Note: Descriptions are shown in the official language in which they were submitted.
~168~95
"PROCESS AND APPARATUS FOR GRAVURE"
FIELD OF THE INVENTION
The present invention is directed to a process
and an apparatus for gravure using an erasable and
reusable gravure form proceeding from a gravure blank
form with a base screen which is designed at least for
the maximum amount of ink to be transferred
BACKGROUND OF THE INVENTION
Gravure refers to a printing process using
printing elements which are depressed relative to the
surface of the form. After the printing form is
completely inked, the printing ink is removed from the
surface. The ink remains only in the depressed areas.
Copper-coated steel cylinders, hollow cylinders
mounted on tensioning cores or, in many cases, copper
plates clamped on cylinders may be used as printing
forms, for example.
Due to the type of inking and the wiping of the
surface of the form with doctor blades, pure surface
printing is not possible. The entire graphic must be
resolved into lines, dots or screen elements. Due to
their differing depth and magnitude, the individual
printing elements take up varying amounts of printing
Z5 ink. Consequently, the impression will have different
ink values at different locations on the image.
Various working methods are currently used for
producing a gravure form. For instance, in the
variable-depth method, the etching principle consists
in a gradual diffusion of concentrated ferric chloride
solutions through a pigment-gelatin layer. The
pigment reproduction on the copper printing form is
formed of a hardened gelatin relief corresponding to
the gradation of tones of the transparencies. The
engraving process is characterized by line-scanning of
the image and text by photocells and simultaneous
engraving of the printing form by engraving heads. It
2 2168595
should be noted in particular that depressions are
made in the copper layer of the printing form a high-
energy electron beam which is directed on the blank
form under vacuum and removes material in conformity
to the intended image. The printing form which is
engraved in this way can be provided with screens with
varying depths and surfaces.
Depressions can also be made using a high-energy
laser beam. In so doing, appropriate steps must be
taken to ensure that the laser energy is coupled to
the substrate, since copper is especially prone to
reflect a laser beam when not subjected to special
preconditioning.
Further, German laid-open patent application
publication 27 48 062 discloses a process for
producing an engraved printing form in which a gravure
blank form is first prepared by providing the smooth
surface with depressions of equal depth and magnitude
in a uniform manner and then covering the engraved
surface with a light-sensitive substance so as to fill
up all of the depressions. The blank form is then
exposed photographically with the desired image so
that the exposed areas are polymerized and the
unexposed portions can be washed off, resulting in a
differentiated image.
It can be asserted in general for all gravure
processes that the depth of image locations on the
printing form is greater than that of non-image
locations. In doctor-blade gravure, in particular,
the screen grid forms webs of uniform height which
define the image locations and form a support surface
for the doctor blade. A special set of printing form
cylinders (for each printing ink there must be one
printing form cylinder with a corresponding number of
printing sides) is required for every printing job.
These cylinders are produced with the required
cylinder circumference depending on the printing
3 2168595
format. When setting up the gravure press or rotary
printing machine, the appropriate printing form
cylinders must be exchanged. A modern cylinder of
this type, e.g., with a width of 200 cm, weighs
approximately 800 kg. The mechanical cost for the
processes described above is very high, since these
processes can only be carried out outside the printing
machine. In addition, each of these production
processes involves steps such as electroplating or
coating, exposure and development, which rules out the
possibility of reusing the same printing form without
extensive processing, in particular chemical
processing. Further, after etching or engraving to
form the image, that is, after removal of material,
chroming is usually carried out to prolong service
life.
If the printing form is to be stored for
subsequent repeated applications, it is generally
necessary to reserve space for the entire cylinder.
For this reason, production of printing forms is very
involved and therefore expensive, particularly when
electroplating is required. Moreover, the resulting
toxic sludge is objectionable in ecological respects.
