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Design Considerations

In general, a small consideration in design can lead to substantial differences in costs. Hence the importance of the communication between the designer and the caster.

Shrinkage

The natural shrinkage of cast metal as it cools and solidifies reduce the workpiece dimensions compared to the size of the mold cavity. It also causes induces stresses and distortion. Luckily, the amount of shrinkage of a given metal is predictable and can be compensated for by making patterns slightly oversized.

Shrinkage allowance for metals commonly used in Sand Casting.

Parting Line

The parting line is a continuous line around a part that separates the two halves of the mold. It is a good design practice to have the parting line on a flat plane. Contoured parting lines usually result in making the mold more difficult to disengage, fewer parts per mold, more costly patterns, less accuracy, higher losses, and a need for more skilled molders, all of which increase costs.

Draft

To facilitate the removal of the pattern form the weak, brittle molding sand, the pattern should have some degree of taper, called draft. The amount of draft needed depends on the method of molding and drawing of the patter, the material the pattern is made from, the degree of precision, and the surface smoothness of the pattern.

Casting Soundness

The liquid-to-solid shrinkage can leave a void at the last point to solidify. Near that point, the caster usually adds a riser, a reservoir, designed to contain metal hotter than the casting. Its function is to supply casting during solidification and to contain the final void.
By considering risering in the original design, the parts can be made easier to produce and rejects can be minimized, leading to lower costs. The designer should visualize the direction of solidification and taper sections to ensure that solidification proceeds toward the direction of risers attached to the heaviest sections. This minimizes the chance for voids to occur.

Corners and Angles

The most common defect in casting design is hot spots. They cause weak zones and points of high stress. At corners and angles, on the outer side heat is radiated in two directions, and the metal cools quickly. On the inner side, the sand is heated from two directions, creating a hot spot that retards solidification. Rounding of corner of all types avoids local structural weakness.

Section Changes

The designer should avoid abrupt section changes. Fillets and tapers are preferred to sharp steps. It a section change of over 2:1 thickness ratio is unavoidable, the designer has the two alternatives: design two separate castings to be bolted together later, or use a wedge form between the unequal sections. The taper of the wedged area should not exceed 1:4.

Source: Bralla, Design for Manufacturing Handbook.