EP2860310A1

EP2860310A1 - Improvements in paper manufacturing

Info

Publication number: EP2860310A1
Application number: EP20140184871
Authority: EP
Inventors: Adam Lister
Original assignee: De la Rue International Ltd
Current assignee: De la Rue International Ltd
Priority date: 2013-10-09
Filing date: 2014-09-16
Publication date: 2015-04-15
Also published as: GB201317860D0; GB2519104A

Abstract

The present invention relates to improvements in methods of manufacturing paper, and in particular to improvements in forming stacks of sheets and cylinder moulds for manufacturing paper incorporating security elements. The method of forming a stack of sheets, comprises the steps of making a continuous web of paper having a plurality of sets of counterpoint watermarks (30) distributed along a longitudinal direction of the web. The sets of counterpoint watermarks (30) comprise at least a first set (45) of counterpoint watermarks having a first pattern and a second set (46) of counterpoint watermarks having a second pattern different from the first pattern. The web is slit in a transverse direction of the web into a plurality of interim sheets (31, 32), each sheet comprising one set of counterpoint watermarks (30). The interim sheets (31, 32) are stacked so that the counterpoint watermarks (30) of the first and second sets (45, 46) on the stacked interim sheets (31, 32) overlap each other at a plurality of overlap points (33) and the counterpoint watermarks comprise watermarks having positive regions.

Description

Technical Field

The present invention relates to improvements in methods of manufacturing paper, and in particular to improvements in forming stacks of sheets and cylinder moulds for manufacturing paper incorporating security elements.

Background

Modern papermaking is typically carried out using known papermaking machines, such as cylinder mould or Fourdrinier machines. In cylinder mould papermaking, paper is formed on a partially submerged wire-cloth covered cylinder mould, which rotates in a vat of aqueous stock containing a dilute suspension of paper fibres. As the cylinder mould rotates, water is drawn through the wire cloth depositing fibres onto the cylinder surface. It is possible to create watermarks, or parts of watermarks, in the resulting paper by forming sunken regions on the cylinder mould. In the sunken regions, the fibres deposit with a greater density or grammage to form a positive watermark, or positive areas within a watermark. Regions of a greater density or grammage are generally darker than the base paper. The sunken regions are generally created by embossing the wire-cloth. Alternatively, or in addition, it is possible to create negative watermarks, or negative areas within a watermark, by limiting the deposition of fibres in certain regions so that there is a lower density or grammage of fibres deposited. The regions of lower density or grammage are generally lighter than the base paper. Such negative regions can be formed by embossing raised regions in the cylinder mould cover or by attaching electrotypes to the cylinder mould cover.
As the paper substrate is formed, it is couched from the cylinder mould cover as a continuous web. Water is extracted from the wet fibrous web and the web of paper undergoes a drying process. The dried web is initially cut along cutting lines to form a plurality of interim sheets, which are stacked in reams of typically 500 sheets. When the paper is required for producing security documents, these interim sheets are typically printed and cut into individual smaller sheets, each of which may form a security document, or a part of a security document, such as a banknote, passport, identification card, and the like.
In such a continuous papermaking process, any watermarks must be produced at predetermined intervals along the length of the continuous web of paper. Therefore, the embossings and/or electrotypes which produce the watermark(s) must be disposed at regularly repeated intervals around the periphery of the cylinder mould cover. For convenience, there may be six sets of embossings and/or electrotypes so disposed, each set each set forming the requisite watermark(s) for one interim sheet.
It is also generally known to use security devices in the form of elongate elements as a security feature in paper. Such security devices can be threads, strips, or ribbons of, for example, plastics film, metal foil, metallised plastic, or metal wire. These security devices may be included in the thickness of the paper substrate to render more difficult the imitation of documents produced therefrom. Alternatively they may be applied to the surface of the paper substrate. These security devices aid in the verification of the documents, as they render the view of the documents in reflected light different from that in transmitted light. To increase the security provided by the use of such a security device, it is also known to endow the security device itself with one or more verifiable properties over and above its presence or absence. Such additional properties include magnetic properties, electrical conductivities, the ability to absorb x-rays, fluorescence, optically variable effects and thermochromic behaviour.
There is an increasing trend towards the use of thicker security devices in security paper. This is in part due to the desire to increase the security and durability of security devices. Security features are typically becoming more complex, which is leading to the use of thicker threads. For example, laminate threads tend to be thicker due to the extra ply/plies which is/are added to the security element. There is also an increased interest in micro-structured features, which again add to the thickness of a security device. A 'thick' security device may have a thickness of greater than 25 microns.
There is also an increasing trend towards the use of wider security devices in security paper, for example security devices having a width of 3mm or more. This trend is due to the fact that wider security devices provide a larger surface area, which enables the provision of more detailed, and clearer, information thereon. A larger surface area also allows for better use of optically variable devices (OVDs).
However, as these types of security devices affect the local thickness of the paper substrate they can result in difficulties with the resulting shape of the ream when the interim sheets are stacked. For example, the ream may sag between the locations of the security devices. This means that, when the ream is stacked in a machine for further processing, e.g. printing, the ream must be passed through the machine more slowly and may be more difficult to print. Such sagging may be due to the thickness differential between the locations of the security devices and the rest of the paper web. An alternative hypothesis is that the sagging may be due to the greater rigidity of the thread with respect to the rigidity of the surrounding paper. A further alternative hypothesis is that the sagging may originate from the drying process of the web, during which the security device may undergo little change but the paper fibres shrinks around it.
Ream shape problems are also encountered when security devices are applied on to the surface of a secure document. The security device could be applied as a label to a secure document which will typically require the application of a heat or pressure sensitive adhesive to the outer surface of the device which will contact the secure document. In addition an optional protective coating/varnish could be applied to the exposed outer surface of the device. The function of the protective coating/varnish is to increase the durability of the device during transfer onto the security substrate and in circulation.
Alternatively a security device could be applied to the surface as a transfer element rather than a label. In this case the security device is preferably prefabricated on a carrier substrate and transferred to the substrate in a subsequent working step. The security device can be applied to the document using an adhesive layer. The adhesive layer is applied either to the security device or the surface of the secure document to which the device is to be applied. After transfer the carrier strip can be removed leaving the security device as the exposed layer or alternatively the carrier layer can remain as part of the structure acting as an outer protective layer.
It is also becoming increasingly common to apply film based security elements on the surface of a secure document over a hole or aperture in a secure document such that the film based security element closes the hole or aperture. Example methods of forming such holes or apertures in paper documents and subsequently applying the film element are described in EP-A-0723501 , EP-A-0724519 , EP-A-1398174 and WO-A-2003/054297 . Application of a film over one side of a hole or aperture results in the mechanical destabilisation of the substrate due to the difference in mechanical properties between the paper substrate and the polymer film. This problem is recognised in EP-A-2077190 where an aperture or hole in a secure document is closed on both sides by separate films or foils in order to stabilise the structure. However the presence of two films superimposed will further increase the thickness of the secure document in a localised region resulting in poor ream shape where stacking sheets of the resultant secure document.
Ream shape problems may also occur in sheets made from paper having other forms of surface applied security features, which may affect the local thickness of the paper or the surface profile. Examples of such features can be those formed by, for example, intaglio printing, screen printing, embossing and the like. One example is described in WO-A-2009/106799 in which a resin, which incorporates tactile elements, is screen printed onto the surface of the paper.
Such problems may also occur in sheets made from paper which is provided with certain types of watermark features. For example, WO-A-03/046282 describes the use of corner or edge reinforcing watermarks. As these are positive watermarks they will cause a similar problem of sagging between the areas where these reinforcing watermarks are located.
Ream shape is a complicated multi-parameter issue and problems can be caused not only by paper or security device profile, but also as a result of differential shrinkage during drying, printing and further processing or moisture variations across a sheet of paper.
One way to overcome the above-mentioned problems in some instances is by using thicker paper. However, such a solution carries the associated disadvantage of increased cost.

