Note: Descriptions are shown in the official language in which they were submitted.
W092/0595, PCT/US90/05546
71~
PLASMA-JET IMAGING APPARATUS AND MET~OD
This application is a continuation-in-part of Serial No.
07/554,089, which is itself a continuation of Serial No.
07/329,979, now abandoned.
I. Backqround of the Invention
A. Field of the Invention
This invention relates to offset lithography and printing
generally. It relates more specifically to method and
apparatus for imaging lithographic and other planographic
plates.
B. Description of the Related Art
There are a variety of known ways to print hard copy in
i 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 major expense 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
.~ :
"'. -:
.
,
. ~ ..................... : , . :
. , . ~ .
. : ' -
, ~ . . ' . .: :
:~ .
W092/05957 PCT/US90/05546
plate or mat as hydrophilic and hydrophobic (and oleophilic, or
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 print~d
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 cylinder by an impression cylinder. -- -
In dry lithography, application of a fountain or dampeningsolution to the plate prior to inking is unnecessary. Instead,
the non-image material of dry plates is itself sufficiently
ink-repellent that ink simply fails to adhere to such material.
Most conventional offset plates are produced
photographically. In a typical negative-working, subtractive
process, the original document is photographed to produce a
photographic negative. This negative, or "mask", is placed on
an aluminum plate having 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 dark or printed areas of the
original) cure to a 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 carries 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 made photographically as aforesaid. In
addition to preparing the appropriate plates for the different
~: '
. .
:,
.~; ~ ,
:~s ~ ' :' .
; ,; . ~ .
.~ .
WOs2/0595, PCT/US90/05546
_3_ 2~7~
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
copie~.
~ he development of lasers has simplified the production of
lithographic plates to some extent. Instead of applying the
original image photographically to the photoresist-coated
printing plate as above, an original document or picture is
scanned line-by-line by an optical scanner which develops
therefrom 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
;1 receive light. That 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 still
i necessary to chemically etch each plate in order to create an
'; image on that plate.
A number of designers ha-e attempted to automate the
platemaking process by etching digitally stored image data onto
a blank lithography plate. one well-known method of
I accomplishing this is known as electro-ero~ion. The type of
plate suitable for imaging in this fashion has an oleophilic
plastic substrate, e.g., Mylar plastic film, having a thin
coating of aluminum metal with an overcoating of conductive
graphite; the latter acts as a lubricant to minimize scratching
of the aluminum coating. A stylus electrode in contact with
the graphite 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
i! electrode and the thin metal coating is, by design, large
enough to erode away the thin metal coating and the overly g
~'` '
~j ~
:.,
~, .,~,
. . :; - . :
', ~
. . ~:
~ ,~, . . .
W092/05957 PCT/US90/05546
conductive graphite surface coating, thereby exposing the
underlying ink-receptive plastic substrate on the areas of the
plate corresponding to the printed portions of the original
document.
This method of making lithographic plates suffers from the
disadvantage that the described electro-erosion process only
works on plates whose conductive surface coatings are very
thin; furthermore, the stylus electrode which contacts the
surface of the plate still 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 that print similarly unwanted marks on the copies.
An alternative to the electro-erosion process is described
in U.s. Patent No. 4,718,340. This reference describes use of
spark-discharge apparatus that do not make contact with the
plate, thereby avoiding the above-noted problem of s~rface
scratching. Because the disclosed apparatus operate at
relatively low power levels, the plates discussed in this
reference all have hydrophilic metal substrates coated with an
oleophilic surface layer, thejlatter being ablated during the
imaging process. This places significant limitations on the
ability to design plates that will perform both durably and
effectively. For example, to be suitable for use with the
disclosed apparatus, the oleophilic surface layer must be
fragile enough to decompose upon exposure to relatively low-
energy sparks, a characteristic that would also result in
limited on-press durability. Furthermore, the approach
d~scribed in this patent would not be suitable for production
of typical dry plates, which feature oleophobic silicone
surface coatings that could not usefully be employed in
conjunction with a hydrophilic substrate, in addition to being
too resilient for removal at low power levels.
Another example of non-contact platemaking is described in
~ .
. ' ,. .
', ~ ' ,
''
~,' ' ' ` , :- ,' ~'
., .' .
,;jr ' ' ."
W092/05957 PCT/US90/0~54h
~5~ 2~,71~
published European Patent Application EP 0167352, which
involves a method for generating a latent image on a blank
plate using low-current electrical discharges. These
discharges do not remove or otherwise alter the physical
characteristics of the substrate. Rather, the apparatus
appears to produce the same chemical changes in the plate-
surface material that would conventionally be produced by
exposure to actinic radiation, as described above. Once again,
the need for chemical response to the low-power discharges
imposes significant limitations on the plate constructions that
can be-imaged with this type of apparatus.
