FR3070895A1 - METHOD FOR EMBOSSING MICROSTRUCTURES ON A SUBSTRATE - Google Patents

Abstract

A method of embossing microstructures on a substrate that reduces resin build-up in an embossing tool. The method includes applying a curable resin layer to at least a portion of the substrate, wherein the curable resin layer has a first edge and a second edge extending beyond an upper edge and a lower edge of the substrate; contacting an embossing tool with the curable resin layer using a rotary embossing roll to form the microstructures in the curable resin layer, the first edge and the second edge of the curable resin layer being substantially aligned with the direction of rotation of the roll; and curing the curable resin layer, wherein the curable resin layer is applied with varying thickness.

Description

METHOD FOR EMBOSSING MICROSTRUCTURES ON A SUBSTRATE

Technical area

[0010] JL ci present invention relates to way general to of the microstruct embossing processes ures on n substrate who reduce resin build-up in one

embossing tool, and devices produced using these methods. The invention is suitable for use in the manufacture of microoptical devices used as security devices for banknotes and similar security documents. The invention should be described in relation to this exemplary, but not limiting, application.

BACKGROUND OF THE INVENTION It is well known that several of the world's banknotes, as well as other security documents, include security devices which produce optical effects allowing visual authentication of the banknote. . Some of these security devices include focusing elements, such as microlenses, which act to sample and enlarge micro-imaging elements and project imagery that can be observed by a user. authentication purposes.

In this case, iormer es microlens substrate by embossing "soft qaufraae" is known.

An example is the

This r- ²¹ soft 'is the gaurrage of others on a gravure photo of a directly on a

Cover curable with polymer strips

UV.

is embossed with

UV by first putting the moving strip in contact with a rotating embossing roller. The resin is clamped between the embossing tool or the block forming part of the roll or affixed to it and the strip, so that under the printing pressure, the resin fills the structures in the recess in the tool embossing. While the resin is still in contact with and under the embossing tool. printing pressure, the resin is cured by UV radiation directed through the transparent polymer strip.

A problem which arises in this “soft embossing” process is the accumulation of UV resin in the embossing tool after a certain period of time, which leads to a modification of the embossing geometry of the driving tool. defects in the embossed microstructures in the UV-cured resin. Such embossing defects can appear as high edge roughness on the perimeter of the UV embossed part.

The Applicant believes that such defects occur (at least in part) because, during each UV embossing cycle, as each UV embossed part is released from the embossing tool, a small amount of UV resin and / or primary material is left on the tool. The embossed geometry thus becomes progressively compromised, particularly in regions corresponding to the perimeter of the UV embossed resin part, as more and more accumulation material is deposited on the embossing tool with, each cycle of embossing.

The accumulation problem is further exacerbated due to the alignment tolerance of the "soft embossing" process. The position of the printed layer of UV curable resin with respect to the structures on the embossing tool changes or varies in the embossing pattern due to the process tolerance. The “soft embossing” process of

Applicant has an alignment tolerance of one unit to the other of approximately +/- 0.25 mm, and therefore the perimeter of the rotogravure part of UV-curable resin will be embossed with a slightly different part from the embossing tool during each repetition. This has the effect of spreading the embossing residue over a larger area, thereby increasing the build-up overall.

The accumulation faults give the appearance of poor quality in the finished bank note, because the

roughness which is dev · eloppe can create a teacher it very edge irregular in the piece of UV resin which is easily visible to the eye bare. [Yes /] To wire time, faults accumulation can become S JL important that they can t encroach well on parties es internal parts UV resin

héliogravée. If the embossed structures are designed to have an optical function, for example if they are diffractive optical elements, diffraction gratings, lenses or any other micro-optical element, their optical performance may be seriously degraded due to the accumulation.

In previous attempts to avoid the build-up problem, the rotogravured UV resin patch was designed to extend beyond the microstructures on the embossing tool. The perimeter portions of the photoetched UV resin part therefore did not fill any structure on the embossing tool and were essentially embossed with a smooth flat surface on the embossing tool. Although this delayed the onset of a problematic build-up, the build-up would still return after a longer period of embossing.

Consequently, it would be desirable to provide a process for embossing microstructures on a substrate which reduces or slows down the rate of resin accumulation in an embossing tool.

It would also be desirable to provide a device and / or a method for embossing microstructures on a substrate which improves / improves or overcomes / overcomes one or more disadvantage (s) or inconvenience (s) of known methods of embossing microstructures .

Summary of the Invention One aspect of the invention relates to a method of embossing microstructures on a substrate comprising: (a) applying a layer of curable resin to at least part of the substrate, where the layer curable resin has a first edge and a second edge extending beyond an upper edge and a lower edge of the substrate; (b) contacting an embossing tool with the curable resin layer using a rotary embossing roller to form the microstructures in the curable resin layer, where the first edge and the second edge of the. layer of curable resin are basically aligned with the direction of roller rotation; and (c) curing the curable resin layer, where the curable resin layer is applied with varying thickness.