Summary

The invention therefore provides a method of forming a stack of sheets, comprising the steps of:

making a continuous web of paper having a plurality of sets of counterpoint watermarks distributed along a longitudinal direction of the web, wherein the sets of counterpoint watermarks comprise at least a first set of counterpoint watermarks having a first pattern and a second set of counterpoint watermarks having a second pattern different from the first pattern;
slitting the web in a transverse direction of the web into a plurality of interim sheets, each sheet comprising one set of counterpoint watermarks; and
stacking the interim sheets; wherein
the counterpoint watermarks of the first and second sets on the stacked interim sheets overlap each other at a plurality of overlap points and the counterpoint watermarks comprise watermarks having positive regions.

The invention further provides a security document made by the aforesaid method.
The invention further provides a cylinder mould cover or dandy roll cover for producing a continuous web of paper, comprising:

a plurality of sets of embossings for producing counterpoint watermarks in the paper, said embossings being distributed along a circumferential direction of the cover, wherein the sets of embossings comprise at least a first set of embossings having a first pattern and a second set of embossings having a second pattern different from the first pattern, wherein if the first set of embossings were superposed on the second set of embossings there would be a plurality of overlap points.

Brief Description of the Drawings

A preferred embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a cross-sectional side elevation of a section of a cylinder mould rotating in a vat;
Figure 2 is a perspective view of a section of a first embodiment of a cylinder mould in accordance with the present invention;
Figure 3 is a plan view of a first interim sheet made from the cylinder mould of figure 2;
Figure 4 is a plan view of a second interim sheet made from the cylinder mould of figure 2;
Figure 5 is a plan view of the sheets of Figure 3 and Figure 4 superposed upon each other;
Figure 6 is a perspective view of a section of a second embodiment of a cylinder mould in accordance with the present invention;
Figure 7 is a plan view of a first interim sheet made from the cylinder mould of figure 6;
Figure 8 is a plan view of a second interim sheet made from the cylinder mould of figure 6;
Figure 9 is a plan view of the sheets of Figure 7 and Figure 8 superposed upon each other;
Figure 10 is a perspective view of a section of a third embodiment of a cylinder mould in accordance with the present invention;
Figure 11 is a plan view of a first interim sheet made from the cylinder mould of figure 10;
Figure 12 is a plan view of a second interim sheet made from the cylinder mould of figure 10; and
Figure 13 is a plan view of the sheets of Figure 11 and Figure 12 superposed upon each other.

