Defective Surface
Flow marks. On the surfaces of otherwise sound castings, the defect appears as lines which trace the flow of the streams of liquid metal.
Possible Causes
Oxide films which lodge at the surface, partially marking the paths of metal flow through the mold.
Remedies
Increase mold temperature;
Lower the pouring temperature;
Modify gate size and location (for permanent molding by gravity or low pressure);
Tilt the mold during pouring;
In die casting: vapor blast or sand blast mold surfaces which are perpendicular, or nearly perpendicular, to the mold parting line.