Our aim is to describe the behaviour of highly filled injection moulding pastes, made of a polymer binder and fine ceramic powders. A specific rheometer has been developed to characterize this behaviour within the injection range of pressures and shear rates. As classical homogeneous models fail to describe the behaviour of the paste, we propose an original model deriving from the mixture theory, taking into account the separate contributions of binder and powder to the global behaviour of the paste. The qualitative and quantitative accuracies of this model are discussed.