Detailed Description

The present invention is based on the use of special watermarks which are located in a particular manner. This has a two fold effect. The first is to bulk up the paper substrate in regions which may be thinner than other regions, for example in the regions between security devices or security features which affect the localised thickness or surface profile of the paper. This bulking up provides a counterpoint to, and offsets, the aforementioned disadvantages when sheets made form the paper substrate are stacked into a ream. In this specification these watermarks will be described as "counterpoint watermarks". This is not a term normally used in the art but has been derived here to distinguish from watermarks which may be used solely as security or decorative features. Having said which, it is possible that the counterpoint watermarks will also act as a security or decorative feature.
The second effect lies in the fact that the counterpoint watermarks are positive watermarks. As a result of the increased grammage, the paper in those areas is stronger and stiffer, so the paper is less likely to distort. Although they are primarily positive watermarks, it may be possible to use a counterpoint watermark which is substantially positive but has some negative regions for a decorative or secure effect. For example the counterpoint watermark may have a positive line with a negative text or pattern within one region.
Figure 1 illustrates a section of a typical cylinder mould papermaking machine used to manufacture a web of security paper. The web (not shown) is subsequently slit into interim sheets 31,32 of security paper (as shown in Figures 3 to 5) and then further slit into smaller sheets for making security documents 38,39 (as shown in Figures 7 to 9 and 11 to 13). The machine comprises a cylinder mould 11 (a section of which is shown in Figure 2), which rotates in a vat 12 of aqueous fibrous stock 13. The cylinder mould 11 is covered with a porous support surface, such as a porous wire mesh, which forms a cylinder mould cover 14.
The fibrous stock 13 may comprise a range of fibre types, including synthetic or natural fibres, or a mixture of both. The actual preparation of the fibres is unrestricted by the invention, and will depend on what effect it is wished to produce in the finished substrate. As a general consideration, security paper used for security documents, such as banknotes, passports, identification cards and so on, needs to be hard wearing, resilient, and self-supporting, and so an appropriate fibre mix must be selected. The substrate may be of any grammage, but a grammage of less than 100gsm is preferred.
The present invention is particularly useful in preventing the aforementioned stacking issues in sheets which incorporate security devices in the form of wholly embedded or partially embedded (windowed) elongate security elements 35.
There are a number of methods known in the art by which an elongate security element 35 may be wholly embedded or partially embedded into a security paper or document. In one method, as described in EP-A-059056 and illustrated in Figure 1, the cylinder mould cover 14 may be embossed with raised portions 15, which are raised with respect to the surrounding level of the cylinder mould cover 14. Recesses 16 between the raised portions 15 may or may not be recessed with respect to the surrounding cylinder mould cover 14, according to the required surface finish of the end product. The raised portions 15 will create windows 36 in the resultant paper web (not shown).
The elongate security element 35 is continuously brought into contact with the raised portions 15 on the cylinder mould cover 14 as the cylinder mould cover 14 rotates, so that the security element 35 is embedded in the paper substrate as it forms. The security element 35 is brought into contact with the cylinder mould cover 14 above the level of the fibrous stock 13. The cylinder mould 11 is rotated in the fibrous stock 13 in the direction shown in Figure 1.
Based on the desire to be able to display as much information or indicia as possible in exposed regions of the security element 35, it has also been recognised that it would be highly advantageous to be able to expose the security element 35 on both sides of the paper, or other substrate, in which it is embedded. Security elements 35 may be incorporated into a paper substrate such that regions of the device are visible on both sides of the substrate. Methods of incorporating a security device such that it is viewable from both sides of the substrate are described in EP-A-1141480 and WO-A-03054297 . In the method described in EP-A-1141480 , one side of the device is wholly exposed at one surface of the substrate in which it is partially embedded, and partially exposed in windows at the other surface of the substrate.
In the embodiment illustrated, a plurality of spaced apart security elements 35 are simultaneously partially embedded in the paper substrate so that they run in the machine direction of the web (not shown) and substantially parallel to each other. The plurality of security elements 35 are fed in parallel to the vat 12 in a known manner. The cylinder mould cover 14 is embossed to accommodate the plurality of security elements 35.