The method can reduce the rate of resin accumulation in an embossing tool, as the curable resin is essentially oriented in the direction of machine movement of the rotary embossing roller.

In one or more embodiment (s), the curable resin comprises a radiation curable resin which is curable by exposure to radiation. For example, the curable resin may include a UV curable resin which is curable by exposure to UV radiation.

The Applicant believes that during the release of the cured resin part from the embossing tool, the adhesion of the perimeter of the part is overcome in the first place and, in doing so, a small amount of residue is left in the embossing tool. A lower force is required to overcome the adhesion of the internal (non-peripheral) parts of the hardened resin part, and therefore there is less build up of residue in these areas of the tool.

The Applicant has observed that the greater the angle between the peripheral tangent of the piece of resin applied and the direction of the machine (the maximum possible angle is 90 degrees, that is to say perpendicular to the machine direction), the higher the rate of increase (over time) in the edge roughness of the embossed hardened resin part, i.e. accumulation occurs more quickly in these peripheral areas , the embossing tool gradually becomes clogged with the build-up material, thereby compromising the geometric integrity of the cured resin part.

The present invention reduces the rate of resin accumulation in the embossing tool by ensuring that the perimeter of the curable resin is essentially aligned with the direction of rotation of the roller. This minimizes the angle between the peripheral tangent of the piece of UV resin applied and the machine direction.

The present invention consequently reduces the appearance of accumulation defects in the embossed hardened resin and / or increases the number of embossing cycles which can be carried out before defects occur and before the tool embossing does require cleaning.

This can lead to savings in production costs and increases in yield due to the reduction in downtime spent in production.

clean the tools of embossing, from the reduction of amount of product wasted at CâU S θ faults of accumulation, and of the reduction of costs tooling resulting from reduction consumption tools - often a wedge or a tool embossing must be replaced when he / she she can not not to be cleaned (e)

properly.

In one or more embodiment (s), the layer of curable resin is applied with a width between the first edge and the second edge greater than a width of an embossing surface profile in the embossing tool corresponding to microstructures. In this case, the first edge and the second edge are not on the surface profile corresponding to the microstructures. This arrangement can further reduce build-up on the embossing tool, as the adhesive force between the embossing tool and the perimeter of the cured resin is less than that in the case where the perimeter is on the profile. surface corresponding to the microstructures (due to the reduced contact area between the tool and the hardened resin in the peripheral area).

In one or more embodiment (s), the substrate is part of a strip. For example, the strip can be a UV-transparent polymer strip. In one or more embodiment (s), the curable resin can be applied in the form of a continuous scratch. The application of the curable resin in the form of a continuous stripe concretely means that for substrates in the strip, the perimeter of the curable resin lies only in the direction of rotation of the roll.

In one or more embodiment (s), the embossing tool has a shim mounted on the embossing roller, and the layer of curable resin has an upper edge located between the upper edge of the substrate and a first line junction between the shim and the embossing roller, and a lower edge located between the lower edge of the substrate and a second junction line between the shim and the embossing roller. A wedge is typically a thin metal plate, (about 100 to 200 microns thick) containing microstructures. The shim can be attached to an embossing roller, for example, with double-sided tape. There may be a small gap between where the top and. the lower part of the shim meets the roller (the first and second shim junction lines). This space can be covered with tape to complete the wedge mounting process. The tape (and associated space) would introduce a significant build-up if the curable resin was applied as a continuous scratch. The arrangement of an upper edge and a lower edge of the curable resin located between the substrate and the shim junction lines avoids contact between the shim junction lines and the applied curable resin, thereby reducing accumulation in the embossing tool in embodiments where a shim is used.

In one or more embodiment (s), the layer of curable resin is applied so that it extends beyond an entire perimeter of the substrate. For example, if a continuous stripe is applied to a strip, the width of the stripe between the first edge and the second edge may be greater than the width of the substrate. In embodiments where a shim is mounted on the embossing roller, the curable resin can be applied so as to obtain a minimum distance between the perimeter of the substrate and the perimeter of the curable resin, and a minimum distance between an upper edge and a bottom edge of the curable resin and the joining lines between the shim and the embossing roller.

In one or more embodiment (s), the layer of curable resin

Θ3Τ, the layer be useful thicker curable structures a substrate edge.

in of in the case of the embossing stripes of

The thickness of the top edge so that it is made secure

Alternatively, and made thinner where it (allowing tolerances' layer thickness along a first edge and the thickness of the curable resin layer may vary in two dimensions and / or over the entire layer .

The thickness of the curable resin layer can vary by varying the geometry of cells (width, length and depth of cells, for example) in an etching cylinder used to apply the curable resin layer. Thicker coats can be applied using etching cells which have wider widths and / or lengths and / or depths. Thinner layers can be applied using etching cells which have smaller widths and / or lengths and / or depths.

