Patent Description:
Security documents and booklets, such as passports, passbooks, identification documents, certificates, licences, cheque books and the like, commonly comprise one or more security sheets for preventing successful counterfeiting. Security threads are particularly suitable security features for security sheets as they can carry yet further security features. Furthermore, security threads are securely embedded within the security sheet such that it is either difficult for a counterfeiter to remove them or any such removal is identifiable to a document inspector. If the security sheet comprises a fibrous substrate or paper, the security thread is commonly at least partially embedded in it during the paper making process, for example as disclosed in <CIT>, <CIT> and <CIT>. The security thread may also be "windowed" by being covered and not visible in areas by "bridges" of fibrous substrate and exposed and visible at "windows". Windowed security threads improve resistance to counterfeiting because they are difficult to remove without tearing the fibrous substrate and are difficult for a counterfeiter to imitate.

If the security sheet comprises a plastic, such as polycarbonate, the security thread is typically introduced between two plastic layers and embedded therebetween by lamination. The security thread may, for example, be adhered to a plastic layer prior to lamination, as disclosed in <CIT>, or comprise an outer plastic layer that fuses with the adjacent plastic layers during lamination, as disclosed in <CIT>. Alternatively, the security thread may be coextruded such that it is integrally formed with one of the plastic layers, as disclosed in <CIT>. However, the incorporation of the security threads is relatively simple in such arrangements, such that they are not particularly difficult for a counterfeiter to imitate.

In addition, the aforementioned methods do not enable the provision of a windowed security thread in a plastic security sheet as in a paper security sheet. <CIT> discloses a security thread located in a transparent region extending between the front surface of a plastic security sheet and an internal opaque layer with a transparent region or window. When viewing the rear surface of the security sheet, the security thread is blocked from view by the opaque layer, but visible through the window. However, again, such an arrangement is not particularly difficult to counterfeit. Furthermore, the need for an additional opaque layer on one side of the security thread to partially hide it severely restricts the design options available for aesthetic purposes and other security features. <CIT> discloses an identification card which has the description and photograph of the party to be identified laminated between sheets of clear plastic material.

An object of the present invention is therefore to provide a method of securely incorporating a security element, and particularly a security thread, into a plastic security sheet with improved counterfeiting prevention. A further object is to provide a security sheet that is more difficult to imitate by a counterfeiter. A yet further object may also be to provide an improved method of providing a windowed security element in a plastic security sheet.

The present invention therefore provides a core structure, stack, security sheet, security document and method of manufacturing a security sheet in accordance with the claims.

The method may preferably comprise laminating together the at least one core layer and security element (i.e. the core structure) and, if present, at least one further plastic layer. Lamination forms a strong bond between the layers such that it is very difficult for a counterfeiter to separate the layers and remove or tamper with the security element. Such difficulty is increased if a layer of material of the security element is selected to fuse with, and thereby form a strong bond with, the at least one core layer and/or at least one further plastic layer.

The method comprises forming at least one perforation as a tab and feeding the security element to extend through the or each at least one perforation twice by locating the first regions over the first core surface of the tab and locating second regions over the second core surface adjacent the tab, or vice-versa. Such an arrangement enables the security element to be fed into the at least one core layer from one side of the at least one core layer. As a result, the process of inserting the security element into the at least one core layer may be performed using a continuous line of security element in a continuous manufacturing process, thereby significantly increasing the speed of manufacture.

The stack is for forming a security sheet comprising the core structure disclosed herein and at least one further layer comprising plastic overlying the first and/or second core surface. Preferably the at least one further layer comprises at least one transparent region through which at least one of the first and/or second regions of the security element is visible in reflected light incident upon the at least one transparent region. The at least one further layer preferably comprises a hinge portion extending from the core structure and, if present, other further layer(s).

The security sheet comprises the core structure or stack disclosed herein, the core structure or stack being laminated together. Preferably the plastic substrate comprises first and second outer surfaces, at least one first region of the security element is not visible in reflected light incident upon the second outer surface and/or at least one second region of the security element is not visible in reflected light incident upon the first outer surface.

