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FLEXO Magazine : March 2011
Technologies & Techniques • Apply a chemical treatment to coat the substrate during film-making. • Apply a primer on press to improve wetting. Overtreated film. Overtreating film will degrade the film’s surface and make the ink appear to have low adhesion. In extreme cases, the upper layer of the film surface will fail and will be removed with the ink during a tape test. Analytical test- ing may be required to pinpoint the actual failure mode. A more subtle characteristic and therefore more dangerous situation is reduced water resistance, especially with solvent based inks. Inks can exhibit very good initial adhesion, but will come off readily when soaked in water. The Solutions • Prevention is paramount because there is no cure for over- treated film. Bracketing the treat level by testing surface energy levels eliminates this: Passing a dyne test at the low end of the specification and failing at the high end of the specification will ensure the film is properly treated. • Remember that surface contamination or slip additives from the film can affect the results of wet chemistry tests, including dyne tests. • Use a goniometer. It provides the most accurate and precise measurement of surface energy. Gauge variation. The surface of a substrate, along with the impression pressure, plate type and sticky back type, will de- termine where the ink contacts the substrate. If the substrate’s surface is not level, the ink will fail to print in the low valleys, leaving unprinted areas. This can lead to dot-gain variation and lack of fine dots in screens. Uneven smoothness, or cali- per, will produce defects such as mottling or overimpression, resulting in dirty or poor print quality. Gauge variation across the web is readily apparent in the roll: It will show up as high and low spots or soft and hard bands. However, gauge variation in the machine direction is not visible in the roll, making it much more difficult to diagnose. It can create variations in dot size be- cause the impression pressure changes as the gauge changes. In extreme cases gauge varia- tion can damage press compo- nents over time. The Solutions • Contact the substrate supplier, who will need to track down the root cause and address it at the mill. • Increase impression pres- sure or use softer plate / sticky back combos to compensate for variations in thickness. • Lower the viscosity of the ink or add solvents or surfactants to make the ink film spread more readily into the substrate’s lower points. Paper acidity. The chemistry involved in making paper can give the paper an acidic pH. If the pH gets much below 4.0 , it can affect the curing properties of litho inks. Litho inks contain a catalyst to enhance the oxidative drying of the oils they con- tain, and low paper pH will halt or impair the curing process. The Solution • This issue has been well-addressed by paper manufac- tures, and should be discussed with the paper supplier to correct the acidity of the paper. Ink Issues A number of ink characteristics can affect substrates and printing. Tackiness. This is a common issue with litho inks that require varied tackiness to wet trap one ink on top of the other properly. But tackiness can also be an issue with flexo and gravure inks, particularly if the paper surface is not well anchored; the tacky ink can pull loose fibers from the paper. These loose fibers, with ink, will build up on the plates, caus- ing dirty printing. They will also recirculate through the inking system and create other defects. The Solutions • Have ink suppliers review the ink’s tackiness and drying rate. Reformulation may help. Inks may become tackier as they dry and solvents are lost, so one option is to adjust the drying rate. Keep in mind that this may affect press speeds. • Change substrates to a paper surface with better fiber adhesion. • Apply a primer or coating prior to ink to seal the substrate surface and reduce fiber pull. 22 FLeXO march 2011 www.flexography.org C M Y CM MY CY CMY K LEC_Flexo_30th.pdf 1 3/10/11 4:59 PM FLX_March11.indd 22 3/18/11 1:32 PM