This thickness gradient can be used to regulate the overall thickness of the finished product and can provide the substrate, for example for essentially uniform thickness.

final

The thickness of the In one or more embodiment (s), the layer of curable resin can be applied with a greater thickness in an area of the substrate on which the microstructures must be positioned. For example, the thickness of the curable resin in areas of the substrate on which the microstructures are to be positioned can exceed a minimum threshold so that an embossing surface profile in the tool, of embossing corresponding to the microstructures is 10 completely filled with curable resin. The curable resin layer can be applied with a reduced thickness in other areas of the substrate. This can reduce the material costs associated with depositing curable resin with a first edge and a second edge extending beyond an upper edge and a lower edge of the substrate, while reducing resin build-up. in the embossing tool and providing sufficient curable resin for embossing the microstructures.

In one or more embodiment (s), the thickness of the layer of curable resin varies from a minimum thickness greater than at the upper edge of the substrate and by a minimum thickness less than at the lower edge of the substrate at a maximum thickness at the area of the substrate on which the microstructures are to be positioned.

In one or more embodiment (s), the thickness of the layer of curable resin varies from a first minimum thickness at the first edge of the layer of curable resin and from a second minimum thickness 30 level of the second edge of the layer of curable resin to a maximum thickness at the level of the area of the substrate on which the microstructures are to be positioned.

In one or more embodiment (s), the thickness of the layer of curable resin varies from a minimum thickness at the first and second edges and the upper and lower edges of the substrate to a maximum thickness at level of the substrate area on which the microstructures are to be positioned.

Such thickness gradient arrangements can promote the adhesion of layers applied to the substrate after embossing as a flatter surface can be presented to subsequent layers. Variations in thickness can also minimize any transparency (visibility) of the perimeter of the curable resin, which may be undesirable. In one or more embodiment (s), the method further comprises applying an opacifying layer to at least part of the substrate with embossed microstructures, the opacifying layer comprising a window zone which essentially coincides with the microstructures . The window zone can be a window or a half-window where the embossed microstructures are positioned in the window or the half-window, possibly with a minimum distance between the embossed microstructures and the perimeter of the window or of the half-window.

In one or more embodiment (s), the microstructures are microlenses and the substrate is transparent. In other embodiments, microstructures can be diffractive optical elements, embossed micro-optical security devices.

Another aspect of 'structures a device having curable microstructures in curable applied to at least the resin layer, curable second edge extending beyond the diffraction gratings, micro-imaging for lenses or the like

1'invention substrate a layer of and resin substrate, a part of the first edge and a lower edge one of where and where are formed in the layer of curable resin by contacting an embossing tool with the layer of curable resin using a rotary embossing roller, where the first edge and the second edge of the curable resin layer are essentially aligned with the direction of rotation of the roller, and. by hardening the layer of curable resin.

In one or more embodiment (s), the curable resin comprises a radiation curable resin which is curable by exposure to radiation. In one or more embodiment (s), the curable resin comprises a UV curable resin which is curable by exposure to UV radiation.

The device of the present invention may incorporate one or more of the features described above in relation to the method. For example, in one or more embodiment (s), the layer of curable resin may have a variable thickness.

The device may include an opacifying layer applied to at least a portion of the substrate with embossed microstructures, the opacifying layer comprising a window zone which essentially coincides with the microstructures.

In one or more embodiment (s), the microstructures are microlenses and the substrate is transparent. The device may further include a layer containing microimaging elements applied to a second side of the substrate, where the microlenses sample and enlarge the microimaging elements. The device can thus be used as a security device or article, for example to allow visual authentication of a bank note.

Another aspect of the invention relates to a security document comprising a device according to any one of the embodiments described above.

Security Token or Document As used herein, the term security tokens and documents includes all types of tokens and 5 valuable documents and identification documents, including, but not limited to, the following: coins such as banknotes and coins, credit cards, checks, passports, ID cards, 10 securities and stock certificates, permits driving, title deeds, travel documents such as plane and train tickets, cards and tickets, birth, death and marriage certificates and transcripts.

The invention can particularly be applied, but not exclusively, to security devices, for authenticating articles, documents or tokens, such as banknotes or identification documents, such as cards identity cards or passports, formed from a substrate to which one or more printing layers are applied.

More generally, the invention can be applied to a micro-optical device which, in various embodiments, is suitable for visual improvement of clothing, skin products, documents, printed matter, manufactured goods, merchandising systems, packaging, point of sale displays, publications, advertising devices, sporting goods, tokens and security documents, 30 financial documents and transaction cards and other products.