The first regions may be at a first distance from the first outer surface and the at least one second region may be at a second distance from the first outer surface, the first and second distances being different. The first regions and at least one second region of the security element are preferably connected by at least one third region of the security element. The first and second regions are preferably substantially parallel to the first and second outer surfaces. The at least one third region is preferably substantially non-parallel to the first and second outer surfaces. The plastic substrate preferably comprises a plurality of plastic layers laminated together. At least one first and/or second region of the security element is preferably separated from the first and second outer surface such that the security element is embedded within the plastic substrate.

The security document comprises the security sheet disclosed herein. The security document may be a passport, driving licence, certificate, passbook, identification document, licence or cheque book. In a plurality of security documents, personal data on the security sheet of each security document is different to personal data on the security sheet of other security documents.

Since the security element extends through the at least one core layer and/or is embedded within the plastic substrate at a variable distance along its length, it is substantially mechanically locked therein and is thus very difficult for a counterfeiter to remove. Any removal is likely to result in destroying the at least one core layer, the security element and/or the plastic substrate. Furthermore, the complexity of manufacturing the security sheet is increased, thereby further discouraging attempted imitation by a counterfeiter.

In preferred embodiments at least part of the at least one core layer is opaque adjacent to at least one first and/or second region of the security element. As a result, the at least one first region is not visible in reflected light incident upon the second core surface (or the second outer surface) and/or at least one second region is not visible in reflected light incident upon the first core surface (or the first outer surface). The opaque area of the at least one core layer partially blocks the security element from view in reflected light at the first and second core surface and the first and second outer surface, thereby resulting in a windowed effect. For instance, when the first core surface or first outer surface is viewed a plurality of first regions (i.e. corresponding to the windows of windowed threads in paper) may be visible with gaps (i.e. corresponding to the bridges of windowed threads in paper), formed of the at least one core layer, between them. The second region(s) between the first regions are blocked from view at the first core surface or first outer surface by the opaque area. As a result, the present invention provides an effective method of providing the windowing effect in a plastic security sheet. In addition, the complexity of imitation by counterfeiters is increased by the windowing effect and an improved aesthetic effect is achieved. Furthermore, the security element may be visible at the first or second core surface when the first or second core or outer surfaces are viewed in transmitted light (i.e. light passing through the core structure or security sheet to the eye).

Preferably the at least one perforation is preformed, cut, die cut and/or punched in the at least one core layer. Preferably the security element is fed to extend through the at least one perforation.

The security element is preferably an elongate security thread. However, the security element may alternatively comprise a security patch, foil, substantially opaque body or the like. Furthermore, the core structure may comprise a plurality of security elements, each extending through the at least one core layer from the first core surface to the second core surface and comprising first regions overlying part of the first core surface and at least one second region overlying part of the second core surface.

By way of example only, embodiments of a core structure, stack, security sheet, security document and method of manufacture in accordance with the present invention are now described with reference to, and as shown in, the accompanying drawings, in which:.

The security sheet according to the present disclosure generally comprises a core structure having at least one core layer, at least one perforation, the or each perforation defining a tab, and a security element extending through the at least one perforation. At least one further layer may be applied to one or both sides of the core structure in a stack. The security sheet is formed of a plastic substrate by laminating the core structure and/or at least one further layer. The security sheet may subsequently form and/or be incorporated into a security document.

<FIG> illustrate an embodiment of a core structure <NUM> comprising at least one core layer <NUM> and a security element <NUM>, in this example a security thread, embedded therein. The at least one core layer <NUM> comprises opposing first and second core surfaces <NUM>, <NUM>, which may be the surfaces <NUM>, <NUM> of a single core layer <NUM> as illustrated or the outer surfaces of outer layers of a plurality of core layers <NUM>. The at least one core layer <NUM> comprises plastic. The at least one core layer <NUM> comprises a thickness between the first and second core surfaces <NUM>, <NUM> and extends across a first width <NUM> and, in a perpendicular direction, a second width <NUM>.

