Design for Casting and Forging
In casting, the liquid material is poured into a cavity (die or mold) corresponding to the desired geometry. The shape obtained in the liquid material is now stabilized by solidification (cooling) and removed from the cavity as a solid component. Casting is the oldest known process for producing metallic components. The main stages are producing a suitable mold cavity; melting the material; pouring the liquid material into the cavity; stabilizing the shape by solidification; removing or extracting the solid component from the mold; cutting off sprues and risers; and cleaning the component.
Forging, or its close cousin, pressing, consists of preheating a solid piece of wrought metal and placing it in a lower die. The upper die is then closed, squeezing the material into the closed die cavity and fomiing the desired shape of the pan. The pressure may be applied slowly, as in pressing, or rapidly, with one or more hammer actions of the upper die, as in forging. Any excess material is squeezed out between the die halves as flash. The die is opened and the part ejected. The main stages are cutting the blank to the correct volume or shape; preheating the blank; forging the part; opening the die and ejecting the part; trimming off the flash (usually in a punch press die); and cleaning the part.
In both processes, depending on the properties of the metal and the desired requirements of the product, the finishing from this point on is similar. Both may require heat treatment, straightening, machining, plating or painting, etc. Both aluminum and steel, as well as most other metals, may be cast or forged. Different alloying ingredients are added to improve the processing characteristics, making the precise alloy selection important in both processes. Some alloys can only be cast and others can only be forged. In general, forging materials require a high ductility at the elevated temperatures required for forging. Forgings also have better mechanical properties in the finished part, since the grain structure of the material becomes somewhat flattened or stretched, as in other wrought alloys. Castings, on the other hand, have a more nonoriented grain structure, with the size of the grains being a function of the rate of cooling. The casting alloying ingredients usually have more effect on the liquidity at the pouring temperature. After heat treatment and the accompanying changes in the internal structure and properties of the metal, castings are generally of lower strength and ductility than forgings.
The design problems of casting and forging involve comparing the cost of one process against another—coupled with the limitations of web thicknesses, radii, strength, and the like. The quantity of parts to be produced is also an important factor. Lead times for various processes differ greatly. For example, the tooling lead time could vary by several months for a sand casting versus a die casting. Subchapter 1.4 discusses some short-time options that are available for use until the final production tooling is available. In nearly all cases, a part that will ultimately be cast or forged can be hogged-out of solid stock,—although the cost may be very high. The designer has more leeway in the physical shape of a part by utilizing one of the casting processes rather than a forging. Castings are usually lower in cost than forgings. Forging are superior in strength, ductility, and fatigue resistance, while castings may be better for absorbing vibration and are generally more rigid.
The introduction to casting and forging processes in this section are not meant to make the manufacturing engineer or product designer an expert in the field. The processes are described in enough detail to provide a reference as to the limitations of the various processes available, in order for the design team to consider the strength, tolerances, surface finish, machinability, lead time for tooling, and cost. Chapter 15 discusses the casting and forging processes in greater detail.

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