EMBOSSING TOOL AND METHOD TO MINIMISE BUBBLE FORMATION IN EMBOSSED STRUCTURES

Abstract

An embossing tool for use with a rotating embossing roller, including: a tool body having a tool surface; and an array of recesses set into the tool surface to form a desired embossing surface profile, wherein at least two of the recesses are interconnected by a passage to enable fluid communication therebetween during embossing.

Claims

1. An embossing tool for use with an embossing roller to form micro-optic security devices, the embossing tool including: a tool body having a tool surface; and an array of recesses set into the tool surface to form a desired embossing surface profile, wherein at least two of the recesses are interconnected by a passage to enable fluid communication therebetween during embossing.

2. An embossing tool according to claim 1, wherein the passage or passages are aligned with the direction of embossing roller rotation or movement.

3. An embossing tool according to claim 1, wherein each recess in the array of recesses is connected to another recess in the array by a passage to enable fluid communication therebetween during embossing.

4. An embossing tool according to claim 1, wherein the embossing surface profile corresponds to a two-dimensional array of micro-lenses forming part of a micro-optic security device.

5. An embossing tool according to claim 1, wherein the embossing surface profile corresponds to a two-dimensional array of micro-imagery elements forming part of a micro-optic security device.

6. A micro-optic security device formed using an embossing tool according to claim 1, including a transparent substrate; one or both of: an array of micro-imagery elements forming a micro-imagery structure on a first side of the substrate, and an array of micro-lenses on a second side of the substrate that image the micro-imagery elements on the first side of the substrate to form an imagery viewable by a viewer; and filled passages interconnecting at least two of the micro-imagery elements and/or two of the micro-lenses.

7. A micro-optic security device according to claim 6, wherein the filled passages and the micro-lenses are offset by a random or non-constant amount to avoid the passages being imaged by the micro-lenses.

8. A micro-optic security device according to claim 6, wherein the micro-imagery elements form one or more of repeating icons, integral imagery and interlaced imagery.

9. A micro-optic security device according to claim 6, wherein the micro-lenses are hexagonal packed and/or rectangular packed.

10. A micro-optic security device according to claim 6, wherein the substrate includes: a transparent layer; and a UV-curable lacquer applied to the transparent layer, wherein the UV-curable lacquer is cured by UV radiation during or after embossing.

11. A security document including a micro-optic security device according to any claim 6 as a security feature.

12. A process for forming a micro-optic security device according to claim 6, including the step of: using a rotating embossing roller to apply the embossing tool to the substrate to form (a) one or both of the array of micro-imagery elements forming a micro-imagery structure on a first side of the substrate, and the array of micro-lenses on a second side of the substrate that image the micro-imagery on the first side of the substrate to form an imagery viewable by a viewer; and (b) the filled passages.

13. A process for forming a micro-optic device according to claim 12, including forming the substrate by applying the UV-curable lacquer to the transparent layer; using a rotating embossing roller to apply the embossing tool to the UV-curable lacquer; and curing the UV-curable lacquer by UV radiation during embossing to form (a) one or both of the array of micro-imagery elements forming a micro-imagery structure on a first side of the substrate, and the array of micro-lenses on a second side of the substrate that image the micro-imagery on the first side of the substrate to form an imagery viewable by a viewer; and (b) the filled passages.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0035] Preferred embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which:

[0036] FIG. 1 is schematic diagram of one embodiment of an apparatus for in-line manufacturing part of a security document;

[0037] FIG. 2 is a cut away side view of a partially manufactured security document manufactured by the apparatus shown in FIG. 1;

[0038] FIG. 3 is an isometric view of a known embossing tool used to manufacture the security document shown in FIG. 2;

[0039] FIG. 4 is an isometric view of a first embodiment of an embossing tool according to the present invention;

[0040] FIG. 5 is an isometric view of a two dimensional array of micro lenses embossed by the embossing tool shown in FIG. 4;

[0041] FIG. 6 is an isometric view of a second embodiment of an embossing tool according to the present invention;

[0042] FIG. 7 is an isometric view of a two dimensional array of micro lenses embossed using the embossing tool of FIG. 6;

[0043] FIG. 8 is an isometric view of a micro optic device including micro lenses and micro imagery elements formed on opposite sides of a substrate;

[0044] FIG. 9 is an isometric view of the micro-optic device shown in FIG. 8 and in addition showing imagery projected to a user;

[0045] FIG. 10 is a plan view of the micro imagery elements of the micro-optic device of FIG. 8;

[0046] FIGS. 11 and 12 are isometric views of the top and bottom of a micro-optic device including an integrated micro lens and micro imagery structure formed by embossing on both sides of the device substrate; and

[0047] FIG. 13 is an isometric view of the micro-optic device shown in FIGS. 11 and 12 and additionally depicting imagery projected to a user from both sides of the micro-optic device.

