Centrifugal Casting – Working Principle, Types & More

Centrifugal casting is a distinct casting process that uses kinetic energy from spinning dies to shape molten metal into cylindrical products. The process offers great reliability and precision and is cost-effective in mass-production scenarios.

In this article, we’ll look into centrifugal casting process to understand its operation in detail, its types, and the benefits it offers.

What Is Centrifugal Casting?

Centrifugal casting is a metal casting process that uses a spinning die to produce cylindrical products like pipes and tubes. During this process, molten metal is poured into a spinning die. As the metal flows into the spinning die, centrifugal force distributes it uniformly along the inner diameter of the die.

The thickness of the casting can be accurately controlled by regulating the amount of metal poured into the cylinder. The liquid metal starts cooling and solidifying from the outside surface in contact with the cylinder.

While cylinders can be cast through other methods, they would have limitations on size, tolerances, seam strength and corrosion resistance. The formation of internal cavities would require cores and the final part may exhibit issues such as gas porosity, oxides and non-metallic inclusions.

Centrifugal casting, on the other hand, has no limitations on the length of the finished part. It allows the casting of very long parts with remarkable accuracy and consistency. Most machines can produce pipes up to 15 meters (49 ft) in length with a diameter of up to 6 meters (20 ft). The wall thickness can vary from 2.5 mm (0.1 in) to 125 mm (~5 in).

The process works best for thin-walled cylinders but it can also be used for thick-walled applications such as gear blanks, ball bearings, railway carriage wheels, and more. Centrifugal casting can also produce non-circular shapes with a near-constant radius.

Multi-layer casting is another unique feature of centrifugal casting. In this casting technique, two dissimilar materials can be combined to form a multi-layer centrifugal casting. This finds many applications in several industries. For example, steel pipes can be coated on the inside or outside with concrete to enhance their corrosion resistance and compressive strength.

Common applications of the centrifugal casting process are in piping, aerospace, automotive and military sectors. Some common products made using this process include engine cylinder liners, jet compressor cases, metal pipes, railway carriage wheels, bearings and many other tubular parts in high-reliability applications.

Centrifugal Casting Process

Centrifugal casting is a fairly straightforward process. Most often, the process is used to create standard-sized products rather than specific ones. Standardisation allows the process to take advantage of economies of scale and reduce part costs.

Let’s explore a typical centrifugal casting process, its stages and their significance. The four stages are as follows:

1. Mould preparation

2. Pouring of molten metal

3. Directional solidification

4. Removal and finishing operations

1. Mould Preparation

The mould preparation begins with cleaning the die’s inner surface to remove contaminants and debris from previous use. Once it is clean, the motors responsible for spinning the die are powered up.

The amount of force applied to the molten metal is proportional to the die’s diameter and spinning RPM. Typically, a die has a rotational speed between 300 and 3000 RPM. The centrifugal force produced by these high speeds can reach up to 100 times the force of gravity.

Then, it is time for preheating and applying a ceramic slurry to the inner surface of the die. The heat dries and cures the ceramic slurry, ensuring that the refractory lining adheres to the die’s surface.

The ceramic slurry is necessary to protect the die and facilitate the removal of the casting upon solidification. An ideal slurry also provides a smoother surface finish for the part.

2. Pouring of Molten Metal

The metal is melted in an external furnace and poured into the pouring basin of the casting machine using a ladle. The pouring basin then transfers the metal to the centre of the die through a spout. The high centrifugal forces ensure that the final product is free of cavities or gas pockets.

The centrifugal force also separates the low and high-density components in the molten metal. All the impurities float towards the inner diameter of the molten metal due to their lower densities. This results in an increased density of the molten metal at the outer diameter compared to the inner diameter.

3. Directional Solidification

The metal casting rotates continuously during the rapid cooling stage until its temperature is below the transition range. Cooling in centrifugal casting begins from the outside diameter towards the inside diameter. This type of controlled solidification in castings is also known as directional solidification.

Directional solidification offers advantages such as improved grain structure, lower concentration of impurities and the development of beneficial microstructures. Solidification shrinkage always occurs at the inner wall of the finished product in centrifugal casting.

4. Removal and Finishing Operations

Once solidification is complete, the casting is removed from the die using extraction equipment if no heat treatment is needed. In some cases, the casting is subjected to annealing before the steel mould is removed.

