Device and method for magnetically transferring indicia to a coating composition applied to a substrate

TECHNICAL FIELD

0001The present invention generally relates to a device and method for magnetically transferring indicia to a coating composition, such as an ink or varnish, applied to at least a part of the surface of a substrate, which coating composition comprises magnetic or magnetizable particles. The present invention also relates to the use of such a device and the application of such a method to produce printed documents, such as banknotes or like valuable and security documents.

BACKGROUND OF THE INVENTION

0002Methods and devices for magnetically transferring indicia to a wet coating composition are already known as such in the art, for instance from International applications Nos. <patcit id="pcit0001" dnum="WO2004007095A"><text>WO 2004/007095</text></patcit>, <patcit id="pcit0002" dnum="WO2004007096A"><text>WO 2004/007096</text></patcit>, <patcit id="pcit0003" dnum="WO2005000585A"><text>WO 2005/000585</text></patcit>, <patcit id="pcit0004" dnum="WO2005002866A"><text>WO 2005/002866</text></patcit> and European patent application No. <patcit id="pcit0005" dnum="EP1650042A"><text>EP 1 650 042</text></patcit>.

0003According to these methods, a layer of coating composition, such as an ink or varnish, is first applied to at least a part of the surface of a substrate, which coating composition comprises at least one type of magnetic or magnetizable particles. While the layer of coating composition is still wet, the layer is exposed to a determined magnetic field generated at a surface of a magnetic-field-generating device, thereby orienting the magnetic or magnetizable particles along field lines of the magnetic field. The layer of coating composition is then dried or cured, thereby fixing the orientation of the magnetic or magnetizable particles.

0004European patent application No. <patcit id="pcit0006" dnum="EP1787728A"><text>EP 1 787 728</text></patcit> (which application was published only after the priority date of the present application) discloses a magnetic plate for printing of optical effects, which plate comprises a magnetizable composite material that is selectively magnetized so that one or more first regions across the surface of the plate provide a first magnetic field having a predetermined direction. These first regions form a logo, indicia or image of an object. Magnetic material in one or more other second regions surrounding the first region are either unmagnetized or magnetized differently from the one or more first regions so as to provide a contrast in magnetic field.

0005Magnetic or magnetizable particles (also designated as "magnetic flakes"), which have the particularity that they can be oriented or aligned by an appropriately-applied magnetic field, are discussed in particular in US Patent No. <patcit id="pcit0007" dnum="US4838648A"><text>US 4,838,648</text></patcit>, European patent application <patcit id="pcit0008" dnum="EP0686675A"><text>EP 0 686 675</text></patcit>, and International applications <patcit id="pcit0009" dnum="WO02073250A"><text>WO 02/073250</text></patcit>, <patcit id="pcit0010" dnum="WO03000801A"><text>WO 03/000801</text></patcit>, <patcit id="pcit0011" dnum="WO2004007095A"><text>WO 2004/007095</text></patcit>, <patcit id="pcit0012" dnum="WO2004007096A"><text>WO 2004/007096</text></patcit> and <patcit id="pcit0013" dnum="WO2005002866A"><text>WO 2005/002866</text></patcit>.

0006Such particles or flakes are in particular used as optically-variable pigments in so-called optically-variable inks, or OVI®'s (OVI® is a registered trademark of SICPA Holding SA, Switzerland) to produce high-level security patterns, especially for banknotes.

0007The most convenient method to apply the above magnetic flakes is by silk-screen printing as discussed in the above-mentioned International application <patcit id="pcit0014" dnum="WO2005000585A"><text>WO 2005/000585</text></patcit>. This is mainly due to the fact that the flakes have a relatively important size which restricts the choice of available printing processes for applying inks or varnishes containing such flakes. In particular, one has to ensure that the flakes are not destroyed or damaged during the printing process, and silk-screen printing constitutes the most convenient printing process to achieve this goal. Furthermore, silk-screen printing has the advantage that the inks or varnishes used in such a process exhibit a relatively low viscosity which favours proper orientation of the magnetic flakes.

0008Nevertheless, other printing processes could be envisaged to apply coating compositions containing magnetic flakes, such as flexographic printing or gravure printing. In European patent application <patcit id="pcit0015" dnum="EP1650042A"><text>EP 1 650 042</text></patcit>, it is even proposed to apply such magnetic flakes in an intaglio printing process, whereby the paste-like intaglio ink containing the magnetic pigments is heated to decrease the viscosity of the ink and thereby allow the flakes to be oriented more easily with a magnetic or electric field. This can be performed in a conventional intaglio printing press, since the plate cylinder of such presses is commonly brought to an operating temperature of approximately 60 to 80 °C during printing operations.

0009Orientation of the magnetic flakes contained in the wet coating composition is carried out by applying an adequate magnetic field to the freshly-applied layer of coating composition. By appropriately shaping the field lines of the magnetic field, the magnetic flakes can be aligned in any desired pattern producing a corresponding optically-variable effect which is very difficult, if not impossible to counterfeit. An adequate solution for orienting the magnetic flakes, as discussed in International application <patcit id="pcit0016" dnum="WO2005000585A"><text>WO 2005/000585</text></patcit> consists in bringing sheets carrying layers of wet coating composition in contact with a rotating cylinder carrying a plurality of magnetic-field-generating devices.

