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 its shape until the metal solidifi es and the casting is removed. Depending on the choice of metal, certain characteris- tics are demanded of the mold. When granular refractory materials, such as
Guide to Casting and Molding Processes
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-
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.
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 cav- ity 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 met- al 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
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: • for many metal applications, green sand processes are the most cost-eff ective of all metal forming operations;
• these processes readily lend themselves to automated systems for high-volume work, as well as short runs and proto- type work;
• in the case of slinging, manual jolt or squeeze molding to form the mold, wood or plastic pattern materials can be used, whereas high-pressure, high-density molding methods almost always require metal pattern equipment;
This automated molding line is used to produce large nobake molds.
• high-pressure, high-density molding normally produces a well-compacted mold, which yields better surface fi nish- es, casting dimensions and tolerances; • the properties of green sand are ad-
6 METAL CASTING DESIGN & PURCHASING 2014 CASTING SOURCE DIRECTORY
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