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Summary:
This is the third of a series of papers in computer simulation of web offset printing process. This paper focuses on the effect of ink feed mechanism on press performance. The following five ink feed mechanisms were simulated: ductor-feed open fountain, continuous-feed open fountain, digital injector, anilox keyless inker, and positive-feed keyless inker. A bar graph test form was used to study the dynamic behavior of various inkers. It was found that both press time constant and incubation period are independent of any change in ink feedrate, if the ink film thickness is used as the control variable. Dynamic properties vary with the magnitude and direction of ink feedrate change, if the optical density is used as the control variable. Differences between the two controlling methods result from the non-linear relationship between optical density and ink film thickness. It was also found that ink feed mechanism influences significantly the dynamic behavior of a press. The press time constant was found to increase with decreasing image coverage for all of the inkers except the positive-feed keyless inker, of which it is independent of image coverage. The press equipped with a continuous-feed open fountain overall responds fastest to a step change of ink feedrate, while the press with a digital injector exhibits the slowest response. Keyless inkers tend to respond faster than conventional, keyed inkers for very light formes. A T-bar/triangle/stripe test form was specifically designed for appraising print quality attributes such as ghosting, starvation, and ink film thickness uniformity. A ditch and ridge pattern is typical of the T-bar image produced by conventional, keyed inkers, but the ridge vanishes when printed by keyless inkers. Ink feed mechanism was found to have a negligible effect on the magnitude of ghost, and the positive feed keyless inker exhibited a lower starvation than the other inkers. The results from the stripe test target demonstrate that ghosting is indeed augmented if a ghost image from one form roller overlaps with that from another. The ink film thickness is the least uniform with the ductor-feed open fountain inker, apparently due to its intermittent feed of ink to the roller train. The ink film thickness of an image produced by the anilox keyless inker is much more strongly dependent upon the image coverage of the plate than the other inkers, because of a poor inker design. The simulation results and the effect of ink feed mechanism on press performance will be presented. Selection of a proper inker for a specific print job will also be discussed.