downsprue

A downsprue is the principal vertical channel cast into a mold that delivers molten metal from the pouring basin directly down toward the cavity or cavities below. It serves as the primary conduit between the surface of the mold and the gates that distribute metal into individual casting impressions. In a two-part mold, the downsprue typically runs through the cope (top half), widening slightly as it descends to accommodate the metal's natural flow resistance and to minimize turbulence and air entrainment.

The geometry of a downsprue is critical to casting quality. A well-designed downsprue tapers gradually from top to bottom, with a diameter usually between 0.5 and 2 inches depending on the size of the casting and the metal being poured. The channel must be smooth and free of sharp corners that could create eddies or slow the metal's descent. If the downsprue is too narrow, the metal cools before reaching the gates; if too wide, it allows excessive turbulence and gas absorption. The depth of the downsprue controls the hydrostatic pressure available to fill thin sections of the casting.

Once the casting has cooled and solidified, the downsprue remains as a solid appendage attached to the finished casting and must be removed. This is done by sawing, grinding, or breaking it off at the parting line, depending on the casting's size and the mold material. The downsprue and gates together are called the gating system, and their combined mass must be calculated into the total mold weight when planning material flow and solidification rates.

Different casting processes use downsprues with varying complexity. In sand casting, the downsprue is formed by a pattern piece and exists only for that one casting. In permanent mold and die casting, the downsprue is machined into the mold itself and must be designed to withstand repeated thermal cycling. Investment casting often uses a wax pattern with a downsprue that burns out with the rest of the tree, leaving a ceramic cavity.

The term "downsprue" distinguishes this primary channel from runner bars and lateral sprues that branch off to feed individual cavities. Some older texts and European sources use "sprue" alone to refer to this element, while "downsprue" has become standard in North American metallurgical practice to avoid confusion with the entire gating assembly. The downsprue, runners, and gates together form the "feeder system" that must be engineered to ensure complete filling and proper solidification of the casting.