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FLEXO Magazine : October 2013
tecHNOLOGIES & tecHNIQUES theoretical & empirical Models of Print gain Analyzing Shoulder Angle & Compression By Dr. Timothy Gotsick Print is fundamentally two-dimensional, in that printed dots are characterized by their size and their posi- tion. But the dots on flexographic plates—the objects that create printed dots on press—are three-dimensional: They have size, position and a support height that can vary in shape—the dot “shoulder. ” T he act of flexo printing is thus one of dimensional transformation and in transforming from 3-D to 2-D, we actually create a larger dot. Here’s a good comparison for the packaging designer: What happens if you unfold a 3-D box into its 2-D shape? It’s a lot larger than the starting box, right? (Although perhaps not the most rigorous analogy, it fits so well I couldn’t resist using it.) Here’s a visual representation: grAsPing gAin The increase in dot size that occurs from plate to print is known almost universally as gain. And although flexo print- ers the world over have come to accept it, compensate for it (somewhat) and generally live with it, it is at best a compro- mise between what we want to print and what we actually do. Surprisingly, for a phenomenon so common, so industrially relevant and so negative, relatively little work has been done on figuring out exactly what causes gain. In recent years, as their ability to engineer a variety of flexo dot shapes has grown, flexographers have found that the shape of the dot shoulder can have a profound effect on print gain. In 2009, presenters described the enormous effect of dot shape on corrugated “fluting” at Flexographic Technical Association’s Forum in Florida. In 2010, they discussed the merits of dot shape on flexo print performance at the legend- ary Louisville, KY, Fall Conference “Great Plate Debate.” As a group, we are convinced the biggest contribution that plate developers can bring to flexo print improvement is better dot shapes that gain less, better highlight dot stability and less change during long printruns. Gain is the enemy and the better we understand it, the better we can fight it. Accordingly, a research team embarked on a program to study the origins of print gain and the factors that promote, or better yet, reduce it. studYing dots There are a lot of factors that contribute to gain, but be- cause it is something that one can directly influence through platemaking, the effect of mechanical dot deformation (the shape change that occurs when someone compresses a dot against a substrate) was where the research started. It is dif- ficult to observe dots in their natural environment (a printing press), so a model system of macroscopic dots that could be studied with the naked eye was created (see Image 2). All dots were 70-mm. tall, had a 10-mm. diameter flat tip and were molded from the same photopolymer. The shoulder angles of the dots were (from left to right in Image 2) 53 de- grees, 62 degrees, 71 degrees and 79 degrees. A material test- ing apparatus was modified to allow compression of the dots at a speed of 1-cm per second to a total compression of 20-mm. The force of compression was measured by the compression apparatus at a 10-Hz. rate. The size of the “print contact patch” of the dot top with the compression surface was measured us- ing a Fuji pressure sensitive paper, allowing accurate quantifi- cation of a dot’s contact area during compression. The force vs. compression curves for the dots are shown in Figure 1 below. It is interesting to note the curves were non-linear, indicating that the force rose faster than the compression distance. The degree of non-linearity, as well as the overall level of force exerted during compression, was clearly dependent upon shoulder angle, with the force curve being higher for lower shoulder angles (i.e., broader dots). It is also noteworthy that the dot with the steepest shoulder angle showed a hiccup in its compression curve, although its force vs. compression progression was the flattest of all the dots tested. The contact patch vs. compression curves (Figure 2) were also non-linear and again showed clear relation of the con- tact patch behavior to the shoulder angle. As with the force generation, contact patch, growth was inversely proportional to dot shoulder and the contact surface of broader dots in- creased more rapidly than that of narrower dots. The steepest shouldered dot once again showed a deviation about halfway through the compression distance, with the contact patch growing rather suddenly in the middle of its compression. Image 3, taken from the compression test of the steepest shouldered dot, explains the hiccup seen in both curves. It is due to the tip of the dot folding over onto its side, which both reduces the force it can exert and drastically increases its contact surface, which is now oblong instead of round. This observation corresponds directly to a phenomenon well known and much disliked in flexo, where the smallest dots start to print like dashes instead of dots. This typically hap- pens at the lowest end of the tone scale and intensifies the dreaded “hard edge” of vignettes. sHouLder AngLe After quantifying the effects of dot shoulder angle on dot defor- mation and observing the phenomenon of dot failure directly, the team directed its attention to more closely studying the effect of shoulder angle on highlight dot failure. A second set of macro- dots was fabricated, but this time with a more closely spaced set of shoulder angles (2-degree increments from 70 degrees to 80 degrees, shown in Image 4) that would allow it to more accu- rately seek the “critical angle” at which the dots would begin to fail under compression. The height and size (70-mm. tall, 10-mm. top diameter) of the second set of dots were identical to the first set, and they were subjected to the same compression tests (20-mm. maxi- STUDY TAKEAWAYS • Critical angle where a dot fails under compression found between 72 degrees and 74 degrees • Steepest shouldered dots failed at lowest compression levels; decreases in shoulder angle allow dots to absorb progressively more compression • Dot with steepest shoulder angle showed hiccup in compression curve • Contact patch growth inversely proportional to dot shoulder • Contact surface of broader dots increased more rapidly than narrower dots image 1 image 2 Figure 1 Figure 2 image 3 60 FLeXo OCTOBER 2013 www.flexography.org www.flexography.org OCTOBER 2013 FLeXo 61 4th Annual Plate/ Platemaking/Sleeve Selection Guide