# Capabilities and Limitations of Using Desktop 3-D Printers in the Laser Sintering Process
## 3 Results and Discussion
### Computed Tomography  
Geometry deviations in the form of a colour map for the manufactured benchmark samples are shown in Figure 8. As can be seen, the highest accuracy is provided by the industrial machine, which is illustrated by the even distribution of deviations. Moreover, the results obtained for the desktop machines differ from each other. The samples produced using the Lisa machine, which had greater mechanical properties, have a significantly lower accuracy than those manufactured using S1. Additionally, deviations of the surface are cumulated in certain areas of the model, which may indicate the greater impact of shrinkage.  
A quantitative comparison of benchmark geometry is shown in the form of deviation distribution (Figure 9a) and the cumulative deviation histogram (Figure 9b) showing the percentage of the actual surface at a given distance from the nominal surface.  
The largest absolute deviations of 0.416 mm were recorded for PA12 Smooth (Lisa). The smallest absolute deviations of 0.143 mm were obtained for the PA2200 sample (Formiga P110). The summary of absolute, positive and negative deviations for 90% of the compared surfaces is presented in Table 7.  
Figure 8: Graphic representation of surface deviation as a result of actual/nominal analysis of (**a**) PA2200, (**b**) PA12 Smooth, (**c**) PA12 Black.  
Figure 9: Actual/nominal analysis: (**a**) histograms of geometry deviations, (**b**) cumulative deviation histogram.  
The porosity occurring in the LS PA12 samples depends on many factors: process parameters such as the energy density supplied by the laser [34], insufficient packing and drying of the powder, low viscosity, coalescence, atmospheric gas trapped between particles and layers, and the off-gassing of degradation products [35].  
The porosity of the samples produced using different devices can therefore vary significantly. The porosity analysis was registered for the region of interest located in the middle of each sample (Figure 10a). Thus, CT reconstruction carried out in this way enabled the detection of pores with diameters larger than 15.1 \(\upmu\)m. Due to the complex structure of pores connected to the surface in the tested samples (Figure 10b), a porosity analysis was performed by taking into account closed and open pores (Figure 10c). This allows for more accurate results of samples produced using different devices when compared to the analysis of only closed porosity that was presented in the literature (56).  
The lowest porosity value was recorded for the samples made from PA 12 Smooth powder. The porosity, in this case, did not exceed 1.9% for the samples produced in all three orientations (Figure 11a). For PA2200, the porosity for all build orientations ranged from 4.6 to 6.1%.  
The highest porosity, exceeding 23%, was recorded for the PA12 Black XZ samples. The largest differences in porosity were observed for this series due to the build direction. For the PA12 Black ZX sample, the porosity was at the level of 6.1%. A comparison of the shape of pores (sphericity) and diameters for the analysed series is presented in Figure 10b-d. Open pores with diameters larger than 5 mm occur in PA12 Black XY and XZ samples. These pores have the most extensive structure (low sphericity) and occupy a significant part of the analysed volumes (Figure 10c). The differences between the shape of the pores occurring in samples built in the ZX plane are shown in Figure 12.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Cumulated Variance Distribution** & **PA2200** & **PA12 Smooth** & **PA12 Black** \\
**of 90\% of the Surface** & **[mm]** & **[mm]** & **[mm]** \\ \hline Absolute & 0.143 & 0.416 & 0.217 \\ Negative & 0.121 & 0.395 & 0.160 \\ Positive & 0.148 & 0.450 & 0.240 \\ \hline \hline \end{tabular}
\end{table}
Table 7: Actual/Nominal Analysis Results of Benchmark’s Computed Tomography (CT) Scans.  
Figure 10: Porosity analysis: (**a**) sample PA12 Black XZ with analysed region of interest (ROI), (**b**) cross-sections in XY and XZ plane, (**c**) 3D view of porosity.