## 3 Results and Discussion
### Taguchi Method  
There were nine experiments used for each material type, each experiment was repeated three times to increase the reliability with four factors and three levels selected from the previous stage shown in Table 2. The PA and PP filaments will be welded on an UPWM and tested for tensile strength using a tensile test machine (TTM). Experimental results will be optimized using the Taguchi method with the larger-is-better formula (Equation (3)) [26]. \[\
icefrac{{s}}{{N}}=-10 ln\left[\frac{1}{x}\sum_{i=1}^{x}\left({n_{i}}^{-2} \right)\right] \tag{3}\]  
where \(x\) is the number of observations and \(n\) is the observed data. Table 3 shows the tensile test results of knots when the Taguchi method was applied using the larger-the-better formula. The standard deviation of the PP knot ranged from 7.97 to 19.56; the durability of knots was greater than 60 Mpa similar to that of the PA knot. However, the difference in the durability of the PA knot was relatively small, at only 1.16, especially in experiment number 6 (A2/P3/T1/S2). Figure 8 shows the control factors affecting the signal-to-noise ratio of PA and PP materials [27]. In particular, for the PA knot with a small difference (ranging from 36.97 to 38.01), the amplitude of the curve tended to decrease from 38.01 to 37.12, while the weld pressure factor peaked at 2 (37.98). At the same time, control factors like hold time and speed troughed at level 2 (37.08 and 37.42). Similarly, for PP knots, the difference rangedfrom 37.37 to 38.71, the curve tended to increase from 37.54 to 38.51 for the amplitude factor, while the weld pressure factor peaked at 2 (37.98). At the same time, control factors like hold time and speed troughed at level 2 (37.08 and 37.42). Figure 8: Process control factors affecting signal-to-noise (S/N) ratios (**a**) polyamide; (**b**) polypropylene. Figure 7: The result of the experiment with the levels of the variables of ultrasonic plastic welding machine for polypropylene material. (**a**) amplitude; (**b**) weld pressure; (**c**) hold time; (**d**) speed. Table 4 and Figure 9 show the percentage influence of control factors on the knots' durability and their ranks. In particular, for PA knots, the most influential factor was welding pressure at 41%, followed by amplitude at 34%, hold time at 24%, while the speed factor had a relatively small influence at 1%. For PP knots, the influence of factors was relatively uniform; the most influential factor being speed at 37%, followed by amplitude and weld pressure at 25% and 22%, respectively, and the last influencing factor was the hold time at 16%. The influence levels of the factors of the two materials were somewhat different; this may be because of the difference in the structure of the fibers (single for PA and group for PP) [28,29]. Specifically, the influence of the speed coefficient of PP material is higher than that of PA material. Because the thickness of the PP weld is relatively small, it affects the strength of the weld (Figure 7d). So, this factor becomes more important for PP material than for PA material. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Material** & \multicolumn{3}{c}{**Polyamide (PA)**} & \multicolumn{3}{c}{**Polypropylene (PP)**} \\ \hline
**Level** & **A** & **P** & **T** & **S** & **A** & **P** & **T** & **S** \\ \hline
1 & 38.01 & 36.97 & 37.89 & 37.68 & 38.38 & 38.54 & 37.54 & 38.27 \\ \hline
2 & 37.48 & 37.98 & 37.08 & 37.42 & 37.52 & 37.56 & 38.29 & 38.71 \\ \hline
3 & 37.12 & 37.66 & 37.64 & 37.52 & 38.45 & 38.24 & 38.51 & 37.37 \\ \hline Delta & 0.89 & 1.00 & 0.81 & 0.26 & 0.92 & 0.98 & 0.98 & 1.34 \\ \hline Rank & 2 & 1 & 3 & 4 & 4 & 2 & 3 & 1 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Response table for signal-to-noise (S/N) ratios.