DETAILED DESCRIPTION OF DRAWINGS

[0048] Embossable ink generally refers to any ink, lacquer, or other coating which may be applied to a suitable substrate in a printing process, and which is embossed while soft to form a desirable relief structure and subsequently cured to retain the relief structure created during the embossing process. The curing process can take place either after embossing or at substantially the same time as the embossing step. Such embossable ink can be cured by radiation such as ultraviolet (UV) radiation, electron beams, X-rays, or heat, chemicals, or any combination of these. Exemplary embodiments of the present invention will now be described using a UV curable lacquer as the embossable ink, but it should be appreciated that other alternative types of embossable inks may also be used.

[0049] With some polymeric substrates, it may be necessary or desirable to apply an intermediate layer to the substrate before the embossable ink is applied to the substrate and embossed, to improve the adhesion of the embossed structure formed on the substrate. Such intermediate layer is generally known as an adhesion promotion layer. For some substrates, an adhesion promotion layer may not be required and the embossable ink may be applied directly onto the substrate.

[0050] FIG. 1 shows an exemplary apparatus 10 for in-line manufacturing part of an exemplary document 12 shown in FIG. 2 and which includes an adhesion promotion layer as mentioned above. 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.

[0051] The 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 adhesion of UV-curable coatings to polymeric surfaces. The adhesion promoting layer may have a UV-curing layer, a solvent-based layer, a water-layer or any combination of these.

[0052] At a third processing station 22 which also includes a roller assembly, an embossable ink coating is applied to the surface of the adhesion promoting layer. The embossable ink can be applied via flexographic printing, gravure printing or a silk screen printing process and variations thereof amongst other printing processes.

[0053] In the embodiment shown in FIG. 2, the embossable ink is only applied to a security element area 24 on a first surface 26 of the security document 12 where a structure 28 is formed. The structure 28 can include micro-imagery elements forming a micro-imagery structure and/or micro-lenses which will be collectively referred to as micro-optic structures herein. The security element area 24 can take the form of a strip, a discrete patch in the form of a simple geometric shape or a more complex geographical design.

[0054] While the embossable ink is still, at least partially, liquid, or soft, it is processed to form the structure 28 at a fourth processing station 30. In one embodiment, the processing station 30 includes an embossing roller 32 for embossing a micro-optic structure, such as the structure 28, into the embossable ink which in this example is provided in the form of a UV-curable lacquer. The cylindrical embossing surface 34 has surface relief formations corresponding to the shape of the structure 28 to be formed. In one embodiment, the surface relief formations can orient the array of micro-imagery elements and/or array of micro-lenses in the machine direction (that is, in the direction of roller rotation), transverse to the machine direction, or in multiple directions at an angle to the machine direction. The apparatus 10 can form micro-lenses and micro-imagery elements in a variety of two-dimensional or three-dimensional shapes.

[0055] The cylindrical embossing surface 34 of the embossing roller 32 may have a repeating pattern of surface relief formations or the relief structure formations may be localised to individual shapes corresponding to the shape of the security element area 24. 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 37 or shims may be attached via adhesive tape, magnetic tape, clamps or other appropriate mounting techniques.

[0056] 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.

[0057] The UV-curable lacquer on the web 14 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 or soft UV-curable lacquer flows into the surface relief formations of the cylindrical embossing surface 34 or the embossing shim 37. At this stage, the UV-curable lacquer is exposed to UV radiation, for example, by transmission through the web 14 to thereby cure the UV-curable lacquer to fix the relief structure formed by the embossing surface 34 and/or the embossing shim 37.

[0058] With the structure 28 now applied to the web 14, 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 a combination of both. In one preferred method, the additional layers are opacifying layers which are applied to one or both surfaces of the web 14 except in the region of this structure 28.

