WO2018176090A1

WO2018176090A1 - Method for manufacturing a cylinder configured for producing microstructure image effects

Info

Publication number: WO2018176090A1
Application number: PCT/AU2018/050285
Authority: WO
Inventors: Karlo Ivan Jolic; Gary Fairless Power; Patrick Swift
Original assignee: Ccl Secure Pty Ltd
Priority date: 2017-03-27
Filing date: 2018-03-27
Publication date: 2018-10-04
Also published as: AU2017100354B4; AU2017100354A4; FR3064210A1

Abstract

A method for manufacturing a cylinder having surface relief structures configured for producing at least one microstructure image effect in one or more security devices applied to a security document is provided. The method includes the steps of providing a blank cylinder having a surface; and forming surface relief structures corresponding to the at least one microstructure image effect directly on the surface of the cylinder.

Description

ME T HOD FO R MANU FACTU RING A CY LIN DE R CO NFIG UR E D F OR P R ODUCING MIC R OST R UCTU R E IMAG E E F F E CTS

Technical F ield

[0001] The invention relates generally to cylinders for producing security devices used as an anti-counterfeiting measure in security devices, and in particular to the manufacture of an embossing cylinder.

Background of Invention

[0002] A variety of security devices are applied to security documents and tokens to deter counterfeiters. For example, bank notes may have a relief structure, such as a diffraction grating or hologram incorporated into a layer of radiation curable ink. One method of incorporating a relief structure into a security document is an embossing process which embosses the desired diffractive relief structure into a radiation curable ink applied to the substrate and irradiating the same whilst the embossing tool is in contact with the radiation curable ink to permanently fix the embossed relief structure.

[0003] While various embossing tools are available, in the past, only shims _ have been found suitable for the production of the finer relief structures required for micro- optical effect imagery than can be produced using an embossing roller. A shim consists of a transfer foil of metallic material bearing the negative of the desired micro-scale or nano-scale diffractive relief structures which is then wrapped around a blank roller.

[0004] It is a disadvantage of the use of such shims, that a protracted set-up time is required to mount the shim on an embossing roller using adhesive tape, taking care to avoid, as much as practicable, malalignment of the shim. The shim must be mounted squarely on the embossing roller using alignment crosses. In some cases, the mounting process will fail due to malalignment, resulting in the shim needing to be discarded and the mounting process recommenced with a fresh shim. Moreover, seams _ tend to form, where the edge of a shim is joined with another edge of the shim or otherwise attached to the roller. In addition, personal injury can occur when handling shims due to their sharp, exposed edges. [0005] The use of a cylinder bearing micro-optical effect image structures has the potential to overcome these problems. Moreover, a cylinder can be removed from the press for cleaning and reuse. This is not possible with a shim which typically sustains irreparable damage on removal from the roller.

[0006] It is therefore desirable to provide a method for manufacturing a cylinder configured for production of the finer relief structures required for micro-optical effect imagery and a cylinder manufactured by such a method.

Definitions

S ecu rity Docu ment or Token

[0007] As used herein the term security document includes all types of documents and tokens of value and identification documents including, but not limited to the following: items of currency such as banknotes and coins, credit cards, cheques, passports, identity cards, securities and share certificates, driver's licenses, deeds of title, travel documents such as airline and train tickets, entrance cards and tickets, birth, death and marriage certificates, and academic transcripts.

[0008] The invention is particularly, but not exclusively, applicable to security documents such as banknotes or identification documents such as identity cards or passports formed from a substrate to which one or more layers of printing are applied. The diffraction gratings and optically variable devices described herein may also have application in other products, such as packaging.

