PRODUCTION OF SECURITY DOCUMENT HAVING A METALLIC SECURITY FEATURE

Abstract: A method of making a reflective security feature on security paper comprises spraying metal onto the surface of the paper, whereby to produce an extensive region (9) of sprayed metal on the surface of the paper (2); and crushing, burnishing or otherwise treating the surface of the sprayed metal to convert it to a more specularly reflective surface, which may be formed into a relief pattern.

Description: PRODUCTION OF SECURITY DOCUMENT HAVING A METALLIC SECURITY FEATURE Field of the Invention This invention relates to the production of a metallic security feature on a printed security document such as a bank note, which may be based on a synthetic plastic or paper substrate. It is known that a reflective surface constituted by, for example, metallic foil is in appropriate circumstances an effective countermeasure against unauthorised reproduction of bank notes or security documents by means of photocopying, digital scanning and reprinting, as well as providing a visibly verifiable security feature. The thickness of an applied surface feature of this kind is in the range, generally, from 1 to 100 micrometres. There are stringent requirements for flexibility, strength of attachment, resistance to abrasion and resistance to chemical agents. Current methods of applying metallic security features to bank notes and security paper include: (i) the thermal transfer of a thin layer of metal to the surface by means of a hot foil embossing tape; (ii) the printing of a layer of adhesive followed by lamination of a metallic layer; (iii) the incorporation of a metallized foil or thread into the fabric of the bank note during the process of paper production in which the foil or thread is brought to the surface of the paper, i.e. the so-called 'windowing' process; (iv) the generation of a releasable metallic feature on a cylinder or flat surface, the metallic feature being then transferred and adhesively laminated to the surface of the bank note; and (v) direct vacuum metallization of selected regions of the surface of the paper. All these methods require rather expensive production equipment and with the exception of (iii) above, tend to be not well adapted to the speed of processing preferably characteristic of high volume security printing. Summary of the Invention The present invention is particularly intended for the production of a metallized stripe on security paper, so as to derive a security feature for a bank note or other security document. However, the invention may be applicable to the production of more complex patterns on paper or other substrates such as plastic cards. In the application of the invention to the production of a security feature on a paper substrate, the invention comprises two principal stages. The most important of these stages consists of the dry spraying of a metallic material on to the surface of the paper. A variety of techniques, such as gas flame spraying, electric arc spraying or plasma spraying may be employed, as further explained hereinafter. It may be desirable to treat a substrate such as paper with a sizing agent before the metal is sprayed. If the invention is employed during a paper-making process, the treatment of the paper with a sizing agent need not require an additional step because the paper may already contain a sizing agent. If a separate stage for a sizing agent were necessary or desirable, a sizing agent may be applied to the paper substrate before the metal is sprayed onto the paper substrate. The metal spraying produces a generally porous dull deposit of metal on the paper. It is found however that such a deposit, which may be in the form of a stripe or a more complex pattern, can readily be converted to a reflective state by for example crushing or burnishing and accordingly the second principal stage comprises the surface treatment of the sprayed metal in a manner which will convert it to a more specularly reflective state. The invention is particularly suitable in combination with the formation of a relief pattern in the manner of intaglio printing. Such a manner of printing, wherein the formation of the relief pattern is followed by the deposition of ink in the valleys of the pattern, has been known for many years past. The present invention allows for the formation of a relief or surface texture pattern, which may be formed by the use of an intaglio press or another pressure mechanism for the surface treatment of the deposited metal so that a single treatment stage may render the metal reflective and form a relief pattern. Brief Description of the Drawings Figure 1 illustrates in simplified schematic form a method according to the invention. Figure 2 illustrates part of a long paper web bearing a sprayed stripe. Figure 3 illustrates in simplified form a security document having a security device according to the invention. Description