At least one perforation <NUM> is formed in the at least one core layer <NUM> extending from the first core surface <NUM> to the second core surface <NUM> through the thickness of the at least one core layer <NUM>. Adjacent perforations <NUM> are at locations separated by a gap along the first and second core surfaces <NUM>, <NUM>. The or each perforation <NUM> defines a tab <NUM> attached to the rest of the at least one core layer <NUM> at a connection <NUM>, which may operate as a hinge or pivot point of the tab <NUM>. The or each tab <NUM> extends from the connection <NUM> in a direction parallel to the second width <NUM>.

The security element <NUM> extends through the at least one core layer <NUM>, through each at least one perforation <NUM>, from the first core surface <NUM> to the second core surface <NUM>. The security element <NUM> thereby comprises first regions <NUM> overlying part of the first core surface <NUM> and at least one second region <NUM> overlying part of the second core surface <NUM>. The security element <NUM> further comprises at least one third region <NUM> extending through the at least one core layer <NUM> and connecting first and second regions <NUM>, <NUM>.

The security element <NUM> preferably comprises a plurality of first, second and third regions <NUM>, <NUM>, <NUM> as in the illustrated embodiment. The security element <NUM> extends through the at least one core layer <NUM> at a plurality of locations, each at a perforation <NUM>, and, between adjacent locations, comprises a first region <NUM> overlying part of the first core surface <NUM> or a second region <NUM> overlying part of the second core surface <NUM>. Therefore, the security element <NUM> overlies the first and second core surfaces <NUM>, <NUM> in an alternating arrangement on either side of each perforation <NUM>. In particular, in the direction along the length of the security element <NUM>, a first region <NUM> extends over the first core surface <NUM>, a third region <NUM> extends through the thickness of the core layer <NUM>, a second region <NUM> extends over the second core surface <NUM>, a third region <NUM> extends through the thickness of the core layer <NUM> and so forth.

In the illustrated embodiment, in which the at least one perforation <NUM> comprises a tab <NUM>, the or each perforation <NUM> defines two locations for the security element <NUM> to pass through the at least one core layer <NUM> and the security element <NUM> extends through the perforation <NUM> twice. The first regions <NUM> of the security element <NUM> overly at least part of the first core surface <NUM> adjacent a tab <NUM> and/or between the tabs <NUM>. The second region(s) <NUM> of the security element <NUM> overlies at least part of the second core surface <NUM> of the tab(s) <NUM>. Alternatively, the first regions <NUM> overlie at least part of the first core surface <NUM> of the tab(s) <NUM> and the second region(s) <NUM> overlie at least part of the second core surface <NUM> adjacent a tab <NUM> and/or between the tabs <NUM>.

Although not falling within the scope of the present invention, <FIG> illustrate a further embodiment of the core structure <NUM> and at least one core layer <NUM> in which the at least one perforation <NUM> comprises a slot <NUM>. The security element <NUM> extends through each slot <NUM> such that it overlies the first core surface <NUM> on one side of the slot <NUM> and the second core surface <NUM> on the other side of the slot <NUM>.

Although not falling within the scope of the present invention, in yet a further embodiment, illustrated in <FIG>, the at least one core layer <NUM> comprises a plurality of abutting or adjacent strips <NUM>. The at least one perforation <NUM> is formed from abutting edges of the adjacent strips <NUM> and the security element <NUM> extends through the at least one core layer <NUM> between the abutting edges. As a result, the security element <NUM> extends over the first core surface <NUM> of one strip <NUM> and over the second core surface <NUM> of the adjacent strip <NUM> or vice-versa.

