AU2018101019A4 - Improvements to methods for security documents including micro image elements - Google Patents

Abstract

Abstract A method of printing a security document including a plurality of micro image elements formed from ink is provided. The ink has at least one variable ink parameter and at least one of the micro image elements has a target image element size. The method includes providing a substrate having a printing surface and at least one variable substrate parameter; and printing an image layer onto at least a portion of the printing surface of the substrate using a printing press having at least one variable printing press parameter; wherein at least one variable ink parameter, variable substrate parameter and/or variable printing press parameter is optimised in order to achieve the target image element size. Figure 4 A _ 504 Figure 5A 502' A' '-r504 Figure 5B

Description

IMPROVEMENTS TO METHODS FOR SECURITY DOCUMENTS INCLUDING

MICRO IMAGE ELEMENTS

Technical Field [0001] The present invention relates to methods for producing micro image elements on a substrate to be viewed through a lens array or other lens microstructure.

Background of Invention [0002] Security devices are applied to security documents or similar articles, such as identity cards, passports, credit cards, bank notes, cheques and the like and may take the form of diffraction gratings and similar optically detectable microstructures. Such security devices are difficult to falsify or modify, and are easily damaged or destroyed by any attempts to tamper with the document. Some of these security devices include focussing elements, such as micro lenses, which act to sample and magnify micro image elements and project imagery which is observable to a user for authentication purposes.

[0003] Micro lenses used in security documents, such as bank notes, must be very small in order to maintain both the requisite thinness of the document and a short focal length, i.e. to enable the micro lenses to focus on micro image elements deployed within their focal plane. Consequently, the micro image elements themselves must be printed using very small ink dot sizes. Typically, these very small ink dot sizes will approach, or in some cases even exceed, the resolution limits of standard printing presses.

[0004] Typically, micro lenses are formed by embossing a layer of UV curable lacquer, having a limited thickness. This limited thickness results in each micro lens having a maximum depth to which it can be embossed, and consequently the micro lens is also limited to enable focus to be maintained on the micro image elements provided on the substrate. The limited width of each micro lens, further places limits on the maximum size of a micro image element that can be placed in the focal plane of the micro lenses, to enable optical effects such as three dimensional images, flips, animations, integral images, moire and other optical effects to be achieved. Various maximum sizes of the micro image elements will apply, depending on the desired optical effect and the micro lens geometry employed.

[0005] The maximum micro image element size is accommodated by contrast switch optical effects, since these effects require the smallest number of image channels to be deployed beneath each micro lens. In a contrast switch optical effect, each micro lens projects a maximum brightness at one viewing angle and a minimum brightness at a complementary viewing angle. This optical effect is optimised using a maximum micro image element size, preferably equal to half the width of a single micro lens. The same effect may still be achieved with a maximum micro image element size greater than half the width of a single micro lens, but less than the full width of a single micro lens. As the size of the micro image element increases, the contrast of the image projected to the viewer will be reduced.

[0006] The ever increasing sophistication of counterfeiting operations requires continuous improvement in the design of security devices for protecting documents against such forgery. One such improvement would be to be able to consistently produce a contrast switch optical effect on a micro lens-based security feature. However, to create a contrast switch optical effect using conventional printing techniques, i.e. having a limited print resolution or ink dot size, typically requires modifications to be made to the micro imagery. For example, in the case of microimagery to be placed beneath micro lenses around 64 microns wide, the micro image element size required to achieve a maximum contrast is around 32 microns. Such sizes are difficult to print consistently with conventional printing techniques such as gravure, since this size is near the practical limit for gravure, and the cell geometry required to produce ink dots of this size cannot be consistently manufactured. Accordingly, significant print defects often result when attempting to print micro image elements of such dimensions.

[0007] Micro image elements that are a little larger, for example, lines around 47 microns wide, can be more consistently printed with gravure, since the cell geometry required to produce ink dots of this size can be manufactured relatively consistently within acceptable tolerances. However, the compromise is that micro image elements having this larger size combined with 64 micron wide lenses produces magnified images having a lower contrast which are therefore undesirable.