Security Article or Device As used herein, the term security article or device includes any one of a large number of security devices, elements or articles intended to protect the token or the security document against counterfeiting, copying, modification or

f a 1 s i f i c a t i ο n Safety items or devices can be supplied in or on the substrate of the document 5 security or in or on one or more layer (s) applied (s) to the base substrate, and can take a

wide variety of forms, such as security threads embedded in layers of the security document; security inks such as fluorescent, luminescent or phosphorescent inks, metallic inks, iridescent inks, photochromatic, thermochromatic, hydrochromic or piezochromic inks; printed or embossed articles, including release structures; interference layers; liquid crystal devices; lenses and lenticular structures; optically variable devices (OVD) such as diffractive devices comprising diffraction gratings, holograms and diffractive optical elements (DOE).

Substrate As used herein, the term substrate refers to the base material from which the token or security document is formed. The base material can be paper or other fibrous materials such as cellulose; plastic or polymeric material including, but not limited to, polypropylene (PP), polyethylene (PE), polycarbonate (PC), polyvinyl chloride (PVC), polyethylene terephthalate (PET), biaxially oriented polypropylene (BOPP); or a composite of two or more materials, such as a laminate of paper and at least one plastic, or of two or more polymeric materials.

Half Windows and Transparent Windows

[0052] Phone that u used here, the window term made reference to a zoned transparent or translucent in the document of SECI jrité compared at the opaque region at

which printing is applied. The window can be completely transparent so as to allow the. essentially unchanged light transmission, or it may be partially transparent or translucent, partially allowing light transmission but without allowing objects to be seen clearly through the window area.

A window area can be formed in a polymer security document which has at least one layer of transparent polymer material and one or more opacifying layer (s) applied to at least one side of a transparent polymer substrate, omitting at least one opacifying layer in the region forming the window area. If opaque layers are applied to both sides of a transparent substrate, a fully transparent window can be formed by omitting the opaque layers on both sides of the transparent substrate in the window area.

A partially transparent or translucent area, hereinafter called "half window", can be formed in a polymeric security document which has opacifying layers on both sides by omitting the opacifying layers on one side of the document. security in the window area so that the "half window" is not completely transparent, but lets sunlight through without allowing objects to be clearly seen through the half window.

As a variant, it is possible that the substrates are formed from an essentially opaque material, such as paper or a fibrous material, with a transparent plastic insert inserted in a cutout or in a recess in the paper. or the fibrous substrate to form a transparent window or a translucent half-window area.

Opacifying layers One or more opacifying layer (s) can / can be applied to a transparent substrate to increase the opacity of the security document. An opacifying layer is such that L _T <L _o , where L _o is the amount of light incident on the document, and L _T is the amount of light transmitted through the document. An opacifying layer may include any one or more of a variety of opacifying coatings. For example, opacifying coatings may include a pigment, such as titanium dioxide, dispersed in a binder or carrier of crosslinkable heat activated polymeric material. Alternatively, a transparent plastic substrate could be nipped between opacifying layers of paper or other partially or essentially opaque material on which indicia can subsequently be printed or otherwise applied.

Curable Resin As used herein, the term curable resin 25 is intended to include, by way of non-limiting example, a resin, ink, coating, or lacquer made of monomers and a photo-initiator dispersed in a liquid. Exposure to radiation, such as ultraviolet radiation, will cause such monomers to undergo radical chain growth polymerization, transforming the liquid into a solid. Other polymerization mechanisms can also be used.

Brief Peseri _p t i ο n Drawings [0058] Embodiments of the invention will now be described with reference to the accompanying drawings. he

J.O should be understood that the embodiments are given only by way of illustration and that the invention is not limited by this illustration. In the drawings:

Figure 1 is a block diagram of an embodiment of an apparatus for the online production of part of a security document;

Figure 2 is a plan view of a roller and an embossing tool and a substrate with a piece of curable resin applied according to existing methods;

Figure 3 is a flowchart showing a method of embossing microstructures on a substrate according to the invention;

Figure 4 is a plan view of a roll and an embossing tool and a substrate with a layer of curable resin applied according to a first embodiment;

Figure 5 is a plan view of a roller and an embossing tool and a substrate with a layer of curable resin applied according to a second embodiment;

Figure 6 is a plan view of a roller and an embossing tool and a substrate with a layer of curable resin applied according to a third embodiment;

Figure 7 is a plan view of the substrate of Figure 6 after embossing, an opacifying layer and the opacifying layer printed on the embossed substrate.

Figure 8 is a plan view of a hardened resin layer after embossing according to a fourth embodiment, and side views of the hardened resin layer showing discrete thickness variations in the resin.

Figure 9 is a plan view of an embossed substrate according to a fifth embodiment using an embossing shim; and Figure 10 is an exploded isometric view of a device according to a sixth embodiment.

Detailed Description [0069] Figure 1 shows an exemplary device 10 for the online production of part of a security document. A continuous strip 14 of translucent or transparent material such as polypropylene or PET is subjected to a process promoting adhesion at a first treatment station 16 comprising a set of rollers. Appropriate adhesion promoting methods include flame treatment, corona discharge treatment, plasma treatment or the like.

