Magnetizing device, cylinder and associated machine

Abstract

The present invention relates in particular to a magnetization device intended to be placed in an exposure and transfer cylinder for orienting particles sensitive to magnetic fields, comprising: • a main assembly of plate-shaped magnets, • a plurality of secondary magnets forming a two-dimensional network, • said secondary magnets being small compared to the main assembly of magnets, characterized in that: • the poles of the magnets of said plurality of secondary magnets are oriented in a direction extending in a plane generally parallel to the mean plane of the main assembly; • in said plurality of secondary magnets: each magnet is separated from the neighboring magnet by a space; • said network of secondary magnets extends along a curved surface;• the magnets of the device are capable of causing the magnetic field lines generated by these magnets to act at a distance greater than their burial distance. Figure 1;

Description

Title of the invention: Magnetization device, associated cylinder and machine FIELD OF THE INVENTION

[0001] The present invention relates to a magnetizing device intended to be placed in an exposure and transfer cylinder to orient particles sensitive to magnetic fields, said particles being contained in a printing material affixed to a support.

[0002] Exposing the printing material to the magnetizing device, while the printing material is in a state allowing a certain degree of freedom of the particles along the magnetic field lines to which they are exposed, makes it possible to control and fix the orientation of these particles.

[0003] The respective positions of the particles are then intended to be fixed in a subsequent step.

[0004] The invention also relates to a sheet exposure and transfer cylinder provided with such a magnetizing device, and to a sheet printing machine for fiduciary use which makes use of at least one such cylinder. STATE OF THE ART

[0005] Inks with remarkable optical effects are widely used in the field of fiduciary printing for the production of anti-counterfeiting security elements. More particularly, there are such inks whose visual effects are obtained through the implementation of magnetic fields, the latter being called “optically variable magnetic inks” (or “OVMI” for “Optically Variable Magnetic Ink” in English).

[0006] The optically variable magnetic pigments (or “OVMP” for “Optically Variable Magnetic Pigment” in English) contained in these inks are in particular in the form of particles similar to thin multi-layer platelets (we speak of “platelet” pigment) which generally have the following characteristics: • A core made of a ferromagnetic material (based on iron, cobalt or nickel) which, thanks to the oblong and flat shape of the plate, allows it to orient itself under the action of a magnetic field due to shape anisotropy; • A reflective layer (often metallic) thanks to which the plate takes on a brilliant shine when the positions of the lighting and the observer respect certain particular geometric conditions, and a tint much darker otherwise; • A possible layer of transparent dielectric material whose precise thickness control allows the main color of the pigment to be fixed; • In connection with the previous characteristic, an absorbent layer allowing the desired color to be obtained.

[0007] Such pigments are described in detail, in particular in patents and patent applications of the family of document US 2002 / 0160194.

[0008] Platelet pigments have a shape anisotropy. They therefore do not have a perfect geometry of a disc, square or rectangle but rather any shape with a long dimension, this long dimension being the diameter of the circle in which the pigment can be inscribed, this dimension being much greater than the thickness of the pigment.

[0009] These pigments thus exposed to magnetic field lines will seek to minimize their energy which implies that the magnetic flux crosses the greatest length of ferro / ferrimagnetic material hence the alignment of the particles, like the needle of a compass. These pigments thus behave individually like so many reflective micro-mirrors whose orientation can be controlled in space remotely by orienting the magnetic field lines, during the time of their exposure to a magnetic field, thanks to their ferro / ferrimagnetic core. These pigments are dispersed in a “vehicle”, in particular a binder, a solvent and / or a resin, and constitute the “active” part of the inks thus obtained.

[0010] The use of these inks is based on a printing system which allows, at the moment when the ink is applied but not yet crosslinked (i.e. not yet dry), the pigments to be oriented by exposure to a magnetic field, to produce with all the pigments patterns and singular optical effects in reflection, before setting the ink, and therefore the patterns it contains, by crosslinking.

[0011] Thus, all the pigments, dispersed in a binder which allows both their maintenance and their orientation before drying, constitute, after their deposition by printing on the support, a mosaic whose components or tesserae are invisible to the eye (the dimension of each pigment being too small for the pigment to be seen individually) but which can be organized into patterns visible to the naked eye, thanks to exposure to particular field lines resulting from a magnet or an assembly of magnets whose characteristics and arrangement aim to obtain a desired visual effect. Their spatial arrangement thus obtained will also generate a variable reflection depending on the direction of illumination or the inclination of the support on which they are located, the position of the observer being fixed.

[0012] The magnetic field is thus generally produced by one or more magnets permanent, arranged to create the desired final pattern. The magnets are distributed in a housing which is inserted into exposure and printing cylinders. This is notably described in document US 2011 / 0168088.

[0013] One of the best known patterns is the one called "rolling bar" (or "rolling bar" in English, the terms "rolling bar" and "bar roulante" being equivalent in this text). The name itself evokes a dynamic effect, it is in fact a pattern in the form of a light bar, which moves giving the impression of "rolling" up and down when the support is tilted along a given axis.

[0014] To produce it, a plate-shaped magnet is used which has a magnetic orientation parallel to the plane of the support and perpendicular to the direction of the desired bar, located at a distance of a few millimeters below the support, and whose horizontal dimensions (length and width) slightly exceed those of the area of ​​the support which is being treated (i.e. the area of ​​the support in which the pattern is desired to extend).

