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1. (WO2019046900) METHOD OF EMBOSSING MICRO-STRUCTURES ON A SUBSTRATE
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METHOD OF EMBOSSING MICRO-STRUCTURES ON A SUBSTRATE

Technical Field

[0010] The present invention relates generally to methods for embossing micro-structures on a substrate that reduce build-up of resin in an embossing tool, and devices produced using these methods. The invention is suitable for use in the manufacture of micro-optic devices used as security devices for bank notes and like security documents. It will be convenient to describe the invention in relation to that exemplary, but non-limiting application.

Background of Invention

[001 1] It is well known that many of the world's bank notes as well as other security documents include security devices which produce optical effects enabling a visual authentication of the bank note. Some of the security devices include focusing elements, such as micro-lenses, which act to sample and magnify micro-imagery elements and project imagery which is observable by a user for authentication purposes.

[0012] In some cases, it is known to form the micro-lenses on a substrate by embossing. One example is the "soft emboss" process used by the Applicant. This "soft emboss" process is a roll-to-roll process that involves embossing micro-lenses or other micro-structures on a substrate formed from gravure printing a layer of UV-curable resin directly onto a UV-transparent polymer web. The applied resin is UV-embossed by firstly placing the moving web in contact with a rotating embossing roller. The resin is sandwiched between the embossing tool or shim forming part of or affixed to the roller and the web, so that under impression pressure, the resin fills the recessed structures in the embossing tool. While the resin is still in contact with the embossing tool and under impression pressure, the resin is cured by UV radiation directed through the transparent polymer web.

[0013] A problem that occurs in this "soft emboss" process is UV resin build-up in the embossing tool after a period of time, which leads to an alteration in the embossing geometry of the tool resulting in defects in the embossed micro-structures in the UV cured resin. Such embossing defects may appear as high edge roughness on the perimeter of the UV embossed patch.

[0014] The Applicant believes that such defects occur (at least partly) because, during each UV embossing cycle, as each UV embossed patch is released from the embossing tool, a small amount of UV resin and/or primer material is left on the tool. The embossed geometry thus progressively becomes compromised, especially in regions corresponding to the perimeter of the UV embossed resin patch, as more and more build-up material is deposited on the embossing tool with each embossing cycle.

[0015] The build-up problem is further exacerbated due to the registration tolerance of the "soft emboss" process. The position of the printed layer of UV curable resin relative to the embossing structures on the embossing tool changes or varies in each embossing cycle due to the registration tolerance of the process. The Applicant's "soft emboss" process has a unit-to-unit registration tolerance of around +/- 0.25 mm, and so the perimeter of the gravure printed patch of UV-curable resin will be embossed with a slightly different portion of the embossing tool during each repetition. This has the effect of distributing the embossing residues over a larger area, thus increasing build-up overall.

[0016] The build-up defects give the appearance of poor quality in the finished bank note, because the roughness that develops can create a very irregular edge profile in the UV resin patch that is easily visible to the naked eye.

[0017] Over time, build-up defects can become so large that they can encroach well into internal portions of the gravure printed UV resin patch. If the structures embossed are designed to have an optical function, for example if they are diffractive optical elements, diffraction gratings, lenses or any other micro-optic, their optical performance can be severely degraded due to build-up.

In previous attempts to avoid the build-up problem, the gravure printed UV resin patch was designed so that it extended beyond the micro-structures on the embossing tool. The perimeter portions of the gravure printed UV resin patch thus did not fill any structures on the embossing tool and were essentially embossed with a smooth flat surface on the embossing tool. Whilst this delayed the occurrence of problematic build-up, the build-up would still return after a longer period of embossing.

[0018] Accordingly, it would be desirable to provide a method of embossing micro-structures on a substrate that reduces or slows the rate of resin build-up in an embossing tool.

[0019] It would also be desirable to provide a device and/or method of embossing micro-structures on a substrate that ameliorates or overcomes one or more disadvantages or inconveniences of known methods of embossing micro-structures.

Summary of Invention

[0020] One aspect of the invention provides a method of embossing micro-structures on a substrate including: (a) applying a layer of curable 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 a top edge and a bottom edge of the substrate; (b) contacting an embossing tool with the layer of curable resin using a rotating embossing roller to form the micro-structures in the layer of curable resin, wherein the first edge and second edge of the layer of curable resin are substantially aligned with the direction of roller rotation; and (c) curing the layer of curable resin, wherein the layer of curable resin is applied with a varying thickness.

