Note: Descriptions are shown in the official language in which they were submitted.
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Method with register between print element and watermark
[0001] The invention relates to a method for manufacturing a security element,
wherein
a substrate is made available which has a front side and a back side, is
translucent at least
to certain radiation and has a watermark modulating the opacity of the
substrate, and a
print element is applied to the front side.
[0002] Watermarks are known to improve the falsification security of protected
objects,
for example of banknotes. They are usually incorporated during the manufacture
of the
substrate from which the security element is produced, usually a paper.
Watermarks are
easily recognizable upon transmissive viewing of the security element, since
they modify
the opacity of the substrate. As a rule, during the manufacture of the paper,
the thickness
of the substrate is so varied that a reduction in thickness leads to a
reduction in opacity.
However, also other approaches to producing a watermark are known, for example
with
the aid of so-called watermark inks, which also modify the opacity of the
substrate
locally.
[0003] Since watermarks are as a rule formed in the substrate during the
manufacture of
the start material for the security element, for example during the production
of a
banknote paper, it is fundamentally difficult in the later printing process to
arrange print
elements in an exact position to the watermark, thus to place them in
register. This
problem occurs independently of the concrete configuration of the print
element, thus not
only upon printing with different printing inks, but also upon coating with or
applying
print elements, upon applying transfer foils or lamination foils or upon
applying a
windowed thread.
[0004] Due to variations in the cutting process of a paper substrate, paper
sheets usually
have variations of the position of the watermark lying in the millimeter
range. In a
printing process further variations in register are added, due to variations
of abutting
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positions, transfer variations in printing apparatus, variations of the
paper's humidity and
a rolling-out effect of the paper in the engraving procedure.
[0005] In order to apply a print element in register with the watermark, it
would be
conceivable to capture the position of the watermark in the substrate by means
of a
transmission sensor, and to adjust the subsequent printing procedure to the
current
position of the watermark, e.g. by a mechanical adjustment of a printing
cylinder or
correction of the printing data of a digital printing system. This results in
an effort caused
by the capture of the watermark's position, an image analysis to be carried
out in real
time to determine the position of the watermark, and a correction of the
printing position
to be carried out in real time. The effort increases in line with the printing
speed.
[0006] It is consequently the object of the invention to specify a method for
structuring a
print element in register with a watermark, without having to precisely detect
the
position of the watermark and adjust the printing system to the position of
the watermark
beforehand.
[0007] This object is achieved according to the invention by a method for
manufacturing
a security element, wherein a substrate is made available which has a front
side and a
back side, is translucent at least to certain radiation and has a watermark
modulating the
opacity of the substrate, and to the front side a print element is applied,
wherein on the
front side the print element covers the watermark at least partially, the
print element can
be modified by means of the certain radiation, and the substrate is penetrated
by the
certain radiation from the back side, wherein the watermark is employed as a
mask with
reference to the modification of the print element, and the print element is
thus modified
in register with the watermark.
[0008] The invention is based on the finding that the energy of radiation is
weakened by
the substrate. The more opaque the substrate is, the stronger is the
weakening. Therefore
advantageous use is made of the property of the watermark of modulating the
opacity of
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the substrate. The watermark is used as a mask for a modification of the print
element by
employing a print element that is modifiable by certain radiation to which the
substrate is
translucent, and that certain radiation is radiated from the back side of the
substrate to the
front side, whereto the print element is applied so as to cover the watermark.
Masked by
the watermark, thereby a modification of the print element takes place, which
is
automatically disposed in perfect register with the watermark. As modification
of the
print element here in particular an ablation comes into question, when a print
element is
employed that can be ablated. However, fundamentally any suitable modification
of a
print element can be used.
[0009] This modification can take place directly, thus by direct action of the
certain
radiation on the print element, or indirectly. In the latter case, the certain
radiation
modifies an element that is in turn used to modify the print element
perceivable on the
front side. For example by means of the certain radiation a layer can be
modified that
influences the adhesion of the print element to the front side. After the
modification of
this layer influencing the adhesion, in a further step then the print element
is removed in
the regions where the layer was modified (or not modified in the inverse
case). The
procedure is similar to the principle to the so-called washing ink. A further
possibility of
indirect modification is the modification of a developer layer which, in
interaction with a
developing agent or fixing agent, modifies the print element in those places
where the
developer layer was suitably exposed. An indirect influencing of the print
element is
possible also in a two-step procedure, when the print element has an adhesive
layer that
is applied to the front side and exposed from the back side with the
cooperation of the
watermark as mask. In a second step then an ink layer of the print element can
be applied
that adheres only to those places where the exposure has taken place.