0010International application <patcit id="pcit0017" dnum="WO2005002866A"><text>WO 2005/002866</text></patcit>, discloses a particular type of magnetic-field-generating device comprising a body, such as a flat plate or a cylindrically curved plate, made of a permanent magnetic material which is permanently magnetized in a direction substantially perpendicular to a surface of the body. The said surface of the body furthermore carries indicia in the form of engravings causing perturbations of its magnetic field.

0011<figref idref="f0001">Figure 1</figref> is a schematic cross-sectional view of a magnetic field simulation taken from the above-mentioned International application which illustrates an example of a vertically magnetized permanent magnetic plate, designated by numerical reference 1, comprising a rectangular engraving 2. In this example, the engraved plate 1 is made of Plastoferrite (such as the Plastoferrite model M100.8 sold by Maurer Magnetic AG, CH-8627 Grüningen, <u>http://www.maurermagnetic.ch</u>) magnetized in a direction perpendicular to the surface of the plate 1.

0012As illustrated in <figref idref="f0001">Figure 1</figref>, as the permanent magnetic body 1 is vertically magnetized, the field lines of the magnetic field are mostly vertical in the region of the surface of the body, except in the region of the vertical walls of the engraving 2. This implies that most of the magnetic pigments contained in the wet composition are aligned in a vertical manner, perpendicularly to the surface of the substrate. In other words, considering the fact that the pigments are mostly reflective when they are aligned substantially horizontally, the resulting pattern induced in the coating composition by means of the device of <figref idref="f0001">Figure 1</figref> is mostly not reflective, when seen and illuminated perpendicularly to the surface of the substrate.

0013<figref idref="f0002 f0003 f0004">Figures 2a to 2c</figref> are greyscale photographs, taken along three different viewing angles, of a magnetically-induced pattern representing the value "50" within an oval shape which was produced by means of a device according to the principle of International application <patcit id="pcit0018" dnum="WO2005002866A"><text>WO 2005/002866</text></patcit> illustrated in <figref idref="f0001">Figure 1</figref>. More precisely, the pattern was produced using a body having engravings representing the value "50" within an engraved oval shape.

0014The layer of coating composition was applied with a silk-screen printing process on top of a black offset background using an OVI® silk-screen ink comprising gold-to-green optically variable magnetic pigment corresponding to the 7-layer pigment design disclosed in <patcit id="pcit0019" dnum="WO0273250A"><text>WO 02/73250</text></patcit>. The purpose of the black (or dark) offset background is to increase the contrast in the induced pattern by making the reflective parts of the pattern (i.e. the portions where the pigments are oriented substantially horizontally with respect to the surface of the substrate) stand out as compared to the less reflective parts of the pattern (i.e. the portions where the pigments are oriented substantially vertically with respect to the surface of the substrate, thereby revealing the underlying background).

0015As already mentioned hereinabove, most part of the induced pattern produced according to the known method disclosed in International application <patcit id="pcit0020" dnum="WO2005002866A"><text>WO 2005/002866</text></patcit> is relatively dark, i.e. the pigments are mostly aligned vertically, thereby making the dark offset background visible through the layer of coating composition. As can be seen from the photographs of <figref idref="f0002 f0003 f0004">Figures 2a to 2c</figref>, the most reflective portions of the magnetically-induced pattern correspond to the position of the walls of the engravings. Looking at the pattern, one basically has the impression that the oval shape and the value "50" stand out in relief above the background as illustrated in <figref idref="f0002 f0003 f0004">Figures 2a to 2c</figref>.

0016The patterns that can be produced according to the known method disclosed in International application <patcit id="pcit0021" dnum="WO2005002866A"><text>WO 2005/002866</text></patcit> discussed above are already quite an improvement as compared to the patterns that could previously be produced. A need has nevertheless arisen for an improved approach which would enable to produce different patterns, albeit with comparable means, especially patterns exhibiting a comparatively lighter, more reflective optical effect.

0017A distinct approach is proposed in International application No. <patcit id="pcit0022" dnum="WO2006114289A"><text>WO 2006/114289</text></patcit> which discloses a method for creating color effect images on a carrier substrate. According to this method, a latent magnetic image comprising magnetic pixels and non-magnetic pixels is created on a magnetizable printing form. A carrier substrate provided with a decorative layer containing non-spherical, preferably needle shaped or lamellar magnetic color effect pigments is guided past the magnetizable printing form such that the orientation of color effect pigments of the decorative layer relative to the carrier substrate changes with the aid of the images of the field lines created by the magnetic pixels of the magnetizable printing form. The color pigments are ultimately fixed in the decorative layer with the orientation thereof modified by the magnetizable printing form.

0018According to <patcit id="pcit0023" dnum="WO2006114289A"><text>WO 2006/114289</text></patcit>, the magnetizable printing form comprises a soft magnetic band and electromagnetic printing heads are used to locally change the magnetic coercivity of the soft magnetic band to form the desired magnetic pixels. A "soft magnetic" material is commonly understood as designating a magnetizable material which has the ability to lose its memory of previous magnetizations, as opposed to "hard" or "permanent" magnetic material which stay magnetized for a long time. According to <patcit id="pcit0024" dnum="WO2006114289A"><text>WO 2006/114289</text></patcit>, each magnetic pixels thus acts as an elementary magnet locally affecting the orientation of the field lines of the magnetic field.

