FIELD OF THE INVENTION
[0001] The present invention relates to a method for making a heat-mode lithographic printing
plate precursor and a lithographic printing master in computer-to-plate and computer-to-press
procedures.
BACKGROUND OF THE INVENTION
[0002] Rotary printing presses use a so-called master such as a printing plate which is
mounted on a cylinder of the printing press. The master carries an image which is
defined by the ink accepting areas of the printing surface and a print is obtained
by applying ink to said surface and then transferring the ink from the master onto
a substrate, which is typically a paper substrate. In conventional lithographic printing,
ink as well as an aqueous fountain solution are fed to the printing surface of the
master, which is referred to herein as lithographic surface and consists of oleophilic
(or hydrophobic, i.e. ink accepting, water repelling) areas as well as hydrophilic
(or oleophobic, i.e. water accepting, ink repelling) areas.
[0003] Printing masters are generally obtained by the so-called computer-to-film method
wherein various pre-press steps such as typeface selection, scanning, colour separation,
screening, trapping, layout and imposition are accomplished digitally and each colour
selection is transferred to graphic arts film using an image-setter. After processing,
the film can be used as a mask for the exposure of an imaging material called plate
precursor and after plate processing, a printing plate is obtained which can be used
as a master.
[0004] In recent years the so-called computer-to-plate method has gained a lot of interest.
This method, also called direct-to-plate method, bypasses the creation of film because
the digital document is transferred directly to a plate precursor by means of a so-called
plate-setter. In the field of such computer-to-plate methods the following improvements
are being studied presently :
(i) On-press imaging. A special type of a computer-to-plate process, involves the
exposure of a plate precursor while being mounted on a plate cylinder of a printing
press by means of an image-setter that is integrated in the press. This method may
be called 'computer-to-press' and printing presses with an integrated image-setter
are sometimes called digital presses. An review of digital presses is given in the
Proceedings of the Imaging Science & Technology's 1997 International Conference on
Digital Printing Technologies (Non-Impact Printing 13). Computer-to-press methods
have been described in e.g. EP-A 770 495, EP-A 770 496, WO 94001280, EP-A 580 394
and EP-A 774 364. The best known imaging methods are based on ablation. A problem
associated with ablative plates is the generation of debris which is difficult to
remove and may disturb the printing process or may contaminate the exposure optics
of the integrated image-setter. Other methods require processing with chemicals which
may damage the electronics and other devices of the press.
(ii) On-press coating. Whereas a plate precursor normally consists of a sheet-like
support and one or more functional coatings, computer-to-press methods have been described
wherein a composition, which is capable to form a lithographic surface upon image-wise
exposure and optional processing, is provided directly on the surface of a plate cylinder
of the press. EP-A 101 266 describes the coating of a hydrophobic layer directly on
the hydrophilic surface of a plate cylinder. After removal of the non-printing areas
by ablation, a master is obtained. However, ablation should be avoided in computer-to-press
methods, as discussed above. US-P 5,713,287 describes a computer-to-press method wherein
a so-called switchable polymer such as tetrahydro-pyranyl methylmethacrylate is applied
directly on the surface of a plate cylinder. The switchable polymer is converted from
a first water-sensitive property to an opposite water-sensitive property by image-wise
exposure. The latter method requires a curing step and the polymers are quite expensive
because they are thermally unstable and therefore difficult to synthesise. EP-A 802
457 describes a hybrid method wherein a functional coating is provided on a plate
support that is mounted on a cylinder of a printing press. This method also needs
processing. A major problem associated with known on-press coating methods is the
need for a wet-coating device which needs to be integrated in the press.
(iii) Thermal imaging. Most of the computer-to-press methods referred to above use
so-called thermal materials, i.e. plate precursors or on-press coatable compositions
which comprise a compound that converts absorbed light into heat. The heat which is
generated on image-wise exposure triggers a (physico-)chemical process, such as ablation,
polymerisation, insolubilisation by cross-linking of a polymer, decomposition, or
particle coagulation of a thermoplastic polymer latex. This heat-mode process then
results in a lithographic surface consisting of ink accepting and ink repelling areas.
