Metal Casting – Definition, Process & Types

Metal casting is a manufacturing process that involves melting metals and pouring them into moulds to form specific shapes. It is primarily used in the manufacturing industry to produce various parts but also in fine arts such as sculpting and jewellery making. In 2020, the iron and steel metal casting market was valued at USD 145.97 billion. The market is estimated to grow at a CAGR of 5.4% from 2021 to 2027.

The process of casting metals dates back over 7000 years. Over the centuries, the casting process has been continuously developed and refined. Today, it enables the repeatable production of highly complex shapes, some of which would be impossible to achieve through any other method.

It is now used so extensively that it is impossible to avoid cast products, no matter what type of environment you are in. Some examples of cast metal products include engine blocks, fire hydrants, motors, tools, traffic lights, manholes, pipelines, valves, and various fittings.

In this article, we’ll explore the metal casting process to understand what it is, explore its types and how they work.

What Is Casting?

The casting process enables the production of metal parts with desired shapes by heating the metals over their melting temperature and pouring the liquid metal into moulds. The molten metal flows into the empty cavities, conforming to the mould’s shape. Once the metal cools and solidifies, it is removed from the mould and sent for post-processing. In post-processing, the surface finish and appearance are enhanced and any necessary protective coatings are applied.

Metal casting is only limited by the manufacturer’s imagination. It can form highly intricate shapes.

Casting is preferred for complex geometries because it is more cost-effective and the process is straightforward compared to, for example, CNC machining in such instances. But it is also widely used for the simplest shapes because of its quick turnaround time and large production capacity.

Modern methods of casting can produce near-net shapes, significantly reducing the necessity for deburring and other post-processing operations.

Metal Casting Process

The reason the metal casting process became so popular was its inherent simplicity. There was no need for modern machinery or complex processes to obtain finished products.

Hence, even ancient armies were able to leverage it for weaponry and tool production. But over the years, the advances in casting have added some steps to the process to ensure that the final products have the best quality possible. Let us look at the typical metal casting process used in industries today.

Step 1: Mould Creation

The mould creation is arguably the most crucial step in the metal casting process. The quality of the mould directly impacts the quality of the finished product. The mould creation method varies depending on the type of metal, part geometry, production capacity, and the desired finish. Once these specifications are decided on, the appropriate materials and the method can be determined.

The mould is created as a negative impression of the desired part. It is made of a material that can not only withstand the high temperatures of molten material but also separate easily from the solid metal part after cooling. Some materials used for mould include sand, plaster, refractory slurry and ceramic liquid. Some moulds can only be used once, while others may be used repeatedly.

Step 2: Melting and pouring of metal

The casting metal is heated above its melting point to create a free-flowing liquid. The liquid must be free of contaminants and dross as much as possible. Arrangements can be added to the mould to prevent the entry of dross into the casting. The molten metal is poured into a pouring basin at the top of the mould. The liquified metal then enters the sprue (vertical channel) and is distributed into the mould cavity through runners (horizontal channels) and gates (entrances for the cavities).

Less viscous metals can flow easily into the casting and form finer patterns. Metals with high viscosity exhibit non-turbulent flow, which helps eliminate defects like porosity. The solidification rate and driving force are also influenced by the viscosity of the molten metal.

Step 3: Removal of the metal casting from the mould

In the case of a single-use mould, the mould is broken to retrieve the metal casting.

For permanent mould casting, there are various opening arrangements on the mould to access and retrieve the casting. The mould can then be reused on the next batches.

Step 4: Finishing and post-processing

The casting retrieved from the mould cannot be used in its current state. When the molten metal is poured into the mould, the liquid metal in the sprue, runners, risers and vents also solidifies. All of these appendages are cut off. Surface finishing may also be necessary.

The part is then sent for cleaning and coating if the application requires it.

Types of Casting Processes

Castings can be created from various metals like aluminium, zinc, steel and iron. Each metal possesses unique characteristics, necessitating adjustments in the casting technique to achieve the best possible product.

The casting technique also depends on parameters such as alloy composition, part geometry, cost, desired final quality, production run size, and performance criteria.

The casting processes are divided in two ways: the first is based on the lifespan of the mould (expendable and non-expendable mould casting) and the second is the method of alloy injection (gravity, pressure or vacuum casting). In this article, we will categorise the casting processes into expendable and non-expendable mould castings.

Expendable Mould Casting Process

Expendable mould casting processes use moulds that can only be used once. These moulds are used in various casting processes such as sand casting, shell mould casting, and investment casting among others. It must be noted, however, that the these moulds may not always be discarded after use. The manufacturers may try to salvage the moulds when mould reform is possible.

Let’s have a closer look at some of the processes using non-reusable mould castings:

Sand Casting

In sand casting, sand is used as the mould material. Sand is cheap, easily available, pliable and can withstand high heat better than most materials. Green sand (moist) and dry sand are the most suitable materials for the casting process.

A sand mould in the negative shape of the desired part is created and then the molten metal is poured into it. Once the metal solidifies, the sand is removed to retrieve the casting.

Sand casting is one of the oldest casting methods. It remains very popular to this day due to its simplicity, versatility and affordability.

Sand casting can produce products ranging in size from a few centimetres to tens of metres. Products can weigh as little as 75g and go up to several tons.

However, sand casting does not provide the best surface finish and is therefore more suitable for larger products that can be refined for a smoother finish later.

