What Is Metal Casting?
Metal casting is one of the most efficient and versatile manufacturing processes today – and with good reason. This technique, which involves pouring molten metal into a mold, is actually thousands of years old.
For the past 6,000 years, metal casting has been used to manufacture everything from swords and wedding bands to sprockets and screws. Today, metal casting is used to effectively make metal parts for a variety of industries.
Castings, or the metal pieces formed during the metal casting process, can vary in size from a few grams to thousands of pounds.
In this article, we’ll break down everything you need to know about metal casting, including what is metal casting, the history of metal casting, and the various types of metal casting.
Table of Contents
How does metal casting work?
Metal casting is a manufacturing process that involves pouring molten metal into a mold to create a 3D metal piece. Once the metal and the mold have both cooled, the metal object is extracted, deburred, machined, and finished. Since the process involves a custom mold, metal casting can be used to create metal objects with a large variety of shapes and sizes.
The basic metal casting process:
The metal casting process begins with a pattern, which is a model of the final part that will be made. Typically, patterns are made of wood, metal, or plastic and can also be made through machining or 3D printing.
Next, a mold is made using the pattern. Molds can be reusable, meaning they are used repeatedly to cast identical parts, or expendable, meaning they are used only once and destroyed in the casting process.
Melting and pouring the metal
Metal is heated until molten and poured into the mold cavity. Steel, stainless steel, aluminum, copper, magnesium, and zinc are the most common types of metal used in metal casting.
Once the metal has cooled and solidified, the final casting is removed from the mold. The last step is finishing, which includes filing, cleaning, and polishing the metal part. Final steps can also include machining and drilling.
The history of metal casting
Today, metal casting is used to make intricate parts for modern equipment, but the process is anything but new. In fact, it dates back over 6,000 years. The oldest known casting is a bronze frog that is estimated to have been made in 3,200 BC in Mesopotamia.
Since its invention, metal casting has made its way around the world, from Egypt to China to Europe and, eventually, America. America’s first casting facility was established in Saugus, Massachusetts, in 1645.
Fast-forward nearly 400 years, and metal casting has come a long way. Today, computer technology and 3D-printed patterns allow for much more intricate, detailed, and precise metal castings.
The different types of metal casting
Metal casting is divided into two main groups: metal casting using reusable molds and metal casting using expendable molds. Reusable and expendable molds are made of different materials, and each is suitable for casting different metal pieces.
Regardless of which is best for a particular project, metal casting is done by pouring molten metal into a mold, letting it cool and harden, then releasing the metal once it’s solidified.
We will discuss a few of the main types of metal casting below, including sand casting, die casting, and investment casting (lost wax casting).
Sand casting is one of the most cost-effective, widely used, and efficient casting processes. It’s also the oldest metal casting method – the earliest recorded use of sand casting is dated circa 1540.
As the name suggests, sand casting involves using sand to create a mold. In the sand casting process, a pattern is immersed in a sand mixture within a special two-part box called a flask.
Sand is packed around the pattern to create a mold. The pattern is then removed, leaving a mold cavity.
Next, molten metal is poured into the mold cavity, where it cools and solidifies.
Pictured above: Motor mount housing manufactured via sand casting
Green sand casting, air set casting, and resin sand molding are the three most common sand casting processes.
- Green sand casting is a method that involves moist sand.
- Air set casting uses chemical binders to bond the molding sand.
- Resin sand molding consists of a mixture of quartz sand and resin.
Almost every industrial business relies on the advantages of die casting for various components, making this one of the most important and most-used types of metal casting for producing high-quality, intricate parts.
Die casting involves forcing molten metal into a metal mold called a die. The metal is forced in at high pressure, which allows for casting extremely detailed parts with intricate features. After the molten metal cools and solidifies, the die is opened, and the part is ejected.
Each die, or mold, is custom-made, resulting in higher initial costs compared to other casting methods. However, die casting tends to result in lower per-unit costs for high-volume production since the molds, or dies, can be used repeatedly to cast identical parts.
Aluminum die casting is one of the more commonly used die casting manufacturing processes for creating high-quality aluminum parts. This process is ideal for creating:
- Extremely complicated shapes
- A great range of surface finishes
- Extremely lightweight components without sacrificing strength
- Corrosion-resistant parts and assemblies
Investment casting (lost wax casting)
Investment casting is a metal casting process that can create very precise metal parts with outstanding dimensional accuracy. Also known as lost wax casting, investment casting is a nearly 5,000-year-old process that’s best for complex shapes. Investment casting also produces a finer surface finish than sand casting.
The investment casting process begins by making a pattern out of wax. The wax pattern is dipped into a wet, clay-like material called a slurry. This is the investment material. To ensure a strong outer layer, the pattern is dipped multiple times.
Once the investment material hardens, the wax is heated to a high temperature so that it melts and drains away from the investment; hence the term “lost wax.” Molten metal is poured into the mold and, once it hardens, the ceramic shell is broken away.
Because of the smooth surface finish and dimensional accuracy, metal parts that are manufactured using investment casting tend to reduce the need for extensive finishing. This, in turn, leads to shorter production times.
What material is used in metal casting?
One of the benefits of metal casting is that a variety of metals can be used. However, the most commonly-used casting metals are steel, stainless steel, aluminum, copper, magnesium, and zinc.
Tin, lead alloys, and iron can also be used in metal casting. The temperature needed to melt the metal depends on the metal being used. Some metals take longer to heat than others.
Why choose metal casting?
Throughout its history, metal casting has been used to create components that would be too difficult or costly to manufacture by forging. By using metal casting, we’re able to accurately and effectively manufacture large numbers of identical metal parts.
Because every metal casting is made using a custom pattern and mold, metal casting is ideal for manufacturing both small and large items.
Additionally, since metal casting involves melting down pieces of metal, this process allows metal scraps to be recycled and reused, therefore reducing waste.
Why is metal casting important?
Metal casting is a vital part of the American industrial economy and provides nearly half a million jobs in the U.S.
Castings are part of almost 90% of all manufactured durable goods and can be found in everything from cars, trains, and construction equipment to household appliances, weapons, toys, and medical devices.
All in all, metal casting is approximately a $44.3 billion industry.