Deeply Understand the Investment Casting I


The latest investment casting is a successful marriage of ancient and modern technology.

Because it was invented 4000 years ago, you might expect investment casting to be a thoroughly mature metal-forming process. However, it is adaptable, readily lending itself to such modern manufacturing technologies as 3-D modeling, numerical solidification simulation, and rapid prototyping that streamline today’s manufacturing cycle. It stands on equal footing with machining and forging, while still retaining its key historical advantage of being able to form highly complex parts.

The ancients cast jewelry, statues, and works of art with such fine detail that they are best appreciated under a jeweler’s loupe. Details such as logos, identifying text and surface textures can be cast into the body of a part. Draft angles that limit other forms of casting do not apply. Shape options are almost limitless.

Using a 3-D CAD model of the part, investment casters machine an injection mold cavity which duplicates the part geometry. Specialized wax is injected into the cavity to create a part pattern. The pattern is replicated as many times as there are parts to be made. The patterns are arranged in “trees,” then coated with multiple layers of ceramic slurries to form a ‘shell’, capable of accepting the molten metal. Once the wax is melted out of the trees, the ceramic is fired to fuse it into a strong, solid shell, leaving behind vacant cavities within ceramic shells. Into these shells are poured steel, cobalt, titanium, nickel, zirconium, and a number of other alloys.

When the molten metal freezes, what comes out of the shells are parts as complex as involutes, manifolds with curved internal passage, spirals, and unique structures such as turbocharger compressors or biomedical tools with projections or connecting struts.

From a manufacturing standpoint, the second most important aspect of investment casting is its efficiency. No near-net process is closer to the net. Most post forming operations are unnecessary. As parts are broken out of shells, often the only remaining task is cut them free of the gates, then sandblast or peen them to remove blemishes resulting from the melt process. As-cast finishes can be smooth, as fine as 60-90 μin. RMS. Some parts may require a slight straightening or heat-treating process, but secondary machining and joining are usually unnecessary since casting creates and integrates complex part geometries. Investment casting is material efficient, generating far less waste than wrought processes. This becomes increasingly important when using costly alloys. Also, the process is accurate. Dimensional tolerances of ± 0.003 in./in. are readily achieved. This tolerance is critical on parts such as turbocharger compressor wheels that spin at speeds greater than 200,000 rpm and have to be finely balanced. Finally, delicate parts with walls as thin as 0.025 in. can be investment cast.