[0015] Many devices using this “rolling bar” have been proposed.

[0016] This is particularly the case of document EP2846932 in which an arrangement is described which makes use of a two-dimensional network of small magnets between the main rolling bar magnet and the printed support and which makes it possible to generate a large number of aesthetic and varied patterns.

[0017] However, the device described in this document has limitations. Thus, it can be seen that the visual effect conferred by the configurations envisaged is not uniform over the entire printed surface.

[0018] Furthermore, as soon as one intends to complicate the shape of the patterns and / or to reduce their size in order to increase their number per unit of surface, one cannot distinguish them from one another, so that the result is an impression of "blur".

[0019] The aim of the present invention is to make it possible to obtain on a medium printed with an ink such as described above very fine visual effects, according to small patterns which are very distinguishable from each other in order to make the visual perception attractive. The invention also aims to make the authentication of such a medium rapid and secure. Statement of the invention

[0020] Thus, the present invention relates to a magnetization device intended to be placed in an exposure and transfer cylinder to orient particles sensitive to magnetic fields, said particles being contained in a printing material affixed to a support, said magnetization device comprising: • a main magnet assembly in the general shape of a plate, said plate defining a mean plane and comprising an upper main face and a lower main face substantially parallel to the upper main face, and whose North and South poles are oriented in a direction extending in a plane generally parallel to said mean plane; • a plurality of secondary magnets extending in a generally two-dimensional array in a general direction which is substantially parallel to said upper surface of the main magnet assembly, said plurality of secondary magnets being intended to be positioned between the upper face of said main magnet assembly and said external surface of the exposure and transfer cylinder, at a burial distance from the external surface of the transfer cylinder, • said secondary magnets being small compared to the main magnet assembly in said mid-plane of said plate;

[0021] characterized by the fact that: • the poles of the magnets of said plurality of secondary magnets are oriented in a direction extending in a plane generally parallel to said mean plane; • in said plurality of secondary magnets: each magnet is separated from the neighboring magnet by a space; • said generally two-dimensional network of secondary magnets extends along a surface curved according to the radius of curvature of said exposure and transfer cylinder and whose concavity is directed towards said main magnet; • the magnets of the magnetization device are capable of causing the magnetic field lines generated by these magnets to act at a distance greater than said burial distance.

[0022] Preferred but non-limiting aspects of such a magnetizing device are as follows: • the upper face of said main magnet assembly is flat and on it rests a wedge, the curved upper face of which supports the secondary magnets of said two-dimensional network, • the upper face of said main magnet assembly is curved, and the secondary magnets of said two-dimensional network rest on this curved upper face or are separated from it by a non-magnetic spacer of the same curvature, • said secondary magnets are identical to each other, • said secondary magnets form at least two groups of magnets, the secondary magnets of each group having the same geometry and secondary magnets of different groups having different geometries, • the plurality of secondary magnets is located at a distance called burial of the external surface of the exposure and transfer cylinder, • the burial distance is less than 1.5 millimeters, • the burial distance is less than 1 millimeter, • the burial distance is less than 0.5 millimeters, • the magnetizing device is contained in a housing, one of the faces of which is rounded to match the curvature of the outer surface of the transfer cylinder,

[0023] Said particles sensitive to magnetic fields are in particular magnetic optically variable pigments as described above and / or reflective particles sensitive to magnetic fields.

[0024] Another aspect of the invention relates to an exposure and transfer cylinder comprising a magnetization device according to one of the above aspects, and / or according to one or more of the aspects set out in this text.

[0025] In such an exposure and transfer cylinder, the curvature of the external surface of the transfer cylinder may in particular be parallel to the curvature of the surface along which the generally two-dimensional network of secondary magnets of the magnetization device extends.

[0026] Finally, another aspect of the invention relates to a printing machine for fiduciary media which comprises at least one cylinder conforming to one of the above characteristics. The media may in particular be sheets of paper and / or plastic, or continuous films or strips of paper and / or plastic. DESCRIPTION OF THE FIGURES

[0027] Other characteristics and advantages of the invention will appear on reading the detailed description which follows. It will be made with reference to the appended drawings in which: • [Fig.l] is a view which schematically illustrates a magnetization box according to the invention; • [Fig.2] is a view that illustrates results of visual patterns obtained by the depositing by varying the spacing between the secondary magnet elements of a magnetizing box; • [Fig.3] shows other results obtained by the applicant, also making vary the distance between the secondary magnets; • [Fig.4a][Fig.4b][Fig.4c][Fig.4d] illustrate different configurations of magnets that can be implemented according to the invention; • [Fig.5a] illustrates the effect obtained on the distribution of particles sensitive to magnetic fields at three different observation angles, by the magnet configuration illustrated in [Fig.4a]; • [Fig.5b] [Fig.5c] [Fig.5d] respectively illustrate the three distributions of magnetic field-sensitive particles, obtained by the respective magnet configurations of Figures 4b, 4c and 4d at a single observation angle; • [Fig.6] illustrates the effect of the magnetic field strength generated by the magnets secondary implemented in the invention.

