Investment Casting
Also: lost wax casting, precision casting, lost-wax, ceramic shell casting, investment cast parts, cire perdue, precision investment casting
Lost-wax investment casting: a ceramic shell around a wax pattern gives fine detail, thin walls and near-net shape in steel, stainless, superalloys and aluminum.
113 shops tagged Investment casting among 514 metal casting companies in the Noramark directory
A wax replica of the part is injected in an aluminum die, assembled with others onto a wax sprue (a tree), dipped repeatedly in ceramic slurry and stucco until a shell 6-10 mm thick has built up, then steamed out in an autoclave so the wax leaves the shell empty. The shell is fired and filled with metal, in air for steel and aluminum and under vacuum for superalloys and titanium, and broken away when the metal is solid. The parts are cut from the tree, the gate witness is ground, and the castings are heat treated, straightened, inspected and machined. For prototypes and small runs the wax is 3D printed instead of molded, which removes the die.
Because the pattern is destroyed, the shell has no parting line and no draft is required, so undercuts, thin walls, fine lettering, and cast-in holes and slots come out as modeled, with a surface of 1.6-3.2 µm Ra (63-125 µin) that equals a machined finish. Tolerances of ±0.13 mm (±0.005") on the first 25 mm (1") mean many surfaces are used as cast. The process pours nearly every alloy: 17-4 PH, 304, 316, 410, 420, 440C, 4140, 4340, 8620, carbon steel, Inconel 718 and 625, cobalt-chrome, titanium, aluminum A356 and C355, and bronze.
It is a labor-intensive process, so the cost per part falls slowly with quantity and the lead time is weeks. Investment casting pays when the alternative is machining a complex shape from a hard alloy, when a weldment can be consolidated into one casting, or when the volume is a few hundred to tens of thousands a year in a shape too detailed for sand casting. Simple shapes, very large parts and high-volume aluminum belong elsewhere.
At a glance
| Typical tolerances | Linear ±0.13 mm (±0.005") on the first 25 mm (1"), plus about ±0.05 mm (±0.002") for each further 25 mm; ±0.08 mm (±0.003") on small features with care. Flatness and straightness about 0.1 mm per 25 mm (0.004"/in), improved by straightening. Hole diameters ±0.1 mm (±0.004"). In ISO 8062 terms, CT4-CT6. Machined features ±0.025 mm (±0.001") after machining. |
|---|---|
| Size limits | From under 1 g to about 50 kg (110 lb) at most foundries, with a few casting to 150 kg; envelopes to about 600 mm (24"). Minimum wall 1.0-1.5 mm (0.040-0.060") over small areas and 2-2.5 mm (0.080-0.100") generally. Cast holes from about 1.5-2 mm (1/16") diameter, blind holes to 1-2 times the diameter deep, through holes to 3-4 times. Lettering and detail down to 0.3-0.5 mm. |
| Surface finish | As cast 1.6-3.2 µm Ra (63-125 µin), and 0.8 µm Ra (32 µin) with a fine first shell coat; gate witnesses are ground and the parts are bead or sand blasted as standard. Surfaces can be used as cast, polished, electropolished or passivated; steel castings are plated, black oxided or painted; machining is reserved for fits and datums. |
| Lead time | Wax die 3-8 weeks, first samples 2-4 weeks after; with printed wax patterns, samples in 2-4 weeks with no die. Production 6-12 weeks, since the shell build alone takes 1-2 weeks; aerospace work with NDT and certification longer. |
Typical of U.S. job shops; confirm with the shop for your part.
Materials
Stainless leads: 17-4 PH (cast as CB7Cu-1, ASTM A747, heat treated to H900-H1150), CF8 and CF8M (cast 304 and 316, ASTM A351 and A743), 410, 420 and 440C for hardened parts, and 2205 duplex. Carbon and low-alloy steels 1020-1050, 4140, 4340 and 8620 (ASTM A732) for hardware, gears and firearm parts. Nickel and cobalt superalloys (718, 625, Waspaloy, Stellite) and titanium Ti-6Al-4V (in specialized foundries) for aerospace and medical parts. Aluminum A356-T6 and C355 (ASTM B618), copper alloys C954 aluminum bronze, silicon brass and bronze for marine and pump parts. Wrought 6061, 7075 and the free-machining grades have no investment-cast equivalent.