In addition to some of the disadvantages of the prior art materials and methods, indicated
above, a major problem associated with all the known non-ablative thermal materials
is the limited shelf life. Because these materials all contain one or more reactive
compounds, the stability is highly dependent on temperature and/or humidity conditions
during storage.
(iv) Elimination of chemical processing. The development of functional coatings which
require no processing or may be processed with plain water, ink or fountain is another
major trend in plate making. WO 90002044, WO 91008108 and EP-A 580 394 disclose such
plates, which are, however, all ablative plates. In addition, these methods require
typically multi-layer materials, which makes them less suitable for on-press coating.
A non-ablative plate which can be processed with plain water is described in e.g.
EP-A 770 497 and EP-A 773 112. Such plates also allow on-press processing, either
by wiping the exposed plate with water while being mounted on the press or by the
ink or fountain solution applied during the first runs of the printing job.
[0005] EP-A 786 337 describes a method wherein dry powder, especially toner, is applied
to a support. The dry powder is then molten image-wise and removed at non-exposed
areas by a mechanical or electrostatic processing device. The latter step is necessary
because the exposure does not convert the powder from a hydrophilic to an oleophilic
state (or vice-versa) but only changes the adherence of the powder to the support
by melting said powder. Such a processing device is difficult to implement in a printing
press.
[0006] Another problem associated with most thermal materials disclosed in the prior art
is that these materials are suitable for exposure with either an internal drum image-setter
(i.e. typically a high-power short-time exposure) or an external drum image-setter
(i.e. relatively low-power long-time exposure). Providing a universal material that
can be exposed with satisfactory results on both these types of laser devices known
in the art is a requirement difficult to fulfil.
SUMMARY OF THE INVENTION
[0007] It is an object of the present invention to provide a cost effective method for preparing
a material which is suitable for making a printing master for conventional lithographic
printing by using computer-to-plate, computer-to-press or on-press coating methods
and which requires no processing or can be processed on-press by applying plain water,
ink or fountain. It is a particular object of the present invention to provide a method
for making a heat-mode material which is characterised by an excellent stability thereby
guaranteeing a long shelf life. It is still another object of the present invention
to provide a method for making a universal material which can be exposed with internal
as well as external drum image-setters. The above objects are realised by the method
specified in claim 1. Preferred embodiments of the method according to the present
invention are specified in the dependent claims.
[0008] Further advantages and embodiments of the present invention will become apparent
from the following description.
DETAILED DESCRIPTION OF THE INVENTION
[0009] Methods have been described in the prior art using heat-mode materials wherein a
light absorbing compound acts as a light-to-heat convertor and wherein the heat generated
upon exposure triggers reactive compounds to undergo a (physico-)chemical reaction.
Due to the presence of reactive compound(s), care must be taken with regard to storage
conditions to guarantee a long shelf life of the material. In such materials the light
absorbing compound is present in a typical amount relative to all the compounds in
the material, excluding the support, of 1 to 10% by weight.
[0010] It is surprising that, according to the present invention, the presence of other
reactive compounds besides the light absorbing compound is not essential and an imaging
material, which is suitable for making a lithographic printing master, may be obtained
by applying on a metal support a dry powder which contains a light absorbing compound
in an amount not less than 50% by weight relative to the dry powder and which is preferably
substantially free from other reactive compounds besides the light absorbing compound.
[0011] In addition to this surprising effect, the materials made by the method of the present
invention require no processing or can be processed by applying plain water, ink or
fountain. Since it is a dry coating method, the method of the present invention is
very suitable for computer-to-press applications and on-press coating procedures.
