The invention relates to a molding tool for producing a composite material part by coating, by means of a matrix, a flexible preform including a skin, and profiled elements delimiting a plurality of three-dimensional meshes of polyhedral shapes. This molding tool comprises a mold and a countermold made of a material having the same expansion coefficient as the preform. Furthermore, includes, for each three-dimensional mesh, a part for filling the latter, formed from a plurality of wedges (12-27) suitable for being interlocked and delimiting a free central volume (28, 29), and from a blocking part (40) suitable for ensuring a relative blocking of said blocking part and of the wedges (12-27), the wedges and blocking parts being produced from a material having an expansion coefficient suitable for obtaining a predetermined compaction of the preform for the polymerization temperature of the matrix.