0019This approach is fundamentally different from that of <patcit id="pcit0025" dnum="WO2005002866A"><text>WO 2005/002866</text></patcit> in that it is not based on the use of an engraved body for influencing the orientation of the field lines of a magnetic field generated by separate electromagnetic means. Moreover, since a soft magnetic material is used according to <patcit id="pcit0026" dnum="WO2006114289A"><text>WO 2006/114289</text></patcit>, there is a high risk that the magnetic configuration of the magnetizable body may become lost or be affected by external magnetic fields. The solutions of <patcit id="pcit0027" dnum="WO2005002866A"><text>WO 2005/002866</text></patcit> and <patcit id="pcit0028" dnum="WO2006114289A"><text>WO 2006/114289</text></patcit> may not therefore be combined together. The solution of <patcit id="pcit0029" dnum="WO2006114289A"><text>WO 2006/114289</text></patcit> is furthermore less robust than that of <patcit id="pcit0030" dnum="WO2005002866A"><text>WO 2005/002866</text></patcit> and is thus not suited for use in a conventional production environment such as that of a printing plant and/or for implementation thereof on a printing press.

SUMMARY OF THE INVENTION

0020An aim of the invention is therefore to improve the known devices and methods for magnetically transferring indicia to a coating composition comprising magnetic or magnetizable particles

0021A further aim of the present invention is to provide a device for magnetically transferring indicia to a coating composition comprising magnetic or magnetizable particles that is relatively easy and cheap to produce.

0022Still another aim of the present invention is to provide a solution that increases the level of security of the resulting magnetically-induced pattern and makes it even more difficult to counterfeit.

0023Yet another aim of the present invention is to provide a solution that is robust and is suited for use in a conventional production environment such as that of a printing plant and/or for implementation thereof on a printing press.

0024These aims are achieved thanks to the solution defined in the claims.

0025According to the invention, there is accordingly proposed a device for magnetically transferring indicia to a layer of coating composition, such as an ink or varnish, applied to at least a part of the surface of a substrate, the coating composition comprising at least one type of magnetic or magnetizable particles. The device comprises a body subjected to a magnetic field, which body carries determined indicia in the form of engravings on a surface of the body, which engravings influence the orientation of field lines of the magnetic field. The body further comprises at least one layer of material of high magnetic permeability in which the engravings are formed. The material of high magnetic permeability is understood as being a material having a magnetic permeability substantially greater than the magnetic permeability of vacuum µ₀ (µ₀ = 4π 10^-1 µN/A²) and which has the ability to concentrate the field lines of the magnetic field. In the unengraved regions of the layer of material of high magnetic permeability, the field lines of the magnetic field extend substantially parallel to the surface of the body inside the layer of material of high magnetic permeability. Due to the presence of the engravings on the body, the field lines of the magnetic field are forced along different routes and orientations outside the layer of material of high magnetic permeability, the field lines of the magnetic field extending, in the regions directly above the engravings, substantially horizontally above the surface of the body where the layer of coating composition applied to the substrate is to be located.

0026Thanks to this solution, a comparatively more reflective pattern can be created, which pattern moreover exhibits a radically different optical-effect than that of the prior art.

0027According to a preferred embodiment, the body further comprises a base plate of material of low magnetic permeability supporting the layer of material of high magnetic permeability. The material of low magnetic permeability is understood as being a material having a magnetic permeability substantially equal to the magnetic permeability of vacuum µ₀ and which does not substantially affect the field lines of the magnetic field and behaves substantially like free space or vacuum. In this context, the layer of material of high magnetic permeability can advantageously be deposited on the base plate by galvanization. Still in the context of this embodiment, the magnetic permeability of the base plate is preferably in the range of 1.25 to 1.26 µN/A². A preferred material for the base plate is a non-ferromagnetic material such as copper, aluminium or alloys thereof.

0028The effect can be maximized when the engravings in the layer of material of high magnetic permeability extend through the whole thickness of the layer.

0029According to an advantageous embodiment, the magnetic permeability of the layer of material of high magnetic permeability is selected to be greater than 100 µN/A² (@ 0.002 T), preferably between 100 to 1000 µN/A² (@ 0.002 T). In this context, a suitable material is a ferromagnetic material such as iron, nickel, cobalt or alloys thereof.

0030The thickness of the layer of material of high magnetic permeability is preferably selected so as to be greater or equal to 50 µm, even more preferably between 50 to 500 µm.

0031As regards the engravings in the layer of material of high magnetic permeability, those preferably comprise engraved rectilinear or curvilinear patterns having a line width and/or a line spacing of 1 millimeter or more.

0032The magnetic field can advantageously be generated by means of at least one permanent magnet or electromagnet, preferably two.

0033The field lines of the magnetic field, seen perpendicular to the surface of the body 20, can extend along substantially one main direction. In this context, it is advantageous to change the main direction of the field lines of the magnetic field during exposure of the layer of coating composition. This change is preferably carried out by rotating the main direction of the magnetic field by 360°.

0034The device of the present invention can advantageously be shaped as a curved plate adapted for mounting onto a rotatable cylinder body of a printing press or as an individual curved plate element adapted for mounting onto a supporting member disposed on the circumference of a cylindrical body of a printing press.

0035Also claimed is a method for magnetically transferring indicia onto a substrate, comprising the steps of : <ol id="ol0001" compact="compact" ol-style=""><li>(a) applying a layer of a coating composition, such as an ink or varnish, onto at least a part of the surface of the substrate, the coating composition comprising at least one type of magnetic or magnetizable particles;</li><li>(b) while the layer of coating composition is still wet, exposing the layer of coating composition to a determined magnetic field generated at a surface of a device according to the invention, thereby orienting the magnetic or magnetizable particles along field lines of the magnetic field; and</li><li>(c) drying or curing the layer of coating composition, thereby fixing the orientation of said magnetic or magnetizable particles.</li></ol>

0036In the context of this method, the coating composition is preferably applied by printing, even more preferably by silk-screen printing, flexographic printing or gravure printing.

