Guide to Casting and Molding Processes
neers and component users fl exibility in their metal forming needs (Fig. 1). Each process off ers 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 aff ect 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 the cavity. T e mold cavity must retain
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.
its shape until the metal solidifi es 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 solidifi cation;
• able to cleanly strip away from the casting after it has cooled;
• economical, since large amounts of refractory material are used.
Green Sand Molding
T e 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. T e additives help to harden and hold the mold shape to withstand the pressures of the molten metal. T e green sand mixture is compacted
This automated molding loop is used to produce nobake molds. The loop is among the largest in pro- duction in North America.
by hand or through mechanical force around a pattern to create a mold. T e me- chanical force can be induced by slinging, jolting, squeezing or impact/impulse. T e following points should be taken
into account when considering the green sand molding process:
6 METAL CASTING DESIGN & PURCHASING 2013 CASTING SOURCE DIRECTORY
he versatility of metalcasting is demonstrated by the number of casting and molding processes currently available. T is range of choices off ers design engi-
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