Lost Wax Casting Process: Steps, Materials and Tolerances

📅 October 5, 2026 💬 By Jeff

Last updated: September 2026

In short: lost wax casting is an investment casting process in which a wax pattern is coated in ceramic, the wax is melted out, and molten metal is poured into the hollow shell that remains. The pattern is destroyed to make the cavity, which is where the name comes from: the wax is lost. Because the cavity comes from a disposable pattern instead of a split die, the process produces undercuts, thin walls and fine surface detail that permanent-mould casting cannot. Typical investment castings hold about ±0.1 mm on small features, reach Ra 1.6 to 6.3 µm as cast, and cover parts from a few grams to tens of kilograms.

Key takeaways

Lost wax casting and investment casting are the same family of process. “Lost wax” is the older craft name, “investment” refers to the ceramic slurry the pattern is invested in.
Seven steps: pattern, tree assembly, shell building, dewaxing, burnout and preheat, pouring, knockout and finishing.
The wax pattern is the tolerance carrier. Every defect that starts in the pattern or the shell shows up in the casting.
Aluminium, stainless steel, carbon steel, copper alloys and superalloys all run in the same process. The alloy changes the shrinkage allowance, not the sequence.
3D printing removes the wax injection die, which is what makes low-volume and prototype investment casting economic.

A 3D printed wax pattern of an intake manifold next to the finished aluminium casting it produced — The pattern on the left, the casting it becomes on the right. Everything the process achieves is decided before the metal is poured.

What lost wax casting actually is

Lost wax casting is one of the oldest metal-forming processes still in industrial use, and the reason it survived is simple: it is the only common casting route that can make a complex internal cavity without breaking the mould apart afterwards.

The sequence is always the same. A pattern is made in wax. The pattern is coated, layer by layer, in a ceramic shell. The wax is melted or burned out, leaving a cavity that is an exact negative of the pattern. Metal is poured in. The shell is broken away.

Two consequences follow from that, and they explain where the process wins and where it loses.

The first is geometric freedom. Because the pattern is melted out rather than pulled out, there is no draft angle to respect and no parting line to design around. Undercuts, internal channels, re-entrant angles and thin sections all come out of the same shell.

The second is cost structure. Every part needs its own pattern, and every pattern needs its own shell. Tooling is therefore per part number, not per batch, and the process only becomes cheap when the part is expensive enough or the geometry is difficult enough to justify it. That trade-off, not the metal, is usually what decides whether a part should be investment cast or sand cast.

A note on the name, because it causes real confusion in search. In English the process is called lost wax casting or investment casting. The Chinese term 低温蜡 refers to low-temperature pattern wax, a material choice inside the process rather than a separate process, and it appears in English as low-temperature wax, not as a different method. Pattern waxes are covered in their own section below.

The lost wax casting process, step by step

The seven steps. Steps 1 to 3 set the dimensions, steps 4 to 7 prove them.

1. The wax pattern

Wax is injected into a machined aluminium or steel die under pressure, then cooled and removed. The die determines the pattern’s dimensions, so the pattern carries the shrinkage allowance for the alloy and the ceramic expansion that follows.

The control points are die temperature, injection pressure, hold time and wax batch consistency. A pattern that flashes, sinks or warps at this stage is scrap, and no amount of care later in the process recovers it.

For prototypes, low volumes and design iterations, the die can be replaced by a printed pattern. That route is covered further down.

Wax pattern for a three-cylinder engine block, with core prints for the cylinder bores and water jacket — A pattern for a three-cylinder block. The core prints and the parting-free geometry are exactly what a split die cannot deliver.

2. Tree assembly

Patterns are attached to a central wax runner, or sprue, to form a tree. The tree does three jobs at once: it feeds metal to every cavity, it controls how the metal cools, and it lets a whole batch be handled as one unit.

Gate size and position decide whether the casting fills before it freezes and whether shrinkage porosity lands in the part or in the runner. On a heavy section, a gate that is too small produces a cold shut. On a thin section, a gate that is too large wastes metal and heat.

3. Shell building

The tree is dipped in ceramic slurry, then coated in dry stucco, and dried. That cycle repeats, typically five to eight times, until the shell is thick enough to survive pouring.

The first coats form the face coat that decides the surface finish and stops metal from reacting with the shell. Later coats add strength. Slurry viscosity, stucco grading, drying temperature and humidity are all controlled, because a shell that dries too fast cracks and a shell that dries too slowly delaminates.

What the shell is made of. The face coat decides the surface finish, the backup coats decide whether the shell survives dewaxing.

4. Dewaxing

The shell is heated in an autoclave, usually with steam under pressure, and the wax melts and runs out. Most of it is recovered and reused.

