Investment Casting
Investment casting builds a ceramic shell around a wax pattern, melts the wax out and pours metal into the cavity, giving fine detail in almost any alloy.
- Part
- Forming
- Revised
- 2026-08-11
At a glance
- Family
- Metal
- Typical tolerances
- About ±0.005 in (±0.13 mm) on the first inch plus roughly ±0.003 in per additional inch; flatness and straightness are usually the limiting characteristics on long or thin parts
- Surface finish
- Roughly 63–125 µin Ra (1.6–3.2 µm) as cast
- Typical volumes
- 100–50,000 parts per year; single prototypes possible with printed wax patterns
- Lead time
- 6–12 weeks for wax tooling and first article, then 4–8 weeks per production run; printed-wax prototypes in 2–4 weeks with no tool
- Materials
- Metal
What it is
Investment casting — lost wax casting — builds a ceramic shell around an injection-molded wax pattern, melts the wax out, and pours metal into the resulting cavity. Because the pattern is destroyed rather than withdrawn, the process needs no draft and tolerates undercuts, so it produces detail no steel die could release.
It runs the full alloy range: carbon and stainless steels, tool steels, cobalt-chrome and nickel superalloys, titanium, aluminum, and bronze. Typical parts are turbine blades, orthopedic implants, firearm and valve components, impellers, and structural fittings from a few grams to about 100 lb (45 kg).
Standard linear tolerance is roughly ±0.005 in (±0.13 mm) on the first inch, and as-cast surfaces run 63–125 µin Ra — good enough that many features never see a cutting tool. The cost is a multi-step, labor-intensive shell build plus a wax injection tool, which keeps per-part cost comparatively flat as volume rises.
How it works
- Wax tooling. An aluminum wax injection die is cut, sized up to account for both wax and metal shrinkage — total allowance is commonly 1.5–2.5% depending on alloy and geometry.
- Pattern injection. Wax is injected at low pressure and modest temperature, typically around 150–200°F (65–95°C). Internal passages come from soluble wax or preformed ceramic cores.
- Assembly. Patterns are welded to a wax sprue with a hot knife to form a tree. Tree layout is a real engineering decision: it determines how metal feeds every part on it.
- Shell build. The tree is dipped in ceramic slurry, drained, rained with refractory stucco, and dried. This repeats 5–8 times over roughly 24–48 hours. The first coat uses the finest flour — it is what reproduces surface detail — and later coats are coarser, for strength.
- Dewax. The shell is flash-dewaxed in a steam autoclave, typically around 300°F (150°C) under pressure, so the wax melts and runs out faster than it can expand and crack the shell. Wax is reclaimed.
- Burnout and preheat. The shell is fired at roughly 1,600–1,900°F (870–1,040°C), burning out residual wax, sintering the ceramic, and leaving the mold hot — which is exactly what lets sections down to 0.030 in (0.75 mm) fill.
- Pour. Metal is poured into the hot shell, by gravity in air for most steels, or under vacuum for superalloys and titanium that would otherwise oxidize.
- Knockout and cutoff. The shell is vibrated and blasted off, parts are cut from the tree with an abrasive wheel, and gate stubs are ground flush.
- Finishing. Heat treatment, straightening, machining of critical features, and inspection — commonly radiographic or fluorescent penetrant on aerospace and medical work.
The hot-shell pour is the whole trick. Because the mold is near red heat when metal enters, fluidity stays high all the way to the last thin web, which is why investment casting fills sections that would misrun in sand.
Design guidelines
Walls
Minimum wall is roughly 0.030–0.060 in (0.75–1.5 mm) over short spans, with 0.090–0.125 in (2.3–3 mm) a comfortable nominal in steel. Large flat thin panels are harder than the numbers suggest — they distort during solidification — so break them up with ribs or a crown.
Draft is optional, not mandatory
Because the wax pattern is melted out rather than pulled, zero draft is castable. Draft still helps in one place: pulling wax from the aluminum injection die. Give 0.5–1° where it is free, and accept that zero-draft or undercut features may need a split or soluble-core wax tool, which costs more.
Fillets and corners
Use a minimum internal fillet of 0.030 in (0.75 mm), and 0.060 in (1.5 mm) or more at load-bearing junctions. Sharp internal corners crack the ceramic shell during dewax; sharp external corners on the wax pattern break off in handling.
Holes and passages
Cast holes down to roughly 0.060 in (1.5 mm) diameter, with blind-hole depth under about 3–4× diameter. Anything smaller or deeper should be drilled. Long internal passages require ceramic cores that must be leached out afterward — a real cost and lead-time adder.