S ubstrate

[0009] As used herein, the term substrate refers to the base material from which the security document or token is formed. The base material may be paper or other fibrous material such as cellulose; a plastic or polymeric material including but not limited to polypropylene (P P), polyethylene (P E ), polycarbonate (P C), polyvinyl chloride (PVC), polyethylene terephthalate (P E T); 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. [0010] The use of plastic or polymeric materials in the manufacture of security documents pioneered in Australia has been very successful because polymeric banknotes are more durable than their paper counterparts and can also incorporate new security devices and features. One particularly successful security feature in polymeric banknotes produced for Australia and other countries has been a transparent area or window_,.

Trans parent Windows and Half Windows

[0011] As used herein the term window refers to a transparent or translucent area in the security document compared to the substantially opaque region to which printing is applied. The window may be fully transparent so that it allows the transmission of light substantially unaffected, or it may be partly transparent or translucent partially allowing the transmission of light but without allowing objects to be seen clearly through the window area.

[0012] 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 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.

[0013] A partly transparent or translucent area, hereinafter 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 the ^"half-windowjs not fully transparent but allows some light to pass through without allowing objects to be viewed clearly through the half-window.

[0014] Alternatively, it is possible for the substrates to be formed from an substantially opaque material, such as paper or fibrous material, with an insert of transparent plastics material inserted into a cut-out, or recess in the paper or fibrous substrate to form a transparent window or a translucent half-window area. Opacifying Layers

[0015] 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<L0 where L0 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. F or 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.

S ecu rity Device or Feature

[0016] As used herein the term security device or feature includes any one of a large number of security devices, elements or features intended to protect the security document or token from counterfeiting, copying, alteration or tampering. S ecurity 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 and phosphorescent inks, metallic inks, iridescent inks, photochromic, thermochromic, hydrochromic or piezochromic inks; printed and embossed features, including relief structures;

interference layers; liquid crystal devices; lenses and lenticular structures; optically variable devices (OV Ds) such as diffractive devices including diffraction gratings, holograms and diffractive optical elements (DO E s).

Radiation C urable Ink

[0017] The term radiation curable ink used herein refers to any ink, lacquer or other coating which may be applied to the substrate in a printing process, and which can be either be embossed while soft to form a relief structure and cured by radiation to fix the embossed relief structure or applied to a die form, such as a shim, and brought into contact with the substrate before being cured by radiation to fix the ^"microprinted_ relief structure. The curing process, generally (some partial curing can occur earlier and full curing can occur after), takes place when the radiation curable ink and die form (either as an embossing step or microprinting step) is in contact with the substrate. The radiation curable ink is preferably curable by ultraviolet (UV) radiation. Alternatively, the radiation curable ink maybe cured by other forms of radiation, such as electron beams or X-rays.

[0018] The radiation curable ink is preferably a transparent or translucent ink formed from a clear resin material. S uch a transparent or translucent ink is particularly suitable for printing light-transmissive security elements such as sub-wavelength gratings, transmissive diffractive gratings and lens structures.

[0019] In one particularly preferred embodiment, the transparent or translucent ink preferably comprises an acrylic based UV curable clear embossable lacquer or coating.

[0020] S uch UV curable lacquers can be obtained from various manufacturers, including Kingfisher Ink Limited, product ultraviolet type UV F-203 or similar.

Alternatively, the radiation curable embossable coatings maybe based on other compounds, e.g. nitro-cellulose.

[0021] The radiation curable inks and lacquers used herein have been found to be particularly suitable for embossing microstructures, including diffractive structures such as diffraction gratings and holograms, and microlenses and lens arrays.

However, they may also be embossed with larger relief structures, such as non- diffractive optically variable devices.

[0022] The ink is preferably microprinted/embossed and cured by ultraviolet (UV) radiation at substantially the same time. In a particularly preferred embodiment, the radiation curable ink is applied and embossed at substantially the same time in a G ravure printing process.

[0023] P referably, in order to be suitable for G ravure printing, the radiation curable ink has a viscosity falling substantially in the range from about 20 to about 175 centipoise, and more preferably from about 30 to about 1 50 centipoise. The viscosity may be determined by measuring the time to drain the lacquer from a Zahn C up #2. A sample which drains in 20 seconds has a viscosity of 30 centipoise, and a sample which drains in 63 seconds has a viscosity of 150 centipoise.