0037Yet another claimed object is the use of the above device for magnetically inducing transfer of indicia to a wet coating composition, such an ink or varnish, applied to at least a part of the surface of a substrate, which coating composition comprises at least one type of magnetic or magnetizable particles.

BRIEF DESCRIPTION OF THE DRAWINGS

0038Other features and advantages of the present invention will appear more clearly from reading the following detailed description of embodiments of the invention which are presented solely by way of non-restrictive examples and illustrated by the attached drawings in which: <ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">Figure 1</figref> is a schematic cross-sectional view of a magnetic field simulation taken from International application <patcit id="pcit0031" dnum="WO2005002866A"><text>WO 2005/002866</text></patcit> mentioned hereinabove, which illustrates an example of a vertically-magnetized permanent magnetic plate comprising a rectangular engraving ;</li><li><figref idref="f0002 f0003 f0004">Figures 2a to 2c</figref> are three greyscale photographs taken along three different viewing angles of an example of magnetically-induced pattern produced according to the known principle disclosed in <patcit id="pcit0032" dnum="WO2005002866A"><text>WO 2005/002866</text></patcit> ;</li><li><figref idref="f0005">Figure 3</figref> is a schematic cross-sectional view of a magnetic-field-generating device according to a preferred embodiment of the present invention ;</li><li><figref idref="f0006">Figure 4</figref> is a magnetic field simulation of the magnetic-field-generating device of <figref idref="f0005">Figure 3</figref> ;</li><li><figref idref="f0007 f0008 f0009 f0010">Figures 5a to 5d</figref> are four greyscale photographs taken along four different viewing angles of an example of magnetically-induced pattern produced according to the invention;</li><li><figref idref="f0011">Figure 5e</figref> is a schematic illustration of a banknote comprising a magnetically-induced pattern as illustrated in <figref idref="f0007 f0008 f0009 f0010">Figures 5a to 5d</figref> ;</li><li><figref idref="f0012 f0013 f0014 f0015">Figures 6a to 6d</figref> are four greyscale photographs taken along four different viewing angles of an example of magnetically-induced pattern, similar to that shown in <figref idref="f0007 f0008 f0009">Figures 5a to 5c</figref> produced according to a variant of the invention ;</li><li><figref idref="f0016 f0017 f0018 f0019">Figures 7a to 7d</figref> are four greyscale photographs taken along four different viewing angles of another example of magnetically-induced pattern produced according to the invention ;</li><li><figref idref="f0020">Figure 7e</figref> is a schematic top view of the engraving pattern of the body used to produce the magnetically-induced pattern of <figref idref="f0016 f0017 f0018 f0019">Figures 7a to 7d</figref> ; and</li><li><figref idref="f0021">Figure 8</figref> is a schematic side view of a silk-screen printing press suitable for carrying out the invention.</li></ul>

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

0039The expression <i>"magnetically transferring indicia"</i> (which expression is also used in International application <patcit id="pcit0033" dnum="WO2005002866A"><text>WO 2005/002866</text></patcit>) is used in the context of the present invention because indicia is virtually "transferred" from an indicia-bearing body to the wet coating composition comprising the magnetic or magnetizable particles thanks to a determined orientation of the field lines of a magnetic field as this will be explained. In this context, the term <i>"transferring"</i> is to be understood as being equivalent to the term <i>"forming"</i> or <i>"inducing"</i> (which terms can therefore also be used to designate the indicia creation process).

0040<figref idref="f0005">Figure 3</figref> is a schematic cross-sectional view of a magnetic-field-generating device, designated globally by reference numeral 10, according to a preferred embodiment of the present invention. According to this embodiment, the device 10 includes a body 20 the purpose of which is to influence the orientation of field lines of a magnetic field, as this will be explained hereinafter. According to this preferred embodiment, the body 20 comprises a layer 21 made of material of high magnetic permeability in which engravings 21 a, 21 b, 21 c are formed, and a base plate 22 made of material of low magnetic permeability which supports the layer 21.

0041Also illustrated in <figref idref="f0005">Figure 3</figref> is a sheet S disposed on top of the surface of the body 20 in contact with the upper surface of layer 21, which sheet S comprises a layer of coating composition P applied on the surface of the sheet S, opposite the surface of layer 21. Coating composition P comprises at least one type of magnetic or magnetizable particles, as discussed hereinabove, that one wishes to orient by means of the magnetic-field-generating device 10.