[0008] Therefore there is a need to develop printing methods that allow smaller micro image elements to be printed consistently.

[0009] A 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.

Summary of Invention [0010] According to an aspect of the present invention, there is provided a method of printing a security document including a plurality of micro image elements formed from ink, the ink having at least one variable ink parameter and at least one of the micro image elements having a target image element size, the method including: a. providing a substrate having a printing surface and at least one variable substrate parameter; and b. printing an image layer onto at least a portion of the printing surface of the substrate using a printing press having at least one variable printing press parameter; wherein at least one variable ink parameter, variable substrate parameter and/or variable printing press parameter is optimised in order to achieve the target image element size.

[0011] According to an embodiment, the at least one variable ink parameter, variable substrate parameter and/or variable printing press parameter is optimised by empirical or theoretical modelling means.

[0012] The substrate may be transparent, translucent or opaque. In some embodiments, the substrate is paper, or plastic or polymeric material.

[0013] In some embodiments, a plurality of focussing elements are provided on the substrate, through which at least some of the micro image elements printed by the method of the present invention are observed.

[0014] The at least one variable ink parameter may be selected from one or more of the following: mass loading, viscosity, shear rate, surface tension; or contact angle to the printing surface.

[0015] The at least one variable substrate parameter may be surface energy.

[0016] In a particular embodiment, the at least one variable ink parameter is surface tension and/or contact angle to the printing surface and the variable ink parameter is optimised by theoretical modelling means based on Young’s Equation: wherein:

YLG = surface energy of the ink/air interface; ΘΕ = contact angle; YSG= surface energy of substrate/air interface; and YSL = surface energy of substrate/ink interface.

[0017] The variable printing press parameters may include one or more of speed or pressure.

[0018] In various embodiments, the printing press is selected from one of the following: a gravure printing press; a flexographic printing press; or an offset printing press.

[0019] In forms of the invention in which the printing press is a gravure printing press or a flexographic printing press, the variable printing press parameter may be selected from one or more of the following: impression roller hardness; wiping blade tip angle; wiping blade tip geometry; wiping blade tip pressure; or cell properties.

[0020] A cell property may be selected from one or more of the following: geometry; aspect ratio; roughness; emptying efficiency; or surface energy.

[0021] Cell geometry may include one or more of cell volume, cell shape, cell width, cell depth or cell side wall angle.

[0022] In forms of the invention in which the printing press is a flexographic printing press, the variable printing press parameter may be selected from one or more of printing plate hardness and printing plate mounting tape hardness.

[0023] In forms of the invention in which the printing press is an offset printing press, the variable printing press parameter may be selected from one or more of the following: blanket hardness; ink tack; ink key settings; plate hardness; ink-to-water balance; or impression pressure.

[0024] In certain embodiments, the substrate surface energy is reduced to achieve a smaller target image element size.

[0025] In other embodiment, the cell volume is increased and the substrate surface energy is reduced to achieve a smaller target image element size.

[0026] In still other embodiments, the cell volume is increased and the ink surface tension is increased to achieve a smaller target image element size.

[0027] The cell volume and ink surface tension may each be increased and the substrate surface energy reduced to achieve a smaller target image element size.

[0028] The ink surface tension may be increased to achieve a smaller target image element size.

[0029] The ink contact angle and the cell volume may be optimised to achieve the target image element size.

[0030] In some embodiments, an intervening layer of ink is applied to the printing surface of the substrate to form a modified printing surface to which the ink forming the image elements is applied, wherein the surface energy of the intervening layer of ink is optimised to achieve the target image element size. The intervening layer of ink may be transparent or translucent.

Definitions

Security Document or Token [0031] 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.

[0032] 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.

Substrate [0033] 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 (PP), polyethylene (PE), polycarbonate (PC), polyvinyl chloride (PVC), polyethylene terephthalate (PET); 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.

[0034] 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”.

