Casting

Casting is both a manufacturing process and the part it produces: molten metal (most often cast iron, aluminum alloys, steel or brass) is poured or injected into a mold, where it solidifies into the required shape. Casting is one of the oldest manufacturing methods and is still cost-effective for geometrically complex parts or large production volumes.

Common metal casting methods include sand casting (large parts, low volumes, inexpensive molds), high-pressure die casting (aluminum and zinc castings, for example for the automotive industry, high volumes), investment casting, also known as lost-wax casting (complex shapes with high accuracy), and gravity casting into permanent metal molds, known as permanent mold casting in the US and gravity die casting in the UK. The choice of method depends on volume, required accuracy, material and geometry.

Castings come with specific design requirements: draft angles for removal from the mold, uniform wall thickness for even solidification, and well-placed gates, risers and vents. Ignoring these rules leads to defects (shrinkage porosity, shrinkage cavities, gas porosity) or to castings that cannot be made at all.

Compared with CNC-machined parts, castings have lower dimensional accuracy without further machining, but they allow complex internal shapes and are economical in series production. The usual practice is to combine a casting with precision machining of the functional surfaces.

Consider a casting for metal parts with complex shapes, internal cavities or ribbing that would otherwise be machined from solid stock with a lot of waste: gearbox and pump housings, valve bodies, flanges, brackets, covers and frames. Die casting is used for aluminum parts in volumes of thousands of pieces, sand casting for larger parts in small batches.

To verify the shape before the mold is built, engineers use prototypes machined from solid stock, or castings made with 3D-printed sand molds or 3D-printed patterns.

A casting rarely works without machining of its functional faces. On the drawing, distinguish as-cast from machined surfaces, specify machining allowances and choose datums for the first setup on the raw casting. The mechanical properties of a casting are lower and less uniform than those of wrought material. Porosity can make pressure-bearing parts leak, or it may be exposed only after machining.

Ask the foundry about achievable tolerances and machining allowances (ISO 8062), the cost and ownership of the pattern equipment or mold, the minimum order quantity and how internal defects are inspected (X-ray, pressure testing). It pays to review the part design with the foundry before the design is finalized.