Do you love metal as much as we do? This guide was created to help you learn more about one of the main metalworking processes - casting. Up to 90% of all manufactured goods and machinery have at least one part that was made using casting, with sizes varying anywhere from a small fastener to important components of industrial-grade engines. In this guide, you will gain an insight into the main features, advantages and disadvantages of casting metal in comparison to other manufacturing technologies.
Metal casting is the process of forming an object from molten metal that was poured into a mold which is then solidified. It’s one of the most ancient technologies for metal forming, dating back thousands of years. Metal casting is often used for forming parts with complex shapes that are difficult to produce using other technologies. It’s one of the most common methods for manufacturing train wheels, gears, and other durable metal components. Some of the leading markets for metal casting are heavy equipment, transportation, and machine tools.
What kind of metal can be used for casting?
Among the most frequently casted metals include iron, aluminum, tin, zinc, magnesium, copper and lead alloys. Gray iron (iron alloy with a graphitic microstructure) is widely used in casting for industrial manufacturing. Ductile iron, as its title suggests, is used for applications that require greater strength than that provided by gray iron. For jewelry, gold, silver and other precious metals are used.
Expendable and non-expendable mold casting
Put simply, a mold is just a hollow container that holds and shapes materials in a liquid state (e.g. wax or metal). Casting molds are generally divided into two main categories: expendable and non-expendable.
Expendable mold casting is a generic classification that includes sand, plastic, shell, plaster, and investment (lost-wax technique) moldings. This method of mold casting involves the use of temporary, non-reusable molds.
Prominent technologies of metal casting
Investment casting (also known as lost wax casting)
Investment casting involves investing (surrounding) a pattern with a refractory material and can be applied for casting stainless steel, brass and aluminum alloys. The liquid metal is poured into a cavity in a refractory material. Investment casting creates products with an exceptionally high-quality surface.
Main features:
- High precision
- Perfect for parts with complex forms
- High-quality surface finish
- Suitable for both ferrous and non-ferrous metals
Die casting
During the die casting process, molten metal is forced into the cavity under high pressure, usually using nonferrous metals such as zinc, copper, and aluminum-based alloys. Die casting is widely used for applications where many small parts with numerous details, as well as a high-quality surface are required.
Main features:
- High precision
- Suitable for small parts with many details
- Does not require extensive machining
- Perfect for high-volume manufacturing
Sand casting, also known as sand molded casting, is a metal casting process characterized by using sand as the primary material for molds. The main part of all metal castings are manufactured via sand casting. Despite the fact that sand molds can only be used just once, they are still much cheaper than other types of molds. For this method, sand is mixed with a bonding agent like clay. Following the spread of 3D printing technologies, sand molds can also be made by forming sand around 3D printed patterns. The most common type of sand casting is called green sand molding and is frequently used to make molds for small to medium-sized metal parts. Green sand is an aggregate of sand, bentonite clay, pulverized coal, and water.
Main features:
- Perfect for large parts
- Can form complex shapes
- Suitable for a wide range of materials
- Relatively low cost
- Scraps can be recycled
Lost Foam Casting
Lost foam casting employs the use of expanded polystyrene (EPS) foam patterns. The first step of this process is creating a foam tool from which foam pieces are made. After the foam is ready, it’s coated in a refractory wash and placed inside a flask. When dry, loose sand is poured around the foam while the flask is vibrated. The liquid metal is then poured into the flask, burning up and replacing the foam. After cooling, the part can be post-processed.
Main features:
- No need for additives, binders or cores
- Minimal waste
- Flexibility in casting design
- Highly cost-effective
- Good for complex forms
- Can be used for parts of various sizes
- Does not require extensive machining
Which method of metal casting is the best?
Investment casting or Die casting?
There is no perfect solution that can be applied to all products because to an extent, all products differ from one another. However, one thing can be said for sure - in cases where design flexibility is required for mass-production, investment casting is the best choice. It also offers a high level of precision, the ability to create complex geometries, as well as thin-walled parts. Generally speaking, the smaller the casting, the higher the dimensional accuracy required. Significantly large investment castings might lose some dimensional accuracy and that’s why die casting could be a better option for relatively large-scale pieces. Investment casting happens to cost more than die casting because it's a highly manual process that is capable of producing parts with superior accuracy and excellent surface finishes. However, die casting comes with higher tooling costs and typically requires at least some secondary machining to adequately finish the product. For these reasons, die casting is the most cost-efficient for high-volume runs.
Sand casting or investment casting?
The typical surface finish of sand castings needs extensive post-processing work to create a surface finish that is acceptable for most applications. Besides, ceramic molds that are used in investment casting can produce a much smoother finish. As a rule, investment castings have better precision, and almost certainly will need less machining than sand casting.
CNC machining or metal casting?
Machining is mostly used for manufacturing simple parts and due to producing a lot of waste, it can result in higher costs. Machining can also be quite a lengthy process, resulting in a slower turnaround and more expensive parts.
Die casting or 3D printing with metals?
Additive manufacturing provides engineers with a high degree of design flexibility by allowing them to create almost any kind of mold with 3D printing. In this area, additive manufacturing is unparalleled by other manufacturing technologies. However, when it comes to high-speed and large-scale productions, metal die casting is second-to-none and is capable of producing increasingly complex parts with high repeatability and accuracy. It offers multiple finishing techniques, achieves tight tolerances, and provides strong and durable parts. On Treatstock, you have access to a wide range of manufacturing technologies to produce molds and metal parts.
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