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FLEXO Magazine : March 2008
www.flexography.org MARCH 2008 FLEXO 21 and shipped. In turn, the cased car- tons of soft drinks become an input to the distribution process as the packaged beverage is transported, distributed to retail outlets, displayed and then purchased by the consumer. The process diagram expands again (Figure 3). ROLE OF CARTON SURFACES At every point in the expanded process, the surface of the folding carton must meet certain levels of performance. In the printing process, a flexo printer/converter needs a specific range of surface performance, typically measured as a slide angle or COF (coefficient of friction) in order to cut and finish (glue) the printed sheets. In end-user operations, the surface performance must enable the filling operation to optimize speeds and minimize waste. In the distribution process, the surface performance must resist the rigors of package move- ment that could rub or scratch. In displays, the surface per- formance must contribute to easy stacking while minimizing slip. In addition to these functional attributes, the coating is ex- pected to provide additional and subjective graphic enhance- ment. In general, the primary function of an overprint varnish or coating (water, UV/EB, or solvent) is to protect the underlying inks and provide the necessary surface performance for the various processes. Additionally, the packaging must convey the appropriate graphic message and its appearance must support the marketing objective of a quality product with high con- sumer appeal. For an overprint varnish, this usually includes a specified level of gloss that is measured by calibrated test equipment and conforms to common descriptors such as high gloss, matte or satin. Coating manufacturers can contribute to evaluation of surface performance through testing methods and the use of sophisti- cated equipment. In some cases, for example, the coat weight may create a film that does not cover the highs and lows of a substrate with a rough surface. Cross-section photos taken by an electronic microscope (Figure 4) can show the interaction between coat- ing and substrate. The challenge for the analyst is to correctly interpret the image and recommend formula changes to achieve desired results. COATING TECHNOLOGY TODAY Today's formulators of graphic arts coatings work with diverse materials to achieve products that provide ideal surface perfor- mance at the lowest cost. In developing a new coating they must consider more than a dozen factors: Monthly or annual volume requirements 1. Substrate 2. Inks 3. Application equipment 4. Drying/curing equipment/method 5. Application speed 6. COF specs 7. Gloss specs 8. Preferred chemistry 9. Cost targets 10. End-use requirements 11. Rub requirements 12. Odor restrictions 13. Wet-block requirements 14. Glue-block requirements 15. Gluing restrictions 16. Customer testing methods 17. Environmental considerations at press side and storage 18. Fortunately for the folding carton industry, basic coating solu- tions for general applications have evolved as the industry has matured. Proven raw materials and design models are now avail- able for most substrates and application methods. How then can we explain the curious unpredictability of the surface performance of an overprint varnish? The fact is, that while the basic chemistries vary widely among the water-based, solvent-based, and energy-curable (UV/EB) overprint varnishes used today, there are problems of surface performance common to all. UNDERSTANDING THE VARIABLES The I-T-O Model described previously assumes that the output is predictable to the extent that the inputs and the transformation remain constant. This is valid in theory. However, as those of us involved in manufacturing know all too well, the real world in- volves variability. This variability is inherent in both the input and transformation segments. Only when these variables can be con- trolled to a specified range, can the output be predictable within a similar specified range. Thus, in order to determine the potential for variation in output, the effect of input and transformation variation for each component must be measured and evaluated. For example, suppose the predictability for COF (output) of a UV overprint varnish on paperboard is affected by both the coat weight applied and the water content of the substrate (input). Yet the measurable effect on COF (output) of varying the coat- ing weight may be 10 times greater than the effect of a similar variation in the water content of the substrate. The challenge for everyone concerned with controlling the process is to identify the input and transformation variables for each phase of the entire process and then, through experimental design, measure the effect of changing the variables. Once this is done, we can con- TECHNOLOGIES & TECHNIQUES Distribution Filled Cartons Inputs Transformation Outputs Store Display FIGURE 3.