Other designers have attempted to use more powerful lasers
to write images on blank plates. However, the use of such
lasers for this purpose has not been entirely satisfactory
because the photoresist coating on the plate must be compatible
with the particular laser, a requirement that limits the choice
of coating materials. Also, the pulsing frequencies of some
lasers used for this purpose are so low as to render the time
required to produce a halftone image on the plate 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 that 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.
We are also aware of a press system that images a
lithographic plate while the plate is actually mounted on the
print cylinder in the press. The cylindrical surface of the
plate, treated to render it either oleophilic or hydrophi ic,
is written on by an ink jetter arranged to scan over the
surface of the plate. The ink jetter is controlled so as to
deposit on the plate surface a thermoplastic image-forming
s,
,s
.,~,...................................................................... .
~" ' ..
~ ';
.~. -,. . . .
.. : .~ . .
.s;' ,. ~
.. ; ., ~ . ~
..,
W092/05957 PCT/US90/05546
~ b -6-
resin or material that 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 in the areas thereof where the
image-forming material is present but is repelled by 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 (philic or phobic)
for all of the inks commonly used for making lithographic
copies. Also, ink jet printers are generally unable to produce
small enough ink dots to allow the production of smooth
continuous tones on the printed copies, i.e. the resolution is
not high enough.
Thus, despite all of the aforementioned efforts to improve
different aspects of lithographic plate production and offset
printing, signi~icant performance and operational limitations
remain.
. .
II. DescriPtion of the Invention
A. Obiects of the Invention
Accordingly, the present invention aims 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 writing both positive and negative or background
images on lithographic plates.
. .,
ï
~: .
.~ . . .
; .
~;
.~ :,'
W092/059~7 PCT/US90/05546
~7~ 2~71~
Still another object of the invention is to provide such a
method which can be used to apply images to a variety of
different kinds of lithographic plates.
A further object of the invention is to provide a method
of producing on lithographic plates half tone images with
variable dot sizes.
A further object of the invention is to provide improved
apparatus for imaging lithographic plates using plasma
discharges.
Another object of the invention is to provide apparatus of
this type which applies the images to the plates efficiently
and with a minimum consumption of power.
Another object of the invention is to provide an imaging
apparatus that can generate a photographic master without
having to chemically develop the image on the master.
A further object of the invention is to provide an
apparatus of this type capable of both indirect writing on
certain polymer coated plates and direct writing on silicone-
based plates.
- Still another object of 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
exe~plified in the constructions described herein and the
several steps and the relation of one or more of 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.
~,....................................................... .
~,.;, .
.
':~
,; .
ff ~
.. :
$
~,
. ',
... . .
w092/0~957 PCT/US90/05~46
B. Brief ~ummarY of the Invention
In accordance with the present invention, images are
applied to a lithographic printing plate by altering or
ablating the physical surface structure of the plate at
selected points or areas of 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
comprises one or more precisely positioned and focused plasma-
jet sources, each of which uses an electric arc to ionize a
working gas into a plasma state. As used herein, a "plasma"
refers to an assembly of ions, electrons, neutral atoms and
molecules in which particle motion is governed primarily by
electromagnetic forces, and the degree of ionization exceeds
5%.
Short duration, high voltage pulses are used to produce
the arc so that the plasma-jet discharges are also of short
duration, and travel a substantially straight-line path from
the discharge source to the point on the plate directly
oppositeO Each such discharge creates, at the point of contact
on the surface of the plate, a precisely controlled and
positioned intense heat zone.
In 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 having precisely controlled voltage and current
proiles are applied to the plasma-jet source electrode or
multiple such sources to produce precisely positioned and
defined plasma-jet or plasma-arc discharges to the plate which
physically transform selected points or areas of the plate
surface to render them either receptive or non-receptive to the
printing ink that will be applied to the plate to make the
' '
,
' '
:,~
W092/0595, PCT/US90/05546
_g_
printed copies. By "transform" we mean either ablation of a
surface layer or significant disruption of physical structure
with loss of material. Such disruption distorts the -
crystallinity of the surface and, we believe, results in pore
spaces being filled in to alter the surface's hydrophilicity.
Preferably, each plasma-jet source operates in a so-called
jet transfer mode wherein the arc and plasma jet extend from a
nozzle in the source to the workpiece being heated, in this
case, the lithographic plate. Plasma-arc discharges operate in
a like manner in an atmosphere of working gas suitable for
conductive arcs. - - - -
Lithographic plates are made ink receptive or oleophilicinitially by providing them with surface areas consisting of
plastic materials to which oil- and rubber-based inks adhere
readily. On the other hand, plates are made ink repellent or
oleophobic initially by providing them with low-surface-energy
coatings to which inks cannot adhere. As will be seen later,
certain ones of these plate embodiments are suitable for wet
printing, others are better suited for dry printing.