[0028] It is specified that these drawings are diagrams illustrating certain aspects of the invention, and that their various elements are not necessarily on the exact scale of the practical embodiments which can be made of the invention. DETAILED DESCRIPTION OF THE INVENTION

[0029] [Fig.l] schematically illustrates a magnetization device (which will also be called a magnetization “box”) B according to the invention and a sheet 30.

[0030] In this figure, the "top" is conventionally defined as the top of the figure, the "bottom" as the bottom of the figure. The terms "upper" and "lower" which will be used in this text are used in reference to these "top" and "bottom" references (for example: the upper face of an element being the face closest to the top).

[0031] The magnetization box is integrated into an exposure and transfer cylinder of a printing machine, not shown here.

[0032] A printing machine is understood to mean a press which makes it possible to: • transferring ink in particular onto a plastic or paper support, said ink comprising particles sensitive to magnetic fields, • exposing this ink to the magnetic field generated by the magnetization box to orient the particles sensitive to magnetic fields, • and to dry this ink in particular by means of electromagnetic radiation and preferably by ultraviolet radiation.

[0033] The printing processes that can be implemented by such a machine include screen printing, intaglio printing, flexography or even rotogravure.

[0034] In one application of the invention, this machine is a machine for printing secure media, particularly security media, preferably for fiduciary use. The "media" to be printed are preferably sheets (on which, for example, a certain number of banknotes are printed). It is also possible for the printing to be continuous. "Fiduciary use" means the act of carrying out a monetary transaction or enabling proof of an identity or a particular right.

[0035] This figure shows: • a main magnet assembly 10 in the general shape of a plate, and • a plurality 20 of secondary magnets 20i (or micro-magnets, the terms secondary magnets and micro-magnets being equivalent in this text) extending in a generally two-dimensional array in a general direction X which is substantially parallel to the upper surface of the main magnet assembly. The main magnet assembly 10 and the plurality 20 of magnets are fixedly assembled within the housing B.

[0036] [Fig. 1] also illustrates a sheet 30 intended to be imaged by the exposure and transfer cylinder (not shown), which carries one or more boxes B, a step of printing the sheet with fresh ink containing magnetic particles having taken place previously.

[0037] By "imaging" is meant here exposing without contact the freshly printed ink layer on the sheet to a magnetic field, each magnetic particle contained in the ink taking an orientation substantially parallel to the field lines which pass through the ink layer. This results, depending on the local modulations of the field lines imposed on the particles sensitive to magnetic fields, in the creation of a bas-relief type effect and therefore in reflecting the light towards an observer in a distinguishable or even evocative pattern. The operating principle of such inks being analogous to that of Fresnel structures, the surface of the inked sheet retains a macroscopic flatness for the observer.Indeed, the orientation of the particles sensitive to magnetic fields within the ink “vehicle” is similar to a “folded” structure such as a Fresnel structure and therefore makes it possible to obtain a bas-relief type effect, in particular similar to an effect obtained using Fresnel mirrors.

[0038] The physical principles implemented in the imaging operation are magnetism and geometric optics.

[0039] It is possible to simulate the visual pattern obtained by a given magnetization, with computer software which restores at any point in space a magnetic flux relative to an emitting magnetic source, as well as the light reflected by a set of particles sensitive to magnetic fields oriented by the magnetic flux, according to the position of the incident light ray and the position of the observer.

[0040] The main magnet assembly 10 may consist of a single magnet, as in [Fig.l]. In this case, the single magnet is the plate defined by the main magnet assembly. This main magnet assembly 10 may also consist of several magnets. In all cases, this main magnet assembly is a main “rolling bar” magnet which will therefore generate the rolling bar background effect on the support to be imaged.

[0041] The plate defined by the main magnet assembly defines a mean plane P. It comprises an upper main face FS and a lower main face FI which are substantially parallel. The North and South poles of the main magnet assembly are oriented in the X direction extending in a plane substantially parallel to said mean plane of the plate.

[0042] The plurality of secondary magnets is positioned between the upper face FS of the main magnet assembly, and the external surface E of the magnetization housing B. To image the sheet, this external surface extends very close to the sheet, at a distance of the order of a few hundredths of a millimeter. This external surface can also be in contact with the sheet and said sheet is then pressed, in particular by suction, onto the cylinder during its processing.

[0043] Since the printed pattern is on the face of the sheet opposite the cylinder, the ink has no contact with any mechanical part of the cylinder or the printing machine that includes it, to avoid any smearing with fresh ink deposited on the sheet.

[0044] This ink will then be crosslinked as soon as exposure to the magnetic field has imaged the desired pattern, to avoid any untimely relaxation of the pigments. Exposure to the magnetic field therefore takes place through the support (in this example: the sheet but this could be a continuous strip of paper or plastic such as PET or BOPP without these examples being limiting). The thickness of the support is generally greater than 20 microns and less than 1 millimeter, in particular between 30 and 120 microns, and preferably between 80 and 110 microns.

[0045] The plurality of secondary magnets is located at a distance D called burial from the external surface of the exposure and transfer cylinder.

[0046] The burial distance D of a secondary magnet is, more precisely, the distance separating the upper face of the secondary magnet from the external surface of the printed support.

[0047] The burial distance is for example less than 1.5 millimeters. Preferably this distance is less than 1 millimeter. More preferably this distance is less than 0.6 millimeters. Each of the secondary magnets is “small” compared to the main magnet assembly in said mid-plane of said plate. This means that the largest dimension of each secondary magnet is smaller than the smallest dimension of the main magnet assembly, in this same plane.

