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FLEXO Magazine : June2010
Technologies & Techniques 56 FLeXO june 2010 www.flexography.org At some point in their career, every press operator has had the unfortunate experience of doing everything right; setting the press up with the proper and cleaned anilox rolls, installing new doctor blades, adjusting inks to the correct viscosity and/or pH, achieving correct densities, setting impression evenly and accurately, and dialing in registration perfectly only to see the image quality “go south” after a short run time. The dot gain rises terribly, halos appear around solids and lines, and text begins to fill in. This scenario begs the questions, “What happened and what do I now?” An industry mentor of mine once told me that troubleshoot- ing flexo printing isn’t always about knowing exactly what the problem is, but rather a matter of eliminating what the problem isn’t. In that context, let’s figure out what happened in the scenario above and what to do about it. What it’s Not Excessive dot gain, halos, and fill-in should send us down the paths of an increased amount of ink being delivered and/ or a drastic change in impression levels. Over a short period of time, does a clean anilox roll typically start to deliver more ink than it did in the beginning? Not likely, so the anilox roll is eliminated. Would a new doctor blade wear out that quickly to allow the added ink through the nip? Again this is highly unlikely. If it was an over pressured doc- tor blade, it would bend at the contact point with the roll and deliver more ink than necessary at the start of the run. We can now take the doctor blade off the list. Next on the agenda of potential culprits is impression-setting changes. While a mechanical problem is always a possibility, it’s much more likely that the impression would back away; yielding voids in the print rather than the heavier print problem we’re dealing with here. Therefore, it’s a safe bet to remove mechanical issues from our list. An unknown or unexpected change in caliper within the roll of stock used could cause this extra “squeeze.” However, the degree of change needed to create the sever- ity of the print problem we have makes this a low probability. Roll changeouts are quick and easy, so this can be confirmed or denied in short order. The new roll of stock yields similar results; so it now comes off the table, too. Mounting tapes do not suddenly increase in thickness. Ex- cept for the very edges that may be exposed to the pressroom elements, it’s completely protected by the printing plate. If anything, a tape may take a “set” during a long run; meaning it loses its ability to rebound to the original gauge. If the initial impression settings were heavy enough to cause the tape to take a set, our print quality would not have been good at the outset. Since our pressrun failed early, and a loss of gauge yields a tape/plate package thickness less than what we started with, a loss of impression and voids in print would be the result. This is the opposite of what we’re experiencing, so the mounting tape isn’t our problem either. We’ve now turned over quite a few stones and still haven’t found the cause of our problem. Go back to the list and determine what could be looked at but hasn’t been. The plate and ink have yet to fall under any scrutiny, so it’s time to look there. In and of itself, ink is not going to cause dot gain, halos, and fill in. So too, can be said of the printing plate. We have systematically erased other possible causes from our list. With respect to the plate and the ink, taken singly, we can- not even print, let alone be it poorly, it’s what they do together is ultimately the cause of our pain on the press. We have determined that there is an incompatibility between the plate and the ink that is causing it to swell on press. Printing plates are made up of chemicals just as inks are. Sometimes, the chemicals in each negatively react with one another, which causes the plate to lose its integrity and dimensional stabil- ity. In our case, this bad reaction made the plate increase in thickness and most probably become softer, both of which troubleshooting Guide: Plate swell What Happened To My image? By Tom Cassano Print defects from plate swelling (left), compared to normal printed dots. Art courtesy MacDermid Printing solutions. • Solvent-based inks contain chemicals that, in high concentrations can, and do, cause plate swelling and softening. • Most photopolymer printing plates, regardless of the process used to make them can handle acetate concentrations of 20 percent or less before adversely affecting the plate. • While a particular plate from your supplier may not be compatible with your UV ink set, in most cases there is another product choice from the same sup- plier that will perform without ill effects. • 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. FLX_June2010_mech.indd 56 6/10/10 9:39 AM