The invention relates to printing generally and more particularly to printing on metal objects such as coins.

It is known to print images on metal substrates. A number of systems and methods are known for applying ink on a metal surface. EP0209896 discloses a method and apparatus for decorating metal or plastic containers. US2002090456 teaches a method to print on strips of metal using computer controlled printing heads. GB2373217 discloses an inkjet printing system having print heads which utilize at least two different print head technologies. WO01920 discloses a paint composition formulation which allows metal coils to be decorated with multi-color complex designs. EP1120248 teaches a printing plate and method for its preparation, which forms a film of a desired image on a substrate medium for printing. EP1629977 describes how a printing plate can be roughened up in order for the polymer film to have good adhesion. US2004194235 describes a process to etch to a defined depth by using a laser and to fill the depth with a colored sealer. Finally EP1084830 discloses a method of roughening a plate surface using an electrochemical method.

Typically, it is necessary to coat the metal substrate with a thermoplastic or thermoset material prior to application of ink. It is further known to transfer the ink first to an intermediate carrier and subsequently transfer the ink from the carrier to the coated substrate. See, for example, U.S. Pat. No. 5,994,264 (Sherman et al.).

In particular, it is known to apply paint or ink to a metal coin using a silk screen process. While high quality images may be produced by such processes, adhesion of the image to the coin surface is not adequate to provide sufficient durability to make such coins suitable for public circulation.

A need exists for an efficient and low-cost method of printing images on metal surfaces such that the image demonstrates good adhesion to the metal surface. There is a particular need for a method of printing images on a face of a metal coin, wherein the image demonstrates sufficient durability to make the printed coin suitable for public circulation.

In a first aspect, the invention is a method of printing on a metal surface comprising the steps of providing a metal surface, forming a plurality of macropores on at least a portion of the metal surface, and forming a plurality of micropores within the macropores. The metal surface is cleaned, and a first ink having a first color is applied to the macropores and micropores to form at least one image on the metal surface. The ink is dried.

Preferably, the metal surface is part of a coin. Additional inks may be applied. Preferably, the first ink and any additional inks are solvent-based and are applied using inkjet printers. The drying process is preferably accomplished by blowing air across the metal surface. The method further preferably comprises a step of inspecting the metal surface to ensure correct orientation of the metal surface and a step of inspecting the metal surface to ensure proper quality of the printed image. The method may include a step of applying a substantially transparent top coat over the first and any additional inks. Preferably, the macropores are in the range of about 0.1 to 0.5 millimeter across and in the range of about 0.01 to 0.05 millimeter deep and the micropores are in the range of approximately 0.01 micrometres (microns) to 15 micrometres (microns) deep. The macropores may be formed in a stamping operation and the micropores may be formed in a sandblasting operation.

In a second aspect, the invention is a method of printing an image on at least a portion of a coin surface. The method comprises a first step of providing a metal coin having a first side and a second side. A plurality of macropores are formed on at least a portion of the first side. A plurality of micropores are formed at least within the macropores. At least the first side is cleaned to provide a substantially clean surface. The first side is inspected to ensure correct orientation of the coin. A first inkjet printer is used to apply a first ink having a first color to the macropores and micropores to form a first image on the first side. The first ink is air dried. A second inkjet printer is used to apply a second ink having a second color to the macropores and micropores. The second ink is air dried. A top coat of lacquer is applied to cover the first and second inks using a third inkjet printer. The top coat is air dried. The method is not limited to two colors, but may involve only one color, or more than two colors, excluding the top coat.

For the purpose of illustrating the invention, there is shown in the drawings a form of the invention which is presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown. In the drawings:

• Figure 1 is a diagram of the steps of a method of printing on a metal surface in accordance with the present invention.
• Figure 2 is a plan view of a first side of a coin having a printed surface produced in accordance with the method of Figure 1.
• Figure 3 is a cross-sectional view of the coin of Figure 2, taken along line 3-3 of Figure 2.
• Figure 4 is an enlarged detail view of the printed surface of the coin of Figure 2, showing macropores and micropores produced in accordance with the method of Figure 1.
• Figures 5 through 13 are plan views of alternative patterns of macropore construction in accordance with the method of Figure 1.
• Figures 14 and 15 are cross-sectional views showing alternative macropore construction in accordance with the method of Figure 1.

