EP2550395A1 - Sheet having multitone watermark, method for manufacturing part for forming multitone watermark, part for forming multitone watermark, and use of said part for forming multitone watermark - Google Patents

Abstract

The present invention relates to the field of paper and relates to a sheet including a high-definition high-contrast multitone watermark having areas of pronounced brightness. Such watermark is particularly obtained by means of a part (4) including a surface (5) having a raised area and perforations (6). The perforations (6) are such that making the cross-section thereof, at a given distance from the draining surface, directly depends on said distance. The present invention also relates to the method for manufacturing said part that is used for forming one such watermark, to the part itself, and to the use of said part when forming a watermark.

Description

SHEET COMPRISING A MULTITON WATERMARK, METHOD FOR MANUFACTURING A PIECE FOR FORMING A MULTITON WATERMARK, PIECE FOR FORMING A MULTITON WATIGRANE AND USE OF SAID PIECE FOR FORMING A MULTITON WATERMARK

The present invention relates to the papermaking field and more particularly to a watermark. It relates to a sheet comprising a watermark and a document comprising such a sheet, the method of manufacturing a part used for the formation of such a watermark, the part obtained according to said method of manufacture and the use of this piece during the formation of a watermark.

 Watermarks are commonly used to mark documents with a specific content or value. These include title deeds, certificates, diplomas, banknotes, passports, identity cards, driving licenses, checks, tickets to cultural or sports events or letterheads with letterheads .

 Watermarks are formed during papermaking, in particular by modifying the dewatering of the fibrous suspension from which the paper is made. Thus, a drip sheet generally comprises flat pieces and therefore waterproof so called galvanos or electrotypes. Such a part 1 is represented in FIG.

 As a remark, a drip sheet (also called forming fabric) generally consists of at least two superposed elementary canvases.

These parts or galvanos locally modify the deposition of the fibers of said fibrous suspension with respect to the zones not provided with said parts. The effect obtained is a clear watermark easily observable in transmitted light. Said watermark obtained _. has clear, uniform areas, that is, one grayscale, and very pronounced.

 "Observable in transmitted light" means an element (here the watermark) that is visible, in particular to the naked eye, when placed between the eye of the observer and a light source so that the eye perceives the rays of the light source through the observed element.

 As a remark, the watermarks are visible in transmitted light but they are not or are not very visible in reflection, that is to say when the eye of the observer and the light source are placed on the same side of the element to be observed, the eye thus perceiving the rays of the light source reflecting on the observed element.

Additional technological developments have resulted in watermarks with several shades of gray. These watermarks, called multitone watermarks or shaded watermarks, are generally obtained by an embossing of the dewatering fabric 2, for example represented in FIG. 2, and especially of at least one of the elementary webs, preferably the upper web, in embossed relief 3 composed of lowered portions 32 and raised portions 31, Such watermarks consist of lighter areas and darker areas than the non-watermarked portion (or velin) of the fibrous substrate. The lighter and darker areas are respectively called light and dark watermark.

 The multiton watermarks are generally obtained according to the following papermaking process: a fibrous suspension comprising water and fibers to which fillers are optionally added, in particular retention agents, opacifying agents, pigments, anti-bacterial agents and or anti-fungal, is brought into contact with said embossed drip cloth. When draining, one. more fiber is deposited with respect to the lowered parts than to the elevated portions relative to the planum of the drip. The "wet" sheet obtained by draining said fibrous suspension on said dewatering fabric is then pressed and then dried according to the current papermaking process. As a remark additional treatments can be applied in or on the sheet during its manufacture, including operations such as impregnations, surfacing, coating and / or smoothing. By observation in transmitted light of the filigree of the sheet thus obtained, there are several gray levels, each level of gray corresponding to a level of relief, lowering or elevation, said parts.

 The manufacture of a drip sheet comprising embossings for the formation of multitone watermarks requires a lot of time and is therefore very expensive. Indeed, such a dewatering fabric may for example comprise several hundred of said parts which must all be identical to allow obtaining identical watermarks. The manufacture of said parts provided with relief therefore requires particular attention, especially with regard to their dimensions and the reproducibility of their manufacturing process.

 The drip sheet of a round-shaped paper machine for forming watermarks generally comprises at least two superposed metal sheets. It is therefore sensitive to mechanical shocks. Such impacts occur in particular on the most exposed parts, for example the raised parts of an embossing. They damage the drip sheet and thus irremediably disrupt the formation of the sheet of paper, and at least a portion of the drip sheet, for example the outer fabric, must then be manufactured again.

 Clearly the limited life of a drip sheet is one of the factors influencing the total cost of producing watermarked paper.