The security element 35 may be a thick and/or wide security element, as is known in the art. For the purposes of the invention, a 'thick' security element is considered to have a minimum thickness of 25 microns and a 'wide' security element is considered to have a minimum width of 3mm. An example of a suitable security element is the 6mm StarChrome® thread available from De La Rue International Limited (described in WO-A-03/061980 ), which has a thickness of around 35 microns when measured on the micro-cal system.
Other options are security elements 35 provided with non-holographic micro-optical structure, which typically have a thickness of greater than 30 microns due to the geometry of the optical structures. Prismatic structures are a preferred type of micro-optical structure. Examples of prismatic structures used in security elements 35 include, but are not limited to, a series of parallel linear prisms with planar facets arranged to form a grooved surface, a ruled array of tetrahedra, an array of square pyramids, an array of corner-cube structures, and an array of hexagonal-faced corner-cubes. An alternative preferred type of micro-optical structure is one which functions as a microlens, including those that refract light at a suitably curved surface of a homogenous material such as plano-convex lenslets, double convex lenslets, plano-concave lenslets, and double concave lenslets. Other suitable micro-optical structures include geometric shapes based on domes, hemispheres, hexagons, squares, cones, stepped structures, cubes, or combinations thereof. Examples of patent specifications describing threads using such non-holographic micro-optical structures include WO-A-2006095161 and WO-A-2005106601 .
It is also becoming increasingly common to combine multiple security features on different layers within one security element 35 using a laminate, leading to security elements with increased thickness; for example, the combination of liquid crystal and magnetic layers is known from WO-A-200905367 .
The cylinder mould cover 14 is also provided with a plurality of embossings 17, for creating one or more counterpoint watermarks 30 in the resultant paper web. The embossings 17 are preferably in the form of lines (straight or curved) or geometric patterns. The embossing may also be designed in the form of indicia such as pictures, portraits and alphanumerics thus providing a secondary decorative or security function. The embossings 17 are at least partially recessed with respect to the surrounding cylinder mould cover 14, to produce positive watermarks 30. This may be achieved by embossing the cylinder mould cover 14. As stated above, a positive watermark is a watermark which has an increased density (grammage) and therefore thickness with respect to the surrounding paper, and hence appears darker than the surrounding paper when viewed in transmitted light. The embossings 17 are located in such a manner that the counterpoint watermarks 30 are located between the security elements 35 in the finished sheets to provide bulk to the paper in the watermarked regions. This will be described in more detail below.
The cylinder mould cover 14 may be provided with further embossings (not shown), to provide security or decorative watermarks. The embossings used to provide these security or decorative watermarks may be in the form of geometric patterns, pictures, portraits, patterns, alphanumerics, or any other form of indicia. The security or decorative watermarks may be positive, negative, or multitonal watermarks.
The cylinder mould cover 14 may also be used to produce an electrotype security or decorative watermark. For this the cylinder mould cover 14 is provided with an electrotype (not shown). An electrotype is created by attaching a metallic electrotype element, or a polymeric sealing compound, to the cylinder mould cover 14. The electrotype results in a significant decrease in drainage of water through the cover which leads to reduced fibre deposition in that area, thereby forming a light mark in the paper.
The relative positioning of the raised portions 15, embossings 17, electrotypes and the like on the cylinder mould cover 14 is determined by the desired location of the security elements 35, counterpoint watermarks 30, and any security or decorative watermarks in the resulting security documents 38,39. The cylinder mould papermaking machine produces a continuous reel of paper (not shown), which is slit longitudinally into a number of webs (not shown). Each web is then slit transversely into interim sheets 31,32, which are intended to be stacked one upon another in a stack of, say, 500 interim sheets 31,32. The interim sheets 31,32 are stacked, for example, in the order 31₁, 32₁, 31₂, 32₂, ... , 31₂₅₀. 32₂₅₀. Thus as the reel is produced continuously, the raised portions 15, embossings 17 are repeated around the perimeter of the cylinder mould cover 14 and along the length of the cylinder mould cover 14.
The length of the cylinder mould cover 14 is determined by the number of webs to be contained in the reel produced from the cylinder mould cover 14, where the width of each web corresponds to the width of one interim sheet 31,32. Figure 2 shows a section of the cylinder mould cover 11 which is the width of one web. Typically the length will be such as to produce three webs.
