Process for the manufacture of sandwich structures

A low density element, formed by a three-dimensional web (1) composed of two sheets of fabric connected mutually by filaments disposed substantially perpendicular to the lying plane of the said fabric sheets, is impregnated with resin, and after having been subjected to a pre-hardening or pre-curing treatment of the resin, is introduced into a mold (11) for the final curing of the resin. Between the two sheets of the three-dimensional web in the mold a pressurized expandible fluid or pressurised gas is fed, before the final curing. The three-dimensional web may be covered, before its introduction into the mold, with two resistant outer sheets of fabric impregnated with resin. Hollow passages for cables, ducts can be formed.