Types of Metal Casting Processes: A Practical Guide

When sourcing cast metal components, the process used to make them affects cost, lead time, surface finish, dimensional accuracy, and minimum order quantity. This guide covers the main casting processes so you can compare them against your requirements.

Investment Casting (Lost Wax Casting)

Investment casting produces parts by building a wax pattern, coating it in ceramic, burning out the wax, and pouring molten metal into the cavity. It delivers the best combination of geometric complexity, surface finish, and dimensional accuracy of any casting method. Tolerances: ±0.1–0.25mm as-cast, tightened to ±0.01–0.05mm with secondary CNC machining. Surface finish: Ra 1.6–6.3μm as-cast — often usable without further finishing. Best for: complex geometry, thin walls, tight tolerances, difficult alloys (stainless steel, nickel alloys, duplex steel, cobalt chrome). Typical industries: aerospace, oil and gas, food machinery, marine, medical. Typical tolerances: ±0.1–0.25mm as-cast, tightened to ±0.01–0.05mm after secondary CNC machining. Common materials: stainless steel (304, 316L, 2205 duplex), carbon steel, nickel-based alloys, aluminium, brass.

Sand Casting

Sand casting uses a compacted sand mould to form the cavity. It is the oldest and most widely available casting process — almost any metal can be sand cast, and there is no practical upper limit on part size. Tolerances: ±0.5–1.5mm depending on part size and complexity. Surface finish: Ra 12.5–25μm — rough, usually requires significant machining. Best for: large parts, low volumes, simple geometry, early prototypes where accuracy is not critical. Limitations: poor surface finish, low dimensional accuracy, not suited to thin walls. Typical tolerances: ±0.5–2mm for hot forging, tighter for cold forging with secondary machining. Common materials: carbon steel, alloy steel, aluminium, titanium.

Permanent Mould Casting (Gravity Die Casting)

Permanent mould casting pours metal by gravity into a reusable steel mould without applied pressure. It sits between sand casting and die casting in terms of accuracy and cost. Tolerances: ±0.2–0.5mm. Surface finish: better than sand casting, inferior to die casting. Best for: medium volumes of aluminium or copper alloy parts where die casting tooling cost is not justified.

Die Casting

Die casting forces molten metal under high pressure into a hardened steel die. It is the fastest casting process for high volumes and produces excellent surface finish and repeatability. Tolerances: ±0.05–0.1mm. Surface finish: Ra 0.8–3.2μm — often requires no further finishing. Best for: high-volume production of small to medium aluminium, zinc, or magnesium parts. Limitations: high tooling cost (£10,000–£50,000+), limited to non-ferrous metals, wall thickness constraints, porosity risk in thick sections.

Shell Moulding

Shell moulding uses a thin resin-bonded sand shell as the mould. It produces better accuracy and surface finish than conventional sand casting at moderate cost. Tolerances: ±0.25–0.5mm. Surface finish: Ra 3.2–6.3μm. Best for: medium-complexity parts in iron or steel where investment casting is too expensive but sand casting is too rough.

Process Comparison

Process	Tolerances	Surface Finish	Tooling Cost	Best Volume	Alloys
Investment casting	±0.1–0.25mm	Ra 1.6–6.3μm	Low–medium	1–5,000	Most alloys
Sand casting	±0.5–1.5mm	Ra 12.5–25μm	Very low	1–500	Most alloys
Die casting	±0.05–0.1mm	Ra 0.8–3.2μm	Very high	10,000+	Al, Zn, Mg only
Permanent mould	±0.2–0.5mm	Ra 3.2–12.5μm	Medium	500–10,000	Al, Cu alloys
Shell moulding	±0.25–0.5mm	Ra 3.2–6.3μm	Low–medium	500–5,000	Iron, steel

Which Process Is Right for Your Part?

Choose investment casting if > your part has complex geometry, requires good as-cast surface finish, is made from stainless steel or a difficult alloy, or is needed at low to medium volumes. Choose sand casting if > your part is large, simple, and needed in small quantities where surface finish and tight tolerances are not critical. Choose die casting if > you are producing tens of thousands of small aluminium or zinc parts and can justify the tooling investment. Choose permanent mould if > you need better quality than sand casting allows but cannot justify die casting tooling costs. If you are unsure which process suits your component, send us a drawing and we will advise — with no obligation.

Frequently Asked Questions

What is the difference between sand casting and investment casting?

Sand casting uses a compacted sand mould that is broken away after each pour. It can handle almost any metal and any part size, but produces a rough surface finish and limited dimensional accuracy. Investment casting uses a ceramic shell built around a wax pattern, which burns out before the metal is poured. It produces significantly better surface finish, tighter tolerances, and is suited to complex geometry and difficult alloys that sand casting handles poorly.

Which casting process gives the best surface finish?

Die casting produces the finest surface finish of the main casting processes, typically Ra 0.8 to 3.2 micrometres, but is limited to non-ferrous metals like aluminium and zinc and requires very high tooling investment. Investment casting produces Ra 1.6 to 6.3 micrometres as-cast, which is often acceptable without further finishing, and works across a much wider range of alloys including stainless steel and nickel alloys.

Why is die casting not suitable for stainless steel or nickel alloys?

Die casting relies on forcing molten metal under high pressure into a hardened steel die. The very high melting temperatures of ferrous metals and nickel alloys would destroy the die tooling rapidly, making the process uneconomical. Die casting is effectively limited to metals with lower melting points, primarily aluminium, zinc, and magnesium.

At what production volume does investment casting make economic sense?

Investment casting tooling costs are relatively low compared to die casting, which makes it viable from very small quantities upward. Single prototype parts can be produced using 3D printed patterns instead of wax injection tooling. For production runs, investment casting is typically economical from around 50 parts up to several thousand, depending on part complexity and size. Above around 10,000 parts of a simple design in aluminium or zinc, die casting usually becomes more cost-effective per unit.

Can investment casting produce large parts?

Investment casting is best suited to small to medium parts, typically up to a few kilograms in weight. For very large parts, sand casting is more practical since there is no upper limit on mould size and the tooling cost is much lower. The ceramic shell process in investment casting becomes increasingly difficult to manage reliably as part size and weight increase.

What does the tolerance figure mean in practice?

Tolerance refers to the acceptable variation from the specified dimension. A tolerance of plus or minus 0.25mm means the finished part can measure up to 0.25mm larger or smaller than the drawing dimension and still be acceptable. Tighter tolerances on critical features can be achieved by CNC machining those surfaces after casting. Investment casting as-cast tolerances of plus or minus 0.1 to 0.25mm are sufficient for most functional surfaces, with machining reserved for mating faces, bores, and other precision interfaces.

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