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FLEXO Magazine : March 2011
Technologies & Techniques 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. Brittleness. Inks that are too brittle and hard can break loose from the substrate surface during printing or converting processes. Brittle inks can also create adhesion problems. The Solutions • Ask your supplier to recommend a more flexible ink system. • Although it is possible to make press side adjustments to the ink to increase flexibility, this is a high-risk approach because of the lack of control over additive concentrations. Highly plasticized inks. Highly plasticized inks cause blocking, penetrate the substrate and soften other construc- tion materials (adhesive, coatings, etc.). Depending on the substrate, they can also migrate, which can contaminate the packaged materials. The Solution • On request the ink supplier can recommend an ink system with different plasticizers or lower/no plasticizer levels. In any case ensure suitability of the ink system for the spe- cific substrate with respect to migration by migration tests. Seal inhibition. Ink components printed on the other side of the web may affect certain types of sealant films when the film is rolled up. These effects are well-understood and there- fore not common today, but examples include reduced seal of cold-seal adhesives or saran sealant surfaces. The Solution • Test and qualify inks for the intended package construc- tion to ensure compatibility with not only the substrate, but all materials of the constructed package. PROCess IMPROVeMenT sTRATeGY Printing can be a complex process with many interconnect- ed inputs and materials. Although materials may be supplied independently, they may affect one another’s performance on the press or in later converting processes. Recognizing the impact inks and substrates have on each other can improve your printing process and enhance problem solving. When confronting a complex problem, or what appears to be a simple problem that never quite gets solved, the team approach may work best. Call in substrate and ink suppliers to work directly with pressroom manage- ment. Together they can look for the root cause and define a permanent solution. n About the Author: Michael Impastato is vice president of stra- tegic marketing for the Flint Group. He is currently chairman of the Foundation of Flexographic Technical Association’s Board of Trustees and sits on Flexographic Technical Associa- tion’s Board of Directors. www.flexography.org march 2011 FLeXO 23 C M Y CM MY CY CMY K LEC_Flexo_30th.pdf 1 3/10/11 4:59 PM FLX_March11.indd 23 3/18/11 1:32 PM