Extended Description: Lost Wax Casting
The following is an extended description of lost wax casting that I wrote as an assignment in the Fall of 2017.
Lost wax casting is a process by which metal objects are made. It begins with a wax object, called a pattern. This wax pattern holds a 1:1 form of what will be a finished object in metal, once cast. A ceramic shell, comprised of a chemical slurry and refractory (heat resistant) sand, is built up around the pattern. Once the wax is eliminated from the shell by means of heating—hence the term “lost wax”—metal can be poured into the shell, assuming the shape left behind by the wax pattern.
To ready the wax pattern for the casting process, other wax structures are attached to the pattern. These structures create the channels in the shell that allow the molten metal to be poured into the cavity left by the pattern, and the channels that allow air to escape from the shell.
- First, a wax cup (a: see diagram above) is attached to the pattern (e) along with wax bars called sprues (c); the cup is where metal will be poured into the empty shell (d), and the sprues form the pathways that direct the metal to the pattern. The sprues are connected to the bottom of the cup, and to the pattern so as to fill the emptied space with metal from the bottom upwards, pushing air out of the cavity as it is filled.
- Second, smaller wax bars called vents (b) are attached to the cup, usually near the lip. These bars are placed in strategic areas where escaping air can get trapped in the shell during the pouring process. These channels serve to allow as much excess air to escape and prevent air pockets from forming in the pattern.
The process of coating the pattern in the ceramic shell can now begin.
First, the entire pattern is dipped into a chemical slurry, with care to ensure that no areas are left uncoated as this can result in weak spots in the shell. Next, the wet slurry-coated pattern is thoroughly sprinkled with a fine grade of refractory sand, and hung to dry; this completes one full layer. To ensure that any detail in the surface of the pattern is fully captured in the construction of the ceramic shell, the first three to four layers of the shell will use the same slurry and fine grade of sand. Once these preliminary layers are complete, the process of building up the shell continues with sand of a larger particle grade. These next layers—anywhere from 10 to 20, depending on the size of the object—create a shell that is strong and less likely to break or crack.
Finally, the dried shell is heated in a foundry oven to cure it and to melt the wax out of the shell. The heated shells are readied for metal by submerging them in sand—which allows them to be held upright securely and to retain their heat—and then the metal is poured into them. This last step is important: the shells must be kept at a temperature that is slightly hotter than the temperature of the molten metal. If they are colder, the metal will chill as soon as it hits the shell, which can result in not enough metal actually filling the shell, creating holes in the pattern.
Once the metal has cooled, the shell can be removed, all of the superstructure can be cut off and the piece can be finished. Lost wax casting is used primarily for production casting, which allows for the efficient casting of multiples.
Images: top image from Read Industrial, diagram my own.