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FLEXO Magazine : September 2014
strokes before the ink was removed from the print sample. The sam- ples for film in both LED UV and conventional UV may be similar because the ink remains on top of the glossy surface of the film and is unable to adhere as well as the ink does to the paper. There is noticeable difference in curing effectiveness on paper when using conventional UV curing, but no conclusions can be drawn about the other test runs at this time. Further testing can be done to draw more detailed conclusions on ink cure effectiveness. SUBSTRATE TEMPERATURE UNDER LAMPS The heat of the substrate was measured during the pressrun using a General IRT206 infrared thermometer. The infrared thermometer measured the heat of the substrate as it exited the second curing unit. The infrared thermometer was held approximately 8-in. away from the substrate and measured the same area of the web in the same unit in each test run. The temperature was read once makeready was com- plete and the press was running in conditions for the test. Both conventional UV and LED UV curing systems emit heat as a byproduct, which transfers to the substrate. Table 5 contains the measured temperatures of both the paper and film substrates at low and high press speeds. The low press speed for the test runs was approximately 150 fpm and the high press speed was approximately 400 fpm. The temperatures in the chart show that LED UV curing sys- tems generate less heat compared to conventional UV curing systems. Because of this, a wider range of heat sensitive substrates may be used with LED UV curing systems. According to the press operator, Scott Shutt, thicker films can be used for conventional UV curing systems, but thinner film may experience a significant distortion, due to the exposure to higher temperatures produced by conventional UV curing systems. The measured tem- peratures for the two tested substrates, paper and film, were slightly higher at the lower press speed. The faster web likely generates more turbulence, subsequently carrying some of the heat away from the lamp housing. Table 5 Substrate Temperature in Degrees Fahrenheit Read at The Cyan Unit Conventional UV LED UV Percent Decrease in Temperature From Conventional UV to LED UV Paper, Low Speed 108 88.5 18.06 Paper, High Speed 104.5 87 16.75 Film, Low Speed 107.5 87 19.07 Film, High Speed 104 86.5 16.83 TYPES OF SUBSTRATES & DISTORTION For this test, two substrates were printed—paper and plastic film— both of which are common in industry. These two substrates have different properties. Paper is known for higher printability and lower heat sensitivity, while film demonstrates higher thermal stability and durability (Sesetyan). Paper is known for higher printability and lower heat sensitivity, while film demonstrates higher thermal stability and durability (Sesetyan). After analyzing the printed substrates, both the 72 FLEXO | SEPTEMBER 2014 Graph 5: Conventional vs. LED on film at high speed (circle = LED value, square = conventional value) Graph 6: Conventional vs. LED on film at low speed (circle = LED value, square = conventional value) Graph 7: Control run paper conventional LV at low speed (circle = LED value, square = conventional value) Graph 8: Control runfilm conventional UV at low speed (circle = LED value, square = conventional value)