This is the step that gives the process its name, and it is also the step that does the most hidden damage. Wax expands roughly ten times faster than the ceramic around it, so the shell has to be permeable and tough enough to survive that expansion. Low-temperature pattern waxes are chosen partly for this reason: a lower melt point means less pressure on the shell during dewaxing, which matters on thin or long shells.

5. Burnout and preheat

The empty shell goes into a kiln. Residual wax and moisture burn off, and the shell is brought to pouring temperature, often in the 800 to 1,100 °C range depending on the alloy.

Two things matter here. The shell must be hot enough that the metal does not freeze on contact, and clean enough that carbon residue does not gas off into the metal. A shell that is pulled from the kiln and left standing loses both advantages.

6. Pouring

Metal is melted and poured into the hot shell. Gravity pouring is the common route. Vacuum assist and pressure assist are used where thin walls or reactive alloys make gravity insufficient.

Pour temperature, pour rate and the time between removing the shell from the kiln and starting the pour are the three variables that decide whether the batch fills. Reactive alloys are usually melted and poured under vacuum or inert gas.

Pouring molten metal from a ladle into preheated ceramic shells on a lost wax casting line — Pouring into shells that have just come out of the kiln. The window between kiln and ladle is one of the variables that decides whether a thin section fills.

7. Knockout and finishing

The shell is broken away, the castings are cut off the tree, and the gates are ground back. From there the part goes through whatever finishing the drawing calls for: heat treatment, machining, shot blasting, polishing, non-destructive testing or surface treatment.

This is also where defects are found. Porosity, inclusions and incomplete fill are usually detected after knockout, which is why the process controls in steps one to six matter more than inspection at the end.

Pattern waxes and shell materials

Wax is not one material. The choice affects surface finish, dimensional stability, how cleanly the pattern burns out and how much stress the shell sees during dewaxing.

Wax type	Where it is used	What it buys you
Filled injection wax	High-volume production patterns	Lower shrinkage and better dimensional stability
Unfilled injection wax	General production, finer detail	Cleaner flow into thin sections
Low-temperature wax	Thin or long shells, patterns where shell stress is a risk	Lower melt point, less expansion pressure during dewaxing
Repair and assembly wax	Pattern touch-up, tree assembly	Workable by hand without disturbing the pattern
Burnable resins (printed)	Prototypes and low-volume patterns	No die required, burns out like wax in a controlled cycle

Shell materials follow the same logic. The face coat is usually zircon or fused silica, chosen for thermal shock resistance and for how little it reacts with the alloy. Backup coats use coarser, cheaper refractories because their only job is mechanical strength.

The castings it produces

Investment casting is used where the part is difficult, the volume is moderate, or the material is expensive enough that near-net shape pays for itself.

Typical parts include pump impellers and housings, valve bodies and trim, turbocharger wheels, medical instruments, firearm components, aerospace brackets and structural fittings, marine hardware, food and dairy machine parts, jewellery, and dental and orthodontic frameworks.

Sizes run from a few grams to tens of kilograms. Wall thicknesses below about 1.5 to 2 mm become difficult to fill reliably, and the practical minimum depends as much on the alloy and the part envelope as on the drawing. Batch sizes are usually described in the hundreds to low tens of thousands per year. Above that, die casting or forging often takes over.

Large thin-wall wax pattern for an aero engine casing, with datum markings, before shell building — A thin-wall pattern for an aero engine casing. Large, low-stiffness patterns like this one are where the pattern, not the pour, decides the tolerance.

Aluminium and other alloys in lost wax casting

Aluminium is the most common alloy family in investment casting, and it behaves differently enough from steel to be worth separating out.

The workhorses are the Al-Si-Mg grades such as A356 and A357, the general-purpose 356, and the higher-strength 319 family. In European specifications the same family appears as LM6 and LM25. They cast well, hold pressure tightness, and respond to heat treatment.

What changes with aluminium is the shrinkage allowance. Aluminium contracts roughly 1.0 to 1.5 % linearly, so the wax die is made oversized by that amount, while steels run closer to 2 %. Get the allowance wrong and the part is dimensionally correct everywhere except the one feature the customer measures.

Other families that run in the same shells:

Stainless and heat-resistant steels, including 304, 316, 17-4PH, duplex grades and heat-resistant cast grades.
Carbon and low-alloy steels, for valve, pump and general engineering parts.
Copper alloys, including bronzes and brasses, common in marine and architectural work.
Nickel and cobalt superalloys, used in turbine and high-temperature applications.
Tool steels, where a complex mould insert is easier to cast near-net than to machine from solid.

