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FLEXO Magazine : March 2008
TECHNOLOGIES & TECHNIQUES 22 FLEXO MARCH 2008 www.flexography.org centrate on minimizing fluctuation of critical input variables and begin to achieve an acceptable and predictable range of variation in the output. TIME, CIRCUMSTANCE, TEST METHODS As scientific as we all hope to be in determining and predict- ing manufactured product output, some demons intervene. First, because we work with paper and chemistry, time plays a role. Although the typical lifecycle of a folding carton is relatively short (a few days to several weeks), time may change the basic composi- tion of the components. What we must determine is the effect of time on every desired output and, in turn, its effect on input for the subsequent process. Second, circumstances change over time. As soon as cartons complete the printing/coating/drying process, the cutting and finishing processes begin to affect the surface per- formance. Circumstances continue to play a role as the converter phase ends and the end user phase be- gins. As the cartons move through the operation, the surface performance is af- fected by conveyer belts, forming equipment, pal- letizers, etc. In order to measure the changes in slip or COF, the researcher must work to isolate the variables and minimize multiple causal effects. All data must reflect both time and circumstance variables. Third, test methods and protocols for conducting slide angle and COF measurements are a virtual "slippery slope" for re- searchers, converters, and suppliers. Equipment manufacturers and testers themselves have established different procedures. Two common methods are ASTM test D 4521-96 and the TAPPI test method 815 om-95. Yet it is obvious that if data are to be useful, test results must correlate between the laboratory and the field or between converting locations. The key to correlation of slide angle and COF test data is to eliminate variation. This means that the test method must be identical, the equipment identical and calibrated, the samples identical, and the conditions identical. Even under the most controlled circumstances, however, a test can show some variation. One example of variation came to our attention last year while qualifying for new business. Some samples were coated on test equipment with a high-gloss UV coating. Several different loca- tions were then given the task of measuring and reporting the COF using Thwing-Albert sled testing devices. All locations used the same equipment, sample pieces, and protocols but the re- sults were very inconsistent. After considerable retesting, it was determined that the coat weight varied across the sheet, side to side. The coater applicator was set unevenly and this created the anomaly! A great deal of time was wasted on critiquing test method variation, when the problem was found in application variation. Surface performance of water-based coatings is inherently less predictable than that of their UV- and EB-cured cousins. The reason is obvious: water, and its absorption by paperboard, creates a set of variables all its own. Recent studies have shown that the minimum measurable range of slide angle variation is 5 degrees, from time of application to the time when the pack- aged goods are placed on the shelf. Additional studies have shown that retesting the same sample will not pro- duce further change. Other studies have confirmed that the effect of relative hu- midity is a critical variable. In the end, the convert- ing industry has learned that to satisfy the many de- mands of the downstream processes, compromise is essential. For example, the additives necessary to maximize rub resistance will negatively affect gloss readings as coat weights increase. Further, the wax or silicones used to im- prove the rub resistance will create a lower COF or slide angle, so as rub resis- tance increases, COF normally decreases. The formulator must therefore know the relative importance of the desired surface performance attributes (rub or COF) in order to proceed. CLOSING THE GAP Our collective involvement in folding carton manufacture and the many industries served is actually only 60 years new. Following WWII, consumer demand dramatically increased and producers filled the gap with everything from TVs to running shoes. Technology has kept pace, but there is still room to grow, and always a search for a better mousetrap. In the grand scheme of things, the surface performance of coatings on folding cartons isn't rocket science, but it is nonetheless complex. Through con- tinued scientific research, experimental design and pressroom testing, we will close the gap and provide the necessary---and desired---predictability. ABOUT THE AUTHOR: This article was adapted from Flint Group's Printer Resource. For more information visit www.flintgrp.com/printers_resource.html. FIGURE 4.