# Recycled PP for 3D Printing: Material and Processing Optimization through Design of Experiment
## 3 Results
### Preliminary Characterization of PPt/r-PP Blends: Optimization of the Material Formulation
as in this case a more prominent yield stress behavior can be observed. To qualitatively evaluate the yield stress behavior of the formulated blends, the complex viscosity curves were fitted using the models that were reported in Equations (1)-(6). More specifically, the experimental complex viscosity curves, that were collected at both 230 and 260 \({}^{\circ}\)C, were fitted with the aforementioned Equations and the model showing the best fit of the experimental data (evaluated through the minimization of the cost function) was selected for each sample for the assessment of the yield stress. The obtained values of yield stress, listed in Table 3, confirmed that higher yield stresses were achieved when the rheological behavior of the samples was assessed at 260 \({}^{\circ}\)C. Besides, among the studied materials, blends containing 30 and 40 wt.% of r-PP showed the highest values of yield stress values at 260 \({}^{\circ}\)C.  
Aiming at assessing the thermal properties of the formulated blends, significantly affecting the quality of the printed part in terms of shrinkage, warpage, and distortion, their melting enthalpy was evaluated, and the obtained values are reported in Table 3. Similarly to what was observed for the rheological behavior, also the melting enthalpy of the blends show intermediate values between those of the starting materials, indicating that the introduction of r-PP is beneficial in minimizing the total melting enthalpy of the samples. Therefore, considering the results of the rheological and thermal characterizations of the blends, 30r-PP was selected for the production of the filament and for the FFF printing, since the presence of 30 wt.% of r-PP allowed maximizing the yield stress of the materials, while minimizing their total melting enthalpy. Taguchi Design of Experiments for the Selection of the Extrusion Process Parameters: Optimization of the Material Compounding  
Before proceeding with the production of the filament and with the 3D printing of the selected materials, the processing conditions of 30r-PP were optimized using the Taguchi method. In particular, the selected blend was processed through melt compounding in a twin-screw extruder, varying the process parameters according to the Taguchi orthogonal array that was reported in Table 2. Since the aim of this step is the obtainment of a Ppt/r-PP blend with an optimal rheological behavior for FFF printing, the yield stress value (which represents the crucial parameter dictating the FFF processability) was selected as a quality factor for the optimization of the processing conditions. Figure 3 reports the main effect plots for yield stress for the 30r-PP system that were evaluated at 260 \({}^{\circ}\)C. In such kinds of representation, the main effect of each considered parameter on the yield stress can be evaluated by observing the slope of the line which links the experimental data. In particular, if the slope of the line is zero, the considered factors and levels have a similar effect on the yield stress; at variance, when the slope is different from zero, the factors and levels strongly affect the selected quality factor. According to the trends that were reported in  
Figure 2: Complex viscosity curves measured at 230 (**a**) and 260 \({}^{\circ}\)C (**b**) for all the investigated materials. Figure 3, the yield stress value (and, hence, the material rheological behavior) appears strongly affected by the screw speed, while the effect of the other considered parameters is quite negligible. More specifically, the higher the screw rotation speed the higher the yield stress, likely due to the beneficial effect of the shear that is experienced by the melt during the compounding at high rotation speeds on the morphology and, hence, on the rheological response of the blend. The parameters that were thus optimized to maximize the yield stress at 260 \({}^{\circ}\)C were: temperature profile centered at 190 \({}^{\circ}\)C, screw speed of 400 rpm, and a flow rate of 700 g/h. With these parameters, a 30r-PP formulation was produced.