CENTRIFUGAL CASTING

The centrifugal casting process, or rotocasting, which involves the pouring of molten metal into a rapidly rotating mould, was patented in England in 1807. The first industrial application of the process was in 1848 in Baltimore, when centrifugal casting was used to produce cast iron pipes. Centrifugal casting is the most economical method of producing a superior-quality tubular or cylindrical castings with regard to the casting yield, cleaning cost, and mould cost.

The essential feature of centrifugal casting is the introduction of molten metal into a pre-heated mould which is kept rotating even during solidification of the casting. The centrifugal force produced by rotation is large when compared with the normal hydrostatic forces, and is utilized in two ways:

• Uniform distribution of the molten metal over the inner surfaces of a mould, forcing them to deposit on the mould walls;
• Development of high pressure in the casting during freezing – this assists the metal feeding and accelerates the separation of non-metallic inclusions and precipitated gases. These have a density lower than the cast metal, and are therefore extruded from it onto the inner surface of the casting

The mould material is determined partly by the shape and partly by the number of the castings required. For simple shapes, the metal moulds are usually preferred on economic grounds, as well as for their effect in inducing directional freezing. For more complex shapes or for the parts required in small quantities, refractory moulds are normally used. These may be based on the high-strength conventional moulding mixtures or on the investment moulds. A steel mould case with inner refractory linings of different thicknesses is usual. Graphite moulds are also used. Graphite has excellent thermal conductivity and resistance to thermal shock, and it is easily machined. Besides, graphite does not react with most of molten metals.
Moulds are usually cooled by a water spray. The machine must be equipped so that to pour the molten metal into the rotating mould. Special tools, either mechanical, hydraulic or pneumatic, are used to remove casting from the mould after solidification. There are two basic types of centrifugal casting machines:

• Horizontal type, rotating around the horizontal axis, and
• Vertical type, rotating around the vertical axis.

In general, horizontal axis machines are preferred for the castings of the high length to diameter ratio, whilst the castings of the diameter exceeding the length are more commonly spun around the vertical axis; although, there are certain exceptions in both cases. Horizontal centrifugal casting machines are generally used to make pipes or other cylindrical or tubular castings that are simple in shape. The application range of the vertical centrifugal casting machines is considerably wider: they enable producing the castings that are neither cylindrical, nor even symmetrical. The centrifugal casting process can be divided into three major groups based on the utilization of the centrifugal force: true centrifugal casting, semi-centrifugal casting and centrifuging.

True Centrifugal Casting

The key feature of this process is production of a symmetrical hollow structure without using any cores. It is achieved by the pure centrifugal force with the mould rotating around its vertical or horizontal axis. The wall thickness is determined by the volume of the metal introduced. The shape of the mould can be circular, square or hexagonal – as long as it is symmetrical around its axis of rotation. The mould is subjected to high-speed rotation to produce the centrifugal force 100 times higher than the gravitational force.

Semi-centrifugal casting

The centrifugal force from the rotation is used only to fill the mould perfectly and to act on the molten metal as a source of pressure during solidification. The castings are shaped wholly by the mould, and the core is placed inside the mould to produce a hollow structure if needed. The vertical spinning axis is normally used at a slower mould rotation speed than it is in the case of the true-centrifugal casting. The process of semi-centrifugal casting is used to produce large, axially symmetrical products.

Centrifuging

The casting cavities are arranged about the central down-runner which forms the centre axis of rotation, thus permitting the production of multiple castings. The rotational axis is usually vertical to facilitate the mould assembly, and the castings are shaped wholly by the mould and cores. Centrifugal force provides the necessary pressure on the molten metal in the same manner as in the semi-centrifugal casting. Of all three methods, the centrifuging finds the widest field of application.