On the other hand, German Patent 38 37 941
discloses a process for producing a gravure form in
which the image can be produced directly in the
printing machine and in which, moreover, the image can
be removed from the gravure form in the printing
machine and the gravure form can be prepared for a new
image. Likewise in this case, a gravure blank form is
produced with a base screen designed at least for the
maximum amount of ink to be transferred. In the
printing machine, an amount of thermoplastic substance
in inverse proportion to the image information is then
introduced into the depressed portions from a nozzle
of the image point transfer unit or by means of image-
correlated ironing so as to reduce the effective
4 216859~
volume of the depressions. In other words, in
contrast to the other methods, the image is formed on
the gravure blank form by image-forming application of
material. After the printing job, the thermoplastic
substance can then be liquefied in the printing
machine by means of a heat source and removed from the
printing form cylinder by a wiping and/or blowing or
suction device.
However, the application of material to form
images raises problems with respect to the positioning
accuracy of the image. Material deposited on the webs
cannot easily be introduced into the depressions
completely. Yet, in order for all of the transferred
material to contribute in a desired manner to the
reduction in the effective volume of the depressions,
this material must be introduced in its entirety.
SUMMARY OF THE INVENTION
Accordingly, the object of the present invention
is to develop a process and an apparatus for gravure
printing in which the gravure printing form can be
produced inexpensively and also directly in the
printing machine and in which the positioning of the
image can be made more accurate.
According to the invention, there is provided a
process of gravure printing of an image using an
erasable and reusable gravure form including a blank
form having a base screen designed for accommodating a
maximum amount of ink to be transferred to a web, the
process comprising the steps of: (a) applying a
liquefiable substance using an applicator device to
uniformly fill depressions within the base screeni (b)
screening the gravure form by removing material from
the depressions in conformity with an image intended
to be printed using an image point transfer device;
(c) inking the screened gravure form using an inking
systemi (d) printing in gravure on the web; and
216853~
(e) regenerating the blank form and repeating step
(a).
The invention also provides a device for gravure
printing of an image using an erasable and reusable
gravure form cylinder rotating therein including a
blank form including a base screen having depression
therein and being designed for accommodating a maximum
amount of ink to be transferred to a web, the
apparatus comprising means for applying a liquefiable
substance to uniformly fill the depression in the base
screen; an image point transfer device for screening
the gravure form cylinder by removing the substance
from the depressions applied by the means for applying
in conformance with the image to be printed; an inking
system for inking the screened gravure form cylinder;
and means for regenerating the base screen of the
inked and screened gravure form cylinder for reusing
the gravure form, the means for applying, image point
transfer device, means for inking and means for
regenerating each being spatially positioned about a
circumference of the gravure form cylinder and
adjustable in a rotating direction about the
circumference of the gravure form.
There is also provided, according to the
invention, an erasable and reusable gravure blank form
for gravure printing of an image according to the
process of: (a) applying a liquefiable substance
using an applicator device to uniformly fill
depressions within the base screen; (b) screening the
gravure form by removing material from the depressions
in conformity with an image intended to be printed
using an image point transfer device; (c) inking the
screened gravure form using an inking system; (d)
printing in gravure on the web; and (e) regenerating
the blank form and repeating step (a), the gravure
blank form comprising: a support cylinder; and a blank
form including a base screen designed for
6 2168595
accommodating a maximum amount of ink to be
transferred to a web positioned about the support
layer, the base screen including depressions therein.
Storage of gravure forms is eliminated since the
cycle of process steps can be carried out repeatedly.
Another special advantage of the process
according to invention and of the apparatus for
carrying out this process consists in that wear on the
gravure blank form is compensated for because the
maximum image-forming depth in the applied substance
on the gravure printing form is appreciably less than
the original depth of the depressions of the
prestructured blank form. That is, if the depth of
the depressions is reduced due to wear on the webs,
the maximum image-forming depth can nevertheless be
achieved by a wide margin. For this reason, the webs
of the blank form are also advantageously constructed
so as to extend vertically to the surface of the
gravure form as far as possible.
DESCRIPTION OF THE DRAWINGS
Preferred embodiment examples and variants of the
invention are explained in the following with
reference to the schematic drawings, in which:
Fig. 1 shows the basic construction for carrying
out the process steps according to the invention;
Fig. 2 shows a detailed view of the surface of a
gravure blank form;
Fig. 3 shows the ablation of the liquefiable
substance from the surface of a gravure form for the
purpose of forming images depending on a given laser
beam intensity per writing line;
Fig. 4 shows an embodiment example of an
apparatus according to the invention;
Fig. 5 shows an applicator device;
Fig. 6 shows an image point transfer device for
image-forming ablation by suction;
7 21 6859s
-
Fig. 7 shows the construction of a micromirror
array for an image point transfer device for image-
forming ablation; and
Figs. 8 and 9 show an arrangement for image-
forming ablation according to Fig. 7.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The image can be formed on the blank form 1
directly in the printing machine with the process and
apparatus according to the invention. The gravure
form on which an image has been formed can also be
erased and prepared for reuse in a simple manner in
the printing machine.
