What is Metal Casting?
Metal casting is a modern process with ancient roots. In the metal casting process, metal shapes are formed by pouring molten metal into a mold cavity, where it is cooled and later extracted from the mold. Metal casting is arguably the earliest and most influential industrial process in history. It’s used to make many of the metal objects used in our daily lives: automotive parts, train wheels, lamp posts, school bus pedals, and much more. Plus, metal casting foundries rely on metal recycling as a cost-efficient source of raw material, significantly reducing wasted scrap metal that might end up in landfills.
The History of Metal Casting
The oldest known metal casting is that of a copper frog, believed to have been produced in 3200 BCE in Mesopotamia when copper was a popularly used material. Later, around 2000 BCE, iron was discovered. But it was not until around 700 BCE that the first production of cast iron was developed in China. Interestingly enough, the sand molding process of casting metals was also invented in China in 645 BCE.
The crucible process, a technique used for producing fine or tool steel, appeared and disappeared in various locations around the globe since the early 1st millennium CE. The technique popped up in India and central Asia first, until it appeared in northern Europe around 800 CE, where it was used to make Viking swords. The technique didn’t resurface until 1750 when Benjamin Huntsman reinvented it in England. Huntsman heated small pieces of carbon steel in a closed crucible, which is a ceramic pot with refractory properties that can withstand a high degree of heat. Huntsman achieved a temperature high enough to melt steel for the first time.
Over the past thousands of years, the casting of metals into usable objects has evolved to become more exact and automated, but the process at its core has essentially remained the same. Innovations in automation of foundry processes like the General Kinematics’ VIBRA-DRUM® Sand Casting Conditioner have made processing large quantities of castings easier as well as improved the quality of the castings. The machine is revolutionary in its high volume mold and sand handling for foundry applications.
Modern Day Castings
Today, nearly every mechanical device we use, from automobiles to washing machines are manufactured using metal parts that were created using the casting process. The difference between today’s cast metal products and those that were manufactured even 100 years ago is the precision and tolerances that can be achieved through the computerized automated design process, and modern methods for producing the detailed cores and molds. Modern-day metal casting represents innovation at work.
Comparing Casting Methods
Metal casting is a diverse process with several methods used to create metal objects. The choice of method depends on factors such as production volume, part complexity, material properties, and cost. Below is a detailed look at the most commonly used casting methods, their advantages, limitations, and best-use scenarios.
1. Sand Casting
Sand casting is one of the oldest and most widely used casting methods. It involves creating molds from a mixture of sand, clay, and water. This method is particularly effective for creating large parts and is well-suited for low-to-medium production volumes. The molds are made by packing the sand around a pattern, and once the mold is created, molten metal is poured into it to form the part. After cooling, the sand is shook out. Sand casting is highly versatile as it can be used for a wide range of metals, including iron, steel, and aluminum.
While sand casting is cost-effective, it does have some drawbacks. The surface finish can be rough, requiring additional post-processing, and the method does not provide the fine tolerances achieved by other methods, like die casting. Sand casting is ideal for large industrial parts where high precision is not a top priority, such as engine blocks, machinery parts, and piping. It’s most commonly used for low-volume, large-scale components.
2. Die Casting
Die casting involves injecting molten metal under high pressure into a steel mold, or “die,” to form precise and uniform parts. This method is highly automated and best suited for producing small to medium-sized parts in large quantities. The high pressure ensures that the metal fills every cavity of the mold, achieving high accuracy and smooth surfaces. Die casting is often used for high-volume production and is ideal for parts like electrical connectors, automobile components, and consumer electronics.
The main advantage of die casting is its speed and the ability to achieve high precision with minimal post-processing. However, it comes with high tooling costs due to the need for custom dies, which makes it more suitable for large production runs. Additionally, die casting is primarily limited to non-ferrous metals, such as aluminum, zinc, and magnesium, which can restrict its application in some industries. Despite these costs, die casting remains one of the most efficient methods for high-volume production.
3. Investment Casting (Lost-Wax Casting)
Investment casting, or lost-wax casting, involves creating a wax pattern of the desired part, coating it with a ceramic shell, and then melting the wax to create a hollow mold. This method is known for its exceptional ability to produce highly detailed, intricate parts with excellent surface finishes. The precision and detail of investment casting make it ideal for producing parts with complex geometries. It is widely used in industries like aerospace, medical devices, and jewelry, where high precision is required.
However, the primary drawback of investment casting is its high cost due to the labor-intensive process of creating the wax pattern and ceramic mold. Additionally, investment casting is most suitable for low to medium production volumes, as the costs associated with mold creation and wax pattern production can be prohibitive for large runs. Despite these drawbacks, investment casting is an ideal choice for parts requiring fine detail and complex designs, where other casting methods would not be effective.
4. Plaster Casting
Plaster casting is similar to investment casting, but instead of a ceramic shell, it uses a plaster-based mold. The pattern is typically made from wax or plastic, which is coated with plaster to form the mold. Once the pattern is removed, molten metal is poured into the mold to form the part. Plaster casting offers a balance between precision and cost-effectiveness, especially for non-ferrous metals like aluminum, brass, and copper.
This method provides good precision and surface quality, making it ideal for low to medium production runs. It’s particularly suitable for applications requiring a fine surface finish without the higher costs associated with investment casting. However, plaster molds are more fragile than ceramic molds, which limits the method’s use in high-volume production. It is most effective for artistic components, decorative items, and small industrial parts where cost efficiency is essential.
