# Injection Molding of Biodegradable Deciduous Teeth Dental Post
## 3 Results and Discussion  
### Characterization of PLA/PCL/ACP Material  
#### 3.1.1 Thermal and Rheological Properties  
The DSC scanning curve of PLA/PCL/ACP is shown in Figure 4. It reveals two melting temperatures: one at around 65 \({}^{\circ}\)C, which is the melting point of PCL, and the other at 172.58 \({}^{\circ}\)C, which is that of PLA. The processing temperature should be higher than 172.58 \({}^{\circ}\)C to process the composite successfully. Thus, 180 \({}^{\circ}\)C was selected as the starting temperature for shear viscosity measurements, with tests performed in 10 \({}^{\circ}\)C increments. When the temperature exceeds 200 \({}^{\circ}\)C, the material flow becomes discontinuous after passing through the rheometer nozzle. Therefore, experimental data at 180 \({}^{\circ}\)C, 190 \({}^{\circ}\)C,  
Figure 3: Formazan crystal formation in a 96-well plate after 24 h incubation with PLA/PCL/ACP composite extract, indicating cell viability based on colorimetric change.  
and 200 \({}^{\circ}\)C were used to calibrate a temperature-dependent rheological model based on the Modified Cross formulation. This model, implemented in Moldex3D, is expressed in Equations (1)-(4):  
\[\eta=\eta_{0}/\left[1+\left(\eta_{0}\dot{\gamma}/\tau*\right)(1-n)\right] \tag{1}\]  
\[\eta_{0}=\mathrm{D}_{1}\times\exp\left[-\mathrm{A}_{1}\left(\mathrm{T}- \mathrm{T}_{\mathrm{c}}\right)/(\mathrm{\AA}_{2}+\left(\mathrm{T}-\mathrm{T}_ {\mathrm{c}}\right))\right] \tag{2}\]  
\[\mathrm{T}_{\mathrm{c}}=\mathrm{D}_{2}+\mathrm{D}_{3}\;P \tag{3}\]  
\[\mathrm{A}_{2}=\mathrm{\AA}_{2}+\mathrm{D}_{3}\;P \tag{4}\]  
where \(\eta\) is the viscosity, \(\dot{\gamma}\) is the shear rate, \(\mathrm{T}\) is the absolute melt temperature, and \(p\) is the absolute melt pressure. \(\eta_{0}\) denotes the zero-shear viscosity, which varies with temperature according to a Williams-Landel-Ferry (WLF)-type relation. The model includes material parameters such as the relaxation stress \(\tau^{*}\), consistency coefficient \(\mathrm{D}_{1}\), and temperature sensitivity constants \(\mathrm{A}_{1}\), \(\mathrm{A}_{2}\), and \(\mathrm{T}_{\mathrm{c}}\), as defined in Table 1.  
From the viscosity model, the melt viscosity at different shear rates can be calculated. Figure 5 shows the shear viscosity diagram between 180 \({}^{\circ}\)C and 200 \({}^{\circ}\)C. In the follow-up of this study, the rheological model and dental post mold design were input into the Moldex3D mold flow analysis software to determine the molding parameter levels before actual injection molding experiments.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Symbol** & **Value** & **Unit** \\ \hline \(n\) & 0.3846 & \\ \hline \(\tau^{*}\) & 1.29 \(\times\) 10\({}^{6}\) & dyne/cm\({}^{2}\) \\ \hline D1 & 2.048 \(\times\) 10\({}^{8}\) & (g/cm\(\cdot\)s) \\ \hline D2 & 373.15 & K \\ \hline D3 & 0 & cm\({}^{2}\) K/dyne \\ \hline A1 & 16.71 & \\ \hline A & 51.6 & K \\ \hline \hline \end{tabular}
\end{table}
Table 1: Rheological parameters used in the Modified Cross model for PLA/PCL/ACP composites. These values were applied to simulate shear viscosity behavior under various processing conditions.  
Figure 4: DSC curve of the PLA/PCL/ACP composite showing a melting peak at 172.6 \({}^{\circ}\)C. This thermal transition was used to guide melt temperature settings for micromolding and rheological testing.