FR3142281A1 - Magnetization device, cylinder and associated machine - Google Patents

Abstract

The present invention relates in particular to a magnetization device intended to be placed in an exposure and transfer cylinder to orient particles sensitive to magnetic fields, comprising: • a main assembly of plate-shaped magnet, • a plurality of secondary magnets forming a two-dimensional network, • said secondary magnets being small relative to the main magnet assembly, characterized in that: • the poles of the magnets of said plurality of secondary magnets are oriented in one direction extending in a plane generally parallel to the average 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 ensuring that the magnetic field lines generated by these magnets act at a distance greater than their burial distance. Figure 1

Description

Magnetization device, cylinder and associated machine FIELD OF THE INVENTION

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.

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

The respective positions of the particles are then intended to be frozen in a subsequent step.

The invention also relates to a sheet exposure and transfer cylinder provided with such a magnetization device, and to a sheet printing machine for fiduciary use which makes use of at least one such cylinder.

STATE OF THE ART

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 use of magnetic fields, the latter being called “optically variable magnetic inks” (or “OVMI” for “Optically Variable Magnetic Ink” in English).

• Un cœur constitué d'un matériau ferromagnétique (à base de fer, de cobalt ou de nickel) qui, grâce à la forme oblongue et plane de la plaquette, permet à celle-ci de s’orienter sous l’action d’un champ magnétique du fait de l’anisotropie de forme ;
• Une couche réfléchissante (souvent métallique) grâce à laquelle la plaquette prend un éclat brillant lorsque les positions de l’éclairage et de l’observateur respectent certaines conditions géométriques particulières, et une teinte beaucoup plus sombre dans le cas contraire ;
• Une éventuelle couche de matériau diélectrique transparent dont le contrôle précis de l’épaisseur permet de fixer la couleur principale du pigment ;
• En lien avec la précédente caractéristique, une couche absorbante permettant l’obtention de la couleur recherchée.

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 comparable to thin multilayer platelets (we speak of “platelet” pigment) which generally present 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 field magnetic 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 much darker shade otherwise;
• A possible layer of transparent dielectric material whose precise control of the thickness makes it possible to fix the main color of the pigment;
• In connection with the previous characteristic, an absorbent layer allowing the desired color to be obtained.

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

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

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. 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.

The use of these inks is based on a printing system allowing, at the time when the ink is applied but not yet crosslinked (i.e. not yet dry), to orient the pigments by exposure to a field magnetic, to create with all the pigments patterns and unique optical effects in reflection, before freezing the ink, and therefore the patterns it contains, by crosslinking.

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 a 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.

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

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

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 treated (that is to say the area of the support in which it is desired for the pattern to extend).

Many devices making use of this “rolling bar” have been proposed.

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 patterns aesthetic and varied.

However, the device described in this document has limits. Thus, we see that the visual effect conferred by the configurations envisaged is not uniform over the entire printed surface.

Furthermore, as soon as we propose to make the shape of the patterns more complex and/or reduce their size in order to increase their number per unit area, we cannot distinguish them from one another, so that this results in an impression of "blurry".

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

• un ensemble principal d’aimant en forme générale de plaque, ladite plaque définissant un plan moyen et comprenant une face principale supérieure et une face principale inférieure sensiblement parallèle à la face principale supérieure, et dont les pôles Nord et Sud sont orientés selon une direction s’étendant dans un plan généralement parallèle audit plan moyen ;
• une pluralité d'aimants secondaires s'étendant selon un réseau généralement bidimensionnel dans une direction générale qui est sensiblement parallèle à ladite surface supérieure de l’ensemble principal d’aimant, ladite pluralité d’aimants secondaires étant destinée à être positionnée entre la face supérieure dudit ensemble principal d’aimant et ladite surface externe du cylindre d’exposition et de transfert, à une distance d’enfouissement de la surface externe du cylindre de transfert,
• lesdits aimants secondaires étant petits par rapport à l’ensemble principal d’aimant dans ledit plan moyen de ladite plaque ;

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 assembly of magnet 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 s 'extending in a plane generally parallel to said average 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 relative to the main magnet assembly in said midplane of said plate;

