In spite of the volume of literature on natural fibres, bio-matrix materials and their composites, the choices of optimum process parameters such as moulding temperature, pressure and compression time are still largely based on experience, rules of thumb and general knowledge of the chemical and physical processes occurring in the melt during consolidation. The moulding process itself is a complex balance between processes that must occur for the composite to successfully consolidate and the onset of thermal degradation of the natural fibre and/or matrix materials. This paper brings together models of thermal penetration, melt infusion, thermal degradation and chemical degradation of natural polymers to construct an ideal processing window for a bio-composite. All processes are mapped in terms of normalized consolidation progress parameters making it easier to identify critical processes and process boundaries. Validation of the concept is achieved by measuring changes in the mechanical properties of a flax/PLA bio-composite formed over a range of processing conditions within and outside of the optimized window.