of Preferred Forms of the Invention The invention is particularly concerned with the production of a security feature in the form of a stripe composed of sprayed metal particles, preferably produced by the dry spraying of hot particles onto the substrate, which may be paper but could be a synthetic plastic, of a security document. Although metallization of paper and synthetic plastic substrates intended for security documents is known, known processes are comparatively expensive and generally are not well adapted to high speed or large volume manufacture. A particularly suitable form of a method according to the invention is one which produces a long, preferably continuous, stripe along the length of a long web of paper such as typically produced by a paper mill. A bank note printing machine or other high volume security printing machine commonly accepts long rolls of paper and prints, in a variety of styles and using a variety of printing techniques, bank notes or other security documents on the roll, typically in multiple rows and columns. Many modern machines or processes perform some printing or finishing stages wherein the web is cut up into individual sheets (usually containing an array of partially printed documents) and further printing processes are performed on the sheets which are, for example, finally cut up to provide the individual documents. It is known to print the documents in at least approximate register with elongate stripes, for example in the form of an embedded security thread running lengthwise of the paper web and embedded in the web during the manufacture of the paper substrate. One advantage of the invention is that it may employ equipment which is well established for the spraying of metal workpieces, with minor modifications, in a paper making process which also uses well established forms of equipment. In the scheme shown in Figure 1, a long web of paper is output from a supply 2. This could be a paper mill but if the invention is practised at the bank note printing stage, the reference numeral 2 may merely represent a supply system for a web of paper. The paper web is traversed at an appropriate speed through a metal spraying stage 3, wherein a spray gun 4 provides a spray 5 of hot metal particles onto the paper substrate. Thereafter the paper is traversed through a treatment stage 6, comprising in this example two rollers 7 and 8 of which the purpose will be described later. The metal spraying may be performed by a spraying machine in principle similar to machines currently used for the spraying of metallic material onto metal workpieces. One spraying process which would be suitable is electric arc spraying, wherein raw metallic material in the form of a pair of metallic wires is melted by an electric arc, atomised by a cone of compressed air, and propelled towards the workpiece. For the purposes of the present invention, the wires may be in the order of 1 or 2 millimetres in diameter. The method may be performed while the substrate is traversed relative to the spraying equipment and typically a traversing speed of 2 metres per minute is obtainable. Metal spraying equipment for this purpose is obtainable, for example, from Metallization Limited, Dudley, England. Another form of spraying which may be suitable is gas flame spraying, wherein raw material in the form of a single wire of cord or even powder is melted in a gas flame, the material being atomized by a cone of compressed air and propelled towards the workpiece. A further alternative is plasma spraying, wherein a plasma is created by an electric arc within the nozzle of a plasma gun and the arc gasses are formed into a plasma jet as it emerges from an anodic nozzle. Powder particles are injected into the jet where they melt and subsequently strike the surface of the workpiece. The workpiece can remain cool because the plasma is localized at the gun. Equipment suitable for these purposes is obtainable from Metallization Limited or from Metco Inc, Westbury, New York State. Using ordinary commercially available metal spraying equipment, the rate of atomization of the raw material tends to be rather high so that the width of deposit of material on a moving substrate would typically 50 to 60 millimetres if the paper were sprayed from a distance of 15 to 25 centimetres. It may therefore be preferable either to reduce the dimensions and adjust the operating conditions of the spraying apparatus to produce a narrower stripe, or to use a sacrificial mask. The width of the stripe may be reduced by using smaller wire diameters in an electric arc spraying process, lower wire feedrates, and/or by modification of dimensions of or pressure in the compressed air nozzle. A mask may be used adjacent to the paper or spaced from it. In the latter case, the stripe of material which is laid down will have a slightly irregular or feathered edge. A feathery edge effect is also obtainable