A process for manufacturing a part made of thermoplastic composite material including a fibrous reinforcement and a matrix of thermoplastic polymer based on a thermoplastic polymer which impregnates said fibrous reinforcement, which process includes: a) a step of impregnating said dry fibrous reinforcement, by injection onto said reinforcement, of a precursor of said polymer, and moulding, with the polymerization step b) being simultaneous with the impregnating and moulding step and with the regulating temperature T2 for said mould being kept constant throughout the moulding cycle, and with reheating of the internal surface of the mould, just before said injection, using a heating means which is external to the mould and removable, with c) demoulding of the part, and d) optional continuation of the polymerization, if said polymerization is partial at demoulding, in a separate step outside the mould, by annealing in an oven at an annealing temperature.