A stack <NUM> is formed, as illustrated in <FIG>, by locating at least one further layer <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> over one or both of the first and second core surfaces <NUM>, <NUM> of the core structure <NUM>. The at least one further layer <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> preferably comprises a plastic. A plastic substrate <NUM> is formed, as illustrated in <FIG>, by laminating the core structure <NUM> and at least one further layer <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> together. In other embodiments the plastic substrate <NUM> may be formed by laminating the core structure <NUM> alone. The stack <NUM> and plastic substrate <NUM> comprise first and second outer surfaces <NUM>, <NUM> formed by the outermost surfaces of the at least one further layer <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and/or core structure <NUM>. The method of forming the stack <NUM> and lamination is discussed in further detail below.

At least one further layer <NUM> may be a hinge layer <NUM> comprising, as illustrated, a hinge portion <NUM> and an attachment portion <NUM>. The attachment portion <NUM> is located within the stack <NUM> and at least partially overlies the rest of the further layers <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and the core structure <NUM> to fix the hinge layer <NUM> in the plastic substrate <NUM> after lamination. The hinge portion <NUM> therefore extends from the main body of the stack <NUM> and plastic substrate <NUM> to enable the plastic substrate <NUM> to be attached in a security document. In the illustrated embodiment the hinge layer <NUM> is located adjacent to and overlies the second core surface <NUM>, although it may be located at any suitable location in the stack <NUM>.

The plastic substrate <NUM> and at least one further layer <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> preferably comprise at least one substantially transparent region <NUM>, <NUM> through which at least one first and/or second region <NUM>, <NUM> of the security element <NUM> is visible in reflected light incident upon the at least one transparent region <NUM>, <NUM>. The at least one substantially transparent region <NUM>, <NUM> thus preferably extends between at least one first and/or second region <NUM>, <NUM> and the first and/or second outer surface <NUM>, <NUM>.

A first transparent region <NUM> may overlie the whole security element <NUM> such that all of the first regions <NUM> are visible in reflected light incident upon the first outer surface <NUM> or all of the second regions <NUM> are visible in reflected light incident upon the second outer surface <NUM>. As in the illustrated embodiment, the first transparent region <NUM> may be formed by one or more first transparent further layers <NUM>, <NUM> overlying the first core surface <NUM> and extending to, and thereby forming, the first outer surface <NUM>.

A second transparent region <NUM> may overlie part of the security element <NUM> to form a window through which part of the security element <NUM> is visible in reflected light incident upon the second transparent region <NUM>. In particular, at least part of at least one of the first or second regions <NUM>, <NUM> of the security element <NUM> is visible in the second transparent region <NUM>. The second transparent region <NUM> is preferably formed, as illustrated, by at least one window layer <NUM> comprising at least one opaque region <NUM> defining at least part of the perimeter of the second transparent region <NUM>. The opaque region <NUM> partially blocks the core structure <NUM> from view when viewed in reflected light incident thereupon whilst the second transparent region <NUM> allows, in the illustrated embodiment, at least one second region <NUM> to be viewed. In the illustrated embodiment the second transparent region <NUM> also comprises a transparent attachment portion <NUM> of the hinge layer <NUM>, which is located between the at least one window layer <NUM> and the core structure <NUM>. The second transparent region <NUM> also comprises one or more second transparent further layers <NUM>, <NUM> overlying the at least one window layer <NUM> and extending to, and thereby forming, the second outer surface <NUM>. However, the hinge layer <NUM> and/or one or more second transparent further layer <NUM>, <NUM> need not be present and the at least one window layer <NUM> may directly overlie the second core surface <NUM>.

In a preferred embodiment at least part of the at least one core layer <NUM> is opaque adjacent to at least one first and/or second region <NUM>, <NUM> of the security element <NUM> such that at least one first region <NUM> is not visible in reflected light incident upon the second core surface <NUM> and/or at least one second region <NUM> is not visible in reflected light incident upon the first core surface <NUM>. The result is that, when viewed in reflected light, the at least one core layer <NUM> creates a windowing effect of the security element <NUM>. In the embodiment of <FIG> the at least one core layer <NUM> is opaque across substantially all of the first and second width <NUM>, <NUM> of the core structure <NUM>, stack <NUM> and plastic substrate <NUM>. If the core structure <NUM> comprises a plurality of core layers <NUM>, only one or more core layers <NUM> may be at least partially opaque.