[0024] With some polymeric substrates, it may be necessary to apply an intermediate layer to the substrate before the radiation curable ink is applied to improve the adhesion of the embossed structure formed by the ink to the substrate. The intermediate layer preferably comprises a primer layer, and more preferably the primer layer includes a polyethylene imine. The primer layer may also include a cross- linker, for example a multi-functional isocyanate. Examples of other primers suitable for use in the invention include: hydroxyl terminated polymers; hydroxyl terminated polyester based co-polymers; cross-linked or uncross-linked hydroxylated acrylates; polyurethanes; and UV curing anionic or cationic acrylates. Examples of suitable cross-linkers include: isocyanates; polyaziridines; zirconium complexes; aluminium acetyl acetone; melamines; and carbodi-imides.

S ummary of Invention

[0025] According to an aspect of the present invention, there is provided a method for manufacturing a cylinder having surface relief structures configured for producing at least one microstructure image effect in one or more security devices applied to a security document, the method including the following steps: providing a blank cylinder having a surface; and forming surface relief structures corresponding to the at least one microstructure image effect directly on the surface of the cylinder.

[0026] What constitutes a microstructure with regard to dimensions will vary depending on the particular type of optical effect That is, the dimensions of microstructures that can be formed using the method of the present invention is limited by the minimum feature size that can be achieved using various techniques suitable for manufacturing cylinders having surface relief structures configured for producing microstructure images.

[0027] According to an embodiment the step of forming one or more surface relief structures directly on the surface of the cylinder involves laser ablation. [0028] For example, in one particular form of the invention, the laser is configured to produce a laser beam width of substantially 5 microns or less.

[0029] The laser may be configured to produce a laser pulse duration sufficient to vaporise the surface of the cylinder. In one exemplary embodiment the laser is configured to produce a laser pulse duration in the range of femtoseconds to nanoseconds. Moreover, selection of a suitable laser will be dependent on the cylinder material to be ablated.

[0030] A laser beam shaping optical element may be applied in an optical path formed between the laser and the surface of the cylinder in order to achieve the requisite uniformity of the laser beam.

[0031] According to another embodiment, the step of forming surface relief structures directly on the surface of the cylinder is carried out by electro-mechanical engraving means using for example, an oscillating stylus formed from a suitable material such as diamond. The electro-mechanical engraving tool may be configured to engrave microstructures of substantially 35 microns or less, or of substantially 5 microns or less.

[0032] According to yet another embodiment the step of forming surface relief structures directly on the surface of the cylinder is carried out by chemical etching. The step of forming surface relief structures directly on the surface of the cylinder by chemical etching may be preceded by one of the following steps: coating the surface the cylinder with a mask material and laser ablating a pattern of proposed surface relief structures into the mask material to expose the pattern on the surface of the cylinder to thereby form an etching mask; coating the surface of the cylinder with a resist, UV exposing the resist through a film containing a pattern of proposed surface relief structures and developing the UV-exposed resist to expose the pattern on the surface of the cylinder; or coating the surface of the cylinder with a resist, scanning a focussed beam of light onto the resist to expose a pattern of proposed surface relief structures in the resist that is subsequently developed to expose the pattern on the surface of the cylinder. [0033] According to still another embodiment the method for manufacturing a cylinder having surface relief structures configured for producing at least one microstructure image effect in one or more security devices applied to a security document further includes the step of depositing layer of material over the surface relief structures to reduce a size of the microstructure.

[0034] P referably, the step of providing a deposited layer comprises

electroforming the layer of material.

[0035] P referably, the electroformed layer of material is a metal.

[0036] The electroformed layer of material may be deposited in a uniform layer.

[0037] Alternatively, the step of providing a deposited layer comprises chemical vapour deposition of the layer of material.

