Casting is a manufacturing process in which molten metal is introduced into a mould or die containing a cavity of the required shape. After the metal solidifies and cools, the casting is removed and may undergo cleaning, heat treatment, machining or other finishing operations as required.
A wide range of ferrous and non-ferrous alloys can be cast, including cast irons, steels, aluminium alloys, copper alloys, magnesium alloys, zinc alloys and other engineering metals. The most suitable casting process depends on factors including the material, component size and geometry, required dimensional accuracy and surface finish, production quantity and tooling cost.
Sand casting is one of the most widely applicable metal casting processes. In the green sand process, moist bonded sand is packed around a pattern to form the mould. The pattern is removed to leave a cavity corresponding approximately to the required component shape.
Cores may be positioned within the mould to form holes, cavities and other internal features. Molten metal is poured into the mould and, after the metal has solidified and cooled sufficiently, the sand mould is broken away and the casting removed.
Sand casting can accommodate a very wide range of component sizes and materials and is particularly useful where the production quantity does not justify expensive permanent tooling. The achievable accuracy and surface finish are generally lower than those obtained using processes such as investment casting or pressure die casting.
Machining allowances may therefore be required on functional surfaces, bearing locations, sealing faces, holes and other features requiring closer dimensional tolerances or improved surface finish.
When designing a component for sand casting, the following general principles should be considered:
A number of variations of sand moulding are used to obtain different mould strength, dimensional capability, production rate and surface characteristics. These include dry sand and chemically bonded moulding systems.
In sodium-silicate/CO2 processes, sand containing a sodium silicate binder can be hardened by passing carbon dioxide through the sand. The resulting mould or core has considerably greater strength than conventional green sand.
Resin-bonded sands are also widely used for mould and core manufacture. The particular binder system and curing process depend on the application, alloy, casting size and foundry practice.
Shell mould casting uses a relatively thin, rigid mould produced from resin-coated sand. The sand is brought into contact with a heated pattern, causing a layer of the resin-coated material to cure and form a shell. The shell halves are subsequently assembled and supported as necessary before the molten metal is introduced.
Compared with conventional green sand casting, shell moulding can provide improved dimensional accuracy and surface finish together with good repeatability. The process requires more specialised tooling and is therefore generally better suited to applications where the improved casting capability and production quantity justify the additional tooling cost.
Lost foam casting, historically also described as full mould casting, uses an expendable pattern manufactured from expanded polymer foam. The pattern remains within the mould and is consumed or displaced as molten metal enters the cavity.
Because the pattern does not have to be withdrawn from the mould, the process can accommodate shapes which would be difficult to produce using a conventional reusable pattern. Complex geometry, bosses, internal features and reduced dependence on conventional pattern draft can therefore be important advantages of the process.
Permanent mould casting uses a reusable mould rather than an expendable sand mould. Metallic moulds are commonly used for non-ferrous alloys, although the mould material and construction depend upon the metal being cast and the required production process.
Reusable tooling can provide improved dimensional consistency, surface finish and production rate compared with many expendable mould processes. The initial tooling cost is relatively high, but this may be offset by lower unit costs when sufficient quantities of components are produced.
Permanent mould processes are generally more restricted by component geometry and tooling requirements than sand casting. The mould must also withstand repeated thermal cycling during production.
Die casting is principally a high-production process in which molten metal is introduced into accurately manufactured reusable dies. Pressure die casting uses pressure to fill the die cavity rapidly and is particularly associated with the quantity production of components in aluminium, zinc and magnesium alloys.
Die casting can provide good dimensional repeatability, relatively thin sections and a good surface finish. Features can often be produced close to their finished dimensions, reducing the amount of subsequent machining required.
The principal disadvantage is the high initial cost of dies and associated production equipment. The process is therefore normally most economical when sufficient production quantity exists to distribute the tooling cost over a large number of components.
Ceramic mould casting uses a refractory ceramic mould or ceramic facing system capable of withstanding the temperature of the molten metal. The process is suited to applications requiring relatively intricate geometry, good surface finish and closer dimensional control than is normally associated with conventional sand casting.
