Investment casting (also called lost-wax casting) is a precision casting process that produces parts with surface finishes of 60-125 microinches Ra and dimensional tolerances of ±0.005 in/in — good enough that many parts ship straight from the foundry with no machining. The process is ancient (Egyptians used it 5,000 years ago for jewelry) but remains the go-to method for turbine blades, surgical implants, and firearm components.

The seven-step process:

• Wax pattern injection: Molten wax is injected into a metal die to form a replica of the final part.
• Tree assembly: Multiple wax patterns are welded onto a central wax sprue, forming a "tree" that lets one shell produce many parts.
• Shell building: The tree is repeatedly dipped in ceramic slurry and coated with refractory stucco (zircon, fused silica). Each layer dries before the next; 6-10 coats build a shell 3/8" thick.
• Dewax: The shell goes into an autoclave or flash-fire oven. Wax melts out (and is recycled), leaving a hollow ceramic mold.
• Burnout and preheat: The shell is fired to 1800°F to burn out residual wax and prevent thermal shock during pour.
• Pour: Molten metal — often stainless, superalloys like Inconel 718, or titanium — fills the hot shell by gravity, vacuum, or centrifugal force.
• Knockout: The ceramic shell is broken away (vibration, water jet), parts are cut from the sprue, and gates are ground flush.

Why it wins for complex parts: Because the pattern is sacrificial, you can cast undercuts, internal passages, and intricate geometry that sand casting (which requires pattern withdrawal) cannot. Jet engine turbine blades have internal cooling channels formed by ceramic cores placed inside the wax pattern — those channels would be impossible to machine.

Cost rule of thumb: Tooling for the wax die runs $5,000-$50,000 (vs. $500-$5,000 for sand-cast patterns), but per-part cost is lower above ~500 units because finishing is minimal. Break-even vs. machining from billet is typically 100-500 parts for complex geometry.

Real-world example: A GE LEAP engine HPT blade is investment cast from a single nickel superalloy crystal (directional solidification keeps grain boundaries from forming perpendicular to the centrifugal load). The blade has 70+ internal cooling holes and serpentine passages — formed by a ceramic core leached out with caustic after casting. No other process can produce that geometry in that alloy.

Limitations: Part size is capped around 75 lb for most foundries (shell weight and pour dynamics), wall thickness minimums are ~0.030", and lead times run 8-16 weeks for first articles because of die fabrication.