[0038] F urther alternatively, the step of providing a deposited layer comprises depositing a ^"Diamond-like carbon_, (DLC) coating as the layer of material.

[0039] According to yet another embodiment of the present invention, there is provided a computer program product storing instructions for manufacturing a cylinder having surface relief structures configured for producing at least one microstructure image effect in at least one security device applied to a security document, the computer program product storing instructions for controlling a processor to control a device to form surface relief structures corresponding to the at least one

microstructure image effect directly on a surface of the cylinder.

[0040] According to a further embodiment of the present invention, there is provided a cylinder having surface relief structures configured for producing at least one microstructure image effect in at least one security device applied to a security document, the surface relief structures corresponding to the at least one

microstructure image effects being formed directly on a surface of the cylinder.

[0041] The surface relief structures may comprise mic restructures having dimensions of substantially 35 microns or less, or of substantially 5 microns or less. B rief Description of Drawings

[0042] E mbodiments 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:

[0043] F igure 1 shows and exemplary apparatus for manufacturing a security document.

[0044] F igure 2 shows an exemplary microstructure that can be formed on a cylinder by the method of the present invention.

[0045] F igure 3 shows an exemplary laser ablation system which is used in one embodiment of the present invention.

[0046] F igure 4 shows an exemplary micro-mechanical engraving system which is used in another embodiment of the present invention.

[0047] F igure 5 show an optical microscope photograph of an array of micro images of the letter ^"A_formed by mask ablation according to an embodiment of the present invention.

[0048] F igures 6A and 6B show an image structure that has been electroplated to reduce the size of the image structure in a post processing step according to one embodiment of the present invention.

Detailed Des cription

[0049] R eferring firstly to F igure 1 there is shown an exemplary apparatus for manufacturing a security document A radiation curable ink, such as a UV curable ink, is applied to a security element area where a surface relief structure configured for producing at least one image effect in a security device applied to a security document is to be positioned. The security element area may take the form of a stripe, a discrete patch in the form of a simple geometric shape, or in the form of a more complex graphical design. [0050] While the radiation curable ink remains at least partially liquid, it is processed at a processing station 100 including a cylinder 1 10 for embossing the security element structure into the radiation curable ink. The cylindrical surface 120 of the cylinder 1 10 may have a repeating pattern of surface relief formations or the relief structures may be localized to individual shapes corresponding to the shape of the security elements area on the substrate.

[0051] The radiation curable ink on the substrate is brought into contact with the cylindrical embossing surface 120 of the cylinder 1 10 by a nip roller 130 at the processing station 100, such that the liquid radiation curable ink flows into the surface relief structures of the embossing surface 120. At this stage, the radiation curable ink is exposed to UV radiation to permanently fix the embossed structures.

[0052] Whilst F ig.1 is described in relation to a cylinder, it is equally possible to generate such microstructures not through embossing but by application of a radiation curable ink to a cylinder and bringing the cylinder into contact with the substrate to be printed, before being exposed to UV to fix the structures. This process only differs in the method of application of the radiation curable ink, and, potentially, other printing parameters, but the cylinder used is identical regardless of the method of application.

[0053] The present invention is directed to the manufacture of a cylinder 1 10 suitable for forming microstructure image effects in security devices in security documents. S uch micro-optical effect imagery is at least in part defined by dimension and includes security features such as micro-text lens based optical effects such as image flips, contrast switches, animations, morphing effects, three dimensional floating and/or receding images, moirH magnification effects and combinations thereof.