[0021] The method may reduce the rate of resin build-up in an embossing tool, as the curable resin is substantially oriented in the direction of machine travel of the rotary embossing roller.

[0022] In one or more embodiments, the curable resin comprises radiation curable resin which is curable by exposure to radiation. For example, the curable resin may comprise UV curable resin which is curable by exposure to UV radiation.

[0023] The Applicant believes that during release of the cured resin patch from the embossing tool, the adhesion of the perimeter of the patch is overcome first, and in doing so a small amount of residue is left in the embossing tool. A lower force is required to overcome adhesion of the internal (non-perimeter) parts of the cured resin patch, and consequently there is less build-up of residue in these areas of the tool. [0024] The Applicant has observed that the greater the angle between the perimeter tangent of the applied resin patch and the machine direction (maximum possible angle is 90 degrees i.e. perpendicular to machine direction) the greater the rate of increase (over time) of the edge roughness of the embossed cured resin patch i.e. the build-up occurs faster in these perimeter areas, the embossing tool progressively becomes fouled with build-up material, thus compromising the geometrical integrity of the cured resin patch.

[0025] The present invention reduces the rate of resin build-up in the embossing tool by ensuring that the perimeter of the curable resin is substantially aligned with the direction of roller rotation. This minimises the angle between the perimeter tangent of the applied UV resin patch and the machine direction.

[0026] The present invention accordingly reduces the occurrence of build-up defects in the embossed cured resin and/or increases the number of embossing cycles that can be performed before defects occur and the embossing tool requires cleaning.

[0027] This may lead to production cost savings and efficiency increases due to less production down time spent cleaning embossing tooling, less product spoiled due to build-up defects, and reduced tooling costs resulting from reduced tooling consumption - often a shim or embossing tool must be replaced when it cannot be adequately cleaned.

[0028] In one or more embodiments, 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 the micro-structures. In this case, the first edge and second edge do not lie on the surface profile corresponding to the micro-structures. This arrangement may further reduce build up on the embossing tool, as the adhesion force between the embossing tool and the perimeter of the cured resin is less than if the perimeter lay on the surface profile corresponding to the micro-structures (due to the reduced contact area between the tool and cured resin in the perimeter area).

[0029] In one or more embodiments, the substrate forms part of a web. For example, the web may be a UV-transparent polymer web. In one or more

embodiments, the curable resin may be applied in a continuous stripe. The application of the curable resin in a continuous stripe effectively means that for substrates in the web, the perimeter of the curable resin lies only in the direction of roller rotation.

[0030] In one or more embodiments, the embossing tool includes a shim mounted to the embossing roller, and the layer of curable resin includes a top edge located between the top edge of the substrate and a first join line between the shim and the embossing roller, and a bottom edge located between the bottom edge of the substrate and a second join line between the shim and the embossing roller. A shim is typically a thin plate of metal (around 100-200 microns thick) containing micro-structures. The shim may be attached to an embossing roller, for example, with double sided tape. There may be a small gap between where the top and bottom of the shim meet the roller (the first and second shim join lines). This gap may be covered over with tape to complete the shim mounting process. The tape (and associated gap) would introduce severe build-up if the curable resin was applied as a continuous stripe. The arrangement of a top edge and a bottom edge of the curable resin located between the substrate and the shim join lines avoids contact between the shim join lines and the applied curable resin, thus reducing build up in the embossing tool in embodiments where a shim is used.

[0031] In one or more embodiments, the layer of curable resin is applied such that it extends beyond an entire perimeter of the substrate. For example, if a continuous stripe is applied to a web, 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 to the embossing roller, the curable resin may be applied so as to provide a minimum distance between the perimeter of the substrate and the perimeter of the curable resin, and a minimum distance between a top and bottom edge of the curable resin and join lines between the shim and embossing roller.

