Introduction to Metalcasting
This introduction to metalcasting provides a brief overview and history of the casting process, as well as an outline of common casting techniques in use today. The goal of this article is to give new manufacturers a better understanding of how metalcasting works and what steps are involved in producing cast products. By the end of the article, the reader should have a clear picture of the opportunities presented by metalcasting, and a sound appreciation for its potential as a modern manufacturing method.
Definition of Metalcasting:
A manufacturing process in which molten metal is poured into a cavity (mold) that holds the metal to a specific shape as it solidifies.
1. Create the master pattern and corebox. The first step in metalcasting is to create an object with a similar shape to that of the desired product, called a pattern. The pattern is a tool used to produce the mold, and must be slightly larger than the final product in order to account for shrinkage during solidification. In metalcasting, patterns are often produced through CNC machining. Similarly, cores, as outlined in Step 2, are created using another type of tooling: coreboxes. Casting methods that utilize reusable molds, like permanent mold casting, do not normally require patterns or coreboxes. In these cases, toolmakers can produce the mold and core directly.
2. Create the mold and core. In order to successfully produce a casting, the manufacturer must first create a mold cavity to contain and form the molten metal. In many cases, they must also produce a core that fits inside the mold, forming hollow cavities inside the final part. Molds and cores can be reusable, as in the case of die casting or permanent molding, or single-use, as in greensand casting or shell mold casting.
3. Melt the alloy. Next, the manufacturer must melt the metal. Often the metal used is an alloy, or a mixture of elements that, together, provide optimal mechanical properties. Melting processes vary: in its simplest form, melting consists of placing alloy in a receptacle with a higher melting point and heating it over a burner or open flame. More contemporary melting methods, like induction melting, rely on the physical properties of alloys to melt them and bring them to casting temperature more efficiently.
4. Pour the molten metal into the mold. Once the manufacturer melts the alloy, they must pour the molten metal into the mold cavity. Normally this is accomplished with a ladle system, which supports the pouring process and allows operators to control pouring speed either manually or automatically. Incorrect pouring techniques can lead to defects, as gases can become trapped inside the mold, leading to holes in the resulting casting.
5. Allow the metal to solidify. After pouring, the manufacturer must create optimal conditions for the metal to solidify within the mold. Again, manufacturers must consider shrinkage of the molten metal as it cools, as well as release of gases that build up inside the mold during pouring.
6. Remove the casting from the mold. When the metal has cooled and solidified sufficiently, the manufacturer must remove the casting from the mold. In methods employing single-use molds, operators simply break the mold away from the casting. For reusable molds, special coatings are often utilized to prevent sticking, and characteristics allowing casting removal (like ejector pins) must be designed into the mold.
7. Finish the casting. Most cast parts must undergo finishing processes like cleaning (removing excess material from the part); gate removal (removing the pouring channels from the part); heat treatment (heating and cooling the castings in a controlled environment to augment physical properties) and inspection for quality control. Many products also pass through CNC machining processes to improve dimensional accuracy and aesthetic value.
History of Metalcasting
Much of what we know today about the origins of metalcasting is based on archaeological findings. Fortunately, castings’ durability means that many casting artifacts have survived for thousands and thousands of years.
Early castings were often artistic statues, tools and weapons, and the first metals used in casting were copper and tin, or an alloy made from the two metals called bronze. Civilizations on the Asian and European continents experimented with other materials and continued to utilize casting for thousands of years. As processes were refined and techniques recorded in precise language, the scope of cast products also expanded. In the Middle Ages, European foundries were tasked largely with producing church bells.
Pictured at right is the Gloucester Candlestick, an elaborate lost-wax (investment) casting produced for the Gloucester Cathedral in England between 1104 and 1113 AD.
In the 20th century, automotive manufacturing–led in part by Ford Motors’ flow production techniques–increased demand for castings and led to further breakthroughs in process. Steel was invented, and by the middle of the 20th century stainless steel was widely used for casting. Both World War I and World War II tested the limits of casting in the United States, Europe and Japan. Old processes, like investment casting, were further refined to produce better castings faster. New processes, like die casting and shell mold casting, were also developed during the 20th century.