[0054] What constitutes a microstructure with regard to dimensions will vary depending on the particular type of optical effect For example, in the case of holograms and diffractive optically variable devices (DOV Ds), the smallest dimension of a microstructure is a couple of microns or less. Using current technology, these would be difficult to directly mechanically engrave. In the case of lens based effects, the smallest dimensions will depend on the size of the lenses used and the particular optical effect For example, moirH magnification effects employ microstructures having the smallest dimensions. For refractive lenses, configured to magnify imagery deployed on the other side of a typical polymer substrate, the smallest dimension is around 2 to 10 microns. The actual size is dependent on the complexity of the moirH magnified image, i.e., the more complex the image, the smaller is the minimum feature size within the microstructure. Particular challenges are encountered in forming microstructures of sufficient resolution and integrity in a cylinder to produce high fidelity and high complexity micro-optical effects. These are addressed by the method of the present invention.

[0055] R eferring now to F igure 2, there is shown an exemplary microstructure formed by the method of the present invention. That is the microstructure is ablated, engraved or otherwise formed directly in the surface 120 of a blank cylinder 1 10 (see Figure 1 ). In the illustrated example, the microstructure 200 consists of an array of the numeral ^"5_ 220 arranged in a hexagonal grid with 55 microns pitch. The numerals 220 are formed by recessing approximately 4 microns deep into the surface 120 of the cylinder 1 10. The smallest feature size of the numerals 220 is the feature width on the cylinder surface 120, at substantially 5 microns. An cylinder 1 10 having such a microstructure 210 formed in its surface by the method of the present invention could be used to create microstructures, for example, moirH effect security features on bank notes that utilise hexagonally packed micro lenses having a similar, but different pitch to that of the correspondingly formed microstructures, e.g. 56 microns pitch lenses could be employed.

[0056] The cylinderl 10 may be manufactured by forming the microstructures 210 that correspond to the microstructure image effects to be created in one or more security devices by direct laser ablation, chemical etching, mask ablation, or by micro- mechanical engraving means or other suitable techniques.

[0057] Now described with reference to F igure 3 is one embodiment of a system 300 for manufacturing a cylinder 1 10 having surface relief structures configured for producing at least one microstructure image effect In this case the surface relief structures are formed on the cylinder by laser ablation, wherein the laser 310 is selected and/or configured to produce a beam width of 5 microns or less. The laser ablation system 300 includes a laser 310, e.g. a picosecond laser, a laser beam 320, an objective lens 330 and a blank cylinder 340. The laser beam 320 width and uniformity is configured to enable homogeneous ablation of the cylinder surface on exposure to the laser beam. To achieve the requisite beam uniformity, optical elements 350 including lenses and/or mirrors and/or beam shapers are placed in the optical pathway between the laser 310 and the cylinder 340 surface.

[0058] The pulse duration and pulse energy of the laser 310 is controlled so as to vaporise the surface of the cylinder 340 to form a cell. The shape of the cell is defined by the material to be ablated, the intensity profile and pulse duration of the laser beam, the number of applied pulses, and the position of each pulse on the cylinder surface. S uitable laser pulse durations are in the femtosecond to nanosecond range, and preferably in the picosecond range. Depending on the material of which the cylinder 340 to be laser ablated is comprised, a single laser pulse typically vaporises around 1 micron or less of the material in depth.

[0059] The cylinders can be formed from a range of materials since the radiation cured process itself causes minimal wear. For example, the blank cylinder may take the form of a hollow steel cylinder that is copper, nickel or zinc plated. Other materials such as chromium or polymers are also envisaged. It will be understood that selection of a suitable laser will be dependent on the cylinder material to be ablated. For example, for materials with high vaporisation temperatures, such as metals, a Q switched YAG laser may be suitable choice. F or example, a Q-switched Nd:YAG multimode laser, with 400W at 1064nm with a repetition rate of 35 KHz. However, for materials requiring a not so high vaporisation temperature, such as polymeric materials, an excimer laser may be a suitable choice.

