A method and device for making integrally formed one-piece molded parts is provided, where the molded parts are to contain a void within their volume. An exemplary such molded part is an arc tube for a discharge lamp. A wax core is first molded onto a core pin wire in the shape of the desired void, the discharge chamber in the case of an arc tube. Then the part is molded over the wax core to provide an integrally formed one-piece part having the appropriately dimensioned void volume therein defined by the wax core. After the part has been hardened around the wax core, the core is removed by conventional means. The holes through the finished molded part left after the core pin wire is removed can be filled in, or in the case of an arc tube they are useful as passageways to accommodate electrodes therethrough.