Abstract:
A cylinder for stamping planar substrates, such as sheets of paper comprises at least a pit with clamping means for holding a stamping medium on said cylinder, said medium comprising at least a row of stamps distributed along the medium. The medium is formed by a stamping plate or by stamping strips and said cylinder further comprises at least a supporting piece forming a bridge over said pit.

Description:
FIELD 
     The present invention concerns a stamping cylinder for planar substrates, such as sheets of paper which carry printed matter. 
     BACKGROUND 
     It is known in the art of printing machines for securities, for example bank notes, to apply security elements with stamping cylinders. For example, PCT application No WO 96/37368, the content of which is incorporated by reference in the present application, discloses a printing method and apparatus using a hot foil stamping. 
     In the field of printing securities, it is known to apply a thin layer of metal foil of predetermined shape or pattern to a substrate, for example bank notes, usually as a feature intended to make counterfeiting more difficult. In a known application, the foil is carried as a continuous lamination between a carrier substrate and a layer of hot melt adhesive, the shape or pattern to be applied being determined by a stamp or press. In prior art arrangements the carrier substrate and the substrate are placed together beneath a platen-type press carrying an array of stamps which are simultaneously applied to cause an array of hot foil imprints to be applied to the substrate. Heat for activating the hot melt adhesive is generally applied from the press. The printing apparatus for applying foil to a substrate comprises means for conveying a carrier for the foil together with an adhesive which is activated by heat in a printing position, means for conveying substrate towards said printing position, printing means at which the carrier and substrate are moved together past said printing position at which pressure is applied to transfer a predetermined pattern or shape of foil from the carrier to the substrate, and pre-heating means for transferring heat to the carrier before it reaches the printing position and/or before it contacts the substrate. Preferably the preheating means is effective to transfer heat to the carrier immediately upstream of the printing position. For example, where the printing means comprises a nip between a print cylinder and a counter cylinder the pre-heating means may be located to be effective as close as possible to said nip. In a preferred arrangement the printing means comprises means for applying both pressure and heat at said printing position. For example, the printing means may comprise a print cylinder carrying one or more stamps defining said predetermined pattern and/or shape, the cylinder carrying heating means in the vicinity of said stamp or stamps. 
     For example, U.S. Pat. No. 6,263,790, the content of which is incorporated by reference in the present application, discloses a machine for security printing on security paper on which security elements carried by a foil are applied to the security prints by a cylinder comprising heated stamps. 
     Another prior art publication, U.S. Pat. No. 6,062,134, the content of which is incorporated by reference in the present application, discloses a stamping cylinder comprising a plurality of stamps, said stamps being mounted on the cylinder through adjustable means allowing each of the stamps to be adjusted in the axial direction and in the circumferential direction of the stamping cylinder. 
