Casting, forging and extrusion: how metal parts are formed

Before a part is machined, it has to get its rough shape. Three processes give most metal parts that shape: casting pours liquid metal into a mould, forging squeezes hot solid metal between dies, and extrusion pushes metal through a shaped opening. Which one a designer picks decides the part’s strength, its cost and even the details that can go on the drawing.

Casting

Casting is the oldest way to shape metal and still the cheapest way to make complex shapes. Molten metal fills a cavity the shape of the part, then freezes. Engine blocks, cylinder heads, exhaust manifolds, pump housings and machine beds are usually cast.

Sand casting, step by step

1. Make a pattern. A replica of the part, slightly oversize to allow for the metal shrinking as it cools (about 1–2 % depending on the metal), with tapered sides (draft) so it can be pulled out of the sand.
2. Make the mould. Sand is packed around the pattern in two boxes: the cope (top) and the drag (bottom). They separate at the parting line so the pattern can be removed. Sand cores form internal passages.
3. Add the gating system. A pouring cup and sprue lead the metal down, a runner feeds it into the cavity, and a riser holds a reserve of liquid metal that feeds the casting as it shrinks.
4. Pour. The metal is melted in a furnace and poured steadily into the cup.
5. Cool and break out. Once it has solidified, the sand is broken away (shake-out).
6. Fettle and finish. The sprue, runners and risers are cut off, the surface is cleaned, and functional faces and holes are machined.

Other casting methods trade cost for accuracy. Die casting forces aluminium or zinc into a steel die at high pressure: very fast and accurate, but the die is expensive, so it suits large quantities. Investment casting (lost wax) gives fine detail and a smooth finish for small, complex parts such as turbine blades and medical implants.

Advantages

• Almost any shape, including internal passages that no other process can make.
• Very large parts are possible (several tonnes in sand).
• Low tooling cost for sand casting; good for one-offs and small batches.
• Works with metals that are hard to machine or forge, such as grey cast iron.

Limitations

• Lower strength and ductility than forged metal, because the grain structure is coarser and porosity can occur.
• Rough surface and loose tolerances in sand casting; functional faces need machining.
• Defects such as shrinkage cavities, blowholes and cold shuts need careful gating design and inspection.

Forging

Forging shapes metal while it is solid, by hammering or pressing it. Steel is usually forged hot, typically between 1100 and 1250 °C, where it flows easily. The big advantage is strength: the grain of the metal follows the shape of the part, like the grain in wood running along a branch, which gives excellent fatigue resistance.

Closed-die forging, step by step

1. Cut and heat a billet of the right volume.
2. Preform it (by rolling or open-die blows) so the metal is roughly where it needs to go.
3. Forge in the dies. The upper die strikes or presses the billet into the lower die until it fills the impression. Excess metal squeezes out between the dies as flash.
4. Trim the flash in a separate press tool.
5. Heat-treat for the required hardness and strength.
6. Machine the bearing surfaces, bores and threads.

Open-die forging, where the metal is shaped between flat or simple dies without being enclosed, is used for large shafts, rings and blocks.

Advantages

• Highest strength, toughness and fatigue life of the three processes.
• No internal porosity.
• Good material use and consistent properties from part to part.

Limitations

• Dies are expensive and wear, so closed-die forging needs reasonable quantities.
• Shapes are limited: no internal cavities or undercuts, and generous radii and draft are needed.
• Scale forms on hot-forged surfaces, so machining allowance is needed.

Where forging is used

Crankshafts and connecting rods, gear blanks, axle shafts, spanners and hand tools, lifting hooks, aircraft landing gear and highly loaded fittings.

Extrusion

In extrusion, a heated billet is pushed by a ram through a die with an opening the shape of the required cross-section, much like toothpaste from a tube. Everything that comes out has exactly the same profile, and it is cut to length afterwards. Aluminium is by far the most common extruded metal, usually at around 450–500 °C, but copper, brass, steel and plastics are extruded too.

Direct extrusion, step by step

1. Heat the billet and the container.
2. Load the billet into the container in front of the die.
3. Push. The ram forces the billet through the die. In direct extrusion the billet slides along the container wall; in indirect extrusion the die moves into a stationary billet, which needs less force.
4. Cool and stretch the profile as it leaves the press to straighten it.
5. Cut to length and age-harden (for alloys such as 6061 and 6063).

Advantages

• Complex cross-sections, including hollow sections, in one step.
• Low die cost compared with forging or die casting, so even small runs of a custom profile are economic.
• Good surface finish and straightness; often used without further machining.

Limitations

• Only constant cross-sections; holes, slots and end features must be machined afterwards.
• Thin walls and very sharp corners are difficult; minimum wall thickness depends on the alloy and profile size.
• Long lengths need careful handling to avoid bending.

Where extrusion is used

Window and door frames, heat sinks, T-slot framing for machines, rails and tracks, tubes and bars, and the bodies of many enclosures.

Comparison table

How the process changes the CAD model

The forming process should be decided before the detailed model, because each one needs its own features:

• Draft on faces parallel to the pull direction (typically 1–3° for castings, 3–7° for forgings).
• Generous fillets instead of sharp corners, which crack castings and stop metal flowing in forging dies.
• Even wall thickness in castings, so the metal cools evenly and doesn’t form shrinkage cavities.
• Machining allowance on faces that will be finished, shown on a separate “as cast” or “as forged” model and drawing.
• A parting line placed where it doesn’t cross critical features.

A good CAD workflow keeps two configurations of the part: the raw casting or forging for the pattern or die maker, and the finished, machined part for the workshop.