Process for manufacturing a structural or semi-structural composite part which may be in complex form, comprising a fibrous reinforcement and a thermoplastic polymer matrix, preferably based on polyamide, characterized in that it comprises the following steps: i) depositing said fibrous reinforcement, which may be in the form of a preform, in a mould comprising two parts, one part being fixed and one mobile, with leaktight closure of said mould by means of a seal compressible by placing said mould under vacuum by means of a vacuum device, the partial pressure in the mould ranging from 1 to 10 mbar, said mould having no vent, ii) filling said closed mould thus under vacuum by injecting, in the molten state, onto said reinforcement in the dry state a pre-metered amount, just necessary to obtain the final part, of a two-component reactive system, which is the precursor of said thermoplastic polymer, based on a reactive thermoplastic prepolymer, preferably on a reactive polyamide prepolymer, it being possible for the mould to be maintained under isothermal conditions at a temperature below the injection temperature and with said injection being carried out using an injection device comprising two distinct compartments for each of the two reactive components in the molten state, said reactive components being mixed before injection in the chamber of a static mixer head with a residence time in said chamber of less than 2 minutes, preferably less than 1 minute and more preferentially less than 10 seconds, at a temperature which can range from 180°C to 400°C, preferably from 200°C to 300°C, with impregnation of said fibrous reinforcement and simultaneous reaction following the impregnation of said two-component reactive system under a pressure of less than 1 mbar, the creep of the resin through the reinforcement being accompanied by a compacting pressure at the end of injection ensuring complete filling of said mould, with no vent, iii) removing said part from the mould without any finishing treatment. i) depositing said fibrous reinforcement, which may be in the form of a preform, in a mould comprising two parts, one part being fixed and one mobile, with leaktight closure of said mould by means of a seal compressible by placing said mould under vacuum by means of a vacuum device, the partial pressure in the mould ranging from 1 to 10 mbar, said mould having no vent, ii) filling said closed mould thus under vacuum by injecting, in the molten state, onto said reinforcement in the dry state a pre-metered amount, just necessary to obtain the final part, of a two-component reactive system, which is the precursor of said thermoplastic polymer, based on a reactive thermoplastic prepolymer, preferably on a reactive polyamide prepolymer, it being possible for the mould to be maintained under isothermal conditions at a temperature below the injection temperature and with said injection being carried out using an injection device comprising two distinct compartments for each of the two reactive components in the molten state, said reactive components being mixed before injection in the chamber of a static mixer head with a residence time in said chamber of less than 2 minutes, preferably less than 1 minute and more preferentially less than 10 seconds, at a temperature which can range from 180°C to 400°C, preferably from 200°C to 300°C, with impregnation of said fibrous reinforcement and simultaneous reaction following the impregnation of said two-component reactive system under a pressure of less than 1 mbar, the creep of the resin through the reinforcement being accompanied by a compacting pressure at the end of injection ensuring complete filling of said mould, with no vent, iii) removing said part from the mould without any finishing treatment.