Aluminum, shown during a pouring operation, is a versatile material that can be cast in most types of molds.
on the other hand, can be made whole, or, in the case of compli- cated applications, as multiple pieces glued together. Benefits of the shell process
include: • an excellent core or mold surface, resulting in good casting finish;
• good dimensional accuracy in the casting because of mold rigidity;
• storage for indefinite periods of time, which improves just-in-time delivery;
• high-volume production; • selection of refractory material other than silica for specialty applications;
• savings in material usage through hol- low cores and thin shell molds. Nobake or Airset Systems—In order
to improve productivity and eliminate the need for heat or gassing to cure mold and core binders, a series of resin systems 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 chemi- cal reaction begins to harden (cure) the binder. The curing time can be lengthened or shortened based on the amount of catalyst used and the temperature of the refractory sand. The mixed sand is placed against the
pattern or into the corebox. Although the sand mixtures have good flowability, some form of compaction (usually vibration) 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. The 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 finish 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. The nobake process provides
the following 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. These include the lost foam process and the less common V-process.
In high-production lost foam casting, foam patterns are molded and assembled on a cluster, coated with a refractory, placed in a flask and surrounded with unbonded sand. Molten metal then is poured over the pattern, evaporating the foam.
8 METAL CASTING DESIGN & PURCHASING 2011 CASTING SOURCE DIRECTORY
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