Recently, the application DE 10 2005 042 344 proposed inserting locally, at the level of an embossing of the outer fabric of the drip screen, a flat piece perforated according to the embossing relief. The perforations of said part are micro-perforations. The advantage described in this application lies in the creation of multitone watermarks including clear areas.

 The application DE 10 064 006 describes how the areas of the dewatering cloth comprising these perforated parts and making it possible to obtain these very light areas may be provided with additional parts by means of a shape memory material.

 Applications DE 2006 058 513 and WO 2008/071325 describe a part used for the formation of watermarks. This injection molded plastic part is provided with a relief comprising perforations made by means of a laser. Thus said perforations are formed in a profiled part, that is to say comprising a relief on one of its faces, and from the face opposite to the profile. According to one variant, the perforations become thinner going from the rear face (dripless side) towards the profiled face. It is explained that the perforations provide a free flow of the fibrous suspension and that the areas of said workpiece having a greater thickness allow the formation of thin points in the paper.

A disadvantage of this method concerns the accuracy of the perforation, and in particular its diameter on the profiled face, which strongly depends on the properties of the material used, and in particular the nature of the plastics and the thickness of the material. The accuracy of the perforation is therefore difficult to control. The laser forms holes that shrink towards the profiled face. Due to the physical process implemented during the perforation of a plastic material by means of a C0 ₂ laser, the shape of the perforation is, in the longitudinal direction of the channel, more or less conical on the laser side due to the Gaussian energy distribution of the laser beam, and the longer the channel (the greater the thickness to be punched), the smaller the conical shape. The decrease of the conical shape does not make it possible to precisely control the different levels of gray in the final watermark observed in transmitted light. The use of a laser to achieve the perforations therefore does not allow to precisely control the diameter of the perforation as a function of the height of the relief. This results in a loss of definition during the formation of the watermark.

 There is a need to obtain a watermark with better definition, high contrast and pronounced clarity, in particular to increase the security of the watermarked substrates and make their authentication or identification easier.

 The Applicant therefore proposes to solve the problems of the prior art by proposing a multiton watermark according to the invention.

The invention relates to a sheet, in particular a security sheet, comprising a multiton watermark having a high definition, a high contrast and pronounced clears. The invention also relates to such a multitone watermark. Said multiton watermark has a high level of detail, including strong shades with a standard deviation of the 8-bit grayscale distribution greater than 10, preferably greater than 15, and / or a difference in grayscale levels. 8-bit greater than 120, preferably greater than 150.

 The distribution of the gray levels is obtained by acquisition by a scanner in transmission at 600 dpi of the image of said multitone watermark.

 The acquisition is done in particular in transparency mode (thus in transmission) with a flatbed scanner and by taking the following precautions:

 colorimetric calibration according to the ICM ("Image Color Management") standard of the scanner and the acquisition chain,

 possibly, limitation of the lighting by polarizing filter to avoid any reflection and any saturation, and

 acquisition any corrections disabled to obtain a raw signal without retouching.

 In particular, the EPSON® Perfection V750 Pro scanner can be used to make this acquisition.

 The limitation of the polarizing filter illumination can be done by the application of a filter on the scanner's acquisition zone, in particular its scanner glass.

 In particular, said multitone watermark comprises pronounced clear zones, in particular characterized by a difference "maximum gray level" - "average gray level" greater than 50, preferably greater than 100, and / or a difference in the gray levels coded on 8 bits greater than 120, preferably greater than 150. Thus, the multiton watermark has very pronounced clear obtained without adding additional specific piece.

 The multiton watermark according to the invention is preferably formed by means of a single piece, in particular fixed to the drip sheet.

 The shades and the different gray levels of the watermark are obtained both by the relief and by the perforations of the said piece used for the formation of the watermark.

 The multiton watermark obtained comprises at least three different thickness values.

 Said watermark also has a very good appearance, that is to say a uniform distribution of fibers and no or few clusters of fibers (also called "flocs"). The epair is a measure of the fomiation of a sheet observed in transmitted light, it is for example measured by a 2D sheet forming sensor, in particular developed by TECHPAP.

In particular, the appearance of the watermark measured at the level of a "flat" is better than the appearance of the vein of the sheet containing said watermark because the formation of the watermark is on a piece provided with a relief and perforated, and not on the drip sheet which has a less regular surface and less precise perforations formed between the son of said dewatering cloth. In this way, the multitone watermarks formed by means of a piece provided with a relief and perforated have a better definition than the watermarks obtained by embossing the drip sheet.

 In particular, said watermark comprises a code formed, positive or negative, by a network of dark spots.