[0032] In one or more embodiments, the layer of curable resin is applied with a varying thickness. For example, the thickness of the layer of curable resin may vary along a top edge and a bottom edge of the substrate. This may be useful in the case of embossing continuous stripes of lens structures. The thickness of the UV resin could be varied along the top and bottom edge of the substrate so that it is made thicker in the areas of the lens structures (to ensure 100% fill) and made thinner where there are no structures (allowing for registration tolerances). Alternatively, the thickness of the layer of curable resin may vary along a first edge and a second edge. Alternatively, the thickness of the layer of curable resin may vary in both dimensions and / or throughout the layer.

[0033] The thickness of the curable resin layer may be varied by varying the cell geometry (cell width, length and depth for example) in a gravure cylinder used to apply the curable resin layer. Thicker layers may be applied by using gravure cells that have larger widths and/or lengths and/or depths. Thinner layers may be applied by using gravure cells that have smaller widths and/or lengths and/or depths.

[0034] This thickness gradient may be used to control the overall thickness of the finished product, and may compensate for other layers that may be applied to the substrate, for example to provide a final product of substantially even thickness. The thickness of the layer of curable resin may vary continuously or discontinuously in discrete steps.

[0035] In one or more embodiments, the layer of curable resin may be applied with a greater thickness in an area of the substrate on which the micro-structures are to be positioned. For example, the thickness of the curable resin in areas of the substrate on which micro-structures are to be positioned may exceed a minimum threshold such that an embossing surface profile in the embossing tool corresponding to the micro-structures is completely filled with curable resin. The layer of curable resin may be applied with less thickness in other areas of the substrate. This may reduce material costs associated with laying down curable resin with a first edge and a second edge extending beyond a top edge and a bottom edge of the substrate, while still reducing resin build-up in the embossing tool and providing enough curable resin for embossing the micro-structures.

[0036] In one or more embodiments, the thickness of the layer of curable resin varies from an upper minimum thickness at the top edge of the substrate and a lower minimum thickness at the bottom edge of the substrate to a maximum thickness at the area of the substrate on which the micro-structures are to be positioned.

[0037] In one or more embodiments, 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 a second minimum thickness at the second edge of the layer of curable resin to a maximum thickness at the area of the substrate on which the micro-structures are to be positioned.

[0038] In one or more embodiments, the thickness of the layer of curable resin varies from a minimum thickness at the first and second edges and the top and bottom edges of the substrate to a maximum thickness at the area of the substrate on which the micro-structures are to be positioned.

[0039] Such thickness gradient arrangements may promote adhesion of layers applied to the substrate after embossing as a flatter surface may be presented to subsequent layers. The thickness variations may also minimise any show-through (visibility) of the perimeter of the curable resin, which may be undesirable. In one or more embodiments, the method further includes applying an opacifying layer to at least a portion of the substrate with embossed micro-structures, the opacifying layer including a window area which substantially coincides with the micro-structures. The window area may be a window or half window where the embossed micro-structures are positioned in the window or half window, optionally with a minimum distance between the embossed micro-structures and the perimeter of the window or half window.

[0040] In one or more embodiments, the micro-structures are micro-lenses and the substrate is transparent. In other embodiments, the micro-structures may be diffraction gratings, diffractive optical elements, micro-imagery for lens-based security devices or other embossed micro-optical structures.

[0041] Another aspect of the invention provides a device including: a substrate; and micro-structures 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 a top edge and a bottom edge of the substrate; wherein the micro-structures are formed in the layer of curable resin by contacting an embossing tool with the layer of curable resin using a rotating embossing roller, wherein the first edge and the second edge of the layer of curable resin are substantially aligned with the direction of roller rotation, and curing the layer of curable resin.

[0042] In one or more embodiments, the curable resin comprises radiation curable resin which is curable by exposure to radiation. In one or more embodiments, the curable resin comprises UV curable resin which is curable by exposure to UV radiation.

[0043] 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 embodiments, the layer of curable resin may have a varying thickness.

[0044] The device may include an opacifying layer applied over at least a portion of the substrate with embossed micro-structures, the opacifying layer including a window area which substantially coincides with the micro-structures.

[0045] In one or more embodiments, the micro-structures are micro-lenses and the substrate is transparent. The device may further include a layer containing micro-imagery elements applied to a second side of the substrate, wherein the micro-lenses sample and magnify the micro-imagery elements. The device may thus be used as a security device or feature, for example, to allow visual authentication of a bank note.

[0046] Another aspect of the invention provides a security document including a device according to any one of the embodiments described above.