Another major benefit of the materials made according to the present invention is
the excellent stability : they can be stored during 2 minutes at 100°C without toning
(accepting ink in non-exposed areas), contrary to conventional thermal lithographic
printing plate precursors which show significant toning when exposed to the above
conditions. Some materials made according to the present invention, especially those
comprising carbon as a light absorbing compound, can even be stored during 2 minutes
at 150°C without noticeable toning.
[0012] The imaging mechanism of the materials that are made according to the present invention
is not known, but may rely on a heat-induced interaction between the light absorbing
compound and the metal support. For instance, it was observed that the aluminium signal
measured by secondary ion mass spectroscopy while sputtering away the upper 2 nm from
the surface of a material, consisting of an anodised aluminium support and a layer
consisting exclusively of a light absorbing compound, drops upon image-wise exposure
down to 50% or even 10% of the signal measured at unexposed areas, the specific value
being highly dependent on the structure of the light absorbing compound used.
[0013] The features of the present invention, as specified in the claims, shall be understood
as indicated hereafter. The word "image" is used herein in the context of lithographic
printing, i.e. a pattern consisting of oleophilic (printing) and hydrophilic (non-printing)
areas. The material that is made according to the present invention is negative working,
which means that the areas, which are exposed to light, are rendered oleophilic and
thus ink accepting due to said exposure. In the context of the present invention,
the feature "negative working" may be considered as an equivalent of the feature "non-ablative",
since in ablative materials the functional layers are completely removed from the
underlying (hydrophilic) metal support upon image-wise exposure so as to obtain a
positive image (exposed areas are hydrophilic, ink repelling). Analysis of the exposed
areas of the material made according to the method of the present invention indeed
showed that the layer or stack of layers is not or only partially removed upon image-wise
exposure but, instead, is converted into a hydrophobic surface on the metal support.
The unexposed areas are hydrophilic or become hydrophilic after processing with plain
water, ink or fountain. The exposed areas are oleophilic and form the printing areas
of the printing master.
[0014] The light absorbing compound is the main compound of the dry powder. The feature
"main compound" designates that the compound is present in an amount not less than
50% by weight relative to all the compounds in the dry powder. This feature distinguishes
the present invention from prior art methods as described in EP-A 786 337 using toner
as a dry powder, since it is well known to the skilled person that toner particles
comprise a low amount of light absorbing compound, which is typically about 5% by
weight. In a preferred embodiment the amount of light absorbing compound is not less
than 70% by weight and even more preferably not less than 90% by weight relative to
all the compounds in the dry powder. In a highly preferred embodiment the dry powder
consists essentially of a light absorbing compound. Mixtures of light absorbing compounds
can also be used, and then, the total amount of all light absorbing compounds relative
to all the compounds in the dry powder is not less than 50% by weight, more preferably
not less than 70% by weight and even more preferably not less than 90 % by weight.
[0015] Though the dry powder may comprise other compounds in addition to the light absorbing
compound, the amount of other reactive compounds besides the light absorbing compound
is preferably less than 20% by weight relative to the dry powder. The feature "reactive
compound" shall be understood as a compound which undergoes a (physico-)chemical reaction
due to the heat generated during image-wise exposure. Examples of such reactive compounds
are thermoplastic polymer latex, diazo resins, naphtoquinone diazide, photopolymers,
resole and novolac resins, or modified poly(vinyl butyral) binders. More examples
can be found in J. Prakt. Chem. Vol. 336 (1994), p. 377-389.
[0016] More preferably the amount of said other reactive compounds in the dry powder is
less than 10% by weight and most preferably, the dry powder is substantially free
from reactive compounds other than the light absorbing compound. The words "substantially
free" shall be understood as meaning that a small ineffective amount of such reactive
compounds may be present in addition to the light absorbing compound. Said small ineffective
amount is not essential for or does not significantly contribute to the imaging process
of the material made according to the present invention. This can be tested easily
by preparing a material without said small amount of reactive compounds and establishing
whether the material thus obtained can still be used to make a printing master. The
treshold value below which the amount of the other reactive compounds, besides the
light absorbing compound, may be regarded as "ineffective" depends on the nature of
the reactive compounds.
