Note: This article is based on "Thinking Thin With Green Sand Cast Iron," first published in Engineered Casting Solutions, Winter 2004.

When design engineers are trying to decide between the use of iron or aluminum in their components, the biggest impediment to using cast iron is that iron components are often thicker than necessary, resulting in added weight and reduced energy efficiency. In automotive applications, where weight reduction is a paramount achievement, this all but eliminates the iron component from contention.

When only sand casting can produce the component you need, surface finishes can turn out a bit rocky. But, metalcasters do have a few tricks up their sleeves.

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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.