While the casting properties section was the metalcasting engineering spectrum of geometry for the benefit of design engineers, the structural properties section is the design engineering spectrum of geometry for the benefit of metalcasting engineers. Geometry found between these two spectrums offers boundless opportunity for castings.

Structural Geometry
Because castings can easily apply shape to structural requirements, most casting designs are used to statically or dynamically control forces. In fact, castings find their way into the most sophisticated applications because they can be so efficient in shape, properties and cost. Examples are turbine blades in jet engines, suspension components (in automobiles, trucks and railroad cars), engine blocks, airframe components, fluid power components, etc.

When designing a component structurally, a design engineer is generally interested in safely controlling forces through choice of allowable stress and deflection. Although choice of material affects allowable stress and deflection, the most significant choice in the designer’s structural arsenal is geometry. As we will see, geometry directly controls stiffness and stress in a structure.

The casting processes are limitless in their combined ability to allow variations in shape. Not many years ago, efficient structural geometry was limited by the designer’s ability to visualize in 3-D. Now, computer generated solid models and rapid prototypes are greatly enhancing the designer’s ability to visualize structural shapes.

Improved efficiency in solid modeling software has led to an interesting design dilemma. Solid models are readily applicable to finite element analysis (FEA) of stress. FEA enables the engineer to quickly evaluate stress levels in the design, and solid models can be tweaked in shape via the software so geometry can be optimized for allowable, uniform stress.

However, optimum geometry for allowable, uniform stress may not be acceptable geometry for castability. When a metalcasting engineer quotes a design that considered structural geometry only, requests for geometry changes are likely. At this point, the geometry adjustments for castability may be more substantial than the solid model software can “tweak.” The result can be no-quotes, higher-than-expected casting prices, or starting over with a new solid model.

A practical solution to this problem is to concurrently engineer geometry considering structural, metalcasting and downstream manufacturing needs. The result can be optimal casting geometry. The most efficient technique is to make engineering sketches or marked sections and/or views on blueprints. The idea is to explore overall geometry before locking in to a solid model too quickly. Engineering sketches or mark-ups are easy and quick to change—even dramatically—in the concurrent brainstorming process; solid models are not. A solid model should be the elegant result, not the knee-jerk start.