As is shown in Fig. 1, a prestructured gravure
blank form 1 with a base screen designed for at least
the maximum amount of ink to be transferred is filled
in a first step at a point 2 during rotation of the
form 1 with a liquefiable substance using an
applicator device. Examples of the substance used for
filling may be a thermoplastic material or wax (hot
melt), lacquer or a cross-linkable polymer melt or
polymer solution. The cross-linkable polymer melt or
polymer solution is also known as a reactive system
and is characterized by an extremely high resistance
to abrasion. The surface of the gravure form is
substantially smooth after filling. The filled in
substance is then removed from the depressions at a
point 3 of rotation of the form 1 so as to form an
image by means of thermal energy applied by an image
point transfer device. The gravure form can now be
inked at a point 4 during rotation of the form 1 by
means of an inking system so that printing stock 5 may
be printed at a point 6 by the gravure blank form 1.
After the printing process on the printing stock
5, the surface of the gravure form 1 is regenerated in
that the ink residues are cleaned off at a point 7 of
rotation of the form 1, the liquefiable substance is
preferably completely removed at a point 8 from the
_ 8 216859~
prestructured depressions, and the depressions are
filled again in a uniform manner at a point 2. The
liquefied substance can be removed from the
prestructured depressions by means of a heat source
and/or by a blowing or suction device.
Fig. 2 shows a prestructured gravure blank form 1
on a cylinder 10 with webs 9 which extend helically
around its cylindrical surface at a defined angle.
The spacing between the webs 9 preferably corresponds
to the spacing of currently used gravure screens. For
an 80 line/cm screen, this spacing would be 125 ~m.
However, the spacing may also be substantially greater
provided that the webs 9 can still guide the doctor
blade dependably without noticeable flexing of the
doctor blade and without resulting in excessive wear
on the webs 9. The gravure blank form 1 is generally
resistant to wear at least at the web surfaces, e.g.,
it is coated with chrome or titanium oxide or is
produced from ceramics and is thus inherently very
hard, and/or is provided with a defined roughness so
that the doctor blade glides on a defined liquid film
during printing.
After the depressions between the webs 9 of the
gravure blank form 1 have been filled with the
liquefied thermoplastic material, the gravure form 20
can be provided with an image by burning off, as shown
in Fig. 3, by the thermal energy of an image point
transfer device, in particular a laser 21 in a manner
analogous to an external drum exposer. Nd YAG or
NDYLF lasers which are switchable between a plurality
of intensity levels 23 via an acousto-optical
modulator are preferably used. The laser beam 22 can
be guided to the gravure blank form 1 and focused
thereon via an optical fiber. It is preferable not to
exceed a cell size of more than approximately 2/10 mm.
That is, approximately after reaching this distance at
most, the image-forming ablation produces a web which
21685g5
does not serve to guide the inking doctor, but rather
to compel the cell to be emptied of ink during
printing. Thus, it is possible in particular to
address surfaces (pixels) which are smaller than an
actual gravure cell so that a cell may be produced by
a plurality of pixels.
Further, the image-forming ablation 3 can be
assisted by setting the filled gravure blank form 1 in
rapid rotation in such a way that some of the material
to be removed is evaporated and some is thrown off.
In an advantageous variant, the gravure blank
form 1 is not constructed as a solid cylinder, so that
a low heating capacity is achieved. Thus, a thermally
insulating layer, e.g., of fiberglass-reinforced
carbon, is provided between a base layer and the
surface layer which carries the base screen of the
gravure blank form 1 and has a thickness of several
tenths of a millimeter. The thermoplastic used as
liquefiable substance can also be a resin or a
synthetic or natural wax.