• les pôles des aimants de ladite pluralité d'aimants secondaires sont orientés selon une direction s’étendant dans un plan généralement parallèle audit plan moyen;
• dans ladite pluralité d'aimants secondaires : chaque aimant est séparé de l’aimant voisin par un espace ;
• ledit réseau généralement bidimensionnel d'aimants secondaires s'étend selon une surface incurvée selon le rayon de courbure dudit cylindre d’exposition et de transfert et dont la concavité est dirigée vers ledit aimant principal ;
• les aimants du dispositif d’aimantation sont aptes à ce que les lignes de champs magnétiques générées par ces aimants agissent à une distance supérieure à ladite distance d’enfouissement.

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 average 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 ensuring that the magnetic field lines generated by these magnets act at a distance greater than said burial distance.

• la face supérieure dudit ensemble principal d’aimant est plane et que sur celle-ci repose une cale, dont la face supérieure incurvée supporte les aimants secondaires dudit réseau bidimensionnel,
• la face supérieure dudit ensemble principal d’aimant est incurvée, et les aimants secondaires dudit réseau bidimensionnel reposent sur cette face supérieure incurvée ou sont séparés d’elle par une entretoise amagnétique de même courbure,

• lesdits aimants secondaires sont identiques les uns aux autres,
• lesdits aimants secondaires forment au moins deux groupes d'aimants, les aimants secondaires de chaque groupe ayant la même géométrie et les aimants secondaires de groupes différents ayant des géométries différentes,
• la pluralité d’aimants secondaires est située à une distance dite d’enfouissement de la surface externe du cylindre d’exposition et de transfert,
• la distance d’enfouissement est inférieure à 1,5 millimètres,
• la distance d’enfouissement est inférieure à 1 millimètre,
• la distance d’enfouissement est inférieure à 0,5 millimètre,
• le dispositif d’aimantation est contenu dans un boîtier dont l’une des faces est arrondie pour épouser la courbure de la surface extérieure du cylindre de transfert,

Preferred but non-limiting aspects of such a magnetization device are the following:

• 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 the secondary magnets of different groups having different geometries,
• the plurality of secondary magnets is located at a so-called burial distance from 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 millimeter,
• the magnetization device is contained in a housing, one of the faces of which is rounded to match the curvature of the exterior surface of the transfer cylinder,

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

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

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.

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

DESCRIPTION OF FIGURES

• est une vue qui illustre schématiquement un boîtier d’aimantation selon l’invention ;
• est une vue qui illustre des résultats de motifs visuels obtenus par le déposant en faisant varier l’espacement entre les éléments d’aimants secondaires d’un boîtier d’aimantation ;
• montre d’autres résultats obtenus par le déposant, en faisant également varier la distance entre les aimants secondaires ;
• illustrent différentes configurations d’aimants pouvant être mises en œuvre selon l’invention ;
• illustre l’effet obtenu sur la répartition de particules sensibles aux champs magnétiques selon trois angles d’observation différents, par la configuration d’aimants illustrée sur la ;
• illustrent respectivement les trois répartitions de particules sensibles aux champs magnétiques, obtenues par les configurations d’aimants respectives des figures 4b, 4c et 4d selon un seul angle d’observation ;
• illustre l’effet de la force du champ magnétique généré par les aimants secondaires mis en œuvre dans l’invention.

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:

• is a view which schematically illustrates a magnetization box according to the invention;
• is a view that illustrates results of visual patterns obtained by the applicant by varying the spacing between secondary magnet elements of a magnetization housing;
• shows other results obtained by the applicant, also varying the distance between the secondary magnets;
• illustrate different configurations of magnets that can be implemented according to the invention;
• illustrates the effect obtained on the distribution of particles sensitive to magnetic fields according to three different observation angles, by the configuration of magnets illustrated on the ;
• respectively illustrate the three distributions of particles sensitive to magnetic fields, obtained by the respective magnet configurations of Figures 4b, 4c and 4d according to a single observation angle;
• illustrates the effect of the strength of the magnetic field generated by the secondary magnets used in the invention.

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 realizations which can be made of the invention.

DETAILED DESCRIPTION OF THE INVENTION

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

In this figure we define by convention the “top” 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 in reference to these “top” and “bottom” marks (for example: the upper face of an element being the face closest to the top).