Security Device or Feature [0035] 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. 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 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 (OVDs) such as diffractive devices including diffraction gratings, holograms and diffractive optical elements (DOEs).

Comprise, Comprises, Comprised or Comprising [0036] 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.

Brief Description of Drawings [0037] 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: [0038] Figurel shows an exemplary apparatus for printing a security document.

[0039] Figure 2 shows a security document including a plurality of security elements.

[0040] Figure 3 shows a security document including a security element including a plurality of micro image elements according to an embodiment.

[0041] Figure 4 shows a section through the security document of Figure 3.

[0042] Figures 5A and 5B each show a droplet of ink of the same volume deposited on a printing surface, with the droplet depicted in Figure 5A producing a larger feature size than the droplet depicted in Figure 5B

[0043] Figure 6 shows a simple geometry formulae for determining the volume of an ink droplet assuming that the ink droplet assumes the shape of a truncated sphere.

[0044] Figures 7A and 7B show example of a printed design comprising parallel lines for implementing a contrast switch optical effect, with the surface energy to the of the printing surface having been optimised for the substrate shown in Figure 7A to achieve a reduced target image element size.

Detailed Description [0045] Referring firstly to Figure 1, there is shown an exemplary rotary printing apparatus 100 for manufacturing a security document 200 (shown in Figure 2). A continuous web of substrate 102 comprising, for example, translucent or transparent material such as polypropylene or PET, i.e. is subject to an adhesion promoting process at a first processing station 104 including a roller assembly. Suitable adhesion promoting processes include flame treatment, corona discharge treatment, plasma treatment or similar.

[0046] An adhesion promoting layer is applied at a second processing station 106 including a roller assembly. A suitable adhesion promoting layer is one specifically adapted for the promotion of an adhesion of radiation curable coatings to polymeric surfaces. The adhesion promoting layer may have a radiation curing layer, a solvent-based layer, a water-based layer or any combinations of these.

[0047] At a third processing station 108 which also includes a roller assembly, the radiation curable coating is applied to the surface of the adhesion promoting layer.

The radiation sensitive coating can be applied via gravure printing, flexographic printing, or a silk screen printing process and variations thereof amongst other printing processes.

[0048] The radiation curable coating is only applied to the security element areas 202 on a surface 204 where a structure including an array of micro lenses (see Figure 4) is to be positioned. The security element areas 202 can 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.

[0049] While the radiation curable ink remains at least partially liquid, it is processed to form the structure (see Figure 4) at a fourth processing station 110. In one embodiment, the processing station 110 includes an embossing roller 112 for embossing a security elements structure, such as the focussing elements (shown in Figure 4) into the radiation curable coating. The cylindrical embossing surface 114 has surface relief formations corresponding to the shape of the focussing elements to be formed. The apparatus 100 can, for example, form micro lenses in a variety of shapes.

[0050] The radiation curable ink on the substrate is brought into contact with the cylindrical embossing surface 114 of the roller 112 by a nip roller 116 at the processing station 110, such that the liquid radiation curable ink flows into the surface relief structures of the embossing surface 114. At this stage, the radiation curable ink is exposed to radiation to permanently fix the embossed structures, by transmission through the substrate layer 206 (see Figure 3).

[0051] With the security element structure 202 applied to the substrate 204, one or more additional layers are applied at a downstream processing station including further roller assemblies 118 and 120. The additional layers may be clear or pigmented coatings and applied as a partial coating, as a contiguous coating or accommodation of both.

[0052] The roller assemblies 118 and 120 at the downstream processing station, comprise a printing press that are used to apply an image layer. The image layer may comprise continuous or discontinuous imagery. The imagery maybe printed using rotary printing processes such as gravure, flexography and offset.

[0053] Referring now to Figure 2, a security document 200 is shown including security element areas 202. It will be appreciated that having more than one security element area 202, is optional. The security elements 202 can be constructed as a separate element to the security document, allowing it to be applied to the security document 200, or may be formed integrally therewith.

