Design guidelines for investment castings

Wall thickness

Uniform walls are the most important single rule. Sections of similar thickness solidify together, feed each other and distort less. As general industry guidance, steel and stainless walls of roughly 2 to 3 mm are commonly achievable over moderate areas, with thinner walls possible locally or in small parts, and aluminium somewhat thinner. The practical minimum depends on alloy fluidity, the distance metal must flow and the area of the wall, and is confirmed for each part.

Maximum thickness is limited by feeding rather than by filling. Isolated heavy sections freeze last and tend towards shrinkage porosity. Where mass is not needed for function, it should be cored out or replaced by ribs.

Transitions, fillets and corners

Where thickness must change, a tapered transition over a length of several times the thickness difference avoids a hot spot at the junction. L, T and X junctions concentrate mass; offsetting ribs or coring the node reduces it.

Internal corners should always be radiused. Sharp corners concentrate stress during contraction, encourage hot tears and are difficult to coat with slurry. A radius of at least about 1 mm is usual, and larger radii in proportion to wall thickness are preferable at loaded junctions. External corners can be sharper, but a small radius improves shell coverage and edge quality.

Draft

Most surfaces of an investment casting need no draft, since only the wax pattern has to leave a die. Small draft angles on deep pockets and long cores in the die make pattern ejection easier and improve consistency; the toolmaker will propose where they are useful.

Holes, slots and passages

Holes can be cast when slurry and stucco can reach, coat and dry inside them. As a general indication, through holes can be longer in relation to their diameter than blind holes, and holes below a few millimetres in diameter are usually drilled. Narrow slots follow the same logic. Long, narrow or curved internal passages need pre-formed ceramic cores, which add cost and require core-removal access. Threads are normally machined; coarse external threads can sometimes be cast.

Undercuts and internal cavities

Undercuts that would be impossible or expensive in die casting or forging are routine: they are formed by slides in the wax die, by soluble cores or by joining pattern segments. Each adds some tooling or labour cost, so the benefit should be weighed, but function should not be compromised to avoid them.

Ribs and bosses

Ribs stiffen thin walls and help to control distortion. They should be somewhat thinner than the wall they support, radiused at the root and arranged to avoid mass accumulation at crossings. Bosses for holes and threads should be connected to walls or ribs so that they can be fed, and their height kept moderate.

Flatness, straightness and distortion

Large flat, thin areas tend to dish or warp. Ribs, slight curvature, windows or a stepped design improve stability. Long slender parts may need straightening, which should be allowed for in the tolerances; see tolerances.

Gating and feeding considerations for the designer

A casting needs at least one surface where a gate can be attached, preferably on a heavy section and on a face that is machined or not functional. Indicating acceptable and forbidden gate areas on the drawing helps the foundry. Gate witness height and location should be agreed.

Machining allowance and datums

Surfaces that will be machined need an allowance sufficient to clean up after casting tolerance, distortion and any surface layer; depending on size this is often of the order of 0.5 to 2 mm per face as an indication, and required machining allowance grades are defined in ISO 8062-3. Datum targets for the first machining operation should lie on as-cast surfaces that are stable, away from gates and die parting lines, and should be the same points used for dimensional inspection.

Part consolidation and weight reduction

Investment casting often pays for itself by combining parts: brackets welded from several plates, housings assembled from machined blocks, or levers with pressed-in bosses can become one casting. Freedom of shape also allows material to be placed along load paths and removed elsewhere.

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