slush-cast
To create hollow castings in which molten metal is poured into a metallic mould and then poured out again, leaving a thin solidified layer of the metal on the walls of the mold.
slush-cast: hollow metal parts from a thin shell
Slush-casting is a gravity-casting method that produces hollow metal parts by exploiting the solidification rate of molten metal against mold walls. Molten metal is poured into a metal mold cavity and allowed to freeze briefly against the walls, typically for 30 seconds to several minutes depending on metal type and wall thickness. Before the interior metal fully solidifies, the still-liquid center is poured back out, leaving behind a hollow shell of solid metal bonded to the mold.
The technique works because metal cools faster at the mold surface than in the interior. Aluminum and its alloys, zinc alloys, and certain magnesium alloys are common choices for slush-casting because they have a narrow freezing range and good wetting behavior on steel mold surfaces. The resulting wall thickness is controlled by dwell time: longer contact produces thicker walls. Typical wall thicknesses range from 3 to 10 millimeters in production parts, though this varies with alloy and mold temperature.
Where it is used: Slush-casting dominates the production of decorative figurines, statuary, and ornamental hardware. It also produces automotive components like grille shells and non-structural trim pieces. The method cannot compete with permanent-mold or pressure-die-casting for high-volume work, but it requires minimal equipment investment compared to those processes.
The main limitation is wall thickness uniformity. Complex mold geometries and internal undercuts create zones where metal cools unevenly, producing weak or oversized sections. Molds require active cooling between pours to maintain consistent solidification rates. If the mold temperature drifts, wall thickness becomes unreliable. Residual interior liquid metal can also slump during ejection, warping the thin shell.
The term slush refers to the partially frozen slurry state of the metal inside the mold at the moment of pouring. Cast denotes the final solid part. The name is literal and has held since the method emerged in the early 20th century, though the process itself predates the term by decades in toy and decorative metalworking shops.