Investment Castings

Investment castings, often called lost wax process casting, is regarded as a precision casting process to fabricate near-net-shaped metal parts from almost any alloy and is typically used for the production of components requiring complex, often thin-wall castings. There is a long and rich history associated with investment castings, dating back thousands of years to the production of bonze, copper and gold jewelry, idols and statues as far back as the ancient Eqypt and Mesopotamia, the Han Dynasty in China, the Aztecs in Mexico, and the Benin civilization in Africa. The earliest known text that described the investment castings process was written by a monk, Theophilius Presbyter around 1100 A.D. His writings were used by sculptor and goldsmith Benvenuto Cellini (1500 - 1571) as descibed in his autobiogaphy, for the production of the Perseus and Head of Medusa sculpture that still stands today in Loggia dei Lanzi in Florence Italy. In fact, by the mid 1500's the investment castings technique was considered "ordinary procedure" for making bronze statues and other works of art.

The technique reemerged in the late 19th century when dentists began using the technique to make crowns and inlays, following the publication of a paper by Dr. D. Philbrook of Council Bluffs, Iowa in 1897. It was Dr. William H. Taggers of Chicago, however, who spearheaded the use and growth of investment casting as a modern industrial process following publication of a paper in 1907 that detailed the development of a technique that utilized a wax pattern compound of excellent properties, the development of an investment material and the invention of an air-pressure casting machine

Use of investment castings accelerated significantly during the 1940's as a result of military demands on the machine tool industry. Investment castings proved to be an efficient, reliable and cost effective method for meeting military demand for near net shape precision parts and use of specialized alloys that could not be shaped by traditional methods, or required too much machining. Following the war, the technique expanded into many commercial and industrial applications that used complex metal parts and it was during this time that Bimac emerged and ultimately evolved into one of the leading investment castings providers in the Mid-West.

List of Key Steps In The Investment Castings Process

Investment Castings Wax Injection
The investment castings process begins with fabrication of a sacrificial pattern with the same basic geometrical shape as the intended finished cast part. Patterns are normally made of investment casting wax that is injected into a metal wax injection die (Fig. 1).

Fig. 1 Wax Injection

Assembly
Fabricating the injection die can often require several months of lead time. Once a wax pattern is produced, it is assembled with other wax components on a central wax stick, called a sprue, to form a casting cluster or assembly (Fig. 2).

Fig. 2. Assembly.

Investment Castings Shell Building
The entire wax assembly is then dipped in a ceramic slurry and covered with a sand stucco (Fig. 3), and allowed to dry.

Fig. 3. Shell Building

Dewaxing
The dipping and stuccoing process is repeated until a shell of ~6-8 mm (1/4-3/8 in) is applied. Once the ceramic has dried, the entire assembly is placed in a steam autoclave to remove most of the wax and the remaining amount of wax that soaked into the ceramic shell is burned out in a furnace (Fig. 4).

Fig. 4. Dewax

Investment Castings Metal Casting Pouring
At this point, all of the residual pattern and gating material is removed, and the ceramic mold remains. The mold is then preheated to a specific temperature and filled with molten metal, creating the metal casting (Fig. 5).

Fig. 5. Metal Casting Pouring

Knockout
Once the casting has cooled sufficiently, the mold shell is chipped away from the casting (Fig. 6).

Fig. 6. Knockout

CutOff
Next, the gates and runners are cut from the casting (Fig. 7).

Fig. 7. CutOff

Finish
After minor final postprocessing (sandblasting, machining), the castings - identical to the original wax patterns - are complete and ready for shipment (Fig. 8).

Fig. 8 Finished Casting