The arrangement of the at least one transparent region <NUM>, <NUM> controls the first or second regions <NUM>, <NUM> visible in reflected light from the first or second outer surfaces <NUM>, <NUM> where the at least one core layer <NUM> is at least partially opaque. As in the illustrated embodiment, only the first regions <NUM> of the security element <NUM> are visible in the first transparent region <NUM> from the first outer surface <NUM> when viewed in reflected light incident thereupon. Only the, or parts of the, second regions <NUM> overlapping the second transparent region <NUM> are visible from the second outer surface <NUM> when viewed in reflected light incident thereupon. However, if the at least one core layer <NUM> is transparent, the first and second regions <NUM>, <NUM> of the security element <NUM> are all visible from the first outer surface <NUM> through the first transparent region <NUM>. All of the first and second regions <NUM>, <NUM> overlapping the second transparent region <NUM> are visible from the second outer surface <NUM> through the second transparent region <NUM>.

Furthermore, the security element <NUM> is preferably at least partially visible when the core structure <NUM>, security sheet and/or plastic substrate <NUM> is viewed in transmitted light. The first or second regions <NUM>, <NUM> of the security element <NUM> not visible at the first or second outer surface <NUM>, <NUM> in reflected light may be visible in transmitted light at the first or second outer surface <NUM>, <NUM>. For example, the first regions <NUM> may not be visible in reflected light incident upon the second outer surface <NUM>. However, the first regions <NUM> may be visible when the second outer surface <NUM> is viewed in light transmitted from the first outer surface <NUM>, through the plastic substrate <NUM>, through the second outer surface <NUM> and into the observer's eye. For instance, the security element <NUM> may be visible as a silhouette as shown in dashed lines in <FIG>, <FIG>, <FIG>, <FIG>.

The plastic substrate <NUM> resulting from the lamination of the plurality of plastic layers <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> is substantially unitary with little or no definition between the at least one core layer <NUM> and at least one further layer <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>. In the plastic substrate <NUM> the security element <NUM> varies in distance from the first or second outer surfaces <NUM>, <NUM>. As illustrated in <FIG>, the first regions <NUM> are at a first distance D1 from the first outer surface <NUM> and the at least one second region <NUM> is at a second distance D2 from the first outer surface <NUM>. The first and second distances D1, D2 are different and, in particular, the first distance is less than the second distance since the first regions <NUM> are closer to the first outer surface <NUM> than the at least one second region <NUM>. It will be appreciated that any alternative reference point may be used instead of the first outer surface <NUM>, such as the second outer surface <NUM>.

Furthermore, in the illustrated plastic substrate <NUM> the first and second regions <NUM>, <NUM> of the security element <NUM> are all separated from the first and second outer surfaces <NUM>, <NUM> such that the security element <NUM> is substantially entirely embedded within the plastic substrate <NUM> (although the ends of the security element <NUM> may be at least partially exposed at the edges of the plastic substrate <NUM>). In general, preferably at least one first and/or second region <NUM>, <NUM> is separated from the first and/or second outer surface <NUM>, <NUM> and thereby embedded in the plastic substrate <NUM> such that a counterfeiter cannot reach at least a portion of the security element <NUM> without destroying at least part of the plastic substrate <NUM>. Such separation is achieved by the existence of at least one further layer <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> overlying the core structure <NUM>. However, in an alternative embodiment the plastic substrate <NUM> may not comprise a further layer <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> on one of both first and second core surfaces <NUM>, <NUM> of the core structure <NUM> such that the security element <NUM> is exposed at the first and/or second outer surface <NUM>, <NUM>. Thus, if there are no further layers <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, the first distance D1 is zero whilst the second distance D2 is the thickness of the core structure <NUM>.

