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Avoid Sharp Corners and Angles

Solidification of molten metal always proceeds from the mold face, forming unbalanced crystal grains that penetrate into the mass at right angles to the plane of cooling surface. A simple section presents uniform cooling and greatest freedom from mechanical weakness. When two or more sections conjoin, mechanical weakness is induced at the junction and free cooling is interrupted, creating a “hot spot,” the most common defect in casting design.

In designing adjoining sections, replace sharp angles with radii and minimize heat and stress concentration. Some examples of improved designs are shown in Fig. 2.

Fig. 2: Designs that incorporate sharp corners will result in structural weaknesses due to localized hot spots at the junctions, however, adding radii that are too large also may result in shrinkage defects (sections that are larger will feed thinner walls, leaving a void in the thick junction). By incorporating small fillet radii, hot spots are avoided, assuring improved strength.

Examples of improved cast iron designs - illustration

Engine cylinder parts such as steam-jacketed cylinders often are ill-designed with localized casting weaknesses due to sharp corners (Fig. 3). When engineering a cored part, avoid designs that do not have a cooling surface.

Fig. 3: These schematics show how a steam-jacketed cylinder design was improved with rounded junctions.

How a steam-jacketed cylinder design was improved with rounded junctions

Streamlining an exterior may result in a heavy section at junctions. Thin, elongated cores forming the cavities within the component result in a heat concentration accentuating the heavy sections. The resulting shrinkage defect causes leakage. A rounded junction will offer uniform strength properties.

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This article was adopted from a section of Casting World Magazine. Reprinted with permission.

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