# The Impact of Underlying Opaque White Coating Parameters on Flexographic Print Quality
## 3 Materials and Methods
### Evaluation  
The main goal of the research was to determine the print quality under the influence of different opaque white ink layer thickness. All measurements were performed on the printed PET film before the varnish printing and lamination stages. The measurement of opacity in the white ink layer (Figure 3) and color ink layer (Figure 4) on transparent (TRN) film was performed using a spherical spectrometer Xrite Ci64 (serial number 16,807), and was calculated based on CIE L*a*b* color space values. Technical specification of the spectrometer and measurement conditions: Measurement Geometrics d/8\({}^{\circ}\) (diffuse 8), Specular excluded, 4 mm measurement area/6.5 mm target window, Illuminant Type D50, and Standard Observer 2\({}^{\circ}\). Diffuse 8 is a principle of geometry where the light source first hits the walls of a highly reflective coated sphere of the measurement device and this diffuse light illuminated the sample. The measurement was detected at 8\({}^{\circ}\) of the sample plane. According to ISO 6504-3, each measurement of samples requires three readings (over-black, over-white, and a measurement of the white backing) which results sample opacity percentage values. The opacity of the color sample is then measured as the difference of a measurement with black background and a measurement with white background taking into account the white backing [32]. The device reads the opacity values in the sample in the form of a percentage value between 0 (completely transparent) and 100 (completely opaque) [33], which follows human visual perception. One layer of uncoated smooth surface black paper board, with a grammage of 240 g/m\({}^{2}\) and thickness of 271 \(\mu\)m, was used as a black background during the measurement (L* = 24.9; a* = 1.5; b* = -0.8). Four-layer GMG Proof paper premium Semimatte, with a grammage of 250 g/m\({}^{2}\), was used as the white background (L* = 96.5; a* = -0.8; b* = -1.2). \begin{table}
\begin{tabular}{l l} \hline \hline \multicolumn{3}{c}{**Printing Specifications**} \\ \hline Flexo printing machine & Uteco ONYX 808 CI Flexo Printing Press \\ Printing speed & 200 m/min \\ Process/Spot flexo inks & HUBER Gecko Bond Top, Solvent based inks \\ White flexo ink & HUBER Gecko Xtreme White, Solvent based ink \\ Matt varnish & HUBER Gecko Frontal Uni \\ Printing width max. & 1200 mm \\ Plate mounting tape & 3M E315, 0.38 mm thickness \\ Print run & 75,000 labels \\ \hline \multicolumn{3}{c}{**Anilox Roller Specifications**} \\ \hline Process color (CMYK) & 470 c/cm (1200 cpi); 3.7 cm\({}^{3}\)/m\({}^{2}\) \\ Spot red P 485 C color & 160 c/cm (400 cpi); 13.0 cm\({}^{3}\)/m\({}^{2}\) \\ Printing Viscosity–Process/Spot ink & 20–25 s DIN 4 \\ White ink–Stage I & 200 c/cm (500 cpi); 10.0 cm\({}^{3}\)/m\({}^{2}\) \\ White ink–Stage II & 160 c/cm (400 cpi); 13.0 cm\({}^{3}\)/m\({}^{2}\) \\ White ink–Stage III & 120 c/cm (300 cpi); 16.0 cm\({}^{3}\)/m\({}^{2}\) \\ Printing Viscosity–White ink & 18–22 s DIN 4 \\ \hline \multicolumn{3}{c}{**Printing Substrate Specifications**} \\ \hline Transparent film &
\begin{tabular}{l} BOPET Film Roll Polyester Film, Polyethylene \\ Terephthalate; thickness: 20 \(\mu\)m \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._Opacity (Equation (1)) is defined in terms of the contrast ratio, the ratio of the relative luminous reflectance of a coating placed over a black and a white substrate [34]:  
\[O(\%)=\frac{R_{b}}{R_{w}}\times 100 \tag{1}\]  
where:  
\(O\)--Opacity;  
\(R_{b}\)--Relative reflectance, black backing;  
\(R_{w}\)--Relative reflectance, white backing.