Industrial & Specialty Printing - November/December 2012 - 34

roll-to-roll printing
Wim Zoomer
Technical Language Roll-to-roll printing plays an important part in aesthetical and industrial applications. Aesthetical commonly signifies the application of graphic decoration, such as labels, wallpapers (Figure 1), textiles, and panels; whereas industrial applications usually involve printing adhesives, RFID antennas, intelligent packaging, flexible solar cells, and flexible circuits. The systems used for these processes are printing lines—practically complete roll-to-roll production lines—that span 1060 meters (and even longer). A roll-to-roll printing line may be completed with an unwinding unit, a corona web pre-treatment, one or more printing stations, a die-cutting station, an embossing unit, hot-melt application, laminating, and a re-winder or a sheeter. Roll-to-roll printing is a continuous production process. A web is guided through the press from the unwinder to the rewinder. The roll-to-roll printing process allows printing at relatively high production speeds and is meant to print very large amounts of the same products efficiently, resulting in a very competitive per-item cost. The web width of a roll-to-roll system may vary from 200 mm to several meters. Print width may vary between approximately 160 mm and several meters, depending on the application. Common roll-to-roll print techniques used may be, among others, flexo, letterpress, offset, rotogravure, digital printing (such as the nonimpact inkjet printing), or screen printing (Figure 2). The application and, therefore, the print technique and corresponding ink applied determine the speed of the roll-toroll print line. In offset, flexo, and letterpress, the ink is transferred from the printing plate’s surface to the substrate’s surface. In screen printing, the ink is transferred through the image carrier onto the substrate. This explains why screen printing is a slower print technology than the others. On the other hand, screen printing is a very versatile technique. Depending on the application, this method is able to deposit wet-ink thicknesses varying from 5-250 μm. Due to this characteristic, screen printing is used for several narrow- to mid-web industrial applications. Roll-to-roll screen printing is substantially faster than sheet-fed printing. Roll to-roll screen-printing speed varies between 10-100 m/min. An important difference is the handling of the printed matter. In sheet-fed printing, each sheet needs to be handled during printing and successive drying. A roll-to-roll printing process allows unwinding the roll, printing, drying, and rewinding the roll without any manual interference. If needed, finishing processes such as lamination, slitting, die-cutting, and the application of hot melt can be added inline. Roll-to-roll logistics are simpler than those associated with the sheet-fed process. Drying a solvent-based ink shows the substantial difference between the two processes. After printing, the sheets may be air dried in a drying rack, which can also be used to collect the sheets after drying using a hotair tunnel. After drying in roll-to-roll printing, the web is guided to the next position or rewound to collect the entire roll. dryIng The ink characteristics and the deposit as well as the capacity of the drying process (Figure 3) determine the speed of the roll-to-roll line. Printing a UV-curable ink

Photos cour tesy of Stork Prints, The Netherlands

Figure 1 Rotary screen printing wallpaper 34 | IndustrIal + specIalt y prIntIng www.industrial-printing.net

prIntIng methods
Figure 2 Basics of rotary screen printing A - Screen B - Squeegee C - Impression Roller D - Substrate

Industrial & Specialty Printing - November/December 2012

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