 Said code can be an image, an alphanumeric character, an ideogram, a symbol, a word or an acronym.

 Said code may be observable in light transmitted to the naked eye, that is to say without a device for enlarging the watermark, or with the aid of a device for enlarging the watermark, for example such that a magnifying glass, an image acquisition device, in particular a scanner in transmission, or a microscope.

 Said code is in particular an authentication element of the watermark and more broadly of the substrate comprising it. In particular, said code may be a serial number corresponding to a manufacture of paper for bank notes.

 The invention also relates to a document comprising or consisting of a sheet according to the invention. Such a document is chosen in particular from property titles, cards, certificates, diplomas, banknotes, passports, identity cards, driving licenses, checks, access tickets to cultural or sports events or letterheads. on your mind.

 The invention extends to the method of manufacturing a part for the formation of a multitone watermark in a substrate by draining a fibrous suspension, said part comprising a relief on one of its faces called "profiled face" and opposed to the "draining face", and perforations, said perforations comprising a through channel with an inlet on the profiled face and an outlet on the drainage face, comprising the steps of forming said relief and forming said perforations on said workpiece. so that each perforation is such that the area of the section of its channel at a given distance from the draining face depends directly on said distance.

By "directly depends", it is meant here that the area of the channel section of a perforation at a given distance from the drainage face is a function of, in particular proportionally, preferably linearly, at this distance. In particular for a part comprising a substantially planar drainage face, there is a proportionality link between the area of the channel section of a perforation at a given distance from the drainage face and this given distance. In this way, the drainage capacity of a perforation depends on the relative position of the relief with respect to this perforation. Preferably, the linear variation is inverse, in other words the greater the distance, the smaller the area of the channel section of a perforation. In other words, the area of the channel section of a perforation decreases from the drainage face to the profiled face.

 The part may comprise perforations all having the same section for a given distance from the drip face.

 The thickness of the part can be variable, so that the thickness of the perforations can be variable. The perforations may be all through, that is to say they open on the profiled face. Alternatively, at least a portion of the perforations, or all, may be non-through, that is to say that the perforations do not reach the profiled face. The part may in particular have a thickness chosen so that the perforations can not lead to the profiled face.

 The majority of perforations, better all, may have a variation of identical section depending on the distance to the draining face.

 When creating a part according to the invention the de draining effect on the formation of the watermark is predictable. It is thus possible to predict the effects of the relief and those of the perforations on the formation of the watermark, and consequently to determine the relief and the perforations of a piece to obtain a given watermark.

 Said "drainage face" is preferably substantially flat. Alternatively, it follows the relief of the drip sheet.

 According to a particular case of the invention, said piece is metallic or ceramic. By "metal part" is meant in particular a part comprising iron, nickel, copper, stainless steel, titanium, tungsten, cobalt, tin and / or their alloys, for example 'stainless steel.

 According to a particular case of the invention, said method comprises at least one sintering step to which said part is subjected.

 According to a particular case of the invention, the relief of the "profiled" face and the perforations are formed during the manufacture of said part, in particular during a single step and in particular simultaneously. This in particular makes it possible not to weaken the part by subsequent perforation operations and to save time during the manufacture of said part by reducing the number of necessary steps.

 In particular, the manufacture of said piece is thus performed without resurfacing or drilling. These operations contribute to the increase in the cost of the part and adversely affect the reproducibility of the process. However, said method must make it possible to obtain identical pieces.

According to a variant of the invention, said process for manufacturing a part for the formation of a multiton watermark in a substrate by draining a suspension fiber, is characterized in that said part is injection molded, said injection molding to obtain said relief and said perforations simultaneously.

 In particular, said manufacturing method is a method which comprises at least the following steps:

use a metal powder or a ceramic powder, and optionally a thermoplastic binder,

 injecting said powder and, if necessary, said binder into a mold allowing the formation of a part comprising said relief and said perforations,

 - possibly debonding said piece, and

- Sinter the room, that is to say, carry out its densification in the solid state.

 Such a method is a process for molding metals or ceramics by injection (MIM process for "Metal Injection Molding" or CI.M for "Ceramic Injection Molding") which makes it possible in particular to produce complex shapes on mechanically resistant parts, with very high precision and high efficiency, and without the need for further machining. In addition, this technology makes it possible to produce appearance parts with left-hand shapes that are difficult to machine and with very high surface qualities.

