A riser, also known as a feeder, is a reservoir built into a metal casting mold to prevent cavities due to shrinkage.
Most metals are less dense as a liquid than as a solid so castings shrink upon cooling, which can leave a void at the last point to solidify. Risers prevent this by providing molten metal to the casting as it solidifies, so that the cavity forms in the riser and not the casting. Without a riser heavier parts of the casting will have shrinkage defects, either on the surface or internally. Risers are not needed for casting processes that utilize pressure to fill the mold cavity.
Just as a pattern is needed to form the shape of the mold cavity, Riser Forms are needed to create the shape of the casting riser in the mold. Below are some examples.