[0048] This dimensional characteristic contributes to the fine resolution of the patterns which will be obtained by magnetization of the particles, during exposure.

[0049] The North and South poles of the secondary magnets are oriented in a direction X extending in a plane generally parallel to said mean plane of the plate, like the North and South poles of the main magnet assembly.

[0050] However, the North and South poles of the secondary magnets are not necessarily aligned with the X direction of the North and South poles of the main magnet assembly. A such alignment of poles corresponds to an embodiment of the invention.

[0051] In other embodiments this alignment is not present. In any event, the axes of the poles of the secondary magnets are not perpendicular to the axis of the poles of the main magnet.

[0052] Each magnet of the plurality of secondary magnets (which may be called “micro-magnets”) is separated from the neighboring magnet of this same plurality by a space.

[0053] The micromagnets and the main magnet assembly generate respective magnetic fields which interact with each other to produce the desired attractive visual effects.

[0054] This, in combination with the fact that each magnet of the plurality of secondary magnets is separated from the neighboring magnet of this same plurality by a space, makes it possible to increase the number and density of active field lines between these magnets. As a result, this makes it possible to obtain additional field lines and therefore richer patterns.

[0055] A space is thus located between each micro-magnet. The applicant studied the influence of this space with the objective of obtaining an aesthetic visual effect (in particular: an effect in which the visual pattern obtained reproduces as faithfully as possible a desired initial graphic pattern, while giving it, depending on the inclination, a dynamic and bas-relief effect). This made it possible to characterize that, in the invention, the smallest distance between two neighboring micro-magnets is preferably less than 4 millimeters, more preferably less than 3 millimeters and even more preferably less than 2 millimeters.

[0056] The main magnet assembly forms, as has been said, a plate (consisting of a single magnet, or of adjacent magnets). This plate has lateral dimensions (i.e. dimensions perpendicular to the height direction, i.e. in the plane (XY) in [Fig.l], in the case of a substantially rectangular plate this will be the length and width of the plate) which are, for example, between 1 and 100 millimeters, preferably between 20 and 50 millimeters, and more preferably between 30 and 40 millimeters.

[0057] The height of the main magnet assembly is for example between 1 and 10 millimeters, preferably between 1.5 and 5 millimeters and more preferably between 1.5 and 4 millimeters.

[0058] The micromagnets have the following dimensions: • the lateral dimensions of each micro-magnet are for example between 0.5 and 20 millimeters, preferably between 0.5 and 15 millimeters and more preferably between 0.5 and 12 millimeters, • the height of each micromagnet is for example between 0.1 and 2 millimeters. Preferably, this height is 1 millimeter.

[0059] In particular configurations, the results of which are illustrated in certain figures, the micromagnets have the following dimensions: • for the micro-magnets of [Fig.4b], the large arcs of the micro-magnets measure 11.017 x 3 millimeters, for a section of 1 x 1 millimeter, • for the micromagnets in figures 4c and 4d, the micromagnets measure 4.24 x 2.83 millimeters, for a height of 1 millimeter.

[0060] The number of micro-magnets in the plurality 20 is for example greater than 4, preferably greater than 16 and more preferably greater than 36.

[0061] The magnets are made of one or more magnetic materials.

[0062] The main magnet assembly and the micromagnets are preferably made of identical magnetic materials.

[0063] This material is for example ferrite, alloys of aluminum, nickel and cobalt, or neodymium, iron and boron, or samarium and cobalt.

[0064] Preferably, the main magnet and / or the secondary magnets do not contain any plastic material, such as an elastomer, and they are in particular free of plastoferrite. In particular, the main magnet does not contain any plastic material, such as an elastomer, and it is in particular free of plastoferrite.

[0065] The magnetic remanence of the magnets is greater than 0.2 Tesla, preferably greater than 0.3 Tesla. The main magnet and the micro-magnets are preferably made of magnetic materials with close magnetic remanences, more preferably of magnetic materials whose magnetic remanences differ by less than 0.4 Tesla, even more preferably of magnetic materials whose magnetic remanences differ by less than 0.2 Tesla, and even more preferably of magnetic materials whose magnetic remanences are equal.

[0066] It is further noted that the generally two-dimensional network formed by the plurality of secondary magnets 20 extends along a surface which is curved.

[0067] The concavity of this curved surface is directed towards the main magnet assembly (downwards).

[0068] This feature helps to ensure that the secondary magnets are all located at substantially the same distance from the external surface of the exposure and transfer cylinder and therefore from the support to be imaged which will be pressed thereon.

[0069] It also makes it possible to optimally use the entire surface of the area of ​​the support which must be treated. This makes it possible to avoid blurring or degradation of the patterns obtained on the support, in particular in the center of the printing area (in a configuration in which the secondary magnets would be arranged along a plane and not along a curved surface, the secondary magnets facing the center of the area to be printed would be further away from said area because the surface E of the housing B respects the curvature of the exposure and transfer cylinder).

[0070] The magnets of the magnetization device are in fact capable of causing the magnetic field lines generated by these magnets to act, i.e. to orient the particles sensitive to the magnetic field, at a distance greater than the burial distance.