[0017] The dry powder used in the present invention may further comprise non-reactive compounds,
i.e. inert components such as e.g. a binder, a matting agent or a filler. The word
"inert" shall not be understood in the meaning of "non-functional", since these inert
compounds may be added to the powder to adjust certain physical properties, such as
e.g. surface roughness and friction coefficient of the applied layer or the rheological
properties of the powder. The word "inert" shall rather be understood as meaning "not
essential for the imaging process", though some inert compounds may have a (minor)
influence on the speed and image quality of the material.
[0018] Examples of such inert compounds are hydrophilic binders, e.g. carboxymethyl cellulose,
homopolymers and copolymers of vinyl pyrrolidone, vinyl alcohol, acrylamide, methylol
acrylamide, methylol methacrylamide, acrylic acid, methacrylic acid, hydroxyethyl
acrylate, hydroxyethyl methacrylate or maleic anhydride/vinylmethylether copolymers.
The amount of hydrophilic binder in the layer applied on the metal support is preferably
less than 40% by weight and more preferably between 5 and 20% by weight.
[0019] The method of the present invention may be used to apply a stack of layers on a metal
support but a single layer is preferred. The light absorbing compound may be present
in all the layers of said stack or may be localised in just a single layer of said
stack. In a method according to the latter embodiment the layer comprising the light
absorbing compound is preferably applied directly on the metal support. The layer
comprising the light absorbing compound is preferably very thin, i.e. having a dry
layer thickness not higher than 1 µm, preferably not higher than 0.5 µm and even more
preferably ranging from 0.1 to 0.25 µm. A layer thickness below 0.1 µm may still give
satisfactory results. For instance, it was observed that an anodised aluminium support
provided with a 0.1 µm layer consisting of finely divided carbon particles, which
was then cleaned by wiping thoroughly with a dry cloth and image-wise exposed with
an infrared laser, still provides an excellent printing master. The latter example
shows that it may be sufficient to fill the pores present in an anodised aluminium
support with light absorbing powder in order to obtain a material having the benefits
of the present invention.
[0020] The support used in the present invention is a metal support. Preferred examples
of said metal support are steel, especially polished stainless steel, and aluminium.
Phosphor bronze (an alloy comprising >90 wt.% of copper, <10 wt.% of tin and small
amounts of phosphor) can also be used. The aluminium support is preferably an electrochemically
grained and anodised aluminium support. Most preferably said aluminium support is
grained in nitric acid, yielding imaging elements with a higher sensitivity. The anodised
aluminium support may be treated to improve the hydrophilic properties of its surface.
For example, the aluminium support may be silicated by treating its surface with sodium
silicate solution at elevated temperature, e.g. 95°C. Alternatively, a phosphate treatment
may be applied which involves treating the aluminium oxide surface with a phosphate
solution that may further contain an inorganic fluoride. Further, the aluminium oxide
surface may be rinsed with a citric acid or citrate solution. This treatment may be
carried out at room temperature or can be carried out at a slightly elevated temperature
of about 30 to 50°C. A further treatment may involve rinsing the aluminium oxide surface
with a bicarbonate solution. Still further, the aluminium oxide surface may be treated
with poly(vinyl phosphonic acid), poly(vinyl methylphosphonic acid), phosphoric acid
esters of poly(vinyl alcohol), poly(vinyl sulphonic acid), poly(vinyl benzenesulphonic
acid), sulphuric acid esters of poly(vinyl alcohol), and acetals of poly(vinyl alcohols)
formed by reaction with a sulphonated aliphatic aldehyde. It is evident that one or
more of these post treatments may be carried out alone or in combination.
