Ceramic core with composite insert for casting airfoils

A method of making a multi-wall, flash-free ceramic core, for casting an airfoil with one or more internal cooling passages, comprising preforming at least one fugitive core insert to have a joint-forming surface with at least one of a male joint feature and a female joint feature, then forming at least one additional fugitive core insert in-situ adjacent and integrally connected to said at least one preformed core insert at said joint-forming surface to form an over-molded, fused joint having an interlocked male joint feature and female joint feature to form a composite core insert that includes features to form internal passage surfaces in the ceramic core when the composite core insert is removed wherein some of said features reside in different planes that the composite core insert cannot be formed as one piece as a result of complex core geometry, placing the composite core insert in a core molding die cavity, introducing a fluid ceramic material into the die cavity to form a ceramic core body incorporating the composite core insert wherein the over-molded, fused joint is hidden internal of the ceramic core body and is so sealed as to prevent internal core body molding flash that cannot be detected or removed, removing the ceramic core body from the die cavity, and selectively removing the composite core insert to yield a flash-free ceramic core having said internal passage surfaces.