[0071] In a variant shown in [Fig.l], the upper face of the main magnet assembly is flat and a shim Ca rests on it, and the upper face of said shim (i.e. the face facing the secondary magnets) is curved to support the secondary magnets of said two-dimensional network, giving this network its curved configuration.

[0072] In another variant, the upper face of the main magnet assembly is itself curved and the secondary magnets of the two-dimensional network rest directly (or indirectly using a non-magnetic spacer of the same curvature) on this curved upper face.

[0073] These two variants are advantageous examples which make it possible to obtain a curved configuration of the network of secondary magnets, while using secondary magnets whose section (in the plane of [Fig.l]) is simply rectangular.

[0074] The secondary magnets are positioned and kept at a distance from each other by any means allowing this, in particular a grid made of non-magnetic material machined or 3D printed with housings accommodating said secondary magnets.

[0075] It is possible that the secondary magnets are identical to each other.

[0076] Alternatively, this is not the case. In particular, it is possible for the secondary magnets to form at least two groups of magnets, the secondary magnets of each group having a specific geometry (i.e., the secondary magnets of each group have the same geometry, and the secondary magnets of different groups have different geometries) and / or a specific magnetization.

[0077] The invention makes it possible to obtain complex patterns with very good sharpness and very good resolution, over the entire area of ​​the support which is treated by the exposure and transfer cylinder.

[0078] In Figures 5a to 5d, and 6, the views representing black squares or trapezoids which represent the results of simulations carried out by the applicant to represent square areas which would be obtained on supports printed with an ink comprising particles sensitive to magnetic fields using the invention, for several configurations of magnets (the trapezoids are representations of the same squares but distorted due to their inclination).

[0079] In the squares or trapezoids of these figures, gray pixelated patterns correspond to the results of digital simulations obtained by the calculation chain used by the applicant to illustrate the patterns obtained on particles sensitive to magnetic fields contained in the ink, by certain configurations of magnets.

[0080] These images show light areas obtained due to the specular reflection by the particles (which act as micro-mirrors) of an incident illumination at a given angle and in a given direction of observation. Conversely, the dark areas of these images correspond to the areas where the conditions of this specular reflection are not met for said angle of illumination and said direction of observation.

[0081] In other words, these figures are faithful representations of the effects of magnetic field lines on magnetic field-sensitive particles, the resulting patterns resulting from different respective configurations of magnets, which act on an ink comprising these particles.

[0082] These figures illustrate in particular the more or less strong contrasts obtained between the light and dark zones, these contrasts corresponding to the sharpness of the pattern observed on the support: the higher the contrast, the sharper the observed pattern. In addition, [Fig.5a] we can see the dynamic effect and the sensation of depth for three different inclinations at fixed angles of illumination and observation.

[0083] The applicant was thus able to identify geometries of secondary magnets of small size and bulk, of any shape without being confined to classic shapes (cube, block, cylinder, ring, etc.) to develop surprising depth and texture effects and this while maintaining clarity of the overall pattern, including at the edge of the treated area on the support.

[0084] The applicant was also able to identify in general the effect on the pattern obtained of the distance between the secondary magnets.

[0085] Each of the variants of the magnetization device described above may be contained in a housing, one of the faces of which is curved to match the shape of the outer surface of the exposure and transfer cylinder, in which the housing is integrated.

[0086] The external surface of the exposure and transfer cylinder thus provided with such a magnetization box, preferably with a plurality of boxes corresponding to the number of exposures to be processed per revolution of said cylinder, may have a curvature which is parallel to the curvature of the surface along which the generally two-dimensional network of secondary magnets of the magnetization device extends.

[0087] Some details are provided below regarding the space between the secondary magnets.

[0088] Numerical simulations carried out by the applicant have shown that for a given support and a given ink comprising particles sensitive to magnetic fields, if the secondary magnets are separated from each other by too great a distance, the pattern obtained on the support is degraded.

[0089] The applicant also analyzed the fact that in a general configuration with a main set of magnets and a two-dimensional array of secondary magnets, the effect of the main magnet assembly is distinct from the effect obtained by the secondary magnets: • the main magnet assembly provides a rolling bar type effect, • secondary magnets, depending on their arrangement, spacing and even their shape, provide specific patterns.

[0090] Thus the combination of the main magnet assembly with the two-dimensional network of secondary magnets makes it possible to obtain specific optically variable patterns, i.e. with a change in visual appearance, in particular in brightness and / or color, preferably in brightness, depending on the angle of observation and / or illumination and the change in visual appearance of which is reinforced by the “rolling bar” type effect, in particular the dynamic effect and / or the depth effect of said specific reinforced spring patterns.

[0091] The simulations carried out by the applicant have notably shown that the distance between two neighboring secondary magnets has an influence on the patterns obtained.

[0092] The simulation results discussed in this text illustrate different reflection patterns by particles sensitive to magnetic fields, which would be obtained on sheets printed with an ink comprising these particles, by a printing machine comprising an exposure and transfer cylinder in which different magnetization boxes have been integrated. Unless otherwise stated, the ink is identical for all simulations. Each simulation corresponds to a different configuration of magnets, in the magnetization boxes.