The circumference of the cylinder mould cover 14 is equivalent to the length of 'n' interim sheets 31,32. 'n' may be any integer. Preferably, 'n' is an even integer. As a non-limiting example, there may be three webs along the cylinder mould cover 14 and 'n' may be equal to 6; in this case, the surface area of the cylinder mould cover 14 would corresponding to the surface area of 18 (= 3 x 6) interim sheets 31,32. As each interim sheet 31,32 is subsequently slit into a plurality of smaller security documents 38,39, the number of counterpoint watermarks 30 produced in each interim sheet 31,32 must be such as to produce the requisite counterpoint watermarks 30 in each of the security documents 38,39. Thus the raised portions 15 and watermark embossings 17 required to produce the requisite pattern of the security element(s) 35and counterpoint watermark(s) 30, for each interim sheet 31,32 are repeated 'n' times around the circumference of the cylinder mould cover 14. Hence there are 'n' sets of raised portions 15 and watermark embossings 17on the cylinder mould cover 14, where each set produces the requisite pattern of counterpoint watermarks 30 for one interim sheet 31,32.
The depth of the recess of the embossings 17 is preferably 2mm. However, it should be noted that the depth of the embossings 17, and hence the thickness of the resultant counterpoint watermarks 30, does not need to be directly related to the thickness of the security elements 35 used. The location of the embossings 17 is such that, when the interim sheets 31,32 have been cut from the web and stacked, the counterpoint watermarks 30 on consecutive interim sheets 31,32 overlap at a plurality of overlap points 33. Any arrangement of embossings 17, and therefore counterpoint watermarks 30, is possible as long as the counterpoint watermarks 30 on the majority of consecutive interim sheets 31,32 overlap each other at the overlap points 33 when the consecutive interim sheets 31,32 are superposed on one another.
Preferably, the arrangement of embossings 17 is such that the overlap points 33 of the counterpoint watermarks 30 are common between all interim sheets 31,32. For example, the embossings 17 on the cylinder mould cover 14 for the 'n' interim sheets 31,32 may alternate between two different patterns resulting in overlap points 33 when the alternating interim sheets 31,32 are stacked. If 'n' corresponds to an even integer, the 'n'th interim sheet 32 and 'n+1'th interim sheet 31 will have different patterns, resulting in overlap points 33 between them when they are stacked one upon the other; hence all interim sheets 31,32 stacked in the resulting ream will have common overlap points 33. If 'n' corresponds to an odd integer, the 'n'th interim sheet 31 and 'n+1'th interim sheet 32 will have the same pattern, and many not have any overlap points 33 between them when they are stacked one upon the other; hence every multiple of the 'n'th interim sheet 31 and 'n+1'th interim sheet 32 stacked in the resulting ream may not have common overlap points 33.
In an alternative example, the embossings 17 on the cylinder mould cover 14 for the 'n' interim sheets 31,32 may vary between two or more different patterns, which result in overlap points 33 between a pre-determined number and sequence of interim sheets 31,32 when the alternating interim sheets 31,32 are stacked in the resulting ream.
In the section of a cylinder mould 11 shown in Figure 2, the length L of the cylinder mould cover 14 corresponds to the width of one web and hence to the width of one interim sheet 31,32. The cylinder mould cover 14 comprises a plurality of raised portions 15 for forming windows 36 in a web produced therefrom. The plurality of raised portions 15 are arranged in a regularly spaced pattern around the circumference of the cylinder mould cover 14, arranged to produce a plurality of 'tracks' spaced along the length of the cylinder mould cover 14 for the introduction of a plurality of security elements 35. The cylinder mould cover 14 further comprises a plurality of recessed embossings 17 for producing positive counterpoint watermarks 30. The embossings 17 extend at least partially between the raised portions 15. The embossings 17 comprise first and second sets 45,46 of embossings 17. In this embodiment, the first set 45 of embossings 17 comprises lines at a first angle with respect to an axis of the cylinder mould cover 14. The second set 46 of embossings 17 comprises lines at a second angle which is different from the first angle. The relative angle between the first set 45 of embossings 17 and the second set 46 of embossings 17 may be in the range of 0 degrees to 180 degrees. A relative angle in the region of 90 degrees would give rise to the greatest number of overlap points 33 in the resulting interim sheets 31,32. The minimum preferred relative angle may be calculated as:- $theta = \tan^{- 1} (g / l)$
where 'g' is the gap between the lines of the embossings 17 and '1' is the length of the lines. Preferably each set 45 of embossings 17 is a symmetrical reflection to the other set 46.
Figure 3 shows a first interim sheet 31 slit from a paper web produced from the section of cylinder mould cover 14 of Figure 2. The first interim sheet 31 has a series of parallel security elements 35 embedded therein. Depending on the embedment process, portions of the security elements 35 may be visible in windows 36 in one or both sides of the interim sheet 31. However, it is equally possible for the security element 35 to be fully embedded in the substrate. The first interim sheet 31 further comprises a series of positive counterpoint watermarks 30 extending at least partially between the security elements 35. The counterpoint watermarks 30 are formed from the first set 45 of embossings 17 on the cylinder mould cover 14. The counterpoint watermarks 30 are thus in the form of lines which are angled with respect to the security elements 35.
