Densification of Fibrous Structures by Resin Transfer Moulding for Making Thick Parts of Composite Material

A fiber structure for forming the reinforcement of a part to be made is placed in a mold ( 22 ) having at least one wall formed by a flexible membrane, and then a resin composition is injected into the mold, the composition having a volatile material content of less than 25% by weight and being at a temperature of a value such that its viscosity lies in the range 0.1 Pa.s to 0.3 Pa.s. The resin is polymerized in the mold placed in an enclosure ( 20 ) with temperature being raised progressively and with polymerization including at least a final stage of polymerization under pressure in order to obtain a composite material part presenting residual porosity of less than 11% by volume.