A research team at the Georgia Institute of Technology has developed a new way to perform investment casting. The basic process dates back thousands of years: Molten metal is poured into an expendable ceramic mold to form a part. The mold is made by creating a wax replica of the part to be cast. The replica gets surrounded or “invested” with a ceramic which dries and hardens to form the mold. The wax is then melted out — or lost — to form a mold cavity into which metal is poured and solidified to produce the casting. However, creating the mold currently involves a sequence of six major operations needing expensive, precision-machined dies and hundreds of tooling pieces.
In contrast, the Georgia Tech approach involves a device that builds ceramic molds from CAD data, completing the task quickly and producing few bad parts. The new technique, dubbed large area maskless photopolymerization (Lamp), builds molds layer by layer (each 100-microns thick) by projecting bitmaps of UV light onto a mixture of photosensitive resin and ceramic particles, then selectively curing the mixture to a solid.
After the mold forms, the cured resin is burned away and the remaining ceramic is sintered in a furnace. The result is a ceramic structure into which molten metal — such as nickel-based superalloys or titanium-based alloys — can be poured, producing highly accurate castings.
“We have developed a proofof- concept system which is turning out complex metal parts. It fundamentally transforms the way high-value castings are made,” says Suman Das, director of the Direct Digital Manufacturing Laboratory in the Manufacturing Research Center. “We’re confident our approach can lower costs by at least 25% and reduce the number of waste parts by more than 90%, while eliminating 100% of the tooling.”
A prototype Lamp machine currently builds six typical turbine-engine airfoil molds in 6 hr. Das predicts that a larger machine — currently being built at Georgia Tech and scheduled for installation at a PCC Airfoils facility in Ohio in 2012 — will create 100 molds at a time in about 24 hr.
Although the current work focuses on turbine-engine airfoils, Das believes the Lamp technique will be effective for making many types of intricate metal parts. He envisions companies sending out designs to “digital foundries” and receiving test castings a short time later, much as integrated-circuit designers send CAD plans to chip foundries today.