Note: Descriptions are shown in the official language in which they were submitted.
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AN ERASABLE PRINTING PLATE AND A PROCESS AND
APPARATUS FOR ERASING AND REGENERATING
THE PRINTING PLATE
TECHNICAL FIELD
The lnvention relates to an erasable printing
plate and to a process and an apparatus for erasing
and regenerating the printing plate.
BACKGROUND ART
A printing plate suitable for the wet offset
method is disclosed in DE 42 35 242 8. This printing
plate has strong microdipoles at least in its outer
layer, and its surface is hydrophobic. After
application of covering material corresponding to an
image to be printed, the non-image parts are
hydrophilized by a hydrophilizing agent. It is true
that these printing plates have the advantage that
they can readily be regenerated, i.e. after completion
of a printing process they can be erased in a simple
manner and provided with further images. However,
these printing plates have the disadvantage that it is
expensive to produce them.
Regeneration processes which are suitable for
smooth as well as rough, porous printing plates of
ceramic, glass or anodized aluminum are disclosed in
DE 41 23 959 C 1. The disadvantage of these
regeneration/erasing processes is that they are
technically relatively complicated and expensive to
carry out.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present
invention to provide a printing plate which can be
directly provided with images and repeatedly erased
and which has the advantage of simpler production
compared with known printing plates.
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Pursuant to this object, one aspect of the
present invention resides in a printing plate which
has a pore-free and smooth surface with a roughness Ra
of less than 1 ~m. The smooth surface is hydrophilic
or can be hydrophilized and plate material contains no
strong microdipoles.
It is a further object of the invention to
provide a process for repeatedly erasing and
regenerating such a printing plate, which can be
carried out within the printing press, without removal
of the plate cylinder or of the printing plate.
Pursuant to this object, another aspect of the
present invention resides in a process for erasing a
print image on the printing plate, which process
includes pressing a cleaning cloth against the surface
of the printing plate using a pressure roller while
the plate cylinder on which the printing plate is
mounted slowly rotates. The cleaning cloth is unwound
from a feed roller and is wound onto a wind-up roller
either in a step wise manner or continuously. The
pressure roller permits the contact pressure of the
cleaning cloth against the plate cylinder to be
varied. Additionally, nozzles supply cleaning
solutions directly to the surface of the printing
plate or to the cleaning cloth.
Another aspect of the present invention resides
in a printing press which includes a plate cylinder on
which the printing plate is mounted, and still further
includes an erasing and hydrophilizing apparatus
positionable at the plate cylinder for cleaning the
printing plate and hydrophilizing the surface thereof.
According to the invention, there is provided a
printing plate a printing plate which can be directly
provided with an image and erased and is suitable for
wet offset printing, the printing plate comprising a
pore-free and smooth surface with a roughness Ra of <
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1 um, which surface is at least one of hydrophilic and
hydrophilizable, the plate also contains no strong
microdipoles.
The invention further provides a printing press
comprising a printing press. The printing press
comprises a plate cylinder; a printing plate mounted
on the plate cylinder; and erasing and hydrophilizing
means arranged to be operatively positionable at the
plate cylinder for cleaning the printing plate and
hydrophilizing a surface of the printing plate.
According to the invention, there is also
provided a process for erasing a print image from a
printing plate mounted on a plate cylinder, comprising
the steps of: pressing a cleaning cloth against a
surface of the printing plate using a pressure roller
while the plate cylinder slowly rotates; unwinding the
cleaning cloth from a feed roller and winding the
cleaning cloth onto a wind-up roller one of
continuously and step wise; and applying a cleaning
solution via nozzles one of directly to the surface of
the printing plate and to the cleaning cloth.
For a better understanding of the invention,
its operating advantages, and specific objects
attained by its use, reference should be had to the
drawing and descriptive matter in which there are
illustrated and described preferred embodiments of the
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
The single figure shows, in cross-section, a
printing unit having an apparatus according to the
invention for erasing and regenerating the printing
plate according to the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
For a printing plate to be capable of being
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readily erased, it must be smooth and pore-free.
