Surface irregularities in titanium marine parts formed by the particulate injection moulding process

Abstract

In this study, a structural hold down component was designed and produced using the particulate injection moulding (PIM) process. The material of choice was titanium due not only to the material properties but also due to the desire to create custom-made components for a state-of-the-art marine vessel, Earthrace 2.
On removal from the mould, the green parts were seen to have an irregular surface on the top face. Known as surface bloom, it can be seen during moulding of single-phase commodity polymers as a result of changes in the polymer density, due to shear stresses and irregularities of turbulent flow.
Literature suggests the surface bloom is a result of a separation between the two phases, but the preliminary findings show little evidence of this within the sectioned profile. The sintered parts were sectioned, and inspection of the surfaces was done using metallographic techniques. The use of CAD models enabled the defect to be modelled and the models provided a more likely scenario. It was further confirmed that there were no through part defects present and although the surface irregularities were caused by separation of the two-phases, the effect was restricted to the outer surface of the parts.