Investment casting is a common industrial process that’s been used for thousands of years. It is rooted in both ancient Egypt and China. In antiquity, beeswax was the material of choice, but more advanced wax materials and alloys are now used. Although investment casting yields just one part per mold, it is highly accurate, versatile, and repeatable. Here, we’ll provide a detailed look into what investment casting is.
Able to create complex shapes and parts, the process involves creating a wax pattern that is dipped into a ceramic slurry. The process is named because of this step, as the pattern is “invested” into the slurry, which then forms a hardened mold. Once the refractory ceramic hardens, it is turned upside down and exposed to heat; the wax drains out to create a shell that molten metal is poured into. When the metal cools, the ceramic mold is broken.
Since the wax pattern is melted out, or lost, in the process, the technique is often referred to as “lost-wax casting”.
The Process of Investment Casting
Used for making jewelry, turbine blades, and industrial parts such as gears, ratchets, and cams, investment casting involves these basic steps:
- Wax Pattern Casting: The wax is formed into the shape of the part. It is often the material of choice because wax melts easily and is reusable. A mold or master die is created to manufacture wax patterns and may have cores to form internal features. To produce multiple parts at once, a wax bar (or runner) may connect the molds and allows molten material to reach many parts simultaneously. The channels and molds create a tree-like central wax gating system.
- Creating the Mold: The wax pattern, runners, and sprues (where molten material is later poured into), are dipped into a ceramic refractory slurry typically consisting of fine silica particles, water, and binders. The layer of slurry that covers the part becomes the mold. Dipped several times, the assembly is coated with a layer of material up to 10 mm thick, and then dried. The part is placed upside down and heated to melt the wax, which empties to form a hollow shell.
- Pouring: The mold is preheated at a very high temperature (up to 2,000℉) to improve flow and prevent the mold from being damaged. Molten metal is poured into the gating system to fill the mold cavity. Gravity is usually sufficient for pouring, but a vacuum or pressure may be applied depending on the application. As the assembly cools, the ceramic and metal shrink, improving accuracy. Cooling time depends on the material and its thickness.
- Breaking the Mold: The mold must be broken carefully as to not damage the cast part. This can be done manually with tools or with water jets. Individual castings may be separated from the gating system with cutting, sawing, and impact tools. Sometimes they’re removed via burning or with liquid nitrogen (cold breaking) before cleaned and finished.
- Finishing: Sandblasting, grinding, or welding or other heat treatment may be applied to remove impurities, straighten the part, or smooth out its surfaces. Hydraulic presses may be used for straightening. Secondary machining adds time to the process and can be unsuited for high volume orders. In general, creating a wax pattern to completing the casting process takes about seven days, with much of that involving forming/drying the ceramic shell mold.
3D Printing Technologies
Resin-based patterns can now be made via 3D printing. Stereolithography (SLA) and digital light processing (DLP) are commonly used. These technologies simplify investment casting by taking the place of traditional steps such as mold creation and wax pattern production.
Contact Laszeray Technology
To learn more about our range of manufacturing capabilities, turnaround times, and what investment casting is and how it works, call 440-582-8430 today.