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FLEXO Magazine : July 2007
hot technologies 2 0 F L E X o J U LY 2 0 0 7 w w w . f l e x o g r a p h y . o r g S ince consumer product companies (CPCs) have been de- manding “no-label-look labels,” printers/converters have been seeking the ability to achieve the opacity and white- ness of rotary screen printing by using the less-costly process of UV flexo technology. Opaque white inks are all made from tita- nium dioxide. It is a difficult pigment to handle in an ink, as it is heavy and “hard.” While it may seem obvious that the opacity of an ink is in di- rect proportion to the percentage of pigment used, practical tests show that this is not completely true. The correlation is not 100 percent linear, because the phenomenon of saturation occurs at a certain level. Formulating an opaque white UV ink is more complex than simply adding more or less opaque pigment. As the white UV ink is cured, it will require a certain amount of UV light to initiate the polymerization; and here is where the amount and type of tita- nium dioxide have a big impact. Opaque whites can be very difficult to cure because they will reflect light, reducing the amount of UV energy reaching the photoinitiators, thus hindering the ink curing process. By careful selection of the right white pigment (TiO 2 ) and photoinitiator, it is possible to formulate fast-curing opaque inks which will not discolor. Discoloring is an important factor for opaque white UV inks that is often forgotten. Incorrectly formulated products will have a yellowish tone. Controlling ink Film Weight Another important factor to consider is the ink film weight ap- plied. A similar correlation between ink and opacity applies: more ink may give more opacity, but will also reach a certain satura- tion point where improvement is no longer linear. Optimum ink film weights on a substrate with the different print processes are roughly illustrated in Figure 1. Research has found that the ink’s ability to flow and form a homogeneous film on the substrate without air bubbles or pin- holes is just as important for the final opacity of the print as the ink film weight and the percentage of pigment used. This leads us to discuss the fundamental difference between UV flexo and UV rotary screen. The key is the way that the ink is applied to the substrate. UV flexo uses an engraved anilox roll to carry the ink up to the raised image plate, and then the plate transfers the ink to the substrate. UV screen uses a mesh. Capillary action transfers the ink direct to the web. (See Figure 2). Flexo laydoWn The engraving on the anilox dictates roughly how much ink will be applied to the substrate. With this said, the engraving method, pattern, angle, channelled, not channelled and open celled can all play a huge part in ink volume transferred from an anilox cell. Unfortunately, what is still an “unknown” is how much of the next generation UV White ink Can UV Flexo Replace Screen? By Mike Buystedt FigUre 1.+#3; Required ink film weight in microns. Photo courtesy XSYS Print Solutiosn. FigUre 2.+#3; Illustration of how flexo transfers ink to substrate (left) versus screen (right). Photo courtesy XSYS Print Solutions. Litho 1.2-1.5 Letterpress 1.8-2.5 UV Flexo/Flexo 2-4 Gravure 6-10 Screen
Golden Anniversary Commemorative Journal