[0054] The security element 202, as shown in Figure 3, includes a viewing side 208 and a printing side or printing surface 210. The viewing side 208 is arranged such that the printing surface 210 can be observed through the substrate 206 from the viewing side 208. For example, an opacifying layer can be applied to the viewing side 208 except in the area of interest to create a window or half-window on the security element 202. In one embodiment, image elements 212 are observable only from the viewing side 208, however, in general the image elements 212 can be observed from either one or both sides 208, 210. Each image element 212 can correspond to part of one or more images. The image elements 212 may be opaque, translucent, or transparent.

[0055] The image elements 212 are formed from a layer of ink applied to the printing surface 210 of the substrate 206 using a rotary printing process, for example gravure printing, offset or flexography printing.

[0056] Referring now to Figure 4, and as described with reference to Figure 1, the viewing side 208 includes a plurality of focusing elements 214. The focusing elements 214 can be micro lenses. The focusing elements 214 may be configured to have a focal length the same as, or substantially the same as, the depth of the substrate 206. Alternatively the focal length can be longer or shorter than the thickness of the substrate 206.

[0057] The present invention is directed to improved methods of printing micro image elements 212 for deployment beneath micro lenses 214 to produce security elements 202 producing a wide variety of optical effects. Such micro image elements 212 are at least in part defined by dimension and when viewed through focussing elements, such as micro lenses 214, produce optical effects such as image flips, contrast switches, animations, morphing effects, three dimensional floating and/or receding images, moire magnification effects and combinations thereof.

[0058] Particular challenges are encountered in printing micro image elements 212 of sufficient resolution and integrity to produce high fidelity and high complexity micro-optical effects. These are addressed by the improvements provided by the printing method of the present invention.

[0059] The improved printing method of the present invention optimises at least one of a variable ink parameter, a variable substrate parameter and/or a variable printing press parameter in order achieve a target image element size. That is, by optimising one or more variable parameters associated with at least one of the ink, the substrate, or the printing press, it is possible to achieve an improvement in print resolution than would otherwise be achievable using a conventional printing press without optimising the selected variable parameters. The target image element size is the size of the micro image element 212, necessary to achieve the desired optical effect with sufficient contrast and fidelity.

[0060] Referring now to Figures 5A and 5B, this may be understood, for example, by considering the exemplary case of a gravure printing press. In this case, the ultimate size of the printed micro image element 212 is dependant, at least in part, on the volume of ink that is released from the gravure cells and/or the contact angle of the ink to the printing surface. That is, for a given volume of ink, provided in a droplet of ink 502 that is deposited on the substrate 504 , the smaller the contact angle 506, the larger will be the extent of the area A, A’ upon which the ink 502 is deposited and consequently, the resulting printed feature size will be larger. This is illustrated in Figures 5A and 5B, which two droplets of ink of identical volume, with the droplet 502 depicted in Figure 5A producing a larger feature size than the droplet 502’ depicted in Figure 5B.

[0061] In a gravure printing process, the contact angle 506 is dependent on the surface energy of the substrate/air interface, the surface energy of the substrate/ink interface, and the surface energy of the ink/air interface. This interdependence can be quantified by Young’s equation as follows: wherein:

YSL = surface energy of substrate/ink interface.

[0062] Improved print resolution can be achieved by optimising a variety of variable parameters. The various parameters, which are exemplified and discussed in more detail below, are optimised or tuned by empirical or theoretical modelling means. Young’s Equation above provides one example of theoretical modelling for the variable ink parameter that is contact angle, and the variable substrate parameter that is surface energy of the substrate.

[0063] A variety of variables ink parameters are applicable to a range of printing processes including gravure, flexographic and offset as well as others that may be employed to print security features on security documents. Examples of ink parameters include mass loading, i.e. the distribution of ink particles, viscosity, shear rate, i.e. the rate of angular deformation, surface tension or contact angle to the printing surface (as exemplified in Figures 5A and 5B).