Fig. 4 shows a preferred embodiment example of an
apparatus for implementing the process according to
the invention.
A device 11 for applying a liquefiable substance
directly to a gravure form cylinder 10 supporting the
gravure blank form 1 is arranged inside the gravure
press so as to be adjustable. A preferred
construction of this device 11 is illustrated in Fig.
5. This device 11 comprises a box lla which opens
toward the surface of the gravure blank form 1 and
contains a heating cartridge llb. The device 11 is
heated and contains the molten thermoplastic llc which
can be filled and refilled in granulated form. The
melt llc is introduced on the surface of the gravure
blank form 1 by gravitational force and capillary
action and penetrates into the depressions of the base
screen. Compressed air or hydraulic pressure generated
lo 2168595
by means of a pump can also be used instead of
gravitational force. Due to the narrow gap between
the gravure blank form 1 and the applicator device 11,
a capillary and hydrodynamic force introduces
precisely the amount of substance required for
filling
In a variant construction of this embodiment
example, two formed strips lld, lle (Fig. 5) are
provided for the device 11. One of them (lle) is
provided in front of the narrow gap between the
gravure form 1 and applicator device 11 while the
other (lld) is arranged after this gap as viewed in
the rotating direction of the gravure form cylinder
10. The formed strip lld subsequent to the gap has a
form corresponding to the form of the cylinder 10 and
is positioned at a very small distance therefrom (some
hundredths of a millimeter) by means of precise
guidance or by supporting cheeks and is designed so as
to be heatable in order to adjust the viscosity of the
filling material so as to promote the effect of the
hydrodynamic forces and ensure a complete filling of
the depressions of the base screen. Further, the rear
edge of this strip lld is sharpened in order to ensure
a clean tearing of the filling material from the gap.
The front formed strip lle is held from the cylinder
10 at a distance greater than the distance between the
back formed strip lld and the cylinder 10 (several
hundredths of a millimeter to several tenths of a
millimeter) so that the gap, which accordingly widens,
is filled with material but the hydrodynamic forces
have an appreciably reduced effect. The actual
filling occurring in the region of the formed strip
lld is accordingly prepared, in particular by means of
heating and pre-filling the thermally insulated
surface of the printing blank form.
An excess amount of the liquefiable substance llc
can also be applied to the gravure blank form 1 in the
ll 21 6859~
heated state. After cooling, the surplus is then
removed, i.e., wiped and/or polished, from the gravure
blank form 1 by means of an adjustable position of the
doctor blade 12. The doctor blade 12 can change for
this purpose. After the thermoplastic material has
cooled, the filled surface of the gravure blank form 1
is preferably polished again in order to adjust the
roughness of the surface in a defined manner.
After ablation 3 of the filled gravure blank form
1 in accordance with the intended image, the gravure
form can be inked by means of an inking system 13. A
chamber doctor is preferably used for this purpose
since it requires less space at the circumference of
the cylinder than a conventional inking system and can
simply be withdrawn from the gravure cylinder 10
during the other process steps. Of course, the
applicator devices 11, doctor blade 12 and image point
transfer unit (e.g., the laser 21) and other devices
can be removed from the gravure cylinder 10 during the
inking so as to protect them from ink and ink mist.
As will be seen from Fig. 4, printing stock 5 can
now be printed against an impression cylinder 14 by
gravure, but preferably by indirect gravure. In
indirect gravure, the paper is not printed upon
directly by the printing form cylinder, but rather a
roller coated with a smooth rubber surface is located
between the printing form cylinder and the paper.
This roller serves as an intermediate substrate and
thus decouples the printing form -cylinder from the
printing stock. In conventional direct gravure, two
hard materials roll off one another in the printing
gap between the printing form cylinder and printing
stock. The printing stock additionally provides an
abrasive action. In order to counter this, hard
materials are required for the printing form. In
indirect gravure, two printing gaps are used instead
of one, wherein a hard material rolls off a soft
12 2I6859~
material in each case. In addition, the printing form
cylinder no longer comes into direct contact with the
abrasive paper medium. This permits substantially
softer materials to be used without decreasing the
service life of the materials. The doctor blade, the
other part which is subject to wear at the printing
form cylinder, is guided by the webs formed of hard
material and thus also does not contact the softer
filling material suitable for thermal ablation. As a
result of this step, the service life of a gravure
form produced according to the invention is
substantially improved.