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

• transférer de l’encre en particulier sur un support en plastique ou en papier, ladite encre comprenant des particules sensibles aux champs magnétiques,
• d’exposer cette encre au champ magnétique généré par le boîtier d’aimantation pour en orienter les particules sensibles aux champs magnétiques,
• et de sécher cette encre notamment par voie de radiations électromagnétiques et préférentiellement par des rayonnements ultra-violet.

By printing machine we mean a press allowing:

• transfer ink in particular onto a plastic or paper support, said ink comprising particles sensitive to magnetic fields,
• to expose 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 electromagnetic radiation and preferably by ultraviolet radiation.

The printing processes that can be implemented by such a machine include screen printing, intaglio, flexography and even rotogravure.

In one application of the invention this machine is a machine for printing secure media, in particular security media, preferably for fiduciary use. The “supports” to be printed are preferably sheets (on which a certain number of bank notes are printed, for example). It is also possible for printing to be continuous. By “fiduciary use” we mean the fact of carrying out a monetary transaction or allowing attestation of an identity or a particular right.

• un ensemble principal d’aimant 10 en forme générale de plaque, et
• une pluralité 20 d’aimants secondaires 20i (ou micro-aimants, les termes aimants secondaires et micro-aimants étant équivalents dans ce texte) s’étendant selon un réseau généralement bidimensionnel dans une direction générale X qui est sensiblement parallèle à la surface supérieure de l’ensemble principal d’aimant. L’ensemble principal d’aimant 10 et la pluralité 20 d’aimants sont assemblés de manière fixe au sein du boîtier B.

In this figure are represented:

• 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 network 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.

There 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 previously taken place.

By “imaging” we mean here exposing the freshly printed ink layer on the sheet without contact 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 maintains macroscopic flatness for the observer. Indeed, the orientation of the particles sensitive to magnetic fields within the “vehicle” of the ink can be compared 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 by means of Fresnel mirrors.

The physical principles implemented in the operation of imaging are magnetism and geometric optics.

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 in relation 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.

The main magnet assembly 10 may consist of a single magnet, as in the . In this case, the single magnet is the plate defined by the main magnet assembly. This main magnet assembly 10 can also be made up 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.

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 direction X extending in a plane substantially parallel to said average plane of the plate.

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 treatment.

The printed pattern being on the side of the sheet which is opposite the cylinder, the ink does not have contact with any mechanical part of the cylinder or of the printing machine which includes it, to avoid any smudging with the fresh ink deposited on the sheet.

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 it 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.

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

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.

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" relative to the main magnet assembly in said midplane 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 that same plane.

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

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

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. Such an alignment of poles corresponds to one embodiment of the invention.

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

Each magnet of the plurality of secondary magnets (which we can call “micro-magnets”) is separated from the neighboring magnet of this same plurality by a space.

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

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.

A space is thus found between each micromagnet. The applicant has 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 a function from the inclination a dynamic and bas-relief effect). This made it possible to characterize that, in the invention, the smallest distance between two neighboring micromagnets is preferably less than 4 millimeters, more preferably less than 3 millimeters and more preferably still less than 2 millimeters.

The main magnet assembly forms, as we have said, a plate (made up of a single magnet, or adjacent magnets). This plate has lateral dimensions (that is to say dimensions perpendicular to the direction of height either in the plane (XY) on the , in the case of a substantially rectangular plate it will be the length and the 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.

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.

• les dimensions latérales de chaque micro-aimant est par exemple entre 0,5 et 20 millimètres, de préférence entre 0,5 et 15 millimètres et de préférence encore entre 0,5 et 12 millimètre,
• la hauteur de chaque micro-aimant est par exemple entre 0,1 et 2 millimètres. De préférence, cette hauteur est de 1 millimètre.

The micromagnets have the following dimensions:

• the lateral dimensions of each micromagnet is 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.

• pour les micro-aimants de la , les grands arcs des micro-aimants mesurent 11,017 x 3 millimètres, pour une section de 1 x 1 millimètre,
• pour les micro-aimants des figures 4c et 4d, les micro-aimants mesurent 4,24 x 2,83 millimètres, pour une hauteur de 1 millimètre.

