The present invention relates to a die for coins or medals, comprising a hardened surface in which a motif is produced. Such a die is known from DE 19741998 A1. In the case of this die a two-dimensional or three-dimensional image or relief is first provided in the usual manner, after which grey tinting is achieved by making indentations by means of radiation with a laser beam. Such indentations are of varying depths, for the purpose of obtaining a more or less dark effect subsequently on the coin. Grey tints can be achieved in this way. However, the die costs for such dies are considerable, since, apart from the machining with the laser beam, the conventional techniques for manufacturing the die are still necessary. Two methods are generally used in this case.

In the case of a first method a drawing is made on a sheet of plaster of Paris, and the sheet of plaster of Paris is modelled in relief. The relief of this sheet is then scanned and a corresponding milling cutter is produced by means of a milling technique. Three-dimensional reliefs are produced in the process.

According to another, simpler technique, only one or two height levels are used in the process. Such a technique can be used directly on a computer.

In the minting of coins and the production of other objects the user is demanding an ever-increasing variety of types. Where large series of coins are being minted, this demand can be met by providing a complex die, since the die costs in this case play an important role only in relation to the other costs.

However, if smaller series of coins or other objects have to be produced, for example for collectors, such die costs are really important.

In addition, there is demand in the market for the ability to show figures with tinting. In the prior art it was envisaged that this could be achieved by a colour coating. However, it has been found that if such an object is handled often, as is the case with coins, the colour wears off. Furthermore, the colour coating produces a relatively artificial effect.

In the prior art of printing webs of material such as paper and textiles, it is known in, for example, screen printing technology to work with certain grids. These techniques have already been known for a number of decades and produce a relatively coarse pattern on the final product.

It is the object of the present invention to provide a die for coins or medals that is simple to produce and by means of which it is possible to apply grey tints and the like to metallic objects.

This object is achieved in the case of a method of the type described above in that the motif is constructed solely of a more or less compact series of indentations, each indentation having substantially the same diameter, lying between 0.1 and 0.3 µm, and each indentation being of substantially the same depth.

According to the invention, not only is the grey tint obtained by means of a pattern of indentations, but so is the entire motif, in other words its peripheral boundary is too. Furthermore, the indentations are no longer of a greater or lesser depth, but are all produced with substantially the same measurement. By varying the degree of compactness, in other words the number of indentations per unit area, a certain image or grey tint is obtained. Owing to the particularly small diameter, lying between 0.1 and 3 µm, a very sharp image can be obtained. In the case of the conventional printing techniques mentioned above for webs of material or fabric, such a measurement is not possible because inking materials are not suitable for it. Besides, it was assumed that, apart from the production of grey tints, it would not be possible to achieve full motifs in coins through suitable patterns or series of indentations.

The indentations described above, with a depth between 1 and 50 µm, and preferably a depth of approximately 10 µm, can be produced in a manner known in the prior art. However, the indentations are preferably produced by means of laser technology. This means that it is possible to provide a very large number of indentations on a very small surface area. For instance, it is possible to make between 40 and 1,600 indentations per linear inch (2.54 cm).

If the laser comprises a CNC-controlled laser, it is possible to control the latter in a simple manner by a central processing unit such as a computer.

In that computer the desired picture can be stored and converted into a pixel image. According to an advantageous embodiment of the invention, the indentations in the die are all of the same depth, and a certain image is obtained through variation of the concentration of indentations. The picture in the computer can be obtained by scanning a photograph or the like, combined with possibly further processing of this picture. It is, of course, also possible to generate the picture in an entirely artificial manner on a computer or the like. The generated picture can be converted into an indentations structure by means of a relatively simple programme.

Through the use of laser technology, it is possible to make indentations even in hardened surfaces, with the result that it is possible to harden the die beforehand. This means that the production can be further optimized, because hardening and further preworking of the die can be carried out in a simple manner before the pattern in question is applied to it. Further preworking operations can comprise polishing. Likewise, a spherical shape may be desired for certain dies, and this spherical shape can also be provided in advance.

The cost of the die can be reduced considerably by carrying out these steps in advance.

