A study of producing smoother gradients in the flexographic process on oriented polypropylene with UV ink by varying screening techniques, gradient lengths and the surrounding

Printers find that producing smooth gradients in the highlight area is a great challenge for flexographic printing. Screening technology vendors claim that hybrid screening technologies produce smoother gradients and enhance reproducible dots in the highlight areas. This study was designed to investigate if hybrid screening technologies can achieve better gradient results than other screening technologies—conventional screening and FM screening—with the flexographic process. A single test form was printed on oriented polypropylene with UV ink, as these are common materials used in flexible packaging. The first objective was to see how different pressure settings impact tone reproduction of each screening technique. There were three pressure settings—kiss impression, moderate pressure and high pressure. Tone reproduction curves of all three screening techniques were evaluated to see the change due to the different pressure settings. The results show that FM screening had a high sensitivity to change in pressure, while AM and hybrid screenings were more forgiving to variations in pressure settings. In the highlight areas, hybrid screening is the least sensitive to changes in pressure. The second objective was to study whether smoother gradients can be produced by altering three variables: screening techniques, gradient lengths and the impact of the surrounding. These variables were used to create a gradient matrix. Printed sheets from different points in the press run were collected for data analysis. There were two types of data analysis, measurement based evaluation and visual evaluation. Because of difficulty in the methodology for analyzing the measured data, the conclusions were then based on the results from the visual evaluation. There are three aspects to the problems with gradient smoothness: highlight breaking in AM screening, graininess of FM screening, and a disjunction at the transition point of hybrid screening. When minimum dot size, transition point, and transfer curve are set correctly, hybrid screening would be the best selection to use with the flexographic process. The surrounding, or solid frames around the gradients, did not truly enhance gradient smoothness at kiss impression