Defense and Aerospace industries drive demand for investment castings
The global investment castings market is projected to grow at an accelerated rate over the next five years, mostly due to its applications in the aerospace and defense industries. In a 2018-2025 market analysis, Grand View Research largely attributes the rising demand to the advantages of the investment casting process which include “smooth surfaces, complex geometries, and cost-savings.”
The report also cites the soaring numbers of global air travelers as a major driver for the increased market for investment castings which are used in both simple and complex aerospace systems including commercial transport, regional jets, military aircraft, helicopters, and launch vehicles.
Also known as “Lost Wax Casting”, the process dates back more than 5000 years and used for art and jewelry production over the centuries. But investment casting is now being seen as the future by many industries seeking lightweight thin-wall metal components.
Superior Surface Finish and Tight Tolerances
The precision investment casting process produces high-strength components with fine detail and greater dimensional accuracy than sand and other casting methods. Typically a linear tolerance of
+/-.005inch/inch is standard for investment casting although it varies depending on the size and complexity of the part. Investment castings also have excellent “as-cast” surface finish, reducing or even eliminating the need for machining, and that means huge reductions in cost and lead times.
More Design Flexibility and Alloy Choice
Investment casting also offers designers more flexibility with alloy selection. Common aerospace and defense investment casting alloys include armor steel, aluminum, stainless steel, cobalt, and nickel-base alloys.
The process begins with the production of a wax model or pattern for each part to be cast. The wax patterns are typically made by injecting wax into a metal tool or “die”. But with today’s 3D printing technology, rapid prototype castings can be manufactured in days.
Because it is uneconomical to make small parts one at a time, wax patterns are typically attached to a wax tree or “sprue”. The wax between the pattern(s) and the sprue are called “Gates. These solid wax branches also guide molten metal in the casting operation to form each final product. Larger items can also be cast on their own.
The wax tree is dipped into a ceramic bath or “slurry” to create a shell. After dipping, fine sand or “stucco” is applied to the wet surface. The mold is allowed to dry, and the process is repeated a number of times resulting in a layered ceramic mold, capable of withstanding the stresses of the casting process.
Before pouring metal into the mold, the wax is removed using a flash fire oven, which melts and burns off the wax. The mold is preheated to a specific temperature to prevent the molten alloy from solidifying before the entire mold is filled.
Alloy is melted in a ceramic crucible using a process known as induction and electric resistance melting. A high-frequency electric current creates a magnetic field around the alloy, generating electric fields inside the metal. When the alloy reaches its specified temperature, it is poured into the mold, and the mold is allowed to cool.
Once cool, the shell material is removed from the metal. This is typically done using a hammer or high-pressure water blast. After the shell material has been removed, the parts are cut off the sprue and the gates are ground off.
The investment castings can be finished using a number of means including vibratory/media finishing, belting or hand grinding, or polishing. Using a ceramic mold, the lost wax process produces a smooth finish, averaging 125Ra surface finish as cast and a.005”/inch tolerance. Depending on the application, investment castings can be used in their “net shape” or undergo machining for precision mating surfaces.