The pattern according to the present invention is particularly wear-resistant, unlike lacquer coatings. Of course, the wear resistance depends on the height of the projecting parts which constitute the mirror image of the indentations in the die.

According to a further advantageous embodiment of the invention, the surface of the die for coins or medals can be comb-like. In other words, it is provided with a series of adjacent ridges, which are always provided with a different pattern, i.e. a different image can be obtained when light is incident upon it from different visual angles. This means that a security feature can be provided. Moreover, certain types of holographic images can be achieved in this way.

The invention also relates to a method for manufacturing a die for coins or medals, comprising starting from a hardened metallic surface and producing in said surface at least part of a motif by making indentations by laser technology, all indentations having substantially the same height and diameter, and a full motif being applied to a surface part solely by a series of indentations.

The invention further relates to a coin/medal, in which said series of elevations in certain areas comprise 40 - 1,600 projecting parts per linear inch (2.54 cm).

The invention will be explained in further detail below with reference to an exemplary embodiment illustrated in the drawing, in which:

• Fig. 1 shows very diagrammatically the production of a die according to the invention;
• Fig. 2 shows the use of a die according to the invention;
• Fig. 3 shows in perspective a coin or token produced with the die according to Fig. 2; and
• Fig. 4 shows section IV-IV from Fig. 3.

In Fig. 1, a die blank or die slug is indicated by 1. The die blank has a hardened die surface that is otherwise completely finished. However, no pattern is yet present in this die surface. All that is achieved in the manner shown diagrammatically in Fig. 1. An image 8, such as a photograph, is placed in a scanner 7. The signal coming from the scanner is fed to a central processor such as a computer 5. This image can be read off screen 6, and corrections can be made in this picture or further details added to it with the aid of means not shown, such as a mouse and keyboard. The picture thus generated is fed to a control unit 4 of a CNC machine, which controls a laser head 3. A large number of indentations are made in the surface of the die blank by means of a laser 3. Such indentations all have substantially the same depth and shape (diameter). The depth lies between 1 and 50 µm, and is preferably approximately 10 µm. The diameter preferably lies between 0.1 and 3 µm.

In the case of such a treatment it does not matter whether or not the surface of the laser blank is hardened. With such a construction it is possible to obtain a die surface within a relatively short time by fully automated means. The die blank is made of a steel material. The number of indentations made depends on the desired image. All this will be explained in greater detail with reference to Fig. 4. Apart from having indentations made in it, the die surface can be provided with a further relief structure in the usual manner, for example by milling. This applies in particular to the peripheral edge if, for example, coins or tokens have to be struck. Other examples are a combination of a relief structure with a pattern structure, as is obtained according to the present invention. The die thus obtained is used for the production of coins or medals.

A die unit is shown in a very diagrammatic way in Fig. 2. The unit is indicated in its entirety by 11 and is composed of a top die 2 that is produced after the treatment of the die blank described with reference to Fig. 1. The bottom die is indicated by 12, and a coin or token blank is indicated by 13. The final coin or token 14 is produced by a stamping action.

The coin or token is shown in detail in Fig. 3, from which it can be seen that a view like a photograph is produced. IV-IV indicates a cross-sectional area of a detail, which is clarified further with reference to Fig. 4. It can be seen from the latter figure that at least a part of the surface of the coin 14 has a structure with projecting parts 16. The projecting parts 16 correspond to the indentations made earlier in the die blank 1 by means of the laser unit 3. The density of the projecting parts 16 relative to the base 17 determines the tint. Images can be produced in this way. If more raised parts are present, a lighter tint will be obtained. It can be seen from Fig. 4 that all raised parts are of substantially the same height. Only the density of the raised parts determines the tint. In fact, this is a case of a relief with two levels, namely base level 17 and the top side of each of the raised parts. It will be understood that the wear resistance of such raised parts is many times greater than that of lacquers, and that the effect obtained is unique.

Although the invention has been described above with reference to a preferred embodiment, the person skilled in the art will understand that numerous modifications can be made to it without going beyond the scope of the present application. For instance, it is possible to generate images directly by means of a computer, without first scanning them in. It is also possible to provide other objects with a pattern. These and further modifications are obvious for the person skilled in the art and lie within the scope of the appended claims.