 Said metal powder may be chosen from powders of iron, nickel, copper, stainless steel, titanium, tungsten, cobalt and their alloys, for example stainless steel. It may be a particle powder whose largest dimension is less than 50 μιτι, preferably between 1 and 40 μηι. Preferably said metal powder is a powder of copper alloy, iron alloy, tin alloy, alloy of stainless metals such as nickel and chromium, or pure metal such as titanium. Copper alloys and stainless metal alloys have the advantage of resisting corrosion during paper production.

 Said ceramic powder may in particular comprise zirconia.

 Said thermoplastic binder can be chosen from polyethylene (PE), polypropylene (PP), paraffin, stearic acid, polyacetal (POM) and mixtures thereof. Preferably said thermoplastic binder is polyacetal (POM).

 Said mixture may in particular comprise 60% of metal powder and 40% of plastic binder.

Mixtures of metal or ceramic powder and thermoplastic binder are commercially available. It may especially be the product Catamold * marketed by BASF. Said. commercially available mixtures may be in the form of powder or granules of metal powder mixed or coated with a thermoplastic binder. Said granules are ready for implementation by injection into a mold. Said mold comprises an upper face and a lower face. Once the closed mold, the interior of the upper face is provided with a relief and the lower and upper faces are connected by several "columns" which serve to form the perforations of said piece. Said columns may be of round or polygonal section, said section having a decreasing area going from said lower face to said upper face.

 The shape of the mold is substantially complementary to the shape of said piece for the formation of a watermark, differences may exist in particular because of the method used.

 While the machining only allows for conical or cylindrical perforations in the material and while the laser does not allow to control the shape of the perforations made, the process of molding the metals by injection makes it possible to produce perforations with polygonal section. which exhibit a behavior, with respect to the dewatering of the fibrous suspension during the formation of the watermark, similar to that of the round-section perforations, for example with a polygon comprising at least six, preferably at least ten sides while allowing a better stiffening of the mold, the polygonal section columns being less subject to torsion and bending phenomena.

 Once the part demolded, the debinding operation aims to remove the thermoplastic binder. It can be carried out using a solvent (in particular elimination by chemical reaction or dissolution) and / or a firing (in particular elimination by evaporation or destruction of the chemical bonds by catalysis), the conditions of atmosphere, of temperature and pressure may vary depending on the mixture used.

The sintering operation performed on the debinding part is a heat treatment for densification in the solid state of the material. It may be carried out under a reduced atmosphere or under vacuum and at high temperatures but below the melting temperature of the metal used, for example greater than 1200 ° C. for certain iron alloys and stainless metals. Sintering is accompanied by a phenomenon of dimensional shrinkage of the part, the metal becomes denser by occupying the free spaces left by the thermoplastic binder eliminated during debinding. This dimensional shrinkage is controlled by the operating conditions and is carried out moreover with a conservation of volume and reliefs. Said part can thus reach a density close to 1, for example greater than 0.98, and it is an exact replica (the tolerances are very small, of the order of 0.03 to 0.07%) on a scale less than 1, for example at a scale of 80%, of the inside of the mold (relief of the upper face and perforations formed by the columns included). The sintering operation being under control through time, temperature and pressure parameters, the density can be controlled and adapted to the alloys used. It will for example be very close to 1 with powders based on titanium and between 0.8 and 1 for copper alloys.

 Thus a density of 0.8 leads to the production of a porous piece. This residual porosity controlled during production gives the part filtration properties.

 Optimum operating conditions, including atmosphere, temperature, and pressure, may be determined by those skilled in the art.

 The invention also extends to a mold as described above and intended to be used for the manufacture of a part for the formation of a multiton watermark according to the invention, in particular by a process for molding metals or ceramics. by injection.

 According to another variant of the invention, said process for manufacturing a part for the formation of a watermark comprises at least the steps of using a powder and densifying said powder, in particular metal, under the action of a laser .

 In particular, said method of manufacturing a part for the formation of a multitone watermark in a substrate by draining a fibrous suspension, is a sintering method using a laser. These include a selective laser melting process (or S.L.M. for "Selective Laser Melting") or selective laser sintering (or S.L.S. for "Selective Laser Sintering"). Said sintering method using a laser makes it possible to form said relief and said perforations in one and the same step.

 The S.L.M. allows the local densification of a material in powder form, by melting it under the action of a high power laser. The material in powder form is, for example, a metal powder.

 The operating conditions of such a process can be as follows:

 - manufacturing chamber under nitrogen atmosphere,

 use of an infrared (IR) laser, having for example a power of between 50 and 100 Watts, and

- use of metals such as stainless or tool steels, titanium, cobalt-chromium mixtures or aluminum.

 The S.L.M. achieves an accuracy of ± 50 μηι per 100 mm, or 0.05%.