Tolerances, surface finish and shrinkage

Investment casting is a precision process compared with sand casting, but it is not a machining process.

Property	Typical investment casting	Notes
Dimensional tolerance	About ±0.1 mm on small features, or roughly 0.5 % of dimension	ISO 8062 grade DCTG 4 to 6 is a common specification
As-cast surface finish	Ra 1.6 to 6.3 µm	Better than sand casting, not equivalent to machining
Minimum wall thickness	About 1.5 to 2 mm	Thinner walls need vacuum or pressure assist
Linear shrinkage allowance	About 1.0 to 1.5 % for aluminium, around 2 % for steel	Built into the wax die
Draft angle	Not required	The pattern is melted out, not pulled

The practical rule is that a feature which has to hold a tolerance tighter than the table above should be machined afterwards, and the casting should be designed with stock for it.

Common defects and how to avoid them

Defect	What causes it	What controls it
Shell cracking	Wax expansion during dewaxing, weak or wet shell	Low-temperature wax, controlled drying, adequate coat count
Gas porosity	Turbulence in the pour, residue in the shell, damp shell	Clean burnout, correct pouring temperature, calm fill
Shrinkage porosity	Feeding path too small or too far from a heavy section	Gate and sprue design, chills, simulated solidification
Misrun or cold shut	Metal freezes before the cavity fills	Higher pour temperature, faster transfer from kiln, vacuum assist
Hot tearing	Restraint during cooling of a thin section	Even section thickness, gate position, alloy choice
Inclusions	Loose shell or refractory carried into the metal	Slurry control, shell handling, filtered pouring
Wax pattern defects	Flash, sink marks, warp, contamination	Die temperature, injection parameters, wax batch discipline

Almost every defect in that table is traceable to a decision made before the metal was melted. That is the honest summary of investment casting quality control: the pour reveals the problem, it rarely creates it.

Where 3D printing replaces the wax die

The wax injection die is the fixed cost in this process. It is also the reason investment casting has historically been hard to justify for a single prototype or a design that is still moving.

Three-dimensional printing changes the economics at that end of the range. A pattern can be printed directly in a wax-like material or a burnable resin, attached to a conventional tree, and processed through the same shell building, dewaxing and pouring steps. No die, no injection cycle, no tooling lead time.

Three routes are in commercial use:

Printed wax patterns, where the printed part behaves like an injected pattern through shell building and dewaxing.
Printed burnable resin patterns, which need a controlled burnout cycle to avoid residue in the shell.
Printed cores, moulds and masters, used alongside conventional wax patterns where internal geometry or rapid tooling is the constraint.

The trade-off is per-part cost. Below a few hundred units, printing the pattern usually wins. Above that, the die amortises and injection takes over. Where exactly that crossover sits depends on part size, wall thickness and how much the design is still changing, which is the calculation we run with customers before quoting a route. Our investment casting with 3D printing guide works through that comparison in detail, and the wax 3D printers page covers the machines used for printed patterns.

If you would rather not run the whole chain yourself, we print the patterns and pour the castings in-house. Alloys, tolerances and lead times are on the precision casting service page.

Frequently asked questions

Is lost wax casting the same as investment casting?
Yes, in almost all industrial usage the two names describe the same process. “Lost wax” is the older term and describes the pattern that is destroyed. “Investment casting” describes the ceramic shell the pattern is invested in. Some foundries reserve “investment casting” for the modern ceramic shell process and “lost wax” for the historical plaster and solid mould versions, but the steps are the same.

How accurate is lost wax casting?
About ±0.1 mm on small features and roughly 0.5 % of dimension on larger ones, with as-cast surface finish in the Ra 1.6 to 6.3 µm range. ISO 8062 grade DCTG 4 to 6 covers most commercial work. Tolerances tighter than that are normally achieved by machining after casting.

Can you 3D print a pattern for lost wax casting?
Yes. Printed wax patterns and burnable resin patterns go through the same shell building, dewaxing and pouring steps as injected patterns. It removes the die, which is what makes single prototypes and low-volume runs practical.

What metals can be lost wax cast?
Aluminium alloys including A356, A357, 356 and 319, stainless and carbon steels, copper alloys, nickel and cobalt superalloys, and tool steels. Reactive alloys such as titanium need vacuum melting and are handled as a separate process route.

Why is it called lost wax if the wax is reused?
The pattern is lost, not the material. Most of the wax that melts out in the autoclave is recovered and reused, but the shape of the individual pattern is destroyed to create the cavity.

How small can a lost wax casting be?
Parts of a few grams are routine, and dental and jewellery work goes smaller. The limit is usually wall thickness and the ability to fill the section rather than overall size.