0042In the context of the present invention, one will understand that the method for magnetically transferring indicia onto the substrate S comprises the steps of : <ol id="ol0002" compact="compact" ol-style=""><li>(a) applying the layer of coating composition P, such as an ink or varnish onto at least a part of the surface of the substrate S (the coating composition P comprising at least one type of magnetic or magnetizable particles such as those described in <patcit id="pcit0034" dnum="WO0273250A"><text>WO 02/73250</text></patcit>) ;</li><li>(b) while the layer of coating composition P is still wet, exposing the layer of coating composition P to a determined magnetic field generated at a surface of the device 10 according to the present invention, thereby orienting the magnetic or magnetizable particles along field lines of the magnetic field ; and</li><li>(c) drying or curing the layer of coating composition P, thereby fixing the orientation of the magnetic or magnetizable particles.</li></ol>

0043Within the scope of the present invention, it will be understood that a material of "high magnetic permeability" is a material that has the ability to concentrate the field lines of a magnetic field (i.e. is "magnetically attractable"), while a material of "low magnetic permeability" is a material that does not substantially affect the field lines of a magnetic field and behaves substantially like free space or vacuum. In other words, a material of "low magnetic permeability" will be understood as a material having a magnetic permeability µ substantially equal to µ₀, where µ₀ is commonly understood to be the magnetic permeability of vacuum and equals the following constant value (1) : <maths id="math0001" num="(1)"><math display="block"><msub><mi mathvariant="normal">μ</mi><mn mathvariant="normal">0</mn></msub><mo>=</mo><mn mathvariant="normal">4</mn><mo>⁢</mo><mi mathvariant="normal">π</mi><mspace width="1em" /><msup><mn mathvariant="normal">10</mn><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup><mo>⁢</mo><mi mathvariant="normal">μN</mi><mo>/</mo><msup><mi mathvariant="normal">A</mi><mn mathvariant="normal">2</mn></msup></math><img file="EP2155498B1_D0001.tif" /></maths>

0044Preferably, the material of low magnetic permeability is selected to be a material having a magnetic permeability in the range of 1.25 to 1.26 µN/A².

0045In contrast, a material of "high magnetic permeability" is a material having a magnetic permeability µ that is substantially greater than µ₀. More precisely, according to the present invention, material of high magnetic permeability will be understood as materials preferably exhibiting a magnetic permeability greater than 100 µN/A² (@ 0.002 T), even more preferably materials exhibiting a magnetic permeability between 100 to 1000 µN/A² (@ 0.002 T). It shall be understood that the magnetic permeability of materials varies with flux density. The above-mentioned values are therefore given considering a flux density of 0.002 T (hence the indication "@ 0.002 T" following the mentioned values).

0046Among materials of high magnetic permeability suitable for forming layer 21, one in particular knows so-called ferromagnetic materials such as iron, nickel, cobalt or alloys thereof (e.g. steel, permalloy, etc.). Within the scope of the present invention, any material of high magnetic permeability is suitable. Tests have however shown that material exhibiting a magnetic permeability comprised between 100 to 1000 µN/A² (@ 0.002 T) are sufficient, and that materials exhibiting a magnetic permeability higher than 1000 µN/A² (@ 0.002 T), while also suitable, are not necessary.

0047A particularly suitable material for layer 21 is nickel, which material has a magnetic permeability of approximately 125 µN/A² (@ 0.002 T). This material is convenient as it is commonly used in the banknote industry to produce intaglio printing plates, especially by galvanization, and is thus readily available to the banknote printer. This material is furthermore very easy to engrave (for instance mechanically by means of a rotating chisel or by means of gaseous or liquid jets of abrasives, by chemical etching, or even by laser ablation using CO₂, Nd-YAG or excimer lasers).

0048Among materials of low magnetic permeability suitable for forming base plate 22, one in particular knows so-called non-ferromagnetic materials such as copper, aluminium or alloys thereof. Within the scope of the present invention, any material of low magnetic permeability is suitable. Glass or plastic could for instance be used as material for the base plate 22.

0049According to an alternative of the invention, one could even do without the base plate 22, for instance by making the layer 21 self-supporting. The base plate 22 of material of low magnetic permeability is therefore not essential but preferred.

0050A particularly suitable material for base plate 22 is copper, which material has a magnetic permeability of approximately 1.2566290 µN/A². This material is also convenient as it is again commonly used in the banknote industry and is thus readily available to the banknote printer. A perfectly suitable alternative is aluminium which exhibits a magnetic permeability of approximately 1.2566650 µN/A².

0051Successful tests have been carried out by the Applicant using a copper base plate 22 and a nickel layer 21 deposited on the copper base plate 22 by galvanization. The copper base plate 22 was approximately 0.5 mm thick and the nickel layer 21 was deposited by galvanization with layer thicknesses ranging from 50 to 500 µm.

0052Turning back to the embodiment illustrated in <figref idref="f0005">Figure 3</figref>, the magnetic field is generated in this example by a pair of permanent magnets 31, 32 (such as samarium-cobalt - SmCo - magnets as supplied by Maurer Magnetic AG) disposed at two ends I, II of the device 10. As illustrated, the permanent magnet 31 is disposed with its north magnetic pole oriented upwards, while the permanent magnet 32 is disposed with its north magnetic pole oriented downwards. The resulting magnetic field is such that field lines of the magnetic field will extend from the north magnetic pole of permanent magnet 31 at end I through the base plate 22, into layer 21, then substantially horizontally through, above and below the layer 21, from end I to end II, back through the base plate 22 and to the south magnetic pole of permanent magnet 32. The remainder of the magnetic circuit is closed through connection of the magnetic field lines at the lower part of the device, via the north magnetic pole of permanent magnet 32 and the south magnetic pole of permanent magnet 31. It will be appreciated that the same magnetic field configuration could alternatively be generated using electromagnets instead of the permanent magnets 31, 32.

0053A simulation of the resulting magnetic field distribution is shown schematically in <figref idref="f0006">Figure 4</figref>. This simulation was produced using the publicly available modelling software Vizimag (<u>http://www.vizimag.com/</u>) and considering a nickel layer as layer 21 and a copper base plate as base plate 22.