Figure 4 shows a second interim sheet 32 slit from a paper web (not shown) produced from the cylinder mould cover 14 of Figure 2. The second interim sheet 32 also has a series of parallel security elements 35 embedded therein. Again, the security elements 35 of this interim sheet 32 may be visible in windows 36, or fully embedded in the substrate. The security elements 35 of the second interim sheet 32 are located at the same positions as the security elements 35 of the first interim sheet 31. The second interim sheet 32 further comprises a series of counterpoint watermarks 30, again extending at least partially between the security elements 35. The counterpoint watermarks 30 are formed from the second set 46 of embossings 17 on the cylinder mould cover 14. The counterpoint watermarks 30 are thus in the form of angled lines with respect to the security elements 35, which are at an angle to the lines of the counterpoint watermarks 30 of the first interim sheet 31.
Figure 5 shows a stack 37 of the first and second interim sheets 31,32, with the second interim sheet 32 superposed on the first interim sheet 31. The security elements 35 in the first and second interim sheets 31,32 substantially overlie each other, as they always need to be located in the same place on each security document (not shown) formed from the interim sheets 31,32. The counterpoint watermarks 30 of the second interim sheet 32 overlap the counterpoint watermarks 30 of the first interim sheet 31 at overlap points 33.
Figure 6 shows a section of a second embodiment of a cylinder mould 11 having a cylinder mould cover 14 in accordance with the present invention. The cylinder mould cover 14 again comprises a plurality of raised portions 15 for supporting the security elements 35 and forming windows 36 in a web produced therefrom. The cylinder mould cover 14 again further comprises a plurality of recessed embossings 17 for producing counterpoint watermarks 30. The embossings 17 extend at least partially between the raised portions 15. The embossings 17 comprise first and second sets 45,46 of embossings 17. The first set 45 of embossings 17 comprises longitudinal lines with respect to an axis of the cylinder mould cover 14. The second set 46 of embossings 17 comprises transverse lines with respect to the same axis of the cylinder mould cover 14.
Figure 7 shows a first security document 38 slit from a first interim sheet which in turn is slit from a paper web produced from the cylinder mould cover 14 of Figure 6. Again, the counterpoint watermarks 30 are formed from the first set 45 of embossings 17 on the cylinder mould cover 14. Thus the only difference between this first security document 38 and a security document slit from the first interim sheet 31 described above is that the counterpoint watermarks 30 are in the form of longitudinal lines extending at least partially across the first security document 38.
Figure 8 shows a second security document 39 slit from a second interim sheet slit from a paper web produced from the cylinder mould cover 14 of Figure 6. Again, the counterpoint watermarks 30 are formed from the second set 46 of embossings 17 on the cylinder mould cover 14. Thus this embodiment of the second security document 39 has counterpoint watermarks 30 in the form of transverse lines extending at least partially across the second security document 39. The transverse lines of the counterpoint watermarks 30 of the second security document 39 are at an angle of substantially 90 degrees to the longitudinal lines of the counterpoint watermarks 30 of the first security document 38.
Figure 9 shows a stack 40 of two security documents 38,39, with the second security document 39 superposed on the first security document 38. The security elements 35 in the first and second security documents 38,39 overlie each other. The transverse counterpoint watermarks 30 of the second security document 39 overlap the longitudinal counterpoint watermarks 30 of the first security document 38 at overlap points 33.
Figure 10 shows a section of a third embodiment of a cylinder mould 11 having a cylinder mould cover 14 in accordance with the present invention. The cylinder mould cover 14 again comprises a plurality of raised portions 15 for supporting the security elements 35 and forming windows 36. The cylinder mould cover 14 again further comprises a plurality of recessed embossings 17 for producing counterpoint watermarks 30. The embossings 17 extend at least partially between the raised portions 15. The embossings 17 comprise first and second sets 45,46 of embossings 17. The first set 45 of embossings 17 comprises arced lines in a first direction. The second set 46 of embossings 17 comprises arced lines in an opposing direction from the first direction.
Figure 11 shows a first security document 38 slit from a third embodiment of the first interim sheet slit from a web of paper substrate produced from the cylinder mould cover 14 of Figure 10. Again, the counterpoint watermarks 30 are formed from the first set 45 of embossings 17 on the cylinder mould cover 14. Thus this embodiment of the first interim sheet 31 has a plurality of counterpoint watermarks 30 in the form of nested arcs extending at least partially across the first security document 38.
Figure 12 shows a second security document 39 slit from a second interim sheet slit from a web of paper substrate produced from the cylinder mould cover 14 of Figure 10. Again, the counterpoint watermarks 30 are formed from the second set 46 of embossings 17 on the cylinder mould cover 14. Thus this embodiment of the second interim sheet 32 also has positive counterpoint watermarks 30 in the form of nested arcs extending at least partially across the second security document 39. The arcs of the counterpoint watermarks 30 of the second security document 39 oppose the arcs of the counterpoint watermarks 30 of the first security document 38.