In the plastic substrate <NUM> the first and second regions <NUM>, <NUM> of the security element <NUM> extend substantially parallel to the first and second outer surfaces <NUM>, <NUM> and the at least one third region <NUM> is substantially non-parallel to the first and second outer surfaces <NUM>, <NUM>. In particular, in the at least one third region <NUM> the security element <NUM> extends substantially orthogonal to the first and second outer surfaces <NUM>, <NUM> due to its extension through the perforation(s) <NUM>.

The plastic from which the plastic substrate <NUM>, at least one core layer <NUM> and, if present, further layer <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> are formed may be a thermoplastic polymer such as polycarbonate, polyester, polyethylene, polypropylene or polyvinyl chloride. Polycarbonate is particularly suitable due to its high durability, its ease of manufacture and the ease with which security features can be incorporated within it. The plastic substrate <NUM> is preferably substantially rigid or at least semi-rigid, this preferably being a result of the thickness of its constituent layers <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>. Each of the at least one core layer <NUM> and at least one further layer <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may be between approximately <NUM> thick and approximately <NUM> thick. The thickness of the plastic substrate <NUM>, which is the distance between the first and second outer surfaces <NUM>, <NUM>, is preferably at least approximately <NUM> and more preferably at least approximately <NUM>. In particular, the plastic substrate <NUM> may be between approximately <NUM> and <NUM> thick and, for example, may be approximately <NUM> thick or approximately <NUM> thick.

The method of formation of the core structure <NUM> and/or stack <NUM> and subsequent lamination thereof is preferably based upon a substantially continuous and in-line method, similar to that of <CIT>, <CIT>, <CIT>, <CIT> and <CIT>. The core structure <NUM> may be formed separately by firstly forming the at least one core layer <NUM> and subsequently inserting the security element <NUM> therein. If the core structure <NUM> comprises a plurality of core layers <NUM>, they may be laminated together prior to or after insertion of the security element <NUM> into them. The at least one perforation <NUM> is preferably formed by cutting, die cutting or punching the at least one core layer <NUM>.

The security element <NUM> is inserted, threaded or fed through the at least one perforation <NUM>, preferably, as illustrated in <FIG> and <FIG>, after it is arranged extend parallel to the first width <NUM> at the first core surface <NUM>. Preferably the security element <NUM> is brought under each tab <NUM> (i.e. brought under the second core surface <NUM> of each tab <NUM>) by raising or pivoting the tab <NUM> above the first core surface <NUM> about the connection <NUM>.

Although not falling within the scope of the present invention, in the embodiment of <FIG>, the security element <NUM> is fed or threaded through each slot <NUM> sequentially. Whilst such an arrangement may, compared to the embodiment of <FIG>, require more complex tools to feed the security element <NUM> through each slot <NUM> sequentially, the slots <NUM> avoid the need for tools to pivot each tab <NUM> to fit the security element <NUM> thereunder. Although not falling within the scope of the present invention, in the embodiment of <FIG>, the strips <NUM> may be brought adjacent to one another before or after the feeding of the security element <NUM> therethrough.

The at least one further layers <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> are also formed separately to the core structure <NUM> prior to being overlaid upon one another. For instance, the hinge layer <NUM> may be formed with the hinge portion <NUM> embedded in the attachment portion <NUM>. The second transparent region <NUM> and at least one opaque region <NUM> may also be formed as described in <CIT> and <CIT>. In particular, the second transparent region <NUM> is formed from a transparent insert located in a correspondingly shaped aperture in an opaque plastic layer. Preferably the aperture and insert are formed and brought together by a single punch tool in line.

The stack <NUM> may be formed by laying one or more of the layers <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> over one another sequentially. A plurality of the at least one further layers <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may be laid over one another first, the core structure <NUM> located thereover and the remainder of the further layers <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> laid over the core structure <NUM>. However, the layers <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may be mounted on feeding rolls or the like and the stack <NUM> may be brought together at substantially the same time as, or just prior to, lamination. Such an arrangement is disclosed in <CIT> and <CIT>.

