The invention relates to a specific process for manufacturing a part made of thermoplastic composite material comprising a fibrous reinforcement and a matrix of thermoplastic polymer based on a thermoplastic polymer which impregnates said fibrous reinforcement, which process comprises: a) a step of impregnating said dry fibrous reinforcement, by injection onto said reinforcement, at an injection temperature T1 above the HDT of said composite material if the matrix is amorphous and above Tm if said material is semi-crystalline, of a reactive composition p) which is the precursor of said polymer, and which is based on a reactive prepolymer of said thermoplastic 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 at a value below the HDT of said composite material when the matrix thereof is composed of an amorphous polymer and at a value between Tc and Tc-20°C for a semi-crystalline matrix, and with reheating of the internal surface of the mould, just before said injection, to a temperature T3 between T1-50°C and T1+50°C, using a heating means which is external to the mould and removable, with c) demoulding of the part when the temperature T4 thereof is equal to T2, 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 Ta