# Injection Molding of Biodegradable Deciduous Teeth Dental Post
## 3 Results and Discussion
### Injection Molding Parameter Study and Taguchi Results
#### 3.2.1 Parameter Setting  
A series of molding trials were conducted to determine the optimal processing window for manufacturing the dental post. The selection of melt temperature, injection speed, and holding speed was based on the processing windows presented in Figures 8 and 9, which provide critical data for optimizing molding conditions. Figure 8: Molding window of PLA/PCL/ACP dental posts, illustrating the influence of melt temperature and injection speed on product quality. Representative molding defects—including short shot, sink mark, and flash—are shown at specific parameter combinations. The blue region indicates the optimal range for defect-free production. Figure 9: Molding window of PLA/PCL/ACP dental posts, showing the effect of melt temperature and holding speed on molding quality. Representative defects—including short shot, sink mark, and ineffective packing—are shown at specific conditions, along with the machine limit. The blue region indicates the optimal range for stable molding. Melt Temperature:  
* If the melt temperature is below 180 \({}^{\circ}\)C, poor melt fluidity leads to short shots, as shown in Figure 8.
* If the melt temperature exceeds 200 \({}^{\circ}\)C, sink marks form due to the high specific volume and shrinkage of the material, as illustrated in Figure 9.  
Thus, the optimal melt temperature range was determined to be 180-200 \({}^{\circ}\)C. Injection Speed:  
* The processing window for injection speed ranges from 40 to 190 mm/s. * Speeds below 40 mm/s resulted in short shots due to an insufficient injection rate. * Speeds above 190 mm/s caused flash formation around the post. Prior to molding trials, Moldex3D simulation was performed to analyze injection speed effects. As shown in Figure 10, injection pressure is lowest at 30 mm/s due to shear thinning effects. When the injection speed exceeds 30 mm/s, the required injection pressure increases with injection speed. To select a range with lower injection pressure while maintaining adequate filling efficiency, this study determined that 40-60 mm/s was optimal for the Taguchi experiment. Holding Speed:  
* When the holding speed exceeds 8 mm/s and continues up to the machine's maximum holding speed (33 mm/s), sufficient pressure retention is achieved. * Based on Figure 9, the optimal holding speed range was selected to be 10-30 mm/s. Figures 8 and 9 serve as the basis for defining these three processing windows, ensuring precise control factor selection. For the Taguchi experiment, an orthogonal array was established based on control factor combinations. This study adopted an L9 orthogonal array, selecting three control factors with three levels. The levels were determined using the processing windows shown in Figures 8 and 9, as outlined in Table 3, rather than relying on trial and error. The L9 Taguchi experiment plan is presented in Table 4.  
Figure 10: Moldex3D simulation showing the effect of injection speed on injection pressure for PLA/PCL/ACP dental posts. Pressure is lowest at 30 mm/s due to shear thinning but increases at higher speeds. A range of 40–60 mm/s was selected to balance filling efficiency and pressure for subsequent molding trials. Other fixed parameters during the experiments are listed in Table 5. The injection material prepared in this study consists of a PLA/PCL/ACP blend, with PLA as the primary component (80%). Due to PLA's significantly higher melting point compared to PCL, the blend must be heated beyond PLA's melting temperature to achieve complete melting. During cooling, if the molten material remains inside the mold for over 6 s, the temperature difference within the cavity stabilizes within 10 \({}^{\circ}\)C, as depicted in Figure 11. If the temperature difference exceeds 10 \({}^{\circ}\)C, warpage and deformation may occur due to uneven cooling after demolding.