Ceramic moulding techniques vary considerably. The achievable section thickness and dimensional capability depend upon the alloy, component geometry, moulding process and foundry practice and should therefore be confirmed for the particular application.
Investment casting, commonly known as the lost-wax process, is used to produce intricate components requiring good dimensional accuracy and surface finish. A wide range of engineering alloys can be processed and, for suitable components, the amount of subsequent machining can be substantially reduced.
Investment casting is particularly useful for relatively complex shapes, components requiring a good as-cast surface and applications where the cost of extensive machining would otherwise be significant.
The achievable tolerance and minimum wall thickness depend on factors including component size, alloy, geometry and foundry capability. These values should therefore be agreed with the selected investment caster rather than treated as universal process limits.
Centrifugal casting uses a rotating mould. Molten metal introduced into the rotating mould is forced towards the outside of the cavity by the rotation of the system.
The process is particularly suitable for rotationally symmetric and hollow components. Typical applications include pipes, tubes, hollow shafts, cylinders and rolls. Depending upon the component geometry, the rotational axis may be horizontal or vertical.
Centrifugal action can assist the production of dense cast material and is especially useful where a hollow cylindrical component can be produced without a conventional central core.
Continuous casting is used for the continuous production of semi-finished metal sections. Molten metal is fed into an open-ended, water-cooled mould where a solid shell forms around the outside of the emerging section.
The partially solidified product is continuously withdrawn from the mould and subjected to further controlled cooling as solidification progresses. The continuous product is subsequently cut to the required length.
Depending on the material and production system, continuous casting is used to produce semi-finished products such as slabs, blooms and billets which are subsequently processed by rolling, forging or other manufacturing operations.
The table below contains comparative process data retained from the original RoyMech casting reference. It is useful for understanding the relative capabilities of the principal casting processes, but the numerical values should be treated as indicative rather than guaranteed manufacturing limits.
| Casting Process | Relative Labour Cost per Unit | Relative Equipment / Tooling Cost | Surface Finish µm CLA |
Indicative Accuracy | Minimum Section mm |
|---|---|---|---|---|---|
| Green Sand | Medium | Low | 500–1000 | ±2.5 mm | 5.0 |
| Shell | Low | Medium | 100–300 | ±0.25 mm | 2.5 |
| Centrifugal | Low | Medium | 100–500 | ±0.7 mm | 8.0 |
| Investment | High | Medium | 25–125 | ±0.06 mm at 25 mm section* | 0.6 |
| Gravity Die Casting | Low | Medium | 100–250 | ±0.4 mm + 0.05 mm per 25 mm* | 2.5 |
| Low-Pressure Die Casting | Low | High | 40–100 | ±0.05 mm + 0.05 mm per 25 mm* | 1.2 |
| Pressure Die Casting | Very Low | Very High | 40–100 | ±0.05 mm + 0.05 mm per 25 mm* | 0.5 |
| Continuous Casting | Low | High | 100–200 | ±0.12 mm per 25 mm* | 8.0 |
* Legacy RoyMech values retained for reference pending independent verification. The original wording of some accuracy values is ambiguous and they should not be interpreted as current guaranteed process capability.
There is no single casting process which provides the best solution for every component. Process selection should begin with the engineering requirements of the component and then identify the casting methods capable of satisfying them at an acceptable manufacturing cost.
Important considerations include:
A casting should not normally be designed independently of the operations which follow it. Machined datums, bearing seats, threaded holes, sealing faces and other functional features need to be considered when establishing the casting geometry and machining allowance.
Where possible, the designer should also consider how the component will be located and supported during machining. Casting features which provide reliable manufacturing datums or clamping locations can simplify subsequent operations and improve repeatability.
Unnecessary machining should nevertheless be avoided. One of the principal advantages of casting is the ability to produce complex shapes close to their final geometry. Good casting design therefore seeks an appropriate balance between as-cast features and accurately machined functional features.