctional silicone, a copolymer incorporating
functionality into the pol~mer chain, or branched structures
terminating with functional groups may be used. It is also
possible to combine linear difunctional polymers with
copolymers and/or branch polymers. The second component will
be a multifunctional monomeric or polymeric component reactive
with the first component. Additional components and types of
functional groups present will be discussed for the coating
chemistries that follow.
a) Condensation Cure Coatinqs are usually based on
silanon (-Si-oH) terminated polydimethylsiloxane polymers (most
commonly linear). The silanol group will condense with a
number of multi~unctional silanes. Some of the reactions are:
Functional Reaction By Product
Group
o o
Acyloxy - Si - OH ~ RCo-Si- - si - o - si- ~ ~OCR
Alkoxy -Si-oH + Ro-si- -si-o-si- ~ HOR
.
Oxime -Si-oH +RlR2C=No-si- -Si-O-Si ~ HON=CRlR2
Catalysts such as tin salts or titanates can he used to
accelerate the reaction. Use of low molecular weight groups
such as CH3- and CH3CH2- for Rl and R2 also help the reaction
rate yielding volatile byproducts easily remov~d from the
coating. The silanes can be difunctional, but trifunctional
and tetrafunctional ~ypes are preferred.
Condensation cure coatings can also be based on a moisture
cure approach. The functional groups of the type indicated
above and others are subject to hydrolysis by water to liberate
.
:::
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~'091/041~4 1~r/~Js9o/o~3~ J3,~
-37-
a silanol functional silane which ca~ then condense with the
silanol yroups of the base polymer. A particularly ~avored
approach is to use acetoxy functional silanes, because the
byproduct, acetic acid, contributes to an acidic environment
favorable for the condensation reaction. A catalyst ~an be
added to promote the condensation when neutral byproducts are
produced by hydrolysis of the silane.
Silanol groups will also react with polymethyl
hydrosiloxanes and polymethylhydrosiloxane copolymers when
catalyzed with a number of metal salt catalysts such as
dibutyltindiacetate, The general reaction is:
-Si-oH + H-SI- catalyst Si-O-Si- ~ H2
This is a preferred reaction because of the requirement
for a catalyst. The silanol terminated polydimethylsiloxane
polymer is blended with a polydimethylsiloxane second component
to produce a coating that can be stored and which is catalyzed
just prior to use~ Catalyzed, the coating has a potlife of
several hours at ambient temperatures, but cures rapidly at
elevated temperatures such as 300-~. Silanes, preferably
acyloxy functional, with an appropriate second functional group
(carboxy phoshonated, and glycidoxy are examples) can be added
to incre.ase coating adhesion. A working example follows.
b) Addition Cure Coatin~ are based on the hydrosilation
reaction; the addition of Si-H to a double bond catalyzed by a
platinum group metal complex. The general reaction is:
-Si-H ~ CH2=CH-Si- catalyst -si-cH2cH2-si-
Coatings are usually formulated as a two part system
composed of a vinyl ~unctional base polymer (or polymer bland)
to which a catalyst such as a chloroplantinic acid complax has
: . ,
: ~: ,.
,
W09l/04l54 rcr/us~)o/oj3()2
-38-
been added alon~ with a reaction modifier~s) when appropriate
(cyclic vinyl-methylsiloxanes are typical modifiers), and a
second part that i5 usually a polymethylhydrosiloxane polymer
or copolymer. The two parts are combined just prior to use to
yield a coating with a potlife of several hours at ambient
temperatures that will cure rapidly at elevated temperatures
(300~F, for example). Typical base polvmers are linear
vinyldimethyl terminated polydimethylsiloxanes and
dimethysiloxane-vinylmethylsiloxane copolvmers. A working
example follows.
c) Radiation Cure Coatinqs can be divided into two
approaches. For U.V. curable coatings, a cationic mechanism is
preferred because the cure i5 not inhibited by oxygen and can
be accelerated by post U.V. exposure application o~ heat.
Silicone polymers for this approach utilize cycloaliphatic
epoxy functional groups. For electron beam curable coatings, a
free radical cure mechanism is used, but requires a high level
of inerting to achieve an adequate cure. Silicone polymers for
this approach utilize acrylate functional groups, and can be
crosslinked effectively by multifunctional acrylate monomers.
