Investment casting, in which molten metal is poured into an expendable ceramic mold, is one of the world’s oldest manufacturing processes, dating back thousands of years. The mold is formed by using a wax pattern or model—a disposable piece in the shape of the desired part.
The wax model or pattern can be used by itself to cast a single piece or attached to a wax “casting tree” (or gating system), with multiple other pieces for higher volume production. The wax model or casting tree is first dipped into a ceramic slurry, then into a ceramic powder, multiple times, until a thick coa forms around the model and/or tree. The resulting ceramic mold is then cured to achieve the required hardness that finally becomes the mold for the investment casting process.
Investment casting is often referred to as “lost-wax casting” because the original wax pattern is melted out of the mold after the ceramic has cured. The base metal being used is then poured into the remaining cavity, filling the inside shape.
Investment Casting for Simple & Complex Shapes
While investment castings made using the lost-wax process are often produced one part at a time, most of the investment castings Deeco creates are produced in multiples for larger volume production. Investment castings with complex geometries and intricate details can be created with little worry about die wear or dimensional changes.
Other advantages of investment castings include:
- Excellent surface finish
- High dimensional accuracy
- Extremely intricate parts can be cast
- Almost any metal can be used
- No flash or parting lines
Investment Casting Materials
A good investment casting foundry like Deeco Metals can produce near-net castings from nearly any metal. The most commonly used materials for investment castings are aluminum alloys, bronze alloys, magnesium alloys, cast iron, carbon steels, stainless steel, and other nickel alloys.
This process is beneficial for casting metals with high melting temperatures that cannot be molded in plaster or metal. Typical investment castings include parts with complex geometries, such as turbine blades, firearm components, marine parts, and other industrial components.
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 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.
Investment Castings for All Applications
Investment castings are used in the aerospace and 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.
Investment castings are also widely used by firearms manufacturers to fabricate receivers, triggers, hammers, and other precision parts at low cost.
Other industries that regularly utilize investment castings include:
- and many others
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.