the efficiency, or "yield", of a casting is defined as the weight of the casting divided by the weight of the total amount of metal poured. risers can add a lot to the total weight being poured, so it is important to optimize their size and shape. risers exist only to ensure the integrity of the casting, they are removed after the part has cooled, and their metal is remelted to be used again; as a result, riser size, number, and placement should be carefully planned to reduce waste while filling all the shrinkage in the casting.