Automation and mass-production help to make lost foam molding an efficient metal parts fabrication solution. Commonly cast metals include cast irons, aluminum alloys, steels, and nickel alloys. Common applications of the lost foam process include housings, pumps, and adaptors.
In a mass production environment, the lost foam process typically involves creating a detailed polystyrene mold inside an aluminum die. Steam is applied in order to heat polystyrene beads placed into the metal mold. As it expands due to moisture and heat, the polystyrene takes the shape and dimensions of the aluminum die’s interior.
The polystyrene patterns are built into clusters. These foam clusters, often created from several patterns, are then coated with a ceramic investment also referred to as a refractory coating. Then these are compacted into a metal flask and backed up with unbonded sand, which is compacted using a vibration table.
The refractory coating captures all of the foam model’s detail and creates a barrier between the smooth foam surface and the coarse sand surface. It controls permeability, allowing gas created by the vaporized foam pattern to escape through the coating and into the sand. Controlling permeability is a vital step to avoid sand erosion. The barrier formed by the refractory coating also ensures that molten metal will not penetrate or cause sand erosion during pouring.
Small or Large Volume Runs
For small volume runs, the polystyrene foam pattern can be hand cut or machined from a solid block of foam. For larger volumes, the pattern can be mass-produced by a process similar to injection molding.
Once the sand is compacted, the mold is ready to be poured. When molten aluminum is introduced, the polystyrene pattern is displaced and its features are duplicated precisely. A new polystyrene pattern must be produced for every casting that is poured. This process allows complex parts, like housings, pumps, and adaptors, to be produced to very tight tolerances. Multiple components can be produced as a single piece with no secondary operations required.
Also, unlike a lost-wax process, there is no bake-out phase. The melt is poured directly into the foam-filled mold, and the foam evaporates. Since foam has a low density—approximately 97.5% air and 2.5% polystyrene—the waste gas produced is minimal and can escape through mold permeability, making lost foam molding an efficient solution.