Guide to Casting and Molding Processes
An MCDP Staff Report
and component users flexibility in their metal forming needs (Fig. 1). Each process offers advantages when
T
matched with the proper alloy and ap- plication. When reviewing these processes 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
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.
metal can be poured into the cavity. The mold cavity must retain its shape until the metal solidifies and the casting is re- moved. Depending on the choice of metal, certain characteristics are demanded of the mold. When granular refractory ma- terials, 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 cast- ing after it has cooled;
• economical, since large amounts of refractory material are used.
Green Sand Molding The most common method used to
This automated molding loop is used to produce nobake molds. The loop is among the largest in production in North America.
make metal castings is green sand mold- ing. In this process, granular refractory sand is coated with a mixture of bentonite clay, water and, in some cases, other ad- ditives. The additives help to harden and hold the mold shape to withstand the pressures of the molten metal. The green sand mixture is compacted
by hand or through mechanical force around a pattern to create a mold. The
6 METAL CASTING DESIGN & PURCHASING 2011 CASTING SOURCE DIRECTORY he versatility of metalcasting is
demonstrated by the number of casting and molding processes currently available. This range of choices offers design engineers
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