In particular configurations, the results of which are illustrated in certain figures, the micromagnets have the following dimensions:

• for the micromagnets of the , the large arcs of the micromagnets 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.

The number of micromagnets in the plurality 20 is for example greater than 4, preferably greater than 16 and even more preferably greater than 36.

The magnets are made from one or more magnetic material(s).

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

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

Preferably, the main magnet and/or the secondary magnets do not contain 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

The magnetic remanence of the magnets is greater than 0.2 Tesla, preferably greater than 0.3 Tesla. The main magnet and the micromagnets are preferably made of magnetic materials with similar 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 less than 0.2 Tesla, and even more preferably magnetic materials whose magnetic remanences are equal.

We also note that the generally two-dimensional network formed by the plurality of secondary magnets 20 extends along a surface which is curved.

The concavity of this curved surface points toward the main magnet assembly (downward).

This characteristic helps to ensure that the secondary magnets are all at approximately the same distance from the external surface of the exposure and transfer cylinder and therefore from the support to be imaged which will be placed there.

It also allows optimal use of the entire surface of the support area that must be treated. This makes it possible to avoid blurring or degradation of the patterns obtained on the support, particularly 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).

The magnets of the magnetization device are in fact capable of causing the magnetic field lines generated by these magnets to act, that is to say, to orient the particles sensitive to the magnetic field, at a distance greater than the burial distance. .

In a variant shown on the , the upper face of the main magnet assembly is planar and a wedge Ca rests thereon, and the upper face of said wedge (that is to say the face facing the secondary magnets) is curved to support the secondary magnets of said two-dimensional network, giving this network its curved configuration.

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 an amagnetic spacer of the same curvature) on this upper face curved.

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 the ) is simply rectangular.

The secondary magnets are positioned and maintained at a distance from each other by any means possible, in particular a grid of non-magnetic material machined or printed in 3D with housings accommodating said secondary magnets.

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

Alternatively, this is not the case. It is notably possible that the secondary magnets form at least two groups of magnets, the secondary magnets of each group having a specific geometry (that is to say that the secondary magnets of each group have the same geometry, and the magnets secondary groups of different groups have different geometries) and/or specific magnetization.

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 processed by the exposure and transfer cylinder.

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 fields magnetic using the invention, for several configurations of magnets (the trapezoids are representations of the same squares but deformed due to their inclination).

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

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

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

These figures illustrate in particular the more or less strong contrasts obtained between the light and dark areas, these contrasts corresponding to the sharpness of the pattern observed on the support: the higher the contrast, the sharper the pattern observed. We further perceive on the the dynamic effect and the sensation of depth for three different tilts at fixed illumination and observation angles.

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

The applicant was also able to generally identify the effect on the pattern obtained of the distance between the secondary magnets.

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

The external surface of the exposure and transfer cylinder thus provided with such a magnetization housing, preferably a plurality of housings corresponding to the number of exposures to be processed per revolution of said cylinder, can 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.

Below we provide some details about the space between the secondary magnets.

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.

• l’ensemble principal d’aimant fournit un effet de type « rolling bar »,
• les aimants secondaires, selon leur disposition, leur espacement et éventuellement leur forme, fournissent des motifs spécifiques.

The applicant also analyzed the fact that in a general configuration with a main magnet assembly 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 effect,
• secondary magnets, depending on their arrangement, spacing and possibly shape, provide specific patterns.

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, that is to say with a change in visual appearance, in particular brightness and/or color. color, preferably brilliance, depending on the angle of observation and/or illumination and whose change in visual appearance is reinforced by the "rolling bar" type effect, in particular the dynamic effect and/or the The depth effect of said specific patterns reinforced spring.

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.

The simulation results commented 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.

There thus shows results of digital simulations illustrating the patterns that 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 D1 to D4 secondary magnets. The four different secondary magnet configurations are shown in column A1. The graphic pattern of the secondary magnets of these configurations is scale-shaped to evoke the skin of a fish or reptile.

This figure shows the effect of variations in the arrangements of the secondary magnets of these different magnet configurations, which all follow the general arrangement of the , with variation only in the spacing between two secondary magnets.