5. Centrifugal Casting
Centrifugal casting uses rotational forces to distribute molten metal evenly within a mold. The mold is spun during the pouring process, which ensures that the metal fills every cavity evenly and forms a strong, uniform part. This method is best suited for cylindrical parts such as pipes, tubes, and rings. The centrifugal force generated during the process helps eliminate air pockets and defects that can occur in traditional casting methods, resulting in a stronger, more consistent product.
While centrifugal casting produces highly uniform parts, it is primarily restricted to cylindrical or hollow shapes, which limits its versatility for other types of components. The main advantage of this method is its ability to create parts with superior strength and uniformity, which makes it ideal for applications like pipes, bearings, and rollers. However, the specialized equipment and molds required for centrifugal casting can be expensive, making this method best suited for high-strength parts with specific shape requirements.
6. Shell Molding
Shell molding is a variation of sand casting where a thin, strong shell mold is created using fine sand mixed with a binder. The pattern is coated with this material, and once the mold hardens, molten metal is poured into it to create the final part. This method is particularly useful for producing parts with high precision and a smooth surface finish. Shell molding is commonly used for complex parts that require fine details and tight tolerances, such as automotive components and aerospace parts.
The key advantage of shell molding over traditional sand casting is its ability to produce parts with better surface quality and greater accuracy. However, this process can be more costly due to the materials required for the shell mold and the labor-intensive nature of the technique. It is most effective for medium-volume production where precision is crucial, such as in aerospace, automotive, and industrial applications.
The Metal Casting Process
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- Patternmaking: A pattern is a replica of the casting’s exterior, made from wood, metal, plastic, or plaster. It’s crucial for precise industrial part-making.
- Coremaking: Cores are used to create hollow spaces in the casting. Made from sand or metal, they are strong yet collapsible for easy removal.
- Molding: Molding forms the cast around the pattern using molding sand. In sand casting, the pattern is surrounded by sand in a frame (flask), and once packed tightly, the pattern is removed, leaving a mold. Alternatively, reusable metal molds can be created for consistent part production.
- Melting and Pouring: Molten metal is poured into the mold cavity to solidify. After solidification, the shakeout process removes sand from the casting, and the sand is collected, cooled, and reused for future castings.
- Cleaning: The cast metal is removed from the mold and cleaned of any rough edges or molding material through fettling.
Common Metals Used In casting
Different metals are selected for casting based on their mechanical properties, resistance to wear and corrosion, and the specific performance requirements of the end product. Each metal has unique characteristics that influence how it behaves during the casting process and its suitability for various applications.
Aluminum
Aluminum is a lightweight, corrosion-resistant metal with excellent thermal conductivity, making it ideal for applications where weight is a concern and heat dissipation is necessary. Its versatility allows it to be used in automotive, aerospace, consumer electronics, and transportation industries. Aluminum’s ability to cast well at high volumes and its relatively low melting point makes it a popular choice for die casting and sand casting. Common products made from aluminum castings include engine components, cylinder heads, heatsinks, and aluminum wheels. Due to its excellent recyclability, aluminum is also a sustainable option for casting.
Iron
Cast iron is known for its durability and resistance to deformation, making it ideal for high-stress applications. The presence of carbon in cast iron gives it remarkable strength and wear resistance, though it can also make it brittle under certain conditions. Cast iron is widely used in applications such as piping, engine blocks, cookware, and machinery parts. It’s often favored in the automotive and construction industries for its ability to withstand high temperatures and heavy loads. Additionally, its excellent castability allows for the creation of intricate shapes with a smooth finish, often using sand casting or shell molding techniques.
Steel
Steel, known for its high strength and toughness, is a versatile material suitable for structural applications, load-bearing parts, and high-performance products. It is often chosen for parts that need to withstand extreme forces or environments that require a high level of mechanical strength. Steel castings are common in automotive engine components, machinery, construction equipment, and military hardware. Steel casting methods, such as investment casting and sand casting, allow for the creation of precise, strong, and durable parts. Alloyed steel castings can also offer additional benefits, such as enhanced resistance to corrosion, wear, or heat.
Copper Alloys (Bronze and Brass)
Copper alloys, including bronze (copper-tin) and brass (copper-zinc), are widely used for decorative, plumbing, and electrical applications due to their excellent conductivity and aesthetic appeal. Bronze is particularly favored for marine applications such as propellers and boat fittings due to its corrosion resistance in seawater. Brass is often used in fittings, fixtures, and musical instruments because of its workability and distinct golden appearance. Both alloys are cast using techniques like sand casting or investment casting and are especially valued for their malleability and resistance to wear.
Zinc
Zinc is a non-ferrous metal commonly used in die casting for parts requiring dimensional accuracy and tight tolerances. Zinc die-casting is popular for producing small, intricate parts such as electrical connectors, automotive components, and hardware products. Zinc alloys offer high strength-to-weight ratios and excellent corrosion resistance, especially when used in outdoor applications. Additionally, zinc has a relatively low melting point, making it well-suited for high-speed, high-volume casting operations. Products such as locks, keyboards, and home appliance parts are commonly made from zinc die castings.
Safety in Metal Casting
The casting process involves extreme heat, heavy equipment, and molten materials, making safety protocols essential. Foundry workers wear specialized personal protective equipment (PPE) such as heat-resistant gloves, face shields, aprons, and respiratory protection. Facilities must be equipped with proper ventilation, spill containment systems, and training protocols to prevent accidents and exposure to harmful fumes. Automation, like General Kinematics’ vibratory equipment, also plays a key role in reducing human exposure to high-risk tasks.
Proud to serve the Foundry Industry
General Kinematics is proud to be a leading manufacturer of automated vibratory processing equipment that has innovated alongside the foundry industry for nearly 60 years. To get more information about all of the General Kinematics foundry equipment, contact us today!