Figure 13 shows a stack 40 of two security documents 38,39, with the second security document 39 superposed on the first security document 38. The security elements 35 in the first and second security documents 38,39 again overlie each other. The arced counterpoint watermarks 30 of the second security document 39 overlap the arced counterpoint watermarks 30 of the first security document 38 at overlap points 33.
It should be noted that most security documents are printed, and that the security documents 38,39 of Figures 7 to 8, and 11 to 12 are merely representative of a limited number of specific arrangement of security elements 35 and counterpoint watermarks 30.
As explained previously, any arrangement of embossings 17 is possible, as long as the counterpoint watermarks 30 on the majority of consecutive interim sheets 31,32 (and hence on the relevant security documents 38,39) overlap each other at the overlap points 33 when consecutive interim sheets 31,32 are superposed upon one another. As a test for this requirement, if the first set 45 of embossings 17 were superposed on the second set 46 of embossings 17 there would be a plurality of overlap points 33.
As will be clear to the skilled person, many arrangements of embossings 17 would fit this criterion. For example, the embossings 17 may be in the form of lines, as previously described; however any arrangement of lines would be possible, with any (or substantially no) angle between the lines of the embossings 17 on consecutive interim sheets 31,32, as long as the lines of the embossings form overlap points 33 on the consecutive interim sheets 31,32 when they are stacked. As a further example, the embossings 17 could be in the form of a series of dots having the same location on all interim sheets 31,32. However, such an arrangement would require a higher manufacturing and stacking tolerance than the previously described arrangements. In a further alternative example, the embossings 17 may be in the form of indicia and may convey information. The indicia may be the same or different on alternating interim sheets 31,32, as long as the counterpoint watermarks 30 on the majority alternating interim sheets 31,32 have overlap points 33. In yet another alternative, the counterpoint watermarks 30 may comprise indicia. For example the lines shown in Figures 3 and 4 could be replaced with a repeating pattern of numerals, letters, pictures or symbols in the form of positive watermarks.
The counterpoint watermarks need to be predominantly positive watermarks which, as a result of the increased grammage, the paper in those areas is stronger and stiffer, so the paper is less likely to distort. Preferably the counterpoint watermarks comprise positive regions which are greater than 75% of the area of the counterpoint watermark and even more preferably greater than 90% of the area and even more preferably 100% of the area. The presence of small areas of negative watermarks within a counterpoint watermark may be used to provide additional decorative effects.
It should also be noted that it is preferred that the overlap points 33 are only a plurality of discrete points on the surface area of the counterpoint watermarks 30 and do not constitute a significant area thereof. The reason for this is to prevent the possibility of interlocking of thicker areas in one layer with thinner areas in an adjacent layer. It is well known that there are limitations on the accuracy of registration and slitting during the papermaking process. Thus if identical counterpoint watermarks were provided in each sheet 31,32, the likelihood of them sitting exactly on top of each other is remote. Thus if the counterpoint watermarks 30 were a series of identical lines in each sheet 31,32 (which were not angularly offset to each other as shown in Figures 3 and 4), there would be a strong possibility that the lines of increased thickness in one sheet would fall in the thinner areas between in an adjacent sheet and risk becoming interlocked.
The present invention may also be employed in a Fourdrinier papermaking machine and method. A typical Fourdrinier papermaking machine and method is described in GB-A-2260772 . In the case of a Fourdrinier papermaking machine and method, the cylinder mould cover 14 of the present invention would be used as the porous wire mesh covering the Fourdrinier dandy roll (which is equivalent to the cylinder in GB-A-2260772 ).
Whilst the aforementioned examples and the Figures only illustrate the method and cylinder mould/dandy roll cover of the present invention in conjunction with security paper which includes an at least partially embedded elongate security element, they are equally applicable to resolving ream issues for paper including all the other types of features mention previously. Thus where reference is made to the relative placement of the embossings 17 and raised portions being determined by the desired location of the security elements 35, the same applies to the placement of the embossings 17 relative to any decorative/security/reinforcing watermarks, surface features (screen printed, embossed, surface applied labels, strips or foils and the like) or any other feature which can cause ream distortion.