A layer promoting adhesion is applied at a second treatment station 20 comprising a set of rollers. A suitable adhesion promoting layer is a layer specifically adapted to promote adhesion of curable coatings (UV curable resin in this example) to polymeric surfaces. The adhesion promoting layer may have a UV curing layer, a layer based on

solvent, one coat to based water or any combination of them. At the level a third post treatment 22 which has equal .ement a set of rolls, one curable resin (resin resin e curable UV) is

applied to the surface of the adhesion promoting layer. The UV curable resin can be applied by a flexography, gravure or screen printing process and variants thereof among other printing processes.

While the UV-curable resin is still, at least partially, liquid, it is treated to form microstructures at a fourth treatment station 30. The treatment station 30 comprises such that a structure curable at for embossing microstructures, of element in the corresponding cylindrical embossing The device elements of

It forms microstructures with diamond orations and can form microimaging microlenses in a variety of forms.

The embossing roller 32 may have surface relief formed by a suitable transverse style, or by laser or chemical etching or in one provided on a surface shim can be provided by the embossing 37 provided on the embossing 32. The embossing shim (s) may be secured with adhesive tape, magnetic tape, clips, or other suitable mounting techniques.

In the context of this specification, the expression "embossing tool" is intended to include both the surface relief formations formed on the embossing surface 34 of the embossing roller 32 and an embossing block 37 which can be affixed to the embossing roller 32.

The UV curable ink or resin on the substrate is brought into contact with the cylindrical embossing surface 34 by an embossing tool roll 38 at the processing station 30 so that the curable resin Liquid ÜV flows into the surface relief formations of the cylindrical embossing surface 34 30 or the embossing shim 37. At this point, the ÜV curable resin is exposed to UV radiation, for example, by transmission through the bandaged.

With the microstructures now applied to the strip, one or more additional layer (s) is / are applied at certain downstream processing stations 40 and 42. The additional layers can be transparent coatings or pigmented and applied as a partial coating, as an adjoining coating or a compromise between the two. In a preferred method, the additional layers are opacifying layers which are applied to one and / or the other of the two surfaces of the strip except in the region of the microstructures.

[0077]

An existing method of embossing micro structures on a substrate which has caused an accumulation of UV resin on the embossing tool will be described with reference to Figure 2. Figure 2 shows an exemplary design of an embossing roll 42 with surface relief formations 44 corresponding to the shape of the microstructures to be formed. The surface relief formations 44 occupy a square shaped area on the embossing tool 46. The embossing tool 46 could be either the roller 42 with microstructures in recess or raised, or a shim mounted on the roller 42 with recessed or raised microstructures.

In this example, the rotogravaged UV resin 4 8 printed on the substrate 49 is in the form of a square piece, which is larger than the surface relief area of square-shaped microstructures 44 on the tool Embossing 46. During embossing, the UV resin patch 48 is brought into contact with and in alignment with the surface relief formations 44, and has sufficient coating weight, so that the relief structures of surface are at least completely filled, i.e. the UV resin patch covers the entire microstructure area and also extends a little beyond the microstructures, for example 1 mm beyond the microstructures when they are positioned correctly in perfect. alignment (in practice, the alignment tolerance will determine the final position of the part of microstructures in relation to the part of UV resin). This configuration has been shown to produce an undesirable build-up, particularly in parts of the embossing tool 46 corresponding to the perimeter of the piece of UV resin 48, in particular along the parts of the perimeter which are perpendicular to the machine direction. (roller rotation direction).

In another configuration similar to Figure 2, the Applicant has observed that the accumulation is produced at an even higher speed if the size of the piece of UV resin of square shape on the substrate is less than the area of Surface relief formations of square microstructures on the embossing roller. The Applicant believes that this is due to the fact that the adhesive force between the embossing tool and the perimeter of the UV-cured resin is now greater because the contact area between the tool and the UV resin, in the peripheral zone has increased, since the perimeter is now located on the microstructures. Since more force is required to release the structures, more residue will remain after release, especially along the parts of the perimeter that are perpendicular to the machine direction.

A method 50 for embossing microstructures on a substrate according to the present invention will now be described with reference to FIG. 3. The method 50 comprises the application 52 of a layer of curable resin to at least part of the substrate, wherein the curable resin layer has a first edge and a second edge extending beyond an upper edge and a lower edge of the substrate; bringing an embossing tool 54 into contact with the curable resin layer using a rotary embossing roller to form the microstructures in the curable resin layer, where the first edge and the second edge of the curable resin layer are essentially aligned with the direction of roller rotation; and curing 56 of the curable resin layer. Various embodiments of this method 50 which have been found by the Applicant to reduce the accumulation of resin in the embossing tool will now be described.