Possibly, an action
on the exposed adhesive layer can also take place in an intermediate step
here, so that
said layer remains only in those regions where a suitable exposure has taken
place
(positive effect) or in those regions where no suitable exposure has taken
place (negative
effect).
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[0010] Within the framework of the present description, the term "print
element" is
understood to mean an element applied to a substrate, realizing possibly after
further
processing an element that can be evaluated visually or by machine. Of course,
print
elements on the basis of printing inks are the most widespread. When mention
is made of
a printing ink in the present description, this is to be understood
exemplarily for a print
element.
[0011] Thus, when mention is made in the present description and in particular
in the
claims of a "print element" or a "printing ink", and the application of said
print element
to the front side above the watermark is mentioned, this also encompasses that
in a print
element layer system only one part of the layer system is applied and exposed
with the
certain irradiation; the other part is then applied subsequently to these
steps and interacts
suitably with the exposed part of the layer system in order to produce a
printed image in
register with the watermark.
[0012] A multilayer system is possible e.g. in such a sense that the print
element
interacts with an additional layer whose visual effect depends on the prior
modification
of the print element. Thus, it is for example known for optically variable
layers that they
are particularly easily recognizable in incident light on a dark or black
background. An
embodiment of the invention therefore provides that the print element is
equipped with a
contrast structure by the certain radiation. Onto the print element thus
modified in
register with the watermark, subsequently an optically variable layer is
applied areally,
i.e. without register requirements, whose visual effect is then automatically
disposed in
register with the watermark due to the precise placing in register of the
print element
acting as background, although the optically variable layer itself was applied
without
register requirements. An example for such an optically variable layer is a
liquid crystal
layer. With regard to this, reference is made to EP 0 435 029 or WO 97/19818,
which
disclose examples of optically variable layers employing liquid crystals. The
disclosure
of these publication prints is included in its entirety here with regard to
the optically
variable layers described there.
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[0013] The term "print element" is thus also understood as a multilayer
system, wherein
a part of the layer system is applied to the front area prior to exposure and
then
illuminated with the certain irradiation. A further part of the layer system
is then applied
only after the exposure.
[0014] Particularly for use in banknotes such watermarks are usual which
modulate the
opacity of the substrate by varying a thickness of the substrate, while the
substrate's
density remains constant, since such watermarks are as a rule hard to imitate.
These
watermarks are well suitable for the printing method according to the
invention, which
then complements the appearance of the watermark by further image information
disposed in register with the watermark. It is thereby made possible for
example to
complement in color the image information made available by the watermark. The
image
information of the watermark, which per se appears only in transmitted light,
can become
recognizable also upon viewing in incident light by means of the printed image
produced
in register according to the invention.
[0015] A watermark modulating the opacity of a substrate can also be produced
by a so-
called watermark ink instead of by varying the thickness of the substrate. The
watermark
ink herein effects a reduction of the scattering of light in the substrate
core.
Fundamentally, scattering is based on a large number of interfaces of fibers,
fillers,
adhesive, etc. with the ambient air. These interfaces with the ambient air are
clearly
reduced by suitable binding agent components in the watermark ink, so that the
opacity
is reduced in these regions. Such a binding agent component is glycerin for
example,
which penetrates into the substrate and remains in the pores of the substrate.
[0016] Watermarks produced by watermark ink are also referred to as "false"
watermarks. Watermarks produced by varying the thickness of the substrate
while the
density of the substrate remains constant, are also referred to as "true"
watermarks.
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[0017] This is applicable analogously when the watermark is effected by a
dyeing that
modulates the opacity of the substrate.
[0018] A watermark can modulate the opacity of a substrate binarily, i.e. it
produces a
structure that is recognizable in transmission by the substrate being less
opaque or more
translucent in individual places. However, also watermarks are known in the
sense of a
grayscale modulation that modulates the opacity or translucence between two
maximum
values. Upon transmissive viewing, a grayscale image is then recognizable.
Either type
of watermark is equally suitable for the present printing method. In a
watermark that is
configured in the fashion of a grayscale image it is of course particularly
advantageous to
employ a printing ink whose modifiability goes beyond a simple threshold value
modification, whose modification thus has more than two states. The multistage
modulation by the watermark is then converted into a corresponding multistage
modification of the print element.
