# Production and Processing of a Spherical Polybutylene Terephthalate Powder for Laser Sintering
## 3 Results and Discussion  
### Extrusion  
The optimal particle size range of polymer powders used for laser sintering lies between 10 and 100 \(\upmu\)m. In order to obtain a size distribution that approaches this ideal range as close as possible, in total 65 trials with different extruder parameters were executed. Amongst the varied parameters were material grade selection, mixing ratio, extruder rotational speed, and nozzle selection. In this paper, only the results of varying rotational speed and nozzle will be shown, to specifically demonstrate the influence of extruder parameters on the particle size. A reduction of mean particle size is expected with higher rotational speeds. The choice of nozzle influences the amount of pressure built up within the extruder, and with this the shear forces the polymer blend is subjected to. Four types of nozzles were used, three of which had a cylindrical geometry. The fourth nozzle was a 2 \(\times\) 100 mm wide slit that caused by far the least amount of back pressure in the extruder. The settings used for selected trials are listed in Table 2.  
The particle sizes measured by dynamic light scattering are summarized in two different ways in Figures 3 and 4a. Figure 3 shows the evolution of the D\({}_{10}\), D\({}_{50}\), and D\({}_{90}\). These numbers represent the diameter of particles at which exactly 10%, 50%, and 90% of total particle volume is smaller than the respective diameter. The D\({}_{50}\) gives the median of the distribution, the D\({}_{10}\) and D\({}_{90}\) give an indication of the distribution width. In Figure 3, the numbers are shown for each trial, i.e. in dependence of extruder rotational speed (rpm), and nozzle selection. The extruder rotational speed has a clear influence on the particle size; smaller particles are generated at higher speeds, as was expected. At 60 rpm speeds, the nozzle selection only has a minor influence on particle size.  
The median value of the particle size (D\({}_{50}\)) lies within the desired range between 10-100 \(\upmu\)m, regardless of process parameters. This observation is supplemented by results shown in figure 4a, which shows the percentage of particles in the total volume that falls within the desired size range. Even though the change in extruder screw speed leads to a clear shift in the distributions as seen in Figure 3, the relative number of particles in the desired range fluctuates only by a few percent.  
Next to particles, also a certain amount of fibrous PBT is generated during extrusion. These fibers are most likely produced due to insufficient mixing time to achieve fiber break-up. Because the system is relatively robust, the choice of definitive process parameters was mainly aimed at minimizing the amount of fibers. Therefore, the amount of PBT fibers relative to the total PBT content was weighed for each trial.  
The generation of fibers strongly depends on the extruder rotation speed, as can be seen in figure 4b. By far the smallest amount of fibrous residue was collected when the wide slit nozzle was used with an extruder rotational speed of 30 rpm. All subsequent powder production trials were conducted with these settings.  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Trial \#** & **Speed [rpm]** & **Nozzle** & **PBT Content [wt. \%]** & **PEG Content [wt. \%]** \\ \hline
1 & 30 & \(\Om=1\) mm; 1 = 20 mm & 40 & 60 \\
2 & 60 & \(\Om=1\) mm; 1 = 20 mm & 40 & 60 \\
3 & 30 & \(\Om=3\) mm; 1 = 20 mm & 40 & 60 \\
4 & 60 & \(\Om=3\) mm; 1 = 20 mm & 40 & 60 \\
5 & 30 & \(\Om=3\) mm; 1 = 10 mm & 40 & 60 \\
6 & 60 & \(\Om=3\) mm; 1 = 10 mm & 40 & 60 \\
7 & 30 & Wide slit nozzle (WSN) & 40 & 60 \\
8 & 60 & Wide slit nozzle (WSN) & 40 & 60 \\ \hline \end{tabular}
\end{table}
Table 2: Extruder settings for selected extrusion trials. PBT: polybutylene terephthalate; PEG: polyethylene glycol.