Definitions

Security Document or Token

[0047] As used herein, the terms security documents and tokens includes all types of documents and tokens of value and identification documents including, but not limited to the following: items of currency such as bank notes and coins, credit cards, cheques, passports, identity cards, securities and share certificates, driver's licences, deeds of title, travel documents such as airline and train tickets, entrance cards and tickets, birth, death and marriage certificates, and academic transcripts.

[0048] The invention is particularly, but not exclusively, applicable to security devices, for authenticating items, documents or tokens, such as bank notes, or

identification documents, such as Identity cards or passports, formed from a substrate to which one or more layers of printing are applied.

[0049] More broadly, the invention is applicable to a micro-optic device which, in various embodiments, is suitable for visual enhancement of clothing, skin products, documents, printed matter, manufactured goods, merchandising systems, packaging, point of purchase displays, publications, advertising devices, sporting goods, security documents and tokens, financial documents and transaction cards, and other goods.

Security Device or Feature

[0050] As used herein, the term security device or feature includes any one of a large number of security devices, elements or features intending to protect security document or token from counterfeiting, copying, alteration or tampering. Security devices or features may be provided in or on the substrate of the security document or in or on one or more layers applied to the base substrate, and may 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, photochromic, thermochromic, hydrochromic, or peizochromic inks; printed or embossed features including release structures; interference layers; liquid crystal devices; lenses and lenticular structures; optically variable devices (OVDs) such as diffractive devices including diffraction gratings, holograms and diffractive optical elements (DOEs).

Substrate

[0051] As used herein, the term substrate refers to the base material from which the security documents or tokens are formed. The base material may be paper or other fibrous materials such as cellulous; a 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 material of two or more materials, such as a laminate of paper and at least one plastic material, or of two or more polymeric materials.

Transparent Windows and Half Windows

[0052] As used herein, the term window refers to a transparent or translucent area in the security document compared to the opaque region to which printing is applied.

The window maybe fully transparent so as to allow the transmission of light substantially unaffected, or it may be partly transparent or translucent, partly allowing the transmission of light but without allowing objects to be seen clearly through the window area.

[0053] A window area may be formed in a polymeric security document which has at least one layer of transparent polymeric material and one or more opacifying layers applied to at least one side of a transparent polymeric substrate, by omitting at least one opacifying layer in the region forming the window area. If opacifying layers are applied to both sides of a transparent substrate, a fully transparent window may be formed by omitting the opacifying layers on both sides of the transparent substrate in the window area.

[0054] A partly transparent or translucent area herein after referred to as a "half-window", may be formed in a polymeric security document which has opacifying layers on both sides by omitting the opacifying layers on one side only of the security document in the window area so that "half-window" is not fully transparent but allows sunlight to pass through without allowing objects to be viewed clearly through the half-window.

[0055] Alternatively, it is possible for the substrates to be formed from a substantially opaque material, such as paper or fibrous material, with an insert of transparent plastics material inserted into a cut out or recessed into the paper or fibrous substrate to form a transparent window or a translucent half-window area.

Opacifying Layers

[0056] One or more opacifying layers may be applied to a transparent substrate to increase the opacity of the security document. An opacifying layer is such that LT<U where U is the amount of light incident on the document, and LT is the amount of light transmitted through the document. An opacifying layer may comprise any one or more of a variety of opacifying coatings. For example, the opacifying coatings may comprise a pigment, such as titanium dioxide, dispersed within a binder or carrier of heat-activated cross-linkable polymeric material. Alternatively, a substrate of transparent plastic material could be sandwiched between opacifying layers of paper or other partially or substantially opaque material to which indicia may be subsequently printed or otherwise applied.

Curable Resin

[0057] As used herein, the term curable resin is intended to include, by way of non-limiting example, a resin, ink, coating or lacquer consisting of monomers and photo-initiator dispersed in a liquid. Exposure to radiation, such as ultraviolet radiation, will cause such monomers to undergo radical chain growth polymerisation, turning the liquid into a solid. Other polymerisation mechanisms may also be used.