Due to the precision and low tolerances of the previously described processes, in particular MIM, CI. M. or SLM is obtained, using the same mold, identical parts and therefore a very high reproducibility multiton watermarks formed by these parts. This precision and these low tolerances also allowing to obtain parts with a relief at a precision between 0.03 and 0.07% and very fine perforations up to 0.2 mm in diameter. The watermarks formed using these parts therefore have a better resolution and a great fineness and precision, especially in terms of the details of the pattern such as the hair on a portrait or the features of a face, even larger than those obtained by previous methods.

 The invention also relates to a piece for the formation of a multitone watermark, as described above, in a substrate by draining a fibrous suspension, characterized in that it comprises a relief on one of its faces called " profiled face "and opposed to the" drainage face ", and perforations, said perforations comprising a through channel with an inlet on the profiled face and an outlet on the drainage face, and each perforation being such that the area of the section of its channel at a given distance from the draining face depends directly on said distance.

 In particular, the dewatering face is substantially flat, and preferably flat. According to a particular case of the invention the section of said perforations is oval, it is preferably a disk.

 According to a particular case of the invention, said perforations have a polygonal section whose area decreases going from the drainage face to the profiled face. Thus the drainage capacity is proportional to the length of the channel, that is to say, the relief, so that the higher the relief is away from the drip side, the less the drainage capacity is important. This proportionality link makes it much easier to design the relief. Thus it is indeed easier to predict the effects of the perforations on the deposition of the fibers on said part during the dewatering, and thus to provide the pattern that will appear during the observation in transmitted light of the watermark formed by means of said room.

 On the other hand, the high zones of the relief correspond to zones where the deposition of fibers will be weak, both because of the relief, as explained above, and because the area of the section of the channel of the perforations situated in this zone high relief is reduced. Their drainage capacities are indeed lower than those of perforations located in lower areas. This has in particular the effect of less suction and consequently a less significant deposition of fibers at said high areas. For these reasons we obtain very pronounced clear, similar to those obtained with galvanos but it is impossible to obtain by simply embossing the drip cloth.

The perforations are preferably sufficiently thin not to allow the fibers of the fibrous suspension to pass while it is being drained. According to a particular case of the invention, the perforations are distributed uniformly. They form for example rows and columns respectively spaced from 0.3 to 0.6 mm. In a variant, the perforations are distributed in a variable manner.

 According to a particular case of the invention, the shape, the dimensions, the distribution and / or the arrangement of the perforations form, in positive or in negative, a code, in particular an image, an alphanumeric sign, an ideogram, a symbol, a word or an acronym.

 Said code is thus an authentication element of said piece but also of a sheet comprising a multiton watermark obtained by means of said piece. Indeed, the perforations are observable on the watermark in the form of dark points.

 Depending on the size of the perforations, the arrangement of the perforations and therefore the said code can be observed in light transmitted to the naked eye, that is to say without any device making it possible to enlarge the watermark, or with the aid of a device for magnifying the watermark, for example such as a magnifying glass, a thread count, an image acquisition device, especially a scanner in transmission, or a microscope.

 Said code can constitute an authentication element or identification of the manufacturer of said piece for the formation of a multiton watermark, the manufacturer of the substrate comprising said multitone watermark, the client or the client. The code is non-erasable and difficult to counterfeit, particularly more difficult to counterfeit than printing.

 According to a variant of the invention, said part is uniformly perforated over its entire surface, in particular along lines and columns, for example regularly spaced, and at least one perforation is not through. In other words, the relief is locally higher, that is to say at a greater distance from the draining face, than the perforations. In this way we obtain clearer more pronounced than before, the suction being zero at the level of said at least one non-through perforation. In addition, during dewatering the perforations located around the at least one non-through perforation draw the fibers above said at least one non-through perforation by suction. This has the effect of creating even more pronounced clear, and especially more pronounced than in the case of a non-perforated piece or uniformly perforated but without non-through perforations.

According to a particular case of the invention, certain zones of said piece for the formation of a watermark are devoid of perforations. The dewatering is thus locally reduced and lighter areas of the contrasting filigree (in particular with a constant relief) are obtained with the surrounding areas provided with perforations. The method according to the invention also makes it possible to create very pronounced reliefs and in particular low areas disposed on the profiled face near high areas. This, combined with the aspiration of the perforations, makes it possible to obtain contrasting and very nuanced watermarks and having in particular clear pronounced close to dark.

