The influence of total base ink coverage on the uniformity of digital prints overprinted with pearlescent inks

Print uniformity is an important parameter that can determine perceived quality of a printed product. If the product is of low print uniformity, its quality is often regarded as non-satisfactory. In this work we were interested in the uniformity of electrophotography prints overprinted with inks containing pearlescent pigments. Our goal was to determine whether the overall print uniformity was influenced by the total base ink coverage i.e. the ink coverage of the printing substrate before pigments were applied to it. Hence, three scenarios were considered: when pearlescent inks were printed over the unprinted paper, previously printed grey, and black colour patch. Nine different types of pearlescent pigments, dispersed in a transparent ink vehicle, were screen printed over the paper and the previously printed patches of grey and black colour. The base colours were printed in electrophotography. The uniformity of prints obtained in such a manner was determined by calculating GLCM parameters that were shown to correlate well with human perception of uniformity. It was shown that overprinting the electrophotography prints with pearlescent inks significantly decreased the uniformity of prints with black base colour and slightly improved the uniformity of those with grey colour. When pearlescent inks were printed over the paper, the uniformity did not change significantly. Observing the print uniformity of overprinted samples, the best results were obtained when pearlescent inks were printed directly to the paper, following the cases when they were printed over the grey and black prints. It is therefore concluded that the total base ink coverage has significant effect on the uniformity of electrophotography prints overprinted with pearlescent inks. The results indicate poor adhesion of the ink vehicle used as a carrier for pearlescent pigments to electrophotographic toner.