15 Investment Casting
The earliest known text that describes the investment casting process (Schedula Diversarum Artium) was written around 1100.
Investment casting is an industrial process based on lost-wax casting, one of the oldest known metal-forming techniques. The term “lost-wax casting” can also refer to modern investment casting processes.
Investment casting has been used in various forms for the last 5,000 years. In its earliest forms, beeswax was used to form patterns or casting necessary for the casting process. Today, more advanced waxes, refractory materials and specialist alloys are typically used for making patterns. Investment casting is valued for its ability to produce components with accuracy, repeatability, versatility, and integrity in a variety of metals and high-performance alloys.
The fragile wax patterns must withstand forces encountered during the mold making. Much of the wax used in investment casting can be reclaimed and reused. Lost-foam casting is a modern form of investment casting that eliminates certain steps in the process.
Investment casting is so named because the process invests (surrounds) the pattern with refractory material to make a mold, and a molten substance is cast into the mold. Materials that can be cast include stainless steel alloys, brass, aluminum, carbon steel and glass. The cavity inside the refractory mold is a slightly oversized but otherwise exact duplicate of the desired part. Due to the hardness of refractory materials used, investment casting can produce products with exceptional surface qualities, which can reduce the need for secondary machine processes.
The process can be used for both small castings of a few ounces and large castings weighing several hundred pounds. It can be more expensive than die casting or sand casting, but per-unit costs decrease with large volumes. Investment casting can produce complicated shapes that would be difficult or impossible with other casting methods. It can also produce products with exceptional surface qualities and low tolerances with minimal surface finishing or machining required.
Process
Castings can be made from an original wax model (the direct method) or from wax replicas of an original pattern that need not be made from wax (the indirect method). The following steps describe the indirect process, which can take two to seven days to complete.
Video explanation
Watch this 5:31 video, Learn about Investment Castings by Niagara Investment Castings (2015).
Investment casting is generally used to make castings which are very intricate and/or difficult to machine. Two examples are gold jewelry items such as rings, and industrial parts, such as turbine blades made from difficult to machine high temperature nickel alloys.
Investment castings are made as follows:
1. A pattern of the exact part is molded out of wax or plastic (image 1).
2. The wax pattern is coated with a ceramic paste that is allowed to dry (image 2).
3. The ceramic is then fired in an oven, allowing the wax to flow out (hence the term “Lost Wax” method which is sometimes used).
4. Molten metal is poured into the ceramic mold and allowed to solidify (image 4).
5. The ceramic shell is broken away, leaving a part that requires only cleanup of the sprue (image 5).
Investment casting parts have a very smooth finish, little or no radii and draft, and no parting lines and flash. Investment cast parts require almost no cleanup machining and can be used almost “as cast”.
Image 1
A pattern of the exact part is molded out of wax or plastic
Image 2
The wax pattern is coated with a ceramic paste that is allowed to dry.
Image 4
Molten metal is poured into the ceramic mold and allowed to solidify.
Image 5
The ceramic shell is broken away, leaving a part that requires only cleanup of the sprue.
Derived from: Investment casting – Wikipedia available and accessed August 3, 2024, and The Virtual Machine Shop available and accessed at http://www.jjjtrain.com/vms/eng_castings/eng_castings_03.html via the WayBack Machine internet archive 16 January 2024.