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FLEXO Magazine : May 2012
Further up the value chain, ink was developed using this new pigment and has been tested in a thermoformable ink system in comparison to other ink systems. During the development, a lab testing procedure used a commercially available, clear PETG film (transverse direction orientation) and the inks were applied with a hand-held proofer utilizing a 300 line, 6.6 BCM anilox. Printed samples were cured using a commercially avail- able laboratory oven with a mercury arc lamp, where the dose level was approximately 175-mJ/ cm2. Prints were then stored at ambient conditions for 24 hours before processing. The prints were then constructed in a way where controlled amounts of shrink could be produced to achieve 10, 20, 30 , 40 and 50 percent shrinkage, but yielding a flat structure that could later be measured. The prints were then shrunk in a water bath held to 80 degrees Celsius in order to get as uniform shrink as possible. To measure the color shift in light- ness as shrink increased, an X-Rite SP62 colorimeter (D50, two-degree, specular light included) was used. The L-value was used for quantifying the shift in lightness and darkness between a virgin print and a shrunken print. The newly devel- oped ink using advanced pigment technology was compared to other commercial inks and plotted. (See Chart 3) As can be seen from the Chart 4, the new UV curable ink pigmented on new advanced pigment has outstanding color stability over other traditional metallic pigmented UV printing inks. This chart only shows the percent change in color, but does not address the perception of how “metallic” the color appears. Characterizing metallic color is a complex subject, and is beyond the scope of this article. While there are many methods to quantify metallic color, there remains a subjective aspect to classifying what is a “good” or “bad” metallic effect. For purposes of this topic we will take for granted that under the measured conditions an L value of 100 is the ideal metallic effect and an L value of 0 has no metallic effect. Measure- ments were made again with a colorimeter on virgin print samples and plotted for comparison. (See Chart 4) As can been seen from the Chart 4, the L-value of the new advanced pigmented UV ink produces a much lighter and whiter metallic color, that is perceived as being more brilliant than the other two pigment types. This is quite significant since the perception of metallic effect is greater than that of a vacuum metallized pigmented ink, which can easily cost five times that of this new pigment technology. BOTTOM LINE We are seeing continued growth in the shrink sleeve market, especially with interest in UV curable flexo metallic effects. While specialty metallic inks have been used in other print media, the available palette of metallic effects in UV curing required improvement. The advent of a specialty ad- vanced milled pigment for UV curable systems has now been developed, opening doors to new opportunities for decorating shrink sleeve labels. Not only does this technology maintain excellent color stability when shrunk, but it raises the bar on the level of metallic effect that can be achieved on reverse printed films in general, something that typically was not ap- proached in the past. n 38 FLEXO may 2012 www.flexography.org Evaluation of color change in metallic silver colored inks