Guide to Casting and Molding Processes
neers and component users flexibility in their metal forming needs (Fig. 1). Each process offers advantages when
T
matched with the proper alloy and application. When reviewing these pro- cesses and determining which best suits your needs, consider the following: • required surface quality; • required dimensional accuracy; • type of pattern/corebox equipment; • cost of making the mold(s); • how the selected casting process will affect casting design. Molding processes can be broken into
four general categories: • sand casting processes; • permanent mold processes; • ceramic processes; • rapid prototyping. Following is a look at the most com- mon casting processes.
SAND CASTING PROCESSES Fundamentally, a mold is produced
by shaping a refractory material to form a cavity of a desired shape such that molten metal can be poured into
In the nobake molding process, refractory sand is coated with binder and a liquid catalyst. As the binder and catalyst combine, a chemical reaction hardens the sand.
the cavity. Te mold cavity must retain its shape until the metal solidifies and the casting is removed. Depending on the choice of metal, certain character- istics are demanded of the mold. When granular refractory materials, such as silica, olivine, chromite or zircon sands, are used, the mold must be:
• strong enough to sustain the weight of the molten metal;
• constructed to permit any gases formed within the mold or mold cavity to escape into the air;
• resistant to the erosive action of molten metal during pouring and the high heat of the metal until the casting is solid;
• collapsible enough to permit the metal to contract without undue restraint during solidification;
• able to cleanly strip away from the casting after it has cooled;
• economical, since large amounts of refractory material are used.
Green Sand Molding
This automated molding loop is used to produce nobake molds. The loop is among the largest in pro- duction in North America.
Te most common method used to make metal castings is green sand mold- ing. In this process, granular refractory sand is coated with a mixture of benton- ite clay, water and, in some cases, other additives. Te additives help to harden and hold the mold shape to withstand the pressures of the molten metal. Te green sand mixture is compacted
by hand or through mechanical force around a pattern to create a mold. Te me- chanical force can be induced by slinging,
6 Metal Casting Design & PurChasing 2012 Casting sourCe DireCtory
he versatility of metalcasting is demonstrated by the number of casting and molding processes currently available. Tis range of choices offers design engi-
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