Investment Casting

The Investment Casting Process

Investment casting — also known as the lost wax process — is one of the oldest and most precise metal forming processes available, producing near-net shape components with tight dimensional tolerances, excellent surface finish, and the full mechanical properties of the cast alloy. The term "investment" refers to the ceramic shell material applied around the wax pattern to form the mold.

Step 1 — Tooling and Wax Pattern Production

The process begins with a precision-machined wax injection die that defines the geometry of the finished casting, engineered to account for shrinkage during wax cooling, shell expansion, and metal solidification. Molten wax is injected into the die under controlled temperature and pressure, producing a pattern that is a precise replica of the finished part. Wax patterns are carefully inspected for dimensional accuracy and surface quality before proceeding.

Step 2 — Pattern Assembly

Individual wax patterns are assembled onto a central wax sprue — or tree — allowing multiple parts to be cast in a single pour. The gating system design is critical, controlling how metal flows into each cavity and how heat is extracted during solidification to ensure directional solidification and avoid shrinkage defects.

Step 3 — Ceramic Shell Building

The wax tree is coated in multiple layers of ceramic slurry and refractory stucco to form the investment mold. A fine prime coat produces the smooth inner surface that determines casting surface finish, while successive backup coats build structural thickness. A typical shell consists of six to ten coats and requires several days to complete under controlled temperature and humidity.

Step 4 — Dewaxing

The ceramic shell is placed in a steam autoclave that rapidly melts and drains the wax before thermal expansion can crack the shell. The empty shell is inspected for cracks or damage before proceeding.

Step 5 — Shell Preheat and Casting

The ceramic shell is preheated to a controlled temperature to burn out residual wax, strengthen the mold, and achieve the thermal conditions required for proper metal flow and solidification. Molten metal — tested and verified to meet the specified chemistry — is then poured into the preheated shell. Engineered Precision Casting Company uses a master heat system to strictly control alloy chemistry across all pours.

Step 6 — Cooling and Shell Removal

Castings cool under controlled conditions to achieve the desired microstructure. The ceramic shell is then removed by mechanical vibration. Individual castings are cut from the sprue tree and gate stubs are ground flush with the casting surface.

Step 7 — Cleaning and Finishing

Castings are shot blasted to remove surface oxides and residual ceramic, and visually inspected for surface defects. In some cases, additional cleaning with molten caustic is requied to remove casting scale.

Step 8 — Heat Treatment

Most investment cast alloys require post-cast heat treatment to develop specified mechanical properties.  All heat treatment is performed in-house under Engineered Precision Casting Company's NADCAP approved processes.

Step 9 — Straightening

Straightening is performed by skilled personnel to correct the dimensional distortion that occurs during solidification, shell removal, and heat treatment.  This process typically requires the use of hydraulic presses, hand tools and checking and strarightening dies.  Engineered Precision Casting Company employees in-house toolmakers to fabricate these tools and fixtures.

Step 10 — Inspection and Nondestructive Testing

Castings undergo comprehensive dimensional inspection and nondestructive testing including fluorescent penetrant inspection (FPI) for non-magnetic alloys, magnetic particle inspection (MPI) for ferromagnetic alloys, and radiographic inspection (X-ray) to detect internal defects. All testing is performed by NADCAP-approved personnel.

Step 11— Secondary Operations and Shipment

Secondary machining achieves tolerances beyond as-cast capability and produces features such as threaded holes and precision bores. Depending on the application and customer requirements, castings may receive additional surface finishing operations after machining and inspection. Common surface finishing operations include: electropolishing, passivation and anodizing. Once all steps have been completed, castings are packaged and ship with a complete documentation package including material certifications, heat treatment records, NDT reports, and dimensional inspection results.

Is Investment Casting the Right Choice for My Project?

The investment casting process is highly versatile and can accommodate many types of complex designs and alloys. It is suitable for various applications and in many cases, it can be more cost-effective and produce better results than other casting or fabrication methods.