Hence, it must not contain cavities, at least not in
its surface, and must not be rough, so that no
mechanical anchoring occurs between the surface and
the image-producing material, printing ink or dirt,
which can no longer be removed once contained in the
surface cavities. To accomplish this the surface
roughness Ra must be < 1 um. To ensure that the
printing plates are suitable for the wet offset
method, their surface must be either hydrophilic or
easily capable of being rendered hydrophilic. The
printing plate is hydrophilized by spraying a
hydrophilizing agent, such as a plate cleaner, onto
it, for example from nozzles. The hydrophilic
character of the surface must be capable of being
restored even after thermal stress, i.e. after a
thermal fixing step for material applied imagewise.
Hence, it must at most become reversibly weaker or
disappear reversibly.
Suitable materials for the production of such
printing plates are, for example, ceramics. The plates
can be are prepared by the sol-gel process, a PVD or a
CVD process, in a thermal spray process, for example
the plasma spray process, or by a sinter process. The
ceramic must then be ground as smooth as glass, i.e.
it may only have a roughness Ra < 0.3 um. If the
printing plates produced in one of the stated
processes are still not pore-free, they are
subsequently sealed.
Ceramic printing plates contain, for example,
alumina, aluminum silicate (mullite), zirconium
silicate or zirconium oxide. It is particularly
advantageous if the hydrophilic character of a ceramic
printing plate not only is achieved by hydrophilizing
the surface but is a volume property of the ceramic
layer.
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Instead of being produced from ceramic,
printing plates having a smooth surface may also be
produced from glass, preferably from a borosilicate-
based glass which is resistant to chemicals and to
heat.
Other materials for the production of the
printing plates are metals, among which alloys having
a good microstructural stability and high resistance
to oxidation even during thermal cycling are
particularly suitable. When used as printing plates,
the metals have a surface smooth as rolled material
with a roughness Ra < 0.2 ~m.
Nickel chromium steels are particularly
suitable for producing printing plates. Nickel-
chromium-iron alloys and nickel-chromium-molybdenum
alloys are preferably used, in particular the alloys
obtainable from the company Heynes International
Incorporated, Kokomo, USA, under the material numbers
2.4665 and 2.4819, 2.4602, 2.4636 and 2.4638,
respectively. Other very suitable nickel-chromium-
molybdenum and nickel-chromium-iron alloys are the
alloys obtainable under the material numbers 2.4856
and 2.4851 from the company Huntington Alloy Product
Division, Huntington, USA. The common feature of all
these alloys is that, like the other alloys not named
more specifically here, they have good resistance to
the thermal cycling caused by the thermal fixing as
well as good resistance to oxidation.
After the end of the printing process, such a
printing plate which has been directly provided with
an image and consists of one of the above-mentioned
materials, a ceramic, a glass, or a metal, or at least
has a surface layer of one of these materials, can be
erased within the printing press in the manner
described next. First, the printing ink residues and
the coating applied imagewise are removed, for example
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wiped off, by means of a cleaning agent. The cleaning
agent is a solvent or solvent mixture that contains no
solid components. In order to be able to remove even
final and slight residues of the coating applied
imagewise to the printing plate, which residues
otherwise cause so-called ghost images, the surface of
the printing plate is then mechanically rubbed. A
polish-containing cleaning agent, for example a plate
cleaner, as generally used for cleaning printing
plates, serves for this purpose. The plate cleaner is
then removed, for example with water, and the printing
plate is rendered hydrophobic again by wetting with a
solvent, in order to prepare it for a further direct-
image-providing step. After provision of an image and
fixing, the printing plate is hydrophilized again, as
also disclosed, for example, in DE 42 35 242 C 1,
unless the material of the printing plate already has
a hydrophilic surface and it is therefore sufficient
if the non-image parts are hydrophilized again only by
the fountain solution during the printing process.
The apparatus shown in the figure is used both
for erasing and for hydrophilizing and for fixing the
printing plate described above, which is directly
provided with an image, and other printing plates, as
disclosed, for example, according to DE 41 23 959 C 1
or DE 36 33 758 A 1. The fixing step comprises heating
the printing plate provided with an image to a surface
temperature between 170C and 210C by means of a
drier 7. Alternatively, the printing plate can be
heated either inductively or by an infrared lamp. In
other fixing processes, W radiation or electron beams
are used. It is also possible to cure the printing
plates by suitable chemicals, for example merely by
moisture.