Preferred base polymers for the surface coatings 184
discussed are based on the coating approach to be used. When a
solvent based coating is formulated, preferred polymers are
medium molecular weight, difunctional polydimethylsiloxanes, or
difunctional polydimethyl-siloxane copolymers with
dimethylsiloxane composing 80% or more of the total polymer.
Preferred molecular weights range from 70,000 to 150,000. When
a 100% solids coating is to be applied, lower molecular weights
are desirable, ranging from 10,000 to 30,000. Higher mole~ular
weight polymers can be added to improve coating properties, but
.~ will comprise less than 20~ of the total coating. When
addition cure or condensation cure coatings are to be
formulated, preferred second components to react with silanol
. -,- . . ..
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~: : : :: , ~: , ; , : :
: :' . ` ,, ,.~ :' - ' -
, , : :: .:. : : . , ,
`~'09l/04154 PCr/US()()/05392~ 3
-39-
or vinyl functional groups are polymethylhydrosiloxane or a
polymethylhydrosiloxane copolymer with dimethylsiloxane.
Preferably, selected filler pigments 188 are incorporated
into the surface layer 184 to support the imaging process as
shown in FIG. 4F. The use~ul pigment materials are diverse,
including:
a) aluminum powders
b) molybdenum disul~ide powders
c) synthetic metal oxides
d) silicon carbide powders
e) graphite
f) carbon black
Preferred particle sizes for these materials are small,
having average or mean particle sizes considerably less than
the thickness o~ the applied coating (as dried and cured). For
example, when an 8 micron thick coating 184 is to be applied,
preferred siæes are less than 5 microns and are preferably, 3
microns or less. For thinner coatings, preferred particle
sizes are decreased accordingly. Particle 188 geometries are
not an important consideration. It is desirable to have all
the particles present enclosed by the coating 184 because
particle surfaces projecting at the coating surface have the
potential to decrease the ink release properties of the
coating. Total pigment content should be 20% or less of the
dried, cured coating 184 and preferably, less than 10% of the
coating. An alu~inum powder supplied by Consolidated
Astronautics as 3 micron sized particles has been found to be
satisfactory. Contributions to the imaging process are
believed to be conductive ions that support the spark (arc)
from electrode sa during its brief existence, and considerable
energy release from the highly exothermic oxidation that is
also believed to occur, the liberated energy contributing to
decomposition and volatili~ation uf material in the region of
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WV 91/0'115~ I'Cr/US~)0/053~2
--~0--
the image forming on the plate~
The ink repellent silicone surface coating 184 may be
applied by any af the available coating processes. One
consideration not uncommon to coating processes in general, is
to produce a highly uniform, smooth, level coating. When this
is achieved, the peaks that are part of the structure of the
base coat will project well into the silicone layer. The tips
of these peaks will be thin points in the silicone layex, which
means the insulating effect of the silicone will be lowest at
these points contributing to a spark jumpiny to these points.
These projections of the base coat 176 peaks due to particle~
177 therein are depicted at P in FIG. 4F.
Workinq Examples of Ink RePellent Silicone Coatinqs
1. Commercial Condensation cure coating supplied by Dow
Corning:
Component Type Parts
Syl-Off 294 Base Coating 40
VM~P Naptha Solvent 110
Methyl Ethyl Ketone Solvent 50
Aliminum Powder Filler Pigment
Blend/Disperse Powder/Then Add:
~; Syl-Off 297 Acetoxy Functional Silane 1.6
Blend/Then Add:
~ ~ XY-176 Catalyst Dib~tyltindiacetate
: ~ :
3 3 3
WO 91/0415q l~cr/ussotos3s2
-~1
Blend~Then use:
Apply with a #10 Wire Wound Rod
Cure at 300F for 1 minute
2. Commercial addition cure coating supplied by Dow Corning:
Component TYpe Parts
Syl-Off 7600 Base Coating 100
VM-P Naptha Solvent 80
Methyl Ethyl Ketone Solvent 40
Aliminum Powder Filler Pigment 7.5
Blend/Disperse Powder/Then Add:
Syl~Off 7601 Crosslinker 4 . B
Blend~Then Use:
.