[0093] [Fig.2] thus shows results of numerical simulations illustrating the patterns which would be obtained on sheets printed with a printing machine comprising an exposure and transfer cylinder in which a magnetization box has been integrated, with four different configurations DI to D4 of secondary magnets. The four different configurations of secondary magnets are illustrated in column AL The graphic pattern of the secondary magnets in these configurations is scale-shaped to evoke fish or reptile skin.

[0094] This figure shows the effect of variations in the arrangements of the secondary magnets of these different magnet configurations, which all repeat the general arrangement of [Fig.l], with a variation only in the spacing between two secondary magnets.

[0095] These spacings are 0.5 mm, 1 mm, 2 mm and 3 mm on the respective lines D1, D2, D3 and D4 of [Fig.2],

[0096] The results obtained for three different inclinations of the support (at fixed illumination and observation angles) with each of these four configurations D1 to D4 of secondary magnets are illustrated on the line corresponding to the respective columns T1, T2 and T3. As shown in the diagrams in the column headers, the illus trations on the same line correspond to a given inclination of the sample support receiving the pattern, relative to the observer: upward inclination in column T1, to the normal (perpendicular) in column T2 and downward in column T3. In column TA we find for each configuration DI to D4 of secondary magnets the graphic superposition of the arrangement of the secondary magnets (column Al) and the effects generated (column T2).

[0097] We see in this figure that beyond a certain spacing between the secondary magnets: • there is less detail in the pattern obtained - this results from the reduction of interactions between the field lines of the different secondary magnets, • a blurring of the pattern also appears - which corresponds to a loss of definition.

[0098] This figure shows that when the distance between the magnets increases to a value of 3 mm, the pattern obtained on the sheet is less and less structured, until it no longer finds the shape of the pattern of the secondary magnets (scale pattern) and thus loses the desired evocation of fish or reptile skin.

[0099] [Fig.3] shows further simulation results obtained by the applicant with a simple secondary magnet pattern, representing a network of symmetrically arranged squares.

[0100] In this figure are superimposed in columns SI and S2 the field line patterns obtained on the surface of a transfer cylinder provided with a magnetization box without a main magnetization assembly but with a network of secondary magnets as generally illustrated in [Fig.l]. In column SI, this is a representation of the assembly and in column S2 an enlarged detail.

[0101] The secondary magnets here are small cubes with 2 mm sides, they are represented by the squares with their two opposite polarities.

[0102] The distance between two neighboring secondary magnets is: • 4 mm on line E3, • 5 mm on line E2, and • 6 mm on the EL line

[0103] It can be seen in this figure that the further the secondary magnets are separated from each other, the more blurred the field line generated on the surface of the printing cylinder is. Indeed, increasing the spacing of the secondary magnets in this configuration leads to a dispersion of the reflected rays, which is detrimental to the sharpness and therefore to the overall understanding of the imaged pattern. This is particularly true at the center of four square magnets where a clear spot is perceived which widens as the magnets move apart and the interactions of the magnetic field lines between them weaken.

[0104] We will now explain in more detail some examples of configurations of magnets, with reference to figures 4a to 4d.

[0105] Figures 4a to 4d show examples of configurations for the main magnet assembly, and the secondary magnets, the overall arrangements of which (said overall arrangements show in top view the superposition of the main magnet assembly and the secondary magnets) are respectively represented in the boxes Ga to Gd.

[0106] In the examples of these figures 4a to 4d all the magnets have a thickness (a height) of 1 millimeter.

[0107] More specifically, Figures 4a and 4b show alternatives for the shapes of the secondary magnets that can be used in the respective examples illustrated by these two figures. In these two figures the alternatives of the secondary magnets can be combined.

[0108] Figures 4c and 4d each show a single secondary magnet geometry but whose respective polarities have undergone a rotation of a quarter of a turn between the secondary magnet illustrated in [Fig.4c], and the secondary magnet illustrated in [Fig.4d].

[0109] In all configurations, the micromagnets and the main magnet assembly generate respective magnetic fields that are in the same plane, as discussed above.

[0110] In these figures 4a to 4d, the North pole of each magnet is shown in black, and its South pole in white.

[0111] In the four examples of Figures 4a to 4d, the main magnet assembly is a parallelepiped whose two lateral dimensions (the two elongated sides in the figures) are 30 millimeters long, and whose height is 3 millimeters.

[0112] The following descriptions are made in the "lateral" plane of the magnets which is orthogonal to the direction of the height of the magnets.

[0113] In [Fig.4a], each of the secondary magnets has a height of 1 millimeter and its lateral geometry is one of the following geometries: • A 401a element whose width and length are 0.71 millimeters, • A large “diamond” 402a made from the crossing of a straight bar crossing the diagonal of a square, the straight bar of which is 4.24 millimeters long and the diagonal of the square crossed is 2.83 millimeters long, the North and South poles of the magnet being distributed on either side of the longitudinal axis of the straight bar, • A large “diamond” 403a made from the crossing of a straight bar crossing the diagonal of a square, the straight bar of which is 4.24 millimeters long and the diagonal of the square crossed is 2.83 millimeters long, the North and South poles of the magnet being distributed on either side of the median transverse axis of the right bar, • A 404a “diamond” with a side length of 1 millimeter, the North and South poles of the magnet being distributed on either side of the diagonal of the square, • An “arrow” 405a formed of a rectangle extended from a point to one of its sides, the long side of the arrow being 2 millimeters and its short side 1 millimeter, the North and South poles of the magnet being distributed on either side of a line perpendicular to a diagonal of the arrow, the South pole being housed in the point of the arrow, • An “arrow” 406a formed of a rectangle extended from a point to one of its sides, the long side of the arrow being 2 millimeters and its short side 1 millimeter, the North and South poles of the magnet being distributed on either side of a line perpendicular to a diagonal of the arrow, the North pole being housed in the point of the arrow.