[0064] Additionally, substrate parameters will be similarly applicable to a range of printing processes. Examples of substrate parameters include surface energy. For example, a higher surface energy substrate will tend to draw more ink from an ink cell compared to lower surface energy substrates.

[0065] For example, one means of varying the surface energy of the substrate is by applying an intervening layer of ink to the printing surface of the substrate to form a modified printing surface. The ink forming the image elements is subsequently applied to the modified printing surface, the surface energy of the intervening layer of ink having been optimised to achieve the target image element size. This enables the surface energy of the substrate to be optimised or otherwise modified, without requiring the composition of substrate itself to be altered in anyway. The intervening layer of ink may be transparent or translucent but may also be opaque if lenses are subsequently applied to the intervening layer.

[0066] Printing press parameters can be classified as either generic, such that they are applicable to a range of printing processes, whilst others are specific to particular print processes. For example, press parameters including speed and pressure are universally applicable to various print processes including gravure, flexographic and offset. For instance, speed determines how much shear is applied to the ink in a cell and will impact on the emptying efficiency of the cell in printing techniques that use cells to deposit ink.

[0067] Other printing parameters will be specific to particular types of printing presses. For examples, cell properties can only be optimised for printing processes which rely on cells to deposit ink on the substrate, e.g. gravure and flexographic.

Blade tip angle, blade tip geometry and blade pressure can only be optimised in printing presses that rely on a blade to wipe excess ink from the roller, e.g. gravure and flexographic.

[0068] More specifically, variable cell parameters include a wide variety of cell properties that can be optimised to improve printing resolution. These include but are not limited to volume, shape, width, depth, side wall angle, roughness, surface energy, aspect ratio (ratio of cell depth to cell width), i.e. high aspect ratio cells require a given volume of ink to be moved a greater distance compared to a lower aspect ratio cell, and emptying efficiency i.e. depends on the surface area to volume ratio of the cell - shallow cells have greater ratios and therefore will have larger adhesive forces between the cell and ink, resulting in lower proportion of ink released being from the cell during the printing process.

[0069] Once the ink is released from the cell, it will spread on the substrate. The contact angle of the wet ink droplet can be calculated using Young’s equation, assuming that the respective surface energy parameters are known. Alternatively, the contact angle may be measured experimentally.

[0070] Referring now to Figure 6, if the volume of ink in the cell is known and the wet ink droplet geometry is assumed to be spherical, this data together with the contact angle data, can be used to determine the size of the wet ink droplet on the substrate. Assuming the wet ink droplet 602 assumes the shape of a truncated sphere, the width of the truncated sphere, i.e. the size of printed image element, is equal to 2a, can be calculated using the known contact angle Θ and the known volume V of the ink droplet using simple geometry formulae as illustrated in Figure 6. The volume of a spherical cap can be calculated as:

[0071] It will be understood that the volume of the ink droplet may be determined empirically, or by using a theoretical model of the printing process. The ink droplet geometry is assumed to be spherical since the ink droplet tends to minimise its surface area, this ink droplet behaviour may also be theoretically modelled or observed empirically. Accordingly, in some embodiments, the invention involves tuning one or both of the ink droplet volume and/or contact angle, using a theoretical model and/or empirically, to achieve a target image element size.

[0072] In various embodiments, the invention involves tuning theoretically and/or empirically, one or more of the following parameters, to print a target image element size: • surface energy of the ink/air interface; and • surface energy of the printing surface/air interface; • surface energy of the printing surface/ink interface; or • cell volume transferred to the substrate [0073] Referring now to Figures 7A and 7B, there are shown examples of a printed design 700, 700’ comprising parallel lines 702, 702’ for implementing a contrast switch optical effect. In Figure 7A, the average line 702 width is 38 microns.