The present apparatus can write images on these different
lithographic plates having either ink-receptive or ink-
repellent surfaces. In other words, if the plate surface is
repellent initially, our apparatus will write a positive image
on the plate by rendering ink receptive or oleophilic the
points or areas of the plate surface corresponding to the
printed portion of the original document. On the other hand,
if the plate surface is ink receptive or oleophilic initially,
the apparatus will apply a background or negative image to the
plate surface by rendering hydrophilic the points or areas of
that surface corresponding to the background or non-printed
portion of the original document. Direct or positive writing
is usually prçferred since the amount of plate surface area
that must be transformed is less because most documents have
.
~..
~,,
..
. .
;, .~ : .
W O 92/05957 P ~ /US90/05546
less printed areas than non-printed areas.
The plate imaging apparatus incorporating our invention i5
preferably implemented as a scanner or plotter whose writing
head consists of one or more plasma-jet sources positioned over
the working surface of the lithographic plate and moved
relative to the plate so as to collectively scan the plate
surface. Each plasma-jet source or electrode is energized 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, telecommunication
apparatus, electronic pre-press system, etc. These signals are
formatted so that the apparatus' plasma-jet source or sources
writes a positive or negative image onto the surface of the
lithographic plate that corresponds to the original document.
If the lithographic plates being imaged by our apparatus
are flat, then the plasma-jet source or sources 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 source (or
sources) is incorporated as a writing head 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 show, our invention can be practiced on a lithographic
plate already mounted in a press to apply an image to that
plate in situ. In this application, then, the print cylinder
itself may comprise the drum component of the scanner or
plotter.
To achieve the requisite relative motion between the
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.
~' .
,
:. . .
; : ,: . :
W092/05957 PCT/US90/05546
--11-- ~,
2~87 ~
Alternatively, the writing head can move parallel to the drum
axis and after each pass of the head, the drum can be
incremented angularly so that the image 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 printing
plate.
As the writing head traverses the plate, it is maintained
at a small distance above the plate surface. In addition to
the working gas that is introduced into the writing head, air
or other gas blends under pressure may also be delivered
between the writing assembly and the plate to provide oxygen or
other reagents for the etching process and to expel residue
from the etching area. That gas flow also provides a cushion
for the writing head to prevent its contacting and, possibly,
scratching the plate surface. In response to the incoming
picture signals, which usually represent a halftone ar screened
image, each plasma-jet source is pulsed or not pulsed at
selected points in the scan depending upon whether, according
to the incoming data, that source is to write or not write at
these locations.
Each time a source is pulsed, there is an accompanying
plasma discharge between the plasma-jet source and the
particular point on the plate o-mosite to that source. Several
features assure that the discharge follows a substantially
straight-line path from the plasma-jet source to the plate
surface -- a critical performance criterion, since a high
degree of accuracy is required to produce an acceptable
lithographic plate. First, we utilize much higher discharge
power than that associated with prior-art systems, thereby
producing a high field gradient that encourages straight-line
travel. This can be explained as follows. The strongest part
of the field on the plate, to which the discharge is most
~ .,.
"
. ~
'- . . '
.. . .. -
. - ,
,', . .
,,; '
:. .
W0~2/0595, PCT/US90/05546
~ 12-
strongly attracted, occurs at the point precisely opposite the
discharge source. However, the strength of the field at this
point must be sufficiently greater than the strength at any
other point to overcome the inherently random nature of the
discharge. The stronger the gradient, the faster the field
strength diminishes as the path from source to plate deviates
from the normal. Accordingly, high discharge power creates a
strong gradient, which, in turn, favors straight-line discharge
travel by emphasizing the recession of the plate field strength
in all directions away from the normal.
Second, the plasma-jet source is precisely configured to
provide a highly focused emission. The electrode is positioned
behind an ejection nozzle, thereby eliminating the electrode as
a possible source of interference with plasma flow, and
preferably terminates in a pointed tip to enhance the focus of
the charge as it builds up. The nozzle has a precise diameter
and length and is supplied with working gas, this can be any
gas that is not too strongly electronegative, such as argon,
neon, xenon, krypton, or helium (radon is usable but not
practical due to its toxicity) or nitrogen. The working gas
can also include one or more electronegative gases (such as
oxygen) as additives; however, in order to maintain adequately
small breakdown times, these should not constitute a major
component of thw working gas. Accordingly, air ~preferably
moist) ~an also be employed as a working gas. The working gas
is delivered at a precisely controlled pressure to provide a
laminar-flow (non-turbulent) discharge of cylindrical cross-
section through the nozzle and onto the plate. The spark,
plasma and accompanying heat transform 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.