[0021] A highly preferred material made according to the present invention comprises an
anodised aluminium support and provided directly thereon a single recording layer
which consists essentially of a light absorbing compound and is substantially free
from other reactive compounds. On top of said recording layer there may be provided
a top layer for protecting the recording layer against moisture, chemicals, oxygen,
mechanical impact, etc.
[0022] The light absorbing compound used in the present invention is a compound which is
capable of converting light into heat. Useful compounds are for example organic dyes,
carbon black, graphite, metal carbides, borides, nitrides, carbonitrides, or oxides.
[0024] In one embodiment of the present invention the dry powder consists of or comprises
soot as a light absorbing compound, i.e. the black carbon obtained from the incomplete
combustion of organic materials such as oils, wood, natural gas, acetylene, coal,
wax or cork. Said soot may even be applied to the metal support by contacting a surface
of said support with a flame obtained by burning said organic material. Preferably
the surface of the metal support is contacted with the colder part of the flame where
combustion is incomplete, e.g. the yellow end of the flame of a candle. Electron microscopic
images of materials made in this way show a uniform coating of submicron soot particles.
[0025] According to the present invention, a metal support can be applied with a dry powder
by rubbing in the surface of said support with a light absorbing compound, e.g. carbon
or an organic dye. Alternative dry coating methods can also be used, e.g. sputter-coating
of carbon on the metal support or direct electrostatic printing (toner jet). The latter
technique can be used to apply the dry powder image-wise on a metal support and after
intense overall heating, e.g. by infrared laser exposure, a printing master is obtained.
Said infrared laser can be mounted on the same carriage as the direct electrostatic
printing head.
[0026] The method of the present invention can be used in computer-to-plate (off-press exposure)
or computer-to-press (on-press exposure) procedures. The method may also involve on-press
coating, i.e. applying a dry powder according to the present invention directly on
the metal surface of a cylinder of a rotary printing press. Said on-press coating
can also be performed indirectly by applying the dry powder on a metal support which
is mounted on a cylinder of a rotary printing press. In still another method according
to the present invention, said composition can be applied on a metal sleeve which,
after image-wise exposure and optional processing, is then transferred to a cylinder
of a rotary printing press.
[0027] The dry powder may also be applied on the metal support by contacting the surface
of said support with another material, which carries a dry layer containing a light
absorbing compound which is then transferred to the metal support. The method of this
embodiment can be automated easily, e.g. by incorporating a supply roll of such a
transfer material, such as a ribbon impregnated with light absorbing compound, in
a print station of a digital press similar to the configuration which is described
EP-A 698 488. The transfer material can be unwound from said supply roll and the layer
containing the light absorbing compound can then be brought in direct contact with
the surface of a plate cylinder by one or more contact rollers. After the transfer
step, which may be carried out by applying pressure and/or heat on said transfer material
while being in contact with the metal support, the used transfer material may be wound
up again on a take-up roll. In the latter embodiment, the transfer of dry power can
be carried out so as to obtain a uniform layer which then can be image-wise exposed.
Alternatively said pressure and/or heat can be applied image-wise, so that the light
absorbing compound is transferred image-wise to the metal support. This step then
may be followed by intense overall heating, e.g. by infrared laser exposure. However,
if sufficient heat is applied during said image-wise transfer, a suitable printing
master may directly be obtained without intense overall heating.
[0028] In an even more preferred embodiment of the automated method, described above, a
dry coating unit as described above, consisting of a supply roll, one or more contact
rollers and a take-up roll, is mounted on the same carriage as the laser exposure
unit of an external drum image-setter. Reference is made to e.g. Figure 1 of US-P
5,713,287 which illustrates a similar device wherein a spray coating unit is mounted
on the same carriage as the laser exposure unit in an external drum configuration.