[0060] A mask or beam shaping optic 350 such as a diffractive optical element may be used to shape the profile of the cross section of the smallest spot size to optimise the process, for example by facilitating efficient expulsion of materials from the ablated profile. Moreover, the laser beam 320 can be otherwise controlled, for example, by dynamically controlling the beam width to increase the beam focus where an area of increased depth ablation is required and conversely, to decrease the beam focus where an area of decreased depth ablation is required. [0061] The laser ablation process can be configured as a step and repeat process using a mask modulated beam. Alternatively, the laser can be controlled by a suitably designed control system that can write a bitmap file or similar, directly on a blank cylinder, wherein the bitmap file represents a simple O N/O F F laser pulse. In another embodiment, the laser is capable of direct writing a greyscale file to the cylinder, wherein each greyscale value corresponds to the ablation depth in the cylinder at the position concerned.

[0062] The micro images may be at least partially hidden by integrating them into a larger or macro image. For example, portions of a 2D array of micro images, that form part of a moirH magnification device, could be placed in non-image areas within the macro image. Alternatively, the density of the array of micro image elements could be varied, for example, by reducing the number of array elements in the array, to simulate grey levels in the macro image.

[0063] R eferring now to F igure 4, the microstructures could similarly be formed on a blank cylinder using a direct micro-mechanical engraving system 400 having an oscillating stylus, typically diamond, which cuts cells directly into the surface of the cylinder, assuming that the selected system provides a suitable engraving resolution. One example of a micro-mechanical engraving machine that can achieve high resolution engraving is the Hell E xtreme engraving System having an engraving resolution of substantially 5 microns. Another is the MicroStar™ MicroE ngraving System offered by Daetwyler R &D C orp. The MicroS tar™ system 400 is

schematically depicted in F igure 4 and is suitable for engraving a blank cylinder at a resolution of 5 microns and less and could accordingly, be employed to engrave functional micro-optical effect imagery micro structures for lenses on security documents such as bank notes. An appropriately selected cutting tool 420 is controlled via interface 430 which enables importing and uploading of microstructure design files.

[0064] Alternative methods suitable for manufacturing an cylinder having surface relief structures configured for producing at least one microstructure image effect, include chemical etching which involves copying the proposed surface relief structures by direct contact exposing of a photoresist from film to the metal surface of the cylinder. After exposure, developing steps and chemical etching are performed to produce recessed structures in the embossing cylinder which will subsequently contain UV-curable ink for producing the microstructure image effects in security devices. T he exposure step is performed by passing UV light through the film, to expose the underlying resist-coated surface of the cylinder. As an alternative method, which does not involve the use of films/contact exposure, the resist-coated cylinder surface may be directly exposed with a focused beam of light or laser light, preferably UV light or UV laser light. Mask ablation methods involve coating the cylinder with a mask using immersion or spray coating techniques. The mask is then removed from areas to be chemically etched by thermal ablation, following this the cylinder is chemically etched to produce recessed microstructures. R eferring now to F igure 5 there is shown an optical microscope photograph 500 of an array of micro images of the letter ^"A_ 510 with a pitch of 50 microns produced using mask ablation followed by chrome plating. T he engraving resolution used was 10,1 60 dots per inch. The engraving artwork was provided in the form of a 10, 160 dpi monochromatic TIF file containing a binary image of the letter ^"A_ array.

[0065] The dimensions of microstructures that can be formed using the method of the present invention is limited by the minimum feature size that can be achieved using various techniques suitable for manufacturing cylinders having surface relief structures configured for producing microstructure images. As previously noted such techniques include film contact exposure, focused beam exposure, mask ablation, chemical etching, micro-electromechanical engraving techniques, and direct laser ablation.