What drives the cost
- Wax die tooling, or printed pattern cost per piece for small runs
- Weight and alloy: superalloys and titanium are vacuum melted and cost many times steel
- Shell complexity: thin walls, cores and deep pockets slow shelling and lower yield
- Heat treatment, hot isostatic pressing and straightening
- Inspection and certification: radiography, penetrant testing, chemistry and test bars, AMS or ASTM certs
- Machining of fits and datums
- Quantity, which spreads the die cost but reduces the labor per part only slowly
When to use it
- Complex shapes in hard-to-machine alloys: 17-4 PH, 316, Inconel, titanium
- Thin walls, fine detail, undercuts and cast-in features that would need many machining setups
- Consolidating a weldment or a machined assembly into one part
- Volumes of a few hundred to tens of thousands a year, or prototypes from printed patterns
- Aerospace, medical, firearms, pump, valve and food-equipment components
When not to
- Simple shapes that machine quickly from bar or plate
- Parts over about 50 kg (110 lb): sand casting or forging
- Very high volumes of aluminum or zinc parts: die casting
- Heavy iron parts with loose tolerances: sand casting
- Tolerances tighter than ±0.05 mm (±0.002") without machining
Design tips
- Keep walls 2-3 mm (0.080-0.120") and uniform; feed thick sections from a gate and avoid isolated heavy bosses.
- Use fillets of 0.8-1.5 mm (0.030-0.060") and radii on every edge; sharp corners crack the shell.
- Avoid deep blind holes and long narrow slots, which need ceramic cores; keep cast holes to 1-2 diameters deep.
- Let the foundry choose the gate location, but say which surfaces are cosmetic and which will be machined.
- Leave 0.75-1.5 mm (0.030-0.060") of machining stock on fits and datums, and cast in locating pads.
- Specify the alloy by the casting spec (ASTM A747 CB7Cu-1, A351 CF8M, A732, B618), the heat treatment, and the acceptance standard for radiography or penetrant testing.
- Dimension from cast datums that the CMM can pick up, not from the machined surfaces.
- Send a STEP model; ask about printed-pattern samples before committing to a wax die.
Frequently asked questions
- How much does investment casting cost?
- Wax dies run $3,000-30,000 depending on size and complexity, or nothing if the patterns are printed. Parts are priced by weight, alloy and complexity: a small stainless bracket might be $5-15 each in hundreds; a 2 kg Inconel part several hundred dollars. It is dearer per part than sand or die casting and cheaper than machining a complex shape from a hard alloy.
- What tolerances does investment casting hold?
- About ±0.13 mm (±0.005") on the first 25 mm (1") and ±0.05 mm (±0.002") for each further 25 mm, with ±0.08 mm (±0.003") on small features when the foundry is told which ones matter. Fits and datums that need better than that are machined.
- Investment casting vs die casting?
- Investment casting pours steel, stainless, superalloys and aluminum with no parting line, fine detail and low tooling cost, at a high labor cost per part; die casting pours only aluminum, zinc and magnesium, needs a $20,000-200,000 steel die, and makes parts in seconds. Below a few thousand parts a year or in any ferrous alloy, investment; above that in aluminum or zinc, die casting.
- Investment casting vs CNC machining?
- Machining wins on tolerance, lead time and simple shapes; investment casting wins on complex geometry in hard alloys, on material yield and on cost above a few hundred pieces. Most investment-cast parts are still machined on their fits, so the comparison is casting plus light machining against machining from solid.
- What is the minimum wall thickness for investment casting?
- About 1.0-1.5 mm (0.040-0.060") over small areas in steel and stainless, 2-2.5 mm (0.080-0.100") in general, a little more in aluminum. Thin walls need short flow paths and good gating; ask the foundry before designing a large thin panel.
