Unbonded Sand Processes
Unlike the sand casting processes that use various binders to hold the sand grains together, two unique processes use unbonded sand as the molding media. These include the lost foam process and the less common V-process.
Lost Foam Casting—In this process, the pattern is made of expendable polystyrene (EPS) beads. For high-production runs, the patterns can be made by injecting EPS beads into a die and bonding them together using a heat source—usually steam. For shorter runs, pattern shapes are cut from sheets of EPS using conventional woodworking equipment and then assembled with glue. In either case, internal passageways in the casting, if needed, are not formed by conventional sand cores but are part of the mold itself.
The polystyrene pattern is coated with a refractory coating, which covers both the external and internal surfaces. With the gating and risering system attached to the pattern, the assembly is suspended in a one-piece flask, which then is placed onto a compaction or vibrating table. As the dry, unbonded sand is poured into the flask and pattern, the compaction and vibratory forces cause the sand to flow and densify. The sand flows around the pattern and into the internal passageways of the pattern.
As the molten metal is poured into the mold, it replaces the EPS pattern, which vaporizes. After the casting solidifies, the unbonded sand is dumped out of the flask, leaving the casting with an attached gating system.
With larger castings, the coated pattern is covered with a facing of chemically bonded sand. The facing sand is then backed up with more chemically bonded sand.
The lost foam process offers the following advantages:
- no size limitations for castings;
- improved surface finish of castings due to the pattern’s refractory coating;
- no fins around coreprints or parting lines;
- in most cases, separate cores are not needed;
- excellent dimensional tolerances.
V-process—In the V-process, the cope and drag halves of the mold are formed separately by heating a thin plastic film to its deformation point. It then is vacuum-formed over a pattern on a hollow carrier plate.
The process uses dry, free-flowing, unbonded sand to fill the special flask set over the film-coated pattern. Slight vibration compacts the fine grain sand to its maximum bulk density. The flask is then covered with a second sheet of plastic film. The vacuum is drawn on the flask, and the sand between the two plastic sheets becomes rigid.
The cope and drag then are assembled to form a plastic-lined mold cavity. Sand hardness is maintained by holding the vacuum within the mold halves at 300-600 mm/Hg. As molten metal is poured into the mold, the plastic film melts and is replaced immediately by the metal. After the metal solidifies and cools, the vacuum is released and the sand falls away.