The pulse duration, current or voltage controlling the
.
,.
.:
- , ,
- ~ .
.
" :
' :
W092/05957 PCT/US90/05546
2 ~
plasma jet at each source may be varied to produce a va~iable
dot on the plate. Also, the polarity of the voltage applied to
the plasma-jet assembly may be made positive or negative
depending upon the nature of the plate surface to be affected
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 facilitate transformation of the
surface at that point. 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 0.0001 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 plasma discharges from the
scanning electrode.
Thus, using our method and apparatus, high-quality images
can be applied to lithographic plates which have a variety of
different plate surfaces suitable for either dry or wet offset
printing. In particular, our use of high-power discharges
permits imaging of plates composed of refractory materials. By
employing strong surface and substrate layers, we are able to
produce lithographic plates that offer longer performance
lifetimes than those proposed in connection with the prior-art
spark-discharge systems.
With the present invention, the image is applied to the
plate relatively 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. A
lithographic plate can be imaged while it is mounted in its
press, thereby reducing set up time considerably. An e _n
greater reduction in setup time results if the invention is
practiced on plates mounted in a color press, because correct
W O 92/0595/ PC~r/US90/05546
~ 14-
color registration between the plates on the various print
cylinders can be accomplished electronically rather than
manually by controlling the timings of the input data applied
to the plasma jet or electrode ~ources that write the images on
the plates.
B. Brief DescriPtion of the Drawinqs
For a fuller understanding of the nature and objects of
the invention, reference should be had to the following
detailed description taken in connection with the accompanying
drawings, in which:
FIG. 1 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. 1 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 and 4B are enlarged sectional views showing
lithographic plates imaged in accordance with our invention.
:
C. Detailed DescriPtion of the Preferred Embodiments
Refer first to FIG. 1 of the drawings which shows a more
or less conventional offset press shown generally at 10 which
can print copies using lithographic plates made in accordance
with this invention.
Press 10 includ~s a print cylinder or drum 12 around which
~; is wrapped a lithographic plate 13 whose opposite edge margins
~,:
, - .
~'
,............................................ .
:$ ` : : - .
., - :
;'~ . '. ' :
W092/05957 PCT/US90/05546
-15-
2 ~
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 15 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 printing. As is customary in presses of this type, the
various cylinders are all geared together so that they are
driven in uniso~ by a single drive motor. This ink train may
be used on a single color press as well as on a multiple color
press.
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 in 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 intermediate roller 22b of ink train 22 as shown in FIG. 1.
When p~ess 10 is operating in its dry printing mode, the
dampening assembly 24 is inactive so that roller 26b is
retracted from roller 22b and the plate as shown in phantom in
FIG. 1 and no water is applied to the plate. The lithographic
plate 13 on cylinder 12, which is described in more detail in
connection with FIG. 4A, is designed for such dry printing. 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
:
,~ .
~'
~.~
WO92/OSg57 PCT/US90/OS~6
~ 16-
rotates, the plate is contacted by the ink-coated roller 22a of
ink train 22. 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 10 is 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 FIG. 1. The lithographic plate in this case is
desi~ned for wet printing. See, for example, plate 152 in FIG.
4B. It has a surface which is ink-receptive or oleophilic
except in the areas thereof which have been written on to make
them hydrophilic. Those areas, which correspond to the
unprinted areas of the original document, accept water. In
this mode of operation, as the cylinder 12 rotates (clockwise
in FIG. 1), water and ink are presented to the surface of plate
13 by the rolls 26b and 22a, respectively. 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
have not been wetted by roller 26 pick up ink from roller 22a,
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 he applied to the lithographic plate 13
can be written onto the plate while the plate is "off press",
our invention lends itself to imaging the plate when the plate
is mounted on the print cylinder 12; the apparatus for
~- ~
.~, ........... ~ , , . .
. .
: : :
~ , .
, ,~; ........ .
W0~2/0s95, PCT/US90/05546
-17- 2Q ~g7 ~
accomplishing this will now be described. 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 rotatively
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, carriage 44 is moved axially with respect to print
cylinder 12.
The cylinder drive motor 34 and step motor 46 are operated
in synchronism by a controller 50 (see FIG. 3), which also
receives signals from detector 36a, so that as the drum
rotates, the carriage 44 moves axially along the drum with the
controller "knowing" the instantaneous relative position of the
carriage and cylinder at any given moment. The control
circuitry required to accomplish this is already very well-
known in the scanner and plotter art.