by omitting the use of a mark entirely. A variety of metals may be employed. It is preferable to employ zinc or an alloy of zinc and tin for the sprayed metal. Other materials, such as aluminium, might be used although the cohesion of the metal particles deposited on paper is less strong than when zinc or an alloy of zinc and tin is used. When the invention is employed in a paper making process, the paper commonly includes a sizing agent which may improve the adhesion of the hot metal particles to the paper. It is possible to employ the invention without any sizing agent, but if it be necessary to employ a sizing agent, typically polyvinyl or other polymeric material which is capable of acting as a adhesive but has a low melting point may be employed. The sizing agent should be activated by the hot metal particles that land on it. The metallized stripe shows excellent adhesion to the paper surface when a sizing agent of the kind mentioned is used, but in a variety of circumstances a special sizing agent would not be necessary. The result of the spraying stage, as shown in Figure 1, is illustrated in Figure 2 wherein 2 denotes the long web of paper and the reference 9 denotes a stripe (which may be one of a plurality of stripes spaced across the width of the paper) extending lengthwise of the web 1. The stripe produced by the metal spraying has a dull, amorphous, appearance. The width d of the stripe and its marginal irregularity have been greatly exaggerated relative to the width W of the paper web. In typical practice the width d would be of the order of 10 millimetres and the width W of the paper would be of the order of 1 metre. The stripe may be subjected to surface treatment to change the surface into a more specularly reflective surface. This may be achieved employing the rollers 7 and 8 as a burnishing or polishing stage, the rollers 7 and 8 operating at different speeds. Alternatively, the rollers 7 and 8 may be constituted by an intaglio printing stage so that at least the stripe and preferably also adjacent regions of the substrate are selectively compressed to form an intaglio or relief pattern. The metallic stripe thereby may be converted into adjacent regions of reflective and dull surface within the borders of the metallic stripe. Figure 3 illustrates schematically a security document 10 having a paper substrate as previously described, a metallic stripe 9 composed of sprayed metal particles and extending across the document, and rendered at least partly reflective, and appropriate lettering 11 and other printing which distinguishes the bank note or security document.

Claims: CLAIMS 1. A method of making a reflective security feature on a substrate intended for a security document, comprising the steps of: spraying metal particles on to the surface of the substrate, whereby to produce an extensive region of sprayed metal on the surface of the substrate; and treating the the sprayed metal to convert it to a partly specularly reflective surface. 2. A method according to claim 1 wherein the spraying is performed by gas, plasma or electric arc spraying. 3. A method according to claim 1 or claim 2 wherein the metal is sprayed through a mask is interposed between the spray and the substrate. 4. A method according to any foregoing claim wherein the region of application of the sprayed metal is varied to produce a stripe of sprayed metal. 5. A method according to any foregoing claim wherein the treating step comprises forming a relief pattern by the selective compression of the region of sprayed metal. 6. A method according to claim 5 wherein the treating step comprises subjecting the region to compression in the manner of intaglio pressing. 7. A method according to any of claims 1 to 4 wherein the treating step comprises burnishing said region. 8. A substrate (2) intended for use as a security document, bearing on a surface of the substrate a security feature constituted by a region (9) of sprayed metal rendered at least partly specularly reflective. 9. A substrate according to claim 8 wherein the region (9) has at least one feathery or irregularly evanescent edge. 10. A substrate according to claim 8 or claim 9 wherein the said region is formed into a relief pattern. 11. A security document (10) carrying on the surface of a substrate of the document a security device (9) comprising a region of sprayed metal rendered at least partly specularly reflective. 12. A security document according to claim 11 wherein the region is formed into a relief pattern.

Inventors: CHORLEY BRIAN, FURLEY ROBERT JOHN, HARMAN ANTHONY DAVID, CHORLEY, BRIAN, FURLEY, ROBERT, JOHN, HARMAN, ANTHONY, DAVID
Applicants: BANK OF ENGLAND, CHORLEY BRIAN, FURLEY ROBERT JOHN, HARMAN ANTHONY DAVID, CHORLEY, BRIAN, FURLEY, ROBERT, JOHN, HARMAN, ANTHONY, DAVID, THE GOVERNOR & COMPANY OF THE BANK OF ENGLAND
Application year: 1996
Main classification: B42D15
IPC codes: B41M3, B42D15