 The invention extends to the use of at least one piece for the formation of a multiton watermark in a substrate by draining a fibrous suspension, said part comprising a relief on one of its faces called "face profiled "and opposed to the" draining face ", and perforations, said perforations comprising a through channel with an inlet on the profiled face and an outlet on the drainage face, the area of the channel section of each perforation at a distance data of the drip face depends directly on said distance. According to this use, said at least one piece is fixed in or on a dewatering web through which a fibrous suspension is drained.

 In particular, said piece is metallic or ceramic.

 "Drainage fabric" is understood to mean a surface comprising openings suitable for dripping during the manufacture of the paper. Such a drip sheet preferably comprises several superposed canvases and in particular a support fabric comprising large openings on which are arranged webs comprising smaller openings. Said drip sheet and said supeiposed fabrics can be woven, electroformed or consist of a perforated plate.

 A drip sheet provided with metal parts according to the invention is less vulnerable than conventional embossed dewatering fabrics, especially with respect to mechanical shocks.

 Said part may in particular be attached to the forming fabric by welding, gluing, clamping or attachment.

 Preferably said dewatering fabric or at least one of said superposed fabrics is a metal fabric. The use of metal parts makes it easy to fix said parts, in particular by welding.

 Draining of the fibrous suspension can be done by means of a flat table or a round shape.

 According to a particular case of the invention the drip sheet, and in particular at least one of said superposed canvases, can be cut or embossed to allow the fixing of said piece.

The position of said piece, in particular the distance of said piece to the planum of the drip sheet which comprises it, can be chosen so as to obtain the average value of thickness (hence of opacity) of the desired watermark. Thus the more the part will be lowered, the greater the average value of the thickness with respect to said part will be important. In particular the part can be placed vertically so that the profiled face, in particular the highest parts, of said piece coincides with the planum of the drip sheet. The piece may for example exceed less than 2 mm, preferably less than 1 mm, of the drip sheet. In this way the part is protected from possible mechanical damage during the formation of the watermark. In addition, because of the rigidity of the part, its ability to withstand a mechanical load on or in the drip sheet will be greater than the area of the watermark (or embossed area) of a conventional drip sheet. The piece can also be positioned below the planum of the drip sheet.

 According to a particular case of the invention, the drainage fabric may further comprise zones permitting the formation of watermarks according to the prior art. These zones allowing the formation of watermarks according to the prior art may, for example, correspond to watermarks. comprising a relief obtained by embossing the drip sheet or "halftone" watermarks comprising two gray levels such as those described in the application EP 1 122 360. In particular the zones allowing the formation of watermarks according to the prior art and according to the invention are arranged on the drip sheet so that the watermarks are juxtaposed or superimposed. They are preferably complementary and can form a single pattern. The securing of the sheet comprising the watermarks is improved because the process for forming the watermarks uses different technologies.

 The invention will be better understood on reading the non-limiting examples and the figures which follow.

 EXAMPLE

 A single-cavity mold is produced whose shape is complementary to the shape of the desired part which comprises a relief on one of its internal faces, said upper face and columns. Said columns depart from the face opposite to the upper face, said lower face, to the upper face that it reaches or not, as the case said columns will therefore be used for the formation, respectively, of conical perforations or non-through. The area of the section of each column at a given distance from the lower face varies with said distance linearly and inversely, in other words the larger the distance, the smaller the area of the section of said column.

Catamold resin granules sold by BASF are introduced into a conventional injection molding machine. These granules consisting of metal powder coated with a thermoplastic binder are heated and then injected into the mold described above to obtain a first part. Said piece is then debianed by cooking in an oven. The thermopiastic binder is thus eliminated.

 The debonded part is then sintered by baking in an oven. This solid densification of the material (temperature below the melting temperature of the metal) by heat gives the piece its final cohesion and density, and is accompanied by a controlled shrinkage but a conservation of volume. So that the part thus obtained P2 is a replica on a smaller scale of the shape of the mold and the part before sintering.

 The image is then acquired by a 600 dpi transmission scanner of the image of two 3 cm x 3 cm watermarks representing the same portrait. The illumination and acquisition conditions are identical for both watermarks. The first watermark F1 was made by embossing the fabric forming a certain relief. The second watermark F2 was produced by means of the P2 piece according to the invention, the manufacture of which was described above. Said piece P2 comprises on its profiled face a relief identical to that formed by the embossing of the fabric used for the first watermark Fl.

 The two watermarks Fl and F2 thus obtained are easily distinguishable by observation in light transmitted to the naked eye, the image of F2 in particular has a better definition, a better contrast, clearer pronounced, a better accuracy of the pattern and a better look-through.

 This results in 8-bit grayscale images, from 0 for black to 255 for white.

 Grayscale measurements are made for each watermark on two areas of the portrait: the face and the hair. The measurements are for example made using the Perfection V750 Pro scanner EPSON® brand, in particular according to the protocol described above.

