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FLEXO Magazine : June2010
Technologies & Techniques 62 FLeXO june 2010 www.flexography.org Photo courtesy Dunwoody College of Technology. Dot Bridging can happen for several different reasons. Approximately 95 percent of all water-based ink issues revolve around pH. That being said, the first thing that I look for when there is dot bridging is the pH of the ink. If the pH dips below 9.0, the resin will start to string out and be less willing to transfer from the plate to the substrate. The ink will, in turn, build up on the plate and “bridge” from one dot to another. By raising the pH, the resin will flow/transfer as it should. If the pH is okay, then the following should be checked: • Anilox to plate screen marriage: the old theory of 4lpi of anilox to 1lpi of plate screen is a good rule of thumb to follow for most issues with bridging. However, with the introduction of high-definition printing or and the desire to print less than 2 percent dots, the ratio of anilox to plate should increase to 6lpi of anilox to 1lpi of plate. If you don’t have this proper ratio, the dots will be overwhelmed with ink and bridge. Since this is a mechanical issue, there is little you can do with the inks to stop the bridging. If you don’t have the anilox to manage this, you will have to raise the minimum dot on your plate in prepress to compensate. • Surface tension of the substrate: If the substrate doesn’t have a high enough surface tension/treat level/printable coating, the inks will not want to transfer to it. To correct this, check the material for dyne level and adjust the treatment applied to be sure that it is in an acceptable range (above 38 on most substrates). If there isn’t an in-line treater available, apply a first down primer/wash coating to allow the inks to transfer. • Inks drying too fast: If the inks are drying on the dots, raise the pH above 9.5 or slow the inks down with glycol or amine. If problems persist, I recommend talking with your ink sup- plier who may recommend modifying the ink. n ABOUT THE AUTHOR: Michael Hoft is narrow web products manager at Color Resolutions International (CRI). He began his career as lab manager and technical representative for Aarberg Printing Inks in Dallas, TX. He later became a technical sales representative for Water Ink Technologies (now ACTEGA WIT), where he advanced to regional techni- cal manager. His extensive experience has allowed him to combine rich laboratory knowledge with field application to help companies solve challenging ink application and label production challenges. CRI is an international packaging ink company focused on water-based and UV curing flexo inks for printing pressure- sensitive labels and security tags as well as flexible packag- ing, envelopes, folding cartons and corrugated. Color Resolu- tions uses proprietary InkChemistry to custom tailor inks that print cleaner, richer, truer colors, giving printed graphics an edge in the marketplace. Food, beverage, HBA and phar- maceutical packages demand high-impact colors with good adhesion, chemical resistance, scratch resistance and fast, economical application on high-speed presses. Contact CRI at 800-346-8570 or visit www.colorresolutions.com. Troubleshooting Guide: Dot Bridging Michael Hoft Preparation for Forum 2011 is already in the works. If you’ve got a hot topic that you would like to see appear on next year’s program we want to hear about it! If you’re an expert in your field with a penchant for public speaking, we want your abstract! If your company is interested in gaining greater visibility at the flexo industry’s most widely anticipated and highly-attended annual gathering, we want to talk to you! Abstract Submission – Deadline: September 10, 2010 All abstracts MUST be submitted online at: www.flexography.org/abstracts.cfm and all fields must be completed in their entirety. Questions? Contact Shelley Rubin, firstname.lastname@example.org to learn more. www.flexography.org Plans for next year’s Forum have already taken flight. FLX_June2010_mech.indd 62 6/10/10 9:40 AM