Claims

A method of forming a stack of sheets, comprising the steps of:
making a continuous web of paper having a plurality of sets of counterpoint watermarks distributed along a longitudinal direction of the web, wherein the sets of counterpoint watermarks comprise at least a first set of counterpoint watermarks having a first pattern and a second set of counterpoint watermarks having a second pattern different from the first pattern;

slitting the web in a transverse direction of the web into a plurality of interim sheets, each sheet comprising one set of counterpoint watermarks; and

stacking the interim sheets; wherein

the counterpoint watermarks of the first and second sets on the stacked interim sheets overlap each other at a plurality of overlap points and the counterpoint watermarks comprise watermarks having positive regions.
A method as claimed in claim 1, wherein at least one or each first set of counterpoint watermarks is adjacent at least one second set of counterpoint watermarks.
A method as claimed in any one of the preceding claims, wherein each second set of counterpoint watermarks is adjacent at least one first set of counterpoint watermarks.
A method as claimed in any one of the preceding claims, wherein the sets of counterpoint watermarks further comprise further sets of counterpoint watermarks having further patterns different from the first pattern and the second pattern.
A method as claimed in any one of the preceding claims, wherein the first and second sets of counterpoint watermarks comprise at least one line, and wherein the lines of the first and second sets of counterpoint watermarks cross each other on the stacked interim sheets at the overlap points.
A method as claimed in claim 5 as dependent on claim 4, wherein the further sets of counterpoint watermarks comprise at least one line, and wherein the lines of the first, second, and further sets of counterpoint watermarks cross each other on the stacked interim sheets at the overlap points.
A method as claimed in claim 5 or claim 6, wherein the lines are straight or curved.
A method as claimed in any one of claims 1 to 4, wherein the first set of counterpoint watermarks comprises at least one line at a first angle with respect to the longitudinal direction of the web and the second set of counterpoint watermarks comprises at least one line at a different angle to the first angle.
A method as claimed in any one of claims 1 to 4, wherein the first set of counterpoint watermarks comprises at least one line in the longitudinal direction of the web and the second set of counterpoint watermarks comprises at least one line in a transverse direction of the web.
A method as claimed in any one of claims 1 to 4, wherein the first set of counterpoint watermarks comprises at least one arced line in a first direction and the second set of counterpoint watermarks comprises at least one arced line in an opposing direction.
A method as claimed in any one of claims 5 to 10, wherein the lines are continuous or non-continued or comprise indicia.
A method as claimed in any one of the preceding claims, wherein the sets of counterpoint watermarks provide information.
A method as claimed in any one of the preceding claims, wherein the positive regions of the counterpoint watermarks comprise preferably greater than 75% by area, preferably greater than 90%, and more preferably comprise 100% of the are of the counterpoint watermark.
A security document made by the method of any one of claims 1 to13.
A security document as claimed in claim 14, the security document being one of a banknote, passport, share certificate, document of value, identity document, voucher, birth certificate, security label, or driving licence.
A cylinder mould cover or dandy roll cover for producing a continuous web of paper, comprising:
a plurality of sets of embossings for producing counterpoint watermarks in the paper, said embossings being distributed along a circumferential direction of the cover, wherein the sets of embossings comprise at least a first set of embossings having a first pattern and a second set of embossings having a second pattern different from the first pattern, wherein if the first set of embossings were superposed on the second set of embossings there would be a plurality of overlap points.
A cylinder mould cover or dandy roll cover as claimed in claim 16, wherein at least one or each first set of embossings is adjacent at least one second set of embossings.
A cylinder mould cover or dandy roll cover as claimed in claim 16 or claim 17, wherein each second set of embossings is adjacent at least one first set of embossings.
A cylinder mould cover or dandy roll cover as claimed in any one of claims 16 to 18, wherein the sets of embossings further comprise further sets of embossings having further patterns different from the first pattern and the second pattern.
A cylinder mould cover or dandy roll cover as claimed in any one of claims 16 to 19, wherein the sets of embossings comprise at least one line.
A cylinder mould cover or dandy roll cover as claimed in claim 20, wherein the lines are straight or curved.
A cylinder mould cover or dandy roll cover as claimed in any one of claims 16 to 19, wherein the first set of embossings comprises at least one line at a first angle with respect to an axis of the cover and the second set of embossings comprises at least one line at a different angle to the first angle.
A cylinder mould cover or dandy roll cover as claimed in any one of claims 16 to 19, wherein the first set of embossings comprises at least one line in the longitudinal direction of the cover and the second set of embossings comprises at least one line in a circumferential direction of the cover.
A cylinder mould cover or dandy roll cover as claimed in any one of claims 16 to 19, wherein the first set of embossings comprises at least one arced line in a first direction and the second set of embossings comprises at least one arced line in an opposing direction.
A cylinder mould cover or dandy roll cover as claimed in any one of claims 20 to 24, wherein the lines are continuous or non-continuous or comprise indicia.
A cylinder mould cover or dandy roll cover as claimed in any one of claims 16 to 25, wherein the sets of embossings provide information.
A cylinder mould cover or dandy roll cover as claimed in any one of claims 16 to 26, wherein the sets of embossings are configured to produce positive counterpoint watermarks.