During lamination a laminator fuses the layers <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> together by the application of heat and pressure to form the plastic substrate <NUM>. Preferably the laminator comprises continuous belts for drawings the layers <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> between sequential heating and cooling devices. During the heating process each of the layers <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> become at least softened or semi-molten (i.e. a liquid of relatively high viscosity) so that the plastic flows and mixes together across the interfaces between them, thereby forming the substantially unitary plastic substrate <NUM>. In order to achieve such fusing the heating devices raise the temperature and pressure applied to the layers <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> to reach the softening point of the plastic. Suitable methods are disclosed in <CIT> and <CIT>.

After the plastic substrate <NUM> has exited the laminator it is cut into a plurality of security sheets. Each security sheet itself may form a security document (e.g. a licence), or it may form part of a security document (e.g. in a passport). The security document may be of any suitable type having the security sheet, including banknotes, cards, such as identity cards, driving licences and the like, and booklets, such as passports, passbooks, identification documents, certificates, licences, cheque books and the like. In the case of booklets, the security sheet may be attached therein by stitching the hinge portion <NUM> to a cover or one or more paper or polymer leaves.

Personal data relating to the holder of the security document is preferably applied to the security sheet or plastic substrate <NUM>, preferably by laser marking. The personal data is typically applied after formation of the security sheet, but it may be applied prior to formation, such as by being printed or marked on one of the layers <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> prior to lamination. Such personal data is known as "variable data" or "biographical data" and, typically, no two security sheets or security documents issued by an issuing authority contain the same personal data. The <NPL> describes such biographical data. The personal data typically includes an image of the holder, text providing the holder's name, nationality and date of birth and a machine readable zone containing machine readable data.

Various alternatives to those described above may also be contemplated. In the illustrated embodiments the locations at which the security element <NUM> extends through the at least one core layer <NUM> are irregularly spaced. However, in other embodiments the locations may be evenly spaced such that at least one first region <NUM> is substantially the same size as at least one second region <NUM>.

As in the illustrated embodiments, the security element <NUM> may be a substantially continuous security thread having any of the forms well known in the art and may comprise an elongate strip, band or ribbon of impermeable material. The security thread may extend between opposing edges of the security sheet and plastic substrate <NUM>. The security thread is preferably between <NUM> and 25pm thick and is preferably between approximately <NUM> and approximately <NUM> wide.

The security element <NUM> may alternatively comprise a discrete band, patch, foil, opaque body or impermeable member. In particular, rather than forming a continuous security thread, the security element <NUM> may comprise a discrete strip or thread having one or more ends inside the edges of the plastic substrate <NUM>. The security element <NUM> may be a patch passing through a relatively large perforation <NUM> such that a second region <NUM> overlies the second core surface <NUM> whilst the first region <NUM> overlies the first core surface <NUM>.

Suitable materials for forming the security element <NUM> include polycarbonate, polyethylene terephthalate (PET), oriented polypropylene (OPP), polyethylene (PE) or polyketone (PK). The security element <NUM> preferably comprises at least one layer of material selected for fusing with the at least one core layer <NUM> and/or further plastic layer <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> during lamination to form a strong bond therebetween. In particular, the at least one layer of material may be the same as the material of the at least one core layer <NUM> and/or further plastic layer <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, such as polycarbonate. The security element <NUM> may further comprise covert or overt security devices disposed in at least one first, second and/or third region <NUM>, <NUM>, <NUM> or continuously along its length. Different security devices may be located in different at least one first, second and/or third region <NUM>, <NUM>, <NUM> such that different security devices are visible at different "windows" on the same or opposing outer surfaces <NUM>, <NUM>. The security devices may be continuous patterns extending along the security element <NUM> in at least one first, second and third regions <NUM>, <NUM>, <NUM>. Suitable overt features include those commonly used on windowing security threads in paper banknotes such as demetallised patterns, microtext, fine line patterns, latent images, venetian blind devices, lenticular devices, moiré interference devices and moiré magnification, holograms and other diffractive based devices, colour-shifting inks and/or luminescent or fluorescent inks. The covert features cannot be seen in visible light by the naked eye, but can be machine read and/or viewed by the naked eye in other forms of light. For example, the security element <NUM> may comprise at least one first, second and/or third region <NUM>, <NUM>, <NUM> having properties selected from at least one of magnetic (which may be coded), luminescent including fluorescent and phosphorescent, conductivity or the like. Such overt or covert features may be provided as logos, symbols, indicia or other recognisable shapes.