The erasing and hydrophilizing apparatus is
installed as a fixed unit in a printing unit of an
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offset printing press and is arranged on the
circumference of a plate cylinder 1 having a printing
plate 2 which can be directly provided with an image.
A rubber blanket cylinder 3 for transferring the print
image to a print medium, such as the print medium web
17 shown here, and rollers 4, 5 of an inking unit or
of a damping unit rest against the plate cylinder 1.
An image-providing unit (not shown here) is
likewise arranged on the circumference of the plate
cylinder 1, for direct provision of an image.
The erasing and hydrophilizing apparatus has a
cleaning apparatus 6 and the drier 7, which can be fed
toward the printing plate 2 and moved away from it
again. The cleaning apparatus 6 operates by means of a
cleaning cloth or cleaning fleece 8, which passes from
a feed roller 9 via a pressure roller 10 which presses
the fleece 8 against the printing plate 2, and is then
wound onto a wind-up roller 11.
The pressure roller 10 is in turn rotatably
mounted in a vibrating head 12. The vibrating head 12
is pressed against the printing plate 2 by means of a
pressure cylinder 13. Nozzles 14, which are present in
front of the cleaning apparatus 6 in the direction of
rotation of the plate cylinder 1, can apply a cleaning
liquid or a cleaning paste, a hydrophilizing agent,
for example a plate cleaner, or another agent which
serves either for erasing the printed image applied to
the printing plate 2 and for removing printing ink
residues or for hydrophilizing the surface of the
printing plate 2, to the printing plate. This agent,
together with the dirt which it removes, can be taken
up by the cleaning cloth 8 if the pressure cylinder 13
presses the pressure roller 10 against the printing
plate 2 and the cleaning cloth 8 is moved past the
printing plate 2. The nozzles 14 can preferably be
adjusted with regard to their angle to spray the agent
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either directly onto the printing plate 2 or onto the
cleaning cloth 8. The pressure roller 10 is, for
example, rubber-coated. The cleaning cloth 8 is wound
either in portions or continuously from the feed
roller 9 onto the wind-up roller 11. Preferably, the
pressure with which the cleaning cloth 8 is pressed
against the printing plate 2 can also be varied.
The printing plate 2 cleaned by the cleaning
apparatus 6 can then be dried by the drier 7, with hot
air via a hot air supply apparatus 15. The hot air
flows subsequently, when it contains, for example,
solvent residues, back through an extraction apparatus
16.
The erasing and hydrophilizing apparatus can,
for example, be positioned alternately at the plate
cylinder 1 or at the rubber blanket cylinder 3 if it
is also intended to clean the rubber blanket by means
of the erasing and hydrophilizing apparatus.
In another embodiment, the erasing and
hydrophilizing apparatus is located outside the
printing press, in order there to clean only printing
plates 2 or rubber blankets.
The cleaning solutions used are solvents for
removing the printing ink residues and the polymer
layer applied imagewise, or other cleaning agents
which contain abrasive media, such as, for example,
some of the plate cleaners conventionally used in the
graphics industry, or water for removing abrasive
residues. The cleaning agents can be used one after
the other in any desired sequence or in a changing
sequence. Also, the printing plate surface to be
erased can also be rubbed off with a dry cleaning
cloth after or between the individual cleaning steps.
In order to increase the efficiency of cleaning agents
containing abrasive media, the cleaning cloth 8
preferably moves in a manner which changes.
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The invention provides a printing plate 2 which
can be repeatedly directly provided with an image and
erased and is suitable for a wet offset printing
method. I-t has a smooth and pore-free surface which is
hydrophilic or can be hydrophilized after being
provided with an image. The printing plate 2 contains
no strong microdipoles and consists of a ceramic, a
glass or a metal, in particular of a metal alloy. By
means of an erasing and hydrophilizing apparatus 6, 7
preferably integrated in the printing press, the
printing plate 2 can be repeatedly erased and
prepared, i.e. rehydrophilized, for a further image-
providing and printing process.
The invention is not limited by the embodiments
described above which are presented as examples only
but can be modified in various ways within the scope
of protection defined by the appended patent claims.
