In the V-Process, the sand mold is vacuum-formed over a pat- tern. This process holds tight tolerances and requires zero draft.
to improve productivity and eliminate the need for heat or gassing to cure mold and core binders, a series of resin sys- tems referred to as nobake or airset binders was developed. In these systems, sand is
mixed with one or two liquid resin components and a liquid catalyst component. When the resin(s) and catalyst combine, a chemical reaction begins to harden (cure) the binder. Te curing time can be lengthened or shortened based on the amount of catalyst used and the temperature of the refractory sand. Te mixed sand is placed against the
pattern or into the corebox. Although the sand mixtures have good flowability, some form of compaction (usually vibra- tion) is used to provide densification of the sand in the mold/core. After a period of time, the core/mold will have cured sufficiently to allow stripping from the corebox or pattern without distortion. Te cores/molds are then allowed to sit and thoroughly cure. After curing, they are covered with a refractory wash or coating that provides a better surface fin- ish on the casting and protects the sand in the mold from the heat and erosive action of the molten metal as it enters the mold cavity. Te nobake process provides the fol-
lowing positive features: • the 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- tion of the casting from the mold after solidification is complete);
• the abiliy to store cores and molds for long periods.
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 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 as- sembled 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 a refractory coating, which covers both the external and internal surfaces. With the gating and risering system at- tached to the pattern, the assem- bly is suspended in a one-piece flask, which is then 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. Te 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 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 chemically 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, un-
bonded sand to fill the special flask set over the film-coated pattern. Slight vibration compacts the fine grain sand to its maxi- mum bulk density. Te flask is then 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 hold- ing 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 by the metal. After the metal solidifies and cools, the vacuum is released and the sand falls away.
PERMANENT MOLD CASTING
At least three families of molding
This robot coldbox core and mold making cell is composed of 11 robot arms and gantries designed to automatically produce, assemble and refractory-coat components.
8 METAL CASTING DESIGN & PURCHASING
and casting processes can be categorized as permanent mold processes. Tese include diecasting (high-pressure di-
2014 CASTING SOURCE DIRECTORY
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