[0019] A particularly finely graded modulation of the opacity can be achieved
by a
combination of a paper-thickness modulating "true" watermark with a "false"
watermark
produced by watermark ink.
[0020] Print elements usually consist of binding agents, accessory agents,
drying agents
or drying accelerators and chromophore components, such as dyes or pigments.
When a
radiation is irradiated that is of a wavelength which is absorbed to a certain
degree by the
chromophore components, it is achieved that the certain radiation ablates the
print
element for modification. In the case of ablation thus the print element and
the certain
radiation are chosen to so match each other that the certain radiation is
absorbed by the
print element in such a fashion that an ablation is possible.
[0021] In order to prevent influences of the certain radiation on other
components of the
substrate or coatings applied to the substrate as far as possible, it is
preferable to irradiate
the certain radiation in an as narrow-band-fashion as possible in the range of
an
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absorption band of the print element. This can be realized particularly easily
with laser
radiation.
[0022] When a print element per se does not have a component making it
suitably
absorbing for an ablation, it is preferable for the purpose of ablation to add
a certain
substance to the print element that absorbs the certain radiation, for example
carbon
black. The employment of an added substance permits in particular to so choose
the
certain radiation that it is in the invisible spectral range, for example IR
radiation, when
the added substance increases the degree of absorption of the print element in
the
corresponding spectral range.
[0023] Instead of an ablation usually leading to a brightening when dark
colors are
employed, it is of course also possible to so design the modification that it
leads to a
color alteration, for example by thermal activation of an ink. This is valid
in particular
when special-effect inks are employed on the basis of inks which show a
viewing-angle
dependent color impression (so-called optically variable inks), and inks that
can be
structured by a magnetic field before drying.
[0024] It is therefore particularly preferred that the printing ink has one or
several of the
following substances: an optically variable ink, a magnetically orientable
ink, a metallic
ink, an ink effective in the invisible spectral range.
[0025] As already mentioned, the principle of the invention provides that the
print
element and the certain radiation are so matched that the certain radiation is
capable of
modifying the print element, wherein the watermark modulating the opacity of
the
substrate is employed as a mask. The term "certain radiation" employed in the
present
description therefore expresses that a print element is used that can be
modified by
means of this radiation. The modification, for example in the form of an
ablation, is
achieved when the certain radiation on the front side has a suitable
intensity. The
modulation of opacity effected by the watermark reduces the intensity of the
radiation on
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the front side where the print element is disposed. The radiation intensity on
the back
side is consequently adjusted in such a fashion that the opacity modulation
effected by
the watermark leads to a radiation modulation on the front side, which
modulates the
modification effect on the print element. In other words, the radiation
intensity on the
back side is so chosen that in the places where the watermark effects the
lowest opacity
in the substrate, a modification of the print element occurs, whereas no or
only a very
small modification is achieved in those places where the watermark effects the
highest
opacity of the substrate. The watermark then serves as mask when the substrate
is
penetrated by the certain radiation. Since the modification of the print
element as a rule
requires a comparatively high radiation intensity, the use of a beam is
advantageous that
is applied in gridded fashion over the back side of the substrate. The beam
can be present
for example in the form of a laser beam. It is then not only easy to make
available the
required radiation intensity, but also the intensity adjustment can take place
in a simple
fashion, since less effort is required to adjust the intensity of a beam than
to effect a
homogeneous intensity adjustment of a fanned out light beam.
[0026] The employment of a gridded beam further has the advantage that the
intensity of
the certain radiation can be varied in addition to the employment of the
watermark as a
mask, in order to effect an additional structuring of the modification of the
print element.
This additional modification is then of course not in register with the
watermark.
[0027] Additionally, it is possible to apply an additional masking to the back
side that
weakens or absorbs the certain radiation. Thereby a further structuring of the
modification of the print element is achieved.
[0028] Examinations by the inventors have shown that usually no visually
visible
marking occurs on the back side, when the printing method according to the
invention is
used to expose and modify, for example ablate, the print element masked by the
watermark on the front side.