0054In the absence of any engravings in layer 21, the magnetic field lines would mostly be concentrated in the layer 21 itself, this layer 21 acting as a magnetic short-circuit. The engravings 21 a, 21 b, 21 c in the layer 21, which form in essence regions of low magnetic permeability (i.e. free space), force the magnetic field lines along different routes and orientations. In other words, the engravings 21 a, 21 b, 21 c influence the orientation of the field lines of the magnetic field in the vicinity of the engravings 21a, 21b, 21c.

0055As illustrated schematically in <figref idref="f0006">Figure 4</figref>, and in contrast to the prior solution disclosed in International application <patcit id="pcit0035" dnum="WO2005002866A"><text>WO 2005/002866</text></patcit> (compare also <figref idref="f0006">Figure 4</figref> and <figref idref="f0001">Figure 1</figref>), in the unengraved regions of the layer 21 of material of high magnetic permeability, the magnetic field lines are mostly concentrated inside the layer 21 and extend substantially parallel to the surface of the body 20. On the other hand, due to the presence of the engravings 21 a, 21 b, 21 c, the magnetic field lines are forced outside the layer 21 and extend, in the regions of the sheet S and of the coating composition P directly above the engravings 21 a, 21 b, 21 c, substantially horizontally rather than vertically as shown in the simulation of <figref idref="f0001">Figure 1</figref>. Conversely, in the regions of the sheet S and of the coating composition P which are not directly above the engravings 21 a, 21 b, 21c (i.e. above the unengraved regions of layer 21), the field lines of the magnetic field tend to be oriented mostly vertically. Consequently, a major part of the particles in the coating composition P above the indicia-forming engravings 21 a, 21 b, 21 c will be aligned almost horizontally, yielding a generally more reflective pattern.

0056This difference is clearly demonstrated by <figref idref="f0007 f0008 f0009 f0010">Figures 5a to 5d</figref> which are greyscale photographs taken from four different viewing angles of a magnetically induced pattern representing the value "50" within an oval shape, similar to the prior pattern illustrated in <figref idref="f0002 f0003 f0004">Figures 2a to 2c</figref>, but which was produced by means of a device according to the above-discussed preferred embodiment of the present invention.

0057More precisely, as far as the engravings are concerned, the body of the magnetic-field-generating device was engraved with exactly the same engraving pattern representing value "50" within an oval shape as that used for producing the prior pattern of <figref idref="f0002 f0003 f0004">Figures 2a to 2c</figref>. Rather than using the vertically-magnetized Plastoferrite plate as proposed in <patcit id="pcit0036" dnum="WO2005002866A"><text>WO 2005/002866</text></patcit>, the above mentioned copper-nickel (Cu-Ni) body 20 was used. In the context of this example, one will in particular understand that the engravings in the layer of material of high magnetic permeability are basically formed of an oval-shaped engraving inside which there remains an unengraved pattern representing the value "50".

0058As illustrated in <figref idref="f0007 f0008 f0009 f0010">Figures 5a to 5d</figref>, the resulting pattern is substantially more reflective and exhibits a radically different optical effect as compared to that illustrated in <figref idref="f0002 f0003 f0004">Figures 2a to 2c</figref>. Indeed, the optical effect created according to the invention is more or less inverted as compared to the optical effect illustrated in <figref idref="f0002 f0003 f0004">Figures 2a to 2c</figref>. More precisely, as illustrated in <figref idref="f0007 f0008 f0009 f0010">Figures 5a to 5d</figref>, the oval shape appears to stand out in relief above the background, like a solid volume, with the value "50" looking like having been engraved into the solid oval shape.

0059<figref idref="f0011">Figure 5e</figref> is a schematic illustration of a possible banknote 50 comprising inter alia a portrait 51 and a magnetically-induced pattern 55 produced according to the present invention, such as the pattern of <figref idref="f0007 f0008 f0009 f0010">Figures 5a to 5d</figref>.

0060While performing tests using the above-described magnetic-field-generating device 10, it has become apparent that the orientation of the engraved patterns on the body 20 had some importance on the resulting effect. Indeed, in the embodiment of the magnetic-field-generating device 10 illustrated in <figref idref="f0005">Figures 3</figref> and <figref idref="f0006">4</figref>, the magnetic field lines, seen perpendicular to the surface of the body 20, are generally oriented along one main direction, that is along the direction I-II in <figref idref="f0005">Figures 3</figref> and <figref idref="f0006">4</figref>. The "main direction" is understood here as referring to the general direction of the field lines of the magnetic field, that is from left to right in <figref idref="f0005">Figures 3</figref> and <figref idref="f0006">4</figref> (this "main direction" extends from bottom to top in the greyscale photographic illustrations of <figref idref="f0007 f0008 f0009">Figures 5a to 5c</figref> and from left to right in the greyscale photographic illustration of <figref idref="f0010">Figure 5d</figref>). Accordingly, portions of the borders of the engravings in the layer of material of high magnetic permeability which are oriented substantially parallel to this main direction I-II will not as such have much influence on the orientation of the magnetic field lines and the corresponding parts of the magnetically induced pattern in the coating composition P will have a tendency to disappear or be attenuated as a result. Looking for instance at the photographs of <figref idref="f0007 f0008 f0009 f0010">Figures 5a to 5d</figref>, one can in particular see that the side portions on the left-hand side and right-hand side of the oval shape are substantially attenuated.

0061In order to overcome this effect, one could design the engraved pattern so as to be devoid of engraved patterns having border portions extending along the main direction of the magnetic field lines and/or make the engraved pattern is such regions wide enough so as to cause a greater influence on the local orientation of the magnetic field lines.