Brief Description of Drawings

[0058] Embodiments of the invention will now be described with reference to the accompanying drawings. It is to be understood that the embodiments are given by way of illustration only and the invention is not limited by this illustration. In the drawings:

[0059] Figure 1 is a schematic diagram of one embodiment of an apparatus for inline manufacturing part of a security document;

[0060] Figure 2 is a plan view of an embossing tool and roller and a substrate with a patch of curable resin applied according to existing methods;

[0061] Figure 3 is a flow chart showing a method of embossing micro-structures on a substrate according to the invention;

[0062] Figure 4 is a plan view of an embossing tool and roller and a substrate with a layer of curable resin applied according to a first embodiment;

[0063] Figure 5 is a plan view of an embossing tool and roller and a substrate with a layer of curable resin applied according to a second embodiment;

[0064] Figure 6 is a plan view of an embossing tool and roller and a substrate with a layer of curable resin applied according to a third embodiment;

[0065] 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.

[0066] Figure 8 is a plan view of a layer of cured resin after embossing according to a fourth embodiment, and side views of the layer of cured resin showing discrete thickness variations in the resin.

[0067] Figure 9 is a plan view of an embossed substrate according to a fifth embodiment using an embossing shim; and

[0068] Figure 10 is an exploded isometric view of a device according to a sixth embodiment.

Detailed Description

[0069] Figure 1 shows an exemplary apparatus 10 for in-line manufacturing part of a security document. A continuous web 14 of translucent or transparent material such as polypropylene or PET is subject to an adhesion promoting process at a first processing station 16 including a roller assembly. Suitable adhesion promoting processes are flame treatment, corona discharge treatment, plasma treatment or similar.

[0070] An adhesion promoting layer is applied at a second processing station 20 including a roller assembly. A suitable adhesion promoting layer is one specifically adapted for the promotion of an adhesion of curable coatings (UV curable resin in this example) to polymeric surfaces. The adhesion promoting layer may have a UV-curing layer, a solvent-based layer, a water-based layer or any combination of these.

[0071] At a third processing station 22 which also includes a roller assembly, a curable resin (UV curable resin) is applied to the surface of the adhesion promoting layer. The UV curable resin can be applied via flexographic printing, gravure printing or a silk screen printing process and variations thereof amongst other printing processes.

[0072] While the UV curable resin is still, at least partially, liquid, it is processed to form micro-structures at a fourth processing station 30. The processing station 30 includes an embossing roller 32 for embossing micro-structures, such as a micro-optic element structure, into the UV curable resin. The cylindrical embossing surface 34 has surface relief formations corresponding to the shape of the micro-structures to be formed. The apparatus 10 can form micro-lenses and micro-imagery elements in a variety of shapes.

[0073] The embossing roller 32 may have the surface relief formations formed by a diamond stylus of appropriate cross section, or by direct laser engraving, or by chemical etching, or the surface relief formations may be provided by at least one embossing shim 37 provided on the embossing roller 32. The embossing shim or shims may be attached via adhesive tape, magnetic tape, clamps or other appropriate mounting techniques.

[0074] In the context of the present specification, the phrase "embossing tool" is intended to embrace both the surface relief formations formed on the embossing surface 34 of the embossing roller 32 and an embossing shim 37 that may be affixed to the embossing roller 32.

[0075] The UV curable resin or ink on the substrate is brought into contact with the cylindrical embossing surface 34 by an embossing tool roller 38 at the processing station 30 such that the liquid UV curable resin flows into the surface relief formations of the cylindrical embossing surface 34 or the embossing shim 37. At this stage, the UV curable resin is exposed to UV radiation, for example, by transmission through the web.

[0076] With the micro-structures now applied to the web, one or more additional layers are applied at some downstream processing stations 40 and 42. The additional layers may be clear or pigmented coatings and applied as partial coating, as a contiguous coating or accommodation of both. In one preferred method, the additional layers are opacifying layers which are applied to one or both surfaces of the web except in the region of the micro-structures.

[0077] An existing method of embossing micro-structures on a substrate that has caused UV resin build-up on the embossing tool will be described with reference to Figure 2. Figure 2 shows an example design of an embossing roller 42 with surface relief formations 44 corresponding to the shape of micro-structures 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 recessed or raised micro- structures, or a shim mounted to the roller 42 with recessed or raised micro-structures.