Figure 4 shows a first embodiment using a roller 62 and an embossing tool 60 and a transparent polymer substrate 64 which is part of a strip. In this embodiment, a nickel roll 62 is used, with an embossing surface profile 65, corresponding to the microstructures to be embossed, etched in the form of gravure printing.

a

Resin in a curable roll at the th edge 7 0 from an edge of an upper edge 72 th substrate (via the continuous stripe). The inlay of the UV curable resin 66 is then brought into contact and alignment with the embossing tool 60 and is then embossed and cured by UV using the "soft embossing" process described above. In this example, the microstructures etched 65 on the embossing tool 60 are lenses which extend continuously around the circumference of the embossing roller 62, and extend beyond the width 25 (in the transverse direction ) of the applied UV resin stripe 66. In this case, the perimeter of the applied UV resin stripe (first edge 68 and second edge 70) extends only in the machine direction 75 (since the UV resin stripe is continuous, it has neither beginning nor end). We have found that this arrangement gives a relatively less accumulation compared to the arrangement shown in Figure 2.

Figure 5 shows a second embodiment using the roller 62 and the embossing tool 35 60 and. a transparent polymer substrate 7 6 which is part of a strip. The second embodiment of Figure 5 is similar to the first embodiment of Figure 4, except that in this case the UV curable resin 78 is applied with a width between the first edge 80 and the second edge 82 greater than a width of an embossing surface profile 65 in the embossing tool 60 corresponding to the microstructures. The curable resin

UV 78 s extends so beyond the width of the lentils engraved (da ns the transverse direction), through example, extends of 1 mm beyond the width of direction

cross section of the etched lens microstructures 65. This configuration gave even less accumulation than the configuration in Figure 4, since the perimeter of the UV-cured resin stripe (first edge 80 and second edge 82) is not found not on microstructures 65, therefore less force is required to release it, leading to cleaner embossing with less accumulation of embossing residue on the perimeter.

The Applicant has thus found that the speed of accumulation can be reduced or minimized by ensuring that the perimeter of the UV photograved resin (more specifically, the part of the perimeter which is in or on the final product) is always oriented in the machine direction / material movement direction 25 75, 83 in the rotary UV embossing process (the closer the perimeter is to the machine direction, the lower the accumulation speed).

Figure 6 shows a third embodiment using an alternative roller 86 and an embossing tool 30 84. In this example, the embossing surface profile 88, corresponding to the microstructures to be embossed, occupies an area of square shape. The UV curable resin 90 is applied as a continuous stripe to the substrate 92, having a width between the first edge 94 and the second edge 96 (i.e. in the transverse direction) beyond the width of the microstructures 88, for example extending by 1 mm beyond the two sides of the microstructures 88. In this configuration, as in FIGS. 4 and 5, the perimeter of the UV-curable resin (ie that is, the first edge 94 and the second edge 96) is aligned with the direction of rotation of the roller 86 (direction of substrate transport 98). There are no perimeter portions perpendicular to the direction of rotation of the roller 86, since the curable resin

UV is a step and therefore the embossing is clean with one of accumulation.

The configuration appropriately used in the Figure to generate nieces of embossed microstructures at

UV in security documents such as banknotes as follows. As shown in Figure 7, microstructures 100 are embossed in UV curable resin 90 on a transparent polymeric substrate 92 as described with reference to Figure 6. The microstructures 100 occupy a square area on the transparent substrate 92. The parts of the UV resin 90 which does not contain microstructures 100 can be overprinted with one or more opacifying layer (s) 102. The opacifying layer 102 contains a square area without ink, defining a window area 104 which must be aligned with the embossed microstructures 100. The end product is a banknote 106 with a piece of UV embossed resin 100 in a window or a half window. As shown in the lower diagram in Figure 7, the opacifying layer 102 has been applied over the polymeric substrate 92, and also over the portions of the UV embossed resin, to form a window or half window in the bill bank (optionally, the opacifying layer is not applied over the embossed microstructures 100, rather a minimum distance between the two is maintained).

In another embodiment, with reference to Figures 6 and 7, the UV curable resin layer 90 on the substrate 92 can be applied with a variable thickness. This can reduce production costs by decreasing the required UV ink and can reduce the impact of the UV 90 curable resin layer on the mechanical properties of the final product.

Optionally, vary across

90.

may be step variations).

of the substrate on which the microstructures 100 must be positioned. The thickness of resin in areas to be embossed with microstructures preferably exceeds a minimum threshold, to ensure that the microstructures 88 on the embossing tool 84 are completely filled. The thickness of the UV-curable resin layer 90 may vary from a minimum thickness greater than at the upper edge 95 of the substrate 92 and from a minimum thickness less than at the lower edge 97 of the substrate 92 to a maximum thickness at level of the zone of the substrate 92 on which the microstructures 100 are to be positioned (optionally allowing alignment tolerance). As a variant or in addition, the thickness of the layer of UV curable resin 90 may vary from a first minimum thickness at the first edge 94 of the layer of UV curable resin 90 and by a second minimum thickness at from the second edge 96 of the UV-curable resin layer 90 to a maximum thickness at the region of the substrate 92 on which the microstructures 100 are to be positioned (optionally allowing alignment tolerance).