     A further prior art is disclosed in U.S. Pat. No. 5,429,044, the content of which is incorporated by reference in the present application. In this patent, a machine for security printing on security papers, especially bank notes, is described, said machine comprising a device for feeding with paper, a device for transporting the paper and a unit for applying optically variable images supported by at least one tape which is brought into contact with the paper such that these images are applied at defined locations on the paper corresponding to the printing marks on security papers. The application unit includes at least one applicator cylinder interacting with a pressure cylinder, the paper moving along between these cylinders. The tape is mounted between two reels arranged in a sector of the applicator cylinder and the tape passes along a generatrix of said applicator cylinder, perpendicularly to the path of the paper, under the action of a servomotor acting intermittently at each application step. 
     Other prior art publications include DE 41 25 996 C1 which relates to a heatable stamping roller with a hollow drum member comprising an internal heating system and a drum mantle with a stamping profile and at least a segment which is interchangeable and located on the drum mantle. The advantage of this design is that the stamping surface can be changed without having to dismantle the heatable roller section. 
     Finally, publication GB 2 255 317, the content of which is incorporated by reference in the present application, discloses a foil deposition machine which transfers the foils from a carrier onto a surface to be decorated. As indicated in this prior art, foil transfer is conventionally carried out as a flat-bed process, with a die being mounted oh a flat bed and the surface to which the decorative layer is to be applied (i.e. print material) being mounted on the surface of an impression cylinder. The flat bed with the die moves backwards and forwards relative to the impression cylinder, along a path which is tangential relative to the cylinder. A band of foil is supported above the die. As the die approaches the cylinder surface, the foil is pressed by the die (which has been heated to a suitable temperature) against the print material on the cylinder, and the decorative layer is transferred to the print material. The print material is usually paper or card, but may be of plastics or other materials. When the die moves away from the cylinder, the release agent allows the carrier layer to be released from the decorative layer. In order to avoid the inherent slowness of a flat-bed process, it has been proposed to carry out foil blocking by a rotary process. Hitherto, this has required the construction of cylinders on which the foil blocking stamps are engraved. The construction of such cylinders is very expensive, and because the die surface is integral with the body of the cylinder, there are often problems in ensuring correct registration of the die surface with the print material. In GB 2 255 317, there is provided a foil blocking die mounted on a substrate, which substrate is adapted to be secured around the circumference of a cylinder. The use of a die which can be mounted on a cylinder surface for operation, but can be dismounted from the cylinder surface has the particular advantage that the higher operating speeds possible with rotary equipment, as compared to reciprocating equipment, can be obtained. 
     A problem with the design of GB 2 255 317 is the presence of a non-continuous surface of the plate carrying the stamps forming an opening, which produces vibrations in the machine when the pressure cylinder passes the opening of the plate. Such vibrations then are the cause of improper application of foil or misregistration of printing. A solution to overcome this problem is to mount the cylinders on laterally displaceable axis, for example with excentrical bearings, to relive the pressure when passing such openings but this design is complicated and expensive. 
     SUMMARY 
     It is therefore an aim of the present invention to improve the known machines. 
     A further aim of the invention is to provide a stamping machine with a stamping cylinder that is easy to operate. 
     A further aim of the present invention is to provide a stamping cylinder that allows retrofitting into existing machines. 
     To this effect, the invention complies with the definition of the claims. 
    
