Introduction to Metal Casting

• Posted by wang jun June 6, 2022 Filed in Business #Metal Casting  74 views
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  This Introduction to Metal Casting provides a brief overview and history of the casting process, as well as an overview of common casting techniques in use today. The purpose of this article is to provide new manufacturers with a better understanding of how metal casting works and the steps involved in producing cast products. At the end of the article, the reader should have a clear understanding of the opportunities presented by metal casting and a full appreciation of its potential as a modern manufacturing method.

   

  What is metal casting?

  The term metal casting refers to the process of forming a product or part from molten metal using any type of mold to control its final shape. Metal castings, on the other hand, are the products or parts that result from the metal casting process. In this article and elsewhere on our website, we use the word casting interchangeably, either as a substitute for the metal casting process or as the object of a metal casting.

   

  Definition of metal casting.

  A manufacturing process in which molten metal is poured into a cavity (mold) that holds the metal in a specific shape while it solidifies.

   

  General steps of metal casting

  1. Create the master pattern and core box. The first step in metal casting is to create an object, called a pattern, that resembles the shape of the desired product. The model is the tool used to produce the mold and must be slightly larger than the final product to account for shrinkage during solidification. In metal casting, the pattern is usually produced by CNC machining. Again, as described in Step 2, the core is created using another tool: the core box. Casting methods that use reusable molds, such as permanent mold casting, usually do not require a model or core box. In these cases, the toolmaker can produce the mold and core directly. 2.

   

  2. Create molds and cores. In order to successfully produce a casting, manufacturers must first create a mold cavity to hold and form the molten metal. In many cases, they must also produce a core that fits inside the mold, thus creating a hollow cavity within the final part. Molds and cores can be reused, such as in die casting or permanent molding, or used on a one-time basis, such as in wet sand casting or shell mold casting.

   

  3. Melt the alloy. Next, the manufacturer must melt the metal. The metals typically used are alloys or mixtures of elements that together provide the best mechanical properties. The melting process varies: In its simplest form, melting consists of placing the alloy in a container with a high melting point and heating it over a burner or open flame. More modern melting methods, such as induction melting, rely on the physical properties of the alloy 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. This is usually done with a ladle system that supports the pouring process and allows the operator to control the pouring rate manually or automatically. Improper pouring techniques can lead to defects, as gases can become trapped inside the mold, causing holes in the final casting.

   

  5. Allow the metal to solidify. After pouring, the manufacturer must create the best conditions for the metal to solidify inside the mold. Again, the manufacturer must consider the shrinkage of the molten metal as it cools, as well as the release of the gases that accumulate inside the mold during the pouring process.

   

  6. Remove the casting from the mold. Once the metal has cooled and solidified sufficiently, the manufacturer must remove the casting from the mold. In the case of methods using disposable molds, the operator simply removes the mold from the casting. For reusable molds, special coatings are often used to prevent sticking, and features in the mold must be designed to allow removal of the casting (e.g., ejector pins).

   

  7. Complete the casting. Most castings must undergo finishing processes such as cleaning (removing excess material from the part); gate removal (removing pour channels from the part); heat treatment (heating and cooling the casting in a controlled environment to enhance physical properties) and quality control checks. Many products also undergo CNC machining processes to improve dimensional accuracy and aesthetic value.

   

  History of Metal Casting

  Gloucester candlestick: an exquisite example of medieval investment casting Much of what we know today about the origins of metal casting is based on archaeological finds. Fortunately, the durability of castings means that many cast artifacts have been preserved for thousands of years.

   

  Early castings were often artistic statues, tools and weapons, and the earliest metals used for casting were copper and tin, or alloys made from these two metals called bronze. Civilizations on the Asian and European continents experimented with other materials and continued to use casting for thousands of years. As processes improved and techniques were recorded in precise language, the range of cast products expanded. In the Middle Ages, the main task of European foundries was to produce church bells.

