The present invention relates to a method of fabricating a mold core (5) for making hollow parts (11) based on a composite material including fibers, fabrics, or textiles together with a resin, the method consisting in mixing a filler material (1) and a binder material (2) in order to obtain a uniform mixture of materials, and in solidifying, compacting, and shaping the resulting mixture in order to constitute said core, the method being characterized in that it further consists in introducing into the mixture at least one complementary material (3) that is expandable under the effect of heat or by applying another physical or chemical principle, and in mixing said materials (1, 2, 3) in order to obtain a uniform distribution of the complementary material (3) in the mixture.