[0114] On the left part of [Fig.4a] we see: • In the upper part, the different forms of secondary magnets whose assembly makes it possible to obtain a complex whole graphically evoking a moucharabieh, • In the lower part, the arrangement Ga of these different secondary magnets in top view superimposed on the main square magnet 410a (also shown in this lower part), forming said complex assembly graphically evoking a moucharabieh.,

[0115] The central part of this same [Fig.4a] illustrates the detailed dimensions of the different parts of each secondary magnet.

[0116] [Fig.4b] represents in its upper right part the basic element 400b of a secondary magnet pattern in “fish scales”, with the dimensions of this basic element. This figure also shows (more particularly in its left part) the secondary magnets used to constitute this basic element, as well as the corresponding overall arrangement Gb.

[0117] As illustrated in the left part of this figure; the secondary magnets used to produce this basic element are of one of the following types: Large Arch / Medium Arch / Small Arch / Half Large Arch / Half Medium Arch / Half Small Arch / Dome.

[0118] The results obtained by a set of secondary magnets which reproduces this pattern linked to this basic element are illustrated in [Fig.2].

[0119] In this [Fig.4b], each of the secondary magnets has a height of 1 millimeter and its lateral geometry is one of the following geometries, as illustrated in the left part of the figure: • An arc describing a portion of a circle, with a length of 11 millimeters and with a total width of 3 millimeters, the North pole being in the middle part of the arc while the two end parts of the arc are South poles, • An arc describing a portion of a circle, with a length of 6.45 millimeters and a total width of 1.6 millimeters, the North Pole being in a portion of the middle part of the arc while the South Pole covers the two end parts of the arc as well as a portion of the middle part, • An arc describing a portion of a circle, with a length of 2.27 millimeters and a total width of 1.24 millimeters, the inner part of the arc being reduced to a point, the North Pole being in a portion of the middle part of the arc while the South Pole is in the remaining portion of the middle part towards the point of the arc, • Half of each of the three geometries just described, retaining only that defined by a median line orthogonal to the arcs described previously which will be used in particular to complete the assembly on the edges. • A “cupola” which is a section of an arc describing a portion of a circle, one elongated edge of which is rounded and the other elongated edge is straight, the total length of the cupola being 6 millimeters and its total width 1 millimeter, the North pole being in a portion of the middle part of the cupola while the South pole covers the two end parts of the cupola as well as a portion of its middle part.

[0120] The right part of [Fig.4b] shows how the secondary magnets are arranged among themselves in the plurality of secondary magnets, to generate the desired basic element.

[0121] In the lower part of [Fig.4b], the arrangement Gb of the different basic elements in top view superimposed on the main square magnet, this complex assembly graphically evoking the desired tiling of fish or reptile skin.

[0122] In [Fig.4c], each of the secondary magnets has a height of 1 millimeter and its lateral geometry is as follows: • A “diamond” made by crossing a straight bar across the diagonal of a square, the straight bar being 4.24 millimeters long and the diagonal of the square crossed being 2.83 millimeters long, the North and South poles of the magnet being distributed on either side of the median transverse axis of the straight bar.

[0123] In [Fig.4d], each of the secondary magnets has a height of 1 millimeter and its lateral geometry is as follows: • A “diamond” made from the crossing of a straight bar crossing the diagonal of a square, the straight bar of which is 4.24 millimeters long and the diagonal of the crossed square is 2.83 millimeters long, the North and South poles of the magnet being distributed on either side of the longitudinal axis of the straight bar.

[0124] [Fig.5a] shows the simulation of the “mashrabiya” graphic pattern as well as the dynamic effect and the sensation of 3D texture generated by the tilting movement of the support, a set of effects obtained by the distribution of particles sensitive to magnetic fields, contained in a printing material affixed to the support by an exposure and transfer cylinder containing a magnetization device in accordance with the configuration of magnets in [Fig.4a].

[0125] In this figure three views of the same support are represented, each view being taken from a different angle of incidence relative to the support. These three views show that the desired complex pattern is obtained, with in addition the deployment of a dynamic effect and the generation of a sensation of 3D texture supported by the "rolling bar" (variation of the view with the incidence, during a rotation which would be around a horizontal axis in the plane of the figure).

[0126] Figures 5b, 5c and 5d show three views respectively illustrating the simulations of the graphic patterns obtained by the respective magnet configurations of Figures 4b, 4c and 4d.

[0127] Each of the simulations represents the effect achieved by means of the respective magnet configuration, the particles being contained in a printing material affixed to a support and oriented by a transfer cylinder containing the magnetizing devices with the respective magnet configuration.

[0128] [Fig.5b] evokes the scales of fish or reptiles.

[0129] [Fig.5c] evokes geometric assemblages referring to Inca iconography and [Fig.5d] to that of ancient Egypt (pyramids).