Numerous variations on this general construction are
possible. For example, a multicolor press can include multiple
printing stations arranged in sequence, each station including
a plate cylinder and a blanket cylinder, so that the paper
':
~:
.
..,
.~ ........................................................................ .
~,.
;: :,
~i, : .
.,.' ' ' : "~
s, ~
'.:,' ~ :
W092/05957 PCT/US90/05546
~ 18-
sheet is guided past each blanket cylinder by a linear
conveyancing mechanism. More than one plate cylinder can also
"share" a single blanket cylinder. Alternatively, the printing
stations can surround a single impression cylinder, as
described in U.S. Patent No. 4,936,211; again, more than one
plate cylinder can be associated with a single blanket
cylinder.
Refer now to FIG. 3 which depicts an illustrative
embodiment of carriage 44. It includes a block 52 having a
threaded opening 52a for threadably 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 writing head 56
made of a suitable rigid electrical insulating material that
supports a plasma-jet source. The illustrated head has only
one such source 58 and is, therefore, capable of imaging only
one point on plate 13 at a time. It should be understood,
however, that the head may carry a plurality of such sources in
which case it would image a corresponding plurality of points
on the plate simultaneously.
Source 58 comprises a vertical passage 60 that extends
down through head 56. The lower end of passage 60 is partially
closed by a nozzle 62 made of a refractory material such as
ceramic, ruby or sapphire. Centered on the axis of passage 60
is an electrode 64 whose upper end 64a is supported by a
conductive socket 66 plugged into the upper end of passage 60.
i Electrode 64 is made of a refractory metal such as tungsten,
nichrome or the like capable of withstanding erosion due to
spark discharges from the electrode. The lower end or tip 64b
of the electrode is preferably pointed and is shown as
- extending slightly into the nozzle orifice 62a; this contrasts
with some of the prior-art systems, which utilize annular
electrodes. In some cases, however, the electrode may be
1~ .
' .
" ,.
..
W092/059~7 PCT/US90/05546
shorter so that its tip 64b is spaced above the nozzle 62. An
insulated conductor 68 connects socket 66 to a termainl 68a at
the top of block 52.
A small gas passage 70 extends from the top of head 56 to
passage 60 at a point below socket 66. The upper end of
passage 70 is connected by a flexible tube 72 to a colinear
passage 74 in block 52 that leads to the top of that block.
The upper end of passage 74 is, in turn, connected by a pipe or
supply tube 76 to a source of working gas such as argon, or one
of the other inert gases. In some cases, the working gas may
also include an oxidizing gas, e.g. oxygen. We have also
achieved successful results using air as a working gas,
particular where the air is moist. Although the breakdown
voltage associated with air is higher than that associated with
argon, air tends to produce more reliable results due to the
greater stability of the ionized species generated therewith.
The gas supply pressure to passage 60 is regulated by a
pressure regulator 78 in supply tube 76 so as to provide a non-
turbulent flow of gas to passage 60 for discharge along the
exposed electrode and through the nozzle orifice 62a.
When the carriage 44 is positioned opposite plate 13 as
shown in FIG. 3, head 56 is spaced a constant distance above
the surface of the plate. This can be accomplished in any of
several ways. one alternative is described in copending
application serial no. 07/553,817, incorporated herein by
reference, which describes a pneumatic sensing device and
associated tracking hardware to maintain the constant spacing.
Alternatively, the head 56 may be provided with a depending
skirt or baffle 56a, with a gas passage 80 extending down from
the top of head 56 into the skirt where it opens into the
r~gion within the skirt. In this embodiment, the upper end of
that passage 80 is connected by a flexible tube 82 to a
vertical gas passage 84 in block 52. The upper end of that
~' ' .
. ' . , , , - .
', '.
., .
W092/0S957 PCT/US90/05546
~ 3~
latter passage is, in turn, connected to a pipe or tube 86
leading from a source of pressured air. Preferably, the tube
86 contains a flow restrictor 88 and a pressure regulator 90 so
that the resultant back pressure from the air flow through the
gap between the plate and the skirt 56, acting over the area
encompassed by the lower edge of skirt 56a, is sufficient to
support the head 56 at a constant distance from the surface of
plate 13. Typically, the head 56 is supported so that a
constant gap in the range of .001 to .015 inch is maintained
between the plate 13 surface and the nozzle 62 at the underside
of the head. The air discharging from passage 80 also performs - -
other functions to be described later.
Other possible means for maintaining a constant head-to-
surface distance include the use of capacitance, optical, eddy-
current or magnetic proximity-monitoring apparatus.