In Figure 7B, the average line 702’ width is 47 microns. The same design was printed with the same ink, with the printing surface used in Figure 7A having a different surface energy to the printing surface used in Figure 7B. The printing parameters and cell geometry of the gravure printing press used to print the designs was the same, and consequently, the volume of ink delivered to the printing surface per ink droplet was consistent. In both examples, the printed ink was air dried. The variation in the appearance of the design in Figure 7A 700 compared with Figure 7B 700’ is therefore a consequence of the difference in surface energy of the printing surface. The surface energy of the printing surface 704 in Figure 7A was lower, at 36 dyne/cm, than the surface energy of the printing surface 704’ in Figure 7B, at 43 dyne/cm. As a result of the difference in surface energy, the contact angle of the wet ink droplets was higher for the printing surface 704 in Figure 7A, thus reducing the extent of ink spreading.

Consequently, a line having line width reduced by 20% was achieved using identical cell geometry and printing parameters. Compared to the lines in Figure 7A, the smaller line widths in Figure 7B produced a contrast switch optical effect with greater contrast in the projected image.

[0074] An alternative approach for reducing the target image element size is to increase the surface tension of the ink. Flowever, care must be taken to ensure the emptying efficiency of the cell is not compromised by the increase in surface tension.

If the emptying efficiency is unsatisfactory, it may be necessary to vary the geometry of the cell. For example, the cell volume may need to be increased, to ensure the volume of ink deposited on the substrate is sufficient to produce an image element of the target size.

[0075] Employing the method of the present invention, it is possible to increase the contrast of a magnified optical image effect, without reducing the printed image element size. Rather the enhanced contrast is achieved by adjusting the ink parameters to increase the ink solid content, preferably without significantly varying the following ink parameters: viscosity, shear rate, transferred ink volume per cell, surface tension and target image element size. In particular, the inventors found that use of a radiation curable ink, and more specifically, a UV-curable ink can significantly increase the contrast of the magnified optical image effect, even if the target image element size is greater than the optimal size.

[0076] For example, to produce a contrast switch optical effect using 64 micron wide lenses, the optimal image element size is 32 microns. Using ink having a 20% solids content to print 47 micron wide lines, the resulting contrast in the magnified optical image effect is less than preferred. Flowever, using a UV-curable ink having a solids content of 77%, and printed at the same viscosity using the same gravure cylinder, i.e. the printing surface and printing press were constant, to print lines of the same width, i.e. around 47 microns wide, it is possible to achieve improved contrast in the resulting magnified optical image effect, even though the line width still exceeds the optimal image element size (32 microns).

[0077] Additionally, the method of the present invention has been found to reduce “drying in” print faults that are typical to lens-based micro-imagery. Specifically, the inventors found that in some circumstances, the use of a radiation curable ink, and more specifically, a UV-curable ink may eliminate “drying in” print defects in microimagery for lens-based optical effects on thin substrates.

[0078] When printing micro image elements and image elements more generally using cell based printing techniques including gravure and flexographic, using conventional air-drying inks, the ink may dry inside the gravure cell during the printing process. This has the consequence of reducing the volume of the cell, which in turn can lead to portions of the image not being printed, or, portions of the image may be printed, but the image element size will be less than it should be, causing defects in the magnified image and a resulting optical image effect that will be perceived as being of poor quality.

[0079] This problem poses a particular challenge when printing micro image elements for lens-based optical effects in thin substrates, since the cell size used, for example in a gravure or flexographic printing press, is relatively small, and even a very small volume of ink that dries inside the cell can significantly impact the filling and emptying efficiency of the cell.

[0080] However, use of radiation curable inks avoids this problem of “drying in” since such inks cannot be dried by air, but rather only by exposure to a radiation light source, e.g. UV.

[0081] It will be understood that the variable parameters that may be optimised, either empirically or via modelling, will depend at least in part on the respective printing process, or the type of printing press that is employed to print the security document including a plurality of micro image elements formed from ink.

