Printing – Processes – Transfer or offset
Patent
1999-02-22
2000-08-08
Eickholt, Eugene
Printing
Processes
Transfer or offset
101119, 101129, 101153, 101154, 101170, B41M 110
Patent
active
060985461
DESCRIPTION:
BRIEF SUMMARY
The invention firstly relates to a method for security printing of documents, in particular banknotes, by printing a sheet or web using intaglio technology, comprising: intaglio plates provided with engravings, which plates are fixed in or on the shell of a rotating plate cylinder, onto a sheet or web pressed against the rotating plate cylinder by a rotating counter pressure cylinder.
When printing banknotes, an image is applied at least on one side of the paper with the aid of intaglio technology. This technology is derived from the old gravure technology. Since 1945 traditional gravure technology has been converted step by step into an industrial process and nowadays production speeds of 10,000 sheets per hour are achieved using what is known as Orlof technology. This speed is comparable to that of commercial offset presses. Intaglio provides the banknote with a relief which is discernible to the touch and offers attractive possibilities for providing the note with security features. Furthermore, a relief offers the possibility of applying additional colour tints using transparent inks, which additional tints are difficult to reproduce, as well as latent images, profiles of different heights being applied. Different images are generated by viewing at an angle. Recently, to protect banknotes against copying by colour copiers, intaglio images in so-called OVI inks (OVI: optical variable ink) have also been applied.
In traditional intaglio, the engravings are completely filled; during the printing process the ink is transferred to the paper, which results in an approximately 20 micron thick ink layer, which is discernible to the touch, on the paper.
In the case of conventional intaglio used on an industrial scale, the engravings are not completely filled, probably as a consequence of combining filling the engravings via a stencil cylinder with wiping clean the areas around the cells with the aid of a wiper cylinder. The relief which is discernible to the touch and usually has a height of 40 microns is made up of an approximately 20 micron thick ink layer and a 20 micron embossing. The embossing is a permanent deformation of the paper, produced by plastic deformation during the intaglio process.
With high speed intaglio only a thin layer of ink is applied in the engravings. As a consequence, a tinted embossing is produced as intaglio image during the intaglio process. The relief discernible to the touch--usually about 50 microns thick--is made up of a 10 micron thick ink layer and an embossing profile of about 40 microns.
It is possible to produce 10,000 sheets per hour with high speed intaglio; with conventional intaglio the maximum number of sheets that can be produced per hour is only 8,000. In the case of high speed intaglio relatively little time is available to introduce ink into the engravings. Moreover, the introduction pressure will have to be increased or the viscosity of the ink will have to be lower or it will have to be accepted that there is less ink in the engravings.
The pressure cannot be infinitely increased and the viscosity has a lower limit which is determined by the fact that the ink must not run out of the engravings.
A simple method for introducing a relatively small layer of ink onto the intaglio plates provided with engravings comprises placing one or more rotary transfer rollers between the stencil cylinder and printing plates mounted on the plate cylinder. Less ink on the intaglio plates provided with engravings means that the time required is shorter and that it is possible to work at a higher speed.
With the aim of applying less ink, it would be possible to produce the stencil image as a halftone. If, however, the halftone has to fill the engravings in the intaglio plates directly, there is a risk that the screen will be visible in the printed image. In order to avoid this, it would be necessary in this case as well to place a transfer roller between the stencil cylinder and the intaglio plate cylinder.
The aim of the invention is to solve the problems indicated above and to pro
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