Referring to the figures, wherein like numerals are used to indicate like elements throughout, there is shown in Figure 1 a diagram of the steps of a method, identified generally by reference numeral 10, of printing on a metal surface in accordance with the present invention. Figures 2-4 illustrate a coin 100 produced by the method 10. The coin 100 includes a printable surface 108 having a plurality of macropores 110 and micropores 112, described in detail below. Figures 5-15 illustrate a few of the various ways in which the macropores 110 may be constructed.

The method of printing on a metal surface 10 will be described in the context of producing the coin 100. However, it will be recognized that the method 10 is applicable to metallic surfaces generally, and is not limited in application to coins. The method 10 is, however, especially well-suited for providing printed images on coins.

With reference to Figures 1-4, the method 10 comprises a first step 12 of providing a metal surface. The metal surface may be, for example, a first side 102 of the metal coin 100. The metal coin 100 includes the first side 102 and a second side 104. In the preferred embodiment illustrated, the first side 102 is provided with embossed indicia 106 surrounding the printable surface 108.

In a second step 14, a plurality of macropores 110 are formed on at least a portion of the metal surface (preferably, the first side 102). Preferably, the macropores 110 are formed using conventional metalworking techniques, such as stamping. With reference to Figures 4, 14, and 15, the macropores have a width A in the range of approximately 0.1 to 0.5 millimeters and a depth C in the range of 0.01 to 0.10 millimeters. Preferably, the macropores 110 are separated from one another by a distance B in the range of 0.0 to 0.3 millimeters.

The macropores 110 may be formed in a wide array of patterns. With reference to Figures 4 and 5, in a first pattern 130, the macropores 110 are formed as separate generally circular indentations of a generally uniform diameter. With reference to Figures 6-13, second through ninth patterns 132 - 146 provide both discrete, localized indentations similar to the first pattern 130, as well as extended indented channels. Other patterns are possible.

Furthermore, the macropores 110 may be formed in a wide array of cross-sectional profiles. With reference again to Figures 4 and 5, in a first cross-sectional profile 150, the macropores 110 are provided with a semi-circular cross-sectional profile. Furthermore, and with reference to Figure 14, in a second cross-sectional profile 152, the macropores 110 may be provided with compound shape having a combination of radiused and linear edges. Still further, with reference to Figure 15, the macropores 110 may be provided with a third, generally triangular, cross-sectional profile 154. The first through third cross-sectional profiles are illustrative of only three of the many geometries possible.

With particular reference to Figures 1 and 4, in a third step 16 a plurality of micropores 112 are formed within and adjacent to the macropores 110. The micropores 112 are characterized by dimensions much smaller than the macropores, ranging in sizes (depth and breadth) from 0.01 micrometre (micron) to 15 micrometres (microns). The micropores 112 average in size from around 0.5 micrometre (micron) to 5.0 micrometre (microns). The micropores 112 are preferably produced by a sandblasting process or by a lasering process.

With continued reference to Figure 1, in a fourth step 18 the metal surface (preferably, the printable surface 108) is cleaned to remove any debris on the surface accumulated during processing, including any lubricants. A clean surface improves adhesion of ink subsequently applied to the surface. The surface is cleaned using conventional techniques known to those of ordinary skill in the art of metalworking.

Preferably, in a fifth step 20 the metal surface is inspected to ensure correct orientation of the metal surface relative to inkjet printing equipment, described further below. The fifth step 20 is necessary for objects such as the coin 100, which is preferably fed to the printing equipment by automated equipment, and which is fed to the printing equipment oriented randomly face up or face down. If there were no uncertainty in the orientation of the object being processed, the fifth step 20 could be omitted.

