spray

In sand casting, a spray is a secondary channel branching from the main runner, designed to feed molten metal evenly into different sections of the mold cavity. Where the runner acts as the primary highway for metal flow, sprays function as distribution roads, ensuring that isolated or distant parts of the mold fill at roughly the same rate and temperature. Without adequate sprays, thin sections cool and solidify before thick sections fill, leaving shrinkage cavities and voids that weaken the casting.

The number and placement of sprays depends on mold geometry. A simple single-cavity mold may need only one or two sprays branching from the runner base. Complex multi-cavity molds, or those with asymmetrical wall thicknesses, require multiple sprays positioned so each section receives metal at balanced pressure and velocity. Sprays are typically smaller in cross-section than the runner itself, narrowing as they extend outward to regulate flow and create backpressure that helps control turbulence and gas entrapment.

Spray diameter and length are calculated based on the weight and distance of metal to be fed. Iron castings may use sprays ranging from 6 mm to 20 mm in diameter depending on pour weight and cavity size. The spray is usually tapered or constricted at its junction with the mold cavity; this point, called the gate, is deliberately made smaller to further slow the incoming metal stream and reduce turbulence that traps air and oxide. Sprays must be tall enough that metal enters the mold under adequate head pressure, yet short enough to minimize heat loss through the sand.

Inadequate spray design causes misruns, where metal reaches only part of the cavity before cooling. Over-sized sprays allow metal to rush in, causing erosion of the mold sand, aspiration of air into the metal, and casting defects. The spray layout is planned during pattern design and transferred to the mold during flask preparation; modern foundries use computer simulation to optimize spray number, size, and angle before pouring.

The term spray is somewhat misleading, as the metal does not atomize or mist; it remains a coherent stream flowing through a solid channel. The name likely derives from the visual effect of metal branching outward through multiple channels, resembling the spray of a water fountain, though in industrial practice it is a controlled, gravity-driven or pressure-driven flow rather than an atmospheric phenomenon.