By Michael Gwyn, Advanced Technology Institute, North Charleston, South Carolina and Alfred Spada, Executive Editor 

How do you spot a conversion opportunity for casting? On the floor of your manufacturing and assembly operation, you undoubtedly have seen components currently made up of several stamped, wrought or machined metal parts. Could they be redesigned to a single cast metal component for improved performance? How do you determine if the potential performance gain or cost savings would make the redesign viable?

The choice of whether a component is best manufactured as welded, assembled, machined or cast component is based on the component’s geometry, production costs and requirements in application. This article looks at these issues and provides a framework for analyzing weldments and assemblies as possible conversion candidates.

By Edward Vinarcik

Product cost stems from design. A skilled manufacturer may be able to reduce scrap and optimize production processes to achieve near ideal efficiency, but the cost of a product never can be reduced further without improving the design. Major reductions in product cost only can be attained through critical and intentional design efforts.

Hot isostatic pressing could augment selected aluminum castings.

Structural design engineers who work successfully with castings commonly design in a narrow group of casting types poured from familiar alloys (like the family of irons or the 300 series of aluminum) and molded from familiar metalcasting processes (like green sand or nobake). Rules of thumb have been developed over the years for common design situations.

Close inspection of these rules reveals that they sometimes recommend conflicting geometries. For example, the use of gusseting instead of mass for stiffness might be labeled “recommended” in one set of design rules and “poor” in another.

Further, when a design engineer leaves a familiar casting design realm for an unfamiliar one, unexpected trouble may result. For example, let’s say we are moving from ductile iron to aluminum bronze while staying in a familiar metalcasting process, nobake molding. No alarms are sounded among the “rules of thumb,” but there’s likely trouble in the usual “ductile iron-style” geometry. Good aluminum bronze geometry is different than typical ductile iron geometry, and the molding process may need to supplement the different geometry with heat transfer techniques. Not suspecting this, the design engineer’s new casting design may suffer from “no-quotes,” or higher-than-expected prices and requests for design changes.

How are design engineers supposed to know that successfully casting geometry for aluminium bronze should somehow be different? And if a design engineer did know that, what would be the proper course of design action?

The answer lies in a better understanding of the relationship among geometry, various metalcasting alloys and structure.

One of metalcasting’s strongest selling points is its ability to encompass several parts, often in the form of  a welded assembly, into one component. This is possible because the nature of the metalcasting process lends itself to complex geometries. At the heart of many of these complex geometries is a core or core assembly.

A core is a shaped body, usually made of sand, which forms the interior part of the casting, like the cavity the pit makes in the flesh of a peach. In metalcasting, the mold provides a space for the molten metal to go, while the core keeps the metal from filling the entire space.