Powder injection moulding is a low-cost manufacturing process that produces very complex parts to net shapes in a wide variety of materials and unique alloys, including superalloys, stainless steels, and carbides. The feedstocks used in this process must meet various requirements. They have to show good flowability for the injection process, but they also need to have high solids loading to reduce shrinkage during sintering. In the present study, we investigated the rheological behaviours of Inconel feedstock with various average particle sizes in a polydisperse population from 6 to 66 gm and solids loading (phi = 50%-72%) at a shear rate of 10-10(5) s(-1). The binder formulation was a polymer-based aqueous system composed of 55 vol% polyethylene glycol (PEG), 40 vol% polypropylene (PP), and 5 vol% stearic acid (SA). In particular, the effects of particle size, flow activation energy and the mouldability index on the rheology of the feedstock were studied in detail. New rheological models that describe the effects of various parameters such as shear rate, particle size, temperature, and solids loading are introduced, and their predictions are compared with the experimental values. These rheological behaviours were also characterised using our empirical models, which can be directly applied to the process optimisation of powder injection moulding for micro-components. As a result, rheological parameters such as flow behaviour index, maximal solids loading, and flow activation energy were obtained. The particular rheological behaviour of the Inconel feedstock with a highly concentrated solids fraction, which deviates from typical pseudoplastic behaviour, has a relevant correlation with the proposed models. Moreover, the models can be used when similar behaviours are observed in other formulation, powder and binder systems. (C) 2017 Elsevier B.V. All rights reserved.