# The Impact of Underlying Opaque White Coating Parameters on Flexographic Print Quality
## 1 Introduction
This is especially important for printing with an opaque white color, which is usually printed first in sequence, although this is not always the rule. A print on which the ink does not adhere well to the substrate has a mottled appearance, that is, the ink film is not uniform [16]. Pinholes formation is mainly caused by bubbles in the white ink liquid when the roller supplement ink [17]. The abrasiveness of the substrate enables better ink adherence and improves ink wetting, and the printing is carried out without any issues [18]. The hardness of the polymer plate in combination with the selection of the mounting adhesive tape has a significant influence on the adherence of the ink and, thus, on the opacity of the opaque white ink [19]. This choice can affect the level of \"mottle\" effect and the appearance of pinholes. When printing full-tone (solid) colors, a less hard polymer plate is usually used in combination with a harder mounting tape. This enables a higher efficiency of ink transfer onto the substrate. The soft polymer plate tends to distribute the ink evenly over its surface, and the hard mounting tape firmly presses the polymer plate against the printing substrate, filling the voids on the surface and reducing the appearance of the \"pinholing\" effect. Furthermore, the smooth surface of the polymer cannot maintain a uniform ink layer thickness due to the surface energy of the plate. The abrasive surface of the polymer plate leads to an increase in white ink opacity; the abrasiveness can be achieved in several ways. The \"old school\" approach is based on solid colors rasterization using values in the 95-97% coverage range, and more recently, advanced screening technologies (Kodak HyperFlex, Esko Microcell, Mac Dermid LUX MicroCell). The mentioned technologies solve this by creating a texture on the surface of the plate that is extremely suitable for creating a uniform ink layer, and the texture itself is invisible on the print. Furthermore, increasing the opacity of the opaque white ink can be achieved by using highly pigmented opaque inks [20]. Such inks are characterized by higher viscosity (particle size of the white titanium dioxide pigment is in a range of 200 to 400 nm), which affects the printing speed. However, with the development of new ink formulas, the viscosity of ink, including white ink, decreased. The new formulas are based on smaller pigment sizes and ink additives that improve printing properties. Although a white underlayer is often required to achieve a high color saturation in attention-grabbing graphics, the cost of adding white ink can be significant, especially when the coverage of the graphic is large [21]. Therefore, the most important thing is to set the optimal acceptable opacity of the opaque white ink that can be easily managed while printing. Any increase in opacity usually requires an increase in the volume of the anilox roller, which ultimately results in increased ink consumption as well as an increase in total costs. Generally, the 50% white ink opacity is acceptable in flexo-printing processes. Considering that the white ink makes for 40-50% of total ink cost, its optimization is very important, providing there is an acceptable level of printing quality. It is, therefore, important to analyze the extent to which the increase in the white ink opacity pays off and how it influences the improvement of the visual effect on a product. Savings in white ink expenses can significantly decrease the production costs. Furthermore, the question arises whether an ordinary observer could even visually detect the difference in color printed over white with higher ink opacity. The primary objective of the research is the optimization of the production process, which includes the relationship between the white ink opacity and the print quality, and can significantly affect the consumption of white ink and, thereby, reduce total production costs.