After the required printing process, ink residues
are cleaned off the gravure form by means of a
regenerating device 15, preferably in the form of an
ultrasonic cleaning installation which is likewise
constructed as an adjustable system similar to a
chamber doctor, and the liquefiable substance is
removed from the depressions of the base screen of the
printing blank form 1 so that the cycle (filling 2,
image-forming ablation 3, inking 4, printing 6,
regeneration 7, 8) can start from the beginning.
The ultrasonic cleaning installation can be
operated at least two different levels, one level has
low sonic energy and/or with a liquid serving only to
loosen the ink serves to remove the remaining ink.
The other levels each have correspondingly higher
sonic pressures and/or other cleaning agents, serve
for partial or complete removal of the filling
material.
Another important advantage of the invention
consists in the noticeable improvement in quality
compared to conventional gravure, particularly with
respect to text reproduction. This is achieved in
that the writing resolution for producing images lies
well below the spacing between two webs, e.g., 500
lines per cm. Accordingly, text can be screened at
13 ~I 6859~
this high resolution and character edges can be
achieved which are substantially sharper than in
conventional gravure. In general, approximately 400
lines per cm are specified as the lower limit for good
text reproduction. Conventional gravure form
production has a resolution of 120 lines per cm
maximum and must therefore simulate sharp edges with
more or less small dots interrupted by blank spaces.
This is why gravure text always has a so-called
sawtooth effect.
In order to achieve the same quantity of gray
steps in the image as the gravure which varies every
dot in up to 200 depth steps, a binary exposer, i.e.,
one working in variable-surface operation, must be
able to write at least 1000 lines per cm. Although
this binary writing mode is also suitable in
principle, the present invention preferably uses a
combination of variable-surface and conventional,
i.e., variable-depth, gravure screening known as a
hybrid screen. This screen is written, for example,
with 500 lines per cm. However, every dot can be
graduated in a plurality of depths. For instance,
five different depths (0%, 25%, 50%, 75% and 100%) at
a writing resolution of 500 lines per cm achieves the
same halftone quality as a writing resolution of 1000
lines per cm and only two depths (0% and 100%) or a
writing resolution of 100 lines per cm and 101
different depths. If 10 different depths are used,
for example, this corresponds to the information
content of 250 gray steps at 100 lines per cm. The
present density information which is typically given
at a resolution of 256 steps is converted into the
hybrid screening model, which has appreciably fewer
than 256 steps per writing point, typically roughly
10, by the known preliminary printing step techniques
of "error diffusion", dithering or stochastic
screening. All of these methods are normally used
14 2168~95
only for binary screening, but can be expanded to more
than two thresholds. In particular, an image pixel
can be ablated in a number of steps of different depth
ranging from 2 to 256.
In order to reduce the necessary maximum depth of
the depressions, between 20 ~m and 40 ~m in
conventional gravure, highly pigmented, particularly
water-based, inks are used. The advantages of this
reduction reside in the lower image-forming output
required for achieving a given ink density and in the
reduced addition of water in the paper, which
considerably accelerates drying.
Wear on the gravure blank form is compensated for
in that the maximum image-forming depth is appreciably
less than the depth of the depressions in the
prestructured gravure blank form. If the depth of the
depressions is reduced as a result of wear of the
webs, the maximum image-forming depth can nevertheless
be easily attained. For this purpose, the webs are to
be structured with vertical walls as far as possible.
Narrowing of the depressions as a result of increasing
web thickness can be compensated for during exposure
by process techniques by determining the volume
characteristic at periodic intervals and compensating
accordingly.
Different advantageous variants of the steps
according to the invention are possible. For example,
a blank form with uniformly arranged depressions, as
used in conventional form production, can be used
instead of the gravure blank form with helically
arranged webs as described above. The magnitude of
the depressions can differ from the fine screens
commonly used today which have cell sizes starting
from 80 ~m to very large depressions with respect to
area, e.g., cell sizes of 1 mm or more. The form can
have stochastically distributed depressions instead of
uniformly distributed depressions in order to prevent
216~5~5
the risk of moiré formation, particularly when
printing with multiple inks. The random distribution
can be produced, e.g., by exposing the gelatins used
for conventional etching with speckles produced from
coherent laser light rather than with a cross-line
screen. In this case, a wax combined with 5% carbon
black is preferably used as filling material.
