Investment casting (lost wax)

Wax copy coated in ceramic, melted out, then filled with metal

Investment casting, also called lost-wax casting, copies a part in wax, coats the wax in ceramic, melts the wax out and pours metal into the hollow shell. Jet-engine turbine blades, golf club heads and hip implants are made this way. You get near-finished parts with fine detail, in almost any alloy.

Photo: 5oclockhustle, CC BY-SA 4.0, via Wikimedia Commons

Investment casting · from wax tree to finished partsRead the top row, then the bottom row; circled numbers match the steps below.

How it works

Make wax copies. Wax is injected into a metal die (a reusable mold) to make a pattern, an exact wax copy of the part. Every casting needs its own pattern.
Build a tree. Many patterns are glued onto a central wax sprue (feeding channel), forming a "tree" that can hold from a few to dozens of parts.
Coat it in ceramic. The tree is dipped in a ceramic slurry (a creamy mix of fine ceramic powder and liquid binder), sprinkled with fine sand and dried. This is repeated about 6 to 10 times, until the shell is several millimeters thick.
Melt out the wax. The shell is turned upside down and heated, usually in a steam autoclave (a pressurized steam oven) or a hot furnace. The wax runs out, often to be reused, leaving a hollow one-piece mold.
Fire, then pour. The shell is fired, often at around 1000 °C, to make it strong and burn off the last wax. Metal is poured in while the shell is still hot, so thin sections fill before the metal freezes.
Break out the parts. After cooling, the shell is knocked off by hammering, vibration or water jets. The parts are cut from the tree, and the gate stubs (where each part joined the tree) are ground smooth.

Key facts

Materials: Steels and stainless steels, nickel and cobalt superalloys, titanium, aluminum, copper alloys, gold and silver
Typical parts: Turbine blades, golf club heads, hip and knee implants, pump impellers, firearm and lock parts, jewelry
Part size: Usually a few grams to a few kilograms; specialist foundries go beyond 100 kg
Precision: About ±0.1 to ±0.25 mm on small features, roughly ±0.5% of a dimension
Surface finish: Smooth: Ra (average roughness) about 1.5 to 3 µm
Batch size it suits: Hundreds to hundreds of thousands of parts a year; also one-offs in art and jewelry
Tooling and machine cost: Wax-injection dies cost a few thousand to tens of thousands of dollars; 3D-printed patterns skip the die for prototypes
Speed: Days to weeks per batch, mostly spent waiting for shell coats to dry; many trees are processed in parallel

Strengths and limits

Strengths

Very fine detail and smooth surfaces, often with little machining
Works with high-melting alloys such as superalloys and stainless steel
Complex shapes in one piece, with little or no seam
Little waste: sprues and runners are remelted

Limits

More steps and labor, so each part costs more than a sand casting
Parts are mostly small; large ones become difficult and expensive
A new wax pattern and a new shell are used up for every part
Long lead time, from days to weeks

Where it's used

AerospaceTurbine blades and vanes, structural brackets, fuel-system parts
MedicalHip and knee implants, surgical instruments
SportsGolf club heads
AutomotiveTurbocharger wheels, rocker arms, small valve parts
IndustrialPump impellers, valve bodies, levers and brackets
Art and jewelryBronze sculptures, rings, dental crowns and bridges

How to recognize it

Crisp detail, sharp lettering and a smooth, slightly satin surface, even in hidden corners.
No mold seam, or only a very faint line left by the wax die.
A small flat, ground spot where the part was cut from the tree (the gate mark).
A complex shape made in one piece that would otherwise need several machined parts.