## 4 Discussion  
The principal aim of this study was to evaluate the use of a relatively coarse Ti powder in the production of MIM parts with the potential for biomedical applications and their subsequent surface modification via PEO. This can be seen by the comparison between the feedstock of a coarse Ti powder  
Figure 14: XRD analysis result for PEO-treated sample for 4 min. Figure 13: Images of the ground surface of a sample before PEO treatment showing four partially-sintered Ti particles and the surface of a specimen (without grinding) after being subjected to PEO. (mean particle size of 75 \(\upmu\)m) with a three-component polymeric binder system (PEG, PMMA and SA), with a commercial feedstock (PolyMIM Grade 2). DSC curves (Figure 3) showed that the current study feedstock requires a lower temperature to be successfully injection moulded than commercial feedstock, where the peak melting temperature of the high viscosity part of the current study binder (PMMA) was 14% lower than that for commercial feedstock. It should be noted that the SA peak could not be detected in the DSC diagram for the current study binder, which could be ascribed to the small amount of SA in the mixture, as has been observed in the literature [30]. In addition, the results of the DSC analysis suggested that the commercial binder consists of more than two parts and does not only contain polypropylene as previously reported, but also high density polyethylene. This was supported by the results of the FTIR analysis, Figure 4. Based on the DSC results, the current study feedstock was injection moulded at a temperature 10% higher than the peak melting temperature of the high viscosity part of the binder, to ensure complete melting. In contrast, the commercial feedstock was injection moulded at a temperature of 185 \({}^{\circ}\)C (chosen based on the specification of 178-186 \({}^{\circ}\)C), which is about 17% higher than the melting point of the high viscosity part of its binder. Viscosity analysis results (Figure 5) demonstrated that both feedstocks exhibit some shear thinning behaviour, which is desirable in MIM. This can arise where shear rate decreases viscosity due to the unfolding, stretching and alignment of the tangled polymer chains and the breakage of the particle agglomerates [13; 31]. The sensitivity with which the shear viscosity changes with shear rate is usually assessed by the flow behaviour index (\(n\)). The lower the \(n\) value, the higher the shear sensitivity and the more the feedstock shows pseudoplasticity or shear thinning [32]. Thus, Table 2 shows that the commercial feedstock exhibits the most pseudoplasticity. However, a low value of \(n\) may also be associated with some moulding defects, including jetting, and leads to the slip flow phenomenon [33]. It was also found that the commercial feedstock has a much higher shear viscosity and yield stress, despite being tested at much higher temperatures. The high shear viscosity, yield stress and shear sensitivity could be attributed to several factors, including the high viscosity of the polymers used in the commercial binder and the high solid content for such commercial feedstocks [33] (usually in the range of 60%-65%), as well as the small Ti particle size used [34]. Sintered samples of the commercial feedstock injection moulded using the vertical plunger-type injection moulded showed relatively large macro-cracks, as shown in Figure 7b. These cracks were up to 2 mm in length and might be formed due to the insufficient injection and backing pressure of the vertical plunger-type injection moulded. Sintered samples processed by the same feedstock using the Arburg 320 all-rounder with a pressure more than three-times higher and a temperature of 195 \({}^{\circ}\)C exhibited smaller cracks, Figure 8. In comparison, sintered samples of the current study feedstock processed using the vertical injection moulded did not show any cracks.