The objective of this work was the development of epoxy-based composites moulds to injectfunctional polymer prototypes and pre-series. This indirect rapid tooling (RT) process (firmtooling) offers a high potential for a faster response to the continuous more demandingmarket needs, creating in this way a new competitive edge.The moulds developed are composed by an epoxy resin and aluminium particles, which wereadded to improve the thermal conductivity of the tool, which is an essential parameter forplastic injection moulding. However, this procedure also lowers the mechanical properties ofthe tool. In order to overcome this problem, fibres were added to the composite.The influence of the particle/resin and fibre/resin interfaces in the mechanical behaviour isanalysed. Quantitative metallography was used to determine the interface extension of thealuminium particles with the resin. The quality of the glass and carbon fibre/resin interfacewas evaluated by the determination of the interfacial shear strength based on Kelly Tyson'smodel, in order to tailor the composite for RT applications