Polymeric materials present mechanical and thermal limitations that disable their use in the mould manufacturing. Nevertheless, an adequate selection of the polymeric matrix and the dispersed materials allows the possibility to achieve performances closer to the metals and their alloys. These new materials are attractive solutions for applications that have less demanding mechanical properties, as is the case of the rapid tooling applications where a small number of parts are required. The composites were obtained from a mixture of an epoxy resin with fine and coarse aluminium particles. One can state that besides the dispersed phase resistance overcome the matrix one, its contribution to the global resistance of the composite is restricted, because the fracture surface lies basically in the matrix and interfaces. As the matrix section under load is reduced, with the increment of the dispersed phase, the composite properties turn out to be dependent on the interfaces quality and resistance. This is particularly true when fine aluminium particles are used. The interface contribution to the global composite properties depends basically on two parameters, the binding quality between the matrix and the dispersed phase, and the interface extension per unit volume. This paper studies the main contribution of the phase-matrix interface in the mechanical behaviour of aluminium filled epoxy.