In this way, said dry coating unit moves in front of the laser exposure unit along
the so-called slow scan axis, parallel to the axis of the plate cylinder. As the plate
cylinder is rotated during image-wise exposure (fast scan movement), the whole surface
of said cylinder passes the dry coating unit and a layer is coated along a spiral
path around the cylinder. Since the laser exposure unit moves together with the dry
coating unit, an area which has been coated during one revolution of the cylinder
is exposed by the laser exposure unit a number of revolutions later, i.e. coating
and image-wise exposing can be carried out almost simultaneously during the same scan
procedure.
[0029] The materials made according to the present invention can be exposed to light by
a light emitting diode or a laser such as a He/Ne or Ar laser. Preferably a laser
emitting near infrared light having a wavelength in the range from about 700 to about
1500 nm is used, e.g. a semiconductor laser diode, a Nd:YAG or a Nd:YLF laser. The
required laser power depends on the pixel dwell time of the laser beam, which is determined
by the spot diameter (typical value of modern plate-setters at 1/e
2 of maximum intensity : 10-25 µm), the scan speed and the resolution (i.e. the number
of distinct pixels per unit of linear distance, often expressed in dots per inch or
dpi; typical value : 1000-4000 dpi). A major benefit of the materials made according
to the present invention is that they can be used as a universal imaging material
which is suitable for exposure by internal (ITD) as well as external drum (XTD) image-setters.
ITD image-setters are typically characterised by very high scan speeds up to 500 m/sec
and may require a laser power of several Watts. Satisfactory results have also been
obtained by using XTD image-setters having a typical laser power from 100 mW to 500
mW at a lower scan speed, e.g. from 0.1 to 10 m/sec.
[0030] The unexposed areas of the material made according to the present invention can be
removed easily by applying plain water, ink or fountain to the material. This step
may be performed on-press, i.e. after mounting the exposed plate on the plate cylinder
of a printing press. The materials can even be used as a printing master immediately
after image-wise exposure without any additional processing because the unexposed
areas are readily removed by the fountain solution or the ink applied during the first
runs of the printing job. It is evident that the step of processing the material can
be omitted when the layer of dry powder is a non-contiguous layer, obtained by applying
said powder image-wise as described above. In the latter method, no powder is present
in non-image areas and as a result, the processing step may be omitted.
[0031] Most printing plates described in the prior art require a so-called post-bake, i.e.
an overall heating treatment after image-wise exposure and optional processing so
as to increase the run length of the plate. The materials made according to the present
invention allow to achieve satisfactory run lengths without a post-bake.
EXAMPLES
[0032] While the present invention will hereinafter be described in connection with preferred
embodiments thereof, it will be understood that it is not intended to limit the invention
to those embodiments.
Example 1
[0033] One surface of an anodised aluminium support was covered with a soot layer by contacting
said surface with the flame of a Bunsen burner fed with natural gas. After coating
the whole support, the layer was rubbed off with a dry cloth so as to obtain a uniform
thin layer of soot. The plate precursor thus obtained was image-wise exposed with
a Nd:YLF (1060 nm) external drum (XTD) laser having a power of 738 mW and a scan speed
of 80 m/sec. The plate was mounted on the cylinder of an AB Dick 360 (trade name)
printing press and cleaned with a sponge that was moistened with plain water. A print
job of 25000 copies was started using Rubber Base Plus VS2329 Universal Black ink,
trade name of Van Son, and Tame EC 7035 fountain, trade name of Anchor, the latter
diluted with water 50-fold. The print quality was very good throughout the press run.
[0034] Comparable results were obtained by applying the carbon layer using the following
alternative methods :
- rubbing in the plate with the ashes of a burned cork; or
- contacting the plate with the flame of an acetylene burner, a cigarette lighter or
a candle; or
- rubbing in the plate with a piece of graphite or even with a pencil.
