sprue

In casting, the sprue is the main vertical or near-vertical passage that carries molten metal from the pouring cup down into the mold cavity. It sits at the top of the mold and acts as the primary feeder, delivering metal at controlled velocity and volume. The sprue connects to smaller branches called runners, which distribute the metal to individual cavities. Without a properly designed sprue, the metal cools, oxidizes, or fills unevenly, ruining the casting.

Sprues vary in diameter and taper depending on the metal, wall thickness, and cavity size. A brass or aluminum casting might use a sprue 0.5 to 1.5 inches in diameter, while large steel castings can demand sprues 3 inches or more. The sprue should taper slightly as it descends, which reduces turbulence and air entrainment. If the sprue is too small, the metal freezes before filling the mold; if too large, the sprue itself becomes expensive scrap and the casting wastes time to cool.

Sand molds versus permanent molds demand different sprue geometry. In green sand casting, the sprue is simply a hole shaped by a rammer or by removing a tapered pattern. In permanent molds, die castings, and investment casting, the sprue is a machined or preformed channel. Ceramic shell molds use thin sprues because the shell is fragile; permanent steel dies tolerate thicker, more robust sprues.

The sprue is cut or broken from the finished casting after it cools and solidifies. This separated piece, along with the runners and gates, becomes scrap metal, typically remelted in the furnace. The volume of sprue and runner scrap can range from 5 percent of the final casting weight (for well-optimized dies) to 50 percent or more in sand molds with multiple cavities. This scrap fraction directly affects material yield and cost.

Common defects originating in the sprue include cold shuts, where two metal fronts meet at different temperatures and fail to fuse; porosity from dissolved gases; and penetration, where metal burns into the mold surface and leaves a rough texture on the casting. Sprue location is chosen to minimize these risks: it is often placed at the thickest section of the casting so metal fills under pressure and heat, rather than at a thin edge where it cools quickly.