Still referring to FIG. 3, the writing head 56, and
particularly the pulsing of electrode 64, 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 fixed or variable resistor 102 to terminal 68a on block
52, which, as noted previously, is connected electrically to
electrode 64. The opposite end of winding 98a is connected to
electrical ground. The transformer primary winding 98b is
connected to a DC voltage source 104 that supplies a voltage in
the order of 1000 volts. The transformer primary circuit
includes a 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
."; . .
; ~
- . . . ,. :, . , .
. . . .
W092/05957 PCT/US90/05S46
-21- 2~7~
electrode 64. 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 ~e applied to the
electrode 64 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 lO
is operated in a non-print or imaging mode with both the ink
and water rollers 22a and 26b (see FIG. 1) being disengaged
from oylinder 12. The imaging of plate 13 in press 10 is
controlled 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 conventional source of
picture signals such as a disk reader 114. The controller 50
sync.~ronizes the image data from disk reader 114 with the
control signals that control rotation of cylinder 12 and
movement of carriage 44 so that when the plasma-jet source 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 is to be written on or not
written on.
If that point is not to be written on, (i.e. in direct
writing it corresponds to a location in the background of the
original document, or in indirect writing it corresponds to a
point in the image area of the document), the source electrode
64 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 on the plate which is to be written on (i.e., the
image area for direct writing, or the background area for
indirect writing), switch 108 is closed. The closing of that
switch discharges capacitor 106 so that a high voltage pulse,
e.g., 1,000 volts, of only about one microsecond duration is
'
W0~2/05957 PCT/US90/05546
~ 22-
applied to transformer 98. The transformer applies a stepped-
up pulse to electrode 64, thereby causing a plasma jet
discharge J between the source tip 64b and plate 13. That is,
each such pulse strikes a spark between the electrode tip 64b
and plate 13 causing ionization and disassociation of the
working gas molecules in passage 60, thereby creating a small
diameter plasma-jet discharge through nozzle orifice 62a to the
plate s~rface.
The source nozzle 62 is provided with an orifice 62a in
the order of .002 to .010 inch in diameter to provide a
sufficient flow of working gas at-the regulated pressure, i.e.,
1 to 4 psi, to deliver a non-turbulent plasma gas jet discharge
to plate 13. This plasma discharge has sufficient mamentum to
function essentially as a compliant conductive path between
electrode 64 and plate 13, and follows a sufficiently strong
field gradient to assure substantially straight-line travel.
~he discharge transforms a small spot of the desired size on
the surface of the plate at the image point I thereon directly
opposite the nozzle orifice 62a. This transformation renders
that point either receptive or non-receptive to ink, depending
upon the type of surface on the plate.
The power of the arc actually reaching the plate (i.e.,
its voltage/current profile) depends on the inherent breakdown ,
voltage associated with the working gas, the voltage (positive
or negative~ of the pulse applied to the electrode and the rise
time of this pulse. The interplay of these variables derives
from the fact that breakdown and arcing is not an instantaneous
process. Although the drop in resistance that accompanies
breakdown would ordinarily prevent maintenance of voltages
above the breakdown threshold, a very fast rise time can
momentarily impose voltage levels across the gap that exceed
this threshold during the finite time required for breakdown to
occur.
.'' '
.. . .
.:
- , . , . ~ :
, " " ' ':, ,.- :, - :.~
.:~ , : , '
,
,: :
.- : ~. , . :
. . .
,
w092/05957 PCT/US90/05546
-23-
The current range, on the other hand, depends both on this
effective arc voltage and the value of current-limiting
resistor 102. Furthermore, the electrical properties of the
layer of conductive material on or within the plate can limit
the maximum useful current, since insuffi~ient conductivity
(e.g., due to use of too thin a layer of material for a given
current level) results in charge buildup that can diminish the
strength of the arc or prevent arcing entirely. Our preferred
apPlied voltage levels -- that is, the voltage actually applied
to electrode 64, not the effective arc voltage -- range from
1,000 to 5,000 volts; potential levels above 2,000 volts are
especially preferred. As stated previously, the effective arc
voltage for a given applied voltage depends on the rise time of
the voltage pulse and the breakdown voltage of the wor~ing gas.
Our preferred working current ranges from 0.1 to 1 amp. Lower
current levels tend to be associated with easily ionized gases
such as argon, and the higher levels with gases having higher
breakdown voltages, such as air.
In addition to providing an air cushion for head 56 as the
head is moved along the surface of plate 13, the air
discharging from passage 80 into the gap between the head and
the plate purges that space of debris produced by the etching
or transformation process. The air is also a source of oxygen
which, in the case of some plates, abets or enhances the
imaging or writing by the plasma-jet source 58.
