This invention relates to the manufacture of printing plates, particularly intaglio printing plates for use in the production of bank notes or other security documents.

The preparation of intaglio printing plates is traditionally a long and complex process, which typically includes the engraving of a steel die, the production of derivative copper dies corresponding to the original steel die, the further engraving of copper dies, the formation of a plastic die using the engraved copper die in a press, the production of a nickel mother die from the plastic assembly. The overall process may take up to nine months from the commencement of engraving.

The main objects of the present invention are, while maintaining a high quality of intaglio printing, to reduce the processing time and preferably the cost of production of intaglio printing plates, to shorten the time necessary for the introduction of changes to the print design, and to exploit computer-aided design so as to reduce the need for highly skilled labour.

Although it has been proposed to use C02 or other lasers under appropriate automatic control to cut metal plates with a required pattern, the proposal has not generally been put into practice owing to the undue production of debris and the lack of definition caused by heating of the material constituting the workpiece.

It is known to use lasers that produce light in the ultraviolet region of the electromagnetic spectrum to produce photoablation of organic polymers. One example is illustrated in US-A-5322986, which describes the use of excimer lasers, producing light at a wavelength of 193 or

248 nanometers. Also described therein is the use of a mask disposed sufficiently apart from the organic polymer workpiece to allow for the removal of debris. Another example is shown in US-A-5359173, which describes the use of an ultraviolet laser which is scanned over a rotating workpiece.

An ultraviolet or excimer laser when ordinarily focussed is not particularly suitable for the production of a polymeric matrix for a printing plate because it produces a fine line which may be satisfactory for remedial work but is not satisfactory for ablating regions which are substantially wider than a focussed spot.

The present invention is based on the use of a polymeric plate as a workpiece into which a relief pattern is formed by photoablation using light from an excimer laser or the like, at a wavelength such as 248 nanometers. A mask, having a pattern composed of parts which are opaque and transparent to the laser light, may be disposed in the path of light from the laser and an image of the mask may be formed on the workpiece by means of an appropriate optical system. Photoablation will occur in those parts of the image corresponding to the transparent parts of the mask.

Since normally the image area of the mask exceeds the area of the beam of light from the laser, the mask should normally be moved in its own plane transverse to the path of the light through the mask. The workpiece therefore needs movement in a parallel plane in synchronous converse movement so that the whole area of the mask is imaged onto the polymeric workpiece.

A suitable mask can readily be made from the output of computer aided design equipment either directly or by way of a photo aster. The detail of a required design can be

incorporated to the required scale in a composite dielectric coating on a quartz plate. This type of mask is suitable for use with excimer lasers, ultraviolet light being transmitted where the dielectric coating has been removed and reflected where it has not been removed.

A plastic master made by photoablation may carry multiple image patterns which are accurately spaced without significant surface imperfections. The plastic assembly may be used to make a nickel mother die in a manner simileυ: to the manufacture of a nickel mother die from a plastic assembly according to the traditional process.

The method which employs photoablation avoids entirely the traditional precursors of an engraved steel die and a copper die and is accordingly much more rapid and adapted to accommodate design changes than the traditional process.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates schematically the customary method of manufacture of intaglio printing plates.

Figure 2 illustrates schematically a method of manufacturing intaglio printing plates according to the invention.

Figure 3 is a diagram illustrating the making of a precursor for the intaglio printing plate by photoablation.

Figure 4 is a diagram illustrating schematically a scanning system incorporating the apparatus shown in Figure 3.

DETAILED DESCRIPTION

Figure 1 illustrates schematically an existing process for producing high quality, multiple image, nickel intaglio plates suitable for example for long print runs of bank notes or other security documents. The process is lengthy and requires highly skilled workmanship.

The first stage 1 in the existing process is the engraving of a steel die. An engraver translates the artistic requirements of original images into a line form suitable for engraving and then manually engraves a steel blank. The engraving process for dies intended for use in the production of bank notes may require up to six months for completion.