The regeneration of the gravure form can also be
carried out with high-pressure water jets. For
example, an arrangement such as that already disclosed
by EP 9 310 798 is used for this purpose. An
arrangement of this kind is formed of a double-walled
chamber which is open toward the gravure form and is
closed off relative to the surroundings by seals
guided along the form. The inner cell contains
nozzles through which water is sprayed at high
pressure on the surface of the gravure form. Suction
is applied to the covered outer chamber region so that
the liquid is removed in particular from the region
which has already been cleaned and the gravure form is
clean and dry after processing.
The high-pressure cleaning arrangement can
operate in at least two different modes. One mode,
using low liquid pressure and/or liquid temperature
serves substantially to remove remaining ink, while
the additional modes each use a correspondingly higher
liquid pressure and/or liquid temperature serve for
partial or complete removal of the filling material.
Different pressure and temperature parameters are
applied depending on whether a first cleaning or
intermediate cleaning is to be carried out. If only
adhering dirt and ink residues are to be cleaned off,
a relatively low temperature in the range below 50C
and low pressure of several bar will be used. If a
first cleaning is to be carried out, temperatures in
the range of the softening or melting temperature and
pressures in the range of 30 bar are to be used.
`` 16 2I 6~595
Agents, such as surfactants as well as particles, can
be added to the cleaning water to improve
effectiveness.
The depressions in the gravure blank form can
also be filled by an applicator roller which draws
from a material reservoir and preferably rotates in
the opposite direction to the rotating direction of
the gravure form cylinder. After application, the
filling material is wiped off by a doctor blade. The
angle of the doctor blade is preferably distinctly
negative, i.e., the doctor blade cuts like a knife. In
particular, the doctor blade can also be heated. The
gravure form can also be heated inductively before and
during filling and during wiping. Regeneration,
filling and wiping can preferably be effected during
one and the same cylinder revolution.
If thermoplastic materials are used, heat may be
applied, for instance, via an infrared radiation
source or heated air and materials which suck the
thermoplastic material out of the depressions by
capillary action or, e.g., a highly absorbent paper or
a blowing or suction device can be used.
It is also possible to clean only adhering dirt
and ink from the gravure form without removing filling
material and to refill the portions of the form
removed during the preceding image formation step.
Complete erasure can then be carried out after a given
number of cycles to produce a blank form.
Further, photopolymers which are hardened by
laser and developed by means of water can also be used
as filling materials. Lacquer can also be applied
successively in multiple layers with intermediate
drying in order to fill the depressions completely, or
the reactive systems already mentioned above can also
be used. The filling materials are sensitized to the
type of radiation used, e.g., by adding carbon black.
_ 17 2168595
The surface of the gravure form can be smoothed
after filling by polishing or wiping with a heated
doctor blade. This can also be effected by means of a
hot-air jet or by the laser beam used for image-
forming ablation at low beam intensity. This can becarried out in the course of normal image formation by
irradiating the non-image areas with a defined but
considerably lower output in relation to the image-
forming ablation so as to result only in melting.
Of course, instead of a laser beam, in particular
a high-energy laser beam, a plurality of parallel
beams can also be used. Any thermal laser source such
as semiconductor lasers, in particular a laser
arrangement formed by a plurality of semiconductor
lasers, Nd YAG lasers, COz lasers or CO lasers, can be
used as sources of radiation waves. A laser radiating
in the ultraviolet or blue range, e.g., an argon
laser, must be used for photopolymer filling.
Further, spark erosion or a water jet can be used
instead of a light source for material removal, e.g.,
if high resolution is not required.
