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FLEXO Magazine : June2010
Technologies & Techniques www.flexography.org june 2010 FLeXO 57 lead to the extra “squeeze” imparted to the substrate, with the resultant dot gain, halos, and fill in. Now that we know what the cause is, what can be done to fix it? ChemiCal ReaCtions Unfortunately, the correct answer to this is, it depends. In some situations a press operator may not be able to do anything about it; in others he/she can certainly correct the problem. Water-based inks, as supplied, are very plate friendly. Rarely is there a compatibility issue between them. Excessive additions of defoamer into the ink at press side can have a nega- tive impact on plate performance, so it needs to be used very sparingly. If ink foaming is an ongoing problem, it’s an indication of something else needing attention, which can be determined by this same process of elimination we used in our printing problem situation. Solvent-based inks contain chemicals that, in high concentrations can, and do, cause plate swelling and soften- ing. As supplied (directly out of the kit), there aren’t typically any issues. It’s the required addition of a solvent blend of normal propyl alcohol and normal propyl acetate for viscosity adjustment at press side that is most often the root cause of the problem. Most photopolymer printing plates, regardless of the process used to make them can handle acetate concentra- tions of 20 percent or less before adversely affecting the plate. Ink main- tenance blends of 90 percent NP (N- propyl) alcohol/10 percent NP acetate or 85 percent/15 percent in virtually all cases will keep the inks adjusted properly. All too many times however, these percentages aren’t adhered to or the additions are made haphazardly, which leads to high concentrations of acetate and loss of plate performance and/or print quality. Ink maintenance and adjustment needs to be done in a systematic and scientific manner to con- sistently print well and optimize plate life and performance. UV curable inks are used as is, require no viscosity adjustment, and the only operator involvement needed is to add some extender to reduce color strength if necessary. As good as this seems, these ink chemistries can be troublesome for photopolymer plates. Many of the chemical constituents can react negatively with those in the plate, resulting in severe swelling and softening of the plate. In this case, there really is nothing an operator can do to solve the problem, although a reduc- tion in inking and substrate impression can mitigate the extraneous dot growth, halos and fill in for a short while. It’s important to note that while a par- ticular plate from your supplier may not be compatible with your ink set, in most cases there is another product choice from the same supplier that will perform without ill effects. There is one thing that every opera- tor can do regardless of the type of ink he/she prints with to eliminate another common cause of plate swell—use the proper plate cleaning product for your specific ink. Warm, soapy water and a soft brush are perfect for cleaning dried water based ink from a plate. A soft brush and NP alcohol works very well for cleanup of solvent based inks. Iso- propyl alcohol should be the cleaner of choice for UV curable ink users. “Home made” recipes of cleaning solutions often lead to unintended consequences, all of which are bad. Avoid using them and remember that more and stronger is definitely not better. Business decisions based on price only—without proper consideration of downstream (press room) conse- quences—are unfortunately a fact of everyday life. Plate suppliers and ink suppliers alike are not immune to those consequences either. The best and true solution to the problem of plate swell is to have compatibility (swell) testing be- fore those business decisions are made. Either supplier can perform these tests which determine the ink/plate combina- tion that doesn’t cause this problem and identifies the combinations that might be a problem. We have now come full circle. The reason why “my image went south” has been identified by logically eliminating what possibilities couldn’t cause it to do so. Why the problem occurs at all has been uncovered, along with ways to prevent it and/or fix it. n ABOUT THE AUTHOR:: Tom Cassano is an applications development specialist for MacDermid Printing Solutions. He has been in the industry for 22 years and is a member of the FTA Awards Committee. Over 40 years of experience in the flexographic industry, design and production of the highest quality systems. innovation is the key to our success. COMPUTER CONTROLLED VISCOSITY CONTROL SYSTEM IN THE ROUND SLEEVEMAKING SYSTEMS IN THE ROUND SLEEVEMAKING Head Office AV Flexologic B.V. H. K . Onnesweg 2 P.O. Box 252 NL-2400 AG Alphen aa n den Rijn The Netherlands Tel +31 (0)172 434221 Fax +31 (0)172 437919 Mail firstname.lastname@example.org Toll Free number from the UK 0800-3892121 (only accessible in the UK) PHOTOPOLYMER PLATEMAKING EQUIPMENT FULLY AUTOMATIC MOUNTING MACHINE for flexo printing plates (for the flexible packaging and corrugated industry) Advanced MOUNTING & PROOFING EQUIPMENT (for the flexible packaging, corrugated and label industry) www.flexologic.nl PRE-PRESS EQUIPMENT FROM MODEST SYSTEMS TO THE MOST ADVANCED HIGH-TECH SOLUTIONS FLX_June2010_mech.indd 57 6/10/10 9:39 AM