Level pour and shielded streams are further methods to avoid reaction of molten metal stream with the oxygen and moisture in the air. Typically, a sprue is designed with a taper to compensate for the acceleration of the stream due to gravity. As the stream falls throught the sprue, it accelerates and the cross section of the stream decreases. If the sprue has a uniform cross section from top to bottom, the stream will pull away from the sprue walls as the stream accelerates and air will be aspirated into the sprue. The sprue is tapered, larger cross section at top, smaller cross section at bottom, so that the sprue will contract faster than the metal stream would contract under the influence of gravity.
Vortices are more likely to form in sprues with round or square cross-sections.
The cross-section of the sprue well is at least twice the cross section of the sprue base and the depth of the sprue well is at least twice the depth of the runner. Often the runner extends beyond the last ingate. Oxide skins and mold debris carried by the tip of the stream will travel past the gates and, hopefully, collect at the end of the runner, which may be enlarged. Copper, iron, steel, brass, bronze are examples of metals and alloys that would be heavier than the dross and oxides that form during pouring. For aluminum, on the other hand, it s common oxides are the same and greater density.