The second stage 2 in the known process is the production of copper dies corresponding to the original steel die. These copper dies are grown by known electrochemical processes, which are quite slow and may require up to a week for completion.

The third stage 3 in the existing process is the engraving of the individual copper dies. Each copper die may have additional engraved detail in order to provide designs for a family of documents, such as bank notes of different denominations. If this stage is included, the further engraving on each copper die may require about two months for completion.

The next stage 4 in the existing method is the manufacture and assembly of a multiplicity of synthetic plastic or polymeric carriers from the or each copper die. In general, an intaglio printing plate comprises a cylindrical plate which carries a multiplicity of relief patterns each embodying the intaglio relief pattern for an individual bank note. The plastic carriers or plates are each formed by pressing, using the copper die. A multiplicity of plastic carriers are then accurately trimmed and assembled by a welding process to produce a plate of a size suited to the intaglio printing press. Great care must be exercised during this stage in order to achieve accurate spacing between adjacent patterns and to minimize surface imperfections caused when the plastic carriers are welded together. Typically, manufacture of the plastic assembly requires two weeks for completion.

The next stage 5 in the existing process is the manufacture of a nickel mother or matrix. Several stages of electro¬ chemical deposition and growing are necessary to proceed from a 'male' relief pattern to a 'female' pattern and vice versa, each growing stage being followed by dressing and polishing before the next growing stage in order to improve surface imperfections. The production stage from the plastic carriers to the nickel matrix requires generally about four weeks for completion.

Overall, the existing process requires about nine or ten

months. The resulting nickel matrix can be used repeatedly for the growth of the final nickel printing plates which can be used for long print runs of bank notes or security documents.

The object of the present invention is the elimination of the first four stages of the existing process. Figure 2 illustrates the principal stages in a process according to the invention.

The first stage in the process illustrated in Figure 2 is the production of a photoablation mask. An intaglio pattern essentially comprises 'lands' and 'valleys'. Depending on whether the workpiece is to be a positive or reverse version of the intaglio plate, one or other of the land or valley regions may be represented in the mask as opaque and the other represented in the mask as regions which are transparent to light at the relevant wavelength.

The mask itself may be made by techniques described by Harvey et al, in Proceedings of The Society of Photo-Optical Instrumentation Engineers, 23-24 October 1995, Austin, Texas (USA), volume 2639, pages 266-277. Briefly, a digital representation of an image corresponding to the intaglio pattern may be made using known computer-aided design techniques and the output of the computer-aided design process may be used either directly or by way of a photomaster, to produce an image composed of transparent and opaque portions, the opaque portions being defined by a dielectric film on a quartz plate. Harvey et al, cited above, describes the use of a laser printer and transparency film to prepare a negative which is transferred by contact exposure into a resist on a chrome-on-quartz blank, the exposed resist being developed and etched by techniques well known to those skilled in the art.

The next stage in a process according to the invention is the use of the mask to produce a photoablated relief pattern corresponding to the mask's image on a workpiece which is typically a synthetic polymer such as rigid polyvinyl chloride. Various materials are known to be suitable for photoablation and the invention is not limited to the use of any particular material.

The photoablation process is more particularly described hereinafter with reference to Figures 3 and 4.

The workpiece is of the same general character as the assembly of plastic carriers made by stage 4 in the existing process except that it is a unitary workpiece without the surface imperfections associated with welding. Typically, the mask may be made with a multiplicity of individual images, each corresponding to the intaglio pattern on a bank note, so that the workpiece is formed with a corresponding multiplicity of relief patterns each pertaining to an individual bank note and being spaced with great accuracy in a manner appropriate for the intaglio printing press in which the final nickel printing plates are to be used.

Accordingly, the workpiece may be used to grow a nickel mother or matrix in a manner corresponding to the stage 5 in the existing process and the nickel matrix may consequently be used for the growth of a multiplicity of final nickel printing plates for use as previously described.

