All heat sinks with fins is extruded alluminum. If they have copper on them then take the copper off. Sheet alluminum is always bent or pressed to form the shape, extruded is drawn through a die to form the piece. If you look at the end of the heat sink you can tell thieres no way that alluminum could have been bent to make the fins. Cast alluminum is always pored into a mold and most of the time you will be able to see mold marks and the inside corrners will be rounded. I hope this helps, I don't know any other way to put it without being there to show you.