## 3 Methodology
### Delphi Research Method and Design  
This study established the Delphi questionnaire aspect content. The interview subject team comprised three categories of professionals: (1) scholars of chemical plastic production, (2) senior engineers developing special machines, and (3) technical executives of production manufacturing. Discussions were made according to their expertise, and the Delphi research questionnaire with the four aspects of specific key quality factors and 49 subitems was formulated. According to the expert interview team's opinion, the Delphi research questionnaire had 13 subitems in dimension 1, \"key factors in PP extrusion sheet production technique\" [47], as shown in Table 2. \begin{table}
\begin{tabular}{l} \hline \hline
1-1 The ratio of PP virgin pellets and reclaimed pellets \\ \hline
1-2 PP extruder main screw output pressure control \\ \hline
1-3 PP extruder polymer temperature control \\ \hline
1-4 T-die lips adjustment depends on sheet thickness \\ \hline
1-5 Cooling rolls pressing stability \\ \hline
1-6 Cooling rolls flatness \\ \hline
1-7 Cooling rolls temperature stability \\ \hline
1-8 T-die output and cooling rolls pressing position \\ \hline
1-9 Processing according to extrusion and raw material characteristics \\ \hline
1-10 PP extruder output (experts complement) \\ \hline
1-11 PP sheet thickness (experts complement) \\ \hline
1-12 PP sheet width (experts complement) \\ \hline
1-13 PP extruder main screw geometric design (experts complement) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Key factors of polypropylene extrusion sheet production (13 subitems). Figure 6: Steps to prepare the questionnaire by the Delphi technique. According to the expert interview team's opinion, there were 12 subitems in dimension 2, \"extrusion sheet manufacturing machinery equipment design\", of the Delphi research questionnaire [48], as shown in Table 3.  
According to the expert interview team's opinion, there were 12 subitems in dimension 3, \"key factors in PP molding [49] and mold manufacturing [50]\", of the Delphi research questionnaire, as shown in Table 4. \begin{table}
\begin{tabular}{l} \hline \hline \multicolumn{2}{l}{3-1 Material of vacuum aluminum alloy forming mold} \\ \hline \multicolumn{2}{l}{3-2 Requirements for mold vacuum holes and pressure holes} \\ \hline \multicolumn{2}{l}{3-3 Feeding stability of forming heater} \\ \hline \multicolumn{2}{l}{3-4 Core/cavity and product modeling design} \\ \hline \multicolumn{2}{l}{3-5 Forming heating controller element stability} \\ \hline \multicolumn{2}{l}{3-6 The deviation of forming heating constant temperature control area} \\ \hline \multicolumn{2}{l}{3-7 Near scenic sport or night markets} \\ \hline \multicolumn{2}{l}{3-8 Collective residential community, enterprise office building, or factory concentration area} \\ \hline \multicolumn{2}{l}{3-9 Mold cooling mode} \\ \hline \multicolumn{2}{l}{3-10 Mold clamping to prevent leakage} \\ \hline \multicolumn{2}{l}{3-11 Product geometric design} \\ \hline \multicolumn{2}{l}{3-12 Product and mold cutter shrinkage} \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Key factors of polypropylene forming and mold manufacturing (12 subitems).