Investment casting is a manufacturing process that produces highly detailed metal parts by pouring molten metal into a ceramic mold built around a wax model. The wax is melted out before casting, which is why the process is also called “lost wax casting.” It’s one of the oldest metalworking techniques in existence, used for at least 6,000 years to create sculpture and jewelry, and now responsible for everything from jet engine turbine blades to hip replacement implants.
The word “investment” doesn’t refer to money. It comes from an older meaning of “invest,” to surround or clothe. The wax pattern is literally “invested” (coated) in layers of ceramic material to form the mold.
How the Process Works
Investment casting starts with a wax replica of the final part. This wax pattern is typically injection-molded using a metal die, so it can be reproduced identically thousands of times. Multiple wax patterns are often attached to a central wax rod called a sprue, forming a tree-like assembly. This lets a foundry cast several parts at once.
The wax assembly is then dipped repeatedly into a ceramic slurry and coated with fine sand or stucco between each dip. Common ceramic materials include silica sand, alumina, fused silica, and zirconium silicate, bound together with agents like ethyl silicate. Each layer dries before the next is applied, gradually building a hard ceramic shell around the wax. The number of layers depends on the size and complexity of the part, but a finished shell is typically several millimeters thick.
Once the shell is complete, it goes into a furnace or autoclave. The heat melts and drains the wax (hence “lost wax”), leaving behind a hollow ceramic mold that perfectly mirrors every detail of the original pattern. The empty mold is then fired at high temperature to cure the ceramic and burn off any remaining wax residue. Molten metal is poured into the hot mold, filling every cavity. After the metal solidifies and cools, the ceramic shell is broken away, the parts are cut from the sprue, and any remaining gate material is ground off. In many cases, the finished parts need little or no additional machining.
Compatible Metals and Alloys
One of investment casting’s biggest strengths is the range of metals it can handle. Because the ceramic mold can withstand extreme temperatures, the process works with alloys that would be difficult or impossible to machine from solid stock. Commonly cast materials include:
- Stainless steel (both 300 and 400 series)
- Carbon and low-alloy steel
- Aluminum alloys
- Nickel-based superalloys (used heavily in aerospace)
- Cobalt alloys
- Copper, brass, and bronze
Nickel and cobalt superalloys are particularly significant. These materials are engineered to perform under extreme heat and stress, which makes them ideal for turbine components but nearly impossible to shape with conventional cutting tools. Investment casting can form them into complex, near-final shapes directly.
Precision and Surface Finish
Investment casting produces parts with tighter tolerances and smoother surfaces than most other casting methods. For dimensions up to 1 inch, the standard linear tolerance is ±0.010 inches. For each additional inch up to 10 inches, you can expect about ±0.003 inches per inch. So a 5-inch dimension would hold ±0.022 inches. Beyond 10 inches, tolerances widen to roughly ±0.005 inches per inch. Wall thickness is an exception: it carries a minimum tolerance of ±0.020 inches regardless of the dimension.
Surface finish on as-cast parts typically falls in the 60 to 200 RMS micro-inch range. That’s smooth enough for many functional applications without any secondary finishing. For context, a machined surface often targets 63 RMS or better, so investment castings can come close to machined quality straight out of the mold.
Wall Thickness and Size Limits
How thin you can cast a wall depends on the metal and the length of the section. Longer, thinner sections are harder to fill completely because the molten metal cools as it flows. For stainless steel (300 series), minimum wall thickness can go as low as 1.0 mm. Cobalt-based alloys and beryllium copper can reach 0.75 mm. Aluminum and brass sit at about 1.0 mm, while low-carbon steel requires at least 1.8 mm. These are general guidelines; a longer section of the same alloy will need a thicker wall to ensure the metal fills the mold completely.
Part weight in investment casting ranges from fractions of an ounce up to several hundred pounds, though most parts fall under about 50 pounds. Extremely large parts push the limits of shell strength and furnace capacity.
How It Compares to Sand Casting
Sand casting is the most common alternative, and the two processes make different tradeoffs. Sand casting packs sand around a reusable pattern, then splits the mold open to remove the pattern before pouring. This leaves a visible seam (parting line) on the finished part, and the rough texture of the sand transfers directly to the casting surface. Sand-cast parts also need draft angles, tapered surfaces that let the pattern pull free from the compacted sand without dragging.
Investment casting avoids all of these issues. The ceramic shell is broken away rather than split, so there’s no parting line. The smooth ceramic produces a smooth casting. And because the wax is melted out rather than physically removed, there’s no need for draft angles. Engineers can design parts with undercuts, internal passages, and thin walls that would be impractical in sand casting. Internal cavities that require multiple sand cores can often be formed directly in the wax pattern.
The tradeoff is cost. Investment casting involves more steps, more materials, and more time per part. Sand casting is cheaper per unit, especially for large or simple parts, and modifying a sand-casting pattern (often made of wood) is faster and less expensive than re-tooling a wax injection die. For lower-volume production of complex, high-precision parts, investment casting usually wins. For higher volumes of simpler shapes, sand casting is often the better choice.
Where Investment Casting Is Used
Modern investment casting took off during World War II and accelerated rapidly with the rise of jet engines. Turbine blades remain one of its signature applications: they require exotic superalloys, complex internal cooling channels, and extremely tight tolerances, all of which play to the process’s strengths. Today, aerospace components account for a large share of the investment casting market, including structural airframe parts, fuel system components, and engine housings.
Medical devices are another major sector. Investment castings appear in surgical instruments, orthopedic implants like hip and knee replacements, MRI machine components, operating room equipment, and wheelchair parts. The process is well suited to medical work because it can produce biocompatible alloys in complex shapes with the smooth surfaces that medical applications demand.
Beyond those two industries, investment casting shows up in turbocharger wheels, golf club heads, firearm components, valve bodies, pump impellers, and electronic enclosures. Any application that needs a complex shape in a high-performance alloy, with good surface finish and minimal machining, is a candidate for the process.