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[0029] It is also possible to utilize the incorporation of the modified
radiation, in
particular of laser radiation, to effect a visible modification of the
substrate on the entry
side of the laser radiation, i.e. before the onset of the masking effect of
the watermark,
possibly after prior application of a radiation-sensitive layer. Thus for
example pulsed
laser radiation can be employed that modifies the substrate, for example
paper, already
on the entry side, e.g. by a blackening or also a foaming of the substrate.
This effect is
then easily visible in transmission. Thus, there advantageously results a
combination of
the watermark, the print element modified in register with the watermark, and
a visual
modification of the surface of the substrate on the entry side of the
modifying radiation.
This modification on the entry side of the radiation of course depends
exclusively on the
application of the radiation. It is thus possible for example to create a
window in which
the watermark and the print element modified in register therewith are
visible.
[0030] Fundamentally, the print element to be modulated can contain both
substances
that are ablatable by means of radiation and substances that are developable
by means of
radiation (for example thermochromic substances preferably having an
irreversible color
change or opacity change), and additionally also substances that are not
ablatable or
developable by means of radiation. Depending on the ink formulation, the last-
mentioned
substances remain on the substrate in an unchanged state or are removed only
partly.
Further, the print element can contain a mixture of substances whose
individual
components are modified in stages in dependence on the impinging energy. For
example
a first component is ablated at a very low opacity, and a second component is
ablated
already at medium opacity.
[0031] According to a preferred embodiment, the printing ink contains a laser-
markable
feature substance with core-shell particles known from EP 1 826 728 A2,
wherein one of
the materials of core and shell absorbs the radiation of a marking laser, and
the other of
the materials of core and shell does not absorb the radiation of the marking
laser.
According to a further preferred embodiment, the core-shell particle contains
a UV-
excitable organic luminescent substance known from EP 1 826 730 A2, with the
organic
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luminescent substance forming the core that is coated with a shell increasing
the
chemical and/or physical resistance of the luminescent substance.
[0032] According to a further preferred embodiment, the printing ink contains
a
modified iron blue pigment known from EP 1 826 246 A2, coated with an alkali-
resistant
and/or acid-resistant shell.
[0033] The term printing ink employed here is intended to express the printing
effect
achieved by the watermark-masked and radiation-based modification. The
printing ink
itself can of course comprise also such structures which exceed conventional
ink
systems, for example a foil coating or a metallization.
[0034] Objects to be protected within the framework of the present description
can be for
example security papers, identity documents and value documents (such as e.g.
banknotes, chip cards, passports, cards, identification cards, identity cards,
shares, bonds,
deeds, vouchers, checks, admission tickets, credit cards, health cards, ...),
as well as
product securing elements, such as e.g. labels, seals, packages.
[0035] The term security paper is understood here to be in particular the not
yet
circulable precursor to a value document (e.g. a card), which can have besides
the print
manufactured according to the invention for example also further
authentication features
(such as e.g. luminescent substances provided within the volume). Value
documents here
are understood to be documents manufactured from security papers, e.g.
banknotes, on
the one hand. On the other hand, value documents can also be other documents
and
objects that are processed using the printing method according to the
invention in order
for the value documents to have uncopiable authenticity features, thereby
making it
possible to check authenticity and at the same time preventing unwanted
copies.
[0036] The substrate consists particularly preferably of paper of cotton
fibers such as is
employed for example for banknotes. Preferably the substrate can also consist
of paper
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of other natural fibers, preferably likewise of synthetic fibers, i.e. a
mixture of natural
and synthetic fibers. Further preferably, the substrate consists of a
combination of at least
two different substrates arranged above each other and connected to each
other, a so-
called hybrid. It can also be for example a combination of plastic foil and
paper or also a
three-layer composite, such as plastic foil - paper - plastic foil, i.e. a
paper substrate is
covered on both of its two sides by a plastic foil, or paper - plastic foil -
paper, i.e. a
plastic-foil substrate is covered on both of its two sides by a substrate of
paper.
[0037] The substrate preferably consists of a plastic foil that is at least
partially
transparent. In this case the watermark is produced by an ink applied to one
side of the
substrate and whose color tone is at least similar to the color tone of the
substrate. Such a
watermark is known for example from DE 10 2009 056 462 Al.
[0038] Translucence or translucent is understood in this description as the
partial light
transmission of a body, thus the property of letting light pass in scattered
fashion.
Translucence has to be seen i.a. delimited from transparency (= image
permeability or
visual permeability). The reciprocal property of translucence is opacity. To
the extent
that it is stated here that a watermark modulates the opacity, it can likewise
be stated that
it modulates translucence inversely thereto.