[0082] For example, wherein the printing process is a gravure printing process, then the variable parameters that are available for optimisation, include the variable gravure printing press parameters including, for example, impression roller hardness, wiping blade tip angle, wiping blade tip geometry, wiping blade tip pressure, or gravure cell properties. Gravure cell properties may include one or more of the following variable parameters: geometry, aspect ratio, roughness, emptying efficiency, or surface energy. Gravure cell geometry includes includes one or more of cell volume, cell shape, cell width, cell depth or cell side wall angle. It will be understood that one or more of the foregoing parameters are optimised to achieve the target image element size.

[0083] In another example, wherein the printing process is a flexographic printing process, then the variable parameters that are available for optimisation, include the variable flexographic printing press parameters including, for example, printing plate hardness and printing plate mounting tape hardness, impression roller hardness, wiping blade tip angle, wiping blade tip geometry, wiping blade tip pressure, or cell geometry or cell volume of the Anilox roller. It will be understood that one or more of the foregoing parameters are optimised to achieve the target image element size.

[0084] In yet another example, wherein the printing process is an offset printing process, then the variable parameters that are available for optimisation, include the variable offset printing press parameters including, for example, blanket hardness, ink tack, ink key settings, plate hardness, ink-to-water balance, or impression pressure. It will be understood that one or more of the foregoing parameters are optimised to achieve the target image element size.

[0085] It will be understood that common to all printing techniques, are variable parameters relating to ink and substrate properties. For example, variable ink parameters include mass loading, viscosity, shear rate, surface tension, or contact angle to the printing surface. A variable substrate parameter is surface energy. It will be understood that one or more of the foregoing parameters are optimised to achieve the target image element size.

[0086] The method of printing a security document including a plurality of micro image elements formed from ink, wherein at least one variable ink parameter, variable substrate parameter and/or variable printing press parameter is optimised in order to achieve the target image element size, has various advantages over conventional printing methods. Specifically, it is possible to provide a method for producing smaller image elements using conventional printing presses, by optimising one or more variable parameters to achieve a target image element size that is reduced when compared to the image element size conventionally produced by the same printing press.

[0087] The cell size that was used can be reliably manufactured, and by simply changing the printing surface to one with a lower surface energy, we have achieved a smaller printed feature size, without having to use smaller cells that cannot be reliably manufactured.

[0088] Additionally, the method of the present invention increases the contrast in magnified optical effect. For example, an increase in contrast may be achieved by increasing the mass loading of the ink in a gravure printing press, without varying the image element size, the cell geometry, ink viscosity, ink shear rate, transferred ink volume per cell, or ink surface tension. One means for increasing the mass loading of the ink, could be, for example, by using radiation curable ink, e.g. UV curable ink. In a particular example, the mass loading of the ink is at least 70%.

[0089] Additionally, the method of the present invention reduces “drying in” print faults in micro image elements. For example, by using radiation curable inks, e.g. UV curable ink, “drying in” print defects can be substantially eliminated.

[0090] 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 alternative, modifications and variations in light of the foregoing description are possible. Accordingly, the present invention is intended to embrace all such alternative, modifications and variations as may fall within the spirit and scope of the invention as disclosed.

[0091] 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 (20)

1. The claims defining the invention are as follows

   1. A method of printing a security document including a plurality of micro image elements formed from ink, the ink having at least one variable ink parameter and at least one of the micro image elements having a target image element size, the method including: a. providing a substrate having a printing surface and at least one variable substrate parameter; and b. printing an image layer onto at least a portion of the printing surface of the substrate using a printing press having at least one variable printing press parameter; wherein at least one variable ink parameter, variable substrate parameter and/or variable printing press parameter is optimised in order to achieve the target image element size.
2. 2. A method of printing a security document including a plurality of micro image elements formed from ink according to claim 1, wherein the at least one variable ink parameter, variable substrate parameter and/or variable printing press parameter is optimised by empirical or theoretical modelling means.
3. 3. A method of printing a security document including a plurality of micro image elements formed from ink according to claim 1 or 2, wherein the at least one variable ink parameter is selected from one or more of the following: a. mass loading; b. viscosity c. shear rate; d. surface tension; or e. contact angle to the printing surface.
4. 4. A method of printing a security document including a plurality of micro image elements formed from ink according to any one of claims 1 to 3, wherein the at least one variable substrate parameter is surface energy.
5. 5. A method of printing a security document including a plurality of micro image elements formed from ink according to claim 4 when dependent on claim 3, wherein the at least one variable ink parameter is surface tension and/or contact angle to the printing surface and the variable ink parameter is optimised by theoretical modelling means based on Young’s Equation: wherein:

