# The Impact of Underlying Opaque White Coating Parameters on Flexographic Print Quality
## 1 Introduction  
Achieving a high level of white ink opacity in printing presents a great challenge for flexographic printing industry today. The white ink printed on transparent substrates due to lack of opacity and nonuniform ink layer reflects less light, resulting in dirty appearance. Multicolor graphics printed on such substrate look blurry, and the colors are less saturated and not faithful. Thus, coverage property (CP) is the most important performance of white ink [1], and the main factor that has the greatest influence on CP is ink dispersion [2]. In order to improve characteristics of white color, inks with a higher viscosity, and anilox rollers with a larger volume, are used and printing with a reduced printing speed or printing with two printing plates is carried out [3]. The question is how much opacity can be achieved with high opaque white ink on transparent materials. It is even more important to determine the optimal value of white ink opacity. In today's flexographic printing environment, opacity above 60% can easily be achieved, as there have been huge advances in all the necessary factors that affect the opacity value, including anilox rollers, inks, doctor blade, mounting adhesive tape, printing plate properties, nip pressure, printing speed, and substrate treatment [4, 5]. The desired level of white ink opacity is determined by the thickness of the layer, which is directly related to the volume of the anilox roller measured in BCM (billions of cubic micrometers per square inch). There are two types of anilox rollers according to their basic purpose, namely printing and coating [6]. Anilox rollers for partial printing have a higher line screen, which means that they have smaller cell openings, as well as a smaller volume. Typical anilox printing rollers have a volume of up to 8 BCM; its final selection dependson the type of work being printed (solids, text, vignettes, or multicolor illustrations) [7]. Anilox rollers intended for coating (complete coloring of the material) have a smaller line screen, meaning relatively thicker cell walls, larger cell openings, and a larger cell volume. Typical anilox coating rollers have a volume of 8 BCM or more. Depending on anilox line screen, as well as the smoothness of its surface, there are differences in the wear and life span of the doctor blade [8]. The role of the anilox roller is to precisely and consistently deliver the ink to the printing plate [9]. The engraving specification of the anilox roller is a key factor in achieving the highest possible opacity of the opaque white ink. The size and frequency of anilox cells control the volume of ink transferred to the printing plate [10]. The thickness of ink layer is directly connected to the speed of printing. At lower printing speeds, around 60 m/min, it is possible to achieve greater ink transfer and over 65% white ink opacity; however, the question of economic benefit can be posed. However, depending on the ink formulation, it may also mean that the increased printing speed will help \"smooth\" or \"wet\" the white ink. The interactions between printing ink and printing substrate are influenced by properties of the printing ink, such as viscosity, surface tension, and wettability [11]. The surface free energy of the film is the basic parameter for good wettability of the substrate and its printing [12; 13]. One of the key parameters that determine the print quality is the adhesion of the ink to the printing substrate [14]. The surface tension of the film significantly affects the white ink lay down and uniformity of the layer [15]. Treating the substrate prior to printing increases its surface tension. However, it has to be higher than the ink surface tension, so that the ink would adhere well to the substrate. Untreated films and foils have a low surface energy level and, as such, are not good for ink adhesion. Once the substrate has been treated, regardless of the type of treatment, any type of ink can adhere to it.