0062Alternatively, a solution might consist in changing the main direction of the magnetic field lines during exposure of the layer of coating composition P. This is preferably carried out by changing, preferably by rotating, advantageously by 360°, the magnetic field with respect to the exposed layer of coating composition P. It shall be understood that the axis of rotation of the magnetic field is to be considered as being substantially perpendicular to the plane where the coating composition P is applied, i.e. substantially perpendicularly to the surface of the body 20 and of the sheet S. <figref idref="f0012 f0013 f0014 f0015">Figures 6a to 6d</figref> are greyscale photographs taken along the same four different viewing angles as in <figref idref="f0007 f0008 f0009 f0010">Figures 5a to 5d</figref> of a magnetically-induced pattern representing the value "50" within an oval shape, identical to that of <figref idref="f0007 f0008 f0009 f0010">Figures 5a to 5d</figref>, with the additional provision that, during exposure of the layer of coating composition P, the main direction of the magnetic field lines was rotated by 360°.

0063As a result of the rotation of the magnetic field during exposure of the coating composition P, the above-mentioned attenuation effect is decreased or completely avoided. This rotation moreover appears to strengthen the embossing/relief effect on the resulting magnetically-induced pattern by making it visible in substantially the same way from all viewing angles, in the manner of a hologram. The difference is in particular visible from a comparison of the photographs of <figref idref="f0010">Figures 5d</figref> and <figref idref="f0015">6d</figref> which are both taken from the same viewing angle, namely from the left-hand side of the coating composition P.

0064<figref idref="f0016 f0017 f0018 f0019">Figures 7a to 7d</figref> are four photographs taken along the same four different viewing angles as those of <figref idref="f0007 f0008 f0009 f0010">Figures 5a to 5d</figref> and <figref idref="f0012 f0015">6a to 6d</figref>, of another example of a magnetically-induced pattern. In this latter example, the main direction of the magnetic field was also rotated by 360° during exposure of the coating composition P.

0065<figref idref="f0020">Figure 7e</figref> is a schematic top view of the engraved body 20 which was used in the context of the example shown in <figref idref="f0016 f0017 f0018 f0019">Figures 7a to 7d</figref>. As illustrated, the layer 21 of the body 20 was engraved with a pattern of engravings 211, 212 representing, on the one hand, a stylised representation of a Pegasus 211 and, on the other hand, the words "KBA GIORI" 212. In this example, the rectilinear or curvilinear patterns 211, 212 where engraved with a line width of approximately 1 millimeter. Tests have shown that a line width of 1 millimeter or more is preferable in the context of the present invention. Similarly, too dense an engraving pattern is preferably to be avoided, i.e. a line spacing of 1 millimeter or more between neighbouring engravings is to be preferred.

0066Preferably, the thickness of layer 21 should be selected to be greater or equal to 50 µm, even more preferably in the range of 50 to 500 µm. The thickness of the base plate 22 on the other hand is not critical.

0067The tests have shown that the distance between the permanent magnets 31, 32 and the body 20 had some influence on the resulting magnetically-induced pattern. Within the scope of the present invention, the permanent magnets (or, alternatively, the electromagnets) could be disposed at a distance from the body 20 (e.g. of the order of a few centimetres) or in close contact with the body 20 depending on the effect one wishes to produce. In that respect, the magnetic force of the magnets also plays a role.

0068As already mentioned, electromagnets could be used in lieu of permanent magnets to create the necessary magnetic field. Electromagnets are particularly advantageous in that the magnetic field can be completely suppressed at the end of the exposure, thereby preventing further modification of the orientation of the magnetic or magnetizable particles, especially during removal of the substrate from the surface of the body 20. In addition, rotation of the main direction of the magnetic field, as discussed above, can easily be carried out using electromagnets disposed in a circular arrangement and by electronically switching the orientation of the magnetic field in a manner similar to that performed in the context of the actuation of electric motors. Rotation of the magnetic field using permanent magnets would have to be performed by physical rotation of the permanent magnets themselves (or of the substrate S carrying the layer of coating composition P) during exposure.

0069The above-described invention can be implemented by designing the above-described magnetic-field-generating device 10 so as to be disposed on the circumference of a cylindrical body of a printing press as generally taught in International application No. <patcit id="pcit0037" dnum="WO2005000585A"><text>WO 2005/000585</text></patcit> in the name of the present Applicant.

0070<figref idref="f0021">Figure 8</figref> schematically illustrates one possible embodiment of a sheet-fed printing press as disclosed in International application No. <patcit id="pcit0038" dnum="WO2005000585A"><text>WO 2005/000585</text></patcit>.

0071This printing press is adapted to print sheets according to the silk-screen printing process and comprises a feeding station 100 for feeding successive sheets to a silk-screen printing group 200 where silk-screen patterns are applied onto the sheets. In this example the printing group 200 comprises an impression cylinder 200a cooperating with two screen cylinders 200b, 200c placed in succession along the printing path of the sheets. Once processed in the printing group 200, the freshly printed sheets are transported by means of a conveyor system 300 to a delivery station 400 comprising a plurality of delivery pile units, three in this example. The conveyor system 300 is typically an endless chain conveyor system comprising a plurality of spaced-apart gripper bars (not shown in <figref idref="f0021">Figure 8</figref>) extending transversely to the sheet transporting direction, each gripper bar comprising clamping means for holding a leading edge of the sheets.