    
     
       BRIEF DESCRIPTION 
       The invention will be best understood with reference to several embodiments and to drawings in which, 
         FIG. 1  shows a perspective view of a first embodiment of the stamping cylinder; 
         FIG. 2  shows a perspective view of a second embodiment of the stamping cylinder and 
         FIG. 3  shows a perspective view of the pit of the cylinder. 
     
    
    
     DETAILED DESCRIPTION 
     In  FIG. 1 , a first embodiment of the stamping cylinder according to the present invention is shown. In this embodiment, the cylinder  1  carries a stamping plate  2  according to the invention. The stamping plate  2  comprises several parallel tracks  3  to  8  on which the stamps  9  are formed. On both sides of the stamps  9  of a track ( 3  to  8 ), there are support borders  10  running along the length of the plate  2 , which have approximately the same height than the stamps  9  and which are used to precisely adjust the position of a pressure cylinder (not shown) with respect to the stamps  9 , said pressure cylinder being used to press the substrate against the foil during the stamping operation, as is known per se in the art. 
     Over the cylinder pit  11  in which the plate  1  is clamped, there is for each track  3  to  8  of stamps  9  a pair of supporting pieces  12 ,  13  which prevent the pressure cylinder to fall into the pit  11  when passing said pit  11 . Said supporting pieces are made preferably of metal and provide a continuous surface of the support borders  10  over the pit  11  thus guiding and maintaining the position of the pressure cylinder over the pit in a smooth manner. 
     Each supporting piece  12 ,  13  is fixed in the cylinder pit  11  by a screw  14  or other equivalent means and comprise a plurality of holes  15  for vacuum (air under depression). The vacuum is brought in the cylinder pit  11  and the supporting pieces  12 ,  13  are mounted and fixed to a support part  16  connected to the vacuum supply and preferably held in the cylinder pit  11  by means of a screw. On the support part  16 , a front alignment stop  17  is mounted in an adjustable manner, and the stop  17  passes through the supporting pieces  12 ,  13 , to provide an alignment reference for the sheet to be embossed which is maintained against the pieces  12 ,  13  by the vacuum. 
     In  FIG. 2 , a second embodiment of the stamping cylinder  1  is shown. In this embodiment, the stamping plate of  FIG. 1  is replaced by several stamping strips  18 , each trip having a track  3 ′ to  8 ′ with support borders  10 ′. Other elements which are similar to the embodiment of  FIG. 1  are referenced in identical manner. Each strip  3 ′ to  8 ′ is mated with a pair of supporting pieces  12 ,  13  to allow a smooth passage of the pressure cylinder over the pit  11  of the stamping cylinder  1 . 
     In a similar manner to  FIG. 1 , each supporting piece  12 ,  13  comprises holes  15  for vacuum, is held on the cylinder through a screw  14  and a support part  16  connected to a vacuum supply and fixed in the pit  11  by a screw. The supporting pieces  12 ,  13  are traversed by an alignment stop  17  for aligning the sheet being embossed. 
       FIG. 3  shows in a more detailed manner an example of a pit  11  of a cylinder  1  on which a stamping plate  2  is mounted without the supporting pieces  12 ,  13 . On the left side of the pit  11 , one end of the plate  2  is cut to form notches with extensions  24  and cutouts  25 . The end of the extensions  24  are bent and are clamped in the pit  11  by clamping means, such as screws  26  and clamp bar  27 . The other end of the plate  1  is shaped in the same manner (extensions  28  and cutouts  29 ) and is clamped with clamping means, i.e. screws  30  and second clamp bar  31  which is also displaceable such as to pull the plate  2  on the cylinder under stress. 
     The vacuum supply for the holes  15  of the supporting pieces  12 ,  13  is indicated by reference  32 . 
     Once the supporting pieces  12 ,  13  are mounted, as shown in  FIGS. 1 and 2 , they form a bridge over the pit  11  extending from cutouts  25  to cutouts  29 . 
     The principle shown for mounting the stamping plate on a cylinder can be also used in the case of stamping strips  18  as in the second embodiment of  FIG. 2 . 
     The heating of the stamps can be made in conventional manner known in the art, for example, as known from GB 2 255 317 or U.S. Pat. No. 6,263,790, enclosed by reference in the present application to this effect. Preferably, the heating is made by heating the entire cylinder  1 , either by a liquid such a water or oil which passes in a circuit in the cylinder  1 , or in a variant by electrical heat resistances known per se in the art. 
     The number of plates per cylinder or of successive strips can be chosen depending on the circumstances or the format of the sheets to be printed. Preferably, there are three successive stamping plates per cylinder or three successive stamping strips around the circumference of the cylinder. In this case, the cylinder comprises three pits equally spaced around the circumference of the cylinder, all pits being similar to the pit  11  described here above. 
     In addition, the cylinder comprises intermediate stamps (referenced  33  in the figures). Theses intermediate stamps are placed in the cylinder pits  11  for each row of stamps  9  and they are used to maintain the pressure between the paper sheets being embossed and the foil and avoid misregistration or relative movement between the sheets being embossed and the foil. 
     The advantage of using a plate according to the first embodiment of the invention is to facilitate the mounting of the plate. On the other hand, the advantage of using strips as in the second embodiment is the fact that the strips can be individually adjusted, for example laterally. 
     A further advantage of the stamping medium according to the present invention with respect to the design known from U.S. Pat. No. 6,062,134 is the fact that there is no need anymore for an individual adjustment of the stamps. 
     As can be readily understood from the present description, the principle of the invention can also be used in planar stamping machines and not only on a cylinder. In this case, the stamping medium (plate or strips) are mounted on a plate replacing the cylinder disclosed in the present application, said plate being heated in a similar manner (for example by a liquid or electrical heating resistances).