It is then sent for finishing operations. The impurities and lighter material that floated inward during the second stage are now machined, leaving a strong, defect-free part with excellent mechanical properties. The outside is shot blasted to remove any traces of the refractory material.

Visual and dimensional inspections are also carried out at this stage.

Types of Centrifugal Casting

Centrifugal casting processes can be classified in different ways. One method is based on the nature of the casting process, where they are divided into true centrifugal casting, semi-centrifugal casting and centrifuging processes. A second, more common method is to classify them based on the die’s orientation, i.e., horizontal and vertical centrifugal casting processes.

The processes between the two classification methods can overlap. For instance, a true centrifugal casting may involve either a horizontal or vertical centrifugal casting process.

Classification Based on the Nature of the Process

True Centrifugal Casting

The true centrifugal casting process is the most popular type of centrifugal casting process. This is the same process described in the previous section. Unlike other processes, this method does not involve a gating system. The material is poured directly into the cylindrical mold, where it contacts the mould’s internal surface and spreads uniformly.

Semi-Centrifugal Casting Method

The semi-centrifugal casting method is similar to the true centrifugal casting process, except that the former creates solid castings, unlike the latter process which produces only tubular shapes. It handles the production of solid components such as spokes, gear blanks and pulleys.

The material away from the rotational axis is denser compared to that around the axis. Impurities, due to their low density, also accumulate around the axis. After solidification, the material around the axis is machined away to make room for the axis on which the produced component will spin.

Semi-centrifugal casting provides us with a product that has excellent mechanical properties and is free of impurities.

Centrifuge Casting

Centrifuging is a unique form of centrifugal casting method that uses centrifugal forces to fill conventional moulds instead of applied pressure. In this process, we use a rotating mould with a central sprue at the axis. The sprue channels the molten material into the rotating mould as it spins.

The centrifugal force from the rotations pushes the molten metal outward and into the cavities spread along the circumference. This process includes all the components of the gating system, such as sprue, runners, gates and risers.

The spinning motion of the die pushes the material into the die until complete distribution and solidification occur. The centrifuging process is used for small-sized products such as jewellery, sleeves and bushings.

Classification Based on Die Orientation

When designing a die for centrifugal casting, we can choose to align it either vertically or horizontally. Each orientation has its benefits and limitations:

Horizontal Centrifugal Casting

Horizontal centrifugal casting refers to the casting process in which the mould’s spin axis is parallel to the ground. The horizontal axis arrangement enables the production of castings with a very high length-to-diameter ratio. Horizontal centrifugal casting is a cost-effective method that delivers high-quality results.

Some common products made using the horizontal centrifugal process include ductile iron pipes, reformer tubes, furnace rolls, sleeves and roller shells.

Vertical Centrifugal Casting

In vertical centrifugal casting, the mould spins along the vertical axis, perpendicular to the ground. This technique is more suitable for products with a short length-to-diameter ratio. Typically, the length should be less than twice the product’s width for optimal results in vertical centrifugal casting.

The effect of gravity needs to be accounted for in vertical centrifugal castings, as it can lead to distortion in the absence of sufficient centrifugal force.

Some examples of products made with this manufacturing process include ball bearings, gear blanks and pulleys.

Advantages of Centrifugal Casting

Centrifugal casting is a special type of casting process that offers unique advantages compared to other casting processes:

• Unrivalled rotational symmetry in parts

• Fine-grained structure

• Ability to seamlessly integrate two materials

• Can work with a variety of materials besides metals, such as glass and concrete

• Eliminates cores and gating system

• Lower manufacturing costs and reduced machining requirements

Limitations of Centrifugal Casting

Each manufacturing process has its limitations, centrifugal casting is no different:

• Restrictions on complex shapes: Centrifugal casting is limited to producing cylindrical castings and, to some extent, other shapes. Even within cylindrical castings, the complexity achievable through this process is restricted.

• Inaccuracies in the inner surface diameter are fairly common. However, it is possible to generate net or near-net shapes with the right controls.

• The process requires a high initial investment. However, similar to die casting, it boasts low per-part costs, making it cost-effective in mass production scenarios.

• Parts with small inside diameters are difficult to cast.

• Operational safety can be a concern with this process because the die operates at high rotational speeds along with other rotating and moving parts.

• The centrifugal process is not particularly effective for small-sized products.