# The Impact of Underlying Opaque White Coating Parameters on Flexographic Print Quality
## 4 Results and Discussion
### Print Quality Analysis of White Ink  
The print quality of white ink throughout the three stages of the experiment, apart from measuring opacity (Section 4.1), was evaluated based on the uniformity of ink layer on the printouts and the appearance of pinholes [40]. Microscopic images captured on printouts were analyzed through ImageJ software (tool: Interactive 3D Surface Plot) that transformed density into proportional height to see just how thick the ink film was [41]. During the transformation process, the \"Invert\" setting was selected, so that the lightest elements, which represent the thickest ink film, would be displayed as relatively high.  
The projection of ink density in the three stages of the experiment clearly showed greater uniformity of the surface as the thickness of the ink film increased, which is visible in Figure 10 starting from left to right. Also, the three samples differed significantly regarding the number and size of pinholes on the print. The far right sample had a significantly smoother surface structure and no visible pinholes, as in the previous two samples. Various factors can cause the appearance of pinholes, and one of the most important ones is the ink film thickness.  
Using ImageJ software (tool: Analyze Particles), the area and the number of pinholes that appeared on the solid color of white were measured. When evaluating the area and the number of pinholes, the color threshold method of image processing was used. Thresholding is a method where the image converts from color or grayscale mode into a binary image mode. The red mask that was thus created in the image was used to measure the area of the pinholes. The following values of color threshold in HSB color space were used: hue 0-255; saturation 80-255; brightness 0-140.  
Figure 10: Topographic view of white ink film thickness at different printing stages.  
Figure 9: Color difference at two last stages of the experiment compared to the initial state.  
Measurement results of area and the number of pinholes at three stages of printing are shown in Table 5. The results were grouped according to pinhole sizes, and the holes with a surface area of less than 100 \(\upmu\)m\({}^{2}\) were not taken into account, since, due to their extremely small size, they do not significantly affect the print quality. In order to compare, that would be the area of one laser dot (10 \(\upmu\)m dot size at resolution of 2540 dpi) and it is also extremely difficult to reproduce it on the printing plate.  
The pinholes usually do not have a regular shape, but for the sake of better understanding the size of these pinholes and in order to compare them, further analysis used more appropriate values. Pinholes over 400 \(\upmu\)m\({}^{2}\) (20 \(\upmu\)m dot size), and especially those over 900 (30 \(\upmu\)m dot size), had a significant impact on print quality. Based on the results, the print obtained with an anilox roller volume of 13 cm\({}^{3}\)/m\({}^{2}\) met the criteria of absence of large pinholes on the impression. However, this does not mean that this choice of anilox roller is acceptable. The opacity of the white ink should also be taken into account for the final selection.