Guide to Casting and Molding Processes
Understanding the metalcasting basics can help you design for manufacturability and utilize processes that meet your specific requirements. An MCDP Staff Report
and component users flexibility in their metalforming needs (Fig. 1). Each process offers advantages when
T
matched with the proper alloy and applica- tion. 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 common
casting processes.
SAND CASTING PROCESSES Fundamentally, a mold is produced by
shaping a refractory material to form a cavity of desired shape such that molten metal can be poured into the cavity. The mold cavity must retain its shape until the metal solidifies
he versatility of metalcasting is
demonstrated by the number of casting and molding processes currently available. This range of choices offers design engineers
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. Shown above is the application of the refractory coating.
and the casting is removed. Depending on the choice of metal, certain characteristics are demanded of the mold. When granular refrac- tory 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 es- cape 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
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
This automated molding loop is used to produce nobake molds. The loop is among the largest in production in North America.
by hand or through mechanical force around a pattern to create a mold. The me- chanical force can be induced by slinging, jolting, squeezing or by impact/impulse. The following points should be taken
into account when considering the green sand molding process: • for many metal applications, green sand processes are the most cost-effec- tive of all metal forming operations; • these processes readily lend themselves to
6 Metal Casting Design & PurChasing 2010 Casting sourCe DireCtory
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