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

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 at the column header, the illustrations on the same line correspond to a given inclination of the support of the sample hosting the pattern, relative to the observer: inclination upwards in column T1, at the normal (perpendicular) in column T2 and downward in column T3. In column TA we find for each configuration D1 to D4 of secondary magnets the graphic superposition of the arrangement of the secondary magnets (column A1) and the effects generated (column T2).

• il y a moins de détails dans le motif obtenu – ceci résultant de la diminution d’interactions entre les lignes de champs des différents aimants secondaires,
• un floutage du motif apparaît en outre – ce qui correspond à une perte de définition.

We see in this figure that beyond a certain spacing between the secondary magnets:

• there is less detail in the pattern obtained – this resulting from the reduction in interactions between the field lines of the different secondary magnets,
• a blurring of the pattern also appears – which corresponds to a loss of definition.

This figure shows that when the distance between the magnets increases until reaching 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 (pattern in scales) and thus lose the desired evocation of fish or reptile skin.

There shows other simulation results obtained by the applicant with a simple secondary magnet pattern, representing a network of squares arranged symmetrically.

On this figure are superimposed in columns S1 and S2 the patterns of field lines obtained on the surface of a transfer cylinder provided with a magnetization box without main magnetization assembly but with a network of secondary magnets as illustrated generally on the . Column S1 is a representation of the whole and column S2 is an enlarged detail.

The secondary magnets here are small cubes which have a side of 2 mm, they are represented by the squares with their two opposite polarities.

• 4 mm sur la ligne E3,
• 5 mm sur la ligne E2, et
• 6 mm sur la ligne E1.

The distance between two neighboring secondary magnets is:

• 4 mm on line E3,
• 5 mm on line E2, and
• 6 mm on line E1.

We see in this figure that the further apart the secondary magnets are from each other, the more blurred the field line generated on the surface of the printing cylinder. Indeed, the increase in the spacing of the secondary magnets of this configuration leads to a dispersion of the reflected rays, which harms the sharpness and therefore the overall understanding of the imaged pattern. This is particularly true at the center of four square magnets where we see a clear spot that widens as the magnets move apart and the interactions of the magnetic field lines between them fade.

We will now present in more detail some examples of magnet configurations, with reference to Figures 4a to 4d.

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

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

More precisely, 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.

Figures 4c and 4d each show a single secondary magnet geometry but whose respective polarities have undergone a rotation of a quarter turn between the secondary magnet illustrated on the , and the secondary magnet shown on the .

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

In these Figures 4a to 4d, the North pole of each magnet is represented in black, and its South pole in white.

In the four examples in 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.

The following descriptions are made in the “lateral” plane of the magnets which is orthogonal to the direction of the height of the magnets.

• Un élément 401a dont la largeur et la longueur sont de 0,71 millimètre,
• Un grand « diamant » 402a fait du croisement d’une barre droite traversant la diagonale d’un carré, dont la barre droite est longue de 4,24 millimètres et la diagonale du carré traversé est longue de 2,83 millimètres, les pôles Nord et Sud de l’aimant étant répartis de part et d’autre de l’axe longitudinal de la barre droite,
• Un grand « diamant » 403a fait du croisement d’une barre droite traversant la diagonale d’un carré, dont la barre droite est longue de 4,24 millimètres et la diagonale du carré traversé est longue de 2,83 millimètres, les pôles Nord et Sud de l’aimant étant répartis de part et d’autre de l’axe transversal médian de la barre droite,
• Un « diamant » 404a de 1 millimètre de côté, les pôles Nord et Sud de l’aimant étant répartis de part et d’autre de la diagonale du carré,
• Une « flèche » 405a formée d’un rectangle prolongé d’une pointe à un de ses côtés, le côté long de la flèche étant de 2 millimètres et son côté court de 1 millimètre, les pôles Nord et Sud de l’aimant étant répartis de part et d’autre d’une ligne perpendiculaire à une diagonale de la flèche, le pôle Sud étant logé dans la pointe de la flèche,
• Une « flèche » 406a formée d’un rectangle prolongé d’une pointe à un de ses côtés, le côté long de la flèche étant de 2 millimètres et son côté court de 1 millimètre, les pôles Nord et Sud de l’aimant étant répartis de part et d’autre d’une ligne perpendiculaire à une diagonale de la flèche, le pôle Nord étant logé dans la pointe de la flèche.