[0035] A suitable printing master was obtained by image-wise exposing the above layer with
the following alternative laser sources :
- the same laser as above with a scan speed of 3.2 m/sec; or
- an XTD diode laser (830 nm) with a laser power of 40 or 80 mW at 1.0 or 2.0 m/sec
(four different combinations exposed on different areas of the plate); or
- an ITD Nd:YLF laser of 7.1 W at 367 m/sec; or
- an XTD laser diode-array (830 nm) having a combined power of 12 W at 1.2 m/sec.
Example 2
[0036] Three plate precursors were prepared by rubbing in the surface of an anodised aluminium
plate with a dry powder consisting of Cpd 1, Cpd 4 or Cpd 9 respectively. The samples
were image-wise exposed with an XTD Nd:YLF laser (1060 nm) with a power of 150 mW
at a scan speed of 2 m/sec. The plates thus obtained were used as a master in a print
job using the same press, ink and fountain as in Example 1. No special measures were
taken to ensure that the layer had a uniform thickness over the whole surface of the
plate and it was observed that the plates were completely hydrophobic at the centre,
where the coating thickness was the highest, regardless whether the plate had been
exposed at that area or not. At the edges, where the layer was much thinner, a good
printing quality was obtained with no toning in the non-exposed areas, indicating
the a low layer thickness is preferred for these light absorbing compounds.
Example 3
[0037] Cpd 2, Cpd 3, Cpd 10 and Cpd 11 were each rubbed in as a dry powder on the surface
of an anodised aluminium plate. The four materials thus obtained were image-wise exposed
with a XTD laser diode (830 nm) with a power of 60 or 80 mW and a scan speed of 1,
2 or 4 m/sec (six combinations exposed at different areas of each plate). The plates
were used as a master in a print job using the same press, ink and fountain as in
Example 1. All masters provided good printing results over the whole area of the plate.
[0038] Having described in detail preferred embodiments of the current invention, it will
now be apparent to those skilled in the art that numerous modifications can be made
therein without departing from the scope of the invention as defined in the appending
claims.
1. A method for making a lithographic printing master having printing and non-printing
areas, said method comprising the steps of
- making a non-ablative imaging material by applying a layer of dry powder to a metal
support;
- exposing said dry powder to heat or light at the printing areas;
- optionally removing the dry powder from the metal support at the non-printing areas
by applying water, ink or fountain;
characterised in that said dry powder comprises not less than 50% by weight of a light
absorbing compound.
2. A method according to claim 1 wherein the dry powder comprises not less than 70% by
weight amount of light absorbing compound.
3. A method according to claim 1 wherein the dry powder comprises not less than 90% by
weight amount of light absorbing compound.
4. A method according to claim 1 wherein the dry powder consists essentially of a light
absorbing compound.
5. A method according to any of the previous claims wherein the amount of other reactive
compounds present in the dry powder, besides the light absorbing compound, is less
than 20% by weight.
6. A method according to any of the previous claims wherein the dry powder is substantially
free from other reactive compounds besides the light absorbing compound.
7. A method according to any of the previous claims wherein the light absorbing compound
is a near infrared light absorbing compound.
8. A method according to any of the previous claims wherein the light absorbing compound
is carbon or soot.
9. A method according to any of the previous claims wherein the step of applying the
layer of dry powder to the metal support is carried out by contacting said support
with a flame.
10. A method according to any of the previous claims wherein the step of applying the
layer of dry powder to the metal support is carried out by contacting said support
with a transfer material having a layer which contains a light absorbing compound.
11. A method according to any of the previous claims wherein the layer of dry powder is
a non-contiguous layer which is image-wise applied to the metal support.
12. A method according to any of the previous claims wherein the metal support is an anodised
aluminium plate.
13. A method according to claim 12 wherein the anodised aluminium plate is mounted on
a cylinder of a rotary printing press.
14. A method according to any of the previous claims wherein the metal support is a sleeve
or a cylinder of a rotary printing press.
15. A method according to any of the previous claims wherein the thickness of the layer
of dry powder is not higher than 1 µm.