 For each watermark, gray-level distribution curves are then established, counting for each of said zones the number of pixels corresponding to each gray level. These curves make it possible to obtain the data grouped in the following table:

The watermark F2 according to the invention has a mean gray level higher than the average gray level of F1. As a whole, it is therefore clearer than the FI watermark.

 The watermark F2 has a standard deviation of the gray levels higher than that of the watermark F1, it is therefore more contrasted than the watermark F1.

 The watermark F2 has a gray scale deviation greater than the gray level difference of the watermark F1, respectively for the same areas. These areas are therefore more contrasted on the watermark F2 than on the watermark Fl.

 The watermark F2 presents a. maximum gray level higher than that of the watermark Fl, clears are therefore more pronounced on the watermark F2.

DESCRIPTION OF THE FIGURES

 Figure 1 shows a galvano for the formation of a watermark according to the prior art.

 FIG. 2 represents the embossing of a fabric for the formation of a watermark according to the prior art.

 FIG. 3 represents a part allowing the realization of a watermark according to the invention.

 Figure 4 illustrates the correspondence between the relief of the used part and the watermark obtained.

 Figure 5 shows a negative code formed by the perforations.

 Figure 6 shows a part according to the invention on a training fabric.

 According to the prior art previously described and allowing the formation of watermarks having a single shade of gray, a galvano 1 as shown in Figure 1 is fixed on the forming fabric of a paper machine.

Furthermore, as described above it is known to emboss the forming wire of a paper machine to carry ^watermarks' multitone. FIG. 2 represents a fabric 2 comprising an embossed zone 3 allowing the formation of a multitone watermark. Area 31 being higher than the rest of the fabric, it allows the formation of a clear area of the watermark. Conversely, the zone 32 allows the fomiation of a dark area of the watermark. The zones 31 and 32 comprising a multi-level relief, they make it possible to obtain a multitone watermark.

A part 4 according to the invention for the formation of a multitone watermark, is shown schematically in FIG. 3. Said part 4 comprises a face 5 provided with a relief and conical perforations 6, and the dewatering capacity. each perforation 6 depends directly on the distance to the dripping face of the section on the profiled face of the channel formed by said perforationô. Due to the relief of the face 5, the perforations 61 are not through. This has the effect of allowing the formation of very clear areas on the watermark.

 As a remark, according to the method implemented for the manufacture of said piece 4 for the formation of a watermark, and in particular for the manufacture of the mold for obtaining said piece 4, it may be easier and more practical to manufacture said mold with a lower face comprising uniformly distributed columns and with a relief on its upper face which may, locally, be higher than said columns so as to form non-through perforations on the part 4. The manufacture of a mold with areas comprising uniformly distributed columns, and other areas devoid of columns, may indeed be more complex according to the method implemented.

 FIG. 4 illustrates the correspondence between a piece 4 according to the invention for the formation of a multitone watermark and the shades of gray obtained on said multitone watermark. Said piece 4 comprises a surface 5 provided with a relief and perforations 6, some of which 61 are not through. The part 4 shown in FIG. 5 makes it possible to form a multitone watermark 8 according to the invention comprising several gray levels.

 The zones 71 and 73 have a low relief and therefore perforations having a large drainage capacity, they therefore correspond to darkness of the watermark 8. The zone 72 has a relief higher than that of the zones 71 and 73, it corresponds therefore to a zone from light to very light (from left to right) of the watermark8. The zone 74 has a high relief and a non-through perforation 61 and corresponds to a very clear zone of the watermark. The method according to the invention which is used to produce the piece 4, by its precision, makes it possible to obtain "steep" reliefs which is reflected on the watermark by the presence of very light areas, such as the zone 74, directly adjacent to very dark areas, such as the zone 73. As previously, the zone 76 corresponds to a clear of the watermark placed between two dark corresponding to the zones 75 and 77. The zones 71, 75 and 77 allow the formation of very dark areas on the watermark 8 because beyond the hollow formed by the relief of the face 5, the perforations are wider in these areas so drainage is more important. This results in greater densification of the fibrous mat in formation and consequently the formation of dark areas.

The arrangement and the distribution of the perforations 6 on the surface of the "profiled" face 5 of the part 4 according to the invention can make it possible to form a code 62. FIG. 5 is a top view of such a part 4, it shows the code 62 "AW" in negative. This code can be produced by means of non-through perforations 61 arranged according to the code 62 or by the fact that the part 4 does not have perforations on a zone forming the code 62. According to a variant not shown, the code 62 may be formed by perforations each having a section on the "profiled" face 5 of area Ai while outside the zone defined by the code 62, the "profiled" face 5 is provided with perforations each having a section area A ₂ greater than Ai. The proportion ratio between Aj and A? is in particular chosen so as to make visible, for example under observation in light transmitted with a magnifying glass of magnification equal to 5, said code 62.

 A part 4 according to the invention is shown in FIG. 6. It can in particular be welded by a weld 9 to an embossed zone 21 of the drip fabric 2. This part 4 comprises a "profiled" face 5, perforations 6 and non-through perforations 61.

 The expression "comprising a" or "comprising a" must be understood as being synonymous with "comprising at least one" or "comprising at least one" unless the contrary is specified.

Claims

Sheet, in particular a security sheet, comprising at least one multiton watermark (8), characterized in that the standard deviation of the distribution of the 8-bit gray levels is greater than 10.

2. Sheet according to claim 1, characterized in that the watermark (8) comprises a code (62), in particular an image, an alphanumeric sign, an ideogram, a symbol, a word or an acronym, formed by a network of dark spots,

3. Document comprising a sheet according to claim 1 or 2, said document being chosen in particular from the titles of ownership, certificates, diplomas, banknotes, passports, identity cards, driving license, checks, tickets access to cultural or sports events, or letterhead with letterhead.

4. A method of manufacturing a part (4) for the formation of a multiton watermark (8) in a substrate by draining a fibrous suspension, said part (4) comprising a relief on one of its faces ( 5) said "profiled face" and opposite the "drainage face", and perforations (6), said perforations (6) comprising a through channel with an inlet on the profiled face (5) and an outlet on its drainage face, said method being characterized in that it comprises the steps of forming said relief and forming said perforations (6) on said piece (4) so that each perforation (6) is such that the area of the section of its channel at a given distance from the draining face depends directly on said distance.

5. Manufacturing process according to the preceding claim, characterized in that said piece (4) is metallic or ceramic.

6. Manufacturing process according to any one of claims 4 to 5, characterized in that said part (4) is subjected to at least one sintering step.

7. Manufacturing process according to any one of claims 4 to 6, characterized in that the relief of the profiled face (5) and the perforations (6) are made during a single step, especially simultaneously.

8. Manufacturing process according to claim 5, characterized in that said piece (4) is metal injection molded.

9. Manufacturing process according to any one of claims 4 to 8, characterized in that it comprises at least the following steps:

 use a metal powder or a ceramic powder, and optionally a thermoplastic binder,

 injecting said powder, and if appropriate said binder, into a mold allowing the formation of a part (4) comprising said relief and said perforations (6),

 - possibly deleanter said piece (4) thus obtained, and

 sinter the piece (4) binder.

10. Manufacturing process according to at least one of claims 4 to 7, characterized in that it comprises at least the steps of using a powder and densify said powder, especially metal, under the action of a laser.

1. Part (4) for the formation of a multiton watermark (8) in a substrate by draining a fibrous suspension comprising a relief on one of its faces (5) called "profiled face" and opposite to the "Draining face", and perforations (6), said perforations (6) comprising a through channel with an inlet on the profiled face (5) and an outlet on the drainage face, characterized in that each perforation (6) is such that the area of the section of its channel at a given distance from the draining face depends directly on said distance.

12. Part (4) according to the preceding claim, characterized in that it is metallic or ceramic.

13. Part (4) according to one of claims 1 1 and 12, characterized in that the dimensions, distribution and / or arrangement of said perforations form a code (62), including an image, an alphanumeric sign, an ideogram , a symbol, a word or an acronym.

14. Part (4) according to any one of claims 11 to 13, the area of the channel section of a perforation (6) decreasing from the drainage face to the profiled face.

15. Part (4) according to any one of claims 1 1 to 14, having perforations (6) all having the same section at a given distance from the drip face.

16. Part (4) according to any one of claims 1 1 to 15, having perforations (6) through and non-through perforations.

17. Part (4) according to any one of claims 1 1 to 16, the majority of the perforations (6) having a variation of identical section depending on the distance to the dripping face.

18. Use of at least one piece (4) for the formation of a multiton watermark (8) in a substrate by draining a fibrous suspension, said part (4) comprising a relief on one of its faces ( 5) said "profiled face" and opposite the "draining face", and perforations (6), said perforations (6) comprising a through channel with an inlet on the profiled face (5) and an outlet on the drain face, characterized in that the area of the channel section of each perforation (6) at a given distance from the drainage face depends directly on said distance, and in that said at least one piece (4) is fixed in or on a dewatering web through which a fibrous suspension is drained.