[0039] It is evident that the features mentioned above and those to be
explained
hereinafter are usable not only in the stated combinations, but also in other
combinations
or in isolation, without going beyond the scope of the present invention.
[0040] In the following the invention is explained in more detail by way of
example with
reference to the accompanying figures that also disclose essential
characteristics of the
invention. The figures are described as follows:
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[0041] Fig. la and lb A schematic representation of a substrate in a sectional
view (Fig.
la) and in plan view (Fig. lb) to illustrate the opacity modulation by
the watermark,
[0042] Fig. 2 a schematic representation of the substrate of Fig. 1 for
producing a
printed image placed in register with the watermark,
[0043] Fig. 3a and 3b a schematic representation similar to Fig. la and lb,
concerning an
embodiment wherein the watermark is produced by a watermark ink,
[0044] Fig. 4 a representation similar to Fig. 2 for the substrate of Fig.
3a and 3b,
[0045] Fig. 5a a schematic representation similar to Fig. 2 for an exemplary
embodiment wherein on the front side of the substrate a non-modifiable
coating is provided,
[0046] Fig. 5b and 5c views of the substrate of Fig. 5a from the back side and
the front
side after a modification, and
[0047] Fig. 6 to 8 plan views (Fig. 6a, 6b, 7, 8) and sectional views (Fig.
6c, d) of an
exemplary embodiment having a windowed thread.
[0048] Fig. la shows a security element 1 in a sectional view, with only one
substrate 2
drawn for the sake of simplification. The substrate 2 has a front side 3
(which is arranged
on the bottom in the figures without any further limitation), as well as a
back side 4. Into
the substrate 2 a watermark 5 is integrated, modulating the thickness of the
substrate 2
and thereby its opacity. This opacity modulation is represented in Fig. lb,
where a plan
view is schematically shown of the front side 3 of the substrate 2. There are
regions 6 of
high opacity in places where the substrate 2 is of the maximal (unreduced)
thickness. In
the regions 7 where the thickness is reduced by a medium degree, the substrate
2 is of
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medium opacity. In a region 8 where the thickness is strongly reduced, the
substrate 2 is
of low opacity or high translucence.
[0049] To produce a printed image placed in register with the watermark, a
printing ink
that is ablatable by means of laser radiation is applied to the front side 3.
Fig. 2 shows
the substrate 2 with printing ink 9 applied to the front side 3. The printing
ink 9 is not
(yet) further structured, but covers at least partially the region where the
watermark 5 is
present in the substrate 2.
[0050] From the front side 3 now a laser beam 10 is guided over the substrate.
It
penetrates the substrate 2. The wavelength of the laser beam is such that the
printing ink
9 absorbs the radiation. The laser beam 10 is weakened in dependence on the
opacity of
the substrate 2 modulated by the watermark 5. In a position 10a, where the
substrate is of
high opacity due to the watermark, the laser beam 10 is greatly weakened. In a
position
10b, where the substrate is of the lowest opacity, the laser beam is the least
weakened. In
Fig. 2 this is schematically illustrated by the thickness of the laser beam
10. In fact, of
course only the intensity of the laser beam 10 is weakened upon passing
through the
substrate 2, but not its beam cross section.
[0051] The intensity of the laser beam 10 is so adjusted that there results a
modulation of
the ablation effect exerted by the laser beam 10 on the printing ink 9 applied
to the front
side 3: in the region 8 of low opacity, the printing ink 9 is maximally
ablated, in regions
7 of medium opacity the printing ink is ablated to a lesser degree, and in
regions 6 of
maximal opacity, the least ablation takes place, or possibly even no ablation.
This is of
course only valid to the degree that the printing ink 9 on the front side 3
covers the
region where the watermark 5 is formed in the substrate 2.
[0052] The described embodiment can be modified so that instead of a watermark
modulating the opacity through a thickness variation of the substrate, a so-
called "false"
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or printed watermark is used, thus a watermark produced on the substrate 2 by
so-called
watermark ink.
[0053] Fig. 3a and 3b schematically show the making available of such a
printed
watermark. On the back side 4 of the substrate 2 a watermark ink ills printed.
The
watermark ink 11 according to Fig. 3b penetrates into the substrate 2, after
drying or
other suitable processing effecting an increase in translucence 12 in the
substrate 2 in the
places where the watermark ink 11 was printed.
