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FLEXO Magazine : February 2012
the team set out to answer was how to get the four to work together without fault in the flexo process. Initially, the group met via conference call to determine how it wanted to approach the testing and analysis phases. In order to get everyone involved, the FQC team spread the " wealth" of testing as much as possible. Initial testing was done at Clemson University, with follow-up trials at locations in Plymouth, MN, and Dayton, OH. EVALUATION CRITERIA The first and most critical step was to identify what to use as the measurements of success. Each "operator" graded the set of print samples from a trial and scaled it on the following: 0: No spitting on the sample (2 print repeats) 1: 1-5 incidences 2: 6-10 incidences 3: Over 10 incidences Based on scoring evaluations, the FQC team was able to then have a statistician determine significance. KEY VARIABLE IDs It was agreed to take a matrix approach so we would not get lost, but rather stay organized in the mountain of potential data. Our matrix consisted of categories that the team felt were very important to the discovery of the causes of UV spit- ting. It decided to measure or control the following variables (examples): a. Anilox (500 cpi vs. 800 cpi) b. Run Speed (Ex. 300 fpm vs. 500 fpm) c. Blade Pressure (light, constant) d. Blade Width (28 mm, 30 mm, 32 mm) e. Blade Tip (standard 15-degree, Lamella, long bevel 4-degree) f. Blade Material (carbon steel, tool steel, plastic) g. Viscosity (low, medium, high, very high, measured in centipoise) h. Operator (how results were judged) The test matrix was designed to allow for adjustment based on what FQC team members wanted to hold constant and what they wanted to evaluate. Headings contained the test parameters and the amount of variation were designated accordingly. RESULTS Many of the trials conducted found no spitting whatsoever to evaluate. Even what was considered to be a verification trial at Clemson, yielded no repeat in the spitting results ex- perienced before. FQC was left to ponder the meaning of this information. It put a lot of effort in making sure there were no outside variables, so the conditions of the blade holders were paramount to the process. Each was meticulously cleaned so interference would not be an issue. Anilox rolls were prepared and verified so the cell condition was at maximum volume. Inks were verified beforehand for viscosity. In the end, the focus on preparation mattered to the outcomes. FQC turned to the statistician, who took the original data from the initial success and evaluated it. Examining the data, the statistician segregated ratings within the parameters (variables FQC chose to examine) if there was a great differ- ence, he used standard deviation of the difference between predicted and actual ratings and finally corrected the ratings, taking the most significant into account. He examined each parameter for the largest effect, one at a time. The most significant was omitted each time, significance meaning the most reduction in variation. The cycle was repeated until the remaining parameters were minimized. In one case, the stat- istician separated out differences within a variable. The sig- nificant difference within operator was classified as "harsh" and "not harsh" based on the subjective evaluation difference of the same print samples by the participants. For this particular trial, the results by significance came in this descending order: a. Anilox Cell b. Blade Material c. Blade Width d. Operator e. Speed The statistician prepared a chart to explain the significance further by placing what he deemed an "up charge" based on the known variables. Essentially, this means that depending on what components you chose within the variables, you could increase or decrease the likelihood of experiencing UV spitting. If a variable was assigned an "up charge" greater than the "up charge" of the current variable employed, the likelihood of spitting increased. In contrast, switching to a lower value for the same variable decreased the chance of spitting. For this particular trial, the chart accompanying this story outlines the "up charge" cost: VARIABLES THAT INFLUENCE UV INK SPITTING Category Option Up Charge Cell 800 cpi 0.0 Cell 500 cpi 1.2 Operator Not Harsh 0.0 Operator Harsh 1.0 Viscosity Low (less than 800 cP) 0.0 Viscosity Medium (800-900 cP) 0.4 Viscosity High (1000-1200 cP) 0.9 Viscosity Very High (1920 cP) 1.0 Blade Material Tool Steel 0.0 Blade Material Carbon Steel 0.5 Blade Width 28or30mm 0.0 Blade Width 32 mm 0.4 Blade Tip Standard 15 deg 0.0 Blade Tip Lamella 0.4 Blade Tip Long Bevel 4 Deg 0.2 Speed 150 fpm 0.0 Speed 300 fpm 0.3 www.flexography.org FEBRUARY 2012 FLEXO 63