injecting EPS beads into a die and bond- ing 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. Te polystyrene pattern is coated with
This automated molding line is used to produce large nobake molds.
lowing positive features: • Te capability to use wood and, in some cases, plastic patterns and coreboxes.
• Good casting dimensional tolerances due to the rigidity of the mold.
• Good casting finishes. • Typically easy shakeout (the separa-
• Te abiliy to store cores and molds for long periods.
tion of the casting from the mold after solidification is complete).
Unbonded Sand Processes Unlike the casting processes that use
various binders to hold the sand grains together, two processes use unbonded sand as the molding media. Tese in- clude the lost foam process and the less
common V-process. Lost Foam Casting—In the lost foam
process, the pattern is made of expendable polystyrene (EPS) beads. For high-pro- duction runs, the patterns can be made by
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 is placed onto a compaction or vibrating table. As the dry, unbonded sand is poured into the flask and pattern, the compaction and vibra- tory forces cause the sand to flow and densify. Te sand flows around the pattern and into the internal passageways. As the molten metal is poured into
the mold, it replaces the EPS pattern, which vaporizes. After the casting solidi- fies, the unbonded sand is dumped out of the flask, leaving the casting with an attached gating system. With larger castings, the coated pat-
tern is covered with a facing of chemi- cally bonded sand. Te facing sand is then backed up with more chemically bonded sand. Te lost foam process offers the fol-
lowing advantages: • No casting size limitations. • Improved casting surface finish. • No fins around coreprints or part- ing lines.
• In most cases, no separate cores are 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. Te mold is then vacuum-formed over a pat- tern on a hollow carrier plate. Te 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. Ten the flask is covered with a second plastic sheet. Te vacuum is drawn on the flask, and the sand between the two sheets becomes rigid. Te 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. As molten metal is poured into the mold, the plastic film melts and is replaced by the metal. After the metal solidifies and cools, the vacuum is released and the sand falls away.
8 METAL CASTING DESIGN & PURCHASING 2016 CASTING SOURCE DIRECTORY
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