[0078] In this example the gravure printed UV resin 48 printed on the substrate 49 is in the shape of a square patch, which is larger than the square-shaped micro-structure surface relief area 44 on the embossing tool 46. During embossing, the UV resin patch 48 is brought into contact with and into register with the surface relief formations 44, and has sufficient coat weight, so that the surface relief structures are at least completely filled i.e. the UV resin patch covers the entire micro-structure area and also extends a little beyond the micro-structures, for example 1 mm beyond the micro-structures when positioned correctly in perfect register (in practice, the registration tolerance will determine the final position of the micro-structure patch relative to the UV resin patch). This configuration was shown to produce undesirable build-up, particularly in parts of the embossing tool 46 corresponding with the perimeter of the UV resin patch 48, especially along the parts of the perimeter which are perpendicular to the machine direction (direction of roller rotation).

[0079] In another similar configuration to Figure 2, the Applicant has observed that the build-up is produced at an even faster rate if the size of the square shaped UV resin patch on the substrate is smaller than the square shaped micro-structure surface relief formation area on the embossing roller. The Applicant believes that this is because the adhesion force between the embossing tool and the perimeter of the UV cured resin is now greater because the contact surface area between the tool and UV resin, in the perimeter area, has increased, since the perimeter now lies on the micro-structures. Since a greater force is required to release the structures there will be more residue left after release particularly along the parts of the perimeter which are perpendicular to the machine direction.

[0080] A method 50 of embossing micro-structures on a substrate according to the present invention will now be described with reference to Figure 3. The method 50 includes applying 52 a layer of curable 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 a top edge and a bottom edge of the substrate; contacting 54 an embossing tool with the layer of curable resin using a rotating embossing roller to form the micro-structures in the layer of curable resin, wherein the first edge and the second edge of the layer of curable resin are substantially aligned with the direction of roller rotation; and curing 56 the layer of curable resin. Different embodiments of this method 50 that have been found by the Applicant to reduce resin build-up in the embossing tool will now be described.

[0081] Figure 4 depicts a first embodiment using an embossing tool 60 and roller 62 and a transparent polymer substrate 64 which forms part of a web. In this embodiment, a nickel roller 62 is used, with an embossing surface profile 65, corresponding to the micro-structures to be embossed, engraved around its entire circumference. The UV curable resin 66 is applied to a portion of the substrate 64 as a continuous stripe using a gravure printing roller. The UV curable resin 66 has a first edge 68 and a second edge 70 extending beyond a top edge 72 and a bottom edge 74 of the substrate (via the continuous stripe). The UV curable resin 66 is subsequently brought into contact with and into register with the embossing tool 60 then embossed and UV-through cured using the above described "soft emboss" process. In this example the engraved micro-structures 65 on the embossing tool 60 are lenses that extend continuously around the circumference of the embossing roller 62, and extend beyond the width (in the cross-direction 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) is running only in the machine direction 75 (since the UV resin stripe is continuous, it has no beginning or end). This arrangement was found to give relatively less build-up compared to the arrangement shown in Figure 2.

[0082] Figure 5 shows a second embodiment using the embossing tool 60 and roller 62 and a transparent polymer substrate 76 which forms part of a web. The second embodiment of Figure 5 is similar to the first embodiment of Figure 4, except 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 micro-structures. The UV curable resin 78 therefore extends beyond the width of the engraved lenses (in the cross-direction) for example extends 1 mm beyond the cross-direction width of the engraved lens micro-structures 65. This configuration gave even less build-up than the configuration in Figure 4, since the perimeter of the UV-cured resin stripe (first edge 80 and second edge 82) does not lie on the micro-structures 65, therefore less force is required to

release it, resulting in a cleaner emboss with less build-up of embossing residues along the perimeter.

[0083] The Applicant thus found that the rate of build-up may be reduced or minimised by ensuring that the perimeter of the gravure-printed UV resin (more specifically, the portion of the perimeter that lies within or on the final product) is always oriented in the machine direction / direction of material travel 75, 83 in the rotary UV embossing process (the closer the perimeter is to the machine direction, the less is the rate of build-up).