An absorbent paper (e.g., blotting paper), which
is cut according to the intended image, can be used
for producing an image forming ablation. This
procedure is explained more fully with reference to
Fig. 6. A multilayer foil 30' is used as a base. An
absorbent material 30a (e.g., blotting paper) is
applied to a non-absorbent substrate 30b. The
absorbent material 3Oa and non-absorbent substrate
30b thus form the multilayer foil 30'. The unneeded
areas are cut out of the absorbent material 30a and
removed using a CAD cutting plotter as is conventional
in foil cutting techniques. The foil 30' is then
applied to the gravure form cylinder 10 which has
already been provided with the filled blank form. The
foil 30' is ironed over the gravure form cylinder by a
heated roller 31. The filling material is then sucked
18 2I 6~595
out by means of capillary forces at those locations
contacted by the absorbent foil material, whereas this
does not occur at the locations in contact with the
nonabsorbent substrate. The image can be
differentiated to produce an image-forming ablation 32
in this way. However, it is only possible to
differentiate substantially between full tones and
paper white.
Image-forming ablation can also be produced using
a micromirror array 40. The construction of such an
array 40 is shown in Fig. 7. A typical array 40 of
this kind is formed of individually electrically
tiltable micromirrors 41 with a typical area of 20~m x
20~m arranged in a matrix of 1000 x 2000 elements.
These micromirrors 41 may be in a rest position, an
off position or an on position as shown in Figure 7.
Figs. 8 and 9 show an example of an arrangement
of an array 40 of this type for an image point
transfer unit for image-forming ablation. The mirror
array 40 is uniformly illuminated by means of a high-
energy arc lamp 42 and is imaged on the surface 44 of
the printing form by an optical system 43 at an
imaging scale of approximately 1 in such a way that
the edge of the array 40 with the 2000 elements is
disposed perpendicularly to the rotating direction of
the form cylinder, i.e. parallel to the axis of
rotation. This edge defines the image lines. One
pixel is defined as the field on which a mirror is
imaged geometrically and calculations are carried out
relative to the surface of a mirror to determine the
half of the non-imaging edge regions adjacent to the
mirror until the next respective adjoining mirror. A
mirror reflects the energy radiated upon it onto the
form and this pixel at a spatial angle determined by
the apertures of the imaging optical system. The
printing form cylinder rotates and 2000 image columns
are written simultaneously. A mirror addresses a
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pixel when more than 50% of its surface is imaged
thereon. Accordingly, a line of pixels which is
stationary with respect to the cylinder travels
through the lines of the mirror array 40, i.e., an
increasing number of lines of the mirror array are
gradually illuminated (Fig. 9).
Suitable electronics (essentially a multielement
shift register) provide for an allocation of image
data synchronized to this travelling. The image data
are filled into the first line. The image data travel
downward line by line synchronously with the rotation
of the cylinder, and the next respective line of image
data is taken over in the first line. During this
traveling, a mirror can always be switched on or off.
A determined pixel can thus obtain 0 to 1000 units of
energy. For instance, in order to act upon a pixel
with 4/10 of the maximum energy dose, 400 mirrors are
switched on and 600 mirrors are switched off during
this wandering, while they address the pixel. Thus,
the addressing of the mirror elements 41 is changed
synchronously with the rotation of the gravure form
surface 44 in a manner analogous to a shift register
so that the allocation of an image pixel to the
printing form surface 44 with its corresponding
exposure data value is maintained on the form surface
44 along the entire imaging surface of the mirror
array 40. The arrangement of the on/off mirror is
optional, but may possibly be predetermined in
conformity to process techniques.
In principle, surfaces (image pixels) which are
smaller than the surface elements of the base screen
of the gravure blank form 1 can be addressed by the
image-forming ablation 3. In particular, the image-
forming ablation 3 can even be carried out
substantially independently from the base screen as
shown in Figure 1. However, the image-forming ablation
3 can also be adapted to the base screen, i.e., can
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have a determined geometric ratio thereto. Ideally,
the image-forming ablation forms the depressions of
the base screen as needed according to process
techniques.
After one revolution of the cylinder, the print
head is displaced by 1000 pixels and the cycle starts
from the beginning. Alternatively, a continuous
forward feed of the print head which displaces the
head by 1000 pixels in one revolution of the printing
form cylinder can also be carried out.
All of the constructions mentioned above relate
to the implementation of the steps according to the
invention in a gravure press. However, the described
steps can, of course, also be carried out outside a
printing machine.
