by clicking the arrows at the side of the page, or by using the toolbar.
by clicking anywhere on the page.
by dragging the page around when zoomed in.
by clicking anywhere on the page when zoomed in.
web sites or send emails by clicking on hyperlinks.
Email this page to a friend
Search this issue
Index - jump to page or section
Archive - view past issues
FLEXO Magazine : February 2010
www.flexography.org FEBRUARY 2010 FLEXO 39 higher press speeds, increased press capacity, less common cause variation and lower overall process waste. As with any new technology there was a learning curve. Digital plate print variables are more finely tuned than those of a conventional analog plate solution. Therefore, the unique dot structure that gives digital photopolymer plates their increased tonal range had to be treated differently. Printers soon realized that press impression, and the individual ele- ments of the print package (plate, tape, ink) needed to work together to achieve optimal quality. MINIMAL IMPRESSION IS KEY If you were to poll today 's printers and converters about how much impression is necessary to be successful, you would have many different answers and opinions. Some believe you must squeeze the plate to adequately transfer the ink knowing full well that high dot gain and premature plate wear are a likely consequence of this action. Others believe that a "kiss impression" is the best way to achieve optimized highlight dot reproduction, but are concerned about the possibility of sacrificing solid ink density. Regardless of their route, most printers agree that minimal impression is the key, and are constantly searching for ways to balance dot gain and solid density. HARD DUROMETER PLATES With the development and introduction of higher durometer digital plates, companies now have the opportunity to print both fine highlights while enjoying an adequate solid ink density. "High durometer digital plates have been gaining in popularity over the last 10 years and are now the plate of choice in Europe" says Jan Scharfenberg, technical manager, DuPont Packaging Graphics Europe. "The current genera- tion of hard digital plates is robust enough to handle higher impression without prematurely slurring the dot or collapsing the cushion." So why don't hard durometer plates enjoy wider accep- tance in North America? A commonly voiced concern is that past experience has shown that a hard durometer plate will deliver excellent dot quality but may not offer a satisfactory solid. Given the capabilities of traditional mounting tapes, this was probably a valid concern at one time. However, the latest advancements in cushion tape technology have changed the rules balancing both dot quality and solid ink density, and an increasing number of North American printers are moving to hard digital plates. FIRMER MOUNTING TAPE As with photopolymer plates, cushion mounting tapes have also gone through an evolution and diversification of their own. As the variety and capability of tapes expanded, our un- derstanding of how to optimize plate and tape combinations expanded as well, and the industry has come to the realiza- tion that a move to a hard durometer digital plate in combina- tion with a firmer tape will yield the best overall results. So how do we know that a hard digital plate and firmer tape will yield the best results? First, let's refer to the charts showing dot gain and density results from wide web film tests conducted with a series of print packages. All conditions were run with the same impression, anilox, sleeve, ink, print deck, doctor blade, press, operator and running speed. The only differences were the choice of plate and/or cushion tape. You'll note that when using a hard digital plate, the shift to a firmer tape had virtually no impact on dot gain, but caused a significant increase in solid density. Compare that to the results with the soft digital plate, where the choice of tape al- lowed you to optimize either dot gain or solid density, but not both simultaneously. Next, refer to Figures 1 and 2. Please note that with a soft tape more plate impression is required, to minimize pinholing in the solids, which compromises highlight dot quality. The bottom line is that a hard durometer digital plate in conjunction with a firmer, not hard, cushion tape will yield you a winning solution! Stan Riches, sales manager, graph- ics from Lohmann Technologies has confirmed, "For several years now, we've seen customers who previously used a soft foam tape for process printing achieve a significant improve- ment in dot gain control when they switched to our medium soft and firmer tape for process and combination printing." Ann Michaud, senior technical service specialist from 3M also commented that the "Mounting tape and plate work together on-press to deliver the desired print quality; moving to a harder durometer plate may also require a change to a firmer mounting tape to achieve the desired print result." FLEXOGRAPHY UNIQUENESS Flexo is a fascinating industry. We have many different size presses and therefore it should come as no surprise that cushion tape parameters for a narrow web press are prob- ably different than a wide web press. And likewise high speed gearless presses have different variables than geared press- es. As press width increases so does the chance of variation. Wide web presses typically require more plate impression due to the changes in caliper (sleeve TIR, tape, plate) over the width of the press. Firmer tapes help minimize this common cause variable enabling you to obtain acceptable print qual- ity with minimal plate and anilox impression. Narrow web presses are also unique in themselves. A hard durometer digital plate and a firmer tape is still the way to FIGURE 1. Print results of a hard plate and soft tape. TECHNOLOGIES & TECHNIQUES