[0066] The minimum feature size will correspondingly impact the minimum image feature size that will be projected by lenses in the case where the microstructure is overlaid with an array of micro lenses. That minimum size is accepted to be substantially 5 microns. However, if the security feature design dictates that smaller microstructures are required to produce the desired image effects, then this may be achieved by electroforming a layer of material, e.g. metal over the surface relief structures in a layer of uniform thickness. [0067] R eferring now to F igures 6A and 6B there is shown schematically how deposition of a layer of material over the microstructure can effectively reduce the size of the surface relief structures. That is, F igure 6A shows the originally formed microstructure having a dimension of 5 microns. F igure 6B shows the same structure 610 after a layer of chrome 620 that is 1 micron thick is electroformed over the surface relief structure. This has the effect of decreasing the dimensions of the microstructure 630 to 3 microns. The decreased dimensions allow a smaller minimum feature size to be achieved. A further advantage of the addition of the chrome layer is that it may assist in extending the lifetime of the cylinder. In a similar manner, alternative deposition techniques can be used instead of electroforming, such as chemical vapour deposition (CV D), P lasma Assisted C hemical Vapor Deposition (PACV D) or P hysical Vapour Deposition (PV D). In particular, one aspect of the present invention includes the addition of a diamond-like carbon (DLC) coating on the cylinder using appropriate deposition techniques.

[0068] Accordingly, design of suitable mic restructures for coating may entail firstly developing the design so that the portions of the design representing the recessed microstructure have dimensions equal to the target dimensions of the recessed structure after coating. S econdly, the design must be adjusted to compensate for the coating by enlarging the portion of the design representing the recessed area so that its perimeter moves away from the recessed area (i.e. perimeter moves perpendicular to its local tangent), by a predetermined distance, so that the recessed feature area will be increased. If the design is represented as a raster image, this enlargement compensation step could be achieved by applying ^"Minimum_, or ^"Maximum_filter functions to the image in P hotoshop (depending on whether the recessed areas are represented by black or white pixels). T he ^"E rode _, or ^"Dilate_ functions known in image processing theory could also be used.

[0069] It is an advantage of the present invention that reducing the reliance on shims having a limited useful life simplifies and reduces the costs of generating micro imagery for security devices. A cylinder having integrated surface relief structures configured for producing at least one microstructure image effect can simply be removed from the processing station for cleaning as required, whilst shims are too fragile to endure the shim removal process that is required in order to clean them. Moreover, the problems encountered when using conventional shims, including alignment issues and the presence of joins/seams, are avoided using a cylinder with integral surface relief structures configured for producing at least one microstructure image effect. Moreover, the angular alignment of the micro imagery to lenses is also improved such that image effects due to lens-to-image skew are reduced, since alignment error due to shim mounting is avoided. This enables larger images implementing multi frame optical effects such as flips, contrast switches, animations and three dimensional designs including interlaced, integral and moirH effects, to be created.

[0070] In the context of a microprint application process of radiation curable ink, where the ink is applied to the cylinder, the present invention enables the ink to be removed from non-recessed areas. In the G ravure printing industry the most common process for achieving this is through the use of a ^"doctor blade _. The blade is set against the cylinder and used to wipe away ink which is not in recesses on the cylinder. It has not been possible to perform this process using shims _ because the nickel surface is not hard enough to provide sufficient pressure and the shim is not provided on the roller in a way which not cause the doctor blade to catch. For example, shims are usually applied using tape or by welding edges together. This leaves an edge, which is either welded or held by tape which the doctor blade would have to pass over and would cause process issued or would damage the shim.

[0071] In addition, through removal of shims, the radiation curable ink application process is simplified and made more efficient by reducing set-up time and

opportunities for errors. Moreover, the risk of personnel injury inherent in handling metal shims due to sharp edges is avoided.

[0072] It will appreciated that any reference to a cylinder in the description above is, equally, a reference to an embossing cylinder, for embossing of radiation curable ink, or a microprinting cylinder, for microprinting of radiation curable ink, unless otherwise specified. [0073] Where the terms "comprise", "comprises", "comprised" or "comprising" are used in this specification (including the claims) they are 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.

[0074] 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.

[0075] The present application may be used as a basis or priority in respect of one or more future applications and the claims of any such future application may be directed to any one feature or combination of features that are described in the present application. Any such future application may include one or more of the following claims, which are given by way of example and are non-limiting in regard to what may be claimed in any future application.