Gates and witness marks
Every part carries at least one gate stub, ground flush and usually leaving a visible witness. Put the gate on a non-critical, non-cosmetic surface and agree it with the foundry early; moving a gate after tooling is expensive.
Tolerances and datums
Standard tolerance is about ±0.005 in (±0.13 mm) on the first inch plus ±0.003 in per additional inch. Flatness and straightness are the weak axes: long or thin parts move during solidification and are often straightened afterward. Machine critical bores and mating faces rather than casting them to a tight tolerance, and locate on cast datum pads. For achievable machined finishes see the surface finish chart; for pour temperature planning see metal melting points.
| Feature | Recommended | Limit | Why |
|---|---|---|---|
| Nominal wall, steel | 0.090–0.125 in (2.3–3 mm) | 0.030 in (0.75 mm) short spans | Sections fill because the shell is poured hot |
| Draft | 0.5–1° where free | 0° is castable | Only needed to pull wax from the injection die |
| Internal fillet | 0.060 in (1.5 mm) | 0.030 in (0.75 mm) | Sharp corners crack the ceramic shell at dewax |
| Cast hole diameter | ≥ 0.090 in (2.3 mm) | 0.060 in (1.5 mm) | Fine ceramic detail has to survive shell handling |
| Blind hole depth | ≤ 3× diameter | 4× diameter | Ceramic in a deep pocket is unsupported |
| Linear tolerance | ±0.005 in first inch | ±0.003 in per added inch | Wax and metal shrinkage stack across length |
| Thin flat panels | Add ribs or machine flat | — | Unsupported flats distort on solidification |
Cost drivers
Three cost centers: the wax tool, the per-part shell build, and finishing. The wax injection die is a fraction of a die-casting die and can be soft aluminum, so entry cost is moderate. But unlike a die or permanent mold, the shell is consumed every cycle and takes 5–8 dip-and-dry passes across a day or two. That makes investment casting's per-part cost high and relatively volume-insensitive — going from 1,000 to 100,000 parts does not collapse unit cost the way it does in die casting.
Volume breakpoints: below roughly 100 parts, printed wax or printed shells skip the tool entirely and prototypes arrive in weeks. From 500 to 50,000 a year is the sweet spot for a hard wax tool. Above roughly 100,000 in an aluminum or zinc geometry, die casting almost always wins on cost.
- Get more parts per tree. Tree density drives shell, metal, and labor cost per part. Compact parts that nest tightly cost less than sprawling ones.
- Avoid ceramic cores. Leachable cores add tooling, cycle time, and a leaching operation. Redesign internal passages to be castable from outside, or drill them.
- Reduce gate count. Each gate is a cutoff and a grind. One well-placed gate on a non-critical face beats three convenient ones.
- Cast near-net and machine only what matters. The 63–125 µin Ra as-cast surface is acceptable for most non-sealing faces.
- Choose an air-melt alloy where you can. Vacuum-melted superalloys and titanium carry a substantial premium over air-melted stainless and carbon steel.
Questions
6 questionsWhat tolerance can investment casting hold?
Roughly ±0.005 in (±0.13 mm) on the first inch, plus about ±0.003 in per additional inch — around six times tighter than green sand casting. Flatness and straightness on long thin parts are usually the harder characteristics and often need post-cast straightening.
Does investment casting need draft?
No. The wax pattern is melted out of the ceramic shell rather than withdrawn, so zero-draft walls and undercuts are castable. Draft only helps when pulling the wax pattern from the aluminum injection die, and even that can be handled with a split or soluble wax tool at extra cost.
What is the minimum wall thickness for investment casting?
About 0.030–0.060 in (0.75–1.5 mm) over short spans, with 0.090–0.125 in (2.3–3 mm) a comfortable nominal in steel. Thin sections fill because the ceramic shell is poured while still near 1,600–1,900°F, keeping the metal fluid to the end of the flow path.
What metals can be investment cast?
Nearly all of them — carbon and stainless steels, tool steels, cobalt-chrome, nickel superalloys, titanium, aluminum, and copper alloys. Reactive alloys such as titanium and many superalloys are poured under vacuum, which adds cost relative to air-melted stainless.
Why doesn't investment casting get much cheaper at high volume?
The ceramic shell is consumed on every cycle and takes 5–8 dip-and-dry coats over 24–48 hours to build. That per-part labor does not amortize with volume, so unit cost stays comparatively flat while die casting's falls. Above roughly 100,000 aluminum or zinc parts a year, die casting normally wins.
Investment casting or machining from billet?
Machining wins for one-offs, simple prismatic shapes, and anything needing tolerances tighter than about ±0.003 in on every feature. Investment casting wins once the geometry has internal cavities, thin curved webs, or undercuts, and once volumes reach a few hundred, because it removes most of the material before the first cut.