A riser is used, depending on the size of the casting, to allow the excess metal to evacuate from the mold.

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A riser (also known as a feed) is a reservoir in a mold where molten metal is stored and from which it is fed into the casting cavity to prevent shrinkage and other defects. The excess metal in the riser solidifies and can be removed from the casting. The size of the riser is dependent on the size of the casting and the amount of metal required to fill any voids created during cooling.

A Riser Is Used, Depending on the Size of the Casting