Design engineers must be famil- iar with what alloys work with the available molding processes. Aluminum, shown during a pouring operation, is a versatile material that can be cast in most types of molds.
than silica for specialty ap- plications;
• savings in material usage through hollow cores and thin shell molds. Nobake or Airset Systems—
In order to improve productiv- ity 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
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 ri- gidity of the mold;
• good casting finishes; •
typically easy shakeout (the separation of the cast- ing from the mold after solidification is complete);
• the abiliy to store cores and molds indefinitely.
Unbonded Sand Processes
catalyst used and the temperature of the refractory sand. The mixed sand is placed against
the pattern or into the corebox. Al- though 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 has 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 can accept a refractory wash or coating that provides a better sur- face 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.
Unlike the sand 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. Lost Foam Casting—In this process,
the pattern is made of expendable poly- styrene (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 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. 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 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
2010 Casting sourCe DireCtory
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