Preferably the fifth step 20 is performed using conventional automated pattern recognition equipment of a type known to those of ordinary skill in the art of automated manufacturing equipment. For example, a digital camera (not illustrated) could be coupled to a computer controller (not illustrated), providing a signal to the controller. The controller is provided with software adapted to determine from the signal the orientation of the object being processed. In particular, with respect to the coin 100, the software would recognize whether the first side 102 or second side 104 was properly oriented relative to the printing equipment. If not properly oriented, the controller would activate machinery of a type well known to those of ordinary skill in the art of automated manufacturing equipment to flip the coin 100 into the desired orientation.

With reference now to Figures 1 and 6, in a sixth step 22, a first ink 116 having a first color is applied to the macropores 110 and micropores 112 to form at least one image 114 on the printable surface 108. Preferably, the first ink 116 is a solvent-based ink having a solvent which evaporates very quickly, preferably within three or four seconds of exposure to air blowing past the printable surface 108 at a moderate velocity (for example, 10 feet per second). In particular, an alcohol ketone based ink, such as the inks available from Imaje France of Bourg Les Valence, France, having product codes FT316 and 5130, have been found to be particularly effective.

The first ink 116 is preferably applied to the printable surface 108 using a first inkjet printer (not illustrated). The first inkjet printer is preferably a piezoelectric-type inkjet printer. The first inkjet printer is conventional. Preferably a Model S8 Series Master available from Image France is used.

In a seventh step 24, the ink is dried in a first air tunnel (not illustrated) where air is circulated at a moderate velocity to evaporate the solvent and dry the first ink 116. As indicated above, the metal object (preferably, the coin 100) moves through the first air tunnel in three to four seconds. The air need not be heated above the ambient temperature within the processing plant.

Preferably in an eighth step 26, a second ink 120 having a second color is applied to the macropores 110 and micropores 112 on the metal surface (the printable surface 108). The second ink 120 forms a second image 118, which preferably cooperates with the first image 114. In the particular embodiment of the coin 100 illustrated, the first image 114 and second image 118 together form a representation of a poppy flower.

Preferably, the second ink 120 is also solvent-based. It has been found to be desirable that the first and second inks 116, 120 be based on different solvents. Otherwise, when the second ink 120 is applied, the solvent contained therein tends to re-wet and blur the first image 114. The second ink 120 is preferably applied with a second inkjet printer (not illustrated) similar to the first inkjet printer (not illustrated). In a ninth step 28, the second ink 120 is air-dried, preferably in a manner similar to the first ink 116, as described in the seventh step 24.

It will be recognized that the method 10 is not limited to application of only two inks. Additional printing stations applying additional inks along with additional drying tunnels could be provided.

With reference now to Figures 1 and 4, in a tenth step 30, a substantially transparent top coat 122 is applied to at least substantially cover the first ink 116, and second ink 120 and other additional inks, if provided. Preferably, the top coat 122 is a substantially transparent lacquer having a nitrocellulose base resin. This lacquer is commercially available from sources including Imaje France, under the product code 5553. The preferred thickness of the top coat 122 is in the range of about 1 to 2 microns. Preferably, the tenth step 30 is performed using a third inkjet printer (not illustrated) of a type similar to the first and second inkjet printers. In an eleventh step 32, the top coat 122 is air-dried, preferably in a manner similar to that described above for the seventh and ninth steps 24 and 28.

Alternatively, it will be recognized that application of the top coat 122 could be accomplished using other materials and techniques. For example, the top coat 122 could be a transparent coating capable of being cured by exposure to ultraviolet radiation. Other types of heat sensitive top coats 122 could be used, including a polyurethane coating. Still further, the top coat 122 could either be colorless, or could be colored, while still being at least substantially transparent.

Finally, in a twelfth step 34, the first and second printed images 114, 118 are inspected for acceptable quality by automated pattern recognition equipment of a type similar to that described above relative to the fifth step 20.

It will be recognized by those of ordinary skill in the pertinent art that one or more of the foregoing twelve steps may be omitted. For example, the twelfth step 34, while desirable, need not be performed.

An efficient and low-cost method of printing images on metal surfaces is thus disclosed. In particular, an efficient and low-cost method of printing images on a face of a metal coin, wherein the image demonstrates sufficient durability and adhesion to the coin surface to make the printed coin suitable for public circulation, is disclosed.