The metal injection moulding (MIM) process combines the well-known thermoplastic injection and powder metallurgy technologies to manufacture small and intricate parts to nearly net shape. The design of injection moulds is a very important stage in such a technology as the mixture of metallic powder and thermoplastic binder is injected in the mould with a viscous behaviour strongly different from the pure polymer injection. To improve the mould design by an efficient numerical tool, an explicit 3D software has been developed to perform efficiently the injection simulation. Taking into account the special behaviour of feedstock in MIM injection, a bi-phasic model is used to describe the flows of metallic powder and plastic binder so as to predict accurately the powder segregation in injection. The volume fractions of metallic powder and plastic binder give directly the evolution of segregation during the injection course. The numerical results are compared with experiments, in which a multi-cavity mould specially designed and realised in our laboratory is used. The segregation zones are well predicted. This specially developed software permits to optimise the mould design and processing parameters to get required components.