In , each of the secondary magnets has a height of 1 millimeter and its lateral geometry is one of the following geometries:

• An element 401a 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 right bar of which is 4.24 millimeters long and the diagonal of the crossed square is 2.83 millimeters long, the North poles and South 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 right bar of which is 4.24 millimeters long and the diagonal of the crossed square is 2.83 millimeters long, the North poles and South of the magnet being distributed on either side of the median transverse axis of the right bar,
• A “diamond” 404a measuring 1 millimeter on each side, the North and South poles of the magnet being distributed on either side of the diagonal of the square,
• An “arrow” 405a formed by a rectangle extended by a point on 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 tip of the arrow,
• An “arrow” 406a formed of a rectangle extended by a point on 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 tip of the arrow.

• En partie haute, les différentes formes d’aimants secondaires dont l’assemblage permet d’obtenir un ensemble complexe évoquant graphiquement un moucharabieh,

• En partie basse, la disposition Ga de ces différents aimants secondaires en vue de dessus en superposition de l’aimant principal carré 410a (également représenté dans cette partie basse), formant ledit ensemble complexe évoquant graphiquement un moucharabieh.,

On the left part of the we see :

• In the upper part, the different shapes of secondary magnets, the assembly of which 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 in superposition of the square main magnet 410a (also represented in this lower part), forming said complex assembly graphically evoking a moucharabieh.,

The central part of this same illustrates the detailed dimensions of the different parts of each secondary magnet.

There 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.

As illustrated in the left part of this figure; the secondary magnets used to create 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.

The results obtained by a set of secondary magnets which reproduces this pattern linked to this basic element are illustrated on the .

• Un arc décrivant une portion de cercle, d’une longueur de 11 millimètres et d’une largeur totale de 3 millimètres, le pôle Nord étant en partie médiane de l’arc tandis que les deux parties d’extrémité de l’arc sont des pôles Sud,
• Un arc décrivant une portion de cercle, d’une longueur de 6,45 millimètres et d’une largeur totale de 1,6 millimètres, le pôle Nord étant dans une portion de la partie médiane de l’arc tandis que le pôle Sud recouvre les deux parties d’extrémité de l’arc ainsi qu’une portion de la partie médiane,
• Un arc décrivant une portion de cercle, d’une longueur de 2,27 millimètres et d’une largeur totale de 1,24 millimètres, la partie intérieure de l’arc étant réduite à une pointe, le pôle Nord étant dans une portion de la partie médiane de l’arc tandis que le pôle Sud est dans la portion restante de la partie médiane vers la pointe de l’arc,
• La moitié de chacune des trois géométries qui viennent d’être décrites, en ne retenant que celle définie par une ligne médiane orthogonale aux arcs décrits précédemment qui servira notamment à compléter l’assemblage sur les bords.
• Une « coupole » qui est une section d’arc décrivant une portion de cercle, dont un bord allongé est arrondi et l’autre bord allongé est droit, la longueur totale de la coupole étant de 6 millimètres et sa largeur totale de 1 millimètre, le pôle Nord étant dans une portion de la partie médiane de la coupole tandis que le pôle Sud recouvre les deux parties d’extrémité de coupole ainsi qu’une portion de sa partie médiane.

On this , 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 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 interior 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 tip of the arc,
• Half of each of the three geometries which have just been 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 “dome” which is an arc section 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 dome being 6 millimeters and its total width of 1 millimeter, the North pole being in a portion of the middle part of the dome while the South pole covers the two end parts of the dome as well as a portion of its middle part.

The right part of the shows how the secondary magnets are arranged together in the plurality of secondary magnets, to generate the desired base element.

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

• Un « diamant » fait du croisement d’une barre droite traversant la diagonale d’un carré, dont la barre droite est longue de 4,24 millimètres et la diagonale du carré traversé est longue de 2,83 millimètres, les pôles Nord et Sud de l’aimant étant répartis de part et d’autre de l’axe transversal médian de la barre droite.

