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FLEXO Magazine : September 2011
nature of all flexo printing. However, since the UV bulb really doesn't emit enough energy to cure the inks per se, an intermediate chemical, called a pho- toinitiator, is used in UV inks to capture the energy from the bulb and pass it to the ink, enabling curing. The photoinitia- tor is usually a little pricey and serves no purpose other than this energy-trans- fer process. After curing, the photoinitia- tor fragments remain in the ink. While the UV inks are fully cured from a functional sense, there may be some trace quantities of uncured material in the ink, in addition to the photoinitiator fragments. Lower cost UV products with a lower performance demand, such as UV coatings used on magazine covers, will have more of these residuals and may have a noticeable smell. High- end UV products will use alternate raw materials that significantly minimize, or even eliminate, these issues. EB products are cured by passing under an electron beam emitter at the end of the press. Yes, there's just one curing station. Since there is no intersta- tion curing on the press, the inks remain uncured, sometimes called ' wet,' and this is normally a problem with flexo, an inherently dry-trapping process. For now, though, let's just say that the EB unit does emit enough power to cure the inks all by itself, without the need to add photoinitators to the inks. Also, since the emitted power is so high, there are fewer residual uncured materials in the ink/coating film. This ' wet- trapping' issue has, until recently, limited EB to the lithographic printing process. Another issue with EB is that the curing must take place in an oxygen-free, nitro- gen-purged zone. This is because oxygen is an inhibitor to the curing process and must be removed for complete curing to occur. This is not generally an issue with UV curing since the photoinitiator over- comes the inhibiting effect of the oxygen. Slightly better/faster curing can be seen with UV with nitrogen-purging, but the improvement level with UV is almost never worth the cost of the nitrogen. WHAT PRINTERS SAY Here, without further embellishment, are some comments from printers as to why they chose UV or EB technology, as com- pared to traditional water-based flexo. • UV flexo has better print quality. • UV flexo requires few-to-no press- side adjustments. • UV flexo causes less waste--you can reuse ink off the press. • UV/EB gives better mileage--"sitting up" rather than "soaking in." • UV flexo has better gloss. • UV flexo has better adhesion to foil substrates. • UV flexo has better print control on process colors due to sharper dots. I don't know of any printers who have switched from solvent flexo to UV flexo, but the reduced press-side adjustments and the less waste statements should still be true. The main issue vs. solvent is the very low, less than 2 percent VOC content of UV flexo inks. PACKAGING'S FIT Not all UV flexo inks that will print well are suitable for food or pharmaceutical packaging. There are many raw materi- als available to the ink formulator that are slow to cure (monomers), or can migrate after printing/curing (residual monomers, or silicones), or are incom- pletely consumed in the curing process www.flexography.org SEPTEMBER 2011 FLEXO 63