   Yiq = surface energy of the ink/air interface; ΘΕ = contact angle; Ysg= surface energy of substrate/air interface; and YSL = surface energy of substrate/ink interface.
6. 6. A method of printing a security document including a plurality of micro image elements formed from ink according to any one of the preceding claims, wherein the variable printing press parameters include one or more of speed or pressure.
7. 7. A method of printing a security document including a plurality of micro image elements formed from ink according to any one of the preceding claims wherein the printing press is one of the following: a. a gravure printing press; b. a flexographic printing press; or c. an offset printing press.
8. 8. A method of printing a security document including a plurality of micro image elements formed from ink according to claim 7, wherein the printing press is a gravure printing press or a flexographic printing press and the variable printing press parameter is selected from one or more of the following: a. impression roller hardness; b. wiping blade tip angle; c. wiping blade tip geometry; d. wiping blade tip pressure; or e. cell properties.
9. 9. A method of printing a security document including a plurality of micro image elements formed from ink according to claim 8, wherein a cell property is selected from one or more of the following: a. geometry; b. aspect ratio; c. roughness; d. emptying efficiency; or e. surface energy.
10. 10. A method of printing a security document including a plurality of micro image elements formed from ink according to claim 9, wherein cell geometry includes one or more of cell volume, cell shape, cell width, cell depth or cell side wall angle.
11. 11. A method of printing a security document including a plurality of micro image elements formed from ink according to claim 7, wherein the printing press is a flexographic printing press and the variable printing press parameter is selected from one or more of printing plate hardness and printing plate mounting tape hardness.
12. 12. A method of printing a security document including a plurality of micro image elements formed from ink according to claim 7, wherein the printing press is an offset printing press and the variable printing press parameter is selected from one or more of the following: a. blanket hardness; b. ink tack; c. ink key settings; d. plate hardness; e. ink-to-water balance; or f. impression pressure.
13. 13. A method of printing a security document including a plurality of micro image elements formed from ink according to claim 4 or 5, wherein the substrate surface energy is reduced to achieve a smaller target image element size.
14. 14. A method of printing a security document including a plurality of micro image elements formed from ink according to claim 10 when dependent on claim 4 or 5, wherein the cell volume is increased and the substrate surface energy is reduced to achieve a target image element size.
15. 15. A method of printing a security document including a plurality of micro image elements formed from ink according to claim 10 when dependent on claim 3, wherein the cell volume is increased and the ink surface tension is increased to achieve a target image element size.
16. 16. A method of printing a security document including a plurality of micro image elements formed from ink according to claim 10 when dependent on claims 3 and 4, wherein the cell volume and ink surface tension are each increased and the substrate surface energy is reduced to achieve a target image element size.
17. 17. A method of printing a security document including a plurality of micro image elements formed from ink according to claim 3, wherein the ink surface tension is increased to achieve a smaller target image element size.
18. 18. A method of printing a security document including a plurality of micro image elements formed from ink according to claim 10 when dependent on claim 3, wherein the ink contact angle and the cell volume are optimised to achieve the target image element size.
19. 19. A method of printing a security document including a plurality of micro image elements formed from ink according to any one of claims 1 to 18, wherein an intervening layer of ink is applied to the printing surface of the substrate to form a modified printing surface to which the ink forming the image elements is applied, wherein the surface energy of the intervening layer of ink is optimised to achieve the target image element size.
20. 20. A method of printing a security document including a plurality of micro image elements formed from ink according to claim 19, wherein the intervening layer of ink is transparent or translucent.