Typically the time required for the completion of a process according to Figure 2 is between one and two months, being much shorter than the duration of the existing process. The new process is better adapted for the accommodation of changes in the design, since the process is adapted for the output of a computer-aided design process and the time

required to produce a plastic carrier which can be used to make the nickel matrix is far shorter than the first four stages in the existing process.

No detailed description will be given herein of the stage 6 of Figure 2 because the production of an etched pattern from the output of a computed-aided design process is well known.

Figures 3 and 4 illustrate the photoablation stage 7 shown in Figure 2 in greater detail.

As is shown in Figure 3, light 11 from a laser 10 (Figure 4) of a wavelength and intensity which when the light impinges on the workpiece is capable of photoablation of a workpiece 14 is directed at the mask 12 which has the image pattern corresponding to the pattern to be photoablated in the workpiece 14. The light, which preferably comes from an excimer laser is directed through a focussing optical system so as to produce on the workpiece 14 an image 24 of that portion of the mask which is illuminated by the light. The intensity or fluence of the light at the workpiece is preferably of the order of 800 to 1000 mJ/cm . The energy density at the exit of the laser is usually substantially less than this and in any case it is desirable to ensure that the energy density in the region of the mask is insufficient to photoablate the mask itself. Consequently, the focussing system normally provides for substantial reduction or demagnification of the image and a consequent increase in the energy density of the light between the region of the mask and the surface region of the workpiece.

The workpiece 14 is, as discussed by Harvey et al, previously cited, typically a rigid polymer such as rigid

polyvinyl chloride. Other materials are known from the documents herein cited.

The area of the image on the workpiece is substantially greater than the area of the beam of light from the excimer laser. In order to reproduce the entire image of the mask on the workpiece by photoablation, it is desirable to produce a relative shift between the mask and the beam so that effectively the beam is caused to scan the entire mask, in typically a raster pattern, and the scanning movement is preferably performed by moving the mask in its own plane. It is consequently appropriate, as noted by Harvey et al previously cited, to produce a synchronous and converse relative movement of the workpiece. As shown, in Figure 3, the beam is impinging on the region of a letter R on the mask and the beam forms a reduced image of that R on the workpiece. The movement between the mask and the beam necessary to bring the F part of the mask pattern into the beam has to be matched by a converse movement of the workpiece to position the F part correctly relative to the part R.

Further, the excimer laser is a pulsed laser and accordingly the rate of ablation at the surface of the workpiece is a function of the energy density, the pulse rate of the laser and the scanning speed. In practice therefore servo- mechanical control of the synchronization of the mask and the workpiece is appropriate, it being also appropriate to vary the pulse rate of the laser so that as the speed of traversal of the mask reduces as the beam is caused to approach the edge of the mask there is a corresponding reduction in the pulse rate of the laser. Since the pulse rate of a typical excimer laser is of the order of one hundred pulses per second, and is reliably controllable, known servo-driven X-Y drives may be employed for controlling the X-Y movements of the mask and the workpiece

and provide a sufficient data rate for information denoting the position and speed of the mask and workpiece to provide control over the pulse rate of the laser.

Accordingly, as particularly shown in Figure 4, the X-Y scanning movement of the mask is controlled by an X-Y servo drive 15 and the corresponding movement of the workpiece is controlled by a similar drive 16, the drives 15 and 16 being conjointly controlled by a synchronizer 17 which also provides control output to a pulse rate controller 18 for the laser 10.

A possible disadvantage of the photoablating technique as has been described is that the slope of the boundaries of the ablated regions tends to be rather steep whereas a shallower slope may be more suitable for regions which are intended for holding the ink used in intaglio printing. However, some control over the degree of the slope may be exerted by varying the numerical aperture of the focussing optics 13 or by fine line scanning around the boundaries of the ablated regions after the completion of the main scanning process. It is also possible to use a multiplicity of masks in succession so that the photoablated regions have a stepped cross-sectional profile.