Designing for economical molding. (Casting Design Notebook).

Editor's Note: Mileta M. Tomovic, associate professor in engineering at Purdue Univ., is the new author for casting Design Notebook. This column serves to provide foundrymen with fundamental casting design information so they can support their customer's casting design needs.

Metalcasting design is an iterative process with the goal of meeting or exceeding the functional and structural requirements for desired parts. To benefit from the potential economies of the casting process, however, casting designers must take into consideration specific features of the manufacturing process and incorporate them into the design.

A casting designer can incorporate certain features into part design to improve product manufacturability and reduce cost and lead-time. These include the location and shape of the bosses and undercuts, and the configuration of the internal passages.

Molding Process Considerations

Castings are manufactured in two general classes--expendable mold and reusable mold processes.

The pattern must not damage the mold cavity while it is being removed from the mold or the resulting casting will not retain the desired shape. Thus, the pattern must satisfy certain geometric conditions for successful molding.

In the expendable mold process, the mold is made typically of sand, but it can be made from other ceramic materials. The sand is packed around a reusable pattern (typically made from wood or metal) that has the shape of the desired casting with certain dimensional additions specific to the casting process and cast metal. At the end of the molding process, the pattern is removed from the mold.

The pattern cannot have any undercuts or it will be locked within the mold. In addition, the draft angle on the pattern must be sufficiently large to minimize friction between the pattern and the mold. Otherwise stresses in the mold induced by the friction between the mold and the pattern can exceed the mold strength and cause mold fractures.

In the reusable molding process, a mold usually is made from metal and it can be reused multiple times. After casting solidification, the reusable mold is opened and the casting is removed from the mold.

Similar to removing a pattern from an expendable mold, reusable mold castings cannot have undercuts and the draft angle must be large enough to allow the removal of the casting from the mold. The draft angle for both molding processes must not be too large, otherwise, it will unnecessarily add material to the casting and possibly increase the machining stock.

The designer also must be aware of the interdependence of the parting line location, the undercuts and the draft angles. The same part design may or may not require undercuts depending on how it is oriented within the mold in relation to the parting line.

Designing for Undercuts, Cores and Bosses

Figure la illustrates a part with four fins spaced apart at 90[degrees] angles. The part is positioned in the mold to allow the pattern (expendable mold casting process) or casting (reusable mold casting process) to be removed easily from the mold. This design succeeds because it has no undercuts on the part and the draft angles are large enough to allow for easy pattern removal. Alternately, if the same part is placed within the mold as shown in Fig. 1b, it will be impossible to remove it without destroying the part or the mold. In this situation, cores will be required to allow for the part or pattern removal, as shown in Fig. 1c.

While this example is affected primarily by the location of the parting line (as determined by the foundry engineer and patternmaker), designers must be aware of the interdependence between the parting line, the undercuts and the draft angles.

Figure 2a illustrates a part with two bosses that have created an undercut. The part cannot be molded without the use of cores. However, by extending the bosses to the parting line (Fig. 2b), the designer can eliminate the need for cores. This may have additional benefits, including improved flow of the molten metal to areas of the casting that need to be filled and increased strength and rigidity of the casting.

The disadvantage of this design alternative is that it increases the part's weight. The designer must decide whether to have an undercut or to eliminate it. In the former case, the required cores will increase the coast, but the part's weight will be reduced. In the later case, the need for cores would be eliminated, reducing the manufacturing cost but increasing the part's weight.

Another class of geometric design features that can cause undercuts are the cross-sectional area of the part. Figure 3a illustrates a casting in which the cross-sectional area increases moving away from the parting line (located at the top surface of the casting), which forces an undercut into the casting process. This design requires two cores--one for the internal geometry and the other for the outside geometry.

An alternative design of the part features a decreasing cross-sectional area of the casting that moves away from the parting line (Fig 3b). This design eliminates the undercut and the need for the outside core, reducing casting cost while improving dimensional accuracy.

One of the major advantages of the metalcasting process is its ability to produce internal cavities and arbitrarily complex shapes. These geometric features are produced in part by the application of cores. However, designers must keep in mind that cores increase the cost of castings and can also affect dimensional accuracy. Whenever possible, designers should design part geometries that will provide the desired functionality, yet eliminate or minimize the number of cores.