[0059] FIG. 2 shows a partially manufactured document 12 formed by the apparatus shown in FIG. 1, including an embossed structure 28 including an array of micro-optic structures such as micro-imagery elements and/or micro-lenses. The document 12 comprises a transparent substrate of polymeric material, preferable a biaxially-orientated polypropylene (BOPP) having a first surface 26 and a second surface 44. Opacifying layers 46, 48 and 50 are applied to the first surface 26, except in the security element area 24. Opacifying layer 54 and 56 are applied to the second surface 44 except in a window area 58. The window area 58 substantially coincides with the security element area 24 on the first surface 26. A printed layer 60 may be applied to the second surface 44 in the window area 58.

[0060] FIG. 3 depicts an example of a known embossing tool 70. The embossing tool 70 includes a tool body 72 having a tool surface 74 into which is set an array of recesses 76 to form a desired embossing surface profile. In the example depicted in FIG. 3, the recesses 76 form a conventional two dimensional array of hexagonally packed concave recesses suitable for embossing a two dimensional array of hexagonally packed spherical convex micro lenses. As an example, the width of each micro lens can be 54 microns and the depth is 12 microns. Each recess 76 is a closed structure separated from adjacent recesses 76. The closed structure of each recess increases the likelihood that bubbles will be formed when the embossing tool 70 is used in a roll-to-roll soft emboss process such as that described above with reference to FIG. 1.

[0061] Investigations by the Applicant have determined that during the embossing process, as the UV curable lacquer is squeezed in between the polymer web 14 and the embossing tool roller 38, UV curable lacquer is pressed into the closed recessed areas 76. During this process, the air present inside the recessed closed areas 76 cannot escape completely. This means that not all of the volume of the recessed closed structure 76 is filled with UV curable lacquer. That portion that is unfilled manifests as a bubble/void in the final cured structure.

[0062] If the embossing tool design consists of a repeating pattern of icons, as is typically the case in moire magnification type designs, and these icons include recessed closed areas, then the bubbles produced tend to be in consistent locations in each icon. This means that the bubbles themselves will be moire magnified by the lenses of the security feature, that is, the bubbles will be clearly visible to a user of the moire magnification design, resulting in a perception of poor quality. A moire magnification design is often employed as a security feature in a security document such as bank notes, ID cards, or cheques. For this reason, it is advantageous to minimise or eliminate the occurrence of such bubbles in the UV embossed imagery structures of moire magnifying security features.

[0063] FIG. 4 depicts an example of an embossing tool 80 that addresses this issue by interconnecting at least two of the array of recesses 82 set into the tool surface 84 by a passage to enable fluid communication therebetween during embossing. Providing such passages 86, 88, 90, 92 enables air present in the recesses 82 to escape during embossing. In the example shown in FIG. 4, all recesses 82 in the array of recesses 82 are connected to another recess by a passage to enable fluid communication therebetween during embossing. The passages, such as those referenced 86, 88, 90, 92 are aligned with the machine direction 94, or in other words the direction of rotation of the embossing roller 32. In this way, the recessed structures in the embossing tool 80 can be completely filled with UV curable lacquer because air is able to be squeezed out of each recess via the passages 86, 88, 90, 92, as the lacquer is pressed into the recessed areas of the embossing tool 80 during embossing, that is, there are no voids or bubbles which are produced.

[0064] FIG. 5 depicts an array 100 of micro lenses formed using the exemplary embossing tool 80. The passages 86, 88, 90, 92 formed between recesses 82 in the embossing tool 80 cause the creation of corresponding filled passages interconnecting adjacent micro lenses. For example, micro lenses 102, 104, 106, 108 and 110 are respectively interconnected by filled passages 112, 114, 116 and 118.

[0065] In the case of an array of micro lenses forming a part of a micro optic device used in bank notes or other security documents, the passages may typically have dimensions of 7 microns wide by 5 microns deep, and may optionally include tapered side walls. It can be seen from FIGS. 4 and 5 that each micro lens has two passages that connect it to two adjacent micro lenses in the machine direction. The interconnecting channels ensure that recessed areas that are aligned in the machine direction are in fluid communication with each other, thereby allowing air trapped in the embossing tool to be squeezed out during the embossing process.

[0066] The introduction of filled passages between micro lenses in the array 100 of micro lenses results in a percentage of the imaging surface of each lens being lost, thereby resulting in the image contrast being proportionally reduced. However, it has been found that the percentage lost in image contrast is small so that the quality of optical effect image remains acceptable.

