## 4 Discussion
The average relative density of the samples sintered using the current study feedstock was found to be 14% lower than that of the samples processed using commercial feedstock with a volume percentage of porosity of about 20%, while the average volume shrinkage for the samples after sintering was 35% lower than that for samples processed by the commercial feedstock. The differences in shrinkage and density between the samples produced by commercial and current study feedstock could be attributed to the relatively large Ti particle size used in the current study feedstock compared to that of the commercial feedstock, as the use of a smaller particle size results in sintering (and therefore, shrinkage) proceeding more rapidly [35], due to the greater amount of surface energy present [36]. This may also explain the fact that pores in the samples made using the commercial feedstock were much more spherical and more regular in shape (Figure 10). The average linear shrinkage of the parts processed by the commercial feedstock (12.51%) lies in the typical range of linear shrinkage for MIM parts in the literature (14%-18%) [37], whereas for samples produced by the current study feedstock, it was significantly lower (7.3%). Hence, the use of a coarse particle size can be advantageous in increasing the competitiveness of MIM among other manufacturing processes in terms of cost, as well as dimensional tolerances and shrinkage. In terms of pore size, the average pore diameter for samples processed by the current study feedstock was found to be three-times higher than that of the commercial feedstock, and the pores were less regular. It should be noted that samples made here using the commercial feedstock had a slightly lower density (higher porosity) and larger pore size compared to that reported for sintered MIM parts using other commercial feedstocks, e.g., for Catamold(r) Ti BASF (Ludwigshafen, Germany), porosity is about 4% with a pore size of less than 10 \(\upmu\)m [38]. These differences might be attributed to the sintering time and temperature (samples were sintered in the previous study at 1300 \({}^{\circ}\)C for more than three hours), as well as the Ti particle size (reported to be 2-35 \(\upmu\)m in the BASF feedstock [38]), finer than that estimated here for PolyMIM feedstock (12-45 \(\upmu\)m). It should however be noted that, as the solid content for both commercial feedstocks is not known, an exact comparison is not possible. Chemical analysis of the interstitial elements (Table 3) showed that samples made by the commercial feedstock had a carbon content approximately half of that detected in samples made by the current study feedstock, even though the finer Ti powder would be more prone to contamination. This could be attributed to the use of PMMA in the current study binder, which decomposes during thermal debinding, releasing large amounts of carbonaceous gases, which may subsequently react with Ti. Nevertheless, the amount of carbon in samples made using the current study feedstock is lower than that reported for BASF feedstock (0.09%-0.11% [38]). The results of the mechanical tests (Figure 11) showed that samples made by the current study feedstock had a yield stress (\(\sigma_{0,2}\)), 60% lower than that for samples made using commercial feedstock, but a much higher ability to absorb strain. The higher porosity and larger micropores of the samples made by the current study feedstock will lead to reduced strength, though the porosity also increases the energy absorption capacity in plastic deformation [39]. PEO treatment of the microporous MIM parts produced by the current study resulted in the formation of a rough oxide coating with micro-craters, as can be seen from Figure 12. During PEO treatment, Ti initially reacts with the oxygen that results from the electrolysis of the aqueous electrolyte solution to form amorphous Ti oxide.