In a particular embodiment the security element <NUM> comprises at least one transparent layer and at least one opaque layer overlying the transparent layer. At least one transparent region, forming a logo, symbol, indicium, alphanumeric character or the like, may be formed in the at least one opaque layer. The at least one transparent region is therefore visible in transmitted light as a light area surrounded by the silhouette of the rest of the at least one opaque layer. In a particular embodiment the at least one opaque layer comprises a metal layer and the at least one transparent region is a demetallised region.

The core structure <NUM>, plastic substrate <NUM> and/or security sheet may comprise a plurality of security elements <NUM>, at least one of which is embedded as disclosed herein. The security element <NUM> may also extend at an acute angle to the first width <NUM> rather than being parallel thereto as disclosed herein, particularly if it is not a security thread. For example, a security patch may be located in any suitable orientation.

The plastic substrate <NUM> and one or more layers <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> may comprise at least one further security device embedded therein or located thereon. Graphical information, colours and/or patterns may be printed on the outer surfaces of the layers <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> prior to or after lamination. Electronic chips and antenna or the like may also be located within the plastic substrate <NUM>, preferably being located in the hinge layer <NUM>. The at least one security device may, for example, comprise laser markings, printed ink, holograms, UV responsive features, optically variable features, windowed or transparent features, laser-perforations and the like. The at least one security device may be visible when viewed in reflected light by virtue of being located in the at least one transparent region <NUM>, <NUM>. Alternatively the at least one security device may be covert, such as by being located in at least one opaque core layer <NUM> and/or the opaque region <NUM> of the window layer <NUM>, such that it is not visible when viewed in reflected light.

The core structure <NUM> is preferably subsequently laminated in order to embed the security element <NUM> in the at least one core layer <NUM>. The materials of the security element <NUM> and at least one core layer <NUM> may be selected such that they fuse together during lamination, such as by comprising the same material, and a strong bond therebetween is formed. The security element <NUM> and at least one core layer <NUM> may alternatively or additionally be fixed together by an adhesive, which may be provided along at least one surface of the security element <NUM> prior to its location in the at least one core layer <NUM>.

Claim 1:
A core structure (<NUM>) for forming a security sheet comprising:
at least one core layer (<NUM>) comprising plastic, first and second core surfaces (<NUM>, <NUM>) and at least one perforation (<NUM>); and
a security element (<NUM>) extending through the at least one perforation (<NUM>) of the at least one core layer (<NUM>) from the first core surface (<NUM>) to the second core surface (<NUM>) and comprising first regions (<NUM>) overlying parts of the first core surface (<NUM>) and at least one second region (<NUM>) overlying part of the second core surface (<NUM>);
characterized in that
the or each perforation (<NUM>) defines a tab (<NUM>) and the security element (<NUM>) extends through the or each perforation (<NUM>) twice such that the at least one second region (<NUM>) of the security element (<NUM>) overlies at least part of the second core surface (<NUM>) of the tab (<NUM>) and first regions (<NUM>) of the security element (<NUM>) overlie at least part of the first core surface (<NUM>) adjacent the tab (<NUM>) or between adjacent tabs (<NUM>), or vice-versa.