[0130] [Fig.6] illustrates the effect of the strength of the magnetic field generated by the secondary magnets implemented in the invention.

[0131] This figure shows five views 61 to 65, which illustrate all the distributions of particles sensitive to magnetic fields, obtained by the configurations of magnets of identical geometries, sizes, and arranged in an identical manner, repeating the configuration 60 already illustrated in the preceding figures 2 line D2 and 4b (with 1 millimeter distance between the secondary magnets).

[0132] For each of the five views, the materials and magnetic characteristics of the magnets are different: • in the left view 61, the secondary magnets as well as the main magnet assembly are made of Neodymium with a remanence of 1.4 Tesla, • in the second view 62 from the left, the secondary magnets as well as the main magnet assembly are made of Neodymium with a remanence of 1 Tesla, • in the third view 63 from the left, the secondary magnets are in Neodymium with remanence 1.4 Tesla and the main magnet assembly is in ferrite with remanence 0.4 Tesla, • in the fourth view from the left 64, the secondary magnets are made of ferrite with a remanence of 0.4 Tesla and the main magnet assembly is made of Neodymium with a remanence of 1.4 Tesla, • in the fifth view 65 from the left (which is the rightmost view), the secondary magnets are and the main magnet assembly are made of 0.4 Tesla remanence ferrite.

[0133] It is noted from the above simulations that it is preferable, in order to promote the interaction between the main magnet and the secondary magnets, that is to say to have very bright reflective zones, to use materials with close magnetic remanence, in particular identical ones, respectively for the main magnet and for the secondary magnets. In particular, the Neodymium 1.4 Tesla / Ferrite 0.4 Tesla combinations lead to more blurred patterns obtained and / or with a loss of details in the particle distribution patterns obtained on the support.

[0134] The geometric designs of magnets according to the invention make possible a particularly fine optical arrangement of the platelets / pigments / saddles or micromirrors contained in the inks and allow the production of complex, well-defined visual effects whose dynamic characteristic, the sensation of 3D or floating images are enhanced compared to the state of the art. This also allows great freedom of graphic design and makes it possible to implement new geometric shapes.

Claims

Claims

1. Magnetizing device intended to be placed in an exposure and transfer cylinder for orienting particles sensitive to magnetic fields, said particles being contained in a printing material affixed to a support, said magnetizing device comprising: a main magnet assembly in the general shape of a plate, said plate defining a mean plane and comprising an upper main face and a lower main face substantially parallel to the upper main face, and whose North and South poles are oriented in a direction extending in a plane generally parallel to said mean plane;a plurality of secondary magnets extending in a generally two-dimensional array in a general direction which is substantially parallel to said upper surface of the main magnet assembly, said plurality of secondary magnets being intended to be positioned between the upper face of said main magnet assembly and said outer surface of the exposure and transfer cylinder, at a burying distance from the outer surface of the transfer cylinder, said secondary magnets being small compared to the main magnet assembly in said mid-plane of said plate; characterized in that: the poles of the magnets of said plurality of secondary magnets are oriented in a direction extending in a plane generally parallel to said mid-plane; in said plurality of secondary magnets: each magnet is separated from the neighboring magnet by a gap;said generally two-dimensional network of secondary magnets extends along a surface curved according to the radius of curvature of said exposure and transfer cylinder and whose concavity is directed towards said main magnet; the magnets of the magnetization device are capable of the magnetic field lines generated by these magnets acting at a distance greater than said burial distance.;

2. Magnetizing device according to claim 1, characterized in that the upper face of said main magnet assembly is flat and that on it rests a wedge, the curved upper face of which supports the secondary magnets of said two-dimensional network.

3. Magnetizing device according to claim 1, characterized in that the upper face of said main magnet assembly is curved, and that the secondary magnets of said two-dimensional network rest on this curved upper face or are separated from it by a non-magnetic spacer of the same curvature.

4. Magnetizing device according to one of the preceding claims, characterized in that said secondary magnets are identical to each other.

5. Magnetizing device according to one of claims 1 to 3, characterized in that said secondary magnets form at least two groups of magnets, the secondary magnets of each group having the same geometry and the secondary magnets of different groups having different geometries.

6. Magnetization device according to one of the preceding claims, characterized in that the plurality of secondary magnets is located at a so-called burial distance from the external surface of the exposure and transfer cylinder.

7. Magnetizing device according to the preceding claim, characterized in that the burial distance is less than 1.5 millimeters.

8. Magnetizing device according to the preceding claim, characterized in that the burial distance is less than 1 millimeter.

9. Magnetizing device according to the preceding claim, characterized in that the burial distance is less than 0.5 millimeters.

10. Magnetizing device according to one of the preceding claims, characterized in that it is contained in a housing one of the faces of which is rounded to match the curvature of the external surface of the transfer cylinder.

11. Exposure and transfer cylinder comprising a magnetization device according to one of the preceding claims.

12. Exposure and transfer cylinder according to the preceding claim, characterized in that the curvature of the external surface of the transfer cylinder is parallel to the curvature of the surface along which the generally two-dimensional network of secondary magnets of the magnetization device extends.

13. Machine for printing secure media, characterized in that it comprises at least one exposure and transfer cylinder according to one of the two preceding claims.

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