Coloured coins – coins with partly coloured minted motifs – are spectacular showpieces that are becoming increasingly popular among collectors as well as other users. Applying colour adds variety to the otherwise limited design options in minting, which is why almost all official mints now offer coloured coins. To achieve perfect printing results on three-dimensional minted reliefs, the coin blanks have to be precisely aligned prior to printing. In addition, the applied ink has to be stable enough to allow the coloured coins to survive the final roller burnishing process intact.
Until now, most coins and other precious metals were printed using screen or pad printing methods. Thieme, working with a German mint, has now developed a new type of digital coin printer that significantly speeds up the printing process. Its one-of-a-kind inkjet printing system employs special tested inks that offer optimum adhesion and abrasion resistance when applied to metal substrates. The system offers a total of seven color channels (CMYK, white, primer and topcoat); a fluorescent clearcoat is also available for special luminous effects.
Efficient loading, automatic alignment
Extensive automation and easy loading are essential for an efficient printing process. The Thieme coin printer uses an entirely new approach with exchangeable adapter trays in which operators can load a large number of coins or bars at a time. The double table design allows parallel loading/unloading and printing with a cycle time of approx. 180 seconds per tray. That translates into 640 coins per hour when each tray is loaded with 32 coins.
The innovative automatic alignment system provides yet another efficiency boost: Digital Image Alignment allows the blanks to be inserted into the press in any position – a scanning process auto-matically adjusts the stored print layout to the orientation of each individual coin. Digital alignment takes only one to two seconds per coin and is therefore much faster than mechanical holder position-ing in conventional printing processes. “We have increased throughput many times over what was previously possible,” ex-plained Oliver Beck, Head of Sales Printing Systems at Thieme. The innovative system also offers unbeatable printing costs, he added: “Ink costs less than one euro cent for each 32-millimeter coin.”
High print quality and productivity
Print quality has not been sacrificed for better productivity. On the contrary, the print resolution of 800 x 1200 dpi can colourise the most delicate minted motifs in high quality. Automatic alignment is also highly precise with a repetition accuracy within ± 0.1 mm. “Our new inkjet printing system revolutionizes the current coin colourisation process,” promised Oliver Beck. “The printing process used to be rather laborious. In the future, coins and metal substrates can be colorized far more efficiently and flexibly while maintaining high quality standards.” This efficiency gain is crucial in a time of rising demand: “For mints, this is a real quantum leap.”