In , each of the secondary magnets has a height of 1 millimeter and its lateral geometry is as follows:

• A “diamond” made from the intersection 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 straight bar.

• Un « diamant » fait du croisement d’une barre droite traversant la diagonale d’un carré, dont la barre droite est longue de 4,24 millimètres et la diagonale du carré traversé est longue de 2,83 millimètres, les pôles Nord et Sud de l’aimant étant répartis de part et d’autre de l’axe longitudinal de la barre droite.

In , each of the secondary magnets has a height of 1 millimeter and its lateral geometry is as follows:

• A “diamond” made from the intersection 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.

There 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 material printing affixed to the support by an exposure and transfer cylinder containing a magnetization device in accordance with the magnet configuration of the .

In this figure three views of the same support are shown, each view being taken at a different angle of incidence relative to the support. These three views show that the desired complex pattern is obtained, with the additional 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 of a rotation which would be around a horizontal axis in the plane of the figure).

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.

Each of the simulations represents the effect obtained 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 magnetization devices with the configuration of respective magnets.

There evokes fish or reptile scales.

There evokes geometric assemblages referring to Inca iconography and to that of ancient Egypt (pyramids).

There illustrates the effect of the strength of the magnetic field generated by the secondary magnets used in the invention.

In this figure are represented five views 61 to 65, which illustrate all the distributions of particles sensitive to magnetic fields, obtained by the configurations of magnets of geometries, of identical sizes, and arranged in an identical manner, taking up the configuration 60 already illustrated on Figures 2 line D2 and 4b above (with 1 millimeter of distance between the secondary magnets).

• dans la vue de gauche 61, les aimants secondaires ainsi que l’ensemble principal d’aimant sont en Neodyme de rémanence 1,4 Tesla,
• dans la deuxième vue 62 en partant de la gauche, les aimants secondaires ainsi que l’ensemble principal d’aimant sont en Neodyme de rémanence 1 Tesla,
• dans la troisième vue 63 en partant de la gauche, les aimants secondaires sont en Neodyme de rémanence 1,4 Tesla et l’ensemble principal d’aimant est en ferrite de rémanence 0,4 Tesla,
• dans la quatrième vue en partant de la gauche 64, les aimants secondaires sont en ferrite de rémanence 0,4 Tesla et l’ensemble principal d’aimant est en Neodyme de rémanence 1,4 Tesla,
• dans la cinquième vue 65 en partant de la gauche (qui est la vue la plus à droite), les aimants secondaires sont et l’ensemble principal d’aimant sont en ferrite de rémanence 0,4 Tesla.

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 starting 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 starting from the left, the secondary magnets are made of Neodymium with a remanence of 1.4 Tesla and the main magnet assembly is made of ferrite with a remanence of 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.

We note from the simulations above 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 similar magnetic remanences, in particular identical, respectively for the main magnet and for the secondary magnets. In particular, the Neodymium 1.4 Tesla / Ferrite 0.4 Tesla associations lead to patterns obtained that are more blurred and/or with a loss of detail in the particle distribution patterns obtained on the support.

The geometric designs of magnets according to the invention make possible a particularly fine optical arrangement of the plates / pigments / tesserae or micro-mirrors contained in the inks and allow the production of complex, well-defined visual effects whose dynamic characteristic, the sensation of 3D or floating images come out reinforced 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 (13)

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 assembly of magnet 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 s 'extending in a plane generally parallel to said average 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 relative to the main magnet assembly in said midplane of said plate;
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 average 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 ensuring that the magnetic field lines generated by these magnets act at a distance greater than said burial distance. Magnetization 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. Magnetization 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. Magnetization device according to one of the preceding claims, characterized in that said secondary magnets are identical to each other. Magnetization 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. 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. Magnetization device according to the preceding claim, characterized in that the burial distance is less than 1.5 millimeters. Magnetization device according to the preceding claim, characterized in that the burial distance is less than 1 millimeter. Magnetization device according to the preceding claim, characterized in that the burial distance is less than 0.5 millimeters. Magnetization 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 exterior surface of the transfer cylinder. Exposure and transfer cylinder comprising a magnetization device according to one of the preceding claims. 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 device extends magnetization. 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.