Die casting technology: getting melt into shape

A central issue regarding the die casting process is the mould. It determines the contours of a die cast part and affects its properties. Mouldmaking still has a huge development potential, particularly in view of using additive manufacturing processes.

Die casting is a forming process for the mass production of parts made from aluminium, zinc, magnesium, copper, lead, tin and their alloys. The casting process takes place in die casting machines which are divided in hot chamber and cold chamber die casting machines. The main difference is that in hot chamber casting machines the container with the molten metal is located inside the machine, while in the other case the container is placed outside the machine. In both types of machines, the molten metal is pressed from a casting chamber through one or more casting channels into the cavity of a permanent steel mould where it takes the shape which is determined by the die and solidifies. These die casting moulds consist of two halves in order that the cast part can be removed from the mould. The feed side mould half is mounted on a fixed plate on the rigid side of the die casting machine, while the ejector side mould half is mounted on a movable plate placed on the other side. Before closing, the halves are sprayed with a release agent so that later the cast part can be easily released from the mould and the plates do not overheat. Depending on the size of cast parts, up to 300 casting cycles per hour can be carried out.

Extreme loads

When the mould is closed, the melt is pressed into the mould under a pressure of up to 1,200 bar, achieving maximum mould filling speeds of 150 m/s (540 km/h). High closing and clamping forces are required to press the mould halves against each other and keep the moulds closed: up to 8,000 kN (800 t) in hot chamber die casting machines and up to 45,000 kN (4,500 t) in cold chamber die casting machines. By using such high forces, large-sized cast parts can be manufactured. Concerning material and design, the moulds that are used for this purpose must be designed in such a way that they can permanently withstand the loads related to the large melt quantities. When the metal has solidified, the mould halves open and the cast part is ejected by bolts or removed by a robot and conveyed for further processing.

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