## 4 Experimental Results and Discussion  
The results of the Cross-Cut and Tape test (DIN ISO 2409) are given in Table 4. The change in free energy of the surface was measured with Plasmatreat test inks. Contact angles were simulated using the COMSOL Multiphysics simulation program. To better understand the differences in CrossCut testing with a PET sample between the classification values GT2 and GT4, Figure 5 is shown. The purpose of the test is to activate the surfaces of different materials to allow adhesion using UV-ink printing with an inkjet printer. The Cross-Cut test shows which \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Test No.** & **Sample No.** & **SE (mN/m)** & \begin{tabular}{c} **Contact Angle** \\ **(\({}^{\circ}\))** \\ \end{tabular} & \begin{tabular}{c} **Cross-Cut Test** \\ **Direct after Printing** \\ \end{tabular} & \
\begin{tabular}{c} **Cross-Cut Test** \\ **after 24 h.** \\ \end{tabular} &
\begin{tabular}{c} **Samples** \\ **Treatment Method** \\ \end{tabular} \\ \hline \multirow{4}{*}{1} & 1 & 32 & 82.3 & GT1 & GT4 & - \\  & 2 & 28 & 83.8 & GT3 & GT4 & - \\  & 3 & 32 & 78.1 & GT4 & GT4 & - \\  & 4 & 38 & 79.7 & GT3 & GT4 & - \\ \hline \multirow{4}{*}{2} & 1 & 32 & - & GT1 & GT4 & Cleaned with IPA \\  & 2 & 28 & - & GT2 & GT4 & Cleaned with IPA \\  & 3 & 32 & - & GT2 & GT4 & Cleaned with IPA \\  & 4 & 38 & - & GT2 & GT4 & Cleaned with IPA \\ \hline \multirow{4}{*}{3} & 1 & 70 & 52.4 & GT0 & GT0 & Plasma treated \\  & 2 & 58 & 55.3 & GT0 & GT0 & Plasma treated \\ \cline{1-1}  & 3 & 72 & 48.7 & GT0 & GT0 & Plasma treated \\ \cline{1-1}  & 4 & 72 & 49.1 & GT0 & GT0 & Plasma treated \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Experiment results. Figure 4: Cross-Cut tool illustration. Figure 5: Cross-Cut testing of PET material. Comparison of IPA cleaned PET polymer (**a**) Cross-Cut test direct after printing (GT2); (**b**) Cross-Cut test 24 h. after printing (GT4); (**c**) Cross-Cut dimensions. material can achieve stable adhesion with a classification value GT0. Excellent adhesion can result when samples are treated with open-air plasma and printed with UV ink, a process that gives all samples a long stable GT0 value. Using COMSOL, the shape of the inkjet droplet (Figure 6) is obtained on non-treated and treated PET material surfaces. Using COMSOL (applying Equations (8) and (9)), the change in droplet velocity at impact on the non-treated and treated surface is illustrated in Figure 7. Here, droplet oscillations were observed during impact and the obtained velocity were higher on the surface of the non-treated poly (ethylene terephthalate). It can be noted that different surfaces play a role in achieving different droplet velocities during impact. At the end of the contact process, when the velocity approaches zero, the droplet takes on its final shape on the surface. It is important to mention that, depending on various surface parameters, droplets on the surface take on different shapes. The droplet diameter can be twice as large and cover four times the area if the treated poly (ethylene terephthalate) surface is considered. During the impact, the droplet velocity changes and decreases to zero. Before reaching a stable phase, several processes of oscillation damping occur on the surface, and their behaviour depends on the properties of the surface. It should be noted that the behaviour of droplet oscillations upon impact depends not only on the characteristics of the surface, but also on the properties of the fluid. The results show that on the non-treated surface of polyethylene terephthalate the level (velocity) of droplet oscillations is twice as high as on the treated surface. In addition, the droplet on the treated surface takes on its final shape faster and is more stable during the printing process. Figure 6: Droplet shape on non-treated (a) and treated (b) poly (ethylene terephthalate) surfaces. Figure 7: The change in inkjet droplet velocity on non-treated and treated poly (ethylene terephthalate) surfaces.