The causes of blister defects often occurring in the paint coating of counterweight castings were investigated. Flake graphite near the casting surface oxidizes to form a tube while casting solidifies slowly. In the paint coating process, solvents sometimes permeate into the oxide tube intergranularly due to capillary attraction. Air in the oxide tube discharges though a part with casting defect such as sand or slag inclusion, resulting in blister defects. Vacuum assist sand mold castings, castings of mold dressed with iron oxide coating and spheroidal graphite castings were found to be appropriate alternatives to conventional seasoning.
