## 3 Results and Discussion  
### Polyamide  
Figure 6 shows the interval plots for the thickness and strength of polyamide welds at levels of control factors. Considering the amplitude factor (Figure 6a), the histogram shows amplitude levels varying from 24 um to 40 um (60% to 100%) when the other factors were fixed (Table 1). According to the results, the method could not be performed on plastic fibers at 60%, and the value of weld strength is not much different at the remaining amplitude levels. In addition, the thickness of the weld fluctuated in the range of 0.05 mm to 0.07 mm and provided the best results at 5 (80%--32 um) with a weld strength and thickness of 70 Mpa and 0.065 mm, respectively. Considering the weld pressure factor (Figure 6b), the welding pressure levels varied from 2.0 kg/cm\({}^{2}\) to 6.0 kg/cm\({}^{2}\). When the welding pressure increased, the thickness of the weld tended to decrease, which partly reduced the durability of weld. For this factor, when the weld strength value did not change much, the thickness of the weld was considered at level 2 (2.5 kg/cm\({}^{2}\)) for the most optimal value. In terms of the hold time factor (Figure 6c), the histogram in this figure shows varying hold time levels from 0.1 s to 0.5 s, while the other factors were fixed. In general, changing the hold time did not affect the strength of the weld much. Similar to the previous factor, at the hold time factor, the weld strength value at level 7 (0.4 s) provided the best results but still ensured its thickness. The mean thickness of the knots seems to be lower than the expected (about 0.07 mm) at level 3 (0.2 s). This may be due to the fact that the mechanical properties of the plastic fibers chosen for the test were not uniform with the rest of the test pieces. In order for the experimental results to be objective, this noise signal is still kept. For the speed factor (Figure 6d), the graph shows that the levels of the speed factor varied from 3.0 mm/s to 7.0 mm/s. When the speed increased, the thickness and weld strength tended to increase markedly. However, if the thickness of the weld is  
\begin{table}
\begin{tabular}{c c c c} \hline
**Amplitude (60–100)** & **Weld Pressure** & **Hold Time (0.1–0.5)** & **Speed (3.0–7.0)** \\
**[\%]** & **(2.0–6.0) [kg/cm\({}^{2}\)]** & **[s]** & **[mm/s]** \\ \hline
**80** & 3.0 & 0.3 & 5.0 \\ \hline \end{tabular}
\end{table}
Table 1: Specification use for the ultrasonic plastic welding machine (UPWM).  
Figure 5: The process of the experiment.  
larger, the strength of the weld will decrease (levels 8 and 9). Therefore, the best results were achieved at level 7 (6 mm/s) with an average weld strength of 68 Mpa and a thickness of 0.17 mm. The best three levels of control factors were selected and they are presented in Table 2.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Materials** & **Factors** & **Symbol** & **Units** & **Level 1** & **Level 2** & **Level 3** \\ \hline \multirow{4}{*}{PA} & Amplitude & A & \% & 80 & 85 & 90 \\ \cline{2-7}  & Weld & pressure & P & kg/cm\({}^{2}\) & 2.0 & 2.5 & 3.0 \\ \cline{2-7}  & Hold time & T & s & 0.35 & 0.40 & 0.45 \\ \cline{2-7}  & Speed & S & mm/s & 5.5 & 6.0 & 6.5 \\ \hline \multirow{4}{*}{PP} & Amplitude & A & \% & 80 & 85 & 90 \\ \cline{2-7}  & Weld & pressure & P & kg/cm\({}^{2}\) & 2.0 & 2.5 & 3.0 \\ \cline{1-1} \cline{2-7}  & Hold time & T & s & 0.15 & 0.20 & 0.25 \\ \cline{1-1} \cline{2-7}  & Speed & S & mm/s & 5.5 & 6.0 & 6.5 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Factors and levels used for process.  
Figure 6: The result of the experiment with the levels of the variables of ultrasonic plastic welding machine for polyamide material. (**a**) amplitude; (**b**) weld pressure; (**c**) hold time; (**d**) speed.