In the embodiment shown in Figure 7, a thickness gradient can be introduced along the upper edge 95 and the lower edge 97, that is to say a variation of the thickness of UV resin on a distance perpendicular to the peripheral tangent line, which varies from zero thickness at the first edge 94 to a non-zero thickness, then returns to zero thickness at the second edge 96, as a continuous function or step. This can facilitate the adhesion of the opacifying layer 102 to the UV-cured resin, since it has a flatter surface to the opacifying layer 102. It can also minimize any transparency (visibility) of the perimeter that may be undesirable.

Figure 8 shows a fourth embodiment, in which a continuous streak of UV cured resin 101 is. embossed with microstructures in two pieces - a circular piece 103 and a square piece 105. In this embodiment, the thickness of the UV-cured resin layer 101 varies in discrete steps.

The thickness of the UV-cured resin 101, as shown in the thickness profile 107 along the line AA, varies from a zero thickness 109 on both sides of the resin 101 to a minimum thickness 111 from the first edge / second edge 113/115 of the resin 101 at a point 117/119 at a distance from the embossed microstructures 105. The thickness of the UV-cured resin 101 has a maximum thickness 121 at an area of the substrate on which the embossed microstructures 105 are positioned and a little beyond the microstructures (that is to say at points 117 and 119, which allow alignment tolerances). In the other dimension, the thickness of the UV-cured resin 101, as shown in the thickness profile 123 along the line BB, has a maximum thickness 125 at the areas of the substrate on which the embossed microstructures 103, 105 are positioned, extending at a distance beyond the microstructures to allow alignment tolerances (as indicated by points 127, 129, 131, 133) and a minimum thickness 135 elsewhere.

This thickness gradient can be applied by varying the geometry of cells in an etching cylinder used to apply the. layer of curable resin before embossing. For example, in the areas to be embossed by microstructures (circular piece 103 and 10 square piece 105) and at a distance beyond these areas to allow alignment tolerances, a maximum thickness 121, 125 of the resin can be applied using etching cells which have wider widths and / or lengths and / or depths.

111 "i · s be areas of etching and / or lengths providing a layer of which have narrower widths and / or depths so UV curable resin having second edge 115 extending beyond an upper edge 137 and a lower edge 139 of the substrate.

The negative effects of the accumulation of UV resin in an embossing tool can be further reduced if the perimeter of the piece of UV cured resin is located outside the product area, for example outside the bank note area. Figure 9 shows a fifth embodiment in which a layer of UV-curable resin 110 is applied so that it extends beyond an entire perimeter of the substrate is

Figure 9 of the

112. The layout

embossing features a wedge climb embossing. In the confi configuration the thickness of the hard resin • cissable

areas outside of the microstructures of useful if

sure the roller of of the figure 9 to the UV 110 in of the 114 to emboss a summer

scaled down. This has the advantage of saving on UV resin and also reduces the thickness of the product in areas which are not embossed with microstructures, thus minimizing the properties of the product in these areas.

In addition, in Figure 9, the part of the perimeter of the UV-cured resin which is oriented in the machine direction 116 (i.e. the first edge 118 and the second edge 120) is located outside the finished product area. This ensures that any accumulation associated with the first edge 118 and the second edge 120 will not have an impact on the finished product (unless, of course, an extremely long period of operation results in an encroachment of accumulation on the product area - at this point the process will have to be stopped and the embossing tools will need to be cleaned, i.e. there will be a production downtime).

In addition, in FIG. 9, the UV resin is not a continuous scratch, rather the layer of UV curable resin 110 has an upper edge 122 situated between an upper edge 124 of the substrate 112 and a first line of junction between the shim and the embossing roller, and a lower edge 126 situated between the lower edge 128 of the substrate 112 and a second junction line between the shim and the embossing roller. That is, there is a part of the perimeter of the UV-cured resin which is oriented in the transverse direction (perpendicular to the machine direction 116) and which is outside the finished product area . This arrangement avoids contact between the shim junction lines and the applied UV curable resin 110.

The configuration of Figure 9 ensures that any accumulation associated with the part of the perimeter extending in the transverse direction (i.e. the upper edge 122 and the lower edge 126 of the UV-curable resin 110 ) will have no impact on the finished product (again, unless, of course, an extremely long period of operation results in an encroachment of accumulation on the product area - at this stage the process should be stopped and the embossing tools will have to be cleaned, i.e. there will be a production downtime).

In Figure 9, the rate of accumulation along the perimeter part oriented in the transverse direction (upper edge 122 and lower edge 126) will be higher than that along the perimeter part oriented in the direction machine edge (first edge 118 and second edge 120), therefore, the accumulation associated with the perimeter part oriented in the transverse direction (upper edge 122 and lower edge 126) will first encroach on the product area.

