By Keith Solomon It is well over 35 years since I became involved with Flexo plates. When DuPont launched Cyrel it was a quantum leap in flexo quality. For 35 or so years we have witnessed the incredible improvements in the quality of flexo printing. I now see that a project has been launched, known as the Revo Team, to further revolutionise digital flexo. The seven members of this dream team have set themselves the goal to drive flexo towards the new digital world of printing and converting. Gurus such as Niklas Olson of Flint, Dan Pulling of Esko (Dan was in South Africa not so long ago), Friedich Wolf of DuPont, Nick Harvey of Apex, Fererico d’Annunzio of Nuova GIDUE, Marco Tiainen of UPM Raflatac and Daragh Whelan of Adare. We are a formidable group and I am sure that we will see a further leap in quality, process consistency, efficiency and cost reduction. It brought to mind an article which was published recently concerning flat top dots and the article is well worth another and closer look. Have a read and let us watch with interest the results from the Revo Team who will be meeting during June in Florence. Background Over the years the flexographic printing industry has used several dot shapes and structures, with varying resultant effects on plate life and consistency. It is generally accepted that the flat top dot structure of analogue or conventional flexo dots enables considerably longer plate life (often 2 to 5 times) compared in like-for-like tests to the rounded dots utilised by traditional digital plates. The introduction of Kodak Flexcel NX Digital Flexographic Plates with Kodak’s flat top dot structure also shows analogue-type improvements in plate life over traditional digital plates. But why? Kodak personnel are often asked why the flat top dot structure is better for plate life. Intuitively the answers seem obvious, but we chose to commission an engineering study to find an independent, science-based answer. Seeking the scientific answer The Welsh Centre for Printing and Coating (WCPC) at Swansea University (SU) is a world-recognised centre of expertise in computer simulations of engineering problems. The technique first developed at SU, and used globally today, is Finite Element Analysis (FEA), which breaks down complex structures and situations into a mesh of smaller elements to allow application of engineering equations to calculate stresses. It is a standard analysis method for analysis of potential structures for buildings, airplanes, cars, etc. The staff at the WCPC, under the guidance of Prof. Tim Claypole MBE and Prof. David Gethin, carried out a series of FEA studies using a commercially available package developed at SU, to study the stresses generated with static conditions at a series of applied deflections and conditions. They used generic dot profiles to represent analogue, traditional digital, and Kodak Flexcel NX Digital Flexographic Plates. The tests assume the same materials and load conditions to provide a like-for-like set of results based on dot shape alone. Reading the results Reading the results is very simple. The blue colours indicate low stresses and probable low wear rates: the brighter the colour higher up the scale, the higher the stress and the faster the probable wear. For the base case WCPC was instructed to use standard photopolymer properties based on data from their flexo material research that started in 1998, and to use dot profiles based on their scientific non-contact measurements. The images below show the standard profiles considered. The lower part of each image represents the dot, and theupper part represents the impression surface. All sets of images in this report are arranged in the same configuration, with the Flexcel NX Plate samples to the upper left and the traditional digital LAMS samples to the lower right. Fixing the Scale To help make the reading of the results as transparent as possible, WCPC were instructed to use the same scale for the stresses on all of the results. This means that the blue, yellow, or red colours represent the same stress levels on ALL of the figures allowing instant reading and simple comparison. As the colour moves from blue to green to yellow, then to orange and red, the stresses get higher. Loading Conditions The model then considers the deflection of the tip of the dot at two pre-set distances, to mimic the dots under two levels of impression setting. The distances used were 3 thousandths of an inch or 75 microns, and 5 thousandths of an inch or 125 microns. Interpretation The author, Dr John Anderson, a 9-year, 3-degree graduate of SU, has been carrying out and reviewing FEA results since 1992, with extensive application throughout his time in Swansea. What follows is Dr Anderson‘s analysis of the results, based on the data supplied by WCPC. The basic premise is that plate areas that undergo the highest stress levels lead to the greatest amount of plate wear. The largest areas of higher stresses indicate the fastest probable wear conditions. The Results 1. Flat tops reduce stress and wear The first case applies the two standard loading conditions to a 30% dot at 150 lpi. The results show that the peak stress on the Flexcel NX Plate is a small point where it transitions rapidly from no load to fully loaded. This is a very small area and should not cause significant wear stresses. Comparing the profiles of the dots, it is clear that the Traditional Digital LAMS dots show very significant stresses over a large area, especially at the tip or contact point. This would be expected to cause much faster rates of wear than either the Flexcel NX Plate or conventional analogue dot structures. It is clear from the image that the flat top dot structures distribute the loading more evenly, reducing the stresses, and the potential for wear. Increasing the engagement to the higher setting shows that the overall stresses experienced increase in all three cases, but again the two flat top dot structures on the left have much smaller increases, and are devoid of large and significant stress concentration throughout the whole contact region. The models have therefore verified a few key points, even with just these two loading cases: • Dot shape has a significant effect on the stress profile. • Stress level increases as the engagement (impression) increases. • The flat top dot structures distribute or share the loading better to achieve lower stresses. • The rounded shape of the traditional digital LAMS dot, results in the highest stress concentrations, suggesting the highest wear rates. • The stresses are focused in the contact zone. Several of these findings are intuitive, and the results of the tests appear to be logical. The following figures show these two conditions in more detail. The results explain some common dot growth issues that occur at the start of production with traditional LAMS dots, and the common practice of cutting the minimum dots off at higher dot sizes or dot percentage to increase plate life. 2. Flat tops still win out as dot size increases This next example considers a 30 percent and 50 percent dot under the same loading conditions. The results, as expected, show that as the dot gets bigger the stresses experienced decrease. What is interesting is the fact that the decrease with the analogue and the Flexcel NX Plate dots is much more significant than that for the traditional digital LAMS dot. These models verify: • Dot size affects stress concentrations. • Stress levels decrease less for rounded dots. 3. The effect of resolution on plate wear The final case considers what happens as resolution increases, and whether there is a trade-off between resolution and potential plate life. In the following two examples, the line screen increases from 150lpi to 200lpi. The results show that the stresses increase, but do not change a great deal. However they are more concentrated than before, and indicate that increasing resolution can potentially negatively affect plate life. These models verify that: • Smaller dots further concentrate the stress and potential wear rates. Conclusions The graphical nature of the stress profile plots means that the figures included in this white paper are relatively easy to read, and demonstrate clear findings that the stresses and the significant concentration of stresses around the tip of the rounded dot of traditional digital LAMS plates, will cause a significantly increased likelihood of accelerated wear. The flat top dot structure of analogue or conventional dots, along with those of Flexcel NX Plates, distributes the stresses more effectively and evenly, reducing the stress concentration and the likelihood of plate wear. Additional feedback gathered from printers has clearly confirmed that flat top dot structures have a positive impact on plate life. Printers are reporting plate life of 2-5 times longer with flat top dot structures, with the differences often higher when more abrasive substrates are used. This provides additional evidence that flat top dot structures are superior for on-press impression and plate life performance.
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