Apply with a #4 Wire Wound Rod
Cure at 300~F for 1 minute
This coating can also be applied as a 100% solids coating (same
formula without soIvents) via o~set gxavure and cured using
the same conditions.
3. Lab coating formulations illustrating condensation aure and
addition cure coatings are gien in the following Table 1.
Identity of indicated components are given in the following
Table 2. All can be applied by coa~ing with wire wound rods
and cured in a convection oven set at 300F using a 1 minute
dwell time. Coat:ing 4 can be applied as a 100% solids coating
and cured under ~he same conditions.
:- : . ... .. .
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WO 91/04154 I'Cr/US')0/05392
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U'091/04154 PCr/US90/053')2
-44-
When plate 172 is subjected to ~ writing operation as
described above, electrode 58 is pulsed, preferably negatively,
at each image point I on the surface of the plate. Each such
pulse creates a spark discharge between the electrode tip 58b
and the plate, and more particularly across the small gap d
between tip 58b and the metallia underlayer 178 at the location
- of a particle 177 in the base coat 176. Where the repellent
outer coat 184 is thinnest. This localizing o~ the discharge
allows close control over the shape of each dot and also over
dot placement to maximi%e image accuracy. The spark discharge
etches or erodes away the ink repellent outer layer 184
(including its primer layer l86, if present) and the metallic
underlayer 17~ at the point I directly opposite the electrode
tip 58b thereby creatiny a well I' at that image point which
exposes the underlying oleophilic surface of base coat or layer
176. The pulses to electrode 58 should be very short, e.g. 0.5
microseconds to avoid arc "fingering" along layer 178 and
conseguent melting of that layer around point I. The total
thickness of layers 178, 182 and l84, i.e. the depth of well
I', should not be so large relative to the width of the image
point I that the well I' will not accept conventional offset
inks and allow those inks to of~set to the blanket cylinder 14
when printing.
Plate 172 is used in press lO with the prPss being
operated in its dry printing mode. The ink from ink roller 22a
will adhere to the plate only ko the image points I thereby
creating an inked image on the plate that is transferred via
blanket roller 14 to tha paper sheet P carried on cylinder 16.
Instead of providing a separate metallic underlayer 178 in
the plate as in FIG. 4F, it is also feasible to use a
conductive plastic film for the conductive layer. A suitable
conductive material for layer 184 should have a volume
.
2~2~3~
`~'O9l/04iS4 l'Cr/US')0/05392
-45-
resistivity o~ loO ohm centimeters or less, Dupont's 200xC600
Kapton brand film beingone example. rrhis i~ an experimental
film in which the normally nonconductive material has been
filled with conductive pigment to create a conductive film.
To facilitate spark discharge to the plate, the base coat
176 may also be made conductive by inclusion of a conductive
pigment such as one o~ the preferred base coat pigments
identified above. ~ -
Also, instead of producing peaks P by particles 177 in thebase coat, the substrate 174 may be a film with a textured
surface that forms those peaks. Polycarbonate films with such
surfaces are available from General Electric Co. Another
possibility is to coat the olephobic surface layer directly
onto a metal or conductive plastic substrate having a textured
surface so that the substrate forms the conductive peaks. For
example, a silicon-coated textured chrome plate has been
successfully imaged in accordance with our process. It is also
feasible to provide a textured surface on the surface layer so
that the spark discharges are localized at the peaks defined by
that texturing.
All of the lithographic plates described above can be
imaged on press lO or~imaged off press by means o~ the spark
discharge imaging apparatus described above. The described
plate constructions~in toto provide both direct and indirect
writing capabilities and they should suit the needs of printers
who wish to make~opies on both wet and dry offset presses with
a variety of conventional inks. In all cases, no subsequent
chemicàl processing is~required to develop or fix the images on
the plates. The coaction and cooperation of the platas and the
imaging apparatus described a~ove thus provide, for the first
time, the potential for a fully automated printing facility
which can print copies in black and white or in color in long
or short runs in a~minimum amount of time and with a minimum
;: : : :
: :
~2~3~
WO9l/04154 PC~/US90/05392
-~6-
amount of effort.
