Optical Topographical Characterisation for Flexographic Printability

Sammanfattning: The flexographic printability is largely dependent on the topographical characteristics of the substrate. OptiTopo is a recently developed optical imaging technique, which provides precise and fast topographic scans of both printed and unprinted paper surfaces. The potential and limitations of the technique have been evaluated by comparing OptiTopo scans from a wide variety of commercial paper grades with those of other imaging techniques. The optical requirements on the substrate for an accurate and precise topographic scan were determined. Detail rendering was substantially improved using a custom-designed restoring filter, and a new enhanced spatial resolution range was established.



Serious deficiencies affecting the quality of flexographic prints may appear in the form of local unprinted areas in a full-tone print, generally caused by incomplete ink transfer. An OptiTopo instrument with an integrated algorithm for the detection of uncovered areas was developed and tested on a range of commercial board samples printed on a laboratory flexographic press. A frequency distribution of uncovered areas, indicating how large a percentage of the surface area with a certain topographical elevation was uncovered, was calculated for the different prints. The topographic characteristics of the uncovered areas clearly indicate that surface depressions are the primary cause of uncovered areas in flexographic printing. Different uncovered area predictability models based on spectral analysis and topographic thresholding are proposed