[0084] Figure 6 shows a third embodiment using an alternative embossing tool 84 and roller 86. In this example, the embossing surface profile 88, corresponding to the micro-structures to be embossed, occupy a square shaped area. The UV curable resin 90 is applied as a continuous stripe to the substrate 92, having a width between the first edge 94 and second edge 96 (i.e. in the cross direction) beyond the width of the micro-structures 88, for example extending 1 mm beyond either side of the micro-structures 88. In this configuration, as in Figures 4 and 5, the perimeter of the UV curable resin 90 (i.e. first edge 94 and second edge 96) is aligned with the direction of roller 86 rotation (direction of substrate transport 98). There are no portions of perimeter perpendicular to the direction of roller 86 rotation, since the UV curable resin is a continuous stripe, not a patch, and so the embossing is clean with minimal build up issues.

[0085] The Figure 6 configuration may be conveniently used to produce patches of UV-embossed micro-structures in security documents such as bank notes as follows. As shown in Figure 7, micro-structures 100 are embossed in a UV curable resin 90 on a transparent polymer substrate 92 as described with reference to Figure 6. The micro-structures 100 occupy a square area on the clear substrate 92. The portions of the UV-resin 90 which do not contain micro-structures 100 may be overprinted with one or more opacifying layers 102. The opacifying layer 102 contains a square area with no ink, defining a window area 104 that is to be registered to the embossed micro-structures 100. The final product is a bank note 106 with a patch of UV embossed resin 100 in a window or a half-window. As shown in the bottom diagram of Figure 7, the opacifying layer 102 has been applied on top of the polymer substrate 92, and also on top of portions of the UV-embossed resin, to form a window or half-window in the bank note (optionally, the opacifying layer is not applied on top of the embossed micro-structures 100, rather a minimum distance between the two is maintained).

[0086] In a further embodiment, with reference to Figure 6 and 7, the layer of UV curable resin 90 on the substrate 92 may be applied with a varying thickness. This may reduce production costs, by decreasing the UV ink required, and may reduce the impact of the layer of UV curable resin 90 on the mechanical properties of the final product.

[0087] Optionally, the ink thickness may be spatially varied across the area covered by the UV curable resin 90. The variations applied may be continuous or discrete (step-wise) thickness variations.

[0088] The layer of UV curable resin 90 may be applied with a greater thickness in an area of the substrate on which the micro-structures 100 are to be positioned. The resin thickness in areas which are to be embossed with micro-structures preferably exceeds a minimum threshold, to ensure the micro-structures 88 on the embossing tool 84 are completely filled. The thickness of the layer of UV curable resin 90 may vary from an upper minimum thickness at the top edge 95 of the substrate 92 and a lower minimum thickness at the bottom edge 97 of the substrate 92 to a maximum thickness at the area of the substrate 92 on which the micro-structures 100 are to be positioned (optionally allowing for a registration tolerance). Alternatively or additionally, 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 a second minimum thickness at the second edge 96 of the layer of UV curable resin 90 to a maximum thickness at the area of the substrate 92 on which the micro-structures 100 are to be positioned (optionally allowing for a registration tolerance).

[0089] In the embodiment shown in Figure 7, a thickness gradient may be introduced along the top edge 95 and bottom edge 97 i.e. a variation in the UV thickness across a distance perpendicular to the perimeter tangent line, which varies from zero thickness at first edge 94 to non-zero thickness then back to zero-thickness at second edge 96, as a continuous or step function. This may assist with adhesion of the opacifying layer 102 to the cured UV resin, since it presents a flatter surface to the opacifying layer 102. It may also minimise any show-through (visibility) of the perimeter which may be undesirable.

[0090] Figure 8 shows a fourth embodiment, in which a continuous stripe of UV cured resin 101 is embossed with micro-structures in two patches - a circular patch 103 and a square patch 105. In this embodiment, the thickness of the layer of UV cured resin 101 varies in discrete steps.

[0091] The thickness of the UV cured resin 101 , as shown in the thickness profile 107 along the line AA, varies from zero thickness 109 on either side of the resin 101 , to a minimum thickness 1 1 1 from the first edge/second edge 1 13/1 15 of the resin 101 to a point 1 17/1 19 at a distance from the embossed micro-structures 105. The thickness of the UV cured resin 101 has a maximum thickness 121 at the area of the substrate on which the embossed micro-structures 105 are positioned and a little beyond the micro-structures (i.e. to the points 1 17 and 1 19, which allow for registration 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 areas of the substrate on which the embossed micro-structures 103, 105 are positioned, extending to a distance beyond the micro-structures to allow for registration tolerances (as indicated by points 127, 129, 131 , 133), and a minimum thickness 135 elsewhere.