Figure 10 shows a sixth embodiment consisting of a transparent polymer substrate 130; a UV cured resin 132 making a waffle microlens piece with a thin solid UV border; an opacifying layer 134 printed over the UV resin 132 to form a window around the lenses; another opacifying layer 136 on the reverse side which forms another window around the lenses and which also prints an opacifying lenticular focal plane microimaging

138 a colored non qur overprinted second color be e mistletoe can

The end result is a lenticular image in the form of a coin, which can be fabricated with minimal accumulation problems and used as a security device in a security document such as a banknote.

Although the invention has been described in conjunction with a limited number of embodiments, those skilled in the art will understand that several variants, modifications and variations in light of the preceding description are possible. Consequently, the present invention aims to encompass all these variants, modifications and variations which may fall within the spirit and the scope of the invention as disclosed.

Any reference here to a patent document or to another matter which is given as prior art should not be considered as an acknowledgment that this document or this matter was known or that the The information it contained was part of general knowledge current at the priority date of one of the claims.

Claims (20)

1. A method of embossing microstructures on a substrate comprising: (a) applying a layer of hardenable resin to At least a portion of the substrate, wherein the layer of curable resin has a first edge and a second edge extending beyond an upper edge and a lower edge of the substrate; (b) contacting an embossing tool with the curable resin layer using a rotating embossing roller to form the microstructures in the curable resin layer, where the first edge and the second edge of the layer of curable resin are essentially aligned with the direction of rotation of the roller; and (c) curing the curable resin layer where the curable resin layer is applied with varying thickness. 2. Method according to claim 1, in which the. Curable resin comprises a radiation curable resin which is curable by exposure to radiation. 3. The method of claim 1 or 2, wherein the curable resin comprises a curable resin 25 UV which is curable by exposure to UV radiation. 4. Method according to any one of the preceding claims, in which the layer of curable resin is applied with a width between the first edge and the second edge greater than a width of a profile of 30 embossing surface in the embossing tool corresponding to the microstructures. 5. Method according to any one of the preceding claims, in which the substrate is part of a strip. 6. Method according to any one of the preceding claims, in which the curable resin is applied in the form of a continuous scratch. 7. Method according to any one of claims 1 5 to 5, in which the embossing tool comprises a shim mounted on the embossing roller, where the layer of curable resin has an upper edge situated between the upper edge of the substrate and a first junction line between the shim and the roller embossing, and a lower edge located between the lower edge of the substrate and a second junction line between the shim and the embossing roller. 8. Method according to any one of the preceding claims, in which the layer of curable resin 15 is applied so that it extends beyond an entire perimeter of the substrate. 9. The method of claim 10, wherein the thickness of the curable resin layer varies along an upper edge and a lower edge of the 20 substrate. 10. Method according to any one of the preceding claims, in which the layer of curable resin is applied with greater thickness in an area of the substrate on which the microstructures are to be 25 positioned. 11. The method of claim 10, wherein the thickness of the layer of curable resin varies from a first minimum thickness at the first edge of the curable resin and from a second minimum thickness at 30 level of the second edge of the curable resin to a maximum thickness at the area of the substrate on which the microstructures are to be positioned. 12. Method according to any one of the preceding claims, in which the thickness of the resin layer 35 curable varies continuously. 13. Method according to any one of claims 1 to 11, in which the thickness of the layer of curable resin varies in discrete steps. 14. Method according to any one of the claims 5 above, further comprising the application of a; opacifying layer on at least part of the substrate with embossed microstructures, the opacifying layer having a window area which essentially coincides with the microstructures. 15. Method according to any one of the preceding claims, in which the microstructures are microlenses and the substrate is transparent. 16. Device comprising: a substrate; and Microstructures embossed in a layer of curable resin applied to at least a portion of the substrate, wherein the layer of curable resin has a first edge and a second edge extending beyond an upper edge and a lower edge substrate; where the microstructures 20 are formed in the layer of curable resin by bringing an embossing tool into contact with the. layer of curable resin using a rotary embossing roller, where the first edge and the second edge of the layer of curable resin are essentially aligned with the direction of rotation of the roll, and curing the layer of curable resin. 17. The device of claim 16, wherein the curable resin comprises a radiation curable resin which is curable by exposure to a 30 radiation. 18. The device of claim 16 or 17, wherein the curable resin comprises a UV curable resin which is curable by exposure to UV radiation. 19. Device according to 1 'a any of the claims 16 to 18, in which, the layer of resin curable to var thickness iable. 20. Device according to 1 'a any of the Claims 16 to 19, further comprising an opacifying layer applied to at least a portion of the substrate with embossed microstructures, the opacifying layer comprising a window region which essentially coincides with the microstructures. 21. Device according to any one of claims 16 to 20, in which the microstructures are microlenses and the substrate is transparent. 22. Device according to claim 21, further comprising a layer containing microimaging elements applied to a second side of the substrate, where the microlenses sample and enlarge the microimaging elements. 23. Security document comprising a device according to any one of claims 16 to 22.