[0067] FIG. 6 depicts another embodiment of an embossing tool 120 identical to the embossing tool 80 in that it includes an array 122 of recesses set into a tool surface 124. However, in this embodiment, passages are provided between each recess and each immediately adjacent recess to optimise the flow of air and/or lacquer between recesses during embossing. By way of example, the recess 126 is connected to each of the six immediately adjacent recesses by six passages 128, 130, 132, 134, 136 and 138. It will be understood however that increasing the number of passages interconnecting each recess to adjacent recesses to improve fluid flow therebetween will have the corresponding effect of proportionally reducing image contrast. A balance must be achieved between the consequent reduction in the quality of the optical effect image produced and the fluid flow during embossing that may be required to minimise or eliminate the occurrence of bubbles or voids in the micro lens structure.

[0068] FIG. 7 depicts an embossed micro lens structure 140 made using the embossing tool 120. The trade-off once again is that a percentage of the imaging surface of each micro lens has been lost, so that the image contrast will be proportionally reduced by a greater amount than the example shown in FIG. 5 which has fewer channels per lens.

[0069] Whilst the embossing tool and embossed structures depicted in FIGS. 3 to 7 relate to a micro lens structure it will be appreciated that the embossing surface profile resulting from the setting of an array of recesses into the embossing tool surface can be used to generate a two dimensional array of micro lenses forming part of a micro optic device but can also be used to produce a two dimensional array of micro imagery elements forming part of a micro optic device.

[0070] FIGS. 8 and 9 depict one example of a micro optic device 160 including a transparent substrate 162, a two dimensional array of micro imagery elements 164 on a first side of the substrate 162 and a two dimensional array of micro lenses 166 on a second side of the substrate 162 that sample and magnify the micro imagery 164 on the first side of the substrate. FIG. 9 depicts imagery 168 that is produced by the micro optic device 160 for observation by a user from a viewing position 170. The imagery produced is a magnified moire type design, and the image elements consist of an array of icons of the numeral 5 corresponding to the array of micro imagery elements forming the micro imagery structure 164 on the first side of the substrate 162.

[0071] In this example, the micro imagery elements (i.e. the icons in the form of the numeral 5) are embossed onto the first side of the substrate 162 such that the background of the numeral 5 is recessed into the surface. Passages aligned with the machine direction 172 interconnecting recesses on the embossing tool have resulted in the embossed micro imagery elements being interconnected by filled passages, such as those referenced 174, 176, 178 in order to minimise or eliminate the production of voids or bubbles in the micro imagery elements that are embossed.

[0072] Preferably, the passages added can be located so that they are not moire magnified by the micro lenses 166 of the micro optic device 160. As can be seen in FIG. 10, the passages 180, 182, 186, 188, 190, 192, 194, 196, 198, 200, 202, 204, 206, 208 are located so that the phase offset between the passages 180 and 208 and the array 166 of micro lenses is random or non-constant.

[0073] FIGS. 11 to 13 depict another embodiment of a micro optic device 220 that includes a transparent substrate 222. An integrated structure 224 of micro imagery elements and micro lenses is formed on a first side of the substrate 222 and a second unitary structure 226 of micro imagery elements and micro lenses is formed on a second side of the substrate 222. Micro lenses from one side of the substrate 222 act to sample and magnify micro imagery elements on the other side of the substrate 222. Imagery 228 produced by moire magnification of the micro imagery elements on a first side of the substrate 222 is viewable from a first viewing position 230, whereas imagery 232 resulting from moire magnification of the micro imagery elements on a second side of the substrate is viewable from a viewing position 234.

[0074] In FIG. 11 the micro imagery shown consists of an array of the numeral 7 wherein each element of the array is recessed below the lens surface. In FIG. 12 the micro imagery shown consists of an array of the numeral 5 wherein each element of the array protrudes above the lens surface.

[0075] As can been seen from FIGS. 11 and 12, passages interconnecting recesses on an embossing tool used to create the unitary structures on both sides of the substrate 222 have resulted in filled passages interconnecting the micro imagery and micro lens elements respectively in the machine direction on both sides of the substrate 222. Once again, where passages interconnect micro imagery and micro lens elements respectively forming a micro imagery and a micro lens structure, the phase offset of the lenses on one side relative to the passages on the other side may be random or non-constant in order to minimise the moire magnification of the passages by the lenses on their opposite side.

[0076] Where the term comprise, comprises, comprised or comprising are used in the specification (including the claims) they are intended to be interpreted as specifying the presence of the stated features, integers, steps or components, but not precluding the presence of one or more other features, integers, steps or components or group thereof.

[0077] It will be understood that the invention is not limited to the specific embodiments described herein, which are provided by way of example only. The scope of the invention is as defined by the claims appended hereto.