The transformations that do occur with our different
lithographic plate constructions will be described in more
detail later. Suffice it to say at this point that the
intensity cf the plasma jet is adjusted for the different plate
embodiments to write thereon a clearly defined image spot on
the order of 0.0001 to 0.005 inch in diameter. This may be
accomplished, for example, by varying resistor 102 either
manually or automatically via controller 50. Dot size may be
,
W092/~5957 PCTtUS90/05546
~ 24-
varied by changing the voltage and/or current flow and/or
duration of the pulses that produce the plasma jet discharges.
Means for doing this are quite well known in the art. Dot size
may also be varied within a single plate by repeated pulsing of
the plasma-jet source at each image point where enlargement is
desired, the number of pulses determining the dot size (pulse
count modulation). The polarity of the voltage applied to the
electrode 64 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 plasma-jet source 58 is scanned across the plate
surface, it can be pulsed at a maximum rate in excess of
100,000 pulses/sec. However, currently preferred rates are
25,000-50,000 pulses/sec. Thus, a broad range of dot densities
can be achieved, e.g. 2,000 dots/inch to 50 dots/inch. The
dots can be printed side-by-side or they may be made to overlap
so that substantially 100~ of the surface area of the plate can
be imaged. Accordingly, in response to the incoming data, an
image corresponding to the original document builds up on the
plate surface constituted by the points or spots on the plate
surface that have been transformed by the plasma-jet discharge
J, as compared with the areas of the plate surface that have
not been so affected by the 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 22a to its inking position shown
in FIG. l, and, in the case of wet printing as with plate 152
in FIG. 4B, by also shifting the water fountain roller 26_ to
its position shown in FIG. 1 and in solid lines in FIG. 2. ~s
the plate rotates, ink will adhere only to the image points
, - , . .
.
,~
- . . . ..
W092/05957 PCT/US90/05~46
-25- 2 ~ ~ 3' 7 t~
written onto the plate that correspond to the printed portion
of the original document (in the case of direct writing), or
the background portion (in the case of indirect writing). That
ink image will then be transferred 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 setup 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 rotation 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 from the foregoing that, since the images may be
applied 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 offset 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. ~hus once imaged, the
plates will automatically print in perfect register on paper
sheet P.
; :
':
W092/05957 PCT/US90/05~46
~ 26-
Refer now to FIGS. 4A and 4B, which illustrate two
lithographic plate embodiments which are capable of being
imaged by the apparatus depicted in FIGS. 1 to 3. The plate
embodiment 13 in FIG. 4A is suitable for direct imaging in a
press without dampening. Plate 13 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 characterized by
low surface energy. One suitable coating material is an
addition-cured silicone release 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 the
plasma-jet discharges J from source 58. The heat from the
associated arc removes the silicone coating and at least some
of the metal from substrate 132 to produce an altered surface
topography of enhanced ink receptivity. The discharge
decomposes the silicone coating at each image point into
silicon dioxide, carbon dioxide, and water; hydrocarbon
fragments in trace amounts are also possible depending on the
chemistry of the silicone polymers used. For other substrate
materials, the presence of an oxidant in the space above image
point I facilitates and abets the decomposition process.
Such decomposition and surface alterations due to the
plasma-jet J renders that surface oleophilic at each image
point I directly opposite the nozzle orifice 62a. Preferably -
that coating 134 is made quite thin, e.g. 0.0003 inch to
minimize the voltage required to break down and remove the
coating material to render the plate ink receptive.
~esultantly, when plate 13 is 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
W092/05957 PCT/US90/05546
-27- 2~ 71~
is then transferred by blanket cylinder 14 to the paper sheet P
as in any conventional offset press.
FIG. 4B illustrates a lithographic plate 152 that can be
designed for wet or dry printing. The plate 152 comprises a
substrate 154 which is oleophobic, mechanically sturdy, and
resistant to extension (stretch) and heat. Polyester film
meets all these requirements well and is readily available.
Dupont's Mylar and ICI's Melinex are two commercially available
films that are suitable for use as substrate 174. Other films
that can be used are those based on polyimides (Dupont's
Xapton) and polycarbonates (GE's Lexan). A preferred thickness -
is 0.005 inch, but thinner and thicker versions can be used
effectively.
There is no requirement for an optically clear film, nor
must the film surface be perfectly smooth. The use of
pigmented films including f ilms pigmented to the point of
opacity are feasible for the substrate, providing mechanical
properties are not lost. It is also possible to provide a
resistive layer beneath substrate 154 to control overburn, as
described in our copending application serial no. 07/410,295
(the disclosure of which is hereby incorporated by reference).