0072In the example illustrated in <figref idref="f0021">Figure 8</figref>, a cylinder body 600 carrying a plurality of magnetic-field-generating devices is located along the path of the sheets carried by the chain conveyor system 300. This cylinder body 600 is designed to apply a magnetic field to selected locations of the sheets for the purpose of orienting magnetic flakes contained in the patterns of coating composition which have been freshly-applied on the sheets in the printing group 200, as discussed above. A drying or curing unit 500 is provided downstream of the cylinder body 600 for drying, respectively curing, the coating composition applied onto the sheets after the magnetic flakes have been oriented and prior to the delivery in the delivery station 400, such unit 500 being typically an infrared drying unit or a UV curing unit depending on the type of coating composition used (e.g. water-based or UV-cured inks/varnishes).

0073Further details regarding silk-screen printing presses, including relevant details of the silk-screen printing press illustrated in <figref idref="f0021">Figure 8</figref>, can be found in European patent applications <patcit id="pcit0039" dnum="EP0723864A"><text>EP 0 723 864</text></patcit>, <patcit id="pcit0040" dnum="EP0769376A"><text>EP 0 769 376</text></patcit> and in International applications <patcit id="pcit0041" dnum="WO9729912A"><text>WO 97/29912</text></patcit>, <patcit id="pcit0042" dnum="WO9734767A"><text>WO 97/34767</text></patcit>, <patcit id="pcit0043" dnum="WO03093013A"><text>WO 03/093013</text></patcit>, <patcit id="pcit0044" dnum="WO2004096545A"><text>WO 2004/096545</text></patcit>, <patcit id="pcit0045" dnum="WO2005095109A"><text>WO 2005/095109</text></patcit> and <patcit id="pcit0046" dnum="WO2005102699A"><text>WO 2005/102699</text></patcit>.

0074As discussed in International application No. <patcit id="pcit0047" dnum="WO2005000585A"><text>WO 2005/000585</text></patcit>, the cylinder body 600 could alternatively be located at the sheet transfer location 300a between the impression cylinder 200a and the conveyor system 300. Still according to another embodiment envisaged in International application No. <patcit id="pcit0048" dnum="WO2005000585A"><text>WO 2005/000585</text></patcit>, the impression cylinder 200a itself could be designed as a cylinder carrying magnetic-field-generating devices.

0075In the embodiment illustrated in <figref idref="f0021">Figure 8</figref>, the cylinder body 600 used to orient the magnetic flakes advantageously cooperates with the non-freshly-printed side of the sheets, thereby preventing smearing problems, the magnetic field being applied from the back side of the sheets through the freshly-printed patterns of coating composition. During orientation of the magnetic flakes, i.e. at the time when a sheet carried by the conveyor system 300 contacts the upper part of the circumference of the cylinder body 600, the cylinder body 600 is rotated at a circumferential speed corresponding to the speed of the transported sheets so that there is no relative displacement between the transported sheets and the circumference of the cylinder. As illustrated, the cylinder body 600 is placed in the path of the chain conveyor system 300 such that the sheets follow a curved path tangential to the outer circumference of the cylinder body 600, thereby enabling part of the surface of the processed sheet to be brought in contact with the outer circumference of the cylinder body 600.

0076In the context of the production of banknotes, in particular, each printed sheet (or each successive portion of a continuous web, in case of web-printing) carries an array of imprints arranged in a matrix of rows and columns, which imprints ultimately form individual securities after final cutting of the sheets or web portions. The cylinder body 600 used to orient the magnetic flakes is therefore typically provided with as many magnetic-field-generating devices as there are imprints on the sheets or web portions.

0077The cylinder body 600 is preferably a cylinder body as further taught in <patcit id="pcit0049" dnum="EP1961559A"><text>EP-A-1 961 559</text></patcit> in the name of the present Applicant. According to this patent application, the cylinder body advantageously comprises a plurality of distinct annular supporting rings distributed axially along a common shaft member, each annular supporting ring carrying a set of magnetic-field-generating devices which are distributed circumferentially on an outer circumference of the annular supporting rings. Thanks to this cylinder body configuration, the position of each magnetic-field-generating device can be adjusted to the corresponding position of the coating composition imprints on the processed sheets or web.

0078Turning back to the magnetic-field-generating devices according to the present invention, it will be appreciated that the body 20 can be shaped as a curved plate adapted for mounting onto a rotatable cylinder body of a printing press (in such a case, a common plate with engravings could be used for all magnetic-field-generating devices) or, alternatively, as an individual curved plate element adapted for mounting onto a supporting member disposed on the circumference of a cylindrical body of a printing press (in such a case, individual plates would be used).

0079Various modifications and/or improvements may be made to the above-described embodiments without departing from the scope of the invention as defined by the annexed claims.

0080For instance, while silk-screen printing is a preferred printing process for applying the coating composition comprising the magnetic or magnetizable particles to be oriented, other printing process might be envisaged, such as flexographic printing, gravure printing, or even intaglio printing as discussed in European patent application <patcit id="pcit0050" dnum="EP1650042A"><text>EP 1 650 042</text></patcit>.

0081In addition, while the layer of coating composition P is preferably to be printed on a dark background, any other background is possible such as for example a structured background as discussed in International application <patcit id="pcit0051" dnum="WO2006061301A"><text>WO 2006/061301</text></patcit>. A mainly dark background is however preferred in order to yield a better contrast in the resulting magnetically-induced pattern.