Claims

The claims defining the invention are as follows

1 . A method for manufacturing a cylinder having surface relief structures

configured for producing at least one microstructure image effect in one or more security devices applied to a security document, the method including the following steps: providing a blank cylinder having a surface; and forming surface relief structures corresponding to the at least one

microstructure image effect directly on the surface of the cylinder.

2. A method according to claim 1 , wherein the step of forming one or more

surface relief structures directly on the surface of the cylinder involves laser ablation.

3. A method according to claim 2, wherein the laser is configured to produce a laser beam width of substantially 5 microns or less.

4. A method according to claim 2 or 3, wherein the laser is configured to produce a laser pulse duration sufficient to vaporise the surface of the cylinder.

5. A method according to claim 4, wherein the laser is configured to produce a laser pulse duration in the range of femtoseconds to nanoseconds.

6. A method according to any one of claims 2 to 5, further including the step of applying a laser beam shaping optical element in an optical path formed between the laser and the surface of the cylinder.

7. A method according to claim 1 , wherein the step of forming surface relief

structures directly on the surface of the cylinder is carried out by electromechanical engraving means.

8. A method according to claim 7, wherein an electro-mechanical engraving tool is configured to engrave microstructures of substantially 5 microns or less.

9. A method according to claim 1 , wherein the step of forming surface relief structures directly on the surface of the cylinder is carried out by chemical etching.

10. A method according to claim 9, wherein the step of forming surface relief

structures directly on the surface of the cylinder by chemical etching is preceded by one of the following steps: a. coating the surface of the cylinder with a mask material and laser ablating a pattern of proposed surface relief structures into the mask material to expose the pattern on the surface of the cylinder; b. coating the surface of the cylinder with a resist, UV exposing the resist through a film containing a pattern of proposed surface relief structures and developing the UV-exposed resist to expose the pattern on the surface of the cylinder; or c. coating the surface of the cylinder with a resist, scanning a focussed beam of light onto the resist to expose a pattern of proposed surface relief structures in the resist that is subsequently developed to expose the pattern on the surface of the cylinder.

1 1 . A method according to any one of the preceding claims, further including the step of depositing a layer of material over the surface relief structures to reduce a size of the microstructure.

12. A method according to claim 1 1 , the step of providing a deposited layer

comprises electroforming the layer of material.

13. A method according to claim 12, wherein the electroformed layer of material is a metal.

14. A method according to claim 12 or 13, wherein the electroformed layer of

material is deposited in a uniform layer.

15. A method according to claim 1 1 , the step of providing a deposited layer comprises chemical vapour deposition of the layer of material.

16. A method according to claim 1 1 , wherein the step of providing a deposited layer comprises depositing a "Diamond-like carbon" (DLC) coating as the layer of material.

17. A method according to any one of the preceding claims, wherein the cylinder is an embossing cylinder.

18. A computer program product storing instructions for manufacturing a cylinder having surface relief structures configured for producing at least one

microstructure image effect in at least one security device applied to a security document, the computer program product storing instructions for controlling a processor to control a device to form surface relief structures corresponding to the at least one microstructure image effect directly on a surface of the cylinder.

19. A computer program product according to claim 18, wherein the surface relief structures comprise microstructures having dimensions of substantially 5 microns or less.

20. A computer program product according to claim 18 or 19, wherein the cylinder is an embossing cylinder.

21 . A cylinder having surface relief structures configured for producing at least one microstructure image effect in at least one security device applied to a security document, the surface relief structures corresponding to the at least one microstructure image effects being formed directly on a surface of the cylinder.

22. A cylinder according to claim 21 , wherein the surface relief structures comprise microstructures having dimensions of substantially 5 microns or less.

23. A cylinder according to claim 21 or 22, wherein the cylinder is an embossing cylinder.

24. A cylinder produced according to the method of any one of claims 1 to 17.