One limitation of arc imaging generally is the tendency of
non-overlapped image points to appear as discrete circular
areas, leaving small poxtions o~ unexposed surface
therebetween. FIG. 5A illustrates t;his e~fect, which is an
inherent consequence of the geometry involved. A spark which
makes contact with a surface at points 201 will produce surface
effects extending radially over a given distance, resulting in
circular imaged areas 200. If these areas barely make contact
with one another, area 202 will remain unexposed despi~e its
presence within the image area.
This difficulty may be overcome by using a more powerful
pulse, thereby producing a laryer imaged area; or by incxeasing
; ~ the number of pulses per unit linear distance as tha electrode
moves along the plate surPace. With either technique, circular
imaged areas 200 are made to overlap as shown in FIG. 5B.
The increase~in the diameter of the imaged areas required
to fill area 202 is~easily calculated. If the distance between
points 201 in the case where circular imaged areas 200 just
touch is defined as D, the minimum increased diameter will be
D~2.
Although increasing the number of image points necessarily
increases imaging time,~ the degree of overlap can similarly be
minimized to~that whlch is~just necessary to eliminate the
unexposed surface.;~
While~either of~the foregoing techniques may be applied
readily~where the plate surface is merely modiPied by the spark
discharge, we have found it difficult to control the amount of
overlap where the spark is used~to actually burn away one or
more plate~layers;~ typiGally, the~edges of the image appear~to
bulge and are ~:unsharp. The microscopic cause of these e~fects
is~shown in FIG.~5C.~ Reference numeral;200a repxesents the
firs~circular image area produced by the spark, which is
2 ~ 3 e~
~'O 91tO4154 PCr/US90/0~3'J2
,
--47--.
burned normally. However, when second circular image area 200b
is burned, the area is found extend over additional area 206
even though the spark has been directed to the center of
circular area 200b.
This und sirable behavior, referred to as "overburn," can
be analogized to a quantum effect; that is, the discrete amount
of energy released in the discharged spark results in removal
of a specific amount o~ material. If the plate substrate is
heat-resi~tant and non-conductive, all of the energy of the
spark will be dissipated at the plate surface, resulting in the
larger-then-intended burn area. This effect is mo~t pronounced
if one of the plate surfaces i8 metal and the oxidation
reaction associated therewith is exothermic. In such cases, an
image point of a given siæe may be produced using a relatively
low spark energy, because the energy released by the oxidation
reaction (triggered by the spark) itself contributes to
formation of the final burn area. Thus, the energy o~ the
spark is more efficiently spread, and decomposition of the
metal is;less retarded by configurational discontinuities such
as the empty overlap area.
We have found that placing a conductive ~ilm beneath the
plate layer or layers~that are burned away can prevent
overburn. The overlapping portion o~ the conductive film
exposed by the previous spark discharge absorbs the excess
energy from the next spark. Thus, referring again to FIG. 5C,
instead of being deflected away from overlap area 204 and
thereby causing burn~at additional area 206, the excess spark
energy is absorb~ed~by~the conductive material exposed at
overlap area 204.
The volume resistivity of the conductive material must be
chosen wlth care.~ If~the resistance~is too great, an
insufficient~amount of energy will be absorbed, resulting in
persistence of the overburn problem. However, if the
' ~
3 3
` WO~1/04154 PCI/US90/053~2
,
resistance is too small, the conductive layer will compete with
the plate surface for spark energy, and deflect the spark from
its intended straight-line path. Hence, the optimum
resistivity of the conductiYe layer is partially a function of
the plate surface layer or layers.
Other factors also influence optimum resistivity,
including the size of the overlap and whether the plate
contains a metal layer with exothermic oxidation
characteristics.
We have found a useful working range of volume
resistivities to be in the range of .5 to 1000 ohm-cm~ This
range has been ~ound effective with aluminum and copper plate
surfaces over a range of image point izes.
It will thus~be seen that the objects set forth above,
among those made apparent from the preceding description, are
efficiently attained and, since certain changes may be made in
carrying out the above process, in the described products, and
in the constructions set forth without departing from the scope
of the invention, it is intended that all matter contained in
the above description~or shown in the accompanying drawings
shall be interpreted~as i1lustrative and not a limiting sense.
It is also to be understDod that the following claims are
intended to cover all~of the generic and specific features of
the- invention herein~described. ~ ~