Plaster mould casting is a type of sand casting method where plaster is used to build the mould instead of sand.

Shell Moulding

Shell moulding is a type of sand casting process. However, instead of loose sand, metal casting occurs inside a thin-walled, hard sand shell.

To create this shell, a pattern must be created first. A pattern is an object that has the shape of the desired part. The pattern is heated and placed into the sand mixed with a thermosetting resin binder. The sand around the pattern takes on the shape of the part, and the resin hardens it. The shell is further cured in the oven. Once the shell is ready, it is placed in the baking material and molten metal is poured into it. The casting is retrieved upon solidification.

Shell moulding offers advantages such as lower manpower requirements (when automated), increased production capacity, and improved dimensional accuracy.

However, shell moulding is slightly more expensive than sand casting due to the resin used. It may also have lower material strength and high porosity issues. The labour costs can also end up being quite high if the process is not automated.

Investment Casting

Investment casting, also known as lost-wax casting or precision casting, uses wax to create accurate castings consistently.

The process starts with a wax pattern obtained through metal dies. Several wax patterns are assembled with gates, runners and a sprue. This allows to cast several parts simultaneously.

The assembly of wax patterns is dipped or “invested” in a refractory slurry that takes on the shape of the assembly.

The wax pattern, along with the slurry shell, is then heated. The wax flows out of the shell and can be reclaimed for reuse.

The slurry shell then forms the mould for the casting process. Molten metal is poured into the mould, where it solidifies into the required part.

Investment casting is highly precise. It is used for complex parts that require accurate parts, such as turbocharger rotors. It also provides a very good surface finish.

On the other hand, the process is more expensive than sand casting and shell moulding. It has limitations on some features, such as the size and depth of holes. Investment casting can be challenging to execute when cores are involved.

Non-Expendable Mould Casting Process

These casting techniques use permanent moulds that can be reused multiple times. There is no need to prepare a new mould for each casting, as is the case with expendable mould casting. This can be advantageous for castings that require a fine surface finish, high dimensional accuracy, and complex shapes.

The following casting processes can be classified as non-expendable:

Die Casting

The die-casting process uses a metal die to cast other metals. The die is usually made out of non-ferrous alloys of aluminium, copper and zinc, but ferrous metal dies are also used.

The metal dies are expensive to make but they last a very long time. This makes die casting more suitable for high-volume production.

Die-casting, compared to other metal casting techniques, is more suitable for small to medium-sized parts that require a high-quality surface finish.

Die casting processes are of two main types: gravity die casting and pressure die casting. Gravity die casting uses the force of gravity to fill the mould with molten metals. The channels and the mould cavities are designed in a way that allows the metal to flow naturally into the pockets.

Pressure die casting injects molten metal at high pressure to fill the mould. This process offers greater flexibility in mould design but is more expensive and skill-based.

Pressure die casting is more suitable for complex geometries, while gravity die casting offers greater value for simpler designs.

Centrifugal Casting

Centrifugal casting uses centrifugal force to cast circular parts. The process is generally used for creating standard-sized castings, such as pipes, and not finished products.

As the process leverages gravity and pressure, it creates very fine designs with remarkable accuracy.

The molten metal is fed through the centre and the spinning motion of the mould pushes the metal to the edge, forming an annular shape. By adding various materials successively, it is possible to produce pipes with multiple layers. For instance, centrifugal casting can create cast iron pipes with a cement lining on the interior to prevent corrosion and subsequent tuberculation.

The centrifugal force can also push viscous molten metal into extremely fine cavities. The metal can be moulded into intricate shapes and form complex designs. Thus, this concept is commonly used to create jewellery as well.

The casting machines may be horizontal or vertical. Horizontal machines are suitable for long products like pipes, whereas vertical axis machines are more appropriate for shorter products such as bearings.

Centrifugal casting offers advantages such as not requiring cores, producing excellent grain structure, and having no limitations on dimensions like length, wall thickness and diameter.

Centrifugal casting is the best alternative for specific applications, such as thin-walled cylinders. However, it has disadvantages such as shape constraints, inability to work with smaller diameters and compatibility with limited alloys. Impurities gather at the centre and are thus generally removed by machining.

Continuous Casting

Continuous casting, also known as strand casting or slab casting, produces a continuous casting of constant cross-section.

Molten metal from a furnace is carefully poured into a tundish, from which the metal is transferred to a water-cooled mould with the desired cross-section. The cooling speeds up the solidification of the molten metal.

While the metal solidifies, it is also pulled out of the mould through rollers, where the casting may be cooled and shaped further to final dimensions. Once the solidification is completed, the casting is cut into standard dimensions.

Continuous casting offers a high production rate, increased accuracy, reduced waste, and enhanced grain structure.

However, the process requires a high initial investment, and the final product may have internal defects and poor surface quality.

Summary

Metal casting is a proficient and cost-effective method for producing dependable metal products. It offers a wide range of processes. Each process has pros and cons that make it suitable for specific applications.

For instance, sand casting is perfect for one-off complex pieces, whereas die casting is more appropriate when intricate products are needed at a high production rate.

Moulds used in casting are categorised as expendable moulds and non-expendable moulds. Expendable moulds are destroyed after each use, such as in sand casting, while non-expendable moulds are more permanent and can last from a few days to a few years.

The versatility of metal casting makes it an essential metalworking technique for various industries, from automotive to aerospace, ensuring the high accuracy and repeatable production of complex parts.