Capillary rheology was used to determine the shear viscosities of low expansion alloy INVAR 36 feedstocks for powder injection moulding (PIM) based on gas atomized powders and cellulose acetate butyrate (CAB) and poly-ethylene glycol (PEG) binders. Different variables that describe the flow behaviour were analysed, in particular, the relationships between viscosity of the melted feedstock with different conditions, such as shear rate (gamma), temperature (T), solid loading (Phi) and particle size (PS). The viscous behaviour was physically interpreted and discussed, and a generalised rheological model was proposed that involves all of the dependant parameters: gamma, T, Phi and PS. The particular rheological behaviour of the feedstocks, which deviates from typical pseudoplastic behaviour, has a relevant correlation with the proposed model. Moreover, the model can be used when similar behaviours are observed in other systems. (C) 2014 Elsevier B.V. All rights reserved.