[0054] When the substrate 2 with the printed watermark 12 is now exposed to
the laser
beam 10, as represented in Fig. 4, again the effect results that the intensity
of the laser
beam 10 is weakened less in positions 10a, where the watermark or the
watermark ink
increases the translucence of the substrate 2, than in positions 10b, where
the watermark
does not have this effect. Again, an ablation of the printing ink 9 is
obtained in exact
register with the watermark, without adjustment of said register upon
application of the
printing ink 9.
[0055] The Figures 5a to 5c relate to a third embodiment, wherein to the front
side 3
additionally a coating 13 not modifiable by the laser radiation 10 was
applied, which is
transparent to the laser radiation 10, thus not weakening it.
[0056] Fig. 5a shows a sectional view similar to Fig. 2, wherein in some
regions the
coating 13 is applied now that is not modifiable by the laser radiation. Fig.
5b shows a
plan view of the back side 4 with the coating 13.
[0057] Fig. Sc shows a plan view of the front side 3 after irradiation with
the laser
radiation 10. Due to the different opacity of the substrate 2 in the region of
the
watermark, the printing ink 9 is influenced by the laser radiation 10 to
different degrees,
so that the different regions 14, 15, 16 result. In the region 16 the printing
ink 9 is almost
uninfluenced, since the energy of the laser beam is greatly weakened due to
the opacity
CA 02862057 2014-06-27
of the substrate 2. In the region 14 the printing ink 9 is modified to a
medium degree, for
example ablated, due to a medium laser-energy weakening by the substrate due
to the
medium opacity of the substrate 2. The laser beam had a strong effect on the
printing ink
9 in the region 15, since the substrate 2 is of low opacity there and weakened
the laser
beam only slightly.
[0058] The exemplary embodiment described by means of the Figures 5a to 5c can
of
course also be realized with a watermark that was produced by a watermark ink
and not
by thickness modulation.
[0059] Of course it is fundamentally possible for all embodiments to employ a
combination of watermark ink and thickness modulation. A watermark ink is
available
for example from Sun Chemical under the designation 669440 Vernes UVSP (art.
no. JV
40000009).
[0060] Of course the procedures described here can also be executed on both
sides of a
substrate, with the printing ink 9 then being applied alternatingly to the
front side and the
back side of the substrate, so that it does not overlap.
[0061] Moreover, also two different printing inks can be used that can be
modified by
radiation of different spectral ranges. It is thus allowed to work with
different intensity
profiles for these two radiations or to print the same region on the front
side and on the
back side with radiation-modified printing inks in register with the
watermark. For this
purpose a first printing ink is applied to the front side and a second
printing ink is applied
to the back side. Subsequently, these printing inks are correspondingly
exposed and
modified by laser radiation applied from the back side (for the printing ink
disposed on
the front side) or from the back side (for the printing ink disposed on the
front side).
[0062] Likewise, it is possible to employ
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= at least two different printing inks requiring different radiation
intensities for
modification, or
= at least one printing ink containing substances requiring different
radiation
intensities for modification, or
= at least one printing ink containing a mixture of substances that are
modifiable and
that are not modifiable.
[0063] For the printing method according to the invention, three examples will
be
described hereinafter:
[0064] Example 1:
[0065] A black offset ink on the basis of carbon black was printed over the
region of a
watermark. The ink was dried for several days and then ablated by means of an
ND
vanadate laser from the back side to the ink disposed on the front side. In
regions of low
paper thickness, i.e. with maximally translucent watermark, 80% to 95% of the
ink were
removed thereby. In regions of maximal paper thickness, i.e. with minimal
translucence
of the watermark, only 10% to 30% of the ink were ablated.
[0066] Example 2:
[0067] The region of a watermark was printed on the front side by means of an
OVI
screen printing ink WP of the producer SICPA. The ink dried for several days.
Subsequently the ink disposed on the front side was ablated from the back side
by means
of a Nd:YAG laser. In regions of low paper thickness 80% to 95% of the ink
were
removed. In the regions of maximal paper thickness 10% to 30% of the ink were
ablated.
[0068] Example 3:
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17
[0069] The region of a watermark was printed on the front side by means of a
magnetic
OVMI screen printing ink of the producer SICPA. The ink dried for several
days.
