Analysis
It was discovered that there was variation between the shoulder angles of smaller dots and larger dots. Dot percentages of 25 percent to 75 percent had similar angles, close to an average of 130 degrees on both sides of the dot, however there is a greater range of variance between angles a and b. Ultimately, it was found that lower reliefs did have slightly greater shoulder angles in highlight dots, compared to the highlight dot shoulder angle of greater reliefs.
Reduced relief did provide less dot gain and more accurate output, even with the use of higher volume anilox rolls. However, lower reliefs did provide less gain overall, compared to greater reliefs across all ink volumes. Dot gain was found to be very similar between reliefs with higher linescreens. As expected, over-impression of each plate provided more dot gain and ink density than the kiss impression.
The data from the second run showed (comparatively) interesting results. SID was greatest at 0.020-in. and 0.023-in., and had similar values within a +/- 0.005-in. margin of error. The plates with 0.009-in. and 0.012-in. had greater average SIDs compared to the baseline 0.018-in. relief, with a difference of 0.18125-in. No difference was found between lpis at kiss impression for each relief.
As for dot gain, the greatest gain in the highlight areas was found with the 0.023-in. relief; the 0.020-in. and 0.018-in. plate performed similarly across all highlight values with the exception of the 1 percent dot, where the 0.020-in. plate had more gain than the latter. The 0.009-in. relief plate had the least amount of dot gain, followed very closely with the 0.012-in. relief plate.
The plate with the 0.009-in. relief value had the most linear gain values, however, there was a spike at the 1 percent dot. There was some difference in the 10 percent dot values across lpis. The HD screening performed better than 150 lpi and 175 lpi, up until the 5 percent dot. The partial percentage patches did not form properly at any lpi or relief.
Extension to Wide Web
Further testing was necessary to see if decreased relief would be a viable option to combat dot gain under industry conditions. Five plate reliefs (see Figure 6) were targeted: 0.007-in., 0.009-in., 0.021-in., 0.024-in. and 0.025-in. Each relief target contained the three conventional screenings of 133 lpi, 150 lpi and 175 lpi, as well as the HD screening used in the last run. Ten plate samples were made overall: Five were kept in the lab and five were printed to compare dot wear across samples at industry conditions. After printing, both plate samples and print samples were compared. The plate samples showed variation in the formation of the dots between reliefs, highlighting the importance of optimizing screening conditions for plate making processes.
The plate samples printed on the wide web press provided similar results to those printed in the lab, where decreased relief provided less dot gain compared to other reliefs. However, there was overall more gain with the wide web samples compared to those run in the lab. Figure 7a illustrates the dot gain measured on wide web samples 1-5, whereas Figure 7b compares the dot gain of each wide web sample to the averaged anilox samples from the second printrun.
Microtype targets were included on each plate. However, they were not sufficiently supported during the print.
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