Full mold casting is an evaporative-pattern casting process which is a combination of sand casting and
lost foam casting. It uses an expanded polystyrene foam pattern which is then surrounded by sand,much like sand casting. The metal is then poured directly into the mold,which vaporizes the foam upon contact.
Process
First,a pattern is made from polystyrene foam,which can be done many different ways. For small volume runs the pattern can be hand cut or machined from a solid block of foam; if the geometry is simple enough it can even be cut using a hot-wire foam cutter. If the volume is large,then the pattern can be mass-produced by a process similar to injection molding. Pre-expanded beads of polystyrene are injected into a preheated aluminium mold at low pressure. Steam is then applied to the polystyrene which causes it to expand more to fill the die. The final pattern is approximately 97.5% air and 2.5% polystyrene. Once the pattern is made pre-made pouring basins,runners,and risers can be hot glued to form the final pattern.
The pattern is then coated with a refractory material. The coated pattern is then placed in a flask and packed carefully with green sand or a chemically bonded sand. Finally,the molten metal is poured into the mold,which vaporizes the foam allowing the metal to fill the entire mold. The casting is allowed to cool and then dumped out of the flask ready to use. The sand does not need to be reprocessed so it can be directly reused.
Details
The minimum wall thickness for a full-mold casting is 2.5 mm (0.10 in). Typical dimensional tolerances are 0.3% and typical surface finishes are from 2.5 to 25 µm (100 to 1000 µin) RMS. The size range is from 400 g (0.88 lb) to several tonnes (tons).
Full-mold casting is often used to produce cylinder heads,engine blocks,pump housings,automotive brake components,and manifolds. Commonly employed materials include aluminium,iron,steel,nickel alloys,and copper alloys.