Abstract: For producing ultra strong lightweight core material for composite structure such as aircraft wings, fairings and stabilizers, preimpregnated thermoplastic fiber yarn is processed according to this invention into blocks of ducted core material having a balanced honeycomb pattern. A tubular thermoplastic fabric sheath is braided along shaped mandrels, positioned end to end, as they pass through a braiding machine. The sheath is cut apart at the mandrel ends and the sheathed mandrels are stacked uniformly into a press, where the stack is compressed to contact all interfacial facets. Then heat is applied to fusion bond the stack together at the interfacial facets. Then after cooling to a setting temperature, the pressure is removed and the stack is removed from the press for mandrel extraction.