Applied to the surface of substrate 154 is a metal layer
156. This layer is important to formation of an image and must
be uniformly present if uniform imaging of the plate is to
occur. The image carrying (i.e. ink receptive) areas of the
plate are created when the plasma-jet discharge J volatizes a
portion of the thin metal layer 156. The size of the feature
formed by the plasma-jet discharge from source tip 58b of a
given energy is a function of the amount of metal that is
volatized. This is, in turn, a function of the amount of .~etal
present and the energy required to volatize the metal used.
Furthermore, the thickness of this layer determines the
necessary penetration depth of the plasma discharge into the
,
''~"
' ' ' ' ' '
~` :
- .
:' ': '
W O 92/05957 P~r~US90/05546
~ 28-
plate surface.
The metal preferred for layer 156 is aluminum, which can
be applied by the process of vacuum metallization (most
commonly used) or sputtering to create a uniform layer 400 +/-
lOO Angstroms thick. Because this layer is so fine, we
typically gauge its thickness using conductivity measurements
(e.g., with standard conductance monitors supplied by Delcom
Instruments, Inc., St. Paul Park, MN); our preferred thickness
range as specified by conductivity measurements is 0.4 to 2
mhos, with readings of 1.0 to 1.5 mhos being especially
preferred. Other suitable metals include titanium, copper and ~
zinc. ln general, any metal or metal mixture, including
alloys, that can be deposited on substrate 154 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. ~hat is, it is possible to image
a plate through a 1000 Angstrom layer of metal, and to image
layers less than 100 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.
Optionally, it is possible to coat metal layer 156 with an
oleophobic surface coating 158 to serve a variety of
objectives. One function of such a coating can be to
facilitate dry printing; a variety of silicone formulations,
well-known in the art, are sufficiently oleophobic to obviate
the need for dampening prior to inking. In addition, surface
coating 158 can serve as a vehicle to carry conductive or
semiconductive pigments that assist the imaging process, such
as those disclosed in copending application serial no.
07/442,317 (the disclosure of which is hereby incorporated by
reference).
,. ,
.,.~ .
.. . . .
, - ~ ..
W092/05957 PCTtUS90/05546
-29- 2~7~
If a silicone surface coating is added, it may also prove
necessary to anchor this coating to the metal layer 156 with an
additional primer layer. 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,
and polyamides-imides are deposited as thin films, typically
less than 1 micron. The techniques for the use of these
materials are well known in the art.
Suitable variations on the above-described plate designs
may be found in U.S. Patent No. 4,911,075, the disclosure of
which is hereby incorporated by reference; see especially FIG.
4F thereof and the supporting discussion.
When the metal layer 156 (and, optionally, the overlying
surface coating 158) is subjected to a plasma-jet discharge J
from nozzle 62, it is ablated at the image point I on the
surface of layer 156 directly opposite the nozzle orifice 62a.
Accordingly, when the plate 152 is coated with water and ink by
the rolls 26_ and 22a, respectively, of press lO, water adheres
to the imaqe points I on plate 152 formed by the discharges J
from the plasma-jet source 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.
It is also feasible to replace metal layer 156 with a
; conductive plastic film. A suitable conductive material for
layer lS6 should have a volume resistivity of 100 ohm
centimeters or less, Dupont's Kapton film being one example.
This is an experimental film in which the normally
,
.
.
W092/05~57 PCT/US90/05~6
~c)~ 30-
nonconductive material has been filled with conductive pigment
to create a conductive film.
All of the lithographic plates described above can be
imaged on press lo or imaged off press by means of the plasma-
jet 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 copies on both wet and dry offset presses with a
variety of conventional inks. In all cases, no subsequent
chemical processing is required to develop or fix the images on
the plates. The coaction and cooperation of the plates and the
imaging apparatus described above thus provide 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 amount of effort.
Furthermore, the imaging system described above may be used in
conjunction with convention presses (e.g., line or web presses)
as well as presses specially configured for ln situ plate
etching.
It will thus be seen that the objects set forth above,
among those made apparent from the preceding description, are
efficiently attained. Also, 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. For example, in the case of certain plates,
it may be possible to operate the plasma-jet source in a non-
transferred mode in which the arc impinges the wall of the
noæzle 62 which functions as an electrode (i.e. is conductive).
In this event, the plasma, but not the arc, is projected as a
jet beyond the nozzle to the surface of the lithographic plate.
Therefore, it is intended that all matter contained in the
above description or shown in the accompanying drawings shall
be interpreted as illustrative and not a limiting sense.
-
W092/059~7 PCT/US90/05546
-31- ~a~7~.
It is also to be understood that the following claims are
intended to cover all of the generic and specific features of
the invention herein described.
- ~ .
: .
....