   

  In the 20th century, automobile manufacturing-led in part by Ford's assembly-line production techniques - increased the demand for castings and led to further breakthroughs in the process. Steel was invented, and by the mid-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. Older processes, such as investment casting, were further improved to produce better castings faster. New processes such as die-casting and shell casting were also developed in the 20th century.

   

  Metal Casting Processes

  

  The following metal casting processes represent some of the most common and versatile techniques in the library of casting facilities. Some methods date back thousands of years, while others are less than a century old, and each of these processes is an ideal casting method for certain types of products. Here, we provide a brief overview of each process. For more in-depth information, please click on the links provided in each section.

   

  Investment casting

  The oldest metal casting technique still used on an industrial scale, investment casting is an ingenious process that involves covering a wax mold shaped like the final product in refractory material, then melting the wax and replacing it with molten metal.

   

  Greensand Casting

  Greensand casting is a versatile metal casting technique with low mold cost in which moist clay-bonded sand is compacted around a pattern to form a mold. The resulting mold can be used to cast almost any alloy, and much of the sand used to form the mold can be recovered and recycled.

   

  Shell Mold Casting

  Shell casting, also known as shell casing or shell casting, was invented by a German engineer in the mid-20th century and subsequently perfected by American and German manufacturers. The process involves coating the pattern with resin-coated sand grains that are bonded to each other by heating. Shell casting provides a high degree of accuracy and excellent repeatability.

   

  Permanent Mold Casting

  As the name implies, permanent mold casting uses reusable molds rather than the disposable molds involved in sand casting. The production of tools can be complex and the alloy is somewhat limited by the melting temperature, but high volume production allows for a lower cost per piece. Reproducibility, surface finish and accuracy are excellent in permanent mold castings.

   

  Air Set Casting

  Another variation of sand casting, air-set casting - also known as self-hardening casting - utilizes chemically bonded sand that solidifies at room temperature. The process can be more time-consuming than wet sand casting or shell molding because of the time required to solidify the mold, but the tooling is inexpensive, and excellent dimensional accuracy and surface finish can be achieved with air set casting.

   

  Die Casting

  Die casting is another casting method that utilizes reusable molds. In contrast to permanent mold casting, die casting uses a high-pressure injection to quickly and completely fill the mold cavity. Die casting can provide excellent casting parameters, but start-up costs typically put it to use for high-volume runs.

   

  Metal casting: A modern manufacturing process with ancient roots

  Today, metal casting is a $33 billion industry, providing nearly 200,000 jobs in the United States alone. Castings are used in 90 percent of all durable goods, from construction equipment to medical devices. According to the American Foundry Association, "Most people are rarely more than 10 feet away from a casting." New technologies and improved processes have made metal castings better and more complex than ever before, which means foundries can expand into new markets and further increase the popularity of the metal casting industry.

   

  Foundry facilities regularly partner with the U.S. government and universities to ensure that the foundry industry benefits from cutting-edge research. One such collaboration with the U.S. Department of Energy has been successful in developing lighter castings, castings that require less post-treatment, and castings that use a wider range of alloys.

   

  Relatively new technologies, such as 3D printing, can be used in conjunction with metal casting to create prototypes, patterns and even molds faster than ever before. Advocacy groups are working to make this and other technologies that can help advance the entire foundry industry more widely available. The Steel Founders Association of America (SFSA) is currently developing a robotic cell that can be studied and adopted by foundry members. Their goal is to increase automation throughout the industry and make more casting work possible.

   

  Meanwhile, on the outreach front, the American Foundry Society (AFS) has undertaken a variety of programs designed to educate the public and U.S. legislators about the potential of metal casting. From working with the National Association of Manufacturers to promote Manufacturing Day to their annual Government Affairs Fly-In, the AFS has made it a priority to train the next generation of foundry professionals.

   

  After a long history of innovation, metal casting remains at the forefront of manufacturing technology. While the basic processes are the same as those used by the first generation of metal casters, casting has transcended its humble origins to meet the needs of modern consumers.

   

  Learn more about purchasing original and machined casting products.