Investment Casting Process
The Investment Casting Process
Investment casting requires the use of a metal die (usually aluminum), wax, ceramic slurry, furnace, molten metal, and any machines needed for sandblasting, vibratory tumbling (with or without a grinding or polishing media), cutting, and grinding. Like any good investment casting foundry, Deeco Metals follows a process that includes the following steps:
- Pattern creation: Wax patterns are typically produced by injecting molten wax into an aluminum alloy die, and are formed as a single piece. Cores may be used to form internal features of the pattern if any. Several of these patterns are attached to a central wax gating system (sprue, runners, and risers), to form a tree-like assembly. The gating system forms the channels through which the molten metal will flow into the mold cavity.
- Mold creation: The “pattern tree” is dipped into a slurry of fine ceramic particles, coated with medium to coarser particles, generally in powder form, and then dried to form a ceramic shell around the patterns and gating system. This process is repeated until the shell is thick enough to hold the weight and heat of the poured metal. The shell is then cured and, once set, is placed into an oven where the wax is melted out, leaving a hollow ceramic shell that acts as a one-piece mold—hence the name “lost wax casting.”
- Pouring: The mold is preheated in a furnace to approximately 1,000°C (1,832°F) before molten metal is poured from a ladle into the gating system of the mold, filling the mold cavity. Pouring metal for investment casting is typically done manually under the force of gravity, but other methods such as vacuum or pressure are sometimes used.
- Cooling: After the mold has been filled, the molten metal is allowed to cool and solidify into the shape of the final casting. Cooling time can vary depending on the thickness of the investment castings, the thickness of the mold, and the metal used.
- Casting removal: After the metal has cooled, the mold is broken and the casting removed. The ceramic mold is broken either manually or with water jets, though several other methods may be used. Once removed, the investment castings are separated from the grating system by either sawing or cold breaking parts from the tree.
- Finishing: Operations such as grinding, sandblasting, or vibratory tumbling is used to smooth the joints at the gates. Heat treatment is also sometimes used to harden the final investment castings. Once this is completed, the parts are ready for any additional processing, such as machining.
The above process is generally used for small investment castings but has been used to produce complete aircraft door frames, steel castings of up to 650 lbs., and aluminum castings up to 100 lbs. Investment casting is generally more expensive per unit than die casting or sand casting, but has lower tooling costs. An experienced investment casting foundry can produce complicated shapes that would be difficult or impossible to create via die casting, yet, like that process, investment casting typically requires little surface finishing and only minor machining.
Surface Finish, Part Weight & Size Parameters for Lost Wax Castings
For industrial applications, the size limits of investment castings are typically between 3 g (≈0.1 oz.) and 5 kg (≈11 lbs.) Cross-sectional limits are generally from 0.6 mm (≈0.024”) to 75 mm (3.0”). Typical tolerances are 0.1 mm for the first 25 mm (0.005” for the first inch) and 0.02 mm for each additional centimeter (0.002” for each additional inch). A standard surface finish is 1.3 to 4 micrometers (≈50-125 µin) RMS. All measurements, of course, depend on the design and complexity of the part.
Deeco Metals requires a design drawing of the component to determine if the weight and size can be produced via investment casting. If investment casting is not the best method by which to create your part, we have the skills and the know-how to produce high quality, high precision parts using any one of our metal casting services.
Custom Investment Castings for All Applications
Investment castings are used in the power generation industries to produce turbine blades with complex shapes or cooling systems. Blades produces via investment casting can include single-crystal (SX), directionally solidified (DS), or conventional equiaxed blades. No matter what the part application, any investment casting foundry worth their salt is capable of producing a near-net shape with very high dimensional accuracy in small castings, although tolerances tend to increase somewhat with casting size.
Because of the relatively high tooling costs and higher than average total costs, investment castings are usually produced in relatively large production runs.
