## 4 Discussion  
The trends seen here correlate well with the other literature focused on powdered polymer additive manufacturing, much of which has been based on selective laser sintering. Research on PA12, presented in [22], identified two peaks in the DSC traces, corresponding to the peaks of the raw powder and SEM images showing unmelted or partially melted particles. The results presented in Table 8 indicate a linear increase in the mechanical properties as energy density increases. This is to be expected based on the existing literature relating to polymer powder bed fusion; early research into the laser sintering process [23] demonstrated increases in the tensile properties with increasing energy densities. Further research [24] showed similar effects, also identifying that the strength and modulus approach a maximum as the energy input exceeds the amount needed to fully melt the applied powder. More recent research into the use of polyether block amide in the HSS process [25] demonstrated an increase in the tensile properties with an increasing energy density, in this case induced via increases in the IR lamp power. In the work presented here, it can be seen that, as would be expected, an increase in the sinter speed, which corresponds to a decrease in the energy input, led to a subsequent decrease in the mechanical properties. While not the key focus of this work, it is worth noting that the significance of this effect would be dependent on the specific application of the parts and that the increase in the sinter speed would also lead to a decrease in the build time. For example, assuming the sinter stroke \(\sim\)40% of the total layer time, an increase of 10% in the sinter speed would lead to a decrease in the build time of 9.1% following warm-up. A closer inspection of the results identified a significant variation in the mechanical properties based on a part's position within the build volume, with centrally located parts outperforming those closer to the edge (Figure 10). Whilst variations in the data are exaggerated compared to what would be expected in a commercial setting (due to the fact that the HSS machine used was a prototype version of a now commercialised machine), this is a well-known phenomenon in PBF caused by variations in the part bed temperature, and it highlights the importance of identifying mechanical properties at as many points as possible throughout the build volume in order to predict part performance with confidence. Figure 9: Correlation between the DPM and the UTS, YM, and EaB. As noted in Section 3, the tensile properties showed strong correlations with the DPM. This matches existing LS research [16], whereby increases in strength and stiffness are seen despite the general appreciation that lower crystallinities (higher DPMs) should result in lower part strength and stiffnesses [15]. Whilst both UTS and YM demonstrate an incredibly strong correlation with the DPM (R\({}^{2}\) values of 0.998 and 0.999, respectively), the EaB shows a weaker (although still >0.9) correlation. This is likely due to the brittle nature of the parts manufactured, with small changes in ductility affecting the average results more significantly. It has also been noted in the literature [26] that samples with less favourable microstructure and crystallinity may also contain higher levels of porosity, exacerbating the effects on the mechanical properties and, in particular, the elongation at break. Research into SLS PA12 [27] identified two melting peaks in samples produced at 0.096 J/mm\({}^{3}\) energy density, transitioning to a single peak at 0.270 J/mm\({}^{3}\), with an XRD analysis showing that higher proportions of the \(\alpha\) crystal form at lower LS energy densities, suggesting a less effective melting of the PA12 particles. Whilst the work presented in this paper focuses on PA12, there is reason to believe that this may extend to other materials; for example, the research in [28] identified residual powder melt peaks at low energy densities in PA11 for SLS.