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FLEXO Magazine : May 2012
SELECTIONS It is important to understand sub- strate choice is an economical decision. A package must fit within a marketable cost model while delivering on per- formance. Performance can be recy- clability, strength, product protection, shelf presence and appearance of the package. Graphics are often a second- ary concern to the functional properties of a substrate. With substrate being the largest cost component to any package, cost savings in substrates can often change graphics potential. Functional properties can also affect graphics potential due to surface properties. Surface properties are what print- ers are interested in. These properties include brightness, whiteness, color, opacity, gloss, absorption, hold out and smoothness. The better the surface and the more stable the substrate, the better the print. Herein begins the thought process for ink metering. It is substrate based. What impact does an absorbent substrate have on ink metering deci- sions? How does this affect graphics? What impact does a non-absorbent substrate have? What can be expected from runnability performance? In flexog- raphy, it comes down to matching the anilox roll to the substrate. This is very true but the anilox is only a component of what is called an ink metering sys- tem. The configuration of the entire ink metering system should be considered when trying to predict performance and graphics quality. The fundamental purpose of any ink metering system is to deliver a uniform ink film thickness to an image carrier. Figure 1 illustrates the basic compo- nents of an ink metering system for flexography. There are three basic ink- metering designs: two roll system, two roll with reverse angle doctor blade and chamber blade system. The set up of an ink metering system needs to be matched to the substrate in order to achieve proper ink coverage, accurate color, clean print and desired dry rate, and press efficiencies. The following is a detailed overview of the metering system and how it fits into the flexographic printing model. TWO-ROLL SYSTEM The two-roll system relies on a rub- ber roller to deliver the ink from the ink pan to the anilox roller. The rubber roll- er is sometimes referred to as a wiper roller or fountain roller. The durometer, or the pressure that squeezes against the anilox roller, and the rotational speed of the rubber roller all create a unique wiping action on the anilox roller. This is where the name “ wiper” roller comes from. Changes in one of these variables, or their settings, can change ink-metering characteristics. This is the reason why two presses with the same configura- tion can print with different results. The two-roll system generally leaves surface ink on the anilox and therefore requires a heavy ink film to mask inconsistent metering. A heavy ink film means this system is only really good for absor- bent substrates and limited graphics capabilities. Additionally, if a heavy ink film is being delivered to the substrate, overprints should be limited, halftone screens should be coarse and dryers will be needed to help productivity. Dry- ers can help but also hinder if opera- tors are not comfortable using them. Because dryers introduce heat, ink can change quicker and the moisture con- tent of paper substrate can be altered. Roll-to-roll systems are commonly used for coatings or low-end graphics on corrugated boxes and bags. The benefit of a roll-to-roll system is there is not a doctor blade for the operators to manage and the operators can vary rubber roller settings to achieve limited print results. A doctor blade is introduced to the two-roll system to get a more uniform wipe on the anilox roller. The doctor blade opposes the direction of rotation of the anilox roller and will significantly increase the graphics potential. A doctor blade can be plastic, metal or composite and its sole purpose is to scrape excess ink off the anilox roller surface. The benefit of using a doctor blade is that it will give smoother solids, cleaner halftone screens and sharper type in the print. www.flexography.org may 2012 FLEXO 49 Harper has devoted an entire division to help flexographic printers and converters achieve unprecedented levels of consistency, quality and profitability. Using our exclusive SHarper SystemTM we eliminate variables that impact quality and increase predictability of press results. Call 800-438-3111 for a free copy. Smart. To learn more, call 704.588.3371 or Toll Free at 800.438.3111 Or visit our website. Graphicsolutions DiVision harpE riM a GE.coM Americas • Europe • Asia ©2011