Subsequently the ink disposed on the front side was ablated from the back side
by means
of a Nd:YAG laser. In regions of low paper thickness 80% to 95% of the ink
were
removed, by the exception of a yellow colorant. In the regions of maximal
paper
thickness 10% to 30% of the ink were ablated.
[0070] Generally, the certain radiation is to be chosen so that no absorption
of the
radiation occurs in the substrate that would lead to an alteration of the
substrate. For this
purpose both continuous-wave lasers and lasers operated in pulsed mode are
possible. An
example for a suitable continuous-wave laser is the model Innoslab, 1S81-E of
the
producer EdgeWave GmbH, which is a Nd:YV04-based laser at a wavelength of
1.064
nm. It provides a power of 100 Watt in continuous-wave operation. The ablation
of an
ink on carbon-black basis can take place at a scanning speed of 2.5 m/s to 5
m/s.
[0071] Employing an IR-modifiable ink as printing ink results in the advantage
that the
printed image and the check of the register is possible by machine-based
infrared
measuring. This ink can be both visually visible and invisible.
[0072] The printing method according to the invention is applicable also to a
watermark
that is provided in connection with a so-called windowed thread in a substrate
of a
security element. Fig. 6a shows a plan view in transmitted light of a front
side 3 of a
substrate 2 having a windowed thread 17. In the substrate 2 here a sequence of
strip-
shaped regions 18 and 19 is provided, forming the watermark: in the regions 18
the
thickness of the substrate is unchanged, so that the watermark is of high
opacity, and in
the regions 19 the thickness of the substrate is reduced, so that the
watermark is of low
opacity (in the exemplary embodiment of the Figures 6 to 8 the watermark is
disposed on
the front side 3 of the substrate, in contrast to the exemplary embodiments of
the Figures
1 to 5). In incident light, this has the effect that the windowed thread 17
has visible
portions 20, where one side of the windowed thread is not covered by the
substrate in
CA 02862057 2014-06-27
18
accordance with Fig. 6d, and invisible portions 21, since these portions are
completely
embedded in the substrate 2 in accordance with Fig. 6c. The effect in incident
light is
shown in Fig. 6b: only the visible portions 20 are bare on the front side 3
and are visible.
[0073] Transmitted light is given within the meaning of the present invention
when an
illumination of the substrate takes place from the side opposite to the
viewer, i.e. the
illumination takes place through the substrate. Incident light is given within
the meaning
of the present invention when an illumination of the substrate takes place
from the side
of the viewer.
[0074] In the region of the watermark 5 formed of the regions 18 and 19, now
printing
ink 9 is applied to the front side 3.
[0075] The Figures 6c and 6d show a section through the substrate 2 of Fig.
6a, wherein
Fig. 6c shows a section disposed in a region 18 and Fig. 6d a section in a
region 19. In
Fig. 6c the thread 17 is disposed below the surface of the front side 3, for
which reason
one of the invisible portions 21 is disposed here. In the region 19, in
contrast, according
to Fig. 6d the windowed thread 17 is disposed on the surface of the front side
3, so that a
visible portion 21 is formed.
[0076] Fig. 7 shows a plan view of the front side 3 after the applied
irradiation of the
laser radiation from the back side 4. In the regions 18 of high opacity the
printing ink 9 is
not ablated, a region 16 like in Fig. 5c is present. In the regions 19 the
printing ink 9 is
ablated in contrast, a region 14 within the meaning of Fig. 2 is present.
[0077] Fig. 8 shows a variation wherein the laser radiation does not effect an
ablation,
but a modification inverse thereto, e.g. by development of thermochromic
effect
pigments. The regions 14 and 16 are exchanged exactly here.
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19
[0078] It is fundamentally unessential for the printing procedure whether the
printing ink
9 is applied to the front side or the back side of the substrate. What is
decisive is merely
that the substrate 2 is penetrated by the radiation from the side that is
opposite the side on
which the printing ink 9 is arranged. Of course the printing ink can be
provided on the
back side and the radiation can be coupled in from the front side in all
exemplary
embodiments.
CA 02862057 2014-06-27
List of reference numbers
1 security element
2 substrate
3 front side
4 back side
5 watermark
6, 7, 8 region
9 printing ink
10 laser beam
10a, 10b position
11 watermark ink
12 translucence increase
13 coating
14, 15, 16 region
17 windowed thread
18, 19 region
20, 21 portion