[0092] This thickness gradient may be applied by varying the cell geometry in a gravure cylinder used to apply the curable resin layer before embossing. For example, in the areas to be embossed by micro-structures (circular patch 103 and square patch 105), and a distance beyond these areas to allow for registration tolerances, a maximum thickness 121 , 125 of the resin may be applied by using gravure cells that have larger widths and/or lengths and/or depths. A minimum thickness 1 1 1 , 135 of resin may be applied to other areas of the substrate by using gravure cells that have smaller widths and/or lengths and/or depths in order to provide a layer of UV curable resin having a first edge 1 13 and a second edge 1 15 extending beyond a top edge 137 and a bottom edge 139 of the substrate.

[0093] The adverse effects of UV resin build-up in an embossing tool can be further reduced if the perimeter of the UV-cured resin patch is located outside the product area, for example outside the area of a bank note. Figure 9 shows a fifth embodiment in which a layer of UV curable resin 1 10 is applied such that it extends beyond an entire perimeter of the substrate 1 12. The Figure 9 arrangement is useful if the embossing tool includes a shim mounted to the embossing roller.

[0094] In the Figure 9 configuration, the thickness of the UV curable resin 1 10 in areas outside the micro-structures 1 14 to be embossed has been reduced. This has the benefit of saving on UV resin, and also reduces the product thickness in areas that are not embossed with micro-structures, thus minimally impacting on the properties of the product in these areas.

[0095] Furthermore in Figure 9, the portion of the perimeter of the UV cured resin which is oriented in the machine direction 1 16 (i.e. the first edge 1 18 and the second edge 120) lies outside the finished product area. This ensures that any build-up associated with the first edge 1 18 and the second edge 120 will not impact on the finished product (unless of course an extremely long period of running results in the build-up encroaching into the product area - at this point the process will need to be stopped and the embossing tooling will need to be cleaned i.e. there will be production down time).

[0096] Furthermore in Figure 9, the UV resin is not a continuous stripe, instead the layer of UV curable resin 1 10 includes a top edge 122 located between a top edge 124 of the substrate 1 12 and a first join line between the shim and the embossing roller, and a bottom edge 126 located between the bottom edge 128 of the substrate 1 12 and a second join line between the shim and the embossing roller. That is, there is a portion of the perimeter of the UV cured resin which is oriented in the cross direction (perpendicular to machine direction 1 16) and this lies outside the finished product area. This arrangement avoids contact between the shim join lines and the applied UV curable resin 1 10.

[0097] The configuration of Figure 9 ensures that any build-up associated with the portion of the perimeter running in the cross direction (i.e. top edge 122 and bottom edge 126 of the UV curable resin 1 10) will not impact on the finished product (again unless of course an extremely long period of running results in build-up encroaching into the product area - at this point the process will need to be stopped and the embossing tooling will need to be cleaned i.e. there will be production down time).

[0098] In Figure 9, the build-up rate along the perimeter portion oriented in cross direction (top edge 122 and bottom edge 126) will be higher than along the perimeter portion oriented in machine direction (first edge 1 18 and second edge 120), therefore build-up associated with the perimeter portion oriented in cross direction (top edge 122 and bottom edge 126) will first encroach into the product area.

[0099] Figure 10 shows a sixth embodiment consisting of a clear polymer substrate 130; a cured UV resin 132 implementing a patch of embossed micro-lenses with a thin solid UV border; one opacifying layer 134 printed on top of the UV resin 132 to form a window around the lenses; another opacifying layer 136 on the reverse side that forms another window around the lenses and which also prints lenticular micro-imagery inside the focal plane of the lenses; and a final non-opacifying coloured layer 138 that overprints the lenticular imagery to create a second contrasting colour in the magnified optical effect image that can be observed through the micro-lenses. The end result is a lenticular image in the shape of a patch, which can be manufactured with minimal build-up issues and used as a security device in a security document such as a bank note.

[0100] While the invention has been described in conjunction with a limited number of embodiments, it will be appreciated by those skilled in the art that many alternatives, modifications and variations in light of the foregoing description are possible. Accordingly, the present invention is intended to embrace all such alternatives, modifications and variations as may fall within the spirit and scope of the invention as disclosed.

[0101] Any reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that that document or matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims.