Patent Publication Number: US-8524039-B2

Title: Papermaking mould for producing two-stage watermarks and method for producing the same

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates primarily to a method for producing a screen for papermaking with watermarks, in particular for producing papers of value, such as bank notes, checks, identification documents and other security papers. The invention in addition relates to a mask for use in the aforementioned method, a papermaking screen to be produced by said method and a paper of value produced by means of such a papermaking screen. 
     2. Description of the Background Art 
     Papermaking is done in continuous fashion on so-called cylinder paper machines or fourdrinier paper machines, whereby paper pulp is deposited on a moving screen and consolidated to the extent that it can be removed from the screen as wet paper web for further processing such as drying, etc. 
     In the production of watermarked paper, a distinction is made between two-level watermarks with a strong light-dark effect and multilevel watermarks with many soft transitions between light and dark. To produce two-level watermarks, metal wires or metal moldings (so-called electrotypes) are soldered onto the screen structure to completely close the screen at these places. Thus, paper deposit is hindered in these partial areas of the screen, and thin places form in the paper that appear very light when viewed in transmission. In the production of multilevel watermarks, however, no holes of the screen are closed. Instead, a three-dimensional relief is embossed into the screen so that the paper thickness of the finished paper varies in accordance with the relief and shows soft transitions between light and dark areas in transmission. 
     WO 99/14425 describes an apparatus for producing absorbent paper and relates in particular to a patterned drying screen on which a paper web is transported or dried. The drying screen comprises not only a two-layer reinforcing structure but also a relief structure of cured photoresist applied thereto. 
     A method for applying a relief structure to a screen is described in U.S. Pat. No. 4,514,345. A thus produced screen is used for providing an existing paper layer with a three-dimensional pattern, the pattern arising by deflecting the fibers into the relief structure. 
     EP-A-0 367 520 describes different methods for fastening flexible watermark designs to a screen or a pressing wire. Said designs serve to produce watermarks in the later papermaking. 
     Unlike two-level watermarks, multilevel watermarks can only be produced on cylinder paper machines since the screens of cylinder paper machines are of rigid form. On fourdrinier paper machines, however, the screen is formed as a moving endless belt and an embossed relief for producing multilevel watermarks would be destroyed by the repeated deflection. But papermaking on cylinder paper machines is time-consuming and cost-intensive, so that embossed, multilevel watermarks are only used in the production of security-relevant papers, such as bank notes or other papers of value. 
     The production of two-level watermarks, although at first glance more versatile, is also problematic in many respects. Thus, soldering the metal wires or electrotypes onto the screen requires elaborate hand labor by highly skilled specialists and is therefore time-consuming and costly. Further, the firmly soldered parts are subjected to great mechanical stress on fourdrinier paper machines and not infrequently come off the screen, which has a negative effect on production reliability and product quality since the loose parts can collect in the screen and destroy the paper. 
     Similar problems also occur with cylinder paper machines, in particular if the two-level watermark is combined with an embossed multilevel watermark, for example to emphasize certain details, writing, coats of arms or the like by special lightness in a multilevel watermark. 
     Above all, the adhesion of electrotypes in the relief-embossed screen areas is critical. In addition, exact positioning of electrotypes in the relief-embossed areas of the screen is difficult. It is accordingly problematic to refasten detached parts at exactly the same position. 
     SUMMARY OF THE INVENTION 
     On these premises, the present invention is based on the problem of proposing a screen and a method for producing a screen for papermaking with a watermark wherein the means responsible for forming the watermark can be produced with little effort and in the accurate position also in embossed screen areas and withstand even strong loads. 
     This problem is solved according to the invention by a screen and a method for producing it having the features described herein. For use in said method, a specially designed mask is proposed in addition. Advantageous embodiments and developments of the invention are described herein. 
     The invention is based on the consideration that it is unnecessary to produce elevations on the screen by means of wires or electrotypes to close areas of the screen therebelow. Rather, it suffices if the screen is closed in said areas in any way, i.e. according to the invention by a sealing compound which is applied partially to the screen and closes the screen completely at these places. The purpose obtained by conventional electrotypes of hindering paper deposit in these areas and thereby producing very light places in the paper is achieved by the inventive measure to the same extent. 
     The sealing compound applied to the screen forms a firm bond with the screen, whereby the sealing compound preferably encloses the screen cloth completely. The sealing compound therefore does not readily come off the screen, thereby avoiding the abovementioned disadvantages of soldered electrotypes. Substances to be used for the sealing compound are ones that can be applied to the screen in a viscous state and bond firmly with the screen after additional treatment or independent curing. For example, one can use viscous adhesives, such as systems based on epoxy resins, or solder paste, which only bonds firmly with the meshes of the screen after heating. It is also possible to use radiation-curing adhesives, such as UV-curable adhesive compounds. 
     The invention can be used on both fourdrinier paper machines and cylinder paper machines. In the case of cylinder paper machines, the choice of material for the sealing compound is relatively uncritical due to the lower mechanical load, and relatively brittle materials can be used. For use in fourdrinier paper machines, the elasticity of the sealing compound must meet higher requirements so that the closed screen areas do not break open due to the repeated deflection. For example, materials such as rubber or other elastomers can be used in this case. 
     The invention is not limited to the production of two-level watermarks. A corresponding rasterization of the sealing compound will also permit multilevel watermarks to be produced. If the different gray tones of image information are assigned a certain coverage of sealing compound, more or less paper pulp is accordingly deposited in the individual areas. 
     Numerous ways of locally applying the sealing compound are imaginable. Thus, application can be effected e.g. by screen printing, ink dabber printing, spray technology or knife-coating. According to a special embodiment of the inventive method, a mask controls the particular application. The mask can be used for partial application of sealing compound or for partial curing of sealing compound applied over a large surface. 
     According to a first embodiment, the mask is positioned over the screen and the sealing compound then applied to the mask. The mask has gaps having the contour of the partial area to be closed, so that sealing compound can only flow into the screen cloth and close it in said partial area. 
     According to a second embodiment, the sealing compound is first applied to the screen over a large surface and then the same mask positioned thereover as in the first embodiment. Then the sealing compound is cured in the gaps of the mask so that the screen is only closed lastingly in these areas. A suitable material for activable sealing compound is for example UV-radiation-curing adhesive. Uncured sealing compound is washed out of the screen in a following step. 
     In a third embodiment, the sealing compound is first applied to the screen over a large surface and then covered with a mask, as in the second embodiment, but in this case the mask has the form of the partial area of the screen to be closed. The areas of the screen not covered by the mask are freed from sealing compound by the sealing compound being blown or sprayed off for example. The remaining sealing compound then cures actively or passively. 
     The use of masks has the advantage that they can be easily designed on the computer and produced by machine. Using the data generated on the computer, the masks can be produced e.g. by milling, laser cutting or laser milling or also by etching according to a previously produced film master. In addition, the application of sealing compound is capable of automation, so that the production of a screen for producing watermarks can be made substantially more efficient by the present invention. Since the positioning of the masks is simple and exact, the invention also has an effect on the quality of the screen and the paper produced therewith. The quality is moreover reproducible anytime when a new identical screen is produced. 
     It is especially advantageous to use the invention for producing an especially light watermark area (a so-called “highlight watermark”) within a multilevel watermark. In this case, said areas are provided with sealing compound within the embossed screen, preferably in the apex area of the relief. A mask can be used here that has a surface structure adapted to the embossed relief of the screen, so that when the mask is laid on the screen it locks into the embossed relief of the screen. The positioning of the sealing compound relative to the multilevel watermark is thus exactly defined and completely repeatable. 
     The sealing compound preferably has added thereto, for further stabilization, fiber-reinforcing fillers, such as glass fibers or carbon fibers if the sealing compound is based on adhesive, or fine copper threads if the sealing compound consists of solder paste. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following the invention will be explained by way of example with reference to the accompanying drawings, in which: 
         FIG. 1  shows a bank note with two-level watermarks  1  and multilevel watermark  2 ; 
         FIG. 2  shows schematically a cross section through a screen after application of sealing compound using a mask; 
         FIG. 3  shows schematically a cross section through a screen after application of sealing compound in a relief-embossed area of the screen using a mask; 
         FIG. 4  shows schematically a cross section through a screen during active, partial curing of sealing compound applied over a large surface, using a mask; and 
         FIG. 5  shows schematically a cross section through a screen after large-surface application of sealing compound and before removal of surplus sealing compound using a mask. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a bank note having several two-level watermarks  1  in the form of the denomination “100” and multilevel watermark  2  in the form of a portrait. Individual two-level watermarks  1  cover multilevel watermark  2  partially or completely. Due to the multilevel watermark the paper for such a bank note is produced on a cylinder paper machine with a relief-embossed screen for producing portrait  2 . 
     With reference to  FIG. 2  an inventive method will be described in the following for producing screen  20  according to a first embodiment to permit production of a strict two-level watermark with the screen. Accordingly, mask  30  is positioned over screen  20 , said mask having gaps  31  defining those partial areas  11  of screen  20  where screen  20  is to be closed with sealing compound  10 . Sealing compound  10  is applied to mask  30 , penetrating through mask  30  into screen  20  only in gaps  31  and completely closing the cloth of screen  20 . This state of the inventive method is shown in  FIG. 2 . 
     In a next step, mask  30  is removed and the sealing compound either cures passively or the curing process is controlled actively, for example by supply of temperature or UV irradiation depending on the sealing compound. Alternatively, sealing compound  10  can first be cured or left to cure and only then mask  30  removed, thereby giving the contour of the two-level watermark an especially exact and sharp boundary. 
     It is to be taken into account that the ratios of size in  FIG. 2  do not correspond to reality. The thickness of mask  30  is shown overdimensioned compared to the thickness of screen  20 . 
     After the sealing compound has bonded with the structure of screen  20  and completely closed it in these areas, the screen is partially water-impermeable, thereby hindering paper deposit at these places during papermaking. These areas appear in transmission as very light areas compared to the surrounding paper material in paper produced with such a screen. 
     While the method described with respect to  FIG. 2  is suitable both for producing a screen for a fourdrinier paper machine and for a cylinder paper machine, the same method will be described in the following with reference to  FIG. 3  specifically for application in a cylinder paper machine, wherein partial areas  11  of relief-embossed screen  20  are provided with sealing compound. As indicated by  FIG. 3 , the surface of mask  30  adjoining screen  20  has surface structure  32  adapted in the manner of a negative relief to the relief-embossed structure of screen  20  and simultaneously also to the curvature thereof, which is due to the use of screen  20  in a cylinder paper machine. Otherwise the method for closing partial areas  11  with sealing compound  10  does not differ from the method described with respect to  FIG. 2 . That is, mask  30  is laid on screen  20  in such a way that its surface structure  32  locks into the relief-embossed structure of screen  20 . Then sealing compound  10  is applied to mask  30  so that sealing compound  10  penetrates into the screen through the gaps of mask  30  and thus encloses the screen cloth only in partial areas  11  of screen  20 . After that, surplus sealing material is removed from mask  30  (the state of  FIG. 3 ) and mask  30  taken off screen  20 . Finally, sealing compound  10  is cured, or, optionally vice-versa, sealing compound  10  first cured and then mask  30  removed. 
     With reference to  FIG. 4  a second embodiment of the inventive method will be described in the following. Accordingly, in a first method step, sealing compound  10  is applied to screen  20  over a large surface so that it completely flows around and closes the screen cloth. “Over a large surface” means in this context a surface larger than partial areas  11  of screen  20  that are to be finally closed lastingly with sealing compound and that will be responsible for the later production of a watermark during papermaking. After screen  20  is impregnated with sealing compound  10 , the same mask  30  that is used in connection with the embodiment of  FIG. 2  is positioned thereover, and the curing of sealing compound  10  in partial areas  11  defined by gaps  31  of mask  30  is activated by UV radiation sources  40  in the shown embodiment. After curing of sealing compound  10  in partial areas  11 , mask  30  is removed and the uncured areas of sealing compound  10  are washed out so that they are permeable again. The result obtained is screen  20  with the same configuration as is obtained with the method described with respect to  FIG. 2 . 
     According to a third embodiment of the inventive method, to be described in the following with respect to  FIG. 5 , sealing compound  10  is applied to screen  20  over a large surface, as described above with respect to  FIG. 4 , and mask  30  positioned thereover, its outside contour  33  corresponding to the form of partial area  11  to be closed. The areas of sealing compound  10  not covered by mask  30  are removed, for example blown out through screen  20 , and the portion of sealing compound  10  remaining in partial areas  11  is then cured. Alternatively, curing in partial areas  11  can be done first, for example by using heated mask  30  and heat-curing sealing compound  10 , and the remaining, uncured part of sealing compound  10  then removed, for example washed out. 
     Especially suitable substances for the sealing compound are ones that can be applied to screen  20  in the viscous state and bond firmly with the screen after their active or passive curing. It is suitable to use viscous adhesives, such as systems based on epoxy resin for example, but also solder paste as is used for example in the SMD technique and bonding firmly with the meshes of the screen only after heating. It is likewise suitable to use sealing compounds based on UV-curable adhesive. This is primarily applicable to the method described with respect to  FIG. 4 . 
     To increase the stability and bond force of the sealing compound, the latter can be mixed with fillers, for example with glass or carbon fibers in the case of adhesives and for example with fine copper threads in the case of solder paste. 
     Sealing compound  10  can be applied to screen  20  by screen printing, ink dabber printing, spray technology, knife-coating and other common methods, the locally exact application being preferably controlled by mask  30 . 
     Mask  30  can be designed on a computer and produced by CAD/CAM technology automatically by milling, laser cutting or laser milling. It is also practicable to etch mask  30  according to a film master.